Insight into the long-term nonoxidative immobilization of thallium on birnessite

Birnessite has been considered as a potential thallium (Tl)-oxidizing mineral and Tl scavenger in natural and engineered systems. However, the birnessite aging that occurs during Tl sorption may severely limit the longterm effectiveness of Tl immobilization on birnessite. In this study, kinetic experiments were performed in batch coupled stirred-flow systems to investigate the effects of Tl(I) loading and pH on Tl(I) retention during short- and long-term Tl-birnessite interactions. Spectral analysis, microscopic observations, and theoretical calculations revealed a change in the Tl(I) binding mode to birnessite with time. In the short term, low levels of Tl(I) could be effectively immobilized through adsorption to birnessite vacancies. Structural disruption and vacancy occupation in birnessite resulted in the migration of adsorbed Tl(I) toward the edges of the mineral during a 24-d reaction period. Under acidic to neutral conditions, the migration of vacancy-bound Tl(I) induced sustained Tl(I) release (10-45%) from birnessite over the long-term. Under alkaline conditions, hydroxide ions contributed to the formation of Tl(I)-OH species with double edge sharing, which enhanced the binding ability (alpha approximate to 7 x 10-4 min-1) and stable retention of Tl(I) on birnessite. Understanding Tl(I) retention dynamics during long-term Tl-birnessite interactions elucidates the Tl cycle in Mn oxide-containing systems.