Role of ZnO and TiO2 nanomaterials in the removal of toxic trace elements from contaminated water: A review

Persistent Organic Pollutants (POP) and trace elements are important contaminants in global water quality management because of their toxicity, non-biodegradability, and bioaccumulation potential. Contaminants are not easily removed, especially when present at low concentrations and in complex natural water situations, where traditional treatment operations may be less effective. This review provides a critical and comprehensive overview of the application of Zinc Oxide (ZnO) and Titanium Dioxide (TiO 2 ) nanoparticles for the removal of trace metals and co-existing organic pollutants from water. It covers the most important adsorption and photocatalytic reactions that take place, as well as the effects of water chemistry, pH, ionic strength, and organic matter on treatment efficiency. The review indicates that both ZnO and TiO 2 nanoparticles exhibit high removal efficiency for Pb 2+ , Cd 2+ , As(V), Cr(VI) and organic contaminants such as dyes, pharmaceuticals, and other micropollutants. TiO 2 is typically more chemically stable, reusable, and persistent in its performance, whereas ZnO tends to have quicker initial removal rates, which is suitable for rapid contaminant reduction. Surface modification, metal or non-metal doping, and composite formation further improve the surface activity, charge separation, photocatalytic response, and aggregation resistance, which enhances pollutant removal. Engineered composites and immobilized nanomaterial systems are especially well suited because they enhance the efficacy of treatment in realistic water conditions but minimize issues associated with nanoparticle leaching, collection, aggregation, and environmental hazards. In general, nanomaterials based on ZnO and TiO 2 potentially become the useful ingredients of the next-generation water treatment system when applied as a part of the hybrid, lifecycle-aware, and real-matrix-validated remediation systems.