Arsenate co-precipitation with Fe(II) oxidation products and retention or release during precipitate aging

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

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

Enhanced Arsenate Immobilization by Kaolinite via Heterogeneous Pathways during Ferrous Iron Oxidation

Clay minerals are ubiquitous in subsurface environments and have long been recognized as having a limited or negligible impact on the fate of arsenic (As) due to their negatively charged surfaces. Here, we demonstrate the significant role of kaolinite (Kln), a pervasive clay mineral, in enhancing As(V) immobilization during ferrous iron (Fe(II)) oxidation at near-neutral pH. Our results showed that Fe(II) oxidation alone was not capable of immobilizing As(V) at relatively low Fe/As molar ratios (≤2) due to the generation of Fe(III)-As(V) nanocolloids that could still migrate easily as truly dissolved As did. In the presence of kaolinite, dissolved As(V) was significantly immobilized on the kaolinite surfaces via forming Kln-Fe(III)-As(V) ternary precipitates, which had large sizes (at micrometer levels) to reduce the As mobility. The kaolinite-induced heterogeneous pathways for As(V) immobilization involved Fe(II) adsorption, heterogeneous oxidation of adsorbed Fe(II), and finally heterogeneous nucleation/precipitation of Fe(III)-As(V) phases on the edge surfaces of kaolinite. The surface precipitates were mixtures of amorphous basic Fe(III)-arsenate and As-rich hydrous ferric oxide. Our findings provide new insights into the role of clay minerals in As transformation, which is significant for the fate of As in natural and engineered systems.

Performance and mechanism of La-Fe metal-organic framework as a highly efficient adsorbent for fluoride removal from mine water

Water fluoride pollution has caused non-negligible harm to the environment and humans, and thus it is crucial to find a suitable treatment technology. In this study, La-Fe@PTA adsorbent was synthesized for the defluoridation of mine water. The results showed that the optimum conditions for defluoridation by La-Fe@PTA were pH close to 7.0, the initial F- concentration of 10 mg/L, the dosage of 0.5 g/L and the adsorption time of 240 min. Compared with SO42‒, Cl‒, NO3‒, Ca2+ and Mg2+, CO32‒ and HCO3‒ presented severer inhibition on fluoride uptake by La-Fe@PTA. The adsorption process fits well with the pseudo-second-order kinetic model and Freundlich model, and the maximum adsorption capacity of Langmuir model was 95 mg/g. Fixed-bed adsorption results indicated that fluoride in practical fluorinated mine water could be effectively removed from 3.6 mg/L to less than 1.5 mg/L within 130 bed volume (BV) by using 1.5 g La-Fe@PTA. Furthermore, the adsorbent still had good adsorption capacity after regeneration, which confirms the great application potential of La-Fe@PTA as a fluoride ion adsorbent. The mechanism analysis showed that La-Fe@PTA adsorption of fluorine ions is a physicochemical reaction driven by electrostatic attraction and ion exchange.

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.

Floc aging: Crystallization and improving low molecular weight organic removal in re-coagulation

Iron coagulants have been used extensively in drinking water treatment. This typically produces substantial quantities of insoluble iron hydrolysis products which interact with natural and anthropogenic organic substances during the coagulation process. Previous studies have shown that the removal of low molecular weight (MW) organics is relatively poor by coagulation, which leads to their presence during disinfection, with the formation of halogenated byproducts, and in treated water supplies as potentially biodegradable material. Currently, there is little knowledge about the changes that occur in the nature of coagulant flocs as they age with time and how such changes affect interactions with organic matter, especially low MW organics. To improve this deficiency, this study has investigated the variation of aged flocs obtained from two commonly used iron salts and their impact on representative organic contaminants, natural organic matter (NOM) and tetracycline antibiotic (TC), in a real surface water. It was found that aging resulted in increasing crystallization of the flocs, which can play a beneficial role in activating persulfate oxidant to remove the representative organics. Furthermore, acidification was also found to further improve the removal of low MW natural organics and tetracycline. In addition, the results showed that the low MW fractions of NOM (<1 K Dalton) were substantially removed by the aging flocs. These results are in marked contrast to the poor removal of low MW organic substances by conventional coagulation, with or without added oxidants, and show that aged flocs have a high potential of reuse for re-coagulation and activation of oxidants to reduce low MW organics, and enhance drinking water quality.

Atomic-Level Structural Differences between Fe(III) Coprecipitates Generated by the Addition of Fe(III) Coagulants and by the Oxidation of Fe(II) Coagulants Determine Their Coagulation Behavior in Phosphate and DOM Removal

In situ Fe(III) coprecipitation from Fe2+ oxidation is a widespread phenomenon in natural environments and water treatment processes. Studies have shown the superiority of in situ Fe(III) (formed by in situ oxidation of a Fe(II) coagulant) over ex situ Fe(III) (using a Fe(III) coagulant directly) in coagulation, but the reasons remain unclear due to the uncertain nature of amorphous structures. Here, we utilized an in situ Fe(III) coagulation process, oxidizing the Fe(II) coagulant by potassium permanganate (KMnO4), to treat phosphate-containing surface water and analyzed differences between in situ and ex situ Fe(III) coagulation in phosphate removal, dissolved organic matter (DOM) removal, and floc growth. Compared to ex situ Fe(III), flocs formed by the natural oxidizing Fe2+ coagulant exhibited more effective phosphate removal. Furthermore, in situ Fe(III) formed through accelerated oxidation by KMnO4 demonstrated improved flocculation behavior and enhanced removal of specific types of DOM by forming a more stable structure while still maintaining effective phosphate removal. Fe K-edge extended X-ray absorption fine structure spectra (EXAFS) of the flocs explained their differences. A short-range ordered strengite-like structure (corner-linked PO4 tetrahedra to FeO6 octahedra) was the key to more effective phosphorus removal of in situ Fe(III) than ex situ Fe(III) and was well preserved when KMnO4 accelerated in situ Fe(III) formation. Conversely, KMnO4 significantly inhibited the edge and corner coordination between FeO6 octahedra and altered the floc-chain-forming behavior by accelerating hydrolysis, resulting in a more dispersed monomeric structure than ex situ Fe(III). This research provides an explanation for the superiority of in situ Fe(III) in phosphorus removal and highlights the importance of atomic-level structural differences between ex situ and in situ Fe(III) coprecipitates in water treatment.

