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Electrochemistry in environment protection

History

Electrochemical water treatment proposed already 1889 in UK

Electrocoagulation – claimed 1904

Formerly electrochemical processes were applied rarely due to high price of electric energy

Increasing press to the quality of waste water causes widespread application of elchem. methods in past 30 years.

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Electrochemical reactions- the transfer of electronsbetween the

electrodesurface and moleculesin the electrolyte.

reactant -electron

rate of electron flow – electric current

oxidation/reduction power -potential (voltage)

amount of electrons – electrical charge

Electroseparation process- the transport forced by electric

potential gradient

Separation of electrode reactions in reactor enable to proceed processes hardly

realizable (or unrealizable) by other way.

Electrochemistry – basic terms

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Electrode– (from technological point of view) piece of electricallyconductive matter (of suitable shape) where electrode reaction takeplace (on its surface)

Anode– electrode with oxidative reaction

Cathode– electrode with reductive reaction

Electrolyte– ion conductive medium. Mainly solutions with dissociated molecules (ions).

Basic terms II

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versatile direct oxidation or reductionmediated oxidation or reductionsimple constructionscalable

selective electrode potentialmaterial of electrode (overpotentials)separation of electrode chambers

ecological electron - „clean reactant“recycling of raw materialswaste treatment

Disadvantages

expensive Faradays law + price of electric energyelectrode materials

mass transfer heterogeneous process - concentrations limited by mass transfer

Advantages

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electrochemical processeseasy automation

low capital cost

electroanalytical methods simple and sensitive (pH – electrode, ASV)

conversion of chemical energy to electrical energy

batteries accumulators

fuel cells

solar cells

electric values (voltage, current)

fast and sensitive response

low cost of reactors (technological scale)

simple peripheral equipment

with exception in case of strongly aggressive environment

Applications of electrochemistry

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conversion of wastes inorganic compounds

organic compounds

construction of zero-emission or low emissions technologies

concentration and recycling

replacement of current technologies by modern with low environmental impact

treatment of old contaminated sites

soils and water contaminated by heavy metals

soils and water contaminated by organic pollutants

Environmental application of electrochem. procs.

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Faraday constant energetic demand related to

F = 96485.3 C/mol

electrode material corrosion stability

electrocatalytic stability

specific surface, hydraulic resistivity

potential efficiency electrolyte conductivity

overpotential

counterelectrode reaction

Limitations

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organic pollutants oxidation

transport electrokinetic methods

electro-membrane methods

electrochemistry as supporting process: gas generation, heat source

direct impact heavy metals removalmine watergalvanoindustry rinsing water

inorganic pollutants reductione.g. nitrates, radioactive waste

indirect impact active agent generated by electrode dissolving

active agent generated on electrode

other

Principles of electrochemical procs.

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direct oxidation of organic pollutants: phenol, chlorphenole , aniline

based on cathodic reduction

based on anodic oxidation

heavy metal electrowinning - galvanoindustry waste water- mine water

Direct impact methods

monopolar and bipolar electrode arrangement

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heterogeneous process

Nernst - Planck mass transfer equation

Electrode reaction take place on electrode-electrolyte interface.

Transport of reactants from solution bulk to electrode surface must be secured

- difficult in diluted solutions

iiiiiii cvcuzcDJrr

+ϕ∇−∇−=diffusionmigrationconvection

cτ

c0

cs

δN

cτ

c0

cs

δN

concentration profile on electrode surface boundary layer

Direct impact methods -Mass transfer limitation

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mass transfer in boundary layer

increase of c0

hydrodynamic flow intensification(δN)

( )s0N

s0 cc kcc

DJ −=δ−= nFJj =

possibilities for intensification:

electrode surface area increase

problematic in case of waste treatmentpossible in combination with separation procs.

local or overallenergetic and material need

specific performancecommon electrode materials

Mass transfer

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commercially available electrode configuration

electrode electrolyte flow

[m s-1]

k

[m s-1]

cmin

[mol m-3]

sheet electrode 1 1x10-5 5

rotating cylindrical electrode 10 1x10-4 5x10-1

high porous three-dimensional(RVC)

0,10 1x10-2 5x10-3

low porosity three-dimensional

packed bed 0,10 2x10-4 5x10-4

fluidized bed 0,01 6x10-3 1x10-2

L.J.J. Janssen, L. Koene, Chem. Eng. J. 85 (2002) 137

Mass transfer as limiting step

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main problem extremely low mass transfer

advantages simple process control

low capital cost

simple maintenance

easy electrode surface treatment (metal deposition)

process enhancement spacers/turbulisers

gas bubbling

electrode surface roughness

inert particles fluidizing bed

typical construction arrangement rectangular flow chanel

„filter – press“

hanging electrodes

electrode movement

Mass transfer as limiting step – 2D electrodes

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Filter-press electrolyzer

Most frequent cell construction

Suitable for processes with “high” electroactive compound concentration Anodic and cathodic chambers separated by membrane or diaphragm

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main pollution sources rinse water from galvanoindustry acidic mine water (abandoned mines)

rinse water from galvanoindustry - well defined solution of constant

composition, minimal risk of contamination by another compounds

acidic mine water - multicomponent composition, variable concentration in

time (rain intensity dependent) - more difficult treatment

Allowed concentration limits for waste water and minimal concentration reachable by precipitation

