Oxidation and incorporation of adsorbed antimonite during iron(II)-catalyzed recrystallization of ferrihydrite

A step towards understanding plastic complexity: Antimony speciation in consumer plastics and synthetic textiles revealed by XAS

We identified the antimony species present in a wide variety of plastic samples by X ray absorption spectroscopy (XAS) at the Sb L3-edge. The samples contained different concentrations of antimony (Sb), ranging from PET bottles in which Sb compounds are used as catalysts, with concentrations around 300 mg/kg, to electrical equipment in which the element is used as a flame retardant, with concentrations of several tens of thousands of mg/kg. Although the shape of the spectra at the L3-edge is quite similar for all Sb reference materials, we were able to identify antimony glycolate or acetate in PET bottles, bound organic Sb in c-PET trays and senarmontite in electrical materials as the main Sb components. In samples with high Ca content (e.g., electrical objects, some c-PET food trays and textiles) the Ca Ka emission line interferes with the Sb La line by introducing a high background which reduces the signal-to-noise ratio in the Sb XAS spectrum, resulting in noisy and distorted spectra. The element-resolved map on a PET bottle sample revealed both Sb and Ca hot spots of around 10-20 microns in size, with no correlation.

Stability and transformation behavior of hydrometallurgical hazardous arsenic-calcium residue in sulfidic anoxic environments

Arsenic-calcium residue (ACR) is a hazardous solid waste generated by the metallurgical industry, posing a significant environmental risk. However, the stability and transformation behavior of ACR in sulfidic conditions remains unclear. Herein, we have investigated the stability and speciation evolution of arsenic (As), sulfur (S), and trace metals during the exposure of ACR to diverse S(-II) concentrations under anoxic conditions at pH of 6 and 11. Our results indicate that environmentally relevant levels of S(-II) (i.e., 1, 10, and 50 mM) significantly enhance the mobilization of As(III) and Cd2+ from ACR, with greater release at pH 6. The main mechanism for the release of As(III) and trace metals from ACR is the reductive dissolution of Ca-arsenate/arsenite and As-trace metals-gypsum. The reductive dissolution of As-trace metals-gypsum leads to the formation of S2O32─ and SO32─. XRD, FE-SEM, FTIR, XPS, and HRTEM analyses reveal that gypsum serves as the host phase for As fixation at pH 6, while calcium-arsenate/arsenite phases predominate at pH 11. Secondary As2S3, CdS, CuS, and symplesite are generated at pH 6, whereas parasymplesite, CdS, and CuS are predominant at pH 11. These results enhance our understanding of the environmental behavior of As, S, and trace metals associated with ACR.

Antimony and arsenic interactions with iron oxides and aluminum oxides in surface environment: A review focused on processes and mechanisms

It has been assumed and widely reported that arsenic (As) and antimony (Sb) share some similarities but also exhibit significant differences in their geochemical behaviors. Their environmental fates are generally controlled by iron (Fe) oxides and aluminum (Al) oxides. The mechanistic differences in their interactions, especially under dynamic environmental conditions, remain poorly understood, which hinders the development and implementation of effective pollution prevention and control measures. Therefore, this review focuses on the processes and mechanisms of interactions between As/Sb and Fe oxides/Al oxides. Antimony exhibits a higher susceptibility to oxidation than As due to its larger atomic radius and lower electronegativity. The property is an important basis for explaining the differences in their interactions in the environment. To obtain a clearer understanding of interactions, a detailed adsorption theory (charge distribution multi-site ion complexation) for the Fe oxides and Al oxides and three primary adsorption mechanisms (electrostatic adsorption, chemical adsorption, and coprecipitation) were explored. Furthermore, the effects of various factors (pH, redox, surface coverage, competing ions, and types of Fe oxides and Al oxides) on the adsorption efficiency were evaluated. We discussed the mechanisms and efficiency of Sb and As adsorption on Fe oxides and Al oxides, and the differences in Sb and As adsorption for various valence states. To efficiently control Sb and As pollution, some differences between Sb and As need to be taken into account.

