Achille De Battisti Carlos Alberto Martinez- Huitle Sergio Ferro Laboratory of Electrochemistry University of Implementation of Advanced Implementation of Advanced Electrodes to the Wastewaters Electrodes to the Wastewaters Treatment Treatment ić, Serbia, 17 – 22 Sept., 2006 ESSEE 4
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Achille De Battisti Carlos Alberto Martinez-Huitle Sergio Ferro Laboratory of Electrochemistry University of Ferrara, Italy Implementation of Advanced.
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Achille De Battisti Carlos Alberto Martinez-Huitle Sergio Ferro
Laboratory of Electrochemistry
University of Ferrara, Italy
Implementation of Advanced Implementation of Advanced Electrodes to the Wastewaters Electrodes to the Wastewaters
TreatmentTreatment
Palić, Serbia, 17 – 22 Sept., 2006
ESSEE 4
Urban legends against the electrochemical Urban legends against the electrochemical wayway
to wastewater remediationto wastewater remediation it’s not useful in case of poorly-conducting electrolytes it requires a huge consumption of electricity electrodes are expensive chemicals are easily available reactors are complex and/or difficult to manage …
1. 1. Cl- + S S-Cl + e Volmer2. S-Cl + S-Cl 2 S + Cl2 Tafel
1. Cl- + S S-Cl + e 2'. S-Cl + Cl- S + Cl2 + e Heyrovsky
1. Cl- + S S-Cl + e 2".S-Cl S-Cl+ + e 3. S-Cl+ + Cl- S + Cl2
in all cases, S-Cl Clads
Krishtalik
the Chlorine evolution the Chlorine evolution reactionreaction
1. S + H2O S-OH + H+ + e 2. S-OH + S-OH S-O + S + H2O
3. S-O + S-O 2 S + O2
1. S + H2O S-OH + H+ + e 2'. S-OH S-O + H+ + e 3. S-O + S-O 2 S + O2
the Oxygen evolution the Oxygen evolution reactionreaction
OO22 and Cl and Cl22 evolution reactions in electrochemical evolution reactions in electrochemical incinerationincineration
major links with the fundamentals of electrocatalysismajor links with the fundamentals of electrocatalysis
a solid background for developments from bench to commercial a solid background for developments from bench to commercial scalescale
Electrochemical incineration: how to follow it?Electrochemical incineration: how to follow it?
Traditional analytical approaches:Traditional analytical approaches: NMR, IR, UV-Vis., mass spectrometry, different NMR, IR, UV-Vis., mass spectrometry, different chromatographies…chromatographies…
The alternative:The alternative:
global analytical parameters, like COD and TOCglobal analytical parameters, like COD and TOC
3.1 3.1 DIRECT ELECTROCHEMICAL OXIDATIONDIRECT ELECTROCHEMICAL OXIDATION
Strongly oxidant hydroxyl radicals are formed at high oxygen overvoltage Strongly oxidant hydroxyl radicals are formed at high oxygen overvoltage anodes (PbOanodes (PbO2 2 , Sb(V) or F, Sb(V) or F-- doped-SnO doped-SnO2 2 , diamond electrodes…), diamond electrodes…)
MOx
MOx+1
H2O
H+ + e
1/2 O2
R
CO2 + z H+ + z e
1/2 O2 + H+ + e
H+ + e
MOx( OH)
R
RO
In case of metal-oxide electrodes, we In case of metal-oxide electrodes, we can distinguish two kind of electrode can distinguish two kind of electrode material: “material: “mineralizingmineralizing” and ” and ““convertingconverting” anodes, depending on ” anodes, depending on the available oxidation state of the the available oxidation state of the metal.metal.When the latter can increase its When the latter can increase its valence, radicals are stabilized by valence, radicals are stabilized by interaction with the electrode surface interaction with the electrode surface and their oxidation power is slower and their oxidation power is slower (effect: partial oxidation). On the (effect: partial oxidation). On the contrary, when the oxide lattice contrary, when the oxide lattice cannot be “expanded”, the hydroxyl cannot be “expanded”, the hydroxyl radicals exhibit larger reactivity radicals exhibit larger reactivity (effect: complete mineralization, i.e. (effect: complete mineralization, i.e. transformation into COtransformation into CO22).).
