Enhanced Removal of Cr(VI) Using Nitrogen-Doped Soy Sauce Residue Biochar: A Comprehensive Study on Preparation, Mechanisms, and Applications

Enhanced Removal of Cr(VI) Using Nitrogen-Doped Soy Sauce Residue Biochar: A Comprehensive Study on Preparation, Mechanisms, and Applications | 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 Enhanced Removal of Cr(VI) Using Nitrogen-Doped Soy Sauce Residue Biochar: A Comprehensive Study on Preparation, Mechanisms, and Applications Ruiping Yan, Yuwei Tang, Yilong Li, Jiahao Tian, Li Zhang, Shuang Liang, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5739941/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 is dedicated to investigating the potential of self-nitrogen enriched biochar derived from soy sauce residue (N-SRB) for the efficient removal of Cr(VI) from wastewater. N-SRB, synthesized from readily available and low-cost soy sauce residue, exhibited a high removal efficiency of Cr(VI). The maximum Cr(VI) removal rate achieved was 99.93% at pH of 2.0. The removal process, under optimized conditions, was most accurately represented by the intra-particle diffusion model and the pseudo-second-order kinetic model. Multiple techniques were employed to verify the role of pyrrole-N, pyridine-N, graphite-N, -COOH, -OH and C=O serving as electron contributors for the reduction of Cr(VI). The primary mechanisms driving the elimination of Cr(VI) using N-SRB were identified as electrostatic attraction, redox reactions, and complexation. Nitrogen-rich bio-carbon eliminates the requirement for supplementary nitrogen sources and ensures the absence of secondary pollution during the pollutant removal process, making it environmentally benign. The development of this low-cost material has significant theoretical and practical implications for the efficient removal of Cr(VI) from contaminated water. Soy sauce residue nitrogen-doped biochar hexavalent chromium reduction Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1. Introduction The rapid growth of the global economy has led to a corresponding increase in the discharge of heavy metal-wastewater. (Li et al., 2022 ; Mokarram et al., 2020 ; Sylwan and Thorin, 2021 ). Heavy metals, characterized by their high toxicity, persistence, and resistance to degradation, pose a significant threat to the environmental safety of water sources and the security of urban drinking water. Chromium and its compounds have a broad range of applications across numerous industries, including steel production, electroplating, tanning, and the fuel sector. The toxicity of Cr(VI) is 100 times greater than that of Cr(III), qualifying it as a potent mutagen that can trigger lung and nasopharyngeal cancers. Chromium pollution, particularly Cr(VI), can result in the death or hindered growth of plants and animals, thereby impacting biodiversity. Once chromium enters aquatic ecosystems through the food chain, it can stimulate excessive algae growth, ultimately leading to algal blooms that jeopardize water quality and the survival of aquatic flora and fauna. Chromium exists primarily in the environment as Cr(III) and Cr(VI), with Cr(VI) being the more toxic form (Ertani et al., 2017 ; Liang et al., 2021 ). Cr(VI) is known to cause skin ulcers and has been classified as a class I human carcinogen by the International Agency for Research on Cancer (IARC) due to its “tri carcinogenic” effect (Costa and Klein, 2006 ; Hessel et al., 2021 ; Wang et al., 2017 ). Consequently, the discharge of Cr(VI)-containing wastewater into the environment poses a serious threat to human health. Current methods for treating Cr(VI)-containing wastewater encompass adsorption, chemical treatment, electrolysis, ion exchange, and membrane separation (Alvarado et al., 2013 ; Islam et al., 2023 ; Peng and Guo, 2020 ). Among these methods, adsorption is particularly noteworthy for its high efficiency, selectivity, ease of operation, wide applicability, and recyclability. Biochar adsorption involves utilizing biochar to remove pollutants from water (Ambaye et al., 2020 ; Cheng et al., 2021 ; Jagadeesh and Sundaram, 2023 ). Biochar is a carbonaceous material produced through the pyrolysis of biomass under oxygen-deprived conditions (Liu et al., 2015 ; Nath et al., 2022 ; Sakhiya et al., 2020 ). Biochar possesses a considerable abundance of micropores and mesopores, resulting in a large specific surface area and porosity that facilitate the adsorption of various organic pollutants, heavy metal ions, and other pollutants present in water (Jia et al., 2021 ; Ling et al., 2017 ). Biochar is derived from natural biomass, making it a renewable and environmentally friendly material. Its regenerative capabilities allow for reuse, further minimizing environmental impact (Kalla et al., 2023 ; Qambrani et al., 2017 ). In addition to its role in water treatment, biochar exhibits multifunctionality, with applications in gas adsorption, energy storage, and other fields, highlighting its broader potential (Bolan et al., 2022 ; Peterson et al., 2013 ). Overall, biochar adsorption presents a promising water treatment technology due to its advantages of high efficiency, environmental friendliness, and low cost. Soy sauce residue, an often-overlooked by-product of soy sauce production typically deemed waste, offers a valuable opportunity for resource reutilization and environmental pollution mitigation when employed as a raw material for biochar. The abundant organic content in soy sauce residue is converted into stable carbon structures during the carbonization process, enhancing the carbon content of the resulting biochar (Yang et al., 2021 ). Furthermore, the high nitrogen content in soy sauce residue contributes to the formation of various nitrogen-containing functional groups, such as amine and hydroxyl groups, within the resulting biochar. These functional groups possess the ability to form complexes with Cr(VI), thereby enhancing the adsorption effect. Furthermore, the utilization of soy sauce residue as a raw material for biochar production offers significant benefits in terms of resource reutilization and the reduction of environmental pollution. Zhang et al. successfully demonstrated the efficacy of soy sauce residue biochar in removing dyes, highlighting its potential for diverse applications (Zhang et al., 2022 ). However, the removal of hexavalent chromium using soy sauce residue biochar has not been extensively studied. This study focuses on the production of nitrogen-rich soy sauce residue biochar (N-SRB) using soy sauce residue as a precursor and its application to the removal of Cr(VI). The surface composition, morphology, and functional groups of N-SRB were thoroughly investigated by various techniques. Furthermore, the study conducts three types of kinetic analyses to evaluate effectiveness of N-SRB in removing Cr(VI) from aqueous solution. The mechanism of Cr(VI) removal explored by systematically examining the influences of factors such as solution pH, N-SRB dosage, and initial Cr(VI) concentration. This research aims to transform soy sauce residue waste into a valuable material for Cr(VI) removal, contributing to the conversion and utilization of waste resources while promoting sustainable development. 2. Materials and methods 2.1 Experimental reagents and materials The soy sauce residue material comes from the Changchun soy sauce factory. All chemicals were used as received. Potassium dichromate (K 2 Cr 2 O 7 ) and sodium hydroxide (NaOH), both of analytical grade, were purchased from Beijing Reagent Co., Ltd. Guaranteed concentrated nitric acid (HNO 3 ) was obtained from National Pharmaceutical Chemical Reagent Co., Ltd. 2.2 Preparation of N-SRB The soy sauce residue was broken into small pieces and soaked in a beaker with deionized water, followed by purification through 3 times. Subsequently, the soy sauce residue was washed by ultrapure water and placed in an oven at 80°C for 12 h. The residue was pyrolyzed in a tube furnace of N 2 at 700°C, with a heating rate of 5°C/min and maintained at the designed temperature for 120 min, and finally the furnace was cooled down to room temperature to obtain N-SRB. 2.3 Characterization methods The chemical changes in the surface of the composite material before and after the removal of Cr(VI) were compared using X-ray photoelectron spectroscopy (XPS, Thermo SCIENTIFIC ESCALAB 250Xi, USA) with Al Kα radiation. Images and elemental compositions of the composites were recorded using a JSM-5600 scanning electron microscope (SEM, HITACHI SU5000, Japan) and energy dispersive X-ray spectroscopy (EDS), respectively. Surface functional groups were identified using a Nicolet iS5 Fourier transform infrared spectrometer (IR tracer 100, Japan). The pore structure was determined using a BET analyzer (Micromeritics 3FLEX). The specific surface area was calculated using BET theory (Ambroz et al., 2018), and the pore size was calculated by the Barrett-Joyner-Halenda (BJH) model (Villarroel-Rocha et al., 2014). 2.4 Methods of Cr(VI) removal A parallel experiment was conducted using 50 mL polypropylene centrifuge tubes, stirred at 130 rpm. 0.2 g of N-SRB was added to 40 mL of 40 mg·L -1 Cr(VI) solution at pH 2.0. Supernatant samples were collected at designated time intervals (5–360 min). To assess the impact of pH, a separate experiment was performed by contacting 0.2 g of N-SRB with a 40 mg·L -1 Cr(VI) solution across a pH range of 2.0 to 8.0. Concentrated HNO 3 and NaOH were used to adjust the pH, and the centrifuge tubes were stirred to ensure the complete equilibrium. Following the reaction, the solid material was washed three times with deionized water and collected for further solid-phase characterization. 