Removing mercury from aqueous solution using sulfurized biochar and associated mechanisms

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

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

Thiol-functionalized cellulose for mercury polluted water remediation: Synthesis and study of the adsorption properties

Mercury pollution poses a global health threat due to its high toxicity, especially in seafood where it accumulates through various pathways. Developing effective and affordable technologies for mercury removal from water is crucial. Adsorption stands out as a promising method, but creating low-cost materials with high selectivity and capacity for mercury adsorption is challenging. Here we show a sustainable method to synthesize low-cost sulfhydrylated cellulose with ethylene sulfide functionalities bonded glucose units. Thiol-functionalized cellulose exhibits exceptional adsorption capacity (1325 mg g-1) and selectivity for Hg(II) over other heavy metals (Co, Cu, Zn, Pb) and common cations (Ca++, Mg++) found in natural waters. It performs efficiently across a wide pH range and different aqueous matrices, including wastewater, and can be regenerated and reused multiple times without significant loss of performance. This approach offers a promising solution for addressing mercury contamination in water sources.

Definitive Review of Nanobiochar

Nanobiochar is an advanced nanosized biochar with enhanced properties and wide applicability for a variety of modern-day applications. Nanobiochar can be developed easily from bulk biochar through top-down approaches including ball-milling, centrifugation, sonication, and hydrothermal synthesis. Nanobiochar can also be modified or engineered to obtain "engineered nanobiochar" or biochar nanocomposites with enhanced properties and applications. Nanobiochar provides many fold enhancements in surface area (0.4-97-times), pore size (0.1-5.3-times), total pore volume (0.5-48.5-times), and surface functionalities over bulk biochars. These enhancements have given increased contaminant sorption in both aqueous and soil media. Further, nanobiochar has also shown catalytic properties and applications in sensors, additive/fillers, targeted drug delivery, enzyme immobilization, polymer production, etc. The advantages and disadvantages of nanobiochar over bulk biochar are summarized herein, in detail. The processes and mechanisms involved in nanobiochar synthesis and contaminants sorption over nanobiochar are summarized. Finally, future directions and recommendations are suggested.

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.

Constructing N,S co-doped network biochar confined CoFe2O4 magnetic nanoparticles adsorbent: Insights into the synergistic and competitive adsorption of Pb2+ and ciprofloxacin

To solve the problem of biochar lack of adsorption sites for heavy metal ions and the difficulty of recycling, CoFe2O4 magnetic nanoparticles confined in nitrogen, sulfur co-doped 3D network biochar matrix (C-CoFe2O4/N,S-BC) was designed and fabricated successfully. The obtained C-CoFe2O4/N,S-BC displays remarkable adsorption performance for both Pb2+ and ciprofloxacin (CIP) removal at the single or binary system due to the role of N,S as metal ion anchoring compared to the N,S-free sample (CoFe2O4/BC). N,S co-doped BC not only participates in adsorption reaction but also effectively inhibites the agglomeration of CoFe2O4 nanoparticles and increases the active sites as a carrier at the same time. In the single system, CoFe2O4/N,S-BC demonstrates a fast adsorption rate (equilibrium time: 30 min) and high adsorption capacity (224.77 mg g-1 for Pb2+, 400.11 mg g-1 for CIP) towards Pb2+ and CIP. The adsorption process is befitted pseudo-second-order model, and the equilibrium data are in great pertinence with Langmuir model. In the binary system, the maximum adsorption capacity of CoFe2O4/N,S-BC for Pb2+ and CIP is 244.80 mg g-1 (CIP: 10.00 mg L-1) and 418.42 mg g-1 (Pb2+: 10.00 mg L-1), respectively. The adsorption mechanism is discussed based on the experimental results. Moreover, C-CoFe2O4/N,S-BC shows good practical water treatment capacity, anti-interference ability and stable reusability (the removal efficiency＞80% after eight cycles). The rapid, multifunctional, reusable, and easily separable adsorption properties make C-CoFe2O4/N,S-BC promising for efficient environmental remediation. This study also offers a viable method for the construction of adsorption material for complex wastewater treatment.

Sulfur functionalized biocarbon sorbents for low-concentration mercury isolation

Sulfur functionalized biocarbons were prepared from naturally abundant lignin alkali with sodium thiocyanate as an activation agent and a sulfur source. The resultant biocarbon sorbents showed a high mercury isolation ability from aqueous solutions, where high surface area and doping of sulfur significantly aid the uptake of mercury, i.e., 0.05 g of biocarbon sorbent removed 99% of mercury from 250 mL of simulated wastewater with an initial concentration of mercury of 10 mg L-1.

Evaluation of novel biomass-derived materials as binding layers for determining labile mercury in water by diffusive gradient in thin-films technique

In this work, several binding gels were successfully prepared in Diffusive Gradient in Thin-film (DGT) that targeted the inclusion of novel biomass-derived materials for the determination of the labile fraction of mercury (Hg) in water. First, five biomass-derived materials were tested and the descending order as a function of the average percentage of Hg removal in solution was feathers > biochar > cork > canola meal > rice husk. The best two materials were treated and pulverized into powder to be embedded in a hydrogel; and so, feathers were pyrolyzed preserving the sulfur contained in their keratin structure (FBC), and biochar (BC) was modified and pyrolyzed with sublimated sulfur (SBC) to increase the Hg sorption sites in its structure. Analysis by Energy Dispersive X-ray fluorescence (EDXRF) spectrometry confirmed that the different pyrolysis procedures increased sulfur absorption successfully. The efficiency of the new gels (BC, SBC and FBC) in agarose was evaluated by comparative Hg uptake tests, showing a larger efficacy in the following order: SBC > BC > FBC. To assess the suitability of their application in freshwater environments, novel DGT devices were also evaluated to determine their diffusion coefficients (D). This test was conducted under controlled laboratory conditions, with particular focus on the potential competence of trace elements (Mn, Cu, Zn, Ni, Pb, Cd and As), which are commonly present in natural waters affected by mining. A stronger linear relationship between the Hg uptake by binding layers and the deployment time were obtained for the DGT devices with SBC (R2 = 0.948) vs. BC (R2 = 0.885). Therefore, the D obtained for Hg were 8.94 × 10-6 cm2 s-1 for DGT-SBC and 5.12 × 10-6 cm2 s-1 for DGT-BC devices at 25 °C, both within the same order of magnitude reported by previous studies. The good performance obtained by DGT-SBC devices is a promising result and indicates the potential for valorization of waste materials in the DGT technique.

