Modulating plant-soil microcosm with green synthesized ZnONPs in arsenic contaminated soil

Biogenic nanoparticle (NP), derived from plant sources, is gaining prominence as a viable, cost-effective, sustainable, and biocompatible alternative for mitigating the extensive environmental impact of arsenic on the interplay between plant-soil system. Herein, the impact of green synthesized zinc oxide nanoparticles (ZnONPs) was assessed on Catharanthus roseus root system-associated enzymes and their possible impact on microbiome niches (rhizocompartments) and overall plant performance under arsenic (As) gradients. The application of ZnONPs at different concentrations successfully modified the arsenic uptake in various plant parts, with the root arsenic levels increasing 1.5 and 1.4-fold after 25 and 50 days, respectively, at medium concentration compared to the control. Moreover, ZnONPs gradients regulated the various soil enzyme activities. Notably, urease and catalase activities showed an increase when exposed to low concentrations of ZnONPs, whereas saccharase and acid phosphatase displayed the opposite pattern, showing increased activities under medium concentration which possibly in turn influence the plant root system associated microflora. The use of nonmetric multidimensional scaling ordination revealed a significant differentiation (with a significance level of p < 0.05) in the structure of both bacterial and fungal communities under different treatment conditions across root associated niches. Bacterial and fungal phyla level analysis showed that Proteobacteria and Basidiomycota displayed a significant increase in relative abundance under medium ZnONPs concentration, as opposed to low and high concentrations, respectively. Similarly, in depth genera level analysis revealed that Burkholderia, Halomonas, Thelephora and Sebacina exhibited a notably high relative abundance in both the rhizosphere and rhizoplane (the former refers to the soil region influenced by root exudates, while the latter is the root surface itself) under medium concentrations of ZnONPs, respectively. These adjustments to the plant root-associated microcosm likely play a role in protecting the plant from oxidative stress by regulating the plant's antioxidant system and overall biomass.

Novel insights into the kinetics and mechanism of arsenopyrite bio-dissolution enhanced by pyrite

Arsenopyrite and pyrite often coexist in metal deposits and tailings, thus simultaneous bioleaching of both sulfides has economic (as well as environmental) significance. Important targets in bio-oxidation operations are high solubilization rates and minimized accumulation of Fe(III)/As-bearing secondary products. This study investigated the role of pyrite bioleaching in the enhancement of arsenopyrite dissolution. At a pyrite to arsenopyrite mass ratio of 1:1, 93.6% of As and 93.0% of Fe were solubilized. The results show that pyrite bio-oxidation can promote arsenopyrite dissolution, enhance S0 bio-oxidation, and inhibit the formation of jarosites, tooeleite, and amorphous ferric arsenate. The dry weight of the pyrite & arsenopyrite residue was reduced by 95.1% after bioleaching, compared to the initial load, while only 5% weight loss was observed when pyrite was absent. A biofilm was formed on the arsenopyrite surface in the presence of pyrite, while a dense passivation layer was observed in the absence of pyrite. As(III) (as As2O3) was a dominant As species in the pyrite & arsenopyrite residue. Novel and detailed findings are presented on arsenopyrite bio-dissolution in the presence of pyrite, and the presented approach could contribute to the development of novel cost-effective extractive bioprocesses. ENVIRONMENTAL IMPLICATION: The oxidation of arsenopyrite presents significant environmental hazards, as it can contribute to acid mine drainage generation and arsenic mobilization from sulfidic mine wastes. Bioleaching is a proven cost-effective and environmentally friendly extractive technology, which has been applied for decades in metal recovery from minerals or tailings. In this work, efficient extraction of arsenic from arsenopyrite bioleaching was presented through coupling the process with bio-oxidation of pyrite, resulting in lowered accumulation of hazardous and metastable Fe(III)/As-bearing secondary phases. The results could help improve current biomining operations and/or contribute to the development of novel cost-effective bioprocesses for metal extraction.

High biodegradability of microbially-derived dissolved organic matter facilitates arsenic enrichment in groundwater: Evidence from molecular compositions and structures

Although biodegradation of organic matter is well-known to trigger enrichment of arsenic (As) in groundwater, the effects of DOM sources and biodegradability on As enrichment remain elusive. In this study, groundwater samples were collected from the Hetao basin to identify DOM source and evaluate biodegradability by using spectral and molecular techniques. Results showed that in the alluvial fan, DOM was mainly sourced from terrestrially derived OM, while DOM in the flat plain was more originated from microbially derived OM. Compared to terrestrially derived DOMs, microbially derived DOMs in groundwater, which had relatively higher H/Cwa ratios, NOSC values and more biodegradable molecules, exhibited higher biodegradability. In the flat plain, microbially derived DOMs with higher biodegradability encountered stronger biodegradation, facilitating the reductive dissolution of Fe(III)/Mn oxides and As enrichment in groundwater. Moreover, the enrichment of As depended on the biodegradable molecules that was preferentially utilized for primary biodegradation. Our study highlights that the enrichment of dissolved As in the aquifers was closely associated with microbially derived DOM with high biodegradability and high ability for primary biodegradation.

Modifying humus-phosphorus-arsenic interactions in sludge composting: The strengthening of phosphorus availability and arsenic efflux detoxification mechanisms

Arsenic (As) in sewage sludge poses a significant threat to environmental and human health, which has attracted widespread attention. This study investigated the value of adding sodium percarbonate (SP) on phosphorus (P) availability and As efflux detoxification through HS-P-As interactions. Due to the unique structure of humus (HS) and the similar chemical properties of P and As, the conditions for HS-P-As interaction are provided. This study discussed the content, morphology and microbial communities of HS, P and As by using metagenomic and correlation analysis. The results showed that the humification index in the experiment group (SPC) was 2.34 times higher than that in the control group (CK). The available phosphorus (AP) content of SPC increased from 71.09 mg/kg to 126.14 mg/kg, and SPC was 1.11 times that of CK. The relative abundance of ACR3/ArsB increased. Pst, Actinomyces and Bacillus commonly participated in P and As conversion. The correlation analysis revealed that the humification process was enhanced, the AP was strengthened, and the As was efflux detoxified after SP amendment. All in all, this study elucidated the key mechanism of HS-P-As interaction and put forward a new strategy for sewage sludge resource utilization and detoxification.

Immobilisation remediation of arsenic-contaminated soils with promising CaAl-layered double hydroxide and bioavailability, bioaccessibility, and speciation-based health risk assessment

Arsenic (As)-contaminated soil poses great health risk to human mostly through inadvertent oral exposure. We investigated CaAl-layered double hydroxide (CaAl-LDH), a promising immobilising agent, for the remediation of As-contaminated Chinese soils. The effects on specific soil properties and As fractionation were analyzed, and changes in the health risk of soil As were accurately assessed by means of advanced in vivo mice model and in vitro PBET-SHIME model. Results showed that the application of CaAl-LDH significantly increased soil pH and concentration of Fe and Al oxides, and effectively converted active As fractions into the most stable residual fraction, guaranteeing long-term remediation stability. Based on in vivo test, As relative bioavailability was significantly reduced by 37.75%. Based on in vitro test, As bioaccessibility in small intestinal and colon phases was significantly reduced by 25.65% and 28.57%, respectively. Furthermore, As metabolism (reduction and methylation) by the gut microbiota inhabiting colon was clearly observed. After immobilisation with CaAl-LDH, the concentration of bioaccessible As(Ⅴ) in the colon fluid was significantly reduced by 61.91%, and organic As (least toxic MMA(V) and DMA(V)) became the main species, which further reduced the health risk of soil As. In summary, CaAl-LDH proved to be a feasible option for immobilisation remediation of As-contaminated soils, and considerable progress was made in relevant health risk assessment.

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

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

On the Existence of Pnictogen Bonding Interactions in As(III) S-Adenosylmethionine Methyltransferase Enzymes

As(III) S-adenosylmethionine methyltransferases, pivotal enzymes in arsenic metabolism, facilitate the methylation of arsenic up to three times. This process predominantly yields trivalent mono- and dimethylarsenite, with trimethylarsine forming in smaller amounts. While this enzyme acts as a detoxifier in microbial systems by altering As(III), in humans, it paradoxically generates more toxic and potentially carcinogenic methylated arsenic species. The strong affinity of As(III) for cysteine residues, forming As(III)-thiolate bonds, is exploited in medical treatments, notably in arsenic trioxide (Trisenox®), an FDA-approved drug for leukemia. The effectiveness of this drug is partly due to its interaction with cysteine residues, leading to the breakdown of key oncogenic fusion proteins. In this study, we extend the understanding of As(III)'s binding mechanisms, showing that, in addition to As(III)-S covalent bonds, noncovalent O⋅⋅⋅As pnictogen bonding plays a vital role. This interaction significantly contributes to the structural stability of the As(III) complexes. Our crystallographic analysis using the PDB database of As(III) S-adenosylmethionine methyltransferases, augmented by comprehensive theoretical studies including molecular electrostatic potential (MEP), quantum theory of atoms in molecules (QTAIM), and natural bond orbital (NBO) analysis, emphasizes the critical role of pnictogen bonding in these systems. We also undertake a detailed evaluation of the energy characteristics of these pnictogen bonds using various theoretical models. To our knowledge, this is the first time pnictogen bonds in As(III) derivatives have been reported in biological systems, marking a significant advancement in our understanding of arsenic's molecular interactions.

