Environ. Sci. Technol. 2001, 35, 3877-3883
Effects of Nonionic Surfactants on Bacterial Transport through Porous Media D E R I C K G . B R O W N A N D P E T E R R . J A F F EÄ * Department of Civil and Environmental Engineering, Princeton University, Princeton, New Jersey 08544
Nonionic surfactants of the form CxEy, where x is the number of carbons in the alkyl chain and y is the number of ethylene oxide units in the polyoxyethylene (POE) chain, were studied for their ability to alter the transport of Sphingomonas pacilimobilis through an aquifer sand. The surfactants C12E4 (Brij 30) and C12E23 (Brij 35) were the focus of this study. Through a systematic study, it was shown that these nonionic surfactants were able to enhance the transport of this bacterial culture through porous media. The magnitude of the enhancement increased with decreasing solution ionic strength and increasing POE chain length. The mechanism of this enhanced transport appears to be due to expansion of the electric double layer about the bacteria and aquifer sand through displacement of the counterions by the sorbed surfactant. This expanded electric double layer increases the electrostatic repulsion, with a resultant reduction in the collision efficiency and an increase in the Langmuirian blocking parameter. Application of the colloid filtration theory with the experimental parameters of this study shows that nonionic surfactants have the potential to significantly enhance the bacterial travel distance, especially for low ionic strength systems.
Introduction Background. The introduction of surfactants to groundwater has the potential to alter the transport of bacteria through porous media. Surfactants readily adsorb onto bacteria and aquifer sands (1). Due to their surface-active properties, the sorbed surfactant can affect bacterial attachment by altering the electrostatic, hydrophobic, and steric interactions. If this results in an increase in the repulsion between the sand grains and the bacteria, the surfactants will enhance the transport of bacteria. Environmental engineers are interested in the potential for surfactants to enhance bacterial transport for two reasons. The first is related to the bioaugmentation of subsurface contaminants. Bacteria readily adhere to porous media, and development of a means to disperse the bacteria is an important consideration when attempting bioaugmentation of contaminated groundwater (2). Surfactants that have the potential to inhibit bacterial attachment onto the porous media would serve to reduce biofouling and enhance the transport of bacteria into a contaminant plume. This may allow bioaugmentation to remain a viable option for biodegrading subsurface contaminants when unfavorable bacterial transport conditions exist. * Corresponding author phone: (609)258-4653; fax: (609)258-2799; e-mail:
[email protected]. 10.1021/es010577w CCC: $20.00 Published on Web 08/24/2001
2001 American Chemical Society
The second reason is the potential threat to public health by surfactant-facilitated transport of pathogenic microorganisms from septic tanks and other waste streams into potable aquifers. Synthetic surfactants from commercial detergents are the major man-made organic contaminant in raw sewage wastewaters and are of concern due to the results of recent studies that have found commercial surfactants in measurable quantities in groundwater aquifers (3). The presence of surfactants in domestic waste is important based on the fact that the dominant source of pathogenic microorganisms is human and animal feces (4) and that the majority of drinking water disease outbreaks between 1972 and 1998 associated with microbial pathogens have been due to consumption of untreated groundwater (4-7). By having a better understanding of how surfactants affect bacterial transport, we may gain an appreciation of how commercial detergents in waste streams may affect the transmission of diseases through groundwater aquifers. While there has been a fairly extensive study on the transport of bacteria through porous media, there is very little data on the effects of surfactants on bacterial transport. A few studies have shown that ionic surfactants can alter bacterial and colloidal transport through surface charge modification by the charged surfactants (8-12). Biosurfactants are beginning to be studied for their potential in enhancing bacterial transport (13). And preliminary studies have shown that nonionic surfactants can enhance bacterial transport (11, 14, 15). While the mechanism for enhanced bacterial transport with charged surfactants is understood, the mechanisms for enhanced bacterial transport with nonionic surfactants are still unknown. It is the purpose of this paper to identify the mechanisms for enhanced transport of bacteria through an aquifer sand with nonionic surfactants. The approach focused on a systematic examination of the effects of nonionic surfactant structure and solution ionic strength on the transport of bacteria through sand columns. The results of these bacterial transport experiments were interpreted through bacterial surface property measurements (16) and through application of the colloid filtration and DLVO theories, which are discussed in the following section.
