Langmuir 2006, 22, 8831-8839
8831
Colloidal Interactions between Langmuir-Blodgett Bitumen Films and Fine Solid Particles Jun Long, Liyan Zhang, Zhenghe Xu, and Jacob H. Masliyah* Department of Chemical and Materials Engineering, UniVersity of Alberta, Edmonton, Alberta, Canada T6G 2G6 ReceiVed April 3, 2006. In Final Form: May 22, 2006 In oil sand processing, accumulation of surface-active compounds at various interfaces imposes a significant impact on bitumen recovery and bitumen froth cleaning (i.e., froth treatment) by altering the interfacial properties and colloidal interactions among various oil sand components. In the present study, bitumen films were prepared at toluene/water interfaces using a Langmuir-Blodgett (LB) upstroke deposition technique. The surface of the prepared LB bitumen films was found to be hydrophobic, comprised of wormlike aggregates containing a relatively high content of oxygen, sulfur, and nitrogen, indicating an accumulation of surface-active compounds in the films. Using an atomic force microscope, colloidal interactions between the LB bitumen films and fine solids (model silica particles and clay particles chosen directly from an oil sand tailing stream) were measured in industrial plant process water and compared with those measured in simple electrolyte solutions of controlled pH and divalent cation concentrations. The results show a stronger long-range repulsive force and weaker adhesion force in solutions of higher pH and lower divalent cation concentration. In plant process water, a moderate long-range repulsive force and weak adhesion were measured despite its high electrolyte content. These findings provide more insight into the mechanisms of bitumen extraction and froth treatment.
1. Introduction Surfactants play a crucial role in numerous natural and industrial processes.1,2 Their unique ability to lower surface/interfacial tensions has been exploited in many practical applications. For example, natural surfactants found in oil sands assist bitumen recovery from oil sand ores in water-based bitumen extraction processes by not only lowering the interfacial tensions but also increasing interfacial charges for all the phases involved in the process (bitumen, water, and solids).3 The increased surface charges on both bitumen droplets and solid particles favor bitumen detachment from the solids, while reduced interfacial tension of bitumen droplets and air bubbles favors attachment of air bubbles to bitumen droplets. Oil sands are a complex mixture of bitumen, sand grains (quartz), clay minerals, water, and electrolytes. In a typical waterbased bitumen extraction process,4 oil sand lumps are mixed with water and process aids such as sodium hydroxide to form a slurry. Bitumen is released or “liberated” from the sand grains during slurry conditioning. The liberated bitumen is aerated by entrained or introduced air bubbles and recovered subsequently as bitumen froth by flotation. During this process, the bitumen droplets can become coated with a layer of fine particles, i.e., the so-called slime-coating phenomenon.5 Slime coating not only reduces the flotation efficiency and bitumen recovery by setting up a steric barrier that retards bitumen droplets from contacting air bubbles but also deteriorates the froth quality by carrying the attached fine solids to the bitumen froth. In the bitumen froth * To whom correspondence should be addressed. E-mail: Jacob.Masliyah@ ualberta.ca. (1) Tadros, T. F. Applied Surfactants: Principles and Applications; WileyVCH: Hoboken, NJ, 2005. (2) Rosen, M. J. Surfactants and Interfacial Phenomena; John Wiley & Sons: Hoboken, NJ, 2004. (3) FTFC. AdVances in Oil Sands Tailings Research; Alberta Department of Energy, Oil Sands and Research Division: Edmonton, Canada, 1995. (4) Masliyah, J.; Zhou, Z.; Xu, Z.; Czarnecki, J.; Hamza, H. Can. J. Chem. Eng. 2004, 82, 628-654. (5) Liu, J.; Xu, Z.; Masliyah, J. Can. J. Chem. Eng. 2004, 84, 655-666.
product, water-in-oil and oil-in-water emulsions are often present. Attachment of fine solids on bitumen or water droplets can set a steric barrier among the droplets and thus stabilizes emulsions, consequently making the bitumen froth cleaning difficult. It is well recognized that the addition of sodium hydroxide in a bitumen extraction process is mainly to produce “natural” surfactants from their precursors present in bitumen.6 The surfactants produced are predominantly aliphatic carboxylates having hydrocarbon chains of at least five carbons (typically C15 to C17) and aliphatic sulfonates having hydrocarbon chains of at least five carbons. These surfactants would adsorb on the surface of bitumen droplets, air bubbles, and fine solids.7 In addition to these surfactants, asphaltenes are also present in crude oils and bitumen. Asphaltenes are macromolecules with an amphiphilic nature. They act like surfactants and tend to stay at water/oil interfaces. The accumulation of surfactants and asphaltenes at the water/oil interfaces makes the surface properties of bitumen droplets significantly different from bulk bitumen properties, thus impacting the interactions between bitumen and other phases, such as fine solids, present in a bitumen extraction system. As a result, bitumen recovery and bitumen froth cleaning (i.e., froth treatment) could be greatly affected. In both the bitumen extraction and froth treatment processes, colloidal interactions between bitumen and fine solid particles play a critical role. For example, the bitumen liberation process is controlled by the interactions between bitumen and sand grains. The interactions between bitumen droplets and fine solid particles control the attachment of these particles to the bitumen droplets, thus controlling the efficiency of bitumen flotation and the stability of oil-in-water emulsions. Knowledge of the colloidal interactions between bitumen and fine solids is therefore essential for understanding the fundamentals of bitumen extraction and froth treatment. It is even more important to identify the key factors (6) Schramm, L. L.; Stasiuk, E. N.; Turner, D. Fuel Process. Technol. 2003, 80, 101-118. (7) Schramm, L. L.; Smith, R. G.; Stone, J. A. Colloids Surf. 1984, 11, 247263.
