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Functional Nanostructured Materials (including low-D carbon)

Enhancement of adsorption performance for organic molecules by combined effect of intermolecular interaction and morphology in porous rGO-incorporated hydrogels Seungmin Lee, Byung Joon Moon, HYUNJUNG LEE, Sukang Bae, TaeWook Kim, Yong Chae Jung, Jong Hyeok Park, and Sang Hyun Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19102 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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ACS Applied Materials & Interfaces

Enhancement of adsorption performance for organic molecules by combined effect of intermolecular interaction and morphology in porous rGO-incorporated hydrogels Seungmin Lee†, ‡,1, Byung Joon Moon †,1, Hyun Jung Lee §, Sukang Bae †, Tae-Wook Kim †, Yong Chae Jung †, Jong Hyeok Park‡, and Sang Hyun Lee*,†

†

Institute of Advanced Composite Materials, Korea Institute of Science and Technology, Chudong-ro

92, Bongdong-eup, Wanju-gun, Jeonbuk 55324, Republic of Korea ‡

Department of Chemical and Biomolecular Engineering, Yonsei University, 50 Yonsei-ro,

Seodaemun-gu, Seoul 03722, Republic of Korea §

BioNano Health Guard Research Center, 125 Gwahak-ro, Yuseong-gu, Daejeon 34141, Republic of

Korea

∗

Corresponding author. Tel.: +82-63-219-8152; Fax: +82-63-219-8129 E-mail address: [email protected] (Dr. S. H. Lee)

1

These authors contributed equally to this paper.

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT

In this study, we developed reduced graphene oxide (rGO)-incorporated porous agarose (ArrGO composites that were prepared via a “one-pot” sol-gel method involving a mixing and vacuum freeze-drying process. These composites represent an easy-to-use adsorbent for organic contaminant removal. Ar-rGOs can efficiently adsorb organic molecules, especially aromatic organic compounds from wastewater, due to the synergistic effect between the agarose bundles, which function as a water absorption site, and the rGO sheets, which function as active sites for pollutant binding. The pore structures and morphology of the ArrGO composites varied according to the added rGO, resulting in effective water infiltration into the composites. The main adsorption mechanism of the aromatic organic compounds onto Ar-rGOs involved π-π interactions with the rGO sheets. The surface interaction was more effective for adsorbing/desorbing the aromatic pollutants than the electrostatic interaction via the O-containing functional groups. In addition, we confirmed that Ar-rGO is highly stable over the entire pH range (1 ~ 13) due to the presence of the rGO sheets.

KEYWORDS Synergetic effects, hybrid aerogels, porous structure, π-π interactions, adsorbent


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1. INTRODUCTION Advanced adsorbents for the efficient and environmentally friendly removal or early detection of diverse forms of organic pollutants from water are being developed. Several porous nanomaterials have been suggested as potential candidates for ideal adsorbents (e.g., expended perlite, zeolites, wood fiber, silica aerogel, activated carbon, and boron nitride).1,2 These materials possess a large number of pores and are hydrophobic, leading to a relatively high adsorption capacity for oil or organic solvents. However, many challenges related to the chemical or mechanical stability, processability, and biocompatibility must be overcome for practical application.3,4 Low-dimensional carbon nanomaterials (e.g., carbon nanotube (CNT), graphene, and fullerene) based on three-dimensional (3D) structures have recently attracted much attention as building blocks for water purification due to their high adsorption capacity, light weight, extraordinary chemical and mechanical durability, and superior electrical and thermal conductivity.5,6 Among these materials, graphene, which consists of a one-atom thick crystal of sp2 carbon atoms in a two-dimensional (2D) honeycomb lattice, is advantageous due to its high adsorption capacity for specific organic compounds or biomaterials with aromatic structures resulting from the strong intermolecular interaction between the delocalized πbonds.7-10 As a graphene derivative, graphene oxide (GO) has become an attractive material because it can be readily mass-produced via the chemical exfoliation of graphite powders.11,12 In addition, due to abundant oxygen (O)-containing functional groups, this material can be easily dispersed in aqueous solutions and strongly interacts with a variety of organic materials via hydrogen bonding and ionic interactions.13 However, despite the many advantages of GO, the performance of GO-based adsorbents is limited due to its intrinsic nature. First, because GO is typically prepared from flaked graphite through a chemical oxidation and cutting process based on Hummers method, GO contains a significant number of structural defects and O-containing functional groups in its structure. The structural defects (i.e., point, line and void defects) in the basal plane of GO can suppress the adsorption process during the removal of aromatic organic compounds via π-π interactions.9,14 In addition, the O-containing groups of GO can easily interact with various ions (i.e., metal, organic and hydrogen ions (H+)) in waste solutions, which can hamper the ion exchange process for waste removal and lead to a decrease in the adsorption ability of adsorbents.15 In terms of physical and chemical stability, the structural defects in GO can be