Practical approach of As(V) adsorption by fabricating biochar with low basicity from FeCl3 and lignin

One of the main challenges of biochar application for environmental cleanup is rise of pH in water or soil due to high ash and alkali metal contents in the biochar. While this intrinsic property of biochar is advantageous in alleviating soil and water acidity, it severely impairs the affinity of biochar toward anionic contaminants such as arsenic. This study explored a technical approach that can reduce the basicity of lignin-based biochar by utilizing FeCl₃ during production of biochar. Three types of biochar were produced by co-pyrolyzing feedstock composed of different combinations of lignin, red mud (RM), and FeCl₃, and the produced biochar samples were applied to adsorption of As(V). The biochar samples commonly possessed porous carbon structure embedded with magnetite (Fe₃O₄) particles. The addition of FeCl₃ in the pyrolysis feedstock had a notable effect on reducing basicity of the biochar to yield significantly lower solution pH values than the biochar produced without FeCl₃ addition. The extent of As(V) removal was also closely related to the final solution pH and the greatest As(V) removal (>77.6%) was observed for the biochar produced from co-pyrolysis of lignin, RM, and FeCl₃. The results of adsorption kinetics and isotherm experiments, along with x-ray spectroscopy (XPS), strongly suggested adsorption of As(V) occurred via specific chemical reaction (chemisorption) between As(V) and Fe-O functional groups on magnetite. Thus, the overall results suggest the use of FeCl₃ is a feasible practical approach to control the intrinsic pH of biochar and impart additional functionality that enables effective treatment of As(V).

Application of a novel Ca-Fe-Si-S composite for the synchronous stabilization of As, Zn, Cu, and Cd in acidic arsenic slag

The control of multiple heavy metals (HMs) pollution in solid wastes, especially the co-contamination of As and other heavy metal cations, is of great importance to ecological and environmental health. To address this problem, the preparation and application of multifunctional materials have attracted wide attention. In this work, a novel Ca-Fe-Si-S composite (CFSS) was applied to stabilize As, Zn, Cu, and Cd in acid arsenic slag (ASS). The CFSS exhibited synchronous stabilization ability for As, Zn, Cu, Cd and owned strong acid neutralization capacity. Under simulated field conditions, the acid rain extracted HMs in ASS successfully decreased below the emission standard (GB 3838-2002-IV category in China) after incubated by 5% CFSS for 90 days. Meanwhile, the application of CFSS promoted the transformation of leachable HMs into less accessible forms, which was conductive to the long-term stabilization for HMs. There was competitive relation among the three heavy metal cations, following the stabilization sequence of Cu > Zn > Cd during incubation. And the stabilization mechanisms of HMs by CFSS were proposed as chemical precipitation, surface complexation, and ion/anion exchange. The research will be greatly conducive to the remediation and governance of field multiple HMs contaminated sites.

Improvement of the Stability of IO3--, SeO32--, and SeO42--Coprecipitated Barite after Treatment with Phosphate

Coprecipitation of radionuclides with barite has been studied to remove radionuclides from radioactive liquid waste because of its excellent removal efficiency; however, little information exists concerning the stability of the ions coprecipitated with barite. This study systematically investigated the stability of iodate, selenite, and selenate coprecipitated with barite via leaching tests. These oxyanions were gradually leached from the oxyanion-bearing barite into ultrapure water over time. Leaching of the oxyanions significantly increased in leaching solutions containing NaCl (pH 5.3), NaNO₃ (pH 5.9), and Na₂SO₄ (pH 5.7). Conversely, leaching of the oxyanions was suppressed in KH₂PO₄ solution (pH 8.5), indicating that phosphate stabilized the oxyanion-bearing barite. The effect of phosphate treatment on oxyanion-bearing barite was further investigated. The results showed that the barite surface was modified with phosphate, and a thin surface layer of a barium phosphate-like structure was formed. The amount of oxyanions leached from the phosphate-treated samples into leaching solutions containing NaCl or NaNO₃ was much lower than the amounts leached from the untreated barite samples into ultrapure water. The barite coprecipitation combined with subsequent phosphate treatment may be a promising method to efficiently remove iodate, selenite, and selenate from wastewater and stabilize them as barite coprecipitates.

