Magnetite Stoichiometry (Fe(II)/Fe(III)) Controls on Trivalent Chromium Surface Speciation

While the elimination of the most toxic form of chromium (Cr(VI)) by its reduction to Cr(III) at the magnetite (Fe₃O₄) surface has widely been documented, elucidating the exact mechanism involved in Cr(III) sorption to magnetite has attracted less attention. Indeed, magnetite stoichiometry (R = Fe(II)/Fe(III)) is rarely controlled or monitored in Cr-magnetite interaction studies, although it was shown to affect not only redox transformation but also adsorption mechanisms of several contaminants. This study examined the interaction of 20 μM (∼1 mg L^-1) Cr(III) with 10 nm magnetites, whose stoichiometries were carefully defined (0.1 ≤ R ≤ 0.5) and preserved under anaerobic conditions in 10 mM NaCl. X-ray absorption spectroscopy showed the formation of a tridentate trinuclear inner-sphere surface complex, but it occurred only on oxidized magnetite (R0.1) or on stoichiometric magnetite (R0.5) under acidic conditions, where H⁺-promoted dissolution generated an Fe(II)-depleted surface. When magnetite stoichiometry increased, Cr surface speciation evolved in favor of a [Fe²⁺Cr_<i>x</i>³⁺Fe³⁺ _1-<i>x</i>]_OhFe_Td³⁺O₄-like solid solution in which Cr(III) partially substitutes Fe(III) in octahedral sites. This study reveals the joint effects of pH and magnetite stoichiometry on the Cr(III) sorption mechanism, demonstrating that Cr(III)-(hydr)oxide precipitation is not necessarily the driving process of Cr(III) elimination from solutions. These results will help predict the fate and transport of chromium as well as develop magnetite-based chromium remediation processes.