A comprehensive survey on the horizontal and vertical distribution of heavy metals and microorganisms in soils of a Pb/Zn smelter

Morphological Enrichment and Environmental Factors Correlation of Heavy Metals in Dominant Plants in Typical Manganese Ore Areas in Guizhou, China

Bioavailable heavy metal and their efficient phytoremediation in mining areas have major implications for environmental and human health. In this study, we investigated 12 dominant plants in a typical Mn ore area of Zunyi, Guizhou Province, China, to determine the heavy metal contents, morphologies, and environmental factors affecting Mn, Cd, Pb, Cu, Zn, and Cr in the plant parts and rhizosphere soil. The bioavailabilities and degrees of metals were evaluated using the ratios of the secondary to primary phase distributions and potential ecological risk indices. Principal component analysis, cluster analysis, positive matrix factorisation modelling, and redundancy analysis were used to trace the origins and correlations among the metals. The results indicate that the bioavailabilities were the highest for Mn and Cd in the study area, and all of the target heavy metals had bioavailabilities above the moderate ecological harm level. Statistical modelling indicates that there are four main pollution sources: mining, smelting, processing operations, and atmospheric deposition. The dominant plants had high heavy metal enrichments, bioconcentration factors, and translocation factors for Mn, Cu, Cr, Cd, and Zn. The redundancy analysis indicates that soil total N, total P, and pH affect metal absorption and distributions in Compositae and non-Compositae plants in low-N, low-P, and slightly alkaline mining environments. This study provides a feasible basis for the screening of heavy metal enrichment plants and the improvement of remediation technology in manganese ore area under the extreme environment of poor nutrition.

A remote sensing analysis method for soil heavy metal pollution sources at site scale considering source-sink relationships

Conventional methods for identifying soil heavy metal (HM) pollution sources are limited to area scale, failing to accurately pinpoint sources at specific sites due to the spatial heterogeneity of HMs in mining areas. Furthermore, these methods primarily focus on existing solid waste polluted dumps, defined as "direct pollution sources", while neglecting existing HM pollution hotspots generated by historical anthropogenic activities (e.g., mineral development, industrial discharges), defined as "potential pollution sources". Addressing this gap, a novel remote sensing analysis method is proposed to identify both direct and potential pollution sources at site scale, considering source-sink relationships. Direct pollution sources are extracted using a supervised classification algorithm on high-resolution multispectral imagery. Potential pollution sources depend on the spatial distribution of HM pollution, mapped using a machine learning model with hyperspectral imagery. Additionally, a source identification algorithm is developed for gridded pollution source analysis. Validated through a case study in a cadmium (Cd)-polluted mine area, the proposed method successfully extracted 21 solid waste polluted dumps with an overall accuracy approaching 90 % and a Kappa coefficient of 0.80. Simultaneously, 4167 HM pollution hotspots were identified, achieving optimal inversion accuracy for Cd (Rv2 = 0.91, RMSEv = 4.27, and RPDv = 3.02). Notably, the spatial distribution patterns of these identified sources exhibited a high degree of similarity. Further analysis employing the identification algorithm indicated that 3 polluted dumps and 258 pollution hotspots were pollution sources for a selected high-value point of Cd content. This innovative method provides a valuable methodological reference for precise prevention and control of soil HM pollution.

Migration of heavy metals in soil-plant system after land use of sewage sludge at high application ratio

As one of the reclamation methods of sewage sludge, land application is commonly used. Because almost all organic waste is supposed to be recycled in land use, higher application ratio is necessary. This study conducted sludge land use experiments under high application ratio, and the migration of heavy metals in soil-plant system were studied. The mixture ratio of sludge to soil was 0:1, 0.00862:1, 0.2:1 (240 DS t/hm2), and 0.75:1 (900 DS t/hm2), which is higher than ISO 19698: 2020 and all the Chinese standards. The results showed that the high ratio of sludge application increased the concentration of heavy metals in soil, but after planting plants, the concentration of heavy metals decreased. And compared to sunflower and black-eyed Susan, ryegrass had the best bioaccumulation and transport capacity for heavy metals. As for the residual heavy metals in the soil, compared to the application ratio of 0.00862:1, increasing the application ratio to 0.2:1 did not significantly increase the risk of heavy metals. And if sludge was applied continuously for 15 years, only Hg may have a cumulative risk at the ratio of 0.2:1, but did not exceed GB 36600-2018. Controlling the maximum application rate at 0.2 and planting ryegrass can be a feasible strategy.

Plant endophyte immobilization technology: A promising approach for chromium-contaminated water and soil remediation

Microbial immobilization technology is considered an efficient bioremediation method for chromium (Cr) pollution. However, it is currently unclear which strain is more beneficial for the remediation of Cr-contaminated water and soil. Therefore, corn straw biochar was used as a carrier to prepare materials for fixing the endophytes Serratia sp. Y-13 (BSR1), Serratia nematodiphila (BSR2), Lysinibacillus sp. strain SePC-36 (BLB1), Lysinibacillus mangiferihumi strain WK63 (BLB2) and the commercial bacteria Shewanella oneidensis MR-1 (BSW). The results demonstrated that, compared with BSW, endophyte-loaded biochar (especially BSR1) was more effective at remediating Cr pollution in water and soil. Endophyte-loaded biochar reduced the abundance of soil pathogenic bacteria, enhanced the number of beneficial plant endophytes, reduced the soil Cr(VI) concentration, improved soil fertility, reduced the plant Cr concentration and improved the yield of lettuce. Redundancy analysis (RDA) and structural equation modelling (PLS-PM) suggested that soil microbes are closely related to soil Cr(VI), plant fresh weight and soil organic matter, whereas endophyte-loaded biochar directly influences plant cell motility pathways by altering plant microbes. This study represents a pioneering investigation into the efficacy of endophyte-loaded biochar as a remediation strategy for Cr pollution.

Health Risk Assessment of Heavy Metals in Agricultural Soils Based on Multi-Receptor Modeling Combined with Monte Carlo Simulation

The spatial characteristics, pollution sources, and risks of soil heavy metals were analyzed on Hainan Island. The results showed that the heavily polluted points accounted for 0.56%, and the number of mildly and above polluted points accounted for 15.27%, respectively, which were mainly distributed in the northern part of the study area. The principal component analysis-absolute principal component score-multiple linear regression (APCS-MLR) and the positive matrix factorization (PMF) revealed four sources of heavy metals: agricultural pollution sources for cadmium, (Cd), industrial and mining pollution sources for arsenic, (As), transportation pollution sources for zinc and lead (Zn and Pb), and natural pollution sources for chromium, nickel, and copper (Cr, Ni, and Cu). The human health risk assessment indicated that the average non-carcinogenic risk (HI) for both adults and children was within the safe threshold (<1), whereas Cr and Ni posed a carcinogenic risk (CR) to human health. In addition, the total non-carcinogenic risk (THI) indicated that heavy metals posed a potential non-carcinogenic risk to children, while the total carcinogenic risk (TCR) remained relatively high, mainly in the northern part of the study area. The results of the Monte Carlo simulation showed that the non-carcinogenic risk (HI) for all heavy metals was <1, but the total non-carcinogenic risk index (THI) for children was >1, indicating a potential health risk above the safe threshold. Meanwhile, nearly 100% and 99.94% of the TCR values exceeded 1 × 10-4 for children and adults, indicating that Cr and Ni are priority heavy metals for control. The research results provide the necessary scientific basis for the prevention and control of heavy metals in agricultural soils.

Microbial-induced calcium precipitation: Bibliometric analysis, reaction mechanisms, mineralization types, and perspectives

Microbial-induced calcium precipitation (MICP) refers to the formation of calcium precipitates induced by mineralization during microbial metabolism. MICP has been widely used as an ecologically sustainable method in environmental, geotechnical, and construction fields. This article reviews the removal mechanisms of MICP for different contaminants in the field of water treatment. The nucleation pathway is explained at both extracellular and intracellular levels, with a focus on evaluating the contribution of extracellular polymers to MICP. The types of mineralization and the regulatory role of enzyme genes in the MICP process are innovatively summarized. Based on this, the environmental significance of MICP is illustrated, and the application prospects of calcium precipitation products are discussed. The research hotspots and development trends of MICP are analyzed by bibliometric methods, and the challenges and future directions of MICP technology are identified. This review aims to provide a theoretical basis for further understanding of the MICP phenomenon in water treatment and the effective removal of multiple pollutants, which will help researchers to find the breakthroughs and innovations in the existing technologies, with a view to making significant progress in MICP technology.

