Antimony and arsenic speciation, redox-cycling and contrasting mobility in a mining-impacted river system

Abiotic aerobic oxidation pathways of stibnite revealed by oxygen and sulfur isotope systematics of sulfate

The environmental threat posed by stibnite is an important geoenvironmental issue of current concern. To better understand stibnite oxidation pathways, aerobic abiotic batch experiments were conducted in aqueous solution with varying δ18OH2O value at initial neutral pH for different lengths of time (15-300 days). The sulfate oxygen and sulfur isotope compositions as well as concentrations of sulfur and antimony species were determined. The sulfur isotope fractionation factor (Δ34SSO4-stibnite) values decreased from 0.8‰ to -2.1‰ during the first 90 days, and increased to 2.6‰ at the 180 days, indicating the dominated intermediate sulfur species such as S2O32-, S0, and H2S (g) involved in Sb2S3 oxidation processes. The incorporation of O into sulfate derived from O2 (∼100%) indicated that the dissociated O2 was only directly adsorbed on the stibnite-S sites in the initial stage (0-90 days). The proportion of O incorporation into sulfate from water (27%-52%) increased in the late stage (90-300 days), which suggested the oxidation mechanism changed to hydroxyl attack on stibnite-S sites promoted by nearby adsorbed O2 on stibnite-Sb sites. The exchange of oxygen between sulfite and water may also contributed to the increase of water derived O into SO42-. The new insight of stibnite oxidation pathway contributes to the understanding of sulfide oxidation mechanism and helps to interpret field data.

Mechanism of extracellular electron transport and reactive oxygen mediated Sb(III) oxidation by Klebsiella aerogenes HC10

Microbial oxidation and the mechanism of Sb(III) are key governing elements in biogeochemical cycling. A novel Sb oxidizing bacterium, Klebsiella aerogenes HC10, was attracted early and revealed that extracellular metabolites were the main fractions driving Sb oxidation. However, linkages between the extracellular metabolite driven Sb oxidation process and mechanism remain elusive. Here, model phenolic and quinone compounds, i.e., anthraquinone-2,6-disulfonate (AQDS) and hydroquinone (HYD), representing extracellular oxidants secreted by K. aerogenes HC10, were chosen to further study the Sb(III) oxidation mechanism. N2 purging and free radical quenching showed that oxygen-induced oxidation accounted for 36.78% of Sb(III) in the metabolite reaction system, while hydroxyl free radicals (·OH) accounted for 15.52%. ·OH and H2O2 are the main driving factors for Sb oxidation. Radical quenching, methanol purification and electron paramagnetic resonance (EPR) analysis revealed that ·OH, superoxide radical (O2•-) and semiquinone (SQ-•) were reactive intermediates of the phenolic induced oxidation process. Phenolic-induced ROS are one of the main oxidants in metabolites. Cyclic voltammetry (CV) showed that electron transfer of quinone also mediated Sb(III) oxidation. Part of Sb(V) was scavenged by the formation of the secondary Sb(V)-bearing mineral mopungite [NaSb(OH)6] in the incubation system. Our study demonstrates the microbial role of oxidation detoxification and mineralization of Sb and provides scientific references for the biochemical remediation of Sb-contaminated soil.

Priority sources identification and risks assessment of heavy metal(loid)s in agricultural soils of a typical antimony mining watershed

Heavy metal(loid) (HM) pollution in agricultural soils has become an environmental concern in antimony (Sb) mining areas. However, priority pollution sources identification and deep understanding of environmental risks of HMs face great challenges due to multiple and complex pollution sources coexist. Herein, an integrated approach was conducted to distinguish pollution sources and assess human health risk (HHR) and ecological risk (ER) in a typical Sb mining watershed in Southern China. This approach combines absolute principal component score-multiple linear regression (APCS-MLR) and positive matrix factorization (PMF) models with ER and HHR assessments. Four pollution sources were distinguished for both models, and APCS-MLR model was more accurate and plausible. Predominant HM concentration source was natural source (39.1%), followed by industrial and agricultural activities (23.0%), unknown sources (21.5%) and Sb mining and smelting activities (16.4%). Although natural source contributed the most to HM concentrations, it did not pose a significant ER. Industrial and agricultural activities predominantly contributed to ER, and attention should be paid to Cd and Sb. Sb mining and smelting activities were primary anthropogenic sources of HHR, particularly Sb and As contaminations. Considering ER and HHR assessments, Sb mining and smelting, and industrial and agricultural activities are critical sources, causing serious ecological and health threats. This study showed the advantages of multiple receptor model application in obtaining reliable source identification and providing better source-oriented risk assessments. HM pollution management, such as regulating mining and smelting and implementing soil remediation in polluted agricultural soils, is strongly recommended for protecting ecosystems and humans.

Antimony(V) sorption and coprecipitation with ferrihydrite: An examination of retention mechanisms and the selectivity of commonly-applied extraction procedures

We investigated the mechanisms that control Sb(V) sorption and coprecipitation with ferrihydrite across a range of Sb(V) loadings, and examined the associated effects on Sb(V) extractability during the commonly-applied 1 M HCl extraction scheme and the BCR and Wenzel sequential extraction schemes. EXAFS spectroscopy reveals that Sb(V) sorption and coprecipitation mainly involved Sb(V) incorporation into the ferrihydrite structure via edge sharing and double-corner sharing between SbO6 and FeO6 octahedra. Large amounts of these linkages partially stabilized ferrihydrite against extraction with 1 M HCl. Negligible (< 0.5 %) ferrihydrite-bound Sb(V) was recovered in the "acid extractable" and "reducible" fractions of the BCR scheme, while 1-16 % was recovered in the "oxidizable" fraction. As such, the BCR scheme risks ferrihydrite-bound Sb(V) being misidentified as Sb residing mainly in "residual" phases. In contrast, in the Wenzel scheme, almost all sorbed- and coprecipitated-Sb(V) was recovered in the "amorphous hydrous oxide-bound" fraction, with only 0.6-3.3 % in the "specifically-bound" fraction (consistent with our finding of Sb(V) retention via incorporation into ferrihydrite, as opposed to adsorption by the ferrihydrite surface). Collectively, the results provide new insights into the retention mechanisms and extraction behaviour of ferrihydrite-bound Sb(V), enhancing our ability to assess Sb contamination in soils, sediments and geogenic wastes.

The impact of gold mining activities: understanding the dynamics of cyanide in river ecosystems in Ecuador

Understanding the behavior of cyanide in rivers is of utmost importance as it has a direct impact on the health of people who depend on these water sources. Cyanide contamination from gold mining activities poses a significant environmental threat to river ecosystems, particularly in southern Ecuador. This study aimed to investigate the behavior of cyanide when it enters contact with other metals in these rivers. Simulations were conducted to determine the speciation of cyanide, mercury, arsenic, lead, and manganese in a study area, taking into account the water temperature and pH at four locations. The findings revealed that CN-and HCN(aq) species were present in the research area. Additionally, mercury-cyanide (Hg(CN)2(aq), Hg(CN)3-), and manganese-cyanide (MnCN+) complexes were identified 3 km downriver from the site where the mining activity is higher. These metal-cyanide complexes tend to dissociate quickly under weak acidic conditions, making them hazardous to the environment. This research is crucial, not only for the environment but also for human health, as it allows to predict toxicity risks for people supplied with this water source, emphasizing the potential harm to human health. This study highlights the importance of stringent regulations and effective monitoring practices to mitigate cyanide contamination and safeguard environmental and occupational health.

Floodplain morphology influences arsenic and antimony spatial distribution in a seasonal acid sulfate soil wetland

Arsenic (As) and antimony (Sb) often co-occur in floodplain depositional environments that are contaminated by legacy mining activities. However, the distribution of As and Sb throughout floodplains is not uniform, adding complexity and expense to management or remediation processes. Identifying floodplain morphology predictor variables that help quantify and explain As and Sb spatial distribution on floodplains is useful for management and remediation. We developed As and Sb risk maps estimating concentration and availability at a coastal floodplain wetland impacted by upper-catchment mining. Significant predictors of As and Sb concentrations included i) distance from distributary channel-wetland intersection and ii) elevation. Distance from channel explained 53 % (P < 0.01) and 28 % (P < 0.01), while elevation explained 42 % (P < 0.01) and 47 % (P < 0.01) of the variability in near-total Sb and As respectively. As had a higher extractability than Sb across all tested soil extractions, suggesting that As is more environmentally available. As and Sb dry mass estimates to a depth of 0.1 m scaled to the lower coastal Macleay floodplain ranged from 113-192 tonnes and 14-24 tonnes respectively. Landscape-scale modelling of metalloid distribution, informed by morphology variables, presented here may be a useful framework for the development of risk maps in other regions impacted by contaminated upper-catchment sediments.

