Influence of Synthesis Methods on the High-Efficiency Removal of Cr(VI) from Aqueous Solution by Fe-Modified Magnetic Biochars

Performances of a novel BAF with ferromanganese oxide modified biochar (FMBC) as the carriers for treating antibiotics, nitrogen and phosphorus in aquaculture wastewater

In this paper, a biological aerated filter (BAF) based on ferromanganese oxide-biochar (FMBC) was constructed to investigated the removal performance and mechanism for conventional pollutants and four kinds of antibiotic, in contrast of conventional zeolite loaded BAF (BAF-A) and bamboo biochar filled BAF (BAF-B). Results showed that the average removal efficiency of total nitrogen (TN), total phosphorus (TP) and antibiotics in a FMBC-BAF (named by BAF-C) were 52.97 ± 2.27%, 51.58 ± 1.92% and 70.36 ± 1.00% ~ 81.65 ± 0.99% respectively in running period (39-100 d), which were significantly higher than those of BAF-A and BAF-B. In the BAF-C, the expression of denitrification enzyme activities and the secretion of extracellular polymeric substance (EPS) especially polyprotein (PN) were effectively stimulated, as well as accelerated electron transfer activity (ETSA) and lower electrochemical impedance spectroscopy (EIS) were acquired. After 100 days of operation, the abundance of nitrogen, phosphorus and antibiotic removal functional bacteria like Sphingorhabdus (4.52%), Bradyrhizobium (1.98%), Hyphomicrobium (2.49%), Ferruginibacter (7.80%), unclassified_f_Blastoca tellaceae (1.84%), norank_f_JG30-KF-CM45 (6.82%), norank_f_norank_o_SBR1031 (2.43%), Nitrospira (2.58%) norank_f_Caldilineaceae (1.53%) and Micropruina (1.11%) were enriched. Mechanism hypothesis of enhanced performances of nutrients and antibiotics removal pointed that: The phosphorus was removed by adsorption and precipitation, antibiotics removal was mainly achieved through the combined action of adsorption and biodegradation, while nitrogen removal was realized by biologic nitrification and denitrification in a FMBC-BAF for aquaculture wastewater treatment.

Silicone-modified black peanut shell (BPS) biochar adsorbents: Preparation and their adsorptions for copper(II) from water

Novel silicone-modified biochar adsorbents (BPS-MBCs) were prepared by utilizing waste black peanut shell (BPS) as a raw biochar and gamma-amino-propyl triethoxysilane (silicone) as an inorganic modifier. The novelty of this work is that the incorporation of silicone into BPS can rise the specific surface area and porosity of BPS-MBCs and elevate their adsorptions for copper (II). Sorption kinetics data for copper (II) were molded using five kinetic equations [i.e. Lagergren 1st-order and 2nd-order, intraparticle diffusion (IN-D), Elovich, and Diffusion-chemisorption]. The equilibrium adsorption data for copper (II) were analyzed using two-parameter isotherm equations [i.e. Langmuir, Freundlich, Dubinin-Radushkevich, and Temkin] and three-parameter Sips, Redlich-Peterson and Toth isotherm models. It was validated that copper (II) sorption on BPS-MBCs matched better with pseudo-2nd-order kinetic, Diffusion-chemisorption and Langmuir isotherm models. The maximal qmLan of BPS-MBC-400 was near 284 mg/g at 45 °C. By multi-phase fitting of IN-D modelling, intra-particle diffusion coefficient (kin-d) and diffusion coefficient of external mass-transfer (DEx-Di) for copper (II) were calculated. The low sorption energy from Temkin and mean free energy from D-R modellings implied that copper (II) sorption was initiated by weak non-covalent bond interactions. Thermodynamic parameters indicated that copper (II) on BPS-MBCs was an endothermic and spontaneous process. Recycling of BPS-MBC-400 for copper (II) suggested it has excellent reusability. The major mechanism of copper (II) on BPS-MBCs is possibly comprised of multiple processes, such as physical adsorption (electrostatic attraction), chemical adsorption (adsorption from functional groups, chelation, and ion exchange) and diffusion-chemisorption. Based on these findings, it is expects that BPS-MBCs are promising sorbents for copper (II) eradication from Cu(II)-including wastewater.

