Poly(methacrylamide) and poly(n-vinyl-2-pyrrolidone) grafted crosslinked chitosan for removal of orange g and selected heavy metal ions from aqueous solutions

Abstract

Natural biopolymers such as chitosan have high potential as sorbents for a wide range of applications. However, these materials present significant limitations that require appropriate modifications. In this study, chitosan was modified by crosslinking and grafting and the newly prepared grafted copolymers were evaluated as sorbents for the removal of a selected dye, Orange G (OG) and several heavy metal ions (Pb(II), Cu(II) and Cd(II)) from aqueous solutions. Chitosan beads were produced from their commercial form and crosslinked with glutaraldehyde to enhance their stability in acidic medium. The crosslinked chitosan beads were then grafted separately with two different monomers, namely methacrylamide and Nvinyl- 2-pyrrolidone by conventional free radical polymerization using ammonium persulfate as an initiator. The modified chitosan beads were characterized using Fourier transform infrared (FTIR) spectroscopy, solid state 13C nuclear magnetic resonance (13C NMR) spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and CHN analysis to provide evidence of successful crosslinking and grafting. The solubility and swelling of the grafted copolymers and chitosan beads were investigated. The optimum percentage grafting (263.5%) for the preparation of crosslinked chitosangraft- poly(methacrylamide) (cts(x)-g-PMAm) was obtained at reaction temperature of 60°C, reaction time of 3 h, 2.63 x 10-1 mol L-1 of ammonium persulfate, 17.62 x 10-1 mol L-1 of methacrylamide and 1 g of crosslinked chitosan beads. Meanwhile, the optimum percentage grafting (138%) for the preparation of crosslinked chitosangraft- poly(N-vinyl-2-pyrrolidone) (cts(x)-g-PNVP), was achieved using reaction temperature of 60°C, reaction time of 2 h, 2.63 x 10-1 mol L-1 of ammonium persulfate, 26.99 x 10-1 mol L-1 of N-vinyl-2-pyrrolidone and 1.5 g of crosslinked chitosan beads. It was found that (cts(x)-g-PMAm) and (cts(x)-g-PNVP) beads showed significantly higher maximum adsorption capacities, qmax, for OG (25.8 mg g-1 and 63.7 mg g-1, respectively) as compared to that of unmodified chitosan beads (1.7 mg g-1). It was also proven that the Langmuir model fitted very well with the experimental adsorption data for OG and the selected heavy metal ions with R2 of nearly unity, while the adsorption kinetics were well described by the pseudo-second order kinetic model. In addition, the adsorbent-adsorbate interactions were elucidated by means of FTIR and X-ray photoelectron spectroscopy (XPS). This study concludes that both cts(x)-g-PMAm and cts(x)-g-PNVP beads are potentially useful as sorbents for the removal of pollutants from water and wastewater.