10.1021/la0608866 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/13/2006
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that affect these colloidal interactions. By doing so, one can control the colloidal interactions to improve bitumen recovery and froth treatment. Surface force measurements using an atomic force microscope (AFM) or surface force apparatus (SFA) provide a direct means to quantitatively study colloidal interactions. Although colloidal force measurements by an AFM or SFA have been reported extensively in the literature, quantitative colloidal force measurements related to crude oil systems are rather limited. Yoon et al.8 and Yoon and Rabinovich9 measured the forces between two bitumen surfaces using an AFM and between two asphaltene surfaces using an SFA. Abraham et al.10 measured the forces between asphaltene and silica using an AFM. Liu et al.5,11 measured the forces between bitumen and fine solids (silica and clay particles) in aqueous solutions with an AFM. They found that the solution pH and divalent cation addition could significantly affect the colloidal interactions of various components of oil sands. Long et al.12 studied the effect of temperature on the interactions between bitumen and fine clays in an aqueous environment. Their results show that the measured adhesion force between clay and bitumen decreases with increasing temperature until a critical value of about 32-35 °C, above which the adhesion force disappears. Li et al.13 also measured the interaction forces between bitumen and fine solids (silica and clays) in an aqueous environment in the presence of a partially hydrolyzed polyacrylamide flocculant. In their study, the flocculant was used as a process aid to recover bitumen from a poor processing oil sand ore. They found that the flocculant is capable of improving bitumen recovery as well as enhancing fine solids settling in the tailing stream. The AFM force measurements were used as a tool to understand the role of the polymer in bitumen recovery. In the studies discussed above, two approaches, spin coating and Langmuir-Blodgett (LB) upstroke, were used to prepare bitumen surfaces for the colloidal/surface force measurements. In the spin-coating technique,5,11-13 bitumen surfaces were prepared by spreading a bitumen-in-toluene solution on silica wafers using a spin coater. The spin-coated bitumen surfaces were found to be acceptably smooth and rigid for colloidal force measurements.11 On the other hand, Yoon et al.8 employed the LB upstroke deposition technique to prepare bitumen surfaces. Using a Langmuir trough, a bitumen-in-chloroform solution was spread at the air/water interface. The interfacial film was then coated onto a mica surface by upstroke. As bitumen does not dissolve in water, all the nonvolatile materials from the solution would stay at the air/water interface. Thus, the bitumen surfaces prepared as such would resemble very much the spin-coated surfaces at least in terms of chemical composition. Because bitumen is a deformable fluid and the surface-active compounds can migrate from the bulk to the surface, in a bitumen extraction system it would thus be anticipated that the surface of bitumen droplets could be enriched by surfactants and asphaltenes. To mimic such surfaces enriched by surface-active compounds, in the current study, the LB deposition technique was used to prepare bitumen films at toluene/water interfaces. As bitumen is soluble in toluene, the hydrocarbons in the bitumen would stay mainly (8) Yoon, R. H.; Guzonas, D.; Aksoy, B. S.; Czarnecki, J.; Leung, A. Processing of Hydrophobic Minerals and Fine Coal, Proceeding of the UBC-McGill BiAnnual International Symposium on Fundamentals of Mineral Processing; CIM: Montreal, Canada, 1995; pp 277-289 (9) Yoon, R. H.; Rabinovich, Y. I. Processing of Hydrophobic Minerals and Fine Coal, Proceeding of the UBC-McGill Bi-Annual International Symposium on Fundamentals of Mineral Processing, 3rd; CIM: Montreal, 1999. (10) Abraham, T.; Christendat, D.; Karan, K.; Xu, Z.; Masliyah, J. Ind. Eng. Chem. Res. 2002, 41, 2170-2177. (11) Liu, J.; Xu, Z.; Masliyah, J. Langmuir 2003, 19, 3911-3920. (12) Long, J.; Xu, Z.; Masliyah, J. H. Energy Fuels 2005, 19, 1440-1446. (13) Li, H.; Long, J.; Xu, Z.; Masliyah, J. H. Energy Fuels 2005, 19, 936-943.
Long et al. Table 1. Some Properties of the Plant Process Watera surface tension, [K+] [Na+] [Mg2+] [Ca2+] [Cl-] [SO42-] [HCO3-] pH mN/m 0.36 21.72 a
0.79
1.20
12.12
0.66
10.61
8.4
68.5
Ion concentrations in mM.
in the toluene phase and the surface-active compounds (asphaltene and surfactants) would adsorb at the toluene/water interface. Thus, the prepared bitumen surface would be more representative of those encountered in water-based bitumen extraction processes. In this study, the surface of such LB bitumen films was characterized using contact angle measurement, AFM imaging, and X-ray photoelectron spectroscopy (XPS) analysis. The colloidal interactions between the LB bitumen films and fine solids (silica and clay particles) in aqueous solutions were directly measured using an AFM. The clay particles were directly chosen from an oil sand tailing stream. The effect of solution pH, divalent cation addition, and industrial plant process water on the interaction forces was studied. The goal of this study is to understand the surface properties of bitumen films enriched by surface-active compounds and their effect on the colloidal interactions between bitumen and fine solids. In general, this investigation will enhance our current understanding of systems involving bitumen and fine solids and help in optimizing the performance of bitumen extraction and bitumen froth treatment. 2. Experimental Section 2.1. Materials. Vacuum distillation feed bitumen, containing 17 wt % asphaltenes, was provided by Syncrude Canada Ltd. HPLCgrade chloroform (99.8%) was purchased from Aldrich. Ultra-highpurity KCl (>99.999%, Aldrich) was used as the supporting electrolyte, while reagent-grade CaCl2 (99.9965%, Fisher) was used as the source of calcium cations. Reagent-grade HCl and NaOH (Fisher) were used as pH modifiers of the aqueous solutions. Deionized water with a resistivity of 18.2 MΩ cm, prepared with an Elix 5 followed by a Millipore-UV Plus water purification system (Millipore Inc., Canada), was used where applicable throughout this study. The plant process water used in this study was the recycle pond water from the Aurora plant of Syncrude Canada Ltd. It had a pH of ∼8.2 and a surface tension of 68.5 mN/m. Calcium, magnesium, and other inorganic cations were found in the process water. Table 1 gives a list of inorganic cations and their concentrations in the process water. Silicon wafers were purchased from MEMC Electronic Materials Inc. (Italy) and used as the substrates for preparation of bitumen films. 2.2. Preparation of Langmuir-Blodgett Bitumen Films. LB bitumen films were deposited onto hydrophilic silicon wafers from a toluene/water interface using the following procedure. Bitumen was dissolved in chloroform at a concentration of 1 g/L. A volume of 100 µL of the prepared bitumen-in-chloroform solution was spread dropwise with the aid of a microsyringe on a water subphase contained in a Langmuir interfacial trough (KSV, Finland). Details of the Langmuir interfacial trough are described elsewhere.14,15 Prior to spreading of the bitumen film, a hydrophilic silicon wafer, precleaned with the procedure of Zhang and Srinivasan,16 was immersed in the water subphase. The spreading solvent was allowed to evaporate from the air/water interface for a period of at least 10 min. The interface was then covered with a top phase consisting of 100 mL of toluene. The toluene/water interface was allowed to equilibrate for 30 min before it was compressed to a desired interfacial pressure of 5 mN/m, at which the LB film was deposited through upstroke at 5 mm/min onto a hydrophilic silicon wafer substrate. In this study, (14) Zhang, L. Y.; Xu, Z.; Masliyah, J. H. Langmuir 2003, 19, 9730-9741. (15) Zhang, L. Y.; Lopetinsky, R.; Xu, Z.; Masliyah, J. H. Energy Fuels 2005, 19, 1330-1336. (16) Zhang, L. Y.; Srinivasan, M. P. Colloids Surf., A 2001, 193, 15-33.