Evaluating the influence of alternating flooding and drainage on antimony speciation and translocation in a soil-rice system

The quantitative evaluation of antimony (Sb) accumulation in rice has garnered significant attention due to the potential risks to human health. A pot experiment was conducted to investigate the essential nodes of Sb transfer in soil-rice system. Seven step extract results showed that during the flooding period, organic matter releasing was the primary factor contributing 14.1 % to the increase in Sb availability, while weakly crystallized Fe-Mn oxides and sulfides respectively accounted for 6.9 % and 1.42 %. During the drainage period, a notable increase in active Sb was observed, coinciding with decrease in Fe-Mn oxides and sulfides bond Sb. The migration rate constant of Sb from the root to the above-ground parts increased dramatically during the early flooding stage, being 2000 times higher than that in the mid-to-late stage. The shoot-to-grain migration rate constant remained low, at 1.07 × 10-2 d-1 and 3.52 × 10-3 d-1 during the flooding and drainage periods, respectively. Consequently, Sb accumulation amount in the grain (11.5 μg) was 2.2 times and 6.24 times lower than that in the roots and shoots, respectively. This study quantitatively evaluates the key processes controlling Sb transformation, uptake and translocation throughout different growth stages of the rice plant.

Incorporation of Shewanella oneidensis MR-1 and goethite stimulates anaerobic Sb(III) oxidation by the generation of labile Fe(III) intermediate

Dissimilatory iron-reducing bacteria (DIRB) affect the geochemical cycling of redox-sensitive pollutants in anaerobic environments by controlling the transformation of Fe morphology. The anaerobic oxidation of antimonite (Sb(III)) driven by DIRB and Fe(III) oxyhydroxides interactions has been previously reported. However, the oxidative species and mechanisms involved remain unclear. In this study, both biotic phenomenon and abiotic verification experiments were conducted to explore the formed oxidative intermediates and related processes that lead to anaerobic Sb(III) oxidation accompanied during dissimilatory iron reduction. Sb(V) up to 2.59 μmol L-1 combined with total Fe(II) increased to 188.79 μmol L-1 when both Shewanella oneidensis MR-1 and goethite were present. In contrast, no Sb(III) oxidation or Fe(III) reduction occurred in the presence of MR-1 or goethite alone. Negative open circuit potential (OCP) shifts further demonstrated the generation of interfacial electron transfer (ET) between biogenic Fe(II) and goethite. Based on spectrophotometry, electron spin resonance (ESR) test and quenching experiments, the active ET production labile Fe(III) was confirmed to oxidize 94.12% of the Sb(III), while the contribution of other radicals was elucidated. Accordingly, we proposed that labile Fe(III) was the main oxidative species during anaerobic Sb(III) oxidation in the presence of DIRB and that the toxicity of antimony (Sb) in the environment was reduced. Considering the prevalence of DIRB and Fe(III) oxyhydroxides in natural environments, our findings provide a new perspective on the transformation of redox sensitive substances and build an eco-friendly bioremediation strategy for treating toxic metalloid pollution.

Investigation of the migration of natural organic matter-iron-antimony nano-colloids in acid mine drainage