Tuning active sites on biochars for remediation of mercury-contaminated soil: A comprehensive review

Mercury (Hg) contamination is acknowledged as a global issue and has generated concerns globally due to its toxicity and persistence. Tunable surface-active sites (SASs) are one of the key features of efficient BCs for Hg remediation, and detailed documentation of their interactions with metal ions in soil medium is essential to support the applications of functionalized BC for Hg remediation. Although a specific active site exhibits identical behavior during the adsorption process, a systematic documentation of their syntheses and interactions with various metal ions in soil medium is crucial to promote the applications of functionalized biochars in Hg remediation. Hence, we summarized the BC's impact on Hg mobility in soils and discussed the potential mechanisms and role of various SASs of BC for Hg remediation, including oxygen-, nitrogen-, sulfur-, and X (chlorine, bromine, iodine)- functional groups (FGs), surface area, pores and pH. The review also categorized synthesis routes to introduce oxygen, nitrogen, and sulfur to BC surfaces to enhance their Hg adsorptive properties. Last but not the least, the direct mechanisms (e.g., Hg- BC binding) and indirect mechanisms (i.e., BC has a significant impact on the cycling of sulfur and thus the Hg-soil binding) that can be used to explain the adverse effects of BC on plants and microorganisms, as well as other related consequences and risk reduction strategies were highlighted. The future perspective will focus on functional BC for multiple heavy metal remediation and other potential applications; hence, future work should focus on designing intelligent/artificial BC for multiple purposes.

Exploring adsorption capacity and mechanisms involved in cadmium removal from aqueous solutions by biochar derived from euhalophyte

Biochar has shown potential as a sorbent for reducing Cd levels in water. Euhalophytes, which thrive in saline-alkali soils containing high concentrations of metal ions and anions, present an intriguing opportunity for producing biochar with inherent metal adsorption properties. This study focused on biochar derived from the euhalophyte Salicornia europaea and aimed to investigate its Cd adsorption capacity through adsorption kinetics and isotherm experiments. The results demonstrated that S. europaea biochar exhibited a high specific surface area, substantial base cation content, and a low negative surface charge, making it a highly effective adsorbent for Cd. The adsorption data fit well with the Langmuir isotherm model, revealing a maximum adsorption capacity of 108.54 mg g-1 at 25 °C. The adsorption process involved both surface adsorption and intraparticle diffusion. The Cd adsorption mechanism on the biochar encompassed precipitation, ion exchange, functional group complexation, and cation-π interactions. Notably, the precipitation of Cd2+ with CO32- in the biochar played a dominant role, accounting for 73.7% of the overall removal mechanism. These findings underscore the potential of euhalophytes such as S. europaea as a promising solution for remediating Cd contamination in aquatic environments.

Chemical and mechanical coating of sulfur on baby corn biochar and their role in soil Pb availability, uptake, and growth of tomato under Pb contamination

In recent ages, industrial revolution and natural weathering processes have been increasing lead (Pb) contamination in agricultural soils, therefore, green remediation technologies are becoming attractive and cost-effective. In the current pot study, 1% and 2% (w/w) application rates of sulfur (S) alone and novel chemo-mechanically S-modified baby corn biochars (CSB and MSB) were applied in a Pb-contaminated (500 mg/kg) soil to evaluate tomato (Lycopersicon esculentum L.) growth, Pb uptake and its soil availability. The results from SEM-EDS and XRD patterns confirmed the S enrichment on the surface of baby-corn biochar. Further, Pb treatment alone imposed a significant reduction in biomass accumulation, photosynthetic pigments, antioxidative mechanism, root traits, and Pb-tolerance index because of increased soil Pb availability and its uptake, translocation and biological accumulation in various tissues of tomato. However, incorporation of lower rate of elemental S (1%) and higher rates of biochars, especially chemically S-modified biochar, CSB (2%) significantly improved dry biomass production, Pb-tolerance index, physiological attributes and antioxidative defense system of tomato plants. These results might be due to a prominent decrease in soil Pb availability by 37.5%, Pb concentration in shoot by 66.7% and root by 58.3%, soil to root transfer by 33.8%, and root to shoot transfer by 20.2% in tomato plants under 2% application rate of CSB, as compared with the Pb treatment without any amendment. Moreover, sulfur treatment induced a significant impact in reduction of soil pH (from 8.97-7.47) as compared to the biochar treatments under Pb-toxicity. The current findings provided an insight that 2% chemically S-modified biochar (CSB) has significant potential to improve the tomato growth by reducing Pb bioavailability in the Pb-contaminated soil, compared to the S alone and MSB amendments.

Aminothiol supported dialdehyde cellulose for efficient and selective removal of Hg(II) from aquatic solutions

The increasingly serious problem of mercury pollution has caused wide concern, and exploring adsorbent materials with high adsorption capacity is a simple and effective approach to address this concern. In the recent study, dialdehyde cellulose (DAC), cyanoacetohydrazide (CAH), and carbon disulfide (CS2) are used as raw materials for the (DAC@CAH@SK2) preparation material through the three-steps method. By utilizing the following characterization techniques; thermogravimetric analysis (TGA), N2 adsorption-desorption isotherm (BET), elemental analysis, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD), 1HNMR and Energy Dispersive X-ray Spectroscopy (EDS) of DAC@CAH@SK2 composite. The point of zero charge (pHPZC) for the prepared DAC@CAH@SK2 also was examined. From the batch experiments, the optimum conditions were found to be pH (5-8), an Hg2+ concentration of 150 mg/L, a DAC@CAH@SK2 dose of 0.01 g, and a contact time of 180 min with a maximum adsorption quantity of 139.6 mg/g. The process of Hg2+ adsorption on the DAC@CAH@SK2 material was spontaneous exothermic, monolayer chemisorption, and well-fitted to Langmuir and pseudo-2nd-order models. The DAC@CAH@SK2 selectivity towards the Hg2+ was examined by investigating the interfering metal ions effect. The DAC@CAH@SK2 was successfully applied for the Hg2+ removal from synthetic effluents and real wastewater samples with a recovery % exceeding 95%. The prepared DAC@CAH@SK2 was regenerated using a mixture of EDTA and thiourea. Also, FT-IR analysis indicates that the synergistic complexation of N and S atoms on DAC@CAH@SK2 with Hg(II) is an essential factor leading to the high adsorption capacity.

