Comparative Study of Graphene Hydrogels and Aerogels Reveals the

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Comparative Study of Graphene Hydrogels and Aerogels Reveals the Important Role of Buried Water in Pollutant Adsorption Jie Ma, Yiran Sun, Mingzhen Zhang, Mingxuan Yang, Xiong Gong, Fei Yu, and Jie Zheng Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b02227 • Publication Date (Web): 29 Sep 2017 Downloaded from http://pubs.acs.org on September 29, 2017

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Environmental Science & Technology

Comparative Study of Graphene Hydrogels and Aerogels Reveals the Important Role of Buried Water in Pollutant Adsorption Jie Maa, Yiran Suna, Mingzhen Zhangd, Mingxuan Yanga,d , Xiong Gongc, Fei Yub*, and Jie Zhengd* a

Prof. Jie Ma, Yiran Sun, Mingxuan Yang State Key Laboratory of Pollution Control and Resource Reuse, College of Environmental Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, China b

Dr. Fei Yu* College of Chemistry and Environmental Engineering, Shanghai Institute of Technology, Shanghai 2001418, China c

Xiong Gong College of Polymer Science and Polymer Engineering, The University of Akron, Akron, Ohio, USA 44325 d

Prof. Jie Zheng*, Mingzhen Zhang Department of Chemical and Biomolecular Engineering, The University of Akron, Akron, Ohio, USA 44325

Keywords: Graphene; Hydrogel; Adsorbent; Adsorption; Water Purification

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Abstract

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Water as the universal solvent has been well demonstrated its ability to dissolve as many as substances, but buried water inside different nanoporous materials always exhibits some unusual behaviors. Herein, 3D porous graphene hydrogel (GH) is developed as a super adsorbent to remove different pollutants (antibiotics, dyes, and heavy ions) for water purification. Due to its highly porous structure and high content of water, GH also demonstrated its super adsorption capacity for adsorbing and removing different pollutants (antibiotics, dyes, and heavy ions) as compared to conventional graphene aerogel (GA). More fundamentally, the buried-water enhanced adsorption mechanism was proposed and demonstrated, where buried water in GH plays the combinatorial roles as (1) supporting media, (2) transport nanochannels, and (3) hydrogen bondings in promoting pollutant adsorption. In parallel, molecular dynamics simulations further confirm that buried water in GH has the stronger interaction with pollutants via hydrogen bonds than other buried alcohols. GH integrates the merit of both graphene (e.g. fine chemical resistance, excellent mechanical property) and hydrogel (e.g. high water content, porous structure, and simple solution-based processability and scalability), making a promising potential for environmental applications.


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1. Introduction

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A great controversy between the huge demand of safe water and the limited resource of fresh water raise a severe global concern for water purification. Water purification has become an urgent global issue for human use, ecosystem, agriculture, and industry[1, 2]. Significant efforts and progress have been made to purify polluted water. However, it is still a challenging task to develop both green materials and effective technologies for water purification at lower cost, less energy, and more environmental safety. Among different chemical, physical, and biological methods for water purification, adsorption method has been considered as one of the most efficient ways, owing to its low cost, high efficiency, simplicity, and insensitivity to toxic substance [3-5]. As an inexpensive resource, can water itself be an adsorbent for water purification and how to utilize it? Graphene is often used as 2D adsorbents to remove organic pollutants from water due to its large specific surface area (SSA) (~2630 m2·g-1), rich physicochemical properties, and ease of synthesis and applicability [6]. However, graphene also suffers from substantial aggregation and poor distribution in aqueous solution, which greatly limits its adsorption capacity for practical applications. Of particular importance for environmental and biomedical applications, if graphene is not separated and recycled appropriately, it likely becomes potential contaminants. Nanoscale graphene was also reported to have toxicological effects on different cell lines of plants, animals, and even human [7]. Thus, use of the assembled graphenes to produce macroscopic, engineered structures with well-controlled configurations is a significant step towards benign environmental applications. To achieve this end, different self-assembly methods were developed to fabricate graphene aerogels (GA) as 3D macroscopic materials for water purification [8, 9]. While graphene aerogels prevent the agglomeration and restack of graphene sheets to some extents [10], GA still has several limits including weak mechanical strength, poor supporting capacity, low density, and excessive hydrophobicity. Moreover, since the GA synthesis process always involves a freeze-drying step to reduce the nano-sheet aggregation, it further increases the cost and complexity of GA synthesis. To overcome these limits, instead to synthesize and use GA as adsorbents for pollutants, we have developed a simple one-step hydrothermal method to synthesize graphene hydrogel granules [11] for pollutant adsorption and removal. We found that graphene hydrogel granules exhibited the much higher adsorption of ciprofloxacin than GA. However, our previous works did not offer answers to the adsorption mechanisms why GH is superior to GA for pollutant adsorption and removal. Motivated by this new phenomena, herein we synthesized bulk graphene hydrogels (GH) with 3D macroscopic structure and high water content. Different from graphene hydrogel granules, bulk graphene hydrogels have much larger macroscopic sizes and nanoscopic porous structures, which allow to examine the effect of buried water in 3 ACS Paragon Plus Environment


