SEARCH

SEARCH BY CITATION

Keywords:

  • Crude oil microdroplet;
  • Oil–water preparation;
  • Polycyclic aromatic hydrocarbon

Abstract

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS AND MODEL DEVELOPMENT
  5. RESULTS AND DISCUSSION
  6. SUMMARY
  7. SUPPLEMENTAL DATA
  8. Acknowledgements
  9. REFERENCES
  10. Supporting Information

Dissolved constituents of crude oil, particularly polycyclic aromatic hydrocarbons (PAHs), can contribute substantially to the toxicity of aquatic organisms. Measured aqueous concentrations of high–molecular weight PAHs (e.g., chrysenes, benzo[a]pyrene) as well as long-chain aliphatic hydrocarbons can exceed the theoretical solubility of these sparingly soluble compounds. This is attributed to the presence of a “microdroplet” or colloidal oil phase. It is important to be able to quantify the dissolved fraction of these compounds in oil-in-water preparations that are commonly used in toxicity assays because the interpretation of test results often assumes that the compounds are dissolved. A method is presented to determine the microdroplet contribution in crude oil-in-water preparations using a comparison of predicted and measured aqueous concentrations. Measured concentrations are reproduced in the model by including both microdroplets and dissolved constituents of petroleum hydrocarbons. Microdroplets were found in all oil–water preparation data sets analyzed. Estimated microdroplet oil concentrations typically ranged from 10 to 700 µg oil/L water. The fraction of dissolved individual petroleum hydrocarbons ranges from 1.0 for highly soluble compounds (e.g., benzene, toluene, ethylbenzene, and xylene) to far less than 0.1 for sparingly soluble compounds (e.g., chrysenes) depending on the microdroplet oil concentration. The presence of these microdroplets complicates the interpretation of toxicity test data because they may exert an additional toxic effect due to a change in the exposure profile. The implications of the droplet model on toxicity are also discussed in terms of both dissolved hydrocarbons and microdroplets. Environ. Toxicol. Chem. 2012; 31: 1814–1822. © 2012 SETAC


INTRODUCTION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS AND MODEL DEVELOPMENT
  5. RESULTS AND DISCUSSION
  6. SUMMARY
  7. SUPPLEMENTAL DATA
  8. Acknowledgements
  9. REFERENCES
  10. Supporting Information

Crude oil is a complex mixture of compounds from several different chemical classes including aromatic and aliphatic hydrocarbons, asphaltenes, as well as oxygen-, sulfur- and nitrogen-containing heterocycles, each with distinct physicochemical properties 1 and widely variable toxic potentials 2. Polycyclic aromatic hydrocarbons (PAHs) are present in water-soluble extracts of petroleum products, and when oil toxicity is investigated, researchers mostly focus on PAHs and benzene, toluene, ethyl benzene, and xylene (BTEX) as the cause of the toxicity 3. Several studies have suggested that microdroplet oil is bioavailable 4–13, though in aquatic toxicity tests the relative contributions from dissolved and microdroplet phases are often unclear.

An oil solubility model has been developed for petroleum products (i.e., gas oils, fuel oils) 2, 14, 15. This solubility model was adapted to calculate the partitioning of refined petroleum products with well-defined physicochemical properties between air (headspace), water, and pure oil phases 15. When this model was applied to a water-accommodated fraction (WAF) prepared from crude oil under typical conditions (e.g., 1:100 oil to water loading) 6, 16, the predicted dissolved concentrations of the more abundant (e.g., >1 µg/L) BTEX and low–molecular weight (MW) PAH constituents were in close agreement with the measured aqueous concentrations (Fig. 1), which is consistent with earlier model validations 3, 14, 15. However, the predicted dissolved concentrations of the less abundant (e.g., 1 µg/L) higher MW PAHs were 10 to 1,000 times lower than the measured aqueous concentrations. This observation—that the measured concentrations of low soluble PAHs are greater than the theoretically predicted dissolved concentrations—suggests the presence of another oil/PAH phase in these mixtures. It is hypothesized that there is a stable, dispersed microdroplet oil phase present in the aqueous mixtures and that the measured concentrations represent the sum of these dispersed microdroplets and the truly dissolved concentrations of PAHs.

thumbnail image

Figure 1. Comparing predicted dissolved concentrations to measured concentrations in a water-accommodated fraction (WAF) 21 for monocyclic aromatics (▵), dicyclic aromatics (◊), and 3+ ring aromatics (○).

Download figure to PowerPoint

It is difficult to experimentally separate and quantify the concentration of crude oil microdroplets. Instead, the model is used to calculate the dissolved-phase concentrations of the individual constituents of the crude oil based on their solubility. The microdroplet concentration of the crude oil WAF is then estimated by collectively comparing the measured concentrations to the predicted dissolved concentrations. The difference is attributed to microdroplets of parent oil. The present study presents the model development and application to crude oil WAFs for estimating crude oil microdroplet concentrations and dissolved-phase concentrations. A preliminary toxicity-modeling framework is also presented for evaluating crude oil toxicity as a combination of dissolved and microdroplet oil constituents.

