ELECTROCHEMICAL REMEDIATION

Achille De BattistiLaboratory of ElettrochemistryDepartment of Chemistryof the University of Ferrara

ESSEE 4

Palić, Serbia, 17 – 22 Sept., 2006

Targets:

Non-destructive removal of pollutants from soils

- Detoxification/sterilization of industrial wastewaters, landfill leachates, swimming-pool water, water in food industry.

- Cold incineration of organic wastes and disgregation of radioactive residuals.

- Quality improvement of potable water, detoxification/sterilization of groundwaters

Methods:Imposition of potential gradients across the soil portion to be remediated polarizing suitably positioned electrodes

Soils: Electrochemical (electrokinetic) extraction of pollutants

Primary effects: displacement of charged species (electromigration, electro-osmosis), and neutral species (electro-osmosis).

Secondary effects: Accumulation of pollutants in electrodic spaces, followed by abatement.

Advantages: in situ methods, substantially non-destructive, no chemicals used, low energy consumption (e.g..: few tens of Volts and currents of the order of few Amperes); flexibility (e.g.: D.C. , A.C. polarization),

Disadvantages: possible incompatibility with the specific features of the site to be reclamated, problems of stability for metallic structures in the soil.

Soils: Electrochemical (electrokinetic) remediation

Electrokinetic phenomena: some general aspects

------------

+

+

+++

++++

+++

+

S

0

2

Outer HelmholtzPlane

Sharing plane

Solution flow

Further information on an electrified interfase interphase

xe 0

/

i i

RTF cz

1 2

12 28

or xe 2

= potential at the distance x from the charged wall 0= potential at the charged wall

2= potential at the Outer Helmholtz Plane (OHP)

= permittivity of the liquid phase-1= diffuse-layer “thickness”

-

-

--

--

-

----

++

+ ++

+++

-1

surface of the immobile solid with charge density -qd

transit plane

eov

1

charge plane (qd)

ve

Flow of electrolyte solutions through capillary channels with charged surface

Elettro-osmosis

Phenomenological equation

ev a P a E1 2Ve=electro-osmotic flow rate; P=pressure gradient; E=campo elettrico

Electrode

ElectrodeElectrolyte solution

Electro-osmotic flow

Electrode

ElectrodeElectrolyte solution

Electrokinetic current

Flow of electrolyte solutions through capillary channels with charged surface

Streaming currents and potentials

Phenomenological equation:

j a P a E3 4j = current density; P=pressure gradient; E=electric field

- - - - - - - - - - - - -

- - - - - - - - - - - - -

v

E

Qualitative representation of the profile of the electro-osmotic flow rate in a capillary channel

+ + ++ ++ ++ ++ + + +

+ + ++ ++ ++ ++ + + +

++

+

++ +

+++

+

+

+

++

+++

+

+ +

+

+++ ++++

-- -

- -- -

--

-

- --- -- -

--- -

---

- ---

+ + +- - -- -

-

-

+

++-

--

- -

+

++-

++ +

Simplified scheme of a particle plug with surface ionogenic groups: fixed solid-mobile solution

Hydrostatic pressure gradients across the plug, cause a displacement of a fraction of the volume-charge in the direction of the fluid flow. As a consequence, streaming currents and potentials are measured

a a

02 3

a a 2 3

In the hypotesis that the shear plane and capillary surface cohincide:

From former definitions and developments we have also:

dqC 1

0

dq

1

0

Transport due to the electric field: electro-osmosis

e ev k EAVe = electro-osmotic flow rateue = electro-osmotic velocityKe = electro-osmotic permeability coefficientE = electric fieldA = total cross-sectional area

eEu

e

EnAv

enk

Some considerations on the electro-osmotic permeability coefficient

The electro-osmotic permeability coefficient, ke is independent from the pore diameter, at variance with the hydraulic permeability coefficient, kh.

The experimental values of ke do not depend on soil nature and change within a very narrow range, between 10-9 e 10-8 m2 V-1 s-1, while kh ranges between 10-13 e 10-5 m s-1.

An electric gradient is more effective than an hydraulic one in fine-grained soils.

Electro-osmotic flow is well controlled, being confined within the area across which electric field is active

Being for a given soil negative, the electro-osmotic flow is addressed toward the cathode.

Transport under the effect of electric field: electromigration

The dependence of ion elettromigrationon potential gradient, in a capillary:

mv EWhere vm is the ion velocity, is the ionic mobility, assuming values typically around 3.10-8 m2 v-1 s-1 (about one order of magnitude higher for H+ e OH- )

Electromigration is affected by electric field, and ionic strength.

Soil chemistry, or soil-contaminant interaction:The kinetics of the removal of contaminants is bound to adsorption phenomena, ion-exchange, buffering capacity

Water content: inhomogeneous distribution of humidiy and consolidation may take place during an electrokinetic

Soil structure: clogging of the soil porous texture and blocking of the electro-osmotic flow may take place due to hydroxide (presence of heavy metals).

