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The surface properties of intrinsic and activated attapulgite were studied using potentiometric titration. The sorption mechanisms of Th(IV) on intrinsic and activated attapulgite were investigated by employing surface complexation model and FITEQL 3.2 code. The results indicated that the sorption sites of intrinsic attapulgite involved in the sorption process were main ion exchange site (  XNa/K), strong site (  S s OH) and weak site (  S w OH), while only strong site (  S s OH) and weak site (  S w OH) were predominant for activated attapulgite. At pH < 2.5, the species of Th(IV) was main Th 4+ in aqueous solution; then the hydrolysis species of Th(IV) (i.e., Th(OH) 2 2+ , Th(OH) 3 + and Th(OH) 4 0 ) gradually formed as pH increasing. For the sorption of Th(IV) on intrinsic attapulgite, both ion exchange species (  X 2 Th) and inner-sphere surface complexes (  S s OTh) were dominant, while only inner-sphere surface complexes (i.e.,  S s OThOH and  S w OTh) were observed for Th(IV) sorption to activated attapulgite. In the presence of humic acid (HA), the sorption of Th(IV) on activated attapulgite was obviously enhanced; and both  S s OH-HA-Th and  S s OTh were the predominant speciation. It was also interesting to find that the addition of HA obviously provoked the desorption behavior of Th(IV).attapulgite, Th(IV), HA, sorption, desorption, surface complexation model