Organic Phase Resistance to Dissolution of Polycyclic Aromatic

a light lubricating oil to paraffin, the governance of mass transfer was shown to vary from water phase control to nonaqueous phase control. An advanc...
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Environ. Sci. Technol. 1999, 33, 235-242

Organic Phase Resistance to Dissolution of Polycyclic Aromatic Hydrocarbon Compounds ENRIQUE ORTIZ, MATTHIAS KRAATZ, AND RICHARD G. LUTHY* Department of Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213

The dissolution of naphthalene, phenanthrene, and pyrene from viscous organic phases into water was studied in continuous-flow systems for time periods ranging from several months to more than 1 year. By selecting nonaqueous phases ranging from low viscosity to semisolid, i.e., from a light lubricating oil to paraffin, the governance of mass transfer was shown to vary from water phase control to nonaqueous phase control. An advancing depleted-zone model is proposed to explain the dissolution of PAHs from a viscous organic phase wherein the formation of a depleted zone within the organic phase increases the organic phase resistance to the dissolution of PAHs. The experimental data suggest the formation of a depleted zone within the organic phase for systems comprising a highviscosity oil (∼1000 cP at 40 °C), petrolatum (petroleum jelly), and paraffin. Organic phase resistance to naphthalene dissolution became dominant over aqueous phase resistance after flushing for several days. Such effects were not evident for low viscosity lubricating oil (86 cP at 40 °C). The transition from aqueous-phase dissolution control to nonaqueous-phase dissolution control appears predictable, and this provides a more rational framework to assess longterm release of HOCs from viscous nonaqueous phase liquids and semisolids.

Introduction Inter-phase mass transfer, e.g., sorption/desorption and dissolution, affects the kinetics of environmental fate processes for hydrophobic organic contaminants (HOCs) such as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls in soil and sediment environments. The mass transfer affects both aqueous concentration and microbial degradation, as it is believed that the microbial degradation of HOCs is governed mainly by the bioavailable fraction in aqueous solution (1, 2). HOC sequestration and mass transfer processes are crucial issues for assessing availability, risk, and remediation end points (3). The rate of HOC dissolution from nonaqueous phase liquids (NAPLs) is frequently modeled as a function of diffusion-dependent mass transfer, with the resistance assumed to reside in a stagnant aqueous layer of defined thickness adjacent to the interface (4-6). The singleresistance approach neglects possible mass transfer limitations within the NAPL phase itself and may be valid only as long as the NAPL phase resistance does not dominate the * Corresponding author phone: (412)268-2948; fax: (412)268-7813; e-mail: [email protected]. 10.1021/es9804417 CCC: $18.00 Published on Web 12/01/1998

 1999 American Chemical Society

overall mass transfer (7). For example, a significant relation between oil viscosity and the oil phase mass transfer coefficient has been observed in experiments of HOC dissolution from coal-derived oils (8). Additionally, emergent interfaces, such as the formation of biofilms between NAPL and aqueous phase (9) and the discovery of semirigid films at the surface of coal tar NAPL blobs (10, 11), may influence the dissolution rate as well. With respect to geosorbents, typical HOC sequestration mechanisms are considered closely related to the nature of the sorbent organic carbon domain (12). In the case of NAPLs, this domain may comprise soft and amorphous organic matter. This seems reasonable if we think of the low aqueous solubilities of organic compounds that usually compose NAPL phases as well as the properties of aged NAPLs that may consist of higher molecular weight compounds and that may form a “polymer-like matrix” (13). For soft or amorphous carbon entities, also referred to as “rubbery polymers”, HOC diffusion coefficients from 10-7 to 10-10 cm2/s are proposed (14). These values are between those for liquids or solids. Similarly, small solutes such as benzene “squeezing through a polymer matrix” are expected to have diffusion coefficients between 10-6 and 10-9 cm2/s (15). The purpose of this paper is to present experimental results on the effect of nonaqueous phase (NAP) viscosity on the diffusivity and the dissolution kinetics of three PAHs and to interpret these results by means of a nonaqueous phase interfacial depletion model. By selecting nonaqueous phases ranging from low viscosity to semisolid, the governance of PAH mass transfer is expected to vary from water phase resistance to nonaqueous phase resistance. If the transition between aqueous phase dissolution control and nonaqueous phase dissolution control were predictable, this would provide a more rational framework to assess long-term release of HOCs from nonaqueous phase liquids and semisolids.