Theory The colloid (clean-bed) filtration theory was developed to understand and predict colloidal removal in packed beds (17). It assumes a constant rate of removal of the form
∂C ) -λ ∂x
(1)
where C is the colloid concentration and λ is the filter coefficient. Solution of eq 1 gives the standard form of the colloid filtration theory
C ) exp[-λL] Co
(2)
where L is the thickness of the packed bed, C is the colloid concentration exiting the packed bed, and Co is the initial colloid concentration entering the packed bed. The filter coefficient is dependent on the porous media, colloid, and fluid properties and the fluid flow conditions. It has been defined as (17)
λ)
3(1 - θ) Rη 2 dc
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where dc is the diameter of the collectors (porous media grains), θ is the porosity, R is the collision efficiency, and η is the collector efficiency. The collector efficiency describes the approach of the colloids to the collector surface, and the collision efficiency describes the attachment of the colloids to the collector. When performing bacterial or colloidal transport experiments, the collision efficiency is typically the empirical parameter being measured, while the collector efficiency is determined from theoretical calculations, described below. A number of expressions for the collector efficiency have been developed, and an excellent discussion of the collector efficiency is given by Logan et al. (18). The most widely used expression was developed using the Happel flow model (19) by Rajagopalan and Tien (20-22) and is written as -2/3 15/8 -0.4 η ) 4A1/3 + AsN1/8 + 0.00338AsN1.2 s NPe Lo NR G NR
(4)
where NPe, NLo, NR, and NG are dimensionless numbers accounting for colloid-collector collisions due to diffusion, London-van der Waals interactions, interception, and sedimentation, respectively. The term As accounts for the influence of neighboring collectors on the flow and is written
As )
2(1 - γ5) 2 - 3γ - 3γ5 - 2γ6
(5)
where
counting for London-van der Waals interactions is written
NLo )
4H 9πµdp2U
where H is the Hamaker constant. When considering bacterial transport, a number of parameters must be estimated in order to calculate the collector efficiency. These are the bacteria diameter, density, and Hamaker coefficient. For nonspherical bacteria, the equivalent diameter can be estimated by comparing the projected area of the bacteria to the area of an equivalent circle (24-26). The density of bacteria is typically very close to that of water (24-32), so the sedimentation term in the colloid filtration theory is very small (NG ∼ 0). Because of this, the bacterial density can be approximated to be that of water (28, 29), with very little resultant error. Finally, since the Hamaker constant is very difficult, if not impossible, to determine for bacteria, a value of 10-20 J is often used as an approximate value (24-26, 31, 32). Now, it should be noted that the form of eq 2 is for a clean-bed, i.e., the colloids already attached to the porous media do not interact with the aqueous colloids approaching and attaching to the porous media. When enough particles are attached to the porous media that they interact with the particles approaching the surface, the filter coefficient will deviate from the logarithmic relationship of eq 1. This suggests that the filter coefficient should be written as (22)
λ ) λoF(σ) γ ) (1 - θ)3
(6)
The diffusion term is given by the Peclet number
NPe )
Udc Dp
(7)
where U is the approach velocity (specific flow or Darcy velocity), and Dp is the diffusion coefficient for the colloidal particles. The diffusion coefficient can be calculated from the Stokes-Einstein eq 23
Dp )
where λo is the filter coefficient given by eq 3, F(σ) is the correction factor for the deviation from the logarithmic relationship, and σ can either be defined as the specific deposit of colloids on the porous media (volume of particles deposited per unit filter volume) (22) or the fractional collector surface coverage (projected area of deposited colloids per unit surface area of collectors) (33-35). When eq 13 is used, the filter effluent will change as a function of time. The filtration rate can then be described as (22)
