Longevity and Recyclability Study of Acid Activated Alkaline Sludge From Photovoltaic Industry Toward Co2 Capture

Longevity and Recyclability Study of Acid Activated Alkaline Sludge From Photovoltaic Industry Toward Co2 Capture | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Longevity and Recyclability Study of Acid Activated Alkaline Sludge From Photovoltaic Industry Toward Co 2 Capture Siti Sarahah Sulhadi, Azizul Hakim Lahuri, Syawal Mohd Yusof, and 5 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8421032/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 6 You are reading this latest preprint version Abstract Recycling industrial wastage offers an effective strategy to mitigate waste discharge, promoting development of low-cost CO 2 adsorbents aligning with the principles of the circular economy. This study utilized waste alkaline sludge (AS) originated from photovoltaic industry, activated with 1-4M of hydrochloric acid (HCl), to study its effect on CO 2 capture performance. Activated AS were characterized with N 2 adsorption-desorption isotherm (BET-N 2 ), X-Ray diffraction (XRD), field emission scanning electron microscopy (FESEM-EDX), CO 2 adsorption-desorption isotherm (BET-CO 2 ) and CO 2 temperature programmed desorption (TPD-CO 2 ). AS activated by 2M HCl exhibited highest surface area of 123.73 m 2 /g and dominated by mesopores which played a significant role in CO 2 adsorption. CO 2 capture by physisorption at 25 ºC exhibited adsorption capacity of 0.44 mg/g, which was 20 times increment than. Meanwhile, CO 2 capture performance by chemisorption was 284 mg/g, with temperature ranging from 207–644 ºC and around 9 times higher than inactivated AS. Longevity study revealed that weight loss after prolonged CO 2 exposure for 24 hours remain around 1.4%, indicating adsorption stop after 24 hours. Longer exposure time induce morphological transformation from irregular ellipsoid into packed and aggregated nano coral, thus lessening its adsorption capacity. It was determined that 25 ºC was the optimal temperature for both adsorption and regeneration process. The adsorbent also demonstrated stable recyclability for 5 cycles, showing only 14% capacity reduction at 2nd cycle and achieving a regeneration efficiency of 88.6%. Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 1. INTRODUCTION Recycle of industrial wastage can help with reduce waste discharge into the environment. Utilization of industrial derived waste can represent significant step towards achieving SDG 12 (responsible consumption and production). In 2017, total production of global solid waste including industrial was 20 billion tonne and projected to increase to 46 billion tonne following the present production patterns (Maalouf & Mavropoulos 2023 ). These wastes are presently managed through various disposal and treatment approaches, including landfilling, open dumping, composting, recycling, and incineration. Some disposal methods such as landfills, dumps and incineration usually will end up polluting the environment and affecting human health. Rather than disposing of this low-cost waste, converting it into high-value products provides sustainable materials for carbon capture, utilization, and storage (CCUS) applications. CCUS applications are crucial tools in meeting the standard set by Paris Agreement that focuses on limiting global warming to 1.5 ºC by reducing greenhouse emissions. These global treaties primarily aim to reduce CO 2 concentration in the atmosphere to limit the greenhouse gases that are affecting the global climate. Based on NOAA observation, CO 2 concentration in the atmosphere reached 430 ppm at May 2025 for the first time ever since it is start recorded in 1958 (Monroe 2025 ). CO 2 gas, acting not only as the biggest greenhouse gas emitter but also identified as amplifier of global warming in 2020 (NOAA 2021 ). Although CO 2 capture technology remains in the developmental stage, it has undergone substantial advancements and global expansion. Several methods such as chemical absorption, physical adsorption and absorption, membrane separation and cryogenic separation are widely recognized in CO 2 capture research. Furthermore, established technologies such as amine scrubbing, oxy combustion capture, direct separation and calcium looping are currently used in cement plants worldwide (Hanifa et al. 2023 ). One of the most widely used CO 2 capture technology, amine scrubbing, present some serious drawback to the environment such as release of ~ 80 tons of MEA into the atmosphere with every million tons of CO 2 captured (Dhanraj & Biswas 2020 ). Since amine scrubbing also uses liquid as absorption medium, degradation and corrosion of equipment was also one of the many concerns. In this research, we proposed utilization of solid alkaline sludge from photovoltaic industry as main material for CO 2 absorbent as possible solution for arising problems stated. Utilizing wastes not only supports circular economy by minimizing waste into landfill, but they also reduce reliance on new raw materials, thereby preserving finite resources. Additionally, low procurement cost of raw materials is an added benefit for waste utilization. Most solid wastes are produced from anthropogenic activity among which are agriculture, industrial, biomass and municipal. Figure 1 summarizes the development of solid waste utilization in global CO 2 capture studies from 2020–2025, illustrating the increasing focus on converting waste into value-added materials. The industrial waste-derived materials are commonly collected from various industries such as explosive, paper manufacturing, polyvinyl chloride (PVC) production and sewage sludge which constitute 16 out of 58 research papers as shown in Fig. 1 . Common industrial waste such as bottom ash has been recycled into CaO-based sorbents, demonstrating superior CO 2 uptake capacity of 140 mg/g that can withstand 20 cycles of adsorption (Zhang et al. 2023 ). Other industrial waste such as fly ash turned zeolite can had sorption capacity ranging from 10.6-183.1 mg/g (Czuma et al. 2020 ). Glass industry that produced waste with high content of SiO 2 and Al 2 O 3 had also been tested for CO 2 capture. CO 2 capture capacity of said waste can reached 42.7 mg/g (Ramos et al. 2022 ). Waste produced from energy explosive industry has also been studied for its application in CO 2 capture. Prepared graphene synthesized from solid waste collected from explosive industry by Aquatar et al. ( 2021 ) presented high surface area and CO 2 capture capacity can reached up to 1040 mg/g (303 K, 20 bar). Alternatively, carbide slag produced from polyvinyl chloride (PVC) industry had also been studied as CO 2 capture material since the waste is composed mainly from Ca(OH) 2 . Ma et al. ( 2019 ) found carbide slag-based sorbent can had CO 2 capacity uptake of 290 mg/g even after 30 cycles of adsorption. Next, paper sludge ash (PSA), an alkaline byproduct from paper manufacturing, utilized for its favorable properties for CO 2 absorption. Kim & Kim ( 2018 ) found that capturing capacity of CO 2 by paper sludge waste can go up to 324 kg CO2 /ton PSA . Other potential waste is sewage sludge ash (SSA) that also contain high amount of Ca and its utilization found CO 2 uptake of 20 kg CO2 /ton SSA (Massa et al. 2025 ). Other similar study that implement sewage sludge found that CO 2 capture capacity was 23.3 mg/g that also possess high surface area and pore volume after chemical activation (Miricioiu et al. 2021 ). Nevertheless, the aforementioned industrial waste-derived materials are subjected to pre-treatment, surface activation and modification to enhance surface properties, improve active sites for CO 2 adsorbate, removal impurities and identify heavy metals presence. Common approaches such as acid and base activation of waste materials have been shown to enhance the surface area and improve overall physical properties. Given the chemical stability of CO 2 , the selection of adsorbent materials depends on several critical factors such as low-cost, high CO 2 /N 2 selectivity, availability, low affinity to moisture, fast kinetics, regeneration ability, mechanical and thermal stability, low heat of adsorption, high surface area and high pore volume (Abuelnoor et al. 2021 ). Industrial waste activated by HCl had considerable success in various applications, especially CO 2 capture study. Past research had found sewage sludge that had been treated by both KOH and HCl had increased surface area from 2.9 m 2 /g to 921 m 2 /g, the highest surface area obtained compared to other 3 samples treatment with only different concentration of KOH (Miricioiu et al. 2021 ). de Andrés et al. ( 2013 ) discovered that sewage sludge activated by NaOH enhanced the surface area of 179 m 2 /g, with CO 2 adsorption capacity of 56 mg/g. The conversion of sewage sludge into biochar reported had CO 2 adsorption capacity ranged from 3.88 mg/g to 4.88 mg/g with surface area of only 10.12 m 2 /g (Xu et al. 2016 ). A similar study reported that carbon derived from waste tire activated by KOH achieved CO 2 sorption capacity that can reached up to 238.7 mg/g (Duduku et al. 2019 ). Fly ash (FA) is another popular waste derived material for CO 2 capture, utilization and usage, which had major component of SiO 2 , Al 2 O 3 , Fe 2 O 3 , CaO, MgO, SO 3 , Na 2 O and K 2 O (Ahmaruzzaman 2010 ). Lee et al. ( 2014 ) applied dry sorbent manufactured with dry mixture of fly ash, NaOH and CaO as CO 2 absorbent. The study found that addition of FA helped increased 9% higher adsorption capacity than samples without FA. The approach proved addition of FA helped dispersion of active site for CO 2 adsorption and optimized surface area. Table 1 Adsorption capacity of CO 2 capture for various wastes activated by different acids Source of waste Activating agent CO 2 capture capacity (mg/g) CO 2 capture condition References Pressure (bar) Temperature (ºC) Carbide slag Acetic acid 290 n.a. 700 (Ma et al. 2019 ) Sewage sludge ash H 2 SO 4 2.4 14 25 (Massa et al. 2025 ) Palm oil fly ash HCl 282.5 4 32 (Kongnoo et al. 2017 ) Molasses H 3 PO 4 2.8 1 0 (Kiełbasa et al. 2022 ) Molasses HCl 1.7 1 0 (Kiełbasa et al. 2022 ) Molasses H 2 SO 4 195.4 1 0 (Kiełbasa et al. 2022 ) Sewage sludge HF 127.6 0–50 30 (Ma et al. 2022 ) Sewage sludge KOH/HCl 11.9 0.34 23 (Miricioiu et al. 2021 ) Entada rheedii shell HCl 193.6 1 50 (Mallesh et al. 2020 ) Municipal solid waste H 2 SO 4 114.4 3 40 (Karimi et al. 2020 ) Rubber-seed shell waste Malic acid 117.3 1.25 25 (Borhan & Yusuf 2020 ) Coal fly ash/CaO Acetic acid 560 1 650 (Nawar et al. 2020 ) Therefore, in this study, we aim to develop alkaline sludge (AS), major wastes from photovoltaic industry (PV) activated by HCl into efficient CO 2 capture material. Based on past research, there is underutilized application of alkaline sludge in CCUS. The enhancement of the textural properties and efficiency in CO 2 uptake capacity of activated sludge was investigated through this study. Besides that, the stability of the AS was evaluated through recyclability study. 2. METHOD 2.1. AS ACTIVATION Alkaline sludge, AS, was collected from photovoltaic solar cell factory located in Bintulu and dried at 110°C for 24 hours following calcination step at 900°C in air prior to activation process. Calcined AS denoted as CAS for the rest of the study. Elemental composition of calcined AS was verified by XRF analysis and was dominated by SiO (56.3%), CaO (30.6%), Na 2 O (3.24%), Fe 2 O 3 (2.61%), Cl (22.5%), MgO (2.14%), and Al 2 O 3 (1.83%). The process followed with activation with HCl (Chemiz, Malaysia). Various concentrations of HCl were prepared (1M-4M) before 10 ml of prepared acid solution mixed with 1 g of CAS. Sample was then thoroughly stirred at 100°C for 24 hours in Teflon bottle. After stirring, samples were cooled, filtered and washed until neutral using distilled water. Samples were then dried in the oven at 110°C overnight before being kept in air-tight container for later use. Samples were denominated following the acid concentration in which CAS was activated, such as AS_HCl_1M, AS_HCl_2M, AS_HCl_3M and AS_HCl_4M. The effect of acid concentration toward activation of AS was analyzed and characterized. 2.2. CHARACTERIZATION The crystal structure and chemical composition of the sample was determined using X-Ray Diffraction (XRD) analysis from a model Bruker D8 Advance with Cu as an x-ray source. The diffraction result was matched with the International Centre for Diffraction Data (ICDD) database to determine the composition of the samples. Determination of surface area and pore size were evaluated from N 2 adsorption-desorption isotherm. The analysis was conducted using a volumetric static technique from a Micromeritics TriStar II Plus. Around 0.2 g of sample was placed in quartz tube and was first degassed at 150°C to remove moisture and trapped gases for 6 hours in N 2 atmosphere. Degassed sample was then immersed in liquid nitrogen circulated bath at temperature of -196°C for the analysis process. Surface area (S BET ) and pore size was obtained from Brunauer-Emmett-Teller (BET) method. Micropore surface area (S mic ) was obtained from t-plot method while pore size distribution obtained from Barrett-Joyner-Halenda (BJH) method. Morphology of the activated samples were observed using field emission scanning electron microscopy (FESEM-EDX) by model Merlin at various magnification. Functional groups of the activated and calcined samples were determined using fourier transform infrared spectroscopy (FTIR), performed with a Perkin Elmer Spectrum 400 FT-IR/NIR. 2.3. CO 2 CAPTURE PERFOMANCE CO 2 uptake performance of the sample was evaluated through adsorption pathways. CO 2 adsorption isotherm at 25°C was conducted to evaluate physical adsorption using Micromeritics ASAP 2020. Samples had the same degas procedure, and instead of N 2 , 99.9% CO 2 gas was used during the analysis at temperature of 25°C under water circulated bath. To further quantify chemical absorption and basic site of the samples, samples were analyzed using Micromeritics Autochem II Plus. Approximately 0.05 g of sample was placed in quartz tube and placed in the furnace microreactor. Sample was then cleaned at 150°C in 15 ml/min of N 2 gas before any analysis procedure. Subsequently, CO 2 gas with a purity of 99.9% was flowed through the sample at a rate of 15 ml/min for 60 min for the adsorption process. After that, sample was cleaned at 50°C using N 2 gas with flow rate of 15 ml/min for 30 min to remove weakly adsorbed CO 2 at sample’s surface. Next step is the desorption process where sample was heated from 50°C to 900°C with a heating rate of 10°C/min. Desorption process occurred in 99.9% N 2 gas atmosphere with flow rate of 15 ml/min. Quantification of CO 2 capture capacity was recorded by using thermocouple detector (TCD) at desorption process. Based on the comparison of the adsorption capacity by physisorption and chemisorption, the most efficient sample was selected for further investigation toward longevity test. The longevity test was performed using the aforementioned instrument of Micromeritics Autochem II Plus. Same analysis method was performed to the sample with slight modification at CO 2 adsorption step. During CO 2 adsorption step, it was prolonged to 6 hours, and the reaction was ended to collect the sample for further analysis. This technique was repeated for 12, 24, and 48 hours to compare the effect of longer CO 2 exposure to the sample. Finally, collected samples were analyzed to identify the formation of functional groups, morphology and thermal analysis. Functional groups were determined using Fourier Transform Infrared Spectroscopy (FTIR) from model Perkin Elmer Spectrum 400 FT-IR/NIR with Imaging System. Meanwhile, the morphology of the carbonate formation was observed using Field Emission Scanning Electron Microscopy, FESEM (Zeiss Merlin). Lastly, sample was then analyzed using Thermogravimetric Analyzer, TGA (NETZSCH, STA 449 F3 Jupiter) to study carbonate decomposition of the sample while determining the adsorption capacity in longer CO 2 exposure. 