Transcript Diapositiva 1 - Serbian Chemical Society

ESSEE 4
Palić, Serbia, 17 – 22 Sept., 2006
ELECTROCHEMICAL REMEDIATION
Achille De Battisti
Laboratory of Elettrochemistry
Department of Chemistry
of the University of Ferrara
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
Soils: Electrochemical (electrokinetic)
extraction of pollutants
Methods:
Imposition of potential gradients across the soil portion to be
remediated polarizing suitably positioned electrodes
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.
Soils: Electrochemical
(electrokinetic) remediation
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.
Electrokinetic phenomena: some general aspects
Outer Helmholtz
Plane
+
+
+
+
S
+
+
+ +
+
+
+
+

0
+
Sharing plane
Solution flow
2
-
Further information on
an electrified interfase interphase
  0e
 x
or
  2e
 x
1/ 2


 RT


1
 
2
2 
8

F
c
z

i i 



 = 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”

transit plane
 v eo
 1
ve
+ + + +
+ + + +
charge plane
(qd)
-1
-
-
-
-
-
surface of the immobile solid
with charge density -qd
Flow of electrolyte solutions through
capillary channels with charged surface
Elettro-osmosis
Electrode
Electroosmotic flow
Electrolyte solution
Phenomenological equation
ve  a1P  a2E
Ve=electro-osmotic flow rate;
P=pressure gradient; E=campo elettrico
Electrode
Flow of electrolyte solutions through
capillary channels with charged surface
Streaming currents and potentials
Electrode
Electrokinetic
current
Electrolyte solution
Phenomenological equation:
j  a3P  a4E
j = current density;
P=pressure gradient; E=electric field
Electrode
Qualitative representation of the profile of the
electro-osmotic flow rate in a capillary channel
- - - - - - - - - - - - + + + + + + + + + + + + +
E
+ + + + + + + + + + + + +
- - - - - - - - - - - - v
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
In the hypotesis that the shear plane
and capillary surface cohincide:
0
a2  a3 

From former definitions and developments we have
also:
qd

C
 1
0 
 qd
0 


a2  a3 

1
Transport due to the electric field:
electro-osmosis
ve  keEA
Ve = electro-osmotic flow rate
ue = electro-osmotic velocity
Ke = electro-osmotic permeability coefficient
E = electric field
A = total cross-sectional area
 E
ue  

 EnA
ve  

 n
ke 

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 elettromigration
on potential gradient, in a capillary:
vm   E
Where 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.
Important factors in electrokinetic remediation
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.
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
u

v

i
 ke  E
R dc
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/2
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
1 LE
TE 
 E
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  k e  / R dc

Electrokinetic 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:
1 LE
TE 
 Id
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
Groundwater treatment
• Definition and
methodology
 PRB normal with
respect to
groundwater
flow.
 Containing
reactive 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 BARRIERS
Reductive degradation of organochloro componds with
oxidation of Fe0 a Fe2+
Fe0 + RCl + H+  Fe2+ + RH + Cl2Fe0 + O2 + 2 H2O  2 Fe2+ + 4 OH–
Fe3+ + 3 OH–  Fe(OH)3 (S)
Fe0+ 2H2O  Fe2+ + H2 + 2OH–
Fe2+ + 2OH–  Fe(OH)2 (S)
Aerobic environment
Anaerobic environment
important!!
Hydraulic permeability and reactivity of the barrier
from PRB to Electrochemical Reactor
Possible efficiency problem: Barrier aging and degradation
Electrochemically generated Fenton reactant
O2 cathodic reduction
in presence of Fe2+
CATHODE
Fe²++Reazione
H2O2  Fe³+ + .OH + OHFe³+ +diH2Fenton
O2  Fe²+ + .OOH + H+
Further
oxidation
ANODE
Oxidative attack to
organic substrates
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 Soils
Lewis Publ., London, 2001