Continuous flow adsorption of ciprofloxacin by using a nanostructured chitin/graphene oxide hybrid material
Graphical abstract
Introduction
Emerging contaminants are defined as chemicals that had not been previously detected in water supplies (or were found in far lesser concentrations) and nowadays are discovered either in surface water or groundwater (Petrie, Barden, & Kasprzyk-Hordern, 2015; Van Doorslaer, Dewulf, Van Langenhove, & Demeestere, 2014). The main groups of emerging contaminants are pharmaceuticals, personal care products and endocrine disrupting compounds which could persist in the environment including the drinking water since the purification processes are not designed to eliminate them (Matamoros, Rodríguez, & Albaigés, 2016). Among the pharmaceutical products, ciprofloxacin is a synthetic antimicrobial derived from nalidixic acid and is one the most common fluoroquinolone used for antibiotic therapies in humans for urinary and respiratory infections. Moreover, the use of ciprofloxacin in veterinary has been raising the last years since it is applied not only as therapeutic but also with prophylactic purposes in cattle, poultry and shrimp farming (Holmström et al., 2003; Teuber, 2001). The main sources of contamination are hospital wastewater and feedlots, being excretions of humans and animals disposed almost intact due to its low metabolism. The contact with the environment represents a threat taking into account that it would induce the proliferation of bacterial resistance (Sharma, Johnson, Cizmas, McDonald, & Kim, 2016). Ciprofloxacin has been determined in water and wastewater at concentrations ranging from <1 μg/L up to 6.5 mg/L in some cases (Fick et al., 2009).
In the last years several research works described different methods for ciprofloxacin decontamination. Some of those techniques are enzymatic degradation, electrochemical and chemical oxidation, photocatalysis and adsorption (Chen, Gao, & Li, 2015; Ji, Ferronato, Salvador, Yang, & Chovelon, 2014; Prieto, Möder, Rodil, Adrian, & Marco-Urrea, 2011; Wang, Shen, Zhang, Zhang, & Yu, 2016; Yan et al., 2013). Most of them present high implementation costs or are not capable to achieve good water quality except for adsorption which has a good performance against different contaminants, for example organic molecules like antibiotics (Rodriguez-Narvaez, Peralta-Hernandez, Goonetilleke, & Bandala, 2017). One of the advantages of adsorption is the opportunity of scale up; also, during the treatment, no co-products are released to the environment. Moreover, either in batch or continuous process, after saturation is reached, many of the adsorbent materials can be recovered and reused (Huang, Fulton, & Keller, 2016).
Currently carbon and its allotropes, such as graphene, carbon nanotubes and fullerene have acquired an important role in the development of new nanostructured materials with several applications in the water remediation field (Ma et al., 2016; Ncibi & Sillanpää, 2015; Rodriguez-Narvaez et al., 2017; Suárez-Iglesias, Collado, Oulego, & Díaz, 2017). Graphene is a one-atom thick layer of graphite where the carbon atoms are distributed in a regular sp2-bonded network and is one of the most studied materials for the research of new materials with uncountable applications including pollutant adsorbents due to its extremely high specific surface (2630 m2/g) (Bonaccorso et al., 2015; Novoselov, 2004). However, its low solubility, reactivity and high production cost represent a drawback when it comes to using it at high scale for the adsorption of polar molecules. The synthesis of graphene oxide nanosheets (nGO) is a facile and low-cost method for increasing the hydrophilicity and reactivity of the nanomaterial because of the addition of oxidized functional groups through the chemical exfoliation of graphite (Sharif, Gagnon, Mulmi, & Roberts, 2017).
Chitin, an abundant biopolymer present in nature, can be found in fungi, the exoskeleton of insects and the shells of crustaceans, including shrimp and crab, as well as other invertebrates, such as marine sponges (Guo, Duan, Zhou, & Zhu, 2014). Its structure consists predominantly of unbranched chains of β-(1 → 4)-2-acetoamido-2-deoxy-d-glucose. The main source of chitin are the disposals of food industry, making it a low-cost and sustainable material for polymeric matrices synthesis since has a good stability against different liquid media, for instance organic solvents and acid or basic conditions (Guo, Duan, Cui, & Zhu, 2015). Considering its use in column adsorption processes, the porous-like structure of this polysaccharide exhibits a good performance against increasing flows and allows to work at low pressure (Ma et al., 2016; McKay, Blair, & Gardner, 1984).
We state the hypothesis that a nanostructured hybrid material composed by chitin loaded with nGO can be synthetized by means of a simple method and successfully applied in a continuous flow system for the adsorption of ciprofloxacin as an emerging contaminant model. Therefore, the aim of the present work is to test the dynamic adsorption behavior of the hybrid and study the main parameters that affect its performance, including real water samples. Although several researchers have already proved the capability of adsorption of ciprofloxacin by using pure nGO or combined in different polymeric matrices most of them only describe the behavior of the material in batch systems. Furthermore, up to our knowledge, there is not any background in literature regarding the implementation of nGO in continuous column systems against emerging contaminants.
Section snippets
Reagents and materials
Chitin from shrimp shells was obtained from Sigma-Aldrich (USA, calculated degree of acetylation, DA: 95%). Graphite fine powder extra pure (particle size <50 μm) was acquired from Merck (Germany). Calcium chloride dihydrate was purchased from Cicarelli (Argentina); Methanol was acquired from Sintorgan (Argentina). Ciprofloxacin hydrochloride was purchased from Saporiti (Argentina). Water was filtered and deionized with a Milli-Q, Millipore system (Milford, MA, USA). All other reagents were of
Spectroscopic characterization
Fig. 1 shows all the spectra of the raw materials, the intermediate components and the final Chi:nGO 3:1 hybrid. In comparison with the graphite FTIR signals, the graphene oxide spectrum shows an increase in the bands corresponding to oxidized groups, which confirms the chemical exfoliation of graphite into. The slight band at 1240 cm−1 is attributed to the COC bond stretching which demonstrates the formation of epoxy groups. Moreover, the presence of carboxyl and carbonyl functional groups can
Conclusions
The Chi:nGO 3:1 hybrid material showed to be applicable for the removal of ciprofloxacin in a continuous system. Its high and wide size porosity endow to the material with a low flow resistance which allows to work at different flow rates. The isotherm assays demonstrated that the adsorption mechanism involves a heterogeneous process since it fitted better to the Sips model with a heterogeneity parameter far from the unit. The tested medium conditions and the real water samples experiments
Acknowledgments
J.A.G. is grateful for his doctoral fellowship granted by Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET). M.E.V. is grateful for her postdoctoral fellowship granted by CONICET. The authors are grateful to the Brazilian Synchrotron Light Laboratory (LNLS) for SAXS facilities and would like to acknowledge INTI Mecánica for their assistance in SEM analysis. This work was supported with grants from Universidad de Buenos Aires (UBACYT 20020130100780BA) and Agencia Nacional de
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