∂σ ) UλoF(σ)C ∂τ
(8) τ)t-
where k is the Boltzmann constant, T is the absolute temperature, µ is the absolute viscosity, and dp is the diameter of the colloidal particle. The interception and sedimentation terms are defined as
dp dc
(9)
(14)
Up U
(10)
∫ Uθ dx x
0
(15)
where t is time. A Langmuirian form of F(σ) has been applied to colloidal particles when there are repulsive forces between the colloids (22)
F(σ) ) 1 - βσ
(16)
where β is the blocking parameter. This form of the correction factor has been successfully applied to bacteria (35). Solution of eq 14 using this blocking expression has been solved and can be expressed as (22)
and
NG )
(13)
where τ is a corrected time, defined as
kT 3πµdp
NP )
(12)
exp(UλoCoβτ) C ) Co exp(λoL) + exp(UλoCoβτ) - 1
(17)
The particle settling velocity, Up, is defined as 2
Up )
g(Fp - Ff)dp 18µ
(11)
Here, Fp and Ff are the particle and fluid densities, respectively, and g is the gravitational constant. Finally, the term ac3878
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When either β or τ is zero (i.e., clean-bed conditions), eq 17 reduces to eq 2. It has been observed experimentally that bacterial transport is strongly influenced by the ionic strength of the system (i.e., R and β in the Colloid Filtration Theory are a function of the ionic strength), indicating that electrostatic interactions play an important role in bacterial attachment to surfaces
TABLE 2. Experimental and Colloid Filtration Theory Parameters FIGURE 1. Surfactants used in this research consist of an alkyl chain (hydrophobic moiety) and a polyoxyethylene chain (hydrophilic moiety).
TABLE 1. Surfactants Used in This Study Are Straight-Chained Polyoxyethylene Alkyl Ethers (See Figure 1) with 12 Carbons in the Alkyl Chain and Varying Polyoxyethylene (POE) Chain Lengtha alkyl chain av POE MW CMC max. POE surfactant length (x) chain (y) (g/mol) (mg/L) chain length (Å) Brij 30 Brij 35
12 12
4 23
362 1198
10.6 39.6
(36, 37). These electrostatic interactions are typically interpreted through the DLVO theory, which considers the balance between the attractive London-van der Waals forces and the repulsive electrostatic interactions between two interacting surfaces (38). The DLVO theory is based on the concept of an electric double layer, which is a layer of counterions that forms around a charged surface. When two surfaces having charges of the same sign are sufficiently close such that the electric double layers interact, a repulsive force arises. This is the case for bacteria interactions with sand grains, as bacteria (39) and typical aquifer materials (40) both have net negative charges at pH values found in typical groundwater systems. The thickness of the electric double layer surrounding a charged surface is given by the Debeye length (κ-1) (41)
x
okT Ie2
(18)
where I is the ionic strength, e is the electron charge, is the relative permittivity, o is the permittivity of free space, k is Boltzmann’s constant, and T is the absolute temperature. For aqueous systems at 20 °C, this can be written as
κ-1 )
0.301 nm xI
value
Actual temperature 20 °C mean sand grain diameter 231 µm column length 13.1 cm column diameter 2.5 cm porosity 0.49 flow rate 10 mL/h (∼1/3 pore vol/h) Estimated bacteria effective diameter 1 µm bacteria density 1 g/cm3 Hamaker constant 10-20 J
24 138
a The maximum possible length of the POE chain assumes that it was fully extended (38).
κ-1 )
description
(19)
where the ionic strength is given in units of mol/L. The Debeye is inversely related to the ionic strength of the system. Thus, as the ionic strength increases, the Debeye length decreases, which in turn reduces the electrostatic repulsion between the two interacting surfaces. For the case of bacterial transport, bacterial attachment increases, and thus transport decreases, as the ionic strength of the system is increased.
