Environ. Sci. Technol. 2004, 38, 2102-2110
Reduction of Benzene and Naphthalene Mass Transfer from Crude Oils by Aging-Induced Interfacial Films SUBHASIS GHOSHAL,* CATHERINE PASION, AND MOHAMMED ALSHAFIE Department of Civil Engineering, McGill University, Macdonald Engineering Building, 817 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6
Semi-rigid films or skins form at the interface of crude oil and water as a result of the accumulation of asphaltene and resin fractions when the water-immiscible crude oil is contacted with water for a period of time or “aged”. The time varying patterns of area-independent mass transfer coefficients of two compounds, benzene and naphthalene, for dissolution from crude oil and gasoline were determined. Aqueous concentrations of the compounds were measured in the eluent from flow-through reactors, where a nondispersed oil phase and constant oil-water interfacial area were maintained. For Brent Blend crude oil and for gasoline amended with asphaltenes and resins, a rapid decrease in both benzene and naphthalene mass transfer coefficients over the first few days of aging was observed. The mass transfer coefficients of the two target solutes were reduced by up to 80% over 35 d although the equilibrium partition coefficients were unchanged. Aging of gasoline, which has negligible amounts of asphaltene and resin, did not result in a change in the solute mass transfer coefficients. The study demonstrates that formation of crude oil-water interfacial films comprised of asphaltenes and resins contribute to time-dependent decreases in rates of release of environmentally relevant solutes from crude oils and may contribute to the persistence of such solutes at crude oil-contaminated sites. It is estimated that the interfacial film has an extremely low film mass transfer coefficient in the range of 10-6 cm/min.
Introduction Uncontrolled discharges of crude oil may cause surface water and groundwater contamination, particularly at oil exploration and production sites, oil refineries, or storage facilities. Knowledge of rates and extent of dissolution of potentially water-soluble pollutants from crude oil is important for evaluating its impact on water quality, for conducting human health and risk assessment studies, and for assessing the potential of water-based technologies in site remediation and for refinery wastewater treatment. In this study, the equilibrium partitioning and mass transfer kinetics of two environmentally significant watersoluble components of crude oil, benzene and naphthalene, have been evaluated in crude oil-water systems. In particular, * Corresponding author e-mail:
[email protected]; telephone: (514)398-6867; fax: (514)398-7361. 2102
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the effects of the presence of a natural viscous, elastic film or “skin” that forms with time at a crude oil-water interface on mass transfer coefficients have been evaluated. The film is formed by surface-active molecules contained in the crude oil, commonly identified as asphaltenes and resins, that partition to the oil-water interface following contact of the two phases (1). Interfacial films have also been reported in coal tar-water systems (2-4) and in creosote-water systems (5). Asphaltenes in crude oil have been studied extensively (6) because they are often problematic for petroleum processing. For example, asphaltenes stabilize water-in-crude oil emulsions by forming interfacial films and hinder oilwater separation, and they often precipitate and impede oil flow in reservoir porous media, in oil pipelines, and in crude oil processing equipment. Several studies have suggested that nonaqueous-phase liquid (NAPL)-water mass transfer of organic solutes may be significantly rate limited, especially under conditions representative of the field (7, 8). NAPL-water mass transfer coefficients of various solutes have been experimentally determined from NAPLs that are pure compounds as well as those that are complex mixtures (9-12). The dissolution kinetics of solutes from NAPL-contaminated porous media have been extensively studied, and various empirical correlations for estimating mass transfer coefficients from the water velocity, NAPL saturation, and grain-size parameters of the porous medium have been proposed (7, 13, 14). Most of the studies have assumed that the aqueous-phase boundary at the NAPL-water interface is the rate-limiting step for mass transfer kinetics. However, several studies have suggested that slow diffusion of solutes within the NAPL or at the oil-side boundary layer of the interface may also limit rates of mass transfer (10, 11, 15). The effects of an interfacial film on crude oil-water mass transfer processes have not been assessed to date. However, certain observations reported in the literature support the hypothesis that crude oil-water interfacial films may cause significant reduction to the mass transfer coefficient of solutes from oil to water. Mohammed et al. (16) found increased magnitudes of elasticity and surface viscosity at the interface of crude oils with aging. Mukherji and Weber (17) reported that the mass transfer coefficients of toluene and several polycyclic aromatic hydrocarbons (PAHs) were reduced by up to a factor of 0.1 because of the presence of biofilms that developed at the interface of a synthesized NAPL and water in a bioreactor. Luthy et al. (2) reported a noticeable difference in the rate of dissolution of naphthalene from 7-d aged and unaged coal tar coated on to microporous silica and suspended in water. However, similar decreases in naphthalene mass transfer coefficients were not observed when the same coal tar was present as larger globules or coated onto larger porous aggregates and aged in similar systems (12). Temporal decreases in solute mass transfer rates may be caused by interfacial films becoming more rigid with time and thus imparting an increasing diffusional resistance to transport of solutes from oil to water. Furthermore, continuous dissolution of the more water-soluble components in the oil may cause changes in the nonidealities experienced by the remaining solutes in the oil phase. This may lead to changes in the oil-water equilibrium partition coefficients of the various oil components. Ignoring changes in the nonidealities experienced by solutes in the NAPL has been shown to result in significant errors in predictions of groundwater plume sizes and solute concentrations at NAPL10.1021/es034832j CCC: $27.50
