Article pubs.acs.org/IECR
Adsorption of Four Representative Biological Compounds onto Graphite Nanofibers Dorothy W. Skaf,*,† Michael A. Smith,† Kevin C. Brodwater,† Maulin N. Gandhi,† Anthony R. DeBiase,† and Alexander J. Zoelle† †
Department of Chemical Engineering, Villanova University, 800 East Lancaster Avenue, Villanova, Pennsylvania 19805, United States S Supporting Information *
ABSTRACT: The present study investigates the adsorption of four compounds important in carbon adsorption hemoperfusion. Graphite nanofibers (GNFs) having different carbon plane orientations have structural features that make them potentially attractive in this application compared to activated carbon. Generally, adsorption capacity increased in the order of ribbon > herringbone > platelet fiber types. All fibers were poor adsorbents for creatinine. Herringbone and platelet fibers had sorption capacities less than 25% of activated carbon for all adsorbates on a mass basis. Except for vitamin B12, behavior of ribbon fibers was similar; vitamin B12 adsorption was nearly 50% greater than activated carbon on a mass basis. In contrast, on the basis of surface area, all of the fibers were comparable to or outperformed activated carbon for the adsorption of all adsorbates. This suggests that in addition to BET surface area, interactions between adsorbate and exposed basal planes in carbon are important in determining adsorption capacity. Chemical treatment with hydrochloric acid or with urea followed by thermal treatment did not significantly change the fiber surface area, micropore volume, or equilibrium adsorption. Nitric acid treatment of herringbone and ribbon fibers slightly decreased the surface area but did not enhance adsorption, whereas this treatment degraded platelet fibers.
1. INTRODUCTION The availability of carbon-based materials with different structures provides an avenue for tailoring the materials for
been ongoing efforts to design improved carbon-based adsorbents for hemoperfusion.2 Graphite nanofibers (GNFs) have a unique combination of moderate surface area, electrical conductivity within the carbon planes, and surface functionalities that may prove advantageous in separations and purifications, specifically hemoperfusion. However, one of the disadvantages of GNFs is that they do not provide the very high pore volume and surface area afforded by microporous materials such as activated carbons and zeolites. Despite this disadvantage, the favorable characteristics of GNFs warrant investigation of their performance in removing uremic toxins. Several investigators have evaluated the hemoperfusionrelated adsorption properties of various forms of carbon using vitamin B12, creatinine, and other model compounds. Yang et al.3 made phenolic-resin-derived activated carbon spheres with various pore sizes through the addition of pore-forming agents and tested these with vitamin B12 and creatinine. The best sorption capacity for vitamin B12 was approximately 9 mg vitamin B12/gram of carbon for an initial vitamin B12 concentration of 25 mg/L. For creatinine, the best sorption capacity was approximately 45 mg creatinine/gram of carbon for an initial creatinine concentration of 100 mg/L. Carbon nanotubes (CNTs) from Ni-catalyzed pyrolysis of propylene have been studied as adsorbents for creatinine and vitamin B12 by Ye et al.4,5 They found that the CNTs had a sorption capacity (24 mg creatinine/gram of carbon) slightly lower than that of an activated carbon sorbent used in a commercial
Figure 1. Sketches of (a) platelet, (b) herringbone, and (c) ribbon fibers. Sketches adapted from Bessel et al.13
specific applications. Hemoperfusion, the direct contact of blood with adsorbent, has application for removal of adsorbates with molecular weights between 500 and 6000 Da, which are too large to pass through dialysis membranes.1 Activated carbon is the adsorbent of choice; however, there have also © 2012 American Chemical Society
Received: Revised: Accepted: Published: 8286
August 11, 2011 May 23, 2012 May 25, 2012 June 8, 2012 dx.doi.org/10.1021/ie201788j | Ind. Eng. Chem. Res. 2012, 51, 8286−8292
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Figure 2. Adsorbates used in experiments.
