Comparison of arsenic and antimony biogeochemical behavior in water, soil and tailings from Xikuangshan, China

Factors driving antimony accumulation in soil-pakchoi and wheat agroecosystems: Insights and predictive models

The accumulation of antimony (Sb) in plants and its potential effects on human health are of increasing concern. Nevertheless, only a few countries or regions have established soil Sb thresholds for agricultural purposes, and soil properties have not been taken into account. This study investigated the accumulation of Sb in the edible parts of pakchoi and wheat grain by adding exogenous Sb to 21 soils with varying properties. The results revealed a positive correlation between bioavailable Sb (Sbava, extracted by 0.1 M K2HPO4) in soil and Sb in the edible parts of pakchoi (R2 = 0.77, p < 0.05) and wheat grain (R2 = 0.54, p < 0.05). Both machine learning and traditional multiple regression analysis indicated Sbava was the most critical feature and the main soil properties that contributed to Sb uptake by pakchoi and wheat were CaCO3 and clay, respectively. The advisory food limits for Sb in pakchoi and wheat were estimated based on health risk assessment, and used to derive soil thresholds for safe pakchoi and wheat production based on Sbtot and Sbava, respectively. These findings hold potential for predicting Sb uptake by crops with different soil properties and informing safe production management strategies.

Sb/As immobilization and soil function improvement under the combined remediation strategy of modified biochar and Sb-oxidizing bacteria at a smelting site

Antimony (Sb) and arsenic (As) lead to soil pollution and structural degradation at Sb smelting sites. However, most sites focus solely on Sb/As immobilization, neglecting the restoration of soil functionality. Here, we investigated the effectiveness of Fe/H2O2 modified biochar (Fe@H2O2-BC) and Sb-oxidizing bacteria (Bacillus sp. S3) in immobilizing Sb/As and enhancing soil functional resilience at an Sb smelting site. Over a twelve-month period, the leaching toxicity of As and Sb was reduced to 0.05 and 0.005 mg L-1 (GB3838-2002) respectively, with 1% (w/w) Fe@H2O2-BC and 2% (v/v) Bacillus sp. S3 solution. Compared to CK, the combination of Fe@H2O2-BC and Bacillus sp. S3 significantly reduced the bioavailable As/Sb by 98.00%/93.52%, whilst increasing residual As and reducible Sb fractions by 210.31% and 96.51%, respectively. The combined application generally improved soil aggregate structure, pore characteristics, and water-holding capacity. Fe@H2O2-BC served as a pH buffer and long-term reservoir of organic carbon, changing the availability of carbon substrates to bacteria. The inoculation of Bacillus sp. S3 facilitated the transformation of Sb(III)/As(III) to Sb(V)/As(V) and differentiated the composition and functional roles of bacterial communities in soils. The combination increased the abundance of soil saprotrophs by 164.20%, whilst improving the relative abundance of N- and S-cycling bacteria according to FUNGuild and FAPROTAX analysis. These results revealed that the integrated application was instrumental in As/Sb detoxification/immobilization and soil function restoration, which demonstrating a promising microbially-driven ecological restoration strategy at Sb smelting sites.

Comprehensive insight into mercury contamination in atmospheric, terrestrial and aquatic ecosystems surrounding a typical antimony-coal mining district

This study comprehensively investigated mercury (Hg) contents of various environmental compartments in a typical antimony-coal mining area with intensive industrial activities over the past 120 years to analyze Hg environmental behaviors and evaluate Hg risks. The total mercury (THg) contents in river water, sediments, soils, PM10, dust falls, vegetables and corns were 1.16 ± 0.63 µg/L, 2.01 ± 1.64 mg/kg, 1.87 ± 3.88 mg/kg, 7.87 ± 18.68 ng/m3, 13.01 ± 14.53 mg/kg, 0.30 ± 0.34 mg/kg and 3.11 ± 0.51 µg/kg, respectively. The δ202Hg values in soils and dust falls were - 1.58 ∼ 0.12‰ and 0.25 ∼ 0.30‰, respectively. Environmental samples affected by industrial activities in the Xikuangshan (XKS) presented higher THg and δ202Hg values. Binary mixing model proved that atmospheric deposition with considerable Hg deposition flux (0.44 ∼ 6.40, 3.12 ± 2.20 mg/m2/y) in the XKS significantly contributed to Hg accumulations on surface soils. Compared with soils, sediments with more frequent paths and higher burst probabilities presented higher dynamic Hg risks. Children were faced higher health risk of multiple Hg exposure than adults. Furthermore, the health risk of THg by consuming leaf vegetables deserved more attention. These findings provided scientific basis for managing Hg contamination.

Influence of dissolved organic matter with different molecular weight from chicken manure on ferrihydrite adsorption and re-release of antimony(V)

Applying organic fertilizer is the main way to enhance soil fertility through the interfacial reaction between mineral and dissolved organic matter (DOM). However, the interfacial reaction between minerals and DOM may influence antimony(V) (Sb(V)) mobility in agricultural soils around antimony mines. In our study the ferrihydrite (Fh) was chosen as a representative mineral, to reveal the effect of its interaction with chicken manure organic fertilizer (CM-DOM) with Fh on Sb(V) migration. In this study, we investigated different organic matter molecular weights and C/Fe molar ratios. Our findings indicated that the addition of CM-DOM decreased the adsorption of Sb(V) by Fh and promoted the re-release of Sb(V) adsorbed on Fh. This effect was enhanced by increasing the C/Fe molar ratio. Fh mainly affects its interaction with Sb(V) through electrostatic gravitational interaction and ligand exchange, but the presence of CM-DOM weakens the electrostatic interaction between Fh and Sb(V) as well as competes with Sb(V) for the hydroxyl reactive site on Fh surface. In addition, the smaller molecular weight fraction (<10 kDa) of CM-DOM has higher aromaticity and hydrophobicity, which potentially leads to more intense competition with Sb(V) for the reaction sites on Fh. Therefore, the application of organic fertilizer may promote Sb(V) migration, posing significant risks to soil ecosystems and human health, which should be a concern in field soil cultivation.

Response strategies of slash pine (Pinus elliottii) to cadmium stress and the gain effects of inoculation with Herbaspirillum sp. YTG72 in alleviating phytotoxicity and enhancing accumulation of cadmium

Phytoremediation using fast-growing woody plants assisted by plant growth-promoting bacteria (PGPB) on cadmium (Cd)-contaminated sites is considered a promising technique; however, its remediation efficiency is still affected by multiple factors. In this study, the mining areas' soil conditions were simulated with different Cd addition levels (0, 3, 6, 9 mg kg-1) in order to investigate the response strategy to Cd stress of fast-growing economic tree species, slash pine (Pinus elliottii), and the effects of inoculation with the PGPB strain Herbaspirillum sp. YTG72 on the physiological activity and Cd accumulation of plants. The main results showed that there were significant (p < 0.05) increases in contents of chlorophyll and nutrient elements (P, K, Ca, and Mg) at low Cd addition level (3 mg kg-1) compared to non-Cd addition treatment. When the additive amount of Cd increased, the growth of plants was severely inhibited and the content of proline was increased, as well as Cd in plants. Besides, the ratios of K:P, Ca:P, and Mg:P in plants were negatively correlated with the contents of Cd in plants and soils. Inoculation of P. elliottii with the PGPB strain Herbaspirillum sp. YTG72 improved the physiological functions of the plants under Cd stress and activated the antioxidant system, reduced the accumulation of proline, and decreased the ratios of K:P, Ca:P, and Mg:P in plant. More importantly, planting P. elliottii in Cd-contaminated soil could significantly (p < 0.05) reduce the Cd content in the rhizosphere soil, and furthermore, inoculation treatment could promote the reduction of soil Cd content and increased the accumulation of Cd by root. The results of the present study emphasized the Cd response mechanism of P. elliottii based on multifaceted regulation, as well as the feasibility of strain Herbaspirillum sp. YTG72 assisted P. elliottii for the remediation on Cd-contaminated sites.

Selenium increases antimony uptake in ramie (Boehmeria nivea L.) by enhancing the physiological, antioxidative, and ionomic mechanisms

Ramie (Boehmeria nivea L.) is a promising phytoremediation candidate due to its high tolerance and enrichment capacity for antimony (Sb). However, challenges arise as Sb accumulated mainly in roots, complicating soil extraction. Under severe Sb contamination, the growth of ramie may be inhibited. Strategies are needed to enhance Sb accumulation in ramie's aboveground parts and improve tolerance to Sb stress. Considering the beneficial effects of selenium (Se) on plant growth and enhancing resistance to abiotic stresses, this study aimed to investigate the potential use of Se in enhancing Sb uptake by ramie. We investigated the effects of Se (0.5, 1, 2, 5, or 10 μM) on ramie growth, Sb uptake and speciation, antioxidant responses, and ionomic profiling in ramie under 10 mg/L of SbIII or antimonate (SbV) stresses. Results revealed that the addition of 0.5 μM Se significantly increased shoot biomass by 75.73% under SbIII stress but showed minimal effects on shoot and root length in both SbIII and SbV treatments. Under SbIII stress, 2 μM Se significantly enhanced Sb concentrations by 48.42% in roots and 62.88% in leaves. In the case of SbV exposure, 10 μM Se increased Sb content in roots by 42.57%, and 1 μM Se led to a 91.74% increase in leaves. The speciation analysis suggested that Se promoted the oxidation of SbIII to less toxic SbV to mitigate Sb toxicity. Additionally, Se addition effectively minimized the excess reactive oxygen species produced by Sb exposure, with the lowest malondialdehyde (MDA) content at 0.5 μM Se under SbIII and 2 μM Se under SbV, by activating antioxidant enzymes including superoxide dismutase, catalase, peroxidase, and glutathione peroxidase. Ionomic analysis revealed that Se helped in maintaining the homeostasis of certain nutrient elements, including magnesium, potassium (K), calcium (Ca), iron (Fe), and copper (Cu) in the SbIII-treated roots and K and manganese (Mg) in the SbV-treated roots. The results suggest that low concentrations of Se can be employed to enhance the phytoremediation of Sb-contaminated soils using ramie.