Highly efficient removal of arsenate and arsenite with potassium ferrate: role of in situ formed ferric nanoparticle

It is well known the capacity of potassium ferrate (Fe(VI)) for the oxidation of pollutants or co-precipitation and adsorption of hazardous species. However, little information has been paid on the adsorption and co-precipitation contribution of the Fe(VI) resultant nanoparticles, the in situ hydrolytic ferric iron oxides. Here, the removal of arsenate (As(V)) and arsenite (As(III)) by Fe(VI) was investigated, which focused on the interaction mechanisms of Fe(VI) with arsenic, especially in the contribution of the co-precipitation and adsorption of its hydrolytic ferric iron oxides. pH and Fe(VI) played significant roles on arsenic removal; over 97.8% and 98.1% of As(V) and As(III) removal were observed when Fe(VI):As(V) and Fe(VI):As(III) were 24:1 and 16:1 at pH 4, respectively. The removal of As(V) and As(III) by in situ and ex situ formed hydrolytic ferric iron oxides was examined respectively. The results revealed that As(III) was oxidized by Fe(VI) to As(V), and then was removed though co-precipitation and adsorption by the hydrolytic ferric iron oxides with the contribution content was about 1:3. For As(V), it could be removed directly by the in situ formed particles from Fe(VI) through co-precipitation and adsorption with the contribution content was about 1:1.5. By comparison, As(III) and As(V) were mainly removed through adsorption by the 30-min hydrolytic ferric iron oxides during the ex situ process. The hydrolytic ferric iron oxides size was obviously different in the process of in situ and ex situ, possessing abundant and multiple morphological structures ferric oxides, which was conducive for the efficient removal of arsenic. This study would provide a new perspective for understanding the potential of Fe(VI) treatment on arsenic control.

Research Progress on Adsorption of Arsenic from Water by Modified Biochar and Its Mechanism: A Review

Arsenic (As) is a non-metallic element, which is widely distributed in nature. Due to its toxicity, arsenic is seriously harmful to human health and the environment. Therefore, it is particularly important to effectively remove arsenic from water. Biochar is a carbon-rich adsorption material with advantages such as large specific surface area, high porosity, and abundant functional groups, but the original biochar has limitations in application, such as limited adsorption capacity and adsorption range. The modified biochar materials have largely enhanced the adsorption capacity of As in water due to their improved physicochemical properties. In this review, the changes in the physicochemical properties of biochar before and after modification were compared by SEM, XRD, XPS, FT-IR, TG, and other characterization techniques. Through the analysis, it was found that the adsorbent dosage and pH are the major factors that influence the As adsorption capacity of the modified biochar. The adsorption process of As by biochar is endothermic, and increasing the reaction temperature is conducive to the progress of adsorption. Results showed that the main mechanisms include complexation, electrostatic interaction, and precipitation for the As removal by the modified biochar. Research in the field of biochar is progressing rapidly, with numerous achievements and new types of biochar-based materials prepared with super-strong adsorption capacity for As. There is still much space for in-depth research in this field. Therefore, the future research interests and applications are put forward in this review.

Anoxic storage to promote arsenic removal with groundwater-native iron

Storage containers are usually used to provide a constant water head in decentralized, community groundwater treatment systems for the removal of iron (Fe) and arsenic (As). However, the commonly practiced aeration prior to storage assists in rapid and complete Fe²⁺ oxidation, resulting in poor As removal, despite sufficient native-Fe²⁺ in the source water. In this study, it was found that application of anoxic storage enhanced As removal from groundwater, containing ≥300 µg/L of As(III) and 2.33 mg/L of Fe²⁺ in an As affected village of Rajshahi district in Bangladesh. Although the oxidation of Fe²⁺ and As(III) during oxic storage was considerably faster, the As/Fe removal ratio was higher during anoxic storage (61-80±5 µgAs/mgFe) compared to the oxic storage (45±5 µgAs/mgFe). This higher As removal efficacy in anoxic storage containers could not be attributed to the speciation of As, since As(V) concentrations were higher during oxic storage due to more favorable abiotic (As(III) oxidation by O₂ and Fenton-like intermediates) and biotic (As(III) oxidizing bacteria, e.g., Sideroxydans, Gallionella, Hydrogenophaga) conditions. The continuous, in-situ hydrous ferric oxide floc formation during flow-through operation, and the favorable lower pH aiding higher sorption capacities for the gradually formed As(V) likely contributed to the improved performance in the anoxic storage containers.

Usage of Si, P, Se, and Ca Decrease Arsenic Concentration/Toxicity in Rice, a Review

Rice is one of the most important routes for arsenic to enter the human food chain and threatens more than half of the world’s population. In addition, arsenic-contaminated soils and waters increase the concentration of this element in various tissues of rice plants. Thus, direct or indirect—infecting livestock and poultry—increase diseases such as respiratory diseases, gastrointestinal tract, liver, and cardiovascular diseases, cancer, and ultimately death in the long term. Therefore, finding different ways to reduce the uptake and transfer of arsenic by rice would reduce the contamination of rice plants with this dangerous element and improve animal and human nutrition and ultimately disease and mortality. In this article, we aim to take a small step in improving sustainable life on earth by referring to the various methods that researchers have taken to reduce rice contamination by arsenic in recent years. Adding micronutrients and macronutrients as fertilizer for rice is one way to improve this plant’s growth and health. In this study, by examining two types of macronutrients and two types of micronutrients, their role in reducing arsenic toxicity and absorption was investigated. Therefore, both calcium and phosphorus were selected from the macronutrients, and selenium and silicon were selected from the micronutrients, whose roles in previous studies had been investigated.

Efficient removal of arsenite by a composite of amino modified silica supported MnO2/Fe-Al hydroxide (SNMFA) prepared from biotite

Developing materials from natural minerals to efficiently remove arsenite (As(Ⅲ)) from solution is vital important for resources comprehensive utilization and environment protection. In this study, biotite containing minerals was used to prepare a novel composite of amino modified silica supported MnO₂/Fe-Al hydroxide (SNMFA composite), which was then applied to remove arsenite. Scanning electron microscope (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) results indicated that many amorphous MnO₂ and Fe-Al hydroxide nano sheets were loaded on the surface of layered silica structure. Batch experiments showed that this composite could efficiently remove As(Ⅲ) from aqueous solution, and the maximal removal capacity was identified as 46.11 mg/g. As(Ⅲ) adsorption behaviours of SNMFA composite were confirmed by the pseudo-second-order kinetic model and Langmuir model, indicating that As(Ⅲ) adsorption on its surface was monolayer adsorption. The adsorption process was a pH and temperature dependent process, and increasing pH and temperature have facilitated the removal of As(Ⅲ). Thermodynamic analysis showed that As(Ⅲ) adsorption process was a spontaneous endothermic reaction. The As(Ⅲ) removal was mainly relied on the stable inner-sphere coordination model, and the corresponding mechanisms were involved in chelation, precipitation, oxidation-adsorption and electrostatic interaction.