Adsorption behavior and the potential risk of As(V) in soils: exploring the effects of representative surfactants

Arsenic contamination in soils poses a critical global challenge, yet the influence of surfactants on arsenic adsorption behavior is often underestimated. This study aims to investigate the effects of three representative surfactants, namely cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and polyethylene glycol anhydrous sugar alcohol monooleate (Tween 80), on arsenic adsorption behavior in soils. The adsorption isotherm shifts from a single Temkin model without surfactants to both the Langmuir and Temkin models in the presence of surfactants, indicating the simultaneous occurrence of monolayer and multilayer adsorption for arsenic in soils. Moreover, the surfactants can inhibit the adsorption and hasten the attainment of adsorption equilibrium. SDS displayed the most inhibitory effect on arsenic adsorption, followed by Tween 80 and CTAB, due to the competitive adsorption, electrostatic interaction, and hydrophobic interaction. Variations in zeta potential with different surfactants further elucidate this inhibitory phenomenon. Through orthogonal experiment analyses, pH emerges as a primary factor influencing arsenic adsorption in soils, with surfactant concentration and type identified as secondary factors. Temperature notably affects CTAB, with the adsorption inhibition rate plummeting to a mere 0.88% at 50 °C. Sequential extraction analysis revealed that surfactants enhanced the bioavailability of arsenic. The FTIR, XRD, SEM, and CA analyses further support the mechanism underlying the effect of surfactants on arsenic adsorption in soil. These analyses indicate that surfactants modify the composition and abundance of functional groups, hinder the formation of arsenic-containing substances, and improve soil compactness, smoothness, and hydrophilicity. This study provides valuable insights into the effect of surfactants in arsenic-contaminated soils, which is often ignored in previous work.

Enhanced attenuation of arsenic by Quaternary agricultural soils of Eastern Punjab, India upon anionic clays and gypsum amendment

Agricultural soil of the Sutlej River basin was evaluated for its natural attenuation efficacy for arsenic (As) under the field variables of pH, competitive anions, contact time and varied As contents. The role of layered double hydroxides (HTLDH) and gypsum on uptake efficiency and long-term stability of entrapped As demonstrates rapid As uptake by both geosorbents without mineral structure altering. Arsenic retention by gypsum is poorer than that by HTLDH and greater uptake (∼100% within 2 h) was achieved in the co-precipitation process than adsorption on HTLDH. Freundlich isotherm and pseudo-second-order kinetic model fits of the data demonstrate the multilayer rate-limiting sorption process. NO3- and PO43- hardly affected As retention capacity of HTLDH and gypsum with greater retention at pH 6 and high sorbate concentrations. Studied soil shows a strong potential for As (0.68 g kg-1) which enhanced upon adding HTLDH, while gypsum lowered As retention efficiency of soil except at pH 6.0. Gypsum exhibited relatively greater desorption than HTLDH where almost no As was desorbed in the latter case within seven days of exposure, but ∼30% sorbed As gets desorbed from gypsum which was further enhanced by NO3-+PO43- and soil mixing. Identical behaviour was observed from the soil and HTLDH/gypsum mixture at variable ratios as well. This study shows that MgFe-based HTLDH can efficiently retard arsenic mobilization from the soil with competitive anions and wide pH ultimately limiting As bioavailability in the environment and can be successfully used as a potential scavenger for As remediation purposes.

Synergistic inhibition of green rust crystallization by co-existing arsenic and silica

Arsenic and silica are known inhibitors of the crystallization of iron minerals from poorly ordered precursor phases. However, little is known about the effects of co-existing As and Si on the crystallization and long-term stability of mixed-valence Fe minerals such as green rust (GR). GR usually forms in anoxic, Fe2+-rich, near-neutral pH environments, where they influence the speciation and mobility of trace elements, nutrients and contaminants. In this work, the Fe2+-induced transformation of As- and/or Si-bearing ferrihydrite (FHY) was monitored at pH 8 ([As]initial = 100 μM, Si/As = 10) over 720 h. Our results showed that in the presence of As(III) + Si or As(V) + Si, GR sulfate (GRSO4) formation from FHY was up to four times slower compared to single species system containing only As(III), As(V) or Si. Co-existing As(III) + Si and As(V) + Si also inhibited GRSO4 transformation to magnetite, contrary to systems with only Si or As(V). Overall, our findings demonstrate the synergistic inhibitory effect of co-existing Si on the crystallization and solid-phase stability of As-bearing GRSO4, establishing an inhibitory effect ladder: As(III) + Si > As(V) + Si > As(III) > Si > As(V). This further highlights the importance of GR in potentially controlling the fate and mobility of As in ferruginous, Si-rich groundwater and sediments such as those in South and Southeast Asia.

Effective adsorption of As(V) from aqueous solution by quaternary ammonium and Zn2+ decorated lignin-based sorbent

Arsenic poses a serious harm to the natural environment and human health. Lignin decorated with quaternary ammonium and metal ion can effectively adsorb arsenic from aqueous solution. Zn2+/quaternary ammonium lignin was synthesized by quaternization and metallization from lignin with 3-Chloro-2-hydroxypropyl trimethylammonium chloride and ZnCl2. The morphology, functional groups and chemical compositions of adsorbent were identified by SEM-EDS, FTIR and XRD. The effects such as pH, initial As(V) concentration, contact time and adsorbent dosage on the adsorption capacity were investigated in batch system. The adsorption mechanism was explored by SEM-EDS, FTIR and XPS. It was shown that the adsorbent was rough and contained a large amount of quaternary ammonium and Zn2+. Zn2+/quaternary ammonium lignin exhibited much strong affinity towards As(V) with the maximum adsorption capacity of 70.38 mg·g-1 at 25 °C, oscillation rate of 180 r·min-1, pH of 5, initial As(V) concentration of 100 mg·L-1, contact time of 30 min and 1 g·L-1 Zn2+/quaternary ammonium lignin. The adsorption could be well described by Langmuir model and quasi-second-order kinetic model, indicating the monolayer homogeneous chemisorption nature. As(V) was adsorbed through electrostatic attraction of Zn2+ and ion exchange between H2AsO4- and Cl-.

Fe/S oxidation-coupled arsenic speciation transformation mediated by AMD enrichment culture under different pH conditions

Arsenic (As) speciation transformation in acid mine drainage (AMD) is comprehensively affected by biological and abiotic factors, such as microbially mediated Fe/S redox reactions and changes in environmental conditions (pH and oxidation-reduction potential). However, their combined impacts on arsenic speciation transformation remain poorly studied. Therefore, we explored arsenic transformation and immobilization during pyrite dissolution mediated by AMD enrichment culture under different acidic pH conditions. The results for incubation and mineralogical transformation of solid residues show that in the presence of AMD enrichment culture, pH 2.0, 2.5, and 3.0 are more conducive to the formation of jarosites and ferric arsenate, which could immobilize high quantities of dissolved arsenic by adsorption and coprecipitation. The pH conditions significantly affect the initial adsorption of microbial cells to the minerals and the evolution of microbial community structure, further influencing the biodissolution of pyrite and the release and oxidation process of Fe/S. The results of Fe/S/As speciation transformation of the solid residues show that the transformation of Fe, S, and As in solution is mainly regulated by pH and potential values, which imposed significantly different effects on the formation of secondary minerals and thus arsenic oxidation and immobilization. The above results indicated that arsenic transformation is closely related to the Fe/S oxidation associated with pyrite bio-oxidation, and this correlation is critically regulated by the pH conditions of the system.

Arsenic(III) sorption on organo-ferrihydrite coprecipitates: Insights from maize and rape straw-derived DOM

The mobility of arsenic (As) specie in agricultural soils is significantly impacted by the interaction between ferrihydrite (Fh) and dissolved organic material (DOM) from returning crop straw. However, additional research is necessary to provide molecular evidence for the interaction of toxic and mobile As (As(III)) specie and crop straw-based organo- Fh coprecipitates (OFCs). This study investigated the As(III) sorption behaviours of OFCs synthesized with maize or rape derived-DOM under various environmental conditions and the primary molecular sorption mechanisms using As K-edge X-ray absorption near edge structure (XANES) spectroscopy. According to our findings, pure Fh adsorbed more As(III) relative to the other two OFCs, and the presence of natural organic matter in the OFCs induced more As(III) adsorption at pH 5.0. Findings from this study indicated a maximum As(III) sorption on Ma (53.71 mg g⁻1) and Ra OFC (52.46 mg g⁻1) at pH 5.0, with a sharp decrease as the pH increased from 5.0 to 8.0. Additionally, As K-edge XANES spectroscopy indicated that ∼30% of adsorbed As(III) on the OFCs undergoes transformation to As(V) at pH 7-8. Functional groups from the DOM, such as O-H, COOH, and CO, contributed to As(III) desorption and its oxidation to As(V), whereas ionic strength analysis revealed inner complexation as the dominant As(III) sorption mechanism on the OFCs. Overall, the results indicate that the interaction of natural organic matter (NOM) with As(III) at higher pH promotes As(III) mobility, which is crucial when evaluating As migration and bioavailability in alkaline agricultural soils.