2.4. BREAKTHROUGH, REGENERATION AND RECYCLABILITY STUDY To investigate the effect of temperature on breakthrough time, adsorption experiments were conducted at temperatures ranging from 25 to 75°C. The most efficient adsorption temperature was then selected to examine desorption performance at two temperatures, 25 and 75°C. Finally, to evaluate the recyclability of the adsorbent, the optimal adsorption and desorption conditions were applied over five consecutive cycles. The CO 2 adsorption capacity for each cycle was recorded to determine the regeneration efficiency of the adsorbent. Regeneration study was conducted to evaluate the reusability of an adsorbent by applying adsorption/desorption process until the adsorbent loses its ability for adsorption. The study was conducted using laboratory bench-scale CO 2 fixed-bed reactor. Procedure for recyclability study started with placing 35 g of the adsorbent in column in packed-bed column. Sample was then purged with N 2 gas at 25 ºC for 10 min to remove other gases in column. Adsorption temperature was conducted at 25 ºC with flow rate 200 ml/min of 15 vol.% CO 2 which was verified using CO 2 analyzer (Alpha Omega, Series 9610). 3. RESULT 3.1. CHARACTERIZATION XRD diffraction pattern of calcined alkaline sludge (CAS) and activated AS was observed in Fig. 2 . After calcination, sludge was dominated with tetragonal CaSiO 3 (ICDD 00-027-0088) at 2θ = 23º, 38º, 41º, 51º, 53º, and 57º. There were also some tetragonal SiO 2 (ICDD 01-082-0512) at 2θ = 21º, 25º, 26º, 28º, 29º and 35º after calcination. There was also unaffected monoclinic CaMg(SiO 3 ) 2 (ICDD 01-083-2016) at 2θ = 39º even after calcination at 900 ºC. Past research showed decomposition of CaMg(SiO 3 ) 2 at higher temperature of 1500 ºC will decomposed into MgSiO 3 and CaSiO 3 which could have potential for CO 2 adsorption (Oguri et al. 1997 ). After activating 1M of HCl, sample presented with more crystalline tetragonal SiO 2 at 2θ = 21º. Activated samples also has less CaSiO 3 peak thus indicated that the HCl reacted with CaSiO 3 in the sample as shown in Eq. 1. H 2 SiO 3 is insoluble in water and remained in the sample, whereas CaCl 2 is water-soluble and was eliminated during washing process thereby explaining the reduced Ca in sample after acid washing. CaSiO 3 + 2HCl ◊ CaCl 2 + H 2 SiO 3 (Eq. 1) It was also observed that cubic Fe 3 Si (ICDD 00-035-0519) was present at 2θ = 45º and CaMg(SiO 3 ) 2 (ICDD 01-083-2016) at 2θ = 56º. The revelation of these compounds demonstrating that even low acid concentration can help release inorganic mineral within the sludge, thereby enhancing adsorption process. However, for sample AS_HCl_2M, SiO 2 becoming less crystalline compared to sample activated with 1M. Our study found similar result from Shang et al. ( 2025 ), in which the acid treatment makes SiO 2 less crystalline. A lower degree of crystallinity in SiO 2 corresponds to a higher density of structural defects, which enhances its oxygen adsorption capacity (Shang et al. 2025 ). It was also observed that Fe 3 Si and CaMg(SiO 3 ) 2 are becoming more amorphous as the concentration of acid increases. It was possible that both compounds reacted with HCl similar with CaSiO 3 above and washed out of sample. For sample AS_HCl_3M, SiO 2 becomes more crystalline after 2M. There was also small peak of CaSiO 3 , Fe 2 Si, CaMg(SiO 3 ) 2 and CaSi 2 (ICDD 00-001-1276) at 2θ = 47º still detected after activation. Sample AS_HCl_4M showed similar crystalline peak of SiO 2 and very small peak of other metal containing compound. CO 2 adsorption performance is highly dependent on textural properties of the adsorbent. Figure 3 showed N 2 adsorption-desorption isotherm and pore size distribution by all samples. As a result, CAS showed type III isotherm in Fig. 3 (a), characteristic of nonporous materials due to absence of monolayer formation adsorption. On the other hand, activated samples were identified to have type IV isotherm identified by knee-shaped at initial relative pressure region indicating a monolayer formation as seen in Fig. 3 (b)-(e). This behavior is consistent with the presence of abundance of mesopores within the sample (Thommes et al. 2015 ). All samples also showed type H3 hysteresis loop indicated that pore network mostly consisted of macropores that did not allow gases to fully adsorbed in the pores (Thommes et al. 2015 ). Sample AS_HCl_2M and AS_HCl_4M were observed to show bigger hysteresis loops compared to other samples. Increment of macropores in these samples could be explained with dissolution of HCl with inorganic element leaving porous material after activation. The smallest hysteresis loops were observed in CAS, since they are not activated, the pores are still filled with inorganic minerals that hinder N 2 adsorption. Pore size distribution of all samples was computed with BJH method are shown in Fig. 3 (f) and (g). All samples showed pore size distribution ranging from 10–125 nm. Sample AS_HCl_2M showed highest distribution of mesopores in region of 30 nm. The inconsistencies of pore development could be fault with various inorganic elements inherent within the sample. Second highest distribution of pores was sample AS_HCl_4M followed by AS_HCl_3M and AS_HCl_1M. These 3 samples showed smooth bell bottom distribution curve that also ranging in region 10–50 nm with the highest pore size in 30 nm. Compared to CAS, activation of sludge by acid increased the pore distribution by 6 times, particularly within mesopore and macropore range. Additionally, CAS did possess the pore size in the similar range, but instead effectively enhanced development of existing pore structure. It is also notable that both 2M and 4M had highest distribution of mesopore that helped contribute to overall S BET of both samples. Using HCl as activator definitely enhance the BET surface area as tabulated in Table 2 . Sample AS_HCl_2M showed the highest S BET which is 123.73 m 2 /g, followed by AS_HCl_4M which had 105.85 m 2 /g. In contrast, S BET of AS is 6.54 m 2 /g and no S micro was detected in inactivated sample. It is obvious observation that the acid had helped with improving surface area and helped with generation of micropore and mesopore in the sample. Increase of S BET had been attributed by micropore and mesopore developed through acid activation. Previous research by Edama et al. ( 2014 ) had shown that acid played a big role in improving surface area of sample. Sample AS_HCl_1M and AS_HCl_2M had the highest micropore surface area, S mic which is 55.79 m 2 /g and 45.83 m 2 /g, respectively which is important criteria for adsorption. However, samples AS_HCl_3M and AS_HCl_4M had been negatively impacted by the higher concentration of acid, demonstrated decrease of S BET in response to SiO species getting more aggregated as confirmed in XRD diffractogram in Fig. 2 . Similar pattern was observed in micropore and mesopore generation, with a decrease in S micro as acid concentration increase. Increment of surface area from using acid was also possibly caused of elimination exchangeable cations and the generation of silica (Barrios et al. 1995 ). Hence, the higher concentration of acid could also accelerate the agglomeration of silica as observed in XRD in Fig. 2 . Meanwhile, average pore diameter had been observed to decrease for all samples compared to the CAS (18.14 nm) and the smallest pore diameter is AS_HCl_1M (5.34 nm). Smaller pore size (< 1 nm) were much preferable for better CO 2 adsorption, nevertheless, the refining of pore size could still help with adsorption. Aggregation of silica also affected pore diameter since inconsistencies of pore diameter refinement was observed for all samples, disregarding the concentration of acid. Additionally, total pore volume, V tot of AS_HCl_2M had drastically increased from 0.023 cm 3 /g to 0.278 cm 3 /g, 12 times increment compared to CAS. All samples experienced enlargement of total pore volume when compared to CAS. Table 2 Textural properties of all adsorbents Samples Surface area Pore volume Average pore diameter (nm) S BET 1 (m 2 /g) S mic 2 (m 2 /g) S meso 3 (m 2 /g) V tot 4 (cm³/g) V mic 5 (cm 3 /g) CAS 6.50 n.a. n.a. 0.023 n.a. 18.14 AS_HCl_1M 96.37 55.79 40.57 0.129 0.0226 5.34 AS_HCl_2M 123.73 45.83 77.89 0.278 0.0178 11.47 AS_HCl_3M 86.37 36.19 50.18 0.181 0.0149 8.39 AS_HCl_4M 105.85 43.00 62.85 0.247 0.0177 9.33 1 surface area by BET method 2 micropore surface area by t-plot method 3 mesopore surface area by t-plot method 4 single point total pore volume 5 micropore volume from t-plot method. n.a.- not available FESEM analysis in Fig. 4 had been applied to understand the differences between internal structure and morphology of inactivated and activated AS. Morphology of CAS in Fig. 4 (a) was quite smooth and plate-like fused together on the surface. After activation, all samples developed rough surface and formed granular-like that shows it separated from main body of the material. Sample AS_HCl_1M as shown in Fig. 4 (b) had almost similar morphology with AS where fused plate-like molecules were beginning to get separated. Sample AS_HCl_2M presented as irregular ellipsoid and generation of pores were observed on the surface. Particles were uniformly well-separated from each other even in larger magnification as seen in Fig. 4 (c). However, in sample AS_HCl_3M, particle was getting more aggregated or not as well separated compared to AS_HCl_2M as shown in Fig. 4 (d). Last sample of AS_HCl_4M in Fig. 4 (e) was also seen to have same morphology as 3M. Particles were aggregated at higher concentration of acid of 3M and 4M. However, some pores were still observed on the surface even though at larger magnification, the particles were aggregated. 3.2. CO 2 CAPTURE PERFORMANCE In order to gain a deeper understanding of CO 2 uptake performance of the sample, two types of analysis were performed. Physical adsorption (physisorption) and chemical absorption (chemisorption) measurements were performed to explore the differences of gases uptake by the sample. Physisorption can be described as physical interaction between gases (adsorptive) and adsorbent through van der Waals force (Lahn et al. 2020 ) and typically require bonding energies of 10–70 Kj/mol for interaction (Gabelman 2017 ). Chemisorption, on the other hand, involves formation of chemical bond between adsorptive and adsorbent (Atif et al. 2022 ) through ionic interactions (Banerjee & Regalbuto 2020 ) or radical processes (Jones et al. 2020 ). Physisorption was measured from CO 2 adsorption isotherm at 25°C and chemisorption was measured from TPD-CO 2 that was performed from 50 ºC to 900 ºC. The results were compared to determine the most efficient adsorbent. Figure 5 (a) showed physisorption by adsorption isotherm at 25 ºC, the quantity of CO 2 adsorbed isothermally while increasing the pressure to reach equilibrium. TPD-CO 2 analysis was used to determine basic site and quantity of CO 2 absorbed by the adsorbent as seen in Fig. 5 (b). CO 2 gases were used in this analysis because acidic gas such as CO 2 are readily attracted to basic sites. Basic site strength of an adsorbent can be evaluated by the desorption temperature corresponding to where CO 2 gas is desorbed. Adsorbents with stronger basicity exhibit higher bond energies, thus require higher temperatures for desorption, whereas weaker basic sites desorb at lower temperatures (Hakim et al. 2016 ). Desorption of gases at certain temperature can elucidate the strength of basic sites on the surface such as weak (250°C), intermediate (250–480°C) and strong (480–700°C) and very strong (700–900°C). CO 2 adsorption isotherm at 25 ºC was shown in Fig. 5 (a) and adsorption capacity performance was listed in Table 3 . It was seen that AS_HCl_2M had the highest adsorption capacity which is 0.44 mg/g compared to CAS which was 0.02 mg/g, increment of almost 20 times after activated by HCl. Adsorption capacity by physisorption increases after activation by 1M and 2M, following that adsorption capacity decreases as the HCl concentration increased. Higher concentration of acid of 3M and 4M was suspected to aggregate SiO 2 species as shown in XRD analysis in Fig. 2 . Both AS_HCl_3M and AS_HCl_4M also possessed the quite high S meso compared to all activated sample, since mesopore can only weakly adsorb CO 2 (Dziejarski et al. 2023 ). TPD –CO 2 analysis was shown in Fig. 5 (b) and adsorption capacity performance was tabulated in Table 3 . Figure 5 (b) showed CAS desorption of CO 2 occurred at 79 ºC and 235 ºC, both at weak basic site, with the second site approaching intermediate strength basic site. CAS was observed did not possess micro- and mesopore on the surface, hence the sample performance on adsorption capacity of physisorption (0.02 mg/g) and chemisorption (29.99 mg/g) was the lowest compared to all activated samples as seen in Table 3 . Inactivated CAS which had abundant of CaSiO 3 is better for absorption that uses high temperature in their reaction (M. Wang & Lee 2009 ). By refining the textural properties through acid activation, the adsorption capacity increased. Sample AS_HCl_1M and AS_HCl_2M exhibited basic site at intermediate strength site, with desorption temperature of 373 ºC and 370 ºC, respectively, in which values bordering at strong basic site. Both samples also presented highest temperature of desorption compared to another sample. Based on textural properties of both samples, they exhibit the highest S mic which according to (L. Chen et al. 2018 ; He et al. 2021 ) is primary contributor to CO 2 adsorption. The narrow channel of micropores provide high surface area creating stronger CO 2 bond to the adsorbent necessitating higher temperature for desorption. Sample AS_HCl_3M and AS_HCl_4M presented two basics sites, weak and intermediate. At weak basic site, sample AS_HCl_3M (86 ºC) and AS_HCl_4M (92 ºC), both samples which also possessed the lowest S mic (36.19 m 2 /g) and (43 m 2 /g), respectively, causing low temperature desorption. Adsorption capacity performance by AS_HCl_2M demonstrated the highest adsorption capacity both from physisorption (0.44 mg/g) and chemisorption (284.06 mg/g). Based on Table 2 , sample AS_HCl_2M also has the second highest amount of S micro which was found to help in stronger CO 2 adsorption (Chen et al. 2018 , (He et al. 2021 ). Furthermore, according to XRD analysis, 2M has relatively high amount of SiO 2 which was found to help CO 2 sorption capacity (Tahari & Yarmo 2014 ) and least crystalline for all inorganic element. Sample AS_HCl_2M also contained Fe 3 Si which may act as an active site for CO 2 adsorption, resulting in almost 9 times higher uptake compared to CAS. Morphology analysis also proven that AS_HCl_2M had better pore generation and particles were well-separated that increases active site. The higher concentration of acid (3M and 4M), however, had washed the metal of the sludge such as Mg, Fe and Ca similar as previous research by Chen et al. ( 2019 ) thus reducing the CO 2 adsorption capacity. Table 3 CO 2 adsorption capacity performance Adsorbents Physisorption by CO 2 adsorption isotherm at 25°C Chemisorption by CO 2 adsorption capacity Adsorption capacity (mg/g) Desorption temperature (°C) Adsorption capacity (mg/g) Total adsorption capacity (mg/g) CAS 0.02 57–106 6.73 29.99 194–303 23.26 AS_HCl_1M 0.29 228–590 180.7 181.02 AS_HCl_2M 0.44 207–644 284.06 284.06 AS_HCl_3M 0.37 65–158 30.18 220.71 160–780 190.53 AS_HCl_4M 0.34 76–160 17.51 229.06 165–754 211.55 According to the analysis, trend for chemisorption adsorption capacity is AS_HCl_2M > AS_HCl_1M > AS_HCl_4M > AS_HCl_3M > CAS. Physisorption adsorption capacity showed that AS_HCl_2M > AS_HCl_3M > AS_HCl_4M > AS_HCl_1M > CAS. Hence, sample AS_HCl_2M was selected owing to its favorable physisorption and chemisorption. Compared to literature summarized in Table 1 , the chemisorption capacity obtained in this study (284.06 mg/g) lower than the highest reported value by Nawar et al. ( 2020 ) (560 mg/g), yet remains in comparable range. However, our study employed physical adsorption as physisorption capacity is crucial indicator of regenerative properties. The longevity test was conducted using AS_HCl_2M, with CAS as standard reference, under different adsorption time (6, 12, 24 and 48 hours) to study the effect of CO 2 exposure time towards adsorbent. 