Synthesis of poly(maleic acid-co-acrylic acid) hydrogel for removing cadmium ion from sewage effluents and evaluation of thermodynamic and kinetic properties of its absorption

PMA-AA hydrogel was synthesized by copolymerizing acrylic acid and maleic acid by free radical polymerization and using methylene bis acrylamide crosslinker and ammonium persulfate initiator. It is an efficient adsorbent for removing Cd2+ ions from wastewater. The resulting hydrogel was evaluated by infrared spectrometry and thermogravimetric analysis. The results of OH stretching vibration of AA and MA carboxylic functional groups showed a large and broad absorption band at 3000 cm-1. The C=O stretching frequency of the carboxyl group at 1631 cm-1 in PMA-AA hydrogel shifted to 1636 cm-1, these changes in frequency indicate that hydrogel adsorption is a chemical process. Thermogravimetric analysis of the unloaded PMA-AA hydrogel showed no weight loss up to 155 °C of the free gel, but at 190 °C, there was a significant weight loss, indicating hydrogel decomposition. This rapid weight loss for gels loaded with Cd2+ at 360°C indicated an increase in thermal stability of the loaded hydrogel. The maximum adsorption capacity for hydrogel -Cd2+ was determined to be 392.51 mg/g. The Langmuir isotherm provided the best explanation for the present result. This is because the surface of the hydrogel is homogeneous so its absorption is monolayer, the pseudo-second-order kinetic model explain the absorption well, and also the thermodynamic analysis shows that the absorption of Cd2+ on hydrogel is exothermic. The PMA-AA hydrogel loaded with Cd2+ ions was desorbed and still maintained the adsorption capacity of 99.30% after four adsorption-desorption cycles.