Adsorption Properties of β- and Hydroxypropyl-β-Cyclodextrins Cross-Linked with Epichlorohydrin in Aqueous Solution. A Sustainable Recycling Strategy in Textile Dyeing Process

Abstract

β-cyclodextrin (β-CD) and hydroxypropyl-β-cyclodextrin (HP-β-CD) were used to prepare insoluble polymers using epichlorohydrin as a cross-linking agent and the azo dye Direct Red 83:1 was used as target adsorbate. The preliminary study related to adsorbent dosage, pH, agitation or dye concentration allowed us to select the best conditions to carry out the rest of experiments. The kinetics was evaluated by Elovich, pseudo first order, pseudo second order, and intra-particle diffusion models. The results indicated that the pseudo second order model presented the best fit to the experimental data, indicating that chemisorption is controlling the process. The results were also evaluated by Freundlich, Langmuir and Temkin isotherms. According to the determination coefficient (R²), Freunlich gave the best results, which indicates that the adsorption process is happening on heterogeneous surfaces. One interesting parameter obtained from Langmuir isotherm is q_max (maximum adsorption capacity). This value was six times higher when a β-CDs-EPI polymer was employed. The cross-linked polymers were fully characterized by nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA). Also, morphology and particle size distribution were both assessed. Under optimized conditions, the β-CDs-EPI polymer seems to be a useful device for removing Direct Red 83:1 (close 90%), from aqueous solutions and industrial effluents. Complementarily, non-adsorbed dye was photolyzed by a pulsed light driven advanced oxidation process. The proposed methodology is environmental and economically advantageous, considering the point of view of a sustainable recycling economy in the textile dyeing process.

Keywords: Direct Red; HP-β-CDs; adsorption; isotherm; kinetics; pulsed light; β-CDs.

Figure 1

Amount of Direct Red 83:1 adsorbed by β-CDs-EPI and HP-β-CDs-EPI as function of contact time, for different dye concentrations. 25 mg/L (●), 50 mg/L (○), 75 mg/L (■), 100 mg/L (□), 150 mg/L (▲), 200 mg/L (Δ), 250 mg/L (♦) and 300 mg/L (⟡).

Figure 2

(A) Pseudo first order model plots and (B) pseudo second order model plots for the Direct Red 83:1 adsorption onto β-CDs-EPI and HP-β-CDs-EPI polymers at different dye concentrations (25 mg/L (●), 50 mg/L (○), 75 mg/L (■), 100 mg/L (□), 150 mg/L (▲), 200 mg/L (Δ), 250 mg/L (♦) and 300 mg/L (⟡)).

Figure 3

(A) Elovich model plots for the Direct Red 83:1 adsorption onto β-CDs-EPI and HP-β-CDs-EPI polymers. (B) Intraparticle diffusion model plots for the Direct Red 83:1 adsorption onto β-CDs-EPI and HP-β-CDs-EPI polymers at different dye concentrations (25 mg/L (●), 50 mg/L (○), 75 mg/L (■), 100 mg/L (□), 150 mg/L (▲), 200 mg/L (Δ), 250 mg/L (♦) and 300 mg/L (⟡)).

Figure 4

Adsorption isotherms for Direct Red 83:1 by β-CDs-EPI (●) and HP-β-CDs-EPI (○). (A) Freundlich isotherm, (B) Langmuir isotherm, (C) Temkin isotherm, (D) Separation factor.

Figure 5

SEM images of cross-linked β-CD-EPI empty polymer (A) and laden with Direct Red 83:1 (B).

Figure 6

β-CD, HP-β CD, β- and HP-β-CD EPI polymers FT-IR spectra.

Figure 7

¹H-NMR spectra of β-CD, HP-β CD, β- and HP-β-CD EPI polymers. Inset: A: β-CD monomer glucose unit chemical structure; B: HP-β-CD spacer arm; C: Possible ways to open oxyrane ring by OH groups.

Figure 8

Thermogravimetric curves for the samples of (a) β-CD, (b) HP-β-CD, (c) β-CD-EPI, and (d) HP-β-CD-EPI polymers.

Figure 9

Spectral changes during the course of the degradation of Direct Red 83:1 by a pulsed light/H₂O₂ advanced oxidation process. Inset: Pseudo-first order kinetic plot.