The translocation of antimony in soil-rice system with comparisons to arsenic: Alleviation of their accumulation in rice by simultaneous use of Fe(II) and NO3. Sci Total Environ 2019;650:633-641. [PMID: 30212692 DOI: 10.1016/j.scitotenv.2018.09.054]

Iron-modified biochar effectively mitigates arsenic-cadmium pollution in paddy fields: A meta-analysis

The escalating problem of compound arsenic (As) and cadmium (Cd) contamination in agricultural soils necessitates the urgency for effective remediation strategies. This is compounded by the opposing geochemical behaviors of As and Cd in soil, and the efficacy of biochar treatment remains unclear. This pioneering study integrated 3780 observation pairs referred from 92 peer-reviewed articles to investigate the impact of iron-modified biochar on As and Cd responses across diverse soil environments. Regarding the treatments, 1) biochar significantly decreased the exchangeable and acid-soluble fraction of As (AsF1, 20.9%) and Cd (CdF1, 24.0%) in paddy fields; 2) iron-modified biochar significantly decreased AsF1 (32.0%) and CdF1 (27.4%); 3) iron-modified biochar in paddy fields contributed to the morphological changes in As and Cd, mainly characterized by a decrease in AsF1 (36.5%) and CdF1 (36.3%) and an increase in the reducible fraction of As (19.7%) and Cd (39.2%); and 4) iron-modified biochar in paddy fields increased As (43.1%) and Cd (53.7%) concentrations in the iron plaque on root surfaces. We conclude that iron-modified biochar treatment of paddy fields is promising in remediating As and Cd contamination by promoting the formation of iron plaque.

How Fe-bearing materials affect soil arsenic bioavailability to rice: A meta-analysis

Arsenic (As) contamination is widespread in soil and poses a threat to agricultural products and human health due to its high susceptibility to absorption by rice. Fe-bearing materials (Fe-Mat) display significant potential for reducing As bioavailability in soil and bioaccumulation in rice. However, the remediation effect of various Fe-Mat is often inconsistent, and the response to diverse environmental factors is ambiguous. Here, we conducted a meta-analysis to quantitatively assess the effects of As in soils, rice roots, and grains based on 673, 321, and 305 individual observations from 67 peer-reviewed articles, respectively. On average, Fe-Mat reduced As bioavailability in soils, rice roots, and grains by 28.74 %, 33.48 %, and 44.61 %, respectively. According to the analysis of influencing factors, the remediation efficiency of Fe-Mat on As-contaminated soil was significantly enhanced with increasing Fe content in the material, in which the industry byproduct was the most effective in soils (-42.31 %) and rice roots (-44.57 %), while Fe-biochar was superior in rice grains (-54.62 %). The efficiency of Fe-Mat in minimizing soil As mobility was negatively correlated with soil Fe content, CEC, and pH. In addition, applying Fe-Mat in alkaline soils with higher silt, lower clay and available P was more effective in reducing As in rice grains. A higher efficiency of applying Fe-Mat under continuous flooding conditions (27.39 %) compared with alternate wetting and drying conditions (23.66 %) was also identified. Our results offer an important reference for the development of remediation strategies and methods for various As-contaminated paddy soils.

Using Fe/H2O2-modified biochar to realize field-scale Sb/As stabilization and soil structure improvement in an Sb smelting site

Antimony (Sb) and arsenic (As) released from the Sb smelting activities pose a major environmental risk and ecological degradation in Sb smelting sites. Here the effects of Fe/H2O2 modified biochar (Fe@H2O2-BC) on the synchronous stabilization of Sb/As and the improvement of soil structure in a typical Sb smelting site in Southern China based on a 1-year field experiment were studied. Application of ≥1 % (w/w) Fe@H2O2-BC could stably decrease the leaching concentrations of Sb and As of the polluted soils to Environmental quality standards for surface water Chinese Level III (GB3838-2002). Compared to the untreated soils, the stabilization efficiency of soil Sb and As treated by Fe@H2O2-BC reached 90.7 % ~ 95.7 % and 89.6 % ~ 90.8 %, respectively. The residue fractions of Sb/As in the soils increased obviously, and the bio-availability of Sb/As decreased by 65.0-95.6 % and 91.1-96.0 %, respectively. Moreover, Fe@H2O2-BC addition elevated soil organic carbon content, increased soil porosity, and improved water retention capacity, indicating the positive effects on soil structure and functions. Advanced mineral identification and characterization systems showed that Sb/As usually occurred in Fe-bearing minerals and stabilized by surface complexation and co-precipitation. The findings demonstrated that 1 % (w/w) Fe@H2O2-BC was appropriate to Sb/As stabilization and soil function recovery following field conditions, which provided potential application for ecological restoration in Sb smelting sites.