Improvement strategy of citrate and biochar assisted nano-palladium/iron composite for effective dechlorination of 2,4-dichlorophenol

Rapid passivation and aggregation of nanoscale zero-valent iron (nZVI) seriously limit its performance in the remediation of different contaminants from wastewater. To overcome such issues, in the present study, nano-palladium/iron (nPd/Fe) was simultaneously improved by biochar (BC) prepared from discarded peanut shells and green complexing agent sodium citrate (SC). For this purpose, a composite (SC-nPd/Fe@BC) was successfully synthesized to remove 2,4-dichlorophenol (2,4-DCP) from wastewater. In the SC-nPd/Fe@BC, BC acts as a carrier with dispersed nPd/Fe particles to effectively prevent its agglomeration, and increased the specific surface area of the composite, thereby improving the reactivity and stability of nPd/Fe. Characterization results demonstrated that the SC-nPd/Fe@BC composites were well dispersed, and the agglomeration was weakened. The formation of the passivation layer on the surface of the particles was inhibited, and the mechanism of SC and BC improving the reactivity of nPd/Fe was clarified. Different factors were found to influence the reductive dichlorination of 2,4-DCP, including Pd loading, Fe:C, SC addition, temperature, initial pH, and initial pollutant concentration. The dechlorination results revealed that the synergistic effect of the BC and SC made the removal efficiency and dechlorination rate of 2,4-DCP by SC-nPd/Fe@BC reached to 96.0 and 95.6%, respectively, which was better than that of nPd/Fe (removal: 46.2%, dechlorination: 45.3%). Kinetic studies explained that the dechlorination reaction of 2,4-DCP and the data were better represented by the pseudo-first-order kinetic model. The reaction rate constants followed the order of SC-nPd/Fe@BC (0.0264 min-1) > nPd/Fe@BC (0.0089 min-1) > SC-nPd/Fe (0.0081 min-1) > nPd/Fe (0.0043 min-1). Thus, SC-nPd/Fe@BC was capable of efficiently reducing 2,4-DCP and the dechlorination efficiency of BC and SC synergistically assisted composite on 2,4-DCP was much better than that of SC-nPd/Fe, nPd/Fe@BC and nPd/Fe. Findings suggested that SC-nPd/Fe@BC can be promising for efficient treatment of chlorinated pollutants.

Synthesizing sulfhydryl-functionalized biochar for effectively removing mercury ions from contaminated water

Biochar is regarded as an effective adsorbent for heavy metal pollution treatment, and functional optimization is still needed to improve its performance. We created raw biochar (BC and BP) from corn straw and pine sawdust, which were modified to produce sulfhydryl-modified biochar (MBC and MBP). Isothermal adsorption experiments and adsorption kinetics experiments as well as the related model fitting were performed to evaluate the adsorption performance of biochar on Hg(II). According to the results of the Langmuir model fitting, the maximum adsorption capacities of sulfhydryl-modified biochar were 193.05 mg/g (MBC) and 178.04 mg/g (MBP), respectively, which were approximately 1.6 times higher than the raw biochar. The results showed that adding sulfhydryl groups to biochar can improve its adsorption performance. The prompt effect resulted from the sulfhydryl modification providing additional functional groups and enhanced chemisorption and physical adsorption properties.

