USING X-RAY PHOTOELECTRON SPECTROSCOPY TO DISCRIMINATE AMONG DIFFERENT SORPTION SITES OF MICAS: WITH IMPLICATIONS FOR HETEROGENEOUS REDUCTION OF CHROMATE AT THE MICA-WATER INTERFACE

Abstract

This contribution uses Cr2p and 3p binding energies (BEs) determined by X-ray photoelectron spectroscopy (XPS) to distinguish CrIII sorbed to different mica sorption sites. The results were used to better understand mechanisms for coupled sorption-reduction of Cr(aq)VI by ferrous micas. The research is important because 1. Cr contamination is a serious and wide spread problem associated with a variety of industries; 2. Micas are important sorbents for Cr because they are ubiquitous, sorb cations and anions over a wide range of pH, and participate in heterogeneous redox reactions via structural FeII and FeIII; and 3. The mobility of cations sorbed by micas will depend, in part, on the dominant sorption site.Micas (two biotites, endmember phlogopite and muscovite) were reacted with CrIII solutions that contained variable concentrations of NaCl and KCl. Samples were extracted at timed intervals and mica edge orientations were analyzed by XPS. Cr(aq)III sorption was greater in CrIII-NaCl solutions relative to CrIII-KCl solutions. Cr2p and 3p BEs were bracketed by those for CrIII in the structure of silicates and in CrIII-oxyhydroxides. The BE of CrIII sorbed to micas was higher by about 0.3 eV after reaction with CrIII-NaCl solutions compared to CrIII-KCl aqueous solutions. At the experimental conditions, Na+ and K+ differentially block permanent charge sites but not variably charged edge sites of micas. Using the constraints inherent in our experimental design, we interpret the difference in CrIII BE between the CrIII-NaCl and CrIII-KCl experiments to reflect a change in the dominant sorption site, where high and low Cr BEs indicate Cr sorbed by the interlamellar region and variably charged functional groups at mica edges, respectively. We conclude that small BE shifts for cations sorbed to silicates can be interpreted with confidence if strict protocols are employed.Biotites were also reacted with a CrVI-NaCl solution. CrIII BEs systematically decrease with increasing sorption-reduction of CrVI(aq), consistent with a model of CrIII accumulation on external sorption sites during heterogeneous reduction of CrVI at the biotite-fluid interface.