Department of Chemical Engineering. Universidad de Castilla La Mancha.

Department of Chemical Engineering. Universidad de Castilla La Mancha.
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 Ciudad Real. Spain. Department of Chemical Engineering. Universidad de Castilla La Mancha. Spain

CONTENTS 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 PROMOTION OF THE ELECTROFLOTATION PROCESS 2.3.4 OTHER PROCESSES 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

Red Ox influent Red Ox effluent M M Mn+ Mn+ Anode Cathode
1.1 What happens inside an electrochemical cell during the electrolysis of a wastewater? Power supply e e - - e e - - Red Ox 5. Migration of anions influent 1. Electrooxidation 2. Electroreduction Anode Red Ox Cathode 5. Migration of cations effluent M M 3. Electrodissolution 4. Electrodeposition Mn+ Mn+

1.2 Types of electrochemical wastewater- treatment technologies
Anions Cations Feed solution Diluted solution Concentrated solution cathode anode Cathionic membrane Anionic membrane Anionic membrane 1.2 Types of electrochemical wastewater- treatment technologies electrodialysis 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

Typical hydraulic residence time of a settler for wastewater treatment
2. ELECTROCOAGULATION 2.1 What is coagulation? Pollutants size 10 micras 100 micras 0.1 nm 10 nm 100 nm 1 micra 1 nm 1 mm 1 cm Dissolved comp. Colloids Suspended solids influent effluent Typical hydraulic residence time of a settler for wastewater treatment Sludge

Distance from the surface
+ - Diffuse layer Negatively charged particle Bulk solution Distance from the surface -(electrostatic potential) Zeta potential Surface potential Interaction energy Ea Ea+Eb Distance between particles Eb Electrostatic repulsion energy: Ea Van der Waals attraction energy: Eb Resulting energy : Ea+Eb 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)

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 Sludge Outlet Inlet sedimentation coagulation flocculation 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.

Typical titration curve for neutralization of aluminium salt solutions
Coagulation by hydrolysing aluminium salts Al3+ Concentration of monomeric hydrolysis products of Al(III) in equilibrium with the amorphous hydroxides at zero ionic strength at 25ºC Al(OH)2+ -2 Al(OH)4- AlT -4 Al(OH)2+ -6 Log [Alx(OH)y 3x-y ] / mol dm-3 Al(OH)3 -8 -10 Precipitate Polymeric species Monomeric species -12 OH-/Al pH 3 4 5 6 7 8 9 2 0.0 0.5 1.5 1.0 2.0 2.5 3.0 Nitrate media Sulphate media z1 z2 z3 z4 2 4 6 8 10 12 14 pH Typical titration curve for neutralization of aluminium salt solutions

Ali/AlT pH 100 80 60 40 20 h= OH/ AlT 3 3,5 4 4,5 monomers
3 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/ AlT 0,25 1 2 2,2 2,25

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 -2 -4 Fe(OH)3 -6 Log [Fe(OH)y 3x-y ] / mol dm-3 -8 Fe3+ Fe(OH)4- -10 Fe(OH)2+ Fe(OH)2+ -12 2 4 6 8 10 12 14 pH

M Electro-dissolution e- Mn+ coagulation +
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. Electrochemical processes involved: Electrodissolution Electrolytic generation of oxygen and hydrogen M Electro-dissolution e- Unstabilized small particles Aggregated particles Mn+ flocculation coagulation + colloids macromolecules emulsions

M Electro-dissolution e- Mn+ coagulation + Aluminium Iron
2.2.1 ANODE MATERIAL Aluminium Iron M Electro-dissolution e- Mn+ coagulation +

Influence of current density
2.2.2 ELECTRODISSOLUTION Influence of pH Influence of current density Faraday’s value Chemical dissolution Experimental Electrochemical process Faradaic Efficiencies can be over 100% Chemical process

Cathode Anode pH profile in the electrochemical cell
Direction of electrolyte flux pH profile in the electrochemical cell Cathode

