Different feedstocks of biochar affected the bioavailability and uptake of heavy metals by wheat (Triticum aestivum L.) plants grown in metal contaminated soil

Jackfruit seed biochar-apatite amendments: investigating changes in lead and zinc's fractionation in the multi-metal-contaminated soil

Multi-metal contamination in soil presents major environmental and agricultural challenges globally, impacting the feasibility of phytoremediation. This study investigated the efficacy of jackfruit seed-derived biochar (JSB) produced at 300 °C (JSB300) and 600 °C (JSB600), combined with apatite, to mitigate potentially toxic elements (PTEs), thus influencing bioavailability, in soils heavily contaminated with lead (Pb) and zinc (Zn). The primary objective was to determine how these amendments altered the chemical fractions of Pb and Zn using Tessier's sequential extraction procedure. Soil samples with initial concentrations of 3052.5 ± 15.6 mg kg-1 Pb and 1531.0 ± 20.2 mg kg-1 Zn were treated with biochar and apatite at 5%, 10%, 2.5:2.5%, and 5:5% (w/w). Results revealed that JSB600 and JSB300 at a 10% ratio, achieved the most significant reduction in exchangeable Pb and Zn fractions, decreasing them by up to 49.3 and 48.6%, respectively, within one month. This substantial decrease in readily available metal fractions, alongside concurrent increases in soil pH (+22.6%), organic carbon (+290.3%), and electrical conductivity (+249.0%), suggests that jackfruit seed biochar and apatite can significantly improve soil conditions for phytostabilization, by reducing metal bioavailability, or potentially for phytoextraction by influencing specific metal chemical fractions, in multi-metal-contaminated environments, enhancing soil conditions for remediation.

Insights into roles of biochar on migration and accumulation of cadmium in Spartina alterniflora Loisel. -sediment systems and their microbial effects

It is widely recognized that remediating cadmium (Cd)-contaminated sediments using biochar (BC) can significantly influence plant growth and development. However, the efficiency of such remediation often diminishes in field trials compared to greenhouse experiments, likely due to limited comprehension of the BC addition on the plant-sediment-microbe interaction. In this study, a 56-day pot experiment demonstrated that BC application offered (i) enhanced plant root length and biomass, (ii) increased proline content, (iii) improved photosynthetic capacity (e.g., total chlorophyll content), and (iv) mitigated oxidative stress (e.g., decreased the peroxidase (POD) and (CAT) activity, and increased superoxide dismutase (SOD) activity). The increased SOD allowed better scavenging of reactive oxygen species (ROS) in leaves (the primary site of ROS generation), thereby alleviating leaf growth retardation. Notably, the translocation factor of Cd significantly reduced to 0.0034 in BC-amended sediments, under high-Cd toxicity. Qualitative and quantitative analysis identified that BC facilitated the Cd binding ability on the wall of the root cells (up to 85.67 ± 0.88 %) by increasing the adsorption capacity of matrix polysaccharides. Furthermore, the bioavailable Cd proportion in sediments was markedly reduced after BC addition. BC also increased the relative abundances of bacteria, such as Desulfuromonadia and Alteromonadaleswhich were involved in Cd immobilization, and enhanced microbial adaptability to Cd-stress by boosting genetic and environmental information processing functions. The mechanisms on how BC reduced the bioavailable Cd in sediment was dependent on both BC and root presence. These findings demonstrated that BC application is an effective strategy for remediating Cd-contaminated coastal wetlands, offering significant benefits for environmental health and human well-being.

Phosphate-modified biochar attenuates cadmium availability in contaminated soil and reduces its transfer to tomato fruits

