Remediation of arsenic-contaminated paddy soil by iron-modified biochar

Reducing arsenic mobilization in sediments: A synergistic effect of oxidation and adsorption with zirconium-manganese binary metal oxides

Remediation of arsenic (As)-contaminated sediments is challenging, due to surface sediment often being subjected to hypoxic/anoxic conditions where As(Ⅲ) is the dominant species. In this study, a novel capping material comprising zirconium-manganese binary oxides (ZMBO) was synthesized and its feasibility in controlling sedimentary As release investigated using high-resolution sampling, X-ray absorption near edge structure (XANES) spectroscopy, and scanning electron microscopy (SEM) techniques. Results showed ZMBO exhibited both high oxidation efficiency (94 %) and strong adsorption capacity (151.8 mg As/g) for As(Ⅲ). Capping As-contaminated sediments with ZMBO resulted in a negative diffusive flux of -0.08 ng/cm2/s, effectively maintaining low As levels in the overlying water over 150 days. XANES spectra showed As in surface sediments existed predominantly As(V), consistent with high-resolution data indicating ∼90 % of labile As(Ⅲ) was oxidized and adsorbed by ZMBO. Furthermore, ZMBO also promoted Fe(Ⅱ) oxidation to stable hematite in sediments, providing additional adsorption sites for As. By comparing with current capping materials, ZMBO exhibited a balanced performance in terms of its cost-effectiveness, adsorption capacity, remediation effects, and environmental adaptability. This study highlights the potential of ZMBO as a promising capping material for remediating As-contaminated sediments through combined chemical oxidation and adsorption mechanisms, offering sustainable solutions for improving water quality management worldwide.

Geochemistry of arsenic in soils with a focus on calcareous soils: control strategies and perspectives

Arsenic (As) contamination has become a significant environmental challenge due to the global expansion of industrial, agricultural, and mining activities, which contribute to the contamination of water, air, soils, and biota with As and other metals and metalloids. This review elucidates the geochemical behavior of As in soils, focusing on the factors influencing its dynamics and the effectiveness of various remediation techniques, particularly in calcareous soils. Calcareous soils, characterized by their unique properties, exhibit intricate interactions with As, necessitating a deeper understanding of the mechanisms driving these processes. Compared to other soil types, the bioavailability of As in calcareous soils is generally lower, largely due to their elevated pH and the presence of calcium carbonate (CaCO3). These factors contribute to the enhanced adsorption of As by soil organic and mineral components, forming less soluble As-CaCO3 complexes and decreasing As solubility. Despite this, research on As geochemistry in calcareous soils and the development of effective removal techniques still needs to be completed, emphasizing the need for further study. Additionally, this review explores future research directions in the context of As contamination and remediation, integrating case studies and advanced technologies to highlight innovative approaches for mitigating As contamination in calcareous soils.

Potentials of urban waste derived biochar in minimizing heavy metal bioavailability: A techno-economic review

Contamination of heavy metals (HMs) in agroecosystem presented an additional dimension of complexity along with the adverse consequences of climate change for the scientific fraternity. The increasing population and urbanization on the other hand are regarded as the main sources of urban waste (UW). Holistic utilization of UW-derived biochar (BC) has shown potential to be utilized as a source of soil supplement in the agriculture sector, fulfilling several sustainable development goals (SDGs). An attempt has been made to evaluate the techno-economical prospect, efficacy of UW-BC in remediation HMs from SDGs and circular bio-economy prospective. Current review has highlighted that biochar, when amended alone/in combination, enhances HMs remediation potential. Economic analysis of UW-BC reinforces its viability as a sustainable solution for waste management. Consequently, the application of UW-BC has the potential to contribute significantly to the achievement of multiple SDGs, warranting further research and increased investment in this field.

A review on As-contaminated soil remediation using waste biomass feedstock-based biochar and metal-modified biochar

Arsenic (As) is a carcinogen that threatens ecosystems and human health. Due to its high adsorption, and microporosity, biochar is widely available for soil remediation. This review significantly summarizes the current status of waste biomass feedstock-based biochar and metal-modified biochar for As-contaminated soil remediation. Firstly, this paper briefly describes the sources and hazards of As in soil, and secondly, lists eleven feedstocks for preparing biochar. Agricultural, domestic, and forestry wastes provide a plentiful source for biochar preparation. Single or multi-metal modifications such as iron (Fe), manganese (Mn), and cerium (Ce) can effectively improve the Arsenite [As(III)] and arsenate [As(V)] adsorption capacity of biochar. The primary mechanisms of As removal by waste biomass feedstock-based biochar and metal-modified biochar include ion exchange, electrostatic attraction, surface complexation, redox transformation, and H-bond formation. In conclusion, this review presents an in-depth discussion on both waste biomass feedstocks and metal modification, providing constructive suggestions for the future development of biochar to remediate As-contaminated soil.

Advancing modified biochar for sustainable agriculture: a comprehensive review on characterization, analysis, and soil performance

Biochar is a carbon-rich material produced through the pyrolysis of various feedstocks. It can be further modified to enhance its properties and is referred to as modified biochar (MB). The research interest in MB application in soil has been on the surge over the past decade. However, the potential benefits of MB are considerable, and its efficiency can be subject to various influencing factors. For instance, unknown physicochemical characteristics, outdated analytical techniques, and a limited understanding of soil factors that could impact its effectiveness after application. This paper reviewed the recent literature pertaining to MB and its evolved physicochemical characteristics to provide a comprehensive understanding beyond synthesis techniques. These include surface area, porosity, alkalinity, pH, elemental composition, and functional groups. Furthermore, it explored innovative analytical methods for characterizing these properties and evaluating their effectiveness in soil applications. In addition to exploring the potential benefits and limitations of utilizing MB as a soil amendment, this article delved into the soil factors that influence its efficacy, along with the latest research findings and advancements in MB technology. Overall, this study will facilitate the synthesis of current knowledge and the identification of gaps in our understanding of MB.

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Optimizing the dual role of biochar for phosphorus availability and arsenic immobilization in soils

Soil Phosphorus (P) fixation and Arsenic (As) contamination pose significant challenges to agriculture and environmental health. Biochar has emerged as a promising soil amendment capable of enhancing P availability while immobilizing As. This review explored the mechanisms by which biochar influences P dynamics and As sequestration. Biochar enhances P availability by reducing fixation, stimulating P-solubilizing microorganisms, and gradually releasing the adsorbed P. Specific biochars, such as Mg-modified and La-modified types, demonstrate high P adsorption capacities, reaching up to 263 mg/g, while cerium and iron-modified biochars show As adsorption efficiencies up to 99 % under certain conditions. Biochar's surface functional groups are essential for P and As adsorption through mechanisms such as surface adsorption, ligand exchange, and inner-sphere complexation. The competitive adsorption between P and As is influenced by pH, biochar modification, and co-existing anions. Under acidic conditions, As shows a higher affinity for biochar, forming stable complexes with metal oxides like iron and aluminum. Biochars modified with calcium, magnesium, lanthanum, zinc, cerium, and iron demonstrate enhanced adsorption capacities. In neutral to alkaline conditions, calcium- and magnesium-modified biochars benefit P retention, while iron-modified biochar is preferable for As adsorption. Additionally, biochar promotes microbial activity and enzymatic processes that facilitate As transformation and P mineralization, enhancing overall soil health. These findings underscore biochar's dual role in increasing nutrient availability and reducing contaminant risks, making it a valuable tool for sustainable agriculture. Field-scale applications should be prioritized in future research to optimize biochar's impact on soil fertility and environmental remediation.

Migration, transformation of arsenic, and pollution controlling strategies in paddy soil-rice system: A comprehensive review

Arsenic pollution in paddy fields has become a public concern by seriously threatening rice growth, food security and human health. In this review, we delve into the biogeochemical behaviors of arsenic in paddy soil-rice system, systemically revealing the complexity of its migration and transformation processes, including the release of arsenic from soil to porewater, uptake and translocation of arsenic by rice plants, as well as transformation of arsenic species mediated by microorganism. Especially, microbial processes like reduction, oxidation and methylation of arsenic, and the coupling of arsenic with carbon, iron, sulfur, nitrogen cycling through microbes and related mechanisms were highlighted. Environmental factors like pH, redox potential, organic matter, minerals, nutrient elements, microorganisms and periphyton significantly influence these processes through different pathways, which are discussed in this review. Furthermore, the current progress in remediation strategies, including agricultural interventions, passivation, phytoremediation and microbial remediation is explored, and their potential and limitations are analyzed to address the gaps. This review offers comprehensive perspectives on the complicated behaviors of arsenic and influence factors in paddy soil-rice system, and provides a scientific basis for developing effective arsenic pollution control strategies.

