Zinc oxide nanoparticles (ZnO-NPs) were successfully synthesized using both biological and chemical techniques. This study is mainly focused on the green synthesis (biological) method of ZnO-NPs using different plant leaf extracts from leaves of guava, olive, fig, and lemon. The plant leaf extracts (polyphenols) act as reductants and zinc nitrate hexahydrate (Zn(NO3)2.6H2O) acts as a precursor. Chemical synthesis method of ZnO-NPs was carried out using prepared sodium hydroxide and zinc nitrate. The produced ZnO-NPs were characterized using X- ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), fourier transform infrared (FTIR), Thermal gravimetric analysis (TGA), and particle size analysis (PSA). The average crystallite size of green-synthesized zinc oxide nanopowders range from 7.1 to 28 nm, while the average crystallite size of chemical-synthesized zinc oxide powders range from 19.6 to 148 nm according to the XRD calculations and TEM observations. Both ZnO nanopowders showed high thermal stability up to 600 oC. The green-synthetized ZnO-NPs were evaluated for phenol decontamination in polluted wastewater. The maximum recorded phenol removal was 99.7% within 250 minutes using 0.1g ZnO-NPs, making ZnO-NPs a promising adsorbent material for phenol decontamination in polluted wastewater