Electrochemical Carbon Nanotube Filter for Adsorption, Desorption

Feb 16, 2011 - + 2h+ f Cl2 (E0 = 1.36 V),38 and the second line crosses 0 mA at 2.3-2.4 V, representing the one-electron oxidation of chloride to a Cl...
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ARTICLE pubs.acs.org/JPCC

Electrochemical Carbon Nanotube Filter for Adsorption, Desorption, and Oxidation of Aqueous Dyes and Anions Chad D. Vecitis,* Guandao Gao, and Han Liu School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States

bS Supporting Information ABSTRACT: An electrochemically active multiwalled carbon nanotube (MWNT) filter is observed to be effective toward the adsorptive removal and electrochemical oxidation of the aqueous dyes, methylene blue and methyl orange, and the oxidation of the aqueous anions, chloride and iodide. In the absence of electrochemistry, the MWNT filter completely removed all dye from the influent solution until a near monolayer of dye molecules adsorbed to the MWNT filter surface. Electrochemical filtration at 2 V resulted in >98% oxidation of the influent dye during a single pass through the 41 μm thin porous MWNT network with a e1.2 s residence time. The electrochemical MWNT filter was also able to oxidize aqueous chloride and iodide with minimal overpotential. However, the oxidation of these anions was limited by the number of electrochemically active MWNT surface sites. These results show the potential of an electrochemical MWNT filter for the adsorptive removal and oxidative degradation of aqueous contaminants.

’ INTRODUCTION Carbon nanotubes (CNTs) have a number of unique physical chemical properties, and the combination of these properties in a single material is optimal for a wide range of applications.1 Carbon nanotubes are graphene sheets seamlessly rolled into singlewalled nanotubes (SWNTs) or coaxial double- and multiwalled nanotubes (DWNTs and MWNTs).2 CNTs have high aspect ratios (103-107), large specific surface areas (50-1000 m2 g-1),3 and exceptional mechanical strength4 and are conducting or semiconducting.5 Thus, CNTs can be formed into mechanically strong and electrically conducting porous thin films or threedimensional networks that have potential for many applications. Carbon nanotubes have large specific surface areas3 and have found utility as adsorbents for both aqueous organic and inorganic contaminants.6 For example, CNTs have been observed to strongly adsorb ionic dyes,7 chlorophenols,8 and natural organic matter via van der Waals interactions with the sp2-conjugated CNT sidewalls.9 CNT oxidation produces a large number of carboxylate surface groups that can bind metal ions, such as Zn2þ and Cd2þ.10 CNTs coated with ceria have been utilized to remove chromium and arsenate from aqueous solutions.11,12 The majority of the CNT adsorption studies utilize batch experiments,7,9-12 with only analytical separation studies8,13 utilizing column experiments that are more applicable to removal of contaminants from aqueous waste streams. The high carbon nanotube aspect ratio and mechanical stability allows for the production of both randomly oriented and aligned porous CNT networks for water purification. Randomly oriented single-walled carbon nanotube (SWNT)14,15 and multiwalled r 2011 American Chemical Society

carbon nanotube (MWNT)16 filters are reported to be effective for complete removal of bacteria by sieving and multilog removal of viruses by depth filtration. The interstitial pore size of CNT networks can be tuned by compression, resulting in mechanically modulated filtration characteristics17 and spongelike properties.18 Aligned MWNT forests produced during synthesis are also effective for removal of heavy petroleum hydrocarbons, bacteria, and viruses from aqueous solution by gravity filtration through their interstitial spaces.19 Aligned MWNTs sealed in a polymer film yield a membrane where the nanotubes themselves serve as pores.20 The hybridization of the CNT adsorptive and filter properties has potential to yield an effective column for removal of aromatic and hydrophobic contaminants from water. The electrical conductivity5 and corrosive stability21 of CNTs have led to their use as three-dimensional conductive substrates22 for a range of energy and/or environmental applications. For example, SWNTs have potential as substitutes for graphite in Liþ batteries,23 and CNTs have been used as catalyst substrates for both the cathode24 and the anode25 in fuel cells and in photoelectrochemical cells for solar energy conversion.26,27 CNTs have also found utility for electroanalytical purposes.28,29 Similarly, hybridization of the CNT adsorption, filtration, and conductive properties may yield an electrochemically active filter for water treatment purposes. Yang et al.30 evaluated MWNTs packed between and supported by two pieces of activated carbon felt for both the porous Received: December 13, 2010 Revised: January 25, 2011 Published: February 16, 2011 3621

