Simultaneous immobilization of arsenic, lead and cadmium by magnesium-aluminum modified biochar in mining soil

Stabilization mechanism and remediation effectiveness of Pb and cd in agricultural soil using nonmetallic minerals

Soil contaminated by Pb and Cd has aroused worldwide concern due to the environmental hazards they pose. The effects, mechanisms, and evaluation of Pb and Cd contaminated agricultural soil remediation by nonmetallic minerals are still poorly understood. In this study, solidification/stabilization experiments were used to screen nonmetallic mineral materials and optimize their dosages. Stabilization mechanisms of Pb and Cd by nonmetallic mineral materials were investigated by adsorption kinetics, X-ray diffraction spectroscopy, and Fourier transform infrared spectroscopy. The effectiveness of soil remediation was further confirmed through a pot experiment with pak choi (Brassica rapa L. subsp. chinensis), an important non-heading leafy vegetable. Results demonstrated that the SL composite (composed of 2.5% sepiolite and 1.5% limestone, with a total dosage of 4.0%) exhibits the optimal stabilization effect on soil contaminated with Pb and Cd. In soils with low, medium, and high contamination levels, SL reduced the bioavailability of Pb by 97.97%, 96.78%, and 95.82%, and the bioavailability of Cd by 92.96%, 91.76%, and 91.02%, respectively. SL surfaces are rich in hydroxyl (-OH) and carbonate (CO32-) groups, enabling binding with Pb and Cd ions to form hydroxide and carbonate precipitates. Such interactions suggest that chemical adsorption primarily drives Pb and Cd ion stabilization, reducing their bioavailability in soil. Pak choi grown in SL-remediated soil exhibited Pb and Cd contents compliant with China's food safety standards. These findings further validate the bioavailability reduction rate as a suitable metric for evaluating the remediation effectiveness of heavy metal pollution in agricultural soils. This study provides a new strategy for evaluating the remediation efficiency of heavy metal-contaminated agricultural soil.

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.

Engineered biochars for simultaneous immobilization of as and Cd in soil: Field evidence

Agricultural soil contamination by potentially toxic elements (PTEs) such as arsenic (As) and cadmium (Cd) poses a serious threat to food security. Immobilization serves as a widely used approach for the remediation of PTEs contaminated soils, nevertheless, the long-term effectiveness for the simultaneous immobilization of both cations and oxyanions remains a challenge. In order to effectively enhance the synergistic immobilization effect of soil As and Cd contaminated by multiple elements and improve the ecological environment of farmland. In this study, a typical polluted tailings area farmland was selected for situ immobilization experiments, and biochar was prepared from cow manure (CMB), rice straw (RSB), and pine wood (PWB) as raw materials. On this basis, the pristine biochar was modified with ferric chloride (F), potassium permanganate (K), magnesium chloride (M), and aluminum chloride (A), respectively. Furthermore, the immobilization effect of modified biochar on As-Cd and the stress effect on soil respiration were investigated. The results showed that CMB and RSB reduced the bioavailability of heavy metals, potassium permanganate has strong oxidizing properties, and the strong oxidability of potassium permanganate stimulated the generation of more oxygen-containing functional groups on the surface of biochar, thereby enhancing the adsorption and complexation effect of modified materials on As and Cd. Among them, the extracted Cd concentration of Diethylenetriamine pentaacetic acid (DTPA) in KCMB and KRSB in 2020 decreased by 8.23-43.12% and 9.67-35.29% compared to other treatments, respectively. Meanwhile, the KCMB and KRSB treatments also reduced the enrichment of As and Cd in plant tissues. In addition, the dissolved organic carbon (DOC) content in KCMB treatment was relatively high, and the carbon stability of the material was weakened. Simultaneously, the soil respiration emission of KCMB treatment was increased by 5.63% and 11.93% compared to KRSB and KPWB treatments, respectively. In addition, the structural equation also shows that DOC has a large positive effect on soil respiration. In summary, the KRSB treatment effectively achieve synergistic immobilization of As-Cd and provide important guiding significance for green and low-carbon remediation of polluted farmland.