A review on sources of soil antimony pollution and recent progress on remediation of antimony polluted soils

Antimony (Sb) is a serious toxic and non-essential metalloid for animals, humans, and plants. The rapid increase in anthropogenic inputs from mining and industrial activities, vehicle emissions, and shoot activity increased the Sb concentration in the environment, which has become a serious concern across the globe. Hence, remediation of Sb-contaminated soils needs serious attention to provide safe and healthy foods to humans. Different techniques, including biochar (BC), compost, manures, plant additives, phyto-hormones, nano-particles (NPs), organic acids (OA), silicon (Si), microbial remediation techniques, and phytoremediation are being used globally to remediate the Sb polluted soils. In the present review, we described sources of soil Sb pollution, the environmental impact of antimony pollution, the multi-faceted nature of antimony pollution, recent progress in remediation techniques, and recommendations for the remediation of soil Sb-pollution. We also discussed the success stories and potential of different practices to remediate Sb-polluted soils. In particular, we discussed the various mechanisms, including bio-sorption, bio-accumulation, complexation, and electrostatic attraction, that can reduce the toxicity of Sb by converting Sb-V into Sb-III. Additionally, we also identified the research gaps that need to be filled in future studies. Therefore, the current review will help to develop appropriate and innovative strategies to limit Sb bioavailability and toxicity and sustainably manage Sb polluted soils hence reducing the toxic effects of Sb on the environment and human health.

Repeated inoculation of antimony resistant bacterium reduces antimony accumulation in rice plants

Applying beneficial bacteria in rice rhizosphere to manage heavy metal behaviour in soil-plant system is a promising strategy. However, colonization/domination of exogenous bacteria in rhizosphere soils remains a challenge. In this study, a bacterium Ochrobactrum anthropi, which showed the potential of transforming soluble Sb^III into Sb₂O₃ mineral, was repeatedly inoculated into the rice rhizosphere weekly throughout the rice growth period, and the colonization of this bacterium in rice rhizosphere soils and its effect on Sb accumulation in rice plants were investigated. Results showed that repeated inoculants changed the native bacterial community in rhizosphere soils in comparison with the control, but the inoculated O. anthropi was not identified as an abundant species. With weekly inoculation, the decrease in Sb in rice roots and straws was maintained throughout the rice growth period, with decrease percentages ranging from 36 to 49% and 33-35%. In addition, decrease percentages of Sb in husks and grains at the maturing stage obtained 34 and 37%, respectively. Furthermore, the XRD identified the formation of valentinite (Sb₂O₃) on rice root in inoculation treatment, and the decrease percentages in aqueous Sb^III in rhizosphere were 53-100% through the growth period. It demonstrated that weekly inoculants performed their temporary activity of valentinite formation, and reduced Sb accumulation in rice plants efficiently. This study suggests that regardless of successful colonization, repeated inoculation of beneficial bacteria is an option to facilitate the positive effects of inoculated bacteria in the management of heavy metal behaviour.

Silicon Actuates Poplar Calli Tolerance after Longer Exposure to Antimony

The presence of antimony (Sb) in high concentrations in the environment is recognized as an emerging problem worldwide. The toxicity of Sb in plant tissues is known; however, new methods of plant tolerance improvement must be addressed. Here, poplar callus (Populus alba L. var. pyramidallis) exposed to Sb(III) in 0.2 mM concentration and/or to silicon (Si) in 5 mM concentration was cultivated in vitro to determine the impact of Sb/Si interaction in the tissue. The Sb and Si uptake, growth, the activity of superoxide dismutase (SOD), catalase (CAT), guaiacol-peroxidase (G-POX), nutrient concentrations, and the concentrations of photosynthetic pigments were investigated. To elucidate the action of Si during the Sb-induced stress, the impact of short and long cultivations was determined. Silicon decreased the accumulation of Sb in the calli, regardless of the length of the cultivation (by approx. 34%). Antimony lowered the callus biomass (by approx. 37%) and decreased the concentrations of photosynthetic pigments (up to 78.5%) and nutrients in the tissue (up to 21.7%). Silicon supported the plant tolerance to Sb via the modification of antioxidant enzyme activity, which resulted in higher biomass production (increased by approx. 35%) and a higher uptake of nutrients from the media (increased by approx. 10%). Silicon aided the development of Sb-tolerance over the longer cultivation period. These results are key in understanding the action of Si-developed tolerance against metalloids.