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The Journal of Physical Chemistry C cathode and the porous anode of a seepage carbon nanotube electrode reactor. The carbon felt-CNT electrode reactor was operated at fluxes characteristic of granular filtration and was found to be more efficient than classical bipolar electrode arrangements due to enhanced mass transfer of aqueous species to the anode surface during filtration for direct oxidation. However, the carbon felt-CNT electrodes utilized in this study may not have optimal electrochemical or filtration characteristics. For example, there are a diverse number of reported CNT filter preparation methods that yield vastly different CNT network structures. The CNT network structure will significantly affect filter properties, including flow rate, average pore size, and specific surface area,14,16-20 which, in turn, will affect the electrochemical CNT filter performance. Here, we design and modify a commercial microfiltration casing to allow for in situ electrochemistry. A perforated stainless steel cathode is selected for low cost, no restriction of water flow, and iron catalysis of hydrogen production. An electrochemically active multiwalled carbon nanotube anodic microfilter is prepared from well-dispersed MWNTs into a free-standing porous thin film, 40-100 μm, with a relatively uniform pore diameter, 50-130 nm. The electrochemical filtration device is evaluated toward adsorption, desorption, and oxidation of the aqueous dyes: the positively charged methylene blue (MB) and the negatively charged methyl orange (MO) and the oxidation of the aqueous anions iodide (I-) and chloride (Cl-). We chose these species because they cover a range of molecular weights/ sizes and thus adsorption characteristics, both positive and negative charges and thus electrostatic interactions, and a range of redox potentials and thus susceptibility to oxidation. Results indicate that electrochemical filter design and electrode selection/preparation can have significant effects on electrochemical filtration performance.

’ MATERIALS AND METHODS Chemicals and Materials. Methylene blue (0.05% in water), methyl orange (>95%), sodium iodide (>99.9%), and sodium chloride (>99.0%) were purchased from Sigma-Aldrich. The multiwalled carbon nanotubes (MWNTs) were purchased from NanoTechLabs as individual nanotubes and in preformed sheets of a range of depths: thin (∼40 μm), medium (∼70 μm), and thick (∼100 μm). The thinnest MWNT sheets, determined to be 41 ( 8 μm by SEM analysis, were used in all experiments unless noted otherwise. The MWNT filters were produced by dispersing the MWNTs in DMSO at 0.5 mg/mL and probe sonicating (Branson) for 15 min. The sonicated MWNTs in DMSO (5-30 mL) were then vacuum-filtered onto a 5 μm PTFE membrane (Millipore, Omnipore, JMWP). The MWNT filters were washed sequentially with 100 mL of EtOH, 100 mL of 1:1 DI-H2O/ EtOH, and 250 mL of DI-H2O before use. All aqueous solutions were made with water from a Barnstead Nanopure Infinity purification system that produced water with a minimal resistivity of 18 MΩ 3 cm-1. All solutions had 10 mM NaCl as a background electrolyte to normalize the ionic strength and conductivity unless otherwise noted. For experiments, influent methylene blue was made to a concentration of 7.0 ( 1.0 μM, influent methyl orange was made to a concentration of 25.0 ( 2.0 μM, and influent iodide was made to a concentration of 1 or 10 mM. Electrochemical Filtration. All filtration experiments were completed using the modified electrochemical filtration casing as described in the text. First, a 5 μm pore PTFE membrane was

