Sorption of U(VI) on dolomite in a dilute carbonate solution: behavior investigation and mechanism probe

Abstract

Abstract The U(VI) sorption behavior on dolomite was systematically investigated in a dilute carbonate solution using batch technique. The distribution coefficients (K d) increased with the increasing time and temperature, but decreased with the increase of initial U(VI) concentration and ionic strength. As the initial pH increased from 2.0 to 10.5, the K d values first increased and then decreased, mainly due to the transition from electrostatic attraction to repulsion caused by the changes in U(VI) species. The sorption process was well-described by pseudo-second-order kinetic and Freundlich isotherm models, and was spontaneous, endothermic, heterogeneous, and pH-dependent. In the presence of HA/FA, at low pH, more HAs/FAs were adsorbed on dolomite surface, providing more sorption sites for the positively charged U(VI) species, and promoting the U(VI) sorption. At high pH, more HAs/FAs were dispersed in solution, encapsulating the negatively charged U(VI) species in aggregates, and inhibiting the U(VI) sorption. The promoting and inhibiting effects of HA on the sorption process are obviously stronger than FA. XPS spectra indicated that the surface ‒OH and CO3 2− on the hydroxylated dolomite and the –OH groups in HA/FA may be involved in U(VI) sorption. The results reported here provide valuable references for further understanding U(VI) migration in geological media.