An efficient biochar synthesized by iron-zinc modified corn straw for simultaneously immobilization Cd in acidic and alkaline soils

Enhanced reduction of Cd uptake by wheat plants using iron and manganese oxides combined with citrate in Cd-contaminated weakly alkaline arable soils

Iron (Fe) and manganese (Mn) oxides can be used to remediate Cd-polluted soils due to their excellent performance in heavy metal adsorption. However, their remediation capability is rather limited, and a higher content of available Mn and Fe in soils can reduce Cd accumulation in wheat plants due to the competitive absorption effect. In this study, goethite and cryptomelane were first respectively used to immobilize Cd in Cd-polluted weakly alkaline soils, and sodium citrate was then added to increase the content of available Mn and Fe content for further reduction of wheat Cd absorption. In the first season, the content of soil-available Cd and Cd in wheat plants significantly decreased when cryptomelane, goethite and their mixture were used as the remediation agents. Cryptomelane showed a better remediation effect, which could be attributed to its higher adsorption performance. The grain Cd content could be decreased from 0.35 mg kg-1 to 0.25 mg kg-1 when the content of cryptomelane was controlled at 0.5%. In the second season, when sodium citrate at 20 mmol kg-1 was further added to the soils with 0.5% cryptomelane treatment in the first season, the content of soil available Cd was increased by 14.8%, and the available Mn content was increased by 19.5%, leading to a lower Cd content in wheat grains (0.16 mg kg-1) probably due to the competitive absorption. This work provides a new strategy for the remediation of slightly Cd-polluted arable soils with safe and high-quality production of wheat.

Synergistic effect of biochar with gypsum, lime, and farm manure on the growth and tolerance in rice plants under different salt-affected soils

Soil salinization and sodication harm soil fertility and crop production, especially in dry regions. To combat this, using biochar combined with gypsum, lime, and farm manure is a promising solution for improving salt-affected soils. In a pot experiment, cotton stick biochar (BC) was applied at a rate of 20 t/ha in combination with gypsum (G), lime (L), and farm manure (F) at rates of 5 and 10 t/ha. These were denoted as BCG-5, BCL-5, BCF-5, BCG-10, BCL-10, and BCF-10. Three different types of soils with electrical conductivity (EC) to sodium adsorption ratio (SAR) ratios of 2.45:13.7, 9.45:22, and 11.56:40 were used for experimentation. The application of BCG-10 led to significant improvements in rice biomass, chlorophyll content, and overall growth. It was observed that applying BCG-10 to soils increased the membrane stability index by 75% in EC:SAR (2.45:13.7), 97% in EC:SAR (9.45:22), and 40% in EC:SAR (11.56:40) compared to respective control treatments. After BCG-10 was applied, the hydrogen peroxide in leaves dropped by 29%, 23%, and 21% in EC:SAR (2.45:13.7), EC:SAR (9.45:22), and EC:SAR (11.56:40) soils, relative to their controls, respectively. The application of BCG-10 resulted in glycine betaine increases of 60, 119, and 165% in EC: SAR (2.45:13.7), EC: SAR (9.45:22), and EC: SAR (11.56:40) soils. EC: SAR (2.45:13.7), EC: SAR (9.45:22), and EC: SAR (11.56:40) soils all had 70, 109, and 130% more ascorbic acid in BCG-10 applied treatment, respectively. The results of this experiment show that BCG-10 increased the growth and physiological traits of rice plants were exposed to different levels of salt stress. This was achieved by lowering hydrogen peroxide levels, making plant cells more stable, and increasing non-enzymatic activity.

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.

Simultaneous immobilization of V and Cr availability, speciation in contaminated soil and accumulation in ryegrass by using Fe-modified pyrolysis char

In this study, municipal waste pyrolytic char (PEWC) was prepared by pyrolysis from municipal solid waste extracted in landfills, and Fe-based modified pyrolytic char (Fe-PEWC) was prepared by modification. Focusing on the evaluation of the stabilization capacity of Fe-PEWC for vanadium (V) and chromium (Cr) in soils, the effects of PEWC addition on soil properties, bioavailability and morphological distribution of V and Cr, ryegrass growth, and V and Cr accumulation were thoroughly investigated. The results of pot experiment showed that the application of PEWC and Fe-PEWC significantly (P < 0.05) improved soil properties (such as pH, EC, total nitrogen, available phosphorus, available potassium, and organic matter). After 42 days of cultivation, Fe-PEWC has a better fixation effect on heavy metals, and the bioavailable V and Cr of 3% Fe-PEWC decreased by 14.96% and 19.48%, respectively. The exchangeable state and reducible state decreased, while the oxidizable state and residual state increased to varying degrees. The Fe-PEWC can effectively reduce the accumulation of V and Cr in ryegrass by 71.25% and 76.43%, respectively, thereby reducing their toxicity to plants. In summary, modified pyrolytic char can effectively solidify heavy metals in soil, improve soil ecology and reduce the toxicity to plants. The use of excavated waste as a raw material for the preparation of soil heavy metal curing agent has the significance of resource recycling, low price, and practical application.

