Abstract
A novel zwitterionic superabsorbent polymer hydrogel poly(MEMA-DMAPS) was synthesized using three different combinations of 2-(N-morpholino) ethyl methacrylate (MEMA) and 3-((2-methacryloyloxy)ethyl) dimethylammonio) propane-1-sulfonate) (DMAPS) monomers in the polymerization mixture. For characterizing its ionic dye adsorption, Congo Red (CR) and Crystal Violet (CV) were used as anionic and cationic dye models, respectively. The pH of the aqueous solution was shown to be the most decisive variable in the adsorption mechanism. The hydrogel (DM-1) possessed a point of zero charge (PZC) at pH 6.9. At pH values above or below the PZC, the hydrogel showed excellent selectivity for oppositely charged dyes. The adsorption capacity was found to be 370 mg g−1 at pH 2, and 139 mg g−1 at pH 10 for CR and CV, respectively. The positive values of enthalpy change calculated at temperatures in the range of 25 °C to 55 °C reflect that the removal processes of both dyes were endothermic. The standard Gibbs free energy (ΔG o) values at all temperatures studied indicated that the adsorption process occurred spontaneously and nonspontaneously for CR and CV, respectively.
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