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CONTENTS
1. ELECTROCHEMICAL WASTEWATER TREATMENT TECHNOLOGIES1.1 What happens inside an electrochemical cell during the electrolysis of a wastewater?1.2 Types of electrochemical wastewater treatment technologies1.3 Advantages of electrochemical technologies in environmental remediation
2. ELECTROCOAGULATION2.1 What is coagulation?2.2 The electrochemically-assisted coagulation: fundamentals
2.2.1 ANODE MATERIALS2.2.2 ELECTRODISSOLUTION2.2.3 ELECTROLYTIC GENERATION OF OXYGEN AND HYDROGEN2.2.4 MAIN PROCESSES INVOLVED IN THE ELECTROCHEMICALLY ASSISTED TECHNOLOGIES FOR COLLOID-POLLUTED WASTES
2.3 Electrochemical cells2.3.1 TANK CELLS2.3.2 FLOW CELLS2.3.3. PROMOTION OF THE ELECTROFLOTATION PROCESS2.3.4 OTHER PROCESSES
2.4 Electrocoagulation of soluble organics and break-up of emulsions. Removal of phosphates2.5 Advantages and disadvantages of electrocoagulation
3. ELECTRO-OXIDATION3.1 Fundamentals3.2 Electrode materials3.3 Electrochemical cell
3.3.1 IS IT RECOMMENDED THE USE OF DIVIDED CELLS?3.3.2 STIRRED-TANK CELLS3.3.3 SINGLE-FLOW CELLS3.3.4 FILTER-PRESS CELLS3.3.5 OTHER CELLS
3.4 Indirect electrochemical oxidation processes3.5 Advantages of the electrooxidation technologies3.6 Combined processes
e- e-
An
od
e
Cath
od
e
Power supply
e- e-
influent
effluentRed
Ox Red
Ox
M
Mn+
Mn+
M
1. Electrooxidation 2. Electroreduction
3. Electrodissolution 4. Electrodeposition
5. Migration of anions
5. Migration of cations
1.1 What happens inside an electrochemical cell during the electrolysis of a wastewater?
Anions
CationsFeed solution
Diluted solutionConcentrated solution
cathodeanode
Cathionic membrane
Cathionic membrane
Anionic membrane
Anionicmembrane
anode
Electrolyte flux
metal
Rotational cathode
electrodialysis
Electro-oxidation electrocoagulation
Electrodeposition
1.2 Types of electrochemical wastewater- treatment technologies
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
Diameter of the particle
(mm) Time needed to settle 1 m
(aprox) 10 1 s 1 10 s
0.1 2 min. 10-2 2 h 10-3 8 d 10-4 2 years
Dissolved comp.Colloids
Suspended solids
0.1
nm
1 n
m
10 n
m
100
nm
1 m
icra
10 m
icra
s
100
mic
ras
1 m
m
1 cm
Pollutants size
Typical hydraulic
residence time of a settler for wastewater treatment
2.1 What is coagulation?
Sludge
effluent
influent
2. ELECTROCOAGULATION
Electrostatic repulsion energy: EaVan der Waals attraction energy: Eb Resulting energy : Ea+Eb
Interaction energy
Distance betweenparticles
Ea
Eb
Ea+Eb
++
+++
++
++
+++
+ + + + +++++
+
+++
++
++
-
-
-
-
-
+
-
Diffuse layer
Negatively chargedparticle
Bulk solution
Distance from the surface
-(el
ectr
osta
tic
pot
enti
al)
Zeta potential
Surface potential
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.
Hydrolysing metal salts (iron, aluminium)
Compression of the diffuse layer by an increase of the ionic strength
Neutralization of superficial charges by adsorption of ions
++
+ +
+
+
+
++
++
+
+
++
+
+
++
+
Precipitation Charge Neutralization
Particles stabilized by electrostatic
repulsion forces
Interparticle bridging
Enmeshment in a precipitate
+
++
++
+
+
Conventional Chemical Coagulation consists of the direct dosing of a
coagulant solution to the wastewater.
flocculation
coagulation
Inlet
Chemical reagent
sedimentation
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.
