Kinetic and mass transfer studies of the reactions between dichlorophenols and ozone in liquid-liquid and gas-liquid systems

The present research concerns the ozonation kinetics of six dichlorophenol (DCP) isomers in aqueous solutions, and mass transport of gaseous ozone accompanied by the reactions in gas-liquid contactors. The kinetic data were obtained from a stopped-flow spectrophotometer system at temperatures varying from 5 to 35°C. The reaction between a dichlorophenol and dissolved ozone is second order overall requiring two moles of ozone per mole of DCP. The oxidation rate increases rapidly with the solution pH (between 2 to 6 investigated), and the Arrhenius equation is applicable to correlate the temperature effect. The reaction of molecular DCP varies from moderate to fast, but the reaction of dichlorophenoxide anion is extremely fast with a rate constant of the order of 1 × 109 M−1s−1.; Ethylacetate, dichlorobenzenediols and other reaction products were detected by a gas chromatograph/mass spectrometer system. A mechanism initiated by electrophilic addition of ozone molecules at the ortho and para positions of the hydroxyl group and/or negative-charged oxygen group on the benzene ring has been proposed to explain the reaction pathways. The mechanism leads to a rate equation which is consistent with the results concluded from the kinetic experiments.; The DCP destruction rate at a fixed absorption time increases significantly with pH in acidic solutions in a gas-liquid contactor. However, the DCP concentration profiles in various alkaline solutions change very little because of limitations of mass transport of gaseous ozone into the liquid phase. The mass transfer and conversion rates are enhanced by the agitation speed, superficial gas velocity, and other factors. Simulated results indicate that the fluids in the stiffed-tank reactor may be considered well mixed. The flow pattern in the bubble column can be represented by the plug flow model though it also can be approximated by the complete mixing model. The results of this research suggest that the ozone injection process is effective for complete destruction of dichlorophenols, and that it is desirable to adjust and maintain a pH of near neutrality in the wastewaters to facilitate DCP removal.