Spatial and temporal variations of groundwater arsenic in South and Southeast Asia

Arsenic mobilization by Bathyarchaeia in subsurface sediments at the Jianghan Plain, China

As one of the most abundant microorganisms on Earth, Bathyarchaeia with diverse abilities to degrade complex organic carbon play a vital role in the global carbon cycle. However, the role of Bathyarchaeia in arsenic (As) metabolism and their contribution to As mobilization in aquifers remain unclear. In this study, we recovered 15 Bathyarchaeota metagenome-assembled genomes (MAGs) from metagenomes of borehole sediments in the Jianghan Plain (JHP), China. Together with 374 representative Bathyarchaeia MAGs from public databases, six As metabolism genes i.e. arrA, arsR, arsA, arsB, arsC (Trx) and arsM were identified, accounting for 4.4, 47.6, 20.3, 38.3, 37.5 and 49.4 % of total Bathyarchaeia MAGs, respectively. Heterologous expression of multiple arsC and arsM genes of Bathyarchaeia MAGs obtained from JHP sediments validated their abilities for As(V) reduction and As(III) methylation at environmentally relevant As concentration. These results indicate that in addition to providing bioavailable carbon sources for other microbial functional populations, Bathyarchaeia directly participate in As mobilization in the JHP aquifer via As(V) reduction and As(III) methylation. The diversified distribution of arsC and arsM in the class Bathyarchaeia suggests that Bathyarchaeia may contribute to As cycling in other As-rich environments, such as hot spring, saline lakes, marine hydrothermal sediments and soils.

Rapid screening of inorganic arsenic in groundwater on-site by a portable three-channel colorimeter

Rapid screening of inorganic arsenic (iAs) in groundwater used for drinking by hundreds of millions of mostly rural residents worldwide is crucial for health protection. Most commercial field test kits are based on the Gutzeit reaction that uses mercury-based reagents for color development, an environmental concern that increasingly limits its utilization. This study further improves the Molybdenum Blue (MB) colorimetric method to allow for faster screening with more stable reagents. More importantly, a portable three-channel colorimeter is developed for screening iAs relative to the WHO drinking water guideline value (10 µg/L). Adding the reducing reagents in sequence not only prolongs the storage time to > 7 days, but also accelerates the color development time to 6 min in conjunction with lowering the H2SO4 concentration in chromogenic reagents. The optimal pH ranges from 1.2 to 1.3 and is achieved by acidifying groundwater to 1% (V/V) HCl. With detection limits of 3.7 µg/L for inorganic arsenate (iAs(V)) and 3.8 µg/L for inorganic arsenite (iAs(III)), testing groundwater with ∼10 µg/L of As has a precision < 20%. The method works well for a range of phosphate concentrations of 48-950 µg/L (0.5-10 µmol/L). Concentrations of total_iAs (6-300 µg/L), iAs(V) (6-230 µg/L) and iAs(III) (0-170 µg/L) for 14 groundwater samples from Yinchuan Plain, Pearl River Delta, and Jianghan Plain, are in excellent agreements (linear regression slope: 0.969-1.029) with the benchmark methods. The improved chemistry here lays the foundation for the MB colorimetric method to become a commercially viable screening tool, with further engineering and design improvement of the colorimeter.

Selenium-carrageenan modulates arsenic bioaccessibility in simulated gastrointestinal bio-fluids: Dual mechanisms of gastric promotion and intestinal inhibition

Inorganic arsenic (As) exposure via oral ingestion poses significant carcinogenic risks, with bioaccessibility in the gastrointestinal tract critical for risk assessment. Selenium (Se), an essential micronutrient, exhibits paradoxical effects on As toxicity, yet its mechanistic role in modulating As bioavailability during digestion remains poorly understood. This study investigates the dual-phase impact of selenated carrageenan (Se-car), a cost-effective organic Se supplement, on As bioaccessibility using an in vitro simulated digestion model. Results show that Se-car (50 g/L) enhances total As, As(III), and As(V) bioaccessibility by 22.28 %, 20.00 %, and 22.53 % during gastric digestion (pH 1.5, 1 h), driven by competitive adsorption of Se-car's anionic groups on hematite surfaces, proton dissociation, and pepsin-mediated reductive dissolution. Conversely, in intestinal digestion (pH 6.5, 8 h), Se-car suppresses total As, As(III), and As(V) bioaccessibility release by 8.51 %, 9.08 %, and 4.71 % through molecular entanglement, enzyme encapsulation, and reduced Fe(II) solubility. Elevated NaCl concentrations (0.01-1 M) synergistically inhibit As release by 7.28 % (gastric) and 2.47 % (intestinal), attributable to ionic shielding-induced Se-car chain contraction. Mechanistic insights indicate gastricization relies on acidic dissolution and Se-car-pepsin interactions, while intestinal inhibition stems from Se-car-trypsin binding and surface passivation. Health risk assessments demonstrate Se-car exacerbates gastric-phase THQ values of As (children: 5.20 → 16.97) but mitigates intestinal-phase risks (children: 9.14 → 3.99). This work elucidates pH- and ionic strength-dependent Se-car behaviors, offering novel insights for optimizing dietary Se interventions in As-endemic regions. The dual-phase regulatory mechanism highlights the importance of digestive-phase-specific risk management and provides a foundation for developing polysaccharide-based As antagonists targeting complex gastrointestinal environments.

Mechanisms and health implications of calcium in reducing arsenic bioaccessibility via in vitro gastrointestinal digestion

Accidental oral ingestion is an important route of exposure to arsenic (As) containing soil and dust. However, the mechanism by which As bioaccessibility is reduced by in vitro digestion of calcium (Ca) remains unknown. In this study, we investigated the effect of Ca intake on the behaviors of As release from simulated gastrointestinal bio-fluids. The results showed that the bioaccessibility of As(III) and As(V) in simulated gastric bio-fluid was reduced by 43.68 % and 37.22 % (p < 0.05), respectively, when the dosage of Ca addition was 10 g/L after 1 h. The bioaccessibility of As(III) and As(V) in simulated intestinal bio-fluids was reduced by 76.13 % and 69.16 % (p < 0.05), respectively, after 8 h. Ca effectively reduced the As bioaccessibility in simulated gastrointestinal bio-fluid, particularly preprandial Ca intake reduced the risk better than postprandial. Ca can achieve As immobilization through electrostatic attraction, surface complexation and other pathways. Ca intake reduces As bioaccessibility through the formation of Fe-As-Ca ternary complexes. These results suggests that Ca-rich supplements could be an effective nutritional strategy to reduce As toxicity. This study deepens our understanding about how Ca interacts with As-containing minerals in simulated gastrointestinal bio-fluids and provides a nutritional strategy to mitigate the health risk of accidental As ingestion.

Utilization of agricultural waste: mango peels and pineapple crown leaves as precursors for nanomaterial production for arsenate remediation

The improper management of agricultural waste can produce particulate matter and greenhouse gases. This study explores a green synthesis approach in which mango peel extract rich in polyphenols is used to produce green nano zero-valent iron (G-NZVI). Furthermore, supporting G-NZVI with nanocellulose isolated from pineapple crown leaves (NCC) enhances the stability and dispersion of NZVI. X-ray diffractometry, zeta potential measurements, scanning electron microscopy with energy-dispersive X-ray spectroscopy, transmission electron microscopy, specific surface area analysis, and X-ray absorption near-edge spectroscopy were used to characterize the properties of the G-NZVI/NCC composites. The adsorption isotherm of arsenate (As5+) on G-NZVI/NCC demonstrated a maximum adsorption capacity of 5.488 mg·g-1, which was higher than that of G-NZVI alone. The isotherm model indicated monolayer As5+ adsorption on the homogeneous G-NZVI/NCC surface without lateral interactions between adsorbed molecules. The results revealed that low pH conditions, a high dosage of G-NZVI/NCC, and a low initial concentration of As5+ increased the removal efficiency. The iron oxide layer of G-NZVI/NCC forms a strong surface complex, facilitating the formation of an inner sphere with As5+ across a normal pH range and providing a cost-effective and eco-friendly solution for arsenic-contaminated water.

Interfacial reactions and speciation identification during arsenic treated with nanoscale zerovalent iron (nZVI) in water: A review

This perspective briefly summarized the progress of inorganic arsenic (As) treated with nanoscale zerovalent iron (nZVI) in water over the past two decades. The intrinsic interfacial reaction between As and nZVI encompassed multiple effects, such as complexation, oxidation, reduction, and co-precipitation, ascribed to core-shell structure of nZVI and environmental behavior of As in water. Surface complexation occurred via ligand exchange of arsenate anions with Fe-OH groups on the iron oxide shell. However, interfacial oxidation of As(III) to As(V) was attributed to form a Fe(III) oxide-Fe(II)-As(III) ternary surface complex under anoxic conditions, as well as generate reactive oxygen species (e.g., H2O2, •OH) from iron reacted with O2 under oxic conditions. Reduction of As(III) to As(0) was followed by subsurface accumulation near the Fe(0) core. Advanced characterization techniques, including high-resolution X-ray photoelectron spectroscopy, in situ X-ray absorption spectroscopy, spherical aberration-corrected scanning transmission electron microscope, and density functional theory combined with quick-scanning extended X-ray absorption fine structure, have unraveled the multi-tiered distributions of As on nZVI at atomic scale. This review highlights critical gaps in understanding As-Fe redox dynamics and advocates for future research to engineer nZVI with tailored surface properties for enhanced As sequestration.

