Using x-ray photoelectron spectroscopy to discriminate among different sorption sites of micas: With implications for heterogeneous reduction of chromate at the mica-fluid interface
This contribution uses Cr2p and 3p binding energies (BEs) determined by X-ray photoelectron spectroscopy (XPS) to distinguish Cr III 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 Fe II and Fe III; 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 Cr III 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 Cr III-NaCl solutions relative to Cr III-KCl solutions. Cr2p and 3p BEs were bracketed by those for Cr III in the structure of silicates and in Cr III-oxyhydroxides. The BE of Cr III sorbed to micas was higher by about 0.3 eV after reaction with Cr III-NaCl solutions compared to Cr III-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 Cr III BE between the Cr III-NaCl and Cr III-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 Cr VI-NaCl solution. Cr III BEs systematically decrease with increasing sorption-reduction of Cr VI(aq), consistent with a model of Cr III accumulation on external sorption sites during heterogeneous reduction of Cr VI at the biotite-fluid interface.