Comparison of antimony and arsenic behaviour at the river-lake junction in the middle of the Yangtze River Basin

As typical metalloid toxic elements widely distributed in environmental media, the geochemical behaviour of antimony (Sb) and arsenic (As) affects environmental safety. We selected the surface waters and sediments at the river-lake junction of Dongting Lake as the research objects, analysed the concentration and chemical partitioning of Sb and As, assessed its contamination and ecological risk levels, and discussed its sources and potential influencing factors. The concentrations of dissolved Sb and As in surface waters were low (< 5.46 µg/L), and the concentrations of Sb and As in surface sediments were 2.49-22.65 mg/kg and 11.10-136.34 mg/kg, respectively. Antimony and As in sediments were mainly enriched in the fraction of residues, but the proportion of As in bioavailability was significantly higher than that of Sb. Although the contamination level of Sb was higher than that of As, the risk assessment code (RAC) showed that the ecological risk level of As was higher than that of Sb. Rainwater erosion and mining activities (in the midstream of Zijiang River) were the main contaminated sources of Sb, while As was affect mainly by rainwater erosion. The contamination and ecological risk of Sb in the inlet of the Zijiang River should receive considerable attention, while those of As in the inlet of the Xiangjiang River should also be seriously considered. This study highlights the need for multi-index-based assessments of contamination and ecological risk and the importance of further studies on the environmental behaviour of metalloids in specific hydrological conditions, such as river-lake junctions.

Washing antimony and arsenic from agricultural soil with eco-friendly organic acids and the relevant bioavailability assessment

Antimony (Sb) and arsenic (As) contamination in agricultural soil poses human health risks through agricultural products. Soil washing with degradable low molecular weight organic acids (LMWOAs) is an eco-friendly strategy to remediate agricultural soils. In this study, three eco-friendly LMWOAs, oxalic acid (OA), tartaric acid (TA), and citric acid (CA), were used to treat Sb and As co-contaminated agricultural soil from Xikuangshan mine area. The OA, TA, and CA washed out 18.4, 16.8, and 26.6% of Sb and 15.3, 19.9, and 23.8% of As from the agricultural soil, with CA being the most efficient reagent for the soil washing. These organic acids also led to pH decline and macronutrients losses. Fraction analysis using a sequential extraction procedure showed that the three organic acids targeted and decreased the specifically sorbed (F2) (by 19.3-37.6% and 2.41-23.5%), amorphous iron oxide associated (F3) (by 49.1-61.2% and 51.2-70.2%), and crystallized iron oxide associated (F4) (by 12.3-26.0% and 26.1-29.1%) Sb and As. The leachability of Sb and As, as well as their concentrations and bioconcentration factor (BCF) in vegetables reduced due to the soil washing. It demonstrated that the bioavailability of both the elements was decreased by the organic acids washing. The concentrations of Sb and As in typical vegetable species cultivated in CA washed soil were less than the threshold value for consumption safety, while those in OA and TA washed soils were still higher than the value, suggesting that only CA is a potential washing reagent in soil washing for Sb- and As-contaminated agricultural soil.

Archaea are better adapted to antimony stress than their bacterial counterparts in Xikuangshan groundwater

Archaea are important ecological components of microbial communities in various environments, but are currently poorly investigated in antimony (Sb) contaminated groundwater particularly on their ecological differences in comparison with bacteria. To address this issue, groundwater samples were collected from Xikuangshan aquifer along an Sb gradient and subjected to 16S rRNA gene amplicon sequencing and bioinformatic analysis. The results demonstrated that bacterial communities were more susceptibly affected by elevated Sb concentration than their archaeal counterparts, and the positive stimulation of Sb concentration on bacterial diversity coincided with the intermediate disturbance hypothesis. Overall, the balance of environmental variables (Sb, pH, and EC), competitive interactions, and stochastic events jointly regulated bacterial and archaeal communities. Linear fitting analysis revealed that Sb significantly drove the deterministic process (heterogeneous selection) of bacterial communities, whereas stochastic process (dispersal limitation) contributed more to archaeal community assembly. In contract, the assembly of Sb-resistant bacteria and archaea was dominated by the stochastic process (undominated), which implied the important role of diversification and drift instead of selection. Compared with Sb-resistant microorganisms, bacterial and archaeal communities showed lower niche width, which may result from the constraints of Sb concentration and competitive interaction. Moreover, Sb-resistant archaea had a higher niche than that of Sb-resistant bacteria via investing on flexible metabolic pathways such as organic metabolism, ammonia oxidation; and carbon fixation to enhance their competitiveness. Our results offered new insights into the ecological adaptation mechanisms of bacteria and archaea in Sb-contaminated groundwater.

In-situ immobilization of arsenic and antimony containing acid mine drainage through chemically forming layered double hydroxides

Acid mine drainage (AMD) rich in arsenic (As) and antimony (Sb) is considered as a significant environmental challenge internationally. However, simultaneous removal of As and Sb from AMD is still inadequately studied. In this study, a highly effective and simple approach was proposed for mitigating As and Sb-rich AMD, which involves in-situ formation of layered double hydroxides (LDHs). Following the treatment, the residual concentrations of iron (Fe), magnesium (Mg), sulfate, As and Sb in field AMD were decreased from their initial concentrations of 1690, 1524, 2055, 7.8 and 10.6 mg L-1, respectively, to 1.3, 12.4, 623, 0.006 and 0.004 mg L-1, respectively. Chemical formula of the resulting As and Sb-loaded LDHs can be identified as Mg4.226Fe2.024OH2SO4AsSb0.006∙mH2O. The dissolution rates of metal(loid)s in As and Sb-loaded LDH were lower than 1% under strongly acidic and alkaline environments. In presence of the mixed adsorbates, the As immobilization capacity by LDHs was significantly decreased, with an apparent intervention from Sb. However, As did not have a significant effect on the immobilization of Sb by LDH. As was immobilized by LDHs through anion exchange and complexation with -OH groups, while Sb was captured by anion exchange and complexation with [Formula: see text] . Density functional theory (DFT) calculations further proved the above conclusions. This novel approach is effective and can be applied for in-situ AMD treatment from abandoned mines.

Sources of antimony contamination and its migration into water systems of Xikuangshan, China: Evidence from hydrogeochemical and stable isotope (H, O, S, and Sr) signatures

The Xikuangshan (XKS) mine was selected for a comprehensive Sb-related hydrogeochemical study because of its significant Sb contamination in water systems. Hydrochemical data, specifically multi-isotope (H, O, S, and Sr) data, were conducted to elucidate the primary sources and migration processes of Sb responsible for water system contamination. At the XKS Sb mine, water is near-neutral to alkaline and is characterized by high concentrations of SO42- and Sb. Sb occurs as Sb(OH)6- (the dominant species) in these oxidized waters. The hydrochemistry is mainly controlled by carbonate dissolution and silicate weathering. δ2HH2O and δ18OH2O values indicate that the infiltration recharge of mine water and mining activities regulate the migration of Sb in groundwater. δ34SSO4 and δ18OSO4 values indicate that dissolved SO42- and Sb primarily come from stibnite oxidation, bacterial SO42- reduction has either not occurred or is extremely weak, and the reductive dissolution of Fe (III) hydroxides does not significantly affect Sb migration in water. The 87Sr/86Sr ratios further indicate that the discharge of solid mine wastes leaching and smelting water is a crucial source of Sb contamination in groundwater. In addition, the relationship between δ34SSO4 and δ87Sr values suggests the complexity of the contamination source and migration of Sb in water. Finally, a robust conceptual hydrogeochemical model was developed using isotopic tools in combination with detailed hydrogeological and hydrochemistry characterization to describe the contamination source and migration of Sb in water systems at the XKS Sb mine.

