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Highly efficient adsorption of fluoroquinolone antibiotics using chitosan derived granular hydrogel with 3D structure
Accepted Manuscript Highly efficient adsorption of fluoroquinolone antibiotics using chitosan derived granular hydrogel with 3D structure
Ningfen Wang, Weilong Xiao, Bihui Niu, Wenzhen Duan, Lei Zhou, Yian Zheng PII: DOI: Reference:
S0167-7322(18)35752-0 https://doi.org/10.1016/j.molliq.2019.02.061 MOLLIQ 10462
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
7 November 2018 11 January 2019 11 February 2019
Please cite this article as: N. Wang, W. Xiao, B. Niu, et al., Highly efficient adsorption of fluoroquinolone antibiotics using chitosan derived granular hydrogel with 3D structure, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.02.061
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ACCEPTED MANUSCRIPT Highly efficient adsorption of fluoroquinolone antibiotics using chitosan derived granular hydrogel with 3D structure
Gansu Key Laboratory for Environmental Pollution Prediction and Control, College of Earth and
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a
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical
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b
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Environmental Sciences, Lanzhou University, Lanzhou, 730000, China
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Engineering, Lanzhou University, Lanzhou, 730000, China
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ABSTRACT
Here, a granular hydrogel prepared from chitosan as the grafted backbone and acrylic
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acid as the polymerizable monomer was used as the adsorbent to remove two typical
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fluoroquinolone antibiotics, i.e. ciprofloxacin and enrofloxacin. The parameters affecting the adsorption capacity were investigated via a series of adsorption
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experiments, including pH (2.0-9.0), contact time (1-180 min), initial concentration
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(10-600 mg/L), and ion strength (1 mM and 10 mM NaCl/CaCl2). The results showed that the granular hydrogel possessed porous 3D structure and, the ionizable characterization of -COO- groups made the adsorbent be pH-sensitive and the maximum adsorption occurred at pH=3.0. Moreover, the presence of abundant -COOgroups within 3D structured network provided the adsorbent with higher adsorption
Corresponding authors. E-mail addresses: [email protected] (L. Zhou), [email protected]
(Y. Zheng) 1
ACCEPTED MANUSCRIPT capacity of 267.7 mg/g for ciprofloxacin and 387.7 mg/g for enrofloxacin. When the initial concentrations were 10, 50 and 250 mg/L, the equilibrium time was determined to be 30, 60 and 120 min, respectively. After five consecutive adsorption-desorption cycles, a higher adsorption percentage (>85%) was still obtained, demonstrating that
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the developed adsorbent could be easily regenerated and reused for several times.
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Keywords: Ciprofloxacin; Enrofloxacin; Fluoroquinolone antibiotics; Adsorption;
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Hydrogel
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1. Introduction
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Antibiotics, treated as effective drugs for treating infections of human and veterinary and growth promoter for livestock and agriculture, are used normally (Kümmerer,
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2009). As reported, their usage approximately is between 1×105 and 2×105 tons
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annually globally, especially in China where the amount of antibiotic consumption is as high as 2.48×105 tons in 2013 (Lu et al, 2018; Wang et al, 2017). However, the
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antibiotics cannot be completely metabolized by users, and also, they are frequently
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treated using some ineffective methods. Accordingly, various antibiotics have been frequently detected in aquatic environment, such as lake, reservoir, river, and even drinking water (Li, Shi, Gao, Liu, & Cai, 2013; Chen, Jing, Teng, & Wang, 2018). Relative to other antibiotics, fluoroquinolones (FQs) are usually considered to be medium to high risk, and their concentration is reported within the range from ng/L up to mg/L in aquatic environment (Li, Zhang, Wu, & Zhao, 2014; Michael et al, 2013). These FQs existing in the water can pose potential threats to aquatic organisms and 2
ACCEPTED MANUSCRIPT human health, such as antibiotic resistance or even genetic mutations in aquatic organisms and human health abnormalities through the enrichment of food chain (Zhang et al., 2018; Mckinney, Loftin, Meyer, Davis, & Pruden, 2010). Since FQs are mainly existed in domestic and medical wastewater, and are easily adsorbed by sludge,
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they are hardly removed by conventional methods (Gao et al., 2012; Narumiya, Nakada,
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Yamashita, & Tanaka, 2013). Therefore, it is essential to find highly efficient methods
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to remove FQs from water environment.
Among many available methods, adsorption technology is featured with simple
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design and easy operation, and thus considered to be one of highly efficient methods
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for removal of different pollutants from aquatic environment. The core using this technology is to find an appropriate adsorbent material. Up to date, a variety of
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adsorbents have been developed, and among available adsorbents, carbonaceous
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materials have been widely reported for their abundant pore structure and high specific surface area, such as activated carbon (Fu et al., 2017), carbon nanotubes and graphene
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oxide (Li, Zhang, Han, & Xing, 2014; Yu, Li, Han, & Ma, 2016). Given the higher
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cation exchange capacity, different clay-based materials have also been developed for removal of FQs, such as kaolinite, montmorillonite and illite (Wang, Li, & Jiang, 2011; Li et al., 2011). Recently, biochar-based materials have also attracted increasing attention for removing FQs (Li et al., 2017; Wang, Jiang, Yi, & Yang, 2017). Nonetheless, most of them cannot separate entirely from water, have the defects of easy agglomeration, or present limited adsorption capacity for FQs. Nowadays, the 3D structured hydrogels have attracted increasing attention in wastewater treatment (Tran, 3
ACCEPTED MANUSCRIPT Park, & Lee, 2018). For instance, chitosan/biochar hydrogel beads had been reported to remove ciprofloxacin (Cip) (Afzal et al., 2018). Nanobeads-based polypyrrole hydrogel was prepared to adsorb and catalyze rhodamine (Yao et al., 2018). Cubic polyhedral oligomeric silsesquioxane nano-cross-linked hybrid hydrogels were
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developed to remove methylene blue (Eftekhari-Sis, Rahimkhoei, Akbari, & Araghi,
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2018). Compared with other reported adsorbents, the hydrogels present the following
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advantages: fast adsorption kinetics, high adsorption capacity, easy separation and regeneration. In addition to wastewater treatment, the 3D structured hydrogels have
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also widely applied to other fields such as biological sensor (Li, Guan, & Zhang, 2018),
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electronic device adhesive (Wang et al., 2018), wound dressing (Lin et al., 2017), tissue engineering (Park, Lee, Lee, Park, & Lee, 2018) and agriculture (Cheng, Liu, Yang, &
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Zhang, 2018).
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Actually, hydrogel itself possesses a 3D structured network that can be formed by any water-soluble or hydrophilic polymer via chemical or physical crosslinking and
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traditionally classified into two major categories (natural and synthetic) depending on
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their source. Among hydrogels, the most important one type comes from natural polysaccharides and their grafted polymers, such as cellulose (Geng, 2018), alginate (Li, Ma, Chen, Huang, & He, 2018), starch (Shahbazi, Majzoobi, & Farahnaky, 2018) and chitosan (CTS) from chitin (Vakili et al, 2014; Vakili et al, 2016; Yan et al., 2018). Chitin is the main component of crustacean shells, and has been discarded as a byproduct of seafood processing industry (Zheng, & Wang, 2012). CTS is the deacetylated chitin and thus has the advantage of being inexpensive, easy to obtain and can be 4
ACCEPTED MANUSCRIPT modified by grafting for application in different fields. Based on above backgrounds, CTS was selected as the backbone in this study to obtain a granular hydrogel with 3D structure by grafting polyacrylic acid (PAA) via free radical solution polymerization. The resulting material was marked as CTS-PAA
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and by combination of scanning electron microscope (SEM) and Fourier transform
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infrared spectrum (FTIR), a well characterization was carried out to reveal its
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morphology and components. Subsequently, its adsorptive behaviors towards Cip and enrofloxacin (Enr) were systematically investigated via a series of adsorption
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experiments, including pH from 2.0 to 9.0, contact time up to 180 min, initial
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concentration from 10 to 600 mg/L, and ion strength in 1 mM and 10 mM NaCl/CaCl2 solutions. Subsequently, the pseudo-first order and pseudo-second order equations, as
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well as three adsorption models including Langmuir, Freundlich and Redlich-Peterson
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equations, were applied to the experimental data to understand better the adsorption process. Finally, the reusability was evaluated for exploring its feasibility in a practical
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application.
