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Removal of oil droplets from contaminated water using magnetic carbon nanotubes
w a t e r r e s e a r c h x x x ( 2 0 1 3 ) 1 e8
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Removal of oil droplets from contaminated water using magnetic carbon nanotubes Haitao Wang a, Kun-Yi Lin a, Benxin Jing b, Galyna Krylova c, Ginger E. Sigmon a, Paul McGinn b, Yingxi Zhu b, Chongzheng Na a,* a
Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, IN 46556, USA b Department of Chemical and Biomolecular Engineering, University of Notre Dame, 182 Fitzpatrick Hall, Notre Dame, IN 46556, USA c Department of Chemistry and Biochemistry, University of Notre Dame, 132 Nieuwland Science Hall, University of Notre Dame, Notre Dame, IN 46556, USA
article info
abstract
Article history:
Water contaminated by oil and gas production poses challenges to the management of
Received 2 June 2012
America’s water resources. Here we report the design, fabrication, and laboratory evalu-
Received in revised form
ation of multi-walled carbon nanotubes decorated with superparamagnetic iron-oxide
31 January 2013
nanoparticles (SPIONs) for oil-water separation. As revealed by confocal laser-scanning
Accepted 11 February 2013
fluorescence microscopy, the magnetic carbon nanotubes (MCNTs) remove oil droplets
Available online xxx
through a two-step mechanism, in which MCNTs are first dispersed at the oil-water interface and then drag the droplets with them out of water by a magnet. Measurements
Keywords:
of removal efficiency with different initial oil concentration, MCNT dose, and mixing time
Pickering emulsion
show that kinetics and equilibrium of the separation process can be described by the
Energy and water security
Langmuir model. Separation capacity qt is a function of MCNT dose m, mixing time t, and
Industrial wastewater
residual oil concentration Ce at equilibrium: 2 1 1 1 1 1 1 ¼ 2 þ1 þ1 þ qt qmax kw m KCe t qmax KCe
Environmental nanotechnology Responsive nanocomposite Fossil fuels
where qmax, kw, and K are maximum separation capacity, wrapping rate constant, and equilibrium constant, respectively. Least-square regressions using experimental data estimate qmax ¼ 6.6(0.6) g-diesel g-MCNT1, kw ¼ 3.36(0.03) L g-diesel1 min1, and K ¼ 2.4(0.2) L g-diesel1. For used MCNTs, we further show that over 80% of the separation capacity can be restored by a 10 min wash with 1 mL ethanol for every 6 mg MCNTs. The separation by reusable MCNTs provides a promising alternative strategy for water treatment design complementary to existing ones such as coagulation, adsorption, filtration, and membrane processes. ª 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
Water plays an important role in oil and gas production, particularly in aging oil fields and unconventional tight-gas,
shale-gas, and coal-bed fields where the injection of highpressure water is needed to improve productivity. When the injected water, together with water already in the geological formation, flows back to the surface, it is contaminated with
* Corresponding author. Tel.: þ1 574 631 5164; fax: þ1 574 631 9236. E-mail address: [email protected] (C. Na). 0043-1354/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2013.02.056
Please cite this article in press as: Wang, H., et al., Removal of oil droplets from contaminated water using magnetic carbon nanotubes, Water Research (2013), http://dx.doi.org/10.1016/j.watres.2013.02.056
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oil droplets and dissolved hydrocarbons from the contact with oil and gas reservoirs. The oil-contaminated water is often referred to as produced water. Surveys have estimated that the United States generates over 2.4 billion gallons per day of produced water, which is equivalent to 7 barrels of contaminated water for each barrel of oil produced onshore (Veil et al., 2004). This amount is equal to nearly half of the public drinking water produced in the country (Clark and Veil, 2009). More oil-contaminated water will likely be produced in the next decade as the country strives to achieve the new energy goal of reducing foreign oil imports. To achieve this goal, the federal government has pledged to increase domestic oil and gas production in addition to improving energy efficiency and developing renewable sources (The White House, 2011). The large quantity of produced water poses serious challenges to the management of America’s future water resources under the pressures of population growth, economic development, and climate change (The, 2030 Water Resources Group, 2009; Tetra Tech Inc., 2010). Before produced water can be discharged into the environment or reused in the production process, oil droplets and dissolved hydrocarbons must be removed to meet environmental requirements or industrial standards. Current practices often use the gun-barrel (aka. knock-put wash) tank or its variations as the first step of oil-water separation under gravity (High-Tech Consultants Inc. 2005). The water effluent of the gravity separation often contains oil droplets less than 10 mm in diameter as an oil-water emulsion. Further reduction of oil content in produced water requires the addition of chemicals to break the emulsion. The chemical treatment, however, increases the operating cost and generates waste that requires disposal. The treatment of produced water costs up to $5 per barrel (Veil et al., 2004), which is a significant expense with the current market price of crude oil at approximately $100 per barrel. Inspired by the Picking-emulsion forming property of carbon nanotubes (Pickering, 1907), we have designed carbon nanotubes (CNTs) decorated with superparamagnetic ironoxide nanoparticles (SPIONs) as agents for oil-water separation. Without surface modification, carbon nanotubes do not disperse well in either water or oil; instead, CNTs are dispersed at the oil-water interface and form a Picking emulsion (Wang et al., 2009). In this study, we took advantage of this century-old phenomenon and designed a new oil-water separation process. We added magnetic carbon nanotubes (MCNTs) into the oil-water emulsion in which MCNTs were attracted to the oil-water interface. To physically separate oil droplets from water, a magnetic field was applied to exert an attractive force on SPIONs, which removed MCNT-wrapped oil droplets from water. Processed water was removed from the reactor by a pipette, mimicking pumping or drainage. Oilsoaked MCNTs were washed with environmentally friendly solvents such as ethanol to recover oil and then put back into reuse. Although magnetic carbon nanotubes and nanofibers have already been proposed as adsorbents of dissolved and dispersed contaminants (Tang et al., 2003; Thostenson et al., 2005; Thanikaivelan et al., 2012), as demulsifiers for destabilizing the oil-water emulsion (Motta et al., 2007), and as agents for drug delivery (de Faria et al., 1997), to our knowledge, the utilization of magnetic carbon nanotubes for rapid physical
separation of the oil-water emulsion has not been reported in the literature. Here we report the design, fabrication, and laboratory evaluation of SPION-decorated CNTs for oil-water separation.
2.
Materials and methods
All the reagents used in this study were of analytical grade in quality (from Sigma Aldrich or Fisher Scientific). Deionized (DI) water (18.2 MU cm1) used in solution making, washing, and rinsing was generated by a Millipore system (Billerica, MA) on site. The neodymium-iron-boron block magnet used in generating the magnetic field for oil-water separation was purchased from K&J Magnetics (Jamison, PA, Model BX0X0C; maximum field strength: 13.2 kOe).
2.1.
Synthesis of magnetic carbon nanotubes
Multi-walled carbon nanotubes were synthesized following a thermal chemical vapor deposition (TCVD) method using CoMg10Mo10O as catalyst (Li et al., 2005). The catalyst was synthesized using a mixture of Co(NO3)2$6H2O, Mg(NO3)2$6H2O, and (NH4)Mo7O24$4H2O with a molar ratio of 1:10:10 for Co:Mg:Mo. The mixture was dissolved in 20 mL DI water with 21 g citric acid as combustion additive. The solution was ignited in an oven (preheated to 600 C) and stayed on fire for 10 min. To perform TCVD, 200 mg catalyst was placed on a quartz boat. The boat was then inserted to the center of a 2-inch quartz tubing. The tubing was flushed with argon at 500 sccm (standard cubic centimeter per minute) and heated in a tube furnace to 1000 C. Methane and hydrogen were introduced at 500 sccm and 100 sccm, respectively. The growth lasted for approximately 30 min, after which the furnace was cooled to the room temperature under a nitrogen atmosphere. To remove the catalyst, CNTs were boiled with 1 M hydrochloric acid and 30% hydrogen peroxide at 80 C for 30 min (Wang et al., 2007). Purified CNTs were collected by filtration, washed with DI water, and freeze-dried before they were used for attaching SPIONs. Magnetite nanoparticles were decorated on CNTs as SPIONs through the co-precipitation of ferrous and ferric irons (Wan et al., 2007). To perform co-precipitation, 0.5 g CNTs and 0.75 g iron trisacetylacetonate were mixed with 90 mL triethylene glycol and then heated to boiling under vigorous stirring and nitrogen protection, and kept at reflux for 30 min. After the mixture was cooled to the room temperature, 30 mL ethyl acetate was added to precipitate MCNTs. MCNTs were thoroughly washed with DI water and freeze-dried before use. As a negative control in studying the mechanism of oil-water separation, SPION suspension was synthesized following the same protocol in the absence of CNTs.
