Alkali Extraction of Arsenic from Groundwater Treatment Sludge: An Essential Initial Step for Arsenic Recovery.

Arsenic (As)-bearing Fe(III) precipitate groundwater treatment sludge has traditionally been viewed by the water sector as a disposal issue rather than a resource opportunity, partly due to assumptions of the low value of As. However, As has now been classified as a Critical Raw Material (CRM) in many regions, providing new incentives to recover As and other useful components of the sludge, such as phosphate (P) and the reactive hydrous ferric oxide (HFO) sorbent. Here, we investigate alkali extraction to separate As from a variety of field and synthetic As-bearing HFO sludges, which is a critical first step to enable sludge upcycling. We found that As extraction was most effective using NaOH, with the As extraction efficiency increasing up to >99% with increasing NaOH concentrations (0.01, 0.1, and 1 M). Extraction with Na2CO3 and Ca(OH)2 was ineffective (<5%). Extraction time (hour, day, week) played a secondary role in As release but tended to be important at lower NaOH concentrations. Little difference in As extraction efficiency was observed for several key variables, including sludge aging time (50 days) and cosorbed oxyanions (e.g., Si, P). However, the presence of ∼10 mass% calcite decreased As release from field and synthetic sludges considerably (<70% As extracted). Concomitant with As release, alkali extraction promoted crystallization of poorly ordered HFO and decreased particle specific surface area, with structural modifications increasing with NaOH concentration and extraction time. Taken together, these results provide essential information to inform and optimize the design of resource recovery methods for As-bearing treatment sludge.

Regenerated Manganese-Oxide Coated Sands: The Role of Mineral Phase in Organic Contaminant Reactivity.

Manganese oxide-coated sand can oxidize electron-rich organic contaminants, but after extended exposure to contaminated water its reactivity decreases. To assess the potential for regenerating geomedia, we measured the ability of passivated manganese-oxide coated sand to oxidize bisphenol A after treatment with oxidants, acid, or methanol. Among the regenerants studied, KMnO4, HOCl, HOBr, and pH 2 or 3 HCl solutions raised the average oxidation state of the Mn, but only HOCl and HOBr restored the reactivity of passivated geomedia to levels comparable to those of the virgin manganese-oxide coated sand. Treatment with HCl restored about one third of the reactivity of the material, likely due to dissolution of reduced Mn. Mn K-edge X-ray absorption spectroscopy data indicated that the reactive manganese oxide phases present in virgin geomedia and geomedia regenerated with HOCl or HOBr had nanocrystalline cryptomelane-like structures and diminished Mn(III) abundance relative to the passivated geomedia. KMnO4-regenerated geomedia also had less Mn(III), but it exhibited less reactivity with bisphenol A because regeneration produced a structure with characteristics of δ-MnO2. The results imply that manganese oxide reactivity depends on both oxidation state and crystal structure; the most effective chemical regenerants oxidize Mn(III) to Mn(IV) oxides exhibiting nanocrystalline, cryptomelane-like forms.

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 (RAs-Fe/Mn = 3.24 ± 0.02 Å) was systematically shorter than the NaOCl solids (RAs-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).

Coprecipitation of Phosphate and Silicate Affects Environmental Iron (Oxyhydr)Oxide Transformations: A Gel-Based Diffusive Sampler Approach.

Sorption of nutrients such as phosphate (P) and silicate (Si) by ferric iron (oxyhydr)oxides (FeOx) modulates nutrient mobility and alters the structure and reactivity of the FeOx. We investigated the impact of these interactions on FeOx transformations using a novel approach with samplers containing synthetic FeOx embedded in diffusive hydrogels. The FeOx were prepared by Fe(III) hydrolysis and Fe(II) oxidation, in the absence and presence of P or Si. Coprecipitation of P or Si during synthesis altered the structure of Fe precipitates and, in the case of Fe(II) oxidation, lepidocrocite was (partly) substituted by poorly ordered FeOx. The pure and P- or Si-bearing FeOx were deployed in (i) freshwater sediment rich in dissolved Fe(II) and P and (ii) marine sediment with sulfidic pore water. Iron(II)-catalyzed crystallization of poorly ordered FeOx was negligible, likely due to surface passivation by adsorption of dissolved P. Reaction with dissolved sulfide was modulated by diffusion limitations and therefore the extent of sulfidation was the lowest for poorly ordered FeOx with high reactivity toward sulfide that created temporary, local sulfide depletion (Fh < Lp). We show that coprecipitation-induced changes in the FeOx structure affect coupled iron-nutrient cycling in aquatic ecosystems. The gel-based method enriches our geochemical toolbox by enabling detailed characterization of target phases under natural conditions.

