Enzymatic treatment of organic micropollutants in municipal wastewater: treatment conditions and reaction kinetics
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Organic micropollutants enter natural waters via treated municipal wastewater discharges, threatening aquatic ecosystems and exposing humans to trace levels of these diverse chemicals through downstream drinking water supplies. Both of these concerns would be addressed simultaneously if micropollutants, which are often recalcitrant to conventional treatment processes, were removed in municipal wastewater treatment plants. Thus, the ultimate goal of this research was to advance the development of a sustainable and cost-effective enzymatic treatment process that oxidizes organic micropollutants in municipal wastewater. The enzymatic treatment process studied, laccase-catalyzed oxidation, involves the oxidation of mediator compounds by the enzyme laccase, generating mediator free radicals that can oxidize target micropollutants. The influences of key treatment conditions (pH, enzyme activity, mediator concentration, wastewater organic content) on the efficacy of the treatment process were evaluated, and the reaction kinetics of enzymatic treatment were investigated. The results demonstrated that enzymatic treatment can transform environmentally relevant concentrations of two representative micropollutants, oxybenzone and sulfamethoxazole, in a primary effluent wastewater matrix under realistic treatment conditions. Experiments with a set of structurally related chlorophenolic target compounds revealed several impacts of target compound structure on the kinetics and mechanisms of the laccase-mediator-target compound reactions. The electronic properties of the target compounds, their acid-base speciation, and steric hindrance to coupling reactions by substitution of the aromatic ring were found to influence their relative reactivities and reaction mechanisms with the free radicals generated by the laccase-mediator system. A kinetic model of the reactions between the enzyme laccase, the mediator acetosyringone, and the chlorophenolic target compounds was developed and fit the experimental data quite well despite the complexity and non-linearity of the system. This model indicated that the rate limiting step was the oxidation of the mediator by the enzyme. The relative rate constants for the reactions of the resulting mediator radical represent the distribution of the radicals among several possible reaction pathways. Although still in its early stages of development, the results of this research indicate that enzymatic treatment could one day be implemented in wastewater treatment plants for the mitigation of micropollutant release into the environment.