Removal performance and mechanisms of toxic hexavalent chromium (Cr(VI)) with ZnCl2 enhanced acidic vinegar residue biochar

Rapid adsorptive removal of emerging and legacy per- and polyfluoroalkyl substances (PFASs) from water using zinc chloride-modified litchi seed-derived biochar

The present study successfully synthesized a novel biochar adsorbent (M-L-BC) using litchi seed modified with zinc chloride for PFASs removal in water. M-L-BC greatly enhanced removal of all examined PFASs (>95 %) as compared to the pristine biochar (<40 %). The maximum adsorption capacity was observed for PFOS, reaching 29.6 mg/g. Adsorption kinetics of PFASs followed the pseudo-second-order model (PSO), suggesting the predominance of chemical adsorption. Moreover, characterization and density functional theory (DFT) calculations jointly revealed involvement of surface complexation, electrostatic interactions, hydrogen bonding, and hydrophobic interactions in PFAS adsorption. Robust PFAS removal was demonstrated for M-L-BC across a wide range of pH (3-9), and coexisting ions had limited impact on adsorption of PFASs except PFBA. Furthermore, M-L-BC showed excellent performance in real water samples and retained reusability after five cycles of regeneration. Overall, M-L-BC represents a promising and high-quality adsorbent for efficient and sustainable removal of PFASs from water.

Efficient adsorption of hexavalent chromium in water by torrefaction biochar from lignin-rich kiwifruit branches: The combination of experiment, 2D-COS and DFT calculation

A biochar (KBC) enriched with O functional groups was prepared by torrefaction using lignin-rich kiwifruit branches (KBM) as a raw material, which was characterized, and then KBC was used to adsorb hexavalent chromium (Cr6+) from water. The results showed that KBC contained more functional groups compared to KBM. The maximum adsorption of Cr6+ by KBC could reach 143.64 mg·g-1 and also had better adsorption performance than other adsorbents reported in some other reports. Cr6+ absorption by KBC was mainly a mechanism of electrostatic interaction and adsorption-reduction coupling. FTIR and XPS revealed that -OH, -COOH, CO and CC on KBC participated in Cr6+ adsorption and new groups (C=O) were generated during the process of adsorption, which implied that a redox reaction occurred. 2D-COS and DFT calculations showed that the order of functional groups on KBC interacting with Cr6+ was -OCH3 > -COOH > -OH > phenolic hydroxyl, and the binding tightness of the different functional groups to Cr6+ was -OCH3 (the shortest displacement of both groups after the adsorption) > -COOH > -OH > phenolic hydroxyl. KBC has good regeneration performance, and it is a good adsorbent for Cr6+.

Applications of engineered biochar in remediation of heavy metal(loid)s pollution from wastewater: Current perspectives toward sustainable development goals

Environmental pollution of heavy metal(loid)s (HMs) caused adverse impacts, has become one of the emerging concerns and challenges worldwide. Metal(loid)s can pose significant threats to living organisms even when present in trace levels within environmental matrices. Extended exposure to these substances can lead to adverse health consequences in humans. Removing HM-contaminated water and moving toward sustainable development goals (SDGs) is critical. In this mission, biochar has recently gained attention in the environmental sector as a green and alternative material for wastewater removal. This work provides a comprehensive analysis of the remediation of typical HMs by biochars, associated with an understanding of remediation mechanisms, and gives practical solutions for ecologically sustainable. Applying engineered biochar in various fields, especially with nanoscale biochar-aided wastewater treatment approaches, can eliminate hazardous metal(loid) contaminants, highlighting an environmentally friendly and low-cost method. Surface modification of engineered biochar with nanomaterials is a potential strategy that positively influences its sorption capacity to remove contaminants. The research findings highlighted the biochars' ability to adsorb HM ions based on increased specific surface area (SSA), heightened porosity, and forming inner-sphere complexes with oxygen-rich groups. Utilizing biochar modification emerged as a viable approach for addressing lead (Pb), cadmium (Cd), arsenic (As), mercury (Hg), and chromium (Cr) pollution in aqueous environments. Most biochars investigated demonstrated a removal efficiency >90 % (Cd, As, Hg) and can reach an impressive 99 % (Pb and Cr). Furthermore, biochar and advanced engineered applications are also considered alternative solutions based on the circular economy.

