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Electro-Treated Carbon Nanotube Membranes for Facile Oil-Water Separations Karen Adie Tankus, Liron Issman, Mikhail Stolov, and Viatcheslav Freger ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00442 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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ACS Applied Nano Materials

Electro-Treated Carbon Nanotube Membranes for Facile Oil-Water Separations Karen Adie Tankus1, Liron Issman2, Mikhail Stolov,1 Viatcheslav Freger1,3,4* 1

Wolfson Department of Chemical Engineering, Technion – IIT, Haifa 32000, Israel Tortech Nano-fibers, Ma’alot Tarshiha 24952, Israel 3 Russel Berrie Nanotechnology Institute, Technion – IIT, Haifa 32000, Israel 4 Grand Technion Energy Program, Technion – IIT, Haifa 32000, Israel 2

KEYWORDS: multiwall carbon-nanotubes, filtration membranes, oil-water separation, electrooxidation, electro-wetting

ABSTRACT Oil-contaminated effluents are widespread, yet purifying them of finely dispersed residual oil presents a significant challenge. Here we report on reactively spun carbon nanotube (CNT) mats, whose high electric conductivity is employed to irreversibly render them highly hydrophilic through electro-oxidation (EO), while retaining the morphology, mesh size, conductivity, and mechanical strength of the pristine CNT material. EO treatment converts natively hydrophobic CNT mats to efficient filtration membranes, whose 30-nm mesh size favorably compromises between the hydraulic permeability and oil breakthrough pressure and allows complete and robust removal of dispersed oil (including surfactant-stabilized) down to its solubility in water.

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Efficient and rapid oil-water separation presents a major challenge in developing environmentfriendly technologies. In particular, oil extraction generates so-called “produced” water as the main by-product that must be thoroughly purified before reuse or discharge.1,

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Conventional

settling, skimming and coagulation still leave unacceptable amounts of fine oil droplets and colloids, stabilized by naturally occurring organics and electrolytes.3 Filtration, conventional or membrane, offers a robust oil removal down to the finest fractions,4 however, the pore size has to be judiciously selected to compromise between energy consumption and rate, degree and robustness of oil removal. Dense hydrophilic membranes, such as nanofiltration, ensure a high oil rejection, yet have a high hydraulic resistance.5 Conversely, porous micro- (MF) and ultrafiltration (UF) membranes offer a higher permeability, but are inherently prone to oil fouling.6, 7 Kota et al. proposed still coarser filters with hydrophilic and oleophobic 20-30 µm pores that separate oil and water using ultra-small hydraulic heads.8 Their downside is the very low breakthrough pressure Pb, above which oil passes the membrane regardless of its surface characteristics. Using a typical value of oil-water interfacial tension9 γ ≈ 20 mN m-1, and pore or mesh radius rp as a measure of curvature of water-oil mensisci, the Young – Laplace equation

Pb ≅

γ

(1)

rp

yields Pb is of the order 10 mbar for 20-30 µm pores. This is equivalent to heads of a few centimeters, which may be readily exceeded in a real process and result in oil breakthrough. On the other hand, eq. 1 suggests that a robust Pb above 1 bar will require rp < 100 nm. Here we report on a novel non-woven multiwall carbon nanotube (MWCNT) mats with a mesh size of ~30 nm in the UF range that meets well this requirement. These large-area fibrous mats, manufactured using a novel reactive CNT-spinning process,10 may function as free-standing