Material and Methods Experimental Setup. Two experimental procedures were used in this study. The first was batch equilibration to assess PAH partitioning equilibria between each of the nonaqueous phases and the aqueous phase. The second experimental procedure was a flow-through experiment to assess the mass transfer of PAHs from the different nonaqueous phases into the aqueous phase and to explore the effects that the viscosity of the nonaqueous phase may have on the release of PAHs into the water phase. Analytical measurements of PAHs in the nonaqueous phase and aqueous phase were performed by hexane dissolution and hexane extraction, respectively, and high performance liquid chromatography employing a HewlettPackard liquid chromatograph series 1050 (including HPChemstation software package) with eluent flow at 1 mL/ min (65% acetonitrile, 35% water, degassed with helium), wavelength at 254 nm, and 20 µL injection volume. The chromatographic column was a 100 mm by 2.1 mm stainless steel filled with ODS Hypersil. The approximate retention times for the PAHs analyzed with this technique were 0.5 min for naphthalene, 0.7 min for phenanthrene, and 1 min for pyrene. Chemicals. Naphthalene, phenanthrene, and pyrene were purchased from Sigma Chemical Co., St. Louis, MO. Solvents were Optima-grade from Fisher Scientific, Fair Lawn, NJ. Nonaqueous Phase Materials. Five nonaqueous phase (NAP) materials were employed in this experiments in order to represent a wide range of viscosity values. The materials selected for this study were as follows: 1, pump oil, SargentVOL. 33, NO. 2, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Schematic of the experimental apparatus used in this study. (a) Batch equilibration system. (b) Flow-through system. Welch Scientific Co., Buffalo Grove, IL (86 cP at 40 °C); 2, transmission oil, T and T Plus, Veedol Mexico, SAE 250, API 6L-4 (800-1400 cP at 40 °C); 3, grease, Grasa Lubricante, Commercial Roshfran, S.A.; Mexico; 4, petrolatum, “Treasury Petroleum Jelly” White Petrolatum (Vaseline); and 5, hard paraffin (P31-500), Fisher Chemicals, Fair Lawn, NJ. These materials were spiked with 5 wt % naphthalene, 5 wt % phenanthrene, and 2.5 wt % pyrene. In all cases, the dissolution was performed in closed glass containers to avoid volatilization. The PAHs were dissolved in the two oils by sonication for approximately 2 h, during which time the temperature of the oils increased to about 80 °C. It was necessary to melt the semisolids (grease, petrolatum, and solid paraffin) in order to dissolve the PAHs. The petrolatum was heated to a liquid at about 75 °C, and then the PAHs were dissolved using sonication for about 1.5 h. Sonication maintained the petrolatum in a liquid state. The temperature necessary to liquefy the grease was about 120 °C, and then the PAHs were added. The dissolution process was approximately 2 h with intermittent sonication and heating as the grease started to solidify when the temperature decreased. A similar procedure was used to dissolve the PAHs in the solid paraffin, with a temperature of about 60 °C being adequate to liquefy the paraffin. Batch Experiments. The experimental setup comprised a flask filled with 125 mL of deionized water into which was inserted a Pyrex glass tube (1 cm diameter with an effective surface area at the interface of 0.79 cm2 at the open end) held in place by tight-fitting, perforated, Teflon-lined septa and locking screw cap. A schematic of the experimental apparatus is shown in Figure 1a. Either two mL of oil or more than the whole tube length in the case of the solid organic phases was placed in the glass tube. The oils were held by buoyant forces. For petrolatum, grease, and paraffin, the Pyrex tube was filled to excess with the molten liquid mixture resulting from dissolution of the PAHs. After this material solidified, excess solid was removed with a knife in order to create a fresh, smooth surface at the interface. This procedure eliminated material that may have experienced loss of PAHs from the exposed surface while the mixture cooled. In the systems containing oil, a syringe needle was positioned within the glass tube before filling in order to take water samples without contacting any oil. Sampling of the systems containing solid organic phases was performed simply by removing the tube containing the solid from the flask. The cap was aluminumwrapped. The flasks were positioned on a horizontal shaker and shaken at 2 rpm, and 15 samples were taken at different 236

