Construction of a Carbon Nanomaterial-Based Nanocomposite

Article Cite This: ACS Appl. Nano Mater. 2018, 1, 4127−4134

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Construction of a Carbon Nanomaterial-Based Nanocomposite Aerogel for the Removal of Organic Compounds from Water Benjamin S. Litts,† Mark K. Eddy,† Paula M. Zaretzky,† Noah N. Ferguson,‡ Anthony B. Dichiara,*,‡ and Reginald E. Rogers, Jr.*,† †

ACS Appl. Nano Mater. 2018.1:4127-4134. Downloaded from pubs.acs.org by QUEEN MARY UNIV OF LONDON on 08/28/18. For personal use only.

Department of Chemical Engineering, Kate Gleason College of Engineering, Rochester Institute of Technology, Rochester, New York 14623, United States ‡ School of Environmental and Forest Sciences, University of Washington, Seattle, Washington 98195, United States ABSTRACT: Overutilization of organic-based compounds (e.g., fertilizers) in agricultural settings continues to cause concern due to potential adverse impacts on the health of humans and the environment. Adsorbents, such as activated carbon, are widely used due to their ability to adsorb various contaminants. Limitations in the effectiveness of activated carbon, where high uptake capacities are desired, require the need for alternatives to current state-of-the-art adsorbents. In this work, the adsorption capability of a lightweight, carbonbased aerogel is presented. The aerogel is composed of 0 to 2 wt % mixture of both graphene nanoplatelets (GnP) and single-walled carbon nanotubes (SWCNT). Two organic compounds, 2,4-dichlorophenoxyacetic acid (2,4-D) and 1-pyrenebutyric acid (PBA), served as the contaminants of interest. Results show that the aerogel containing 0.2 wt % SWCNT-GnP achieved an uptake capacity of 0.22 mg of 2,4-D/mg of aerogel and 0.083 mg of PBA/mg of aerogel. Compared to activated carbon, from a kinetic standpoint, the aerogels produced in this work demonstrated 39-fold higher adsorption of 2,4-D and 5-fold higher adsorption of PBA, making them a viable candidate as a next generation adsorbent. KEYWORDS: adsorption, aerogel, carbon nanotubes, contaminant, graphene, water treatment

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INTRODUCTION A persistent problem that is expected to continue over the next several decades is the continued contamination of water in heavily populated areas.1 Extensive industrial and agricultural activities are the main sources of these aromatic compounds, although they also arise from natural sources.1−4 Human health and the environment are adversely affected due to the relatively high mobility, persistence in aqueous media, and the relative toxicity of these substances. To combat the negative impacts of contaminants, various methods for their removal have been developed.2−9 Of particular interest is the separation technique of adsorption, which employs materials of high binding affinity and uptake capacity to remove undesirable chemicals across a broad range of concentrations.10 The advantages of adsorption, including simplicity of implementation, plays an important role in environmental remediation efforts.11 Alternative adsorbents to activated carbon, the most widely used material for water purification, require rational selection of materials that can exceed the capabilities of the current state-ofthe-art adsorbents. Efforts in the nanoscale science and engineering community focus on the carbon-nanomaterial family to develop cost-effective adsorbents with improved performance. Within this family are graphene, a two-dimensional material composed of hexagonally packed carbon atoms, and carbon nanotubes. Carbon nanotubes (CNTs) are graphene sheets rolled into seamless cylinders. Both graphene and CNTs © 2018 American Chemical Society