relatively easily chemically oxidized because their local energy states are unstable. This behavior can lead to a degradation of the performance and durability of adsorbents.16-18 However, rGO exhibits an improved absorption capacity for hydrophobic aromatic compounds as well as enhanced chemical stability due to the restoration of sp2-hybridized carbons and the reduction of oxygen-related functional groups and their charges. In general, the water solubility of rGOs is poor owing to the lack of hydrophilic functional groups, leading to their severe aggregation in aqueous solutions.19 The self-aggregation of rGO will most likely cause various problems, such as decreased surface areas for intermolecular interactions, poor stability, and difficulty in handling and in limiting its application. In addition, the hydrophobic nature of rGO can hinder the flow of wastewater into pores, leading to a decline in intraparticle diffusion and adsorption of organic pollutants.20 Recently, as a solution to these problems, it has been demonstrated that GO can be chemically reduced after forming polysaccharide-based composite aerogels.21 The adsorption ability of these composites is relatively superior to that of pristine GO (or rGO) adsorbents due to the synergistic effect of the hydrocarbon polymers and graphitic carbon materials. However, the degree of reduction of GO is limited due to the low temperature process required to avoid deconstruction of the hydrogel structure. Therefore, a method to fabricate aerogel composites that contain high-quality rGO (high sp2/sp3 ratio and low O/C ratio) uniformly distributed throughout the polymer matrix is desirable. In this study, we fabricated porous agarose-rGOs (Ar-rGOs) prepared via a “one-pot” sol-gel method involving an in situ mixing and vacuum freeze-drying process. These composites represent an easy-to-use adsorbent for organic contaminant removal. During the gelation process, the shape of Ar-rGOs is easily controlled and scaled using molds or injectors. In addition, the presence of rGO sheets in the composite simultaneously controls its pore size and morphology in the hybrid aerogel due to their hydrophobic nature hindering intermolecular interactions between water and agarose molecules during both the gelation and drying processes. Ar-rGOs is very efficient and stable over the entire pH range (1 ~ 13) due to the synergistic effect of the agarose bundles, which allow for water absorption, and rGO sheets, which function as the contaminant-binding substrate. In addition, our adsorbent can be regenerated and reused effectively by washing in DI water and facilitates the disposal of wastewater contamination by various organic aromatic compounds across all industries.


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2. EXPERIMENTAL SECTION 2.1 Materials. Agarose polymer (molecular formula: (C12H14O5(OH)4)n) was purchased from Calbiochem Co., Ltd. Reduced graphene oxide (rGO) was purchased from Standard Graphene Co., Ltd (Korea). Silicone oil (viscosity ~ 100 cs) was purchased from Shinetsu silicon Co., Ltd. n-Hexane (95%), Rhodamine-B (denoted as RhB, ≥ 95%, HPLC), NaOH (≥ 97%) and HCl (35%) were purchased from Sigma-Aldrich Co., Ltd. Ethanol (95%), 2propanol (99.5%) and acetone (99.5%) were purchased from Samchun pure chemical Co., Ltd. 2.2 Preparation of agarose/rGO composite aerogels. The agarose and reduced graphene oxide (rGO) composite aerogel (Ar-rGOx) was prepared by simple sol-gel process. Agarose powder was first dissolved in DI-water at various concentrations (20.4 ~ 52.6 mg/mL), with stirring at room temperature for 2 hr. And then, this solution was heated up to about 90 ℃ for 10 min while maintaining stirring at 200 rpm. Also, 500 mg rGO powder was dispersed in a 50 mL aqueous solution by direct sonication for 1 hr. After each homogeneous solution was prepared, a specific amount of rGO-dispersed solution was added to each agarose solution, and then they were heated at 90 ℃ for 20 min, with constant stirring. Thereafter, the resulting mixtures were dropped into the silicon oil using a syringe. After being gelled in the oil bath for 30 min, the as-prepared beads were filtered and rinsed repeatedly with excess nhexane. 2.3 Materials characterizations. Scanning electron microscopic (SEM) observations were performed using a Nova NanoSEM 450 microscope. X-ray photoelectron spectroscopy (XPS) measurements were carried out using a Thermo Scientific K-Alpha with a focused-beam monochromated Al K-alpha (hν = 15 KeV) at approximately 10−8 Torr. N2 sorption isotherms were recorded at 77 K with a Belsorp mini II analyzer (MicrotracBEL, Japan). Before measuring, the samples were degassed in a vacuum at 120 °C for at least 12 hr. The average total run time was 10 hr. The Brunauer–Emmett–Teller (BET) and non-localized density functional theory (NLDFT) methods were used to quantitatively calculate the specific surface areas (SSAs), the pore volume and pore size distributions. The BET SSAs were calculated from the adsorption isotherms at the relative pressure (P/P0) range from 0.05 to 0.15. The NLDFT equilibrium model method was performed to calculate the pore size distribution of the samples (Program: BEL Master). Mercury (Hg) porosimetry was performed using a



MicroActive AutoPore V 9600 mercury intrusion porosimeter (Micromeritics). The contact angle (θ) was measured by a contact angle measurement equipment (Phoenix 300). The Raman spectroscopic analysis of the samples were performed on a multichannel bench Renishaw InVia Reflex spectrometer laser (Renishaw plc, Wootton-under-Edge, UK) coupled with a 514 nm laser. The UV-vis spectra were collected using a Jasco V670 spectrometer. The Fourier transform infrared (FT-IR) spectra were obtained on a FT-IR spectrophotometer (Jasco FT/IR-6600).

