Chromium Oxidation‐Reduction Chemistry at Soil Horizon Interfaces Defined by Iron and Manganese Oxides

Soil interfaces, including those defined by horizons, are a largely unstudied area in soils research, especially in terms of oxidation–reduction properties and processes. Despite this lack of research, understanding these interfaces is essential to the complete understanding of chromium chemistry in soils. Chromium is most commonly found in its non‐toxic trivalent [Cr(III)] and its toxic hexavalent [Cr(VI)] forms under normal environmental conditions. In soils, both Cr(III) and Cr(VI) can undergo oxidation–reduction processes as a result of microbes, organic C, or Fe(III)/Mn(III,IV) (hydr)oxides, and can exist as soluble ions in the soil solution, sorbed ions on the soil surface, soluble organic complexes, or precipitated compounds. Quantitatively determining each of these fractions of Cr can refine our understanding of how soil chemical properties and their differences across horizon interfaces may influence element speciation. This study addresses the question of how oxidation/reduction of Cr changes in mineralogically different soil horizons. The A and B horizons from seven soils encompassing a wide range of soil properties, were used in a series of leaching experiments which were set up as follows. Using a mechanical vacuum extractor, Cr(III) and Cr(VI) solutions were leached through A horizons, B horizons, and A horizons stacked on B horizons; the A horizon leachate was also leached through the B horizons in an attempt to dissect the mechanisms of interfacial oxidation–reduction processes. The Cr speciation results of the study indicate that there are significant interfacial properties affecting reduction of Cr(VI) in soils, but no interfacial effects on Cr(III) oxidation.