Rapid and Efficient Arsenic Removal by Iron Electrocoagulation Enabled with in Situ Generation of Hydrogen Peroxide.

Millions of people are exposed to toxic levels of dissolved arsenic in groundwater used for drinking. Iron electrocoagulation (FeEC) has been demonstrated as an effective technology to remove arsenic at an affordable price. However, FeEC requires long operating times (∼hours) to remove dissolved arsenic due to inherent kinetics limitations. Air cathode Assisted Iron Electrocoagulation (ACAIE) overcomes this limitation by cathodically generating H2O2 in situ. In ACAIE operation, rapid oxidation of Fe(II) and complete oxidation and removal of As(III) are achieved. We compare FeEC and ACAIE for removing As(III) from an initial concentration of 1464 µg/L, aiming for a final concentration of less than 4 µg/L. We demonstrate that at short electrolysis times (0.5 min), i.e., high charge dosage rates (1200 C/L/min), ACAIE consistently outperformed FeEC in bringing arsenic levels to less than WHO-MCL of 10 µg/L. Using XRD and XAS data, we conclusively show that poor arsenic removal in FeEC arises from incomplete As(III) oxidation, ineffective Fe(II) oxidation and the formation of Fe(II-III) (hydr)oxides at short electrolysis times (<20 min). Finally, we report successful ACAIE performance (retention time 19 s) in removing dissolved arsenic from contaminated groundwater in rural California.

Direct Visualization of Arsenic Binding on Green Rust Sulfate.

"Green rust" (GR), a redox-active Fe(II)-Fe(III) layered double hydroxide, is a potential environmentally relevant mineral substrate for arsenic (As) sequestration in reduced, subsurface environments. GR phases have high As uptake capacities at circum-neutral pH conditions, but the exact interaction mechanism between the GR phases and As species is still poorly understood. Here, we documented the bonding and interaction mechanisms between GR sulfate and As species [As(III) and As(V)] under anoxic and circum-neutral pH conditions through scanning transmission electron microscopy (STEM) coupled with energy-dispersive X-ray (EDX) spectroscopy and combined it with synchrotron-based X-ray total scattering, pair distribution function (PDF) analysis, and As K-edge X-ray absorption spectroscopy (XAS). Our highly spatially resolved STEM-EDX data revealed that the preferred adsorption sites of both As(III) and As(V) are at GR crystal edges. Combining this data with differential PDF and XAS allowed us to conclude that As adsorption occurs primarily as bidentate binuclear (2C) inner-sphere surface complexes. In the As(III)-reacted GR sulfate, no secondary Fe-As phases were observed. However, authigenic parasymplesite (ferrous arsenate nanophase), exhibiting a threadlike morphology, formed in the As(V)-reacted GR sulfate and acts as an additional immobilization pathway for As(V) (∼87% of immobilized As). We demonstrate that only by combining high-resolution STEM imaging and EDX mapping with the bulk (differential) PDF and extended X-ray absorption fine structure (EXAFS) data can one truly determine the de facto As binding nature on GR surfaces. More importantly, these new insights into As-GR interaction mechanisms highlight the impact of GR phases on As sequestration in anoxic subsurface environments.

Concrete stabilization of arsenic-bearing iron sludge generated from an electrochemical arsenic remediation plant.

In this study, concrete stabilization is adopted to sustainably manage hazardous arsenic-iron sludge near the vicinity of a community-based arsenic water treatment plant for potential use as material for local construction. The strength and workability of the sludge mixed with fresh concrete were investigated to determine the suitability of the concrete mixture for building materials. We found that over 25% sludge (with respect to cement weight) can be incorporated safely into different grades of concrete (M15 and M20). Structural characterization of the concrete mixtures by Fe and As K-edge X-ray absorption spectroscopy indicated a structural transformation of Fe in the sludge from a hydrous ferric oxide to a less ordered phase consistent with Fe siliceous hydrogarnet. Differences in the As K-edge XAS data of samples before and after stabilization in concrete were interpreted as a decrease in As-Fe coordination after concrete stabilization in favor of As-Ca coordination. The leaching of arsenic in the stabilized concrete was examined by the Toxicity Characteristics Leaching Procedure (TCLP) and found to produce < 15 µg/L As, even at the highest sludge mixture fraction (40% sludge with respect to cement weight). The formation of calcite in concrete stabilized arsenic sludge, which was detected by X-ray diffraction (XRD), contributes to the low leachability of arsenic in the sludge for a variety of reasons, including decreasing pore size. In addition, the formation of poorly soluble calcium arsenates can also be responsible for the low mobility of arsenic. Overall concrete stabilization of arsenic-iron sludge can be an effective pre-treatment to safe landfill disposal and, when the arsenic-iron sludge is mixed in specific proportions to achieve desired strength, we propose this concrete can be used locally in nearby construction.