display properties that are significantly better than those of activated carbon. These include large surface area, high hydrophobicity, and strong noncovalent interactions with organic compounds. To date, the adsorption uptake of various aromatic compounds onto CNTs (including both single-wallled and multiwalled nanotubes) and graphene nanoplatelets (GnPs) have reported higher adsorption capacities versus activated carbon (AC).2−4,6−9,11−30 While these studies demonstrate the efficacy of carbon-nanomaterial adsorbents, improved adsorption performance may be realized by using a different architecture. A previous study has shown that the combination of these materials allows for an optimization of performance and cost.2 Though CNTs and graphene share some commonalities with regards to structure and properties, both exhibit different affinities toward given molecules. Therefore, a combination of these nanomaterials can significantly increase the number and variety of pollutants that may be adsorbed. Recent work demonstrates that graphene nanoplatelet single-walled carbon nanotube papers offer fast adsorption rates, improved stability, and large capacity.2,4,8 These papers also exhibit a very high regeneration efficiency, making them attractive for many water purification systems.7 While 2-D, flat papers have been shown to Received: May 25, 2018 Accepted: July 6, 2018 Published: July 6, 2018 4127

DOI: 10.1021/acsanm.8b00884 ACS Appl. Nano Mater. 2018, 1, 4127−4134

Article

ACS Applied Nano Materials Scheme 1. Process for Making Carbon-Nanomaterial-Based Aerogel

weight loadings of carbon nanotubes incorporated within them. Using UV−visible spectroscopy, batch adsorption experiments are completed to understand the kinetic and equilibrium behavior of the aerogel toward the removal of PBA and 2,4-D. It will be shown that the aerogels show a high affinity toward the removal of these organic compounds in a relatively short period of time. Given the high interest in reducing the footprint of nanomaterials on the environment, creation of a lightweight aerogel may lead to improved techniques for extending the life of these specific nanomaterials, thereby decreasing the waste generated after the adsorbent is exhausted.

yield significant improvements in the adsorption uptake of organic material, a 3-D structure could produce a more effective and efficient means by which to complete the separation of the contaminant from the specific environment. Another major advantage to using carbon-nanotube-based aerogels is the capability of utilizing less mass of carbon nanomaterials per volume of aerogel. Such a fact correlates directly to the need for reducing costs associated with carbon nanomaterials. On the basis of continued efforts to reduced production costs of carbon nanomaterials, being able to effectively demonstrate a high uptake of adsorbate using less carbon nanomaterials will advance the capabilities of this particular adsorbent system. Within the agricultural community, the compound 2,4dichlorophenoxyacetic acid (2,4-D) is of high interest, since it is one of the most extensively employed agricultural herbicides for the control of broad-leaved weeds.2,7 This pesticide is highly toxic, and it is important to remove it from hydrological systems. 2,4-D cannot be biodegraded effectively and rapidly at concentrations higher than 1 ppm, and its maximum allowable concentration in drinking water is 0.1 mg/L. There is, however, a potential for human exposure to this herbicide through residue on crops. Additionally, during application on crops, aerial drift and field runoff can occur to aquatic systems. A compound, which is capable of serving as a model standard for adsorption studies, is 1-pyrenebutyric acid (PBA). PBA is unique given the large number of noncovalent interactions capable of being generated with carbon nanotubes. Previous studies have shown the adsorption potential of carbon nanomaterials based on the removal of PBA from aqueous systems.2,31 2,4-D was selected because of its frequent application as an agricultural herbicide, and PBA was selected because of its model polyaromatic structure.31 The ability to remove these specific compounds from watersheds is important for helping to prevent potential impacts on humans and wildlife at high concentration levels. Here, we report, for the first time, an experimental study of the adsorption of various aromatic compounds from aqueous solutions onto a 3-D architectural structure composed of carbon nanomaterials integrated within a carbon-based aerogel. Through a stepwise, additive initiation reaction, aerogels composed of an amorphous carbon shell are grown with various

