β-Cyclodextrin hydroxypropyl methylcellulose hydrogels for bisphenol A adsorption☆
Graphical abstract
Introduction
Water is one of the most important substances on Earth because all living beings depend on it to survive. Due to human activities, increasing concentration of contaminants has been detected in aquatic environments. Endocrine-disrupting compounds (EDCs) are an important class of emerging contaminants because they can cause many hormonal disorders. Bisphenol A (BPA), an EDC, is largely applied as monomer and as plasticizer in the polymer industry. In the last decade concerns about its effect as endocrine disruptor led to restricted use in food packing [1]. The BPA concentration range of 1 mg L−1 to 10 mg L−1 is acutely toxic for aquatic organisms [2]. The concentration ranges of BPA in wastewater and biosolids were reported as 0.416 to 2.050 μg L−1 and from 0.06 to 1.37 mg kg−1, respectively [3]. Thus, monitoring the BPA concentration in aquatic environment is important to avoid undesired estrogenic activity. In some cases, when the analyte concentration is too low for the detection limit of conventional techniques, adsorption is a low cost efficient strategy that allows preconcentration of analytes. The adsorbents should be stable and insoluble in water and recyclable. A recent review [4] compiled the adsorption behavior of BPA on a plethora of adsorbents, including clays, zeolites, rice husk, bagasse, carbon and graphene based materials, agricultural wastes, molecular imprinted polymers, nanocomposites, polysaccharides and cyclodextrins.
Cyclodextrins (CD) or cycloamyloses are water soluble cyclic oligosaccharides obtained from starch. They may be composed of six, seven or eight glucose units, named as αCD, βCD and γCD, respectively [5]. They have a remarkable capacity to interact with other molecules by host-guest interactions, forming inclusion complexes [6]. Among the CDs, βCD is the most frequently used because it is the cheapest one; its dimensions are 0.7 nm internal diameter and 0.8 nm depth [7]. The common hosts are aromatics, phthalates, surfactants, dyes, polycyclic hydrocarbons and metals [8]. For adsorption purposes, βCD molecules must be crosslinked to become insoluble in water. The most used crosslinkers are epichlorohydrin (EPI), ethylene glycol diglycidyl ether (EGDE), glutaraldehyde, isocyanates, ethylenediaminetetraacetic acid (EDTA) and carboxylic acids [9]. The polymerization of βCD and the modification of polysaccharides by grafting βCD molecules to their structure mediated by crosslinkers are successful strategies to create efficient adsorbents for BPA. For instance, βCD molecules polymerized with EPI, poly(βCD), presented maximum adsorption capacity (qmax) of 84 mg g−1 [10]. Benzylated βCD (BnCD) was crosslinked with dichloroxylene (DCX) via a Friedel-Crafts alkylation route, creating a benzylated poly(βCD), which presented qmax of 278 mg g−1 [11]. Chitosan was successfully crosslinked with βCD via carbodiimide chemistry [12], displaying qmax of 85 mg g−1. Hydrogels of carboxymethylcellulose sodium salt (CMC) and βCD presented qmax of 38 mg g−1 [13]. βCD molecules were grafted to cellulose nanofibril (CNF) aerogels using epichlrohydrine (EPI) as crosslinkers, they were applied as adsorbent for p-chlorophenol and the maximum adsorption capacity (qmax) was 34 mg g−1 [14]. In this work, βCD molecules were grafted to hydroxypropyl methylcellulose (HPMC) cryogels using citric acid as crosslinkers. HPMC is a cellulose ether widely used in food, cosmetics and drug formulations due to its high biocompatibility [15]. There is no report in the literature so far, to the best of our knowledge, which describes the formation of βCD and HPMC networks mediated by citric acid. Upon dipping the HPMC-βCD cryogels in water, they swell, behaving as stable hydrogels. The adsorption kinetics and isotherm of BPA on HPMC-βCD hydrogels, as well as the possibility of recycling, were investigated. In order to gain insight about the adsorption process, the inclusion equilibrium constant of BPA into bare βCD in solution was assessed by means of 1H NMR spectroscopy.
Section snippets
Materials
Commercial HPMC J5MS sample (USP 1828) kindly supplied by The Dow Chemical Company (Brazil) presented degree of substitution (DS) of 1.5, molar substitution (MS) of 0.75, and weight-average molar mass (Mw) 3.0 × 105 g mol−1 [16]. Citric acid (Labsynth, Brazil, 192.12 g mol−1), sodium hypophosphite (Labsynth, Brazil, 87.98 g mol−1), β-cyclodextrin (βCD, Sigma Aldrich 7585-39-9, 1134.98 g mol−1), bisphenol A (BPA, Sigma Aldrich 80-05-7, 228.29 g mol−1), Trizma® base (Sigma Aldrich 77-86-1,
Determination of the inclusion constant for the βCD-BPA complex in solution
Fig. 2 shows typical 1H NMR spectra for pure BPA and mixture of BPA and βCD at molar ratio 1:1000. Regardless of the BPA: βCD molar ratio in the mixture, the most pronounced changes in the chemical shifts were always observed for the protons belonging to the BPA aromatic ring, probably it is due to the polarity change caused by the inclusion of BPA in the βCD cavity. The 1H NMR spectra obtained for pure BPA and βCD with the corresponding chemical shifts assignments were provided as
Conclusions
This work demonstrated the easy formation of HPMC-βCD hydrogels crosslinked with citric acid. Despite of the content of 5.6 wt% βCD in the networks, the hydrogels were able to adsorb up to 14.6 mg g−1 of BPA and could be reused at least five times without losing performance. The kinetics study revealed a two-step mechanism for the adsorption of BPA on HPMC-βCD hydrogels: a fast one, when the BPA molecules diffuse freely from the bulk to the hydrogel surface and a slow one, corresponding to the
Acknowledgements
Authors gratefully acknowledge financial support from Brazilian Funding Agencies Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP Grant 2015/25103-2) and Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq Grants 155431/2016-1, 448497/2014-0 and 305178/2013-0). The authors thank Paulo V. O Toledo for his assistance during the MicroCT measurements. We thank LMN-LNNano/CNPEM (Campinas, Brazil) for the MicroCT facility (Grant # 22728).
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The article was accepted for publication under the special issue on Polymers in Solution and in Liquids - 14th Brazilian Polymer Congress.