Due to a lower oxygen overvoltage (Due to a lower oxygen overvoltage (higher catalytic activity towards the higher catalytic activity towards the OEROER), other anodic materials generally exhibit low faradaic yields. It is ), other anodic materials generally exhibit low faradaic yields. It is the case of galvanic Pt and IrOthe case of galvanic Pt and IrO22-based DSA’s-based DSA’s®®. The performance of these . The performance of these
stable anodes can be improved by using stable anodes can be improved by using inorganic mediatorsinorganic mediators of the of the oxidationoxidation..
Active chlorineActive chlorine is of particular is of particular interest: oxidation of chlorides interest: oxidation of chlorides requires lower anode potentials, requires lower anode potentials, compared with those necessary compared with those necessary for OHfor OH formation. formation.The contemporaneous formation The contemporaneous formation of the two reactive species (Clof the two reactive species (Cl and OHand OH radicals) may produce radicals) may produce hypochlorous acid, which is a hypochlorous acid, which is a strong oxidant.strong oxidant.
Cl-, ClO3-, ClO-
ads
Cl-
CO2 + H2O + Cl-
Cl-
OH- + Cl2
MOx (HOCl)
e
e
OH-
R
MOx( OH)
MOx
A. De Battisti et al., J. Electrochem. Soc., 147 (2000) 592
A possible consequence of A possible consequence of chloride mediation: more chloride mediation: more electrode materials for electrode materials for electrochemical incinerationelectrochemical incineration
A. De Battisti et al., J. Electrochem. Soc., 147 (2000) 592
glucose 10 g/l in Na2SO4 1M and NaOH 0.01N
0 3 6 9 12 15 18 21 240
2000
4000
6000
8000
10000
12000
NaCl 3g/lNaCl 5g/l
NaCl 1g/lwithout NaCl
electrolysis time (h)
CO
D (
mg
/l o
f O
2)
3.2 IN3.2 INDIRECT OXIDATION DIRECT OXIDATION –– Role of the different Role of the different ParametersParameters
Glucose 10 g/l in 1M Na2SO4 + 0.1M NaOH; Ti/Pt at 1200 A/m2 and 25 °C
S. Ferro et al., Electrochim. Acta, 46 (2000) 305
Glucose 10 g/l in 1M Na2SO4 + NaCl 5g/l; Ti/Pt at 1200 A/m2 and 25 °C
3.2 IN3.2 INDIRECT OXIDATION DIRECT OXIDATION –– Role of the different Role of the different ParametersParameters
S. Ferro et al., Electrochim. Acta, 46 (2000) 305
Glucose 10 g/l in 1M Na2SO4 + 0.1M NaOH + NaCl 3 g/l; Ti/Pt at 25 °C
3.2 IN3.2 INDIRECT OXIDATION DIRECT OXIDATION –– Role of the different Role of the different ParametersParameters
S. Ferro et al., Electrochim. Acta, 46 (2000) 305
Glucose 10 g/l in 1M Na2SO4 + 0.1M NaOH + NaCl 3 g/l; Ti/Pt at 1200 A/m2
3.2 IN3.2 INDIRECT OXIDATION DIRECT OXIDATION –– Role of the different Role of the different ParametersParameters
S. Ferro et al., Electrochim. Acta, 46 (2000) 305
S. Ferro et al., Electrochim. Acta, 46 (2000) 305
3.2 IN3.2 INDIRECT OXIDATION DIRECT OXIDATION – – WHAT HAVE WE WHAT HAVE WE LEARNT?LEARNT?
relatively small amounts of chloride ions relatively small amounts of chloride ions may inhibitmay inhibit the OER, the OER, causingcausing an increase of the anode potential and therefore a higher reactivity an increase of the anode potential and therefore a higher reactivity ofof adsorbed hydroxyl and chloride/oxychloride radicalsadsorbed hydroxyl and chloride/oxychloride radicals
increasing the chloride concentration above a certain critical value increasing the chloride concentration above a certain critical value would causewould cause a potentiostatic buffering by the chlorine redox a potentiostatic buffering by the chlorine redox system,system, and consequently a decrease of the anode potentialand consequently a decrease of the anode potential
…may inhibit……would cause…
Further investigation is needed!