2.5 Methods of analysis Total Cr concentration in the filtrate was analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES) (Prodigy XP, Leeman Labs). Residual Cr(VI) concentration was quantified using a spectrophotometer (Prodigy XP, Leeman Laboratories) at 540 nm. The removal rate (R, %) and removal capacity (q, mg·g -1 ) of for Cr(VI) and N-SRB were obtained from the following equations (Hu et al., 2007). C0, Ct, and Ce are used to represent the initial concentration of Cr(VI), the residual concentration of Cr(VI) at a specific time t (in minutes), and the equilibrium concentration of Cr(VI), respectively. The volume of the solution is denoted by V (in liters), while m signifies the mass of the material (in grams). 3. Results and discussion 3.1 Characterization The surface area specificity and pore size properties of the materials prior to and subsequent to the adsorption of Cr(VI)were quantitatively assessed through the analysis of isothermal adsorption-desorption profiles of nitrogen gas (N 2 ), as depicted in Fig.1. Table 1 indicated that the specific surface area, the pore volume, and the pore size of N-SRB were 0.6906m²·g -1 , 0.002259 m 3 ·g -1 , 137.155 nm, respectively. As shown in Fig. 1a, the isothermal curves of adsorption and desorption of N-SRB exhibited a significant hysteresis phenomenon, with a steep increase in adsorption capacity at higher pressures. This behavior was characteristic of type IV isotherms, primarily dominated by microporous adsorption. In Fig. 1b, it could be observed that the material contained a large number of pores exceeding 400 nm in size, with the pore size distribution ranging from 400 to 1200 nm. The FTIR spectra of biochar before and after Cr(VI) removal were presented in Fig. 1c. Analysis of the spectra revealed the presence of a significant number of functional groups on the surface of the biochar. The vibrational peak at 673 cm -1 represented the deformation vibration of C-H bonds (Sun et al., 2022). The vibrational peaks observed at the positions of 1524 cm -1 (Yin et al., 2015) and 1691 cm -1 (Akgül et al., 2016) correspond to the tensile vibrations of the C=O bond. The peaks at 3613 cm -1 (Hong et al., 2016) and 3736 cm -1 (Sheng et al., 2014) represented the stretching vibrations of the C=O bond. The vibrational peaks at 3613 cm -1 (Hong et al., 2016) and 3736 cm -1 (Sheng et al., 2014) corresponded to the stretching vibrations of the -OH bond. The peak located at 927 cm -1 was associated with the deformation vibration of the C-O bond. Although there was no obvious shift in the position of peaks before and after the reaction, a notable decrease in intensity suggested that the material's structure was compromised to some extent. In Fig.2, a comparative analysis of the morphology, elemental surface distribution, and elemental content of N-SRB before and after the removal of Cr(VI) was presented Fig. 2a highlighted the original morphology of the biochar, revealing an irregular, block-like structure with a relatively rough surface. Numerous fine particles and pores, approximately 10 µm in size, were visible on the surface of the blocks. An elemental surface distribution map of the pristine biochar was presented in Fig. 2c The map highlighted a dominant presence of carbon (C), alongside a significant amount of oxygen (O) and a clearly visible nitrogen (N) signal. Notably, there was almost no trace of Cr in the map. The corresponding full-spectrum analysis (Fig. 2e) confirmed this observation, indicating that the pristine biochar sample contained 73.01% carbon, 20.42% oxygen, and 6.56% nitrogen. Based on these findings, it could be concluded that the pristine biochar sample was essentially free of Cr and rich in N. XPS was employed to investigate the elemental variations in the biochar before and after Cr(VI) elimination. Fig. 3provided a comparative analysis of the changes in the core level spectra of four elements: C, N, O, and Cr. In Fig. 3a, the pristine biochar exhibited three elements: C, N, and O. In Fig. 3b, C 1s spectrum was back-convoluted into four peaks attributed to O-C=O, C=O, C-O, and C-C, respectively, from high to low binding energies. N-SRBs typically featured more basic structural units dominated by C-C and C-O bonds. Fig. 3c depicted that N 1s spectrum, identifying three types of nitrogen functional: graphitic nitrogen at 401.6 eV (Hu et al., 2017), pyrrole nitrogen at 400.4 eV (Li et al., 2019), pyridine nitrogen at 399.0 eV (Imamura and Saiki, 2011). The presence of these nitrogen functional groups could enhance the surface electrophilicity, provide coordination sites, and play a crucial role in Cr(VI) elimination by N-SRB. In Fig. 3d, O 1s spectrum was deconvoluted into three peaks, attributed to O-C=O, C-O, and C=O, respectively. These results indicated that N-SRB possessed a rich variety of functional groups, contributing to its adsorption capability. 3.2 Cr(VI) removal performance 3.2.1 Kinetics of Cr(VI) removal by N-SRB Fig. 4a illustrated the variation of the three different forms of Cr over time. The concentration of Cr(VI) decreased sharply, reaching only 0.025 mg·L⁻¹ at 240 min. The process consisted of three distinct phases: a rapid removal from 0 to 120 min, which was attributed to the availability of the active sites; a slower phase from 120 to 240 min, as the active sites became depleted; and finally, an equilibrium phase at 240 min. The study also revealed that Cr(III) was produced during the Cr(VI) removal process by N-SRB. The concentration of Cr(III) increased from below the detection limit (0.01 mg·L -1 ) to 30.12 mg·L -1 in the first 30 min. During the reduction of Cr(VI) to Cr(III), electrons were transferred from the N-SRB to Cr(VI) ions, which led to a decrease in the oxidation state of the Cr and an increase in the oxidation state of Cr ions. The kinetic mechanism of Cr(VI) removal by N-SRB was analyzed by four kinetic models. As shown in Fig. 4b, the experimental data most accurately fitted the pseudo-second-order (PSO) model, evidenced by a high correlation coefficient (R²=0.9999). This strong correlation indicated that the main mechanism driving Cr(VI) removal was the formation of a chemical bond between the Cr(VI) and the adsorbent (N-SRB) (Zou et al., 2021). In Fig. 4c, the intraparticle diffusion (IPD) model was applied to determine the removal stages of Cr(VI) removal by N-SRB. The process was divided into three stages: the first stage exhibited a higher removal rate, where Cr(VI) ions were directly reduced by the functional groups on the surface of N-SRB. In the second stage, the reduction ability diminished, mainly due to the electrostatic attraction of Cr(III) and the decrease of available active sites. The third stage of the IPD model represented the adsorption equilibrium, where the removal rate of Cr(VI) was in equilibrium with the desorption rate, indicating a steady state in the process.. As depicted in Fig. 4d, the Elovich model produced a correlation coefficient (R²) of 0.83, suggesting a moderate fit while also revealing a significant variation in the activation energy. This observation implied that the removal process involved a multi-layered mechanism, encompassing not only interfacial adsorption but also dynamic processes of activation and deactivation. These findings highlighted the presence of multiple interactions and energy barriers that influenced the adsorption kinetics, further demonstrating that the activation energy of the adsorption process was relatively high (Sun et al., 2022). 3.2.2 Effect of pH The effect of initial solution pH on Cr(Ⅵ) removal efficiency was examined, as pH is a key parameter influencing the removal of Cr(VI) by materials (Zhang et al., 2019). The influence of pH on the removal of Cr(VI) using N-SRB was investigated over a pH range of 2.0 to 8.0, maintaining a constant initial Cr(VI) concentration of 40 mg·L -1 , N-SRB dosage of 5.0 g L -1 , and reaction time of 240 min. The results demonstrated a pronounced dependence of Cr(VI) removal on pH of the solution. In Fig. 5, the maximum removal rate achieved was 99.93% at pH 2.0. However, as the pH increased, the removal efficiency declined sharply: to 48.31% at pH 4.0, 37.6% at pH 6.0, and 35.88% at pH 8.0. This trend underscored the importance of acidic conditions in enhancing Cr(VI) removal, likely due to the increased protonation of functional groups on the N-SRB surface and the greater availability of Cr(VI) species in lower pH environments. At lower pH levels, Cr(VI) primarily existed as HCrO 4 - , whereas at higher pH values, the dominant form is Cr 2 O 7 2- . The higher adsorption free energy of HCrO 4 - compared to Cr 2 O 7 2- suggested that HCrO 4 - was more readily adsorbed by N-SRB at lower pH values. This suggested that the efficiency of Cr(VI) removal using N-SRB was significantly influenced by the speciation of Cr(VI), with the acidic form, HCrO 4 - , being more effectively removed (Wang et al., 2009). The adsorption free energy of Cr(VI) surpassed that of Cr(III), further enhancing the removal process at lower pH. 3.2.3 Effects of N-SRB dosage and initial Cr(VI) concentration The initial concentration of Cr(Ⅵ) is another important factor affecting the removal performance of the material (Miretzky and Cirelli, 2010). As illustrated in Fig. 6a, a high Cr(VI) removal rate was achieved at an initial concentration of 40 mg·L -1 . However, as the initial concentration increased to 100, 200, and 400 mg·L -1 , Cr(VI) removal significantly declined, with the efficiencies dropping to 39.45%, 16.59%, and 7.66%, respectively. Conversely, when the initial concentration was reduced to 25 mg·L -1 , Cr(VI) was nearly undetectable in the aqueous solution. This inverse relationship between initial Cr(VI) concentration and removal efficiency was attributed to the limited number of active sites available on the surface of the N-SRB. At higher Cr(VI) concentrations, the available active sites quickly became saturated, reducing the material's capacity to adsorb additional Cr(VI). This highlighted the importance of optimizing the material dosage or potentially modifying the N-SRB surface to increase the number of active sites for better performance at higher Cr(VI) concentrations (Akram et al., 2017; Singha and Das, 2011). The adsorption capacity (Q e ) of N-SRB for Cr(VI) was investigated at varying initial