Arsenic Release Research Articles

Colloidal fraction on pomelo peel-derived biochar plays a dual role as electron shuttle and adsorbent in controlling arsenic transformation in anoxic paddy soil

Arsenic release and reduction in anoxic environments can be mitigated or facilitated by biochar amendment. However, the key fractions in biochars and how they control arsenic transformation remain poorly understood. In this study, a biochar produced from pomelo peel was rich in colloids and was used to evaluate the roles of the colloidal and residual fractions of biochar in arsenic transformation in anoxic paddy soil. Bulk biochar showed a markedly higher maximum adsorption capacity for As(III) at 1732 mg/kg than for As(V) at 75.7 mg/kg, mainly because of the colloidal fraction on the surface. When compared with the control and treatments with the colloidal/residual fraction, the addition of bulk biochar facilitated As(V) reduction and release in the soil during days 0–12, but decreased the dissolved As(III) concentration during days 12–20. The colloidal fraction revealed significantly higher electron donating capacity (8.26 μmole−/g) than that of bulk biochar (0.88 μmole−/g) and residual fraction (0.65 μmole−/g), acting as electron shuttle to promote As(V) reduction. Because the colloidal fraction was rich in aliphatic carbon, fulvic acid-like compounds, potassium, and calcium, it favored As(III) adsorption when more As(III) was released, probably via organic-cation-As(III) complexation. These findings provide deeper insight into the role of the colloidal fraction of biochar in controlling anaerobic arsenic transformation, which will be helpful for the practical application of biochar in arsenic-contaminated environments.

Sulfate-reducing bacteria (SRB) mediated carbonate dissolution and arsenic release: Behavior and mechanisms

Carbonate bound arsenic act as an important reservoir for arsenic (As) in nature aquifers. Sulfate-reducing bacteria (SRB), one of the dominant bacterial species in reductive groundwater, profoundly affects the biogeochemical cycling of As. However, whether and how SRB act on the migration and transformation of carbonate bound arsenic remains to be elucidated. Batch culture experiment was employed using filed collected arsenic bearing calcite to investigate the release and species transformation of As by SRB. We found that arsenic in the carbonate samples mostly exist as inorganic As(V) (93.92 %) and As(III). The present of SRB significantly facilitated arsenic release from carbonates with a maximum of 22.3 μg/L. The main release mechanisms of As by SRB include 1) calcite dissolution and the liberate of arsenic in calcite lattices, and 2) the break of H-bonds frees arsenic absorbed on carbonate surface. A redistribution of arsenic during culture incubation took place which may due to the precipitation of As2Sx or secondary FeAl minerals. To our best knowledge, it is the first experimental study focusing on the release of carbonate bound arsenic by SRB. This study provides new insights into the fate and transport of arsenic mediated by microorganism within high arsenic groundwater-sediment system.

Nitrate-induced mobilization of trace elements in reduced groundwater environments

Groundwater is an essential source for drinking water production. Nitrate infiltration into groundwater due to over-fertilization can cause a potential risk for groundwater quality. Pyrite and other geogenic minerals can be oxidized and trace metals consequently released into water, e.g., nickel and uranium.To achieve a better understanding of the nitrate-induced mobilization of metals, this study investigated the release of antimony, arsenic, chromium, cobalt, molybdenum, uranium, and vanadium from three different reduced sediments after nitrate addition. The experiments were conducted as batch and soil column tests under oxygen-free conditions. In addition to the ORP, the pH value was a relevant driver for the metal mobilization due to pH dependent adsorption and ion exchange processes. Uranium concentrations in the water increased with increasing redox potential. Also, antimony and, to a lesser extent, molybdenum showed higher mobilization at higher ORP as well as at higher pH values. On the contrary, arsenic and cobalt was immobilized with increasing redox potential. Pourbaix diagrams demonstrated very complex species distributions even in synthetic water. The mobilization of trace metals is expected to be also influenced by the type of surrounding rocks and water quality parameters such as dissolved organic carbon.

Toxic Effects of Arsenic on Four Freshwater Aquatic Species and Its Transformation Metabolism in Crucian Carp (Carassius auratus).

