presentación3 def
TRANSCRIPT
Study of the transport of heavy metal ions through cation-exchange membranes applied to
the treatment of industrial effluents
Manuel César Martí Calatayud
Dr. D. Valentín Pérez Herranz
Dra. Dª. Montserrat García Gabaldón
FUNDING
Ayuda predoctoral: Formación de Personal Investigador de la Universitat Politècnica de València (Ref. FPI-UPV 2010-12)
Ayuda para la realización de estancias en centros de investigación de prestigio de la Universitat Politècnica de València:
(PAID-00-12)
Proyecto del Ministerio de Economía y Competitividad: “Caracterización electroquímica de membranas cerámicas nanoestructuradas de intercambio iónico para su aplicación en reactores electroquímicos y sistemas electrodialíticos” (CTQ2012-37450-C02-01/PPQ)
Proyecto del Ministerio de Ciencia e Innovación: “Desarrollo de nuevos reactores electroquímicos basados en membranas cerámicas para la recuperación de cromo hexavalente de los efluentes de las industrias de tratamiento de superficies” (CTQ2008-06750-C02-01/PPQ)
Predoctoral funding
Projects
Programa Oficial de posgrado en Ingeniería y Producción Industrial
RESEARCH ARTICLES IN SCIENTIFIC JOURNALS
Determination of transport properties of Ni(II) through a Nafion cation-exchange membrane in chromic acid solutions.M.C. Martí-Calatayud, M. García-Gabaldón, V. Pérez-Herranz, E. OrtegaJournal of Membrane Science, 379 (2011) 449-458.
Study of the effects of the applied current regime and the concentration of chromic acid on the transport of Ni2+ ions through Nafion 117 membranes.M.C. Martí-Calatayud, M. García-Gabaldón, V. Pérez-HerranzJournal of Membrane Science, 392-393 (2012) 137-149.
Effect of the equilibria of multivalent metal sulfates on the transport through cation-exchange membranes at different current regimes.M.C. Martí-Calatayud, M. García-Gabaldón, V. Pérez-HerranzJournal of Membrane Science, 443 (2013) 181-192.
Ion transport through homogeneous and heterogeneous ion-exchange membranes in single salt and multicomponent electrolyte solutions.M.C. Martí-Calatayud, D.C. Buzzi, M. García-Gabaldón, A.M. Bernardes, J.A.S. Tenório, V. Pérez-HerranzJournal of Membrane Science, 466 (2014) 45-57.
Results related to the Doctoral Thesis
Layer-by-Layer modification of cation exchange membranes controls ion selectivity and water splitting.S. Abdu, M.C. Martí-Calatayud, J.E. Wong, M. García-Gabaldón, M. WesslingACS Applied Materials & Interfaces, 6 (2014) 1843-1854.
Research stay at Chemische Verfahrenstechnik – RWTH Aachen University (Germany)
Synthesis and electrochemical behavior of ceramic cation-exchange membranes based on zirconium phosphate.M.C. Martí-Calatayud, M. García-Gabaldón, V. Pérez-Herranz, S. Sales, S. MestreCeramics International, 39 (2013) 4045-4054.
Chronopotentiometric study of ceramic cation-exchange membranes based on zirconium phosphate in contact with nickel sulfate solutions.M.C. Martí-Calatayud, M. García-Gabaldón, V. Pérez-Herranz, S. Sales, S. MestreDesalination and Water Treatment, 51 (2013) 597-605.
Collaboration with the Instituto de Tecnología Cerámica – UJI Castelló
Collaboration with the Departamento de Ingeniería de Materiales – Universidade do Rio Grande do Sul (Brasil) Sulfuric acid recovery from acid mine drainage by means of electrodialysis.
M.C. Martí-Calatayud, D.C. Buzzi, M. García-Gabaldón, E. Ortega, A.M. Bernardes, J.A.S. Tenório, V. Pérez-HerranzDesalination, 343 (2014) 120-127.
10th European Symposium on Electrochemical Engineering, 2014, Sardinia (Italy)
65th Annual Meeting of the International Society of Electrochemistry, 2014, Lausanne (Switzerland)
IX Ibero-americal Congress on Membrane Science and Technology, 2014, Santander.
13th International Conference on Environmental Science and Technology (CEST 2013), Atenas
XXXIV Reunión bienal de la RSEQ 2013, Santander 2 contribuciones XXXIV Reunión de electroquímica de la RSEQ, 2013, València 1st International Conference on Desalination using Membrane Technology,
2013, Sitges (Spain) 63rd Annual Meeting of the International Society of Electrochemistry, 2013,
Prague (Czech Republic) VIII Simposio Internacional de Qualidade Ambiental, 2012, Porto Alegre
(Brazil) Conference on Desalination for the Environment, Clean Water and Energy,
2012, Barcelona. 13 Network of Young Membrains, 2011, Enschede (The Netherlands) 9th European Symposium on Electrochemical Engineering, 2011, Chania
(Greece) 2nd Regional Symposium on Electrochemistry, 2011, Belgrade (Serbia)
Premio en el VII Certamen València Idea 2013, sección Energía y Medio Ambiente“Desarrollo de membranas de intercambio iónico con funcionalidad óptima para su utilización en baterías de flujo redox de elevada eficiencia energética”.
CONTRIBUTIONS IN CONFERENCES
AWARDS
LIST OF CONTENTS
Transport of single salt solutions
Transport of multicomponent mixtures
Mechanisms of transport at overlimiting currents
Galvanostatic electrodialysis experiments
3. EXPERIMENTAL TECHNIQUES
2. OBJECTIVE
5. CONCLUSIONS
1. INTRODUCTION
4. RESULTS & DISCUSSION
2. Objetive 3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Results and discussion
1.Introduction 1
Heavy metals have high specific density (>5), and are highly persistent. Most of them (Cr, Ni, Cd, …) are carcinogenic.