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placed on the bottom piece of the casing and wetted. Next, the 47 mm diameter multiwalled carbon nanotube (MWNT) filter (NanoTechLabs) was placed on top of the PTFE membrane and wetted. A layer of water was then spread on the MWNT filter and allowed to sit for 10-15 min until the water had seeped through the filter. The filtration casing was then sealed, and the top half of the casing was primed with DI water using a needle syringe to remove any air that could restrict flow. Water was then peristaltically pumped (Masterflex) through the filter at 1.5 ( 0.1 mL min-1 to compact and rinse the MWNT filter and to calibrate the flow rate, which was measured gravimetrically (Pinnacle, Denver Instruments). After the water rinse was complete, the pump was first primed with the appropriate influent solution and then the experiment was started. Sample aliquots were collected directly from the filter casing outlet and analyzed immediately after collection. UV-vis Analysis. The quantification of aqueous methylene blue, methyl orange, and triiodide was completed on an Agilent 8453 UV-visible spectrophotometer. Aliquots (0.5-0.75 mL) of filter effluent were collected in a 1 mL glass cuvette with a 1 cm path length. Methylene blue was quantified by its absorption at λmax = 665 nm (ε = 74 100 M-1 cm-1). For high-concentration methylene blue solutions, absorption at 550 nm (ε = 6190 M-1 cm-1) was used for quantification. Methyl orange was quantified by its absorption at λmax = 464 nm (ε = 26 900 M-1 cm-1). Triiodide was quantified by its adsorption at 287 nm (ε = 40 000 M-1 cm-1) or 353 nm (ε = 26 400 M-1 cm-1) for higher concentrations. High aqueous triiodide concentrations were diluted 10 times with DI water prior to analysis. BET Surface Area Analysis. The specific surface area of the MWNT nanotube filters was measured with a Beckman Coulter SA 3100 surface area and pore size analyzer. Approximately 0.1 g of filter sample was placed into a glass analysis tube. The sample was dried at 120 °C for 1 h prior to analysis. SEM Analysis. Scanning electron microscopy was completed in Harvard’s Center for Nanoscale Systems on a Zeiss FESEM Supra55VP. Scanning electron micrographs were analyzed with ImageJ software.

’ RESULTS AND DISCUSSION Design and Operation of the Electrochemical Filter. Scheme 1 contains a schematic (A) and images (B-E) of the electrochemical filtration device. A commercial 47 mm polycarbonate filtration casing (Whatman) was modified to allow for simultaneous electrochemistry (Scheme 1B,C). Two holes were drilled in the upper piece of the filtration casing as openings for the cathodic and anodic leads. The main components of the electrochemical filter casing are the perforated stainless steel cathode (1) separated with an insulating silicone rubber seal (2) from the titanium anodic ring-connector (3). When the filtration casing is sealed, the anodic Ti ring (3) is pressed into the carbon nanotube filter (4) for electrical connectivity. Images of the electrochemical filtration setup and the MWNT filters mats used in this study can be found in Figure S1 (Supporting Information). Figures 1 and S2 (Supporting Information) show both aerial and cross-sectional SEM images of the MWNT filter. The MWNT mat is composed of randomly oriented MWNTs ( = 15 nm, = 100 μm, 4-5% residual Fe catalyst, NanoTechLabs). The SEM images were analyzed by ImageJ to determine the average pore size, 115.2 ( 46.7 nm, and height, 41.1 ( 7.6 μm. The MWNT network has an effective filtration 3622

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Scheme 1. Depiction and Images of the Electrochemical Filtration Apparatusa

(A) Design of the modified commercial polycarbonate filtration casing consisting of (1) a perforated stainless steel cathode, (2) an insulating silicone rubber separator and seal, (3) a titanium anodic ring that is pressed into the carbon nanotube anodic filter, and (4) the MWNT anodic filter supported by a PTFE membrane. (B, C) Images of the modified filtration casing. (D, E) Images of the MWNT network before and after electrochemical filtration, respectively. a

Figure 1. Scanning electron micrographs of the MWNT filter. (A) Aerial image of the MWNT filter with an average pore size of 115 ( 47 nm. (B) Cross-sectional image of the MWNT filter with an average height of 41 ( 8 μm.

area of 706 mm2. The total volume of the filter (not excluding CNTs) is 0.029 mL; thus, an upper limit for liquid residence time at 1.5 mL min-1 in the filter is τ e 1.2 s. The density of the filter is