2004 American Chemical Society Published on Web 02/24/2004
contaminated sites (18). In the context of this study, ignoring changes in the nonidealities experienced by solutes in the oil phase may lead to erroneous estimation of mass transfer coefficients. Thus, the specific objectives of this study were (i) to assess the changes in oil-water mass transfer coefficients and equilibrium partition coefficients of two target solutes, benzene and naphthalene, as a function of contact time in a crude oil-water system and (ii) to assess how the presence of asphaltene and resin fractions that comprise the interfacial film alter the oil-water mass transfer coefficients and equilibrium partition coefficients of the target solutes. The latter objective was achieved by comparing the timevarying patterns of equilibrium partition coefficients and mass transfer coefficients of the target solutes from a sample of gasoline, a crude oil derivative that contains no asphaltenes and resins, and from an oil synthesized by adding asphaltenes or a mixture of asphaltenes and resins extracted from a crude oil, to the same gasoline sample. Characteristics of Crude-Oil Water Interfacial Films. Crude oil-water interfacial films have been reported to form under both aerobic and anaerobic conditions (19, 20), and the general consensus is that the interfacial films are formed by the partitioning of surface-active fractions of crude oil termed asphaltenes and resins to the oil-water interface. Asphaltenes are functionally defined as the insoluble matter that precipitates from a solution of oil in n-pentane, and resins are the material retained on adsorbent clay from the n-pentane eluent. Chemical structures of asphaltenes and resins have not been precisely determined, but it is generally accepted that asphaltene molecules are polymers comprised of a flat sheet of condensed aromatic moieties containing occasional “holes” filled with metals such as vanadium and nickel coordinated with heterocyclic atoms. Alkyl chains are present at the edges of these flat sheets. Up to eight flat sheets are often interconnected by sulfide, ether, aliphatic chains, or naphthenic ring linkages forming asphaltene aggregates (21). Resins are very similar to surfactant molecules with a hydrophilic end comprised of polar functional groups such as hydroxyls, acids, and esters, and a hydrophilic tail comprised of alkyl chains (22). In nonpolar oil environments, resin molecules associate with the exposed polar holes in asphaltene molecules through electron donor-acceptor complexes or through hydrogen bonding and facilitate solvation of asphaltenes into aggregates or other colloidal structures 20-100 Å in diameter (23, 24). Asphaltene aggregates accumulate at the oil-water interface over time and rearrange to form a “network structure” that gets stronger with time as more and more of these compounds pack in causing continuous rearranging of the network structure (25). The asphaltene aggregates are held at the oil-water interface by hydrogen-bonding interactions between the polar holes of asphaltenes and water and the interfacial tension forces (24). The interaction of water with asphaltenes is essential to the formation of interfacial films. Using 1H NMR and FTIR analysis, weak bonds between coal tar components and water in aged coal tar-water interfaces have been observed by Nelson et al. (3). Somewhat different mechanisms for interfacial film formation at the interface of a toluene-asphaltene mixture and water have been suggested in other studies. For example, Andersen et al. (26) suggested that interfacial films between a tolueneasphaltene mixture and water were a result of the formation of a microemulsion zone at the interface created by the flux of water into the organic phase. Horvath-Szabo et al. (27) have suggested that naphthenates and other polar molecules may form liquid crystal phases at oil-water interfaces and in conjunction with asphaltene aggregates contribute to the formation of interfacial films. The formation of interfacial films has been shown to be dependent on aqueous-phase pH. In crude oil-water
TABLE 1. Mass Fractions of BTEX and Naphthalene in Brent Blend Crude Oil and Gasoline
NAPLs gasoline Brent Blend crude oil
ethylbenzene toluene benzene xylenes naphthalene (wt %) (wt %) (wt %) (wt %) (wt %) 2.11 0.48
8.65 0.98
2.02 0.22
6.54 0.85
0.34 0.11
systems, interfacial films have been observed in systems with aqueous-phase pH up to 8 but not at higher pH (20). Similarly, in coal tar-water systems, interfacial films were observed to form in acid and neutral solutions but not at a pH of 12 (4). Crude oil-water interfacial films are formed in the temperature range relevant to groundwater and surface waters. At temperatures above 45 °C, interfacial films become more liquid-like and are less rigid (16).
