Predicting the efficiency of arsenic immobilization in soils by biochar using machine learning.

Arsenic (As) pollution in soils is a pervasive environmental issue. Biochar immobilization offers a promising solution for addressing soil As contamination. The efficiency of biochar in immobilizing As in soils primarily hinges on the characteristics of both the soil and the biochar. However, the influence of a specific property on As immobilization varies among different studies, and the development and application of arsenic passivation materials based on biochar often rely on empirical knowledge. To enhance immobilization efficiency and reduce labor and time costs, a machine learning (ML) model was employed to predict As immobilization efficiency before biochar application. In this study, we collected a dataset comprising 182 data points on As immobilization efficiency from 17 publications to construct three ML models. The results demonstrated that the random forest (RF) model outperformed gradient boost regression tree and support vector regression models in predictive performance. Relative importance analysis and partial dependence plots based on the RF model were conducted to identify the most crucial factors influencing As immobilization. These findings highlighted the significant roles of biochar application time and biochar pH in As immobilization efficiency in soils. Furthermore, the study revealed that Fe-modified biochar exhibited a substantial improvement in As immobilization. These insights can facilitate targeted biochar property design and optimization of biochar application conditions to enhance As immobilization efficiency.

Optimization of desorption parameters using response surface methodology for enhanced recovery of arsenic from spent reclaimable activated carbon: Eco-friendly and sorbent sustainability approach.

Desorption and adsorbent regeneration are imperative factors that are required to be taken into account when designing the adsorption system. From the environmental, economic, and practical points of view, regeneration is necessary for evaluating the efficiency and sustainability of synthesized adsorbents. However, no study has investigated the optimization of arsenic species desorption from spent adsorbents and their regeneration ability for reuse as well as safe disposal. This study aims to investigate the desorption ability of arsenic ions adsorbed on hybrid granular activated carbon and the optimization of the independent factors influencing the efficient recovery of arsenic species from the spent activated carbon using central composite design of the response surface methodology. The activated carbon before the sorption process and after the adsorption-desorption of arsenic ions have been characterized using SEM-EDX, FTIR, and TEM. The study found that all the investigated independent desorption variables greatly influence the retrievability of arsenic ions from the spent activated carbon. Using the desirability function for the optimization of the independent factors as a function of desorption efficiency, the optimum experimental conditions were solution pH of 2.00, eluent concentration of 0.10 M, and temperature of 26.63 ℃, which gave maximum arsenic ions recovery efficiency of 91 %. The validation of the quadratic model using laboratory confirmatory experiments gave an optimum arsenic ions desorption efficiency of 97 %. Therefore, the study reveals that the application of the central composite design of the response surface methodology led to the development of an accurate and valid quadratic model, which was utilized in the enhanced optimization of arsenic ions recovery from the spent reclaimable activated carbon. More so, the desorption isotherm and kinetic data of arsenic were well correlated with the Langmuir and the pseudo-second-order models, while the thermodynamics studies indicated that arsenic ions desorption process was feasible, endothermic, and spontaneous.

Nanoconfined Fe(II) releaser for long-term arsenic immobilization and its sustainability assessment.

Ferrous (Fe(II))-based oxygen activation for pollutant abatements in soil and groundwater has attracted great attention, while the low utilization and insufficient longevity of electron donors are the primary challenges to hinder its practical applications. Herein, we propose a nanoconfined Fe(II) releasing strategy that enables stable long-term electron donation for oxygen activation and efficient arsenic (As) immobilization under oxic conditions, by encapsulating zero-valent iron in biomass-derived carbon shell (ZVI@porous carbon composites; ZVI@PC). This strategy effectively enhances the generation of reactive oxygen species, enabling efficient oxidation and subsequent immobilization of As(III) in soils. Importantly, this Fe(II) releaser exhibits strong anti-interference capability against complex soil matrices, and the accompanying generation of Fe(III) enables As immobilization in soils, effectively lowering soil As bioavailability. Soil fixed-bed column experiments demonstrate a 79.5 % reduction of the total As in effluent with a simulated rainfall input for 10 years, indicating the excellent long-term stability for As immobilization in soil. Life cycle assessment results show that this Fe(II) releaser can substantially mitigate the negative environmental impacts. This work offers new insights into developing green and sustainable technologies for environmental remediation.

Pb/As simultaneous removal from soil leachate of Pb/Zn smelting sites by magnetic biochar.