Effects of Transporter Inhibitors and Chemical Analogs on the Uptake of Antimonite and Antimonate by Boehmeria nivea L

Antimony (Sb) is a non-essential metalloid that can be taken up by plants from contaminated soils and thus enter the food chain and threaten human health. Boehmeria nivea L. (ramie) is a promising phytoremediation plant for Sb-polluted soils. However, the mechanisms of antimonite (SbIII) and antimonate (SbV) uptake by ramie remain unclear. In this study, a hydroponic system was established to investigate how different substances affect the uptake of SbIII or SbV by ramie, including an energy inhibitor (malonic acid), an aquaglyceroporin inhibitor (silver nitrate), an SbV analog (phosphate-PV), and SbIII analogs (arsenite-AsIII, glycerol, silicic acid-Si, and glucose). The results indicated that ramie primarily transported Sb by increasing the Sb concentration in the bleeding sap, rather than increasing the weight of the bleeding sap. After 16 h of Sb exposure, the absolute amount of transported Sb from the roots to the aboveground parts was 1.90 times higher under SbIII than under SbV. The addition of malonic acid significantly inhibited the uptake of SbV but had limited effects on SbIII, indicating that SbV uptake was energy dependent. PV addition significantly reduced SbV uptake, while the addition of AsIII, glycerol, and Si obviously inhibited SbIII uptake. This suggested that the uptake of SbV might be via low-affinity P transporters and SbIII might use aquaglyceroporins. These findings deepen the understanding of Sb uptake pathways in ramie, contribute to a better comprehension of Sb toxicity mechanisms in ramie, and establish a foundation for identifying the most effective Sb uptake pathways, which could further improve the efficiency of phytoremediation of Sb-polluted soils.

Effects of microorganisms on the migration and transformation of typical heavy metal (loid)s in mercury-thallium mining waste slag during the combined application of fish manure and natural minerals

Mercury-thallium mining waste slag has the characteristics of extremely acidic, low fertility and highly toxic polymetallic composite pollution, making it difficult to be treated. We use nitrogen- and phosphorus-rich natural organic matter (fish manure) and calcium- and phosphorus-rich natural minerals (carbonate and phosphate tailings) individually or in combination to amend the slag, analyze their effects on the migration and transformation of potentially toxic elements (Tl and As) in the waste slag. We set up sterile and non-sterile treatments specifically to further investigate the direct or indirect effect of microorganisms attached to added organic matter on Tl and As. The results showed that addition of fish manure and natural minerals to the non-sterile treatments promoted the release of As and Tl, resulting in an increase in As and Tl concentrations in the tailing lixiviums from 0.57 to 2.38-6.37 μg/L and from 69.92 to 107.51-157.21 μg/L, respectively. Sterile treatments promoted the release of As (from 0.28 to 49.88-104.18 μg/L) and inhibited the release of Tl (from 94.53 to 27.60-34.50 μg/L). Use of fish manure and natural minerals alone or in combination significantly reduced the biotoxicity of the mining waste slag, in which the combination was more efficient. XRD analysis showed that microorganisms in the medium promoted the dissolution of jarosite and other minerals, which indicated that the release and migration of As and Tl in Hg-Tl mining waste slag were closely related to microbial activities. Furthermore, metagenomic sequencing revealed that microorganisms such as Prevotella, Bacteroides, Geobacter, and Azospira, which were abundant in the non-sterile treatments, had remarkable resistance to a variety of highly toxic heavy metals and could affect the dissolution of minerals and the release and migration of heavy metals through redox reactions. Our results may aid in the rapid soilless ecological restoration of related large multi-metal waste slag dumps.

Assessment of industrial by-products as amendments to stabilize antimony mine wastes

The spread of antimony from mine wastes to the environment represents a matter of great concern due to its adverse effects on impacted ecosystems. There is an urgent need for developing and adopting sustainable and inexpensive measures to deal with this type of wastes. In this study the Sb leaching behavior of mine waste rocks and mine tailings derived from the exploitation of Sb ore deposits was characterized using standard batch leaching tests (TCLP and EN-12457-4) and column leaching essays. Accordingly, these mine wastes were characterized as toxic (>0.6 mg Sb L^-1) and not acceptable at hazardous waste landfills (>5 mg Sb kg^-1), showing also an ongoing Sb release under prolonged leaching conditions. Two industrial by-products were evaluated as amendments to stabilize them, namely deferrisation sludge (DFS) and a by-product derived from the treatment of aluminum salt slags (BP-Al). Mine wastes were amended with different doses (0-25%) of DFS or BP-Al and the performance of these treatments was evaluated employing also batch and column leaching procedures. The effectiveness of DFS to immobilize Sb was much higher than that exhibited by BP-Al. Thus, treatments with 25% BP-Al showed Sb immobilization levels of approximately 33-53%, whereas treatments with 5 and 25% DFS already attained Sb immobilization levels up to approximately 80-90 and 90-99%, respectively. Mine tailings amended with 5% DFS and mine waste rocks amended with 25% DFS decreased their leachable Sb contents below the limit for non-hazardous waste landfill acceptance (<0.7 mg Sb kg^-1). Likewise, these DFS treatments were able to revert their toxic characterization. Moreover, the 25% DFS treatment showed to be a long-lasting stabilizing system, efficient at least during a leaching period equivalent to 10-year rainfall with a great Sb leaching reduction (close to 98%). After this long-term leaching process, DFS-treated mine wastes kept their non-hazardous and non-toxic characterization. The amorphous Fe (oxyhydr)oxides composing DFS were responsible for the important Sb removal capacity showed by this by-product. Thus, when DFS was applied to mine wastes mobile Sb was importantly fixed as non-desorbable Sb, showing also a considerable Sb removal capacity in presence of strong competing anions such as phosphate. The application of DFS as amendment presents a great potential to be used as a sustainable long-term stabilizing system of Sb mine wastes.

Soil microbial responses to simultaneous contamination of antimony and arsenic in the surrounding area of an abandoned antimony smelter in Southwest China

Soil contamination with heavy metal(loid)s may influence microbial activities in the soil, and consequently jeopardize soil health. Microbial responses to soil contamination play an important role in ecological risk assessment. This study investigated the effect of heavy metal(loid)s contamination on microbial community structure and abundance in the surrounding soil of an abandoned antimony (Sb) smelter in Qinglong county, Guizhou province, Southwest China. A total of 46 soil samples were collected from ten sampling sites (labelled as A-I, and CK) across the study area at depths of 0-2, 2-10, 10-20, 20-30, 30-40, and 40-50 cm. The soil samples were analyzed for total and bioavailable heavy metal(loid) concentrations, bacterial, fungal, and archaeal community structures, diversities, and functions, together with soil basic physicochemical properties. Much greater ecological risk of Sb and arsenic (As) was present in the surface soil (0-2 cm) compared to that in the subsoils. The activities of dominant microorganisms tended to be associated with soil pH and heavy metal(loid)s (i.e., Sb, As, lead (Pb), cadmium (Cd), and chromium (Cr)). Bacteria associated with IMCC26256, Rhizobiales, Burkholderiales, and Gaiellales, and archaea associated with Methanocellales were estimated to be tolerant to high concentrations of Sb and As in the soil. In addition, the magnitude of soil microbial responses to Sb and As contamination was in the order of archaea > bacteria > fungi. In contrast to the negligible response of fungi and negative response of bacteria to Sb and As contamination, there was a strongly positive correlation between archaeal activity and total Sb and As concentrations in the soil. Our findings provide a theoretical basis for the remediation of Sb smelter-affected soil.

Elevated antimony concentration stimulates rare taxa of potential autotrophic bacteria in the Xikuangshan groundwater

Microbial communities composed of few abundant and many rare species are widely involved in the biogeochemical cycles of elements. Yet little is known about the ecological roles of rare taxa in antimony (Sb) contaminated groundwater. Groundwater samples were collected along an Sb concentration gradient in the Xikuangshan antimony mine area and subjected to high through-put sequencing of 16S rRNA genes to investigate the bacterial communities. Results suggested that both abundant and rare sub-communities were dominated by Betaproteobacteria, Gammaproteobacteria, and Alphaproteobacteria, whereas rare sub-communities showed higher alpha-diversities. Multivariate analysis showed that both the abundant and rare taxa were under the stress of Sb, but the impact on rare taxa was greater. Nitrate explained a large part for the variation of the abundant sub-communities, indicating the critical role of nitrate for their activities under anoxic conditions. In contrast, bicarbonate significantly impacted rare sub-communities, suggesting their potential autotrophic characteristics. To further explore the role of rare taxa in the communities and the mechanism of affecting the community composition, a network was constructed to display the co-occurrence pattern of bacterial communities. The rare taxa contributed most of the network nodes and served as keystone species to maintain the stability of community. Abiotic factors (mainly Sb and pH) and bacterial interspecific interactions (interactions between keystone species and other bacterial groups) jointly affect the community dynamics. Functional prediction was performed to further reveal the ecological function of rare taxa in the Sb-disturbed groundwater environment. The results indicated that the rare taxa harbored much more diverse functions than their abundant counterparts. Notably, elevated Sb concentration promoted some potential autotrophic functions in rare taxa such as the oxidation of S-, N-, and Fe(II)-compounds. These results offer new insights into the roles of rare species in elemental cycles in the Sb-impacted groundwater.

