A review on the fabrication and applicability of ZnO Nanoparticles for adsorption of heavy metals from wastewater: A prospect towards sustainability

The rapid advancement of economic and industrial activities has significantly increased the concentration of toxic heavy metals in aquatic systems, posing serious risks to human health and ecosystems even at low concentrations. Consequently, the removal of heavy metals from contaminated water has become critical environmental challenge. Among various treatment techniques, adsorption has emerged as an efficient and sustainable approach due to its operational simplicity, cost-effectiveness, and minimal secondary pollution. In recent years, nanoadsorbents have attracted considerable attention owing to their high surface area to volume ratio, which provides abundant active sites for metal ion removal. Among them, zinc oxide nanoparticles (ZnO NPs) have been widely explored due to their biocompatibility, low toxicity, tunable surface properties, and unique physicochemical characteristics, including surface charge behavior and photoinduced reactivity. However, existing studies and reviews remain largely fragmented, with limited efforts to systematically correlate synthesis methods with structural properties and adsorption performance. In this context, the present review critically analyzes differently fabricated ZnO nanoadsorbents including commercial, conventionally synthesized, hybrid, and green synthesized by establishing comprehensive synthesis, structure and performance relationship. The influence of key parameters such as morphology, defect density, and surface hydroxylation on adsorption behavior is systematically evaluated. Furthermore, this review provides a comparative assessment of adsorption capacities reported in the literature and discusses dominant adsorption mechanisms under varying environmental conditions. The regeneration and reusability of ZnO nanoadsorbents are also critically assessed, along with associated challenges such as stability and potential environmental risks. Finally, future perspectives are outlined with an emphasis on sustainable strategies and circular economy integration for practical wastewater treatment applications.