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GH on pollutant adsorption and removal in a large scale. The resultant GH was then used as super adsorbents to remove different pollutants including antibiotics, dyes, and heavy ions from aqueous solutions, with a more focus on ciprofloxacin antibiotics as a representative case. Unlike dry GA without any water, the high adsorption capacity of GH for pollutants was largely due to its large amounts of water (95 wt %) buried in the GH structures. Yu et al. synthesized the polyelectrolyte complexes (PECs)-loaded agarose hydrogel beads for the removal of heavy metal ions and ionic organic dyes from water. The resulting PEC-based adsorbents demonstrated not only the largest maximum adsorption capacity for ionic pollutants, but also easy regeneration for reuse by simple acidic or alkaline treatment [12]. The increase of buried water contents in GH led to the stronger adsorption capacity for pollutants. More importantly, the design and use of GH as super adsorbents offer several new aspects for pollutant removal: First, we developed a simple facile method, which is a time-saving and easily controlled process, to synthesize GH containing high water content inside macroporous graphene structures. In this way, we propose and demonstrate a new enhancement mechanism of pollutant adsorption, where buried water in GH acts as a highly active adsorbent to enhance pollutant adsorption. Second, GH not only makes full use of the specific surface area of graphene, but also prevents graphene agglomeration, both of which promote pollutant adsorption. Third, unlike conventional GA, GH can be more easily collected after the adsorption process via filtration or decantation, thus minimizing environmental issues. Apart from the super adsorption capacity of GH, GH also possessed high electrical and mechanical properties. Taken together, we believe that multifunctional GH system, accompanied with new synthesis method and new adsorption mechanisms, would facilitate the more fundamental and practical discoveries towards different environmental applications.

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2.1. Preparation of graphene hydrogel

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The graphite oxide was prepared according to a modified Hummer’s method[13]. Graphite oxide was dispersed in deionized water and sonicated in an ultrasound bath for 6 h to obtain graphene oxide (GO) aqueous dispersion (2.0 mg·ml-1). Then, ascorbic acid，was added to get a concentration of 2.0 mg·ml-1. After ultrasonic dispersion for 15 min to dissolve the ascorbic acid, 2.5 ml as-prepared dispersion was loaded in a glass vial and heated at 95 °C for 12 h without any disturbance. After the chemical reduction, the obtained GH was placed in deionized water for 12 h to remove the excess ascorbic acid. Then, graphene aerogel (GA) was made by a freezedrying process using GH. The as-prepared GH was soaked in t-butanol to exchange the buried water in GH, and then t-GA was made by freeze-drying process.

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2.2. Batch adsorption experiments

2. Materials and Methods


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Batch adsorption experiments were carried out in 50 ml volumetric capacity batch bottles with a GH and 20 ml ciprofloxacin solution of different initial concentrations. The timing of the sorption period started as soon as the solution was poured into the bottle. Sample bottles were shaken on a shaker (TS-2102C, Shanghai Tensuclab Instruments Manufacturing Co., Ltd., China) and operated at a constant temperature of 25 °C and 150 rpm for 5 days. The blank experiments without the addition of adsorbents were conducted to ensure that the decrease in the concentration was due to the adsorption, rather than other factors. After adsorption, adsorbent samples were filtered and diluted for UV-Visible spectroscopy measurements. The ionic strength experiments were conducted in 100 mg/L ciprofloxacin solutions with varying concentrations of NaCl solution (0, 0.001, 0.01, 0.1, 0.5, 1.0 mol/L). The effect of solution pH on ciprofloxacin removal was studied in the range of 2-12 with 100 mg/L initial concentrations of ciprofloxacin solutions. The initial pH values of all the solutions were adjusted using 0.1 mol·L-1 HCl or 0.1 mol·L-1 NaOH solution with desired concentrations. The absorption capacity of ciprofloxacin (qe, mg·g-1) was calculated as follows[14] qe 

