Surface reactions of chromium in soils and waters

Abstract Chromium is a redox active metal that persists as either Cr(III) or Cr(VI) in the environment. These two oxidation states have opposing toxicities and mobilities: Cr(III) is rather benign and immobile in soils while Cr(VI) is toxic and readily transported. Reactions influencing Cr chemistry in soils and waters must be known in order to predict and understand the fate of this potentially hazardous element. Reactions at the solid-water interface have important consequences on the bioavailability (sorption reactions) and hazard (redox reactions) of Cr. Accordingly, this paper describes surface reactions that influence Cr chemistry in soils. Specifically, retention reactions of Cr(III) and Cr(VI) are described, e.g., adsorption of Cr(III) and Cr(VI) on goethite, along with interfacial redox reactions, e.g., Cr(III) oxidation by manganese oxides and Cr(VI) reduction by Fe(ll). The influences of organic chelates on these reactions are also detailed. Direct evidence on the specific reactions of Cr at the solid-water interface are provided; techniques used in this paper to detail the reactions of Cr include X-ray absorption fine structure spectroscopy, scanning probing microscopies, and high-resolution transmission electron microscopy.