Sludge
Outlet
pH
0 2 64 8 10 12 14
Log
[A
l x(O
H) y
3x-
y ]
/ m
ol d
m-3
Al3+
Al(OH)4-
Al(OH)3
0
-2
-4
-6
-8
-10
-12
Al(OH)2+
Al(OH)2+ AlT
OH-/Al
pH
3
4
5
6
7
8
9
2
0.0 0.5 1.51.0 2.0 2.5 3.0
Nitrate media
Sulphate media
z1 z2 z3 z4
Coagulation by hydrolysing aluminium salts
Concentration of monomeric hydrolysis products of Al(III) in equilibrium with the amorphous
hydroxides at zero ionic strength at 25ºC
Typical titration curve for neutralization of aluminium salt
solutionsM
onom
eric
spe
cies
Poly
mer
ic s
peci
es
Prec
ipita
te
Al i/
Al T
100
80
60
40
20
03 3,5 4 4,5
monomers
[Al13O4(OH)24]7+
[Al2(OH)2]4+
[Al(OH)3]*
[Al2(OH)x](6-x)+
pH
h= OH/ AlT0,25 1 2 2,2 2,25
pH
0 2 64 8 10 12 14
Log
[F
e(O
H) y
3x-
y ]
/ m
ol d
m-3
Fe3+ Fe(OH)4-
Fe(OH)3
0
-2
-4
-6
-8
-10
-12
Fe(OH)2+
Fe(OH)2+
Coagulation by hydrolysing iron salts
Concentration of monomeric hydrolysis products of Fe(III) in equilibrium with the amorphous
hydroxides at zero ionic strength at 25ºC
Electrochemical processes involved: Electrodissolution Electrolytic generation of oxygen and hydrogen
coagulation
Electro-dissolutione-
+
Mn+
M
colloidsmacromoleculesemulsions
Unstabilized small particles flocculation
Aggregatedparticles
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.
Electrocoagulation
2.2 The electrochemically assisted coagulation: fundamentals
coagulation
Electro-dissolutione-
+
Mn+
M
2.2.1 ANODE MATERIAL
Aluminium
Iron
2.2.2 ELECTRODISSOLUTION
Faradaic Efficiencies can be over 100%
Electrochemical process
Chemical process
Influence of pHInfluence of current density
0
2
4
6
8
10
12
0 0,005 0,01 0,015
Specific electrical charge, A h dm-3
Alu
min
ium
, mg
dm-3
0
5
10
15
20
0 2 4 6 8 10 12 14
pH
Alu
min
ium
, mg
dm-3
Faraday’s valueChemical dissolutionExperimental
Anode
pH profile
Direction of electrolyte flux
pH profile in the electrochemical cell
Cathode
e-
Anodic processes
H20
O2
H2O
H2
Cathodic processes
+ -
e-
2.2.3 ELECTROLYTIC GENERATION OF OXYGEN AND HYDROGEN
Air-dissolved flotation
Bubbles diminish the overall density of the system and the particle floats
turbulence
Oxygen and hydrogen bubbles
Promotes soft mixing conditions and improves flocculation processes
Electrochemically assisted flocculation (electroflocculation)
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)
adhesion
e -
e -
Anodic processes
e -
e -
e -
e -
Cathodic processes
e-
Al(III) species
pollutants
flocsElectroflotation
Electrocoagulation
Electroflocculation
Electrodissolution
H2O
H+ + O2
H2O
H2 + OH-
2.2.4. MAIN PROCESSES INVOLVED IN THE ELECTROCHEMICALLY ASSISTED TECHNOLOGIES FOR COLLOID-POLLUTED WASTES
2.3 Electrochemical cells
Type of cells
Only electrodissolution
Electrocoagulation/electroflocculation
Electrocoagulation/electroflocculation electroflotation
purpose
Inlet
OutletH2OH2O
Precipitated
An
od
e
Cath
od
e
Power supply
(Pollutant)
e-
Mn+
hydratedOH-
H2
Settling
Flotation
Settled sludge
Sludge
Sludge
Mn+
Floated sludge
e-
The process combines Coagulation/flocculation
Sedimentation/flotation
2.3.1 TANK CELLS
Mixing can be accomplished either by mechanical stirrers or by the evolved gases
Contrarily to electrooxidation processes, mass transport does not control the overall rate of the process
Hydrogen evolution can disturb the sedimentation process. For this reason, if possible, it is better to separate the cathodic process from the sedimentation
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
Inlet
OutletH2OH2O
Precipitated
An
od
e
Cath
od
e
Power supply
(Pollutant)
e-
Mn+
hydratedOH-
H2
Settling
Flotation
Settled sludge
Sludge
Sludge
Mn+
Floated sludge
e-
HydroShock™ ElectroCoagulation
+-+-+-+-
Multiple channels
+-+-+-+-
Single channelElectrode configuration in cells for aluminium dose
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…
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
Cathodes (-)
Anodes (+)
+ +- - + +- -
Bipolar electrodes
anode
+-
cathode
… and both, monopolar and bipolar connections, allow this change of polarity!
