Colloids and Surfaces B: Biointerfaces 67 (2008) 265–271

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Colloids and Surfaces B: Biointerfaces

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Bacterial interactions and transport in unsaturated porous media

Gang Chen ∗

Department of Civil and Environmental Engineering, FAMU-FSU College of Engineering, 2525 Pottsdamer Street, Tallahasse, FL 32310, United States

a r t i c l e i n f o

Article history:Received 16 May 2008Received in revised form 16 August 2008Accepted 8 September 2008Available online 16 September 2008

Keywords:BacteriaTransport

a b s t r a c t

The convection-dispersion transport model, which can well define solute transport, has been introduced todescribe bacterial transport. Due to different interaction natures within the porous media, bacterial trans-port in the subsurface, especially in the vadose zone is a complex scenario. When transported in the vadosezone, bacteria may be captured on the media surface, at the air–water interface, or at the media–air–waterthree-phase interface depending upon the predominant interactions of concerned bacteria within the poresystem. In this study, transport of Echerichia coli, Pseudomonas fluorescens and Bacillus subtilis in silica sandunder water unsaturated conditions was investigated using column experiments. Bacterial interactionswithin the system were characterized based on bacterial and media surface thermodynamic proper-

ModelingInteractionEcherichia coliPB

ties, which were determined independently by means of contact angle measurements. These calculatedinteractions provided solid evidence of the bacterial retention mechanisms in the pore system, whichserved as the bases for suitable assumptions of bacterial transport modeling. The micro-scale interaction

inate

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crUaastweedaii[tiPstt

ttmtioasuis

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0d

seudomonas fluorescensacillus subtilis

investigations helped elim

. Introduction

Bacterial transport in the subsurface, which is controlled byomplex interactions between bacteria and the surrounding envi-onment, has been assessed using different transport models [1–3].nder saturated conditions, three-phase (porous media, bacteria,nd aqueous phase) transport models developed based on thessumption that the movement of bacteria follows rules governingolute transport, and movement and mass transfer within thesehree phases exist for bacteria are popularly used [4–7]. Underater unsaturated conditions, bacterial transport is significantly

ffected by the air–water interface. Bacteria are assumed to beither captured at the solid–liquid interface by physicochemicaleposition or captured at the air–water interface [8,9]. Bacterialttachment to the air–water interface is usually assumed to berreversible and the attachment is determined by the bacterialnteractions with the air–water interface including capillary forces8,9]. At the same time, they may also be captured in small porehroats at the media–air–water three phase interfaces due to phys-cal constraints, which are too small for bacteria to pass through.

hysical constraints are greatly affected by the bacterial size, mediaize distribution, as well as water saturation. Thus, for bacterialransport in the vadose zone environment, mathematical modelshat incorporate first-order law or second-order law to account bac-

∗ Tel.: +1 850 4106303; fax: +1 850 4106142.E-mail address: gchen@eng.fsu.edu.

Itdatpta

927-7765/$ – see front matter. Published by Elsevier B.V.oi:10.1016/j.colsurfb.2008.09.004

uncertainties arising with bacterial transport modeling.Published by Elsevier B.V.

erial partitioning at the air–water interface have been proposedo describe bacterial transport [1,2,10]. However, careful attention

ust be taken when applying these models to unsaturated bac-erial transport modeling. Failure of bacterial transport modelingn the vadose zone may arise from the indistinct understandingf mutual interactions between bacteria, soil matrices and their–water interface, which determine bacterial retention in theystem and are reflected by the sink terms in the model. A clearnderstanding of mutual interactions in the pore water system

s the key for the successful bacterial transport modeling in theubsurface.

