Distribution and speciation of iron in Fe-modified biochars and its application in removal of As(V), As(III), Cr(VI), and Hg(II): An X-ray absorption study

Ferrate (VI) oxidation of sulfamethoxazole enhanced by magnetized sludge-based biochar: Active sites regulation and degradation mechanism analysis

Developing non radical systems for antibiotic degradation is crucial for addressing the inefficiency of conventional radical systems. In this study, novel magnetic-modified sludge biochar (MASBC) was synthesized to significantly enhance the oxidative degradation of sulfamethoxazole (SMX) by ferrate (Fe (VI)). In the Fe (VI)/MASBC system, 90.46% of SMX at a concentration of 10 μM and 49.34% of the total organic carbon (TOC) could be removed under optimal conditions of 100 μM of Fe (VI) and 0.40 g/L of MASBC within 10 min. Furthermore, the Fe (VI)/MASBC system was demonstrated with broad-spectrum removal capability towards sulfonamides in single or mixture. Quenching experiments, EPR analyses, and electrochemical experiments revealed that direct electron transfer (DET) and •O2- were mainly responsible for the removal of SMX, with functional groups (e.g., -OH, C=O) and Fe-O (redox of Fe (III)/Fe (II)) acting as the active sites, while the probe experiments showed that Fe (IV)/Fe (V) made a minor contribution to the degradation of SMX. Benefiting from the DET, the Fe (VI)/MASBC system exhibited a wide pH adaptation range (e.g., from 5.0 to 10.0) and strong anti-interference ability. The N atoms and their neighboring atoms in SMX were the prior degradation sites, with the cleavage of bond and ring opening. The degradation products showed low or non-toxicity according to ECOSAR program assessment. The removal of SMX remained within a reasonable range of 71.33%-90.46% over five consecutive cycles. Also, the Fe (VI)/MASBC system was demonstrated to be effectively applied for successful SMX removal in various water matrices, including ultrapure water, tap water, lake water, Yangtze River water, and wastewater. Therefore, this study offered new insights into the mechanism of Fe (VI) oxidation and would contribute to the efficient treatment of organic pollutants.

Exploring nanomaterial-modified biochar for environmental remediation applications

Environmental pollution, particularly from heavy metals and toxic elements, poses a significant threat to both human health and ecological systems. While various remediation technologies exist, there is an urgent need for cost-effective and sustainable solutions. Biochar, a carbon-rich product derived from the pyrolysis of organic matter, has emerged as a promising material for environmental remediation. However, its pristine form has limitations, such as low adsorption capacities, a relatively narrow range of pH adaptability which can limit its effectiveness in diverse environmental conditions, and a tendency to lose adsorption capacity rapidly in the presence of competing ions or organic matters. This review aims to explore the burgeoning field of nanomaterial-modified biochar, which seeks to overcome the limitations of pristine biochar. By incorporating nanomaterials, the adsorptive and reactive properties of biochar can be significantly enhanced. Such modifications, especially biochar supported with metal nanoparticles (biochar-MNPs), have shown promise in various applications, including the removal of heavy metals, organic contaminants, and other inorganic pollutants from aqueous environments, soil, and air. This review provides a comprehensive overview of the synthesis techniques, characterization methods, and applications of biochar-MNPs, as well as discusses their underlying mechanisms for contaminant removal. It also offers insights into the advantages and challenges of using nanomaterial-modified biochar for environmental remediation and suggests directions for future research.

Is Pyrolysis Treatment a Viable Solution to Detoxify Metal(loid)s in Sewage Sludge toward Land Application? Case Studies of Chromium and Zinc

Metal(loid)s in sewage sludge (SS) are effectively immobilized after pyrolysis. However, the bioavailability and fate of the immobilized metal(loid)s in SS-derived biochar (SSB) following land application remain largely unknown. Here, the speciation and bioavailability evolution of SSB-borne Cr and Zn in soil were systematically investigated by combining pot and field trials and X-ray absorption spectroscopy. Results showed that approximately 58% of Cr existing as Cr(III)-humic complex in SS were transformed into Fe (hydr)oxide-bound Cr(III), while nano-ZnS in SS was transformed into stable ZnS and ferrihydrite-bound species (accounting for over 90% of Zn in SSB) during pyrolysis. All immobilized metal(loid)s, including Cr and Zn, in SSB tended to be slowly remobilized during aging in soil. This study highlighted that SSB acted as a dual role of source and sink of metal(loid)s in soil and posed potential risks by serving a greater role of a metal(loid) source than a sink when applied to uncontaminated soils. Nevertheless, SSB could impede the translocation of metal(loid)s from soil to crop compared to SS, where coexisting elements, including Fe, P, and Zn, played critical roles. These findings provide new insights for understanding the fate of SSB-borne metal(loid)s in soil and assessing the viability of pyrolyzing SS for land application.

Effective elimination of Hg(II) from water bodies with acid-modified magnetic biomass spent coffee grounds: conditional optimization and application

To maximize the efficiency of biomass waste utilization and waste management, a novel acid-modified magnetic biomass spent coffee grounds (NiFe2O4/SCG) was obtained by pyrolysis at 473 K and co-precipitation methods and employed to eliminate bivalent mercury (Hg(II)) in water bodies. The prepared NiFe2O4/SCG adsorbent exhibits remarkable magnetism with a strength of 45.78 emu/g and can easily be separated from water via a magnetic force. The adsorption of Hg(II) over the NiFe2O4/SCG has an optimal conditions of pH = 8, T = 39 ℃, and dosage of 0.055 g/L, and the maximal adsorption capacity for Hg(II) is 167.44 mg/g via Response Surface Methodology optimization. The removal of Hg(II) over NiFe2O4/SCG primarily involves ion exchange, electrostatic attraction, and chelation; conforms to the pseudo-second-order kinetic and Langmuir models; and is an endothermic reaction. Additionally, the magnetic biomass NiFe2O4/SCG has good regeneration capability and stability. The application research reveal that inorganic salt ions, nitrogen fertilizer urea, humus, and other contaminants in different actual water bodies (river water, lake water, and the effluent of sewage treatment plant) have little effect on the adsorption of Hg(II) over the NiFe2O4/SCG. The prepared adsorbent NiFe2O4/SCG has practical application value for removing Hg(II) from water bodies.

Antimony and arsenic migration in a heterogeneous subsurface at an abandoned antimony smelter under rainfall

While antimony (Sb) and arsenic (As) co-contamination in subsurface soil systems due to the legacy of Sb smelting wastes has been documented, the role of inherent heterogeneity on pollutant migration is largely overlooked. Herein this study investigated Sb and As migration in a slag impacted, vertically stratified subsurface at an abandoned Sb smelter. A 2-dimensional flume was assembled as a lab-scale analogue of the site and subject to rainfall and stop-rain events. Reactive transport modeling was then performed by matching the experimental observations to verify the key factors and processes controlling pollutant migration. Results showed that rainfall caused Sb and As release from the shallow slag layer and promoted their downward movement. Nevertheless, the less permeable deeper layers limited physical flow and transport, which led to Sb and As accumulation at the interface. The re-adsorption of Sb and As onto iron oxides in the deeper, more acidic layers further retarded their migration. Because of the large difference between Sb and As concentrations, Sb re-adsorption was much less effective, which led to higher mobility. Our findings overall highlight the necessity of understanding the degree and impacts of physicochemical heterogeneity for risk exposure assessment and remediation of abandoned Sb smelting sites.