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EXPERIMENTAL METHOD

Carbon-Nanomaterial Purification. Single-wall carbon nanotubes (SWCNTs) were obtained from Cheap Tubes, Inc. (90% purity, USA) and Carbon Solutions (60−70% purity, USA). The characteristic length of the SWCNTs was below 30 μm, and the individual tube inner diameter ranged between 0.8 and 1.6 nm. Graphene nanoplatelets (GnP), (Angstron Materials (N002- PDR), USA) were approximately 10 μm in size with a thickness lower than 1 nm and purity of 95%. Similar to previous work,3,7 the GnP and SWCNTs were soaked in hydrochloric acid (37% Sigma-Aldrich, USA) for 17 h at 20 °C to remove metal particles and ultimately purify both species. The mixture was then filtered through a 90 mm, 30 μm pore size aqueous membrane (Whatman, USA) and thoroughly washed with deionized water (18.3 MΩ). The SWCNT-GnPs were placed in a Thermolyne 1300 muffle furnace (Thermo-Scientific, USA) and heated to 560 °C to remove residual amorphous carbon. After holding for 5 min at this temperature, the oven was turned off, and the SWCNT-GnPs were allowed to cool to room temperature (20 °C) in the oven prior to use in an aerogel preparation. SWCNT-GnP Aerogel Preparation. Scheme 1 shows how the carbon-nanomaterial-based aerogel was prepared. SWCNT-GnP aerogels were made with varying mass contents of carbon nanomaterial (SWCNTs and GnP), ranging from 0 to 2% (w/w). A mass ratio of SWCNTs to GnP was set at 1:1 based on prior work establishing an optimal hybridization to achieve maximum adsorption uptake.3,7 In a typical experiment, approximately 2.50 g of sucrose (Domino, USA) was dissolved into a 20 mL scintillation vial marked “A” with 5 mL of deionized water (PURELAB flex, Veolia) using an ultrasonication bath at room temperature for 60 min. On the basis of prior work,32 gum arabic (Ward’s Science+, USA) was placed into scintillation vial “A” and also in a 20 mL scintillation vial marked “B” according to Table 1 to prevent aggregation of carbon nanomaterial in aqueous solutions. Both scintillation vials were bath sonicated at room temperature for 30 min to 4128


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ACS Applied Nano Materials

PBA in a 1% (v/v) ammonium hydroxide (30%, Sigma-Aldrich) solution were prepared at a 20 μg/mL concentration in aqueous solution having a pH of 9.8. The stock solutions were diluted with deionized water to achieve concentrations of 2.5, 5, 10, 15, and 20 μg/ mL. Stock solutions of 2,4-D were prepared at a 350 μg/mL concentration in aqueous solution having a pH of 6.4 The stock solutions were diluted with deionized water to achieve concentrations of 2,4-D at 12.5, 25, 37.5, 50, 100, 150 , 200, and 300 μg/mL. Characterization of Aerogels. The microstructure of the asproduced carbon-based aerogel was characterized by electron microscopy and Raman spectroscopy. A Hitachi field emission scanning electron microscope (Hitachi Corporation, S-900) was used to analyze the surface morphology of the aerogel, while transmission electron microscopy observations were performed using a FEI TECNAI F20 STWIN high resolution TEM with a beam acceleration voltage of 200 keV. Raman spectral shifts were recorded over the range of 100−3000 cm−1 with a spectral resolution of 1 cm−1 by a Jobin Yvon Horiba LabRAM spectrophotometer using the 632.8 nm emission of a He−Ne laser source. The specific surface area of the aerogels was analyzed on the basis of nitrogen adsorption and desorption isotherms measured at 77 K with a Micromeritics TriStar II Plus instrument (Norcross, GA, USA). All samples were outgassed in vacuum at 200 °C for 4 h to remove any possible adsorbed impurities prior to data collection. The Brunauer−Emmett−Teller (BET) method was used for the specific surface area calculations as per the IUPAC standards. Adsorption Measurements. SWCNT-GnP aerogels were cut into three even sections. The three sections were designated as top, middle, or bottom based upon their orientation in the 50 mL beaker. Batch adsorption studies were performed on all three individual sections to

Table 1. Mass Contents of Gum Arabic and Carbon Nanomaterials Used in the Aerogel Synthesis scintillation vial A weight loading

gum arabic (mg)

carbon nanomaterial (mg)

