Re-evaluating the role of iron and manganese oxides in thallium retention: Insights from laboratory adsorption and soil pot experiments.

Iron (Fe) and manganese (Mn) oxides play a crucial role in thallium (Tl) retention. However, the specific roles they play in this process remain unclear. Here, Fe oxides (goethite/hematite), Mn oxides (birnessite/MnO2), and Fe-Mn binary oxides (Goe-MnOx/Hem-MnOx) were synthesized and used in single and mixed adsorption systems. Goethite and birnessite were then selected as amendments for the Lactuca sativa pot experiment. Results showed that Mn oxides exhibited higher adsorption capacity for Tl(I) than Fe-Mn binary oxides or Fe oxides. Adsorption energies calculated by density-functional-theory (DFT) further confirm the stronger affinity of birnessite for Tl(I). In mixed systems, as the mass ratio of birnessite-to-goethite/hematite increased from 1:0 to 1:200, Tl(I) adsorption capacities of the mixture decreased significantly by 65.5 % and 76.9 %. X-ray photoelectron spectroscopy analysis confirmed the oxidative uptake of Tl(I) by birnessite. Additionally, the distribution of Fe associated with the Mn phase collected by transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDS) indicated its inhibitory effect during the Tl(I) retention process by Mn(IV) oxides. Furthermore, in the pot experiment, goethite showed no discernible effect on lettuce growth or Tl uptake. In contrast, birnessite significantly lowered shoot Tl concentrations (84.7 %-94.4 %) and this was associated with the changes in the speciation of soil Mn and Tl as revealed by principal-component-analysis (PCA) and correlation analysis. This study clarifies the role of Fe/Mn oxides in the environmental behavior of Tl, offering valuable implications for the development of effective remediation strategies and ensuring safe crop production in Tl-contaminated soils.