Biobased Nanographene Oxide Creates Stronger Chitosan Hydrogels

Research Article pubs.acs.org/journal/ascecg

Biobased Nanographene Oxide Creates Stronger Chitosan Hydrogels with Improved Adsorption Capacity for Trace Pharmaceuticals Zhaoxuan Feng, Antonio Simeone, Karin Odelius, and Minna Hakkarainen* Department of Fibre and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 58, 100 44 Stockholm, Sweden ABSTRACT: A promising green strategy for the fabrication of fully biobased chitosan adsorbents for wastewater purification is presented. Nanographene oxide (nGO)-type carbon dots were derived from chitosan (nGOCS) or from cellulose (nGOCL) through a two-step process including microwave-assisted hydrothermal carbonization and oxidation. Finally, nGO were evaluated as biobased property enhancers in chitosan hydrogel adsorbents. Macroporous chitosan hydrogels were synthesized by cross-linking with genipin, and the incorporation of nGO into these hydrogels was shown to facilitate the cross-linking reaction leading to more robust 3D cross-linked networks. This was evidenced by the increased storage modulus and by the swelling ratio that decreased from 5.7 for pristine chitosan hydrogel to 2.6 for hydrogel with 5 mg/mL nGOCS and 3.3 for hydrogel with 5 mg/mL nGOCL. As a further proof of the concept the hydrogels were shown to be effective adsorbent for the common anti-inflammatory drug diclofenac sodium (DCF). Here, the addition of nGO promoted the DCF adsorption process leading to 100% removal of DCF after only 5 h. The synergistic effect of electrostatic interactions, hydrogen bonding, and π−π stacking could explain the high adsorption of DCF on the hydrogels. The developed biobased CS/nGO hydrogels are thus promising adsorbents with great potential for purification of trace pharmaceuticals from wastewater. KEYWORDS: Graphene oxide, Carbon dot, Hydrogel, Diclofenac sodium, Genipin, Microwave, Waste water purification, Adsorbent

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INTRODUCTION Transformation and valorization of widely available, abundant, renewable waste/biomass into energy, green chemicals, and functional materials is emerging as a notable trend as well as a prerequisite for sustainable, low-carbon, and circular economy.1 Chitosan (CS), a natural polysaccharide consisting of Dglucosamine and N-acetyl-D-glucosamine units, is derived in massive quantities from the exoskeleton of crustaceans. As a functional biopolymer with characteristics including nontoxicity,2 biocompatibility,3 biodegradability,4 and chelation ability,5 chitosan is used to prepare hydrogels, sponges, capsules, or films with focus in the biomedical and environmental domains. Graphene oxide (GO), a derivative of graphene, contains abundant hydrophobic π-domains and hydrophilic oxygen functionalities such as carboxyl, epoxy, and hydroxyl groups in the basal plane, and these functionalities are especially abundant at the edges of the GO sheet.6 The amphiphilic character has enabled good compatibility of GO with various polymer matrices, imparting GO with multifunctionality in polymer composites. Nanographene oxide, as its name suggests, is defined as the GO sheets with lateral width in the nanoscale.7 Compared to two-dimensional (2D) GO, the zero-dimensional (0D) nanographene oxide is especially suitable for biomedical applications such as cellular imaging8 and drug delivery9−11 due to its ultrasmall size. The common approach for synthesizing nanographene oxide was inspired from the modified Hummer’s © 2017 American Chemical Society

method with graphite as the precursor under chemical exfoliation.7,9−12 Recently, chitosan-based composites in the form of hydrogels, beads, or films with 2D GO as property enhancer has been explored. CS/GO composite films were reported to have better tensile strength and storage modulus than the pristine CS films.13 A self-assembly method has also been applied to fabricate CS/GO hydrogels with good adsorption performance toward organic dyes including Eosin Y, methylene blue, and heavy metals such as Cu2+ and Pb2+.14 Sponge-like reduced GO/CS/montmorillonite hydrogels were also synthesized and applied as effective Cr5+ adsorbents.15 In addition, CS/GO aerogels have been fabricated and were shown to be good adsorbents for copper ions.16 Previously, our group developed a microwave-assisted hydrothermal carbonization strategy to synthesize carbon nanospheres (CN) from cellulose,17 starch,18 and waste paper.19 We also demonstrated that these CN can be further transformed to 0D nanographene oxide-type carbon dots (nGO) through a simple oxidation route.20,21 nGO was also proven as an effective property enhancer for bioplastics22 and biomedical scaffolds.23,24 Further, knowing that conventional 2D GO is an effective adsorbent for dyes,25 heavy metals,26 and Received: August 15, 2017 Revised: October 2, 2017 Published: October 30, 2017 11525