Review on microwave immobilization of soil heavy metals: Processes and mechanisms

Soil contamination with heavy metals (HMs) is still a global issue. The maintenance of long-term stability of HMs in soil during immobilization remediation is a challenge. Microwave (MW) technology can promote the immobilization of HMs in the form of crystals and minerals, thus enhancing their resistance of corrosion. This review provides a comprehensive introduction to the basics of MW irradiation through 177 papers, and reviews the research progress of MW involvement in the immobilization of soil HMs in 10 years. The effects of MW parameter settings, absorber/fixative types and soil physicochemical properties on immobilized HMs are investigated. The immobilization mechanisms of HMs are discussed, high-temperature physical encapsulation and chemical stabilization are the two basic mechanisms in the immobilization process. MW has a unique heating method to achieve efficient remediation by shortening remediation time, reducing the activation energy of reactions and promoting the transformation of stabilization products. Finally, the current limitations of MW in the remediation of HMs contaminated soils are systematically discussed and the corresponding proposed solutions are presented which may provide directions for further laboratory studies. There are still serious problems in taking the results obtained in the laboratory to the full scale. Thus, process optimization, scale-up, design and demonstration are strongly desired. In summary, this review may help new researchers to seize the research frontier in MW and can serve as a reference for future development of MW technology in soil remediation.

Effects of active silicon amendment on Pb(II)/Cd(II) adsorption: Performance evaluation and mechanism

In this study, a high-Si (Si) adsorbent (APR@Sam) was prepared by acid leaching slag (APR) from lead-zinc (Pb-Zn) tailings based on high-temperature alkali melting technology. The synthesized Si-based materials were applied to aqueous solutions contaminated with Pb and cadmium (Cd) to investigate the crucial role of active Si in sequestering heavy metals. The adsorption capacities of APR@Sam and the Si-depleted material (APR@Sam-NSi) were studied under different pH and temperature conditions. The results showed that as the pH increased from 3 to 7, the adsorption capacity increased, the active Si content in the solution increased by 63 %, and the maximum pH of the solution after adsorption was 7.12. After the removal of active Si, the Pb (II) and Cd (II) adsorption capacities of APR@Sam decreased by 45 % and 11.96 %, respectively. OH- promoted the release of Si into the solution, enhancing the material's adsorption efficiency. The reaction mechanism is mainly attributed to surface complexation guided by Si-O and Si-O-Si bonds, metal cation exchange, and bidentate coordination. The results indicated that the Si component is critical for the removal of Pb (II) and Cd (II) by APR@Sam and provide valuable insights into resource recovery strategies from leaching residues.

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.

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.

A review of amendments for simultaneously reducing Cd and As availability in paddy soils and rice grain based on meta-analysis

Arsenic (As) and cadmium (Cd) accumulation in rice grains is a global food safety issue, and various methods and materials have been used to remove or reduce As and Cd in agricultural soils and rice grains. Despite the availability of synthesized materials capable of simultaneous As and Cd reduction from soil and rice grains, the contributions, efficiency, and main ingredients of the materials for As and Cd immobilization remain unclear. The present study first summarized the biogeochemistry of As and Cd in paddy soils and their transfer in the soil-food-human continuum. We also reviewed a series of reported inorganic and organic materials for simultaneous immobilization of As and Cd in paddy soils, and their reduction efficiency of As and Cd bioavailability were listed and compared. Based on the abovementioned materials, the study conducted a meta-analysis of 38 articles with 2565 observations to quantify the impacts of materials on simultaneous As and Cd reduction from soil and rice grains. Meta-analysis results showed that combining organic and inorganic amendments corresponded to effect sizes of -62.3% and -67.8% on As and Cd accumulation in rice grains, while the effect sizes on As and Cd reduction in paddy soils were -44.2% and -46.2%, respectively. Application of Fe based materials significantly (P < 0.05) reduced As (-54.2%) and Cd (-74.9%), accounting for the highest immobilization efficiency of As and Cd in rice grain among all the reviewed materials, outweighing S, Mn, P, Si, and Ca based materials. Moreover, precipitation, surface complexation, ion exchange, and electrostatic attraction mechanisms were involved in the co-immobilization tactics. The present study underlines the application of combined organic and inorganic amendments in simultaneous As and Cd immobilization. It also highlighted that employing Fe-incorporated biochar material may be a potential strategy for co-mitigating As and Cd pollution in paddy soils and accumulation in rice grains.

