Synthesis, swelling and adsorption studies of a pH-responsive sodium alginate–poly(acrylic acid) superabsorbent hydrogel

Abstract

Polyacrylic acid grafted sodium alginate (SA-cl-PAA)-based hydrogel was synthesized by an aqueous polymerisation method. An acrylic acid (AA) monomer was grafted onto sodium alginate (SA) using N,N′-methylene-bisacrylamide (MBA) and potassium persulfate (KPS) as a crosslinker–initiator system. The impact of various parameters such as reaction time, the amount of solvent, pH, crosslinker amount, initiator concentration and monomer concentration on the swelling behavior of the synthesized hydrogel was investigated. We obtained a hydrogel with swelling percentage 41,298% which is quite high. Swelling studies were carried out under acidic (pH 2 buffer) and basic (pH 10 buffer) conditions and evaluated kinetically. The results revealed that the swelling process follows second order kinetics, and the water transport inside the hydrogel supports a Fickian mechanism. The swelling results also indicate that the swelling properties of the synthesized hydrogel showed an on- and off-switchable behavior under basic and acidic conditions, respectively. The synthesized hydrogel was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetric analysis (TGA). The hydrogel was used in the victoria blue R (VB) and rhodamine 6G (RG6) dye adsorption from wastewater. It was found that hydrogel adsorbed 95.8 and 99% of VB and RG6 (132 ppm), respectively, within 77 min. Adsorption of victoria blue is due to electrostatic interaction only, and removal of rhodamine 6G is explained on the basis of electrostatic interaction and hydrogen bonding.