Cadmium (Cd) is a toxic heavy metal for plant growth and development. Biochar has been proposed as an effective approach to increase immobilized cadmium fractions in soil and reduce its availability and accumulation by plants. In the present study, the enrichment of biochar with water-soluble polyphosphate (PLB) was tested to assess its ability to modify cadmium immobilization capacities in soils compared with standard biochar (SB) and orthophosphate enriched biochar (OLB) under Cd stress. In a pot experiment, the impact of these three biochar forms (SB, PLB, and OLB) on soil properties, soil Cd fractions, photosynthesis, tomato plant growth, yield, fruit quality, and nutrient uptake was assessed under two levels of Cd (0, and 5 mg kg-1). The obtained results showed that the enrichment of biochar with both forms of phosphorus fertilizers (orthophosphate and polyphosphate) significantly and positively impacted P and K contents in the final enriched biochar compared to SB. Results demonstrate that applying PLB under cadmium stress significantly increased phosphorus availability in soil (+32 %) and reduced Cd exchangeable fraction compared to the SB and OLB treatments. These findings suggest that, under cadmium stress, the slow-release properties as well as the organometallic chelation and precipitation capacities of the PLB can help in improving phosphorus, calcium, and iron uptake by plants and reducing Cd uptake and translocation to the shoot tissues, which resulted in significant enhancement of plant photosynthesis efficiency (PIabs: +144 %), shoot dry biomass (+117 %), and fruit yield (308 %) and quality, compared to standard biochar. Therefore, the enrichment of biochar with polyphosphate fertilizers could be proposed as an efficient strategy to enhance plant growth, physiology, and nutrition and mitigate cadmium stress and toxicity in contaminated soils.

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

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

Effects of Mg-Palygorskite Modified Biochar on the Growth of Sedum alfredii Hance in Heavy Metal Contaminated Soil

Heavy metal contamination of soil poses a serious threat to agricultural production and human health. Biochar modified with Mg and palygorskite can reduce the content of available heavy metals in soil; however, its passivation effect is affected by the modification method, and there is a lack of research on its impact on plant growth in heavily polluted soil. In this study, four types of modified biochar were prepared using MgCl2, palygorskite, and wood as raw materials, including MBC and MPB prepared by pre-modification and BCM and BPM prepared by post-modification. Sedum alfredii Hance was selected as the test plant, and a pot experiment was conducted to explore the effects of unmodified and modified biochar on the growth of Sedum alfredii Hance in heavily polluted soil with Cu, Pb, Zn, and Cd. Compared with the original biochar, the modified biochar, especially the pre-modified biochar, significantly increased the ash content, pH, O/C ratio, surface functional group count, and mineral content. The adsorption capacity for heavy metals was also significantly enhanced, with the main adsorption mechanisms being precipitation, complexation, and ion exchange. The four types of modified biochar promoted the growth and biomass of Sedum alfredii Hance to varying degrees, with the promotion effect in the order of MPB > MBC > BPM > BCM, and the effect was more significant with a 3% addition. The modified biochar significantly reduced the content of available heavy metals in the rhizosphere soil, with a passivation effect in the order of MPB > MBC > BPM > BCM, and the 3% addition had the greatest effect. Further analysis via the Mantel test and structural equation modeling confirmed that modified biochar promoted the growth of Sedum alfredii Hance by reducing the available heavy metal content in the rhizosphere soil and increasing the NO3--N and AP contents. This study provides data support for the development of functionalized biochar for the remediation of heavy metal pollution in soil.

Analysis of growth physiological changes and metabolome of highland barley seedlings under cadmium (II) stress

This study aims to investigate the physiological changes in growth and metabolic response mechanisms of highland barley under different concentrations of cadmium. To achieve this, cadmium stress was applied to green barley at levels of 20, 40, and 80 mg/L. The results revealed that, under Cd(II) stress, the chlorophyll content and photosynthesis in leaves of highland barley seedlings were inhibited to some extent. Additionally, the malondialdehyde (MDA) content and superoxide dismutase activity increased significantly, indicating that the seedlings were affected by oxidative stress. In addition, Cd(II) stress also significantly affected the accumulation of metabolites in highland barley seedlings, resulting in an increase in lipids and lipid molecules, organic heterocyclic compounds, and phenylpropanoids. Cd(II) stress also significantly interfered with phenylalanine metabolism, fructose and mannose metabolism, amino acid, sugar, and nucleotide sugar metabolism, and biosynthetic metabolic pathways of isoquinoline alkaloids. The increase in Cd(II) stress also resulted in elevated levels of soluble sugars, soluble proteins, and proline as defense mechanisms against the stress. Overall, barley has a very good ability to resist adversity, and the mechanism of barley's resistance to adversity has not been deeply investigated. Therefore, in this paper, we systematically investigated the stress resistance mechanism of barley to cadmium stress and found that the growth physiology and metabolism of barley seedlings were significantly affected by cadmium stress. Differential changes in metabolites and enrichment of metabolic pathways may be the main mechanisms for barley seedlings to cope with Cd(II) stress. This provides direction for selecting better varieties of barley.