concentrations. At lower initial concentrations (up to 50 mg·L -1 ), Q e exhibited a continuous increase with increasing initial concentration, indicating that the available adsorption sites on the N-SRB surface were readily accessible. However, as the initial Cr(VI) concentration was further increased to 100 mg·L -1 , 200 mg·L -1 , and 400 mg·L -1 , the change in Q e became less significant, suggesting that the adsorption sites on the N-SRB surface were approaching saturation. At these higher concentrations, the available active sites were rapidly occupied by Cr(VI), limiting further adsorption. This saturation effect is evident in the plateauing of Q e , indicating that the adsorption process was nearing equilibrium with all active sites effectively occupied. In general, moderate amount of N-SRB can improve the efficiency of removing Cr(VI). However, both excessive and insufficient dosages can negatively affect the Cr(VI) removal process.. As evidenced in Fig. 6b, when the dosage of N-SRB was increased from 4 g·L⁻¹ to 5 g·L⁻¹, the removal rate surged from 91.71% to 99.93%. This observation underscored the significance of employing an optimal dosage of N-SRB, as it could furnish an abundance of available adsorption sites and augment the interfacial contact area between Cr(VI) and N-SRB, thereby enhancing the overall removal efficiency. When the dosage was 2 g·L -1 , the removal rate was 51.43%, and at even lower dosage of 1 g·L -1 and 0.5 g·L -1 , the removal rates were only 28.5% and 22.04%, respectively. This indicated that the adsorption performance of N-SRB might not be fully utilized if the dosage was too low, which affected the removal effect. 3.3 Mechanisms As shown in Table 1, all parameters increased after the reaction: the specific surface area increased to 1.1595 m²·g -1 , the pore volume rose to 0.006254 m 3 ·g -1 , the pore size increased to 312.762 nm. In Fig. 1a, we observed that the area of BET after adsorption was larger than that before adsorption, with the emergence of additional pores around 200 nm. This suggested a strong interaction between the biochar and Cr(VI), with Cr(VI) possibly exhibiting a corrosive effect on the biochar. This corrosive interaction could be responsible for the increased number of pores around 200 nm on the biochar. The results confirmed that N-SRB possessed a porous structure with a large specific surface area, which was able to adsorb Cr(VI) to its surface by physical adsorption. In (Fig. 1c), a new stronger vibrational peak was observed at the 1189 cm -1 position of biochar after Cr(VI) removal, which might be a signal from the Cr(VI) (Dong et al., 2021), indicating that the biochar successfully adsorbed a large amount of Cr(VI). After the adsorption of Cr(VI), the C-H bond was blue shifted to the position of 704 cm -1 , with a noticeable decrease in intensity, which proved that the structure of biochar might have been damaged. Following the reaction, notable blue shifts were observed in the vibrational frequencies of the C=O and -OH bonds, indicative of an increase in their respective bond strengths. This augmentation suggested that the biochar, upon adsorbing Cr(VI), exhibited a significantly enhanced capacity for the adsorption of oxygen-containing species and water. This enhancement was likely attributed to the defects that emerged as a consequence of structural alterations incurred during the adsorption process. After contacted with Cr(VI) (Fig. 2b), the volume of the material was obviously reduced, and the plate-like block became lamellar superposition with more pronounced gaps between the layers. Additionally, a large number of small-scale aggregated floating materials appeared on the surface of the material, alongside a large number of bumps and depressions. These morphological changes might be related to the strong oxidizing properties of Cr(VI) which corroded the biochar and cracked it into small-sized layered lumpy morphology. After exposure to Cr(VI), Fig. 2d illustrated that elemental Cr was uniformly distributed throughout the sample and the signal of elemental N became weakened. Correspondingly, Fig. 2f indicated that the content of elemental Cr increases to 0.61% and the content of elemental N decreased to 5.42%, which proved that elemental N was involved in the removal of Cr(VI), and that Cr binds to N-SRB after the reaction. In the full XPS spectrum (Fig. 3), when adsorbed Cr(VI), a new distinct peak attributed to the Cr signal appeared, indicating that the biochar adsorbed a large amount of Cr. This observation confirmed the effectiveness of the biochar in removing Cr(VI) from the solution. It was noteworthy that the ratio of C/O was significantly reduced after the reaction, suggesting that Cr(VI) oxidized the biochar, leading to structural changes in the material. In addition, the C-O content was significantly reduced in Fig. 3b, indicating that a large number of structural units of the material were destroyed. In addition, the decrease in the content of C=O bonds, coupled with the increase in the content of O-C=O bonds, indicated that the strong oxidative properties of Cr(VI) caused C=O to be oxidized to O-C=O, while Cr(VI) was simultaneously reduced to the more innocuous Cr(III). This phenomenon proved the effective adsorption performance of N-SRB. In addition, the positions of every Peak after the reaction were obviously shifted negatively, indicating the obvious electron transfer between the N-SRB and Cr(VI). The N 1s peaks in Fig. 3c were back-convoluted into pyridine nitrogen, pyrrole nitrogen, and graphite nitrogen. After the reaction, a noticeable decrease in the proportion of graphitic nitrogen and pyridine nitrogen was observed, accompanied by an increase in the amount of pyrrole nitrogen. This shift in nitrogen types suggested that the type of nitrogen altered during the reaction. And it was noticed that the binding energy of the main peak of N 1s produced a significant negative shift, indicating that N gained electrons during the reaction process, which might be the reason for the shift in the type of nitrogen. Fig. 3e illustrated that the biochar prior to the reaction contained almost no Cr element. However, upon adsorption, the adsorbed Cr element was back-convoluted into three peaks, representing Cr species at various oxidation states: Cr 6+ , Cr(OH) 3 and Cr 2 O 3 . The presence of these peaks confirmed that some of the initially present Cr(VI) was reduced to less toxic forms (Cr(III)). This finding demonstrated the ability of the N-SRB to effectively reduce Cr(VI) during the adsorption process. This dual functionality—adsorption and reduction—highlighted the potential of N-SRB as a sustainable material for treating Cr(VI)-contaminated water. According to our analysis, the potential mechanism by which N-SRB facilitated the removal of Cr(VI) was illustrated in Fig. 7. This process unfeld in three distinct stages: Cr(VI) molecules were attracted to the N-SRB surface due to the electrostatic forces between the negatively charged Cr(VI) and the positively charged functional groups presented on the adsorbent; certain Cr(VI) ions underwent a reduction to Cr(III), facilitated by nearby electron-rich entities, including nitrogen and oxygen-containing groups; Cr(III) was then adsorbed by C=O group on the N-SRB. 4. Conclusion This research explored the efficacy of nitrogen-enriched soy sauce residue biochar (N-SRB) as an adsorbent for the elimination of Cr(VI) from aqueous environments. Findings indicated that N-SRB exhibits superior Cr(VI) removal performance, particularly under acidic conditions. The adsorption equilibrium was established within 240 min, reflecting a rapid adsorption kinetics. The kinetic study aligned with a pseudo-second-order model, implying a chemical adsorption process that included chelation, ion exchange, and interactions with active functional groups on the biochar's surface. The Cr(VI) removal mechanism by N-SRB was multifaceted, commencing with electrostatic interactions between the negatively charged Cr(VI) species and the positively charged biochar surface, reduction of Cr(VI) to Cr(III) by nitrogen and oxygen-containing groups, succeeded by complexation with trivalent chromium (Cr(III)) and biochar's functional groups (C = O). Ultimately, N-SRB was identified as a promising, economical, and sustainable adsorbent alternative to activated carbon for the remediation of Cr(VI)-laden wastewater. Declarations Author Contribution Ruiping Yan: First author &Writing original draft &Investigation. Yuwei Tang: Second author &Investigation. Yilong Li: Investigation. Jiahao Tian: Formal analysis. Li Zhang: Formal analysis. Shuang Liang: Writing review &editing &Funding acquisition. Yuting Zhang: Funding acquisition &Conceptualization.All authors are Jilin Jianzhu University. Acknowledgement This work was funded by the Science and Technology Planning Project of Jilin Province (20220508008RC and 20220505018ZP), and the Science and Technology Projects of the Ministry of Housing and Urban-Rural Development (2018-K6-003). References Aigbe, U.O., Osibote, O.A., 2020. A review of hexavalent chromium removal from aqueous solutions by sorption technique using nanomaterials. J. Environ. Chem. 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Acs Publications. 11, 193-205. https://doi.org/10.1021/bk-2013-1143.ch011. Qambrani, N.A., Rahman, M.M., Won, S., Shim, S., Ra, C., 2017. Biochar properties and eco-friendly applications for climate change mitigation, waste management, and wastewater treatment: A review. Renewable Sustainable Energy Rev. 79, 255-273. https://doi.org/10.1016/j.rser.2017.05.057. Sakhiya, A.K., Anand, A., Kaushal, P., 2020. Production, activation, and applications of biochar in recent times. Biochar. 