Electrochemical Ox-Red.pptx

8/10/2019 Electrochemical Ox-Red.pptx

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Electrochemical Oxidation / Reduction

Prepared by Ernest Nsabimana,Graduate Student, Geotechnical Eng. Lab / Dept. of Civil Engineering/Kyung Hee University

Physicochemical Processes Class

Submitted to Professor: Seoktae (Steve) Kang

2012.12.04 1


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Content

Introduction

Oxidation/Reduction Reactions

Electrochemical cells

Electrochemical in wastewater Treatment

Conclusion


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Electrochemistry is the branch of chemistry concernedwith the interrelation of electrical and chemical effects.A large part of this field deals with the study ofchemical changes caused by the passage of an electriccurrent and the production of electrical energy bychemical reactions.

Introduction

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Characterizing oxidation/reduction reactions

In an oxidation/reduction reactions, electrons are transferred

from one reactant to another.For example:

Ce4+ + Fe2+ Ce3++ Fe3+

Cerium Ce4+: an oxidizing agent/oxidant, electron acceptor.

Iron Fe2+: an reducing agent/reductant, electron donor.

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Oxidation/Reduction reactions are sometimes called redox reactions.So we can write a generalized equation for a redox reaction as:

(1)Aredreductant, reduced form, having given upelectrons, becomes

an oxidizing agent, Aox.

(2) Box, the oxidized formof species B, accepts electrons from Ared

to form the new reductant Bred.


Ared + Box Aox+ Bred

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Types of electrochemical cells:

1Galvanic (or voltaic) cellsstore electrical energy.Batteries.

2Electrolytic cellrequires an external source of

electrical energy for operation

Electrochemical Cells

Cathodes and Anodesgalvanic or electrolytic cell

The cathodereduction

The anode oxidation

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An electrochemical cellis a device capable of either deriving electrical

energy from chemical reactions, or facilitating chemical reactions throughthe introduction of electrical energy
http://en.wikipedia.org/wiki/Electricityhttp://en.wikipedia.org/wiki/Electrochemistryhttp://en.wikipedia.org/wiki/Electrochemistryhttp://en.wikipedia.org/wiki/Electricity


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Galvanic Cells

An electrochemical cell that releases energy is called a galvaniccell. The electrochemical reaction has a negative value of theGibbs free energy and a positive cell potential difference.

E0: Standard Electrode Potential

Galvanic cell in which the reaction between A+and Bis exothermic, with a G of -10 kJ/mol under standard conditions and a value of E oof 0.10 V.

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Gibbs Free Energy

When all substances are at unit activity

E0rnxis called the standard emf of the cell reaction


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Electrolytic Cells

An electrolytic cell has an endothermic chemical reaction. Thereaction is not spontaneous so a power source is required. Energy

is stored in an electrochemical cell. That energy can be releasedwhen the cell is allowed to run in a galvanic mode.

Above left the molecules A and B+are produced and energy is released. The electrode on the left side ofthe cell is the cathode because A+is reduced to A at that electrode.

Above right is the same cell in an electrolytic mode. An energy source causes the reaction to go in thedirection opposite to the spontaneous direction. Now, A+and B are produced. The electrode on the rightis the cathode becuase B+is reduced to B there.

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Electrolytic Cell

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The Nernst Equation

0E=E + ln

o

R

RT

nF

Oxne- Red

E0

: Standard Electrode Potential

R: ideal gas constant, 8.314J K -1 mol-1

T: temperature, K n: number of moles of electrons

F: the faraday=96,485C ln: natural logarithm=2.303log

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Nernst Equation is used to calculate all potential under non-standardconditions

reaction quotient.
http://en.wikipedia.org/wiki/Reaction_quotienthttp://en.wikipedia.org/wiki/Reaction_quotient


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E0: Standard Electrode Potential

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Electrochemical in Wastewater

