Combined apatite, biochar, and organic fertilizer application for heavy metal co-contaminated soil remediation reduces heavy metal transport and alters soil microbial community structure

Soil resource availability regulates the response of micro-food web multitrophic interactions to heavy metal contamination

The effects of heavy metal contamination on soil biomes have been of considerable interest. However, the effects of heavy metal pollution on the interactions between soil multi-trophic biota in soil food webs and the regulatory mechanisms still need more research, especially in different soil situations. This study examined the impact of heavy metal contamination on soil micro-food web in two distinct soil resource availability situations. Under low soil resources availability situation, heavy metals mainly affected the community structure of soil bacteria and nematodes, with the number of edges of the bacterial network and network complexity reduced by 60.5% and 187%, respectively. In addition, the presence of heavy metals led to a significant reduction in the energy flow from soil resources to bacterivores in the nematode food web. For micro-food webs, heavy metal contamination increased the network average degree by 18.8% and 11.56% in the low and high resource availability situations, respectively. However, in high soil resource availability, heavy metal contamination decreased micro-food web stability and eased competitive relationships among multitrophic organisms and increased microbial carbon limitation and mitigates nitrogen limitation. In low soil resource availability, it increased network stability and shifted relationships among micro-food web organisms from cooperative to competitive and decreased microbial carbon limitation and aggravated nitrogen limitation. This study offers new research insights into the feedback discrepancy between resource availability and pollution stress from the perspective of multitrophic level interactions and further deepens the understanding of the environmental impacts of heavy metal pollution at the ecosystem level.

Interactive effects of soil dissolved organic matter (DOM) and Per- and polyfluoroalkyl substances on contaminated soil site: DOM molecular-level perspective

Dissolved organic matter (DOM), as the most active soil component, plays a crucial role in regulating the transport of contaminants. Per- and polyfluoroalkyl substances (PFAS) have been found to be widespread contaminants in the soil environment, and their migration would be also affected by DOM. Herein, the surface and subsurface soil samples collected from two PFAS manufacturing factories were studied for the variation characteristics of DOM under PFAS contamination, and the interaction between DOM and PFAS in soil was further explored. The results showed that PFAS contamination significantly reduced the richness of surface soil DOM. For the specific DOM components, the potential transformation of DOM in subsurface soil indicates that the presence of PFAS promotes the transformation of other DOM components to PA compounds. Moreover, a strong positive relationship was observed between the concentration of most perfluoroalkyl sulfonic acids (PFSAs) and the average unsaturation (DBE) and aromaticity index (AImod) of DOM, while no such relationship for perfluoroalkyl carboxylic acids (PFCAs), suggesting DBE and AImod may be a potential contributor influencing the distribution and transport of PFSAs. These findings highlight the interaction between DOM and the PFAS in the soil environment, which may enhance our understanding of the release and fate of PFAS in the soil environment.

Exploring the phytostabilization potential of ryegrass (Lolium perenne L.) upon synergistic application of Cd-tolerant Pseudomonas fluorescens and organic amendments

Soil Cd contamination limits plant productivity by affecting their key functions and reducing yields. In-situ immobilization of heavy metals (HMs) can achieve 'green' and 'sustainable' ways of HM remediation owing to its lower life cycle environmental footprints. This study explored the effects of Cd-resistant P. fluorescens and OAs such as rice straw, wheat straw and cow dung (1% w/w) on the Cd tolerance of ryegrass under Cd contamination (2.2 mg kg-1). In our findings, Cd exposure reduced plant height (22%), root length (38%), chlorophyll 'a' and 'b' (36 and 38%), and relative water content (32%) in uninoculated plants. However, combined use of P. fluorescens and OAs mitigated these effects by immobilizing Cd in soil, with a 56% increase in residual Cd and higher Cd retention in roots and shoots (77 and 87%). Co-application enhanced plant height (96%), root length (158%), chlorophyll content (90 and 98%), relative water content (168%), flavonoids and phenols (151 and 68%) and NPK uptake (104, 73 and 71%) as compared to uninoculated control. Integration of P. fluorescens and OAs not only reduced Cd uptake but also improved growth and yield. Thus, this approach mitigates Cd stress in ryegrass, improving growth and physiology by reducing Cd uptake.

The restoration of zinc pollution in smelting site soil using nanohydroxyapatite-modified cyanobacterial biochar and its mechanism

Heavy metal pollution in the soils at smelting sites must be effectively controlled. Recent advancements in stabilization technology have shown promising results in the remediation of heavy metal-contaminated soils. In this study, nanohydroxyapatite (nHAP) and cyanobacterial biochar were co-pyrolyzed to produce nHAP-modified cyanobacterial biochar (nHAP-CBC), which was applied to remediate Zn contamination of soils at smelting sites. The remediation effect of nHAP-CBC on Zn-contaminated soil was evaluated using batch experiments, and the materials were characterized using XRD, SEM, TEM, BET, and FTIR. These analyses confirmed the uniform dispersion of nHAP on the CBC to form a stable nHAP-CBC material. The results demonstrated that nHAP-CBC effectively converted Zn from an unstable state to a stable state, achieving a 65.79 % conversion rate and a 64.24 % stabilization rate during toxicity characteristic leaching after 45 days of treatment. nHAP-CBC was the most effective at fixing Zn and significantly increased the organic matter (OM) content, suggesting that OM played a key role in Zn fixation. In conclusion, the nHAP-CBC developed in this study can effectively stabilize heavy metals in smelting site soils and offers promising potential for expanding cyanobacterial resource utilization.

Potassium permanganate-hematite-modified biochar enhances cadmium and zinc passivation and nutrient availability and promotes soil microbial activity in heavy metal-contaminated soil

Modified biochar has garnered considerable attention for its versatile applications in remediating soils contaminated with heavy metals. However, most existing studies have primarily focused on the stabilisation of heavy metals, with limited research exploring the broader environmental effects following the application of modified biochar. In this study, we developed a potassium permanganate (KMnO4)-hematite-modified biochar (MnFeB) as a passivating agent for heavy metals, specifically targeting cadmium (Cd) and zinc (Zn)-contaminated soils. We examined the effects of MnFeB on the biotoxicity of Cd and Zn, soil properties, enzyme activities, heavy metal resistance genes (czcA, czcC, and czcD), and the soil microbial community in contaminated soils. Treatment with MnFeB markedly reduced the soil diethylenetriaminepentaacetic acid (DTPA)-extractable Zn and Cd contents by 18.79% and 43.65%, respectively. Furthermore, soil organic carbon (SOC), cation exchange capacity (CEC), and the availability of nitrogen, phosphorus, and potassium were found to be increased. MnFeB application also enhanced the activities of catalase, urease, and alkaline phosphatase while reducing the expression of czcA by 23.63%. Moreover, changes in the composition and diversity of soil bacterial and fungal communities were observed. These findings highlight the effects of environmental changes induced by MnFeB application on Cd/Zn-contaminated soil and offer theoretical support for employing passivation strategies in the remediation of heavy metal-contaminated soils.