Methods and Materials The surfactants used in these experiments consist of an alkyl chain and a polyoxyethylene (POE) chain, as shown in Figure 1. The surfactant series utilized for these experiments consists of a fixed alkyl chain length (12 carbons) and POE chains of 4 and 23 units (Table 1). These surfactants are designated CxEy, where x is the number of carbons in the alkyl chain and y is the number of ethylene oxide (EO) units in the POE chain. The surfactants were obtained from Aldrich and were used as received without any further processing. The critical micelle concentrations (CMC) of the surfactants were determined previously from surface tension measurements (42).
The bacterium used in these experiments was a Sphingomonas sp., which was isolated from a mixed culture that is capable of degrading polycyclic aromatic hydrocarbons (42, 43). This species is an aerobic, rod-shaped bacterium, with approximate dimensions of 2 µm × 0.5 µm. The bacteria is moderately hydrophobic, with a 1-bromonaphthalene contact angle of 37 degrees, and it is negatively charged at neutral pH, with an electrophoretic mobility ranging from approximately -0.7 to -0.2 µm-cm/V-s under the solution conditions used in these experiments (16). The bacteria were maintained on slants of R2A agar (Difco) at 4 °C (44). For each experiment, bacteria were removed from the slants, plated on Petri dishes with R2A agar, and allowed to grow for 5 days at 20 °C. They were then harvested and suspended in a baseline solution of CaCl2 at a specified ionic strength and a pH of 7 (adjusted with NaOH). CaCl2 was chosen as the salt due to its capabilities in stabilizing the bacterial cell wall to the effects of surfactants (i.e., increasing the cell wall permeability and removal of cell surface proteins) (45). The bacteria solution was placed on a magnetic stirrer and allowed to mix at room temperature for a minimum of 1 h. The solution was then centrifuged at 1500g for 30 min, decanted, resuspended in identical CaCl2 solution, and placed on a magnetic stirrer at 20 °C to mix overnight. The next morning the bacteria were centrifuged and resuspended as before. The bacterial solution was then diluted to an absorbance of 1.5 at 220 nm, giving final bacteria concentration of approximately 1.6 × 108 CFU/mL. The bacterial transport apparatus consisted of a glass column, a peristaltic pump, and a UV/visible spectrophotometer with a microflowcell. The column was 2.5 cm diameter and 13.1 centimeters long (ACE Glass). The end fittings were Teflon with glass frits (average pore size 160 µm). Experiments showed that the glass frits did not retain bacteria. The column was packed with F-75 Ottawa sand (U.S. Silica), according to the preparation procedure defined below. The column was operated in an upflow mode, and the outflow was passed through a Spectronic Genesys 2 spectrophotometer with a 0.25 mL microflowcell (Thomas Scientific). The spectrophotometer was used to determine the bacteria concentration from predetermined calibration curves at 220 nm. The flow system and spectrophotometer were operated in an environmentally controlled chamber at 20 °C. The sand was sized between #70 and #60 sieves (212 µm and 250 µm openings, respectively), using a combined wetsieving and mechanical-sieving method (46). As shown in Brown et al. (46), for the experimental conditions given in Table 2, the 60/70 fraction allows determination of collision efficiencies (R) over approximately 3 orders of magnitude, from O(10-3) to O(10-1). The columns were prepared using the 60/70 fraction sand as follows. Details of the development of this column VOL. 35, NO. 19, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Breakthrough curves for culture #1(o) in a CaCl2 solution as a function of solution ionic strength. Solid symbols show repeatability between different bacteria harvests. preparation procedure and the reproducibility of bacterial transport experiments using columns prepared with this procedure are provided elsewhere (46). For each experiment, the column was dry-packed with fresh 60/70-fraction sand. Once packed, the column was flushed with CO2(g) (>250 pore volumes) and then flushed with a mild HCl solution (pH ) 2) for 3 h (∼6 pore volumes). This was followed by a deionized water flush at 10 mL/h for 4 days (∼38 pore volumes). After flushing with deionized water, the column inflow was switched to the baseline solution for the specific experiment being conducted. This solution flowed through the column overnight (minimum of six pore volumes), allowing the column to equilibrate with the surfactant prior to injection of the bacteria. Prior to beginning the experiment, the