Ball milling nano-sized biochar: bibliometrics, preparation, and environmental application

Nano-sized biochar, which is a small structure prepared from biochar by grinding, has surpassed traditional biochar in performance, showing enhanced effects and potential for a wide range of environmental applications. Firstly, this paper visualizes and analyzes the literature in this field by CiteSpace to clarify the development trend of nano-sized biochar. The review intuitively shows the most influential countries, the most productive institutions, and the most concerned hot spots in the field of nano-sized biochar. Secondly, these hotspots in environment management are summarized by keywords and clustering: (1) The application of ball milling is a modification scheme that researchers have paid attention to, and it is also a key method for preparing biochar nanomaterials. It has a more dispersed structure and can support more modified materials. (2) Nano-sized biochar in the comprehensive utilization of water, soil, and plants was discussed and is a small range of application modification methods. (3) The bidirectional effects of nano-sized biochar on plants were analyzed, and the challenges in its application were listed. Finally, the economic management of nano-sized biochar and the relationship between microorganisms are the focus of the next research.

Colloidal fraction on pomelo peel-derived biochar plays a dual role as electron shuttle and adsorbent in controlling arsenic transformation in anoxic paddy soil

Arsenic release and reduction in anoxic environments can be mitigated or facilitated by biochar amendment. However, the key fractions in biochars and how they control arsenic transformation remain poorly understood. In this study, a biochar produced from pomelo peel was rich in colloids and was used to evaluate the roles of the colloidal and residual fractions of biochar in arsenic transformation in anoxic paddy soil. Bulk biochar showed a markedly higher maximum adsorption capacity for As(III) at 1732 mg/kg than for As(V) at 75.7 mg/kg, mainly because of the colloidal fraction on the surface. When compared with the control and treatments with the colloidal/residual fraction, the addition of bulk biochar facilitated As(V) reduction and release in the soil during days 0-12, but decreased the dissolved As(III) concentration during days 12-20. The colloidal fraction revealed significantly higher electron donating capacity (8.26 μmole-/g) than that of bulk biochar (0.88 μmole-/g) and residual fraction (0.65 μmole-/g), acting as electron shuttle to promote As(V) reduction. Because the colloidal fraction was rich in aliphatic carbon, fulvic acid-like compounds, potassium, and calcium, it favored As(III) adsorption when more As(III) was released, probably via organic-cation-As(III) complexation. These findings provide deeper insight into the role of the colloidal fraction of biochar in controlling anaerobic arsenic transformation, which will be helpful for the practical application of biochar in arsenic-contaminated environments.

Sb/As immobilization and soil function improvement under the combined remediation strategy of modified biochar and Sb-oxidizing bacteria at a smelting site

Antimony (Sb) and arsenic (As) lead to soil pollution and structural degradation at Sb smelting sites. However, most sites focus solely on Sb/As immobilization, neglecting the restoration of soil functionality. Here, we investigated the effectiveness of Fe/H2O2 modified biochar (Fe@H2O2-BC) and Sb-oxidizing bacteria (Bacillus sp. S3) in immobilizing Sb/As and enhancing soil functional resilience at an Sb smelting site. Over a twelve-month period, the leaching toxicity of As and Sb was reduced to 0.05 and 0.005 mg L-1 (GB3838-2002) respectively, with 1% (w/w) Fe@H2O2-BC and 2% (v/v) Bacillus sp. S3 solution. Compared to CK, the combination of Fe@H2O2-BC and Bacillus sp. S3 significantly reduced the bioavailable As/Sb by 98.00%/93.52%, whilst increasing residual As and reducible Sb fractions by 210.31% and 96.51%, respectively. The combined application generally improved soil aggregate structure, pore characteristics, and water-holding capacity. Fe@H2O2-BC served as a pH buffer and long-term reservoir of organic carbon, changing the availability of carbon substrates to bacteria. The inoculation of Bacillus sp. S3 facilitated the transformation of Sb(III)/As(III) to Sb(V)/As(V) and differentiated the composition and functional roles of bacterial communities in soils. The combination increased the abundance of soil saprotrophs by 164.20%, whilst improving the relative abundance of N- and S-cycling bacteria according to FUNGuild and FAPROTAX analysis. These results revealed that the integrated application was instrumental in As/Sb detoxification/immobilization and soil function restoration, which demonstrating a promising microbially-driven ecological restoration strategy at Sb smelting sites.

Iron-modified biochar effectively mitigates arsenic-cadmium pollution in paddy fields: A meta-analysis

The escalating problem of compound arsenic (As) and cadmium (Cd) contamination in agricultural soils necessitates the urgency for effective remediation strategies. This is compounded by the opposing geochemical behaviors of As and Cd in soil, and the efficacy of biochar treatment remains unclear. This pioneering study integrated 3780 observation pairs referred from 92 peer-reviewed articles to investigate the impact of iron-modified biochar on As and Cd responses across diverse soil environments. Regarding the treatments, 1) biochar significantly decreased the exchangeable and acid-soluble fraction of As (AsF1, 20.9%) and Cd (CdF1, 24.0%) in paddy fields; 2) iron-modified biochar significantly decreased AsF1 (32.0%) and CdF1 (27.4%); 3) iron-modified biochar in paddy fields contributed to the morphological changes in As and Cd, mainly characterized by a decrease in AsF1 (36.5%) and CdF1 (36.3%) and an increase in the reducible fraction of As (19.7%) and Cd (39.2%); and 4) iron-modified biochar in paddy fields increased As (43.1%) and Cd (53.7%) concentrations in the iron plaque on root surfaces. We conclude that iron-modified biochar treatment of paddy fields is promising in remediating As and Cd contamination by promoting the formation of iron plaque.

Systematic evaluation of methods for iron-impregnation of biochar and effects on arsenic in flooded soils

There is a need for innovative strategies to decrease the mobility of metal(loids) including arsenic (As) and cadmium (Cd) in agricultural soils, including rice paddies, so as to minimize dietary exposure to these toxic elements. Iron (Fe)-modified biochars (FBCs) are used to immobilize As and Cd in soil-water systems, but there is a lack of clarity on optimal methods for preparing FBCs because there are only limited studies that directly compare BCs impregnated with Fe under different conditions. There is also a lack of information on the long-term performance of FBCs in flooded soil environments, where reductive dissolution of Fe (oxy)hydroxide phases loaded onto biochar surfaces may decrease the effectiveness of FBCs. This study uses material characterization methods including FTIR, SEM-EDX, BET, and adsorption isotherm experiments to investigate the effects of Fe-impregnation methods (pH, pyrolysis sequence, and sonication) on the morphology and mineralogy of Fe loaded onto the biochar surface, and to FBC adsorbent properties for arsenate (As(V)), arsenite (As(III)), and Cd. Acidic impregnation conditions favored the adsorption of As(III) onto amorphous Fe phases that were evenly distributed on the biochar surface, including within the biochar pore structure. The combination of sonication with acidic Fe-impregnation conditions led to the best adsorption capacities for As(V) and As(III) (4830 and 11,166 μg As g-1 biochar, respectively). Alkaline Fe-impregnation conditions led to the highest Cd adsorption capacity of 3054 μg Cd g-1 biochar, but had poor effectiveness as an As adsorbent. Amending soil with 5% (w/w) of an acid-impregnated and sonicated FBC was more effective than an alkaline-impregnated FBC or ferrihydrite in decreasing porewater As concentrations. The acid-impregnated FBC also had greater longevity, decreasing As by 54% and 56% in two flooded phases, probably due to the greater stability of Fe(III) within the biochar pore structure that may have a direct chemical bond to the biochar surface. This study demonstrates that FBCs can be designed with selectivity towards different As species or Cd and that they can maintain their effectiveness under anaerobic soil conditions. This is the first study to systematically test how impregnation conditions affect the stability of FBCs in soils under multiple drying-rewetting cycles.