- Anodic processes Cathodic processes H20 e- H2O e- O2 H2 +
2.2.3 ELECTROLYTIC GENERATION OF OXYGEN AND HYDROGEN Anodic processes Cathodic processes H20 e- H2O e- O2 H2 + -

Air-dissolved flotation
Bubbles diminish the overall density of the system and the particle floats

Oxygen and hydrogen bubbles
turbulence adhesion Promotes soft mixing conditions and improves flocculation processes Gaseous microbubbles link to pollutant particles. Consequently, the density of the new species decreases and this promotes the flotation of the particle Electrochemically assisted flocculation (electroflocculation) Electrochemically assisted flotation (electroflotation)

MAIN PROCESSES INVOLVED IN THE ELECTROCHEMICALLY ASSISTED TECHNOLOGIES FOR COLLOID-POLLUTED WASTES e - Anodic processes Cathodic processes e- Al(III) species pollutants flocs Electroflotation Electrocoagulation Electroflocculation Electrodissolution H2O H+ + O2 H2 + OH-

2.3 Electrochemical cells
purpose Only electrodissolution Electrocoagulation/electroflocculation Electrocoagulation/electroflocculation electroflotation Type of cells

Coagulation/flocculation Sedimentation/flotation The process combines
TANK CELLS Inlet Outlet H 2 O Precipitated Anode Cathode Power supply (Pollutant) e - M n + hydrated OH Settling Flotation Settled sludge Sludge Floated sludge Mixing can be accomplished either by mechanical stirrers or by the evolved gases Coagulation/flocculation Sedimentation/flotation The process combines Contrarily to electrooxidation processes, mass transport does not control the overall rate of the process

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 Outlet H 2 O Precipitated Anode Cathode Power supply (Pollutant) e - M n + hydrated OH Settling Flotation Settled sludge Sludge Floated sludge Hydrogen evolution can disturb the sedimentation process. For this reason, if possible, it is better to separate the cathodic process from the sedimentation

HydroShock™ ElectroCoagulation

Electrode configuration in cells for aluminium dose
2.3.2 FLOW CELLS 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… 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 + - Multiple channels + - Single channel Electrode configuration in cells for aluminium dose

… and both, monopolar and bipolar connections, allow this change of polarity!
Cathodes (-) Anodes (+) Bipolar electrodes - + cathode + + - - + + - - anode 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

Divided electrocoagulation/ electroflotation
And also that the electroflotation can be carried out in the same or in a different cell Power supply Efluent EF Divided electrocoagulation/ electroflotation EC Separator Power supply Efluent Combined electrocoagulation/ electroflotation EF EC Separator

2.3.4 OTHER PROCESSES air influent

2. 4 Electrocoagulation of soluble organics and break-up of emulsions
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
HO OH SO3 - NO2 N M3+ Dissolved organic matter Binding of monomeric cationic species to anionic sites of the organic molecules, neutralising their charge and resulting in reduced solubility compounds HO OH SO3 - NO2 N HO OH SO3 - NO2 N HO OH SO3 - NO2 N HO OH SO3 - NO2 N HO OH SO3 - NO2 N Binding of polymeric cationic species to anionic sites of the organic molecules, neutralising their charge and resulting in reduced solubility compounds HO OH SO3 - NO2 N HO OH SO3 - NO2 N HO OH SO3 - NO2 N HO OH SO3 - NO2 N HO OH SO3 - NO2 N HO OH SO3 - NO2 N HO OH SO3 - NO2 N HO OH SO3 - NO2 N HO OH SO3 - NO2 N HO OH SO3 - NO2 N HO OH SO3 - NO2 N + + HO OH SO3 - NO2 N + HO OH SO3 - NO2 N HO OH SO3 - NO2 N + HO OH SO3 - NO2 N NO2 HO OH SO3 - NO2 N + + HO OH SO3 - NO2 N SO3 - HO OH SO3 - NO2 N HO OH SO3 - NO2 N + + HO N N + OH HO OH SO3 - NO2 N HO OH SO3 - NO2 N HO OH SO3 - NO2 N Enmeshment in a precipitate Adsorption on a superficially charged precipitate

