Using Gypsum-modified Biochar to Adsorb Phosphate from Water: Adsorption Performance and Mechanism

Using Gypsum-modified Biochar to Adsorb Phosphate from Water: Adsorption Performance and Mechanism | 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 Using Gypsum-modified Biochar to Adsorb Phosphate from Water: Adsorption Performance and Mechanism Guanli Xu, Di Wu, Xiang Ao This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5660141/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract This study explored the use of gypsum-modified biochar (GBC) for phosphate adsorption from water. GBC, composed of an equal weight mixture of gypsum (CaSO4·2H2O) and straw biochar subjected to pyrolysis, demonstrated strong resistance to interfering anions (NO 3 − , Cl − , and HCO 3 − ) and high adsorption efficiency across a wide pH range (7–11). The adsorption behavior was well described by the Pseudo-Second-Order kinetic and Langmuir models, with a maximum capacity of 247 mg∙g −1 at 303 K. The adsorption mechanisms involved ligand exchange, electrostatic attraction, and micro-precipitation. These findings highlight GBC as a simple and effective adsorbent for phosphate removal. Gypsum-modified biochar Phosphate Adsorption Micro-precipitation Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1. Introduction Phosphorus is essential to biology. It is crucial to the growth of plants and is a contributory factor to causes of eutrophication. Its excessive presence in bodies of water stimulates the rising of harmful algae and aquatic plants, whose subsequent death and decomposition deplete the water’s oxygen(Khan and Mohammad 2014). Removing excessive phosphorus from water is therefore essential to protect the aquatic environment. Phosphorus usually exists in water as phosphate, so efforts to control phosphorus have sought to reduce aqueous phosphate concentrations. Applied methodologies include chemical precipitation(Shaddel et al. 2020), adsorption(Zhu et al. 2020) and biological techniques (L. G. Wu et al. 2021), among which adsorption is particularly attractive for offering the advantages of less sludge, cost-effective, convenient operation and wider applicability. Biochar, a solid with a lot of carbon by the pyrolysis of organic materials, is a popular adsorbent because of its ample specific surface area, well-developed pores and ability to host various surface functional groups. However, biochar’s negative surface charge impedes its adsorption of phosphate anions; the phosphorous adsorption capacity of unmodified biochar is 5.45–16.1 mg∙g − 1 . (Eduah et al. 2020; Trazzi et al. 2016; Zhao et al. 2017). Metallic elements in biomass-based adsorbents have an essential influence on the adsorption capacity of anions (Li, Wang, Zhou, Awasthi, Ali, Zhang, Lahori, et al. 2016), but the low metal cation content of biochar(Li, Wang, Zhou, Awasthi, Ali, Zhang, Gaston, et al. 2016) limits its capacity to adsorb anions. Modifying biochar with cations (Ca 2+ , Mg 2+ , La 3+ , Fe 2+ and Al 3+ ) can significantly improve its ability to adsorb phosphate (Cui et al. 2020; C. Fang et al. 2014; Jia et al. 2020; X. N. Liu et al. 2019). Among these, calcium (Ca) is ecologically harmless and easy to obtain (Cao et al. 2020), thus making it a suitable cation for modifying biochar. It had been proven to be an effective way to prepare modified adsorbent by pyrolyzing Ca(OH) 2 or CaCO 3 with various raw materials to greatly improve the phosphorus adsorption capacity (Wang et al., 2018; Liu et al., 2019; Cao et al., 2020). Traditional Ca-modified biochar is made of chemical reagents (L. Fang et al. 2020) or other biomass wastes containing calcium (X. N. Liu et al. 2019) to load into the biochar. Controlling the cost of adsorbents is essential to their industrial applicability. Gypsum is a common and cheap calcium mineral. It consists of calcium sulfate dihydrate (CaSO 4 ·2H 2 O), which is a suitable calcium source to enhance biochar’s absorption of phosphorus. This work makes biochar from rice straw and uses gypsum as the Ca source. This study focused on the feasibility and mechanism of milling and pyrolysing rice straw and gypsum to create modified biochar that was suitable for adsorbing phosphate. The influencing factors of temperature, pH and anions on phosphate adsorption were assessed. The adsorption capacity was determined, and adsorption mechanisms were elucidated by considering the experiments and related adsorption models. This study demonstrates a promising functionalised biochar adsorbent made from agricultural waste and a low-cost calcium mineral, which should have broad applicability to phosphate adsorption. This study aims to explore the feasibility and mechanisms of developing a functionalized biochar adsorbent from rice straw and gypsum for efficient phosphate removal from water. It seeks to achieve a cost-effective solution by utilizing agricultural waste and a low-cost calcium source while investigating key factors affecting phosphate adsorption, such as temperature, pH, and competing anions. Through experimental analysis and adsorption modeling, the study elucidates the interaction mechanisms between modified biochar and phosphate. Ultimately, it demonstrates the practicality of a sustainable and efficient approach to addressing water pollution and mitigating eutrophication. 2. Materials and methods 2.1 Materials Gypsum (CaSO 4 ·2H 2 O; particle size < 75 µm) and chemical reagents were analytical-grade reagents (AR). Rice straw collected from a farm in China’s Sichuan Province was used as feedstock for biochar production. 2.2 Preparation of GBC Rice straw was cut (5–8 cm) and rinsed using deionised water. After being put in a constant temperature drying oven at 50℃ for 12 h, the feedstocks were pyrolysed in a vacuum tube furnace at a ramping rate of 10℃∙ min − 1 to a target temperature of 700, 600, or 500°C under a nitrogen flow of 1 L∙min − 1 , holding for 1 h before cooling. The resulting biochar were crushed and sieved (0.1–0.5 mm) and were denoted 700BC, 600BC and 500BC, respectively. The gypsum-modified biochar (GBC) was prepared by mixing the different biochar specimens with an equal mass of gypsum; the resulting specimens were named 700GBC, 600GBC and 500GBC, respectively. Specimens were stored in sealed containers until they were used in further tests. 2.3 Batch experiments Batch adsorption experiments were performed in 50 ml centrifuge tubes by transferring 40 mg GBC into 40 ml phosphate solutions made by dissolving known quantities of potassium phosphate monobasic (KH 2 PO 4 , AR) in deionised water. These containers were tightly sealed and vibrated at 180 r∙min − 1 in a constant temperature shaker. After adsorption, solutions were centrifuged at 3000 rpm for 10 min, the supernatants were collected and passed through a nylon filter membrane (pore size < 0.45 µm). The phosphorus concentration in the supernatants was measured by ammonium molybdate tetrahydrate spectrophotometry on a spectrophotometer (UV -2700i Shimadzu) set at a wavelength of 660 nm (Murphy and Riley 1962). Each experiment was repeated three times and reported average values. Kinetic adsorption experiments were tested at pH 6.5 and 303 K using 100 mg∙L − 1 phosphorus solution with 40 mg of each GBC under different residence times (0.5, 1, 2, 4, 6, 8, 12, 24, 48, 72, 120, 168, 216 and 360 h). Isothermal adsorption experiments were conducted, using different initial concentrations of phosphorus (1, 5, 10, 25, 50, 100, 150, 200, 300, 500, 1000, 1500 and 2000 mg∙L − 1 ). The containers oscillated for 216 h to reach equilibrium. To explore the impact of initial pH on adsorption results, the initial pH was adjusted to 3, 5, 7, 9 or 11 by HCl and NaOH solutions. A 40 mg portion of 700GBC was added to the phosphorus solutions (100 mg∙L − 1 ) and shaken at 180 r∙min − 1 for 216 h at 303 K. The final pH values and phosphorus concentrations were then measured. To analyse the influence of accompanying anions on phosphorus adsorption, 40 mg 700GBC was added to 100 mg∙L − 1 phosphorus solution at initial pH of 6.5 and holding various concentrations of anions (Cl − , NO 3− and HCO 3− ). After being shaken at 180 r∙min − 1 for 216 h at 303 K, measuring the phosphorus concentration and final pH value of each filtrate. 2.4 Analytical method The microstructure and element distributions of the samples were detected by Scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The specificity surface area of samples was determined by the Brunauer–Emmett–Teller standard method of measuring N 2 absorption. Fourier transform infrared (FTIR) spectroscopy identified changes of functional groups in the wavenumber range of 400 to 4000 cm − 1 . The mineralogical characteristics and crystal structures were analysed by powder X-ray diffractometry (XRD) at a scan speed of 0.02°/s in a scanning 2θ range of 10°–60°. 