Cationic cellulose hydrogels cross-linked by poly(ethylene glycol): Preparation, molecular dynamics, and adsorption of anionic dyes
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 1–3 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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2022, Colloids and Surfaces A: Physicochemical and Engineering AspectsCitation Excerpt :Organic dye pollution that comes from paper-making, printing industries, paints, cosmetics, has become a global environmental problem [1–3]. Due to its high toxicity and carcinogenicity, it has causes serious damage to the surrounding environmental balance and human health [4,5]. With the increasing concern on environmental pollution, a growing number of researchers devote themselves to the research of sewage treatment.