A review of pristine and modified biochar immobilizing typical heavy metals in soil: Applications and challenges

Research progress on environmental behavior of arsenic in Qinghai-Tibet Plateau soil

The Qinghai-Tibet Plateau, with its high altitude and cold climate, is one of the most fragile ecological environments in China and is distinguished by its naturally elevated arsenic (As) levels in the soil, largely due to its rich mineral and geothermal resources. This review provides a comprehensive analysis of As content, focusing on its distribution, environmental migration, and transformation behavior across the plateau. The review further evaluates the distribution of As in different functional areas, revealing that geothermal fields (107.2 mg/kg), mining areas (53.8 mg/kg), and croplands (39.3 mg/kg) have the highest As concentrations, followed by river and lake sediments and adjacent areas (33.1 mg/kg). These elevated levels are primarily attributed to the presence of As-rich minerals, such as arsenopyrite and pyrite. Additionally, human activities, including mining and geothermal energy production, exacerbate the release of As into the environment. The review also highlights the role of local microorganisms, particularly those from the phyla Proteobacteria and Actinobacteria, which possess As metabolic genes that facilitate As translocation. Given the unique climatic conditions of the plateau, conventional methods for As control may not be fully effective. However, the review identifies promising remediation strategies that are environmentally adaptable, such as the use of local microorganisms, specific adsorbents, and integrated technologies, which offer potential solutions for managing and utilizing As-contaminated soils on the plateau.

Reclamation of available phosphorus and separation of heavy metals from sewage sludge via FeCl3-assisted electrokinetic treatment and pyrolysis

Recycling of sewage sludge and the endogenous phosphorus (P) is a promising strategy for sustainable development, while the disposal of heavy metals (HMs) in sewage sludge and the recovery of targeted P species remain challenges. An innovative method coupling electrokinetic treatment with pyrolysis was proposed in the present study to achieve the effective reclamation of available P and the separation of HMs from sewage sludge. The pristine and FeCl3-assisted electrokinetic treatment were employed for the removal of HMs from sewage sludge and to modify the P species, and the subsequent pyrolysis (300-700 °C) was conducted for the recovery of available P along with the production of biochar. The X-ray absorption near-edge spectroscopy (XANES), 31P liquid nuclear magnetic resonance (NMR) spectroscopy, and sequential chemical extraction were used to systematically determine the evolution of P during the combined treatment of sewage sludge. 19.69-24.80 % of Ni, Cu, and Zn were removed from sewage sludge after pristine electrokinetic treatment, and the HM removal efficiency was further elevated to 47.01-56.86 % with the assistance of FeCl3. Consequently, in comparison with the raw sewage sludge-derived biochars (SBs), the biochars derived from FeCl3-assisted electrokinetic treated sewage sludge (FESBs) contained much lower HM contents and showed higher stability of HMs. The FeCl3-assisted electrokinetic treatment converted alkaline biochars dominated by poorly soluble Ca-phosphates into neutral to slightly acidic biochars dominated by Al/Fe-associated phosphates. This transformation greatly improved the available P concentrations determined by diffusive gradients in thin film in FESBs by 0.6-1.3 folds compared to untreated SBs. Therefore, coupling FeCl3-assisted electrokinetic treatment with pyrolysis could be a promising strategy to achieve the reclamation of available P and the separation of HMs from sewage sludge.

Review of Fe/Mn-based chemical stabilizers for remediating arsenic and antimony co-contaminated soil

Arsenic (As) and antimony (Sb) frequently co-occur in soil contaminated by mining, smelting and traffic emissions, creating an urgent need for effectively simultaneous remediation strategies. Although chemical stabilization has garnered significant attention for its high remediation efficiency, a systematic comparison of stabilization effects and mechanisms for As and Sb in co-contaminated soil remained unexplored. Iron-based materials are widely recognized as the most effective stabilizers for As and Sb in soil. Meanwhile, manganese-based materials, owing to their superior oxidizing capacity that maintains As and Sb in the less toxic pentavalent species, have also attracted considerable interest. Iron-manganese-based materials provide an efficient solution for stabilizing As and Sb in soil by synergistically combining the advantages of both iron and manganese components. This review therefore elaborated on the core stabilizers, including iron-, manganese-, and iron-manganese-based materials. The stabilization efficiencies and underlying mechanisms of As and Sb in soil were comprehensively examined, with key environmental factors also discussed in relation to their stabilization performance. As widely used soil amendments, raw biochar and clay materials demonstrate limited efficacy in stabilizing As and Sb in soil. However, they can be employed as functional modifiers to enhance the dispersion of iron or manganese particles, thereby improving stabilization performance. While current progress is systematically evaluated, the development of stabilizers enabling simultaneous immobilization of As and Sb remains a critical research priority for future research.

Ecological effects of biochar in heavy metal-contaminated soils from multidimensional perspective: Using meta-analysis

The application of biochar in the remediation of heavy metal-contaminated soil shows great potential, but its comprehensive impacts on metal dynamics and the soil ecosystem have not been quantified. This study conducted a meta - analysis of 496 observations from 41 studies. The results indicated that biochar can significantly decrease the bioavailability of cationic metals, but has limited effect on anionic heavy metals. After application, soil nutrient content and enzyme activity significantly increased. Microbial network analysis revealed enhanced interactions between species, and the reconstruction of the core microbial community indicated a shift in microbial survival strategies from resisting heavy metal stress to nutrient cycling and plant symbiosis. FAPROTAX analysis showed that microbial communities related to nitrogen and sulfur cycles were significantly stimulated. This study explores the role of biochar in restoring soil multifunctionality while addressing metal pollution issues from multiple perspectives, offering key insights for developing targeted soil remediation strategies.

Multifunctional Sites for Enhanced Adsorption of Arsenic Using Sulfydryl-Modified Biochar/MgFe-Layered Double Hydroxides

Arsenic contamination in water poses a significant threat to the environment and human health due to the high toxicity of arsenic. Therefore, the development of functionalized materials with an enhanced adsorption capacity for arsenic remains a key research focus in water purification. In this study, straw powder was hydrothermally pretreated and subsequently pyrolyzed with zinc chloride at 700 °C to produce hydrothermal biochar with tailored pores. The hydrothermal biochar was then modified with sulfhydryl groups, and Sulfhydryl-Modified Biochar/MgFe-Layered Double Hydroxides (SH@HB/MgFe-LDH) composites were synthesized using the coprecipitation method. By utilizing HB with a high surface area, a composite material with a high specific surface area of 479.3677 m2/g was prepared. The experimental results indicated that the SH@HB/MgFe-LDH composites exhibited excellent arsenic adsorption performance across a wide pH range, achieving an arsenic adsorption capacity as high as 388.01 mg/g. The adsorption process and mechanism of the SH@HB/MgFe-LDH composites were investigated through adsorption kinetics, adsorption isotherms, thermodynamic analysis, and X-ray photoelectron spectroscopy. Additionally, recycling studies demonstrated that the composites maintained stable performance over three reuse cycles, showing good potential for practical applications. Overall, the SH@HB/MgFe-LDH composites offer an effective solution for arsenic pollution control in water while promoting the high-value utilization of agricultural and forestry waste.

Advancements in Biochar for Soil Remediation of Heavy Metals and/or Organic Pollutants

The rapid industrialization and economic growth have exacerbated the contamination of soils with both heavy metals and organic pollutants. These persistent contaminants pose substantial threats to ecosystem integrity and human health due to their long-term environmental persistence and potential for bioaccumulation. Biochar, with its high specific surface area, well-developed pore structure, and abundant surface functional groups, has emerged as a promising material for remediating soils contaminated by heavy metals and organic pollutants. While some research has explored the role of biochar in soil remediation, several aspects remain under investigation. Fully harnessing the potential of biochar for soil contamination remediation is of critical importance. This review provides an overview of the preparation methods and physicochemical properties of biochar, discusses its application in soils contaminated by organic compounds and/or heavy metals, and examines the mechanisms underlying its interaction with pollutants. Additionally, it summarizes the toxicity assessments of biochar during soil remediation and outlines future research directions, offering scientific insights and references for the practical deployment of biochar in soil pollution remediation.

Influence of sewage sludge compost on heavy metals in abandoned mine land reclamation: A large-scale field study for three years

Using sewage sludge compost (SSC) for abandoned mine land reclamation supports ecological sustainability, but the environmental behavior of heavy metals in this process lacks systematic field validation. Here we analyzed the dynamic changes in heavy metal composition in topsoil, surface runoff, and subsurface infiltration after large-scale reclamation. Results show that SSC application promoted plant growth by 2-4 times, enhanced the physicochemical structure of the topsoil, and increased the levels of organic matter and inorganic nutrients. Most heavy metals exhibited higher retention in SSC-treated areas compared to non-SSC areas; nonetheless, they remained within low toxicity risk levels overall. Surface runoff from areas with high SSC content exhibited elevated concentrations of heavy metals. In the 2020-M225 sample, Cd, Cu, Pb, and Zn concentrations were at least 1.5 times that of M0. Mixing application of SSC further mitigated the subsurface migration of Cr, Cu, Pb, and Zn compared to S120, with concentrations of As, Cr, Pb, and Zn in 2020-M225 being less than 1/10 of those in M0. Correlation analysis demonstrates that SSC regulated topsoil pH and the contents of organic matter, phosphorus, and Fe and Al (hydr)oxides, which synergistically enhanced the adsorption and complexation of most toxic heavy metals, thereby reducing their migratory pollution over time. This study suggests that practical SSC application (up to 225 t/ha) results in long-term effects on heavy metals characterized by in-situ multi-effect stabilization, rather than increasing overall environmental risks, and provides a technological foundation for ensuring the safe use of SSC in mine reclamation.