1.1 Scope / Background
Effects on the environment Effects on the human health
They have multitude of applications. Their extraction, use and price are continuously growing.
Effects on the economy. Unsustainable growth. Geopolitical problems.
Metals imports
Mineral depletion
2
1.2 Electromembrane processes
Fixed membrane ion Counterion
Coion
-
-
-
-
-
+
+
+
+
+
+ -M+n
X-y
M+n
X-y
X-y
M+n
AEM CEM
An
od
e
Cat
ho
de
The presence of heavy metals in natural watercourses is mainly consequence of mining activities and due to the discharge of industrial effluents.
Electromembrane processes, such as electrodialysis, represent a sustainable alternative for the treatment of these effluents, since they allow the selective recovery and reuse of valuable metals.
They permit the separation of ions from aqueous solutions by using:
Driving force: an electric field
Selective barriers: ion-exchange membranes
Principle of electromembrane
processes
Structure of a cation-exchange membrane
2. Objetive 3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Results and discussion
1.Introduction
3
1.3 Characteristics / LimitationsThey are selective processes which allow the separation of cationic and anionic species.
The addition of reagents is not required.
Continuous and modular processes (stacks)
Membrane fouling.
Concentration polarization phenomena: The mass transfer becomes limited with the increase in the driving force
Desalted solution
Brine
Feed solution
An
od
e
Cat
hod
e
AEM AEM AEM AEMCEM CEM CEMCEM AlkaliAcid
0.0
0.2
0.4
0.6
0.8
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Um (V)
i (m
A·c
m-2
)
ilim
Overlimiting current densities
2. Objetive 3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Results and discussion
1.Introduction
LIST OF CONTENTS
Transport of single salt solutions
Transport of multicomponent mixtures
Mechanisms of transport at overlimiting currents
Galvanostatic electrodialysis experiments
3. EXPERIMENTAL TECHNIQUES
2. OBJECTIVE
5. CONCLUSIONS
1. INTRODUCTION
4. RESULTS & DISCUSSION
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Results and discussion
1.Introduction 4
2. Objective and structure of the Thesis
Identificar e investigar los fenómenos de transferencia de materia implicados en procesos electroquímicos de membrana con el fin de optimizar el tratamiento de efluentes industriales que contienen metales pesados
To identify and investigate the mass transfer phenomena involved in electromembrane processes in order to optimize the treatment of industrial effluents containing heavy metals.
Ch. 4.1. Transport in single salt solutions
Ch. 4.2. Transport in mixture solutions
Monovalent ions Multivalent ionsIon competition between H+ and
M+n ions
Ion competition between M+ and M+n ions (n>1)
Ion uptakeElectrical resistance
SelectivityMass transfer phenomena
Ch. 4.3. Evaluation of the mechanisms of overlimiting current
Ch. 4.4. Galvanostatic experiments at different current regimes
Comparison:Transport of monovalent ions vs.
Transport of multivalent ions
Generation of new current carriers
Coupled convection
Effect of current regime
Effect of electrolyte
composition
Comparison:Transport of single salt solutions vs.
Transport of mixture solutions
Ch. 5. Conclusions
Ch. 1. Introduction
Ch. 2. Objectives and scope
Ch. 3. Methodology and experimental techniques
Ion uptakeElectrical resistance
SelectivityMass transfer phenomena
Ion uptakeElectrical resistance
SelectivityMass transfer phenomena
Ion uptakeElectrical resistance
SelectivityMass transfer phenomena
LIST OF CONTENTS
Transport of single salt solutions
Transport of multicomponent mixtures
Mechanisms of transport at overlimiting currents
Galvanostatic electrodialysis experiments
3. EXPERIMENTAL TECHNIQUES
2. OBJECTIVE
5. CONCLUSIONS
1. INTRODUCTION
4. RESULTS & DISCUSSION
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Results and discussion
1.Introduction 5
3. Experimental techniques
Cation-exchange membranes NAFION 117Properties:
High conductivity (low electrical resistance) High selectivity
High mechanical and chemical resistance (durability).