Manganese promotes stability of natural arsenic sinks in a groundwater system with arsenic-immobilization minerals: Natural remediation mechanism and environmental implications

Arsenic (As)-immobilizing iron (Fe)-manganese (Mn) minerals (AFMM) represent potential As sinks in As-enriched groundwater environments. The process and mechanisms governing As bio-leaching from AFMM through interaction with reducing bacteria, however, remain poorly delineated. This study examined the transformation and release of As from AFMM with varying Mn/Fe molar ratios (0:1, 1:5, 1:3, and 1:1) in the presence of As(V)-reducing bacteria specifically Shewanella putrefaciens CN32. Notably, strain CN32 significantly facilitated the bio-reduction of As(V), Fe(III), and Mn(IV) in AFMM. In systems with Mn/Fe molar ratios of 1:5, 1:3, and 1:1, As bio-reduction decreased by 28%, 34%, and 47%, respectively, compared to the system with a 0:1 ratio. This Mn-induced inhibition of Fe/As bio-reduction was linked to several concurrent factors: preferential Mn bio-reduction, reoxidation of resultant Fe(II)/As(III) due to Mn components, and As adsorption onto emergent Fe precipitates. Both the reductive dissolution of AFMM and the bio-reduction of As(V) predominantly controlled As bio-release. Structural equation models indicated that reducing bacteria destabilize natural As sinks more through As reduction than through Mn(II) release, Fe reduction, or Fe(II) release. Systems with Mn/Fe molar ratios of 1:5, 1:3, and 1:1 showed a decrease in As bio-release by 24%, 41%, and 59%, respectively, relative to the 0:1 system. The observed suppression of As bioleaching was ascribed to both the inhibition of As/Fe bio-reduction by Mn components and the immobilization of As by freshly generated Fe precipitates. These insights into the constraining effect of Mn on the biotransformation and bioleaching of As from AFMM are crucial for grasping the long-term stability of natural As sinks in groundwater, and enhance strategies for in-situ As stabilization in As-afflicted aquifers through Nature-Based Solutions.

Techno-economic feasibility and life cycle assessment analysis for a developed novel biosorbent-based arsenic bio-filter system

Significant aquifers around the world is contaminated by arsenic (As), that is regarded as a serious inorganic pollution. In this study, a biosorbent-based bio-filter column has been developed using two different plant biomasses (Colocasia esculenta stems and Artocarpus heterophyllus seeds) to remove total As from the aqueous system. Due to its natural origin, affordability, adaptability, removal effectiveness, and possibility for integration with existing systems, the biosorbent-based bio-filter column presents an alluring and promising method. It offers a practical and eco-friendly way to lessen the damaging impacts of heavy metal contamination on ecosystems and public health. In this system, As (III) is oxidized to As (V) using chlorine as an oxidant, after this post-oxidized As-contaminated water is passed through the bio-filter column to receive As-free water (or below World Health Organization permissible limit for As in drinking water). Optimization of inlet flow rate, interference of co-existing anions and cations, and life cycle of the column were studied. The maximum removal percent of As was identified to be 500 µg L-1 of initial concentration at a flow rate of 1.5 L h-1. Furthermore, the specifications of the biosorbent material was studied using elemental analysis and Zeta potential. The particle size distribution, morphological structures, and chemical composition before and after binding with As were studied using dynamic light scattering (DLS), scanning electron microscope-energy dispersive X-Ray spectroscopy (SEM-EDX), and fourier's transform infrared spectroscopy (FTIR) analysis, respectively. SuperPro 10 software was used to analyze the techno-economic viability of the complete unit and determine its ideal demand and potential. Life cycle assessment was studied to interpret the environmental impacts associated alongside the process system. Therefore, this bio-filtration system could have a potential application in rural, urban, and industrial sectors.

Application of dirty-acid wastewater treatment technology in non-ferrous metal smelting industry: Retrospect and prospect

Dirty-acid wastewater (DW) originating from the non-ferrous metal smelting industry is characterized by a high concentration of H2SO4 and As. During the chemical precipitation treatment, a significant volume of arsenic-containing slag is generated, leading to elevated treatment expenses. The imperative to address DW with methods that are cost-effective, highly efficient, and safe is underscored. This paper conducts a comprehensive analysis of three typical methods to DW treatment, encompassing technical principles, industrial application flow charts, research advancements, arsenic residual treatment, and economic considerations. Notably, the sulfide method emerges as a focal point due to its minimal production of arsenic residue and the associated lowest overall treatment costs. Moreover, in response to increasingly stringent environmental protection policies targeting new pollutants and carbon emissions reduction, the paper explores the evolving trends in DW treatment. These trends encompass rare metal and sulfuric acid recycling, cost-effective H2S production methods, and strategies for reducing, safely disposing of, and harnessing resources from arsenic residue.

Removal of arsenic from jarosite waste using hydrometallurgical treatment

The jarosite waste used during this study consists of minute amount of arsenic that has a potential to be leached into environment when kept in open area. This study tried to recover arsenic from jarosite waste using hydrometallurgical treatment. The comprehensive characterization of jarosite samples was performed using various analytical techniques, including X-ray diffraction (XRD), Fourier transform Infrared (FTIR), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX), and it was characterized as natrojarosite. For optimal removal of arsenic, the response surface methodology (RSM) was applied with the key factors, including dosage (A), time (B), temperature (C), and acid concentration (D) on the recovery of arsenic. The results indicated that the dosage (A) and acid concentration (D) demonstrated significant positive effects on arsenic recovery. As expected, the higher dosage and acid concentration was associated with increased recovery percentages for the arsenic from jarosite. Whereas time (B) and temperature (C) did not exhibit statistically significant recovery of arsenic within the specified experimental range. The contour plots showed the optimal operating conditions for the highest recovery percentage was approximately 52.61% when 2.5 g of jarosite was treated with 10 mol/L acid for 150 min at operating temperature of 80°. Although our study showed very moderate recovery of arsenic, it is first report where arsenic has been removed from jarosite waste. Readjustment of range of operating parameters would provide more insight into the further optimization of the yield.

Deep-dive into iron-based co-precipitation of arsenic: A review of mechanisms derived from synchrotron techniques and implications for groundwater treatment

The co-precipitation of Fe(III) (oxyhydr)oxides with arsenic (As) is one of the most widespread approaches to treat As-contaminated groundwater in both low- and high-income settings. Fe-based co-precipitation of As occurs in a variety of conventional and decentralized treatment schemes, including aeration and sand filtration, ferric chloride addition and technologies based on controlled corrosion of Fe(0) (i.e., electrocoagulation). Despite its ease of deployment, Fe-based co-precipitation of As entails a complex series of chemical reactions that often occur simultaneously, including electron-transfer reactions, mineral nucleation, crystal growth, and As sorption. In recent years, the growing use of sophisticated synchrotron-based characterization techniques in water treatment research has generated new detailed and mechanistic insights into the reactions that govern As removal efficiency. The purpose of this critical review is to synthesize the current understanding of the molecular-scale reaction pathways of As co-precipitation with Fe(III), where the source of Fe(III) can be ferric chloride solutions or oxidized Fe(II) sourced from natural Fe(II) in groundwater, ferrous salts or controlled Fe(0) corrosion. We draw primarily on the mechanistic knowledge gained from spectroscopic and nano-scale investigations. We begin by describing the least complex reactions relevant in these conditions (Fe(II) oxidation, Fe(III) polymerization, As sorption in single-solute systems) and build to multi-solute systems containing common groundwater ions that can alter the pathways of As uptake during Fe(III) co-precipitation (Ca, Mg bivalent cations; P, Si oxyanions). We conclude the review by providing a perspective on critical knowledge gaps remaining in this field and new research directions that can further improve the understanding of As removal via Fe(III) co-precipitation.

Exploring the interference mechanisms of surface and aqueous complexes with groundwater arsenate and arsenite adsorption

Ca2+, Mg2+, and HCO3- are extremely common coexisting ions with arsenic (As) in geogenic As-polluted groundwaters. Although extensive research has improved our knowledge of groundwater As removal techniques and mechanisms, there is still a lack of a definite explanation of the distinct influences of Ca2+ and Mg2+ on As immobilization. Furthermore, the question of whether the occurrence of metal-As aqueous complexes has positive or detrimental effects on As adsorption is still open, which hinders our ability to predict the effectiveness of groundwater As removal. The goal of our present work was to investigate the molecular-level interference mechanisms of Ca2+, Mg2+, and HCO3- on arsenic adsorption with batch/column filtration experiments and spectroscopic techniques. The results showed that the co-presence of Ca2+ and As significantly increased As(V) and As(III) adsorption by 22.1 and 12.2% in batch studies and by 20.1 and 16.7% in column adsorptive filtrations, which could be explained by forming a ternary Ca-As-TiO2 complex. Without the surface complex, Mg2+ only had a slightly positive effect on As removal. Co-existence of Ca2+ and HCO3- prevented the generation this surface complex, which significantly decreased the elimination of As(III). Inversely, the As(V) ternary complex and adsorption were not interfered by HCO3-. Moreover, an aqueous Ca-As(V) complex was detected without surface, which facilitated generation of the surface complex and As(V) adsorption. The results of this work clarified the distinct effects of Ca2+ and Mg2+ and As(V) and As(III) adsorption, which was critical in predicting the As adsorption efficiency in column filtration processes.

