**1. Introduction**

Antibiotic residues in the environment is pose a major public health challenge [1]. Fluoroquinolones (FQs) are a class of environmentally stable broad spectrum antibiotics, which inhibits the enzymes DNA topoisomerase II (Gyrase) and DNA topoisomerase IV in bacteria thus interfering with their DNA replication machinery [2,3]. FQs are effective against both Gram positive and Gram negative bacteria and are used both in humans and animals. Ciprofloxacin is the most commonly

used FQ, Studies report the occurrence of FQs, including ciprofloxacin, in water bodies worldwide [4]. FQ reaches water bodies through excretion after incomplete metabolism within the human/animal gut [5]. Their presence at up to 87 microgram/L and 31 mg/L has been demonstrated in wastewater discharge [6]. Conventional wastewater treatment including biological oxidation and other chemical and physical process leads to only partial removal of these compounds [7]. As a consequence, the presence of broad spectrum antibiotics like FQs, even at very minute concentrations, poses a threat to the surrounding ecosystem and human health through the development of antibiotic resistance amongst environmental bacteria [8], which can potentially lead to further spread of resistance to other bacterial populations including human and animal pathogens through processes such as ingestion of untreated or partially purified water or horizontal gene transfer [9].

With the immediate necessity for substantive degradation of such organic environmental pollutants, semiconductor photocatalysis more appropriately, Advanced Oxidation Processes (AOPs) have proven quite useful [10]. They normally use a semiconductor metal oxide or one of its doped variants as a photo-oxidant which in presence of light charges up and leads to the generation of highly reactive oxidative species like hydroxyl radicals (OH·), superoxide anion (O2·−) and hydrogen peroxide (H2O2) for remediation of organic pollutants. The basic principle behind their action is shown in Figure 1. To date TiO2 and ZnO has been reported to be the best catalysts for photocatalytic applications because of their optical properties, thus having a much better quantum efficiency under visible light [11]. Moreover, owing to their high chemical stability, high oxidation efficiency, low toxicity, less cost, easy availability and being abundant in Nature they are excellent photocatalysts for the mineralization of organic pollutants in both acidic and basic media [12]. ZnO absorbs a substantial amount in the UV range [12] and UV accounts for only 3–5% of the sunlight, thus there is insufficient usage of the total sunlight available, so efforts are needed to design catalysts which will show better photocatalytic efficiency in the visible region of sunlight [11]. In order to address such problems, modifying the metal oxide semiconductor with transition, alkaline and rare earth metals like Mn, Fe, Co, Ni, Ag, Mg, Pb, N, C, S, P, is done [11], which will shift the light absorption towards the visible range.

**Figure 1.** Schematic representation showing generation of reactive oxygen species (ROS) by Fe ZnO nanoparticles on activation with sunlight, and how these ROS attack active components of FQ to degrade them and reduce their anti-bacterial activity.

Photocatalysis with ZnO for the degradation of antibiotics like ciprofloxacin, amoxicillin, ampicillin, cloxacillin using different sources of light was performed earlier [13,14]. Nearly 50% degradation of antibiotics was achieved with high rate constant and maximium degradation was reported at pH 10–11. It has been previously reported in one of our studies that using Fe-doped ZnO for photocatalytic applications majorly contributes towards the generation of H2O2 in the system, which ultimately is detrimental for the photocatalytic oxidation. Moreover the presence of Fe in

the system, serves as an added advantage for the photocatalytic oxidation, since it comes in contact with H2O2 in the system to generate more of hydroxyl radicals via the Fenton process [15]. This will ultimately magnify the oxidation of antibiotic-containing water. Thus doping the catalyst with iron has some added benefits as far as increasing the photocatalytic efficiency of ZnO are concerned. Earlier such Fe-doped ZnO has been used for the successful degradation of wastewater containing dye molecules [16]. The aim of this study was to evaluate sunlight-assisted photocatalytic degradation of ciprofloxacin using Fe-doped ZnO nanoparticles. Further, the residual antibacterial activity of the treated water was assessed against a Gram positive (*Staphylococcus aureus*) and a Gram negative (*Escherichia coli*) bacterium.