However, it is more complex for cells that combine electrocoagulation and electroflotation in different compartments
Horizontal flowVertical flow
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.
e-
-
Current density (j) influences on both: number of bubbles and the average size of bubbles
e-
-
Flow rate can also be used to control the average bubble size
If electroflotation processes have to be promoted it has to be taken into account that:
2.3.3. PROMOTION OF THE ELECTROFLOTATION PROCESS
Power supply
Separator
Efluent
EC
EF
Divided electrocoagulation/
electroflotation
Combinedelectrocoagulation/
electroflotation
And also that the electroflotation can be carried out in the same or in a different cell
Power supply
Separator
Efluent
EC
EF
air
influent
2.3.4 OTHER PROCESSES
++
+ +
+
+
+
++
++
+
+
++
+
+
++
+
+
++
++
+
+
Coalescence of phases
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
Inter-droplet bridging
Dissolved organic matter
Enmeshment in a precipitate
HO OH SO3 -
NO2
NN
HO OH SO3 -
NO2
NN
HO OH SO3 -
NO2
NN
HO OH SO3 -
NO2
NN
HO
O
HS
O3
-
NO
2
NN
HO OH SO3 -
NO2
NN
HO OHSO3 -
NO2
NN
HO
OH
SO3 -
NO
2
NN
HO OHSO3 -
NO2
NN
HO
OH
SO3 -
NO2 NN
HO
OH
SO3 -
NO2NN
HO OH SO3
-
NO2
NN
HO OH
SO3 -
NO2
NN
HO OHSO3 -
NO2
NN
HO OHSO3 -
NO2
NN
HO OH
SO3 -
NO2
NN
HO
OH
SO3 -
NO2
NN
HO
OH
SO3 -
NO2 NN
HO OH
SO3 -
NO2
NN
HO
OH
SO 3 -
NO 2
N N
HO
OH
SO
3 -
NO
2N
N
M3+
M3+HO
OH
SO3
-
NO2
NN
HO OH
SO3 -
NO2
NN
HO
O
HS
O3
-
NO
2
NN
HO
OH
SO3
-
NO2
NN
HO OH SO3 -
NO2
NN
HO
O
HS
O3
-
NO
2
NN
HO
OH
SO 3 -
NO 2
N N
HO
OH
SO3 -
NO2 NN
HO
OH
SO
3 -NO
2N
N
HO
OH
SO3
-
NO2
NN
HO OH
SO3 -
NO2
NN
Binding of monomeric cationic species to anionic
sites of the organic molecules, neutralising
their charge and resulting in reduced solubility
compounds
Binding of polymeric cationic species to anionic
sites of the organic molecules, neutralising
their charge and resulting in reduced solubility
compounds
Adsorption on a superficially charged precipitate
HO
OH
SO
3 -
NO
2
NN
HO OH
SO3 -
NO2
NN
HO
O
HS
O3
-
NO
2
NN
HO
O
HS
O3
-
NO
2
NN
HO OH SO3 -
NO2
NN
HO
OH
SO 3 -
NO 2
N N HO
OH
SO3 -
NO
2
NN
HO
OH
SO3 -
NO
2
NN
+
+
+
+
+
+
+
+
+
HO
O
HS
O3
-
NO
2
NN
Precipitation of phosphates
clarifier
Treated wastewater
wastewater
Electrodissolution cell
AlPO4
FePO4
-2
-4
-6
2 4 6 8 10pH
Log dissolved P
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 WWTP6) Lower operating cost. However, higher investment
In literature some advantages are reported for electrocoagulation processes including:
2.5 Advantages and disadvantages of electrocoagulation
3.ELECTRO-OXIDATION
Wastewater polluted with soluble organic pollutants
Is it possible the recovery of the pollutant as a valuable product?