Correlation analysis and predicting modeling of pyrolysis gas based on landfill excavated waste pyrolysis characteristics

The contribution of excavated waste to waste management is multifaceted, including minimization, non-hazardous disposal, access to useable land resources, improved waste management techniques and public environmental awareness, consistent with recent circular economy initiatives. Pyrolysis can be converted into tar, pyrolysis gas and char with recyclable utilization, enriching the application of pyrolysis technology in the field of excavation waste. In this study, the pyrolysis system includes horizontal tube furnace, gas collection device and Micro GC. The excavated waste was pyrolyzed at a temperature of 500∼900 °C with a heating rate of 10 °C/min. Pyrolysis gases include H2, CO, CO2, CH4, C2H4, C2H6 and C3H8. Pyrolysis was divided into four stages, the main decomposition range is 230∼500 °C, with a weight loss rate of 68.49% and a co-pyrolysis behavior. As the temperature increases, the tar and char decreased and the gas production increased significantly, and the pyrolysis gas reached 47.02% at 900 °C. According to Pearson correlation coefficient analysis, the generation of H2 and CO is positively correlated with temperature. Therefore, the target products can be influenced by changing the parameters, when considering the practical utilization of the excavated waste pyrolysis products. On this basis, the prediction models were built by polynomial fitting method. This model can reduce the experimental exploration cycle, reduce the cost, and accurately predict the pyrolysis gas, which has practical guidance for the application of pyrolysis industry, and provides a theoretical basis for the resource recycling and energy recovery of landfill.

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.

Biochar application for the remediation of soil contaminated with potentially toxic elements: Current situation and challenges

Recently, biochar has garnered extensive attention in the remediation of soils contaminated with potentially toxic elements (PTEs) owing to its exceptional adsorption properties and straightforward operation. Most researchers have primarily concentrated on the effects, mechanisms, impact factors, and risks of biochar in remediation of PTEs. However, concerns about the long-term safety and impact of biochar have restricted its application. This review aims to establish a basis for the large-scale popularization of biochar for remediating PTEs-contaminated soil based on a review of interactive mechanisms between soil, PTEs and biochar, as well as the current situation of biochar for remediation in PTEs scenarios. Biochar can directly interact with PTEs or indirectly with soil components, influencing the bioavailability, mobility, and toxicity of PTEs. The efficacy of biochar in remediation varies depending on biomass feedstock, pyrolysis temperature, type of PTEs, and application rate. Compared to pristine biochar, modified biochar offers feasible solutions for tailoring specialized biochar suited to specific PTEs-contaminated soil. Main challenges limiting the applications of biochar are overdose and potential risks. The used biochar is separated from the soil that not only actually removes PTEs, but also mitigates the negative long-term effects of biochar. A sustainable remediation technology is advocated that enables the recovery and regeneration (95.0-95.6%) of biochar from the soil and the removal of PTEs (the removal rate of Cd is more than 20%) from the soil. Finally, future research directions are suggested to augment the environmental safety of biochar and promote its wider application.

Effect of zinc-biochar composite aging on its physicochemical and ecotoxicological properties

The stability of Zn-biochar composites is determined by environmental factors, including the aging processes. This paper focused on the ecotoxicological evaluation of Zn-biochar (Zn-BC) composites subjected to chemical aging. Pristine biochars and composites produced at 500 or 700 °C were incubated at 60 and 90 °C for six months. All biochars were characterized in terms of their physicochemical (elemental composition, Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and porous structure), ecotoxicological properties (tested with Folsomia candida and Aliivibrio fischeri) and contaminant content (polycyclic aromatic hydrocarbons (PAH), heavy metals (HM) and environmentally persistent free radicals (EPFR)). An increase in the number of surface oxygen functional groups and increased hydrophilicity and polarity of all Zn-BC composites were observed due to oxidation during aging. It was also found that Zn-BC aging at 90 °C resulted in a 28-30% decrease in solvent-extractable PAHs (Ʃ16 Ctot PAHs) compared to nonaged composites. The aging process at both temperatures also caused a 104 fold reduction in EPFRs in Zn-BC composites produced at 500 °C. The changes in the physicochemical properties of Zn-BC composites after chemical aging at 90 °C (such as pH and HM content) caused an increase in the toxicity of the composites to Folsomia candida (reproduction inhibition from 19 to 24%) and Aliivibrio fischeri (luminescence inhibition from 96 to 99%). The aging of composites for a long time may increase the adverse environmental impact of BC-Zn composites due to changes in physicochemical properties (itself and its interactions with pollutants) and the release of Zn from the composite.

Study on improvement of copper sulfide acid soil properties and mechanism of metal ion fixation based on Fe-biochar composite

In this study, Fe modification of bamboo biochar (BC) with ferrate was used to construct a composite soil amendment based on K2FeO4-biochar (Fe-BC) system. Based on soil culture experiments, Fe-BC combined with organic-inorganic materials at the application levels of 3%, 5% and 10% to copper sulfide contaminated acid soil was studied. Adsorption kinetics experiment was used to investigate the adsorption capacity of Fe-modified biochar to heavy metal Cu. The results showed that the pH value of bamboo biochar could be increased by 1.12 units after K2FeO4 modification. Compared with the BC, the adsorption capacity of Cu2+ increased from 190.48 to 276.12 mg/g, which was mainly reflected in single-layer surface adsorption and chemisorption. Pore diffusion, electrostatic interaction and surface interaction are the possible mechanisms of Fe-BC interaction with Cu2+ ions. And the contents of Pb, Cu and Zn in soil leaching state decreased by 59.20%, 65.88% and 57.88%, respectively, at the 10% application level of Fe-BC. In general, the composite modifier based on ferrate and biochar has a positive effect on improving the characteristics of acidic soil in copper mining area.