IEC (ion exchange capacity) : 0.90 meq. SO3
- / g membrane
6
B) ChronopotentiometryElectrochemical technique used to analyze the dynamics of ion transport processes through a system composed by a membrane and an electrolyte: Concentration polarization
AEM CEM
+ -H+
X-yH+ M+n
H+
M+n
M0
Um
OH-
OH-
OH-X-y
H2O
H2O
Application of current pulses Concentration polarization Chronopotentiograms
CEM
Um
i<ilim
i>ilim
i≈ilim
Electric double layer
DBL
Bulk electrolyte
c0 c0
t (s)
Um
(V
) i ≈ ilim
i < ilim
i > ilim
t (s)
i (m
A·c
m-2
)
i ≈ ilim
i < ilim
i > ilim
DBL
A) Ion sorption experiments
3. Experimental techniques
2
2
0 1
4 itT
FzcD
Sand’s equation:
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Results and discussion
1.Introduction
7
C) Polarization curves: i vs. Um
They provide an idea about the membrane behavior at different current regimes:
1. Quasi-ohmic region:linear dependence between i and Um
2. Plateau region:mass transfer limitations
3. Region of overlimiting currents:The transfer of current carriers toward the membrane surface is activated
)(lim
tT
FDczi
0Peers’
equation:
3. Experimental techniques
B) Chronopotentiometry
A) Ion sorption experiments
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Results and discussion
1.Introduction
0.0
0.2
0.4
0.6
0.8
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Um (V)
i (m
A·c
m-2
)
ilim
Overlimiting region
quasi-ohmic region
plateau
8
D) Galvanostatic experiments in 3-compartment electrochemical cell Parameters to evaluate the
efficiency in the mass transfer and energy usage in an electromembrane reactor:
AEM CEM
+ -H+
X-yH+ M+n
H+
M+n
M0
Ucell
OH-
OH-
OH-X-y
H2O
H2O
1000
0 c
cctX t)(
100
0
0
tt
dtI
ccnFVt)(
t
ccMt t 0)(
XcVM
dtIUtE
tcell
s
0
0
3600)( (kW·h·kg-1)
(g·l-1·h-1)
(%)
(%)
3. Experimental techniques
C) Polarization curves: i vs. Um
B) Chronopotentiometry
A) Ion sorption experiments
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Results and discussion
1.Introduction
LIST OF CONTENTS
Transport of single salt solutions
Transport of multicomponent mixtures
Mechanisms of transport at overlimiting currents
Galvanostatic electrodialysis experiments
3. EXPERIMENTAL TECHNIQUES
2. OBJECTIVE
5. CONCLUSIONS
1. INTRODUCTION
4. RESULTS & DISCUSSION
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction 9
4. Results and discussion
4.1. Transport in single salt solutions
Monovalent metal:
Na+
4. Resultados y discusión4. Results and discussion
Tabla 1. Electrolyte compositions used for the experiments conducted with single salt solutions.
[M+n] Na2SO4 NiSO4 Cr2(SO4)3 Fe2(SO4)3
10-3M 5·10-4M 10-3M 5·10-4M 5·10-4M
5·10-3M 2.5·10-3M 5·10-3M 2.5·10-3M 2.5·10-3M
10-2M 5·10-3M 10-2M 5·10-3M 5·10-3M
2·10-2M 10-2M 2·10-2M 10-2M 10-2M
Ni(II), Cr(III) y Fe(III)Present in spent
baths generated in the metal finishing
industry
Cr(III)Present in effluents
generated in leather tanneries
Fe(III)Present in acid mine drainage solutions
10
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.0E+00 5.0E-03 1.0E-02 1.5E-02 2.0E-02 2.5E-02
[M+n] (mol/L)
c m (
mm
ol/g
)
Na(I)
Ni(II)
Fe(III)
Cr(III)
Ni2
+
Cr3
+
Fe3
+
10-3M
5·10-
3M
10-2M
2·10-
2M
The membrane becomes saturated in counterions
4.1. Transport in single salt solutions
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
11
Na2SO4
5·10-4M
2.5·10-3M
5·10-3M
10-2M
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0 100 200 300 400 500
t (s)
Um
(V
)
0.028 mA·cm-2
0.014 mA·cm-2
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0 100 200 300 400 500
t (s)
Um
(V
)
0.044 mA·cm-2
0.028 mA·cm-2
0.014 mA·cm-2
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0 100 200 300 400 500
t (s)
Um
(V
)0.051 mA·cm-2
0.044 mA·cm-2
0.028 mA·cm-2
0.014 mA·cm-2
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0 100 200 300 400 500
t (s)
Um
(V
)
0.054 mA·cm-2
0.051 mA·cm-2
0.044 mA·cm-2
0.028 mA·cm-2
0.014 mA·cm-2
0.00
0.15
0.30
0.45
0.60
0.75
0 100 200 300 400 500
t (s)
Um
(V
)
0.055 mA·cm-2
0.00
0.15
0.30
0.45
0.60
0.75
0 100 200 300 400 500
t (s)
Um
(V
)0.059 mA·cm-2
0.055 mA·cm-2
0.00
0.15
0.30
0.45
0.60
0.75
0 100 200 300 400 500
t (s)
Um
(V
)
0.071 mA·cm-2
0.059 mA·cm-2
0.055 mA·cm-2
0.00
0.15
0.30
0.45
0.60
0.75
0 100 200 300 400 500
t (s)
Um
(V
)
0.085 mA·cm-2
0.071 mA·cm-2
0.059 mA·cm-2
0.055 mA·cm-2
0.00
0.15
0.30
0.45
0.60
0.75
0 100 200 300 400 500
t (s)
Um
(V
)
0.113 mA·cm-2
0.085 mA·cm-2
0.071 mA·cm-2
0.059 mA·cm-2
0.055 mA·cm-2
i < ilimi > ilim
4.1. Transport in single salt solutions
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
12
y1 = 7.252x - 7.405
R2 = 0.999y2 = 5.132x + 3.209
R2 = 0.994
y3 = 4.717x - 5.445
R2 = 0.995
y4 = 3.058x - 0.815
R2 = 0.990
0
20
40
60
80
100
120
0 5 10 15 20 25
(c0/I)2 (mol/L·A)2
(s
)
10-2M Na2SO4
5·10-3M Na2SO4
2.5·10-3M Na2SO4
5·10-4M Na2SO4
Agreement with the
Sand’s eq.
