Reaction path modelling of serpentinite dissolution in meteoric water was performed, varying Fe2O3/(FeO + Fe2O3) weight ratios of serpentine and reproducing the analytical concentrations of relevant solutes, including Cr(VI), in the Mg-HCO3 groundwaters hosted in the ophiolitic aquifers of Italy. The occurrence of geogenic Cr(VI) in these groundwaters appears to be potentially controlled by the oxidation of trivalent Cr to the hexavalent redox state, driven by the reduction of trivalent Fe to the divalent redox state. In fact, trivalent Fe is the only oxidant present in suitable amounts in serpentinite rock, and even serpentine contains a high content of trivalent Fe as proven by recent studies. In contrast, the generally accepted hypothesis that geogenic Cr(VI) in waters interacting with serpentinites is driven by the reduction of trivalent and tetravalent Mn is questionable, since serpentinite rock has a low Mn content and it is necessary to invoke adsorption of trivalent Cr ions onto the surface of Mn oxides, which are oxidised as a surface complex to hexavalent Cr. Moreover, Mn oxides are present in the aqueous solution as suspended particles or in rocks as coatings or nodules.
Keywords: Hexavalent chromium; Ophiolite; Reaction path modelling; Serpentinite; Water-rock interaction.
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