METHODS AND MODEL DEVELOPMENT

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS AND MODEL DEVELOPMENT
  5. RESULTS AND DISCUSSION
  6. SUMMARY
  7. SUPPLEMENTAL DATA
  8. Acknowledgements
  9. REFERENCES
  10. Supporting Information

Oil–water preparations

All of the studies analyzed in the present report (Table 1) 17–20 used Alaskan North Slope crude oil and included measured concentrations of parent and alkylated PAHs (including dibenzothiophenes) in both the oil and aqueous exposure waters. Exposure waters were prepared either with direct oil–water contact in the usual WAF preparation 19, 21 or through water contact with oiled gravel 17, 18, 20.

Table 1. Description of the studies used in the present analysis
Study IDStudy typeState of North Slope Alaskan crude oilExposure temperature (°C)Duration (d)Source
  • a

    LWO = Less weathered oil, first 21 d of water contact; MWO = More weathered oil, from day 22 to 38 of water contact with oiled gravel.

1Oiled gravelArtificially weathered, LWOa42117
2Oiled gravelArtificially weathered, MWOa41717
3Oiled gravelArtificially weathered46518
4Oiled gravelArtificially weathered128320
5Oiled gravelNaturally weathered128320
6Static WAFNeat25419
7Static WAFNaturally weathered25419
8Flow-through WAFNeat105121

Carls et al. 17 conducted a toxicity exposure where seawater was pumped through an oiled-gravel column over the course of 38 d. The study was performed in two phases, in which the period of sample collection between 5 and 21 d of pumping was referred to as the less-weathered oil (LWO) phase, and the collection period between 22 and 38 d comprised the more-weathered oil (MWO) phase. Measurements of oil-phase PAHs and saturates (e.g., alkanes) were made at the beginning of the study. Aqueous PAH measurements were made four to six times during the LWO exposure and four times during the MWO exposure. Flow to the columns was stopped for approximately 14 d between LWO and MWO treatments.

Heintz et al. 18 performed a similar, but longer-term, oiled-gravel experiment over the course of 177 d, collecting samples at days 1, 35, 36, 63, and 177. Only oil- and aqueous-phase PAH and saturates data between days 0 and 65 were used in the present analysis to be consistent with the toxicological end point (e.g., fertilization to eyeing) in the present study. Data from oiled-gravel experiments from Brannon et al. 20 were also analyzed. This study used artificially and naturally weathered oils in two separate experiments and collected oil-phase and aqueous-phase PAH measurements at days 0, 11, 23, 40, and 83.

The ENSR study 19 performed a 4-d acute toxicity test using WAFs prepared with neat and weathered Alaskan North Slope crude oil from the Exxon Valdez. Concentrations of oil- and aqueous-phase monoaromatic hydrocarbons (MAHs), PAHs, and saturated hydrocarbons (SHCs) were measured. A long-term flow-through experiment was conducted, in which neat Alaskan North Slope crude oil WAFs were prepared for use in the analysis presented below. The concentration data used in this study are presented in the Supplemental Data, Table S1. Table 1 summarizes the preparation methods employed by the various studies used in the present analysis.

Temperature-controlled oil–water preparations were performed 21 to investigate the effect of temperature on the solubility of MAHs and PAHs, as well as the formation of oil microdroplets. Briefly, a measured volume of dilution water (3,465 ml) was stirred with a measured volume of oil (35 ml) at a ratio of 1:100 after the oil and dilution water had reached designated test temperatures (i.e., 5, 15, and 25°C). The solutions were mixed for 30 h on a stir plate within a temperature-controlled environmental chamber. Efforts were made to control mixing to minimize vortex formation. Water and oil layers were allowed to settle for 2 h. Selected concentration data from this study are presented in the Supplemental Data, Table S1.

Solubility model

A three-phase model is used to calculate the partitioning of the petroleum hydrocarbons between the air, water, and oil phases. This model has been validated elsewhere for use with refined petroleum products such as gasoline 14, kerosene, gas oil, heavy fuel oil 15, and crude oil 2. The model is based on Raoult's law 22 to describe the solubility of complex mixtures and Henry's law to describe the partitioning of hydrocarbons to the headspace of the chambers where the WAFs are prepared. McGrath et al. 14 and Di Toro et al. 2 present the details of the model formulation and terminology.

A critical component of the model is the subcooled liquid solubility (Si) for MAHs, PAHs, and SHCs. The subcooled liquid solubility refers to the aqueous solubility of a chemical in its liquid form, accounting for the cosolubility effect that occurs in hydrocarbon mixtures 2. Values were calculated with the physicochemical properties calculator SPARC 23 as compiled in the PETROTOX model 15 and compared to experimental values that were available for only a few PAHs and MAHs. Therefore, the SPARC-derived values were used for all compounds to maintain internal consistency among the model parameters. Several of the studies in the present analysis used seawater, and the subcooled liquid solubilities were adjusted for the salting-out effect in marine waters with Setschenow's empirical relationship 24, 25. A temperature correction was applied to the aqueous solubility (see below), but all other physicochemical properties (e.g., log(KOW), MW, Henry's law constant) were estimated using SPARC at standard temperature and pressure. Another important parameter in the model is the molecular weight of the residual fraction (MWResidual) of the oil. The residual fraction could contain asphaltenes or other high-MW compounds that are difficult to characterize but important for determining the weighted-average molecular weight (MWavg), mole fractions, and resulting dissolved concentrations for all of the oil constituents. The exact nature of these constituents is not important, and they are assumed to be dissolved in the oil phase. This assumption does not appear to impact the activity of the MAHs and PAHs in crude oils and is not expected to introduce substantial error into the present analysis 2. The MWResidual was determined for neat and weathered oils by calibration to a temperature-controlled oil-solubility experiment using a least squares minimization approach by simultaneously optimizing the MWResidual and droplet concentrations against the measured data.

