We investigated the adsorption of Tl+ onto purified Illite du Puy (IdP). Distribution coefficients (Kd) for trace Tl adsorption indicated a moderate pH-dependence from pH 2.5 to 11. Adsorption isotherms measured at Tl+ concentrations from 10–9 to 10–2 M at near-neutral pH on illite saturated with Na+ (100 mM), K+ (1 and 10 mM), NH4+ (10 mM) or Ca2+ (5 mM) revealed a high adsorption affinity of Tl+ in Na+- and Ca2+-electrolytes and strong competition with K+ and NH4+. Cation exchange selectivity coefficients for Tl+ with respect to Na+, K+, NH4+, and Ca2+ were derived using a 3-site sorption model. They confirmed the strong adsorption of Tl+ at the frayed edges of illite, with Tl selectivity coefficients between those reported for Rb+ and Cs+. X-ray absorption spectra of Tl adsorbed onto Na-exchanged IdP indicated a shift from adsorption of (dehydrated) Tl+ at the frayed edges at low loadings to adsorption of (hydrated) Tl+ on planar sites at the highest loadings. Our results suggest that illite is an important adsorbent for Tl in soils and sediments, considering its often high abundance and its stability relative to other potential adsorbents and the selective nature of Tl+ uptake by illite.
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.