Sorption of Th(IV) onto two-line ferrihydrite and magnetite in NaClO4 solutions has been studied as a function of pH and ionic strength revealing that sorption onto both solids increases with pH while it is independent on ionic strength. Sorption capacity of both solids is high, the maximum sorption (almost 100% of Th(IV)) occurs at pH higher than 3.5 for ferrihydrite, and higher than 3.0 for magnetite. Sorption variation with pH was modeled with three different models using the FITEQL 4.0 code: non-electrostatic model, constant capacitance model, and diffuse-double layer model. In all cases, good fit to the experimental data is obtained with one-species: a corner-sharing bidentate-mononuclear surface complex, (triple bond; length of mdashFeO)2Th2+, which coincides with the surface complex postulated on these solids surface in previous spectroscopic studies; however, the monodentate species triple bond; length of mdashFeOThOH2+ also gives a satisfactory fit. Under the experimental conditions of the present study, any effect of possible thorium colloid formation is negligible.

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