Efficient removal of Cr(VI) and As(V) from an aquatic system using iron oxide supported typha biochar

The removal of Cr(VI) and As(V) from aqueous solutions has been a worldwide concern. In this study, Typha biochar (FBC) with magnetic iron oxide was prepared by impregnating Typha with FeCl₃ and performing pyrolysis, and the possible mechanism of Cr(VI) and As(V) removal was investigated by combining characterization means and adsorption experiments. The results showed that the modified Typha biochar is rich in pores and has the potential to eliminate Cr and As through processes such as exchange and reduction. The single molecule uptake capacities of FBC for Cr(VI) and As(V) were 32.82 and 21.56 mg g^-1, respectively. The adsorption process is spontaneous heat absorption, and the adsorption results are also consistent with the proposed secondary kinetic model. FBC still had >60% removal efficiency in the second and third reuse of Cr(VI), indicating its good recyclability. Therefore, this study confirms that FBC can effectively remove both Cr(VI) and As(V).

Adsorption of the non-steroidal anti-inflammatory drug (ibuprofen) onto biochar and magnetic biochar prepared from chrysanthemum waste of the beverage industry

Biochar and magnetic biochar prepared from chrysanthemum waste of the beverage industry are effective adsorbents for the removal of the non-steroidal anti-inflammatory drug, ibuprofen (IBP), from aqueous systems. The development of magnetic biochar using iron chloride, overcame poor separation characteristics from the liquid phase of the powdered biochar after adsorption. Characterisation of biochars was achieved through Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), N₂ adsorption/desorption porosimetry, scanning electron microscopy (SEM), electron dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometer (VSM), moisture and ash content, bulk density, pH and zero-point charge (pH_pzc). The specific surface area of non-magnetic and magnetic biochars was 220 and 194 m² g^-1, respectively. Adsorption of ibuprofen was optimised with respect to contact time (5-180 min), solution pH (2-12) and initial drug concentration (5-100 mg L^-1), with equilibrium being reached in 1 hour, and the maximum ibuprofen removal occurred at pH 2 and 4 for biochar and magnetic biochars, respectively. Investigation of the adsorption kinetics was achieved through application of the pseudo-first order, pseudo-second order, Elovich and intra-particle diffusion models. Adsorption equilibrium was evaluated using Langmuir, Freundlich and Langmuir-Freundlich isotherm models. The adsorption kinetics and isotherms for both biochars are well described by pseudo-second order kinetic and Langmuir-Freundlich isotherm models, respectively, with the maximum adsorption capacity of biochar and magnetic biochar being 167 and 140 mg g^-1, respectively. Chrysanthemum derived non-magnetic and magnetic biochars exhibited significant potential as sustainable adsorbents toward the removal of emerging pharmaceutical pollutants such as ibuprofen from aqueous solution.

Review on arsenic environment behaviors in aqueous solution and soil

Arsenic pollution in environment has always been an important environmental problem that has attracted wide attention in recent years. Adsorption is one of the main methods of treatment for arsenic in the aqueous solution and soil because of the advantages of high efficiency, low cost and wide application. Firstly, this report summarizes the commonly and widely used adsorbent materials such as metal-organic frameworks, layered bimetallic hydroxides, chitosan, biochar and their derivatives. The adsorption effects and mechanisms of these materials are further discussed, and the application prospects of these adsorbents are considered. Meanwhile, the gaps and deficiencies in the study of adsorption mechanism was pointed out. Then, this study comprehensively evaluated the effects of various factors on arsenic transport, including (i) the effects of pH and redox potential on the existing form of As; (ii) complexation mechanism of dissolved organic matter and As; (iii) factors affecting the plant enrichment of As. Finally, the latest scientific researches on microbial remediation of arsenic and the mechanisms were summarized. The review finally enlightens the subsequent development of more efficient and practical adsorption material.