Acidic stability and mechanisms of soil cadmium immobilization by layered double hydroxides intercalated with mercaptosuccinic acid

Layered double hydroxide intercalated with mercaptosuccinic acid (MSA-CFA) holds considerable promise for remediating cadmium (Cd)-contaminated soils through selective immobilization; however, its stability under acidic conditions has yet to be investigated. The acidic stability of MSA-CFA was investigated by acid stability investigation and simulated soil acidification. In the immersion test, the cadmium dissolution rate (DR) for the Cd immobilized products of MSA-CFA (MSA-CFA-Cd) was significantly lower (2.06% in nitric acid and 5.91% in malic acid) compared to 53.74% and 62.76%, respectively, for the Cd immobilized products of lime (lime-Cd). Furthermore, the CaCl2-Cd in soils immobilized with the MSA-CFA increased by 8.49% and 17.4%, respectively, after the soils were treated with inorganic acid solution of H₂SO₄ and HNO₃ and organic acid solution of p-hydroxybenzoic acid, coumaric acid, benzoic acid, and cinnamic acid. The study demonstrated that in the MSA-CFA treatment, bioavailable Cd was transformed into a more stable Fe/Mn oxide-bound state. This was attributed to the formation of Cd-containing hydrotalcite through isomorphous substitution and the formation of high-energy S-Cd complexes via sulfhydryl binding. In comparison, lime treatment resulted primarily in precipitation, which was less effective at stabilizing the Cd. The replacement of interlamellar S-Cd by hydrolyzed organic acid ions increased the remobilization risk (△CaCl2-Cd/△pH) of the soil Cd compared to inorganic acid treatment. This study preliminarily elucidated the resistance mechanisms of MSA-CFA in a complex acidic environment, providing insights into its long-term stability for soil Cd remediation.

Co-application of organic phosphate fertilizer, manure, and biochar synergistically improves chemical and biological properties of Pb-Zn mine tailings: Insights from a pot trial

The incorporation of amendments is a commonly employed strategy to enhance the effectiveness of phytostabilization in remediating mine tailings. Although the remediation effectiveness of organic phosphate fertilizer (P), manure (M), and biochar (B) as individual amendments for heavy metal pollution has been extensively investigated, yet, the effectiveness of combined amendments for mine tailings remains unknown. The objective of this study is to assess the potential effectiveness of the co-application of P, M, and B in remediating Pb-Zn mine tailings by examining the alterations of chemical and biological properties. In this study, the mine tailings were amended with single and combined P, M, and B at a dosage of 5 % (w/w) followed by a two-month pot trial involving the seeding of ryegrass (Lolium perenne L.). In comparison to the control, all amendments effectively elevated the levels of organic matter, total nitrogen, and total phosphorus in the mine tailings while reducing diethylenetriamine-pentaacetic acid (DTPA)-extractable Pb, Cd, and As; Zn did not show significant changes except in combined treatment with P + M + B (i.e. PMB combination). The combined amendment treatments, particularly PMB, resulted in a substantial enhancement in both microbial biomass and the activities of alkaline phosphatase, urease, invertase, and dehydrogenase. Moreover, the combined amendments facilitated ryegrass growth while concurrently mitigating heavy metal uptake. Pearson's correlation analysis demonstrated a positive association between biological properties of the mine tailings (microbial biomass, enzyme activities, and ryegrass biomass) and nutritional levels while showing a negative correlation with DTPA-extractable heavy metals. Principal component analysis revealed that the addition of PMB exhibited superior efficacy in remediating mine tailings. Overall, our findings suggest that the application of PMB shows promise for in-situ remediation of Pb-Zn mine tailings; however, further research is required to validate these findings under field conditions.

Molecular mechanistic insights towards aggregation of nano-biochar moderated by aromatic components in dissolved organic matter

Nano-biochar (NBC) is a promising tool for sustainable remediation of contaminants in aquatic environments. However, the presence of ubiquitous ions and dissolved organic matter (DOM) can impact NBC aggregation, resulting in reduced application efficacy and potential ecological risks. Understanding and regulating NBC aggregation offers valuable insights for its deployment. This study integrated batch aggregation experiments, theoretical models, Fourier transform ion cyclotron resonance-mass spectrometry (FTICR-MS), and density functional theory (DFT) calculations to explore the behaviors and mechanisms of NBC aggregation with coexisting ions and model DOM. NBC aggregation kinetics followed the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory in both NBC-ions and NBC-ions-fulvic acid (FA) solutions, indicating that the aggregation process is controlled by Van der Waals forces and electrostatic repulsion. Mono/di-valent electrolytes promoted NBC aggregation, whereas FA moderated it, with higher molecular weight FA fractions exhibiting superior performance. Three-dimensional excitation-emission (3D-EEM) fluorescence spectra and Parallel factor analysis (PARAFAC) analyses revealed that HA-like substances, followed by FA-like substances, can form a complex with ions, thereby moderating NBC aggregation. FTICR-MS scans identified lignin substances with aromatic structures as key components that effectively reduce the promoted NBC aggregation with coexisting mono/di-valent electrolytes. DFT calculations confirmed that the aromatic structures in FA spontaneously form complexes with electrolytes, thereby potentially regulating NBC aggregation. This research highlights potential strategies for regulating NBC applications and offers insights into the behavior of nanoparticles in aquatic environments.

An efficient fungi-biochar-based system for advancing sustainable management of combined pollution

Heavy metal (HM) contamination poses significant global environmental threats, impacting ecosystems, public health, and sustainable development. Fungi, as eco-friendly alternatives to chemical treatments, have the potential to reduce HM bioavailability in contaminated soils while promoting plant growth. However, current fungal remediation methods face limitations in efficiency, long-term effectiveness, and the ability to address combined contamination, particularly with naturally occurring strains. Herein, we developed a Trichoderma reesei-Laccase (LAC)-Biochar coupling system (TLBS), based on the structural and electrostatic analyses of LAC's metal-chelated active site (T1 Cu), for the sustainable remediation of combined pollutants, including HMs. In the TLBS, genetically engineered T. reesei produces a mutated LAC with enhanced binding capability for HMs (Ni and Cd). The TLBS enables high-efficiency remediation through three steps. First, lignin-derived biochar serves as both a supportive carrier and an inducer, initiating LAC expression. Second, natural mediators are released due to the interaction between biochar and T. reesei, and LAC is activated by environmental HMs and natural mediators. Finally, TLBS achieved significant reductions in the available concentrations of Ni (93.63%) and Cd (89.68%) and efficiently remediated multiple organic pollutants (71.41-96.79%), including antibiotics and pesticides. Furthermore, the synergistic interaction among TLBS components ensures long-term remediation effects in environments rich in agricultural biomass, making it ideal for eco-friendly farming practices. This in situ amendment strategy, utilizing only green, biodegradable lignocellulosic wastes and environmentally friendly fungi, offers new pathways for the sustainable management of combined contamination and the improvement of human health.

Can N-Doped Biochar Achieve Safe Vegetable Production in Soil Heavily Contaminated by Heavy Metals?

Although the cultivation of food crops in farmland heavily contaminated by heavy metals is prohibited in China, vegetables can still be planted on a small-scale due to their short growth cycles and flexible sale models, posing a significant threat to local consumers. In this study, a pot culture experiment was conducted to investigate the feasibility of safe production through the in-situ stabilization of heavy metals in heavily contaminated soil. The remediation efficiency of wheat straw biochar and N-doped biochar, the growth of spinach, the heavy metal accumulation in spinach, and potential health risks were also explored. The results indicated that both biochar and N-doped biochar significantly affected the soil pH, cation exchange capacity, organic matter, available phosphorus, available potassium, alkaline nitrogen content, and spinach biomass, but the trends were variable. Additionally, the diethylenetriaminepentaacetic-extractable Pb, Cd, Cu, Zn, and Ni concentrations decreased 9.23%, 7.54%, 5.95, 7.44%, and 16.33% with biochar, and 10.46%, 12.91%, 21.98%, 12.62%, and 12.24% with N-doped biochar, respectively. Furthermore, N-doped biochar significantly reduced the accumulation of Pb, Cd, and Ni in spinach by 35.50%, 33.25%, and 30.31%, respectively. Health risk assessment revealed that the non-carcinogenic risk index for adults and children decreased from 17.0 and 54.8 to 16.3 and 52.5 with biochar and 11.8 and 38.2 with N-doped biochar, respectively, but remained significantly higher than the acceptable range (1.0). The carcinogenic risk assessment revealed that the risk posed by Cd in spinach exceeded the acceptable value (10-4) for both adults and children across all treatments. These results may imply that biochar and N-doped biochar cannot achieve the safe production of vegetables in soil heavily contaminated by heavy metals through in-situ stabilization.