Impact of iron-modified biochars on soil nitrous oxide emissions: Variations with iron salts and soil fertility

Nitrous oxide (N2O) emissions from soils are a significant environmental concern due to their contribution to greenhouse gas emissions. Biochar has been considered as a promising soil amendment for its potential to influence soil processes. Iron modification of biochar has been extensively discussed for its ability to enhance adsorption of pollutants, yet its impact on mitigating soil N2O emissions remains poorly understood. In the present study, corn straw (CB) and wood (WB) biochars were treated with FeSO4/FeCl3 (SCB and SWB) and Fe(NO3)3 (NCB and NWB). The effects of these biochars on soil N2O emissions were investigated using soils with varying fertility levels over a 35-day incubation period at 20 °C. Results revealed significant variations in biochar surface chemistry depending on biochar feedstock and iron salts. Compared to pristine biochars, NWB and NCB exhibited higher pH, total N content, and dissolved NO3-N concentrations (246 ± 17 and 298 ± 35 mg kg-1, respectively), but lower bulk and surface C content. In contrast, SWB and SCB demonstrated acidic pH and elevated dissolved NH4-N concentrations (5.38 ± 0.43 and 4.19 ± 0.22 mg kg-1, respectively). In forest soils, NWB and NCB increased cumulative N2O emission by 28.5% and 67.0%, respectively, likely due to the introduction of mineral nitrogen evidenced by significant positive correlation with NO3-N or NH4-N. Conversely, SWB and SCB reduced emissions in the same soil by 28.5% and 6.9%, respectively. In agricultural soil, most biochars, except SWB, enhanced N2O emissions, possibly through the release of labile organic carbon facilitating denitrification. These findings underscore the significance of changes in biochar surface chemistry and the associated potential risk in triggering soil N2O emissions. This study highlights the need for a balanced design of biochar that considers both engineering benefits and climate change mitigation.

Rapid immobilization of arsenic in contaminated soils by microwave irradiation combined with magnetic biochar: Microwave-induced electron transfer for oxidation and immobilization of arsenic (III)

Biochar with adjustable redox activity is an effective strategy for immobilization of excess arsenic (As(III)) contaminated soil. However, biochar exhibits limitations in terms of electron transfer efficiency and immobilization efficiency due to insufficient activation energy. In this study, As(III) in the soil was rapidly immobilized by adding magnetic biochar (Fe-BC) and introducing microwave irradiation energy to enhance electron transport efficiency. The results showed that the pore structure and iron species (ZVI, Fe3O4) loaded onto the biochar could be modulated by controlling the temperature and time of microwave pyrolysis, which enhanced the microwave absorption capacity and the immobilization performance of As. After adding Fe-BC (10 wt%) and treating with microwave irradiation for 3 h, the content of As(III) in the soil was reduced to 54.56 %. Compared with the conventional heating treatment, the percentage of stabilized As (residual form) increased by 11.21 %. The localized hot spots formed through the absorption of microwave energy by biochar promote the formation of arsenic-containing mineral crystals (FeAsO4 and Fe3AsO7), thus enhancing the immobilization efficiency. In addition, microwave-induced electron transfer facilitated the oxidation of As(III) to As(V) by surface quinone and carbonyl groups on the Fe-BC. Density functional theory calculation further proved that the surface groups of the Fe-BC had a stronger electron-withdrawing ability under microwave irradiation, thereby promoting the adsorption and immobilization of As(III). This work provided a new perspective on the technology of rapid remediation of heavy metals contaminated soil using biochar.

Evaluation of iron-modified biochar on arsenic accumulation by rice: a pathway to assess human health risk from cooked rice

Arsenic (As) contamination of rice grain poses a serious threat to human health. Therefore, it is crucial to reduce the bioavailability of As in the soil and its accumulation in rice grains to ensure the safety of food and human health. In this study, mango (Mangifera indica) leaf-derived biochars (MBC) were synthesized and modified with iron (Fe) to produce FeMBC. In this study, 0.5 and 1% (w/w) doses of MBC and FeMBC were used. The results showed that 1% FeMBC enhanced the percentage of filled grains/panicle and biomass yield by 17 and 27%, respectively, compared to the control. The application of 0.5 and 1% FeMBC significantly (p < 0.05) reduced bioavailable soil As concentration by 33 and 48%, respectively, in comparison to the control. The even higher As flux in the control group as compared to the biochar-treated groups indicates the lower As availability to biochar-treated rice plant. The concentration of As in rice grains was reduced by 6 and 31% in 1% MBC and 1% FeMBC, respectively, compared to the control. The reduction in As concentration in rice grain under 1% FeMBC was more pronounced due to reduced bioavailability of As and enhanced formation of Fe-plaque. This may restrict the entry of As through the rice plant. The concentrations of micronutrients (such as Fe, Zn, Se, and Mn) in brown rice were also improved after the application of both MBC and FeMBC in comparison to the control. This study indicates that the consumption of parboiled rice reduces the health risk associated with As compared to cooked sunned rice. It emphasizes that 1% MBC and 1% FeMBC have great potential to decrease the uptake of As in rice grains.

The Molecular Mechanism of the Response of Rice to Arsenic Stress and Effective Strategies to Reduce the Accumulation of Arsenic in Grain

Rice (Oryza sativa L.) is the staple food for more than 50% of the world's population. Owing to its growth characteristics, rice has more than 10-fold the ability to enrich the carcinogen arsenic (As) than other crops, which seriously affects world food security. The consumption of rice is one of the primary ways for humans to intake As, and it endangers human health. Effective measures to control As pollution need to be studied and promoted. Currently, there have been many studies on reducing the accumulation of As in rice. They are generally divided into agronomic practices and biotechnological approaches, but simultaneously, the problem of using the same measures to obtain the opposite results may be due to the different species of As or soil environments. There is a lack of systematic discussion on measures to reduce As in rice based on its mechanism of action. Therefore, an in-depth understanding of the molecular mechanism of the accumulation of As in rice could result in accurate measures to reduce the content of As based on local conditions. Different species of As have different toxicity and metabolic pathways. This review comprehensively summarizes and reviews the molecular mechanisms of toxicity, absorption, transport and redistribution of different species of As in rice in recent years, and the agronomic measures to effectively reduce the accumulation of As in rice and the genetic resources that can be used to breed for rice that only accumulates low levels of As. The goal of this review is to provide theoretical support for the prevention and control of As pollution in rice, facilitate the creation of new types of germplasm aiming to develop without arsenic accumulation or within an acceptable limit to prevent the health consequences associated with heavy metal As as described here.

How Fe-bearing materials affect soil arsenic bioavailability to rice: A meta-analysis

Arsenic (As) contamination is widespread in soil and poses a threat to agricultural products and human health due to its high susceptibility to absorption by rice. Fe-bearing materials (Fe-Mat) display significant potential for reducing As bioavailability in soil and bioaccumulation in rice. However, the remediation effect of various Fe-Mat is often inconsistent, and the response to diverse environmental factors is ambiguous. Here, we conducted a meta-analysis to quantitatively assess the effects of As in soils, rice roots, and grains based on 673, 321, and 305 individual observations from 67 peer-reviewed articles, respectively. On average, Fe-Mat reduced As bioavailability in soils, rice roots, and grains by 28.74 %, 33.48 %, and 44.61 %, respectively. According to the analysis of influencing factors, the remediation efficiency of Fe-Mat on As-contaminated soil was significantly enhanced with increasing Fe content in the material, in which the industry byproduct was the most effective in soils (-42.31 %) and rice roots (-44.57 %), while Fe-biochar was superior in rice grains (-54.62 %). The efficiency of Fe-Mat in minimizing soil As mobility was negatively correlated with soil Fe content, CEC, and pH. In addition, applying Fe-Mat in alkaline soils with higher silt, lower clay and available P was more effective in reducing As in rice grains. A higher efficiency of applying Fe-Mat under continuous flooding conditions (27.39 %) compared with alternate wetting and drying conditions (23.66 %) was also identified. Our results offer an important reference for the development of remediation strategies and methods for various As-contaminated paddy soils.

