Removal of phenol and 4-nitrophenol from wastewater using a composite prepared from clay and Cocos nucifera shell: Kinetic, equilibrium and thermodynamic studies

Wastewater generated from industries contains a large number of toxic substances, including phenolic compounds, that are harmful to the aquatic life and the environment. In this work, a composite was prepared from locally sourced materials (coconut shell and clay) for adsorption of phenol and 4-nitrophenol from aqueous solutions. The prepared composite was activated using KOH and characterised using Scanning Electron Microscopy-Energy Dispersive X-ray spectroscopy, Fourier Transforms Infrared spectroscopy, X-ray diffraction, nitrogen adsorption-desorption isotherm, and pH of point of zero charge. Variations of time, temperature, initial pH and the initial concentrations of adsorbate on the adsorption capacity of the composite were explored using batch adsorption procedure. Kinetic models (pseudo-first order, pseudo-second order, general order, Avrami and intra-particle diffusion), equilibrium models (Langmuir, Freundlich, Temkin, Dubinin- Radushkevich (D-R), Liu and Sips) and thermodynamic equations were employed to explain the adsorption data. Avrami fractional order and Liu isotherm gave the best fit of the respective kinetic and equilibrium experimental data for both phenol and 4-nitrophenol. The maximum adsorption capacities calculated from the Liu isotherm are 1665 mg g-1 (25 degrees C) and 476.9 mg g-1 (30 degrees C) for phenol and 4-nitrophenol, respectively. The adsorption process was endothermic and spontaneous. The composite removed at least 86% of the two types of simulated effluents tested. This shows that the composite is a promising adsorbent for treatment of industrial wastewater.