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

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

Enhancing the effect of novel cd mobilization bacteria on phytoremediation and microecology of cadmium contaminated soil

The efficacy of phytoextraction for remediating heavy-metal contaminated soil depends on the bioavailability of the heavy metals and plant growth. In this study, we employed a synergistic system comprising water-soluble chitosan and the novel Cd mobilization bacteria, Serratia sp. K6 (hereafter K6), to enhance cadmium (Cd) extraction by Lolium perenne L. (ryegrass). The application of chitosan and K6 resulted in an increase in the biomass of ryegrass by 11.81% and Cd accumulation by 73.99% and effective-state Cd by 43.69% and pH decreased by 4.67%, compared to the control group. Microbiome and metabolomics analyses revealed significant alterations in the inter-root microbial ommunity, with rhizobacteria such as Sphingomonas, Nocardioides, and Bacillus likely contributing to enhanced plant growth and Cd accumulation in response to chitosan and K6 addition. Additionally, the contents of various organic acids, amino acids, lipids, and other metabolites exhibited significant changes under different additive treatments, suggesting that ryegrass can regulate its own metabolites to resist Cd stress. This study provides valuable insights into the effects of additives on phytoextraction efficiency and the soil bacterial community, offering a promising approach for phytoremediation of Cd-contaminated soils.

Exploiting the synergistic influence of AgNPs-TiO2NPs: enhancing phytostabilization of Pb and mitigating its toxicity in Vigna unguiculata

In this study, a composite of silver and titanium dioxide nanoparticles (AgNPs-TiO2NPs) was examined for its synergistic effects on phytostabilization of lead (Pb) and mitigation of toxicity in cowpea (Vigna unguiculata (L) Walp). Seeds of V. unguiculata were wetted with water, 0.05 and 0.1 mgL-1 Pb and 25 mgmL-1 each of AgNPs, TiO2NPs, and AgNPs-TiO2NPs. Root lengths of V. unguiculata were reduced by 25% and 44% at 0.05 and 0.1 mgL-1 Pb, respectively, while shoot lengths were reduced by 2% and 7%. In V. unguiculata, AgNPs and TiO2NPs significantly improved physiological indicators and mitigated Pb effects, with TiO2NPs modulating physiological parameters more effectively than AgNPs. The composite (AgNPs-TiO2NPs) synergistically regulated V. unguiculata physiology better than individual nanoparticles. Compared to individual AgNPs and TiO2NPs, the composite (AgNPs-TiO2NPs) synergistically increased antioxidant activity by 12% and 9%, and carotenoid contents by 88%. Additionally, AgNPs-TiO2NPs effectively reduced malondialdehyde levels by 29%, thereby mitigating the effects of Pb on V. unguiculata better than individual nanoparticles. AgNPs-TiO2NPs enhanced Pb immobilization by 57%, reducing its translocation from soil to shoots compared to V. unguiculata wetted with water. The bioconcentration and translocation factors of Pb indicate that phytostabilization was most effective when the composite was used.

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.

Rapid humification of cotton stalk catalyzed by coal fly ash and its excellent cadmium passivation performance

Due to industrialization, soil heavy metal pollution is a growing concern, with humic substances (HS) playing a pivotal role in soil passivation. To address the long duration of the compost humification problem, coal fly ash (CFA) in situ catalyzes the rapid pyrolysis of the cotton stalk (CS) to produce HS to address Cd passivation. Results indicate that the highest yield of humic acid (HA) (8.42%) and fulvic acid (FA) (1.36%) is obtained when the CS to CFA mass ratio is 1:0.5, at 275 ℃ for 120 min. Further study reveals that CFA catalysis CS humification, through the creation of alkaline pyrolysis conditions, Fe2O3 can stimulate the protein and the decomposition of hemicellulose in CS, and then, through the Maillard and Sugar-amine condensation reaction synthesis HA and FA. Applying HS-CS&CFA in Cd-contaminated soil demonstrates a 26.69% reduction in exchangeable Cd within 30 days by chemical complexation. Excellent maize growth effects and environmental benefits of HS products are the prerequisites for subsequent engineering applications. Similar industrial solid wastes, such as steel slag and red mud, rich in Fe2O3, can be explored to identify their catalytic humification effect. It could provide a novel and effective way for industrial solid wastes to be recycled for biomass humification and widely applied in remediating Cd-contaminated agricultural soil.

Protaetia brevitarsis larvae produce frass that can be used as an additive to immobilize Cd and improve fertility in alkaline soils

Bioconversion of agricultural waste by Protaetia brevitarsis larvae (PBL) holds significant promise for producing high-quality frass organic amendments. However, the effects and mechanisms of PBL frass on Cd immobilization in an alkaline environment remain poorly understood. In this study, three types of frass, namely maize straw frass (MF), rice straw frass (RF), and sawdust frass (SF), were produced by feeding PBL. The Cd immobilization efficiencies of three frass in alkaline solutions and soils were investigated through batch sorption and incubation experiments, and spectroscopic techniques were employed to elucidate the sorption mechanisms of Cd onto different frass at the molecular level. The results showed that MF proved to be an efficient sorbent for Cd in alkaline solutions (176.67-227.27 mg g-1). X-ray absorption near-edge structure (XANES) spectroscopy indicated that Cd immobilization in frass is primarily attributed to the association with organic matter (OM-Cd, 78-90%). And MF had more oxygen-containing functional groups than the other frass. In weakly alkaline soils, MF application (0.5-1.5%) significantly decreased Cd bioavailability (5.65-18.48%) and concurrently improved soil nutrients (2.21-56.79%). Redundancy analysis (RDA) unveiled that pH, CEC, and available P were important factors controlling Cd fractions. Path analysis demonstrated that MF application affected Cd bioavailability directly and indirectly by influencing soil chemical properties and nutrients. In summary, MF, the product of PBL-mediated conversion maize straw, demonstrated promise as an effective organic amendment for Cd immobilization and fertility improvement in alkaline soils.

