Electrokinetics-enhanced Biodegradation of Heavy Polycyclic Aromatic Hydrocarbons in Soil Around Iron and Steel Industries

Abstract

Bioremediation is a safe and cost-effective technology for the removal of polycyclic aromatic hydrocarbons (PAHs) contaminated soils, but its remediation rate is usually very slow at soils contaminated with heavy PAHs in high concentrations. This paper describes the feasibility of using electrokinetics to enhance the degradation of heavy PAHs in soil around iron and steel industries. Three bench-scale experiments were conducted for 90 days using historically polluted soil with a total PAHs content of 220.01 mg/kg dry soil. All of the experiments were inoculated with PAHs degrading bacteria, but experiments II and III were performed using constant polarity and alternating polarity electrokinetic conditions, respectively. Results were compared with those from the control experiment (experiment I), which did not receive any electrokinetic treatment. The results demonstrated that the electrokinetic process could enhance the biodegradation extent of total PAHs and heavy PAHs in the soil. The final degradation extents of total PAHs were 9.5% and 13.5% higher in experiments II and III, respectively, as compared to experiment I. Under the electrokinetic and bacteria conditions, the relative enhancement in the degradation of four- to six-ring PAHs compared to the control experiment was much stronger and increased with increasing ring number. The final degradation extents of four- to six-ring PAHs increased by 7.9–8.6%, 11.0–18.4% and 17.2–25.6% in experiments II and III compared to experiment I, respectively. The results also showed that the electrokinetic operation mode could affect not only the degradation extent of total PAHs but also bacterial counts and soil moisture of different regions in soils. The use of alternating polarity electrokinetics was favorable to the bacterial growth and kept the soil properties uniform. In addition, there was a positive correlation between the degradation extent of PAHs, bacterial counts and moisture content by Pearson correlation analysis under electrokinetics. The results of this work demonstrate that the use of electrokinetics can significantly enhance the degradation of PAHs by influencing soil conditions. Therefore, the use of electrokinetic technology may provide a useful tool for enhancing the bioremediation of heavy PAHs in soil.

Full text

Electrochimica
Acta
85 (2012) 228–
234
Contents
lists
available
at
SciVerse
ScienceDirect
Electrochimica
Acta
jou
rn
al
h
om
epa
ge:
www.elsevier.com/locate/electacta
Electrokinetics-enhanced
biodegradation
of
heavy
polycyclic
aromatic
hydrocarbons
in
soil
around
iron
and
steel
industries
Fengmei
Lia,
Shuhai
Guoa,∗,
Niels
Hartogb
aInstitute
of
Applied
Ecology,
Chinese
Academy
of
Sciences,
Shenyang
110016,
China
bKWR
Watercycle
Research
Institute,
3433
PE
Nieuwegein,
The
Netherlands
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
19
April
2012
Received
in
revised
form
9
August
2012
Accepted
15
August
2012
Available online xxx
Keywords:
Electrokinetics
Heavy
polycyclic
aromatic
hydrocarbons
Biodegradation
a
b
s
t
r
a
c
t
Bioremediation
is
a
safe
and
cost-effective
technology
for
the
removal
of
polycyclic
aromatic
hydrocar-
bons
(PAHs)
contaminated
soils,
but
its
remediation
rate
is
usually
very
slow
at
soils
contaminated
with
heavy
PAHs
in
high
concentrations.
This
paper
describes
the
feasibility
of
using
electrokinetics
to
enhance
the
degradation
of
heavy
PAHs
in
soil
around
iron
and
steel
industries.
Three
bench-scale
experiments
were
conducted
for
90
days
using
historically
polluted
soil
with
a
total
PAHs
content
of
220.01
mg/kg
dry
soil.
All
of
the
experiments
were
inoculated
with
PAHs
degrading
bacteria,
but
experiments
II
and
III
were
performed
using
constant
polarity
and
alternating
polarity
electrokinetic
conditions,
respectively.
Results
were
compared
with
those
from
the
control
experiment
(experiment
I),
which
did
not
receive
any
electrokinetic
treatment.
The
results
demonstrated
that
the
electrokinetic
process
could
enhance
the
biodegradation
extent
of
total
PAHs
and
heavy
PAHs
in
the
soil.
The
final
degradation
extents
of
total
PAHs
were
9.5%
and
13.5%
higher
in
experiments
II
and
III,
respectively,
as
compared
to
experiment
I.
Under
the
electrokinetic
and
bacteria
conditions,
the
relative
enhancement
in
the
degradation
of
four-
to
six-ring
PAHs
compared
to
the
control
experiment
was
much
stronger
and
increased
with
increasing
ring
number.
