Chapter 19 - Transport

1. Chapter 19 - Transport • Objectives • Be able to explain the possible interactions of a bacterium with soil pores • Be able to explain the relationship between bacterial size and soil pore size and effective transport • Be able to list and understand the four factors that affect microbial transport • For a given a set of conditions (bacterial shape and size, ionic strength, soil texture) be able to provide an educated prediction of whether microbial transport will occur

2. Transport of microorganisms in soil • Distribution of microorganisms in nature • preference is shown for attachment mountain streams • sediments • subsurface environments • microorganisms tend to be found in “patches” or small colonies rather than evenly distributed on soil surfaces • soil often “filters out” microorganisms as they move with water flow

3. Importance of understanding transport of microorganisms • To determine the fate of added micoorganisms (either selected or GEM) • life vs. death • proliferation vs. maintenance • adhesion vs. transport • To determine the facilitated transport of pollutants

4. Pore spaces in microaggregates with neck diameters less than 6 um have more activity than pore spaces with larger diameters. Bacteria within the former are protected from protozoal predation.

5. solid macroaggregate pore 2000 um microaggregate root hypha 200 um aggregrates or soil particles Assume that 50% of the aggregate is pore space and the pores are 15 um in diameter, there will be 1,000,000 pores Assume the pores are 30um in diameter, there will be 150,000 pores

6. Pore size distribution for three porous media 0.05 – 0.5 um 0.5 – 3 um Hayhook: 10% clay, 5% silt, 85% sand Vinton Mix: 5% clay, 10% silt, 85% sand Bacteria can be no more than 5% of the average pore diameter to get effective transport. 2 um 40 um

7. large flow limited flow Factors affecting microbial transport in soil Advection - movement with bulk fluid Convective flux velocity Q = K DH A t where: z Q = volume of water moving through the column (l3) K = hydraulic conductivity (l/t) DH = hydraulic head difference between inlet and outlet (l) A = cross sectional area of column (l2) t = time (t) z = length of column (l)

8. Dispersion • mechanical mixing – path tortuosity creates velocity differences depending on pore sizes • molecular diffusion – random movement of very small particles in a fluid generally due to a concentration gradient. Usually not important for bacteria but might affect virus transport Factors that cause mechanical mixing

9. Adsorption loss of cells from the solution phase due to interaction with surfaces (ranges from reversible to irreversible) • There are several ways a cell can approach a surface. • Active movement (chemotaxis) is in response to a chemical gradient • Diffusion – brownian motion allows random interactions with a surface • Convective transport - • due to water movement, • usually several orders of • magnitude > than diffusion

10. Once at the surface several different forces govern the interaction Electrostatic interactions – repulsive forces Hydrophobic interactions – attractive forces Van der Waals forces – attractive forces Electrostatic interactions Coulomb’s Law: F = k q1. q2 where: e . r2 F = force between the particles q1, q2 = charged particles k = constant e = dielectric constant (depends on ionic strength and type)

11. Is F expected to be positive or negative between a bacterial cell and the soil? ? Electrostatic forces are repulsive What is the effect of increasing the ionic strength of the medium? Electrostatic repulsion is reduced.

12. Transport of Pseudomonas aeruginosa 9027 through sand C0 = 5 x 107 cells/ml CEC = 0.03% Experimental data deionized water Model prediction 2 mM NaCl Bai et al., 1997. Appl. Environ. Microbiol. 63:1266-1273

13. Modeling was performed using a one-dimensional advection-dispersion model that includes combined instantaneous and rate-limited sorption and two first-order irreversible retention terms. retardation dispersion advection first-order retention terms • where: • C = bacterial concentration (M V-1) • S = sorbed phase bacterial concentration (M V-1) • R = retardation factor • T = time • = fraction of instantaneous retardation P = Peclet number ,  = dimensionless first order cell sticking rate constants

14. van der Waals Forces Occurs between neutral molecules. Electron motion is such as to produce net electrostatic attraction at every instant. van der Waals forces are attractive Hydrophobic interactions Nonpolar molecules attract each other What is the effect of increasing ionic strength? Electrostatic repulsion is reduced and hydrophobic interactions can increase

15. Polystyrene Glass

16. When a cell is right next to the surface the attraction is very strong due to attractive forces creating a primary minimum (H-bonding and dipole interactions). As the two surfaces separate slightly (several nm) repulsive forces grow quickly. At slightly longer distances another, smaller minimum exists. At the secondary minimum the cell is not in actual contact with the surface and so the cells can be removed by increasing water velocity or by changing the chemistry of the system. DLVO theory – Gibbs free energy between a sphere and a flat surface = electrostatic interaction = separation distance = van der Walls interaction

17. Reversible vs. irreversible attachment

18. Factors affecting microbial transport in soil • Advection - movement with bulk fluid • Dispersion • mechanical mixing – path tortuosity creates velocity differences depending on pore sizes • molecular diffusion – random movement of very small particles in a fluid generally due to a concentration gradient. Usually not important for bacteria but might affect virus transport • Adsorption – loss of cells from the solution phase due to interaction with surfaces (ranges from reversible to irreversible) • Decay – loss of cells from the solution phase due to death (irreversible)

19. A short pulse of cells have been added to a column and this is a snapshot of the distribution of cells along the length of the column at some time later. Advection only Advection, dispersion Advection, dispersion, adsorption Advection, dispersion, adsorption, decay

20. Summary and Homework (predict relative recoveries) 14.5% 80.4% 2.8% 49.3% 3.9% 43.6 0.3% 4.3% W6 a coccus with radius 0.75 um W8 a bacillus with dimensions 0.75 x 1.8 um

21. A series of experiments were performed in glass bead columns to determine the impact of injected cells on the permeability of the column. 500 PV of Klebsiella pneumoniae (108 CFU/mL) were injected. The cells were either vegetative, starved for 2 weeks, or starved for 4 weeks. MacLeod et al., 1988. Appl. Environ. Microbiol. 54:1365-1372.

22. Differences in the DNA-derived cell distribution in glass bead cores injected with vegetative or starved cells. MacLeod et al., 1988. Appl. Environ. Microbiol. 54:1365-1372.