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.

Simultaneous immobilization strategy of anionic metalloids and cationic metals in agricultural systems: A review

Concurrent heavy metals remediation in natural environments poses significant challenges due to factors like metal speciation and interactions with soil moisture. This review focuses on strategies for immobilizing both anionic and cationic metals simultaneously in soil-crop systems. Key approaches include water management, biochar utilization, stabilizing agents, nanotechnology, fertilization, and bioremediation. Sprinkler or intermittent irrigation combined with soil amendments/biochar effectively immobilizes As/Cd/Pb simultaneously. This immobilization occurs through continuous adsorption-desorption, oxidation-reduction, and precipitation mechanisms influenced by soil pH, redox reactions, and Fe-oxides. Biochar from sources like wine lees, sewage sludge, spent coffee, and Fe-nanoparticles can immobilize As/Cd/Pb/Cr/Co/Cu/Zn together via precipitation. In addition, biochar from rice, wheat, corn straw, rice husk, sawdust, and wood chips, modified with chemicals or nanoparticles, simultaneously immobilizes As and Cd, containing higher Fe3O4, Fe-oxide, and OH groups. Ligand exchange immobilizes As, while ion exchange immobilizes Cd. Furthermore, combining biochar especially with iron, hydroxyapatite, magnetite, goethite, silicon, graphene, alginate, compost, and microbes-can achieve simultaneous immobilization. Other effective amendments are selenium fertilizer, Ge-nanocomposites, Fe-Si materials, ash, hormone, and sterilization. Notably, combining nano-biochar with microbes and/or fertilizers with Fe-containing higher adsorption sites, metal-binding cores, and maintaining a neutral pH could stimulate simultaneous immobilization. The amendments have a positive impact on soil physio-chemical improvement and crop development. Crops enhance production of growth metabolites, hormones, and xylem tissue thickening, forming a protective barrier by root Fe-plaque containing higher Fe-oxide, restricting upward metal movement. Therefore, a holistic immobilization mechanism reduces plant oxidative damage, improves soil and crop quality, and reduces food contamination.

Bioassessment of Cd and Pb at Multiple Growth Stages of Wheat Grown in Texturally Different Soils Using Diffusive Gradients in Thin Films and Traditional Extractants: A Comparative Study

The bioavailability of heavy metals in soil is a crucial factor in determining their potential uptake by plants and their subsequent entry into the food chain. Various methods, including traditional chemical extractants and the diffusive gradients in thin films (DGT) technique, are employed to assess this bioavailability. The bioavailability of heavy metals, particularly cadmium (Cd) and lead (Pb), is also influenced by soil texture and their concentrations in the soil solution. The primary objectives of this experiment were to compare and correlate the assessment of the Cd and Pb bioavailability using the DGT technique and traditional extractants across two soil textural classes: sandy clay loam (SCL) and clay loam (CL) at two contamination levels: aged contaminated (NC) and artificially contaminated (AC). The specific objectives included assessing the bioavailability of Cd and Pb at different growth stages of the wheat plant and correlating the DGT-based bioassessments of Cd and Pb with their concentrations in various plant parts at different growth stages. This study also compared the effectiveness of the DGT method and traditional extraction techniques in assessing the bioavailable fractions of Cd and Pb in soil. The regression analysis demonstrated strong positive correlations between the DGT method and various extraction methods. The results showed that the wheat plants grown in the AC soils exhibited lower root, shoot, and grain weights compared to those grown in the NC soils, indicating that metal contamination negatively impacts plant performance. The concentrations of Cd and Pb in the wheat tissues varied across different growth stages, with the highest levels observed during the grain filling (S3) and maturity (S4) stages. It is concluded that the in situ assessment of Cd and Pb though DGT was strongly and positively correlated with the Cd and Pb concentration in wheat plant parts at the maturity stage. A correlation and regression analysis of the DGT assessment and traditional extractants showed that the DGT method provides a reliable tool for assessing the bioavailability of Cd and Pb in soils and helped in developing sustainable soil management strategies to ensure the safety of agricultural products for human consumption.