Antimony (Sb) isotopic signature in water systems from the world's largest Sb mine, central China: Novel insights to trace Sb source and mobilization

The Xikuangshan (XKS) mine, the world's largest antimony (Sb) mine, was chosen for a detailed Sb isotopic signature study owing to its historical Sb contamination of water systems. Hydrochemical data, in particularδ¹²³Sb values, were analyzed to identify the Sb source and predominant geochemical processes that affect Sb mobilization in different waters. The δ¹²³Sb values of waters from the XKS Sb mine range from - 0.20‰ to + 0.73‰. In particular, the δ¹²³Sb values of the main Feishuiyan stream do not significantly vary (+0.19‰-+0.24‰), while those of groundwater in different aquifers (-0.08‰ to +0.73‰) and mine water in different adits (-0.20‰ to +0.37‰) vary over a wide range. The relationships between δ¹²³Sb values and Sb concentrations indicate that a simple dilution of Sb and a weak Sb adsorption onto Fe/Mn suspended particles and sediments in the Feishuiyan stream may occur, oxidative weathering and leaching infiltration of Sb-containing waste rocks and slags may cause variations in the δ¹²³Sb values in groundwater, and Sb mobilization in the mine water is influenced by a combination of processes (oxidative dissolution, adsorption of Fe/Mn (hydr)oxides, and mixing). A conceptual hydrogeochemical model was summarized to elucidate the Sb source and mobilization in water systems from the XKS Sb mine.

Geochemical impact of dissolved organic matter on antimony mobilization in shallow groundwater of the Xikuangshan antimony mine, Hunan Province, China

Dissolved organic matter (DOM) is widely used in aquatic systems to control the environmental fate of As. However, similar to the behavior of As, Sb mobilization driven by DOM is poorly understood. A total of 25 samples were collected from shallow groundwater in the Xikuangshan mine to compare the spectroscopic characteristics and chemical properties of DOM between high- and low-Sb groundwater and to determine the roles of DOM in Sb mobility. The concentrations of Sb and DOM varied from 0.003 to 18.402 mg/L (mean: 3.407 mg/L) and 0.38 to 9.90 mg/L (mean: 2.49 mg/L), respectively. The DOM of the D₃x⁴ water was primarily dominated by terrestrial and microbial humic-like and fulvic acid substances, with a relatively small contribution of tryptophan-like components. Complexing agents, competitive adsorption, and photopromoted oxidation under sunlight were considered as the formation mechanisms for DOM-controlled Sb(V)-dominated Sb species in D₃x⁴ water. The weakly alkaline and oxidizing conditions, and the presence of Fe hydroxides facilitated the promotion of Sb(V) concentration. The findings of this study further enhance our understanding of the Sb migration mechanism in oxic groundwater.

Chemolithotrophic Biological Nitrogen Fixation Fueled by Antimonite Oxidation May Be Widespread in Sb-Contaminated Habitats

Nitrogen (N) deficiency in mining-contaminated habitats usually hinders plant growth and thus hampers tailing revegetation. Biological N fixation (BNF) is an essential biogeochemical process that contributes to the initial accumulation of N in oligotrophic mining-contaminated regions. Previous studies reported that chemolithotrophic rather than heterotrophic diazotrophs frequently dominated in the mining-contaminated regions. Chemolithotrophic diazotrophs may utilize elements abundant in such habitats (e.g., sulfur (S), arsenic (As), and antimony (Sb)) as electron donors to fix N₂. BNF fueled by the oxidation of S and As has been detected in previous studies. However, BNF fueled by Sb(III) oxidation (Sb-dependent BNF) has never been reported. The current study observed the presence of Sb-dependent BNF in slurries inoculated from Sb-contaminated habitats across the South China Sb belt, suggesting that Sb-dependent BNF may be widespread in this region. DNA-stable isotope probing identified bacteria associated with Rhodocyclaceae and Rhizobiaceae as putative microorganisms responsible for Sb-dependent BNF. Furthermore, metagenomic-binning demonstrated that Rhodocyclaceae and Rhizobiaceae contained essential genes involved in Sb(III) oxidation, N₂ fixation, and carbon fixation, suggesting their genetic potential for Sb-dependent BNF. In addition, meta-analysis indicated that these bacteria are widespread among Sb-contaminated habitats with different niche preferences: Rhodocyclaceae was enriched in river sediments and tailings, while Rhizobiaceae was enriched only in soils. This study may broaden our fundamental understanding of N fixation in Sb-mining regions.

Occurrence, distribution, and migration of antimony in the Zijiang River around a superlarge antimony deposit zone

Under the environmental changes associated with mine tributaries entering mainstream rivers, differences in the distributions and migration behaviors of metal(loid)s can be found, but the behavior of antimony (Sb) is still poorly understood in this regard. We analyzed the occurrence, distribution, migration, and influencing factors of the Sb concentration in the water body of the Zijiang River (ZR) around a superlarge Sb deposit zone. The total Sb concentrations were 1.45-15.66 μg/L, 3.16-133.63 mg/kg, and 0.83-41.82 μg/L in the ZR surface waters, sediments, and pore waters, respectively; Sb(V) was the predominant form of Sb found in the surface waters. Mining and smelting were the main sources of Sb in the ZR. Spatially, the Sb concentration showed a decreasing trend from the tributaries to the ZR mainstream. In the ZR, the surface-water Sb concentration showed an increasing trend from the upstream to the downstream, while in the sediments and pore waters, the midstream Sb concentrations were higher than the upstream and downstream concentrations; this finding was related to the sediment retention and accumulation behaviors of reservoirs and dams resulting in the secondary release of Sb in sediments. Sb tended to be bound to the solid phase, dominated by amorphous iron (Fe)/aluminum (Al) oxides and calcium in sediments. This study highlights that, based on current Sb migration patterns, the accumulation of sediments carried by tributaries near Sb mines in the midstream ZR and the Sb pollution caused by sediment release will be long-term, and the related environmental consequences need to be further predicted.

In Vitro Bioaccessibility and Health Risk Assessment of Arsenic and Zinc Contaminated Soil Stabilized by Ferrous Sulfate: Effect of Different Dietary Components

Iron-based materials have good stability in reducing the mobility and toxicity of heavy metals, but the behavior and human health risks of heavy metals could be affected by dietary components. This study investigated the effect of typical diets (lettuce, cooked rice and apples) on the bioaccessibility and morphological changes of arsenic (As) and zinc (Zn) in contaminated site after stabilization by ferrous sulfate (FeSO4). The results showed that the bioaccessibility of As and Zn were increased in a co-digestion system of food. The augmented effect on As bioaccessibility mainly occurred in the gastric phase: apple > lettuce > cooked rice (p < 0.05), while the augmented effect on Zn bioaccessibility mainly occurred in the intestinal phase: lettuce > apple > cooked rice (p < 0.05). FeSO4 weakened the dissolution effect of dietary components on As bioaccessibility, and reduced As bioaccessibility in the gastric and intestinal phases by 34.0% and 37.9% (p < 0.05), respectively. Dietary components and Fe fractions influenced the speciation and distribution of As and Zn. FeSO4 reduced the hazard quotient (HQ) and carcinogenic risk (CR) values of the contaminated soil by 33.97% and 33.59%, respectively. This study provides a reference for a better understanding of more realistic strategies to modulate exposure risks of heavy metal-contaminated sites.

Response of soil protists to antimony and arsenic contamination

Microorganisms can mediate antimony (Sb) and arsenic (As) transformation and thus change their mobility and toxicity. Having similar geochemical behavior, Sb and As are generally considered to exert similar environmental pressure on microbiome. However, it needs further validation, especially for protists. In this study, the responses of protistan communities to Sb and As were investigated by collecting soils from Xikuangshan Sb mine and Shimen As mine in China. Antimony and As contamination taxonomically and functionally (consumer and phototroph) changed the alpha and beta diversities of protistan communities, but exerted different impacts on the parasitic community. Based on multiple statistical tools, As contamination had a greater impact on protistan communities than Sb. The ecological networks of highly contaminated sites were less complex but highly positively connected compared to less contaminated sites. High As contamination raised the ratio of consumers and decreased the ratio of phototrophs in ecological networks, while the opposite tendency was observed in Sb contaminated soils. High Sb and As contamination enriched different keystone taxa resistant to Sb and As. These results demonstrate that protistan community respond differently to Sb and As.

The leaching of antimony and arsenic by simulated acid rain in three soil types from the world's largest antimony mine area

A simulated acid rain (SAR) experiment on leaching of antimony (Sb) and arsenic (As) in three soil types including paddy soils (PS), vegetable soils (VS) and slag based soils (SS) from Xikuangshan (XKS) Sb mine area was conducted. The SAR at pH 2.5, 3.5, 4.5 and 5.6 were sprayed to soil columns with intermittent pattern in a period of 50 days. Through the spraying duration, leaching Sb in PS, VS and SS showed decreasing trends regardless of pH values in SAR and were in the ranges of 0.026-0.064 mg L^-1, 0.19-2.18 mg L^-1 and 11.8-32.4 mg L^-1, respectively. By contrast, leaching As in these three soil types continuously increased at the initial five spraying times and then deeply decreased afterward, with ranges being 0-0.007 mg L^-1, 0.001-0.071 mg L^-1 and 0.17-1.07 mg L^-1, respectively. The leaching Sb in all the three soil types were extremely higher than the reference value in grade IV (0.01 mg L^-1) for groundwater quality of China (GB/T 14,848-2017). For leaching As, peck values in VS and all the values in SS were also greater than the corresponding reference value (0.05 mg L^-1). This indicated that leaching Sb and As could pollute the groundwater in XKS Sb mine area, especially those in slag based soils. The total leaching losses of Sb and As were affected by pH ambiguously, such as SAR at pH 2.5, 5.6 and 2.5 induced the greatest losses of Sb in PS, VS and SS, and pH 3.5, 5.6 and 2.5 resulted in the greatest leaching losses of As in these soils. After SAR treatment, the specific sorbed and Fe/Mn oxide-associated Sb and As significantly decreased. It demonstrated that these two fractions of both Sb and As were involved in leaching losses. The present study also found that the SAR treatment resulted in soil acidification in all the three soil types. In addition, available N, P and K in all the SAR treatments decreased regardless of pH values, except for available N and P in PS.