High calorific power?
Non AOP oxidationAOP oxidationElectrochemical oxidation
Biodegradable?
no
no
no
When can be applied?
3.1 Fundamentals
Electro-oxidation technologies: use of an electrolytic cell to oxidize the pollutants contained in a wastewater
1. Direct electrolysis Oxidation of the pollutant on the electrode surface
2. Advanced oxidation processes
With some anode materials it is possible the generation of OH·
3. Chemical oxidation
On the electrode surface several oxidants can be formed from the salts contained in the salt
e-
+
OH·
pollutant
H2O
PO43-
P2O84-
pollutant
pollutant
e-
e-
Organic pollutant
intermediates(aromatics, carboxylic acids)
+
...CO2
H2O
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
e-
e-
Organic pollutant
intermediates(aromatics, carboxylic acids)
+
...CO2
H2O
O2
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
Cl2
Cl-
e-
e-
Organic pollutant
intermediates(aromatics, carboxylic acids)
+
...CO2
H2O
O2
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
Cl2
Cl-
3. The presence of compounds in the wastewater that can be transformed into oxidants, promoting mediated electrochemical oxidation processes
3. The presence of compounds in the wastewater that can be transformed into oxidants, promoting mediated electrochemical oxidation processes
e-
e-
Organic pollutant
+
...CO2
H2O
O2
Cl2
Organic pollutant
2. Mass transport, which can be promoted by a proper cell design
2. Mass transport, which can be promoted by a proper cell design
1. Electrode material, which influences on the nature of the products and on the importance of the side reactions
1. Electrode material, which influences on the nature of the products and on the importance of the side reactions
Cl-
3.2 Electrode material
MECHANICAL STABILITY. CHEMICAL STABILITY MORPHOLOGY. ELECTRICAL CONDUCTIVITY CATALYTIC PROPERTIES RATIO PRICE/ LIFETIME.
MECHANICAL STABILITY. CHEMICAL STABILITY MORPHOLOGY. ELECTRICAL CONDUCTIVITY CATALYTIC PROPERTIES RATIO PRICE/ LIFETIME.
DESIRABLE PROPERTIES
Typical materials include
low efficiency electrodes
High efficiency electrodes
material
Metals
Carbon
oxides
PlatinumStainless stell
GrafiteDoped diamond
DSATi/SnO2
Ti/PbO2
e-
+
SOFT OXIDATION CONDITIONS
Many intermediatesSmall conversion to carbon dioxideSlow oxidation ratesSmall current efficienciesFormation of polymers from aromatic pollutants is favoured
phenol
Quinones, polymers, carboxylic acids
Mediated oxidation by a higher oxidation state of the species that conforms the electrode surface?
