Spatial Distribution of Toxic Metal(loid)s and Microbial Community Analysis in Soil Vertical Profile at an Abandoned Nonferrous Metal Smelting Site

Exploring the relationship between metal(loid) contamination rate, physicochemical conditions, and microbial community dynamics in industrially contaminated urban soils

Increasing metal(loid) contamination in urban soils and its impact on soil microbial community have attracted considerable attention. In the present study, the physicochemical parameters and the effects of twelve metal(loid) pollution on soil microbial diversity, their ecotoxic effects, and human health risk assessment in urban soils with different industrial background were studied in comparison with an unpolluted forest soil sample. Results showed that urban soils were highly contaminated, and metal(loid) contamination significantly influenced structure of the soil microbial communities. In all samples the bacterial community was dominated by Proteobacteria, and on the level of phyla characteristic differences were not possible to observe between polluted and control sampling sites. However, clear differences emerged at class and genus level, where several rare taxa disappeared from contaminated urban soils. Simper test results showed that there is 71.6 % bacterial OTU and 9.5 % bacterial diversity dissimilarity between polluted and control samples. Ratio of Patescibacteria, Armatimonadetes, Chlamydiae, Fibrobacteres, and Gemmatimonadetes indicated a significant (p < 0.05) positive correlation with soil Zn, Cr, Pb, Sn, Cu, Mn content, suggest that metal(loid)s strongly influence the structure of microbial community. In contrast, the presence of metal(loid) contamination in urban soils has been found to significantly reduce the population of Archaeal communities. This can be attributed to the depletion of organic matter caused by contamination that reached a minimum of 0.5 m/m% for nitrate and 0.9 m/m% for total organic carbon. The values of urban soil pH were close to neutral, ranging from 5.9 to 8.3. The findings of ecotoxicology test are alarming, as all the studied urban soil sites were cytotoxic to soil microorganisms, and in one site metal(loid) contamination reached genotoxic level. Moreover, all the metal(loid) contaminated sites pose severe and persistent health risk to children, highlighting the urgent need for effective measures to mitigate metal(loid) pollution in urban areas.

Systematic characterization of sediment microbial community structure and function associated with anaerobic microbial degradation of PBDEs in coastal wetland

As the widely used flame retardant, polybrominated diphenyl ethers (PBDEs) have been ubiquitously detected in wetland sediments. Microbial degradation is the importantly natural attenuation process for PBDEs in sediments. In this study, the microbial degradation of PBDEs and inherent alternation of microbial communities were explored in anaerobic sediments from coastal wetland, North China. BDE-47 and BDE-153 could be degraded by the indigenous microbes, with biodegradation following pseudo-first-order kinetic. In sediments, the major genera for BDE-47 and BDE-153 degradation were Paeisporosarcina and Gp7, respectively, in single exposure. However, Marinobacter was dominant genera in the combined exposure to BDE-47 and BDE-153, and competition against Marinobacter existed between BDE-47 and BDE-153 degradation. Analysis of bacterial metabolic function indicated that membrane transport, amino acid and carbohydrate metabolism were included in degradation. This study provides the systematic characterization of the sediment microbial community structure and function associated anaerobic microbial degradation of PBDEs in coastal wetland.

Microbial communities in the rhizosphere of maize and cowpea respond differently to chromium contamination

Chromium (Cr) contamination can affect microorganisms in the soil, but the response of the microbial community in the rhizosphere of plants grown in Cr-contaminated soils is poorly understood. Therefore, this study assessed the microbial community, by amplicon sequencing, in the rhizosphere of maize and cowpea growing in uncontaminated (∼6.0 mg kg^-1 Cr) and Cr-contaminated soils (∼250 mg kg^-1 Cr). Comparing Cr-contaminated and uncontaminated soils, the microbial community in the maize rhizosphere clustered separately, while the microbial community in the cowpea rhizosphere did not present clear clustering. The microbial richness ranged from ∼5000 (rhizosphere in Cr-contaminated soil) to ∼8000 OTUs (in uncontaminated soil). In the comparison of specific bacterial groups in the rhizosphere of maize, Firmicutes were enriched in Cr-contaminated soil, including Bacilli, Bacillales, and Paenibacillus. Cowpea rhizosphere showed a higher abundance of six microbial groups in Cr-contaminated soil, highlighting Rhizobiales, Pedomicrobium, and Gemmatimonadetes. The microbial community in both rhizospheres presented a similar proportion of specialists comparing uncontaminated (2.2 and 3.4% in the rhizosphere of maize and cowpea, respectively) and Cr-contaminated soils (1.8 and 3.2% in the rhizosphere of maize and cowpea, respectively). This study showed that each plant species drove differently the microbial community in the rhizosphere, with an important effect of Cr-contamination on the microbial community assembly.