Progress of gaseous arsenic removal from flue gas by adsorption: Experimental and theoretical calculations

Because of its high mobility and difficult capture, gaseous arsenic pollution control has become the focus of arsenic pollution control. It mainly exists in the form of highly toxic As2O3 in the flue gas. Therefore, removing gaseous As2O3 from flue gas is of great practical significance for arsenic pollution control. Stabilizing gaseous As2O3 on the surface of adsorbents by physical or chemical adsorption is an effective way to reduce the content of arsenic in the flue gas and alleviate arsenic pollution. Over the past few decades, various adsorbents have been developed to capture gaseous As2O3 in the flue gas, and their adsorption mechanisms have been studied in detail. Thus, it is necessary to review the strategies of arsenic removal from flue gas by adsorption, which can inspire further research. Based on summarizing the morphological distribution of gaseous As2O3 in the flue gas, this review further summarizes the removal of gaseous As2O3 by several adsorbents and the effect of temperature and the main components of the flue gas on arsenic adsorption. In addition, the mechanism of arsenic removal based on adsorption in the flue gas is discussed in depth through theoretical calculations, which is the particular focus of this review. Finally, prospects based on the present research state of arsenic removal by adsorption are proposed to provide ideas for developing effective and stable adsorbents for arsenic removal from flue gas.

Comparative study of Fe(II)/sulfite, Fe(II)/PDS and Fe(II)/PMS for p-arsanilic acid treatment: Efficient organic arsenic degradation and contrasting total arsenic removal

As a widely used feed additives, p-arsanilic acid (p-AsA) frequently detected in the environment poses serious threats to aquatic ecology and water security due to its potential in releasing more toxic inorganic arsenic. In this work, the efficiency of Fe(II)/sulfite, Fe(II)/PDS and Fe(II)/PMS systems in p-AsA degradation and simultaneous arsenic removal was comparatively investigated for the first time. Efficient p-AsA abatement was achieved in theses Fe-based systems, while notable discrepancy in total arsenic removal was observed under identical acidic condition. By using chemical probing method, quenching experiments, isotopically labeled water experiments, p-AsA degradation was ascribed to the combined contribution of high-valent Fe(IV) and SO4•-in these Fe(II)-based system. In particular, the relative contribution of Fe(IV) and SO4•- in the Fe(II)/sulfite system was highly dependent on the molar ratio of [Fe(II)] and [sulfite]. Negligible arsenic removal was observed in the Fe(II)/sulfite and Fe(II)/PDS systems, while ∼80% arsenic was removed in the Fe(II)/PMS system under identical acidic condition. This interesting phenomenon was due to that ferric precipitation only occurred in the Fe(II)/PMS system. As(V) was further removed via adsorption onto the iron precipitate or the formation of ferric arsenate-sulfate compounds, which was confirmed by particle diameter measurements, fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. Through tuning solution pH, complete removal of total arsenic could achieve in all three systems. Among these three Fe-based technologies, the hybrid oxidation-coagulation Fe(II)/PMS system demonstrated potential superiority for arsenic immobilization by not requiring pH adjustment for coagulation and facilitating the in-situ generation of ferric arsenate-sulfate compounds with comparably low solubility levels like scorodite. These findings would deepen the understanding of these three Fe-based Fenton-like technologies for decontamination in water treatment.

Interaction mechanism of water-soluble inorganic arsenic onto pristine nanoplastics

Nanoplastics (NPLs) persist in aquatic habitats, leading to incremental research on their interaction mechanisms with metalloids in the environment. In this regard, it is known that plastic debris can reduce the number of water-soluble arsenicals in contaminated environments. Here, the arsenic interaction mechanism with pure NPLs, such as polyethylene terephthalate (PET), aliphatic polyamide (PA), polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), and polystyrene (PS) is evaluated using computational chemistry tools. Our results show that arsenic forms stable monolayers on NPLs through surface adsorption, with adsorption energies of 9-24 kcal/mol comparable to those on minerals and composite materials. NPLs exhibit varying affinity towards arsenic based on their composition, with As(V) adsorption showing higher stability than As(III). The adsorption mechanism results from a balance between electrostatics and dispersion forces (physisorption), with an average combined contribution of 87%. PA, PET, PVC, and PS maximize the electrostatic effects over dispersion forces, while PE and PP maximize the dispersion forces over electrostatic effects. The electrostatic contribution is attributed to hydrogen bonding and the activation of terminal O-C, C-H, and C-Cl groups of NPLs, resulting in several pairwise interactions with arsenic. Moreover, NPLs polarity enables high mobility in aqueous environments and fast mass transfer. Upon adsorption, As(III) keeps the NPLs polarity, while As(V) limits subsequent uptake but ensures high mobility in water. The solvation process is destabilizing, and the higher the NPL polarity, the higher the solvation energy penalty. Finally, the mechanistic understanding explains how temperature, pressure, pH, salinity, and aging affect arsenic adsorption. This study provides reliable quantitative data for sorption and kinetic experiments on plastic pollution and enhances our understanding of interactions between water contaminants.

Metabolic coupling of arsenic, carbon, nitrogen, and sulfur in high arsenic geothermal groundwater: Evidence from molecular mechanisms to community ecology

Groundwater arsenic (As) poses a global environmental problem and is regulated by complex biogeochemical processes. However, the As biogeochemistry and its metabolic coupling with carbon (C), nitrogen (N), and sulfur (S) in high As geothermal groundwater remain unclear. Here, we reported significant shifts in the geothermal groundwater microbiome and its functional ecological clusters along the flow path with increased As levels and dynamic As-C-N-S biogeochemical cycle from the Guide Basin, China. Strong associations among As(III), NH4+, HCO3-, and corresponding functional microbial taxa suggest that microbe-mediated As transformation, ammonification, and organic carbon biodegradation potentially contributed to the As mobilization in the discharge area. And As oxidizers (coupling with denitrification or carbon fixation) and S oxidizers were closely linked to the transformation of As(III) to immobile As(V) in the recharge area. Our study provides a comprehensive insight into the complex microbial As-C-N-S coupling network and its potential role in groundwater As mobilization under hydrological disturbances.

Arsenite S-Adenosylmethionine Methyltransferase Is Responsible for Antimony Biomethylation in Nostoc sp. PCC7120

Antimony (Sb) biomethylation is an important but uninformed process in Sb biogeochemical cycling. Methylated Sb species have been widely detected in the environment, but the gene and enzyme for Sb methylation remain unknown. Here, we found that arsenite S-adenosylmethionine methyltransferase (ArsM) is able to catalyze Sb(III) methylation. The stepwise methylation by ArsM forms mono-, di-, and trimethylated Sb species. Sb(III) is readily coordinated with glutathione, forming the preferred ArsM substrate which is anchored on three conserved cysteines. Overexpressing arsM in Escherichia coli AW3110 conferred resistance to Sb(III) by converting intracellular Sb(III) into gaseous methylated species, serving as a detoxification process. Methylated Sb species were detected in paddy soil cultures, and phylogenetic analysis of ArsM showed its great diversity in ecosystems, suggesting a high metabolic potential for Sb(III) methylation in the environment. This study shows an undiscovered microbial process methylating aqueous Sb(III) into the gaseous phase, mobilizing Sb on a regional and even global scale as a re-emerging contaminant.

A Comprehensive Review on Molecular Mechanism Involved in Arsenic Trioxide Mediated Cerebral Neurodegenerative and Infectious Diseases

Arsenic is an environmental toxicant and its toxicity is a global health problem affecting millions of people. Arsenic exposure occurs from natural geological sources leaching into aquifers, contaminating drinking water and may also occur from mining and other industrial processes. Both cancerous, noncancerous and immunological complications are possible after arsenic exposure. The many other target organs like lungs, thymus, spleen, liver, heart, kidney, and brain. Arsenic-mediated neuro, as well as immunotoxicity, is the main concern of this review. Long-term arsenic exposure can lead to various neurological dysfunctions, which may cause neurobehavioral defects and biochemical impairment in the brain, this might negatively affect one's quality of life in later stages. Arsenic also alters the levels of various neurotransmitters such as serotonin, dopamine and norepinephrine in the brain which produces neurotoxic effects and immunological deficiency. So, it is crucial to understand the neurotoxic mechanism of arsenic trioxide-mediated cerebro neurodegenerative and immunerelated alterations. One of the major mechanisms by which it exerts its toxic effect is through an impairment of cellular respiration by inhibition of various mitochondrial enzymes, and the uncoupling of oxidative phosphorylation. This review focuses on the various toxic mechanisms responsible for arsenic-mediated neurobehavioral and immune-related changes. Therefore, this review provides a critical analysis of mitochondrial dysfunctions, oxidative stress, glutamate excitatory, inflammatory and apoptosis-related mechanistic aspects in arsenic-mediated immunotoxicity, neurotoxicity, and neurodegenerative changes.