In-situ registration subtraction image segmentation algorithm for spatiotemporal visualization of copper adsorption onto corn stalk-derived pellet biochar by micro-computed tomography

The non-homogeneous structure and high-density ash composition of biochar matrix pose significant challenges in characterizing the dynamic changes of heavy metal adsorption onto biochar with micro-computed tomography (Micro-CT). A novel in-situ registration subtraction image segmentation method (IRS) was developed to enhance micro-CT characterization accuracy. The kinetics of Cu(II) adsorption onto pellet biochar derived from corn stalks were tested. Respectively, the IRS and traditional K-means algorithms were used for image segmentation to the in-situ three-dimensional (3D) visual characterization of the Cu(II) adsorption onto biochar. The results indicated that the IRS algorithm reduced interference from high-density biochar composition, and thus achieved more precise results (R2 = 0.95) than that of K-means (R2 = 0.72). The visualized dynamic migration of Cu(II) from surface adsorption to intraparticle diffusion reflexed the complex mechanism of heavy metal adsorption. The developed Micro-CT method with high generalizability has great potential for studying the process and mechanism of biochar heavy metal adsorption.

Pristine and modified biochar applications as multifunctional component towards sustainable future: Recent advances and new insights

Employing biomass for environmental conservation is regarded as a successful and environmentally friendly technique since they are cost-effective, renewable, and abundant. Biochar (BC), a thermochemically converted biomass, has a considerably lower production cost than the other conventional activated carbons. This material's distinctive properties, including a high carbon content, good electrical conductivity (EC), high stability, and a large surface area, can be utilized in various research fields. BC is feasible as a renewable source for potential applications that may achieve a comprehensive economic niche. Despite being an inexpensive and environmentally sustainable product, research has indicated that pristine BC possesses restricted properties that prevent it from fulfilling the intended remediation objectives. Consequently, modifications must be made to BC to strengthen its physicochemical properties and, thereby, its efficacy in decontaminating the environment. Modified BC, an enhanced iteration of BC, has garnered considerable interest within academia. Many modification techniques have been suggested to augment BC's functionality, including its adsorption and immobilization reliability. Modified BC is overviewed in its production, functionality, applications, and regeneration. This work provides a holistic review of the recent advances in synthesizing modified BC through physical, chemical, or biological methods to achieve enhanced performance in a specific application, which has generated considerable research interest. Surface chemistry modifications require the initiation of surface functional groups, which can be accomplished through various techniques. Therefore, the fundamental objective of these modification techniques is to improve the efficacy of BC contaminant removal, typically through adjustments in its physical or chemical characteristics, including surface area or functionality. In addition, this article summarized and discussed the applications and related mechanisms of modified BC in environmental decontamination, focusing on applying it as an ideal adsorbent, soil amendment, catalyst, electrochemical device, and anaerobic digestion (AD) promoter. Current research trends, future directions, and academic demands were available in this study.

Applications and synergistic degradation mechanisms of nZVI-modified biochar for the remediation of organic polluted soil and water: A review

Increasing organic pollution in soil and water has garnered considerable attention in recent years. Nano zero-valent iron-modified biochar (nZVI/BC) has been proven to remediate the contaminated environment effectively due to its abundant active sites and unique reducing properties. This paper provides a comprehensive overview of the application of nZVI/BC in organic polluted environmental remediation and its mechanisms. Firstly, the review introduced primary synthetic methods of nZVI/BC, including in-situ synthesis (carbothermal reduction and green synthesis) and post-modification (liquid-phase reduction and ball milling). Secondly, the application effects of nZVI/BC were discussed in remediating soil and water polluted by antibiotics, pesticides, polycyclic aromatic hydrocarbons (PAHs), and dyes. Thirdly, this review explored the mechanisms of the adsorption and chemical degradation of nZVI/BC, and synergistic degradation mechanisms of nZVI/BC-AOPs and nZVI/BC-Microbial interactions. Fourth, the factors that influence the removal of organic pollutants using nZVI/BC were summarized, encompassing synthesis conditions (raw materials, pyrolysis temperature and aging of nZVI/BC) and external factors (reagent dosage, pH, and coexisting substances). Finally, this review proposed future challenges for the application of nZVI/BC in environmental remediation. This review offers valuable insights for advancing technology in the degradation of organic pollutants using nZVI/BC and promoting its on-site application.

Fabrication of Iron-Containing Biochar by One-Step Ball Milling for Cr(VI) and Tetracycline Removal from Wastewater

Simple ball milling technology can simultaneously improve the adsorption performance of adsorbents for heavy metals and organic pollutants and has attracted increasing attention. Iron-modified biochar (Fe@MBC) was prepared by one-step ball milling, and the characterization results proved that FeCl3 was successfully loaded on biochar. The removal rates of Cr(VI) and tetracycline hydrochloride (TC) by Fe@MBC were increased by 88.27% and 82.64% compared with BC. The average pore size, oxygen-containing functional groups and graphitization degree of Fe@MBC are higher than those of BC, which is more conducive to promoting adsorption. The adsorption isotherms show that the adsorption of Cr(VI) and TC on the Fe@MBC surface conforms to the Langmuir type of single-layer adsorption and the Freundlich model of multilayer adsorption, respectively. The maximum adsorption capacities of Cr(VI) and TC are 25.46 and 66.91 mg·g-1, respectively. Kinetic experiments show that the adsorption process is more consistent with the pseudo-second-order model of chemical adsorption. The adsorption process of Cr(VI) and TC on the Fe@MBC surface is a spontaneous endothermic process that becomes more obvious as the temperature increases. The increase in solution pH has a significant impact on the removal rate of Fe@MBC. When the pH value increased from 3 to 11, the adsorption rates decreased by 53.74% and 17.16%, respectively. The presence of PO43-, CO32-, K+, and Cu2+ significantly affects the adsorption of TC by Fe@MBC, and PO43- and CO32- also affect the adsorption of Cr(VI). Mechanistic studies show that ion exchange, electrostatic interaction, pore filling, and hydrogen bonding contribute to the removal of Cr(VI) and TC by Fe@MBC. The removal mechanism of Cr(VI) also involves complexation and redox reactions, and the removal mechanism of TC involves π-π bonds and van der Waals forces. The results show that Fe@MBC is a green and efficient adsorbent.

Removal of antimonite and antimonate in aqueous solution by mugwort biochar modified by Acidithiobacillus ferrooxidans after pyrolysis

In the research, iron oxides-biochar composites (ALBC) were prepared from pristine biochar modified by Acidithiobacillus ferrooxidans (A. ferrooxidans) and pyrolyzed at 500 °C and 700 °C in order to remove antimonite (Sb(III)) and antimonate (Sb(V)) from water. The results indicated that biochar prepared at 500 °C and 700 °C (ALBC500 and ALBC700) were loaded with Fe₂O₃ and Fe₃O₄, respectively. In bacterial modification systems, ferrous iron and total iron concentrations decreased continuously. The pH values of bacterial modification systems including ALBC500 increased first and then decreased to a stable state, while the pH values of bacterial modification systems with ALBC700 continued to decrease. The bacterial modification systems can facilitate the formation of more jarosites by A. ferrooxidans. ALBC500 had optimal adsorbing capacities for Sb(III) (18.81 mg·g^-1) and Sb(V) (14.64 mg·g^-1). The main mechanisms of Sb(III) and Sb(V) adsorption by ALBC were electrostatic interaction and pore filling.

