Peroxidase-catalysed oxidation of natural and synthetic hormones

One of the fastest growing concerns in the wastewater treatment industry is the presence of estrogenic compounds in wastewater effluents that may threaten the reproductive success and development of aquatic organisms. The fact that such contaminants have been detected in surface waters at potentially detrimental levels indicates that conventional treatment plants are unable to effectively remove them; hence, supplementary treatment technologies must be considered. As one of these, enzymatic treatment offers numerous advantages which make such a treatment methodology almost tailor-made for the selective removal of such contaminants. In order to evaluate the potential for enzymatic treatment to be used to target important estrogenic compounds, the enzyme-catalysed oxidation of selected potent estrogens including estradiol, estriol and ethinylestradiol was studied. Horseradish peroxidase (HRP) enzyme was selected as the candidate enzyme since it is amongst the most studied oxidative enzymes and has been investigated extensively in waste treatment applications. Experiments were conducted at concentrations higher than those found in typical effluents in order to characterize the reactivity of the enzymes toward these compounds without being hampered by analytical difficulties. In order to gauge the importance/relevance of the data obtained for the enzymatic oxidation of estrogens parallel experiments were also conducted with phenol, which has been the subject of many past studies as a substrate of HRP and for which the technical feasibility of enzymatic treatment has been well established.; HRP was able to effectively catalyse the oxidation of estrogens over a wide range of pHs, with optimal performance in the pH range typically experienced in wastewaters. Measurements of the stoichiometry of the reaction between estrogens and peroxide and also the enzyme dose required to achieve certain target levels of substrate removal suggested that the enzymatic oxidation of estrogens consistently had lower peroxide and enzyme requirements than phenol. In fact, phenol required between 2.5 and 5 times more enzyme than the estrogens to achieve various target levels of removal. For all substrates studied, similar kinetics of removal were found, provided that sufficient enzyme was added to reactions to compensate for differences in substrate affinity. In contrast to earlier studies conducted with HRP, minimal inactivation of the enzyme was observed during the treatment of all substrates. The lesser inactivation observed in the present study was probably due to the very low concentration of target substrates used. Collectively, the results indicate that the removal of estrogens is likely to be more feasible than phenol itself. It is also suggested that since the estrogen concentrations utilized here were an order of magnitude higher than environmentally-relevant concentrations and since the enzyme dose required and the level of inactivation observed are directly related to the amount of substrate targeted for treatment, the feasibility of removing estrogens may be more favourable at environmentally relevant values, except where kinetic limitations may dominate.