Diffusive Gradients in Thin Films Reveals Differences in Antimony and Arsenic Mobility in a Contaminated Wetland Sediment during an Oxic-Anoxic Transition

A microcosm evaluation of metal cycling in an urbanized contaminated estuary varying with oxic-hypoxic-anoxic-reoxic transition: Behavior, fluxes, and mechanism

Water hypoxia and metal pollution are commonly co-existed in urbanized estuaries. This study focuses on the effect of an extended dissolved oxygen (DO) full-life dynamics (86 days) on metal behavior across the sediment-water interface through laboratory microcosms from two typical zones in Pearl River Estuary. Combining our time-series results of concentrations and fluxes, it showed that Co, Ni, and Zn consistently presented a release-precipitation-release trajectory with an oxic-hypoxic-anoxic-reoxic transition, characterized with highly variable behavior in the hypoxic-anoxic hotmoments. In parallel, changing DO dynamics significantly activated a repartitioning process of Co, Ni, and Zn among several species and elevated their risk in sediments, promoting the formation of more labile species in the 0-10 mm hotspots, where metals sensitively responded. Over DO transition, metal cycling was tightly co-related with Fe, Mn, and S elements. It was found that Mn was dominated in low oxygen-hypoxic period, but switched to S and Fe in anoxic stage, limiting sustained metal liberation to overlying water. Enlarging this experiment to practice, released Zn fluxes from sediments in hypoxic summer could contribute about ∼2.0% to their stocks in water column, while increase to 20% (1 m bottom water) in highly-stratified zones. This study has certain significance in understanding the long-term metal behavior and fate in estuarine regions, even lakes and reservoirs.

Antimony release and volatilization from organic-rich and iron-rich submerged soils

Antimony (Sb) is an poorly understood, increasingly common pollutant, especially in soils susceptible to waterlogging. We investigated the impact of waterlogging on Sb release, methylation, and volatilization from an organic-rich wetland soil and an iron (Fe)-rich floodplain soil in a 27-day microcosm experiment. The release of Sb into the porewaters of the organic-rich soil was environmentally relevant and immediate with waterlogging (3.2 to 3.5 mg L-1), and likely associated with a complex interplay of sulfide precipitation, sorption with organic matter and manganese (Mn) (oxyhydr)oxides in the soil. The release of Sb from the Fe-rich soil was likely associated with Fe-(oxyhydr)oxide reduction and immobilized due to co-precipitation with Fe-sulfides or as Sb-sulfides. Volatile Sb was produced from the soils after waterlogging. The organic-rich soil produced more volatile Sb (409 to 835 ng kgsoil-1), but the Fe-rich soil volatilized Sb more efficiently. The negligible association of Sb volatilization with soil parameters indicates a more complex underlying, potentially microbial, mechanism and that antimony volatilization could be ubiquitous and not dependent on specific soil properties. Future works should investigate the microbial and physiochemical drivers of Sb volatilization in soils as it may be an environmentally relevant part of the biogeochemical cycle.

Biogeochemical cycling in paddy soils controls antimony transformation: Roles of iron (oxyhydr)oxides, organic matter and sulfate

In paddy fields, periodic flooding and drainage phases can significantly affect the availability of antimony (Sb), but the underlying mechanisms remain unclear. In this study, Sb-contaminated paddy soil was incubated under anaerobic (40 day) and subsequently aerobic (40-55 day) conditions. The Sb fractions was investigated and a kinetic model was established to quantitatively evaluate the main processes controlling Sb transformation. Under anaerobic conditions, the reductive dissolution of iron (Fe) (oxyhydr)oxides, the release of soil colloids, and dissolved organic carbon (DOC) could facilitate the release of Sb(V), while newly released Sb(V) were synchronously reduced to Sb(III) that could be incorporated into the solid phase (34.1%, 40 day) or precipitated as Sb2S3 (9.7%, 40 day). After soil aeration, a significant increase in dissolved and extracted Sb(V) (34.7%, 45 day) was observed due to the Sb(III) oxidization by the reactive oxygen species (ROS) generated from Fe(II) oxidization. The dissolved and extracted Sb(V) were simultaneously incorporated into the solid phase as the re-aggregation of soil colloids and DOC, and only contributed to 17.1% of the total Sb content at the end of aerobic phase (55 day). Our results elucidated the mechanisms about how biogeochemical Fe/S/C cycling jointly controlled Sb transformation in paddy systems.

Seasonal variations in spatial distribution, mobilization kinetic and toxicity risk of arsenic in sediments of Lake Taihu, China

This study investigated seasonal variations in spatial distribution, mobilization kinetic and toxicity risk of arsenic (As) in sediments of three representative ecological lakes in Lake Taihu. Results suggested that the bioavailability and mobility of As in sediments depended on the lake ecological types and seasonal changes. At the algal-type zones and macrophyte-type zones, elevated As concentrations were observed in April and July, while these occurred at the transition areas in July and October. The diffusion flux of soluble As ranged from 0.03 to 3.03 ng/cm2/d, indicating sediments acted as a source of As. Reductive dissolution of As-bearing iron/manganese-oxides was the key driver of sediment As remobilization. However, labile S(-II) caused by the degradations of algae and macrophytes buffered sediment As release at the algal-type and macrophyte-type zones. Furthermore, the resupply ratio was less than 1 at three ecological lakes, indicating the resupply As capacity of sediment solid phase was partially sustained case. The risk quotient values were higher than 1 at the algal-type zones and transition areas in July, thereby, the adverse effects of As should not be ignored. This suggested that it is urgently need to be specifically monitored and managed for As contamination in sediments across multi-ecological lakes.

Seaward alteration of arsenic mobilization mechanisms based on fine-scale measurements in Pearl River estuarine sediments

Identification of key As mobilization processes in estuarine sediments is challenging due to the transitional hydrodynamic condition and the technical restriction of obtaining fine-scale results. Herein, high-resolution (μm to mm) and in situ profiling of As with associated elements (Fe, Mn, and S) by the diffusive gradients in thin-film (DGT) technique were applied and coupled with pore water and solid phase analysis as well as microbial high-throughput sequencing, to ascertain the driving mechanisms of As mobilization in the sediments of Pearl River Estuary (PRE). Significant diffusion fluxes of As from sediment to water were observed, particularly in the upper estuary. With the seaward increase of salinity, the driving mechanism of As mobilization gradually shifted from microbial-induced dissimilatory Fe reduction to saltwater-induced ion exchange. Correspondingly, the dominant Fe-reducing bacteria (FeRB) in sediments changed from the genera Clostridium_sensu_stricto_1 and Bacillus to Ferrimonas and Deferribacter. The presence of dissolved sulfide in deeper sediments contributes to As removal through the formation of As-S precipitates as supported by theoretical calculations. Fine-scale findings revealed seaward changes of As mobilization mechanism in the sediments of a human-impacted estuary and may benefit the understanding of As biogeochemical behavior in estuaries worldwide.

