Download Factors affecting microbial transport in soil

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. death
proliferation vs. maintenance
adhesion 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.
macroaggregate
solid
pore
2000 um
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
microaggregate
root
hypha
200 um
aggregrates or soil particles
Pore size distribution for three porous media
Pore Radius
Sand
%
Hayhook
%
Vinton Mixture
%
< 1 um
0.025
6.51
11.70
1 – 10 um
0.35
16.05
17.68
0.05 – 0.5 um
10 - 60 um
17.13
30.93
49.9
0.5 – 3 um
> 60 um
82.5
46.51
20.83
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
Factors affecting microbial transport in soil
Advection - movement with bulk fluid
103
Sandy soil
1
large flow
Hydraulic conductivity (cm/hr)
10
limited flow
Convective flux velocity
Q = K DH A t
z
-1
10
10-3
Clay soil
-5
10
where:
10-7
1
(l3)
Q = volume of water moving through the column
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)
-10
3
-105
-10
Matric potential head (cm)
A
Dispersion
Pore
size
•
•
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
B
Longer path
Path
length
Shorter path
C
Factors that cause mechanical mixing
Slower
Slower
Faster
Slower
Faster
Slower
Friction
in pore
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
Active movement
Convection
Diffusion
Diffusion layer
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
e . r2
where:
F = force between the particles
q1, q2 = charged particles
k = constant
e = 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.
Transport of Pseudomonas aeruginosa 9027 through sand
0.4
C0 = 5 x 107 cells/ml
CEC = 0.03%
C/Co
0.3
Experimental data
deionized water
0.2
Model prediction
0.1
2 mM NaCl
0.0
0
1
2
3
4
5
6
7
Pore volume
Bai et al., 1997. Appl. Environ. Microbiol. 63:1266-1273
Modeling was performed using a one-dimensional advectiondispersion model that includes combined instantaneous and ratelimited sorption and two first-order irreversible retention terms.
C *
S * 1 2C * C *
R
 (1   ) R


 C * S *
2
T
T
PX
X
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
Hydrophobic interactions
Nonpolar molecules attract each other
What is the effect of increasing ionic strength?
Electrostatic repulsion is reduced and hydrophobic interactions can
increase
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
Adhesion
(% coverage)
A.
100
75
Polystyrene
50
25
0
70
60
50
40
30
Contact angle ( o)
B.
0
-1.0 etic
)
r
c
o
e
-2.0
ph ty - s
-3.0
rt o bili V
20
ec mo er /
l
E
et
-8 m
0
(1
Adhesion
(% coverage)
100
75
50
25
ic
)
et
r
c
o
-2.0
e
ph ity - s
-3.0
rt o bil V
20
ec o r /
E l m et e
-8 m
0
(1
-1.0
0
70
60
50
40
30
Contact angle ( o)
0
Glass
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.
Reversible
“secondary minimum”
Irreversible
“primary minimum”
DLVO theory – Gibbs free energy
between a sphere and a flat surface
GE = electrostatic interaction
H = separation distance
Gtot
Secondary
minimum
GA = van der Walls interaction
Primary
minimum
Reversible vs. irreversible attachment
Polymers
Fibrils
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)
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.
1.0
Advection only
Concentration (C/Co)
Advection, dispersion
0.5
Advection, dispersion, adsorption
Advection, dispersion, adsorption, decay
0.0
Distance (m)
Summary and Homework (predict relative recoveries)
Organism
Ionic strength
Mineral grain size
% Recovery
W6
W6
W6
W6
low
low
high
high
fine
coarse
fine
coarse
14.5%
80.4%
2.8%
49.3%
W8
W8
W8
W8
low
low
high
high
fine
coarse
fine
coarse
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
A series of experiments were
performed in glass bead
columns to determine the impact
of injected cells on the
permeability of the column.
MacLeod et al., 1988. Appl.
Environ. Microbiol. 54:13651372.
60
% Initial permeability
500 PV of Klebsiella
pneumoniae (108 CFU/mL) were
injected. The cells were either
vegetative, starved for 2 weeks,
or starved for 4 weeks.
100
Cells starved for 4 weeks
40
20
Cells starved for 2 weeks
Vegetative cells
0
Pore volumes
Differences in the DNA-derived
cell distribution in glass bead
cores injected with vegetative or
starved cells.
1000
MacLeod et al., 1988. Appl.
Environ. Microbiol. 54:13651372.
Cells starved for 2 weeks
8
10 Cells/gram
100
10
Vegetative cells
Core depth (cm)