2.2.
Characterization of magnetic carbon nanotubes
Physical properties of MCNTs were characterized using transmission electron microscopy (TEM), the Brunauer-EmmettTeller (BET) surface area analysis, and the superconducting quantum interference device (SQUID). For the TEM analysis of
Please cite this article in press as: Wang, H., et al., Removal of oil droplets from contaminated water using magnetic carbon nanotubes, Water Research (2013), http://dx.doi.org/10.1016/j.watres.2013.02.056
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morphology and composition, MCNTs were dissolved in ethanol, deposited on copper grids, and imaged using a Titan 80e300 microscope (FEI, Hillsboro, OR). Specific surface areas of MCNTs, compared to unattached SPIONs and pristine CNTs, were measured using an ASAP 2000 BET analyzer (Micromeritics, Norcross, GA) with nitrogen. The magnetic property of MCNTs was characterized using an MPMS SQUID system (Quantum Design, San Diego, CA). All experimental magnetic data were corrected for the diamagnetism of sample holders and constituent atoms.
2.3.
Visualization of oil-water separation in action
Direct, real-time observation of MCNTs in the oil-water emulsion required that oil droplets had diameters in the range of tens of micrometers in order to counter Brownian motion and comply with resolution (i.e., one half of the laser wavelength, 271.5 nm). To increase the sizes of oil droplets, 5 mg sodium 1,2-dioleoyl-sn-glycero-3-phosphate was added as the stabilizing surfactant to an emulsion prepared with 1 mL diesel and 4 mL DI water. One tenth of a gram of ammonium 1,2-dioleoyl-sn-glycero-3-phosphoethanolamineN-(lissamine rhodamine B sulfonyl) was also added to the emulsion as a fluorescent probe to illuminate oil droplets. Unattached SPIONs, pristine CNTs, or SPION-attached MCNTs were added to the emulsion at a dose level of 2, 2, or 5 g L1, respectively. The interactions between nanomaterials and oil droplets, with or without an external magnetic field, were visualized using a confocal laser-scanning microscope (LSM 5 Pascal; Carl Zeiss Microscopy, Thornwood, NY) equipped with a 100 objective lens (numerical aperture ¼ 1.4 in oil). Confocal images and movies were processed using ImageJ (Rasband, 1997e2011).
2.4.
Evaluation of oil-water separation by MCNTs
The performance of MCNTs in oil-water separation was evaluated using emulsions made of diesel (Martin’s Gas Station, South Bend, IN; ASTM Grade No. 2-D: density, 0.84 g mL1 at 25 C; viscosity, 2.7 mPa s at 40 C) and DI water. Diesel-inwater emulsions were prepared by adding 1e2.5% of diesel by volume to water under 10 min sonication (Misonix S3000, Farmingdale, NY). The oil-in-water emulsion was stable for more than 24 h, which was confirmed by its visual opacity, and used in experiments within 12 h. The size of oil droplets was confirmed to be in the micrometer range by placing a drop of the emulsion on a glass slide and observed under an optical microscope calibrated with a standard grid (DC Imaging BA300, Mason, OH). The diesel concentration was measured as chemical oxygen demand (COD; unit: mg-O2 L1, detection limit: 3 mg L1) after oxidizing the emulsion with potassium dichromate (Cuenca et al., 2006; American Public Health Association et al., 1995). The diesel concentration was computed from COD using an average diesel formula of C12H23 and with the assumption that diesel had been completely converted to carbon dioxide and water. It is worth noting that no surfactant was used in evaluation experiments. To separate oil droplets from water, dry MCNTs were weighed and dispersed in 5 mL water in a 40 mL glass vial representing a batch reactor. An aliquot of concentrated diesel-in-
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water emulsion was added to the vial, creating a 30 mL emulsion with a preset diesel concentration. The contents were mixed on a shaking table at 300 rpm. The effect of mixing on MCNT performance was assessed by varying the shaking time from several minutes to 1.5 h. MCNTs and the associated oil droplets were then separated from water by placing the magnet on one side of the vial. Water was removed from the vial using a pipette, mimicking a pumping or draining process. The remaining diesel concentration was analyzed by the COD method with three replicates. To regenerate oil-soaked MCNTs, 10 mL ethanol was added to the MCNT aggregate in the vial. After shaking by hand, the ethanol was decanted and the regenerated MCNTs were used again for oil-water separation.
3.
Results
Fig. 1 summarizes the physical properties of magnetic carbon nanotubes obtained by transmission electron microscopy and the SQUID magnetometer. As shown in Fig. 1a, MCNTs are well separated long tubes decorated with SPIONs on the outside. The contrast between them suggests that nanotubes are made of the lighter element of carbon whereas SPIONs are made of heavier elements of iron and oxygen. The TEM image in Fig. 1b taken under an increased magnification reveals lattice fringes for both CNTs (spacing in the 002 direction: 3.49 A) and SPIONs (spacing in the 400 direction: 2.08 A), which are in good agreement with reports in the literature (Kiang et al., 1998; Sun et al., 2004). The microscopic results further provide quantitative information on the dimensions and composition of MCNTs. By making direct measurements on approximately 50 representative MCNTs in TEM images, we have obtained estimates of the outer diameter and length for CNTs to be 14.5(6.1) nm and 0.69(0.45) mm, respectively. The diameter and interparticle distance for SPIONs are estimated to be 9.6(0.8) nm and 48(23) nm, respectively. Based on these parameters, we further estimate that MCNTs consist of 75(19)% of CNTs and 25(27)% of SPIONs in mass, have a density of 2.9(1.5) g cm3, and possess a specific surface area of 146(68) m2 g1. The estimated specific surface area is in good agreement with the BET measurement of 150.0(0.3) m2 g1. Under a varying external magnetic field, MCNTs exhibit a typical superparamagnetic behavior with little histeresis, as shown in Fig. 1c. Coercity, which is defined as the external field needed for demagnetization, is only 8.3(0.5) Oe for MCNTs, indicating that they are superparamagnetic materials with negligible remanence of magnetization once the external field is removed. The saturation magnetization of MCNTs is 15.5(0.1) emu g1. Since the mass fraction of SPIONs in MCNTs is 25%, the saturation magnetization of SPIONs is 62 emu g1, which is approximately 70% of the saturated magnetization of bulk magnetite (Tao et al., 2008). The reduction of saturated magnetization can be attributed to the size effect (Kodama et al., 1996). Fig. 2 shows a series of snapshots taken by a confocal laserscanning microscope that reveal the mechanism of oil-water separation by MCNTs. As shown in Fig. 2a, in the absence of MCNTs, diesel droplets with sizes up to tens of micrometers are stabilized in water by surfactants. Under the microscope,
Please cite this article in press as: Wang, H., et al., Removal of oil droplets from contaminated water using magnetic carbon nanotubes, Water Research (2013), http://dx.doi.org/10.1016/j.watres.2013.02.056
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Fig. 1 e Physical properties of magnetic carbon nanotubes. (a) Transmission electron micrograph (TEM) of MCNTs. (b) TEM showing lattice fringes for carbon nanotubes and superparamagnetic iron-oxide nanoparticles (SPIONs). (c) Superparamagnetic loop for MCNTs at 300 K with the inset showing a negligible coercivity. Scale bars: a, 200 nm; b, 5 nm.