2. Materials and methods 2.1. Materials
Ciprofloxacin (Cip, 98%) and enrofloxacin (Enr, 98%) were purchased from J&K Scientific, Beijing, China, and their molecular structures were shown in Fig. 1. Chitosan (CTS, with an degree of deacetylation of 0.85 and average molecular weight of 3.0× 105) was provided by Yuhuan Ocean Biology Co., Ltd., Zhejiang, China. Acrylic acid 5
ACCEPTED MANUSCRIPT (AA, chemically pure) was obtained from Shanghai Shanpu Chemical Factory, Shanghai, China. Ammonium persulfate (APS, analytical grade) was received from Xi’an Chemical Reagent Factory, Shanxi, China. N,N’-methylene-bisacrylamide (MBA, chemically pure) was the product of Shanghai Chemical Reagent Factory, Shanghai,
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China. Other reagents including acetic acid, HCl, NaOH, NaCl and CaCl2 were
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analytical purity and used as received. Distilled water was used for preparing the
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solutions throughout the experiment. 2.2. Adsorbent preparation
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The preparation process of chitosan grafted poly(acrylic acid) (marked as CTS-PAA)
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can refer to our previous study (Zheng, Zhang, & Wang, 2009). Typically, in a 5L double-layer glass reactor equipped with a stirrer, a condenser, a thermometer and a
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nitrogen line, an appropriate amount of CTS dissolving in 1% (v/v) acetic acid was
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added. When nitrogen was released in above reactor for 30 min, the CTS solution was heated to 60oC under oxygen-free atmosphere conditions, followed by adding APS
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solution. As CTS was initiated to produce radicals, the mixture consisting
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polymerizable monomer AA and crosslinker MBA was introduced into the solution when the mixture was heated to 70oC and reacted for 3h. When the polymerization process was finished, the resulting granular product was immersed in NaOH solution for neutralization to pH 6.0-7.0, and then dehydrated with industrial alcohol and finally dried at 70oC in an oven to a constant weight. During the experiment, the weight percentage of CTS, APS and MBA was fixed to 10 wt%, 2 wt% and 3 wt% in the feed, respectively. Prior to use, the product was milled and the sample through 40-80 mesh 6
ACCEPTED MANUSCRIPT was collected. 2.3. Batch adsorption experiments Typically, 20 mg as-prepared adsorbent was contacted with 20 mL solution containing different FQs in 50 mL Erlenmeyer flask and the mixture was then placed
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in a shaker (THZ-100, Shanghai Yiheng Scientific Instrument Co., Ltd.) for a certain
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time at 30oC/120 rpm. When the adsorption process was complete, the adsorbent was
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separated from the solution by direct filtration and the residual concentration of FQs was obtained by the changes in the maximum absorbance at 275 nm by using a UV-Vis
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spectrophotometer (TU-1810PC). After that, the corresponding adsorption capacity for
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different FQs was calculated according to the differences in the concentration before and after the adsorption.
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Subsequently, a series of experiments were performed to investigate the influences
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of different variables on the adsorption properties. Firstly, the pH-dependence was studied at the initial concentration of 50 mg/L and pH ranging from 2.0 to 9.0 by
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adjusting with 0.1 and 1.0 mol/L HCl or NaOH solutions. After 180 min, the
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supernatant of different FQs was sampled and the pH value was recorded with a pH meter (FE20, Mettle-Toledo Instrument, Co., Ltd.). Next, the effect of contact time on the adsorption capacity was carried out to investigate the adsorption kinetics, by varying the contact time up to 180 min at pH of 3.0 and three concentration levels of 10, 50 and 250 mg/L. Then, the adsorption isotherms were investigated by varying the initial concentration of FQs from 10 mg/L to 600 mg/L at pH of 3.0 and contact time of 180 min. Finally, the influence of ion strength on the adsorption capacity was performed by 7
ACCEPTED MANUSCRIPT using 1 mM and 10 mM NaCl and CaCl2 solution as the background solvents. In this case, the initial concentration of FQs was 50 mg/L and pH was fixed at 3.0. 2.4. Reusability When an adsorption process was finished, the adsorbent was separated and the
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supernatant was analyzed to obtain the initial adsorption capacity, i.e. 1st adsorption
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capacity. Then, the adsorbent was immersed into 20 mL of 0.1 mol/L HCl to desorb
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these FQs molecules at 30oC/120 rpm/120 min. Followed, the adsorbent was separated from the desorbing solution and neutralized in 0.1 mol/L NaOH for 10 min. After
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regeneration, the recovered adsorbent was washed for several times with distilled water
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and then used for the next adsorption. Here, the reusability was performed for completing adsorption-desorption experiments for at least five times.
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2.5. Characterization method
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The adsorbent samples before and after the adsorption were analyzed by scanning electron microscopy (SEM) and Fourier transform infrared spectrum (FTIR) to reveal
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their surface morphology, element mapping in the micro-area of adsorbent, and
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functional groups attached onto the adsorbent.
3. Results and discussion 3.1. SEM analysis Fig. 2 shows the surface morphology of granular hydrogel before and after the adsorption for Cip and Enr. Significantly, original hydrogel presents a sponge-like 3D structured morphology, and such a pore structure will contribute to the adsorption 8
ACCEPTED MANUSCRIPT kinetics and then facilitate the adsorption process. After the adsorption, the surface morphology of adsorbent becomes significantly different. Contrastively, the adsorbent surface appears quite compact and the pore size is significantly reduced at ×1000 magnification, and also, the particle size is evidently diminished at ×100 magnification.
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This is reasonable for that with the adsorption of Cip and Enr, the 3D structured network
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of CTS-PAA will be occupied by these antibiotic molecules. Furthermore, the
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interaction between the adsorbent and adsorbates will result in decreasing ionic functional groups of CTS-PAA via free counter ions, by which the repulsion between
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size and a diminishing particle size.
