Three-year field experiments revealed the immobilization effect of natural aging biochar on typical heavy metals (Pb, Cu, Cd)

Long-term soil remediation using layered double hydroxides: Field evidence for simultaneous immobilization of both cations and oxyanions

Layered double hydroxides (LDHs) have great potential for immobilizing potentially toxic elements in soil. Nevertheless, their practical effectiveness under field conditions remains largely unknown. In this study, we conducted a 2.5-year field trial using pristine Mg-Al LDHs, Ca-Al LDHs, and iron (Fe)-modified LDHs to simultaneously immobilize both oxyanions (including As and Sb) and cations (including Cd and Pb) in historically contaminated soil affected by mining activities since the 1950s. The immobilization performance of LDHs was examined using various batch tests, including water and DTPA extraction, and by measuring metal(loid) concentrations in Coriandrum sativum (coriander). We found that both pristine and Fe-modified LDHs showed promising initial immobilization performance 7 days after application, achieving significant reductions in DTPA-extractable concentrations of As, Sb, Cd, and Pb by 45.6%-68.3%, 55.4%-94.2%, 11.2%-50.9%, and 62.9%-64.9%, respectively, compared to the control soil without amendment. Notably, pristine LDHs showed diminished immobilization performance in the long term, while Fe-modified LDHs exhibited long-term stability over 2.5 years. A conditional probability-based model was used to depict long-term metal(loid) leaching characteristics in LDH-amended soils. Temporal changes in metal(loid) concentrations in the aboveground edible parts (namely, stems and leaves) of coriander corroborated well with DTPA extraction results. Coriander grown in Fe-modified LDH-amended soils had much lower metal(loid) concentrations compared to those grown in pristine LDH-amended soils. As a result, reductions of 35.1%-42.2% for As, 54.4%-66.2% for Sb, 8.5%-22.8% for Cd, and 56.0%-62.7% for Pb concentrations in coriander were still observed 2.5 years after soil amendment with Fe-modified LDHs. To the best of our knowledge, this is the first field-based evidence using LDHs to simultaneously stabilize both cations and oxyanions in soil. The findings support the potential of LDHs for long-term immobilization of metal(loid)s in soil.

Differences in soil Cd immobilization and blockage of rice Cd uptake by biochar derived from crop residue and bone - A 2-year field experiment

Biochar is widely recognized as an effective amendment for soils contaminated with cadmium (Cd). However, the properties and elemental compositions of biochar derived from different feedstocks may significantly impact the transfer of Cd in the soil-rice system. This study conducted a two-year field trial in Cd-contaminated paddy soil. Rapeseed straw biochar (REB), rice husk biochar (RHB), and bone char (BOC) were applied once at rates of 0 t ha-1 (CK) and 15 t ha-1, respectively. The results indicated that biochar significantly decreased grain Cd concentrations by on average of 60.1 % and 22.9 % in 2021 and 2022, respectively. BOC significantly decreased CaCl2-Cd concentration by on average of 52.1 % and 64.7 % during two rice growing seasons, which was higher than that of crop biochar (22.7 % and 17.8 %). Soil exchangeable Ca and dissolved P in BOC treatment were higher, and had significantly negatively correlated with CaCl2-Cd (r = -0.50; r = -0.724). REB with higher S content efficiently increased the proportion of organics and sulfides bounding Cd. Except for BOC, REB and RHB significantly enhanced Cd fixation in IP by 44.4 %-92.0 % and 42.8 %-59.5 % in two years, in which IP-bound Fe and IP-bound Cd in REB were by 10.9 % and by 9.11 %-27.4 % higher than those of RHB respectively. The electron donating capacity of REB was 2.21-folds higher than that of RHB, which could promote IP formation by enhancing Fe(Ⅲ) reduction. RHB decreased Cd transformation from roots to shoots by 20.6 %-30.3 % compared to REB and BOC. Higher Si content in rice root in RHB treatment may promote complexation and deposition of Si hemicellulose-bound Cd in the root cell walls. This study reveals the important role of biochar's elemental composition and properties in soil Cd immobilization and the mitigation of rice Cd uptake.

