MECHANISM OF PHOTODEGRADATION OF AQUEOUS ORGANIC POLLUTANTS .1. EPR SPIN-TRAPPING TECHNIQUE FOR THE DETERMINATION OF .OH RADICAL RATE CONSTANTS IN THE PHOTOOXIDATION OF CHLOROPHENOLS FOLLOWING THE PHOTOLYSIS OF H2O2

A technique for the determination of the rate constants for the reaction of .OH radicals with organic substrate is described. This technique is based on the use of spin trapping with electron paramagnetic resonance (EPR) detection of spin adducts. The desired rate constants are obtained by measuring either the initial rate of production of the EPR signal of the spin adduct or its amplitude after a fixed time, as a function of the concentration of the substrate. By using the known rate constant for the reaction of .OH radicals with the spin trap (DMPO), rate constants for the reaction of .OH radicals with a given substrate can be obtained within a competition kinetic scheme. The method has been validated by determining the rate constants for the reaction of .OH radicals with phenol and with formate, two reactions well studied by using pulse radiolysis. Our results agree with literature values within experimental error. The method is then applied to several chlorophenols. The rate constants for 3-chloro-substituted phenols are significantly less than for 4-chloro or 2-chloro-substituted phenols. Some rate constants are significantly larger than the diffusion-controlled rate constant. This is explained by proposing a Grotthus-type mechanism for the movement of .OH radicals through water.