3.3. LONGEVITY STUDY Carbonate formation and CO 2 adsorbed species were identified from FTIR spectra in Fig. 6 and Table 4 listed the assignment of functional group to the frequencies. AS was observed to have sharp absorption bands associated with fingerprints peak of Ca-O at 710 and 873 cm − 1 (Araújo et al. 2021 ). The sample also shown broad peak at 1421 cm − 1 which is corresponding to symmetry O-C-O stretching bicarbonate (Hakim et al. 2016 ) possibly of its exposure to atmospheric air. The disappearance of Ca–O bands in 710 and 873 cm − 1 in activated AS_HCl_2M suggesting the formation of carbonate during activation process. Considering during preparation, adsorbent was dried in non-inert environment that could cause carbonate formation before analysis. There was smaller peaks at 1421 cm − 1 indicating formation of bicarbonate even to the exposure to the atmosphere. This also can be confirmed from small peaks at 1511 and 1639 cm − 1 assigned to asymmetry O-C-O bidentate carbonate. Small peak at 3728 cm − 1 assigned to –OH, possibly of established hydrogen bonds between hydroxyl groups and Si after preparation. After 6 hours of CO 2 exposure, Si-O-Si stretching peak detected at 792 and 1022 cm − 1 (Akopyan et al. 2022 ; Gwon et al. 2010 ) was becoming more prominent. Contribution of broad peak at 1061 cm − 1 was assigned to Si-OH bending (Akopyan et al. 2022 ) which also becoming more broad as exposure time increased. This could be related to absorption of moisture from the gases from the tank. Next, formation of carbonate was detected at 1639 cm − 1 which is associated with asymmetry O-C-O bidentate carbonate (Lahuri & Yarmo 2022 ; W. Wang et al. 2020 ). After 12 hours of exposure, Si-O-Si was detected and Si-OH becoming broader and sharper. Small peak of asymmetry O-C-O bidentate carbonate remained unchanged even after 12 hours of exposure. Formation of \(\:\text{C}\equiv\:\text{C-Si}\) stretching vibration was detected at 2155 cm −1 (Mei et al. 2020 ). Organosilicon with alkyne group can function as nucleophiles that will likely have reaction with CO 2 (Zhang et al. 2020 ). After 24 hours of exposure, both Si-O-Si and Si-OH were detected and broad peak stay the same as before. Shoulder peak of asymmetry O-C-O bidentate carbonate was also detected and becoming more prominent with longer CO 2 exposure. Small peak of of \(\:\text{C}\equiv\:\text{C-Si}\) stretching vibration becoming more noticeable at longer CO 2 exposure. Absorption band around 2311 cm −1 corresponds to multilayer physisorbed CO 2 (Yang & Wöll 2017 ). CO 2 was found to absorb weakly on the surface and does not have any reaction to form carbonate. Small absorption peak at 3645 cm − 1 was associated to bicarbonate from O-H stretch (Hakim et al. 2016 ). Another small peaks around 3728 cm − 1 which is commonly associated with –OH group (Satapute et al. 2017 ). At 48 hours of exposure, Si-O-Si and Si-OH peak stay similar even at the longest contact of CO 2 . Asymmetry O-C-O bidentate carbonate peak was observed becoming smaller compared to 24 hours. Peak at 2155 cm − 1 corresponded to multilayer physisorbed CO 2 was also becoming smaller and barely noticeable –OH group at 3600–3700 cm − 1 . Table 4 Assignments of experimental frequencies compared with reference frequencies Assignments Experimental frequencies Reference frequencies References Ca-O bonding 710, 873 710, 872 (Araújo et al. 2021 ) Si-O-Si stretching 792 792 (Akopyan et al. 2022 ) Si-O-Si stretching and/or Si-O-C band 1022 1150 − 950 (Gwon et al. 2010 ) Si-OH bending 1061 1055 (Akopyan et al. 2022 ) Symmetry O-C-O bicarbonate 1421 1396–1500 (Lahuri & Yarmo 2022 ) Asymmetry O-C-O bidentate carbonate 1511, 1639 1446–1590, 1555–1720 (Lahuri & Yarmo 2022 ) C \(\:\equiv\:\) C-Si stretching vibration 2155 2150 (Mei et al. 2020 ) Physisorption CO 2 2311 2240–2390 (Yang & Wöll 2017 ) Bicarbonate O-H 3645 3600–3627 (Hakim et al. 2016 ) -OH group 3728 3728 (Satapute et al. 2017 ) Figure 8 presented TGA analysis after AS_HCl_2M was exposed to CO 2 for 6, 12, 24 and 48 H to study carbonate formation by adsorbent. Before activation, broad decomposition range indicated the moisture and multiple components are tightly bound thus slowing the decomposition rate of the materials. After activation, adsorbent exhibit two degradation zone at lower temperature range of 42–159 ºC and 170–1000 ºC. First stage weight loss was 1.5% and second stage exhibit 2.3% for quite broad decomposition range. Broad decomposition range usually indicate multiple decomposition of multiple components with overlapping decomposition temperature. After exposure to different range of time, the adsorbent exhibited two stages of degradation ranging from 51–300 ºC and 530–1000 ºC. After 6 hours of CO 2 adsorption, first stage decomposition occurred at 51–300 ºC which commonly associated with moisture loss (Nassar et al. 2022 ). The first decomposition zone displayed weight loss of 2.6% could be attributed from breaking of hydroxyl group from Si-OH. This observation could be explained from FTIR analysis, in which the broadest band of all exposed adsorbents at 1160 cm − 1 was assigned to Si-OH bending. Second stage of decomposition occurred at 530–1000 ºC, which is early decomposition of CaCO 3 ◊ CaO + CO 2 . Previous research also found decomposition of CaCO 3 can started from 500 to 900 ºC (Al-Fateh & Fakeeha 2012 ). Table 5 listed weight loss from this stage was around 1.3% which can also be associated with efficiency of CO 2 adsorption. After 12 hour, weight loss dropped to 0.8%, increased again to 1.4% at 24 hour and decreased again to 1.3% at 48 hour inferring the adsorption process stop at 24 hours. Table 5 Decomposition temperature and weight loss after timely exposure of CO 2 Sample Temperature (ºC) Weight loss (%) CAS 45–509 0.8 707–991 1.1 AS_HCl_2M 45–159 1.5 170–1000 2.3 6H 51–302 2.6 500–1000 1.3 12H 50–301 2.6 500–1000 0.8 24H 57–300 2.4 505–1000 1.4 48H 55–308 2.3 507–1000 1.3 3.4. RECYCLABILITY STUDY Based on CO 2 capture performance obtained from physisorption and chemisorption method, adsorbent that had the highest adsorption capacity was selected for breakthrough adsorption and regeneration study. Recyclability is an important indicator for commercialization for adsorbent, while efficient desorption process can lower the energy and economic cost (Dutta et al. 2019 ). In addition, breakthrough time served as important parameter for characterizing adsorbent before it can be considered for industrial application. 3.4.1 Effect of adsorption temperature Figure 9 (a) showed effect of adsorption temperature from 25°C to 75°C towards breakthrough time. Breakthrough curve is plotted with duration of test against concentration of adsorbate mixed in air with the adsorptive. Breakthrough time is reached when the bed filled with adsorbate no longer has capacity to absorb and equilibrium saturation has been reached. It is observed as temperature increased, breakthrough time become shorter. At 25 ºC, breakthrough time is around 3.4 min while increasing it to 35 ºC, reduce the breakthrough time to 2 min. At higher temperature of 65 and 75ºC, shifted the breakthrough curve to the left reducing the breakthrough time to less than 1 min. High temperature had elevated kinetic energy of CO 2 gas leading to less residence time in adsorbent explaining shorter breakthrough time at 75 ºC (Akpasi & Isa 2022 ). It was also observed steeper slope of breakthrough curve as temperature increased quickly reaching breakthrough time. Since adsorption capacity is proportional to the area above the breakthrough curve bounded by C t /C o = 1, shorter breakthrough time shifted the curve the left thus reduces the area bounded. This explains the shorter breakthrough time to have lower adsorption capacity. This relationship is consistent with the observed trend: 25°C (12.36 mg/g) > 35°C (7.44 mg/g) > 45°C (5.94 mg/g) > 55°C (5.32 mg/g) > 65°C (4.75 mg/g) > 75°C (4.49 mg/g). These results clearly indicate that 25 ºC is the most favorable operating temperature, providing the highest adsorption capacity and longest breakthrough time. This finding is consistent with the work of Hussin et al. ( 2024 ), who also reported that higher temperature reduced adsorption capacity of adsorbent. This pattern of adsorption suggested that the adsorption is dominantly controlled by physisorption (Tan et al. 2014 ). As adsorption is an exothermic process, temperature plays an important role in influence performance (Gabelman 2017 ). In this study, the adsorption capacity at the highest temperature (75°C) was reduced to one-third of that observed at the lowest temperature (25°C), further supporting the physisorption mechanism. Since physisorption relies on weak van der Waals’ interactions, these forces easily disrupted at high temperature (Hussin et al. 2024 ). In our study, two regenerations temperatures were used to compare low and relatively high temperatures in finding the best regeneration condition. Adsorbent was subjected to five cycles of adsorption – desorption cycle at 25°C and then regenerated at two different temperatures of 25 and 75°C. Figure 9 (b) showed that during room temperature regeneration (25°C), the 1st cycle exhibited the highest adsorption capacity of 12.32 mg/g, followed with minor drop to 10.56 mg/g and remain persisted until the 5th cycle. The minor decline of adsorption capacity is contributed by chemically bound CO 2 in adsorbent’s pore that cannot be desorbed at low temperature (Hussin et al. 2024 ). In contrast, regeneration at 75 ºC significantly reduced the adsorption capacity to 5.72 mg/g, approximately half of that achieved at 25 ºC. The elevated temperature, even at 75 ºC, may have accelerated the geopolymerization process affecting material containing aluminosilicate (Chindaprasirt & Rattanasak 2017 ), resulting in entrapment of previously adsorbed CO 2 and affecting the adsorption capacity. Despite this reduction, adsorption capacity remained stable throughout all 5 cycles, suggesting higher regeneration temperature hindered further decline in performance. Regeneration efficiency, RE of adsorbent regenerated at 25 and 75°C was shown in Fig. 10 . At 25°C, RE was 88.6% whereas regeneration at 75°C, RE achieved 100%. At first glance, it was seen that regeneration at 75°C exhibited superior RE performance since there is no loss of adsorption capability. However, adsorption capacity of 75°C remained 5.72 mg/g for all of its 5 cycles. Even though RE value of regeneration at 25°C is 12% lower than 75°C, the adsorption capacity at 25°C is still nearly twofold higher, reaching 12.32 mg/g in the first cycle. These findings indicate that room-temperature regeneration is still viable, as AS_HCl_2M retained its adsorption capacity and demonstrated reliable recyclability performances. 4. CONCLUSION In this work, CO 2 adsorption performance of activated alkaline sludge (AS) from photovoltaic waste using low to high acid concentration was compared with calcined AS. While all activated waste showed significant improvement in their physicochemical properties, AS_HCl_2M had stand out in terms of CO 2 adsorption and its recyclability attribute. Surface area of CAS was improved from 6.50 m 2 /g to 123.73 m 2 /g (AS_HCl_2M) which is 20 times higher than the original value. XRD analysis also revealed that AS_HCl_2M had SiO species with lower degree of crystallinity that can help increase oxygen adsorption capacity. Highest CO 2 adsorption capacity in this study is 0.44 mg/g for physical adsorption and 284.06 mg/g for chemical absorption. It was observed that low concentration acid (1 and 2M) had substantially improved the textural properties which are the crucial traits for adsorption, while the higher concentration acid (3 and 4M) led to a slight decline of adsorption performance. The study followed with longevity test of the adsorbent. FTIR analysis confirmed that CO 2 exposure saturated after 6 hours, as the O-C-O bidentate carbonate peak at 1639 cm − 1 remained unchanged at 12, 24 and 48 hours. TGA also confirmed the saturation time since the weight loss of adsorbent maintained the same with only 0.1% differences even after 6 hours. Morphology analysis observed the change from irregular ellipsoid particle to rough coral framework-like after CO 2 exposure. Recyclability study had also shown that straightforward regeneration process at 25°C had shown adsorption capacity of 10.56 mg/g remain stable for 4 cycles. Considering its reusability, utilization of unused waste and the simplicity of its preparation, this waste-derived adsorbent shows strong potential as viable CO 2 adsorbent. Declarations ETHICAL APPROVAL This is not applicable. CONSENT TO PARTICIPATE This is not applicable. CONSENT TO PUBLISH This is not applicable. COMPETING INTEREST The authors declare no competing interests. FUNDING The authors wish to express gratitude for the research work supported by OCI TerraSus Sdn. Bhd. (6300940) awarded to Universiti Putra Malaysia and (9795800) awarded by Universiti Putra Malaysia. AUTHORS’ CONTRIBUTION Conceptualization and writing—original draft: Siti Sarahah Sulhadi. Data collection, formal analysis, and investigation: Siti Sarahah Sulhadi. Writing—review and editing: Siti Sarahah Sulhadi, Azizul Hakim Lahuri, Syawal Mohd Yusof, Farihahusnah Hussin, Mohamed Kheireddine Aroua, Taufiq Yap Yun Hin, Nur Farhana Jaafar, Syazreen Nadia Sulaiman. Resources and supervision: Azizul Hakim Lahuri. All authors read and approved the final manuscript. ACKNOWLEDGEMENTS The authors thank Dr. Azizul Hakim Lahuri for technical advice and constructive feedback. The authors also acknowledge Syawal Mohd Yusof and Assoc. Prof. Ts. Dr Farihahusnah Hussin for assistance with data analysis. In addition, the author are grateful for Prof. Mohamed Kheireddine Aroua, Prof. Taufiq Yap Yun Hin, Nur Farhana Jaafar and Syazreen Nadia Sulaiman for providing the facilities for this research. This research was supported by OCI TerraSus Sdn. Bhd. (6300940) and Universiti Putra Malaysia (9795800). Any errors remain our own. DATA AVAILABILITY STATEMENT All data generated or analyzed during this study are included in this article. References Abuelnoor N, AlHajaj A, Khaleel M, Vega LF, Abu-Zahra MRM (2021) Activated carbons from biomass-based sources for CO2 capture applications. Chemosphere 282(June):131111. 10.1016/j.chemosphere.2021.131111 Ahmaruzzaman M (2010) A review on the utilization of fly ash. 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Separation and Purification Technology 307(November 2022). 10.1016/j.seppur.2022.122795 Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Major Revision 18 Feb, 2026 Reviewers agreed at journal 18 Jan, 2026 Reviewers invited by journal 08 Jan, 2026 Editor invited by journal 08 Jan, 2026 Editor assigned by journal 31 Dec, 2025 First submitted to journal 27 Dec, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. 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after being exposed to CO\u003csub\u003e2\u003c/sub\u003e for 6, 12, 24 and 48 H\u003c/p\u003e","description":"","filename":"8.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8421032/v1/0b7a1bb81ef33d012922affd.jpg"},{"id":100368777,"identity":"b3888b54-c4a7-48d7-a4d5-e670a10b9c78","added_by":"auto","created_at":"2026-01-16 07:58:20","extension":"jpg","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":136106,"visible":true,"origin":"","legend":"\u003cp\u003e(a) Breakthrough adsorption curve at different temperatures (b) Adsorption cycle after regenerated at 25 °C and 75 °C\u003c/p\u003e","description":"","filename":"9.