Kinetics and Thermodynamic Study on Rapid Removal of Cationic Dyes Using TiO2-Poly(3-chloro-2-hydroxypropyl methacrylate) Nanocomposite
Authors: Hossan M. Shamim , Ochiai Bungo 
Published: February 2020

Various synthetic dyes are used in a wide range of applications including textiles, leather, papers, plastics and fibers. However, most of these dyes are toxic, mutagenic, carcinogenic, and allergenic. Discharging waste water of dyes has caused a severe problem of water pollution, and these dyes consisting of stable aromatic rings are highly tolerant to degradation under natural conditions [1]. Accordingly, materials for removal of such dyes are highly demanded, and quick and complete removal is a significant requirement. Various methods, such as adsorption, coagulation, filtration, chemical oxidation, photodegradation, biodegradation, reverse osmosis, and ion-exchange have been used for removal of these dyes [2-4]. Among of them, adsorption is advantageous in efficiency, cost effectiveness, and recyclability of adsorbents. In this study, we prepared in highly effective adsorbent, TiO2-poly(3-chloro-2-hydroxypropyl methacrylate) (TiO2-PCHPMA) nanocomposite, for rapid removal of cationic dyes, and investigated its kinetic and thermodynamic study.
TiO2-PCHPMA was synthesized by free radical polymerization of CHPMA in the presence of TiO2 modified with 3-(trimethoxysilyl)propyl methacrylate (MPS) at 70 °C with magnetic stirring for 24 h in 1,4-dioxane (Figure 1).
TiO2-PCHPMA with an average diameter of 340 nm was obtained by the coating of surface modified TiO2 by radical polymerization. The structure of TiO2-PCHPMA nanocomposite was confirmed by SEM, EDX, FTIR, TGA, XRD, and DLS. We examined adsorption of cationic dyes, namely Basic Red 2 (BR2), Basic Red 5 (BR5), Basic Blue 17 (BB17) and Methylene Blue (MB) in aqueous solutions using TiO2-PCHPMA (0.6 mg/mL for BR2 and BR5, and 0.8 mg/mL for BB17 and MB; and dye concentrations= 20-40 mg/L) at 20 °C. The adsorption efficiencies for BR2, BR5, BB17 and MB were 99.2, 99.0, 98.8, and 98.7%, respectively, and adsorption reached equilibria within 10 seconds (Figure 3a). The rapid adsorption of these cationic dyes by the nanocomposite is ascribable to the electrostatic interaction between the -Cl moieties of the polymer coating and the cationic structures in the dyes.
The isotherm model was examined (Figure 2a and 2b). The Langmuir isotherm model (r2
>0.999) better fitted with the experimental data than the Freundlich isotherm (r2
=0.892-0.998), indicating that the adsorption is occurred by chemical adsorption of one dye molecule on one -Cl function. The adsorption kinetics was also examined. The pseudo-second-order kinetic model (r2
> 0.999) fits better than pseudo-first-order kinetic model (r2
= 0.306-0.719) (Figure 3b), indicating that the chemical adsorption of the cationic dyes on TiO2-PCHPMA is the rate determining step of the adsorption.
The adsorption experiment was also conducted at different pH (4 to 13). At pH 4, the adsorption efficiency was lower probably due to the competition of protonated species such as H3O+ with the cationic dyes. The adsorption efficiency was increased with increasing pH up to 10 by weakening of the competition. The adsorption efficiency under basic conditions (pH> 10) was lower probably owing to the electrostatic interaction between bases and the dyes.
Separation of cationic dyes from mixtures by TiO2-PCHPMA was also conducted. When a mixed aqueous solution of BB17 and MO was treated with TiO2-PCHPMA, the color of the supernatant after centrifugation was identical to the yellow color of the aqueous solution of MO. On the other hand, TiO2-PCHPMA was stained blue by BB17 through the selective adsorption.
The adsorbed cationic dyes could be washed out by washing with MeOH, and TiO2-PCHPMA maintained 82.4% of adsorption efficiency towards the original one after 4 successive cycles the adsorption.

Reference:

Z. Aksu, Process Biochem.,
40, 998 (2005).

N. Danishvar, A. Oladegaragoze, N. Djafarzadeh, J. Hazard. Mater.,
129, 116 (2006).

E. Fororgacs, T. Cserhati, G. Oros, Environ. Int.
30, 953 (2004).

Y. Anjaneyulu, N. S. Chary, D. S. S. Raj, Rev. Environ. Sci. Bio-Technol.
4, 245 (2005)

Figure 1