Lead (Pb) and arsenic (As) contaminated soils, caused by Pb and zinc (Zn) smelting activities, pose an urgent environmental issue. Magnetic biochar (MB) has been regarded as an increasingly appealing candidate for the remediation of multi-metals in contaminated soils or their leachate. Finding economically feasible preparation methods for MB and demonstrating its remediation potential is desperately required for the remediation of such complex smelting sites. In this study, a modified MB was prepared using an optimized co-precipitation method, and its application potential for Pb/As simultaneous removal based on the basic properties of a typical Pb/Zn smelting site was evaluated. The surface modifications of MB facilitated the encapsulation of various ultrafine iron oxide particles, predominantly γ-Fe2O3 and Fe3O4, whilst notably enhancing the presence of oxygen-containing surface functional groups. The adsorption of Pb(II) and As(III) by MB was well-described using the pseudo-second-order adsorption and Langmuir models. The existence of SO42- and Ca2+ in the soil leachate competed with the adsorption sites for Pb(II) and As(III). Notably, within the pH range of 5-9, the adsorption efficiency of Pb(II) by MB increased with the rising solution pH, whereas alterations in pH minimally affected the removal rate of As(III), maintaining a consistent removal rate exceeding 95%. Furthermore, dissolved organic matter (DOM) abundant in organic functional groups, particularly CO and CC groups, significantly augmented the adsorption affinity for both Pb(II) and As(III). An application rate of 2 g/L could effectively reduce the concentration of Pb(II) and As(III) in soil leachate to <0.05 mg/L. The findings demonstrated the potential of the prepared MB for simultaneous removal of As(III) and Pb(II) in soil leachate, which should be beneficial to multi-metals polluted soil remediation in Pb/Zn smelting sites.

The effect of thermal treatment on the transformation and transportation of arsenic and cadmium in soil.

Thermal treatment can effectively decontaminate soils but alter their properties. Previous research mainly focused on volatile organic compounds and metals, i.e. Hg, neglecting non-volatile metal(loid)s. This study aimed to investigate Cd and As transformation during aerobic and anaerobic calcination. The results showed that both aerobic and anaerobic calcination increased soil pH by reducing soil organic matter (SOM) content, which also influenced the cation exchange capacity (CEC) and the leaching behavior of Cd and As in the soil. The total concentrations of Cd and As in the calcined soils varied depending on the calcination temperature and atmosphere. When the aerobic calcination temperature exceeded 700 °C, Cd volatilized as CdCl2, while anaerobic calcination at relatively low temperatures (600 °C) involved reductive reactions, resulting in the formation of metallic Cd with a lower boiling point. Similarly, As volatilized at 800 °C aerobically and 600 °C anaerobically. The formation of As-based minerals, particularly Ca3(AsO4)2, hindered its gasification, whereas anaerobic calcination promoted volatilization efficiency through the generation of C-As(III) based gaseous components with lower boiling points. Contrasting trends were observed in the TCLP-extractable Cd and As contents of the calcined soils. Over 70% of TCLP-extractable Cd contents were suppressed after thermal treatment, attributed to the elevated pH and reduced CEC of the soil, as well as volatilization. However, TCLP-extractable As contents increased with elevated temperatures, likely due to the desorption of AsO43- and re-adsorption of gaseous As2O3 during cooling. These findings have implications for assessing the environmental impact of thermal treatment and provide insights for remediation strategies concerning Cd and As-contaminated soils.

Microparticles of arsenic water treatment media and water softener resin observed in treated water at private wells.

Observation of arsenic water treatment adsorption media in the treated water of several homes with high arsenic private wells led to the hypothesis that treatment media was escaping the treatment systems and entering the plumbing and drinking water. Our research at 62 homes identified that microparticles of arsenic water treatment media and/or water softener resin had escaped the treatment system in 71% of the homes. This is a potential health hazard as ingesting arsenic treatment media or water softener resin may lead to an elevated ingestion exposure to arsenic and other contaminants. Potential causes of media escape from the treatment systems include media observed to be smaller in size than specifications and media breaking into smaller pieces. One interim solution to media escape is installation of a post-treatment sediment filter. New developments in media durability or treatment system design and maintenance may be needed to prevent media escaping into drinking water. PRACTITIONER POINTS: Arsenic in private wells is often treated with point-of-entry whole house adsorption systems. Arsenic adsorption treatment media and/or water softener resin was observed in treated water at 44 of 62 homes inspected. Water treatment media escaping into treated water is a potential hazardous exposure pathway. Potential causes and solutions are discussed.