Bioavailable fractions of heavy metals in the road dust during infrastructure construction at urban Coimbatore and its potential health implications, India

Over the past two decades the Global South is witnessing unprecedented economic transformation and Asian Cities in particular have a remarkable upsurge. Coimbatore, an industrial city in Southern India with an estimated population of 2 million (in 2022) is witnessing a rapid transition in terms of infrastructure development. In this context, the present study attempts to assess the particulate matter (PM10 and PM2.5) emissions at road network construction sites and the heavy metal fractionation in the road dust/sediment samples with a core focus to quantify the bioavailable fraction of metals (Fe, Cu, Cr, Cd, Pb and Ni) and its source apportionment in the road side dust/sediment samples. About 60 composite road dust/sediment samples were collected for heavy metal fractionation analysis in the six arterial roads that undergo core developments like construction of road over bridges, additional road incorporation and street expansions. PM monitoring revealed that 24 h average PM2.5 (47 µg/m3) and PM10 (69 µg/m3) concentrations at many construction sites exceeded 24 h average recommended by WHO guidelines [PM2.5 (15 µg/m3) and PM10 (45 µg/m3), respectively]. The bioavailable fractions of Fe, Cu, Cr and Cd are notably higher in the roadside sediment samples at road construction sites. Health Risk assessment, such as carcinogenic risks (Children-4.41 × 10-2, Adult-3.598 × 10-6) and non-carcinogenic risks, inferred substantial risks at high intensity construction sites with statistical analyses, including PCA and cluster analysis, indicating considerable anthropogenic influences in the heavy metal fractions.

Driving factors for distribution and transformation of heavy metals speciation in a zinc smelting site

Heavy metal pollution at an abandoned smelter pose a significant risk to environmental health. However, remediation strategies are constrained by inadequate knowledge of the polymetallic distribution, speciation patterns, and transformation factors at these sites. This study investigates the influence of soil minerals, heavy metal occurrence forms, and environmental factors on heavy metal migration behaviors and speciation transformations. X-ray diffraction analysis revealed that the minerals associated with heavy metals are mainly hematite, franklinite, sphalerite, and galena. Sequential extraction results suggest that lead and zinc are primarily present in the organic-sulfide fractions (F4) and residual form (F5) in the soil, accounting for over 70% of the total heavy metal content. Zinc displayed greater instability in carbonate-bound (16%) and exchangeable (2%) forms. The migration and diffusion patterns of heavy metals in the subsurface environment were visualized through the simulation of labile state heavy metals, demonstrating high congruence with groundwater pollution distribution patterns. The key environmental factors influencing heavy metal stable states (F4 and F5) were assessed by integrating random forest models and redundancy analysis. Primary factors facilitating Pb transformation into stable states were available phosphorus, clay content, depth, and soil organic matter. For Zn, the principal drivers were Mn oxides, soil organic matter, clay content, and inorganic sulfur ions. These findings enhance understanding of the distribution and transformation of heavy metal speciation and can provide valuable insights into controlling heavy metal pollution at non-ferrous smelting sites.

Effect of humic substances on the fraction of heavy metal and microbial response

Contamination of soils by Molybdenum (Mo) has raised increasing concern worldwide. Both fulvic acid (FA) and humic acid (HA) possess numerous positive properties, such as large specific surface areas and microporous structure that facilitates the immobilization of the heavy metal in soils. Despite these characteristics, there have been few studies on the microbiology effects of FA and HA. Therefore, this study aimed to assess the Mo immobilization effects of FA and HA, as well as the associated changes in microbial community in Mo-contaminated soils (with application rates of 0%, 0.5% and 1.0%). The result of the incubation demonstrated a decrease in soil pH (from 8.23 ~ 8.94 to 8.05 ~ 8.77). Importantly, both FA and HA reduced the exchangeable fraction and reducible fraction of Mo in the soil, thereby transforming Mo into a more stable form. Furthermore, the application of FA and HA led to an increase in the relative abundance of Actinobacteriota and Firmicutes, resulting in alterations to the microbial community structure. However, it is worth noting that due to the differing structures and properties of FA and HA, these outcomes were not entirely consistent. In summary, the aging of FA and HA in soil enhanced their capacity to immobilization Mo as a soil amendment. This suggests that they have the potential to serve as effective amendments for the remediation of Mo-contaminated soils.

Biochar and nano-hydroxyapatite combined remediation of soil surrounding tailings area: Multi-metal(loid)s fixation and soybean rhizosphere soil microbial improvement

The soil near tailings areas is relatively barren and contaminated by multi-metal(loid)s, seriously threatening the safety of crop production. Here, biochar and nano-hydroxyapatite (nHAP) were combined to improve the sterilized and unsterilized polymetallic contaminated soil, and soil incubation and soybean pot experiments were designed. Results showed that biochar and nHAP not only increased soil C, N, and P but also effectively reduced multi-metal bioavailability, wherein the combined application of the two amendments had the best effect on metal immobilization. The synergistic effect of the two amendments decreased the acid-soluble contents of Co, Cu, Fe, and Pb in rhizosphere soils up to 86.75%, 80.69%, 89.09%, and 96.70%, respectively. The ameliorant reduced the accumulation of metal(loid)s in soybean plants, and rhizosphere microorganisms inhibited the migration of soil metals to plants. Additionally, biochar and nHAP regulated the rhizosphere soil microbial community. The rhizosphere soil of the sterilization group tended to prioritize the restoration of the original dominant bacteria. As, Pb, Fe, Urease, OM, TN, and TP were the critical environmental variables affecting rhizosphere soil bacterial communities. Therefore, combining biochar and nHAP is an environmentally friendly strategy to reduce polymetallic mobility in tailings soil and crops and improve soil microbial community structure.

Food waste compost and digestate as novel fertilizers: Impacts on antibiotic resistome and potential risks in a soil-vegetable system

As a novel agricultural practice, the reuse of food waste compost and digestate as fertilizers leads to a circular economy, but inevitably introduces bio-contaminants such as antibiotic resistance genes (ARGs) into the agroecosystem. Moreover, heavy metal and antibiotic contamination in farmland soil may exert selective pressures on the evolution of ARGs, posing threats to human health. This study investigated the fate, influencing mechanisms and potential risks of ARGs in a soil-vegetable system under different food waste fertilization and remediation treatments and soil contamination conditions. Application of food waste fertilizers significantly promoted the pakchoi growth, but resulted in the spread of ARGs from fertilizers to pakchoi. A total of 56, 80, 84, 41, and 73 ARGs, mobile genetic elements (MGEs) and metal resistance genes (MRGs) were detected in the rhizosphere soil (RS), bulk soil (BS), control soil (CS), root endophytes (RE), and leaf endophytes (LE), respectively. Notably, 7 genes were shared in the above five subgroups, indicating a specific soil-root-endophytes transmission pathway. 36 genes were uniquely detected in the LE, which may originate from airborne ARGs. The combined application of biochar and fertilizers reduced the occurrence of ARGs and MGEs to some extent, showing the remediation effect of biochar. The average abundance of ARGs in the RS, BS and CS was 3.15 × 10-2, 1.31 × 10-2 and 2.35 × 10-1, respectively. Rhizosphere effects may reduce the abundance of ARGs in soil. The distribution pattern of ARGs was influenced by the types of soil, endophyte and contaminant. MGEs is the key driver shaping ARGs dynamics. Soil properties and pakchoi growth status may affect the bacterial composition, and consequently regulate ARGs fate, while endophytic ARGs were more impacted by biotic factors. Moreover, the average daily doses of ARGs from pakchoi consumption is 107-109 copies/d/kg, and its potential health risks should be emphasized.

