Zn(OH)₂ nanoparticles (Zn(OH)₂-NPs) were sonochemically synthesized. A small amount of Zn(OH)₂-NPs was loaded onto activated carbon with a weight ratio of 1 : 10 followed by characterization using scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and diffuse reflectance spectroscopy (DRS). Both activated carbon (AC) and Zn(OH)₂ nanoparticle-loaded activated carbon (Zn(OH)₂-NP-AC) as safe, green and cost-effective adsorbents were used for the removal of malachite green (MG). Response surface methodology as a cost-effective and time-saving approach was applied to model and optimize dye removal versus adsorbent mass, pH, initial dye concentration and sonication time as well as to investigate the possible interaction among these variables. Zn(OH)₂-NP-AC, even at a small nanoparticle loading (with a weight ratio of 1 : 10), was found to be more efficient than AC. For the Zn(OH)₂-NP-AC, the optimum values were found to be 0.019 g, 4.5, 20 mg L⁻¹ and 8.6 min for the adsorbent mass, pH, initial dye concentration and sonication time, respectively. The experimental equilibrium data were then fitted to the conventional isotherm models such as Langmuir, Freundlich, Temkin and Dubinin–Radushkevich. The Langmuir isotherm was found to be the best model for the explanation of the experimental data. The adsorption monolayer capacity of Zn(OH)₂-NP-AC was obtained to be 74.63 mg g⁻¹, which is comparable to published reports. Adsorption kinetics was studied at various initial MG concentrations, which showed that the adsorption of MG follows the pseudo-second-order rate equation, in addition to the intraparticle diffusion model. The adsorbent was shown to be highly regenerable over several iterations. The short-time adsorption process, high adsorption capacity and good regenerability of the safe, green and cost-effective Zn(OH)₂-NP-AC make it advantageous and promising for wastewater treatment.