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 100 200 300 400
t (s)
Um
(V
)
0.E+00
1.E-02
2.E-02
3.E-02
4.E-02
DU
m/D
t
0.99 mA·cm-2
0.71 mA·cm-2
0.62 mA·cm-2
0.58 mA·cm-2
Calculation of the
transition time
Indicates the fraction of current associated with the transport of a specific ion with respect to the total current passed through the membrane
Sand’s equation: 2
2
0 1
4 itT
FzcD
T+
4.1. Transport in single salt solutions
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
0.0
0.2
0.4
0.6
0.8
1.0
0.0E+00 5.0E-03 1.0E-02 1.5E-02 2.0E-02 2.5E-02
[M+n] (mol/L)
T+
Na+
Ni2+
13
0.0
0.2
0.4
0.6
0.8
0 100 200 300 400
t (s)
Um
(V
)
0.000
0.005
0.010
0.015
DU
m/D
t
0.538 mA·cm-2
0.0
0.2
0.4
0.6
0.8
0 100 200 300 400
t (s)
Um
(V
)
0.000
0.005
0.010
0.015
DU
m/D
t
0.878 mA·cm-2
0.538 mA·cm-2
0.0
0.2
0.4
0.6
0.8
0 100 200 300 400
t (s)
Um
(V
)
0.000
0.005
0.010
0.015
DU
m/D
t0.992 mA·cm-2
0.878 mA·cm-2
0.538 mA·cm-2
0.0
0.2
0.4
0.6
0.8
0 100 200 300 400
t (s)
Um
(V
)
0.000
0.005
0.010
0.015
DU
m/D
t
1.133 mA·cm-2
0.992 mA·cm-2
0.878 mA·cm-2
0.538 mA·cm-2
0.0
0.2
0.4
0.6
0.8
0 100 200 300 400
t (s)
Um
(V
)
0.000
0.005
0.010
0.015
DU
m/D
t
1.346 mA·cm-2
1.133 mA·cm-2
0.992 mA·cm-2
1
1
1
2
2
2
0.878 mA·cm-2
0.538 mA·cm-2Multiple transition times
Cr2(SO4)3
2.5·10-3M
4.1. Transport in single salt solutions
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
14
Speciation diagram of 2.5·10-3M Cr2(SO4)3
0.0
0.2
0.4
0.6
0.8
1.0
0 1 2 3 4 5 6 7
pH
ai
CrSO4+
Cr(OH)2+
CrOH2+Cr3+
pHeq
4.1. Transport in single salt solutions
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
15
NiSO4
ilim (mA·cm-2)
10-2M 0.85
0.0
0.2
0.4
0.6
0.8
1.0
0 100 200 300 400 500
t (s)
Um
(V
)
0.99 mA·cm-2
0.57 mA·cm-2
0.0
0.2
0.4
0.6
0.8
1.0
0 100 200 300 400 500
t (s)
Um
(V
)
0.99 mA·cm-2
0.57 mA·cm-2
1.56 mA·cm-2
1.70 mA·cm-2
0.0
0.2
0.4
0.6
0.8
1.0
0 100 200 300 400 500
t (s)
Um
(V
)
2.27 mA·cm-2
1.98 mA·cm-2
0.99 mA·cm-2
0.57 mA·cm-2
1.56 mA·cm-2
1.70 mA·cm-2
0
1
2
3
4
5
0 100 200 300 400 500
t (s)
Um
(V
)
7.08 mA·cm-2
5.67 mA·cm-2
Fe2(SO4)3
ilim (mA·cm-2)
10-2M 7.08
4.1. Transport in single salt solutions
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
0.0
0.2
0.4
0.6
0.8
1.0
0 100 200 300 400 500
t (s)
Um
(V
)
2.27 mA·cm-2
1.98 mA·cm-2
0.99 mA·cm-2
0.57 mA·cm-2
1.56 mA·cm-2
1.70 mA·cm-2
Formation of precipitates
Plateau in the relaxation of Um
0
1
2
3
4
5
0 100 200 300 400 500
t (s)
Um
(V
)
7.79 mA·cm-2
7.37 mA·cm-2
7.08 mA·cm-2
5.67 mA·cm-2
Formation of precipitates
0
1
2
3
4
5
0 100 200 300 400 500
t (s)
Um
(V
)
7.79 mA·cm-2
7.37 mA·cm-2
7.08 mA·cm-2
5.67 mA·cm-2
0.0
0.4
300 350t (s)
Um
(V
)
Plateau in the relaxation of Um
Formation of precipitates
16
The formation of precipitates is due to pH changes in the vicinities of the membrane
The difference between pHeq - pH↓ is low for the solutions of Fe(III)
The difference between pHeq - pH↓ is higher in the case of Cr(III)
4.1. Transport in single salt solutions
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
1.E+00
0 2 4 6 8 10 12 14
pH
s (m
ol/L
)
NiSO4 2·10-2M
Cr2(SO4)3 10-2 M
Fe2(SO4)3 10-2 M
pHeq pHeq
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
17
0
50
100
150
200
250
300
0.0E+00 5.0E-03 1.0E-02 1.5E-02 2.0E-02 2.5E-02
[Na(I)] (mol/L)
R1 (W
·cm
2)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
i lim
(m
A·c
m-2
)
3 regions of membrane
behavior:
Quasi-ohmic regime
Plateau ilim
Region of overlimiting currents
Good agreement with the Peers’
equation
R1 decreases with the electrolyte concentration.