Rice husk-derived biochar can aggravate arsenic mobility in ferrous-rich groundwater during oxygenation

Elevated As(III) and Fe(II) in shallow reducing groundwater can be frequently re-oxidized by introducing O₂ due to natural/anthropogenic processes, thus leading to oxidative precipitation of As as well as Fe. Nevertheless, the geochemical process may be impacted by co-existing engineered black carbon due to its considerable applications, which remains poorly understood. Taking rice husk-derived biochar prepared at 500 °C as an example, we explored its impact on the process particularly for the As(III) oxidation and (im)mobilization during the oxygenation. The presence of the biochar had a negligible effect on the As(III) oxidation and immobilization extents within 1 d, while accelerating their rates. However, the immobilized As(III) was significantly liberated from the formed Fe(III) minerals afterward within 21 d, which was 2.2-fold higher than that in the absence of the biochar. The enhanced As(III) liberation was attributed to the presence of the surface silicon-carbon structure, consisting of the outer silicon and inner carbon layers, of the rice husk-derived biochar. The outer silicon components, particularly for the dissolved silicate primarily promoted the As(III) release via ligand exchange, while significantly impeding the transformation of ferrihydrite to lepidocrocite and goethite still resulted secondarily in the As(III) release. Our findings reveal the possible impact of biochar on the environmental behavior and fate of As(III) in the Fe(II)-rich groundwater during the oxygenation. This work highlights that biochar, particularly for its structural features should be a concern in re-mobilizing As in such scenarios when the oxygenation time reaches several days or weeks.

Arsenic removal from iron-containing groundwater by delayed aeration in dual-media sand filters

Generally, abstracted groundwater is aerated, leading to iron (Fe²⁺) oxidation to Fe³⁺ and precipitation as Fe³⁺-(hydr)oxide (HFO) flocs. This practice of passive groundwater treatment, however, is not considered a barrier for arsenic (As), as removal efficiencies vary widely (15-95%), depending on Fe/As ratio. This study hypothesizes that full utilization of the adsorption capacity of groundwater native-Fe²⁺ based HFO flocs is hampered by rapid Fe²⁺ oxidation-precipitation during aeration before or after storage. Therefore, delaying Fe²⁺ oxidation by the introduction of an anoxic storage step before aeration-filtration was investigated for As(III) oxidation and removal in Rajshahi (Bangladesh) with natural groundwater containing 329(±0.05) µgAs/L. The results indicated that As(III) oxidation in the oxic storage was higher with complete and rapid Fe²⁺ oxidation (2±0.01 mg/L) than in the anoxic storage system, where Fe²⁺ oxidation was partial (1.03±0.32 mg/L), but the oxidized As(V)/Fe removal ratio was comparatively higher for the anoxic storage system. The low pH (6.9) and dissolved oxygen (DO) concentration (0.24 mg/L) in the anoxic storage limited the rapid oxidation of Fe²⁺ and facilitated more As(V) removal. The groundwater native-Fe²⁺ (2.33±0.03 mg/L) removed 61% of As in the oxic system (storage-aeration-filtration), whereas 92% As removal was achieved in the anoxic system.

Managed aquifer recharge implementation criteria to achieve water sustainability

Depletion of groundwater is accelerated due to an increase in water demand for applications in urbanized areas, agriculture sectors, and energy extraction, and dwindling surface water during changing climate. Managed aquifer recharge (MAR) is one of the several methods that can help achieve long-term water sustainability by increasing the natural recharge of groundwater reservoirs with water from non-traditional supplies such as excess surface water, stormwater, and treated wastewater. Despite the multiple benefits of MAR, the wide-scale implementation of MAR is lacking, partly because of challenges to select the location for MAR implementation and identify the MAR type based on site conditions and needs. In this review, we provide an overview of MAR types with a basic framework to select and implement specific MAR at a site based on water availability and quality, land use, source type, soil, and aquifer properties. Our analysis of 1127 MAR projects shows that MAR has been predominantly implemented in sites with sandy clay loam soil (soil group C) and with access to river water for recharge. Spatial analysis reveals that many regions with depleting water storage have opportunities to implement MAR projects. Analyzing data from 34 studies where stormwater was used for recharge, we show that MAR can remove dissolved organic carbon, most metals, E. coli but not efficient at removing most trace organics, and enterococci. Removal efficiency depends on the type of MAR. In the end, we highlight potential challenges for implementing MAR at a site and additional benefits such as minimizing land subsidence, flood risk, augmenting low dry-season flow, and minimizing salt-water intrusion. These results could help identify locations in the water-stressed regions to implement specific MAR for water sustainability.