The role of superoxide anion to Cr(VI) reduction by pine biochar

It has been reported that Cr(VI) can be reduced by biochar because of its redox activity. Considering the anionic form of Cr(VI), we hypothesize that the reduction in aqueous phase is significant. However, the contribution of different reactive oxygen species in the biochar-Cr(VI) reaction system has not been distinguished. Herein, we quantitatively identified Cr(VI) adsorption and reduction in biochar systems. The reduction content of Cr(VI) was 1.5 times higher in untreated conditions than in anaerobic conditions. The disappearance of·O2- under anaerobic conditions illustrated that·O2- may be involved in the reduction of Cr(VI). Quenching of·O2- resulted in a decrease of Cr(VI) reduction by 34%, while 1O2 was negligible, probably due to the stronger electron-donating capacity of·O2-. The degradation of nitrotetrazolium blue chloride (quenching agent of·O2-) confirmed that the reduction process of·O2- mainly occurred in the liquid-phase. Boehm titration and quantification of·O2- further elucidated the significant correlation (P < 0.05) between phenolic groups and the formation of·O2-, which implied that phenolic groups acted as the primary electron donors in generating·O2-. This study highlights the importance of the liquid-phase reduction process in removing Cr(VI), which provides theoretical support for biochar conversion of Cr(VI).

Sodium hydroxide/magnesium chloride multistage activated sludge biochar: interfacial chemical behavior and Cd(II) adsorption performance

To enhance the adsorption performance of municipal sludge biochar on Cd(II), modified sludge biochar was prepared by sodium hydroxide/magnesium chloride (NaOH/MgCl2) graded activation, and the Cd(II) adsorption performance on sludge biochar (BC), NaOH-activated sludge biochar (NBC) and NaOH/MgCl2 activated sludge biochar (NBC-Mg) was investigated. The results showed that NaOH/MgCl2 graded activation upgraded the surface structure and enhanced the graphitization of sludge biochar. The adsorption experiments indicated that the adsorption kinetic and adsorption isotherm for Cd(II) were in accordance with the pseudo second-order kinetic and Langmuir model. The adsorption capacity of NBC-Mg (143.49 mg/g) for Cd(II) was higher than that of BC (50.40 mg/g) and NBC (85.20 mg/g). The mechanism of Cd(II) adsorption included ion exchange, complexation, cation-π interaction, and mineral precipitation. After five regeneration, the removal efficiency of Cd(II) by NBC-Mg remained above 90%. This work indicated that sludge biochar prepared by multistage activation could be an effective material for Cd-containing wastewater treatment.

Biochar loaded with root exudates of hyperaccumulator Leersia hexandra Swartz facilitated Cr(VI) reduction by shaping soil functional microbial communities

Cr(VI) contamination is widely recognized as one of the major environmental hazards. To address the problem of remediation of soil Cr(VI) contamination and utilization of waste peanut shells, this study comprehensively investigated the effects of peanut shell-derived biochar loaded with root exudates of hyperaccumulator Leersia hexandra Swartz on Cr(VI) reduction and microbial community succession in soil. This study confirmed that root exudate-loaded peanut shell biochar reduced soil pH while simultaneously increasing DOC, sulfide, and Fe(II) concentrations, thereby facilitating the reduction of Cr(VI), achieving a reduction efficiency of 81.8%. Based on XPS and SEM elemental mapping analyses, Cr(VI) reduction occurred concurrently with the Fe and S redox cycles. Furthermore, the microbial diversity, abundance of the functional genera (Geobacter, Arthrobacter, and Desulfococcus) and the metabolic functions associated with Cr(VI) reduction were enhanced by root exudate-loaded biochar. Root exudate-loaded biochar can promote both direct Cr(VI) reduction mediated by the Cr(VI)-reducing bacteria Arthrobacter, and indirect Cr(VI) reduction through Cr/S/Fe co-transformation mediated by the sulfate-reducing bacteria Desulfococcus and Fe(III)-reducing bacteria Geobacter. This study demonstrates the effectiveness of peanut shell biochar loaded with root exudates of hyperaccumulator Leersia hexandra Swartz to promote soil Cr(VI) reduction, reveals the mechanism how root exudate-loaded biochar shapes functional microbial communities to facilitate Cr(VI) reduction, and proposes a viable strategy for Cr(VI) remediation and utilization of peanut shell.

Utilization of biochar for remediation of heavy metals in aqueous environments: A review and bibliometric analysis

Biochar usage for removing heavy metals from aqueous environments has emerged as a promising research area with significant environmental and economic benefits. Using the PICO approach, the research question aimed to explore using biochar to remove heavy metals from aqueous media. We merged the data from Scopus and the Web of Science Core Collection databases to acquire a comprehensive perspective of the subject. The PRISMA guidelines were applied to establish the search parameters, identify the appropriate articles, and collect the bibliographic information from the publications between 2010 and 2022. The bibliometric analysis showed that biochar-based heavy metal remediation is a research field with increasing scholarly attention. The removal of Cr(VI), Pb(II), Cd(II), and Cu(II) was the most studied among the heavy metals. We identified five main clusters centered on adsorption, water treatment, adsorption models, analytical techniques, and hydrothermal carbonization by performing keyword co-occurrence analysis. Trending topics include biochar reusability, modification, acid mine drainage (AMD), wastewater treatment, and hydrochar. The reutilization of heavy metal-loaded spent biochar includes transforming it into electrodes for supercapacitors or stable catalyst materials. This study provides a comprehensive overview of biochar-based heavy metal remediation in aquatic environments and highlights knowledge gaps and future research directions.