Highly efficient removal of hexavalent chromium by magnetic Fe-C composite from reed straw and electric furnace dust waste

Reed straw and electric furnace dust (EFD) waste were used to prepare magnetic Fe-C composite (EFD&C) by co-precipitation and high-temperature activation method to remove Cr(VI) from water. The magnetic EFD&C owned a large specific surface (536.61 m²/g) and a porous structure (micropores and mesopores), and had an efficient removal capacity for Cr(VI). Under conditions of pH (2), the addition amount of EFD&C (1 g/L), the adsorption time (760 min), and the temperature (45 °C), the maximum adsorption capacity reached 111.94 mg/g. The adsorption mechanism mainly attributed to chemical adsorption (redox), Cr(VI) reduced to Cr(III) by Fe(II) and Fe(0) (from Fe₃O₄ and Fe components in EFD) and surface functional groups of -OH, C = C, C-C and O-C = O (from biochar), and secondary attributed to physical adsorption, Cr(VI) and Cr(III) (from reduced Cr(VI)) adsorbed into the porous structure of EFD&C. This study provided a feasible solution for the preparation of adsorbents for adsorbing heavy metals from iron-containing metallurgical solid waste and biomass waste, which contributed to reducing the environmental pollution and lowering the cost of adsorbent preparation.

Multifunctional interfaces for multiple uses: Tin(II)-hydroxyapatite for reductive adsorption of Cr(VI) and its upcycling into catalyst for air protection reactions

Evidence collected to date by our group has demonstrated that tin(II)-functionalized hydroxyapatites (Sn/HAP) are a newly discovered class of ecofriendly reductive adsorbents for Cr(VI) removal from wastewaters. In this work an upgraded series of Sn/HAP materials assured a maximum removal capacity of ≈ 20 mgCr/g, doubling the previously reported value for Sn/HAP materials, thanks to higher Sn-dispersion as proved by X-ray photoelectron spectroscopy and electron microscopy. Insights on kinetics and thermodynamics of the reductive adsorption process are provided and the influence of pH, dosage, and nature of Cr(VI) precursors on chromium removal performances have been investigated. Pseudo-second-order kinetics described the interfacial reductive adsorption process on Sn/HAP, characterized by low activation energy (21 kJ mol-1), when measured in the 278-318 K range. Tests performed in the 2-6 pH interval showed similar efficiency in terms of Cr(VI) removal. Conventional procedures of recycling and regeneration resulted ineffective in restoring the pristine performances of the samples due to surface presence of both Sn(IV) and Cr(III). To overcome these weaknesses, the used samples (Sn + Cr/HAP) were upcycled into catalysts in a circular economy perspective. Used samples were tested as catalysts in gas-phase catalytic processes for air pollution remediation: selective catalytic reduction of NOx (NH3-SCR), NH3 selective catalytic Oxidation (NH3-SCO), and selective catalytic oxidation of methane to CO2. Catalytic tests enlightened the interesting activity of the upcycled Sn + Cr/HAP samples in catalytic oxidation processes, being able to selectively oxidize methane to CO2 at relatively low temperature.

Acute myocardial infarction therapy using calycosin and tanshinone co-loaded; mitochondrion-targeted tetrapeptide and cyclic arginyl-glycyl-aspartic acid peptide co-modified lipid-polymer hybrid nano-system: preparation, characterization, and anti myocardial infarction activity assessment

Acute myocardial infarction (AMI) is one of the most common ischemic heart diseases. However, lack of sufficient drug concentration (in the ischemic heart) is the major factor of treatment failure. It is urgent for researchers to engineer novel drug delivery systems to enhance the targeted delivery of cardioprotective agents. The aim of the present study was to investigate the anti-AMI ability of calycosin (CAL) and tanshinone (TAN) co-loaded; mitochondrion-targeted tetrapeptide (MTP) and cyclic arginyl-glycyl-aspartic acid (RGD) peptide co-modified nano-system.: We prepared CAL and TAN combined lipid-polymer hybrid nano-system, and RGD was modified to the system to achieve RGD-CAL/TAN NS. MTP-131 was conjugated with PEG and modified onto the nanoparticles to achieve dual ligands co-modified MTP/RGD-CAL/TAN NS. The physicochemical properties of nano-systems were characterized. The AMI therapy ability of the systems was investigated in AMI rats' model. The size of MTP/RGD-CAL/TAN NS was 170.2 ± 5.6 nm, with a surface charge of -18.9 ± 1.9 mV. The area under the curve (AUC) and blood circulation half-life (T_1/2) of MTP/RGD-CAL/TAN NS was 178.86 ± 6.62 μg·min/mL and 0.47 h, respectively. MTP/RGD-CAL/TAN NS exhibited the most significant infarct size reduction effect of 22.9%. MTP/RGD-CAL/TAN NS exhibited the highest heart accumulation and best infarct size reduction effect, which could be used as a promising system for efficient treatment of cardiovascular diseases.

