Smart and regeneratable Xanthan gum hydrogel adsorbents for selective removal of cationic dyes

Poly-anionic Xanthan gum-based hydrogels were synthesized and used as potent adsorbents for the removal of dye pollutants from waste water effluents. The adsorbents were synthesized by esterification of Xanthan gum with maleic anhydride, followed by thiol-ene cross-linking chemistry with 2,2'-(ethylenedioxy)diethanethiol. Methylene blue was used as model cationic dye to mimic dye polluted water. The adsorption kinetics of methylene blue by the Xanthan gum-based hydrogels was modeled by pseudo-first-order, pseudo-second-order and intraparticle diffusion models. The experimental adsorption data at 25 degrees C was modeled by the Langmuir isotherm. The effects of adsorption operation parameters such as the initial dye concentration, pH, ionic strength, and adsorbent dose were also investigated. The pseudo-second-order and Langmuir models were found to be the most appropriate models for the description of the adsorption kinetics and isotherm data, respectively. The Xanthan gum-based hydrogel exhibited a high adsorption capacity of up to q(max) = 435 mg/g (1.35 mmol/g) towards methylene blue. The incorporation of carboxylic acid groups in the Xanthan gum backbone afforded appreciable charge density at pH > 5, which can effectively facilitate the binding of cationic dye molecules. Consequently, the uptake mechanism of methylene blue by the polyanionic Xanthan hydrogel is attributed mainly to the electrostatic interactions. The Xanthan gum-based hydrogel had a remarkable selectivity for the cationic dye in binary and ternary mixtures of methylene blue with methyl orange or sunset yellow FCF. Finally, the hydrogel showed a potential to be re-used for at least for twenty times after regeneration and maintaining over 95% efficiency dye removal as well as recovery.