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

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

Understanding the dependence of biochar properties on different types of biomass

Owing to the diversity of biomasses and many variables in pyrolysis process, the property of biochar from varied biomass feedstock or even same biomass could differ significantly. Since the property of biochar governs the further application of biochar, this review paid particular attention to the correlation between the nature of biomass feedstock and the specifications of biochar in terms of yield, elemental composition, pH, functionalities, heating value, pore structures, morphologies, etc. The property of the biochar from the pyrolysis of cellulose, hemicellulose, lignin, woody biomass (pine, mallee, poplar, acacia, oak, eucalyptus and beech), bark of woody biomass, leaves of woody biomass, straw, algae, fruit peels, tea waste was compared and summarized. In addition, the differences of the biochar of these varied origins were also analyzed. The remaining questions, about the correlation of biomass nature with biochar characteristics, to be further investigated are analyzed in detail. The deduced information about the relationship of the nature of biochar and biomass feedstock as well as key pyrolysis parameters is of importance for further development of the methods for tailoring or production of the biochar of desirable properties. The results from this study could be interesting technically and commercially for the technology developer using biochar as the source of carbon in different applications.

Effects of biochar types on seed germination, growth, chlorophyll contents, grain yield, sodium, and potassium uptake by wheat (Triticum aestivum L.) under salt stress

Soil salinity is a significant challenge in agriculture, particularly in arid and semi-arid regions such as Pakistan, leading to soil degradation and reduced crop yields. The present study assessed the impact of different salinity levels (0, 25, and 50 mmol NaCl) and biochar treatments (control, wheat-straw biochar, rice-husk biochar, and sawdust biochar applied @ 1% w/w) on the germination and growth performance of wheat. Two experiments: a germination study and a pot experiment (grown up to maturity), were performed. The results showed that NaCl-stress negatively impacted the germination parameters, grain, and straw yield, and agronomic and soil parameters. Biochar treatments restored these parameters compared to control (no biochar), but the effects were inconsistent across NaCl levels. Among the different biochars, wheat-straw biochar performed better than rice-husk and sawdust-derived biochar regarding germination and agronomic parameters. Biochar application notably increased soil pHs and electrical conductivity (ECe). Imposing NaCl stress reduced K concentrations in the wheat shoot and grains with concomitant higher Na concentrations in both parts. Parameters like foliar chlorophyll content (a, b, and total), stomatal and sub-stomatal conductance, and transpiration rate were also positively influenced by biochar addition. The study confirmed that biochar, particularly wheat-straw biochar, effectively mitigated the adverse effects of soil salinity, enhancing both soil quality and wheat growth. The study highlighted that biochar application can minimize the negative effects of salinity stress on wheat. Specifically, the types and dosages of biochar have to be optimized for different salinity levels under field conditions.

Biochar as a partner of plants and beneficial microorganisms to assist in-situ bioremediation of heavy metal contaminated soil

Synergistic remediation of heavy metal (HM) contaminated soil using beneficial microorganisms (BM) and plants is a common and effective in situ bioremediation method. However, the shortcomings of this approach are the low colonisation of BM under high levels of heavy metal stress (HMS) and the poor state of plant growth. Previous studies have overlooked the potential of biochar to mitigate the above problems and aid in-situ remediation. Therefore, this paper describes the characteristics and physicochemical properties of biochar. It is proposed that biochar enhances plant resistance to HMS and aids in situ bioremediation by increasing colonisation of BM and HM stability. On this basis, the paper focuses on the following possible mechanisms: specific biochar-derived organic matter regulates the transport of HMs in plants and promotes mycorrhizal colonisation via the abscisic acid signalling pathway and the karrikin signalling pathway; promotes the growth-promoting pathway of indole-3-acetic acid and increases expression of the nodule-initiating gene NIN; improvement of soil HM stability by ion exchange, electrostatic adsorption, redox and complex precipitation mechanisms. And this paper summarizes guidelines on how to use biochar-assisted remediation based on current research for reference. Finally, the paper identifies research gaps in biochar in the direction of promoting beneficial microbial symbiotic mechanisms, recognition and function of organic molecules, and factors affecting practical applications.