Materials and Methods Crude Oil and Gasoline Samples. Samples of two crude oils, Menemota Venezuela and Brent Blend, were supplied by the Petro Canada Refinery in Montreal, Canada. Unleaded gasoline (87 octane) used in the experiments was purchased from an Esso (Imperial Oil Inc.) gas station in Montreal. The crude oils and gasoline were stored in glass bottles capped with lids lined on the inside with aluminum foil. The concentrations of BTEX compounds and naphthalene in the Brent Blend crude oil and the gasoline sample are presented in Table 1. Naphthalene measurement was made on a Hewlett-Packard 6890 gas chromatograph (GC) coupled to a Hewlett-Packard 5973 quadrupole mass spectrometer in the single-ion monitoring mode according to U.S. EPA Method 8270. Stock solutions of the pure petroleum fractions were prepared by diluting approximately 0.1 g of the crude oil in 10 mL of dichloromethane. Subsequent dilutions were then prepared in hexane. A total of 2 µL of the diluted solutions was injected in the splitless mode into a MDN-5S capillary column that was initially maintained at 55 °C for 3 min and then raised to 280 °C at a rate of 4 °C/min and held at that temperature for 25 min. Helium was used as the carrier gas. The injector and the detector were maintained at 275 and 285 °C, respectively. Analyses for BTEX compounds in gasoline were made on a Perkin-Elmer Sigma 2000 gas chromatograph, “DBpetro100” capillary column. An initial temperature of 50 °C was maintained for 3 min, raised to 100 °C at a rate of 2 °C/min, and then raised to 310 °C at a rate of 4 °C/min. Helium was used as the carrier gas. The injector and the detector were maintained at 325 °C. BTEX analysis for Brent Blend crude oil was performed by Bodycote Materials Testing Canada Inc. (Montreal, Canada) using GC headspace analysis. The BTEX and naphthalene concentrations in the gasoline and the Brent Blend crude oil reported in Table 1 are similar to those reported in other studies (28-30). Preparation of Gasoline Amended with Asphaltenes and Resins. Solutions of (i) gasoline with asphaltenes and (ii) gasoline with asphaltenes and resins were prepared for use in the equilibrium partitioning, dissolution, and pendant drop experiments. Asphaltenes and resins that were added to gasoline were extracted from the Menemota Venezuela crude oil to determine if, in the presence of these fractions, films form at the oil-water interface and if the films had any effect on the rates of dissolution of solutes. Asphaltenes and resins were extracted according to ASTM Method D2007. Briefly, 10 g of a Menemota Venezuela crude oil sample was dissolved in 100 mL of n-pentane. The asphaltenes precipitated out of the solution and were collected on 11-µm rapid filter paper. The filter paper was allowed to dry to a constant weight, and VOL. 38, NO. 7, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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the asphaltene fraction collected on it was stored in a glass bottle with an aluminum foil-lined cap for future use. The filtrate was diluted with 25 mL of n-pentane and was passed though a chromatographic tube containing 100 g of 250500-µm particle size Attapulgus clay (Forcoven Products Inc., Humble, TX). The resins were retained on the adsorbent clay subsequently washed out of the column using a 1:1 by volume mixture of toluene-acetone into a separatory funnel. Anhydrous calcium chloride granules were added to the funnel. The funnel was shaken for 30 s and allowed to settle for at least 10 min. The contents were filtered, and the toluene and acetone were evaporated off from the eluent under a fume hood, leaving behind the resins. Several studies have used the approach of extracting asphaltenes and resins from crude oils and adding them to various NAPLs to evaluate the effects of those fractions on changes to the rheological behavior of the NAPLs or to study the solvency characteristics of asphaltenes (25, 31-34). Menemota Venezuela crude asphaltenes could be dissolved in the gasoline at up to 1.2 wt %, and beyond this concentration, precipitation of the asphaltenes was observed. A gasoline-asphaltene solution containing 1.2% asphaltenes by weight and a gasoline-asphaltene-resin solution containing 2.3% of asphaltenes and resins in a 1:1 mass ratio were prepared. The 1:1 ratio of asphaltenes and resins was chosen based on results of Mohammed et al. (25), who reported that this ratio yielded the greatest interfacial rigidity, thus maximizing interfacial film stability. Adding Menemota Venezuela asphaltenes to gasoline at a concentration approximately equivalent to the point of precipitation resulted in an oil where the asphaltenes were flocculated, poorly solvated, but most surface active, and thus likely to form the most rigid films that would maximize mass transfer reduction of solutes (34). It is for this reason that Menemota Venezuela crude oil asphaltenes and resins rather than those extracted from the Brent Crude, which were significantly more soluble in gasoline, were employed for amending gasoline. Asphaltenes extracted from Brent Blend crude oil could be solubilized in gasoline at concentrations even greater than 10.4 wt %, and thus achieving a concentration equivalent to the point of precipitation was relatively difficult. Observations of Oil-Water Interfacial Films. The presence of interfacial films was identified by visual observation of the surface of a pendant drop of oil in water. The pendant drop method has been used to identify physical changes to crude oil-water interfaces (1, 20) and to coal tar-water interfaces (2). In this method, the oil is expelled from a syringe immersed in water to form a stable, spherical drop at the syringe tip. Because the oil samples used were lighter than water, the syringe needle was bent in a U-shape, and the oil drop emerged vertically from the syringe immersed in water. A freshly formed drop of the oil can be withdrawn into the syringe without loss of shape or without any observable textural change at the oil-water interface. However, when interfacial films are formed after prolonged contact with water (interfacial aging), they can be visible if the oil drops are slowly retracted into the syringe and re-expelled into water. The formation of interfacial films is indicated by textural changes in the drop surface and loss of spherical shape when retracted into the syringe and re-expelled into water. Determination of Solute Concentrations Using Radiolabeled Tracer Techniques. Radiolabeled tracer techniques were employed for determining the benzene and naphthalene concentrations in the aqueous phase and oil samples in equilibrium partitioning and mass transfer experiments. Stock solutions of 1-14C-labeled benzene and naphthalene (Sigma-Aldrich Chemical Co.) were prepared in methanol. Prior to transfer to the experimental systems, desired amounts (up to 10 µL) of stock solutions were added to 1 mL of the 2104