oleophilic fluorescent molecules added to the emulsion give oil droplets a yellow color against the black water background. Carbon nanotubes added to the oil-water emulsion, as shown in Fig. 2b, are dispersed at the oil-water interface as dark spots because they can quench fluorescence by their p-stacking
structures (Arai et al., 2007). MCNTs can also quench fluorescence because the attachment of SPIONs does not disturb the integrity of CNTs’ p stacking. As shown in Fig. 2c, when MCNTs are mixed with the oil-water emulsion, they are either dispersed at the oil-water interface or immersed in the oil
Fig. 2 e Oil-water separation in action captured by confocal laser-scanning fluorescence microscopy, presented in snapshots. (a) Oil-in-water emulsion made of diesel. (b) Pristine CNTs dispersed at the oil-water interface. (c) MCNTs dispersed at the oil-water interface and immersed inside the oil droplet. Examples are marked by red arrows. (d) MCNTwrapped oil droplets under the influence of an external magnetic field placed at the lower right corner. The snapshot reveals the change of droplet shape in the direction of their movement toward the magnetic source. (e) SPIONs dispersed at the oilwater interface. (f) SPIONs removed from the oil-water interface by an external magnetic field. Examples of removed SPIONs are marked by red arrows in b. Note that shapes and positions of oil droplets are marginally influenced by the external magnetic field. Scale bars: 5 mm. See movies S1eS5 in the Supplementary materials for details. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Please cite this article in press as: Wang, H., et al., Removal of oil droplets from contaminated water using magnetic carbon nanotubes, Water Research (2013), http://dx.doi.org/10.1016/j.watres.2013.02.056
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phase as bundles. The MCNT-wrapped oil droplets can be readily removed from water by an external magnetic field, as manifested by their ellipsoidal shapes in Fig. 2d. In comparison, oil droplets experience little magnetic force when unattached SPIONs is added to the emulsion. Comparison between images taken in the absence (Fig. 2e) and presence (Fig. 2f) of an external magnetic field shows that magnetite nanoparticles can be moved out of oil droplets by the field. The unattached SPIONs cannot, however, drag the oil droplets along with them due to the lack of sufficient affinity. The mechanism can be better appreciated with the movies in the Supplementary materials that provide 3-dimensional slicing of oil droplets and time sequences for the movements of oil droplets and SPIONs in external magnetic fields. Supplementary video related to this article can be found at http://dx.doi.org/10.1016/j.watres.2013.02.056. Fig. 3 provides a qualitative assessment of the performance of MCNTs in oil-water separation. MCNTs are prepared as dry black powders (Fig. 3a). MCNTs do not disperse well in either water or diesel oil but preferentially reside at their interface (Fig. 3b). The diesel-water emulsion prepared by sonication without using surfactants is opaque due to the light-scattering property of oil droplets (Fig. 3c). The opacity blocks the view of the Notre Dame logo placed behind the emulsion. MCNTs are readily dispersed in the diesel-water emulsion by wrapping themselves around oil droplets (Fig. 3d). A magnet (in the middle of Fig. 3e) attracts MCNTs, together with the wrapped oil droplets, to a side of the vial and thus effectively moves them out of water. Water becomes clean as evident from the clear Notre Dame logo in the background. The optical micrographs underneath Fig. 3cee show that after treatment with MCNTs, there is no longer any visible oil droplet. Oil trapped by MCNTs can be recovered by ethanol wash (Fig. 3f and g) and
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the cleaned MCNTs can be reused for oil-water separation as illustrated by Fig. 3cee. Fig. 4 presents the quantitative evaluation on the kinetics and equilibrium of oil-water separation using MCNTs. Each experiment was performed with a diesel-water emulsion having a volume of 30 mL and an initial diesel concentration of C0 (in g L1) and an MCNT dose of m (in g L1). After mixing for a period of time t, the remaining diesel concentration Ct (in g L1) was measured and the separation capacity of MCNTs in g-diesel g-MCNT1 was computed as: qt ¼
C0 Ct : m
(1)
We first investigated the relationship between qt and t with constant C0 and m. An example is shown in Fig. 4a, revealing that qt increases rapidly with increasing t. qt approaches an equilibrium capacity qe as Ct approaches an equilibrium residual concentration Ce. According to Fig. 4a, qt reaches >99% of qe after 90 min of mixing. We then investigated the equilibrium of oil-water separation by MCNTs using 90 min as the mixing time and with increasing C0 or decreasing m. As shown in Fig. 4b, both qe and Ce increase either with increasing C0 from 0.85 to 3.85 g L1 at fixed m values (colored in blue, green, and red) or with increasing m from 0.5 to 4 g L1 at fixed C0 values (colored in gold and brown). The experimental data agree well with each other at low qe and Ce but become increasingly scattered at high qe and Ce. We suspect that the scattering is in part due to the dilution required for measuring high concentrations of initial and residual diesel. Because oil is dispersed in water as droplets, taking a representative fraction for dilution can be challenging. Nonetheless, a general trend of qe approaching a limiting capacity qmax at high Ce can be seen in Fig. 4b.
Fig. 3 e Digital pictures of experimental setups for evaluating the efficiency of oil-water separation. (a) Dry magnetic carbon nanotube (MCNT) powder. (b) MCNTs residing at the interface of bulk oil (yellow) and bulk water (white). (c) Oil-in-water emulsion made of diesel. (d) Mixture of MCNTs with the oil-water emulsion. (e) Removal of MCNTs, together with trapped oil droplets, by a magnet to the side of the vial. (f) Extraction of trapped oil from spent MCNTs in ethanol. (g) Removal of MCNTs from ethanol by a magnet for reuse. The removal of oil droplets is also shown in the optical micrographs underneath b, c, and d. Scale bar: 10 mm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Please cite this article in press as: Wang, H., et al., Removal of oil droplets from contaminated water using magnetic carbon nanotubes, Water Research (2013), http://dx.doi.org/10.1016/j.watres.2013.02.056
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Fig. 4 e Kinetics (a) and equilibrium (b) of oil-water separation using magnetic carbon nanotubes (MCNTs). Insets are linearregression plots based on the Langmuir model. Conditions for experiments in a: 1 g LL1 MCNTs and 0.5 g LL1 diesel. Conditions for experiments in b: blue, 0.5 g LL1 MCNTs with 0.85, 1.36, 1.70, 2.03, 2.37, and 3.05 g LL1 diesel; green and red, 0.75 and 1 g LL1 MCNTs, respectively, with 3.85, 3.78, 3.55, 3.32, 2.92, and 2.60 g LL1 diesel; gold, 1.83 g LL1 diesel with 4, 2, 1.5, and 1 g LL1 MCNTs; brown, 2.14 g LL1 diesel with 1.5, 1.0, 0.75, 0.5 and 0.25 g LL1 MCNTs; mixing time for all, 90 min. Standard deviations are represented by error bars. The solid curves are least-square regressions according to the Langmuir model. The dashed curves bracket 95% confidence intervals. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5 summarizes the experimental results for MCNT regeneration and oil recovery in ethanol. We first assessed the effect of wash time on the performance of regenerated MCNTs. As shown in Fig. 5a, the ethanol wash can recover over 80% of the separation capacity within the first 10 min and the recovery, defined as the ratio of reusable capacity qr to equilibrium capacity qe, increases with increasing regeneration time tr. Based on the kinetic results, 10 min wash is sufficient for recovering most of MCNTs’ separation capacity. The reusability of MCNTs is further demonstrated in Fig. 5b for 10 cycles of regeneration with 10 min ethanol wash, which shows a consistent performance with a recovery of 81.2(0.6)%.
4.