Adsorption characteristics and mechanisms of Cd(II) from wastewater by modified chicken manure biochar

Due to the threat to food supply and human health posed by cadmium-contaminated wastewater, a highly effective adsorbent is under necessary development to remove cadmium from wastewater. In this study, four new types of modified biochars with different modifier concentrations were prepared from chicken manure using K2FeO4 as a modifier, and the modified biochar KFBC1 with the best adsorption effect was obtained through optimal experiments. Various characterization analyses have shown that KFBC1 has a rough surface structure, abundant pore structure, and a large number of functional groups. Additionally, iron oxides are introduced on the surface of the biochar, which provided a favorable condition for the adsorption of Cd(II) in wastewater. The adsorption performance of Cd(II) on the biochar before and after modification was investigated through batch adsorption experiments. The adsorption kinetic model of KFBC1 to Cd(II) in solution was in accordance with the quasi-secondary kinetic model, and the adsorption isothermal model was in accordance with the Langmuir model, with a maximum adsorption capacity of 330.06 mg/g, which was 5.15 fold of pristine BC. Meanwhile, the adsorption rate of Cd(II) by KFBC1 was positively correlated with dosage and pH. Pore adsorption, ion exchange, surface precipitation, interaction with -π electrons, and complexation of oxygen-containing functional groups on the surface were considered as important mechanisms for the removal of Cd(II) by KFBC1. According to the results, KFBC1 is a novel and efficient adsorbent that can be used as a treatment agent for cadmium-contaminated wastewater.

Maize straw application reduced cadmium and increased arsenic uptake in wheat and enhanced the rhizospheric bacterial communities in alkaline-contaminated soil

Many fields where wheat is grown in northern China are co-polluted by arsenic (As) and cadmium (Cd). Thus, remediation of As and Cd-contaminated alkaline soils is crucial for safe wheat production. In this study, a pot experiment was carried out to investigate the impact of 1% and 2% maize straw (MS) incorporation on As and Cd bioavailability, binding forms, uptake by winter wheat (Triticum aestivum L.), and bacterial communities in smelter (SS) and irrigation (IS) alkaline contaminated soils. The results indicated that 2% MS incorporation significantly (p < 0.05) increased bioavailable-As by 37% (SS) and 39% (IS) with no significant change in the bioavailable-Cd in SS2% (31.95%) from 31.95% (SSCK) and IS2% (33.33%) from 32.82% (ISCK). Incorporation of 2% MS increased the grain As concentration from 0.22 mg kg-1 (SSCK) to 0.51 mg kg-1 (SS2%) and from 0.59 mg kg-1 (ISCK) to 0.84 mg kg-1 (IS2%) which is above the acceptable standard of 0.5 mg kg-1 (GB2726-2017). In contrast, the Cd content in grains was maintained at 0.09 (SS1%), 0.04 (SS2%) and 0.03 (IS1%), 0.02 (IS2%) below the acceptable standard of 0.10 mg kg-1 (GB2762-2017). The amendment through dissolved organic carbon mediated As desorption enhanced As transfer to wheat grain, decreasing DTPA-Cd in the soils and its consequent translocation to wheat leaves and grain. The 2% MS incorporation increased the active As fractions, reduced mobile Cd into immobile fractions, and promoted the abundance of Actinobacteria, Bacteroidetes, and Firmicutes in the two soils. These attributes of MS in decreasing the accumulation of Cd in wheat leaves and grains signified its potential as a suitable ingredient for Cd sequestration and food safety in Cd-contaminated soils.

Biochar may alter plant communities when remediating the cadmium-contaminated soil in the saline-alkaline wetland

It is thought remediating cadmium pollution with biochar can affect plant traits. However, the potential impact of this practice on plant communities is poorly understood. Here, we established natural-germinated plant communities using soil seed bank from a saline-alkaline wetland and applied a biochar treatment in Cd-polluted wetland soil. The outcomes illustrated that Juglans regia biochar (JBC), Spartina alterniflora biochar (SBC), and Flaveria bidentis biochar (FBC) promoted exchangeable Cd transform into FeMn oxide bound Cd. Additionally, most biochar addition reduced species abundance, root-shoot ratio, biomass, diversity, and community stability, yet enhanced community height. Among all treatments, the 5 % SBC demonstrated the most significant reduction in species abundance, biomass, species richness and functional richness. Specifically, it resulted in a reduction of 92.80 % in species abundance, 73.80 % in biomass, 66.67 % in species richness, and 95.14 % in functional richness compared to the CK. We also observed changes in root morphological traits and community structure after biochar addition. Soil pH, salinity, and nutrients played a dominant role in shaping plant community. These findings have implications for biodiversity conservation, and the use of biochar for the remediation of heavy metals like cadmium should be approached with caution due to its potential negative impacts on plant communities.

The impact of maize straw incorporation on arsenic and cadmium availability, transformation and microbial communities in alkaline-contaminated soils

Increasing evidence of the uncertainty of crop straw returning in heavy metal-contaminated soil is a significant concern. The present study investigated the influence of 1 and 2% maize straws (MS) amendment on As and Cd bioavailability in two different alkaline soils (A-industrial and B-irrigation) after 56 days of ageing. Adding MS to the two soils decreased the pH by 1.28 (A soil) and 1.13 (B soil) and increased the concentration of dissolved organic carbon (DOC) by 54.40 mg/kg (A soil) and 100.00 mg/kg (B soil) during the study period. After 56 days of ageing, the overall NaHCO3-As and DTPA-Cd increased by 40% and 33% (A) and 39% and 41% (B) soils, respectively. The MS additions increased the alteration of As and Cd exchangeable and residual fractions, whereas advanced solid-state 13C nuclear magnetic resonance (NMR) revealed that alkyl C and alkyl O-C-O in A soil and alkyl C, Methoxy C/N-alkyl, and alkyl O-C-O in B soil significantly contributed to the As and Cd mobilisation. Collectively, 16 S rRNA analyses revealed Acidobacteria, Firmicutes, Chloroflexi, Actinobacteria and Bacillus promoted the As and Cd mobilisation following the MS addition, while principle component analysis (PCA) demonstrated that bacterial proliferation significantly influenced MS decomposition, resulting in As and Cd mobilisation in the two soils. Overall, the study highlights the implications of applying MS to As- and Cd-contaminated alkaline soil and offers the framework for conditions to be considered during As- and Cd-remediation efforts, especially when MS is the sole remediation component.

