Tuning ZnO/GO p-n heterostructure with carbon interlayer supported on clay for visible-light catalysis: Removal of steroid estrogens from water

This study demonstrates the efficiency of a new visible-light p-n ZnO/GO heterostructured composite catalyst supported on clay with carbon interlayer. This photocatalyst was prepared via microwave assisted technique for the removal of four steroid estrogens in water: Estrone (E1), 17-beta-estradiol (E2), Estriol (E3) and the synthetic estrogen 17-alpha ethinylestradiol (EE2). The prepared catalyst was characterized by different techniques: FE-SEM, EDX, RAMAN, ATR-FTIR, XPS, BET, UV-VIS, and PL. Studies confirmed that the presence of carbon interlayer (from carica papaya seeds) and graphene oxide (GO) were important for the visible-light efficiency of the photocatalyst. In single solute systems, estrogen removal was >89% and as high as 98% and this was not significantly different in a competitive system. In real wastewater samples, efficiency was 63-78% estrogen removal. A reuse study suggested that the photocatalyst efficiency was slightly >80% after 3 reuse cycles. The presence of humic acid reduced the efficiency to >= 70% for all estrogens while the addition of 1% H2O2 raised photodegradation of estrogens to 100% in 10 min. However, using the chemical oxidation demand test, the actual oxidation level of steroid estrogens after photodegradation was 51-77% for the various steroid estrogens. Important reactive oxygen species responsible for photodegradation was hydroxyl radical (HO center dot) via superoxide radical (center dot O2-) and hole (h+) formation from the photocatalytic composite. Test with ceriodaphnia silvestrii suggests very mild toxicity from treated water which is below the acute level of these estrogens (LC50 = ca. 0.89 mg/L).