Discussion
Establishing rational design principles is essential for transforming exciting properties of nanomaterials to cost-effective applications. In our design of magnetic carbon nanotubes, several guidelines have been followed. First, we select multiwalled carbon nanotubes as wrapping agents because they are less expensive and less technically challenging to synthesize compared to single- and few-walled nanotubes. In addition, single- and few-walled CNTs tend to bundle and collapse radially (Motta et al., 2007), which makes it difficult to control product quality. Second, CNTs’ diameter is selected to be 10e20 nm in order to accommodate the 10 nm magnetite nanoparticles used as SPIONs. Increasing CNTs’ diameter beyond the nanoparticle size is undesirable because of the resulting increase of MCNT density and decrease of specific surface area. Third, 10 nm magnetite nanoparticles are chosen as magnetic agents because they are superparamagnetic (Wang et al., 2009). Having no residual magnetization in the
absence of an external field, MCNTs will not aggregate in storage or during mixing with the oil-water emulsion. Finally, we select nanoparticle coverage of ca. 10% to minimize their agglomeration on CNTs and keep the MCNT density low. Further lowering the coverage may result in a non-uniform distribution of nanoparticles. The separation of oil and water using MCNTs follows the kinetics and equilibrium described by the Langmuir model (Hiemenz and Rajagopalan, 1997). We propose that the process can be represented by a reversible reaction: kw
O þ MCNT % O MCNT; ku
(2)
where O represents oil droplets that are not sufficiently wrapped by MCNTs for removal, O-MCNT represents droplets that are sufficiently wrapped for removal, kw is the wrapping rate constant, and ku is the unwrapping rate constant. We further assume the wrapping and unwrapping processes are pseudo second-order and pseudo first-order, respectively, with regard to the reactants. The equilibrium constant of the overall reaction is: K¼
½O MCNT kw ¼ : ½O½MCNT ku
(3)
When the overall reaction is near equilibrium, a pseudo second-order rate law can be derived with respect to the separation capacity qt defined by Eq. (1) (Liu and Shen, 2008): 1 1 1 1 þ ; ¼ qt q2e kw m t qe
(4)
where qe is the equilibrium capacity, and m is the MCNT dose. The Langmuir model further predicts the relationship between qe, maximum separation capacity qmax, and residual oil
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experimental data. Using Eq. (3), we further estimate ku ¼ 1.4(0.1) min1. Combining Eqs. (4) and (5) gives: 2 1 V 1 1 1 1 þ ¼ 2 þ1 þ1 : qt qmax kw m KCe t qmax KCe
(6)
Compared to the kinetics of oil-water separation, the regeneration of used MCNTs in ethanol is a fast process. We find that the relationship of recovered capacity qr, equilibrium capacity qe, and regeneration time tr is well represented by the power law proposed for the rapid dissolution of a solute from a confining matrix (Peppas, 1985): qr ¼ kr tnr ; qe
Fig. 5 e Regeneration of magnetic carbon nanotubes (MCNTs) and recovery of separated oil by ethanol wash. (a) Kinetics of ethanol wash. (b) Multi-cycle reuse. Conditions for experiments in a: 5 mL ethanol and 30 mg LL1 MCNTs for regeneration; 1 g LL1 MCNTs and 2 g LL1 diesel for the evaluation of recovered capacity qr in comparison to the equilibrium capacity qe. Conditions in b: 10 min mixing and otherwise the same as the conditions used in a. Standard deviations are represented by error bars. The solid curves in a are least-square regressions. The solid line in b represents an estimated average. The dashed curves bracket 95% confidence intervals.
concentration Ce at equilibrium to be (Hiemenz and Rajagopalan, 1997): 1 1 1 1 ¼ þ : qe qmax K Ce qmax
(5)
According to Eq. (4), a linear regression of the kinetic data in Fig. 4a gives kw ¼ 3.36(0.03) L g-diesel1 min1 for m ¼ 10 gMCNT L1, as illustrated by the figure inset. According to Eq. (5), a linear regression of the equilibrium data in Fig. 4b gives qmax ¼ 6.6(0.6) g-diesel g-MCNT1 and K ¼ 2.4(0.2) L g-diesel1. Both regressions have R2 > 0.99, indicating that the Langmuir kinetics and equilibrium described by Reaction 2 can explain over 99% of the variation associated with the
(7)
where kr and n are empirical parameters. In this case, oil droplets are confined in the matrix of MCNT networks. The least-square regression of the experimental data inFig. 5a to the linearized format of Eq. (7) gives kr ¼ 0.84(0.01) min0.023 and n ¼ 0.023(0.002). The rapid kinetics represented by these values suggests that immersion in ethanol causes immediate rapture of the MCNT network wrapped around oil droplets. To develop the separation technique into a practical engineering process, several critical factors must be further considered. First, although CNTs are mechanically robust and chemically stable (Salvetat et al., 1999; Balasubramanian and Burghard, 2005), the stability of SPIONs should be evaluated for long-term, repeated use. The oxidation of ferrous iron into dissolved ferric iron is proposed as the main driving force for magnetite dissolution (Bohnsack, 1987; Matei et al., 2011), which can strip all the magnetic nanoparticles on MCNTs and make them useless. Alternatively, the oxidation of ferrous ion can transform magnetite to maghemite, which is also superparamagnetic and thus does not alter the functions of MCNTs (Tang et al., 2003). Distinguishing between the two pathways under engineering conditions is crucial for the design and use of MCNTs. In our experiments, particularly during the 10 cycle reuse of MCNTs, no change of the response of MCNTs to the external magnetic field was noticed, suggesting that there was negligible dissolution of magnetite nanoparticles. Second, the influence of constituents in the emulsion other than oil droplets should be evaluated. For instance, dissolved salts may crystalize on MCNTs and thus alter the physicochemical properties of MCNTs and eventually their interaction with oil droplets. Third, a practical electromagnet should be designed to replace the permanent magnet used in laboratory experiments. By generating an alternating magnetic field, the electromagnet can not only remove oil-trapping MCNTs from water but also drive the mixing of MCNTs with the emulsion during wrapping. Finally, advances in material synthesis, application, and reuse must be made to improve the economic competitiveness of the treatment process.
5.
Conclusions
Magnetic carbon nanotubes designed by utilizing the unique physicochemical properties of multi-walled carbon nanotubes and superparamagnetic magnetite nanoparticles are promising for separating oil droplets from water contaminated by
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oil and gas production. Direct observation of the separation process using confocal laser-scanning fluorescence microscopy reveals the mechanism of oil-water separation using magnetic carbon nanotubes. The kinetics and thermodynamics of the separation process conforms to the Langmuir model. After separation, spent MCNTs can be rapidly regenerated by ethanol wash, which also recovers the separated oil. The oil-water separation by reusable MCNTs provides a promising alternative strategy for water treatment design complementary to existing ones such as coagulation, adsorption, filtration, and membrane processes.
Acknowledgment This project is in part supported by the Notre Dame Sustainable Energy Initiative. C.N. also acknowledges financial support from the donors of the American Chemical Society Petroleum Research Fund (Award 50379DNI2), the National Science Foundation Environmental Engineering Program (Grant No. 1033502), and the Department of Energy Office of Nuclear Energy’s Nuclear Energy University Programs (Grant CFP-123923). The authors thank the Center for Sustainable Energy at Notre Dame Material Characterization Facility, Notre Dame Center for Environmental Science and Technology, and Notre Dame Integrated Imaging Facility for technical assistance.
references
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Li, Y., Zhang, X.B., Tao, X.Y., Xu, J.M., Huang, W.Z., Luo, J.H., Luo, Z.Q., Li, T., Liu, F., Bao, Y., Geise, H.J., 2005. Mass production of high-quality multi-walled carbon nanotube bundles on a Ni/Mo/MgO catalyst. Carbon 43 (2), 295e301. Liu, Y., Shen, L., 2008. From Langmuir kinetics to first- and second-order rate equations for adsorption. Langmuir 24 (20), 11625e11630. Matei, E., Predescu, C., Berbecaru, A., Predescu, A., Trusca, R., 2011. Leaching tests for synthesized magnetite nanoparticles used as adsorbent for metal ions from liquid solutions. Digest Journal of Nanomaterials and Biostructures 6 (4), 1701e1708. Motta, M., Moisala, A., Kinloch, I.A., Windle, A.H., 2007. High performance fibres from ‘dog bone’ carbon nanotubes. Advanced Materials 19 (21), 3721e3726. Peppas, N.A., 1985. Analysis of Fickian and non-Fickian drug release from polymers. Pharmaceutica Acta Helvetiae 60 (4), 110e111. Pickering, S.U., 1907. Emulsions. Journal of the Chemical Society 91, 2001e2021. Rasband, W.S., 1997e2011. ImageJ. U. S. National Institutes of Health, Benthesda, MD. Salvetat, J.P., Bonard, J.M., Thomson, N.H., Kulik, A.J., Forro´, L., Benoit, W., Zuppiroli, L., 1999. Mechanical properties of carbon nanotubes. Applied Physics A: Materials Science & Processing 69 (3), 255e260. Sun, S.H., Zeng, H., Robinson, D.B., Raoux, S., Rice, P.M., Wang, S.X., Li, G.X., 2004. Monodisperse MFe2O4 (M ¼ Fe, Co, Mn) nanoparticles. Journal of the American Chemical Society 126 (1), 273e279. Tang, J., Myers, M., Bosnick, K.A., Brus, L.E., 2003. Magnetite Fe3O4 nanocrystals: spectroscopic observation of aqueous oxidation kinetics. Journal of Physical Chemistry B 107 (30), 7501e7506. Tao, K., Dou, H., Sun, K., 2008. Interfacial coprecipitation to prepare magnetite nanoparticles: concentration and temperature dependence. Colloids and Surfaces A: Physicochemical and Engineering Aspects 320 (1e3), 115e122. Tetra Tech Inc, 2010. Evaluating Sustainability of Projected Water Demands under Future Climate Change Scenarios. The Natural Resources Defense Council., High-Tech Consultants Inc (2005) Oilfield Water-Oil-Solids Separation. Thanikaivelan, P., Narayanan, N.T., Pradhan, B.K., Ajayan, P.M., 2012. Collagen Based Magnetic Nanocomposites for Oil Removal Applications. Scientific Reports 2. The 2030 Water Resources Group, 2009. Charting Our Water Future: Economic Frameworks to Inform Decision-making (accessed 31.01.13.). http://www.mckinsey.com/client_service/ sustainability/latest_thinking/charting_our_water_future. The White House, 2011. Blueprint for a Secure Energy Future (accessed 31.01.13.). http://www.whitehouse.gov/sites/ default/files/blueprint_secure_energy_future.pdf. Thostenson, E.T., Li, C.Y., Chou, T.W., 2005. Nanocomposites in context. Composites Science and Technology 65 (3e4), 491e516. Veil, J.A., Puder, M.G., Elcock, D., Redweik, R.J., 2004. A White Paper Describing Produced Water from Production of Crude Oil, Natural Gas, and Coal Bed Methane. Argonne National Laboratory. Wan, J., Cai, W., Feng, J., Meng, X., Liu, E., 2007. In situ decoration of carbon nanotubes with nearly monodisperse magnetite nanoparticles in liquid polyols. Journal of Materials Chemistry 17 (12), 1188e1192. Wang, Y., Shan, H., Hauge, R.H., Pasquali, M., Smalley, R.E., 2007. A highly selective, one-pot purification method for singlewalled carbon nanotubes. The Journal of Physical Chemistry B 111 (6), 1249e1252. Wang, H., Cao, L., Yan, S., Huang, N., Xiao, Z., 2009. An efficient method for decoration of the multiwalled carbon nanotubes with nearly monodispersed magnetite nanoparticles. Materials Science and Engineering B-advanced Functional Solid-state Materials 164 (3), 191e194.
