Defect-Engineered ZnO Nanostructures for Photocatalytic Water Purification

Contamination of water by pesticides, antibiotics, pharmaceuticals and other persistent organic compounds creates environmental and food-safety concerns. Zinc oxide (ZnO) is a promising semiconductor photocatalyst, but its practical performance is limited by ultraviolet-dominated absorption, electron–hole recombination and photocorrosion. This narrative and critical review examines how defect engineering, elemental doping, heterojunctions and carbon coupling influence ZnO structure, surface chemistry and charge-carrier behavior in water purification. The supplied literature was assessed for material composition, synthesis, irradiation conditions, pollutant removal, mechanistic evidence, catalyst reuse, transformation products, toxicity and leaching where reported. Studies of Ag-, Pd-, Ce-, Tb- and Er-modified ZnO and oxide/carbon composites demonstrate that modified systems can improve degradation of selected contaminants, although results depend strongly on irradiation, catalyst dosage, pH, pollutant concentration and water matrix. The evidence also shows that disappearance of a parent contaminant does not necessarily establish mineralization or detoxification; transformation products may retain or increase toxicity. Reliable translation to water treatment therefore requires complementary defect characterization, standardized performance reporting, transformation-product and toxicity analysis, catalyst recovery, leaching assessment and energy evaluation. The review identifies ZnO/CuO, ZnO/TiO₂ and ZnO/carbon systems as research directions whose practical advantages must be validated under realistic water-treatment conditions.