Separation and recovery of arsenic, germanium and tungsten from toxic coal ash from lignite by sequential vacuum distillation with disulphide

Large amount of coal ash is produced as industrial waste during the electricity generation through the combustion of lignite. Toxic elements arsenic exists in the coal ash, which hinders the subsequent recycling processes. Moreover, coal ash could be recycled further to retrieve scattered metals germanium and tungsten. It is believed that traditional recycling methods present barriers to scaled application, especially serious secondary pollution, such as toxic residue and waste liquid. In this work, a novel sequential vacuum distillation with disulphide method is proposed to separate arsenic, germanium and tungsten from coal ash. First, arsenic can be volatilized completely out of the reaction system at temperatures below 550 °C. Subsequently, Ge and W volatilized in the form of sulfide in the presence of Na2S2O3. The optimal condition was 1050 °C, the mass ratio of 0.6 with reaction a pressure of 1 Pa and a time duration of 120 min demonstrated the best evaporation ratio. For coal fly ash, chemical species As2S3, GeS, and WOx (x < 3)/WS2 were the main condensed products. For coal bottom ash, As2S3, GeS, and WO3/WS2 were dominant chemical components. Mechanisms for the process of release and evaporation of As, Ge, and W from coal ash, vacuum reaction, evaporation, and condensation were analyzed. In summary, the vacuum distillation method deserves to be further developed as it provides an eco-friendly method to recycle coal ash.

Arsenic removal adsorbent using limonite-polyethersulfone composite fiber via continuous flow column process

This research introduces an enhanced limonite-based composite fiber adsorbent for arsenic (As) removal. The modification involves creating polyethersulfone (PES)-limonite composite fibers loaded with 60 wt% limonite powders, designed to be applicable in water flow environments. The fibers were prepared using a wet-spinning process based on phase inversion, with varying concentrations (10, 20, and 30 wt%) of PES in NMP solution. The composite fiber with 10 wt% NMP exhibited a porous structure and demonstrated efficient absorption of both As(III) and As(V). Adsorption followed the Langmuir model, with qm values of 1.5 mg/g for As(III) and 3.2 mg/g for As(V) at pH 6. In column experiments, As removal rates increased with contact time, attributed to decreased flow rates (1 mL/min). Moreover, increasing fiber column height led to enhanced removal rates, as indicated by the Adams-Bohart model. The mechanism for As(V) removal involved the formation of an inner-sphere complex through ion exchange between α-FeOOH and HAsO4 - and H2 AsO4 2- in an aqueous solution at pH 6.8. PRACTITIONER POINTS: Changing the polyethersulfone ratio in the composite leads to variations in the appearance of limonite within each composite fiber. Limonite composite fibers effectively remove As(III) and As(V) at neutral pH. The adsorption behavior follows Langmuir kinetic model, the qm of 1.5 mg/g for As(III) and 3.2 mg/g for As(V). Longer columns and contact times enhance arsenic (As) removal in practical water treatment systems. Adam-Bohart model aids in predicting breakthrough and saturation time in As adsorption column design.

Biodegradable chitosan‑zirconium composite adsorptive membranes for potential arsenic (III/V) capture electrodialysis

Combining adsorption with other technologies holds great potential in fast and deep arsenic ion removal. Herein, chitosan‑zirconium composite adsorptive membranes (CS-Zr CM) were successfully prepared using simple casting and sodium hydroxide coagulation strategies, which was demonstrated the use in arsenic ion-capture electrodialysis based on their good adsorption performance. In the batch adsorption tests, the maximum adsorption capacities of CS-Zr CM for As(III) and As(V) were 134.2 mg/g and 119.5 mg/g, respectively. CS-Zr CM also exhibited satisfying adsorption selectivity and good reusability toward As(III) and As(V). However, the adsorption kinetics showed that they needed 48 h to reach the adsorption equilibrium and the adsorption ability toward trace arsenic ion was ineffective. Furthermore, CS-Zr CM was applied as the adsorptive membrane in the electrodialysis process. Under the influence of electric field, the As(III) and As(V) removal equilibrium time was shortened to 12 h and the concentrations of As(III) and As(V) ions could be efficiently reduced to below the WHO limit in drinking water (10 μg/L), which far surpassed the physicochemical adsorption method. Such good arsenic ion removal ability of CS-Zr CM together with the ease scalable fabrication, low cost, and biodegradable properties shows its huge prospects in arsenic-containing wastewater treatment.

Mechanistic insights into the leaching and environmental safety of arsenic in ceramsite prepared from fly ash

Utilizing fly ash to prepare ceramsite is a promising way to immobilize heavy metals and recycle industrial solid waste. However, traditional preparation method of fly ash ceramsite has the disadvantages of large ignition loss. Therefore, the present study applied the pressure molding method to enhance solid content and improve the strength of ceramsite. The optimal preparation conditions of ceramsite were suggested as preheating at 450 °C for 25 min followed by sintering at 1050 °C for 30 min. Under such conditions, ceramsite with high compressive strength of 10.8 Mpa, bulk density of 878 kg m-3, and 1-h water absorption of 18.5% was fabricated, in compliance with Chinese standard (GB/T 1743.1-2010). The arsenic leaching concentration from the resulting product was considerably lower than Chinese standard (GB 5085.3-2007). Moreover, arsenic volatilization during ceramsite calcination was insignificant, and the vast majority of arsenic remained in resulting ceramsite. A geochemical speciation model developed for the multiple component system in ceramsite suggested that FeAsO4, Ca5(OH) (AsO4)3, and hydrous ferric oxide adsorption are the primary mechanisms retaining arsenic in ceramsite. Additionally, based on density functional theory calculations and biotoxicity test, the binding site of arsenic atom on mineral components and the environmental safety of ceramsite was determined and evaluated.

Synergistic mechanism of iron manganese supported biochar for arsenic remediation and enzyme activity in contaminated soil

This study prepared and characterized bamboo-derived biochar loaded with different ratios of iron and manganese; evaluated its remediation performance in arsenic-contaminated soil by studying the changes in various environmental factors, arsenic speciation, and arsenic leaching amount in the soil after adding different materials; proposed the optimal ratio and mechanism of iron-manganese removal of arsenic; and explained the multivariate relationship between enzyme activity and soil environmental factors based on biological information. Treatment with Fe-Mn-modified biochar increased the organic matter, cation exchange capacity, and N, P, K, and other nutrient contents. During the remediation process, O-containing functional groups such as Mn-O/As and Fe-O/As were formed on the surface of the biochar, promoting the transformation of As from the mobile fraction to the residual fraction and reducing the phytotoxicity of As, and the remediation ability for As was superior to that of Fe-modified biochar. Mn is indispensable in the FeMn-BC synergistic remediation of As, as it can increase the adsorption sites and the number of functional groups for trace metals on the surface of biochar. In addition to electrostatic attraction, the synergistic mechanism of ferromanganese-modified biochar for arsenic mainly involves redox and complexation. Mn oxidizes As(Ⅲ) to more inert As(V). In this reaction process, Mn(Ⅳ) is reduced to Mn(Ⅲ) and Mn(II), promoting the formation of Fe(Ⅲ) and the conversion of As into Fe-As complexes, while As is fixed due to the formation of ternary surface complexes. Moreover, the effect of adding Fe-Mn-modified biochar on soil enzyme activity was correlated with changes in soil environmental factors; catalase was correlated with soil pH; neutral phosphatase was correlated with soil organic matter; urease was correlated with ammonia nitrogen, and sucrase activity was not significant. This study highlights the potential value of FM1:3-BC as a remediation agent in arsenic-contaminated neutral soils.

An impressive pristine biochar from food waste digestate for arsenic(V) removal from water: Performance, optimization, and mechanism

Anaerobic digestion has become a global practice for valorizing food waste, but the recycling of the digestate (FWD) remains challenging. This study aimed to address this issue by utilizing FWD as a low-cost feedstock for Ca-rich biochar production. The results demonstrated that biochar pyrolyzed at 900 °C exhibited impressive As(V) adsorption performance without any modifications. Kinetic analysis suggested As(V) was chemisorbed onto CDBC9, while isotherm data conformed well to Langmuir model, indicating monolayer adsorption with a maximum capacity of 76.764 mg/g. Further analysis using response surface methodology revealed that pH value and adsorbent dosage were significant influencing factors, and density functional theory (DFT) calculation visualized the formation of ionic bonds between HAsO42- and CaO(110) and Ca(OH)2(101) surfaces. This work demonstrated the potential of using FWD for producing Ca-rich biochar, providing an effective solution for As(V) removal and highlighting the importance of waste material utilization in sustainable environmental remediation.