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FIGURE 1. Reactor employed in mass transfer experiments. oil sample to be used. It was assumed that the behavior of the 14C-labeled compound is representative of the unlabeled compound naturally present in the oil samples, and the small amount of methanol in the oil did not affect asphaltene solubility in the oils. The 14C activity in oil and aqueous samples was measured using a Beckman LS6500 (Beckman Coulter Canada Inc.) liquid scintillation counter (LSC) with automatic quench compensation capability. 14C activity in the aqueous phase was measured by dissolving 1 mL of aqueous sample into 5 mL of Beckman Ready Organic scintillation cocktail (Beckman Coulter Canada Inc.). Due to the opacity and color of the crude oils and gasolineasphaltene/resin solutions, 14C activity in these NAPL samples was measured by diluting the sample in a known volume of toluene and introducing a 1-mL volume of the diluted solution in 15 mL of the scintillation cocktail. Samples were stored overnight to reduce errors due to chemiluminescence and electrostatic charge. The activity of 14C was recorded as disintegrations per minute (dpm). The concentration of the target compounds in aqueous samples was calculated from the radioactivity of a unit volume of the sample, the initial mass of the target unlabeled compound in the NAPL that was introduced into the experimental system, and the radioactivity added to the NAPL. The radioactivity of benzene and naphthalene could not be measured reliably in the Menemota Venezuela crude oils using liquid scintillation counting due to its high viscosity and dark color, and thus this crude oil was not employed in the equilibrium partitioning and dissolution kinetics experiments. Equilibrium Partitioning Experiments. Experiments for determining the equilibrium partitioning of naphthalene and benzene between oil samples and water were conducted in 40-mL glass vials sealed with caps and Teflon-lined septa. One mL of oil was added as a thin layer above 25 mL of deionized water. Equilibration was achieved by allowing the systems to mix on a gyratory shaker at a rate of 50 rpm over a period of 6 d. The oil and water phases remained nondispersed during the mixing process, and each phase was sampled and analyzed in the LSC. Mass Transfer Experiments. Mass transfer experiments were conducted in flow-through reactors where the aqueous phase was gently stirred to maintain the segregation of the oil and aqueous phases. The aqueous concentration of naphthalene and benzene in the reactor effluent was determined in samples collected at different time points and was used to compute the oil-water mass transfer coefficients. A schematic of the reactor that was employed for these experiments is shown in Figure 1. Threaded glass connectors were fused to the flask to create inlet and outlet arms. Stainless steel tubing was used as the inlet and outlet for water, and the tubing was passed through adapters that screwed onto the glass connectors with PTFE O-rings to form a leak-proof seal. The positions of the inlet and outlet tubes were vertically staggered inside the reactor to facilitate mixing and to minimize the possibility of hydraulic short-circuiting. Each
reactor was filled with approximately 45 mL of water. The amount of headspace in the reactors was minimal. One mL of NAPL sample was carefully placed as a thin layer on the surface of the water. Care was taken to avoid smearing of the oil along the sides of the vial or to avoid formation of oil droplets. The oil-water interfacial area was approximately the cross-sectional area of the neck of the reactor, which was 2.4 cm2. With aging there was a slight increase in the curvature of the meniscus, but the increase in the interfacial area as computed from the shape of the meniscus was negligible. During the mass transfer experiments, water was pumped by a HPLC pump (Waters model 501) at a rate of 0.6 mL/min. The flow of water across the oil-water interface in the reactor created a driving force for continuous dissolution of potentially water-soluble compounds contained in the oil. The aqueous phase was gently stirred with a PTFE-coated stir bar to ensure a completely mixed aqueous phase, and completely mixed conditions were confirmed by dye washout tests. Adequate internal mixing of the thin oil layer was imparted by the mixing of the aqueous phase, and this was verified by visual observation of the distribution of oil-red-o (Fisher Scientific Co.) dye added to a 1-mL gasoline layer in the reactor. The reactor was placed in a water bath and maintained at a temperature of 25 ( 2 °C. The aqueousphase solute concentration was monitored by collecting 1-mL samples from the outlet of the reactor at various time points. The eluent from the outlet was collected in a capped flask and analyzed in the LSC. The concentration of solutes remaining in the oil phase at the end of the flushing period was calculated from the knowledge the total mass of the target solutes contained in the effluent. Upon first contact (day 0) of the oil and water, the water was flushed through the reactor for about 70 min to estimate the mass transfer coefficient at day 0, during which time several 1-mL aqueous samples from the outlet of the reactor were collected and analyzed for aqueous concentrations of the target compound in the LSC. The flow was then stopped, the oil-water interface was aged for several days, and mass transfer tests were repeated. After a predetermined aging time, the flushing of the aqueous phase was resumed for 70 min, and eluent samples were collected to determine the mass transfer coefficient at that aging time. Volatilization losses of the target solutes from the reactor during the 70-min flushing period were found to be negligible based on 14C mass balance assessments. Thus the effluent solute concentration changes during any flushing period were free from volatilization loss effects. Losses of