Electrochemistry and Molecular Compositions Reflect Electron Shuttling of Dissolved Organic Matter in High Arsenic Groundwater

Little is known about the electron shuttle ability of dissolved organic matter (DOM) and its effects on arsenic (As) mobilization, which makes the underlying mechanism of groundwater As enrichment elusive. In this study, both the electrochemical properties and molecular compositions of DOM in high As groundwater were quantified in the Hetao Basin, China. We found that, along the flow path, the average electron-transferring capacity (ETC) of DOM, including the capacities of electron-accepting and electron-donating, continuously increased from 2.85 to 3.59 mmole-/gC along with As concentrations. The increasing ETC reflected an increase in electron shuttle ability of DOM. Furthermore, the increasing electron shuttle ability was mainly attributed to the recalcitrant compounds in DOM, especially CHOS and CHONS formulas in highly unsaturated structures with high oxygen (HUSHO) and CHO and CHON formulas in aromatic structures (AS). The significantly positive correlation between As concentration and ETC indicated that recalcitrant DOM promoted groundwater As enrichment through electron shuttling for inducing the reductive dissolution of As-containing Fe(III) oxide minerals, which was further supported by our culture experiments showing that goethite was more reduced [133 μM Fe(II)] in the presence of DOM with a higher ETC (3.35 mmole-/gC) as electron shuttling than that [65.2 μM Fe(II)] with a relatively lower ETC (2.41 mmole-/gC). Our study highlights that recalcitrant DOM compounds with unsaturated and AS have high electron shuttle ability, promoting As enrichment in groundwater.

Hydrogen gas oxidation-driven reductive mobilization of arsenic in solid phase contributing to arsenite contamination in groundwater: Insights from metagenomic and microcosm analyses

Hydrogen gas (H2) is naturally produced by biological and non-biological reactions in various environmental niches. However, the influence of H2 on microbial processes that cause the mobilization and release of arsenic from solid phase into groundwater remains to be resolved. Given that dissimilatory arsenate [As(V)]-respiring prokaryotes (DARPs) have been demonstrated to significantly contribute to the formation of As-contaminated groundwater, our study specifically examined the interactions between H2 and DARPs. We prepared an enriched DARP population from As-contaminated soils. Metagenomic analyses of the DARP population revealed that approximately 46.7 % of the qualified DARPs' MAGs contain at least one type I Ni-Fe hydrogenase. The Ni-Fe hydrogenase proteins in DARPs show unique diversity. Functional assays indicate that the DARP population exhibited notable activity in oxidizing H2 while concurrently reducing As(V) under strictly anaerobic conditions. Arsenic release assays indicate that the DARP population is highly proficient at catalyzing the reductive mobilization of arsenic in scorodite, using hydrogen as the electron donor. These findings offer the initial evidence that H2 can directly promote the formation of arsenic-contaminated groundwater mediated by DARPs, a biogeochemical process that has long been overlooked. Therefore, this study increases our insight into the microbial mechanisms involved in the formation of arsenic-contaminated groundwater.

Colloids fractionation and characterization of arsenic (As) and dissolved organic matter (DOM) in surface water around a closed arsenic mine

The speciation and mobility of arsenic (As) in waters are largely influenced by the colloids; however, the impacts of colloids with different molecular weights (MWs) in water fractions remain largely unknown. Herein, the surface water was fractionated into three colloidal fractions and truly dissolved fraction via cross-flow ultrafiltration. Total As (As(T)) presented mainly as As(V) and existed primarily in the truly dissolved fraction. Arsenic(III) proportions of 3.1 %-8.0 % in various colloids sizes indicated reducing conditions within the colloids particles in surface water around the mine. Negative correlations between EC, TDS and As(T) and As(V) in colloids were found, indicating water properties favored As mobilization in surface water. The aromaticity and humification of DOM reflected endogenous sources in truly dissolved fraction and large-MW colloids and exogenous sources in medium-small-MW colloids. Fourier transform infrared spectroscopy detected As-DOM and As-Fe-DOM formation in colloids, highlighting new aspects of colloids in surface water. Arsenic(T) and As(V) concentrations were positively correlated with terrestrial humic-like components and negatively correlated with microbial humic-like component in colloids, suggesting that in the closed As mine, As tend to be transported from land to surface water together with terrestrial DOM, while endogenous DOM in water affected As mobilization.

Arsenic toxicity exacerbates China's groundwater and health crisis

Arsenic (As) contamination is considered a major threat to groundwater quality and human health. The uneven distribution of arsenic contributes to regional variations, creating discrimination related to arsenic enrichment and carcinogenic risk. Here, we have analyzed 2,737 groundwater samples across China, which spans a broad range of geo-environments, climates and land use types. We find that regional inequality of groundwater arsenic concentration is caused by ontology environment. By mapping the groundwater arsenic distribution across China and conducting a global meta-analysis, the spatial response of arsenic concentration to different cancer risks was revealed, and neglected As(V) should be given attention. A random forest analysis identified chemical properties (including oxidation-reduction potential, pH, total manganese ion, total iron ion, total dissolved solids, and sulfate ion) as the most influential drivers, contributing 56% to the model's explanatory power, followed by geographical factors at 28%, climatic factors at 10%, and human activities at 6%. Additionally, reducing the proportion of groundwater supply with high arsenic concentration in drinking water in regions without water treatment may help lower the potential carcinogenic risk. This study emphasizes the potential health risk associated with high arsenic groundwater, making it particularly important to roll out efficient water purification technologies given the natural enrichment of arsenic, especially rural regions.

Geothermal Arsenic Threats to Intensive Groundwater Utilization in an Arid Basin

Groundwater quality is critical for safe drinking water and irrigation supplies but can be threatened by geogenic toxins that are difficult to predict. In the arid, high desert San Luis Valley (SLV), Colorado, a groundwater basin serves as the primary water supply with observed arsenic concentrations exceeding the maximum contaminant level (MCL) of 10 μg/L set by the U.S. EPA. However, the sources and processes responsible for As occurrence are unclear. Through a community-engaged sampling effort, we collected 244 groundwater samples and measured major/trace element concentrations. Long-term land subsidence and depth-resolved sediment texture were computed at the same locations. We tested three plausible geochemical processes responsible for As release: (1) overpumping-induced dewatering of As-bearing clays (proxied by land subsidence), (2) pH-promoted desorption as well as reductive dissolution of As(V)/Fe(III) (hydr)oxides, and (3) incursion of higher-As geothermal fluids (proxied by lithium, boron, tungsten, and molybdenum) into groundwater. We find that statistics, statistical/machine learning, and aqueous thermodynamics all agree that geothermal fluid mixing within the aquifer is the main source of dissolved As. Our findings suggest that overpumping draws higher-As thermal fluid from the bottom of the aquifer to pumping depth, leading to increased concentrations of As in drinking/irrigation water supplies at wells.

A review on arsenic contamination in drinking water: sources, health impacts, and remediation approaches

Arsenic (As) contamination in groundwater has become a global concern, and it poses a serious threat to the health of millions of people. Groundwater with high As concentrations has been reported worldwide. It is widely recognized that the toxicity of As largely depends on its chemical forms, making As speciation a critical issue. Numerous studies on As speciation have been conducted, extending beyond the general knowledge on As to the toxicity and health issues caused by exposure to various As species in water. This article reviews various As species, their sources and health effects, and treatment methods for the removal of As from contaminated water. Additionally, various established and emerging technologies for the removal of As contaminants from the environment, including adsorption (using rocks, soils, minerals, industrial by-products, biosorbents, biochars, and microalgal and fungal biomass), ion exchange, phytoremediation, chemical precipitation, electrocoagulation, and membrane technologies, are discussed. Treating As-contaminated drinking water is considered the most effective approach to minimize the associated health risks. Finally, the advantages and disadvantages of various remediation and removal methods are outlined, along with their key advantages. Among these techniques, the simplicity, low cost, and ease of operation make adsorption techniques desirable, particularly with the use of novel functional materials like graphite oxides, metal-organic frameworks, carbon nanotubes, and other emerging functional materials, which are promising future alternatives for As removal.

The potential for glacial flour to impact soil fertility, crop yield and nutrition in mountain regions

Novel sustainable agricultural strategies that enhance soil nutrients and human nutrition are crucial for meeting global food production needs. Here, we evaluate the potential of "glacial flour," a naturally crushed rock produced by glaciers known to be rich in nutrients (P, K, and micronutrients) needed for plant growth. Our proof-of-concept study, investigated soybean (Glycine max. var. Black jet) growth, yield, and nutrient content with soil supplementation from glacial flour sourced from Himalayan glaciers (meta-sediment gneiss bedrock) and Icelandic glaciers (basaltic bedrock). Glacial flour treatment enhanced crop yields by 85% (Himalayan) and 135% (Icelandic), compared to controls. Additionally, glacial flour fortified crops with beneficial micronutrients zinc and selenium. However, the application of Himalayan flour led to arsenic bioaccumulation in the crop, underscoring the importance of catchment geology. This study supports using glacial flour as a soil remediation strategy for sustainable agriculture but emphasizes the need to consider potential toxicity risks.