Colloids can potentially affect the efficacy of traditional acid mine drainage (AMD) treatment methods such as precipitation and filtration. However, it is unclear how colloids affect antimony (Sb) migration in AMD, especially when natural organic matter (NOM) is present. To conduct an in-depth investigation on the formation and migration behavior of NOM, iron (Fe), Sb and NOM-Fe-Sb colloids in AMD, experiments were performed under simulated AMD conditions. The results demonstrate significant variations in the formation of NOM-Fe-Sb colloids (1-3-450 nm) as the molar ratio of carbon to iron (C/Fe) increases within acidic conditions (pH = 3). Increasing the C/Fe molar ratio from 0.1 to 1.2 resulted in a decrease in colloid formation but an increase in particulate fraction. The distribution of colloidal Sb, Sb(III), and Fe(III) within the NOM-Fe-Sb colloids decreased from 68 % to 55 %, 72 % to 57 %, and 68 % to 55 %, respectively. Their distribution in the particulate fraction increased from 28 % to 42 %, 21 % to 34 %, and 8 % to 27 %. XRD, FTIR, and SEM-EDS analyses demonstrated that NOM facilitates the formation and crystallization of Fe3O4 and FeSbO4 crystalline phases. The formation of the colloids depended on pH. Our results indicate that NOM-Fe-Sb colloids can form when the pH ≤ 4, and the proportion of colloidal Sb fraction within the NOM-Fe-Sb colloids increased from 9 % to a maximum of 73 %. Column experiments show that the concentration of NOM-Fe-Sb colloids reaches its peak and remains stable at approximately 3.5 pore volumes (PVs), facilitating the migration of Sb in the porous media. At pH ≥ 5, stable NOM-Fe-Sb colloids do not form, and the proportion of colloidal Sb fraction decreases from 7 % to 0 %. This implies that as pH increases, the electrostatic repulsion between colloidal particles weakens, resulting in a reduction in the colloidal fraction and an increase in the particulate fraction. At higher pH values (pH ≥ 5), the repulsive forces between colloidal particles nearly disappear, promoting particle aggregation. The findings of this study provide important scientific evidence for understanding the migration behavior of NOM-Fe-Sb colloids in AMD. As the pH gradually shifts from acidic to near-neutral pH during the remediation process of AMD, these results could be applied to develop new strategies for this purpose.

Sorption of antimony(V) to naturally formed multicomponent secondary iron minerals: Sorption behavior and a comparison with synthetic analogs

Antimony (Sb) pollution in water has attracted extensive attention due to the biotoxicity of Sb. Secondary iron minerals readily sorb heavy metal(loid)s and critically affect their cycling in terrestrial environments. However, compared with synthetic pure iron mineral phases, little is known about the Sb sorption behavior and mechanism on natural secondary iron minerals (nSIMs) composed of various mineral phases. In this study, sorption experiments were conducted to investigate the Sb(V) sorption properties of nSIMs from an acid mine drainage zone and corresponding single-component synthetic secondary mineral phases and to compare their sorption behaviors and mechanisms. Spectroscopic analyses indicated that the nSIMs structurally resembled a hybrid of schwertmannite, jarosite and goethite. Sb(V) sorption on nSIMs, schwertmannite, goethite and jarosite was controlled by chemisorption, with maximum Sb(V) sorption capacities of 217.39, 233.65, 32.17 and 35.61 mg/g, respectively. nSIMs demonstrated an excellent Sb(V) sorption capacity equivalent to or greater than that of the single-component phases. XRD, FTIR and Raman analyses indicated that Sb(V) was immobilized on nSIMs mainly through ion exchange with structural SO42- and complexation interactions with surface FeO and FeOH; then, an FeOSb surface phase formed during the dissolution and further transformation of schwertmannite and jarosite into goethite. SEM revealed that nSIMs had an advantage in surface microstructure over the single components. These results suggested that despite the similarities in Sb(V) binding mechanism between nSIMs and schwertmannite, nSIMs might be more reactive for Sb(V) sorption since the nSIM components could mutually influence each other and facilitate Sb(V) sorption. This research suggests that nSIMs have potential for Sb(V) removal and helps elucidate the environmental behavior of Sb(V) associated with nSIMs.

Unveiling the crucial role of iron mineral phase transformation in antimony(V) elimination from natural water

Antimony (Sb) in natural water has long-term effects on both the ecological environment and human health. Iron mineral phase transformation (IMPT) is a prominent process for removing Sb(V) from natural water. However, the importance of IMPT in eliminating Sb remains uncertain. This study examined the various Sb-Fe binding mechanisms found in different IMPT pathways in natural water, shedding light on the underlying mechanisms. The study revealed that the presence of goethite (Goe), hematite (Hem), and magnetite (Mag) significantly affected the concentration of Sb(V) in natural water. Elevated pH levels facilitated higher Fe content in iron solids but impeded the process of removing Sb(V). To further our understanding, polluted natural water samples were collected from various locations surrounding Sb smelter sites. Results confirmed that converting ferrihydrite (Fhy) to Goe significantly reduced Sb levels (<5 μg/L) in natural water. The emergence of secondary iron phases resulted in greater electrostatic attraction and stabilized surface complexes, which was the most likely cause of the decline of Sb concentration in natural water. The comprehensive findings offer new insights into the factors governing IMPT as well as the Sb(V) behavior control.

