Sustainable water treatment using thermally stable natural clay: dual adsorption–thermolysis approach for organic pollutants and nitrate removal

This work introduces a sustainable and innovative water purification technique that uses thermally stable natural clay to remove hazardous organic contaminants and nitrate ions from water. The method integrates adsorption and thermal regeneration to ensure both high removal efficiency and reusability. The contaminants investigated include pharmaceutical compounds (phenazopyridine and tetracycline), industrial dyes (methyl orange and methylene blue), and nitrate ions, which are selected on the basis of their persistence, toxicity, and resistance to conventional treatment methods. Batch adsorption experiments conducted under optimized conditions (pH ≈ 4, 25 °C, and pollutant concentrations ranging from 10 to 40 mg/L) revealed high removal efficiencies, ranging between 84% and 93% for organic pollutants and 89% for nitrate. To assess practical applicability, a continuous-flow column system was employed, confirming the robust performance of the clay under dynamic conditions. A thermal decomposition step at 600 °C effectively regenerated the spent clay by completely mineralizing the adsorbed pollutants into harmless byproducts such as carbon dioxide and water vapor, with no observable secondary waste. The regenerated clay maintained adsorption efficiencies above 85% through five successive reuse cycles, and the reuse experiments were conducted in triplicate to ensure the repeatability and reliability of the application. Comprehensive characterization via X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, and elemental analysis confirmed the high crystallinity, porous structure, and thermal robustness of the clay-properties essential for its dual adsorption-thermolysis functionality. The novelty of this work lies in combining low-cost, thermally stable natural clay with complete thermal decomposition of adsorbed contaminants, achieving both high reusability and pollutant mineralization without secondary waste. This dual-action purification method offers a cost-effective, reusable, and environmentally friendly alternative to conventional water treatment technologies. The results position thermally regenerated natural clay as a promising candidate for large-scale applications in water purification, particularly in resource-limited settings.