Treatment

The use of electricity in wastewater treatment has been startedin the late 1900s

The capital investment and the electricity costs ofelectrochemical treatment have been high and applications were

not used widely in the 20th century

However, electrochemical technologies have been started toinvestigate and use again during the past two decades

Nowadays, the costs of the electrochemical treatment are

comparable with other technologies and in some cases,electrochemical treatment may be more efficient and compactthan other technologies

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Electrochemical wastewater treatment technologies are:

Electrodeposition,

Electrocoagulation (EC),

Electroflotation, Electrooxidation (Electrochemical

Oxidation) (EO), Electrodisinfection and Electroreduction.

In this Presentation, focus is on electrooxidationandElectroreduction

Electrochemical in Wastewater

Treatment

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Electro-oxidation

Electrooxidation is a mediated reaction and occurs viaoxygen atoms transfer from water in the solvent phase tothe oxidation product

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Electro-oxidation

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Electrochemical reactor

Electro-oxidation

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Electrochemical reactor

Electro-oxidation

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Electrochemical oxidation of pollutants can take place throughtwo different oxidation mechanisms.

(1) Direct anodic oxidation, where the pollutants are destroyed at the anodesurface

(2) indirect oxidation where a mediator (HClO,H2S2O8and others) iselectrochemically generated to carry out the oxidation

Electro-oxidation

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Direct oxidation of pollutants takes place in two steps:

(1) Diffusion of pollutants from the bulk solution to the anode

surface and

Consequently, the efficiency of the electrochemical process will depend onthe relationship between mass transfer of the substrate andelectron transfer at the electrode surface. The rate of electrontransfer is determined by the electrode activity and current density.

Electro-oxidation

Direct oxidation

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(2) oxidation of pollutants at the anode surface.


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Anodic oxidation of organic pollutants: two different pathways

Electrochemical conversion. Organic compounds are onlypartially oxidized. Therefore, a subsequent treatment may be

required

Electrochemical combustion. Organic compounds are transformed

into water, carbon dioxide and other inorganic components:

Electro-oxidation

Direct oxidation

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Design of an electrochemical-oxidation process

Electrode material

High physical and chemical stability; resistance to erosion, corrosion

and formation of passivation layers. High electrical conductivity. Catalytic activity and selectivity. Low cost/life ratio. The use of electrode materials that areinexpensive and durable must be favored.

The oxidation of water to oxygen occurs at approximately 1.2 V vs. NHE(Normal Hydrogen Electrode). However, in fact, a higher voltage has to beapplied for electrochemical oxidation of water to occur at the anode.

Electro-oxidation

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Low O2 overvoltage anodes are characterized by a high electrochemical activity toward oxygen evolution

and low chemical reactivity toward oxidation of organics

In view of the afore mentioned facts, high O2overvoltage anodes are usually preferred. In particular,boron-doped diamond (BDD) anodes have been reported to yield higher organic oxidation rates andgreater current efficiencies than other commonly used metal oxides such as PbO2 andTi/SnO2-Sb2O5

Design of an electrochemical-oxidation process(Contn)

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Cell configuration

Classification of electrochemical reactors in terms of cell configuration, electrode geometry and flow type.

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The rate of oxidation is determined from the concentration ofpollutant vs. time plot whereas the representation against Qis a measure of the efficiency of the electro-oxidation process

The evolution of the total organic carbon (TOC) concentration withtime and with Q during electrochemical oxidation of phenol bymeans of a BDD (Boron-Doped Diamond)anode

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There are specific energetic parameters (Instantaneous CurrentEfficiency (ICE), Average Current Efficiency (ACE), ElectrochemicalOxidation Index(EOI),and Specific Energy Consumption(W)) that are

used to assess the energy efficiency of the process

Among these, ICEis often used and measures the amount of currentintensity used on the destruction of the target pollutants, and theoccurrence and importance of side reactions

An ICE value of 1 indicates that the applied intensity is beingused for the oxidation of the target pollutant completely.