3 Result and discussion 3.1. Characterisation of biochar Characteristics of biochar specimens (Table 1 ) indicated that the pore size and specific surface area generally broadened with rising pyrolysis temperature but decreased when the temperature reached 700°C. This could have been due to the high temperature causing pyrogenic volatiles to be released from cellulose, the main substance structurally supporting cell walls in the rice straw (Chabannes et al. 2001),which broadened the pore size and specific surface area. However, higher pyrolysis temperature damaged the structure carbon (Tao et al. 2020) and decreased the pore size and specific surface area. The increase of carbon content was caused by an increasing rate of aromatisation (W. X. Wu et al. 2012), which reflected the degree of structural modification of the biochar (Zheng et al. 2013). Silicon, which has a high uptake rate in rice (Mehrabanjoubani et al. 2019), was the second most abundant element in biochar, followed by potassium, calcium, nitrogen, magnesium and phosphorus. These elements could considerably influence the mineral composition of biochar and likely affect its adsorption of phosphate. Table 1 Characteristics of biochar specimens Sample Yield (%) Surface area (m 2 g − 1 ) Pore size (nm) C N Si Al P Ca Mg K Na Cl 500BC 38.82 6.13 20.8 42.44 0.47 12.38 0.04 0.15 0.64 0.24 5.68 0.048 1.8 600BC 38.72 9.08 29.3 44.8 0.42 12.65 0.018 0.18 0.77 0.3 5.64 < 0.03 1.8 700BC 36.9 4.07 16.4 41.16 0.54 14.2 0.022 0.14 0.74 0.24 5.91 0.088 1.8 According to the FTIR spectra (Fig. S1 ), All bands appeared with the highest intensities for 500BC but gradually fell with increasing pyrolysis temperature, reflecting aromatization and dehydration causing a decrease in acidic and basic groups as temperature rises. (Novak et al. 2013). XRD spectra (Fig. S2) show that mineral composition of biochar changed slightly with increasing pyrolysis temperature. The peaks at 2θ of 24.2°, 28.3° and 29.4° confirmed the presence of kalicinite, sylvite and calcite, respectively. The peak intensity of calcite at 600BC and 700BC is lower than that at 500BC, indicating the decomposition of calcite during pyrolysis. The hump observed around 22° reveals the presence of an amorphous-like phase, which comprised a stacked matrix of graphitic planes, suggesting the formation of graphitic platelets(K. K. Zhang et al. 2018). 3.2 Adsorption performance 3.2.1 Adsorption kinetics The adsorption kinetics of phosphorus by different GBC samples were researched. The overall outcomes are depicted in Fig. 1 . The quantity of adsorption was calculated using the adsorption equation (Eq. (1)). $$\:{\text{q}}_{\text{t}}=\frac{{\text{C}}_{\text{i}}-{\text{C}}_{\text{e}}}{\text{m}}\times\:\text{V}$$ q t (mg∙g − 1 ) is the quantity of adsorption at time t (h); The concentrations C e and C i (mg∙L − 1 ) refer to the equilibrium and initial phosphorus concentrations, respectively; V (L) and m (g) represent the volume of solution and mass of GBC, respectively. The porosity of biochar may cause it take longer to reach equilibrium than non-porous adsorbents (Tran et al. 2017), so the adsorption time was set to 360 h. Figure 1 shows the changes in the different specimens’ phosphate adsorption capacity and pH value with time. Each GBC shows a similar trend in phosphate adsorption capacity, which can be separated into five stages (Fig. 1 ). The first stage was external diffusion. The adsorption rate was the fastest, and the pH changed rapidly after the beginning of adsorption. The fast adsorption rate of GBC could be caused by the impetus that came from the difference of phosphate concentration between adsorbent and solution. Phosphate could be quickly adsorbed through the liquid membrane of the adsorbent. At the same time, Ca reacted with phosphate to form Hydroxyapatite (HAP), which was adsorbed on GBC and caused the rapid decline of pH value and increase of q t value. The second stage was internal diffusion. The adsorbed phosphate was transported to the internal pores of the GBC, therefore adsorption rate was slow. The third stage was temporary adsorption equilibrium. The adsorbed phosphate reached a temporary saturation in GBC, and the q t of GBC remained essentially stable, as did the pH value. The fourth stage was partial desorption. The q t decreased and the phosphate content in the solution increased, but the pH value was generally stable. This might be caused by the physical desorption of phosphate: some phosphate might have experienced electrostatic association with the surface of the GBC, where it could then be released back into the solution because the prolonged fast stirring was stronger than the phosphate’s relatively weak interaction with the surface(Shepherd et al. 2017). The fifth stage was the final adsorption equilibrium. q t remained finally balanced and pH remained stable, which indicated the phosphate adsorbed by GBC reached final equilibrium in about 216 h. The results of adsorption data were fitted to two kinetic models (Eq. 2 and Eq. 3) Pseudo-First-Order: \(\:{\text{q}}_{\text{t}}={\text{q}}_{\text{e}}(1-{\text{e}}^{{-\text{k}}_{1}\text{t}})\) ( 2 ) Pseudo-Second-Order: \(\:{\text{q}}_{\text{t}}=\frac{{\text{q}}_{\text{e}}^{2}{\text{k}}_{2}\text{t}}{1+{\text{k}}_{2}{\text{q}}_{\text{e}}^{\text{t}}}\) ( 3 ) q e (mg·g − 1 ) refers to the quantity of adsorption at equilibrium time; k 1 (h − 1 ) and k 2 (g∙mg − 1 ∙h − 1 ) are adsorption rate constants. Table 2 shows the corresponding fitting results. The Pseudo-Second-Order model better fits the adsorption data on the three GBC adsorbents (Fig. 2 ), thus reflecting that chemical adsorption is the primary mechanism of phosphate adsorption (Lalley et al. 2016). Comparing the q e values of different GBC adsorption equilibria reveals that 700GBC had the highest adsorption capacity, so further experiments considered only 700GBC. Table 2 Adsorption kinetic parameters Sample Pseudo-first-order model Pseudo-second-order model K 1 (h − 1 ) q e (mg∙g − 1 ) R 2 K 2 (g∙mg − 1 ∙h − 1 ) q e (mg∙g − 1 ) R 2 700GBC 0.602 ± 0.0898 22.9 ± 0.589 0.934 0.0419 ± 0.00651 23.9 ± 0.488 0.965 600GBC 0.373 ± 0.0779 22.0 ± 0.895 0.858 0.0323 ± 0.00749 22.5 ± 0.768 0.912 500GBC 0.351 ± 0.0392 20.5 ± 0.573 0.938 0.0285 ± 0.00392 21.3 ± 0.433 0.972 3.2.2 Adsorption isotherms The Langmuir (Eq. 4) and Freundlich (Eq. 5) models were used to fit 700GBC's adsorption isotherms, as shown in Fig. 3 and Table 3 . Table 3 Adsorption isotherm parameter Tempter Langmuir Freundlich K l ×10 − 3 (L∙mg − 1 ) q m (mg∙g − 1 ) R 2 K f (mg (1−1/n) ∙L 1/n ∙g − 1 ) 1/n R 2 313K 1.55 ± 0.157 265 ± 10.9 0.995 3.44 ± 0.874 0.546 ± 0.0358 0.983 303K 1.53 ± 0.159 247 ± 10.4 0.995 3.02 ± 0.818 0.553 ± 0.0380 0.982 293K 1.47 ± 0.110 212 ± 5.11 0.998 2.84 ± 0.876 0.548 ± 0.043 0.974 Langmuir: \(\:{\text{q}}_{\text{e}}=\frac{{\text{K}}_{\text{l}}{\text{q}}_{\text{m}}{\text{C}}_{\text{e}}}{(1+{\text{K}}_{\text{l}}{\text{C}}_{\text{e}})}\) ( 4 ) Freundlich: \(\:{\text{q}}_{\text{e}}={\text{K}}_{\text{f}}{\text{C}}_{\text{e}}^{1/\text{n}}\) ( 5 ) Where K l (L∙mg − 1 ) and K f (mg (1−1/n) L 1/n ∙g − 1 ) are the constants of the Langmuir and Freundlich adsorption isothermal equations, respectively; q m (mg∙g − 1 ) represents the maximum adsorption capacity; 1/n indicates the relevant reaction strength between the adsorbent surface and adsorbed molecules. The fitting results showed that both models fit the phosphorus adsorption well (R 2 > 0.96). The surface heterogeneity (1/n) is < 1, showing a chemical adsorption process (Foo and Hameed 2010; Q. Zhang et al. 2021). The Langmuir model showed a better fit to the results, suggesting homogeneous monolayer adsorption (Tran et al. 2017; H. J. Yin et al. 2020). K l , adsorption affinity, increased with temperature. Higher temperature-induced random thermal movement of ions may make collisions between the phosphate and adsorption sites of the biochar more likely. (M. Zhang et al. 2020), thus increased the capacity of GBC to absorb phosphorus. 3.2.3 Effect of initial pH The process of adsorption was influenced by the pH of the solution, which also affected the phosphorus's form and the amounts of active components in the adsorbent. (Z. R. Zhang et al. 2019). With the initial pH of the solution adjusted from 3 to 9, q e changed rapidly from 8.12 to 56.50 mg/g. Further adjusting the pH to 11, q e changed gradually to 66.88 mg/g (Fig. 4 ). The main form of phosphorus was H 2 PO 3− at pH 3–7, which was electrostatically attracted by the protonated surface hydroxyl groups (–OH + )(Z. R. Zhang et al. 2019) and removed from the solution via ligand exchange (Eduah et al. 2020), but the exitance of H + hindered the micro-precipitation reaction of Ca with phosphate (H. Yin et al. 2011). The main form of phosphorus was HPO 3 2− at pH 7–11, which was favourable for the reaction of Ca and phosphate to result in the precipitation of HAP. In this process, the precipitation reaction was the main form of adsorption, and any change in pH affected the amount of adsorption, mainly by affecting the precipitation reaction. 