Stabilization and solidification of arsenic contaminated silty sand using alkaline activated slag

Contamination of soils by arsenic represents a significant environmental and public health risk, making effective remediation strategies a pressing concern. One commonly employed technique is stabilization and solidification, which involves the addition of stabilizing binders such as cement to immobilize arsenic. This study investigates the potential of alkaline activated slag for stabilization and solidification of arsenic-contaminated soil, employing the toxic characteristic leaching procedure (TCLP) and unconfined compressive strength (UCS) tests. To assess the strength and leachability behavior, the research examines the effect of several factors, including binder content, curing time, curing conditions, alkaline activator solution to slag ratio, sodium silicate to sodium hydroxide ratio, and sodium hydroxide concentration. Additionally, field emission scanning electron microscopes (FE-SEM) in combination with energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) tests are employed to analyze the stabilization and solidification mechanism. The study shows that increasing the slag content to 20% by weight after 28 days of curing at ambient temperature leads to a decrease of almost 92% in the concentration of leached arsenic and an increase in UCS from 80 kPa to approximately 19 MPa. The formation of albite and anorthite crystals, along with gels such as (N, C)-A-S-H and C-S-H, contributes to enhanced strength and reduced leachability. As a result, the use of alkaline activated slag is identified as an effective and environmentally friendly approach for the stabilization/solidification of arsenic-contaminated soils. This study highlights the use of alkaline activated slag as an effective solution for remediating arsenic-contaminated soils while simultaneously reducing waste, greenhouse gas emissions, and energy consumption. Slag, a byproduct of metal production, is often wasted due to a lack of value and landfill space. However, alkaline activated slag demonstrates the potential to stabilize soil, immobilize heavy metals, and provide efficient and sustainable soil remediation.

Characterization of histone chaperone MCM2 as a key regulator in arsenic-induced depletion of H3.3 at genomic loci

Arsenic exposure is associated with an increased risk of many cancers, and epigenetic mechanisms play a crucial role in arsenic-mediated carcinogenesis. Our previous studies have shown that arsenic exposure induces polyadenylation of H3.1 mRNA and inhibits the deposition of H3.3 at critical gene regulatory elements. However, the precise underling mechanisms are not yet understood. To characterize the factors governing arsenic-induced inhibition of H3.3 assembly through H3.1 mRNA polyadenylation, we utilized mass spectrometry to identify the proteins, especially histone chaperones, with reduced binding affinity to H3.3 under conditions of arsenic exposure and polyadenylated H3.1 mRNA overexpression. Our findings reveal that the interaction between H3.3 and the histone chaperon protein MCM2 is diminished by both polyadenylated H3.1 mRNA overexpression and arsenic treatment in human lung epithelial BEAS-2B cells. The increased binding of MCM2 to H3.1, resulting from elevated H3.1 protein levels, appears to contribute to the reduced availability of MCM2 for H3.3. To further investigate the role of MCM2 in H3.3 deposition during arsenic exposure and H3.1 mRNA polyadenylation, we overexpressed MCM2 in BEAS-2B cells overexpressing polyadenylated H3.1 or exposed to arsenic. Our results demonstrate that MCM2 overexpression attenuates H3.3 depletion at several genomic loci, suggesting its involvement in the arsenic-induced displacement of H3.3 mediated by H3.1 mRNA polyadenylation. These findings suggest that changes in the association between histone chaperone MCM2 and H3.3 due to polyadenylation of H3.1 mRNA may play a pivotal role in arsenic-induced carcinogenesis.

Co-adsorption mechanisms of As(V) and Cd(II) by three-dimensional flower-like Mg/Al/Fe-CLDH synthesized by "memory effect"

Due to the different physical and chemical properties such as surface charge and ion morphology between As(V) and Cd(II), it is challenging to remove As(V) and Cd(II), especially at low concentrations. This study constructed a novel three-dimension nanocomposite adsorbent Mg/Al/Fe-CLDH (CFMA) by "hydrothermal + calcination method". And different initial concentration ratios (Cd: As=1: 2, 1: 1, 2: 1) were used to investigate the removal performance of CFMA for Cd(II) and As(V). When the concentration ratio Cd: As=1: 2, the residual concentrations of As(V) and Cd(II) were 8.7 μg/L and 4.2 μg/L, respectively, which met the drinking water standard; In the co-adsorption system, As(V) and Cd(II) influence each other's adsorption behavior due to the anionic bridge and shielding effect of As(V) on Cd(II), As(V) gradually changed from monolayer adsorption to multi-layer adsorption dominant, while Cd(II) gradually changed from multi-layer adsorption to monolayer adsorption dominant. In this paper, the structure-activity relationship between material structure and synchronous removal of arsenic and cadmium was clarified, and the mechanism of synchronous removal was revealed, which provided technical guidance for synchronous removal of As(V) and Cd(II) from non-ferrous metal smelting wastewater.

Identification of Monomethylmonothioarsonic Acid as the Major Thioarsenical Generated During Extraction Processes for Arsenic Species Analysis

Acid extraction is commonly used to analyze arsenic species in rice. During the extraction process, spiked monomethylarsonic acid (MMA) is often transformed into different compounds. A similar phenomenon is observed in the arsenic speciation analysis of seafood. To identify these compounds, we analyzed a previously prepared extract using liquid chromatography-time-of-flight/mass spectrometry in differential analysis and liquid chromatography-inductively coupled plasma-MS. The compound was identified as monomethylmonothioarsonic acid (MMMTA), a thioarsenical, which is estimated to be more cytotoxic than MMA. As MMMTA was readily produced by bubbling hydrogen sulfide through MMA, this suggests that MMA reacts with sulfur in rice during the extraction process. Our data also suggested that dimethylarsinic acid could be transformed into another compound, although the generation rate was low. For reliable arsenic speciation analyses, the transformation of arsenic compounds during extraction must be avoided. This study demonstrates that arsenic compounds can be transformed by dilute acid extraction.

Insights into the selectivity of metallic oxides for arsenic and phosphate from EXAFS and DFT calculations

Phosphate is the biggest competitor for arsenic removal. Nanoscale metal oxides (NMOs) are commonly used to treat arsenic-contaminated water, yet their selective adsorption mechanisms for arsenic and phosphate are poorly understood. We quantified the selectivity of iron oxide (Fe2O3), zinc oxide (ZnO), and titanium dioxide (TiO2) nanosheets for arsenic in systems containing arsenic and phosphate, and determined the interaction of phosphate and arsenate/arsenite on metal oxide surfaces through batch experiments, spectroscopic techniques, and DFT calculations. We found that Fe2O3, TiO2, and ZnO nanosheets exhibit selectivity for arsenate/arsenite in the presence of phosphate, with Fe2O3 the most selective, followed by TiO2 and ZnO. The bonding mechanism on these metallic oxide surfaces dominates the selectivity. The more stable inner-sphere complexes of arsenate on the surfaces of Fe2O3 (bidentate binuclear), TiO2 (bidentate binuclear), and ZnO nanosheets (tridentate trinuclear) contribute to their preference for arsenate over phosphate. This difference in arsenate selectivity can be reflected in the difference in adsorption energy, net electron transfer number, and M - O bond length of the most stable inner sphere complexes. Overall, our study elucidated the selective adsorption mechanisms of arsenate/arsenite on Fe2O3, TiO2, and ZnO surfaces and highlighted the need to consider the competition between arsenate and phosphate during their removal from contaminated water.

Solutions for an efficient arsenite oxidation and removal from groundwater containing ferrous iron

Manganese (Mn) oxides are extensively used to oxidize As(III) present in ground, drinking, and waste waters to the less toxic and more easily removable As(V). The common presence of multiple other cations in natural waters, and more especially of redox-sensitive ones such as Fe2+, may however significantly hamper As(III) oxidation and its subsequent removal. The present work investigates experimentally the influence of Mn(III) chelating agents on As(III) oxidation process in such environmentally relevant complex systems. Specifically, the influence of sodium pyrophosphate (PP), an efficient Mn(III) chelating agent, on As(III) oxidation by birnessite in the presence of Fe(II) was investigated using batch experiments at circum-neutral pH. In the absence of PP, competitive oxidation of Fe(II) and As(III) leads to Mn oxide surface passivation by Fe(III) and Mn(II/III) (oxyhydr)oxides, thus inhibiting As(III) oxidation. Addition of PP to the system highly enhances As(III) oxidation by birnessite even in the presence of Fe(II). PP presence prevents passivation of Mn oxide surfaces keeping As and Fe species in solution while lower valence Mn species are released to solution. In addition, reactive oxygen species (ROS), tentatively identified as hydroxyl radicals (•OH), are generated under aerobic conditions through oxygen activation by Fe(II)-PP complexes, enhancing As(III) oxidation further. The positive influence of Mn(III) chelating agents on As(III) oxidation most likely not only depend on their affinity for Mn(III) but also on their ability to promote formation of these active radical species. Finally, removal of As(V) through sorption to Fe (oxyhydr)oxides is efficient even in the presence of significant concentrations of PP, and addition of such Mn(III) chelating agents thus appears as an efficient way to enhance the oxidizing activity of birnessite in large-scale treatment for arsenic detoxification of groundwaters.