Comparative efficacy of Parthenium hysterophorus (L.) derived biochar and iron doped zinc oxide nanoparticle on heavy metals (HMs) mobility and its uptake by Triticum aestivum (L.) in chromite mining contaminated soils

In this study we investigated the efficacy of a novel material parthenium weed (Parthenium hysterophorus L.) biochar (PBC), iron doped zinc oxide nanoparticles (nFe-ZnO), and biochar modified with nFe-ZnO (Fe-ZnO@BC) to adsorb heavy metals (HMs) and reduce their uptake by wheat (Triticum aestivum L.) in a highly chromite mining contaminated soil. The co-application of the applied soil conditioners exhibited a positive effect on the immobilization and restricted the HMs uptake below their threshold levels in shoot content of wheat. The maximum adsorption capacity was because of large surface area, cation exchange capacity, surface precipitation, and complexation of the soil conditioners. The scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) showed porous smooth structure of parthenium weed derived biochar that helped in HMs adsorption, increase the efficiency of soil fertilizers and nutrients retention which help in the enhancement soil condition. Under different application rates the highest translocation factor (TFHMs) was obtained at 2 g nFe-ZnO rate followed the descending order: Mn > Cr > Cu > Ni > Pb. The overall TFHMs was found <1.0 indicating that low content of HMs accumulation in roots from soil slight transferred to shoot, thus satisfying the remediation requirements.

Effects of oxygen on the adsorption/oxidation of aqueous Sb(III) by Fe-loaded biochar: An X-ray absorption spectroscopy study

Fe-loaded biochar (FeBC) has been considered for Sb(III) adsorption, but the effects of oxygen (O₂) on the adsorption need further investigation. Liquid-/solid-phase analyses were conducted to investigate the role of O₂ in the Sb(III) adsorption by FeBC. The adsorption was best described by the pseudo-second-order (PSO) model for kinetic results and by the Langmuir model for thermodynamic results. More than 96.8 % of Sb(III) was adsorbed by FeBC, and available O₂ increased the liquid-phase Sb(III) oxidation efficiency by 2.1-7.5 times. The peak changes at ~1640 and 3450 cm^-1 in FTIR spectra indicated the occurrence of inner-sphere complexation between Sb(III)/Sb(V) and hydroxyl (-OH)/carboxyl (-COOH) groups in FeBC under aerobic and anaerobic conditions. Fe/Sb X-ray absorption spectroscopy (XAS) analysis results showed aqueous Sb(III) complexed to the edge-sharing Fe(III)-O-Fe(III) in FeBC. Regardless of whether O₂ was available or not, solid-phase edge-sharing Fe(III)-O-Sb(V) complexes (~3.05 Å), which had lower toxicity and migration ability than aqueous Sb(III), formed through a ligand-to-metal charge-transfer (LMCT) process. More than 91 % of adsorbed Sb(III) was oxidized to edge-sharing Fe(III)-O-Sb(V) complexes in 3 h. Additionally, the Sb(V) from liquid-phase oxidation could also directly complex to the Fe(III)-O-Fe(III) and form edge-sharing Fe(III)-O-Sb(V) complexes. These results provide evidence to inform further FeBC application for the Sb-contaminated water treatment.

Iron-modified biochar-based bilayer permeable reactive barrier for Cr(VI) removal

Iron (Fe)-modified biochar (FeBC) has been developed to remove hexavalent chromium (Cr(VI)) from groundwater and is suitable for use in permeable reactive barriers (PRBs). However, Cr(VI) removal behavior and chemical processes in FeBC-based PRBs are not fully understood, and the potential for Fe release has not been addressed. In this study, three FeBC-based PRBs were assessed in column experiments for 563 days with respect to their ability to remove Cr(VI). Bilayer column filled with FeBC+limestone and BC+limestone in two separate layers (FeBC_Ca_BC) showed the best performance in terms of Cr(VI) removal with a low treatment cost. The corrosion of FeBC was mainly related to pH and Cr(VI) concentration rather than flow rate. Leached Fe was attenuated by BC and limestone and reutilized in FeBC_Ca_BC. Cr(VI) was reduced to Cr(III) and then adsorbed or precipitated on the biochars. Cr and Fe formed inner-sphere complexes and then transformed from double corner sharing to edge sharing. During the reaction, Cr penetrated from the surface to the interior of the biochars and became a more stable species. This study provides evidence of the effectiveness of a new combination of biochars for Cr(VI) removal and insights into the reaction mechanisms.

Effect of organic substrate and Fe oxides transformation on the mobility of arsenic by biotic reductive dissolution under repetitive redox conditions

The mobility of arsenic (As) in soil is highly affected by the change in the form of iron oxides present in the soil, which has a strong correlation with the change in redox potential. In this study, the altered mobility of As under repetitive redox conditions and the effect of organic substrates (i.e., glucose) on such change during four anoxic-oxic cycles were studied. During the 1st anoxic period, 37.1% of soil As was released into the soil solution, but the As in the soil solution decreased to 25.2% after the 1st oxic period. Moreover, the As in the soil solution further decreased during the 2nd to 4th oxic periods, indicating further re-adsorption of aqueous As. The analysis of As speciation revealed that inorganic arsenate (As(V)) increased under the redox-oscillating conditions, probably due to the depletion of electron donors. When glucose was re-spiked at the beginning of the 4th cycle, aqueous As increased to 47.3% again in the anoxic period and decreased to 27.6% in the subsequent oxic period, indicating inhibition of As re-adsorption. During the same period, the amount of highly sorptive As(V) in the solution decreased sharply to less than 3.3%. The X-ray absorption near edge structure analysis with linear combination fitting confirmed that the transformation of Fe oxides to poorly crystalline structures such as ferrihydrite occurred during repetitive cycles. These results imply that the mobility of As can be increased in As-contaminated redox transition zones by the introduction of rainfall with labile organics or by the fluctuation of organic-rich groundwater.

Tailoring wood waste biochar as a reusable microwave absorbent for pollutant removal: Structure-property-performance relationship and iron-carbon interaction

This study innovated the concept in designing an efficient and reusable microwave (MW) absorbent through concurrent exploitation of carbon graphitization, oxygen functionalization, and carbothermal iron reduction underpinned by an endothermic co-pyrolysis of wood waste and low-dosage iron. A powerful MW assimilation was accomplished from nanoscale amorphous magnetic particles as well as graphitized microporous carbon-iron skeleton in the biochar composites. Relative to a weak magnetic loss derived from the iron phase, the graphitic carbon architecture with abundant surface functionalities (i.e., CO and CO) exhibited a strong dielectric loss, which was thus prioritized as major active sites during MW reuse. The MW-absorbing biochar demonstrated a fast, robust, and durable removal of a refractory herbicide (2,4-dichlorophenoxy acetic acid) under mild MW irradiation with zero chemical input, low electricity consumption, and negligible Fe dissolution. Overall, this study will foster carbon-neutral industrial wastewater treatment and wood waste valorization.

Low-cost treated lignocellulosic biomass waste supported with FeCl3/Zn(NO3)2 for water decolorization

Dye pollution has always been a serious concern globally, threatening the lives of humans and the ecosystem. In the current study, treated lignocellulosic biomass waste supported with FeCl₃/Zn(NO₃)₂ was utilized as an effective composite for removing Reactive Orange 16 (RO16). SEM/EDAX, FTIR, and XRD analyses exhibited that the prepared material was successfully synthesized. The removal efficiency of 99.1% was found at an equilibrium time of 110 min and dye concentration of 5 mg L^-1 Adsorbent mass of 30 mg resulted in the maximum dye elimination, and the efficiency of the process decreased by increasing the temperature from 25 to 40 °C. The effect of pH revealed that optimum pH was occurred at acidic media, having the maximum dye removal of greater than 90%. The kinetic and isotherm models revealed that RO16 elimination followed pseudo-second-order (R² = 0.9982) and Freundlich (R² = 0.9758) assumptions. Surprisingly, the performance of modified sawdust was 15.5 times better than the raw sawdust for the dye removal. In conclusion, lignocellulosic sawdust-Fe/Zn composite is promising for dye removal.

