*3.1. Photocataltytic Degradation of Ciprofloxacin and Process Optimization*

Figure 2 shows the decrease in the C/Co absorption spectrum of ciprofloxacin (C = concentration at a particular time, Co = initial concentration of ciprofloxacin) at three catalyst concentrations (100, 150, 200 mg L<sup>−</sup>1), during sun-assisted photocatalysis by Fe-doped ZnO nanoparticles. The values were calculated on the basis of intensity of the absorbance peaks at 280 and 320 nm. At both these λmax, the absorbance showed a decreasing trend at all the three catalyst concentrations.

**Figure 2.** Photocatalytic degradation of antibiotic ciprofloxacin (10 mg/L) in water, in the presence of Fe-ZnO nanoparticles (at different concentrations of 100, 150 and 200 mg/L) irradiated with sunlight light intensity of 80,000 ± 3000 lux compared to photolysis (light control) and degradation in the absence of light (dark control). C0 represents initial concentration of ciprofloxacin and C represents concentration of ciprofloxacin at a particular time point. C/C0 denotes the time dependent change in ciprofloxacin concentration with respect to initial concentration.

A catalyst concentration of 150 mgL−<sup>1</sup> caused a significant degradation of ciprofloxacin (10 mg L<sup>−</sup>1) of up to 66% in 210 min and was found to be optimum. The other two concentrations were not as effective. The 100 mg L−<sup>1</sup> catalyst may not have the capability for substantial generation of reactive oxygen species, while the 200 mg L−<sup>1</sup> catalyst concentration may be high enough to create a catalyst shielding effect. Moreover the 200 mg L−<sup>1</sup> may possess slow or improper degradation kinetics of only 51%. For further experiments, therefore all the degradation experiments were carried out with 150 mg/L of Fe-doped ZnO. There was no significant change in concentration of the ciprofloxacin due to the direct sunlight assisted photolysis (light control) which was found to be only 14% [14]. The decrease in C/Co value (up to 25%) of the antibiotic when subjected to dark control reaction (at the optimum photocatalyst concentration of 150 mg/L), may be attributed to direct adsorption of the antibiotic in the presence of doped ZnO nanoparticles [11].

The concentration of antibiotic in the wastewater system is a key parameter to optimize the photocatalytic degradation process. A study was performed with ciprofloxacin concentrations of 5, 10 and 15 mg L−1. Figure 3 shows the photocatalytic degradation pattern of different concentrations of ciprofloxacin with the optimized concentration of Fe-doped ZnO nanoparticles. At 10 mg L−<sup>1</sup> concentration no peaks were observed at 280 and 320 nm after 210 min of photocatalytic treatment, suggesting complete degradation of the quinolone ring. Five mg L−<sup>1</sup> concentrations of ciprofloxacin were also completely degraded. Since studies with 10 mg L−<sup>1</sup> concentrations were previously done and reported, the rest of the photocatalytic study were done with 10 mg L−<sup>1</sup> concentration. With 15 mg L−<sup>1</sup> ciprofloxacin concentration the degradation kinetics were a bit slower. Possible reasons could be a catalyst shielding effect and over-occupied catalyst active sites at 15 mg/L concentration [11,19].

**Figure 3.** Photocatalytic degradation of antibiotic ciprofloxacin in water at different antibiotic concentration between 5, 10, 15 mg/L with optimum Fe-doped ZnO nanoparticles concentration of 150 mg/L and irradiated with sunlight intensity of 80,000 ± 3000 lux. C0 represents initial concentration of ciprofloxacin and C represents concentration of ciprofloxacin at a particular time point. C/C0 denotes, time dependent change in ciprofloxacin concentration with respect to initial concentration.

pH modifies the surface charge properties of Fe-doped ZnO and possibly the chemical structure of the antibiotic, therefore the influence of pH on the photocatalytic activity of Fe-doped ZnO nanoparticles was studied by altering the pH of the reaction mixture in both the acidic and basic range. Figure 4 shows the effect on the photocatalytic degradation on ciprofloxacin of different pHs in the presence of Fe-doped ZnO nanoparticles. The best degradation efficiency of ciprofloxacin with Fe-doped ZnO nanoparticles, nearly 65%, was seen at pH 9, while the lowest degradation, only 10%, was observed at pH 2 [14]. The maximum ciprofloxacin degradation was thus obtained at basic pH values between 9 and 11 under solar light, where the available hydroxyl ions in the system can react with the valence band holes (h+) to form reactive hydroxyl radicals (OH·), which possesses high

oxidation capability under photocatalytic conditions, subsequently enhancing the rate of photocatalytic degradation of ciprofloxacin. Similar results for the degradation of aromatic compounds were reported earlier [20]. At an acidic pH value of 2, the solar photocatalytic degradation of ciprofloxacin was hindered due to the high proton concentration, which possesses higher attraction for the hydroxyl anions, quenching the formation of hydroxyl radicals. As free hydroxyl ions in the system are decreased, the formation of hydroxyl radicals becomes limiting. Thus photocatalytic degradation of ciprofloxacin decreased at lower pH. It may also be possibly due to dissolution of Fe-doped ZnO under acidic conditions. Similar observations were previously made in the photocatalytic degradation of azo dyes [16].