Advances in green materials derived from wood for detecting and removing mercury ions in water

The issue of mercury pollution in environmental remediation has garnered significant attention due to its severe health hazards to humans. Various strategies have been devised to mitigate the impact of toxic mercury ions, including coagulation, ion exchange, adsorption, membrane technology, and electrochemical treatment. Among these approaches, adsorption has emerged as an efficient and widely employed method for the uptake of low concentrations of mercury ions. It offers convenient operation, high removal efficiency, and facile regeneration of the adsorbent. Wood, being the most abundant renewable and sustainable bioresource, has garnered attention as a promising material for treating heavy metal wastewater. This is attributed to its unique physical and chemical characteristics, encompassing hierarchical pores, aligned channels, active functional groups, biodegradability, and cost-effectiveness. However, a comprehensive examination of the cutting-edge applications of wood and wood-derived biopolymers in the detection and removal of mercury ions from wastewater has yet to be undertaken. Consequently, this article presents a chronological overview of recent advancements in materials and structures derived from bulk wood and its constituents, including cellulose, lignin, hemicellulose, and tannin, with a specific focus on their utility in detecting and eliminating mercury from water sources. Subsequently, the most promising techniques and strategies involving wood and wood-derived biopolymers in addressing the predicament of mercury pollution are explored. Furthermore, this piece offers insights into the existing challenges and future prospects concerning environmentally friendly materials derived from wood, aiming to foster the development of cost-effective mercury adsorbents and detection devices.

A 50-year systemic review of bioavailability application in Soil environmental criteria and risk assessment

Bioavailability has become a critical factor in improving ecological risk assessment and environmental remediation efficiency in contaminated soil research. However, the soil environmental quality standards and risk assessment procedures used in most countries are still based on the total amount of pollutants for lacking sufficient understanding of the exposure pathways and action mechanisms of pollutants. we collected relevant literature from the Web of Science database, spanning the period from 1950 to 2021 by using Citespace to analyze the scientific development of bioavailability. As of January 09, 2022, the database contained 118,813 publications on bioavailability. The review summarizes the progress in bioavailability research and emerging trends, including exploring advanced analytical techniques, advancing modeling approaches, and integrating interdisciplinary approaches to better understand the fate and behavior of pollutants in complex environmental matrices. In particular, the review emphasizes the need for better integration of bioavailability concepts into soil environmental reference, risk assessment procedures, and environmental remediation strategies. Overall, this review emphasized the necessity of incorporating the concept of bioavailability into soil environmental reference, risk assessment procedures, and environmental remediation strategies.

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.

Mechanisms and biological effects of organic amendments on mercury speciation in soil-rice systems: A review

Mercury (Hg) pollution is a well-recognized global environmental and health issue and exhibits distinctive persistence, neurotoxicity, bioaccumulation, and biomagnification effects. As the largest global Hg reservoir, the Hg cumulatively stored in soils has reached as high as 250-1000 Gg. Even more concerning is that global soil-rice systems distributed in many countries have become central to the global Hg cycle because they are both a major food source for more than 3 billion people worldwide and the central bridge linking atmospheric and soil Hg circulation. In this review, we discuss the form distribution, transformation, and bioavailability of Hg in soil-rice systems by focusing on the Hg methylation and demethylation pathways and distribution, uptake, and accumulation in rice plants and the effects of Hg on the community structure and ecological functions of microorganisms in soil-rice systems. In addition, we clarify the mechanisms through which commonly used humus and biochar organic amendments influence Hg and its environmental effects in soil-rice systems. The review also elaborates on the advantages of sulfur-modified biochars and their critical role in controlling Hg migration and bioavailability in soils. Finally, we provide key information about Hg pollution in soil-rice systems, which is of great significance for developing appropriate strategies and mitigation planning to limit Hg bioconcentration in rice crops and achieving key global sustainable development goals, such as the guarantee of food security and the promotion of sustainable agriculture.

Using recoverable sulfurized magnetic biochar for active capping to remediate multiple heavy metal contaminated sediment

Due to anthropogenic activities, heavy metals are discharged into the hydrosphere and deposit onto the sediment. Heavy metals remobilize through physical disturbance and change in environmental conditions, posing a risk to environments and human health. Among several remediation methods, active layer capping is considered to be more feasible due to its financial and technical advantages; however, its long-term effects remain unknown. To overcome this problem, this work applied a novel, recoverable amendment, sulfurized magnetic biochar (SMBC), to remediate multiple heavy metal (Cu, Ni, Zn, Cr, Hg, and MeHg) contaminated sediment. Physiochemical characterization shows magnetite (Fe₃O₄) crystalline in both magnetic biochar (MBC) and SMBC, with such characteristics resulting in a greater surface area (324.9 and 346.3 m²/g) than BC (39.6 m²/g) and SBC (65.0 m²/g). FeS crystalline was also observed in SMBC, which plays an important role in controlling heavy metal release from sediment. Microcosm experiments indicated the effectiveness of SMBC in lowering aquatic Cu, Ni, Zn, Hg, and MeHg releases was significantly greater than the other three biochar materials. Notably, the recovery of SMBC by magnetism was 87%, demonstrating the exceptional recoverability of SMBC from seawater and sediment. Based on its robust capability in lowering Cu, Ni, Zn, Hg, and MeHg release and excellent recoverability from seawater and sediment, this technique represents a practical alternative to conventional approaches for heavy metal immobilization from sediment.