Insights into chromium removal mechanism by Ca-based sorbents from flue gas

Chromium is a typical toxic pollution in sewage sludge incineration flue gas. Cr removal from flue gas is a challenge due to the high toxicity and valence variability of chromium. Ca-based sorbents, including CG-CaO, CA-CaO, and CCi-CaO, were developed for Cr capture by calcining calcium D-gluconate monohydrate, calcium acetate hydrate, and calcium citrate tetrahydrate, respectively. CG-CaO, CA-CaO, and CCi-CaO exhibit better Cr removal performance than traditional CaO. CA-CaO shows superior Cr adsorption ability due to the large BET surface area and pore volume. The Cr adsorption efficiency of CA-CaO is up to 94.79 % at 1000 °C. XRD and XPS results reveal that the adsorbed Cr contains Cr(III) and Cr(VI), and exists in the form of CaCr2O4 and CaCrO4. Cr adsorption on Ca-based sorbents is mainly controlled by adsorption and oxidation mechanism. The adsorption process of Cr on different Ca-based sorbents was described by four typical adsorption kinetic models. For CaO and CG-CaO, pseudo-first order model and Elovich model are suitable for the description of Cr adsorption. For CA-CaO and CCi-CaO, pseudo-second order model, Elovich model and Weber and Morris model fit well with the experimental values of Cr adsorption, suggesting that Cr adsorption on CA-CaO and CCi-CaO is controlled by a combined mechanism of chemisorption and intraparticle diffusion. The saturated adsorption capacity of CaO, CG-CaO, CA-CaO and CCi-CaO are evaluated to be 39.77, 48.98, 102.22 and 104.52 mg/g, respectively. The effects of incineration flue gas components on Cr adsorption were also explored. O2 shows no obvious influence on Cr adsorption over CA-CaO. HCl, SO2, NO and CO2 can inhibit Cr adsorption because of the competitive adsorption, and the inhibitory effect of SO2 is the strongest.

The ultramicropore biochar derived from waste distiller's grains for wet-process phosphoric acid purification: Removal performance and mechanisms of Cr(VI)

Solid waste and heavy metal pollution are long-term and challenging subjects in the field of environmental engineering. In this study, we propose a sustainable approach to "treating waste with waste" by utilizing the ultramicropore biochar derived from solid waste distiller's grains as a means to remove Cr(VI) from simulated wastewater and wet phosphoric acid. The biochar prepared in this research exhibit extremely high specific surface areas (up to 2973 m2/g) and a well-developed pore structure, resulting in a maximum Cr(VI) adsorption capacity of 426.0 mg/g and over 99% removal efficiency of Cr(VI). Furthermore, the adsorbent can be reused for up to eight cycles without significant reduction in its Cr(VI) adsorption performance. Mechanistic investigations suggest that the exceptional Cr(VI) adsorption capacity can be attributed to the synergistic effect of electrostatic interaction and reduction adsorption. This study offers an alternative approach for the resource utilization of solid waste distiller's grains, and the prepared biochar holds promise for the removal of Cr(VI) from wastewater and wet-process phosphoric acid.

Pyrolysis of Mixed Date Stones and Pistachio Shells: Identification of Bio-Oil and Utilization of Bio-Char as Activated Carbon Precursor

Thermal pyrolysis of mixed date stones and pistachio shells in a semi-batch reactor was addressed in this study. The highest yield of liquids (51.20 %) was produced at 500 °C, 90 min, 20 °C/min heating rate, and 50 mesh particle size. Under these conditions, yield of liquid from date stones and pistachio shells separately was 49.12 % and 47.67 %, respectively. The FT-IR results confirmed the presence of multiple oxygen-containing compounds in the bio-oil. Results from GC-MS declared that it was predominately composed of acids (57.57 %), esters (21.35 %), phenols (4.63 5), and alcohols (3.49 5). The obtained biochar was transformed into activated carbon (AC) by the optimized ZnCl2 activation method. The ideal AC was synthesized at 600 °C for 60 min using a 2 : 1 ZnCl2 : biochar impregnation ratio. FESEM and XRD measurements showed that the AC was amorphous. The prepared AC was effective in eliminating dibenzothiophene (DBT) from model fuel (200 ppm DBT/hexane) with a maximum performance 95.26 % at 40 °C for 1h using 0.35 g of the AC. The exhausted AC was regenerated and reutilized 4 times, and removal efficiency reached 88.23 % in the 4th cycle under ideal working conditions.

