3.4. Photocatalytic Activity

The photocatalytic activity of Ce_2−xSb_x(MoO₄)₂ (0.00 ≤ x ≤ 0.09) was investigated against potent anti-inflammatory drug diclofenac potassium using 8 W UV lamp as light irradiation source. Each of the synthesized material (20 mg) was added as photocatalyst into the aqueous solution of diclofenac potassium (5 mg L⁻¹, 50 mL) in separate photocatalytic runs. The working solution was equilibrated by stirring it continuously for half an hour in the dark, prior to the light exposure. Afterward, the suspension was irradiated with light under constant stirring and uniform conditions. The analytical samples were taken out from the reaction mixture after equal intervals of 30 min, followed by centrifugation to separate the photocatalyst from drug solution. The supernatant was analyzed through UV–Vis spectrophotometer and concentration of residual drug was measured. The obtained data were utilized to calculate the photodegradation efficiency (%) using the following equation (Equation (21)) [80]:

where C_t is the concentration of drug solution at certain time interval of light irradiation, whereas C_o is the equilibrium concentration of drug solution obtained before light exposure.

Catalyst dosage, initial concentration of pollutant, and pH of the reaction mixture are some of the crucial factors which affect the efficiency of photocatalytic process; therefore, it is essential to study these parameters to obtain maximum photodegradation efficiency. Amid various synthesized compositions, the material having maximum photocatalytic efficiency was selected as the preferred catalyst to perform further photocatalytic studies. The influence of pH was examined by changing the solution pH from 2 to 11 while keeping other parameters constant, i.e., initial drug concentration = 5 mg/L and catalyst loading = 20 mg. HCl (1 M) and NaOH (1 M) were used to maintain pH. The effect of catalyst dosage was investigated by varying the catalyst amount from 15 mg to 35 mg at pH = 4 and an initial drug concentration of 5 mg/L. In the following series of experiments, initial drug concentration was varied from 5 mg/L to 25 mg/L at optimum catalyst dosage of 5 mg and pH = 4.

For reusability test, the used photocatalyst was recovered from the drug solution via centrifugation after the first photocatalytic experiment. The photocatalyst was washed with deionized water, followed by ethanol, and then oven-dried at 60 °C for 24 h. The dried photocatalyst was used for the second photocatalytic run under the same experimental conditions and the same procedure was repeated until a total of five photocatalytic cycles were completed.

The dominant active species involved in the photodegradation of diclofenac drug was identified via radical scavenging experiments. Each scavenger (5 mL) was separately mixed with the drug solution, and the whole catalytic process was repeated under identical conditions. The scavengers used were isopropyl alcohol (IPA, 0.01 M), triethanolamine (TEOA, 0.01 M) and benzoquinone (BQ, 0.01 M) to quench hydroxyl radicals (OH^•), holes (h⁺), and superoxide anions (^•O₂⁻²), respectively.