Co-exposure of decabromodiphenyl ethane and cadmium increases toxicity to earthworms: Enrichment, oxidative stress, damage and molecular binding mechanisms

The increase of electronic waste worldwide has resulted in the exacerbation of combined decabromodiphenyl ethane (DBDPE) and cadmium (Cd) pollution in soil, posing a serious threat to the safety of soil organisms. However, whether combined exposure increases toxicity remains unclear. Therefore, this study primarily investigated the toxic effects of DBDPE and Cd on earthworms at the individual, tissue, and cellular levels under single and combined exposure. The results showed that the combined exposure significantly increased the enrichment of Cd in earthworms by 50.32-90.42 %. The toxicity to earthworms increased with co-exposure, primarily resulting in enhanced oxidative stress, inhibition of growth and reproduction, intensified intestinal and epidermal damage, and amplified coelomocyte apoptosis. PLS-PM analysis revealed a significant and direct relationship between the accumulation of target pollutants in earthworms and oxidative stress, damage, as well as growth and reproduction of earthworms. Furthermore, IBR analysis indicated that SOD and POD were sensitive biomarkers in earthworms. Molecular docking elucidated that DBDPE and Cd induced oxidative stress responses in earthworms through the alteration of the conformation of the two enzymes. This study enhances understanding of the mechanisms behind the toxicity of combined pollution and provides important insights for assessing e-waste contaminated soils.

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.

Comparative role of charcoal, biochar, hydrochar and modified biochar on bioavailability of heavy metal(loid)s and machine learning regression analysis in alkaline polluted soil

Pot experiment was performed aimed to assess the comparative role of charcoal, biochar, hydrochar and thiourea-vegetable modified biochar at 1 and 2 % doses, and <1 mm particle size on the bioavailability of Cd, Pb, As, Ni, Cu and Zn, and enhance NPK, and mustard growth in a slightly alkaline polluted soil. Furthermore, machine learning method was used to examine the systematic evaluation of the impact of feature selection based on Pearson's correlation on the performance of the linear regression model. The results revealed that maximum fresh and dry biomass of mustard was observed by 26.38 and 38.18 % with hydrochar 1 %, whereas lemon biochar at 2 % reduced fresh and dry biomass up to 34.0 and 53.0 % than control. The immobilization of Cd and Pb was observed by 83.70 and 71.15 % with thiourea-vegetable modified biochar at 2 %, As 71.62 % with hydrochar 2 %, Ni 80.84 % with thiourea-vegetable modified biochar 2 %, Cu 66.32 % with and Zn 36.30 % with thiourea-vegetable modified biochar at 2 % than control. However, the maximum mobilization of Cu in soil was observed by 30.3 % with lemon biochar 2 %, similarly for Zn 37.36 % with hydrochar 2 % as compared with other treatments. The phyto-availability of Cd, Pb, As and Cu in the mustard shoot and root biomass was reduced except Ni and Zn in soil than control. It was observed that using the machine learning regression analysis approach, variability in treatments effectiveness is evident across different feature correlation thresholds. This study clearly shows that the beneficial role of studied amendments on mustard growth and reduced bioavailability of heavy metal(loid)s and enhance primary macronutrients in alkaline polluted soil. It is suggested that future studies may be conducted on combined application of studies amendments on plant growth, immobilization of heavy metal(loid)s in multi-metal polluted soil under different field conditions.