2, 253-285. https://doi.org/10.1007/s42773-020-00047-1. Sheng, J., Ji, D., Yu, F., Cui, L., Zeng, Q., Ai, N., Ji, J., 2014. Influence of chemical treatment on rice straw pyrolysis by TG-FTIR. IERI Procedia 8, 30-34. https://doi.org/10.1016/j.ieri.2014.09.006. Singha, B., Das, S.K., 2011. Biosorption of Cr(VI) ions from aqueous solutions: kinetics, equilibrium, thermodynamics and desorption studies. Colloids Surf. B Biointerfaces 84, 221-232. https://doi.org/10./j.colsurfb.2011.01.004. Sun, P., Wang, Z., An, S., Zhao, J., Yan, Y., Zhang, D., Wu, Z., Shen, B., Lyu, H., 2022. Biochar-supported nZVI for the removal of Cr(VI) from soil and water. Advances in experimental research and engineering applications. J. Environ. Manage. 316, 115211. https://doi.org/10.1016/j.jenvman. Sylwan, I., Thorin, E., 2021. Removal of heavy metals during primary treatment of municipal wastewater and possibilities of enhanced removal: A review. Water 13, 1121. https://doi.org/10./w13081121. Villarroel-Rocha, J., Barrera, D., Sapag, K., 2014. Introducing a self-consistent test and the corresponding modification in the Barrett, Joyner and Halenda method for pore-size determination. Microporous Mesoporous Mater. 200, 68-78. https://doi.org/10.1016/j.micromeso.2014.08.017. Wang, S.L., Chen, C.C., Tzou, Y.M., Hsu, C.L., Chen, J.H., Lin, C.F., 2009. A mechanism study of light-induced Cr(VI) reduction in an acidic solution. J. Hazard. Mater. 164, 223-228. https://doi.org/10./j.jhazmat.2008.07.145. Wang, Y., Su, H., Gu, Y., Song, X., Zhao, J., 2017. Carcinogenicity of chromium and chemoprevention: A brief update. Onco Targets Ther. 10, 4065-4079. https://doi.org/10.2147/OTT.S139262. Yang, Y., Zhang, Y., Wang, G., Yang, Z., Xian, J., Yang, Y., Li, T., Pu, Y., Jia, Y., Li, Y., 2021. Adsorption and reduction of Cr (VI) by a novel nanoscale FeS/ chitosan/biochar composite from aqueous solution. J. Environ. Chem. Eng. 9, 105407. https://doi.org/10.1016/j.jece.2021.105407. Yin, Z., Chen, B., Chen, M., Hu, S., Cheng, H., 2015. Recovery of chromium (III) ions from aqueous solution by carboxylate functionalized wool fibres. J. Soc. Leather Technol. Chem. 99, 101-106. https://www.researchgate.net/publication/281690504. Zhang, X., Tian, J., Wang, P., Liu, T., Ahmad, M., Zhang, T., Guo, J., Xiao, H., Song, J., 2022. Highly-efficient nitrogen self-doped biochar for versatile dyes' removal prepared from soybean cake via a simple dual-templating approach and associated thermodynamics. J. cleaner Prod. 332, 130069. https://doi.org/10.1016/j.jclepro.2021.130069. Zhang, Y., Tian, Z., Jing, Q., Chen, Y., Huang, X., 2019. Removal of Cr(VI) by modified diatomite supported NZVI from aqueous solution: evaluating the effects of removal factors by RSM and understanding the effects of pH. Water Sci. Technol. 80, 308-316. https://doi.org/10.2166/wst.2019.275. Zou, H., Zhao, J., He, F., Zhong, Z., Huang, J., Zheng, Y., Zhang, Y., Yang, Y., Yu, F., Bashir, M.A., Gao, B., 2021. Ball milling biochar iron oxide composites for the removal of chromium (Cr(VI)) from water: performance and mechanisms. J. Hazard. Mater. 413, 125252. https://doi.org/10.1016/j.jhazmat.2021.125252. Table Table 1. BET characteristics of N-SRB before and after the reaction Specific surface area (m²·g -1 ) Pore volume (m 3 ·g -1 ) Pore size (nm) Before the reaction 0.6906 0.002259 137.155 After the reaction 1.1595 0.006254 312.762 Additional Declarations No competing interests reported. Supplementary Files Highlights.docx Supplementarymaterial.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. 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. 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-5739941","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":406186627,"identity":"80ffc884-6872-4ef8-b352-552b5b3c53c6","order_by":0,"name":"Ruiping Yan","email":"","orcid":"","institution":"Ministry of Education, Jilin Jianzhu University","correspondingAuthor":false,"prefix":"","firstName":"Ruiping","middleName":"","lastName":"Yan","suffix":""},{"id":406186628,"identity":"fd2661ec-044b-4f9d-b244-876556e172d5","order_by":1,"name":"Yuwei Tang","email":"","orcid":"","institution":"Ministry of Education, Jilin Jianzhu University","correspondingAuthor":false,"prefix":"","firstName":"Yuwei","middleName":"","lastName":"Tang","suffix":""},{"id":406186629,"identity":"405504a7-db17-4eae-bbdc-664508882ca1","order_by":2,"name":"Yilong Li","email":"","orcid":"","institution":"Ministry of Education, Jilin Jianzhu University","correspondingAuthor":false,"prefix":"","firstName":"Yilong","middleName":"","lastName":"Li","suffix":""},{"id":406186630,"identity":"8f7bcd56-c9d4-46cf-9865-d75ea565ab22","order_by":3,"name":"Jiahao Tian","email":"","orcid":"","institution":"Ministry of Education, Jilin Jianzhu University","correspondingAuthor":false,"prefix":"","firstName":"Jiahao","middleName":"","lastName":"Tian","suffix":""},{"id":406186631,"identity":"cbffacac-b829-46a7-bd61-be4d7692c7aa","order_by":4,"name":"Li Zhang","email":"","orcid":"","institution":"Ministry of Education, Jilin Jianzhu University","correspondingAuthor":false,"prefix":"","firstName":"Li","middleName":"","lastName":"Zhang","suffix":""},{"id":406186632,"identity":"cafd22d1-1da1-42d1-b3aa-63dee71c3bd4","order_by":5,"name":"Shuang Liang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA4ElEQVRIiWNgGAWjYBACPmYwdYCHn7354IOEihrCWtigWmQke44lGzw4c4wILRDqgI3BjRwzyYctzERoYWd+9vDLnzs8BmfOmFUkNrAx8Ld3JxBwGJu5sWzbMx7J421lNxJ3yDBInDm7gZBfzKQlGw7z8J05vO1G4hk2BgOJXEJa2L9JS/w5zMNwI8GsILGNmRgtPGaSH9gO8wjcSDFjIFZLmTRj22EeUCBLJJw5xkPQL/z8x7dJ/vhz2B4UlR9/VNTI8bf34tcCAsw8SBwenMqQAeMPopSNglEwCkbBiAUAf3RIqlpFIE8AAAAASUVORK5CYII=","orcid":"","institution":"Ministry of Education, Jilin Jianzhu University","correspondingAuthor":true,"prefix":"","firstName":"Shuang","middleName":"","lastName":"Liang","suffix":""},{"id":406186633,"identity":"01aa94fb-efd8-4e52-9b1e-561043015d74","order_by":6,"name":"Yuting Zhang","email":"","orcid":"","institution":"Ministry of Education, Jilin Jianzhu University","correspondingAuthor":false,"prefix":"","firstName":"Yuting","middleName":"","lastName":"Zhang","suffix":""}],"badges":[],"createdAt":"2024-12-31 07:08:28","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5739941/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5739941/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":80274190,"identity":"39b9c67c-106e-4459-8125-4b939716fe95","added_by":"auto","created_at":"2025-04-10 04:33:59","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":213147,"visible":true,"origin":"","legend":"\u003cp\u003e(a) N\u003csub\u003e2\u003c/sub\u003e adsorption-desorption isotherm at 77 K of N-SRB; (b) pore size distribution of N-SRB; (c) FTIR spectra of N-SRB after Cr(VI) removal and before Cr(VI) removal.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5739941/v1/7ba876d3fe3cf558e077eb53.png"},{"id":80273479,"identity":"36177fe7-7006-46c8-8b33-602d0f4ef903","added_by":"auto","created_at":"2025-04-10 04:17:59","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":430758,"visible":true,"origin":"","legend":"\u003cp\u003e(a) and (b) Scanning electron microscope (SEM) images of N-SRB before and after the reaction; (c) and (d) elemental maps of the spatial distribution of C, O, N, and Cr in N-SRB before and after the reaction; (e) and (f) EDS spectra of N-SRB before and after the reaction.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5739941/v1/05de7e7ba1f9865b2dc197fb.png"},{"id":80273481,"identity":"37582bc1-db06-47e8-a611-3b1c4c687833","added_by":"auto","created_at":"2025-04-10 04:17:59","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":288342,"visible":true,"origin":"","legend":"\u003cp\u003eXPS survey of N-SRB and the reacted N-SRB (a); XPS spectra: C1s (b), N 1s (c), O 1s (d), and Cr 2p (e).\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5739941/v1/b52c6908752005c3a4a2a5b6.png"},{"id":80273996,"identity":"24689561-cd8b-4016-a41c-1db97c287ed5","added_by":"auto","created_at":"2025-04-10 04:25:59","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":126614,"visible":true,"origin":"","legend":"\u003cp\u003eCr(VI) removal kinetics: (a) analysis of the remaining Cr ions in solution; (b) PFO and PSO kinetic models; (c) IPD kinetic model; (d) Elovich kinetic model.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5739941/v1/30f7d0038e62d5bc38e703e8.png"},{"id":80273485,"identity":"b1bf9581-17f6-493b-b2c4-896aa1e5e96e","added_by":"auto","created_at":"2025-04-10 04:17:59","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":55901,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of pH value on N-SRB removal of Cr(VI).\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-5739941/v1/ff23bc816d0670f66c12fbb9.png"},{"id":80273488,"identity":"9d83a145-456c-4a84-bf86-9f8585eba57e","added_by":"auto","created_at":"2025-04-10 04:17:59","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":108335,"visible":true,"origin":"","legend":"\u003cp\u003e(a) N-SRB removal efficiency of Cr(VI) as a function of the initial Cr(VI) concentration and (b) the effect of dosage on the removal efficiency of Cr(VI).\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-5739941/v1/bb61117759c97623725f96a5.png"},{"id":80273493,"identity":"6ba1ab46-505c-4cc2-b240-b6526c100ab9","added_by":"auto","created_at":"2025-04-10 04:18:00","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":286875,"visible":true,"origin":"","legend":"\u003cp\u003eMechanism diagram of Cr(VI) removal by N-SRB.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-5739941/v1/3cf65df4c2eeb39a7d602871.png"},{"id":80274850,"identity":"80f6d5fa-e571-4174-a50d-16f988b4c9b6","added_by":"auto","created_at":"2025-04-10 04:42:00","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1960955,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5739941/v1/eb5db1ef-7ffe-424f-ba0f-d634a708cc98.pdf"},{"id":80274191,"identity":"5fe43562-35b8-4eb2-8754-a1e97471e552","added_by":"auto","created_at":"2025-04-10 04:33:59","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":15700,"visible":true,"origin":"","legend":"","description":"","filename":"Highlights.docx","url":"https://assets-eu.researchsquare.com/files/rs-5739941/v1/7d1c73a587fc2edab9f32cc2.docx"},{"id":80273483,"identity":"5253e6b5-4fdb-41a7-bb83-db5eec3c88ba","added_by":"auto","created_at":"2025-04-10 04:17:59","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":34996,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementarymaterial.docx","url":"https://assets-eu.researchsquare.com/files/rs-5739941/v1/e255bcd9f2e162faa123968a.