Chemical Regeneration of Manganese Oxide-Coated Sand for Oxidation of Organic Stormwater Contaminants.

Urban stormwater, municipal wastewater effluent, and agricultural runoff contain trace amounts of organic contaminants that can compromise water quality. To provide a passive, low-cost means of oxidizing substituted phenols, aromatic amines, and other electron-rich organic compounds during infiltration of contaminated waters, we coated sand with manganese oxide using a new approach involving the room-temperature oxidation of Mn2+ with permanganate. Manganese oxide-coated sand effectively oxidized bisphenol A under typical infiltration conditions and sustained reactivity longer than previously described geomedia. Because geomedia reactivity decreased after extended operation, chlorine was evaluated for use as an in situ geomedia regenerant. Geomedia regenerated by HOCl demonstrated similar reactivity and longevity to that of virgin geomedia. Chemical analyses indicated that the average manganese oxidation state of the coatings decreased as the geomedia passivated. X-ray absorption spectroscopy and X-ray diffraction showed that the reactive virgin and regenerated geomedia coatings had nanocrystalline manganese oxide structures, whereas the failed geomedia coating exhibited greater crystallinity and resembled cryptomelane. These results suggest that it is possible to regenerate the oxidative capacity of manganese oxide-coated sands without excavating stormwater infiltration systems. These results also suggest that manganese oxide geomedia may be a cost-effective means of treating urban stormwater and other contaminated waters.

Mn(II) Oxidation in Fenton and Fenton Type Systems: Identification of Reaction Efficiency and Reaction Products.

Efficient and low-cost methods of removing aqueous Mn(II) are required to improve the quality of impacted groundwater supplies. In this work, we show that Fe(0) electrocoagulation (EC) permits the oxidative removal of Mn(II) from solution by reaction with the reactive oxidant species produced through Fe(II) oxidation. Manganese(II) removal was enhanced when the accumulation of aqueous Fe(II) was minimized, which was achieved at low Fe(II) production rates, high pH, the presence of H2O2 instead of O2 as the initial Fe(II) oxidant, or a combination of all three. In addition, in the EC-H2O2 system, Mn(II) removal efficiency increased as pH decreased from 6.5 to 4.5 and as pH increased from 6.5 to 8.5, which implicates different reactive oxidants in acidic and alkaline solutions. Chemical analyses and X-ray absorption spectroscopy revealed that Mn(II) removal during Fe(0) EC leads to the formation of Mn(III) (0.02 to >0.26 Mn·Fe-1 molar ratios) and its incorporation into the resulting Fe(III) coprecipitates (lepidocrocite and hydrous ferric oxide for EC-O2 and EC-H2O2, respectively), regardless of pH and Fe(II) production rate. The Mn(II) oxidation pathways elucidated in this study set the framework to develop kinetic models on the impact of Mn(II) during EC treatment and in other Fenton type systems.

Escherichia coli Attenuation by Fe Electrocoagulation in Synthetic Bengal Groundwater: Effect of pH and Natural Organic Matter.