Dual regulation of As release and soil environment by Fe(Ⅱ) assisted steel slag and coal fly ash: Effects and potential mechanisms

Multiple solid waste-based amendments are used for arsenic (As)-contaminated soil remediation, but their mechanisms in inhibiting As release and the effects on soil health in real sites remain poorly understood. Here, an amendment consisting of steel slag (SS), coal fly ash (CFA) and Fe(Ⅱ), namely, Fe(Ⅱ) assisted SS and CFA, was applied to an As-contaminated mining soil. 120 days field experimental results revealed that amendment addition in low-As soil (LA soil) and high-As soil (HA soil) significantly increased amorphous Fe(Ⅲ) (hydro)oxides content and decreased dissolved organic carbon (DOC), and thus inhibited As mobilization. More importantly, the soil microbial community activity was improved in HA soil, while it significantly decreased in LA soil. Correlation analyses demonstrated that the activation of fungal and bacterial communities was directly correlated with soil pH, amorphous Fe(Ⅲ) (hydro)oxides, soil organic matter (SOM), and DOC. The C-containing functional groups, newly generated Fe(Ⅲ) (hydro)oxides and Fe-As-SOM complexes inhibit As release, while the Fe(Ⅲ) reduction drove the As release. This work highlighted the importance of Fe(Ⅱ) assisted SS and CFA in inhibiting As release and regulating soil microbial communities, providing a new strategy for the remediation of heavy metals contaminated mining soil using solid waste-based amendment.

Synergistic effects of allantoin and Achyranthes japonica-biochar profoundly alleviate lead toxicity during barley growth

Lead (Pb), a toxic metal, causes severe health hazards to both humans and plants due to environmental pollution. Biochar addition has been efficiently utilized to enhance growth of plants as well as yield in the presence of Pb-induced stress. The present research introduces a novel use of biochar obtained from the weed Achyranthes japonica to enhance the growth of plants in Pb-contaminated soil. An experiment was performed with 7 treatments: Control, Pb2+ (10 mg kg-1) only, biochar (4 %) only, allantoin (4 g kg-1) only, biochar combined with Pb2+, allantoin combined with biochar, as well as a combination of allantoin and biochar with Pb2+. Lead toxicity alone markedly diminished plant growth metrics, including root and shoot length, biomass (wet and dry), chlorophyll concentration, and grain production. The application of biochar, allantoin, or their joint administration markedly enhanced the length of shoots (by 50.3 %, 29 %, and 70 %), length of roots (by 69 %, 50 %, and 69 %), and fresh biomass of shoots (by 5 %, 29 %, and 5 %), respectively. This enhancement is ascribed to improved soil characteristics and more efficient absorption of nutrients. The application of biochar, allantoin and their combination improved the tolerance against Pb2+ by increasing the total chlorophyll level by 12 %, 16 %, and 17 %, respectively, vs. the control. Likewise, these amendments significantly (p < 0.05) improved the activity of antioxidant enzymes, including SOD, POD, and CAT by 49 %, 29 %, and 49 %, respectively. The resistance towards Pb2+ was enhanced by biochar, allantoin, and their combined application, with lower Pb2+ concentrations in shoots (59.9 %, 40.1 %, and 49.8 %), roots (48.2 %, 24.1 %, and 58.3 %), and grains (60.2 %, 29.7 %, and 40.1 %) compared to solely Pb-stress, respectively. In summary, converting the weed Achyranthes japonica into biochar and integrating it with allantoin provides an eco-friendly approach to control its proliferation while efficiently alleviating Pb-induced toxicity in plants.

Effects of Different Nitrogen Fertilizer Application Rates on Soil Microbial Structure in Paddy Soil When Combined with Rice Straw Return

Metagenomic sequencing of the microbial soil community was used to assess the effect of various nitrogen fertilizer treatments in combination with constant rice straw return to the soil in the tiller layer of Northeast China's black paddy soil used for rice production. Here, we investigated changes in the composition, diversity, and structure of soil microbial communities in the soil treated with four amounts of nitrogen fertilizers (53, 93, 133, and 173 kg/ha) applied to the soil under a constant straw return of 7500 kg/ha, with a control not receiving N. The relationships between soil microbial community structure and soil physical and chemical properties were determined. The results showed that the available K content of the soil significantly (p < 0.05) increased in soil receiving the lowest N-fertilizer dose. When applied at high amounts, N-fertilizer changed the Chao1 and ACE indices of the soil microorganisms (p < 0.05), and the treatments resulted in significant differences in the β-diversity of the soil microorganisms. By NMDS analysis it was demonstrated that the treatment significantly affected the structure of the soil microbial communities. Redundancy analysis showed that the main physicochemical drivers behind these differences were total nitrogen, total potassium, ammonium nitrogen, total phosphorus, and available potassium. The soil microbial communities in the control treatment were negatively correlated with nitrate and ammonium nitrogen; the lowest N-fertilizer treatment produced positive correlations with total nitrogen, total potassium, and total phosphorus and negative correlations with ammonium nitrogen; the highest dose negatively correlated with total nitrogen, available potassium, available phosphorus, total phosphorus, and pH. This study showed that moderate N fertilizer application is an effective way to increase soil microbial diversity and improve soil quality. This experiment provides technical support for the application of the alternative fertilizer technology of straw return to the field and provides a theoretical basis for rational fertilization of paddy fields in a cold climate.

Differentiated effects and mechanisms of N-, P-, S-, and Fe-modified biochar materials for remediating Cd- and Pb-contaminated calcareous soil

To investigate the remediation effects of various modified biochar materials derived from different impregnation agents on Cd- and Pb-contaminated calcareous soil, nitrogen (N-), phosphorus (P-), sulfur (S-), and iron (Fe-) modified biochar materials (NBC, PBC, SBC, FBC) were fabricated through the impregnation-pyrolysis method and employed to immobilize Pb and Cd in the calcareous soil. The characterization results showed that NBC exhibited an uneven pore size distribution and increased aromaticity, while PBC and SBC had increased pH and ash content. Pot experiments demonstrated significantly different effects of various modified biochar materials on soil immobilization and plant uptake of Cd and Pb. With regard to soil pH, FBC caused a notable decrease in both rhizosphere and non-rhizosphere areas, while the other materials showed an increase. NBC, PBC, and SBC effectively immobilized Cd and Pb in the soil and significantly reduced their accumulation in Chinese cabbage by 34.4 %-58.9 % for Cd and 9.2 %-53.1 % for Pb, with PBC having the best effect, attributed to complexation, precipitation, and adsorption. However, FBC had strong acidity and poor immobilization ability, which increased the available concentrations of Cd and Pb in the soil. Additionally, PBC promoted the growth, enzyme activity, and tolerance to Cd- and Pb-contaminated soil of Chinese cabbage. Overall, NBC and PBC were identified as the most effective modified biochar materials for stabilizing Cd and Pb in the soil, reducing heavy metal uptake by Chinese cabbage, and boosting enzyme activity.

Pristine/magnesium-loaded biochar and ZVI affect rice grain arsenic speciation and cadmium accumulation through different pathways in an alkaline paddy soil

Cadmium (Cd) and arsenic (As) co-contamination has threatened rice production and food safety. It is challenging to mitigate Cd and As contamination in rice simultaneously due to their opposite geochemical behaviors. Mg-loaded biochar with outstanding adsorption capacity for As and Cd was used for the first time to remediate Cd/As contaminated paddy soils. In addition, the effect of zero-valent iron (ZVI) on grain As speciation accumulation in alkaline paddy soils was first investigated. The effect of rice straw biochar (SC), magnesium-loaded rice straw biochar (Mg/SC), and ZVI on concentrations of Cd and As speciation in soil porewater and their accumulation in rice tissues was investigated in a pot experiment. Addition of SC, Mg/SC and ZVI to soil reduced Cd concentrations in rice grain by 46.1%, 90.3% and 100%, and inorganic As (iAs) by 35.4%, 33.1% and 29.1%, respectively, and reduced Cd concentrations in porewater by 74.3%, 96.5% and 96.2%, respectively. Reductions of 51.6% and 87.7% in porewater iAs concentrations were observed with Mg/SC and ZVI amendments, but not with SC. Dimethylarsinic acid (DMA) concentrations in porewater and grain increased by a factor of 4.9 and 3.3, respectively, with ZVI amendment. The three amendments affected grain concentrations of iAs, DMA and Cd mainly by modulating their translocation within plant and the levels of As(III), silicon, dissolved organic carbon, iron or Cd in porewater. All three amendments (SC, Mg/SC and ZVI) have the potential to simultaneously mitigate Cd and iAs accumulation in rice grain, although the pathways are different.

Engineered biochar for in-situ and ex-situ remediation of contaminants from soil and water

Tailoring physical and chemical properties of biochar enhances its selectivity, treatability, and efficiency in contaminant remediation. Thus, engineered biochar has emerged as a promising remedy for both in-situ and ex-situ remediation of polluted soil and water. Several factors influence the effectiveness of engineered biochar, including feedstock sources, pyrolysis conditions, surface functionalization, mode of application, and site characteristics. The advantages and disadvantages of different modification approaches to engineered biochar and their specific treatability for in-situ and ex-situ remediation are obscure and must be adequately addressed. This review critically evaluates the application of engineered biochar for on/off-spot contamination management, taking into account the long-term stability and biocompatibility prospects. The properties of engineered biochar resulting from modification with clay minerals, nanoparticles, polymers, surfactants, and oxidants/reductants were critically reviewed. Recent progress and advances in remediation mechanisms and modes of application were elaborated for the effective removal of organic and inorganic contaminants, including heavy metals, pesticides, dyes, polycyclic aromatic hydrocarbons, per- and poly-fluoroalkyl substances, and agrochemicals. Several crucial parameters influence in-situ remediation, including the distribution of contaminants, background electrolytes, hydraulic conductivity, as well as dispersion and stability of adsorbents. Ex-situ remediation of pollutants relies heavily on adsorption or degradation kinetics, background electrolytes, adsorbent dose, and pollutant concentrations. In addition, factors restricting the application of engineered biochar were highlighted for long-term sustainable contaminant management and maintaining low environmental impact. Finally, the challenges and future perspectives of utilizing engineered biochar for field-scale demonstration of contaminated sites are proposed.

