Tuning the surface charge of silica for efficient phenol removal from aqueous solution

Phenol is a hazardous aromatic compound commonly found in industrial effluents. Although silica-based materials are widely used for pollutant removal due to their high surface area and tunable porosity, their negatively charged and hydrophilic surfaces limit their effectiveness for phenol adsorption. In this study, we developed a strategy to overcome this limitation by synthesizing CuO/SiO₂ nanocomposites and further modifying them with cetyltrimethylammonium bromide (CTAB) to enhance surface charge and introduce hydrophobicity. The zeta potential of silica shifted from −37 mV to + 61.8 mV upon CuO incorporation, and further to + 65.9 mV after CTAB functionalization. Both materials were characterized using XRD, SEM, FTIR, DLS, and zeta potential analysis. Adsorption experiments were conducted under varying pH, contact time, adsorbent dosage, phenol concentration, and temperature. The CTAB modified CuO/SiO₂ exhibited a significantly higher adsorption capacity (83.5 mg/g) compared to unmodified CuO/SiO₂ (19 mg/g). Adsorption followed pseudo second order kinetics, indicating that the process is governed by adsorbent–adsorbate interactions. Thermodynamic analysis revealed that the adsorption is spontaneous, endothermic, and entropy driven, with improved performance at elevated temperatures. Isotherm modeling showed that the Freundlich model best described adsorption on CuO/SiO₂, while the Temkin model better fit the CTAB modified system, suggesting multilayer adsorption and uniform energy distribution. Compared to other reported adsorbents, the CTAB modified CuO/SiO₂ nanocomposite demonstrated superior performance for phenol removal, highlighting the effectiveness of surface charge tuning and surfactant modification.