Immobilization of Thallium(I) Using Ferromanganese Sulfides

Thallium (Tl) is a toxic element typically enriched in sulfide minerals and ferromanganese oxides, and its immobilization depends largely on the stability of thallium sulfide (Tl2S) under various environmental conditions. This study examines Tl(I) immobilization using ferromanganese sulfides, focusing on the effects of Fe/Mn/S molar ratios, oxygenation levels, and stirring intensity on Tl2S stability. Under anaerobic conditions, Tl(I) immobilization efficiency reached 96.1 ± 0.3% in 14 days and increased to 99.4 ± 0.2% over 6 months. Under microaerobic and aerobic conditions, efficiencies decreased to 85.7 ± 0.4 and 83.4 ± 0.8%, respectively. Oxygen facilitated the formation of Fe/Mn (oxyhydr)oxides, a sink for Tl(I), primarily present as ≡FeOTl and ≡MnOTl. Continuous stirring enhanced the removal of Tl(I) under anaerobic conditions, whereas static conditions favored Tl(I) immobilization in aerobic environments. Under aerobic conditions, sulfides were oxidized into elemental sulfur (77.2%) and sulfate (11.7%), leading to Tl(I) dissolution and an impact on its immobilization dynamics. Pb2+, Hg2+, Cu2+, Ni2+, and Zn2+ further promoted Tl(I) dissolution through competitive adsorption and a reduction in solution pH. Key strategies for Tl(I) immobilization include maintaining low dissolved oxygen and redox potential levels, enhancing surface hydroxyl complexation, and promoting sulfide-induced precipitation and electrostatic adsorption. This study provides insights into Tl(I) immobilization dynamics within complex Fe–Mn–S systems subjected to redox cycling and varied environmental conditions.