Bacterial deposition in porous medium as impacted by solution chemistry

Research in Microbiology 155 (2004) 467–474 www.elsevier.com/locate/resmic

Bacterial deposition in porous medium as impacted by solution chemistry Gang Chen a,∗ , Honglong Zhu b a Crop and Soil Sciences, Washington State University, Pullman, WA 99164, USA b Qingdao Haichuan Biological Renovation Research Center of Natural Medicine, 316 East Hongkong Road, Qingdao 266061, PR China

Received 11 February 2004; accepted 12 February 2004 Available online 4 March 2004

Abstract Bacterial transport in porous medium was investigated by means of column experiments using typical rod-shaped bacteria of Escherichia coli and Pseudomonas fluorescens. Mobility of E. coli and P. fluorescens in silica gel decreased with increasing ionic strength of the solution. In the presence of nonionic surfactants, the mobility of E. coli and P. fluorescens increased, and this was more pronounced at lower than at higher ionic strength. Bacterial transport in the porous medium was described by the equilibrium-kinetic two-region model and bacterial deposition was assumed to occur in the kinetic adsorption region only. Quantified bacterial deposition from bacterial column breakthrough curves was related to electrostatic and Lifshitz–van der Waals interactions between bacterial cells and medium surfaces. It was found that electrostatic interactions played a more important role than Lifshitz–van der Waals interactions in determining bacterial deposition in the porous medium, and were actually the barrier for bacteria to attach to the porous medium.  2004 Elsevier SAS. All rights reserved. Keywords: Escherichia coli; Pseudomonas fluorescens; Deposition; Ionic strength; Nonionic surfactant

1. Introduction Bacterial transport in the subsurface has received considerable attention either because it may contaminate drinking water supplies or because of its role in in situ bioremediation [9]. Bacterial transport is governed by adherence propensity to soil matrices [24], which is influenced by solution chemistry, i.e., pH and ionic strength [4], bacterial and abiotic surface physicochemical properties [5,7,8,20,22] and transport conditions [1]. Bacterial strains with different cell surface properties show different adhesion kinetics and affinity for substrate [6]. Bacterial surface physicochemical properties can be chemically modified to stimulate or impede bacterial adhesion to the substratum [21,26,29]. It has been proven that initial adhesion plays an important role in bacterial transport [3,11,12]. Meinders et al. [19] further concluded that initial bacterial adhesion could be explained in terms of overall physicochemical surface properties. Physicochemical processes of bacterial adhesion to abiotic surfaces have been extensively studied [3,11,12,19] * Corresponding author.

E-mail address: [email protected] (G. Chen). 0923-2508/$ – see front matter  2004 Elsevier SAS. All rights reserved. doi:10.1016/j.resmic.2004.02.004

and have been described by the DLVO theory [6]. Bacteria approach the substratum through random diffusion and bulk fluid transport, apart from settling and the possibility of chemotaxis of motile bacteria. Bacterial adhesion is tampered by long-range, nonspecific electrostatic interactions between bacterial cells and the substratum. Once bacterial cells overcome the electrostatic interaction barrier with the help of hydrodynamic forces, and get close to the medium surface, Lifshitz–van der Waals interactions dependent on surface thermodynamic properties of the bacterial cell and substratum surfaces, as well as the intervening medium, begin to act. Both electrostatic and Lifshitz–van der Waals interactions are strongly dependent upon the separation distance between bacterial cells and the substratum. Electrostatic interactions drop, while Lifshitz–van der Waals interactions increase significantly with the decrease in separating distance. Lifshitz–van der Waals interactions dominate electrostatic interactions at the equilibrium distance, which stabilizes bacterial cells on abiotic surfaces, resulting in the final bacterial attachment. Electrostatic and Lifshitz–van der Waals interactions are greatly affected by the solution chemistry in terms of solution ionic strength and solution surface free energy, respectively. Variations in the ionic strength of the solution and so-