0.36 g cm-3 and the bulk density of the MWNTs is 2.3-2.4 g cm-3, resulting in an ∼85% pore volume. The specific surface was measured to be 88.5 ( 4.3 m2 g-1; thus, each filter with a thickness of 41 μm has ∼1.05 m2 of total MWNT surface area. I-V curves for sodium chloride electrolyte solutions can be found in Figure 2. In Figure 2A, the instantaneous current of the aqueous solution flowing at 1.5 mL min-1 is plotted as a function of applied voltage and NaCl concentration, where “instantaneous” is defined as the initial current value displayed. At all potentials, the current increases with increasing electrolyte concentration. When NaCl is present, the current increases linearly with increasing potential above 2.3 V. This corresponds to the one-electron oxidation of chloride: Cl- þ hþ f Cl• (E0 = 2.4 V).31 At the higher NaCl concentrations (10 and 100 mM), there is broad current peak from 0.7 to 1.7 V. The MWNTs utilized in this study contain 4-5% residual iron catalyst (NanoTechLabs). Thus, this broad peak may correspond to iron oxidation. This is consistent with Figure 2B that compares the “instantaneous” to “steady-state” current, where “steady-state” is the current after 10 s at a chosen potential. In the “steady-state” I-V curve, the broad peak has disappeared, indicating that there is a finite amount of a current generating species at the MWNT surface, such as the residual iron catalyst. The electrochemical filtration process at 3 V decreases the unbuffered influent pH 6.3 slightly to 5.3. The effect of the liquid flow rate on the electrochemical MWNT filter I-V curves is presented in Figure 2C. The current is observed to slightly increase with increasing flow rate, but the magnitude of the effect is relatively small. Dye Adsorption to the MWNT Filter. Figure 3A shows the methylene blue adsorption breakthrough curve, [MB]eff/[MB]in versus t, in the absence of electrochemistry for three MWNT filters of varying physical dimensions ([MB]in = 7.0 ( 1.0 μM, [NaCl]in = 10 mM, J = 1.5 ( 0.1 mL min-1). The black squares, red circles, and blue triangles represent filters of average height (h) and diameter (d) of h = 41 μm and d = 30 mm, h = 68 μm and 3623

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Figure 3. MWNT filter dye adsorption isotherms. (A) Methylene blue adsorption breakthrough curves for MWNT filters of various dimensions; [MB]in = 7.0 ( 1.0 μM, [NaCl] = 10 mM, and 1.5 ( 0.1 mL min-1: (black squares) thickness (h) = 41 μm and diameter (d) = 30 mm, (red circles) h = 68 μm and d = 30 mm, and (blue triangles) h = 41 μm and d = 40 mm. (B) Methyl orange adsorption breakthrough curves for three MWNT filters of similar dimensions; [MO]in = 25.0 ( 2.0 μM, [NaCl] = 10 mM, 1.5 ( 0.1 mL min-1: h = 41 μm, and d = 30 mm. Representative plots are shown, and all experiments were completed in at least duplicate.

Figure 2. Electrochemical MWNT filter I-V curves as a function of NaCl concentration and liquid flow rate. (A) “Instantaneous” current (mA) as a function of applied potential (V) for [NaCl]in = 0, 1, 10, and 100 mM, where “instantaneous” is described as the first current reading displayed after setting to a specific voltage. (B) Comparison of “instantaneous” vs “steady-state” I-V curves for 10 mM NaCl at 1.5 mL min-1, where “steady-state” occurs after sufficient electrolysis time such that the current does not change, 10-15 s. (C) “Instantaneous” I-V curves for 10 mM NaCl at flow rates of 0, 0.5, 1.5, 2.5, and 3.5 mL min-1.

d = 30 mm, and h = 41 μm and d = 40 mm, respectively. In all cases, the effluent methylene blue concentration was below the limit of detection prior to breakthrough, indicating that all MB molecules had at least one collision that could result in sorption with the MWNT surface during a single pass through the filter of e1.2 s. Images of the filtration setup during the MB adsorption process can be found in Figure S3 (Supporting Information). The MB sorption capacities of the three filters were 28.5, 29.0, and 26.4 mg g-1, lower than previous reports for dye adsorption to MWNTs.7 The specific BET surface area of the MWNT filter was independent of filter physical dimensions and determined to be 88.5 ( 4.3 m2 g-1, and the area per MB molecule adsorbed for