Exploring the feasibility of Zr-based metal-organic frameworks for the recovery of Sb (V) and Sb (III) from mining waste

The present study investigates the efficacy of newly developed Zr-based metal-organic frameworks, specifically MIP-206, and its amine-modified counterpart, MIP-206-NH2, for the re-covery of antimony (Sb) from both synthetic and actual mining wastewater. Batch method studies were employed to examine the effect of waste media pH, Sb concentration, process kinetics, and the performance of the regeneration solution. MIP-206-NH2 exhibited impressive separation capabilities, achieving 102.18 mg/g and 63.23 mg/g for Sb (V) and Sb (III), respectively. In contrast, the pristine MIP-206 reached maximum values of 26.26 mg/g for Sb (V) and 16.95 mg/g for Sb (III). The separation process was well-described by the Langmuir equation, and the kinetics followed the pseudo-second-order model. Although the amine modification resulted in a decrease in the surface area of MIP-206 from 1345.21 to 1169.86 m2/g, SEM and XRD analyses confirmed that the structural integrity of MIP-206-NH2 remained unchanged. In terms of reusability, MIP-206-NH2 maintained up to 90% of its separation performance over 9 cycles, while MIP-206 demonstrated effectiveness for 7 cycles. The regeneration solution exhibited a capacity of approximately 0.63 mol/L for Sb (V) and 0.71 mol/L for Sb (III). Furthermore, MIP-206 and MIP-206-NH2 demonstrated successful application in selectively separating Sb from real mining wastewater.

Legacy effects of historical gold mining on floodplains of an Australian river

The gold rush at the end of the nineteenth century in south-eastern Australia resulted in the mobilization and re-deposition of vast quantities of tailings that modified the geomorphology of the associated river valleys. Previous studies of contamination risk in these systems have either been performed directly on mine wastes (e.g., battery sand) or at locations close to historical mine sites but have largely ignored the extensive area of riverine alluvial deposits extending downstream from gold mining locations. Here we studied the distribution of contaminant metal(loids) in the Loddon River catchment, one of the most intensively mined areas of the historical gold-rush period in Australia (1851-1914). Floodplain alluvium along the Loddon River was sampled to capture differences in metal and metalloid concentrations between the anthropogenic floodplain deposits and the underlying original floodplain. Elevated levels of arsenic up to 300 mg-As/kg were identified within the anthropogenic alluvial sediment, well above sediment guidelines (ISQG-high trigger value of 70 ppm) and substantially higher than in the pre-mining alluvium. Maximum arsenic concentrations were found at depth within the anthropogenic alluvium (plume-like), close to the contact with the original floodplain. The results obtained here indicate that arsenic may pose a significantly higher risk within this river catchment than previously assessed through analysis of surface floodplain soils. The risks of this submerged arsenic plume will require further investigation of its chemical form (speciation) to determine its mobility and potential bioavailability. Our work shows the long-lasting impact of historical gold mining on riverine landscapes.

A novel magnetic graphene-loaded biochar gel for the remediation of arsenic- and antimony-contaminated mining soil

Metalloid co-contamination such as arsenic (As) and antimony (Sb) in soils has posed a significant threat to ecological balance and human well-being. In this study, a novel magnetic graphene-loaded biochar gel (FeBG) was developed, and its remediation potential for the reclamation of AsSb spoiled soil was assessed through a six-month soil incubation experiment. Results showed that the incorporation of iron substances and graphene imparted FeBG with enhanced surface characteristics, such as the formation of a new FeO bond and an enlarged surface area compared to the pristine biochar (BC) (80.5 m2 g-1 vs 57.4 m2 g-1). Application of FeBG significantly decreased Na2HPO4-extractable concentration of As in soils by 9.9 %, whilst BC addition had a non-significant influence on As availability, compared to the control. Additionally, both BC (8.2 %) and FeBG (16.4 %) treatments decreased the Na2HPO4-extractable concentration of Sb in soils. The enhanced immobilization efficiency of FeBG for As/Sb could be attributed to FeBG-induced electrostatic attraction, complexation (Fe-O(H)-As/Sb), and π-π electron donor-acceptor coordination mechanisms. Additionally, the FeBG application boosted the activities of sucrase (9.6 %) and leucine aminopeptidase (7.7 %), compared to the control. PLS-PM analysis revealed a significant negative impact of soil physicochemical properties on the availability of As (β = -0.611, P < 0.01) and Sb (β = -0.848, P < 0.001) in soils, in which Sb availability subsequently led to a suppression in soil enzyme activities (β = -0.514, P < 0.01). Overall, the novel FeBG could be a potential amendment for the simultaneous stabilization of As/Sb and the improvement of soil quality in contaminated soils.

Effect of freeze-thaw frequency plus rainfall on As and Sb metal(loid)s leaching from the solidified/stabilized soil remediated with Fe-based composite agent

The composite agent of ferrous sulfate, fly ash, and calcium lignosulfonate (FFC) can remediate the soil contaminated by As and Sb under cyclic freeze-thaw (F-T) via stabilization/solidification (S/S). However, the impact of high-frequency F-T cycles on the leaching behavior and migration of As and Sb in FFC-treated soils remains unclear. Here the leaching concentrations, heavy metal speciation (Wenzel's method), and Hydrus-1d simulations were investigated. The results showed that FFC effectively maintained the long-term S/S efficiency of arsenic remediation subject to an extended rainfall and freeze-thaw cycles, and stabilized the easily mobile form of As. The short-term S/S effect on Sb in the remediated soils suffering from F-T cycles was demonstrated in the presence of FFC. In a 20-year span, the mobility of Sb was affected by the number of F-T cycles (FT60 > FT20 > FT40 > FT0) in soil with a depth of 100 cm. As leaching progressed, FFC slowed the upward proportion of adsorbed As fractions but converted parts of the residual Sb to the form of crystalline Fe/Al (hydro) oxide. Moreover, the adsorption rate and capacity of As also preceded that of Sb. Long-term curative effects of FFC could be observed for As, but further development of agents capable of remedying Sb under cyclic F-T and long-term rainfall was needed. The predictive results on the migration and leaching behavior of heavy metals in S/S remediated soils may provide new insight into the long-term assessment of S/S under natural conditions.

Antimony and arsenic migration in a heterogeneous subsurface at an abandoned antimony smelter under rainfall

While antimony (Sb) and arsenic (As) co-contamination in subsurface soil systems due to the legacy of Sb smelting wastes has been documented, the role of inherent heterogeneity on pollutant migration is largely overlooked. Herein this study investigated Sb and As migration in a slag impacted, vertically stratified subsurface at an abandoned Sb smelter. A 2-dimensional flume was assembled as a lab-scale analogue of the site and subject to rainfall and stop-rain events. Reactive transport modeling was then performed by matching the experimental observations to verify the key factors and processes controlling pollutant migration. Results showed that rainfall caused Sb and As release from the shallow slag layer and promoted their downward movement. Nevertheless, the less permeable deeper layers limited physical flow and transport, which led to Sb and As accumulation at the interface. The re-adsorption of Sb and As onto iron oxides in the deeper, more acidic layers further retarded their migration. Because of the large difference between Sb and As concentrations, Sb re-adsorption was much less effective, which led to higher mobility. Our findings overall highlight the necessity of understanding the degree and impacts of physicochemical heterogeneity for risk exposure assessment and remediation of abandoned Sb smelting sites.