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8421032/v1/74833695d8576eba9adc8adc.jpg"},{"id":100369654,"identity":"e912a572-8e0a-4593-b98d-24a04329daae","added_by":"auto","created_at":"2026-01-16 07:59:14","extension":"jpg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":147370,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of regeneration temperature towards regeneration efficiency\u003c/p\u003e","description":"","filename":"10.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8421032/v1/e63d704ffe1b10fed65e993f.jpg"},{"id":100382657,"identity":"8bf3701d-bb23-4a7e-b8d2-94922d03f170","added_by":"auto","created_at":"2026-01-16 10:43:33","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3975281,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8421032/v1/8a223c19-2fce-4dc0-940b-a1b4d62ba6b2.pdf"}],"financialInterests":"","formattedTitle":"\u003cp\u003eLongevity and Recyclability Study of Acid Activated Alkaline Sludge From Photovoltaic Industry Toward Co\u003csub\u003e2\u003c/sub\u003e Capture\u003c/p\u003e","fulltext":[{"header":"1. INTRODUCTION","content":"\u003cp\u003eRecycle of industrial wastage can help with reduce waste discharge into the environment. Utilization of industrial derived waste can represent significant step towards achieving SDG 12 (responsible consumption and production). In 2017, total production of global solid waste including industrial was 20\u0026nbsp;billion tonne and projected to increase to 46\u0026nbsp;billion tonne following the present production patterns (Maalouf \u0026amp; Mavropoulos \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). These wastes are presently managed through various disposal and treatment approaches, including landfilling, open dumping, composting, recycling, and incineration. Some disposal methods such as landfills, dumps and incineration usually will end up polluting the environment and affecting human health. Rather than disposing of this low-cost waste, converting it into high-value products provides sustainable materials for carbon capture, utilization, and storage (CCUS) applications. CCUS applications are crucial tools in meeting the standard set by Paris Agreement that focuses on limiting global warming to 1.5 \u0026ordm;C by reducing greenhouse emissions. These global treaties primarily aim to reduce CO\u003csub\u003e2\u003c/sub\u003e concentration in the atmosphere to limit the greenhouse gases that are affecting the global climate. Based on NOAA observation, CO\u003csub\u003e2\u003c/sub\u003e concentration in the atmosphere reached 430 ppm at May 2025 for the first time ever since it is start recorded in 1958 (Monroe \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). CO\u003csub\u003e2\u003c/sub\u003e gas, acting not only as the biggest greenhouse gas emitter but also identified as amplifier of global warming in 2020 (NOAA \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eAlthough CO\u003csub\u003e2\u003c/sub\u003e capture technology remains in the developmental stage, it has undergone substantial advancements and global expansion. Several methods such as chemical absorption, physical adsorption and absorption, membrane separation and cryogenic separation are widely recognized in CO\u003csub\u003e2\u003c/sub\u003e capture research. Furthermore, established technologies such as amine scrubbing, oxy combustion capture, direct separation and calcium looping are currently used in cement plants worldwide (Hanifa et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). One of the most widely used CO\u003csub\u003e2\u003c/sub\u003e capture technology, amine scrubbing, present some serious drawback to the environment such as release of ~\u0026thinsp;80 tons of MEA into the atmosphere with every million tons of CO\u003csub\u003e2\u003c/sub\u003e captured (Dhanraj \u0026amp; Biswas \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Since amine scrubbing also uses liquid as absorption medium, degradation and corrosion of equipment was also one of the many concerns. In this research, we proposed utilization of solid alkaline sludge from photovoltaic industry as main material for CO\u003csub\u003e2\u003c/sub\u003e absorbent as possible solution for arising problems stated. Utilizing wastes not only supports circular economy by minimizing waste into landfill, but they also reduce reliance on new raw materials, thereby preserving finite resources. Additionally, low procurement cost of raw materials is an added benefit for waste utilization.\u003c/p\u003e \u003cp\u003eMost solid wastes are produced from anthropogenic activity among which are agriculture, industrial, biomass and municipal. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e summarizes the development of solid waste utilization in global CO\u003csub\u003e2\u003c/sub\u003e capture studies from 2020\u0026ndash;2025, illustrating the increasing focus on converting waste into value-added materials.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe industrial waste-derived materials are commonly collected from various industries such as explosive, paper manufacturing, polyvinyl chloride (PVC) production and sewage sludge which constitute 16 out of 58 research papers as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Common industrial waste such as bottom ash has been recycled into CaO-based sorbents, demonstrating superior CO\u003csub\u003e2\u003c/sub\u003e uptake capacity of 140 mg/g that can withstand 20 cycles of adsorption (Zhang et al. \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Other industrial waste such as fly ash turned zeolite can had sorption capacity ranging from 10.6-183.1 mg/g (Czuma et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Glass industry that produced waste with high content of SiO\u003csub\u003e2\u003c/sub\u003e and Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e had also been tested for CO\u003csub\u003e2\u003c/sub\u003e capture. CO\u003csub\u003e2\u003c/sub\u003e capture capacity of said waste can reached 42.7 mg/g (Ramos et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Waste produced from energy explosive industry has also been studied for its application in CO\u003csub\u003e2\u003c/sub\u003e capture. Prepared graphene synthesized from solid waste collected from explosive industry by Aquatar et al. (\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2021\u003c/span\u003e) presented high surface area and CO\u003csub\u003e2\u003c/sub\u003e capture capacity can reached up to 1040 mg/g (303 K, 20 bar). Alternatively, carbide slag produced from polyvinyl chloride (PVC) industry had also been studied as CO\u003csub\u003e2\u003c/sub\u003e capture material since the waste is composed mainly from Ca(OH)\u003csub\u003e2\u003c/sub\u003e. Ma et al. (\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) found carbide slag-based sorbent can had CO\u003csub\u003e2\u003c/sub\u003e capacity uptake of 290 mg/g even after 30 cycles of adsorption. Next, paper sludge ash (PSA), an alkaline byproduct from paper manufacturing, utilized for its favorable properties for CO\u003csub\u003e2\u003c/sub\u003e absorption. Kim \u0026amp; Kim (\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) found that capturing capacity of CO\u003csub\u003e2\u003c/sub\u003e by paper sludge waste can go up to 324 kg\u003csub\u003eCO2\u003c/sub\u003e/ton\u003csub\u003ePSA\u003c/sub\u003e. Other potential waste is sewage sludge ash (SSA) that also contain high amount of Ca and its utilization found CO\u003csub\u003e2\u003c/sub\u003e uptake of 20 kg\u003csub\u003eCO2\u003c/sub\u003e/ton\u003csub\u003eSSA\u003c/sub\u003e (Massa et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Other similar study that implement sewage sludge found that CO\u003csub\u003e2\u003c/sub\u003e capture capacity was 23.3 mg/g that also possess high surface area and pore volume after chemical activation (Miricioiu et al. \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Nevertheless, the aforementioned industrial waste-derived materials are subjected to pre-treatment, surface activation and modification to enhance surface properties, improve active sites for CO\u003csub\u003e2\u003c/sub\u003e adsorbate, removal impurities and identify heavy metals presence.\u003c/p\u003e \u003cp\u003eCommon approaches such as acid and base activation of waste materials have been shown to enhance the surface area and improve overall physical properties. Given the chemical stability of CO\u003csub\u003e2\u003c/sub\u003e, the selection of adsorbent materials depends on several critical factors such as low-cost, high CO\u003csub\u003e2\u003c/sub\u003e/N\u003csub\u003e2\u003c/sub\u003e selectivity, availability, low affinity to moisture, fast kinetics, regeneration ability, mechanical and thermal stability, low heat of adsorption, high surface area and high pore volume (Abuelnoor et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIndustrial waste activated by HCl had considerable success in various applications, especially CO\u003csub\u003e2\u003c/sub\u003e capture study. Past research had found sewage sludge that had been treated by both KOH and HCl had increased surface area from 2.9 m\u003csup\u003e2\u003c/sup\u003e/g to 921 m\u003csup\u003e2\u003c/sup\u003e/g, the highest surface area obtained compared to other 3 samples treatment with only different concentration of KOH (Miricioiu et al. \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). de Andr\u0026eacute;s et al. (\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2013\u003c/span\u003e) discovered that sewage sludge activated by NaOH enhanced the surface area of 179 m\u003csup\u003e2\u003c/sup\u003e/g, with CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity of 56 mg/g. The conversion of sewage sludge into biochar reported had CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity ranged from 3.88 mg/g to 4.88 mg/g with surface area of only 10.12 m\u003csup\u003e2\u003c/sup\u003e/g (Xu et al. \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). A similar study reported that carbon derived from waste tire activated by KOH achieved CO\u003csub\u003e2\u003c/sub\u003e sorption capacity that can reached up to 238.7 mg/g (Duduku et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). Fly ash (FA) is another popular waste derived material for CO\u003csub\u003e2\u003c/sub\u003e capture, utilization and usage, which had major component of SiO\u003csub\u003e2\u003c/sub\u003e, Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, CaO, MgO, SO\u003csub\u003e3\u003c/sub\u003e, Na\u003csub\u003e2\u003c/sub\u003eO and K\u003csub\u003e2\u003c/sub\u003eO (Ahmaruzzaman \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). Lee et al. (\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2014\u003c/span\u003e) applied dry sorbent manufactured with dry mixture of fly ash, NaOH and CaO as CO\u003csub\u003e2\u003c/sub\u003e absorbent. The study found that addition of FA helped increased 9% higher adsorption capacity than samples without FA. The approach proved addition of FA helped dispersion of active site for CO\u003csub\u003e2\u003c/sub\u003e adsorption and optimized surface area.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eAdsorption capacity of CO\u003csub\u003e2\u003c/sub\u003e capture for various wastes activated by different acids\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSource of waste\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eActivating agent\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e capture capacity (mg/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c5\" namest=\"c4\"\u003e \u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e capture condition\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eReferences\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ePressure (bar)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eTemperature (\u0026ordm;C)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCarbide slag\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAcetic acid\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e290\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003en.a.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e700\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e(Ma et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2019\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSewage sludge ash\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eH\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e(Massa et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2025\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePalm oil fly ash\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eHCl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e282.5\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e32\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e(Kongnoo et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2017\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMolasses\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eH\u003csub\u003e3\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e(Kiełbasa et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2022\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMolasses\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eHCl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e(Kiełbasa et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2022\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMolasses\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eH\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e195.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e(Kiełbasa et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2022\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSewage sludge\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eHF\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e127.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0\u0026ndash;50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e(Ma et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2022\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSewage sludge\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eKOH/HCl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e11.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e23\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e(Miricioiu et al. \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2021\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e\u003cem\u003eEntada rheedii\u003c/em\u003e shell\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eHCl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e193.6\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e(Mallesh et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2020\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eMunicipal solid waste\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eH\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e114.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e(Karimi et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2020\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eRubber-seed shell waste\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMalic acid\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e117.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e(Borhan \u0026amp; Yusuf \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2020\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCoal fly ash/CaO\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAcetic acid\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e560\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e650\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e(Nawar et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2020\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eTherefore, in this study, we aim to develop alkaline sludge (AS), major wastes from photovoltaic industry (PV) activated by HCl into efficient CO\u003csub\u003e2\u003c/sub\u003e capture material. Based on past research, there is underutilized application of alkaline sludge in CCUS. The enhancement of the textural properties and efficiency in CO\u003csub\u003e2\u003c/sub\u003e uptake capacity of activated sludge was investigated through this study. Besides that, the stability of the AS was evaluated through recyclability study.