Xanthan Gum Hydrogels as High-Capacity Adsorbents for Dye Removal

Xanthan gum-based hydrogels were synthesized and investigated in an attempt to develop efficient adsorbents for removal of dye pollutants from wastewaters. The hydrogels were obtained by first esterifying Xanthan gum with maleic anhydride and subsequent cross-linking of the pendant vinyl groups via thiol-ene click chemistry with oligoethyleneglycol dithiols. The modified hydrogels were systematically characterized to assess the swelling ratio, thermal properties, point of zero charge, and chemical composition. The adsorption capacity (qmax) and the corresponding adsorption kinetics and isotherms of the hydrogels were determined using the cationic dye Gentian Violet. The adsorption efficiency was optimized by varying the initial dye concentration, pH, ionic strength, contact time, and adsorbent dosage. The dye removal mechanism was attributed predominantly to electrostatic interactions of the polyanionic hydrogel with the cationic dye and hydrogel swelling. The Langmuir model of monolayer adsorption explained the isotherms very well. At pH 8 to 9, the hydrogel exhibited a high theoretical qmax = 502 mg/g (1.23 mmol/g) toward Gentian Violet. The kinetics of adsorption and desorption were best described with pseudo-second-order kinetics and pseudo-first-order model kinetics, respectively. The Xanthan gum-based adsorbent can be regenerated with 0.1 M HCl and reused at least 4 times maintaining over 99% dye removal.