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lution surface free energy can be achieved by the addition of NaCl and surfactants. Effects of variations in solution ionic strength on Lifshitz–van der Waals interactions and the presence of nonionic surfactants on electrostatic interactions can be ignored [27]. In the present work, the role of electrostatic and Lifshitz–van der Waals interactions in bacterial adhesion to abiotic medium surfaces was investigated using column experiments. Typical representatives of rod-shaped bacteria of Enterobacteriaceae and Pseudomonadaceas were used as model bacteria. Bacterial adhesion was quantified by means of model calculations with the support of numerical simulation techniques from bacterial column breakthrough curves, and was related to electrostatic and Lifshitz–van der Waals interactions between bacterial cells and porous medium surfaces.

2. Materials and methods 2.1. Bacterial strains Bacterial strains used in this research, Escherichia coli K12 (ATCC 29181) and Pseudomonas fluorescens (ATCC 17559), were cultured in 250 ml Erlenmeyer flasks containing 100 ml minimal salt medium. The medium had a composition of KH2 PO4 , 160 mg/l; K2 HPO4 , 420 mg/l; Na2 HPO4 , 50 mg/l; NH4 Cl, 40 mg/l; MgSO4 ·7H2 O, 50 mg/l; CaCl2 , 50 mg/l; FeCl3 ·6H2 O, 0.5 mg/l; MnSO4 · 4H2 O, 0.05 mg/l; H3 BO3 , 0.1 mg/l; ZnSO4 ·7H2 O, 0.05 mg/l; (NH4 )6 Mo7 O24 , 0.03 mg/l; glucose, 0.2 g/l and ammonia chloride, 0.06 g/l. The medium was adjusted to 7.4 with 1 N HCl or 1 N NaOH and sterilized by autoclaving (121 ◦ C and 1 atm) for 20 min before use for culturing. Glucose was filter-sterilized and aseptically added to the autoclaved minimal salt media. Both Escherichia and Pseudomonas strains were quantified using adenosine triphosphate (ATP) analysis [6]. Bacterial hydrodynamic radii were measured using a Malven Zetasizer 3000 Hsa (Malvern Instruments Ltd., Malvern, Worcs, UK) in a buffer solution (potassium phosphate monobasic-sodium hydroxide buffer, Fisher Scientific, Pittsburgh, PA) as described by Meinders et al. [19]. For column experiments, bacterial cells collected from the stationary phase of growth (predetermined by ATP assay) were centrifuged at 2500 rpm (Damon/IEC Divison, Needham Heights, MA) and washed twice with the sterilized buffer solution. Then they were resuspended in sterilized NaCl solutions at ionic strength of 0.001, 0.005, 0.01 and 0.1 M, respectively, at a concentration of 5 × 108 cell/ml and used as injectants for column experiments. After the buffer wash, exopolysaccharide (if any) was stripped off the bacteria [14]. During the transport process, bacterial growth was assumed to be minimal due to the lack of substrate or nutrients, and bacterial surface properties were assumed to remain unchanged. Nonionic surfactants used in this study were pentaethylene glycol monododecyl ether (C12 E5 ) and decaethylene