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Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
Removal of oil droplets from contaminated water using magnetic carbon nanotubes Haitao Wang a, Kun-Yi Lin a, Benxin Jing b, Galyna Krylova c, Ginger E. Sigmon a, Paul McGinn b, Yingxi Zhu b, Chongzheng Na a,* a
Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, IN 46556, USA b Department of Chemical and Biomolecular Engineering, University of Notre Dame, 182 Fitzpatrick Hall, Notre Dame, IN 46556, USA c Department of Chemistry and Biochemistry, University of Notre Dame, 132 Nieuwland Science Hall, University of Notre Dame, Notre Dame, IN 46556, USA
article info
abstract
Article history:
Water contaminated by oil and gas production poses challenges to the management of
Received 2 June 2012
America’s water resources. Here we report the design, fabrication, and laboratory evalu-
Received in revised form
ation of multi-walled carbon nanotubes decorated with superparamagnetic iron-oxide
31 January 2013
nanoparticles (SPIONs) for oil-water separation. As revealed by confocal laser-scanning
Accepted 11 February 2013
fluorescence microscopy, the magnetic carbon nanotubes (MCNTs) remove oil droplets
Available online xxx
through a two-step mechanism, in which MCNTs are first dispersed at the oil-water interface and then drag the droplets with them out of water by a magnet. Measurements
Keywords:
of removal efficiency with different initial oil concentration, MCNT dose, and mixing time
Pickering emulsion
show that kinetics and equilibrium of the separation process can be described by the
Energy and water security
Langmuir model. Separation capacity qt is a function of MCNT dose m, mixing time t, and
Industrial wastewater
residual oil concentration Ce at equilibrium: 2 1 1 1 1 1 1 ¼ 2 þ1 þ1 þ qt qmax kw m KCe t qmax KCe
Environmental nanotechnology Responsive nanocomposite Fossil fuels
where qmax, kw, and K are maximum separation capacity, wrapping rate constant, and equilibrium constant, respectively. Least-square regressions using experimental data estimate qmax ¼ 6.6(0.6) g-diesel g-MCNT1, kw ¼ 3.36(0.03) L g-diesel1 min1, and K ¼ 2.4(0.2) L g-diesel1. For used MCNTs, we further show that over 80% of the separation capacity can be restored by a 10 min wash with 1 mL ethanol for every 6 mg MCNTs. The separation by reusable MCNTs provides a promising alternative strategy for water treatment design complementary to existing ones such as coagulation, adsorption, filtration, and membrane processes. ª 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
Water plays an important role in oil and gas production, particularly in aging oil fields and unconventional tight-gas,
shale-gas, and coal-bed fields where the injection of highpressure water is needed to improve productivity. When the injected water, together with water already in the geological formation, flows back to the surface, it is contaminated with
* Corresponding author. Tel.: þ1 574 631 5164; fax: þ1 574 631 9236. E-mail address: [email protected] (C. Na). 0043-1354/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2013.02.056
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oil droplets and dissolved hydrocarbons from the contact with oil and gas reservoirs. The oil-contaminated water is often referred to as produced water. Surveys have estimated that the United States generates over 2.4 billion gallons per day of produced water, which is equivalent to 7 barrels of contaminated water for each barrel of oil produced onshore (Veil et al., 2004). This amount is equal to nearly half of the public drinking water produced in the country (Clark and Veil, 2009). More oil-contaminated water will likely be produced in the next decade as the country strives to achieve the new energy goal of reducing foreign oil imports. To achieve this goal, the federal government has pledged to increase domestic oil and gas production in addition to improving energy efficiency and developing renewable sources (The White House, 2011). The large quantity of produced water poses serious challenges to the management of America’s future water resources under the pressures of population growth, economic development, and climate change (The, 2030 Water Resources Group, 2009; Tetra Tech Inc., 2010). Before produced water can be discharged into the environment or reused in the production process, oil droplets and dissolved hydrocarbons must be removed to meet environmental requirements or industrial standards. Current practices often use the gun-barrel (aka. knock-put wash) tank or its variations as the first step of oil-water separation under gravity (High-Tech Consultants Inc. 2005). The water effluent of the gravity separation often contains oil droplets less than 10 mm in diameter as an oil-water emulsion. Further reduction of oil content in produced water requires the addition of chemicals to break the emulsion. The chemical treatment, however, increases the operating cost and generates waste that requires disposal. The treatment of produced water costs up to $5 per barrel (Veil et al., 2004), which is a significant expense with the current market price of crude oil at approximately $100 per barrel. Inspired by the Picking-emulsion forming property of carbon nanotubes (Pickering, 1907), we have designed carbon nanotubes (CNTs) decorated with superparamagnetic ironoxide nanoparticles (SPIONs) as agents for oil-water separation. Without surface modification, carbon nanotubes do not disperse well in either water or oil; instead, CNTs are dispersed at the oil-water interface and form a Picking emulsion (Wang et al., 2009). In this study, we took advantage of this century-old phenomenon and designed a new oil-water separation process. We added magnetic carbon nanotubes (MCNTs) into the oil-water emulsion in which MCNTs were attracted to the oil-water interface. To physically separate oil droplets from water, a magnetic field was applied to exert an attractive force on SPIONs, which removed MCNT-wrapped oil droplets from water. Processed water was removed from the reactor by a pipette, mimicking pumping or drainage. Oilsoaked MCNTs were washed with environmentally friendly solvents such as ethanol to recover oil and then put back into reuse. Although magnetic carbon nanotubes and nanofibers have already been proposed as adsorbents of dissolved and dispersed contaminants (Tang et al., 2003; Thostenson et al., 2005; Thanikaivelan et al., 2012), as demulsifiers for destabilizing the oil-water emulsion (Motta et al., 2007), and as agents for drug delivery (de Faria et al., 1997), to our knowledge, the utilization of magnetic carbon nanotubes for rapid physical
separation of the oil-water emulsion has not been reported in the literature. Here we report the design, fabrication, and laboratory evaluation of SPION-decorated CNTs for oil-water separation.
2.
Materials and methods
All the reagents used in this study were of analytical grade in quality (from Sigma Aldrich or Fisher Scientific). Deionized (DI) water (18.2 MU cm1) used in solution making, washing, and rinsing was generated by a Millipore system (Billerica, MA) on site. The neodymium-iron-boron block magnet used in generating the magnetic field for oil-water separation was purchased from K&J Magnetics (Jamison, PA, Model BX0X0C; maximum field strength: 13.2 kOe).
