Hydroxyl Radical/Ozone Ratios During Ozonation Processes. I. The Rct Concept

The ozonation of model systems and several natural waters was examined in bench-scale batch experiments. In addition to measuring the concentration of ozone (O₃), the rate of depletion of an in situ hydroxyl radical probe compound was monitored, thus providing information on the transient steady-state concentration of hydroxyl radicals (√OH). A new parameter, R_ct , representing the ratio of the √OH-exposure to the O₃-exposure was calculated as a function of reaction time. For most waters tested, including pH-buffered model systems and natural waters, R_ct was a constant value for the majority of the reaction. Therefore, R_ct corresponds to the ratio of the √OH concentration to the O₃ concentration in a given water (i.e. R_ct = [√OH]/[O₃]). For a given water source, the degradation of a micropollutant (e.g. atrazine) via O₃ and √OH reaction pathways can be predicted by the O₃ reaction kinetics and R_ct .     

Comparison of Different Advanced Oxidation Processes Involving Ozone to Eliminate Atrazine

The oxidation of the herbicide atrazine by advanced oxidation processes (AOP) has been studied. The experiments were carried out in a tubular photoreactor, 2.5 L capacity, capable of providing good contact between the liquid and gas reactants. The decomposition rate of atrazine was determined at different pH using UV radiation, Hydrogen Peroxide, Ozone, Ozone/UV, Ozone/H₂O₂, H₂O₂/UV and Ozone/H₂O₂/UV processes. The effect of three different pH values was studied (4.7, 6.8, 11.7).     

Ozonation and Advanced Oxidation of Wastewater: Effect of O3 Dose,pH, DOM and HO•-Scavengers on Ozone Decomposition and HO• Generation

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 R_ct (=∫[HO^?]dt/∫[O₃]dt > 10^?6) are larger than in most advanced oxidation processes (AOP) in natural waters, but rapidly decrease over time. R_ct 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.     

The Chemistry of Water Treatment Processes Involving Ozone,Hydrogen Peroxide and Ultraviolet Radiation

Advanced oxidation processes are defined as those which involve the generation of hydroxyl radicals in sufficient quantity to affect water purification. The theoretical and (practical yield of OH from O₃ at high pH, 0₃/H₂0₂, O₃/UV and H₂O₂/UV systems is reviewed. New data is presented which illustrates the importance of direct photolysis in the O₃/UV process, the effect of the H₂0₂:0₃ ratio in the O₃/H₂O₂ process, and the impact of the low extinction coefficient of H₂O₂ in the H₂0₂/UV process.