Research on the performance of modified blue coke in adsorbing hexavalent chromium

To solve the issue of hexavalent chromium (Cr(VI)) contamination in water bodies, blue coke powder (LC) was chemically changed using potassium hydroxide to create the modified material (GLC), which was then used to treat a Cr(VI)-containing wastewater solution. The differences between the modified and unmodified blue coke's adsorption characteristics for Cr(VI) were studied, and the impact of pH, starting solution concentration, and adsorption period on the GLC's adsorption performance was investigated. The adsorption behavior of the GLC was analyzed using isothermal adsorption models, kinetic models, and adsorption thermodynamic analysis. The mechanism of Cr(VI) adsorption by the GLC was investigated using characterization techniques such as Fourier Transform Infrared Spectroscopy (FTIR), Field Emission Scanning Electron Microscope (FE-SEM), X-Ray Diffraction (XRD), and X-Ray Photoelectron Spectroscopy (XPS). With the biggest difference in removal rate at pH = 2, which was 2.42 times that of LC, batch adsorption experiments revealed that, under the same adsorption conditions, the GLC always performed better than LC. With a specific surface area that was three times that of LC and an average pore diameter that was 0.67 times that of LC, GLC had a more porous structure than LC. The alteration significantly increased the number of hydroxyls on the surface of GLC by altering the structural makeup of LC. The ideal pH for removing Cr(VI) was 2, and the ideal GLC adsorbent dosage was 2.0 g/L. Pseudo-second-order kinetic (PSO) model and Redlich-Peterson (RP) model can effectively describe the adsorption behavior of GLC for Cr(VI). Physical and chemical adsorption work together to remove Cr(VI) by GLC in a spontaneous, exothermic, and entropy-increasing process, with oxidation-reduction processes playing a key role. GLC is a potent adsorbent that can be used to remove Cr(VI) from aqueous solutions.

Enhanced removal of aqueous Cr(VI) by the in situ iron loaded activated carbon through a facile impregnation with Fe(II) and Fe(VI) two step method: Mechanism study

In this study, a novel in situ iron-loaded activated carbon (AFPAC) was prepared by a FeSO₄/K₂FeO₄ impregnation and oxidation combination two-step supported on activated carbon for enhanced removal of Cr(VI) from aqueous solutions. Cr(VI) removal efficiency greatly increased by AFPAC more than 70% than that of fresh activated carbon (AC), which is due to rich iron oxides formed in situ and the synergistic effect between iron oxides and activated carbon. Cr(VI) adsorption behaviors on AFPAC under different water quality parameters were investigated. The maximum monolayer adsorption capacities for Cr(VI) by AFPAC are as high as 26.24 mg/g, 28.65 mg/g, and 32.05 mg/g at 25 °C, 35 °C and 45 °C at pH 4, respectively. Density functional theory (DFT) results showed that the adsorption energy of K₂Cr₂O₇ on the surface of FeOOH was - 2.52 eV, which was greater than that on the surface of bare AC, and more charge transfer occurred during the adsorption of K₂Cr₂O₇ on the surface of FeOOH, greatly promoting the formation of Cr = O-Fe. Cr(VI) removal by AFPAC included electrostatic attraction, redox reaction, coordinate complexation, and co-precipitation. Cr(VI) adsorption process on AFPAC consisted of the three reaction steps: (1) AFPAC was fast protonation and Cr₂O₇^2- would electrostatically attract to the positively charged AFPAC surface. (2) Cr₂O₇^2- was reduced into Cr₂O₃ by the carbons bond to the oxygen functionalities on activated carbon and the redox reaction process of FeSO₄ and K₂FeO₄. (3) The inner-sphere complexes were formed, and adsorbed on AFPAC by iron oxides and then co-precipitation.