Impact of soil physicochemical factors and heavy metals on co-occurrence pattern of bacterial in rural simple garbage dumping site

Rural waste accumulation leads to heavy metal soil pollution, impacting microbial communities. However, knowledge gaps exist regarding the distribution and occurrence patterns of bacterial communities in multi-metal contaminated soil profiles. In this study, high-throughput 16 S rRNA gene sequencing technology was used to explore the response of soil bacterial communities to various heavy metal pollution in rural simple waste dumps in karst areas of Southwest China. The study selected three habitats in the center, edge, and uncontaminated areas of the waste dump to evaluate the main factors driving the change in bacterial community composition. Pollution indices reveal severe contamination across all elements, except for moderately polluted lead (Pb); contamination severity ranks as follows: Mn > Cd > Zn > Cr > Sb > V > Cu > As > Pb. Proteobacteria, Actinobacteria, Chloroflexi, and Acidobacteriota predominate, collectively constituting over 60% of the relative abundance. Analysis of Chao and Shannon indices demonstrated that the waste dump center boasted the greatest bacterial richness and diversity. Correlation data indicated a predominant synergistic interaction among the landfill's bacterial community, with a higher number of positive associations (76.4%) compared to negative ones (26.3%). Network complexity was minimal at the dump's edge. RDA analysis showed that Pb(explained:46%) and Mn(explained:21%) were the key factors causing the difference in bacterial community composition in the edge area of the waste dump, and AK(explained:42.1%) and Cd(explained:35.2%) were the key factors in the center of the waste dump. This study provides important information for understanding the distribution patterns, co-occurrence networks, and environmental response mechanisms of bacterial communities in landfill soils under heavy metal stress, which helps guide the formulation of rural waste treatment and soil remediation strategies.

Temporal and spatial evolution of different heavy metal fractions and correlation with environmental factors after prolonged acid mine drainage irrigation: A column experiment

Acid mine drainage (AMD) has global significance due to its low pH and elevated heavy metal content, which have received widespread attention. After AMD irrigation in mining areas, heavy metals are distributed among soil layers, but the influencing factors and mechanisms remain unclear. AMD contamination of surrounding soil is primarily attributed to surface runoff and irrigation and causes significant environmental degradation. A laboratory soil column experiment was conducted to investigate the temporal and spatial distribution of the heavy metals Cd and Cu, as well as the impact of key environmental factors on the migration and transformation of these heavy metals following long-term soil pollution by AMD. After AMD addition, the soil exhibited a significant increase in acidity, accompanied by notable alterations in various environmental parameters, including soil pH, Eh, Fe(II) content, and iron oxide content. Over time, Cd and Cu in the soil mainly existed in the exchangeable and carbonate-bound fractions. In spatial terms, exchangeable Cu increased with increasing depth. Pearson correlation analysis indicated significant negative correlations between pH and Cu, Cd, and Eh in pore water, as well as negative correlations between pH and the exchangeable fraction of Cd (F1), carbonate-bound fraction of Cd (F2), and exchangeable fraction of Cu (F1) in the solid phase. Additionally, a positive correlation was observed between pH and the residual fraction of Cu (F5). Furthermore, the soil total Cd content exhibited a positive correlation with pyrophosphate-Fe (Fep) and dithionite-Fe (Fed), while CdF1, CdF2, total Cu, and CuF1 displayed positive correlations with Fep. Our findings indicate that the presence of AMD in soil leads to alterations in the chemical fractions of Cd and Cu, resulting in enhanced bioavailability. These results offer valuable insights for developing effective remediation strategies for soils near mining sites.

Straw removal reduces Cd availability and rice Cd accumulation in Cd-contaminated paddy soil: Cd fraction, soil microorganism structure and porewater DOC and Cd

The impacts of straw removal on rice Cd absorption, behaviour of Cd and microbial community in rhizosphere soil were investigated in paddy fields over two consecutive seasons. The results of the experiments in two fields revealed that straw removal promoted the transformation of soil Cd from acid-extractable and oxidisable fraction to residual fraction and reduced soil DTPA-Cd content with the reduction in DOC and Cd ions in soil porewater, thereby decreasing Cd content in rice. Specifically, the Cd content in brown rice of early rice was below 0.2 mg·kg-1 when all rice straw and roots were removed in the slightly Cd-contaminated soils. The α-diversity of soil microbial communities was less influenced by continuous straw removal, β-diversity was altered and the relative abundances of Anaeromyxobacter, Methylocystis and Mycobacterium microbes were increased. Redundancy analysis and network analysis exhibited that soil pH predominantly influenced the microbial community. Path analysis revealed that the Cd content in brown rice could be directly influenced by the soil Total-Cd and DTPA-Cd, as well as soil pH and OM. Straw removal, including roots removal, is an economical and effective technique to reduce Cd accumulation in rice plants.

Effects of dissolved organic matter on distribution characteristics of heavy metals and their interactions with microorganisms in soil under long-term exogenous effects

Long-term waste accumulation (LTWA) in soil not only alters its physical and chemical properties but also affects heavy metals and microorganisms in polluted soil through the dissolved organic matter (DOM) it produces. However, research on the impact of DOM from LTWA on heavy metals and microorganisms in polluted soil is limited, which has resulted in an incomplete understanding of the mechanisms involved in LTWA soils remediation. This study focuses on the DOM generated by waste accumulation and analyses the physicochemical properties, microbial community structure, and vertical distribution of heavy metals in four types of LTWA soils at different depths (0-100 cm). A causal analysis is conducted using structural equation modelling. The results indicate that due to the retention effect of the soil and microorganisms, heavy metal pollution is concentrated on the soil surface layer (>30 cm). With increasing depth, there is a decrease in heavy metal concentration and an increase in microbial diversity and abundance. DOM plays a significant role in regulating the concentration of soil heavy metals and the diversity and abundance of microorganisms. The DOM from different soils gradually transforms into substances dominated by tyrosine, tryptophan, and fulvic acid, which sustain the normal life activities and gene expression of microorganisms. Bacteria such as Pseudarthrobacter, Desulfurivibrio, Thiobacillus, and Sulfurimonas, which are involved in energy transformation, along with genes such as water channel protein and YDIF, which enhance heavy metal metabolism, ensure that microbial communities can maintain basic life processes in polluted environments and gradually select for dominant species that are adapted to heavy metal pollution. These novel discoveries illuminate the potential for modulating the composition of DOM to amplify microbial activity, while concurrently offering insights into the migration patterns of various long-term exogenous pollutants. This foundational knowledge provides a foundation for the development of efficacious remediation strategies.

Bacterial abundant taxa exhibit stronger environmental adaption than rare taxa in the Arctic Ocean sediments

Marine bacteria influence Earth's environmental dynamics in fundamental ways by controlling the biogeochemistry and productivity of the oceans. However, little is known about the survival strategies of their abundant and rare taxa, especially in polar marine environments. Here, bacterial environmental adaptation, community assembly processes, and co-occurrence patterns between abundant and rare taxa were compared in the Arctic Ocean sediments. Results indicated that the diversity of rare taxa is significantly higher than that of abundant taxa, whereas the distance-decay rate of rare taxa community similarity is over 1.5 times higher than that of abundant taxa. Furthermore, abundant taxa exhibited broader environmental breadth and stronger phylogenetic signals compared to rare taxa. Additionally, the community assembly processes of the abundant taxa were predominantly governed by 81% dispersal limitation, while rare taxa were primarily influenced by 48% heterogeneous selection. The co-occurrence network further revealed the abundant taxa formed a more complex network to enhance their environmental adaptability. This study revealed the differences in environmental responses and community assembly processes between bacterial abundant and rare taxa in polar ocean sediments, providing some valuable insights for understanding their environmental adaptation strategies in marine ecosystems.

Anthropogenic effects on soils in the eastern Tibetan Plateau revealed by geochemical elemental characteristics

The Tibetan Plateau (TP) constitutes a fragile and sensitive ecological environment, which is vulnerable to global climate change and human activities. To investigate the anthropogenic effects on the TP's environmental system is valuable for guiding human responses and adaptations to future environmental changes. In this study, we detailedly analyzed the geochemical elements of four representative soil sections developed on loess from Ganzi, Jinchuan, Aba, and Chuanzhusi in the eastern TP. The chemical elemental profiles distinctly indicated the presence of typical anthropogenic elements (Cu, Zn, Ni, Cr, Pb, Mn, and Fe), underscoring the substantial influence of human activities on TP soil, and showing spatial variance. Our results indicate that anthropogenic impacts were relatively low at Aba and Ganzi, resulting in a deficit of anthropogenic elements at the surface layer. Whereas at Jinchuan and Chuanzhusi, relatively intense anthropogenic impacts have led to the enrichment of anthropogenic elements in the topsoil. We infer that agricultural activities, increased traffic, and expansion of tourism activities were the major factors affecting the anthropogenic elements of TP soils. Our study highlights the impact of human activities on soil geochemical processes in the Tibetan Plateau.