Novel insight into ammonium, phosphate, and iron(II) dynamics in the sediment porewater of a constructed wetland under artificial aeration through the diffusive equilibrium in thin films technique

The analysis of porewater concentrations in constructed wetland sediments could help to understand biogeochemical processes, the sources and sinks of nutrients, and their effect on overlying water quality. In this study, we measured high-resolution porewater concentration profiles of ammonium (NH4+-N), nitrate (NO3N), phosphate (PO43--P), and ferrous iron (Fe(II)) in-situ in the Laratinga constructed wetland in Mount Barker (South Australia) using diffusive equilibration in thin films (DET) techniques. Measurements were taken under light and dark conditions, and non-aerated and aerated conditions to determine the effect on sediment porewater nutrient concentrations. Baseline surface water nutrient concentrations (NH4+-N > 36 mg L-1, PO43--P > 0.43 mg L-1) greatly exceeded water quality guideline criteria. Aeration of the water column alleviated night-time hypoxic conditions (i.e. dissolved oxygen increased from a minimum of 0.7 mg L-1 to a minimum of 4 mg L-1), and increased the redox potential in the sediment. Significant differences were present for NH4+-N, PO43--P, and Fe(II) concentrations with depth in the sediment. Ammonium concentrations in the sediment reduced under aerated conditions, presumably due to enhanced nitrification. However it was observed that PO43--P and Fe(II) concentrations increased significantly with aeration, especially under dark conditions, and were strongly correlated (R2>0.8). This was not what was hypothesised and points to complex interactions between Fe and P in the sediment. Nitrate concentrations in the sediment were below the detection limit (<0.9 mg L-1) which suggests limited nitrification-denitrification is occurring. Overall the results suggest that DET techniques are useful tools for quantifying porewater concentrations of nutrients in constructed wetlands under various environmental conditions.

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

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

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

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

High-resolution imaging of O2 dynamics and metal solubilization in the rhizosphere of the hyperaccumulator Leersia hexandra Swartz

The mobilization of trace metals in the rhizosphere can be affected by the redox potential, which is closely related to the O₂ dynamics. This study examined the distributions of O₂ and trace metals in the rhizosphere of the subaquatic hyperaccumulator Leersia hexandra Swartz under chromium (Cr) stress using planar optodes and the diffusive gradients in thin films technique coupled with laser ablation inductively coupled plasma mass spectrometry. The O₂ concentrations and oxidized areas in the rhizosphere significantly increased with increases in the light intensity, air humidity, and atmospheric CO₂ concentrations (p < 0.05). The O₂ concentration first increased with increasing ambient temperatures, then decreased when the temperature increased from 25 to 32 ℃. The O₂ concentration in the rhizosphere was significantly decreased under Cr stress (p < 0.05), with a prolonged response time to the altered ambient temperature. Cr stress led to decreased mobilities of As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Sb, V, W, and Zn in the rhizosphere, which were negatively correlated with the concentrations of O₂. These results provide new insights into the role of changes in the O₂ concentration induced by the roots of hyperaccumulator plants in controlling the mobility of trace metals in soils.

Mechanisms of antimony release from lacustrine sediments with increasing temperature

Antimony (Sb) is more mobile in lacustrine sediments with seasonal warming. However, the mechanisms of Sb mobility in sediments are still unclear, especially considering the interactions among Sb, iron (Fe), manganese (Mn), and dissolved organic matter (DOM). In this study, high-resolution dialysis (HR-Peeper) and multi-spectral techniques simultaneously investigated changes in Sb, Fe, Mn, and DOM in two different ecological types (algal and grass) sediments with increasing temperature. We found that the dissolved Sb rapidly increased with the increase in temperature. The oxidation of Sb(III) to Sb(V) by Fe/Mn oxides in oxygen (O₂) rich overlying water and surface sediment layers was one of the reasons for Sb concentration enhancement in pore water. Further, using excitation-emission matrix and parallel factor analysis (EEM-PARAFAC), synchronous fluorescence (SF) spectroscopy, fourier transform infrared (FTIR) spectroscopy, and two-dimensional correlation spectroscopy (2D-COS) revealed that complexation with DOM was the other reasons for Sb concentration increasing in sediments. This was demonstrated by the similar distribution pattern and significant correlation between Sb and tryptophan-like components. Titration experiments further revealed that Sb was more stably bound to tryptophan-like components in the aromatic C-H (660 cm^-1), alcoholic C-O (1115 cm^-1), alkene CC (1615 cm^-1), and carboxylic acid OH (3390 cm^-1) groups. The tryptophan-like components from the algae region had a higher binding force than that from the macrophytes region. Our study effectively promotes an understanding of Sb mobilization in lacustrine sediments.

Integrated effects of bioturbation, warming and sea-level rise on mobility of sulfide and metalloids in sediment porewater of mangrove wetlands

Global warming and sea-level rise exert profound impacts on coastal mangrove ecosystems, where widespread benthic crabs change sediment properties and regulate material cycles. How crab bioturbation perturbs the mobilities of bioavailable arsenic (As), antimony (Sb) and sulfide in sediment-water systems and their variability in response to temperature and sea-level rise is still unknown. By combining field monitoring and laboratory experiments, we found that As was mobilized under sulfidic conditions while Sb was mobilized under oxic conditions in mangrove sediments. Crab burrowing greatly enhanced oxidizing conditions, resulting in enhanced Sb mobilization and release but As sequestration by iron/manganese oxides. In control experiments with non-bioturbation, the more sulfidic conditions triggered the contrasting situation of As remobilization and release but Sb precipitation and burial. Moreover, the bioturbated sediments were highly heterogeneous for spatial distributions of labile sulfide, As and Sb as presented by 2-D high-resolution imaging and Moran's Index (patchy at the <1 cm scale). Warming stimulated stronger burrowing activities, which led to more oxic conditions and further Sb mobilization and As sequestration, whilst sea-level rise did the opposite via suppressing crab burrowing activity. This work highlights that global climate changes have the potential to significantly alter element cycles in coastal mangrove wetlands by regulating benthic bioturbation and redox chemistry.

Simultaneous measurement of labile As (III) and As (V) in soils combining DGT and HPLC-ICP-MS

The speciation of arsenic (As) determines its biogeochemistry and ecotoxicity in soils. However, the approach to in situ monitor labile As (III) and As (V) in soils still requires more exploration. In this study, we developed a method for simultaneously obtaining in-situ data on labile As (III) and As (V) in soils using diffusive gradients in thin films (DGT) and high performance liquid chromatography-inductively coupled plasma mass spectrometry. The Fe₂O₃∙xH₂O DGT sampler exhibited rapid and simultaneous accumulation of As (III) and As (V) in solutions within 90 min. The high efficiency of simultaneous elution of As (III) (~84 %) and As (V) (~97 %) was achieved using 0.8 % H₃PO₄ as eluent at 90 °C for 80 min. The method detection limits for As (III) and As (V) were 0.01 and 0.005 μg/L, respectively. This method was applied to reveal the labile As (III) and As (V) in soils in the water level fluctuation zones of the Three Gorges Reservoir, which is the largest reservoir in China. The concentrations of As (III) and As (V) measured by DGT varied with different sampling sites, ranging from 0.01 μg/L to 1.20 μg/L and from 0.01 μg/L to 0.26 μg/L, respectively. The labile As (III) exhibited the higher resupply rate from soil solid phase to soil solution than labile As (V). This study helps to achieve simultaneous in-situ quantification of labile As (III) and As (V) in soils, and will improve the understanding of As mobilization and ecotoxicity in soils.

