KINETICS OF HYDROXYL RADICAL SPIN-TRAPPING IN PHOTOACTIVATED HOMOGENEOUS (H2O2) AND HETEROGENEOUS (TIO2, O-2) AQUEOUS SYSTEMS

Homogeneous (H2O2)and heterogeneous (TiO2 + Oz)aquatic photochemical s ystems were compared in their ability to generate hydroxyl radicals,an a to decompose-p-toluenesulfonic acid (PTS) in buffered and unbuffered solutions. The kinetics of hydroxyl radical formation were monitored with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin trap. The zero zeta potential measured in aqueous TiO2 suspensions was-found at pH = 6, bu t zeta potentials shifted to considerably more negative values in phop hate-borate buffers. Consequently, a well-defined PTS adsorption isoth erm on the TiO2 surface was found in unbuffered systems, and no PTS an d DMPO adsorption was measurable in the phosphate-borate buffer, due t o the competitive adsorption of buffer ions. The identical dependence of (.)DMPO-OH formation on PTS concentrations in both homogeneous and heterogeneous buffered systems along with zeta potential and adsorptio n measurements suggests that the reaction of (OH)-O-. radicals, their addition to DMPO and the oxidative degradation of PTS, is taking place (in the presence of phosphate-borate buffer) in the homogeneous phase , with radicals leaving the TiO2 surface.