The composition and differences of antimony isotopic in sediments affected by the world's largest antimony deposit zone

Antimony (Sb) isotopic fingerprinting is a novel technique for stable metal isotope analysis, but the use of this technique is still limited, especially in sediments. In this study, the world's most important Sb mineralization belt (the Xikuangshan mineralization belt) was taken as the research object and the Sb isotopic composition and Sb enrichment characteristics in the sediments of water systems from different Sb mining areas located in the Zijiang River (ZR) Basin were systematically studied. The results showed that the ε123Sb values in the sediments of the ZR and its tributaries, such as those near the Longshan Sb-Au mine, the Xikuangshan Sb mine, and the Zhazixi Sb mine, were 0.50‒3.13 ε, 2.31‒3.99 ε, 3.12‒5.63 ε and 1.14‒2.91 ε, respectively, and there were obvious changes in Sb isotopic composition. Antimony was mainly enriched in the sediments due to anthropogenic sources. Dilution of Sb along the river and adsorption of Sb to Al-Fe oxides in the sediment did not lead to obvious Sb isotopic fractionation in the sediment, indicating that the Sb isotopic signature was conserved during transport along the river. The Sb isotopic signatures measured in mine-affected streams may have differed from those in the original Sb ore, and further investigation of Sb isotopic fingerprints from other possible sources and unknown geochemical processes is needed. This study reveals that the apparent differences in ε123Sb values across regions make Sb isotopic analysis a potentially suitable tool for tracing Sb sources and biogeochemical processes in the environment.

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.

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.

Novel insights into the mechanisms for Sb mobilization in groundwater in a mining area: A colloid field study

Colloids may play an important role in the geochemical cycle of antimony (Sb). However, the controlling behaviors of colloids on Sb fate in contaminated groundwater are not available. To investigate the effects of colloids on Sb mobility, groundwater samples from Xikuangshan Sb Mine's two main aquifers (the D3s2 aquifer and the D3x4 aquifer) were successively (ultra)filtered through progressively decreasing pore sizes (0.45 µm, 100 kDa, 50 kDa and 5 kDa). The results showed that 0.1-84.1% of Sb was adsorbed or carried by colloids, which corresponded to Sb concentration ranging between 0 and 2973 μg/L in the colloids (0.45 µm - 5 kDa). In both aquifers, Sb was closely associated with organic colloids (r = 0.72 p < 0.05 for the D3x4 aquifer, r = 0.94 p < 0.01 for the D3s2 aquifer). Parallel factor analysis of the three-dimensional fluorescence spectra determined that the protein-like substances in the D3x4 aquifer and the humus-like substances in the D3s2 aquifer controlled Sb behavior. X-ray absorption spectroscopy confirmed Sb complexing with organic substances. Competitive adsorption of As and Sb suppressed the complexation of colloids with Sb, particularly in the D3x4 aquifer (r = -0.71, p < 0.05). Sb mobility was also influenced by the redox of the groundwater system. As the oxidation-reduction potential and dissolved oxygen increased, Sb in the colloidal fractions decreased. These findings provide new insights into the mechanisms involved in Sb fate affected by colloids, establishing the theoretical basis for developing effective Sb and even metalloid pollution remediation strategies.

Monte Carlo simulation and delayed geochemical hazard revealed the contamination and risk of arsenic in natural water sources

Due to its ubiquity and carcinogenicity, the geochemical behavior and health risks of arsenic (As) have been a research focus worldwide. A comprehensive investigation was conducted on the contamination and ecological and health risks of As in the Zijiang River (ZR)-a natural water source. The concentration ranges of As were separately 1.36-6.23 μg/L, 11.42-74.53 mg/kg, and 1.26-130.68 μg/L in surface waters (dissolved), sediments, and pore waters. The concentrations of As in the midstream pore waters and sediments were relatively high, which was related to mining, dam interception, and sediment resuspension. The Monte Carlo simulation results showed that the occurrence probability of As contamination and static risk in sediments was low, however, in the midstream, the secondary risk caused by the release of As should be given more consideration. In the sediments, the transformation paths and the dynamic risk of As were explored based on the delayed geochemical hazard model, showing that there was a probability of a potential burst of 26.47% - 55.88% in the sediments of the ZR. Although at the detected surface waters, the total risk of the noncarcinogenicity and carcinogenicity of As were low, overall adults have lower health risks than children, and As exposure in children should be of concern. This study complements the further understanding of the geochemical behavior of arsenic, which can be extended to other toxic metal(loid)s.

Toxic response of antimony in the Comamonas testosteroni and its application in soil antimony bioremediation

Antimony (Sb) is toxic to ecosystems and potentially to public health via its accumulation in the food chain. Bioavailability and toxicity of Sb have been reduced using various methods for the remediation of Sb-contaminated soil in most studies. However, Sb-contaminated soil remediation by microbial agents has been rarely evaluated. In this study, we evaluated the potential for the use of Comamonas testosteroni JL40 in the bioremediation of Sb-contamination. Strain JL40 immobilized more than 30 % of the Sb(III) in solution and oxidized over 18 % to Sb(V) for detoxification. Meanwhile, strain JL40 responds to Sb toxicity through such as Sb efflux, intracellular accumulation, biofilm production, and scavenging of reactive oxygen species (ROS), etc. The results of the pot experiment showed the average Sb content of the brown rice was decreased by 59.1%, 38.8%, and 48.4%, for 1.8, 50, and 100 mg/kg Sb spiked soils, respectively. In addition, the results of plant, soil enzyme activity, and rice agronomic trait observations showed that the application of strain JL40 could maintain the health of plants and soil and improve rice production. The single-step and sequential extraction of Sb from rhizosphere soil showed that strain JL40 also plays a role in Sb immobilization and oxidation in the soil environment. During rice potted cultivation, bacterial community analysis and plate counting showed that the strain JL40 could still maintain 103 CFU/g after 30 days of inoculation. With phenotypic and differential proteomics analysis, strain JL40 conferred Sb(III) tolerance by a combination of immobilization, oxidation, efflux and scavenging of ROS, etc. Our study demonstrates the application of Sb-immobilizing and oxidizing bacteria to lower soil Sb and reduce accumulation of Sb in rice. Our results provide guidance for bacterial remediation of Sb-contaminated soil.

Urbanization-driven soil degradation; ecological risks and human health implications

Urban soils contaminated with heavy metals and pesticide residues are of great concern because of their adverse impact on human health. A total of 66 agricultural topsoil samples (15 cm) were collected to represent the study area and determine how anthropogenic activities adversely affect soil quality and human health. Sampling was conducted in the summer, when it was dry and hot, and in the winter, after atmospheric deposition. Seventeen potentially hazardous metals/metalloids (Ag, As, Al, B, Ba, Cd, Cr, Cu, Fe, Hg, Mn, Ni, Mo, Pb, Se, Zn, and V) were measured in the soils. The mean concentrations of metals ranged between 0.05 and 8080 mg/kg, and their distribution was site-specific, with high pollution at the sampling sites owing to proximity to human activities. In agricultural areas, the greatest arsenic concentration was recorded at 48 mg/kg. The potential ecological risk index (PERI) and health hazard index (HI) were calculated, as well as metal contamination indices including contamination factor (Cf), geo-accumulation index (Igeo), and pollution load index (PLI). The mean PLI was calculated to be 4.89, indicating that the area is highly polluted. The potential ecological risk index showed remarkably high risks for As, Cd, and Hg, and moderate risks for Ni and Pb. The arsenic hazard index (HI) was greater than one (2.41) in children, indicating a risk of exposure through ingestion. Pesticide residue analyses were performed in areas where the metal intensity was high. Banned or restricted organochlorine pesticide (OCPs) residues, including, dieldrin, endrin ketone, endosulfan I, II, heptachlor, heptachlor epoxide, lindane (γ-HCH), PP-DDD, and methoxychlor, were detected between 0.002 and 1.45 mg/kg in the soil samples.

Remobilization of legacy arsenic from sediment in a large subarctic waterbody impacted by gold mining

Arsenic contamination from mining poses an environmental challenge due to the mobility of this redox-sensitive element. This study evaluated arsenic mobility in sediments of Yellowknife Bay (Canada), a large subarctic water body impacted by gold mining during the 20th century. Short-term measurements of arsenic flux from sediment, arsenic profiling of the water column and sediment porewater, and mass balance modelling were conducted to assess the importance of sediment as an arsenic source. Sediment arsenic fluxes were highly variable throughout Yellowknife Bay and ranged from - 65-1520 µg m^-2 day^-1. Elevated fluxes measured near the mine site were among the highest published for well-oxygenated lakes. Redox boundaries were typically 2-3 cm below the sediment surface as indicated by porewater profiles of iron, manganese, and arsenic, with arsenic maxima of 65-3220 µg L^-1 predominately as arsenite. Sediment arsenic flux was positively related to its solid-phase concentration. Modelling indicated sediment was a principal source of arsenic to the water column. Adsorption and precipitation processes in the oxidizing environment of near-surface sediments did not effectively attenuate arsenic remobilized from contaminated sediments. Internal recycling of legacy arsenic between sediment and surface water will impede a return to background conditions in Yellowknife Bay for decades.