0.0 0.2 2.0

17 17 34

0 1.7 17

scintillation vial B gum arabic (mg)

carbon nanomaterial (mg)

33 33 66

0 3.4 34

unwind the long gum arabic chains. Carbon nanomaterials were added into both scintillation vials based upon the desired mass loading listed in Table 1. The resulting mixtures were dispersed by an ultrasonication bath at room temperature for 60 min. The solution in scintillation vial A was poured into a 100 mL beaker. The solution in scintillation vial B and 10 mL of sulfuric acid (93% by weight, Theochem Laboratories) were simultaneously poured into the same 100 mL beaker. A spontaneous, exothermic reaction was observed with the growth of the aerogel taking place in the beaker. Note that the morphology of the aerogel can be adjusted by changing the volume and shape of the reactor. The reaction temperature was monitored using an infrared pyrometer and when the beaker cooled to under 40 °C, it was soaked in water to remove unreacted chemicals and placed in a drying oven (Binder, USA) at 120 °C for 20 h. Adsorbate Solution Preparation. 2,4-Dichlorophenoxyacetic acid [2,4-D] (98%, Sigma-Aldrich) and 1-pyrenebutyric acid [PBA] (97%, Sigma-Aldrich) were chosen as adsorbates. Stock solutions of

Figure 1. SEM images of aerogel taken at (a) 500× and (b) 3000× and (c) representative TEM image showing the amorphous nature of the plain aerogel with the micrograph in the inset revealing the presence of graphitic nanomaterials in the composite aerogel. (d) Raman shift spectra of aerogel highlighting area between 1000 and 2500 cm−1. Inset of (d) is final aerogel product after synthesis. 4129


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noneffective adsorbent as was demonstrated previously.7 Knowing this, experimental studies proceeded to understand the kinetic and equilibrium behavior of the aerogels. Uniformity of Aerogel. To confirm the uniform adsorption properties of the aerogel throughout its volume, each sample was cut into three sections, as illustrated in Figure 2. We sampled five

determine the uniformity of adsorption throughout the aerogels. SWCNT-GnP aerogels were prepared by drying foam formed from the dehydration of sucrose by sulfuric acid as previously described. The dehydration was a spontaneous reaction causing the carbon nanomaterials to be dispersed at random throughout the material. Measuring the adsorption capacity of the three sections showed how uniformly the carbon nanomaterial was dispersed throughout the SWCNT-GnP aerogels. Batch adsorption studies were performed by placing 2.0 mg of SWCNT-GnP aerogel into a 20 mL scintillation vial with 20 mL of the PBA and 2,4-D solutions. Short-term (≤1.5 h) adsorption studies and measurements of equilibrium concentrations of the batch systems were performed. Short-term adsorption studies were performed by measuring the concentration of a system at 15 min intervals. The batch system was agitated on an orbital shaker (Bel-Art Spindrive) operated at 120 rpm. The final concentration of a solution was measured after it was deemed to have reached equilibrium. The systems were determined to have reached equilibrium when there was no appreciable change in solution concentration over a period of 1 h. Adsorption of 2,4-D and PBA onto the surface of SWCNT-GnP aerogels reached equilibrium within 1.5 h. Longer term (1.5 to 6 h) studies were also completed to show the effect of the adsorption over time. These longer trials were shown to achieve static equilibrium after 3 h. As a result of the mechanical integrity of the aerogel, each sample was broken down into smaller pieces to account for solution agitation imposed during adsorption tests. Before measuring solution concentrations, solutions were filtered through a 90 mm, 30 μm pore size aqueous membrane (Whatman, USA) into a 20 mL scintillation vial. Filtered solutions (3 mL) were transferred into a quartz cuvette using a volumetric pipet. The concentration in the quartz cuvette was measured by absorption spectroscopy on a Lambda 950 UV/vis/NIR spectrophotometer (PerkinElmer, USA) at the maximum absorption peaks for PBA and 2,4-D (λ = 341 nm, 283 nm, respectively). A mass balance on the bulk solution was used to calculate all adsorption capacities. Adsorption capacity, q, is the mass of adsorbate adhered to the surface of the adsorbent per mass of adsorbent.

