Successive grafting of poly(hydroxyethyl methacrylate) brushes and melamine onto chitosan microspheres for effective Cu(II) uptake

Cross-linked chitosan (CCS) microspheres tethered with melamine-conjugated poly(hydroxyethyl methacrylate) (PHEMA) brushes were synthesized by combination of surface-initiated atom transfer radical polymerization (ATRP) of HEMA and subsequent covalent immobilization of melamine onto the chain ends of PHEMA brushes. The as-synthesized CCS microsphere was used as a novel adsorbent for effective uptake of Cu(II) ions from aqueous solution. Success in each functionalization step was ascertained by SEM, ATR-FTIR and XPS characterization. Batch adsorption experimental results demonstrated that the adsorption equilibrium of Cu(II) ions on the melamine-grafted CCS microsphere was rapidly established within 20 min, and the adsorption process was found to be governed by intra-particle diffusion and chemisorption processes. The Langmuir-fitted maximum adsorption capacity of Cu(II) ions on the as-synthesized CCS microspheres was as high as circa 4.67 mmol L−1 (299 mg g−1). The calculated thermodynamic parameters revealed an endothermic and spontaneous adsorption process of Cu(II) ions on the melamine-grafted CCS microspheres. XPS spectra revealed that the adsorption mechanism was attributed to coordination (or chelation) interactions between amino (or hydroxyl) groups with cationic Cu(II) ions.