Impacts of multiple environmental factors on soil bacterial community assembly in heavy metal polluted paddy fields

Soil microorganisms play pivotal roles in driving essential biogeochemical processes in terrestrial ecosystems, and they are sensitive to heavy metal pollution. However, our understanding of multiple environmental factors interaction in heavy metal polluted paddy fields to shape microbial community assembly remain limited. In the current study, we used 16S rRNA amplicon sequencing to characterize the microbial community composition in paddy soils collected from a typical industry town in Taihu region, eastern China. The results revealed that Cd and Pb were the major pollutant, and Proteobacteria, Acidobacteria and Chloroflexi were the dominate indigenous bacterial phyla. Linear regression and random forest analysis demonstrated that soil pH was the most important predictor of bacterial diversity. Mantel analysis showed that bacterial community structure was mainly driven by pH, CEC, silt, sand, AK, total Cd and DTPA-Cd. The constructed bacterial co-occurrence network, utilizing a random matrix theory-based approach, exhibited non-random with scale-free and modularity features. The major modules within the networks also showed significant correlations with soil pH. Overall, our study indicated that soil physiochemical properties made predominant contribution to bacterial community diversity, structure and their association in Cd/Pb polluted paddy fields. These findings expand our knowledge of the key environmental drivers and co-occurrence patterns of bacterial community in polluted paddy fields.

Omics technology draws a comprehensive heavy metal resistance strategy in bacteria

The rapid industrial revolution significantly increased heavy metal pollution, becoming a major global environmental concern. This pollution is considered as one of the most harmful and toxic threats to all environmental components (air, soil, water, animals, and plants until reaching to human). Therefore, scientists try to find a promising and eco-friendly technique to solve this problem i.e., bacterial bioremediation. Various heavy metal resistance mechanisms were reported. Omics technologies can significantly improve our understanding of heavy metal resistant bacteria and their communities. They are a potent tool for investigating the adaptation processes of microbes in severe conditions. These omics methods provide unique benefits for investigating metabolic alterations, microbial diversity, and mechanisms of resistance of individual strains or communities to harsh conditions. Starting with genome sequencing which provides us with complete and comprehensive insight into the resistance mechanism of heavy metal resistant bacteria. Moreover, genome sequencing facilitates the opportunities to identify specific metal resistance genes, operons, and regulatory elements in the genomes of individual bacteria, understand the genetic mechanisms and variations responsible for heavy metal resistance within and between bacterial species in addition to the transcriptome, proteome that obtain the real expressed genes. Moreover, at the community level, metagenome, meta transcriptome and meta proteome participate in understanding the microbial interactive network potentially novel metabolic pathways, enzymes and gene species can all be found using these methods. This review presents the state of the art and anticipated developments in the use of omics technologies in the investigation of microbes used for heavy metal bioremediation.

Microbial assisted multifaceted amelioration processes of heavy-metal remediation: a clean perspective toward sustainable and greener future

Rapidly increasing heavy metal waste has adversely affected the environment and the Earth's health. The lack of appropriate remediation technologies has worsened the issue globally, especially in developing countries. Heavy-metals contaminants have severely impacted the environment and led to devastating conditions owing to their abundance and reactivity. As they are nondegradable, the potential risk increases even at a low concentration. However, heavy-metal remediation has increased with the up-gradation of technologies and integration of new approaches. Also, of all the treatment methodologies, microbial-assisted multifaceted approach for ameliorating heavy metals is a promising strategy for propagating the idea of a green and sustainable environment with minimal waste aggregation. Microbial remediation combined with different biotechniques could aid in unraveling new methods for eradicating heavy metals. Thus, the present review focuses on various microbial remediation approaches and their affecting factors, enabling recapitulation of the interplay between heavy-metals ions and microorganisms. Additionally, heavy-metals remediation mechanisms adapted by microorganisms, the role of genetically modified (GM) microorganisms, life cycle assessment (LCA), techno-economic assessment (TEA) limitations, and prospects of microbial-assisted amelioration of heavy-metals have been elaborated in the current review with focus toward "sustainable and greener future."

Field-scale assessment of soil, water, plant, and soil microbiome in and around Rania-Khan Chandpur Chromium contaminated site, India

Rania-Khan Chandpur site, (Kanpur Dehat, Uttar Pradesh, India), one of the highly Chromium (Cr) contaminated sites in India due to Chromite Ore Processing Residue (COPR), has been investigated at the field-scale. We found that the area around the COPR dumps was hazardously contaminated with the Cr where its concentrations in the surface water and groundwater were > 40 mgL-1, its maximum contents in the COPRs and in the soils of the adjoining lands were 9.6 wt% and 3.83 wt%, respectively. By exploring the vegetation and microbial distribution across the site, we advocate the appropriateness of Cynodon dactylon, Chrysopogon zizanioides, Cyperus sp., and Typha angustifolia as the most suitable phytoremediation agent because their association with Cr remediating bacterial species (Pseudomonas sp., Clostridium sp. and Bacillus sp.) was strong. Using this remarkable information for the bioremediation projects, this site can be re-vegetated and bioaugmented to remediate Cr in soils, waterlogged ditches, surface water, and in groundwater systems.

Exploiting the role of plant growth promoting rhizobacteria in reducing heavy metal toxicity of pepper (Capsicum annuum L.)

Microorganisms are cost-effective and eco-friendly alternative methods for removing heavy metals (HM) from contaminated agricultural soils. Therefore, this study aims to identify and characterize HM-tolerant (HMT) plant growth-promoting rhizobacteria (PGPR) isolated from industry-contaminated soils to determine their impact as bioremediators on HM-stressed pepper plants. Four isolates [Pseudomonas azotoformans (Pa), Serratia rubidaea (Sr), Paenibacillus pabuli (Pp) and Bacillus velezensis (Bv)] were identified based on their remarkable levels of HM tolerance in vitro. Field studies were conducted to evaluate the growth promotion and tolerance to HM toxicity of pepper plants grown in HM-polluted soils. Plants exposed to HM stress showed improved growth, physio-biochemistry, and antioxidant defense system components when treated with any of the individual isolates, in contrast to the control group that did not receive PGPR. The combined treatment of the tested HMT PGPR was, however, relatively superior to other treatments. Compared to no or single PGPR treatment, the consortia (Pa+Sr+Pp+Bv) increased the photosynthetic pigment contents, relative water content, and membrane stability index but lowered the electrolyte leakage and contents of malondialdehyde and hydrogen peroxide by suppressing the (non) enzymatic antioxidants in plant tissues. In pepper, Cd, Cu, Pb, and Ni contents decreased by 88.0-88.5, 63.8-66.5, 66.2-67.0, and 90.2-90.9% in leaves, and 87.2-88.1, 69.4-70.0%, 80.0-81.3, and 92.3%% in fruits, respectively. Thus, these PGPR are highly effective at immobilizing HM and reducing translocation in planta. These findings indicate that the application of HMT PGPR could be a promising "bioremediation" strategy to enhance growth and productivity of crops cultivated in soils contaminated with HM for sustainable agricultural practices.

Efficient stabilization of arsenic migration and conversion in soil with surfactant-modified iron-manganese oxide: Environmental effects and mechanistic insights

The use of iron-manganese oxide (FMO) as a promising amendment for remediating arsenic (As) contamination in soils has gained attention, but its application is limited owing to agglomeration issues. This study aims to address agglomeration using surfactant-modified FMO and investigate their stabilization behavior towards As and resulting environmental changes upon amendments. The results confirmed the efficacy of surfactants and demonstrated that cetyltrimethylammonium-bromide-modified FMO significantly reduced the leaching concentration of As by 92.5 % and effectively suppressed the uptake of As by 85.8 % compared with the control groups. The ratio of the residual fraction increased from 30.5-41.6 % in unamended soil to 67.9-69.2 %. The number of active sites was through the introduction of surfactants and immobilized As via complexation, ion exchange, and redox reactions. The study also revealed that amendments and the concentration of As influenced the soil physicochemical properties and enriched bacteria associated with As and Fe reduction and changed the distribution of C, N, Fe, and As metabolism genes, which promoted the stabilization of As. The interactions among cetyltrimethylammonium bromide, FMO, and microorganisms were found to have the greatest effect on As immobilization.

Alleviating the adverse effects of Cd-Pb contamination through the application of silicon fertilizer: Enhancing soil microbial diversity and mitigating heavy metal contamination

The use of silicon fertilizer (SF) as a means of remediating cadmium (Cd) and lead (Pb) pollution has proven to be beneficial. However, the mechanism via which SF enhances soil quality and crop productivity under Cd- and Pb-contaminated soil (S) remains unclear. This study investigated the impacts of chemical fertilizer, mineral SF (MSF), and organic SF (OSF) on microbial community structure, activity of nutrient acquisition enzymes, and growth of tobacco in the presence of S condition. SF significantly reduced the contents of Cd and Pb in soil under S condition by 6.92-42.43% and increased plant height and leaf area by 15.27-81.77%. Moreover, the use of SF was observed to increase the efficiency of soil carbon and phosphorus cycling under S condition by 6.88-23.08%. Concurrently, SF was found to play a crucial role in facilitating the establishment of a complex, efficient, and interdependent molecular ecological network among soil microorganisms. In this context, Actinobacteriota, Bacteroidota, Ascomycota, and Basidiomycota were observed to be integral components of this network. SF was found to have a substantial positive impact on the metabolic functions and organismal systems of soil microorganisms. Moreover, the combined utilization of the Mantel test and partial least squares path model provided empirical evidence supporting the assertion that the administration of SF had a positive impact on both soil nutrient acquisition enzyme activity and tobacco growth, which was attributed to the enhancement of soil microbial diversity resulting from the application of SF. Furthermore, compared with MSF, OSF has advantages in reducing soil Pb and Cd content, promoting tobacco agronomic traits, increasing the number of key microbial communities, and maintaining the structural stability of microbial networks. The aforementioned findings, therefore, suggest that the OSF played a pivotal role in alleviating the adverse impacts of S, thereby demonstrating its efficacy in this particular process.