introduced basic surface functionalities which enhanced adsorption of phenol.10 Chemical treatments have also included coating carbon to enhance adsorption and to improve biocompatibility. Mesoporous carbon with pore diameters ranging from 2.5 to 3.4 nm showed improved uptake of vitamin B12 after coating with poly(methyl methylacrylate), despite a reduction in pore volume and surface area. This was attributed to hydrogen bonding between the coating and the solute.11 Additionally, the use of structural groups and antibodies on polymer-based adsorbents has improved selectivity for middle molecular weight solutes.1,12 In this work, adsorption studies were performed using platelet, herringbone and ribbon carbon nanofibers, named for their carbon plane orientations.13 Sketches in Figure 1 illustrate the as-grown fibers, with a dark core representing the metal catalyst particle from which the fibers grow. These fibers have been tested for adsorption of solutes related to hemoperfusion. Creatinine and uridine were selected as representative low molecular weight molecules. Vitamin B12 was selected as a representative middle molecular weight molecule. Acetaminophen was selected as a molecule with a smaller conjugated aromatic structure than vitamin B12. Figure 2 shows the chemical structure of each adsorbate.14
hemoperfusion unit, but higher than that of a commercial macroporous resin. However, the CNTs reached equilibrium with creatinine faster than activated carbon. The CNTs were excellent adsorbents for vitamin B12, having a 10-fold higher adsorption capacity than activated carbon and a faster approach to equilibrium. Chemical treatment of fibers has also been tested as a means to enhance adsorption, whether through physical changes, such as increased porosity and surface area, or chemical changes, such as increased density of oxygenated moieties. Since adsorption is associated with both the rejection of an incompatible solute from the solvent and the attraction of the solute to the adsorbent surface,6 adding oxygen- or nitrogencontaining groups to the exposed edges of the carbon planes could potentially change the capacity and selectivity of the adsorbent. Li et al.7 pretreated carbon fibers using 5 M HNO3 and studied the effect of adsorption of aniline, pyridine, and phenol. The acid treatment significantly increased the concentration of oxygenated surface groups, such as C−OH, CO, and C−O−R. For all solutes evaluated, the pretreated carbon fibers were poorer adsorbents, which was attributed to increased hydrophilicity of the adsorbent. An earlier study using platelet and herringbone fibers showed that the impact of acid treatment on alcohol adsorption varied considerably depending on fiber type, acid, and solute and that harsh treatment with oxidizing acids was not beneficial.8 Cuervo et al.9 observed that oxidative treatment with HNO3 decreased the adsorption capacity of commercial carbon nanofibers for a variety of chemical families. Urea treatment of activated carbon
2. EXPERIMENTAL SECTION 2.1. Materials. Creatinine, uridine (>99%), vitamin B12 (∼ 99%), and acetaminophen (min 99%) were purchased from Sigma Aldrich. All chemicals were used as received and solutions were made in deionized water. Herringbone, platelet, 8287
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Figure 3. SEM images of (a) activated carbon and (b) herringbone fiber morphology.
Figure 4. SEM images of herringbone fibers before and after chemical treatment: (a) untreated, (b) HNO3 treated, (c) HCl treated, (d) urea and heat treated.
days, filtered, rinsed with deionized water, and dried in warm air. (2) Fibers were added to 1.0 M HCl, stirred for 15 min, then stored without stirring at room temperature for 6 days,8 filtered, rinsed with deionized water, and dried in warm air. (3) Fibers were stirred in saturated urea solution at 60 °C for 1 h, rinsed with distilled water, dried in air, and subsequently heated under helium flow at 400 °C for 1 h.10 The solutes were also tested with Calgon F400 activated carbon that was sieved to pass through a Tyler 24 mesh ( herringbone > platelet. Ribbon, herringbone, and platelet fibers had sorption capacities that were generally less than 25% of the activated carbon capacity for all adsorbate/adsorbent combinations, with the lone exception of vitamin B12 and ribbon fiber. For this 8290
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introduced during treatments. Low adsorption for these fiber types relative to the ribbon form suggests edge functionality is not an important factor in determining adsorption capacity. This view is also consistent with the observation that chemical treatments did not substantially alter adsorption capacity. Due to the lower cost of activated carbon and its superior adsorption capacity compared to GNFs on a mass basis, GNFs are not an economical replacement for activated carbon in hemoperfusion applications. However, the superior performance of GNFs per surface area, especially ribbon fibers with their exposed basal planes, suggest development of carbons with more graphitic structure may result in improved adsorbents for hemoperfusion. These results also suggest that the substantial microporosity typical of activated carbon is not necessarily advantageous in this application.