Positioning of the electrodes and electrode structure: Solidity of the structure, easy workability, chemical stability, costs, are major actors. graphite, activated Titanium are electrode materials of practical interest.

Important factors in electrokinetic remediation

Optimization of the electro-osmotic process

-use of “washing” liquids , like water, diluite NaCl solutions.

-control of soil pH by means of buffering substances.

-washing of electrodic compartments, by addition of water/water solutions.

-addition of complexing agents, for heavy-metal removal

-weak alkalinization and/or adition of surfactants for the removal of strongly adsorbed organic contaminants

Simple equipment for Studies in electrokinetic soil remediation

Electrode distribution (bench scale to commercial installations

Electrode distribution (bench scale to commercial installations

Electrokinetic soil remediation: time required/1

i e

dc

u k Ev R

V = rate of species transport (or velocity: LT-1)i*= effective ionic mobility L2T-1V-1

Ke = coefficient of electroosmotic permeability L2T-1V-1

Rdc= delaying factor (dimensionless, depends on soil type, pH, and contaminant nature. Accounts for contaminant desorption and dissolution.E = the electric gradient V L-1

Electrokinetic soil remediation: time required/2Electrokinetic soil remediation: time required/2

EE

L1T E

The remediation time TE required for a given contaminant in a soil may be expressed as: LE/v, where LE is the spacing between electrodes of opposite polarity

σ* = effective soil conductivity (siemens m-1)

β is a lumped property of the contaminant and of the soil (L3 C-1), similar to transference numbers in electrochemistry, but it also accounts for electroosmosis, soil conditions and retardation caused by geochemical reaction:

i e dck / R

Electrokinetic soil remediation: time required/3Electrokinetic soil remediation: time required/3

β is a fundamental parameter in electrokinetic soil remediation. Its values typically range within 1.10-9 and 1.10-6 m3 C-1

If the remediation time has to be calculated on the basis of current density applied across the soil, then we have:

EE

d

L1T I

Where Id is the electric current density = I/A (amp L-2), I is the total current and A the cross-sectional area of the soil treated.

PERMEABLE REACTIVE BARRIERS PERMEABLE REACTIVE BARRIERS Groundwater treatmentGroundwater treatment

• Definition andDefinition and methodologymethodology

PRB normal with PRB normal with respect to respect to groundwater groundwater flow.flow.

Containing Containing reactive materialreactive material

Veduta schematica dell’installazione di una PRB Funnel-and-gate and continuous PRB

Encouraging number of commercial installations:the situation in USA

PERMEABLE REACTIVE BARRIERSPERMEABLE REACTIVE BARRIERSReductive degradation of organochloro componds with oxidation of Reductive degradation of organochloro componds with oxidation of

FeFe00 a Fe a Fe2+2+

FeFe00 + RCl + H + RCl + H++ Fe Fe2+2+ + RH + Cl + RH + Cl--

2Fe0 + O2 + 2 H2O 2 Fe2+ + 4 OH–

Fe3+ + 3 OH– Fe(OH)3 (S)

Aerobic environmentAerobic environment

important!!Hydraulic permeability and reactivity of the barrierHydraulic permeability and reactivity of the barrier

Anaerobic environmentAnaerobic environmentFe0+ 2H2O Fe2+ + H2 + 2OH–

Fe2+ + 2OH– Fe(OH)2 (S)

from PRB to Electrochemical Reactorfrom PRB to Electrochemical ReactorPossible efficiency problem: Barrier aging and degradationPossible efficiency problem: Barrier aging and degradation

Electrochemically generated Fenton reactantElectrochemically generated Fenton reactant

Reazione Reazione di di FentonFenton

Oxidative attack to Oxidative attack to organic substratesorganic substrates

OO2 2 cathodic reductioncathodic reductionin presence of Fein presence of Fe2+2+

CATHODE ANODE

Further Further oxidationoxidation

Fe²Fe²+++ H+ H22OO22 Fe³ Fe³++ + + ..OH + OHOH + OH--

Fe³Fe³++ + H + H22OO22 Fe² Fe²++ + + ..OOH + HOOH + H++

Hybrid Processes

- Synergism has been observed in the application of electric fields and hydraulic pressure gradients (removal of heavy metals from da mixtures sand/process-sludge

-Similar observations in the use of acoustic treatments coupled with the electro-osmotic one (removal of decane, Zn, Cd, from clays)

-Electrodialysis: The efficiency of removal of heavy metals and maintenance of treated soil pH can be improved making use of ion-exchange membranes.

-Bioremediation: The displacement of nutrients and colonies of microorganisms under the action of electric fields may favour the removal of complex organic species.-Fitoremediation: the transfer of the pollutant to the treated area accelerates the removal process.

Suggested reference:

I.K. Iskander, Environmental Restoration of Metals-Contaminated SoilsLewis Publ., London, 2001