Evaluating the release risk of potentially toxic elements from sediments in the New Zhuzhao River Estuary of Nansi Lake, using high-resolution technology and sequential extraction

Potentially toxic elements (PTEs) re-release from sediment is an essential process in the sediment-water interface (SWI), especially for the influent river estuary as an important accumulation site. In this study, the diffusive gradient in thin films (DGT), high-resolution dialysis (HR-peeper) technique, and BCR sequential extraction were employed to evaluate the release risk of PTEs (As, Cu, Pb, Zn, Cd) in the New Zhuzhao River Estuary of Nansi Lake. Results showed that Cd existed primarily in the non-residual fraction (accounting for 59.87%), and the residual fractions of As, Cu, Pb, and Zn accounted for a greater proportion (12.65 to 33.07%). The mobility of Cd was the highest with a risk assessment code of 33.53% reaching the medium risk category. The resupply capacity calculated by C_DGT/C_Dis showed that As was the largest, with an average value of 0.43, indicating the strongest release capacity of As from the sediment to pore water. Furthermore, the diffusive fluxes using DGT and HR-peeper showed that As possesses a much higher potential to release upward overlying water than other elements.

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

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

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

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

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

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

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.

In Situ Simultaneous Analysis of Nitrogen and Phosphorus Migration in Urban Black Odorous Runoff

The extremely serious urban runoff eutrophication and black odorous phenomenon pose a significant threat to the lake aquatic ecosystem, resulting in a significantly increased frequency, magnitude, and duration of algal blooms in lakes. However, few investigations focus on small tributaries of the lakes, despite the ubiquity and potential local importance of these runoffs. Thus, the labile sediments NH₄⁺-N, NO₃^--N, PO₄^3-, Fe²⁺, and S^2- in black odorous runoff at Wuxi were overall analyzed at high resolution using diffusive gradients in thin films (DGT). The variations in labile N, P, Fe, and S distribution profiles at different sampling sites indicated high heterogeneity in sediments. The concentrations of labile P, Fe, and S showed synchronous variation from the sediment-water interface (SWI) up to -20 mm along sediment profiles. Moreover, there existed a significant positive correlation among labile P, Fe, and S concentrations (p < 0.05), which might represent typical odor compounds' FeS and H₂S synchronous release process in urban runoff. Furthermore, the apparent diffusion fluxes of labile P, Fe, and S across the SWI were all released upward, while fluxes of NH₄⁺-N and NO₃^--N release downward, indicating the sediments act as source and sink of P and N, respectively. Sediments' potential for endogenous P and N fractions release results in the black-odorous water, and sediment finally abouchement the Taihu, which intensifies further lake eutrophication phenomenon.

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

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

The variations of labile arsenic diffusion driven by algal bloom decomposition in eutrophic lake ecosystems

The vertical labile arsenic (As) concentration and diffusion pattern variations in eutrophic lakes were investigated using in situ techniques of diffusive gradients in thin films (DGT) and high-resolution dialysis (HR-Peeper) in the typical eutrophic system of Lake Taihu in China. In addition, simulation experiments were used to reveal labile As distributions in sediment profiles under the influence of algae blooms and wind fluctuations. Our results indicated that eutrophication could lead to the migration and transformation of As fractions, including increased As bioavailability, as well as varied diffusion patterns. The sulfate released from algae decomposition reduced to H₂S and formed FeS, which weak adsorbability contributed to the increased mobility of the As fractions. Meanwhile, further decomposition released a large quantity of algae-derived organic matter which competed with the adsorbed As, leading to more endogenous As migrating to the overlying water. Accordingly, the H₂S production presented a likely explanation for the changed distribution of labile As and contributed to labile As concentrations in the sediment profiles significantly increasing at depths of -20 mm to -60 mm in the early stages of the simulation experiment. Moreover, the areas of enhanced diffusion patterns with high concentrations of As obviously expanded. However, following the complete decomposition of the algae, the organic matter component significantly changed, suggesting an explanation for the variations in distribution of labile As. All the diffusion pattern variations showed similar trends. Consequently, variation of labile As diffusion patterns could indicate the decomposition and eutrophication levels of freshwater ecosystems.

Multiple geochemical and microbial processes regulated by redox and organic matter control the vertical heterogeneity of As and Cd in paddy soil

The heterogeneity of arsenic (As) and cadmium (Cd) in paddy soils seriously hinders the assessment of contamination status and prediction of rice uptake. Their vertical patterns across different environmental conditions and the underlying mechanisms remain largely unexplored. In this study, maximum vertical differences of bioavailable As and Cd within 0-30 cm depth in paddy soils were 4.1-fold and four orders of magnitude, respectively. The vertical patterns of As and Cd followed the vertical redox gradient in long-term reduced paddies, but were shaped by the vertical pH gradient derived from acidic wastewater irrigation in partly oxidized soils. Iron(III)- and sulfate-reducing bacteria played key roles in the formation of vertical pH gradient and the immobilization of As and Cd by iron (hydr)oxides and sulfides under varied redox conditions. Soil redox and organic matter determined the transition between these two mechanisms via regulating microbial iron(III) and sulfate reduction processes. The work proposes that soil vertical As and Cd patterns directly affect the accumulation of As and Cd in different rice cultivars with different vertical root patterns. This is the first study elucidating the controlling mechanisms governing the vertical As and Cd patterns in paddy fields, providing important references to identify, manage and remediate contaminated paddy fields.

Partitioning behavior and ecological risk of arsenic and antimony in the sediment-porewater profile system in the Three Gorges Reservoir, China

Arsenic and antimony are widely distributed toxic metalloids in aquatic environments. However, their partitioning behaviors in the sediment profile remain not well understood. Here, partitioning behaviors, diffusive fluxes, as well as the ecological risks of As and Sb in the sediment-porewater profile system in the tributaries of the Three Gorges Reservoir (TGR) were investigated. As and Sb showed markedly different spatial variations in the longitudinal profiles of both porewater and sediment samples. Specifically, the concentration of As showed an accumulation trend with depth, while that of Sb showed a relatively complicated trend. Further, As showed lower sediment-porewater partitioning coefficient (K_d) values, suggesting that it had a relatively lower sediment affinity and a higher mobility than Sb. Its residual fraction (30%-60%) was also lower than that of Sb. This phenomenon could be attributed to the chemical fractions of the trace metals and the pH value of the sediments. Furthermore, the K_d values corresponding to As were influenced by both the residual fraction (r = 0.338, p < 0.05) and the exchangeable fraction (r = -0.643, p < 0.01), while those corresponding to Sb were only influenced by pH. Additionally, even though these two trace metals showed low ecological and mobility risks, the diffusive fluxes at the sediment-water interface suggested that the sediment acted as a source of As and a sink for Sb relative to the overlying water. This study indicated that As and Sb had different partitioning behaviors and release risks in the sediment-porewater profile system, enhanced the understanding the transport and fate of As and Sb in the aquatic environment.