the three filters was 163, 161, and 176 Å2 per molecule. The molecular area of methylene blue has been estimated to be 160 Å2,32 indicating that MB adsorption to the MWNT filters occurs until monolayer coverage. Figure 2B shows the methyl orange adsorption breakthrough curve, [MO]eff/[MO]in versus t, in the absence of electrochemistry for three MWNT filters of similar physical dimensions ([MO]in = 25.0 ( 2.0 μM, [NaCl]in = 10 mM, J = 1.5 ( 0.1 mL min-1). The black squares, red circles, and blue triangles represent adsorption experiments completed on three different MWNT filters of average height (h) and diameter (d) of h = 41 μm and d = 30 mm to display the repeatability of the procedure. In all cases, the effluent methyl orange concentration was below the limit of detection prior to breakthrough, indicating that all MO molecules had at least one collision that could result in sorption with the MWNT surface during a single pass through the filter of e1.2 s. The results of the three runs were quite similar, showing the reproducibility of the adsorption process. Images of the filtration setup during the MO adsorption process can be found in Figure S4 (Supporting Information). The filter MO sorption capacity was 30.0 mg g-1, slightly higher than the MB, but still lower than previous reports on dye adsorption to MWNTs that were in the range of 80-250 mg g-1.7 The lower MWNT sorption capacities observed in this study are likely due 3624

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The Journal of Physical Chemistry C to a lower MWNT filter specific surface area, 88.5 m2 g-1, as compared with that of other CNTs that can have >500 m2 g-1.3 The filter surface area per adsorbed MO molecule was 144 Å2 per molecule, slightly smaller than that observed for methylene blue and also indicative of monolayer formation. The complete removal of both influent methylene blue and methyl orange during a single pass (e1.2 s) though the thin (h = 41 μm, d = 30 mm) MWNT filter shows the potential of the filter for adsorptive removal of aquatic contaminants from solution. The efficient adsorptive removal at microfiltration flow rates (130 L m-2 h-1) is due to the dyes' strong affinity to the MWNT surface,7 the large MWNT surface area (88.5 m2 g-1, 1.05 m2), and the MWNT filter pore size. For example, the average MWNT filter pore diameter is 115 ( 47 nm; thus, if a dye molecule is at the center of the largest pore, the maximum distance to an MWNT surface is (115 þ 47)/2 = 81 nm. The maximum dye diffusion time, td = ld2/(2D), to the MWNT surface can be estimated using this distance and a diffusion coefficient, D = 10-5 cm2 s-1 = 103 nm2 μs-1.33 Thus, an influent dye molecule will collide with an MWNT surface in the filter with a maximal characteristic time of 3.3 μs, and thus, during the filter residence time, τ e 1.2 s, a single dye molecule could have 100 s of collisions with an MWNT interface. Because of the planar dye’s affinity for the MWNT surface, one of these collisions will be adsorptive, in agreement with results presented in Figure 3. Because of the thin film nature of the filter, the total adsorptive capacity of the MWNT filter is relatively low; that is, dye breakthrough occurs in 1, suggesting that the adsorbed MB is electrostatically desorbed. This is expected because methylene blue is positively charged at the unbuffered pH 6.3 used for the experiments, and anodic operation of the MWNT filter will result in accumulation of positively charged holes at the anode surface and/or generation of protons near the MWNT interface. The increase in effluent MB concentration was correlated with applied potential, 3 V ([MB]eff/[MB]in = 20) > 2 V ([MB]eff/[MB]in = 6) > 1 V ([MB]eff/[MB]in = 2), consistent with an electrostatic desorption mechanism. In all cases, upon continued electrolysis, the [MB]eff/[MB]in quickly decreased until it achieved an equilibrium value of 1 for 1 V and 98% and >93% of the influent dye at all points in time, respectively. The absence of dye breakthrough at 2 and 3 V indicates that the primary MB loss mechanism is oxidation. Once again, 2 V is observed to be more effective than 3 V toward dye oxidation, indicating that application of 3 V may be detrimental to the operation of the electrochemical MWNT filter.

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Figure 5. Electrochemical filtration of dyes as a function of applied potential. Experimental conditions are the same as those in Figure 1. (A) Electrochemical methylene blue filtration at potentials of 0 V (black squares), 1 V (red circles), 2 V (blue triangles, pointed up), and 3 V (green triangles, pointed down). (B) Electrochemical methyl orange filtration at potentials of 0 V (black squares), 1 V (red circles), 2 V (blue triangles, pointed up), and 3 V (green triangles, pointed down). Representative plots are shown, and all experiments were completed in at least duplicate.