Removal performance and mechanisms of aqueous Cr (VI) by biochar derived from waste hazelnut shell

Cr (VI) is still of great concern due to its high toxicity, solubility, and mobility. The transformation of waste biomass to biochar is favorable for sustainable development. Hazelnut shell, an agriculture waste, was utilized as precursor to prepare biochar at 700 °C and firstly conducted for Cr (VI) removal. Nearly all 50 mg L-1 of Cr (VI) was removed from aqueous media in 180 min under the optimal conditions. The best compliance with pseudo-second-order kinetic model (R2 = 0.999) and Langmuir isotherm model (R2 = 0.999) indicated Cr (VI) removal was a monolayer chemisorption process. The hazelnut shell biochar exhibited superior performance on Cr (VI) removal at low pH (2.0) and Cr (VI) concentrations (≤ 50 mg L-1). Various techniques illustrated that the predominant mechanism of Cr (VI) removal by hazelnut shell biochar involved electrostatic attraction, reduction, and complexation. This study provides a promising low-cost alternative for Cr (VI) elimination from acidic wastewater and groundwater after extraction following by pH adjustment to 2.0.

The biochar-based nanocomposites improve seedling emergence and growth of dill by changing phytohormones and sugar signaling under salinity

Biochar-based nanocomposites (BNCs) with a high level of sodium sorption capacity may improve salinity tolerance and seedling establishment of dill. Thus, a pot experiment was conducted to evaluate the effects of solid biochar (30 g solid biochar kg^-1 soil) and biochar-based nanocomposites of iron (BNC-FeO) and zinc (BNC-ZnO) in individual (30 g BNC kg^-1 soil) and a combined form (15 g BNC-FeO + 15 g BNC-ZnO kg^-1 soil) on dill seedling growth in different levels of salt stress (non-saline, 6 and 12 dSm^-1). Salinity caused a decrease in emergence percentage and emergence rate of seedlings. Increasing salinity of soil up to 12 dSm^-1 decreased the biomass of dill seedlings by about 77%. Application of biochar and particularly BNCs increased the content of potassium, calcium, magnesium, iron, and zinc, reducing and non-reducing sugars, total sugars, invertase and sucrose synthase activities, leaf water content, gibberellic acid, and indole-3-acetic acid in dill plants, leading to an improvement in seedling growth (shoot length, root length, and dry weight) under saline conditions. Sodium content was noticeably decreased by BNC treatments (9-21%), which reduced mean emergence rate and stress phytohormones such as abscisic acid (31-43%), jasmonic acid (21-42%), and salicylic acid (16-23%). Therefore, BNCs especially in combined form can potentially improve emergence and growth of dill seedlings under salt stress, through reducing sodium content and endogenous stress hormones, and enhancing sugars and growth promoting hormones.

Application of ferromanganese functionalized biochar simultaneously reduces Cd and Pb uptake of wheat in contaminated alkaline soils

The reduction of Cd and Pb accumulation in wheat grains grown on Cd and Pb contaminated alkaline soils is a pressing issue that needs to be solved. In this study, ferromanganese functionalized biochar (FM-BC) was used to remediate Cd and Pb contaminated alkaline soils and mitigate Cd and Pb accumulation in wheat grains. The immobilization capacity and mechanism of FM-BC were investigated by Fourier transform infrared spectroscopy (FT-IR) and X-ray diffraction (XRD) characterization and chemical analysis. Fe and Mn loaded on FM-BC improved the removal efficiencies of DTPA-Cd and DTPA-Pb in soil with DTPA-Cd removal of 22.99%- 52.04% (JM22) and 25.54%- 53.32 (AK58) and DTPA-Pb removal of 11.39%- 22.36% (JM22) and 5.38%- 13.00% (AK58). The FT-IR and XRD results indicated that the complexation and precipitation of Cd and Pb with the Fe-Mn oxides and the oxygen-containing functional groups on biochar surface stabilized the Cd and Pb in soil for the observation of Cd₂Mn₃O₈, PbHPO₄, CdCO₃, and PbO₂ on FM-BC isolated from contaminated soils. FM-BC with excellent adsorption capacity reduced the available Cd and Pb in the soil, therefore, thereby inhibiting the Cd and Pb accumulation in wheat. In the 3% FM-BC treatment, Cd and Pb contents in wheat grains were lower than 0.10 mg/kg and 0.20 mg/kg, respectively, reaching the national safety standards. And FM-BC increased the Fe, Mn, Na and Zn contents in wheat grains, and improved the growth and yield of wheat. These findings suggest that FM-BC can be considered a prospective and effective material for remediation of alkaline soils contaminated with Cd and Pb.