The
final
degradation
extents
of
four-
to
six-ring
PAHs
increased
by
7.9–8.6%,
11.0–18.4%
and
17.2–25.6%
in
experiments
II
and
III
compared
to
experiment
I,
respectively.
The
results
also
showed
that
the
electrokinetic
operation
mode
could
affect
not
only
the
degradation
extent
of
total
PAHs
but
also
bacterial
counts
and
soil
moisture
of
different
regions
in
soils.
The
use
of
alternating
polarity
electroki-
netics
was
favorable
to
the
bacterial
growth
and
kept
the
soil
properties
uniform.
In
addition,
there
was
a
positive
correlation
between
the
degradation
extent
of
PAHs,
bacterial
counts
and
moisture
content
by
Pearson
correlation
analysis
under
electrokinetics.
The
results
of
this
work
demonstrate
that
the
use
of
electrokinetics
can
significantly
enhance
the
degradation
of
PAHs
by
influencing
soil
conditions.
There-
fore,
the
use
of
electrokinetic
technology
may
provide
a
useful
tool
for
enhancing
the
bioremediation
of
heavy
PAHs
in
soil.
© 2012 Elsevier Ltd. All rights reserved.
1.
Introduction
Polycyclic
aromatic
hydrocarbons
(PAHs)
are
a
class
of
poten-
tially
carcinogenic
organic
substances
composed
of
more
than
two
fused
benzene
rings
[1,2].
Natural
and
anthropogenic
processes
generate
PAHs
during
the
incomplete
combustion
of
all
types
of
organic
matter
[3].
Coal-
and
coke-burning
are
the
major
sources
of
PAHs
in
the
environment,
causing
soils
around
iron
and
steel
industries
to
become
heavily
contaminated
with
PAHs
[4],
typically
with
high
concentrations
of
heavy
PAHs
(>3
aromatic
rings).
Low
molecular
weight
PAHs,
with
two
or
three
fused
aromatic
rings,
have
a
relatively
higher
water
solubility
and
thus
degrade
more
easily,
while
high
molecular
weight
PAHs,
with
four
to
six
fused
∗Corresponding
author.
Tel.:
+86
024
83970449;
fax:
+86
024
83970448.
E-mail
addresses:
shuhaiguo@yahoo.com,
fmli2010@hotmail.com
(S.
Guo).
aromatic
rings,
are
quite
hydrophobic
and
more
difficult
to
break
down
[5].
As
heavy
PAHs
are
more
recalcitrant
and
may
persist
in
the
environment
for
a
long
time,
the
remediation
of
soils
around
iron
and
steel
industries
poses
a
great
challenge.
Bioremediation
is
typically
regarded
as
an
attractive
technology
for
removing
PAHs
from
soils
because
it
is
cost-effective
and
environmentally
accept-
able
[6].
However,
many
factors
hinder
the
biodegradation
of
PAHs
in
soils,
such
as
insufficient
bioavailability
of
the
contaminant
and
nutrient
[7].
In
recent
years
there
has
been
increasing
interest
in
employing
electrokinetic
remediation
to
overcome
the
problems
associated
with
bioremediation
[8–11].
Electrokinetic
remediation
is
an
in
situ
technology
that
con-
sists
of
the
controlled
application
of
a
low
intensity
direct
electric
current
through
the
contaminated
soil
between
appro-
priately
distributed
electrodes.
The
technique,
which
relies
on
three
process
– electromigration
(movement
of
charges),
electro-
osmosis
(water),
and
electrophoresis
(charged
particles)
– has
been
0013-4686/$
–
see
front
matter ©
2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2012.08.055

F.
Li
et
al.
/
Electrochimica
Acta
85 (2012) 228–
234 229
successfully
applied
to
remove
heavy
metals
from
the
soil
[12–14].
In
recent
years,
there
has
been
increasing
interest
in
the
treat-
ment
of
soil
contaminated
with
organic
pollutants.
Several
studies
have
indeed
demonstrated
improved
removal
of
organic
pollutants
such
as
gasoline
hydrocarbons,
aromatic
compounds,
herbicides
or
trichloroethylene
in
electric
fields
applied
to
soil
[15–17].
During
the
electrokinetic
treatment
of
a
soil,
nutrients
and
microorganisms
are
spread
throughout
the
soil
[18,19]
and
the
organic
pollutants
can
be
transported
to
the
area
harboring
microbial
populations
able
to
degrade
the
pollutants
[20–22].
The
electric
current
also
induces
redox
reactions
on
the
electrode
surfaces,
resulting
in
the
destruction
of
organic
compounds
[23,24].