Effective reduction and recovery of As(III) and As(V) from alkaline wastewater by thiourea dioxide: Efficiency and mechanism

For alkaline wastewater with high arsenic concentration, the traditional lime precipitation inevitably produces large amounts of hazardous waste. Herein, a heat-activated reduction method employing thiourea dioxide (TDO) as the reductant was proposed to efficiently remove and recover As(III)/As(V) from alkaline wastewater in the form of valuable As(0). More than 99.9% of As(III)/As(V) (2-400 mM) were reduced to As(0) with a high purity of more than 99.5 wt% by TDO within 30 min. The highly reductive eaq- and SO2- radical generated during TDO decomposition contribute to the arsenic reduction, and the contribution ratios of eaq- and SO2- radical were estimated to be approximately 57.6% and 42.4% for As(III) removal and 62.2% and 37.8% for As(V) removal, respectively. The arsenic reduction was greatly improved by increasing pH and temperature, which could accelerate the cleavage of C-S bond in TDO for the eaq- and SO2- formation. The presence of dissolved oxygen, which can not only scavenge eaq-/SO2- but also directly oxidize SO22-, had a negative effect on the arsenic removal. The presence of CO32- slightly suppressed the arsenic removal due to the eaq- scavenging effect while SiO32-, PO43-, Cl-, SO42- and NH4+ had negligible effects. The proposed method was a potential technology for the efficient removal and reduction of arsenic in alkaline wastewater.

The key roles of Fe oxyhydroxides and humic substances during the transformation of exogenous arsenic in a redox-alternating acidic paddy soil

Arsenic (As) from mine wastewater is a significant source for acidic paddy soil pollution, and its mobility can be influenced by alternating redox conditions. However, mechanistic and quantitative insights into the biogeochemical cycles of exogenous As in paddy soil are still lacking. Herein, the variations of As species in paddy soil spiking with As(III) or As(V) were investigated in the process of 40 d of flooding followed 20 d of drainage. During flooding process, available As was immobilized in paddy soil spiking As(III) and the immobilized As was activated in paddy soil spiking As(V) owing to deprotonation. The contributions of Fe oxyhydroxides and humic substances (HS) to As immobilization in paddy soil spiking As(III) were 80.16% and 18.64%, respectively. Whereas the contributions of Fe oxyhydroxides and HS to As activation in paddy soil spiking As(V) were 47.9% and 52.1%, respectively. After entering drainage, available As was mainly immobilized by Fe oxyhydroxides and HS and adsorbed As(III) was oxidized. The contribution of Fe oxyhydroxides to As fixation in paddy soil spiking As(III) and As(V) was 88.82% and 90.26%, respectively, and of HS to As fixation in paddy soil spiking As(III) and As(V) was 11.12% and 8.95%, respectively. Based on the model fitting results, the activation of Fe oxyhydroxides and HS bound As followed with available As(V) reduction were key processes during flooding. This may be because the dispersion of soil particles and release of soil colloids activated the adsorbed As. Immobilization of available As(III) by amorphous Fe oxyhydroxides followed with adsorbed As(III) oxidation were key processes during drainage. This may be ascribe to the occurrence of coprecipitation and As(III) oxidation mediated by reactive oxygen species from Fe(II) oxidation. The results are beneficial for a deeper understanding of As species transformation at the interface of paddy soil-water as well as an estimation pathway for the impacts of key biogeochemical cycles on exogenous As species under a redox-alternating condition.

Facet-dependent electron transfer induces distinct arsenic reallocations on hematite

The interfacial electron transfer (ET) between electron shuttling compounds and iron (Fe) oxyhydroxides plays a crucial role in the reductive dissolution of Fe minerals and the fate of surface-bound arsenic (As). However, the impact of exposed facets of highly crystalline hematite on reductive dissolution and As immobilization is poorly understood. In this study, we systematically investigated the interfacial processes of the electron shuttling compound cysteine (Cys) on various facets of hematite and the reallocations of surface-bound As(III) or As(V) on the respective surfaces. Our results demonstrate that the ET process between Cys and hematite generates Fe(II) and leads to reductive dissolution, with more Fe(II) generated on {001} facets of exposed hematite nanoplates (HNPs). Reductive dissolution of hematite leads to significantly enhanced As(V) reallocations on hematite. Nevertheless, upon the addition of Cys, a raipd release of As(III) can be halted by its prompt re-adsorption, leaving the extent of As(III) immobilization on hematite unchanged throughout the course of reductive dissolution. This is due to that Fe(II) can form new precipitates with As(V), a process that is facet-dependent and influenced by water chemistry. Electrochemical analysis reveals that HNPs exhibit higher conductivity and ET ability, which is beneficial for reductive dissolution and As reallocations on hematite. These findings highlight the facet-dependent reallocations of As(III) and As(V) facilitated by electron shuttling compounds and have implications for the biogeochemical processes of As in soil and subsurface environments.

Potential use of nanoparticles produced from byproducts of drinking water industry in stabilizing arsenic in alkaline-contaminated soils

The stabilization of heavy metals in soils is considered a cost-effective and environmentally sustainable remediation approach. In the current study, the applicability of water treatment residual nanoparticles (nWTRs) with the particle size ranged from 45 to 96 nm was evaluated for its efficacy in reducing arsenic mobility in clayey and sandy contaminated alkaline soils. Sorption isotherms, kinetics, speciation and fractionation studies were performed. Sorption equilibrium and kinetics studies revealed that As sorption by nWTRs-amended soils followed Langmuir and second-order/power function models. The maximum As sorption capacity (qmax) of Langmuir increased up to 21- and 15-folds in clayey and sandy soils, respectively, as a result of nWTRs application at 0.3% rate. A drastic reduction in non-residual (NORS) As fraction from 80.2 and 51.49% to 11.25 and 14.42% for clayey and sandy soils, respectively, at 0.3% nWTRs application rate was observed, whereas residual (RS) As fraction in both studied soils strongly increased following nWTRs application. The decline in percentage of As mobile form (arsenious acid) in both soils after nWTRs application indicated the strong effect of nWTRs on As immobilization in contaminated soils. Furthermore, Fourier transmission infrared spectroscopy analysis suggested reaction mechanisms between As and the surfaces of amorphous Fe and Al oxides of nWTRs through OH groups. This study highlights the effective management approach of using nWTRs as soil amendment to stabilize As in contaminated alkaline soils.

Mobility and bioaccessibility of arsenic (As) bound to titanium dioxide (TiO2) water treatment residuals (WTRs)

This work systematically describes arsenic mobility and potential bioaccessibility of arsenic-enriched titanium dioxide water treatment residuals (TiO₂ WTRs) by employing a suite of wet chemical experiments and spectroscopic measurements. Specifically, Environmental Protection Agency (EPA) digestion method 3051a indicated <3% of total arsenic in the solid phase was released, and arsenic assessed by EPA method 1340 for bioaccessibility was below detection limits. A novel finding is while the arsenic appeared to be stable under highly acidic digestion conditions, it is in fact highly mobile when exposed to simple phosphate solutions. On average, 55% of arsenic was extracted from all samples during a 50-day replenishment study. This was equivalent to 169 mg kg^-1 arsenic released from the solid phase. Macroscopic desorption experiments indicated arsenic likely formed inner-sphere bonds with the TiO₂ particles present in the samples. This was confirmed with X-ray absorption spectroscopy (XAS), where an interatomic distance of 3.32 Å and a coordination number (CN) of 1.79 titanium atoms were determined. This translates to a configuration of arsenic on TiO₂ surfaces as a bidentate binuclear inner-sphere complex. Thus, both macroscopic and spectroscopic data are in agreement. During incubation experiments, arsenic(V) was actively reduced to arsenic(III); the amount of arsenic(III) in solution varied from 8 to 38% of total dissolved arsenic. Lastly, elevated concentrations and mobility of vanadium in these systems merit further investigation. The high mobility of arsenic and its potential for reduction when reintroduced into the environment, particularly in agriculturally important areas, presents an important risk when waste products are not properly managed.

Removal and environmentally safe disposal of As(III) and As(V)-loaded ferrihydrite/biosilica composites

Pure ferrihydrite and ferrihydrite-biosilica composite were synthesized and studied for the removal of As(III) and As(V). The synthesized materials have an adsorption capacity higher than some reported materials in the literature - 140 and 90 mg g^-1 for As(III) and As(V), respectively. The pH of the solution was shown to impact greatly on As(V) adsorption, but not on As (III), which is stable as a protonated, uncharged oxyanion, at pH < 9.2. The adsorption products were subjected to thermal treatment (500 °C for 2 h), promoting ferric arsenate formation. The adsorbed As on ferrihydrite (Fh) was shown to inhibit the phase transformation of Fh to hematite. More so, thermal treatment was shown to oxidize As(III) to As (V). The changes in the adsorption residues after thermal treatment also had an impact on As mobility. The As (III) associated with the Fh phase increased from 42 to 95%, according to a sequential extraction protocol. Therefore, this work presents a process for As removal, followed by thermal treatment of arsenic-loaded ferrihydrites which enables environmentally safe disposal of As residues.