Impact of Vanadium-Containing Stone Coal Smelting on Trace Metals in an Agricultural Soil-Vegetable System: Accumulation, Transfer, and Health Risks

Dietary exposure to trace metals (TMs) through vegetable consumption has been identified as a potential risk to human health. Fifty-one paired agricultural soil and leaf vegetable samples were collected around V-containing stone coal smelting sites in Hunan Province, China, to study the contamination and transfer characteristics of TMs (Cd, Cr, Cu, Pb, V, and Zn) in the soil-vegetable system. The health risk to local residents through vegetable ingestion was evaluated using Monte Carlo simulations. The results showed that 96.2%, 23.1%, 53.8%, 30.8%, 96.2%, and 69.2% of the soil samples had Cd, Cr, Cu, Pb, V, and Zn contents exceeding their related maximum allowable values, respectively. Cadmium and V were the primary pollutants based on the I_geo values. Moreover, 46.9% and 48.4% of vegetable samples exceeded the maximum permissible levels for Cd and Pb, respectively. There was a negative correlation between the bioaccumulation factors for Cd and V of the vegetable and soil physicochemical properties, including pH, organic matter, and free Fe₂O₃ content. Ingestion of garland chrysanthemum and pak choi posed high health risks, and Cd, V, and Pb were the primary contributors. These findings will help design strategies to minimize contamination and human exposure to soil-vegetable systems caused by V-containing stone coal smelting.

Potential Mechanism of Long-Term Immobilization of Pb/Cd by Layered Double Hydroxide Doped Chicken-Manure Biochar

Layered double hydroxide (LDH)-doped chicken-manure biochar (CMB) with long-term stability was synthesized to immobilize Pb/Cd. MgAl-Cl-LDH-doped CMB (MHs) showed prominent long-term oxidation resistance and the least biodegradation sensitivity. Efficient Pb/Cd adsorption was observed on MHs, and the maximum adsorption capacities of Pb(II)/Cd(II) reached 1.95 mmol/g and 0.65 mmol/g, respectively. Precipitation and isomorphous substitution were identified as the key adsorption mechanisms, which formed highly stable Pb/Cd species (PbAl-CO₃-LDH, Pb₃(OH)₂CO₃, CdAl-Cl-LDH and CdCO₃). Pb(II) and Cd(II) precipitated with CO₃^2- in MHs; meanwhile, Mg(II) and Ca(II) in LDH layers were substituted by Pb(II) and Cd(II) respectively. Therefore, MHs had the potential for long-term stability of Pb/Cd. Moreover, complexation and electrostatic adsorption also contributed to the Pb/Cd immobilization to a certain extent. When 5% MHs (w/w) was applied to Pb/Cd contaminated smelting site soils, the soil pH increased from 5.9 to 7.3. After applying MHs for 25 d, the content of bioavailable Pb(II) and Cd(II) decreased by 98.8% and 85.2%, respectively, and the content of soluble Pb and Cd dropped by 99.5% and 96.7%. This study paves the way for designing a novel LDH doped CMB as efficient Pb/Cd immobilizers for smelting site soils.

Biogeochemical behavior and pollution control of arsenic in mining areas: A review

Arsenic (As) is one of the most toxic metalloids that possess many forms. As is constantly migrating from abandoned mining area to the surrounding environment in both oxidation and reducing conditions, threatening human health and ecological safety. The biogeochemical reaction of As included oxidation, reduction, methylation, and demethylation, which is closely associated with microbial metabolisms. The study of the geochemical behavior of arsenic in mining areas and the microbial remediation of arsenic pollution have great potential and are hot spots for the prevention and remediation of arsenic pollution. In this study, we review the distribution and migration of arsenic in the mining area, focus on the geochemical cycle of arsenic under the action of microorganisms, and summarize the factors influencing the biogeochemical cycle of arsenic, and strategies for arsenic pollution in mining areas are also discussed. Finally, the problems of the risk control strategies and the future development direction are prospected.