Figure 2. Cartoon illustration showing how aerogel is removed and sectioned into three pieces for characterization.

pieces of 2 mg each for statistical purposes. Figure 3 shows the adsorption isotherms of 2,4-D and PBA onto the 2.0 and 0.2 wt % SWCNT-GnP aerogel sections at 20 °C. The data shows that the sections have the same adsorption capacity within error at each concentration and that the percentage of carbon nanomaterial loaded into the aerogel does not have an effect on the consistency of SWCNT-GnP aerogel adsorption capacity. With the adsorption capacity across all sections of the SWCNT-GnP aerogels being the same, the random dispersion of carbon nanomaterials in each of the aerogels was assumed to be uniform. Regardless of what section of the SWCNT-GnP aerogel was used for the adsorption experiments, it will be representative of the whole material. Effect of Carbon-Nanomaterial Mass Loadings on Adsorption Kinetics for Short Time Periods. After preparation of the adsorbate solution, the aerogel was introduced to observe the adsorption uptake of the selected adsorbate. It should be noted that the volume of the solution did not change after introducing the aerogel because of the hydrophobicity of the aerogel. Short-time (90 min) adsorption of 2,4-D with an initial adsorbate concentration of 100 μg/mL and PBA with an initial adsorbate concentration of 15 μg/mL onto the surface of SWCNT-GnP aerogels at 20 °C under constant agitation is shown in Figure 4. All adsorbents reach steady state within 30 min, and the uptake values of the aerogel composites are up to 104 and 82 mg/mg for 2,4-D and PBA, respectively. The corresponding steady state adsorption values of the plain carbonaceous aerogels without any graphitic nanomaterials are 20 and 32% lower for 2,4-D and PBA, respectively. Those steady state adsorption capacities were the same as the equilibrium adsorption capacities for the same initial adsorbate concentrations, suggesting that all of the available adsorption sites are filled within 30 min. This rate of adsorption is significantly faster than those previously reported.2 Table 2 summarizes the maximum adsorption uptake for the three weight loadings of the graphitic nanomaterials in the aerogels. Strong interactions between the aerogels and organic chemicals are expected due to the nonpolar nature of the sorbent materials, limiting the competitive sorption of water molecules on the carbon surface. Besides hydrophobic interactions, the presence of benzene rings in the 2,4-D and PBA molecular structure is likely promoting π−π stacking with

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RESULTS AND DISCUSSION Morphology of Aerogel. Figure 1 shows the resulting aerogel after synthesis and cooling. The density of the aerogels is estimated to be 0.414 g/mL based on the measured mass and volume taken up in the beaker. SEM images of as-produced aerogels are shown in Figure 1. To gain a complete understanding of the structure, images were taken at 500× (a) and 3000× (b) magnification. As can be seen from the SEM images, the aerogels are nonuniform and exhibit a rough surface with irregular edges, cracks, and cavities. The porous structure of the aerogels can be attributed to the presence of steam generated during the exothermic dehydration of sucrose by sulfuric acid. Nitrogen sorption analysis of the template (0.0 wt %) and the sample with CNT/GnP at 0.2 wt % were completed to gain additional information on the aerogels. The corresponding specific surface area of the template was 7.83 m2/g and that of the sample with 0.2 wt % nanomaterials was 32.84 m2/g. The presence of graphitic nanomaterials in the aerogel composites was revealed by TEM observations, as indicated by the vertical lines contrasting from the amorphous carbon background in the inset of Figure 1c. Raman spectroscopy was completed to further confirm the presence of CNTs and GnPs within the aerogel structure. Figure 3d shows the Raman shift for the aerogel with 2 wt % carbon nanotubes. The spectral shift is consistent with a low content of carbon nanomaterials with a slightly stronger D peak around 1350 cm−1 compared to the G peak around 1600 cm−1. Though a slightly disordered structure exists, it does not necessarily translate into a 4130


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Figure 3. Adsorption isotherms of 2,4-D onto (a) 0.2 wt % sections of SWCNT-GnP aerogel and (b) 2.0 wt % sections of SWCNT-GnP aerogel at 20 °C. Adsorption isotherms of PBA onto (c) 0.2 wt % sections of SWCNT-GnP aerogel and (d) 2.0 wt % sections of SWCNT-GnP aerogel at 20 °C.