Many of the experiments from the literature studies used in this analysis were conducted at temperatures below 25°C. To model the effect of temperature on the subcooled liquid solubility for MAHs and PAHs, the van't Hoff equation was applied

  • equation image(1)

where S2 and S1 are subcooled liquid solubility values for temperatures T2 and T1 in degrees Kelvin, respectively; R is the universal gas constant (8.314 × 10−3 kJ K−1 mol−1); and equation image is the standard enthalpy of the dissolution reaction (kJ mol−1). Enthalpy values were available for four MAHs and 14 PAHs 26. These reported values were used to develop regressions of enthalpy versus log(KOW) to estimate enthalpy values for other compounds (Fig. 2).

  • equation image(2)
  • equation image(3)
thumbnail image

Figure 2. Reported enthalpy values 26 used for temperature corrections in solubility model. Polycyclic aromatic hydrocarbon (PAH) (▴) and monoaromatic hydrocarbon (MAH) (▵) regressions used for chemicals without reported values (Eqns. 2 and 3).

Download figure to PowerPoint

Oil microdroplet model

The concentration of microdroplets is estimated by comparing model-predicted dissolved concentrations to measured aqueous concentrations. For each component i, measured concentrations are assumed to be the sum of both dissolved and microdroplet phases

  • equation image(4)

where Caq,i is the total measured aqueous concentrations (moles/L) and CW,i and CPO,i are the dissolved- and particulate oil-phase concentrations (moles/L) of component i, respectively.

The dissolved-phase concentration (CW,i) is calculated with Raoult's law

  • equation image(5)

where xi is the mole fraction of component i (unitless) and Si (moles/L) is the aqueous solubility of component i in the liquid state. The concentration of material in the particulate oil phase (CPO,i) is expressed as

  • equation image(6)

where Coil,i is the concentration of component i in the oil (moles of component i/g oil) and CMD is the concentration of colloidal microdroplets (g oil/L) in the exposure water. The oil-phase concentration of the i-th component (Coil,i, moles i/g oil) can also be expressed in terms of its mole fraction (xi) and the MWavg of the oil

  • equation image(7)

The total aqueous concentration of component i is found by substituting Equations 5 to 7 into Equation 4

  • equation image(8)

The subcooled liquid solubility (Si) is known, and the mole fraction (xi) is calculated using Equations A6 to A8 in McGrath et al. 14. The microdroplet concentration (CMD) is the only unknown variable in this equation, and it is determined with an optimization program (e.g., Solver) by minimizing the residuals between model-calculated total aqueous concentrations (Eqn. 8) and the measured aqueous concentrations for only the heavy PAH (log[KOW] > 5.3), as discussed below. Once the microdroplet oil concentration is determined, the dissolved fraction of the measured components can be computed

  • equation image(9)

in terms of the oil droplet concentrations (CMD) by substituting Equations 6 to 8 so that

  • equation image(10)

which simplifies to

  • equation image(11)

The dissolved concentrations (Cw,i) are calculated by multiplying the measured total aqueous concentrations with the dissolved fraction (Eqn. 11)

  • equation image(12)

Evaluating the potential role of microdroplet toxicity is outside the scope of the present study. The target lipid model 3, 14, 15 can be used to estimate toxicity from dissolved-phase concentrations. However, evaluations on the basis of dissolved hydrocarbons may result in underprediction of toxicity in instances where microdroplets may be contributing to the overall toxic effects. This proposed toxicity-modeling framework for addressing both dissolved-phase and microdroplet oil constituents will be discussed below.

RESULTS AND DISCUSSION

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS AND MODEL DEVELOPMENT
  5. RESULTS AND DISCUSSION
  6. SUMMARY
  7. SUPPLEMENTAL DATA
  8. Acknowledgements
  9. REFERENCES
  10. Supporting Information

Solubility model

The presence of higher-MW, less soluble PAHs (i.e., chrysenes, benzo[e]pyrene) in excess of their computed solubilities is used to quantify the microdroplet phase. It is critical to have high-quality subcooled liquid solubility data for these compounds. Subcooled liquid solubilities estimated with SPARC at standard temperature and pressure compare favorably with available literature values (Fig. 3) 27–31, in particular with the values from Mackay et al. 26, a critically reviewed database. The available data span the range of compounds commonly found in crude oils from MAHs such as benzene and toluene to lower-MW PAHs such as naphthalene and phenanthrene to higher-MW, less soluble PAHs such as chrysene and benzo[a]pyrene.

thumbnail image

Figure 3. Comparison of literature values and SPARC-calculated subcooled solubilities for commonly measured monocyclic aromatic (○), polycyclic aromatic (●), and saturated (equation image) hydrocarbons. Median and range of measurements are presented.