The aim of this study was to address the uncertainty associatedith bacterial transport model structures and formulations. It was

lso the goal of this research to validate that surface thermody-amic characterizations of the pore system can help eliminate theacterial transport modeling uncertainty. We present here resultsf transport of three different types of bacteria that had differ-nt interactions with the surrounding environment in a laboratoryolumn packed with model media of silica sand. The transportas carried out under unsaturated, steady-state flow conditions.

nteractions of bacteria with the media matrices as well as withhe air–water interface was investigated based on their indepen-ently determined surface characteristic properties. Traditional

nd extended DLVO theory was utilized to calculate the interac-ions, which were then used in describing bacterial adhesion in theorous media [11–13]. The structure of the kinetic sink terms of theransport model was formulated based on these quantified inter-ctions to account for bacterial capturing or partitioning on the

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66 G. Chen / Colloids and Surfaces

edia surface and at the air–water interface. In addition, the effectf the air–water interface on bacterial retention was also discussed.he results of this research will lead to a better understanding ofacterial interaction mechanisms within the pore system and willrovide conclusive solutions to address the uncertainty of bacterialransport modeling in the vadose zone.

. Materials and methods

.1. Bacterial strains and growth conditions

Three bacteria, Echerichia coli, Pseudomonas fluorescens, andacillus subtilis were selected for this study. These three bacte-ial strains were typical representatives of rod-shaped bacterialtrains of Enterobacteriaceae, Pseudomonadaceas, and Bacillaceae.hey represented the typical indigenous bacteria in the soil.hese three strains were obtained from ATCC (33694, 17559nd 6051a) and were cultured in 250 ml Erlenmeyer flasksontaining 100 ml minimal salt medium, which had a composi-ion of KH2PO4, 160 mg/l; K2HPO4, 420 mg/l; Na2HPO4, 50 mg/l;H4Cl, 40 mg/l; MgSO4·7H2O, 50 mg/l; CaCl2, 50 mg/l; FeCl3·6H2O,.5 mg/l; MnSO4·4H2O, 0.05 mg/l; H3BO3, 0.1 mg/l; ZnSO4·7H2O,.05 mg/l; (NH4)6Mo7O24, 0.03 mg/l; and glucose, 200 mg/l at 37 ◦C,6 ◦C and 30 ◦C, respectively. All these three strains were grownntil the stationary state as determined by Adenosine TriphosphateATP) analysis [13]. At stationary state, the bacterial strains wereollected and centrifuged at 2500 RPM for 20 min. After washedwice with a sterilized buffer solution (potassium phosphate

onobasic-sodium hydroxide buffer, Fisher Scientific, Pittsburgh,A), they were re-suspended in sterilized nano-pure de-ionizedater to make a bacterial suspension. During the washing process,

oluble exopolysaccharide (if any) was stripped off the bacteria14]. For bacterial transport in the column, the growth of bacte-ia was assumed to be minimal due to the lack of substrate orutrient and short retention time. Consequently, bacterial surfaceroperties should remain unchanged during transport and could beescribed by their surface thermodynamic properties. The size ofhese bacteria strains was measured using a Malven Zetasizer 3000sa (Malvern Instrument Ltd., Malvern, Worcs, UK) as described byeinders et al. [15], which was 1.0 �m, 0.6 �m and 2.6 �m for E.

oli, P. fluorescens and B. subtilis, respectively.

.2. Measurement of surface thermodynamics

Surface thermodynamic properties of the bacterial strains wereeasured using contact angle measurements [11]. An apolar liquid

f diiodomethane and two polar liquids of formamide and waterere used as the measuring standard liquids. As weakly polar sur-

aces do not bind strongly with polar liquids, and strongly polarurfaces attract many types of impurities and are more solublen polar liquids, formamide and water were chosen as the polariquids. Contact angle measurements were performed using a Con-act Angle Meter (Tantec, Schaumburg, IL) following the methodescribed by Grasso et al. [11]. Bacterial suspensions prepareds described above was vacuum filtered on silver metal mem-rane filters (0.45 �m, Osmonic, Inc., Livermore, CA) and air-driedo produce a lawn. The amount of cells on the silver filter waspproximately 13 mg to ensure a multi-layer membrane coverage.ach contact measurement was repeated 30 times and the average

esults were fitted in the van Oss–Chaundhury–Good equation tostimate bacterial surface thermodynamic properties [16]:

1 + cos �)�L = 2(√�LW

S �LWL +

√�+

S �−L +

√�−

S �+L ) (1)

aocrs

interfaces 67 (2008) 265–271

here � is the contact angle (degree); L is the surface tension ofhe liquid that is used for the measurement (J/m2); �LW is theiftshitz–van der Waals component of surface tension (subscriptdenotes solid, i.e., bacteria and L denotes the measuring liq-

id) (J/m2); �+ is the electron-acceptor parameter and �− is thelectron-donor parameter of Lewis acid/base component of surfaceension (J/m2). In addition, the measuring liquid surface tension �Lan be expressed in terms of Liftshitz–van der Waals and Lewiscid/base components of surface tension �LW

L , �L+, and �L

−:

L = �LWL + 2

√�−

L �+L (2)

.3. Porous media

The porous medium used for this research was the same sil-ca sand (Fisher Scientific, 8 mesh) as reported by Chen and Zhu17]. Silica sand was first rinsed with de-ionized water and thenreated with sodium acetate, hydrogen peroxide, sodium dithion-te and sodium citrate to remove organic matters. Silica sand wasxtensively flushed with sterilized nanopure de-ionized water untilhe electrical conductivity was less than 1 dS/m. Surface thermody-amic properties of silica sand were measured using the wickingethod [18,19]. Similar as the contact angle measurements, an apo-

ar liquid of diiodomethane and two polar liquids of formamide andater were used. The contact angles of silica sand were estimatedased on the Washburn equation [20]:

2 = Ret�L cos �2�

(3)

here h is the height (m) of capillary rise of the wicking liquid atime t (s); Re is the average interstitial pore radius (m), obtainedy using low surface tension liquid of decane or hexadecane withos � = 1; and � is the viscosity of the measuring liquid (N·s/m2).he measurements were conducted using a Kruss K100 tensiometerKrüss GmbH, Hamburg, Germany) by dry packing silica sand intoKruss powder sample holder in a closed chamber. Each measure-ent was repeated 30 times and the average results were reported.

.4. Column experiments

The transport of bacteria through the porous medium of silicaand was evaluated in column experiments. The experiments wereonducted in a vertically oriented custom-made column (5.0-cmD × 25.0-cm length). Silica sand was packed in the column throughO2 solvation to eliminate air pockets. The inflow was applied usingsprinkler from the top by a peristaltic pump (Masterflex, Cole-

armer, Vernon Hills, IL). During the column experiments, matrixotential inside the column was monitored using three tensiome-ers mounted evenly along the length of the column and recordedsing a Campbell Scientific CR-7X datalogger (Campbell Scien-ific, Inc.). Water content within the column was predicted by thean Genuchten equation in term of effective water saturation, Se

21,22]:

e = [1 + (˛h)n](1/n−1)

here ˛ is the inverse of the air-entry potential (m−1); h is theater potential (m-H2O); and n is the parameter related to pore sizeistribution (-). During the transport experiments, water saturationithin the column was controlled by the flow rate up to 3.0 ml/min

nd the water suction of the hanging water column. The uniformityf water saturation was achieved by adjusting the hanging waterolumn at the outlet until the three tensiometers had consistenteadings, which were then translated to the desired effective wateraturation according to the van Genuchten equation.

G. Chen / Colloids and Surfaces B: Biointerfaces 67 (2008) 265–271 267

Table 1Bacterial and silica sand contact angles and surface thermodynamic properties.

