Thallium Adsorption onto Illite

We investigated the adsorption of Tl⁺ onto purified Illite du Puy (IdP). Distribution coefficients (K_d) 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.

Cesium migration in Hanford sediment: a multisite cation exchange model based on laboratory transport experiments

Cs+ transport experiments carried out in columns packed with uncontaminated Hanford formation sediment from the SX tank farm provide strong support for the use of a multisite, multicomponent cation exchange model to describe Cs+ migration in the Hanford vadose zone. The experimental results indicate a strong dependence of the effective Cs+ Kd on the concentrations of other cations, including Na+ that is present at high to extremely high concentrations in fluids leaking from the Hanford SX tanks. A strong dependence of the Cs+ Kd on the aqueous Cs+ concentration is also apparent, with retardation of Cs+ increasing from a value of 41 at a Cs+ concentration of 10(-4) M in the feed solution to as much as 282 at a Cs+ concentration of 5x10(-7) M, all in a background of 1 M NaNO3. The total cation exchange capacity (CEC) of the Hanford sediment was determined using 22Na isotopic equilibrium exchange in a flow-through column experiment. The value for the CEC of 120 microeq/g determined with this method is compatible with a value of 121.9 microeq/g determined by multi-cation elution. While two distinct exchange sites were proposed by Zachara et al. [Geochim. Cosmochim. Acta 66 (2002) 193] based on binary batch exchange experiments, a third site is proposed in this study to improve the fit of the Cs+-Na+ and Cs+-Ca+ exchange data and to capture self-sharpened Cs+ breakthrough curves at low concentrations of Cs+. Two of the proposed exchange sites represent frayed edge sites (FES) on weathered micas and constitute 0.02% and 0.22% of the total CEC. Both of the FES show a very strong selectivity for Cs+ over Na+ (K(Na-Cs)=10(7.22) and 10(4.93), respectively). The third site, accounting for over 99% of the total CEC, is associated with planar sites on expansible clays and shows a smaller Na+-Cs+ selectivity coefficient of 10(1.99). Parameters derived from a fit of binary batch experiments alone tend to under predict Cs+ retardation in the column experiments. The transport experiments indicate 72-90% of the Cs+ sorbed in experiments targeting exchange on FES was desorbed over a 10- and 24-day period, respectively. At high Cs+ concentrations, where sorption is controlled primarily by exchange on planar sites, 95% of the Cs+ desorption was desorbed. Most of the difficulty in desorbing Cs+ from FES is a result of the extremely high selectivity of these sites for Cs+, although truly irreversible sorption as high as 23% was suggested in one experiment. The conclusion that Cs+ exchange is largely reversible in a thermodynamic sense is supported by the ability to match Cs+ desorption curves almost quantitatively with an equilibrium reactive transport simulation. The model for Cs+ retardation developed here qualitatively explains the behavior of Cs+ in the Hanford vadose zone underneath a variety of leaking tanks with differing salt concentrations. The high selectivity of FES for Cs+ implies that future desorption and migration is very unlikely to occur under natural recharge conditions.