3.2.4 Effect of other anions Some accompanying anions in solutions may impede phosphate adsorption. (J. Liu et al. 2019; S. Yang et al. 2021). Figure 5 shows the effects of NO 3− , Cl − and HCO 3 2− on adsorption processes. With increasing concentrations of NO 3− and Cl − , the q e of biochar decreased because these ions either increase the Coulombic repulsion forces or compete with phosphate for the active site (Cheng et al. 2021). Therefore, the existence of these anions would hinder the adsorption of phosphate. Compared to the control group, the final pH tends to rise. This was because the accompanying anions hinder the generation of HAP, which would otherwise consume OH − and reduce the pH of the solution. In contrast, q e of 700GBC and its solution pH were greatly increased by the presence of HCO 3 2− . This was because the lower pH caused by phosphate adsorption led to the escape of CO 2 from the reaction between H + and HCO 3 2− and consequently increased the pH of the solution, which advanced the q e through Ca–P precipitation. However, increasing the HCO 3 2− concentration reduced the q e of 700GBC compared with that at the lower concentration, as excessive anions will hinder adsorption, as noted above. In general, the adsorption capacity of GBC did not decrease substantially in the presence of a specific concentration of anions. 3.3 Mechanism analysis To elucidate the specific adsorption mechanism, the specimens were analysed after adsorption using FTIR, XRD and SEM. The FTIR spectra recognized the types of groups. The emerging peak of 700GBC after adsorption processes at 1039 cm − 1 (Fig. S1 ) belongs to the P–O bending vibration of adsorbed phosphate (K. Yang et al. 2014), showing that phosphate was stably adsorbed onto 700GBC. The XRD spectra of 700GBC before and after adsorption (Fig. S2) showed that diffraction peaks of calcium hydroxyapatite (HAP) appeared after adsorption. Besides, the diffraction peaks after adsorption for 72 and 216 h were essentially the same, indicating that 700GBC can stably adsorb phosphate as (HAP) for extended periods. The SEM images (Fig. S3) show apparent porous structure and various biological microstructures are clearly observable on the surface of biochar. After adsorption, many flocculent precipitates appeared on the surface of the GBC. The EDS results suggested that the concentrations of Ca and P were correlated indicating phosphate adsorbed to the GBC primarily by micro-precipitation to form HAP. Furthermore, the crystal nuclei for HAP formed in the lacunes of the biochar: the strong attractive forces on the hollow surfaces facilitated HAP nucleation. The SEM element mapping sequence of GBC after adsorption (Fig. S4) showed P-rich and Ca-rich regions on the concave surface, indicating the association of P and Ca. Based on the evidence presented in this paper, the mechanisms of phosphate adsorption on GBC were ligand exchange, electrostatic attraction and micro-precipitation (Fig. 6 ). At low pH, the hydroxyl group (–OH) was protonated, which attracted phosphate through electrostatic attraction and ligand exchange. At high pH, phosphate adsorbed mainly through micro-precipitation, whereby the hydroxyl group (–OH) would be deprotonated and attracted Ca, which then reacted with phosphate on the surface of GBC and decreased the pH. 4. Conclusion GBC has excellent performance for the adsorption of phosphate in solution. The Langmuir isotherm and Pseudo-Second-Order model are appropriately described the adsorption of phosphate by 700GBC, whose phosphorous adsorption capacity is 247 mg∙g − 1 at 303 K. During the long-term adsorption of GBC, micro-precipitation is the main adsorption mechanism and can stably adsorb phosphate through the formation of flocculent precipitates of HAP. Besides, ligand exchange and electrostatic attraction can also affect the adsorption process. This paper demonstrates the potential of GBC to alleviate phosphorus pollution by efficiently and conveniently utilising gypsum and agricultural waste. Declarations Conflict of Interest The authors declare no competing interests. Author Contribution Based on their respective contributions to the study, the following author contributions are stated:Author G and Author D were involved in the experimental process and the writing of the manuscript. Author X contributed to the editing of Figures 1, 2, 5, and Tables 1, 2. All authors reviewed and approved the final version of the manuscript. Data Availability The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request. References Cao, H. L., Wu, X. S., Syed-Hassan, S. S. A., Zhang, S., Mood, S. H., Milan, Y. J., & Garcia-Perez, M. (2020). 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G., Wei, Q. T., Zhang, Y. Y., Fan, Y. X., Li, M., Rong, L. L., et al. (2021). Effects of antibiotics on enhanced biological phosphorus removal and its mechanisms. Science of the Total Environment , 774 . https://doi.org/10.1016/j.scitotenv.2021.145571 Wu, W. X., Yang, M., Feng, Q. B., McGrouther, K., Wang, H. L., Lu, H. H., & Chen, Y. X. (2012). Chemical characterization of rice straw-derived biochar for soil amendment. BIOMASS & BIOENERGY , 47 , 268–276. https://doi.org/10.1016/j.biombioe.2012.09.034 Yang, K., Yan, L. G., Yang, Y. M., Yu, S. J., Shan, R. R., Yu, H. Q., et al. (2014). Adsorptive removal of phosphate by Mg-Al and Zn-Al layered double hydroxides: Kinetics, isotherms and mechanisms. SEPARATION AND PURIFICATION TECHNOLOGY , 124 , 36–42. https://doi.org/10.1016/j.seppur.2013.12.042 Yang, S., Katuwal, S., Zheng, W., Sharma, B., & Cooke, R. (2021). Capture and recover dissolved phosphorous from aqueous solutions by a designer biochar: Mechanism and performance insights. Chemosphere , 274 . https://doi.org/10.1016/j.chemosphere.2021.129717 Yin, H. J., Liu, L., Lv, M. Z., Feng, L. J., & Zhou, J. H. (2020). Metal-Modified Mussel Shell for Efficient Binding of Phosphorus in Eutrophic Waters. INTERNATIONAL JOURNAL OF ENVIRONMENTAL RESEARCH , 14 (2), 135–143. https://doi.org/10.1007/s41742-020-00250-9 Yin, H., Yun, Y., Zhang, Y., & Fan, C. (2011). Phosphate removal from wastewaters by a naturally occurring, calcium-rich sepiolite. Journal of Hazardous Materials , 198 , 362–369. https://doi.org/10.1016/j.jhazmat.2011.10.072 Zhang, K. K., Sun, P., Faye, M., & Zhang, Y. R. (2018). Characterization of biochar derived from rice husks and its potential in chlorobenzene degradation. CARBON , 130 , 730–740. https://doi.org/10.1016/j.carbon.2018.01.036 Zhang, M., Song, G., Gelardi, D. L., Huang, L., Khan, E., Masek, O., et al. (2020). Evaluating biochar and its modifications for the removal of ammonium, nitrate, and phosphate in water. Water Research , 186 . https://doi.org/10.1016/j.watres.2020.116303 Zhang, Q., Ding, Y., Lu, L., Li, J., Liang, M., & Zhu, Y. (2021). Phosphate Adsorption onto Bagasse Iron Oxide Biochar: Parameter Optimization, Kinetic Analysis, and Study of Mechanisms. Bioresources , 16 (1), 1335–1357. https://doi.org/10.15376/biores.16.1.1335-1357 Zhang, Z. R., Yan, L. G., Yu, H. Q., Yan, T., & Li, X. G. (2019). Adsorption of phosphate from aqueous solution by vegetable biochar/layered double oxides: Fast removal and mechanistic studies. BIORESOURCE TECHNOLOGY , 284 , 65–71. https://doi.org/10.1016/j.biortech.2019.03.113 Zhao, S., Wang, B., Gao, Q., Gao, Y., & Liu, S. (2017). Adsorption of phosphorus by different biochars. Spectroscopy Letters , 50 (2), 73–80. https://doi.org/10.1080/00387010.2017.1287091 Zheng, H., Wang, Z. Y., Deng, X., Zhao, J., Luo, Y., Novak, J., et al. (2013). Characteristics and nutrient values of biochars produced from giant reed at different temperatures. BIORESOURCE TECHNOLOGY , 130 , 463–471. https://doi.org/10.1016/j.biortech.2012.12.044 Zhu, D. C., Chen, Y. Q., Yang, H. P., Wang, S. H., Wang, X. H., Zhang, S. H., & Chen, H. P. (2020). Synthesis and characterization of magnesium oxide nanoparticle-containing biochar composites for efficient phosphorus removal from aqueous solution. CHEMOSPHERE , 247 . https://doi.org/10.1016/j.chemosphere.2020.125847 Additional Declarations No competing interests reported. Supplementary Files Supplementaryinformation.docx Cite Share Download PDF Status: Posted Version 1 posted 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. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5660141","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":391778014,"identity":"62f7f41b-a99d-4c16-9a04-5ee9b616c0f0","order_by":0,"name":"Guanli Xu","email":"","orcid":"","institution":"Chengdu University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Guanli","middleName":"","lastName":"Xu","suffix":""},{"id":391778015,"identity":"bb0163d1-fc4f-4feb-b76c-893e2af451fe","order_by":1,"name":"Di Wu","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA30lEQVRIiWNgGAWjYLCCBAYJOX6GwweATAkZYrVYGEs2HksAaeEh1p6KxA2HzxiAWIS18PefMd3woEIiccOxM59f3aix4GFgP3x0Az4tEjdyzG4knJEwnnnm7DbrnGNAh/Gkpd3Aa80NHrMbiW0Ssn03zm4zzmEDapEAiuDTIX/+DFgLY8P9N8+Mc/4RocXgQA5Yi+KEA2eYH+e2EaHF8EZaGdgvkg3HzJhz+yR42Aj5Re784W03f1TUgaLy8eecb0AG++Fj+L2PBNgkwCSxykGA+QMpqkfBKBgFo2DkAAB9N09T1YekwwAAAABJRU5ErkJggg==","orcid":"","institution":"Chengdu University of Technology","correspondingAuthor":true,"prefix":"","firstName":"Di","middleName":"","lastName":"Wu","suffix":""},{"id":391778017,"identity":"5c980798-568a-4dd1-af26-4edadb486d27","order_by":2,"name":"Xiang Ao","email":"","orcid":"","institution":"Chengdu University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Xiang","middleName":"","lastName":"Ao","suffix":""}],"badges":[],"createdAt":"2024-12-17 09:08:33","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5660141/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5660141/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":71985597,"identity":"22ee161c-a93a-472d-8d1c-a43ea96ff263","added_by":"auto","created_at":"2024-12-20 10:41:00","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":298066,"visible":true,"origin":"","legend":"\u003cp\u003e(a–c) Changes of q\u003csub\u003et\u003c/sub\u003e with time for (a) 700GBC, (b) 600GBC and (c) 500GBC. (d–f) Changes of pH with time for (d) 700GBC, (e) 600GBC and (f) 500GBC.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5660141/v1/47407abfaa9b5c7f53b56722.png"},{"id":71985598,"identity":"9a525efe-d6f4-4242-a438-c821a3bb43f2","added_by":"auto","created_at":"2024-12-20 10:41:00","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":132481,"visible":true,"origin":"","legend":"\u003cp\u003eKinetic curves of phosphate adsorption by biochar: (a) 700GBC, (b) 600GBC and (c) 500GBC.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5660141/v1/dc2e08eadc8471d373d4a176.png"},{"id":71984567,"identity":"dfab7510-edb7-433d-aa52-af73b6933591","added_by":"auto","created_at":"2024-12-20 10:33:00","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":103009,"visible":true,"origin":"","legend":"\u003cp\u003eAdsorption isotherms of 700GBC at (a) 293 K, (b) 303 K and (c) 313 K.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5660141/v1/dcba39ce6890e5771c8c6646.png"},{"id":71984187,"identity":"a6b2741b-195f-4b17-9adc-b5d49fd7a71a","added_by":"auto","created_at":"2024-12-20 10:25:00","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":39072,"visible":true,"origin":"","legend":"\u003cp\u003eInfluence of initial pH on phosphate adsorption on 700GBC.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5660141/v1/0c1733494f5bc4cf80f44488.png"},{"id":71984190,"identity":"c69b60eb-00eb-4843-87b6-4ebbefc33684","added_by":"auto","created_at":"2024-12-20 10:25:00","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":62138,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of accompanying anions on 700GBC’s (a) adsorption of phosphate and (b)final pH.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-5660141/v1/511ea088cd8ccf3398ed160d.png"},{"id":71984574,"identity":"e7e06822-2978-4f04-b755-afbcdceaa7e4","added_by":"auto","created_at":"2024-12-20 10:33:01","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":202130,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of possible adsorption mechanisms by GBC.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-5660141/v1/157bc7dab7531970359c4ee4.png"},{"id":77071599,"identity":"5863869d-4d85-4307-aea3-8e4f639b00d3","added_by":"auto","created_at":"2025-02-24 22:01:33","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1454265,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5660141/v1/2cb70a59-6062-4e47-8a4d-fef760f8eaee.pdf"},{"id":71984212,"identity":"2777b6dd-9bcd-4c27-b14c-d8477535e9dd","added_by":"auto","created_at":"2024-12-20 10:25:01","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":11743893,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementaryinformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-5660141/v1/c79f531cad5d7dc9bc94c11e.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Using Gypsum-modified Biochar to Adsorb Phosphate from Water: Adsorption Performance and Mechanism","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003ePhosphorus is essential to biology. It is crucial to the growth of plants and is a contributory factor to causes of eutrophication. Its excessive presence in bodies of water stimulates the rising of harmful algae and aquatic plants, whose subsequent death and decomposition deplete the water\u0026rsquo;s oxygen(Khan and Mohammad 2014). Removing excessive phosphorus from water is therefore essential to protect the aquatic environment.\u003c/p\u003e \u003cp\u003ePhosphorus usually exists in water as phosphate, so efforts to control phosphorus have sought to reduce aqueous phosphate concentrations. Applied methodologies include chemical precipitation(Shaddel et al. 2020), adsorption(Zhu et al. 2020) and biological techniques (L. G. Wu et al. 2021), among which adsorption is particularly attractive for offering the advantages of less sludge, cost-effective, convenient operation and wider applicability.\u003c/p\u003e \u003cp\u003eBiochar, a solid with a lot of carbon by the pyrolysis of organic materials, is a popular adsorbent because of its ample specific surface area, well-developed pores and ability to host various surface functional groups. However, biochar\u0026rsquo;s negative surface charge impedes its adsorption of phosphate anions; the phosphorous adsorption capacity of unmodified biochar is 5.45\u0026ndash;16.1 mg∙g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. (Eduah et al. 2020; Trazzi et al. 2016; Zhao et al. 2017). Metallic elements in biomass-based adsorbents have an essential influence on the adsorption capacity of anions (Li, Wang, Zhou, Awasthi, Ali, Zhang, Lahori, et al. 2016), but the low metal cation content of biochar(Li, Wang, Zhou, Awasthi, Ali, Zhang, Gaston, et al. 2016) limits its capacity to adsorb anions. Modifying biochar with cations (Ca\u003csup\u003e2+\u003c/sup\u003e, Mg\u003csup\u003e2+\u003c/sup\u003e, La\u003csup\u003e3+\u003c/sup\u003e, Fe\u003csup\u003e2+\u003c/sup\u003e and Al\u003csup\u003e3+\u003c/sup\u003e) can significantly improve its ability to adsorb phosphate (Cui et al. 2020; C. Fang et al. 2014; Jia et al. 2020; X. N. Liu et al. 2019). Among these, calcium (Ca) is ecologically harmless and easy to obtain (Cao et al. 2020), thus making it a suitable cation for modifying biochar. It had been proven to be an effective way to prepare modified adsorbent by pyrolyzing Ca(OH) \u003csub\u003e2\u003c/sub\u003e or CaCO\u003csub\u003e3\u003c/sub\u003e with various raw materials to greatly improve the phosphorus adsorption capacity (Wang et al., 2018; Liu et al., 2019; Cao et al., 2020).\u003c/p\u003e \u003cp\u003eTraditional Ca-modified biochar is made of chemical reagents (L. Fang et al. 2020) or other biomass wastes containing calcium (X. N. Liu et al. 2019) to load into the biochar. Controlling the cost of adsorbents is essential to their industrial applicability. Gypsum is a common and cheap calcium mineral. It consists of calcium sulfate dihydrate (CaSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;2H\u003csub\u003e2\u003c/sub\u003eO), which is a suitable calcium source to enhance biochar\u0026rsquo;s absorption of phosphorus. This work makes biochar from rice straw and uses gypsum as the Ca source.\u003c/p\u003e \u003cp\u003eThis study focused on the feasibility and mechanism of milling and pyrolysing rice straw and gypsum to create modified biochar that was suitable for adsorbing phosphate. The influencing factors of temperature, pH and anions on phosphate adsorption were assessed. The adsorption capacity was determined, and adsorption mechanisms were elucidated by considering the experiments and related adsorption models. This study demonstrates a promising functionalised biochar adsorbent made from agricultural waste and a low-cost calcium mineral, which should have broad applicability to phosphate adsorption.\u003c/p\u003e \u003cp\u003eThis study aims to explore the feasibility and mechanisms of developing a functionalized biochar adsorbent from rice straw and gypsum for efficient phosphate removal from water. It seeks to achieve a cost-effective solution by utilizing agricultural waste and a low-cost calcium source while investigating key factors affecting phosphate adsorption, such as temperature, pH, and competing anions. Through experimental analysis and adsorption modeling, the study elucidates the interaction mechanisms between modified biochar and phosphate. Ultimately, it demonstrates the practicality of a sustainable and efficient approach to addressing water pollution and mitigating eutrophication.