chapter 19 - transport
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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 - PowerPoint PPT PresentationTRANSCRIPT
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
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
Importance of understanding transport of microorganisms
• To determine the fate of added micoorganisms (either selected or GEM)
life vs. deathproliferation vs. maintenanceadhesion vs. transport
• To determine the facilitated transport of pollutants
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.
200 um
microaggregate root
hypha
aggregrates or soil particles
2000 um
macroaggregate solidpore
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
Pore Radius
Sand
%
Hayhook
%
Vinton Mixture
%
< 1 um
1 – 10 um
10 - 60 um
> 60 um
0.025
0.35
17.13
82.5
6.51
16.05
30.93
46.51
11.70
17.68
49.9
20.83
Hayhook: 10% clay, 5% silt, 85% sand
Vinton Mix: 5% clay, 10% silt, 85% sand
Pore size distribution for three porous media
Bacteria can be no more than 5% of the average pore diameter to get effective transport.
2 um 40 um
0.05 – 0.5 um
0.5 – 3 um
Convective flux velocity
Q = K DH A t where: zQ = 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)
103
101
10 -1
10 -3
10 -5
10 -7
Matric potential head (cm)
Hyd
raul
ic c
ondu
ctiv
ity (
cm/h
r)
-10 5-103-101
Sandy soil
Clay soil
large flow
limited flow
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
Faster
Poresize
Longer path
Shorter path
Pathlength
Slower
Slower
Slower
SlowerFaster
Faster
Frictionin pore
A
B
C
Factors that cause mechanical mixing
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
Diffusion layer
Active movement
Convection
Diffusion
Surface
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 particlesq1, q2 = charged particlesk = constante = dielectric constant (depends on ionic strength and type)
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.
Pore volume
0 1 2 3 4 5 6 7
C/C
o
0.0
0.1
0.2
0.3
0.4
C0 = 5 x 107 cells/ml
CEC = 0.03%
Transport of Pseudomonas aeruginosa 9027 through sand
deionized water
2 mM NaCl
Bai et al., 1997. Appl. Environ. Microbiol. 63:1266-1273
Model prediction
Experimental data
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.
where: C = bacterial concentration (M V-1)S = sorbed phase bacterial concentration (M V-1)R = retardation factorT = time= fraction of instantaneous retardationP = Peclet number, = dimensionless first order cell sticking rate constants
****1*
)1(*
2
2
SCX
C
XP
C
T
SR
T
CR
retardation first-orderretention terms
advectiondispersion
Hydrophobic interactions
Nonpolar molecules attract each other
Electrostatic repulsion is reduced and hydrophobic interactions can increase
What is the effect of increasing ionic strength?
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
Polystyrene
Glass
70
70
60
60
50
50
40
40
30
30
20
2020
0
0
-1.0
-1.0
-2.0
-2 .0
-3.0
-3.0
100
100
75
75
50
50
25
25
0
0
Contact angle ( )o
Contact angle ( )o
Electro
phoretic
mobilit
y
(10
mete
r / V
- se
c )
-8
Electrophore
tic
mobilit
y
(10
mete
r / V
- se
c )
-8
Ad
hesi
on(%
cov
era
ge)
Adh
esio
n(%
co
vera
ge)
A.
B .
Reversible“secondary m inimum”
Irreversible“primary m inimum”
G E
G tot
G A
H
P r i m a r y m i n i m u m
S e c o n d a r ym i n i m u m
= electrostatic interaction
= separation distance
= van der Walls interaction
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
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)
1.0
0.5
0.0
Distance (m)
Con
cent
ratio
n (C
/C) o
Advection only
Advection, dispersion
Advection, dispersion, adsorption
Advection, dispersion, adsorption, decay
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.
Organism Ionic strength Mineral grain size % Recovery
W6
W6
W6
W6
W8
W8
W8W8
low
low
high
high
low
low
high
high
fine
coarse
fine
coarse
fine
coarse
fine
coarse
W6 a coccus with radius 0.75 um
W8 a bacillus with dimensions 0.75 x 1.8 um
Summary and Homework (predict relative recoveries)
14.5%
80.4%
2.8%
49.3%
3.9%
43.6
0.3%
4.3%
100
60
40
20
0
Pore volumes
% In
itia
l per
me
abili
ty
Cells starved for 4 weeks
Cells starved for 2 weeks
Vegetative cells
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.