3. RESULTS AND DISCUSSION The preparation procedures for the rGO filled aerogels consisted of dissolving and mixing two main components (agarose and rGO) in DI water followed by heating to 90°C and then cooling to room temperature (Figure 1a). The detailed experimental conditions are described in the Supporting Information. During the mixing process, the aqueous rGO solution is slowly added to the agarose solution with constant stirring to avoid self-aggregation of rGO. The resulting dispersion of the rGO sheets that is shown in Figure S1 is stable for several days, indicating that the agarose polymer may serve as a good dispersing (or stabilizing) agent. The photographic images of the as-prepared agarose-rGO aerogel are shown in Figures 1b, S2 and S3. The surface color of the hybrid aerogels is matt black even when only a small amount of rGO sheets (less than 1 wt%) is added (Figures S2 and S3). The agarose-rGO aerogel can be formed in various shapes using a mold or injector, as shown in Figures 1b and S3. The aerogel fiber is very light and sufficiently flexible to be knotted. The internal morphology of the as-prepared agarose-rGO structures containing different amounts of rGO was investigated by scanning electron microscopy (SEM), as shown in Figures 2 and S4. Each sample is named Ar-rGOx, where x denotes the weight ratio of the added rGO sheets (Table S1). All types of Ar-rGOs have well-ordered and 3D porous structures containing hierarchical pore structures with interconnected nanoscopic (inner pores) and microscopic cavities (outer pores). It is important to note that agglomerates of the rGO sheets are not observed even at a high rGO concentration (30 wt%). Interestingly, depending on the amount of rGO in the hybrid structure, two major differences can be observed for each sample. First, as the rGO content increases, the density of the wrinkles formed on the macropore wall increases, which may be caused by the ‘pilling-up’ of randomly oriented rGO


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sheets on the surface through a variety of non-covalent interactions (Figure S4). Furthermore, the pore size of the samples gradually decreases as the rGO concentration increases. These observations were quantitatively confirmed by N2 adsorption/desorption (nanometer-scale pore size) and mercury porosimetry analysis (wide-scale pore size) as summarized in Tables S2 and S3. As shown in Figure S5, all samples exhibit a type IV isotherm with H3-type hysteresis loop. The most remarkable feature is that an increase in rGO content leads to an increase in hysteresis, suggesting that the addition of rGO sheets has a critical effect on the formation of pore structure. It can be also seen that, as the rGO content increases, the Brunauer–Emmett–Teller (BET) specific surface area (SSA) and the pore volume of the samples increase (Ar: 8.296 m2/g and 0.0304 cm3/g → Ar-rGO30: 35.629 and 0.0798 cm3/g). From the pore size distributions obtained from non-local density functional theory (NLDFT) calculations, it can be determined that all the samples have bimodal mesopores, which appear as one narrow peak centered at 4.4 nm (relatively small size) and another broad peak centered around 50 ~ 130 nm (relatively large size). The total pore area of the adsorbents, measured by mercury porosimetry, also increases with increasing the amount of rGO sheets (Ar: 33.765 m2/g → Ar-rGO30: 60.402 m2/g). Meanwhile, their average pore size (4V/A) decreases (Ar: 2.18 µm → Ar-rGO30: 1.43 µm). The morphological variation of the Ar-rGOs can be explained by a previously reported gelation mechanism22 that involves the agarose chains being reconstructed from randomly coiled polysaccharide chains to form a more ordered system with the multidimensional intermolecular formation of 1) helical structures and 2) less disordered coiling during the heating and cooling steps of the gel preparation. Then, the branching and aggregation of helix threads, which are connected by hydrogen bonds, produce long stiff rods with water-filled cavities (pores) between them. Water molecules that occupy the cavity are able to contribute to the stability of the double helix via hydrogen bonding. During this process, the multistacked rGO sheets that bind to the acetal groups (C-O-C) or the hydroxyl groups (-OH) of the agarose molecules via intermolecular hydrogen bonds may reduce the size of the waterfilled voids because the hydrophobic nature of the rGO sheets can inhibit the chemical interaction between water and the polymer, as illustrated in Scheme 1a. For the same reason, during the freeze-drying process, the presence of rGO sheets in Ar-rGOs may accelerate water loss in the cavity, leading to pore shrinkage (Scheme 1b). Therefore, based on the two proposed mechanisms, the incorporation of rGO sheets into the agarose-based aerogel can effectively improve the performance of the adsorbent by increasing the surface area for water ACS Paragon Plus Environment