�Dii (◦) �F (◦) �W (◦) �LW (mJ/m2) �+ (mJ/m2) �− (mJ/m2) �-Potential (mV)

E. coli 49.8 ± 0.6 24.1 ± 0.3 13.3 ± 0.3 39.1 0.59 58.9 −10.6 ± 0.4P. fluorescens 52.6 ± 0.7 35.5 ± 0.5 25.9 ± 0.5 35.4 0.42 56.9 −10.2 ± 0.1B. subtilis 29.1 ± 0.4 25.5 ± 0.4 14.2 ± 0.4 44.6 0.08 59.9 −14.7 ± 0.4S 2

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ilica sand 70.3 ± 1.2 59.9 ± 0.7 70.8 ± 0.8

uperscript Dii denotes contact angles measured with diiodomethane. Superscript Feasured with water.

For each series of the column experiment, a fresh column wasacked and prior to starting each experiment, approximately 100ore volumes of nanopure de-ionized water was eluted throughhe column by the peristaltic pump to stabilize the column. A con-ervative pulse tracer (chloride) breakthrough curve was generatedeparately before the introduction of bacterial suspension. For eacholumn run, 1 pore volume of bacterial suspension at a concentra-ion of 10 × 108 mg/l was introduced to the column, after which theolumn was continuously flushed with nanopure de-ionized waterntil the background signal was detected from the elution collectedy a fraction collector. Collected elution was measured for bacte-ial concentration using ATP assay. For each column run, three runsere performed, and the inconsistency of breakthrough curves wasithin 5% (95% CI).

. Results and discussion

.1. Bacterial interactions in the pore system

Bacterial and silica sand surface thermodynamic propertiesere calculated according to Eq. (1) based on their contact angleseasured with diiodomethane, formamide and water (Table 1).

LW was found to be 39.1 mJ/m2, 35.4 mJ/m2, and 44.6 mJ/m2 for E.oli, P. fluorescens and B. subtilis, respectively. This is in consistenceith van Oss’s prediction that �1

LW typically equals to 40 mJ/m2

ith minor variability for a considerable number of bacterial strains16]. Besides, these three bacterial strains exhibited a monopolarurface, i.e., their �− was at least one order in magnitude greaterhan �1

+ (58.9 mJ/m2, 56.9 mJ/m2 and 59.9 mJ/m2 as compared to.59 mJ/m2, 0.42 mJ/m2 and 0.08 mJ/m2 for E. coli, P. fluorescens and. subtilis, respectively) [16].

Assuming that the bacterial strains behave as inert particles andheir adhesion can be understood by a physicochemical approach,raditional and extended DLVO theory can be used in describingacterial interactions in the pore system. Bacterial interactions thathould be considered include the Liftshitz–van der Waals, Lewiscid-base and electrostatic interactions, which can be calculateds:

GLWy0 132 = −4�Ry0[(

√�LW

3 −√�LW

2 )(√�LW

3 −√�LW

1 )] (5)

GABy0 132 = 4�Ry0[(

√�+

1 −√�+

2 )(√�−

1 −√�−

2 )

− (√�+

1 −√�+

3 )(√�−

1 −√�−

3 )

− (√�+

2 −√�+

3 )(√�−

2 −√�−

3 )] (6)

[ (1 + e−y)

GELy0132 = �εε0R 2 01 02Ln

1 − e−y

+( 201 + 2

02)Ln(1 − e−2y)

](7)

ctEep

2.7 1.57 15.4 −27.5 ± 0.7

es Contact angles measured with formamide. Superscript W denotes Contact angles

here �GLWy0132, �GAB

y0132, and �GELy0132 are the free energy of the

iftshitz–van der Waals, Lewis acid/base, and electrostatic interac-ions between the bacteria, 1 and the porous media or air–waternterface, 2 immersed in the water, 3 evaluated at the equilibriumistance, y0 of 1.57 × 10−10 m (J); R is the bacterial radius (m); ε and0 are the relative dielectric permittivity of water (78.55 for watert 25 ◦C) and permittivity under vacuum (8.854 × 10−12 C/V·m)espectively; 01 and 02 are the potentials at the surfaces ofhe bacteria and the media or air–water interface; and 1/ ishe Debye–Hückel length and also an estimation of the effectivehickness of the electrical double layer [23]. 01 and 02 can bealculated based on the following equation:

0 = �(

1 + z

a

)exp(z) (8)

here � is the zeta potential measured at the slipping plate (V); z ishe distance from the bacterial or silica sand surface to the slippinglate (m), which is generally on the order of 5 Å [16]; and a is theadius of the bacteria or the silica sand (m).