 C0  Ce  V m

(1)

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where C0 and Ce are the initial and residual concentrations of ciprofloxacin (mg·L-1), V is the initial solution volume (L), and m is the adsorbent weight (g).

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2.3. Electro-adsorption tests

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The Cyclic Voltammetry (CV) test on the GH and GA were performed in 15 ml NaCl solution with used as electrolyte a concentration around 1M. The specific capacitance of a supercapacitor cell (Ct) was calculated using the equations

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Ct 

I m(dV / dt )

(2)

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where I is the constant discharge current, m is the total mass of GH in both electrodes, and dV/dt is the voltage scan rate[15].

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2.4. Characterization methods

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The microstructure and morphology of the samples from each step were observed by Scanning Electron Microscopy (SEM, Hitachi S-4800, Japan) and Environmental scanning electron microscopy (ESEM, FEI, QUANTA250, Netherlands). The functional groups and the related oxidation state on the surface of materials, X-ray photoelectron spectroscopy (XPS) analysis was carried out in a Kratos Axis Ultra DLD spectrometer, using monochromated Al Ka X-rays, at a base 5 ACS Paragon Plus Environment


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pressure of 1×10-9Torr. Survey scans determined between 1100 and 0 eV revealed the overall elemental compositions of the sample and regional scans for specific elements were performed. The peak energies were calibrated by placing the major C1s peak at 284.6 eV. Raman spectroscopy (JOBIN-YVON T64000) was used to characterize further the structural integrity of adsorbents. The concentration of ciprofloxacin was determined using UV-Vis spectrophotometer (Techcomp UV2310 II) at wavelengths of 270 nm. The surface functional groups were observed by FTIR (NEXUS, 670). The Cyclic Voltammetry (CV) measurements were conducted by using an electrochemical workstation (CHI Instruments 660D) in the range of -0.4 ~0.6 V.

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3. Results and Discussion

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3.1. Characterization of graphene hydrogels

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Figure 1 shows the macroscopic and microscopic structural morphologies of both GH and GA. Macroscopically, the as-prepared GH possessed high mechanical strength and free-shapeable plasticity (Figure 1a), while the GA was extremely light enough to stand on the top of pine needles (Figure 1b). Figure 1c displays the contrasting behaviors of GH and GA in both density and hydrophobicity, i.e. dense, hydrophilic GH sunk in water solution, while light, hydrophobic GA floated on water, suggesting that GH should have a better adsorption potential for contaminant dissolved in water than GA. Microscopically, GH displayed a porous structure via graphene nano-sheets staking (Figure 1d). Such porous structure in GH provides sufficient space for the accommodation of water and pollutants. Figure 2a shows and compares the X-ray diffraction (XRD) patterns of graphite, GO, GH, and GA samples. As a control, GO exhibited a sharp peak at 2θ=10.9° corresponding to the average interlayer space of ~0.81 nm between the stacked GO sheets. A large interlayer space tended to weaken the van der Waals interactions between GO sheets. After the chemical reduction and self-assembly process was conducted to convert GO into GH, this peak at 10.9° disappeared, instead a broad peak appeared at ~26.2°. This indicates that upon GO reduction, removal of oxygencontaining functional groups enhances π-π interactions and π-conjugated structure, both leading to a reduced interlayer space of ~0.33 nm[16]. Meanwhile, the broad peak reflected that graphene sheets were self-assembled into a largely random packing network[16, 17]. Different from GH, GA (2θ = 24.2°, d-spacing of ~0.370 nm) showed a slightly larger interlayer space than GH, which was presumably caused by solvent expansion during the freeze-drying process, thus resulting in smaller pores in GA. Figure 2b shows the Raman spectra of GO and GH. Both GO and GH displayed two major peaks: a D peak at ~1244 cm-1 was an indicator of the defective structures of carbon materials, while a G peak at ~1603 cm-1 was an indicator for the 6 ACS Paragon Plus Environment