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absorption as well as the adsorption of organic contaminants. However, from the viewpoint of interfacial chemistry, the hydrophobic nature of the rGO sheets may inhibit the adsorption property of adsorbents by suppressing water absorption. Therefore, to confirm the effect of the rGO sheets on the amount of water absorption, we measured the contact angle as a function of the rGO sheet content (Figure 3a). As the rGO sheet content increases, a gradual increase in the water-contact angle is observed, revealing that the addition of a very small amounts of rGO sheets to agarose is sufficient to tune the hydrophobicity of the composite owing to their hydrophobic nature. As a result, for the first 20 sec, the amount and rate of water sorption are affected by the hydrophobicity of the composites (Figures 3b and S7). By contrast, after 20 sec, the water sorption capacity (qeq, water)

increases in proportion to the rGO content, as shown in Figure 3b. This interesting

behavior can be explained in terms of a combination of surface tension and adhesive forces. The capillary pressure (PC), which is governed by pore size and surface wettability, can affect the microfluidic flow. In detail, the unbalanced pressure difference between the bulk interfacial surface and the interior pore structure is typically a major driving force for the capillary penetration of water into porous materials.23-25 Therefore, a reduction in the pore channels due to the addition of rGO sheets can enhance the capillary pressure for penetration of water into Ar-rGOs. In addition, the SSA of the composites is another crucial factor that influences the qeq,

water

value because the wettability of solid surfaces depends on the surface

charge and roughness.26,27 Therefore, Ar-rGO30 exhibits the highest qeq,

water

value of 35.3

mL/g due to a synergistic effect between the pore size and the SSA. The chemical configuration of each sample was confirmed by X-ray photoelectron spectroscopy (XPS). The high-resolution C 1s spectra of agarose and rGO can be deconvoluted into three and six components, respectively (see Figure S8 for details). As shown in Figure 3c, two distinct peaks, which correspond to agarose and rGO, are observed in the spectra of the composites. Based on these spectra, the relative peak intensities are calculated to be located at binding energies of 284.6 eV and 286.6 eV (Ic 1s, rGO/Ic 1s, agarose), as shown in Figure S9. As expected, the relative ratio of as-prepared composites tends to be proportional to the rGO/agarose composition ratio, indicating that the difference in the hydrophobicity of each aerogel is derived from the composition ratio of the hybrid structures. To confirm the synergistic effect of the agarose bundles and the rGO sheets on the steady state adsorption of organic molecules in a batch-type process, liquid-phase adsorption


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experiments were performed without agitation. Adsorption is a physicochemical process that involves the mass transfer of the adsorbate from the liquid to the surface of the adsorbent. For the adsorption study, Rhodamine B (RhB) with various adsorption behaviors, such as ionic and π–π stacking interactions with host materials, was used to investigate the adsorption performance of our samples (Figure 4a). As described in the experimental part, the adsorption experiments with RhB were carried out by adding 10 mg of agarose foams to 10 mL of an aqueous solution. In addition, these experiments were performed in the dark to prevent the dye molecules from being photodegraded by rGO sheets. We confirmed the adsorption of RhB on Ar-rGOx using various analysis techniques (i.e., UV-vis absorption, Fourier-transform infrared and X-ray photoemission spectroscopy, representative data shown in Figures S10~S11). Among them, the adsorption activity of the adsorbent was evaluated by UV-vis absorption spectroscopy because it is the most accurate method to quantitatively analyze the amount of dye adsorbed onto them. The amount of the dye adsorbed with change in solution concentration is displayed in Figure S12. The adsorption of RhB molecules onto the adsorbent increases with increasing the concentration of the dye in aqueous solution, indicating that the C0 plays an important role in the adsorption process of dye. Also, the adsorption capacities (qt) of each sample as a function of time are shown in Figure 4b. One of the notable results is that the qt value gradually improved as the rGO sheet content increased. Although pristine agarose adsorbent (Ar) barely extracts RhB molecules from the solution even over a long period of time (2000 min), all types of Ar-rGO were highly capable of adsorbing RhB (i.e., up to approximately 500 mg/g) (Figure S13). Moreover, Ar-rGOs can adsorb more than 30% of the maximum RhB molecules adsorption value within 120 min. These phenomena may be due to two reasons. First, the morphological variation of agarosebased aerogels has a significant effect on their dye adsorption ability. Specifically, an enhancement of the capillary force by reducing the pore size (Ar: 2.19 µm → Ar-rGO30: 1.43 µm, as summarized in Table S3) will most likely improve the probability that RhB molecules come into contact with the rGO sheets or agarose molecules for adsorption into the aerogel. In addition, an increase in the SSA due to the increased surface roughness or the number of pores can influence the dye adsorption ability of Ar-rGOs. As the SSA of the composites increases, the probability that the RhB molecules become non-covalently bound to the rGO sheets or agarose molecules will also increase.