Antimony Isotope Fractionation Revealed from EXAFS during Adsorption on Fe (Oxyhydr)oxides

A lack of knowledge about antimony (Sb) isotope fractionation mechanisms in key geochemical processes has limited its environmental applications as a tracer. Naturally widespread iron (Fe) (oxyhydr)oxides play a key role in Sb migration due to strong adsorption, but the behavior and mechanisms of Sb isotopic fractionation on Fe (oxyhydr)oxides are still unclear. Here, we investigate the adsorption mechanisms of Sb on ferrihydrite (Fh), goethite (Goe), and hematite (Hem) using extended X-ray absorption fine structure (EXAFS) and show that inner-sphere complexation of Sb species with Fe (oxyhydr)oxides occurs independent of pH and surface coverage. Lighter Sb isotopes are preferentially enriched on Fe (oxyhydr)oxides due to isotopic equilibrium fractionation, with neither surface coverage nor pH influencing the degree of fractionation (Δ¹²³Sb_{aqueous-adsorbed}). Limited Fe atoms are present in the second shell of Hem and Goe, resulting in weaker surface complexes and leading to greater Sb isotopic fractionation than with Fh (Δ¹²³Sb_{aqueous-adsorbed} of 0.49 ± 0.004, 1.12 ± 0.006, and 1.14 ± 0.05‰ for Fh, Hem, and Goe, respectively). These results improve the understanding of the mechanism of Sb adsorption by Fe (oxyhydr)oxides and further clarify the Sb isotope fractionation mechanism, providing an essential basis for future application of Sb isotopes in source and process tracing.

Arbuscular mycorrhizal fungi community analysis revealed the significant impact of arsenic in antimony- and arsenic-contaminated soil in three Guizhou regions

Introduction

The lack of systematic investigations of arbuscular mycorrhizal fungi (AMF) community composition is an obstacle to AMF biotechnological applications in antimony (Sb)- and arsenic (As)-polluted soil.

Methods

Morphological and molecular identification were applied to study the AMF community composition in Sb- and As-contaminated areas, and the main influencing factors of AMF community composition in Sb- and As-contaminated areas were explored.

Results

(1) A total of 513,546 sequences were obtained, and the majority belonged to Glomeraceae [88.27%, 193 operational taxonomic units (OTUs)], followed by Diversisporaceae, Paraglomeraceae, Acaulosporaceae, Gigasporaceae, and Archaeosporaceae; (2) the affinity between AMF and plants was mainly related to plant species (F = 3.488, p = 0.022 < 0.050), which was not significantly correlated with the total Sb (TSb) and total As (TAs) in soil; (3) the AMF spore density was mainly related to the available nitrogen, available potassium, and total organic carbon; (4) The effect of soil nutrients on AMF community composition (total explanation: 15.36%) was greater than that of soil Sb and As content (total explanation: 5.80%); (5) the effect of TAs on AMF community composition (λ = -0.96) was more drastic than that of TSb (λ = -0.21), and the effect of As on AMF community composition was exacerbated by the interaction between As and phosphorus in the soil; and (6) Diversisporaceae was positively correlated with the TSb and TAs.

Discussion

The potential impact of As on the effective application of mycorrhizal technology should be further considered when applied to the ecological restoration of Sb- and As-contaminated areas.

Natural attenuation and remobilization of arsenic in a small river contaminated by the volcanic eruption of Mount Iou in southern Kyushu Island, Japan

The active volcano Mount Iou, in the southern part of Japan, erupted in 2018 for the first time in approximately 250 years. Geothermal water discharged from Mount Iou had high concentrations of toxic elements, such as arsenic (As), which could seriously contaminate the adjacent river. In this study, we aimed to clarify the natural attenuation of As in the river through daily water sampling for approximately eight months. The risk of As in the sediment was also evaluated using the sequential extraction procedures. The highest As concentration (2000 μg/L) was observed upstream but typically remained below 10 μg/L downstream. Dissolved As was the main form in the river water on non-rainy days. Arsenic concentration in the river naturally decreased through dilution and sorption/coprecipitation with Fe, Mn, and Al (hydr)oxides during flow. However, peaks in As concentration were frequently observed during rainfall events, possibly due to sediment resuspension. Furthermore, the range of pseudo-total As in the sediment was 4.62-14.3 mg/kg. Total As content was highest upstream before decreasing further along the flow. When using the modified Keon method, 44-70% of the total As existed as more reactive fractions associated with (hydr)oxides.

Microbial community composition and cooccurrence patterns driven by co-contamination of arsenic and antimony in antimony-mining area

In the current study, a typical Sb mine was selected to explore the microbial community composition and assembly driven by the cocontamination of As/Sb with geographic distance. Our results showed that environmental parameters, especially pH, TOC, nitrate, total and bioavailable As/Sb contents largely affected the microbial community diversity and composition. The total and bioavailable As/Sb levels were significantly positively correlated with the relative abundance of Zavarzinella, Thermosporothrix and Holophaga, while the pH presented a significant negative correlation with the three genera, potentially implying they are important taxonomic groups in acid-mining soils. The cooccurrence network analysis indicated the environmental stress dominated by pH and As/Sb co-contamination affected the microbial modularity and interaction. Meanwhile, Homogeneous selection (HoS, 26.4-49.3%), and drift and others (DR, 27.1∼40.2%) were the most important assembly processes for soil bacterial, and the importance of HoS decreased and the DR increased with geographic distance to the contamination source respectively. Soil pH, nutrient availability, total and bioavailable As/Sb contents significantly affected the HoS and DR processes. This study provides theoretical support for microbial remediation in metal(loid)-contaminated soils.

Pollution characteristics and risk assessment of antimony and arsenic in a typical abandoned antimony smelter

Antimony (Sb) and arsenic (As) co-contamination occurs in Sb smelting areas and is harmful to the surrounding ecological environment. The purpose of this study is to explore the spatial distribution characteristics of Sb and As in abandoned Sb smelting area and carry out risk assessments. Soil samples were collected from the smelting area profile and background points, and groundwater samples were also collected. Samples from two geological background sections were collected to understand the geological background characteristics of Sb and As. The spatial distribution was drawn via the inverse distance weighted interpolation method. The hazard assessment was carried out by the geo-accumulation index and potential ecological hazard methods. The results showed that special high geological background value of Sb and As in study area. Sb and As co-contamination is one of the characters in soil. And the contents of Sb and As decrease as depth increases, reflecting the weak migration capacity. The spatial distribution of Sb and As is affected by slag distribution and rainfall leaching. The Sb content in groundwater was higher in the wet and normal seasons than in the dry season, slag leaching may be one of the elements. The potential ecological hazards of Sb and As are high and considerable, respectively. In abandoned smelting area with high geological background values, it is necessary to focus on the pollution abatement and protection of ecological health.

Effect of phosphate on ferrihydrite transformation and the associated arsenic behavior mediated by sulfate-reducing bacterium

Although PO₄^3- is commonly found in association with iron (oxyhydr)oxide, the effect of PO₄^3- on ferrihydrite reduction, mineralogical transformation, and associated As behavior in sulfate-reducing bacteria (SRB)-rich environments remains unclear. In this study, batch experiments, together with geochemical, mineralogical, and biological analyses, were conducted to elucidate these processes. The results showed that SRB can reduce ferrihydrite via direct and indirect processes, and PO₄^3- promoted ferrihydrite reduction by supporting SRB growth at low and medium PO₄^3- loadings. However, at high loadings, PO₄^3- stabilized the ferrihydrite. PO₄^3- shifted the transformation of ferrihydrite from magnetite and mackinawite to vivianite, which scavenges As effectively by incorporating As into its particle. In systems with 0.5 mM SO₄^2-, PO₄^3- exerted a weak effect on As mobilization. However, in systems with 10 mM SO₄^2-, substantial amounts of As were released into the solution, and PO₄^3- impacted As behavior strongly. Low PO₄^3- loadings increased the mobilization of As because of the competitive adsorption of PO₄^3- on mackinawite. Medium and high PO₄^3- loadings were beneficial for As immobilization because of the substitution of mackinawite by vivianite. These findings have important implications for understanding the biogeochemistry of iron (oxyhydr)oxide and As behavior in SRB-containing sediments.

