A study on microwave removal of pyridine from wastewater

Abstract

Pyridine is toxic and volatile N-containing organic pollutant. It occurs in the environment as effluents from different industries such as herbicides and pesticides manufacturing. Pyridine contaminated wastewater presents a great threat on water resources safety. In this study, a fundamental research had been carried out to explore the removal of pyridine in wastewater by microwave (MW) radiation. The effects of pH, radiation time, aeration, post irradiation and initial pyridine concentration on the removal were investigated. pH and radiation time showed significant influence on the removal efficiency. The largest removal was obtained at pH 9 in 5 min with initial pyridine-concentration of 20 ppm. The mechanism of pyridine removal was proposed as both thermal and non-thermal effects, which were responsible for the removal. In the end, it could be proposed that MW radiation was an effective method for the removal of pyridine from wastewater.

Full text

A
study
on
microwave
removal
of
pyridine
from
wastewater
O.A.
Zalat,
M.A.
Elsayed *
Egyptian
Armed
Forces,
Cairo,
Egypt
Introduction
Pyridine
and
its
derivatives
are
volatile,
toxic,
and
flammable
with
a
pungent
and
unpleasant
odor.
Exposure
to
pyridine
has
harmful
effects
on
the
liver,
kidneys,
immune
systems
and
reproductive
functions,
and
has
potential
carcinogenicity
[1–7].
Industrial
wastewaters
containing
pyridine
and
its
derivatives
show
toxicity
to
aquatic
life
and
create
nuisance
because
of
their
malodorous
and
unpleasant
smell
[6,8,9].
Pyridine
is
a
basic
heterocyclic
organic
with
the
chemical
formula
C
5
H
5
N.
It
is
structurally
related
to
benzene,
with
one
C–H
group
replaced
by
a
nitrogen
atom
as
shown
in
Fig.
1.
Pyridine
is
a
colorless
liquid
that
boils
at
115.2
8C
and
freezes
at
41.6
8C.
Its
density,
0.9819
g/cm
3
,
is
close
to
that
of
water.
Table
1
shows
the
physical
properties
of
pyridine.
Most
chemical
properties
of
pyridine
are
typical
of
a
heteroaromatic
compound.
In
organic
reactions,
pyridine
behaves
both
as
a
tertiary
amine,
undergoing
protonation,
alkylation,
acylation,
and
N-oxidation
at
the
nitrogen
atom,
and
as
an
aromatic
compound,
undergoing
nucleophilic
substitutions
[1,10].
Historically,
pyridine
was
produced
from
coal
tar
and
as
a
by-
product
of
the
coal
gasification.
However,
increased
demand
for
pyridine
resulted
in
the
development
of
more
economical
methods
of
synthesis
from
acetaldehyde
and
ammonia,
and
more
than
20,000
tons
per
year
are
manufactured
worldwide.
Pyridine
and
its
derivatives
may
enter
the
environment
as
a
consequence
of
their
extensive
use
as
insecticides
and
herbicides
in
agriculture
and
through
industrial
activities
associated
with
pharmaceutical
and
textile
manufacture
and
chemical
synthesis
[1].
The
pyridine
concentration
in
the
wastewaters
emanating
from
the
plants
manufacturing
pyridine
and
its
derivatives
is
generally
in
the
range
of
20–300
mg
dm
3
.
During
emergency
spills,
the
concentration
can
be
as
high
as
600–1000
mg
dm
3
.
At
a
pyridine
concentration
of
0.82
mg
dm
