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 Ag1.1Hg0.9 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