Differences in soil bacterial community structure during the remediation of Cd-polluted cotton fields by biochar and biofertilizer in Xinjiang, China

Introduction

Heavy metal pollution is a major worldwide environmental problem. Many remediation techniques have been developed, these techniques have different performance in different environments.

Methods

In this study, soil sampling was conducted in multiple cotton fields in Xinjiang, China, and found that cadmium (Cd) was the most abundant soil heavy metal. Then, to find the most suitable technique for the remediation of Cd pollution in cotton fields, a two-year study was conducted to explore the effects of cotton straw-derived biochar (BC, 3%) and Bacillus-based biofertilizer (BF, 1.5%) on cotton Cd uptake and transport and soil microbial community structure under Cd exposure conditions (soil Cd contents: 1, 2, and 4 mg·kg-1).

Results

The results showed that the bioaccumulation coefficients (Cd content of cotton organs / soil available Cd content) of cotton roots, stems, leaves, and buds/bolls reduced by 15.93%, 14.41%, 23.53%, and 20.68%, respectively after the application of BC, and reduced by 16.83%, 17.15%, 22.21%, and 26.25%, respectively after the application of BF, compared with the control (no BC and BF). Besides, the application of BC and BF reduced the transport of Cd from soil to root system, and enhanced the diversity of soil bacterial communities (dominant species: Alphaproteobacteria and Actinobacteria) and the metabolic functions related to amino acid synthesis. It was worth noting that the differential species for BF group vs BC group including Alphaproteobacteria, Gemmatimonadetes, Bacilli, and Vicinamibacteria were associated with the enrichment and transport of Cd, especially the transport of Cd from cotton roots to stems.

Discussion

Therefore, the application of BC and BF changed the soil bacterial diversity in Cd-polluted cotton field, and then promoted the transport of Cd in cotton, ultimately improving soil quality. This study will provide a reference for the selection of soil heavy metal pollution remediation techniques in Xinjiang, China.

Unlocking the potential of biochar in the remediation of soils contaminated with heavy metals for sustainable agriculture

Agricultural soils contaminated with heavy metals (HMs) impose a threat to the environmental and to human health. Amendment with biochar could be an eco-friendly and cost-effective option to decrease HMs in contaminated soil. This paper reviews the application of biochar as a soil amendment to immobilise HMs in contaminated soil. We discuss the technologies of its preparation, their specific properties, and effect on the bioavailability of HMs. Biochar stabilises HMs in contaminated soil, enhance the overall quality of the contaminated soil, and significantly reduce HM uptake by plants, making it an option in soil remediation for HM contamination. Biochar enhances the physical (e.g. bulk density, soil structure, water holding capacity), chemical (e.g. cation exchange capacity, pH, nutrient availability, ion exchange, complexes), and biological properties (e.g. microbial abundance, enzymatic activities) of contaminated soil. Biochar also enhances soil fertility, improves plant growth, and reduces the plant availability of HMs. Various field studies have shown that biochar application reduces the bioavailability of HMs from contaminated soil while increasing crop yield. The review highlights the positive effects of biochar by reducing HM bioavailability in contaminated soils. Future work is recommended to ensure that biochars offer a safe and sustainable solution to remediate soils contaminated with HMs.