Bitumen Film and Fine Solid Particle Interactions the interfacial pressure of 5 mN/m was chosen to ensure that the area between the two compressing barriers is sufficiently large to cover both sides of the silicon substrate. After deposition, the substrate was held in the air for 1 h for the entrained water or toluene to evaporate. The LB bitumen films were kept in a desiccator until being used for subsequent characterization and force measurements. 2.3. Surface Composition Analysis. XPS17,18 was used to analyze surface elemental compositions of the prepared LB bitumen films using an Axis Ultra spectrometer (Kratos). The excitation source was monochromatic Al KR (energy 1486.71 eV) radiation. The spectrometer was calibrated by the binding energy (84.0 eV) of Au 4f7/2 with reference to the Fermi level. Energy analysis of photoelectrons was made with a hemispheric electrostatic analyzer using the FAT (fixed analyzer transmission) mode. For each specimen, a survey spectrum within the range of binding energies from 0 to 1100 eV was collected at a pass energy of 160 eV. Corelevel spectra of C1s, O1s, Si2p, N1s, S2s, and S2p were collected at a pass energy of 20 eV and a scanning step of 0.1 eV. It should be noted that the XPS instrument probes 2-3 nm of the surface layer with an elliptical scanning area of 1 mm in width and 2 mm in length. For each sample, two specimens were measured, and the results presented are the averaged values. For comparison, the elemental compositions of bulk bitumen and asphaltene were measured in this study using a CHNS-O EA 1108 elemental analyzer (Carlo Erba, Milan, Italy). 2.4. Contact Angle Measurement. Advancing and receding contact angles of deionized water on the bitumen films were measured using a Kru¨ss drop shape analysis system (DSA 10-MK2) at room temperature. A sessile drop of deionized water was placed on an LB bitumen film deposited on a hydrophilic silicon wafer. A computerized dosing system was used to pump the liquid into or out of the drop to make the three-phase contact line advance or recede on the surface. Images were captured during the process. Contact angles were obtained from the captured images using a tangent method. 2.5. Colloidal Force Measurements. A Nanoscope E atomic force microscope (AFM) with a vendor-supplied fluid cell (Digital Instruments, Santa Barbara, CA) was used for surface force measurements. Gold-coated silicon nitride cantilevers also from Digital Instruments were chosen. Model silica particles (Duke Scientific Co., ∼8 µm in diameter) or clay particles having a pseudospherical shape (∼8-16 µm in diameter) were used as the probe for the force measurements by attaching them to the apex of a cantilever (lever type 100 µm wide) with a spring constant of 0.58 N/m. Figure 1 shows SEM micrographs of a prepared silica probe (a) and a clay probe (b). The clay particles were chosen under an optical microscope from a large number of particles, which were directly obtained from the tailing slurry of a bitumen extraction experiment using a poor processing ore.13 Prior to each set of force measurements, the prepared probes were thoroughly rinsed with deionized water and ethanol, followed by blow-drying with ultrapure-grade nitrogen. The probes were then exposed to an ultraviolet light for more than 5 h to remove any possible organic contaminants. The details of preparing such colloid probes and using an AFM for force measurements have been given elsewhere.11,19 Briefly, in the AFM force mode, a triangular waveform is applied to the AFM Z piezotube. As a result, the sample surface attached to the piezotube moves toward and away from the cantilever tip (the colloid probe) by the extension and retraction of the piezotube. The force acting between the probe and the surface is determined from the deflection of the cantilever by using Hooke’s law (force ) spring constant × cantilever deflection). Each force plot represents a complete extension-retraction cycle of the piezotube. When a sample surface approaches a probe, the long-range interaction force between the two surfaces is measured while the adhesion (or pull-off) force can be obtained during the retraction process after contact has been (17) Watts, J. F.; Wolstenholme, J. An Introduction to Surface Analysis by XPS and AES; John Wiley and Sons Ltd.: Hoboken, NJ, 2003. (18) Briggs, D. Surface Analysis of Polymers by XPS and Static SIMS; Cambridge University Press: Cambridge, U.K., 1998. (19) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Langmuir 1992, 8, 18311836.
