Elsevier

Carbohydrate Polymers

Volume 152, 5 November 2016, Pages 170-180
Carbohydrate Polymers

Cationic cellulose hydrogels cross-linked by poly(ethylene glycol): Preparation, molecular dynamics, and adsorption of anionic dyes

https://doi.org/10.1016/j.carbpol.2016.07.011Get rights and content

Highlights

  • Hydrogels composed of poly(ethylene glycol) and quaternized cellulose were prepared.

  • The hydrogels exhibited water-swelling behavior in solutions with a wide range of pH.

  • The prepared hydrogels exhibit high adsorption ability towards anionic dyes.

  • The dynamics of the hydrogels was investigated by the solid relaxation analyses.

  • The hydrogels could be regenerated and proved to be recyclable adsorbents.

Abstract

Cationic cellulose hydrogels (CCGs) were prepared from quaternized celluloses with degrees of substitution (DS) of 0.56, 0.84, and 1.33, by the cross-linking reaction with poly(ethylene glycol) diglycidyl ether as a cross-linker. The CCGs exhibited swelling behavior in aqueous solutions, which was not affected by pH and temperature of the solution because of the presence of quaternary ammonium groups in their structures. The CCGs showed adsorption ability toward anionic dyes in aqueous solution, which increased with increasing DS. The dye adsorption was found to follow the pseudo-second order kinetic model and the equilibrium isotherm data can be described by the Langmuir adsorption model. In addition, the CCGs could be regenerated and proved to be recyclable adsorbents for wastewater treatment.

Introduction

The presence of dyes in wastewater is an important environmental problem because of their high visibility, resistance, and toxic impact (Pushpa et al., 2015). Even very low concentrations of dye can reduce the photosynthetic activity in aquatic environments by preventing the penetration of light and oxygen (Khatri, Peerzada, Mohsin, & White, 2015). Because of their complex aromatic structures, dyes are non-biodegradable substances that are stable under various conditions; they have direct and indirect toxic effects on humans and are associated with diseases such as cancer, tumors, and skin irritation (Vakili et al., 2014).

Various treatment methods have been investigated for the removal of dyes from wastewaters, e.g., coagulation/flocculation (Verma, Dash, & Bhunia, 2012), chemical precipitation (Pan, Wang, Sun, Liu, & Zhang, 2016), reverse osmosis (Zheng, Wang, & Wang, 2015), and membrane filtration (Guo, Zhang, Cai, & Zhao, 2016). However, these methods have several limitations, including high capital and operating costs and low removal efficiency. Adsorption is one of the preferred techniques because it is relatively rapid, convenient, and easy to perform (Yagub, Sen, Afroze, & Ang, 2014; Mu & Wang, 2016).

The adsorption efficiency is generally affected by the nature of the adsorbent. Many adsorbents, such as activated carbon, zeolites, silica gel, and alumina, have been tested for their potential to lower dye concentrations in aqueous solutions. However, despite their relatively low cost, their wider use is restricted by the difficult recycling (Yagub et al., 2014). An ideal adsorbent for dye removal should have properties such as ease of regeneration, environmental safety, low-cost productivity, and high adsorption capacity. Hence, recently, attention has been directed toward natural polysaccharide-based hydrogels because of their biocompatibility and biodegradability (Crini, 2006).

Cellulose, the most abundant natural polymer on earth, is one of the most environmentally friendly non-food sources for the production of a wide range of eco-friendly products. The numerous potential chemical modifications of cellulose also make it an attractive adsorbent hydrogel candidate. For example, the hydrogel prepared from sodium carboxymethyl cellulose, which is a water-soluble anionic cellulose derivative, has shown promising results as adsorbent for cationic compounds (Zhang, Yi, Deng, & Sun, 2014), whereas a quaternary ammonium sodium salt derivative of cellulose, prepared by the homogeneous reaction of cellulose with 2,3-epoxypropyl trimethyl ammonium chloride (EPTMAC), proved effective for anionic compounds (Quinlan, Tanvir, & Tam, 2015). Quaternized cellulose derivatives are advantageous for biomedical, pharmaceutical, and environmental applications because their tertiary ammonium groups exhibit intermediate basicity and are permanently charged regardless of the pH of the solution (Saini, Falco, Belgacem, & Bras, 2016). In addition, they are reported to be excellent flocculants for kaolin suspensions because of the charge neutralization between the negatively charged surface of kaolin particles and the cationic groups of quaternized cellulose (Yan, Tao, & Bangal, 2009). Thus, quaternized cellulose is expected to be a powerful candidate for forming the matrix of hydrogels for cationic dye adsorption.