In
addition,
the
electrical
current
can
cause
heating
of
the
soil,
which
can
be
ben-
eficial
for
contaminant
remediation
in
cold
climate
areas
[10].
Biodegradation
may
also
be
affected
indirectly
by
the
alteration
of
soil
properties
such
as
pH
and
moisture
content
caused
by
the
applied
electrical
field
[25].
So,
several
mechanisms
contribute
to
the
potential
of
electrokinetics
to
enhance
the
bioremediation
of
organic
contaminants.
While
some
researchers
have
successfully
applied
electrokinetics
to
enhance
the
biodegradation
of
vari-
ous
organic
compounds
[21,25–27],
studies
on
PAHs
have
mainly
focused
on
the
degradation
of
individual
PAHs
or
light
PAHs
[28–30].
Furthermore,
these
studies
have
mainly
been
based
on
experiments
using
artificially
contaminated
soils
[31];
mixed
and
heavy
PAHs
in
naturally
contaminated
soils
have
received
less
attention.
The
main
objectives
of
this
study
were
to
experimentally
inves-
tigate
the
enhancement
effect
of
electrokinetics,
in
particular
the
differences
under
conditions
of
constant
and
alternating
polarity
electrical
fields,
on
the
biodegradation
of
total
PAHs
and
heavy
PAHs
in
contaminated
soils
and
to
determine
the
possible
mecha-
nism(s)
involved.
We
used
historically
polluted
soil
around
Benxi
Iron
and
Steel
Group
Corporation
in
which
heavy
PAHs
make
up
the
majority
of
the
total
PAHs
contamination.
Benxi
is
an
old
industrial
city
in
northeastern
China
that
mainly
produces
iron
and
steel.
The
city
has
a
serious
PAH
pollution
problem
because
of
an
inadequate
industrial
structure,
as
well
as
special
climatic,
geographical
and
environmental
conditions
[4].
The
outcomes
of
this
study
are
aimed
at
providing
essential
reference
information
for
future
remediation
efforts
of
heavy
PAHs
in
soils
surrounding
iron
and
steel
industries.
2.
Experimental
2.1.
Soil
The
soil
used
in
this
study
was
a
sandy
loam,
obtained
from
the
topsoil
layer
(0–10
cm)
of
woodland
around
Benxi
Iron
and
Steel
Group
Corporation,
China.
In
order
to
keep
the
test
consis-
tent,
the
sample
was
prepared
by
air
drying
for
2
weeks,
and
sieved
(2
mm
mesh).
The
resulting
soil
sample
was
thoroughly
mixed
and
its
initial
characteristics
were
analyzed
according
to
standard
methods
for
soil
analysis
[32,33].
The
characteristics
were
as
fol-
lows:
20%
moisture;
pH
8.36;
1.77%
organic
mater;
0.073%
total
nitrogen;
0.046%
total
phosphorus;
a
particle
size
distribution
of
64.9%
sand,
21.9%
fine,
13.3%
clay;
and
a
total
PAH
concentration
of
220.01
mg/kg
dry
soil
(sum
of
11
EPA
PAHs).
The
concentration
and
composition
of
individual
PAHs
are
listed
in
Table
1.
Heavy
(four-
to
six-ring)
PAHs
accounted
for
92.19%
of
total
PAHs
in
the
study.
The
light
PAHs
phenanthrene
accounted
for
the
remaining
7.81%.
2.2.
Bacteria
The
PAHs-degrading
bacteria
were
isolated
from
contaminated
soil
samples
around
Benxi
Iron
and
Steel
Group
Corporation.
PAHs
were
also
extracted
from
this
soil,
which
were
then
Table
1
Concentration
and
composition
of
individual
PAHs.
Polycyclic
aromatic
hydrocarbons
Rings
Content
(mg/kg)
Percentage
(%)
Phenanthrene
(Phe)
3
17.19
±
0.034
7.81
Fluoranthene(Flu) 4 36.49 ±
0.225
16.59
Pyrene(Pyr) 4
29.50
±
0.541
13.41
Benzo(a)anthracene
(BaA)
4
9.41
±
0.336
4.28
Chrysene
(Chr) 4
18.93
±
0.185
8.61
Benzo(b)fluoranthene(BbF)
5
29.52
±
0.346
13.42
Benzo(k)fluoranthene(BkF)
5
11.91
±
0.432
5.41
Benzo(a)pyrene(BaP)
5
21.92
±
0.234
9.96
Indeno(1,2,3-cd)pyrene(IcdP) 5 3.00 ±
0.196
1.36
Dibenzo(a,h)anthracene(DahA)
6
21.66
±
0.098
9.85
Benzo(g,h,i)perylene(BghiP) 6
20.47
±
0.076
9.30
added
to
the
mineral
media
to
provide
the
sole
carbon
source
(500
mg/L).