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these ionic groups from CTS-PAA will be reduced, leading thus a shrinking 3D network
3.2. Element mappings
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The element mappings of granular hydrogel are presented in Fig. 3, corresponding
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respectively to the samples before and after the adsorption for Cip and Enr. It is observed that C and O elements are visible on the surface of original hydrogel, while
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additional N and F elements are found for hydrogel after the adsorption, testifying the
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presence of Cip or Enr attached onto the hydrogel, i.e. Cip and Enr have been successfully adsorbed by CTS-PAA. 3.3. FTIR analysis
Grafting modification is a promising way to obtain various utilizations of polysaccharides, such as CTS in this study, and more details on the grafting reaction between the reactants can refer to our previous study by using FTIR analysis (Zheng, Huang, & Wang, 2011). Here, only the main characteristic bands that can reflect the 9
ACCEPTED MANUSCRIPT adsorption of Cip and Enr are presented, as shown in Fig. 4. For original hydrogel, the characteristic absorption bands, assigning respectively to asymmetric and symmetric vibration of -COO- groups, have been found at 1565 and 1407 cm-1. After the adsorption, above two absorption bands are getting weaken, while the absorption band indexing to
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-COOH groups at ca. 1720 cm-1 is strengthened (Movasaghi, Yan, & Niu, 2019). This
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increase in the absorption intensity is originated from (i) the transform of -COO- groups
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to -COOH groups for the hydrogel, for the adsorption experiment is conducted at pH 3.0, and (ii) the adsorption of Cip and Enr onto the 3D structured network of CTS-PAA,
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for their structures contain the functional -COOH groups. Further, some new absorption
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bands are observable in the spectra of Cip- and Enr-loaded hydrogel. The band at 1629 cm-1 is arisen from the vibration of phenyl framework conjugated to -COOH groups
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attached onto the antibiotic molecules. The band at ca. 1270 cm-1 is assigned to coupling
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of the carboxyl C–O stretching and O–H deformation (Trivedi, & Vasudevan, 2007). Besides, several small bands in the range of 1000–500 cm-1 are also ascribed to the
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presence of FQs, such as 802 and 892 cm-1 (Gunasekaran, Rajalakshmi, & Kumaresan,
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2013). Based on the changes in characteristic absorption bands, it can be deduced that Cip and Enr have been successfully adsorbed by CTS-PAA. 3.4. Adsorption properties In current study, we use CTS as the backbone to graft PAA, obtaining a granular hydrogel CTS-PAA via free radical solution polymerization. CTS is the second polysaccharide ranked only second to cellulose, and then it is a low-cost, biocompatible, biodegradable and nontoxic natural polymer and has been focused on by researchers 10
ACCEPTED MANUSCRIPT for many years (Wang et al., 2019). However, CTS shows the defects of non-acidbase resistance property and weak mechanical strength (Zeng et al., 2019). Nevertheless, CTS can be easily modified by grafting reaction for its highly reactive amino and hydroxyl groups. Here, AA is selected as the functional monomer because
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of fast polymerization speed and easily occurred addition reaction (Pakdel, &
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Peighambardoust, 2018), and a general reaction mechanism between CTS and AA can
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be proposed as three stages: (i) the initiator APS is decomposed under the heating to produce a sulfate anion radical, (ii) the resulting anion radical abstracts hydrogen atom
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from –NH2 or –OH groups attached onto the backbone of CTS to form macro-radicals,
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(iii) the resulting macro-radicals initiate the polymerization reaction of AA to graft the PAA chains on the backbone of CTS. By then, we obtain the product CTS-PAA that has
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crosslinked polymeric network and abundant carboxylic groups. This kind of adsorbent
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material is thus expected to show higher affinity to Cip+ and Enr+ via electrostatic interaction, and then presents excellent adsorption capacity towards them.
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3.4.1. Effect of pH
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The pH of the antibiotic solution is a critical parameter influencing the adsorption process. As shown in Fig. 5a, the adsorption capacity showed a remarkable increase by increasing the initial pH from 2.0 to 3.0, beyond which a gradual decrease in the adsorption efficiency was observable. At pH=3.0, the maximum adsorption capacity was obtained. One can speculate that the electrostatic interaction is responsible for this phenomenon. According to the experimental data, when an initial pH from 3.0 to 9.0 was used, the equilibrium pH was determined to be 4.3-6.7 for Cip and 5.5 and 7.1 for 11
ACCEPTED MANUSCRIPT Enr after the adsorption (Fig. 5b). Then, when CTS-PAA is immersed in the solution, the functional groups within the 3D structured network can be ionized easily by water permeation. With increasing the pH values, the degree of deprotonation is getting higher, by which the carboxylate ions are dominant in the solution (Pakdel, & Peighambardoust,
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2018). But for Cip and Enr, the two FQs have two relevant ionizable functional groups,
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the 3-carboxyl group and N-4 of the piperazine substituent, and accordingly, two pKa
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values are commonly reported for each antibiotic, with 5.90 and 8.89 for Cip and 6.27 and 8.30 for Enr (Picó, & Andreu, 2007). At pH 3.0, there is predominantly a cationic
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form in the solution for both types of antibiotic molecules, while -COO- groups are
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predominant within polymeric network of CTS-PAA. At that time, a strong electrostatic attraction occurs between the negatively charged -COO- groups from CTS-PAA and
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positively charged adsorbates, i.e. Cip+ and Enr+. At pH 2.0, the available functional -
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COO- groups within the 3D polymeric network of CTS-PAA are completely converted into -COOH groups, by which the repulsion between the negatively charged -COO-
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groups within the 3D polymeric network of CTS-PAA will be minimized, making the
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3D network size of hydrogel shrink. And accordingly, the interaction between CTSPAA and adsorbates is greatly weakened and then a sudden decrease in the adsorption capacity is observable. With increasing pH from 3.0 to 9.0, the electrostatic attraction is decreased between the adsorbent and adsorbate, while the electrostatic repulsion is gradually dominated, leading also a lower adsorption capacity. 3.4.2. Effect of contact time Under three concentration levels, the influences of contact time on the adsorption 12
ACCEPTED MANUSCRIPT capacity were investigated, as shown in Fig. 6. Observably, when the initial concentrations were 10, 50 and 250 mg/L, a fast adsorption stage was all within 30 min, and thereafter the time to reach equilibrium were found to be 30, 60 and 120 min, respectively.
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Further, the experimental data was analyzed using pseudo-first order and pseudo-
ln(𝑞e − 𝑞t ) = ln 𝑞e − 𝑘1 𝑡
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Pseudo-first order kinetic equation:
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second order kinetic models. Their linear equations can be expressed as below:
(2)
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Pseudo-second order kinetic equation:
(1)
where qe and qt (mg/g) are the adsorption capacity at equilibrium and at time t (min),
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respectively. k1 (min-1) and k2 (g·mg-1·min-1) correspond to rate constant of these two 𝑞t
against t, the kinetic parameters can be
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obtained, as listed in Table 1.
𝑡
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equations. By plotting ln(𝑞e −𝑞t ) and
Obviously, the pseudo-second order kinetic model is more suitable for analyzing the
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experimental data for higher regression coefficients (R2>0.998) and smaller differences between calculated adsorption capacity (expressed as qe,cal) and experimental one
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(expressed as qe,exp). Further, the initial adsorption rate increases as the initial concentration increases, and this fact is mainly attributed to that increasing the concentration is equivalent to increasing the driving force, by which the adsorption rate is also accelerated. Here, it should be mentioned that CTS-PAA shows higher adsorption capacity to Cip at lower concentration (10 mg/L), comparable adsorption
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ACCEPTED MANUSCRIPT capacity for Cip and Enr at medium concentration (50 mg/L) and higher affinity to Enr at higher concentration (250 mg/L). 3.4.3. Effect of concentration Equilibrium adsorption isotherms that are used to reflect the relationship between
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equilibrium concentration and adsorption capacity are important for understanding the
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mechanism of an adsorption processes. For Cip and Enr, when the initial concentration
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of the adsorbate increased from 10 to 600 mg/L, the adsorption capacity gradually reached a maximum (Fig. 7a), when the maximum adsorption capacity for Cip and Enr
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were found to be 229.7 and 339.6 mg/g, respectively. Fig. 7b showed the adsorption
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isotherm by plotting adsorption capacity against equilibrium concentration, from which the saturated adsorption capacity was gradually achieved with increasing the
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concentration.
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To clearly understand the adsorption process, three adsorption models including Langmuir, Freundlich and Redlich-Peterson equations were applied to the experimental
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data, as shown as follows:
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Langmuir equation:
qe
qmbCe 1 bCe
(3)
Freundlich equation:
qe KCe1/ n
(4)
Redlich-Peterson equation:
qe Kr C/e ( 1 a bCe )
(5)
where 𝑞e (mg/g) and 𝐶e (mg/L) are the adsorption capacity and equilibrium concentration, respectively. 𝑞m (mg/g) is the maximum adsorption capacity. Other parameters are the adsorption constants of different isotherms. 14
ACCEPTED MANUSCRIPT All the parameter and regressions coefficients are summarized in Table 2, from which the Redlich-Peterson model showed the highest correlation coefficient R2, it is, however, not a suitable isotherm for describing the experimental data due to lager b values. Comparatively, the correlation coefficient R2 from Langmuir isotherm is close to that
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from Redlich-Peterson model, indicating that the adsorption behavior of Cip and Enr is
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closer to the Langmuir model. That is, such an adsorption process occurs mainly in the
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form of monolayer adsorption on the homogeneous surface of an adsorbent. From the Langmuir isotherm, the maximum adsorption capacity is calculated to be 267.7 mg/g
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for Cip and 387.7 mg/g for Enr.
