Mechanism of adsorption affinity and capacity of Mg(OH)2 to uranyl revealed by molecular dynamics simulation

Abstract

Because of its remarkably high adsorption affinity to uranyl ions, Mg(OH)₂ can effectively extract trace-level uranyl and has been exploited for the treatment of field water samples. In this work, we used molecular dynamics simulation to systematically study the dynamics, energetics and structure aspects of uranyl adsorption on the Mg(OH)₂ (001) surface. The approach of the uranyl cation causes the redistribution of surface OH groups and the emergence of a negatively charged surface region, which accommodates the adsorption of uranyl. The adsorption stability of uranyl is largely attributed to the coordination interaction with surface OH groups, and the calculated adsorption free energy is in quantitative agreement with experimental results. On the other hand, the adsorbed uranyl affects the orientation of surrounding OH groups, which may hinder the additional uranyl adsorption to the adjacent region and limit the adsorption capacity. The estimation of monolayer surface coverage is also well consistent with the experiments. Taken together, our results reveal the mechanisms of both adsorption affinity and capacity of Mg(OH)₂. As suggested by this work, comprehensive studies about uranyl adsorption can provide insight into the adsorption properties and should be helpful for the further development of uranyl adsorbents.