Download figure to PowerPoint

The physicochemical property estimates with SPARC compare favorably to measurements of subcooled solubilities for saturated aliphatic chemicals 32, 33. This is an important feature of the model because the presence of long chain aliphatic hydrocarbons in water is a strong indication of the presence of microdroplet oil as the solubility of these materials is very low (e.g., <10 ng/L). Reported subcooled solubilities 32, 33 for aliphatics range from approximately 900 µg/L for a C9 paraffin to approximately 0.010 µg/L for a C19 aliphatic. Of the 26 total entries for aliphatics, 10 are within a factor of 2, another 12 are within a factor of 5, and the remaining 4 are within a factor of 30. No detectable bias was apparent in the model-data comparisons along the range of C9 to C19 aliphatics, though the greatest deviations occur with the highest–carbon number chemicals. This suggests that additional work may be required to refine the solubility estimates for high–carbon number aliphatics whose behavior in aqueous environments is complex 34, 35. Because both the model and measured solubilities are very low and the model-data comparisons do not have any particular bias, the error in the solubility estimates of these materials is not likely to substantially impact the microdroplet estimates. Furthermore, microdroplet concentrations are determined by fitting the entire suite of hydrocarbons (see Methods, Eqns. 4–8), so the impact of the model error for any one constituent is moderated by the model performance for other constituents.

The SPARC-estimated subcooled liquid solubilities for the two least soluble PAHs in this model (benzo[a]pyrene and benzo[e]pyrene) are 1.93 and 1.94 µg/L and compare quite favorably with the values reported by Mackay et al. 26 of 2.11 and 2.14 µg/L, respectively. The SPARC estimates for other PAHs are within a factor of 2 of the values found in Mackay et al. 26 for a total of 19 compounds including 11 parent and three alkyl PAHs. A similar level of agreement was found for the five MAHs in the data set. Consequently, the SPARC estimates were used for all of the 46 compounds to maintain internal consistency with other SPARC-derived properties in this solubility model.

Highly soluble constituents found in WAFs are not affected by the presence of microdroplet oil since the relative concentrations of these constituents in droplet oil is small relative to their solubility in water. Therefore, due to their low solubility, only compounds with log(KOW) >5.3 were used to estimate microdroplet oil concentrations. An example of the computed dissolved fraction (Eqn. 10) for a WAF 21 is shown in Figure 4A. The estimated dissolved fraction for each PAH (Eqn. 10) is plotted against the log(KOW) for each PAH in a WAF. At a log(KOW) of approximately 5.3, the fraction dissolved is near 0.75 and decreases to 0.1 at a log(KOW) approximately equal to 7, indicating that concentrations of compounds with a log(KOW) >5.3 are more likely affected by entrained microdroplets of oil. The estimated microdroplet concentration for this sample was 68 µg/L.

thumbnail image

Figure 4. Determination of log(KOW) cutoff for oil droplet concentration estimates. (A) The fraction of dissolved hydrocarbons (monoaromatic hydrocarbon [MAH] and polycyclic aromatic hydrocarbon [PAH]) in a water-accommodated fraction (WAF) 21 with 68 µg/L of microdroplets (CMD) versus log(KOW). (B) The ratio of measured to predicted total aqueous PAH in an oiled-gravel test 17 from the following selected sampling times: 13 d (●), 22 d (▿), 26 d (♦), and 38 d (□).

Download figure to PowerPoint

The data sets evaluated in the present study include standard WAFs and oiled-gravel tests. Due to the longer-term (>10 d) oil–water contact in the oiled-gravel experiments, which do not benefit from static renewals such as in WAF, biodegradation can be an important process. This process is not included in this model; as a result, the ratios of predicted total (droplet and dissolved phases) to measured total aqueous concentrations of individual constituents are quite small (<0.001–0.1) for compounds with log(KOW) <5.3, whereas for compounds with log(KOW) >5.3 the ratios are within a factor of 2 of the model prediction (Fig. 4B). Additionally, it appears that compounds with log(KOW) <5.3 are more prone to loss by biodegradation, as evident by the decreasing ratio with increasing duration. Therefore, the microdroplet concentration was estimated using only compounds with log(KOW) >5.3 based on the observation that the concentration of low-solubility constituents is most affected by the presence of microdroplets and that biodegradation has a more substantial effect on the more soluble constituents.

In the calculation of dissolved hydrocarbon concentrations (Eqn. 11) MWavg is important and substantially affected by MWResidual. The MWResidual value was determined by calibration to a data set of slow-stir, temperature-controlled WAFs performed for the present analysis.

The predicted versus observed total aqueous concentrations for all the measured components are presented in Figure 5. The top panels show the temperature series for naturally weathered oil calibrations and the lower panels are the neat oil calibration. The estimated MW of the residual fraction is higher in weathered North Slope Alaskan crude oil than neat oil, and the microdroplet concentrations in these test solutions varied. However, the higher viscosity of naturally weathered oil seems to result in a smaller concentration of oil droplets compared to neat oils (Table 2), though the processes that control the formation of oil droplets in these test systems are not well understood.

thumbnail image

Figure 5. Calibration data set (predicted vs measured total polycyclic aromatic hydrocarbon [PAH] and monoaromatic hydrocarbon [MAH] concentrations) for naturally weathered (top panels) and neat (bottom panels) Exxon Valdez crude oil at three temperatures (5, 15, and 25°C). Data are from 21.

Download figure to PowerPoint

Table 2. Summary of calibration data set
 MWResidual (g/mol)Microdroplet concentrations (µg/L)
5°C15°C25°C
  1. MWResidual = Molecular weight of the residual fraction.

