Effect of low-molecular-weight organic acids on the leaching of thallium and accompanying cations from soil – A model rhizosphere solution approach

Abstract

The kinetic batch leaching of Tl-bearing mineral soil in 500 μM solutions of citric, oxalic and acetic acid was performed to simulate the release of Tl and main cations (Al, Fe, Si, K, Ca, Mn) in the rhizosphere-like environment. The obtained data demonstrate that low-molecular-weight organic acids (LMWOAs) significantly contribute to soil alteration accompanied by the release of lithogenic Tl (if present). The highest mobilization rates for Tl were observed after 0.5 h of leaching (with maximal values obtained for oxalate) followed by a substantial decrease. Thallium extractability in the organic acid solutions was up to 2.8-fold higher compared to water. Based on the experimental and speciation modeling data, Tl release is mainly pH-driven and can be attributed to acid ion exchange and/or acid leaching, as the formation of Tl-LMWOA complexes is negligible. The main Tl solubility-controlling phases predicted include illite and the identified Mn(III,IV) oxide due to their ability to efficiently sorb Tl + on the mineral surfaces from which Tl can be potentially mobilized. The role of primary silicates (i.e., orthoclase and muscovite) in the total process of Tl mobilization seems to be of lesser importance because the supposed alteration/dissolution of these phases during LWMOA leaching was limited.