Transcript Diapositiva 1 - Serbian Chemical Society

ESSEE 4
4th European Summer School on Electrochemical Engineering
Palić, Serbia and Montenegro
17 – 22 September, 2006
ELECTROCOAGULATION / ELECTROOXIDATION.
Dr. Manuel A. Rodrigo
Department of Chemical Engineering. Facultad de Ciencias Químicas.
Universidad de Castilla La Mancha. Campus Universitario s/n. 13071
Ciudad Real. Spain.
Department of Chemical Engineering.
Universidad de Castilla La Mancha.
Spain
CONTENTS
1.
2.
ELECTROCHEMICAL WASTEWATER TREATMENT TECHNOLOGIES
1.1 What happens inside an electrochemical cell during the electrolysis of a wastewater?
1.2 Types of electrochemical wastewater treatment technologies
1.3 Advantages of electrochemical technologies in environmental remediation
ELECTROCOAGULATION
2.1 What is coagulation?
2.2 The electrochemically-assisted coagulation: fundamentals
2.2.1 ANODE MATERIALS
2.2.2 ELECTRODISSOLUTION
2.2.3 ELECTROLYTIC GENERATION OF OXYGEN AND HYDROGEN
2.2.4 MAIN PROCESSES INVOLVED IN THE ELECTROCHEMICALLY ASSISTED TECHNOLOGIES FOR COLLOID-POLLUTED
WASTES
2.3 Electrochemical cells
2.3.1 TANK CELLS
2.3.2 FLOW CELLS
2.3.3. PROMOTION OF THE ELECTROFLOTATION PROCESS
2.3.4 OTHER PROCESSES
3.
2.4 Electrocoagulation of soluble organics and break-up of emulsions. Removal of phosphates
2.5 Advantages and disadvantages of electrocoagulation
ELECTRO-OXIDATION
3.1 Fundamentals
3.2 Electrode materials
3.3 Electrochemical cell
3.3.1 IS IT RECOMMENDED THE USE OF DIVIDED CELLS?
3.3.2 STIRRED-TANK CELLS
3.3.3 SINGLE-FLOW CELLS
3.3.4 FILTER-PRESS CELLS
3.3.5 OTHER CELLS
3.4 Indirect electrochemical oxidation processes
3.5 Advantages of the electrooxidation technologies
3.6 Combined processes
1.1 What happens inside an electrochemical cell during the electrolysis of a
wastewater?
Power supply
e-
e-
Ox
Red
M
Red
2. Electroreduction
Ox
5. Migration of cations
3. Electrodissolution
4. Electrodeposition
Mn+
Mn+
M
Cathode
1. Electrooxidation
Anode
influent
5. Migration of anions
effluent
Diluted solution
1.2 Types of
electrochemical
wastewatertreatment
technologies
Anionic
membrane
Cathionic
membrane
anode
Anionic
membrane
Cathionic
membrane
Concentrated solution
cathode
Anions
electrodialysis
Feed solution
Cations
Electro-oxidation
electrocoagulation
Electrodeposition
metal
Rotational cathode
anode
Electrolyte flux
1.3 Advantages of electrochemical technologies in environmental remediation
Environmental compatibility: “the main reagent used is the electron” No
residues are formed.
Versatility:
Many processes occur simultaneously in any electrochemical cell. Plethora
of reactors, electrode materials, shapes, configuration can be utilized and
allow to promote different kinds of treatment technologies.
Point-of-use production of chemicals is facilitated by electrochemical
technology
Volumes of fluid from microliters to thousand of cubic meters can be
treated
Processes work at room temperature and atmospheric pressure
Selectivity: the applied potentials can be controlled to selectively attack specific
compounds.
Easy operation. Amenability to automation.
Cost effectiveness
2. ELECTROCOAGULATION
2.1 What is coagulation?
1 cm
1 mm
100 micras
10 micras
1 micra
100 nm
10 nm
1 nm
0.1 nm
Pollutants size
Dissolved comp.
Colloids
Suspended solids
influent
Diameter of the particle
(mm)
10
1
0.1
10-2
10-3
10-4
Time needed to settle 1 m
(aprox)
1s
10 s
2 min.
