Isotopic evidence of Cr(III) oxidation in soils and the occurrence of hexavalent chromium in the High Friuli Plain aquifer (northern Italy).

The Cr(III)-Cr(VI) cycling in soils and groundwaters is of great environmental concern. In particular, Cr(III) is an essential micro-nutrient and is considered to be the stable form in soils where it forms precipitates and is strongly sorbed on the charged surface of solids. Cr(VI) is toxic and a suspect carcinogen, stable under oxidizing conditions in aqueous forms and mobile as soluble oxyanions at the pH of natural waters. Under the redox and pH conditions usually found in the environment, Cr(VI) has the tendency to be reduced to Cr(III), e.g. on the surface of Fe(II)-bearing minerals or in the presence of aqueous Fe(II) or organic matter, acting as electron donors, and in this form it is scavenged from solution. However, it has been documented that in the presence of various forms of Mn-oxyhydroxides in soils Cr(III) is readily oxidized and the conversion of Cr(III) to Cr(VI) represents a potential pollution hazard, critical to predict the fate of hexavalent Cr contamination in waters. Cr isotopes have been demonstrated to be sensitive tracers of the Cr redox changes in aqueous media through both equilibrium and kinetic (Rayleigh-type) fractionations occurring during Cr(VI) reduction. A limited isotopic fractionation has been so far reported during Cr(III) oxidation experiments and small effects are also expected during oxidation of Cr(III) in soils, even if a larger fractionation was inferred from BIF in ancient redox environments and attributed to Cr(III) oxidation catalyzed by Mn oxides. However, the changes in the Cr isotope composition during Cr(III) oxidation require further investigations, due to the different pathways reactions with possible intermediate Cr(IV) and Cr(V) species. In the present study the Cr-isotope systematics has been applied to groundwaters from the phreatic aquifer in the High Friuli Plain (northern Italy), an area which underwent extensive Cr(VI) contamination from manufacturing during 1997 with Cr(VI) concentrations in waters reaching 4 mg/l. Afterward, the industrial activity was made environmentally protective and a natural decay of contamination occurred over the years due to Cr(VI) reduction. Nevertheless, since 2002 and until the last survey at the beginning of 2011, new spikes in the Cr(VI) content reaching 1.6 mg/l have been observed in groundwaters. The measured 53Cr/52Cr isotopic ratio in contaminated waters, expressed in delta notation, show uncommon negative values ranging between -0.60 and -3.00 per mil. A negative value of -2.99 was also measured in Cr(VI) extracted from the sediments which characterize the contaminated site. The isotopic data are interpreted as tracing the oxidation of Cr(III) not from geogenic sources but originated by the reduction of Cr(VI) released during the pristine anthropogenic contamination, which accumulated onto Mn-oxide rich alluvial sediments. The mechanism of Cr(III) oxidation on Mn-oxides has been investigated by using HRTEM and micro-Raman techniques. These results indicate that the conditions favourable for Cr oxidation are likely to occur in the field soils of the study area, favoring the Cr(VI) plume migration and implying that suitable management activities are necessary for remediation.