Biochar loaded with root exudates of hyperaccumulator Leersia hexandra Swartz facilitated Cr(VI) reduction by shaping soil functional microbial communities

Cr(VI) contamination is widely recognized as one of the major environmental hazards. To address the problem of remediation of soil Cr(VI) contamination and utilization of waste peanut shells, this study comprehensively investigated the effects of peanut shell-derived biochar loaded with root exudates of hyperaccumulator Leersia hexandra Swartz on Cr(VI) reduction and microbial community succession in soil. This study confirmed that root exudate-loaded peanut shell biochar reduced soil pH while simultaneously increasing DOC, sulfide, and Fe(II) concentrations, thereby facilitating the reduction of Cr(VI), achieving a reduction efficiency of 81.8%. Based on XPS and SEM elemental mapping analyses, Cr(VI) reduction occurred concurrently with the Fe and S redox cycles. Furthermore, the microbial diversity, abundance of the functional genera (Geobacter, Arthrobacter, and Desulfococcus) and the metabolic functions associated with Cr(VI) reduction were enhanced by root exudate-loaded biochar. Root exudate-loaded biochar can promote both direct Cr(VI) reduction mediated by the Cr(VI)-reducing bacteria Arthrobacter, and indirect Cr(VI) reduction through Cr/S/Fe co-transformation mediated by the sulfate-reducing bacteria Desulfococcus and Fe(III)-reducing bacteria Geobacter. This study demonstrates the effectiveness of peanut shell biochar loaded with root exudates of hyperaccumulator Leersia hexandra Swartz to promote soil Cr(VI) reduction, reveals the mechanism how root exudate-loaded biochar shapes functional microbial communities to facilitate Cr(VI) reduction, and proposes a viable strategy for Cr(VI) remediation and utilization of peanut shell.

Amendment-driven soil health restoration through soil pH and microbial robustness in a Cd/Cu-combined acidic soil: A ten-year in-situ field experiment

Soil health arguably depends on biodiversity and has received wide attention in heavy-metal (HM) contaminated farmland remediation in recent years. However, long-term effects and mechanisms of soil amendment remain poorly understood with respect to soil microbal community. In this in-situ field study, four soil amendments (attapulgite-At, apatite-Ap, montmorillonite-M, lime-L) at three rates were applied once only for ten years in a cadmium (Cd)-copper (Cu) contaminated paddy soil deprecated for over five years. Results showed that after ten years and in compared with CK (no amendment), total Cd concentration and its risk in plot soils were not altered by amendments (p > 0.05), but total Cu concentration and its risk were significantly increased by both Ap and L, especially the former, rather than At and M (p < 0.05), through increased soil pH and enhanced bacterial alpha diversity as well as plant community. Soil microbial communities were more affected by amendment type (30%) than dosage (11%), microbial network characteristics were dominated by rare taxa, and soil multifunctionality was improved in Ap- and L-amended soils. A structural equation model (SEM) indicated that 57.3% of soil multifunctionality variances were accounted for by soil pH (+0.696) and microbial network robustness (-0.301). Moreover, microbial robustness could be potentially used as an indicator of soil multifunctionality, and Ap could be optimized to improve soil health in combined with biomass removal. These findings would advance the understanding of soil microbial roles, especially its network robustness, on soil multifunctionality for the remediation of metal contaminated soils and metal control management strategies in acidic soils. ENVIRONMENTAL IMPLICATION: Farmland soil contamination by heavy metals (HMs) has been becoming a serious global environmental challenge. However, most studies have been conducted over the short term, leading to a gap in the long-term remediation efficiency and ecological benefits of soil amendments. For the successful deployment of immobilization technologies, it is critical to understand the long-term stability of the immobilized HMs and soil health. Our study, to the best of our knowlege, is the first to state the long-term effects and mechanisms of soil amendments on soil health and optimize an effective and eco-friendly amendment for long-term Cd/Cu immobilization.

Biotoxicity attenuation and the underlying physicochemical mechanism of biochar aged under simulated natural environmental conditions

Biochar (BC), with the benefits of enhancing soil fertility, absorbing heavy metals, carbon sequestration, and mitigating the greenhouse effect, has been extensively used for soil remediation. However, the long-term changes in the biotoxicity of BC under complex environmental conditions, which are the key factors influencing the sustainable application of BC in soil, are still unclear. Herein, the biotoxicity of BC aged with various processes, including dry‒wet cycle (DW) aging, freeze‒thaw cycle (FT) aging, ultraviolet irradiation (UV) aging, and low molecular weight organic acid (OA) aging, was systematically investigated by Escherichia coli (E. coli) culture experiments. The toxicity attenuation rate (%·week-1) was proposed to more concisely and clearly compare the influence of different aging methods on BC toxicity. The results indicated that after 5 weeks of aging, the toxicity attenuation rate during the four aging modes followed the order OA aging > FT aging > UV aging > DW aging. BC was nontoxic after 1 week of OA aging, 4 weeks of FT aging, 7 weeks of UV aging, and 14 weeks of DW aging. Spectroscopic characterizations revealed that humic acids in the dissolved organic matter of BC were the main reason for the biotoxicity. In addition, the attenuation of environmentally persistent free radicals on BC during aging was also an important factor for reducing environmental toxicity. This work provides insight into the detoxification mechanism of the BC aging process under ordinary environmental conditions and guidance for the safe application of BC in soil.