4.1. Transport in single salt solutions
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
0.0
0.2
0.4
0.6
0.8
0.0 0.2 0.4 0.6 0.8 1.0
Um (V)
i (m
A·c
m-2
)
2.5·10-3M Na2SO4
5·10-4M Na2SO4
1/R1
ilim
Quasi-ohmic region
Plateau
Overlimiting region
18
0.0
0.5
1.0
1.5
2.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Um (V)
i (m
A·c
m-2
)
ilim1
2.5·10-3M Cr2(SO4)3
5·10-4M Cr2(SO4)3
ilim2
4.1. Transport in single salt solutions
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
LIST OF CONTENTS
Transport of single salt solutions
Transport of multicomponent mixtures
Mechanisms of transport at overlimiting currents
Galvanostatic electrodialysis experiments
3. EXPERIMENTAL TECHNIQUES
2. OBJECTIVE
5. CONCLUSIONS
1. INTRODUCTION
4. RESULTS & DISCUSSION
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction 19
4. Results and discussion
4.2. Transport in multicomponent mixture solutions
Baths from the metal finishing industry:
H+ vs. Ni2+
4. Resultados y discusión4. Results and discussion
Acid mine drainage solutions:Na+ vs. Fe(III)
CrO3 + H2O H2CrO4 ↔ 2H+ + CrO42-
NiSO4
0.0
0.1
0.2
0.3
0.4
0.5
0.00E+00 2.50E-03 5.00E-03 7.50E-03 1.00E-02 1.25E-02
[Ni(II)] (mol/L)
c m (
mm
ol/g
)
0M CrO3
10-3M CrO3
10-2M CrO30%
25%
50%
75%
100%
Electrolyte composition
Fra
ctio
n o
f m
emb
ran
efi
xed
ch
arg
es
10-3M CrO3 10-2M CrO30M CrO3
5·10
-4M
NiS
O4
5·10
-4M
NiS
O4
5·10
-4M
NiS
O4
5·10
-3M
NiS
O4
5·10
-3M
NiS
O4
5·10
-3M
NiS
O4
H+
Ni2+
10-3
M N
iSO
4
10-3
M N
iSO
4
10-3
M N
iSO
4
10-2
M N
iSO
4
10-2
M N
iSO
4
10-2
M N
iSO
4
20
CrO3 NiSO4
10-3M
5·10-4M
10-3M
5·10-3M
10-2M
10-2M
5·10-4M
10-3M
5·10-3M
10-2M
Analogous behavior to that obtained with single salt solutions of NiSO4 (without CrO3)
4.2. Transport in multicomponent mixture solutions
Baths from the metal finishing industry:
H+ vs. Ni2+
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 100 200 300 400 500
t (s)
Um
(V
)
3.12 mA·cm-2
2.83 mA·cm-2
2.55 mA·cm-2
2.27 mA·cm-2
1.35 mA·cm-2
1.16 mA·cm-2
Formation of precipitates
Plateau in the relaxation of Um
21
CrO3 NiSO4
10-3M
5·10-4M
10-3M
5·10-3M
10-2M
10-2M
5·10-4M
10-3M
5·10-3M
10-2M
With 10-2M CrO3 solutions the formation of precipitates did not occur.
0.0
0.2
0.4
0.6
0.8
0 100 200 300 400
t (s)
Um
(V
)
5.38 mA·cm-2
8.50 mA·cm-2
4.25 mA·cm-2
3.97 mA·cm-2
3.26 mA·cm-2
Baths from the metal finishing industry:
H+ vs. Ni2+4.2. Transport in multicomponent mixture solutions
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
22
4.2. Transport in multicomponent mixture solutions
Baths from the metal finishing industry:
H+ vs. Ni2+
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
1.0E-07
1.0E-06
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
0 2 4 6 8 10 12 14pH
s (m
ol/L
)The formation of
precipitates starts
pHeq
10-2M CrO3pHeq
10-3M CrO3
pHeq
0M CrO3
Increasing current densities
23
0.0
0.2
0.4
0.6
0.8
1.0
0.00E+00 2.50E-03 5.00E-03 7.50E-03 1.00E-02 1.25E-02
[Ni(II)] (mol/L)
TN
i2+
0M CrO3
10-3M CrO3
10-2M CrO30.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Um (V)
i (m
A·c
m-2
)
10-2M NiSO4
5·10-3M NiSO4
10-3M NiSO4
5·10-4M NiSO4
0
2
4
6
8
10
12
14
16
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Um (V)
i (m
A·c
m-2
)
10-2M NiSO4
5·10-3M NiSO4
10-3M NiSO4
5·10-4M NiSO4
4.2. Transport in multicomponent mixture solutions
Baths from the metal finishing industry:
H+ vs. Ni2+
10-3 M CrO3
10-2 M CrO3
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
24
The attractive forces between the SO3
- groups and Fe3+ are higher than those with Na+
Fe2(SO4)3 Na2SO4
2·10-2M
0M
10-2M
2·10-2M
0%
25%
50%
75%
100%
Electrolyte composition
Fra
ctio
n o
f m
emb
ran
e fi
xed
ch
arg
es
0.02M Fe2(SO4)3
0.02M Fe2(SO4)3
+0.01M Na2SO4
0.02M Fe2(SO4)3
+0.02M Na2SO4
0.01M
Na2SO4
FeSO4+
Fe3+
Na+
4.2. Transport in multicomponent mixture solutions
Acid mine drainage solutions:Na+ vs. Fe(III)
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
25
Fe2(SO4)3 Na2SO4
2·10-2M
0M
10-2M
2·10-2M
0.00
0.05
0.10
0.15
0 100 200 300 400 500t (s)
Um
(V
)
8.50 mA·cm-2
5.67 mA·cm-2
0.00
0.05
0.10
0.15
0 100 200 300 400 500t (s)
Um
(V
)
11.33 mA·cm-2
8.50 mA·cm-2
5.67 mA·cm-2
0.00
0.05
0.10
0.15
0 100 200 300 400 500t (s)
Um
(V
)14.16 mA·cm-2
11.33 mA·cm-2
8.50 mA·cm-2
5.67 mA·cm-2
0.00
0.05
0.10
0.15
0 100 200 300 400 500t (s)
Um
(V
)
17.00 mA·cm-2
14.16 mA·cm-2
11.33 mA·cm-2
8.50 mA·cm-2
5.67 mA·cm-2
i < ilimi > ilim
4.2. Transport in multicomponent mixture solutions
Acid mine drainage solutions:Na+ vs. Fe(III)
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
0.0
1.0
2.0
3.0
0 100 200 300 400 500t (s)
Um
(V
)
17.28 mA·cm-2
17.14 mA·cm-2
depletion of
Fe3+ ions
0.0
1.0
2.0
3.0
0 100 200 300 400 500t (s)
Um
(V
)
17.56 mA·cm-2
17.28 mA·cm-2
17.14 mA·cm-2
depletion of
Fe3+ ions
formation of Fe(OH)3
precipitates
26
Change in electrical resistance
ilim associated with the depletion of Na+
0
3
6
9
12
15
18
0.0 0.5 1.0 1.5 2.0 2.5 3.0
Um (V)
i (m
A·c
m-2
)
0.02M Fe2(SO4)3
0.02M Fe2(SO4)3 + 0.01M Na2SO4
0.02M Fe2(SO4)3 + 0.02M Na2SO4
0
5
10
15
20
0.0 0.1 0.2
Um (V)
i (m
A·c
m-2
)
1/R1
1/R2
1/R1
1/R2
0.02M Fe2(SO4)3
ilim1
ilim2
4.2. Transport in multicomponent mixture solutions
Acid mine drainage solutions:Na+ vs. Fe(III)
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
27
Table 6. Diffusion coefficients and ionic conductivities at infinite dilution for various species present in solutions of Fe2(SO4)3 +
Na2SO4.