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"\u003cp\u003eEnhanced Removal of Cr(VI) Using Nitrogen-Doped Soy Sauce Residue Biochar: A Comprehensive Study on Preparation, Mechanisms, and Applications\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe rapid growth of the global economy has led to a corresponding increase in the discharge of heavy metal-wastewater. (Li et al., \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Mokarram et al., \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Sylwan and Thorin, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Heavy metals, characterized by their high toxicity, persistence, and resistance to degradation, pose a significant threat to the environmental safety of water sources and the security of urban drinking water. Chromium and its compounds have a broad range of applications across numerous industries, including steel production, electroplating, tanning, and the fuel sector. The toxicity of Cr(VI) is 100 times greater than that of Cr(III), qualifying it as a potent mutagen that can trigger lung and nasopharyngeal cancers. Chromium pollution, particularly Cr(VI), can result in the death or hindered growth of plants and animals, thereby impacting biodiversity. Once chromium enters aquatic ecosystems through the food chain, it can stimulate excessive algae growth, ultimately leading to algal blooms that jeopardize water quality and the survival of aquatic flora and fauna. Chromium exists primarily in the environment as Cr(III) and Cr(VI), with Cr(VI) being the more toxic form (Ertani et al., \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Liang et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Cr(VI) is known to cause skin ulcers and has been classified as a class I human carcinogen by the International Agency for Research on Cancer (IARC) due to its \u0026ldquo;tri carcinogenic\u0026rdquo; effect (Costa and Klein, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2006\u003c/span\u003e; Hessel et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Wang et al., \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Consequently, the discharge of Cr(VI)-containing wastewater into the environment poses a serious threat to human health. Current methods for treating Cr(VI)-containing wastewater encompass adsorption, chemical treatment, electrolysis, ion exchange, and membrane separation (Alvarado et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Islam et al., \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Peng and Guo, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Among these methods, adsorption is particularly noteworthy for its high efficiency, selectivity, ease of operation, wide applicability, and recyclability. Biochar adsorption involves utilizing biochar to remove pollutants from water (Ambaye et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Cheng et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Jagadeesh and Sundaram, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Biochar is a carbonaceous material produced through the pyrolysis of biomass under oxygen-deprived conditions (Liu et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Nath et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Sakhiya et al., \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Biochar possesses a considerable abundance of micropores and mesopores, resulting in a large specific surface area and porosity that facilitate the adsorption of various organic pollutants, heavy metal ions, and other pollutants present in water (Jia et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Ling et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Biochar is derived from natural biomass, making it a renewable and environmentally friendly material. Its regenerative capabilities allow for reuse, further minimizing environmental impact (Kalla et al., \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Qambrani et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). In addition to its role in water treatment, biochar exhibits multifunctionality, with applications in gas adsorption, energy storage, and other fields, highlighting its broader potential (Bolan et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Peterson et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Overall, biochar adsorption presents a promising water treatment technology due to its advantages of high efficiency, environmental friendliness, and low cost.\u003c/p\u003e \u003cp\u003eSoy sauce residue, an often-overlooked by-product of soy sauce production typically deemed waste, offers a valuable opportunity for resource reutilization and environmental pollution mitigation when employed as a raw material for biochar. The abundant organic content in soy sauce residue is converted into stable carbon structures during the carbonization process, enhancing the carbon content of the resulting biochar (Yang et al., \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Furthermore, the high nitrogen content in soy sauce residue contributes to the formation of various nitrogen-containing functional groups, such as amine and hydroxyl groups, within the resulting biochar. These functional groups possess the ability to form complexes with Cr(VI), thereby enhancing the adsorption effect. Furthermore, the utilization of soy sauce residue as a raw material for biochar production offers significant benefits in terms of resource reutilization and the reduction of environmental pollution. Zhang et al. successfully demonstrated the efficacy of soy sauce residue biochar in removing dyes, highlighting its potential for diverse applications (Zhang et al., \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). However, the removal of hexavalent chromium using soy sauce residue biochar has not been extensively studied.\u003c/p\u003e \u003cp\u003eThis study focuses on the production of nitrogen-rich soy sauce residue biochar (N-SRB) using soy sauce residue as a precursor and its application to the removal of Cr(VI). The surface composition, morphology, and functional groups of N-SRB were thoroughly investigated by various techniques. Furthermore, the study conducts three types of kinetic analyses to evaluate effectiveness of N-SRB in removing Cr(VI) from aqueous solution. The mechanism of Cr(VI) removal explored by systematically examining the influences of factors such as solution pH, N-SRB dosage, and initial Cr(VI) concentration. This research aims to transform soy sauce residue waste into a valuable material for Cr(VI) removal, contributing to the conversion and utilization of waste resources while promoting sustainable development.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cp\u003e\u003cstrong\u003e2.1 Experimental reagents and materials\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe soy sauce residue material comes from the Changchun soy sauce factory. All chemicals were used as received. Potassium dichromate (K\u003csub\u003e2\u003c/sub\u003eCr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e) and sodium hydroxide (NaOH), both of analytical grade, were purchased from Beijing Reagent Co., Ltd. Guaranteed concentrated nitric acid (HNO\u003csub\u003e3\u003c/sub\u003e) was obtained from National Pharmaceutical Chemical Reagent Co., Ltd.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.2 Preparation of N-SRB\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe soy sauce residue was broken into small pieces and soaked in a beaker with deionized water, followed by purification\u0026nbsp;through 3 times. Subsequently, the soy sauce residue was washed by ultrapure water and placed in an oven at 80\u0026deg;C for 12 h. The residue was pyrolyzed in a tube furnace of N\u003csub\u003e2\u003c/sub\u003e at 700\u0026deg;C, with a heating rate of 5\u0026deg;C/min and maintained at the designed temperature for 120 min, and finally the furnace was cooled down to room temperature to obtain N-SRB.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.3 Characterization methods\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe chemical changes in the surface of the composite material before and after the removal of Cr(VI) were compared using X-ray photoelectron spectroscopy (XPS, Thermo SCIENTIFIC ESCALAB 250Xi, USA) with Al K\u0026alpha; radiation. Images and elemental compositions of the composites were recorded using a JSM-5600 scanning electron microscope (SEM, HITACHI SU5000, Japan) and energy dispersive X-ray spectroscopy (EDS), respectively. Surface functional groups were identified using a Nicolet iS5 Fourier transform infrared spectrometer (IR tracer 100, Japan). The pore structure was determined using a BET analyzer (Micromeritics 3FLEX). The specific surface area was calculated using BET theory (Ambroz et al., 2018), and the pore size was calculated by the Barrett-Joyner-Halenda (BJH) model (Villarroel-Rocha et al., 2014).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.4 Methods of Cr(VI) removal\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eA parallel experiment was conducted using 50 mL polypropylene centrifuge tubes, stirred at 130 rpm. 0.2 g of N-SRB was added to 40 mL of 40 mg\u0026middot;L\u003csup\u003e-1\u003c/sup\u003e Cr(VI) solution at pH 2.0. Supernatant samples were collected at designated time intervals (5\u0026ndash;360 min).\u003c/p\u003e\n\u003cp\u003eTo assess the impact of pH, a separate experiment was performed by contacting 0.2 g of N-SRB with a 40 mg\u0026middot;L\u003csup\u003e-1\u003c/sup\u003e Cr(VI) solution across a pH range of 2.0 to 8.0. Concentrated HNO\u003csub\u003e3\u003c/sub\u003e and NaOH were used to adjust the pH, and the centrifuge tubes were stirred to ensure the complete equilibrium. Following the reaction, the solid material was washed three times with deionized water and collected for further solid-phase characterization.