PtIrO2
Fouling by polymers
Low efficiency electrodes
e-
+
HARD OXIDATION CONDITIONS
few intermediatesLarge conversion to carbon dioxideLarge current efficiencies only limited by mass transfer
phenol
Carbon dioxide
OH· generation? Confirmed for conductive-diamondSuggested for PbO2/SnO2
High efficiencies electrodes
BDDTi/PbO2
Active electrodes
Non-active electrodes
Ti/SnO2
Ti/ PbO2
Doped diamond
PtStainless steelDSA
Drawbacks of non-active electrodes:
Conductive diamond: large price >6000 euros/sqmPbO2/SnO2: Dissolution of toxic species
Anode (+)
e-
R
RO
R
RO
Mass Transport
Electrochemical Reaction
Interfase
Electrolyte
Anode (+)
e-
Mass Transport
Electrochemical Reaction
Interfase
Electrolyte
R
RO
R
RO
H2O
OH·
Anode (+)
e-
Mass Transport
Electrochemical Reaction
Interfase
Electrolyte
RO
RCred
Cox Cox
Direct oxidation process Mediated oxidation process
Ele
ctro
che
mic
al o
xid
atio
n
ROLE OF THE HYDROXYL RADICALS
Kinetic or mass transport controlled
Kinetic controlled
e- e-
cath
ode
anod
e
H2O
0.5H2+ OH-
Cathodic material
Deposit of carbonatesOH- + HCO3
- Increase in the cell potential
Increase in the energy consumption
e- e-
cath
ode
anod
e
e- e-
anod
e
cath
ode
H2O
0.5 O2+ 2H+
The organic-oxidation processes that occur in an electrochemical cell are usually irreversible. Hydrogen evolution is the main cathodic reaction.
Polarity reversal
SIMPLE MECHANICAL DESIGN. SMALL PRICE. EASY TO USE. LOW MAINTENANCE COST.
ENHANCED MASS TRANSFER.
HOMOGENEOUS CURRENT DISTRIBUTION ON THE ELECTRODES.
LARGE DURABILITY
SAFETY
DESIRED CHARACTERISTICS FOR A ELECTROCHEMICAL CELL
3.3 Electrochemical cell
+ -
Power supply
e- e-
Anode Cathode
Anolite Catholite
Membrane
Turbulence promoters
3.3.1 IS IT RECOMMENDED THE USE OF DIVIDED CELLS?
1. The membrane increases the cell potential and consequently the operating cost.2. Most organic-oxidation processes are irreversible
1. The membrane increases the cell potential and consequently the operating cost.2. Most organic-oxidation processes are irreversible
Direction of charge flux
V
Cel
l pot
entia
l
ElectrolyteANODE CATHODE
hW
ea + h
hdiff
hW
ea + h + hreaction
hW
3.3.2 STIRRED-TANK CELLS
+ -
Power supply
e- e-
anode cathode
Turbulence promoters
ADVANTAGE: Simplest cell
DRAWBACK: Low mass transfer coefficients
ANODE CATHODETURBULENCE PROMOTER
Membrane?
INLET ANOLYTE INLET CATHOLYTE
OUTLET ANOLYTE OUTLET CATHOLYTE
3.3.3 SINGLE FLOW CELL
3.3.4 FILTER PRESS CELL
Large electrode surfaces / volume ratiosSmall interelectrode gapPlane electrodes
Electrolyte flow
+ +
Packed bed cell
Steel cathode
Steel anode
Activated carbon
polyuretane
Cell with continuous regeneration of the adsorbent
-
3.3.5. OTHER CELLS
Rotating electrode cell
CATHODE
ANODE
e-
Electrod
o
Power supply
e-
pollutant
product
inert1
electroactive
pollutant
Product
inert
electroactive
pollutant
Product
a) Direct electrolysis
b) Indirect electrolysis
inert2
3.4 Indirect electrochemical oxidation processes
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
anode cathode
Homogeneous reactions
Heterogeneous
reactions
A B
C D
e-
e-
e-
IV
A
B
D
Ce-
e-
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
Types of mediated electrochemical oxidation processes
Production of reagents and treatment of the waste in the same cell
Production of reagents and treatment of the waste in different cells
Separation of the oxidant or of its reduction product
Electrosynthesis of the oxidant
Oxidation and electroxidation of the pollutants
Electrosynthesis of the oxidant
Oxidation and electrooxidation of the pollutants
wastewater
Treated wastewastewater
Treated waste
Dosing of reagent
Dosing of reagent