Turning waste into valuables: In situ deposition of polypyrrole on the obsolete mask for Cr(VI) removal and desalination

The global mask consumption has been exacerbated because of the coronavirus disease 2019 (COVID-19) pandemic. Simultaneously, the traditional mask disposal methods (incineration and landfill) have caused serious environmental pollution and waste of resources. Herein, a simple and green mass-production method has been proposed to recycle carbon protective mask (CPM) into the carbon protective mask/polydopamine/polypyrrole (CPM/PDA/PPy) composite by in situ polymerization of PPy. The CPM/PDA/PPy composite was used for the removal of Cr(VI) and salt ions to produce clean water. The synergistic effect of PPy and the CPM improved the removal capability of Cr(VI). The CPM/PDA/PPy composite provided high adsorption capacity (358.68 mg g^-1) and economic value (811.42 mg $^-1). Consequently, the CPM/PDA/PPy (cathode) was combined with MnO₂ (anode) for desalination in CDI cells, demonstrated excellent desalination capacity (26.65 mg g^-1) and ultrafast salt adsorption rate (6.96 mg g^-1 min^-1), which was higher than conventional CDI cells. Our work proposes a new low-carbon strategy to recycle discarded masks and demonstrates their utilization in Cr(VI) removal and seawater desalination.

Chemically activated carbon preparation from natural rubber biosludge for the study of characterization, kinetics and isotherms, thermodynamics, reusability during Cr(VI) and methylene blue adsorption

Alkaline leachate, dust generation, and foul smell during the stacking process of natural rubber biosludge (NRBS) can pollute surrounding water, soil, and air. In this study, natural rubber chemically activated carbon (NRCAC) has been synthesized for the first time from NRBS by pyrolysis using ZnCl₂ at 700 °C for adsorptive removal of Cr(VI) and methylene blue (MB) from aqueous solutions. Both NRBS and NRCAC were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier transform infrared (FTIR), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), Brunauer-Emmett-Teller (BET), and thermogravimetric analyzer (TGA). FTIR and SEM-EDS suggested significant functional and morphological transformations in NRCAC. Experimental investigations of different process parameters, such as pH, concentration, contact time, salt concentration, etc., were conducted to study their influences on adsorption. Adsorption and desorption kinetics followed a pseudo-second-order model, while adsorption equilibrium followed Liu isotherm. Maximum uptake calculated from the Liu model was 81.28 and 211.90 mg/g for Cr(VI) and MB, respectively. Thermodynamic analysis established spontaneous and endothermic adsorption. Up to five adsorption/desorption cycles were conducted using eluents such as 1 M NaOH and water for Cr(VI) and MB, respectively. Electrostatic attraction and ion-exchange favored Cr(VI)/MB adsorption, while hydrogen bonding and π-π stacking were significant in MB uptake. Overall findings suggest that NRBS (a renewable agro-industrial, abundant, and freely available) could be employed to synthesize biochar for adsorptive removal of wastewater containing Cr(VI)/MB.

Properties and mechanism of Cr(VI) removal by a ZnCl2-modified sugarcane bagasse biochar-supported nanoscale iron sulfide composite

A ZnCl₂-modified biochar-supported nanoscale iron sulfide composite (FeS-ZnBC) was successfully prepared to address the easy oxidization of FeS and enhance Cr(VI) removal from water. The material was characterized by SEM, XRD, FTIR, and XPS. The effects of FeS:ZnBC mass ratio, FeS-ZnBC dosage, solution pH, initial Cr(VI) concentration, and reaction time on the adsorption performance were investigated. The results revealed that the optimum adsorption capacity of FeS-ZnBC (FeS:ZnBC = 1:2) for Cr(VI) was 264.03 mg/g at 298 K (pH = 2). A Box-Behnken design (BBD) was applied to optimize the input variables that affected the adsorption of Cr(VI) solution. The results revealed that the highest removal (99.52%) of Cr(VI) solution was achieved with a Cr(VI) initial concentration of 150.59 mg/L, FeS-ZnBC adsorbent dosage of 2 g/L, and solution pH of 2. The sorption kinetics could be interpreted using a pseudo-second-order kinetic model. The isotherms were simulated using the Redlich-Peterson isotherm model, indicating that Cr(VI) removal by the FeS-ZnBC composites was a hybrid chemical reaction-sorption process. The main mechanisms of Cr(VI) removal by FeS-ZnBC were adsorption, chemical reduction, and complexation. This study demonstrated that FeS-ZnBC has potential application prospects in Cr(VI) removal.