The amount of adsorption sites on the adsorbent is another important factor to determine its dye adsorption ability. As shown in Scheme 1a, the RhB molecule can be adsorbed via π-π interactions on the basal plane of the rGO sheets (Route 1). This behavior corresponds well with the experimental result, whereby the addition of only 2 wt% rGO to the agarose foam (Ar-rGO2) results in a substantial improvement in the equilibrium adsorption capacity (qe,exp) even though the SSA of Ar-rGO2 is nearly the same to that of Ar (Scheme 1b). In addition, the O-containing functional groups of both the agarose molecule and rGO sheets can ionically bind dye molecules through electrostatic interactions (Route 2).28 However, because the content of O atoms in rGO is very low (Figure S5b) and the considerable amounts of acetal groups (C-O-C) and the hydroxyl groups (-OH) in agarose molecules are bounded to the rGO sheets via intermolecular hydrogen bonds, the adsorption of RhB molecules on Ar-rGOs is expected to primarily proceed via aromatic π-π stacking interactions. Consequently, Ar-rGOs exhibit substantially better adsorption performance compared with pristine agarose adsorbent due to a synergistic effect of the morphological modification (i.e., pore size and SSA) and additional binding site by rGO sheets. To precisely determine the rate of the adsorption process and investigate the possible adsorption mechanisms of RhB, as shown in Figures 4c and 4d, the experimental data was fitted using two kinetic models: pseudo-first-order and pseudo-second-order kinetic models29 (see Supporting Information, Eqs. (1) and (2)). Based on these parameters (Table S4), the adsorption system of Ar-rGOs is better described by the pseudo-second-order kinetic model, and the rate-limiting step for RhB adsorption on the adsorbent may involve a chemisorption process.32 As the content of rGO increases, the R2 value approaches 1 (Ar-rGO2: 0.97659 → Ar-rGO30: 0.99985) because the second-order kinetic model corresponds to two reactive substances or two types of adsorption sites. In addition, the k2 value decreases as the rGO sheet content in the composite increases (Ar: 8.78 → Ar-rGO30: 1.95 [unit: 10-3 g/mg/min]), indicating that the adsorption rate of RhB molecules is significantly dependent on the availability of adsorption sites. However, these two kinetic models cannot identify the diffusion mechanism, thereby the rate-limiting step of our adsorption process cannot be determined. Therefore, to gain insight into the diffusion mechanisms and rate-limiting steps that influence the kinetics of adsorption, the kinetic experimental results were also fitted to the intraparticle diffusion model based on the theory proposed by Weber and Morris31,32 (see


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Supporting Information, Eq. (3)). As shown in Figure 4e, two steps occur during the adsorption of RhB molecules onto the adsorbent, and these steps consist of 1) external diffusion (film diffusion) and 2) intraparticle diffusion (diffusion in the pores).33,34 Interestingly, we can observe that each sample has a different rate-limiting step. For the pristine agarose adsorbent, the RhB adsorption process is dominated by the diffusion into the pores. By contrast, the adsorption process for Ar-rGO30 is strongly governed by the external diffusion step with macropore or interparticle diffusion. Another remarkable feature is that the k1d value for Ar-rGO30 is significantly higher than that for Ar (Table S5). However, the k2d values of both adsorbents are nearly the same. Multiple factors can affect the adsorption process of RhB. First, the change in the adsorption kinetics for the removal of organic small molecules from aqueous solutions may be due to the existence of different SSAs, pore sizes and capillary pressures that could be triggered by structural differences in the composites. In addition, the external diffusion process would be greatly influenced by the intermolecular interaction between RhB molecules and aerogels, such as π-π interactions, H-bond interactions and ionic interaction (physical or host-guest chemical interactions). Moreover, as shown in Figure S14, internal and external particle diffusion processes can be facilitated by agitation, resulting in the enhancement of adsorption rate (180.2 mg/g → 457.78 mg/g at 60 min). This observation indicates that shaking or stirring can positively influence the diffusion in the boundary layer, which may lead to an increase in the adsorption rate and capacity. In order to further evaluate the adsorption capacity of adsorbents as well as elucidate the interactions between adsorbent and adsorbate, we examined the isothermal behavior for RhB adsorption on Ar-rGO30. The equilibrium adsorption data have been subjected to different adsorption isotherms, such as Langmuir, Freundlich and Dubinin-Kaganer-Radushkevich (DKR), which are detailedly described in Supporting Information (Eqs. 4~9). The experimental results and fitting curves are depicted in Figure S15 and the related calculation parameters are summarized in Table S6. The average R2 value obtained from Langmuir model for adsorption of RhB by Ar-rGO30 (approximately 0.97) is a little higher than that from Freundlich model (approximately 0.95). Also, the calculated values of the dimensionless factor (RL) for all the three temperatures are between 0 and 1. These observations indicate that the experimental adsorption system is more appropriately described by Langmuir isotherm, suggesting that adsorption sites are energetically homogeneous.