3
in
wastewaters,
unpleasant
pyri-
dine
odor
is
easily
detectable
[11].
Although
no
pyridine
concentration
limit
has
been
prescribed
in
the
industrial
waste-
waters
for
their
safe
discharge
into
sewers
or
on
land,
it
is
recommended
that
the
pyridine
concentration
in
wastewater
should
not
exceed
1
mg
dm
3
.
This
is
to
minimize
its
toxicity
and
to
control
the
odor
[1].
Wastewater
that
contains
pyridine
at
low
concentration
is
treated
in
multiple-effect
evaporators
and
incinerators.
However,
the
process
is
energy
intensive
[12].
Adsorption
has
been
used
for
the
removal
of
pyridine
and
its
derivatives
from
wastewater.
Adsorbents
such
as:
Rundle
oil
shale
[12],
montmorillonite
and
kaolinites
[13],
Al
2
O
3
and
iron
powders
[14],
zeolite
[15],
sepiolite
[16],
granular
activated
carbon
(GAC)
[6],
activated
carbons
from
coconut
fibers
and
shells
[17–19],
ion-
exchange
and
porous
resins
[20]
and
bagasse
fly
ash
(BFA)
[7–9]
have
been
used
for
the
removal
of
pyridine.
Mohan
et
al.
[17–19]
have
been
reported
detailed
to
remove
pyridine.
Since
several
decades
the
potential
use
of
microwave
technol-
ogy
as
an
energy-efficient
alternative
to
current
heating
technolo-
gies
in
waste-streams
was
investigated
[21–23].
In
recent
years,
microwave
(MW)
radiation
has
attracted
a
great
deal
of
attention
Journal
of
Environmental
Chemical
Engineering
1
(2013)
137–143
A
R
T
I
C
L
E
I
N
F
O
Article
history:
Received
8
February
2013
Received
in
revised
form
7
April
2013
Accepted
14
April
2013
Keywords:
Pyridine
Microwave
radiation
Wastewater
treatment
Post
irradiation
Non-thermal
effects
A
B
S
T
R
A
C
T
Pyridine
is
toxic
and
volatile
N-containing
organic
pollutant.
It
occurs
in
the
environment
as
effluents
from
different
industries
such
as
herbicides
and
pesticides
manufacturing.
Pyridine
contaminated
wastewater
presents
a
great
threat
on
water
resources
safety.
In
this
study,
a
fundamental
research
had
been
carried
out
to
explore
the
removal
of
pyridine
in
wastewater
by
microwave
(MW)
radiation.
The
effects
of
pH,
radiation
time,
aeration,
post
irradiation
and
initial
pyridine
concentration
on
the
removal
were
investigated.
pH
and
radiation
time
showed
significant
influence
on
the
removal
efficiency.
The
largest
removal
was
obtained
at
pH
9
in
5
min
with
initial
pyridine-concentration
of
20
ppm.
The
mechanism
of
pyridine
removal
was
proposed
as
both
thermal
and
non-thermal
effects,
which
were
responsible
for
the
removal.
In
the
end,
it
could
be
proposed
that
MW
radiation
was
an
effective
method
for
the
removal
of
pyridine
from
wastewater.
ß
2013
Elsevier
Ltd
All
rights
reserved.
*
Corresponding
author
at:
Department
of
Chemical
Engineering,
Military
Technical
College,
Egyptian
Armed
Forces,
Cairo,
Egypt.
Tel.:
+20
1141750544;
fax:
+20
222621918.
E-mail
address:
aboelfotoh@gmail.com
(M.A.
Elsayed).
Contents
lists
available
at
SciVerse
ScienceDirect
Journal
of
Environmental
Chemical
Engineering
jou
r
n
al
h
o
mep
ag
e:
w
ww
.elsevier
.co
m
/loc
ate/jec
e
2213-3437/$
–
see
front
matter
ß
2013
Elsevier
Ltd
All
rights
reserved.
http://dx.doi.org/10.1016/j.jece.2013.04.010