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time intervals from 2 h up to 6 weeks. Each sampling included two separate 30-mL water samples taken from each system. PAHs were extracted from each 30 mL water samples with 1 mL of hexane by vortexing extraction tubes for 2 min. Control samples showed that extraction recoveries were over 97% for each PAH. Flow-Through Experiments. A similar experimental setup as in the batch extraction tests was used in the flow-through experiments with approximately 125-mL aqueous volume and with an inflow and an outflow capillary stainless steel tube replacing the sampling needle in the septa. These tubes provided a continuous flow of deionized water through the system; precision HPLC-type pumps were used to maintain the flow. Figure 1b shows a schematic of this apparatus. The flushing rate was 1 mL/min for a detention time in the flask of about 0.09 day. The PAHs dissolved in the water were collected during sampling periods by installing an extraction cartridge at the end of the outflow capillary. The extraction cartridge (Waters Sep-Pak Environmental Cartridge) contained about 1 g of C18-bound silica. The sampling periods used in the experiment were 1 h for short-term sampling and 24 h for long-term sampling. Short-term sampling of the aqueous PAH concentrations was performed every 3 h for up to 1 day of operation, after which long-term samples of the aqueous phase were collected weekly. The PAHs were recovered from the solid-phase extraction cartridges by flushing with hexane in three consecutive steps. The first and second extraction steps were performed by passing 2 mL of hexane through the cartridge and collecting the extracts in a glass vial. A third extraction was performed with 1 mL of hexane. For all extractions, the flow rate of the hexane through the cartridge was about 1 mL/min (one drop per second) as the manufacturer recommended. The efficiency of the extraction procedure was determined to be over 95%. The first extraction was responsible for about 80-90% removal of the PAHs from the cartridge. The third extraction contained in all cases nondetectable levels of naphthalene and pyrene and about 1% or less of the phenanthrene extracted. This last extract was used as a quality control for the extraction procedure in all sampling events. The flow through experiments were conducted from times ranging from several months to more than 1 year depending on the system being tested. Temperature was 21 ( 1 °C.

Modeling Approach The release flux of HOCs from a nonaqueous phase (NAP) into water is taken as the mass of HOC transferred into the water per unit of time per unit surface area of the nonaqueous phase in contact with the aqueous phase. This release flux is commonly expressed in terms of an overall mass transfer coefficient and a driving force:

flux ) ko(C* - Cw) )

C* - Cw 1/ko

(1)

where flux is the release of HOC in g/cm2‚min; ko is an overall mass transfer coefficient in cm/min, and when expressed as an inverse value (1/ko), it represents the overall resistance to the mass transfer of HOC; C* is the aqueous concentration of HOC at equilibrium with the nonaqueous phase in g/cm3; and Cw is the aqueous concentration of HOC in g/cm3. The term (C* - Cw) is known as the driving force for the mass transfer process (15). This equation is derived from Fick’s First Law, flux ) D(dC/dx) where D is the diffusivitiy of the compound with the gradient assumed constant with distance x. The equilibrium concentration in water is C* ) Co/Kp where Co is the concentration in the bulk NAP and Kp is the dimensionless partition coefficient expressed on a concentration basis, (g of HOC/cm3 of NAP)/(g of HOC/cm3 of water).

The overall resistance to the mass transfer is commonly explained by analogy with electrical resistances in series, i.e., the total resistance is equal to the sum of the individual values for each resistance in the series. When this is applied to inter-phase mass transfer processes, the overall resistance to the mass transfer is equal to the sum of the individual resistances produced for each of the individual phases, in this case the nonaqueous phase and water (15, 16) as shown in eq 2. The partition coefficient, Kp, for the HOC between the nonaqueous phase and water has been invoked to eliminate interfacial concentrations, where equilibrium is assumed to prevail (15).

1 1 1 ) + ko KpkNAP kw

(2)

Here kNAP is the mass transfer coefficient within the nonaqueous phase (cm/min), and kw is the mass transfer coefficient in the aqueous phase (cm/min). The largest value of the individual mass transfer resistances will control the value of the overall mass transfer resistance. Thus, when the resistance in the nonaqueous phase is larger than the resistance in the aqueous phase, the overall mass transfer will be controlled by the mass transfer in the nonaqueous phase. The mass transfer coefficient can be expressed as the ratio of the diffusion coefficient of the HOC in the respective phase and an interfacial length over which the diffusive transfer occurs. This is presented in eq 3, where Dj is the diffusion coefficient of the HOC in the phase j (i.e., nonaqueous phase or water) in cm2/min, and δj is the diffusion length over which diffusional transfer is occurring (nonaqueous phase or water) in cm:

Dj kj ) δj

(3)

