Arsenic sorption onto iron oxide spinels such as magnetite could contribute to immobilization of arsenite (AsO33-), the reduced, highly toxic form of arsenic in contaminated anoxic ground waters, 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 similar to 20 mu mol/m(2) 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 ill average particle diameter of 34 nm produced by reduction of lepidocrocite (gamma-FeOOH) by Shewanella putrefaciens and (2) a synthetic, abiotic, nanocrystalline magnetite with,it) 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 mu mol/m(2) indicate that As(III) forms dominantly inner-sphere, tridentate, hexanuclear, corner-sharing surface complexes (C-3) in which AsO3 pyramids occupy vacant tetrahedral sites oil octahedrally terminated {111}surfaces of magnetite. Formation of this type of surface complex results in a decrease in dissolved As(III) concentration below file maximum concentration level recommended by the World Health Organization (10 mu g/L), which corresponds to As(III) surface coverages of 0.16 and 0.19 mu mol/m(2) 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 mu mol/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/nm(2) or 5.3 mu mol/m(2)). This finding helps to explain the exceptional As(III) sorption capacity of nanocrystalline magnetite particles ( > 10 mu mol/m(2)). However, the higher solubility of the amorphous surface precipitate compared to the C-3 surface complexes causes a dramatic increase of dissolved As concentration for coverages above 1.9 mu mol/m(2).