Zn-doped hydroxyapatite@g-C3N4: a novel efficient visible-light-driven photocatalyst for degradation of pharmaceutical pollutants

Heterojunction formation has been shown to be an effective technique for tuning nanomaterial features such as chemical reactivity and optical performance. In this study, we discuss the synthesis of Zn metal-doped hydroxyapatite (Zn-HAp) via ion-exchange method followed by the formation of a heterojunction with g-C3N4 by ultrasonication method (Zn-HAp@g-C3N4). This novel heterogeneous Zn-HAp@g-C3N4 photocatalyst was employed in the degradation of pharmaceutical pollutants like antibiotics through photocatalysis under solar light. The morphological, optical, structural and thermal characteristics of the prepared heterojunctions were investigated by SEM-EDS, TEM, UV-DRS, XPS, PXRD, FT-IR and TGA. The optical analysis revealed that visible light harvesting of Zn-HAp@g-C3N4 was improved, and the band gap of pristine HAp was lowered from 3.7 eV to 2.6 eV in the hybrid Zn-HAp@g-C3N4 nanocomposite, indicating improved charge carrier mobility. It was found that, in comparison to Zn-HAp, pristine HAp, and pristine g-C3N4, Zn-HAp@g-C3N4 showed better photocatalytic performance for the degradation of pharmaceutical pollutants, namely ciprofloxacin and levofloxacin, in natural solar light. Moreover, the effects of various reaction parameters, such as initial concentration of antibiotic solution, the amount of catalyst, and effect of solution pH on the rate of photodegradation were examined. The photodegradation of antibiotics with Zn-HAp@g-C3N4 obeyed pseudo-first-order kinetics, with rate constants of 0.05113 min(-1) and 0.0613 min(-1) for ciprofloxacin and levofloxacin, respectively. Quenching tests were also performed to establish the role of the produced superoxide radicals in the degradation process. A plausible photocatalytic mechanism has been proposed for the increased degradataion of ciprofloxacin and levofloxacin by the Zn-HAp@g-C3N4 nanocomposite under visible light irradiation.