naphthalene during the entire 35-d aging period were less than 5%. However, in some reactors, particularly those containing gasoline or amended gasoline samples, losses of benzene and other similarly volatile compounds occurred during the aging periods between flushing events, even though efforts were made to completely seal the reactor. The losses of these volatile fractions caused changes to oil-phase solute concentrations and oil volumes and, if ignored, could result in erroneous estimates of mass transfer coefficients in aged systems. The effects of volatilization losses on the oil-phase concentrations of benzene and naphthalene were accounted for by determining their concentrations in the oil before each flushing period by analyzing three, 2-µL, replicate samples of the oil phase. The fractional decrease in the oil volume due to volatilization was determined from the ratio of the oil-phase naphthalene concentrations at the end of the aging period and at the beginning of the aging period. Estimation of Mass Transfer Coefficients. The change in solute concentration with time in the bulk aqueous phase in the mass transfer test reactors is given by the difference of the rate of change of solute concentration caused by the dissolution flux of the solute from oil and by the
advective flux through the reactor and thus:
(
)
i dCiw Co,t A q ) Kit i - Ciw,t - Ciw,t dt Vw P Vw o/w
(1)
The first term on the right-hand side of the equation is a function of the concentration difference or the driving force term that incorporates the time-dependent concentrations of solute i in the bulk oil phase, Cio,t (M/L3), and in water, Ciw,t (M/L3). As a result of continuous dissolution of the target solute from the oil phase, Cio,t may change significantly over time. Pio/w (dimensionless) is the equilibrium partition coefficient for the solute i between the oil and water and is defined as the ratio of the equilibrium molar concentration of the solute in the oil and water. The magnitude of the partition coefficient, Pio/w, was obtained from equilibrium partitioning tests. Pio/w could change over time if a significant change in the composition of the oil occurred as a result of continuous dissolution (weathering). In the dissolution experiments reported in this study, changes in the magnitude of Pio/w did not occur with aging or with repeated flushing, and this is discussed in a later section. Kit (L/T) is the overall mass transfer coefficient, which is a measure of the resistance to mass transport across the oil-water interface. For the reactor system described above, the oil-water interfacial area (A; L2), aqueous volume (Vw; L3), and flow rate (q; L3/T) were known. The overall mass transfer coefficient, Kit, at any day was obtained by fitting, using least squares regression, the integrated form of eq 1 to the measured aqueous concentrations of i over a 70-min dissolution period. An optimization computer program subroutine BCLSF, available from the IMSL Math Library, was employed to solve the least squares problem using a modified Levenberg-Marquardt algorithm (35). The objective function, which was minimized, was given by n
Fobj )
∑ k)1
(
)
(Ciw,t)k - (Ciw,t)k′ (Ciw,t)k
2
(2)
where (Ciw,t)k is the kth effluent concentration data point, and (Ciw,t)k′ is the corresponding model prediction. Equation 1 was integrated using Euler’s method to solve for Ciw,t with the knowledge of the initial aqueous concentration and oilphase concentration of i. Cio,t was determined by computing the mass difference between the initial mass of the target compound in the oil and the mass of the solute that was lost from the oil phase into the aqueous phase in the reactor, given by the integral of the first term on the right-hand side of eq 1. It was assumed that during each flushing period the change in oil volume was negligible. The goodness of fit for each set of aqueous concentration data was evaluated by the value of the relative residual sum of squares (RRSS), which is the magnitude of the objective function at the optimal value of Kit. Additional details on the regression and computing methods are provided in Ghoshal and Luthy (36). The accuracy of the parameter estimation program was evaluated by verifying the mass transfer coefficients estimated from synthetic data sets of aqueous solute concentrations in the eluent that were generated using assigned sets of initial conditions and mass transfer coefficients. The difference between the assumed mass transfer coefficient used for generating the synthetic data set and the predicted mass transfer coefficient was less than (7% in all cases, and this was significantly smaller than the changes in mass transfer coefficients determined at different aging times. VOL. 38, NO. 7, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Partition Coefficient (Pio/w) for Benzene and Naphthalene in Different Oil Samples
oil sample Brent Blend crude oil gasoline gasoline with 1.2 wt % asphaltenesb gasoline with 2.3 wt % asphaltenes & resinsb
FIGURE 2. Photographs of 14-day aged oil pendant drops showing wrinkled oil-water interfacial films. (A) Gasoline amended with asphaltenes and resins at 2.3 wt % (1:1 ratio). A transparent interfacial film is visible when the oil is retracted into the syringe. (B) Brent Blend crude oil drop showing wrinkling of the interface due to the presence of a rigid film. The outer diameter of the syringe needle is 2.1 mm in both photographs. Sorption of Naphthalene on Extracted Asphaltenes. Asphaltenes extracted from Menemota Venezuela crude oil were air-dried to a granular form and employed in experiments to determine its sorptive capacity for naphthalene in aqueous solutions. The asphaltene-water partition coefficient for naphthalene was used to compute the extent of partitioning of naphthalene in the interfacial film phase, as described later. Naphthalene was added to suspensions of asphaltenes (0.5-2 wt %) in water contained in 40-mL vials with minimal headspace and capped with Teflon-lined septa. The vials were mixed for 7 d on end-over-end rotators for equilibration, after which the aqueous phase was filtered using 0.2-µm Teflon filters and analyzed for naphthalene using a HPLC. The asphaltenes were extracted with methanol and analyzed for naphthalene. The equilibrium aqueous concentration of naphthalene ranged from 0.3 to 5 mg/L, and a linear fit to the equilibrium naphthalene concentration in the aqueous and asphaltene phases was obtained.