Selective adsorption of copper by amidoxime modified low-temperature biochar: Performance and mechanism

Biochars prepared at 300-700 °C were functionalized with amidoxime groups to evaluate their selective adsorptive removal capabilities towards Cu(II), Cd(II), and Pb(II). The results show that the amidoxime modification significantly enhanced the the Cu(II) adsorption capacity of the biochar prepared at 300 °C (AOBC300) by 1.6 times, reaching 0.61 mmol/g. In binary and ternary heavy metal solutions, AOBC300 exhibited preferential adsorption of Cu(II), followed by Pb(II) and Cd(II). High salinity, alkaline earth metal ions, humic substances, and other metal cations had minimal interference on the adsorption of heavy metals by AOBC300. Sample characterization revealed that amidoxime modification reduced the zeta potential and increased the hydrophilicity of the biochar. XPS analysis demonstrated that both N and O atoms of the amidoxime group are involved in the adsorption process, contributing to AOBC300's strong affinity for heavy metal ions. DFT calculations further confirmed the adsorption preference of different heavy metals on AOBC300. This study demonstrates that amidoxime grafting is an effective protocol for refining low-temperature biochar, aimed at efficiently eliminating heavy metals from waste streams.

Influence of biochar derived from Cd polluted silkworm excrement on the phytoavailability of Cd in a paddy soil and its accumulation in mulberry

Developing sericulture industry is a promising model for the utilization of soils heavily contaminated with cadmium (Cd), but the management of polluted silkworm excrement (SE) becomes challenging. This study aimed to evaluate the effects of the SE biochar (SB) with Cd (SB-Cd) and without Cd (SB-Cd free) on the chemical properties of paddy soil, the mulberry leaf quality and the accumulation of Cd in mulberry. The soil incubation experiments showed that the two SBs all raised the acidic soil pH (20.24 %∼49.97 %) significantly (P < 0.05) with the increasing SB addition rates. The two SBs increased the soil cation exchange capacity (CEC) and played an essential role in reducing the phytoavailability of Cd. The pot experiment elucidated the two SBs all promoted the growth of mulberry, increased the crude protein content and the chlorophyll content, reduced the total sugar content in leaves. The Cd concentrations in root, stem, leaf were decreased with the increase of SB respectively, but no significant differences were found between the same additions of SB-Cd free and SB-Cd. The use of SB-Cd for remediation of the Cd polluted soils could be a reasonable method to address the Cd polluted SE.

Critical role of soil-applied molybdenum dioxide composite biochar material in enhancing Cr(VI) remediation process: The driver of Fe(III)/Fe(II) redox cycle

Heavy metal contamination of agricultural land due to sewage irrigation, over-application of fertilizers and pesticides, and industrial activities. Biochar, due to its rich functional groups and excellent electrochemical performance, is used for the remediation of heavy metal-contaminated farmland. However, the remediation mechanism remains uncertain due to the influence of minerals and multi-element composite pollution on soil. Therefore, introducing transition metal oxide MoO2 to prepare biochar composite remediation materials enhances the adsorption and reduction of soil Cr (Ⅵ). This study compared the differences in Cr (Ⅵ) improvement under different pollution systems and pH conditions and explored the potential mechanism of Fe (Ⅲ)/Fe (Ⅱ) redox cycling in Cr (Ⅵ) remediation. The results showed that both biochar MoO2 ball-milling composite (BC + M) and biochar-loaded MoO2 (BC/M) retained the original biochar (BC) remediation method for Cr (Ⅵ). Among them, the remediation of BC/M was the most stable, with the maximum remediation value ranging from approximately 6.52 to 58.58 mg/kg. In different pollution systems, Cd and Pb exhibited competitive adsorption toward Cr (Ⅵ), but they enhanced Cr (Ⅵ) remediation by promoting adsorption and self-complexation. In acidic conditions (pH = 4), BC/M showed the best remediation effect, with a reduction kinetic constant of 34.61 × 10-3 S-1 and a maximum adsorption capacity of 61.64 mg/g. Fe (Ⅲ)/Fe (Ⅱ) redox cycling accelerated the reduction of Cr (Ⅵ) (R2 = 0.81), and MoO2 promoted the Fe (Ⅲ)/Fe (Ⅱ) redox cycle. BC/M enhanced the Fe (Ⅱ) formation efficiency by 66.39% and 71.81% compared to BC + M and BC at pH = 4. The introduction of MoO2 and biochar composite materials enhanced the reduction process of Cr (Ⅵ), with BC/M achieving the optimal remediation level. This study reveals the potential mechanisms of MoO2 and biochar composite materials in soil Cr (Ⅵ) remediation, providing a reference and insight for the preparation of Cr (Ⅵ) remediation materials and the treatment of contaminated farmland.