Di (m2·s-1) i (mS·m2·mol-1)
Na+ 1.334 5.01
Fe3+ 0.604 20.40
SO42- 1.065 16.00
FeSO4+ 0.201 0.76
Fe3+
FeSO4+
Fe3+SO42-
SO42-
Reaction layerformed next to
the ion-exchange membrane
ion-exchange membrane
4.2. Transport in multicomponent mixture solutions
Acid mine drainage solutions:Na+ vs. Fe(III)
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
LIST OF CONTENTS
Transport of single salt solutions
Transport of multicomponent mixtures
Mechanisms of transport at overlimiting currents
Galvanostatic electrodialysis experiments
3. EXPERIMENTAL TECHNIQUES
2. OBJECTIVE
5. CONCLUSIONS
1. INTRODUCTION
4. RESULTS & DISCUSSION
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction 28
Investigate the origin of the overlimiting currents
Evaluate the role of heavy metals and electrolyte composition on the overlimiting currents
4. Resultados y discusión4. Results and discussion
4.3. Mechanisms of overlimiting current transfer
Transmembrane pressure, DP
Pe
rme
ate
flu
x,
J
Jlim
Transmembrane voltage drop, Um
Cu
rre
nt
de
ns
ity
, i
ilim
Overlimiting current
densities
Water splittingCurrent-induced
convection
Generation of new current carriers
(H+ and OH-)Exaltation effect
Gravitationalconvection
Electroconvection
Overlimiting current transfer
29
Water splittingGravitational convection
Mass transfer driven by density differences within the fluid:
Bulk solution membrane depleting surface (reduction of the thickness of the diffusion boundary layer)
H2O H⇄ + + OH-
Exaltation effect
AH + H2O ⇄ A- + H3O+
A- + H2O ⇄ AH + OH-
Protons are transferred through the CEMHidroxyls remain in the diffusion boundary layer
Electroconvection
Hydrodynamic instabilities generated due to the coexistence of intense electric fields with very diluted electrolytes
Vortex generation Distortion of the diffusion boundary layer
R. Kwak, G. Guan, W.K. Peng, J. Han, Desalination 308 (2013) 138-146
0 100 200 300t (s)
Um
(V
)
Decrease of the thickness of the
diffusion boundary layer
Maximum in Um
4.3. Mechanisms of overlimiting current transfer
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
0 100 200 300t (s)
Um
(V
)
0 100 200 300t (s)
Um
(V
)
0 100 200 300t (s)
Um
(V
)
0 100 200 300t (s)
Um
(V
)
0 100 200 300t (s)
Um
(V
)
0 100 200 300t (s)
Um
(V
)
0 100 200 300t (s)
Um
(V
)
0 100 200 300t (s)
Um
(V
)
0 100 200 300t (s)
Um
(V
)
0 100 200 300t (s)
Um
(V
)
30
Water dissociation
NiSO4
10-3M
5·10-3M
10-2M
2·10-2M
NiSO4
10-3M
5·10-3M
10-2M
2·10-2M
H+OH-
Cation-exchange membrane
-+
-----------
+
+
+
+
+
+
+
+
+
+
+
Metal hydroxide
Electric field
4.3. Mechanisms of overlimiting current transfer
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0.0 0.2 0.4 0.6 0.8 1.0
Um (V)
i (m
A·c
m-2
)
0
1
2
3
4
5
6
7
8
9
10
pH
pHcathode
pHanode
pHcentral
overlimiting region due to convective
phenomena
0
1
2
3
4
5
6
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Um (V)
i (m
A·c
m-2
)
0
1
2
3
4
5
6
7
8
9
10
pH
pHcathode
pHanode
pHcentral
catalyzed water splitting
overlimiting region due to enhanced
water splitting
31
Fe(OH)3 precipitates are more dense, they are formed inside the membrane.
Transport blockage
Ni(OH)2 precipitates are not compact, at the membrane surface.
