Thorium(IV) sorption onto hematite (α-Fe2O3) was examined as a function of pH and ionic strength. Sorption behaved Langmuirian over an eleven order of magnitude range in adsorption densities, Γ: 10−12 to 10−1 moles Th sorbed per mole hematite sites, indicating that the overall free energy of Th adsorption is independent of adsorption density. Modeling of Th sorption was conducted with the Triple Layer Model of Davis and Leckie; reactions considered included solution-phase hydroxy and carbonato complexes of thorium, and carbonate/hematite surface complexes. The entire Th sorption isotherm can be modeled with a single surface complex formation reaction
≡Fe–OH + Th4+ =*KThint ≡Fe–OTh3+ + H+
Inverse modeling of pH-fractional Th adsorption data yielded a value of *KThint of 21.2. The relatively large value of the surface complex formation constant is required for hematite surface sites to effectively compete against solution-phase hydroxide and carbonate ions at high system pHs. Insensitivity of fractional thorium sorption to changes in ionic strength suggests the formation of inner-sphere surface complexes. Sorption and desorption rate experiments showed: (1) Th sorption onto hematite suspended in a simple electrolyte solution is rapid with equilibrium attained within a few minutes; (2) sorption is irreversible on the time scale of the experiments (4 days). Coagulation and sedimentation experiments demonstrate the utility of Th isotopes asin situ ‘coagulometers’: even at relatively low particle concentrations (e.g., 5 ppm), Th acts as a surrogate for particle mass transfer.
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