Micro-nanobubble-assisted As(III) removal from water by Ni-doped MOF materials.

Micro-nanobubbles (MNBs) can form reactive oxygen species (ROS) with high oxidizing potential. In this study, nickel-doped metal-organic framework materials (MOFs) capable of activating molecular oxygen were synthesized using trivalent arsenic (As(III)) as a target pollutant and combined with peroxymonosulfate (PMS) to construct a MOF/MNB/PMS system. The results included the rapid oxidation of As(III), the successful absorption of oxidized As(V), and finally the efficient removal of As. The effects of pH, amount of PMS used, and preparation time of MNBs on the As removal performance of the MOF/MNB/PMS system were investigated experimentally. The changes in the properties of the materials before and after the reaction were analyzed by XPS, and it was found that the main active sites on the surface of the MOFs were the metal elements and the pyridine nitrogen near the carbon atom. The regular morphology and elemental composition of the MOFs were determined by TEM scanning and EDS test, which indicated the presence of nickel. XRD tests before and after the reaction showed that the MOFs were structurally stable. The results of the free radical burst experiments show that the single linear oxygen (1O2) is the main active substance in the system, and that the MNBs are key factors by which the system achieves efficient oxidation performance. In addition to providing a sustainable supply of molecular oxygen to the MOFs during the reaction process, coupling the MNBs with PMS was found to improve the oxidation capacity of the system. The results of this study thus provide a new concept for As removal and advanced oxidation in water bodies.

The redistribution process of As(â ¢) and Fe(â ¡) 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.

Anodic stripping voltammetry of arsenic determination with edible mushroom-nafion-modified glassy carbon electrode.

An edible Mushroom-Nafion modified glassy carbon electrode (M2N5-GCE) was prepared using a homogeneous mixture varying the concentrations of these, in addition to the origin of the mushroom (Shiitake, Lentinula edodes, M1 and Abrantes, Agariscus bisporus, M2) and applied to the As(III) determination by anodic stripping voltammetry. After choosing the optimal conditions in the preparation of the electrode, the second stage was to study the effects of various parameters such as supporting electrolyte, pH, accumulation potential, and time (Eacc, tacc). The optimum experimental conditions chosen were Britton Robinson buffer 0.01 mol L-1 pH:4.6; Eacc: -1.0 and tacc: 60 s obtaining a signal of oxidation of As(0) to As(III) about 0.08 V. Peak current was proportional to arsenic concentration over the 19.6-117.6 µg L-1 range, with a 3σ detection limit of 13.4 µg L-1. The method was validated using As(III) spiked tap water from the laboratory with satisfactory results (RE:3.0 %). Finally, the method was applied to the determination of As(III) in water samples from the Loa River (Northern Chile) in the presence of As(V) in a concentration >20 times higher (RE: 2.3 %).

Arsenite oxidation and adsorptive arsenic removal from contaminated water: a review.

Arsenic poisoning of groundwater is one of the most critical environmental hazards on Earth. Therefore, the practical and proper treatment of arsenic in water requires more attention to ensure safe drinking water. The World Health Organization (WHO) sets guidelines for 10 µg/L of arsenic in drinking water, and direct long-term exposure to arsenic in drinking water beyond this value causes severe health hazards to individuals. Numerous studies have confirmed the adverse effects of arsenic after long-term consumption of arsenic-contaminated water. Here, technologies for the remediation of arsenic from water are highlighted for the purpose of understanding the need for a single-point solution for the treatment of As(III)-contaminated water. As(III) species are neutral at neutral pH; the solution requires transformation technology for its complete removal. In this critical review, emphasis was placed on single-step technologies with multiple functions to remediate arsenic from water.

Dynamic performance of combined biochar from co-pyrolysis of pig manure with invasive weed: Effect of natural aging on Pb and As mobilization in polluted mining soil.

