Influence of anions on the adsorption of uranyl on hydroxylated α-SiO2(001): A first-principles study

Abstract

The adsorption of uranyl on hydroxylated α-SiO2(001) in the presence of a series of anionic ligands, i.e. OH−, CO32-, NO3-, H2PO4-, HPO42-, CH3COO− (Ac−), C6H5COO− (PhCO2-), C6H5O− (PhO−), was studied by the periodic density functional theory (DFT) implemented in the Vienna ab initio simulation package (VASP). For the ligands other than OH− and PhO−, only the bidentate coordination modes to the uranyl were considered. The excess charge effect of a charged system was first evaluated by constructing models with net charge as is or neutralized by creating defect at the bottom of silica, and the results show that a neutralized model, even with defects, is more realistic than the charged ones. All uranyl species prefer to bind with the deprotonated site (O−) rather than the protonated one (OH), which suggests that the increase of pH, which leads to the deprotonation of the surface, may enhance the uranyl adsorption. On the other hand, the anionic ligands, which are formed at higher pH, have negative effects. The weaker acidic ligands, such as H2CO3, H3PO4 and H2O, whose speciation in solutions is sensitive to the fluctuation of pH, have more complex effect on the uranyl adsorption than strong acids or bases. Humic substances may coordinate with uranyl through carboxyl and phenolic groups, with the carboxyl group bound stronger. The ternary complexes with one bidentate (or monodentate) anion and one (or two) H2O as ligands, which leads to the uranyl penta-coordinated in its equatorial plane, are more favorable than other configurations when bound to the same anionic ligand. Both the charged nature and the coordination behavior of an anionic ligand are relevant to its ability to influence the adsorption of uranyl on the mineral surface. In addition, the uranyl species adsorbed at the surface functionalized by anionic ligands were also addressed, and the functionalized surfaces have weaker interaction with hydrated uranyl dication.