Removal of malachite green using carboxymethyl cellulose-g-polyacrylamide/montmorillonite nanocomposite hydrogel

Abstract

In this study, carboxymethyl cellulose based graft poly(acrylamide) hydrogel (CMC-g-P(AAm)) and its nanocomposite with montmorillonite (CMC-g-P(AAm)/MMT) were produced by the free radical method and it was used to malachite green (MG) dye removal from aqueous solution. The properties and characterization of the adsorbents were investigated using FTIR, SEM, TGA, and XRD analyzes, and the results showed that MMT nanoparticles were successfully distributed in the hydrogel system. MMT nanoparticles were loaded into the hydrogel system with different weight percentages and the maximum adsorption efficiency of MG dye was determined at 10 wt% MMT. Also, the effect of temperature, contact time, initial concentration of MG dye and initial pH on the adsorption efficiency of MG dye was studied in a batch. Equilibrium behavior investigation of the adsorption process showed that the equilibrium data determined are in good agreement with the Langmuir isotherm model and the monolayer surfaces play an effective role in the adsorption process. The maximum monolayer adsorption capacity (q_max) determined using the Langmuir isotherm model for CMC-g-P(AAm) and CMC-g-P(AAm)/MMT were determined to be 158.1 mg/g and 172.4 mg/g, respectively. Also, the kinetic study showed that the pseudo-second- order kinetic model is more capable of describing the kinetic behavior of the process than other models. Also, the α parameter values for the MG dye adsorption process using CMC-g-P(AAm) and CMC-g-P(AAm)/MMT were determined to be 0.6337 mg/g·min and 31.04 mg/g·min, respectively. This indicates that the produced composite has a high adsorption value. Gibbs free energy (ΔG°) was negative for the adsorption processes in the range of 25-50 °C, indicating that the process was spontaneous. In addition, the enthalpy parameter (ΔH°) was determined for the adsorption process using CMC-g-P(AAm) and CMC-g-P(AAm)/MMT nanocomposite hydrogels at 39.859 KJ/mol and 74.736 KJ/mol, respectively. Positive ΔH° indicates that the process is endothermic in the range of 25-50 °C using both adsorbents. Also, the concentrations effect of Na+ and K+ ions on adsorption efficiency was investigated and it was concluded that the efficiency of the adsorption process decreased with the increase of ions concentration. Adsorption efficiency decrease can be due to the occupation of active sites and repulsive electrostatic interactions at the adsorbent surface with the MG dye molecule.

Keywords: Adsorption capacity; Carboxymethyl cellulose; Malachite green; Montmorillonite; Nanocomposite hydrogel; Thermodynamic study.