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.5 Methods of analysis\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTotal Cr concentration in the filtrate was analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES) (Prodigy XP, Leeman Labs). Residual Cr(VI) concentration was quantified using a spectrophotometer (Prodigy XP, Leeman Laboratories) at 540 nm. The removal rate (R, %) and removal capacity (q, mg\u0026middot;g\u003csup\u003e-1\u003c/sup\u003e ) of for Cr(VI) and N-SRB were obtained from the following equations (Hu et al., 2007).\u003c/p\u003e\n\u003cp\u003e\u003cimg 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\"\u003e\u003c/p\u003e\n\u003cp\u003eC0, Ct, and Ce are used to represent the initial concentration of Cr(VI), the residual concentration of Cr(VI) at a specific time t (in minutes), and the equilibrium concentration of Cr(VI), respectively. The volume of the solution is denoted by V (in liters), while m signifies the mass of the material (in grams).\u0026nbsp;\u003c/p\u003e"},{"header":"3. Results and discussion","content":"\u003ch2\u003e3.1 Characterization\u003c/h2\u003e\n\u003cp\u003eThe surface area specificity and pore size properties of the materials prior to and subsequent to the adsorption of Cr(VI)were quantitatively assessed through the analysis of isothermal adsorption-desorption profiles of nitrogen gas (N\u003csub\u003e2\u003c/sub\u003e), as depicted in Fig.1. Table 1 indicated that the specific surface area, the pore volume, and the pore size of N-SRB were 0.6906m\u0026sup2;\u0026middot;g\u003csup\u003e-1\u003c/sup\u003e, 0.002259 m\u003csup\u003e3\u003c/sup\u003e\u0026middot;g\u003csup\u003e-1\u003c/sup\u003e, 137.155 nm, respectively.\u0026nbsp;As shown in Fig. 1a, the isothermal curves of adsorption and desorption of N-SRB exhibited a significant hysteresis phenomenon, with a steep increase in adsorption capacity at higher pressures.\u0026nbsp;This behavior was characteristic of type IV isotherms, primarily dominated by microporous adsorption. In Fig. 1b, it could be observed that the material contained a large number of pores exceeding 400 nm in size, with the pore size distribution ranging from 400 to 1200 nm.\u003c/p\u003e\n\u003cp\u003eThe FTIR spectra of biochar before and after Cr(VI) removal were presented in Fig. 1c. Analysis of the spectra revealed the presence of a significant number of functional groups on the surface of the biochar. The vibrational peak at 673 cm\u003csup\u003e-1\u003c/sup\u003e represented the deformation vibration of C-H bonds (Sun et al., 2022). The vibrational peaks observed at the positions of 1524 cm\u003csup\u003e-1\u003c/sup\u003e (Yin et al., 2015) and 1691 cm\u003csup\u003e-1\u003c/sup\u003e (Akg\u0026uuml;l et al., 2016) correspond to the tensile vibrations of the C=O bond. The peaks at 3613 cm\u003csup\u003e-1\u003c/sup\u003e (Hong et al., 2016) and 3736 cm\u003csup\u003e-1\u003c/sup\u003e (Sheng et al., 2014) represented the stretching vibrations of the C=O bond. The vibrational peaks at 3613 cm\u003csup\u003e-1\u003c/sup\u003e (Hong et al., 2016) and 3736 cm\u003csup\u003e-1\u003c/sup\u003e (Sheng et al., 2014) corresponded to the stretching vibrations of the -OH bond. The peak located at 927 cm\u003csup\u003e-1\u003c/sup\u003e was associated with the deformation vibration of the C-O bond. Although there was no obvious shift in the position of peaks before and after the reaction, a notable decrease in intensity suggested that the material\u0026apos;s structure was compromised to some extent.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn Fig.2, a comparative analysis of the morphology, elemental surface distribution, and elemental content of N-SRB before and after the removal of Cr(VI) was presented Fig. 2a highlighted the original morphology of the biochar, revealing an irregular, block-like structure with a relatively rough surface. Numerous fine particles and pores, approximately 10 \u0026micro;m in size, were visible on the surface of the blocks. An elemental surface distribution map of the pristine biochar was presented in Fig. 2c The map highlighted a dominant presence of carbon (C), alongside a significant amount of oxygen (O) and a clearly visible nitrogen (N) signal. Notably, there was almost no trace of Cr in the map. The corresponding full-spectrum analysis (Fig. 2e) confirmed this observation, indicating that the pristine biochar sample contained 73.01% carbon, 20.42% oxygen, and 6.56% nitrogen. Based on these findings, it could be concluded that the pristine biochar sample was essentially free of Cr and rich in N.\u003c/p\u003e\n\u003cp\u003eXPS was employed to investigate the elemental variations in the biochar before and after Cr(VI) elimination. Fig. 3provided a comparative analysis of the changes in the core level spectra of four elements: C, N, O, and Cr. In Fig. 3a, the pristine biochar exhibited three elements: C, N, and O. In Fig. 3b, C 1s spectrum was back-convoluted into four peaks attributed to O-C=O, C=O, C-O, and C-C, respectively, from high to low binding energies. N-SRBs typically featured more basic structural units dominated by C-C and C-O bonds. Fig. 3c depicted that N 1s spectrum, identifying three types of nitrogen functional: graphitic nitrogen at 401.6 eV (Hu et al., 2017), pyrrole nitrogen at 400.4 eV (Li et al., 2019), pyridine nitrogen at 399.0 eV (Imamura and Saiki, 2011). The presence of these nitrogen functional groups could enhance the surface electrophilicity, provide coordination sites, and play a crucial role in Cr(VI) elimination by N-SRB. In Fig. 3d, O 1s spectrum was deconvoluted into three peaks, attributed to O-C=O, C-O, and C=O, respectively. These results indicated that N-SRB possessed a rich variety of functional groups, contributing to its adsorption capability.\u003c/p\u003e\n\u003ch2\u003e3.2 Cr(VI) removal performance\u003c/h2\u003e\n\u003ch3\u003e3.2.1 Kinetics of Cr(VI) removal by N-SRB\u003c/h3\u003e\n\u003cp\u003eFig. 4a illustrated \u0026nbsp;the variation of the three different forms of Cr over time. The concentration of Cr(VI) decreased sharply, reaching only 0.025 mg\u0026middot;L⁻\u0026sup1; at 240 min. The process consisted of three distinct phases: a rapid removal from 0 to 120 min, which was attributed to the availability of the active sites; a slower phase from 120 to 240 min, as the active sites became depleted; and finally, an equilibrium phase at 240 min. The study also revealed that Cr(III) was produced during the Cr(VI) removal process by N-SRB. The concentration of Cr(III) increased from below the detection limit (0.01 mg\u0026middot;L\u003csup\u003e-1\u003c/sup\u003e) to 30.12 mg\u0026middot;L\u003csup\u003e-1\u003c/sup\u003e in the first 30 min. During the reduction of Cr(VI) to Cr(III), electrons were transferred from the N-SRB to Cr(VI) ions, which led to a decrease in the oxidation state of the Cr and an increase in the oxidation state of Cr ions.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe kinetic mechanism of Cr(VI) removal by N-SRB was analyzed by four kinetic models. As shown in Fig. 4b, the experimental data most accurately fitted the pseudo-second-order (PSO) model, evidenced by a high correlation coefficient (R\u0026sup2;=0.9999). This strong correlation indicated that the main mechanism driving Cr(VI) removal was the formation of a chemical bond between the Cr(VI) and the adsorbent (N-SRB)\u0026nbsp;(Zou et al., 2021).\u0026nbsp;In Fig. 4c, the intraparticle diffusion (IPD) model was applied to determine the removal stages of Cr(VI) removal by N-SRB. The process was divided into three stages: the first stage exhibited a higher removal rate, where Cr(VI) ions were directly reduced by the functional groups on the surface of N-SRB. In the second stage, the reduction ability diminished, mainly due to the electrostatic attraction of Cr(III) and the decrease of available active sites. The third stage of the IPD model represented the adsorption equilibrium, where the removal rate of Cr(VI) was in equilibrium with the desorption rate, indicating a steady state in the process..\u003c/p\u003e\n\u003cp\u003eAs depicted in Fig. 4d, the Elovich model produced a correlation coefficient (R\u0026sup2;) of 0.83, suggesting a moderate fit while also revealing a significant variation in the activation energy. This observation implied that the removal process involved a multi-layered mechanism, encompassing not only interfacial adsorption but also dynamic processes of activation and deactivation. These findings highlighted the presence of multiple interactions and energy barriers that influenced the adsorption kinetics, further demonstrating that the activation energy of the adsorption process was relatively high (Sun et al., 2022).\u0026nbsp;\u003c/p\u003e\n\u003ch3\u003e3.2.2 Effect of pH\u003c/h3\u003e\n\u003cp\u003eThe effect of initial solution pH on Cr(Ⅵ) removal efficiency was examined, as pH is a key parameter influencing the removal of Cr(VI) by materials\u0026nbsp;(Zhang et al., 2019). The influence of pH on the removal of Cr(VI) using N-SRB was investigated over a pH range of 2.0 to 8.0, maintaining a constant initial Cr(VI) concentration of 40 mg\u0026middot;L\u003csup\u003e-1\u003c/sup\u003e, N-SRB dosage of 5.0 g L\u003csup\u003e-1\u003c/sup\u003e, and reaction time of 240 min. The results demonstrated a pronounced dependence of Cr(VI) removal on pH of the solution. In Fig. 5, the maximum removal rate achieved was 99.93% at pH 2.0. However, as the pH increased, the removal efficiency declined sharply: to 48.31% at pH 4.0, 37.6% at pH 6.0, and 35.88% at pH 8.0. This trend underscored the importance of acidic conditions in enhancing Cr(VI) removal, likely due to the increased protonation of functional groups on the N-SRB surface and the greater availability of Cr(VI) species in lower pH environments. At lower pH levels, Cr(VI) primarily existed as HCrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, whereas at higher pH values, the dominant form is Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e. The higher adsorption free energy of HCrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e compared to Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e suggested that HCrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e was more readily adsorbed by N-SRB at lower pH values. This suggested that the efficiency of Cr(VI) removal using N-SRB was significantly influenced by the speciation of Cr(VI), with the acidic form, HCrO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e-\u003c/sup\u003e, being more effectively removed (Wang et al., 2009). The adsorption free energy of Cr(VI) surpassed that of Cr(III), further enhancing the removal process at lower pH.