Functionalized biochars: Synthesis, characterization, and applications for removing trace elements from water

Biochar (BC) has been recognized as an effective adsorbent to remove trace elements (TEs) from water. However, low surface functionality and small pore size can limit the adsorption ability of pristine biochar. These limitations can be addressed by using functionalized biochars which are developed by physical, chemical, or biological activation of biochar to improve their physico-chemical properties and adsorption efficiency. Despite the large amount of research concerning functionalized biochars in recent decades, to our knowledge, no comprehensive review of this topic has been published. This review focuses solely on the synthesis, characterization, and applications of functionalized/engineered biochars for removing TEs from water. Firstly, we evaluate the synthesis of functionalized biochars by physical, chemical, and biological strategies that yield the desired properties in the final product. The following section describes the characterization of functionalized biochars using various techniques (SEM, TEM, EDS, XRD, XANES/NEXAFS, XPS, FTIR, and Raman spectroscopy). Afterward, the role of functionalized biochars in the adsorption of different TEs from water/wastewater is critically evaluated with an emphasis on the factors affecting sorption efficiency, sorption mechanisms, fate of sorbed TEs from contaminated environments and associated challenges. Finally, we specifically scrutinized the future recommendations and research directions for the application of functionalized biochar. This review serves as a comprehensive resource for the use of functionalized biochar as an emerging environmental material capable of removing TEs from contaminated water/wastewater.

Research Progress on Adsorption of Arsenic from Water by Modified Biochar and Its Mechanism: A Review

Arsenic (As) is a non-metallic element, which is widely distributed in nature. Due to its toxicity, arsenic is seriously harmful to human health and the environment. Therefore, it is particularly important to effectively remove arsenic from water. Biochar is a carbon-rich adsorption material with advantages such as large specific surface area, high porosity, and abundant functional groups, but the original biochar has limitations in application, such as limited adsorption capacity and adsorption range. The modified biochar materials have largely enhanced the adsorption capacity of As in water due to their improved physicochemical properties. In this review, the changes in the physicochemical properties of biochar before and after modification were compared by SEM, XRD, XPS, FT-IR, TG, and other characterization techniques. Through the analysis, it was found that the adsorbent dosage and pH are the major factors that influence the As adsorption capacity of the modified biochar. The adsorption process of As by biochar is endothermic, and increasing the reaction temperature is conducive to the progress of adsorption. Results showed that the main mechanisms include complexation, electrostatic interaction, and precipitation for the As removal by the modified biochar. Research in the field of biochar is progressing rapidly, with numerous achievements and new types of biochar-based materials prepared with super-strong adsorption capacity for As. There is still much space for in-depth research in this field. Therefore, the future research interests and applications are put forward in this review.

Electroactive Fe-biochar for redox-related remediation of arsenic and chromium: Distinct redox nature with varying iron/carbon speciation

Electroactive Fe-biochar has attracted significant attention for As(III)/Cr(VI) immobilization through redox reactions, and its performance essentially lies in the regulation of various Fe/C moieties for desired redox performance. Here, a series of Fe-biochar with distinct Fe/C speciation were rationally produced via two-step pyrolysis of iron minerals and biomass waste at 400-850 °C (BCX-Fe-Y, X and Y represented the first- and second-step pyrolysis temperature, respectively). The redox transformation of Cr(VI) and As(III) by Fe-biochar was evaluated in simulated wastewater under oxic or anoxic conditions. Results showed that more effective Cr(VI) reduction could be achieved by BCX-Fe-400, while a higher amount of As (III) was oxidized by BCX-Fe-850 under the anoxic environment. Besides, BCX-Fe-400 could generate more reactive oxygen species (e.g.,^•OH) by reducing the O₂, which enhanced the redox-related transformation of pollutants under the oxic situation. The evolving redox performance of Fe-biochar was governed by the transition of the redox state from reductive to oxidative related to the Fe/C speciation. The small-sized amorphous/low-crystalline ferrous minerals contributed to a higher electron-donating capacity (0.43-1.28 mmol g^-1) of BCX-Fe-400. In contrast, the oxidative surface oxygen-functionalities (i.e., carboxyl and quinoid) on BCX-Fe-850 endowed a stronger electron-accepting capacity (0.71-1.39 mmol g^-1). Moreover, the graphitic crystallites with edge-type defects and porous structure facilitated the electron transfer, leading to a higher electron efficiency of BCX-Fe-850. Overall, we unveiled the roles of both Fe and C speciation in maneuvering the redox reactivity of Fe-biochar, which can advance our rational design of electroactive Fe-biochar for redox-related environmental remediation.

Effects of Fe(II)-induced transformation of scorodite on arsenic solubility

Scorodite (FeAsO₄·2H₂O) is a pivotal secondary ferric arsenate that immobilizes most of arsenic (As) in acidic As-contaminated environments, but secondary As pollution may occur during dissolution of scorodite in environments involving redox changes. Reductive dissolution of scorodite by coexisting dissolved Fe²⁺ (Fe(II)_aq) under anaerobic conditions and its effects on the behavior of As have yet to be examined. Here, this study monitored the changes in mineralogy, solubility and speciation of As during scorodite transformation induced by Fe(II) under anaerobic conditions at pH 7.0 and discussed the underlying mechanisms. Mössbauer and X-ray diffraction (XRD) analysis showed the formation of parasymplesite and ferrihydrite-like species during scorodite transformation, which was highly controlled by Fe(II)_aq concentrations. 1 mM Fe(II)_aq enhanced As mobilization into the solution, whereas As was repartitioned to the PO₄^3--extractable and HCl-extractable phases with 5 and 10 mM Fe(II). The neo-formed parasymplesite and ferrihydrite-like species immobilized dissolved As(V) through adsorption and incorporation. Additionally, As(V) reduction occurred during Fe(II)-induced scorodite transformation. Our results provide new insights into the stability and risk of scorodite in anaerobic environments as well as the geochemical behavior of As in response to Fe cycling.

Effective removal of Hg(II) and MeHg from aqueous environment by ball milling aided thiol-modification of biochars: Effect of different pyrolysis temperatures

In order to synthesize biochar with the enhanced adsorption of inorganic mercury (Hg(II)) and organic mercury (Methylmercury, MeHg), biochars pyrolyzed at different pyrolysis temperatures (300, 500, and 700 °C) were ball milled with 3-trimethoxysilylpropanethiol (3-MPTS). Characterization results showed that 3-MPTS acted as an activator which further enlarged pores in biochars and ball-milling increased surface area of biochars. During ball milling, oxygen-containing functional groups increased which facilitated the loading of -SH group. The maximum adsorption capacities for Hg(II) adsorption in ball mill sulfhydryl modified biochars of 300, 500, and 700 °C were 401.8, 379.6 and 270.6 mg/g, respectively; simultaneously, the maximum adsorption capacity of MeHg was 108.16, 85.27 and 39.14 mg/g, respectively, which showed preferential increasing of 5.54 times on MeHg compared to the non-thiol modified biochar at low pyrolysis temperature of 300 °C. Results of kinetic adsorption experiments suggested that sorption data fitted well with Pseudo-second-order kinetic model, which proved that the mainly rate-limiting adsorption step was surface diffusion. Langmuir isotherm model fitting result showed that ligand exchange, surface complexation, surface adsorption and electrostatic attraction are dominant removal mechanisms. The 3-MPTS content and ball milling time are crucial during ball milling, and 2% 3-MPTS and 30 h of ball milling were found to be the most suitable conditions for both Hg(II) and MeHg adsorption. The result suggests that ball milling aided thiol-modification has great potential in synthesis of -SH modified biochar with prioritized MeHg adsorption.

