EXAFS and HRTEM evidence for As(III)-containing surface precipitates on nanocrystalline magnetite: Implications for as sequestration

Arsenic sorption onto iron oxide spinels such as magnetite could contribute to immobilization of arsenite (AsO₃^3-), the reduced, highly toxic form of arsenic in contaminated anoxic groundwaters, as well as to putative remediation processes. Nanocrystalline magnetite (< 20 nm) is known to exhibit higher efficiency for arsenite sorption than larger particles, sorbing as much as ∼20/μmol/m² of arsenite. To improve our understanding of this process, we investigated the molecular level structure of As(III)-containing sorption products on two types of fine-grained magnetite: (1) a biogenic one with an average particle diameter of 34 nm produced by reduction of lepidocrocite (y-FeOOH) by Shewanella putrefaciens and (2) a synthetic, abiotic, nanocrystalline magnetite with an average particle diameter of 11 nm. Results from extended X-ray absorption spectroscopy (EXAFS) for both types of magnetite with As(III) surface coverages of up to 5 μmol/m² indicate that As(III) forms dominantly inner-sphere, tridentate, hexanuclear, corner-sharing surface complexes (³C) in which AsO₃ pyramids occupy vacant tetrahedral sites on octahedrally terminated {111} surfaces of magnetite. Formation of this type of surface complex results in a decrease in dissolved As(III) concentration below the maximum concentration level recommended by the World Health Organization (10 μg/L), which corresponds to As(III) surface coverages of 0.16 and 0.19 μmol/m² in our experiments. In addition, high-resolution transmission electron microscopy (HRTEM) coupled with energy dispersive X-ray spectroscopy (EDXS) analyses revealed the occurrence of an amorphous As(III)-rich surface precipitate forming at As(III) surface coverages as low as 1.61 μmol/m ². This phase hosts the majority of adsorbed arsenite at surface coverages exceeding the theoretical maximum site density of vacant tetrahedral sites on the magnetite {111} surface (3.2 sites/nm² or 5.3 μmol/m²). This finding helps to explain the exceptional As(III) sorption capacity of nanocrystalline magnetite particles (>10 μmol/m2). However, the higher solubility of the amorphous surface precipitate compared to the ³C surface complexes causes a dramatic increase of dissolved As concentration for coverages above 1.9 μmol/m².