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

ESSEE 44th European Summer School on Electrochemical Engineering

Palić, Serbia and Montenegro17 – 22 September, 2006

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-

Anode

Cathode

Power supply

e- e-

influent

effluentRed

Ox Red

OxM

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 technologyVolumes 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 nm

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

pote

ntia

l)

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

[Al x(

OH

) y 3x

-y ]

/ mol

dm

-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

3456789

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

spec

ies

Polym

eric

spe

cies

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

[Fe(

OH

) y 3x

-y ]

/ mol

dm

-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 cellCathode

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

Anode

Cathode

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

Anode

Cathode

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 flow Vertical 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

HSO

3 -

NO

2

NN

HO OH SO3 -

NO2

NN

HO OHSO3 -

NO2

NN

HO

OH

SO3 -

NO2

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

SO3 -

NO2

NN

M3+

M3+HO

OHSO

3 -

NO2

NN

HO OH

SO3 -

NO2

NN

HO

O

HSO

3 -

NO

2

NN

HO

OH

SO3

-

NO2

NN

HO OH SO3 -

NO2

NN

HO

O

HSO

3 -

NO

2

NN

HO

OH

SO 3 -

NO 2

N N

HO

OH

SO3 -

NO2 NN

HO

OH

SO3 -N

O2

NN

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

SO3

-

NO2

NN

HO OH

SO3 -

NO2

NN

HO

O

HSO

3 -

NO

2

NN

HO

O

HSO

3 -

NO

2

NN

HO OH SO3 -

NO2

NN

HO

OH

SO3

-

NO 2

N N HO

OH

SO3 -

NO2

NN

HO

OH

SO3 -

NO2

NN

+

+

+

+

+

+

+

+

+

HO

O

HSO

3 -

NO

2

NN

Precipitation of phosphates

clarifier

Treated wastewater

wastewater

Electrodissolution cell

AlPO4FePO4

-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

e-

e-

Organic pollutant

+

...CO2

H2O

O2

Cl2

Organic pollutant2. 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

Cl-

3.2 Electrode material

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

Elec

troc

hem

ical

oxi

datio

nEl

ectr

oche

mic

al o

xida

tion

ROLE OF THE HYDROXYL RADICALS

Kinetic or mass transport controlled

Kinetic controlled

e- e-

catho

de

anod

e

H2O

0.5H2+ OH-

Cathodic material

Deposit of carbonatesOH- + HCO3

- Increase in the cell potential

Increase in the energy consumption

e- e-

catho

de

anod

e

e- e-

anod

eca

thode

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

Direction of charge flux

V

Cel

l pot

entia

l

ElectrolyteANODE CATHODE

a +

diff

a + + reaction

3.3.2 STIRRED-TANK CELLS

+ -Power supply

e- e-

anode cathode

Turbulence promoters

ADVANTAGE: Simplest cellDRAWBACK: 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 anodeActivated carbon

polyuretane

Cell with continuous regeneration of the adsorbent

-

3.3.5. OTHER CELLS

Rotating electrode cell

CATHODE

ANODE

e-

Electrodo

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 productschlorides can reduce the efficiencies due to precipitates formationsilver 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(CoSHE.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 wastewatersPowerful oxidant (more selective than persulphate). The oxidation carried out by this reagent depends importantly on the pHLess 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 organicsgood efficiencies are obtained for high temperatures and low current densitiesElectrocoagulation 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

1.00.80.60.40.20.0

% 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

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-

catho

de

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?

a) pre-treatment

b) post-treatment