glycol monododecyl ether (C12 E10 ). They were obtained from Sigma–Aldrich Corp. (Saint Louis, MO) in solid form and used in experiments without further purification. The measured critical micelle concentrations (CMCs) of these nonionic surfactants were 0.065 and 0.098 mM, respectively. Porous medium used for this research was silica gel from Fisher Scientific (8 mesh, 200–500 µm). After rinsing using deionized water, it was treated with sodium acetate, hydrogen peroxide, sodium dithionate and sodium citrate to remove organic matter. Silica gel was saturated with Na+ using 1 M phosphate-buffered saline (pH 7.0). After packing, the packed column was sterilized at 121 ◦ C for 20 min. Before column experiments, the column was stabilized by extensive flushing with sterilized deionized water until the electrical conductivity of the outflow was less than 1 dS/m. 2.2. Surface thermodynamic property determination ζ -potentials of bacterial cells and silica gel were determined from their electrophoretic mobility as measured in sterilized NaCl solutions at ionic strengths of 0.001, 0.005, 0.01, and 0.1 M, respectively, by dynamic light scanning (Zetasizer 3000HAS, Malvern Instruments Ltd., Malvern, UK). Silica gel was ground first before being suspended in NaCl solutions. Besides the above measurements, ζ -potentials of bacterial cells and silica gel suspended in sterilized NaCl solution at ionic strength of 1 M were also determined. Each measurement was repeated 5 times and average results were reported. The van der Waals component surface tension of bacterial cells was determined by contact angle measurements [23] using an apolar liquid of diiodomethane (Contact Angle Meter, Tantec, Schaumburg, IL) following the method described by Grasso et al. [13]. Bacterial cells collected from the stationary growth state were first centrifuged and suspended in the phosphate buffer solution and then vacuum-filtered on silver metal membrane filters (0.45 µm, Osmonic, Inc., Livermore, CA). Each measurement was repeated 5 times and average results were fitted to the Young–Dupré equation [27]: (1 + cos θ )γL = −GSL ,

(1)

where θ is the measured contact angle (degree); γL diiodomethane surface tension (J/m2 ); and GSL adhesion free energy between diiodomethane and bacterial cells. In the above equation, subscript “L” denotes diiodomethane and “S” stands for bacterial cells. As diiodomethane is an apolar liquid, only Lifshitz–van der Waals interactions were exerted, thus,
GSL = −2 γSLW γLLW , (2) where γ LW is van der Waals component surface tension (J/m2 ).

G. Chen ,H. Zhu / Research in Microbiology 155 (2004) 467–474

The van der Waals component surface tension of silica gel was measured using the wicking method based on the Washburn equation [27,28]: h2 = (Re tγL cos θ )(2η)−1 ,

(3)

where h is the height (m) of capillary rise of diiodomethane at time t (s); η diiodomethane viscosity (N s/m2 ) and Re the average interstitial pore size (m), obtained by using low surface tension liquid of decane or hexadecane with cos θ = 1. The measurements were conducted using a Krüss K100 tensiometer (Krüss GmbH, Hamburg, Germany) by dry-packing silica gel into a Krüss powder sample holder in a closed chamber. Temperature was controlled at 20.0 ◦ C by circulating thermostatted water through a jacketed vessel containing the sample. The van der Waals component surface tension of nonionic surfactant solutions was determined from contact angle measurements on an apolar solid surface of polypropylene (Aldrich Chemical Co., Milwaukee, WI), whose van der Waals component surface tension was measured using diiodomethane in advance following the method described above. 2.3. Interaction free energy calculations The electrostatic interaction free energy between a spherical bacterium, 1, and a flat plate medium grain, 2, immersed in water, 3 can be evaluated by [27]:    1 + e−κy EL G(y)132 = πεε0 R 2ψ01 ψ02 ln 1 − e−κy   2  2 −2κy + ψ01 + ψ02 ln(1 − e ) , (4) where ε and ε0 are the relative dielectric permissivity of water (78.55 for water at 25 ◦ C) and permissivity under a vacuum (8.854 × 10−12 C/V m), respectively; R the bacterial radius (m); 1/κ the Debye–Hückel length that is also an estimation of the effective thickness of the electrical double layer [18]; y the distance between the bacterial surface (sphere) and the medium surface (flat plate) measured from the outer edge of the sphere (m); and ψ01 , ψ02 the potentials at bacterial and medium surfaces, which can be calculated by: ψ0 = ζ (1 + z/a) exp(κz),

GLW 132 (sphere − plate) =

(5)

where ζ is the zeta potential measured at the slipping plate (V); z the distance from the particle (bacterial cell or silica gel grain) surface to the slipping plate (m); and a the radius of the particle. The distance-dependent Lifshitz–van der Waals interaction free energy between a spherical bacterium, 1, and a flat plate medium grain, 2, immersed in water, 3, is given by [27]:

469

 2R 2 2R 2 A + 6 y(4R + y) (2R + y)2 y(4R + y) , (6) + ln (2R + y)2

where A is the Hamaker constant, which can be obtained from the Gibbs free energy at the equilibrium distance, i.e., A = 12πy02GLW y0 ,

(7)

where y0 is the equilibrium distance of 1.57 Å, which was obtained by comparison of a sizable number of liquid and solid compounds [27]; and GLW y0 the Lifshitz–van der Waals interaction free energy of two parallel plates, 1 and 2, immersed in water 3 at y0 , which can be calculated based on a plate van der Waals component surface tension [27]:





LW − γ LW LW − γ LW . (8) GLW = −2 γ γ y0 ,132 3 2 3 1 2.4. Column experiments Column experiments were conducted using an acrylic column (Kimble–Kontes, 2.5 × 15 cm). The column was oriented vertically and sealed at the bottom with a custom fit to permit the flow of water and retain the medium. Silica gel was packed in the column through CO2 solvation to eliminate air pockets. Bacterial suspensions were introduced into the column by a peristaltic pump from the bottom at a flow rate of 0.33 ml/min. For each run, six pore volumes of bacteria suspended in NaCl solutions at ionic strengths of 0.001, 0.005, 0.01 and 0.1 M (5 × 108 cell/ml as determined by ATP analysis) were pumped into the column. The column was then flushed with sterilized NaCl solutions alone at corresponding ionic strength for up to 50 pore volumes until no cells could be detected from the elution. These runs were repeated in the presence of C12 E5 and C12 E10 at a concentration of half their CMCs, respectively. Elution was collected by a fraction collector and was measured for bacterial concentration using ATP analysis. After each run, a breakthrough curve was generated and mass balance was performed. Under saturated conditions, bacterial transport is controlled by kinetic adsorption instead of equilibrium adsorption processes, which has been proven to be true for bacterial transport in sand columns [2,10,15–17]. Generally speaking, bacterial transport through saturated porous media can be described by the equilibrium-kinetic two-region concept based on the assumption that bacteria are deposited in the kinetic region only [25]:  fρb Kd ∂C 1+ θ ∂t

∂ 2C ∂C αρb − (1 − f )Kd C − Sk , −v ∂x 2 ∂x θ

∂Sk = α (1 − f )Kd C − Sk ) − µs,k Sk , ∂t =D

(9) (10)

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where C is the bacterial concentration in the aqueous phase (cell/m3 ); Sk the bacterial concentration on kinetic adsorption sites (cell/g); t elapsed time (s); f the fraction of adsorption sites that equilibrate with the bacteria in the aqueous phase (–); ρb the bulk density (g/m3); Kd the partitioning coefficient of bacteria to the equilibrium adsorption sites of the medium (m3 /g); θ the porosity (m3 /m3 ); D the longitudinal dispersion coefficient (m2 /s); x the coordinate parallel to the flow (m); v the pore velocity (m/s); α the first order mass transfer coefficient (s−1 ) governing the rate of bacterial exchange between equilibrium and kinetic adsorption sites; and µs,k the first order bacterial deposition coefficient on kinetic adsorption sites (s−1 ). Transport parameters in Eqs. (9) and (10) were obtained by fitting experimentally obtained bacterial breakthrough data using CXTFIT 2.1 [25]. All these parameters were optimized by minimizing the sum of the squared differences between observed and fitted concentrations using the nonlinear least-square method.