Langmuir, Vol. 22, No. 21, 2006 8833
Figure 1. SEM micrographs of silica (a) and clay (b) probes. made. For quantitative comparison, the measured long-range interaction force (F) and adhesion force (pull-off force) were normalized by the probe radius (R). Force measurements were performed in a fluid cell where clay (or silica) probes interacted with a bitumen surface in aqueous solutions. All force measurements were conducted after an incubation time of 30 min. Preliminary experiments showed that 30 min was sufficient for the two surfaces immersed in the aqueous medium to equilibrate. As the surface of the clay probes was quite irregular, force measurements under each set of conditions were performed several times with different clay probes to obtain representative results. All force measurements were conducted at a room temperature of 22 ( 1 °C.
3. Theoretical Analysis of the Measured Forces To understand the origin of the measured forces, the classical DLVO theory20,21 was used in the current study to fit the experiment data. The DLVO theory has been widely regarded as a cornerstone for understanding colloidal systems and forces on the molecular scale and for predicting macroscopic properties of colloidal dispersions. The DLVO theory considers only electrostatic double-layer22,23 and Lifshitz-van der Waals interactions.24,25 In the current study, the van der Waals force was calculated by Hamaker’s microscopic approach.23 For a spherical probe of radius R interacting with a substrate plate, the van der Waals forces (Fv) can be expressed as a function of the (20) Derjaguin, B. V.; Landau, L. Acta Physicochim., URSS 1941, 14, 633662. (21) Verwey, E. J. W.; Overbeek, J. T. G. Theory of Stability of Lyophobic Colloids; Elsevier: Amsterdam, 1948. (22) Russel, W. B.; Saville, D. A.; Schowalter, W. R. Colloidal Dispersions; Cambridge University Press: Cambridge, U.K., 1992. (23) Israelachvili, J. N. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992. (24) Lifshitz, E. M. Eksp. Teor. Fiz. 1955, 29, 94-110. (25) Hamaker, H. C. Pysica 1937, 4, 1058-1072.
8834 Langmuir, Vol. 22, No. 21, 2006
Long et al.
Table 2. Parameters Used in the DLVO Fittings system
Hamaker constant A, J
silica-water-bitumen clay-water-bitumen
2.5 × 10-21 5.0 × 10-21
debye length (1/κ), nm
solution 1 mM KCl at pH 3.2 1 mM KCl at pH 8.4 1 mM KCl + 1 mM CaCl2 at pH 8.4 Aurora process water (pH ≈ 8.4)
Table 3. Elemental Compositions of the LB Bitumen Films, Bitumen, and Asphaltenea bulk bitumen asphaltene LB bitumen film
surface potentials, mV bitumen silica clay
9.4 9.1 4.8
-25 -50 -35
-25 -59 -25
-10 -35 -10
2.4
-42
-28
-18
Fv A )- 2 R 6D
(1)
where A is the Hamaker constant. For the LB bitumen films and clay particles, the Hamaker constants of n-tetradecane and mica, 5.4 × 10-20 and 10 × 10-20 J,23 respectively, were used. For water and silica, the Hamaker constants are 3.7 × 10-20 and 6.5 × 10-20 J,23 respectively. The combined Hamaker constants for the silica-water-bitumen and clay-water-bitumen systems were calculated23 to be ca. 2.5 × 10-21 and 5.0 × 10-21 J, respectively. The electrostatic double-layer forces, on the other hand, were calculated by numerically solving the nonlinear Poisson-Boltzmann equation
d2ψ dx2
∑i
) -e
( )
zini∞ exp -
zieψ
(2)
kT
where and 0 are the dielectric constant of the liquid medium and permittivity of the vacuum (8.854 × 10-12 CV-1 m-1), respectively, e the elementary charge (1.602 × 10-19 C), z the valence of the electrolyte, ni∞ the electrolyte bulk concentration (molecules‚m-3), k the Boltzmann constant (1.381 × 10-23 J‚K-1), T the temperature (K), and ψ the potential (V) as a function of point x (m). For the case of constant surface charge density, the electric double-layer forces (Fe) are given by
Fe R
[ ( ( )) ( ) ]
) -2π kTni∞
∑i 1 - exp -
zieψ kT
-
0 dψ 2 dx
2
(3)
From eqs 1-3, the total forces (or the DLVO forces, Ftotal) of a system can be obtained:
Ftotal ) Fe + Fv
N
O
S
0.56 1.24 1.40
0.88 1.74 3.30
5.44 8.74 9.70
Relative weight percentages of carbon, nitrogen, oxygen, and sulfur.