Inorganic arsenic is a well-known carcinogen that is much more toxic than its organic counterpart. While much is known about the accumulation and transformation of arsenic in marine organisms, little is known regarding these processes in freshwater aquatic species. In this study, the acute toxicity and toxicological effects of inorganic arsenic on four freshwater organisms (Cyprinus carpio, Misgurnus anguillicaudatus, Pseudorasbora parva, Eriocheir sinensis) commonly found in rice-fish farming systems were investigated. The organisms exhibited different levels of sensitivity to inorganic arsenic, with crustaceans being more sensitive than fish. Fish were found to be more tolerant to As(V) than As(III). The study also investigated the accumulation, transformation, and release of inorganic arsenic in crucian carp, an omnivorous species with high environmental tolerance. The fish accumulated As(III) rapidly in various tissues, and were able to transport it to other tissues through gills, intestines, and skin. The accumulated As(III) was converted into less toxic forms, such as monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA), via methylation. The fish also converted As(III) into arsenate (AsV) via enzymatic and oxidative reactions. After the transferal to clean water, the forms of arsenic in the various tissues decreased rapidly, but the rates of excretion of the four forms of arsenic were not the same among the different tissues. Our results suggest that crucian carp can reduce the environmental toxicity of As(III) at certain concentrations by transforming it into less toxic forms within their bodies.

The impact of microbial community structure changes on the migration and release of typical heavy metal (loid)s during the revegetation process of mercury-thallium mining waste slag

The effect of changes in microbial community structure on the migration and release of toxic heavy metal (loid)s is often ignored in ecological restoration. Here, we investigated a multi-metal (mercury and thallium, Tl) mine waste slag. With particular focus on its strong acidity, poor nutrition, and high toxicity pollution characteristics, we added fish manure and carbonate to the slag as environmental-friendly amendments. On this basis, ryegrass, which is suitable for the remediation of metal waste dumps, was then planted for ecological restoration. We finally explored the influence of changes in microbial community structure on the release of Tl and As in the waste slag during vegetation reconstruction. The results show that the combination of fish manure and carbonate temporarily halted the release of Tl, but subsequently promoted the release of Tl and arsenic (As), which was closely related to changes in the microbial community structure in the waste slag after fish manure and carbonate addition. The main reason for these patterns was that in the early stage of the experiment, Bacillaceae inhibited the release of Tl by secreting extracellular polymeric substances; with increasing time, Actinobacteriota became the dominant bacterium, which promoted the migration and release of Tl by mycelial disintegration of minerals. In addition, the exogenously added organic matter acted as an electron transport medium for reducing microorganisms and thus helped to reduce nitrate or As (Ⅴ) in the substrate, which reduced the redox potential of the waste slag and promoted As release. At the same time, the phylum Firmicutes, including specific dissimilatory As-reducing bacteria that are capable of converting As into a more soluble form, further promoted the release of As. Our findings provide a theoretical basis for guiding the ecological restoration of relevant heavy-metal (loid) mine waste dumps.

The alternation of flood and ebb tide induced arsenic release and migration from coastal tidal flat sediments in Yellow Sea wetlands: An ex-situ study

Periodic floods and ebb tides driven by seawater affect arsenic (As) behavior in coastal wetlands, which is not fully understood. In this study, we combined DGT, high-throughput sequencing, and real-time quantitative polymerase chain reaction (qPCR) techniques for the first time to investigate the effects of flood and ebb tide on As transformation and mobility in a typical coastal tidal flat wetland by laboratory experiments. In the flood-tide stage, more As was released into the water solution, and As(III) concentration in water solution was notably higher (51 μg/L) than in the ebb-tide stage, especially in SA (sodium acetate addition) treatment. As-methylation also occurred in this stage. The release of As was attributed to the reductive dissolution of amorphous and crystalline Fe oxides bound As induced by microbes. Fusibacter, Sva1033, Bradyrhizobium, Ralstonia, and Desulfobacter were primarily involved in the reduction of As, Fe, and S. The copy numbers of arsC were 20–148 times higher than arrA, suggesting that the detoxification reduction pathway was the primary mechanism of direct As-reduction in the flood-tide stage. However, 2-D high-resolution DGT techniques revealed that the concentrations of As(III) and total As in pore water of sediments in the ebb-tide stage were higher than those in the flood-tide stage. The detected aioA gene indicated that As was oxidized in the sediment surface, resulting in As stabilization by crystalline iron minerals, whereas more As(III) were accumulated in pore water. The reduction pathway mediated by arrA may play a prominent role in the ebb-tide stage. This study provides new insights into As release processes in coastal wetlands in seawater environments, which further deepens our understanding of As biogeochemical cycling behavior in different natural environments.