2.1.
Synthesis of magnetic carbon nanotubes
Multi-walled carbon nanotubes were synthesized following a thermal chemical vapor deposition (TCVD) method using CoMg10Mo10O as catalyst (Li et al., 2005). The catalyst was synthesized using a mixture of Co(NO3)2$6H2O, Mg(NO3)2$6H2O, and (NH4)Mo7O24$4H2O with a molar ratio of 1:10:10 for Co:Mg:Mo. The mixture was dissolved in 20 mL DI water with 21 g citric acid as combustion additive. The solution was ignited in an oven (preheated to 600 C) and stayed on fire for 10 min. To perform TCVD, 200 mg catalyst was placed on a quartz boat. The boat was then inserted to the center of a 2-inch quartz tubing. The tubing was flushed with argon at 500 sccm (standard cubic centimeter per minute) and heated in a tube furnace to 1000 C. Methane and hydrogen were introduced at 500 sccm and 100 sccm, respectively. The growth lasted for approximately 30 min, after which the furnace was cooled to the room temperature under a nitrogen atmosphere. To remove the catalyst, CNTs were boiled with 1 M hydrochloric acid and 30% hydrogen peroxide at 80 C for 30 min (Wang et al., 2007). Purified CNTs were collected by filtration, washed with DI water, and freeze-dried before they were used for attaching SPIONs. Magnetite nanoparticles were decorated on CNTs as SPIONs through the co-precipitation of ferrous and ferric irons (Wan et al., 2007). To perform co-precipitation, 0.5 g CNTs and 0.75 g iron trisacetylacetonate were mixed with 90 mL triethylene glycol and then heated to boiling under vigorous stirring and nitrogen protection, and kept at reflux for 30 min. After the mixture was cooled to the room temperature, 30 mL ethyl acetate was added to precipitate MCNTs. MCNTs were thoroughly washed with DI water and freeze-dried before use. As a negative control in studying the mechanism of oil-water separation, SPION suspension was synthesized following the same protocol in the absence of CNTs.
2.2.
Characterization of magnetic carbon nanotubes
Physical properties of MCNTs were characterized using transmission electron microscopy (TEM), the Brunauer-EmmettTeller (BET) surface area analysis, and the superconducting quantum interference device (SQUID). For the TEM analysis of
Please cite this article in press as: Wang, H., et al., Removal of oil droplets from contaminated water using magnetic carbon nanotubes, Water Research (2013), http://dx.doi.org/10.1016/j.watres.2013.02.056
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morphology and composition, MCNTs were dissolved in ethanol, deposited on copper grids, and imaged using a Titan 80e300 microscope (FEI, Hillsboro, OR). Specific surface areas of MCNTs, compared to unattached SPIONs and pristine CNTs, were measured using an ASAP 2000 BET analyzer (Micromeritics, Norcross, GA) with nitrogen. The magnetic property of MCNTs was characterized using an MPMS SQUID system (Quantum Design, San Diego, CA). All experimental magnetic data were corrected for the diamagnetism of sample holders and constituent atoms.
2.3.
Visualization of oil-water separation in action
Direct, real-time observation of MCNTs in the oil-water emulsion required that oil droplets had diameters in the range of tens of micrometers in order to counter Brownian motion and comply with resolution (i.e., one half of the laser wavelength, 271.5 nm). To increase the sizes of oil droplets, 5 mg sodium 1,2-dioleoyl-sn-glycero-3-phosphate was added as the stabilizing surfactant to an emulsion prepared with 1 mL diesel and 4 mL DI water. One tenth of a gram of ammonium 1,2-dioleoyl-sn-glycero-3-phosphoethanolamineN-(lissamine rhodamine B sulfonyl) was also added to the emulsion as a fluorescent probe to illuminate oil droplets. Unattached SPIONs, pristine CNTs, or SPION-attached MCNTs were added to the emulsion at a dose level of 2, 2, or 5 g L1, respectively. The interactions between nanomaterials and oil droplets, with or without an external magnetic field, were visualized using a confocal laser-scanning microscope (LSM 5 Pascal; Carl Zeiss Microscopy, Thornwood, NY) equipped with a 100 objective lens (numerical aperture ¼ 1.4 in oil). Confocal images and movies were processed using ImageJ (Rasband, 1997e2011).
2.4.
Evaluation of oil-water separation by MCNTs
The performance of MCNTs in oil-water separation was evaluated using emulsions made of diesel (Martin’s Gas Station, South Bend, IN; ASTM Grade No. 2-D: density, 0.84 g mL1 at 25 C; viscosity, 2.7 mPa s at 40 C) and DI water. Diesel-inwater emulsions were prepared by adding 1e2.5% of diesel by volume to water under 10 min sonication (Misonix S3000, Farmingdale, NY). The oil-in-water emulsion was stable for more than 24 h, which was confirmed by its visual opacity, and used in experiments within 12 h. The size of oil droplets was confirmed to be in the micrometer range by placing a drop of the emulsion on a glass slide and observed under an optical microscope calibrated with a standard grid (DC Imaging BA300, Mason, OH). The diesel concentration was measured as chemical oxygen demand (COD; unit: mg-O2 L1, detection limit: 3 mg L1) after oxidizing the emulsion with potassium dichromate (Cuenca et al., 2006; American Public Health Association et al., 1995). The diesel concentration was computed from COD using an average diesel formula of C12H23 and with the assumption that diesel had been completely converted to carbon dioxide and water. It is worth noting that no surfactant was used in evaluation experiments. To separate oil droplets from water, dry MCNTs were weighed and dispersed in 5 mL water in a 40 mL glass vial representing a batch reactor. An aliquot of concentrated diesel-in-
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water emulsion was added to the vial, creating a 30 mL emulsion with a preset diesel concentration. The contents were mixed on a shaking table at 300 rpm. The effect of mixing on MCNT performance was assessed by varying the shaking time from several minutes to 1.5 h. MCNTs and the associated oil droplets were then separated from water by placing the magnet on one side of the vial. Water was removed from the vial using a pipette, mimicking a pumping or draining process. The remaining diesel concentration was analyzed by the COD method with three replicates. To regenerate oil-soaked MCNTs, 10 mL ethanol was added to the MCNT aggregate in the vial. After shaking by hand, the ethanol was decanted and the regenerated MCNTs were used again for oil-water separation.
3.
Results
Fig. 1 summarizes the physical properties of magnetic carbon nanotubes obtained by transmission electron microscopy and the SQUID magnetometer. As shown in Fig. 1a, MCNTs are well separated long tubes decorated with SPIONs on the outside. The contrast between them suggests that nanotubes are made of the lighter element of carbon whereas SPIONs are made of heavier elements of iron and oxygen. The TEM image in Fig. 1b taken under an increased magnification reveals lattice fringes for both CNTs (spacing in the 002 direction: 3.49 A) and SPIONs (spacing in the 400 direction: 2.08 A), which are in good agreement with reports in the literature (Kiang et al., 1998; Sun et al., 2004). The microscopic results further provide quantitative information on the dimensions and composition of MCNTs. By making direct measurements on approximately 50 representative MCNTs in TEM images, we have obtained estimates of the outer diameter and length for CNTs to be 14.5(6.1) nm and 0.69(0.45) mm, respectively. The diameter and interparticle distance for SPIONs are estimated to be 9.6(0.8) nm and 48(23) nm, respectively. Based on these parameters, we further estimate that MCNTs consist of 75(19)% of CNTs and 25(27)% of SPIONs in mass, have a density of 2.9(1.5) g cm3, and possess a specific surface area of 146(68) m2 g1. The estimated specific surface area is in good agreement with the BET measurement of 150.0(0.3) m2 g1. Under a varying external magnetic field, MCNTs exhibit a typical superparamagnetic behavior with little histeresis, as shown in Fig. 1c. Coercity, which is defined as the external field needed for demagnetization, is only 8.3(0.5) Oe for MCNTs, indicating that they are superparamagnetic materials with negligible remanence of magnetization once the external field is removed. The saturation magnetization of MCNTs is 15.5(0.1) emu g1. Since the mass fraction of SPIONs in MCNTs is 25%, the saturation magnetization of SPIONs is 62 emu g1, which is approximately 70% of the saturated magnetization of bulk magnetite (Tao et al., 2008). The reduction of saturated magnetization can be attributed to the size effect (Kodama et al., 1996). Fig. 2 shows a series of snapshots taken by a confocal laserscanning microscope that reveal the mechanism of oil-water separation by MCNTs. As shown in Fig. 2a, in the absence of MCNTs, diesel droplets with sizes up to tens of micrometers are stabilized in water by surfactants. Under the microscope,
Please cite this article in press as: Wang, H., et al., Removal of oil droplets from contaminated water using magnetic carbon nanotubes, Water Research (2013), http://dx.doi.org/10.1016/j.watres.2013.02.056
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Fig. 1 e Physical properties of magnetic carbon nanotubes. (a) Transmission electron micrograph (TEM) of MCNTs. (b) TEM showing lattice fringes for carbon nanotubes and superparamagnetic iron-oxide nanoparticles (SPIONs). (c) Superparamagnetic loop for MCNTs at 300 K with the inset showing a negligible coercivity. Scale bars: a, 200 nm; b, 5 nm.