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graphitization of carbon materials based on the E2g graphite mode [18]. So, the intensity ratio of the D peak to the G one (ID/IG) quantitatively defines the extent of defects of carbon materials (i.e. the larger ID/IG value, the poorer graphite structure of carbon materials). Two sharp spikes at 1300 and 1500 cm-1 demonstrated a high crystal degree of GO. It can also be seen that GH (1.15) had a higher value of ID/IG than GO (0.483). The increase of ID/IG in GH suggests the existence of some defects after the removal of a large amount of oxygen-containing functional groups during the GO reduction process [19]. Moreover, the appearance of 2D and D+G peaks in GH indicates that GH has the more compact, stacked sheet structure than GA [20].

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3.2. Adsorption properties of buried water in graphene hydrogels

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Ciprofloxacin, one of the most common antibiotics found in wastewater, was selected as a representative pollutant in this study. Figure 3a shows the equilibrium adsorption isotherms of ciprofloxacin on GH and GA. GH (~235.6 mg·g-1) had the much higher adsorption capacity for ciprofloxacin than GA (~42.2 mg·g-1). By fitting the experimental equilibrium adsorption data to the Langmuir and Freundlich models, the correlation coefficients (R2) were 0.985 for the Langmuir model and 0.919 for the Freundlich model, respectively (Table S1). The Langmuir model with the relatively higher R2 value is more appropriate to describe the adsorption of ciprofloxacin on GH. We also found that GH adsorption capacity can also be improved by changing the initial GO synthesis concentrations, resulting in the highest value of 308.3 mg/g. Figure 3c and Table S2 present a comparison of the maximum adsorption capacities (qm) of ciprofloxacin between our GH and other adsorbents obtained from the

To characterize reduction process of GO, Figure 2c-d shows the C1s deconvolution spectrum of GH and GO, respectively. Different carbon bonds of C-C (~284.6 eV), C-O (~286.5 eV), and C=O (~288.1 eV) were assigned to different functional groups of sp2 aromatic rings, epoxy/alkoxy, and carbonyl, respectively. Overall, the C1s spectra of GO and GH displayed similar peaks for C-C and C=C groups, but the oxygen-containing groups (especially for the C-O peaks) in GH were significantly decreased as compared to those in GO. Consistently, atomic element ratio of C/O increased from 2.7 for GO to 10.3 for GH. In summary, all spectrum data including FT-IR (Fourier Transform Infrared Spectroscopy) data (Fig. S1 in supplemental materials) demonstrate the successful deoxidization and reduction process by converting GO to GH [21]. Upon reduction, GH displays the more compact, π-conjugated graphene structures that help generate the smaller nanopores inside GH. GH also possessed excellent electric conductivity [22] (Figure S2a), and GH (43.61 F·g-1) had the two-time higher charge capacitance than GA (18.65 F·g-1) (Figure S2b) [23]. Since high water content in GH provides the better charge transport behavior, GH has a promising potential for a supercapacitor due to its excellent ion capacity and conductivity.