Results and Discussion Observations of Aged Oil-Water Interfaces of Pendant Drops. Noticeable changes occurred to the Brent Blend crude oil-water interface on retracting the 2-week aged pendant oil drop into the syringe. The interface of the pendant drop exhibited significant wrinkling and crumpling as shown in Figure 2A, suggesting the presence of a semi-solid interfacial film or skin. These changes were observed even when the pendant drop was immersed in water under quiescent conditions for as little as 1 d. The crumpling of the surface indicates resistance to surface deformations and is indicative of a large surface viscosity. Similar characteristics of aged crude oil as well as coal tar pendant drops in water have been reported in other studies (1, 2, 20). The oil phase beneath the semi-solid interface is liquid even after several weeks of aging, and liquid oil is released when the interface is ruptured. Pendant drops of gasoline amended with asphaltenes or a mixture of asphaltenes and resins, when aged in water, resulted in changes to the interface similar to the crude oil. Moreover, pendant drops of amended gasoline samples when retracted after aging for 14 d revealed a transparent membranous film encasing a residual oil phase as shown in Figure 2B. This membranous film is the interfacial film or skin comprised of the asphaltene and resin material added to gasoline. Gasoline to which asphaltenes and resins were not added did not produce any wrinkling or crumpling of the interface in the pendant drop, nor did it produce membranous films or other changes indicative of surface viscosity even when aged for extended time periods. 2106
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Po/w [(mol/Loil)/ (mol/Lwater)]
SD in Po/w
ta
Benzene 155.6 219.1 222.9
17.4 25.15 8.56
nac 0 0.79
235.5
25.15
1.11
Naphthalene Brent Blend crude oil 1946 gasoline 2445 gasoline with 1.2 wt % 2375 asphaltenesb gasoline with 2.3 wt % 2439 asphaltenes & resinsb
383.8 103.3 652.6
na 0 -0.24
352.3
-0.028
a t values for test of significance. At 0.05 level of significance, a t value larger in magnitude than 2.89 indicates a significant difference from the gasoline partitioning results. b Asphaltene/resin source is Menemota Venezuela Crude oil. c na, not applicable.
Equilibrium Partition Coefficients. The equilibrium partition coefficients, Pio/w, for benzene and naphthalene presented in Table 2were calculated from measurements of the solute concentrations in the oil and aqueous phases in equilibrated batch systems. The extent of partitioning of naphthalene to water from all oil samples was much lower than for benzene because naphthalene is more hydrophobic than benzene (octanol-water partition coefficient for naphthalene is 103.35 and for benzene is 102.13) (37). The coefficients determined in this study are comparable to those reported by Cline et al. (28), and the minor differences in magnitudes are attributable to the variability in composition of the gasoline samples tested. The t test was performed at a 0.05 level of significance to determine if the addition of asphaltenes and resins to gasoline produced a significant change in the partitioning of benzene and naphthalene between the different NAPLs and water. The gasoline amended with Menemota Venezuela crude asphaltenes or asphaltenes and resins in the 1.2-2.3 wt % concentration range did not show a significant difference in the partition coefficient of benzene or naphthalene. 14C mass balances in the equilibrium partitioning batch systems were checked by measuring the radioactivity in the NAPL and aqueous phases at the end of the test and comparing the sum of those to the radioactivity initially added. The losses of target compounds were negligible in all cases. To investigate if there were significant changes to the equilibrium partition coefficients with increasing aging time and with repeated flushing of the gasoline and crude oil samples in the reactors employed for the mass transfer experiments, the equilibrium partitioning of the solutes were determined using oil samples that were subjected to flushing and aging. At day 0, contact between crude oil or gasoline and water was commenced, and quiescent conditions were maintained for the most part to encourage rapid film formation. Three days was the minimum time required to achieve equilibrium in those reactors under intermittently mixed conditions. Thus at day 3, the oil and water phases were sampled and analyzed for naphthalene concentrations to determine the oil-water equilibrium partition coefficient. The aqueous phase was then flushed for a period of 2 h to cause changes in composition of the oil phase as a result of dissolution of the water-soluble components. Following this, the flushing was stopped, and the oil-water system was
TABLE 3. Partition Coefficient of Naphthalene (Pnaph o/w ) at Different Aging Periods and after Flushing Events Pnaph o/w
reactor condition no flushing, equilibrated after first flush and requilibration after second flush and requilibration after third flush and requilibration
aging period (d)
gasolinewater systems
crude oil-water systems
3 6
2449 2456
1945 1952
9
2442
1945
12
2444
1941
allowed to equilibrate for another 3 d to assess equilibrium partitioning at day 6, and this cycle was repeated until day 12. Table 3 presents the equilibrium partition coefficients at days 3, 6, 9 and 12. After three flushing events and aging periods, there was no significant change in the equilibrium partition coefficient of naphthalene. It is assumed that the partition coefficient of benzene would also be unchanged for similar aging periods and number of flushing events. Changes in Mass Transfer Coefficients with Aging. The oil-water mass transfer tests were performed at predetermined aging periods. Aqueous concentrations of the target solute, benzene or naphthalene, were measured in the effluent samples at frequent time intervals during the 70-min flushing period. Figure 3 shows effluent concentrations during flushing of the Brent Blend crude oil at three different aging times from a typical mass transfer test. The solid lines represent the best fit to the data by the integrated form of eq 1. An increasing trend in aqueous naphthalene concentrations is observed during the flushing period on day 0 as naphthalene dissolves into the aqueous phase after contact of the water with the crude oil. The aqueous naphthalene concentrations