A comprehensive review on arsenic contamination in groundwater: Sources, detection, mitigation strategies and cost analysis

While groundwater is commonly perceived as safe, the excessive presence of trace metals, particularly arsenic (As), can pose significant health hazards. This review examines the current scenario of pollutants and their mitigations focusing on As contamination in groundwater across multiple nations, with a specific emphasis on the Indian Peninsula. Arsenic pollution surpasses the WHO limit of 10 ppb in 107 countries, impacting around 230 million people worldwide, with a substantial portion in Asia, including 20 states and four union territories in India. Analysis of the correlation between the aquifer and arsenic poisoning highlights severe contamination in groundwater originating from loose sedimentary aquifer strata, particularly in recently formed mountain ranges with geological sources presumed to contribute over 90% of arsenic pollution, i.e. a big environmental challenge. A myriad of techniques, including chromatographic, electrochemical, biological, spectroscopic, and colorimetric methods among others, are available for the detection and removal of arsenic from groundwater. Removal strategies encompass a wide array of approaches such as bioremediation, adsorption, coagulation/flocculation, ion exchange, biological processes, membrane treatment, and oxidation techniques specifically tailored for affected areas. Constructed wetlands help to eliminate heavy metal impurities such as As, Zn, Cd, Cu, Ni, Fe, and Cr. Their efficiency is influenced by design and environmental factors. Nanotechnology and nanoparticles have recently been studied to remove arsenic and toxic metal ions from water. Cost-effective solutions including community-based mitigation initiatives, alongside policy and regulatory frameworks addressing arsenic contamination, are essential considerations.

Characteristics of boron isotopes and their indicative significance in groundwater arsenic mobilization from an alluvial basin

Groundwater with high arsenic (As) concentration is widely distributed all over the world and seriously threatens human health. Due to the similar chemical properties, boron (B) would be used to understand the formation mechanism of high As groundwater. Thirty groundwater samples were collected from alluvial fan, transition area, and flat plain generally along the flow path in the northwestern Hetao Basin, China. Groundwater As concentration generally showed an increasing trend along the path. The δ11B values ranged from 3.36 ‰ to 26.19 ‰, and exhibited a decreasing trend (from alluvial fan to transition area; Stage I) followed by an increasing trend (from transition area to flat plain; Stage II). Boron release from incongruent dissolution of silicate minerals and B adsorption onto secondary clay minerals resulted in high δ11B values in groundwater with low As concentrations from the alluvial fan, where As was adsorbed and immobilized on Fe(III) oxides or clay minerals. During Stage I, B concentrations increased slightly and the δ11B values decreased, as the result of B desorption. A negative correlation between As concentrations and δ11B values illustrated that desorption was an important process of As enrichment. During Stage II, degradation of organic matter and reductive dissolution of Fe(III) oxides increased concentrations of B and As. However, the decreasing trend of B/Cl and the increasing trend of δ11B showed that co-precipitation of B and carbonates removed B from groundwater. The positive correlation between As and SIcalcite+dolomite supported that secondary Ca precipitation decreased As adsorption by directly isolating As from Fe(III) oxides, which promoted As enrichment. This study provides insights into hydrogeochemical processes associated with As and B enrichment in groundwater.

Redox Dynamic Interactions of Arsenic(III) with Green Rust Sulfate in the Presence of Citrate

Arsenic is a global pollutant. Recent studies found that Fe(II) can oxidize As(III), but the extent of oxidation with mixed-valent iron minerals and the mechanisms involved are unknown. In this study, we investigated whether As(III) can be oxidized under reducing conditions using green rust sulfate (GR-SO4), an Fe mineral containing both Fe(II) and Fe(III). Batch sorption experiments showed that GR-SO4 (1 g L-1) effectively sorbs environmentally relevant concentrations of As(III) (50-500 μg L-1) under anoxic, neutral pH conditions with and without citrate (50 μM). X-ray absorption near-edge structure spectroscopy analysis at the As K-edge demonstrated that approximately 76% of As(III) was oxidized to As(V) by GR-SO4. Complete oxidation of As(III) was observed in the presence of citrate. As(III) oxidation can be linked to the phase transformation of GR-SO4 to goethite, resulting in new reactive Fe(III) species that plausibly drive oxidation. Citrate enhanced this process by stabilizing Fe on the mixed GR-SO4/goethite surface, preventing its reduction back to Fe(II) and facilitating further As(III) oxidation without significant Fe loss to the solution. This study highlights the cryptic As(III) oxidation that occurs under reducing conditions, providing new insights into the cycling of arsenic in mixed phases of iron-rich, anoxic environments.

Identification of key factors and mechanism determining arsenic mobilization in paddy soil-porewater-rice system

Arsenic (As) mobilization in paddy fields poses significant health risks, necessitating a thorough understanding of the controlling factors and mechanisms to safeguard human health. We conducted a comprehensive investigation of the soil-porewater-rice system throughout the rice life cycle, focusing on monitoring arsenic distribution and porewater characteristics in typical paddy field plots. Soil pH ranged from 4.79 to 7.98, while porewater pH was weakly alkaline, varying from 7.2 to 7.47. Total arsenic content in paddy soils ranged from 6.8 to 17.2 mg/kg, with arsenic concentrations in porewater during rice growth ranging from 2.97 to 14.85 μg/L. Specifically, arsenite concentrations in porewater ranged from 0.48 to 7.91 μg/L, and arsenate concentrations ranged from 0.73 to 5.83 μg/L. Through principal component analysis (PCA) and analysis of redox factors, we identified that arsenic concentration in porewater is predominantly influenced by the interplay of reduction and desorption processes, contributing 43.5 % collectively. Specifically, the reductive dissolution of iron oxides associated with organic carbon accounted for 23.3 % of arsenic concentration dynamics in porewater. Additionally, arsenic release from the soil followed a sequence starting with nitrate reduction, followed by ferric ion reduction, and subsequently sulfate reduction. Our findings provide valuable insights into the mechanisms governing arsenic mobilization within the paddy soil-porewater-rice system. These insights could inform strategies for irrigation management aimed at mitigating arsenic toxicity and associated health risks.

Hydrogeochemical controls on contrasting co-occurrence of geogenic Arsenic (As) and Fluoride (F-) in complex aquifer system of Upper Indus Basin, (UIB) western Himalaya

Arsenicosis and fluorosis have become severe health hazards associated with the drinking of Arsenic (As) and Fluoride (F-) contaminated groundwater across south-east Asia. Although, significant As and F- concentration is reported from major Himalayan river basins but, the hydrogeochemical processes and mechanisms controlling their contrasting co-occurrence in groundwater is still poorly explored and understood. In the present study, groundwater samples were collected from phreatic and confined aquifers of Upper Indus Basin (UIB), India to understand the hydrogeochemical processes controlling the distribution and co-occurrence of geogenic As and F- in this complex aquifer system. Generally, the groundwater is circum-neutral to alkaline with Na+-HCO3-, Ca2+-Na+-HCO3- and Ca2+-Mg2+-HCO3- water facies signifying the dominance of silicate and carbonate dissolution. The poor correlation of As and F- in groundwater depicted that these geogenic elements have discrete sources of origin with distinct mechanisms controlling their distribution. As enrichment in groundwater is associated with high pH, Fe, Mn and NH4-N suggesting dominance of metal oxide/hydroxide reduction with organic matter degradation. However, F- enrichment in groundwater is associated with high pH, HCO3- and Na+, which is assisted by the incessant dissolution of fluorinated minerals. The study also revealed that high HCO3- facilitates the exchange of hydroxides (OH-) with As and F- on sediment surfaces that contribute to As and F- enrichment in groundwater through desorption. 70% groundwater samples have As and F- concentration above the permissible limit given by WHO. Therefore, continuous exposure to these contaminants may pose severe health hazard of arsenicosis and fluorosis to people living in the region and downstream. The study provides insights into geological sources, hydrogeochemical processes and mechanisms controlling distribution of As and F- in groundwater that will help in developing the appropriate measures to mitigate the impact these contaminants on human health.

Loss of microbial diversity increases methane emissions and arsenic release in paddy soils

Microorganisms are vital to the emission of greenhouse gases and transforming pollutants in paddy soils. However, the impact of microbial diversity loss on anaerobic methane (CH4) oxidation and arsenic (As) reduction under flooded conditions remains unclear. In this study, we inoculated microbial suspensions into natural As-contaminated paddy soils using a dilution approach (untreated, 10-2, 10-4, 10-6, 10-8 dilutions) to manipulate microbial diversity levels. The results revealed that the 10-4 and 10-6 dilutions resulted in the highest CH4 emissions (97.0 μmol and 102.3 μmol) compared to untreated groups (27.6 μmol). However, anaerobic CH4 oxidation was not observed in 10-4 dilution groups and higher dilutions, suggesting the loss of diversity inhibited the natural reduction of CH4. Moreover, the porewater As concentration in the dilution groups was 1.8-8.2 times greater than in the untreated groups. The loss of microbial diversity promoted the reductive dissolution of iron (Fe) minerals bearing As, leading to increased concentrations of Fe(II) and dissolved organic carbon (DOC), which further enhanced As release (Fe(II), R = 0.9, p < 0.001) (DOC, R = 0.8, p < 0.001) from soil to porewater. However, CH4-dependent As(V) reduction was almost entirely inhibited under diversity loss. The decline in microbial diversity increased the relative abundances of methanogens (e.g., Methanobacterium and Methanomassiliicoccus), Fe(III)/As(V)-reducing bacteria (e.g., Bacillus, Clostridium_sensu_stricto_10, and Geobacter), and the related functional genes (i.e., mcrA and Geo). These findings suggest that microbial diversity is critical for specialized soil processes, highlighting the detrimental effects of biodiversity loss on CH4 emissions and As release in As-contaminated paddies.