2h
8d
2 years
effluent
Typical
hydraulic
residence time
of a settler for
wastewater
treatment
Sludge
+
Negatively charged
particle
Bulk solution
+
+
+
++ + +
+
+
+ +
+
- +
+
+
+
+
+
+
++
++ +
+
+
+
+
+
Ea
Ea+Eb
Surface potential
-(electrostatic
potential)
Diffuse layer
Zeta potential
Distance from the surface
Coagulation is a chemical
treatment which consists of
the addition of chemical
reagents to reduce the
electrical repulsion forces
that inhibit the aggregation
of particles.
Interaction energy
Eb
Distance between
particles
Electrostatic repulsion energy: Ea
Van der Waals attraction energy: Eb
Resulting energy : Ea+Eb
Hydrolysing metal salts (iron, aluminium)
Particles
stabilized by
electrostatic
repulsion forces
Compression of the diffuse layer by
an increase of the ionic strength
Neutralization of superficial
charges by adsorption of ions
Precipitation Charge
Neutralization
Interparticle bridging
Enmeshment in a precipitate
Conventional Chemical Coagulation
consists of the direct dosing of a
coagulant solution to the wastewater.
Chemical
reagent
Outlet
Inlet
sedimentation
coagulation
flocculation
Sludge
Flocculation is a physical treatment in which the
collision of coagulated colloids is promoted in order
to make possible the formation of larger particles.
The result of both processes is a wastewater in
which the size of the particles is enough to be
separated by a settler or a flotation unit.
Coagulation by hydrolysing aluminium salts
Log [Alx(OH)y 3x-y ] / mol dm-3
Al3+
0
Al(OH)2+
-2
-4
Al(OH)4-
AlT
Al(OH)2+
-6
Al(OH)3
Concentration of monomeric
hydrolysis products of Al(III) in
equilibrium with the amorphous
hydroxides at zero ionic strength at
25ºC
-8
-10
-12
0
2
4
6
8
10
12
14
pH
pH
Typical titration curve for
neutralization of aluminium salt
solutions
9
8
7
6
5
4
3
2
z1
z2
z3
z4
Nitrate media
Sulphate media
0.0
0.5
1.0
1.5
OH-/Al
2.0
2.5
3.0
100
monomers
[Al13O4(OH)24]7+
Ali/AlT
80
60
40
[Al(OH)3]*
20 [Al2(OH)2]4+
[Al2(OH)x](6-x)+
pH
0
3
3,5
0,25
4
1
2
4,5
2,2 2,25
h= OH/ AlT
Log [Fe(OH)y 3x-y ] / mol dm-3
Coagulation by hydrolysing iron salts
0
Concentration of monomeric
hydrolysis products of Fe(III) in
equilibrium with the amorphous
hydroxides at zero ionic strength at
25ºC
-2
Fe(OH)3
-4
-6
-8
Fe3+
-10
Fe(OH)4Fe(OH)2+
Fe(OH)2+
-12
0
2
4
6
pH
8
10
12
14
2.2 The electrochemically assisted coagulation: fundamentals
Electrocoagulation
An alternative to the direct use of a solution containing the coagulant salts, is the in situ
generation of coagulants by electrolytic oxidation of an appropriate anode material (e.g.
iron or aluminium). This process is called electrocoagulation or electrochemically
assisted coagulation.