Biotransformation of roxarsone by earthworms and subsequent risk of soil arsenic release: The role of gut bacteria

The organoarsenical feed additive roxarsone (ROX) is a ubiquitous threat due to the unpredictable levels of arsenic (As) released by soil bacteria. The earthworms representing soil fauna communities provide hotspots for As biotransformation genes (ABGs). Nonetheless, the role of gut bacteria in this regard is unclear. In this study, the changes in As speciation, bacterial ABGs, and communities were analyzed in a ROX-contaminated soil (50 mg/kg As in ROX form) containing the earthworm Eisenia feotida. (RE vs. R treatment). After 56 d, earthworms reduced the levels of both ROX and total As by 59 % and 17 %, respectively. The available As content was 10 % lower in the RE than in R treatment. Under ROX stress, the total ABG abundance was upregulated in both earthworm gut and soil, with synergistic effects observed following RE treatment. Besides, the enrichment of arsM and arsB genes in earthworm gut suggested that gut bacteria may facilitate As removal by enhancing As methylation and transport function in soil. However, the bacteria carrying ABGs were not associated with the ABG abundance in earthworm gut indicating the unique strategies of earthworm gut bacteria compared with soil bacteria due to different microenvironments. Based on a well-fit structural equation model (P = 0.120), we concluded that gut bacteria indirectly contribute to ROX transformation and As detoxification by modifying soil ABGs. The positive findings of earthworm-induced ROX transformation shed light on the role of As biomonitoring and bioremediation in organoarsenical-contaminated environments.

Sea level rise from climate change is expected to increase the release of arsenic into Bangladesh's drinking well water by reduction and by the salt effect.

Over 165,000,000 people live in Bangladesh; approximately 97% of Bangladeshis drink well water. Approximately 49% of Bangladesh's area has drinking well water with arsenic (As) concentrations that exceed the 10 micrograms per liter (μg/L) World Health Organization (WHO) guideline. This exposure to a potent carcinogen is a significant threat to public health. About 21% of Bangladesh is flooded each year during a typical monsoon season. As climate change progresses, sea levels will continue to rise, and the area and duration of these annual floods will increase. We hypothesize that these consequences of climate change can increase the release of arsenic from sediments into Bangladesh's drinking well water. Drinking well water samples were collected during a national-scale survey in Bangladesh. The dissolved oxygen concentration, oxidation-reduction potential, specific conductance, pH, and temperature were measured at sampling with calibrated portable electronic sensors. The arsenic concentration was measured by the silver diethyldithiocarbamate method. As the concentration of dissolved oxygen decreases, the concentration of arsenic increases (p-value = 0.0028). Relatedly, as the oxidation-reduction potential decreases, the concentration of arsenic increases (p-value = 1.3×10-5). This suggests that arsenic is released from sediments into Bangladesh's drinking well drinking water by reduction. As the specific conductance increases, the concentration of arsenic increases (p-value = 0.023). This suggests that arsenic is also released from sediments into water by the salt effect. Rising sea levels can cause a decrease in the dissolved oxygen concentration and oxidation-reduction potential of the underlying aquifer; this should increase the dissolution of insoluble arsenate (H3-xAs(V)O4x-) in sediments by reduction. This, in turn, should release soluble arsenite (H3-xAs(III)O3x-) into the drinking well water. Rising sea levels can cause an increase in the salt concentration of the underlying aquifer; this should increase the release of arsenic from sediments into the drinking well water by the salt effect.

Nitrogen addition accelerates litter decomposition and arsenic release of Pteris vittata in arsenic-contaminated soil from mine

The arsenic (As) release from litter decomposition of As-hyperaccumulator (Pteris vittata L.) in mine areas poses an ecological risk for metal dispersion into the soil. However, the effect of atmospheric nitrogen (N) deposition on the litter decomposition of As-hyperaccumulator in the tailing mine area remains poorly understood. In this study, we conducted a microcosm experiment to investigate the As release during the decomposition of P. vittata litter under four gradients of N addition (0, 5, 10, and 20 mg N g−1). The N10 treatment (10 mg N g−1) enhanced As release from P. vittata litter by 1.2–2.6 folds compared to control. Furthermore, Streptomyces, Pantoea, and Curtobacterium were found to primarily affect the As release during the litter decomposition process. Additionally, N addition decreased the soil pH, subsequently increased the microbial biomass, as well as hydrolase activities (NAG) which regulated N release. Thereby, N addition increased the As release from P. vittata litter and then transferred to the soil. Moreover, this process caused a transformation of non-labile As fractions into labile forms, resulting in an increase of available As concentration by 13.02–20.16% within the soil after a 90-day incubation period. Our findings provide valuable insights into assessing the ecological risk associated with As release from the decomposition of P. vittata litter towards the soil, particularly under elevated atmospheric N deposition.