Weathered2488315280
Neat187388387197

Temperature corrections

Temperature corrections are important for estimating oil droplet concentrations because the analysis depends on the difference between predicted dissolved and measured aqueous concentrations of high-MW PAHs. Higher enthalpy values lower the predicted dissolved concentrations at lower temperatures (Eqn. 1), which results in relatively higher estimates of oil droplet concentrations. Enthalpy values were obtained for four MAHs and 14 parent and alkyl PAHs from Mackay et al. 26. These values and the regressions (Eqns. 2 and 3) that were used to estimate the enthalpy are displayed in Figure 2. Enthalpy values for MAHs (2.1–4.4 kJ mol−1) and PAHs (16.7–50.6 kJ mol−1) resulted in a decrease in the subcooled liquid solubility by a factor of 2 to 9 between 25 and 5°C.

Example application

An example of the observed enrichment of measured high-log(KOW) compounds relative to the predicted dissolved concentrations is shown in Figure 6, which compares the predicted and observed total (i.e., sum of microdroplet and dissolved phases) concentrations of aqueous PAHs and MAHs and aliphatics for WAFs prepared with weathered crude oil in panels A and B 19 and a fresh oil in panels C and D 21. The data in panels C and D are the same measured data that are shown in Figure 1. Data-model comparisons shown with open symbols (upper panels A and C) do not consider the presence of microdroplets. The comparisons shown with filled symbols include the microdroplets. Measured values >10 µg/L are MAHs and are close to the predicted values; they show little impact of the concentration of microdroplets. Compounds with measured aqueous concentrations less than 0.5 to 0.002 µg/L are 3+ ring PAHs, including parent and alkyl (C1–C4 homologs) phenanthrene, chrysene, fluoranthene, and others. The lowest two points are benzo[b]fluoranthene and benzo[ghi]perylene, with measured aqueous concentrations near 0.002 µg/L. The ratio between the predicted dissolved and measured aqueous concentrations for the 3+ ring PAHs range from 0.1 to 0.001, which is similar to the predicted dissolved concentrations of the aliphatics (stars in panel A) that are, also, more than 1,000-fold lower than the measured concentrations.

thumbnail image

Figure 6. Demonstration of solubility calculations with predicted dissolved-phase (open symbols) and predicted total, for example, microdroplet-adjusted, (filled symbols) aqueous concentrations of monocyclic aromatics (▵, ▴), dicyclic aromatics (◊, ♦), 3+ ring aromatics (○, ●), and aliphatics (equation image), open symbols (A, C), and with adjustments for oil droplets, filled symbols (B, D). A and B contain data from a water-accommodated fraction (WAF) with weathered Exxon Valdez cargo crude oil 19. C and D contain the same measured data as Figure 1 from a WAF with fresh oil 21. Data are in Supplemental Data, Table S1.

Download figure to PowerPoint

The inclusion of a microdroplet phase (filled circles) dramatically improves the agreement of predicted and measured concentrations in both panels B and D. The estimated microdroplet oil concentration (CMD) was 374 µg/L of oil for the WAF with weathered oil and 430 µg/L for the WAF with fresh oil. The aqueous-phase concentration of each PAH is increased proportionally to the microdroplet concentration and the concentration of the individual oil-phase constituents. The composition of the higher-MW components in the WAF is similar to the composition of the parent oil, consistent with the presence of microdroplet oil in the WAF. The inclusion of microdroplet oil in the predicted total concentrations allowed us to explain the observed enrichment in both the 3+ ring PAHs and the sparingly soluble aliphatics.

As mentioned, the presence of aliphatic hydrocarbons in water is a strong indication of stable microdroplets. Also, the enrichment in the observed heavy PAHs relative to the model predictions is another strong indication. An alternate explanation might be the presence of dissolved organic carbon, which acts as a third phase that can show enrichment in the heavy PAHs. This hypothesis was considered insufficient to explain the observations for PAHs and SHCs. For example, the fraction of dissolved chrysene (log(KOW) 5.8) in the presence of typical dissolved organic carbon concentrations (2–10 mg/L) is expected to be around 90% (Eqn. 1 in 36), which is not sufficient to explain the observed enrichment in the model-data comparison that can be as high at 1,000-fold. Extrapolation of the dissolved organic carbon partitioning model to long chain aliphatics is uncertain due to a lack of validation data. Therefore, because the presence of microdroplets in oil–water preparations is considered likely, the modeling framework presented here provides a method for systematically estimating microdroplet concentrations and the fraction of dissolved material that is measured by chemical analysis.

Sensitivity analysis on microdroplet estimation method

Measurements of aqueous and oil-phase petroleum hydrocarbons, including saturates from an acute oil toxicity exposure to juvenile fathead minnows 19, were used to validate the microdroplet estimation method. For weathered and neat Exxon Valdez crude oil, WAFs were prepared and analyzed for concentrations of MAHs and PAHs in addition to a series of saturated C9 to C40 hydrocarbons. Saturated hydrocarbons are very insoluble in water, and their presence is a strong indication of the presence of colloidal oil in the exposure waters 37. Three different microdroplet concentration estimation methods were compared. The first used only PAHs with log(KOW) >5.3, the second approach used only SHCs (e.g., alkanes), and the third approach combined all three chemical classes. The three methods give similar microdroplet estimates (Table 3). The droplet estimates based on PAH-only data are approximately within a factor of 2 of the estimates using SHCs. These results support the use of measured aqueous PAHs if these are the only data available and demonstrate the usefulness of measuring long chain saturates in exposure waters.

