Functionalization of biosourced silica and surface reactions with mercury in aqueous solutions

This study presents the production of pure silica from rice husk and its stepwise functionalization with triethoxysilane (TES) and silver nanoparticles. The derived TES-SiO₂ (0.97 mmol TES per g of SiO₂) and Ag⁰@SiO₂ (3.70–21.60 mg Ag per g of SiO₂) materials were used for the removal of Hg²⁺ from aqueous solutions in the presence of chloride ions. The removal of Hg²⁺ was rapid and the equilibrium solid phase loading was between 21 and 101 mg/g. TES-SiO₂ was more effective than Ag⁰@SiO₂ in removing mercury due to the highly reactive hydride groups. In the case of Ag⁰@SiO₂ samples the removal of Hg²⁺ is proportional to the amount of Ag⁰. The materials were characterized before and after the removal of Hg²⁺ to allow insights into the interaction mechanism. The reaction products for TES-SiO₂ sample were calomel (Hg₂Cl₂) and AgCl. On the surface of Ag⁰@SiO₂ samples besides these products the silver amalgams Ag_1.1Hg_0.9 and possibly Ag₂Hg₃ were identified. These results show that redox reactions between silicon-hydride group, Ag⁰ and Hg²⁺ take place on the surface of the materials. The coexistence of calomel and silver amalgams on amorphous silica is observed for the first time while the calomel formation in heterogeneous Hg²⁺ redox systems is a rare observation. Also, the samples showed extraordinary efficiency in terms of Hg:Ag molar ratio (2.28–3.02), considerably higher than those reported before. Besides the beneficial effect of the Cl^- presence on the removal of Hg²⁺ from the solution there is evidence of a hyperstoichiometric interaction between Ag⁰ and Hg²⁺ at the nanoscale.