decrease with time during the 70-min flushing period on days 4 and 12. This is because during the first and second aging periods between day 0 and day 4 and between day 4 and day 12, respectively, when no flushing of the aqueous phase occurred, equilibrium partitioning of naphthalene was approached. Thus initial aqueous naphthalene concentrations were near equilibrium. The magnitudes of the mass transfer coefficients of naph benzene, Kben , at different t , and that of naphthalene, Kt oil-water aging times are shown in Figures 4 and 5. For the unaged oil samples, Kben is in the range of 0.038-0.065 cm/ t min, and Knaph is in the range of 0.042-0.073 cm/min. These t values are comparable to reported mass transfer coefficients for benzene and naphthalene in various NAPL-water systems (9-12). The minor variations in Kben and Knaph from one t t system to another are attributable in part to the different composition and physical properties of crude oil, gasoline, and asphaltene- and resin-amended gasoline. A decrease in Kben and Knaph with time was consistently t t observed from oils that form interfacial films and contain asphaltenes and or resins (i.e., the Brent Blend crude oil, gasoline amended with 1.2% asphaltenes, and gasoline amended with 2.3% asphaltenes and resins). The decreases in the mass transfer coefficients are more rapid in the initial time period of 0-10 d. Figure 4B shows an absence of a decrease in Kben and Knaph for gasoline, which does not t t contain significant asphaltenes and resins. The pattern of decrease in Kben and Knaph with time for the Brent Blend t t crude oil (Figure 4A), which contains naturally occurring
FIGURE 3. Naphthalene concentrations in the eluent from the mass transfer reactors for the 70-min flushing period upon initial contact (day 0) and after two aging periods (days 4 and 12). The solid line is fitted to the data using the integrated form of eq 1, and the goodness of each fit is given by the relative residual sum of squares (RRSS). asphaltenes at 1 wt % and resins at 4 wt % (38), is similar to the decreases in mass transfer coefficients with time observed for gasoline amended with asphaltenes or asphaltenes and resins (Figure 5A,B). Due to the presence of asphaltenes and resins in the oil phase, the interfacial films starts to form on contact with water and continues to increase in viscosity and rigidity with time (16, 25). The mass transfer coefficients decrease with time, likely because of the increase in diffusional resistance to solute mass transfer through an increasingly denser and more viscous interfacial film. After a period of 10 d, the rate of change in the mass transfer coefficient for benzene and naphthalene seemed to decrease. This is likely because a critical concentration of asphaltenes and resins in the interface is reached, and as a result the resistance posed by the interfacial film begins to stabilize and approach a maximum. It is interesting to note that the time scales over which changes in rhelogical properties of crude oil-water interfaces have been observed is much shorter than the time periods over which change in mass transfer coefficient are observed to occur. Mohammed et al. (16) observed the interfacial shear viscosity in crude oil-water systems to increase 4-100-fold in a period of 8 h. The interfacial tension in crude oil-brine systems was observed to change rapidly over 4 h and thereafter level off (39). The presence of resins in addition to asphaltenes in the gasoline had no significant effect on the rates and extent of and Knaph , suggesting that the resins did not decrease of Kben t t VOL. 38, NO. 7, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Mass transfer coefficients (Kit) at different aging periods for (A) Brent Blend crude oil that naturally contains asphaltenes and resins and (B) gasoline that does not contain asphaltenes and resins. Error bars denote 95% confidence intervals. Some error bars are smaller than the symbol size.
FIGURE 5. Mass transfer coefficients (Kit) at different aging periods for (A) gasoline amended with asphaltenes at 1.2 wt % and (B) gasoline amended with asphaltenes and resins (1:1) at 2.3 wt %. Error bars denote 95% confidence intervals. Some error bars are smaller than the symbol size.
significantly alter the structure of the interfacial film in a way that changed its diffusional resistance or path length to the target solutes. Resins tend to diminish the surface active nature of asphaltenes by solubilizing them to a greater extent in oil. Thus, the mass of resins relative to asphaltenes influences the solubility of asphaltenes in crude oil, their tendency to aggregate, and their tendency to adsorb at oilwater interfaces (32, 34). However, resin-solvated asphaltenic colloidal aggregates may remain surface-active and partitioned at the oil-water interface if the solvating resins are present on one side of the colloid to form a partially solvated aggregate (34). Mohammed et al. (25) found that the rigidity of an model oil (xylene and heptane)-water interfacial film composed of Buchanan crude oil asphaltenes and resins increased with asphaltene-resin mass ratio, with a maximum rigidity at observed at the 1:1 mass ratio. Further increases in the mass ratio however resulted in a steady decrease in rigidity. Additional studies are required to ascertain how the mass ratio of asphaltenes and resins influence solute mass transfer coefficients. Mass Transfer Resistance of Interfacial Film. The mass flux of solutes from one bulk phase to another is described by models such as the stagnant two-film model and the surface renewal model (37). These models state that the flux of a solute, Ji (M L-2 T-1), from an oil phase to the aqueous phase is
, and this equation has been incorporated in eq 1. The twofilm model is mathematically the simpler of the two and states that interphase mass transfer involves steady-state diffusion of the solute through two stagnant films on either side of the interface. At the interface, equilibrium between the interfacial concentrations is instantaneously achieved, and thus the interface itself, does not provide a resistance to mass transfer (Kit)-1; the overall mass transfer resistance is related to the individual film resistances as follows:
Ji ) Kit 2108
9
(
i Co,t
Pio/w
- Ciw,t
)
(3)
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 7, 2004
1 1 1 ) + Kit Pio/wkio kiw
(4)
