Thallium Sorption onto Manganese Oxides

Abstract

The sorption of thallium (Tl) onto manganese (Mn) oxides critically influences its environmental fate and geochemical cycling and is also of interest in water treatment. Combined quantitative and mechanistic understanding of Tl sorption onto Mn oxides, however, is limited. We investigated the uptake of dissolved Tl(I) by environmentally relevant phyllo- and tectomanganates and used X-ray absorption spectroscopy to determine the oxidation state and local coordination of sorbed Tl. We show that extremely strong sorption of Tl onto vacancy-containing layered Δ-MnO2 at low dissolved Tl(I) concentrations (log Kd ≥ 7.4 for ≤10-8 M Tl(I); Kd in (L/kg)) is due to oxidative uptake of Tl and that less specific nonoxidative Tl uptake only becomes dominant at very high Tl(I) concentrations (>10-6 M). Partial reduction of Δ-MnO2 induces phase changes that result in inhibited oxidative Tl uptake and lower Tl sorption affinity (log Kd 6.2-6.4 at 10-8 M Tl(I)) and capacity. Triclinic birnessite, which features no vacancy sites, and todorokite, a 3 × 3 tectomanganate, bind Tl with lower sorption affinity than Δ-MnO2, mainly as hydrated Tl+ in interlayers (triclinic birnessite; log Kd 5.5 at 10-8 M Tl(I)) or tunnels (todorokite). In cryptomelane, a 2 × 2 tectomanganate, dehydrated Tl+ replaces structural K+. The new quantitative and mechanistic insights from this study contribute to an improved understanding of the uptake of Tl by key Mn oxides and its relevance in natural and engineered systems.