Diffusion coefficients are a function of temperature, solvent viscosity, and the molecular size of solute and solvent. Among these variables, the solvent viscosity is the parameter that has the greater effect on the diffusivity value (15). Several relationships for estimation of the diffusion coefficient in liquids are summarized by Cussler (15). The diffusion coefficient is inversely proportional to the solvent viscosity. As explained below, the mass transfer coefficient is a variable in time due to the change in the diffusion length with time. Several studies have shown that the aqueous phase resistance is the limiting step in dissolution of PAHs from fluid NAPLs (9, 17). Those studies considered a relatively well-mixed phase for both the water and the NAPL. However, where the nonaqueous phase is very viscous or semisolid or solid, and thus unmixed, a “depleted zone” may form as HOCs are transferred to the water. This depleted zone will increase with time as progressively more HOCs are moved to the aqueous phase, causing the mass transfer resistance within the nonaqueous phase to increase as the diffusion length δ increases with time. Eventually, the resistance in the nonaqueous phase may become greater than the resistance in the aqueous phase, making the mass transfer within the nonaqueous phase the controlling step in the overall release of HOCs. Figure 2 is a schematic representation of the release of HOCs from a viscous or semisolid nonaqueous phase, which shows the formation of a depleted zone with time. With time, this depleted-zone length increases from δNAP,t ) n to δNAP,t ) n+1, etc. To model the HOC release in the system represented in Figure 2, a concentration profile of the HOC in the nonaqueous phase-depleted zone needs to be defined. Here, a linear profile between the bulk nonaqueous phase HOC

FIGURE 2. Schematic of the release of HOC from a nonaqueous phase. (a) The nonaqueous phase-water interface showing concentration profiles in each boundary layer. (b) The progression of a depleted zone in the nonaqueous phase. The cross-hatched region is the rectangular zone that defines the region for HOC mass balance, for which a linear concentration profile is considered. concentration, Co, and the interface HOC concentration in the nonaqueous phase, Co,i, was assumed. Based on this assumption, the change in depth of the depleted zone with time can be estimated with a HOC mass balance. The rectangle represents the original HOC mass contained in the now-depleted zone. The zone of depletion of HOC is described as a triangle comprising half of the rectangle. One triangle represents the mass of HOC remaining, and the other triangle with the same area represents the mass of HOC removed. The depth of the rectangle is the depleted-zone length in the nonaqueous phase, δNAP, which increases with time. Based on this analogy, the HOC mass removed from the depleted-zone in the nonaqueous phase is equal to the HOC mass remaining in the depleted-zone nonaqueous phase. The mathematical expression of the mass of HOC remaining (or removed) in the nonaqueous phase (NAP) depleted-zone is

dMHOC 1 dδNAP dV 1 ) (Co - Co,i) ) (C - Co,i)ANAP (4) dt 2 dt 2 o dt where MHOC is the mass of HOC remaining in the nonaqueous phase depleted zone in g; V is the volume of the depleted zone in the nonaqueous phase in cm3; t is time in minutes; ANAP is the transverse area of the depleted zone nonaqueous phase volume in cm2; and δNAP is the depleted-zone length (penetration) in the nonaqueous phase in cm. Equation 4 can be used to obtain a relationship for the value of δNAP as a function of time if the mass of HOC removed from the nonaqueous phase is accounted as the mass of HOC transferred through the nonaqueous phase interface. This mass balance is

dδNAP 1 (C - Co,i)ANAP ) kNAPANAP(Co - Co,i) 2 o dt

(5)

where kNAP is the mass transfer coefficient for the HOC in the VOL. 33, NO. 2, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. PAH Equilibrium Concentrations in the Nonaqueous and the Aqueous Phases PAH concn in water, C* (ppm)

PAH concn in NAP, CNAP (ppm) NAP

NA

PHE

PY

NA

PHE

PY

pump oil SAE 250 grease paraffin petrolatum

42 260 41 200 48 460 50 600 51 350

42 610 52 310 47 220 45 950 49 000

22 930 18 340 23 300 27 600 28 100

9.87 11.2 12.0 3.20 8.20

0.852 0.662 0.700 0.847 0.880

0.103 0.047 0.160 0.067 0.120

nonaqueous phase, cm/min. This mass transfer coefficient can be expressed as a function of the diffusion coefficient and the depleted-zone length by using eq 3. Equation 5, modified by eq 3, can be solved with the initial condition t ) 0, δNAP ) 0 to obtain

δNAP ) x4DHOC,NAPt

(6)

where DHOC,NAP is the diffusion coefficient for the HOC in the nonaqueous phase in cm2/min. Substituting eq 3 in eq 2 in order to express kNAP as the diffusion coefficient divided by δNAP and then using the expression obtained in eq 6 for δNAP , the overall mass transfer resistance 1/ko can be expressed in terms of time and diffusivity of the HOC in the nonaqueous phase. The resulting expression is presented in eq 7.

(

1 1 4t ) ko Kp DHOC,NAP

)

1/2

+

1 kw

(7)

Equation 7 predicts a linear relationship between the overall mass transfer resistance 1/ko and the square root of time; thus, the overall mass transfer coefficient should decrease with time if the resistance in the organic phase is significant. Substitution of eq 7 in eq 1 provides a model to estimate HOC release flux from a nonaqueous phase into the aqueous phase. The experimental data on measurement of the flux release of three PAHs (naphthalene, phenanthrene, and pyrene) versus time was used with measured values of the nonaqueous phase-water PAH concentrations at equilibrium to estimate the overall mass transfer resistance 1/ko in eq 1. The values of 1/ko were plotted against the square root of time, and eq 7 was fitted to these data in order to assess whether the advancing depleted-zone model was able to represent the release of PAHs for the different nonaqueous phases tested herein.

