Effects of Surface Functional Groups of SiO₂ on Mercury Adsorption in Aqueous Solution: DFT Calculation and Adsorption Measurement

The energy differences between aqueous Hg2+ species and acidic functional groups on model adsorbent surfaces were evaluated using density functional theory (DFT) calculations. The carboxyl group on the model adsorbent surface exhibits the strongest affinity for Hg2+ species among the four acidic functional groups: alkoxy, ether, carbonyl, and carboxyl. The Hg2+ adsorption capacity of SiO2 modified with carboxyl groups (SiO2–COOH) increased markedly in the pH range of 5-7. Analysis of the pH dependence of the zeta potential of SiO2–COOH in water revealed that the surface potential approaches zero near pH 5, indicating that the optimum Hg2+ adsorption occurs under conditions with minimum electrostatic interactions. These experimental observations agreed well with the DFT results.