The
mineral
media
contained
KH2PO4(0.9
g/L),
K2HPO4
(0.1
g/L),
NH4NO3(0.1
g/L),
MgSO4·7H2O
(0.1
g/L),
CaCl2(0.08
g/L),
FeCl3·6H2O
(0.01
g/L),
and
1
mL/L
of
microelements
stock
to
solu-
tion
with
a
pH
value
of
7.0
[23].
All
the
chemicals
used
were
of
analytical
grade.
The
bacterial
cells
were
grown
in
the
mineral
media
on
a
shaker
at
28 ◦C
and
160
rpm
and
collected
by
cen-
trifugation
when
they
were
in
the
exponential
growth
phase.
After
washing
twice
with
sterilized
water,
they
were
resuspended
in
the
mineral
media
to
obtain
a
highly
concentrated
bacterial
suspension
for
the
experiments.
2.3.
Experimental
setup
Fig.
1
shows
a
schematic
diagram
of
the
experimental
test
setup
used.
It
consisted
of
a
soil
cell,
two
pairs
of
electrodes,
an
elec-
trode
control
system
and
a
power
supply.
The
soil
cell
was
made
of
Perspex
with
inner
dimensions
of
24
cm
length,
12
cm
width
and
10
cm
height.
Column-shaped
stainless
steel
electrodes
with
a
length
of
12
cm
and
a
diameter
of
0.5
cm
were
used
to
generate
the
electric
field.
The
lateral
distance
between
the
two
electrode
pairs
was
6
cm.
The
electrode
autocontrol
system
(Siemens)
was
capable
of
alternating
the
polarity
of
the
electric
field,
thus
allow-
ing
testing
of
different
operation
modes
during
the
experiments.
The
power
supply
provided
a
constant
direct
current
(DC)
electri-
cal
potential
difference
of
24
V
for
the
experiments
(1
V/cm).
During
operation
the
experimental
set-up
was
covered
by
a
lid
to
prevent
evaporation.
2.4.
Testing
procedure
The
bacterial
suspension
was
mixed
directly
into
the
soil
while
preparing
the
soil
specimen
at
an
initial
inoculation
in
the
order
of
5.21
×
107cfu
(colony-forming
units)/g
dry
soil.
The
soil
was
then
rehydrated
to
moisture
content
about
20%
(w/w)
with
sterilized
mineral
media.
The
wet
soil
was
tamped
into
the
soil
cell
in
layers
with
a
pressure
of
0.1
kg/cm2to
a
height
of
8
cm
[34]
(Fig.
1).
The
Fig.
1.
Schematic
of
the
experimental
setup
(
,
sampling
location).

230 F.
Li
et
al.
/
Electrochimica
Acta
85 (2012) 228–
234
electrodes
were
immersed
directly
into
the
soil
and
connected
to
the
power
supply.
This
procedure
was
repeated
for
all
three
exper-
iments.
In
experiment
I,
no
electrokinetic
treatment
was
applied
(control).
In
experiment
II,
the
electrical
field
was
provided
using
constant
polarities
and
in
experiment
III
the
electrical
field
was
provided
using
polarities
alternating
at
the
electrodes
every
12
h.
Each
experiment
was
conducted
at
room
temperature
(25
±
1◦C),
which
was
controlled
by
air
conditioning.
Three
parallel
sampling
lines
along
the
length
of
the
soil
cell,
each
with
six
equally
spaced
sampling
points,
provided
a
total
of
eighteen
soil
samples
per
samp-
ling
round
at
0
day,
7
days,
14
days,
28
days,
60
days
and
90
days
(Fig.
1).
Samples
(10
mm
diameter
×
8
cm
height)
were
collected
by
a
metal
tubing
(10
mm
inner
diameter).
For
each
sampling
round,
the
samples
at
the
same
distance
from
the
three
sampling
lines
were
thoroughly
mixed
together
to
form
one
composite
sample
before
analysis.
2.5.
Analytical
procedure
The
concentration
of
PAHs,
bacterial
concentration,
soil
pH,
temperature
and
moisture
content
were
analyzed,
all
analytical
determinations
were
performed
in
triplicate,
and
the
results
were
calculated
as
means.
All
data
obtained
in
the
study
are
presented
as
mean
±
standard
deviation
(SD).
Statistical
analysis
was
performed
with
SPSS
for
Windows
Ver.
11.5.
Bacteria
concentration
was
determined
by
preparing
serial
dilu-
tions
of
soil
suspensions
in
sterilized
water,
plating
onto
LB-agar
plates,
and
incubating
at
30 ◦C
for
3
days.