Comprehensive distribution characteristics and factors affecting the migration of chromium in a typical chromium slag-contaminated site with a long history in China

The contamination of abandoned chromium slag-contaminated sites poses serious threats to human health and the environment. Therefore, improving the understanding of their distribution characteristics and health risks by multiple information is necessary. This study explored the distribution, accumulation characteristic, and the role in the migration process of chromium. The results showed that the contents of total Cr and Cr (VI) ranged from 12.00 to 7400.00 mg/kg, and 0.25 to 2160.00 mg/kg, respectively. The average contents of both total Cr and Cr (VI) reached the highest value at the depth of 7-9 m, where the silt layer retaining total Cr and Cr (VI) was. The spatial distribution analysis revealed that the total contamination area percentages of total Cr and Cr (VI) reached 7.87% and 90.02% in the mixed fill layer, and reduced to 1.21% and 34.53% in the silty layer, and the same heavily polluted areas were located in the open chromium residue storage. Soil pH and moisture content were the major factors controlling the migration of total Cr and Cr(VI) in soils. Results of probabilistic health risk assessment revealed that carcinogenic risk was negligible for adults and children, and the sensitive analysis implied that the content of Cr(VI) was the predominant contributor to carcinogenic risk. The combination of chemical reduction and microbial remediation could be the feasible remediation strategy for soil Cr(VI) pollution. Overall, this study provides scientific information into the chromium post-remediation and pollution management for various similar chromium-contaminated sites.

Removal of thallium by MnOx coated limestone sand filter through regeneration of KMnO4: Combination of physiochemical and biochemical actions

Treatment of industrial thallium(Tl)-containing wastewater is crucial for mitigating environmental risks and health threats associated with this toxic metal. The incorporation of Mn oxides (MnOx) into the filtration system is a promising solution for efficient Tl(I) removal. However, further research is needed to elucidate the underlying mechanism behind MnOx-enhanced filtration and the rules of its stable operation. In this study, limestone, a cost-effective material, was selected as the filter media. Raw water with Mn(II), Tl(I), and other pollutants was prepared after a thorough investigation of actual industrial wastewater conditions. KMnO4 was added to induce the formation of MnO2 on limestone surfaces, while long-term operation led to enrichment of manganese oxidizing microorganisms (MnOM). Results revealed a dual mechanism. Firstly, most Mn(II) were oxidized by KMnO4 to form MnO2 attaching to limestone sands, and both Tl(I) and residual Mn(II) were adsorbed onto the newly formed MnO2. Subsequently, enzymes secreted by MnOM facilitated oxidation of remaining Mn(II), resulting in the generation of biogenic manganese oxides (BioMnOx) with numerous vacancies during long-term operation. The generated BioMnOx not only adsorbed Mn(II) and Tl(I) but also promoted their oxidation process. This approach offers an effective and sustainable method for removing both Mn(II) and Tl(I) from industrial wastewater, thereby addressing the challenges posed by thallium-contaminated effluents.

Metals and microorganisms in a Maar lake sediment core indicating the anthropogenic impact over last 800 years

A closed Maar lake, receiving mostly atmospheric deposition, offers a unique setting for investigating the impact of human activities on the environment. In this study, we aimed to investigate the historical record of metals in core sediments of Maar Lake in Huguangyan (HGY), Southeast China, and elucidate the possible microbial responses to anthropogenic metal stress. Five stages were divided according to the historical record of metals and corresponding distribution of microbial community, among which Pb and Sn showed a peak value around 1760 CE, indicating the ancient mining and smelting activities. Since the 1980s, a substantial enrichment of metals such as Zn, As, Mo, Cd, Sn, Sb, and Pb was observed, due to the rapid industrial growth in China. In terms of microorganisms, Chloroflexi phylum, particularly dominated by Anaerolineales, showed significant correlations with Pb and Sn, and could potentially serve as indicator species for mining and smelting-related contamination. Desulfarculales and Desulfobacterales were found to be more prevalent in recent period and exhibited positive correlations with anthropogenic metals. Moreover, according to the multivariate regression modeling and variance decomposition analysis, Pb and Sn could regulate Anaerolineales and further pose impact on the carbon cycle; while sulfate-reducing bacteria (SRB) could response to anthropogenic metals and influence sulfur cycle. These findings provide new insights into the interaction between metals and microbial communities over human history.

Performance and mechanism of a bioelectrochemical system for reduction of heavy metal cadmium ions

This study explores the removal of Cd(ii) from wastewater using a microbial electrolysis cell (MEC) to investigate the electrochemical performance and removal kinetics of an anodic polarity reversal biocathode and the mechanism of action of electrochemically active bacteria. Comparative electrochemical methods showed that using an anodic polarity reversal biocathode resulted in greater than 90% removal of different concentrations of Cd(ii) within three days, which may be related to the catalytic effect of anodic electrochemically active bacteria. However, due to the ability of bacteria to regulate, up to nearly 2 mg L-1 of Cd(ii) ions will remain in solution. As shown by the linear fitting relationship between scanning speed and peak current, the removal process was dominated by adsorption control for 20-80 mg L-1 Cd(ii) and diffusion control for 100 mg L-1 Cd(ii). The analysis of raw sludge and sludge containing Cd(ii) showed that Arcobacter and Pseudomonas were the primary cadmium-tolerant bacteria, and that the ability to remove Cd(ii) was the result of a synergistic collaboration between autotrophic and heterotrophic Gram-negative bacteria.

Geochemical properties, heavy metals and soil microbial community during revegetation process in a production Pb-Zn tailings

Lead-zinc (Pb-Zn) tailings pose a significant environmental threat from heavy metals (HMs) contamination. Revegetation is considered as a green path for HM remediation. However, the interplay between HM transport processes and soil microbial community in Pb-Zn tailings (especially those in production) remain unclear. This study investigated the spatial distribution of HMs as well as the crucial roles of the soil microbial community (i.e., structure, richness, and diversity) during a three-year revegetation of production Pb-Zn tailings in northern Guangdong province, China. Prolonged tailings stockpiling exacerbated Pb contamination, elevating concentrations (from 10.11 to 11.53 g/kg) in long-term weathering. However, revegetation effectively alleviated Pb, reducing its concentrations of 9.81 g/kg. Through 16 S rRNA gene amplicon sequencing, the dominant genera shifted from Weissella (44%) to Thiobacillus (17%) and then to Pseudomonas (comprising 44% of the sequences) during the revegetation process. The structural equation model suggested that Pseudomonas, with its potential to transform bioavailable Pb into a more stable form, emerged as a potential Pb remediator. This study provides essential evidence of HMs contamination and microbial community dynamics during Pb-Zn tailings revegetation, contributing to the development of sustainable microbial technologies for tailings management.

Roots recruited distinct rhizo-microbial communities to adapt to long-term Cd and As co-contaminated soil in wheat-maize rotation

Cd and As accumulation in staple crops poses potential risks to food safety and human health. Rhizo-microbial communities are involved in their behaviors from soil to crops. However, the responses of rhizo-microbial communities to different Cd and As co-contaminated soils in wheat‒maize rotation are still unclear. This study explored whether wheat or maize could recruit distinct rhizo-microbial communities to adapt to long-term co-contaminated soils with low or high levels of Cd and As (LS or HS). It was apparent that the average wheat grain-Cd/As concentrations were 17.96-fold/4.81-fold in LS and 5.64-fold/7.70-fold in HS higher than those in maize grains, significantly depending on the mobility of Cd/As in soil-crop system, especially from soil to root and from straw to grain. Meanwhile, wheat or maize roots recruited specific bacteria and fungi in LS and HS, which were substantially associated with Cd/As bioavailability in rhizosphere. Wheat roots recruited specific bacterial genera norank_c__MB-A2-108 (Actinobacteria), norank_f__JG30-KF-CM45 (Chloroflexi), and norank_o__Rokubacteriales (Methylomirabilota) and fungal genera Metarhizium and Olpidium under HS, and their relative abundances were positively correlated with soil Cd/As bioavailability and were resistant to Cd and As co-contamination. However, bacterial genera Arthrobacter, Nocardioides, Devosia, Skermanella, and Pedobacter were sensitive to Cd and As co-contamination and were specifically enriched in wheat rhizospheres under LS. Meanwhile, the bacterial genus norank_c__KD4-96 (Chloroflexi) was resistant to Cd and As co-contamination under HS and was distinctly enriched in maize rhizosphere. Furthermore, the roots of wheat and maize recruited the bacterial genus Marmoricola in LS, which was sensitive to Cd and As co-contamination, and recruited specific fungal genus Fusicolla in HS, which was tolerant to Cd and As co-contamination. These results confirmed that HS and LS shifted the composition and structure of the rhizo-microbial communities in the wheat-maize rotation to promote crops survival in different long-term Cd and As co-contaminated soils.