Effects of Light-Emitting Diode Illumination on Sediment Surface Biological Activities and Releases of Nutrients and Metals to Overlying Water in Eutrophic Lake Microcosms

The release of nutrients and metals from the sediment to the overlying water induced by oxygen depletion is an important issue in eutrophic aquatic systems. Effects of light-emitting diode (LED) illumination on oxygen conditions and release of nutrients and metals from the sediment were examined by comparing with those effects of aeration in microcosms using water and sediment of Lake Taihu, China. Periphyton with filamentous algae developed on the sediment surface in the LED (blue wavelength) treatment. Dissolved oxygen became rapidly saturated and gradually supersaturated in the aeration and LED treatments, respectively, but remained low in the control. A thicker oxic layer developed on the sediment for the LED than aeration but was poorly developed with a blackened surface in the control. Invertebrate burrows were distributed deeper and the bacterial community was more dominated by aerobic species in the LED, indicating deeper penetration of oxygen into the sediment. Nutrients (e.g., N and P) and some metals (e.g., Hg, As, and Mn) in water were lower for the LED and aeration than in the control; nutrients and other solutes that increased electric conductivity (e.g., Ca, Mg) were lower for the LED than aeration. These results suggest that LED can effectively oxygenate the bottom water by stimulating algal photosynthesis and benthic invertebrate activity, resulting in greater retention of nutrients and metals in/on sediment compared to aeration.

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.

Mechanistic Study for Antimony Adsorption and Precipitation on Hematite Facets

Heterogeneous reactions at the mineral-water interface are of paramount importance in controlling the transport of contaminants. Herein, antimony (Sb) adsorption and subsequent precipitation on Fe₂O₃ facets were explored to understand its partitioning mechanisms by multiple complementary techniques. Our extended X-ray absorption fine structure spectroscopy and density functional theory results provided a consensus on the local coordination environment of Sb(III) and Sb(V) on Fe₂O₃ facets. We observed that Sb adsorption and the following precipitation are associated with both Sb concentrations and Fe₂O₃ facets, and a change in the Sb surface-binding mode from edge-sharing to corner-sharing is preferred in precipitation. Fe₂O₃ facets determine Sb binding structures, resulting in a facet-dependent transformation of adsorption to precipitation. The preferred corner-sharing complexes on the {001} facet facilitated the formation of Sb₂O₃ and NaSb(OH)₆ precipitates at a lower Sb concentration compared with other two {110} and {214} facets. In addition, the facet-specific binding configuration renders a heterogeneous epitaxial growth of Sb₂O₃. Our study provides a molecular understanding of facet effects on Sb adsorption and precipitation on minerals.

Mechanistic insights into trace metal mobilization at the micro-scale in the rhizosphere of Vallisneria spiralis

Mobilization of trace metals in the rhizosphere of macrophytes is controlled by root-driven chemical changes, especially the steep gradients of O₂ and pH from the rhizosphere to bulk sediments. Here, the O₂ and pH dynamics, and the distribution of trace metal, in the rhizosphere of Vallisneria spiralis were obtained using planar optodes and diffusive gradients in thin films, respectively. Radial O₂ loss (ROL) and acidification occurred on all visible roots of V. spiralis and exhibited highly spatiotemporal dynamics depending on the root growth and various environmental conditions. Trace metals showed different mobilization mechanisms in the rhizosphere. ROL and produced Fe(III) (oxyhydr)oxides decreased the mobility of Fe, As, Co, V and W in the rhizosphere. However, Mn, Ni and Cu exhibited greater mobility in the rhizosphere than bulk sediments as a result of the oxidation of metal sulfide and proton-induced dissolution of minerals. In particular, Co and Ni presented increased activity at the interface between rhizosphere and bulk sediment, which was attributed to the redox dissolution processes of Fe and Mn as a result of ROL and rhizosphere acidification. These results provide new insights into the roles of macrophyte root-induced O₂ and pH changes in controlling trace metal mobility in sediments.

Establishing the state-of-the-art on the adsorption of coexisting pnictogens in water: A literature review

The occurrence of pnictogens, namely phosphorus, arsenic, and antimony, can be observed in soils, sediments and mining areas, and their coexistence requires a multifaceted approach to the design of adsorption systems to maximize their simultaneous removal efficiency. Therefore, this work aims to provide an extensive literature review of P, As, and Sb adsorption in multicomponent systems and the statistical treatment of the quantitative results. Binary As-P systems have been the most studied in the literature. The oxidation state did not significantly affect the P influence in As adsorbed amount (p = 0.955), but this influence was correlated with the As:P ratio (p < 0.05). A few works have explored As-Sb and Sb-P systems, demonstrating that effective treatments for As do not always reveal a good removal efficiency of the other pnictogens. The Sb adsorbed amount was significantly less affected in the trivalent than in the pentavalent state in both As-Sb and Sb-P systems (p < 0.05). Most of the interactions were competitive, with a few studies reporting synergistic effects for Sb due to the presence of the other elements. Many topics have been identified as lacking in-depth research: ternary As-Sb-P systems, the effect of concentration ratios, pH, and redox conditions (namely those that lead to trivalent species' prevalence), the surface interactions with materials other than iron oxides, and the influence of other aqueous components. This review provides a first step in gathering the relevant literature and approaching the study of adsorption treatment methodology as a complex subject involving many factors.

Arsenic contamination, impact and mitigation strategies in rice agro-environment: An inclusive insight

Arsenic (As) contamination and its adverse consequences on rice agroecosystem are well known. Rice has the credit to feed more than 50% of the world population but concurrently, rice accumulates a substantial amount of As, thereby compromising food security. The gravity of the situation lays in the fact that the population in theAs uncontaminated areas may be accidentally exposed to toxic levels of As from rice consumption. In this review, we are trying to summarize the documents on the impact of As contamination and phytotoxicity in past two decades. The unique feature of this attempt is wide spectrum coverages of topics, and that makes it truly an interdisciplinary review. Aprat from the behaviour of As in rice field soil, we have documented the cellular and molecular response of rice plant upon exposure to As. The potential of various mitigation strategies with particular emphasis on using biochar, seed priming technology, irrigation management, transgenic variety development and other agronomic methods have been critically explored. The review attempts to give a comprehensive and multidiciplinary insight into the behaviour of As in Paddy -Water - Soil - Plate prospective from molecular to post-harvest phase. From the comprehensive literature review, we may conclude that considerable emphasis on rice grain, nutritional and anti-nutritional components, and grain quality traits under arsenic stress condition is yet to be given. Besides these, some emerging mitigation options like seed priming technology, adoption of nanotechnological strategies, applications of biochar should be fortified in large scale without interfering with the proper use of biodiversity.

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.

Arsenic and mercury contamination and complex aquatic bioindicator responses to historical gold mining and modern watershed stressors in urban Nova Scotia, Canada

Beginning in the late-1800s, gold mining activities throughout Nova Scotia, eastern Canada, released contaminants, notably geogenic arsenic from milled ore and anthropogenic mercury from amalgamation, to local environments via surface water flows through tailings fields. We investigated recovery from and legacy effects of the tailings field at the Montague Gold District (~1863-1940) on nearby urban lake ecosystems using geochemical measures and zooplankton remains archived in dated sediment cores from an impact (Lake Charles) and a reference (Loon Lake) lake. Sedimentary levels of total arsenic and total mercury were used to assess mining-related inputs. Arsenic concentrations remain elevated at nearly 300 times above sediment guidelines in Lake Charles surface sediments, due to its upward mobilization from enriched sediment intervals and sequestration by iron oxyhydroxides in surficial sediments. Peak mercury concentrations at Lake Charles were eight times above sediment guidelines during the mining period, and since ~1990 have recovered to levels observed before mining began. Legacy mining impacts at Lake Charles and non-mining related environmental changes in the post-1950 sediments at both lakes have thus combined to structure assemblage compositions of primary consumers. At both lakes, assemblages of pelagic-dominated Cladocera differed (p ≤ 0.05) during the mining period compared to periods before and after mining. Taxon richness differed (p ≤ 0.01) only between the pre- and post-mining periods at mining-impacted Lake Charles and reflects long-term declines of substrate-dwelling littoral taxa. Geochemical and biological recovery have not completely occurred at Lake Charles despite the mine district's closure ~80 years ago. Our findings demonstrate that impacts of ore processing and amalgamation from historical gold mining, combined with recent watershed stressors, continue to affect sedimentary arsenic geochemistry and intermediate trophic levels of nearby, downstream aquatic habitats.