Bacterial
counts
were
calculated
per
gram
of
air-dried
soil
and
log-transformed
(log10 x)
to
improve
the
homogeneity
of
the
variance
of
the
data.
Moisture
content
was
measured
by
heating
to
105 ◦C
until
constant
weight
was
achieved
and
soil
pH
was
measured
by
pH
meter
under
a
soil-
to-water
ratio
of
1:2.5
[35].
The
soil
temperature
was
monitored
and
taken
down
by
digital
thermometer
every
day.
The
total
PAHs
was
extracted
according
to
the
EPA
Standard
Method
3550C
(USEPA,
1996)
[36].
The
concentrations
of
16
PAHs
(US
EPA
priority
PAHs)
were
determined
by
high
performance
liquid
chromatography
(HPLC,
Waters)
equipped
with
a
vari-
able
wavelength
fluorescence
detector
(FLD,
waters
2475)
and
a
Waters
PAHs
Column
(250
mm
×
4.6
mm
i.d.,
5
␮m
particle
size).
Prior
to
injection,
the
extraction
of
PAHs
was
filtered
through
a
0.22
␮m
Teflon
filer.
The
injection
volume
was
set
at
10.0
␮l
and
the
column
temperature
was
25.0 ◦C.
The
gradient
elution
pro-
gram
used
consisted
of
60%
water
and
40%
acetonitrile
for
2
min,
then
programmed
to
100%
acetonitrile
in
12
min
at
a
flow
rate
of
1.0
ml/min.
3.
Results
and
discussion
3.1.
Changes
in
soil
pH
The
changes
of
soil
pH
in
the
three
experiments
are
presented
in
Fig.
2.
The
average
values
of
soil
pH
did
not
show
significant
changes
during
the
experiment
period
(Fig.
2A).
The
change
extents
of
soil
pH
were
between
0.1%
and
1.4%
in
three
experiments.
Fig.
2B
shows
the
changes
of
soil
pH
with
distance
from
the
anode
at
90
days.
The
results
showed
that
the
soil
pH
remained
broadly
constant
in
experiments
I
(control)
and
III
(alternating
polarity
electrokinetics).
But
the
soil
pH
had
a
little
change
at
the
anode
and
the
cathode
in
experiment
II
(constant
polarity
electrokinetics).
The
pH
of
the
soils
decreased
to
8.13
near
the
anode
while
increased
to
8.40
near
the
cathode
(Fig.
2B),
and
this
was
attribute
to
the
electrolysis
reac-
tion.
The
electrolysis
of
water
results
in
the
formation
of
H+at
the
anode
(low
pH
region)
and
OH−at
the
cathode
(high
pH
region)
under
constant
polarity
electrokinetics
[37–39].
However,
the
H+
and
OH−generated
at
electrodes
may
be
neutralized
automatically
under
alternating
polarity
electrokinetics.
Gee
et
al.
proposed
that
soil
buffer
capacity
increased
as
the
amount
of
calcium
carbonates
in
the
soil
increased
[40].
The
test
soils
had
a
pH
of
8.36
and
a
high
buffering
capacity
because
of
the
high
carbonate
mineral
con-
tent
[41].
Thus,
test
soils
kept
the
weak
alkaline
all
the
time
under
electric
fields.
3.2.
Changes
in
moisture
content
Applying
an
electric
field
may
induce
changes
in
soil
pH,
mois-
ture
content
and
soil
temperature,
further
influencing
the
growth
and
distribution
of
soil
microorganisms
[42–44].
During
the
exper-
imental
period,
the
soil
temperature
presented
little
change
and
was
consistent
with
room
temperature
(25
±
1◦C).
Although
the
tests
showed
that
the
electric
field
did
not
influence
soil
pH
and
temperature,
it
did
affect
soil
moisture
content
and
distribution
(Figs.
3
and
4).
Over
the
duration
of
study,
the
soil
moisture
content
of
experiments
II
and
III
(with
electrokinetics)
decreased
by
7.0%
and
6.6%,
about
one
time
greater
than
in
the
control
experiment
(I)
(3.8%)
(Fig.
3).
Under
constant
polarity
electrokinetics,
soil
mois-
ture
content
decreased
at
the
anode
and
increased
at
the
cathode,
an
indication
of
pore
liquid
transport
due
to
electro-osmosis
[38].
After
running
for
14
days,
the
soil
moisture
content
near
the
anode
dropped
by
5.1%
while
near
the
cathode
it
increased
by
4.5%
in
experiment
II
(Fig.
4A).
Fig.
4B
shows
that
under
alternating
polarity
electrokinetics
soil
moisture
content
distribution
remained
even.
