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

Distinct mechanisms of stibnite (Sb2S3) oxidative dissolution mediated by acid-generating and alkali-generating microorganisms within a wide pH range

Indigenous microorganisms in antimony mining areas facilitate stibnite dissolution over a wide pH range, yet their mechanisms remain poorly studied. Herein, the acid-generating Bosea sp. AS-1 and the alkali-generating Pseudomonas sp. PS-3 were selected to interact with stibnite (Sb2S3) under initial acidic (pH value = 5) and alkaline (pH value = 8) conditions, respectively. Results indicated that AS-1 and PS-3 promoted stibnite dissolution compared to the sterile control irrespective of initial pH conditions, but through distinct mechanisms. AS-1 oxidized sulfide to sulfate by regulating the expression of SoxB and SoxC genes, thereby driving the stibnite dissolution and oxidation. Up to 18.63 mg/L total antimony (Sb(tot)) was released and the dissolved Sb(III) was completely oxidized to Sb(V). Moreover, AS-1 prevented the passivation layer formation by inhibiting sulfur oxidation intermediates accumulation. Conversely, PS-3 could not oxidize sulfur, but produced more extracellular polymeric substances which bound microorganisms closely to stibnite. The stibnite dissolution was facilitated through pH elevation from 5.0 to above 9.0 mediated by PS-3, releasing up to 35.56 mg/L Sb(tot). However, the accumulated sulfur oxidation intermediates facilitated the passivation layer formation, inhibiting further dissolution of stibnite. Additionally, less than 44 % of the soluble Sb(III) was oxidized to Sb(V). These results contribute to understanding the microbial-mediated transformation, mobilization and oxidation of antimony.

Identification of the sources and migration of antimony in karst terraces via Sb stable isotopes

The provenance and migration pathways of antimony (Sb) in karst terraced soils remain poorly constrained. This study employs Sb isotopic compositions (ε¹²³Sb values ranging from -1.91ε to 4.55ε), major-trace element geochemistry, and mass balance modeling to quantitatively resolve Sb input fluxes and translocation mechanisms in typical Guizhou karst terrace systems. The results demonstrate that approximately 67 % of the soil Sb is derived from limestone weathering, with 27 % and 6 % attributable to wet deposition and dry deposition, respectively. Within the 0-0.59 m soil layer, the Sb isotopic composition is relatively heavier (1.96ε-3.18ε), which is likely influenced by atmospheric deposition and organic matter adsorption. Within the 0.59-8.7 m soil layer, the Sb isotopic composition becomes lighter (0.17ε-2.45ε), which is attributed to the preferential adsorption of lighter Sb isotopes by iron oxides and manganese oxides. Within the 8.7 m valley bottom layer, leaching drives the migration of heavier isotopes toward the valley bottom, resulting in the heaviest Sb isotopic composition in this layer (with a maximum value of 4.55ε).

Application of Antimony Stable Isotopes in Revealing the Source and Vertical Migration of Sb in Soil

Antimony (Sb) is a widespread contaminant that poses potential carcinogenic risks worldwide. Many countries are grappling with significant historical Sb-containing waste, leading to soil contamination, which is of international concern. The lack of clarity on Sb sources and migration mechanisms in soil limits the effective prevention and control of soil pollution. This study utilized Sb stable isotope techniques to analyze soil profiles from two pollution scenarios, with the aims of quantifying Sb sources, understanding migration processes, and elucidating isotopic fractionation mechanisms in soil. The results reveal that surface soil Sb originates primarily from atmospheric dry deposition (31.7-56.3%), wet deposition (19.6-32.7%), and rock weathering (<21.0%). In subsurface soils, Sb mainly comes from solutions resulting from desorption in upper soil layers, with adsorption by iron minerals, manganese (hydr-)oxides, and organic matter impeding its downward movement. Deeper soil layers contain Sb derived from parent rock weathering, with migration impacted by secondary mineral adsorption. Groundwater aids in Sb migration and influences isotopic fractionation during water-soil exchange. This research introduces a novel framework for quantitatively tracking Sb pollution and enhances the scientific understanding of the geochemical behavior of Sb in soil.

