A Kinetic Study of the Reactions of Sulfate Radicals at the Silica Nanoparticle−Water Interface

Sulfate radicals, SO 4 • -, were generated using flash photolysis of aqueous S 2 O 8 2- solutions and the reactions of the inorganic radicals with the surface of suspended silica nanoparticles (NP) investigated. In the presence of colloidal silica no absorption traces due to SO 4 • - radicals are observed at 100 μs after the flash of light. However, two transient species with absorption maxima around 320 and 600 nm are formed. A kinetic analysis of the experimental results indicate that SO 4 • - radicals are adsorbed on the NP surface, leading to the formation of an adduct, with λ max ≈ 320 nm (ε ≈ 7000 cm -1 M -1 ), and showing similar reactivity to that observed for the sulfate radical in aqueous solutions. The NP−sulfate radical adducts react with adsorbed water, and with single and geminal SiO - sites with reaction rate constants of 1.5 × 10 14 × e -(58±12)kJ/mol)/ RT s -1, < 10 3 × e -(2±17)kJ/mol/ RT s -1 and < 10 11 × e -(46±13)kJ/mol/ RT s -1, respectively. Two different SiO • surface defects, showing similar spectra (λ max ≈ 600 nm) but different reactivities, are formed from the reaction of NP−sulfate radicals and deprotonated geminal and single silanols.