Figure 4. Time dependent adsorption of (a) 2,4-D with initial adsorbate concentration of 100 μg/mL and (b) PBA with initial adsorbate concentration of 15 μg/mL onto SWCNT-GnP aerogels for a time period of 90 min at 20 °C.

the delocalized monoelectronic π orbitals of the graphitic nanomaterials. Adsorption Kinetics Studied over Extended Time Period. The carbon nanotube aerogels were also studied over a longer period of time in order to fully establish an equilibrium

point and to ensure that the aerogel would not leech over time after it reached its peak adsorption capacity. The middle layer of the 0.2 wt % SWCNT-GnP aerogel was used for this elongated time study. The methodology for these trials follows exactly that described above for the short time period isotherm data 4131


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ACS Applied Nano Materials Table 2. Maximum Adsorption Uptakea of 2,4-D and PBA onto Aerogels with and without Graphitic Nanomaterials 0 wt % 0.2 wt % 2 wt %

2,4-D

PBA

79 104 104

55 82 82

q=

K1C 1 + K 2C

(1)

where K1 and K2 are equilibrium constants (mL/μg), and C is the concentration of the adsorbate solution (μg/mL). Table 3 presents the results of the data fitting for both adsorbates. The isotherms in Figure 6 are consistent with other reported isotherms for the adsorption of aromatic compounds into carbonaceous materials. The data from Figure 6 show that aerogels containing 0.2 and 2.0 wt % carbon nanomaterials had a larger adsorption capacity than the aerogel with zero percent loading of carbon nanomaterials. When the SWCNT/GnP content varied from 0 to 0.2%, the adsorption capacities for 2,4D and PBA increased by 25 and 45%, respectively. However, no additional increase in adsorption capacity was observed with greater loadings of carbon nanomaterials. This indicates that above 0.2%, additional carbon nanomaterials are wasted and do not generate new adsorption sites, possibly because of selfaggregation or entrapment in amorphous carbon during the dehydration process. The maximum adsorption capacities of 2,4-D and PBA onto the surface of the adsorbents at equilibrium were 103.5 mg of 2,4-D per mg of 2.0 wt % SWCNT-GnP aerogel, 104 mg of 2,4-D per mg of 0.2 wt % SWCNT-GnP aerogel, 83 mg of 2,4-D per mg of 0.0 wt % SWCNT-GnP aerogel, 82 mg of PBA per mg of 2.0 wt % SWCNT-GnP aerogel, 80.5 mg of PBA per mg of 0.2 wt % SWCNT-GnP aerogel, and 56 mg of PBA per mg of 0.0 wt % SWCNT-GnP aerogel. By subtracting the maximum adsorption capacity of the plain aerogel from that of the composite containing 0.2% SWCNT/ GnP and dividing by the mass of carbon nanomaterials, the contribution of carbon-nanomaterial adsorption can be quantified. The differences in the uptakes of 2,4-D and PBA between the plain aerogel and the composite with 0.2% SWCNT/GnP are 21 and 25 mg/g, respectively. Considering that 2 mg of aerogel (i.e., 4 μg of SWCNT/GnP hybrids) was used in the adsorption experiments, the adsorption capacities of the SWCNT/GnP hybrids for 2,4-D and PBA were 10.5 and 12.5 mg/mg, respectively. These values reveal that whereas the adsorption capacities of the aerogel composites are lower than previously reported values,2,3,33 the carbon nanomaterials are employed in a much more efficient way. Although the presence of carbon nanomaterials might induce changes in the structure