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CNT-only UF membranes. Absence of other components makes them distinct from other promising CNT-based membranes, proposed for oil-water11-14 and other separations,15-20 most of which, with very few exceptions,11 integrate nanotubes with other modifying and supporting materials within composite structures. As expected of fibrous membranes,21 the mats have a high permeability, on par with conventional asymmetric UF membranes of similar pore size. They also exhibit excellent thermal, chemical and mechanical stability, far exceeding that of polymeric counterparts. Unfortunately, pristine mats are hydrophobic, while oil rejection requires hydrophilicity and oleophobicity .1, 2, 5, 8, 22 To convert the mats to efficient filters, we propose to utilize yet another unique feature of CNTs, namely, their high electrical conductivity and use electro-treatment to alter reversibly or irreversibly their wetting characteristics. This approach, conceptually illustrated in Figure 1a, may be conveniently applied in situ, i.e., within a working filtration unit. For instance, electro-oxidation (EO) under anodic potentials generates oxygen-containing polar groups on the CNT surface, similar to changes that take place upon conversion of graphene to graphene oxide.23 MWCNT structure is presumably less vulnerable to electro-treatment than single-wall CNTs,11 since oxidation should predominantly affect out-most walls, preserving their mechanical and electrical characteristics.


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Figure 1. (a) Illustration of the electro-oxidative conversion of natively hydrophobic CNT mats to hydrophilic oil-rejecting ultrafilters; (b) a CNT coupon used for modificationa and filtration experiments; a SEM image (c) and pure water and oil permeability results (d) for the pristine membranes; (d) the setup for in situ electro-treatment and filtration; a SEM image (f) and pure water and oil permeability results (g) for electro-oxidized CNT membranes. Insets in (d) and (g) show contact angles of a sessile drop of water on corresponding CNT mats.

Prior to experiments, non-woven MWCNT mats (Figure 1b and c), obtained from Tortech (Ma‘alot, Israel), were immersed in petroleum ether (PE, boiling range 100-120 ºC), used as a model oil, and dried. This treatment was found to reduce water permeation and produce more


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reproducible oil permeability. Presumably, the negative capillary pressure, developed in menisci receding between nanotubes upon drying, pulled CNTs together and shrank larger pores with a low Pb, through which water could leak. Filtration of pure PE and water, as well as water contact angle, clearly indicate that pretreated mats are hydrophobic and oil-permeable but barely permeable to water (Figure 1d). The PE permeability, i.e., the slope of the flux JV versus applied pressure ΔP, is related to the effective pore radius rp through a Hagen-Poiseulle-like relation for fully wetted porous media24

rp2 φ JV = ∆P 8t µ α

(2)

where µ is the fluid viscosity and α is the tortuosity. Using the results for PE in Figure 1d, known thickness (t ~ 50 µm) and porosity (ϕ ~ 0.7, for which α ~ 124) of the mats, eq. 2 yields rp ~ 30 nm, consistent with SEM micrographs (Figure 1c). In situ electro-oxidation of CNT mats was performed within an air-pressurized Amicon filtration cell fitted with a Pt counter-electrode, schematically shown in Figure 1e. Positive potentials ≥5 V applied for a sufficient time left the membrane morphology unaltered (cf. Figures 1c and f), but water permeation sharply increased, while contact angle and oil permeability of water-wetted membrane dropped dramatically, i.e., the membrane turned hydrophilic and (underwater) oleophobic (Figure 1g). High voltages were apparently required to overcome competition with water splitting, starting under 1 V. The water permeability was about half the pristine membrane oil permeability (Figures 1d and g), due to different viscosities of water and oil, thereby eq. 2 yielded identical pore size estimates before and after modification.


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Figure 2. (a) TGA diagrams of CNT membranes before modification (as is), after pre-immersed in PE, and after electro-oxidation using indicated voltages and times; (b) elastic modulus and electrical conductivity of CNT membranes before (pristine) and after electro-oxidation using indicated voltages and times; (c) representative Raman spectra of CNT membranes before (pristine) and after electro-oxidation at 5 V for inidicated times; (d) variation of G/D intensity ratio for membranes electro-oxidized for 30 min at different voltages; (e) variation of G/D intensity ratio for membranes electro-oxidized at 5 V for different times.