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Materials\u003c/h2\u003e \u003cp\u003eGypsum (CaSO\u003csub\u003e4\u003c/sub\u003e\u0026middot;2H\u003csub\u003e2\u003c/sub\u003eO; particle size\u0026thinsp;\u0026lt;\u0026thinsp;75 \u0026micro;m) and chemical reagents were analytical-grade reagents (AR). Rice straw collected from a farm in China\u0026rsquo;s Sichuan Province was used as feedstock for biochar production.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Preparation of GBC\u003c/h2\u003e \u003cp\u003eRice straw was cut (5\u0026ndash;8 cm) and rinsed using deionised water. After being put in a constant temperature drying oven at 50℃ for 12 h, the feedstocks were pyrolysed in a vacuum tube furnace at a ramping rate of 10℃∙ min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e to a target temperature of 700, 600, or 500\u0026deg;C under a nitrogen flow of 1 L∙min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, holding for 1 h before cooling. The resulting biochar were crushed and sieved (0.1\u0026ndash;0.5 mm) and were denoted 700BC, 600BC and 500BC, respectively. The gypsum-modified biochar (GBC) was prepared by mixing the different biochar specimens with an equal mass of gypsum; the resulting specimens were named 700GBC, 600GBC and 500GBC, respectively. Specimens were stored in sealed containers until they were used in further tests.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Batch experiments\u003c/h2\u003e \u003cp\u003eBatch adsorption experiments were performed in 50 ml centrifuge tubes by transferring 40 mg GBC into 40 ml phosphate solutions made by dissolving known quantities of potassium phosphate monobasic (KH\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e, AR) in deionised water. These containers were tightly sealed and vibrated at 180 r∙min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in a constant temperature shaker. After adsorption, solutions were centrifuged at 3000 rpm for 10 min, the supernatants were collected and passed through a nylon filter membrane (pore size\u0026thinsp;\u0026lt;\u0026thinsp;0.45 \u0026micro;m). The phosphorus concentration in the supernatants was measured by ammonium molybdate tetrahydrate spectrophotometry on a spectrophotometer (UV -2700i Shimadzu) set at a wavelength of 660 nm (Murphy and Riley 1962). Each experiment was repeated three times and reported average values.\u003c/p\u003e \u003cp\u003eKinetic adsorption experiments were tested at pH 6.5 and 303 K using 100 mg∙L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e phosphorus solution with 40 mg of each GBC under different residence times (0.5, 1, 2, 4, 6, 8, 12, 24, 48, 72, 120, 168, 216 and 360 h).\u003c/p\u003e \u003cp\u003eIsothermal adsorption experiments were conducted, using different initial concentrations of phosphorus (1, 5, 10, 25, 50, 100, 150, 200, 300, 500, 1000, 1500 and 2000 mg∙L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). The containers oscillated for 216 h to reach equilibrium.\u003c/p\u003e \u003cp\u003eTo explore the impact of initial pH on adsorption results, the initial pH was adjusted to 3, 5, 7, 9 or 11 by HCl and NaOH solutions. A 40 mg portion of 700GBC was added to the phosphorus solutions (100 mg∙L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and shaken at 180 r∙min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for 216 h at 303 K. The final pH values and phosphorus concentrations were then measured.\u003c/p\u003e \u003cp\u003eTo analyse the influence of accompanying anions on phosphorus adsorption, 40 mg 700GBC was added to 100 mg∙L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e phosphorus solution at initial pH of 6.5 and holding various concentrations of anions (Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e, NO\u003csup\u003e3\u0026minus;\u003c/sup\u003e and HCO\u003csup\u003e3\u0026minus;\u003c/sup\u003e). After being shaken at 180 r∙min\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for 216 h at 303 K, measuring the phosphorus concentration and final pH value of each filtrate.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e \u003ch2\u003e2.4 Analytical method\u003c/h2\u003e \u003cp\u003eThe microstructure and element distributions of the samples were detected by Scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The specificity surface area of samples was determined by the Brunauer\u0026ndash;Emmett\u0026ndash;Teller standard method of measuring N\u003csub\u003e2\u003c/sub\u003e absorption. Fourier transform infrared (FTIR) spectroscopy identified changes of functional groups in the wavenumber range of 400 to 4000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The mineralogical characteristics and crystal structures were analysed by powder X-ray diffractometry (XRD) at a scan speed of 0.02\u0026deg;/s in a scanning 2θ range of 10\u0026deg;\u0026ndash;60\u0026deg;.\u003c/p\u003e \u003c/div\u003e"},{"header":"3 Result and discussion","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Characterisation of biochar\u003c/h2\u003e \u003cp\u003eCharacteristics of biochar specimens (Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) indicated that the pore size and specific surface area generally broadened with rising pyrolysis temperature but decreased when the temperature reached 700\u0026deg;C. This could have been due to the high temperature causing pyrogenic volatiles to be released from cellulose, the main substance structurally supporting cell walls in the rice straw (Chabannes et al. 2001),which broadened the pore size and specific surface area. However, higher pyrolysis temperature damaged the structure carbon (Tao et al. 2020) and decreased the pore size and specific surface area. The increase of carbon content was caused by an increasing rate of aromatisation (W. X. Wu et al. 2012), which reflected the degree of structural modification of the biochar (Zheng et al. 2013). Silicon, which has a high uptake rate in rice (Mehrabanjoubani et al. 2019), was the second most abundant element in biochar, followed by potassium, calcium, nitrogen, magnesium and phosphorus. These elements could considerably influence the mineral composition of biochar and likely affect its adsorption of phosphate.\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\u003eCharacteristics of biochar specimens\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"14\"\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=\"char\" char=\".\" 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 \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c9\" colnum=\"9\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c10\" colnum=\"10\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c11\" colnum=\"11\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c12\" colnum=\"12\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c13\" colnum=\"13\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c14\" colnum=\"14\"\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\u003eYield (%)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eSurface area (m\u003csup\u003e2\u003c/sup\u003e g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003ePore size\u003c/p\u003e \u003cp\u003e(nm)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eC\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eN\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eSi\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c8\"\u003e \u003cp\u003eAl\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c9\"\u003e \u003cp\u003eP\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c10\"\u003e \u003cp\u003eCa\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c11\"\u003e \u003cp\u003eMg\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c12\"\u003e \u003cp\u003eK\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c13\"\u003e \u003cp\u003eNa\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c14\"\u003e \u003cp\u003eCl\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e500BC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e38.82\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e6.13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e20.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e42.44\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.47\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e12.38\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e0.04\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c10\"\u003e \u003cp\u003e0.64\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c11\"\u003e \u003cp\u003e0.