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literature. It is can be clearly seen that GH had a superior adsorption capacity as compared to other carbon-based adsorbents[24],[25],[26],[27],[28],[29],[30],[31]. Besides ciprofloxacin, we also applied GH to adsorb and remove (i) two dyes of methylene blue (MB) and methyl orange (MO) in Figure S3, (ii) two heavy ions of CrO42- and Cu2+ in Figure S4, and (iii) additional tetracycline antibiotics (antibacterial substances isolated from Streptomyces aureofaciens) in Figure S5. Figure S3-S5 clearly showed that GH always achieves much higher adsorption capacity for different pollutants at different concentrations than GA. Among a total of 12 cases tested (i.e. six different pollutants, each with two different concentrations), 7 cases showed that GH promoted the adsorption capacity by 113%-995%. In all cases we tested, GH showed significant advantages over GA as adsorbents for the removal of pollutants from aqueous solution. However, such adsorption enhancement was different for different pollutant simply because GH applies different adsorption mechanisms to different pollutants. For example, GH adsorbs Cu2+ mainly by electrostatic interaction and complexing formation, while GH adsorbs CIP by multiple interactions of hydrogen bonding, π-π electron donor–acceptor (EDA) interaction, hydrophobic interaction, and electrostatic interaction. Even for dyes, anionic dyes (MO) and cationic dyes (MB) need different active sites for adsorption. As a result, the three roles of water in GH as supporting media, (2) transport nanochannels, and (3) hydrogen bondings showed different levels of enhanced adsorption capacity for adsorbing and removing different pollutants (antibiotics, dyes, and heavy ions) as compared to graphene aerogel. It is well known that the environmental conditions (e.g. pH and ion strength) are significant factors for the removal of antibiotics in aqueous media. Figure 3b shows the effects of NaCl and pH on ciprofloxacin adsorption by GH. At low NaCl concentrations (0-0.1 M), GH retained its adsorption capacity for ciprofloxacin at a similar level of 210 mg·g-1. As NaCl concentrations increased to 0.5-1.0 M, the adsorption capacity for ciprofloxacin increased slightly by ~10%. It appears that the adsorption capacity of GH is not very sensitive to ionic strength over a wide range of NaCl concentrations (0-1M). Different from the electrostatic attraction-driven adsorption process by GO that shows a strong salt dependence on adsorption capacity[14], the electrostatic interactions between ciprofloxacin and GH are not a major driven force for adsorption. Instead, the addition of NaCl increases the osmotic pressure of adsorbate, which drives more ciprofloxacin transferring from the solution into GH, thus leading to the ciprofloxacin adsorption. Figure 3b showed that at an optimal pH of 6, GH achieved the maximal adsorption of ciprofloxacin (247.5 mg·g-1). This is not surprising, because the adsorption of ciprofloxacin on the surface of GH strongly depends on their charge states (i.e. pKa value). Ciprofloxacin is known to have two pKa values (pKa1=6.1 and 8 ACS Paragon Plus Environment

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pKa2=8.7) [32]. In highly acidic solutions (pH150 mg·g-1).

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3.3. Molecular dynamics simulations of ciprofloxacin adsorption on GH

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To better understand why the buried water in GH enhances ciprofloxacin adsorption on GH, we performed all-atom molecular dynamics (MD) simulations to study the adsorption process of ciprofloxacin on GH in three different solvents (i.e. water, methanol, and ethanol) using the NAMD program with the CHARMM force field [34, 35]. Simulation details can be found in Supporting Information. As shown in Figure 4a, ciprofloxacin was initially placed at ~14 Å above the graphene with a random orientation to mimic a desorption state. In water and methanol solutions, 9

Since most of adsorbents have finite adsorption capacity for pollutants in a fluid phase, it is equally important to regenerate the adsorbents for reuse. We also investigated the regenerability of GH for reuse as an adsorbent. GH loaded with pollutants were placed into a distillation water without adding any chemical reagent for 24 h. Then, GH was reused to adsorb the pollutants again. This simple process was repeated five times to test the regenerable adsorption ability of GH. The regenerability of GH for ciprofloxacin adsorption was shown in Figure S6. Starting with the saturated adsorption capacity of GH for ciprofloxacin at the first cycle (211.6 mg/g), adsorption capacity rapidly decreased to 77 mg/g at the second cycle and finally reached to 26.2 mg/g, leading to 12.4 % recovery of the original adsorption capacity. The relatively poor regenerability of GH is attributed to two factors that (i) the buried water in GH has the strong interaction with ciprofloxacin via hydrogen bonds than other buried alcohols, as shown in our MD (molecular dynamics) simulations and (ii) the regeneration capacity of GH is mainly driven by the concentration gradientinduced diffusion mechanism, which is a simple but inefficient method. Due to hydrogen bonding characteristics of the buried water in GH, we designed different experiments to improve the regeneration properties of GH using chemical agents (carbamide etc.) and heating treatment. As shown in the Table S8, heating treatment can improve desorption and regeneration properties of GH. Particularly, when the heating temperature was 95°C, the desorption capacity increased by ~300%, while the re-adsorption capacity increased by more than ~50%.