Effect of Phanerochaete chrysosporium induced phosphate precipitation on bacterial diversity during the soil remediation process

Biomineralization by phosphate minerals and phosphate solubilizing fungi (PSF) has attracted great interest as a novel remediation method for heavy metal(loid) co-contaminated soil. It was very essential to investigate the microenvironment response with the application of amendments. In this study, three grain sizes of hydroxyapatites (HAP) and Phanerochaete chrysosporium (P. chrysosporium) were used to investigate the change in heavy metal(loid) fractions, soil physicochemical properties, and bacterial community during the remediation of Mangchang and Dabaoshan acidic mine soils. The results showed that the residual fractions in the two soils increased significantly after 35 days of remediation, especially that of As and Zn in Dabaoshan soils were presented at over 87%. In addition, soil pH, organic matter (OM), and available phosphorous (AP) were almost improved. 16S rRNA sequencing analysis indicated that the introduction of culture medium and P. chrysosporium alone changed bacterial abundance, but the addition of HAP changed the bacterial diversity and community composition by altering environmental conditions. The amendments in the research showed good performance on immobilizing heavy metal(loid)s and reducing their bioavailability. Moreover, the research suggested that environmental factors and soil inherent properties could influence the microbial community structure and composition.

Mechanistic insights into mitigating Cd stress in plants using typical organic waste fermentation solutions

Finding practical solutions for utilizing agricultural organic wastes has always been a challenge. To address this, our study investigated the effects and mechanisms of different exogenous organic waste fermentation solutions on alleviating Cd stress in plants using hydroponic experiments. Out of the seven fermentation solutions examined, pea fermentation liquid (T3), chicken manure (T5), molasses (T6), and chitosan oligosaccharide broth (T9) exhibited positive effects. They increased shoot fresh weight by 1.17%, 26.83%, 7.94%, and 15.59%, and root fresh weight by 50.00%, 12.21%, 81.19%, and 19.47%, respectively. Conversely, amino acid mother liquid (T7) and potassium polyaspartate liquid (T8) reduced shoot fresh weight by 34.21% and 24.74%, and root fresh weight by 27.06% and 7.10%, respectively. All organic waste liquids reduced Cd concentration in shoots and roots. Corn fermentation liquid (T4) reduced Cd in shoots from 87.91 to 19.20 mg/kg, while molasses (T6) reduced Cd in roots from 980.94 to 260.47 mg/kg. SEM-EDX results revealed that molasses (T6) effectively repaired Cd damage on root surfaces. In addition, several waste liquids mitigated microelement absorption disturbances. All waste liquids reduced MDA, corn fermentation liquid (T4), chicken manure (T5), molasses (T6), potassium polyaspartate liquid (T8), and chitosan oligosaccharide liquid (T9) significantly decreased H2O2 by 21.6-38.3%. Structural equation model (SEM) and correlation analysis highlighted the importance of root Mg, Cu, and Zn content and CAT activity in relieving Cd stress and promoting plant growth. Overall, molasses (T6) and chicken manure (T5) demonstrated the most beneficial combined effects, while amino acid mother liquid (T7) and chitosan oligosaccharide liquid (T9) should be exercised with caution due to their weaker effects.

The influence of cysteine in transformation of Cd fractionation and microbial community structure and functional profile in contaminated paddy soil

Remediating cadmium (Cd) contaminated paddy soil is vital for agroecology, food safety, and human health. Soil washing is more feasible to reduce remediation method due to its high efficiency. However, green, low-cost and more efficient washing agents are still required. In this study, we investigated the ability of cysteine as a washing agent for soil washing to remove Cd from contaminated paddy soil. Through a batch experiment, we evaluated the removal efficiency of cysteine as a washing agent by comparing their removal rate with that of a microbial inoculant and sulphuric acid as other washing agents. The transformation of Cd fractionation and microbial community structure and functional profile in paddy soils after cysteine leaching was studied by using sequential extraction and high-throughput sequencing. Results showed that cysteine had better efficiency in the removal of Cd from paddy soil in comparison to sulphuric acid and the microbial inoculant, and could achieve a maximum removal rate of 97 % Cd in paddy soil. Cysteine decreased the proportion of Cd in the exchangeable fraction, carbonate bound fraction, iron and manganese bound fraction, and organic matter bound fraction and was best for the removal of the residual fraction, which contributed to its higher Cd removal ability. Considering the economic benefits of the reagents used, cysteine was shown to be economically feasible for use as a leaching agent. In addition, cysteine could significantly increase the relative abundance of Thermochromatium, Sideroxydans, Streptacidiphilus, and Frankia which promoted the nitrogen and sulfur metabolism in the paddy soil. In summary, this study revealed that cysteine was readily available, cheap, non-toxic, highly efficient, and even has fertilizing properties, making it eco-friendly and ideal for remediation of Cd-contaminated paddy soils. Besides, the health of paddy soils would also benefit from cysteine's promotion of microbial nitrogen and sulfur metabolism.

Using Mediterranean Native Plants for the Phytoremediation of Mining Sites: An Overview of the Past and Present, and Perspectives for the Future

Mining exploitation in the Mediterranean Basin has left evident scars on the environment, and poses serious risks for human health and biodiversity, especially when mine wastes are left abandoned. This review analysed the main issues of metal(loid)s pollution related to mine exploitation in the Mediterranean Basin. Here, a list of Mediterranean native plant species studied for phytoremediation is given and, considering their biological forms, vegetational types, and ecology, we categorised them into halotolerant and hydro/hygrophilous vegetation, annual and perennial meadows, garrigues and maquis, and high maquis and woods. The main conclusions of the review are as follows: (1) plant communities established on mine environments are often rich in endemic taxa which ensure a high biodiversity and landscape value, and can help in the psychophysical health of local inhabitants; (2) political and land management should take greater account of the use of native plants for the remediation of contaminated soils; (3) a multidisciplinary approach that includes, among others, studies on biochemical response to metal(loid)s as well as the application of innovative soil amendments gives better results; (4) phytoextraction applications require a detailed recovery plan that takes into consideration several issues, including the negative influence on biodiversity due to extensive use of monotypic plantations, disposal of harvested hazardous plants, and the risk of phytoextracts entering the food chain; and (5) more studies are necessary to increase knowledge and to detect suitable species-especially halophytic ones-for phytoremediation purposes.