All values in μg/mg.

a

collection. The aerogel was allowed to adsorb material over a 5 h period for 2,4-D. Figure 5 shows that the maximum uptake by

Figure 5. Time dependent adsorption of 2,4-D with initial adsorbate concentration of 100 μg/mL for a time period of 5 h at 20 °C.

the aerogel of 2,4-D was 107.8 mg per mg of 0.2 wt % SWCNTGnP aerogel. The adsorption uptake reached steady state for the 2,4-D system at 245 min. Equilibrium Isotherm Data. Figure 6 shows the equilibrium isotherms for the adsorption of 2,4-D and PBA examined at 20 °C. The Langmuir isotherm was a good fit for the data

Figure 6. Adsorption isotherms of (a) 2,4-D and (b) PBA onto SWCNT-GnP aerogels at 20 °C. The curve fits correspond to Langmuir isotherms with equilibrium constants shown in Table 3. 4132


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ACS Applied Nano Materials Table 3. Langmuir Parameters 2,4-D

PBA

adsorbents

K1 (mL/μg)

K2 (mL/μg)

R2

K1 (mL/μg)

K2 (mL/μg)

R2

0.0% 0.2% 2.0%

1.405 1.358 1.467

4.94 × 10−3 2.43 × 10−3 3.17 × 10−3

0.99 0.99 0.99

21.453 48.251 64.166

0.3797 0.6343 0.8552

0.98 0.99 0.99

(4) Dichiara, A. B.; Webber, M. R.; Gorman, W. R.; Rogers, R. E. Removal of Copper Ions from Aqueous Solutions Via Adsorption on Carbon Nanocomposites. ACS Appl. Mater. Interfaces 2015, 7, 15674− 15680. (5) Arthur, D.; Prada Silvy, R.; Wallis, P.; Tan, Y.; Rocha, J. D. R.; Resasco, D. E.; Praino, R.; Hurley, W. Carbon Nanomaterial Commercialization: Lessons for Graphene from Carbon Nanomaterials. MRS Bull. 2012, 37, 1297−1306. (6) Ayranci, E.; Hoda, N. Adsorption of Bentazon and Propanil from Aqueous Solutions at the High Area Activated Carbon-Cloth. Chemosphere 2004, 57, 755−762. (7) Dichiara, A. B.; Benton-Smith, J.; Rogers, R. E. Enhanced Adsorption of Carbon Nanocomposites Exhausted with 2,4-Dichlorophenoxyacetic Acid after Regeneration by Thermal Oxidation and Microwave Irradiation. Environ. Sci.: Nano 2014, 1, 113−116. (8) Dichiara, A. B.; Harlander, S. F.; Rogers, R. E. Fixed Bed Adsorption of Diquat Dibromide from Aqueous Solution Using Carbon Nanotubes. RSC Adv. 2015, 5, 61508−61512. (9) Wu, C. H. Adsorption of Reactive Dye onto Carbon Nanotubes: Equilibrium, Kinetics and Thermodynamics. J. Hazard. Mater. 2007, 144, 93−100. (10) Cussler, E. L. Diffusion: Mass Transfer in Fluid Systems, 3rd ed.; Cambridge University Press, 2009. (11) Dichiara, A. B.; Weinstein, S. J.; Rogers, R. E. On the Choice of Batch or Fixed Bed Adsorption Processes for Wastewater Treatment. Ind. Eng. Chem. Res. 2015, 54, 8579−8586. (12) Pan, B.; Xing, B. Adsorption Mechanisms of Organic Chemicals on Carbon Nanotubes. Environ. Sci. Technol. 2008, 42, 9005−9013. (13) Peng, X. J.; Li, Y. H.; Luan, Z. K.; Di, Z. C.; Wang, H. Y.; Tian, B. H.; Jia, Z. P. Adsorption of 1,2-Dichlorobenzene from Water to Carbon Nanotubes. Chem. Phys. Lett. 2003, 376, 154−158. (14) Kuo, C. Y.; Lo, S. L. Adsorption of Aqueous 4-Chlorobiphenyl and Treatment with Uv-Illuminated Titanium Dioxide. J. Colloid Interface Sci. 1997, 196, 199−206. (15) Wang, F.; Yao, J.; Sun, K.; Xing, B. Adsorption of Dialkyl Phthalate Esters on Carbon Nanotubes. Environ. Sci. Technol. 2010, 44, 6985−6991. (16) Wang, F.; Yao, J.; Sun, K.; Xing, B. Adsorption of Dialkyl Phthalate Esters on Carbon Nanotubes. Environ. Sci. Technol. 2010, 44, 6985−91. (17) Kuo, C. Y.; Wu, C. H.; Wu, J. Y. Adsorption of Direct Dyes from Aqueous Solutions by Carbon Nanotubes: Determination of Equilibrium, Kinetics and Thermodynamics Parameters. J. Colloid Interface Sci. 2008, 327, 308−315. (18) Huang, B.; Li, Z. Y.; Liu, Z. R.; Zhou, G.; Hao, S. G.; Wu, J.; Gu, B. L.; Duan, W. H. Adsorption of Gas Molecules on Graphene Nanoribbons and Its Implication for Nanoscale Molecule Sensor. J. Phys. Chem. C 2008, 112, 13442−13446. (19) Niu, C. M.; Wu, W. H.; Wang, Z.; Li, S. M.; Wang, J. Q. Adsorption of Heavy Metal Ions from Aqueous Solution by Crosslinked Carboxymethyl Konjac Glucomannan. J. Hazard. Mater. 2007, 141, 209−214. (20) Kaiser, A.; Leidlmair, C.; Bartl, P.; Zottl, S.; Denifl, S.; Mauracher, A.; Probst, M.; Scheier, P.; Echt, O. Adsorption of Hydrogen on Neutral and Charged Fullerene: Experiment and Theory. J. Chem. Phys. 2013, 138, 074311. (21) Cruz, F. J. A. L.; Esteves, I. A. A. C.; Mota, J. P. B. Adsorption of Light Alkanes and Alkenes onto Single-Walled Carbon Nanotube Bundles: Langmuirian Analysis and Molecular Simulations. Colloids Surf., A 2010, 357, 43−52.

