Goethite Mediated Electron Transfer between Cr(VI) and AH2DS Under Anoxic Conditions.

Quinones are redox active, organic molecules, often present in soils as exudates from plants or microbial species, or quinoid functional groups on natural organic matter (NOM). 9,10-anthraquinone-2,6-disulfonic acid (AQDS) is commonly used as a model species to examine the influence of quinones on different biogeochemical cycles. When AQDS is used as an electron acceptor by microbes, increased rates and extents of electron transfer to transitions metals such as Cr(VI) are observed. Cr(VI) is a soluble, known carcinogen. Therefore, increasing Cr(VI) removal from solution by reduction to the less soluble species, Cr(III) is a desirable process. Due to the environmental abundance of Fe(III) (oxyhydr)oxides, such as goethite (α-FeOOH), it is important to study the influence of these mineral phases on the electron transfer reaction between AH₂DS and Cr(VI). In this study, this electron transfer reaction is examined in the presence and absence of goethite at four different ratios of AH₂DS:Cr(VI). Possible production of semi-quinone radical species may limit electron transfer, causing a decrease the percent yields of Fe(II) and Cr(III), both in the presence and absence of goethite. In the presence of goethite, the extent of Cr(VI) reduction to Cr(III) is inhibited, especially at the highest ratios of AH₂DS:Cr(VI) investigated. Additionally, X-ray absorption spectroscopy is performed to assess the binding environment of Cr with goethite and identify Cr(III) phases that form. Understanding abiotic electron transfer reactions that occur in systems with multiple redox active species is important to elucidate the contribution of abiotic redox reactions to biogeochemical cycling in natural soils.