Table 3. Summary of oil droplet concentration estimates using different approaches (µg/L)
ApproachWeatheredNeat
  1. PAH = polycyclic aromatic hydrocarbon.

PAH only534.4194.1
Saturates only591.9415.4
Both581.8373.9

Applications

The median microdroplet concentrations in the highest oil loadings among all studies range from 567 to 64 µg/L, with smaller to nonquantifiable concentrations at lower loadings (Table 4). In flow-through oiled-gravel tests, the effluent from the first few days of use had higher microdroplet concentrations that diminished quickly during the 10 to 20 d until microdroplet concentrations are no longer quantifiable. For example, data from each of the four oil-loading levels in Carls et al. 17 over the duration of the experiment are shown in Figure 7 as time-variable oil droplet concentrations in the aqueous effluents. The individual data series decrease generally with decreasing loading (see caption). Generally, higher microdroplet concentrations are observed in the higher oil-loading tests, which remain relatively constant during the first 21 d, corresponding to the LWO portion of the test, after which the concentrations begin to decline. This method estimated that there were no microdroplets in the effluents of the lower loadings by the end of the experiment. These patterns are consistent with the droplet estimates in the other oiled-gravel tests 18, 20 evaluated in the present study.

Table 4. Summary of microdroplet results and other calculated parameters
Study IDMedian microdroplet concentration at highest loading (µg/L)aResidual fractionb (%)Weighted-averaged molecular weight (g/mol)Source
  • a

    95% confidence intervals.

  • b

    Computed by difference of total mass less the sum of all measured components.

  • c

    Computed with measured polycyclic aromatic hydrocarbon (PAH), monoaromatic hydrocarbon (MAH), and saturated hydrocarbon concentrations (SHC).

  • d

    Computed with measured PAH and MAH concentrations.

145.3 (±47)93.83c247.317
246.7 (±26)93.83c247.317
371.6 (±42)93.91c247.018
4301 (±105)99.81c247.920
593.8 (±20)99.48c247.620
6311 (±430)91.82c185.819
7567 (±354)89.50c246.619
8216 (±249)96.56d183.121
thumbnail image

Figure 7. Time-variable oil droplet concentration (CMD) patterns in oiled gravel 17. Filled symbols are less-weathered oil (LWO) and open symbols are more-weathered oil (MWO) portions of study for high (equation image), medium (▵, ▴), low (▿, ▾), and trace (○, ●) oil loadings. Microdroplet oil droplet concentration estimates at or below the method detection limit are also plotted (<).

Download figure to PowerPoint

Figure 8 summarizes the median and range of microdroplet concentrations of the highest oil loading for the various studies analyzed. Though the magnitude and occurrence of the microdroplets varied widely, they were estimated to be present in many different testing environments (i.e., standard WAF preparations to oiled-gravel exposures). This suggests that microdroplets were present in most crude oil exposures. The microdroplets appear to be stable, having remained in suspension even though there were several hours to days between sample collection and analysis.

thumbnail image

Figure 8. Summary of microdroplet concentration estimates in highest loading or dilution stream from the studies in this analysis (Table 1). Symbols are plotted at median microdroplet concentration, whiskers represent the range, and the box is the inner two quartiles ( ±25%) of the concentration estimates over the duration of the experiment.

Download figure to PowerPoint

SUMMARY

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS AND MODEL DEVELOPMENT
  5. RESULTS AND DISCUSSION
  6. SUMMARY
  7. SUPPLEMENTAL DATA
  8. Acknowledgements
  9. REFERENCES
  10. Supporting Information

The presence and concentration of microdroplets in WAFs resulting from different exposure scenarios were determined by comparing measured concentrations of aqueous PAHs and saturates to predicted dissolved concentrations. The microdroplet concentration was found by mathematically adding the oil-phase PAHs to the predicted dissolved PAHs to match the observed aqueous concentrations. Surprisingly, substantial microdroplet concentrations (>10 µg oil/L water) were estimated for a large fraction of the exposure scenarios (e.g., loadings, sampling times, media) analyzed. This suggests that stable oil droplets are likely present in the water exposures used in oil toxicity experiments. The presence of microdroplets complicates the interpretation of these studies. The present study focused on physically dispersed crude oil, though presumably this framework can be applied to assess the role of chemically dispersed oil in aquatic toxicity tests as well. This framework could be applied to field settings for estimating the presence of microdroplets by focusing on the high-MW PAHs, which are relatively less affected by volatilization and biodegradation.

A model is proposed for evaluating crude oil toxicity in the presence of microdroplet oil. Toxicity in a crude oil exposure is assumed to be affected by both dissolve-phase constituents (CW) and microdroplet oil (MD).

  • equation image(13)

The present study provides a methodology for differentiating between dissolved- and oil-phase oil constituents. The toxicity of dissolved-phase hydrocarbons (CW) is often evaluated using the toxic unit approach 2, 14, 15

  • equation image(14)

where the contribution of individual dissolved hydrocarbons to overall toxicity is evaluated by normalizing the dissolved-phase concentrations (equation image) to their respective critical effect levels (e.g., equation image). The target lipid model provides an ideal framework for this evaluation 3.