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

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

Assessing pesticide, trace metal, and arsenic contamination in soils and dam sediments in a rapidly expanding horticultural area in Australia

Industrial horticulture can release pesticides and trace metals/metalloids to terrestrial and aquatic environments. To assess long-term and more recent land contamination from an expanding horticultural region, we sampled soils from chemical mixing, crop production, and drainage areas, as well as retention reservoirs (dam) sediments, from 3 blueberry farms with varying land-use history in subtropical Australia. Soils were analysed for 97 different pesticides and trace metal/metalloid contents. The most recent farm had fungicides propiconazole and cyprodinil contents that may compromise soil invertebrate survival and/or nutrient recycling (5-125 mg kg^-1). A site previously used to cultivate bananas had 6 dam sediment subsamples with arsenic contents over sediment quality guidelines (SQG); however, the soil content values were just below Australian health investigation levels (100 mg kg^-1). Arsenic is suspected to originate from pesticide application during previous banana cultivation in the region. Dam sediment cores at all sites had mercury contents over the SQG likely due to fungicides or fertiliser impurities. Mean contents of mercury from dam sediments (141 ± 15.5 µg kg^-1) were greater than terrestrial soils (78 ± 6.5 µg kg^-1), and sediment profiles suggest mercury retention in anoxic sediments. Soils in chemical mixing areas at two sites were contaminated with copper and zinc which were above the national soil ecological investigation levels. Based on toxicity data, distribution, persistence, and mobility, we identified the fungicide cyprodinil, mercury, and phosphorus as contaminants of the greatest concern in this intensive horticulture area of Australia. Additional sampling (spatial, chemical speciation, biotic) is required to support mitigation efforts of the emerging contamination in the rapidly expanding blueberry farms of this region of Australia.

Evaluation of the Chelex-DGT technique for the measurement of rare earth elements in the porewater of estuarine and arine sediments

This study describes the validation of a diffusive gradients in thin film (DGT) technique for determining lanthanide rare earth elements (REEs) and in situ measurements of REEs in sediment pore waters. Laboratory experiments demonstrated that Chelex-100 binding layers had uptake efficiencies ranging from 78.0% to 92.3% for all REEs. An eluent of 1 mol L^-1 HNO₃ was optimal with elution efficiencies >80% for all REEs. Mass versus time experiments confirmed that DGT uptake was linear for all REEs at pH 8.1, 6.6 and 3.9 over a period of 3-4 days. Diffusion coefficients (D) for all REEs were derived from these experiments using the slopes of the linear regressions. D values varied with pH but were generally similar to values reported previously. The Chelex-100 DGT technique from this study is highly sensitive for the measurement of REE concentrations with detection limits ranging from 1.8 to 45 ng L^-1 based on 72 h deployments allowing measurements of natural trace REE levels. Chelex-100 DGT devices were deployed in estuarine and marine sediments over a period of 72 h and most REE porewater concentrations (50-10,410 ng L^-1) were successfully measured. Individual depth profiles of REEs showed a complex response, with many peaks and troughs suggesting a high degree of sediment heterogeneity. Depth-averaged REE concentrations showed a typical zig-zag distribution, although patterns varied between sediment types, after the REEs were normalised using the Queensland Mud Composite shale reference. The Chelex-100 DGT technique therefore shows promise for REE measurements in sediments.

In Situ DGT Sensing of Bioavailable Metal Fluxes to Improve Toxicity Predictions for Sediments

An increased risk of adverse biological effects of metals in sediments may be accompanied by high labile metal fluxes as measured by the diffusive gradients in thin films (DGT) technique. To improve the usefulness of the DGT technique for sediment quality risk assessments, we used the simpler and more cost-effective piston DGTs rather than planar DGT probes to measure bioavailable metal fluxes in naturally contaminated sediments with widely varying composition (properties, metals and concentrations) and assessed their prediction of toxicity to amphipod reproduction in a flow-through microcosm. DGT pistons were deployed in sediments under different conditions, both in the field (in situ) and in the laboratory in sediment cores (lab-equilibrated) and in homogenized sediments (lab-homogenized). We demonstrated that the metal flux toxic units, DGT_TU, measured in situ best predicted the magnitude of toxicity to amphipod reproduction. For sediments that had been highly disturbed before testing, DGT_TU were less predictive for observed toxicity, but the copper flux alone (DGT_TU-Cu) was effective, indicating copper was the primary cause of toxicity in these highly perturbed sediments. Overall, our study highlighted that the adverse effects induced by excessive bioavailable metals in contaminated sediments can be consistently sensed by the DGT pistons.

Arsenic behavior across soil-water interfaces in paddy soils: Coupling, decoupling and speciation

The sharp redox gradient at soil-water interfaces (SWI) plays a key role in controlling arsenic (As) translocation and transformation in paddy soils. When Eh drops, As is released to porewater from solid iron (Fe) and manganese (Mn) minerals and reduced to arsenite. However, the coupling or decoupling processes operating within the redox gradient at the SWI in flooded paddy soils remain poorly constrained due to the lack of direct evidence. In this paper, we reported the mm-scale mapping of Fe, As and other associated elements across the redox gradient in the SWI of five different paddy soils. The results showed a strong positive linear relationship between dissolved Fe, Mn, As, and phosphorus (P) in 4 out of the 5 paddy soils, indicating the general coupling of these elements. However, decoupling of Fe, Mn and As was observed in one of the paddy soils. In this soil, distinct releasing profiles of Mn, As and Fe were observed, and the releasing order followed the redox ladder. Further investigation of As species showed the ratio of arsenite to total As dropped from 100% to 75.5% and then kept stable along depth of the soil profile, which indicates a dynamic equilibrium between arsenite oxidization and arsenate reduction. This study provides direct evidence of multi-elements' interaction along redox gradient of SWI in paddy soils.

Simultaneous measurement of aqueous redox-sensitive elements and their species across the soil-water interface

The redox-sensitive elements, such as iron, manganese, sulfur, phosphorus, and arsenic, shift their speciation every millimeter (mm) across the soil-water interface in the flooded soil environments. Monitoring of element speciation at this high-resolution (HR) within the SWI is still difficult. The key challenge lies in obtaining sufficient porewater samples at specific locations along the soil gradient for downstream analysis. Here with an optimized inductively coupled plasma mass spectrometry (ICP-MS) method and a HR porewater sampler, we demonstrate mm-scale element profiles mapping across the SWI in paddy soils. High-concentrations of iron and manganese (> 10 mg/L) were measured by ICP-MS in an extended dynamic range mode to avoid signal overflow. The iron profile along the SWI generated by the ICP-MS method showed no significant difference (p < 0.05) compared to that measured independently using a colorimetric method. Furthermore, four arsenic (arsenite, arsenate, monomethylarsonic and dimethylarsinic acid), two phosphorus (phosphite and phosphate) and two sulfur (sulfide and sulfate) species were separated in 10 min by ion chromatography -ICP-MS with the NH₄HCO₃ mobile phase. We verified the technique using paddy soils collected from the field, and present the mm-scale profiles of iron, manganese, and arsenic, phosphorus, sulfur species (relative standard deviation < 8%). The technique developed in this study will significantly promote the measurement throughput in limited samples (e.g. 100 μL) collected by HR samplers, which would greatly facilitate redox-sensitive elements biogeochemical cycling in saturated soils.