Figure 5B shows the results of the electrochemical filtration of methyl orange over a range of applied potentials (0-3 V) under conditions of [MO]in = 25.0 ( 2.0 μM, [NaCl]in = 10 mM, and J = 1.5 ( 0.1 mL min-1. The application of 1 V results in a slight delay in the dye breakthrough as compared to 0 V conditions and a slight decrease in [MO]eff/[MO]in = 0.8. This is similar to the equilibrium MO effluent concentration in Figure 3B. A recent study37 evaluated the MO oxidation potential as a function of pH, which was observed to increase with decreasing pH: 0.3 V at pH 7 to 0.7 V at pH 3. The influent MO solution is unbuffered at pH 6.3, and the estimated MO oxidation potential, EpH 6.3 = 0.37, is at variance with the observed extent of MO oxidation. That is, most of the influent MO should have been oxidized at pH 6.3 and 1 V. The variance between the theoretical and experimental results suggests that the solution near the anodic MWNT surface has a lower effective pH than the bulk water and is consistent with the observed pH decrease during electrochemical MO monolayer oxidation in Figure S4 (Supporting Information). There was no shift in the effluent MO UV-vis adsorption peak, suggesting that this is a surface phenomenon. The application of 2 and 3 V during MO filtration resulted in the removal and/or oxidation of >98% and >93% of the influent dye at all points in time, respectively. The absence of dye breakthrough at 2 and 3 V indicates that the primary MO loss mechanism during electrochemical filtration is 3626

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The Journal of Physical Chemistry C oxidation. The results are in agreement with the methylene blue electrochemical filtration results. The >90% oxidation of influent methylene blue and methyl orange in a single pass through the MWNT filter (h = 41 μm, d = 30 mm) is an impressive result because the characteristic solution residence time within the filter is e1.2 s. The efficient oxidation of these dyes shows the potential of the electrochemical MWNT filter for degradation of aqueous organic contaminants. The efficacy of the anodic MWNT filter toward dye oxidation is likely enhanced by the strong affinity of these planar aromatic molecules for the sp2-conjugated nanotube surface.6,7 Thus, it is of interest to compare the reactivity of these dyes to aqueous species that will have a weak affinity for the MWNT surface to examine the importance of adsorption to the oxidation process. Here, the electrochemical filtration of the aqueous anions, chloride, Cl-, and iodide, I-, is examined. Electrochemical Anion Filtration. Figures 1 and 6 show I-V curves and the electrochemical filtration of the aqueous chloride and iodide solutions. The steady-state I-V curves for 10 mM NaCl (black squares) and 10 mM NaCl-10 mM NaI (red circles) flowing at 1.5 mL min-1 in Figure 6A can both be described with two straight lines. Regarding the “steady-state” I-V curve for NaCl, the first line crosses 0 mA at 1.25 V, representing the onset of the two-electron oxidation of chloride to chlorine, 2Cl- þ 2hþ f Cl2 (E0 = 1.36 V),38 and the second line crosses 0 mA at 2.3-2.4 V, representing the one-electron oxidation of chloride to a Cl atom, Cl- þ hþ f Cl•. Similar to chloride, the first iodide line represents a two-electron oxidation process yielding iodine, 2I- þ 2hþ f I2 (E0 = 0.55 V),38 and the second line represents a one-electron oxidation process yielding an I atom, I- þ hþ f I• (E0 = 1.5 V).31 The point where the extrapolation of these lines crosses 0 mA represents the threshold potential for anodic MWNT oxidation of ions. In both cases, there is minimal oxidation overpotential at the MWNT anode. The current peak for the NaCl-NaI solution occurs at 2.0 V, indicating an optimal potential for iodide oxidation during electrochemical filtration. Upon increasing the applied potential above 2 V, the NaCl-NaI current decreases until the onset of Cloxidation at 2.5 V, where the current begins to increase again. Figure 6B shows the results of the electrochemical MWNT filtration at 1.5 mL min-1 of the 10 mM NaCl-10 mM NaI solution over a range of applied potentials (0-3 V). The percent iodide oxidized, [I-]ox/[I-]in  100, is plotted as a function of time, where [I-]ox = 2[I3-]. In the absence of applied potential, I- is not oxidized during filtration. Application of 1 V results in the gradual increase of iodide oxidation with time until a plateau of ∼0.3% oxidation is achieved after 60 min of filtration. At 1 V, the two-electron process is the only thermodynamically allowed oxidation pathway and thus requires 2I- to be in close proximity to each other on the MWNT filter surface. Thus, the lag in achieving the steady-state oxidation value may be a result of the slow adsorption of I- to the MWNT surface. Application of both 2 and 3 V results in the steady-state oxidation of 1-2% (or 100200 μM) of the influent iodide. At 1.5 mL min-1, the maximum rate of I- oxidation is 3  1015 molecules s-1, which can be compared to the average current (3-6 mA at 2 V, 5-10 mA at 3 V) to determine average anodic iodide oxidation current efficiencies of 8-16% at 2 V and 5-10% at 3 V. The MWNT area per I- oxidation site can be estimated by dividing the total MWNT surface area of 1.05 m2 by the maximum iodide oxidation rate and multiplying by the liquid retention time of 1.2 s to yield 45 500 Å2 per molecule. This is significantly greater