Aluminum adsorption and antimonite oxidation dominantly regulate antimony solubility in soils

Soil antimony (Sb) contamination occurs globally due to natural processes and human activities. Total Sb concentration in soils fails to assess its ecological risk, while determined by the concentration of available Sb, which is readily for biological uptake. Available Sb in different soils varied significantly according to soil properties. However, so far it is unknown how soil properties regulate Sb availability, and no model has been established to predict it through soil properties. In this study, 19 soils spiked with antimonite [Sb(III)] were used to identify the major factors controlling Sb availability and establish its predicting models. The results showed that available Sb in different soils varied largely depending on the contents of free aluminum (fAl), free iron (fFe) and electric conductivity (EC), which explained 33%, 27% and 24.9% of the total variation, respectively. During the first 42 days of soil aging, fAl and EC effectively predicted the concentrations of available Sb with R² = 0.64, while during the later stages (70-150 d) of soil aging, fAl content was the unique parameter employed into the predicting model (R² = 0.53). These results firstly demonstrate that the content of free aluminum (fAl) is the most important factor regulating Sb availability in soils, although the content of fAl is much lower than that of fFe. This finding can help to develop new remediation materials for Sb-contaminated soils. The prediction models can provide promising tools of assessing the ecological risk. In addition, Sb availability was also affected by the oxidation of Sb(III). After 150 days aging, 1-61% of Sb(III) was oxidized to pentavalent Sb [Sb(V)], which was significantly positively correlated with available Sb, suggesting that Sb(III) oxidization mobilizes Sb in soils. All these findings would help to understand Sb migration and transformation in soils, and to develop new strategies for remediating Sb-contaminated soils.

Impact of Biochars on the Iron Plaque Formation and the Antimony Accumulation in Rice Seedings

Biochar was a kind of restoration material for soil pollution. Investigation about biochar amendment on the Sb transformation in rice plants is scarce. The pot experiment was conducted to evaluate the impact of biochar on the iron plaque formation in Sb-contaminated soil, and the translocation and accumulation of Sb in rice seedings. After the straw and husk biochar amendments (5% by weight), the levels increased on average by 20.0% and 16.0% for exchangeable Sb in soil, and by 233.3% and 74.8% for soluble Sb in pore water, respectively; but the residual fractions of Sb decreased by 18.5% and 15.1%. The iron plaque formation on rice root surface was enhanced, but its sequestration capacity for Sb decreased due to increasing competition for binding sites led by the elevated phosphorus and silicon levels in pore water after biochar application. The shoot Sb content sharply increased by 215.8% upon straw biochar application.

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

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

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.

Impacts of typical engineering nanomaterials on the response of rhizobacteria communities and rice (Oryza sativa L.) growths in waterlogged antimony-contaminated soils

The combined eco-risks of Sb (widely presented in soils, especially nearing mining areas) and the engineering nanomaterials (ENMs) (applied in agriculture and soil remediation) still remain uncovered. The current study investigated the impacts of single and combined exposure of CuO, CeO₂ nanoparticles (NPs) and multi-walled carbon nanotube (MWCNTs) with Sb on rice growths and rhizosphere bacterial communities. The results showed that co-exposure of CuO NPs (0.075 wt%) with Sb (III) posed the most adverse impacts on root biomass and branches (up to 66.59% and 70.00% compared to other treatments, respectively). Treatments containing MWCNTs showed insignificant dose-dependent effects, while CeO₂ NPs combined with Sb (III) showed significant synergistic stimulating effects on the fresh weights of root and shoot, by 68.30% and 73.48% (p < 0.05) compared to single Sb exposure, respectively. The rice planting increased the percentage of non-specifically sorbed Sb in soils by 1.50-14.49 than the no-planting stage. Analysis on microbial communities revealed that co-exposure of CuO NPs with Sb (III) induced the greatest adverse impacts on rhizobacteria abundances and community structures at both phylum and genus levels. Therein, significant decrease of Bacteroidetes, Acidobacteria and increase of Firmicutes abundance at the phylum level were observed. This study provided information about the risks of different ENMs released to Sb-contaminated soils under flooded condition on both crops and bacterial communities.

Sb(III) resistance mechanism and oxidation characteristics of Klebsiella aerogenes X