due
to
the
molecular-level
heating,
which
leads
to
homogeneous
and
quick
thermal
reactions
[24].
MW
radiation
has
been
applied
in
the
field
such
as
organic
and
inorganic
synthesis
[25,26],
polymerization
processes
[27],
biological
aspects
[28]
and
extrac-
tion
in
analytical
chemistry
[29].
Researchers
have
attempted
the
use
of
MW
radiation
in
wastewaters
treatment.
It
has
been
applied
to
remove
dyes
from
wastewaters
[16].
After
extensive
search,
no
reports
on
the
removal
of
pyridine
from
wastewater
by
MW
radiation
have
been
determined.
Applying
MW
radiation
for
waste
destruction
is
attracting
due
to
its
molecular-level
heating
which
leads
to
homogeneous
and
quick
thermal
reactions
[9,30].
The
aim
of
this
work
is
to
use
MW
radiation
to
remove
pyridine
from
pyridine-bearing
wastewater
and
to
optimize
the
operating
conditions.
A
proposed
mechanism
of
pyridine
removal
from
wastewater
is
presented
in
this
work
as
well.
Experimental
Chemicals
Pyridine
standard
solution
was
supplied
by
Fluka
with
purity
better
than
98.0%.
It
was
used
to
prepare
a
synthetic
wastewater.
Aqueous
solutions
were
made
using
deionized
water,
which
was
prepared
by
an
Elga
B114
Deionizer
using
C114
cartridges
(EC
=
5
m
S
cm
@
25
8C
and
TDS
=
3.5
ppm).
Procedures
and
equipments
The
schematic
diagram
of
the
applied
experimental
apparatus
to
remove
pyridine
is
shown
in
Fig.
2.
A
modified
domestic
microwave
oven
(800
W,
2450
MHz,
Hot
Plait
Co.,
Korea)
with
different
power
setting
was
used
as
MW
source.
A
300-ml
glass
column
reactor
was
placed
in
the
oven.
The
reactor
was
filled
with
50
ml
of
wastewater
and
radiated
by
MW
under
different
conditions.
At
the
bottom
of
the
MW
oven,
a
Teflon
plate
was
fixed
to
act
as
a
cell
holder.
The
top
back
of
the
MW
oven
was
adapted
to
be
connected
to
gas
flow
system.
The
aeration
rate
was
maintained
at
1
L/min
by
an
air
compressor.
All
the
system
was
placed
in
a
suction
cupboard
to
collect
the
generated
vapors
during
the
process.
The
initial
pH
of
the
solution
was
adjusted
by
adding
1–3
drops
of
NaOH
(0.1
M)
and
HCl
(0.1
M).
A
primarily
experiments
showed
that,
the
addition
of
a
diluted
solution
of
NaOH
and
HCl
without
MW
radiation
are
inert
in
the
pyridine
solution
and
cause
no
change
in
its
concentration
measured
by
GC.
A
thermometer
was
used
for
the
measurement
of
temperature
at
the
end
of
radiation.
The
top
of
the
column
was
connected
to
a
condensing
system
to
control
wastewater
volume.
The
generated
contaminant
vapor
passed
through
two
absorption
vessels
containing
H
2
O
solution
to
collect
the
generated
vapors
during
the
process.
On
the
other
hands,
during
the
MW
treatment
process,
the
volume
of
wastewater
slightly
decreased
due
to
evaporation
of
water.
When
the
wastewater
was
cooled
to
room
temperature,
deionized
water
was
added
into
the
reactor
to
keep
the
same
initial
volume
of
the
wastewater.
Pyridine
concentration
was
determined
by
measuring
its
absorbance
at
l
max
=
254
nm
using
UV–vis
spectrophotometer.
Solutions
irradiated
for
different
lengths
of
time
were
analyzed
by
GC–MS
and
HPLC.
Analysis
GC–MS
analyses
were
performed
on
a
Shimadzu
QP
2000
instrument,
equipped
with
an
Equity-5
column
(Supelco)
(30
m

