BACKGROUND: Recently, the fate of emerging micro-contaminants in environmentally relevant samples has attracted considerable attention. Semiconductor photocatalysis may offer an appealing methodology to treat such contaminants; in this respect, the degradation of synthetic estrogen 17a-ethynylestradiol (EE2) employing simulated solar radiation and ZnO as the photocatalyst was investigated. RESULTS: A factorial design approach was adopted to evaluate the effect of estrogen concentration (100-500 mu g L-1), ZnO concentration (50-500 mg L-1 in suspension), treatment time (2-10 min), photon flux (4.93 x 10(-7)5.8 x 10(-7) einstein L-1 s-1) and the water matrix (ultrapure water and treated wastewater) on EE2 removal. The first four variables had a statistically important, positive effect on degradation, while the water matrix introduced a negative effect presumably due to the competition between EE2 and the effluent organic and inorganic matter for hydroxyl radicals and other oxidizing species. Moreover, second-order interactions of estrogen concentration with time and the water matrix were also significant. EE2 degradation follows first-order kinetics with the respective rate constants in wastewater and water being 9.2 +/- 0.7 x 10-2 and 41 +/- 8 x 10-2 min-1 at the maximum ZnO concentration and photon flux. On the other hand, the removal rate of effluent's overall estrogenicity (as assessed by the yeast estrogen screening bioassay) was an order of magnitude lower than that of EE2, implying the presence of persistent estrogenic compounds in the photocatalyzed effluent. CONCLUSIONS: An effective treatment process is demonstrated which benefits from the use of renewable energy and a stable and highly active photocatalyst. Copyright (C) 2012 Society of Chemical Industry