Resistant bacteria are potential natural materials for the bioremediation of soil metalloid pollution. A strain isolated from farmland soil chronically exposed to Sb was identified as K. aerogenes X with high antimonite [Sb(III)] tolerance and oxidation ability. The resistance mechanism of K. aerogenes X and its extracellular polymeric substances (EPS), antioxidant enzymes, and oxidation characteristics in Sb(III) stress were investigated in this study by stress incubation experiments and FTIR. The biotoxicity of Sb was limited by the binding of the organic compounds in EPS, and the anionic functional groups (e.g., amino, carboxyl and hydroxyl groups, etc.) present in the cell envelope were the components primarily responsible for the metalloid-binding capability of K. aerogenes X. The K. aerogenes X can oxidize Sb(III), and its metabolites induce changes in reactive oxygen species (ROS), catalase (CAT), total superoxide dismutase (SOD) and glutathione s-transferase (GSH-S) activity, indicating that the resistance mechanisms of K. aerogenes X are mediated by oxidative stress, EPS restriction and cell damage. Oxidation of Sb(III) is driven by interactions in intracellular oxidation, cell electron transport, extracellular metabolism including proteins and low molecular weight components (LMWs). LMWs (molecular weight <3 kDa) are the main driving factor of Sb(III) oxidation. In addition, Sb resistance genes arsA, arsB, arsC, arsD and acr3 and potential oxidation gene arsH were identified in K. aerogenes X. Owing to its natural origin, high tolerance and oxidation ability, K. aerogenes X could serve as a potential bioremediation material for the mitigation of Sb(III) in contaminated areas.

Kinetics of antimony biogeochemical processes under pre-definite anaerobic and aerobic conditions in a paddy soil

While the transformation of antimony (Sb) in paddy soil has been previously investigated, the biogeochemical processes of highly chemical active Sb in the soil remain poorly understood. In addition, there is a lack of quantitative understanding of Sb transformation in soil. Therefore, in this study, the kinetics of exogenous Sb in paddy soils were investigated under anaerobic and aerobic incubation conditions. The dissolved Sb(V) and the Sb(V) extracted by diffusive gradient technique decreased under anaerobic conditions and then increased under aerobic conditions. The redox reaction of Sb occurred, and Sb bioavailability significantly decreased after 55 days of incubation. The kinetics of Fe and the scanning transmission electron microscopy analysis revealed that the Fe oxides were reduced and became dispersed under anaerobic conditions, whereas they were oxidized and re-aggregated during the aerobic stage. In addition, the redox processes of sulfur and nitrogen were detected under both anaerobic and aerobic conditions. Based on these observations, a simplified kinetic model was established to distinguish the relative contributions of the transformation processes. The bioavailability of Sb was controlled by immobilization as a result of S reduction and by mobilization as a result of Fe reductive dissolution and S oxidation, rather than the pH. These processes coupled with the redox reaction of Sb jointly resulted in the complex behavior of Sb transformation under anaerobic and aerobic conditions. The model-based method and findings of this study provide a comprehensive understanding of the Sb transformation in a complex soil biogeochemical system under changing redox conditions.

Unique feature of Fe-OM complexes for limiting Cd accumulation in grains by target-regulating gene expression in rice tissues

The excessive accumulation of cadmium (Cd) in rice grains is highly determined by the expression of specific genes in different tissues. Targeted gene regulation in rice plants is a long-standing challenge. Herein, a new strategy for regulating target gene expression responsible for Cd absorption and translocation in roots and leaves was developed by complexing Fe(II) with organic matter (i.e., Fe-OM) with the optimal mass ratio of 1. Results showed that Fe-OM noticeably reduced the grain Cd content from 0.48 ± 0.04 mg kg^-1 to 0.25 ± 0.03 mg kg^-1, exhibiting a significantly higher capacity in mitigating Cd accumulation in grains than Fe(II) or OM alone. The translocation factor (TF) was reduced from 0.14 (control) to 0.08 by Fe-FA from root to grain, which could be due to the preferential Cd translocation to leaves (i.e., TF_{root to leaves} was enhanced four times by the complex of Fe(II) with fulvic acid (Fe-FA). Further gene analysis revealed that the cooperative effects of OsNramp1 and OsNramp5 downregulation in roots/stems and OsLCT1 upregulation in leaves contributed to the mitigation of Cd in grains. This work provides a new strategy to regulating target gene expression in specific tissues to alleviate Cd accumulation in grains.

Tin and Antimony as Soil Pollutants along Railway Lines—A Case Study from North-Western Croatia

The aim of this study was to determine the extent and patterns of antimony and tin contamination in soils along railway lines, as there are very few data in the literature on this subject. The study was conducted in north-western Croatia. Total and bioavailable concentrations of Sn and Sb were detected using high resolution inductively coupled plasma mass spectrometry (HR-ICP-MS). The following results were obtained: total concentrations of Sb ranged from 0.98 to 52.0 mg/kg and of Sn from 3.04 mg/kg to 97.6 mg/kg. The soil samples showed pronounced Sb and Sn enrichment, up to 87 and 33 times the median value for European soils, respectively. In contrast to the total concentrations, the bioavailable concentrations showed relatively low values. For Sn, the percentage of total content ranged from 0.001 to 0.021%, while for Sb it ranged from 0.001 to 0.136%. Statistical data analysis suggests that the distribution of Sb and Sn in soils near railway lines is influenced by the functional use of the site, distance from the tracks, topography, age of the railway line, and also by soil properties such as soil texture, humus content, and soil pH. This study demonstrates that rail transport is a source of soil pollution with Sn and Sb. The origin of Sb and Sn enrichment is abrasion by brakes, rails, wheels, freight losses, exhaust gasses, etc. Both elements in soils along railway lines pose an environmental risk to humans, agricultural production, and wildlife, and therefore further detailed studies are required.