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c12\"\u003e \u003cp\u003e5.68\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c13\"\u003e \u003cp\u003e0.048\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c14\"\u003e \u003cp\u003e1.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e600BC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e38.72\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e9.08\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e29.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e44.8\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.42\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e12.65\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e0.018\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c10\"\u003e \u003cp\u003e0.77\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c11\"\u003e \u003cp\u003e0.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c12\"\u003e \u003cp\u003e5.64\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c13\"\u003e \u003cp\u003e\u0026lt;\u0026thinsp;0.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c14\"\u003e \u003cp\u003e1.8\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e700BC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e36.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e4.07\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e16.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e41.16\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e0.54\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e14.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e \u003cp\u003e0.022\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e \u003cp\u003e0.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c10\"\u003e \u003cp\u003e0.74\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c11\"\u003e \u003cp\u003e0.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c12\"\u003e \u003cp\u003e5.91\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c13\"\u003e \u003cp\u003e0.088\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c14\"\u003e \u003cp\u003e1.8\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 FTIR spectra (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e), All bands appeared with the highest intensities for 500BC but gradually fell with increasing pyrolysis temperature, reflecting aromatization and dehydration causing a decrease in acidic and basic groups as temperature rises. (Novak et al. 2013).\u003c/p\u003e \u003cp\u003eXRD spectra (Fig. S2) show that mineral composition of biochar changed slightly with increasing pyrolysis temperature. The peaks at 2θ of 24.2\u0026deg;, 28.3\u0026deg; and 29.4\u0026deg; confirmed the presence of kalicinite, sylvite and calcite, respectively. The peak intensity of calcite at 600BC and 700BC is lower than that at 500BC, indicating the decomposition of calcite during pyrolysis. The hump observed around 22\u0026deg; reveals the presence of an amorphous-like phase, which comprised a stacked matrix of graphitic planes, suggesting the formation of graphitic platelets(K. K. Zhang et al. 2018).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Adsorption performance\u003c/h2\u003e \u003cdiv id=\"Sec10\" class=\"Section3\"\u003e \u003ch2\u003e3.2.1 Adsorption kinetics\u003c/h2\u003e \u003cp\u003eThe adsorption kinetics of phosphorus by different GBC samples were researched. The overall outcomes are depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The quantity of adsorption was calculated using the adsorption equation (Eq.\u0026nbsp;(1)).\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:{\\text{q}}_{\\text{t}}=\\frac{{\\text{C}}_{\\text{i}}-{\\text{C}}_{\\text{e}}}{\\text{m}}\\times\\:\\text{V}$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eq\u003csub\u003et\u003c/sub\u003e(mg∙g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) is the quantity of adsorption at time t (h); The concentrations C\u003csub\u003ee\u003c/sub\u003e and C\u003csub\u003ei\u003c/sub\u003e (mg∙L\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) refer to the equilibrium and initial phosphorus concentrations, respectively; V (L) and m (g) represent the volume of solution and mass of GBC, respectively.\u003c/p\u003e \u003cp\u003eThe porosity of biochar may cause it take longer to reach equilibrium than non-porous adsorbents (Tran et al. 2017), so the adsorption time was set to 360 h. Figure\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e shows the changes in the different specimens\u0026rsquo; phosphate adsorption capacity and pH value with time. Each GBC shows a similar trend in phosphate adsorption capacity, which can be separated into five stages (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe first stage was external diffusion. The adsorption rate was the fastest, and the pH changed rapidly after the beginning of adsorption. The fast adsorption rate of GBC could be caused by the impetus that came from the difference of phosphate concentration between adsorbent and solution. Phosphate could be quickly adsorbed through the liquid membrane of the adsorbent. At the same time, Ca reacted with phosphate to form Hydroxyapatite (HAP), which was adsorbed on GBC and caused the rapid decline of pH value and increase of q\u003csub\u003et\u003c/sub\u003e value.\u003c/p\u003e \u003cp\u003eThe second stage was internal diffusion. The adsorbed phosphate was transported to the internal pores of the GBC, therefore adsorption rate was slow.\u003c/p\u003e \u003cp\u003eThe third stage was temporary adsorption equilibrium. The adsorbed phosphate reached a temporary saturation in GBC, and the q\u003csub\u003et\u003c/sub\u003e of GBC remained essentially stable, as did the pH value.\u003c/p\u003e \u003cp\u003eThe fourth stage was partial desorption. The q\u003csub\u003et\u003c/sub\u003e decreased and the phosphate content in the solution increased, but the pH value was generally stable. This might be caused by the physical desorption of phosphate: some phosphate might have experienced electrostatic association with the surface of the GBC, where it could then be released back into the solution because the prolonged fast stirring was stronger than the phosphate\u0026rsquo;s relatively weak interaction with the surface(Shepherd et al. 2017).\u003c/p\u003e \u003cp\u003eThe fifth stage was the final adsorption equilibrium. q\u003csub\u003et\u003c/sub\u003e remained finally balanced and pH remained stable, which indicated the phosphate adsorbed by GBC reached final equilibrium in about 216 h.\u003c/p\u003e \u003cp\u003eThe results of adsorption data were fitted to two kinetic models (Eq.\u0026nbsp;2 and Eq.\u0026nbsp;3)\u003c/p\u003e \u003cp\u003ePseudo-First-Order: \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{q}}_{\\text{t}}={\\text{q}}_{\\text{e}}(1-{\\text{e}}^{{-\\text{k}}_{1}\\text{t}})\\)\u003c/span\u003e\u003c/span\u003e (\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e)\u003c/p\u003e \u003cp\u003ePseudo-Second-Order:\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{q}}_{\\text{t}}=\\frac{{\\text{q}}_{\\text{e}}^{2}{\\text{k}}_{2}\\text{t}}{1+{\\text{k}}_{2}{\\text{q}}_{\\text{e}}^{\\text{t}}}\\)\u003c/span\u003e\u003c/span\u003e (\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e)\u003c/p\u003e \u003cp\u003eq\u003csub\u003ee\u003c/sub\u003e (mg\u0026middot;g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) refers to the quantity of adsorption at equilibrium time; k\u003csub\u003e1\u003c/sub\u003e (h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and k\u003csub\u003e2\u003c/sub\u003e (g∙mg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e∙h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) are adsorption rate constants.\u003c/p\u003e \u003cp\u003eTable\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e shows the corresponding fitting results. The Pseudo-Second-Order model better fits the adsorption data on the three GBC adsorbents (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), thus reflecting that chemical adsorption is the primary mechanism of phosphate adsorption (Lalley et al. 2016). Comparing the q\u003csub\u003ee\u003c/sub\u003e values of different GBC adsorption equilibria reveals that 700GBC had the highest adsorption capacity, so further experiments considered only 700GBC.