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3.4.4. Effect of ion strength
To evalute the effect of ion strength, 1 mM and 10 mM NaCl and CaCl2 were used
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to prepare antibiotic solution to perform a comparative study in different media. Firstly,
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the conductivity was determined to be 136 μs/cm for 1 mM NaCl, 1222 μs/cm for 10 mM NaCl, 233 μs/cm for 1 mM CaCl2 and 2030 μs/cm for 1 mM CaCl2. That, the ion
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strength follows the order of 1 mM NaCl<1 mM CaCl2<10 mM NaCl<10 mM CaCl2.
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As shown in Fig. 8, the adsorption capacity of two antibiotics showed a monotonic decrease by increasing the ion strength. This can be attributed to the competition of adsorbate molecules and ions from background solution for the available adsorption sites on the adsorbent surface. This also proves that the electrostatic interaction is the principal adsorption mechanism between the adsorbent and adsorbate, consistent with pH-dependent results. In addition, the divalent cations, i.e. Ca (II) in this study can form 1:1 complex with FQs by ion–dipole interaction with the 4-keto oxygen and the ionized 15
ACCEPTED MANUSCRIPT 3-carboxylic acid group, and by then, the adsorption capacity is also greatly affected (Duan, Wang, Xiao, Zhao, & Zheng, 2018). 3.4.5. Reusability To evaluate the reusability, five consecutive adsorption-desorption experiment was
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carried out, and the experimental results were exhibited in Fig. 9. Clearly, the as-
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prepared hydrogel was recyclable for removal of Cip and Enr in this study, rendering
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that after five cycles, the as-prepared adsorbent showed still higher adsorption percentage for these two antibiotics. As a consequence, this hydrogel is promising as
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an alternative adsorbent for application in the field of FQs-containing wastewater
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treatment. 3.4.6. Comparative study
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For removal of FQs, different adsorbents have been developed. Here, a comparative
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study was preliminarily performed to compare the adsorption capacity. It is reported that commonly, the carbon-based materials showed higher adsorption capacity, due to
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their large specific surface area and oxygen-containing groups, such as activated carbon
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with the adsorption capacity of 289 mg/g for Cip and 275 mg/g for Enr (Fu et al., 2017) and ZIF-8 derived nanoporous carbon with 416.7 mg/g for Cip (Li, Zhang, & Huang, 2017). However, most of the adsorbents could not be comparable with the as-prepared hydrogel. For example, polyacrylic acid-grafted-carboxylic graphene/titanium nanotube showed the adsorption capacity of 13.40 mg/g for Enr (Anirudhan, Shainy, & Christa, 2017), functionalized magnetic microsphere NiFe2O4 had the adsorption capacity of 14.45 mg/g for Cip and 14.49 mg/g for Enr (Liu, Liu, & Zhang, 2018), and 16
ACCEPTED MANUSCRIPT sodium alginate/graphene oxide composite beads exhibited the adsorption capacity of 86.12 mg/g for Cip (Yu, Li, Han, & Ma, 2016). Generally, the adsorption capacity is obtained under different operation conditions, and then it is not an absolute indicator for evaluating the adsorption performance. However, such a comparative study may
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provide the data support and reference for designing an adsorbent with excellent
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performance.
4. Conclusions
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A 3D structured hydrogel CTS-PAA was developed as the adsorbent for efficient
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removal of Cip and Enr by a batch of experiments. The experimental results indicated that CTS-PAA had the best adsorption capacity at pH 3.0 and, the electrostatic attraction
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was predominant during the adsorption process. Further, the pseudo-second order
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kinetic equation and the Langmuir equation were suitable for analyzing the adsorption kinetics and isotherms. Finally, the as-developed adsorbent could be easily regenerated
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and the recovered adsorbent showed a comparable adsorption capacity during five
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consecutive adsorption-desorption cycles. In conclusion, CTS-PAA can be used as a promising adsorbent to remove Cip and Enr from aqueous solution.
Acknowledgements This work was jointly supported by the National Natural Science Foundation of China (No. 21477135); the Gansu Provincial Natural Science Foundation of China (No. 17JR5RA205); and the Fundamental Research Funds for the Central Universities (No. 17
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lzujbky-2017-209).
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ACCEPTED MANUSCRIPT Figure captions: Fig. 1. Molecular structure of Cip and Enr. Fig. 2. SEM images of CTS-PAA before (a, a’) and after the adsorption for Cip (b, b’) and Enr (c, c’). a-c, ×100 magnification with the scale bar of 100 μm; a’-c’, ×1000
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magnification with the scale bar of 10 μm.
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Fig. 3. SEM image and corresponding element mappings of CTS-PAA before (a) and
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after the adsorption for Cip (b) and Enr (c).
Fig. 4. FTIR spectra of CTS-PAA before (a) and after the adsorption for Cip (b) and
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Enr (c).
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Fig. 5. Adsorption capacity (a) and equilibrium pH (b) as a function of initial pH. Adsorption conditions: C0, 50 mg/L; t, 180 min; pH, 2.0-9.0; adsorbent amount, 1.0 g/L.
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Fig. 6. Effect of contact time on the adsorption capacity at three initial concentrations.
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Adsorption conditions: C0, 10 mg/L (a), 50 mg/L (b) and 250 mg/L (c); t, 1-180 min; pH, 3.0; adsorbent amount, 1.0 g/L.
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Fig. 7. Adsorption capacity as a function of initial concentration (a) and equilibrium
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concentration (b). Adsorption conditions: C0, 10-600 mg/L; t, 180 min; pH, 3.0; adsorbent amount, 1.0 g/L. For the fitting curves, the Langmuir, Freundlich and Redlich-Peterson isotherms are expressed by solid line, dash line and dot line, respectively. Fig. 8. Effects of ion strength on the adsorption capacity. Adsorption conditions: C0, 50 mg/L; t, 180 min; pH, 3.0; adsorbent amount, 1.0 g/L. Fig. 9. Reusability of CTS-PAA for five consecutive adsorption-desorption cycles. 28
ACCEPTED MANUSCRIPT Adsorption conditions: C0, 50 mg/L; t, 180 min; pH, 3.0; adsorbent amount, 1.0 g/L.
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Desorption conditions: 0.1 mol/L HCl for 120 min at 30oC/120 rpm.
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ACCEPTED MANUSCRIPT Table 1. Adsorption kinetic parameters of two antibiotics using CTS-PAA as the adsorbent. Pseudo-first order kinetic
Pseudo-second order
equation
kinetic equation
Initial qe,exp
k2
concentration qe,cal
k1
qe,cal
(mg/L)
R (mg/g)
40.34
250
6.56
0.9222
18.99
0.0303
116.8
0.0302
2.16
0.0341
0.8222
50
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Enr
219.3
∙min
-
1
)
5.17× 0.9985 10-2 4.30×
41.67
0.9997 10-3 7.08×
0.9852
185.2
0.9989 10-4 7.93×
0.9029
6.68
0.9989 10-2 6.14×
22.45
0.0694
0.8547
39.37
0.9994 10-3 6.15×
129.5
0.0335
0.9245
227.3
0.9998 10-4
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250
38.16
R2
1
7.33
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10
178.6
0.0380
(mg/g)
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50
2.97
(min )
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Cip
7.18
-1
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10
(g∙mg-
2
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(mg/g)
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Adsorbates
30
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Table 2. Adsorption isotherm parameters of two antibiotics using CTS-PAA as the adsorbent. Langmuir equation qm
b
K R2
(mg/g)
n
267.7
0.9802
26.26
2.592
3.43× Enr
387.7
0.9306
46.84
2.655
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10-2
31
0.8875
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10-2
Kr
a
(L/g)
(L/mg)
R2
((mg/g)(mg/L)n)
(L/mg) 1.97×
Cip
Redlich-Peterson equation
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Adsorbates
Freundlich equation
0.7905
b
R2
1.170
0.9819
1.219
0.9335
5.67×
4.132 10-3 8.29× 10.42 10-3
ACCEPTED MANUSCRIPT Highlights > 3D hydrogel by grafting chitosan with functional monomer acrylic acid was prepared. > Influencing factor and adsorption mechanism of FQs on hydrogel were investigated.