Bacteria, silica sand and the air–water interface were foundo be negatively charged. Consequently, electrostatic interactionsetween bacteria and silica sand and between bacteria and their–water interface were positive, which served as the barriero prevent bacteria to get close to the media and the air–waternterface. These repulsive electrostatic interactions operated in theange of several tens of nanometers. Once bacteria strains over-ame the repulsive barrier and got close to the media surfacer the air–water interface with the aid of hydrodynamic forces,lectrostatic interactions dropped dramatically owing to the super-mposition of the double layers, and Liftshitz–van der Waals andewis acid/base interactions became the actual driving forces of theacteria to interact with the surrounding environment. Therefore,

nteractions evaluated at the equilibrium distance where physicalontact between bacteria and the media and between bacteria andhe air–water interface occurred were used to interpret bacterialdhesion in the porous media [15].

Interaction free energy between bacteria and silica sand andetween bacteria and the air–water interface evaluated at the equi-

ibrium distance were calculated according to Eqs. (5)–(7) (Table 2).ll these three strains had attractive Liftshitz–van der Waals andepulsive Lewis acid/base interactions with silica sand, and attrac-ive Liftshitz–van der Waals and Lewis acid/base interactions withhe water–air interface. �GLW+AB+EL

y0132 , sum of the interaction freenergy of Liftshitz–van der Waals, Lewis acid/base and electrostaticnteractions evaluated at the equilibrium distance, was positiveetween bacteria and the media surface, demonstrating repulsiveotentials of these strains to silica sand.�GLW+AB+EL

y0132 , however, wasegative between these strains with the air–water interface, indi-

ating adhesion potentials. Among these three strains, B. subtilis hadhe greatest potential to attach to air–water interface followed by. coli and P. fluorescens as demonstrated by their interaction freenergy values. At the same time, B. subtilis also had the greatestotential to be repelled from the media surface.

268 G. Chen / Colloids and Surfaces B: Biointerfaces 67 (2008) 265–271

Table 2Bacterial interaction free energy with silica sand and with air–water interface.

�GLW132 (kT)a �GAB

132 (kT) �GEL132 (kT) �GLW+AB+EL

132 (kT)

With silica sandE. coli −36.3 1234.8 17.8 1216.3P. fluorescens −17.6 649.6 0.48 632.5B. subtilis −119.6 3023.4 467.0 3370.8

With air–water interfaceE. coli −7146.9 −3030.0 1481.1 −8695.8P. fluorescens −4080.2 −1726.5 917.3 −4889.4B. subtilis −19845.9 −6740.9 2611.6 −23975.2

�GLW132,�GAB

132 and�GEL132 were evaluated at the equilibrium distance where physical

ci

A

tmtstcir

3

cdaBmf

wDtpfl

rrtctuo

ptlktattbb

sec

ontact between bacteria and media surface and between bacteria and air–waternterface occurred.

a k is the Boltzmann constant (1.38 × 10−23 J/K) and T is absolute temperature (K).t 25 ◦C, 1kT = 4.11 × 10−21 J.

Based on the analysis of bacterial interactions within the sys-em, it could be concluded that bacteria were retained at the

edia–air–water three phase interface (Fig. 1). Bacteria diffusedo the media–air–water interface and were retained there. Repul-ive forces between bacteria and the media aided bacteria attacho the air–water interface. Bacterial retention in these areas shouldorrespond to the total “effective” forces exerted on the bacteria,.e., sum of attractive interactions with the air–water interface andepulsive interactions with the media surface (absolute values).

