Effective Adsorption of Cr(VI) by High Strength Chitosan/Montmorillonite Composite Hydrogels Involving Spirulina Biomass/Microalgae

The 3-in-1 type biopolymer composite (chitosan/montmorillonite clay/biosorbent) hydrogels were produced and used as adsorbents for Cr(VI) ion. Na-Montmorillonite (NaMMT) clay was modified with Spirulina (Sp) biosorbent by using lyophilization based cryoscopic expansion (C-XP) method. The Sp immobilized MMT (SpMMT) clay containing hydrogels were found to have an open/extended form of Sp structure on their pores' walls, presenting all possible receptor groups for adsorption of Cr(VI) ions. SpMMT loaded hydrogels showed higher adsorption capacities than NaMMT loaded ones. The physically crosslinked hydrogel including only 1% SpMMT (1SpM-H) clay exhibited 150% higher adsorption capacity as compared to neat chitosan hydrogel even in 50ppm Cr(VI) solution. The same composite hydrogel was found to adsorp about 780% Cr(VI) with respect to the clay's weight while individual uses of Sp and MMT can remove only about 4.80 and 0.36% Cr(VI) with respect to their weights. The pseudo-first order model was found to be the most suitable for the kinetic data of NaMMT loaded hydrogels while that of SpMMT containing hydrogels followed the pseudo-second order kinetics. The isotherm data of all the hydrogels exhibited a better fit to the Freundlich and Sips model. The maximum adsorption capacity (3333mgg(-1)) calculated by Sips model was achieved via the hydrogel having 1% SpMMT which is in good agreement with the experimental kinetic data. The highest adsorption with the lowest amount of SpMMT clay could be attributed to its looser Sp network structure whose functional groups are in long-distance, releasing more adsorption sites for the Cr(VI). The highest compression modulus and toughness were also obtained with the 1SpM-H hydrogel which is probably due to increased physical and reversible interactions between chitosan molecules and SpMMT clay layers at optimum clay loading (1%).