and porosity of the aerogel leading to higher uptake values, these estimations suggest that more sites are available for the adsorption of organic compounds when carbon nanomaterials are embedded in a 3-D network compared to when they are implemented as powders or buckypapers.2,8

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CONCLUSIONS SWCNT-GnP aerogels have been prepared by the dehydration of sucrose in the presence of SWCNT and GnP. All SWCNTGnP aerogels displayed much faster adsorption rates and higher adsorption capacities of 2,4-D and PBA when compared to previous studies involving adsorption onto hybrid SWCNTGnP papers. At both short- and long-time scales, adsorption was observed to be favorable, with PBA uptake being significantly better as a result of its unique structure with excess π−π noncovalent bonds. Adsorption uptake was observed to proceed at a faster rate even with a small quantity of carbon nanomaterial in the aerogel. This gives an economical advantage that only a small amount of carbon nanomaterials is required to significantly increase the adsorption capacity of these aromatic compounds. To further validate the economic viability of such an aerogel, more research is required to determine the range of compounds that these SWCNT-GnP aerogels can adsorb. Overall, however, the attributes shown by these aerogels give it potential to be used in aqueous purification systems.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (R.E.R.) *E-mail: [email protected] (A.B.D.) ORCID

Anthony B. Dichiara: 0000-0002-3118-7029 Reginald E. Rogers, Jr.: 0000-0002-2499-9686 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Kate Gleason College of Engineering for undergraduate student financial support during the course of this project. We also thank the RIT Office of the Vice President for Research for support during this project.

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REFERENCES

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