Mechanism of arsenic release process from arsenopyrite chemical oxidation

Oxidation of arsenopyrite is one of the main causes of arsenic pollution in the environment. This study, examind changes in the surface properties of arsenopyrite in the presence of oxygen. Furthermore, X-ray photoelectron spectroscopy, in situ Raman analysis, high-resolution transmission electron microscopy, and ab initio molecular dynamics were carried out on arsenopyrite, and the oxidation properties and processes of the arsenopyrite surface, along with the Fe/As/S oxidation processes, were analysed In situ-Raman spectroscopy data clearly showed that in the presence of oxygen, Fe2+ ions were transformed into Fe3+ ions on the surface of arsenopyrite at 207 cm−1, and the content of iron oxides on the surface of arsenopyrite increased significantly over time. The presence of iron promoted the oxidation of As(III), and the oxidation process was found to affect the oxidation of As atoms on the surface of arsenopyrite due to the presence of FeS bonds. The presence of As3+ intensified the oxidation of arsenopyrite surface (139 cm−1).The main reason was the presence of O2·-/HO2· on the surface of arsenopyrite, the change of Fe2+ → FeOH2+ → Fe3+ on the surface of arsenopyrite, and the change of As3+ → As4+ → As5+, which strengthened the overflow of As, leading to reconstruction and changes on the surface of arsenopyrite.

Macrophage-hitchhiked arsenic/AB bionic preparations for liver cancer.

Macrophage-hitchhiked arsenic/AB bionic preparations were developed to improve the therapeutic effect on liver cancer by means of the tumor-targeting ability of macrophages in vivo. In vitro and in vivo cellular uptake assays demonstrated that arsenic/AB, with negatively charged particles of around 100-200 nm size, could hitchhike to macrophages. Dissolution experiments of arsenic/AB showed that arsenic/AB could delay the release of arsenic and ensure the safety of macrophages during its transport. Histological examination confirmed the safety of the preparations for major organs. In vivo distribution experiment showed that the arsenic/AB bionic preparations could rapidly accumulate in tumors, and in vivo treatment experiment showed a significant tumor inhibition of arsenic/AB. The therapeutic mechanism of liver cancer might be that the arsenic/AB bionic preparations could inhibit tumor growth by reducing inflammatory response and inhibiting CSF1 secretion to block CSF1R activation to induce more differentiation of tumor-associated macrophages (TAMs) towards the anti-tumor M1 phenotype. Therefore, we concluded that the arsenic/AB bionic preparations could improve the distribution of arsenic in vivo by hitchhiking on macrophages as well as make it have tumor targeting and deep penetration abilities, thus increasing the therapeutic effect of arsenic on liver cancer with reduced side effects.

Degradation of algae promotes the release of arsenic from sediments under high-sulfate conditions

Sulfate concentrations in eutrophic waters continue to increase; however, the transformations of arsenic (As) in sediments under these conditions are unclear. In this study, we constructed a series of microcosms to investigate the effect of algal degradation on As transformations in sediments with high sulfate concentrations. The results showed that both the elevated sulfate levels and algal degradation enhanced the release of As from sediments to the overlying water, and degradation of algal in the presence of elevated sulfate levels could further contribute to As release. Sulfate competed with arsenate for adsorption in the sediments, leading to As desorption, while algal degradation created a strongly anaerobic environment, leading to the loss of the redox layer in the surface sediments. With high sulfate, algal degradation enhanced sulfate reduction, and sulfur caused the formation of thioarsenates, which may cause re-dissolution of the arsenides, enhancing As mobility by changing the As speciation. The results of sedimentary As speciation analysis indicated that elevated sulfur levels and algal degradation led to a shift of As from Fe2O3/oxyhydroxide-bound state to specifically adsorbed state at the sediment water interface. This study indicated that algal degradation increases the risk of As pollution in sulfate-enriched eutrophic waters.