3. Results At the same ionic strength, silica gel had smaller ζ potentials than bacterial cells, and compared to E. coli, P. fluorescens had slightly smaller ζ potentials (Table 1). Both bacterial cells and silica gel had smaller ζ potentials at lower ionic strength, which increased with the increase in ionic strength. The ζ potential increase with increasing ionic strength became moderate when ionic strength reached 0.1 M [log(I ) = −1.0] (Fig. 1). The increase in ζ potentials with increasing solution ionic strength was attributed to the fact that the stern layer moved closer to the medium surface due to the compression of the double layer. Surface potentials of bacterial cells and silica gel cannot be determined Table 1 ζ potential, ψ0 potential, diiodomethane contact angle θ and van der Waals component surface tension γ LW Ionic strength (M) 0.001 0.005 0.01 0.1 1

E. coli

P. fluorescens

Silica gel

−50.6 ± 5.4 −30.4 ± 3.5 −21.6 ± 1.7 −10.6 ± 0.9 −10.1 ± 0.6

ζ potential (mV) −62.6 ± 4.2 −42.3 ± 4.5 −29.6 ± 2.3 −11.7 ± 1.6 −10.8 ± 0.7

−84.3 ± 5.1 −55.4 ± 4.2 −40.2 ± 1.3 −14.6 ± 1.6 −13.5 ± 0.8

−26.1

ψ0 potential (mV) −27.9

−34.9

θ (◦ )

γ LW (mJ/m2 )

Polypropylene E. coli P. fluorescens Silica gel

65.0 ± 0.1 41.0 ± 0.2 46.5 ± 0.3 70.3 ± 0.4

25.7 39.1 36.2 22.7

Solution C12 E5 C12 E10

94.1 ± 0.3 105.0 ± 0.2

22.3 22.0

Solid

without knowing the distance from the particle surface to the slipping plate (z). At high ionic strength, z can have a value of 3 ∼ 5 Å when the double layer is highly compressed to have a similar range of z. At an ionic strength of 1 M, the double layer (1/κ) was 3.16 Å, and thus z should be in the range of 3 ∼ 5 Å. Calculated surface potentials (ψ0 ) of E. coli, P. fluorescens and silica gel according to Eq. (5) were −26.1, −27.9, and −34.9 mV, respectively, with z taken as 3 Å (Table 1). Electrostatic interactions between bacteria and silica gel were thus calculated according to Eq. (4) using these ψ0 values. Contact angle measurements provided stable diiodomethane contact angles for E. coli and P. fluorescens cells owing to the osmotic protection from the cell wall, which provided an experimentally practicable way of carrying out bacterial contact angle measurements. Bacterial van der Waals component surface tension was thus calculated according to Eqs. (1) and (2) based on these contact angles and so was silica gel (Table 1). The van der Waals component surface tension of nonionic surfactant solutions was calculated using contact angles of the solutions on polypropylene. Lifshitz–van der Waals interaction free energies between bacteria and silica gel in different surfactant solutions were calculated according to Eqs. (6)–(8) using van der Waals component surface tensions of bacteria, silica gel and the intervening nonionic surfactant solutions. The mobility of E. coli and P. fluorescens in silica gel decreased with the increase in ionic strength, i.e., E. coli and P. fluorescens had smaller peak value breakthrough curves with increasing ionic strength (Fig. 2). Both E. coli and P. fluorescens breakthrough curves displayed a narrow selfsharpening front, which became broader and diffuser at the elution limb. Changes in breakthrough curves with increasing ionic strength manifested dispersion and kinetic adsorption increase. Compared to P. fluorescens, E. coli had greater dispersion and kinetic adsorption as manifested by the broader, more diffuse, and smaller peak-value breakthrough curves. Both E. coli and P. fluorescens had higher mobility in the presence of nonionic surfactants than in the absence of them (Fig. 3). E. coli and P. fluorescens had less diffusive

Fig. 1. ζ potential as a function of solution ionic strength.

G. Chen ,H. Zhu / Research in Microbiology 155 (2004) 467–474

Fig. 2. Breakthrough curves of E. coli and P. fluorescens at ionic strengths of 0.001, 0.005, 0.01 and 0.1 M. Symbols are measured bacterial breakthrough data and lines are model fits.

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Fig. 3. Breakthrough curves of E. coli and P. fluorescens in the presence of C12 E5 and C12 E10 at variable ionic strength. Symbols are measured bacterial breakthrough data and lines are model fits.