Influence of temperature change on the immobilization of soil Pb and Zn by hydrochar: Roles of soil microbial modulation

Considering the potential effect of the ambient temperature on soil microorganisms during heavy metal immobilization by hydrochar, 60 days of soil incubation was conducted to explore the impact of ambient temperature (5, 25, and 35 °C) on the immobilization of Pb and Zn by chitosan-magnetic sawdust hydrochar (CMSH) and magnetic chitosan hydrochar (MCH). The results showed that soil pH was relatively high and total organic carbon (TOC) was slightly lower in the 35 °C treatment. The diethylenetriaminepentaacetic acid (DTPA) available state content decreased significantly with the temperature increasing. Meanwhile, the ratios of stable Pb and Zn in the sequential extraction method proposed by the European Community Bureau of Reference (BCR) gradually increased with increasing temperature. The heatmap based on microbial community showed that elevated temperature not only favored the enrichment of metal-stable phyla, such as Chloroflexi, but was also involved in inhibiting the growth of Firmicutes, Actinobacteriota, and Proteobacteria. Meanwhile, different genera (Fonticella and Bacillus) in the Firmicutes phylum had distinct responses to temperature as well as to heavy metal immobilization effects. Subsequently, redundancy analysis confirmed that Chloroflexi and Fonticella were positively correlated with temperature and stable state metal content, while Actinobacteriota and Bacillus were negatively correlated with temperature and were positively correlated with DTPA available metal content. Moreover, Pb and Zn indicators displayed significant correlations for the dominant genera (R² > 0.8, p < 0.02).

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

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

Decreased in vitro bioaccessibility of Cd and Pb in an acidic Ultisol through incorporation of crop straw-derived biochar

Studies analyzing the in vitro bioaccessibility (BAc) of heavy metals in biochar-amended soils are currently lacking. The present study aimed to assess the metal BAc in Cd- and Pb-spiked acidic Ultisol samples treated individually with 2% (w/w) maize, rice, wheat, soybean, and pea straw-derived biochar. The results indicate that the Cd-BAc simulated in gastric phase (GP) decreased from 78.4% to 66.5-72.3% and the Pb-BAC decreased from 74.3% to 67.2-69.2%; however, the Cd-BAc in the intestinal phase (IP) decreased from 35.6% to 27.9-33.5% and the Pb-BAc decreased from 34.7% to 29.7-32.9% after 120 d of incubation with biochar application compared to the un-amended Ultisol. The Cd- and Pb-BAc in both GP and IP were significantly negatively correlated with soil pH, CEC, and organic carbon (P < 0.05), which increased after biochar application. The soybean straw-derived biochar amendment has the greatest potential to decrease the BAc of Cd and Pb in the GP and IP, owing to the highest level of CEC, SOC, TC and TN among all soil samples. Moreover, the BAc was positively correlated with the exchangeable, and exchangeable + carbonate-bound Cd and Pb fractions (P < 0.05), indicating these fractions had a dominant influence on the BAc of cationic heavy metals. Therefore, crop straw-derived biochar amendment can decrease the BAc of Cd and Pb in acidic Ultisol, and thus mitigate the health risks posed by these metals from incidental ingestion.

Pros and Cons of Biochar to Soil Potentially Toxic Element Mobilization and Phytoavailability: Environmental Implications

While the potential of biochar (BC) to immobilize potentially toxic elements (PTEs) in contaminated soils has been studied and reviewed, no review has focused on the potential use of BC for enhancing the phytoremediation efficacy of PTE-contaminated soils. Consequently, the overarching purpose in this study is to critically review the effects of BC on the mobilization, phytoextraction, phytostabilization, and bioremediation of PTEs in contaminated soils. Potential mechanisms of the interactions between BC and PTEs in soils are also reviewed in detail. We discuss the promises and challenges of various approaches, including potential environmental implications, of BC application to PTE-contaminated soils. The properties of BC (e.g., surface functional groups, mineral content, ionic content, and π-electrons) govern its impact on the (im)mobilization of PTEs, which is complex and highly element-specific. This review demonstrates the contrary effects of BC on PTE mobilization and highlights possible opportunities for using BC as a mobilizing agent for enhancing phytoremediation of PTEs-contaminated soils.

A critical review on biochar-assisted free radicals mediated redox reactions influencing transformation of potentially toxic metals: Occurrence, formation, and environmental applications

Potentially toxic metals have become a viable threat to the ecosystem due to their carcinogenic nature. Biochar has gained substantial interest due to its redox-mediated processes and redox-active metals. Biochar has the capacity to directly adsorb the pollutants from contaminated environments through several mechanisms such as coprecipitation, complexation, ion exchange, and electrostatic interaction. Biochar's electron-mediating potential may be influenced by the cyclic transition of surface moieties and conjugated carbon structures. Thus, pyrolysis configuration, biomass material, retention time, oxygen flow, and heating time also affect biochar's redox properties. Generally, reactive oxygen species (ROS) exist as free radicals (FRs) in radical and non-radical forms, i.e., hydroxyl radical, superoxide, nitric oxide, hydrogen peroxide, and singlet oxygen. Heavy metals are involved in the production of FRs during redox-mediated reactions, which may contribute to ROS formation. This review aims to critically evaluate the redox-mediated characteristics of biochar produced from various biomass feedstocks under different pyrolysis conditions. In addition, we assessed the impact of biochar-assisted FRs redox-mediated processes on heavy metal immobilization and mobility. We also revealed new insights into the function of FRs in biochar and its potential uses for environment-friendly remediation and reducing the dependency on fossil-based materials, utilizing local residual biomass as a raw material in terms of sustainability.