Recent advances of carbon-based nano zero valent iron for heavy metals remediation in soil and water: A critical review

Heavy metal pollution in soil and water has presented a new challenge for the environmental remediation technology. Nano zero valent iron (nZVI) has excellent adsorbent properties for heavy metals, and thus, exhibits great potential in environmental remediation. Used as supporting materials for nZVI, carbon-based materials, such as activated carbon (AC), biochar (BC), carbon nanotubes (CNTs), and graphene (GNs) with aromatic rings formed by carbon atoms as the skeleton, have a large specific surface area and porous structure. This paper provides a comprehensive review on the advancement of carbon-based nano zero valent iron (C-nZVI) particles for heavy metal remediation in soil and water. First, different types of carbon-based materials and their combination with nZVI, as well as the synthesis methods and common characterization techniques of C-nZVI, are reviewed. Second, the mechanisms for the interactions between contaminants and C-nZVI, including adsorption, reduction, and oxidation reactions are detailed. Third, the environmental factors affecting the remediation efficiency, such as pH, coexisting constituents, oxygen, contact time, and temperature, are highlighted. Finally, perspectives on the challenges for utilization of C-nZVI in the actual contaminated soil and water and on the long-term efficacy and safety evaluation of C-nZVI have been proposed for further development.

Active biochar-supported iron oxides for Cr(VI) removal from groundwater: Kinetics, stability and the key role of FeO in electron-transfer mechanism

Chromium (Cr), especially in forms of hexavalent chromium (Cr(VI)) remains a serious threat to public health and environmental safety for its high toxicity. Herein, two types of iron-modification methods adopting co-pyrolysis and surface-deposition respectively were carried out to prepare active Fe-biochar composites (FeBC) for Cr(VI) removal in the simulated groundwater environment. The systematic characterization demonstrated that larger BET surface area and diversified iron oxides of FeBC-1 obtained from the co-pyrolysis method contributed to higher adsorption and reduction activity towards Cr(VI) degradation in comparison with FeBC-2 produced from surface-deposition method. Further, FeO was evidenced to be a main active component for transforming Cr(VI) to lower-toxicity Cr(III) uniting XRD and XPS analysis. Also, the designed batch experiments aiming at deeper clarifying FeBC-1 revealed that the pseudo-second-order kinetic and intra-particle diffusion model could well describe the Cr(VI) sorption behaviors, suggesting that a single-layer, chemical adsorption process as well as internal particle diffusion both controlled the removal process of Cr(VI) using FeBC-1. Finally, the stability experiments stated that FeBC-1 was basically stable at acidic and neutral conditions. Thus, it was found that co-pyrolysis of FeBC-1 is a potential technology for Cr(VI) remediation.

Phytoremediator Potential of Ipomea asarifolia in Gold Mine Waste Treated with Iron Impregnated Biochar

Growing environmental pollution in recent decades has been generating potentially toxic elements (PTE) which pose an ongoing threat to terrestrial and aquatic ecosystems and human health, especially in mining areas. Biochar and PTE-tolerant species have been used in soil remediation as they are environmentally friendly alternatives. This study aimed to assess the influence of açaí seed biochar (Euterpe oleracea Mart), impregnated with iron (BFe) or not (BC), on the bioavailability of PTEs, in a multi-contaminated soil from a gold (Au) mining area in the Amazon, using Ipomea asarifolia as a plant test since it was naturally growing on the tailings. BC increased the soil pH while BFe reduced. Biochars increased PTEs in the oxidizable fraction (linked to soil organic matter). The use of BC and BFe improved the immobilization of PTEs and BC increased arsenic (As) in the easily soluble fraction in the soil. Moreover, plants grown with biochars showed lower dry matter yield, higher concentrations of PTEs and lower nutrient content than the control treatment. According to the phytoextraction and translocation factors, Ipomea asarifolia can be classified as a species with potential for phytostabilization of Zn and tolerant to other PTEs, mainly As.

Elucidating the redox-driven dynamic interactions between arsenic and iron-impregnated biochar in a paddy soil using geochemical and spectroscopic techniques

Iron (Fe)-modified biochar, a renewable amendment that synthetizes the functions of biochar and Fe materials, demonstrates a potential to remediate arsenic (As)-contaminated soils. However, the effectiveness of Fe-based biochar to immobilize As in paddy soils under varying redox conditions (Eh) has not been quantified. We tested the capability of the raw (RBC) and Fe-impregnated (FeBC) biochars to immobilize As in a paddy soil under various Eh conditions (from -400 to +300 mV) using a biogeochemical microcosm system. In the control, As was mobilized (686.2-1535.8 μg L^-1) under reducing conditions and immobilized (61.6-71.1 μg L^-1) under oxidizing conditions. Application of FeBC immobilized As at Eh < 0 mV by 32.6%-81.1%, compared to the control, because of the transformation of As-bound Fe (hydro)oxides (e.g., ferrihydrite) and the formation of complexes (e.g., ternary As-Fe-DOC). Application of RBC immobilized As at Eh < -100 mV by 16.0%-41.3%, compared to the control, due to its porous structure and oxygen-containing functional groups. Mobilized As at Eh > +200 mV was caused by the increase of pH after RBC application. Amendment of the Fe-modified biochar can be a suitable approach for alleviating the environmental risk of As under reducing conditions in paddy soils.

Responses of bacterial taxonomic attributes to mercury species in rhizosphere paddy soil under natural sulphur-rich biochar amendment

Biochar and sulphur (S) are important factors regulating the level, speciation and transformation of mercury (Hg), leading to alterations in the assemblage of the soil microbial community. However, variations in the taxonomic attributes of the rhizosphere soil bacterial community arising from the Hg speciation in paddy soil, amended with natural S-rich biochar (NSBC) derived from the pyrolysis of S-rich oilseed rape straw, remain unclear. Herein, a rice pot experiment was conducted. Hg-polluted paddy soils were amended with NSBC and low-S biochar (LSBC) to evaluate the role of Hg chemical form affected by NSBC in regulating the taxonomic attributes of rhizosphere soil, including microbial abundance, composition, and ecological clusters within the co-occurrence network of microbial communities. Results showed that microbial abundance was higher in soils with lower Hg levels, and mean increases of 149 observed operational taxonomic units (OTUs) and 238 predicted OTUs (Chao 1) were observed, with a 1 mg kg^-1 decrease in the total Hg (T-Hg) content. Among the 13 predictor variables, the T-Hg content was the strongest and most consistent predictor of the bacterial taxonomic attributes. This finding may be attributed to the fact that the drastic reduction in T-Hg and Hg bioavailability induced by NSBC results in the decrease of Hg stress on the soil microbiome. Moreover, NSBC amendment shifted the ecological clusters toward the amelioration of Hg pollution.

Magnetic porous biochar with high specific surface area derived from microwave-assisted hydrothermal and pyrolysis treatments of water hyacinth for Cr(Ⅵ) and tetracycline adsorption from water

Magnetic porous water hyacinth-derived biochar (MPBC_MW3) was synthesized via two-step Microwave (MW)-assisted processes. Characterization results not only testified high specific surface area (2097.50 m²/g) of the MPBC_MW3 assisted by MW-assisted pyrolysis, but also revealed its favorable magnetism derived from MW-assisted hydrothermal process. The MPBC_MW3 possessed pH-dependent monolayer adsorption capacities of 202.61 and 202.62 mg/g for Cr(VI) and TC with quick attainments of uptake equilibrium within 150 and 200 min. Moreover, the Cr(VI) and TC uptake were substantially steady under the interference from multifarious co-existing ions with slight decline after three adsorption-desorption cycles. Furthermore, the MPBC_MW3 was demonstrated to achieve excellent Cr(VI) binding primarily through complexation, electrostatic interaction, reduction and ion exchange, while presenting outstanding TC removal via pore filling, π-π stacking, hydrogen bonding force, electrostatic interaction and complexation. All these findings suggested the MPBC_MW3 synthesized by MW-assisted processes as an excellent adsorbent for purification of Cr(VI) and TC-contaminated water.