Evaluation of the effectiveness of ex-situ stabilization for arsenic and antimony contaminated soil: Short-term and long-term leaching characteristics

Ex-situ stabilization for As and Sb co-contaminated soil was conducted through an iron-based stabilizer, PFSC (a mixture of polymerized ferric sulfate (PFS) and hydrated lime (Ca(OH₂)) with a dry mass ratio of 2:1). After field aging for one week, the stabilized contaminated soil was subjected to a horizontal vibration leaching test (HJ 557), Wenzel's sequential extraction, and a semi-dynamic leaching test (ANS 16.1). By assessing the cumulative fractions of As and Sb, the observed diffusion coefficients (D^obs) and leachability indices (LX) of metalloids released from the soil specimens were calculated. The PFSC ex-situ stabilization was effective to immobilize metalloids, and the As and Sb leached concentrations of stabilized contaminated soil samples were lower than remediation targets. Nonspecifically bound As and Sb in the stabilized contaminated soil samples decreased from 4.5 - 9.2 % to 1.5-2.5 % and from 2.2 - 5.8 % to 1.1-1.5 %, respectively. The mechanisms controlling the leaching behaviors of As and Sb included wash-off and diffusion and they were changed with the leaching interval. The mean D^obs of As and Sb released from stabilized contaminated soil specimen were 3.46 × 10^-12 and 2.99 × 10^-13 cm² s^-1, in the which were two orders of magnitude lower than that of untreated contaminated soil specimen. The mean LX of stabilized contaminated soil specimen for As and Sb releases were 11.40 and 12.83, respectively, indicating that the stabilized contaminated soil was acceptable for "controlled utilization".

Spatial distribution of heavy metals in rice grains and human health risk assessment in Hunan Province, China

Rice is the main food in China, and its pollution by heavy metals has attracted widespread attention. In this study, rice grain samples were collected from 14 prefecture-level cities in Hunan Province, China. The contents of 9 heavy metals (i.e., As, Cr, Co, Ni, Cu, Zn, Cd, Pb, and Sb) were measured using graphite digestion-inductively coupled plasma mass spectrometry (ICP-MS). Pearson correlation analysis and principal component analysis were performed to evaluate the correlation among these heavy metals. In addition, ordinary kriging interpolation were applied to investigate the spatial distribution pattern of the heavy metals. Results showed that the average concentrations of these heavy metals were 0.48 (As), 1.28 (Cr), 0.03 (Co), 0.84 (Ni), 2.39 (Cu), 15.73 (Zn), 0.28 (Cd), 0.66 (Pb), and 0.0043 (Sb) mg/kg, respectively. The single-factor pollution index (SFPI) contamination assessment showed that As, Pb, Cr, Ni, and Cd accumulated significantly in the rice grain, with over-standard rates of 100%, 100%, 64.70%, 47.05%, and 44.12%, respectively. The Sb concentrations at the sampling sites were low, and there was no obvious pollution. Health risk assessment showed that the target hazard quotient followed the order of As> Cr> Cd> Pb> 1.0> Co> Cu> Zn> Ni> Sb, and the carcinogenic risk value was in the order of Cd> Ni> As> Cr> 1.0×10^-4> Pb. In particular, quick actions should be taken to regulate As, Cr, and Cd contents in rice because they posed greater non-carcinogenic and carcinogenic health risks than the others to the local residents.

Stimulated leaching of metalloids along 3D-printed fractured rock vadose zone

Fractured rock aquifers are susceptible to contamination, with metal(loid)s rapidly migrating from poorly developed overburden to the fractured rock vadose zone and thus into groundwater. Compared to typical porous aquifers, retention effects within the rock matrix are small, and rapid advection along fractures leads to a higher risk of groundwater contamination. However, the highly complex anisotropic pathways of natural fractures hinder research in this field. To construct reproducible fractures, this study used 3D printing following Computed X-ray Microtomography (μCT) scans of a fractured rock collected in a natural limestone aquifer. Stimulated metalloid release was observed in the fractured rock during column leaching, and the leachate concentrations of arsenic (As) and antimony (Sb) increased by up to 17.5 and 36.4 times, respectively, compared with the porous vadose zone. Fluctuations in fracture metalloid release patterns in dissolved and adsorbed phases were attributed to retention and filtration effects induced by soil particles within fractures. Geophysical properties of the porous overburden, especially the aggregation characteristics, greatly affected the non-equilibrium leaching behavior of As, but had a limited effect on the near-equilibrium leaching of Sb, which was explored by modifying the surficial soil layer with either montmorillonite clay or charcoal. The results of this study provide a novel method and useful information for modeling and risk assessment of fractured rock aquifers.

Antimony release and volatilization from rice paddy soils: Field and microcosm study

The fate of antimony (Sb) in submerged soils and the impact of common agricultural practices (e.g., manuring) on Sb release and volatilization is understudied. We investigated porewater Sb release and volatilization in the field and laboratory for three rice paddy soils. In the field study, the porewater Sb concentration (up to 107.1 μg L^-1) was associated with iron (Fe) at two sites, and with pH, Fe, manganese (Mn), and sulfate (SO₄^2-) at one site. The surface water Sb concentrations (up to 495.3 ± 113.7 μg L^-1) were up to 99 times higher than the regulatory values indicating a potential risk to aquaculture and rice agriculture. For the first time, volatile Sb was detected in rice paddy fields using a validated quantitative method (18.1 ± 5.2 to 217.9 ± 160.7 mg ha^-1 y^-1). We also investigated the influence of two common rice agriculture practices (flooding and manuring) on Sb release and volatilization in a 56-day microcosm experiment using the same soils from the field campaign. Flooding induced an immediate, but temporary, Sb release into the porewater that declined with SO₄^2-, indicating that SO₄^2- reduction may reduce porewater Sb concentrations. A secondary Sb release, corresponding to Fe reduction in the porewater, was observed in some of the microcosms. Our results suggest flooding-induced Sb release into rice paddy porewaters is temporary but relevant. Manuring the soils did not impact the porewater Sb concentration but did enhance Sb volatilization. Volatile Sb (159.6 ± 108.4 to 2237.5 ± 679.7 ng kg^-1 y^-1) was detected in most of the treatments and was correlated with the surface water Sb concentration. Our study indicates that Sb volatilization could be occurring at the soil-water interface or directly in the surface water and highlights that future works should investigate this potentially relevant mechanism.

Stabilization of arsenic and antimony Co-contaminated soil with an iron-based stabilizer: Assessment of strength, leaching and hydraulic properties and immobilization mechanisms

Soils with relatively high concentrations of arsenic (As) and antimony (Sb) in mining areas would impose significant risks to human health and ecosystem. A new stabilizer PFSC composed of polymerized ferric sulfate (PFS) and calcium hydroxide (Ca(OH)₂) is proposed to stabilize the soil with co-existed As and Sb sampled at an abandoned arsenic factory site. The effects of stabilizer dosage on the properties of the stabilized soil including leached concentrations of As and Sb, unconfined compressive strength (UCS), and hydraulic conductivity (k_w) were investigated. The mechanisms of As and Sb immobilization in the soils were interpreted by Tessier's sequential extraction procedure (SEP), scanning electron microscope (SEM), and X-ray diffraction (XRD) results. The results showed increasing PFSC dosage was effective for reducing leached concentrations of As and Sb. When the PFSC dosage increased from 2% to 10%, the UCS and k_w increased from 84 to 206 kPa and decreased from 6.48 × 10^-8 to 6.33 × 10^-9 m s^-1, respectively. Tessier's SEP results showed that the leachable As and Sb fractions decreased from 12% to 5.6% and 7.5% to 3.8%, while the Fe-Mn oxides bound fractions increased from 22.3% to 29.4% and 13.2% to 19.5%. The SEM images and XRD patterns of untreated and PFSC stabilized contaminated soils indicated that hematite and calcite (CaCO₃) were the main products of PFSC stabilization processes. Adsorption on ferrihydrite, entrapment in hematite lattices, and co-precipitate with calcite might were the main mechanisms of As and Sb immobilization.

Keystone taxa and functional analysis in arsenic and antimony co-contaminated rice terraces

Both arsenic (As) and antimony (Sb) are primary environmental contaminants that often co-exist at contaminated sites. Though the microbial community compositions of As- and Sb-contaminated sites have been previously described, the changes in microbial community interactions and community functions remain elusive. In the current study, several key metabolic processes, such as As/Sb detoxification and carbon fixation, were enriched under heavily contaminated conditions. Furthermore, the identified keystone taxa, which are associated with the families Nitrosomonadaceae, Pedosphaeraceae, Halieaceae, and Latescibacterota, demonstrated positive correlations with As and Sb concentrations, indicating that they may be resistant to As and Sb toxicities. Accordingly, arsenic resistance-related functions, along with several functions such as carbon fixation, were found to be enriched in heavily contaminated sites. The current study elucidated the key microbial populations in As- and Sb-contaminated rice terraces and may provide useful information for remediation purposes.

