Ozonation and Advanced Oxidation of Wastewater: Effect of O3Dose, pH, DOM and HO•-Scavengers on Ozone Decomposition and HO•Generation

Abstract

The decomposition of ozone in wastewater is observed starting 350 milliseconds after ozone addition. It seems not to be controlled by the autocatalytic chain reaction, but rather by direct reactions with reactive moieties of the dissolved organic matter (DOM). A larger ozone dose increases ozone consumption prior to 350 milliseconds but decreases the rate of ozone decomposition later on; this effect is predicted by a second-order kinetic model. Transferred Ozone Dose (TOD) is poorly correlated with ozone exposure (= ∫[O3]dt) indicating that TOD is not a suitable parameter for the prediction of disinfection or oxidation in wastewater. HO• concentrations (> 10−10 M) and Rct (=∫[HO•]dt/∫[O3]dt > 10−6) are larger than in most advanced oxidation processes (AOP) in natural waters, but rapidly decrease over time. Rct also decreases with increasing pre-ozonation doses. An increase in pH accelerates ozone decomposition and HO• generation; this effect is predicted by a kinetic model taking into account deprotonation of reactive moieties of the DOM. DOC emerges as a crucial water quality parameter that might be of use to normalize ozone doses when comparing ozonation in different wastewaters. A rapid drop of absorbance in the water matrix—with a maximum between 255–285 nm—is noticeable in the first 350 milliseconds and is directly proportional to ozone consumption. The rate of absorbance decrease at 285 nm is first order with respect to ozone concentration. A kinetic model is introduced to explore ozone decomposition induced by distributions of reactive moieties at sub-stoichiometric ozone concentrations. The model helps visualize and comprehend the operationally-defined “instantaneous ozone demand” observed during ozone batch experiments with DOM-containing waters.