Using Fe/H2O2-modified biochar to realize field-scale Sb/As stabilization and soil structure improvement in an Sb smelting site

Antimony (Sb) and arsenic (As) released from the Sb smelting activities pose a major environmental risk and ecological degradation in Sb smelting sites. Here the effects of Fe/H2O2 modified biochar (Fe@H2O2-BC) on the synchronous stabilization of Sb/As and the improvement of soil structure in a typical Sb smelting site in Southern China based on a 1-year field experiment were studied. Application of ≥1 % (w/w) Fe@H2O2-BC could stably decrease the leaching concentrations of Sb and As of the polluted soils to Environmental quality standards for surface water Chinese Level III (GB3838-2002). Compared to the untreated soils, the stabilization efficiency of soil Sb and As treated by Fe@H2O2-BC reached 90.7 % ~ 95.7 % and 89.6 % ~ 90.8 %, respectively. The residue fractions of Sb/As in the soils increased obviously, and the bio-availability of Sb/As decreased by 65.0-95.6 % and 91.1-96.0 %, respectively. Moreover, Fe@H2O2-BC addition elevated soil organic carbon content, increased soil porosity, and improved water retention capacity, indicating the positive effects on soil structure and functions. Advanced mineral identification and characterization systems showed that Sb/As usually occurred in Fe-bearing minerals and stabilized by surface complexation and co-precipitation. The findings demonstrated that 1 % (w/w) Fe@H2O2-BC was appropriate to Sb/As stabilization and soil function recovery following field conditions, which provided potential application for ecological restoration in Sb smelting sites.

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.

Risk assessment of toxic and hazardous metals in paddy agroecosystem by biochar-for bio-membrane applications

Toxic and carcinogenic metal (loid)s, such arsenic (As) and cadmium (Cd), found in contaminated paddy soils pose a serious danger to environmental sustainability. Their geochemical activities are complex, making it difficult to manage their contamination. Rice grown in Cd and As-polluted soils ends up in people's bellies, where it can cause cancer, anemia, and the deadly itai sickness. Solving this issue calls for research into eco-friendly and cost-effective remediation technology to lower rice's As and Cd levels. This research delves deeply into the origins of As and Cd in paddy soils, as well as their mobility, bioavailability, and uptake mechanisms by rice plants. It also examines the current methods and reactors used to lower As and Cd contamination in rice. Iron-modified biochar (Fe-BC) is a promising technology for reducing As and Cd toxicity in rice, improving soil health, and boosting rice's nutritional value. Biochar's physiochemical characteristics are enhanced by the addition of iron, making it a potent adsorbent for As and Cd ions. In conclusion, Fe-BC's biomembrane properties make them an attractive option for remediating As- and Cd-contaminated paddy soils. More efficient mitigation measures, including the use of biomembrane technology, can be developed when sustainable agriculture practices are combined with these technologies.

Remediation of cadmium contaminated soil using K2FeO4 modified vinasse biochar

The remediation of cadmium (Cd) contaminated soil is challenging for agricultural practices. In this study, a novel vinasse biochar modified by potassium ferrate (K₂FeO₄) was synthesized to immobilize Cd in agricultural soil. Three biochars [i.e., vinasse biochar (BC), KMnO₄ modified vinasse biochar (MnBC), and K₂FeO₄ modified vinasse biochar (FeBC)] were applied to compare their efficiencies of Cd immobilization. The results showed that the orders of pH, ash content, and functional groups in different biochar were the same following BC < MnBC < FeBC. Scanning electron microscope images showed that the FeBC has more micropores than MnBC and BC. X-ray diffraction identified manganese oxides and iron oxides within MnBC and FeBC, indicating that Mn and Fe were well loaded on the biochar. In the soil-based pot experiment, both MnBC and FeBC significantly reduced soil available Cd by 23-38% and 36-45% compared with the control, respectively (p < 0.05). In addition, the application of BC, MnBC, and FeBC significantly increased the yield, chlorophyll, and vitamin C of Chinese cabbage (p < 0.05), and decreased its Cd uptake compared with the control. Notably, shoot Cd significantly reduced when 2% FeBC was applied (p < 0.05). Overall, using K₂FeO₄ to modify vinasse biochar enriched the surface functional groups and minerals as well as reduced Cd availability in soil and its uptake by the plant. Our study showed that K₂FeO₄ modified vinasse biochar could be used as an ideal amendment for the remediation of Cd-contaminated soil.

The preparation of paddy soil amendment using granite and marble waste: Performance and mechanisms

The wastes generated from the mining and processing of granite and marble stone are generally regarded as useless. However, these waste materials were used as the soil amendments for the first time. The functional groups, crystalline structure and micro-morphology of granite and marble wastes amendments (GMWA) were different from the original wastes demonstrated by X-ray diffractometer (XRD), Fourier transform infrared spectrometer (FT-IR) and Scanning electron microscope-energy dispersive spectrometer (SEM-EDS) analyses. With the addition of the amendments, the cation exchange capacity, electrical conductivity and nutrient availability of the soil increased, and the extractable heavy metals of the soil reduced significantly. Under the condition of the addition of 3% amendments, 7.0%, 99.9%, 99.7% and 70.5% of Cu, Pb, Zn and Cd in exchangeable fractions in soil were transformed to the more stable Fe-Mn oxides- or carbonates-bounded fractions. Tessier method and correlation analysis showed that the reduction of extractable metals in the acidic paddy soil can be attributed to the adsorption of available SiO₂, the co-precipitation induced by the elevated pH value, the complexation induced by Fe-Mn oxides and the cation exchange induced by mineral nutrients. This study provides a new strategy for resource recovery of waste stones and remediation of heavy metal-contaminated soil.

A chrysotile-based Fe/Ti nanoreactor enables efficient arsenic capture for sustainable environmental remediation

Iron-based materials for arsenic (As) immobilization in practical groundwater and soil remediation suffer from a low removal capacity and an insufficient long-term stability. Herein, a unique chrysotile-based nanoreactor has been developed by incorporating iron/titanium oxides into the cylindrical cavity of chrysotile (TiFe-Chy), providing sufficient internal reaction sites for As immobilization. Results reveal that the adsorption capacities of TiFe-Chy for As(III) and As(V) are considerably higher than the commonly used amendments, i.e., layered double hydroxide (LDH) and Phoslock®, respectively. More importantly, TiFe-Chy exhibits a strong anti-interference capability of As immobilization in soils compared to those commercial products due to this unique incorporation approach. Fixed-bed leaching experiments indciate that this TiFe-Chy nanoreactor can efficiently decrase the As(III) and As(V) concentrations by 81.8-87.3% within a period of ten years, significantly improving the long-term stability of As immobilization in soils. Life cycle assessment analysis reveals that TiFe-Chy can reduce negative environmental impacts (such as carbon emissions), resulting in a low cost for soils and groundwater remediation. The findings of this work open a new avenue for sustainable heavy metal(loid)s remediation in groundwater and soils.

Recent advances in biochar amendments for immobilization of heavy metals in an agricultural ecosystem: A systematic review

Over the last several decades, extensive and inefficient use of contemporary technologies has resulted in substantial environmental pollution, predominantly caused by potentially hazardous elements (PTEs), like heavy metals that severely harm living species. To combat the presence of heavy metals (HMs) in the agrarian system, biochar becomes an attractive approach for stabilizing and limiting availability of HMs in soils due to its high surface area, porosity, pH, aromatic structure as well as several functional groups, which mostly rely on the feedstock and pyrolysis temperature. Additionally, agricultural waste-derived biochar is an effective management option to ensure carbon neutrality and circular economy while also addressing social and environmental concerns. Given these diverse parameters, the present systematic evaluation seeks to (i) ascertain the effectiveness of heavy metal immobilization by agro waste-derived biochar; (ii) examine the presence of biochar on soil physico-chemical, and thermal properties, along with microbial diversity; (iii) explore the underlying mechanisms responsible for the reduction in heavy metal concentration; and (iv) possibility of biochar implications to advance circular economy approach. The collection of more than 200 papers catalogues the immobilization efficiency of biochar in agricultural soil and its impacts on soil from multi-angle perspectives. The data gathered suggests that pristine biochar effectively reduced cationic heavy metals (Pb, Cd, Cu, Ni) and Cr mobilization and uptake by plants, whereas modified biochar effectively reduced As in soil and plant systems. However, the exact mechanism underlying is a complex biochar-soil interaction. In addition to successfully immobilizing heavy metals in the soil, the application of biochar improved soil fertility and increased agricultural productivity. However, the lack of knowledge on unfavorable impacts on the agricultural systems, along with discrepancies between the use of biochar and experimental conditions, impeded a thorough understanding on a deeper level.

