Microwave-assisted hydrogels for removing heavy metals: synthesis and characterization

The microwave irradiation technique was used to synthesize poly (acrylamide-co-2-hydroxyethyl methacrylate) / poly(vinyl alcohol) (P(AM-co-HEMA)/PVA) semi-IPN hydrogels, which were separately synthesized by using one-pot polymerization (HEMA-PM) and two-step polymerization (HEMA-PM-t) techniques. The semi-IPN hydrogels were crosslinked by glutaraldehyde (GA) and Ethylene glycol dimenthacrylate (EGDMA) using ammonium persulfate (APS) as initiator. For the first step, (P(AM-co-HEMA)) network was synthesized in PVA aqueous solution, then followed by glutaraldehyde crosslinking reagent, it formed to semi-IPN structured. Several analytical techniques such XRD, SEM, FT-IR, AFM, DSC, TGA and tensile testing were used to characterize the formation and the resulting gel structure. The FTIR spectroscopy was used to identify the functional groups in the structure of hydrogel films and XRD testing was carried out to identify crystallized the hydrogel structure. The tensile testing, DSC and TGA measurement were performed to investigate the effect of amount of monomers on mechanical properties and thermal stability of the membranes. SEM and AFM were used to characterize surface microstructure of the hydrogel. In this study, the properties, including water adsorption and heavy metals adsorption ability of hydrogels obtained from different techniques were compared. The swelling behaviors of the hydrogel were measured gravimetrically in distilled water at the temperature of 27 C, and a rapid response. In additions, the results showed that the hydrogel prepared by one-pot polymerization technique exhibited the highest swelling ratio (the percentage swelling has higher than 1,300%), comparing with the hydrogels prepared by two-step polymerization technique. The adsorption capacities and rates of Cu(II) and Pb(II) ions onto semi-IPN hydrogel were evaluated. Maximum adsorption capacity of both metals was found to occur at pH 4, initial metal concentration 1000 mg/L and the temperature of 27 C. The Langmuir and Freundlich adsorption models were applied to describe the isotherms and isotherm constants. Adsorption isotherms data of metal ions were likely to follows the Langmuir model. The pseudo-first order and pseudo-second order kinetics were applied to examine the of adsorption behavior. The kinetic experimental data properly correlated with pseudo-second order kinetic model more than pseudo-first order. By comparing the adsorption capacity among the hydrogels obtained from different techniques, HEMA-PM showed higher removing efficiency of heavy metal ions than HEMA-PM-t hydrogel.