layer near a charged surface and can be given by the following expression:23
∑i ni∞e2zi2/0kT)-1/2
κ-1 ) (
(5)
4. Results and Discussion
separation distance (D) between the probe and plate:
0
a
C 93.12 88.28 85.60
(4)
A Visual Basic program running on an Excel spreadsheet developed in our laboratory by Liu et al.11 was used for the theoretical calculation of the DLVO forces. During the fitting exercise, surface potentials of the probe and substrate and the Debye decay length (κ-1) were set as adjustable parameters. In some cases where these parameters are already known, they are directly used to calculate the DLVO forces, and then the calculated forces are compared with the corresponding experimental results. Table 2 gives all the parameters used for the DLVO fittings in the present study. The Debye decay length represents the characteristic length or “thickness” of the diffuse electric double
4.1. Characteristics of LB Bitumen Films. 4.1.1. Surface Elemental Compositions. Bitumen is a mixture of high-molecularweight hydrocarbon compounds. It contains asphaltene (17 wt % in the bitumen used in this study) and various surfactants. In general, bitumen is composed of five elements (to the extent of about 99.4-99.9%): carbon, hydrogen, nitrogen, oxygen, and sulfur. The rest (up to about 0.6%) is made up of metals in the form of organometallics. The amount of hydrogen and carbon in bitumen is normally over 90%. The combined amount of the other three elements is up to about 9.9%.26 Among the various factors that determine the physicochemical properties of a surface or an interface, surface chemical composition is the most important one. To understand the colloidal interactions between bitumen and solids, it is essential to know the surface chemical composition of bitumen. In the current study, XPS was employed to determine the surface elemental composition of the LB bitumen films. XPS is a surface-sensitive method with a typical information depth of 1-5 nm, corresponding to 4-20 atomic or molecular monolayers. As XPS does not detect hydrogen, the elemental composition results given in Table 3 represent the relative weight percentages of carbon, oxygen, nitrogen, and sulfur. For comparison, the elemental compositions of bulk bitumen and asphaltene obtained are given in Table 3. As anticipated, the results in Table 3 show that asphaltenes contain a higher content of oxygen, nitrogen, and sulfur than bitumen. The content of these elements is further elevated in the LB bitumen films. As asphaltenes are amphiphilic and behave like surfactants, it is expected that the LB bitumen films contain mainly asphaltenes.27 However, the elevated oxygen, nitrogen, and sulfur content indicates that, in addition to asphaltenes, the LB bitumen films were enriched also by surfactants, such as aliphatic carboxylates and aliphatic sulfonates. These results suggest that, during the preparation of the LB bitumen films, asphaltenes and surfactants were adsorbed at the toluene/water interface. Thus, the prepared films are not bitumen anymore. Instead, these films mainly contain surface-active compounds derived from the bitumen. For the convenience of discussion, in this study we still refer to these films as LB bitumen films although they do not completely represent bitumen. 4.1.2. Surface Hydrophobicity. The water contact angles confirmed a successful deposition of LB bitumen films from the toluene/water interface. In contrast to a contact angle value of nearly zero for a bare hydrophilic silicon wafer before deposition, (26) Strausz, O. P.; Lown, E. M. The Chemistry of Alberta Oil Sands, Biutmens and HeaVy Oils; Alberta Energy Research Institute: Edmonton, Canada, 2003; p 695. (27) Solovyev, A.; Zhang, L. Y.; Xu, Z.; Masliyah, J. Submitted for publication to Energy Fuels.
Bitumen Film and Fine Solid Particle Interactions
Langmuir, Vol. 22, No. 21, 2006 8835
Figure 3. Effect of solution pH on the long-range interaction and adhesion forces between silica particles and LB films in 1 mM KCl solutions. Symbols are the measured long-range interaction forces. For each condition, different symbols represent the results of different runs. Solid curves represent the results of DLVO fitting. The inset shows the distributions of adhesion forces.
Figure 2. (a) Topographic image (1 × 1 µm2) of an LB bitumen film obtained by AFM tapping-mode imaging in air. (b) Section analysis of the topographic image in panel a.
the advancing contact angle for the LB bitumen films obtained through upstroke deposition was about 90° (the receding angle is about 70°). This contact angle value is very close to the values reported for deposited asphaltene films from a heptane/water interface.15 The contact angle results indicate a successful deposition of the LB bitumen films on the silicon substrates and their hydrophobic nature. 4.1.3. Surface Topography. Imaging of the LB bitumen films was carried out with a Nanoscope IIIa (Digital Instruments) AFM in tapping mode. The imaging was performed in air at room temperature, using a multimode scanning probe microscopy (MMSPM) head and a J scanner. A silicon tip (Digital Instruments) with a resonance frequency of 300-400 kHz was used for tappingmode imaging at a scan rate of 1-2 Hz. Figure 2a shows a height image of an LB bitumen film deposited from the toluene/water interface at 5 mN/m. This image shows that the LB bitumen film is composed of wormlike aggregates of a few nanometers in diameter. Similar aggregates were observed on LB asphaltene surfaces by Liu et al.28 This would indicate that the LB bitumen films were enriched by surface-active compounds, predominantly asphaltenes. A section profile (Figure 2b) of the AFM image (Figure 2a) shows that the deposited LB bitumen film is relatively smooth, with a surface average roughness less than 2 nm. 4.2. Effect of the Solution pH on Interactions between LB Bitumen Films and Solids. The slurry pH in a bitumen extraction process controls the surface charges of various components in the slurry, and it is thus a critical operating parameter. The effect of pH on the interaction forces between the LB bitumen films (28) Liu, J.; Zhang, L.; Xu, Z.; Masliyah, J. Langmuir 2006, 22, 1485-1492.
and silica particles in 1 mM KCl solutions is shown in Figure 3. At pH 8.4, the long-range interaction force was purely repulsive. A weak long-range repulsive force was still observed at pH 3.2. At this pH, a jump-in at a separation distance of ca. 4 nm, as indicated by the arrow in the figure, appeared on the force profiles, indicating attractive forces between the probe and substrate. Attractive forces of similar range were also reported for silica and silica nitride probes interacting with mica in aqueous solutions of electrolytes. Bremmell et al.29 speculated that the attraction of the silica to mica at 5 nm separation is caused by a hydrous silica gel layer surrounding the silica surface. The silica/aqueous interface is known to generate a gel layer that can cause such interaction forces at small surface separations.30 Teschke et al.,31 on the other hand, simply attributed such short-range attractions to van der Waals forces. In Figure 3, the solid curves represent the DLVO fitting curves. At separations greater than ca. 2-3 nm for pH 3.2 and 4-5 nm for pH 8.4, the measured force profiles are fitted reasonably well with the classical DLVO theory, suggesting that the long-range repulsive forces are predominantly from the electrostatic doublelayer interactions. The ζ potentials of silica particles experimentally measured by Liu et al.,11 -59 and -25 mV for pH 8.4 and 3.2, respectively, were used in the fitting. At pH 8.4, the fitted ζ potential of the LB bitumen films is -50 mV, which is identical to that of asphaltenes measured by Abraham et al.10 As the LB bitumen films contained mainly asphaltenes, such a consistency is anticipated. At pH 3.2, the fitted ζ potential of the LB bitumen films, -25 mV, is close to that of the bitumen surface measured by Liu et al.11 In this case, the reasonable match of the jump-in peaks of the measured force profiles with the fitted curve indicates that such attractive forces at small separations could be contributed to the van der Waals interactions between the probes and substrates. The fitted Debye decay length (κ-1) is 9.1 and 9.4 nm for pH 8.4 and 3.2, respectively. These values are close to the calculated value (9.6 nm) for 1 mM KCl solutions used in the experiment, further confirming the electrostatic nature of the long-range repulsive forces. At very small separation distances, e.g., less than 2-3 nm, the DLVO theory predicts a strong attractive force regime due to the (29) Bremmell, K. E.; Jameson, G. J.; Biggs, S. Colloids Surf., A 1998, 139, 199-211. (30) Chan, D. Y. C.; Pashley, R. M.; White, L. R. J. Colloid Interface Sci. 1980, 77, 283-285. (31) Teschke, O.; Ceotto, G.; Souza, E. F. d. Phys. ReV. E 2001, 64, 011605011615.