Results Nonaqueous Phase-Water Equilibrium Measurements. During these equilibration experiments, measurements of the PAH concentrations in the aqueous phase were taken at different times until equilibrium was attained. The equilibrium state was selected when the PAH concentrations in two or more consecutive water samples were essentially constant. Equilibration times for these experiments ranged from about 5 to 20 days for the different nonaqueous phases tested. The PAH concentrations measured at equilibrium for the nonaqueous and aqueous phases are presented in Table 1. These values were used to calculate the observed nonaqueous phase-water partition coefficients (dimensionless, concentration basis) for the three PAHs used in the experiments. The values of the nonaqueous phase-water partition coefficients obtained from the experimental data are presented in Table 2, and they are compared with reported values of octanol-water partition coefficients reported in the literature (18). Flow-Through Dissolution Experiments. The release flux for each PAH was estimated as the mass of each PAH released 238

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TABLE 2. Nonaqueous Phase-Water PAH Partition Coefficients, Kp, Estimated from Equilibrium Data log Kpa NAP

NA

PHE

PY

pump oil SAE 250 grease paraffin petrolatum octanol (Kow)b

3.63 3.56 3.61 4.20 3.80 3.36

4.70 4.90 4.83 4.73 4.75 4.57

5.35 5.59 5.16 5.61 5.37 5.13

a Concentration basis. ref 18.

b

Octanol-water partition coefficients from

during the sampling period divided by the nonaqueous phase surface area in contact with the aqueous phase (0.79 cm2), giving an average measurement of the flux for each PAH during the sampling period expressed as g/cm2‚d. The overall mass transfer coefficients for each sampling event for each PAH were computed with eq 1, using the equilibrium aqueous concentrations for each PAH (C*) reported in Table 1, the measured release flux, and the computed aqueous concentrations (C). Based on eq 7, at times close to zero, the overall mass transfer coefficient may be governed by the aqueous phase mass transfer coefficient (kw), so short-term measurements of the release of PAH were necessary in order to estimate the value of kw adequately. These short-term measurements were performed with sampling times of about 1 h at the beginning of the experiment and experimental times no longer than 1 day. After this, the long-term measurements were initiated in order to measure the effects of the depletion of the nonaqueous phase on the overall mass transfer of PAHs into the aqueous phase. As the experiment progressed, 24-h composite samples were collected in the sorbing cartridges, weekly during approximately the first 3 months and then every other week through the completion of the experiments. Because of the experimental setup, at the beginning of the experiment the water in the flask did not contain any PAHs, and after the experiment was started the concentration of PAHs in the water increased. In a system not impacted by depletion of PAH from the nonaqueous phase, the aqueous concentration will increase until it reaches a maximum and then remain constant. It was for these reasons that shortterm measurements were necessary in order to identify when the system had attained a maximum concentration and when effects due to mass transfer resistance from the nonaqueous phase may be apparent. The observed system behavior was as follows: at time zero, the aqueous concentration of PAHs was zero; as the time increased, the concentration of PAHs in the aqueous phase increased attaining a maximum value that may remain so for some time; the aqueous concentration may then start to decrease due to the depletion of PAHs from the interfacial region of the nonaqueous phase. This latter point defines the transition between the short-term and longterm experiments, and the experimental data used to fit the model parameters were taken after this time. In all cases, the maximum PAH concentration was observed between 4 and

FIGURE 3. Concentration of phenanthrene in the aqueous phase vs time as observed in the dissolution experiment for high-viscosity transmission oil (SAE 250). The time scale is logarithmic in order to show small times.