Effects of biochar layer position on treatment performance and microbial community in subsurface flow constructed wetlands for removal of cadmium and lead

Levels of cadmium (Cd) and lead (Pb) correspond to common composition in acid mine wastewater of Hunan Province of China. The removal path of Cd and Pb and the structure of microbial community were investigated by developing constructed wetlands (CWs) with different layer positions of biochar. The biochar as a layer at the bottom of CW (BCW) system exhibited maximum Cd and Pb removal efficiencies of 96.6-98.6% and 97.2-98.9%, respectively. Compared with original soil, BCW increased the relative proportions of Proteobacteria, Firmicutes, Acidobacteriota, Verrucomicrobiota, Desulfobacterota, Armatimonadota, Bacteroidota, Patescibacteria, Basidiomycota (phylum level) and Burkholderia-Caballeronia-Paraburkholderia, Citrifermentans, Chthonomonadales, Cellulomonas, Geothrix, Terracidiphilus, Gallionellaceae, Microbacterium, Vanrija, Apiotrichum, Saitozyma, Fusarium (genus level). The concentrations of Cd and Pb were positively correlated with the abundance of Verrucomicrobiota, Basidiomycota (phylum level), and Methylacidiphilaceae, Meyerozyma, Vanrija (genus level). This study demonstrates that BCW system can improve removal performance toward Cd and Pb, as well as alter microbial community.

Physiological characteristics, rhizosphere soil properties, and root-related microbial communities of Trifolium repens L. in response to Pb toxicity

Trifolium repens L. (T. repens) is considered a potential phytoremediation species due to its large biomass and ability to accumulate and tolerate heavy metals. Lead (Pb) is an important heavy metal pollutant that can affect plant growth, photosynthesis, and enzyme activity. However, response mechanism of microorganisms in three root niches of metal tolerant plants to Pb is not completely understood. Therefore, in this study, a Pb poisoning model of T. repens was established with a Pb gradient (0, 1000 mg/kg, 2000 mg/kg, and 3000 mg/kg), and was used to evaluate growth and physiological responses, as well as enrichment and transport coefficients in T. repens, and explore the characteristics of rhizosphere soil and microbial composition of three root niches. We found that Pb stress caused oxidative injury, and inhibited photosynthesis in T. repens. 16S rDNA sequencing analysis showed that the richness of microbial communities in bulk soil was higher than that in rhizosphere soil both under Pb stress and Pb nonstress conditions. Moreover, Proteobacteria was dominant phylum in bulk and rhizosphere soils, and Proteobacteria and Cyanobacteria were dominant phylum in endophytic bacteria. For the first time, we systematically investigated the response of Pb from bulk soil to plant leaves. The results showed that microbial interaction existed between bulk and rhizosphere soil. Rhizosphere bacterium Haliangium was positively correlated with urease activity and soil nutrients. Endophytic bacterium Pseudomonas was positively correlated with plant biomass and played an important role in Pb tolerance of T. repens. In addition, endophytic bacteria formed complex correlation networks with growth and physiological indexes of both root and shoot, moreover the network in root was more complicated. Taken together, Pb stress dose-dependently inhibited the growth of plants. This study provided a theoretical basis for the further development of microbial cooperation with plant remediation of heavy metal contaminated soil.

Insight into microbial functional genes' role in geochemical distribution and cycling of uranium: The evidence from covering soils of uranium tailings dam

There exists a research gap on microbial functional genes' role in U geochemical behavior and cycling in U contaminated soils, which has been poorly understood. Herein, 16S rRNA sequencing gene amplifiers and metagenome analysis were applied to probe microbial community structure and functional metabolism of different depth layers of covering soils in U tailings dam. Results showed that the soils were highly enriched with U (47.5-123.3 mg/kg) and a remarkable portion of 35-70% was associated with the labile fractions. It was found that U geochemical distribution was notably interacted with functional genes from N, S, Fe and P related microbes. Importantly, diminution in gene NirK and amplification in nrfH involving in nitrate reduction could induce microbial tolerance to U. Moreover, gene Sat in microbial sulfate reduction, NosZ and NorB in nitrate reduction, phnD, upgA and upgC in P transportation and phnI, phnK, phoA and opd in microbial organic P mineralization, were all closely linked to U geochemical distribution, species and cycling. All these findings disclose the functional genes that may control the transfer and transformation behavior of U in soil environment, which provides important and novel indications for the bio-remediation strategies towards U polluted sites.

Occurrence of polyhalogenated carbazoles and the combined effects with heavy metals on variation in bacterial communities in estuarine sediments

Carbazole (CZ) and eight polyhalogenated carbazoles (PHCZs) were quantified by GC-MS in sediments of 12 estuaries, the interface linking large industrial and living areas to the Bohai Sea, China. These pollutants, heavy metals, and environmental factors caused integrated exposure to sediment bacteria. Four PHCZ congeners were detectable, with ΣPHCZs ranging from 0.56 to 15.94 ng/g dw. The dominant congeners were 3,6-dichlorocarbazole (36-CCZ) and 3-chlorocarbazole (3-CCZ), with a mean contribution of 72.6 % and 20.2 %. Significant positive correlations were found between 36-CCZ and both total organic carbon and heavy metals. Redundancy analysis of microbial variation implicated no impacts from PHCZs. Correlation analysis demonstrated an increase in abundance of Rhodocyclaceae but a decrease in Bacteroides-acidifaciens-JCM-10556 with presence of PHCZs, suggesting that these bacteria can be used as potential contamination indicators. The combined exposure of heavy metals, nutrients, and PHCZs may also increase toxicity and biological availability, adversely affecting the ecosystem and human health.

Impacts of land use/cover and slope on the spatial distribution and ecological risk of trace metals in soils affected by smelting emissions

The soil contamination around smelting sites shows high spatial heterogeneity. This study investigated the impacts of distance, land use/cover types, land slopes, wind direction, and soil properties on the distribution and ecological risk of trace metals in the soil around a copper smelter. The results demonstrated that the average concentrations of As, Cd, Cu, Pb, and Zn were 248.0, 16.8, 502.4, 885.6, and 250.2 g mg kg-1, respectively, higher than their background values. The hotspots of trace metals were primarily distributed in the soil of smelting production areas, runoff pollution areas, and areas in the dominant wind direction. The concentrations of trace metals decreased with the distance to the smelting production area. An exponential decay regression revealed that, depending on the metal species, the influence distances of smelting emissions on trace metals in soil ranged from 450 to 1000 m. Land use/cover types and land slopes significantly affected trace element concentrations in the soil around the smelter. High concentrations of trace metals were observed in farmland, grassland, and flatland areas. The average concentrations of trace metals in the soil decreased in the order of flat land > gentle slope > steep slope. Soil pH values were significantly positively correlated with Cd, Cu, Pb, Zn, and As, and SOM was significantly positively correlated with Cd, Pb, and Zn in the soil. Trace metals in the soil of the study area posed a significant ecological risk. The primary factors influencing the distribution of ecological risk, as determined by the Ctree analysis, were land slope, soil pH, and distance to the source. These results can support the rapid identification of high-risk sites and facilitate risk prevention and control around smelting sites.

Research progress and hotspots on microbial remediation of heavy metal-contaminated soil: a systematic review and future perspectives

Microbial remediation technology has received much attention as a green, ecological, and inexpensive technology, and there is great potential for the application of microbial remediation technology for heavy metals (HMs) contaminated soil alone and in conjunction with other technologies in environmental remediation. To gain an in-depth understanding of the latest research progress, research hotspots, and development trends on microbial remediation of HMs-contaminated soil, and to objectively reflect the scientific contributions and impacts of relevant countries/regions, institutions, and individuals of this field, in this manuscript, ISI Web of Knowledge's Web of Science™ core collection database, data visualization, and analysis software Bibliometrix, VOSviewer, and HistCite Pro were used to collect and analyze the relevant literature from 2000 to 2022, and 1409 publications were subjected to scientometric analyses. It involved 327 journals, 5150 authors, 75 countries/regions, and 2740 keywords. The current progress and hotspots on microbial remediation of HMs-contaminated soil since the twenty-first century were analyzed in terms of the top 10 most productive countries (regions), high-yielding authors, source journals, important research institutions, and hotspots of research directions. Over the past 22 years, China, India, and the USA have been the countries with the most articles. The institution and author with the most publications are the Chinese Acad Sci and Zhu YG, respectively. Journal of Hazardous Materials is the most productive journal. The keywords showed 6 co-occurrence clusters. These findings revealed the research hotspots, knowledge gaps, and future exploration trends related to microbial remediation of HMs-contaminated soil.