Contrasted speciation distribution of toxic metal(loid)s and microbial community structure in vanadium-titanium magnetite tailings under dry and wet disposal methods

Tailing disposal technologies such as dry and wet disposal methods have a profound effect on the ecosystem of mining areas. However, the chemical speciation of metal(loid)s and microbial community structure in tailings under different disposal methods are still poorly understood. Here we compared the bioavailable fraction of metal(loid)s and the microbial community in vanadium-titanium (V-Ti) magnetite tailing profiles derived from dry and wet stockpiled methods. In wet tailings, the bioavailability of Cr, Cu, Mn, Ni, V, and Zn was higher than that in dry tailings as identified by BCR sequential extraction. Especially for Cu and Ni, the oxidizable fraction was the predominant fraction except the residual fraction, accounting for 37.2-59.0% and 23.2-36.6% of the total concentration in wet tailings, respectively. Based on 16 S rRNA high-throughput sequencing, totally 12 indicator bacterial taxa were detected in dry tailings against 68 in wet tailings. As the biomarkers in wet tailings, genera Sulfuricurvum, Geobacter, and Pseudomonas were expected to be applied to the transformation of metal(loid)s in the tailings. Our results emphasize the importance of dehydration treatment of tailings before stockpiling to minimize the environmental risks caused by toxic metal(loid)s, and provide insights into the engineering application of microbial technologies in V-Ti magnetite tailing area.

Adsorption of Cd on Soils with Various Particle Sizes from an Abandoned Non-ferrous Smelting Site: Characteristics and Mechanism

Soil particle size could intensively impact the Cd adsorption in soils. The adsorption characteristics of Cd on miscellaneous fill (MF) and weathered slate (WS), collected from a zinc smelting site, were studied by batch experiments under conditions of different initial Cd concentrations and soil particle sizes. The results showed that the adsorption kinetics of Cd for soil particles from MF and WS were well fitted with the pseudo-first-order model, and the Cd adsorption isotherms well conformed to the Freundlich model. Soil particle size had an inconspicuous influence on adsorption rate, while the adsorption capacity decreased with particle size increase. The Cd adsorption on soil particles could be due to the exchange with Fe/Al, and -OH/C=O sites were the predominant adsorption sites. The MF may cause secondary pollution risk due to its low adsorption ability for Cd in smelting sites.

The impact of the mountain barrier on the spread of heavy metal pollution on the example of Gorce Mountains, Southern Poland

The main objective of this study was to determine the content, mobility, and the variability of concentration of zinc, lead, and cadmium in soils from the Gorce Mountains (south Poland), located over 100 km south-east from the potential industrial sources of contamination-zinc-lead sulfide ore mine and smelter in Bukowno, as well as hard coal mines of Silesia region and Kraków Nowa-Huta steelwork. The abovementioned problem is crucial in the context of the traditional mountain farming still extant in the region, as well as intensively developing tourism. The geoaccumulation index and potential ecological risk index were adopted to evaluate soil pollution in the study area and the BCR sequential extraction technique to assess mobility of the abovementioned elements. The obtained results clearly show that the pollution from distant industrial sources in the mountains is detectable. Apart from the increased concentrations of the tested metals in the soil (especially available forms), there is also a strong correlation between the concentrations of lead, zinc, and cadmium, which proves their common source of origin. The main evidence is the fact that differences in the concentrations of the tested metals on the windward and leeward sides were statistically significant. This also means that the studied mountain area, despite relatively low altitudes (up to 1310 m above sea level), constitutes a measurable barrier to the spread of atmospheric pollutants.

Insight into functional microorganisms in wet–dry conversion to alleviate the toxicity of chromium fractions in red soil