A metal-organic framework surrounded with conjugate acid-base pairs for the efficient capture of Cr(VI) via hydrogen bonding over a wide pH range

Given the large amount of toxic Cr(VI) wastewater from various industries, it is urgent to take effective treatment measures. Adsorption has been regarded as highly desirable for Cr(VI) removal, but the effectiveness of most adsorbents is significantly dependent on pH value, in which precipitous performance drop and even structural collapse generally occur in strong acidic/alkaline aqueous. Thus, maintaining high adsorption performance and structural integrity over a wide pH range is challenging. To efficiently remove Cr(VI), we designed and prepared of an acid-base resistant metal-organic framework (MOF) Zr-BDPO, by introducing weak acid-base groups (-NH-, -N= and -OH) onto the ligand. Zr-BDPO achieved a maximum adsorption capacity of 555.6 mg·g^-1 and retained skeletal structure at pH= 1-11. Interestingly, all these groups can generate conjugate acid-base pairs by means of H⁺ and OH^- in the external solution and then form buffer layer. The removal of Cr(VI) at a broad range of pH values primarily via hydrogen bonds between -NH- and -OH, and the oxoanion species of Cr(VI) is unusual. This strategy that insulating high concentrations of acids and bases and relying on hydrogen bonds to capture Cr(VI) oxoanions provides a new perspective for actual Cr(VI) wastewater treatment.

Enhanced adsorption capacity of tetracycline on porous graphitic biochar with an ultra-large surface area†

Excessive tetracycline in the water environment may lead to the harming of human and ecosystem health. Removing tetracycline antibiotics from aqueous solution is currently a most urgent issue. Porous graphitic biochar with an ultra-large surface area was successfully prepared by a one-step method. The effects of activation temperature, activation time, and activator dosage on the structural changes of biochar were investigated by scanning electron microscopy, Brunauer–Emmett–Teller, X-ray powder diffraction, and Raman spectroscopy. The effect of the structure change, adsorption time, temperature, initial pH, and co-existing ions on the tetracycline removal efficiency was also investigated. The results show that temperature had the most potent effect on the specific surface area, pore structure, and extent of graphitization. The ultra-large surface area and pore structure of biochar are critical to the removal of tetracycline. The q_e of porous graphitic biochar could reach 1122.2 mg g⁻¹ at room temperature. The calculations of density functional theory indicate that π–π stacking interaction and p–π stacking interaction can enhance the tetracycline adsorption on the ultra-large surface area of graphitic biochar.

1. A ultra-large surface area of porous graphitic biochar was successfully using corn starch and ZnCl₂ by a one-step method. 2. The adsorption capacity of tetracycline on the biochar could get 1122.2 mg g⁻¹ at room temperature.

Improved Cr (VI) adsorption performance in wastewater and groundwater by synthesized magnetic adsorbent derived from Fe3O4 loaded corn straw biochar

This work developed an easy method to utilize corn straw (CS) waste for sustainable development and reduce the volume of waste volume as well as bring value-added. The magnetic adsorbent was prepared by loading Fe₃O₄ onto biochar derived from corn straw (Fe@CSBC), then used for capturing Cr (VI) in groundwater and wastewater samples. The characterization of adsorbents showed that Fe3O4 was successfully loaded on corn straw biochar (CSBC) and contributed to the improvement of the surface area, and surface functional groups like Fe-O, Fe-OOH, CO, and O-H. The presence of iron oxide was further confirmed by XPS and XRD analysis and a magnetization value of 35.6 emu/g was obtained for Fe@CSBC. The highest uptake capacity of Cr (VI) onto Fe@CSBC and CSBC by monolayer were 138.8 and 90.6 mg/g, respectively. By applying magnetic adsorbent Fe@CSBC for the treatment of groundwater and wastewater samples, the chromium could be removed up to 90.3 and 72.6%, respectively. The remaining efficiency of Cr (VI) was found to be 84.5% after four times reused Fe@CSBC, demonstrating the great recyclable ability of the adsorbent. In addition, several interactions between Cr (VI) and Fe@CSBC like ion exchange, complexation, and reduction reaction were discussed in the proposed adsorption mechanism. This study brings an efficient method to turn corn straw biomass into an effective magnetic adsorbent with high adsorption performance and good reusability of Cr (VI) in groundwater as well as in wastewater.

Amine-functionalized magnetic biochars derived from invasive plants Alternanthera philoxeroides for enhanced efficient removal of Cr(VI): performance, kinetics and mechanism studies

In this study, novel magnetic biochars derived from Alternanthera philoxeroides and modified by different amines (hexanediamine, melamine, and L-glutathione) were successfully prepared by hydrothermal carbonization and employed as an efficient adsorbent for Cr(VI). When pH = 2.0, T = 25 °C, c₀ = 100 mg/L, and the dosage of biochars is 0.05 g, the maximum adsorption capacity of Cr(VI) by pristine biochar (BAP) was 42.47 mg/g and modified biochars (MFBAP, MEBAP, LBAP) was 80.58, 62.26, and 55.66 mg/g, respectively. It was found that hexanediamine and melamine could enhance the S_BET of biochars, while L-glutathione could reduce its S_BET, which could be supported by BET measurement and SEM images. Adsorption kinetics and isotherm studies showed that the Cr(VI) adsorption process of MFBAP followed Elovich kinetic model and Langmuir isotherm, respectively, which means that it was mainly a chemical adsorption process. The characterization results proved that -NH₂ derived from amines plays a significant role in removing Cr(VI), which is mainly degraded by complexation reaction, electrostatic interaction, and reduction. In sum, the biochar modified by amines has excellent Cr(VI) adsorption performance, highly enhanced S_BET, and excellent recyclability, which is a promising candidate for solving the problem of invasive plants and wastewater treatment.