Technologies addressing both arsenic and microbial contamination of Bengal groundwater are needed. Fe electrocoagulation (Fe-EC), a simple process relying on the dissolution of an Fe(0) anode to produce Fe(III) precipitates, has been shown to efficiently remove arsenic from groundwater at low cost. We investigated Escherichia coli (E. coli) attenuation by Fe-EC in synthetic Bengal groundwater as a function of Fe dosage rate, total Fe dosed, pH, and presence of natural organic matter (NOM). A 2.5 mM Fe dosage simultaneously achieved over 4-log E. coli attenuation and arsenic removal from 450 to below 10 µg/L. E. coli reduction was significantly enhanced at pH 6.6 compared to pH 7.5, which we linked to the decreased rate of Fe(II) oxidation at lower pH. 3 mg/L-C of NOM (Suwanee River fulvic acid) did not significantly affect E. coli attenuation. Live-dead staining and comparisons of Fe-EC with chemical coagulation controls showed that the primary mechanism of E. coli attenuation is physical removal with Fe(III) precipitates, with inactivation likely contributing as well at lower pH. Transmission electron microscopy showed that EC precipitates adhere to and bridge individual E. coli cells, resulting in large bacteria-Fe aggregates that can be removed by gravitational settling. Our results point to the promising ability of Fe-EC to treat arsenic and bacterial contamination simultaneously at low cost.

Production and transformation of mixed-valent nanoparticles generated by Fe(0) electrocoagulation.

Mixed-valent iron nanoparticles (NP) generated electrochemically by Fe(0) electrocoagulation (EC) show promise for on-demand industrial and drinking water treatment in engineered systems. This work applies multiple characterization techniques (in situ Raman spectroscopy, XRD, SEM, and cryo-TEM) to investigate the formation and persistence of magnetite and green rust (GR) NP phases produced via the Fe(0) EC process. Current density and background electrolyte composition were examined in a controlled anaerobic system to determine the initial Fe phases generated as well as transformation products with aging. Fe phases were characterized in an aerobic EC system with both simple model electrolytes and real groundwater to investigate the formation and aging of Fe phases produced in a system representing treatment of arsenic-contaminated ground waters in South Asia. Two central pathways for magnetite production via Fe(0) EC were identified: (i) as a primary product (formation within seconds when DO absent, no intermediates detected) and (ii) as a transformation product of GR (from minutes to days depending on pH, electrolyte composition, and aging conditions). This study provides a better understanding of the formation conditions of magnetite, GR, and ferric (oxyhydr)oxides in Fe EC, which is essential for process optimization for varying source waters.

Fe(III) nucleation in the presence of bivalent cations and oxyanions leads to subnanoscale 7 Å polymers.

Highly disordered Fe(III) phases formed in the presence of bivalent cations and oxyanions represent important components of the global Fe cycle due to their potential for rapid turnover and their critical roles in controlling the speciation of major and trace elements. However, a poor understanding of the formation pathway and structure of these Fe phases has prevented assessments of their thermodynamic properties and biogeochemical reactivity. In this work, we derive structural models for the Fe(III)-As(V)-Ca and Fe(III)-P-Ca polymers formed from Fe(II) oxidation and Fe(III) polymerization in the presence of As(V)/P and Ca. The polymer phase consists of a less than 7 Å coherent network of As(V)/P coordinated to Fe(III) polyhedra, with varying amounts of Ca bound directly and indirectly to the oxyanion. This phase forms at the onset of Fe(II) oxidation and, because of its large oxyanion:Fe solids ratio, depletes the oxyanion concentration with only small amounts of Fe. Our results demonstrate that when a steady supply of Fe(III) is provided from an Fe(II) source, these Fe(III) polymers, which dominate oxyanion uptake, form with little dependence on the initial oxyanion concentration. The formation mechanisms and structures of the oxyanion-rich Fe(III) polymers determined in this study enable future thermodynamic investigations of these phases, which are required to model the interrelated biogeochemical cycles of Fe, As(V)/P, and Ca.

Embedding Fe(0) electrocoagulation in a biologically active As(III) oxidising filter bed.

Long-term consumption of groundwater containing elevated levels of arsenic (As) can have severe health consequences, including cancer. To effectively remove As, conventional treatment technologies require expensive chemical oxidants to oxidise neutral arsenite (As(III)) in groundwater to negatively charged arsenate (As(V)), which is more easily removed. Rapid sand filter beds used in conventional aeration-filtration to treat anaerobic groundwater can naturally oxidise As(III) through biological processes but require an additional step to remove the generated As(V), adding complexity and cost. This study introduces a novel approach where As(V), produced through biological As(III) oxidation in a sand filter, is effectively removed within the same filter by embedding and operating an iron electrocoagulation (FeEC) system inside the filter. Operating FeEC within the biological filter achieved higher As(III) removal (81 %) compared to operating FeEC in the filter supernatant (67 %). This performance was similar to an analogous embedded-FeEC system treating As(V)-contaminated water (85 %), confirming the benefits of incorporating FeEC in a biological bed for comparable As(III) and As(V) removal. However, operating FeEC in the sand matrix consumed more energy (14 Wh/m3) compared to FeEC operated in a water matrix (7 Wh/m3). The efficiency of As removal increased and energy requirements decreased in such embedded-FeEC systems by deep-bed infiltration of Fe(III)-precipitates, which can be controlled by adjusting flow rate and pH. This study is one of the first to demonstrate the feasibility of embedding FeEC systems in sand filters for groundwater arsenic removal. Such systems capitalise on biological As(III) oxidation in aeration-filtration, effectively eliminating As(V) within the same setup without the need for chemicals or major modifications.

