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

Manganese dioxides (MnO2) with a high affinity for thallium (Tl), strongly influence its fate and toxicity in aquatic environments. The crystal structure of MnO2 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 MnO2. In this study, the fundamental data on the sorption and oxidative reactivity of Tl(I) by MnO2 with five known crystal structures (?-, ?-, ?-, ?-, ?-MnO2) was obtained at different pH values (5, 7, 12). The results showed that the uptake of Tl followed the order of ?-MnO2 > ?-MnO2 > ?-MnO2 > ?-MnO2 > ?-MnO2, and increased with increasing pH for each MnO2. Moreover, ?-MnO2 exhibited the strongest oxidation ability for Tl(I) at all the tested pH values, while ?-MnO2 could scarcely oxidize aqueous Tl(I). However, for ?-MnO2, ?-MnO2 and ?-MnO2, 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 ?-MnO2 and ?-MnO2 but slightly impacted the structure of ?-MnO2, ?-MnO2 and ?-MnO2. These findings might contribute to understanding the immobilization processes of Tl by different crystal structured MnO2 at various pH conditions.

Automated summary

The study found that different crystal structured MnO2 have varying adsorption and oxidation reactivity towards Tl(I), with α-MnO2 showing the strongest oxidation ability and β-MnO2 hardly oxidizing Tl(I). Additionally, the sorption of Tl(I) affected the crystallinity of MnO2.