8836 Langmuir, Vol. 22, No. 21, 2006
Figure 4. Effect of solution pH on the long-range interaction and adhesion forces between clay particles and LB bitumen films in 1 mM KCl solutions. Symbols are the measured long-range interaction forces. For each condition, different symbols represent the results of different runs. Solid curves represent the results of DLVO fitting. The inset shows the distributions of adhesion forces.
prevalence of van der Waals forces against electrostatic repulsive forces. This is inconsistent with the strong repulsive forces measured at such small separation distances, suggesting the presence of additional repulsive forces. Although the exact reason for this contradiction is not clear, at such a short range of 2-3 nm, these additional repulsive forces could originate from surface roughness,32 small protrusions from the bitumen/water interface,11 or the hydration of the surfaces.33-36 To fully understand the colloidal interaction in a dynamic system, such as the bitumen extraction system, the adhesion forces between the LB bitumen films and the fine particles have to be considered. The inset of Figure 3 shows the distributions of the adhesion forces (or pull-off forces) between the LB bitumen films and silica particles. At pH 8.4, no adhesion force (or zero adhesion) was detected. In most cases, the retracting force profiles obtained at this pH were the reverse of the approaching force profiles. At pH 3.2, the adhesion forces were centered at 2.5 mN/m. The observed increase in adhesion forces with decreasing pH could be contributed to the pH-dependent dissociation of cationic surfactants at the bitumen/water interface. The effect of pH on the interaction forces between the LB bitumen films and clay particles in 1 mM KCl solutions is shown in Figure 4. Similar to the case between the LB bitumen films and silica particles (Figure 3), the long-range forces between the LB films and clay particles were repulsive at both pH 8.4 and pH 3.2. However, in this case the repulsion at pH 8.4 was not as strong as that between the LB bitumen films and silica particles (Figure 3). For example, the repulsion was ∼0.3 mN/m at a separation distance of ∼10 nm as shown in Figure 4, but it was ∼0.6 mN/m between the LB bitumen films and silica particles at the same separation distance (Figure 3). This is mainly because clay particles are less negatively charged than silica particles in solutions of the same pH. At pH 3.2, the long-range interaction forces were still repulsive albeit very weak. At both pH 8.4 and pH 3.2, the measured force profiles fit reasonably well with the DLVO theory as shown by the solid curves in Figure 4. The fitted ζ potentials for the clay particles are -35 and -10 mV for pH 8.4 and 3.2, respectively. These values are close to the (32) Considine, R. F.; Drummond, C. J. Langmuir 2001, 17, 7777-7783. (33) Vigil, G.; Xu, Z.; Steinberg, S.; Israelachvili, J. J. Colloid Interface Sci. 1994, 165, 367-385. (34) Rabinovich, Y. I.; Derjaguin, B. V.; Churaev, N. V. AdV. Colloid Interface Sci. 1982, 16, 63-78. (35) Pashley, R. M. J. Colloid Interface Sci. 1981, 83, 531-546. (36) Grabbe, A.; Horn, R. G. J. Colloid Interface Sci. 1993, 157, 375-383.
Long et al.
Figure 5. Effect of calcium addition on the long-range interaction and adhesion forces between silica particles and LB bitumen films in 1 mM KCl solutions at pH 8.4. Symbols are the measured longrange interaction forces. For each condition, different symbols represent the results of different runs. Solid curves represent the results of DLVO fitting. The inset shows the distributions of adhesion forces.
measured ζ potentials of clays.37,38 Other parameters used in the fitting procedure are the same as used for Figure 3. The inset of Figure 4 shows the effect of the solution pH on the adhesion forces between the LB bitumen films and clay particles. At pH 8.4, no adhesion was detected. However, at pH 3.2, adhesion forces ranging from 0 to ∼1.3 mN/m as shown by the distribution in the inset were observed. The average adhesion force is about 0.33 mN/m with a deviation of ∼0.27 mN/m. It should be noted that although pseudospherical clay particles were chosen as probes for the force measurements, the irregularity of the clay particles resulted in scattered data for both long-range and adhesion forces. Nevertheless, the trend of the measured forces with decreasing pH is clear, and thus, conclusions derived from the measured force profiles are valid. For example, from the results presented in Figure 4, one can easily conclude that a decrease in solution pH would depress the long-range interaction forces and cause an increase in the adhesion force between the LB bitumen films and clay particles. 4.3. Effect of Divalent Cation Addition on Bitumen-Solid Interactions. Figure 5 shows the effect of 1 mM calcium addition on the long-range and adhesion forces between the LB bitumen films and silica particles in 1 mM KCl solutions at pH 8.4. For comparison, the results obtained both in the absence and in the presence of calcium at pH 8.4 are presented in this figure. For the case without calcium addition, the results (the upper symbols and curve) are the same as those plotted in Figure 3 (pH 8.4). Although the long-range interaction forces in the presence of 1 mM calcium, as shown by the lower symbols, were still repulsive, the repulsion was clearly depressed by calcium addition. A jumpin, or a plateau to be precise, at a separation distance of ca. 3 nm, as indicated by the arrow in the figure, was observed in the presence of calcium. In this case, the measured forces can be well fitted by the DLVO theory as indicated by the lower solid curve. This excellent fitting shows that the electrostatic forces were still the dominant forces between the LB bitumen films and silica particles. The fitted ζ potentials of the LB bitumen films and silica particles are -35 and -25 mV, respectively. These values are nearly identical to the ζ potentials measured by Liu (37) Liu, J.; Zhou, Z.; Xu, Z.; Masliyah, J. J. Colloid Interface Sci. 2002, 252, 409-418. (38) Ding, X. Effects of DiValent Ions, Illite Clays and Temperature on Bitumen RecoVery; University of Alberta: Edmonton, Canada, 2004.