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Figure 6. I-V curves and electrochemical filtration of NaCl and NaI. Experimental conditions are J = 1.5 mL min-1. (A) Steady-state I-V curves for [NaCl]in = 10 mM (black squares) and [NaCl]in = 10 mM and [NaI]in = 10 mM (red circles). (B) Electrochemical iodide filtration at potentials of 0 V (black squares), 1 V (red circles), 2 V (blue triangles, pointed up), and 3 V (green triangles, pointed down). Representative plots are shown, and all experiments were completed in at least duplicate. (C) Electrochemical iodide filtration over a range of [NaCl] and [NaI]. In the legend, X-Y is representative of salt concentrations in mM, where X is [NaCl] and Y is [NaI]. Open symbols represent experiments run at 2 V, and closed symbols represent experiments run at 3 V. All experiments were completed in at least duplicate.

than areas observed for MB, 165 Å2, and MO, 144 Å2, adsorption and the estimated iodide molecular area33 of 20 Å2. The significant difference between electrocatalytic site area, 45 500 Å2, and I- molecular area, 20 Å2, indicates that only a fraction of the MWNT surface sites is active toward iodide oxidation. To confirm this, the electrochemical filtration of various NaCl-NaI mixtures (10 mM NaCl-1 mM NaI, 100 mM NaCl-10 mM NaI, and 10 mM NaI) was completed (Figure 6C). In all cases, 3627

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The Journal of Physical Chemistry C the steady-state percent of iodide oxidized fell between 0.5 and 2.0%, confirming that I- oxidation is a limited by electrocatalytically active MWNT surface sites. It is of note that, in all cases in Figures 6B, 6C and S6 (Supporting Information), there is variation in the steady-state iodide oxidation values. This was correlated with observations of oscillations in both the effluent flow rate and the steady-state current. At 2 V, the steady-state current oscillated between 5 and 20 mA, and at 3 V, the steady-state current oscillated between 0 and 25 mA. The low current values corresponded to points in time when the effluent flow was significantly reduced, and the high current values corresponded to points in time when the effluent flowed as expected. It was hypothesized that the oscillating flow rate may have been due to electrolytic gas formation that resulted in blockage of MWNT filter pores. To test this hypothesis, the gas was vented by removing the influent tubing, which resulted in a jet or spray of liquid out of the filter casing. After replacement of the tubing, a significantly higher current value of ∼30-50 mA at 2 V and ∼80-100 mA at 3 V was observed for a brief period, < 5 s, and soon thereafter, the current and flow oscillation resumed. Future investigations of the electrochemical MWNT filter will require a practical solution to this issue; a few possibilities are listed here. One solution would be the incorporation of a pressure release valve to continually vent electrolytically produced gas. Another solution would be to operate the system gravimetrically such that the top of the system was open to the atmosphere for gas release. A third solution would be to place the cathode after the anode such that hydrogen produced at the cathode would be hydrodynamically carried out of the system rather than driven into the porous MWNT anode. Comparison to Previously Reported Electrochemical Wastewater Treatment Systems. Yang et.al.30 reported on an activated carbon felt-carbon nanotube seepage electrode reactor for the oxidation of Brilliant Red X-3B, which will be compared to the electrochemical filter and oxidation of methyl orange observed in this study Table S1 (Supporting Information). There are a number of differences between the two experimental designs (Yang et.al. vs this study), including the cathode selection, (carbon felt-CNT vs perforated stainless steel), anode preparation (carbon felt-CNT vs free-standing CNT thin film), and filtration regime due to the CNT anode preparation method (granular vs micro). In the previous work, the anode was produced by packing MWNTs between two pieces of activated carbon felt, resulting in a granular filter that could operate at a flux of 1700 L m-2 h-1. In this work, welldispersed MWNTs were prepared into a free-standing thin CNT film, 20-50 μm, with a pore size distribution of d = 90 ( 40 nm, resulting in a microfilter with a maximum operating flux of around 300 L m-2 h-1. For a quantitative comparison, we will assume that the flow rate is proportional to the filter area and then normalize the flow rate in this study by the relative areas, yielding an equivalent flow rate of 6 mL min-1. Yang et.al. reported that, after 90 min, >95% of the initial 300 mL of 60 μM X-3B had been decolorized or a total of 18 μmoles of X-3B had been partially oxidized. In a single pass in the system described here, >98% of the 24 μM methyl orange had been decolorized. That is, after 90 min of flow at 6 mL min-1, a total of 13 μmoles of methyl orange had been decolorized, slightly less than Yang et.al.’s system. However, a significantly lower applied potential, 2 versus 10 V, was utilized in this study. A single pass as compared with >10 passes through the filter was utilized in this study. A significantly lesser mass of