\u003c/p\u003e\n\u003ch3\u003e3.2.3 Effects of N-SRB dosage and initial Cr(VI) concentration\u003c/h3\u003e\n\u003cp\u003eThe initial concentration of Cr(Ⅵ) is another important factor affecting the removal performance of the material (Miretzky and Cirelli, 2010). \u0026nbsp;As illustrated in Fig. 6a, a high Cr(VI) removal rate was achieved at an initial concentration of 40 mg\u0026middot;L\u003csup\u003e-1\u003c/sup\u003e. However, as the initial concentration increased to 100, 200, and 400 mg\u0026middot;L\u003csup\u003e-1\u003c/sup\u003e, Cr(VI) removal significantly declined, with the efficiencies dropping to 39.45%, 16.59%, and 7.66%, respectively. Conversely, when the initial concentration was reduced to 25 mg\u0026middot;L\u003csup\u003e-1\u003c/sup\u003e, Cr(VI) was nearly undetectable in the aqueous solution.\u003c/p\u003e\n\u003cp\u003eThis inverse relationship between initial Cr(VI) concentration and removal efficiency was attributed to the limited number of active sites available on the surface of the N-SRB. At higher Cr(VI) concentrations, the available active sites quickly became saturated, reducing the material\u0026apos;s capacity to adsorb additional Cr(VI). This highlighted the importance of optimizing the material dosage or potentially modifying the N-SRB surface to increase the number of active sites for\u0026nbsp;better performance at higher Cr(VI) concentrations (Akram et al., 2017; Singha and Das, 2011).\u0026nbsp;The adsorption capacity (Q\u003csub\u003ee\u003c/sub\u003e) of N-SRB for Cr(VI) was investigated at varying initial concentrations. At lower initial concentrations (up to 50 mg\u0026middot;L\u003csup\u003e-1\u003c/sup\u003e), Q\u003csub\u003ee\u003c/sub\u003e exhibited a continuous increase with increasing initial concentration, indicating that the available adsorption sites on the N-SRB surface were readily accessible. However, as the initial Cr(VI) concentration was further increased to 100 mg\u0026middot;L\u003csup\u003e-1\u003c/sup\u003e, 200 mg\u0026middot;L\u003csup\u003e-1\u003c/sup\u003e, and 400 mg\u0026middot;L\u003csup\u003e-1\u003c/sup\u003e, the change in Q\u003csub\u003ee\u0026nbsp;\u003c/sub\u003ebecame less significant, suggesting that the adsorption sites on the N-SRB surface were approaching saturation. At these higher concentrations, the available active sites were rapidly occupied by Cr(VI), limiting further adsorption. This saturation effect is evident in the plateauing of Q\u003csub\u003ee\u003c/sub\u003e, indicating that the adsorption process was nearing equilibrium with all active sites effectively occupied.\u003c/p\u003e\n\u003cp\u003eIn general, moderate amount of N-SRB can improve the efficiency of removing Cr(VI). However, both excessive and insufficient dosages can negatively affect the Cr(VI) removal process.. As evidenced in Fig. 6b, when the dosage of N-SRB was increased from 4 g\u0026middot;L⁻\u0026sup1; to 5 g\u0026middot;L⁻\u0026sup1;, the removal rate surged from 91.71% to 99.93%. This observation underscored the significance of employing an optimal dosage of N-SRB, as it could furnish an abundance of available adsorption sites and augment the interfacial contact area between Cr(VI) and N-SRB, thereby enhancing the overall removal efficiency. When the dosage was 2 g\u0026middot;L\u003csup\u003e-1\u003c/sup\u003e, the removal rate was 51.43%, and at even lower dosage of \u0026nbsp;1 g\u0026middot;L\u003csup\u003e-1\u003c/sup\u003e and 0.5 g\u0026middot;L\u003csup\u003e-1\u003c/sup\u003e, the removal rates were only 28.5% and 22.04%, respectively. This indicated that the adsorption performance of N-SRB might not be fully utilized if the dosage was too low, which affected the removal effect.\u003c/p\u003e\n\u003ch2\u003e3.3 Mechanisms\u003c/h2\u003e\n\u003cp\u003eAs shown in Table 1, all parameters increased after the reaction: the specific surface area increased to 1.1595 m\u0026sup2;\u0026middot;g\u003csup\u003e-1\u003c/sup\u003e, the pore volume rose to 0.006254 m\u003csup\u003e3\u003c/sup\u003e\u0026middot;g\u003csup\u003e-1\u003c/sup\u003e, the pore size increased to 312.762 nm. In Fig. 1a, we observed that the area of BET after adsorption was larger than that before adsorption, with the emergence of additional pores around 200 nm. This suggested a strong interaction between the biochar and Cr(VI), with Cr(VI) possibly exhibiting a corrosive effect on the biochar. This corrosive interaction could be responsible for the increased number of pores around 200 nm on the biochar. The results confirmed that N-SRB possessed a porous structure with a large specific surface area, which was able to adsorb Cr(VI) to its surface by physical adsorption.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn (Fig. 1c), a new stronger vibrational peak was observed at the 1189 cm\u003csup\u003e-1\u003c/sup\u003e position of biochar after Cr(VI) removal, which might be a signal from the Cr(VI) (Dong et al., 2021), indicating that the biochar successfully adsorbed a large amount of Cr(VI). After the adsorption of Cr(VI), the C-H bond was blue shifted to the position of 704 cm\u003csup\u003e-1\u003c/sup\u003e, with a noticeable decrease in intensity, which proved that the structure of biochar might have been damaged. Following the reaction, notable blue shifts were observed in the vibrational frequencies of the C=O and -OH bonds, indicative of an increase in their respective bond strengths. This augmentation suggested that the biochar, upon adsorbing Cr(VI), exhibited a significantly enhanced capacity for the adsorption of oxygen-containing species and water. This enhancement was likely attributed to the defects that emerged as a consequence of structural alterations incurred during the adsorption process.\u003c/p\u003e\n\u003cp\u003eAfter contacted with Cr(VI) (Fig. 2b), the volume of the material was obviously reduced, and the plate-like block became lamellar superposition with more pronounced gaps between the layers. Additionally, a large number of small-scale aggregated floating materials appeared on the surface of the material, alongside a large number of bumps and depressions. These morphological changes might be related to the strong oxidizing properties of Cr(VI) which corroded the biochar and cracked it into small-sized layered lumpy morphology. After exposure to Cr(VI), Fig. 2d illustrated that elemental Cr was uniformly distributed throughout the sample and the signal of elemental N became weakened. Correspondingly, Fig. 2f indicated that the content of elemental Cr increases to 0.61% and the content of elemental N decreased to 5.42%, which proved that elemental N was involved in the removal of Cr(VI), and that Cr binds to N-SRB after the reaction.\u003c/p\u003e\n\u003cp\u003eIn the full XPS spectrum (Fig. 3), when adsorbed Cr(VI), a new distinct peak attributed to the Cr signal appeared, indicating that the biochar adsorbed a large amount of Cr. This observation confirmed the effectiveness of the biochar in removing Cr(VI) from the solution. It was noteworthy that the ratio of C/O was significantly reduced after the reaction, suggesting that Cr(VI) oxidized the biochar, leading to structural changes in the material. In addition, the C-O content was significantly reduced in Fig. 3b, indicating that a large number of structural units of the material were destroyed. In addition, the decrease in the content of C=O bonds, coupled with the increase in the content of O-C=O bonds, indicated that the strong oxidative properties of Cr(VI) caused C=O to be oxidized to O-C=O, while Cr(VI) was simultaneously reduced to the more innocuous Cr(III). This phenomenon proved the effective adsorption performance of N-SRB. In addition, the positions of every Peak after the reaction were obviously shifted negatively, indicating the obvious electron transfer between the N-SRB and Cr(VI). The N 1s peaks in Fig. 3c were back-convoluted into pyridine nitrogen, pyrrole nitrogen, and graphite nitrogen. After the reaction, a noticeable decrease in the proportion of graphitic nitrogen and pyridine nitrogen was observed, accompanied by an increase in the amount of pyrrole nitrogen. This shift in nitrogen types suggested that the type of nitrogen altered during the reaction. And it was noticed that the binding energy of the main peak of N 1s produced a significant negative shift, indicating that N gained electrons during the reaction process, which might be the reason for the shift in the type of nitrogen. Fig. 3e illustrated that the biochar prior to the reaction contained almost no Cr element. However, upon adsorption, the adsorbed Cr element was back-convoluted into three peaks, representing Cr species at various oxidation states: Cr\u003csup\u003e6+\u003c/sup\u003e, Cr(OH)\u003csub\u003e3\u003c/sub\u003e and Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. The presence of these peaks confirmed that some of the initially present Cr(VI) was reduced to less toxic forms (Cr(III)). This finding demonstrated the ability of the N-SRB to effectively reduce Cr(VI) during the adsorption process. This dual functionality\u0026mdash;adsorption and reduction\u0026mdash;highlighted the potential of N-SRB as a sustainable material for treating Cr(VI)-contaminated water.