Reductive Transformation of MnO<sub>2</sub> Controls Thallium Remobilization: Differential Effects of Layered and Tunneled Structures
Wanpeng Chen, Juan Liu, Xiaoliu Huangfu, Yan Chen, Wenye Zhong, Yu Liu, Yuheng Huang, Hongxia Liu
Abstract
Mn oxides play a critical role in Tl scavenging and accumulation in the epibiotic environment. However, the effect of Mn oxide reduction in the Mn/Fe cycle on Tl mobilization is not clear. Herein, the influence of Mn oxide configuration, oxygen environment, and degree of reduction on MnO 2 transformation and associated Tl species distribution is investigated. In oxic environments, both typical δ-MnO 2 and α-MnO 2 structures (i.e., layered and tunneled, respectively) can immobilize Tl(I) for a long time. In mild-to-moderate reducing anoxic environments, the drastic reductive transformation of δ-MnO 2 results in Tl binding, mainly in an exchangeable form. In highly reducing environments, δ-MnO 2 or α-MnO 2 is converted to Manganite, resulting in the release of more Tl. Tl-L III edge X-ray absorption spectroscopy indicates that oxidized Tl(III) (54–62%) is converted to structural Tl(I) (67–80%) and bound to interlayer/tunnel centers during the reductive transformation of MnO 2, which enhances Tl exchange in δ-MnO 2 and leads to Tl immobilization in α-MnO 2 . Our results show that anoxic Tl(I)-/Mn(II)-/Fe(II)-induced MnO 2 transformation can enhance Tl mobilization, and tunneled MnO 2 may have a more sustainable Tl immobilization potential than layered MnO 2, which improves the general understanding of the geochemical behavior of Tl in different Mn-related reducing environments.