\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\u003eAdsorption kinetic parameters\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=\"\u0026plusmn;\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" 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=\"\u0026plusmn;\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" 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\u003eSample\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e \u003cp\u003ePseudo-first-order model\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c7\" namest=\"c5\"\u003e \u003cp\u003ePseudo-second-order model\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eK\u003csub\u003e1\u003c/sub\u003e(h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eq\u003csub\u003ee\u003c/sub\u003e(mg∙g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eK\u003csub\u003e2\u003c/sub\u003e(g∙mg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e∙h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e )\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eq\u003csub\u003ee\u003c/sub\u003e(mg∙g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e700GBC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e0.602\u0026thinsp;\u0026plusmn;\u0026thinsp;0.0898\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e22.9\u0026thinsp;\u0026plusmn;\u0026thinsp;0.589\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.934\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e0.0419\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00651\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e23.9\u0026thinsp;\u0026plusmn;\u0026thinsp;0.488\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.965\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e600GBC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e0.373\u0026thinsp;\u0026plusmn;\u0026thinsp;0.0779\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e22.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.895\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.858\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e0.0323\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00749\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e22.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.768\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.912\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e500GBC\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e0.351\u0026thinsp;\u0026plusmn;\u0026thinsp;0.0392\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e20.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.573\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.938\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e0.0285\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00392\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e21.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.433\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.972\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=\"Section3\"\u003e \u003ch2\u003e3.2.2 Adsorption isotherms\u003c/h2\u003e \u003cp\u003eThe Langmuir (Eq.\u0026nbsp;4) and Freundlich (Eq.\u0026nbsp;5) models were used to fit 700GBC's adsorption isotherms, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e and Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e.\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\u003eAdsorption isotherm parameter\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=\"\u0026plusmn;\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" 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=\"\u0026plusmn;\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\"\u0026plusmn;\" 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\u003eTempter\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e \u003cp\u003eLangmuir\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"3\" nameend=\"c7\" namest=\"c5\"\u003e \u003cp\u003eFreundlich\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eK\u003csub\u003el\u003c/sub\u003e\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e(L∙mg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eq\u003csub\u003em\u003c/sub\u003e(mg∙g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eK\u003csub\u003ef\u003c/sub\u003e(mg\u003csup\u003e(1\u0026minus;1/n)\u003c/sup\u003e ∙L \u003csup\u003e1/n\u003c/sup\u003e∙g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003e1/n\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eR\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e313K\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e1.55\u0026thinsp;\u0026plusmn;\u0026thinsp;0.157\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e265\u0026thinsp;\u0026plusmn;\u0026thinsp;10.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.995\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e3.44\u0026thinsp;\u0026plusmn;\u0026thinsp;0.874\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e0.546\u0026thinsp;\u0026plusmn;\u0026thinsp;0.0358\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.983\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e303K\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e1.53\u0026thinsp;\u0026plusmn;\u0026thinsp;0.159\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e247\u0026thinsp;\u0026plusmn;\u0026thinsp;10.4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.995\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e3.02\u0026thinsp;\u0026plusmn;\u0026thinsp;0.818\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e0.553\u0026thinsp;\u0026plusmn;\u0026thinsp;0.0380\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.982\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e293K\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e1.47\u0026thinsp;\u0026plusmn;\u0026thinsp;0.110\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e \u003cp\u003e212\u0026thinsp;\u0026plusmn;\u0026thinsp;5.11\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.998\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e \u003cp\u003e2.84\u0026thinsp;\u0026plusmn;\u0026thinsp;0.876\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c6\"\u003e \u003cp\u003e0.548\u0026thinsp;\u0026plusmn;\u0026thinsp;0.043\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\"\u003e \u003cp\u003e0.974\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\u003eLangmuir:\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{q}}_{\\text{e}}=\\frac{{\\text{K}}_{\\text{l}}{\\text{q}}_{\\text{m}}{\\text{C}}_{\\text{e}}}{(1+{\\text{K}}_{\\text{l}}{\\text{C}}_{\\text{e}})}\\)\u003c/span\u003e\u003c/span\u003e (\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e)\u003c/p\u003e \u003cp\u003eFreundlich:\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\text{q}}_{\\text{e}}={\\text{K}}_{\\text{f}}{\\text{C}}_{\\text{e}}^{1/\\text{n}}\\)\u003c/span\u003e\u003c/span\u003e (\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e)\u003c/p\u003e \u003cp\u003eWhere K\u003csub\u003el\u003c/sub\u003e (L∙mg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and K\u003csub\u003ef\u003c/sub\u003e (mg\u003csup\u003e(1\u0026minus;1/n)\u003c/sup\u003eL \u003csup\u003e1/n\u003c/sup\u003e∙g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) are the constants of the Langmuir and Freundlich adsorption isothermal equations, respectively; q\u003csub\u003em\u003c/sub\u003e (mg∙g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) represents the maximum adsorption capacity; 1/n indicates the relevant reaction strength between the adsorbent surface and adsorbed molecules.\u003c/p\u003e \u003cp\u003eThe fitting results showed that both models fit the phosphorus adsorption well (R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;\u0026gt;\u0026thinsp;0.96). The surface heterogeneity (1/n) is \u0026lt;\u0026thinsp;1, showing a chemical adsorption process (Foo and Hameed 2010; Q. Zhang et al. 2021). The Langmuir model showed a better fit to the results, suggesting homogeneous monolayer adsorption (Tran et al. 2017; H. J. Yin et al. 2020). K\u003csub\u003el\u003c/sub\u003e, adsorption affinity, increased with temperature. Higher temperature-induced random thermal movement of ions may make collisions between the phosphate and adsorption sites of the biochar more likely. (M. Zhang et al. 2020), thus increased the capacity of GBC to absorb phosphorus.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003e3.2.3 Effect of initial pH\u003c/h2\u003e \u003cp\u003eThe process of adsorption was influenced by the pH of the solution, which also affected the phosphorus's form and the amounts of active components in the adsorbent. (Z. R. Zhang et al. 2019).\u003c/p\u003e \u003cp\u003eWith the initial pH of the solution adjusted from 3 to 9, q\u003csub\u003ee\u003c/sub\u003e changed rapidly from 8.12 to 56.50 mg/g. Further adjusting the pH to 11, q\u003csub\u003ee\u003c/sub\u003e changed gradually to 66.88 mg/g (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe main form of phosphorus was H\u003csub\u003e2\u003c/sub\u003ePO\u003csup\u003e3\u0026minus;\u003c/sup\u003e at pH 3\u0026ndash;7, which was electrostatically attracted by the protonated surface hydroxyl groups (\u0026ndash;OH\u003csup\u003e+\u003c/sup\u003e)(Z. R. Zhang et al. 2019) and removed from the solution via ligand exchange (Eduah et al. 2020), but the exitance of H\u003csup\u003e+\u003c/sup\u003e hindered the micro-precipitation reaction of Ca with phosphate (H. Yin et al. 2011). The main form of phosphorus was HPO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e at pH 7\u0026ndash;11, which was favourable for the reaction of Ca and phosphate to result in the precipitation of HAP. In this process, the precipitation reaction was the main form of adsorption, and any change in pH affected the amount of adsorption, mainly by affecting the precipitation reaction.