TABLE 3. Values of Aqueous-Phase Mass Transfer Coefficient (kw) and Nonaqueous Phase Diffusivity (DHOC,NAP) for Naphthalene, Phenanthrene, and Pyrene in Different Systems NAP/compound petrolatum naphthalene phenanthrene pyrene transmission oil (SAE 250) naphthalene phenanthrene pyrene paraffin naphthalene phenanthrene pyrene

kw (cm/min)

DHOC,NAP (cm2/s)

MAPE (%)

0.035 0.040 0.053

1.45 × 10-8 6.17 × 10-10 4.53 × 10-11

28.8 29.5 32.1

0.014 0.073 0.050

3.35 × 10-9 3.80 × 10-11 5.25 × 10-12

31.5 29.9 89.0

0.038 0.052 0.038

1.82 × 10-9 2.97 × 10-10 7.18 × 10-12

22.3 23.8 40.4

20 h after the experiment was initiated. After 24 h, the concentration of PAHs in the aqueous phase started to decrease, except for the pump oil and grease, which stayed approximately constant. Figure 3 shows the aqueous concentration for phenanthrene versus time for the experiment with viscous transmission oil (SAE 250). In this figure, it is observed that the maximum concentration of phenanthrene in the aqueous phase is attained within the first 2 or 3 h and then the aqueous concentration for phenanthrene drops with time after the first day of operation. Similar behavior was observed also for naphthalene and pyrene and with petrolatum and paraffin. Statistical fitting was used to estimate the values of the parameters in eq 7. The mean absolute percent of error (MAPE) was used as the goodness-of-fit parameter. The MAPE is a relative measurement of error that quantifies the differences between predicted values and measured values as compared with the magnitude of the values rather than quantifying just the difference between values (i.e., sum of square errors). Equation 7 was fitted to the long-term data for each PAH and the various nonaqueous phase materials used in this study. The optimization procedure was based on iterative search of the minimum value of MAPE when the values for the aqueous phase mass transfer coefficient (kw) and the diffusivity of the PAH in the nonaqueous phase (DHOC,NAP) were changed. The values obtained with this procedure for kw and DHOC,NAP for each PAH for petrolatum, transmission oil (SAE 250), and paraffin are presented in Table 3. Figure 4 presents the experimental data and model predictions obtained for the different PAHs in petrolatum. Similar behaviors were observed for paraffin and the transmission oil (SAE 250). Pump oil and grease exhibited a different behavior, giving an approximately constant overall

FIGURE 4. Experimental and predicted values of the overall mass transfer coefficient for dissolution of naphthalene, phenanthrene, and pyrene from petrolatum.

FIGURE 5. Experimental values of the overall mass transfer coefficient for dissolution of naphthalene, phenanthrene, and pyrene from pump oil. mass transfer coefficient versus time. The data obtained for pump oil are shown in Figure 5. Figure 5 shows that the overall mass transfer coefficient exhibited by the pump oil does not follow the same trend observed for petrolatum, paraffin, and high-viscosity transmission oil (SAE 250). The dissolution of PAHs from the pump oil exhibits the behavior of a constant source during the 80 days of the experiment. A similar behavior was observed for release of PAHs from the grease. Because the overall mass transfer coefficient did not show a proportional decrease with the square root of time for pump oil and grease, the concept of depleted zone within the nonaqueous phase does not apply. An average value of the overall mass transfer coefficient is reported as kw for pump oil and grease in Table 4 for each PAH. The contribution of the individual mass transfer resistances to the overall mass transfer resistance was derived as eq 7, which incorporates a depletion model for the nonaqueous phase. The overall mass transfer resistance, Ro ) 1/ko, will increase with time if a depleted zone is being formed in the nonaqueous phase material. This behavior was evident for petrolatum, paraffin, and the high-viscosity transmission oil. Figure 6 shows the relative contribution versus time for the individual mass transfer resistances (RNAP and Rw) to the VOL. 33, NO. 2, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 4. Average Values of the Overall Mass Transfer Coefficients Observed for Naphthalene, Phenanthrene, and Pyrene in Systems Containing Pump Oil and Grease NAP/compound pump oil naphthalene phenanthrene pyrene grease naphthalene phenanthrene pyrene

average kw (cm/min)

SD (cm/min)

0.008 0.012 0.007

0.003 0.005 0.009

0.013 0.023 0.005

0.002 0.003 0.002

dominate Ro after approximately 3 days. The contribution of the nonaqueous phase to the overall mass transfer resistance was eventually over 80% for petrolatum, paraffin, and transmission oil. In the case of naphthalene and petrolatum, the contribution of the nonaqueous phase resistance was more than 95% at the completion of the experiment after 370 days. A simulation was performed using the model proposed in eq 7 to assess the behavior of the dissolution of PAHs into water from petrolatum. The parameters employed in the simulation were those obtained from the fitting process shown in Tables 2 and 3. A total of 200 days were simulated. The total mass transfer resistance, Ro (1/ko), and the individual resistances of the aqueous (Rw) and nonaqueous phases (RNAP) were computed versus time. The results obtained from the simulation are presented in Figure 7. In this figure, the total mass transfer resistance (Ro ) 1/ko) is shown on the right vertical axes for the different PAH compounds, and the relative contribution (%) of the nonaqueous material to the total mass transfer resistance is shown on the left vertical axes for each PAH. Naphthalene exhibits the largest value of overall mass transfer resistance. This is a consequence of its greater solubility, which results in a greater diffusion length in the NAP with time.