.2. Bacterial transport modeling

Bacterial transport through homogeneously packed sandolumns can be mathematically described using the one-imensional transport model with air–water interface representingn additional sink for bavteria under unsaturated conditions [3].ased on the interaction analysis, bacteria are retained at theedia–air–water interface. One single sink term is used to account

or bacterial retention at the interface:

∂

∂t[�mC] = ∂

∂z[Dz�m

∂C

∂z] − ∂

∂z[qC] − k1�mC + kdes

�bS

SeCr (9)

∂Cr

∂t= k1

�mSe

�bSC − kdesCr (10)

3

here C is the bacterial concentration in the liquid phase (cells/m );z is the apparent dispersion coefficient (m2/s); �m is the mois-

ure content (moisture volume divided by the total volume of theorous media) (m3/m3); q is the specific discharge (Darcian fluidux) (m/s); k1 is the deposition coefficient accounting for bacterial

Fig. 1. Bacterial retention mechanism illustration.

uEtdtbf

TB

P864

E864

B864

Fig. 2. Tracer breakthrough curves at variable water saturation.

etention at the media–air–water interface (s−1); kdes is the bacte-ial desorption coefficient (s−1); �b is the bulk density (g/m3); S ishe air–water interfacial area (m2/m3); Cr is the retained bacterialoncentration [cells/(g)(m2/m3)]; z is the axial coordinate (m); andis time (s). For each series of the column experiment, a fresh col-mn was used. Owing to the low bacterial input, the variation of k1wing to the previous bacterial attachment can be ignored.

Tracer (chloride) transport was studied before bacterial trans-ort experiments. Nearly all the input tracer was eluted fromhe column (Fig. 2). The tracer breakthrough curve was simu-ated with the proposed models. During the model simulation,1 and kdes were set to 0. This was based on the considerationhat chloride should not be retained in the media as chloride wasssumed not to adsorb in the media. This is true since nearly allhe inputted chloride was eluted from the column at the end ofhe tracer experiments. After the simulation, Dz was determined toe 11.96 cm2/min, which was then used for all the simulations ofacterial transport.

The bacterial breakthrough curves were characterized by aelf-sharpening front, which became broader and diffuser at thelution limb (Fig. 3). The long-lasting tails of the breakthroughurves indicated kinetic-controlled bacterial retention in the col-mn. The bacterial breakthrough curves were simulated againstqs. (9) and (10) using an implicit, finite-difference scheme. Allhe parameters were optimized by minimizing the sum of squared

ifferences between observed and fitted concentrations usinghe nonlinear least-square method [24]. Based on the models,acterial retention was assumed to occur at media–air–water inter-ace. With the decrease of water saturation, bacterial retention

able 3acterial transport parameters at variable water saturation.

k1 (min−1) kdes (min−1)

. fluorescens0 0.341 0.5230 0.706 0.5890 0.846 0.586

. coli0 0.378 0.6240 1.045 0.7250 1.460 0.712

. subtilis0 1.228 0.6830 2.155 0.8270 3.416 0.895

G. Chen / Colloids and Surfaces B: Biointerfaces 67 (2008) 265–271 269

curve

iBflvswbo(“iw

�w

3

increased accordingly and more bacteria were retained in the sys-

Fig. 3. Bacterial breakthrough

ncreased accordingly (Table 3). For the same water saturation,. subtilis had the greatest retention followed by E. coli and P.uorescence, which was also evidenced by the corresponding peak-alues of the breakthrough curves (Fig. 3). Bacterial retention alsouffered from desorption during transport. Bacterial desorptionas found to be determined by the repulsive interactions of theacteria with the media surface, �GTOT

132 (media). Bacterial des-rption coefficient increased linearly with the increase of �GTOT

132media) (Fig. 4). Bacterial retention was determined by the totaleffective” forces exerted on the bacteria, i.e., sum of attractiventeractions with the air–water interface and repulsive interactions

ith the media surface (absolute values), �GTOT132 (air − water) +

Fig. 4. Bacterial desorption coefficient as a function of bacterial interactions.

tr

s at variable water saturation.