Dynamic crosstalk between silicon nanomaterials and potentially toxic trace elements in plant-soil systems

Agricultural soil pollution with potentially toxic trace elements (PTEs) has emerged as a significant environmental concern, jeopardizing food safety and human health. Although, conventional remediation approaches have been used for PTEs-contaminated soils treatment; however, these techniques are toxic, expensive, harmful to human health, and can lead to environmental contamination. Nano-enabled agriculture has gained significant attention as a sustainable approach to improve crop production and food security. Silicon nanomaterials (SiNMs) have emerged as a promising alternative for PTEs-contaminated soils remediation. SiNMs have unique characteristics, such as higher chemical reactivity, higher stability, greater surface area to volume ratio and smaller size that make them effective in removing PTEs from the environment. The review discusses the recent advancements and developments in SiNMs for the sustainable remediation of PTEs in agricultural soils. The article covers various synthesis methods, characterization techniques, and the potential mechanisms of SiNMs to alleviate PTEs toxicity in plant-soil systems. Additionally, we highlight the potential benefits and limitations of SiNMs and discusses future directions for research and development. Overall, the use of SiNMs for PTEs remediation offers a sustainable platform for the protection of agricultural soils and the environment.

Effects of fresh and field-aged holm-oak biochar on As, Cd and Pb bioaccumulation in different rice growing environments

Arsenic, Cd, and Pb environmental fate is influenced when the traditional permanent flooding rice production systems are replaced by water-saving and soil conservation practices, urging for additional strategies that avoid their bioaccumulation in rice grain. The aim of this two-years field study was to evaluate the effects of fresh and field-aged biochar on As, Cd, and Pb bioaccumulation, and on As speciation, in rice grain produced in different growing environments (flooding versus sprinkler and conventional tillage versus direct seeding). Biochar produced from holm-oak pruning residues (pyrolysis at 550 °C, 48 h), in a single application (28 Mg ha^-1), reduced As bioaccumulation in rice grain in the permanent flooding system to non-quantifiable concentrations (e.g., from 0.178 mg kg^-1 to <0.04 mg kg^-1, for inorganic-As, respectively), an effect which remained under field-aging conditions, increasing rice commercial value. When adopting sprinkler irrigation, the undesirable increase in Cd bioaccumulation in rice, relatively to the anaerobic system, was counteracted by biochar application, reducing its bioaccumulation in kernels between 32 and 80 %, allowing a simultaneous control of metals and metalloids bioaccumulation in rice. The bioaccumulation of Pb was also prevented with biochar application, with a reduction in its concentration four- to 13-times, in all the management systems, relatively to the non-amended plots, under fresh biochar effects. However, Pb immobilization decreased with biochar field-aging, indicating that the biochar application may have to be repeated to maintain the same beneficial effect. Therefore, the present study shows that the implementation of sprinkler irrigation with holm-oak biochar could reduce the risk of heavy metals(loids) bioaccumulation in rice grains and, thereby, ensuring food safety aspects, particularly under fresh biochar effects.

Biochar as a Green Sorbent for Remediation of Polluted Soils and Associated Toxicity Risks: A Critical Review

Soil contamination with organic contaminants and various heavy metals has become a global environmental concern. Biochar application for the remediation of polluted soils may render a novel solution to soil contamination issues. However, the complexity of the decontaminating mechanisms and the real environment significantly influences the preparation and large-scale application of biochar for soil ramification. This review paper highlights the utilization of biochar in immobilizing and eliminating the heavy metals and organic pollutants from contaminated soils and factors affecting the remediation efficacy of biochar. Furthermore, the risks related to biochar application in unpolluted agricultural soils are also debated. Biochar production conditions (pyrolysis temperature, feedstock type, and residence time) and the application rate greatly influence the biochar performance in remediating the contaminated soils. Biochars prepared at high temperatures (800 °C) contained more porosity and specific surface area, thus offering more adsorption potential. The redox and electrostatic adsorption contributed more to the adsorption of oxyanions, whereas ion exchange, complexation, and precipitation were mainly involved in the adsorption of cations. Volatile organic compounds (VOCs), dioxins, and polycyclic aromatic hydrocarbons (PAHs) produced during biochar pyrolysis induce negative impacts on soil alga, microbes, and plants. A careful selection of unpolluted feedstock and its compatibility with carbonization technology having suitable operating conditions is essential to avoid these impurities. It would help to prepare a specific biochar with desired features to target a particular pollutant at a specific site. This review provided explicit knowledge for developing a cost-effective, environment-friendly specific biochar, which could be used to decontaminate targeted polluted soils at a large scale. Furthermore, future study directions are also described to ensure a sustainable and safe application of biochar as a soil improver for the reclamation of polluted soils.