Recycling paper sludge into hydrochar and ZnO nanocomposite for enhanced ammonium adsorption in aqueous solutions

This study investigates the ammonium (NH₄ ⁺) adsorption capabilities of hydrochar derived from paper waste sludge (PWSH) and its modified variant with ZnO (PWSH@ZnO).This study investigates the ammonium (NH₄ ⁺) adsorption capabilities of hydrochar derived from paper waste sludge (PWSH) and its modified variant with ZnO (PWSH@ZnO). The adsorption behaviors were analyzed by varying parameters such as pH, contact time, initial NH₄ ⁺ concentration, and ZnO modification ratios. The results indicate that ZnO modification significantly enhances the NH₄ ⁺ adsorption capacity, with PWSH@ZnO achieving a maximum capacity of 23.08 mg g^-1, compared to 20.09 mg g^-1 for unmodified PWSH. The optimal pH for NH₄ ⁺ removal was found to be 8, at which PWSH@ZnO demonstrated a superior removal efficiency of 80%, compared to 68.03% for PWSH. Kinetic studies revealed that the adsorption process followed a pseudo-first-order model for both materials, with PWSH@ZnO exhibiting faster adsorption rates. Isotherm analysis further indicated that the adsorption is best represented by the Langmuir model, suggesting monolayer adsorption on a homogeneous surface. Overall, the incorporation of ZnO nanoparticles enhanced the adsorption capacity and improved the material's stability, positioning PWSH@ZnO as a promising candidate for NH₄ ⁺ removal in wastewater treatment applications. The synthesized PWSH@ZnO also demonstrates commendable reusability, maintaining nearly 50% of its initial adsorption capacity after five cycles.