Synthesis and application of starch-stablized Fe-Mn/biochar composites for the removal of lead from water and soil

Starch-stablized and Fe/Mn bimetals modified biochar derived from corn straw (SFM@CBC and SFM@CBC-350) were firstly prepared, characterized (FTIR, XRD, SEM, EDS, BET and XPS), and applied in Pb removal from water and soil. SFM@CBC and SFM@CBC-350 displayed highly effective adsorption performance of Pb²⁺ from wastewater with the maximum adsorption capacity of 170.91 mg g^-1 and 190.17 mg g^-1, respectively, which were much greater than that of FM@CBC (149.25 mg g^-1) and CBC (101.10 mg g^-1). Studies of adsorption kinetics, isotherms and thermodynamics indicated that the absorption of Pb²⁺ by SFM@CBC and SFM@CBC-350 was spontaneous and endothermic reaction, and it was controlled by monolayer chemisorption. The mechanism studies indicated that Pb²⁺ removal involved with multiple mechanism, including complexation (dominant process confirmed by XPS analysis), physical adsorption, electrostatic attraction, and cation exchange. The reusability test demonstrated that SFM@CBC and SFM@CBC-350 had very good stability and reusability. In addition, in order to further explore Pb removal performance of the modified biochar, SFM@CBC-350 was used in soil-ryegrass pot systems. Compared with the controls, the addition of SFM@CBC-350 reduced Pb content in soil and ryegrass, increased the biomass and total chlorophyll content, reduced the activity of antioxidant enzymes (CAT, SOD, MDA and POD) and ROS fluorescence intensity of ryegrass, thus alleviating Pb stress of ryegrass. Besides, the addition of SFM@CBC-350 could increase the richness and diversity of soil microorganisms, which was beneficial to the growth of ryegrass. Hence, SFM@CBC-350 has the potential of being used as a green, efficient and promising adsorbent in Pb removal from wastewater and soil.

Impact of sulfur-impregnated biochar amendment on microbial communities and mercury methylation in contaminated sediment

S-impregnation of biochar through elemental S streaming is known to increase its sorption performance against Hg and methyl mercury (MeHg). However, the effects of %S-loading on biochar's mechanism and sorption capacities for MeHg, and its consequent impact when used as an amendment material for Hg-contaminated sediments, are poorly understood, and thus, were investigated in this work. Our results showed that a minimum sulfur loading of 1% was the most effective in reducing MeHg levels in sediments. At higher %S-loading (3-20%), the reduction in surface area, pore blockage due to unreacted sulfur particles, and presence of poorly bound sulfur species resulted in lowered effectiveness for MeHg control. Increasing S-functionalization during impregnation shifted the sorption process of MeHg from Hg-O to Hg-S in S-impregnated biochar (BCS). Our 60-day slurry experiment showed a significant reduction in pore water THg (40-70%) and MeHg (30-55%), as well as sediment MeHg (50-60%) in biochar-amended sediments. The reduction in the bioavailable Hg resulted in lowered Hg methylation, as supported by the suppression of both the Fe- and SO₄^2--reduction activities in the amended sediments. The microbial community structure in BCS-amended sediments showed a shift towards sulfur-consuming, iron-reducing, thiosulfate-oxidizing, and sulfate-reducing bacterial populations. At the genus level, the overall relative abundance of principal Hg methylators was also lower in the BCS treatment than in the unamended sediments. This study highlights the application of BCS as a promising strategy for remediation of Hg-contaminated sediments.

Nanoporous biochar with high specific surface area based on rice straw digestion residue for efficient adsorption of mercury ion from water

The unreasonable disposal of residue after anaerobic digestion seriously affects the stability of the ecosystem, and the preparation of adsorbent is an effective way to value-added utilization of the residue. In this study, a high adsorption capacity (209.65 mg/g) biochar-based adsorbent was prepared by hydrothermal carbonization and alkali modification using rice straw biogas residue. The lignocellulosic structure was destroyed after anaerobic digestion, forming porous biochar with larger specific surface area (2372.51 m²/g) and richer pore structure. Besides, the mercury ion complexed on the adsorbent surface in monovalent and divalent forms and possessed favorable selectivity in the presence of other examples of interference. The adsorption process is consistent with pseudo second-order kinetics and the Langmuir isotherm, indicating a predominance of chemisorption. This study provides a methodology for use of rice straw biogas residue and treatment of mercury containing wastewater, which offers a fresh direction for resource utilization of biogas residue.

Hierarchical porous activated carbon from waste Zanthoxylum bungeanum branches by modified H3PO4 activation for toluene removal in air

Activated carbon adsorbents were prepared by chemical activation with waste Zanthoxylum bungeanum branches as raw materials and H₃PO₄/H₂SO₄ as composite activator under different dosages of the auxiliary activator H₂SO₄. The prepared samples were characterized by scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) specific surface area test, Fourier transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD). The adsorption/desorption performances of low concentration toluene in the air were evaluated, and its reusability was evaluated by the adsorption/desorption cycle. Adsorption results were fitted using the quasi-first, quasi-second, and Bangham models. The adsorption properties of activated carbon adsorbent for toluene in the air show a "volcanic-type change trend" with the increase of H₂SO₄ dosage. The toluene adsorption properties of the prepared activated carbon adsorbents from high to low are as follows: BAC02 > BAC05 > BAC01 > BAC10 > BAC00. When the mass fraction of auxiliary activator H₂SO₄ was 2.0%, the adsorption amount of toluene on the prepared BAC02 activated carbon adsorbent increased by 51%, reaching 511 mg/g. After thermal desorption at 200℃, the adsorption performance of toluene was regenerated. The adsorption process of toluene conforms to the quasi-first-order model and Bangham model. The whole adsorption process can be divided into three stages: outer surface adsorption, intra-channel diffusion, and adsorption equilibrium. The addition amount of H₂SO₄ significantly affected the specific surface area, pore volume, and pore size distribution of the prepared activated carbon adsorbent.