DOI: 10.1021/acssuschemeng.7b02809 ACS Sustainable Chem. Eng. 2017, 5, 11525−11535

Research Article

ACS Sustainable Chemistry & Engineering pharmaceutical antibiotics,27 we hypothesized that biobased nGO could improve the properties and adsorption capacity of chitosan hydrogels to enable fabrication of fully biobased highly effective adsorbents. Therefore, we aimed to expand the versatility and functionality of nGO by utilizing chitosan as a raw material source for nGO production and to compare this novel nGO to a cellulose-derived nGO analogue. As a subsequent step we created a “proof-of-concept” for biobased nGO as a property enhancer in genipin cross-linked chitosan hydrogel adsorbents. This was done by evaluating the effect of nGO on the cross-linking reaction, hydrogel properties, and their adsorption capacity. Diclofenac sodium (DCF) was utilized as a model pharmaceutical for the adsorption study. This methodology offers a green and eco-friendly strategy that begins with the preparation of fully biobased CS/nGO hydrogels and ends with the application of the hydrogels for removal of harmful pharmaceutical contaminants from wastewater.

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Table 1. Composition of the Prepared Hydrogels sample denomination

CS [mg/mL]

GP [mg/mL]

nGOCS [mg/mL]

GP2/nGO0 GP2/nGOCS5 GP2/nGOCL5

10 10 10

2 2 2

5

nGOCL [mg/mL]

5

spectrometer (Norwalk, CT) equipped with an attenuated total reflectance (ATR) crystal accessory (Golden Gate). The FTIR spectra were recorded with 20 scans in the range between 600 and 4000 cm−1. Morphological characterization of nGOCS and nGOCL was performed by ultra-high-resolution FE-SEM scanning electron microscopy (Hitachi S-4800). The samples were dried and sputtered with ∼7 nm gold layers. Surface elementary information on nGOCS and nGOCL was provided by energy-dispersive X-ray spectroscopy (EDS) equipped with an 80 mm2 X-Max large area silicon drift detector sensor (Oxford Instrument Nanotechnology). X-ray photoelectron spectroscopy (XPS) analysis was performed on a Kratos Axis Ultra DLD electron spectrometer with a monochromated Al Kα source to collect information concerning the chemical composition of the surface of CNCS, CNCL, nGOCS, and nGOCL. For the wide spectra analyzer and individual photoelectron line, pass energies of 160 and 20 eV were applied, respectively. A spectrometer charge neutralization system was utilized to stabilize the surface potential. The binding energy (BE) scale was set at 285.0 eV for referencing the C 1s line of aliphatic carbon. The thermal stability of nGOCS and nGOCL was examined by a Mettler-Toledo TGA/SDTA 851e with thermogravimetric analysis (TGA). A 3−4 mg amount of each sample was placed into a 70 mL alumina cup. The samples were heated from 30 to 500 °C at a rate 10 °C min−1 in a N2 atmosphere. Dynamic Light Scattering (DLS) analysis of nGOCS and nGOCL was performed on a Zetasizer Nano Series from Malvern Instruments (Malvern, UK) to get information on the size and zeta potential of nGOCS and nGOCL. Samples were prepared by dispersing the nGO dots in milli-Q water (0.1 mg/mL) with mild sonication for 10 min. The average sizes of nGOCS and nGOCL were also measured from the transmission electron microscopy (TEM) images by virtue of the ImageJ software. Around 500 particles were chosen for each measurement. The TEM images were obtained by a Hitachi HT7700 (high-contrast mode). Aqueous solutions of nGOCS and nGOCL (0.1 mg/mL nGO were dispersed in milli-Q water) were drop casted on ultrathin carbon-coated copper grids. Characterization of Hydrogels. To investigate the cross-linking reaction between the genipin and the chitosan matrix as well as to investigate how nGOCS and nGOCL influenced the cross-linking of the hydrogel system, FTIR spectra of the samples were obtained by a PerkinElmer Spectrum 2000 FTIR spectrometer (Norwalk, CT). The morphology of the cross section of the lyophilized hydrogel samples was investigated with FE-SEM (Hitachi S-4800). Samples were sputtered with 7 nm gold on the surface. The swelling behavior of the hydrogels in deionized water was evaluated by immersing 70−80 mg of sample into a vial with 10 mL of deionized water. The test was performed in room temperature for 48 h until a constant weight was reached. For each hydrogel, three parallel samples were tested. The equilibrium swelling ratio was calculated by considering the average value of three measurements according to eq 1