Acute myocardial infarction therapy using calycosin and tanshinone co-loaded mitochondria targeted lipid-polymer hybrid nano-system: Preparation, characterization, and anti myocardial infarction activity assessment

BACKGROUND

Acute myocardial infarction (AMI) is one of the most common ischemic heart diseases. However, lack of sufficient drug concentrations in the ischemic heart may led to treatment failure. It is urgent for researchers to engineer novel drug delivery systems to enhance the targeted delivery of cardioprotective agents.

OBJECTIVE

The aim of the present study was to investigate the anti-AMI ability of calycosin (CAL) and tanshinone (TAN) co-loaded mitochondria targeted lipid-polymer hybrid nano-system.

METHODS

CAL and TAN combined lipid-polymer hybrid nano-systems were prepared and MTP-131 was conjugated with PEG and modified onto the nanoparticles to achieve MTP-CAL/TAN NS. The physicochemical properties of nano-systems were characterized, the AMI therapy ability of the systems was investigated in AMI rats' model.

RESULTS

The size of MTP-CAL/TAN NS was 168.7 ± 5.1 nm, with a surface charge of - 21.3 ± 2.3 mV. The area under the curve (AUC) and blood circulation half-life (T_1/2) of MTP-CAL/TAN NS was 178.86 ± 6.62 μg·min/mL and 0.47 h, respectively. MTP-CAL/TAN NS exhibited the most significant infarct size reduction effect of 23.9 %.

CONCLUSION

MTP-CAL/TAN NS exhibited the highest heart accumulation and best infarct size reduction effect, which could be used as a promising system for efficient treatment of cardiovascular diseases.

Fate and mechanistic insights into nanoscale zerovalent iron (nZVI) activation of sludge derived biochar reacted with Cr(VI)

While nanoscale zero-valent iron modified biochar (nZVI-BC) have been widely investigated for the removal of heavy metals, the corrosion products of nZVI and their interaction with heavy metals have not been revealed yet. In this paper, nZVI-BC was synthesized and applied for the removal of Cr(VI). Batch experiments indicated that the adsorption of Cr(VI) fit Langmuir isotherm, with the maximum removal capacity at 172.4 mg/g at pH 2.0. SEM-EDS, BET, XRD, FT-IR, Raman and XPS investigation suggested that reduction of Cr(VI) to Cr(III) was the major removal mechanism. pH played an important role on the corrosion of nZVI-BC, at pH 4.5 and 2.0, FeOOH and Fe₃O₄ were detected as the major iron oxide, respectively. Therefore, FeOOH-BC and Fe₃O₄-BC were further prepared and their interaction with Cr were studied. Combining with DFT calculations, it revealed that Fe₃O₄ has higher adsorption capacity and was responsible for the effective removal of Cr(VI) through electrostatic attraction and reduction under acidic conditions. However, Fe₃O₄ will continue to convert to the more stable FeOOH, which is the key to for the subsequent stabilization of the reduced Cr(III). The results showed that the oxide corrosion products of nZVI-BC were subjected to the environment, which will eventually affect the fate and transport of the adsorbed heavy metal.