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

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

First "unsaturated soils" view towards quantitative adsorption and immobilization mechanisms of Cd by biochar in soils during aging

Instead of traditional batch and column experiments with large water-soil ratios, this study investigated the behaviors and mechanisms of Cd adsorption and immobilization by biochar in unsaturated soils, in which the soil moisture conditions were closer to those in the actual field. The transport, transformation, and immobilization of cadmium (Cd) by pristine and KMnO₄-modified biochars in unsaturated soils were investigated during a 48-week mild aging process. Biochar acidified with HCl solution was employed to quantify the contributions of mineral and non-mineral components in biochar to Cd adsorption and immobilization in unsaturated soils with a three-layer mesh method. The behaviors and mechanisms of Cd adsorption by biochar in unsaturated soils significantly differed from those in aqueous solutions. The equilibrium times of Cd adsorption by biochar in unsaturated soils (weeks) were much longer than those in aqueous solutions (hours). The percentages of the Cd adsorbed by pristine and modified biochar remained relatively constant relative to the total Cd in unsaturated soils, which accounted for 39.50-49.39 % and 57.35-68.94 %, respectively. The contribution of mineral components to Cd adsorption dominated in both unsaturated soils (45.00-94.09 %) and aqueous solutions (70.73-95.51 %). The process of Cd immobilization in unsaturated soils was that biochar firstly adsorbed the exchangeable Cd from the soil, and then converted it to relatively stable Cd. After aging for 48 weeks, the contributions of non-mineral components to Cd immobilization dominated in unsaturated soil with a low concentration (1.23 mg·kg^-1), and the contributions of mineral components to Cd immobilization dominated in unsaturated soil with medium-high concentrations (4.08-51.26 mg·kg^-1).

Remediation of Cr(VI)-Contaminated Soil by Biochar-Supported Nanoscale Zero-Valent Iron and the Consequences for Indigenous Microbial Communities

Biochar/nano-zero-valent iron (BC-nZVI) composites are currently of great interest as an efficient remediation material for contaminated soil, but their potential to remediate Cr-contaminated soils and effect on soil microecology is unclear. The purpose of this study was to investigate the effect of BC-nZVI composites on the removal of Cr(VI) from soil, and indigenous microbial diversity and community composition. The results showed that after 15 days of remediation with 10 g/kg of BC-nZVI, 86.55% of Cr(VI) was removed from the soil. The remediation of the Cr-contaminated soil with BC-nZVI resulted in a significant increase in OTUs and α-diversity index, and even a significant increase in the abundance and diversity of indigenous bacteria and unique bacterial species in the community by reducing the toxic concentration of Cr, changing soil properties, and providing habitat for survival. These results confirm that BC-nZVI is effective in removing Cr(VI) and stabilizing Cr in soil with no significant adverse effects on soil quality or soil microorganisms.

Efficient Remediation of Cadmium Contamination in Soil by Functionalized Biochar: Recent Advances, Challenges, and Future Prospects

Heavy metal pollution in soil seriously harms human health and animal and plant growth. Among them, cadmium pollution is one of the most serious issues. As a promising remediation material for cadmium pollution in soil, functionalized biochar has attracted wide attention in the last decade. This paper summarizes the preparation technology of biochar, the existing forms of heavy metals in soil, the remediation mechanism of biochar for remediating cadmium contamination in soil, and the factors affecting the remediation process, and discusses the latest research advances of functionalized biochar for remediating cadmium contamination in soil. Finally, the challenges encountered by the implementation of biochar for remediating Cd contamination in soil are summarized, and the prospects in this field are highlighted for its expected industrial large-scale implementation.

Immobilization of cadmium by mercapto-functionalized palygorskite under stimulated acid rain: Stability performance and micro-ecological response

The interaction of cadmium (Cd) pollution and acid rain stress has seriously threatened soil ecosystem and human health. However, there are still few effective amendments for the in-situ remediation in the Cd-contaminated acidified soil. In this study, the performance and mechanisms of palygorskite (PAL) and mercapto-functionalized PAL (MPAL) on Cd immobilization were investigated, and the stability as well as effects on soil micro-ecology under stimulated acid rain were also explored. Results showed that MPAL could react with Cd to form stable Cd-sulfhydryl and Cd-O complexes. The reduction of bioavailable Cd by MPAL was 121.19-164.86% higher than that by PAL. Notably, the Cd immobilization by MPAL remained stable within 90 days in which the concentrations of HOAc-extractable Cd were reduced by 18.28-25.12%, while the reducible and residual fractions were increased by 9.26-18.53% and 54.16%-479.01%, respectively. The sequential acid rain leaching demonstrated that soil after MPAL treatments had a strong H⁺ resistance, and the immobilized Cd showed prominent stability. In addition, activities of acid phosphatase, catalase and invertase in MPAL treated soil were significantly enhanced by 34.60%, 22.09% and 48.87%, respectively. After MPAL application, bacterial diversity was further improved with diversified sulfur metabolism biomarkers. The decreased abundance of Cd resistance genes including cadA, cadC, czcA, czcB, czcR and zipA also indicated that soil micro-ecology was improved by MPAL. These results showed that MPAL was an effective and eco-friendly amendment for the immobilization of Cd in contaminated soil.

