Solar photocatalytic decomposition of estrogens over immobilized zinc oxide

The photocatalytic degradation of synthetic estrogen 17α-ethynylestradiol (EE2) in environmental samples was investigated. Zinc oxide immobilized onto a glass substrate was prepared and used as the photocatalyst, while radiation was provided by a solar simulator. EE2 in the range 50–200 μg/L was treated in various matrices, i.e. ultrapure water, wastewater and drinking water, and treatment efficiency was assessed as a function of photon flux, ZnO loading and addition of hydrogen peroxide. Degradation follows apparent first-order kinetics and increases with increasing photon flux (4.93·10−7–5.8·10−7 einstein/(L s)) and H2O2 concentration (up to 100 mg/L), while ZnO loading (1.2–16.3 mg) has a marginal effect. Reaction in ultrapure water is twice as fast as in wastewater (e.g. the respective apparent rate constants are 17.3·10−3 and 9.4·10−3 min−1 at maximum photon flux and 3.7 mg ZnO) due to the competition for oxidants between EE2 and the wastewater components (organic matter and ions). The catalyst retained most of its activity upon repeated use (i.e. 21 consecutive runs of 31.5 h duration) although it was partially dissolved in the liquid phase; leached zinc can trigger homogeneous reactions, thus contributing to the overall photocatalytic degradation.