PH responsive adsorption/desorption studies of organic dyes from their aqueous solutions by katira gum-cl-poly(acrylic acid-co-N-vinyl imidazole) hydrogel

Abstract

A pH responsive hydrogel was prepared by grafting of a mixture of acrylic acid (AA) and N-vinyl imidazole (NVI) onto the katira gum (KG) by free radical co-polymerization technique using KPS as initiator and N,N′-methylene-bis-acrylamide (MBA) as a cross-linking agent in water medium. The pH sensitive hydrogel was characterized by FTIR, XRD and SEM analysis. Swelling studies with water were carried out at different pH in which hydrogel showed two maxima at pH = 4.5 and at pH = 9. The hydrogel containing poly-ampholytic network intelligently responds in the swelling behaviour at different pH. The so prepared hydrogel was used as an adsorbent for the removal of cationic [methylene blue (MB), methyl violet (MV)] and anionic [Tartrazine (TA), Carmoisine-A (CR-A)] dyes from their aqueous solutions. The adsorption showed pH dependence. The cationic dyes were adsorbed at pH = 7 and anionic dyes were adsorbed at pH = 2 for TA and pH = 3 for CR-A. The adsorption followed pseudo second order kinetics and Langmuir isotherm model. The amount of interaction between the hydrogel (adsorbent) and dye molecules (adsorbate) was studied using Gile’s model. The maximum adsorption capacity was found to be 331.5, 286.01, 273.5 and 201.53 mg/g for the MB, MV, CR-A and TA respectively. The cationic dyes were adsorbed strongly than anionic dyes by hydrogel. Various thermodynamics parameters such as Gibbs free energy change (ΔG°), enthalpy change (ΔH°), and entropy change (ΔS°) were also investigated. The removal of dyes by the hydrogel followed in the order MB > MV > CR-A > TA.