Reduction of Sb(V) by coupled biotic-abiotic processes under sulfidogenic conditions

Increasing use and mining of antimony (Sb) has resulted in greater concern involving its fate and transport in the environment. Antimony(V) and (III) are the two most environmentally relevant oxidation states, but little is known about the redox transitions between the two in natural systems. To better understand the behavior of antimony in anoxic environments, the redox transformations of Sb(V) were studied in biotic and abiotic reactors. The biotic reactors contained Sb(V) (2 mM as KSb(OH)₆), ferrihydrite (50 mM Fe(III)), sulfate (10 mM), and lactate (10 mM), that were inoculated with sediment from a wetland. In the abiotic reactors, The interaction of Sb(V) with green rust, magnetite, siderite, vivianite or mackinawite was examined under abiotic conditions. Changes in the concentrations of Sb, Fe(II), sulfate, and lactate, as well as the microbial community composition were monitored over time. Lactate was rapidly fermented to acetate and propionate in the bioreactors, with the latter serving as the primary electron donor for dissimilatory sulfate reduction (DSR). The reduction of ferrihydrite was primarily abiotic, being driven by biogenic sulfide. Sb and Fe K-edge X-ray absorption near edge structure (XANES) analysis showed reduction of Sb(V) to Sb(III) within 4 weeks, concurrent with DSR and the formation of FeS. Sb K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy analysis indicated that the reduced phase was a mixture of S- and O-coordinated Sb(III). Reduction of Sb(V) was not observed in the presence of magnetite, siderite, or green rust, and limited reduction occurred with vivianite. However, reduction of Sb(V) to amorphous Sb(III) sulfide occurred with mackinawite. These results are consistent with abiotic reduction of Sb(V) by biogenic sulfide and reveal a substantial influence of Fe oxides on the speciation of Sb(III), which illustrates the tight coupling of Sb speciation with the biogeochemical cycling of S and Fe.

Study on antimony mobility in a contaminated shallow lake sediment using the diffusive gradients in thin films technique

Antimony is a priority environmental contaminant. Increasing attention is being paid to the behaviors and mobilities of the various Sb species in the environment. Sb speciation in the environment and the mobilities of Sb species at mining sites have been studied well, but Sb speciation and mobility in shallow lakes requires further study. Here, we studied Sb behavior in sediment of a shallow lake in the plain rivers network in Taihu Basin that suffers continual Sb inputs from textile plants. The diffusive gradients in thin films techniques (DGT) made of zirconium oxide based binding resin gel (ZrO-Chelex), agarose diffusive gel and polyvinylidene fluoride filter were deployed in water and sediment to obtain a high-resolution record in situ. The results indicated that (1) pollutants released by textile plants caused relatively high Sb(Ⅲ), Sb(Ⅴ) and organoantimony concentrations in the eutrophic shallow lake, (2) Sb was seldomly mobile in the oxic layer where Sb(Ⅲ) was sorbed on Fe(Ⅲ) oxides and gradually formed Fe-Sb complexes in the sediment, but in the anoxic environment (oxidation-reduction potential: 366 - -344 mv) Sb(V), Fe(Ⅱ) and P (V) were simultaneously released to resupply the porewater, (3) the release of Sb from solid phase is decided by the redox condition, and the rate of release is dependent on the labile Sb content of the sediment. The mobility of Sb should be given sufficient attention when the potential ecological risk of metal(loid)s in shallow lakes and wetlands sediment are evaluated.

Fine scale measurements in Belgian coastal sediments reveal different mobilization mechanisms for cationic trace metals and oxyanions

Belgian coastal sediment serves as an important sink for trace elements, yet a systematic study covering a wide range of elements including redox-sensitive metals (Fe, Mn, and Co), cationic trace metals (Cd, Pb, Ni, Cu, and Zn), oxyanions (P, V, As, and Mo), and sulfide has not been performed and the mechanisms controlling their mobilization were not investigated. Here, a passive sampling technique, Diffusive Gradients in Thin-films (DGT), was used in situ to obtain high resolution concentration profiles of these elements in the sediment porewater. Our results revealed two mobilization mechanisms of cationic trace metals and oxyanions in Belgian coastal sediments, both strongly linked to the cycling of Fe. Mobilization of Co, Pb, Ni, and Cu is controlled by electrogenic sulfur oxidation, acidification of the porewater and dissolution of FeS, while that of oxyanions (P, V, and As) is controlled by reductive dissolution of Fe oxyhydroxides. Constant cationic trace metal to Fe molar ratios were established in FeS, while the oxyanion to Fe ratios in Fe oxyhydroxides differ significantly between sampling stations, which is primarily caused by competing effects. We found no evidence that cationic trace metal mobilization was related to Fe oxyhydroxides, or oxyanion mobilization to FeS. This suggests that particulate organic matter forms the major pathway for cationic trace metal input in coastal sediments and that oxyanions will not be incorporated in FeS but form their own oxyanion-sulfide compound. These findings will contribute to a better understanding of the mobilization mechanisms of cationic trace metals and oxyanions in coastal sediments, and of their biogeochemical cycling in coastal ecosystems.

Spatiotemporal dynamics of labile Cd in soil during rice growth

Labile soil Cd is susceptible to changes over space and time due to the physical and chemical properties of the soil as well as biological processes. In this study, non-disruptive, in situ monitoring was used to explore these changes. We analyzed the mechanism(s) by which Cd is morphologically transformed and its migration patterns, with the goal of preventing soil Cd from becoming labile. The results showed that the concentration of labile Cd in the soil exhibited spatiotemporal variability throughout the rice growth period. Over time, it increased and then declined, while over space, the concentration of labile Cd in rhizosphere is higher than that in non-rhizosphere. As the depth increased, the concentration of Cd increased and then declined, especially during the flowering stage. The change of soil labile Cd concentration showed significant negative correlation with the change in soil pH and easy dynamic S(II) and Fe(II) content. It was also found that root action changed the pH of the soil during rice growth, thereby affecting the morphologies of S(II) and Fe(II), which governed the transformation of Cd and its soil mobility.