On the basis of the adsorption constant in the Langmuir isotherm, the thermodynamic parameters (∆H0, ∆S0 and ∆G0) for the adsorption of RhB molecules were also determined by using the Van’t Hoff equation (see Supporting Information, Eqs. 10 and 11). As shown in Table S7, in both Ar and Ar-rGO30, the values of ∆H0 are positive and similar, indicating endothermic processes. However, the ∆S0 value of Ar-rGO30 is nearly twenty-three times higher than that of Ar (Ar: 2.03 J mol-1 K-1 → Ar-rGO30: 45.71 J mol-1 K-1), implying that RhB molecules are more easily adsorbed onto the Ar-rGO30 because the positive value of ∆S0 indicates the increased randomness at the solid/solution interface during the adsorption process. The ∆G0 value of Ar-rGO30, which is calculated from the values of ∆H0 and ∆S0, is negative (-6.41 KJ mol-1 K-1) indicating a spontaneous process, while that of Ar is positive (5.88 KJ mol-1 K-1, non-spontaneous process). Also, the ∆G0 value of Ar-rGO30 is less than 40 kJ mol-1 K-1, indicating that RhB molecules are adsorbed onto the Ar-rGO30 mainly through weak intermolecular interaction such as π-π interaction. Additionally, the apparent energy of adsorption (E) obtained from DKR isotherm indicates that the adsorption mechanism of RhB onto the Ar-rGO30 is somewhat related to the C0 of the adsorbate (RhB). Therefore, we can infer that, although the contribution of chemical adsorption forces is by no means negligible when the adsorption process is performed at relatively low C0, the adsorption process occurs mainly through a physical adsorption process driven by π-π interaction between rGO sheets and RhB molecules. To further explore the chemical mechanism of dye adsorption on the composite, we investigated the effect of pH on the adsorption of RhB molecules. The initial pH value of the solution is one of the most crucial factors that affects the adsorption amount of dye molecule because this factor can affect the surface charge of the adsorbent (or adsorbate) species in solution. Generally, the qeq value tends to significantly decrease as the pH decreased (to ~ 3.0), which is primarily due to 1) strengthening of the electrostatic repulsion and 2) an increase in the competition between the protonated forms of O-containing functional groups (i.e., –OH and –COOH) and RhB cations for the available adsorption sites.35 By contrast, as the pH value increases from 7.0 to 13.0, the O-containing groups can be deprotonated due to the electrostatic attraction between the adsorbate and the adsorbent, leading to an enhancement of the adsorption capacity. Therefore, the ionic interaction is considered as the major adsorption mechanism for RhB adsorption on carbon-based aerogels or their composites. However, for the Ar-rGO composite, the qeq value and its shape remain nearly


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constant over the entire pH range (from pH 3 to pH 13), as shown in Figure 4f, indicating that the adsorption property of Ar-rGOs is less determined by the nature and concentration of the surface functional groups compared to conventional graphitic carbon-based adsorbents. In order to further determine the adsorption mechanism of organic dye molecules on ArrGOs, the desorption experiments of RhB molecules from the Ar-rGO30 composites were performed by using DI-water. Based on previous studies, organic dye molecules (i.e., RhB, methylene blue (MB) and malachite green (MG)) can be easily desorbed by strong acids (or bases) while adsorbed dyes are difficult to be desorbed by dilution with neutral pH water (< 10%), indicating that the attachment of the organic molecules onto the adsorbent is primarily due to electrostatic interactions or ion exchange. By contrast, as shown in Figures S16 and S17, the desorption efficiency was approximately 67% even though the Ar-rGO30 adsorbed with RhB molecules was soaked for only approximately 120 min in DI water (10 mL, same volume as the adsorption experiment). This finding indicates that, compared to conventional adsorbents, a relatively low activation energy is required to remove contaminants from the studied Ar-rGOs. Furthermore, this also implies that several problems which may arise when using an acid or base solution as the removal agent (i.e., low desorption efficiency (< 70%), the decomplexation reaction of the hydrogel structure, and economic and environmental issues) can be avoided with the use of Ar-rGO composite. This issue will be addressed in detail later on. Additionally, to directly compare the difference in the adsorption behavior of RhB molecules based on the chemical configuration of graphitic carbon-based fillers, adsorption experiments with RhB molecules (initial concentration of RhB solution: 75 mg/L) were conducted using two different types of adsorbents (Ar-GO30 and Ar-rGO30), and these adsorbents were prepared by adding the same amount of fillers (30 wt% of GO or rGO). The initial absorption rate of Ar-rGO30 as well as its qeq (Ar-GO30: ~ 630 mg/g & Ar-rGO30: ~ 680 mg/g) are superior to those of Ar-GO30 (Figure S18), suggesting that the adsorption efficiency of adsorbents with graphitic carbon materials is closely associated with the chemical configuration of the fillers in the composite. To confirm this hypothesis, the chemical configuration of GO and rGO was analyzed using XPS and Raman spectroscopy (Figure S19). The full range XPS analysis of each material indicates that the C/O atomic ratio of rGO (approximately 11.5) is much higher than that of GO (approximately 2.0). Based on these results, it can be expected that the lack of O-containing functional groups at the edge of rGO