\u003c/p\u003e"},{"header":"2. METHOD","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1. AS ACTIVATION\u003c/h2\u003e \u003cp\u003eAlkaline sludge, AS, was collected from photovoltaic solar cell factory located in Bintulu and dried at 110\u0026deg;C for 24 hours following calcination step at 900\u0026deg;C in air prior to activation process. Calcined AS denoted as CAS for the rest of the study. Elemental composition of calcined AS was verified by XRF analysis and was dominated by SiO (56.3%), CaO (30.6%), Na\u003csub\u003e2\u003c/sub\u003eO (3.24%), Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (2.61%), Cl (22.5%), MgO (2.14%), and Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (1.83%). The process followed with activation with HCl (Chemiz, Malaysia). Various concentrations of HCl were prepared (1M-4M) before 10 ml of prepared acid solution mixed with 1 g of CAS. Sample was then thoroughly stirred at 100\u0026deg;C for 24 hours in Teflon bottle. After stirring, samples were cooled, filtered and washed until neutral using distilled water. Samples were then dried in the oven at 110\u0026deg;C overnight before being kept in air-tight container for later use. Samples were denominated following the acid concentration in which CAS was activated, such as AS_HCl_1M, AS_HCl_2M, AS_HCl_3M and AS_HCl_4M. The effect of acid concentration toward activation of AS was analyzed and characterized.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2. CHARACTERIZATION\u003c/h2\u003e \u003cp\u003eThe crystal structure and chemical composition of the sample was determined using X-Ray Diffraction (XRD) analysis from a model Bruker D8 Advance with Cu as an x-ray source. The diffraction result was matched with the International Centre for Diffraction Data (ICDD) database to determine the composition of the samples. Determination of surface area and pore size were evaluated from N\u003csub\u003e2\u003c/sub\u003e adsorption-desorption isotherm. The analysis was conducted using a volumetric static technique from a Micromeritics TriStar II Plus. Around 0.2 g of sample was placed in quartz tube and was first degassed at 150\u0026deg;C to remove moisture and trapped gases for 6 hours in N\u003csub\u003e2\u003c/sub\u003e atmosphere. Degassed sample was then immersed in liquid nitrogen circulated bath at temperature of -196\u0026deg;C for the analysis process. Surface area (S\u003csub\u003eBET\u003c/sub\u003e) and pore size was obtained from Brunauer-Emmett-Teller (BET) method. Micropore surface area (S\u003csub\u003emic\u003c/sub\u003e) was obtained from t-plot method while pore size distribution obtained from Barrett-Joyner-Halenda (BJH) method.\u003c/p\u003e \u003cp\u003eMorphology of the activated samples were observed using field emission scanning electron microscopy (FESEM-EDX) by model Merlin at various magnification. Functional groups of the activated and calcined samples were determined using fourier transform infrared spectroscopy (FTIR), performed with a Perkin Elmer Spectrum 400 FT-IR/NIR.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3. CO\u003csub\u003e2\u003c/sub\u003e CAPTURE PERFOMANCE\u003c/h2\u003e \u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e uptake performance of the sample was evaluated through adsorption pathways. CO\u003csub\u003e2\u003c/sub\u003e adsorption isotherm at 25\u0026deg;C was conducted to evaluate physical adsorption using Micromeritics ASAP 2020. Samples had the same degas procedure, and instead of N\u003csub\u003e2\u003c/sub\u003e, 99.9% CO\u003csub\u003e2\u003c/sub\u003e gas was used during the analysis at temperature of 25\u0026deg;C under water circulated bath. To further quantify chemical absorption and basic site of the samples, samples were analyzed using Micromeritics Autochem II Plus. Approximately 0.05 g of sample was placed in quartz tube and placed in the furnace microreactor. Sample was then cleaned at 150\u0026deg;C in 15 ml/min of N\u003csub\u003e2\u003c/sub\u003e gas before any analysis procedure. Subsequently, CO\u003csub\u003e2\u003c/sub\u003e gas with a purity of 99.9% was flowed through the sample at a rate of 15 ml/min for 60 min for the adsorption process. After that, sample was cleaned at 50\u0026deg;C using N\u003csub\u003e2\u003c/sub\u003e gas with flow rate of 15 ml/min for 30 min to remove weakly adsorbed CO\u003csub\u003e2\u003c/sub\u003e at sample\u0026rsquo;s surface. Next step is the desorption process where sample was heated from 50\u0026deg;C to 900\u0026deg;C with a heating rate of 10\u0026deg;C/min. Desorption process occurred in 99.9% N\u003csub\u003e2\u003c/sub\u003e gas atmosphere with flow rate of 15 ml/min. Quantification of CO\u003csub\u003e2\u003c/sub\u003e capture capacity was recorded by using thermocouple detector (TCD) at desorption process. Based on the comparison of the adsorption capacity by physisorption and chemisorption, the most efficient sample was selected for further investigation toward longevity test.\u003c/p\u003e \u003cp\u003eThe longevity test was performed using the aforementioned instrument of Micromeritics Autochem II Plus. Same analysis method was performed to the sample with slight modification at CO\u003csub\u003e2\u003c/sub\u003e adsorption step. During CO\u003csub\u003e2\u003c/sub\u003e adsorption step, it was prolonged to 6 hours, and the reaction was ended to collect the sample for further analysis. This technique was repeated for 12, 24, and 48 hours to compare the effect of longer CO\u003csub\u003e2\u003c/sub\u003e exposure to the sample. Finally, collected samples were analyzed to identify the formation of functional groups, morphology and thermal analysis. Functional groups were determined using Fourier Transform Infrared Spectroscopy (FTIR) from model Perkin Elmer Spectrum 400 FT-IR/NIR with Imaging System. Meanwhile, the morphology of the carbonate formation was observed using Field Emission Scanning Electron Microscopy, FESEM (Zeiss Merlin). Lastly, sample was then analyzed using Thermogravimetric Analyzer, TGA (NETZSCH, STA 449 F3 Jupiter) to study carbonate decomposition of the sample while determining the adsorption capacity in longer CO\u003csub\u003e2\u003c/sub\u003e exposure.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4. BREAKTHROUGH, REGENERATION AND RECYCLABILITY STUDY\u003c/h2\u003e \u003cp\u003eTo investigate the effect of temperature on breakthrough time, adsorption experiments were conducted at temperatures ranging from 25 to 75\u0026deg;C. The most efficient adsorption temperature was then selected to examine desorption performance at two temperatures, 25 and 75\u0026deg;C. Finally, to evaluate the recyclability of the adsorbent, the optimal adsorption and desorption conditions were applied over five consecutive cycles. The CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity for each cycle was recorded to determine the regeneration efficiency of the adsorbent.\u003c/p\u003e \u003cp\u003eRegeneration study was conducted to evaluate the reusability of an adsorbent by applying adsorption/desorption process until the adsorbent loses its ability for adsorption. The study was conducted using laboratory bench-scale CO\u003csub\u003e2\u003c/sub\u003e fixed-bed reactor. Procedure for recyclability study started with placing 35 g of the adsorbent in column in packed-bed column. Sample was then purged with N\u003csub\u003e2\u003c/sub\u003e gas at 25 \u0026ordm;C for 10 min to remove other gases in column. Adsorption temperature was conducted at 25 \u0026ordm;C with flow rate 200 ml/min of 15 vol.% CO\u003csub\u003e2\u003c/sub\u003e which was verified using CO\u003csub\u003e2\u003c/sub\u003e analyzer (Alpha Omega, Series 9610).\u003c/p\u003e \u003c/div\u003e"},{"header":"3. RESULT","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.1. CHARACTERIZATION\u003c/h2\u003e \u003cp\u003eXRD diffraction pattern of calcined alkaline sludge (CAS) and activated AS was observed in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. After calcination, sludge was dominated with tetragonal CaSiO\u003csub\u003e3\u003c/sub\u003e (ICDD 00-027-0088) at 2θ\u0026thinsp;=\u0026thinsp;23\u0026ordm;, 38\u0026ordm;, 41\u0026ordm;, 51\u0026ordm;, 53\u0026ordm;, and 57\u0026ordm;. There were also some tetragonal SiO\u003csub\u003e2\u003c/sub\u003e (ICDD 01-082-0512) at 2θ\u0026thinsp;=\u0026thinsp;21\u0026ordm;, 25\u0026ordm;, 26\u0026ordm;, 28\u0026ordm;, 29\u0026ordm; and 35\u0026ordm; after calcination. There was also unaffected monoclinic CaMg(SiO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e (ICDD 01-083-2016) at 2θ\u0026thinsp;=\u0026thinsp;39\u0026ordm; even after calcination at 900 \u0026ordm;C. Past research showed decomposition of CaMg(SiO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e at higher temperature of 1500 \u0026ordm;C will decomposed into MgSiO\u003csub\u003e3\u003c/sub\u003e and CaSiO\u003csub\u003e3\u003c/sub\u003e which could have potential for CO\u003csub\u003e2\u003c/sub\u003e adsorption (Oguri et al. \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e1997\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eAfter activating 1M of HCl, sample presented with more crystalline tetragonal SiO\u003csub\u003e2\u003c/sub\u003e at 2θ\u0026thinsp;=\u0026thinsp;21\u0026ordm;. Activated samples also has less CaSiO\u003csub\u003e3\u003c/sub\u003e peak thus indicated that the HCl reacted with CaSiO\u003csub\u003e3\u003c/sub\u003e in the sample as shown in Eq.\u0026nbsp;1. H\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e is insoluble in water and remained in the sample, whereas CaCl\u003csub\u003e2\u003c/sub\u003e is water-soluble and was eliminated during washing process thereby explaining the reduced Ca in sample after acid washing.\u003c/p\u003e \u003cp\u003eCaSiO\u003csub\u003e3\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;2HCl \u0026loz; CaCl\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;H\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e3\u003c/sub\u003e (Eq.\u0026nbsp;1)\u003c/p\u003e \u003cp\u003eIt was also observed that cubic Fe\u003csub\u003e3\u003c/sub\u003eSi (ICDD 00-035-0519) was present at 2θ\u0026thinsp;=\u0026thinsp;45\u0026ordm; and CaMg(SiO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e (ICDD 01-083-2016) at 2θ\u0026thinsp;=\u0026thinsp;56\u0026ordm;. The revelation of these compounds demonstrating that even low acid concentration can help release inorganic mineral within the sludge, thereby enhancing adsorption process.\u003c/p\u003e \u003cp\u003eHowever, for sample AS_HCl_2M, SiO\u003csub\u003e2\u003c/sub\u003e becoming less crystalline compared to sample activated with 1M. Our study found similar result from Shang et al. (\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2025\u003c/span\u003e), in which the acid treatment makes SiO\u003csub\u003e2\u003c/sub\u003e less crystalline. A lower degree of crystallinity in SiO\u003csub\u003e2\u003c/sub\u003e corresponds to a higher density of structural defects, which enhances its oxygen adsorption capacity (Shang et al. \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). It was also observed that Fe\u003csub\u003e3\u003c/sub\u003eSi and CaMg(SiO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e are becoming more amorphous as the concentration of acid increases. It was possible that both compounds reacted with HCl similar with CaSiO\u003csub\u003e3\u003c/sub\u003e above and washed out of sample. For sample AS_HCl_3M, SiO\u003csub\u003e2\u003c/sub\u003e becomes more crystalline after 2M. There was also small peak of CaSiO\u003csub\u003e3\u003c/sub\u003e, Fe\u003csub\u003e2\u003c/sub\u003eSi, CaMg(SiO\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003e and CaSi\u003csub\u003e2\u003c/sub\u003e (ICDD 00-001-1276) at 2θ\u0026thinsp;=\u0026thinsp;47\u0026ordm; still detected after activation. Sample AS_HCl_4M showed similar crystalline peak of SiO\u003csub\u003e2\u003c/sub\u003e and very small peak of other metal containing compound.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e adsorption performance is highly dependent on textural properties of the adsorbent. Figure\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e showed N\u003csub\u003e2\u003c/sub\u003e adsorption-desorption isotherm and pore size distribution by all samples. As a result, CAS showed type III isotherm in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(a), characteristic of nonporous materials due to absence of monolayer formation adsorption. On the other hand, activated samples were identified to have type IV isotherm identified by knee-shaped at initial relative pressure region indicating a monolayer formation as seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(b)-(e). This behavior is consistent with the presence of abundance of mesopores within the sample (Thommes et al. \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). All samples also showed type H3 hysteresis loop indicated that pore network mostly consisted of macropores that did not allow gases to fully adsorbed in the pores (Thommes et al. \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Sample AS_HCl_2M and AS_HCl_4M were observed to show bigger hysteresis loops compared to other samples. Increment of macropores in these samples could be explained with dissolution of HCl with inorganic element leaving porous material after activation. The smallest hysteresis loops were observed in CAS, since they are not activated, the pores are still filled with inorganic minerals that hinder N\u003csub\u003e2\u003c/sub\u003e adsorption.\u003c/p\u003e \u003cp\u003ePore size distribution of all samples was computed with BJH method are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e(f) and (g). All samples showed pore size distribution ranging from 10\u0026ndash;125 nm. Sample AS_HCl_2M showed highest distribution of mesopores in region of 30 nm. The inconsistencies of pore development could be fault with various inorganic elements inherent within the sample. Second highest distribution of pores was sample AS_HCl_4M followed by AS_HCl_3M and AS_HCl_1M. These 3 samples showed smooth bell bottom distribution curve that also ranging in region 10\u0026ndash;50 nm with the highest pore size in 30 nm. Compared to CAS, activation of sludge by acid increased the pore distribution by 6 times, particularly within mesopore and macropore range. Additionally, CAS did possess the pore size in the similar range, but instead effectively enhanced development of existing pore structure. It is also notable that both 2M and 4M had highest distribution of mesopore that helped contribute to overall S\u003csub\u003eBET\u003c/sub\u003e of both samples.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eUsing HCl as activator definitely enhance the BET surface area as tabulated in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Sample AS_HCl_2M showed the highest S\u003csub\u003eBET\u003c/sub\u003e which is 123.73 m\u003csup\u003e2\u003c/sup\u003e/g, followed by AS_HCl_4M which had 105.85 m\u003csup\u003e2\u003c/sup\u003e/g. In contrast, S\u003csub\u003eBET\u003c/sub\u003e of AS is 6.54 m\u003csup\u003e2\u003c/sup\u003e/g and no S\u003csub\u003emicro\u003c/sub\u003e was detected in inactivated sample. It is obvious observation that the acid had helped with improving surface area and helped with generation of micropore and mesopore in the sample. Increase of S\u003csub\u003eBET\u003c/sub\u003e had been attributed by micropore and mesopore developed through acid activation. Previous research by Edama et al. (\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2014\u003c/span\u003e) had shown that acid played a big role in improving surface area of sample. Sample AS_HCl_1M and AS_HCl_2M had the highest micropore surface area, S\u003csub\u003emic\u003c/sub\u003e which is 55.79 m\u003csup\u003e2\u003c/sup\u003e/g and 45.83 m\u003csup\u003e2\u003c/sup\u003e/g, respectively which is important criteria for adsorption. However, samples AS_HCl_3M and AS_HCl_4M had been negatively impacted by the higher concentration of acid, demonstrated decrease of S\u003csub\u003eBET\u003c/sub\u003e in response to SiO species getting more aggregated as confirmed in XRD diffractogram in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Similar pattern was observed in micropore and mesopore generation, with a decrease in S\u003csub\u003emicro\u003c/sub\u003e as acid concentration increase.