Optimization of Chitosan Glutaraldehyde-Crosslinked Beads for Reactive Blue 4 Anionic Dye Removal Using a Surface Response Methodology

Abstract

The use of dyes at an industrial level has become problematic, since the discharge of dye effluents into water disturbs the photosynthetic activity of numerous aquatic organisms by reducing the penetration of light and oxygen, in addition to causing carcinogenic diseases and mutagenic effects in humans, as well as alterations in different ecosystems. Chitosan (CS) is suitable for removing anionic dyes since it has favorable properties, such as acquiring a positive charge and a typical macromolecular structure of polysaccharides. In this study, the optimization of CS beads crosslinked with glutaraldehyde (GA) for the adsorption of reactive blue dye 4 (RB4) in an aqueous solution was carried out. In this sense, the response surface methodology (RSM) was applied to evaluate the concentration of CS, GA, and sodium hydroxide on the swelling degree in the GA-crosslinked CS beads. In the same way, RSM was applied to optimize the adsorption process of the RB4 dye as a function of the initial pH of the solution, initial concentration of the dye, and adsorbent dose. The crosslinking reaction was investigated by scanning electron microscopy (SEM), Fourier transformed infrared spectroscopy (FTIR), and X-ray diffractometry (XRD). The design described for the swelling degree showed an R² (coefficient of determination) adjusted of 0.8634 and optimized concentrations (CS 3.3% w/v, GA 1.7% v/v, and NaOH 1.3 M) that were conveniently applied with a concentration of CS at 3.0% w/v to decrease the viscosity and facilitate the formation of the beads. In the RB4 dye adsorption design, an adjusted R² (0.8280) with good correlation was observed, where the optimized conditions were: pH = 2, adsorbent dose 0.6 g, and initial concentration of RB4 dye 5 mg/L. The kinetic behavior and the adsorption isotherm allowed us to conclude that the GA-crosslinked CS beads' adsorption mechanism was controlled mainly by chemisorption interactions, demonstrating its applicability in systems that require the removal of contaminants with similar structures to the model presented.

Keywords: adsorption; crosslinking chitosan beads; experimental design; glutaraldehyde; reactive blue 4 dye; removal efficiency; swelling degree.

Figure 1

Pareto chart of standardized effects for response variable swelling degree.

Figure 2

Three-dimensional (3D) response surface plot for optimizing the swelling degree of GA-crosslinked CS beads. NaOH = 1.5 M.

Figure 3

The appearance of CS in the process of obtaining the beads; (a) CS–CH₃COOH solution, (b) CS beads precipitated in NaOH_(aq); (c) final product of the beads cross-linked with GA.

Figure 4

Calibration curve for RB4 dye measured at 599 nm. Concentration range of 1 mg/L to 70 mg/L.

Figure 5

Pareto chart of standardized effects for response variable RB4 dye removal efficiency. AD: adsorbent dose.

Figure 6

The 3D response surface plot for optimization of RB4 dye removal efficiency. Adsorbent dose: 0.35 g/25 mL.

Figure 7

SEM images. Morphology of non-crosslinked CS beads, cross-section: (1a) at 100×, surface area: (1b) at 100×, (1c) at 500×, (1d) at 25,000×; GA-crosslinked CS beads, cross-section (2a) at 100×, surface area: (2b) at 100×, (2c) at 500×, (2d) at 25,000×; GA-crosslinked CS beads after adsorption of the RB4 dye, cross-section (3a) at 100×, surface area: (3b) at 100×, (3c) at 500×, (3d) at 25,000×.

Figure 8

FT-IR spectrum of non-crosslinked CS beads (A) and GA-crosslinked CS beads (B).

Figure 9

X-ray diffractogram of non-crosslinked CS beads (upper-black line) and GA-crosslinked CS beads (lower-red line).

Figure 10

(a) Adsorption kinetics adjusted to different models for RB4 dye; (b) aqueous solution of RB4 dye before and after the adsorption process with GA-crosslinked CS beads. (Initial concentration of RB4 dye = 35 mg/L; pH = 3.0; adsorbent dose = 0.4 g; solution volume = 25 mL; constant stirring = 400 rpm; temperature = 25 °C).

Figure 11

Illustration of the possible interactions between GA-crosslinked CS beads (CS/GA) and RB4 dye: (A) electrostatic attraction, (B) hydrogen bonds dipole-dipole interactions, (C) Yoshida hydrogen bonding interactions, (D) n– π interactions.

Figure 12

Equilibrium adsorption adjusted to different isothermal models for RB4 dye adsorption on GA-crosslinked CS beads (Initial concentration of RB4 dye = 35 mg/L; pH = 3.0; adsorbent dose = 0.4 g/25 mL; constant stirring = 400 rpm; temperature = 25 °C).