Risk assessment of toxic and hazardous metals in paddy agroecosystem by biochar-for bio-membrane applications

Toxic and carcinogenic metal (loid)s, such arsenic (As) and cadmium (Cd), found in contaminated paddy soils pose a serious danger to environmental sustainability. Their geochemical activities are complex, making it difficult to manage their contamination. Rice grown in Cd and As-polluted soils ends up in people's bellies, where it can cause cancer, anemia, and the deadly itai sickness. Solving this issue calls for research into eco-friendly and cost-effective remediation technology to lower rice's As and Cd levels. This research delves deeply into the origins of As and Cd in paddy soils, as well as their mobility, bioavailability, and uptake mechanisms by rice plants. It also examines the current methods and reactors used to lower As and Cd contamination in rice. Iron-modified biochar (Fe-BC) is a promising technology for reducing As and Cd toxicity in rice, improving soil health, and boosting rice's nutritional value. Biochar's physiochemical characteristics are enhanced by the addition of iron, making it a potent adsorbent for As and Cd ions. In conclusion, Fe-BC's biomembrane properties make them an attractive option for remediating As- and Cd-contaminated paddy soils. More efficient mitigation measures, including the use of biomembrane technology, can be developed when sustainable agriculture practices are combined with these technologies.

Comparative analysis of metabolic mechanisms in the remediation of Cd-polluted alkaline soil in cotton field by biochar and biofertilizer

To screen environmentally friendly and efficient Cd pollution remediation material, the effects of BC and BF on soil Cd bio-availability and cotton Cd absorption were analyzed under Cd exposure. Besides, the differences in metabolic mechanisms by which biochar (BC) and biofertilizer (BF) affect Cd-contaminated soil and cotton were also analyzed. The results showed that the application of BC and BF increased cotton dry matter accumulation, boll number, and single boll weight, and reduced the Cd content in cotton roots, stems, leaves, and bolls. At harvest, the Cd content in cotton roots in the BC and BF groups reduced by 15.23% and 16.33%, respectively, compared with that in the control. This was attributed to the conversion of carbonate-bound Cd (carbon-Cd) and exchangeable Cd (EX-Cd) by BC and BF into residual Cd (Res-Cd). It should be noted that the soil available Cd (Ava-Cd) content in the BF group was lower than that in the BC group. The metabolomic analysis results showed that for BC vs BF, the relative abundance of differential metabolites Caffeic acid, Xanthurenic acid, and Shikimic acid in soil and cotton roots were up-regulated. Mantel test found that cotton root exudate l-Histinine was correlated with the enrichment of Cd in various organs of cotton. Therefore, the application of BC and BF can alleviate Cd stress by reducing soil Ava-Cd content and cotton's Cd uptake, and BF is superior to BC in reducing Cd content in soil and cotton organs. This study will provide a reference for the development of efficient techniques for the remediation of Cd-polluted alkaline soil, and provide a basis for subsequent metagenomics analysis.

In Situ Toxicity Reduction and Food Safety Assessment of Pak Choi (Brassica campestris L.) in Cadmium-Contaminated Soil by Jointly Using Alkaline Passivators and Organic Fertilizer

An economical and effective method is still lacking for cadmium (Cd) toxicity reduction and food product safety improvement in soil-vegetable systems. Therefore, this study aimed to reduce the Cd toxicity to pak choi (Brassica campestris L.) by jointly using passivators and organic fertilizer, highlighting food products' safety based on pot experiments. The results showed that compared with the control, organic fertilizer decreased the Cd content in edible parts and the soil's available Cd by 48.4% and 20.9% on average, respectively, due to the 0.15-unit increases in soil pH. Once jointly applied with passivators, the decrements increased by 52.3-72.6% and 32.5-52.6% for the Cd content in edible parts and for the soil's available Cd, respectively, while the pH increment increased by 0.15-0.46 units. Compared with the control, the transport factor of Cd was reduced by 61.9% and 50.9-55.0% when applying organic fertilizer alone and together with the passivators, respectively. The combination treatment of biochar and organic fertilizer performed the best in decreasing the Cd content in the edible parts and the soil's available Cd. The combination treatment of fish bone meal and organic fertilizer induced the greatest increases in soil pH. The grey relational analysis results showed that the combination treatment of biochar and organic fertilizer performed the best in reducing the potential Cd pollution risk, thereby highlighting the vegetable food safety. This study provides a potential economical and effective technology for toxicity reduction and food safety in Cd-polluted soil.

Synergistic interplay between ABA-generating bacteria and biochar in the reduction of heavy metal accumulation in radish, pakchoi, and tomato

Heavy metal (HM) contamination is an environmental concern that threatens the agricultural product safety and human health. To address this concern, we developed a novel strategy involving the synergistic application of Azospirillum brasilense, a growth-promoting rhizobacterium which produces abscisic acid (ABA), and biochar to minimize HM accumulation in the edible parts of vegetable crops. Compared to A. brasilense or biochar alone, the concentrations of Cd, Ni, Pb, and Zn in radish (Raphanus sativus L.), pakchoi (Brassica chinensis L.), and tomato (Lycopersicon esculentum L.) decreased by 18-63% and 14-56%, respectively. Additionally, the synergistic treatment led to a 14-63% decrease in the bioconcentration factor. The biomass of the edible parts of the three crops increased by 65-278% after synergistic treatment, surpassing the effects of single treatments. Furthermore, the synergistic application enhanced the SPAD values by 1-45% compared to single treatments. The MDA concentrations in stressed plants decreased by 16-39% with the bacteria-biochar co-treatment compared to single treatments. Co-treatment also resulted in increased soluble protein and sugar concentrations by 8-174%, and improvements in flavonoids, total phenols, ascorbic acid, and DPPH levels by 2-50%. Pearson correlation analysis and structural equation modeling revealed that the synergistic effect was attributed to the enhanced growth of A. brasilense facilitated by biochar and the improved availability of HMs in soils. Notably, although ABA concentrations were not as high as those achieved with A. brasilense alone, they were maintained at relatively high levels. Overall, the synergistic application of A. brasilense-biochar might have remarkable potential for reducing the accumulation of HMs while promoting growth and improving nutritional and antioxidant qualities in tuberous, leafy, and fruit crops.

Coupling effect of DOM and microbe on arsenic speciation and bioavailability in tailings soil after the addition of different biologically stabilized sludges

Dissolved organic matter (DOM) and microbes co-mediate the transformation of heavy metals in soil. However, few previous studies have investigated the effects of interaction between DOM and microbes on the transformation and bioavailability of heavy metals in tailings soil at the molecular level after the addition of organic wastes. This study used co-occurrence network analysis based on Fourier-transform ion cyclone resonance mass spectrometry and high-throughput sequencing to investigate the molecular mechanisms of different bio-stabilized sludge addition on arsenic fraction transformation and bioavailability in tailings soil. It was found that sludge amendments decreased the arsenic bioavailable fraction from 3.62% to 1.74% and 1.68% and promoted humification of DOM in soil. The extra inorganic salt ions introduced with sludge desorb the adsorbed As(V) into soil solution. Specifically, bio-stabilized sludge increased the contents of labile compounds that provided nutrients for microbial metabolism and shaped the microbial community composition into a more copiotrophic state, which increased the abundance of As(V)-reducing bacteria and then converted the As(V) into As(III) and precipitated as As₂S₃. This work innovatively explores the transformation mechanisms of As fractions through the perspectives of microbial community and DOM molecular characterization, providing an important basis for the remediation of As-contaminated soil using biosolids.

