Congo red attached poly(EGDMA–HEMA) microspheres as specific sorbents for removal of cadmium ions

Poly[ethyleneglycoldimethacrylate (EGDMA)–hydroxyethylmethacrylate (HEMA)] microspheres (150–200 μm in diameter) were produced by suspension copolymerization of EGDMA and HEMA in an aqueous medium. Toluene was included in the formulations in order to produce water-swellable microspheres. Poly(vinyl alcohol) and benzoyl peroxide were used as stabilizer and initiator, respectively. Congo red was chemically attached to the microspheres as a metal chelating ligand for specific adsorption of heavy metal ions. These sorbents were characterized by an optical microscopy and a FTIR. Adsorption/desorption of cadmium (Cd2+) ions from aqueous solutions on these sorbents were investigated in batch equilibrium experiments by using an atomic absorption spectroscopy with a graphite furnace atomizer. The maximum cadmium adsorption on to the dye-attached microspheres (i.e., by complex formation) was about 18.3 mg Cd2+ ions/g polymer, which was observed at pH 6.8. While adsorption onto the plain poly(EGDMA–HEMA) microspheres (i.e., nonspecific adsorption) was about 0.93 mg Cd2+ ions/g polymer at the same conditions. More than 90% of the adsorbed cadmium was desorbed in 1 h by using 2M NaCl as an eluant. The resorption capacity of the sorbent did not significantly decrease during repeated sorption–desorption cycling.