A novel agrosinters support growth, photosynthetic efficiency and reduce trace metal elements accumulation in oilseed rape growing on metalliferous soil

Soil conditioners to fertilize, improve soil structure and support the phytostabilization of trace metal elements (TMEs) are being used more and more frequently. One of the options are agrosinters - slow-release ceramic fertilizers consisting mainly of SiO2, CaO, P2O5 and K2O, with an alkaline pH and high impact strength. The effect of two different agrosinters, A1 and A2, on the growth and physiological condition of Brassica napus grown in uncontaminated and Pb-, Cd- and Zn-contaminated soil was investigated in a pot experiment. In vivo data were collected using an infrared gas analyzer, a fluorimeter, a pigment content meter and a thermal camera. Elemental composition of the biomass was also investigated. The tested agrosinters promote biomass yield and have an effect on improving leaf chlorophyll content, phenomenological energy fluxes and plant gas exchange. The effect of the agrosinters on the plants was dose- and amendment-specific. An immobilization effect was observed not only in the soil but also in the plants. A reduction in the Cd (22%) and Zn (40%) content in the biomass was measured. All this was related to the effect of increasing the available form of P (50%), K (300%) and Ca (50%) in the soil, which improves soil fertility and reduces the bioavailable forms of the studied TMEs, due to the increase in soil pH and/or the complexation of these with phosphate compounds. The multidimensional analysis of A2 agrosinter shows the most positive effects on plant conditions, indicating that fly ash as a mixed substrate benefits the plants.

Influence of sludge biochar at different carbonization temperatures on anammox process

Adding biochar can expedite the establishment of the anaerobic ammonia oxidation (anammox) process and improve the nitrogen removal efficiency of the anammox reactor. However, the optimization research of biochar derived from dewatered sludge on anammox is relatively limited. In this study, four sequencing batch reactors (SBRs) were compared for the enrichment of anammox bacteria using synthetic wastewater with sludge biochar carbonized at temperatures of 300°C, 550°C, and 800°C, and without biochar (CK). The start-up and the nitrogen removal performance of anammox process were evaluated, as well as the effect of organic carbon on nitrogen removal. The results showed that the addition of sludge biochar at different pyrolysis temperatures all can accelerate the start-up of the anammox process, improve the nitrogen removal efficiency, and reduce the total nitrogen (TN) in the effluent. Although the reactor with biochar carbonized at 800°C showed the fastest increase in the nitrogen loading, the best TNRE occurred in the reactor with biochar carbonized at 300°C, which was 8.0% higher than those of the control (CK, p < 0.05). The predominant genus of anammox in SBRs differed between the sludge biochar reactor and the control reactor (without biochar), which were Candidatus Brocadia and Candidatus Jettenia, respectively. Additionally, the total abundances of anammox bacteria and denitrifiers increased with the addition of sludge biochar.

Activated carbon-mediated arsenopyrite oxidation and arsenic immobilization: ROS formation and its role

The oxidative dissolution of arsenopyrite (FeAsS) is a significant source of arsenic contamination in nature. Activated biochar (AC), a widely used environmental remediation agent, is prevalent in ecosystems and participated in various geochemical processes of arsenic and iron-containing sulfide minerals. However, the impact of AC-arsenopyrite association on reactive oxidation species (ROS) generation and its contribution to As transformation were rarely explored. Here, ROS formation and the redox conversion of As during the interaction between AC and arsenopyrite were investigated. AC-mediated arsenopyrite oxidation was a two-stage process. At stage I, the heterogeneous electron transfer from arsenopyrite facilitated O2 reduction on AC, enhancing arsenopyrite dissolution and ROS formation. TBA, PBQ and catalase inhibited 86.40 %, 79.39 % and 49.66 % of As(III) oxidation, respectively, indicating indicated that HO˙, (O2•)- and H2O2 were responsible for As(III) oxidation. However, at stage II, the mobility of As was highly restricted, especially increasing AC addition. Besides adsorption, AC retained appreciable As through catalyzing insoluble ferric arsenate formation and growth by promoting Fe(II) and As(III) oxidation and functioning as nuclei. These findings deepen our understanding of the coupling behavior of AC-arsenopyrite and its influence on geochemical cycling of arsenic in mined surroundings, which has important implications for mitigating arsenic pollution.

Endogenous silicon-activated rice husk biochar prepared for the remediation of cadmium-contaminated soils: Performance and mechanism

Biochar is widely used for the remediation of heavy metal-contaminated soils. However, pristine biochar generally has limited active functional groups and adsorption sites, thereby exhibiting low immobilization performance for heavy metals. In addition to carbon (C), silicon (Si) is another common macro-element present in rice husk biochar, but it often exists in the form of amorphous oxide and therefore contributes little to the adsorption performance for heavy metals. The transformation of amorphous Si oxide to dissolved silicate through a precipitation effect can significantly improve its heavy metal immobilization capability. Herein, the amorphous Si oxide in rice husk biochar was activated by sodium hydroxide and then the dissolved silicate was immobilized by calcium salt. The as-synthetized Si-activated biochar was used to remediate cadmium (Cd)-contaminated soils. The results indicated that Si-activated rice husk biochar could reduce Cd migration and environmental risks by the transformation from exchangeable Cd into carbonate-bound and residual Cd. With increasing Ca: Si molar ratio, the content of CaCl2 and H2O-extractable Cd exhibited a decreasing trend. Moreover, a higher addition amount of Si-activated biochar improved the Cd immobilization efficiency. The application of 1.0% Ca/Si molar ratio of 2: 2 Si-activated rice husk biochar decreased the CaCl2-Cd and H2O-Cd concentration by a maximum of 83.7% and 90.5% compared with pristine rice husk biochar, respectively. The present work proposes an approach for highly efficient remediation of Cd-polluted soils by biochar.

Water-extractable metals as indicators of wheat metal accumulation: Insights from Cd, Pb, Mn, Cu, and Zn

There is a long-standing debate over the effectiveness of chemical extraction methods in assessing soil metal phytoavailability. This study addresses the limitations of widely-used chemical extraction methods and presents the water-extractable pool as a more reliable indicator based on wheat pot experiments using homogenized agricultural soil amended with lime materials, phosphate, and biochar. Over 120 days' pot experiments, Cd accumulation in whole wheat plants and tissues exhibited positive relationships with water-extractable Cd concentrations at heading and maturity stage (Spearman's rho: 0.521-0.851; P < 0.05), revealing that the water-extractable pool instead of other pools better indicates wheat metal accumulation. Water-extractable metal concentrations are effective in assessing phytoavailability of metals primarily in ionic forms in soil solution (e.g, Zn, Cd), but less reliable for metals strongly complexed with dissolved organic matter (DOM) or sensitive to redox conditions. It demonstrated that water-extractable metal concentrations and chemical forms are key factors, fundamentally determined by metal properties and impacted by environmental factors. This study clarifies a more direct link between chemical extraction and plant metal uptake mechanisms. Given the extensive application of chemical extraction methods over several decades, this study will help advance soil metal risk assessment and remediation practices.

Role of key microbial modules for soil carbon sequestration effects in biochar-based remediation of cadmium-contaminated soil

How changes in soil properties and heavy metal toxicity induced during biochar remediation of cadmium (Cd) contaminated soils induce changes in microbial communities, and further, how this process affects the soil carbon sequestration capacity by microorganisms, has not been explored. We prepared virgin biochar (named BC400 and BC600) using rice straw at 400 °C and 600 °C and modified biochar (named MNT-BC400 and MNT-BC600) using co-pyrolysis of montmorillonite and rice straw at the same two temperatures, in an attempt to create different CO2 emission backgrounds during the remediation of cadmium contamination in soil, and to explore the mechanism of soil carbon sequestration capacity during remediation. The results showed that MNT-BC600 was effective in reducing the soil available Cd during incubation without increasing soil carbon emission, whereas soil carbon emission was elevated by 83.10%, 50.19%, and 21.53% in BC400, MNT-BC400, and MNT-BC600 treatments compared to the control group (CK). Microbial carbon use efficiency (CUE) was increased by 20.68%, 18.78%, and 12.73% in BC400, BC600, and MNT-BC600 treatments, respectively. We found that the bacterial module (Bmod#2), dominated by the eutrophic bacteria Proteobacteria and Actinobacteriota, controlled the increase of soil carbon emissions; the bacterial module (Bmod#3) composed of both oligotrophic and eutrophic flora were closely related to soil CUE after remediation; the bacterial module (Bmod#4) consisting of oligotrophic and eutrophic bacteria alone was easily replaced by eutrophic bacteria module (Bmod#2) under eutrophic conditions and had a negative linear relationship with soil CO2 emission, while the switch in Cd form was more likely to affect the community structure of modules containing eutrophic bacteria. These findings provide new insights into the use of biochar for soil remediation and balancing soil carbon sequestration.