Adsorption of pollutants in wastewater via biosorbents, nanoparticles and magnetic biosorbents: A review

Adsorption has gained much attention as one of the efficient approaches to remediate the contaminants in wastewater. Herein, this critical review focuses on the preparation, modification, application and regeneration of the biosorbents, nanoparticles and magnetic biosorbents for the wastewater treatment in recent 5 years (2017-2021). Among these materials, the development of magnetic biosorbents is attractive owing to their variable active sites, high specific surface area, easy separation and low cost. To improve the adsorption performance of biosorbents, the chemical activations such as acid, alkali and salt activations of biosorbents are discussed. In general, the oxidation reaction in acid, alkali and salt activations increases the porosity of biosorbents. The surface characteristics, surface chemistry of the biosorbents and magnetic biosorbents such as electrostatic interaction, π-π interaction and hydrogen bonding are highlighted. Ionic compounds are separated through ion exchange, surface charge and electrostatic interactions while the organic pollutants are removed via hydrophobicity, π-π interactions and hydrogen bonding. The effect of solution pH, adsorbent dosage, initial concentration of pollutants, adsorption duration and temperature on the adsorption capacity, and removal efficiency are discussed. Generally, an increase in adsorbent dosage resulted in a decrease in adsorption capacity due to the excessive active sites. On the other hand, a higher initial concentration or an increase in contact time of adsorbent increased the driving force, subsequently enhancing the adsorption capacity. Finally, this review will be concluded with a summary, challenges and future outlook of magnetic biosorbents. It is anticipated that this review will provide insights into engineering advanced and suitable materials to achieve cost-effective and scalable adsorbents for practical and sustainable environmental remediation.

Modified and pristine biochars for remediation of chromium contamination in soil and aquatic systems

Chromium (Cr) contamination in soil and water poses high toxicity risks to organisms and threatens food and water security worldwide. Biochar has emerged as a promising material for cleaning up Cr contamination owing to biochar's strong capacity to immobilize Cr. This paper synthesizes information on biochar modification for the efficient remediation of Cr contamination in soil and water, and critically reviews mechanisms of Cr adsorption on pristine and modified biochars. Biochar modification methods include physical activation via ball milling or ultraviolet irradiation, chemical activation via magnetization, alkali/acid treatment, nano-fabrication or loading of reductive agents, and biological activation via integrating biochars with microorganisms and their metabolites. Modified biochars often have multi-fold enhancement in Cr adsorption/reduction capacity than pristine biochars. Iron (Fe)-supported magnetic biochars have the most promising Cr removal abilities with high reusability of the biochars. Pre-pyrolysis modification with Fe could load Fe₃O₄ micro-/nanoparticles on biochars, and increase the surface area and electrostatic attraction between chromate anions and biochar surfaces, and reduce Cr(VI) to Cr(III). Post-pyrolysis modification could enrich oxygen-containing functional groups such as CO and -OH on biochar surfaces and promote Cr reduction and adsorption. Future research directions for Cr mitigation using advanced biochar products are discussed in this review.

The Preparation of Eco-Friendly Magnetic Adsorbent from Wild Water Hyacinth (Eichhornia crassipes): The Application for Removing Lead Ions from Industrial Wastewater