Recent progresses, challenges, and opportunities of carbon-based materials applied in heavy metal polluted soil remediation

The application of carbon-based materials (CBMs) for heavy metal polluted soil remediation has gained growing interest due to their versatile properties and excellent remediation performance. Although the progresses on applications of CBMs in removing heavy metal from aqueous solution and their corresponding mechanisms were well known, comprehensive review on applications of CBMs in heavy metal polluted soil remediation were less identified. Therefore, this review provided insights into advanced progresses on utilization of typical CBMs including biochar, activated carbon, graphene, graphene oxide, carbon nanotubes, and carbon black for heavy metal polluted soil remediation. The mechanisms of CBM remediation of heavy metals in soil were summarized, mainly including physical adsorption, precipitation, complexation, electrostatic interaction, and cationic-π coordination. The key factors affecting the remediation effect include soil pH, organic matter, minerals, microorganisms, coexisting ions, moisture, and material size. Disadvantages of CBMs were also included, such as: potential health risks, high cost, and difficulty in achieving co-passivation of anions and cations. This work will contribute to our understanding of current research advances, challenges, and opportunities for CBMs remediation of heavy metal-contaminated soils.

Effects of biochar on the transformation of cadmium fractions in alkaline soil

To investigate the chemical properties in the biochar-mediated transformation of soil cadmium (Cd) fractions, the effects of biochar applied at different pyrolysis temperatures on soil Cd-fractions, pH value, and soil organic matter (SOM) were studied through an in-lab incubation experiment on contaminated soil. The results showed that the dissolved organic carbon (DOC) of CsBC300 (biochar prepared at 300 °C) was significantly higher (up to 1.31 times) than that of CsBC600 (biochar prepared at 600 °C). However, CsBC600 was more aromatic. Due to the difference in pyrolysis temperatures, the Cd deactivation mechanism of CsBC300 and CsBC600 was mainly to provide a large amount of organic matter and aromatic functional groups to the soil, respectively. The addition of these two biochar types significantly reduced the acid-extracted Cd content, by 76.56-83.52% and 70.48-76.81%, respectively. Contrastingly, it increased the residual Cd content by 2.26-2.36 and 2.08-2.29 times, respectively, which promoted the Cd transformation from the unstable to the stable state. However, CsBC300 had slightly better deactivation effect than CsBC600 on the 120th day, which was due to the decrease of soil pH and the increased SOM content. These study results can provide a theoretical reference for the remediation of Cd-contaminated alkaline soil.

Remediation of cadmium or arsenic contaminated water and soil by modified biochar: A review

Biochar has a high specific surface area with abundant pore structure and functional groups, which has been widely used in remediation of cadmium or arsenic contaminated water and soil. However, the bottleneck problem of low-efficiency of pristine biochar in remediation of contaminated environments always occurs. Nowadays, the modification of biochar is a feasible way to enhance the performance of biochar. Based on the Web of science™, the research progress of modified biochar and its application in remediation of cadmium or arsenic contaminated water and soil have been systematically summarized in this paper. The main modification strategies of biochar were summarized, and the variation of physicochemical properties of biochar before and after modification were illustrated. The efficiency and key mechanisms of modified biochar for remediation of cadmium or arsenic contaminated water and soil were expounded in detail. Finally, some constructive suggestions were given for the future direction and challenges of modified biochar research.

Biochar-plant interaction and detoxification strategies under abiotic stresses for achieving agricultural resilience: A critical review

The unpredictable climatic perturbations, the expanding industrial and mining sectors, excessive agrochemicals, greater reliance on wastewater usage in cultivation, and landfill leachates, are collectively causing land degradation and affecting cultivation, thereby reducing food production globally. Biochar can generally mitigate the unfavourable effects brought about by climatic perturbations (drought, waterlogging) and degraded soils to sustain crop production. It can also reduce the bioavailability and phytotoxicity of pollutants in contaminated soils via the immobilization of inorganic and/or organic contaminants, commonly through surface complexation, electrostatic attraction, ion exchange, adsorption, and co-precipitation. When biochar is applied to soil, it typically neutralizes soil acidity, enhances cation exchange capacity, water holding capacity, soil aeration, and microbial activity. Thus, biochar has been was widely used as an amendment to ameliorate crop abiotic/biotic stress. This review discusses the effects of biochar addition under certain unfavourable conditions (salinity, drought, flooding and heavy metal stress) to improve plant resilience undergoing these perturbations. Biochar applied with other stimulants like compost, humic acid, phytohormones, microbes and nanoparticles could be synergistic in some situation to enhance plant resilience and survivorship in especially saline, waterlogged and arid conditions. Overall, biochar can provide an effective and low-cost solution, especially in nutrient-poor and highly degraded soils to sustain plant cultivation.

Co-incorporation of Chinese milk vetch (Astragalus sinicus L.), rice straw, and biochar strengthens the mitigation of Cd uptake by rice (Oryza sativa L.)

Soil cadmium (Cd) contamination is becoming a widespread concern because of its threat to global ecosystem health and food security. Co-incorporation of Chinese milk vetch (MV) and rice straw (RS) is a common agricultural practice in Southern China; however, the effects of combining these two materials with biochar on Cd bioavailability remain unclear. This study investigated the effects of MV, RS, rape straw biochar (RB), iron-modified biochar (FB), and their combinations on Cd uptake by rice through incubation and field experiments. The results showed that compared with the control without material input (CK), MV + RS (MR), MV + RS + RB (MRRB), and MV + RS + FB (MRFB) considerably reduced the Cd concentration in brown rice by 61.20 %, 65.38 %, and 62.65 %, respectively. Furthermore, the treatments increased the formation of iron‑manganese plaque (IMP) at different growth stages; MRRB and MRFB exhibited the highest increase rates among the treatments. Quantitatively, the Fe plaque and Mn plaque were increased by 20.61 %-47.23 % and 80.18 %-172.74 %, respectively. Compared with CK, the MRRB and MRFB treatments reduced the soil available Cd by 35.09 %-54.45 % and 38.20 %-50.20 %, respectively, at all stages. This decrease was substantially lower than that observed in the MV, RS, and MR treatments. Similar trends were observed in the incubation experiment. Additionally, the Community Bureau of Reference Sequential Extraction Analysis indicated that the MRRB and MRFB treatments converted the bioavailable Cd fractions into a stable form. Partial least squares path model and redundancy analysis revealed that pH was the major factor influencing Cd bioavailability. This study emphasized that the dual impact factors from the enhancement of Cd passivation capability and IMP formation jointly result in the reduction of Cd uptake by rice. Consequently, the co-incorporation of MV, RS, and biochar is promising for remediating Cd-contaminated paddy soils in Southern China.

Recent developments in modification of biochar and its application in soil pollution control and ecoregulation

Soil pollution has become a real threat to mankind in the 21st century. On the one hand, soil pollution has reduced the world's arable land area, resulting in the contradiction between the world's population expansion and the shortage of arable land. On the other hand, soil pollution has seriously disrupted the soil ecological balance and significantly affected the biodiversity in the soil. Soil pollutants may further affect the survival, reproduction and health of humans and other organisms through the food chain. Several studies have suggested that biochar has the potential to act as a soil conditioner and to promote crop growth, and is widely used to remove environmental pollutants. Biochar modified by physical, chemical, and biological methods will affect the treatment efficiency of soil pollution, soil quality, soil ecology and interaction with organisms, especially with microorganisms. Therefore, in this review, we summarized several main biochar modification methods and the mechanisms of the modification and introduced the effects of the application of modified biochar to soil pollutant control, soil ecological regulation and soil nutrient regulation. We also introduced some case studies for the development of modified biochars suitable for different soil conditions, which plays a guiding role in the future development and application of modified biochar. In general, this review provides a reference for the green treatment of different soil pollutants by modified biochar and provides data support for the sustainable development of agriculture.