An inconvenient impact: Unveiling the overlooked differences in crystalline forms of iron (hydro)oxides on anaerobic digestion

Iron (hydro)oxides are commonly used to enhance anaerobic digestion due to their cost-effectiveness and versatility. However, the influence of crystalline structure on digestion performance is often overlooked despite their unique characteristics. In this study, we investigated how different crystalline forms of FeOOH affect substrate utilization, sludge activity, and the microbiomes in up-flow anaerobic sludge blanket (UASB) reactors. The crystalline structure of FeOOH impacted reactor performance, with γ-FeOOH, β-FeOOH, and α-FeOOH showing decreasing effectiveness, as reflected in chemical oxygen demand (COD) removal efficiencies of 99.0 %, 98.3 % and 97.1 %, respectively. FeOOH crystals influenced the secretion of extracellular polymeric substances (EPS) and sludge activity by releasing Fe ions at varying rates, leading to Fe accumulation in EPS in the order of β-FeOOH > γ-FeOOH > α-FeOOH. Additionally, γ-FeOOH supported the most stable microbial community structure, as indicated by the highest Alpha diversity index. This stability was associated with increased levels of Mesotoga and Syntrophus, along with the highest coenzyme F420 activity, which was approximately twice as high as in other groups. These findings underscore the crucial role of the crystalline structure of iron oxides in enhancing anaerobic digestion, emphasizing that biocompatibility should be a priority when optimizing digestion performance.

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.

Sedimentary antimony stable isotope record of anthropogenic contamination in a karst lake in southwestern China

Anthropogenic sources of antimony (Sb) are an important driver of pollution in the Earth environment, but their roles in the historical changes of Sb pollution in lake ecosystems are currently poorly understood. This study documents the sedimentary Sb deposition fluxes in Hongfeng lake (HFL), in southwestern China during 1958-2021 and quantifies the changes of anthropogenic contributions to sediments using Sb stable isotopes. Mean Sb concentration (mean: 1.89 mg kg-1) and deposition flux (mean: 302.1 ng cm-2 a-1) in lake sediments remained relatively stable from 1958 to 1980. Sb deposition fluxes increased rapidly since 1980, peaked at 990.8 ng cm-2 a-1 in 2000, and then decreased consistently, reaching 306.9 ng cm-2 a-1 in 2021. Generally, the historical changes in Sb isotopes were anticorrelated with Sb deposition fluxes and enrichment factors, suggesting a lower ε123Sb signature in anthropogenic loading sources, and highlight the ability of Sb isotopes to distinguish anthropogenic signatures from natural processes in complex hydrological systems. Using a binary end-member mixing model, the contributions of anthropogenic sources to the accumulated of Sb in the lake sediments were estimated to be 20 % before 1980s and increased approximate 58 % during 2000-2015, then decreased to 24 % in 2021, likely reflecting the changes of degree in regional industrial activities. Our results help to better understand the response of Sb pollutions to anthropogenic activities and would in turn benefit the controls Sb contamination in lake ecosystems.

Sensitive and isotopic interference-free analysis of Sb using hydride generation-microwave plasma torch-mass spectrometry under ambient condition

A sensitive and isotopic interference-free analysis method for Sb was developed based on hydride generation-microwave plasma torch-mass spectrometry (HG-MPT-MS). Compared to the conventional ICP-MS, MPT coupled to an ion trap mass spectrometer enabled much "softer" ionization of Sb under ambient condition, which provided multi-detection modes and various ion forms, such as Sb+, SbO+, SbO2-, SbO++H2O and so on. These ion formations can be easily regulated by tuning capillary voltage and tube lens voltage, which facilitated elimination of isotopic interference during analysis, for instance the interference of 123Te on 123Sb could be effectively excluded by optimizing parameters of capillary voltage and tube lens voltage. The potential application of HG-MPT-MS for Sb isotope ratio analysis was also demonstrated, which could be determined in different forms, e.g., 123Sb/121Sb or 123Sb16O/121Sb16O. The value of 123Sb/121Sb was determined to be 0.75110 ± 0.00038 (2σ, n > 50). In addition, the detection limit, linearity and spike recovery were also studied. Overall, HG-MPT-MS performed equally well on detection limit (0.05 μg/L) with ICP-MS or HG-AFS. The linearity (R2 = 0.998) was checked in the concentration range of 10-500 μg/L. Spike recovery were evaluated with two soil samples, and the obtained spike recovery ranged 90-100 %. In general, HG-MPT-MS was expected to be a versatile tool for study the biochemical or geochemical behaviors of Sb and other hydride forming elements under ambient condition in a much simpler and more efficient way.

Sulfidation-reoxidation enhances heavy metal immobilization by vivianite

Iron minerals in nature are pivotal hosts for heavy metals, significantly influencing their geochemical cycling and eventual fate. It is generally accepted that, vivianite, a prevalent iron phosphate mineral in aquatic and terrestrial environments, exhibits a limited capacity for adsorbing cationic heavy metals. However, our study unveils a remarkable phenomenon that the synergistic interaction between sulfide (S2-) and vivianite triggers an unexpected sulfidation-reoxidation process, enhancing the immobilization of heavy metals such as cadmium (Cd), copper (Cu), and zinc (Zn). For instance, the combination of vivianite and S2- boosted the removal of Cd2+ from the aqueous phase under anaerobic conditions, and ensured the retention of Cd stabilized in the solid phase when shifted to aerobic conditions. It is intriguing to note that no discrete FeS formation was detected in the sulfidation phase, and the primary crystal structure of vivianite largely retained its integrity throughout the whole process. Detailed molecular-level investigations indicate that sulfidation predominantly targets the Fe(II) sites at the corners of the PO4 tetrahedron in vivianite. With the transition to aerobic conditions, the exothermic oxidation of CdS and the S sites in vivianite initiates, rendering it thermodynamically favorable for Cd to form multidentate coordination structures, predominantly through the Cd-O-P and Cd-O-Fe bonds. This mechanism elucidates how Cd is incorporated into the vivianite structure, highlighting a novel pathway for heavy metal immobilization via the sulfidation-reoxidation dynamics in iron phosphate minerals.