sheets may be unfavorable for RhB adsorption in terms of ionic interactions. However, according to the empirical Tuinstra-Koenig relationship, the sp2 cluster size (La) of rGO (10.2 nm) is much larger than that of GO (6.3 nm), suggesting that rGO is more suited for RhB adsorption via π-π interactions compared to GO.36 Therefore, the adsorption mechanism for RhB adsorption onto Ar-rGO30 is likely to be more dependent on π-π interactions than ionic interactions due to the chemical structure of rGOs. In addition, the use of rGO as a filler is a more efficient approach for extracting RhB molecules from aqueous solutions than utilizing GO because the initial absorption rate and qeq of Ar-rGO30 are superior to those of Ar-GO30. To investigate the reusability and recyclability of the Ar-rGO30 composite, the desorption experiments of RhB molecules were carried out by using 10 mL of ethanol (EtOH), which is less toxic for the human body than other solvents. The adsorbent was regenerated for five consecutive cycles. As shown in Figure S20, the adsorption capacity of Ar-rGO30 is still high in the fifth regeneration cycle, and is over 95 % (5th cycle) of the initial adsorption amount. Also, there is no structural deformation after repeated cycle test (Figure S21). Therefore, we anticipate that our adsorbent can be repeatedly used in various places without any environmental, physical or economic issues. Furthermore, we briefly determined the solvent effect on the desorption of RhB molecules from the adsorbent. These experiments were performed with a larger amount of solvent than previous ones (10 mL → 30 mL). As shown in Figure S22, acetone shows the highest initial desorption rate (approximately 78.4 % at 1 min) and equilibrium desorption efficiency (approximately 99.5 % at 60 min) among various solvents. These observations imply that, in absence of competition from other adsorbates, adsorbed molecules are relatively easy to desorb when diluted with weak base solvent (pKa > 15.4), such as EtOH, 2-propanol (IPA) and acetone. As previously mentioned, the adsorption properties and shape of Ar-rGOs remain nearly constant over the entire pH range. Therefore, we can infer that our adsorbent can be used in extreme environments without the adsorption efficiency decreasing. In order to confirm the acid (or alkali) durability of our adsorbents, a pH stability test was performed by preincubating Ar-rGO30 at various pH values (range: 1 ~ 14). As shown in Figure S23, the qeq values of Ar-rGO30 remain nearly constant under acidic (or basic) conditions. This result is caused by some O-containing groups in the rGO sheets being able to act as traps for H+ (or OH-) ions. Another plausible reason is that the aggregation and branching of helix threads, which are primarily connected via hydrogen bonds, are nearly intact. However, for Ar, H+


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ions have a relatively strong influence on helix threads, leading to a significant decrease in the qeq value, as shown in Figure S24. These results indicate that the presence of rGO sheets improves the chemical stability in the agarose-based aerogel under acidic and basic conditions. In addition, these data imply that the adsorption property of Ar-rGOs is determined less by the nature and concentration of the surface functional groups than conventional graphitic carbon-based adsorbents. Finally, to confirm the general suitability of Ar-rGO composites, additional adsorption experiments were performed using various organic small molecules (i.e., phenol, 1naphtylamine, bisphenol A and tetracycline) that are considered hazardous pollutants (Figures 5a and S25) with an initial solution concentration of 0.005 mol/L. As shown in Figure 5b, more than 90% of the molar adsorption capacities at equilibrium (qmmol,eq) is removed within the first 6 hr of contact with the adsorbent, demonstrating that our aerogel composite is a promising candidate for use as highly efficient macrosponges that are able to remove a wide range of organic pollutants from water. Notably, each qmmol,eq value is significantly different and depends on the type of organic pollutant. In addition, qmmol,eq increases as the number of phenyl units increases. In particular, the degree of ring fusion (i.e., phenol: 1, bisphenol A: 1, naphthylamine: 2, tetracycline: 4) is a strong influence on the π-π interaction between organic molecules.37-39 In our batch experiments, Ar-rGO30 exhibits the highest adsorption efficiency when used for tetracycline adsorption (4.50 mmol/g). In addition, the equilibrium times (teq) are slightly different among the pollutants due to variation in the chemical configurations. As previously mentioned, each adsorbate contains a different number of phenyl units as well as degrees of ring fusion. Therefore, it may induce different van der Waals interaction energies between organic pollutants and adsorbents. Furthermore, the variation in electrostatic (and polarization) interaction energies due to the different edge-functional groups (i.e., –OH, COOH, -CONH2, -C=O, etc.) of organic pollutants can have a positive (synergetic effect) or negative (competition with van der Waals interaction) effect on the teq value of each adsorption process.