A molecular level understanding of antimony immobilization mechanism on goethite by the combination of X-ray absorption spectroscopy and density functional theory calculations

A molecular level understanding of antimony (Sb) immobilization mechanism on Fe oxides is required to clarify the fate of Sb in the soil. In this study, macroscopic sorption experiments, combined with extended X-ray absorption fine structure (EXAFS) spectroscopy and density functional theory (DFT), were utilized to explore the interaction between Sb and goethite. The ion strength has no effect on Sb sorption on goethite, indicating the inner-sphere complex Sb formed on goethite. Goethite has the higher sorption potential to Sb(III) than Sb(V), consistent with the higher thermodynamic stability of the geometry for Sb(III) formed on goethite than Sb(V) revealed by DFT calculations. By comparing the Sb-Fe distances obtained by EXAFS spectroscopy and DFT, eight kinds of Sb(III) surface complexes and nine kinds of Sb(V) surface complexes were considered to be the possible geometries Sb formed on different crystal planes of goethite, including monodentate mononuclear, bidentate mononuclear, bidentate binuclear, tridentate mononuclear, tridentate binuclear, tridentate four-nuclear complexes. The structural and energetic details of these filtered geometries provide comprehensive information on Sb immobilization mechanism on goethite, helpful in clarifying the fate of Sb in soils.

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.

Fate of antimony contamination generated by road traffic - A focus on Sb geochemistry and speciation in stormwater ponds

Although antimony (Sb) contamination has been documented in urban areas, knowledge gaps remain concerning the contributions of the different sources to the Sb urban biogeochemical cycle, including non-exhaust road traffic emissions, urban materials leaching/erosion and waste incineration. Additionally, details are lacking about Sb chemical forms involved in urban soils, sediments and water bodies. Here, with the aim to document the fate of metallic contaminants emitted through non-exhaust traffic emissions in urban aquatic systems, we studied trace element contamination, with a particular focus on Sb geochemistry, in three highway stormwater pond systems, standing as models of surface environments receiving road-water runoff. In all systems, differentiated on the basis of lead isotopic signatures, Sb shows the higher enrichment factor with respect to the geochemical background, up to 130, compared to other traffic-related inorganic contaminants (Co, Cr, Ni, Cu, Zn, Cd, Pb). Measurements of Sb isotopic composition (δ¹²³Sb) performed on solid samples, including air-exposed dusts and underwater sediments, show an average signature of 0.07 ± 0.05‰ (n = 25, all sites), close to the δ¹²³Sb value measured previously in certified reference material of road dust (BCR 723, δ¹²³Sb = 0.03 ± 0.05‰). Moreover, a fractionation of Sb isotopes is observed between solid and dissolved phases in one sample, which might result from Sb (bio)reduction and/or adsorption processes. SEM-EDXS investigations show the presence of discrete submicrometric particles concentrating Sb in all the systems, interpreted as friction residues of Sb-containing brake pads. Sb solid speciation determined by linear combination fitting of X-Ray Absorption Near Edge Structure (XANES) spectra at the Sb K-edge shows an important spatial variability in the ponds, with Sb chemical forms likely driven by local redox conditions: "dry" samples exposed to air exhibited contributions from Sb(V)-O (52% to 100%) and Sb(III)-O (<10% to 48%) species whereas only underwater samples, representative of suboxic/anoxic conditions, showed an additional contribution from Sb(III)-S (41% to 80%) species. Altogether, these results confirm the traffic emission as a specific source of Sb emission in surface environments. The spatial variations of Sb speciation observed along the road-to-pond continuum likely reflect a high geochemical reactivity, which could have important implications on Sb transfer properties in (sub)surface hydrosystems.

Behavior of Sb and As in the hydrogeochemistry of adjacent karst underground river systems and the responses of such systems to mining activities

Through the investigation of Qinglong mining area and adjacent karst underground river system, mining activities and water-rock interactions are found to control the hydrogeochemical evolution of karst underground water. Along the flow direction of the karst underground river, the hydro-chemical type is converted from HCO₃-Ca type to SO₄-Ca type. The concentrations of Sb and As also gradually decrease. Using PHREEQC to calculate the SI shows that: in the karst underground river system, both gypsum and fluorite are unsaturated, indicating a high degree of water-rock interaction. LogPCO₂ is negatively correlated with pH, indicating that the karst underground river systems are both open systems. The dissolution of carbonate minerals and the alternate adsorption of ions are the main water-rock interactions that lead to the rapid decline of Sb and As concentrations. This research also applies principal component analysis to identify the types of pollution in adjacent karst underground river systems. The results show that the LongBaiwei underground river was mainly affected by coal mining activities, and Fe was more prominent; the ShuiYa underground river was more significantly affected by the leachate from the antimony tailings yard. This study provides a scientific basis for the evolution of the water environment as well as strategies for pollution prevention and control in typical karst underground river systems owing to the influence of mining activities.

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.

Evaluation of antimony availability in a mining context: Impact for the environment, and for mineral exploration and exploitation

This work aims to establish Sb mobility, its transfer to biota and its effect on soil health in a semi-arid climate. The results show the presence of stibnite (Sb2S3) as the main primary Sb compound, bindhemite (Pb2Sb2O6(O,OH)), and minor proportions of stibiconite (Sb3+(Sb5+)2O6(OH)) as oxidised Sb species. This research also observes very high total Sb contents in mining materials (max: 20,000 mg kg-1) and soils (400-3000 mg kg-1), with physical dispersion around mining materials restricted to 450 m. The soil-to-plant transfer is very low, (bioaccumulation factor: 0.0002-0.1520). Most Sb remains in a residual fraction (99.9%), a very low fraction is bound to Fe and Mn oxy-hydroxides or organic matter, and a negligible proportion of Sb is leachable. The higher Sb mobility rates has been found under oxidising conditions with a long contact time between solids and water. The main factors that explain the poor Sb mobility and dispersion in the mining area are the low annual rainfall rates that slow down the Sb mobilisation process and the scarce formation of oxidised Sb compounds. All these data suggest poor Sb (III) formation and a low toxicological risk in the area associated with past mining activities. The low mobility of Sb suggests advantages for future sustainable mining of such ore deposits in a semi-arid climate and is also indicative of the limitations of geochemical exploration in the search for new Sb deposits.

Elevated Urbanization-Driven Plant Accumulation of Metal(loid)s Including Arsenic Species and Assessment of the Kłodnica River Sediment Contamination

The impact of water and bottom sediment pollution of a river subjected to a strong industrial anthropogenic pressure of metal(loid) (including arsenic and its species) accumulation in riverbank plants such as Solidago virgaurea L., Phragmites L. and Urtica dioica L. was investigated. The high-performance liquid chromatography-inductively coupled plasma-mass spectrometry (HPLC-ICP-MS) technique was used to study organic and inorganic arsenic species in selected plants and their response to heavy metal and arsenic contamination. The modified BCR extraction results showed that arsenic was mainly bound to the mobile reducible and organic-sulfide fractions in the Kłodnica River bottom sediments. Research has shown that the bottom sediments of the Kłodnica River are contaminated with metals, including Pb, Zn, Ni, As, and among arsenic species, the As(V) form dominated quantitatively, with its highest concentration being 49.3 mg kg^-1 and the organic species occurred extremely rarely. The highest concentration of arsenic, among the tested plants, occurred in Phragmites communis L. The evaluation of the bottom sediment pollution was performed using Sb/As factor, geoaccumulation index (I_geo), enrichment factor (EF) and pollution load index (PLI). The ability of the plant to assimilate metals from the substrate was studied by calculation of the bioaccumulation factor (BAF). Values of the I_geo change in a wide range from class 1 (uncontaminated to moderately polluted for Cu and Zn) at the first sampling point, to 5 (highly to extremely polluted for Ba and Fe) at the K4 sampling point. The I_geo results show an increase in the contamination with elements toward the runoff of the Kłodnica River.