Cadmium Isotope Fractionation during Adsorption onto Edge Sites and Vacancies in Phyllomanganate

Cadmium (Cd) geochemical behavior is strongly influenced by its adsorption onto natural phyllomanganates, which contain both layer edge sites and vacancies; however, Cd isotope fractionation mechanisms at these sites have not yet been addressed. In the present work, Cd isotope fractionation during adsorption onto hexagonal (containing both types of sites) and triclinic birnessite (almost only edge sites) was investigated using a combination of batch adsorption experiments, extended X-ray absorption fine structure (EXAFS) spectroscopy, surface complexation modeling, and density functional theory (DFT) calculations. Light Cd isotopes are preferentially enriched on solid surfaces, and the isotope fractionation induced by Cd2+ adsorption on edge sites (Δ114/110Cdedge-solution = -1.54 ± 0.11‰) is smaller than that on vacancies (Δ114/110Cdvacancy-solution = -0.71 ± 0.21‰), independent of surface coverage or pH. Both Cd K-edge EXAFS and DFT results indicate the formation of double corner-sharing complexes on layer edge sites and mainly triple cornering-sharing complexes on vacancies. The distortion of both complexes results in the negative isotope fractionation onto the solids, and the slightly longer first Cd-O distances and a smaller number of nearest Mn atoms around Cd at edge sites probably account for the larger fractionation magnitude compared to that of vacancies. These results provide deep insights into Cd isotope fractionation mechanisms during interactions with phyllomanganates.

Antimony isotopic fractionation induced by Sb(V) adsorption on β-MnO2

Antimony (Sb) isotopes hold immense promise for unraveling Sb biogeochemical cycling in environmental systems. Mn oxides help control the fate of Sb via adsorption reactions, yet the behavior and mechanisms of Sb isotopic fractionation on Mn oxides are poorly understood. In this study, we examine the Sb isotopic fractionation induced by adsorption on β-MnO2 in different experiments (kinetic, isothermal, effect of pH). We observe that adsorption on β-MnO2 surfaces preferentially enriches lighter Sb isotopes through equilibrium fractionation, with Δ123Sbaqueous-adsorbed of 0.55-0.79 ‰. Neither the pH or surface coverage affects the fractionation magnitude. The analysis of extended X-ray absorption fine structure (EXAFS) demonstrates that the enrichment of light isotope results from the adsorption of inner-sphere complexation on solids. Our finding of this study enhances our comprehension of the impact of β-MnO2 on Sb isotopic fractionation behavior and mechanism and facilitate the applicability of Sb isotopes as effective tracers to elucidate the origins and pathways of Sb contamination in environmental systems, as well as provide a new insight into forecasting the isotopic fractionation of other similar metals adsorbed by manganese oxides.

Antimony Isotope Fractionation during Kinetic Sb(III) Oxidation by Antimony-Oxidizing Bacteria Pseudomonas sp. J1

Antimony (Sb) isotopic fractionation is frequently used as a proxy for biogeochemical processes in nature. However, to date, little is known about Sb isotope fractionation in biologically driven reactions. In this study, Pseudomonas sp. J1 was selected for Sb isotope fractionation experiments with varying initial Sb concentration gradients (50-200 μM) at pH 7.2 and 30 °C. Compared to the initial Sb(III) reservoir (δ123Sb = 0.03 ± 0.01 ∼ 0.06 ± 0.01‰), lighter isotopes were preferentially oxidized to Sb(V). Relatively constant isotope enrichment factors (ε) of -0.62 ± 0.06 and -0.58 ± 0.02‰ were observed for the initial Sb concentrations ranging between 50 and 200 μM during the first 22 days. Therefore, the Sb concentration has a limited influence on Sb isotope fractionation during Sb(III) oxidation that can be described by a kinetically dominated Rayleigh fractionation model. Due to the decrease in the Sb-oxidation rate by Pseudomonas sp. J1, observed for the initial Sb concentration of 200 μM, Sb isotope fractionation shifted toward isotopic equilibrium after 22 days, with slightly heavy Sb(V) after 68 days. These findings provide the prospect of using Sb isotopes as an environmental tracer in the Sb biogeochemical cycle.

