The present inquiry studied the phytochemical composition in the aqueous extract of Pelargonium, which grows in Iran. Aluminum chloride methods, Folin–Ciocalteu, and GC/MS technique were applied to determine the total flavonoid and phenolic contents and volatile constituents in the extract, accordingly. The stabilizing and reducing potentials of phytochemical composition in the extract were examined for an eco-friendly, simple, and fast biosynthesis of cerium oxide nanoparticles (CeO2-NPs). DLS, XRD, UV–Vis, BET, and FESEM were applied to analyze and characterize the synthesized nanoparticles. In the end and as a practical application, the photocatalyst and antioxidant activities of the biosynthesized CeO2-NPs were evaluated by the reduction of hexavalent chromium {Cr(VI)} ions in aqueous solutions and the scavenging capability of 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radicals, respectively. The phytochemical analysis found the total phenolic and flavonoid contents in the extract to be 136.5 mg GA/g and 63.9 mg RU/g, respectively. Besides, the study identified 22 compounds, including 17 (about 93.5%) of the total volatile constituents, with the major components being α-pinene (25.28%) and fenchyl acetate (20.63%). The characterization results demonstrated that the prepared nanoparticles had a spherical shape, average size of 28 nm, BET surface area of 33.84 m2/g, zeta potential of −25.8 mV, 9.31 nm mane diameter pore, and total pore volume of 0.078 cm3/g. Moreover, the results of the practical applications indicated that the CeO2-NPs have a good scavenging capability of DPPH free radical with the IC50 value of 42.7 μg/mL and good reduction ability of Cr(VI) ions with a reduction efficiency of 94% in aqueous solution containing 10 mg/L of Cr(VI) of ions with natural pH of 5.5 and CeO2-NPs dose of 200 mg/L during 60 min.