It is possible that microdroplets could exert a toxic effect in addition to the dissolved-phase constituents. In this case, dissolved-phase toxicity may not be sufficient to explain the observed toxicity. Microdroplet toxicity can be estimated by evaluating the difference between the observed and dissolved-phase predicted toxicity as a function of the properties and concentrations of microdroplets. Applications of this model should also account for other dissolved constituents that may not be characterized by standard methods for MAHs, PAHs, and SHCs.

The work discussed in the present study provides a quantitative framework for evaluating the concentration and potential effects of microdroplets in crude oil exposures to aquatic life. Further work is needed to characterize the toxicological properties of microdroplet oil as well as to characterize the processes that control the formation and fate of microdroplets in the laboratory and in the field.

REFERENCES

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS AND MODEL DEVELOPMENT
  5. RESULTS AND DISCUSSION
  6. SUMMARY
  7. SUPPLEMENTAL DATA
  8. Acknowledgements
  9. REFERENCES
  10. Supporting Information
  • 1
    Clark RC Jr, Brown DW. 1977. Petroleum: Properties and analyses in biotic and abiotic systems. In Malins DC, ed, Effects of Petroleum on Arctic and Subarctic Marine Environments and Organisms, Vol 1—Nature and Fate of Petroleum. Academic Press, New York, NY, USA, pp 189.
  • 2
    Di Toro D, McGrath J, Stubblefield W. 2007. Predicting the toxicity of neat and weathered crude oil: Toxic potential and the toxicity of saturated mixtures. Environ Toxicol Chem 26: 2436.
  • 3
    McGrath JA, Di Toro DM. 2009. Validation of the target lipid model for toxicity assessment of residual petroleum constituents: Monocyclic and polycyclic aromatic hydrocarbons. Environ Toxicol Chem 28: 11301148.
  • 4
    Ramachandran SD, Sweezy MJ, Hodson PV, Boudreau M, Courtenay SC, Lee K, King T, Dixon JA. 2006. Influence on salinity and fish species on PAH uptake from dispersed crude oil. Mar Pollut Bull 42: 226236.
  • 5
    Payne JR, Driskell WB. 2003. The importance of distinguishing dissolved- versus oil-droplet phases in assessing the fate, transport and toxic effects of marine oil pollution. Proceedings, the International Oil Spill Conference, Vancouver, BC, Canada. American Petroleum Institute, Washington, DC, pp 14031409.
  • 6
    Singer MM, George S, Lee I, Jacobson S, Weetman LL, Blondina G, Tjeerdema RS, Aurand S, Sowby ML. 1998. Effects of dispersant treatment on the acute aquatic toxicity of petroleum hydrocarbons. Arch Environ Contam Toxicol 34: 177187.
  • 7
    Gulec I, Holdway DA. 2000. Toxicity of crude oil and dispersed crude oil to ghost shrimp Palaemon serenus and larvae of Australian bass Maquaria novemaculeata. Environ Toxicol 15: 9198.
  • 8
    Perkins RA, Rhoton S, Behr-Andres C. 2003. Toxicity of dispersed and undispersed, fresh and weathered oil to larvae of a cold-water species, Tanner crab (C. bairdi) and standard warm-water test species. Cold Reg Sci Technol 36: 129140.
  • 9
    Fuller C, Bonner J, Page C, Ernest A, McDonald T, McDonald S. 2004. Comparative toxicity of oil, dispersant and oil plus dispersant to several marine species. Environ Toxicol Chem 23: 29412949.
  • 10
    Ramachandran SD, Hodson PV, Khan CW, Lee K. 2004. Oil dispersant increases PAH uptake by fish exposed to crude oil. Ecotoxicol Environ Saf 59: 300308.
  • 11
    Couillard CM, Lee K, Legare B, King TL. 2005. Effect of dispersant on the composition of the water accommodated fraction of crude oil and its toxicity to larval marine fish. Environ Toxicol Chem 24: 14961504.
  • 12
    Perkins RA, Rhoton S, Behr-Andres C. 2005. Comparative marine toxicity testing: A cold-water species and standard warm-water test species exposed to crude oil and dispersant. Cold Reg Sci Technol 42: 226236.
  • 13
    Schein A, Scott JA, Mos L, Hodson PV. 2009. Oil dispersion increases the apparent bioavailability and toxicity of diesel to rainbow trout (Oncorhynchus mykiss). Environ Toxicol Chem 28: 595602.
  • 14
    McGrath J, Parkerton T, Hellweger F, Di Toro D. 2005. Validation of the narcosis target lipid model for petroleum products: Gasoline as a case study. Environ Toxicol Chem 24: 23822394.
  • 15
    CONCAWE. 2011. PETROTOX User's Manual, Ver 3.06. Study CONC.006.
  • 16
    Anderson JW, Neff JM, Cox BA, Tatem HE, Hightower CM. 1974. Characteristics of dispersions and water-soluble extracts of crude and refined oils and their toxicity to estuarine crustaceans and fish. Mar Biol 27: 7588.
  • 17