Significance of humic matters-soil mineral interactions for environmental remediation: A review

Industrial and human activities have led to serious soil and water pollution. Traditional remediation techniques have problems such as high treatment costs and the tendency to cause secondary pollution. Soil minerals and humic matters are common active components in soils. Both play vital roles and are frequently bound together to form humic matters-mineral complexes, which are considered sustainable and eco-friendly materials for environmental remediation and improvement. However, due to the complexity of humic matters-mineral interactions and the wide variation in the removal of different pollutants, there is a lack of research in this area. This paper provides a comprehensive introduction and summary of the interaction mechanisms between humic matters and typical soil minerals such as layered phosphate minerals and iron oxides, and their applications in environmental remediation, especially for the treatment of heavy metals (lead, mercury, chromium and cadmium) and organic pollutants (antibiotics, pesticides and polycyclic aromatic hydrocarbons) in water and soil. The humic matters-mineral complex can reduce the toxicity and migration rate of pollutants through adsorption, electrostatic attraction, together with H-bonding and hydrophobic interactions, reducing the harm of these pollutants to soil and water environments and realizing the efficient remediation of soil and water environments. And compared with the traditional treatment technology, this method is more green and environmental protection, and the treatment cost is greatly reduced. Finally, the deficiencies of using humic matters-mineral complex to achieve soil and water remediation were summarized and also proposed directions for future endeavors as well as concrete measures.

Effect of metal-modified sewage sludge biochar tubule on immobilization of chromium in unsaturated soil: Groundwater table fluctuations induced by rainfall

Many studies have studied biochar immobilizing chromium (Cr) in soil. However, few studies were conducted to reduce the environmental risks due to biochar aging in soil. In this study, we adopt FeCl3, MgCl2, and AlCl3 to activate sewage sludge to form modified biochar and produce biochar tubules. Then, the column experiments were carried out to study the effect of fluctuating groundwater table on Cr leaching behavior, total Cr, and fractions distribution with the insertion of biochar tubule. Results showed that the Cr immobilization performance was improved by metal-modification biochar, the biochar tubules can significantly decrease the Cr leaching amounts, retard the Cr downward migration in the soil, and there was a better effect with slightly Cr-contaminated soil. In addition, the immobilization effect is also impacted by the biochar's application mode and the hydrodynamic conditions. Detailedly, the Cr leaching amounts maximally decreased by 33.39%, the residual amounts maximally increased by 10.05% in the soil column, and the exchangeable (EX) and carbonates-bound (CB) fractions were maximally increased by 85.18%, 151.78% at the equal depth of soil column, respectively. BET, SEM-EDS, XRD, and FTIR analyses revealed that biochars' immobilization mechanisms on Cr involved reduction(predominately), physisorption, precipitation, and complexation. Risk assessment demonstrated that the sewage sludge biochar has very low environmental risk. This study indicates that the biochar tubule applied to immobilize Cr in soil has potential and provides new insights into reducing environmental risks due to biochar aging.

Biochar with KMnO4-hematite modification promoted foxtail millet growth by alleviating soil Cd and Zn biotoxicity

The excessive accumulation of Cd and Zn in soil poisons crops and threatens food safety. In this study, KMnO4-hematite modified biochar (MnFeB) was developed and applied to remediate weakly alkaline Cd-Zn contaminated soil, and the heavy metal immobilization effect, plant growth, and metal ion uptake of foxtail millet were studied. MnFeB application reduced the phytotoxicity of soil heavy metals; bioavailable acid-soluble Cd and Zn were reduced by 57.79% and 35.64%, respectively, whereas stable, non-bioavailable, residual Cd and Zn increased by 96.44% and 32.08%, respectively. The chlorophyll and total protein contents and the superoxide dismutase (SOD)activity were enhanced, whereas proline, malondialdehyde, the H2O2 content, glutathione reductase (GR), ascorbate peroxidase (APX) and catalase (CAT) activities were reduced. Accordingly, the expressions of GR, APX, and CAT were downregulated, whereas the expression of MnSOD was upregulated. In addition, MnFeB promoted the net photosynthetic rate and growth of foxtail millet plants. Furthermore, MnFeB reduced the levels of Cd and Zn in the stems, leaves, and grains, decreased the bioconcentration factor of Cd and Zn in shoots, and weakened the translocation of Cd and Zn from roots to shoots. Precipitation, complexation, oxidation-reduction, ion exchange, and π-π stacking interaction were the main Cd and Zn immobilization mechanisms, and MnFeB reduced the soil bacterial community diversity and the relative abundance of Proteobacteria and Planctomycetota. This study provides a feasible and effective remediation material for Cd- and Zn-contaminated soils.

Biochar regulates the functions of keystone taxa to reduce p-coumaric acid accumulation in soil

Introduction

Applying biochar (BC) to reduce toxic substance accumulation in soil, either through direct adsorption or modulation of the microbial community, has received considerable attention. However, a knowledge gap exists regarding how BC regulates microbial community structure and functions to mitigate toxic substance accumulation.

Methods

We previously identified p-coumaric acid (p-CA) as a representative autotoxin in tobacco rhizosphere soil. On this basis, this study simulated a soil environment with p-CA accumulation to investigate the impacts of BC on p-CA, soil physicochemical properties, and microbial community structure and function.

Results

The results showed that p-CA could be directly adsorbed onto BC, which followed the pseudo-second-order kinetic model (R 2 = 0.996). A pot experiment revealed that BC significantly reduced soil p-CA, altered soil microbial composition, and enhanced bacterial community diversity. A weighted correlation network analysis showed a close association between taxon 1 in the microbial network and p-CA, suggesting a pivotal role for this taxon in reducing p-CA, with Devosia and Nocardioides identified as potential key contributors to this process. The prediction of possible keystone taxa functions showed that BC increased the relative abundances of aromatic compound degraders. Mantel tests indicated that soil organic matter exerted the greatest influence on keystone taxa functions and hub genera.

Discussion

These findings suggest that BC may either directly chemisorb p-CA or indirectly facilitate p-CA degradation by regulating the functioning of keystone taxa. The results of this study provide a novel perspective for further investigation of the mechanisms through which BC reduces the accumulation of toxic substances in soil.

Unveiling biochar potential to promote safe crop production in toxic metal(loid) contaminated soil: A meta-analysis

Biochar application emerges as a promising and sustainable solution for the remediation of soils contaminated with potentially toxic metal (loid)s (PTMs), yet its potential to reduce PTM accumulation in crops remains to be fully elucidated. In our study, a hierarchical meta-analysis based on 276 research articles was conducted to quantify the effects of biochar application on crop growth and PTM accumulation. Meanwhile, a machine learning approach was developed to identify the major contributing features. Our findings revealed that biochar application significantly enhanced crop growth, and reduced PTM concentrations in crop tissues, showing a decrease trend of grains (36.1%, 33.6-38.6%) > shoots (31.1%, 29.3-32.8%) > roots (27.5%, 25.7-29.2%). Furthermore, biochar modifications were found to amplify its remediation potential in PTM-contaminated soils. Biochar application was observed to provide favorable conditions for reducing PTM uptake by crops, primarily through decreasing available PTM concentrations and improving overall soil quality. Employing machine learning techniques, we identified biochar properties, such as surface area and C content as a key factor in decreasing PTM bioavailability in soil-crop systems. Furthermore, our study indicated that biochar application could reduce probabilistic health risks associated with of the presence of PTMs in crop grains, thereby contributing to human health protection. These findings highlighted the essential role of biochar in remediating PTM-contaminated lands and offered guidelines for enhancing safe crop production.

Metal-doped biochar for selective recovery and reuse of phosphate from water: Modification design, removal mechanism, and reutilization strategy

Phosphorus (P) plays a crucial role in plant growth, which can provide nutrients for plants. Nonetheless, excessive phosphate can cause eutrophication of water, deterioration of aquatic environment, and even harm for human health. Therefore, adopting feasible adsorption technology to remove phosphate from water is necessary. Biochar (BC) has received wide attention for its low cost and environment-friendly properties. However, undeveloped pore structure and limited surface groups of primary BC result in poor uptake performance. Consequently, this work introduced the synthesis of pristine BC, parameters influencing phosphate removal, and corresponding mechanisms. Moreover, multifarious metal-doped BCs were summarized with related design principles. Meanwhile, mechanisms of selective phosphate adsorption by metal-doped BC were investigated deeply, and the recovery of phosphate from water, and the utilization of phosphate-loaded adsorbents in soil were critically presented. Finally, challenges and prospects for widespread applications of selective phosphate adsorption were proposed in the future.

Efficacy of Fe-Mg-bimetallic biochar in stabilization of multiple heavy metals-contaminated soil and attenuation of toxicity in spinach (Spinacia oleracea L.)