0.25
mm

0.25
m
m),
coated
with
5%
phenyl
95%
methyl
poly-siloxane.
Separation
of
the
by-products
was
conducted
under
the
following
chromatographic
conditions:
injector
temperature
240
8C,
oven
temperature
program
50
8C
ramped
at
5
8C
min
1
–
250
8C
followed
by
another
ramp
of
10
8C
min
1
–290
8C
held
for
2
min.
Helium
was
used
as
carrier
gas
at
a
flow
of
1
ml
min
1
.
The
temperatures
of
the
ion
source
and
the
interface
were
set
at
240
8C
and
290
8C,
respectively.
The
MS
operated
in
electron
ionization
mode
with
a
potential
of
70
eV
and
the
spectra
were
obtained
at
a
scan
range
from
m/z
50–450
(full
scan
mode).
The
scan
time
was
46
min
and
1.0
m
l
injections
were
made
using
a
split
ratio
varying
from
2
to
20.
High-performance
liquid
chromatography
(HPLC)
has
also
been
used
to
measure
pyridine
concentration
as
per
described
protocol.
This
method
has
the
advantage
of
compatibility
with
the
liquid
matrix
samples.
In
order
to
identify
the
degradation
of
pyridine,
different
samples
were
analyzed
by
HPLC
direct
injection.
The
samples
were
analyzed
using
a
SielcPrimesep
A,
4.6
mm

250
mm,
5
m
particle
size,
100
A
˚pore
size,
reverse
phase,
acidic
column.
A
5:95
acetonitrile:water
solution
was
used
as
an
isocratic
mobile
phase
at
1
ml
min
1
flow-rate
at
30
8C.
The
Fig.
1.
The
chemical
structure
of
pyridine.
Table
1
The
physical
properties
of
pyridine.
Properties
Molecular
formula
C
5
H
5
N
Molar
mass
79.1
g
mol
1
Appearance
Colorless
liquid
Density
0.9819
g/cm
3
,
liquid
Melting
point
41.6
8C,
232
K,
43
8F
Boiling
point
115.2
8C,
388
K,
239
8F
Solubility
in
water
Miscible
Vapor
pressure
18
mmHg
Acidity
(pK
a
)
5.25
Reflective
index
1.5093
Viscosity
0.88
cP
Dipole
moment
2.2
D
Fig.
2.
Schematic
diagram
of
the
microwave
reactor
system.
(1)
MW
oven,
(2)
air
compressor,
(3)
glass
reactor,
(4)
condenser,
(5)
absorption
vessel.
O.A.
Zalat,
M.A.
Elsayed
/
Journal
of
Environmental
Chemical
Engineering
1
(2013)
137–143
138

injection
volume
was
20
m
l
and
detection
was
through
a
LaChrom
L-7400
UV
detector
set
at
250
nm.
Results
and
discussion
Optimization
of
operation
parameters
In
order
to
achieve
the
maximal
removal
of
pyridine
from
wastewater
by
MW
radiation,
the
operation
conditions
were
first
optimized.
Four
factors
were
considered
to
evaluate;
initial
pH,
MW
radiation
time,
post
irradiation
and
initial
pyridine
concen-
tration.
Effect
of
pH
Fig.
3
illustrates
the
removal
of
pyridine
at
different
pH
initial
values.
The
optimal
pH
was
found
to
be
9
which
resulted
in
97.5%
pyridine
removal
after
5
min.
Pyridine
MW
radiation
at
pH
5
and
7
shows
only
50%
and
80%
removal
efficiencies,
respectively.
No
significant
increase
of
pyridine
removal
was
observed
when
solution
pH
was
further
increased
over
9.
However,
at
acidic
medium
(pH
<
5)
the
removal
efficiency
decreased
considerably.
This
could
be
attributed
to
that,
pyridine
is
a
heterocyclic
nitrogenous
compound
and
during
its
degradation,
the
N-atom
in
the
pyridine
ring
upon
mineralization
is
released
as
ammonia
which
easily
observed
by
its
unpleasant
odor.
However,
because
pyridine
contains
an
N
atom,
which
is
more
electronegative
than
an
SP
2
hybridized
C
atom,
it
is
suggested
that
at
higher
acidity
values,
formation
of
ammonium
salt
predominate
which
is
more
stable
in
solution.
In
addition
to,
in
alkaline
medium,
the
anionic
state
of
compound
favors
the
microwave
absorption
and
produc-
tion
of
more
hydroxyl
radical
from
hydroxyl
ion
(OH