The dual role of oxygen in redox-mediated removal of aqueous arsenic(III/V) by Fe-modified biochar

The Fe-modified biochar (FeBC) was used to remove aqueous As(III/V), and the role of oxygen (O₂) in As removal was investigated by integrating aqueous and solid analyses. The removal efficiencies for As(III) and As(V) increased from 86.4% and 99.2% under anoxic conditions, respectively, to >99.9% when O₂ was available. FeBC removed As(III) from As(III)-spiked systems by surface-oxidation following adsorption, where oxidation of As(III) was promoted by O₂. As(V) was first reduced, re-oxidized in solutions, and then adsorbed to FeBC in As(V)-spiked systems, where reduction of As(V) was inhibited at the presence of O₂. Both As(III) and As(V) were bidentate corner-sharing complexed to Fe oxides/hydroxides on FeBC, with As coordinated to Fe at ~3.4 Å according to As extended X-ray absorption fine structure (EXAFS) modeling. These findings identified the effect of ambient O₂ in As(III/V) redox transformations and removal, guiding the further application of FeBC in environmental treatment.

Conversion of cattle manure into functional material to remove selenate from wastewater

Herein, pyrolysis of cattle manure was conducted to synthesize an effective material for removing heavy metals (e.g., selenium) from water environments. To remove selenate from aqueous solution, iron-impregnated cattle manure biochar (Fe/CM-biochar) was synthesized. The Fe-impregnation was performed by pre-treating cattle manure before its pyrolysis. The pretreatment increased the biochar yield. Influence of various factors such as contacting time, initial selenate concentration, reaction temperature, pH, and presence of coexisting anions were explored by performing batch adsorption experiments. The selenate adsorption reached equilibrium within 15 min. The Langmuir model was better fitted to equilibrium adsorption data than the Freundlich model. The maximum adsorption capacity of Fe/CM-biochar was calculated to be 52.56 mg-Se/g, which is superior to other adsorbents reported in the literature. As the reaction temperature increased in the range (15-35) °C, selenate adsorption on Fe/CM-biochar showed an endothermic and nonspontaneous reaction. The enthalpy change during selenate adsorption was 18.44 kJ/mol, which ranges in physical adsorption. The increase of solution pH (3-11) reduced the selenate adsorption (46.4-37.7 mg-Se/g). The extent of co-existing anion impact on selenate adsorption followed an order of HPO₄^2- > HCO₃^- > SO₄^2- > NO₃^-. These results indicate that Fe/CM-biochar is an effective functional material for the removal of selenate from wastewater.

As(III) adsorption onto Fe-impregnated food waste biochar: experimental investigation, modeling, and optimization using response surface methodology

Biochar derived from food waste was modified with Fe to enhance its adsorption capacity for As(III), which is the most toxic form of As. The synthesis of Fe-impregnated food waste biochar (Fe-FWB) was optimized using response surface methodology (RSM), and the pyrolysis time (1.0, 2.5, and 4.0 h), temperature (300, 450, and 600 °C), and Fe concentration (0.1, 0.3, and 0.5 M) were set as independent variables. The pyrolysis temperature and Fe concentration significantly influenced the As(III) removal, but the effect of pyrolysis time was insignificant. The optimum conditions for the synthesis of Fe-FWB were 1 h and 300 °C with a 0.42-M Fe concentration. Both physical and chemical properties of the optimized Fe-FWB were studied. They were also used for kinetic, equilibrium, thermodynamic, pH, and competing anion studies. Kinetic adsorption experiments demonstrated that the pseudo-second-order model had a superior fit for As(III) adsorption than the pseudo-first-order model. The maximum adsorption capacity derived from the Langmuir model was 119.5 mg/g, which surpassed that of other adsorbents published in the literature. Maximum As(III) adsorption occurred at an elevated pH in the range from 3 to 11 owing to the presence of As(III) as H₂AsO₃^- above a pH of 9.2. A slight reduction in As(III) adsorption was observed in the existence of bicarbonate, hydrogen phosphate, nitrate, and sulfate even at a high concentration of 10 mM. This study demonstrates that aqueous solutions can be treated using Fe-FWB, which is an affordable and readily available resource for As(III) removal.

Simultaneous reduction and immobilization of Cr(VI) in seasonally frozen areas: Remediation mechanisms and the role of ageing

Among the toxic metals, hexavalent chromium [Cr(VI)] has attracted much attention due to its high mobility and toxicity, rendering considerable challenges for long-term remediation. In this study, the soil was collected from a dichromate contaminated industrial site in Liaoning Province, a seasonally frozen area in northern China, and subjected to frequent freeze-thaw cycles. Three additives, including (i) ferrous sulfate; (ii) calcium polysulfide; and (iii) combined biochar and calcium polysulfide were applied to reduce and immobilize Cr(VI) in the soils. The samples underwent 28 days of incubation followed by 16 freeze-thaw cycles. The toxicity characteristic leaching procedure (TCLP) and simulated acid rain leaching were adopted to test the remediation performances. It was observed that all three treatments can significantly reduce and immobilize Cr(VI) after short-term incubation, while biochar with abundant functional groups could adsorb and reduce Cr(VI) effectively. Notably, the concentration of Cr(VI) in TCLP leachates after incubation in combined treatment decreased by 67.87% and 37.27%, respectively, compared with the application of ferrous sulfate or calcium polysulfide alone. Freeze-thaw cycles induced the disintegration of soil particles and increased the risk of contaminant mobilization. Conversely, biochar particles has become finer and even produced nanoparticles with ageing, accompanied by the increase in oxygen-containing surface functional groups. Additionally, the specific surface area increased with the pyrolysis of biochar, which further enhanced the retention of soil colloidal particles and suppressed the migration of contaminants. Therefore, the cumulative release of Cr(VI) in the combined treatment (i.e., 10.97 ~ 32.97 mg/kg) was much lower than that of the other two treatments after freeze-thaw ageing. Overall, the combination of biochar and calcium polysulfide displayed advantages in the reduction and immobilization of Cr(VI), and offered a long-term, effective strategy for the remediation of Cr(VI) contaminated soils in cold regions.

Preparation and application of Fe/biochar (Fe-BC) catalysts in wastewater treatment: A review

The removal of organic pollutants from water environments is a challenging problem. Fe-based BC (Fe-BC) composites are promising catalysts for generating reactive oxygen species (ROS) for environmental remediation considering their low costs and excellent physicochemical surface characteristics. The synthesis methods, properties, applications, and the mechanism of Fe-BC for removing pollutants are reviewed. Various methods have been used to prepare Fe-BC composites, and the synthetic methods and conditions used affect the properties of the Fe-BC material, thereby influencing its pollutant removal performance. The mechanisms of pollutant removal by Fe-BC are intricate and include adsorption, degradation and reduction. Fe loading on BC could improve the performance of BC by affecting its surface area, surface functional groups and electron transfer rate. Moreover, research gaps and uncertainties that exist in the use of Fe-BC were identified. Finally, the problems that need to be solved to make Fe-BC suitable for future applications are described.