Carbon materials in persulfate-based advanced oxidation processes: The roles and construction of active sites

Many researchers have paid more attention to the progress of carbon materials owing to their advantages, such as high activity, low cost, large surface area, high conductivity and high stability. Carbon materials have been widely used in persulfate-based advanced oxidation processes (PS-AOPs), especially for graphene (G), carbon nanotubes (CNTs) and biochar (BC). Various strategies are applied to promote their activity, however, up to now, the relationship between the structures of carbon materials and their activities in PS-AOPs has not been specifically reviewed. The methods to switch reaction pathway (radical and nonradical pathways) in carbon-persulfate-based AOPs have not been systematically explored. Hereon, this review illustrated the active sites of G, CNTs, BC and other carbon materials, and generalized the modification methods to promote the activity of carbon materials and to switch reaction pathway in PS-AOPs. The roles of carbon materials in PS-AOPs were discussed around reactive oxygen species (ROS) and the structures. ROS are frequently complex in AOPs, but main ROS generation is related to the active sites on carbon materials. The structures of carbon materials (e.g., metal-carbon bonds, the electron-deficient C atoms, unbalanced electron distribution and graphitized structures) play a decisive role in the nonradical pathway. Finally, future breakthroughs of carbon materials were proposed for practical engineering and multi-field application.

Synergetic Enhancement of Pb2+ and Zn2+ Adsorption onto Size-Selective Sludge Biochar Portions in Multiple Ion Solution Systems

Particle size, one of the predominant factors that affect the adsorption capacity of biochar, has been widely investigated. However, correlative studies on a coexistence system containing various ions together with differentiated particle sizes are scarce. In this study, samples of municipal solid waste (sludge) biochar (SB) with different particle sizes were separated and examined for the adsorption performance in bi-cation (Pb2+/Zn2+) and multi-ion (Pb2+, Zn2+ and Cl-) systems. The results showed that the adsorption capacity is influenced by both particle size and ion configurations. The effective stabilization ability of a small size group can be attributed to the most non-bioavailable fraction. Meanwhile, the acidic soluble and non-bioavailable fraction of Pb2+/Zn2+ reached more than 90%. The mixed adsorption experiment showed that Pb2+ would compete for the adsorption sites of biochar with Zn2+, and Cl- intervention could improve the adsorption of Pb2+ (2.33-6.93%) and Zn2+ (16.52-18.01%) on biochar. Further, X-ray diffraction spectra and phosphorus concentration dynamics and kinetics simulations revealed that more abundant active sites in the formatted pyromorphite were able to be exposed in the presence of Cl-. The small-size portion of SB therefore exhibited excellent potential for the long-term heavy metal remediation under practical conditions of multi-ion systems in an actual environment.

Modification of naturally abundant resources for remediation of potentially toxic elements: A review

Water and soil contamination due to potentially toxic elements (PTEs) represents a critical threat to the global ecosystem and human health. Naturally abundant resources have significant advantages as adsorbent materials for environmental remediation over manufactured materials such as nanostructured materials and activated carbons. These advantages include cost-effectiveness, eco-friendliness, sustainability, and nontoxicity. In this review, we firstly compare the characteristics of representative adsorbent materials including bentonite, zeolite, biochar, biomass, and effective modification methods that are frequently used to enhance their adsorption capacity and kinetics. Following this, the adsorption pathways and sites are outlined at an atomic level, and an in-depth understanding of the structure-property relationships are provided based on surface functional groups. Finally, the challenges and perspectives of some emerging naturally abundant resources such as lignite are examined. Although both unamended and modified naturally abundant resources face challenges associated with their adsorption performance, cost performance, energy consumption, and secondary pollution, these can be tackled by using advanced techniques such as tailored modification, formulated mixing and reorganization of these materials. Recent studies on adsorbent materials provide a strong foundation for the remediation of PTEs in soil and water. We speculate that the pursuit of effective modification strategies will generate remediation processes of PTEs better suited to a wider variety of practical application conditions.

Artificial intelligence (AI) applications in adsorption of heavy metals using modified biochar

The process of removal of heavy metals is important due to their toxic effects on living organisms and undesirable anthropogenic effects. Conventional methods possess many irreconcilable disadvantages pertaining to cost and efficiency. As a result, the usage of biochar, which is produced as a by-product of biomass pyrolysis, has gained sizable traction in recent times for the removal of heavy metals. This review elucidates some widely recognized harmful heavy metals and their removal using biochar. It also highlights and compares the variety of feedstock available for preparation of biochar, pyrolysis variables involved and efficiency of biochar. Various adsorption kinetics and isotherms are also discussed along with the process of desorption to recycle biochar for reuse as adsorbent. Furthermore, this review elucidates the advancements in remediation of heavy metals using biochar by emphasizing the importance and advantages in the usage of machine learning (ML) and artificial intelligence (AI) for the optimization of adsorption variables and biochar feedstock properties. The usage of AI and ML is cost and time-effective and allows an interdisciplinary approach to remove heavy metals by biochar.

Mesoporous ball-milling iron-loaded biochar for enhanced sorption of reactive red: Performance and mechanisms

In order to solve the low sorption capacity of pristine biochar for anionic pollutants, e.g., reactive red 120 (RR120), a novel mesoporous Fe-biochar composite was fabricated in this study by combination of Fe-loading and ball-milling methods. The ball-milling Fe-biochar composite could effectively remove RR120 by up to 90.1 mg g^-1 at pH of 7.5, and slightly alkaline condition was preferred. Adsorption kinetics showed that ball-milling Fe-biochar composite could quickly sorb RR120 with the rate constant (k₂) of 2.07 g mg^-1 min^-1 (pH = 7.5). Positive surface charge and large surface area were responsible for the outstanding removal performance of RR120 by ball-milling Fe-biochar composite: (1) The adscititious Fe would be converted to β-FeOOH during pyrolysis, which significantly improved the zeta potential of biochar and thus facilitated the electrostatic adsorption for RR120, which contributed to 42.3% and 85.5% at pH of 3 and 7.5, respectively; (2) Ball-milling effectively increased the specific surface area and uniformed the pore size distribution, which could provide more sorption sites and expedite the diffusion of RR120 molecules, shortening the time from several hours to less than 15 min. Findings of this study not only provide a feasible modification method for biochar to adsorb anionic pollutants efficiently and rapidly, but also help to reveal the roles of Fe-loading and ball-milling in enhancing adsorption capacity.

