Adsolubilization phenomenon perceived in chitosan beads leading to a fast and enhanced malachite green removal

Abstract

Chitosan (CS) is a well recognized cationic polyelectrolyte, widely used as an adsorbent. However, CS hydrogel bead lacks its ability to adsorb cationic dyes because of the obvious repulsion between cationic CS and the dye of same charge. In this work CS hydrogel beads are modified by sodium dodecyl sulfate (SDS), an anionic surfactant, under various conditions to improve its efficiency for the adsorption of malachite green (MG), a cationic dye. In the first part, the complex forming ability of surfactant (at <<CMC) with counter ion polyelectrolyte is utilized to fabricate modified chitosan beads, which are designated as surfactant–polyelectrolyte-complex (SPEC). In the second part of the adsorbent preparation, micelle aggregation behavior of surfactant (at >>CMC) is used to modify the surface charges and solubilization properties of the modified adsorbent. These beads are called as chitosan–surfactant–core–shell (CSCS). The mechanism of formation for both is discussed. Besides these modified beads, the CS–SDS composite material (CSC) is also prepared. The adsorptive removal of MG by all the prepared beads has been evaluated. The CSCS beads are found to be the best (adsorption capacity: 360mg/g) for MG removal. Such an enhanced adsorption is believed to be due to ‘adsolubilization’ of MG on surfactant bilayer, which has been realized for the first time in CS bead support. The kinetics of removal also is much faster compared to CS beads. The efficiency of CSC for MG removal is higher compared to that of CS beads. The adsorption behavior of CS beads in binary mixture of MG and SDS has also been examined.