Experimental investigation of short-term warming on arsenic flux from contaminated sediments of two well-oxygenated subarctic lakes

Legacy arsenic (As) contamination from past mining operations remains an environmental concern in lakes of the Yellowknife area (Northwest Territories, Canada) due to its post-depositional mobility in sediment and potential for continued remobilization to surface waters. Warmer temperatures associated with climate change in this subarctic region may impact As internal loading from lake sediments either by a direct effect on sediment porewater diffusion rate or indirect effects on microbial metabolism and sediment redox conditions. This study assessed the influence of warmer temperatures on As diffusion from contaminated sediment of two lakes with contrasting sediment characteristics using an experimental incubation approach. Sediments from Yellowknife Bay (on Great Slave Lake) contained predominately clay and silt with low organic matter (10%) and high As content (1675 μg/g) while sediments of Lower Martin Lake had high organic matter content (~70%) and approximately half the As (822 μg/g). Duplicate sediment batches from each lake were incubated in a temperature-controlled chamber, and overlying water was kept well-oxygenated while As flux from sediment was measured during four weekly temperature treatments (7°C to 21°C, at ~5°C intervals). During the experiment, As diffused from sediment to overlying water in all cores and temperature treatments, with As fluxes ranging from 48-956 μg/m2/day. Arsenic fluxes were greater from Yellowknife Bay sediments, which had higher solid-phase As concentrations, compared to those of Lower Martin Lake. Short-term warming did not stimulate As flux from duplicate cores of either sediment type, in contrast with reported temperature enhancement in other published studies. We conclude that warmer temperatures were insufficient to strongly enhance sediment As diffusion into overlying oxic waters. These observations are relevant for evaluating climate-warming effects on sediment As mobility in subarctic lakes with little or no thermal stratification and a well-oxygenated water column.

Antimony(V) Incorporation into Schwertmannite: Critical Insights on Antimony Retention in Acidic Environments

This study examines incorporation of Sb(V) into schwertmannite─an Fe(III) oxyhydroxysulfate mineral that can be an important Sb host phase in acidic environments. Schwertmannite was synthesized from solutions containing a range of Sb(V)/Fe(III) ratios, and the resulting solids were investigated using geochemical analysis, powder X-ray diffraction (XRD), dissolution kinetic experiments, and extended X-ray absorption fine structure (EXAFS) spectroscopy. Shell-fitting and wavelet transform analyses of Sb K-edge EXAFS data, together with congruent Sb and Fe release during schwertmannite dissolution, indicate that schwertmannite incorporates Sb(V) via heterovalent substitution for Fe(III). Elemental analysis combined with XRD and Fe K-edge EXAFS spectroscopy shows that schwertmannite can incorporate Sb(V) via this mechanism at up to about 8 mol % substitution when formed from solutions having Sb/Fe ratios ≤0.04 (higher ratios inhibit schwertmannite formation). Incorporation of Sb(V) into schwertmannite involves formation of edge and double-corner sharing linkages between Sb^VO₆ and Fe^III(O,OH)₆ octahedra which strongly stabilize schwertmannite against dissolution. This implies that Sb(V)-coprecipitated schwertmannite may represent a potential long-term sink for Sb in acidic environments.

Antimony and arsenic particle size distribution in a mining contaminated freshwater river: Implications for sediment quality assessment and quantifying dispersion

Assessments of antimony (Sb) and arsenic (As) contamination in sediments are reported on a wide range of different particle size fractions, including <63 μm, < 180 μm and <2 mm. Guidelines vary between jurisdictions which limits comparative assessment between contamination events and complicates ecotoxicity assessment, and almost no information exists on Sb size distribution in contaminated sediments. This study quantified and compared the size distribution of Sb and As in 11 sediments (and 2 floodplain soils) collected along 320 km of waterway contaminated by historic mining activity. Sediment particle size distribution was the primary determinant of total metalloid load in size fractions across the varying substrates of the waterway. Minerals and sorption complexes influenced metalloid particle distribution but relative importance depended on location. Arsenic concentrations were greatest in the fine <63 μm fraction across all the different river environments (7.3-189 mg kg^-1, or 1-26% of total sample As), attributed to fine-grained primary arsenopyrite and/or sorption of As(V) to fine solid-phases. The Sb particle size concentrations were greatest in mid-size fractions (205-903 mg kg^-1) in the upper catchment and up to 100 km downstream to the mid-catchment as a result of remnant Sb minerals. Antimony concentrations in the lower catchment were greatest in the <63 μm fraction (8.8-12.1 mg kg^-1), reflecting the increasing importance of sorption for Sb particle associations. This work demonstrates the importance of particle size analysed for assessment of sediment quality, and provides support for analysis of at least the <250 μm fraction for Sb and As when comparing pollutant distribution in events impacted by primary contamination. Analysis of the <63 μm fraction, however, provides good representation in well-dispersed contaminated sediments.

Tooeleite Transformation and Coupled As(III) Mobilization Are Induced by Fe(II) under Anoxic, Circumneutral Conditions

Tooeleite [Fe^III₆(As^IIIO₃)₄SO₄(OH)₄.4H₂O] is an important As(III) host phase in diverse mining-impacted environments. Tooeleite has also received attention as a target phase for immobilizing As(III) in environmental and engineered settings. However, little is known regarding tooeleite's environmental stability, with no previous research examining the possible role of Fe(II) in inducing tooeleite transformation (as occurs for Fe(III) oxide minerals). We investigated shifts in solid-phase Fe and As speciation and associated As mobilization into the aqueous phase during exposure of tooeleite to aqueous Fe(II) under anoxic conditions at pH 4 to 8. Our results demonstrate that environmentally relevant concentrations of aqueous Fe(II) (i.e., 1 to 10 mM) induce significant mobilization of As(III) from tooeleite under near-neutral pH conditions, with greater As(III) mobilization occurring at higher pH. Extended X-ray absorption fine structure spectroscopy at both the As and Fe K-edge reveals that the observed As(III) mobilization was coupled with partial Fe(II)-induced transformation of tooeleite to As(III)-bearing ferrihydrite at pH 6 to 8. These results provide new insights into the environmental stability of tooeleite and demonstrate a novel pathway for As(III) mobilization in tooeleite-bearing systems.

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.

Antimony isotope fractionation during adsorption on aluminum oxides

The environmental fate of antimony (Sb) is often strongly affected by adsorption, and the Sb isotope fractionation mechanism during adsorption has not been reported. Four batch experiments (kinetic, isothermal, effect of pH, and effect of coexisting anions) were conducted to evaluate the mechanism of Sb(V) adsorption to γ-Al₂O₃ and the fractionation of Sb isotopes. Extended X-ray absorption fine structure (EXAFS) analyses show Sb(V) adsorption on γ-Al₂O₃ occurs via outer-sphere surface complexation. The triple-layer model (TLM) effectively predicted the theoretical Sb adsorption amount under different pH conditions. The Sb isotope fractionation in the adsorption process can be divided into an initial kinetic stage (Rayleigh model, α_{adsorbed-aqueous} = 0.99975 ± 0.00003) and subsequent isotopic equilibrium stage due to isotope exchange; however, no significant equilibrium isotope fractionation (Δ¹²³Sb_{aqueous-adsorbed} = ~0 ± 0.08‰) was evident by the end of the experiments. We propose the lack of significant equilibrium isotope fractionation in the effect of pH and isothermal experiments is due to Sb forming an outer-sphere complex on γ-Al₂O_3. This study reveals Sb equilibrium isotope fractionation does not occur during Sb(V) adsorption onto γ-Al₂O₃, providing a reference for the future study of Sb isotopes and furthering understanding of the Sb isotope fractionation mechanism.