EXPERIMENTAL SECTION

Materials. Chitosan (CS) (medium molecular weight), α-cellulose (CL) (commercial grade containing 3% of pentose), diclofenac sodium (DCF, 99%), sulfuric acid (H2SO4, 95%−98%), and nitric acid (HNO3, 70%) were obtained from Sigma-Aldrich. Acetic acid (CH3COOH, 99.5%) was purchased from Acros Organics. Genipin (≥98%) was provided by Linchuan Zhixin Bio-Tech. Ethanol (CH3CH2OH, 96%) was received from VWR Chemicals. All chemicals were used without any further purification. Synthesis of Nanographene Oxide (nGO) Type Carbon Dots from Chitosan and α-Cellulose. Microwave-assisted hydrothermal carbonization of chitosan or α-cellulose was carried out in a Milestone flexiWAVE multivessel microwave platform. The process was a modification of a previous process used for α-cellulose,17 starch,18 and spent coffee grounds.28 Six vessels were prepared with 2 g of chitosan or α-cellulose and 20 mL of H2SO4 (0.1 g/mL) in each. The vials were closed airtight and thereafter placed into the microwave platform. The temperature was initially set up to reach 200 °C over a 20 min time period and then kept isothermally at 200 °C for 2 h. The power used was 1200 W. After cooling, the vials were opened and the carbon nanosphere (CN) intermediates were separated by filtration in a Bushnell funnel and purified by washing with deionized water several times. Finally, CN was dried in the vacuum oven at 25 °C for 3 days. CN derived from chitosan and α-cellulose was denoted as CNCS and CNCL, respectively. For further oxidation of CN to nGO, a solution of CN in nitric acid (10:1, w/v) was kept in a round-bottom flask and sonicated in a sonication bath for 30 min. The solution was heated at 90 °C for 60 min with continuous magnetic stirring, after which 150 mL of deionized water was added to the solution to stop the reaction. Vacuum distillation was applied to remove the aqueous solution, and finally, the orange-brown nGO was obtained after a freeze-drying process. nGO derived from chitosan and α-cellulose was denoted as nGOCS and nGOCL, respectively. Preparation of Chitosan/nGO Hybrid Hydrogels. The hydrogels were prepared by first sonicating nGOCS or nGOCL suspensions in deionized water for 10 min and then mixing a given amount of CS solution in acetic acid (2.5% v/v), genipin (GP) solution in ethanol (60% v/v), and nGO solution in deionized water by magnetic stirring for 20 min. After that the mixed solution was poured into a circular Petri dish (50 mm in diameter and 20 mm in height), which was covered with Parafilm. The cross-linking took place in a 37 °C oven for 48 h. The compositions and abbreviations of all of the prepared hydrogels are shown in Table 1. Characterization of nGOCS and nGOCL. The changes in functional groups after the carbonization process and oxidation reaction were monitored by Fourier transform infrared spectroscopy (FTIR). Neat chitosan, α-cellulose, CNCS, CNCL, nGOCS, and nGOCL were analyzed by a PerkinElmer Spectrum 2000 FTIR

Q eq = (meq − m0)/m0

(1)

where m0 is defined as the initial weight of the hydrogel and meq refers to the weight of the hydrogel after swelling for 48 h. The rheological properties of the hydrogels were characterized by the TA Discovery Hybrid 2 (DHR-2) rheometer equipped with an 8 mm stainless steel Peltier plate. All hydrogels underwent a frequencysweep test from 0.01 to 10 Hz at 25 °C. The strain rate was set to be 1% to ensure all samples were tested in the linear viscoelastic region. Adsorption of Diclofenac Sodium (DCF) as a Model Pharmaceutical Compound. For the hydrogel adsorption study a 11526


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Scheme 1. Schematic Route Starting from the Synthesis of nGOCS and nGOCL and the Hydrogel Preparation and Ending at the Application for Adsorption of Trace Pharmaceuticals

