Eco‐Friendly Synthesis of ZnO Nanoparticles From Spent Zinc–Carbon Batteries for Dye Water Treatment

ABSTRACT Recycling waste materials is essential for environmental and economic sustainability. This study presents the recovery of zinc from spent zinc–carbon batteries and its conversion into zinc oxide nanoparticles (ZnO NPs) via a wet‐chemical process. The NPs were characterized using various techniques, confirming spherical, homogeneous, and well‐dispersed particles containing 58 wt% Zn, with a crystallite size of 33 nm, thermal stability up to 800 °C, and a pH PZC of 6.8, indicating abundant active surface sites. The NPs were tested as adsorbents for Congo Red (CR) dye, achieving a maximum removal of 98.09% and an adsorption capacity of ∼60 mg g −1 under optimal conditions (40 mg L −1 dye, 60 mg adsorbent, pH 6, 303–308 K, 180 min). Adsorption decreased with increasing ionic strength. Kinetic data fitted the pseudo‐second‐order model, indicating chemisorption, while isotherm analysis followed the Freundlich model ( R 2 = 0.9999), suggesting heterogeneous multilayer adsorption with high capacity ( q m = 500 mg g −1 ). Thermodynamic parameters (negative Δ H °, Δ G °, and Δ S °) confirmed a spontaneous, exothermic, and enthalpy‐driven process. Adsorption mechanisms involved electrostatic attraction, hydrogen bonding, π–π interactions, and weak complexation. The ZnO NPs maintained good performance over four regeneration cycles, highlighting their potential as low‐cost, sustainable adsorbents for dye‐contaminated wastewater.