Environ. Sci. Technol. 1998, 32, 2317-2324
Bioavailability of Mixtures of PAHs Partitioned into the Micellar Phase of a Nonionic Surfactant S A U M Y E N G U H A , P E T E R R . J A F F EÄ , * A N D CATHERINE A. PETERS Department of Civil Engineering and Operations Research, Princeton University, Princeton, New Jersey 08544
Recent work has shown that a fraction of a contaminant solubilized in the micellar phase of some nonionic surfactants is directly available for biodegradation, meaning that the contaminant can be transferred directly from the core of the micelle to cell without having to transfer to the water phase first. This study extends the understanding of the bioavailability of the micellar phase for a single compound to a multicomponent system of contaminants. Biodegradation experiments were conducted with binary and ternary mixtures of naphthalene, phenanthrene, and pyrene in the presence of a nonionic surfactant, Triton X-100. A mixed bacterial culture, isolated and enriched from a PAHcontaminated soil at the Wurstsmith Air Force Base, MI, was used for the biodegradation experiments. In the absence of the surfactant and at surfactant concentrations below cmc, the multisubstrate Monod kinetics adequately simulated the biodegradation kinetics of the binary and ternary mixtures. In the multicomponent systems, as in single solute systems, the solutes in the micelle were found to be directly bioavailable, and the bioavailability of each compound in the micellar phase decreased with increasing surfactant concentration. For a given surfactant concentration, the bioavailability was higher for the lower molecular weight PAHs. There was little difference in the bioavailability of the same compound as a single solute or in different binary and ternary mixtures. To predict the bioavailability of the micellar phase substrates, a mass transfer-based model was formulated that describes the transfer of substrate from the micellar phase to the microorganisms. The predictions matched the experimental observations well, indicating the validity of the model and its potential for applications in remediation designs.
Introduction It is well-known that surfactants can increase the solubility of hydrophobic organic compounds by solubilizing them in the hydrophobic core of micelles. Numerous studies have investigated the enhanced solubilization of a hydrophobic contaminant in the presence of surfactants above their critical micelle concentrations (cmc) (1-6) and its potential to expedite pump-and-treat groundwater remediation or surfactant-enhanced bioremediation of contaminated soils and aquifer materials (7-9). The reported effects of surfactants on biodegradation can be divided into two groups: (i) empirical experimental observations, showing either inhibi* Corresponding author phone: 609-258-4653; fax: 609-258-2799; e-mail:
[email protected]. S0013-936X(97)01093-6 CCC: $15.00 Published on Web 06/27/1998
1998 American Chemical Society
tion (10) or enhancement (11-17) of biodegradation in the presence of surfactant; and (ii) systematic experimental and modeling studies, some of which do not account for the direct bioavailability of the micellar phase (18, 19) and others that take into account the micellar dynamics, biomass characteristics, and direct bioavailability of the micellar phase (20, 21). The last group of studies described above has shown that a PAH such as phenanthrene partitioned into the micellar phase of some nonionic surfactants is, to some degree, directly bioavailable (21). Direct bioavailability means that the contaminant or substrate partitioned into the micellar phase is biodegradable without having to be transferred to the dissolved phase first. A bioavailability factor (f) was defined that indicates the degree of direct bioavailability of a micellar phase substrate for a given surfactant concentration. When f ) 0, the substrate partitioned into the micellar phase may still be bioavailable, but only if it is transferred to the dissolved phase first. A value of f ) 1 indicates that the micellar phase substrate is degraded at the same rate as the dissolved substrate, and a value of f > 1 indicates that the transfer of the substrate to the cell is more effective from the micellar phase than from the dissolved phase. This direct bioavailability of the micellar phase contaminant depends on the surfactant type, surface characteristics of the biomass, surfactant concentration, and mixing conditions. The variation of the bioavailability factor with the surfactant concentration was observed for the biodegradation of phenanthrene in the presence of four different surfactants and was explained by a model describing the direct mass transfer of the micellar phase contaminant to bacteria (21). The present study applies the insight gained from single compound systems to multicomponent systems. Aqueous solutions of PAHs are dilute, and the interactions between the solutes are negligible. However, inside the micelle, the PAHs reside in an organic liquid-like environment at much higher concentrations. Interactions between the solutes in the micellar core can affect the solubilization behavior of individual compounds in the presence of others (6). The presence of multiple solutes also affects the biodegradation kinetics of the individual components (22). The solute interactions observed for solubilization