Globally, soil contamination with heavy metals (HMs) pose serious threats to soil health, crop productivity, and human health. The present investigation involved synthesis and analysis of biochar with bimetallic combination of iron and magnesium (Fe-Mg-BC). Our study evaluated how Fe-Mg-BC affects the absorption of cadmium (Cd), lead (Pb), and copper (Cu) in spinach (Spinacia oleracea L.) and remediation of soil contaminated with multiple HMs. Results demonstrated the successful loading of iron (Fe) and magnesium (Mg) onto pristine biochar (BC) derived from peanut shells. The addition of Fe-Mg-BC (3%) notably increased spinach biomass, enhancing photosynthesis, transpiration, stomatal conductance, and intercellular CO2 levels by 22%, 21%, 103%, and 15.3%, respectively. Compared to control, Fe-Mg-BC (3%) suppressed metal-induced oxidative stress by boosting levels of superoxide dismutase (SOD), ascorbate peroxidase (APX) and catalase (CAT) in roots by 40.9%, 57%, 54.8 %, and in shoots by 55.5%, 65.5%, and 37.4% in shoots, respectively. The Fe-Mg-BC effectively reduced the uptake of Cd, Pb, and Cu in spinach tissues by transforming their bioavailable fractions to non-bioavailable forms. The Fe-Mg-BC (3%) significantly reduced the mobility of Cd, Pb and Cu in soil and limited the concentration of Cd, Pb, and Cu in plant roots by 34.1%, 79.2%, 47%, and shoots by 56.3%, 43.3%, and 54.1%, respectively, compared to control. These findings underscore the potential of Fe-Mg-BC as a promising amendment for reclaiming soils contaminated with variety of HMs, thereby making a significant contribution to the promotion of safer food production.

Mitigation of thallium threat in paddy soil and rice plant by application of functional biochar

Thallium (Tl) is a highly toxic metal, and its contamination in soils entails high risks to human health via food chain. It remains largely unknown of the effects of applying biochar on Tl uptake in paddy systems despite that few studies have shown that biochar exhibits great potential for decreasing Tl bioavailability in soils. Herein, we examined the mitigating effects of the application of biochar (5 and 20 g/kg pristine biochar; 5 and 20 g/kg Fe/Mn-modified biochar) on Tl uptake in paddy soil and rice plant after an entire rice growth period. The results suggested that the application of Fe/Mn-modified biochar (FMBC) considerably mitigated the accumulation of Tl in different tissues of rice plants. Specifically, total Tl content in rice plants treated with FMBC-20 decreased by over 75% compared with control experiment. In addition, the amendment of FMBC in Tl-rich paddy soils can enhance the communities of microorganisms (Actinobacteria and Proteobacteria). Further analysis of the soil microbial symbiosis network revealed that FMBC promotes the living microorganisms to play modular synergistic interactions, which is crucial for FMBC-induced Tl stabilization in soils. All these findings indicated that FMBC is an efficient and environmentally friendly Tl-immobilization alternative material and can be potentially used in the remediation of Tl-contaminated paddy soils and/or cropland.

Investigating the carcinogenic and non-carcinogenic health hazards of heavy metal ions in Spinacia oleracea grown in agricultural soil treated with biochar and humic acid

This study addressed the bioaccumulation and human health risk among the consumption of Spinacia oleracea grown in agricultural soil treated with humic acid (189-2310 ppm) and biochars (0.00-5.10%.wt). The biochars came from two local feedstocks of rice-husk (RH) and sugar-beet-pulp (SBP) pyrolyzed at temperatures 300 and 600 °C. Total concentrations of Cu, Cd, and Ni found in both the soil and biomass/biochar exceeded global safety thresholds. The bioaccumulation levels of HMs in spinach leaves varied, with Fe reaching the highest concentration at 765.27 mg kg-1 and Cd having the lowest concentration at 3.31 mg kg-1. Overall, the concentrations of Zn, Cd, Pb, and Ni in spinach leaves exceeded the safety threshold limits, so that its consumption is not recommended. The assessment of hazard quotient (HI) for the HMs indicated potential health hazards for humans (HI > 1) from consuming the edible parts of spinach. The biochar application rates of 4.35%wt and 0.00%.wt resulted in the highest (3.69) and lowest (3.15) HI values, respectively. The cumulative carcinogenic risk (TCR) ranged from 0.0085 to 0.0119, exceeding the cancer risk threshold. Introducing 5.10%wt biomass/biochar resulted in a 36% rise in TCR compared to the control. The utilization of humic acid alongside HMs-polluted biochars results in elevated levels of HMs bioaccumulation exceeding the allowable thresholds in crops (with a maximum increase of 49% at 2000 ppm humic acid in comparison to 189 ppm). Consequently, this raised the HI by 46% and the TCR by 22%. This study demonstrated that the utilization of HMs-polluted biochars could potentially pose supplementary health hazards. Moreover, it is evident that the utilization of HMs-polluted biochars in treating metal-contaminated soil does not effectively stabilize or reduce pollution.

Eliminating the stabilizer antagonistic effects for efficiently stabilizing Pb and As co-contaminated soil by innovative stepwise steam flash heating

Chemical stabilization is frequently used to stabilize lead (Pb) or arsenate (As), but faces challenges in Pb-As co-contaminated soils because of the antagonistic reactions between chemical stabilizers and contaminants. In this work, we innovated an effective and cost-efficient stepwise steam flash heating (SSFH) strategy to simultaneously immobilize Pb and As, and unraveled the underlying mechanisms. The combination of 1.5% Ca(H2PO4)2 and 2% Fe2(SO4)3 only decreased 1.99% Pb by toxicity characteristic leaching procedure (TCLP-Pb) but increased 17.8% of TCLP-As due to the antagonistic effects. SSFH with Ca(H2PO4)2 in the first step and Fe2(SO4)3 in the second step achieved the minimal TCLP-Pb and TCLP-As of 0.778 and 0.327 mg/L, respectively. It also reduced 69.8% of leachable As in 100-year acid rain simulation, indicating a favorable long-term stabilization performance. Additionally, SSFH approach reduced 43.2% stabilizer dosage and 14.9% cost. X-ray absorption near edge structure (XANES) documented that the stepwise SFH promoted the transformation of Pb(NO3)2 and NaAsO2/NaAsO3/As2O3/As2O5 into stable Pb3(PO4)2 and FeAsO4, preventing the formation of AsO43- and FePO4. Our findings proved the state-of-the-art SSFH approach and unraveled its mechanisms to stabilize Pb and As co-contamination in soils, offering a green and sustainable remediation alternative for the management of heavy metal contaminated sites. ENVIRONMENTAL IMPLICATION: A novel stepwise SFH approach can be applied to overcome the stabilizer antagonist effects by separately immobilizing Pb and As in two sequential steps. It also decreased 43.2% of stabilizer dosage and 14.9% of cost comparing to conventional chemical stabilization. This approach can be used for other metal co-contaminated soils facing similar antagonistic challenges, and our work raises a state-of-the-art solution for cost-effective, green and sustainable remediation practices.

A systematic review of biochar aging and the potential eco-environmental risk in heavy metal contaminated soil

Biochar is widely accepted as a green and effective amendment for remediating heavy metals (HMs) contaminated soil, but its long-term efficiency and safety changes with biochar aging in fields. Currently, some reviews have qualitatively summarized biochar aging methods and mechanisms, aging-induced changes in biochar properties, and often ignored the potential eco-environmental risk during biochar aging process. Therefore, this review systematically summarizes the study methods of biochar aging, quantitatively compares the effects of different biochar aging process on its properties, and discusses the potential eco-environmental risk due to biochar aging in HMs contaminated soil. At present, various artificial aging methods (physical aging, chemical aging and biological aging) rather than natural field aging have been applied to study the changes of biochar's properties. Generally, biochar aging increases specific surface area (SSA), pore volume (PV), surface oxygen-containing functional group (OFGs) and O content, while decreases pH, ash, H, C and N content. Chemical aging method has a greater effect on the properties of biochar than other aging methods. In addition, biochar aging may lead to HMs remobilization and produce new types of pollutants, such as polycyclic aromatic hydrocarbons (PAHs), environmentally persistent free radicals (EPFRs) and colloidal/nano biochar particles, which consequently bring secondary eco-environmental risk. Finally, future research directions are suggested to establish a more accurate assessment method and model on biochar aging behavior and evaluate the environmental safety of aged biochar, in order to promote its wider application for remediating HMs contaminated soil.

Co-pyrolysis of alkali-fused fly ash and corn stover to synthesize biochar composites for remediating lead-contaminated soil

Fly ash (FA) is mainly composed of silica, alumina, and other metal oxide components, and has a positive stabilizing effect on soil heavy metals. Biochar composites produced from FA and corn stover (CS) can improve its remediation performance. Therefore, a batch of biochar composites (alkali-fused FA-CS biochars, ABs), synthesized via co-pyrolysis of CS and alkali-fused FA (AFFA) at different temperatures of 300, 500, and 700 °C (AB300-1, AB500-1, and AB700-1) and CS to AFFA mass ratios of 10:1, 10:2, and 10:5 (AB500-1, AB500-2, and AB500-5), was used to remediate lead (Pb)-contaminated soil. Compared with pristine biochars (BCs), ABs were enriched with oxygen-containing functional groups (Si-O-Si and Si-O) and aromatic structures. The ABs prepared at lower pyrolytic temperature (≤500 °C) and lower ratio of CS to AFFA (10:1) showed higher yield and stability. The contents of Toxicity Characteristic Leaching Procedure (TCLP)-extractable Pb and DTPA-CaCl2-triethanolamine (DTPA)-extractable Pb were generally lower in the soils amended with ABs than BCs. Compared with other ABs such as AB300-1, AB500-2, AB500-5, and AB700-1, the soil amended with AB500-1 had lower contents of TCLP and DTPA-extractable Pb (24% reduction), exhibiting superior performance in stabilizing Pb in the soil. The gradual decrease of DTPA-extractable Pb content in the soil with increasing dosage of AB500-1 amendments suggests that AB500-1 facilitated the conversion of bioavailable Pb to the stable and less toxic residual fractions. Specifically, the highest percentage of residual fraction of Pb in soil amended with AB500-1 was 14%. Correlation analyses showed that the soil DTPA-extractable Pb content decreased with the increase of soil pH and cation-exchange capacity (CEC) value. ABs stabilize Pb in the soils mainly via electrostatic attraction, precipitation, cation-π interaction, cation exchange, and complexation. These findings provide insights for producing functionalized biochar composites from industrial waste like FA and biomass waste for remediating the soils polluted by heavy metals.

