Redox reactions and the influence of natural Mn oxides on Cr oxidation in a contaminated site in northern Italy: evidence from Cr stable-isotopes and EPR spectroscopy

Hexavalent chromium-contaminated groundwaters and sediments in northern Italy have been studied using the Cr stable-isotope systematics and electron spin resonance spectroscopy (ESR), in order to explore redox changes and soil-groundwater interactions. The isotopic data indicate a possible Cr(VI) source released into the environment from an industrial plant. EPR spectra on the sediments which constitute the aquifers show a broad asymmetric absorption due to coupled Fe(III) and coupled Cr(III) ions and a well resolved hyperfine structure due to manganese ions, resulting from Mn(IV) and Mn(II). The isotopic and EPR data support the hypothesis of Cr(III) being oxidized by Mn oxides which are widespread in the aquifer, possibly related to the oscillation of the phreatic level. The obtained results highlight the usefulness of chromium stable isotopes as environmental tracers and support the observations that naturally occurring Mn oxides in soils may catalize Cr oxidation from the stable Cr(III) form to the toxic Cr(VI) soluble form, yielding valuable information in planning remediation interventions.