Nitrate, arsenic and fluoride removal by electrodialysis from brackish groundwater

Nitrate, arsenic and fluoride are some of the most hazardous elements contaminating groundwater resources. In this work, the impact of operative (flowrate, electricpotential) and water quality (salinity, contaminant feed concentration, pH) parameters on brackish water decontamination was investigated using a batch electrodialysis (ED) system. Electrodialysis at low electric potentials (5 V) was more selective toward monovalent ions, at higher potentials (>15 V) removal of all ions increased and selectivity approached one, meaning removal of all ions. Changing the flowrate from 30 to 70 L/h, increased nitrate and fluoride removal slightly, while arsenic(V) removal was maximum at 50 L/h. Rising salinity delayed removal of ions with low ionic mobility and diffusivity (i.e. fluoride, arsenic(V)). Increased feed concentration of contaminants had no impact on removal values. pH variations did not impact the nitrate, fluoride and salinity removal, yet arsenic(V) removal was greatly pH dependent. This was explained in part by lower diffusivity and higher hydration number of bi- and trivalent species of arsenic(V) at basic pH. The results of this work showed the significance of ionic characteristics (diffusivity, ionic mobility, hydration number) in ED. Nitrate concentrations satisfied guideline threshold in all experiments with concentrations below 50 mg/L. Lowest arsenic(V) concentration was 35 µg/L at the highest electric potential, 25 V. Using ionic characteristics makes separation of different ions possible, providing new opportunities for ED in environmentally friendly processes (e.g. resource recovery and zero liquid discharge).

Enhanced Arsenite Removal from Silicate-containing Water by Using Redox Polymer-based Fe(III) Oxides Nanocomposite

The efficient removal of arsenite [As(III)] from groundwater remains a great challenge. Nanoscale oxides of Fe(III), Zr(IV), and Al(III) can selectively remove arsenic from groundwater through inner-sphere complexation. However, owing to polysilicate coatings formation on nanoparticles surface, the ubiquitous silicate exerts remarkably adverse effects on As(III) removal. Herein, we propose a new strategy to enhance silicate resistance of nanoscale oxides by embedding them inside the redox polymer host. As a proof-of-concept, the nanocomposite HFO@PS-Cl was employed to remove As(III) from silicate-containing water. The polymer host (PS-Cl) contains active chlorine to oxidize As(III) into arsenate [As(V)], and the embedded Fe(III) oxides enabling specific adsorption toward arsenic. Silicate exerts negligible effects on As(III) removal by HFO@PS-Cl in pH 3-7, but increasing the residual arsenic concentration from 49 µg/L to 166 µg/L for the solutions treated by HFO@PS-N, i.e., the nanoscale Fe(III) oxides embedded inside the polymer host without active chlorine. During the six cyclic decontamination-regeneration assays, HFO@PS-Cl steadily reduces As(III) below 10 µg/L. As for HFO@PS-N, however, the residual arsenic increases to ~57 µg/L in the sixth run. In column mode, HFO@PS-Cl column generates >3200-bed volume (BV) clean water ([As]<10 µg/L) from the simulated As(III)-contaminated groundwater. In contrast, the values for As(V)-contaminated water and HFO@PS-N column are only ~650 BV and ~608 BV, respectively. The stoichiometric assays, XPS, and in-situ ATR-FTIR analysis demonstrate that silicate polymerization is intensively suppressed by the protons produced during As(III) oxidation, thus rendering HFO@PS-Cl with excellent silicate resistant properties.

Characterization of Coprecipitates of As(III) and Fe(II) in the Presence of Phyllosilicate Nanoparticles

Phyllosilicate nanoparticles play an important role in regulating the biogeochemical processes of Fe(II) and As(III) in paddy soils due to their high mobility and activity. In the present work, two prepared muscovite nanoparticles with different sizes (LNPs and SNPs) were used to investigate the effect of the size of phyllosilicate nanoparticles on the coprecipitation of Fe(II) and As(III) during oxidation process. The results showed that muscovite nanoparticles could significantly promote the removal of Fe(II) and As(III) during coprecipitation process. The formation of crystalline iron oxide and oxidation of As(III) tended to be suppressed by the two muscovite nanoparticles, and the suppression increased as muscovite nanoparticle size decrease. The findings of this study provide a contribution to understanding the roles of the natural phyllosilicate nanoparticles in regulating the biogeochemical processes of Fe and As elements in polluted paddy soils.

Kanchan arsenic filters in the lowlands of Nepal: mode of operation, arsenic removal, and future improvements

In the lowlands of Nepal (Terai), the WHO drinking water guideline concentration of 10 μg/L for arsenic (As) is frequently exceeded. Since their introduction in 2006, iron-assisted bio-sand filters (Kanchan filters) are widely used to treat well water in Nepal. The filters are constructed on the basis of As-removal with corroding zero-valent iron (ZVI), with water flowing through a filter bed of iron nails placed above a sand filter. According to several studies, the performance of Kanchan filters varies greatly and depends on the size of the iron nails, filter design, water composition, and operating conditions, leading to concerns about their actual efficiency. This study examined 38 Kanchan household filters for which insufficient As-removal was reported, to evaluate the reasons for limited removal efficiency and to define measures for improved performance. The measured arsenic removal ranged from 6.3% to 98.5%. The most relevant factors were the concentrations of As and Fe in the raw water, with the best removal efficiency observed for water with low As (123 µg/L) and high Fe (5.0 mg/L). Although the concentrations of other elements, pH, flow rates, and contact time with ZVI also played a role, the combined evidence indicated that the reactivity of the frequently drying nail beds between filtrations was insufficient for efficient As-removal. Optimized filters with added top layers of sand and raised water outlets with flow restrictions to keep nails permanently immersed and to increase contact times, should be able to achieve higher and more consistent arsenic removal efficiencies.