Table 2 Tow-region model parameters for E. coli and P. fluorescens Bacteria E. coli I = 0.001 M I = 0.001 M + C12 E5 I = 0.001 M + C12 E10 I = 0.005 M I = 0.005 M + C12 E5 I = 0.005 M + C12 E10 I = 0.01 M I = 0.01 M + C12 E5 I = 0.01 M + C12 E10 I = 0.1 M I = 0.1 M + C12 E5 I = 0.1 M + C12 E10 P. fluorescens I = 0.001 M I = 0.001 M + C12 E5 I = 0.001 M + C12 E10 I = 0.005 M I = 0.005 M + C12 E5 I = 0.005 M + C12 E10 I = 0.01 M I = 0.01 M + C12 E5 I = 0.01 M + C12 E10 I = 0.1 M I = 0.1 M + C12 E5 I = 0.1 M + C12 E10

D (cm2 /min)

α (min−1 )

Kd (cm3 /g)

µs,k (min−1 )

5.55 3.24 4.51 7.07 5.47 6.27 10.32 8.80 9.80 12.29 11.14 11.93

0.08 0.04 0.06 0.12 0.11 0.12 0.34 0.24 0.27 0.58 0.44 0.47

2.79 1.48 2.39 3.50 3.23 3.42 8.41 7.90 8.04 16.59 12.36 14.45

1.13 0.82 0.93 1.60 1.34 1.49 2.31 2.01 2.10 3.47 3.03 3.13

7.62 5.89 5.14 9.87 8.40 8.93 11.56 9.64 10.28 14.36 13.32 13.79

0.06 0.04 0.04 0.09 0.07 0.08 0.23 0.16 0.18 0.39 0.31 0.35

1.25 0.89 1.10 2.14 1.78 1.54 7.14 5.34 6.08 10.52 8.11 8.87

0.69 0.43 0.59 0.94 0.72 0.81 1.93 1.72 1.83 2.78 2.56 2.63

f was averaged from initial fittings (0.19) and then used as a fixed value for final fittings (E. coli). f was averaged from initial fittings (0.21) and then used as a fixed value for final fittings (P. fluorescens).

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Fig. 4. Maximum electrostatic interaction free energy as a function of separation distance.

Fig. 6. Deposition coefficient as a function of maximum electrostatic interaction free energy.

and greater peak-value breakthrough curves in the presence of surfactants, which was more pronounced for C12 E5 than C12 E10 . Changes in breakthrough curves in the presence of nonionic surfactants were more obvious at lower ionic strength than at higher ionic strength.

4. Discussion

Fig. 5. Lifshitz–van der Waals interaction free energy as a function of separation distance.

The equilibrium-kinetic two-region concept model was successful in describing bacterial transport (Figs. 2 and 3) and model parameters obtained by optimization to fit experimental data are listed in Table 2. It was hypothesized that bacterial deposition occurred only in the kinetic adsorption region that was limited by mass transfer between the equilibrium region and the kinetic region. The E. coli mass transfer coefficient increased from 0.08 min−1 at an ionic strength of 0.001 M to 0.58 min−1 at an ionic strength of 0.1 M, and P. fluorescens went from 0.06 to 0.39 min−1 . In the presence of nonionic surfactants of C12 E5 and C12 E10 , the bacterial mass transfer coefficient decreased, which was more pronounced for C12 E5 than C12 E10 . The E. coli partitioning coefficient at the equilibrium adsorption site increased from 2.79 cm3 /g at an ionic strength of 0.001 M to 16.59 cm3 /g at an ionic strength of 0.1 M, and P. fluorescens went from 1.25 to 10.52 cm3 /g. Like the mass transfer coefficient, the bacterial partitioning coefficient decreased in the presence of nonionic surfactants.