\u003c/p\u003e \u003cp\u003eIncrement of surface area from using acid was also possibly caused of elimination exchangeable cations and the generation of silica (Barrios et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e1995\u003c/span\u003e). Hence, the higher concentration of acid could also accelerate the agglomeration of silica as observed in XRD in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Meanwhile, average pore diameter had been observed to decrease for all samples compared to the CAS (18.14 nm) and the smallest pore diameter is AS_HCl_1M (5.34 nm). Smaller pore size (\u0026lt;\u0026thinsp;1 nm) were much preferable for better CO\u003csub\u003e2\u003c/sub\u003e adsorption, nevertheless, the refining of pore size could still help with adsorption. Aggregation of silica also affected pore diameter since inconsistencies of pore diameter refinement was observed for all samples, disregarding the concentration of acid. Additionally, total pore volume, V\u003csub\u003etot\u003c/sub\u003e of AS_HCl_2M had drastically increased from 0.023 cm\u003csup\u003e3\u003c/sup\u003e/g to 0.278 cm\u003csup\u003e3\u003c/sup\u003e/g, 12 times increment compared to CAS. All samples experienced enlargement of total pore volume when compared to CAS.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eTextural properties of all adsorbents\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSamples\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e \u003cp\u003eSurface area\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c6\" namest=\"c5\"\u003e \u003cp\u003ePore volume\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eAverage pore diameter (nm)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eS\u003csub\u003eBET\u003c/sub\u003e\u003csup\u003e1\u003c/sup\u003e (m\u003csup\u003e2\u003c/sup\u003e/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eS\u003csub\u003emic\u003c/sub\u003e\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e(m\u003csup\u003e2\u003c/sup\u003e/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eS\u003csub\u003emeso\u003c/sub\u003e\u003csup\u003e3\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e(m\u003csup\u003e2\u003c/sup\u003e/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eV\u003csub\u003etot\u003c/sub\u003e\u003csup\u003e4\u003c/sup\u003e (cm\u0026sup3;/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eV\u003csub\u003emic\u003c/sub\u003e\u003csup\u003e5\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e(cm\u003csup\u003e3\u003c/sup\u003e/g)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCAS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e6.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003en.a.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003en.a.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.023\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003en.a.\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e18.14\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAS_HCl_1M\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e96.37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e55.79\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e40.57\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.129\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.0226\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e5.34\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAS_HCl_2M\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e123.73\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e45.83\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e77.89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.278\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.0178\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e11.47\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAS_HCl_3M\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e86.37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e36.19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e50.18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.181\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.0149\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e8.39\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAS_HCl_4M\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e105.85\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e43.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e62.85\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.247\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.0177\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e9.33\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003ctfoot\u003e \u003ctr\u003e\u003ctd colspan=\"7\"\u003e\u003csup\u003e1\u003c/sup\u003esurface area by BET method \u003csup\u003e2\u003c/sup\u003emicropore surface area by t-plot method \u003csup\u003e3\u003c/sup\u003emesopore surface area by t-plot method \u003csup\u003e4\u003c/sup\u003esingle point total pore volume \u003csup\u003e5\u003c/sup\u003emicropore volume from t-plot method. n.a.- not available\u003c/td\u003e\u003c/tr\u003e \u003c/tfoot\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eFESEM analysis in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e had been applied to understand the differences between internal structure and morphology of inactivated and activated AS. Morphology of CAS in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(a) was quite smooth and plate-like fused together on the surface. After activation, all samples developed rough surface and formed granular-like that shows it separated from main body of the material. Sample AS_HCl_1M as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(b) had almost similar morphology with AS where fused plate-like molecules were beginning to get separated. Sample AS_HCl_2M presented as irregular ellipsoid and generation of pores were observed on the surface. Particles were uniformly well-separated from each other even in larger magnification as seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(c). However, in sample AS_HCl_3M, particle was getting more aggregated or not as well separated compared to AS_HCl_2M as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(d). Last sample of AS_HCl_4M in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e(e) was also seen to have same morphology as 3M. Particles were aggregated at higher concentration of acid of 3M and 4M. However, some pores were still observed on the surface even though at larger magnification, the particles were aggregated.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.2. CO\u003csub\u003e2\u003c/sub\u003e CAPTURE PERFORMANCE\u003c/h2\u003e \u003cp\u003eIn order to gain a deeper understanding of CO\u003csub\u003e2\u003c/sub\u003e uptake performance of the sample, two types of analysis were performed. Physical adsorption (physisorption) and chemical absorption (chemisorption) measurements were performed to explore the differences of gases uptake by the sample. Physisorption can be described as physical interaction between gases (adsorptive) and adsorbent through van der Waals force (Lahn et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) and typically require bonding energies of 10\u0026ndash;70 Kj/mol for interaction (Gabelman \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Chemisorption, on the other hand, involves formation of chemical bond between adsorptive and adsorbent (Atif et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) through ionic interactions (Banerjee \u0026amp; Regalbuto \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) or radical processes (Jones et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Physisorption was measured from CO\u003csub\u003e2\u003c/sub\u003e adsorption isotherm at 25\u0026deg;C and chemisorption was measured from TPD-CO\u003csub\u003e2\u003c/sub\u003e that was performed from 50 \u0026ordm;C to 900 \u0026ordm;C. The results were compared to determine the most efficient adsorbent.\u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a) showed physisorption by adsorption isotherm at 25 \u0026ordm;C, the quantity of CO\u003csub\u003e2\u003c/sub\u003e adsorbed isothermally while increasing the pressure to reach equilibrium. TPD-CO\u003csub\u003e2\u003c/sub\u003e analysis was used to determine basic site and quantity of CO\u003csub\u003e2\u003c/sub\u003e absorbed by the adsorbent as seen in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b). CO\u003csub\u003e2\u003c/sub\u003e gases were used in this analysis because acidic gas such as CO\u003csub\u003e2\u003c/sub\u003e are readily attracted to basic sites. Basic site strength of an adsorbent can be evaluated by the desorption temperature corresponding to where CO\u003csub\u003e2\u003c/sub\u003e gas is desorbed. Adsorbents with stronger basicity exhibit higher bond energies, thus require higher temperatures for desorption, whereas weaker basic sites desorb at lower temperatures (Hakim et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Desorption of gases at certain temperature can elucidate the strength of basic sites on the surface such as weak (250\u0026deg;C), intermediate (250\u0026ndash;480\u0026deg;C) and strong (480\u0026ndash;700\u0026deg;C) and very strong (700\u0026ndash;900\u0026deg;C).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e adsorption isotherm at 25 \u0026ordm;C was shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(a) and adsorption capacity performance was listed in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. It was seen that AS_HCl_2M had the highest adsorption capacity which is 0.44 mg/g compared to CAS which was 0.02 mg/g, increment of almost 20 times after activated by HCl. Adsorption capacity by physisorption increases after activation by 1M and 2M, following that adsorption capacity decreases as the HCl concentration increased. Higher concentration of acid of 3M and 4M was suspected to aggregate SiO\u003csub\u003e2\u003c/sub\u003e species as shown in XRD analysis in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. Both AS_HCl_3M and AS_HCl_4M also possessed the quite high S\u003csub\u003emeso\u003c/sub\u003e compared to all activated sample, since mesopore can only weakly adsorb CO\u003csub\u003e2\u003c/sub\u003e (Dziejarski et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2023\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eTPD \u0026ndash;CO\u003csub\u003e2\u003c/sub\u003e analysis was shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b) and adsorption capacity performance was tabulated in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e(b) showed CAS desorption of CO\u003csub\u003e2\u003c/sub\u003e occurred at 79 \u0026ordm;C and 235 \u0026ordm;C, both at weak basic site, with the second site approaching intermediate strength basic site. CAS was observed did not possess micro- and mesopore on the surface, hence the sample performance on adsorption capacity of physisorption (0.02 mg/g) and chemisorption (29.99 mg/g) was the lowest compared to all activated samples as seen in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. Inactivated CAS which had abundant of CaSiO\u003csub\u003e3\u003c/sub\u003e is better for absorption that uses high temperature in their reaction (M. Wang \u0026amp; Lee \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). By refining the textural properties through acid activation, the adsorption capacity increased. Sample AS_HCl_1M and AS_HCl_2M exhibited basic site at intermediate strength site, with desorption temperature of 373 \u0026ordm;C and 370 \u0026ordm;C, respectively, in which values bordering at strong basic site. Both samples also presented highest temperature of desorption compared to another sample. Based on textural properties of both samples, they exhibit the highest S\u003csub\u003emic\u003c/sub\u003e which according to (L. Chen et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; He et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2021\u003c/span\u003e) is primary contributor to CO\u003csub\u003e2\u003c/sub\u003e adsorption. The narrow channel of micropores provide high surface area creating stronger CO\u003csub\u003e2\u003c/sub\u003e bond to the adsorbent necessitating higher temperature for desorption. Sample AS_HCl_3M and AS_HCl_4M presented two basics sites, weak and intermediate. At weak basic site, sample AS_HCl_3M (86 \u0026ordm;C) and AS_HCl_4M (92 \u0026ordm;C), both samples which also possessed the lowest S\u003csub\u003emic\u003c/sub\u003e (36.19 m\u003csup\u003e2\u003c/sup\u003e/g) and (43 m\u003csup\u003e2\u003c/sup\u003e/g), respectively, causing low temperature desorption.\u003c/p\u003e \u003cp\u003eAdsorption capacity performance by AS_HCl_2M demonstrated the highest adsorption capacity both from physisorption (0.44 mg/g) and chemisorption (284.06 mg/g). Based on Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, sample AS_HCl_2M also has the second highest amount of S\u003csub\u003emicro\u003c/sub\u003e which was found to help in stronger CO\u003csub\u003e2\u003c/sub\u003e adsorption (Chen et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2018\u003c/span\u003e, (He et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Furthermore, according to XRD analysis, 2M has relatively high amount of SiO\u003csub\u003e2\u003c/sub\u003e which was found to help CO\u003csub\u003e2\u003c/sub\u003e sorption capacity (Tahari \u0026amp; Yarmo \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2014\u003c/span\u003e) and least crystalline for all inorganic element. Sample AS_HCl_2M also contained Fe\u003csub\u003e3\u003c/sub\u003eSi which may act as an active site for CO\u003csub\u003e2\u003c/sub\u003e adsorption, resulting in almost 9 times higher uptake compared to CAS. Morphology analysis also proven that AS_HCl_2M had better pore generation and particles were well-separated that increases active site. The higher concentration of acid (3M and 4M), however, had washed the metal of the sludge such as Mg, Fe and Ca similar as previous research by Chen et al. (\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) thus reducing the CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e adsorption capacity performance\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eAdsorbents\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003ePhysisorption by CO\u003csub\u003e2\u003c/sub\u003e adsorption isotherm\u003c/p\u003e \u003cp\u003eat 25\u0026deg;C\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c5\" namest=\"c3\"\u003e \u003cp\u003eChemisorption by CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eAdsorption capacity (mg/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eDesorption temperature (\u0026deg;C)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eAdsorption capacity (mg/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eTotal adsorption capacity (mg/g)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eCAS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e0.02\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e57\u0026ndash;106\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e6.73\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e29.99\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e194\u0026ndash;303\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e23.26\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAS_HCl_1M\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.29\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e228\u0026ndash;590\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e180.