Water hyacinth (Eichhornia crassipes) is a wild floating plant that can be found widely in pond or river areas. The plant grows fiercely and causes many harmful issues to the ecosystem around its covered area. This work provides a utilization method that converts wild water hyacinth to reliable magnetic biochar which can be used as a very effective adsorbent for the removal of lead ion Pb(II) in industrial wastewater. The mentioned magnetic biochar can be prepared via a modified pyrolysis process at 550°C with the support of cobalt sulfates as magnetite precursors and limited oxygen from the sweeping gas (the gas mixture ratio is 4 : 1 nitrogen/oxygen). The produced samples were hydrophobic biochar with high oxygen-containing functional groups that are suitable for the removal of inorganic contaminants. The impregnation of cobalt (II, III) oxides provided high magnetic separation performance and additional adsorption sites on the produced magnetic biochar. As indicated by the obtained result, the WHB-Co2M sample possesses a highly porous structure (0.126 cc/g), higher thermal stability (thermal durability reaches 900°C), relatively stable magnetic properties (14.74 emu/g), and a larger surface area (192 m2/g). These beneficial properties led to its suitability to serve as an adsorbent in removing lead ions in the contaminated effluent, recording 95% of removal efficiency and adsorption capacity of 67.815 mg/g. As indicated in the result, all prepared magnetic biochar samples were fitted to two-parameter (Langmuir models) and three-parameter (Sips model) isotherm models. Therefore, the adsorption process in this work could be carried out on both homogeneous and heterogeneous adsorbent surfaces. The adsorption kinetics of the removal process also was described by the pseudo-first-order, pseudo-second-order, and Elovich models to reveal the adsorption and desorption rate of the as-prepared magnetic biochar. This work indicates a successful waste refinery route of converting lignocellulosic biomass such as water hyacinth into value-added material for use as promising heavy metal adsorbents.

Remediation of Chromium (VI) from Groundwater by Metal-Based Biochar under Anaerobic Conditions

Iron salt-modified biochar has been widely used to remove Cr(VI) pollution due to the combination of the generated iron oxides and biochar, which can bring positive charge and rich redox activity. However, there are few comprehensive studies on the methods of modifying biochar with different iron salts. In this study, two iron salt (FeCl3 and Fe(NO3)3) modification methods were used to prepare two Fe-modified biochar materials for removing Cr(VI) in simulated groundwater environment. It was revealed by systematic characterization that FeCl3@BC prepared via the FeCl3 modification method, has larger pore size, higher zeta potential and iron oxide content, and has higher Cr(VI) adsorption-reduction performance efficiency as compared to Fe(NO3)3@BC prepared via Fe(NO3)3 modification method. Combined with XRD and XPS analyses, Fe3O4 is the key active component for the reduction of Cr(VI) to Cr(III). The experimental results have shown that acidic conditions promoted Cr(VI) removal, while competing ions (SO42− and PO43−) inhibited Cr(VI) removal by FeCl3@BC. The Elovich model and intra-particle diffusion model of FeCl3@BC can describe the adsorption behavior of Cr(VI) well, indicating that both the high activation energy adsorption process and intra-particle diffusion control the removal process of Cr(VI). The Freundlich model (R2 > 0.999) indicated that there were unevenly distributed chemisorptions centers on the FeCl3@BC surface. Stability experiments exposed that FeCl3@BC was stable under neutral, acidic, and alkaline conditions. Furthermore, the main mechanisms of FeCl3@BC removal of Cr(VI) include electrostatic adsorption, chemical reduction, ion exchange, and co-precipitation. In conclusion, our findings provide a new insight for the selection of iron salt-modified biochar methods, and will also be beneficial for the preparation of more efficient Fe-modified biochars in the future.

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

Acidic vinegar residue (VR) and toxic hexavalent chromium (Cr(VI)) are unfavorable substances due to their toxicity against the environment. In this study, modified biochar was prepared to investigate the removal mechanisms of Cr(VI). The results showed that ZnCl₂ could yield highly aromatic products with improved pore structures. The adsorption capacity of modified biochar reached the highest efficiency (236.81 mg g^-1) when the mass ratio of ZnCl₂/VR was 1, which is higher than the control (9.96 mg g^-1). In addition, Cr(VI) adsorption coexisted with physical and chemical adsorption. The mechanisms of modified biochar to Cr(VI) removal included electrostatic attraction, pore filing, reduction and surface complexation. Notably, as a fermented product, VR biochar was a nitrogen-rich product; the formation of the amino group could provide a direct solid site for Cr(VI) adsorption. Subsequently, amorphous silica could be converted into silanol to provide additional adsorption sites. This work establishes the theoretical basis for efficient Cr(VI) removal and VR reuse.