Evaluating the metabolic functional profiles of the microbial community and alfalfa (Medicago sativa) traits affected by the presence of carbon nanotubes and antimony in drained and waterlogged sediments

Antimony (Sb) is the ubiquitous re-emerging contaminant greatly accumulated in sediments which has been revealed risky to ecological environment. However, the impacts of Sb (III/V) on microbes and plants in sediments, under different water management with presence of engineering materials are poorly understood. This study conducted sequential incubation of sediments (flooding, draining and planting) with presence of multiwall carbon nanotubes (MWCNTs) and Sb to explore the influence on microbial functional diversity, Sb accumulation and alfalfa traits. Results showed that water management and planting led to greater impacts of sediment enzyme activities and microbial community metabolic function and bioavailable Sb fractions (defined as sum of acid-soluble fraction and reducible fraction, F1 + F2). Available fractions of Sb (V) showed higher correlation to microbial metabolism (r = 0.933) than that of Sb (III) (r = -0.480) in planting stage. MWCNTs with increasing concentrations (0.011%, w/w) positively correlated to microbial community metabolic function in planting stage whereas resulted in decreasing of Sb (III/V) concentrations in alfalfa, although 0.01% MWCNT led to increase of Sb (V) and decrease of Sb (V) by 50.97% and 32.68% respectively. This study provided information for investigating combined ecological impacts of heavy metal and engineering materials under different water managing sediments.

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.

Multiple effects of nitrate amendment on the transport, transformation and bioavailability of antimony in a paddy soil-rice plant system

Nitrate (NO₃^-) is known to be actively involved in the processes of mineralization and heavy metal transformation; however, it is unclear whether and how it affects the bioavailability of antimony (Sb) in paddy soils and subsequent Sb accumulation in rice. Here, the effects of NO₃^- on Sb transformation in soil-rice system were investigated with pot experiments over the entire growth period. Results demonstrated that NO₃^- reduced Sb accumulation in brown rice by 15.6% compared to that in the control. After amendment with NO₃^-, the Sb content in rice plants increased initially and then gradually decreased (in roots by 46.1%). During the first 15 days, the soil pH increased, the oxidation of Sb(III) and sulfides was promoted, but the reduction of iron oxide minerals was inhibited, resulting in the release of adsorbed and organic-bound Sb from soil. The microbial arsenite-oxidizing marker gene aoxB played an important role in Sb(III) oxidation. From days 15 to 45, after NO₃^- was partially consumed, the soil pH decreased, and the reductive dissolution of Fe(III)-bearing minerals was enhanced; consequently, iron oxide-bound Sb was transformed into adsorbed and dissolved Sb species. After day 45, NO₃^- was completely reduced, Sb(V) was evidently reduced to Sb(III), and green rust was generated gradually. Thus, the available Sb decreased due to its enhanced affinity for iron oxides. Moreover, NO₃^- inhibited the reductive dissolution of iron minerals, which ultimately caused low Sb availability. Therefore, NO₃^- can chemically and biologically reduce the Sb availability in paddy soils and alleviate Sb accumulation in rice. This study provides a potential strategy for decreasing Sb accumulation in rice in the Sb-contaminated sites.

Bacterially mediated release and mobilization of As/Fe coupled to nitrate reduction in a sediment environment

Metal-reducing bacteria play an important role in the release and mobilization of arsenic from sediments into groundwater. This study aimed to investigate the influence of nitrate on arsenic bio-release. Microcosm experiments consisting of high arsenic sediments and indigenous bacterium Bacillus sp. D2201 were conducted and the effects of nitrate on the mobilization of As/Fe determined. The results show arsenic release is triggered by iron reduction, which is regulated by nitrate. Increasing the nitrate concentration from 0 to 1 and 3 mM decreased Fe(III) reduction by 62.5% and 16.9% and decreased As(V) bio-release by 41.5% and 85.5%, respectively. Moreover, the results of step-wise Wenzel sequential extractions indicate nitrate addition prevents the transformation of poorly crystalline iron oxides to well crystalline iron oxides. Overall, nitrate appears to have a dual effect, inhibiting both iron reduction and arsenic release by incubation strain D2201. This study offers new insights regarding the biogeochemistry of arsenic in groundwater systems.