A Review of the Current State and Future Prospects in Resource Recovery of Chinese Cereal Vinegar Residue

Vinegar residue (VR) is a typical organic solid waste in Chinese cereal vinegar production. It is characterized by high yield, high moisture and low pH and is rich in lignocellulose and other organic matter. To avoid the environmental pollution caused by VR, it should be properly treated. The industry’s existing treatment processes, landfills and incineration, cause secondary pollution and waste of resources. Therefore, there is an urgent demand for environmentally friendly and cost-effective resource recovery technologies for VR. To date, a considerable amount of research has been performed in the area of resource recovery technologies for VR. This review summarizes the reported resource recovery technologies, mainly anaerobic digestion, feed production, fertilizer production, high-value product production and soil/water remediation. The principles, advantages and challenges of these technologies are highlighted. Finally, as a future perspective, a cascade and full utilization model for VR is proposed by considering the inherent drawbacks and economic-environmental feasibility of these technologies.

Remediation of Cr(VI)-Contaminated Soil by Biochar-Supported Nanoscale Zero-Valent Iron and the Consequences for Indigenous Microbial Communities

Biochar/nano-zero-valent iron (BC-nZVI) composites are currently of great interest as an efficient remediation material for contaminated soil, but their potential to remediate Cr-contaminated soils and effect on soil microecology is unclear. The purpose of this study was to investigate the effect of BC-nZVI composites on the removal of Cr(VI) from soil, and indigenous microbial diversity and community composition. The results showed that after 15 days of remediation with 10 g/kg of BC-nZVI, 86.55% of Cr(VI) was removed from the soil. The remediation of the Cr-contaminated soil with BC-nZVI resulted in a significant increase in OTUs and α-diversity index, and even a significant increase in the abundance and diversity of indigenous bacteria and unique bacterial species in the community by reducing the toxic concentration of Cr, changing soil properties, and providing habitat for survival. These results confirm that BC-nZVI is effective in removing Cr(VI) and stabilizing Cr in soil with no significant adverse effects on soil quality or soil microorganisms.

Removal of hexavalent chromium via biochar-based adsorbents: State-of-the-art, challenges, and future perspectives

Chromium originates from geogenic and extensive anthropogenic activities and significantly impacts natural ecosystems and human health. Various methods have been applied to remove hexavalent chromium (Cr(VI)) from aquatic environmental matrices, including adsorption via different adsorbents, which is considered to be the most common and low-cost approach. Biochar materials have been recognized as renewable carbon sorbents, pyrolyzed from various biomass at different temperatures under limited/no oxygen conditions for heavy metals remediation. This review summarizes the sources, chemical speciation & toxicity of Cr(VI) ions, and raw and modified biochar applications for Cr(VI) remediation from various contaminated matrices. Mechanistic understanding of Cr(VI) adsorption using different biochar-based materials through batch and saturated column adsorption experiments is documented. Electrostatic interaction and ion exchange dominate the Cr(VI) adsorption onto the biochar materials in acidic pH media. Cr(VI) ions tend to break down as HCrO₄^-, CrO₄^2-, and Cr₂O₇^2- ions in aqueous solutions. At low pH (∼1-4), the availability of HCrO₄^- ions attributes the electrostatic forces of attraction due to the available functional groups such as -NH₄⁺, -COOH, and -OH₂⁺, which encourages higher adsorption of Cr(VI). Equilibrium isotherm, kinetic, and thermodynamic models help to understand Cr(VI)-biochar interactions and their adsorption mechanism. The adsorption studies of Cr(VI) are summarized through the fixed-bed saturated column experiments and Cr-contaminated real groundwater analysis using biochar-based sorbents for practical applicability. This review highlights the significant challenges in biochar-based material applications as green, renewable, and cost-effective adsorbents for the remediation of Cr(VI). Further recommendations and future scope for the implications of advanced novel biochar materials for Cr(VI) removal and other heavy metals are elegantly discussed.