Cadmium immobilization in lake sediment using different crystallographic manganese oxides: Performance and mechanism

Cd pollution in sediments poses severe threats to environmental safety and human health. Mn oxides have potential merit for the remediation of Cd pollution in sediment but have not received enough attention. Although Mn oxides have proven effective as adsorbents for removing heavy metals from water/wastewater, the performance and the underlying mechanism of Cd immobilization in sediments by Mn oxides remain unclear. Here, three crystallographic Mn oxides δ-MnO₂, γ-MnOOH, and Mn₃O₄ were used as amendments to investigate their potential for the in situ immobilization of Cd in lake sediment. Experimental data showed that when the sediment samples were treated with synthesized Mn oxides at dosages of 2% and 6% (w/w) for 56 days, the TCLP (toxicity characteristic leaching procedure) leachable Cd in the sediment decreased by 43.9-66.81%, and the PBET (physiologically based extraction test) extractable Cd decreased by 45.16-99.40%. Additionally, the acid-soluble fraction of Cd was partially transformed to a residual fraction, resulting in a 27.55-35.49% decrease in acid-soluble Cd and a 25.16-30.36% increase in the residual Cd fraction. Sediment pH and oxidation-reduction potential were important factors affecting the bioavailability of Cd in the remediation process. Furthermore, scanning electron microscopy, X-ray diffractometer, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy analysis illustrated that the interaction between the amendment and Cd mainly involved complexation with O-containing groups, ion-exchange as > OCd⁺, and precipitation with carbonate. The efficient remediation capacity and associated mechanism for Mn oxides provide insights for the improved restoration of heavy metal-contaminated sediment.

Biochar produced from the straw of common crops simultaneously stabilizes soil organic matter and heavy metals

The simultaneous stabilization of heavy metals and organic matter in polluted soil has received little research attention. In this study, we studied the immobilization of Cu and Cd and the mineralization of organic matter in the acidic soil amended with biochar produced from rice, wheat, corn, and rape straws through incubation experiments. Compared with that in the control treatment, the availability of Cu and Cd in the biochar amended soils decreased by 17-31% and 3-17%, respectively. The cumulative amount of CO₂ released from each treatment in 60 days of incubation followed the order: control treatment (399 mg CO₂-C kg^-1) > rape straw biochar treatment (388 mg CO₂-C kg^-1) > rice straw biochar treatment (374 mg CO₂-C kg^-1) > corn straw biochar treatment (355 mg CO₂-C kg^-1) > wheat straw biochar treatment (288 mg CO₂-C kg^-1). The information implied that biochar produced from the straw of common crops can simultaneously stabilize both heavy metals and organic matter in the acidic soil. The transformation of Cu and Cd from acid soluble fraction to residual fraction was the potential mechanism of biochar in facilitating soil heavy metal immobilization. The significant decrease in soil β-glucosidase activity, which controlled the degradation of soil organic matter, was an important potential pathway of biochar in decreasing soil organic matter mineralization. A significant decrease in the content and a substantial increase in the structural complexity of soil dissolved organic matter could further the decrease of wheat straw biochar in soil organic matter mineralization. Thus, biochar produced from the straw of common crops is a promising amendment for simultaneously stabilizing both heavy metals and organic matter in the acidic soil.

The performance and mechanism of cadmium availability mitigation by biochars differ among soils with different pH: Hints for the reasonable choice of passivators

The performances of passivation materials mitigating Cadmium (Cd) bioavailability considerably vary with the pH condition of Cd-contaminated soils. However, less information was available for the method of improving Cd passivation efficiency taking into account the pH of the targeted soil. Furthermore, the underlying mechanism of Cd availability mitigation in soils with different pH has not been clearly explored. In this study, cotton straw biochar (CSB) and its modified products using NaOH (CSB-NaOH) were prepared and applied in two kinds of Cd-contaminated soils with different pH. It was found that CSB-NaOH was more effective than CSB in regulating the Cd bioavailability in the acid soil, while the opposite tendency was observed in alkaline soil. The difference of the Cd passivation efficiency is correlated with contributions of various Cd-biochar binding mechanisms, which cation exchange mechanism is largely eliminated for CSB-NaOH. The interaction of Cd with CSB/CSB-NaOH was further evidenced through characterization results of Scan Electron Microscopy (SEM), X-Ray Diffraction (XRD), Fourier-transformed infrared spectroscopy (FTIR) and X-ray Photoelectron spectroscopy (XPS). Characterization results proved that carboxyl, hydroxyl and ethyl groups were the key functional groups involved in Cd passivation. XPS results showed that Cd binding methods varied between CSB and CSB-NaOH, which Cd²⁺ and Cd-O were the main form of Cd binding to CSB while Cd-O was the main form on CSB-NaOH. In this work, it was demonstrated that in acid soil, pH change caused by biochar plays a more significant role in controlling the Cd bioavailability, while in alkaline soil, the strength of the Cd-biochar interaction is more decisive for the Cd passivation efficiency. This work provides information on how to select the suitable passivator to decrease the Cd bioavailability in terms of different soil pH and property.

Cadmium long-term immobilization by biochar and potential risks in soils with different pH under combined aging