M
e-
Electrochemical processes involved:
 Electrodissolution
 Electrolytic generation of oxygen and hydrogen
Electro-dissolution
Mn+
coagulation
+
Unstabilized
small
flocculation
particles
colloids
macromolecules
emulsions
Aggregated
particles
2.2.1 ANODE MATERIAL
Aluminium
M
e-
Electro-dissolution
Mn+
coagulation
+
Iron
2.2.2 ELECTRODISSOLUTION
20
-3
10
Aluminium, mg dm
Aluminium, mg dm-3
12
8
6
4
2
0
0
0,005
0,01
15
10
5
0,015
0
Specific electrical charge, A h dm-3
0
2
Faraday’s value
Chemical dissolution
Experimental
Influence of current density
Faradaic
Efficiencies
can be over
100%
4
6
8
10
pH
Influence of pH
Electrochemical process
Chemical process
12
14
pH profile in the electrochemical cell
A
n
o
d
e
pH profile
Direction of electrolyte flux
C
a
t
h
o
d
e
2.2.3 ELECTROLYTIC GENERATION OF OXYGEN AND HYDROGEN
Anodic processes
e-
H2 0
Cathodic processes
H2 O
e-
O2
+
H2
-
Air-dissolved flotation
Bubbles diminish the
overall density of the
system and the particle
floats
Oxygen and hydrogen bubbles
turbulence
Promotes soft mixing conditions and
improves flocculation processes
Electrochemically
assisted flocculation
(electroflocculation)
adhesion
Gaseous microbubbles link to pollutant
particles. Consequently, the density of the
new species decreases and this promotes
the flotation of the particle
Electrochemically assisted
flotation (electroflotation)
2.2.4. MAIN PROCESSES INVOLVED IN THE ELECTROCHEMICALLY ASSISTED
TECHNOLOGIES FOR COLLOID-POLLUTED WASTES
Anodic processes
Cathodic processes
pollutants
eElectrodissolution
Al(III) species
Electrocoagulation
flocs
Electroflotation
Electroflocculation
e-
H2 O
H+ + O2
H2O
H2 + OH-
e-
2.3 Electrochemical cells
purpose
Only electrodissolution
Type of cells
Electrocoagulation/electroflocculation
Electrocoagulation/electroflocculation electroflotation
2.3.1 TANK CELLS
Power supply
e-
e-
Sludge
Floated sludge
Inlet
Flotation
M n+
hydrated
(Pollutant) H2
Precipitated
OH-
Cathode
Anode
H2O
M n+
Settling
Settled sludge
Sludge
Outlet
Mixing can be
accomplished either by
mechanical stirrers or by
the evolved gases
Coagulation/flocculation
The process combines
Sedimentation/flotation
Contrarily to electrooxidation processes, mass transport does not
control the overall rate of the process
Power supply
e-
e-
Flotation
H2O
M n+
hydrated
(Pollutant) H2
OH-
Cathode
M
n+
Precipitated
Settling
Settled sludge
Sludge
Hydrogen evolution can disturb the
sedimentation process. For this
reason, if possible, it is better to
separate the cathodic process from
the sedimentation
Sludge
Floated sludge
Inlet
Anode
The activity of the anode
can decrease with time due
to the formation of insoluble
hydroxides or sludge layer.
These can be avoid by using
motion electrodes or by
using turbulence promoters
Outlet
HydroShock™
ElectroCoagulation
2.3.2 FLOW CELLS
Normally, these cells do not
promote the electroflocculation
and the electroflotation processes
except for especial designs.
Hence its main goal is the
electrodissolution and the
electrocoagulation
The activity of the electrodes can be
decreased by passivation. To solve
this problem reverse of polarity (the
anode acts as a cathode during a
small period) are advised. This can
be easily done in a cell designed with
the only purpose of aluminium
dosing…
+
+
+
+
-
Multiple channels
Single channel
Electrode configuration in
cells for aluminium dose
+
+
+
+
-
… and both, monopolar and bipolar connections, allow this change of polarity!
Cathodes (-)
cathode
Bipolar
electrodes
+- +- +- + -
+
anode
Anodes (+)
However, it is more complex for cells that combine electrocoagulation and
electroflotation in different compartments
The turbulence generated by the
evolved gases can be used in both
types of flow. However, vertical flow
allows to improve the separation by
electroflotation as compared with
horizontal flow.
Horizontal flow
Vertical flow
2.3.3. PROMOTION OF THE ELECTROFLOTATION PROCESS
If electroflotation processes have to be promoted it has to be taken into account that:
e-
e-
-
Current density (j)
