The present study demonstrates the removal of hazardous metal ions such as lead (Pb), arsenic (As), cadmium (Cd), and chromium (Cr) from industrial wastewater by adsorption of ZnO nanoparticles. The ZnO nanoparticles are prepared using the green synthesis route, in which the leaves of the cassia fistula (Amaltas) plant are used as a plant extract. Pure hexagonal wurtzite crystal structure is obtained in X-ray diffraction (XRD) measurements for both 200 and 750 °C annealed ZnO samples. The morphological study using field emission scanning electron microscopy (FESEM) indicates that the particle size of 200 °C annealed ZnO nanoparticles is much smaller (10–15 nm) than the 750 °C annealed ZnO nanoparticles (100–150 nm). The optical band gaps of 2.79 eV and 2.99 eV are determined for 200 and 750 °C annealed ZnO samples, respectively. Maximum adsorption of only 3 ions is noted for various concentrations of the ZnO nanoadsorbents. For 750 °C annealed ZnO nanoparticles “Pb”, “Cd”, and “Cr” ions are easily adsorbed, while for 200 °C annealed ZnO nanoparticles “Pb” and “Cr” are easily adsorbed. Further, for low concentrations of 200 °C annealed ZnO nanoparticles “Cd” ion is easily adsorbed, while the high concentration favors “As” adsorption. Out of various ions, the highest adsorption capacity is noted for the “Cr” ion with an adsorption capacity value of 9.16 µg/g for 750 °C annealed ZnO and 5.20 µg/g for 200 °C annealed ZnO nanoadsorbents. Consequently, both 200 and 750 °C annealed ZnO nanoadsorbents showed a removal efficiency of 75.25% and 73.69% for “Cr” ions.