Heavy metals removal

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advantages mass transfer enhancement

application cathodic metal electrowinning

diluted solutions with electrode surface inhibition

mechanical electrode surface treatment

advantages of 2D electrodes

increase of k (mass transfer coeff.) by one order - 10 times lower outflow concentrations

ChemelecR BEWT

Cell with fluidized bed of inert particles

http://www.p2pays.org/ref/04/03361/

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advantages enhanced mass transfer

application cathodic metal electrowinningfrom organics solutions

formation of electric insulation layer on electrode surface(continuous layer cutting)

simple construction

easy control

Electrolyzer with moving 2D electrode

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treatment of coolant from copper machining

Electrolyser with moving 2D electrode

treatment of thalium solution

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disadvantages complicated process control

advantages high specific surface

high mass transfer

high specific yield

treatment of very diluted solutions

construction arrangement “flow-through”

“flow-by”

fluidized bed

discontinuous process

electrode price (some cases)

j j

v

v

3D electrodes

electrolyte flowel. current flow

j

position

flow-by

flow through

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Swiss roll cella- Ni anodec – SS cathodeb d - PE separatore – current feeder

3D electrodes - static

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enViro cell

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arrangement moving particle bed

still discontinuous process

pulsating particle bed

fluidizing bed

static 3D electrodes – potential risk of pore blocking by electrodeposited metal or by mechanical impurities

solution – moving 3D electrodes

3D electrodes - moving

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Rota-CatTM, Trionetics, Inc.

fluidizing bed – charge transferred by particle touching

moving particle bed – cathode particles still in contact

3D electrodes - moving

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rotating cascade of 3D particle bed cathodes with self-ordered particle distribution for continuous galvanic rinse water treatment

3D electrodes – moving with self-ordering effect (VMPB)

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electrochemical treatment – low efficiency in highly diluted

solutions

combination with separation process (e.g. IX, RO) significant

improvement of pollutant removal

Combination with separation method

6 BV.h-1 3 BV.h-1

treatedwater

watersource

elec

trol

yser

tank I.

tank II.

colu

mn

scheme of IX and electrochemical treatment combination

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basic methods direct electrooxidation on anode

anodic oxidation

indirect oxidation by agent generated on anode

high energy consuming

possible final mineralization or only toxicity decerase

Organic pollutants removal

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organic molecule oxidation mechanism

anode requirements

most frequent anode materials

reduced compounds

high oxygen evolution overvoltage

corrosion stability

Ni, glassy carbon, ATA (Pt, IrO2)

M + H2O → M-OH + H+ + e-

M-OH + R → M + RO + H+ + e-

M-OH + H2O → M + O2 + 3 H+ + 3 e-

adsorbed hydroxyl production

transfer of oxygen atom

parasitic reaction

phenol, chlorphenol, organic acids

Direct oxidation of organic pollutants

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Various application of direct oxidation

Direct oxidation of organic pollutants

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FeO42- production

oxidation agents generated electrochemicaly

production of OH•

common

new

diamond anode

Cl2, ClO-

O3, Ag2+/Ag1+, Co3+/Co2+, Ce4+/Ce3+, FeO42-, OH•

Fenton’s reaction

chemical

electrochemical

in solutionin molten salts

photocatalytic, photoelectrocatalytic

diamond electrode

Indirect oxidation of organic pollutants

in solutionin molten salts

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Indirect oxidation of organic pollutants

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Electrocoagulation

- classical coagulation alternative

- in-situ generation of coagulation agent by anode dissolving

- Al anode:

- Fe anode:

- H2 produced on cathode enhances flakes separation form water

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Electrocoagulation

horizontal and vertical arrangement of electrocoagulation unit

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Electrocoagulation

PARS ENVIRONMENTAL INC.

JOULE ECTM System

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Electrokinetic's soil decontamination

Direct electric current application on contaminated soils causes

migration of charged compounds to the electrode with

reversed charge.

Heavy metals (Cu, Zn, Pb, As, Cd, Cr, ....) are concentated

near electrodes.

Concentrated pollutants in ground water can be transported for

further treatment or immobilized by suitable vitrification agent.

Electric current can serve also as source of heat to ensure

vitrification.

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Electrokinetic's soil decontamination

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Electroflotation

Indentical process with flotation, but gas is produced by water

electrolysis (H2 and O2 )

Low current densities prevents formation of explosive mixture.

Electrochemically produced bubbles has significantly smaller

size than from mechanical bubblers – better separation

efficiency.

No danger of jet blocking.

electrode connection in electroflotation unit (top and side view)

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Electroflotation

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Electroflotation

Airport waste water treatment (airplanes washing, galvanic and dyeing units) www.dr-baer.de

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Combined Electrocoagulation and Electroflotation

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Nuclear waste treatment

main advantage of electrochemical process is no volume

increase by addition of acting chemicals.

In case of need of waste volume minimization the

electrochemical reduction/oxidation is suitable. Nuclear waste

treatment: nuclear power plants old nuclear weapons

PUREX system• Griding of used nuclear fuel pills

• Dissolution in hot HNO3

• Reduction / oxidation and subsequent extractionof Pu4+/3+

• Volume decrease by N2H4 oxidation

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PUREX

electrochemical steps in PUREX system

Pulsating electroextraction column head a b- cathode, c-anode, d-izolation, e-gas separators , f-water solution inlet, g-organic solution outlet, h-overflow,

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Cell for electrooxidation of Pu3+ and hydrazinea- cathode, b-anode, c-gas, d- electrolyte flow, e-insulation

PUREX system