Separation of the oxidant or of its reduction product
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
To take in mind…
Ag(I) / Ag(II)
Co(II) / Co(III)
Ce(III) / Ce (IV)
Fe(II) / Fe (III)
SO4 2- / S2O8
2-
Reversible oxidant
The oxidant can be reduced in the cathode. A divided cell may be considered
Irreversible (killers)Cl2
O3
H2O2The oxidant is not reduced on the cathode. Non-divided cells are used for their production
PO4 3- / S2O8
4-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)
Main drawbacks
ions Ag+ are harmful products chlorides can reduce the efficiencies due to
precipitates formation silver is very expensive
eAgAg 2
SHE.Vvs98.1E0
AgeAg2
2COR 2
·2 OOHOH
Some pollutants efficiency removed by this technology: Ethylene glycol, isopropanol, acetone, organic acids, benzene, kerosene
e)III(Co)II(Co
SHE.Vvs82.1E0
)II(Coe)III(Co
2COR
Co(II) / Co (III)
2·
2 OOHOH
Some pollutants successfully treated: Organic radioactive waste materials, dichloropropanol, ethylene glycol
This process has to be carried out in divided cells (Co can be electrodeposited on the cathode surface)
Main drawback
Phosphate/peroxodiphosphate
e 2OP PO 2 482
34
SHEVvsE .01.20
Large efficiencies with diamond electrodes
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
Sulphate/peroxodisulphate
e 2OS SO 2 282
24
SHE.Vvs06.2E0
Large efficiencies with diamond electrodes
2242
282 O 2
1H2SO 2 OH OS
H2SOSO OH OS 24
252
282
24222
25 SOOH OH SO
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
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
eFeFe 32 SHE.Vvs77.0E0
23 FeeFe
2COR
Some pollutants treated by this technology: Celluloid materials, fats, urea, cattle manure, sewage sludge, meat packing wastes, ethylene glycol
Fe(II) / Fe (III)
ROR
)VI(Fe ?
2·
2 OOHOH
Chloride/ Chlorine
e2Cl lC 2 2-
HClOHClOHCl 22 It can lead to the formation
of organochlorinated compounds
The chlorine speciation depends on the pH
-
Its presence is very common: chloride salts are frequently present in industrial wastewaters.
hypochlorite
hypochlorite
Dosing in channel
Dosing in pipe
+
-
Electrochemical cell
+Electrochemical cell
NaCl
NaCl
5.0 6.0 7.0 8.0 9.0 10.0
1.0
0.8
0.6
0.4
0.2
0.0
% HClO
pH
Hydrogen peroxide
OHHOe2OH2O 222
22 OOH2HO
E0=-0.065 V
It can be formed on the cathode by reduction of oxygen
However, the main drawback is the decomposition of the hydroperoxide anion that it is favoured at alkaline conditions.
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
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
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
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- e-
cath
ode
anod
e
O2
H2O2
Anodic oxidation
processes
Ozone
OH3e6H6O 23
OH2e4H4O 22
E0=1.51 V
E0=1.23 V
The oxidation of water to ozone can occur on the electrode surface but it is less favoured than that of oxygen
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 ozoneanode electrolyte current density yieldB-PbO2 HPF6 (2M) 750 mA cm-2 21%Active carbon HBF4 (7.3 M) 600 mA cm-2 35%Active carbon HBF4 (62% w/w) 200 mA cm-2 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.
0
500
1000
1500
2000
2500
3000
3500
0 500 1000 1500 2000 2500 3000
W / kWh m-3
CO
D /
mg
dm
-3Lower 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
+ -
Comportment of adjust of the pH
Filter
Electrochemical Reactor
Absorber
H2
S(S)
NaOH Solution
PoorGas H2S
RichGas H2S
Solution
3.6. Combined processes
Treatment of gaseous effluents
Combination of electrochemical oxidation with bio-oxidation
electrooxidation biooxidation
biooxidation electrooxidation
Main drawback: When to change?Main drawback: When to change?
a) pre-treatment
b) post-treatment