oleophilic fluorescent molecules added to the emulsion give oil droplets a yellow color against the black water background. Carbon nanotubes added to the oil-water emulsion, as shown in Fig. 2b, are dispersed at the oil-water interface as dark spots because they can quench fluorescence by their p-stacking
structures (Arai et al., 2007). MCNTs can also quench fluorescence because the attachment of SPIONs does not disturb the integrity of CNTs’ p stacking. As shown in Fig. 2c, when MCNTs are mixed with the oil-water emulsion, they are either dispersed at the oil-water interface or immersed in the oil
Fig. 2 e Oil-water separation in action captured by confocal laser-scanning fluorescence microscopy, presented in snapshots. (a) Oil-in-water emulsion made of diesel. (b) Pristine CNTs dispersed at the oil-water interface. (c) MCNTs dispersed at the oil-water interface and immersed inside the oil droplet. Examples are marked by red arrows. (d) MCNTwrapped oil droplets under the influence of an external magnetic field placed at the lower right corner. The snapshot reveals the change of droplet shape in the direction of their movement toward the magnetic source. (e) SPIONs dispersed at the oilwater interface. (f) SPIONs removed from the oil-water interface by an external magnetic field. Examples of removed SPIONs are marked by red arrows in b. Note that shapes and positions of oil droplets are marginally influenced by the external magnetic field. Scale bars: 5 mm. See movies S1eS5 in the Supplementary materials for details. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Please cite this article in press as: Wang, H., et al., Removal of oil droplets from contaminated water using magnetic carbon nanotubes, Water Research (2013), http://dx.doi.org/10.1016/j.watres.2013.02.056
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phase as bundles. The MCNT-wrapped oil droplets can be readily removed from water by an external magnetic field, as manifested by their ellipsoidal shapes in Fig. 2d. In comparison, oil droplets experience little magnetic force when unattached SPIONs is added to the emulsion. Comparison between images taken in the absence (Fig. 2e) and presence (Fig. 2f) of an external magnetic field shows that magnetite nanoparticles can be moved out of oil droplets by the field. The unattached SPIONs cannot, however, drag the oil droplets along with them due to the lack of sufficient affinity. The mechanism can be better appreciated with the movies in the Supplementary materials that provide 3-dimensional slicing of oil droplets and time sequences for the movements of oil droplets and SPIONs in external magnetic fields. Supplementary video related to this article can be found at http://dx.doi.org/10.1016/j.watres.2013.02.056. Fig. 3 provides a qualitative assessment of the performance of MCNTs in oil-water separation. MCNTs are prepared as dry black powders (Fig. 3a). MCNTs do not disperse well in either water or diesel oil but preferentially reside at their interface (Fig. 3b). The diesel-water emulsion prepared by sonication without using surfactants is opaque due to the light-scattering property of oil droplets (Fig. 3c). The opacity blocks the view of the Notre Dame logo placed behind the emulsion. MCNTs are readily dispersed in the diesel-water emulsion by wrapping themselves around oil droplets (Fig. 3d). A magnet (in the middle of Fig. 3e) attracts MCNTs, together with the wrapped oil droplets, to a side of the vial and thus effectively moves them out of water. Water becomes clean as evident from the clear Notre Dame logo in the background. The optical micrographs underneath Fig. 3cee show that after treatment with MCNTs, there is no longer any visible oil droplet. Oil trapped by MCNTs can be recovered by ethanol wash (Fig. 3f and g) and
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the cleaned MCNTs can be reused for oil-water separation as illustrated by Fig. 3cee. Fig. 4 presents the quantitative evaluation on the kinetics and equilibrium of oil-water separation using MCNTs. Each experiment was performed with a diesel-water emulsion having a volume of 30 mL and an initial diesel concentration of C0 (in g L1) and an MCNT dose of m (in g L1). After mixing for a period of time t, the remaining diesel concentration Ct (in g L1) was measured and the separation capacity of MCNTs in g-diesel g-MCNT1 was computed as: qt ¼
C0 Ct : m
(1)
We first investigated the relationship between qt and t with constant C0 and m. An example is shown in Fig. 4a, revealing that qt increases rapidly with increasing t. qt approaches an equilibrium capacity qe as Ct approaches an equilibrium residual concentration Ce. According to Fig. 4a, qt reaches >99% of qe after 90 min of mixing. We then investigated the equilibrium of oil-water separation by MCNTs using 90 min as the mixing time and with increasing C0 or decreasing m. As shown in Fig. 4b, both qe and Ce increase either with increasing C0 from 0.85 to 3.85 g L1 at fixed m values (colored in blue, green, and red) or with increasing m from 0.5 to 4 g L1 at fixed C0 values (colored in gold and brown). The experimental data agree well with each other at low qe and Ce but become increasingly scattered at high qe and Ce. We suspect that the scattering is in part due to the dilution required for measuring high concentrations of initial and residual diesel. Because oil is dispersed in water as droplets, taking a representative fraction for dilution can be challenging. Nonetheless, a general trend of qe approaching a limiting capacity qmax at high Ce can be seen in Fig. 4b.
Fig. 3 e Digital pictures of experimental setups for evaluating the efficiency of oil-water separation. (a) Dry magnetic carbon nanotube (MCNT) powder. (b) MCNTs residing at the interface of bulk oil (yellow) and bulk water (white). (c) Oil-in-water emulsion made of diesel. (d) Mixture of MCNTs with the oil-water emulsion. (e) Removal of MCNTs, together with trapped oil droplets, by a magnet to the side of the vial. (f) Extraction of trapped oil from spent MCNTs in ethanol. (g) Removal of MCNTs from ethanol by a magnet for reuse. The removal of oil droplets is also shown in the optical micrographs underneath b, c, and d. Scale bar: 10 mm. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Please cite this article in press as: Wang, H., et al., Removal of oil droplets from contaminated water using magnetic carbon nanotubes, Water Research (2013), http://dx.doi.org/10.1016/j.watres.2013.02.056
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Fig. 4 e Kinetics (a) and equilibrium (b) of oil-water separation using magnetic carbon nanotubes (MCNTs). Insets are linearregression plots based on the Langmuir model. Conditions for experiments in a: 1 g LL1 MCNTs and 0.5 g LL1 diesel. Conditions for experiments in b: blue, 0.5 g LL1 MCNTs with 0.85, 1.36, 1.70, 2.03, 2.37, and 3.05 g LL1 diesel; green and red, 0.75 and 1 g LL1 MCNTs, respectively, with 3.85, 3.78, 3.55, 3.32, 2.92, and 2.60 g LL1 diesel; gold, 1.83 g LL1 diesel with 4, 2, 1.5, and 1 g LL1 MCNTs; brown, 2.14 g LL1 diesel with 1.5, 1.0, 0.75, 0.5 and 0.25 g LL1 MCNTs; mixing time for all, 90 min. Standard deviations are represented by error bars. The solid curves are least-square regressions according to the Langmuir model. The dashed curves bracket 95% confidence intervals. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5 summarizes the experimental results for MCNT regeneration and oil recovery in ethanol. We first assessed the effect of wash time on the performance of regenerated MCNTs. As shown in Fig. 5a, the ethanol wash can recover over 80% of the separation capacity within the first 10 min and the recovery, defined as the ratio of reusable capacity qr to equilibrium capacity qe, increases with increasing regeneration time tr. Based on the kinetic results, 10 min wash is sufficient for recovering most of MCNTs’ separation capacity. The reusability of MCNTs is further demonstrated in Fig. 5b for 10 cycles of regeneration with 10 min ethanol wash, which shows a consistent performance with a recovery of 81.2(0.6)%.