Figure 1. Appearance and morphological characterization of the obtained carbon nanospheres (CN) and nanographene oxide (nGO). (a) Photos of chitosan, α-cellulose, CNCS, CNCL, nGOCS, and nGOCL; (b) SEM images of CNCS, CNCL, nGOCS, and nGOCL; (c) SEM images of nGOCS sheets with self-assembled corrugated structure. stock DCF solution with a concentration of 0.1 mg/mL was prepared by dissolving 100 mg of DCF powder into 1 L of milli-Q water. This solution was then diluted into a DCF solution of 0.01 mg/mL for further use. A 50 mg amount of hydrogel samples was evenly cut into small pieces by a stainless steel sieve with 18 meshes and added into 10 mL of DCF solutions with a concentration 0.01 mg/mL. Solutions were shaken at a speed of 400 rpm at room temperature. After predetermined time intervals, solutions were immediately filtered through 0.45 μm syringe filters, and the DCF concentration was determined by the absorbance at 276 nm in the ultraviolet−visible (UV−vis) spectrum. The removal efficiency of DCF was calculated by eq 2

Removal efficiency(%) = (C0 − C t)/C0 × 100

systems for trace pharmaceutical adsorption (Scheme 1). The synthesis and structure of nGOCS and nGOCL and their function in the hydrogel systems for purification of trace pharmaceuticals will be discussed. Diclofenac sodium (DCF), an anti-inflammatory drug commonly present in wastewater, was utilized as a model compound for the adsorption study. Transformation of Chitosan and α-Cellulose to nGOType Carbon Dots. Chitosan and α-cellulose were first successfully carbonized to carbon nanospheres (CNCS and CNCL) by a microwave-assisted process. As illustrated by the photos and SEM images in Figure 1, the solid black CN formed are observed as clusters of smooth spherical particles with heterogeneous sizes. After oxidation in the microwave using nitric acid for oxidation and exfoliation under the O-rich acidic environment, the 3D CN were further broken down into 0D nGO-type carbon dots, scaling in the nanometer region in all dimensions (Figure 1b). The conventional 2D graphene oxide sheets naturally possess a hierarchical structure endorsing them as “building blocks” for high-performance graphene products through self-assembly.29 In accordance it can be assumed that the nGO dots can self-associate into wrinkled GO sheets with

(2)

where C0 is the initial DCF concentration (mg/mL) at 0 min and Ct is the DCF concentration (mg/mL) at a certain time t min. To identify how nGO influences the adsorption efficiency and to understand the adsorption mechanism, the adsorption kinetics was studied.

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RESULTS AND DISCUSSION Chitosan- and cellulose-derived nGO, nGOCS and nGOCL, respectively, were synthesized from chitosan and α-cellulose and evaluated as property enhancers in chitosan-based hydrogel 11527


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Figure 2. FTIR spectra of (a) chitosan, CNCS, and nGOCS and (b) α-cellulose, CNCL, and nGOCL.

Figure 3. EDS spectra of chitosan, CNCS, nGOCS, α-cellulose, CNCL, and nGOCL.

“‘peak and valley”’ undulations30−32 as evidenced from Figure 1c. nGOCS was dark brown in color, and nGOCL portrayed a slightly lighter brown color which could indicate a higher amount of oxygen functionalities in nGOCL (Figure 1a)33,34 as the higher oxidation progress decreases the conjugation of the π system making the color lighter. Changes in the chemical structures during transformation from chitosan and α-cellulose to nGOCS and nGOCL were verified by FTIR spectra (Figure 2a) and EDS images (Figure 3). After the microwave carbonization reaction, CNCS and CNCL were created from chitosan and α-cellulose. This was reflected by clear changes in the corresponding FTIR spectra. The broad peak in the range from 3500 to 3000 cm−1 indicating a hydroxyl peak became broader and weaker as compared to the spectra of the original biopolymers (Figure 2a), which is ascribed to the decrease of aliphatic hydroxyl functionalities and formation of some carboxylic acid units

during the formation process of 5-HMF and its further polymerization process to carbon nanospheres (CN). As found previously, the side group −NH2 on the glucosamine can be replaced with OH ions to form glucose in an acid environment with H2O as catalyst.35 The formed glucose will further degrade to 5-HMF. EDS spectra of neat chitosan and CNCS also support this theory due to the decrease in the relative amount of N atoms after carbonization (Figure 3). The IR spectra of CNCS and CNCL also showed the formation of aromatic structures by the appearance of sp2hybridized CC stretching in the range from 1600 to 1500 cm−1. For CNCS, some acetyl glucosamine units remained after microwave treatment as evidenced from the peaks at 1691 cm−1 corresponding to the amide CO stretching, and this was ascribed to the incomplete deacetylation process from chitin to chitosan. For CNCL, the appearance of carboxylic groups was seen by peaks at 1300 and 1700 cm−1, which belong to C−O 11528


