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

[Display omitted] •Synthesis of new ZnO/GO p-n heterostructured visible-light catalyst on kaolinite.•Catalyst photodegradation efficiency of the estrogens, >90%.•New photocatalyst efficient for the treatment of real wastewater.•Mild toxicity from treated water which is below the acute level of estrogens.•Efficiency of photocatalyst stable over three cycles. 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-β-estradiol (E2), Estriol (E3) and the synthetic estrogen 17-α 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) via superoxide radical (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).