In
experiment
III,
the
soil
moisture
content
stayed
within
the
range
Fig.
2.
Changes
in
soil
pH
in
the
three
experiments.
(A)
During
the
experimental
period
and
(B)
at
90
days.

F.
Li
et
al.
/
Electrochimica
Acta
85 (2012) 228–
234 231
Fig.
3.
Changes
in
soil
moisture
content
in
the
three
experiments.
18.4–22.6%
within
14
days,
indicating
that
alternating
polarity
elec-
trokinetics
could
overcome
pore
liquid
single-direction
transport,
thus
causing
soil
moisture
content
to
be
subject
to
little
change
(Fig.
4B).
In
addition,
alternating
polarity
electrokinetics
may
also
have
helped
stabilize
homogenous
distribution
and
availability
of
ionic
nutrients
[45,46].
From
the
results
of
these
two
experiments,
over
the
period
28–90
days
the
moisture
content
have
the
same
tendency
as
the
changes
during
the
period
0–14
days.
The
result
is
identical
to
the
findings
of
Luo
et
al.
[39].
Therefore,
alternating
polarity
electrokinetics
may
be
favorable
for
in
situ
bioremediation.
3.3.
Changes
in
culturable
bacterial
counts
The
counts
of
culturable
bacteria
(colony-forming
unit,
cfu)
in
soils
changed
during
the
tests,
as
shown
in
Fig.
5.
Bacterial
counts
remained
constant
in
the
control
experiment
(I)
over
the
experi-
mental
period
(i.e.
7.67–7.82
log10 cfu/g
dry
soil).
In
experiments
II
and
III,
bacteria
counts
increased
rapidly
within
the
first
28
days
of
testing
and
then
started
to
decrease
gradually.
The
bacterial
counts
of
experiments
II
and
III
(with
electrokinetics)
increased
by
an
order
of
magnitude
of
0.26–2.58
compared
to
the
control
experiment
(I).
In
experiment
III
(alternating
polarity
electrokinetics),
the
counts
of
bacteria
were
higher
by
9.3%
compared
to
experiment
II
(con-
stant
polarity
electrokinetics).
These
findings
show
that
the
activity
Fig.
5.
Changes
in
bacterial
counts
as
the
log
of
colony
forming
units
(cfu)
in
the
three
experiments.
of
PAHs-degrading
bacteria
is
stimulated
when
exposed
to
appro-
priate
electrokinetics
and
an
alternating
polarity
approach
is
better
for
bacterial
growth.
This
observed
influence
of
electrokinetic
treat-
ment
on
microbial
activity
and
community
is
in
line
with
the
results
of
Kim
et
al.
and
Lear
et
al.
[47,9].
The
bacterial
counts
also
varied
spatially
under
electrokinetic
conditions,
as
shown
in
Fig.
6.
In
the
experiment
II
with
constant
polarity
electrokinetics,
the
bacterial
counts
gradually
increased
from
anode
to
cathode
(Fig.
6A).
Peak
bacterial
counts
were
observed
at
18
cm
from
the
anode
with
counts
increasing
from
7.71
to
10.31
log10 cfu/g
dry
soil,
while
near
the
anode
counts
dropped
to
about
6.24
log10 cfu/g
dry
soil.
This
result
suggests
that,
under
con-
stant
polarity
electrokinetics,
proximity
to
the
cathode
as
opposed
to
the
anode
is
more
beneficial
for
bacterial
growth.
Under
alter-
nating
polarity
electrokinetics,
bacterial
counts
in
experiment
III
increased
from
7.71
to
10.93
log10 cfu/g
dried
soil
and
were
higher
in
the
middle
region
rather
than
near
the
electrodes
(Fig.
6B).
The
test
soils
had
a
pH
of
8.36
and
a
high
acid
buffering
capacity
because
of
the
high
carbonate
mineral
content
[41].
During
the
tests,
the
soil
average
pH
did
not
show
any
apparent
changes
and
stayed
weakly
alkaline
in
all
experiments
(Fig.
2).
Thus,
the
changes
of
the
soil
moisture
content
and
ionic
nutrients
in
water
may
be
the
factors
for
influencing
bacterial
activity
and
counts
(Figs.
4
and
6)
[48].
Fig.
4.
Changes
in
soil
moisture
content
under
different
electrokinetic
operation
modes.
(A)
Experiment
II
and
(B)
experiment
III.

232 F.
Li
et
al.
/
Electrochimica
Acta
85 (2012) 228–
234
Fig.
6.
Changes
in
bacterial
counts
under
different
electrokinetic
operation
modes.
(A)
Experiment
II
and
(B)
experiment
III.