Dynamics of Spatiotemporal Variation of Groundwater Arsenic in Central Rift Vally of Ethiopia: A Serial Cross-Sectional Study

Background

Arsenic is a well-known, highly poisonous metalloid that affects human health and ecosystems and is widely distributed in the environment. Nevertheless, data on the spatiotemporal distribution of arsenic in groundwater sources in Ethiopia are scarce.

Objective

The principal aim of this study was to assess the extent of arsenic in groundwater sources and analyze the spatiotemporal variations in the central rift valley of Ethiopia.

Methods

The study employed a serial cross-sectional study design and census sampling methods. The concentrations of arsenic in the groundwater samples were determined using inductively coupled plasma mass spectrometry (ICP-MS) at the Ethiopian Food and Drug Authority laboratory. Descriptive statistical analyses were performed using IBM SPSS version 29 software. Additionally, ArcGIS software was utilized to map the spatiotemporal distribution of arsenic. Furthermore, Minitab statistical software version 21.4 was employed to assess the correlation between spatiotemporal variations of arsenic concentrations in groundwater sources.

Results

The mean values of arsenic in the groundwater samples were 11.2 µg/L during the dry season and 10.7 µg/L during the rainy season. The study results showed that 18 wells (42.2%) and 22 wells (48.8%) had higher arsenic concentrations (>10 µg/L) during the dry and rainy seasons, respectively. Thus, arsenic levels in 42.2% and 48.8% of the samples exceeded the maximum threshold limit set by WHO, USEPA, and Ethiopian standards (10 µg/L), respectively, during the dry and rainy seasons. Furthermore, our analysis revealed a significant positive correlation between arsenic in groundwater and well depth (r = .75, P < .001), indicating a strong association between higher arsenic concentrations and deeper wells. Similarly, we observed a substantial positive correlation between arsenic concentration in groundwater and season (r = .9, P < .001), suggesting notable variations in arsenic levels between dry and rainy seasons.

Conclusions

The majority of the groundwater sources in the studied area are unfit for human consumption because they contain high amounts of arsenic, which poses a significant risk to human health. Moreover, the arsenic concentration varied spatially and temporally. Therefore, special attention is needed to reduce arsenic exposure and associated health risks.

Characteristics and Mechanism of Hematite Dissolution and Release on Arsenic Migration in Heterogeneous Materials

The migration of arsenic in groundwater is influenced by the heterogeneity of the medium, and the presence of iron minerals adds complexity and uncertainty to this effect. In this study, a stratified heterogeneous sand column with an embedded hematite lens at the coarse-to-medium sand interface was designed. We introduced an arsenic-laden solution and controlled groundwater flow to investigate the spatiotemporal characteristics of arsenic migration and the impact of hematite dissolution. The results showed that the medium structure significantly influenced the arsenic migration and distribution within the lens-containing sand column. The clay layers directed the lateral migration of arsenic, and the arsenic concentrations in deeper layers were up to seven times greater than those on the surface. The extraction experiments of solid-phase arsenic revealed that the main adsorption modes on quartz sand surfaces were the specific adsorption (F2) and adsorption on weakly crystalline iron-aluminum oxides (F3), correlating to the specific and colloidal adsorption modes, respectively. Monitoring the total iron ions (Fe(aq)) revealed rapid increases within the first 14 days, reaching a maximum on day 15, and then gradually declining; these results indicate that hematite did not continuously dissolve. This study can aid in the prevention and control of arsenic contamination in groundwater.

Remediation of arsenic- and nitrate-contaminated groundwater through iron-dependent autotrophic denitrifying culture

Groundwater contamination with arsenic and nitrate poses a pressing concern for the safety of local communities. Bioremediation, utilizing Fe(II)-oxidizing nitrate reducing bacteria, shows promise as a solution to this problem. However, the relatively weak environmental adaptability of a single bacterium hampers practical application. Therefore, this study explored the feasibility and characteristics of a mixed iron-dependent autotrophic denitrifying (IDAD) culture for effectively removing arsenic and nitrate from synthetic groundwater. The IDAD biosystem exhibited stable performace and arsenic resistance, even at a high As(III) concentration of 800 μg/L. Although the nitrogen removal efficiency of the IDAD biosystem decreased from 71.4% to 64.7% in this case, the arsenic concentration in the effluent remained below the standard (10 μg/L) set by WHO. The crystallinity of the lepidocrocite produced by the IDAD culture decreased with increasing arsenic concentration, but the relative abundance of the key iron-oxidizing bacteria norank_f_Gallionellaceae in the culture showed an opposite trend. Metagenomic analysis revealed that the IDAD culture possess arsenic detoxification pathways, including redox, methylation, and efflux of arsenic, which enable it to mitigate the adverse impact of arsenic stress. This study provides theoretical understanding and technical support for the remediation of arsenic and nitrate-contaminated groundwater using the IDAD culture.

The hippo-YAP1/HIF-1α pathway mediates arsenic-induced renal fibrosis

Epidemiological evidence reveals that arsenic increases the risk of chronic kidney disease (CKD) in humans, but its mechanism of action has so far been unclear. Fibrosis is the manifestation of end-stage renal disease. Hypoxia is recognized as a vital event accompanying the progression of renal fibrosis. KM mice were exposed to 0, 20, 40, and 80 mg/L NaAsO2 for 12 weeks. HK-2 cells were treated with 1 μM NaAsO2 for 4 weeks. The results showed that arsenic increased the expression of hypoxia-inducible factor 1α (HIF-1α) (P < 0.05), which is involved in inorganic arsenic-induced renal fibrosis. The Hippo signaling pathway is the upstream signal of HIF-1α and the kinase cascade of Large tumor suppressor kinase 1 (LATS1) and Yes-associated protein 1 (YAP1) is the heart of the Hippo pathway. Our results showed that protein expressions of LATS1 and phosphorylated YAP1 were decreased, and dephosphorylated YAP1 expression increased in arsenic-treated mouse kidneys and human HK-2 cells (P < 0.05). Our research manifested that arsenic treatment suppressed the Hippo signaling and induced high expression of YAP1 into the nucleus. We also found that YAP1 was involved in arsenic-induced renal fibrosis by forming a complex with HIF-1α and maintaining HIF-1α stability. Our findings indicate that YAP1 is a potential target for molecular-based therapy for arsenic-mediated renal fibrosis.

Spectroscopic and modeling approaches of arsenic (III/V) adsorption onto Illite

Illite plays an essential role in arsenic (As) transportation in the subsurface. Despite extensive investigations into As adsorption onto illite, debates persist due to the absence of direct evidence revealing the underlying processes. In this research, we conducted batch experiments and employed spherical aberration-corrected scanning transmission electron microscope, X-ray absorption spectroscopy, and density functional theory-based calculations to elucidate the mechanisms for the adsorption of two major inorganic As species (As(III) and As(V)) onto illite. Experimental results indicate adsorption capacities of 0.251 and 0.667 μmol/g for As(III) and As(V) onto illite, respectively. As(III) adsorption occurs within 300 min, whereas As(V) is rapidly adsorbed within 500 min, after which it tends to stabilize. Both As species can adsorbed onto the basal surface via electrostatic forces, where cations act as a bridge, leading to specific-cation effects. Conversely, As adsorption onto the edge surface can be ascribed to inner-sphere complexes via As-O-Al bonds, causing a negatively shifted isoelectric point of illite. These mechanisms collectively account for the partially reversible adsorption and two-stage kinetics pattern. Finally, a process-based surface complexation model was developed to predict As adsorption onto illite, which includes the inner/outer-sphere complexation and monodentate/bidentate complexes.

Floodplain morphology influences arsenic and antimony spatial distribution in a seasonal acid sulfate soil wetland

Arsenic (As) and antimony (Sb) often co-occur in floodplain depositional environments that are contaminated by legacy mining activities. However, the distribution of As and Sb throughout floodplains is not uniform, adding complexity and expense to management or remediation processes. Identifying floodplain morphology predictor variables that help quantify and explain As and Sb spatial distribution on floodplains is useful for management and remediation. We developed As and Sb risk maps estimating concentration and availability at a coastal floodplain wetland impacted by upper-catchment mining. Significant predictors of As and Sb concentrations included i) distance from distributary channel-wetland intersection and ii) elevation. Distance from channel explained 53 % (P < 0.01) and 28 % (P < 0.01), while elevation explained 42 % (P < 0.01) and 47 % (P < 0.01) of the variability in near-total Sb and As respectively. As had a higher extractability than Sb across all tested soil extractions, suggesting that As is more environmentally available. As and Sb dry mass estimates to a depth of 0.1 m scaled to the lower coastal Macleay floodplain ranged from 113-192 tonnes and 14-24 tonnes respectively. Landscape-scale modelling of metalloid distribution, informed by morphology variables, presented here may be a useful framework for the development of risk maps in other regions impacted by contaminated upper-catchment sediments.