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e181.02\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAS_HCl_2M\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.44\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e207\u0026ndash;644\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e284.06\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e284.06\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eAS_HCl_3M\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e0.37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e65\u0026ndash;158\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e30.18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e220.71\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e160\u0026ndash;780\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e190.53\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eAS_HCl_4M\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e0.34\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e76\u0026ndash;160\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e17.51\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e229.06\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e165\u0026ndash;754\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e211.55\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eAccording to the analysis, trend for chemisorption adsorption capacity is AS_HCl_2M\u0026thinsp;\u0026gt;\u0026thinsp;AS_HCl_1M\u0026thinsp;\u0026gt;\u0026thinsp;AS_HCl_4M\u0026thinsp;\u0026gt;\u0026thinsp;AS_HCl_3M\u0026thinsp;\u0026gt;\u0026thinsp;CAS. Physisorption adsorption capacity showed that AS_HCl_2M\u0026thinsp;\u0026gt;\u0026thinsp;AS_HCl_3M\u0026thinsp;\u0026gt;\u0026thinsp;AS_HCl_4M\u0026thinsp;\u0026gt;\u0026thinsp;AS_HCl_1M\u0026thinsp;\u0026gt;\u0026thinsp;CAS. Hence, sample AS_HCl_2M was selected owing to its favorable physisorption and chemisorption.\u003c/p\u003e \u003cp\u003eCompared to literature summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, the chemisorption capacity obtained in this study (284.06 mg/g) lower than the highest reported value by Nawar et al. (\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) (560 mg/g), yet remains in comparable range. However, our study employed physical adsorption as physisorption capacity is crucial indicator of regenerative properties. The longevity test was conducted using AS_HCl_2M, with CAS as standard reference, under different adsorption time (6, 12, 24 and 48 hours) to study the effect of CO\u003csub\u003e2\u003c/sub\u003e exposure time towards adsorbent.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.3. LONGEVITY STUDY\u003c/h2\u003e \u003cp\u003eCarbonate formation and CO\u003csub\u003e2\u003c/sub\u003e adsorbed species were identified from FTIR spectra in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e and Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e listed the assignment of functional group to the frequencies. AS was observed to have sharp absorption bands associated with fingerprints peak of Ca-O at 710 and 873 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Ara\u0026uacute;jo et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). The sample also shown broad peak at 1421 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e which is corresponding to symmetry O-C-O stretching bicarbonate (Hakim et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) possibly of its exposure to atmospheric air. The disappearance of Ca\u0026ndash;O bands in 710 and 873 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in activated AS_HCl_2M suggesting the formation of carbonate during activation process. Considering during preparation, adsorbent was dried in non-inert environment that could cause carbonate formation before analysis. There was smaller peaks at 1421 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e indicating formation of bicarbonate even to the exposure to the atmosphere. This also can be confirmed from small peaks at 1511 and 1639 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e assigned to asymmetry O-C-O bidentate carbonate. Small peak at 3728 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e assigned to \u0026ndash;OH, possibly of established hydrogen bonds between hydroxyl groups and Si after preparation.\u003c/p\u003e \u003cp\u003eAfter 6 hours of CO\u003csub\u003e2\u003c/sub\u003e exposure, Si-O-Si stretching peak detected at 792 and 1022 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Akopyan et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Gwon et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2010\u003c/span\u003e) was becoming more prominent. Contribution of broad peak at 1061 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was assigned to Si-OH bending (Akopyan et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2022\u003c/span\u003e) which also becoming more broad as exposure time increased. This could be related to absorption of moisture from the gases from the tank. Next, formation of carbonate was detected at 1639 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e which is associated with asymmetry O-C-O bidentate carbonate (Lahuri \u0026amp; Yarmo \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; W. Wang et al. \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eAfter 12 hours of exposure, Si-O-Si was detected and Si-OH becoming broader and sharper. Small peak of asymmetry O-C-O bidentate carbonate remained unchanged even after 12 hours of exposure. Formation of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\text{C}\\equiv\\:\\text{C-Si}\\)\u003c/span\u003e\u003c/span\u003e stretching vibration was detected at 2155 cm\u003csup\u003e\u0026minus;1\u003c/sup\u003e (Mei et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Organosilicon with alkyne group can function as nucleophiles that will likely have reaction with CO\u003csub\u003e2\u003c/sub\u003e (Zhang et al. \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eAfter 24 hours of exposure, both Si-O-Si and Si-OH were detected and broad peak stay the same as before. Shoulder peak of asymmetry O-C-O bidentate carbonate was also detected and becoming more prominent with longer CO\u003csub\u003e2\u003c/sub\u003e exposure. Small peak of of \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\text{C}\\equiv\\:\\text{C-Si}\\)\u003c/span\u003e\u003c/span\u003e stretching vibration becoming more noticeable at longer CO\u003csub\u003e2\u003c/sub\u003e exposure. Absorption band around 2311 cm\u003csup\u003e\u0026minus;1\u003c/sup\u003e corresponds to multilayer physisorbed CO\u003csub\u003e2\u003c/sub\u003e (Yang \u0026amp; W\u0026ouml;ll \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). CO\u003csub\u003e2\u003c/sub\u003e was found to absorb weakly on the surface and does not have any reaction to form carbonate. Small absorption peak at 3645 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was associated to bicarbonate from O-H stretch (Hakim et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Another small peaks around 3728 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e which is commonly associated with \u0026ndash;OH group (Satapute et al. \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2017\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eAt 48 hours of exposure, Si-O-Si and Si-OH peak stay similar even at the longest contact of CO\u003csub\u003e2\u003c/sub\u003e. Asymmetry O-C-O bidentate carbonate peak was observed becoming smaller compared to 24 hours. Peak at 2155 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e corresponded to multilayer physisorbed CO\u003csub\u003e2\u003c/sub\u003e was also becoming smaller and barely noticeable \u0026ndash;OH group at 3600\u0026ndash;3700 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eAssignments of experimental frequencies compared with reference frequencies\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAssignments\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eExperimental frequencies\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eReference frequencies\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eReferences\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCa-O bonding\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e710, 873\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e710, 872\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e(Ara\u0026uacute;jo et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2021\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSi-O-Si stretching\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e792\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e792\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e(Akopyan et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2022\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSi-O-Si stretching and/or Si-O-C band\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1022\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1150\u0026thinsp;\u0026minus;\u0026thinsp;950\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e(Gwon et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2010\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSi-OH bending\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1061\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1055\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e(Akopyan et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2022\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSymmetry O-C-O bicarbonate\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1421\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1396\u0026ndash;1500\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e(Lahuri \u0026amp; Yarmo \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2022\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAsymmetry O-C-O bidentate carbonate\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1511, 1639\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e1446\u0026ndash;1590, 1555\u0026ndash;1720\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e(Lahuri \u0026amp; Yarmo \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2022\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\equiv\\:\\)\u003c/span\u003e\u003c/span\u003eC-Si stretching vibration\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2155\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2150\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e(Mei et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2020\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePhysisorption CO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e2311\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e2240\u0026ndash;2390\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e(Yang \u0026amp; W\u0026ouml;ll \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2017\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eBicarbonate O-H\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3645\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3600\u0026ndash;3627\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e(Hakim et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2016\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e-OH group\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e3728\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e3728\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e(Satapute et al. \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2017\u003c/span\u003e)\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e presented TGA analysis after AS_HCl_2M was exposed to CO\u003csub\u003e2\u003c/sub\u003e for 6, 12, 24 and 48 H to study carbonate formation by adsorbent. Before activation, broad decomposition range indicated the moisture and multiple components are tightly bound thus slowing the decomposition rate of the materials. After activation, adsorbent exhibit two degradation zone at lower temperature range of 42\u0026ndash;159 \u0026ordm;C and 170\u0026ndash;1000 \u0026ordm;C. First stage weight loss was 1.5% and second stage exhibit 2.3% for quite broad decomposition range. Broad decomposition range usually indicate multiple decomposition of multiple components with overlapping decomposition temperature. After exposure to different range of time, the adsorbent exhibited two stages of degradation ranging from 51\u0026ndash;300 \u0026ordm;C and 530\u0026ndash;1000 \u0026ordm;C. After 6 hours of CO\u003csub\u003e2\u003c/sub\u003e adsorption, first stage decomposition occurred at 51\u0026ndash;300 \u0026ordm;C which commonly associated with moisture loss (Nassar et al. \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). The first decomposition zone displayed weight loss of 2.6% could be attributed from breaking of hydroxyl group from Si-OH. This observation could be explained from FTIR analysis, in which the broadest band of all exposed adsorbents at 1160 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was assigned to Si-OH bending. Second stage of decomposition occurred at 530\u0026ndash;1000 \u0026ordm;C, which is early decomposition of CaCO\u003csub\u003e3\u003c/sub\u003e \u0026loz; CaO\u0026thinsp;+\u0026thinsp;CO\u003csub\u003e2\u003c/sub\u003e. Previous research also found decomposition of CaCO\u003csub\u003e3\u003c/sub\u003e can started from 500 to 900 \u0026ordm;C (Al-Fateh \u0026amp; Fakeeha \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). Table\u0026nbsp;\u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e listed weight loss from this stage was around 1.3% which can also be associated with efficiency of CO\u003csub\u003e2\u003c/sub\u003e adsorption. After 12 hour, weight loss dropped to 0.8%, increased again to 1.4% at 24 hour and decreased again to 1.3% at 48 hour inferring the adsorption process stop at 24 hours.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab5\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 5\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDecomposition temperature and weight loss after timely exposure of CO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"3\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTemperature (\u0026ordm;C)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eWeight loss (%)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eCAS\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e45\u0026ndash;509\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e707\u0026ndash;991\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.1\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eAS_HCl_2M\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e45\u0026ndash;159\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.5\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e170\u0026ndash;1000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e6H\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e51\u0026ndash;302\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e500\u0026ndash;1000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e12H\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e50\u0026ndash;301\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.6\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e500\u0026ndash;1000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e24H\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e57\u0026ndash;300\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e505\u0026ndash;1000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.4\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e48H\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e55\u0026ndash;308\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e2.