Soil contamination with potentially toxic element such as chromium (Cr) poses a threat to the environment and human health. The environmental toxicity of Cr is related not only to the total Cr content but also to the distribution of Cr fractions. In this study, laboratory simulation experiments were conducted to explore the characteristics of Cr fractions and responses of the functional microbial community during dynamic leaching and static drying processes. The results showed that acid-soluble Cr and reducible Cr transformed into other relatively stable fractions under dry conditions, and ammonium nitrogen promoted the transformation. Nitrate-nitrogen was significantly positively correlated with Cr fractions in the wet stage (p < 0.05), while ammonium nitrogen showed the same relation in the dry process. Analysis of the microbial community showed that the bacterial and fungal genera Flavihumibacter, Altererythrobacter, Methylobacillus, Flavisolibacter, Lysobacter, and Cladosporium were related to the Cr fractions (acid-soluble Cr, reducible Cr, and oxidizable Cr) under wet conditions, while the microbial genera Ellin6067, MND1, and Ramlibacter were related to Cr fractions under dry conditions. Moreover, the proliferation of the functional microbial genera Methylobacillus, Ellin6067, and MND1 related to Cr fractions in the wet–dry conversion process alleviated the environmental toxicity of Cr. These findings provide useful information for the remediation of Cr-contaminated soils by monitoring the distribution fractions of Cr and the functional microbial community under wet–dry conditions.

Microbial community structures and their driving factors in a typical gathering area of antimony mining and smelting in South China

This study investigated soil microbial community in a typical gathering area of antimony mining and smelting in South China. The physical and chemical properties of different soils (mining waste dumps, flotation tailings, and smelting slag) and depths (0-20 cm, 40-60 cm, and 80-100 cm) were compared. The results showed that antimony (Sb) and arsenic (As) were the main pollutants, and their concentrations were 5524.7 mg/kg and 3433.7 mg/kg, respectively. Xanthates were found in the flotation tailings and smelting slag, and the highest concentration was 585.1 mg/kg. The microbial communities were analyzed by high-throughput sequencing, and it was shown that Proteobacteria, Acidobacteria, Chlorobacterium, Bacteroides, and Actinomycetes were the dominant taxa at the phylum level. There were obvious differences in microbial community structure in different sites. The dominant microorganism in the mining site was Chujaibacter. Subgroup_2_unclassified and Gemmatimonadaceae_unclassified were the prevalent microorganisms in the flotation and smelting sites, respectively. As, Sb, and xanthates were the main factors affecting the diversity and composition of bacteria in the flotation tailings and smelting slag areas. Therefore, this study provides experimental guidance and a theoretical basis for soil antimony pollution quality assessment, biological treatment, and environmental remediation.

Heavy metals speciation and distribution of microbial communities in sediments from the abandoned Mo-Ni polymetallic mines, southwest of China

Chemical fractions of heavy metals (Mo, Ni, Cu, Zn, Fe, Mn, Pb, Cd, and Cr) and compositions of bacteria and fungi in surface sediments from the Mo-Ni polymetallic mine area were analyzed. The results indicated that the mean concentrations of Mo, Ni, Cu, Zn, and Cd were higher than their background values. The mean percentage of Cr in residual fraction was much higher than that of other heavy metals. Mo, Cu, Zn, Fe, and Pb were mainly associated with oxidizable fraction. The dominant proportions of Mn and Cd were found in exchangeable fraction with mean percentages of 93.46% and 54.50%, respectively. According to RAC classification and potential ecological risk index (PERI), the Cd with high bioavailability had a very high environmental risk. The MisSeq sequencing results of bacteria and fungi revealed that microbial communities discrepantly respond to different sampling sites. The most abundant phylum of bacteria and fungi were Proteobacteria and Ascomycota, respectively. The bioavailable heavy metals including Mo-B, Pb-B, and Cd-B were recognized to have important influences on both dominant bacterial and fungal communities. The present study manifested that the bioavailability of heavy metal is very important to assess the potential environmental risk and plays a key role in shaping microbial structure.

Ecological responses of bacterial assembly and functions to steep Cd gradient in a typical Cd-contaminated farmland ecosystem

The response of soil bacterial communities from farmland ecosystems to cadmium (Cd) pollution, in which a steep concentration gradient of more than 100 mg/kg has naturally formed, has not previously been fully reported. In this study, a field investigation was conducted in a typical severe Cd-polluted farmland ecosystem, and the bacterial community response to the steep Cd gradient was analyzed. The results showed that Cd concentration sharply decreased from 159.2 mg/kg to 4.18 mg/kg among four sampling sites alongside an irrigation canal over a distance of 150 m. Bacterial diversity and richness were significantly lower in highly polluted sites, and random forest analysis indicated that Cd gradient played a decisive role in reducing alpha diversity. Redundancy analysis (RDA) and co-occurrence network indicated that the synergistic effects of pH, Cd, and phosphorus were the main drivers shaping community structure. The functional results predicted by BugBase suggested that the bacterial community may adapt to the harsh environment by recruiting Cd-resistant microbes and improving oxidative stress tolerance of the whole community. Cd-resistant microorganisms such as Burkholderia, Bradyrhizobium, and Sulfurifustis, which directly or indirectly participate in diminishing oxidative damage of Cd, may play essential roles in maintaining community stability and might be potential bacterial resources for the bioremediation of Cd pollution.