Metagenomic and FT-ICR MS insights into the mechanism for the arsenic biogeochemical cycling in groundwater

Arsenic (As) is a groundwater contaminant of global concern. The degradation of dissolved organic matter (DOM) can provide a reducing environment for As release. However, the interaction of DOM with local microbial communities and how different sources and types of DOM influence the biotransformation of As in aquifers is uncertain. This study used optical spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), metagenomics, and structural equation modeling (SEM) to demonstrate the how the biotransformation of As in aquifers is promoted. The results indicated that the DOM in high-As groundwater is dominated by highly unsaturated low-oxygen(O) compounds that are quite humic and stable. Metagenomics analysis indicated Acinetobacter, Pseudoxanthomonas, and Pseudomonas predominate in high-As environments; these genera all contain As detoxification genes and are members of the same phylum (Proteobacteria). SEM analyses indicated the presence of Proteobacteria is positively related to highly unsaturated low-O compounds in the groundwater and conditions that promote arsenite release. The results illustrate how the biogeochemical transformation of As in groundwater systems is affected by DOM from different sources and with different characteristics.

Influence of pH on the leaching behavior of a solidified arsenic contaminated soil

Stabilization/solidification is widely used for treatment of arsenic (As)-contaminated soils. The stability of the soil may deteriorate significantly when exposed to acid or alkaline leachate. In this study, semi-dynamic leaching tests under different pH were carried out to investigate the leaching behavior of As from the solidified soils. Spectroscopic and microscopic analyses were performed to reveal the related mechanisms. The results showed that the leaching of As was closely correlated with the pH of the leachate, because the encapsulation effect of the cementitious matrix and the chemical speciation and valence of As were all significantly influenced by pH. In the strongly acidic leachant (pH 3.0), the leached As concentration increased by an order of magnitude, and the effective diffusion coefficient of As reached 3.71 × 10-13 m2/s. This is because that pores and cracks increased owing to the acidic corrosion of CSH, such that the physical encapsulation effect was reduced and the mobility of As increased. The leachability index showed that the solidified soil was unsuitable for 'controlled utilization' under strongly acidic conditions. The leached As concentration was the lowest in the weakly alkaline leachant (pH 9.0) because under weakly alkaline conditions the hydration process of the cement was facilitated, and more CSH gels were attached to the surface of the soil particles, forming a tighter structure for As encapsulation. However, as pH increased from 9.0-11.0 the leached As concentration increased due to an increased content of As(III)-O in the solidified soil.

Marine microplastics enhance release of arsenic in coastal aquifer during seawater intrusion process

Microplastics (MPs), omnipresent contaminants in the ocean, could be carried by seawater intrusion into coastal aquifers, which might affect the fate of heavy metals existing in aquifers. Herein, we investigated the release behavior of arsenic (As) in coastal aquifers during MPs-containing seawater intrusion by applying laboratory experiment and numerical simulation. We found that seawater with marine MPs enhanced the release of As in aquifers, especially for dissolved As(V) and colloidal As. Negatively charged MPs competed with As(V) for the adsorption sites on iron (hydr)oxides in aquifers, resulting in the desorption of As(V). In addition, MPs could promote the release of Fe-rich colloids by imparting negative charge to its surface and providing it with sufficient repulsive force to detach from the matrix, thereby leading to the release of As associated with Fe-rich colloid. We also developed a modeling approach that well described the transport of As in coastal aquifer under the impact of MPs, which coupled variable density flow and kinetically controlled colloids transport with multicomponent reactive transport model. Our findings elucidated the enhancement of MPs on the release of As in aquifers during seawater intrusion, which provides new insights into the risk assessment of MPs in coastal zones.

The redistribution process of As(Ⅲ) and Fe(Ⅱ) caused by As/Fe ratio, organic matter, and co-existing ions: Co-precipitation and co-oxidation

The contamination of arsenic (As) in aqueous environments has drawn widespread attention, and iron compounds may largely alter the migration ability of As. However, the stability of As(III) in Fe-As system with the intervention of organic matter (OM) remains unclear. Herein, we had explored the co-precipitation and co-oxidation processes of As-Fe system by using batch experiments combined with Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS) in this research. The precipitation quantity of As(III) increased (28.85-92.41 %) when the As/Fe ratio decreased, and increased (24.20-64.20 %) with pH increased. The main active substance for oxidizing As(III) was H2O2, which was produced in the As-Fe system. FTIR and XPS revealed that As(III) was first oxidized in neutral, and then absorbed and enteredthe interior of Fe(OH)3 colloids. But under alkaline conditions, As(III) was adsorbed by Fe (Oxyhydr) oxides firstly, and then oxidized. The intervention of OM would inhibit the redistribution process of As(III) in aqueous environments. Functional groups and unsaturation of the carbon chain were the dominant factors that affected the precipitation and oxidation processes of As(III), respectively. Co-existing ions (especially PO43-) also signally affected the precipitation quantity of As(Ⅲ) in the system and, when coexisting with OM, could exacerbate this process. The influence of co-existing ions on the redistributive process of As(III) in the As-Fe system with/without OM were as follows: PO43- > SO42- > mixed ions > SiO32-. Moreover, high concentration of OM and PO43- might lead to morphological alterations of As, acting as a threat to aqueous environments. In summary, the present findings were to further understand and appreciate the changes of As toxicity in the aqueous environments. Particularly, the coexistence of OM and As can potentially increase the risk to drinking water safety.

Artificial neural network modeling for the oxidation kinetics of divalent manganese ions during chlorination and the role of arsenite ions in the binary/ternary systems

This study investigated the coexistence and contamination of manganese (Mn(II)) and arsenite (As(III)) in groundwater and examined their oxidation behavior under different equilibrating parameters, including varying pH, bicarbonate (HCO3-) concentrations, and sodium hypochlorite (NaClO) oxidant concentrations. Results showed that if the molar ratio of NaClO: As(III) was >1, the oxidation of As(III) could be achieved within a minute with an extremely high oxidation rate of 99.7 %. In the binary system, the removal of As(III) prevailed over Mn(II). The As(III) oxidation efficiency increased from 59.8 ± 0.6 % to 70.8 ± 1.9 % when pH rose from 5.7 to 8.0. The oxidation reaction between As(III) and NaClO releases H+ ions, decreasing the pH from 6.77 to 6.19 and reducing the removal efficiency of Mn(II). The presence of HCO3- reduced the oxidation rate of Mn(II) from 63.2 % to 13.9 % within four hours. Instead, the final oxidation rate of Mn(II) increased from 68.1 % to 87.7 %. This increase can be attributed to HCO3- ions competing with the free Mn(II) for the adsorption sites on the sediments, inhibiting the formation of H+. Moreover, kinetic studies revealed that the oxidation reaction between Mn(II) and NaClO followed first-order kinetics based on their R2 values. The significant factors affecting the Mn(II) oxidation efficiency were the initial concentration of NaClO and pH. Applying an artificial neural network (ANN) model for data analysis proved to be an effective tool for predicting Mn(II) oxidation rates under different experimental conditions. The actual Mn(II) oxidation data and the predicted values obtained from the ANN model showed significant consistency. The training and validation data sets yielded R2 values of 0.995 and 0.992, respectively. Moreover, the ANN model highlights the importance of pH and NaClO concentrations in influencing the oxidation rate of Mn(II).

Promoting the transformation of green rust for As immobilization with Acidovorax sp. strain BoFeN1

Microbially mediated Fe(II) oxidation has a great potential for attenuating arsenic (As) mobility in an anoxic groundwaters. Green rust (GR), a common Fe(II)-bearing phase in such environments, could be easily oxidized into Fe (oxyhydr)oxides through microbial activity. This study focused on Acidovorax sp. strain BoFeN1, an anaerobic nitrate-reducing Fe(II)-oxidizing (NRFO) bacterium, to promote the transformation of GR. In biotic GR transformation experiments, magnetite formation occurred at [As]ini = 5 mg/L while lepidocrocite and goethite were formed at [As]ini = 10 mg/L. In the absence of bacterium, the GR persisted throughout the 120-h experiment. Meanwhile, with the addition of strain BoFeN1, the final aqueous As concentration significantly decreased from 0.237 to 0.004 mg/L (C0 = 5 mg/L) and from 1.457 to 0.096 mg/L (C0 = 10 mg/L) at 120 h. It was indicated that strain BoFeN1 played a crucial role in promoting the GR transformation and enhancing As immobilization. Further investigations revealed that the role of strain BoFeN1 extended beyond Fe-oxidation. With nitrite (the intermediate of nitrate bioreduction) as oxidizer, lepidocrocite/goethite were formed in the chemical-oxidation system, excluding magnetite. In the Bio - [As]ini = 5 mg/L, the occurrence of lepidocrocite via the bio-oxidation of Fe(II) in GR at 24 h, along with the metabolism of strain BoFeN1 reducing nitrate accompanied with H+ consumption, it should be reasonably deduced that the alkaline micro-environment of periplasm induced by strain BoFeN1 were vital for the transformation of lepidocrocite to magnetite triggered by trace Fe(II). However, in the Bio - [As]ini = 10 mg/L, more As adsorbed on GR inhibiting the adsorption of bacterium, so the alkaline micro-environment had no obvious effect on such transformation. This study helps to understand the interdependence between GR and anaerobic NRFO bacterium, and provides a new perspective for more effective As remediation strategies in anoxic groundwaters.

