Thallium adsorption onto phyllosilicate minerals.

The adsorption of thallium (Tl) onto phyllosilicate minerals plays a critical role in the retention of Tl in soils and sediments and the potential transfer of Tl into plants and groundwater. Especially micaceous minerals are thought to strongly bind monovalent Tl(I), in analogy to their strong binding of Cs. To advance the understanding of Tl(I) adsorption onto phyllosilicate minerals, we studied the adsorption of Tl(I) onto Na- and K-saturated illite and Na-saturated smectite, two muscovites, two vermiculites and a naturally Tl-enriched soil clay mineral fraction. Macroscopic adsorption isotherms were combined with the characterization of the adsorbed Tl by X-ray absorption spectroscopy (XAS). In combination, the results suggest that the adsorption of Tl(I) onto phyllosilicate minerals can be interpreted in terms of three major uptake paths: (i) highest-affinity inner-sphere adsorption of dehydrated Tl+ on a very low number of adsorption sites at the wedge of frayed particle edges of illite and around collapsed zones in vermiculite interlayers through complexation between two siloxane cavities, (ii) intermediate-affinity inner-sphere adsorption of partially dehydrated Tl+ on the planar surfaces of illite and muscovite through complexation onto siloxane cavities, (iii) low-affinity adsorption of hydrated Tl+, especially in the hydrated interlayers of smectite and expanded vermiculite. At the frayed edges of illite particles and in the vermiculite interlayer, Tl uptake can lead to the formation of new wedge sites that enable further adsorption of dehydrated Tl+. On the soil clay fraction, a shift in Tl(I) uptake from frayed edge sites (on illite) to planar sites (on illite and muscovite) was observed with increasing Tl(I) loading. The results from this study show that the adsorption of Tl(I) onto phyllosilicate minerals follows the same trends as reported for Cs and Rb and thus suggests that concepts to describe the retention of (radio)cesium by different types of phyllosilicate minerals in soils, sediments and rocks are also applicable to Tl(I).

Thallium Sorption onto Manganese Oxides.

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.

Thallium Adsorption onto Illite.

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.