Cd long-term immobilization by biochar and potential risk in soils with different pH were quantified under a combined artificial aging, which simulated five years of aging in the field based on local climate. Two biochars (original and KMnO₄-modified) and five soils with different pH were tested, and an improved three-layer mesh method was employed in this study. Five aging cycles were carried out (Cycle 1-Cycle 5), and each aging cycle quantitatively simulated 1 year of natural aging. As the aging time increased, Cd leaching loss in all soils gradually increased from Cycle 1 to Cycle 5; for relatively stable Cd fraction, it decreased firstly and then stabilized in acidic and neutral soils (S1-S4), while it decreased firstly and then increased in alkaline soil (S5). Biochars significantly promoted Cd immobilization in strongly acidic soil (S1) by increasing relatively stable fractions and decreasing leaching loss. For weakly acidic and neutral soils (S2-S4), although biochars still had positive effects, the immobilization effects were weakened to certain extents compared with S1. The percentage of Cd leaching loss decreased by 19.12% in strongly acidic soil (S1) and by 1.12-11.35% in weakly acidic and neutral soils (S2-S4) after modified biochar treatment. For alkaline soil (S5), the application of biochars had negative effects on Cd immobilization by decreasing relatively stable fractions and increasing leaching loss, and posed risks to the environment. For strongly acidic soil (S1) and weakly acidic and neutral soils (S2-S4), the percentages of relatively stable fractions increased from 6.09-19.93% to 24.98-36.70% after modified biochar treatment. However, for alkaline soil, the percentage of relatively stable fractions decreased from 55.27% to 53.93% after biochar treatment. The more acidic the soil, the more effective the Cd immobilization by biochar. Biochars with high pH level are not suitable for the remediation of alkaline Cd contaminated soil.

Modified Biochars and Their Effects on Soil Quality: A Review

Biochar (BC) has attracted attention due to its impacts on soil quality by enhancing soil fertility, carbon storage and contaminants immobilization. BC also induces changes in microbial community structure and enhances crop productivity in long term scenarios compared to many other organic amendments. However, information related to the role of modified BCs in altering the soil quality is still scarce. BC can be modified by using physical, chemical and microbial methods. Modified BC can change the functional groups, pore size, pore structure, surface area and chemical properties of soil, which plays a key role in changing the soil quality. The addition of modified BCs as soil amendment increased soil CEC (cation exchange capacity), EC (electron conductivity), pH, organic matter, hydraulic conductivity, soil porosity, infiltration rate, microbial activities (enzymes and community), nutrient profile and gas exchange properties, but it varies according to the soil structure and pervading environmental conditions. This study provides a basis for effective practical approaches to modifying BCs for improving soil quality.

Efficient removal of Cd(II) by phosphate-modified biochars derived from apple tree branches: Processes, mechanisms, and application

Phosphate (P)-modified biochar is a good material for cadmium (Cd) immobilization, and the pore-forming effect of potassium ions (K⁺) can favor the P loading on biochar. However, few studies have been done specifically on Cd(II) removal by composites of potassium phosphates with biochar, and the removal potential and mechanisms are not clear. Herein, apple tree branches, a major agricultural waste suitable for the development of porous materials, were pyrolyzed individually or together with KH₂PO₄, K₂HPO₄·3H₂O, or K₃PO₄·3H₂O to obtain biochars to remove Cd(II), denoted as pristine BC, BC-1, BC-2, and BC-3, respectively. The results showed that the orthophosphates containing more K⁺ enlarged the specific surface area, total pore volume and phosphorus loading of biochar. Co-pyrolysis of apple tree branches and P promoted the thermochemical transformation of P species. Only weak signal of orthophosphate was observed in the pristine BC, while the presence of orthophosphate, pyrophosphate and metaphosphate were detected in BC-1, and BC-2 and BC-3 showed the presence of orthophosphate and pyrophosphate. The maximum Cd(II) adsorption capacities of pristine BC, BC-1, BC-2 and BC-3 were 10.4, 88.5, 95.8, and 116 mg·g^-1, respectively. Orthophosphate modification enhanced the Cd(II) adsorption capacity due to the formation of Cd-P-precipitates, namely Cd₅(PO₄)₃Cl, Cd₅(PO₄)₃OH, Cd₃(PO₄)₂, Cd₂P₂O₇, and Cd(PO₃)₂. Furthermore, higher cation exchange efficiencies between Cd(II) and K⁺ in P-modified biochars also contributed to their high Cd(II) adsorption capacity. Cd(II) removal by BC-3 from artificially polluted water bodies showed more than 99.98% removal rates. Application of BC-3 also reduced the diethylene triamine pentaacetic acid-extracted Cd(II) in soil by 69.1%. The co-pyrolysis of apple tree branches and potassium phosphates shows great prospect in Cd(II) wastewater/soil treatment and provide a promising solution for agricultural waste utilization and carbon sequestration.

Stabilization of lead and cadmium in soil by sulfur-iron functionalized biochar: Performance, mechanisms and microbial community evolution

Sulfur-iron functionalized biochar (BC-Fe-S) was designed by simultaneously supporting Fe₂O₃ nanoparticles and grafting sulfur-containing functional groups onto biochar to stabilize Pb and Cd in soil. The BC-Fe-S exhibited excellent stabilization performance for Pb and Cd with fast kinetic equilibrium within 5 days associating with pseudo-second-order model. The bioavailable-Pb and -Cd contents decreased by 59.22% and 70.28% with 3% BC-Fe-S treatment after 20 days of remediation. Speciation transformation analysis revealed that the increase of stabilization time and BC-Fe-S dosage with appropriate soil moisture and pH promoted toxicities decrease of Pb and Cd with transformation of labile fractions to more steady fractions. The labile fractions of Pb and Cd decreased by 12.22% and 16.21% with 3% BC-Fe-S treatment, and transformed to the residual speciation. Meanwhile, wetting-drying and freezing-thawing aging did not markedly alter the bioavailability of Pb and Cd, proving that the BC-Fe-S holds promise for stabilization of Pb and Cd in varying environmental conditions. 16S rRNA sequencing analysis demonstrated that the BC-Fe-S significantly improved diversity and composition of microbial community, especially increasing the relative abundance of heavy metal-resistant bacteria. Overall, these results suggested BC-Fe-S as a high-performance and environmental-friendly amendment with stability to remediate heavy metals polluted soil.