Rhizosphere Microbial Communities and Geochemical Constraining Mechanism of Antimony Mine Waste-Adapted Plants in Southwestern China

Antimony (Sb) and arsenic (As) are two hazardous metalloid elements, and the biogeochemical cycle of Sb and As can be better understood by studying plant rhizosphere microorganisms associated with Sb mine waste. In the current study, samples of three types of mine waste—Sb mine tailing, waste rocks, and smelting slag—and associated rhizosphere microorganisms of adapted plants were collected from Qinglong Sb mine, southwest China. 16S rRNA was sequenced and used to study the composition of the mine waste microbial community. The most abundant phylum in all samples was Proteobacteria, followed by Bacteroidota, Acidobacteriota, and Actinobacteriota. The community composition varied among different mine waste types. Gammaproteobacteria was the most abundant microorganism in tailings, Actinobacteria was mainly distributed in waste rock, and Saccharimonadia, Acidobacteriae, and Ktedonobacteria were mainly present in slag. At the family level, the vast majority of Hydrogenophilaceae were found in tailings, Ktedonobacteraceae, Chthoniobacteraceae, and Acidobacteriaceae (Subgroup 1) were mostly found in slag, and Pseudomonadaceae and Micrococcaceae were mainly found in waste rock. Actinobacteriota and Arthrobacter are important taxa for reducing heavy metal(loid) mobility, vegetation restoration, and self-sustaining ecosystem construction on antimony mine waste. The high concentrations of Sb and As reduce microbial diversity.

Synergistic Impacts of Arsenic and Antimony Co-contamination on Diazotrophic Communities

Nitrogen (N) shortage poses a great challenge to the implementation of in situ bioremediation practices in mining-contaminated sites. Diazotrophs can fix atmospheric N₂ into a bioavailable form to plants and microorganisms inhabiting adverse habitats. Increasing numbers of studies mainly focused on the diazotrophic communities in the agroecosystems, while those communities in mining areas are still not well understood. This study compared the variations of diazotrophic communities in composition and interactions in the mining areas with different extents of arsenic (As) and antimony (Sb) contamination. As and Sb co-contamination increased alpha diversities and the abundance of nifH encoding the dinitrogenase reductase, while inhibited the diazotrophic interactions and substantially changed the composition of communities. Based on the multiple lines of evidence (e.g., the enrichment analysis of diazotrophs, microbe-microbe network, and random forest regression), six diazotrophs (e.g., Sinorhizobium, Dechloromonas, Trichormus, Herbaspirillum, Desmonostoc, and Klebsiella) were identified as keystone taxa. Environment-microbe network and random forest prediction demonstrated that these keystone taxa were highly correlated with the As and Sb contamination fractions. All these results imply that the above-mentioned diazotrophs may be resistant to metal(loid)s.

Immobilization mechanism of antimony by applying zirconium-manganese oxide in soil

Antimony (Sb) accumulation in soil poses great potential risk to ecological environment, and its mobilization, transformation and bioavailability are controlled by its fractions and species. Hence, it is important to develop functional materials with both adsorption and oxidation that achieve detoxification and control the mobilization of Sb. In this study, the synthesized zirconium‑manganese oxide (ZrMn) could extremely promoted the transformation of antimonite [Sb(III)] to antimonate [Sb(V)], induced the bioavailable Sb shift to well-crystallized (hydr)oxides of Mn and residual fractions, and further reduced mobility and bioavailability Sb in soil. The sorption of ZrMn to Sb(III) and antimonate Sb(V) were affected by interfering ions, and to Sb(III) was a heterogeneous adsorption process. Spectroscopic characterization of XPS and FTIR suggested exchange between the hydroxyl groups and Sb was crucial in its retain and forming an electronegative inner-sphere mononuclear or binuclear bridging compound. The oxidation induced the transformation of Mn species in ZrMn, generated Mn(II) and Mn(III) exposing more reactive sites conducive to oxidation and adsorption, thus Mn oxides has a higher adsorption capacity for Sb(III). However, the Zr oxides of ZrMn presented adsorption rather than oxidation. The application of ZrMn could realize the dual effect of Sb oxidation detoxification and adsorption immobilization in soil, which provided references for Sb contaminated soil remediation.

Stabilization of heavy metals in biochar derived from plants in antimony mining area and its environmental implications

Heavy metals pollution in mining soils seriously threatens the ecological environment and human health worldwide. Phytoremediation is considered to be an ideal method to reduce the toxicity, mobility, and bioavailability of heavy metals in the soils. However, the disposal of plant-enriched heavy metals has become a thorny problem. To estimate the effect of pyrolysis on the stabilization of heavy metals in post-phytoremediation plant residues, different biochars were prepared from Conyza canadensis (CC), Gahnia tristis (GT), and Betula luminifera (BL) at different pyrolysis temperatures (300, 450, and 600 °C). Results indicated that pyrolysis was effective in the stabilization of heavy metals (Cr, Ni, As, Sb, Hg, and Pb) in plants and significantly (P < 0.05) decreased the bioavailability of most heavy metals. Among them, GT₆₀₀ prepared by pyrolysis of GT at 600 °C has the best stabilization effect on Sb, which increases the residual fraction by 7.32 times, up to 82.05%. The results of environmental risk assessment show that pyrolysis of biomass at high temperature (600 °C) can effectively mitigate the environmental impact of As, Sb, and Hg. Additionally, the reutilization potential of biochar produced by post-phytoremediation plant residues as adsorbents was investigated. The results of adsorption experiments revealed that all biochars have an excellent performance to adsorb Pb(II), and the maximum adsorption capacity is 139.16 mg g^-1 for CC₄₅₀. The adsorption mechanism could be attributed to complexation, electrostatic attraction, and cation exchange. This study demonstrates that pyrolysis is an effective and environment-friendly alternative method to stabilize heavy metals in plants, and their pyrolysis products can be reused for heavy metal adsorption.

Spatial Distribution and Migration Characteristics of Heavy Metals in Grassland Open-Pit Coal Mine Dump Soil Interface

The open-pit coal mine dump in the study area contains many low-concentration heavy metal pollutants, which may cause pollution to the soil interface. Firstly, statistical analysis and geostatistical spatial interpolation methods described heavy metal pollution's spatial distribution. The mine dump heavy metal pollution distribution is strongly random due to disorderly piles, but it is closely related to slope soil erosion. Furthermore, the soil deposition area is where pollutants accumulate. For example, all heavy metal elements converge at the bottom of the dump. Usually, the pollution in the lower part is higher than that in the upper part; the pollution in the lower step is higher than the upper step; the pollution in the soil deposition locations such as flat plate and slope bottom is higher than the soil erosion locations such as slope tip and middle slope. Finally, the hyperspectral remote sensing method described heavy metals pollution's migration characteristics, that the pollutants could affect the soil interface by at least 1 km. This study provides a basis for preventing and controlling critical parts of mine dump heavy metal pollution and pollution path control.

Bioavailability of antimony and arsenic in a flowering cabbage-soil system: Controlling factors and interactive effect

Soil contamination with antimony (Sb) and arsenic (As) has become a well-recognized environmental and human health issue. Consumption of vegetables, especially leafy vegetables, is one of the most important sources of Sb and As exposure in humans. Accordingly, it is necessary to understand the behaviors of Sb and As in the vegetable-soil system. Moreover, although Sb and As are often assumed to have similar biogeochemical behavior, identified differences in the controlling factors affecting mobility and bioavailability of Sb and As in soils need further investigation. In this study, 112 pairs of soil and flowering cabbage samples were collected from typical farmland protection areas and vegetable-producing regions across the Pearl River Delta (PRD), South China. The contamination levels of Sb and As in soils and harvested cabbages across the PRD were investigated. The main factors affecting the mobility and bioavailability of Sb and As in the cabbage-soil system were disentangled using a random forest model. The contamination levels of Sb in the cabbages and soils of the PRD were generally low, but the soils were moderately polluted by As. Increased concentrations of Fe oxides could decrease Sb accumulation in cabbages but increased the mobilization of As in soils to some extent. In contrast, Al oxides contributed strongly to the mobilization of Sb and the immobilization of As. Moreover, an increased sand content promoted the mobility of Sb and As, whereas increased silt and clay contents showed inhibitory effects. The interactions of As and Sb with Fe oxides decreased the mobility of Sb but moderately increased the mobility of As in soils. Overall, the behaviors of Sb and As in the cabbage-soil system under the effect of several important environmental factors showed some differences indicating that these differences should be considered in the remediation of co-contaminated soils.

