Thalliummobilität in der kritischen Zone - Sorption von Thallium an ausgewählten Tonmineralen
Zusammenfassung der Projektergebnisse
An understanding of Thallium (Tl) dispersion in the critical zone, which is the starting point to transfer this highly toxic element to the hydro-, bio- and anthroposphere, is of high societal importance. It requires a characterization of mineral phases serving as sinks or sources, due to either incorporation/adsorption on one hand or desorption on the other hand. Smectite, as one of the most abundant and relevant minerals controlling transfer of Tl in the critical zone was selected in this project. The common procedure for purifying smectites was optimized leading to substantial improvement of calcium balances of isotherm data. Complete isotherms between the end members Na-, K-, and Casmectite, respectively, were measured. For the system K-Ca-smectite, a temperature dependence of cation exchange was observed, but for the Ca-smectite side, this dependence was negligible. Temperature effects of Tl exchange can therefore expected to be minor. Adsorption of Tl and Rb to homoionic Ca-smectite was investigated in the system Tl-Ca, Rb-Ca and Rb-Tl-Ca. Rubidium and Tl show similar behavior, although distribution coefficients for Rb are slightly higher in line with an increased selectivity correlated to the slightly smaller effective radius of the hydrated Rb+ compared to the radius of Tl+. In addition to published studies with a focus on geogenic Tl enrichment (e.g. Erzmatt, Switzerland) this project delivers a case study on anthropogenic Tl enrichment derived from atmospheric deposition of emissions from Pb-Zn processing industry in an area close to Aachen (Germany). Anthropogenic Tl shows a low vertical mobility and is dominantly fixed in Mn oxide phases, humic matter and in clay mineral interlayer sites. Its’ mobility seems mainly controlled by exchange sites on clay minerals showing that it occurs as monovalent ionic species. It must have been emitted either as Tl+ or fastly transfers to this species and must be incorporated in soil phases. Due to its similar selectivity to Tl and its absence from anthropogenic sources, Rb is shown to be a highly suitable reference element to express Tl enrichment.
