Lithogenic thallium behavior in soils with different land use

Abstract

In order to evaluate the effect of different land use on lithogenic Tl geochemistry, two forest and grassland soils developed on an identical Tl-rich substrate were examined. For this purpose a complex soil-plant investigation supplemented by mineralogical methods was performed. The modified BCR sequential extraction combined with X-ray diffraction analysis (XRD) and voltammetry of microparticles (VMP) were performed for a detailed insight on lithogenic Tl speciation and availability in both contrasting soils. It was revealed that soil forming processes like bioturbation and probably dust deposition may influence the increased input of lithogenic Tl into the forest floor. Thallium was predominantly bound within the residual fraction (up to 95%) corresponding to primary silicates (mainly orthoclase and muscovite) and probably secondary illite, which were detected by XRD in all studied horizons. Thus, stable silicates can be thought as the phases controlling the solubility of lithogenic Tl in both the forest and grassland soils. The highest portion (~ 5%) of “labile” Tl was found in the organic horizons of the forest soil indicating a distinct role of forest soil organic matter (SOM) on Tl mobilization and availability. Thallium adsorption was dominated by an identified non-crystalline Mn(III,IV) oxide detected by VMP proving thus its strong affinity for Mn oxides in mineral soils. On the contrary, Tl adsorption by more abundant Fe(III) oxides (goethite and ferrihydrite) was evaluated to be negligible. Organically bound Tl in the forest floor was found to be associated with primary SOM corresponding to the raw and partially decomposed litter of Norway spruce ( Picea abies L.). Moreover, a relatively high Tl uptake was recorded by this species. In contrast, lithogenic Tl uptake by common grasses like red clover ( Trifolium pratense L.) or timothy grass ( Phleum pratense L.) seems to be very low.