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ciprofloxacin quickly approached to and adsorbed on the graphene surface within only 5 ns, and remained its strong adsorption state for the rest of simulation time (40 ns). Due to the strong polarity of ethanol, it took much longer time (~22 ns) for ciprofloxacin to be adsorbed on the graphene. In all cases, upon adsorption, ciprofloxacin oriented its aromatic rings parallel to the graphene surface, and such orientation allowed ciprofloxacin to be adsorbed on the graphene via strong π-π interactions between ciprofloxacin and graphene [36]. Since hydrogen bonds between ciprofloxacin and solvent play an important role in stabilizing the adsorption state of ciprofloxacin, Figure 4b showed that the average number of hydrogen bonds was 11.9 between ciprofloxacin and water, 3 between ciprofloxacin and methanol, and 2.8 between ciprofloxacin and ethanol, respectively. This indicates that water has the stronger interactions with ciprofloxacin than methanol and ethanol due to its smaller size and higher polarity. The relationship between additional adsorption capacity and MD computational simulation results were shown in Figure S7. Overall, GH in water exhibited the much higher adsorption capacity for ciprofloxacin than GH in alcohols. This implies that (i) water-filling GH has the stronger interactions with ciprofloxacin than methanol- and ethanol-filling GH and (ii) the solvent plays a complement role in ciprofloxacin adsorption.

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3.4. Adsorption mechanisms of buried water in GH

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A simple comparison of adsorption capacity for ciprofloxacin on GA and GH reveals a significant difference (Figure 3a). Since the only difference between GH and GA is that GH contains a large content of buried water (>95 wt %), while GA does not. So, it is reasonable to derive that the buried water in GH may play certain roles in the adsorption of ciprofloxacin on GH: (a) the buried water in GH acts as a skeleton to support and expand three-dimensional porous network and (b) buried water in GH provides abundant active sites to interact with CIP via hydrogen bonds. (c) buried water in GH builds nanochannels for promoting the transportation and adsorption of contaminants. The primary proof experiments were conducted, the results and analysis were shown in Figure S8. To in-depth discuss this hypothesis, two series of control experiments were designed and conducted as below. Adsorption mechanism by buried water in GH. Ciprofloxacin contains one fluorine group as π-electron acceptor and two hydrophilic C=O groups as π-electron donors, while GH surface contains −OH and −COOH as n-electron donors and the πelectron-rich benzene ring as π-electron donors [37]. Our MD simulations showed that the n−π or π–π electron donor–acceptor (EDA) interaction is considered to be a predominant driving force for the sorption of ciprofloxacin on GH [38-40]. Similar results were also reported where ciprofloxacin molecules were adsorbed onto other carbon-based materials (i.e. activated carbon, carbon nanotubes) via π–π interactions [24, 32]. Due to the presence of hydrophilic groups in both ciprofloxacin and GH surface (-OH, COOH, C=O), hydrogen bond interaction between ciprofloxacin and 10 ACS Paragon Plus Environment