G. Chen ,H. Zhu / Research in Microbiology 155 (2004) 467–474

473

Fig. 8. Deposition coefficient as functions of maximum electrostatic and Lifshitz–van der Waals interaction free energy. Table 3 Interaction free energy between bacterial cells and the medium Maximum electrostatic interaction free energy at different ionic strength: Ionic strength (M)/

1 GEL 131

2 GEL 131

separation distance (nm)

(kT )

(kT )

0.001/20 0.005/8.94 0.01/6.32 0.1/0.632

151.4 129.0 104.9 97.2

111.2 106.2 87.8 78.6

Lifshitz–van der Waals interaction free energy evaluated at the equilibrium distance: Fig. 7. Deposition coefficient as a function of Lifshitz–van der Waals interaction free energy.

The repulsive electrostatic interactions between bacteria and the porous medium prevent bacteria from getting close to medium surfaces. The maximum electrostatic interactions occur when the separation distance is in the range of the sum of the double layer thickness of the bacteria and the porous medium. At this distance, van der Waals interactions can be ignored (Figs. 4 and 5). Thus the maximum electrostatic interactions actually make up the barrier of bacterial adhesion to medium matrices and should be the determining factor in bacterial deposition on medium matrices. The deposition coefficient of both E. coli and P. fluorescens in silica gel decreased with the increase in maximum electrostatic interactions (Fig. 6a). Specifically, they decayed exponentially with increasing electrostatic interactions (Fig. 6b). With the decrease in the separation distance, electrostatic interactions decrease and Lifshitz–van der Waals interactions increase. Once bacteria are in close proximity to silica gel surfaces, Lifshitz–van der Waals interactions be-

Control C12 E5 C12 E10

1 GLW y0,132

2 GLW y0,132

(kT )

(kT )

−362.5 −154.8 −277.5

−185.0 −78.6 −141.3

1 E. coli. 2 P. fluorescens.

k is the Boltzmann constant (1.38 × 10−23 J/K) and T is absolute temperature (K). At 25 ◦ C, kT = 4.11 × 10−21 J.

gin to dominate over electrostatic interactions. Lifshitz–van der Waals interactions, as the driving forces for bacteria adhesion on medium surfaces, can be quantified at the equilibrium distance at which physical contact between bacteria and medium matrices actually occurs. Lifshitz–van der Waals interactions are dependent upon physicochemical properties of the bacterial and medium surfaces as well as the intervening medium. Thus, Lifshitz–van der Waals interactions between bacteria and the porous medium can be modified by varying the solution chemistry, i.e., the addition of nonionic surfactants. The deposition coefficient of both E. coli and P. fluorescens in silica gel decreased exponentially with the increase in Lifshitz–van der Waals interac-

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tions evaluated at the equilibrium distance (Fig. 7). The effect of the presence of nonionic surfactants on ionic strength and consequent electrostatic interactions was ignored [27]. The bacterial deposition coefficient as a function of electrostatic interactions and Lifshitz–van der Waals interactions was plotted in Fig. 8. It was obvious that electrostatic interactions played a more important role than Lifshitz–van der Waals interactions in determining bacterial deposition coefficient in the porous medium, i.e., the slope of the deposition coefficient versus the electrostatic interaction free energy curve was much steeper than versus the Lifshitz–van der Waals interaction free energy. 4.1. Concluding remarks Bacterial adhesion to the porous medium is prevented by repulsive electrostatic interactions between negatively charged bacteria and negatively charged porous medium, which operate in the range of several tens of nanometers. Once bacteria overcome the repulsive barrier and get close to the medium surface with the help of hydrodynamic forces, electrostatic interactions decrease and Lifshitz–van der Waals interactions begin to dominate. The final bacterial attachment is determined by Lifshitz–van der Waals interactions between bacteria and the porous medium evaluated at the equilibrium distance at which physical contact occurs. Electrostatic interactions play a more important role than Lifshitz–van der Waals interactions in determining bacterial adhesion in the porous medium, and actually form the barriers of bacterial adhesion to medium matrices.

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