Designing AgFeO2-graphene/Cu2(BTC)3 MOF heterojunction photocatalysts for enhanced treatment of pharmaceutical wastewater under sunlight

In this work, nanoscale silver ferrite (AgFeO2) was successively impregnated onto graphene(G) and Cu-2(BTC)(3)MOF to construct binary and tertiary heterojunction photocatalysts with engineered bandgap energies (E-g = 1.95 to 1.72 eV) for photocatalytic removal of pharmaceutical drug pollutants under sunlight irradiation. Response surface methodology was employed to investigate and optimize the prepared photocatalysts for visible-driven photodegradation of amoxicillin (AMC) and diclofenac (DCF) (as model drug pollutants) under the effect of irradiation time (X-T), drug concentrations (X-C), and water salinity (X-S). At optimum conditions (X-T: 100 min, X-C: 5 mg/L, and X-S: 250 mg/L), all photocatalysts exhibited high sensitivity for solution pH due to the ionization of AMC/DCF molecules, which influences their adsorbability on the photocatalysts surfaces. Under sunlight irradiation, AgFeO2/G@Cu-2(BTC)(3) outperformed other binary photocatalysts (AgFeO2/G and AgFeO2/Cu-2(BTC)(3)) towards AMC/DCF drug pollutants removal (up to 97 % after 150 min), with apparent photo-kinetic (k(1)) rates of 6.4-8.7 x 10(-2) min(-1) and excellent stability up to four cycles. More importantly, AgFeO2/G@Cu-2(BTC)(3) heterojunction showed high efficacy to remediate real pharmaceutical wastewater with an efficiency of 87.1 +/- 2.9% reduction in chemical oxygen demand (COD) after 7 h under direct sunlight. The enhanced photocatalytic activity of the tertiary heterojunction photocatalyst is attributed to promoted charges transfer mechanism via direct Z-scheme from the conduction band of Cu-2(BTC)(3) to the valence band of AgFeO2 in the presence of graphene as a solid electron mediator/acceptor. Hence, the photogenerated electrons in the conduction band of AgFeO2 reacted with O-2 molecules to generate O-2(center dot-) anion (as predominant oxidative species), which successively enhances the degradation/mineralization of adsorbed drug molecules on the catalyst surfaces.