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.