Mass transfer and kinetics study of the ozonation of refractory organics in waste waters
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.