While EO preserved the morphology and porosity of the membranes, thermogravimetric analysis (TGA), mechanical and electrical characterization, Raman and FTIR spectroscopies, and surface analysis using X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary-ion mass


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spectroscopy (TOF-SIMS) revealed a clear change. TGA showed that decomposition temperature decreased and its distribution widened (Figure 2a), consistent with an increased defect rate. Curiously, dry electro-oxidized mats showed elastic modulus increased by 11-34% and electrical conductivity by 70% compared to pristine counterparts (see Figure 2b and Table S1 and S2 in Supporting Information). Since defects are supposed to reduce electrical conductivity of individual CNTs, the observed increase suggests enhanced bonding between nanotubes, presumably, since oxidation increased surface polarity and removed amorphous impurities, which strengthened inter-tube contacts. The ratio of the Raman G and D bands (Figure 2c) is a common indicator of chemical defects in carbon nanomaterials.25 Figure 2d and 2e show that, as the voltage or time increased, the G/D ratio declined, consistent with increased defect rate. FTIR spectra are featureless for pristine CNTs, as dictated by IR selection rules for symmetric CNT structure.25 Yet, oxidation breaks the symmetry thereby weak but distinct bands assigned to various oxygen-containing groups and increased water adsorption emerge in the 1000-1700 cm-1 region (Fig. S1 in Supporting Information). This was confirmed by surface analysis using time-of-flight secondary-ion mass spectroscopy (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS), which showed an increased O and reduced C and H surface content after oxidation (Supporting Information, Figs. S2 and S3 and Table S3). TOF-SIMS probes essentially the first atomic layer thereby attenuated H- and strongly enhanced O2- and O2H- signals indicate increased content of oxygen-rich groups such as carboxyl. Conversely, XPS penetrates the entire MWCNT diameter and high-resolution XPS of carbon binding energies (C1s band) shows only a moderate increase in carboxyl bands (see Supporting Information for details). This might suggest that oxidation indeed affected primarily the outmost MWCNT walls.


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Figure 3. (a) Permeate flux (a) and TOC (b) vs. feed pressure for indicated electro-oxidation times; feed solution 1% PE with and without 0.4 mM Triton X100; (c) feed and permeate during filtration experiment, (d) a phase-contrast optical micrograph of the 1% PE feed solution stabilized with 0.4 mM Triton, (e) forces acting on oil droplets next to the membrane surface during filtration of a stirred emulsion.

Following in situ electro-oxidation, oil-water separation performance was tested in the same setup (Figure 1d) using PE-water dispersions, containing 5 mM Na2SO4 (to simulate tap-water salinity) and optionally stabilized with 0.4 mM Triton X100 surfactant (twice its critical micellization concentration). Figure 3a shows that the fluxes were somewhat lower than for pure water due to build-up of a thin blocking layer of rejected oil on the membrane surface (concentration polarization, see next). Nevertheless, only dissolved organics could reach the permeate, while oil droplets were totally rejected, as was confirmed by measuring chemical


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oxygen demand of the entire collected permeate volume. Total organic carbon (TOC) analysis supplied a more accurate measure of the organics in permeate. Without surfactant in the feed, it contained around 3-4 ppm TOC (Figure 3b), which closely corresponds to solubility of n-heptane (boiling point 98 ˚C), representative of the PE used (dashed line in Figure 3b). Surfactant was added to model realistic situations, when oil emulsion may be stabilized by organic impurities. The feed solution became milky due to presence of micron and submicron droplets, however, permeate remained transparent (Figure 3c and d). Compared to surfactant-free oil emulsions, the permeation rates dropped and tended to plateau with increasing pressure, indicating a stronger concentration polarization, resulting in surface blockage by rejected oil. Note the flux varied reversibly upon pressure cycling, which rules out irreversible fouling as the reason for reduced flux. However, the enhanced blockage in presence of surfactant is consistent with the change of force balance for reduced droplet size. Indeed, the balance mainly involves inertial lift force26 that arises in a stirred fluid next to a solid surface and drives droplets away from the surface and the convective drag towards the membrane by the trans-membrane flow, as illustrated in Figure 3e. Since lift force drops more rapidly than drag when droplet size decreases (as 4th power of size vs 1st power for drag),26,

