Poly(acrylamide-co-maleic acid) hydrogels for removal of Cr(VI) from aqueous solutions, part 1:: Synthesis and swelling characterization

The swelling behavior of poly (acrylamide-co-maleic acid) hydrogels has been investigated in distilled water at 30 degrees C. The gels were characterized with respect to structural parameters, Fourier transform infrared, and thermogravimetric analysis. The gels showed fair pH-dependent swelling and exhibited double "s"-shaped curve between equilibrium water uptake and pH of the swelling media. The two pK(a) values, as determined from the curve, were found to be 2.46 and 6.58. The activation energy of the water uptake process for plain and acid containing gels Was found to be 7.93 and 3.26 kJ mol(-1) respectively. Similarly, the enthalpy of mixing between dry polymer and solvent showed positive values, thus indicting endothermic nature of the process, and the values increased from 10.06 to 16.29 kJ mol(-1) with increase in acid content from 2.1 X 10(-1) to 4.7 X 10(-1) mM respectively. There was an optimum initiator concentration 24.0 X 10(-2) mM and reaction temperature 60 degrees C at which gels synthesized showed maximum absorbency. The dilution of the reaction mixture resulted in the formation of hydrogels with enhanced absorbency. Finally, the gels with varying content of monomer acid in the feed mixture showed different swelling behavior when studied in the medium of pH 1.0 and 7.0. (c) 2006 Wiley Periodicals, Inc.