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e507\u0026ndash;1000\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.3\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.4. RECYCLABILITY STUDY\u003c/h2\u003e \u003cp\u003eBased on CO\u003csub\u003e2\u003c/sub\u003e capture performance obtained from physisorption and chemisorption method, adsorbent that had the highest adsorption capacity was selected for breakthrough adsorption and regeneration study. Recyclability is an important indicator for commercialization for adsorbent, while efficient desorption process can lower the energy and economic cost (Dutta et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). In addition, breakthrough time served as important parameter for characterizing adsorbent before it can be considered for industrial application.\u003c/p\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003e3.4.1 Effect of adsorption temperature\u003c/h2\u003e \u003cp\u003eFigure \u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(a) showed effect of adsorption temperature from 25\u0026deg;C to 75\u0026deg;C towards breakthrough time. Breakthrough curve is plotted with duration of test against concentration of adsorbate mixed in air with the adsorptive. Breakthrough time is reached when the bed filled with adsorbate no longer has capacity to absorb and equilibrium saturation has been reached. It is observed as temperature increased, breakthrough time become shorter.\u003c/p\u003e \u003cp\u003eAt 25 \u0026ordm;C, breakthrough time is around 3.4 min while increasing it to 35 \u0026ordm;C, reduce the breakthrough time to 2 min. At higher temperature of 65 and 75\u0026ordm;C, shifted the breakthrough curve to the left reducing the breakthrough time to less than 1 min. High temperature had elevated kinetic energy of CO\u003csub\u003e2\u003c/sub\u003e gas leading to less residence time in adsorbent explaining shorter breakthrough time at 75 \u0026ordm;C (Akpasi \u0026amp; Isa \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). It was also observed steeper slope of breakthrough curve as temperature increased quickly reaching breakthrough time. Since adsorption capacity is proportional to the area above the breakthrough curve bounded by \u003cem\u003eC\u003c/em\u003e\u003csub\u003e\u003cem\u003et\u003c/em\u003e\u003c/sub\u003e\u003cem\u003e/C\u003c/em\u003e\u003csub\u003e\u003cem\u003eo\u003c/em\u003e\u003c/sub\u003e = 1, shorter breakthrough time shifted the curve the left thus reduces the area bounded. This explains the shorter breakthrough time to have lower adsorption capacity. This relationship is consistent with the observed trend: 25\u0026deg;C (12.36 mg/g)\u0026thinsp;\u0026gt;\u0026thinsp;35\u0026deg;C (7.44 mg/g)\u0026thinsp;\u0026gt;\u0026thinsp;45\u0026deg;C (5.94 mg/g)\u0026thinsp;\u0026gt;\u0026thinsp;55\u0026deg;C (5.32 mg/g)\u0026thinsp;\u0026gt;\u0026thinsp;65\u0026deg;C (4.75 mg/g)\u0026thinsp;\u0026gt;\u0026thinsp;75\u0026deg;C (4.49 mg/g). These results clearly indicate that 25 \u0026ordm;C is the most favorable operating temperature, providing the highest adsorption capacity and longest breakthrough time.\u003c/p\u003e \u003cp\u003eThis finding is consistent with the work of Hussin et al. (\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), who also reported that higher temperature reduced adsorption capacity of adsorbent. This pattern of adsorption suggested that the adsorption is dominantly controlled by physisorption (Tan et al. \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). As adsorption is an exothermic process, temperature plays an important role in influence performance (Gabelman \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). In this study, the adsorption capacity at the highest temperature (75\u0026deg;C) was reduced to one-third of that observed at the lowest temperature (25\u0026deg;C), further supporting the physisorption mechanism. Since physisorption relies on weak van der Waals\u0026rsquo; interactions, these forces easily disrupted at high temperature (Hussin et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eIn our study, two regenerations temperatures were used to compare low and relatively high temperatures in finding the best regeneration condition. Adsorbent was subjected to five cycles of adsorption \u0026ndash; desorption cycle at 25\u0026deg;C and then regenerated at two different temperatures of 25 and 75\u0026deg;C. Figure\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e9\u003c/span\u003e(b) showed that during room temperature regeneration (25\u0026deg;C), the 1st cycle exhibited the highest adsorption capacity of 12.32 mg/g, followed with minor drop to 10.56 mg/g and remain persisted until the 5th cycle. The minor decline of adsorption capacity is contributed by chemically bound CO\u003csub\u003e2\u003c/sub\u003e in adsorbent\u0026rsquo;s pore that cannot be desorbed at low temperature (Hussin et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn contrast, regeneration at 75 \u0026ordm;C significantly reduced the adsorption capacity to 5.72 mg/g, approximately half of that achieved at 25 \u0026ordm;C. The elevated temperature, even at 75 \u0026ordm;C, may have accelerated the geopolymerization process affecting material containing aluminosilicate (Chindaprasirt \u0026amp; Rattanasak \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2017\u003c/span\u003e), resulting in entrapment of previously adsorbed CO\u003csub\u003e2\u003c/sub\u003e and affecting the adsorption capacity. Despite this reduction, adsorption capacity remained stable throughout all 5 cycles, suggesting higher regeneration temperature hindered further decline in performance.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eRegeneration efficiency, RE of adsorbent regenerated at 25 and 75\u0026deg;C was shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e10\u003c/span\u003e. At 25\u0026deg;C, RE was 88.6% whereas regeneration at 75\u0026deg;C, RE achieved 100%. At first glance, it was seen that regeneration at 75\u0026deg;C exhibited superior RE performance since there is no loss of adsorption capability. However, adsorption capacity of 75\u0026deg;C remained 5.72 mg/g for all of its 5 cycles. Even though RE value of regeneration at 25\u0026deg;C is 12% lower than 75\u0026deg;C, the adsorption capacity at 25\u0026deg;C is still nearly twofold higher, reaching 12.32 mg/g in the first cycle. These findings indicate that room-temperature regeneration is still viable, as AS_HCl_2M retained its adsorption capacity and demonstrated reliable recyclability performances.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"4. CONCLUSION","content":"\u003cp\u003eIn this work, CO\u003csub\u003e2\u003c/sub\u003e adsorption performance of activated alkaline sludge (AS) from photovoltaic waste using low to high acid concentration was compared with calcined AS.\u003c/p\u003e \u003cp\u003eWhile all activated waste showed significant improvement in their physicochemical properties, AS_HCl_2M had stand out in terms of CO\u003csub\u003e2\u003c/sub\u003e adsorption and its recyclability attribute. Surface area of CAS was improved from 6.50 m\u003csup\u003e2\u003c/sup\u003e/g to 123.73 m\u003csup\u003e2\u003c/sup\u003e/g (AS_HCl_2M) which is 20 times higher than the original value. XRD analysis also revealed that AS_HCl_2M had SiO species with lower degree of crystallinity that can help increase oxygen adsorption capacity. Highest CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity in this study is 0.44 mg/g for physical adsorption and 284.06 mg/g for chemical absorption. It was observed that low concentration acid (1 and 2M) had substantially improved the textural properties which are the crucial traits for adsorption, while the higher concentration acid (3 and 4M) led to a slight decline of adsorption performance. The study followed with longevity test of the adsorbent. FTIR analysis confirmed that CO\u003csub\u003e2\u003c/sub\u003e exposure saturated after 6 hours, as the O-C-O bidentate carbonate peak at 1639 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e remained unchanged at 12, 24 and 48 hours. TGA also confirmed the saturation time since the weight loss of adsorbent maintained the same with only 0.1% differences even after 6 hours. Morphology analysis observed the change from irregular ellipsoid particle to rough coral framework-like after CO\u003csub\u003e2\u003c/sub\u003e exposure. Recyclability study had also shown that straightforward regeneration process at 25\u0026deg;C had shown adsorption capacity of 10.56 mg/g remain stable for 4 cycles. Considering its reusability, utilization of unused waste and the simplicity of its preparation, this waste-derived adsorbent shows strong potential as viable CO\u003csub\u003e2\u003c/sub\u003e adsorbent.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eETHICAL APPROVAL\u003c/h2\u003e \u003cp\u003eThis is not applicable.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eCONSENT TO PARTICIPATE\u003c/strong\u003e \u003cp\u003eThis is not applicable.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eCONSENT TO PUBLISH\u003c/strong\u003e \u003cp\u003eThis is not applicable.\u003c/p\u003e \u003c/p\u003e \u003cp\u003e \u003cstrong\u003eCOMPETING INTEREST\u003c/strong\u003e \u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFUNDING\u003c/h2\u003e \u003cp\u003eThe authors wish to express gratitude for the research work supported by OCI TerraSus Sdn. Bhd. (6300940) awarded to Universiti Putra Malaysia and (9795800) awarded by Universiti Putra Malaysia.\u003c/p\u003e\u003ch2\u003eAUTHORS\u0026rsquo; CONTRIBUTION\u003c/h2\u003e \u003cp\u003eConceptualization and writing\u0026mdash;original draft: Siti Sarahah Sulhadi. Data collection, formal analysis, and investigation: Siti Sarahah Sulhadi. Writing\u0026mdash;review and editing: Siti Sarahah Sulhadi, Azizul Hakim Lahuri, Syawal Mohd Yusof, Farihahusnah Hussin, Mohamed Kheireddine Aroua, Taufiq Yap Yun Hin, Nur Farhana Jaafar, Syazreen Nadia Sulaiman. Resources and supervision: Azizul Hakim Lahuri. All authors read and approved the final manuscript.\u003c/p\u003e\u003ch2\u003eACKNOWLEDGEMENTS\u003c/h2\u003e \u003cp\u003eThe authors thank Dr. Azizul Hakim Lahuri for technical advice and constructive feedback. The authors also acknowledge Syawal Mohd Yusof and Assoc. Prof. Ts. Dr Farihahusnah Hussin for assistance with data analysis. In addition, the author are grateful for Prof. Mohamed Kheireddine Aroua, Prof. Taufiq Yap Yun Hin, Nur Farhana Jaafar and Syazreen Nadia Sulaiman for providing the facilities for this research. This research was supported by OCI TerraSus Sdn. Bhd. (6300940) and Universiti Putra Malaysia (9795800). Any errors remain our own.\u003c/p\u003e\u003ch2\u003eDATA AVAILABILITY STATEMENT\u003c/h2\u003e \u003cp\u003eAll data generated or analyzed during this study are included in this article.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAbuelnoor N, AlHajaj A, Khaleel M, Vega LF, Abu-Zahra MRM (2021) Activated carbons from biomass-based sources for CO2 capture applications. 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Matter 3:558\u0026ndash;570\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eZhang Z, Yang Z, Zhang S, Zhang D, Shen B, Li Z, Ma J, Liu L (2023) Fabrication of robust CaO-based sorbent via entire utilization of MSW incineration bottom ash for CO2 capture. \u003cem\u003eSeparation and Purification Technology\u003c/em\u003e 307(November 2022). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1016/j.seppur.2022.122795\u003c/span\u003e\u003cspan address=\"10.1016/j.seppur.2022.122795\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"environmental-science-and-pollution-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"espr","sideBox":"Learn more about [Environmental Science and Pollution Research](https://www.springer.com/journal/11356)","snPcode":"11356","submissionUrl":"https://submission.nature.com/new-submission/11356/3","title":"Environmental Science and Pollution Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-8421032/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8421032/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eRecycling industrial wastage offers an effective strategy to mitigate waste discharge, promoting development of low-cost CO\u003csub\u003e2\u003c/sub\u003e adsorbents aligning with the principles of the circular economy. This study utilized waste alkaline sludge (AS) originated from photovoltaic industry, activated with 1-4M of hydrochloric acid (HCl), to study its effect on CO\u003csub\u003e2\u003c/sub\u003e capture performance. Activated AS were characterized with N\u003csub\u003e2\u003c/sub\u003e adsorption-desorption isotherm (BET-N\u003csub\u003e2\u003c/sub\u003e), X-Ray diffraction (XRD), field emission scanning electron microscopy (FESEM-EDX), CO\u003csub\u003e2\u003c/sub\u003e adsorption-desorption isotherm (BET-CO\u003csub\u003e2\u003c/sub\u003e) and CO\u003csub\u003e2\u003c/sub\u003e temperature programmed desorption (TPD-CO\u003csub\u003e2\u003c/sub\u003e). AS activated by 2M HCl exhibited highest surface area of 123.73 m\u003csup\u003e2\u003c/sup\u003e/g and dominated by mesopores which played a significant role in CO\u003csub\u003e2\u003c/sub\u003e adsorption. CO\u003csub\u003e2\u003c/sub\u003e capture by physisorption at 25 \u0026ordm;C exhibited adsorption capacity of 0.44 mg/g, which was 20 times increment than. Meanwhile, CO\u003csub\u003e2\u003c/sub\u003e capture performance by chemisorption was 284 mg/g, with temperature ranging from 207\u0026ndash;644 \u0026ordm;C and around 9 times higher than inactivated AS. Longevity study revealed that weight loss after prolonged CO\u003csub\u003e2\u003c/sub\u003e exposure for 24 hours remain around 1.4%, indicating adsorption stop after 24 hours. Longer exposure time induce morphological transformation from irregular ellipsoid into packed and aggregated nano coral, thus lessening its adsorption capacity. It was determined that 25 \u0026ordm;C was the optimal temperature for both adsorption and regeneration process. The adsorbent also demonstrated stable recyclability for 5 cycles, showing only 14% capacity reduction at 2nd cycle and achieving a regeneration efficiency of 88.6%.\u003c/p\u003e","manuscriptTitle":"Longevity and Recyclability Study of Acid Activated Alkaline Sludge From Photovoltaic Industry Toward Co2 Capture","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-13 19:25:49","doi":"10.21203/rs.3.rs-8421032/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major Revision","date":"2026-02-18T07:17:46+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2026-01-18T10:52:45+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-01-08T17:43:20+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Environmental Science and Pollution Research","date":"2026-01-08T12:06:33+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-12-31T05:30:43+00:00","index":"","fulltext":""},{"type":"submitted","content":"Environmental Science and Pollution Research","date":"2025-12-28T03:27:42+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"environmental-science-and-pollution-research","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"espr","sideBox":"Learn more about [Environmental Science and Pollution Research](https://www.springer.com/journal/11356)","snPcode":"11356","submissionUrl":"https://submission.nature.com/new-submission/11356/3","title":"Environmental Science and Pollution Research","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"b198eaf9-d61a-4ce7-80ff-d493248d4503","owner":[],"postedDate":"January 13th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-05-06T11:22:46+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-13 19:25:49","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8421032","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8421032","identity":"rs-8421032","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}