Transformation pathways of arsenic species: SRB mediated mechanism and seasonal patterns

Sulfate reducing bacteria (SRB) mediated reduction plays a key role in the biological cycling of As, which inherently associates with the transformation of As species. However, the potential pathways of As species transformation, the predominant driving process and their explanatory factors regulating seasonal As mobility mediated by SRB remains poorly understood. This study explored the possible pathways of seasonal As species transformation mediated by SRB, and identified the predominant driving process and key environmental factors in response to As mobilization in different seasons. SRB-mediated reduction governed the seasonal mobilization of As, significantly promoted reduction of As (V) and endogenous release of As, and exhibited strong seasonal variability. The flux of As(III) and TAs in group SRB in summer were 1.92-3.53 times higher than those during the ice-bound period. The results showed two distinct stages namely release and re-immobilization both in summer and ice-bound period. While As was easier to be gradually transformed into a more stable state in SRB reduction process during ice-bound period. Both in summer and ice-bound period, SRB presented significant regulating effects on As behavior by influencing loosely adsorbed As, pyrite and As sulfides in sediments as well as the formation of sulfide during the process of SRB reduction. The main effecting pathways on As mobilization were the direct effects of SRB, S^2- and Fe²⁺ in summer, but IP was also an important pathway affecting As mobility during ice-bound period. This work provides new insights into mechanisms responsible for seasonal As mobilization.

Abiotic mediation of common ions on the co-exposure of CeO2 NPs with Sb (III) or Sb (V) to Glycine max (Linn.) Merrill. (Soybean): Impacts on uptake, accumulation and physiochemical characters

With the most active Sb mines, the "dominance" on Sb production of China lead to increasingly release and omnipresence of Sb in environment through mining activities as well as the life cycle of Sb-containing productions. The introduction of engineered nanoparticles (ENPs) accidentally or intentionally (such as NP-containing sludge as fertilizer) might increase the probability of co-exposed with Sb to plants. In this study, CeO2 NPs, one of the most widely used nanomaterials in industries with potential oxidizing or reducing properties, was selected and co-exposed with Sb (III) or Sb (V) to investigate their mutual effects on uptake, accumulation and physiological effects in soybeans. The results showed that CeO2 NPs increased the Sb (III) and Sb (V) concentrations in roots by 36.7% and 14.0% respectively, while Sb (III) and Sb (V) inhibited the concentration of Ce in roots by 97.1% and 86.9% respectively. In addition, the impacts of extra common ions (Mn2+, Cu2+, Fe3+ and Zn2+) on the fate of Ce and Sb in soybeans in co-exposure of CeO2 NPs with Sb were investigated as well. Mn2+ and Fe3+ increased the accumulations of Ce and Sb (III) in the co-exposure of CeO2 NPs with Sb (III), but reduced that in the co-exposure of CeO2 NPs with Sb (V). Notably, the addition of Cu2+ and Zn2+ consistently increased the uptake and accumulation of Ce and Sb in the co-exposure treatments. Moreover, the effects of Sb on the dissolved portion of CeO2 NPs in soybean roots were also investigated. This study provided a perspective that extra ingredient (mineral elements, organic element or other nutrients) might regulated the interactions in ENPs-heavy metals-plants system which need further explorations.

The effect of straw-returning on antimony and arsenic volatilization from paddy soil and accumulation in rice grains

Pollution by antimony (Sb) and arsenic (As) in soil can pose a great threat to human health. Straw-returning is widely applied to paddy fields for improving and remediating soil. A pot experiment was conducted to investigate the effect of straw-returning on Sb and As transformation and translocation in a soil-rice system. In this study, Sb and As co-contaminated soil was thoroughly mixed with different proportions (0, 0.5, 1, and 2%) of straw and used for growing rice plants through the entire growing stage in a pot experiment and 4 weeks in a microcosm experiment. The straw application significantly increased Sb and As mobility. The concentrations of total Sb and As in soil-pore water increased after the application of straw in most growing stages. The Sb volatilization in the pot and microcosm experiments was also stimulated by straw application. With the high dose of straw application (2%), the concentration of Sb in brown grain was reduced by 72% compared with the control, but As concentrations increased by around 77%. These findings provide a new perspective in that straw-returning could affect the behavior of both Sb and As in soil and reduce the Sb accumulation in brown grain and some guidance in the use of straw-returning in Sb-contaminated paddy soil.