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Figure 4. (a) High-resolution XPS deconvoluted C 1s spectra of CNCS, nGOCS, CNCL, and nGOCL. (b) High-resolution XPS spectra of CNCS, nGOCS, CNCL, and nGOCL.

cm−1 for nGOCL) became stronger as compared to CNCS and CNCL. Complementary evidence can be seen from EDS spectra (Figure 3), which show that the C/O ratio decreased when CNCS and CNCL were oxidized to nGOCS and nGOCL indicating incorporation of oxygen functionalities. Meanwhile, the aromatic structure (sp2 CC stretching at 1546 cm−1 for nGOCS and at 1549 cm−1 nGOCL) was retained during the

stretching and CO stretching vibration on the carboxylic group. During the oxidation of CNCS and CNCL to nGOCS and nGOCL, the amount of oxygen-containing functional groups increased, which is evident by the IR spectra of nGOCS and nGOCL showing that peaks belonging to CO stretching (1716 cm−1 for nGOCS and 1717 cm−1 for nGOCL) and C− O−C stretching vibration (1217 cm−1 for nGOCS and 1222 11529


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comparison, 26% ash residue remained from nGOCL, whereas 33% ash residue remains from nGOCS at 500 °C, which could further be a reflection of the higher initial carbon content. Average nGOCS and nGOCL Size. The average size of nGOCS and nGOCL was measured statistically from TEM and SEM images with the help of ImageJ software (500 particles were counted in each measurement). Additionally, DLS was used to measure the hydrodynamic diameter by distributing nGOCS and nGOCL in milli-Q water (0.1 mg/mL). As measured from TEM and SEM, nGOCS and nGOCL showed average particle sizes in the range of 80−90 and 60−70 nm, respectively (Figures 1b2, 6a, and 6b). From DLS measure-

oxidation step. Although EDS spectra are not fully accurate for determining C/O ratios, Figure 3 clearly shows the trends where the oxygen functionalities decrease during the carbonization to produce CNCS and CNCL and then the oxygen functionalities increase again during the oxidation step to produce nGOCS and nGOCL. Figure 4a shows the XPS analysis performed on CNCS, CNCL, nGOCS, and nGOCL revealing further surface elemental information. The comparison of the XPS C 1s before and after oxidation shows a clearly increased amount of oxygen functionalities, which is in correlation with previous studies on cellulose- and starch-based CN and nGO20,21 and agrees with the changes in the relation between carbon and oxygen elements indicated by EDS (Figure 3). The calculated C/O ratios decreased from 5.1 to 3.2 when transferring from CNCS to nGOCS and from 4.3 to 1.6 when CNCL was oxidized to nGOCL. Components such as sp3 C−C, sp2 CC, C−O−C, C−OH, carbonyl >CO, carboxylic COOH, and ester OC−O were shown on the high-resolution XPS C 1s of both CN and nGO with different intensities and percentages. It is worth mentioning that there was a large increase in the carboxylic −COOH functionalities when CN was transformed from CN to nGO. This change was even more significant for nGOCL, again indicating a higher degree of oxygenation. At around 290 eV, both CNCS and CNCL demonstrated a π−π* shake-up satellite peak, which indicates delocalized π conjugation structures. Some nitrogen remained after carbonization of chitosan to CNCS due to the incomplete substitution of −NH2 by OH ion, while no nitrogen element was found on the surface of CNCL (Figure 4b). After the oxidation process, the trace amount of nitrogen element found on both nGOCS and nGOCL might originate from nitric acid used for oxidation. The thermal stability of nGOCS and nGOCL was evaluated by thermogravimetry. Thermogravimetric analysis showed that the major weight loss took place between 150 and 200 °C for both nGOCS and nGOCL (Figure 5). This is explained by the