Removing arsenic from synthetic groundwater with iron electrocoagulation: an Fe and As K-edge EXAFS study.

Electrocoagulation (EC) using iron electrodes is a promising arsenic removal strategy for Bangladesh groundwater drinking supplies. EC is based on the rapid in situ dissolution of a sacrificial Fe(0) anode to generate iron precipitates with a high arsenic sorption affinity. We used X-ray absorption spectroscopy (XAS) to investigate the local coordination environment (<4.0 Å) of Fe and As in EC precipitates generated in synthetic Bangladesh groundwater (SBGW). Fe and As K-edge EXAFS spectra were found to be similar between samples regardless of the large range of current density (0.02, 1.1, 5.0, 100 mA/cm(2)) used to generate samples. Shell-by-shell fits of the Fe K-edge EXAFS spectra indicated that EC precipitates consist of primarily edge-sharing FeO(6) octahedra. The absence of corner-sharing FeO(6) octahedra implies that EC precipitates resemble nanoscale clusters (polymers) of edge-sharing octahedra that efficiently bind arsenic. Shell-by-shell fits of As K-edge EXAFS spectra show that arsenic, initially present as a mixture of As(III) and As(V), forms primarily binuclear, corner-sharing As(V) surface complexes on EC precipitates. This specific coordination geometry prevents the formation of FeO(6) corner-sharing linkages. Phosphate and silicate, abundant in SBGW, likely influence the structure of EC precipitates in a similar way by preventing FeO(6) corner-sharing linkages. This study provides a better understanding of the structure, reactivity, and colloidal stability of EC precipitates and the behavior of arsenic during EC. The results also offer useful constraints for predicting arsenic remobilization during the long-term disposal of EC sludge.

Modeling As(III) oxidation and removal with iron electrocoagulation in groundwater.

Understanding the chemical kinetics of arsenic during electrocoagulation (EC) treatment is essential for a deeper understanding of arsenic removal using EC under a variety of operating conditions and solution compositions. We describe a highly constrained, simple chemical dynamic model of As(III) oxidation and As(III,V), Si, and P sorption for the EC system using model parameters extracted from some of our experimental results and previous studies. Our model predictions agree well with both data extracted from previous studies and our observed experimental data over a broad range of operating conditions (charge dosage rate) and solution chemistry (pH, co-occurring ions) without free model parameters. Our model provides insights into why higher pH and lower charge dosage rate (Coulombs/L/min) facilitate As(III) removal by EC and sheds light on the debate in the recent published literature regarding the mechanism of As(III) oxidation during EC. Our model also provides practically useful estimates of the minimum amount of iron required to remove 500 µg/L As(III) to <50 µg/L. Parameters measured in this work include the ratio of rate constants for Fe(II) and As(III) reactions with Fe(IV) in synthetic groundwater (k(1)/k(2) = 1.07) and the apparent rate constant of Fe(II) oxidation with dissolved oxygen at pH 7 (k(app) = 10(0.22) M(-1)s(-1)).

Groundwater-native Fe(II) oxidation prior to aeration with H2O2 to enhance As(III) removal.