Removal of arsenic from realgar-containing water by Spirogyra: Implications for in situ remediation of arsenic in the mine area

Realgar (As4S4) is an arsenic sulfide mineral, occurring widely in metamorphic-hydrothermal deposits, reductive soil and wetland sediments. The dissolution of realgar releases arsenic, which is toxic to the environment, so the process that causes this problem deserves attention. Native Spirogyra sp. was collected and cultured from a realgar mine area and used to create varying concentrations of algae to investigate the dissolution of realgar using a photosynthesis microcosm. Separately the dynamic adsorption and uptake processes of arsenic (III) and arsenic (V) were investigated. The dissolution rate equation of realgar under Spirogyra sp. is R = 10−6.36[DO]1.84[H+]−0.60. The diurnal growth of Spirogyra sp. increases arsenic release by as much as three times than the control group and significantly changes aqueous arsenic species by varying the dissolved oxygen and pH. The amounts of arsenic (III) and arsenic (V) removed from living Spirogyra sp. were 14605 μg⋅g−1⋅h−1 and 6511 μg⋅g−1⋅h−1. Increasing amounts of –COOH, CO and carbonyl groups from the live Spirogyra sp. also promoted the dissolution of realgar. The results suggest that filament Spirogyra sp. would be an effective method for in situ acceleration of arsenic sulfide dissolution and remediation of arsenic pollution in environments such as paddy soil.

Effect of sulfate on arsenic migration and transformation in micro-cosmic experiments simulating mangrove sediment environment

Fragile mangrove wetlands are threatened by sea level rise, which results in the introduction of increased sulfate concentrations into mangrove sediment. Mangroves are also susceptible to heavy metal contamination, such as arsenic (As) contamination. However, little is known about whether sulfate affects the bioavailability of As in mangrove sediment. This study focuses on the effects of sulfate on arsenic mobilization and transformation in As-contaminated mangrove sediment, and modeled changes in sulfate concentration, As contamination levels, and pH conditions. The As release from sediment treated with sulfate (30 mM) significantly decreased (P < 0.05). In the moderate arsenic-contaminated sediment, the mean arsenic release was 181.95 ± 33.05 μg kg−1 without sulfate and 157.55 ± 40.98 μg kg−1 with sulfate. In addition, the release of As was affected by both pH and sulfate when pH conditions were changed. Arsenic release was promoted at pH 11 (P < 0.05), but sulfate still mitigating As release at this pH value (P < 0.001). Furthermore, the release of As after sulfate input was always accompanied by the release of iron (Fe) and aluminum (Al). Sulfate altered the distribution of forms of As in the sediment, with a significant increase in the proportion of strongly adsorbed As and silicate-bound states of As. Overall, sulfate intrusion mitigates As release, but it also increases the bioavailability of As in the sediment. This fact makes the toxic element most susceptible to long-term mobilization via biological processes. Therefore, the effect of persistent sulfate intrusion caused by sea level rise on toxic elements in mangrove sediment deserves long-term attention. The significance of this study is that it contributes to understanding the behavior of heavy metals in coastal wetland ecosystems affected by sea level rise.

A case study on regional arsenic sources and its distribution in Mekong River groundwater

Arsenic contamination in the Mekong River is a well-known environmental issue yet to be resolved due to its transboundary nature which further limits its access and data collection. Other than that, the key mechanisms that controlling the arsenic release in Mekong sub-region groundwater was heterogeneously distributed and can be varied from region to region. The main purpose of this project is to identify the regional arsenic contamination levels in the hope of helping the government integrate regional groundwater arsenic risk reduction policy in their near future planning. Sampling was conducted during May and August 2022 in Cambodia and Laos, respectively. The findings revealed that the topography of certain areas in Cambodia has exposed significantly high concentrations of arsenic in groundwater compared to those in Laos. About 33% of the sampling sites in Cambodia had shown a high arsenic contamination (&gt; 10 µg/L, WHO guideline) with its concentration ranging from 47.7 ± 0.8 to 696.9 ± 5.6 ppb. The physicochemical properties revealed that the arsenic controlling mechanisms were totally different between both study area. More regional and site-specific arsenic contamination research related to climate change and arsenic hydrology at regional levels should be carried out to ensure the water safety plan in specific regions. Further, we believe that the findings of this study will be beneficial to policy and regional water safety plans for the Mekong River, especially in Cambodia.

