Polymer nanocomposite adsorbents for synthetic dye removal from water and wastewater: a critical review of structure-performance relationships and practical challenges

Synthetic dyes discharged from textile and related industries pose a major water-quality threat due to their chemical stability, structural complexity, and resistance to conventional treatment. This review critically evaluates polymer nanocomposite adsorbents for dye removal, focusing on polymer-matrix chemistry, nanofiller properties, fabrication, dispersion, surface modification, porosity, and adsorption mechanisms. Emphasis is placed on the interplay among polymer–nanofiller interactions, surface chemistry, pore accessibility, dye structure/charge, and solution conditions. Beyond maximum adsorption capacity (Qmax), performance is assessed through pH, contact time, temperature, kinetics, isotherms, regeneration, structural stability, nanofiller leaching, and water-matrix complexity. Azo dyes are emphasized as the dominant class, while methylene blue and malachite green serve as non-azo benchmark adsorbates; cross-linked polymers are used as reference materials to isolate nanofiller effects from those of the polymer matrix. The review highlights the persistent gap between laboratory-scale results and practical wastewater treatment, proposing a research roadmap centered on standardized reporting, authentic wastewater testing, repeated regeneration cycles, leaching assessment, continuous-flow validation, and preliminary cost/life-cycle evaluation. Overall, key factors governing adsorption are identified, and the evidence still required to establish real-world treatment potential and sustainability is defined.