Polyethylene glycol (PEG), which is a synthetic polymer that is amphiphilic (Bailey and Koleske, 1991, Bailey and Koleske, 1976) and nontoxic, can be frequently used as an excipient or as a carrier in different pharmaceutical formulations, foods, and cosmetics (Fuertges, & Abuchowski, 1990). PEGs are readily available in a range of molecular weights (Mw); most of those that have low Mw (<104) can be rapidly removed from the human body after consumption (Working, Newman, Johnson, & Cornacoff, 1997), making them widely used for biomedical research, drug delivery, tissue engineering, and surface functionalization of biomaterials (Craig, 2002, Torchilin, 2002). The wide range of available PEGs with functionalized end-groups (e.g., azides, thiols, carboxylic acids, hydroxyls, and epoxides) makes them increasingly attractive for use in biomedical and biomaterial research (Zalipsky, 1995). In particular, poly(ethylene glycol) diglycidyl ethers (PEGDEs) which are epoxy-functionalized PEGs are well known for their reactivity toward hydroxyl groups, and are thus used to polysaccharides to form the ether crosslinkage (Cesteros, Ramírez, Peciña, & Katime, 2006; Nielsen, Wintgens, Larsen, & Amiel, 2009).

In this study, a series of cationic cellulose hydrogels (CCGs) with different degree of substitution (DS) were prepared from quaternized cellulose by use of PEGDE as a crosslinking agent (Fig. 1) and their adsorption capacity towards three kinds of anionic dyes, namely, AR13, AB92, and AR112 (Fig. S1), was investigated. In addition, the effect of the DS and pH on the adsorption of anionic dyes on the hydrogels, and the kinetics and isotherms of the adsorption process were evaluated and compared in detail for the elucidation of the adsorption mechanism. Moreover, the recycling efficiency of the adsorbents was determined.

Section snippets

Materials

Powdered cellulose with an average polymerization degree of 320 was obtained from Wako Pure Chemicals Industries (Japan). EPTMAC and PEGDE with an average polymerization degree of 7.2 (Kono, Nakamura, Hashimoto, & Shimizu, 2015) were purchased from Sigma-Aldrich Inc. (USA). AR13, AB92, and AR112 were purchased from Tokyo Chemical Industry Co., Ltd. (Japan). Other chemicals were of chemically pure grade and all solutions were prepared with pure water.

Preparation of CCs 1–3

A series of CCs (1–3) were prepared following

DS determination of CCs 1–3

CCs 1 and 2 were prepared by setting the molar feed ratios of EPTMAC to AGU of cellulose to 7.5:1 and 10:1, respectively, and the CC 3 was prepared by performing twice the preparation procedure of CC 2 (Table S1). All the homogeneous cationization reactions of cellulose were performed in the urea/NaOH solution. The structures of CCs 13 were confirmed by the quantitative 13C NMR spectra (Fig. S2), which show four resonances for the quaternary ammonium salt groups at 71, 68, 63, and 57 ppm (Kono

Conclusion

In the present study, cationic hydrogels prepared from native cellulose by a two-step reaction proved to be efficient and recyclable adsorbents with high adsorption capacity for anionic dyes. As the adsorption reaches the equilibrium, the fast kinetics of the process allows efficient decontamination of pollutants. Because their adsorption performance is hardly altered by changes in temperature and pH, the present hydrogels can be used under different operating conditions. The efficient

Acknowledgments

This work was supported in part by a Grant-in-Aid for Scientific ResearchC25410134 and by the Japan Society for the Promotion of Science (JSPS). The authors would like to express our appreciation to Ms. Atsumi Ozaki, JASCO International Co., Ltd. (Japan), for the SEM/EDS measurement.

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