Free Radical and Direct Ozone Reaction Competition to Remove Priority and Pharmaceutical Water Contaminants with Single and Hydrogen Peroxide Ozonation Systems

<p>From the application of concepts derived from the gas-liquid absorption film model, the competition between ozone reactions with 72 water emerging or priority contaminants (pharmaceuticals, pesticides, polynuclear aromatic hydrocarbons, etc.) and the initiation steps of the hydroxyl radical decomposition of ozone in ozone alone and combined with hydrogen peroxide oxidation systems has been studied. With this information, the ozone preferential reaction, that is, the ozone direct reaction or the formation of free radicals and the kinetic absorption regime are known. In a second step, the ratio of removal rates of the contaminants studied by reacting with hydroxyl radical and ozone has also been estimated. With this, the way contaminants are preferentially removed (from their reaction with ozone or from the reaction with free radicals) can also be known and, hence, whether or not an ozone advanced oxidation system is convenient to be applied. For instance, most of the contaminants studied in this work at concentrations lower than 50 μgL⁻¹ and hydrogen peroxide at concentrations lower than 50 mgL⁻¹ react with ozone under chemical control regime so that both direct and free radical reactions theoretically compete. However, under chemical control, typical concentration of scavengers present in wastewater or surface water would inhibit the free radical reactions and, at least theoretically, for many contaminants studied here, the direct ozone reaction is the principal removal way. When mass transfer controls the process rate only contaminants with a hydroxyl and ozone rate constant ratio ≥ 1.6x10⁶ M^-1s^-1 would be preferentially removed through free radical way.