Enhanced methylene blue adsorption using single-walled carbon nanotubes/chitosan-graft-gelatin nanocomposite hydrogels

Abstract

In the present study, single-walled carbon nanotubes (SWCNTs) incorporating chitosan-graft-gelatin (CS-g-GEL/SWCNTs) hydrogels were fabricated with multiple advantages, including cost-effectiveness, high efficiency, biodegradability, and ease of separation for methylene blue (MB) dye from aqueous solution. To verify the successful formulation of the prepared hydrogels, various characterization methods such as NMR, FTIR, XRD, FE-SEM, TGA, BET, and EDX were employed. The removal efficiency of CS-g-GEL/SWCNTs nanocomposite hydrogel increased significantly to 98.87% when the SWCNTs percentage was increased to 20%. The highest adsorption was observed for pH = 9, an adsorbent dose = 1.5 g L^-1, a temperature = 25 °C, a contact time = 60 min, and a contaminant concentration = 20 mg L^-1. Based on the thermodynamic results, spontaneous adsorption occurred from a negative Gibbs free energy (ΔG°). In addition, the thermodynamic analysis of the adsorption process revealed an average enthalpy of - 21.869 kJ mol^-1 for the adsorption process at a temperature range of 25-45 °C, which indicates its spontaneous and exothermic behavior. The Langmuir isotherm model was successfully used to describe the equilibrium behavior of adsorption. The pseudo-first-order model better described adsorption kinetics compared to the pseudo-second-order, intra-particle, and Elovich models. CS-g-GEL/SWCNTs hydrogels have improved reusability for five consecutive cycles, suggesting that they may be effective for removing anionic dyes from aquatic environments.

Figure 1

Synthesis pathway of CS-g-GEL/SWCNTs hydrogel.

Figure 2

¹H NMR spectrum (500 MHz, D₂O, 25 °C) of CS-g-GEL.

Figure 3

FTIR spectra of the CS, GEL, SWCNTs, CS-g-GEL, CS-g-GEL/SWCNTs, and CS-g-GEL/SWCNTs containing dye hydrogels.

Figure 4

TGA and DTG curves of CS-g-GEL (a), and CS-g-GEL/SWCNTs (b), nitrogen sorption- desorption (c), and Pore size distribution of CS-g-GEL and CS-g-GEL/SWCNTs hydrogels (d).

Figure 5

FESEM images of the freeze-dried of CS-g-GEL (a), and CS-g-GEL/SWCNTs (b) hydrogels, the cross-sectional of CS-g-GEL (c), CS-g-GEL/SWCNTs (d) and CS-g-GEL/SWCNTs containing dye (e) hydrogels.

Figure 6

EDX analysis of CS-g-GEL (a) CS-g-GEL/SWCNTs (b), and CS-g-GEL/SWCNTs containing dye (c) hydrogels, and XRD analysis of SWCNTs and CS-g-GEL/SWCNTs hydrogel (d).

Figure 7

The effect of SWCNTs percentage (%) (a), photograph of visual changes at different of SWCNTs percentage (%) (b), the effect of different pH (c), photograph of visual changes at different pH (d), the effect of different amount of adsorbent dosage (e), and photograph of visual changes at different adsorbent dosage (f) on removal of MB dye.

Figure 8

The effect of temperature on MB dye removal (a), the photograph of visual changes at different of temperatures on MB dye removal (b), and the Van-t-Hoff linear relationship for determining thermodynamic parameters (c).

Figure 9

The effect of contact time (a), the photograph of visual changes at different contact time (b), the non-linear relationship of kinetic models for MB dye adsorption process (c), and linear relationship for intra-particle diffusion model (d) (at pH: 10, optimal adsorbent dose, temperature:25 °C, dye concentration: 20 mg L⁻¹, mixing speed of 500 rpm).

Figure 10

The effect of initial concentration of MB dye on the removal efficiency, the photograph of visual changes of different initial concentration (b), the relation between non-linear isotherm models and MB dye adsorption using CS-g-GEL/SWCNTs (c), and reusability of CS-g-GEL/SWCNTs hydrogel.

Figure 11

FT-IR spectra CS-g-GEL/SWCNTs nanocomposite hydrogel: (a) fresh hydrogel, (b) recovered hydrogel after 5th runs.

Figure 12

Adsorption mechanism of CS-g-GEL/SWCNTs hydrogel and MB dye.