Adsorption Behavior of Azo Dyes on Carbon Nanotubes Grown on Alumina: Process Optimization, Kinetics, and Equilibrium Study

Abstract

In the present study, carbon nanotubes (CNTs) are synthesized using the chemical vapor deposition method modified by growing the CNTs on alumina (CNT/alumina). The maximum Congo red (CR) dye removal of 96.4% and 94% is obtained using CNT/alumina and CNTs, respectively, as adsorbents for 10 mg/L initial dye concentration and an adsorbent dose of 15 mg at pH 2 and 30°C. For Direct red (DR) 80 dye, the maximum removal of 90.5% and 87% is achieved using CNT/alumina and CNTs as adsorbents, respectively, for an initial dye concentration of 10 mg/L with an adsorbent dose of 25 mg at pH 4 and 303 K. The adsorption process fits the Langmuir isotherm. For CR dye, the enthalpy (ΔH) is 13.39 kJ·mol−1 for CNTs and 25.9 kJ·mol−1 for CNT/alumina. For DR 80 dye, the value of ΔH (12.446 kJ·mol−1 for CNTs and 24.086 kJ·mol−1 for CNT/alumina) indicates that adsorption of both the dyes onto both adsorbents is endothermic. The values of Gibbs free energy for both dyes are negative for the temperature range between 303 and 323 K, confirming that adsorption is spontaneous and thermodynamically favorable.