influences on both: number
of bubbles and the average
size of bubbles
-
Flow rate can also be used to
control the average bubble size
And also that the
electroflotation can be carried
out in the same or in a
different cell
Power supply
Efluent
Divided
electrocoagulation/
electroflotation
EF
Separator
EC
Power supply
Efluent
Combined
electrocoagulation/
electroflotation
EF
EC
Separator
2.3.4 OTHER PROCESSES
influent
air
2.4 Electrocoagulation of soluble organics and break-up of emulsions.
Removal of phosphates
Emulsion
stabilized by
electrostatic
repulsion forces
Compression of the diffuse layer by
an increase of the ionic strength
Neutralization of superficial
charges by adsorption of ions
Coalescence of
phases
Inter-droplet bridging
Dissolved
organic matter
OH
HO
Binding of monomeric
cationic species to anionic
sites of the organic
molecules, neutralising
their charge and resulting in
reduced solubility
compounds
SO3 -
N
N
OH
HO
N
NO2
SO3 -
N
NO2
OH
HO
N
SO3 -
N
OH
HO
N
NO2
SO3 -
N
NO2
OH
HO
N
SO3 -
N
NO2
Binding of polymeric
cationic species to anionic
sites of the organic
molecules, neutralising
their charge and resulting in
reduced solubility
compounds
NO2
N
NO2
NO2
SO3 -
Enmeshment in a precipitate
HO
Adsorption on a superficially
charged precipitate
N
+
OH
+
HO
SO3 -
N
N
+
HO
N
SO3 -
+
N
NO2
OH
N
N
+
OH
SO3
+
+
+
NO2
SO3 N
-
HO
HO
N
OH
OH
+
Precipitation of phosphates
Log dissolved P
-2
FePO4
-4
AlPO4
-6
2
4
8
6
pH
Electrodissolution
cell
Treated wastewater
clarifier
wastewater
10
2.5 Advantages and disadvantages of electrocoagulation
In literature some advantages are reported for electrocoagulation processes
including:
1) A promotion in the flocculation process due to the movement of the smallest
charged colloids inside the electric field generated in the electrochemical cell and
also to the turbulence created by the bubbles (electroflocculation process)
2) A promotion in the separation process due to the hydrogen bubbles produced in
the cathode during the electrolysis, which can carry the solids to the top of the
solution, where they can be easily collected and removed (electroflotation
process)
3) A more compact residue, as it is reported that the electrocoagulation process
produces a smaller amount of sludge that the chemical coagulation, and that the
solids produced are more hydrophobic
4) A more easy operation mode as no mixing of chemicals is required, the dosing
of coagulants can be easily controlled by manipulating the cell voltage (or the
current density), and thus the operating costs are much lower compared with most
of the conventional technologies
5) Very simple. Suitable for small WWTP
6) Lower operating cost. However, higher investment
3.ELECTRO-OXIDATION
3.1 Fundamentals
When can be applied?
Wastewater polluted with soluble
organic pollutants
Is it possible the
recovery of the pollutant
as a valuable product?
no
High
calorific
power?
Biodegradable?
no
no
Non AOP oxidation
AOP oxidation
Electrochemical oxidation
Electro-oxidation technologies: use of an electrolytic cell to
oxidize the pollutants contained in a wastewater
pollutant
1. Direct electrolysis
Oxidation of the pollutant on the
electrode surface
H2 O
pollutant
With some anode materials it is
possible the generation of OH·
OH·
e-
2. Advanced oxidation processes
PO433. Chemical oxidation
+
P2O84pollutant
On the electrode surface several
oxidants can be formed from the
salts contained in the salt
Organic
pollutant
e-
intermediates
(aromatics,
carboxylic
acids)
...
e-
CO2
+
H2 O
O2
Cl-
Cl2
Direct electrolysis consists of the direct oxidation of a
pollutant on the surface of the anode. To be oxidized
the organic must arrive to the anodic surface and
interact with this surface. This means that
electrocatalytic properties of the surface towards
the oxidation of organics can play an important role in
the process. Likewise, it means that in certain
conditions mass transfer can control the rate and the
efficiency of the electrochemical process
Organic
pollutant
e-
intermediates
(aromatics,
carboxylic
acids)
...
e-
CO2
+
H2 O
O2
Cl-
Cl2
The potentials required for the oxidation of organics
are usually high. This implies that water can be
oxidized and the generation of oxygen is the main
side reaction. This is a non desired reaction and it
influences dramatically on the efficiencies
Organic
pollutant
e-
intermediates
(aromatics,
carboxylic
acids)
...
e-
CO2
+
H2 O
O2
Cl-
Cl2
Frequently the potential is high enough to promote the
formation of stable oxidants, through the oxidation of
other species contained in the wastewater. This can
have a beneficial effect on the efficiency as these
oxidants can oxidize the pollutant in all the volume of
wastewater
Organic
pollutant
e-
2. Mass transport, which can
be promoted by a proper cell
design
...
e-
CO2
+
1. Electrode material, which
influences on the nature of the
products and on the importance of
theHside
O reactions
2
O2
Cl-
Cl2
3. The presence of compounds in the
wastewater that can be transformed
into oxidants, promoting mediated
electrochemical oxidation processes
Organic
pollutant
3.2 Electrode material
DESIRABLE PROPERTIES
MECHANICAL STABILITY.