\u003c/p\u003e\n\u003cp\u003eAccording to our analysis, the potential mechanism by which N-SRB facilitated the removal of Cr(VI) was illustrated in Fig. 7. This process unfeld in three distinct stages: Cr(VI) molecules were attracted to the N-SRB surface due to the electrostatic forces between the negatively charged Cr(VI) and the positively charged functional groups presented on the adsorbent; certain Cr(VI) ions underwent a reduction to Cr(III), facilitated by nearby electron-rich entities, including nitrogen and oxygen-containing groups; Cr(III) was then adsorbed by C=O group on the N-SRB.\u0026nbsp;\u003c/p\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eThis research explored the efficacy of nitrogen-enriched soy sauce residue biochar (N-SRB) as an adsorbent for the elimination of Cr(VI) from aqueous environments. Findings indicated that N-SRB exhibits superior Cr(VI) removal performance, particularly under acidic conditions. The adsorption equilibrium was established within 240 min, reflecting a rapid adsorption kinetics. The kinetic study aligned with a pseudo-second-order model, implying a chemical adsorption process that included chelation, ion exchange, and interactions with active functional groups on the biochar's surface. The Cr(VI) removal mechanism by N-SRB was multifaceted, commencing with electrostatic interactions between the negatively charged Cr(VI) species and the positively charged biochar surface, reduction of Cr(VI) to Cr(III) by nitrogen and oxygen-containing groups, succeeded by complexation with trivalent chromium (Cr(III)) and biochar's functional groups (C\u0026thinsp;=\u0026thinsp;O). Ultimately, N-SRB was identified as a promising, economical, and sustainable adsorbent alternative to activated carbon for the remediation of Cr(VI)-laden wastewater.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eRuiping Yan: First author \u0026amp;Writing original draft \u0026amp;Investigation. Yuwei Tang: Second author \u0026amp;Investigation. Yilong Li: Investigation. Jiahao Tian: Formal analysis. Li Zhang: Formal analysis. Shuang Liang: Writing review \u0026amp;editing \u0026amp;Funding acquisition. Yuting Zhang: Funding acquisition \u0026amp;Conceptualization.All authors are Jilin Jianzhu University.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e \u003cp\u003eThis work was funded by the Science and Technology Planning Project of Jilin Province (20220508008RC and 20220505018ZP), and the Science and Technology Projects of the Ministry of Housing and Urban-Rural Development (2018-K6-003).\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eAigbe, U.O., Osibote, O.A., 2020. A review of hexavalent chromium removal from aqueous solutions by sorption technique using nanomaterials. J. Environ. Chem. 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Influence of chemical treatment on rice straw pyrolysis by TG-FTIR. IERI Procedia 8, 30-34. https://doi.org/10.1016/j.ieri.2014.09.006.\u003c/li\u003e\n \u003cli\u003eSingha, B., Das, S.K., 2011. Biosorption of Cr(VI) ions from aqueous solutions: kinetics, equilibrium, thermodynamics and desorption studies. Colloids Surf. B Biointerfaces 84, 221-232. https://doi.org/10./j.colsurfb.2011.01.004.\u003c/li\u003e\n \u003cli\u003eSun, P., Wang, Z., An, S., Zhao, J., Yan, Y., Zhang, D., Wu, Z., Shen, B., Lyu, H., 2022. Biochar-supported nZVI for the removal of Cr(VI) from soil and water. Advances in experimental research and engineering applications. J. Environ. Manage. 316, 115211. https://doi.org/10.1016/j.jenvman.\u003c/li\u003e\n \u003cli\u003eSylwan, I., Thorin, E., 2021. Removal of heavy metals during primary treatment of municipal wastewater and possibilities of enhanced removal: A review. Water 13, 1121. https://doi.org/10./w13081121.\u003c/li\u003e\n \u003cli\u003eVillarroel-Rocha, J., Barrera, D., Sapag, K., 2014. Introducing a self-consistent test and the corresponding modification in the Barrett, Joyner and Halenda method for pore-size determination. Microporous Mesoporous Mater. 200, 68-78. https://doi.org/10.1016/j.micromeso.2014.08.017.\u003c/li\u003e\n \u003cli\u003eWang, S.L., Chen, C.C., Tzou, Y.M., Hsu, C.L., Chen, J.H., Lin, C.F., 2009. A mechanism study of light-induced Cr(VI) reduction in an acidic solution. J. Hazard. Mater. 164, 223-228. https://doi.org/10./j.jhazmat.2008.07.145.\u003c/li\u003e\n \u003cli\u003eWang, Y., Su, H., Gu, Y., Song, X., Zhao, J., 2017. Carcinogenicity of chromium and chemoprevention: A brief update. Onco Targets Ther. 10, 4065-4079. https://doi.org/10.2147/OTT.S139262.\u003c/li\u003e\n \u003cli\u003eYang, Y., Zhang, Y., Wang, G., Yang, Z., Xian, J., Yang, Y., Li, T., Pu, Y., Jia, Y., Li, Y., 2021. Adsorption and reduction of Cr (VI) by a novel nanoscale FeS/ chitosan/biochar composite from aqueous solution. J. Environ. Chem. Eng. 9, 105407. https://doi.org/10.1016/j.jece.2021.105407.\u003c/li\u003e\n \u003cli\u003eYin, Z., Chen, B., Chen, M., Hu, S., Cheng, H., 2015. Recovery of chromium (III) ions from aqueous solution by carboxylate functionalized wool fibres. J. Soc. Leather Technol. Chem. 99, 101-106. https://www.researchgate.net/publication/281690504.\u003c/li\u003e\n \u003cli\u003eZhang, X., Tian, J., Wang, P., Liu, T., Ahmad, M., Zhang, T., Guo, J., Xiao, H., Song, J., 2022. Highly-efficient nitrogen self-doped biochar for versatile dyes\u0026apos; removal prepared from soybean cake via a simple dual-templating approach and associated thermodynamics. J. cleaner Prod. 332, 130069. https://doi.org/10.1016/j.jclepro.2021.130069.\u003c/li\u003e\n \u003cli\u003eZhang, Y., Tian, Z., Jing, Q., Chen, Y., Huang, X., 2019. Removal of Cr(VI) by modified diatomite supported NZVI from aqueous solution: evaluating the effects of removal factors by RSM and understanding the effects of pH. Water Sci. Technol. 80, 308-316. https://doi.org/10.2166/wst.2019.275.\u003c/li\u003e\n \u003cli\u003eZou, H., Zhao, J., He, F., Zhong, Z., Huang, J., Zheng, Y., Zhang, Y., Yang, Y., Yu, F., Bashir, M.A., Gao, B., 2021. Ball milling biochar iron oxide composites for the removal of chromium (Cr(VI)) from water: performance and mechanisms. J. Hazard. Mater. 413, 125252. https://doi.org/10.1016/j.jhazmat.2021.125252.\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Table","content":"\u003cp\u003eTable 1. BET characteristics of N-SRB before and after the reaction\u003c/p\u003e\n\u003cdiv align=\"center\"\u003e\n \u003ctable border=\"1\" cellspacing=\"0\" cellpadding=\"0\" width=\"100%\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd valign=\"top\" style=\"width: 21px;\"\u003e\n \u003cp\u003e\u0026nbsp;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 33px;\"\u003e\n \u003cp\u003eSpecific surface area (m\u0026sup2;\u0026middot;g\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 26px;\"\u003e\n \u003cp\u003ePore volume (m\u003csup\u003e3\u003c/sup\u003e\u0026middot;g\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd valign=\"top\" style=\"width: 18px;\"\u003e\n \u003cp\u003ePore size (nm)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 21px;\"\u003e\n \u003cp\u003eBefore the reaction\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 33px;\"\u003e\n \u003cp\u003e0.6906\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 26px;\"\u003e\n \u003cp\u003e0.002259\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 18px;\"\u003e\n \u003cp\u003e137.155\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 21px;\"\u003e\n \u003cp\u003eAfter the reaction\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 33px;\"\u003e\n \u003cp\u003e1.1595\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 26px;\"\u003e\n \u003cp\u003e0.006254\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 18px;\"\u003e\n \u003cp\u003e312.762\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"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":"Soy sauce residue, nitrogen-doped biochar, hexavalent chromium, reduction","lastPublishedDoi":"10.21203/rs.3.rs-5739941/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5739941/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study is dedicated to investigating the potential of self-nitrogen enriched biochar derived from soy sauce residue (N-SRB) for the efficient removal of Cr(VI) from wastewater. N-SRB, synthesized from readily available and low-cost soy sauce residue, exhibited a high removal efficiency of Cr(VI). The maximum Cr(VI) removal rate achieved was 99.93% at pH of 2.0. The removal process, under optimized conditions, was most accurately represented by the intra-particle diffusion model and the pseudo-second-order kinetic model. Multiple techniques were employed to verify the role of pyrrole-N, pyridine-N, graphite-N, -COOH, -OH and C=O serving as electron contributors for the reduction of Cr(VI). The primary mechanisms driving the elimination of Cr(VI) using N-SRB were identified as electrostatic attraction, redox reactions, and complexation. Nitrogen-rich bio-carbon eliminates the requirement for supplementary nitrogen sources and ensures the absence of secondary pollution during the pollutant removal process, making it environmentally benign. The development of this low-cost material has significant theoretical and practical implications for the efficient removal of Cr(VI) from contaminated water.\u003c/p\u003e","manuscriptTitle":"Enhanced Removal of Cr(VI) Using Nitrogen-Doped Soy Sauce Residue Biochar: A Comprehensive Study on Preparation, Mechanisms, and Applications","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-10 04:17:55","doi":"10.21203/rs.3.rs-5739941/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":"4c8d402b-419c-4714-bc65-f9746f02f421","owner":[],"postedDate":"April 10th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-04-10T04:17:55+00:00","versionOfRecord":[],"versionCreatedAt":"2025-04-10 04:17:55","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-5739941","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5739941","identity":"rs-5739941","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}