and biodegradation in the above studies were incorporated to examine the bioavailability of micellar-phase compounds in multicomponent systems. The objective of this research was to (i) obtain experimental results for the direct bioavailability (i.e., the bioavailability factor) of binary and ternary mixtures of PAHs, solubilized in the micellar phase of a nonionic surfactant, Triton X-100; (ii) formulate a generalized mathematical model that describes the multicomponent mass transfer and biodegradation, both from the aqueous phase and the micellar phase; (iii) evaluate the performance of the model for prediction of the bioavailability factor for a given surfactant and contaminant by comparing values of the bioavailability factor obtained from experimental observations and predicted theoretically. The model formulation is similar to that described by Guha and Jaffe´ (21) for the single compound biodegradation. In the present study the masstransfer model parameters were estimated theoretically, and the model was used only to estimate values of the bioavailability factor. The final goal was to formulate a model that may facilitate the optimization of surfactant-enhanced bioremediation schemes of a system contaminated with multiple PAHs. VOL. 32, NO. 15, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Materials and Methods Materials. Biomass. An enrichment culture was maintained in the laboratory, which was isolated from a PAH-contaminated soil sample at the Fire Training Area 02 (FTA-02) of the Wurtsmith Air Force Base, MI. The culture was periodically replenished with fresh nutrient solution (BOD dilution water prepared from from the pillows obtained from HACH company) and a mixture of naphthalene, phenanthrene, and pyrene dissolved in methanol. Prior to the experiments, the culture was washed 5 times with ∼1000 mg/L of Brij 35 followed by three washes with BOD dilution water so that the Brij 35 was reduced below detection. Before commencing the experiments, the culture was starved for 24 h. Details of the biomass maintenance and inoculum preparation are given elsewhere (22). Contaminant. The PAHs used in this study were naphthalene, phenanthrene, and pyrene. All the compounds were obtained from Aldrich Chemical Company. The purity of the compounds were 99+%, 98%, and 99% for naphthalene, phenanthrene, and pyrene, respectively. They were used without further purification. Surfactant. A polydispersive synthetic nonionic surfactant, Triton X-100 (octylphenol polyoxyethylene), was obtained from the Aldrich Chemical Company. The solution contained cmc 0 for S e cmc
}
(6)
where S is the total concentration of surfactant in aqueous solution (mg/L), cmc is the critical micelle concentration of surfactant (mg/L), Cbio,i is the bioavailable concentration of solute i ) Ci + fiCmc,i (mg/L of carbon), Cmc,i is the concentration of solute i in micellar phase expressed as an equivalent concentration in solution (mg/L of carbon), Ci is the concentration of solute i in aqueous phase (mg/L of carbon), fi is the bioavailability factor for component i (dimensionless), Ks,i is the half saturation constant for solute i (mg/L), Yi is the yield coefficient for component i (dimensionless), bendo is the first-order endogenous respiration coefficient (h-1), Kbms,i is the partition coefficient of the component i onto biomass (L/mg), Kdg,i is the partition coefficient of component i onto reaction vessel (dimension* is the less), kwg,i is the volatilization rate parameter (h-1), KH volatilization equilibrium parameter (dimensionless), Cg,i is the concentration in gas phase expressed as equivalent concentration in the aqueous phase (mg/L of carbon), fw is the fractional volume of the experimental setup occupied by the liquid phase (dimensionless), fg is the fractional volume of the experimental setup occupied by the gas phase (dimensionless), and X is the biomass concentration as carbon (mg/L). These equations are identical to the mass-balance equations used to analyze the experimental observations in single substrate experiments (20, 21), except that the biodegradation kinetics describe multisubstrate effects through the multisubstrate Monod kinetics (22) and the mass balance of the biomass allows for growth on multiple substrates. The partition and biological parameters were estimated from independent batch experiments and are listed in Table 1 (22). Values of the micelle-water partition coefficients of the PAHs studied vary significantly, depending on the concentrations of other components present in the mixture
where
TABLE 1. Partition and Biological Parameter Values parameters
naphthalene
phenanthrene
pyrene
Kdg Kbms (L/mg) kwg (h) K* H fw fg bendo (h-1) µmax (h-1) Ks (mg/L) Y
-a 0.002 0.5 0.53 0.47 0.0039 0.23 23.75 0.485
0.0914 0.01 1.0 0.53 0.47 0.0039 0.037 0.8 0.497
0.191 0.015 3.0 0.53 0.47 0.0039 0.8 × 10-3 0.11 0.502
a
ms,i )
hs,iab
(10)
f cFbνb
where f c is the weight fraction of carbon in a cell (dimensionless), νb is volume of one cell (L3); Fb is the bulk density of biomass (M/L3). If the rate of biodegradation is completely controlled by this mass-transfer process, this mass-transfer rate can be related to a specific growth rate of µmt,i which in turn can be related to the substrate concentration via the yield coefficient (21, 24):
-, experiments did not show any significant positive values.
