Low-Cost Removal of Phenol from Pharmaceutical Wastewater Using Carbonized Snail Shell Adsorbent: Kinetic, Isotherm, and Thermodynamic Evaluation

Abstract

Phenol-containing pharmaceutical wastewater presents environmental and health concerns because of the toxicity and persistence of phenol. This study evaluated activated carbonised snail shell (SS) as a low-cost adsorbent for phenol removal from pharmaceutical wastewater. The adsorbent was prepared through thermal carbonisation followed by phosphoric-acid activation and was characterised using scanning electron microscopy and Fourier transform infrared spectroscopy. Batch adsorption experiments were conducted to assess the effects of adsorbent dosage, contact time, solution pH, temperature and initial phenol concentration. The highest phenol removal efficiency was 86.19% under the investigated optimum conditions of a 2.5 g adsorbent dosage, pH 5, a temperature of 35°C and a contact time of 35 min. Equilibrium data were best described by the Langmuir isotherm model, with R² = 0.9841 and a maximum adsorption capacity of 58.48 mg/g, indicating predominantly monolayer adsorption within the evaluated concentration range. The pseudo-second-order model provided the best kinetic fit, with R² = 0.9790. The observed pH dependence, surface characteristics and spectral changes suggested that hydrogen bonding, electrostatic interactions, surface-site interactions and pore filling contributed to phenol uptake. Negative Gibbs free-energy values indicated thermodynamic favourability, while an enthalpy change of −36.88 kJ/mol indicated an exothermic process. These findings support further evaluation of activated snail-shell waste for phenol removal from industrial wastewater.