Arsenic and cadmium availability and its removal in paddy farming areas

Arsenic (As) and cadmium (Cd) accumulation in rice grain is a global concern threatening food security and safety to the growing population. As and Cd are toxic non-essential elements poisonous to animal and human at higher levels. Its accumulation in agro-ecosystems pose a public health risk to consumers of agro-ecosystem products. Due to their hazards, As and Cd sources should be cleared, avoiding entering plants and the human body. As and Cd removal in soils and grains in agro-ecosystems has been conducted by various materials (natural and synthesized), however, there are little documentation on their contribution on As and Cd removal or reduction in rice grains. This identified knowledge gap necessitate a systematically review to understand efficiency and mechanisms of As and Cd availability reduction and removal in paddy farming areas through utilization of various synthetic and modified materials. To achieve this, published peer reviewed articles between 2010 and 2024 were collected from various database i.e., Science Direct, Web of Science, Google Scholar, and Research Gate and analyzed its content in respect to As and Cd reduction and removal. Furthermore, collected data were re-analyzed to determine standardized mean differences (SMD) with 95% confidence intervals (CI). Based on 96 studies with 228 observations involving Fe, Ca, Si, and Se-based materials were identified, it was found that application of Fe, Ca, Si, and Se-based materials potentially reduced As and Cd in rice grains among various study sites and across studies. Among the studied materials, Fe-based materials observed to be more efficient compared to other utilized materials. However, there little or no information on performance of materials when used in combination and how they can improve crop productivity and soil health, thus requiring further studies. Thus, this study confirm Fe, Ca, Si, and Se modified materials have significant potential to reduce As and Cd availability in paddy farming areas and rice grains, thus necessary effort must be made to ensure materials access and availability for farmers utilization in paddy fields to reduce As and Cd accumulation.

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.

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.

Silicon-nanoparticles loaded biochar for soil arsenic immobilization and alleviation of phytotoxicity in barley: Implications for human health risk

Arsenic (As)-induced environmental pollution and associated health risks are recognized on a global level. Here the impact of cotton shells derived biochar (BC) and silicon-nanoparticles loaded biochar (nano-Si-BC) was explored on soil As immobilization and its phytotoxicity in barley plants in a greenhouse study. The barley plants were grown in a sandy loam soil with varying concentrations of BC and nano-Si-BC (0, 1, and 2%), along with different levels of As (0, 5, 10, and 20 mg kg-1). The FTIR spectroscopy, SEM-EDX, and XRD were used to characterize BC and nano-Si-BC. Results revealed that As treatment had a negative impact on barley plant development, grain yield, physiology, and anti-oxidative response. However, the addition of nano-Si-BC led to a 71% reduction in shoot As concentration compared to the control with 20 mg kg-1 of As, while BC alone resulted in a 51% decline. Furthermore, the 2% nano-Si-BC increased grain yield by 94% compared to control and 28% compared to BC. The addition of 2% nano-Si-BC to As-contaminated soil reduced oxidative stress (34% H2O2 and 48% MDA content) and enhanced plant As tolerance (92% peroxidase and 46% Ascorbate peroxidase activity). The chlorophyll concentration in barley plants decreased due to oxidative stress. Additionally, the incorporation of 2% nano-Si-BC resulted in a 76% reduction in water soluble and NaHCO3 extractable As. It is concluded that the use of BC or nano-Si-BC in As contaminated soil for barley resulted in a low human health risk (HQ < 1), as it effectively immobilized As and promoted higher activity of antioxidants.

Quantification of the effect of biochar application on heavy metals in paddy systems: Impact, mechanisms and future prospects