CHEMICAL STABILITY
MORPHOLOGY.
ELECTRICAL CONDUCTIVITY
CATALYTIC PROPERTIES
RATIO PRICE/ LIFETIME.
Typical materials include
Metals
material
Carbon
oxides
Platinum
Stainless stell
Grafite
Doped diamond
DSA
Ti/SnO2
Ti/PbO2
low efficiency electrodes
High efficiency electrodes
Low efficiency electrodes
phenol
SOFT OXIDATION CONDITIONS
Quinones, polymers, carboxylic acids
e-
Fouling by
polymers
+
Many intermediates
Small conversion to carbon dioxide
Slow oxidation rates
Small current efficiencies
Formation of polymers from aromatic pollutants is favoured
Pt
IrO2
Mediated oxidation by a higher oxidation state of
the species that conforms the electrode surface?
High efficiencies electrodes
phenol
HARD OXIDATION CONDITIONS
Carbon dioxide
few intermediates
Large conversion to carbon dioxide
Large current efficiencies only limited by mass transfer
e-
+
BDD
Ti/PbO2
OH· generation?
Confirmed for conductive-diamond
Suggested for PbO2/SnO2
Active electrodes
Pt
Stainless steel
DSA
Non-active electrodes
Ti/SnO2
Ti/ PbO2
Doped diamond
Drawbacks of non-active electrodes:
Conductive diamond: large price >6000 euros/sqm
PbO2/SnO2: Dissolution of toxic species
ROLE OF THE HYDROXYL RADICALS
Electrochemical oxidation
Direct oxidation process
Mediated oxidation process
Electrolyte
e-
eRO
Electrochemical
Reaction
RO
Mass
Transport
Electrolyte
H2O
R
R
Anode (+)
Interfase
Interfase
Interfase
OH ·
Anode (+)
R
R
Kinetic or mass
transport controlled
R
eCox
Cox
RO
Electrochemical
Reaction
Electrolyte
Cred
Mass
Transport
RO
Anode (+)
RO
Electrochemical
Reaction
Mass
Transport
Kinetic
controlled
Cathodic material
The organic-oxidation processes that occur in an
electrochemical cell are usually irreversible.
Hydrogen evolution is the main cathodic reaction.
e-
e-
e-
e-
H2 O
0.5H2+ OH-
Deposit of carbonates
OH- + HCO3e-
Increase in the cell potential
e-
Increase in the energy consumption
H2 O
0.5 O2+ 2H+
Polarity reversal
3.3 Electrochemical cell
DESIRED CHARACTERISTICS FOR A ELECTROCHEMICAL CELL
SIMPLE MECHANICAL DESIGN. SMALL PRICE. EASY TO USE. LOW
MAINTENANCE COST.
ENHANCED MASS TRANSFER.
HOMOGENEOUS CURRENT DISTRIBUTION ON THE ELECTRODES.
LARGE DURABILITY
SAFETY
3.3.1 IS IT RECOMMENDED THE USE OF DIVIDED CELLS?
Power supply
-
+
e-
Turbulence promoters
e-
1. The membrane increases the cell
potential and consequently the operating
cost.
2. Most organic-oxidation processes are
irreversible
Membrane
V
Anolite
Catholite
hW
Anode
Cathode
Cell potential
ea + h
hdiff
hW
ea + h + hreaction
hW
Electrolyte
ANODE
CATHODE
Direction of
charge flux
3.3.2 STIRRED-TANK CELLS
Power supply
-
+
Turbulence promoters
e-
anode
e-
cathode
ADVANTAGE: Simplest cell
DRAWBACK: Low mass transfer coefficients
3.3.3 SINGLE FLOW CELL
ANODE
TURBULENCE
PROMOTER
OUTLET ANOLYTE
INLET ANOLYTE
Membrane?
