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.

We apply Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and pair distribution function (PDF) analysis of high-energy X-ray scattering to investigate the effects of bivalent cation-oxyanion pairs on the structure of Fe(III) precipitates formed from the oxidation of Fe(II) generated by the electrolytic dissolution of Fe(0) electrodes. We found that Fe(II) oxidation in the presence of weakly adsorbing electrolytes (NaCl, CaCl 2 , MgCl 2 ) leads to pseudo-lepidocrocite (Lp; c-FeOOH), a poorly crystalline version of Lp with low sheet-stacking coherence. In the absence of bivalent cations, P and As(V) have similar uptake behavior, but different effects on the average Fe(III) precipitate structure: pseudo-Lp dominates in the presence of P, whereas a disordered ferrihydrite-like precipitate akin to hydrous ferric oxide (HFO) is the dominant phase that forms in the presence of As(V). Despite its lower affinity for Fe(III) precipitates, Si leads to Si-HFO in all conditions tested. The presence of 1 mM Ca 2+ or Mg 2+ enhances oxyanion uptake, destabilizes the colloidally stable oxyanion-bearing particle suspensions and, in some P and As(V) electrolytes, results in more crystalline precipitates. The trends in oxyanion uptake and Fe(III) precipitate structure in the presence of Ca 2+ /Mg 2+ suggest a systematic decrease in the strength of bivalent cation:oxyanion interaction in the order of Ca 2+ > Mg 2+ and P > As(V) ) Si. Using the PDF technique, we identify the polyhedral linkages that contribute to the intermediate structures (>6 A ˚) of disordered, nanoscale oxyanion-bearing Fe(III) precipitate samples. Our results suggest that oxyanions present during Fe(III) polymerization bind to corner-sharing Fe surface sites leading to a precipitate surface deficient in corner-sharing Fe, whereas the edge-and corner-sharing Fe sites in the precipitate core likely remain intact.

The photoreductive dissolution of Mn(IV) oxide minerals in sunlit aquatic environments couples the Mn cycle to the oxidation of organic matter and fate of trace elements associated with Mn oxides, but the intrinsic rate and mechanism of mineral dissolution in the absence of organic electron donors is unknown. We investigated the photoreduction of δ-MnO 2 nanosheets at pH 6.5 with Na or Ca as the interlayer cation under 400-nm light irradiation and quantified the yield and timescales of Mn(III) production. Our study of transient intermediate states using time-resolved optical and X-ray absorption spectroscopy showed key roles for chemically distinct Mn(III) species. The reaction pathway involves (i) formation of Jahn-Teller distorted Mn(III) sites in the octahedral sheet within 0.6 ps of photoexcitation; (ii) Mn(III) migration into the interlayer within 600 ps; and (iii) increased nanosheet stacking. We propose that irreversible Mn reduction is coupled to hole-scavenging by surface water molecules or hydroxyl groups, with associated radical formation. This work demonstrates the importance of direct MnO 2 photoreduction in environmental processes and provides a framework to test new hypotheses regarding the role of organic molecules and metal species in photochemical reactions with Mn oxide phases. The timescales for the production and evolution of Mn(III) species and a catalytic role for interlayer Ca 2+ identified here from spectroscopic measurements can also guide the design of efficient Mn-based catalysts for water oxidation.manganese oxide | photoreduction | band-gap excitation | pump-probe spectroscopy | water oxidation M anganese is a key element in environmental processes, catalytic materials, and biological systems due to its rich redox chemistry and ability to form species with a high oxidizing potential. Photochemical processes can enhance significantly the cycling of Mn between the +4, +3, and +2 valence states (1-3). Photoreduction of Mn(IV) is the first step in the reductive dissolution of birnessite minerals in the euphotic zone of marine and lacustrine environments (4-6). This process couples the biogeochemical cycle of Mn to the redox cycling of carbon and trace metals associated with Mn oxide phases. In addition, the greater role of Mn(IV) photoreduction relative to microbial Mn(II) oxidation leads to the predominance of dissolved over particulate Mn in the photic zone of natural waters (1). Thermodynamic calculations predict that direct photoexcitation of Mn oxides in water by visible light will lead to net metal reduction over a wide range of environmentally relevant pH values (7). However, experimental evidence of direct photoexcitation of MnO 2 and subsequent photoreduction of Mn(IV) in the absence of organic electron donors is currently lacking. Experimental studies on the photochemical cycling of Mn have incorporated natural organic ligands that can enhance metal reduction via multiple pathways (5,8,9). These studies have identified aqueous Mn(II) as a reaction end product but have not inve...

Birnessite minerals are layer-type manganese oxides characterized by large surface areas, the presence of cation vacancy sites and varying amounts of structural and adsorbed Mn(III). In this study, we identify the conditions that favor trace metal adsorption on the edge surfaces of birnessite nanoparticles by using Ni as a probe ion for Ni K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy and geometry optimizations based on density function theory (DFT). In d-MnO 2 nanoparticles free of Mn(II,III) at pH 6.6, Ni was adsorbed primarily at vacancy sites, with a minor fraction of Ni present as a double-edge sharing (DES) or a double-corner sharing (DCS) complex at surface loadings exceeding the vacancy content. In Mn(III)-rich d-MnO 2 nanoparticles, about 80% of the adsorbed Ni formed a mixture of DES and DCS complexes at particle edges in samples with loadings ranging from 0.01 to 0.08 mol Ni mol À1 Mn, with only a small fraction of vacancy sites available to adsorb Ni. The presence of Mn(III) at the nanoparticle edges also changed the architecture of the DES complex, causing the Ni octahedra to adsorb onto the cavity formed between two Mn(III) octahedra at the particle edges. The EXAFS-derived Ni-Mn interatomic distances of 3.01-3.05 Å for this "flipped" Ni-DES complex were in excellent agreement with those obtained by DFT geometry optimization. Edge surfaces on birnessite nanoparticles have a lower affinity for trace metals than vacancy sites, but have a moderate sorption capacity (ca. 0.14 mol Ni mol À1 Mn at vacancies vs. 0.06 mol Ni mol À1 Mn at edge surfaces). Finally, although Mn(III) increases the relative proportion of Ni adsorbed at particle edges by blocking sorption sites on the basal surface, the overall sorption capacity of the mineral diminishes significantly.

Bacteriogenic birnessite nanoparticles have a large capacity to adsorb metal cations due to their high proportion of cation vacancy defects. In the current study, a synergistic experimental-computational approach was used to study the molecular-scale mechanisms of Ni sorption at varying loadings and at pH 6-8 using the hexagonal birnessite produced by Pseudomonas putida GB-1. We found that Ni is scavenged effectively by the biomass-