Groundwater As Removal by As(III), Fe(II), and Mn(II) Co-Oxidation: Contrasting As Removal Pathways with O2, NaOCl, and KMnO4

Effective arsenic (As) removal from groundwater is a pressing need in view of increasingly stringent As drinking water limits in some US states and European countries. In this study, we compared the addition of weak (O₂), intermediate (NaOCl), and strong (KMnO₄) groundwater oxidants on the fate of As during As(III), Fe(II), and Mn(II) co-oxidation. Experiments were performed with 50 μg/L As(III), 5 mg/L Fe(II), and 0.5 mg/L Mn(II) in solutions containing relevant groundwater ions, with the reaction products characterized by As K-edge X-ray absorption spectroscopy (XAS). Adding O₂ by aeration was the least effective method, unable to decrease As to below 10 μg/L, which was attributed to inefficient As(III) oxidation. Dosing NaOCl (55 μM) consistently removed As to <10 μg/L (and often <5 μg/L). The As K-edge XAS data of the NaOCl samples indicated complete As(III) oxidation and As(V) sorption to coprecipitated hydrous ferric oxide (HFO) in the binuclear, bridging (²C) complex. The most effective As removal was observed with KMnO₄ (40 μM), which completely oxidized As(III) and yielded residual As concentrations that were less than (by as much as 50%) or equal to the NaOCl experiments. Furthermore, the average As-metal bond length of the KMnO₄ solids (R_As-Fe/Mn = 3.24 ± 0.02 Å) was systematically shorter than the NaOCl solids (R_As-Fe/Mn = 3.29 ± 0.02 Å), consistent with As(V) sorption to both MnO₂ and HFO. These findings can be used to optimize groundwater As treatment to meet relevant drinking water guidelines, while considering the As uptake mode and characteristics of the particle suspension (i.e., colloidal stability and filterability).

Simultaneous Removal of Arsenate and Chromate from Ground- and Surface- Waters by Iron-Based Redox Assisted Coagulation

Arsenic (As) and chromate (Cr(VI)) contamination of ground and surface waters is a major problem worldwide. Given that a new drinking water limit is anticipated for Cr(VI) and that the limit of arsenic in drinking water is quite low (10 μg/L), there is an urgent need for evaluating technologies that could be efficient for removal of both contaminants simultaneously. In this work, the use of Fe(II) redox assisted coagulation was investigated to simultaneously remove the contaminants of interest. The basic principle of this technology is that Fe(II) could react with Cr(VI) and form Fe(III)-hydroxides and insoluble Cr(III) species, while the freshly formed Fe(III) hydroxides are very efficient adsorbents for As(V). The effect of pH, the water matrix composition, Fe(II) dose, initial contaminant concentrations, NOM presence and phosphate concentration were the examined parameters. The results revealed that with a dose of 2 mg/L Fe(II), residual As(V) and Cr(VI) concentrations were both below 10 μg/L, from initial concentrations of 50 μg/L. Though, this is effective only at circumneutral pH values. This is however not a big obstacle, since most natural waters, especially groundwaters, have near neutral pH values. At these pH values, residual iron concentration was far below 200 μg/L. The presence of phosphate anions inhibited As(V) removal but had no effect on Cr(VI) removal. Increasing Fe(II) concentrations eliminated the effect of phosphate and provided simultaneous phosphate removal. Therefore, Fe(II) coagulation can be applied, with secured results, for simultaneous As(V), Cr(VI) and phosphate removal from waters.

As(V) sorption from aqueous solutions using quaternized algal/polyethyleneimine composite beads

Composite beads (APEI*), obtained by the controlled interaction of algal biomass with PEI, followed by ionotropic gelation and crosslinking processes using CaCl₂/glutaraldehyde solution, constitute efficient supports for metal binding. The quaternization of algal/PEI beads (Q-APEI*) significantly increases the sorption properties of the composite beads (APEI*) for As(V). The materials are characterized by SEM/EDX, TGA, BET, elemental analysis, FTIR, XPS, and titration. The sorption of As(V) is studied in function of pH while sorption mechanism is discussed in function of metal speciation and surface characteristics of the sorbent. Optimum sorption occurs at pH close to 7. Fast uptake kinetics, correlated to textural properties are successfully fitted by pseudo-first order rate equation and the Crank equation (for resistance to intraparticle diffusion); equilibrium is reached with 45-60 min. The Langmuir equation finely fits sorption isotherms; maximum sorption capacity reaches 1.34 mmol As g^-1. Arsenic can be completely eluted using 0.5 M CaCl₂/0.5 M HCl solutions; the sorbent maintains high sorption and desorption efficiencies for a minimum of 5 cycles. The sorbent is tested for the removal of As(V) from mining effluents containing high concentration of iron and traces of zinc. At pH 3, the sorbent shows remarkable selectivity for As(V) over Fe. After controlling the initial pH to 5, a sorbent dosage of 2 g L^-1 is sufficient for achieving the complete recovery of As(V) from mining effluent (corresponding to initial concentration of 1.295 mmol As L^-1).

Adsorptive removal of trivalent and pentavalent arsenic from aqueous solutions using iron and copper impregnated melanin extracted from the marine bacterium Pseudomonas stutzeri

The metalloid arsenic is one of the most conspicuous groundwater contaminants in the Indian subcontinent and its removal from aqueous medium is the main focus of this study. The study aims at functionalising melanin using iron and copper for the efficient removal of arsenic and rendering water fit for consumption. Melanin obtained from the marine bacteria Pseudomonas stutzeri was functionalised by iron impregnation (Fe-melanin) and copper impregnation (Cu-melanin). Morphological studies using FESEM portrayed the impregnated iron and copper granules on the surface of melanin, while XRD analysis confirmed the presence of Fe₂O₃ and CuO on melanin. Adsorption studies on As (V) and As (III) were conducted using Fe-melanin and Cu-melanin for different operating variables like pH, temperature and contact time. More than 99% per cent of As (III) and As (V) from water was removed at a pH range between 4 and 6 within 50 min in the case of Fe-melanin and 80 min for Cu-melanin. Adsorption equilibrium studies showed better fit with Langmuir adsorption isotherm and had good agreement with Redlich-Peterson's three-parameter model. The maximum adsorption capacities of Fe-melanin and Cu-melanin obtained from Langmuir adsorption model are 50.12 and 20.39 mg/g, respectively, for As (V) and similarly 39.98 and 19.52 mg/g, respectively, for As (III). Arsenic-binding to the functionalised melanin was confirmed using FT-IR and the XPS analysis. Reuse of the adsorbent was effectively done by desorbing the iron and copper together with the bound As (III) and As (V) and further re-impregnation of iron and copper in melanin. Re-functionalised melanin showed 99% adsorption efficiency up to four cycles of adsorption/desorption.