Transport of OH- and H+
4.3. Mechanisms of overlimiting current transfer
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
0
1
2
3
0 0.5 1 1.5 2
Um (V)
i/ilim
Catalyzed water splitting
Formation of a blocking layer of precipitates
32
Convective phenomena
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0 100 200 300 400
t (s)
Um
(V
)
0.32 mA·cm-2
0.40 mA·cm-2
0.85 mA·cm-2
0.99 mA·cm-2
1.28 mA·cm-2
0.64 mA·cm-2
0.60 mA·cm-2
overlimiting instabilities
intensification of concentration
polarization
0.0
0.2
0.4
0.6
0.8
1.0
0 100 200 300 400
t (s)
Um
(V
)
5.10 mA·cm-2
4.53 mA·cm-2
3.12 mA·cm-2
2.69 mA·cm-2
2.34 mA·cm-2
2.13 mA·cm-2
coupled gravitational convection and electroconvection
Cr2(SO4)3
5·10-4M
2.5·10-3M
5·10-3M
10-2M
Cr2(SO4)3
5·10-4M
2.5·10-3M
5·10-3M
10-2M
4.3. Mechanisms of overlimiting current transfer
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
33
0.2
0.4
0.6
0.8
1.0
1.2
1.4
0 100 200 300
t (s)
Um
(V
)
increasing current densities
NiSO4 CrO3
5·10-4M 10-3M
4.3. Mechanisms of overlimiting current transfer
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
34
Convective phenomena favor the transport of ions at i>ilim: they imply a reduction in lplateau
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0.0 0.2 0.4 0.6 0.8 1.0
Um (V)
i/ilim
lplateau1
lplateau2
2.5·10-3M Cr2(SO4)3
10-2M Cr2(SO4)3
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 0.2 0.4 0.6 0.8 1 1.2
Um (V)
i/ilim
0M CrO3
10-3M CrO3
10-2M CrO3
lplateau1lplateau2
lplateau3
lplateau[M+n] lplateau[H+]
4.3. Mechanisms of overlimiting current transfer
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
35
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.000 0.005 0.010 0.015 0.020 0.025
[M+n] (mol/L)
l pla
teau
(V
)
Na(I)Ni(II)Fe(III)Cr(III)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.000 0.002 0.004 0.006 0.008 0.010 0.012
Ni(II) (mol/L)
l pla
tea
u (
V)
0M CrO3
10-3M CrO3
10-2M CrO3
Gravitational convection is favored at high concentrations of heavy metals.
The height of electroconvective vortices increases with the size of the involved cations.
The Grotthuss mechanism of H+ ions does not involve the distortion of the diffusion boundary layer.
4.3. Mechanisms of overlimiting current transfer
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
LIST OF CONTENTS
Transport of single salt solutions
Transport of multicomponent mixtures
Mechanisms of transport at overlimiting currents
Galvanostatic electrodialysis experiments
3. EXPERIMENTAL TECHNIQUES
2. OBJECTIVE
5. CONCLUSIONS
1. INTRODUCTION
4. RESULTS & DISCUSSION
2. Objective3. Techniques Single salt solutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction 36
4. Results and discussion
4.4. Galvanostatic electrodialysis experiments
Effect of CrO3
4. Resultados y discusión4. Results and discussion
Evaluate the membrane selectivity in long-term experimentsEvaluate the effect of the applied current density in long-term experiments
Objetives
Table 7. Compositions tested and applied current densities selected for conducting the galvanostatic electrodialysis experiments.
[NiSO4] [CrO3]ilim (mA·cm-
2)Applied current densities
10-3M
0 M 0.064
75%·ilim
(10-3M NiSO4)
75%·ilim
100%·ilim125%·il
im
10-3M 0.25575%·ili
m
100%·ilim125%·il
im
10-2M 3.26075%·ili
m
100%·ilim125%·il
im
10-2M
0M 0.850
75%·ilim (10-2M NiSO4)
75%·ilim
100%·ilim125%·il
im
10-3M 1.13075%·ili
m
100%·ilim125%·il
im
10-2M 4.24075%·ili
m
100%·ilim125%·il
im
Effect of CrO3 and i
Effect of the range of
applied current density
37
4.4. Galvanostatic electrodialysis experiments
i = 0.048 mA·cm-2
Effect of CrO3
0
10
20
30
40
50
0 1 2 3 4 5 6 7
t (h)
X (
%)
0M CrO3
10-3M CrO3
10-2M CrO3
0
20
40
60
80
100
0 1 2 3 4 5 6 7
t (h)
(%
)
0 M CrO3
10-3 M CrO3
10-2 M CrO3
NiSO4 CrO3
10-3M
0M
10-3M
10-2M
Effect of CrO3 and i
0
20
40
60
80
100
0 1 2 3 4 5 6 7t (h)
X (
%)
0M CrO3
10-3M CrO3
10-2M CrO3
0
10
20
30
40
50
0 1 2 3 4 5 6 7t (h)
(%
)
0M CrO3
10-3M CrO3
10-2M CrO3
NiSO4 CrO3 75%·ilim
10-2M
0M0.64 mA·cm-
2
10-3M0.85 mA·cm-
2
10-2M3.18 mA·cm-
2
2. Objective3. Techniques Single salt solutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
38
Ni2+ transport is improved
General improvement in all parameters
0
20
40
60
80
100
0 1 2 3 4 5 6 7
t (h)
X (
%)
125% ilim100% ilim 75% ilim
10
14
18
22
26
0 1 2 3 4 5 6 7
t (h)
Uce
ll (
V)
125% ilim100% ilim 75% ilim
Development of concentration polarization
Onset of electroconvection
0
10
20
30
40
50
60
70
0 1 2 3 4 5 6 7
t (h)
(%
)
125% ilim100% ilim 75% ilim
0
20
40
60
80
100
0 1 2 3 4 5 6 7
t (h)
Es
(kW
·h/k
g)
125% ilim100% ilim 75% ilim
NiSO4 CrO3 iaplicada
10-2M 0M
75%·ilim
100%·ilim
125%·ilim
4.4. Galvanostatic electrodialysis experiments
2. Objective3. Techniques Single salt solutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
Effect of the range of applied currents
39
Ni2+ transport is improved
The energy consumption increases
0
20
40
60
80
100
0 1 2 3 4 5 6 7
t (h)X
(%
)
125% ilim100% ilim 75% ilim
5
10
15
20
25
30
35
40
0 1 2 3 4 5 6 7
t (h)
Uce
ll (
V)
125% ilim100% ilim 75% ilim
0
5
10
15
20
25
30
35
0 1 2 3 4 5 6 7
t (h)
(%
)
125% ilim100% ilim 75% ilim