    Carls MG, Rice SD, Hose JE. 1999. Sensitivity of fish embryos to weathered crude oil: Part I. Low level exposure during incubation causes malformations, genetic damage, and mortality in larval pacific herring (Clupea pallasi). Environ Toxicol Chem 18: 481493.
  • 18
    Heintz RA, Short JW, Rice SD. 1999. Sensitivity of fish embryos to weathered crude oil: Part II. Increased mortality of pink salmon (Oncorhynchus gorbuscha) embryos incubating downstream from weathered Exxon Valdez crude oil. Environ Toxicol Chem 18: 494503.
  • 19
    ENSR. 2001. Acute toxicity of water accommodated fractions of Alaska North Slope crude oil and weathered Alaska North Slope crude oil to the fathead minnow (Pimephales promelas) under static renewal conditions. March 2, 2001. Project 2620-184.
  • 20
    Brannon EL, Collins KM, Brown JS, Neff JM, Parker KR, Stubblefield WA. 2006. Toxicity of weathered Exxon Valdez crude oil to pink salmon embryos. Environ Toxicol Chem 25: 962972.
  • 21
    Stubblefield WA, Brannon EL, Maki AW, Brown JS, Di Toro DM, McGrath JA, Redman A, Parker KR, Vangenderen E. 2010. Evaluation of the chronic toxicity of Alaska North Slope crude oil to pink salmon (Oncorhynchus gorbuscha). Research Bulletin 10-1. University of Idaho Center for Salmonid and Freshwater Species at Risk, Moscow, Idaho, USA.
  • 22
    Prausnitz JM, Rudiger NL, Gomez de Acevado E. 1999. Molecular Thermodynamics of Fluid-Phase Equilibria. Prentice Hall, Upper Saddle River, NJ, USA.
  • 23
    Karickhoff SW, McDaniel VK, Melton C, Vellino AN, Nute DE, Carreira LA. 1991. Predicting chemical reactivity by computer. Environ Toxicol Chem 10: 14051416.
  • 24
    Setschenow JZ. 1889. Uber di konstitution der salzlosungen aur grund ihres verhaltens zu kohlensaure. Z Physik Chem 4: 117.
  • 25
    U.S. Environmental Protection Agency. 2003. Procedures for the derivation of equilibrium partitioning sediment benchmarks (ESBs) for the protection of benthic organisms: PAH mixtures. EPA 600/R-02/013. Office of Research and Development. Washington, DC.
  • 26
    Mackay D, Hiu WY, Ma KC, Lee SC. 2006. Illustrated Handbook of Physical–Chemical Properties and Environmental Fate for Organic Chemicals, 2nd ed, Vol 1—Introduction and Hydrocarbons. Taylor and Francis, Boca Raton, FL, USA.
  • 27
    Lee L, Rao P, Okuda I. 1992. Equilibrium partitioning of polycyclic aromatic hydrocarbons from coal tar into water. Environ Sci Technol 26: 21102115.
  • 28
    Lee L, Hagwell M, Delfino J, Rao P. 1992. Partitioning of polycyclic aromatic hydrocarbons from diesel fuel into water. Environ Sci Technol 26: 21042110.
  • 29
    De Maagd PG, Ten Hulscher DTEM, Van Den Heuval H. 1998. Physicochemical properties of polycyclic aromatic hydrocarbons: Aqueous solubilities, n-octanol/water partition coefficients and Henry's law constants. Environ Toxicol Chem 17: 251257.
  • 30
    May WE, Wasik SP, Freeman DH. 1978. Determination of the solubility behavior of some polycyclic aromatic hydrocarbons in water. Anal Chem 50: 9971000.
  • 31
    Huibers P, Katritzky A. 1998. Correlation of the aqueous solubility of hydrocarbons and halogenated hydrocarbons with molecular structure. J Chem Inf Comput Sci 38: 283292.
  • 32
    ExxonMobil Biomedical Sciences. 2011. Water solubility of C12-C16 alkanes using slow stir method. Final report. Study 0545084. Annandale, NJ, USA.
  • 33
    Tolls J, van Dijk J, Verbruggen EJM, Hermens JLM, Loeprecht B, Schuurmann G. 2002. Aqueous solubility–molecular size relationships: A mechanistic case study using C10 to C19 alkanes. J Phys Chem A 106: 27602765.
  • 34
    Underwood R, Tomlinson-Phillips J, Ben-Amotz D. 2010. Are long-chain alkanes hydrophilic? J Phys Chem B 114: 86468651.
  • 35
    Ferguson AL, Debenedetti PG, Panagiotopoulos AZ. 2009. Solubility and molecular conformations of n-alkane chains in water. J Phys Chem B 113: 64056414.
  • 36
    Burkhard LP. 2000. Estimating dissolved organic carbon partition coefficients for nonionic organic chemicals. Environ Sci Technol 34: 46634668.
  • 37
    Faksness L, Grini PG, Daling PS. 2004. Partitioning of semi-soluble organic compounds between the water phase and oil droplets in produced water. Mar Pollut Bull 48: 731742.

Supporting Information

  1. Top of page
  2. Abstract
  3. INTRODUCTION
  4. METHODS AND MODEL DEVELOPMENT
  5. RESULTS AND DISCUSSION
  6. SUMMARY
  7. SUPPLEMENTAL DATA
  8. Acknowledgements
  9. REFERENCES
  10. Supporting Information

Additional Supporting Information may be found in the online version of this article.

FilenameFormatSizeDescription
etc_1882_sm_SupplData.pdf61KSupplementary Data

Please note: Wiley Blackwell is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.