Thallium isotopic fractionation in soil: the key controls

Abstract

We studied the key geochemical and mineralogical factors that could affect the fractionation of stable thallium (Tl) isotopes in soil. A set of grassland soil samples enriched in geogenic Tl in combination with selected Tl-containing mineral materials from the Czech Republic (Kluky) were investigated for this purpose. The results demonstrate significant incorporation of Tl in pedogenic (specific) Mn-oxide, which led to a large accumulation of the heavy ²⁰⁵Tl isotope (∼+14 ε²⁰⁵Tl units), presumably resulting from oxidative Tl sorption. Consequently, we concluded that the Mn-oxide-controlled Tl uptake is the primary cause of the observed ²⁰⁵Tl enrichment in the middle profile zone, at the A/B soil horizon interface, with up to +4 of ε²⁰⁵Tl. Furthermore, our results displayed a clear relationship between the Tl isotopic fractionation degree and the Mn-oxide soil concentration (R² = 0.6), as derived from the oxalate-extractable data. A combination of soil and mineralogical considerations suggests that ²⁰⁵Tl enrichment in respective soil samples is also partly due to the Tl present in micaceous clay minerals, mainly illite, which is the predominant pedogenic Tl host phase. In line with our previous results, this Tl behavior can be inferred from systematic Mn-oxide degradation and the associated Tl (enriched in ²⁰⁵Tl) cycling in the studied soils and thus, presumably in the redoximorphic soils in general.