RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

The equilibrium adsorption of Th by the hydrous oxides goethite (α-FeOOH) and δ-MnO₂ in marine electrolytes is not affected by the major cations Ca²⁺ and Mg²⁺, relative to NaCl electrolyte, while SO^2-₄ decreased adsorption through competitive ion pairing with Th in solution. A triple layer model of surface speciation in NaCl electrolyte suggested that adsorption of Th mainly involves the hydrolysed forms Th(OH)²⁺₂, Th(OH)⁺₃ and Th(OH)₄. Intrinsic surface binding constants for these species were determined by fitting the model calculations to experimental adsorption data. In sulphate-containing electrolytes, including seawater, the model correctly predicted the slope of the adsorption curves for both oxides, with the pH of the adsorption edge depending on values used for Th-sulphate ion-pairing equilibrium constants. Competition experiments using synthetic organic ligands to buffer the Th concentrations with δ-MnO₂ lowered the concentration free Th available for adsorption to within the range of oceanic dissolved Th concentrations. This gave rise to shifts in the adsorption edge to higher pH close to that of seawater, which were consistently predicted by the model. Application of the model to the deep ocean water column suggested that δ-MnO₂ would not be a powerful scavenger for Th because of the development of a strong negative surface charge at pH 8, while adsorption on goethite should be significant.