Due to the natural biochar aging, the improvement of soil quality and immobilization of soil pollutants achieved by biochar may change; understanding the dynamic evolution of the in situ performance of biochar in these roles is essential to discuss the long-term sustainability of biochar remediation. Therefore, in this study, combined biochar from co-pyrolysis of pig manure and invasive Japanese knotweed - P1J1, as well as pure pig manure - PM - and pure Japanese knotweed - JK - derived biochar were applied to investigate their remediation performance in a high As- and Pb-polluted soil with prolonged incubation periods (up to 360 days). Biochar application, especially P1J1 and PM, initially promoted soil pH, dissolved organic carbon, and EC, but the improvements were not constant through time. The JK-treated soil exhibited the highest increase of soil organic matter (OM), followed by P1J1 and then PM, and OM did not change with aging. Biochar, especially P1J1, was a comprehensive nutrient source of Ca, K, Mg, and P to improve soil fertility. However, while soluble cationic Ca, K, and Mg increased with time, anionic P decreased over time, indicating that continuous P availability might not be guaranteed with the aging process. The total microorganism content declined with time; adding biochars slowed down this tendency, which was more remarkable at the later incubation stage. Biochar significantly impeded soil Pb mobility but mobilized soil As, especially in PM- and P1J1-treated soils. However, mobilized As gradually re-fixed in the long run; meanwhile, the excellent Pb immobilization achieved by biochars was slightly reduced with time. The findings of this study offer fresh insights into the alterations in metal(loid)s mobility over an extended duration, suggesting that the potential mobilization risk of As is reduced while Pb mobility slightly increases over time.

AsIII Selectively Induces a Disorder-to-Order Transition in the Metalloid Binding Region of the AfArsR Protein.

Arsenic is highly toxic and a significant threat to human health, but certain bacteria have developed defense mechanisms initiated by AsIII binding to AsIII-sensing proteins of the ArsR family. The transcriptional regulator AfArsR responds to AsIII and SbIII by coordinating the metalloids with three cysteines, located in a short sequence of the same monomer chain. Here, we characterize the binding of AsIII and HgII to a model peptide encompassing this fragment of the protein via solution equilibrium and spectroscopic/spectrometric techniques (pH potentiometry, UV, CD, NMR, PAC, EXAFS, and ESI-MS) combined with DFT calculations and MD simulations. Coordination of AsIII changes the peptide structure from a random-coil to a well-defined structure of the complex. A trigonal pyramidal AsS3 binding site is formed with almost exactly the same structure as observed in the crystal structure of the native protein, implying that the peptide possesses all of the features required to mimic the AsIII recognition and response selectivity of AfArsR. Contrary to this, binding of HgII to the peptide does not lead to a well-defined structure of the peptide, and the atoms near the metal binding site are displaced and reoriented in the HgII model. Our model study suggests that structural organization of the metal site by the inducer ion is a key element in the mechanism of the metalloid-selective recognition of this protein.

Systematic evaluation of methods for iron-impregnation of biochar and effects on arsenic in flooded soils.

There is a need for innovative strategies to decrease the mobility of metal(loids) including arsenic (As) and cadmium (Cd) in agricultural soils, including rice paddies, so as to minimize dietary exposure to these toxic elements. Iron (Fe)-modified biochars (FBCs) are used to immobilize As and Cd in soil-water systems, but there is a lack of clarity on optimal methods for preparing FBCs because there are only limited studies that directly compare BCs impregnated with Fe under different conditions. There is also a lack of information on the long-term performance of FBCs in flooded soil environments, where reductive dissolution of Fe (oxy)hydroxide phases loaded onto biochar surfaces may decrease the effectiveness of FBCs. This study uses material characterization methods including FTIR, SEM-EDX, BET, and adsorption isotherm experiments to investigate the effects of Fe-impregnation methods (pH, pyrolysis sequence, and sonication) on the morphology and mineralogy of Fe loaded onto the biochar surface, and to FBC adsorbent properties for arsenate (As(V)), arsenite (As(III)), and Cd. Acidic impregnation conditions favored the adsorption of As(III) onto amorphous Fe phases that were evenly distributed on the biochar surface, including within the biochar pore structure. The combination of sonication with acidic Fe-impregnation conditions led to the best adsorption capacities for As(V) and As(III) (4830 and 11,166 µg As g-1 biochar, respectively). Alkaline Fe-impregnation conditions led to the highest Cd adsorption capacity of 3054 µg Cd g-1 biochar, but had poor effectiveness as an As adsorbent. Amending soil with 5% (w/w) of an acid-impregnated and sonicated FBC was more effective than an alkaline-impregnated FBC or ferrihydrite in decreasing porewater As concentrations. The acid-impregnated FBC also had greater longevity, decreasing As by 54% and 56% in two flooded phases, probably due to the greater stability of Fe(III) within the biochar pore structure that may have a direct chemical bond to the biochar surface. This study demonstrates that FBCs can be designed with selectivity towards different As species or Cd and that they can maintain their effectiveness under anaerobic soil conditions. This is the first study to systematically test how impregnation conditions affect the stability of FBCs in soils under multiple drying-rewetting cycles.