where kio (L/T) and kiw (L/T) are the film mass transfer coefficients for the oil-side film and water-side film, respectively, and are directly proportional to the diffusion coefficient of the solute. Generally, for NAPL-water mass transfer of hydrophobic organic compounds (Pio/w > 200), the ratelimiting step is considered to be the aqueous-phase boundary layer at the interface (10). The oil-side resistance is dominant for hydrophobic solutes only for NAPLs of very high viscosity (∼1000 cP) and for organic semi-solids such as petroleum jelly and paraffin (11). The viscosity of the Brent Blend crude oil and gasoline is approximately 6 and 1 cP, respectively. Thus, the oil-side boundary layer resistance may be considered negligible. When a viscous, semi-rigid interfacial film is present between the oil and water phases, an additional resistance to mass transport of solutes by the film may be incorporated
FIGURE 6. Three-film mass transfer model for an oil-water interface separated by an interfacial film. in eq 4. The overall mass transfer rate resistance may be expressed as
1 1 1 1 ) i + i i+ i i i Kt Po/wko Ps/wks kw
(5)
i where Ps/w is the partition coefficient of a solute between interfacial film (skin) and water, oil and skin, and oil and water, respectively. The term (Pis/wkis)-1 represents the resistance caused by the interfacial film. Figure 6 shows an idealized interface of oil and water separated by an interfacial film. Three-film models have been used to describe the mass transfer of solutes across the air-water interface when thin oil films exist between the two phases (37). A significant change in the overall mass transfer coefficient for benzene and naphthalene would occur when the magnitude of the interfacial film resistance is large in comparison to the oil-side and water-side boundary layer resistances. With reference to eq 5, this translates to a decreasing value of kis with time. It is assumed that the boundary layer thicknesses in the aqueous phase and in the NAPL do not change with time (i.e., the mixing conditions and turbulence in each bulk phase, and oil-water density and viscosity are invariant with time). The magnitude of Pis/w is difficult to determine directly, as the interfacial film material cannot be isolated from the oil-water system. The interfacial film material spread on the air-water interface during attempts to physically isolate it. Asphaltenes could however be extracted from the crude naph oil, as described earlier. Thus Ps/w was estimated from the asphaltene-water equilibrium partition coefficient of naphthalene, Knaph (L3/M), determined from isotherm experid ments employing a suspension of solid, extracted asphaltenes in water and a reported value of the density of solubilized asphaltenes in an interfacial film, Fasph (M/L3), as
naph Pnaph Fasph s/w ) Kd
(6)
The above equation assumes that asphaltenes in the interfacial film have the same affinity for naphthalene as solid, extracted asphaltenes suspended in water. The Knaph d in the two systems may be considered to be similar in magnitude given that the interfacial film phase is largely comprised of asphaltene micelles bonded to water. It was found that the extracted asphaltenes sorbed naphthalene strongly (Kd ) 3.9 L/g) from aqueous solutions. The interfacial film-phase density of asphaltenes extracted from Athbasca bitumen and dissolved in toluene was found to be 1162 Kg/ m3 (40). Using these values, the magnitude of Pnaph is s/w estimated to be 4530. It should be noted however that, because the mass of asphaltenes at the interface is very small,
there is no significant accumulation of naphthalene in the interfacial film phase. The water-side film mass transfer coefficients, kiw, for each oil-water system employed in this study was estimated to be equal to Kit for the corresponding solute in unaged systems (i.e., equal to Kit at day 0) because the magnitude of the oil-side boundary layer resistance, (Pio/w kio)-1, in eq 5 is considered to be negligible. The resistance due to the interfacial film is absent in unaged samples. The values of knaph in amended gasoline samples and the Brent Blend w crude oil were in the range of 0.042-0.062 cm/min. These values are similar to reported values of knaph that are in the w range of 0.035-0.04 cm/min for gasoline-water systems (10) and in the range of 0.008-0.038 cm/min for naphthalene dissolution from various petroleum oils (11). naph With the values of Ps/w and knaph estimated above, the w naphthalene film transfer coefficient for the interfacial film, knaph , is estimated to be in the range of 2.24 × 10-6-7.24 × s 10-6 cm/min. This small value of the film transfer coefficient suggests that the interfacial film provides a very significant resistance to solute mass transfer from the oil phase to the water. Thus, interfacial films may significantly contribute to the persistence and slow release of organic compounds from NAPLs such as crude oils, heavy oils, coal tars, and creosotes into surface waters as well as groundwaters. If solid films such as those formed in coal tars aged for 1 yr (3) are present in NAPL-water systems, it is likely that mass transfer limitations will be more significant than those observed in this study. Further study is needed to determine if the patterns of reduction in mass transfer of solutes from crude oils reported here are consistent when the oils are spread onto porous medium. NAPLs containing asphaltenes may become more wetting with time (41), and the reduction in mass transfer due to diffusional resistance caused by the interfacial films may be compensated to some extent by increases in NAPLwater interfacial area. In this study, the increases in NAPLwater interfacial areas were negligible and did not influence solute mass transfer rates.
Acknowledgments This study was funded by a Petro Canada Young Innovator Award and by a grant from the NSERC to S.G. C.P. was supported by a post-graduate scholarship from NSERC, and M.A. was supported by a post-graduate scholarship from FQRNT. T. El Ramahi assisted with some of the equilibrium partitioning experiments. The assistance of J. Hawari, NRC Biotechnology Research Institute, Montreal, in analyzing BTEX and naphthalene in some of the crude oil and gasoline samples is gratefully acknowledged.
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Received for review July 25, 2003. Revised manuscript received December 23, 2003. Accepted January 6, 2004. ES034832J
