Removal of chemical oxygen demand from thin-film transistor liquid-crystal display wastewater using chitosan-coated bentonite: Isotherm, kinetics and optimization studies

Abstract

In this study, real thin-film transistor liquid-crystal display wastewater with an initial chemical oxygen demand (COD) concentration of 1348.00 ppm was treated using chitosan-coated bentonite (CCB). Characterization analysis of the CCB adsorbent was performed using Brunauer-Emmett-Teller surface area analysis, scanning electron microscopy, and Fourier-transform infrared spectrometer. The effect of parameters such as contact time, CCB dosage, pH and temperature on the COD removal was examined. Results show that increasing the contact time and CCB dosage increases COD removal efficiency while no considerable change was observed in removal efficiency with varying temperature and pH. Adsorption experiments showed that the removal of COD using CCB best fits the Langmuir isotherm (R2 ≥ 0.9821) while kinetic data was best described by the pseudo-second order equation (R2 ≥ 0.9980), which implies that chemisorption is the rate-determining step. Thermodynamic studies revealed that adsorption of COD onto CCB was spontaneous, exothermic (ΔH° = 5.95 kJ/mol) and decreased randomness in the system (ΔS° = -0.88 J/mol·K). Optimization studies using response surface methodology with central composite design was performed to determine the operating parameters that would yield the maximum COD removal. It was determined that the optimum conditions of 20.32 h, 0.8 g CCB, pH 4.0, and 30 °C would yield a maximum removal of COD of 73.34%.