Accumulation characteristics, driving factors, and model prediction of cadmium in soil-highland barley system on the Tibetan Plateau

Cadmium (Cd) poses major human health problems due to its high toxicity and organ bioaccumulation potential. This study collected and analysed 130 pairs of representative soil-highland barley samples on the Tibetan Plateau. The total soil Cd content (Cd-soil), available soil Cd (Cd-ava), and highland barley Cd contents (Cd-barley) ranged from 0.03 to 0.46 mg kg-1, 0.006-0.185 mg kg-1, and 0.57-13.62 μg kg-1, with mean values of 0.19 ± 0.01 mg kg-1, 0.045 ± 0.003 mg kg-1, and 4.57 ± 0.17 μg kg-1, respectively. Redundancy analysis (RDA) demonstrated that geographic factors and soil properties explained 28.46% of the variation in Cd-soil and Cd-ava, and precipitation (14.6%) and pH (9.1%) were the dominant factors. The structural equation model (SEM) indicated that Cd-soil and Cd-ava were predominantly controlled by pH. Furthermore, the Cd-soil, Cd-ava, and Cd-barley with significantly different environmental conditions were more accurately predicted by conditional inference trees-multiple linear regression (CITs-MLR). When Cd-soil is more than 0.376 mg kg-1, Cd-ava obtains the most accurate predictor (R2 =0.64, P < 0.01). This study provides new scientific insight into understanding the environmental biogeochemical nexus of Cd in the complex and fragile plateau environment and evaluating food security on the Tibetan Plateau under the self-sufficiency model of highland barley.

Evaluation of the effectiveness of amendments derived from vermicompost combined with modified shell powder on Cd immobilization in Cd-contaminated soil by multiscale experiments

The widespread heavy metal contamination of agricultural soils poses an enormous challenge to food safety. To evaluate the Cd immobilization potential of vermicompost combined with modified shell powder (VMSP) on Cd-contaminated soil, batch adsorption tests and field experiments were conducted. First, the Cd²⁺ removal characteristics and adsorption mechanisms of vermicompost (V), vermicompost combined with shell powder (VSP), and VMSP in an aqueous solution were investigated by batch tests. Then, 3 kg·m² V, VSP, and VMSP doses were applied to Cd-contaminated farmland soils as soil amendments to plant green garlic (Allium sativum L.) and investigate their Cd immobilization effects in Cd-contaminated soils. Batch adsorption tests showed that VMSP was most effective for Cd²⁺ removal, with adsorption rates as high as 85.7-99.79% and desorption rates of approximately 1.25-1.34%. Combining further characterization analysis of VMSP, it was demonstrated that the adsorption mechanism of Cd²⁺ was monolayer chemisorption, mainly involving the complexation reaction of Cd²⁺ with organic functional groups and the precipitation reaction of Cd²⁺ with mineral elements. The field experiment showed that adding V, VSP, and VMSP effectively inhibited the enrichment of Cd in green garlic, and the Cd content was reduced by 42.18%, 46.88%, and 68.75%, respectively. However, only the Cd content of green garlic treated with VMSP was lower than the national standard for food safety in China (Cd≤ 0.2 mg·kg^-1). V, VSP, and VMSP additions improved soil fertility and reduced Cd bioavailability in the soil by 15.5%, 18.9%, and 36.3%, respectively. In addition, V, VSP, and VMSP addition increased bacterial diversity and improved bacterial communities and functions in the soil by improving basic soil properties and reducing Cd-related toxicity. The results indicated that VMSP is a promising amendment for Cd immobilization in Cd-contaminated farmland soils.

Facilitated remediation of heavy metals contaminated land using Quercus spp. with different strategies: Variations in amendments and experiment periods

Phytoremediation using trees combined with soil amendments has gained much attention for its highly cost-effective trait. In natural field conditions, however, the results may not reflect the true performance of amendments based on short-term laboratory studies. In this three-year field trial, various soil amendments such as rice straw biochar, palygorskite, a combined biochar of rice straw biochar and palygorskite, and hydroxyapatite were used to systematically study the potential of the low-accumulator (Quercus fabri Hance) and high-accumulator (Quercus texana Buckley) for cadmium (Cd) and zinc (Zn) to remediate severely contaminated soils. Soil amendments enhanced the dendroremediation capacity of Quercus as the growth period prolonged. In 2021, the rice straw biochar treatment increased Cd and Zn accumulation by 1.76 and 2.09 times in Q. fabri, respectively, compared to the control. Cd and Zn accumulation increased to 1.78 and 2.10 times, respectively, under combined biochar treatment for Q. texana compared to the control. Metals accumulation was mainly enhanced by soil amendments through increasing the growth biomass of Q. fabri and improving the biomass and bioconcentration ability of Q. texana. Overall, soil amendments effectively improved the phytoremediation efficiency of Quercus in the long term, and selecting suitable amendments should be fully considered in phytoremediation.

Organic-inorganic composite modifiers enhance restoration potential of Nerium oleander L. to lead-zinc tailing: application of phytoremediation

Lead-zinc tailings are complex heavy metal solid wastes produced in the mining process. In this study, two kinds of organic-inorganic mixed improvers mushroom residue + calcium carbonate (M + C) and peat soil + calcium carbonate (N + C) were selected. Then, the effect of two improvers and a woody plant, Nerium oleander L., on the combined remediation of lead-zinc tailings was compared, respectively. The results showed that two combined improvers can slightly improve the pH of tailing, significantly increase the activity of phosphatase and catalase, effectively reduce the contents of DTPA-extractable Pb and Zn, and significantly improve the structure of tailing. However, the improvement effect of M + C was better than that of N + C on tailings' physical and chemical properties. Two improvers can reduce the enrichment and the stress degree of Pb and Zn on the N. oleander and increase the accumulation of Pb and Zn while promoting the growth of the N. oleander. The content of Pb and Zn showed the trend of root > stem > leaf under the two improvers, and the content of Zn was basically higher than that of Pb. To sum up, the combination of two modifiers and N. oleander has a good effect on the remediation of lead-zinc tailings, and the remediation effect of M + C was better than N + C.