Electrodissolution cell
Precipitation of phosphates Log dissolved P -2 FePO4 AlPO4 -4 -6 2 4 6 8 10 pH Electrodissolution cell Treated wastewater clarifier wastewater

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? High calorific power? no Biodegradable? no Non AOP oxidation AOP oxidation Electrochemical oxidation no

pollutant H2O pollutant e- OH· PO43- + P2O84- pollutant
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 H2O pollutant 2. Advanced oxidation processes With some anode materials it is possible the generation of OH· e- OH· PO43- 3. Chemical oxidation + On the electrode surface several oxidants can be formed from the salts contained in the salt P2O84- pollutant

... e- Organic pollutant e- CO2 + H2O O2
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- Organic pollutant intermediates (aromatics, carboxylic acids) ... e- CO2 + H2O O2 Cl- Cl2

... e- Organic pollutant e- CO2 + H2O O2
intermediates (aromatics, carboxylic acids) ... e- CO2 + 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 H2O O2 Cl- Cl2

... e- Organic pollutant e- CO2 + H2O O2 Cl-
intermediates (aromatics, carboxylic acids) ... e- CO2 + H2O O2 Cl- 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

... Organic pollutant e- Organic pollutant e- CO2 + H2O O2
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 the side reactions + H2O 3. The presence of compounds in the wastewater that can be transformed into oxidants, promoting mediated electrochemical oxidation processes O2 Cl- Cl2

DESIRABLE PROPERTIES 3.2 Electrode material MECHANICAL STABILITY.
CHEMICAL STABILITY MORPHOLOGY. ELECTRICAL CONDUCTIVITY CATALYTIC PROPERTIES RATIO PRICE/ LIFETIME.

Typical materials include
Platinum Stainless stell Metals Carbon oxides Grafite Doped diamond material DSA Ti/SnO2 Ti/PbO2 low efficiency electrodes High efficiency electrodes

SOFT OXIDATION CONDITIONS phenol
Low efficiency electrodes SOFT OXIDATION CONDITIONS phenol Quinones, polymers, carboxylic acids Many intermediates Small conversion to carbon dioxide Slow oxidation rates Small current efficiencies Formation of polymers from aromatic pollutants is favoured e- Fouling by polymers + Pt IrO2 Mediated oxidation by a higher oxidation state of the species that conforms the electrode surface?

HARD OXIDATION CONDITIONS phenol
High efficiencies electrodes HARD OXIDATION CONDITIONS phenol Carbon dioxide few intermediates Large conversion to carbon dioxide Large current efficiencies only limited by mass transfer e- + Confirmed for conductive-diamond Suggested for PbO2/SnO2 BDD Ti/PbO2 OH· generation?

Non-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

Electrochemical oxidation
ROLE OF THE HYDROXYL RADICALS Direct oxidation process Mediated oxidation process Interfase Interfase Interfase R RO H 2 O OH Electrolyte RO R Cred Cox Electrolyte R RO e - e - Electrolyte Electrochemical oxidation e - Anode (+) Anode (+) Anode (+) Electrochemical Reaction Mass Transport Electrochemical Reaction Mass Transport Electrochemical Reaction Mass Transport Kinetic or mass transport controlled Kinetic controlled

H2O 0.5H2+ OH- H2O 0.5 O2+ 2H+ 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- H2O 0.5H2+ OH- cathode anode cathode anode Deposit of carbonates OH- + HCO3- Increase in the cell potential e- e- Increase in the energy consumption H2O 0.5 O2+ 2H+ Polarity reversal anode cathode

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

2. Most organic-oxidation processes are irreversible
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 Power supply + - Turbulence promoters e- e- Membrane Direction of charge flux V Cell potential Electrolyte ANODE CATHODE hW ea + h hdiff ea + h + hreaction Anolite Catholite Anode Cathode