!
OH

),
which
cause
the
enhancement
in
degradation
efficiency
as
will
be
discuss
in
a
later
section
[30].
Effect
of
MW
radiation
time
with
and
without
aeration
Fig.
4
demonstrates
that
the
removal
of
pyridine
increased
with
radiation
time
until
attaining
removal
value
of
97.5%
after
5
min.
With
longer
MW
radiation
time,
more
heat
could
be
generated;
thus
the
solution
temperature
became
higher.
It
is
believed
that
degradation
is
not
due
to
heat
effect
only;
however
it
is
due
to
more
vigorous
and
rapid
molecular
motion
induced
by
MW
radiation.
The
result
shows
the
effect
of
aeration
on
the
removal
of
pyridine
by
MW
radiation
as
well.
It
could
be
seen
that
the
removal
was
enhanced
by
aeration
to
some
extent.
Aeration
brought
a
lot
of
air
bubbles
into
the
solution.
This
might
result
in
turbulence
and
agitation.
Therefore,
the
mass
transfer
in
the
solution
was
enhanced,
which
benefited
the
volatile
of
molecular
decomposi-
tion
product.
MW
radiation
without
aeration
was
sufficient
to
remove
almost
all
the
pyridine
in
wastewater.
The
removal
of
pyridine
was
increased
by
only
10–20%
when
aeration
was
applied.
When
the
wastewater
was
radiated
by
MW,
the
polar
molecules
in
solution
rotated
rapidly
(2450
million
times/s),
which
resulted
in
rapid
heating
of
the
solution
[31].
Consequently,
the
molecular
motion
in
wastewater
was
greatly
enhanced,
which
was
highly
advantageous
for
the
evaporation
of
volatile
molecular
ammonia
and
others
decomposition
product
from
liquid
to
gas.
In
addition
to,
these
products
could
be
stripped
from
the
solution
by
the
gas
bubbles
produced.
The
additional
aeration
was
beneficial
for
the
escape
of
volatile
matter
from
solution.
Besides,
MW
radiation
might
reduce
the
activation
energy
of
reaction
system
and
weaken
the
intensity
of
molecular
chemical
bond
[32].
This
effect
was
also
advantageous
for
the
removal
of
pyridine.
Fig.
5
shows
the
relationship
between
temperature
and
pyridine
removal
with
respect
to
MW
radiation
time.
The
pyridine
removal
was
minute
at
low
temperature
and
increased
sharply
at
temperature
above
80
8C,
particularly
when
the
wastewater
was
boiling.
It
could
be
speculated
that
the
pyridine
was
removed
mostly
at
high
temperature.
Regarding
the
mechanism
of
MW
radiation,
thermal
and
non-thermal
effects
are
responsible
for
the
removal
enhancement
by
MW
radiation.
Thermal
effect
is
related
min
0
1
2
3
4
5
% of Concentration
0
20
40
60
80
100
120
pH 5
pH 7
pH 9
Fig.
3.
Influence
of
initial
pH
on
the
pyridine
removal
(20
mg/l
pyridine,
750
W
MW
power,
5
min
radiation
time).
0
20
40
60
80
100
120
0
1
2
3
4
5
6
aeration
no aeration
% of Concentration
min
Fig.
4.
Influence
of
radiation
time
on
the
pyridine
removal
(20
mg/l
pyridine,
pH
=
9,
750
W
MW
power
level,
1–5
min
radiation
time
and
aeration
rate
1
L/min).
0
20
40
60
80
100
120
0
20
40
60
80
100
120
0
1
2
3
4
5
Temperature ( C) % of Concentration
Temperature ( oC)
% of Concentration
MW radiation time (min)
Fig.
5.
Relation
between
temperature
and
pyridine
removal
(20
mg/l
pyridine,
pH
=
9,
750
W
MW
power
level
with
aeration).
O.A.
Zalat,
M.A.
Elsayed
/
Journal
of
Environmental
Chemical
Engineering
1
(2013)
137–143
139