Biochar (BC) has shown great potential in remediating heavy metal(loid)s (HMs) contamination in paddy fields. Variation in feedstock sources, pyrolysis temperatures, modification methods, and application rates of BC can result in great changes in its effects on HM bioavailability and bioaccumulation in soil-rice systems and remediation mechanisms. Meanwhile, there is a lack of application guidelines for BC with specific properties and application rates when targeting rice fields contaminated with certain HMs. To elucidate this topic, this review focuses on i) the effects of feedstock type, pyrolysis temperature, and modification method on the properties of BC; ii) the changes in bioavailability and bioaccumulation of HMs in soil-rice systems applying BC with different feedstocks, pyrolysis temperatures, modification methods, and application rates; and iii) exploration of potential remediation mechanisms for applying BC to reduce the mobility and bioaccumulation of HMs in rice field systems. In general, the application of Fe/Mn modified organic waste (OW) derived BC for mid-temperature pyrolysis is still a well-optimized choice for the remediation of HM contamination in rice fields. From the viewpoint of remediation efficiency, the application rate of BC should be appropriately increased to immobilize Cd, Pb, and Cu in rice paddies, while the application rate of BC for immobilizing As should be <2.0 % (w/w). The mechanism of remediation of HM-contaminated rice fields by applying BC is mainly the direct adsorption of HMs by BC in soil pore water and the mediation of soil microenvironmental changes. In addition, the application of Fe/Mn modified BC induced the formation of iron plaque (IP) on the root surface of rice, which reduced the uptake of HM by the plant. Finally, this paper describes the prospects and challenges for the extension of various BCs for the remediation of HM contamination in paddy fields and makes some suggestions for future development.

Enhancing rice quality and productivity: Multifunctional biochar for arsenic, cadmium, and bacterial control in paddy soil

The perilousness of arsenic and cadmium (As-Cd) toxicity in water and soil presents a substantial hazard to the ecosystem and human well-being. Additionally, this metal (loids) (MLs) can have a deleterious effect on rice quality and yield, owing to the existence of toxic stress. In response to the pressing concern of reducing the MLs accumulation in rice grain, this study has prepared magnesium-manganese-modified corn-stover biochar (MMCB), magnesium-manganese-modified eggshell char (MMEB), and a combination of both (MMCEB). To test the effectiveness of these amendments, several pot trials were conducted, utilizing 1% and 2% application rates. The research discovered that the MMEB followed by MMCEB treatment at a 2% rate yielded the most significant paddy and rice quality, compared to the untreated control (CON) and MMCB. MMEB and MMCEB also extensively decreased the MLs content in the grain than CON, thereby demonstrating the potential to enrich food security and human healthiness. In addition, MMEB and MMCEB augmented the microbial community configuration in the paddy soil, including As-Cd detoxifying bacteria, and decreased bioavailable form of the MLs in the soil compared to the CON. The amendments also augmented Fe/Mn-plaque which captured a considerable quantity of As-Cd in comparison to the CON. In conclusion, the utilization of multifunctional biochar, such as MMEB and MMCEB, is an encouraging approach to diminish MLs aggregation in rice grain and increase rice yield for the reparation of paddy soils via transforming microbiota especially enhancing As-Cd detoxifying taxa, thereby improving agroecology, food security, and human and animal health.

Coal gangue-based magnetic porous material for simultaneous remediation of arsenic and cadmium in contaminated soils: Performance and mechanisms

Remediation of arsenic (As) and cadmium (Cd) co-contaminated soil is a challenge in environmental remediation. In this study, coal gangue-based magnetic porous material (MPCG) was designed for simultaneous immobilization of As and Cd in contaminated soil. After the incubation experiment, the effects of CG and MPCG on the availability and fractions of As and Cd and the related microbial functional genes were analyzed to explore the potential remediation mechanisms of MPCG for As and Cd in contaminated soil. The results showed that the stabilization effect of MPCG on As and Cd was significantly higher than that of coal gangue. It reduced the available As and Cd by 17.94-29.81% and 14.22-30.41%, respectively, and transformed unstable As/Cd to stable. The remediation mechanisms of MPCG on As included adsorption, oxidation, complexation and precipitation/co-precipitation. Meanwhile, the remediation mechanisms of MPCG for Cd included adsorption, ion exchange, complexation and precipitation. In addition, MPCG increases the abundance of sulfate-reducing bacteria (dsrA) by 43.39-381.28%, which can promote sulfate reduction. The sulfide can precipitate with As and Cd to reduce the availability of As and Cd in soil. Thus, MPCG is a promising amendment for achieving the remediation of As and Cd co-contaminated soil.