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.

Effect of ferrous sulfate modified sludge biochar on the mobility, speciation, fractionation and bioaccumulation of vanadium in contaminated soil from a mining area

In contaminated soil, pristine biochar has poor applicability for immobilizing vanadium (V), which mainly exists as oxyanions in soil. To elucidate the immobilization potential and biotic/abiotic stabilizing mechanisms of a ferrous sulfate (FS)-modified sludge biochar in a V-contaminated soil from a mining area, we investigated the effects of biochar addition on the soil characteristics, growth of alfalfa, leachability, bioavailability, speciation, and fractionation of V, and changes in the microbial community structure and metabolic response. The results showed that the water extractable, acid-soluble (F1), and pentavalent fractions of V in soil decreased by up to 99 %, 95 %, and 55 %, respectively, whereas the reducible and (F2) oxidizable (F3) fractions increased by up to 45 % and 76 %, respectively. After the soil was treated with the FS-modified biochar for 90 d, the V concentration in the roots and shoots of alfalfa (Medicago sativa L.) decreased by up to 81.5 % and 96 %, respectively. The changes in the speciation, fractionation, and efficient immobilization of V in the studied soil were due to the combined effects of the biochar-induced decrease in soil pH, adsorption and precipitation by elevated iron concentrations, reduction and complexation due to an increase in the organic matter content, and microbial reduction by Proteobacteria.

Biochar, Ochre, and Manure Maturation in an Acidic Technosol Helps Stabilize As and Pb in Soil and Allows Its Vegetation by Salix triandra

Past mining extraction activities still have a negative impact in the present time, the resulting metal(loid) contaminated soils affecting both the environment and human health. Assisted phytostabilization technology, combining soil conditioner application to immobilize metal(loid)s and plant growth to reduce erosion and leaching risks, is a useful strategy in the restoration of metal(loid) contaminated lands. However, contaminants will respond differently to a particular amendment, having their own specific characteristics. Therefore, in multi-contaminated soils, soil conditioner combination has been suggested as a good strategy for metal(loid) immobilization. In the present study, in a mesocosm experiment, organic (biochar and manure) and inorganic (ochre) amendments were evaluated in single and combined applications for their effect on metal(loid) stabilization and Salix triandra growth improvement, in an arsenic and lead highly contaminated soil. Specifically, the effects of these amendments on soil properties, metal(loid) behavior, and plant growth were evaluated after they aged in the soil for 6 months. Results showed that all amendments, except biochar alone, could reduce soil acidity, with the best outcomes obtained with the three amendments combined. The combination of the three soil conditioners has also led to reducing soil lead availability. However, only ochre, alone or combined with the other soil fertilizers, was capable of immobilizing arsenic. Moreover, amendment application enhanced plant growth, without affecting arsenic accumulation. On the contrary, plants grown on all the amended soils, except plants grown on soil added with manure alone, showed higher lead concentration in leaves, which poses a risk of return of lead into the soil when leaves will shed in autumn. Considering that the best plant growth improvement, together with the lowest increase in lead aerial accumulation, was observed in manure-treated soil, the addition of manure seems to have potential in the restoration of arsenic and lead contaminated soil.

Iron-Modified Biochar Strengthens Simazine Adsorption and Decreases Simazine Decomposition in the Soil

Currently, modified biochar has been successfully used in the remediation of soil polluted with heavy metals. However, the effects of the modified biochar on pesticides (such as simazine) are still unclear. Herein, the environmental fate of simazine, such as decomposition, leaching, and adsorption in unamended soil, in the soil amended with unmodified and modified biochar (biochar + FeCl₃, biochar + FeOS, biochar + Fe) were evaluated. In addition, an incubation experiment was also performed to observe the influence of modified biochar on the microbial community and diversity in the soil. The results showed that modified biochar significantly decreased the decomposition of simazine in the soil compared to its counterpart. Modified biochar also reduced the concentration of simazine in the leachate. Compared with the control, soil microbial biomass in the soil amended with unmodified biochar, biochar + FeCl₃, biochar + Fe, and biochar + FeOS was decreased by 5.3%, 18.8%, 8.7%, and 18.1%, respectively. Furthermore, modified biochar changed the structure of the microbial community. This shows that modified biochar could increase the soil adsorption capacity for simazine and change the amount and microbial community that regulates the fate of simazine in the soil. This study concludes that iron-modified biochar has positive and negative effects on the soil. Therefore, its advantages and side effects should be considered before applying it to the soil.

Assessment of goethite-combined/modified biochar for cadmium and arsenic remediation in alkaline paddy soil

The opposed transformation of arsenic (As) and cadmium (Cd) in paddy soil postures numerous challenges for their simultaneous remediation. An incubation study was conducted on the immobilization of Cd and As by biochar (BC), goethite (G), goethite-combined biochar (BC + G), and goethite-modified biochar (GBC). The results showed that biochar effectively immobilized Cd while significantly increasing As mobility, whereas goethite effectively immobilized As more than Cd. BC + G treatment significantly decreased toxicity characteristics leaching procedure (TCLP) and CaCl₂-extractable Cd by 22.70% and 40.15%; meanwhile, TCLP and NaHCO₃-As were significantly reduced by 38.25% and 31.87%, respectively, compared with the control. This study found that GBC was the optimum amendment within the immobilization efficiency for CaCl₂-Cd (57.03%) and TCLP-As (61.11%). BC + G and GBC applications showed some interactions between biochar and goethite, which played an essential role in immobilizing Cd and As simultaneously. Therefore, GBC showed a great benefit in being a low-cost and efficient environmental amendment for Cd and As remediation in alkaline co-contaminated paddy soil.

Remediation of arsenic-contaminated paddy field by a new iron oxidizing strain (Ochrobactrum sp.) and iron-modified biochar

Iron-oxidizing strain (FeOB) and iron modified biochars have been shown arsenic (As) remediation ability in the environment. However, due to the complicated soil environment, few field experiment has been conducted. The study was conducted to investigate the potential of iron modified biochar (BC-FeOS) and biomineralization by a new found FeOB to remediate As-contaminated paddy field. Compared with the control, the As contents of GB (BC-FeOS), GF (FeOB), GFN (FeOB and nitrogen fertilizer), GBF (BC-FeOS and FeOB) and GBFN (BC-FeOS, FeOB and nitrogen fertilizer) treatments in pore water decreased by 36.53%-80.03% and the microbial richness of iron-oxidizing bacteria in these treatments increased in soils at the rice maturation stage. The concentrations of available As of GB, GF, GFN, GBF and GBFN at the tillering stage were significantly decreased by 10.78%-55.48%. The concentrations of nonspecifically absorbed and specifically absorbed As fractions of GB, GF, GFN, GBF and GBFN in soils were decreased and the amorphous and poorly crystalline hydrated Fe and Al oxide-bound fraction was increased. Moreover, the As contents of GB, GF, GFN, GBF and GBFN in rice grains were significantly decreased (*P < 0.05) and the total As contents of GFN, GBF and GBFN were lower than the standard limit of the National Standard for Food Safety (GB 2762-2017). Compared with the other treatments, GBFN showed the greatest potential for the effective remediation of As-contaminated paddy fields.

Removal of Arsenic by Wheat Straw Biochar from Soil

Biochar prepared from wheat straw (Triticum aestivum) at different pyrolysis temperatures was screened, followed by its application to soil for arsenic removal in the present study. Characterization of biochar by Field emission scanning electron microscope studies and Fourier thermal Infrared imaging showed smooth and porous biochar surface and abundance of surface functional groups. A low value of H/C was obtained by CHNS analyzer, indicating high stability of biochar. The surface area was 15.86 m²/g on an average. Batch sorption experiments were carried out to optimize conditions for arsenic sorption. Maximum arsenic removal of 83.7% was obtained when applied at a 7.5% dose for a contact time of 60 min at 25 °C. Isotherm, kinetic and thermodynamic studies revealed the feasibility of sorption and removal of arsenic through physisorption, chemisorption, ion exchange, and diffusion.