Antimony distribution and mobility in different types of waste derived from the exploitation of stibnite ore deposits

Wastes derived from the exploitation of stibnite ore deposits were studied to determine their mineralogical, chemical, and environmental characteristics and establish the Sb distribution and the current and long-term risks of Sb mobilization. Representative samples of mine waste rocks, mine tailings, and smelting waste were studied by X-ray powder diffraction, polarized light microscopy, electron microprobe analysis, and digestion, leaching, and extraction procedures. The main Sb-bearing minerals and phases identified in the smelting waste were natrojarosite, iron (oxyhydr)oxides, mixtures of iron and antimony (oxyhydr)oxides, and tripuhyite; those in the mine tailings and mine waste rocks were iron (oxyhydr)oxides and/or mixtures of iron and antimony (oxyhydr)oxides. Iron (oxyhydr)oxides and natrojarosite had high Sb contents, with maximum values of 16.51 and 9.63 wt% Sb₂O₅, respectively. All three types of waste were characterized as toxic; the mine waste rocks and mine tailings would require pretreatment to decrease their leachable Sb content before they would be acceptable at hazardous waste landfills. Relatively little of the Sb was in desorbable forms, which accounted for <0.01 and <0.8% of the total Sb content in the smelting waste and mine waste rocks/mine tailings, respectively. Under reducing conditions, further Sb mobilization from mine waste rocks and mine tailings could occur (up to 4.6 and 3.3% of the total content, respectively), considerably increasing the risk that Sb will be introduced into the surroundings. Although the smelting waste had the highest total Sb content, it showed the lowest risk of Sb release under different environmental conditions. The significant Fe levels in the smelting waste facilitated the formation of various Fe compounds that greatly decreased the Sb mobilization from these wastes.

Study on the Arsenate Removal from Raw As(V)-Rich Wastewater Using Zero-Valent Iron

Due to the large volumes of solid waste produced by the traditional arsenic-rich lime iron salt precipitation method treatment produced during wet-smelting by precious metal workshops, raw As(V)-rich wastewater from a domestic metallurgical enterprise was chosen as the research object. Zero-valent iron (ZVI) was used to remove arsenate (As(V)) from raw wastewater. Factors affecting the adsorption of As(V), such as the ZVI size and adsorption time, were investigated. The As(V) removal percentage was >98.2% when using 40, 100, 250, or 300 mesh ZVI in a 2.8 mg·L−1 As(V) solution at pH 7, with an iron mass–wastewater ratio of 5 g/100 mL, and 12 h reaction time. The As(V) removal percentage was >86.5% when using 40 mesh ZVI after 50 min of reaction. A comprehensive evaluation was performed on the effects of factors such as cost and water head loss. Here, 40 mesh ZVI was used for column-based separation, in which the mass of solid waste was very small. Column experiments indicated that the adsorbent more efficiently eliminated arsenate in comparison to the earlier reported adsorbents. High bed volumes (BV) of 3200 BV, 6300 BV, and 8400 BV up to a breakthrough concentration of 100 μg·L−1 were achieved for arsenate removal in the presence of 2.8 mg·L−1 of arsenic. The empty bed contact times (EBCTs) were 2.6 min, 5.1 min, and 9.8 min, respectively. Furthermore, the concentrations of other pollutants such as Cu2+, Zn2+, F−, Cd2+, Cr6+, Pb2+, and F- met the national discharge standard. The elimination of As(V) and other heavy metals from solutions employing ZVI is efficient, cheap, and produces no secondary environmental pollution, making it an ideal candidate for heavy metal removal from wastewater.

Reduction of antimony mobility from Sb-rich smelting slag by Shewanella oneidensis: Integrated biosorption and precipitation

The dissimilatory Fe(III)-reducing bacteria play a significant role in the mobility of antimony (Sb) under reducing environment. Sb-rich smelting slag is iron (Fe)-containing antimonic mine waste, which is one of the main sources of antimony pollution. In this study, the soluble antimony reacted with Fe(III) by S. oneidensis (Shewanella oneidensis strain MR-1) was performed in reduction condition, then the dissolution behavior of the Sb-rich smelting slag with S. oneidensis was investigated. The results showed that the released Sb was immobilized by S. oneidensis and the strain adsorbed Sb(III) preferentially. Sb(V) can be reduced by S. oneidensis without aqueous Fe. In the presence of Fe(III), S. oneidensis mediated Sb bio-adsorption and the chemical redox of Sb-Fe occurred simultaneously. Sb was co-precipitated with Fe to form the Sb(V)-O-Fe(III) secondary mineral, which was identified as the bidentate mononuclear edge-sharing structure by extended X-ray absorption fine structure (EXAFS) analysis. These results suggest that S. oneidensis has a positive effect on the immobilization and minimizing toxicity of antimony in anoxic soil and groundwater, which provides a theoretical basis for the treatment of antimony contamination.

Innovative in situ remediation of mine waters using a layered double hydroxide-biochar composite

The current demand for alternative water sources requires the incorporation of low-cost composites in remediation technologies. These represent a sustainable alternative to more expensive, commercially used adsorbents. The main objective of this comprehensive field-scale study was to incorporate the layered double hydroxides (LDHs) into the hybrid biochar-based composites and apply an innovative material to remediate As/Sb-rich mine waters. The presence of hydrous Fe oxides (HFOs) within the composite enhanced the total adsorption efficiency of the composite for As(V) and Sb(V). The kinetic data fitted a pseudo-second order model. Equilibrium experiments confirmed that the composite had a stronger interaction with As(V) than with Sb(V). The efficient removal of As(V) from mine water was achieved in both batch and continuous flow column systems, reaching up to 98% and 80%, respectively. Sb(V) showed different behavior to As(V) during mine water treatment, reaching adsorption efficiencies of up to 39% and 26% in batch and column experiments, respectively. The migration of Sb(V) in mine water was mostly attributed to its dispersion before it was able to show affinity to the composite. In general, the proposed column technology is suitable for the field remediation of small volumes of contaminated water, and thus has significant commercial potential.

Spatial distribution, geochemical behaviors and risk assessment of antimony in rivers around the antimony mine of Xikuangshan, Hunan Province, China

Pollutants derived from antimony (Sb) mining can easily cause pollution of surrounding water bodies. However, qualitative source analysis of river pollution is mostly conducted, and quantitative source analysis is still lacking. A total of 21 water samples were collected to analyze the pollution status of the heavy metal element Sb, explore the Xikuangshan (XKS) area river heavy metals pollution mechanism, undertake quantitative analysis of the sources of pollution, and carry out irrigation water suitability assessment and potential ecological risk assessment. The results showed that, compared with the mining non-affected area, the maximum excess multiple of Sb in surface water and rivers in Hunan XKS area is 411.31. When the river fluid flows through the mining-affected area, the heavy metal element Sb content increases rapidly, and then decreases due to dilution process. Positive matrix factorization (PMF) source analysis showed that the main source of Sb pollution in the rivers is the impact of mining and smelting (83.60%), followed by the role of waste rock leaching (16.40%). After irrigation, 27.78% of the river water had strong ecological risks, and 16.67% had extremely strong ecological risks. This achievement provides a theoretical basis and technical guarantee for protecting and using the local water body of the mining area.

Adsorption-Desorption Behavior of Arsenate Using Single and Binary Iron-Modified Biochars: Thermodynamics and Redox Transformation

Arsenic (As) is a dangerous contaminant in drinking water which displays cogent health risks to humans. Effective clean-up approaches must be developed. However, the knowledge of adsorption-desorption behavior of As on modified biochars is limited. In this study, the adsorption-desorption behavior of arsenate (As^V) by single iron (Fe) and binary zirconium-iron (Zr-Fe)-modified biosolid biochars (BSBC) was investigated. For this purpose, BSBC was modified using Fe-chips (FeBSBC), Fe-salt (FeCl₃BSBC), and Zr-Fe-salt (Zr-FeCl₃BSBC) to determine the adsorption-desorption behavior of As^V using a range of techniques. X-ray photoelectron spectroscopy results revealed the partial reduction of pentavalent As^V to the more toxic trivalent As^III form by FeCl₃BSBC and Zr-FeCl₃BSBC, which was not observed with FeBSBC. The Langmuir maximum As^V adsorption capacities were achieved as 27.4, 29.77, and 67.28 mg/g when treated with FeBSBC (at pH 5), FeCl₃BSBC (at pH 5), and Zr-FeCl₃BSBC (at pH 6), respectively, using 2 g/L biochar density and 22 ± 0.5 °C. Co-existing anions reduced the As^V removal efficiency in the order PO₄ ^3- > CO₃ ^2- > SO₄ ^2- > Cl^- > NO₃ ^-, although no significant inhibitory effects were observed with cations like Na⁺, K⁺, Mg²⁺, Ca²⁺, and Al³⁺. The positive correlation of As^V adsorption capacity with temperature demonstrated that the endothermic process and the negative value of Gibbs free energy increased (-14.95 to -12.47 kJ/mol) with increasing temperature (277 to 313 K), indicating spontaneous reactions. Desorption and regeneration showed that recycled Fe-chips, Fe-salt, and Zr-Fe-salt-coated biochars can be utilized for the effective removal of As^V up to six-repeated cycles.