Bioaccessibility and human health risks of arsenic from geological origin in lateritic red soil on construction land.

Current human health risk assessments of soil arsenic (As) contamination rarely consider bioaccessibility (IVBA), which may overestimate the health risks of soil As. The IVBA of As (As-IVBA) may differ among various soil types. This investigation of As-IVBA focused As from geological origin in a typical subtropical soil, lateritic red soil, and its risk control values. The study used the SBRC gastric phase in vitro digestion method and As speciation sequential extraction based upon phosphorus speciation extraction method. Two construction land sites (CH and HD sites) in the Pearl River Delta region were surveyed. The results revealed a high content of residual As (including scorodite, mansfieldite, orpiment, realgar, and aluminum arsenite) in the lateritic red soils at both sites (CH: 84.9%, HD: 91.7%). The content of adsorbed aluminum arsenate (CH: 3.24%, HD: 0.228%), adsorbed ferrum arsenate (CH: 8.55%, HD: 5.01%), and calcium arsenate (CH: 7.33%, HD: 3.01%) were found to be low. The bioaccessible As content was significantly positively correlated with the As content in adsorbed aluminum arsenate, adsorbed ferrum arsenate, and calcium arsenate. A small portion of these sequential extractable As speciation could be absorbed by the human body (CH: 14.9%, HD: 3.16%), posing a certain health risk. Adsorbed aluminum arsenate had the highest IVBA, followed by calcium arsenate, and adsorbed ferrum arsenate had the lowest IVBA. The aforementioned speciation characteristics of As from geological origin in lateritic red soil contributed to its lower IVBA compared to other soils. The oxidation state of As did not significantly affect As-IVBA. Based on As-IVBA, the carcinogenic and non-carcinogenic risks of soil As in the CH and HD sites decreased greatly in human health risk assessment. The results suggest that As-IVBA in lateritic red soil should be considered when assessing human health risks on construction land.

Progress of photocatalytic oxidation-adsorption synergistic removal of organic arsenic in water.

Photocatalytic oxidation-adsorption synergistic treatment of organic arsenic pollutants is a promising wastewater treatment technology, which not only degrades organic arsenic pollutants by photocatalytic degradation but also removes the generated inorganic arsenic by adsorption. This paper compares the results of photocatalytic oxidation-adsorption co-treatment of organic arsenic pollutants such as monomethylarsonic acid, dimethylarsinic acid, phenylarsonic acid, p-arsanilic acid, and 3-nitro-4-hydroxyphenylarsonic acid on titanium dioxide, goethite, zinc oxide, and copper oxide. It examines the influence of the morphology of organic arsenic molecules, pH, coexisting ions, and the role of natural organic matter. The photocatalytic oxidation-adsorption co-treatment mechanism is investigated, comparing the hydroxyl radical oxidation mechanism, the hydroxyl radical and superoxide anion radical cooxidation mechanism, and the hydroxyl radical and hole cooxidation mechanism. Finally, the future prospects of metal oxide photocatalytic materials and the development of robust and efficient technologies for removing organic arsenic are envisioned.

Synthesis of novel magnetic hydroxyapatite-biomass nanocomposite for arsenic and fluoride adsorption.

A magnetic nanocomposite of hydroxyapatite and biomass (HAp-CM) was synthesized through a combined ultrasonic and hydrothermal method, aiming for efficient adsorption of arsenic (As) and fluoride (F-) from drinking water in natural environments. The characterization of HAp-CM was carried out using TG, FTIR, XRD, SEM, SEM-EDS, and TEM techniques, along with the determination of pHpzc charge. FTIR analysis suggested that coordinating links are the main interactions that allow the formation of the nanocomposite. XRD data indicated that the crystalline structure of the constituent materials remained unaffected during the formation of HAp-CM. SEM-EDS analysis revelated a Ca/P molar ratio of 1.78. Adsorption assays conducted in batches demonstrated that As and F- followed a PSO kinetic model. Furthermore, As adsorption fitting well to the Langmuir model, while F- adsorption could be explained by both Langmuir and Freundlich models. The maximum adsorption capacity of HAp-CM was found to be 5.0 mg g-1 for As and 10.2 mg g-1 for F-. The influence of sorbent dosage, pH, and the presence of coexisting species on adsorption capacity was explored. The pH significantly affected the nanocomposite's efficiency in removing both pollutants. The presence of various coexisting species had different effects on F- removal efficiency, while As adsorption efficiency was generally enhanced, except in the case of PO43-. The competitive adsorption between F- and As on HAp-CM was also examined. The achieved results demonstrate that HAp-CM has great potential for use in a natural environment, particularly in groundwater remediation as a preliminary treatment for water consumption.

