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.

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.

Groundwater-native Fe(II) oxidation prior to aeration with HO 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 (O). 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)).

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, KMnO, 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.

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

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 (O), intermediate (NaOCl), and strong (KMnO) 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.

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.

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.

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.

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.

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.

Characteristics of Fe and Mn bearing precipitates generated by Fe(II) and Mn(II) co-oxidation with O, MnO and HOCl in the presence of groundwater ions.

In this work, we combined macroscopic measurements of precipitate aggregation and chemical composition (Mn/Fe solids ratio) with Fe and Mn K-edge X-ray absorption spectroscopy to investigate the solids formed by co-oxidation of Fe(II) and Mn(II) with O, MnO, and HOCl in the presence of groundwater ions. In the absence of the strongly sorbing oxyanions, phosphate (P) and silicate (Si), and calcium (Ca), O and HOCl produced suspensions that aggregated rapidly, whereas co-oxidation of Fe(II) and Mn(II) by MnO generated colloidally stable suspensions. The aggregation of all suspensions decreased in P and Si solutions, but Ca counteracted these oxyanion effects.

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.

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).

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 Mn with permanganate. Manganese oxide-coated sand effectively oxidized bisphenol A under typical infiltration conditions and sustained reactivity longer than previously described geomedia.

Bacterial bioreporter detection of arsenic associated with iron oxides.

Bacterial bioreporters are engineered microorganisms that have found recent application as a low-cost method of detecting arsenic (As) in environmental systems. However, no assessment exists of bioreporter detection of particle-bound As. We applied an Escherichia coli-based bioreporter to assess the bioavailability of As(v) adsorbed by goethite (α-FeOOH), 2-line ferrihydrite and As(v) co-precipitated with Fe(iii).

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 HO instead of O as the initial Fe(II) oxidant, or a combination of all three.

Bacteria attenuation by iron electrocoagulation governed by interactions between bacterial phosphate groups and Fe(III) precipitates.

Iron electrocoagulation (Fe-EC) is a low-cost process in which Fe(II) generated from an Fe(0) anode reacts with dissolved O2 to form (1) Fe(III) precipitates with an affinity for bacterial cell walls and (2) bactericidal reactive oxidants. Previous work suggests that Fe-EC is a promising treatment option for groundwater containing arsenic and bacterial contamination. However, the mechanisms of bacteria attenuation and the impact of major groundwater ions are not well understood.

Sorption selectivity of birnessite particle edges: a d-PDF analysis of Cd(ii) and Pb(ii) sorption by δ-MnO2 and ferrihydrite.

Birnessite minerals (layer-type MnO2), which bear both internal (cation vacancies) and external (particle edges) metal sorption sites, are important sinks of contaminants in soils and sediments. Although the particle edges of birnessite minerals often dominate the total reactive surface area, especially in the case of nanoscale crystallites, the metal sorption reactivity of birnessite particle edges remains elusive. In this study, we investigated the sorption selectivity of birnessite particle edges by combining Cd(ii) and Pb(ii) adsorption isotherms at pH 5.

Formation of macroscopic surface layers on Fe(0) electrocoagulation electrodes during an extended field trial of arsenic treatment.

Extended field trials to remove arsenic (As) via Fe(0) electrocoagulation (EC) have demonstrated consistent As removal from groundwater to concentrations below 10 μg L(-1). However, the coulombic performance of long-term EC field operation is lower than that of laboratory-based systems. Although EC electrodes used over prolonged periods show distinct passivation layers, which have been linked to decreased treatment efficiency, the spatial distribution and mineralogy of such surface layers have not been investigated.

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.

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(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.

Electro-chemical arsenic remediation: field trials in West Bengal.

Millions of people in rural South Asia are exposed to high levels of arsenic through groundwater used for drinking. Many deployed arsenic remediation technologies quickly fail because they are not maintained, repaired, accepted, or affordable. It is therefore imperative that arsenic remediation technologies be evaluated for their ability to perform within a sustainable and scalable business model that addresses these challenges.

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.