Advances in As contamination and adsorption in soil for effective management

Arsenic (As) is a heavy metal that causes widespread contamination and toxicity in the soil environment. This article reviewed the levels of As contamination in soils worldwide, and evaluated how soil properties (pH, clay mineral, organic matter, texture) and environmental conditions (ionic strength, anions, bacteria) affected the adsorption of As species on soils. The application of the adsorption isotherm models for estimating the adsorption capacities of As(III) and As(V) on soils was assessed. The results indicated that As concentrations in contaminated soil varying significantly from 1 mg/kg to 116,000 mg/kg, with the highest concentrations being reported in Mexico with mining being the dominating source. Regarding the controlling factors of As adsorption, soil pH, clay mineral and texture had demonstrated the most significant impacts. Both Langmuir and Freundlich isotherm models can be well fitted with As(III) and As(V) adsorption on soils. The Langmuir adsorption capacity varied in the range of 22-42400 mg/kg for As(V), which is greater than 45-8901 mg/kg for As(III). The research findings have enhanced our knowledge of As contamination in soil and its underlying controls, which are critical for the effective management and remediation of As-contaminated soil.

Removal of Antimony(V) from Drinking Water Using nZVI/AC: Optimization of Batch and Fix Bed Conditions

Antimony (Sb) traces in water pose a serious threat to human health due to their negative effects. In this work, nanoscale zero-valent iron (Fe0) supported on activated carbon (nZVI) was employed for eliminating Sb(V) from the drinking water. To better understand the overall process, the effects of several experimental variables, including pH, dissolved oxygen (DO), coexisting ions, and adsorption kinetics on the removal of Sb(V) from the SW were investigated by employing fixed-bed column runs or batch-adsorption methods. A pH of 4.5 and 72 h of equilibrium time were found to be the ideal conditions for drinking water. The presence of phosphate (PO43-), silicate (SiO42-), chromate (CrO42-) and arsenate (AsO43-) significantly decreased the rate of Sb(V) removal, while humic acid and other anions exhibited a negligible effect. The capacity for Sb(V) uptake decreased from 6.665 to 2.433 mg when the flow rate was increased from 5 to 10 mL·min-1. The dynamic adsorption penetration curves of Sb(V) were 116.4% and 144.1% with the weak magnetic field (WMF) in fixed-bed column runs. Considering the removal rate of Sb(V), reusability, operability, no release of Sb(V) after being incorporated into the iron (hydr)oxides structure, it can be concluded that WMF coupled with ZVI would be an effective Sb(V) immobilization technology for drinking water.

Effect of Municipal Solid Waste Compost on Antimony Mobility, Phytotoxicity and Bioavailability in Polluted Soils

The effect of a municipal solid waste compost (MSWC), added at 1 and 2% rates, on the mobility, phytotoxicity, and bioavailability of antimony (Sb) was investigated in two soils (SA: acidic soil; SB: alkaline soil), spiked with two Sb concentrations (100 and 1000 mg kg−1). The impact of MSWC on microbial activity and biochemical functioning within the Sb-polluted soils was also considered. MSWC addition reduced water-soluble Sb and favored an increase in residual Sb (e.g., by 1.45- and 1.14-fold in SA-100 and SA-1000 treated with 2% MSWC, respectively). Significant increases in dehydrogenase activity were recorded in both the amended soils, as well as a clear positive effect of MSWC on the metabolic activity and catabolic diversity of respective microbial communities. MSWC alleviated Sb phytotoxicity in triticale plants and decreased Sb uptake by roots. However, increased Sb translocation from roots to shoots was recorded in the amended soils, according to the compost rate. Overall, the results obtained indicated that MSWC, particularly at a 2% rate, can be used for the recovery of Sb-polluted soils. It also emerged that using MSWC in combination with triticale plants can be an option for the remediation of Sb-polluted soils, by means of assisted phytoextraction.

A study on the effects of anion, cation, organic compounds, and pH on the release behaviors of As and Sb from sediments

The trace element of As and Sb have resulted in serious threats to ecological sustainability and human health in the Xijiang River basin, China. The geoaccumulation index (I_geo) results showed the sediment of Xijiang River was heavily contaminated by Sb as well as moderately contaminated by As. The effect of inorganic anions on the released As and Sb from the sediment increases in the following sequence: Cl^- < SO₄^2- < CO₃^2- < PO₄^3-. The presence of PO₄^3- and CO₃^2- had the greater effect over As mobility from the sediment compared to Sb. The effect of Ca(II), Al(III), and Mg(II) on the released amount of Sb from the sediment is negligible. Meanwhile, in the case of As, Ca(II) and Mg(II) had small impacts, but the released amount of As increased slightly with an increase of Al(III) concentration. The stability of As and Sb in the sediment was found to be the best at pH 5. The effect of organic compounds (≤ 0.05 mol/L) on the dissolved As percentages from the sediment increased in the following sequence: ethylenediaminetetraacetate (EDTA) < oxalate < citrate, and the effect on Sb release was oxalate < EDTA < citrate at concentrations (≤ 0.025 mol/L). This study provides the basic theoretical basis to manage the mobilization of co-contamination of sediments with As and Sb under the influence of external perturbations.

Insights into the fate of antimony (Sb) in contaminated soils: Ageing influence on Sb mobility, bioavailability, bioaccessibility and speciation

The effect of long-term ageing (up to 700 days) on the mobility, potential bioavailability and bioaccessibility of antimony (Sb) was investigated in two soils (S1: pH 8.2; S2: pH 4.9) spiked with two Sb concentrations (100 and 1000 mg·kg^-1). The Sb mobility decreased with ageing as highlighted by sequential extraction, while its residual fraction significantly increased. The concentration of Sb (C_DGT), as determined by diffusive gradients in thin films (DGT), showed a reduction in potential contaminant bioavailability during ageing. The DGT analysis also showed that Sb-C_DGT after 700 days ageing was significantly higher in S1-1000 compared to S2-1000, suggesting soil pH plays a key role in Sb potential bioavailability. In-vitro tests also revealed that Sb bioaccessibility (and Hazard Quotient) decreased over time. Linear combination fitting of Sb K-edge XANES derivative spectra showed, as a general trend, an increase in Sb(V) sorption to inorganic oxides with ageing as well as Sb(V) bound to organic matter (e.g. up to 27 and 37% respectively for S2-100). The results indicated that ageing can alleviate Sb ecotoxicity in soil and that the effectiveness of such processes can be increased at acidic pH. However, substantial risks due to Sb mobility, potential bioavailability and bioaccessibility remained in contaminated soils even after 700 days ageing.

Impact of Antimony(V) on Iron(II)-Catalyzed Ferrihydrite Transformation Pathways: A Novel Mineral Switch for Feroxyhyte Formation

The environmental mobility of antimony (Sb) is controlled by interactions with iron (Fe) oxides, such as ferrihydrite. Under near-neutral pH conditions, Fe(II) catalyzes the transformation of ferrihydrite to more stable phases, thereby potentially altering the partitioning and speciation of associated Sb. Although largely unexplored, Sb itself may also influence ferrihydrite transformation pathways. Here, we investigated the impact of Sb on the Fe(II)-induced transformation of ferrihydrite at pH 7 across a range of Sb(V) loadings (Sb:Fe(III) molar ratios of 0, 0.003, 0.016, and 0.08). At low and medium Sb loadings, Fe(II) induced rapid transformation of ferrihydrite to goethite, with some lepidocrocite forming as an intermediate phase. In contrast, the highest Sb:Fe(III) ratio inhibited lepidocrocite formation, decreased the extent of goethite formation, and instead resulted in substantial formation of feroxyhyte, a rarely reported FeOOH polymorph. At all Sb loadings, the transformation of ferrihydrite was paralleled by a decrease in aqueous and phosphate-extractable Sb concentrations. Extended X-ray absorption fine structure spectroscopy showed that this Sb immobilization was attributable to incorporation of Sb into Fe(III) octahedral sites of the neo-formed minerals. Our results suggest that Fe oxide transformation pathways in Sb-contaminated systems may strongly differ from the well-known pathways under Sb-free conditions.