Biochar and arbuscular mycorrhizal fungi promote rapid-cycling Brassica napus growth under cadmium stress

PURPOSE

To explore the mechanisms of tolerance of Brassica napus to ultra-high concentration cadmium pollution and the synergistic effects of biochar (BC) and Arbuscular mycorrhizal fungi (AMF) on plant growth under cadmium (Cd) stress.

RESULTS

The application of 5 % BC and inoculation with 10 g AMF significantly promoted the growth and development of B. napus. The combined application of BC and AMF (BC1A and BC2A) was better than the single application. At the Cd 200 mg/kg level, BC1A increased the fresh weight and Cd content of the above-ground parts of B. napus by 35.5 % and decreased by 21.20 %. The SOD and POD activities increased by 30.63 % and 73.37 %. The MDA and H2O2 contents decreased by 40.8 % and 69.99 %, soluble sugar content increased by 37.96 %. At the Cd 300 mg/kg level, BC1A increased the fresh weight and Cd content of the above-ground parts of B. napus by 32.8 % and decreased by 15.99 %. The SOD and POD activities increased by 39.06 % and 93.56 %. The MDA and H2O2 contents decreased by 28.39 % and 72.45 %, and the soluble sugar content increased by 21.16 %. Overall, both BC and AMF treatments alone or in combination (BC1A) were able to alleviate Cd stress and promote plant growth, with the combination of biochar and AMF being the most effective. Furthermore, transcriptome analyses indicated that BC may improve cadmium resistance in B. napus by significantly up-regulating the expression of genes related to peroxidase, photosynthesis, and plant MAPK signaling pathways. AMF may alleviate the toxicity of Cd stress on B. napus by up-regulating the expression of genes related to peroxisomes, phytohormone signaling, and carotenoid biosynthesis. The results of the study will provide support for ecological restoration technology in extremely heavy metal-polluted environments and provide some reference for the application and popularization of BC and AMF conjugation technology.

Effect of natural aging on biochar physicochemical property and mobility of Cd (II)

This project utilized both field experiment and laboratory analyses to address the gap in understanding regarding the alterations in properties and functions of biochar, and the impact of heavy metal passivation in soil over long-term natural field aging. The study aimed to examine the changes in the physical and chemical characteristics of biochar over an extended period of natural aging. Additionally, it sought to analyze the impact and mechanisms of biochar in reducing of the harmful effects of the heavy metal cadmium (Cd) during the aging process. Both original and aged biochar conformed to the pseudo-second-order kinetics model and the Langmuir model. The aging process enhanced the adsorption of Cd by biochar and mitigated the leaching of Cd2+ into the soil. These findings provide a scientific basis for evaluating biochar's environmental behavior and its potential use in the remediation of soil contaminated with heavy metals.

Effective amendment of cadmium in water and soil before and after aging of nitrogen-doped biochar: Preparation optimization, removal efficiency and mechanism

Nitrogen-doped biochar (NBC) is a green material for remediating heavy metal pollution, but it undergoes aging under natural conditions, affecting its interaction with heavy metals. The preparation conditions of NBC were optimized using response surface methodology (RSM), and NBC was subjected to five different aging treatments to analyze the removal efficiency of Cd(II) and soil remediation capability before and after aging. The results indicated that NBC achieved optimal performance with a mass ratio of 5:2.43, an immersion time of 10.66 h, and a pyrolysis temperature of 900 °C. Aging diminished NBC's adsorption capacity for Cd(II) but did not change the main removal mechanism of monolayer chemical adsorption. Freeze-thaw cycles (FT), UV aging (L), and composite aging (U) treatments increased the proportion of bioavailable-Cd, and all aging treatments facilitated the conversion of potentially bioavailable-Cd to non-bioavailable-Cd. The application of NBC and five aged NBCs reduced the proportion of bioavailable-Cd in the soil through precipitation and complexation, increasing the proportion of non-bioavailable-Cd. Aging modifies the physicochemical properties of NBC, thus influencing soil characteristics and ultimately diminishing NBC's ability to passivate Cd in the soil. This study provides reference for the long-term application of biochar in heavy metal-contaminated environments.