Acidity-dependent mobilization of antimony and arsenic in sediments near a mining area

Coexisting antimony (Sb) and arsenic (As) have raised worldwide concerns, but the factors controlling the mobilization of Sb and As in sediments near mining areas are not fully understood. Herein, multiple leaching methods and complementary spectroscopic analyses were used to investigate the mobility of Sb and As and its controlling factors in sediments around the Xikuangshan tailings pond over a wide range of acidity. The general acid neutralizing capacity (GANC) test showed that the leachability of Sb and As exhibited a V-shape pattern with a minimum concentration at 1.6 eq H⁺/kg. The result of MINTEQ simulation agreed well with our GANC results, and demonstrated that the decrease of Sb and As in the range 0-1.6 eq H⁺/kg and the increase in 1.6-4 eq H⁺/kg were mainly controlled by the adsorption and dissolution of iron oxyhydroxide, respectively. Based on the V-shaped leaching trend, Sb and As were predicted to be immobilized in sediments when the acidity accumulated to 1.6 eq H⁺/kg for a long term up to 61 years. This study provides insights in assessing the leaching risks and predicting the mobilization of Sb and As in sediments.

Extracellular proteins of Desulfovibrio vulgaris as adsorbents and redox shuttles promote biomineralization of antimony

Biomineralization is the key process governing the biogeochemical cycling of multivalent metals in the environment. Although some sulfate-reducing bacteria (SRB) are recently recognized to respire metal ions, the role of their extracellular proteins in the immobilization and redox transformation of antimony (Sb) remains elusive. Here, a model strain Desulfovibrio vulgaris Hildenborough (DvH) was used to study microbial extracellular proteins of functions and possible mechanisms in Sb(V) biomineralization. We found that the functional groups (N-H, CO, O-CO, NH₂-R and RCOH/RCNH₂) of extracellular proteins could adsorb and fix Sb(V) through electrostatic attraction and chelation. DvH could rapidly reduce Sb(V) adsorbed on the cell surface and form amorphous nanometer-sized stibnite and/or antimony trioxide, respectively with sulfur and oxygen. Proteomic analysis indicated that some extracellular proteins involved in electron transfer increased significantly (p < 0.05) at 1.8 mM Sb(V). The upregulated flavoproteins could serve as a redox shuttle to transfer electrons from c-type cytochrome networks to reduce Sb(V). Also, the upregulated extracellular proteins involved in sulfur reduction, amino acid transport and protein synthesis processes, and the downregulated flagellar proteins would contribute to a better adaption under 1.8 mM Sb(V). This study advances our understanding of how microbial extracellular proteins promote Sb biomineralization in DvH.

Immobilization of Sb in a smelting residue by micro-sized zero-valent iron: Long-term performance under accelerated exposure to strong acid rain

This study investigated the long-term leachability of antimony (Sb) in a smelting residue immobilized by three commercial micro-sized zero-valent iron (ZVI) products. Effect of oxic incubation time (14 days and 120 days) on the immobilization efficiency of Sb were compared, and the long-term leaching risk was evaluated by an accelerated exposure test, in which the slag was consecutively extracted by simulated strong acid rain (SSAR, HNO₃: H₂SO₄ = 1:2, pH = 3.20). Notably, all ZVI treatments efficiently immobilized the Sb in this slag in a short term (14 days); the one-step SSAR-leached Sb was reduced by 89%-91% compared to the original slag (5.9 mg/L) and was far below the environmental standard (0.6 mg/L) established by the US EPA. The sequential SSAR leaching results reflected that the 14-d incubated slags after ZVI treatments had strong H⁺ resistance, and the immobilized Sb was not easily activated by continuous SSAR corrosion. The binding of Sb with amorphous phase Fe oxyhydroxides (e.g. ferrihydrite) derived from ZVI corrosion played a dominant role in the Sb immobilization efficiency. However, the longer aging process (120 days) easily resulted in the reduction of Sb immobilization by ZVI treatments. The changes in crystallinity of Fe oxyhydroxides (transformation from poorly-crystalline to crystalline ones) and the pH elevation to alkaline range might explain the weakening of the immobilization of Sb in ZVI-amended slags with 120 days of incubation. In total, the effectiveness of Sb immobilization in smelting residue greatly depended on the type of ZVI and the aging process. Our work has demonstrated that the ZVI treatment was potentially feasible to mitigate the Sb leaching risk from smelting slags; however, the ZVI type needs to be carefully selected and its long-term performance should be adequately verified before practical application.

Antimony reduction by a non-conventional sulfate reducer with simultaneous bioenergy production in microbial fuel cells

Environmental toxicity of antimony (Sb) is significantly increased through the widespread industrial application. The extended release of Sb above the regulatory level became a risk to humans habituated in the ecosystem. Conventional methods to remediate Sb demand high energy or resource input, which further leads to secondary pollution. The bio-electrochemical system offers a promising bioremediation strategy to remove or reduce toxic heavy metals. Thus, this research explores the possibilities of simultaneous metal sulfide (MeS) precipitation and electricity production using a full biological Microbial fuel cell (MFC). A non-conventional sulfate-reducing bacteria (SRB) Citrobacter freundii SR10 was used for this investigation, where the MFC was operated for lactate utilization in the bio-anode and Sb reduction at the bio-cathode. This study observed 81% of coulombic efficiency (bio-anode) and 97% of sulfate reduction with 99.3% Sb (V) reduction (bio-cathode), and it was concluded that the MeS precipitation entirely depends on sulfide concentration via SR10 sulfate reduction. The MFC-SR10 offers a maximum power density of 1652.9 ± 32.1 mW/m^3, and their performance was depicted using cyclic voltammetry and electrochemical impedance spectroscopy. The Sb reduction was evaluated through fluorescence spectroscopy, and the Sb (V) MeS precipitation was confirmed as stibnite (Sb₂S₃) by Raman spectroscopy and X-ray photoelectron spectroscopy. Furthermore, the matured anodic and cathodic biofilm formation was confirmed by Scanning electron microscopy with Energy-dispersive X-ray spectroscopy. Thus the MFC with SRB bio-cathode can be used as an alternative to simultaneously remove sulfate and Sb from the wastewater with electricity production.

Multiple pollution sources unravelled by environmental forensics techniques and multivariate statistics

Industrial sites affected by anthropogenic contamination, both past and present-day, commonly have intricate pollutant patterns, and source discrimination can be thus highly challenging. To this goal, this paper presents a novel approach combining multivariate statistics and environmental forensic techniques. The efficiency of this methodology was exemplified in a severely polluted estuarine area (Avilés, Spain), where factor analysis and clustering were performed to identify sub-areas with distinct geochemical behaviour. Once six clusters were defined and a pollution index applied, forensic tools revealed that the As speciation, Pb isotopes, and PAHs molecular ratios were useful to categorise the cluster groups on the basis of distinct pollution sources: Zn-smelting, coaly particles and waste disposal. Overall, this methodology offers valuable insight into pollution sources identification, which can be extended to comparable scenarios of complexly polluted environmental compartments. The information gathered using this approach is also important for the planning of risk assessment procedures and potential remediation strategies.

Accumulation, regional distribution, and environmental effects of Sb in the largest Hg-Sb mine area in Qinling Orogen, China

In this study, knowledge gaps on Sb concentration in rocks, ores, tailings, soil, river water, sediments, and crops of mine areas were identified and discussed in terms of contamination levels, spatial distribution, and environmental effects. Accordingly, Xunyang Hg-Sb mine, the largest Hg-Sb deposit in China as research region in this study, field sampling and laboratory analysis were conducted. The results showed elevated concentrations of Sb in the soil, sediment, and river water. The X-ray diffraction analysis indicated that the main minerals of the rocks were quartz, dolomite, calcite, and margarite. Based on the TESCAN integrated mineral analyzer analysis, the main ore minerals in the Gongguan mine were dolomite (93.97%), cinnabar (2.50%), stibnite (2.48%), calcite (0.38%), and quartz (0.38%). The μ-XRF analysis indicated that Sb distribution was similar to those of S and O, instead of those of Hg and As. The clear spatial variation of Sb concentration in environmental media, mines, tailings, and settling ponds affected Sb accumulation. Actinobacteriota, Proteobacteria, Acidobacteriota, and Chloroflexi were the dominant phyla in the soil. Patescibacteria, Proteobacteria, and Bdellovibrionota were negatively correlated with Sb in the soil (p < 0.05). Exposure to Sb through maize grain and cabbage consumption poses serious non-carcinogenic health risk for residents. This work provides a scientific basis for the environmental quality assessment of Sb mine areas and development of applicable guidelines.

Long-term leachability of Sb in smelting residue stabilized by reactive magnesia under accelerated exposure to strong acid rain

This study investigated the long-term leachability of antimony (Sb) in a smelting residue (39519 mg/kg) solidified/stabilized by reactive magnesia (MgO). Different dosages of MgO (0% as control, 2%, 5%, and 10% on a dry basis) were compared, and the long-term performance was evaluated by an accelerated exposure test consist of 20 consecutive leaching steps with simulated strong acid rain (SAR, HNO₃: H₂SO₄ = 1:2, pH = 3.20) as the extractant. Notably, the MgO treatments efficiently reduced the Sb leachability. Compared to the original slag (8.3 mg/L), the leaching concentrations based on a Chinese standard HJ/T299-2007 were reduced by 58%, 79%, 85%, and 86% at MgO dosages of 0%, 2%, 5%, and 10%, respectively. Because the studied slag was rich in oxides like SiO₂, CaO, and MgO, the hydration reactions probably happened during the aging processes with oxic water. It was inferred that the formed hydration products have a self-solidification/stabilization function to suppress the Sb leaching from the solid phase. The mineralogical characterization results proved that the hydrated Mg(OH)₂ played an essential role in the decrease of Sb leachability. Besides, the MgO addition promoted the hydration of this smelting slag and formed new hydrate gels that immobilize Sb in this slag. Our results confirmed that MgO-amended slags were resistant to continuous SAR corrosion. Compared to the control, the dosage of 5% MgO could effectively reduce the cumulatively released Sb by 57%, with only 0.46% of total Sb could be leached. The decomposition of Mg(OH)₂ and hydrate gels determined the re-release of Sb in a long term. Our work has demonstrated that reactive MgO amendment could be potentially selected as an effective strategy for the treatment of Sb-containing smelting residues in field conditions.