Chemical processes of Cr(VI) removal by Fe-modified biochar under aerobic and anaerobic conditions and mechanism characterization under aerobic conditions using synchrotron-related techniques

Fe-modified biochar (FeBC) has been considered for aqueous hexavalent chromium (Cr(VI)) removal, but a better understanding is needed with respect to the removal behavior, chemical processes, and removal mechanisms under aerobic and anaerobic conditions. Aqueous Cr(VI) removal was evaluated using unmodified (BC) and FeBC. The Cr(VI) was completely removed in a pH range of 2-10. The removal behavior was properly depicted using pseudo-second-order (PSO) and Langmuir models under aerobic conditions, and using PSO and Freundlich models under anaerobic conditions. Removal rate and capacity were enhanced by up to 3.8 times under anaerobic conditions. Desorption experiments indicated removed Cr in FeBC was stable except under strong acid condition. X-ray absorption spectroscopy (XAS) analysis suggested removed Cr in FeBC was 100% in Cr(III) form and bound to Fe with a bond length of 3.01 Å in the stable form of Fe(III)_nCr(III)_(1-n)(OOH). The removal mechanisms of Cr(VI) under aerobic conditions by FeBC mainly included electrostatic adsorption, chemical reduction, and complex precipitation.

Application of iron-biochar composite in topsoil for simultaneous remediation of chromium-contaminated soil and groundwater: Immobilization mechanism and long-term stability

This study was aimed to prove the effectiveness and practicability of an integrated technology for simultaneous remediation of Cr-contaminated soil and groundwater. The remediation system was built by pumping Cr-contaminated groundwater into top contaminated soil round after round, enabling the pre-applied iron-biochar composite (Fe-BC) in topsoil to stabilize Cr from both groundwater and soil. Immobilization ability and mechanism of Cr in soil were explored by toxicity characteristic leaching procedure test, scanning electron microscopy-elemental mapping, and X-ray photo spectroscopy. Hydrus-1D software was used to examine the long-term stability of immobilized Cr in soil. Results showed that Fe-BC-amended soil could remove about 71% Cr from contaminated groundwater. Meanwhile, Cr from both groundwater and soil was simultaneously immobilized in topsoil, leachability of Cr in which was reduced by over 81%. The immobilization of Cr in soil was attributed to the reduction of Cr(VI) into Cr(III) to form stable Cr_xFe_(1-x)(OH)₃. After remediation, the average transport rate of Cr in the soil profile was only 0.420 cm y^-1 along with the local rainfall. Our study demonstrated that integrated technology could effectively remove Cr from groundwater and stabilize Cr in soil and the simultaneous remediation target for both soil and groundwater reached.

Cysteine chemical modification for surface regulation of biochar and its application for polymetallic adsorption from aqueous solutions

Biochar (BC) has been widely used to remove heavy metals from wastewater. However, due to the hydrophobicity of BC and the lack of its surface functional groups, the effect of metal ions adsorption onto BC is limited. In order to improve the adsorption efficiency, L-cysteine was used to modify biochar derived from pomelo peel (PP) to regulate surface structure. The characteristics of BC and cysteine/biochar composite (cys/BC) were analyzed by various characterization methods. Results showed that the hydrophilicity of biochar was enhanced, and the number of surface functional groups was increased, resulting to strong adsorption ability of Ag(I) (618.9 mg/g), Pb(II) (274.5 mg/g), and As(V) (34.7 mg/g) for cys/BC, which increased approximately by 15%, 35%, and 29% compared with that of BC, respectively. The adsorption process of Pb(II) onto cys/BC was fitted better by the Freundlich isotherm model and for Ag(I) and As(V) by the Langmuir isotherm model. Moreover, the adsorption kinetics followed pseudo-second-order equation and the adsorption process was controlled by the intraparticle diffusion for Ag(I), Pb(II), and As(V) adsorption onto cys/BC. In addition, the adsorption capacities of cys/BC for Ag(I), Pb(II), and As(V) decreased slightly after five adsorption/desorption cycles. Finally, the multiple adsorption mechanisms including functional groups, pore adsorption, surface complexation, and cations-π were analyzed. The paper demonstrated that the cys/BC composite could be reused as effective adsorbents for removing contaminants in the environment.

Modification on biochars for applications: A research update

Biochars are the solid product of biomass under pyrolysis or gasification treatment, whose wholesale prices are lower than commercial activated carbons and other fine materials now in use. The employment of biochars as a renewable resource for field applications, if feasible, would gain apparent economic niche. Modification using physical or chemical protocol to revise the surface properties of biochar for reaching enhanced performances of target application has attracted great research interests. This article provided an overview of biochar application, particularly with the respect to the use of modified biochar as preferred soil amendment, adsorbent, electrochemical material, anaerobic digestion promotor, and catalyst. Based on literature works the current research trends and the prospects and research needs were outlined.

Removal of Cadmium from Contaminated Groundwater Using a Novel Silicon/Aluminum Nanomaterial: An Experimental Study

Cadmium (Cd) is a harmful element to human health and biodiversity. The removal of Cd from groundwater is of great significance to maintain the environmental sustainability and biodiversity. In this work, a novel low-temperature roasting associated with alkali was applied to synthesize an eco-friendly adsorbent using coal fly ash. Scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray fluorescence, and X-ray photoelectron spectroscopy were applied to analyze the physical and chemical characteristics of the adsorbent. The experiments show that a significant improvement in specific surface area and activity of adsorbent was observed in this study. The functional groups of Na-O and Fe-O were verified to be beneficial in the removal of Cd²⁺. The material capacity to adsorb Cd²⁺ was considerably improved, and the maximum uptake capacity was 61.8 mg g^-1 for Cd²⁺ at 25 °C. Furthermore, pH and ionic strength play critical roles in the adsorption process. The Langmuir and pseudo-second-order models can appropriately describe the adsorption behavior, and the enhanced adsorption ability of Cd²⁺ by modified coal fly ash was attributed to ion-exchange, co-precipitation, and complexation. Higher sorption efficiency was maintained after two regeneration cycles. These results offer valuable insights to develop high-performance adsorbent for Cd²⁺ removal.

Review: Efficiently performing periodic elements with modern adsorption technologies for arsenic removal

Arsenic (As) toxicity is a global phenomenon, and it is continuously threatening human life. Arsenic remains in the Earth's crust in the forms of rocks and minerals, which can be released into water. In addition, anthropogenic activity also contributes to increase of As concentration in water. Arsenic-contaminated water is used as a raw water for drinking water treatment plants in many parts of the world especially Bangladesh and India. Based on extensive literature study, adsorption is the superior method of arsenic removal from water and Fe is the most researched periodic element in different adsorbent. Oxides and hydroxides of Fe-based adsorbents have been reported to have excellent adsorptive capacity to reduce As concentration to below recommended level. In addition, Fe-based adsorbents were found less expensive and not to have any toxicity after treatment. Most of the available commercial adsorbents were also found to be Fe based. Nanoparticles of Fe-, Ti-, Cu-, and Zr-based adsorbents have been found superior As removal capacity. Mixed element-based adsorbents (Fe-Mn, Fe-Ti, Fe-Cu, Fe-Zr, Fe-Cu-Y, Fe-Mg, etc.) removed As efficiently from water. Oxidation of AsO₃^3- to AsO₄^3-and adsorption of oxidized As on the mixed element-based adsorbent occurred by different adsorbents. Metal organic frameworks have also been confirmed as good performance adsorbents for As but had a limited application due to nano-crystallinity. However, using porous materials having extended surface area as carrier for nano-sized adsorbents could alleviate the separation problem of the used adsorbent after treatment and displayed outstanding removal performances.

