PROBING ADSORPTION OF AQUEOUS SELENATE AT MINERAL SURFACES VIA TOTAL INTERNAL REFLECTION RAMAN SPECTROSCOPY

The mobility and fate of environmental aqueous contaminants are governed in part by interactions which occur near and at environmental surfaces such as minerals. An elucidation of these interactions, adsorption etc., is necessary in order to predict the behavior for aqueous contaminants within natural environments. Selenate (SeO₄^2-) is an environmentally relevant inorganic contaminant of interest due to the toxicity of selenium compounds and the high aqueous solubility of the selenate anion. The adsorption of selenate was investigated at the solution/solid surface for two minerals, silica and hematite, at a near neutral pH as a function of concentration via total internal reflection (TIR) Raman spectroscopy. TIR-Raman spectroscopy is a linear optical vibrational spectroscopic technique which allows for enhanced probing of surface regions due to the total internal reflection phenomenon where the detected signal originates only from the first ~100 nm of the solution/solid interface. The use of TIR-Raman spectroscopy to examine selenate behavior at mineral surfaces which feature different surface charges at a near neutral pH due to differing isoelectric points, silica is negatively charged and hematite is positively charged at pH ~6, allows for the direct observation of anion adsorption speciation as a function of surface charge.