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GH surface and between ciprofloxacin and the buried water in GH may be another mechanism for the sorption of ciprofloxacin on GH because ciprofloxacin reached the maximum adsorption capacity on GH at neutral pH (Figure 3b). To further investigate the role of the buried water in GH as additional adsorbate in ciprofloxacin, we designed comparative tests for ciprofloxacin adsorption on GH using different solvents. The as-prepared GH was first soaked in either methanol or ethanol to exchange the buried water in GH, followed by the adsorption tests under the exchanged solvent conditions. The adsorption capacities for ciprofloxacin on GH and GA in three solvents were in a decreased order of water>methanol>ethanol (Figure 5a-b). Considering that the graphene is a major adsorbent in GH, to account for the contribution from the buried solvent in GH, we used t-butanol GA (t-GA) to exclude the contribution of graphene to adsorption capacity. In a sharp contrast, the adsorption capacity for CIP on t-GA in three solvents changed in a small range, illustrating that the adsorption capacity of t-GA was much less affected by solvents than GH. In addition, we have conducted experiments to test the bulk adsorption capacity of CIP in three solvents of water, methanol, and ethanol. As shown in Table S7, three different solvents showed the similar bulk solubility of CIP. However, when the solvents are introduced to GH, the buried water in GH exhibited much higher adsorption capacity for CIP than the buried ethanol and methanol. Therefore, after successfully excluding the influence of graphene sheets by using t-GA and bulk solubility, the main factor that affects adsorption capacity of GH is considered to be solvent buried in GH structure. Differences in adsorption capacity for ciprofloxacin between GH and GA and between different solvents reveal an important role of buried water in GH, indicating that the buried water in GH acts as an adsorbent to promote ciprofloxacin adsorption. Buried water in GH can be classified into two different states of free water and bound water [41]. Bound water in GH can be considered as surface functional groups, which provide a large amount of -OH to strongly interact with ciprofloxacin via hydrogen bondings and thus greatly enhance the adsorption capacity. More evidence was found by analyzing hydrogen bonds of different solvents in GH. The average number of hydrogen bonds of water, methanol, and ethanol in GH were 18.33, 4.67, and 3.26, respectively, all of which are significantly higher than that of hydrogen bonds in bulk water (~3.3), methanol (~1.9), and ethanol (~1.9) [42]. Figure 5c shows the solventinduced adsorption capacity for ciprofloxacin and reveals a linear relationship between the number of hydrogen bonds contributed by solvents and the additional adsorption capacity for ciprofloxacin. Buried water-enhanced adsorption capacity for ciprofloxacin can also be affected by the water content in GH. We further prepared the GH with different moisture contents by changing initial GO concentrations[43]. t-GA samples were also prepared at the same conditions for comparison. Figure 6a clearly showed that GH 11 ACS Paragon Plus Environment


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decreased its size as moisture. Figure 6b-c show equilibrium adsorption isotherms of GH and t-GA for ciprofloxacin, fitted by the Langmuir model as well. Table S6 shows the Langmuir isotherm parameters of GH and t-GA with different moisture contents as prepared with different GO concentrations. Overall, GH had the higher adsorption capacity for ciprofloxacin than t-GA at all GO concentrations tested (Figure 6b vs. 6c). As GO concentration increased, the adsorption capacity of ciprofloxacin on both GH and t-GA decreased. Moreover, the additional adsorption capacity as contributed by buried solvent in GH (qe, mg·g-1) is calculated by additional adsorption capacity= qe(GH) - qe(t-GA) where qe(GH) and qe(t-GA) are the maximum absorption capacity of ciprofloxacin (mg·g-1) on GH and t-GA, obtained from by the Langmuir model. Consistently, additional adsorption capacity showed a linear correlation with the moisture content of GH (Figure 6d), further supporting the adsorption enhancement by buried water in GH. Graphene surface is electron-rich π-system that can enhance the hydrogen bond or behave as acceptors with respect to most H-bond donors. Supportive skeleton network by buried water in GH. The preparation of GA involves a freeze-drying process, which would cause the aggregation and collapse of the 3D graphene network (Figure 2a). To avoid the potential effect of removal of freezing water on the network change of GH, t-butanol was selected as a solvent for freeze-drying because it experiences a very small volume change upon freezing [44]. Visual inspection of t-GA, GA, and GH (Figure 7e) showed that t-GA retained the almost same volume as GH, while GA shrunk its size due to the network collapse upon removal of freezing water. Consistent with optical images, t-GA showed the higher adsorption capacity of ciprofloxacin than GA, but still much less than GH (Figure 7a and Table S3). The relatively high adsorption capacity of t-GA was attributed to two factors: (i) the larger N2 adsorption/desorption amount of t-GA at all pressures tested (Figure 7c) and (ii) the higher internal and external surface areas, and the larger pore volume of t-GA (Figure 7d and Table S3). Water expansion by freezing indeed induces the collapse and destruction of 3D porous structures (Table S3), which indirectly demonstrates a supportive role of buried water in GH. Nevertheless, GH still removed more ciprofloxacin than t-GA, again confirming the enhanced adsorption capacity induced by the buried water in GH. Buried water in GH acting as the supportive media can also be supported by CV tests (Figure S2a), for GH showed a significant difference in specific capacitance from GA. The aggregation of graphene nanosheets in GA during freeze drying decreased the SSA and reduced active sites for electrolyte ions and water, while the buried water in GH props up a three-dimensional porous network which increased the SSA and realizes greater specific capacitances. Furthermore, The continuously filling 12 ACS Paragon Plus Environment

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liquid in GH offers highly efficient ion transport channels[45] during the capillary compression process, due to the fluid nature of the liquid.