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

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

Combined contribution of biochar and introduced AM fungi on lead stability and microbial community in polluted agricultural soil

Introduction

Lead (Pb) pollution in agricultural soil has been accelerated by industrial development and human activities, and poses a major threat to agricultural ecosystems. Both biochar and arbuscular mycorrhiza (AM) fungi are considered to play an important role in remediation of Pb contaminated soil.

Methods

The combined remediation effects of introduced AM fungi and biochar on soil properties, Pb availability, microbial community and functional profiles were systematically investigated in unsterilized Pb-polluted agricultural soil.

Results

Results indicated that soil nutrients were significantly improved through the combined application of biochar and introduced AM fungi. The introduced AM fungi combined with biochar prepared at 400°C and 500°C promoted the transformation of Pb to a more stable state with low bioavailability. Moreover, the addition of AM fungi and biochar affected the relative abundances of dominant bacteria and fungi at the phylum and genus levels. Biochar mainly affected soil bacterial community and obviously increased the relative abundance of Actinobacteria and Blastococcus. The interactions between biochar and introduced AM fungi mainly affected fungal community, and increased the abundance of Ascomycota and Botryotrichum. Further, PICRUSt analysis indicated biochar amendment supported stronger bacterial metabolic functional potentials.

Discussion

Therefore, the combined application of biochar and Therefore, the combined application of biochar and introduced AM fungi could improve soil nutrients, reduce Pb introduced AM fungi could improve soil nutrients, reduce Pb availability, availability, and show and show a positive effect on a positive effect on indigenous microbial communities and indigenous microbial communities and metabolic functions in metabolic functions in farmland soil.

Changes of heavy metal concentrations in farmland soils affected by non-ferrous metal smelting in China: A meta-analysis

Long-term human smelting activities have resulted in substantial heavy metals (HMs) pollution of farmland soils around smelting sites, and the safety of farmland products is critical for human health. The current study focuses on HMs in farmland soils surrounding a single smelter, therefore the impact of smelting on a national scale needs to be investigated further. This study was based on 116 papers and 1143 sets of relevant data for meta-analysis, and a hierarchical mixed-effects model was used to quantify the changes of HMs concentrations in farmland soils affected by non-ferrous metal smelting on a national scale, as well as their relationships with relevant explanatory variables in China. Results showed that: (i) non-ferrous metal smelting substantially increased farmland soils HMs concentrations (323%), with each HM concentration increasing in the following order: Cd (2753%) > Pb (562%) > Hg (455%) > Zn (228%) > Cu (158%) > As (107%) > Ni (52%); (ii) the highest increase of HMs in vegetable fields (361%), but not significant in comparison to other farmland categories, and the increase of Pb, Zn, Cu and As concentrations were significantly different in different types of smelting areas; (iii) the increase of Hg was significantly higher in the northern region than in the southern region, and the opposite increase of Cu; (iv) the soil depth from 0 to 40 cm was significantly affected by smelting, and the increase of multiple HMs were significantly positively correlated with soil pH and negatively correlated with distance; (v) the other explanatory variables (farmland category and soil organic matter) were not significantly related to the effect of smelting. The results can provide some reference for protecting and restoring farmland soils around smelting areas.

Remediation potential of magnetic biochar in lead smelting sites: Insight from the complexation of dissolved organic matter with potentially toxic elements

Magnetic biochar has been widely used in potentially toxic elements (PTEs) polluted soils due to its magnetic separation capability and synchronous immobilization for multiple metals. However, the contribution of magnetic biochar to soil dissolve organic material (SDOM) and its binding behavior with PTEs needs to be further clarified prior to its remediation application on lead smelting sites. In this study, multi-spectral techniques of excitation-emission matrix (EEM) fluorescence spectroscopy and two-dimensional FTIR correlation spectroscopy (2D-FTIR-COS) were used to explore the evolution characteristics of SDOM in the lead smelting site under the remediation of magnetic biochar, and to further analyze its affinity and binding behavior with Pb and As. Results showed that magnetic biochar significantly increased SDOM content and decreased Pb and As available content. EEM and parallel factor analysis (EEM-PARAFAC) and Self-Organizing map analysis showed that humus-like and aromatic DOM increased and microbial-derived SDOM decreased after magnetic biochar cultivation. Furthermore, 2D-FTIR-COS correlation spectroscopy analysis indicated that BDOM had a stronger binding affinity to Pb, while SDOM has a stronger binding affinity to As. The binding sequences of different DOMs to PTEs varied greatly, the carboxyl and amide groups of SDOM and BDOM showed a remarkable and rapid response. Our results enhance the insights of magnetic biochar on soil function and PTEs remediation potential, providing novel information for its environmental remediation application.

Model development and probabilistic risks of cadmium transport in slag-soil-groundwater systems with heterogeneous conditions

Prediction of the long-term risk of trace metals leaching from soils at smelting sites is essential for groundwater protection. Herein, a mass balance-based stochastic model was developed to simulate the transport and probabilistic risks of trace metals in heterogeneous slag-soil-groundwater systems. The model was applied to a smelting slag yard with three stacking scenarios, including (A) fixed stacking amount, (B) stacking amount increasing yearly, and (C) slag removal after 20 years. The simulations suggested that the leaching flux and net accumulation of Cd in soils of the slag yard and abandoned farmland were greatest for scenario (B), which was followed by scenarios (A) and (C). In the slag yard, a plateau occurred in the Cd leaching flux curves, followed by a sharp increase. After 100 years of leaching, only scenario (B) had a high probabilistic risk (>99.9 %) of threatening groundwater safety under heterogeneous conditions. <11.1 % of the exogenous Cd may leach into groundwater under the worst scenario. The key parameters affecting Cd leaching risk include runoff interception rate (IRCR), input flux from slag release (I), and stacking time (ST). The simulation results were consistent with the values measured in a field investigation and laboratory leaching experiments. The results should help guide remediation objectives and measures to minimize the leaching risk at smelting sites.

Spatial distribution, source apportionment and potential ecological risk assessment of trace metals in surface soils in the upstream region of the Guanzhong Basin, China

The health of ecosystems and safety of agricultural products are correlated with trace metal pollutionin in the soil, which eventually affects mankind. For this research, topsoil (0-20 cm) was sampled from 51 locations in the upstream area of the Guanzhong Basin to determine the level of pollution, spatial distribution characteristics and origins of 15 trace metals (V, Cr, Mn, Fe, Ni, Cu, Zn, As, Se, Rb, Sr, Y, Zr, Cd, Pb). The pollution index and potential ecological risk index were adopted for the accurate analyses of contamination degree and ecological risk that trace elements cause. The identification of potential sources of trace metals pollution was carried out using the APCS-MLR model and multivariate statistical analysis. Findings demonstrated that the most contaminated elements in the topsoil of the designated areas were Cr, Cu, Cd and Pb, and the average levels of all trace metal elements exceeded their respective local background values. However, most of the sampling points showed slight pollution, and a few demonstrated moderate and severe pollution. The southern, south-western and eastern parts in the research zone were relatively seriously contaminated, especially near Baoji City and Wugong County. Fe, Cu, Zn, Ni, Se were mainly caused by combination of agricultural and industrial production, the primary sources of Mn, Y, and Zr were the process of mining and industrial production, Cd and Pb originated mainly from traffic emission and agricultural pollution, and Cr mainly came from mining and metal smelting processes. Meanwhlie, some unknown pollution sources were also disclosed. This study has a reliable reference value for determining the source of trace metals in this region. To further determine the pollution sources of trace elements, long-term monitoring and management is necessary.

Insights on the assembly processes and drivers of soil microbial communities in different depth layers in an abandoned polymetallic mining district

Soil microbes, which play crucial roles in maintaining soil functions and restoring degraded lands, are impacted by heavy metal pollution. This study investigated the vertical distribution of bacterial communities along the soil profiles across four types of areas (heavy metal pollution level: tailings heap area > phytoremediation area > natural restoration area > original forest area) in an abandoned polymetallic mining district by 16S rRNA sequencing, and aimed to disentangle the assembly mechanisms and key drivers of the vertical variation in bacterial community structure. Bacterial diversity and composition were found to vary remarkably between the depth layers in all types of areas, with heterogeneous selection dominated the vertical distribution pattern of soil bacterial communities. Pearson correlation analysis and partial Mantel test revealed that soil nutrients mainly shaped the vertical distribution of bacterial microbiota along soil profiles in the original forest and natural restoration areas. Ni, As, and bioavailable As were the key drivers regulating the vertical variation of bacterial assemblages in the phytoremediation area, whereas Pb, pH, soil organic carbon, and available nitrogen were crucial drivers in the tailings heap area. These findings reveal the predominant assembly mechanisms and drivers governing the vertical distribution of soil bacterial microbiota and indicate the efficiency of phytoremediation and ecological restoration on ameliorating edaphic micro-ecosystems in heavy metal-contaminated areas.

