Study of selenium sorption processes in volcanic ash using Total Reflection X-ray Fluorescence (TXRF)

Abstract : Selenium (Se) cycling around volcanoes has implications for human health given the high population density close to volcanoes and the narrow range between essential and toxic Se intake. To study the Se mobility during interaction between volcanic derived acid rain and volcanic ash, new analytical approaches are required. This paper explores the Total Reflection X-Ray Fluorescence (TXRF) for Se determination using sorption processes on volcanic ash as a geochemical application. Our experiments reveal the importance of the anion content of the rainwater for Se mobility in volcanic ash samples. Desorption studies of a weathered volcanic soil using the application of a dispersive liquid-liquid microextraction procedure (DLLME) to isolate Se prior to TXRF analysis (LOD 0.7 mu g/L) revealed much higher level of released Se in sulfuric acid rain compared to hydrochloric acid rain. Additionally, the dominance of selenate in the leaching solution suggests adsorption competition with sulfate as a Se release mechanism. Kinetic adsorption experiments using direct TXRF with as little as 10 mu L. solution (LOD 0.4-8 mu g/L depending on the sample characteristics) showed that the anions present play a key role in the selenite adsorption on volcanic ash by competitive behavior and/or changes in the pH conditions promoted by glass dissolution processes. Our experiments show the high potential of TXRF in such applications. Similarly, TXRF can be used as an analytical tool to study the mobility of trace elements in other geochemical studies.