to
the
heat
generated
by
the
absorption
of
microwave
energy
by
water
and
other
polar
molecules,
both
characterized
by
a
permanent
or
induced
polarization
[31].
Whereas,
non-thermal
effect
is
claimed
to
change
the
chemical,
biochemical,
or
physical
behaviors
of
systems
while
temperature
and
other
parameters
remain
unaltered.
There
was
a
fundamental
difference
between
MW
radiation
and
conventional
heating
process.
In
conventional
heating
process,
energy
was
transferred
from
the
surface
to
material
inside
via
convection,
conduction
and
radiation
[31].
However,
heat
was
generated
simultaneously
in
the
whole
system
under
MW
radiation.
The
mechanism
of
heat
generation
was
a
partial
dispersion
of
the
electromagnetic
field
energy
and
its
conversion
into
heat.
The
alternating
electromagnetic
field
induced
the
rotation
of
the
dipoles
of
polar
substances,
such
as
H
2
O
or
ammonia
molecule
resulting
from
degradation.
The
intermolecular
interaction
resulted
in
the
generation
of
substantial
amount
of
heat
and
then
the
solution
temperature
in
the
container
rapidly
rose
to
a
high
level.
This
was
the
thermal
effect
of
MW
radiation
[31].
Extensive
researches
had
been
conducted
to
study
the
non-
thermal
effect
associated
with
MW
radiation
[31,32,26,33].
Non-
thermal
effect
was
the
reason
to;
enhance
the
crystallization
rate
of
SnO
under
MW
radiation
by
Wu
et
al.
[34]
and
in
activation
of
two
thermophilic
and
thermostable
enzymes
by
Porcelli
et
al.
[35].
However,
Zhang
et
al.
[26]
and
Shazman
et
al.
[33]
reported
that
no
non-thermal
effects
could
be
found
in
their
works.
Critics
of
the
non-thermal
effect
often
claimed
that
differences
of
the
effect
could
be
attributed
to
poor
temperature
measurement
and
control
of
experimental
conditions
that
resulted
in
systematic
error.
In
general,
the
existence
or
not
of
non-thermal
effects
continued
to
be
an
area
of
considerable
debate
and
research
[35–38].
min
0
1
2
3
4
5
% of Consentration
0
20
40
60
80
100
120
10 pp
m
20 pp
m
30 pp
m
Fig.
6.
Influence
of
initial
concentration
on
the
pyridine
removal
(10,
20,
30
mg/l
pyridine,
750
W
MW
power
level,
1–5
min
radiation
time).
Table
2
Removal
of
pyridine
at
different
high
initial
concentrations.
Concentration
(ppm)
300
500
800
1000
%
of
removal
95.2
95
94.12
94.01
0
20
40
60
80
100
120
0
1
2
3
4
5
6
% of concentration
Time (hr)
Fig.
7.
Influence
of
post
irradiation
on
the
pyridine
removal
(20
mg/l
pyridine,
750
W
MW
power
level
1,
1
min
radiation
time).
Fig.
8.
Tentative
degradation
pathway
proposed
for
microwave
degradation
of
pyridine.
(a)
HPLC
analysis
(20
mg/l
pyridine,
750
W
MW
power
level,
pH
9,
1
min
radiation
time).
(b)
HPLC
analysis
(20
mg/l
pyridine,
750
W
MW
power
level,
pH
9,
3
min
radiation
time).
O.A.
Zalat,
M.A.
Elsayed
/
Journal
of
Environmental
Chemical
Engineering
1
(2013)
137–143
140

