This study deals with the biosynthesis of selenium (Se) nanoparticles and their impregnation as co-catalyst on ZnO to prepare Se-ZnO nano-catalyst for studying its photocatlytic activity. Selenium nanoparticles was isolated from Se-tolerant bacteria using trypton soya broth supplemented with Na2SeO4. The prepared Se-ZnO phtocatalyst was characterized for its physicochemical properties using UV-DRS, DLS, TEM, SEM-EDS and XRD. The potential of biosynthesized SeZnO was tested against the photocatalytic degradation of recalcitrant compound 4-chloroguaiacol and real pharmaceutical industry effluent under sunlight irradiation (Fig. 1). Various concentrations of Se were doped on ZnO semiconductor, and 0.5wt% was found to be the most optimum. The effect of photocatalyst on different concentrations of model compound was also evaluated. Kinetic study of degradation reaction was performed and it was observed to follow first-order reaction. Evolution of 88% of CO2 under solar radiations further confirms the complete mineralization of the compound. Scheme elucidating the proposed mechanism of photocatalytic reaction. https://s3-euw1-ap-pe-df-pch-content-public-u.s3.eu-west-1.amazonaws.com/9780429423482/fb4dd114-ce13-4e5c-aae2-028ecb69317b/content/fig99_1.tif" xmlns:xlink="https://www.w3.org/1999/xlink"/>