3.4.
Total
PAHs
degradation
Fig.
7
presents
the
average
degradation
extent
of
total
PAHs
for
all
samples
(18
samples)
in
the
three
experiments.
As
is
evident,
all
three
experiments
were
similar
insofar
as
their
degradation
extent
was
fastest
in
the
initial
stages
and
then
stalled
toward
the
end
of
each
experiment
at
90
days.
On
the
other
hand,
results
differed
in
that
the
degradation
extent
of
total
PAHs
was
significantly
higher
in
experiments
II
and
III
(electrokinetics
and
bacteria)
than
in
the
control
experiment
(I)
(bacteria
only).
Under
continuous
operation
for
90
days,
the
degradation
extent
of
total
PAHs
in
experiments
II
and
III
reached
24.3%
and
28.3%,
respectively,
which
was
about
9.5%
and
13.5%
higher
than
in
experiment
I
(14.8%).
In
addition,
the
total
PAHs
degradation
extent
in
experiment
III
was
higher
than
that
in
experiment
II,
indicating
that
alternating
polarity
electrokinetics
enhanced
the
biodegradation
of
PAHs
more
effectively
than
with
constant
polarity
electrokinetics.
Electric
fields
have
been
applied
in
soil
to
increase
contact
opportunities
among
bacteria,
nutrients
and
pollutants
[24].
When
an
alternating
polarity
electrokinetics
is
imposed
upon
soil,
it
can
change
the
movement
direction,
and
hence
may
produce
more
opportunities
for
the
bacteria,
nutrients
and
contaminants
in
soils
to
contact
and
interact
with
each
other
[29].
This
result
is
similar
to
those
found
by
Luo
et
al.
and
Fan
et
al.
when
they
applied
an
alternating
direct
current
to
phenol-
contaminated
soil
[34,39].
Fig.
7.
Progression
of
the
total
PAHs
degradation
extents
during
the
experimental
period.
Fig.
8
shows
a
comparison
of
PAHs
degradation
extents
in
rela-
tion
to
distance
from
the
anode
at
different
times
throughout
the
course
of
experiments
II
and
III.
The
results
show
that
the
elec-
trokinetic
operation
mode
could
affect
the
degradation
extent
of
total
PAHs
of
different
regions
in
soils.
For
experiment
II,
which
used
constant
polarity
electrokinetics,
the
degradation
of
PAHs
was
limited
near
the
anode,
but
gradually
increased
and
peaked
at
a
distance
of
18
cm,
before
decreasing
slightly
toward
the
cathode
(Fig.
8A).
However,
under
the
alternating
conditions
of
experiment
III,
the
degradation
extent
of
PAHs
was
basically
equal
from
anode
to
cathode,
albeit
slightly
lower
near
to
both
electrodes
(Fig.
8B).
Compared
to
Figs.
4,
6
and
8,
there
was
the
same
tendency
among
soil
moisture
content,
culturable
bacterial
counts
and
degradation
extent
of
PAHs.
These
results
indicated
that
the
changes
of
soil
moisture
content
and
bacterial
counts
affected
the
degradation
extents
of
PAHs.
These
observations
are
similar
to
those
of
Harbottle
et
al.
who
studied
the
use
of
alternating
polarity
electrokinetics
to
enhance
the
biodegradation
of
pentachlorophenol
in
unsaturated
soil
[27].
3.5.
Degradation
of
heavy
PAHs
Fig.
9
shows
the
differences
in
degradation
extents
for
three-
to
six-ring
PAHs
in
the
three
experiments.
Compared
to
the
con-
trol
experiment
(I),
the
degradation
extents
were
higher
in
the
electrokinetic
experiments
(II
and
III)
for
all
ring
numbers.
Further-
more,
for
four-
to
six-ring
PAHs,
degradation
extents
were
more
enhanced
at
90
days,
indicating
that
electrokinetics
could
promote
the
biodegradation
of
heavy
PAHs.
For
the
three-ring
PAHs,
the
degradation
extents
were
higher
in
experiments
II
and
III
than
experiment
I
from
14
days
until
the
conclusion
of
the
experiments
at
90
days,
with
values
being
almost
the
same
in
all
three
exper-
iments
at
90
days
(Fig.
9A).
However,
for
four-
to
six-ring
PAHs,
the
degradation
extents
increased
significantly
from
14
days
up
until
90
days
in
experiments
II
and
III,
and
the
degradation
extent
increased
as
the
ring
number
increased
(Fig.
9B–D).
It
may
be
due
to
varying
resonance
properties
(be
induced
by
electrokinet-
ics)
of
different
size
PAHs
and
degradation
microorganisms,
and
that
further
research
into
these
particular
effects
is
required.