Facile fabrication of Fe/Zr binary MOFs for arsenic removal in water: High capacity, fast kinetics and good reusability

A water-stable bimetallic Fe/Zr metal-organic framework [UiO-66(Fe/Zr)] for exceptional decontamination of arsenic in water was fabricated through a facile one-step strategy. The batch adsorption experiments revealed the excellent performances with ultrafast adsorption kinetics due to the synergistic effects of two functional centers and large surface area (498.33 m²/g). The absorption capacity of UiO-66(Fe/Zr) for arsenate [As(V)] and arsenite [As(III)] reached as high as 204.1 mg/g and 101.7 mg/g, respectively. Langmuir model was suitable to describe the adsorption behaviors of arsenic on UiO-66(Fe/Zr). The fast kinetics (adsorption equilibrium in 30 min, 10 mg/L As) and pseudo-second-order model implied the strong chemisorption between arsenic ions and UiO-66(Fe/Zr), which was further confirmed by DFT theoretical calculations. The results of FT-IR, XPS analysis and TCLP test demonstrated that arsenic was immobilized on the surface of UiO-66(Fe/Zr) through Fe/Zr-O-As bonds, and the leaching rates of the adsorbed As(III) and As(V) from the spent adsorbent were only 5.6% and 1.4%, respectively. UiO-66(Fe/Zr) can be regenerated for five cycles without obvious removal efficiency decrease. The original arsenic (1.0 mg/L) in lake and tap water was effectively removed in 2.0 hr [99.0% of As(III) and 99.8% of As(V)]. The bimetallic UiO-66(Fe/Zr) has great potentials in water deep purification of arsenic with fast kinetics and high capacity.

Comparative study for sorption of arsenic on peanut shell biochar and modified peanut shell biochar

In this study, arsenite [As(III)] and arsenate [As(V)] removal efficiency of peanut shell biochar (PSB) and modified peanut shell biochar (MPSB) was compared in aqueous solutions. The modification was carried out with KMnO₄ and KOH. Sorption efficiency of MPSB was relatively higher than PSB at pH 6 for As(III) (86%) and for As(V) (91.26%) for initial concentration of 1 mg/L, adsorbent dose of 0.5 g/L and 240 min equilibrium time at 100 rpm. Freundlich isotherm and pseudo-second order kinetic model suggested possible multilayer chemisorption. Fourier transform infrared spectroscopy showed that -OH, C-C, CC and C-O-C groups contributed significantly in adsorption for both PSB and MPSB. Thermodynamic study showed that the adsorption process was spontaneous and endothermic. Regeneration studies revealed that PSB and MPSB can be successfully used for three cycles. This study established that peanut shell is a low-cost, environment friendly and efficient biochar for removal of arsenic from water.

Chemical speciation determines combined cytotoxicity: Examples of biochar and arsenic/chromium

As both electron donors and acceptors, biochars (BCs) may interact with multivalent metal ions in the environment, causing changes in ionic valence states and resulting in unknown combined toxicity. Therefore, we systematically investigated the interaction between BCs and Cr (Cr(III) & Cr(VI)) or As (As(III) & As(V)) and their combined cytotoxicity in human colorectal mucosal (FHC) cells. Our results suggest that the redox-induced valence state change is a critical factor in the combined cytotoxicity of BCs with Cr/As. Specifically, when Cr(VI) was adsorbed on BCs, 86.4 % of Cr(VI) was reduced to Cr(III). In contrast, As(III) was partially oxidized to As(V) with a ratio of 37.2 %, thus reaching a reaction equilibrium. Meanwhile, only As(V) was released in the cell, which could cause more As(III) to be oxidized. As both Cr(III) and As(V) are less toxic than their corresponding counterparts Cr(VI) and As(III), different redox interactions between BCs and Cr/As and release profiles between BCs and Cr/As together lead to reduced combined cytotoxicity of BP-BC-Cr(VI) and BP-BC-As(III). It suggests that the valence state changes of metal ions due to redox effects is one of the parameters to be focused on when studying the combined toxicity of complexes of BCs with different heavy metal ions.

Nitrogen-doped carbon dots/Fe3+-based fluorescent probe for the "off-on" sensing of As(V) in seafood

To better satisfy the application of rapid detection methods in the detection of As(V) in complex food substrates, we developed an "off-on" fluorescence assay to detect As(V) based on the competition between the electron transfer effect of nitrogen-doped carbon dots (N-CDs)/Fe³⁺ and the complexation reaction of As(V)/Fe³⁺, using N-CDs/Fe³⁺ as a fluorescence probe. Solid-phase extraction (SPE) was used to eliminate matrix interference during sample pretreatment. The detection limit was 7.6 ng g^-1, with a linear range of 10-100 ng g^-1. The method was further used to determine As(V) in different seafood products including snapper, shrimp, clams, and kelp. At the same time, the recovery of the method was validated by high-performance liquid chromatography-inductively coupled plasma mass spectrometry (HPLC-ICP/MS), indicating that the developed method had good recoveries from 86% to 117% and met the needs for accurate determination of As(V). This approach has shown excellent application potential in the field of As(V) detection in various seafood products.

Mechanistic insights into the removal of As(III) and As(V) by iron modified carbon based materials with the aid of machine learning

The machine learning (ML) technique was used to examine the effects of different microscopic material features on the ability of iron modified carbon-based materials (Fe-CBMs) to remove As(V) and As(III). The findings showed that specific CBMs and Fe-CBMs features (such as surface functionality) from sophisticated microscopic and spectroscopic techniques led to models that were more accurate than those constructed using more basic information, such as bulk elemental composition and surface area (the root-mean-square error fell by 44.7% for As(V) and 56.9% for As(III), respectively). The high non-polar carbon (NPC) content of CBMs and Fe-CBMs had a detrimental influence on As(V) and As(III) removal capability, whereas surface oxygen-containing functional groups (SOFGs) contents on CBMs and Fe-CBMs played an essential role in arsenic removal based on ML approaches. The relative importance of CO was greater by 77.8% and 40.6% than that of C-O on the elimination of As(V) and As(III), respectively. The accurate ML models are helpful for the future design of Fe-CBMs and the relative importance and partial dependence plot analysis can direct the use of Fe-CBMs for arsenic removal in a sensible manner under different application situations.

Application of infrared spectroscopy and its theoretical simulation to arsenic adsorption processes

Accurate detection and analysis of arsenic pollutants are an important means to enhance the ability to manage arsenic pollution. Infrared (IR) spectroscopy technology has the advantages of fast analysis speed, high resolution, and high sensitivity and can be monitored by real-time in situ analysis. This paper reviews the application of IR spectroscopy in the qualitative and quantitative analysis of inorganic and organic arsenic acid adsorbed by major minerals such as ferrihydrite (FH), hematite, goethite, and titanium dioxide. The IR spectroscopy technique cannot only identify different arsenic contaminants but also obtain the content and adsorption rate of arsenic contaminants in the solid phase. The reaction equilibrium constants and the degree of reaction conversion can be determined by constructing adsorption isotherms or combining them with modeling techniques. Theoretical calculations of IR spectra of mineral adsorbed arsenic pollutant systems based on density functional theory (DFT) and analysis and comparison of the measured and theoretically calculated characteristic peaks of IR spectra can reveal the microscopic mechanism and surface chemical morphology of the arsenic adsorption process. This paper systematically summarizes the qualitative and quantitative studies and theoretical calculations of IR spectroscopy in inorganic and organic arsenic pollutant adsorption systems, which provides new insights for accurate detection and analysis of arsenic pollutants and arsenic pollution control. PRACTITIONER POINTS: This paper reviews the application of infrared spectroscopy in the qualitative and quantitative analyses of inorganic and organic arsenic acid adsorbed by major minerals such as ferrihydrite, hematite, goethite, and titanium dioxide, which can help identify and evaluate the type and concentration of arsenic pollutants in water bodies. In this paper, theoretical calculations of infrared spectra of mineral adsorbed arsenic pollutant systems based on density functional theory reveal the adsorption mechanism of arsenic pollutants in water at the solid-liquid interface and help to develop targeted arsenic pollution control technologies. This paper provides a new and reliable analytical detection technique for the study of arsenic contaminants in water bodies.

(Bio)dissolution of arsenopyrite coupled with multiple proportions of pyrite: Emphasis on the mobilization and existential state of arsenic

The formation of arsenic-bearing acid mine drainage (AMD) via the oxidation of arsenopyrite refuse ore has attracted significant attention. Pyrite, as main a concomitant mineral, is a crucial factor that affects the (bio)dissolution of arsenopyrite, but there are still some points on the detailed action mechanism under normal environmental conditions that need further study. In this study, the effect mechanism of pyrite with a systematic pyrite content (0, 10, 25, 50, 75, 90, and 100 wt %) on arsenopyrite oxidation and arsenic release in the presence of Acidithiobacillus ferrooxidans was investigated. The X-ray diffraction (XRD), scanning election microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and electrochemical analyses were also carried out. Results showed that the existence of pyrite and Acidithiobacillus ferrooxidans significantly accelerated the dissolution of arsenopyrite and the oxidation of As (Ⅲ) to As (Ⅴ), resulting from the galvanic effect, an increase in the Fe³⁺/Fe²⁺ ratio and the oxidation-reduction potential (Eh) value, and a decrease in pH level. As the detected main intermediate products, element sulphur was considered as the dominating obstructive factor during arsenopyrite oxidation, while the added pyrite could accelerate its oxidation. Moreover, a close relationship between different mineral proportions and the galvanic effect was also observed and discussed. Finally, suggestions on AMD governance and source control are proposed.