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section3\"\u003e \u003ch2\u003e3.2.4 Effect of other anions\u003c/h2\u003e \u003cp\u003eSome accompanying anions in solutions may impede phosphate adsorption. (J. Liu et al. 2019; S. Yang et al. 2021). Figure\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e shows the effects of NO\u003csup\u003e3\u0026minus;\u003c/sup\u003e, Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e and HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e on adsorption processes. With increasing concentrations of NO\u003csup\u003e3\u0026minus;\u003c/sup\u003e and Cl\u003csup\u003e\u0026minus;\u003c/sup\u003e, the q\u003csub\u003ee\u003c/sub\u003e of biochar decreased because these ions either increase the Coulombic repulsion forces or compete with phosphate for the active site (Cheng et al. 2021). Therefore, the existence of these anions would hinder the adsorption of phosphate. Compared to the control group, the final pH tends to rise. This was because the accompanying anions hinder the generation of HAP, which would otherwise consume OH\u003csup\u003e\u0026minus;\u003c/sup\u003e and reduce the pH of the solution.\u003c/p\u003e \u003cp\u003eIn contrast, q\u003csub\u003ee\u003c/sub\u003e of 700GBC and its solution pH were greatly increased by the presence of HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e. This was because the lower pH caused by phosphate adsorption led to the escape of CO\u003csub\u003e2\u003c/sub\u003e from the reaction between H\u003csup\u003e+\u003c/sup\u003e and HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e and consequently increased the pH of the solution, which advanced the q\u003csub\u003ee\u003c/sub\u003e through Ca\u0026ndash;P precipitation. However, increasing the HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e concentration reduced the q\u003csub\u003ee\u003c/sub\u003e of 700GBC compared with that at the lower concentration, as excessive anions will hinder adsorption, as noted above. In general, the adsorption capacity of GBC did not decrease substantially in the presence of a specific concentration of anions.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Mechanism analysis\u003c/h2\u003e \u003cp\u003eTo elucidate the specific adsorption mechanism, the specimens were analysed after adsorption using FTIR, XRD and SEM. The FTIR spectra recognized the types of groups. The emerging peak of 700GBC after adsorption processes at 1039 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e) belongs to the P\u0026ndash;O bending vibration of adsorbed phosphate (K. Yang et al. 2014), showing that phosphate was stably adsorbed onto 700GBC.\u003c/p\u003e \u003cp\u003eThe XRD spectra of 700GBC before and after adsorption (Fig. S2) showed that diffraction peaks of calcium hydroxyapatite (HAP) appeared after adsorption. Besides, the diffraction peaks after adsorption for 72 and 216 h were essentially the same, indicating that 700GBC can stably adsorb phosphate as (HAP) for extended periods.\u003c/p\u003e \u003cp\u003eThe SEM images (Fig. S3) show apparent porous structure and various biological microstructures are clearly observable on the surface of biochar. After adsorption, many flocculent precipitates appeared on the surface of the GBC. The EDS results suggested that the concentrations of Ca and P were correlated indicating phosphate adsorbed to the GBC primarily by micro-precipitation to form HAP. Furthermore, the crystal nuclei for HAP formed in the lacunes of the biochar: the strong attractive forces on the hollow surfaces facilitated HAP nucleation. The SEM element mapping sequence of GBC after adsorption (Fig. S4) showed P-rich and Ca-rich regions on the concave surface, indicating the association of P and Ca.\u003c/p\u003e \u003cp\u003eBased on the evidence presented in this paper, the mechanisms of phosphate adsorption on GBC were ligand exchange, electrostatic attraction and micro-precipitation (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). At low pH, the hydroxyl group (\u0026ndash;OH) was protonated, which attracted phosphate through electrostatic attraction and ligand exchange. At high pH, phosphate adsorbed mainly through micro-precipitation, whereby the hydroxyl group (\u0026ndash;OH) would be deprotonated and attracted Ca, which then reacted with phosphate on the surface of GBC and decreased the pH.\u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eGBC has excellent performance for the adsorption of phosphate in solution. The Langmuir isotherm and Pseudo-Second-Order model are appropriately described the adsorption of phosphate by 700GBC, whose phosphorous adsorption capacity is 247 mg∙g\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at 303 K. During the long-term adsorption of GBC, micro-precipitation is the main adsorption mechanism and can stably adsorb phosphate through the formation of flocculent precipitates of HAP. Besides, ligand exchange and electrostatic attraction can also affect the adsorption process. This paper demonstrates the potential of GBC to alleviate phosphorus pollution by efficiently and conveniently utilising gypsum and agricultural waste.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eConflict of Interest\u003c/h2\u003e \u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eBased on their respective contributions to the study, the following author contributions are stated:Author G and Author D were involved in the experimental process and the writing of the manuscript. Author X contributed to the editing of Figures 1, 2, 5, and Tables 1, 2. All authors reviewed and approved the final version of the manuscript.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e \u003cp\u003eThe datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eCao, H. L., Wu, X. S., Syed-Hassan, S. S. A., Zhang, S., Mood, S. H., Milan, Y. J., \u0026amp; Garcia-Perez, M. (2020). Characteristics and mechanisms of phosphorous adsorption by rape straw-derived biochar functionalized with calcium from eggshell. \u003cem\u003eBioresource Technology\u003c/em\u003e, \u003cem\u003e318\u003c/em\u003e. https://doi.org/10.1016/j.biortech.2020.124063\u003c/li\u003e\n \u003cli\u003eChabannes, M., Ruel, K., Yoshinaga, A., Chabbert, B., Jauneau, A., Joseleau, J. P., \u0026amp; Boudet, A. M. (2001). 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Synthesis and characterization of magnesium oxide nanoparticle-containing biochar composites for efficient phosphorus removal from aqueous solution. \u003cem\u003eCHEMOSPHERE\u003c/em\u003e, \u003cem\u003e247\u003c/em\u003e. https://doi.org/10.1016/j.chemosphere.2020.125847\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Gypsum-modified biochar, Phosphate, Adsorption, Micro-precipitation","lastPublishedDoi":"10.21203/rs.3.rs-5660141/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5660141/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study explored the use of gypsum-modified biochar (GBC) for phosphate adsorption from water. GBC, composed of an equal weight mixture of gypsum (CaSO4·2H2O) and straw biochar subjected to pyrolysis, demonstrated strong resistance to interfering anions (NO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e−\u003c/sup\u003e, Cl\u003csup\u003e−\u003c/sup\u003e, and HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e−\u003c/sup\u003e) and high adsorption efficiency across a wide pH range (7–11). The adsorption behavior was well described by the Pseudo-Second-Order kinetic and Langmuir models, with a maximum capacity of 247 mg∙g\u003csup\u003e−1\u003c/sup\u003e at 303 K. The adsorption mechanisms involved ligand exchange, electrostatic attraction, and micro-precipitation. These findings highlight GBC as a simple and effective adsorbent for phosphate removal.\u003c/p\u003e","manuscriptTitle":"Using Gypsum-modified Biochar to Adsorb Phosphate from Water: Adsorption Performance and Mechanism","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-12-20 10:24:56","doi":"10.21203/rs.3.rs-5660141/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"f0a06db5-316e-4541-9b1a-200e29201087","owner":[],"postedDate":"December 20th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-02-24T21:53:21+00:00","versionOfRecord":[],"versionCreatedAt":"2024-12-20 10:24:56","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5660141","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5660141","identity":"rs-5660141","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}