The
four-
to
six-ring
PAHs
degradation
extents
increased
by
7.9–8.6%,
11.0–18.4%
and
17.2–25.6%
in
experiments
II
and
III
compared
to
experiment
I,
respectively,
which
were
far
higher
than
that
for
three-ring
PAHs
(2.2%)
at
90
days
(Fig.
9B–D).
These
results
indi-
cate
that
electrokinetics
could
promote
the
biodegradation
of
heavy
PAHs
and
electro-bioremediation
is
a
potentially
useful
approach
for
the
treatment
of
soil
contaminated
with
heavy
PAHs.
The
results

F.
Li
et
al.
/
Electrochimica
Acta
85 (2012) 228–
234 233
Fig.
8.
Changes
in
total
PAHs
degradation
extents
under
different
modes
of
electrokinetic
operation.
(A)
Experiment
II
and
(B)
experiment
III.
also
show
that
alternating
polarity
electrokinetics
is
more
favorable
for
PAHs
degradation
than
constant
polarity
electrokinetics.
3.6.
Relationships
among
degradation
extent,
bacterial
counts
and
soil
moisture
content
The
present
study
has
shown
that
the
electrokinetic
operation
mode
could
affect
soil
moisture
content
(Figs.
3
and
4),
bacterial
counts
(Figs.
5
and
6)
and
the
degradation
extent
(Figs.
7
and
8).
Moreover,
the
distribution
change
profiles
of
the
above
three
fac-
tors
have
the
same
tendency
under
electrokinetics.
In
order
to
further
study
the
relationships,
Pearson
correlation
analysis
among
degradation
extent,
bacterial
counts
and
moisture
content
was
per-
formed,
as
shown
in
Table
2.
The
results
show
that
degradation
extent
and
bacterial
counts,
as
well
as
degradation
extent
and
moisture
content,
have
significant
positive
correlations,
reaching
0.919
and
0.787,
respectively
(significant
at
the
0.01
level).
Further-
more,
correlation
between
bacterial
counts
and
moisture
content
also
0.643
(significant
at
the
0.05
level).
Electrokinetics
inevitably
impacts
upon
soil
conditions
[42],
such
as
pH
and
soil
moisture
content,
which
are
critical
for
soil
microbial
community
compo-
sition
and
processes
[37].
In
this
study,
electrokinetics
resulted
Fig.
9.
Degradation
extents
of
three-
to
six-ring
PAHs
under
different
operation
modes.
(A)
Three
rings;
(B)
four
rings;
(C)
five
rings;
(D)
six
rings.

234 F.
Li
et
al.
/
Electrochimica
Acta
85 (2012) 228–
234
Table
2
Pearson
correlation
analysis
among
degradation
extent,
bacteria
counts
and
moisture
content.
Degradation
extent
(%)
Bacteria
counts
(log
cfu/g
w.d.)
Moisture
content
(%)
Degradation
extent
(%)
1
0.919** 0.787**
Bacteria
counts
(log
cfu/g
w.d.) 0.919*1
0.643*
Moisture
content
(%) 0.787** 0.643*1
*Correlation
is
significant
at
the
0.05
level
(2-tailed).
** Correlation
is
significant
at
the
0.01
level
(2-tailed).
in
the
movement
of
soil
moisture
content
which
transported
the
nutrients
(e.g.
K+and
NH4+)
[44,45,49],
thus
affecting
the
soil
bacterial
counts
and
activity,
which
in
turn
influenced
the
degra-
dation
extent
of
PAHs.
4.
Conclusions
This
study
has
shown
that
electrokinetics
may
enhance
the
biodegradation
of
heavy
PAHs.
Moreover,
alternating
polarity
electrokinetics
has
been
found
to
be
more
efficient
for
the
biodegradation
of
heavy
PAHs
than
constant
polarity
electroki-
netics.
Under
alternating
polarity
electrokinetics,
the
properties
of
soil
are
less
impacted
upon
and
adapt
to
the
growth
of
microorganisms.
Therefore,
electro-bioremediation
is
a
poten-
tial
approach
for
the
treatment
of
soil
contaminated
with
heavy
PAHs.
Acknowledgments
We
gratefully
acknowledge
the
constructive
comments
of
the
three
reviewers
and
their
suggestions
for
improving
the
paper.
The
funding
for
this
work
was
provided
by
the
project
supported
by
the
National
Natural
Science
Foundation
of
China
(21047006)
and
the
Knowledge
Innovation
Project
Key-Direction
Project
Sub-project
of
Chinese
Academy
of
Sciences
(No.
KZCX2-EW-407).
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