CATHODE
OUTLET CATHOLYTE
INLET CATHOLYTE
3.3.4 FILTER PRESS CELL
Large electrode surfaces / volume ratios
Small interelectrode gap
Plane electrodes
Electrolyte flow
3.3.5. OTHER CELLS
+
-
+
Steel cathode
polyuretane
Activated carbon
Steel anode
Packed bed cell
Cell with continuous
regeneration of the
adsorbent
Rotating electrode cell
CATHODE
ANODE
3.4 Indirect electrochemical oxidation processes
Power supply
e-
pollutant
a) Direct electrolysis
product
Electrodo
inert1
inert2
pollutant
electroactive
Product
inert
pollutant
electroactive
Product
b) Indirect electrolysis
The oxidation is carried out in the whole reaction volume (not limited to the electrode
surface)
No mass transfer control
higher efficiency
Both direct and indirect electro-oxidation develop simultaneously in the cell
Power supply
Homogeneous
reactions
V
I
A
e
B
e
-
-
e
-
C
A
anode
B
D
D
e
e
C-
-
Heterogeneous
reactions
cathode
Types of mediated electrochemical oxidation processes
Without addition of reagents: changes in the pH and temperature to promote
the generation of oxidants from the direct oxidation of salts present in the
wastewater (in some cases throught hydroxyl radicals)
With additions of reagents: in addition to changes in pH and temperature,
some salts are added to promote the generation of oxidants
Production of reagents and treatment of the waste in the same cell
Production of reagents and treatment of the waste in different cells
Dosing of reagent
Electrosynthesis of
the oxidant
wastewater
Oxidation and
electroxidation of
the pollutants
Separation of the
oxidant or of its
reduction product
Treated waste
Dosing of reagent
Electrosynthesis of
the oxidant
Oxidation and
electrooxidation of
the pollutants
wastewater
Separation of
the oxidant or of
its reduction
product
Treated waste
To take in mind…
The potential at which the electrogenerated oxidants are
produced must not be near the potential for water oxidation, since
then a large portion of the current will be employed in the side
reaction
The rate of generation of the electrogenerated oxidant should be
large
The rate of oxidation of pollutant by the electrogenerated oxidant
must be higher than the rates of any competing reactions.
The electrogenerated oxidant must not be a harmful product
Ag(I) / Ag(II)
Reversible oxidant
The oxidant can be reduced in the
cathode. A divided cell may be considered
Co(II) / Co(III)
Ce(III) / Ce (IV)
Fe(II) / Fe (III)
SO4 2- / S2O8 2PO4 3- / S2O8 4-
Irreversible (killers)
The oxidant is not reduced on the
cathode. Non-divided cells are used
for their production
Cl2
O3
H2 O 2
These oxidants
are generated
from anions
typically present
in a wastewater
It can be formed
by a cathodic
process. Extra
oxidation
efficiency!
Ag(I) / Ag(II)
Ag  Ag2  e
E 0  1.98Vvs.SHE
Some pollutants efficiency removed by
this technology: Ethylene glycol,
isopropanol, acetone, organic acids,
benzene, kerosene
Ag2  e  Ag
Main drawbacks
R  CO2
H2O  OH·  O2
 ions Ag+ are harmful products
chlorides can reduce the efficiencies due to
precipitates formation
silver is very expensive
Co(II) / Co (III)
Co(II)  Co(III)  e
E 0  1.82Vvs.SHE
Some pollutants successfully treated:
Organic radioactive waste materials,
dichloropropanol, ethylene glycol
Co(III)  e  Co(II)
Main drawback
R  CO2
H2O  OH·  O2
This process has to be carried out in divided
cells (Co can be electrodeposited on the
cathode surface)
Its presence is very common: Sulphate salts
are frequently present in industrial wastewaters.
