Anoxic Oxidation of Chromium

Abstract

Naturally occurring Cr(VI) has been ascribed to terrestrial Cr(III) oxidation by Mn (di)oxides, generated through reaction of Mn(II) with molecular oxygen (O2). However, hydrogen peroxide (H2O2) is a potential oxidant of Cr(III) that may form in serpentinization (high H2, low O2) systems where chromite [i.e., the main mineralogical source of Cr(III)] is abundant. Accordingly, here we evaluate H2O2 and chromite interactions in serpentinization systems to determine pathways of Cr(III) oxidation that alters the current paradigm of O2-dependent oxidation. Field observations support that metastable H2O2 and Cr(VI) are present in serpentinization-related fluids relatively absent of O2. Further, laboratory experiments demonstrate and support that H2O2 is a kinetically facile oxidant of chromite, especially under alkaline conditions, which provides a variety of alternative means by which Cr(VI) may be generated and supplied to the oceans not directly linked to atmospheric O2. Thus, Cr(III) oxidation pathways, and their influence on the Cr isotopic record, must account for anoxic Cr(III) oxidation in serpentinization systems as well as a variety of H2O2-induced Cr(III) oxidation pathways that may occur in both terrestrial and marine systems.