column flow rate was set to 10 mL/h, giving a rate of approximately one pore volume every 3 h. When specified by the experimental method, surfactant was added to the bacteria solution just prior to the start of the experiment to a concentration of 4xCMC. The column inlet was then switched to the bacteria solution, and the experiment was run for 20 h. Spectrophotometer readings of the column effluent were taken at 30-s intervals over the course of the experiment. The UV/visible absorbance spectrum of the stock bacteria solution was measured before and after the experiments in order to monitor for possible cell aggregation, exudates formation, and alteration of the cell structure due to the presence of the surfactants (16, 45). No alteration in the absorbance spectrum was observed with surfactant-free solutions and with C12E23. Minor shifts in the spectrum were observed with C12E4 at high ionic strengths and long time durations. While this affected the determination of the blocking parameter, it had no effect on measurement of the initial clean-bed breakthroughs. This effect on the blocking parameter estimation will be discussed with the experimental results.
Results and Discussion Figure 2 presents breakthrough curves of culture #1(o) without surfactant at five different ionic strengths ranging from 0.3 mM to 2.0 mM. The curves follow the general form predicted by eq 17, where the clean-bed breakthrough is followed by particle blocking by the attached bacteria. The trend of decreasing breakthrough with increasing ionic strength follows the DLVO theory, suggesting that electrostatic interactions are the main mechanism controlling the transport of this bacterial culture through the aquifer sand. The curves are also repeatable, as indicated by the solid symbols in Figure 2. These repeatability experiments were conducted with bacteria grown and harvested on different weeks. 3880
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FIGURE 3. Breakthrough curves for culture #1(o) in a CaCl2 solution with C12E23 at 4xCMC as a function of solution ionic strength.
FIGURE 4. Breakthrough curves for culture #1(o) in a CaCl2 solution with C12E4 at 4xCMC as a function of solution ionic strength. Figure 3 shows breakthrough curves for culture #1(o) in the presence of C12E23 at 4xCMC at five different ionic strengths ranging from 0.5 mM to 4.0 mM. Comparison of this figure to Figure 2 shows there is a significant shift in the clean-bed breakthrough values due to the presence of the surfactant. For example, at an ionic strength of 2.0 mM, the clean-bed breakthrough has increased from approximately zero to approximately 0.45. Figure 4 shows similar results for the case of C12E4 at 4xCMC. This surfactant also increased the clean-bed breakthrough values but to a lesser extent than the longer-chained C12E23. These results suggest that the shift in the clean-bed breakthrough values in the presence of nonionic surfactants is a function of the POE chain length. Comparison of Figures 2-4 also shows that the surfactants appear to have had an effect on the blocking region. However, the blocking region with the C12E4 surfactant is difficult to interpret, as the absorbance of the bacterial solution at 220 nm dropped over 10% during the course of the experiments, with the magnitude of the drop increasing with ionic strength. This change in bacterial absorbance with C12E4 was insignificant over the first few pore volumes, and measurement of the clean-bed filtration was not affected. A similar drop in absorbance was observed in a separate study that examined the effects of nonionic surfactants on the bacterial absorbance spectrum of culture #1(o) and is most likely due to removal of surface proteins by the surfactant (45). These results presented above suggest that the effectiveness of nonionic surfactants for enhancing bacterial transport is a function of the solution ionic strength and the POE chain length. To examine this further, the colloid filtration theory with Langmuirian blocking (eqs 2-17) was fit to the experimental data using the nonlinear parameter estimation routine in Mathcad (Mathsoft, Inc.). From this analysis, the clean-bed collision efficiencies (R) and blocking parameters (β) were determined for the breakthrough curves in Figures
TABLE 3. Experimental Parameters from Figures 2-4 Calculated Using the Colloid Filtration Theory (Eqs 2-17) with the Experimental Conditions Given in Table 2a ionic strength (mM)
Debeye length (Å)
C/ Co
no surfactant r
β
4.0 2.0 1.0 0.7 0.5 0.3
48 67 95 114 135 174
0.00 0.13 0.31 0.52 0.95
0.223 0.133 0.068 0.043 0.012
0.16 NDb 0.59 0.91 25.8
a
Surfactants present at 4xCMC.