Very powerful oxidant (non selective oxidation)
It decomposes at temperatures above 60ºC
Sulphate/peroxodisulphate
Large efficiencies with diamond
electrodes
2 SO 4
2
2
 S2 O8  2 e

E 0  2.06Vvs.SHE
Phosphate/peroxodiphosphate
Large efficiencies with diamond
electrodes
2 PO4
3
4
 P2O8  2 e
E 0  2.01Vvs.SHE
2
S2O8  H 2O  2 SO 24   2H   1
2
2
O2
S2O8  H2O  SO 52  SO 24  2H
2
SO 5  H2O  H2O2  SO 24
Its presence is very common: Phosphate
salts are frequently present in industrial
wastewaters
Powerful oxidant (more selective than
persulphate). The oxidation carried out by
this reagent depends importantly on the pH
Less sensitive to temperature
Fe(II) / Fe (III)
Fe2  Fe3  e 
E 0  0.77Vvs.SHE
 Fe(VI )
?
Fe3  e  Fe2
R  CO2
R  RO
H2O  OH·  O2
Some pollutants treated by this
technology: Celluloid materials, fats,
urea, cattle manure, sewage sludge,
meat packing wastes, ethylene glycol
selectivity depends on operating
conditions. Carbon dioxide can be the
final product in the oxidation of organics
good efficiencies are obtained for high
temperatures and low current densities
Electrocoagulation can occur
simultaneously
Its presence is very
common: chloride salts are
frequently present in
industrial wastewaters.
Chloride/ Chlorine
2 Cl-  Cl 2  2e
The chlorine speciation
depends on the pH
Cl 2  H2O  HCl  HClO
It can lead to the formation
of organochlorinated
compounds
hypochlorite
Dosing in channel
NaCl
% HClO
+
1.0
0.8
Electrochemical cell
0.6
NaCl
Dosing in pipe
0.4
0.2
0.0
hypochlorite
5.0
6.0
7.0
8.0
9.0
10.0
pH
+
Electrochemical cell
-
Hydrogen peroxide
It can be formed on the cathode by reduction of oxygen
O2  2H2O  2e  HO2  OH
E0=-0.065 V
However, the main drawback is the decomposition of the
hydroperoxide anion that it is favoured at alkaline
conditions.
HO2  2OH  O2
To promote the efficiencies it is required :
a cathode material with a high overpotential for the
reduction of the hydroperoxide anion to water (graphite)
Good oxygen transfer rates to the cathode surface
e-
Combination of electrooxidation with cathodic
generation of hydrogen peroxide allows to
obtain current efficiencies over 100%. It is
the best way of obtaining a valuable
compound from the cathodic reaction in
wastewater treatment processes
e-
O2
H2O2
Anodic
oxidation
processes
Ozone
The oxidation of water to ozone can occur on the electrode surface but it is less favoured than that
of oxygen
O3  6H  6e  3H2O
E0=1.51 V
O2  4H  4e  2H2O
E0=1.23 V
To promote the formation of ozone:
Use of anode material with large overpotentials for oxygen evolution
Use of very high current densities
Use of an adsorbate to block the oxygen evolution process (f.i.F-, BF4-, BF6-)
Some examples of electrochemical generation of ozone
anode
electrolyte
current density
B-PbO2
HPF6 (2M)
750 mA cm-2
Active carbon
HBF4 (7.3 M)
600 mA cm-2
Active carbon
HBF4 (62% w/w)
200 mA cm-2
yield
21%
35%
45%
3.5. Advantages of the electro-oxidation technology
Environmental compatibility: “the main reagent used is the electron” No
residues are formed.
Can be a complementary treatment or a final treatment
Operation at room temperature and atmospheric pressure
High efficiency if proper anode material is used.
The efficiency can be easily increased by promoting indirect processes
Easy operation. Amenability to automation.
3500
Lower operating cost
compared with other AOP
Energy consumption during the treatment of
an actual industrial waste. Electrochemical
oxidation j:30 mA cm-2; natural pH; T: 25ºC
; Ozonation pH 12; T: 25ºC 
COD / mg dm-3
3000
2500
2000
1500
1000
500
0
0
500
1000
1500
2000
W / kWh m-3
2500
3000
3.6. Combined processes
Treatment of gaseous effluents
Poor
Gas
H2S
NaOH
Solution
H2
Rich
Gas
H2S
+
-
Filter
Comportment of
adjust of the pH
Electrochemical
Reactor
Absorber
Solution
S(S)
Combination of electrochemical oxidation with bio-oxidation
a) pre-treatment
electrooxidation
biooxidation
b) post-treatment
biooxidation
electrooxidation
Main drawback: When to change?