Vertical distribution of the toxic metal(loid)s chemical fraction and microbial community in waste heap at a nonferrous metal mining site

Over the past few decades, nonferrous mining has produced numerous waste rock and part of the waste that has not been properly treated was generally dumped at roadsides and hill slopes. However, the vertical distributions of toxic metal(loid)s and composition of microbial communities in waste heap and the under-laid pristine soil are rarely studied. In this work, the fraction-related distributions of toxic metal(loid)s were investigated at a waste heap profile and the indigenous microbial assemblages were also analyzed by Illumina sequencing of 16 s rRNA genes. Results showed that compared to the under-laid pristine soil, content of toxic metal(loid)s, especially Cd, As and Pb, in waste rock layer were higher. Most of As in subsoil existed as non-specifically sorbed and specifically-sorbed fractions, which could be ascribed to the migration from the upper layer. The mobility was significantly correlated with Eh, EC, clay content, CEC and the total content of metal(loid)s. Phyla Proteobacteria, Acidobacteria and Firmicutes dominated the microbial communities. The microbial community compositions at the genus level were similar, but their relative abundances were mainly influenced by pH, CEC, Eh, SOM, and bioavailability content of toxic metal(loid)s. Besides, microbial functions of elements (S, Fe, Mn and As) oxidation/reduction and metabolites (siderophore, biosurfactant, organic acid, phosphatase and urease) potentially were used for pollutants bioremediation.

Changes in the Composition of the Soil Bacterial Community in Heavy Metal-Contaminated Farmland

The structural changes of microorganisms in soil are the focus of soil indicators research. The purpose of this study was to investigate the changes in the composition of the soil bacterial community in heavy metal-contaminated soil. A total of six soil samples (two sampling times) were collected from contaminated farmland at three different depths (surface, middle, and deep layer). The pH value was measured. The concentrations of heavy metals (Cr, Ni, Cu, Zn, Cd, and Pb) and the soil bacterial community were analyzed using ICP-OES and 16S rRNA gene sequencing. The results of the two samplings showed that the pH value in the deep layer decreased from 6.88 to 6.23, and the concentrations of Cu, Zn, Cd, and Pb, with a smaller ion radius, increased by 16-28%, and Shannon, Chao1 increased by ~13%. The bacteria community composition at the three depths changed, but Proteobacteria, Acidobacteria, and Actinobacteria were the dominant phyla. In the copper and zinc tolerance test, the isolated bacterium that was able to tolerate copper and zinc was Bacillus sp. We found that, the longer the heavy metal pollution was of concern, the higher the tolerance. These results can be used as references for the microbial remediation of heavy metal-contaminated farmland.

The Response of the Soil Microbiome to Contamination with Cadmium, Cobalt and Nickel in Soil Sown with Brassica napus

Soil fertility is determined by biological diversity at all levels of life, from genes to entire biocenoses. The aim of this study was to evaluate bacterial diversity in soil contaminated with Cd2+, Co2+ and Ni2+ and sown with Brassica napus. This is an important consideration because soil-dwelling microorganisms support phytoremediation and minimize the adverse effects of heavy metals on the environment. Microbial counts, the influence (IFHM) of Cd2+, Co2+ and Ni2+ on microorganisms, the colony development (CD) index, the ecophysiological diversity (EP) index and genetic diversity of bacteria were determined under controlled conditions. Soil contamination with Cd2+, Co2+ and Ni2+ significantly influenced microbial diversity and increased the values of CD and EP indices. The tested heavy metals decreased the genetic diversity of bacteria, in particular in the phyla Actinobacteria and Proteobacteria. Bacteria of the genera Arthrobacter, Devosia, Kaistobacter, Paenibacillus, Phycicoccus, Rhodoplanes and Thermomonas were identified in both contaminated and non-contaminated soil. These bacteria are highly resistant to soil contamination with Cd2+, Co2+ and Ni2+.