Impact of Bacterial Biomass on Contaminant Sorption and Transport in a Subsurface Soil

Mobile bacterial particles can act as carriers and enhance the transport of hydrophobic contaminants in groundwater by reducing retardation effects. Because of their colloidal size and favorable surface conditions, bacteria act as efficient contaminant carriers. When such carriers exist in a water-saturated porous medium, the system can be thought of as three phases: an aqueous phase, a carrier phase, and a stationary solid matrix phase. Contaminant may be present in either or all of these phases. In this study, a mathematical model based on mass balances is developed to describe the simultaneous transport and fate of a biodegradable contaminant and bacteria in a porous medium. The Monod model describes the relation between the growth rate of bacteria and concentration of the contaminant which serves as a growth substrate. The bacterial decay is assumed to be a first-order reaction. The mechanisms of bacterial mass transfer between aqueous and solid matrix phases, and contaminant mass transfer between aqueous and bacterial phases are represented by kinetic models. Governing equations are nondimensionalized and solved to analyze the bacteria-facilitated contaminant transport. The model is compared with a similar model obtained with local equilibrium assumption to understand the characteristics of kinetic and local equilibrium descriptions. Results show that the contaminant transport as well as bacterial transport can be described by local equilibrium assumption when Damk6hler numbers are larger than 10. Significant sensitivities to model parameters, particularly bacterial growth rate and influent bacterial concentration, were observed. The numerical results of the bacteria carrier effect match favorably with experimental data reported in the literature. Bacteria are colloidal-sized particles and naturally exist in groundwater aquifers. They carry negative charge in most natural systems [Harden and Harris, 1953]. In addition, since their density is near unity, they tend to form stable colloidal suspensions [Marshall, 1986]. Changes in groundwater chemistry caused by contaminant migration or changes in groundwater velocity can cause the detachment and mobilization of indigenous subsurface bacteria. Furthermore, introduction of exogenous bacteria in a bioremediation operation can enhance the transport of contaminants. Harvey et al. [1989] reported that larger microorganisms within bacterial size range migrate more rapidly through aquifer sediment than do smaller microorganisms. Smaller microorganisms should contact and therefore sorb to the solid matrix surface with greater frequency because of their higher rates of diffusional motion. Bacterial surfaces have hydrophobic chai'acteristics depending on their surface constituents, which enable hydrophobic substances to adsorb on their surfaces. Canton et al. [1977] reported that hydrophobic contaminants adsorb on the bacterial surface and can also desorb to some extent. In a review paper, Baughman and Paris [1981] concluded that sorption of organic solutes occurs on...

supporting

confidence: 66%

A Kinetic Approach to Modeling Mobile Bacteria‐Facilitated Groundwater Contaminant Transport

Kim¹,

Çorapçıoğlu²

1996

“…Furthermore, the introduction of genetically engineered exogenous bacteria in a bioremeCliation operation can short-circuit the effort by biocolloid-facilitated contaminant transport. Recently, Bellin and Rao [1993] noted that the presence of bacteria in subsurface environments may affect polynuclear aromatic hydrocarbon (PAH) migration by altering the sorption characteristics of soil solids. Jenkins and Lion [1993] have indicated that bacteria may enhance the transport of PAHs in the subsurface.…”