Effects of dissolved organic matter derived from cow manure on heavy metal(loid)s and bacterial community dynamics in mercury-thallium mining waste slag

Organic amendments in aided phytostabilization of waste slag containing high levels of heavy metal (loid)s (HMs) are an important way to control the release of HMs in situ. However, the effects of dissolved organic matter (DOM) derived from organic amendments on HMs and microbial community dynamics in waste slag are still unclear. Here, the effect of DOM derived from organic amendments (cow manure) on the geochemical behaviour of HMs and the bacterial community dynamics in mercury (Hg)-thallium (Tl) mining waste slag were investigated. The results showed that the Hg-Tl mining waste slag without the addition of DOM continuously decreased the pH and increased the EC, Eh, SO42-, Hg, and Tl levels in the leachate with increasing incubation time. The addition of DOM significantly increased the pH, EC, SO42-, and arsenic (As) levels but decreased the Eh, Hg, and Tl levels. The addition of DOM significantly increased the diversity and richness of the bacterial community. The dominant bacterial phyla (Proteobacteria, Firmicutes, Acidobacteriota, Actinobacteriota, and Bacteroidota) and genera (Bacillus, Acinetobacter, Delftia, Sphingomonas, and Enterobacter) were changed in association with increases in DOM content and incubation time. The DOM components in the leachate were humic-like substances (C1 and C2), and the DOC content and maximum fluorescence intensity (FMax) values of C1 and C2 in the leachate decreased and first increased and then decreased with increasing incubation time. The correlations between HMs and DOM and the bacterial community showed that the geochemical behaviours of HMs in Hg-Tl mining waste slag were directly influenced by DOM-mediated properties and indirectly influenced by DOM regulation of bacterial community changes. Overall, these results indicated that DOM properties associated with bacterial community changes increased As mobilization but decreased Hg and Tl mobilization from Hg-Tl mining waste slag.

Genome-resolved metagenomics provides insights into the ecological roles of the keystone taxa in heavy-metal-contaminated soils

Microorganisms that exhibit resistance to environmental stressors, particularly heavy metals, have the potential to be used in bioremediation strategies. This study aimed to explore and identify microorganisms that are resistant to heavy metals in soil environments as potential candidates for bioremediation. Metagenomic analysis was conducted using microbiome metagenomes obtained from the rhizosphere of soil contaminated with heavy metals and mineral-affected soil. The analysis resulted in the recovery of a total of 175 metagenome-assembled genomes (MAGs), 73 of which were potentially representing novel taxonomic levels beyond the genus level. The constructed ecological network revealed the presence of keystone taxa, including Rhizobiaceae, Xanthobacteraceae, Burkholderiaceae, and Actinomycetia. Among the recovered MAGs, 50 were associated with these keystone taxa. Notably, these MAGs displayed an abundance of genes conferring resistance to heavy metals and other abiotic stresses, particularly those affiliated with the keystone taxa. These genes were found to combat excessive accumulation of zinc/manganese, arsenate/arsenite, chromate, nickel/cobalt, copper, and tellurite. Furthermore, the keystone taxa were found to utilize both organic and inorganic energy sources, such as sulfur, arsenic, and carbon dioxide. Additionally, these keystone taxa exhibited the ability to promote vegetation development in re-vegetated mining areas through phosphorus solubilization and metabolite secretion. In summary, our study highlights the metabolic adaptability and ecological significance of microbial keystone taxa in mineral-affected soils. The MAGs associated with keystone taxa exhibited a markedly higher number of genes related to abiotic stress resistance and plant growth promotion compared to non-keystone taxa MAGs.

Anthropogenic processes drive heterogeneous distributions of toxic elements in shallow groundwater around a smelting site

Smelting activities have a far-reaching influence on the quality of soil and groundwater, while most studies have neglected the information on the pollution characteristics of groundwater. The hydrochemical parameters of shallow groundwater and the spatial distributions of toxic elements were investigated in this study. Correlations analysis and groundwater evolution revealed that the major ions were primarily determined by silicate weathering and calcite dissolution process, and anthropogenic processes had a significant effect on groundwater hydrochemistry. Almost 79%, 71%, 57%, 89%, 100%, and 78.6% of samples exceeded the standards of Cd, Zn, Pb, As, SO₄^2-, and NO₃^-, and their distribution is closely related to the production process. Analysis of soil geochemistry indicated that the relatively mobile forms of toxic elements strongly influence the origin and concentration in shallow groundwater. Besides, rainfall with high magnitude would lead to a decrease of toxic elements in shallow groundwater, whereas the area once stacked waste residue was the opposite. It is recommended to strengthen risk management of the limited mobility fraction while devising a plan for waste residue treatment in accordance with the local pollution conditions. The research on controlling the mechanism of toxic elements in shallow groundwater, along with sustainable development in the study area and other smelting zones may benefit from this study.

Distribution of bound-PAH residues and their correlations with the bacterial community at different depths of soil from an abandoned chemical plant site

The situ pollutant residue and microbial characteristics in contaminated environments are crucial for ecological restoration and soil utilization. This work reported the variation of polycyclic aromatic hydrocarbon (PAH) residues and the bacterial community at different depths in an aged-abandoned site. These results unveiled that over 90% of low molecular weight (LMW) and medium molecular weight (MMW), 52.84-76.88% of high molecular weight (HMW) bound-PAH (BP) residues were sequestrated in humin (HM). The stresses of PAH and soil depth enhanced the frequency of bacteria associations, especially positive associations. We enriched and cultured PAH degradation bacteria (PDB) from the sampling site mainly consisting of Pseudomonas and Acinetobacter, which were originally 0.39-0.52% abundant in the sampling site. The abundances of PDB and PAH-degradation genes (PDGs) were higher at shallower depths and increased with high PAH concentration. Simultaneously, Pearson correlation analysis and experimental verification found that the process of PAH binding with SOM limited the further increase of PDB and PDGs in PAH-contaminated sites. These findings may illustrate possible ecological risks of contaminated soils and provide guidance for the isolation and application of PDB.

The spatial distribution, health risk, and cytotoxicity of metal(loid)s in contaminated field soils: The role of Cd in human gastric cells damage

The spatial distribution and pollution level of heavy metal(loid)s in soil (0-6 m) from a typical industrial region in Jiangmen City, Southeast China was investigated. Their bioaccessibility, health risk, and human gastric cytotoxicity in topsoil were also evaluated using an in vitro digestion/human cell model. The average concentrations of Cd (87.52 mg/kg), Co (106.9 mg/kg), and Ni (1007 mg/kg) exceeded the risk screening values. The distribution profiles of metal(loid)s showed a downward migration trend to reach a depth of 2 m. The highest contamination was found in topsoil (0-0.5 m), with the concentrations of As, Cd, Co, and Ni being 46.98, 348.28, 317.44, and 2395.60 mg/kg, respectively, while Cd showed the highest bioaccessibility in the gastric phase (72.80 %), followed by Co (21.08 %), Ni (18.27 %), and As (5.26 %) and unacceptable carcinogenic risk. Moreover, the gastric digesta of topsoil suppressed the cell viability and triggered cell apoptosis, evidenced by disruption of mitochondrial transmembrane potential and increase of Cytochrome c (Cyt c) and Caspases 3/9 mRNA expression. Bioaccessible Cd in topsoil was responsible for those adverse effects. Our data suggest the importance to reduce Cd in the soil to decrease its adverse impacts on the human stomach.

Spatial-vertical variations of energetic compounds and microbial community response in soils from an ammunition demolition site in China

Ammunition-related activities have caused severe energetic compound (EC) contamination and pose serious risks to ecosystems. However, little is known regarding the spatial-vertical variations of ECs or their migration in soils at ammunition demolition sites. Although the toxic effect of some ECs to microorganisms have been reported through laboratory simulations, the responses of indigenous microbial communities to ammunition demolition activities are unclear. In this study, the spatial-vertical variations of ECs in 117 topsoil samples and three soil profiles from a typical ammunition demolition site in China were studied. Heavy contamination of ECs was concentrated in the top soils of the work platforms, and ECs were also detected in the surrounding area and nearby farmland. ECs showed different migration characteristics in the 0-100 cm soil layer of the different soil profiles. Demolition activities and surface runoff play critical roles in the spatial-vertical variations and migration of ECs. These findings suggest that ECs are able to migrate from the topsoil to the subsoil and from the core demolition area to further ecosystems. The work platforms exhibited lower microbial diversity and different microbiota compositions compared to the surrounding areas and farmlands. Using the random forest analysis, pH and 1,3,5-trinitrobenzene (TNB) were characterized as the most important factors affecting microbial diversity. Network analysis revealed that Desulfosporosinus was highly sensitive to ECs and may be a unique indicator of EC contamination. These findings provide key information in understanding EC migration characteristics in soils and the potential threats to indigenous soil microorganisms in ammunition demolition sites.