Effect of production temperature and particle size of rice husk biochar on mercury immobilization and erosion prevention of a mercury contaminated soil

Mercury (Hg) contaminated soil is a potential hazardous material especially under soil erosion and surface runoff. This work aims to use rice husk biochar to immobilize Hg and prevent erosion, and find the optimal production temperature and particle size of the biochar. The biochars were produced at 300, 500, and 700 °C and sieved to three particle sizes ~20, < 2, and < 0.15 mm. They were applied to a Hg contaminated loamy sand (20.2 mg/kg) and undergone simulated rainfall erosion representing 7 years of heavy rain events in Beijing. All biochar amendments reduced the runoff volume by 5.1-15.4%. Hg amount in runoff were significantly reduced by 36.7-48.8% after the amendments of biochar. The Hg concentration of infiltration was reduced by biochar treatments except that produced at 300 °C, while its amount was increased due to larger infiltration volume. All biochar amendments significantly reduced soil loss in runoff by 43.5-77.2%. Hg was enriched in the sediments (39.7-46.8 mg/kg) compared with the parent soil (20.2 mg/kg) regardless of biochar treatment, but its bioavailability was low. Higher pyrolysis temperature of the rice husk biochars resulted in less runoff, more infiltration, and better erosion prevention, while the effect of biochar particle size is less significant.

Sulfur-anchored palm shell waste-based activated carbon for ultrahigh sorption of Hg(II) for in-situ groundwater treatment

This study utilized a facile and scalable one-pot wet impregnation method for Hg(II) adsorption to prepare sulfur-anchored palm shell waste activated carbon powder (PSAC-S). The experimental results revealed that the sulfur precursors promote the surface charge on the PSAC and enhance Hg(II) removal via the Na₂S > Na₂S₂O₄ > CH₃CSNH₂ sequence. PSAC-S prepared using Na₂S had significant Hg(II) sorption efficiencies, achieving a maximum sorption capacity of 136 mg g^-1 from the Freundlich model. Compared to PSAC, PSAC-S had an enhancement in Hg(II) sorption behavior for heterogeneous interactions with sulfur. PSAC-S also demonstrated high Hg(II) sorption capacities over a wide range of solution pH, while ionic strength had an insignificant impact on Hg(II) removal efficiencies. Through various spectroscopic analyses, we identified the mechanisms of Hg(II) removal by PSAC-S as electrostatic interactions, Hg-Cl complexation, and precipitation as HgSO₄. Moreover, PSAC-S unveiled high adsorption affinity and Hg(II) stability in actual groundwater (even in µg L^-1 level). These overall results show the potentials of PSAC-S as an alternative, easily scalable material for in-situ Hg(II) remediation.

Metagenomic analysis revealed the microbiota and metabolic function during co-composting of food waste and residual sludge for nitrogen and phosphorus transformation

This paper used bagasse as a composting additive and bulking agent in order to investigate the aerobic composting process of food waste and residual sludge. Accordingly, the variations of nitrogen and phosphorus nutrients, microbiota and metabolic function during the composting process were systematically explored. Three piles with residual sludge, food waste and bagasse mass ratios of 1:1:1, 2:1:1 and 4:1:1 were set. The ammonia nitrogen content in the three compost piles were 3.18 mg/g, 4.68 mg/g and 5.84 mg/g at the end of composting. The final available phosphorus content of the three piles were 3.42 mg/g, 6.70 mg/g and 11.21 mg/g, respectively. X-ray photoelectron spectroscopy (XPS) analysis showed that absorption peaks attributed to amines, amino acids and amides appeared in the 1:1:1 pile. Metagenomic analysis of the glycolysis and ammonia transformation pathways showed that the total relative abundance of key enzyme genes for the conversion of glucose to glucose-6-phosphate in the three plies were 0.326%, 0.213% and 0.248%, respectively. The total relative abundance of 2 glutamate dehydrogenase (GDH2), glud1-2 and E1,4,1,4 dehydrogenases in the three piles was 0.125%, 0.151% and 0.160%, respectively, as the main enzymes for the mutual conversion of ammonia and glutamate.

Laboratory and simulation study on the Cd(Ⅱ) adsorption by lake sediment: Mechanism and influencing factors

Sediments are the major sinks for Cd(Ⅱ) in the aquatic environment. Here, the detailed binding mechanisms and effects of environmental factors on Cd(Ⅱ) adsorption onto lake sediment were tested by a batch of adsorption and characteristic experiments. Sediment samples and sediment-Cd complexes were characterized using Scanning electron microscopy, Energy dispersive spectroscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffraction spectral analyses. The interactive and main effect of parameters such as pH, flow velocity, Cd(II) concentration, sediment particle size, humic acid, fulvic acid and adsorption time involved in the adsorption process were determined using two models based on response surface methodology (RSM) and a back-propagation neural network with genetic algorithm (GABP). Results showed that Cd(II) adsorption onto sediment was mainly achieved through surface complexation with O-containing groups and precipitation with carbonate and sulfide. RSM was favorable for modeling Cd(II) adsorption in lake systems because it intuitively reflected the influence of the factors and had a good fitting precision (R² = 0.8838, RSME = 2.5496) close to that of the GABP model (R² = 0.8959, RSME = 2.5410). pH, sediment particle size, and humic acid exerted strong influences on Cd(II) immobilized by the sediment. Overall, our findings facilitate a better understanding of Cd(II) mobility in lakes and provide a reference for controlling heavy metals derived from both aqueous and sediment sources.

