Effects of MnO2 crystal structure on the sorption and oxidative reactivity toward thallium(I)

Abstract

• Tl(I) uptake affinity decreased as: δ-MnO 2 > α-MnO 2 > γ-MnO 2 > β-MnO 2 > λ-MnO 2 . • α-MnO 2 exhibited the strongest oxidation ability for Tl(I). • Tl(I) was scarcely oxidized by β-MnO 2 over a wide range of pH values. • TlOH was involved in the redox process at alkaline pH conditions. Manganese dioxides (MnO 2 ) with a high affinity for thallium (Tl), strongly influence its fate and toxicity in aquatic environments. The crystal structure of MnO 2 plays a critical role in their activity for the uptake of heavy metals. However, few studies have examined the differences in interactions between Tl and various crystal structured MnO 2 . In this study, the fundamental data on the sorption and oxidative reactivity of Tl(I) by MnO 2 with five known crystal structures (δ-, α-, γ-, β-, λ-MnO 2 ) was obtained at different pH values (5, 7, 12). The results showed that the uptake of Tl followed the order of δ-MnO 2 > α-MnO 2 > γ-MnO 2 > β-MnO 2 > λ-MnO 2 , and increased with increasing pH for each MnO 2 . Moreover, α-MnO 2 exhibited the strongest oxidation ability for Tl(I) at all the tested pH values, while β-MnO 2 could scarcely oxidize aqueous Tl(I). However, for δ-MnO 2, γ-MnO 2 and λ-MnO 2 , the extent of Tl(I) oxidation was much larger at pH 12 than at pH 5 or 7, which was ascribed to the active species of TlOH involved in the redox process. Besides, the sorption of Tl(I) led to a decrease in crystallinity of δ-MnO 2 and α-MnO 2 but slightly impacted the structure of γ-MnO 2 , β-MnO 2 and λ-MnO 2 . These findings might contribute to understanding the immobilization processes of Tl by different crystal structured MnO 2 at various pH conditions.