The effect of pH on the U(VI) sorption on graphene oxide (GO): A theoretical study

Environmental contamination of radioactive waste water has drawn major public concerns because of its serious hazard in chemical and dynamic bio-toxicities. In present work, the interaction mechanism of the important radionuclide uranium (U) and graphene oxide (GO) under aqueous conditions with varying pH levels was scrutinized by means of high-level density functional theory (DFT) together with the implicit water model. A closer look at the bonding structures and adsorption energies of 12 GO/uranyl complexes indicated that the main adsorption sites on GO were the oxygen-containing functional groups such as epoxy group, carboxyl group, and hydroxyl group. More importantly, high pH level was more favorable for the adsorption process of uranyl species on the graphene oxide due to the stronger electrostatic interaction between negatively charged O atom and uranyl ion. This conclusion was further verified by comprehensive analysis from density of state (DOS) and charge density difference. Compared to the other forms of graphene oxide, the GO containing bi-hydroxyl groups had the highest adsorption capacity towards the uranyl species. The present theoretical view point gave supplement to the experimental observations and the proposed intrinsic mechanisms may bring new insight into the environmental management of radioactive pollution.