GTOT132 (media). Bacterial deposition coefficient increased linearly

ith the increase of the total “effective” forces (Fig. 5).

.3. Effect of air–water interface on bacterial transport

With the decrease in system saturation, the air–water interface

em. The presence of the air–water interface played an importantole in controlling unsaturated bacterial retention, the area of which

Fig. 5. Bacterial deposition coefficient as a function of bacterial interactions.

270 G. Chen / Colloids and Surfaces B: Bio

c

S

wdwm˛ttaftsfCwawwraw

F

4

lsndpdattemttt

parbrrcwmfwmdesitt

A

t

Fig. 6. Air–water interfacial area as a function of water saturation.

an be estimated from pore size radia [25]:

= �g

˛�

∫ �0

�

(�/�0)n/1−n

− 1]1/n d� (11)

here S is the air–water interfacial area (cm2/cm3); � is the waterensity (kg/m3); g is the gravitational constant (9.8 m/s2); � is theater surface tension (72.69 mJ/m2 at 20 ◦C); and �0 is the porousedia’s volume fraction of pore space or porosity of the column.and n are defined previously and can be estimated by fitting

he volumetric water content versus metric potential of the sys-em using the van Genuchten fitting [26], which was 0.136 cm−1

nd 4.776 cm−1 respectively for this research. The air–water inter-acial area increased with decreasing water saturation (Fig. 6). Forhis research, all the column experiments were conducted at wateraturation ranging from 0.4 to 0.8, within which the air–water inter-acial area displayed a linear relationship with water saturation.onsequently, the increase of bacterial retention with decreasingater saturation should show the same trend as with increasing

ir–water interfacial area. To reflect increased bacterial retentionith decreasing water saturation, bacterial deposition coefficientas plotted against the air–water interface (Fig. 7). The linear

elationship indicated that the increased bacterial retention wasttributed to the increased air–water interface with decreasingater saturation.

ig. 7. Bacterial deposition coefficient as a function air–water interfacial area.

v

R

[

[

[

[[

[

interfaces 67 (2008) 265–271

. Conclusions

Uncertainty of bacterial transport modeling has been a chal-enge for accurately estimating bacterial fate and transport in theubsurface. Without further investigating the interaction mecha-isms, uncertainties exist for transport model applications sinceifferent models have different assumptions. For bacterial trans-ort modeling, the most important part is the sink terms thatescribe bacterial retention in the pore system. To include theppropriate sink terms, mechanistic investigation of the interac-ions between bacteria and the sediment and between bacteria andhe air–water interface is usually required. Over the past decade,fforts have been made to characterize bacterial attachment toedia surfaces thermodynamically by using the DLVO theory. The

hermodynamic investigation of the bacterial interactions withinhe system is one of the options to eliminate modeling uncertain-ies.

To investigate bacterial retention and model bacterial trans-ort in the pore system, interactions of E. coli, P. fluorescencend B. subtilis within the pore system were evaluated in thisesearch to provide evidence for the proper sink terms ofacterial transport modeling. Based on the analysis of bacte-ial interactions within the system, bacteria were found to beetained at the media–air–water three-phase interface. Specifi-ally, bacteria were attached to the air–water interface, whichas aided by the repulsive forces between bacteria and theedia. Bacterial retention was determined by the total “effective”

orces exerted on the bacteria, i.e., sum of attractive interactionsith the air–water interface and repulsive interactions with theedia surface. To account for increased bacterial retention with

ecreasing water saturation, the air–water interfacial area wasxamined as a function of water saturation. The linear relation-hip between bacteria retention and the air–water interfacial areandicated that the increased bacterial retention was attributed tohe increased air–water interface with decreasing water satura-ion.

cknowledgements

The work was supported by the National Research Initiative ofhe USDA Cooperative State Research, Education and Extension Ser-ice, Grant No. 2007-35102-18111 to Florida A&M University.

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