Analytical study on heavy metal output fluxes and source apportionment of a non-ferrous smelter in southwest China

Abandoned Pb/Zn smelters are often accompanied by a large amount of smelting slag, which is a serious environmental problem. Previous studies have demonstrated that slag deposits pose an environmental threat even if the smelters are shut down. Herein, a Pb/Zn smelter and its impacted zone in GeJiu, Yunnan, China were selected as the study area. The risk and source apportionment of heavy metals (HMs) in the soil of the impacted zone were systematically studied. Based on the hydrogeological features, the migration path and output fluxes of the HMs released from smelting slag to the impacted zone were investigated. The HM contents (Cd, As, Zn, Pb, and Cu) in the soil substantially exceeded the screening values of the Chinese soil standard (GB15618-2018). Based on the results of the Pb isotopic and statistical analyses for source apportionment, the contaminated sites and agricultural irrigation water had a large impact on the HMs of soil. The hydrological analysis results showed that runoff, as an HM migration path under rainfall, continued to affect the environment. The water balance calculations using the Hydrologic Evaluation of Landfill Performance model showed that the rainfall was distributed on site as follows: evaporation (57.35%), runoff (32.63%), and infiltration (10.02%). Finally, the output fluxes were calculated in combination with the leaching experiment. As, Zn, Cd, Pb, and Cu runoff had the output fluxes of 6.1 × 10^-3, 4.2 × 10^-3, 4.1, 1.4 × 10^-2, and 7.2 × 10^-4 mg/kg/y, and infiltration of 1.9 × 10^-3, 1.3 × 10^-3, 1.3, 4.0 × 10^-4, and 2.2 × 10^-4 mg/kg/y, respectively. Therefore, this study offers theoretical and scientific recommendations for effective environmental management and engineering remediation.

A double-edged sword: Reductive soil disinfestation changes the fates of trace metal elements in soil

Although the effects of reductive soil disinfestation (RSD) in soil sterilization have been proven in several countries, the potential risks of trace metal elements (TMEs) caused by RSD require further assessment. Here, freshly Cd-spiked soil and historically contaminated greenhouse soil were exposed to RSD and the fates of TMEs, Cd, Co, Cu, Ni, Pb, and Zn, were investigated. All RSD treatments lasted for 21 days and subsamples were collected at different time intervals. Samples were open-air incubated for another 7 days until day 28 to simulate the situation after drainage. The bioavailability and geochemical fractionation of TMEs were investigated based on single and sequential extraction procedures and the environmental risks were assessed. The results showed that RSD increased the relative abundance of Firmicutes and Bacteroidetes, and the content of functional groups, including Fe, Mn, and S compounds respirations increased after RSD, highlighting the possible reductive dissolution of FeMn oxides and precipitation of TMEs. The dissolution decreased the reducible fractions of TMEs and increased the acid-soluble fractions of Co, Ni, Pb, and Zn, in the European Community Bureau of Reference results, reflecting the activation of TMEs in soils. However, the precipitation of sulfate resulted in the stabilization of Cd and Cu in two types of soils, increased their residual fractions, and decreased their acid-soluble fractions and bioavailabilities. After drainage, because the influence caused by precipitation rapidly disappeared and the impact of FeMn oxides dissolution remained, the acid-solubility of TMEs was greater than their initial status in the two soils. Furthermore, as a highly toxic metal, the activation of Cd at 28 days caused the rapid increase of ecological risks, which is particularly concerning. The results suggest that RSD temporarily increases the potential risks of TMEs and that certain measures must be taken.

Spatial Distribution and Risk Assessment of Heavy Metal(oid)s Contamination in Topsoil around a Lead and Zinc Smelter in Henan Province, Central China

A total of 137 farmland soil samples were collected around a lead/zinc smelter within 64 km². The concentration, spatial distribution, and potential source of nine heavy metal(oid)s (As, Cd, Co, Cr, Cu, Ni, Pb, V, and Zn) in soils and their potential ecological risk were investigated in detail. The results showed that the average concentrations of Cd, Pb, Cr and Zn in these soils were higher than their background value in Henan Province, and the average content of Cd was 2.83 times of the risk screening values in the national standard of China (GB 15618-2018). According to the distribution of different heavy metal(oid)s in soils, Cd and Pb in soil decrease gradually with the increase of distance from the smelter to the surrounding area. This indicates that the Pb and Cd originate from smelters via airborne practices according to the typical air pollution diffusion model. The distribution of Zn, Cu, and As were similar to Cd and Pb. However, Ni, V, Cr, and Co were mainly affected by soil parent materials. The potential ecological risk of Cd was higher than those of other elements, and the risk grade of the other eight elements was mainly low. The polluted soils with significantly high and high potential ecological risk covered 93.84% of all the studied regions. This should be of serious concern to government. The results of a principal component analysis (PCA) and cluster analysis (CA) show that Pb, Cd, Zn, Cu, and As were the elements mainly stemmed from smelter and other types of plants, with a contribution rate of 60.08%, while Co, Cr, Ni, and V are mainly caused by nature, with a contribution rate of 26.26%.

Effect of potentially toxic elements on soil multifunctionality at a lead smelting site

Contaminated soil at smelting sites affects land utilization and environmental regulation, resulting in soil degradation. However, the extent to which potentially toxic elements (PTEs) contribute to site soil degradation and the relationship between soil multifunctionality and microbial diversity in the process remains poorly understood. In this study, we investigated changes in soil multifunctionality and the correlation between soil multifunctionality and microbial diversity under the influence of PTEs. The change in microbial community diversity was closely related to changes in soil multifunctionality caused by PTEs. Microbial diversity, not richness, drives the delivery of ecosystem services in smelting site PTEs-stressed environments. Structural equation modeling identified that soil contamination, microbial taxonomic profile and microbial functional profile could explain 70% of the variance in soil multifunctionality. Furthermore, our findings demonstrate that PTEs limit soil multifunctionality by affecting soil microbial communities and functionality, whilst the positive effect of microorganisms on soil multifunctionality was mainly driven by the fungal diversity and biomass. Finally, specific fungal genera closely related to soil multifunctionality were identified, with saprophytic fungi being particularly important for maintaining multiple soil functions. The results of the study provide potential guidance for the remediation, pollution control practices and mitigation of degraded soils at smelting sites.

Spatial distribution, contamination characteristics and ecological-health risk assessment of toxic heavy metals in soils near a smelting area

Soil heavy metals (HMs) contamination stemming from smelting and mining activities is becoming a global concern due to its devastating impacts on the environment and human health. In this study, 128 soil samples were investigated to assess the spatial distribution, contamination characteristics, ecological and human health risk of HMs in soils near a smelting area by using BP artificial neural network (BP-ANN) and Monte Carlo simulation. The results showed that the concentrations of all five HMs in the soil greatly exceeded the background value of study area with a basic trend: Pb > As > Cr > Cd > Hg, indicating a high pollution level. Arsenic and lead were the major pollutants in the study area with an exceedance rate of 78.95% and 28.95%, respectively. The toxic fume and dust emitted during the smelting process were identified as the major sources of HMs pollution in soil, while Cd pollution was mainly caused by agricultural activities near the study area. The probabilistic risk assessment suggested that the average HQ values of five HMs for children and adults exceeded the acceptable threshold with a trend: As > Pb > Cr > Cd > Hg. The average CR values of As, Cr and Pb for all population were greatly larger than the acceptable threshold (CR ≥ 1), indicating a high cancer risk. However, the CR values of Cd for adults and children were within the acceptable threshold (CR < 1), implying no cancer risk. The results of the present study can provide some insight into the contamination characteristics, ecological and human health risk of HMs in contaminated soils by mining and smelting activities, which can help prevent and control soil pollution and environmental risk.

The relationships between heavy metals and bacterial communities in a coal gangue site

Coal is the main source of energy for China's economic development, but coal gangue dumps are a major source of heavy metal pollution. Bacterial communities have a major effect on the bioremediation of heavy metals in coal gangue dumps. The effects of different concentrations of heavy metals on the composition of bacterial communities in coal gangue sites remain unclear. Soil bacterial communities from four gangue sites that vary in natural heavy metal concentrations were investigated using high-throughput sequencing in this study. Correlations among bacterial communities, heavy metal concentrations, physicochemical properties of the soil, and the composition of dissolved organic matter of soil in coal gangue dumps were also analyzed. Our results indicated that Actinobacteriota, Proteobacteria, Chloroflexi, Acidobacteriota, and Gemmatimonadota were the bacterial taxa most resistant to heavy metal stress at gangue sites. Heavy metal contamination may be the main cause of changes in bacterial communities. Heavy metal pollution can foster mutually beneficial symbioses between microbial species. Microbial-derived organic matter was the main source of soil organic matter in unvegetated mining areas, and this could affect the toxicity and transport of heavy metals in soil. Polar functional groups such as hydroxyl and ester groups (A_226-400) play an important role in the reaction of cadmium (Cd) and lead (Pb), and organic matter with low molecular weight (S_R) tends to bind more to mercury (Hg). In addition to heavy metals, the content of nitrogen (N), phosphorus (P), and total organic carbon (TOC) also affected the composition of the bacterial communities; TOC had the strongest effect, followed by N, SOM, and P. Our findings have implications for the microbial remediation of heavy metal-contaminated soils in coal gangue sites and sustainable development.