0
20
40
60
80
100
120
0 1 2 3 4 5 6 7
t (h)
Es
(kW
·h/k
g)
125% ilim100% ilim 75% ilim
NiSO4 CrO3 iaplicada
10-3M 10-3M
75%·ilim
100%·ilim
125%·ilim
4.4. Galvanostatic electrodialysis experiments
2. Objective3. Techniques Single salt solutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
Effect of the range of applied currents
40
Effect of the range of applied currents
4.4. Galvanostatic electrodialysis experiments
2. Objective3. Techniques Single salt solutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Resultados y discusión4. Results and discussion
0
10
20
30
40
50
60
70
0 0.1 0.2 0.3 0.4 0.5 0.6
lplateau (V)
%
10-3M NiSO4
10-2M NiSO4
10-3M NiSO4 + 10-3M CrO3
10-2M NiSO4 + 10-3M CrO3
10-2M NiSO4 + 10-2M CrO3
100% ilim
100% ilim
100% ilim
100% ilim
125% ilim
125% ilim
125% ilim
125% ilim 75% ilim
75% ilim
75% ilim
75% ilim
125% ilim
75% ilim
100% ilim
LIST OF CONTENTS
Transport of single salt solutions
Transport of multicomponent mixtures
Mechanisms of transport at overlimiting currents
Galvanostatic electrodialysis experiments
3. EXPERIMENTAL TECHNIQUES
2. OBJECTIVE
5. CONCLUSIONS
1. INTRODUCTION
4. RESULTS & DISCUSSION
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction 41
5. Conclusions
Single salt solutions The membrane fixed charges become saturated in solution counterions (Na+, Ni2+, Cr3+,
Fe3+).
The chronopotentiometric curves obtained with multivalent metals show various transition times, which evidence the depletion of positively charged complex species (NiOH+, FeSO4
+, CrSO4+, etc.)
In systems with NiSO4 and Fe2(SO4)3 after surpassing the ilim the formation of precipitates could take place due to an increase of the local pH at the depleting membrane surface. This originates an increased membrane resistance and extended plateaus in the current-voltage curves.
In general, the current-voltage curves obtained show the three characteristic regions of membrane behavior. However, two ilim appear with diluted Cr2(SO4)3 solutions due to the depletion of different species.
4. Results and discussi0n
42
5. ConclusionsMulticomponent solutions
When immersed in mixtures of NiSO4 and CrO3, the membrane fixed charges are preferentially balanced with multicharged Ni2+ ions.
The chronopotentiometric and current-voltage curves obtained for the mixtures are similar to those obtained for single salt solutions. However, the H+ provided by CrO3 compete with the Ni2+ ions for the transport through the membranes.
The transport number of Ni2+ through the membrane is significantly high in single salt solutions of NiSO4, reaching values around 0.9. In the mixture solutions, the TNi
2+ decreases for increasing concentrations of CrO3.
The membrane fixed charges show a higher affinity for Fe(III) with respect to that for Na+ ions.
At low underlimiting currents the concentration of FeSO4+ ions is predominant in the
electrolyte. However, as the polarization of the membrane is intensified, they dissociate into Fe3+ and SO4
2- ions, which implies a reduction in the electrical resistance of the membrane system.
The curves obtained for mixtures of Fe2(SO4)3 and Na2SO4 denote a preferential transport of Na+ ions at low current densities, whereas the transport of Fe3+ is favored at higher currents, when the depletion of Na+ ions has already occurred.
NiSO4
+CrO3
Na2SO4
+Fe2(SO4)
3
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Results and discussi0n
43
5. ConclusionsMechanisms of overlimiting current densities The dissociation of water is the main mechanism originating the overlimiting
currents when the formation of precipitates occurs at the anodic membrane surface.
Electroconvection is the main mechanism of overlimiting current densities in diluted solutions of all the metals tested. The magnitude of the oscillations in Um increases with the applied current density.
The role of gravitational convection becomes important in concentrated solutions of Cr2(SO4)3 and NiSO4. At high current densities gravitational convection and electroconvection have a synergic effect on the reduction of the electrical resistance of the membrane system.
The length of the plateau region of the current-voltage curves decreases for high concentrations of multivalent metals due to their participation on the motion of large volumes of fluid when coupled convection is initiated. On the contrary, H+ ions are transported via the Grotthuss mechanism and hamper electroconvection.
Overlimiting current densities lead to higher nickel transport rates. In terms of energy consumption, this increased transfer of Ni2+ is positive when the concentration of Ni2+ is relatively higher than that of H+ ions. On the contrary, for high concentrations of CrO3, the increased Ni2+ transfer rates are achieved at the cost of an important increase in the specific energy consumption.
Ele
ctro
chem
ical
tech
niq
ues
Galv
anost
ati
c experi
men
ts
2. Objective3. Techniques Single saltsolutions
Multicomponentsolutions
Mechanismsi>ilim
Galvanostaticexperiments
5. Conclusions4. Resultados y discusión
1.Introduction4. Results and discussi0n
¡Thank you very much for your attention!