Desilification of phytolith exacerbates the release of arsenic from rice straw

Arsenic (As) turnover in rice paddy agro-ecosystems has received much attention because As can enter the food chain through its accumulation in rice, thereby affecting human health. Returning straw to soil is a common practice to retain nutrients for soil and crops, but it also cycles As within the rice paddy field ecosystems. However, there is still a lack of detailed understanding of the fate of As in rice straw, and how or to what extent it is recycled back into the soil environment. This study aims to elucidate the relationship between the microstructure of rice straw and the release of As during rice straw decomposition. The microstructure of rice straw was found to comprise both organic and silica (phytolith) components. These two constituents are inter-embedded to form a composite-like structure that contains up to 6.48 mg As Kg−1. The 30-day batch experiments revealed that the biochemical release of As simultaneously depends upon the decomposition of the organic component and the desilicification of the silica component. Accompanying the release of As was the release of other elements such as Fe, Al, P and S. These elements can further interact with As to form less mobile compounds. The introduction of either Trichoderma harzianum or Bacillus velezensis was expected to accelerate the decomposition of rice straw, and enhance the silica dissolution, hence contributing to an increase in the As release. Despite these expectations, our observations showed the opposite effects. Microorganisms presumably have facilitated the change in solution chemistry or the inclusion of As into the newly-formed precipitates. The biochemical decomposition process can reduce straw particle size, while the negatively-charge surface will involve microsized straw particles in the electrostatic interaction, thereby favoring the dispersibility state. Therefore, the co-transport of micro-sized straw particles with As under field conditions should not be neglected.

New clues to the mechanism in controlling arsenic leaching from flash copper smelter flue dusts by using X-ray absorption spectroscopy

This paper characterizes the leaching of the toxic element (As) and other elements (Cu, Pb, Zn, Fe, Al, Cd, Mg, K, Ca, Mn, Co, Ni, Mo, Bi, and Sb) from the fresh and washed copper smelter flue dusts (FDs) over a wide pH range (3–12) based on the batch pH-static leaching experiment in combination with geochemical modeling. Furthermore, the leaching control mechanism of As was also explored on a microscale by using an X-ray absorption spectroscopy. Results revealed that the leached concentration of As showed a strong pH dependency, resulting in “L-shaped” leaching curves, similar to the leaching of Cu and Zn, whose leaching contents decreased dramatically with the increase of pH values. It should be noted that a partial of As was also released at the alkali conditions, especially from the washed FDs. Arsenic phases in the leachate were modeled by Visual MINTEQA 3.1 incorporated adsorption to Fe/Al hydroxides. The results were well in line with the actual leached content of As and showed that H3AsO3 is prevailing in the leachates at the pH range of 3–11, and AsO3- 4 is the dominant species in the alkali condition (pH = 12). Furthermore, the decreasing mineralogical composition of arsenolite and koritnigite in the washed FDs indicated that the above two arsenic-bearing phases primarily accounted for its release. According to the EXAFS analysis, ferric arsenates with a poorly crystalline scorodite structure in the raw FDs are important arsenic carriers, which could easily convert into crystalline scorodite at low pH. Furthermore, secondary Al hydroxides also have the potential to retain arsenate by forming a bidentate complex. Thus, As3+ species are the major resources for arsenic release from FDs and should be taken a crucial consideration when assessing its environmental risk. In contrast, arsenate was easily fixed by converting it into crystalline scorodite or adsorbing it onto Al hydroxides. This work may have important implications for the development of new eco-friendly technologies to reclaim valuable elements from the copper smelter flue dusts.