Arsenate removal from aqueous solutions by cuttlebone/copper oxide nanobiocomposite

This study aims to illustrate the preparation of a new nanobiocomposite by incorporating copper oxide nanoparticles into cuttlebone matrix (CB/CuO NPs), and it was tested to define how effective it was to adsorb and remove arsenate from aqueous systems. CB is the bone tissue of cuttlefish with high porosity, permeability, and low cost. CuO NPs have been introduced as an effective arsenate adsorbent. Producing nanocomposite by introducing of CuO NPs in the structure of CB enhanced their stability and facilitated their separation from solution. Incorporation of CuO NPs in the structure of CB enhanced the adsorption capacity of CB. The adsorption data were fitted with both Langmuir and Freundlich isotherms, but Langmuir isotherm exhibited better matching rather than Freundlich isotherm. The maximum adsorption capacity (q_max) was calculated from Langmuir adsorption isotherm which was around 25.13 mg g^-1. Kinetic data fitted well to the pseudo-second-order reaction model. The results indicate that the possible mechanism of arsenate adsorption on CB/CuO is through development of inner sphere complex. Simple preparation and abundant and good adsorption capacity in the presence of calcium ions indicate that the CB/CuO is suitable for removal of arsenate from contaminated drinking water.

Long-term stability of the Fe(III)-As(V) coprecipitates: Effects of neutralization mode and the addition of Fe(II) on arsenic retention

The coprecipitation of arsenic with Fe(III) by lime neutralization is widely used in industrial practices to treat arsenic-containing waste waters generated from mineral processing operations. In this work, coprecipitation was conducted directly at pH 8 to simulate the operations in hydrometallurgical practices, which differed from the conventional laboratory operations. Moreover, although ferric is the major species of iron in arsenic-containing waste waters, the coexistence of ferrous ions cannot be ignored. Therefore, the effect of different neutralization modes, as well as the effect of ferrous ions on the removal of arsenic and the stability of the generated arsenic-bearing wastes, was systematically investigated. The result showed that arsenic was still released back into the liquid phase under alkaline conditions even for the samples formed directly at alkaline pH. It was found that the extra addition of Fe(II) may exert negative effect on the stability of the as-formed Fe(II)-Fe(III)-As(V) coprecipitates at pH 7 - 10. The concentration of ferrous ions in the liquid/solid phase decreased with increasing pH for each sample formed at different Fe(II)/Fe(tot). The results indicated that complete oxidation of the ferrous ions before coprecipitation with arsenic should be conducted to achieve optimal stability of the arsenic-bearing wastes for hydrometallurgical practice and waste disposal.

Transformation of arsenic during realgar tailings stabilization using ferrous sulfate in a pilot-scale treatment

Massive realgar tailings abandoned in mining areas in China have caused serious arsenic (As) pollution seeking for urgent disposal. Stabilization treatment is a feasible strategy, however, stabilization technologies for realgar tailings, that are Fe-deficient, Ca-rich and S-rich, have not been well developed to date. In this study, we conducted a pilot-scale stabilization treatment of realgar tailings via ferrous sulfate addition to evaluate the transformation of As during stabilization. We found that Si, As, Ca, and S were the predominant elements in the raw realgar tailings with a low content of Fe, and realgar (As^II₄S₄) and pharmacolite (CaHAs^VO₄·2H₂O) were the main As-bearing minerals. After the ferrous sulfate treatment, the As leaching concentration of realgar tailings was successfully reduced from 135 mg/L to a level below the Chinese regulatory limit (2.5 mg/L). Based on the results of leaching tests, sequential extraction analysis, XRD, SEM-EDS, XPS, and thermodynamic modeling, we concluded that ferrous sulfate addition enhanced the transformation of Ca-As and S-As species to more stable Fe-As species, e.g., crystalline symplesite and amorphous Fe-As complex. Dissolution of pharmacolite was facilitated by H⁺ and SO₄^2- derived from the hydrolysis and oxidation of ferrous sulfate, and oxidation of realgar could be promoted by reactive oxygen species (ROSs) from Fe(II) oxygenation. This study improved our understanding of As transformation pathways in realgar tailings during ferrous sulfate treatment, which could serve as an alternative scheme for realgar tailings stabilization.