Antimony transformation and mobilization from stibnite by an antimonite oxidizing bacterium Bosea sp. AS-1

Soils and waters are heavily contaminated by antimony in Xikuangshan (XKS) mine area. It is widely accepted that oxidative dissolution of sulfide minerals and aqueous dissolution are the most prevalent geochemical mechanisms for the release of Sb to the environment. Bosea sp. AS-1 is an antimonite-oxidizer isolated from the mine slag in Xikuangshan Sb mine. Whole genome sequencing revealed the presence of multiple sulfur-oxidizing genes, antimony (Sb) metabolism genes and carbon fixation genes in AS-1's genome. We therefore hypothesized that under oxic conditions, AS-1 could mediate the oxidation of sulfide and Sb(III) in stibnite (Sb₂S₃) and lead to the release of Sb. Indeed, strain AS-1 was discovered as an autotrophic Sb(III)-oxidizer. Antimony mobilization studies conducted with strain AS-1 showed significantly enhanced mobilization of Sb, and complete oxidation of released Sb and sulfur to Sb(V) and sulfate. In addition, AS-1 induced a faster release of Sb under heterotrophic condition, and new acicular minerals might form. These findings support the hypothesis that microorganisms play an important role in the mobilization and transformation of Sb in XKS mine area and may contribute to our further understanding of the Sb biogeochemical redox cycle in natural environment.

Antimony contamination and its risk management in complex environmental settings: A review

Antimony (Sb) is introduced into soils, sediments, and aquatic environments from various sources such as weathering of sulfide ores, leaching of mining wastes, and anthropogenic activities. High Sb concentrations are toxic to ecosystems and potentially to public health via the accumulation in food chain. Although Sb is poisonous and carcinogenic to humans, the exact mechanisms causing toxicity still remain unclear. Most studies concerning the remediation of soils and aquatic environments contaminated with Sb have evaluated various amendments that reduce Sb bioavailability and toxicity. However, there is no comprehensive review on the biogeochemistry and transformation of Sb related to its remediation. Therefore, the present review summarizes: (1) the sources of Sb and its geochemical distribution and speciation in soils and aquatic environments, (2) the biogeochemical processes that govern Sb mobilization, bioavailability, toxicity in soils and aquatic environments, and possible threats to human and ecosystem health, and (3) the approaches used to remediate Sb-contaminated soils and water and mitigate potential environmental and health risks. Knowledge gaps and future research needs also are discussed. The review presents up-to-date knowledge about the fate of Sb in soils and aquatic environments and contributes to an important insight into the environmental hazards of Sb. The findings from the review should help to develop innovative and appropriate technologies for controlling Sb bioavailability and toxicity and sustainably managing Sb-polluted soils and water, subsequently minimizing its environmental and human health risks.

Adsorption Capacities of Iron Hydroxide for Arsenate and Arsenite Removal from Water by Chemical Coagulation: Kinetics, Thermodynamics and Equilibrium Studies

Arsenic (As)-laden wastewater may pose a threat to biodiversity when released into soil and water bodies without treatment. The current study investigated the sorption properties of both As(III, V) oxyanions onto iron hydroxide (FHO) by chemical coagulation. The potential mechanisms were identified using the adsorption models, ζ-potential, X-ray diffraction (XRD) and Fourier Transform Infrared Spectrometry (FT-IR) analysis. The results indicate that the sorption kinetics of pentavalent and trivalent As species closely followed the pseudo-second-order model, and the adsorption rates of both toxicants were remarkably governed by pH as well as the quantity of FHO in suspension. Notably, the FHO formation was directly related to the amount of ferric chloride (FC) coagulant added in the solution. The sorption isotherm results show a better maximum sorption capacity for pentavalent As ions than trivalent species, with the same amount of FHO in the suspensions. The thermodynamic study suggests that the sorption process was spontaneously exothermic with increased randomness. The ζ-potential, FT-IR and XRD analyses confirm that a strong Fe-O bond with As(V) and the closeness of the surface potential of the bonded complex to the point of zero charge (pHzpc) resulted in the higher adsorption affinity of pentavalent As species than trivalent ions in most aquatic conditions. Moreover, the presence of sulfates, phosphates, and humic and salicylic acid significantly affected the As(III, V) sorption performance by altering the surface properties of Fe precipitates. The combined effect of charge neutralization, complexation, oxidation and multilayer chemisorption was identified as a major removal mechanism. These findings may provide some understanding regarding the fate, transport and adsorption properties onto FHO of As oxyanions in a complex water environment.

Sustainable Applications for Utilizing Antimony Tailing Coarse Aggregate (ATCA) in Concrete: Characteristic of ATCA and Toxicity Risks of Concrete

In this research, the sustainable applications for utilizing antimony tailing coarse aggregate (ATCA) in concrete is investigated. Comprehensive verifications were performed by a series of experiments on the characteristic of ATCA and the toxicity risks of concrete. Firstly, a real case study of utilization of ATCA as a complete substitute for natural coarse aggregate (NCA) in high strength concrete was conducted. Then, chemical composition of ATCA was tested. It is demonstrated that the essential mineral is SiO₂ and the lithology of ATCA is quartzite. The mechanical properties, coarse quality of ATCA, and NCA were studied and compared. The compressive strength, splitting tensile strength, and compressive elastic modulus of ATWR are 221.51 MPa, 5.93 MPa, and 3.33 × 10⁴ MPa, which are 1.31, 2.22, 1.40 times of that of NR, respectively. All of the quality control indices of ATCA meet the requirements of the current industry standards of China. Finally, the toxicity risks of ATCA concrete were investigated. It is illustrated that the leaching of main heavy metals including Sb, As, Hg, Pb, Cd, and Zn in the ATCA concrete under different pH conditions are below the regulatory limits. The utilization of antimony tailing has significant environmental and economic benefits.

Implications of Soil Potentially Toxic Elements Contamination, Distribution and Health Risk at Hunan’s Xikuangshan Mine

A field survey was conducted to determine the pollution grade, sources, potential ecological risk, and health risk of soil potentially toxic elements (PTEs) in Xikuangshan Mine (XKS), the largest antimony (Sb) deposit in the world. A total of 106 topsoil samples were collected from 6 sites in XKS to measure the concentrations of PTEs Cr, Zn, Cd, Pb, As, Hg, and Sb. The results show that the average concentrations of these elements at all six sites were generally greater than their corresponding background values in Hunan province, especially Sb, Hg, and As. Correlation and principal component analyses suggested that Cd, Zn, Pb, Hg, and Sb were primarily released from mining and other industrial and human activities, while Cr and As were mainly impacted by the parent material from pedogenesis. A risk index analysis showed that, overall, sites were at very high ecological risk, and Sb is the highest ecological risk factor, followed by Cd and Hg. According to health risk assessment, oral ingestion is the main non-carcinogenic and carcinogenic risk exposure route. The higher potentially non-carcinogenic and carcinogenic risks happen to the local children who live in the vicinity of mining area. It revealed that the mining and smelting processes of XKS have negatively influenced the local people, therefore, we should pay increasing attention to this practical issue and take effective measures to protect the ecology of XKS.

Arsenic and antimony co-contamination influences on soil microbial community composition and functions: Relevance to arsenic resistance and carbon, nitrogen, and sulfur cycling

Microorganisms can mediate arsenic (As) and antimony (Sb) transformation and thus change the As and Sb toxicity and mobility. The influence of As and Sb on the innate microbiome has been extensively characterized. However, how microbial metabolic potentials are influenced by the As and Sb co-contamination is still ambiguous. In this study, we selected two contrasting sites located in the Shimen realgar mine, the largest realgar mine in Asia, to explore the adaptability and response of the soil microbiome to As and Sb co-contamination and the impact of co-contamination on microbial metabolic potentials. It is observed that the geochemical parameters, including the As and Sb fractions, were the driving forces that reshaped the community composition and metabolic potentials. Bacteria associated with Bradyrhizobium, Nocardioides, Sphingomonas, Burkholderia, and Streptomyces were predicted to be tolerant to high concentrations of As and Sb. Co-occurrence network analysis revealed that the genes related to C fixation, nitrate/nitrite reduction, N fixation, and sulfate reduction were positively correlated with the As and Sb fractions, suggesting that As and Sb biogeochemical cycling may interact with and benefit from C, N, and S cycling. The results suggest that As and Sb co-contamination not only influences As-related genes, but also influences other genes correlated with microbial C, N, and S cycling.