Preparation of chitosan-graft-polyacrylamide magnetic composite microspheres for enhanced selective removal of mercury ions from water

A novel magnetic composite microsphere based on polyacrylamide (PAM)-grafted chitosan and silica-coated Fe3O4 nanoparticles (CS-PAM-MCM) was successfully synthesized by a simple method. The molecular structure, surface morphology, and magnetic characteristics of the composite microsphere were characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), vibrating-sample magnetometer (VSM), and scanning electron microscopy (SEM). The prepared CS-PAM-MCM was applied as an efficient adsorbent for the removal of copper(II), lead(II), and mercury(II) ions from aqueous solutions in respective single, binary, and ternary metal systems. Compared with chitosan magnetic composite microsphere (CS-MCM) without modification, CS-PAM-MCM showed improved adsorption capacity for each metal ion and highly selective adsorption for Hg from Pb and Cu. This improvement is attributed to the formation of stronger interactions between Hg and the amide groups of PAM branches for chelating effects. The adsorption isotherms of Hg/Cu and Hg/Pb binary metal systems onto CS-PAM-MCM are both well-described by extended and modified Langmuir models, indicating that the removal of the three aforementioned metal ions may follow a similar adsorption manner; that is, through a homogeneous monolayer chemisorption process. Furthermore, these magnetic adsorbents could be easily regenerated in EDTA aqueous solution and reused virtually without any adsorption capacity loss.