Discussion of Results

FIGURE 6. Relative contribution of the aqueous and nonaqueous phases to the overall mass transfer resistance, Ro (1/ko), for petrolatum: (a) naphthalene, (b) phenanthrene, and (c) pyrene. overall mass transfer resistance (Ro) for petrolatum. Similar behaviors were observed for paraffin and transmission oil where the resistance in the nonaqueous material started to 240

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Batch Experiments. The partitioning information obtained from the batch equilibration tests was consistent with expected behavior for partitioning of PAHs into organic matrixes. The logarithm of the nonaqueous phase-water partition coefficients (log Kp) for the materials tested herein ranged from 3.56 to 4.20 for naphthalene, from 4.70 to 4.90 for phenanthrene, and from 5.16 to 5.61 for pyrene. These values were slightly higher than reported values of octanolwater partition coefficients and may reflect the greater hydrophobicity of the nonaqueous phases tested as compared to octanol. The partitioning values were consistent with the range values of octanol-water partition coefficients, and this provides some confirmation in the measured PAH equilibrium concentrations in the aqueous phase. Flow-Through Dissolution Experiments. Measurements of the overall mass transfer coefficient for the dissolution of PAHs from the nonaqueous phases into water were performed for times as long as 1 year. The decrease in the overall mass transfer coefficients observed with time for petrolatum, high-viscosity transmission oil, and paraffin supported the hypothesis established in this study. Dissolution from highviscosity nonaqueous phase materials may result in the formation of a depleted zone that increases with time as progressively more PAHs (or HOCs) are transferred to the aqueous phase. This depleted zone may then increase the diffusion length (resistance) for the mass transfer of PAHs from the nonaqueous phase to the aqueous phase. Other studies have shown that the mass transfer of PAHs from NAPLs to water is dominated by the aqueous phase (9, 17). In those studies, either the NAPLs were continuously mixed (9) or the viscosity of the NAPL was not very high (17). In this study, it was observed for the mass transfer to be nonaqueous phase-dominated that the viscosity of the nonaqueous phase must be very high, close to a semisolid material. Pump oil (86 cP at 40 °C) was not sufficiently viscous to cause a decrease with time in the overall mass transfer of PAHs from the oil into the water. A high viscosity transmission oil (∼1000 cP at 40 °C) exhibited a decrease in the release of PAHs similar to that exhibited by petrolatum and paraffin. Lubricating grease (oily semisolid) did not exhibit a decrease in the release of PAHs during the experiment (80 days). However, in this case it was observed over time that the grease separated in two phases, liquid oil and a semisolid. Lubricating greases are manufactured by dissolving a polymer

FIGURE 7. Simulation of the overall mass transfer resistance Ro vs time for the dissolution of naphthalene, phenanthrene, and pyrene from petrolatum. material (low-density polyethylene, nylon, etc.), referred to as a thickening agent, in a lubricant oil having a viscosity range similar to motor oils. This likely explains the observed constant rate of mass transfer of PAHs from the grease into the water, as the PAHs were diffusing through the more fluid oil phase contained in the grease. This was not the case for paraffin and petrolatum where the texture results from a true solid-semisolid mixture of alkane hydrocarbons (19). The overall mass transfer resistance for the release of PAHs into the aqueous phase was expressed as the combination of the mass transfer resistance in the aqueous and nonaqueous phases with a linear concentration profile representing the depleted zone for PAH in the nonaqueous materials. By fitting the proposed model to the experimental data, values for the mass transfer coefficient in the aqueous phase and the diffusivity of each PAH in the nonaqueous phase were estimated for each system tested. The estimated values of the mass transfer coefficient in the aqueous phase (kw) for the different PAHs tested ranged from 0.014 to 0.073 cm/min in systems with petrolatum, high-viscosity transmission oil, and paraffin; in systems containing pump oil and grease, where no depletion effects were observed, the values for kw ranged from 0.005 to 0.023 cm/min. Although the mass transfer coefficients are system dependent, the values of kw obtained herein are consistent with values of mass transfer coefficients observed for dissolution of PAHs to water from NAPL mixtures. Reported values are (i) 0.0480.18 cm/min for various PAHs including naphthalene, 1-methylnaphthalene, 2-ethylnaphthalene, acenaphthene, fluorene, phenanthrene, fluoranthene, and pyrene (9); (ii) 0.1 cm/min (kla ) 26.1 day-1 in a system with NAPL-water interfacial area of 9 cm2 and a system volume of 50 mL) reported by Ghoshal et al. (17) for dissolution of naphthalene in water from heptamethylnonane; and (iii) about 0.04 cm/ min for various HOCs including naphthalene and 1-methylnaphthalene (8). A typical mass transfer coefficient for chemicals diffusing into water is about 0.05 cm/min (15). In the systems with petrolatum, paraffin, and high-viscosity transmission oil, the dissolution of the PAHs was limited by the aqueous phase at the beginning of the experiment (0-3 days), and as dissolution progressed the release became limited by the mass transfer within the nonaqueous phase (after 3-30 days). The diffusivity values obtained for the PAHs in the different nonaqueous phases were on the order of 10-8-10-9 cm2/s