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> Fast, higher and reusable adsorption characteristics of hydrogel were obtained.
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Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Ningfen Wang, Weilong Xiao, Bihui Niu, Wenzhen Duan, Lei Zhou, Yian Zheng PII: DOI: Reference:
S0167-7322(18)35752-0 https://doi.org/10.1016/j.molliq.2019.02.061 MOLLIQ 10462
To appear in:
Journal of Molecular Liquids
Received date: Revised date: Accepted date:
7 November 2018 11 January 2019 11 February 2019
Please cite this article as: N. Wang, W. Xiao, B. Niu, et al., Highly efficient adsorption of fluoroquinolone antibiotics using chitosan derived granular hydrogel with 3D structure, Journal of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.02.061
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Highly efficient adsorption of fluoroquinolone antibiotics using chitosan derived granular hydrogel with 3D structure
Gansu Key Laboratory for Environmental Pollution Prediction and Control, College of Earth and
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a
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical
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b
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Environmental Sciences, Lanzhou University, Lanzhou, 730000, China
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Engineering, Lanzhou University, Lanzhou, 730000, China
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ABSTRACT
Here, a granular hydrogel prepared from chitosan as the grafted backbone and acrylic
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acid as the polymerizable monomer was used as the adsorbent to remove two typical
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fluoroquinolone antibiotics, i.e. ciprofloxacin and enrofloxacin. The parameters affecting the adsorption capacity were investigated via a series of adsorption
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experiments, including pH (2.0-9.0), contact time (1-180 min), initial concentration
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(10-600 mg/L), and ion strength (1 mM and 10 mM NaCl/CaCl2). The results showed that the granular hydrogel possessed porous 3D structure and, the ionizable characterization of -COO- groups made the adsorbent be pH-sensitive and the maximum adsorption occurred at pH=3.0. Moreover, the presence of abundant -COOgroups within 3D structured network provided the adsorbent with higher adsorption
Corresponding authors. E-mail addresses: [email protected] (L. Zhou), [email protected]
(Y. Zheng) 1
ACCEPTED MANUSCRIPT capacity of 267.7 mg/g for ciprofloxacin and 387.7 mg/g for enrofloxacin. When the initial concentrations were 10, 50 and 250 mg/L, the equilibrium time was determined to be 30, 60 and 120 min, respectively. After five consecutive adsorption-desorption cycles, a higher adsorption percentage (>85%) was still obtained, demonstrating that
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the developed adsorbent could be easily regenerated and reused for several times.
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Keywords: Ciprofloxacin; Enrofloxacin; Fluoroquinolone antibiotics; Adsorption;
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Hydrogel
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1. Introduction
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Antibiotics, treated as effective drugs for treating infections of human and veterinary and growth promoter for livestock and agriculture, are used normally (Kümmerer,
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2009). As reported, their usage approximately is between 1×105 and 2×105 tons
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annually globally, especially in China where the amount of antibiotic consumption is as high as 2.48×105 tons in 2013 (Lu et al, 2018; Wang et al, 2017). However, the
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antibiotics cannot be completely metabolized by users, and also, they are frequently
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treated using some ineffective methods. Accordingly, various antibiotics have been frequently detected in aquatic environment, such as lake, reservoir, river, and even drinking water (Li, Shi, Gao, Liu, & Cai, 2013; Chen, Jing, Teng, & Wang, 2018). Relative to other antibiotics, fluoroquinolones (FQs) are usually considered to be medium to high risk, and their concentration is reported within the range from ng/L up to mg/L in aquatic environment (Li, Zhang, Wu, & Zhao, 2014; Michael et al, 2013). These FQs existing in the water can pose potential threats to aquatic organisms and 2
ACCEPTED MANUSCRIPT human health, such as antibiotic resistance or even genetic mutations in aquatic organisms and human health abnormalities through the enrichment of food chain (Zhang et al., 2018; Mckinney, Loftin, Meyer, Davis, & Pruden, 2010). Since FQs are mainly existed in domestic and medical wastewater, and are easily adsorbed by sludge,
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they are hardly removed by conventional methods (Gao et al., 2012; Narumiya, Nakada,
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Yamashita, & Tanaka, 2013). Therefore, it is essential to find highly efficient methods
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to remove FQs from water environment.
Among many available methods, adsorption technology is featured with simple
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design and easy operation, and thus considered to be one of highly efficient methods
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for removal of different pollutants from aquatic environment. The core using this technology is to find an appropriate adsorbent material. Up to date, a variety of
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adsorbents have been developed, and among available adsorbents, carbonaceous
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materials have been widely reported for their abundant pore structure and high specific surface area, such as activated carbon (Fu et al., 2017), carbon nanotubes and graphene
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oxide (Li, Zhang, Han, & Xing, 2014; Yu, Li, Han, & Ma, 2016). Given the higher
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cation exchange capacity, different clay-based materials have also been developed for removal of FQs, such as kaolinite, montmorillonite and illite (Wang, Li, & Jiang, 2011; Li et al., 2011). Recently, biochar-based materials have also attracted increasing attention for removing FQs (Li et al., 2017; Wang, Jiang, Yi, & Yang, 2017). Nonetheless, most of them cannot separate entirely from water, have the defects of easy agglomeration, or present limited adsorption capacity for FQs. Nowadays, the 3D structured hydrogels have attracted increasing attention in wastewater treatment (Tran, 3
ACCEPTED MANUSCRIPT Park, & Lee, 2018). For instance, chitosan/biochar hydrogel beads had been reported to remove ciprofloxacin (Cip) (Afzal et al., 2018). Nanobeads-based polypyrrole hydrogel was prepared to adsorb and catalyze rhodamine (Yao et al., 2018). Cubic polyhedral oligomeric silsesquioxane nano-cross-linked hybrid hydrogels were
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developed to remove methylene blue (Eftekhari-Sis, Rahimkhoei, Akbari, & Araghi,
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2018). Compared with other reported adsorbents, the hydrogels present the following
SC
advantages: fast adsorption kinetics, high adsorption capacity, easy separation and regeneration. In addition to wastewater treatment, the 3D structured hydrogels have
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also widely applied to other fields such as biological sensor (Li, Guan, & Zhang, 2018),
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electronic device adhesive (Wang et al., 2018), wound dressing (Lin et al., 2017), tissue engineering (Park, Lee, Lee, Park, & Lee, 2018) and agriculture (Cheng, Liu, Yang, &
D
Zhang, 2018).
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Actually, hydrogel itself possesses a 3D structured network that can be formed by any water-soluble or hydrophilic polymer via chemical or physical crosslinking and
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traditionally classified into two major categories (natural and synthetic) depending on
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their source. Among hydrogels, the most important one type comes from natural polysaccharides and their grafted polymers, such as cellulose (Geng, 2018), alginate (Li, Ma, Chen, Huang, & He, 2018), starch (Shahbazi, Majzoobi, & Farahnaky, 2018) and chitosan (CTS) from chitin (Vakili et al, 2014; Vakili et al, 2016; Yan et al., 2018). Chitin is the main component of crustacean shells, and has been discarded as a byproduct of seafood processing industry (Zheng, & Wang, 2012). CTS is the deacetylated chitin and thus has the advantage of being inexpensive, easy to obtain and can be 4
ACCEPTED MANUSCRIPT modified by grafting for application in different fields. Based on above backgrounds, CTS was selected as the backbone in this study to obtain a granular hydrogel with 3D structure by grafting polyacrylic acid (PAA) via free radical solution polymerization. The resulting material was marked as CTS-PAA
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and by combination of scanning electron microscope (SEM) and Fourier transform
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infrared spectrum (FTIR), a well characterization was carried out to reveal its
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morphology and components. Subsequently, its adsorptive behaviors towards Cip and enrofloxacin (Enr) were systematically investigated via a series of adsorption
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experiments, including pH from 2.0 to 9.0, contact time up to 180 min, initial
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concentration from 10 to 600 mg/L, and ion strength in 1 mM and 10 mM NaCl/CaCl2 solutions. Subsequently, the pseudo-first order and pseudo-second order equations, as
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well as three adsorption models including Langmuir, Freundlich and Redlich-Peterson
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equations, were applied to the experimental data to understand better the adsorption process. Finally, the reusability was evaluated for exploring its feasibility in a practical
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application.