Silver nanoparticles strengthen Zea mays against toxic metal-related phytotoxicity via enhanced metal phytostabilization and improved antioxidant responses

This study investigated the phytostabilization and plant-promoting abilities of silver nanoparticles (AgNPs). Twelve Zea mays seeds were planted in water and AgNPs (10, 15 and 20 mg mL-1) irrigated soil for 21 days on soil containing 0.32 ± 0.01, 3.77 ± 0.03, 3.64 ± 0.02, 69.91 ± 9.44 and 13.17 ± 0.11 mg kg-1 of As, Cr, Pb, Mn and Cu, respectively. In soil treated with AgNPs, the metal contents were reduced by 75%, 69%, 62%, 86%, and 76%. The different AgNPs concentrations significantly reduced accumulation of As, Cr, Pb, Mn, and Cu in Z. mays roots by 80%, 40%, 79%, 57%, and 70%, respectively. There were also reductions in shoots by 100%, 76%, 85%, 64%, and 80%. Translocation factor, bio-extraction factor and bioconcentration factor demonstrated a phytoremediation mechanism based on phytostabilization. Shoots, roots, and vigor index improved by 4%, 16%, and 9%, respectively in Z. mays grown with AgNPs. Also, AgNPs increased antioxidant activity, carotenoids, chlorophyll a and chlorophyll b by 9%, 56%, 64%, and 63%, respectively, while decreasing malondialdehyde contents in Z. mays by 35.67%. This study discovered that AgNPs improved the phytostabilization of toxic metals while also contributing to Z. mays' health-promoting properties.

Application of phosphorus amendments reduces metal uptake and increases yield of Oryza saliva L. (rice) in Cd/Cu-contaminated paddy field

To alleviate worldwide food safety issues caused by metal contamination, an easily available material is urgently needed for extensive application. In this study, calcium magnesium phosphate fertiliser (Pcm) was applied to a Cd/Cu co-contaminated paddy field in comparison with limestone and organic fertiliser. The results showed that only Pcm is effective in simultaneously reducing Cd uptake by 56.7% and Cu uptake by 36.2% in Oryza saliva L. (rice). The rice yield, reduced mainly by Cu, also increased by 30.1% with respect to the enhancement of soil pH, cation exchange capacity and availability of phosphorus, as well as the reduction in availabilities of Cd and Cu. Additionally, Pcm dramatically shaped the bacterial community structure, with Proteobacteria and Firmicutes predominant in the soils. The beneficial genera Exiguobacterium, Citrobacter, and Acinetobacter, which are vital for phosphate dissolution and Cd/Cu immobilisation, were also enriched. The results demonstrated that the application of Pcm at 0.4% (w:w) was able to enhance both crop quantity and quality in Cd/Cu co-contaminated paddy fields by reducing Cu/Cd availability, promoting rice yield, and reshaping bacterial community structures.

Risk assessment of Artemia egg shell-Mg-P composites as a slow-release phosphorus fertilizer during its formation and application in typical heavy metals contaminated environment

Artemia egg shell loaded with nano-magnesium (shell-Mg) can be used to recover phosphorus from wastewater. The exhausted Artemia egg shell-Mg (denoted as shell-Mg-P) can be used as a slow-release fertilizer for phosphorus reuse. However, due to the coexistence of heavy metal ions in the environment, the application of slow-release fertilizer for phosphorus removal and reuse may have potential risks. In this paper, the potential risks of Pb²⁺, Cd²⁺, Zn²⁺ and Cu²⁺ in phosphorus wastewater and soil were studied from the formation and application process of shell-Mg-P. The result showed that shell-Mg adsorbed Pb²⁺, Cd²⁺, Zn²⁺ and Cu²⁺ in phosphate wastewater during the formation of shell-Mg-P and became shell-Mg-P-metal hybrid biomaterial. Although the experiment proved that the existence of heavy metal ions did not affect the phosphorus slow-release behavior of slow-release fertilizer, but the heavy metal ions in the shell-Mg-P-metal were also slow released. The pot experiment results confirmed that the slow-release phosphorus fertilizers (shell-Mg-P and shell-Mg-P-metal) in the soil polluted in low concentration of heavy metals can reduce the amount of heavy metals in whole wheat seedlings and promote wheat seedling growth. However, the application of slow-release fertilizers increased the translocation efficiency (T_{FR to SL}) of metal from root (R) to aboveground part (stem and leaves, SL), promoted the transportation of heavy metals from roots to the stems and leaves, and increased the safety risk of the wheat seedling edible. Therefore, besides the positive role of slow-release fertilizers in retaining heavy metals and reducing the amount of heavy metals in whole seedlings, the risk that it may aggravate the translocation of heavy metals to stems and leaves should be paid more attention, so as to ensure the safe and reliable application of slow-release fertilizers.

Rhizosphere activity induced mobilization of heavy metals immobilized by combined amendments in a typical lead/zinc smelter-contaminated soil

The persistence of the stabilization effect of amendments on heavy metals (HMs) is of great concern when they are used for remediating HM-contaminated soil. Here, pot experiments were conducted to investigate the effects of two consecutive seasons of vegetable cultivation on the mobilization of HMs (Cu, Pb, Zn, and Cd) immobilized by different application ratios (0, 20, 40, and 80 g kg^-1, labelled C0, C2, C4, and C8) of a combined amendments (lime: sepiolite: biochar: humic acid = 2:2:1:1). The results showed that HM bioavailability decreased with increasing application ratios of the combined amendments in control (CK) treatments. The DOC contents, HM bioavailability, and HM contents in the leaves of vegetables increased, but the pH decreased during two consecutive seasons of vegetable cultivation; however, the HM bioavailability in the C2, C4, and C8 treatments was lower than that in the C0 treatments with vegetables. Catalase, urease, alkaline phosphatase, and dehydrogenase activities in the combined amendment treatments with and without vegetables were decreased compared to those in the C0 treatments. The relative abundances of the dominant bacterial phyla in the different treatments were Actinobacteria > Proteobacteria > Chloroflexi > Acidobacteria > Gemmatimonadetes > Bacteroidetes for the first season and Proteobacteria > Actinobacteria > Chloroflexi > Acidobacteria > Bacteroidetes > Gemmatimonadetes for the second season. Correlations showed that the pH and DOM properties during two consecutive seasons of vegetable cultivation were important factors influencing HM bioavailability, enzyme activity, and bacterial community composition. The bacterial community composition shift indirectly influenced the mobilization of HMs immobilized by the combined amendments. Thus, rhizosphere activity induced the mobilization of HMs immobilized by combined amendments during two consecutive seasons of vegetable cultivation.