Scaling effects on arsenic release from excavated hydrothermally altered rocks in column experiments

The excavation of hydrothermally altered rocks from construction sites in Japan has raised concerns over environmental pollution due to the arsenic (As) release beyond the regulatory limit. An accurate assessment of As leaching from these rocks is imperative to understanding potential environmental implications and formulating efficient containment measures. However, the conduction of column leaching experiments to evaluate As leaching from these rocks encounters a lack of well-established protocols primarily due to the ambiguity surrounding scaling effects resulting from alterations in particle sizes and the corresponding column dimensions. Our study aimed to address this critical issue by conducting column percolation experiments on hydrothermally altered rocks of two distinct particle size ranges and rock layer thicknesses. The pH value was found to be proportional to the specific surface area (SSA) of rocks and the rock layer thickness in terms of H+ concentrations. Furthermore, the concentration and leachability of As showed a similar proportionality with the SSA. In contrast, the concentration of As remained relatively unaffected by the increased rock layer thickness, while the leachability of As was noticeably diminished in the column with a thicker rock layer. The absence of elevated As concentration and the decrease in leachability can be attributed to the enhanced As onto Fe/Al oxyhydroxides/oxides within the half-bottom part of the column with a thicker rock layer. Our findings underscore the importance of considering the SSA of rocks and rock layer thickness in the column experiments and help in the design of effective strategies to mitigate environmental contamination.

Application of geomicrobial techniques to constrain mechanisms of arsenic mobilisation in anoxic aquifers

Geogenic arsenic contamination of groundwater in South and South-East Asia poses a significant human health threat, causing a range of health conditions including but not limited to cardio-vascular disease, cancer and skin lesions (Argos et al. 2010, Pienkowska et al. 2021). Arsenic contamination also hosts a range of dire socioeconomic implications for the affected areas. A variety of mechanisms for arsenic release in anoxic aquifers have been proposed, however, the most widely accepted mechanism is the microbial reduction of As-bearing Fe(III) (oxyhydr)oxide minerals coupled with the oxidation of organic carbon (Glodowska et al. 2020, Gnanaprakasam et al. 2017). Recent research has implicated methane as a possible carbon source in the reduction of Fe(III) (oxyhydr)oxide minerals and the subsequent release of arsenic into the groundwater (Gnanaprakasam et al. 2017, Pienkowska et al. 2021). The research suggests that methanotrophs have the ability to drive anaerobic oxidation of methane, AOM, coupled to Fe(III) (oxyhydr)oxide reduction. In this study, we aim to provide unequivocal evidence for the occurrence of AOM coupled to Fe(III) (oxyhydr)oxide as well to further explore the exact mechanism(s) involved which is yet to be characterised. Here we present an overview of our work so far.

The effect of biogeochemical redox oscillations on arsenic release from legacy mine tailings

Exposed and un-remediated metal(loid)-bearing mine tailings are susceptible to wind and water erosion that disperses toxic elements into the surrounding environment. Compost-assisted phytostabilization has been successfully applied to legacy tailings as an inexpensive, eco-friendly, and sustainable landscape rehabilitation that provides vegetative cover and subsurface scaffolding to inhibit offsite transport of contaminant laden particles. The possibility of augmented metal(loid) mobility from subsurface redox reactions driven by irrigation and organic amendments is known and arsenic (As) is of particular concern because of its high affinity for adsorption to reducible ferric (oxyhydr)oxide surface sites. However, the biogeochemical transformation of As in mine tailings during multiple redox oscillations has not yet been addressed. In the present study, a redox-stat reactor was used to control oscillations between 7 d oxic and 7 d anoxic half-cycles over a three-month period in mine tailings with and without amendment of compost-derived organic matter (OM) solution. Aqueous and solid phase analyses during and after redox oscillations by mass spectrometry and synchrotron X-ray absorption spectroscopy revealed that soluble OM addition stimulated pyrite oxidation, which resulted in accelerated acidification and increased aqueous sulfate activity. Soluble OM in the reactor solution significantly increased mobilization of As under anoxic half-cycles primarily through reductive dissolution of ferrihydrite. Microbially-mediated As reduction was also observed in compost treatments, which increased partitioning to the aqueous phase due to the lower affinity of As(III) for complexation on ferric surface sites, e.g. ferrihydrite. Oxic half-cycles showed As repartitioned to the solid phase concurrent with precipitation of ferrihydrite and jarosite. Multiple redox oscillations increased the crystallinity of Fe minerals in the Treatment reactors with compost solution due to the reductive dissolution of ferrihydrite and precipitation of jarosite. The release of As from tailings gradually decreased after repeated redox oscillations. The high sulfate, ferrous iron, and hydronium activity promoted the precipitation of jarosite, which sequestered arsenic. Our results indicated that redox oscillations under compost-assisted phytostabilization can promote As release that diminishes over time, which should inform remediation assessment and environmental risk assessment of mine site compost-assisted phytostabilization.