Soil migration of antimony and arsenic facilitated by colloids in lysimeter studies

The migration behaviors of antimony (Sb) and arsenic (As) and its influence factors have not been well understood among the different soils. In this study, we used lysimeter experiments to investigate the migration behavior of Sb compared with that of As in four representative soil materials from China. All the experiments processes and management measures were conducted to simulate the actual natural environmental conditions. Results indicated that after two years of leaching, the concentrations of Sb and As at the soil surface had decreased, whereas they increased in the deep soil profiles. In the polluted soil materials, 28.5%-39.2% of Sb and 0.4%-1.3% of As existed in the stable fraction, respectively. As and Sb levels were higher in the surface soil layer, and decreased with the soil depth in the different soil profiles. In soil leachate, Sb was mainly found in particle sizes smaller than 0.45 μm with the organic colloids, which had a peak in the spring and summer. On contrast, As was found in particle sizes larger than 0.45 μm with the inorganic colloids such as iron (Fe) and aluminum (Al) oxides. Pearson correlation results showed that the concentrations of Sb in the soil leaching solution and 0.45-μm-filltered solution were all positively correlated with Fe and Al. The results confirmed that Sb was combined with Fe and Al in the solution, and As posed a greater environmental risk than Sb during the leaching process. This study will help us to describe and predict As and Sb pollution in the soil environment, providing a basis for managing soil contaminated by these pollutants.

Effect of redox variation on the geochemical behavior of Sb in a vegetated Sb(V)-contaminated soil column

This study examined the geochemical behavior of antimony (Sb) in a vegetated contaminated soil column consisting of unsaturated rhizosphere and a waterlogging layer. The results showed a reducing condition (Oxidation-Reduction Potential (ORP) of -171 mV) was formed in about 5 days in the waterlogging zone. The amount of Sb released was higher under the oxidizing unsaturated-rhizosphere compared to that in the waterlogging zone possibly because of the weaker affinity of Sb(V) to Mn- and/or Fe-oxides in soil. The fraction of Sb(III) in the dissolved total Sb increased with time when soil redox states were subjected to a further reduction. Solid phase Sb K-edge X-ray absorption spectroscopy (XAS) of soils showed that Sb(III) fraction of the deeper layer soil increased while the unsaturated upper soil solely composed Sb(V). In this study, 250 mg/kg of Sb pollution did not significantly affect plant growth and no significant transport of Sb occurred from the soil to plant. However, changes in redox conditions within the soil column induced a shift in soil microbial communities. Consequently, the importance of redox states of soil on geochemical behavior of Sb and the effects of soil flooding or waterlogging deserve attention in the management of Sb-contaminated soil.

Determining the effects of past gold mining using a sediment palaeotoxicity model

Ore processing techniques used in Yellowknife's largest mining operation, Giant Mine, is responsible for the atmospheric release of approximately 20,000 t of particulate arsenic trioxide and other heavy metal(loids). This rapid deposition of heavy metal(loids) may have caused ecological disturbances to aquatic food webs. Here we use ²¹⁰Pb and ¹³⁷Cs dated lake sediment cores from 20 lakes within a 40 km radius of Yellowknife to examine the spatial-temporal distribution of arsenic, antimony and lead. Further, we model the toxicity of the sediment to aquatic biota pre-, during, and post-mining using palaeotoxicity modelling, enrichment factor assessment, and comparisons to national sediment quality guidelines. We found that metal(loid) profiles in sediment peaked during the height of mining operations. These peak metal(loid) concentrations were highest in lakes near the mine's roaster stack, and decreased with distance from the historic mine. Palaeotoxicity modelling of lake sediment archives indicate that there is no significant difference in the mean predicted toxicity of pre- and post-mining samples (p = 0.14), however mining activities in the region significantly increased the predicted toxicity of sediments to aquatic organisms during mining operations (p < 0.001). In the years since roasting processes ceased, the mean palaeotoxicity of all lakes has decreased significantly (p < 0.05), indicating a projected pattern of biological recovery. Importantly, some lakes remain at an elevated risk, indicating that aquatic ecosystems in Yellowknife may continue to have lingering effects on aquatic biota despite the closure of the mine two decades ago.

Evaluating the mobility and labile of As and Sb using diffusive gradients in thin-films (DGT) in the sediments of Nansi Lake, China

Arsenic (As) and antimony (Sb) contamination in the aquatic environment have received significant attention recently due to the potential risks they pose. However, there have been few studies about the simultaneous behaviors of As and Sb, resulting in a poor understanding of their occurrence at the sediment-water interface (SWI), especially at the millimeter scale. In this study, soluble and labile concentrations of As and Sb were investigated using high-resolution dialysis (HR-Peeper) and diffusive gradients in thin films technique (DGT) in Nansi Lake, China, respectively. Results showed mean soluble concentrations of As and Sb were 5.00 μg/L and 2.05 μg/L, respectively. DGT-labile concentrations of As and Sb ranged from 0 to 0.80 μg/L and from 0.50 to 0.67 μg/L, respectively. In the vertical profile, different tends for DGT-labile concentration As and Sb were observed. The reductive dissolution of Fe/Mn (hydr)oxides was considered as a crucial driver for As release and mobility, which was supported by its significant correlation (r = 0.348, p < .05) with Fe. While DGT-labile Sb concentration was negatively correlated with DGT-labile Fe (r = -0.24, p < .05) and Mn (r = -0.324, p < .05), this may be attributed to the absorption of the Sb(III) by the green rusts in sub-oxic and mildly alkaline environments. The significant differences between DGT-labile concentration and community Bureau of Reference (BCR) sequential extraction were shown using a linear regression relationship, indicating that BCR chemical fractions cannot reflect the mobility of As and Sb in the sediment. Furthermore, the net diffusive fluxes of As and Sb based on DGT-labile concentration were 0.24 and - 0.56 μg∙m^-2∙day^-1, respectively. There was a potential risk of toxicity to the overlying water from As.

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

The Macleay River in eastern Australia is severely impacted by historic stibnite- and arsenopyrite-rich mine-tailings. We explore the partitioning, speciation, redox-cycling, mineral associations and mobility of antimony and arsenic along >70 km reach of the upper Macleay River. Elevated Sb/As occur throughout the active channel-zone and in floodplain pockets up to the regolith margin, indicating broad dispersal during floods. Sb concentrations in bulk-sediments decay exponentially downstream more efficiently than As, likely reflecting sediment dilution, hydraulic sorting and comparatively greater leaching of (more mobile) Sb(V) species. However, Sb in bulk-sediments becomes proportionally more bio-available downstream. Sb(V) and As(V) species dominate stream fine-grained (<180 μm) bulk-sediments, reflecting oxidative weathering downstream. Increasing poorly-crystalline Fe(III) [Fe(III)_HCl] in bulk-sediments also indicates progressive oxidative weathering of Fe(II)-bearing minerals downstream and significant (P < .05) correlations exist between PO₄^-3-exchangeable As and Sb fractions and Fe(III)_HCl. Accumulations of poorly-crystalline Fe(III) precipitates (mainly ferrihydrite/feroxyhyte) occur intermittently in hyporheic-zone seeps and are enriched in As relative to Sb and contain some As(III) and Sb(III) (~30-40%). There is dynamic in-stream redox-cycling of both Sb and As, with localised S-coordinated As and Sb species re-forming in organic-rich, hyporheic sediments subject to contemporary sulfidogenesis. Sb [mainly Sb(V)] is comparatively more mobile in hyporheic and surface waters under oxic conditions, whereas As [mainly As(III)] is more mobile in hyporheic porewaters subject to reducing/sulfidogenic conditions. Repeat water-leaching of bulk-sediments confirms that Sb is proportionally more mobile than As. Mean concentrations of Sb in river water 168 km downstream from the mine are significantly (P < .05) higher than As, while K_d data indicate Sb is more strongly partitioned to the aqueous phase than As. Although the (mainly) oxic flow path of this river favours aqueous Sb mobility compared to As, localised redox-driven shifts in speciation of both elements strongly influence their respective mobility and partitioning.

