Reactions between Cr(VI) solutions and pyrite: chemical and surface studies

Geochemical processes that result in the reduction of hexavalent chromium in natural waters with pyrite (FeS2) have been studied at varying degrees of pH (from 1.2 to 12.6) and solution concentration (from 0.001 to 0.3 M of Cr6+) in order to illustrate the differences in the proportions of elements between the aqueous and solid phases and to infer mechanisms that limit the Cr6+ concentration in pore-waters in iron sulfide-rich environments. The experiments were carried out in the absence of oxygen and on pyrite grains previously treated to remove any oxide or sulfur layer at the mineral surface. The methods used to characterize reacting solutions and mineral surface comprised: chemical analyses (microprobe analyses and inductively coupled plasma analyses), scanning electron microscopy, atomic force microscopy and X-ray single crystal analysis. The results suggest that: 1) mineral dissolution increases with decreasing pH, whereas it is close to zero at pH > 7; at alkaline pH, the Cr6+ reduction is very low and the decrease in total Cr probably indicates the formation of precipitated phases, like FeCrO4, on the pyrite surface; 2) Cr6+ reduction is significant at pH < 2.3. Cr6+ to Cr3+ reduction involves the oxidation of Fe2+ and S22- on the pyrite surfaces, following the reaction 2FeS2+5Cr2O72-+32H+2Fe(OH)3 +4(SO4)2- +10Cr3+ +13H2O; 3) at acidic pH all the pyrite crystals show a variable Cr content on the surfaces.