Co-production of spirosiloxane and biochar adsorbent from wheat straw by a low-cost and environment-friendly method

To enhance the value of wheat straw derivatives, wheat straw ash (WSA) was used as a reactant for the first time to synthesize spirocyclic alkoxysilane, an important organosilicon raw material, using an energy-saving and environmentally friendly non-carbon thermal reduction method. After spirocyclic alkoxysilane extraction, the biochar in the wheat straw ash prepared an adsorbent for Cu²⁺. The maximum copper ion adsorption capacity (Q_m) of silica-depleted wheat straw ash (SDWSA) was 31.431nullmg/g, far exceeding those of WSA and similar biomass adsorbents. The effects of the pH, adsorbent dose, and contact time on the adsorption behaviour of the SDWSA for Cu²⁺ adsorption were systematically investigated. The adsorption mechanism of Cu²⁺ by the SDWSA was investigated using the Langmuir, Freundlich, pseudo-first-order kinetic, pseudo-second-order kinetic, and Weber and Morris models by combining the preliminary experimental data and characterization results. The adsorption isotherm and Langmuir equation matched perfectly. The Weber and Morris model can describe the mass-transfer mechanism of Cu²⁺ adsorption by SDWSA. Both film and intraparticle diffusion are rapid control steps. Compared to WSA, SDWSA has a larger specific surface area and a higher content of oxygen-containing functional groups. A large specific surface area provides more adsorption sites. Oxygen-containing functional groups react with Cu²⁺ through electrostatic interactions, surface complexation, and ion exchange, which are the possible adsorption mechanisms for SDWSA. These methods improve the added value of wheat straw derivatives and promote wheat straw ash recovery and centralized treatment. This makes it possible to use the thermal energy of wheat straw and facilitates the treatment of exhaust gases and carbon capture.

Effect of biochar derived from co-pyrolysis of sewage sludge and rice straw on cadmium immobilization in paddy soil

The remediation of cadmium (Cd) contaminated cropland has been related to food safety and public health. While biochar derived from sewage sludge (SS) has been widely used for soil remediation due to its high efficiency of Cd immobilization, it has a low specific surface area and the ecological risk of heavy metals. Co-pyrolysis of straws and SS could resolve these issues. To date, little is known about the effect of biochar from SS/rice straw (RS) on Cd immobilization in soils. Here, we explored the soil remediation efficiency and mechanism of biochar derived from different mixing ratios (1:0, 3:1, 2:1, 1:1, 1:2, 1:3 and 0:1) of RS and SS named as RBC, R3S1, R2S1, R1S1, R1S2, R1S3 and SBC. It was shown that R1S2 amendment had the most efficiency of Cd immobilization among all amendments, which decreased the bioavailable Cd by 85.61% and 66.89% compared with RBC and SBC amendments. Results of biochar after soil remediation revealed that cation-π interaction, complexation, ion exchange and precipitation were the key mechanisms of Cd immobilization by biochar. All biochar amendments indirectly promoted Cd immobilization through increasing soil pH values, cation exchange capacity (CEC), soil organic carbon (SOC) and available phosphorous (AP). Compared with RBC, R1S2 reduced bioavailable Cd mainly through the increased soil pH, CEC and AP. However, the enhanced efficiency of Cd immobilization in R1S2 amendment than that in SBC amendment because of the more developed pore structure, functional groups and larger specific surface area of R1S2. Overall, our study showed a new type of biochar for the effective remediation of Cd-contaminated soil.

Effect of goethite-loaded montmorillonite on immobilization of metal(loid)s and the micro-ecological soil response in non-ferrous metal smelting areas

In this work, the immobilization stabilization and mechanism of heavy metal(loid)s by goethite loaded montmorillonite (GMt) were investigated, and the soil microbial response was explored. The simulated acid rain leaching experiment showed that GMt had a higher acid tolerance and the more stable heavy metal(loid)s fixation ability. The soil incubation demonstrated that GMt significantly decreased the available Cd, Zn, Pb and As concentration. Interestingly, higher immobilization of heavy metals was observed by GMt in highly acid leached and acidic soils. The richness and diversity of bacterial communities improved after the addition of GMt. GMt induced the enrichment of the excellent functional bacteria of the phylum Proteobacteria as well as the genus Massilia and Sphingomonas. The main immobilization mechanisms of heavy metal(loid)s by GMt include electrostatic interaction, complexation, precipitation and oxidation. The addition of the GMt also optimizes the soil bacterial community structure, which further facilitates the immobilization of heavy metal(loid)s. Our results confirm that the novel GMt has a promising application in the immobilization and stabilization of heavy metal(loid)s contaminated soils in non-ferrous metal smelting areas.