**Figure 4.** Photocatalytic degradation of antibiotic ciprofloxacin (10 mg/L) in water in the presence of Fe-ZnO nanoparticles (150 mg/L) irradiated with sunlight intensity of 80,000 ± 3000 lux at different reaction pH of 2, 3, 5.5, 7, 9, 10, 11. C0 represents initial concentration of ciprofloxacin and C represents concentration of ciprofloxacin at a particular time point. C/C0 denotes the time dependent change in ciprofloxacin concentration with respect to initial concentration.

Ciprofloxacin is an ampholytic compound with a pKa value of 6.09 for the carboxylic group and 8.74 for the nitrogen on the piperazinyl ring. The isoelectric or zwitterionic point is at pH 7.4. Thus ciprofloxacin seemed to be most sensitive to photocatalytic degradation at a pH closer to its zwitterionic form, i.e. at basic pH 9. It has earlier been reported that the maximum stability of the molecule was observed in reaction solution of pH 4.0 [21], where the carboxylic group is un-ionized and basic nitrogen is completely protonated. This adds an advantage to the ciprofloxacin pharmaceutically, because most of the pharmaceutical formulation possess pH between 3.5 and 5.5. This seems good from a pharmaceutical perspective but photocatalytic degradation at such low pH will be a challenge. Interestingly, it has been previously reported that, hospital wastewater flowing to drains has an pH in between 6.7 to 7.7 throughout the year, Moreover the pH of surface waters (mainly lakes and rivers) in India is between 6.5 to 8.5 [22]. The current study thus finds it application for degradation of antibiotics in hospital wastewater and surface water, since at this pH range the photocatalytic degradation was more than 60%, as shown in Figure 3.

From experimental observations and previous reports on the photocatalytic degradation of organic molecules like dyes [23] and antibiotics [24], we assumed that upon irradiation with solar light, within the Fe-doped ZnO nanoparticles, excitation of electrons takes place from the valence band into the conduction band. Photogenerated holes in the conduction band upon reacting with water molecules in the system generate hydroxyl radicals which possess oxidative nature and can get rid of antibiotics adsorbed on the Fe-doped ZnO surface. Moreover the high oxidative potential of valence band holes can also lead to the direct and indirect oxidation of antibiotics. The presence of Fe in the system possesses an added advantage to this photocatalytic degradation process. The presence of Fe delays the electron whole recombination, acting as one of the terminal acceptors of electrons, which eventually increases the generation of hydroxyl radicals and reactive species in the system. Also Fe as a Fenton agent is capable of producing reactive oxygen species like OH· radicals through the Fenton process, adding more ROS to the system for subsequent degradation of ciprofloxacin [11,25].

Temperature was found to modulate the degradation kinetics (Figure 5). Generally it has been reported that with an increase in temperature the degradation kinetics are enhanced [11], but in the current study, the opposite trend was observed. With increasing temperature, the degradation kinetics decreased up to 60 ◦C. A possible reason could be the increase in the stability of fluoroquinolones on exposure to heat stress. It has been reported by Roca et. al. [26], that FQs can be stable at temperatures up to 120 ◦C. In a country like India, where the atmospheric temperature can reach up to 50 ◦C, the technique presented in this paper can be employed for successful degradation of ciprofloxacin and maybe other fluoroquinolones also, in wastewater matrices. The technique presented in this paper may also find its application for the treatment of hospital, pharmaceutical or industrial wastewater for the degradation of many organic molecules.

**Figure 5.** Photocatalytic degradation of antibiotic ciprofloxacin (10 mg/L) in water in the presence of Fe-ZnO nanoparticles (150 mg/L) irradiated with sunlight intensity of 80,000 ± 3000 lux and pH 9 with different reaction temperature. C0 represents initial concentration of ciprofloxacin and C represents concentration of ciprofloxacin at a particular time point. C/C0 denotes the time dependent change in ciprofloxacin concentration with respect to initial concentration.