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CNTs per filter area, 28.5 versus 178.5 g m-2, was utilized in this study. Thus, the filter design, cathode selection, and anode preparation method have a dramatic effect on the electrochemical filtration efficiency. Recent research into the electrochemical oxidation for water treatment has focused on the design of new anode materials and structures based on boron-doped diamond (BDD),39-41 Sbdoped SnO2,42,43 and Bi-doped TiO2.44,45 These anode materials are exemplary due to a combination of properties, such as high O2 overpotential, oxidative/corrosion stability, high conductivity, and high yield of surface-bound hydroxyl radicals. Research is also occurring in the area of hybrid electrooxidation technologies, such as microwave-assisted BDD electrooxidation,46 photoelectrocatalysis,47,48 and electro-Fenton processes.49,50 BDD anodes are reported to be superior to platinum and glassy carbon toward phenol and formate oxidation39 and are able to mineralize the atrazine,51 a recalcitrant pesticide. Improvements in BDD anode performance toward 2,4-dichlorophenoxyacetate oxidation and mineralization are observed when the BDD is coated with Sb-doped SnO2 nanoparticles due to their superior electrocatalytic properties.41 BDD methanol electrooxidative performance has also been improved by addition of a porous, 3D platinum structure perpendicular to the BDD surface.40 The high surface area porous Pt increases the number of electrocatalytically active surface sites, whereas the BDD anodic surface acts to limit Pt passivating products. Following these examples may lead to improvements in the electrochemical CNT filter presented here. For example, one strategy would be to coat the CNT filter with doped-SnO2 nanoparticles to improve its electrocatalytic activity.

’ CONCLUSIONS In summary, an electrochemical MWNT filter has been shown to be effective for the adsorptive removal and electrochemical desorption and oxidation of the aqueous dyes methylene blue and methyl orange. At 2 and 3 V, a single pass through the 41 μm thin, 30 mm diameter MWNT filter in e1.2 s results in oxidation of >90% of the influent dye. The efficient removal and oxidation of these dyes are due to their planar aromatic structure that promotes adsorption to the anodic MWNT surface. The aqueous anions, chloride and iodide, were also oxidized while passing through the anodic MWNT filter with minimal overpotential. The electrochemical oxidation of the influent anions was limited by the number of MWNT surface sites active toward their oxidation. These results show the potential of an electrochemical carbon nanotube filter for the removal and oxidation of aqueous contaminants. Investigations are currently underway to better understand the electrochemical filtration process. ’ ASSOCIATED CONTENT

bS

Supporting Information. Images of the electrochemical filtration setup, modified filtration casing, and MWNT filters; additional SEM images of the MWNT filter mat; and images of MB and MO adsorption and electrochemically driven desorption and oxidation. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. 3628

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