Effect of Ionic Strength, Crosslinking Degree and Copolymerization on Properties of Poly(Acrylamide) Hydrogel, Application as Adsorbent of Heavy Metal Ions

Abstract

Hydrogels are hydrophilic macromolecular systems that swell in water without dissolving. The nature of the solvent has a significant effect on the amount of water absorbed. Poly(acrylamide) hydrogel and its copolymer with diprotic maleic acid are prepared in this work by copolymerization and chemical crosslinking with methylene bisacrylamide; at different mass percentages of acrylamide (AAm, %) and maleic acid (MA, %), respectively: (100, 0%) and (80, 20%). The obtained results show the formation of copolymers with intermolecular hydrogen bonding between carboxylic groups and amide groups. The content of maleic acid within the crosslinked polymer (acid value) is determined. Two acid values are obtained; 14 and 57.2 mg g–1. Thermal stability of hydrogels is examined through TGA analysis. The thermal degradation of poly(acrylamide) occurs via two stage process; (204–330°C) and at temperature higher than 330°C. The thermal stability of the copolymer increases with incorporates of MA. The copolymer degrades in a multistage process; less than 230°C, (230–350°C) and above 350°C. Different percentages of crosslinker between 1–5 wt % and copolymer with maleic acid are used to study their swelling behavior in different aqueous solutions (NaCl, pH 9.18 and pH 2.2). The swelling is reduced with increasing degree of the cross-linking agent. The obtained hydrogels have shown substantial percent swelling in water and basic buffer and shrinking in saline solution and acidic buffer. The results show that MA20 hydrogel has pH and ionic sensitive characters. These sensitivities exhibit hydrogels an important utility in water treatments, especially as adsorbent of heavy metal ions. The study of oscillatory swelling of hydrogels at pH 4.01 and 9.18 as function of time are demonstrated in this work. The result shows that the synthesized hydrogels exhibit excellent reversible pH-sensibility character.