Sediment–water interactions of thallium under simulated estuarine conditions

Abstract

Thallium(I) has been added to estuarine sediment suspended in various natural and artificial aqueous samples in order to examine its reactivity under simulated estuarine conditions. In river water and sea water, adsorption of Tl to sediment was so rapid that a period of desorption–relaxation succeeded instantaneous adsorption. Entire time-courses could not be fitted with a conventional kinetic model, but pseudo-first-order forward and reverse rate constants of 0.0044 and 0.30 h −1, respectively, were derived for river water by omitting measurements defining the adsorption “overshoot” observed at the onset of the experiment. The extent of adsorption after a 16 h equilibration period was considerably greater in river water than in sea water, and displayed a quasi-linear increase with increasing pH over the range 2–9 in the former but no clear dependence on pH in the latter. A logarithmic reduction in the sediment–water distribution coefficient, K D, was observed on estuarine mixing from river water to sea water. Experiments conducted in electrolyte solutions coupled with inorganic equilibrium speciation modeling revealed that the effect was the combined result of a reduction in the activity of Tl +, an increase in the proportion of TlCl 0 and increasing competition for adsorption sites from K + with an increase in salinity. Overall, there was little experimental evidence for either the oxidation of Tl + or its complexation by dissolved organic matter. The findings of the investigation are discussed in terms of the likely behavior of Tl in estuaries.