Bitumen Film and Fine Solid Particle Interactions
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Figure 6. Effect of calcium addition on the long-range interaction and adhesion forces between clay particles and LB bitumen films in 1 mM KCl solutions at pH 8.4. Symbols are the measured longrange interaction forces. For each condition, different symbols represent the results of different runs. Solid curves represent the results of DLVO fitting. The inset shows the distributions of adhesion forces in the presence of 1 mM calcium.
Figure 7. Long-range interaction and adhesion forces between silica particles and LB bitumen films in the plant process water. Open symbols are the measured long-range interaction forces. Different symbols represent the results of different runs. The solid curve represents the DLVO profile. The dots represent the measured force profiles after shifting of the separation distance by 1.2 nm. The inset shows the distributions of adhesion forces.
et al.11 The fitted Debye decay length (κ-1) is 4.8 nm, which equals the calculated value for 1 mM KCl and a 1 mM CaCl2 solution. The inset of Figure 5 shows the distributions of measured adhesion forces at pH 8.4. In the presence of 1 mM calcium, the adhesion forces were centered at 0.52 mN/m with a standard deviation of 0.13 mN/m. Compared with a zero adhesion measured in the absence of calcium, the results show that calcium addition not only depressed the long-range interactions between the LB bitumen films and silica particles but also induced an adhesion between them. This could be attributed to the fact that calcium cations strongly adsorbed on both silica and bitumen surfaces. The adsorbed calcium cations could act as a bridge linking the negatively charged sites on both surfaces. The effect of calcium addition on the interaction forces between the LB bitumen films and clay particles in 1 mM KCl solutions at pH 8.4 is shown in Figure 6. In the presence of 1 mM calcium, the long-range interaction forces, as indicated by the lower symbols, were still repulsive, albeit very weak. Again, the data were quite scattered because of the irregularity of the clay probes. The measured force profiles fit reasonably well with the DLVO theory as shown by the lower solid curve in Figure 6. The fitted ζ potentials for the LB bitumen films and clay particles are -35 and -10 mV, respectively. The inset of Figure 6 shows the distribution of adhesion forces between the LB bitumen films and clay particles in the presence of 1 mM calcium. Without calcium addition, the adhesion force between them in KCl solutions at pH 8.4 was zero (Figure 4). However, a wide range of adhesion forces from 0 to 2.6 mN/m were measured in the presence of 1 mM calcium as shown by the distribution in the inset of Figure 6. The average adhesion force is about 0.56 mN/m with a deviation of ∼0.27 mN/m. These results show that calcium addition causes an adhesion between the LB bitumen films and clay particles. Similar to the case in Figure 4, the scattered nature of both the long-range and adhesion forces could originate from the surface irregularity of the clay particles. 4.4. Bitumen-Solid Interactions in Plant Process Water. By using prepared simple electrolyte solutions, the effect of various factors, such as solution pH and calcium addition, on the interactions between bitumen and solids as discussed above (Figures 3-5) can be clearly revealed. However, considering the fact that the water used in industrial operations (i.e., plant process water) is more complex than these simple electrolyte
solutions in terms of their physiochemical properties, it is necessary to measure the interaction and adhesion forces between bitumen and solids in industrial plant process water. By doing so, the forces obtained could better predict the colloidal state of a bitumen extraction system because the measurement conditions are closer to those in the extraction process. The measured long-range interaction and adhesion forces between the LB bitumen films and silica particles in plant process water are shown in Figure 7. The plant process water used was the recycle pond water from a commercial operation site (the Aurora plant of Syncrude Canada Ltd.). The pH of this water was about 8.4. Atomic adsorption spectroscopy (AAS) analysis showed that there were 1.2 mM calcium and 0.6 mM magnesium in the water.39 The process water also contained a high concentration of bicarbonates and various surfactants released during the extraction process.11 As shown in Figure 7, the longrange forces are purely repulsive. Compared with the forces measured in deionized water at pH 8.4 (Figure 3), the repulsive forces in the plant process water were quite small and were not detected until a separation distance of less than 10 nm, in contrast to 30 nm in deionized water. The depression in repulsion could be attributed to the presence of various cations in the plant process water as shown in Table 1. Using the measured ζ potentials of bitumen and silica in the same plant process water,39 -42 and -28 mV, respectively, and a Debye decay length (κ-1) of 2.4 nm,12 a DLVO force profile was obtained (the solid curve in Figure 7). There is a deviation between the measured and theoretically calculated force profiles. In the range of separation from ∼3 to 10 nm, the calculated forces are higher than the measured forces. Such a deviation could result from the use of inaccurate separation distances obtained between the probe and substrate in the AFM force measurements. As various surfactants or residual hydrocarbons in the plant process water could adsorb onto the probe and substrate, the zero distance obtained in the force measurements may not represent a true contact between the probe and substrate because of the presence of adsorbed materials between them. For the case of Figure 7, we estimated that, at the zero separation, the real distance between the probe and substrate was about 1.2 nm. By shifting all separation distances by 1.2 nm, the modified experimental results as shown by the dots agree reasonably well with the theoretical profile. (39) Zhao, H. Y.; Long, J.; Xu, Z.; Masliyah, J. Ind. Eng. Chem. Res. 2006.
