2.5. Photocatalytic experiment for degradation of aqueous pollutants

The photocatalytic activity of synthesized biogenic Ag–ZnO nanocomposites (1 and 3 wt% Ag–ZnO NPs) in comparison to that of bare ZnO NP powder was evaluated for degradation of Rh-B (pink colored dye) and chlorpyrifos (colorless toxic pesticide) as representative pollutants. Initially, in a photoreactor of 200 mL capacity, 100 mL Rh-B (10 μM) and photocatalyst (20 mg) were added and stirred vigorously under dark for about 30 min to achieve adsorption–desorption equilibrium. Thereafter, the mixture solution was exposed to direct sunlight. The photocatalytic degradation tests were conducted in sunny days of May 2018 between 11:00 am and 3:00 pm. To prevent heating of the reaction mixture, cold water was circulated in the outer jacket around the photoreactor. For determining the dye degradation rate, a small sample solution of around 3 mL was withdrawn after every 3 min and centrifuged to remove the catalyst. After photodegradation, the filtrate was analyzed using a UV-vis spectrophotometer to determine the Rh-B concentration in the solution. Furthermore, for degradation of chlorpyrifos, 20 mg of photocatalyst was taken with 100 mL aqueous solution of chlorpyrifos (50 ppm) in photoreactor and the procedure was identical to that used for Rh-B degradation. To analyze the pesticide degradation process, small quantity of reaction mixture was withdrawn and centrifuged and the filtrate was analyzed through a UV-vis spectrophotometer at periodic intervals of 10 min.