(6). Table 2 shows values of these partition coefficients in different binary and ternary mixtures. The above mass-balance equations were solved and fitted to the experimental observations using an Euler backward time-stepping scheme and Picard iteration to obtain the bioavailability factors. The bioavailability factors (f) were the only unknown parameters in this system of equations, and a unique value of f was obtained for each set of experimental conditions. The bioavailability factors were obtained by minimizing the following squared sum of error (θ): n
θ)
N
∑∑ i)1 j)1
(
)
Ci,j - C ˆ i,j Ci,j
2
(7)
where C ˆ i,j is the predicted concentration of the ith component in the mixture at time j and Ci,j represents the observed value of the same; n is the number of components in the mixture and N is the number of observations. The optimization problem was solved using the Broyden-Fletcher-GoldfrabShanno algorithm to obtain the fi values that minimize θ. The values of the bioavailability factor obtained in this manner were compared to theoretically computed values as described in the following section.
Model of the Bioavailability Factor Biodegradation of Aqueous Phase Compounds. The aqueous solubilities of PAHs like naphthalene, phenanthrene, and pyrene are very low. In an aqueous solution saturated with these three PAHs, the combined solute volume fraction is about 0.005%. The aqueous solution can be treated as dilute and the mass-transfer of one is not affected by the concentration of the other. In a completely mixed system, the rate of mass transfer of compound i from the bulk aqueous phase to one cell can be expressed as (24):
Ji ) hs,i(Ci - C0,i)ab
(8)
where hs,i is the overall mass transfer coefficient that includes mass transfer through the diffusive layer around the cell and through the cell membrane (L/T), ab is the surface area of a cell (L2), Ci is the concentration of contaminant in the bulk solution (M/L3), and C0,i is the concentration of contaminant at the location where biodegradation is occurring (M/L3). During active biodegradation, the concentration C0,i can be assumed to be negligible as compared to the concentration in the bulk solution, Ci. The rate of change of solute concentration due to net mass transfer of the contaminant to the cells, for a biomass concentration of X, can then be written as
-
dCi ) ms,iXCi dt
(9)
(dX)i Yi ) ) dCi
(dX dt )
i
( ) dCi dt
)
µmt,iX ms,iXCi
(11)
where (dX)i is the differential biomass growth on substrate i. This gives the specific growth rate as
µmt,i ) ms,iYiCi
(12)
where Yi is the microbial yield coefficient for the component i Once the contaminant has reached the site of biodegradation, the enzyme reaction is assumed to proceed at a constant rate depending upon the enzyme concentration. This is given by the maximum specific growth rate for single substrate (21, 24). In a mixture, the maximum growth rate due to one substrate will be affected by the concentration of the other substrates. The growth rate due to the ith substrate in a mixture was assumed to be zero order with respect to the ith substrate and inversely proportional to the concentration of the other substrates:
µmax,i
µm,i )
j)n
1+
∑
βij
j)1,j*i
(13)
C0,j C0,i
A similar expression can be obtained from the multicomponent Michaelis-Menten enzyme kinetics (25) by employing the assumption that the half-saturation constant is much smaller than the substrate concentration (Ks,i , C0,i). We make a further assumption that the ratio of the concentrations (C0,j/C0,i) inside the cells are proportional to the ratio of concentrations in the bulk (Cj/Ci). The above equation then becomes
µmax,i
µm,i )
i)n
1+
∑
j)1,j*i
β*ij
(14)
Cj Ci
where βij and β*ij are binary substrate interaction coefficients. The inverse of the specific growth rates given by eqs 12 and 14 can be viewed as resistances for the respective processes. When the mass transfer of substrate from the bulk solution into the cell and the degradation within the cell both control the overall rate of biodegradation, the observed rate of biodegradation (µi) is the equivalent resistance of the above two resistances in series (21): VOL. 32, NO. 15, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Micellar Partition Coefficients at Different Experimental Conditions Kmc,i (L/mg)
description of the experiments