for naphthalene, 10-10-10-11 cm2/s for phenanthrene, and 10-11-10-12 cm2/s for pyrene. Although this experiment was not designed as a method for measuring diffusivity, the coefficients obtained herein are in the range of values for chemicals in polymer-like materials, between that for liquids and solids (15). An important assumption in the model presented in this work is that the nonaqueous phase material may be considered an infinite source for the solute, in this case the PAHs. A mass balance was performed for each PAH in order to assess the amount removed from the nonaqueous phase and to verify the validity of this assumption. The mass percent removed during each experiment for naphthalene, phenanthrene, and pyrene respectively was (i) petrolatum (370 days), 5.7%, 1.2%, and 0.4%; (ii) paraffin (190 days), 1.2%, 0.4, and 0.1%; and (iii) high-viscosity transmission oil (190 days), 6.0%, 0.8%, and 0.2%. The mass balance calculations show that the infinite source assumption may be acceptable for the duration of these experiments, as the total removal was 6% or less for naphthalene and about 1% or less for phenanthrene and pyrene. The decrease in flux due to loss of material by flushing could not account for a change in mass transfer coefficient in proportion to the change in overall mole fraction in the NAP. At most, this could only account for a 7-8% decrease in flux for naphthalene and a 1-2% decrease for phenanthrene and pyrene. These values are too small to explain the observed decrease in flux. It is also assumed that the equilibrium HOC concentration in water is unchanging even after aging has occurred. No interfacial films were observed with any of the highly viscous materials, nor for pump oil, which showed a constant flux with time, indicating no change in equilibrium concentration. Interfacial films have been observed for coal tars (10) and crude oils (11) but were not evident in this work as the NAPs comprised inert alkane compounds. It has also been observed with synthetic PAH-NAPL mixtures that the liquid phase may become more viscous and sold-like with time as certain soluble constituents are depleted. This was not the case with paraffin, heavy lubricating oil, and petrolatum. Here the bulk phase is essentially insoluble and changes little as a result of dissolution of small amounts of PAHs. The relative importance of the mass transfer resistance in the nonaqueous phase to the overall mass transfer resistance depends on the value of the partition coefficient Kp and the diffusivity of the compound in the nonaqueous VOL. 33, NO. 2, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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phase, as was shown in eq 7. Therefore, to have a dissolution process dominated by the diffusion of the HOC within the nonaqueous phase, the combined value of Kp and diffusivity in the nonaqueous phase resistance term (eq 7) needs to be large enough to overcome the value of the resistance in the aqueous phase (Rw ) 1/kw). The magnitude of Kp can be approximated by the octanol-water partition coefficient. For substantially hydrophobic compounds, the value of log Kow is greater than about 3. Assuming a typical value of the mass transfer coefficient in the aqueous phase, kw, equal to 0.05 cm/min, it is possible to estimate a diffusivity value where the mass transfer resistance in the nonaqueous phase dominates over that resistance in the aqueous phase. We consider that the nonaqueous phase is the limiting step in the dissolution process when RNAP is greater than 80% of the total mass transfer resistance, and thus RNAP needs to be about 100 min/cm if Rw is equal to 20 min/cm. Considering naphthalene (log Kow ) 3.36) and a required time of 20 days (28 800 min, see Figure 6) for the formation of a depleted zone that may account for a resistance in the nonaqueous phase greater than 80% of the overall mass transfer resistance, then from eq 7 with RNAP ) 100 min/cm, the value of the diffusivity needs to be less than 4 × 10-8 cm2/s for a nonaqueous phase-dominated dissolution process.

Acknowledgments Support for this study was provided by the U.S. Army Waterways Experiment Station, the Mexican Council for Science and Technology (CONACyT), Instituto Tecnologico y de Estudios Superiores de Monterrey (Mexico), and Deutsche Forschungsgemeinschaft (DFG, Germany).

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Received for review April 30, 1998. Revised manuscript received October 13, 1998. Accepted October 19, 1998. ES9804417