Geochemical insights of arsenic mobilization into the aquifers of Punjab, Pakistan

It is well known that groundwater arsenic (As) contamination affects million(s) of people throughout the Indus flood plain, Pakistan. In this study, groundwater (n = 96) and drilled borehole samples (n = 87 sediments of 12 boreholes) were collected to investigate geochemical proxy-indicators for As release into groundwater across floodplains of the Indus Basin. The mean dissolved (μg/L) and sedimentary As concentrations (mg/kg) showed significant association in all studied areas viz.; lower reaches of Indus flood plain area (71 and 12.7), upper flood plain areas (33.7 and 7.2), and Thal desert areas (5.3 and 4.7) and are indicative of Basin-scale geogenic As contamination. As contamination in aquifer sediments is dependent on various geochemical factors including particle size (3-4-fold higher As levels in fine clay particles than in fine-coarse sand), sediment types (3-fold higher As in Holocene sediments of floodplain areas vs Pleistocene/Quaternary sediments in the Thal desert) with varying proportion of Al-Fe-Mn oxides/hydroxides. The total organic carbon (TOC) of cored aquifer sediments yielded low TOC content (mean = 0.13 %), which indicates that organic carbon is not a major driver (with a few exceptions) of As mobilization in the Indus Basin. Alkaline pH, high dissolved sulfate and other water quality parameters indicate pH-induced As leaching and the dominance of oxidizing conditions in the aquifers of upper flood plain areas of Punjab, Pakistan while at the lower reaches of the Indus flood plain and alluvial pockets along the rivers with elevated flood-driven dissolved organic carbon (exhibiting high dissolved Mn and Fe and a wide range of redox conditions). Furthermore, we also identified that paired dissolved AsMn values (instead of AsFe) may serve as a geochemical marker of a range of redox conditions throughout Indus flood plains.

Groundwater irrigation induced variations in DOM fluorescence and arsenic mobility

Dissolved organic matter (DOM) plays a predominant role in groundwater arsenic (As) mobility. However, the temporal-spatial variations in DOM fluorescent characteristics and their effects on As mobility induced by groundwater irrigation remain unclear. To address these issues, groundwater from multilevel and irrigation wells in Zones I and II (with low- and high-As groundwater irrigation, respectively) from the Hetao Basin, China, were monitored in both non-irrigation (NIG) and irrigation (IG) seasons. Upon irrigation, the irrigation return increased the relative abundance of protein- and humic-like DOM in shallow groundwater from Zone I with Ca-type groundwater and Zone II with Na-type groundwater irrigation, respectively. The introduced dissolved oxygen by irrigation return decreased As concentrations by 22 % and 6 % on average in shallow groundwater from Zones I and II, respectively. However, the pumping-induced lateral recharge of lower- and higher-As groundwater led to an average 17 % decrease and 38 % increase in As concentrations in deeper groundwater from the two zones, respectively. The increased degradation of protein-like DOM may also contribute to the elevated As concentrations in deep groundwater from Zone II. The study provides insights into the dependence of irrigation-induced variations in DOM fluorescence and As concentrations on geochemicals of irrigation groundwater and aquifer hydrogeological conditions.

Sustaining Irrigation Supplies through Immobilization of Groundwater Arsenic In Situ

Geogenic arsenic (As) in groundwater is widespread, affecting drinking water and irrigation supplies globally, with food security and safety concerns on the rise. Here, we present push-pull tests that demonstrate field-scale As immobilization through the injection of small amounts of ferrous iron (Fe) and nitrate, two readily available agricultural fertilizers. Such injections into an aquifer with As-rich (200 ± 52 μg/L) reducing groundwater led to the formation of a regenerable As reactive filter in situ, producing 15 m3 of groundwater meeting the irrigation water quality standard of 50 μg/L. Concurrently, sediment magnetic properties were markedly enhanced around the well screen, pointing to neo-formed magnetite-like minerals. A reactive transport modeling approach was used to quantitatively evaluate the experimental observations and assess potential strategies for larger-scale implementation. The modeling results demonstrate that As removal was primarily achieved by adsorption onto neo-formed minerals and that an increased adsorption site density coincides with the finer-grained textures of the target aquifer. Up-scaled model simulations with 80-fold more Fe-nitrate reactants suggest that enough As-safe water can be produced to irrigate 1000 m2 of arid land for one season of water-intense rice cultivation at a low cost without causing undue contamination in surface soils that threatens agricultural sustainability.

Activated carbon as a strong DOM adsorbent mitigates antimony and arsenic release in flooded mining-impacted soils

In Southern China, the co-occurrence of arsenic (As) and antimony (Sb) contamination in soils around Sb mines presents an environmental challenge. During the flooding period of mining-impacted soils, anaerobic reduction of iron (Fe) oxides enhances the mobilization and bioavailability of Sb and As, further elevating the risk of Sb and As entering the food chain. To address this problem, activated carbon (AC) and biochar (BC) were applied to remediate flooded mining-impacted soils. Our results explored that AC can significantly decrease mobilization by 9-97 % for Sb and 9-67 % for As through inhibiting Fe(III) mineral reduction and dissolution in flooded soils. In contrast, there was no significant effect of BC. This was attributed to the strong adsorption of soil dissolved organic matter (DOM) by AC compared to BC, while DOM as electron shuttle is crucial for microbial Fe(III) reduction. Consequently, the DOM sequestration by AC effectively mitigates Sb and As leaching in contaminated mining soils.

Recent developments in speciation and determination of arsenic in marine organisms using different analytical techniques. A review

Marine organisms play a vital role as the main providers of essential and functional food. Yet they also constitute the primary pathway through which humans are exposed to total arsenic (As) in their diets. Since it is well known that the toxicity of this metalloid ultimately depends on its chemical forms, speciation in As is an important issue. Most relevant articles about arsenic speciation have been investigated. This extended not only from general knowledge about As but also the toxicity and health related issues resulting from exposure to these As species from the food ecosystem. There can be enormous side effects originating from exposure to As species that must be measured quantitatively. Therefore, various convenient approaches have been developed to identify different species of As in marine samples. Different extraction strategies have been utilized based on the As species of interest including water, methanol and mixtures of both, and many other extraction agents have been explained in this article. Furthermore, details of hyphenated techniques which are available for detecting these As species have been documented, especially the most versatile and applied technique including inductively coupled plasma mass spectrometry.

Co-occurrence of elevated arsenic and fluoride concentrations in Wuliangsu Lake: Implications for the genesis of poor-quality groundwater in the (paleo-)Huanghe (Yellow River) catchment, China

The co-occurrence of high As and F concentrations in saline groundwater in arid and semi-arid regions has attracted considerable attention. However, the factors determining the elevated concentrations of the two elements in surface water in these regions have not been sufficiently studied, and their implications for the poor-quality of local groundwater (high levels of As, F, and salinity) are unknown. A total of 18 water samples were collected from Wuliangsu Lake, irrigation/drainage channels, and the Huanghe (i.e., Yellow River) in the Hetao Basin, China. The pH, concentrations of As and F as well as those of other major elements, and stable isotope (H and O) compositions were analyzed. The water samples had a high pH (7.85-9.01, mean 8.25) and high TDS (402-9778 mg/L, mean 1920 mg/L) values. In six of the 10 lake samples, As concentration was above 10 μg/L (maximum 69.1 μg/L) and, in one of them, F concentration was above 1.5 mg/L. Interestingly, the high As, F, and TDS values simultaneously detected in the lake water were similar to those previously reported in local groundwater, and all water samples showed a significant positive correlation between As and F concentrations (R2 = 0.96, p < 0.01), except for two samples with abnormally high Ca2+ levels. The results of stable isotope analysis and Cl/Br ratios suggested that the lake experienced strong evaporation, which is consistent with the high TDS values. Evaporative concentration is suggested as the main factor contributing to the elevated As and F concentrations in the lake water. In addition, the major ions (e.g., Na+, Cl-, HCO3-, and OH-) and pH in the lake water increased during evaporation, leading to desorption of As and F. Thus, the evaporation process, including evaporative concentration and desorption, was considered primarily responsible for the elevated As and F in the lake water. Based on the results of this study, we presume that the paleolakes in the study area have experienced intense evaporation process. As a result, As, F, and major elements accumulated in sediments (or residual lake water) and were buried in the fluvial basins; then, they were released into the groundwater through multiple (bio)hydrogeochemical processes. By combining the results of this study with those obtained from previous groundwater analyses, we propose a new hypothesis explaining the origin of elevated As and F concentrations in saline groundwater in arid and semi-arid regions.