naphthalene ) 0.0025 ( 0.00005 phenanthrene ) 0.023 ( 0.0023 pyrene ) 0.148 ( 0.016 naphthalene ) 0.0019 ( 0.0002 phenanthrene ) 0.044 ( 0.004 phenanthrene and pyrene, each present in excess of its apparent solubility limit phenanthrene ) 0.041 ( 0.005 pyrene ) 0.132 ( 0.012 naphthalene and pyrene, each present in excess of its apparent solubility limit naphthalene ) 0.0017 ( 0.00045 pyrene ) 0.126 ( 0.013 naphthalene, phenanthrene, and pyrene, each present in excess of its apparent solubility limit naphthalene ) 0.0022 ( 0.00013 phenanthrene ) 0.04 ( 0.003 pyrene ) 0.18 ( 0.025 phenanthrene, present in excess of its solubility limit and 8.41 ( 0.49 mg/L of naphthalene in solution phenanthrene ) 0.027 ( 0.004 phenanthrene, present in excess of its solubility limit and 19.28 ( 1.5 mg/L of naphthalene in solution phenanthrene ) 0.032 ( 0.005 pyrene, present in excess of its solubility limit and 2.44 ( 0.66 mg/L of naphthalene in solution pyrene ) 0.105 ( 0.016 pyrene, present in excess of its solubility limit and 7.82 ( 0.71 mg/L of naphthalene in solution pyrene ) 0.105 ( 0.013 pyrene, present in excess of its solubility limit and 18.71 ( 0.95 mg/L of naphthalene in solution pyrene ) 0.106 ( 0.014
naphthalene, present in excess of its apparent solubility limit phenanthrene, present in excess of its apparent solubility limit pyrene, present in excess of its apparent solubility limit naphthalene and phenanthrene, each present in excess of its apparent solubility limit
1 1 1 ) + µi µmt,i µm,i
(15)
From eqs 12, 14, and 15 we obtain
µi )
µmax,iCi µmax,i ms,iYi
(16)
j)n
+ Ci +
∑
β*j Cj
j)1,j*i
where βa is the capture efficiency that depends on the steric repulsion between the micelles and the monolayer around the cell (dimensionless), γ˘ is mixing shear rate (T-1), Rij is the collision radius ) ai + aj ) sum of the radii of the particles i and j (L), nmic ) SmcNA/φsNagg is the number concentration of particles of radius ai, micelle in this case (no./L3), and J is the rate of collision (no./T). The rate of collision (J) can be related to the rate of change of the concentration of contaminant i due to mass transfer to nbac number of cells by the transport of the micelle, as follows:
By comparing eq 4 and eq 16, one obtains the following equivalencies:
Ks,i )
µmax,i Ks,j and β*j ) ms,iYi Ks,i
(17)
The maximum specific growth rate (µmax,i), the apparent halfsaturation constants (Ks,i), and the biomass yield coefficient (Yi) were estimated from single-component biodegradation experiments for each of the three compounds studied here (22). The mass-transfer coefficients ms,i were then computed for the individual compounds using these values (µmax,i, Ks,i, and Yi). Biodegradation of the Micellar Phase Components. The direct biodegradation of a micellar phase substrate is assumed to involve the following steps (21): (a) Transport of the micellar-phase substrate to the direct proximity of the cell. (b) Breakdown of the micelle in the proximity of the cell. This is represented by a probability factor that is the ratio of the diffusion relaxation time to the micellar breakdown relaxation time. The product of this probability factor and the flux due to step a yields a net flux of substrate from the micellar phase in the bulk solution to the direct proximity of the cell surface. (c) The diffusion of the substrate released due to steps a and b above, into the cell. (d) Biodegradation of the substrate within the cell. The mass transfer of solute in the micellar phase involves two resistances in series. The first one is due to steps a and b, and the second one is due to step c described above. Mass Transfer of Solute in Micellar Phase from the Bulk Solution to the Cell Surface. The rate of collision between micelles and a cell can be expressed based on the analysis by Smoluchowski (27) and expanded and verified by several authors (28-33):
J) 2320
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32 β γ˘ R 3n 3 a ij i
(18)
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 32, NO. 15, 1998
dCi JNaggKnc,iCi ) ηsnbac dt NA
-
(19)
where nbac ) X/f cFbνb is the number concentration of cells (no./L3), Nagg is the aggregation number of the micelles (no.), NA is Avogadro number (no./mol), and ηs is the conversion factor for surfactant (M/mol). We obtain the rate of change of contaminant concentration (Ci) in the bulk aqueous phase due to transport of micelles to a biomass concentration of X, as follows:
-
dCi ) mmicSmcKmc,iCiX dt
(20)
where
mmic )
3 32 Raγ˘ Rij 3 f cF ν b b
(21)
The mass transfer of contaminant i due to steps a and b is the transport of the contaminant solubilized in micelles from the bulk solution to the cell multiplied by the probability of micellar breakdown. Therefore, the rate of change of contaminant concentration in the bulk solution due to steps a and b can be obtained by multiplying eq 20 by the probability factor of the micellar breakdown:
-
dCi ) mmicPrKmc,iSmcCiX dt
(22)
where Pr is the probability factor for the micellar breakdown (dimensionless):
Pr )
∆2 2Dmτ
(23)
where ∆ is the average diameter of the micelles (L), Dm is the
diffusion coefficient of the micelle (L2/T), and τ is the relaxation time for micellar breakdown (T). The relaxation time is related to the surfactant concentration as follows (21, 34-37):
resistances in series, which is given by the sum of the inverses of eqs 25 and 28. The specific growth rate for the component i due to the mass transfer resistance of the micellar phase can then be obtained as follows:
Smc 1 )a+b τ cmc
1 1 1 ) break + mc µc,i µmt,i µmc,i
(24)
where a and b are the rate parameters relating the forward and backward reaction rate of the monomer-micelle equilibria. For Triton X-100, the following parameter values have been reported: a ) 2 × 103/s (37), b ) 104/s (37) Dm ) 7 × 10-7 cm2/s (38), ∆ ) 10 nm (38). The above mass transfer involves the transport of the micelle as a whole and is unlikely to be affected by the interaction between the solutes present inside the micelle. The limiting specific growth rate due to this mass transfer step can then be written as follows (21):
µbreak ) mmicYiPrKmc,iSmcCi mc
(25)
where µbreak is the limiting specific growth rate due to mc micellar transport and breakdown (1/T). Transfer of Solute from the Admicelle to the Cell. Once the micelle breaks down in the vicinity of a cell, it may adsorb to the cell surface forming an admicelle. The next step (c) is then the diffusion of contaminant from the admicelle into the cell. This step is similar to that described for the transport of the water-phase contaminant in eq 1 except that the contaminant inside the admicelles exists at a much higher concentration than in the aqueous phase, and likely even higher than its solubility in water (6). This mass transfer rate can be expressed as follows:
-
dCi dt
Cmic,j
eff ) hc,i
n
νsurfχsSmc +
∑ν χ K
k k mc,kSmcCk
X (φab) ) f cFbνb
k)1
mc,iKmc,iCiX (26)
After some algebraic manipulations, we obtain an expression mc similar to eq 9 as for µmt,i
( ) ( )
mc µmt,i ) ms,iYi
mc,i P ms,i r
mc,i + PrSmc mmic
mc,i )
fi )
n
(χsνsurf +
∑χ ν K
k k mc,kCk)f
c
Fbνb
eff hc,i is an effective mass transfer coefficient in the multicomponent mixture, νsurf is the molar volume of the alkyl chain of a surfactant molecule (L3/mol), νk is the molar volume of the solubilizate k (L3/mol), χs and χk are the conversion factors for the surfactant and contaminants, respectively (mol/M), and φ is the fraction of the biomass surface covered by monolayer. The term in parentheses in the denominator is the volume of the micellar core per unit volume of solution. This is needed to convert Cmc,i to the local concentration in the core of the micelle. A multicomponent form of Fick’s law can be written explicitly and the mass transfer coefficient related to the multicomponent diffusion matrix that considers solute interaction (39). Due to the lack of data, we estimated the mass transfer coefficient from the infinite dilution diffusion coefficients. The limiting specific growth rate due to this step is given by
µc,i ) mc,iYiKmc,iCi
(28)
The net resistance due to mass transfer from the micellar phase to the cell is the equivalent resistance provided by two
(31)
mc,i + PrSmc mmic
total mc µmt,i ) µmt,i + µmt,i ) ms,iYi(Ci + fiCmc,i) ) ms,iYiCbio,i
(32) where Cbio,i is the total bioavailable concentration (M/L3). The specific growth rate due to the enzyme reaction kinetics of component i for a bioavailable concentration of Cbio,i can be obtained from eq 14 by replacing Ci with Cbio,i. The total resistances for complete biodegradation of component i will be given by the equivalent resistance provided by the two resistances in series, one due to total mass transfer resistance (eq 32) and the other due to the enzyme reaction (µm,i). The net observed specific growth rate for component i can then be written as
