CHROMIUM SORPTION BY PHLOGOPITE AND BIOTITE IN ACIDIC SOLUTIONS AT 25-DEGREES-C - INSIGHTS FROM X-RAY PHOTOELECTRON-SPECTROSCOPY AND ELECTRON-MICROSCOPY

Large single crystals of biotite and near endmember phlogopite were reacted with acidic Cr(VI)-and Cr(III)-bearing solutions, at 24-27-degrees-C and 1 atm. After reaction of the micas with Cr(VI) solutions, XPS analyses of biotite and phlogopite edges indicated little or no chromium on phlogopite but elevated concentrations of chromium on biotite. The binding energies for Cr2p are consistent with a Cr(III) species. XPS and SEM of biotites reacted with Cr(VI) solutions indicate that chromium is strongly sorbed by the edges of mica books relative to the basal plane. In fact, much of the chromium associated with the basal plane is sorbed by steps and micron-sized biotite flakes, or, in other words, by layer edges. TEM and SEM of biotite basal planes and edges show no Cr-rich precipitates. After reaction of the micas with a Cr(III) solution at pH = 3, XPS shows similar concentrations of a Cr(III) species on both biotite and phlogopite, and on the edges and basal planes. These results stand in strong contrast to the results from Cr(VI) solutions. XPS indicates that near-surface iron in biotite is more oxidized after reaction with Cr(VI)-bearing relative to Cr(III)-bearing solutions. A comparison of biotites and phlogopites from the Cr(VI) and Cr(III) experiments provides convincing evidence for coupled sorption and reduction of Cr(VI)aq at the biotite edge-fluid interface. Experiments with additions of potassium and sodium perchlorate confirm the importance of potassium in the heterogeneous coupled cation-electron transfer reaction. Initial observations indicate that biotite concentrated Cr(III) more strongly in Cr(VI) solutions than in the Cr(III) solution, at pH = 3 and ionic strength almost-equal-to 10(-3) M, despite the fact that there were orders-of-magnitude more Cr(III)aq in the Cr(III) solution than in the Cr(VI) solutions.