Removal of Congo red dye from aqueous solution using a clay based nanocomposite

Abstract

In this study, the efficacy of bentonite clay, pre-treated magnesite and their nanocomposite on the removal of Congo red dye from aqueous solution was explored. Batch experimental approach was a technique used to fulfil the goals of this study. A number of operational parameters were optimised, and they include effects of shaking time, adsorbent dosage, initial CR dye concentration, initial solution pH and temperature. Findings of the study revealed that the optimum conditions that are suitable for the removal of CR dye are 20 minutes, 0.5 g of dosage, 120 mg/L, 250 rpm, and pH = 7. This has achieved > 99% removal efficacy of CR dye for the nanocomposite and reduced it to below the South African National Standard (SANS) 241 water quality specifications. Furthermore, kinetic studies revealed that bentonite clay, pre-treated magnesite, and their nanocomposite fitted very well to pseudo-second-order kinetics than pseudo-first-order kinetics. The regression analysis was observed to be 1, 0.9, and 0.9 for bentonite clay, pre-treated magnesite, and their nanocomposite respectively. Adsorption isotherms indicated that CR removal by bentonite clay, pre-treated magnesite, and their nanocomposite fitted well to Langmuir adsorption isotherm than the Freundlich adsorption isotherm hence indicating mono-layer adsorption. Thermodynamic values for CR removal were observed to be: ΔH0 (kJ mol-1) = 43.86, 30.67, and 24.88 for bentonite clay, pre-treated magnesite, and their nanocomposite respectively. This indicates that the reaction is endothermic. The positive ΔS0 (kJ mol-1 K-1) values for bentonite clay and 25 °C for pre-treated magnesite confirms that there is an increase in the degree of randomness at solid/solution interface during the removal of CR ions from aqueous solution. The negative values of ΔG0 (kJ mol-1) for 40 – 70 °C on bentonite and the entire range for the nanocomposite suggest the spontaneity and feasibility of CR adsorption whereas the positive ΔG0 (kJ mol-1) for bentonite clay suggest a non-spontaneous nature of adsorption. As such, pre-treated magnesite/bentonite clay nanocomposite demonstrated superior adsorption capacity in relation to individual materials and other materials reported in literature.