2. Materials and methods 2.1. Materials
Ciprofloxacin (Cip, 98%) and enrofloxacin (Enr, 98%) were purchased from J&K Scientific, Beijing, China, and their molecular structures were shown in Fig. 1. Chitosan (CTS, with an degree of deacetylation of 0.85 and average molecular weight of 3.0× 105) was provided by Yuhuan Ocean Biology Co., Ltd., Zhejiang, China. Acrylic acid 5
ACCEPTED MANUSCRIPT (AA, chemically pure) was obtained from Shanghai Shanpu Chemical Factory, Shanghai, China. Ammonium persulfate (APS, analytical grade) was received from Xi’an Chemical Reagent Factory, Shanxi, China. N,N’-methylene-bisacrylamide (MBA, chemically pure) was the product of Shanghai Chemical Reagent Factory, Shanghai,
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China. Other reagents including acetic acid, HCl, NaOH, NaCl and CaCl2 were
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analytical purity and used as received. Distilled water was used for preparing the
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solutions throughout the experiment. 2.2. Adsorbent preparation
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The preparation process of chitosan grafted poly(acrylic acid) (marked as CTS-PAA)
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can refer to our previous study (Zheng, Zhang, & Wang, 2009). Typically, in a 5L double-layer glass reactor equipped with a stirrer, a condenser, a thermometer and a
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nitrogen line, an appropriate amount of CTS dissolving in 1% (v/v) acetic acid was
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added. When nitrogen was released in above reactor for 30 min, the CTS solution was heated to 60oC under oxygen-free atmosphere conditions, followed by adding APS
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solution. As CTS was initiated to produce radicals, the mixture consisting
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polymerizable monomer AA and crosslinker MBA was introduced into the solution when the mixture was heated to 70oC and reacted for 3h. When the polymerization process was finished, the resulting granular product was immersed in NaOH solution for neutralization to pH 6.0-7.0, and then dehydrated with industrial alcohol and finally dried at 70oC in an oven to a constant weight. During the experiment, the weight percentage of CTS, APS and MBA was fixed to 10 wt%, 2 wt% and 3 wt% in the feed, respectively. Prior to use, the product was milled and the sample through 40-80 mesh 6
ACCEPTED MANUSCRIPT was collected. 2.3. Batch adsorption experiments Typically, 20 mg as-prepared adsorbent was contacted with 20 mL solution containing different FQs in 50 mL Erlenmeyer flask and the mixture was then placed
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in a shaker (THZ-100, Shanghai Yiheng Scientific Instrument Co., Ltd.) for a certain
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time at 30oC/120 rpm. When the adsorption process was complete, the adsorbent was
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separated from the solution by direct filtration and the residual concentration of FQs was obtained by the changes in the maximum absorbance at 275 nm by using a UV-Vis
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spectrophotometer (TU-1810PC). After that, the corresponding adsorption capacity for
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different FQs was calculated according to the differences in the concentration before and after the adsorption.
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Subsequently, a series of experiments were performed to investigate the influences
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of different variables on the adsorption properties. Firstly, the pH-dependence was studied at the initial concentration of 50 mg/L and pH ranging from 2.0 to 9.0 by
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adjusting with 0.1 and 1.0 mol/L HCl or NaOH solutions. After 180 min, the
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supernatant of different FQs was sampled and the pH value was recorded with a pH meter (FE20, Mettle-Toledo Instrument, Co., Ltd.). Next, the effect of contact time on the adsorption capacity was carried out to investigate the adsorption kinetics, by varying the contact time up to 180 min at pH of 3.0 and three concentration levels of 10, 50 and 250 mg/L. Then, the adsorption isotherms were investigated by varying the initial concentration of FQs from 10 mg/L to 600 mg/L at pH of 3.0 and contact time of 180 min. Finally, the influence of ion strength on the adsorption capacity was performed by 7
ACCEPTED MANUSCRIPT using 1 mM and 10 mM NaCl and CaCl2 solution as the background solvents. In this case, the initial concentration of FQs was 50 mg/L and pH was fixed at 3.0. 2.4. Reusability When an adsorption process was finished, the adsorbent was separated and the
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supernatant was analyzed to obtain the initial adsorption capacity, i.e. 1st adsorption
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capacity. Then, the adsorbent was immersed into 20 mL of 0.1 mol/L HCl to desorb
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these FQs molecules at 30oC/120 rpm/120 min. Followed, the adsorbent was separated from the desorbing solution and neutralized in 0.1 mol/L NaOH for 10 min. After
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regeneration, the recovered adsorbent was washed for several times with distilled water
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and then used for the next adsorption. Here, the reusability was performed for completing adsorption-desorption experiments for at least five times.
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2.5. Characterization method
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The adsorbent samples before and after the adsorption were analyzed by scanning electron microscopy (SEM) and Fourier transform infrared spectrum (FTIR) to reveal
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their surface morphology, element mapping in the micro-area of adsorbent, and
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functional groups attached onto the adsorbent.
3. Results and discussion 3.1. SEM analysis Fig. 2 shows the surface morphology of granular hydrogel before and after the adsorption for Cip and Enr. Significantly, original hydrogel presents a sponge-like 3D structured morphology, and such a pore structure will contribute to the adsorption 8
ACCEPTED MANUSCRIPT kinetics and then facilitate the adsorption process. After the adsorption, the surface morphology of adsorbent becomes significantly different. Contrastively, the adsorbent surface appears quite compact and the pore size is significantly reduced at ×1000 magnification, and also, the particle size is evidently diminished at ×100 magnification.
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This is reasonable for that with the adsorption of Cip and Enr, the 3D structured network
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of CTS-PAA will be occupied by these antibiotic molecules. Furthermore, the
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interaction between the adsorbent and adsorbates will result in decreasing ionic functional groups of CTS-PAA via free counter ions, by which the repulsion between
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size and a diminishing particle size.
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these ionic groups from CTS-PAA will be reduced, leading thus a shrinking 3D network
3.2. Element mappings
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The element mappings of granular hydrogel are presented in Fig. 3, corresponding
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respectively to the samples before and after the adsorption for Cip and Enr. It is observed that C and O elements are visible on the surface of original hydrogel, while
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additional N and F elements are found for hydrogel after the adsorption, testifying the
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presence of Cip or Enr attached onto the hydrogel, i.e. Cip and Enr have been successfully adsorbed by CTS-PAA. 3.3. FTIR analysis
Grafting modification is a promising way to obtain various utilizations of polysaccharides, such as CTS in this study, and more details on the grafting reaction between the reactants can refer to our previous study by using FTIR analysis (Zheng, Huang, & Wang, 2011). Here, only the main characteristic bands that can reflect the 9
ACCEPTED MANUSCRIPT adsorption of Cip and Enr are presented, as shown in Fig. 4. For original hydrogel, the characteristic absorption bands, assigning respectively to asymmetric and symmetric vibration of -COO- groups, have been found at 1565 and 1407 cm-1. After the adsorption, above two absorption bands are getting weaken, while the absorption band indexing to
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-COOH groups at ca. 1720 cm-1 is strengthened (Movasaghi, Yan, & Niu, 2019). This
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increase in the absorption intensity is originated from (i) the transform of -COO- groups
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to -COOH groups for the hydrogel, for the adsorption experiment is conducted at pH 3.0, and (ii) the adsorption of Cip and Enr onto the 3D structured network of CTS-PAA,
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for their structures contain the functional -COOH groups. Further, some new absorption
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bands are observable in the spectra of Cip- and Enr-loaded hydrogel. The band at 1629 cm-1 is arisen from the vibration of phenyl framework conjugated to -COOH groups
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attached onto the antibiotic molecules. The band at ca. 1270 cm-1 is assigned to coupling
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of the carboxyl C–O stretching and O–H deformation (Trivedi, & Vasudevan, 2007). Besides, several small bands in the range of 1000–500 cm-1 are also ascribed to the
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presence of FQs, such as 802 and 892 cm-1 (Gunasekaran, Rajalakshmi, & Kumaresan,
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2013). Based on the changes in characteristic absorption bands, it can be deduced that Cip and Enr have been successfully adsorbed by CTS-PAA. 3.4. Adsorption properties In current study, we use CTS as the backbone to graft PAA, obtaining a granular hydrogel CTS-PAA via free radical solution polymerization. CTS is the second polysaccharide ranked only second to cellulose, and then it is a low-cost, biocompatible, biodegradable and nontoxic natural polymer and has been focused on by researchers 10
ACCEPTED MANUSCRIPT for many years (Wang et al., 2019). However, CTS shows the defects of non-acidbase resistance property and weak mechanical strength (Zeng et al., 2019). Nevertheless, CTS can be easily modified by grafting reaction for its highly reactive amino and hydroxyl groups. Here, AA is selected as the functional monomer because
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of fast polymerization speed and easily occurred addition reaction (Pakdel, &
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Peighambardoust, 2018), and a general reaction mechanism between CTS and AA can
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be proposed as three stages: (i) the initiator APS is decomposed under the heating to produce a sulfate anion radical, (ii) the resulting anion radical abstracts hydrogen atom
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from –NH2 or –OH groups attached onto the backbone of CTS to form macro-radicals,
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(iii) the resulting macro-radicals initiate the polymerization reaction of AA to graft the PAA chains on the backbone of CTS. By then, we obtain the product CTS-PAA that has
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crosslinked polymeric network and abundant carboxylic groups. This kind of adsorbent
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material is thus expected to show higher affinity to Cip+ and Enr+ via electrostatic interaction, and then presents excellent adsorption capacity towards them.
