Principles and Application of Electrical
Impedance Spectroscopy
Membranes & Water Systems- Fouling and
Monitoring, Singapore, February 2015
Hans G. L. Coster
School of Chemical and Biomolecular Engineering
Membrane Dielectric Structure
Skin
Layer
Sub layer
What is involved in Impoedance Spectroscopy ?
• Injection of sinusoidal AC currents through the membrane
• Measurement of the current and voltage across the membrane
• Measurement of the phase shift between the voltage & current
Current
voltage sample
Contact solution/material
Phase shift
Sinewave signal generator
Electrical Circuit Equivalents
Impedance and Phase
Phase
Sine θ
Capacitance, C = ω |z|
current
I
V
voltage
V
Impedance, |Z|=
I
Electrical representation of films
Homogeneous
film
Film with 2
substructural
layers
C1
C
G
G1
d
d1
C1= 0.006 F/m2
C2=0.059 F/m2
G1 = 0.003 S/m2
G2 =4.35 S/m2
C2
G2
d2
Capacitance dispersion with frequency
This requires accurate determination of phase
and impedance magnitude
0.0060
Capacitance – mF m-2
0.0059
J
0.0058
0.0057
J
0.0056
F
J
0.0055
F
F
0.1
1
10
100
Frequency - Hz
1000
Conductance dispersion with frequency
This requires accurate determination of phase
and impedance magnitude
0.040
0.035
J
Conductance – S m-2
0.030
J
0.025
0.020
F
F
0.015
J
0.010
0.005
F
0.1
1
10
100
Frequency - Hz
1000
Help to discern various processes
and layers with different time
constants
Nyquist Plots
J
J
J
F
F
- Imaginary impedance
F
Real impedance
additional arcs appear for each element with a different time constant
EIS Membrane characteristics
Fitted data
Each dielectric element or transport process will have a
characteristic electrical time-constant.
These various elements/processes can be readily distinguished in
a Nyguist plot of the imaginary vs the real impedance.
Data for RO with NaCl 2000ppm and silica 200 ppm, crossflow 0.15 m/s: from Ho, Sim, Gu,
Webster, Fane & Coster. (2015)
-Imaginary Impedance (Ohms.m2)
0.5
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ůĞŵĞŶƚϯ͗DĞŵďƌĂŶĞƐŬŝŶ
ůĞŵĞŶƚϰ͗DĞŵďƌĂŶĞƐŬŝŶ
ůĞŵĞŶƚϱ͗ŝĨĨƵƐŝŽŶƉŽůĂƌŝnjĂƚŝŽŶůĂLJĞƌ
ůĞŵĞŶƚϲ͗ŝĨĨƵƐŝŽŶƉŽůĂƌŝnjĂƚŝŽŶůĂLJĞƌ
0.4
0.3
Fitting of EIS
Data
Fitting
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;tͲϯϬͿŵĞŵďƌĂŶĞ
0.2
0.1
0
0
0.1
0.2
0.3
0.4
Real Impedance (Ohms.m2)
ϭ
0.5
Ϯ
Conducta
Capacitance,
ϲ
ϰ ϱ Frequency,
Elements nce, Gn(Ȧ)
Cn (Ȧ)ϯ (F/m2)
Ȧ (Hz)
2
(S/m )
n=1
n=2
n=3
n=4
n=5
12.4
5.79
35.3
21.1
15.4
4.124 x 10-7
2.41 x 10-6
2.53 x 10-3
4.34 x 10-2
0.26
4.792 x 106
3.83 x 105
2.22 x 103
77.1
9.32
n=6
14.6
3.1
<1Hz
DĞŵďƌĂŶĞ
ƚŚŝĐŬŶĞƐƐΕ
ϭϱϬƵŵ
dielectric
constant, İ
į (m)
78
36
5
5
78
78
1.67 x 10-3
1.32 x 10-4
1.75 x 10-8
1.02 x 10-9
2.66 x 10-9
2.23 x 10-10
Data from Wang, Sim, Gu, Coster & Fane J. Mem Sci
Non-linear Effects: Concentration polarization
membrane
Build up in
time of
solute at
surface
Pressure
driven flux
+
-
Build up
and
depletion in
time of
ionic
solutes at
surface
Electrically
driven
transport of
ionic
solutes
AC Effects
+
-
-
+
With AC
currents, the
profiles undergo
an inversion
during each
cycle
At low frequencies the concentration polarization will
be much larger than at high frequencies of the AC.
At very high frequencies there is insufficient time for
concentration polarization to manifest
Impedance Monitoring Cross-flow Module
Cross section of module
Voltage electrodes
Current injecting
plate electrodes
An Impedance Cross-flow Module
for monitoring membrane fouling in situ
Insulating
spacers and
gasket
Pressure plate
Membrane
Chamb
plates/c
electrod
Membrane Signatures in early stages of Filtration
5 hrs silica
2 hrs Silica
TMP (bar)
Saline –
no silica
25
20
15
10
5
0
Wang, Sim, Gu, Coster & Fane J Mem Sci
20
40
Time (hour)
60
Initial stages of filtration
Concentration
polarization and
surface
accumulation
Data from Wang, Sim, Gu, Coster & Fane J. Mem Sci
Signatures of Membrane Fouling
5 hrs silica
15 hrs silica
TMP (bar)
48 hrs silica
25
20
Cake enhanced
15
concentration polarization
10
The EIS signatures changed well
ahead of fouling revealed by TMP
Data from Wang, Sim, Gu, Coster & Fane J. Mem Sci
5
0
20
40
Time (hour)
60
Signatures of fouling
Cake
formation
The impedance signatures for fouling with Inorganics (eg
silica) are very different from those with organic foulants.
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ŵĞĐŚĂŶŝƐŵ
• Shift to the right in the Nyquist plots: slow
built up of the silica layer on the membrane
surface; Conductance in the concentration
polarization layer drops.
• Shift to the left: Impact of the cake enhanced
concentration polarization (CECP) effect; The
increased concentration polarization of NaCl
at the membrane surface increases the
overall conductance. More NaCl permeates
through the membrane which shows up in a
decrease in rejection.
20
Locating a “Canary” Membrane Fouling Monitor
in Water Treatment Plants
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EIS
Fouling Monitor
&ĞĞĚ
EIS
Fouling Monitor
WĞƌŵĞĂƚĞ
WƌŽĚƵĐƚ
Collaborators
SMTC, NTU
A.G. Fane
L. N. Sim
Z.J. Wang
C. Tang
Jia Shin
J. Gu
W. Lo
A. Yeo
Chemical Eng., USYD
T. Chilcott
J. Kavanagh
G. Barton
J. Cien
S. Hussain
T. Handelsman
UNESCO CMST, UNSW
G. Leslie
A. Anthony
Yuang Wang
Siao Yien Yeo
INPHAZE, Pty Ltd
Sydney
M. Darestani
D. Wang
T. Kausmann