Influence of agro-wastes derived biochar and their composite on reducing the mobility of toxic heavy metals and their bioavailability in industrial contaminated soils

The agro-waste derived valuable products are prime interest for effective management of toxic heavy metals (THMs). The present study investigated the efficacy of biochars (BCs) on immobilization of THMs (Cr, Zn, Pb, Cu, Ni and Cd), bioaccumulation and health risk. Agro-wastes derived BCs including wheat straw biochar (WSB), orange peel biochar (OPB), rice husk biochar (RHB) and their composite biochar (CB) were applied in industrial contaminated soil (ICS) at 1% and 3% amendments rates. All the BCs significantly decreased the bioavailable THMs and significantly (p < 0.001) reduced bioaccumulation at 3% application with highest efficiency for CB followed by OPB, WSB and RHB as compared to control treatment. The bioaccumulation factor (BAF), concentration index (CI) and ecological risk were decreased with all BCs. The hazard quotient (HQ) and hazard index (HI) of all THMs were <1, except Cd, while carcer risk (CR) and total cancer risk index (TCRI) were decreased through all BCs. The overall results depicted that CB at 3% application rate showed higher efficacy to reduce significantly (p < 0.001) the THMs uptake and reduced health risk. Hence, the present study suggests that the composite of BCs prepared from agro-wastes is eco-friendly amendment to reduce THMs in ICS and minimize its subsequent uptake in vegetables.

Dynamic performance of combined biochar from co-pyrolysis of pig manure with invasive weed: Effect of natural aging on Pb and As mobilization in polluted mining soil

Due to the natural biochar aging, the improvement of soil quality and immobilization of soil pollutants achieved by biochar may change; understanding the dynamic evolution of the in situ performance of biochar in these roles is essential to discuss the long-term sustainability of biochar remediation. Therefore, in this study, combined biochar from co-pyrolysis of pig manure and invasive Japanese knotweed - P1J1, as well as pure pig manure - PM - and pure Japanese knotweed - JK - derived biochar were applied to investigate their remediation performance in a high As- and Pb-polluted soil with prolonged incubation periods (up to 360 days). Biochar application, especially P1J1 and PM, initially promoted soil pH, dissolved organic carbon, and EC, but the improvements were not constant through time. The JK-treated soil exhibited the highest increase of soil organic matter (OM), followed by P1J1 and then PM, and OM did not change with aging. Biochar, especially P1J1, was a comprehensive nutrient source of Ca, K, Mg, and P to improve soil fertility. However, while soluble cationic Ca, K, and Mg increased with time, anionic P decreased over time, indicating that continuous P availability might not be guaranteed with the aging process. The total microorganism content declined with time; adding biochars slowed down this tendency, which was more remarkable at the later incubation stage. Biochar significantly impeded soil Pb mobility but mobilized soil As, especially in PM- and P1J1-treated soils. However, mobilized As gradually re-fixed in the long run; meanwhile, the excellent Pb immobilization achieved by biochars was slightly reduced with time. The findings of this study offer fresh insights into the alterations in metal(loid)s mobility over an extended duration, suggesting that the potential mobilization risk of As is reduced while Pb mobility slightly increases over time.

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

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

Feasibility and solidification mechanism study of self-sustaining smoldering remediation for copper and lead-contaminated soil

Soil heavy metal pollution is an important issue that affects human health and ecological well-being. In-situ thermal treatment techniques, such as self-sustaining smoldering combustion (SSS), have been widely studied for the treatment of organic pollutants. However, the lack of fuel in heavy metal-contaminated soil has hindered its application. In this study, we used corn straw as fuel to investigate the feasibility of SSS remediation for copper and lead in heavy metal-contaminated soil, as well as to explore the remediation mechanism. The results of the study showed that SSS increased soil pH, electrical conductivity (EC), total phosphorus (TP), total potassium (TK), rapidly available phosphorus (AP), and available potassium (AK), while decreasing total nitrogen (TN), alkali-hydrolyzed nitrogen (AN), and cation exchange capacity (CEC). The oxidation state of copper (Cu) increased from 10% to 21%-40%, and the residual state of lead (Pb) increased from 18% to 51%-73%. The Toxicity characteristic leaching procedure (TCLP) of Cu decreased by a maximum of 81.08%, and the extracted state of Diethylenetriaminepentaacetic acid (DTPA) decreased by 67.63%; the TCLP of Pb decreased by a maximum of 81.87%, and DTPA decreased by a maximum of 85.68%. The study indicates that SSS using corn straw as fuel successfully achieved remediation of heavy metal-contaminated soil. However, SSS does not reduce the content of copper and lead; it only changes their forms in the soil. The main reasons for the fixation of copper and lead during the SSS process are the adsorption of biochar, complexation with -OH functional groups, binding with π electrons, and the formation of crystalline compounds. This research provides a reference for the application of SSS in heavy metal-contaminated soil and has potential practical implications.

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

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