Section: Introductionmentioning

confidence: 99%

Modeling Facilitated Contaminant Transport by Mobile Bacteria

Çorapçıoğlu¹,

Kim²

1995

Introduction of exogenous biocolloids such as genetically engineered bacteria in a bioremediation operation can enhance the transport of contaminants in groundwater by reducing the retardation effects. Because of their colloidal size and favorable surface conditions, bacteria are efficient contaminant carriers. In cases where contaminants have a low mobility in porous media because of their high partition with solid matrix, facilitated contaminant ti'ansport by mobile bacteria can create high contaminant fluxes. When metabolically active mobile bacteria are present in a subsurface environment, the system can be treated as consisting of three phases: water phase, bacterial phase, and stationary solid matrix phase. In this work a mathematical model based on mass balance equations is developed to describe the facilitated transport and fate of a contaminant and bacteria in a porous medium. Bacterial partition between the bulk solution and the stationary solid matrix and contaminant partition among three phases are represented by expressions in terms of measurable quantities. Solutions were obtained to provide estimates of contaminant and bacterial concentrations. A dimensional analysis of the transport model was utilized to estimate model parameters from the experimental data and to assess the effect of several parameters on model behavior. The model results matched favorably with experimental data of Jenkins and Lion (1993). The presence of mobile bacteria enhances the contaminant transport. However, bacterial consumption of the contaminant, which serves as a bacterial nutrient, can attenuate the contaminant mobility. The work presented in this paper is the first three-phase model to include the effects of substrate metabolism on the fate of groundwater contaminants.

“…For some problems, such as pollution of drinking water by pathogenic bacteria from leaking septic tanks and landfills [Gerba and Bitton, 1984;Yates and Yates, 1988], limited microbial mobility may be preferred, whereas for other applications, such as in situ bioremediation of polluted aquifers, high mobility is advantageous, since the success of remediation procedures depends on the ability to transfer bacteria to the contaminated areas in the aquifer [Harvey, 1991]. However, mobile bacteria may enhance the transport of, for example, hydrophobic trace contaminants [Lindqvist and Enfield, 1992a], ionizable organic bases [Bellin and Rao, 1993], and radionuclides [Champ, 1986], so predictive models of bacterial dispersal in porous media are needed to estimate both the efficiencies and risks of aquifer bioremediation schemes [Tiedje et aI., 1989] and the risks of bacterial contamination of drinking water.…”

Section: Introductionmentioning

confidence: 99%

A kinetic model for cell density dependent bacterial transport in porous media

Lindqvist¹,

Cho²,

Enfield³

1994

A kinetic transport model with the ability to account for variations in cell density of the aqueous and solid phases was developed for bacteria in porous media. Sorption kinetics in the advective-dispersive-sorptive equation was described by assuming that adsorption was proportional to the aqueous cell density and the number of available sites on the solid phase, whereas desorption was proportional to the density of sorbed cells. A numerical solution to the model was tested against laboratory column data, and the performance was compared with that of a two-site model. The kinetic model described the column data as well as the two-site model did, but the highest efficiencies of both models were associated with experiments with the smallest sorption. Furthermore, the kinetic model accounted for cell density dependent sorption, as demonstrated by fair predictions of bacterial transport at one cell density when using parameters obtained at another cell density. 1978; Shales and Kumarasingham, 1987; Reynolds et al., 1989; Gannon et al., 1991; Fontes et al., 1991; Tan et al., 1991; Trevors et al., 1991], and some have used mathemat-Paper number 94WR01725. 0043-1397/94/94WR-01725505.00 ical models to generalize the results [Taylor and Jaffd, 1990; Lindqvist and Bengtsson, 1991; Hornberger et al., 1992; Lindqvist and Enfield, 1992b]. The models describe bacterial movement by the advective-dispersive equation modified to account for retention processes, growth, and mortality (see Corapciouglu and Haridas [1984] and Harvey [1991] for a discussion of these and additional processes). The cell velocity may be retarded compared with the water velocity by several retention processes, namely, sorption, physical straining, sedimentation, and interception. These processes lead to a distribution of cells between the solid phase of the medium and the pore water. The growth and mortality rates may be different in the solid and aqueous habitats [DeIaquis et al., 1989], and the distribution of cells is affected by organic matter content on the sand and in the water [Scholl and Harvey, 1992], nutrient concentrations [Bengtsson, 1989; Lindqvist and Bengtsson, 1991], cell density [Lindqvist and Enfield, 1992b], physiological status [Fletcher, 1977], ionic strength [Gannon et al., 1991; Fontes et al., 1991], pH [Scholl and Harvey, 1992], grain size [Fontes et al., 1991; Lindqvist and Enfield, 1992b], and oxyhydroxide coatings