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The main drawback of this process is its high operating cost dueto the high energy consumption. Additionally, potential for theformation of chlorinated organics during indirect oxidation byactive chlorine needs to be considered. Also, in order to applythis technology, the effluent has to be conducting. Unfortunately,

not all waste streams will have sufficient conductance and theaddition of an electrolyte may be necessary. Moreover, electrodefouling may occur due to deposition of material on the electrodesurface.

ADVANTAGES AND DISADVANTAGES

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Electro-Reduction:

Case of Wastewater Treatment

Electrochemical reduction of nitrate has been studied extensively during lastyears in numerous works.In this respect electrochemical reduction catalysis could be advantageouslyapplied to the treatment of potable and industrial wastewater, wherebynitrate species were transformed into harmless products at various cathodematerials

From a practical and an environmental point of view, it is highly desirable

that the electrochemical process transform nitrate efficiently and selectivelyinto the harmless N2gas. Using the Electrochemical Reduction of Nitrate(ERN)

Nitrate removal

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The final composition depends mainly on the pH, the applied potentialand the used cathode material. On the other hand, electrochemistryprovides promising solutions when it is combined with ionexchange, the last one being capable of NO-3

selective removalfrom the waste water. The electrochemical alternative represents anattractive and promising solution for the removal of nitrate ions

Electrochemical reduction of Nitrate

power

supply

anode(+)

cathode(-)

NO NNH NO NO

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El t h i l d ti f Nit t


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Materials used in electrochemical reactors for nitrate reduction

Cathode electrode materials

carbon electrodes;

monometallic electrodes, which are composed entirely of a single metalcomponent;

multimetallic electrodes, which are composed of two or more metalcomponents.

Anode materials

Two requirements should ideally be met by potential anode materials: first,the material should be stable under actual electrolysis conditions andsecond, it would be an advantage if the anode reaction did not interferewith the reduction process so that the cell may be operated in anundivided configuration


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Analysis methods of the nitrate products


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Types of electrochemical reactors

Several types of electrochemical reactors (ER) are used for nitrate destruction.The electrochemical process for removal of nitrate can be operated in eitherundivided or membrane-divided cells

Both types of cells share the following characteristics

- capable of destroying >99% of nitrate;

- compatible with caustic recycle processes

-when no chromate is present, best results are obtained with a nickel

cathode and a nickel anode;

-presence of chromate inhibits nitrate reduction at high current density;

-ammonia gas generated at the cathode requires further treatment

-before release to the environment.


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El t h i l d ti f Nit t


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Direct Reduction in an Electrochemical flow reactor


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Effect of applied current density and flow rate on the ERN

The nitrate removal rate is proportional to the concentration ofnitrate and to the hydrogen concentration

The kinetic equation for nitrate removal


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In the used flow reactor the nitrate/nitrite destruction efficiency was improvedwith an increase in the current density and operation of the cell in a dividedconfiguration. On the two investigated types of cathode materials (Cu and CuSn), the concentration of nitrate was reduced electrochemically to the maximumpermissible limit (50 mg/L in drinking water).

An identical quantity of nitrate (8.5 g) was reduced at the CuSn cathode, belowthe permissible limit (99.5% conversion), in 18 h, and at the Cu cathode in 20h.

In both cases, as the current density increases the removal efficiency increases,less time being needed to reduce nitrate successfully.

Conclusion

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References:


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References:

1. ELECTROCHEMICAL METHODS Fundamentals and Applications, Allen J. Bard, Larry R. Faulkner,2001

2. Electrochemical removel of Nitrate from waste water, PhD Thesis, 2012

3. Contributions of electrochemical oxidation to waste-water treatment:

fundamentals and review of applications, Angela Anglada, Ane Urtiaga, 2009

4. Google: wikipedia

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Electrochemical Ox-Red.pptx

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