XANES reflects coordination change and underlying surface disorder of zinc adsorbed to silica.

Zinc K-edge X-ray absorption near-edge structure (XANES) spectroscopy of Zn adsorbed to silica and Zn-bearing minerals, salts and solutions was conducted to explore how XANES spectra reflect coordination environment and disorder in the surface to which a metal ion is sorbed. Specifically, XANES spectra for five distinct Zn adsorption complexes (Zn) on quartz and amorphous silica [SiO] are presented from the Zn-water-silica surface system: outer-sphere octahedral Zn on quartz, inner-sphere octahedral Zn on quartz, inner-sphere tetrahedral Zn on quartz, inner-sphere octahedral Zn on SiO and inner-sphere tetrahedral Zn on SiO. XANES spectral analysis of these complexes on quartz versus SiO reveals that normalized peak absorbance and K-edge energy position generally decrease with increasing surface disorder and decreasing Zn-O coordination. On quartz, the absorption-edge energy of Zn ranges from 9663.0 to 9664.1 eV for samples dominated by tetrahedrally versus octahedrally coordinated species, respectively. On SiO, the absorption-edge energy of Zn ranges from 9662.3 to 9663.4 eV for samples dominated by tetrahedrally versus octahedrally coordinated species, respectively. On both silica substrates, octahedral Zn presents a single K-edge peak feature, whereas tetrahedral Zn presents two absorbance features. The energy space between the two absorbance peak features of the XANES K-edge of tetrahedral Zn is 2.4 eV for Zn on quartz and 3.2 eV for Zn on SiO. Linear combination fitting of samples with a mixture of Zn complex types demonstrates that the XANES spectra of octahedral and tetrahedral Zn on silica are distinct enough for quantitative identification. These results suggest caution when deciphering Zn speciation in natural samples via linear combination approaches using a single Zn standard to represent sorption on a particular mineral surface. Correlation between XANES spectral features and prior extended X-ray absorption fine structure (EXAFS) derived coordination environments for these Zn on silica samples provides insight into Zn speciation in natural systems with XANES compatible Zn concentrations too low for EXAFS analysis.