Effect of humic acid, fulvic acid, pH and temperature on the sorption-desorption of Th(IV) on attapulgite

Abstract

Summary. Sorption of Th(IV) on attapulgite as a function ofpH and temperature in the presence and absence of humic acidandfulvicacidwasstudiedunderambientconditionsusingbatchtechnique. The results indicated that sorption of Th(IV) on at-tapulgite was strongly affected by pH values. The presence ofhumic acid and fulvic acid enhanced the sorption of Th(IV) atlow pH values and no significant influence was observed at highpH values. Sorption of Th(IV) was mainly dominated by sur-facecomplexation. Sorption ofTh(IV)increased withincreasingtemperature of the system. Enthalpy (∆ H 0 ), entropy (∆ S 0 )andGibbs free energy (∆ G 0 ) were calculated from the temperaturedependent sorption data, and the results indicated that the sorp-tion of Th(IV) on attapulgite was a spontaneous process. Theresults of sorption and desorption of Th(IV) on HS bound atta-pulgite indicated that the sorption of Th(IV) on HS-attapulgitehybrids was reversible. 1. Introduction The fate of Th(IV) in clay minerals and in the environment iscontrolled by sorption, desorption, diffusion, migration andcomplexation on all kinds of clay minerals and oxides [1–5].Sorption of Th(IV) on minerals is one of most importantparameters determining the mobility and bioavailability ofTh(IV). Sorption of Th(IV) on clay minerals and oxides hasbeen studied extensively, and many mechanisms have beenpostulated for Th(IV) sorption, such as surface complex-ation (outer-sphere and inner-sphere), complexation, sorp-tion (chemisorption and physicosorption), cation exchange,precipitation (surface precipitation and inner precipitation),colloid formation at surface and diffusion into the particlemicropores [6]. The results of the previous studies indicatedthat sorption of Th(IV) is strongly pH dependent and weaklyionic strength dependent. Xu