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3.4.1. Effect of pH
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The pH of the antibiotic solution is a critical parameter influencing the adsorption process. As shown in Fig. 5a, the adsorption capacity showed a remarkable increase by increasing the initial pH from 2.0 to 3.0, beyond which a gradual decrease in the adsorption efficiency was observable. At pH=3.0, the maximum adsorption capacity was obtained. One can speculate that the electrostatic interaction is responsible for this phenomenon. According to the experimental data, when an initial pH from 3.0 to 9.0 was used, the equilibrium pH was determined to be 4.3-6.7 for Cip and 5.5 and 7.1 for 11
ACCEPTED MANUSCRIPT Enr after the adsorption (Fig. 5b). Then, when CTS-PAA is immersed in the solution, the functional groups within the 3D structured network can be ionized easily by water permeation. With increasing the pH values, the degree of deprotonation is getting higher, by which the carboxylate ions are dominant in the solution (Pakdel, & Peighambardoust,
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2018). But for Cip and Enr, the two FQs have two relevant ionizable functional groups,
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the 3-carboxyl group and N-4 of the piperazine substituent, and accordingly, two pKa
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values are commonly reported for each antibiotic, with 5.90 and 8.89 for Cip and 6.27 and 8.30 for Enr (Picó, & Andreu, 2007). At pH 3.0, there is predominantly a cationic
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form in the solution for both types of antibiotic molecules, while -COO- groups are
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predominant within polymeric network of CTS-PAA. At that time, a strong electrostatic attraction occurs between the negatively charged -COO- groups from CTS-PAA and
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positively charged adsorbates, i.e. Cip+ and Enr+. At pH 2.0, the available functional -
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COO- groups within the 3D polymeric network of CTS-PAA are completely converted into -COOH groups, by which the repulsion between the negatively charged -COO-
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groups within the 3D polymeric network of CTS-PAA will be minimized, making the
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3D network size of hydrogel shrink. And accordingly, the interaction between CTSPAA and adsorbates is greatly weakened and then a sudden decrease in the adsorption capacity is observable. With increasing pH from 3.0 to 9.0, the electrostatic attraction is decreased between the adsorbent and adsorbate, while the electrostatic repulsion is gradually dominated, leading also a lower adsorption capacity. 3.4.2. Effect of contact time Under three concentration levels, the influences of contact time on the adsorption 12
ACCEPTED MANUSCRIPT capacity were investigated, as shown in Fig. 6. Observably, when the initial concentrations were 10, 50 and 250 mg/L, a fast adsorption stage was all within 30 min, and thereafter the time to reach equilibrium were found to be 30, 60 and 120 min, respectively.
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Further, the experimental data was analyzed using pseudo-first order and pseudo-
ln(𝑞e − 𝑞t ) = ln 𝑞e − 𝑘1 𝑡
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Pseudo-first order kinetic equation:
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second order kinetic models. Their linear equations can be expressed as below:
(2)
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Pseudo-second order kinetic equation:
(1)
where qe and qt (mg/g) are the adsorption capacity at equilibrium and at time t (min),
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respectively. k1 (min-1) and k2 (g·mg-1·min-1) correspond to rate constant of these two 𝑞t
against t, the kinetic parameters can be
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obtained, as listed in Table 1.
𝑡
D
equations. By plotting ln(𝑞e −𝑞t ) and
Obviously, the pseudo-second order kinetic model is more suitable for analyzing the
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experimental data for higher regression coefficients (R2>0.998) and smaller differences between calculated adsorption capacity (expressed as qe,cal) and experimental one
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(expressed as qe,exp). Further, the initial adsorption rate increases as the initial concentration increases, and this fact is mainly attributed to that increasing the concentration is equivalent to increasing the driving force, by which the adsorption rate is also accelerated. Here, it should be mentioned that CTS-PAA shows higher adsorption capacity to Cip at lower concentration (10 mg/L), comparable adsorption
13
ACCEPTED MANUSCRIPT capacity for Cip and Enr at medium concentration (50 mg/L) and higher affinity to Enr at higher concentration (250 mg/L). 3.4.3. Effect of concentration Equilibrium adsorption isotherms that are used to reflect the relationship between
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equilibrium concentration and adsorption capacity are important for understanding the
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mechanism of an adsorption processes. For Cip and Enr, when the initial concentration
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of the adsorbate increased from 10 to 600 mg/L, the adsorption capacity gradually reached a maximum (Fig. 7a), when the maximum adsorption capacity for Cip and Enr
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were found to be 229.7 and 339.6 mg/g, respectively. Fig. 7b showed the adsorption
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isotherm by plotting adsorption capacity against equilibrium concentration, from which the saturated adsorption capacity was gradually achieved with increasing the
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concentration.
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To clearly understand the adsorption process, three adsorption models including Langmuir, Freundlich and Redlich-Peterson equations were applied to the experimental
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data, as shown as follows:
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Langmuir equation:
qe
qmbCe 1 bCe
(3)
Freundlich equation:
qe KCe1/ n
(4)
Redlich-Peterson equation:
qe Kr C/e ( 1 a bCe )
(5)
where 𝑞e (mg/g) and 𝐶e (mg/L) are the adsorption capacity and equilibrium concentration, respectively. 𝑞m (mg/g) is the maximum adsorption capacity. Other parameters are the adsorption constants of different isotherms. 14
ACCEPTED MANUSCRIPT All the parameter and regressions coefficients are summarized in Table 2, from which the Redlich-Peterson model showed the highest correlation coefficient R2, it is, however, not a suitable isotherm for describing the experimental data due to lager b values. Comparatively, the correlation coefficient R2 from Langmuir isotherm is close to that
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from Redlich-Peterson model, indicating that the adsorption behavior of Cip and Enr is
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closer to the Langmuir model. That is, such an adsorption process occurs mainly in the
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form of monolayer adsorption on the homogeneous surface of an adsorbent. From the Langmuir isotherm, the maximum adsorption capacity is calculated to be 267.7 mg/g
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for Cip and 387.7 mg/g for Enr.