Figure 6. Size of nGOCS and nGO-CL measured from (a and b) TEM and (c and d) DLS.

ment, the Z-average hydrodynamic diameters of nGOCS and nGOCL were around 100 and 80 nm, respectively (Figure 6c and 6d). These values show good correlations, although some particle size deviations were observed and are expected when different measuring methods and environments are applied. The hydrodynamic diameter of nGOCS and nGOCL in aqueous solution was slightly larger than the diameter of the dry sample particles, which is in agreement with previous studies when particle diameters were measured in solid and aqueous states.38,39 In addition, particle agglomeration could influence both measurements. Preparation and Characterization of CS/nGO Hydrogels. As the next step nGOCS and nGOCL were evaluated as property enhancers for chitosan hydrogels. Chitosan was crosslinked with genipin with or without inclusion of nGOCS or nGOCL. The prepared hydrogels demonstrated darkening blue-black color, especially in the presence of nGOCS or nGOCL, and gained solidity after 48 h reaction in the oven at 37 °C (Figure 7a1, 7a2, and 7a3). It should be noted that the change in color during the reaction is not caused by genipin itself, since it is colorless. Instead, the spontaneous formation of blue pigment is an inherent indication of a successful crosslinking reaction between genipin and the primary amine groups in chitosan.40,41 The postulated cross-linking mechanism is shown in Scheme 2.42 It was obvious that when nGOCS or nGOCL was included, a much deeper blue color and less transparent hydrogel was attained. In addition to using a Petri

Figure 5. Thermogravimetric analysis curves for nGOCS and nGOCL tested in N2 environment.

release of CO2, CO, and steam from the most labile oxygen functional groups.36,37 The somewhat higher weight loss for nGOCL during this temperature interval compared to nGOCS further supports the analyses showing a larger amount of oxygen functionalities in nGOCL.34 The minor weight loss exhibited at temperatures below 100 °C corresponds with the evaporation of the trapped water molecules in the nGO structure. Since nGOCL contained more oxygen functional groups, it is reasonable that its water content was slightly higher, which reflects the slightly larger initial weight loss. In 11530


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Figure 7. Photos and SEM images of the prepared hydrogel samples. (a1−a3) Photos of the prepared hydrogel GP2/nGO0, GP2/nGOCS5, and GP2/nGOCL5. (b1−b3) Photos of the initial liquid solutions for preparing the above hydrogels. (a4) Hydrogels prepared in the test tubes: (from left to right) GP2/nGO0, GP2/nGOCS5, GP2/nGOCL5. (b4) Photos of the initial liquid solutions for preparing the above hydrogels in the test tubes. (c1 and c2) SEM images of the lyophilized hydrogels GP2/nGOCS5 and GP2/nGOCL5, respectively.

Scheme 2. Mechanism of Genipin Cross-Linking Reaction with Primary Amine Groups on Chitosan

dish as hydrogel preparation mold, hydrogels were also prepared in test tubes as shown in Figure 7b4 to evaluate the homogeneity of the cross-linking reaction. Interestingly, after being incubated in a 37 °C oven for 2 days, the pristine chitosan hydrogel GP2/nGO0 (Figure 7a4) demonstrated a vertical color gradient along the test tube, transiting from light gray at the bottom to dark blue on the top. In comparison, the hydrogels GP2/nGOCS5 and GP2/nGOCL5 (Figure 7a4) showed evenly distributed dark blue color along the whole test tube. It has been indicated that the genipin-cross-linking reaction with the primary amine groups on chitosan is induced by the presence of oxygen radicals leading to a blue coloration.42 On the basis of this mechanism, we hypothesize that oxygen groups present in the nGO particles helped to catalyze the crosslinking reaction, resulting in a deeper and more homogeneous formation of the blue color and cross-linking. The interior architectures of the lyophilized hydrogels were characterized by SEM. As seen from Figure 7c, the hydrogels GP2/nGOCS5 and GP2/nGOCL5 showed 3D honeycomb networks consisting of interconnected macro pores. The large-volume pore structure endowed the hydrogel with large surface area and high adsorbent permeability which should benefit the subsequent adsorption process. To further monitor the structure and functionalities of the formed hydrogels, genipin and the hydrogel samples were examined by FTIR (Figure 8). The spectra of pristine chitosan, nGOCS, and nGOCL are illustrated in Figure 2. Chitosan shows its typical adsorption peaks at 3354, 1650, and 1576 cm−1, which correspond to the N−H stretching vibration of the

