Evaluation of a pH-responsive guar gum-based hydrogel as adsorbent for cationic dyes: kinetic and modelling study

Abstract

Interpenetrating network of guar gum and polyacrylamide has been made by simultaneous polymerization and crosslinking. Two different crosslinkers have been used to control the network structure. The amide groups in the network have been subsequently hydrolyzed by controlled saponification to obtain different degrees of ionic content in the gel. The gels containing –NH2 and –COOH functionalities have been characterized by FTIR, elemental analysis, DSC and TGA techniques. The swelling of the gel and pH responsiveness have been evaluated. The suitability of the gel as adsorbent material for removal of cationic dyes from aqueous solution has been investigated with crystal violet and azure B. The results revealed high degree of pH responsiveness and high dye adsorption capacity of gels. The adsorption–desorption processes were observed to be effective in repeated cycles, indicating the reusability of the gels for dye removal applications. The adsorption is observed to occur by Fickian diffusion and pseudo-second-order kinetic model appears to describe the adsorption. Crystal violet was absorbed to higher extent and bound stronger than azure B by the gel network. The adsorption is described by Langmuir model in case of crystal violet and Freundlich model for azure B. The thermodynamic parameters of adsorption indicate exothermic adsorption leading to ordered arrangement of dye molecules on the gel.