Reductive dissolution of As(V)-bearing Fe(III)-precipitates formed by Fe(II) oxidation in aqueous solutions

Iron(III)-precipitates formed by the oxidation of dissolved Fe(II) are important sorbents for major and trace elements in aquatic and terrestrial systems. Their reductive dissolution in turn may result in the release of associated elements. We examined the reductive dissolution kinetics of an environmentally relevant set of Fe(II)-derived arsenate-containing Fe(III)-precipitates whose structure as function of phosphate (P) and silicate (Si) content varied between poorly-crystalline lepidocrocite, amorphous Fe(III)-phosphate, and Si-containing ferrihydrite. The experiments were performed with 0.2-0.5 mM precipitate-Fe(III) using 10 mM Na-ascorbate as reductant, 5 mM bipyridine as Fe(II)-complexing ligand, and 10 mM MOPS/5 mM NaOH as pH 7.0 buffer. Times required for the dissolution of half of the precipitate (t_50%) ranged from 1.5 to 39 h; spanning a factor 25 range. At loadings up to ~ 0.2 P/Fe (molar ratio), phosphate decreased the t_50% of Si-free precipitates, probably by reducing the crystallinity of lepidocrocite. The reductive dissolution of Fe(III)-phosphates formed at higher P/Fe ratios was again slower, possibly due to P-inhibited ascorbate binding to precipitate-Fe(III). The slowest reductive dissolution was observed for P-free Si-ferrihydrite with ~ 0.1 Si/Fe, suggesting that silicate binding and polymerization may reduce surface accessibility. The inhibiting effect of Si was reduced by phosphate. Dried-resuspended precipitates dissolved 1.0 to 1.8-times more slowly than precipitates that were kept wet after synthesis, most probably because drying enhanced nanoparticle aggregation. Variations in the reductive dissolution kinetics of Fe(II) oxidation products as reported from this study should be taken into account when addressing the impact of such precipitates on the environmental cycling of co-transformed nutrients and contaminants.

Effect of supernatant water level on As removal in biological rapid sand filters

Current groundwater treatment facilities, mostly relying on aeration-filtration configurations, aim at the removal of iron (Fe), ammonia (NH₄ ⁺) and manganese (Mn). However, recently water companies expressed the ambition to also reduce arsenic (As) concentrations in these rapid sand filters. The aim of this study was to investigate the effect of the Fe oxidation state entering a biological filter bed on As removal. By varying supernatant water level, either Fe(II) or Fe(III) in the form of hydrous ferric oxides (HFO) could be stimulated to enter the filter bed at alkaline groundwater pH (7.6). The experimental pilot column filters showed that once the As(III) oxidation stabilised in the top layer of the filter sand, As removal reached its maximum (±75% at 120 cm supernatant level and 1.5 m/h filtration velocity). The increase in supernatant level from 5 to 120 cm resulted in additional HFO production prior to rapid filtration (1.5, 5 and 10 m/h), i.e. homogeneous Fe(II) oxidation and flocculation, and subsequently, HFO ending up deeper into the filter bed (120 cm filter depth). At a low supernatant water level of 5 cm, Fe(II) oxidised heterogeneously and was removed within the top 20 cm of the filter bed. Consequently, filters with high supernatant levels removed As to lower levels (by 20%) than in filters with low supernatant water levels. The benefits of Fe(II) oxidation prior to filtration for As removal was confirmed by comparing Fe(III) to Fe(II) additions in the supernatant water or in the filter bed. Overall it is concluded that in biological groundwater filters, the combination of a higher supernatant level and/or Fe(III) addition with biological As(III) oxidation in the top of the filter bed promotes As removal.

Monothioarsenate Occurrence in Bangladesh Groundwater and Its Removal by Ferrous and Zero-Valent Iron Technologies

In most natural groundwaters, sulfide concentrations are low, and little attention has been paid to potential occurrence of thioarsenates (As^VS _n^-IIO_{4- n}^3- with n = 1-4). Thioarsenate occurrence in groundwater could be critical with regard to the efficiency of iron (Fe)-based treatment technologies because previous studies reported less sorption of thioarsenates to preformed Fe-minerals compared to arsenite and arsenate. We analyzed 273 groundwater samples taken from different wells in Bangladesh over 1 year and detected monothioarsenate (MTA), likely formed via solid-phase zero-valent sulfur, in almost 50% of all samples. Concentrations ranged up to >30 μg L^-1 (21% of total As). MTA removal by locally used technologies in which zero-valent or ferrous Fe is oxidized by aeration and As sorbs or coprecipitates with the forming Fe(III)hydroxides was indeed lower than for arsenate. The presence of phosphate required up to three times as much Fe(II) for comparable MTA removal. However, in contrast to previous sorption studies on preformed Fe minerals, MTA removal, even in the presence of phosphate, was still higher than that of arsenite. The more efficient MTA removal is likely caused by a combination of coprecipitation and adsorption rendering the tested Fe-based treatment technologies suitable for As removal also in the presence of MTA.

Review of interactions between phosphorus and arsenic in soils from four case studies

Arsenic is a non-essential element that poses risks in many environments, including soil, groundwater, and surface water. Insights into the environmental biogeochemistry of As can be gained by comparing As and P reaction processes. Arsenic and P are chemical analogues, and it is proposed that they have similar chemical behaviors in environmental systems. However some chemical properties of As and P are distinct, such as redox reactions, causing the biogeochemical behavior of the two elements to differ. In the environment, As occurs as either As(V) or As(III) oxyanions (e.g., AsO₄^3- or AsO₃^3-). In contrast, P occurs predominantly as oxidation state five plus; most commonly as the orthophosphate ion (PO₄^3-). In this paper, data from four published case studies are presented with a focus on P and As distribution and speciation in soil. The goal is show how analyzing P chemistry in soils can provide greater insights into As reaction processes in soils. The case studies discussed include: (1) soil developed from shale parent material, (2) mine-waste impacted wetland soils, (3) phosphate-amended contaminated soil, and (4) plants grown in biochar-amended, mine-contaminated soil. Data show that while P and As have competitive reactions in soils, in most natural systems they have distinct biogeochemical processes that create differing mobility and bioavailability. These processes include redox reactions and rhizosphere processes that affect As bioavailability. Results from these case studies are used as examples to illustrate how studying P and As together allows for enhanced interpretation of As biogeochemical processes in soils.