Deciphering the spatial heterogeneity of groundwater arsenic in Quaternary aquifers of the Central Yangtze River Basin

Significant spatial variability of groundwater arsenic (As) concentrations in South/Southeast Asia is closely associated with sedimentogenesis and biogeochemical cycling processes. However, the role of fine-scale differences in biogeochemical processes under similar sedimentological environments in controlling the spatial heterogeneity of groundwater As concentrations is poorly understood. Within the central Yangtze Basin, dissolved organic matter (DOM) and microbial functional communities in the groundwater and solid-phase As-Fe speciation in Jianghan Plain (JHP) and Jiangbei Plain (JBP) were compared to reveal mechanisms related to the spatial heterogeneity of groundwater As concentration. The optical signatures of DOM showed that low molecular terrestrial fulvic-like with highly humified was predominant in the groundwater of JHP, while terrestrial humic-like and microbial humic-like with high molecular weight were predominant in the groundwater of JBP. The inorganic carbon isotope, microbial functional communities, and solid-phase As-Fe speciation suggest that the primary process controlling As accumulation in JHP groundwater system is the degradation of highly humified OM by methanogens, which drive the reductive dissolution of amorphous iron oxides. While in JBP groundwater systems, anaerobic methane-oxidizing microorganisms (AOM) coupled with fermentative bacteria, iron reduction bacteria (IRB), and sulfate reduction bacteria (SRB) utilize low molecular weight DOM degradation to drive biotic/abiotic reduction of Fe oxides, further facilitating the formation of carbonate associated Fe and crystalline Fe oxides, resulting in As release into groundwater. Different biogeochemical cycling processes determine the evolution of As-enriched aquifer systems, and the coupling of multiple processes involving organic matter transformation‑iron cycling‑sulfur cycling-methane cycling leads to heterogeneity in the spatial distribution of As concentrations in groundwater. These findings provide new perspectives to decipher the spatial variability of As concentrations in groundwater.

Vertical distribution of arsenic and bacterial communities in calcareous farmland amending by organic fertilizer and iron-oxidizing bacteria: Field experiment on concomitant remediation

The migration and transformation mechanisms of arsenic (As) in soil environments necessitate an understanding of its influencing processes. Here, we investigate the subsurface biogeochemical transformation of As and iron (Fe) through amended in the top 20 cm with iron oxidizing bacteria (FeOB) and organic fertilizer (OF). Our comprehensive 400-day field study, conducted in a calcareous soil profile sectioned into 20 cm increments, involved analysis by sequential extraction and assessment of microbial properties. The results reveal that the introduction of additional OF increased the release ratio of As/Fe from the non-specific adsorption fraction (136.47 %) at the subsoil depth (40-60 cm), underscoring the importance of sampling at various depths and time points to accurately elucidate the form, instability, and migration of As within the profile. Examination of bacterial interaction networks indicated a disrupted initial niche in the bottom layer, resulting in a novel cooperative symbiosis. While the addition of FeOB did not lead to the dominance of specific bacterial species, it did enhance the relative abundance of As-tolerant Acidobacteria and Gemmatimonadetes in both surface (39.2 % and 38.76 %) and deeper soils (44.29 % and 23.73 %) compared to the control. Consequently, the amendment of FeOB in conjunction with OF facilitated the formation of poorly amorphous Fe (hydr)oxides in the soil, achieved through abiotic and biotic sequestration processes. Throughout the long-term remediation process, the migration coefficient of bioavailable As within the soil profile decreased, indicating that these practices did not exacerbate As mobilization. This study carries significant implications for enhancing biogeochemical cycling in As-contaminated Sierozem soils and exploring potential bioremediation strategies. ENVIRONMENTAL IMPLICATION: The long-term exposure of sewage irrigation has potential adverse effects on the local ecosystem, causing serious environmental problems. Microorganisms play a vital role in the migration and transformation of arsenic in calcareous soil in arid areas, which highlights the necessity of understanding its dynamics. The vertical distribution, microbial community and fate of arsenic in calcareous farmland soil profile in northwest China were studied through field experiments. The results of this work have certain significance for the remediation of arsenic-contaminated soil in arid areas, and provide new insights for the migration, transformation and remediation of arsenic in this kind of soil.

Spatio-temporal evolution of groundwater quality and its health risk assessment in Punjab (India) during 2000-2020

The state known as the bread basket of India has now been defamed as the cancer capital of the country. The toxicity of groundwater associated with the declining water level is reported in recent years. However, an extensive temporal and spatial analysis is required to identify hotspots. In this study, spatial tools are utilized to understand the evolution of groundwater in Punjab (> 315 sites) for the last two decades (2000-2020) for drinking purposes using the water quality index (WQI). The data for pH, electric conductivity (EC), bicarbonate (HCO3¯), chloride (Cl¯), sulfate (SO42¯), nitrate (NO3¯), fluoride (F¯), calcium (Ca2+), magnesium (Mg2+), sodium (Na+), and potassium (K+) collected from the Central Groundwater Board (CGWB) were analyzed. The results show that the average cation abundance is in declining order of Na > Mg > Ca > K, and anion abundance is in order of HCO3¯ > SO42¯ > Cl¯ > NO3 > F. The ions are compared with water quality standards defined by BIS and WHO. The study shows that in the year 2000, 69.52% of locations are above the acceptable limit for EC, 68.89% for Mg2, 84.76% for Na+, 51.75% for HCO3¯, 38.41% for NO3¯, and 17.20% for F¯. While in the year 2020, 48.89% exceed the acceptable limit for EC, 57.78% for Mg2+, 68.25% for Na+, 34.92% for HCO3¯, 27.30% for NO3¯, and 8.88% for F¯. WQI shows that in the year 2000, 13.01% of sampling locations are categorized as very poor and 20% as unsuitable for drinking. Meanwhile, in 2020, 6.35% of locations are categorized as very poor and 12.38% as unsuitable for drinking in the study area. In addition to the effect on plant growth, consumption of contaminated water can adversely affect human health. The health hazards for F¯ (HQF) and NO3¯ (HQN) and their total health index (THI) are also evaluated that depicts 244 groundwater sampling sites in the year 2000, and 152 sampling sites in the year 2020 show high non-carcinogenic effects on adults, children, and infants. Southwestern Punjab is found to be the worst affected, while north-eastern regions drained by the Himalayan rivers show better quality water. Shifting in agricultural practices in the last two decades and declining water levels due to excess pumping of water from deeper water tables deteriorated the quality of water in the Southern region as observed from the geospatial analysis.

Chronic exposure of arsenic among children in Asia: A current opinion based on epidemiological evidence

The health effects of arsenic (As) exposure are a major global environmental issue affecting millions of people around the globe. Although adult epidemiological studies on As-induced health consequences have been extensively reviewed, but not much comprehensive review has been done targeting children. In this epidemiological review, 64 human subject studies on children were identified after applying exclusion criteria, which addressed an array of health effects of As exposure in early life stages in South and Southeast (S-SE) Asian countries, where a great variability in As exposures has been reported. The present review identified neurocognitive impairment linked to As exposure in early life stages. In utero and childhood As exposures were also associated with genetic and metabolic alteration, elevated pneumonia risk, and skin lesions in several populations in S-SE Asia. Significant associations of As with epigenetic changes, DNA damages, abnormal birth outcomes, and elevated mortality were also reported in epidemiological studies. The findings of this review article may help public health policymakers and clinicians develop early-life intervention strategies to reduce the burden of diseases in As-exposed populations.

Environmental arsenic (As) and its potential relationship with endemic disease in southwestern China

Many cases of an unknown disease exhibiting the clinical features of limb gangrene, blisters, ulceration, and exfoliation have been reported in Daping village (DV) in southwestern China. However, the pathogenesis is unknown and has puzzled doctors for many years. A preliminary study on heavy metals and symptoms indicated that arsenic might pose the greatest threat to the health of local residents. Here, to explore the sources of and factors influencing arsenic enrichment in DV, whose residents exhibit signs of arsenic poisoning, the As contents in soil, water, and plants were systematically measured. The results indicated high As contents in plant and soil samples obtained from the area, and the source of As may be linked to the weathering of black shale rock. Ingestion of soil and consumption of plants were the two main As exposure pathways among children and adults, respectively, and children exhibited a higher health risk than adults. We presume and emphasize that when extreme drought events occur, humans might face unusual risks resulting from exposure to toxic elements and the direct consumption of highly polluted water. Our study provides a new perspective and sheds light on the environmental geochemistry and health links of this disease.

Elemental Characterization of Ambient Particulate Matter for a Globally Distributed Monitoring Network: Methodology and Implications

Global ground-level measurements of elements in ambient particulate matter (PM) can provide valuable information to understand the distribution of dust and trace elements, assess health impacts, and investigate emission sources. We use X-ray fluorescence spectroscopy to characterize the elemental composition of PM samples collected from 27 globally distributed sites in the Surface PARTiculate mAtter Network (SPARTAN) over 2019-2023. Consistent protocols are applied to collect all samples and analyze them at one central laboratory, which facilitates comparison across different sites. Multiple quality assurance measures are performed, including applying reference materials that resemble typical PM samples, acceptance testing, and routine quality control. Method detection limits and uncertainties are estimated. Concentrations of dust and trace element oxides (TEO) are determined from the elemental dataset. In addition to sites in arid regions, a moderately high mean dust concentration (6 μg/m3) in PM2.5 is also found in Dhaka (Bangladesh) along with a high average TEO level (6 μg/m3). High carcinogenic risk (>1 cancer case per 100000 adults) from airborne arsenic is observed in Dhaka (Bangladesh), Kanpur (India), and Hanoi (Vietnam). Industries of informal lead-acid battery and e-waste recycling as well as coal-fired brick kilns likely contribute to the elevated trace element concentrations found in Dhaka.

