In this study, Copper (II) adsorption from an aqueous solution with nanoparticles synthesized using organic waste was investigated. For this purpose, the extract obtained from Allium cepa (A. cepa) peel was used as a reducing agent, and zinc oxide nanoparticles (ZnO NPs) were synthesized by the green synthesis method. The synthesized ZnO NPs were obtained as composite adsorbents with halloysite (HNT), a type of clay. The adsorbents were characterized by Fourier transform infrared spectroscopy (FT-IR), energy dispersive X-ray (EDX), scanning electron microscopy (SEM), X-ray diffractometer (XRD), dynamic light scattering spectrometry (DLS), N2 adsorption–desorption analysis and zero charge point (pHPZC) analysis. Parameters affecting the adsorption process such as pH, adsorbent dose, contact time, initial metal concentration, and temperature were investigated. According to the correlation coefficient, the data obtained with ZnO NPs were best summarized by the Langmuir isotherm with an adsorption capacity of 500 mg g−1. The kinetic description of the adsorption system was performed by pseudo-second-order kinetic and intraparticle diffusion kinetic models. Thermodynamic parameters were calculated and the adsorption process was found to be spontaneous and exothermic.