Simultaneous reduction in cadmium and arsenic accumulation in rice (Oryza sativa L.) by iron/iron-manganese modified sepiolite

It is challenging to reduce the cadmium (Cd) and arsenic (As) contents of brown rice simultaneously due to their converse chemical behaviors in the paddy soil. Clay minerals, such as sepiolite (SEP), have significant advantages in remediating Cd-contaminated soil. Moreover, iron or manganese oxide loaded SEP can improve the As adsorption efficiency. Herein, ferric nitrate modified sepiolite (NIMS) and iron‑manganese modified sepiolite (FMS) were prepared to study their effects on Cd and As accumulation in rice using pot experiments. The results showed that NIMS and FMS had a larger specific surface area than SEP. The application of SEP only decreased Cd content (by 45%), while NIMS and FMS treatments reduced both Cd (by 57% and 87%) and As (by 30% and 25%) contents in brown rice compared with the control. The X-ray photoelectron spectroscopy (XPS) analysis results indicated that MnO₂ and MnOOH^⁎ in FMS enhanced the adsorption and co-precipitation of Cd as well as the oxidation of As(III) to As(V). The NIMS, as well as the FMS application, increased soil pH, decreased the exchangeable Cd and non-specifically and specifically adsorbed As fractions in soil, and reduced the level of Cd in the pore water. Moreover, NIMS and FMS addition limited the transfer of As from the soil to the roots by enhancing its sequestration in the iron plaque. On the other hand, FMS treatment significantly promoted the uptake of Mn by rice (P < 0.05). The results suggested that both NIMS and FMS were promising materials for simultaneous reduction of Cd and As accumulation in rice. Notably, FMS had better performance in reducing the Cd content in rice than that of NIMS.

Adsorption of Arsenic on Fe-Modified Biochar and Monitoring Using Spectral Induced Polarization

This work demonstrates the potential of Fe-modified biochar for the treatment of arsenic (As) simulated wastewater and the monitoring of adsorption in real-time. Specifically, we propose the utilization of date-palm leaves for the production of biochar, further modified with Fe in order to improve its adsorption function against inorganic pollutants, such as As. Both the original biochar and the Fe-modified biochar were used for adsorption of As in laboratory batch and column experiments. The monitoring of the biochar(s) performance and As treatment was also enhanced by using the spectral induced polarization (SIP) method, offering real-time monitoring, in addition to standard chemical monitoring. Both the original and the Fe-modified biochar achieved high removal rates with Fe-modified biochar achieving up to 98% removal of As compared to the 17% by sand only (control). In addition, a correlation was found between post-adsorption measurements and SIP measurements.

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.

Remediation of arsenic-contaminated paddy soil by iron oxyhydroxide and iron oxyhydroxide sulfate-modified coal gangue under flooded condition

Flooded condition enhances arsenic (As) mobility in paddy soils, posing an imminent threat to food safety and human health. Hence, iron oxyhydroxide and iron oxyhydroxide sulfate-modified coal gangue (CG-FeOH and CG-FeOS) were synthesized for remediation of As-contaminated paddy soils under a flooded condition. Compared to the control, CG-FeOH and CG-FeOS application decreased the soil pH by 0.10-0.80 and 0.13-1.63 units, respectively. CG-FeOH and CG-FeOS application significantly (P < 0.05) decreased available As concentration by 13.46-43.44% and 21.31-54.37%, respectively. CG-FeOH and CG-FeOS significantly (P < 0.05) reduced the non-specifically adsorbed and specifically adsorbed As fractions and increased As(V) proportion by 22.61-26.53% and 29.10-36.51%, respectively. Our results showed that CG-FeOH and CG-FeOS could change As geochemical fraction and valence state, consequently reducing available As concentration in paddy soils. Moreover, the sulfate could enhance the oxidation and co-precipitation of As with CG-FeOH. Compared to CG-FeOH, CG-FeOS was more effective in decreasing available As concentration and oxidizing As(III) to As(V). This study revealed that CG-FeOS is a potential amendment for As immobilization in paddy soils.

Pig carcass-derived biochar caused contradictory effects on arsenic mobilization in a contaminated paddy soil under fluctuating controlled redox conditions

Contamination of paddy soils by arsenic (As) is of great concern for human health and the environment. The impact of animal-derived biochar on As mobilization under fluctuating redox conditions in paddy soils has not been studied. Consequently, we investigated the effects of pig carcass-derived biochar (PB) on As (im)mobilization in a contaminated paddy soil under controlled redox potential (Eh) using a biogeochemical microcosm-setup. The addition of PB decreased the concentration of dissolved As at Eh = +100 and +200 mV by 38.7% and 35.4%, respectively (compared to the control), because of the co-precipitation of As with Fe-Mn oxides and the complexation between As and aromatic organic molecules. However, under reducing conditions (Eh = -300 mV), PB increased the dissolved As by 13.5% through promoting reduction and decomposition of As-bearing Fe minerals (e.g., ferrihydrite-As, Fe-humic-As). Under oxidizing conditions (Eh = +250 mV), PB increased the dissolved As by 317.6%, due to the associated increase of pH. We conclude that As mobilization in PB-treated paddy soils is highly affected by Eh. PB can be used to reduce the environmental risk of As under moderately reducing conditions, but it may increase the risk under highly reducing and oxidizing conditions in paddy soils.

Applications of Biochar and Modified Biochar in Heavy Metal Contaminated Soil: A Descriptive Review

Given that the problem of contaminated soil continues to grow, the development of effective control and remediation measures has become imperative, especially for heavy-metal-contaminated soil. Biochar and modified biochar are eco-friendly and cost-effective remediation materials that are widely used in the remediation of contaminated soil. This review provides an overview of the different raw materials used in the preparation of biochar as well as the modification of biochar using various synthesis methods, highlighting their differences and providing recommendations for biochar and modified biochar as applied toward ameliorating pollution in soil contaminated by heavy metals. We also explore the effects of the physicochemical properties of raw materials, pyrolysis temperature, additives, and modification methods on the properties of the resulting biochar and modified biochar, and systematically present the types of soil and operating factors for repair. Moreover, the mechanisms involved in remediation of heavy-metal-contaminated soil by biochar and modified biochar are outlined in detail, and include adsorption, complexation, precipitation, ion exchange, and electrostatic attractions. Finally, the corresponding monitoring technologies after remediation are illustrated. Future directions for studies on biochar and modified biochar in the remediation of contaminated soil are also proposed to support the development of green environmental protection materials, simple preparation methods, and effective follow-up monitoring techniques.

Short-Term Aging of Pod-Derived Biochar Reduces Soil Cadmium Mobility and Ameliorates Cadmium Toxicity to Soil Enzymes and Tomato

Contamination of agricultural soil with cadmium (Cd) has become a global concern because of its adverse effects on ecohealth and food safety. Soil amendment with biochar has become one of the phytotechnologies to reduce soil metal phyto-availability and its potential risks along the food chain. Biochar, derived from cocoa pod, was evaluated in soil Cd fractions (exchangeable, reducible, oxidizable, and residual) by modified Commission of the European Communities Bureau of Reference sequential extraction and its efficacy to ameliorate Cd toxicity to soil enzymes and leaf bioactive compounds. A pot experiment was conducted using Cd-spiked soil at 10 mg/kg with tomato (Solanum lycopersicum L.) at a biochar application rate of 1 and 3% (w/w) for 6 wk. The addition of biochar significantly reduced (p < 0.05) the exchangeable, reducible, and residual fractions by at least approximately 23%, with a consequential decrease in Cd root uptake and transport within tomato tissues. The activity of soil enzymes (catalase, dehydrogenase, alkaline phosphatase, and urease) was affected by Cd toxicity. However, with the exception of dehydrogenase, biochar application significantly enhanced the activity of these enzymes, especially at the 3% (w/w) rate. As for the secondary metabolites we studied, Cd toxicity was observed for glutathione, terpenoids, and total phenols. However, the biochar application rate of 1% (w/w) significantly ameliorated the effects of toxicity on the secondary metabolites. In conclusion, biochar demonstrated the potential to act as a soil amendment for Cd immobilization and thereby reduce the bioavailability of Cd in soil, mitigating food security risks. Environ Toxicol Chem 2021;40:3306-3316. © 2020 SETAC.

