The pharmaceutical oxytetracycline dihydrate (OTC) was obtained from Sigma-Aldrich, Chemie GmbH, Kappelwegl, Germany. ZnO (99.99%, product# 30N-0801, Lot# IAM10229ZNON), BaTiO₃ (99.95%, 100 nm, Catalog# 5622ON-01, Lot# IAM6070NBTO1) were collected from Inframat^® Advanced Materials^TM, USA. 3 mg BaTiO₃ with 97 mg ZnO, 18 mg BaTiO₃ with 82 mg ZnO, 33 mg BaTiO₃ with 67 mg ZnO and 48 mg BaTiO₃ with 52 mg ZnO, respectively, to produce 3 BZ, 18 BZ, 33 BZ and 48 BZ. An ISO Laboratory (Institute of National Analytical Research and Service, BCSIR, Dhaka, Bangladesh) provided the deionized water that was used in this experiment. A Nylon membrane pore size—0.22 µm, dia—25 mm; PTFE; chrodisc by CHM (CHMLAB Group, Barcelona, Spain) was used to filter the samples after a certain experimental time. Ethanol was used to clean all of the glassware used in this experiment. No additional process was demonstrated for further purification of the chemicals.

At 2˚ min⁻¹ of scan rate from 25˚ to 65˚, the powder XRD patterns of the catalysts were characterized using a Bruker D8 Advance XRD (Bruker AXS, Germany) with CuKα radiation, λ = 1.5418 Å.

Under UV irradiation, OTC antibiotics were degraded using a simple photocatalytic reactor. Three ultraviolet lights, each of 8W and 254 nm wavelength were used as UV irradiation source. A spacing of 4.5 cm between the radiation source and solution was maintained to utilize the maximum amount of UV light which was explained by the experimental setup represented in our previous work [26] .

A stock solution of OTC which was of 30 mg/L concentration, was prepared using deionized water. In this research, the dosage of each catalyst was 0.5 g/L. The photocatalysts were added to a 100 mL OTC solution at the room temperature. Before exposure to the UV radiation, an adsorption control equilibrium of each catalyst was performed. In the dark, the OTC solution mixture was stirred for 20 minitues using an orbital shaker at 250 rpm. After the adsorption and UV experiment, 5 mL of the sample was collected using a syringe after a specific experimental time. The solutions were then kept in the dark for 24 h to settle down the nanoparticles. After removing the nanoparticles via filtration using a Nylon Membrane Filter, the solutions were analyzed. The UV-vis absorption spectra of OTC were measured using a UV1800 spectrophotometer (Shanghai UPG International Trading Co., LTD, China). By investigating the removal of OTC antibiotics, the impact of different photocatalysts was investigated.

For measuring the removal efficiency of OTC antibiotics, the following equation was used—

% Removalefficiency = C 0 − C t C 0 × 100 % (1)

where C₀ is the initial concentration of OTC and C_t is the concentration of OTC after a distinct time interval.

The photocatalytic degradation kinetics was determined using first-order kinetic model.

First-order kinetic equation:

ln ( C 0 C t ) = K o b s t (2)

where K_obs is the first-order kinetic rate constant.

Figure 1 shows the XRD patterns of ZnO, BaTiO₃ and 48 BZ. The diffraction peaks occurred at 31.66˚, 34.30˚, 36.21˚, 47.49˚, 56.59˚ and 62.89˚ for ZnO. These results matched with JCPS No (36-1451). For BaTiO₃, the diffraction peaks were at 31.51˚, 38.99˚, 45.29˚ and 56.29˚. These results matched with JCPDS No (05-0626). For 48 BZ, a group of diffraction peaks were at 31.66˚, 34.44˚ and 36.35˚ indicating the presence of ZnO and another group of diffraction peaks were at 38.84˚, 45.29˚ and 56.14˚ indicating the presence of BaTiO₃.

The adsorption-desorption activities of all catalysts were evaluated after stirring in the dark for 20 min. The concentrations of OTC were 12.55, 14.21, 7.95, 13.82 and 10.46 ppm after 20 min using ZnO, 3 BZ, 18 BZ, 33 BZ and 48 BZ respectively. 58.15%, 52.62%, 73.47%, 53.9% and 65.10% removal of OTC were done via adsorption process using ZnO, 3 BZ, 18 BZ, 33 BZ and 48 BZ respectively. The percentages are higher due to the destruction of the outer layer. However, the highest efficiency was found using 18 BZ in the dark.

Figures 2(a)-(e) show the evolutions of UV-vis absorption spectra of photodegradation of OTC within irradiation time. The topmost curve indicates the initial concentration of OTC (30 mg/L) which has a maximum absorbance at 347.5 nm. As time elapsed, the absorbance peaks were decreased that is, the concentrations of antibiotics were decreased. The peaks were shifted to the right side with the flow of time due to the red shift.

5, 10, 15, 20, 25 and 30 ppm standard solutions were prepared for having calibration curve. The solutions were investigated by using a UV-vis spectrophotometer. The wavelengths were recorded at which maximum absorbances were gained. All of the absorbances of OTC solutions were taken at the wavelength of 347.5 nm. Therefore, in Figure 3, a calibration curve of OTC was drawn by graphing the values of absorbance at 347.5 nm against the standard concentrations of OTC solution. The curve was further used for the determination of unknown concentrations of OTC solutions.

70.21% of OTC was degraded after 60 min using ZnO catalyst which can be seen in Figure 4. This reached 99.57% after 420 min whereas 97.87% of OTC was degraded by using 3 BZ. Again, more than 420 min of experiment time was required to get more than 95% efficiency using 18 BZ, 33 BZ and 48 BZ. The use of 18 BZ showed an efficient photocatalytic degradation after 540 min which had 99.72% removal efficiency. Figure 4 shows that when the irradiation time rose, the removal efficiency improved as well. After 600 min of the exposure time, the

removal efficiencies were 99.86% and 99.87% for using 33 BZ and 48 BZ respectively which were almost the same. Enhancing the amount of BaTiO₃ causes a decrease in degradation efficiency. Hence, more than 420 min is required to get almost 99% efficiency. Qinghua Chen et al. investigated that almost 96% of OTC can be from aqueous solution after 1 hr of exposure to the simulated solar solution using Au-CuS-TiO₂ nanobelts [27] .

Kinetic study plays a pivotal role in understanding the degradation rate. Figure 5 depicts the experimental data of the first-order kinetic curves of OTC. The applicability and validation of the first-order model were determined following the values of the determination of coefficient (R²). The values of R² observed using the zero and second-order kinetic models were much lower and it can be determined that the degradation of antibiotics does not follow those models. The kinetic reaction rate constant was measured from the slope of the plot of −ln(C_t/C₀) versus experiment time (t). It is evident that the highest K_obs value was found for the ZnO photocatalyst which had the value of 0.00933 min⁻¹. The high value of the reaction rate constant indicates high degradation of OTC. Furthermore, the lowest value of reaction rate constant was 0.00532 min⁻¹ which was observed for 48 BZ photocatalyst which means that BaTiO₃ can be used for tailoring the activity of ZnO photocatalyst. Previously, Shuai Zhang et al. showed that for the photodegradation of OTC antibiotics using visible light, the first-order reaction rate constant was 0.00765 min⁻¹ using nanohybrids of CoFe alloy nanoparticles along with mesoporous carbon codoped with nitrogen-/sulfur where the flow of N₂ was at 1000˚C for 3 hr [28] .