Roles of nanoparticles in arsenic mobility and microbial community composition in arsenic-enriched soils

Environmental effects of nano remediation engineering of arsenic (As) pollution need to be considered. In this study, the roles of Fe2O3 and TiO2 nanoparticles (NPs) on the microbial mediated As mobilization from As contaminated soil were investigated. The addition of Fe2O3 and TiO2 NPs restrained As(V) release, and stimulated As(III) release. As(V) concentration decreased by 94% and 93% after Fe2O3 addition, and decreased by 89% and 45% after TiO2 addition compared to the Biotic and Biotic+Acetate (amended with sodium acetate) controls, respectively. The maximum values of As(III) were 20.5 and 27.1 µg/L at 48 d after Fe2O3 and TiO2 NPs addition, respectively, and were higher than that in Biotic+Acetate control (12.9 µg/L). The released As co-precipitated with Fe in soils in the presence of Fe2O3 NPs, but adsorbed on TiO2 NPs in the presence of TiO2 NPs. Moreover, the addition of NPs amended with sodium acetate as the electron donor clearly promoted As(V) reduction induced by microbes. The NPs addition changed the relative abundance of soil bacterial community, while Proteobacteria (42.8%-70.4%), Planctomycetes (2.6%-14.3%), and Firmicutes (3.5%-25.4%) were the dominant microorganisms in soils. Several potential As/Fe reducing bacteria were related to Pseudomonas, Geobacter, Desulfuromonas, and Thiobacillus. The addition of Fe2O3 and TiO2 NPs induced to the decrease of arrA gene. The results indicated that the addition of NPs had a negative impact on soil microbial population in a long term. The findings offer a relatively comprehensive assessment of Fe2O3 and TiO2 NPs effects on As mobilization and soil bacterial communities.

Synergetic effects of Mn(II) production and site availability on arsenite oxidation and arsenate adsorption on birnessite in the presence of low molecular weight organic acids

Manganese oxides and organic acids are key factors affecting arsenic mobility, but As(III) oxidation and adsorption in the coexistence of birnessite and low molecular weight organic acids (LMWOAs) are poorly understood. Herein, As(III) immobilization by birnessite was investigated with/without LMWOAs (including tartaric (TA), malate (MA), and succinic acids (SA) with two, one and zero hydroxyl groups, respectively). In the low-As(III) system with less Mn(II) production, LMWOAs generally inhibited As(III) oxidation. The slower decrease in As(III) concentration in TA-amended batches resulted from stronger bonding interaction between TA and edge sites, evidenced by higher removal of TA than MA and SA in solutions and the higher proportion of shifted C-OH component in solids. In high-As(III) systems with abundant Mn(II) production, higher concentrations of dissolved Mn and Mn(III) in LMWOA-amended batches than in LMWOA-free batches revealed that LMWOA-induced complexing dissolution caused the release of adsorbed Mn(II), which was conducive to As(III) oxidation and As(V) adsorption onto the edge sites. The lowest concentrations of dissolved Mn and Mn(III) in TA-amended batches indicated that the hydroxyl group constrained complexing dissolution. This study reveals that concentrations of produced Mn(II) determined the roles of LMWOAs in As(III) behavior and highlights the impacts of the hydroxyl group on arsenic mobility.

Inorganic Hydrogeochemistry in the 21st Century

Chemical and isotopic processes occur in every segment of the hydrological cycle. Hydrogeochemistry-the subdiscipline that studies these processes-has seen a transformation from "witch's brew" to credible science since 2000. Going forward, hydrogeochemical research and applications are critical to meeting urgent societal needs of climate change mitigation and clean energy, such as (1) removing CO2 from the atmosphere and storing gigatons of CO2 in soils and aquifers to achieve net-zero emissions, (2) securing critical minerals in support of the transition from fossil fuels to renewable energies, and (3) protecting water resources by adapting to a warming climate. In the last two decades, we have seen extensive activity and progress in four research areas of hydrogeochemistry related to water-rock interactions: arsenic contamination of groundwater; the use of isotopic and chemical tracers to quantify groundwater recharge and submarine groundwater discharge; the kinetics of chemical reactions and the mineral-water interface's control of contaminant fate and transport; and the transformation of geochemical modeling from an expert-only exercise to a widely accessible tool. In the future, embracing technological advances in machine learning, cyberinfrastructure, and isotope analytical tools will allow breakthrough research and expand the role of hydrogeochemistry in meeting society's needs for climate change mitigation and the transition from fossil fuels to renewable energies.

Mechanistic Insights into the Selectivity for Arsenic over Phosphate Adsorption by Fe3+-Cross-Linked Chitosan Using DFT

Fe3+-cross-linked chitosan exhibits the potential for selectively adsorbing arsenic (As) over competing species, such as phosphate, for water remediation. However, the effective binding mechanisms, bond nature, and controlling factor(s) of the selectivity are poorly understood. This study employs ab initio calculations to examine the competitive binding of As(V), P(V), and As(III) to neat chitosan and Fe3+-chitosan. Neat chitosan fails to selectively bind As oxyanions, as all three oxyanions bind similarly via weak hydrogen bonds with preferences of P(V) = As(V) > As(III). Conversely, Fe3+-chitosan selectively binds As(V) over As(III) and P(V) with binding energies of -1.9, -1, and -1.8 eV for As(V), As(III), and P(V), respectively. The preferences are due to varying Fe3+-oxyanion donor-acceptor characteristics, forming covalent bonds with distinct strengths (Fe-O bond ICOHP values: - 4.9 eV/bond for As(V), - 4.7 eV/bond for P(V), and -3.5 eV/bond for As(III)). Differences in pKa between As(V)/P(V) and As(III) preclude any preference for As(III) under typical environmental pH conditions. Furthermore, our calculations suggest that the binding selectivity of Fe3+-chitosan exhibits a pH dependence. These findings enhance our understanding of the Fe3+-oxyanion interaction crucial for preferential oxyanion binding using Fe3+-chitosan and provide a lens for further exploration into alternative transition-metal-chitosan combinations and coordination chemistries for applications in selective separations.

Elevated arsenic concentrations in groundwater of the Upper Indus Plain of Pakistan across a range of redox conditions

Groundwater of the Ravi River floodplain is particularly elevated in arsenic (As) on both sides of the Pakistan-India border. To understand this pattern, 14 sites were drilled to 12-30 m depth across floodplains and doabs of Pakistan after testing over 20,000 wells. Drill cuttings were collected at 1.5 m intervals, 132 of which were sand overlain by 77 intervals of clay and/or silt. Radiocarbon dating of clay indicates deposition of the aquifer sands tapped by wells 20-30 kyr ago. Most (85 %) of the sand samples were gray in color, indicating partial reduction to Fe(II) oxides, whereas most (92 %) of the clay and/or silt samples were orange. Associations between groundwater electrical conductivity, dissolved Fe, sulfate, and nitrate suggest that wells can be elevated (>10 μg/L) in As in the region due to either reductive dissolution of Fe oxides, evaporative concentration, or alkali desorption. In the Ravi floodplain, 47 % of 6445 wells tested contain >10 μg/L As compared to only 9 % of 14,165 tested wells in other floodplains and doabs. The As content of aquifer sands in the Ravi floodplain of Pakistan averages 4 ± 4 mg/kg (n = 66) and is higher than the average of 2 ± 2 mg/kg (n = 51) for aquifer sands outside the Ravi. Synchrotron spectroscopy and column-based speciation indicate predominance of As(V) over As(III) in both aquifer sands and groundwater. Whereas multiple processes may be responsible for elevated levels of As in groundwater across the region, spatial heterogeneity in groundwater As concentrations in the Ravi floodplain seems linked to variations in As concentrations in aquifer sands. Regulation by the solid phase may limit variations in groundwater As over time in response to natural and human-induced changes in hydrology. This means spatial heterogeneity could be taken advantage of to lower the exposure across the region with more testing and targeted drilling.

Understanding spatial heterogeneity of groundwater arsenic concentrations at a field scale: Taking the Datong Basin as an example to explore the significance of hydrogeological factors

The spatial heterogeneity of arsenic (As) concentration exceeding the 10 μg/L WHO limit at the field scale poses significant challenges for groundwater utilization, but it remains poorly understood. To address this knowledge gap, the Daying site was selected as a representative case (As concentration ranged from 1.55 to 2237 μg/L within a 250 × 150 m field), and a total of 28 groundwater samples were collected and analyzed for hydrochemistry, As speciation, and stable hydrogen and oxygen isotope. Principal component analysis was employed to identify the primary factors controlling groundwater hydrochemistry. Results indicate that the spatial heterogeneity of groundwater As concentration is primarily attributed to vertical recharge and competitive adsorption. Low vertical recharge introduces reductive substances, such as dissolved organic matter, which enhances the reductive environment and facilitates microbial-induced reduction and mobilization of As. Conversely, areas with high vertical recharge introduce oxidizing agents like SO42- and DO, which act as preferred electron acceptors over Fe(III), thus inhibiting the reductive dissolution of Fe(III) oxides and the mobilization of As. PCA and hydrochemistry jointly indicate that spatial variability of P and its competitive adsorption with As are important factors leading to spatial heterogeneity of groundwater As concentration. However, the impacts of pH, Si, HCO3-, and F- on As adsorption are insignificant. Specifically, low vertical recharge can increase the proportion of As(III) and promote P release through organic matter mineralization. This process further leads to the desorption of As, indicating a synergistic effect between low vertical recharge and competitive adsorption. This field-scale spatial heterogeneity underscores the critical role of hydrogeological conditions. Sites with close hydraulic connections to surface water often exhibit low As concentrations in groundwater. Therefore, when establishing wells in areas with widespread high-As groundwater, selecting sites with open hydrogeological conditions can prove beneficial.