Adsorption of Thorium(IV) and Uranium(VI) onto Azraq Humic Acid, Jordan

Abstract

Humic Acid (HA) is an important constituent of soil organic matter that was the subject of several environmental studies. In this work, the adsorption isotherms of Thorium (IV) and Uranium(VI) onto Azraq Humic acid (AZHA) were studied at different temperature and pH values. The results indicate that Thorium(IV) has higher adsorptivity than Uranium(VI). The increasing of pH resulted in higher adsorptivity of Thorium(IV) and Uranium(VI). In contrast to transition metals and lanthanides, suc h as Ni(II), Cu(II), Zn(II), Cd(II), Hg(II), Gd(III), Ce(III) and Yb(III), the adsorption was entropy driven, while the adsorption of actinides on HA was found to be exothermic and so enthalpy driven. This leads to the hypothesis that Thorium(IV) and Urani um(IV) are encapsulated in the interior of HA helix with their water of hydration because the interior is full with water. However, transition metals and lanthanides are adsorbed on the exterior of HA helix.

Keywords: Humic acid; Adsorption; Thorium(IV) and Uranium(VI) metal ions; Adsorption isotherms; Enthalpy; Entropy.