Batch sorption–photodegradation of Alizarin Red S using synthesized TiO2/activated carbon nanocomposite: an experimental study and computer modelling

Abstract

Photocatalysis is an alternative wastewater treatment method for the removal of toxic pollutants. The present work describes the hydrothermal synthesis of a new photocatalytic composite material involving activated carbon (PAC) and titanium dioxide nanoparticles (TiO2-NP) and its application for removal of Alizarin Red S (ARS) dye from wastewater. Characterization of the TiO2-NP/PAC composite was done by different analytical methods like SEM–EDX, XRD, FT-IR, photoluminescence, pHZPC and UV–visible diffuse reflectance spectroscopy. In order to evaluate the efficacy of the composite in the simultaneous adsorption and degradation of an anionic dye, ARS aqueous solution at varying TiO2-NP/PAC dosage, contact time, ARS concentrations and pH was evaluated. A composite dosage of 0.05 g shows 99.10% degradation of 20 mg l−1 ARS dye concentration in 80 min contact time at pH 2. It was observed that photocatalytic degradation of the dye followed pseudo-first-order rate kinetics and the adsorption isotherm was found to be best fitted with Freundlich isotherm model. The negative enthalpy and free energy indicated an exothermic and spontaneous process. DFT studies were used to explain the mechanism of formation of the composite, and results revealed a favourable immobilization of TiO2-NP on PAC via O-linkage of the O–Ti–O bond resulting in the formation of TiO2-NP/PAC composite. Chemical descriptors such as dipole moment, ionization energy, chemical softness and hardness and HOMO–LUMO energy obtained from DFT studies helped in understanding the comparative efficiency and reactivity of PAC and TiO2-NP/PAC towards ARS degradation.