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Acknowledgements

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This research was supported by the National Natural Science Foundation of China (grant nos. 21777118, 21577099, and 51408362). J.Z. thanks a fully financial support

Overall, graphene hydrogel was fabricated using a simple facile method, and its physicochemical properties were characterized to show large specific surface area (~334.8 m2·g-1), highly porous structure, and high water content (~90 wt%). GH as super-adsorbents can achieve an excellent adsorption capacity for different pollutants including antibiotics, dyes, and heavy ions (e.g. ciprofloxacin adsorption as high as 308.3 mg·g-1), superior to other carbon-based adsorbents. Such high adsorption capacity is attributed to a combination of its 3D porous network structure, buried water in GH, and underlying different adsorption mechanisms including π-π EDA interaction, hydrogen bonding, and hydrophobic interaction. More importantly, different from the traditional viewpoint, we conclude that buried water in GH plays three roles in contaminant adsorption, namely (1) supporting media, (2) building transport nanochannel and (3) hydrogen bondings. Additional evidence for the functions of buried water in GH is found in their strong interactions with ciprofloxacin by MD simulations and weak adsorption capacity for ciprofloxacin in alcohols as alternative solvents in GH. Compared to other carbon-based nano materials, GH can be easily collected after the adsorption process via filtration or decantation, minimizing environmental issues. This work highlights the great potential of graphene hydrogels as a unique nano scaffolds for pollutant removal, and offers a new viewpoint to better understand the important role of the buried water in nanostructures in pollutant adsorption. Supporting Information Supporting Information includes Figure S1-S8 and Table S1-S9. Materials, adsorption isotherm model, molecular dynamics simulation methods, FtIR and ATR-IR spectrum, electric conductivity and CV curves of GA, adsorption capacity of dyes and heavy metal ions on GH, recyclability of GH, relationship(left) between additional adsorption capacity and MD computational simulation results, adsorption capacity of GH and GA after different preparation methods, comparison of the adsorption capacities of ciprofloxacin onto various adsorbents, Langmuir, Freundlich models parameters of GH, GA and t-GA in different solvents, solubility data of CIP in different solvents. Author Information F.Y., email:[email protected]; J.Z., email:[email protected] The authors declare no competing ﬁnancial interest.



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from NSF (DMR-16074 75) and partial supports from NSF (CBET-1510099), and National Natural Science Foundation of China (NSFC-21528601).

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Figure 1. Optical images of (a, b) GA and (c) GA/GH, (d) ESEM of GH, (e) SEM of GA.



Figure 2. (a) XRD patterns of graphite, GO, and GH. (b) Raman spectra of GO, graphite, and GH. C1s deconvolution spectrum of (c) GO and (d) GH.


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Figure 3. (a) Equilibrium adsorption isotherms of GH, GA, and GH-0.1.GH-0.1 was prepared by 1mg/mL GO aqueous dispersion and exhibits the best adsorption capacity for CIP. (b) Effects of ionic strength and pH on the adsorption capacity for ciprofloxacin on GH. (c) Comparison of the adsorption capacities of ciprofloxacin onto various adsorbents collected from the literature.



Figure 4. (a) Time-dependent separation distances between ciprofloxacin and graphene surfaces. (b) Intermolecular hydrogen bonds between ciprofloxacin and different solvents of water, methanol, and ethanol.


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Figure 5. Equilibrium adsorption isotherms of (a) GH and (b) t-GA in different solvents. (c) Linear regression between additional adsorption capacity and hydrogen bonding number.



Figure 6. (a) Optical images of GH prepared with different solution concentrations of GO. Adsorption isotherms of (b) GH and (c) t-GA at different moisture conditions. (d) Linear regression between additional adsorption capacity and moisture content.


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Figure 7. (a) Equilibrium adsorption isotherms of GA and t-GA. (b) Comparison of adsorption capacity of GH, GA and t-GA. (c) N2 adsorption/desorption isotherms and (d) pore size distribution of GA and t-GA. (e) Optical images of GH, t-GH, and GA.



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Table of Content


Comparative Study of Graphene Hydrogels and Aerogels Reveals the

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