Soil organic matter affects arsenic and antimony sorption in anaerobic soils

Soil organic matter (SOM) affects arsenic (As) and antimony (Sb) mobility in soils under waterlogged conditions by acting as an electron donor, by catalyzing redox-cycling through electron shuttling and by acting as a competing ligand. This study was set up to disentangle these different effects of SOM towards As and Sb sorption in anaerobic soils. Nine samples were taken at different depths in an agricultural soil profile to collect samples with a natural SOM gradient (<1-40 g soil organic carbon kg^-1). The samples were incubated either or not under waterlogged conditions in an anaerobic chamber for 63-70 days, and glucose (5 g C kg^-1) was either or not added to the anaerobic incubated samples as an electron donor that neither acts as an electron shuttle nor as a competing ligand. The solid-liquid distribution coefficients (K_D) of As and Sb were measured at trace levels. The K_D values of As decreased ∼2 orders of magnitude upon waterlogging the SOM rich topsoil, while no additional changes were observed when glucose was added. In contrast, smaller changes in the As K_D values were found in the low SOM containing subsoil samples, unless glucose was added that mobilised As. The Sb K_D values increased upon reducing conditions up to factor 20, but again only in the high SOM topsoil samples. Surprisingly, the Sb immobilisation during waterlogging only occurred in Sb amended soils whereas the geogenic Sb was mobilised upon reducing conditions, although total dissolved Sb concentrations remained low (<10 nM). The change in As and Sb sorption upon waterlogging was similar in the SOM rich topsoil as in the low SOM subsoil amended with glucose. This suggests that the SOM dependent changes in As and Sb mobility in response to soil waterlogging are primarily determined by the role of SOM as electron donor.

Antimony speciation and mobility during Fe(II)-induced transformation of humic acid-antimony(V)-iron(III) coprecipitates

Antimony, as the Sb(V) species, often occurs in oxic soils and sediments as coprecipitates with poorly-crystalline Fe(III)-bearing minerals. It is common for these Sb(V)-Fe(III) coprecipitates to also contain varying quantities of co-occurring humic acid (HA). When exposed to reducing conditions, the production of Fe(II) may cause the initial metastable HA-Sb(V)-Fe(III) phases to undergo rapid transformations to more stable phases, thereby potentially influencing the geochemical behavior of coprecipitated Sb(V). However, little is known about the impacts of this transformation on the mobility and speciation of Sb. In this study, we reacted synthetic HA-Sb(V)-Fe(III) coprecipitates (Fe:Sb ratio = 4, and C:Fe molar ratios = 0, 0.3, 0.8 and 1.3) with 0, 1 or 10 mM Fe(II) under O₂-free conditions at pH 7.0 for 15 days. Fe K-edge EXAFS spectroscopy revealed that solid-phase Fe(III) in the initial coprecipitates contained a mixture of ∼4/5 ferrihydrite (Fe₁₀O₁₄(OH)₂) and ∼1/5 tripuhyite (FeSbO₄), regardless of the corresponding amount of coprecipitated HA. Tripuhyite persisted throughout the full experiment duration, while ferrihydrite was partially replaced by goethite (FeOOH) when either 1 or 10 mM Fe(II)_aq was added to the coprecipitates. The greatest level of goethite formation (∼55% of solid-phase Fe) was observed in the HA-free/10 mM Fe(II)_aq treatment, with ferrihydrite transformation being partially attenuated at higher levels of HA. Mobilisation of aqueous Sb was the greatest for 1 mM Fe(II) treatments at high HA:Fe ratios. Sb K-edge XANES spectroscopy showed that the largest reduction of Sb(V) to Sb(III) (∼37%) and the greatest repartitioning of Sb to the mineral surface (∼7.9-9.8%) occurred in the coprecipitates with the highest HA contents in the presence of 10 mM Fe(II). The results indicate that the amount of HA in HA-Sb(V)-Fe(III) coprecipitates can greatly influence mobility and speciation of Sb in Fe(II)-rich conditions. The results of this study provide new insights into alterations in Sb mobility and retention in response to Fe cycling under organic matter-rich reducing conditions.

Arsenic mobilization from soils in the presence of herbicides

Arsenic (As) mobilization in soils is a fundamental step controlling its transport and fate, especially in the presence of the co-existing components. In this study, the effect of two commonly used herbicides, glyphosate (PMG) and dicamba, and two competing ions including phosphate and humic acid, on As desorption and release was investigated using batch and column experiments. The batch kinetics results showed that As desorption in the presence of competing factors conformed to the pseudo-second order kinetics at pH range of 5-9. The impact of phosphate on desorption was greatest, followed by PMG. The competitive effect of dicamba and humic acid was at the same level with electrolyte solution. In situ flow cell ATR-FTIR analysis was performed to explore the mechanism of phosphate and PMG impact on As mobilization. The results showed that PMG promoted As(III) desorption by competiting for available adsorption sites with no change in As(III) complexing structure. On the other hand, phophate changed As(III) surface complexes from bidentate to monodentate structures, exhibiting the most siginficant effect on As(III) desorption. As(V) surface complexes remained unchanged in the presence of PMG and phosphate, implying that the competitive effect for As(V) desorption was primarily determined by the available adsorption sites. Long-term (10 days) soil column experiments suggested that the effect of humic acid on As mobilization became pronounced from 3 days (18 PVs). The insights of this study help us understand the transport and fate of As due to herbicides application.

Seasonal variation of arsenic and antimony in surface waters of small subarctic lakes impacted by legacy mining pollution near Yellowknife, NT, Canada

The seasonal variation in lake water arsenic (As) and antimony (Sb) concentrations was assessed in four small (<1.5km²) subarctic lakes impacted by As and Sb emissions from legacy mining activities near Yellowknife, Northwest Territories, Canada. Substantial variation in As concentrations were measured over the two-year period of study in all but the deepest lake (maximum depth 6.9m), including a four-fold difference in As in the shallowest lake ([As]: 172-846μgL^-1; maximum depth 0.8m). Arsenic concentrations were enriched following ice cover development in the three shallowest lakes (50-110%) through a combination of physical and biogeochemical processes. Early winter increases in As were associated with the exclusion of solutes from the developing ice-cover; and large increases in As were measured once oxygen conditions were depleted to the point of anoxia by mid-winter. The onset of anoxic conditions within the water column was associated with large increases in the concentration of redox sensitive elements in lake waters (As, iron [Fe], and manganese [Mn]), suggesting coupling of As mobility with Fe and Mn cycling. In contrast, there was little difference in Sb concentrations under ice suggesting that Sb mobility was controlled by factors other than Fe and Mn associated redox processes. A survey of 30 lakes in the region during fall (open-water) and late-winter (under-ice) revealed large seasonal differences in surface water As were more common in lakes with a maximum depth <4m. This threshold highlights the importance of winter conditions and links between physical lake properties and biogeochemical processes in the chemical recovery of As-impacted subarctic landscapes. The findings indicate annual remobilization of As from contaminated lake sediments may be inhibiting recovery in small shallow lakes that undergo seasonal transitions in redox state.