Effect
of
initial
pyridine
concentration
Fig.
6
shows
the
removal
of
pyridine
at
different
initial
concentrations
(10–30
ppm).
It
could
be
seen
that
pyridine
could
be
largely
removed
by
MW
radiation,
even
at
the
highest
initial
concentrations.
For
higher
range
of
concentration
(300–
1000
ppm),
Table
2
summarizes
the
removal
of
pyridine
for
this
range
which
also
indicates
effective
degradation
of
pyridine.
When
the
initial
concentration
increased
from
300
to
1000
ppm,
a
slight
decrease
of
removal
efficiency
was
observed.
However,
the
removal
efficiencies
were
still
above
94%.
When
the
initial
concentration
increased,
the
removal
concentration
of
pyridine
from
wastewater
increased,
but
the
removal
efficiency
was
almost
unchanged.
Summarily,
results
suggested
a
significant
effect
of
pH
and
MW
radiation
time
on
pyridine
removal.
The
effect
of
the
initial
concentration
and
aeration
on
the
removal
was
minute.
The
removal
of
pyridine
increased
with
the
increase
of
pH
and
MW
radiation
time.
The
optimal
condition
was
obtained
as
pH
9
and
5
min
MW
radiation.
The
effect
of
post
irradiation
The
effect
of
post-irradiation
was
studied
in
which
the
sample
was
radiated
for
1
min
and
then
concentration
was
determined
every
1
h
for
a
total
period
of
5
h.
Fig.
7
shows
that
the
removal
efficiency
increased
during
this
period
without
irradiation.
This
result
is
of
great
interest
and
may
suggest
presence
of
some
Fig.
9.
(a)
GC/MS
analysis
(20
mg/l
pyridine,
pH
9,
750
W
MW
power
level,
1
min
radiation
time).
(b)
GC/MS
analysis
(20
mg/l
pyridine,
750
W
MW
power
level,
pH
9,
5
min
radiation
time).
O.A.
Zalat,
M.A.
Elsayed
/
Journal
of
Environmental
Chemical
Engineering
1
(2013)
137–143
141

radiolytic
products
that
is
responsible
for
this
post
degradation.
This
phenomenon
may
suggest
that
non
thermal
effects
are
responsible
for
the
enhancement
of
pyridine
degradation.
This
idea
needs
more
investigation
which
is
considered
to
be
performed
in
the
near
future
through
a
separate
work.
However
this
result
could
be
interpreted
as,
heating
by
MW
radiation
depended
on
the
dipole
relaxation
time
and
ionic
conductivity
of
the
solution.
This
non-
ionizing
electromagnetic
radiation
was
absorbed
at
molecular
level
and
can
be
recognized
as
changes
in
vibrational
energy
of
the
molecules
[38].
Pyridine
wastewater
consisted
of
a
large
amount
of
molecular
H
2
O
and
ammonia.
In
addition
to,
the
N–HO
and
O–HN
intermolecular
hydrogen
bond
can
be
existed
in
the
wastewater
[37,38].
Molecular
H
2
O
and
ammonia
were
both
polar
molecules
and
could
be
polarized
by
MW
radiation.
MW
radiation
caused
dipoles
to
rotate
and
line
up
rapidly
(2450
million
times/s).
Therefore,
the
frequent
pendulum
vibration
of
molecular
H
2
O
and
ammonia
led
to
the
break
and
weakening
of
the
intermolecular
hydrogen
bond
between
ammonia
and
H
2
O,
which
was
beneficial
for
the
escape
of
pyridine
decomposition
product
from
liquid
phase
to
gas
phase.
It
was
clear
that
thermal
effect
could
not
lead
to
the
molecular
rotation
or
vibration.
As
a
result,
it
could
be
concluded
that
thermal
effect
played
a
key
role
on
the
removal
of
pyridine
by
MW
radiation,
and
non-thermal
effect
enhanced
the
removal
to
some
extent.
Having
a
look
at
the
literature
on
degradation
of
pyridine
and
pyridine
derivatives
there
are
inconsistency
concerning
the
reaction
mechanism
through
holes
or
hydroxyl
radicals.
Therefore,
Agrios
and
Pichat
[39]
suggest
that
pyridine
reacts
over
TiO
2
predominantly
via
formation
of
a
radical
centered
on
the
pyridine
ring.
Some
researchers
reported
that
free
radicals
would
be
generated
by
applying
microwave
irradiation
simultaneously
with
oxidants
(air
bubbling)
[40,41].
Quan
et
al.
[42]
showed
that
hydroxyl
radicals
(OH