Behaviors of heavy metal(loid)s in a cocontaminated alkaline paddy soil throughout the growth period of rice

A pot experiment was conducted to investigate uptake of cadmium (Cd), arsenic (As) and antimony (Sb) by rice from a lime-treated paddy soil contaminated with the three pollutants. The results showed that the content of Cd in the total rice plants decreased as the plant grew, whereas the As and Sb contents increased steadily. The concentration of As in the pore water showed steady increase throughout the growth period, likely due to the reductive dissolution of iron (Fe)-bearing minerals and the reduction of As(V). In contrast, the concentrations of Cd and Sb in the pore water increased initially, likely attributable to the reductive dissolution of Fe-bearing minerals, and then decreased likely due to their adsorptions onto carbonate and Fe sulfides, the reduction of Sb(V), and the formation of CdS. A random forest model was used to quantitatively evaluate the relative contributions of environmental factors to the accumulation of Cd, As, and Sb in the rice plants. The results suggest that sulfides produced through sulfate reduction and the formation of Cd forms associated with sulfur (S) might significantly affected the Cd content in the rice plants. In addition, the dissolved Fe species, the oxidation-reduction potential, and the abundance of the As(V)-respiring gene were major contributors to the As content in the rice plants, suggesting the important role of the reduction of Fe-bearing minerals and As(V). The results also showed that the Sb content in the rice plants was correlated with Fe species, Sb(V) reduction, and acid volatile S. The environmental behaviors of Cd, As, and Sb in the cocontaminated paddy soil exhibited significant differences. Such differences should be considered in remedy of soils contaminated with multiple heavy metals and metalloids.

The effect of an antimony resistant bacterium on the iron plaque fraction and antimony uptake by rice seedlings

Iron plaque (IP) is crucial in mitigating antimony (Sb) uptake and accumulation in rice plants, while, few studies focused on the effect of the iron plaque-associated Sb resistant bacteria on IP and Sb uptake into rice plants. Here, the effect of a Sb resistant bacterium (GenBank accession No. MH345840, with potential of conversion soluble Sb(III) into insoluble Sb₂O₃) on IP and Sb(III)/Sb(V) uptake under hydroponic condition was investigated. The results showed that in the presence of Sb(III), a large quantity of bacterial cells consorted with IP on rice roots, the bacterial inoculum altered the IP fraction distribution without enhancing its amount. However, it reduced Sb(III) uptake into rice roots. On contrary, seldom bacterial cells associated with the IP on rice roots in the presence of the Sb(V), the bacterial inoculum increased the IP amount slightly, and did not decline the Sb(V) uptake into rice roots. It also showed that the bacterial inoculum decreased Sb concentrations in rice shoots greatly in both Sb(III) and Sb(V) supplied treatments.

Combined Effect of Ferrous Ion and Biochar on Cadmium and Arsenic Accumulation in Rice

Excessive accumulation of cadmium (Cd) and arsenic (As) in rice (Oryza sativa L.) poses a potential health risk to populations. Cd and As exhibit opposite geochemical behavior in paddy soil, using appropriate remediation materials to reduce their migration and inhibit their uptake by rice is a great challenge. A pot culture experiment was conducted to investigate the effects of application of silkworm excrement biochar (BC) and ferrous sulfate (Fe(II)) on available Cd and As in paddy soils and their uptake by rice. Results showed that the application of BC + Fe significantly accelerated the tillering of rice plants, and the addition of BC alone to soil did not have a significant effect on the pH of soil, while applied 1% (w/w) BC and 1% (w/w) Fe(II) (1BC-1 Fe(II)) treatment could markedly reduce the soil pH. BC+Fe(II) could reduce the content of available Cd (reduced by 10%–23%) and As (reduced by 6%–33%) in soil. BC+Fe(II) has a distinct decreasing effect on the available As, thus inhibiting As uptake in rice tissues, and the effect was more obvious with an increasing mass ratio of Fe in BC+Fe(II) treatment. BC+Fe(II) decreased bioaccumulation factors (BF) of As compared to control and BC alone treatments. Compared with As, Cd was more readily transferred from the root to the shoot and accumulated in rice eventually. These findings provide a safe and reliable remediation strategy though application of BC+Fe(II) in Cd and As co-contaminated soil. However, the improvement effect of amendments should be paid a special attention on soil pH.

Risk Assessment of Potentially Toxic Elements Pollution from Mineral Processing Steps at Xikuangshan Antimony Plant, Hunan, China

