Rapid removal of rhodamine dye from aqueous solution using casein-surfactant complexes: role of casein-surfactant interaction

Abstract

We report here a new approach to remove rhodamine B dye, a toxic contaminant in industrial effluents, from aqueous solution using colloidal casein protein and its complexes with surfactants. The dye removal efficiency of casein is studied in the absence and in the presence of anionic, cationic, and nonionic surfactants. The anionic, cationic, and nonionic surfactants used in this study are sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), and Triton X-100, respectively. The presence of Triton X-100 is found to improve the dye removal efficiency of casein from 21 to 84% with a contact time of 5 minutes. The effect of contact time, pH, and temperature on the dye removal efficiency of casein is also studied here. The adsorption of rhodamine B on casein micelles is found to follow the Freundlich adsorption isotherm and a pseudo second order kinetics model. The variation in hydrodynamic diameter of casein in whole and skimmed milk exhibited similar trend for a given surfactant at different concentrations, which suggests that the presence of fat globules, does not interfere with the interaction of casein for a given surfactant. The hydrodynamic diameter of casein micelles in reconstituted aqueous casein solution is found to increase with an increase in the concentration of SDS, which is attributed to the interaction of SDS with micelles at calcium phosphate sequestration centers, which results in micelle swelling. In the presence of CTAB at varying concentration, the diameter of casein micelles in reconstituted aqueous casein is found to decrease. This is attributed to the interaction of cationic group of CTAB with negatively charged κ-casein which makes the micelles more compact as a result of reduced electrostatic repulsion between the micelles. Similarly, the hydrodynamic diameter of casein micelles in reconstituted casein containing varying concentrations of Triton X-100 is found to decrease which suggests that large micelles break down into smaller micelles. Thus, this work gives new insights to the interaction of casein with anionic, cationic, and nonionic surfactants when present in whole milk, skimmed milk, and reconstituted aqueous solution. Further, casein-Triton X-100 complexes are found to effectively remove rhodamine B dye rapidly due to the synergistic interaction between casein, rhodamine B, and Triton X-100.