b
C /C o
C12E23 r
β
C /C o
C12E4 r
β
0.32 0.45 0.55 0.70 0.95
0.076 0.049 0.042 0.021 0.005
0.23 0.24 0.65 0.85 4.46
0.13 0.31 0.50 0.68 0.82
0.138 0.079 0.045 0.026 0.017
0.51 0.27 0.40 1.46 4.17
ND ) not determinable (see text for discussion).
FIGURE 5. Collision efficiencies as a function of the Debeye length for the experiments shown in Figures 2-4. Surfactants present at 4xCMC. Vertical lines indicate maximum possible length of the POE chain (Table 1). 2-4 using the experimental parameters given in Table 2. The results of this analysis are tabulated in Table 3. Figure 5 shows the experimental collision efficiencies as a function of the Debeye length. The surfactants had a large effect on the collision efficiency, with the collision efficiency decreasing with increasing POE chain length. In a separate study examining the effects of surfactants on the cell surface hydrophobicity of culture #1(o), it was found that C12E4 at 4xCMC increased the contact angle of 1-bromonaphthalene from 37° to 60° (16), indicating that the bacteria became more hydrophobic due to the sorbed surfactant. The surfactant C12E23 made culture #1(o) slightly more hydrophilic, where the contact angle decreased to 33°. If hydrophobic interactions were the dominant mechanism affecting bacterial transport, these contact angle results suggest that the collision efficiency should increase in the presence of C12E4 and remain relatively unchanged with C12E23. Since the collision efficiency decreased appreciably with both C12E4 and C12E23, it appears that hydrophobic interactions are not dominant for this system. The decrease in collision efficiency with increasing POE chain length suggests that steric effects may be the cause of the increased bacterial transport. However, when comparing the maximum possible extension of the POE chains from the bacterial surface (vertical lines in Figure 5, e.g., C12E4 can extend 24 Å from the cell surface) with the Debeye length, it is seen that the surfactants reduce the collision efficiency even when the electric double layer is much thicker than the extended POE chains (i.e., steric effects should be negligible). This is very evident for C12E4 and is also evident for C12E23 when considering that due to conformation of the POE chain, its actual length from the surface will be much less than that depicted in Figure 5 (38). In a study of the electrophoretic mobility of polystyrene colloids in the presence of C12Ey surfactants, it was found that the electrophoretic mobility decreased linearly as a function of the POE chain length when the POE chains of the sorbed surfactant layer could extend
beyond the electric double layer (16). And when the POE chains could not extend beyond the electric double layer, the electrophoretic mobility remained unchanged. This is in agreement with a model developed by Brooks describing the effects of absorbed polymer on the electric double layer (47) and supports the hypothesis that steric effects are unimportant for this system, as the POE chains will remain well within the electric double layer. In addition, it is unlikely that steric forces due to sorbed surfactants are important with bacteria, as the molecular weights of the surfactants used in this study are much lower than those of bacterial surface proteins and polysaccharides (48). A similar conclusion has been reported for bacterial transport in the presence of the nonionic surfactant Tween 20 (14). This conclusion is supported by the results of a separate study, where the electrophoretic mobility of culture #1(o) remained unchanged in the presence of C12E4 and C12E23 at 4xCMC, even when the POE chains could extend well beyond the electric double layer (16). This is in contrast to the experiments discussed above, where the electrophoretic mobility of polystyrene colloids was reduced when the surfactant POE chains could extend beyond the electric double layer (these colloids have surfactant sorption isotherms very similar to those for culture #1(o) (1)). This indicates that the bacterial surface structures are much larger than the sorbed surfactant molecules and suggests that steric forces due to the sorbed surfactant are insignificant compared to the steric contributions of the bacterial surface components. Since steric and hydrophobic interactions due to the sorbed surfactants appear to be insignificant for this system, and Figure 5 