case, ribbon fibers had a capacity nearly 50% higher than activated carbon for vitamin B12. Comparisons on the basis of solute adsorbed per surface area of adsorbent, as shown in Figure 6b, show that all fiber types outperformed the activated carbon for use with acetaminophen and vitamin B12. For uridine, herringbone and platelet fibers were comparable to activated carbon, whereas ribbon fibers outperformed activated carbon. The high capacity of the ribbon fibers, despite their low area and porosity, could potentially be attributed to interactions between the functional groups of the adsorbates and exposed (001) basal planes of the ribbon fiber surface.18 Because carbon nanotubes have similar exposed graphitic planes, the high vitamin B12 adsorption observed with ribbon fibers is consistent with Ye et al.’s observed high adsorption of vitamin B12 onto carbon nanotubes.4,5 The improved fiber performance relative to activated carbon on the basis of mg solute adsorbed per surface area of adsorbent is most likely because the surface area associated with at least some of the micropores of activated carbon is inaccessible to the solutes in this study. The time dependence of adsorption for each solute is shown in Figure 7. The fibers typically reach their equilibrium capacity within 15−30 min, as opposed to the slower equilibrium approach demonstrated by the activated carbon. This observation is consistent with the low fiber porosity and absence of a pore diffusion limitation for adsorption. The disadvantage of low adsorption capacity for the fibers may be offset in certain applications by the rapid equilibration with solute afforded by the fibers. The equilibrium adsorption isotherms for each solute with ribbon fibers were also obtained and are shown in Figure 8. The vitamin B12 adsorption onto ribbon fibers was comparable to the vitamin B12 adsorption reported for carbon nanotubes that had BET surface area of 122 m2/g and total pore volume of 0.607 cm3/g.4 The ribbon fibers adsorbed significantly less creatinine than was reported for the same carbon nanotubes, which may be a reflection of the uncertainty in our creatinine data. The untreated fibers were especially poor adsorbents for creatinine and uridine, important solutes for hemoperfusion, therefore, further studies were performed to determine whether chemical treatment of the fibers would improve their adsorption. Results for adsorption of uridine and creatinine onto chemically treated fibers are presented in Figure 9. In general, chemical treatment did not alter the relative adsorption performance between fiber types, with ribbon > herringbone > platelet. The exception to this is the improved performance of herringbone fibers after HCl treatment; HCl-treated herringbone fibers outperformed similarly treated ribbon fibers for both uridine and creatinine adsorption. For the strongly adsorbing ribbon fibers, HNO3 and urea treatment had negligible effect on sorption capacity, whereas HCl treatment decreased uridine and increased creatinine uptake. As noted above, HNO3 treatment degraded the platelet fibers to the point that they could not be recovered by gravity filtration. As illustrated in Figure 1, ribbon fibers have the highest fraction of exposed basal planes. The observation that ribbon fibers have the highest adsorption capacity per surface area is consistent with our suggestion that interactions between adsorbate and exposed basal planes in carbon impact adsorption. In contrast, herringbone and platelet fibers show primarily exposed basal plane edges, these edges must be terminated by heteroatoms such as hydrogen from initial fiber synthesis, or oxygen or nitrogen containing functional groups
4. CONCLUSIONS In this study, we compare and contrast three types of graphite nanofibers with activated carbon for the adsorption of four model compounds relevant to hemoperfusion. In general, fiber adsorption capacity consistently increased in the order of ribbon > herringbone > platelet. Under test conditions, untreated herringbone and platelet fibers had sorption capacities (mg adsorbate/mg adsorbent) for all adsorbates less than 25% of the activated carbon capacity on a mass basis. Behavior of ribbon fibers was similar, with the exception of vitamin B12, which was nearly 50% greater than for activated carbon on a mass basis. When compared on the basis of surface area, all of the fibers were either comparable to or outperformed activated carbon for the adsorption of all adsorbates. On this basis, ribbon fibers have higher capacity for uridine than the other adsorbents and all four adsorbents had a comparable low capacity for creatinine. The superior performance of GNFs over activated carbon on a surface area basis was attributed to the complete availability of the BET surface area for adsorption. In contrast, micropores in activated carbon that contribute to BET surface area are likely too small to be accessible to these adsorbates. Additionally, the kinetics of adsorption on fibers was found to be qualitatively superior to kinetics of adsorption on activated carbon. For the GNFs tested, there is a strong relationship between fiber type and adsorption capacity, with ribbon fibers exhibiting superior performance. This could be attributed to interactions between the function groups on the adsorbates and the carbon basal planes of the fibers. Chemical treatment of the fibers with HNO3, urea and heating, or HCl had little effect on the BET surface area and pore volume, and a modest effect on sorption capacity. The exception was HNO3 treatment of platelet fibers which caused fiber degradation.
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ASSOCIATED CONTENT
S Supporting Information *
The data presented in Figures 4 and 6 in tabular format; additional SEM images of fibers, including the three fiber types after HCl treatment. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 8291
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ACKNOWLEDGMENTS We are grateful for financial support from the National Science Foundation (Grant 0416040) and the Chemical Engineering Department of Villanova University. We thank Charles Coe for helpful discussions and review of this manuscript.
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