Groundwater contaminated with arsenic (As) must be treated prior to drinking, as human exposure to As at toxic levels can cause various diseases including cancer. Conventional aeration-filtration applied to anaerobic arsenite (As(III)) contaminated groundwater can remove As(III) by co-oxidizing native iron (Fe(II)) and As(III) with oxygen (O2). However, the As(III) removal efficiency of conventional aeration can be low, in part, because of incomplete As(III) oxidation to readily-sorbed arsenate (As(V)). In this work, we investigated a new approach to enhance As(III) co-removal with native Fe(II) by the anaerobic addition of hydrogen peroxide (H2O2) prior to aeration. Experiments were performed to co-oxidize Fe(II) and As(III) with H2O2 (anaerobically), O2 (aerobically), and by sequentially adding of H2O2 and O2. Aqueous As(III) and As(V) measurements after the reaction were coupled with solid-phase speciation by Fe and As K-edge X-ray absorption spectroscopy (XAS). We found that complete anaerobic oxidation of 100 µM Fe(II) with 100 µM H2O2 resulted in co-removal of 95% of 7 µM As(III) compared to 44% with 8.0-9.0 mg/L dissolved O2. Furthermore, we found that with 100 µM Fe(II), the initial Fe(II):H2O2 ratio was a critical parameter to remove 7 µM As(III) to below the 10 µg/L (0.13 µM) WHO guideline, where ratios of 1:4 (mol:mol) Fe(II):H2O2 led to As(III) removal matching that of 7 µM As(V). The improved As(III) removal with H2O2 was found to occur partly because of the well-established enhanced efficiency of As(III) oxidation in Fe(II)+H2O2 systems relatively to Fe(II)+O2 systems. However, the XAS results unambiguously demonstrated that a large factor in the improved As(III) removal was also due to a systematic decrease in crystallinity, and thus increase in specific surface area, of the generated Fe(III) (oxyhydr)oxides from lepidocrocite in the Fe(II)+O2 system to poorly-ordered Fe(III) precipitates in the Fe(II)+H2O2 system. The combined roles of H2O2 (enhanced As(III) oxidation and structural modification) can be easily overlooked when only aqueous species are measured, but this dual impact must be considered for accurate predictions of As removal in groundwater treatment.

Integrating biological As(III) oxidation with Fe(0) electrocoagulation for arsenic removal from groundwater.

Arsenic (As) is a toxic element present in many (ground)water sources in the world. Most conventional As removal techniques require pre-oxidation of the neutral arsenite (As(III)) species to the negatively charged arsenate (As(V)) oxyanion to optimize As removal and minimize chemical use. In this work, a novel, continuous-flow As removal system was developed that combines biological As(III) oxidation by bacteria with Fe electrocoagulation (EC), an Fe(0)-based electrochemical technology that generates reactive Fe(III) precipitates to bind As. The bio-integrated FeEC system (bio-FeEC) showed effective oxidation and removal of 150 µg/L As(III), without the need of chemicals. To remove As to below the WHO guideline of 10 µg/L, 10 times lower charge dosage was required for the bio-FeEC system compared to conventional FeEC. This lower Fe dosage requirement reduced sludge production and energy consumption. The As(III) oxidizing biomass was found to consist of bacteria belonging to Comamonadaceae, Rhodobacteraceae and Acidovorax, which are capable of oxidizing As(III) and are common in drinking water biofilms. Characterization of the As-laden Fe solids by X-ray absorption spectroscopy indicated that both bio-FeEC and conventional FeEC produced solids consistent with a mixture of lepidocrocite and 2-line ferrihydrite. Arsenic bound to the solids was dominantly As(V), but a slightly higher fraction of As(V) was detected in the bio-FeEC solids compared to the conventional FeEC.

Long-term electrode behavior during treatment of arsenic contaminated groundwater by a pilot-scale iron electrocoagulation system.

Iron electrocoagulation (Fe-EC) is an effective technology to remove arsenic (As) from groundwater used for drinking. A commonly noted limitation of Fe-EC is fouling or passivation of electrode surfaces via rust accumulation over long-term use. In this study, we examined the effect of removing electrode surface layers on the performance of a large-scale (10,000 L/d capacity) Fe-EC plant in West Bengal, India. We also characterized the layers formed on the electrodes in active use for over 2 years at this plant. The electrode surfaces developed three distinct horizontal sections of layers that consisted of different minerals: calcite, Fe(III) precipitates and magnetite near the top, magnetite in the middle, and Fe(III) precipitates and magnetite near the bottom. The interior of all surface layers adjacent to the Fe(0) metal was dominated by magnetite. We determined the impact of surface layer removal by mechanical abrasion on Fe-EC performance by measuring solution composition (As, Fe, P, Si, Mn, Ca, pH, DO) and electrochemical parameters (total cell voltage and electrode interface potentials) during electrolysis. After electrode cleaning, the Fe concentration in the bulk solution increased substantially from 15.2 to 41.5 mg/L. This higher Fe concentration led to increased removal of a number of solutes. For As, the concentration reached below the 10 µg/L WHO MCL more rapidly and with less total Fe consumed (i.e. less electrical energy) after cleaning (128.4 µg/L As removed per kWh) compared to before cleaning (72.9 µg/L As removed per kWh). Similarly, the removal of P and Si improved after cleaning by 0.3 mg/L/kWh and 1.1 mg/L/kWh, respectively. Our results show that mechanically removing the surface layers that accumulate on electrodes over extended periods of Fe-EC operation can restore Fe-EC system efficiency (concentration of solute removed/kWh delivered). Since Fe release into the bulk solution substantially increased upon electrode cleaning, our results also suggest that routine electrode maintenance can ensure robust and reliable Fe-EC performance over year-long timescales.