Adsorption behavior and surface complexation modeling of oxygen anion Sb(V) adsorption on goethite

Simulating the adsorption behavior of Sb(V) on goethite is of great significance for predicting the mobility of Sb(V) in soil. However, there is still a lack of charge distribution and multisite surface complexation (CD-MUSIC) models that conform to the actual adsorption mechanism. Therefore, our research combined the previous EXAFS results with the SCMs and established two CD-MUSIC models (Model I, which introduced bidentate binuclear and bidentate mononuclear complexes, and Model II, which introduced bidentate mononuclear and monodentate mononuclear complexes). It is also one of the rare cases that bidentate edge- and corner-sharing adsorption complexes have been distinguished by introducing different amounts of H⁺. The results showed that Model I was more suitable for predicting the adsorption behavior of Sb(V) on the goethite surface than Model II. However, at low pH and high Sb(V) loading, the presence of the monodentate complex ≡FeOHSb(OH)₅^0.5- was possible due to the fast and slow two-step mechanism. Using the optimized CD-MUSIC model parameters, the contribution of surface species to the equilibrium adsorption under different conditions was determined. In the adsorption edge and adsorption isotherm simulation process, the edge-sharing bidentate mononuclear complex ≡(FeO)₂H₂Sb(OH)₄ was always the most important product, which was consistent with the results of extended X-ray absorption fine structure (EXAFS) analysis. The applicability of the model parameters was verified by predicting the competitive adsorption between PO₄^3- and Sb(V). The CD-MUSIC model established in the present study could be a useful tool for evaluating the equilibrium distribution behavior and environmental risk of Sb(V) in different types of soils.

The fate of co-existent cadmium and arsenic during Fe(II)-induced transformation of As(V)/Cd(II)-bearing ferrihydrite

Ubiquitous co-existence of arsenic (As) and cadmium (Cd) in smelting operations and mine drainage presents a major challenge to the environment. Fe(II)-induced ferrihydrite transformation into secondary, more crystalline minerals often controls the geochemical behavior of associated contaminants including arsenate (As(V)) and Cd(II) in natural and contaminated environments. However, the fate of co-existent As(V) and Cd(II) and the underlying mechanism during this transformation process remain unclear. In this contribution, ferrihydrite containing co-precipitated Cd(II) and As(V) with Fe(II) under diverse pH conditions has been investigated. Results from powder X-ray diffraction (PXRD), Fourier transform infrared spectroscopy (FTIR), and Raman spectra show that the co-existence of As(V) and Cd(II) significantly retards the transformation rates of As(V)/Cd(II)-bearing ferrihydrite to more stable iron oxides and reduces that from the newly formed lepidocrocite to goethite. Compared to Cd(II), the co-existent As(V) has stronger influence on the compositions of the transformation products. Chemical analysis shows that phosphate-unextractable As(V) and 0.4 M HCl unextractable Cd(II) both increase as the reaction proceeds during the recrystallization of As(V)/Cd(II)-bearing ferrihydrite, indicating that both As(V) and Cd(II) partially transform to a more stable phase. The co-existent Cd(II) has negligible effects on the As(V) redistribution, but the co-existent As(V) at high loadings has a significant modification in the distribution of Cd(II) during the transformation, which reduces the liberation of Cd(II) into solution, thus decreasing the mobility of Cd(II). These findings have important implications for understanding the mobility and fate of the co-existent As(V) and Cd(II) under natural anoxic environments, remediating the co-existent contaminants, and predicting the long-term behavior of As(V) and Cd(II) in natural and contaminated environments.