Figure 8. FTIR spectra of genipin (GP) and the hydrogels GP2/ nGO0, GP2/nGOCS5, and GP2/nGOCL5. FTIR spectra of neat CS and nGO are shown in Figure 2.

primary amine group, carbonyl (CO) stretching of the secondary amide, and N−H bending vibration of the Nacetylated residues.43 The formation of cross-linked networks was confirmed by the newly formed C−N stretching peaks of aromatic amines around 1300 cm−1 on GP2/nGO0, GP2/ nGOCS5, and GP2/nGOCL5. This indicates nucleophilic substitution of genipins C-3 carbon atom by the nitrogen atom from the primary amine on chitosan to form the heterocyclic tertiary amine. It is observed that the intensity of C−N stretching increased slightly for the hydrogels containing nGOCS or nGOCL as compared with GP2/nGO0. This further indicates that nGOCS and nGOCL can promote the cross-linking reaction. The appearance of a peak at 825 cm−1 in the spectrum of GP2/nGOCS5 corresponds to C−H stretching vibration associated with the heterocyclic ring. Compared with 11531


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ACS Sustainable Chemistry & Engineering pristine chitosan, the peaks representing the amide N−H bending were downshifted from the initial 1576 to 1542 cm−1 for GP2/nGOCS5 and 1545 cm−1 for GP2/nGOCL5 and increased in the intensity, which provided information about the formation of secondary amide through the nucleophilic substitution of the ester group on genipin by chitosan’s primary amine group. The peak at 1633 cm−1 in all three hydrogels corresponds to the CO stretching of the secondary amide group.43 Additionally, the disappearance of N−H stretching in all three hydrogels further evidence that chitosan’s primary amine had participated in the cross-linking reaction. Swelling Behavior of the Hydrogels in Deionized Water (DI Water). As shown in Figure 9, all three hydrogels

Figure 10. Storage modulus (G′) and loss modulus (G″) versus frequency curve of the hydrogel GP2/nGO0, GP2/nGOCS5, and GP2/nGOCL5.

Figure 9. Swelling ratios of GP2/nGO0, GP2/nGOCS5, and GP2/ nGOCL5 after 48 h in the DI water. Photos show the appearance of the hydrogel before and after swelling.

led to promotion of G′, which can be explained by formation of secondary cross-linking (intermolecular interactions) between nGO and chitosan. These could include the electrostatic forces between protonated amine side groups in chitosan chain and carboxyl groups on nGO and hydrogen bonding formed between amino groups on chitosan and hydroxyl groups on nGO. The slight differences in G″ between GP2/nGO0, GP2/ nGOCS5, and GP2/nGOCL5 are almost within the experimental error, but possibly the addition of nGO slightly increased the loss modulus. Adsorption of DCF by the Hydrogels. All three hydrogels exhibited fast and effective performance toward DCF adsorption, Figure 11. For the neat chitosan hydrogel GP2/nGO0, the adsorption percentage reached 59.5% after 1 h and 97.3% after 48 h. The addition of nGO promoted the DCF adsorption process and resulted in a removal percentage of

demonstrated significant water adsorption ability. The incorporation of nGOCS or nGOCL into the chitosan-based hydrogel systems enhanced the network stability and decreased the swelling ratio (Figure 9). This might be due to the improved cross-linking reaction in the presence of nGOCL and nGOCS which is evidenced by the color change difference when compared to the neat hydrogel with nGO-containing hydrogels as described above. Further, there could be secondary intermolecular interactions such as hydrogen bonding and electrostatic attraction between nGO and chitosan chains, thus increasing the cross-linking density of the hydrogel network. Rheological Behavior of the Hydrogels. The rheological properties of the hydrogels were evaluated by frequency-sweep oscillation tests with frequencies varying from 0.01 to 10 Hz. As shown in Figure 10, all three hydrogels demonstrated typical gel performance. Over the entire frequency sweeps, the storage modulus (G′) was always higher than the loss modulus (G″), which indicated that the elastic behavior was dominant for the hydrogels. Compared with previous studies, the values of moduli fall in the range typical for chitosan hydrogels.44,45 In general, except the slight fluctuating with increasing oscillating frequency, G′ and G″ were roughly constant without crossing over each other, which indicates formation of stable and consistent 3D networks.46 The addition of nGOCS or nGOCL