Iron modification to silicon-rich biochar and alternative water management to decrease arsenic accumulation in rice (Oryza sativa L.)

Production of rice grains at non-toxic levels of arsenic (As) to meet the demands of an ever-increasing population is a global challenge. There is currently a lack of investigation into integrated approaches for decreasing As levels in rice agro-ecosystems. By examining the integrated iron-modified rice hull biochar (Fe-RBC) and water management approaches on As dynamics in the paddy agro-ecosystem, this study aims to reduce As accumulation in rice grains. The rice cultivar, Ishikari, was grown and irrigated with As-containing water (1 mg L^-1 of As(V)), under the following treatments: (1) Fe-RBC-flooded water management, (2) Fe-RBC-intermittent water management, (3) conventional flooded water management, and (4) intermittent water management. Compared to the conventional flooded water management, grain weight per pot and Fe and Si concentrations in the paddy pore water under Fe-RBC-intermittent and Fe-RBC-flooded treatments increased by 24%-39%, 100%-142%, and 93%-184%, respectively. The supplementation of Fe-RBC decreased the As/Fe ratio and the abundance of Fe(III) reducing bacteria (i.e. Bacillus, Clostridium, Geobacter, and Anaeromyxobacter) by 57%-88% and 24%-64%, respectively, in Fe-RBC-flooded and Fe-RBC-intermittent treatments compared to the conventional flooded treatment. Most importantly, Fe-RBC-intermittent treatment significantly (p ≤ 0.05) decreased As accumulation in rice roots, shoots, husks, and unpolished rice grains by 62%, 37%, 79%, and 59%, respectively, compared to the conventional flooded treatment. Overall, integrated Fe-RBC-intermittent treatment could be proposed for As endemic areas to produce rice grains with safer As levels, while sustaining rice yields to meet the demands of growing populations.

Performance evaluation of crop residue and kitchen waste-derived biochar for eco-efficient removal of arsenic from soils of the Indo-Gangetic plain: A step towards sustainable pollution management

Biochar was produced from wheat straw (Triticum aestivum), rice straw (Oryza sativa), and kitchen waste at varying pyrolysis temperatures (300°C-700 °C). The biochars were screened depending on their production and physicochemical properties for the adsorptive removal of arsenic (As). The morphological analysis by Field emission scanning electron microscope revealed a porous biochar surface. Spectroscopic characterization of biochars indicated the co-existence of minerals, carboxyl, carbonyl, amide, and hydroxyl groups, which implies the suitability of biochar to immobilize metal (loid)s from soils. Changes in peaks were observed in Fourier-transform infrared and X-ray diffraction images after As sorption indicating the involvement of chemisorption. The thermogravimetric analysis and a low H/C value derived from the CHNS analyzer confirmed the high stability of biochar. The BET analysis was used to estimate the surface areas of wheat straw (15.8 m² g^-1), rice straw (12.5 m² g^-1), and kitchen waste (2.57 m² g^-1) -derived biochars. Batch sorption studies were performed to optimize experimental parameters for maximum removal of As. Maximum removal of As was observed for wheat straw-derived biochar (pyrolyzed at 500 °C) at 8 mg L^-1 initial concentration (IC), 7.5 % dose, 25 °C temperature, and 60 min contact time (83.7 ± 0.06 %); in rice straw-derived biochar (pyrolyzed at 500 °C) at 8 mg L^-1 IC, 7.5 % dose, 25 °C temperature, 90 min contact time (83.6 ± 0.37 %); and in kitchen waste-derived biochar (pyrolyzed at 500 °C) at 8 mg L^-1 IC, 5 % dose, 25 °C temperature, 60 min contact time (76.7 ± 0.16 %). The sorption model parameters suggested the possibility of chemisorption, physisorption, diffusion, and ion exchange for the removal of As. Therefore, it could be recommended to farmers that instead of disposing or burning straws and waste openly, they could adopt the process of charring to generate livelihood security and mitigation of geogenic contaminants from the soil/water dynamic systems.

Biochar-templated surface precipitation and inner-sphere complexation effectively removes arsenic from acid mine drainage

Treatment of aqueous leachate from acid mine tailings with pristine biochar (BC) resulted in the removal of more than 90% of the dissolved arsenic with an attendant rapid and sustained pH buffering from 3 to 4. Pine forest waste BC was transformed to a highly effective adsorbent for arsenic remediation of acid mine drainage (AMD) because the dissolved iron induced "activation" of BC through accumulation of highly reactive ferric hydroxide surface sites. Physicochemical properties of the BC surface, and molecular mechanisms of Fe, S, and As phase transfer, were investigated using a multi-method, micro-scale approach (SEM, XRD, FTIR, XANES, EXAFS, and STXM). Co-located carbon and iron analysis with STXM indicated preferential iron neo-precipitates at carboxylic BC surface sites. Iron and arsenic X-ray spectroscopy showed an initial precipitation of ferrihydrite on BC, with concurrent adsorption/coprecipitation of arsenate. The molecular mechanism of arsenic removal involved bidentate, binuclear inner-sphere complexation of arsenate at the surfaces of pioneering ferric precipitates. Nucleation and crystal growth of ferrihydrite and goethite were observed after 1 h of reaction. The high sulfate activity in AMD promoted schwertmannite precipitation beginning at 6 h of reaction. At reaction times beyond 6 h, goethite and schwertmannite accumulated at the expense of ferrihydrite. Results indicate that the highly functionalized surface of BC acts as a scaffolding for the precipitation and activation of positively charged ferric hydroxy(sulf)oxide surface sites from iron-rich AMD, which then complex oxyanion arsenate, effectively removing it from porewaters. Graphical abstract.

Towards a Soil Remediation Strategy Using Biochar: Effects on Soil Chemical Properties and Bioavailability of Potentially Toxic Elements

Soil contamination with potentially toxic elements (PTEs) is considered one of the most severe environmental threats, while among remediation strategies, research on the application of soil amendments has received important consideration. This review highlights the effects of biochar application on soil properties and the bioavailability of potentially toxic elements describing research areas of intense current and emerging activity. Using a visual scientometric analysis, our study shows that between 2019 and 2020, research sub-fields like earthworm activities and responses, greenhouse gass emissions, and low molecular weight organic acids have gained most of the attention when biochar was investigated for soil remediation purposes. Moreover, biomasses like rice straw, sewage sludge, and sawdust were found to be the most commonly used feedstocks for biochar production. The effect of biochar on soil chemistry and different mechanisms responsible for PTEs' immobilization with biochar, are also briefly reported. Special attention is also given to specific PTEs most commonly found at contaminated soils, including Cu, Zn, Ni, Cr, Pb, Cd, and As, and therefore are more extensively revised in this paper. This review also addresses some of the issues in developing innovative methodologies for engineered biochars, introduced alongside some suggestions which intend to form a more focused soil remediation strategy.

Role of the biochar modified with ZnCl2 and FeCl3 on the electrochemical degradation of nitrobenzene

The Zn/Fe-modified biochar on nitrobenzene (NB) removal during the electrolysis was investigated in this study. Both the Fe and Zn-modified biochar enhanced the NB adsorption compared with the un-modified biochar due to their greater specific surface area and more abundant surface function groups, respectively. The electrolysis under 2-11 V with the assist of both Fe/Zn-modified biochar achieved effective NB removal (>93%). The removal rate under 2 V using Zn/Fe-modified biochar (∼94%) was higher than that of the un-modified biochar (∼80%), whereas the removal was similar for those under 5, 8 and 11 V. The NB removal under 2 and 5 V was attributed to both adsorption and electrochemical decomposition of NB molecules. Electrolysis under 5 V by Fe-modified biochar had a higher degree of NB mineralisation than that using un-modified and Zn-modified biochar. This was likely that the Fe-modified biochar exhibited higher electrocatalytic properties, facilitating the further NB mineralisation. The ∙OH played significant roles in the degradation of NB by Fe-modified and un-modified biochar but did not significantly participated for the test using Zn-modified biochar. This was possibly because the Zn-modified biochar could adsorb greater amounts of ∙OH into the inner pores of Zn-modified biochar via its greater porosity and specific surface area, which may prevent the contact between ∙OH and NB molecules.