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3.4.4. Effect of ion strength
To evalute the effect of ion strength, 1 mM and 10 mM NaCl and CaCl2 were used
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to prepare antibiotic solution to perform a comparative study in different media. Firstly,
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the conductivity was determined to be 136 μs/cm for 1 mM NaCl, 1222 μs/cm for 10 mM NaCl, 233 μs/cm for 1 mM CaCl2 and 2030 μs/cm for 1 mM CaCl2. That, the ion
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strength follows the order of 1 mM NaCl<1 mM CaCl2<10 mM NaCl<10 mM CaCl2.
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As shown in Fig. 8, the adsorption capacity of two antibiotics showed a monotonic decrease by increasing the ion strength. This can be attributed to the competition of adsorbate molecules and ions from background solution for the available adsorption sites on the adsorbent surface. This also proves that the electrostatic interaction is the principal adsorption mechanism between the adsorbent and adsorbate, consistent with pH-dependent results. In addition, the divalent cations, i.e. Ca (II) in this study can form 1:1 complex with FQs by ion–dipole interaction with the 4-keto oxygen and the ionized 15
ACCEPTED MANUSCRIPT 3-carboxylic acid group, and by then, the adsorption capacity is also greatly affected (Duan, Wang, Xiao, Zhao, & Zheng, 2018). 3.4.5. Reusability To evaluate the reusability, five consecutive adsorption-desorption experiment was
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carried out, and the experimental results were exhibited in Fig. 9. Clearly, the as-
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prepared hydrogel was recyclable for removal of Cip and Enr in this study, rendering
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that after five cycles, the as-prepared adsorbent showed still higher adsorption percentage for these two antibiotics. As a consequence, this hydrogel is promising as
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an alternative adsorbent for application in the field of FQs-containing wastewater
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treatment. 3.4.6. Comparative study
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For removal of FQs, different adsorbents have been developed. Here, a comparative
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study was preliminarily performed to compare the adsorption capacity. It is reported that commonly, the carbon-based materials showed higher adsorption capacity, due to
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their large specific surface area and oxygen-containing groups, such as activated carbon
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with the adsorption capacity of 289 mg/g for Cip and 275 mg/g for Enr (Fu et al., 2017) and ZIF-8 derived nanoporous carbon with 416.7 mg/g for Cip (Li, Zhang, & Huang, 2017). However, most of the adsorbents could not be comparable with the as-prepared hydrogel. For example, polyacrylic acid-grafted-carboxylic graphene/titanium nanotube showed the adsorption capacity of 13.40 mg/g for Enr (Anirudhan, Shainy, & Christa, 2017), functionalized magnetic microsphere NiFe2O4 had the adsorption capacity of 14.45 mg/g for Cip and 14.49 mg/g for Enr (Liu, Liu, & Zhang, 2018), and 16
ACCEPTED MANUSCRIPT sodium alginate/graphene oxide composite beads exhibited the adsorption capacity of 86.12 mg/g for Cip (Yu, Li, Han, & Ma, 2016). Generally, the adsorption capacity is obtained under different operation conditions, and then it is not an absolute indicator for evaluating the adsorption performance. However, such a comparative study may
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provide the data support and reference for designing an adsorbent with excellent
SC
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performance.
4. Conclusions
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A 3D structured hydrogel CTS-PAA was developed as the adsorbent for efficient
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removal of Cip and Enr by a batch of experiments. The experimental results indicated that CTS-PAA had the best adsorption capacity at pH 3.0 and, the electrostatic attraction
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was predominant during the adsorption process. Further, the pseudo-second order
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kinetic equation and the Langmuir equation were suitable for analyzing the adsorption kinetics and isotherms. Finally, the as-developed adsorbent could be easily regenerated
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and the recovered adsorbent showed a comparable adsorption capacity during five
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consecutive adsorption-desorption cycles. In conclusion, CTS-PAA can be used as a promising adsorbent to remove Cip and Enr from aqueous solution.
Acknowledgements This work was jointly supported by the National Natural Science Foundation of China (No. 21477135); the Gansu Provincial Natural Science Foundation of China (No. 17JR5RA205); and the Fundamental Research Funds for the Central Universities (No. 17
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lzujbky-2017-209).
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ACCEPTED MANUSCRIPT Figure captions: Fig. 1. Molecular structure of Cip and Enr. Fig. 2. SEM images of CTS-PAA before (a, a’) and after the adsorption for Cip (b, b’) and Enr (c, c’). a-c, ×100 magnification with the scale bar of 100 μm; a’-c’, ×1000
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magnification with the scale bar of 10 μm.
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Fig. 3. SEM image and corresponding element mappings of CTS-PAA before (a) and
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after the adsorption for Cip (b) and Enr (c).
Fig. 4. FTIR spectra of CTS-PAA before (a) and after the adsorption for Cip (b) and
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Enr (c).
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Fig. 5. Adsorption capacity (a) and equilibrium pH (b) as a function of initial pH. Adsorption conditions: C0, 50 mg/L; t, 180 min; pH, 2.0-9.0; adsorbent amount, 1.0 g/L.
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Fig. 6. Effect of contact time on the adsorption capacity at three initial concentrations.
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Adsorption conditions: C0, 10 mg/L (a), 50 mg/L (b) and 250 mg/L (c); t, 1-180 min; pH, 3.0; adsorbent amount, 1.0 g/L.
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Fig. 7. Adsorption capacity as a function of initial concentration (a) and equilibrium
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concentration (b). Adsorption conditions: C0, 10-600 mg/L; t, 180 min; pH, 3.0; adsorbent amount, 1.0 g/L. For the fitting curves, the Langmuir, Freundlich and Redlich-Peterson isotherms are expressed by solid line, dash line and dot line, respectively. Fig. 8. Effects of ion strength on the adsorption capacity. Adsorption conditions: C0, 50 mg/L; t, 180 min; pH, 3.0; adsorbent amount, 1.0 g/L. Fig. 9. Reusability of CTS-PAA for five consecutive adsorption-desorption cycles. 28
ACCEPTED MANUSCRIPT Adsorption conditions: C0, 50 mg/L; t, 180 min; pH, 3.0; adsorbent amount, 1.0 g/L.
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Desorption conditions: 0.1 mol/L HCl for 120 min at 30oC/120 rpm.
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ACCEPTED MANUSCRIPT Table 1. Adsorption kinetic parameters of two antibiotics using CTS-PAA as the adsorbent. Pseudo-first order kinetic
Pseudo-second order
equation
kinetic equation
Initial qe,exp
k2
concentration qe,cal
k1
qe,cal
(mg/L)
R (mg/g)
40.34
250
6.56
0.9222
18.99
0.0303
116.8
0.0302
2.16
0.0341
0.8222
50
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Enr
219.3
∙min
-
1
)
5.17× 0.9985 10-2 4.30×
41.67
0.9997 10-3 7.08×
0.9852
185.2
0.9989 10-4 7.93×
0.9029
6.68
0.9989 10-2 6.14×
22.45
0.0694
0.8547
39.37
0.9994 10-3 6.15×
129.5
0.0335
0.9245
227.3
0.9998 10-4
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250
38.16
R2
1
7.33
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10
178.6
0.0380
(mg/g)
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50
2.97
(min )
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Cip
7.18
-1
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10
(g∙mg-
2
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(mg/g)
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Adsorbates
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Table 2. Adsorption isotherm parameters of two antibiotics using CTS-PAA as the adsorbent. Langmuir equation qm
b
K R2
(mg/g)
n
267.7
0.9802
26.26
2.592
3.43× Enr
387.7
0.9306
46.84
2.655
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10-2
31
0.8875
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10-2
Kr
a
(L/g)
(L/mg)
R2
((mg/g)(mg/L)n)
(L/mg) 1.97×
Cip
Redlich-Peterson equation
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Adsorbates
Freundlich equation
0.7905
b
R2
1.170
0.9819
1.219
0.9335
5.67×
4.132 10-3 8.29× 10.42 10-3
ACCEPTED MANUSCRIPT Highlights > 3D hydrogel by grafting chitosan with functional monomer acrylic acid was prepared. > Influencing factor and adsorption mechanism of FQs on hydrogel were investigated.
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> Fast, higher and reusable adsorption characteristics of hydrogel were obtained.
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Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9