Figure 11. Study of DCF adsorption kinetics. (a) DCF removal efficiency (%) as a function of time. (b) Comparison of the DCF adsorption capacity of the three types of hydrogels over 30 min. Initial DCF concentration was 0.01 mg/mL, and experiment was carried out at room temperature. 11532


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sodium (DCF), a common pharmaceutical in wastewater. The adsorption kinetics demonstrated that the maximum percentage of DCF removal by the neat CS hydrogel reached 97.3% after 48 h, while the addition of either nGOCS or nGOCL greatly promoted the adsorption efficiency, and almost 100% of the DCF was removed after only 5 h. The synergistic effect of electrostatic interactions, hydrogen bonding, and π−π stacking could explain the high adsorption of DCF on the hydrogels. The developed biobased CS/nGO hydrogels are, thus, promising adsorbents with great potential for purification of trace pharmaceuticals from wastewater.

67.6% (for GP2/nGOCS5) and 72.3% (for GP2/nGOCL5) after 1 h and almost 100% after 5 h for both hydrogels with nGO (Figure 11a). The DCF adsorption curves rise steeply at the initial stage, indicating numerous active adsorption sites. As shown in Figure 11b, over one-half of the original DCF was removed by the hydrogels after only 30 min. DCF is actually a type of sodium salt, and when dissolved in the water, it will ionize to anion DCF− and Na+. Due to the presence of protonated amine groups in chitosan, DCF with negatively charged COO− can be strongly adsorbed and bound through electrostatic attraction. Moreover, the free hydroxyl groups on the chitosan can serve as the active adsorption sites toward DCF by hydrogen bonding.46−48 The presence of delocalized π domains and oxygen functional groups on the basal plane of nGO seem to have a synergetic effect on the interactions between adsorbents and DCF, which could be explained by the formation of π−π stacking49−51 and hydrogen bonding.52 A simple comparison concerning DCF removal efficiency for different adsorbents mentioned in the literature is presented in Table 2. Although a direct comparison may be subjective to

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Corresponding Author

*E-mail: [email protected]. ORCID

Minna Hakkarainen: 0000-0002-7790-8987 Notes

The authors declare no competing financial interest.

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Table 2. DCF Removal Percentage by Different Adsorbent Found in the Literature adsorbent precursor

DCF removal percentage (%)

contact time (h)

chitosan or cellulose cocoa shell

100 97.05

5 2

graphite powder cyclamen persicum tuber isable grape bagasse

75 72

6 2

22.8

35

oxidized activated carbon olive waste cake

40

24

93.7

26

AUTHOR INFORMATION

ACKNOWLEDGMENTS Zhaoxuan Feng gratefully acknowledges the financial support from the China Scholarship Council (CSC). Duo Wu and Nejla Erdal are thanked for the help with TEM analysis. Geng Hua and Karin H. Adolfsson are acknowledged for discussions during the research.

ref present work Saucier et al.53 Nam et al.54 Jodeh et al.55

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multiple parameters and different experimental conditions, it still gives some indication for the effectiveness of the DCF adsorption by the CS/nGO hybrid hydrogels. Among all of the compared adsorbents, the CS/nGO hybrid hydrogel synthesized in the present study demonstrates higher DCF removal percentage, which makes it a promising adsorbent for purification of DCF and other similar trace pharmaceuticals from wastewater.

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CONCLUSIONS Biobased nGO derived from chitosan (nGOCS) and αcellulose (nGOCL) was successfully synthesized through the microwave-assisted carbonization method. Green macroporous CS/nGO hybrid hydrogels were then fabricated by incorporating nGO into the genipin cross-linked chitosan hydrogel system. The results indicated that both types of nGO increased the cross-linking efficiency of genipin, thereby increasing the homogeneity of the formed hydrogels. nGOCS and nGOCL both promoted the robustness of the three-dimensional hydrogel network by demonstrating lower swelling ratios and increasing moduli as compared with pristine chitosan hydrogel. This is explained by strong secondary interactions such as hydrogen bonding and electrostatic interactions formed between nGO and chitosan matrix in combination with higher cross-linking density of the nGO-containing hydrogels. The hydrogels were shown to be effective adsorbents for diclofenac 11533


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Biobased Nanographene Oxide Creates Stronger Chitosan Hydrogels

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