µmax,iCbio,i µmax,i ms,iYi
k)1
mc,i P ms,i r
The total mass transfer resistance for the component i will be the equivalent resistance of two parallel resistances given by the sum of eqs 12 and 30:
µi ) (27)
Kmc,iSmcCi ) ms,iYifiCmc,i (30)
where the bioavailability factor fi is given by
where eff φab hc,i
(29)
(33)
n
+ Cbio,i +
∑
R*jCbio,j
j)1,j*i
Equation 33 is analogous to the multicomponent Monod kinetics in the absence of surfactant (eq 16) with Ci replaced by Cbio,i.
Results and Discussion Biodegradation below the Critical Micelle Concentration. To determine that the surfactant is not toxic to the bacteria, biodegradation experiments were conducted in the absence of surfactant and at a surfactant concentration just below cmc with three binary mixtures and a ternary mixture of naphthalene, phenanthrene, and pyrene. The simulation of biodegradation dynamics was done by solving eqs 1-3 with the parameters shown in Table 1, which were estimated from independent batch experiments. At surfactant concentrations below or near cmc, the bioavailable concentration (Cbio,i) is equal to the aqueous concentration (Ci), since the concentration of the micellar phase (Smc) is negligible. The presence of the surfactant below cmc did not affect the rates of biodegradation, and the simulations predicted the biodegradation dynamics accurately. This was also observed for all three binary mixtures. These observations lead to the following assertions: (a) the estimated model parameter values are robust; (b) the surfactant is not toxic to the bacteria VOL. 32, NO. 15, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Biodegradation of naphthalene in the ternary mixture in the absence of surfactant and in the presence of Triton X-100 at 116 mg/L.
FIGURE 1. Biodegradation of phenanthrene in a binary mixture of naphthalene and phenanthrene at two different surfactant concentrations. Initial biomass concentration, 1.2 mg/L carbon. Initial total concentrations are 16.05 and 0.81 mg/L for naphthalene and phenanthrene, respectively. (a) 251 mg/L of Triton X-100, (b) 1004 mg/L of Triton X-100. Symbols are experimental values, and lines are predicted concentrations. The error bars are the pooled standard errors estimates from duplicate samples and are in some cases smaller than the symbols. and does not alter the biodegradation rate parameters. However, this latter statement does not rule out the possibility of cell lysing at very high surfactant concentration (>1%) by solubilizing the cell membrane lipids (40). Experiments with surfactant concentrations above cmc were restricted to a maximum surfactant concentration of about 0.1%. Biodegradation above Critical Micelle Concentration. The kinetics of biodegradation of binary and ternary mixtures of naphthalene, phenanthrene, and pyrene were studied in the presence of four different concentrations of surfactant above cmc, ∼2.5 × cmc, ∼6 × cmc, ∼12 × cmc, and ∼25 × cmc. Figure 1 shows results of the biodegradation of phenanthrene in a binary mixture of naphthalene and phenanthrene. The key observation is a slower rate of biodegradation with increasing concentration of surfactant beyond cmc. This was observed in all binary and ternary mixtures. Similar observations were reported for the degradation rate of phenanthrene in the presence of four different nonionic surfactants above their cmc (20, 21). The results further demonstrate that the experimental observations can be simulated adequately by selecting an appropriate bioavailability factor for each substrate/surfactant combination. Similar fitting of the bioavailability factor was done for all other binary and ternary mixtures. Figure 2 shows the biodegradation of naphthalene in the ternary mixture in the absence of a surfactant and in the presence of surfactant at 2.5 × cmc. For these experimental conditions, the addition of the surfactant at 2.5 × cmc increased the degradation rate of naphthalene as compared to that in the absence of the surfactant. This is because the 2322
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FIGURE 3. Variation of the bioavailability factor with the surfactant concentration for naphthalene. Symbols are the values obtained by fitting the experimental data. The line is theoretically predicted.