Concurrent sorption of antimony and lead by iron phosphate and its possible application for multi-oxyanion contaminated soil

Concurrent stabilization of oxyanions such as antimony (Sb), arsenic (As), and heavy metals including lead (Pb) and manganese (Mn) in contaminated soils is difficult because of their diverse chemical properties. Antimony and As are stabilized by sorption with iron oxides while heavy metals are stabilized by phosphate. Hence, iron phosphate can be used to simultaneously stabilize Sb and Pb. Therefore, this study aimed to evaluate the possibility of simultaneous stabilization of Sb and Pb using iron phosphate. A single and a mixed solution of Sb and Pb were reacted with synthesized iron phosphate. Contaminated soil by Sb, As, Mo, Cr, and Mn was treated with iron phosphate, and bioavailable metal concentrations were evaluated by extracting the soil with 0.05 M ammonium sulfate. In a single solution, Sb(III) and Sb(V) sorption rate ranged up to 97% and 65%, respectively. In a mixed metal solution, Sb sorption increased compared to the single solution and Pb removal reached more than 95% in all cases. The sorption of Sb increased as the pH decreased, but the Sb(III) sorption was less affected by the pH than Sb(V). In various pH ranges, Sb(III) and Sb(V) sorption rates increased by 26 ~ 32% and 38 ~ 68%, respectively, compared to the single solution. Especially, Sb(V) sorption significantly increased in the presence of Pb at lower pH because of the lower solubility of iron phosphate. In soil, iron phosphate slightly decreased bioavailable As, Cr, Mo, Sb, and Mn concentrations. Therefore, metalloids and metals can be simultaneously stabilized by iron phosphate both in solution and soil.

Simultaneous immobilization of arsenic, lead, and cadmium in soil by magnesium-aluminum modified biochar: Influences of organic acids, aging, and rainfall

Magnesium-aluminum modified biochar (MABs) has an outstanding effect on the simultaneous immobilization of arsenic (As), lead (Pb), and cadmium (Cd) in soil, but the stability of remediation effect of MAB under various natural conditions is still unknown. In this study, we investigated the effects of organic acids, dry-wet cycles (DW), freeze-thaw cycles (FT), and rainfall (pH 4, 7, and 8) on the immobilization of As, Pb, and Cd by MAB. The results showed that oxalic acid decreased the immobilization efficiencies of As, Pb, and Cd by 15.5%-38.5%; meanwhile, humic acid reduced the immobilization efficiency of Pb by 89.7%, but elevated that of Cd by 19.5%. The immobilization mechanisms of MAB-5 on three metals were mainly involved in ion exchange and surface-complexation. Compared with the 7th round, the immobilization efficiencies of As, Pb, and Cd by MAB in the 28th round was decreased by 17%-28% in DW, but was increased by 11%-18% in FT. In addition, MAB was transformed into hydrotalcite after FT and DW. After experiencing simulated rainfall, MAB caused more As, Pb, and Cd to be retained in the upper soil layer, and the immobilization effect of MBA was more significant under the stimulated rainfall with higher pH. The study provides a more theoretical basis for the application of MAB in the actual site remediation.