ADVANTAGE: Simplest cell
3.3.2 STIRRED-TANK CELLS Power supply + - Turbulence promoters e- e- anode cathode ADVANTAGE: Simplest cell DRAWBACK: Low mass transfer coefficients

3.3.3 SINGLE FLOW CELL ANODE TURBULENCE PROMOTER Membrane? CATHODE
OUTLET ANOLYTE OUTLET CATHOLYTE INLET ANOLYTE INLET CATHOLYTE

3.3.4 FILTER PRESS CELL Large electrode surfaces / volume ratios Small interelectrode gap Plane electrodes

Electrolyte flow

Cell with continuous regeneration of the adsorbent
OTHER CELLS + - + Steel cathode Steel anode Activated carbon polyuretane Packed bed cell Cell with continuous regeneration of the adsorbent

Rotating electrode cell
CATHODE ANODE

Electrodo 3.4 Indirect electrochemical oxidation processes e e
Power supply e e - - pollutant a) Direct electrolysis product inert1 inert2 pollutant Electrodo electroactive Product b) Indirect electrolysis inert pollutant electroactive Product

Homogeneous reactions
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 I V A B C D e- e- e- A B D C e- anode cathode Heterogeneous reactions

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 wastewater Dosing of reagent Treated waste
Electrosynthesis of the oxidant Separation of the oxidant or of its reduction product Oxidation and electroxidation of the pollutants wastewater Treated waste Dosing of reagent Electrosynthesis of the oxidant Separation of the oxidant or of its reduction product Oxidation and electrooxidation of the pollutants Treated waste wastewater

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

It can be formed by a cathodic process. Extra oxidation efficiency!
Ag(I) / Ag(II) Co(II) / Co(III) Reversible oxidant Ce(III) / Ce (IV) Fe(II) / Fe (III) The oxidant can be reduced in the cathode. A divided cell may be considered SO4 2- / S2O8 2- These oxidants are generated from anions typically present in a wastewater PO4 3- / S2O8 4- Cl2 Irreversible (killers) O3 It can be formed by a cathodic process. Extra oxidation efficiency! The oxidant is not reduced on the cathode. Non-divided cells are used for their production H2O2

Ag(I) / Ag(II) Co(II) / Co (III)
Some pollutants efficiency removed by this technology: Ethylene glycol, isopropanol, acetone, organic acids, benzene, kerosene Main drawbacks ions Ag+ are harmful products chlorides can reduce the efficiencies due to precipitates formation silver is very expensive Co(II) / Co (III) Some pollutants successfully treated: Organic radioactive waste materials, dichloropropanol, ethylene glycol Main drawback This process has to be carried out in divided cells (Co can be electrodeposited on the cathode surface)

Sulphate/peroxodisulphate
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 Phosphate/peroxodiphosphate 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 Large efficiencies with diamond electrodes

Fe(II) / Fe (III) 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

Chloride/ Chlorine Its presence is very common: chloride salts are frequently present in industrial wastewaters. The chlorine speciation depends on the pH It can lead to the formation of organochlorinated compounds hypochlorite Dosing in channel 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 + Electrochemical cell NaCl Dosing in pipe hypochlorite Electrochemical cell + -

Anodic oxidation processes
Hydrogen peroxide It can be formed on the cathode by reduction of oxygen E0= V 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 e- 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 O2 Anodic oxidation processes H2O2 cathode anode

Ozone The oxidation of water to ozone can occur on the electrode surface but it is less favoured than that of oxygen E0=1.51 V 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 yield B-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.

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 

H2 S(S) 3.6. Combined processes Treatment of gaseous effluents
PoorGas H2S NaOH Solution H2 RichGas H2S + - Filter Comportment of adjust of the pH Electrochemical Reactor Absorber S(S) Solution

Main drawback: When to change?
Combination of electrochemical oxidation with bio-oxidation a) pre-treatment electrooxidation biooxidation b) post-treatment electrooxidation biooxidation Main drawback: When to change?

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