FIGURE 4. Variation of the bioavailability factor with the surfactant concentration for phenanthrene. Symbols are the values obtained by fitting the experimental data. The line is theoretically predicted. bioavailability factor for naphthalene in the micellar phase of Triton X-100 at that surfactant concentration is larger than 1, as can be seen in Figure 3. The Bioavailability Factor. Figures 3-5 show the bioavailability factor obtained by fitting the biodegradation model to the experimental observations (symbols) for each of the three compounds. Also shown are the theoretically
concentration of the micellar phase solubility and the micellar phase bioavailability indicates that the net bioavailable contaminant concentration has a maximum value along the surfactant concentration axis. The model proposed here could be useful to predict the optimum surfactant concentration based on different criteria, which may be the maximum degradation rate of a target component in a mixture or the maximum disappearance rate of a group of compounds.
Acknowledgments
FIGURE 5. Variation of the bioavailability factor with the surfactant concentration for pyrene. Symbols are the values obtained by fitting the experimental data. The line is theoretically predicted.
TABLE 3. Estimated Mass-Transfer Rate Coefficients mass-transfer rate coefficient (L mg-1 h-1) compound naphthalene phenanthrene pyrene Triton X-100
mmic
ms
mc
0.02 0.09 0.01
12 11 5
6
predicted values of the bioavailability factor (lines) obtained by evaluating eq 31 using the proper surfactant parameters for Triton X-100 given above and the estimated mass transfer coefficients (see Supporting Information) summarized in Table 3. The following observations can be made: (a) for all compounds, once the surfactant concentration is slightly above cmc, the micellar-phase substrate bioavailability is inversely related to the surfactant concentration; (b) the values of the bioavailability factor vary substantially between the different compounds (note the difference in y-axis scale) but are rather similar for the same compound in different mixtures; (c) the bioavailability factor at any surfactant concentration is higher for the lower molecular weight compounds; (d) the mass transfer model predicts the bioavailability factor well, regardless of the fact that the mass transfer parameters are only rough approximations. For a very small surfactant concentration range, just above cmc, the theoretical estimates of the bioavailability factor indicate an increase with the surfactant concentration. This is because Pr is proportional to the micellar phase surfactant concentration, Smc, which makes eq 31 non-monotonic. This concentration range is too narrow to verify the trend in the bioavailability factor experimentally. The fact that the bioavailability factor seems to remain constant for a given PAH-surfactant combination, either if it is a single compound solution or a in multicomponent solution, greatly facilitates the design of complex multi-PAH surfactant-enhanced bioremediation schemes. In the absence of toxic effects, appropriate mass transferbased models are capable of predicting a surfactant’s effect on the biodegradation of low solubility substrates, based on physical properties of the substrate, surfactant, and biomass. An approximate estimate of the physical properties based on molecular information may be sufficient for such predictions. If properly used, such models may not replace but minimize time consuming experimental evaluations. A similar mass transfer-based model as described here for single compound biodegradation in the presence of a surfactant has been used to simulate the performance of a soil-slurry reactor (41). The opposing trend with surfactant
This research was funded in part by the National Center for Integrated Bioremediation Research and Development (NCIBRD) through the Department of Defense Strategic Environment Research and Development Program (SERDP) under Cooperative Agreement CR822922 by the U.S. Environmental Protection Agency and by a grant from the EPA Northeast Hazardous Substance Research Center, Subcontract 9-92610 (R-64). The content of this publication does not necessarily represent the views of any of these agencies.
Supporting Information Available Additional information on the estimated mass transfer coefficients and references (5 pages). Ordering information is given on any current masthead page.
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Received for review December 18, 1997. Revised manuscript received May 4, 1998. Accepted May 12, 1998. ES971093W
