Adsorption of anionic dyes on a cationic amphiphilic dextran hydrogel: equilibrium, kinetic, and thermodynamic studies

An amphiphilic cationic dextran hydrogel with quaternary ammonium pendent groups was prepared, and its performances in adsorption of anionic dyes were studied as a function of pH, contact time, initial dye concentration, dye type, and temperature and evaluated by equilibrium, kinetic, and thermodynamic analysis. Adsorption equilibrium data fitted well with Langmuir model over the entire range of dyes' concentrations, while Freundlich model was appropriate only after 30-60% of gel cationic sites were occupied by dye molecules. Pseudo-second-order kinetic model was the best fit for the experimental data. The values of calculated adsorption thermodynamic parameters and activation energies suggest an adsorption process governed by diffusion (Rose Bengal), chemisorption (Indigo Carmine), or combined physical and chemical interactions (Methyl Orange and Orange II). The maximum adsorption capacity of dextran hydrogel for anionic dyes was in the range 650-730mg/g and depended on dye chemical structure and molecular weight. The most rapid desorption of Methyl Orange from the dye-loaded hydrogel was achieved under dynamic conditions with a successive addition of water, NaCl 0.5M, and methanol as eluents.