4.
Discussion
Establishing rational design principles is essential for transforming exciting properties of nanomaterials to cost-effective applications. In our design of magnetic carbon nanotubes, several guidelines have been followed. First, we select multiwalled carbon nanotubes as wrapping agents because they are less expensive and less technically challenging to synthesize compared to single- and few-walled nanotubes. In addition, single- and few-walled CNTs tend to bundle and collapse radially (Motta et al., 2007), which makes it difficult to control product quality. Second, CNTs’ diameter is selected to be 10e20 nm in order to accommodate the 10 nm magnetite nanoparticles used as SPIONs. Increasing CNTs’ diameter beyond the nanoparticle size is undesirable because of the resulting increase of MCNT density and decrease of specific surface area. Third, 10 nm magnetite nanoparticles are chosen as magnetic agents because they are superparamagnetic (Wang et al., 2009). Having no residual magnetization in the
absence of an external field, MCNTs will not aggregate in storage or during mixing with the oil-water emulsion. Finally, we select nanoparticle coverage of ca. 10% to minimize their agglomeration on CNTs and keep the MCNT density low. Further lowering the coverage may result in a non-uniform distribution of nanoparticles. The separation of oil and water using MCNTs follows the kinetics and equilibrium described by the Langmuir model (Hiemenz and Rajagopalan, 1997). We propose that the process can be represented by a reversible reaction: kw
O þ MCNT % O MCNT; ku
(2)
where O represents oil droplets that are not sufficiently wrapped by MCNTs for removal, O-MCNT represents droplets that are sufficiently wrapped for removal, kw is the wrapping rate constant, and ku is the unwrapping rate constant. We further assume the wrapping and unwrapping processes are pseudo second-order and pseudo first-order, respectively, with regard to the reactants. The equilibrium constant of the overall reaction is: K¼
½O MCNT kw ¼ : ½O½MCNT ku
(3)
When the overall reaction is near equilibrium, a pseudo second-order rate law can be derived with respect to the separation capacity qt defined by Eq. (1) (Liu and Shen, 2008): 1 1 1 1 þ ; ¼ qt q2e kw m t qe
(4)
where qe is the equilibrium capacity, and m is the MCNT dose. The Langmuir model further predicts the relationship between qe, maximum separation capacity qmax, and residual oil
Please cite this article in press as: Wang, H., et al., Removal of oil droplets from contaminated water using magnetic carbon nanotubes, Water Research (2013), http://dx.doi.org/10.1016/j.watres.2013.02.056
w a t e r r e s e a r c h x x x ( 2 0 1 3 ) 1 e8
7
experimental data. Using Eq. (3), we further estimate ku ¼ 1.4(0.1) min1. Combining Eqs. (4) and (5) gives: 2 1 V 1 1 1 1 þ ¼ 2 þ1 þ1 : qt qmax kw m KCe t qmax KCe
(6)
Compared to the kinetics of oil-water separation, the regeneration of used MCNTs in ethanol is a fast process. We find that the relationship of recovered capacity qr, equilibrium capacity qe, and regeneration time tr is well represented by the power law proposed for the rapid dissolution of a solute from a confining matrix (Peppas, 1985): qr ¼ kr tnr ; qe
Fig. 5 e Regeneration of magnetic carbon nanotubes (MCNTs) and recovery of separated oil by ethanol wash. (a) Kinetics of ethanol wash. (b) Multi-cycle reuse. Conditions for experiments in a: 5 mL ethanol and 30 mg LL1 MCNTs for regeneration; 1 g LL1 MCNTs and 2 g LL1 diesel for the evaluation of recovered capacity qr in comparison to the equilibrium capacity qe. Conditions in b: 10 min mixing and otherwise the same as the conditions used in a. Standard deviations are represented by error bars. The solid curves in a are least-square regressions. The solid line in b represents an estimated average. The dashed curves bracket 95% confidence intervals.
concentration Ce at equilibrium to be (Hiemenz and Rajagopalan, 1997): 1 1 1 1 ¼ þ : qe qmax K Ce qmax
(5)
According to Eq. (4), a linear regression of the kinetic data in Fig. 4a gives kw ¼ 3.36(0.03) L g-diesel1 min1 for m ¼ 10 gMCNT L1, as illustrated by the figure inset. According to Eq. (5), a linear regression of the equilibrium data in Fig. 4b gives qmax ¼ 6.6(0.6) g-diesel g-MCNT1 and K ¼ 2.4(0.2) L g-diesel1. Both regressions have R2 > 0.99, indicating that the Langmuir kinetics and equilibrium described by Reaction 2 can explain over 99% of the variation associated with the
(7)
where kr and n are empirical parameters. In this case, oil droplets are confined in the matrix of MCNT networks. The least-square regression of the experimental data inFig. 5a to the linearized format of Eq. (7) gives kr ¼ 0.84(0.01) min0.023 and n ¼ 0.023(0.002). The rapid kinetics represented by these values suggests that immersion in ethanol causes immediate rapture of the MCNT network wrapped around oil droplets. To develop the separation technique into a practical engineering process, several critical factors must be further considered. First, although CNTs are mechanically robust and chemically stable (Salvetat et al., 1999; Balasubramanian and Burghard, 2005), the stability of SPIONs should be evaluated for long-term, repeated use. The oxidation of ferrous iron into dissolved ferric iron is proposed as the main driving force for magnetite dissolution (Bohnsack, 1987; Matei et al., 2011), which can strip all the magnetic nanoparticles on MCNTs and make them useless. Alternatively, the oxidation of ferrous ion can transform magnetite to maghemite, which is also superparamagnetic and thus does not alter the functions of MCNTs (Tang et al., 2003). Distinguishing between the two pathways under engineering conditions is crucial for the design and use of MCNTs. In our experiments, particularly during the 10 cycle reuse of MCNTs, no change of the response of MCNTs to the external magnetic field was noticed, suggesting that there was negligible dissolution of magnetite nanoparticles. Second, the influence of constituents in the emulsion other than oil droplets should be evaluated. For instance, dissolved salts may crystalize on MCNTs and thus alter the physicochemical properties of MCNTs and eventually their interaction with oil droplets. Third, a practical electromagnet should be designed to replace the permanent magnet used in laboratory experiments. By generating an alternating magnetic field, the electromagnet can not only remove oil-trapping MCNTs from water but also drive the mixing of MCNTs with the emulsion during wrapping. Finally, advances in material synthesis, application, and reuse must be made to improve the economic competitiveness of the treatment process.
5.
Conclusions
Magnetic carbon nanotubes designed by utilizing the unique physicochemical properties of multi-walled carbon nanotubes and superparamagnetic magnetite nanoparticles are promising for separating oil droplets from water contaminated by
Please cite this article in press as: Wang, H., et al., Removal of oil droplets from contaminated water using magnetic carbon nanotubes, Water Research (2013), http://dx.doi.org/10.1016/j.watres.2013.02.056
8
w a t e r r e s e a r c h x x x ( 2 0 1 3 ) 1 e8
oil and gas production. Direct observation of the separation process using confocal laser-scanning fluorescence microscopy reveals the mechanism of oil-water separation using magnetic carbon nanotubes. The kinetics and thermodynamics of the separation process conforms to the Langmuir model. After separation, spent MCNTs can be rapidly regenerated by ethanol wash, which also recovers the separated oil. The oil-water separation by reusable MCNTs provides a promising alternative strategy for water treatment design complementary to existing ones such as coagulation, adsorption, filtration, and membrane processes.
Acknowledgment This project is in part supported by the Notre Dame Sustainable Energy Initiative. C.N. also acknowledges financial support from the donors of the American Chemical Society Petroleum Research Fund (Award 50379DNI2), the National Science Foundation Environmental Engineering Program (Grant No. 1033502), and the Department of Energy Office of Nuclear Energy’s Nuclear Energy University Programs (Grant CFP-123923). The authors thank the Center for Sustainable Energy at Notre Dame Material Characterization Facility, Notre Dame Center for Environmental Science and Technology, and Notre Dame Integrated Imaging Facility for technical assistance.
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