Physical chemistry of
bacterial leaching
Lecture no.3
Physical chemistry of
bacterial leaching
Lecture no.3
Cell adhesion
• Adhesion of microorganisms to the
mineral surface is ever present in the
natural environment.
• Microbial adhesion occurs and is
detrimental in wide range of areas
such as:
Biocorrosion
Biofouling
Bioleaching
Bacteria attachment
• The attachment of bacteria to the
mineral surface is affected by
hydrophobic and electrostatic
parameters.
• The
surface chemistry
of bacteria
attachment.
• The colonization of a surface by
bacteria is occurring in four distinct
steps
Steps in the colonization of
the mineral surface
Theory of adhesion
The contact angle
The adhesion of bacteria to the polystyrene
surface was strongly
correlated with the contact angle
Young equation
Thomas Young proposed the equation
for a drop of liquid placed on a solid
surface
Surface energy
1.Fowkes pioneered the surface component
approach He divided the total surface energy
in 2 parts.
=
d
+
p
d
– dispersive part (Londona- van der Waalsa)
p
- non-dispersive part – polar part
2. Van Oss and Good diveted polar component of
surface energy into electron accepting (
+
)
and
electro-donating (
-
) parameters.
=
VdW
+
AB
AB
= 2√
+
-
Surface energy and
hydrophobicity
Contact angle Hydrophobicit
y
Surface
energy
0
0
- 90
0
hydrophilic
High
(up to 100 mJ)
90
0
- 180
0
hydrophobic
low
(10- 30mJ)
Bacteria cell as colloid
particle
• Bacteria in solution have been
frequently described as colloidal
suspension.
• The colloid particles interaction is the
combination of attraction and repulsion
forces.
• Attraction force
– van der Waals force
• Repulsion force
– electrostatic force
Van der Waals force
• Van der Waals force is responsible for
long-range
attractive forces
between
colloid particles and between colloid
particle and the surface.
• Hydrophobicity arises when the
magnitude of van der Waals
interaction between water molecules
is greater
than the interactions of the
water molecules and the surface.
Attraction energy
• The
attraction energy V
A
arising from the van
der Waals force between colloidal particles of
radius r and separated by a distance H, is
given:
where: A
123
is the Hamaker constant of the
system.
The Hamaker constant can be calculated from
contact angle by a method outlined by Fowkes
H
r
A
V
A
6
132
Interaction energy curve
Electrostatic force
• The electrostatic forces are generally
repulsive in bacteria surface
interaction.
• The electrostatic forces arise
because both the colloid particles
and surface are charged.
Double electric layer
Schematic diagram of the
electric double layer
Surface potential of e.d.l.
The surface
potential of e.d.l
cannot be directly
measured
The zeta potential
represents the
potential between
the plane of shear
and the bulk
solution.
Theory of D.L.V.O.
(Derjaguin, Landau, Verwey,
Ovrbeek)
• The total interaction energy is presented
as a function of the distance of separation.
• Total interaction energy is given by:
V
TOT
= V
A
+ V
E
+ A
AB
• Initial bacterial adhesion can be described
by the DLVO theory in which adhesion is
predicted as interplay of Lifshitz-van der
Waals interaction, electrostatic
interactions and Lewis acid –base
interactions.
DLVO theory curves
Bacterial adhesion to solid
surfaces – disjoining pressure
The interplay between electrostatic forces and attractive van der
Waals surface forces and the charge of the bacterium cell determines
the minimum approach distance (position of the energy barrier/well).
Most bacteria in soils are negatively charged (otherwise, immobile).
Electrokinetic behavior of bacteria
cells
• Zeta potential of unadapted and
adapted A. ferrooxidans cells were
determined using a Zeta-meter.
• The tests were carried out at 22
0
C
under the required pH.
• Bacteria cells were dispersed in 10
-2
M
KCl,
• The cell concentration was 3 x 10
8
cells/ml
Zeta potential
(a) pyrite, (b) chalcopyrite-after interaction
with
A. ferrooxidans
Square-mineral alone
Circle-mineral interacted with ferrous grown cells
Triangle-mineral interacted with sulfur grow cells
Adhesion of bacteria cells
pokrycie, %
kąt zwilżania,
potencjał
dzeta, mV
100
75
50
25
0
60
40
20
-30
-20
-10
0
The relationship between the contact angel of the
bacterial cell and zeta potential. Results for adhesion of
bacteria
Adhesion of Bacillus suptilis onto the
calcite surface
Biofilm (EPS)
• Most bacteria grow attached to
mineral surface in form of a biofilm.
• Bacterial attachment predominantly
is mediated by EPS (biopolymers),
which surround the bacterial cells.
• Bacteria are able to adapt their EPS
(biopolymer) according to the solid
surface.
Attachment side
• AFM imges demonstrate that cells of
A.ferrooxidans preferentially attach
to sites with visible surface
imperfections.
A
tomic
F
orce
M
icroscope
Atomic Force Microscope
(AFM)
AFM image
AFM image
Pyrite surface after 24 h of incubation with A.
ferrooxidans
Cells attached preferentially to the sites with surface
defects
AFM-imige of cells of Leptospirillum
ferrooxidans attached to pyrite after
24h of incubation
Cell of A.ferrooxidans attached to
the pyrite surface
Electron coming from pyrite to the iron(III) complex at
EPS and iron(III) is reduced to iron(II).
Adhesion of A.ferrooxidans
cells onto the mineral surface
red-pyrite; blue-chalcopyrite; green-sphalerite;
yellow-galena;
yellow cross-quartz
A-strain R-1 and B-strain SPm/3
Summary
• The cells of leaching bacteria are
attracted to the mineral surface by
van der Waals force.
• EPS generate microenvironment for
bacteria cells attached to the mineral
surface