4. CONCLUSION In this study, Ar-rGOs were successfully fabricated through a facile “one-pot” sol-gel method involving a mixing and vacuum freeze-drying process. The as-prepared aerogels,



which can be formed in various shapes using molds or injectors, exhibit a high SSA with a porous structure. In batch adsorption experiments, Ar-rGOs, especially Ar-rGO30, is very efficient and stable for the adsorption of RhB molecules from polluted water due to the synergistic effect of the agarose bundles, which aid in water absorption, and rGO sheets, which function as active sites for pollutant binding. For example, Ar-rGO30 can adsorb RhB molecules at more than 50% of qeq within 120 min at an initial concentration of 0.05 mg/mL without any agitation. In addition, the qeq values of Ar-rGO30 remain nearly constant under acidic (or basic) conditions (pH range: 1 ~ 13). The adsorption mechanism of the RhB molecules onto Ar-rGO30 is most likely more dependent on the π-π interactions than ionic interactions due to the chemical structure of rGO. Because π-π interactions are relatively weak intermolecular interactions, more than 90% of adsorbate molecules are desorbed by immersing Ar-rGO30 in DI water for 2 hr. Finally, we thoroughly confirmed the general suitability of the Ar-rGO composites for the removal of various kinds of toxic and hazardous aromatic molecules. Based on the obtained results, we anticipate that our hybrid aerogels will be beneficial for the effective removal of organic pollutants in wastewater or purification of other industrial waste solutions.

ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.XXXXXXX. Detailed information for visual dispersibility test of rGO sheets, photograph images of ArrGO composite aerogels, cross-sectional SEM images of agarose and Ar-rGO30, composition ratios of Ar-rGOx, characterization of porous structure of the adsorbent, static water contact angle analysis of Ar-rGOx composites, XPS analysis of agarose and rGO matrix, FT-IR, XPS and UV-vis absorption spectra of Ar and Ar-rGO30 before/after adsorption of RhB, kinetic study of RhB adsorption over agarose based adsorbents, adsorption isotherm study of the


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removal of RhB molecules, kinetic study of RhB desorption over Ar-rGO30, effect of additives on the adsorption properties of adsorbents, XPS and Raman analysis of rGO and GO, reusability and recyclability test of Ar-rGO30, pH-Stability test of Ar-rGOs at various pH values and batch adsorption experiment for the removal of various organic pollutants.

AUTHOR INFORMATION Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This work was financially supported by the Korea Institute of Science and Technology (KIST) Institutional Program.



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TOC GRAPHICS


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Scheme LEGEND Scheme 1. (a) Schematic illustration of the Ar (or Ar-rGOx) network (left) and the chemical interaction between water molecule and agarose polymer (right). (b) Schematic illustration of the mechanism for pore size control of Ar (or Ar-rGOx) during the freeze-drying process.

FIGURE LEGENDS Figure 1. (a) Schematic representation of the preparation of Ar-rGOx via the “one-pot” solgel method. (b) Optical photographs of Ar-rGOs with various shapes.

Figure 2. Cross-sectional FE-SEM images of (a) intrinsic agarose, (b) Ar-rGO2, (c) Ar-rGO5, (d) Ar-rGO10, (e) Ar-rGO20 and (f) Ar-rGO30 (inset: the inner pore structure of Ar-rGO30).

Figure 3. (a) Contact angles of DI water on aerogel matrix as a function of the content of rGO sheets. (b) Amount of absorbed water on each adsorbent as a function of the retention time. (c) C 1s high-resolution XPS spectra of Ar-rGOx.

Figure 4. (a) Schematic illustration of RhB adsorption on the rGO surface. (b) Effect of the adsorption time on the adsorption capacity of each adsorbent for RhB. (c) Pseudo-first-order and (d) pseudo-second-order kinetic models for the adsorption of RhB by each adsorbent. (e) Intraparticle diffusion model for RhB adsorption onto intrinsic agarose adsorbent and ArrGO30. (f) Effect of initial pH solution on the adsorption of RhB onto Ar-rGO30. All experiments are performed under the following conditions: 500 mg/L (Initial Conc.); 1 mg/mL (adsorbent dose); 25 °C (temperature).

Figure 5. (a) Chemical structures of rhodamine B, phenol, bisphenol A, 1-naphthylamine and tetracycline. (b) Kinetic data for the adsorption of various organic pollutants by Ar-rGO30. The initial concentration of solutions is 0.0005 mol/L.



Scheme 1


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Figure 1



Figure 2


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Figure 3



Figure 4


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Figure 5


Enhancement of adsorption performance for ... - ACS Publications

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