Antimony redox processes in the environment: A critical review of associated oxidants and reductants

The environmental behavior of antimony (Sb) has recently received greater attention due to the increasing global use of Sb in a range of industrial applications. Although present at trace levels in most natural systems, elevated Sb concentrations in aquatic and terrestrial environments may result from anthropogenic activities. The mobility and toxicity of Sb largely depend on its speciation, which is dependent to a large extent on its oxidation state. To a certain extent, our understanding of the environmental behavior of Sb has been informed by studies of the environmental behavior of arsenic (As), as Sb and As have somewhat similar chemical properties. However, recently it has become evident that the speciation of Sb and As, especially in the context of redox reactions, may be fundamentally different. Therefore, it is crucial to study the biogeochemical processes impacting Sb redox transformations to understand the behavior of Sb in natural and engineered environments. Currently, there is a growing body of literature involving the speciation, mobility, toxicity, and remediation of Sb, and several reviews on these general topics are available; however, a comprehensive review focused on Sb environmental redox chemistry is lacking. This paper provides a review of research conducted within the past two decades examining the redox chemistry of Sb in aquatic and terrestrial environments and identifies knowledge gaps that need to be addressed to develop a better understanding of Sb biogeochemistry for improved management of Sb in natural and engineered systems.

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.

Removal of Sb(V) from wastewater via siliceous ferrihydrite: Interactions among ferrihydrite, coprecipitated Si, and adsorbed Sb(V)

Although ferrihydrite (Fh) exhibits good Sb(V) adsorption behavior, the instability of its amorphous structure limits its engineering applications. In this study, siliceous ferrihydrite (SiFh) was prepared via coprecipitation to resolve these limitations. X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and SiFh aging tests revealed that the growth of Fh particles covered with Fe-O-Si links was inhibited while maintaining their amorphous structure. Meanwhile, the XRD patterns indicated that SiFh maintained excellent stability after five adsorption-desorption cycles. During the aging process, the added Si decreased the electrostatic interaction between SiFh and Sb(V), which weakened the affinity between Sb(V) and Fh; however, most of the Sb(V) still entered the Fe lattice after seven days of aging, which was favorable for Sb(V) recovery during reutilization. Furthermore, Sb(V) adsorbed from the simulated textile wastewater onto SiFh had the highest adsorption energy (E_ads), which meant its unstable inner-sphere complexation on the surface of SiFh. Meanwhile, the presence of SO₄^2-, NO₃^-, Ca²⁺, and Mg²⁺ contributed to Sb(V) outer-sphere adsorption. Both of these factors were conducive to Sb(V) desorption. Hence, SiFh is a promising adsorbent owing to its facile preparation process, stability, and optimal regeneration properties.

New Mobilization Pathway of Antimonite: Thiolation and Oxidation by Dissimilatory Metal-Reducing Bacteria via Elemental Sulfur Respiration

Antimony (Sb) mobilization is widely explored with dissimilatory metal-reducing bacteria (DMRB) via microbial iron(III)-reduction. Here, our study found a previously unknown pathway whereby DMRB release adsorbed antimonite (Sb^III-O) from goethite via elemental sulfur (S⁰) respiratory reduction under mild alkaline conditions. We incubated Sb^III-O-loaded goethite with Shewanella oneidensis MR-1 in the presence of S⁰ at pH 8.5. The incubation results showed that MR-1 reduced S⁰ instead of goethite, and biogenic sulfide induced the formation of thioantimonite (Sb^III-S). Sb^III-S was then oxidized by S⁰ to mobile thioantimonate (Sb^V-S), resulting in over fourfold greater Sb release to water compared with the abiotic control. Sb^IV-S was identified as the intermediate during the oxidation process by Fourier transform ion cyclotron resonance mass spectrometry and electron spin resonance analysis. The existence of Sb^IV-S reveals that the oxidation of Sb^III-S to Sb^V-S follows a two-step consecutive one-electron transfer from Sb to S atoms. Sb^V-S then links with Sb^III-S by sharing S atoms and inhibits Sb^III-S polymerization and Sb^III₂S₃ precipitation like a "capping agent". This study clarifies the thiolation and oxidation pathway of Sb^III-O to Sb^V-S by S⁰ respiration and expands the role of DMRB in the fate of Sb.