indicates that electrostatic interactions dominate, the nonionic surfactants must be increasing the electrostatic repulsion between the bacteria and the sand grains through some physical means. Numerous researchers have reported that neutral polymers adsorbed to the surface of colloids displace the counterions, causing the electric double layer to expand (49-53). This should be related to (1) the amount of sorbed surfactant, where more specifically adsorbed counterions will be displaced as the number of sorbed surfactant molecules increases, and (2) the length of the POE chain, where the electric double layer is displaced due to volume exclusion effects. Brooks has described this extension of the electric double layer as (47)
κB ) κ exp
(B2)
(20)
where κB-1 is the expanded double layer thickness and B is related to the difference between the volume fractions of the sorbed and bulk polymer and to the interaction energies between the solvent, ions, and polymer. Brooks states that B accounts for the generalized excluded volume effects interpreted in an energetic, rather than a geometric, sense. This expansion of the electric double layer would increase the electrostatic repulsion and thus decrease the collision VOL. 35, NO. 19, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Blocking parameter of eq 16 as a function of the Debeye length for the experiments shown in Figures 2-4. Surfactants present at 4xCMC. Vertical lines indicate maximum possible length of the POE chain (Table 1). See comment on C12E4 data in text. efficiency, as a function of both the POE chain length and solution ionic strength, as is observed in Figure 5. The importance of these results is that even when the electric double layer is much thicker than the sorbed surfactant layer, nonionic surfactants are still able to affect the attachment and transport of bacteria by altering the structure of the electric double layer. Next, the blocking parameters for the experimental data are presented in Table 3. There are two issues to note about these data. The first is that the blocking parameter for the breakthrough curve with no surfactant at an ionic strength of 1.0 mM could not be determined. The Langmuirian form of the blocking parameter given by eq 16 is not able to simulate the break in the curve at ∼2.5 pore volumes (Figure 2). This suggests that a nonlinear dependence of the blocking function on the surface coverage, rather than the linear function given by eq 16, may be required (33) and that the deviation between the linear and nonlinear forms becomes much more apparent near this ionic strength. The second issue is with the calculation of the blocking data for the C12E4 surfactant. An attempt was made to determine the blocking parameter by utilizing the initial slope of the breakthrough curves between one and two pore volumes, and this is the data presented in Table 3 for C12E4. However, the blocking parameters for C12E4 are speculative due to the short section of the breakthrough curves used to determine them. Because of this, the C12E4 data should be considered qualitative and is used to show overall trends and to support the C12E32 data. The experimental blocking parameters as a function of the Debeye length are presented in Figure 6. It is evident in this figure that there is a clear trend of the blocking parameter increasing with increasing Debeye length (i.e., decreasing solution ionic strength), indicating that electrostatic forces play a major role in the blocking effect. Similar relationships between the blocking parameter and solution ionic strength have been observed with polystyrene colloids (33, 34). Figure 6 also indicates that for a given Debeye length, the surfactants increased the blocking parameter over that with no surfactant present. As with the collision efficiency data in Figure 5, the surfactants increase the blocking parameter even when the electric double layer is thick compared to the POE chain length. This is in agreement with the hypothesis that the surfactants expand the electric double layer, increasing the electrostatic repulsion between the attached bacteria and the bacteria approaching the sand grain surface. To visualize regions with enhanced or inhibited bacterial transport, the collision efficiency was plotted against the blocking parameter, and this is shown in Figure 7. This figure shows that the blocking parameter increases as the collision efficiency decreases. This trend is due to the expansion of 3882