8838 Langmuir, Vol. 22, No. 21, 2006
Long et al. Table 4. Summary of Adhesion Forces Measured between Fine Solids and LB Bitumen Films in Various Waters adhesion force (mN/m) solution
silica probes clay probes
1 mM KCl solution at pH 8.4 1 mM KCl solution at pH 3.2 1 mM KCl + 1 mM Ca solution at pH 8.4 industrial plant process water
Figure 8. Long-range interaction and adhesion forces between clay particles and LB bitumen films in plant process water. Symbols are the measured long-range interaction forces. Different symbols represent the results of different runs. The solid curve represents the DLVO force profile. The inset shows the distribution of adhesion forces measured in the process water.
A distribution of adhesion forces between the LB bitumen films and silica particles measured in the plant process water is shown in the inset of Figure 7. In most cases, a weak adhesion force was detected. The average adhesion force is ∼0.32 mN/m with a standard deviation of ∼0.14 mN/m. Figure 8 shows the measured long-range interaction and adhesion forces between the LB bitumen films and clay particles in the plant process water. The long-range forces are repulsive. Similar to the case of silica particles (Figure 7), there is a clear deviation between the measured and DLVO force profiles, which could also be due to the interpretation of the separation distance as discussed above. In this fitting procedure, the measured ζ potential of clay particles in the plant process water, -20 mV,13 was used, and other parameters are given in Table 2. The inset of Figure 8 shows the distribution of measured adhesion forces between the LB bitumen films and clay particles in the plant process water. Similar to the case of silica particles (inset of Figure 7), the measured adhesion forces are also weak, with an average value of ∼0.29 ( 0.18 mN/m. 4.5. Comparison of Average Adhesion Forces Measured in Various Solutions. In general, long-range forces described by the DLVO theory can be used satisfactorily to interpret the stability of colloidal systems. The long-range forces between bitumen and solids were repulsive in all the liquid media used in the present study, although sometimes the repulsive force was weak. Thus, no coagulation between bitumen and fine solids would be anticipated. This anticipation is in contrast to the visual observations of bitumen attachment to a sand grain made by other researchers.40,41 To reconcile the observed discrepancy, the adhesive forces between bitumen and solids have to be considered. In a dynamic colloidal system such as a bitumen extraction process, adhesion forces determine how well colloidal particles can hold each other. Table 4 compares the average adhesion forces measured in various solutions. In a KCl solution at pH 8.4, the adhesion forces between bitumen and silica/clay particles were zero. When calcium was added to the simple electrolyte solution or its pH was decreased to ∼3.2, adhesion forces were detected. However, the long-range forces measured in these cases were repulsive albeit weak. For such a system with a weak repulsive long-range interaction but with an adhesion (40) Zhou, Z. A.; Xu, Z. H.; Masliyah, J. H.; Czarnecki, J. Colloids Surf., A 1999, 148, 199-211. (41) Dai, Q.; Chung, K. H. Fuel 1995, 74, 1858-1864.
0 2.55 ( 0.80 0.52 ( 0.13 0.32 ( 0.14
0 0.33 ( 0.27 0.56 ( 0.27 0.29 ( 0.18
force, heterocoagulation between bitumen and solids could occur. This may explain the observed bitumen attachment to a sand grain. It also shows that the presence of calcium and a lower pH could deteriorate the separation of bitumen from solids in oil sands by inducing heterocoagulation between bitumen and solids. Another important point that needs to be noted here is that the average adhesion forces between the LB bitumen films and silica/ clay particles measured in the plant process water (0.32/0.29 mN/m) were slightly lower than or close to those measured in the 1 mM calcium solutions (0.52/0.56 mN/m). In general, a higher divalent cation concentration would result in a higher adhesion force due to the bridging effect of the divalent cations.11,42 The amount of divalent cations (calcium and magnesium) in the plant process water was ∼2 mM (Table 1), which was nearly double the calcium concentration in the prepared calcium solutions (1 mM). Therefore, one would anticipate higher adhesion forces to be measured in the process water. This anticipation seems contradictory with the lower adhesion forces actually obtained in the process water. As the pH values of the process water and prepared calcium solutions were the same (∼8.4), other factors (other than solution pH) have to be considered to account for such a “contradiction”. It is well-known that the chemical composition of industrial plant process water is complicated. Besides the cations listed in Table 1, there were a variety of surfactants present in the plant process water.11 The complexation of divalent cations with other species, e.g., anionic surfactants and bicarbonates, reduces the effective amount of free divalent cations in the plant process water, and thus, only part of these divalent cations would contribute to the measured adhesion forces. Therefore, it is possible that lower adhesion forces can be manifested despite a higher content of divalent cations in the plant process water. This feature shows that the presence of anionic species such as bicarbonates in the plant process water can to some extent overcome the effect of divalent cations and thus benefit the detachment of solids from the bitumen surface.
5. Conclusions In the present study, bitumen films were prepared at the toluene/ water interface using an LB upstroke deposition technique. The surface of the prepared LB bitumen films was characterized by various techniques. Colloidal interactions between the LB bitumen films and fine solids (silica and clay) in the prepared simple electrolyte solutions and in the industrial plant process water were directly measured using an atomic force microscope. It was found that the surface of the prepared LB bitumen films was hydrophobic, comprised of wormlike aggregates with a high content of oxygen, sulfur, and nitrogen, indicating an accumulation of surface-active compounds in the films. The results of the force measurements in simple electrolyte solutions show a stronger long-range repulsive force and weaker adhesion in solutions having a higher pH and a lower divalent cation concentration, (42) J. Long, H. L. Z. X. J. H. M. AIChE J. 2006, 52, 371-383.
Bitumen Film and Fine Solid Particle Interactions
suggesting that these solution conditions would favor the separation of bitumen from solids. In the plant process water, a moderate long-range repulsive force and a slightly weaker adhesion force were measured despite its high electrolyte content. The presence of anionic species in the plant process water may reduce the adhesion force between fine solids and bitumen, thus benefiting the liberation of bitumen from the fine solids.
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Acknowledgment. Financial support from the NSERC Industrial Research Chair in Oil Sands Engineering (held by J.H.M.) is gratefully acknowledged. We also thank Dr. Guoxing Gu for assistance in the analysis of the elemental composition of bulk bitumen and asphaltenes. LA0608866