ZnS modified N, S dual-doped interconnected porous carbon derived from dye sludge waste as high-efficient ORR/OER catalyst for rechargeable zinc-air battery

Facile and rational design of high-efficient oxygen reduction reaction (ORR)/oxygen evolution reaction (OER) bifunctional electrocatalysts is significant for rechargeable Zinc-air batteries. In this study, ZnS modified N, S dual-doped interconnected porous carbon (ZnS/NSC) derived from the dye sludge waste is successfully fabricated via a facile ZnCl₂-assisted pyrolysis process. The effect of ZnCl₂ and carbonization temperature on the microstructure and electrocatalytic performance is systematically investigated. By virtue of the synergistic effect between ZnS nanoparticles and N, S dual-doped porous carbon network, the obtained catalyst ZnS/NSC calcined at 1000 °C exhibits a decent bifunctional electrocatalytic performance with potential gap (ΔE=E_OER,10-E_ORR,1/2) of 0.76 V comparable with commercial electrocatalysts (Pt/C and RuO₂). In addition, a rechargeable zinc-air battery employed ZnS/NSC-1000 as the air cathode also displays the favorable electrochemical performance, in which the power density is 125 mW cm^-2, the specific capacity is 763.27 mAh g^-1 and the cycling stability at 10 mA cm^-2 is more than 85 h, indicating a promising application prospect in rechargeable Zinc-air batteries.

Treated activated carbon as a metal-free catalyst for effectively catalytic reduction of toxic hexavalent chromium

In this work, activated carbon treated in N₂ atmosphere, as a non-metallic catalyst, exhibits excellent catalytic performance in reduction of Cr (VI) to Cr (III) using HCOOH as the reducing agent at room temperature. A series of characterizations and control experiments were carried out to deduce the possible reaction mechanism. The results showed that the improved catalytic performance can be attributed to the enhanced graphitization degree and basic sites such as pyrone-like, which favor electron transferring and activation of reactant. The reaction rate constant observed herein for the C-800 was 22 and 6 times more than that for C-0 and Pd/C catalyst, respectively. In addition, C-800 showed good recycle performance, and no loss of activity was observed after 5 cycles. This study broadens the application of nonmetallic catalyst and provides an easy-available and cost-effective catalytic material for removing toxic Cr (VI).

Remediation of emerging metal pollutants using environment friendly biochar- Review on applications and mechanism

Bioremediation of heavy metals has become a major environmental concern due to their bio resistant nature and tendency to accumulate. Application of various technologies, involving physical and chemical working principles are applied and passive uptake using sorption involving eco-friendly substrates gained significant attention. Biochar, a cheaper and efficient material, offers good potential due to the greater ease of production, treatment and disposal. This review focuses on the effective application of biochar to treat water contaminated by three specific heavy metals: chromium, lead and arsenic. The on-field applications like soil amendment, industrial wastewater treatment and groundwater treatment using biochar are highlighted. The review article describes the feedstock available for biochar production, various production processes and the importance of optimum conditions like pyrolysis temperature, rate and retention time for various feedstocks reported in literature. The energy requirement of the production process can be supplied by its own energy output. Various modifications that are suitable for the biochar from distinct feedstocks are also discussed. The removal performance of biochar at different working conditions like pH, initial concentration of pollutant and adsorbent dose are consolidated. The highest removal efficiencies reported were by coconut shell biochar (Cr - 99.9%), canola straw biochar (Pb - 100%) and perilla leaf biochar (As - 100%). The adsorption mechanism is explained with reference to kinetics, isotherms, and molecular dynamics. Adsorption mechanism of most of the biochars was found to fit either Freundlich or Langmuir isotherm.

High-performance biochar derived from the residue of Chaga mushroom (Inonotus obliquus) for pollutants removal

A high-performance biochar derived from the residue of Chaga mushroom (Inonotus obliquus) was reported in this study. Inonotus obliquus residues were used to prepare biochar, and the optimal synthesis conditions were obtained by response surface methodology. The specific surface area, pore volume, and average pore size of the optimal biochar (Zn-IORBC) was 1676.78 m²/g, 1.87 cm³/g, and 3.88 nm, respectively. Methylene blue (MB) and tetracycline (TC) were selected to estimate the adsorption performance of Zn-IORBC. The adsorption process was suitable for the pseudo-second-order model and Langmuir model. Zn-IORBC could maintained a large amount of TC adsorption (the lowest value was 686.20 mg/g in mountain spring water) in different natural water. The maximum adsorption capacity of TC and MB was 947.42 and 1033.66 mg/g. The adsorption mechanism was contributed to the electrostatic attraction, hydrogen bonding, π-π interactions, and pore-filling. Zn-IORBC is an effective adsorbent for high-performance pollutants removal.