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FIGURE 7. Clean bed collision efficiency as a function of the blocking parameter. For maximum bacterial transport, low collision efficiencies and high blocking parameters are required, while for minimal bacterial transport, high collision efficiencies and low blocking parameters are required. See comment on C12E4 blocking parameter (β) in text. the electric double layer with decreasing ionic strength. This simultaneously decreases the collision efficiency (Figure 5) and increases the blocking parameter (Figure 6), enhancing the bacterial transport. Examination of the data in Figure 7 also shows that there is a shift in the R-β relationship in the presence of the surfactants. This indicates that the sorbed surfactants affect the bacteria-sand interactions (clean-bed filtration) differently than the bacteria-bacteria interactions (blocking effect). This may be due to differences between the bacteria and sand grain surface properties, such as surface charges, levels of surfactant sorption, and surface structure (i.e., hard, irregular surface of the sand grain versus the soft, deformable, gellike bacterial membrane). Finally, Figures 5 and 6 show that the highest bacterial mobility is achieved when surfactants are used in conjunction with a low ionic strength solution. The field-scale implications of this are demonstrated by utilizing the colloid (clean-bed) filtration theory without blocking (eqs 2-12) with the experimental results of this study to predict the travel distance of bacteria with and without surfactants present. For this analysis, the travel distance of bacteria for a dilution of 106 was calculated (i.e., C/Co ) 10-6). This dilution is analogous to a starting bacteria concentration of 104/mL, which is a conservative estimate of fecal coliform bacteria in domestic wastewater (fecal coliform range from 104 to 105/mL and total coliform range from 105 to 106/mL) (54), and a final bacterial concentration of 1/100 mL, which is the USEPA maximum level for coliform bacteria in drinking water. The results are shown in Figure 8, for ionic strengths of 2.0 mM and 0.5 mM, with CaCl2 as the salt. This figure shows surfactants can have a significant effect on the travel distance of bacteria and that the effect is much greater for the lower ionic strength solution and for larger sand grains. For example, at the high ionic strength and sand grain diameter of 230 µm (i.e., the sand size used in this study), C12E23 only has the potential to increase the bacterial transport distance from 0.3 to 2 m. While this may help reduce biofouling near an injection well, it would have little impact on the transport of pathogenic organisms. However, at the low ionic strength, C12E23 has the potential to increase bacterial transport from 2 to 30 m, which can become significant when considering transport of pathogenic organisms to drinking water wells. It has been stated that the collision efficiency must be less than 0.01 in order to transport bacteria distances greater than 100 m at typical groundwater conditions (14), and this statement is in general agreement with the analysis shown in Figure 8. Most importantly, the results of this study show that nonionic surfactants can reduce
FIGURE 8. Potential travel distance of bacteria as a function of the clean-bed collision efficiency and sand grain diameter, for a bacterial concentration reduction of 106. Lines from top to bottom represent sand grain diameters of 750 µm, 500 µm, 230 µm, and 150 µm, respectively. All other parameters are identical to the experimental conditions. Circles represent the data points from this study. Ionic strength is (A) 2.0 m M and (B) 0.5 mM, both with CaCl2 as the salt. the collision efficiency to below 0.01, and, as such, they have the potential to greatly enhance the travel distance of bacteria through groundwater aquifers.
Acknowledgments The authors would like to acknowledge the support of National Science Foundation grant BES-9710301.
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Received for review January 26, 2001. Revised manuscript received July 3, 2001. Accepted July 6, 2001. ES010577W VOL. 35, NO. 19, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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