Mechanistic and future prospects in rhizospheric engineering for agricultural contaminants removal, soil health restoration, and management of climate change stress

Climate change, food insecurity, and agricultural pollution are all serious challenges in the twenty-first century, impacting plant growth, soil quality, and food security. Innovative techniques are required to mitigate these negative outcomes. Toxic heavy metals (THMs), organic pollutants (OPs), and emerging contaminants (ECs), as well as other biotic and abiotic stressors, can all affect nutrient availability, plant metabolic pathways, agricultural productivity, and soil-fertility. Comprehending the interactions between root exudates, microorganisms, and modified biochar can aid in the fight against environmental problems such as the accumulation of pollutants and the stressful effects of climate change. Microbes can inhibit THMs uptake, degrade organic pollutants, releases biomolecules that regulate crop development under drought, salinity, pathogenic attack and other stresses. However, these microbial abilities are primarily demonstrated in research facilities rather than in contaminated or stressed habitats. Despite not being a perfect solution, biochar can remove THMs, OPs, and ECs from contaminated areas and reduce the impact of climate change on plants. We hypothesized that combining microorganisms with biochar to address the problems of contaminated soil and climate change stress would be effective in the field. Despite the fact that root exudates have the potential to attract selected microorganisms and biochar, there has been little attention paid to these areas, considering that this work addresses a critical knowledge gap of rhizospheric engineering mediated root exudates to foster microbial and biochar adaptation. Reducing the detrimental impacts of THMs, OPs, ECs, as well as abiotic and biotic stress, requires identifying the best root-associated microbes and biochar adaptation mechanisms.

Remediation effect of walnut shell biochar on Cu and Pb co-contaminated soils in different utilization types

Biochar, with its dual roles of soil remediation and carbon sequestration, is gradually demonstrating great potential for sustainability in agricultural and ecological aspects. In this study, a porous biochar derived from walnut shell wastes was prepared via a facile pyrolysis coupling with in-situ alkali etching method. An incubation study was conducted to investigate its performance in stabilizing copper (Cu) and lead (Pb) co-contaminated soils under different utilization types. The biochar effectively decreased the bioavailable Cu (8.5-91.68%) and Pb (5.03-88.54%), while increasing the pH, CEC, and SOM contents in both soils. Additionally, the results of sequential extraction confirmed that biochar promoted the transformation of the labile fraction of Cu and Pb to stable fractions. The mechanisms of Cu and Pb stabilization were found to be greatly dependent on the soil types. For tea plantation yellow soil, the main approach for stabilization was the complexation of heavy metals with abundant organic functional groups and deprotonation structure. Surface electrostatic adsorption and cation exchange contributed to the immobilization of Cu and Pb in vegetable-cultivated purple soil. This research provides valuable information for the stabilization of Cu and Pb co-contaminated soils for different utilization types using environmentally-friendly biochar.

Mechanism of clay mineral modified biochar simultaneously immobilizes heavy metals and reduces soil carbon emissions

Heavy metal pollution in farmland soil has become increasingly severe, and multi-element composite pollution has brought enormous harm to human production and life. Environmental changes in cold regions (such as freeze-thaw cycles and dry-wet alternations) may increase the potential physiological toxicity of heavy metals and exacerbate pollution risks. In order to reveal the effectiveness of sepiolite modified biochar in the remediation of the soil contaminated with lead (Pb), cadmium (Cd), and chromium (Cr), the rice husk biochar pyrolyzed at 500 and 800 °C were selected for remediation treatment (denoted as BC500 and BC800). Meanwhile, different proportions of sepiolite were used for modification (biochar: sepiolite = 1: 0.5 and 1: 1), denoted as MBC500/MBC800 and HBC500/HBC800, respectively. The results showed that modified biochar with sepiolite can effectively improve the immobilization of heavy metals. Under natural conservation condition, the amount of diethylenetriaminepentaacetic acid (DTPA) extractable Pb in BC500, MBC500, and HBC500 decreased by 5.95, 12.39, and 13.55%, respectively, compared to CK. Freeze-thaw cycles and dry-wet alternations activated soil heavy metals, while modified biochar increased adsorption sites and oxygen-containing functional groups under aging conditions, inhibiting the fractions transformation of heavy metals. Furthermore, freeze-thaw cycles promoted the decomposition and mineralization of soil organic carbon (SOC), while sepiolite hindered the release of active carbon through ion exchange and adsorption complexation. Among them, and the soil dissolved organic carbon (DOC) content in HBC800 decreased by 49.39% compared to BC800. Additionally, the high-temperature pyrolyzed biochar (BC800) enhanced the porosity richness and alkalinity of material, which effectively inhibited the migration and transformation of heavy metals compared to BC500, and reduced the decomposition of soil DOC.

Soil amendments alter cadmium distribution and bacterial community structure in paddy soils

Soil amendments play a pivotal role in ensuring the safety of food production by inhibiting the transfer of heavy metal ions from soils to crops. Nevertheless, their impact on soil characteristics and the microbial community and their role in reducing cadmium (Cd) accumulation in rice remain unclear. In this study, pot experiments were conducted to investigate the effects of three soil amendments (mineral, organic, and microbial) on the distribution of Cd speciation, organic components, iron oxides, and microbial community structure. The application of soil amendments resulted in significant reductions in the soil available Cd content (16 %-51 %) and brown rice Cd content (16 %-78 %), facilitating the transformation of Cd from unstable forms (decreasing 10 %-20 %) to stable forms (increasing 77 %-150 %) in the soil. The mineral and organic amendments increased the soil cation exchange capacity (CEC) and plant-derived organic carbon (OC), respectively, leading to reduced Cd accumulation in brown rice, while the microbial amendment enhanced OC complexity and the abundances of Firmicutes and Bacteroidota, contributing to the decreased rice Cd uptake. The synchrotron radiation-based Fourier transform infrared (SR-FTIR) spectroscopy indicated that soil amendments regulated soil Cd species by promoting iron oxides and OC coupling. Moreover, both organic and microbial amendments significantly reduced the diversity and richness of the bacterial communities and altered their compositions and structures, by increasing the relative abundances of Bacteroidota and Firmicutes and decreasing those of Acidobacteria, Actinobacteria, and Myxococcota. Soil microbiome analysis revealed that the increase of Firmicutes and Bacteroidota associated with Cd adsorption and sequestration contributed to the suppression of soil Cd reactivity. These findings offer valuable insights into the potential mechanisms by which soil amendments regulate the speciation and bioavailability of Cd, and improve the bacterial communities, thereby providing guidance for agricultural management practices.

Heavy metals immobilization and bioavailability in multi-metal contaminated soil under ryegrass cultivation as affected by ZnO and MnO2 nanoparticle-modified biochar

Pollution by heavy metals (HMs) has become a global problem for agriculture and the environment. In this study, the effects of pristine biochar and biochar modified with manganese dioxide (BC@MnO2) and zinc oxide (BC@ZnO) nanoparticles on the immobilization and bioavailability of Pb, Cd, Zn, and Ni in soil under ryegrass (Lolium perenne L.) cultivation were investigated. The results of SEM-EDX, FTIR, and XRD showed that ZnO and MnO2 nanoparticles were successfully loaded onto biochar. The results showed that BC, BC@MnO2 and BC@ZnO treatments significantly increased shoots and roots dry weight of ryegrass compared to the control. The maximum dry weight of root and shoot (1.365 g pot-1 and 4.163 g pot-1, respectively) was reached at 1% BC@MnO2. The HMs uptake by ryegrass roots and shoots decreased significantly after addition of amendments. The lowest Pb, Cd, Zn and Ni uptake in the plant shoot (13.176, 24.92, 32.407, and 53.88 µg pot-1, respectively) was obtained in the 1% BC@MnO2 treatment. Modified biochar was more successful in reducing HMs uptake by ryegrass and improving plant growth than pristine biochar and can therefore be used as an efficient and cost effective amendment for the remediation of HMs contaminated soils. The lowest HMs translocation (TF) and bioconcentration factors were related to the 1% BC@MnO2 treatment. Therefore, BC@MnO2 was the most successful treatment for HMs immobilization in soil. Also, a comparison of the TF values of plant showed that ryegrass had a good ability to accumulate all studied HMs in its roots, and it is a suitable plant for HMs phytostabilization.