Chitosan crosslinked with polyamine-co-melamine for adsorption of Hg2+: Application in purification of polluted water

A batch experiment was carried out in order to remove Hg²⁺ from the aqueous solution as well as the polluted water using modified chitosan (CS) with polyamine compounds (triethylenetetramine (TETA), tetraethylenepentamine (TEPA)), and melamine. The obtained polyamine-co-melamine crosslinked CS derivatives (MCS-4N and MCS-5N) were characterized and used as adsorbents. In comparison to the raw CS, the modification significantly promoted the adsorption of Hg²⁺ ions. The results of the pseudo-second-order kinetic model revealed that pH-dependent derivatives adsorbents achieved the equilibrium state within 12 h. The Langmuir model was best fitted with the Hg²⁺ adsorption isotherm and showed the highest adsorption capacities of 140.3 and 109.7 mg/g for MCS-4N and MCS-5N, respectively. A slight decrease in the adsorption efficiency of Hg²⁺ was noticed with the increment of the ionic strength of the solution. However, the studied adsorbents were easily regenerated and presented adequate reusability. The Hg²⁺ adsorption was regulated by the combined process of coordination reaction and electrostatic attraction as well. The as-prepared polyamine-co-melamine crosslinked CS derivatives were found potential adsorbents for the adsorptive capture of Hg²⁺ ions from aqueous solutions and polluted waters.

Efficient removal of Hg2+ from aqueous solution by a novel composite of nano humboldtine decorated almandine (NHDA): Ion exchange, reducing-oxidation and adsorption

Efficient removal of Hg²⁺ from aqueous solution is key for environmental protection and human health. Herein, a novel composite of nano humboldtine decorated almandine was synthesized from almandine for the removal of Hg²⁺. Results showed that the Hg²⁺ removal process followed pseudo-second-order kinetic model and Langmuir equation, and the maximum adsorption capacity was 575.17 mg/g. Furthermore, Hg²⁺ removal by the composite was pH-dependent and low pH value facilitated the removal of Hg²⁺. SEM and HADDF-STEM results suggested a new rod morphology was generated and the adsorbed mercury was mainly enriched into this structure after reaction with Hg²⁺ solution. The removal mechanisms of Hg²⁺ by the composite was pH dependent, and included ion exchange, surface complexation, reduction and oxidation. Our results demonstrated that the composite was an ideal material for Hg²⁺ removal and the transformation ways of mercury related species could be a significant but currently underestimated pathway in natural and engineered systems.

Co-existing TiO2 nanoparticles influencing adsorption/ desorption of tetracycline on magnetically modified kaolin

Interaction of coexisting nanoparticles (NPs) and other pollutants may affect their behavior in the environment. In this study, we investigated the effects of TiO₂ NPs on the adsorption and desorption of tetracycline (TC) by magnetized kaolin (MK). The interactions among TC, TiO₂ NPs, and MK were then discussed through their morphology and characteristics by using scanning electron microscopy, X-ray energy dispersive spectrometry, Transmission electron microscope, X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy analyses. Results showed that TiO₂ NPs increased TC adsorption on MK by 2.02% and increased TC desorption by 45.26%, TC increased the maximum adsorption of TiO₂ on MK from 31.32 to 49.42 mg g^-1 and decreased the amount of stable adsorption state TiO₂ NPs on MK from 17.92 to 12.71 mg g^-1. The characterization results demonstrated that TC molecules combined on MK though hydrogen bonding, π-π bonding and hydrophobic interaction. The adsorption of TiO₂ NPs on MK can provide additional hydrogen-bonding sites for TC adsorption by increasing the number of hydroxyl groups. However, TC can decrease the electrostatic attraction sites for TiO₂ NPs adsorption on the MK surface. The complexation of TC and TiO₂ NPs weakened the electrical attraction between MK and TiO₂ NPs and decreased the amount of stable adsorption state TiO₂ NPs.

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.

Effect of excess sludge and food waste feeding ratio on the nutrient fractions, and bacterial and fungal community during aerobic co-composting

The effect of excess sludge and food waste feeding ratio on the co-composting process was explored using 5% bagasse biochar as an additive and conditioner. Results showed that when the mass ratio was 1:1, nitrogen fixation ability was the strongest and ammonia nitrogen increment in the pile reached 2.31 mg/g. The increase in excess sludge content/food waste ratio during composting was conducive to the accumulation of H₂O-P, BD-P, HCl-P, NaOH-P and NaOH₈₅-P. When the ratio of excess sludge to food waste mass was 1:1, the relative abundance of Firmicutes was the largest in the compost, which corresponded to 72.77% at the phylum level. Food waste mass was more beneficial to the growth and reproduction of microorganisms and to the metabolic activities related to membrane transport. Considering the fungal content, Ascomycota and Basidiomycota were maximum, with relative abundance of 69.53% and 20.91%, respectively, at the mass ratio of 1:1.

Application of biochar prepared from ethanol refinery by-products for Hg stabilization in floodplain soil: Impacts of drying and rewetting

This study evaluated three biochars derived from bioenergy by-products - manure-based anaerobic digestate (DIG), distillers' grains (DIS), and a mixture thereof (75G25S) - as amendments to stabilize Hg in contaminated floodplain soil under long-term saturated (up to 200 d) and cyclic drying and rewetting conditions. Greater total Hg (THg) removal (72 to nearly 100%) and limited MeHg production (<65 ng L^-1) were observed in digestate-based biochar-amended systems under initial saturated conditions. Drying and rewetting resulted in limited THg release, increased aqueous MeHg, and decreased solid MeHg in digestate-based biochar-amended systems. Changes in Fe and S chemistry as well as microbial communities during drying and rewetting potentially affected MeHg production. Digestate-based biochars may be more effective as amendments to control Hg release and minimize MeHg production in floodplain soils under long-term saturated and drying and rewetting conditions compared to distillers' grains biochar.

