University of California Water Resources Center

Treatment of industrial and municipal wastewater containing refractory organic compounds is of primary concern. In this research, the feasibility of ozonation as a treatment process for the removal of 2,4,6-trichlorophenol was studied. The chemical kinetics of the reaction between 2,4,6-trichlorophenol (TCP) and ozone were determined.

A wetted-sphere absorber was used to measure rates of absorption of ozone into aqueous buffered solutions of TCP. Gas consisting of approximately 2.5% ozone in oxygen was contacted with aqueous buffered TCP solution flowing over a sphere in a laminar liquid film. Absorption data were obtained by measuring the change in the liquid phase concentration of TCP from the inlet to the outlet of the absorber. A rigorous numerical model for this diffusion/reaction process was used to analyze the absorption data in order to determine the second-order rate constant of the reaction between ozone and TCP. Results were obtained over the temperature range of 15 to 35°C at pH values of 2 and 7. The kinetic data indicate that the rate limiting step for the reaction of aqueous TCP with ozone is the same at pH 2 and 7. Four reaction products which occur early in the reaction of ozone with aqueous TCP were identified by GCMS, including: 2,3,4,6- tetrachlorophenol (TRCP), 4,6-dichlorocatechol (DCC), 2,6-dichlorohydroquinone (DCHQ), and 2,6-dichloroquinone (DCQ). Only DCHQ and DCQ were found to occur in significant concentrations. Over the pH range of 2 to 5, both DCHQ and DCQ were detected in the reacted solutions, while for pH 6 and 7 only DCHQ was detected. Over longer ozone exposure times, it was found by carbon-13 NMR analysis that short chain carboxylic acids are the dominant species in the reacted TCP solutions.

The kinetics of the ozonation of DCHQ were also determined at 25°C.