)
could
be
generated
indirectly
from
application
of
microwave
energy.
Therefore,
the
degradation
relies
on
the
generation
of
reactive
free
radicals,
especially
hydroxyl
radicals
(

OH).
It
is
highly
powerful
oxidizing
agent
having
an
oxidation
potential
of
2.33
V,
which
can
undergo
rapid
and
non-
selective
reaction
with
most
organic
and
many
inorganic
solutes
[22].
In
our
case,
detection
of
pyridine
intermediate
decomposition
product
by
GC/MS
and
HPLC
would
support
mechanism
through
holes,
as
shown
in
Fig.
8,
whereas
the
increase
in
the
degradation
rate
on
the
bubbling
of
air
through
the
solution
evidences
the
important
role
played
by
hydroxyl
radicals.
Analysis
of
pyridine
degradation
by
HPLC
and
GC/MS
In
order
to
confirm
the
degradation
of
pyridine,
different
samples
were
analyzed
by
HPLC
(direct
injection).
Fig.
9(a)
shows
the
HPLC
response
of
pyridine
sample
injection
which
was
subjected
to
MW
radiation
1
min.
The
pyridine
retention
time
is
at
4.435
min.
Fig.
9(b)
shows
the
response
after
3
min
radiation
time
of
the
same
sample.
The
chromatogram
shows
two
other
peaks
which
could
be
attributed
to
the
Pyridine
degradation
intermediates
products
according
to
what
have
been
discussed
before.
The
results
show
that
the
concentration
of
pyridine
decreased
by
80%
with
3
min
MW
radiation
time
at
the
same
retention
time.
On
the
other
hand,
as
can
be
seen
in
Fig.
9(a)
and
(b),
the
degradation
was
confirmed
by
GC–MS
analysis,
as
well.
The
abundance
of
pyridine
(concentration)
decreased
by
98%
after
irradiated
by
MW
to
5
min.
This
can
be
attributed
to,
during
MW
irradiation
of
pyridine;
the
N-atom
in
the
pyridine
ring
is
released
as
NH
3
upon
mineralization
and
then
stripped
from
solution
by
aeration
[44].
The
ammonia
nitrogen
(NH
3
-N)
is
often
monitored
as
a
measure
of
pyridine
degradation
[43,44].
Conclusions
A
fundamental
research
had
been
carried
out
to
explore
the
removal
of
pyridine
in
wastewater
by
MW
radiation.
The
influence
of
operating
parameters
and
the
mechanism
of
pyridine
removal
were
investigated.
Conclusions
were
drawn
as
follows:
MW
radiation
had
been
proved
to
be
an
effective
technique
and
considered
as
an
alternative
approach
for
the
removal
of
pyridine
in
wastewater.
Large
removal
of
pyridine
was
achieved
by
MW
radiation
in
a
short
time.
pH
and
MW
radiation
time
showed
heavy
influence
on
pyridine
removal,
while
initial
pyridine
concentration
presents
minute
effect.
pH
9
and
longer
MW
radiation
time
resulted
in
larger
removal
efficiencies.
5
min
MW
radiation
time
could
reduce
pyridine
concentration
from
20
to
0.5
mg/l
at
pH
9
and
MW
power
750
W.
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