Exploration of the potential capacity of fly ash and bottom ash derived from wood pellet-based thermal power plant for heavy metal removal

This study was conducted to explore the potential capacity for the removal of heavy metals from the fly ash (FA) and bottom ash (BA) emitted by wood pellet thermal power plants. Fly ash consists of inorganic compounds such as CaSiO₃, P₂O₅, and K₂O, whereas BA shows properties very similar to the biochar derived from organic biomass. The adsorption properties of both FA and BA for Cd were described well by the Langmuir and pseudo-second-order models, and the maximum adsorption capacity of FA for Cd was 4.2 times higher than that of BA. The results indicate that FA can be applied to the treatment of wastewater that contains heavy metals over pH range from 2-6; however, BA is considered to be most effective for application with wastewater that contains heavy metals at a pH of 5-6. Study of the mechanism concluded that the adsorption of Cd by FA is dominated by the formation of Cd₂SiO₄ complexes by chemical reactions between CaSiO₃ and Cd ions as well as via the precipitation of Cd(OH)₂ in the neutral and alkaline solutions that is caused by the dissolution of K. It was found that the adsorption of Cd by BA was influenced by the binding of functional groups (CC and COH), coupled CaCO₃ dissolution-CdCO₃ precipitation reaction and ion exchange between some minerals with Si and Cd ions in weakly acidic conditions. Results indicate that the FA and BA emitted from wood pellet thermal power plants have high potential for heavy metal removal, and their practical use in the purification and restoration of heavy metals could be an effective way to reduce the waste produced by power plants and clean the environment.

Enhanced heavy metal removal from an aqueous environment using an eco-friendly and sustainable adsorbent

Thiol-lignocellulose sodium bentonite (TLSB) nanocomposites can effectively remove heavy metals from aqueous solutions. TLSB was formed by using -SH group-modified lignocellulose as a raw material, which was intercalated into the interlayers of hierarchical sodium bentonite. Characterization of TLSB was then performed with BET, FTIR, XRD, TGA, PZC, SEM, and TEM analyses. The results indicated that thiol-lignocellulose molecules may have different influences on the physicochemical properties of sodium bentonite, and an intercalated-exfoliated structure was successfully formed. The TLSB nanocomposite was subsequently investigated to validate its adsorption and desorption capacities for the zinc subgroup ions Zn(II), Cd(II) and Hg(II). The optimum adsorption parameters were determined based on the TLSB nanocomposite dosage, concentration of zinc subgroup ions, solution pH, adsorption temperature and adsorption time. The results revealed that the maximum adsorption capacity onto TLSB was 357.29 mg/g for Zn(II), 458.32 mg/g for Cd(II) and 208.12 mg/g for Hg(II). The adsorption kinetics were explained by the pseudo-second-order model, and the adsorption isotherm conformed to the Langmuir model, implying that the dominant chemical adsorption mechanism on TLSB is monolayer coverage. Thermodynamic studies suggested that the adsorption is spontaneous and endothermic. Desorption and regeneration experiments revealed that TLSB could be desorbed with HCl to recover Zn(II) and Cd(II) and with HNO3 to recover Hg(II) after several consecutive adsorption/desorption cycles. The adsorption mechanism was investigated through FTIR, EDX and SEM, which demonstrated that the introduction of thiol groups improved the adsorption capacity. All of these results suggested that TLSB is an eco-friendly and sustainable adsorbent for the extraction of Zn(II), Cd(II) and Hg(II) ions in aqueous media.

Adsorption Mechanisms and Characteristics of Hg2+ Removal by Different Fractions of Biochar

The adsorption mechanisms of mercury ion (Hg2+) by different fractions of biochar were studied, providing a theoretical basis and practical value for the use of biochar to remediate mercury contamination in water. Biochar (RC) was prepared using corn straw as the raw material. It was then fractionated, resulting in inorganic carbon (IC), organic carbon (OC), hydroxyl-blocked carbon (BHC), and carboxyl-blocked carbon (BCC). Before and after Hg2+ adsorption, the biochar fractions were characterized by several techniques, such as energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Obtained results indicate that the reaction mechanisms of RC for Hg2+ removal mainly include electrostatic adsorption, ion exchange, reduction, precipitation, and complexation. The equilibrium adsorption capacity of RC for Hg2+ is 75.56 mg/g, and the adsorption contribution rates of IC and OC are approximately 22.4% and 77.6%, respectively. Despite the lower rate, IC shows the largest adsorption capacity, of 92.63 mg/g. This is attributed to all the mechanisms involved in Hg2+ adsorption by IC, with ion exchange being the main reaction mechanism (accounting for 39.8%). The main adsorption mechanism of OC is the complexation of carboxyl and hydroxyl groups with Hg2+, accounting for 71.6% of the total OC contribution. BHC and BCC adsorb mercury mainly via the reduction–adsorption mechanism, accounting for 54.6% and 54.5%, respectively. Among all the adsorption mechanisms, the complexation reaction of carboxyl and hydroxyl groups with Hg2+ is the dominant effect.

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.

The mechanism of cadmium sorption by sulphur-modified wheat straw biochar and its application cadmium-contaminated soil

Cadmium (Cd) pollution in soils has received considerable research attention globally, and sulphur-modified biochar (SBC) could combine the advantages of biochar and the sulphur element for Cd remediation. Biochar from agricultural waste is feasible, which has a low preparation cost. However, there are few studies regarding the effects of the sulphur modification of biochar on the Cd immobilization mechanism. This study aimed to research the Cd immobilization mechanism of pristine wheat straw biochar (BC) and sulphur-modified biochar (SBC), and the Cd immobilization effects of BC and SBC in Cd-contaminated soils. Elemental and SEM analysis confirmed that sulphur was successfully loaded on the pristine biochar. XPS analysis confirmed that there was a considerable discrepancy between adsorption mechanisms of Cd on BC and SBC. In particular, cadmium sorption on BC was due to Cd(OH)₂ and CdCO₃ precipitation formation and interaction with carbonyl and carboxyl groups, whereas on SBC, sorption was mainly due to CdS and CdHS⁺ formation and interaction with organic sulphide. In the incubation experiment, SBC and BC additions increased pH value and also reduced the available Cd concentrations in the soil. Compared with the control, the contents of available Cd in soil were significantly decreased by 15.86% ~ 22.10% and 22.72% ~ 27.90%, following treatments with BC and SBC, respectively.