One-step synthesis of a core-shell structured biochar using algae (Chlorella) powder and ferric sulfate for immobilizing Hg(II)

Mercury (Hg) pollution poses a significant environmental challenge. One promising method for its removal is the sorption of mercuric ions using biochar. FeS-doped biochar (FBC) exhibits effective mercury adsorption, however may release excess iron into the surrounding water. To address this issue, a novel magnetic pyrrhotite/magnetite-doped biochar with a core-shell structure was synthesized for the adsorption of 2-valent mercury (Hg(II)). The proposed synthesis process involved the use of algae powder and ferric sulfate in a one-step method. By varying the ratio of ferric sulfate and alga powder (within the range of 0.18 - 2.5) had a notable impact on the composition of FBC. As the ferric sulfate content increased, the FBC exhibited a higher concentration of oxygen-containing groups. To assess the adsorption capacity, Langmuir and Freundlich adsorption models were applied to the experimental data. The most effective adsorption was achieved with FBC-4, reaching a maximum capacity (Qm) of 95.51 mg/g. In particular, at low Hg(II) concentrations, FBC-5 demonstrated the ability to reduce Hg(II) concentrations to less than 0.05 mg/L within 30 min. Additionally, the stability of FBC was confirmed within the pH range of 3.8 - 7.2. The study also introduced a model to analyze the adsorption preference for different Hg(II) species. Calomel was identified in the mercury saturated FBC, whereas the core-shell structure exhibited excellent conductivity, which most likely contributed to the minimal release of iron. In summary, this research presents a novel and promising method for synthesizing core-shell structured biochar and provides a novel approach to explore the adsorption contribution of different metal species.

Respective evolution of soil and biochar on competitive adsorption mechanisms for Cd(II), Ni(II), and Cu(II) after 2-year natural ageing

To reveal the respective evolution of soil and biochar on competitive heavy metal adsorption mechanisms after natural ageing, three soils and two biochars were tested in this study. The soil-biochar interlayer samples were buried in the field for 0.5, 1, and 2 years, for which competitive adsorption characteristics and mechanisms of soils and biochars in four systems (Cd, Cd+Ni, Cd+Cu, and Cd+Ni+Cu) were investigated. Results showed that physicochemical properties, adsorption capacity and mechanisms of soils and biochars all changed the most in the first 0.5 years. The properties and adsorption capacity of biochars gradually weakened with the ageing time, meanwhile, those of soils gradually enhanced. After co-ageing with acidic soil for 0.5 years, the Cd(II) adsorption capacity of modified biochar decreased by 86.59% in the ternary system; meanwhile, that of acidic soil increased by 65.52%. The contributions of mineral mechanisms decreased significantly, while non-mineral mechanisms were slightly affected by ageing. This study highlighted that when using biochar to remediate heavy metal-contaminated soils, biochar should be applied at least half a year in advance before planting crops so that biochar can fully contact and react with the soil.

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.

Effect of composite organic amendment on Cd(II) ions stabilization and microbial activity under various ammonium sulfate levels

To attenuate the risk of Cadmium(Cd) contamination and the deterioration of soil quality caused by excessive nitrogen fertilizer application in greenhouse, a composite organic amendment (spend mushroom substrate and its biochar) was prepared to remedy Cd(II) ions contaminated soil (0.6 mg/kg) under different N fertilizer levels. The results showed that in the absence of a composite organic amendment, the soil pH decreased by 0.15 when the N level increased from 0.1 to 0.8 g N⋅kg-1. However, the pH increased by 0.86-0.91, the exchangeable Cd(II) ions content decreased by 26.0%-26.7%, the microbial biomass increased by 34.34%-164.46%, and the number of copies of the AOB gene increased by 13-20 times with the application of composite organic amendment and the increase of N level. Both Pearson correlation analysis and Mantel test demonstrated the reduction in Cd(II) ions availability, the restoration of soil properties and the increase in microbial biomass all contributed to the composite organic amendment, which is of importance for soil remediation under excessive N fertilizer.

A review of solid wastes-based stabilizers for remediating heavy metals co-contaminated soil: Applications and challenges

The remediation of heavy metals/metalloids (HMs) co-contaminated soil by solid wastes-based stabilizers (SWBS) has received major concern recently. Based on the literature reported in the latest years (2010-2023), this review systematically summarizes the different types of solid wastes (e.g., steel slag, coal fly ash, red mud, and sewage sludge, etc.) employed to stabilize HMs contaminated soil, and presents results from laboratory and field experiments. Firstly, the suitable solid wastes for soil remediation are reviewed, and the pros and cons are presented. Thereafter, the technical feasibility and economic benefit are evaluated for field application. Moreover, evaluation methods for remediation of different types of HMs-contaminated soil and the effects of SWBS on soil properties are summarized. Finally, due to the large specific surface, porous structure, and high reactivity, the SWBS can effectively stabilize HMs via adsorption, complexation, co/precipitation, ion exchange, electrostatic interaction, redox, and hydration process. Importantly, the environmental implications and long-term effectiveness associated with the utilization of solid wastes are highlighted, which are challenges for practical implementation of soil stabilization using SWBS, because the aging of soil/solid wastes has not been thoroughly investigated. Future attention should focus on modifying the SWBS and establishing an integrated long-term stability evaluation method.

Insight into the mechanism of nano-TiO2-doped biochar in mitigating cadmium mobility in soil-pak choi system

Soil cadmium (Cd) pollution poses severe threats to food security and human health. Previous studies have reported that both nanoparticles (NPs) and biochar have potential for soil Cd remediation. In this study, a composite material (BN) was synthesized using low-dose TiO2 NPs and silkworm excrement-based biochar, and the mechanism of its effect on the Cd-contaminated soil-pak choi system was investigated. The application of 0.5 % BN to the soil effectively reduced 24.8 % of diethylenetriaminepentaacetic acid (DTPA) Cd in the soil and promoted the conversion of Cd from leaching and HOAc-extractive to reducible forms. BN could improve the adsorption capacity of soil for Cd by promoting the formation of humic acid (HA) and increasing the cation exchange capacity (CEC), as well as activating the oxygen-containing functional groups such as CO and CO. BN also increased soil urease and catalase activities and improved the synergistic network among soil bacterial communities to promote soil microbial carbon (C) and nitrogen (N) cycling, thus enhancing Cd passivation. Moreover, BN increased soil biological activity-associated metabolites like T-2 Triol and altered lipid metabolism-related fatty acids, especially hexadecanoic acid and dodecanoic acid, crucial for bacterial Cd tolerance. In addition, BN inhibited Cd uptake and root-to-shoot translocation in pak choi, which ultimately decreased Cd accumulation in shoots by 51.0 %. BN significantly increased the phosphorus (P) uptake in shoots by 59.4 % by improving the soil microbial P cycling. This may serve as a beneficial strategy for pak choi to counteract Cd toxicity. These findings provide new insights into nanomaterial-doped biochar for remediation of heavy metal contamination in soil-plant systems.

Adsorption of Pb(II) by UV-aged microplastics and cotransport in homogeneous and heterogeneous porous media

To investigate the adsorption effects of aged microplastics (MPs) on Pb(II) and their co-transport properties in homogeneous (quartz sand) and heterogeneous (quartz sand with apple branches biochar) porous media, we explored the co-transport of UV-irradiated aged MPs and coexisting Pb(II) along with their interaction mechanisms. The UV aging process increased the binding sites and electronegativity of the aged MPs' surface, enhancing its adsorption capacity for Pb(II). Aged MPs significantly improved Pb(II) transport through homogeneous media, while Pb(II) hindered the transport of aged MPs by reducing electrostatic repulsion between these particles and the quartz sand. When biochar, with its loose and porous structure, was used as a porous medium, it effectively inhibited the transport capacity of both contaminants. In addition, since the aged MPs cannot penetrate the column, a portion of Pb(II) adsorbed by the aged MPs will be co-deposited with the aged MPs, hindering Pb(II) transport to a greater extent. The transport experiments were simulated and interpreted using two-point kinetic modeling and the DLVO theory. The study results elucidate disparities in the capacity of MPs and aged MPs to transport Pb(II), underscoring the potential of biochar application as an effective strategy to impede the dispersion of composite environmental pollutants.

Hierarchically porous magnetic biochar as an amendment for wheat (Triticum aestivum L.) cultivation in alkaline Cd-contaminated soils: Impacts on plant growth, soil properties and microbiota

Hierarchically porous magnetic biochar (HMB) had been found to act as an effective amendment to remediate cadmium (Cd) in water and soil in a previous study, but the effects on wheat growth, Cd uptake and translocation mechanisms, and soil microorganisms were unknown. Therefore, soil Cd form transformation, soil enzyme activity, soil microbial diversity, wheat Cd uptake and migration, and wheat growth were explored by adding different amounts of HMB to alkaline Cd-contaminated soil under pot experiments. The results showed that application of HMB (0.5 %-2.0 %) raised soil pH, electrical conductivity (EC) and available Fe concentration, decreased soil available Cd concentration (35.11 %-50.91 %), and promoted Cd conversion to less bioavailable Cd forms. HMB treatments could reduce Cd enrichment in wheat, inhibit Cd migration from root to stem, rachis to glume, glume to grain, and promote Cd migration from stem to leaf and stem to rachis. HMB (0.5 %-1.0 %) boosted antioxidant enzyme activity, reduced oxidative stress, and enhanced photosynthesis in wheat seedlings. Application of 1.0 % HMB increased wheat grain biomass by 40.32 %. Besides, the addition of HMB (0.5 %-1.0 %) could reduce soil Cd bioavailability, increase soil enzyme activity, and increase the abundance and diversity of soil bacteria. Higher soil EC brought forth by HMB (2.0 %) made the wheat plants and soil bacteria poisonous. This study suggests that applying the right amount of HMB to alkaline Cd-contaminated soil could be a potential remediation strategy to decrease Cd in plants' edible parts and enhance soil quality.