Data mining of arsenic-based small molecules geometrics present in Cambridge structural database.

Arsenic, ubiquitous in various industrial processes and consumer products, presents both essential functions and considerable toxicity risks, driving extensive research into safer applications. Our investigation, drawing from 7182 arsenic-containing molecules in the Cambridge Structural Database (CSD), outlines their diverse bonding patterns. Notably, 51% of these molecules exhibit cyclic connections, while 49% display acyclic ones. Arsenic forms eight distinct bonding types with other elements, with significant interactions observed, particularly with phenyl rings, O3 and F6 moieties. Top interactions involve carbon, nitrogen, oxygen, fluorine, sulfur, and arsenic itself. We meticulously evaluated average bond lengths under three conditions: without an R-factor cut-off, with R-factor ≤0.075, and with R-factor ≤0.05, supporting the credibility of our results. Comparative analysis with existing literature data enriches our understanding of arsenic's bonding behaviour. Our findings illuminate the structural attributes, molecular coordination, geometry, and bond lengths of arsenic with 68 diverse atoms, enriching our comprehension of arsenic chemistry. These revelations not only offer a pathway for crafting innovative and safer arsenic-based compounds but also foster the evolution of arsenic detoxification mechanisms, tackling pivotal health and environmental challenges linked to arsenic exposure across different contexts.

Mechanism of efficient adsorption for arsenic in aqueous solution by zeolitic imidazolate framework­8.

Arsenic (As) is one extremely hazardous and carcinogenic metalloid element. Due to mining, metal smelting, and other human activities, the pollution of water (especially groundwater) and soil caused by As is increasingly serious, which badly threatens the environment and human health. In this study, a zeolite imidazolate framework (ZIF-8) was synthesized at room temperature and employed as an adsorbent to facilitate the adsorption of As(III) and As(V) from the solution. The successful synthesis of ZIF-8 was demonstrated by X-ray diffraction (XRD), and scanning electron microscopy (SEM) revealed that its particle size was approximately 80 nm. The adsorption kinetics, adsorption isotherm, solution pH, dose, coexisting ions, and the synonymous elements antimony (Sb) were conducted to study the adsorption of As by ZIF-8 nanoparticles. The maximum saturation adsorption capacity was determined to be 101.47 mg/g and 81.40 mg/g for As(III), and As(V) at initial pH = 7.0, respectively. Apparently, ZIF-8 had a good removal effect on As, and it still maintained a good performance after four cycles. The coexisting ions PO43- and CO32- inhibited the adsorption of both As(III) and As(V). ZIF-8 performed well in removing both As and Sb simultaneously, although the presence of Sb hindered the adsorption of both As(III) and As(V). Both FTIR and XPS indicated the adsorption mechanism of As on ZIF-8: ZIF-8 generates a large amount of Zn-OH on the surface through hydrolysis and partial fracture of Zn-N, both of which form surface complexes with As.

Extracellular polymeric substances altered ferrihydrite (trans)formation and induced arsenic mobilization.

The behavior of As is closely related to trans(formation) of ferrihydrite, which often coprecipitates with extracellular polymeric substances (EPS), forming EPS-mineral aggregates in natural environments. While the effect of EPS on ferrihydrite properity, mineralogy reductive transformation, and associated As fate in sulfate-reducing bacteria (SRB)-rich environments remains unclear. In this research, ferrihydrite-EPS aggregates were synthesized and batch experiments combined with spectroscopic, microscopic, and geochemical analyses were conducted to address these knowledge gaps. Results indicated that EPS blocked micropores in ferrihydrite, and altered mineral surface area and susceptibility. Although EPS enhanced Fe(III) reduction, it retarded ferrihydrite transformation to magnetite by inhibiting Fe atom exchange in systems with low SO42-. As a result, 16% of the ferrihydrite was converted into magnetite in the Fh-0.3 treatment, and no ferrihydrite transformation occurred in the Fh-EPS-0.3 treatment. In systems with high SO42-, however, EPS promoted mackinawite formation and increased As mobilization into the solution. Additionally, the coprecipitated EPS facilitated As(V) reduction to more mobilized As(III) and decreased conversion of As into the residual phase, enhancing the potential risk of As contamination. These findings advance our understanding on biogeochemistry of elements Fe, S, and As and are helpful for accurate prediction of As behavior.

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.