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.

Antimony mobility in soil near historical waste rock at the world's largest Sb mine, Central China

Soil near waste rock often contains high concentrations of antimony (Sb), but the mechanisms that mobilize Sb in a soil closely impacted by the waste rock piles are not well understood. We investigated these mobility mechanisms in soils near historical waste rock at the world's largest Sb mine. The sequential extraction (BCR) of soil reveal that over 95 % Sb is present in the residual fraction. The leached Sb concentration is related to the surface protonation and deprotonation of soil minerals. SEM-EDS shows Sb in the soil is associated with Fe and Ca. Moreover, X-ray absorption spectroscopy (XAS) results show Sb is predominantly present as Sb(V) and is associated with Fe in the form of tripuhyite (FeSbO4) as well as edge- and corner-sharing complexes on ferrihydrite and goethite. Thus, Fe in soils is important in controlling the mobility of Sb via surface complexation and co-precipitation of Sb by Fe oxides. The initially surface-adsorbed Sb(V) or co-precipitation is likely to undergo a phase transformation as the Fe oxides age. In addition, Sb mobility may be controlled by small amounts of calcium antimonate. These results further the understanding of the effect of secondary minerals in soils on the fate of Sb from waste rock weathering and inform source treatment for Sb-contaminated soils.

Antimony(V) behavior during the Fe(II)-induced transformation of Sb(V)-bearing natural multicomponent secondary iron mineral under acidic conditions

Fe(II)-induced phase transformations of secondary iron minerals have attracted considerable attention due to their influence on antimony (Sb) mobility. However, Fe(II)-induced natural multicomponent secondary iron mineral (nmSIM) transformations and the corresponding repartitioning of Sb on nmSIM under acidic conditions upon Fe(II) exposure have not been systematically examined. Herein, we investigated the effect of Fe(II) on nmSIM mineralogy and Sb mobility in Sb(V)-bearing nmSIM at pH 3.8 and 5.6 at various Fe(II) concentrations over 15 d. The Sb(V)-bearing nmSIM phase transformation occurred under both strongly and weakly acidic conditions without Fe(II) exposure, while the presence of Fe(II) significantly intensified the transformation, and substantial amounts of intermediary minerals, including jarosite, ferrihydrite, lepidocrocite and fougerite, formed during the initial reaction stage, especially at pH 5.6. X-ray diffraction (XRD) analyses confirmed that goethite and hematite were the primary final-stage transformation products of Sb(V)-bearing nmSIM, regardless of Fe(II) exposure. Throughout the Sb(V)-bearing nmSIM transformation at pH 3.8, Sb release was inversely related to the Fe(II) concentration in the initial stage, and after maximum release was achieved, Sb was gradually repartitioned onto the nmSIM. No Sb repartitioning occurred in the absence of Fe(II) at pH 5.6, but the introduction of Fe(II) suppressed Sb release and improved Sb repartitioning on nmSIM. This transformation was conducive to Sb reimmobilization on Sb(V)-bearing nmSIM due to the structural incorporation of Sb into the highly crystalline goethite and hematite generated by the Sb(V)-bearing nmSIM transformation, and no reduction of Sb(V) occurred. These results imply that Fe(II) can trigger mineralogical changes in Sb(V)-bearing nmSIM and have important impacts on Sb partitioning under acidic conditions. These new insights are essential for assessing the mobility and availability of Sb in acid mine drainage areas.

Antimony Isotope Fractionation during Adsorption on Iron (Oxyhydr)oxides

The fate of antimony (Sb) is strongly affected by adsorption, yet Sb isotope fractionation and the associated mechanism have not been widely reported. Here we experimentally investigated the process of Sb(V) adsorption on iron (oxyhydr)oxides and the associated isotope effects. Sb isotope fractionation occurs during adsorption (Δ123Sbsolution-mineral = 1.20 ± 0.02‰ for ferrihydrite and 2.35 ± 0.04‰ for goethite). Extended X-ray absorption fine structure (EXAFS) analysis shows that Sb(V) adsorption on iron (oxyhydr)oxides occurs via inner-sphere surface complexation, including mononuclear bidentate edge-sharing (2E) and binuclear bidentate corner-sharing (2C) complexes. A longer atom distance of Sb-Fe in ferrihydrite leads to less Sb isotope fractionation during Sb adsorption than in goethite. The Gibbs free energy and Mayer bond order were calculated based on density functional theory (DFT) and suggested that the strength of the bonding environment can be summarized as Sb(OH)6- > 2E > 2C. In turn, the bonding environment indicates the mechanism of Sb isotope fractionation during the process. This study reveals that Sb isotope fractionation occurs during Sb(V) adsorption onto iron (oxyhydr)oxides, providing a basis for the future study of Sb isotopes and further understanding of the fractionation mechanism.