We evaluated the direct release to the environment of a number of potentially toxic elements (PTEs) from various processing nodes at Xikuangshan Antimony Mine in Hunan Province, China. Sampling wastewater, processing dust, and solid waste and characterizing PTE content (major elements Sb, As, Zn, and associated Hg, Pb, and Cd) from processing activities, we extrapolated findings to assess wider environmental significance using the pollution index and the potential ecological risk index. The Sb, As, and Zn in wastewater from the antimony benefication industry and a wider group of PTEs in the fine ore bin were significantly higher than their reference values. The content of Sb, As, and Zn in tailings were relatively high, with the average value being 2674, 1040, and 590 mg·kg−1, respectively. The content of PTEs in the surface soils surrounding the tailings was similar to that in tailings, and much higher than the background values. The results of the pollution index evaluation of the degree of pollution by PTEs showed that while dominated by Sb, some variation in order of significance was seen namely for: (1) The ore processing wastewater Sb > Pb > As > Zn > Hg > Cd, (2) in dust Sb > As > Cd > Pb > Hg > Zn, and (3) surface soil (near tailings) Sb > Hg > Cd > As > Zn > Pb. From the assessment of the potential ecological risk index, the levels were most significant at the three dust generation nodes and in the soil surrounding the tailings reservoir.

Speciation and location of arsenic and antimony in rice samples around antimony mining area

Arsenic (As) and antimony (Sb) are considered as priority environmental pollutants and their accumulation in crop plants particularly in rice has posed a great health risk. This study endeavored to investigate As and Sb contents in paired soil-rice samples obtained from Xikuangshan, the world largest active Sb mining region, situated in China, and to investigate As speciation and location in rice grains. The soil and rice samples were analyzed by coupling the wet chemistry, laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS), synchrotron-based micro X-ray fluorescence mapping (μ-XRF) and micro X-ray absorption near-edge structure (μ-XANES) spectroscopy. The results of field survey indicated that the paddy soil in the region was co-polluted by Sb (5.91-322.35 mg kg^-1) and As (0.01-57.21 mg kg^-1). Despite the higher Sb concentration in the soil, rice accumulated more As than Sb indicating the higher phytoavailability of As. Dimethylarsinic acid (DMA) was the predominant species (>60% on average) in the rice grains while the percentage of inorganic As species was 19%-63%. The μ-XRF mapping of the grain section revealed that the most of As was distributed and concentrated in rice husk, bran and embryo. Sb was distributed similarly to As but was not in the endosperm of rice grain based on LA-ICP-MS. The present results deepened our understanding of the As/Sb co-pollution and their association with the agricultural-product safety in the vicinity of Sb mining area.

Bacterial response to antimony and arsenic contamination in rice paddies during different flooding conditions

Rice is more vulnerable to arsenic (As) and antimony (Sb) contamination than other cereals due to the special cultivation methods, during which irrigation conditions are adjusted depending upon the growth stages. The changes in irrigation conditions may alter the oxidation states of Sb and As, which influences their mobility and bioavailability and hence uptake by rice. In this study, bacterial responses to As and Sb contamination in rice fields were investigated during two different stages of rice growth: the vegetative stage (flooded conditions), and the ripening stage (drained conditions). The substantial changes in the irrigation conditions caused a variation in geochemical parameters including the As- and Sb-extractable fractions. As and Sb were more mobile and bioaccessible during the flooded than under drained conditions. The microbial communities varied during two irrigation conditions, suggesting that the geochemical conditions may have different effects on the innate paddy microbiota. Therefore, various statistical tools including co-occurrence network and random forest (RF) were performed to reveal the environment-microbe interactions in two different irrigation conditions. One of the notable findings is that Sb- and As-related parameters exerted more influences during the flooded than under drained conditions. Furthermore, a detailed RF analysis indicated that the individual bacterial taxa may also respond differently to contaminant fractions during the two irrigation conditions. Notably, RF indicated that individual taxa such as Clostridiaceae and Geobacter may be responsible for biotransformation of As and Sb (e.g., As and Sb reduction). The results provided knowledge for As and Sb transformation during contrasting irrigation conditions and the potential mitigation strategy for contaminant removal.

Antimony accumulation and iron plaque formation at different growth stages of rice (Oryza sativa L.)

To better understand the Sb phytoavailability in rice, we studied Sb accumulation in rice (Zhongjiazao-17, widely cultivated in Hunan province) at different growth stages based on adding Sb^III and Sb^V to waterlogged soils in 10, 50 and 100 mg kg^-1 treatment levels. Proportional exogenous Sb^III and Sb^V remained in the soil solution after equilibration. In Sb^III treatments, the iron plaque (IP) amounts and Sb in rice roots sharply increased from tillering to jointing stages and then reduced at the following stages. However, in Sb^V treatments, they increased continuously from tillering to maturing stages. The accumulation trends of Sb in straws, ears and grains were consistent in Sb^III and Sb^V treatments, rising from tillering to jointing stages followed with reducing from jointing to flowering stages slightly, and rising again significantly from flowering to maturing stages. The Tfsoil-grain values in all the Sb treatments were low (0.77 × 10^-3-5.1 × 10^-3), However, when Sb in waterlogged soils were higher than 50 mg kg^-1, it could pose human health risk for residents.