Ternary Ca-Mg-Al layered double-hydroxides for synergistic remediation of As, Cd, and Pb from both contaminated soil and groundwater: Characteristics, effectiveness, and immobilization mechanisms

Layered double hydroxides (LDH) are the cost-effective and high-efficiency materials for remediation of potentially toxic elements (PTEs) in contaminated soil and groundwater. Herein, the effectiveness and mechanisms of a ternary Ca-Mg-Al LDH (CMAL) for the synergistic remediation of As, Cd, and Pb were investigated in contaminated soils and simulative groundwaters for the first time. The immobilization efficiencies of As, Cd, and Pb in both black soil (BS) and red soil (RS) amended by CMAL at 5 wt% were all > 75%. CMAL amendment transferred more mobile As, Cd, and Pb fractions in soils to immobile species than did Ca-Al LDH and Mg-Al LDH treatments. Furthermore, using a pump-and-treat technology, 82-98% of these 3 PTEs from contaminated groundwater were successfully immobilized in both CMAL treated BS and RS top-soils. Meanwhile, leaching of Ca, Mg, and Al from CMAL was minimal indicating the material was stable. The excellent immobilization performance of CMAL for these PTEs was attributed to the coating of soil microparticles by CMAL nanosheets that allowed complexation of Ca-O-As/Cd or Mg-O-As/Cd/Pb formation, co-precipitation of Ca/Fe-As and Cd(OH)₂, and formation of Ca-bridged ternary complex (FeO-Ca-As/Cd). The adverse effect of oppositive pH/Eh-dependence between As and Cd/Pb was overshadowed by these mechanisms and thus allowed As immobilization. Immobilization of As, Cd, and Pb by CMAL amendment was more favorable for RS soil due to its lower reduction potential and more participation of metal-(hydr)oxides for complexation. Overall, the ternary-LDH is a promising synergistic remediation strategy for multi-PTEs contaminated soil and groundwater.

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.

Remediation of cadmium and lead contaminated soils using Fe-OM based materials

Iron oxide-lignin composites (GLS) were prepared based on the significant role of Fe-OM in the environmental behaviour of heavy metals and lignin binding with iron oxide preferentially in soil. GLS was applied in Cd/Pb immobilization and the stability under acid rain was investigated. The results show that the iron oxide appeared weakly crystalline or amorphous similar to 2-line ferrihydrite after the addition of lignin. Agglomerates of nanoparticles with higher adsorption capacity were observed for GLS. The mobility factor (MF) of Cd/Pb in the soil decreased rapidly after adding GLS. At the 3% dosage, the MF of Cd and Pb in the soil was decreased by 58.94% and 78.15% respectively, which was approximately 5 times that of goethite (GE). The mobile and exchangeable Cd/Pb were converted to organic, amorphous Fe oxide-bound and residue fractions. Under acid rain conditions, MF continues to decline for the GLS group, increasing the organic and amorphous Fe oxide-bound fractions, while for control group (CK) and GE, the trend was the opposite. Lignin could inhibit iron oxide dissolution and stabilize the combination of Cd/Pb and iron oxides in soil. The better stability performance of GLS for Cd/Pb may be related to the higher adsorption capacity and microstructural difference after iron oxide combined with lignin.

Immobilization on anionic metal(loid)s in soil by biochar: A meta-analysis assisted by machine learning

Metal pollution in soil has become one of the most serious environmental problems in China. Biochar is one of the most widely used remediation agents for soil metal pollution. However, the literature does not provide a consistent picture of the performance of biochar on the immobilization of anionic metal(loid)s, especially arsenic, in soil. To obtain a baseline understanding on the interactions of metals and biochar, 597 data records on four metal(loid)s (As, Cr, Sb and V) were collected from 70 publications for this meta-analysis, and the results are highlighted below. Biochar has a significant immobilization effect on anionic metal(loid)s in soil and reduces the bioavailability of these metals to plants. Subgroup analysis found that biochar could decrease the potential mobility of Cr, Sb and V, but the immobilization effect on As was not always consistent. Meanwhile, biochar pH and soil pH are the most key factors affecting the immobilization effect. To summarize, biochar can effectively immobilize Cr, Sb and V in soil, but more attention should be given to As immobilization in future applications. By regulating the properties of biochar and appropriate modification, anionic metal(loid)s in soil can be immobilized more effectively. Hence, both of the soil quality and crop quality can be improved.