Investigation of Cross-Linked Chitosan-Based Membranes as Potential Adsorbents for the Removal of Cu2+ Ions from Aqueous Solutions

Abstract

Rapid industrialization has led to huge amounts of organic pollutants and toxic heavy metals into aquatic environment. Among the different strategies explored, adsorption remains until the most convenient process for water remediation. In the present work, novel cross-linked chitosan-based membranes were elaborated as potential adsorbents of Cu²⁺ ions, using as cross-linking agent a random water-soluble copolymer P(DMAM-co-GMA) of glycidyl methacrylate (GMA) and N,N-dimethylacrylamide (DMAM). Cross-linked polymeric membranes were prepared through casting aqueous solutions of mixtures of P(DMAM-co-GMA) and chitosan hydrochloride, followed by thermal treatment at 120 °C. After deprotonation, the membranes were further explored as potential adsorbents of Cu²⁺ ions from aqueous CuSO₄ solution. The successful complexation of copper ions with unprotonated chitosan was verified visually through the color change of the membranes and quantified through UV-vis spectroscopy. Cross-linked membranes based on unprotonated chitosan adsorb Cu²⁺ ions efficiently and decrease the concentration of Cu²⁺ ions in water to a few ppm. In addition, they can act as simple visual sensors for the detection of Cu²⁺ ions at low concentrations (~0.2 mM). The adsorption kinetics were well-described by a pseudo-second order and intraparticle diffusion model, while the adsorption isotherms followed the Langmuir model, revealing maximum adsorption capacities in the range of 66-130 mg/g. Finally, it was shown that the membranes can be effectively regenerated using aqueous H₂SO₄ solution and reused.

Keywords: adsorption capacity; chemical cross-linking; chitosan; copolymers; copper sulfate; glycidyl methacrylate; polymeric membranes.

Figure 1

Schematic representation of the chemical synthesis of (a) P(DMAM-co-GMA) copolymer and (b) cross-linked chitosan-based membranes.

Figure 2

¹H NMR spectrum of the copolymer P(DMAM-co-GMA) in CDCl₃ in comparison with the spectra of the two homopolymers, PDMAM and PGMA.

Figure 3

ATR-FTIR spectrum in the 500–2500 cm⁻¹ region of the copolymer P(DMAM-co-GMA) in comparison with the spectra of the homopolymers PDMAM and PGMA.

Figure 4

ATR-FTIR spectra in the 500–2500 cm⁻¹ region of P(DMAM-co-GMA), Ch-NH₃⁺Cl⁻ and the membrane Ch-NH₃⁺Cl⁻/P(DMAM- GMA) 9:1 after thermal treatment at 60 °C and 120 °C.

Figure 5

Water uptake of Ch-NH₃⁺Cl⁻/P(DMAM-co-GMA) membranes with different molar ratio of Ch-NH₃⁺Cl⁻, which were thermally treated at 60 °C and 120 °C. The appearance of the membranes with the highest Ch-NH₃⁺Cl⁻ content after immersion in the solvents is shown in the inset.

Figure 6

Representative images of the adsorption of copper ions by the membrane (Ch-NH₂/P(DMAM-co-GMA) 8:2) compared to the appearance of the protonated Ch-NH₃⁺Cl⁻/P(DMAM-co-GMA) 8:2 membrane, after immersion in aqueous 1 mM CuSO₄ solution for 24 h.

Figure 7

UV-vis spectrum of aqueous CuSO₄ 1 mM solution before and after immersion of the membranes Ch-NH₃⁺Cl⁻/P(DMAM-co-GMA₃₀) 8:2 and Ch-NH₂/P(DMAM-co-GMA) 8:2 for 24 h.

Figure 8

UV-vis spectra of aqueous CuSO₄ 1.5 mM solution before and after immersion of the membrane Ch-NH₂/P(DMAM-co-GMA) 9:1 with increasing contact time.

Figure 9

(a) Effect of contact time on adsorption capacity of the membranes Ch-NH₂/P(DMAM-co-GMA) 9:1 and 8:2 in aqueous 1 mM CuSO₄ solution, with or without magnetic stirring. Fitting of the experimental data with the (b) pseudo-second order, (c) Elovich and (d) Weber–Morris intraparticle diffusion model.

Figure 10

UV-vis spectra of a series of aqueous CuSO₄ solutions, before (solid lines) and after (dotted lines) immersion of Ch-NH₂/P(DMAM-co-GMA) 9:1 membranes. Characteristic images of the membranes during this study are inserted in the figure.

Figure 11

Adsorption isotherms of Cu²⁺ ions for membranes Ch-NH₂/P(DMAM-co-GMA) 9:1 (green squares), 8:2 (magenta circles) and 5:5 (orange triangles), immersed in aqueous CuSO₄ solutions for 2 days, without magnetic stirring. The results derived using the bathocuproine method are shown as semifilled symbols.

Figure 12

Freundlich (a) and Langmuir (b) isotherm models for the adsorption of Cu²⁺ for the studies reported in Figure 11.

Figure 13

UV-vis spectra of adsorption–desorption studies of membrane Ch-NH₂/P(DMAM-GMA) 8:2 for (a) first, (b) second and (c) third cycle.

Figure 13

UV-vis spectra of adsorption–desorption studies of membrane Ch-NH₂/P(DMAM-GMA) 8:2 for (a) first, (b) second and (c) third cycle.

Figure 14

The values of C_e (mmol/L) during three cycles of Cu²⁺ ions adsorption–desorption study of Ch-NH₂/P(DMAM-co-GMA) 8:2 membrane.