Sustaining efficient production of aqueous iron during repeated operation of Fe(0)-electrocoagulation.

Iron-electrocoagulation is a promising contaminant (e.g. arsenic) removal technology that is based on electrochemical Fe(II) production from steel electrodes and subsequent transport of Fe(II) to the bulk solution, where contaminant removal occurs. Although Fe-electrocoagulation systems have been shown to effectively remove contaminants in extended field trials, the efficiency of field systems can be lower than in laboratory studies. One hypothesis for this disparity is that the Faradaic efficiency of short-term laboratory experiments is higher than field systems operated over extended periods. The Faradaic efficiency is a pivotal performance indicator that we define as the measured Fe dosage normalized by the theoretical Fe dosage calculated by Faraday's law. In this work, we investigated the Faradaic efficiency in laboratory experiments for up to 35 operating cycles (>2 months) with varied Fe(0) anode purity, charge dosage rate, and electrolyte composition. Our results showed that the Faradaic efficiency decreased continuously during repeated operation under typical field conditions (charge dosage rate = 4 C/L/min, synthetic groundwater) regardless of the Fe(0) anode purity, leading to a Faradaic efficiency ≈ 0.6 after 2 months. By contrast, increasing the charge dosage rate to ≥15 C/L/min produced a Faradaic efficiency >0.85 over the entire experiment for both Fe(0) anode purities. Electrolyte solutions free of oxyanions also resulted in sustained Faradaic efficiency >0.85, regardless of the charge dosage rate. Our results confirm a previously proposed relationship between low Faradaic efficiency and the formation of macroscopic electrode surface layers, which consist of Fe (oxyhydr)oxides on the anode and a mixture of Fe (oxyhydr)oxides and calcite on the cathode. Based on these results, we discuss potential strategies to maintain a high Faradaic efficiency during Fe-electrocoagulation field treatment.

Sorption of phosphate and silicate alters dissolution kinetics of poorly crystalline iron (oxyhydr)oxide.

Iron (oxyhydr)oxides (FeOx) control retention of dissolved nutrients and contaminants in aquatic systems. However, FeOx structure and reactivity is dependent on adsorption and incorporation of such dissolved species, particularly oxyanions such as phosphate and silicate. These interactions affect the fate of nutrients and metal(loids), especially in perturbed aquatic environments such as eutrophic coastal systems and environments impacted by acid mine drainage. Altered FeOx reactivity impacts sedimentary nutrient retention capacity and, eventually, ecosystem trophic state. Here, we explore the influence of phosphate (P) and silicate (Si) on FeOx structure and reactivity. Synthetic, poorly crystalline FeOx with adsorbed and coprecipitated phosphate or silicate at low but environmentally relevant P/Fe or Si/Fe ratios (0.02-0.1 mol mol-1) was prepared by base titration of Fe(III) solutions. Structural characteristics of FeOx were investigated by X-ray diffraction, synchrotron-based X-ray absorption spectroscopy and high-energy X-ray scattering. Reactivity of FeOx was assessed by kinetic dissolution experiments under acidic (dilute HCl, pH 2) and circum-neutral reducing (bicarbonate-buffered ascorbic acid, pH 7.8, Eh ∼ -300 mV) conditions. At these loadings, phosphate and silicate coprecipitation had only slight impact on local and intermediate-ranged FeOx structure, but significantly enhanced the dissolution rate of FeOx. Conversely, phosphate and silicate adsorption at similar loadings resulted in particle surface passivation and decreased FeOx dissolution rates. These findings indicate that varying nutrient loadings and different interaction mechanisms between anions and FeOx (adsorption versus coprecipitation) can influence the broader biogeochemical functioning of aquatic ecosystems by impacting the structure and reactivity of FeOx.