Aqua regia digestion cannot completely extract Hg from biochar: A synchrotron-based study

Mercury (Hg) is commonly extracted from solid phase samples using aqua regia for total Hg (tHg) analysis. However, uncertainties exist regarding the complete extraction of Hg by aqua regia, especially from carbonaceous materials. To investigate whether aqua regia can completely extract Hg from biochars, batch-style experiments were carried out to evaluate extraction efficiency of aqua regia with respect to Hg-loaded biochar and to characterize the residual Hg speciation and spatial distribution. Different types of biochars (raw, FeCl₃-modified, and FeSO₄-modified, prepared at different temperatures) were reacted with Hg-spiked solution before the digestion experiments. Adsorption analyses indicate the biochars were successfully loaded with Hg and that the Hg content was higher in biochars pyrolyzed at higher temperature (900 versus 300 or 600 °C). The results of digestion experiments indicate Hg could not be completely extracted from the biochars tested, with a greater percentage of residual Hg in biochars pyrolyzed at 600 (60 ± 15%) and 900 (75 ± 22%) than 300 °C (7 ± 2%). Furthermore, the fraction of residual Hg in FeSO₄-modified biochars after aqua regia digestion was significantly lower than in FeCl₃-modified and unmodified biochars. Confocal micro-X-ray fluorescence imaging (CMXRFI) showed residual Hg in biochars is concentrated on surfaces prior to digestion, but more homogeneously distributed after digestion, which indicates Hg on biochar surface is more easily digested. Hg extended X-ray absorption fine structure (EXAFS) spectra modelling showed residual Hg in biochars mainly exists as Hg(II)-Cl. These results indicate extra caution should be paid for tHg determinations using aqua regia digestion method in soil (especially in forest), sediment, and peat samples containing black carbon, activated carbon, or biochar.

Synthesis of Fe-impregnated biochar from food waste for Selenium(Ⅵ) removal from aqueous solution through adsorption: Process optimization and assessment

Iron-impregnated food waste biochar (Fe-FWB) was synthesized for Se(Ⅵ) removal from aqueous solution. The effect and interactive effects of different parameters including pyrolysis time, temperature, and Fe concentration were explored using response surface methodology (RSM) to enhance conditions to achieve the highest Se(Ⅵ) removal using Fe-FWB. Pyrolysis time was not significant for Se(Ⅵ) adsorption capacity of Fe-FWB, but temperature and Fe concentration were found to be significant. The highest adsorption was achieved at 3.47 h and 495.0 °C with an Fe concentration of 0.44 M. Fe-FWB synthesized under optimum conditions were used to investigate the kinetic, equilibrium, and thermodynamic adsorption of Se(Ⅵ). Se(Ⅵ) adsorption reached equilibrium within 6 h, and both pseudo-second order and pseudo-first order models were suitable for describing kinetic Se(Ⅵ) adsorption. The Freundlich model was found to suitably fit the equilibrium adsorption data than the Langmuir model. The highest adsorption capacity of Fe-FWB for Se(Ⅵ) was 11.7 mg g^-1. Se(Ⅵ) adsorption on Fe-FWB was endothermic and spontaneous. The enthalpy change for Se(Ⅵ) adsorption was 54.4 kJ mol^-1, and the entropy change was negative at 15-35 °C. The increment of solution pH from 3 to 11 decreased the Se(Ⅵ) adsorption from 19.2 to 7.4 mg g^-1. The impact of interfering anions on Se(Ⅵ) adsorption followed the lineup: HCO₃^- > HPO₄^2- > SO₄^2- > NO₃^-. When compared to some adsorbents, the adsorption capacity of Se(Ⅵ) onto Fe-FWB was comparable even at neutral pH and the Fe-FWB was granular. These results indicate that Fe-FWB has prospective application in the removal of Se(Ⅵ) from aqueous solutions.

Mechanistic investigation of mercury removal by unmodified and Fe-modified biochars based on synchrotron-based methods

Improved surface characteristics and incorporated Fe, S, and Cl species are reported in Fe-modified biochar, which makes it a prospective material for Hg(II) removal. In this study, aqueous Hg(II) was removed from solution by unmodified, FeCl₃-modified, and FeSO₄-modified biochars pyrolyzed at 300, 600, or 900 °C. Higher pyrolytic temperature resulted in higher removal efficiency, with the biochars pyrolyzed at 900 °C removing >96% of Hg(II). Fe-modification enhanced Hg(II) removal for biochars pyrolyzed at 600 °C (from 88% to >95%) or 900 °C (from 96% to 99%). Based on synchronous extended X-ray absorption fine structure (EXAFS) analysis, Hg coordinated to S in modified and unmodified biochars pyrolyzed at 900 °C, where thiol was reported, and in FeSO₄-modified biochars pyrolyzed at 600 or 900 °C, where sulfide was recognized; in other biochars, Hg bound to O or Cl. Additionally, confocal micro-X-ray fluorescence imaging (CMXRFI) demonstrated Hg was distributed in agreement with S in biochars where HgS was formed; otherwise, Hg distribution was influenced by Hg species in solution and the pore characteristics of the biochar. This investigation provides information on the effectiveness and mechanisms of Hg removal that is critical for evaluating biochar applications and optimizing modification methods in groundwater remediation.

As(III) and As(V) removal mechanisms by Fe-modified biochar characterized using synchrotron-based X-ray absorption spectroscopy and confocal micro-X-ray fluorescence imaging

Batch experiments followed by solid-phase analyses were conducted to explore As(III) and As(V) removal mechanisms by Fe-modified biochars (FeBC) pyrolyzed at different temperatures (300, 600, and 900 °C). Arsenic removal by FeBC, best described by pseudo-second order kinetic and Langmuir isotherm models, increased from 73.8 to 99.9% for As(III) and 86.8 to 99.9% for As(V) as the pyrolysis temperature increased. The addition of calcite enhanced the removal efficiency (all > 99%). Confocal micro-X-ray fluorescence imaging (CMXRFI) analyses indicated As co-located with Fe and diffused deeper into the particles as the pyrolysis temperature increased. For As(III)-spiked systems, X-ray absorption near-edge structure (XANES) data indicated 20.2 to 81.5% of As(III) was oxidized to As(V) as the pyrolysis temperature increased; an increase of oxidation efficiency was observed after adding calcite. For As(V)-spiked systems, no As(V) reduction was observed. Overall, As(III/V) removal using FeBC was affected by the spatial distribution and species of As.

Preparation, environmental application and prospect of biochar-supported metal nanoparticles: A review

Biochar is a low-cost, porous, and carbon-rich material and it exhibits a great potential as an adsorbent and a supporting matrix due to its high surface activity, high specific surface area, and high ion exchange capacity. Metal nanomaterials are nanometer-sized solid particles which have high reactivity, high surface area, and high surface energy. Owing to their aggregation and passivation, metal nanomaterials will lose excellent physiochemical properties. Carbon-enriched biochar can be applied to overcome these drawbacks of metal nanomaterials. Combining the advantages of biochar and metal nanomaterials, supporting metal nanomaterials on porous and stable biochar creates a new biochar-supported metal nanoparticles (MNPs@BC). Therefore, MNPs@BC can be used to design the properties of metal nanoparticles, stabilize the anchored metal nanoparticles, and facilitate the catalytic/redox reactions at the biochar-metal interfaces, which maximizes the efficiency of biochar and metal nanoparticles in environmental application. This work detailedly reviews the synthesis methods of MNPs@BC and the effects of preparation conditions on the properties of MNPs@BC during the preparation processes. The characterization methods of MNPs@BC, the removal/remediation performance of MNPs@BC for organic contaminants, heavy metals and other inorganic contaminants in water and soil, and the effect of MNPs@BC properties on the remediation efficiency were discussed. In addition, this paper summarizes the effect of various parameters on the removal of contaminants from water, the effect of MNPs@BC remediation on soil properties, and the removal/remediation mechanisms of the contaminants by MNPs@BC in water and soil. Moreover, the potential directions for future research and development of MNPs@BC have also been discussed.