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stabilized emulsions. Added surfactant also increased permeate TOC to 20-25 ppm, but the increase was mainly associated with surfactant, as verified by filtering solutions with reduced PE content (0.1%) and oil-free 0.4 mM Triton X100 solutions (Figure 3b and Figure S4 in Supporting Information). This confirms that only dissolved molecules could reach permeate, while all colloids and larger droplets were effectively rejected. The results were unaltered when the membrane and feed were first equilibrated overnight, and then the filtration was run for longer times (Supporting


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Information, Figure S5). The observed stable performance confirms that the observed oil removal was a result of steady-state rejection rather that adsorption of organics on CNTs. Finally, we also examined another way of altering the wetting properties of the CNT membranes using an electric potential. Unlike irreversible electro-oxidation, surface hydrophilicity could also be induced reversibly, by polarization of the membrane-solution interface without an electrode reaction via effects such as electrowetting,28 electric-field-induced phase separation,29 or forces induced by mismatch in dielectric properties. The latter was used by Kwon et al. to elegantly separate oil-water mixture in an on-demand manner by exerting a 1500 V/cm electric field across a non-conductive porous membrane, forcing selective water permeation.30 In the present case, membrane conductivity precludes any trans-membrane electric field gradients, yet a voltage applied directly to CNT mats could potentially promote displacement of low-dielectric oil by high-dielectric water from the adjacent solution layer, either in the form of electrowetting or near-surface phase transitions. Notably, such effects are roughly independent of the sign of the applied potential,28,

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and may then be produced by both positive and negative as well as

alternating (AC) potentials. The use of AC instead of DC could then differentiate between such effects and electro-oxidation. Surprisingly, our tests showed that AC voltages, even as high as 50 V, applied to both pristine and electro-oxidized CNT mats across water or 1% PE solutions concurrently with filtration had no noticeable effect on water permeation and oil rejection (see Supporting Information, Figure S6). As compared to other studies,30 the applied AC voltage might be too low, however, higher voltages were unusable as well. When voltage was increased to 200 V, the tap-water-level solution conductivity was already enough to make the solution heat up and boil within minutes. It appears that, while on-demand reversible electro-treatment may be attractive for primary


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separation of crude oil-rich mixtures, it may be problematic for removal of fine oil residues from predominantly aqueous media. In such a case, irreversibly electro-oxidized CNT mats reported here may offer a simple, safe, and robust way of producing highly water-permeable and oilrejecting CNT membranes. We envision that the reported approach may suit numerous applications, requiring purification of oil-contaminated aqueous streams at scales spanning from micro- and laboratory to large industrial operations.

Supporting Information: The file is available free of charge and describes filtration and modification setup and procedures, FTIR , TOF-SIMS, XPS, and electrical characterization results, and additional filtration data, including experiments with an AC voltage applied concurrently with filtration.

Corresponding Author *E-mail: [email protected]

Author Contributions The manuscript was written by KAT and VF through contributions of all authors. All authors have given approval to the final version of the manuscript.


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Conflict of interest VF and KAT declare a potential interest through a submitted patent application and LI declares interest through Tortech, the manufacturer of the CNT mats.

Funding Sources This work was supported by the Nofar program of the Chief Scientist Office of the Israel Ministry of Industry and Trade, grant No 56794.

Acknowledgment This work was supported by the Nofar program of the Chief Scientist Office of the Israel Ministry of Industry and Trade, grant No 56794. The authors thank Dr. Tatyana Kravchuk and Dr. Kamira Weinfeld-Cohen of the Surface Science Laboratory of Solid State Institute of Technion for help with TOF-SIMS and XPS measurements.

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TOC graphic

Electrotreatment

Filtration


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