The electrode material is becoming the main character
…It means one more “tough” variable
We have to simplify the experimental approach
Let’ study a simpler substrate: Oxalic Acid
3.3 Electrochemical3.3 Electrochemical Oxidation Oxidation – – The electrode The electrode materialmaterial
Oxalic Acid – direct oxidationOxalic Acid – direct oxidationsubstrate concentration: 0.12M ; background electrolyte: 1N H2SO4
Polarization curves for a Ti/IrO2-Ta2O5 electrode, at different OA concentrations.Inset: elaboration of data in terms of Tafel plot.
Anodic oxidation of Oxalic Acid (OA) at different electrode materials: IrO2-Ta2O5 active coatings
Anodic oxidation of Oxalic Acid (OA) at different electrode materials: IrO2-2SnO2 active coatings
Polarization curves for a Ti/IrO2-2SnO2 electrode, at different OA concentrations
. Polarization curves for a Ti/Ir0.67Ru0.33O2-2SnO2 electrode,at different OA concentrations. Inset: elaboration of data in terms of Tafel plot.
Anodic oxidation of Oxalic Acid (OA) at different electrode materials: Ti/Ir0.67Ru0.33O2-2SnO2 electrode
Anodic oxidation of Oxalic Acid (OA) at different electrode materials: IrO2-2SnO2 active coatings
Polarization curves for a Ti/IrO2-2SnO2 electrode, at different OA concentrations
3.3 Electrochemical3.3 Electrochemical Oxidation Oxidation – – The electrode The electrode materialmaterial
Tafel plot for Oxalic acid electroxidation, in HClO4, at different electrode materials
[OA] = 750mM
3.3 Electrochemical3.3 Electrochemical Oxidation Oxidation – – The electrode The electrode materialmaterial
Considering the BDD anode material, Comninellis et al. [ref] have proposed a mechanism of OA oxidation that involves the participation of hydroxyl radicals generated at the electrode surface:
HH22O O ••OH + HOH + H++ + e + e- - (r.d.s.)(r.d.s.)
HH22CC22OO44 + + ••OHOH HCHC22OO44•• + H+ H22OO
HCHC22OO44•• COCO22 + + ••COOHCOOH
••COOH + COOH + ••OHOH COCO22 + H + H22OO
H2O•OH
•OH
•OH
•OH
HOOC-COOH
•OH
•OH
Ch. Comninellis et al., J. Appl. Electrochem., 30 (2000) 1345
3.3 Electrochemical3.3 Electrochemical Oxidation Oxidation – – The electrode The electrode materialmaterial
3.3 Electrochemical3.3 Electrochemical Oxidation Oxidation – – The electrode The electrode materialmaterial
Oxalic Acid – direct and mediated oxidation at Oxalic Acid – direct and mediated oxidation at bulkbulk Pt Ptsubstrate concentration: 0.12M ; NaX concentration: 5 g/l
background electrolyte: 0.25M NaOH + 0.5M Na2SO4A. D
e B
att
isti
et
al., Ele
ctro
chem
. S
olid
-Sta
te L
ett
., 8
(200
5)
D3
5
3.3 Electrochemical3.3 Electrochemical Oxidation Oxidation – – The electrode The electrode materialmaterial
Tartaric Acid – direct and mediated oxidation at Tartaric Acid – direct and mediated oxidation at Ti/PtTi/Pt
background electrolyte: 0.5M H2SO4 or 0.25M NaOH + 0.5M Na2SO4
3.3 Electrochemical3.3 Electrochemical Oxidation Oxidation – – The electrode The electrode materialmaterial
Effect of NaCl concentration Effect of NaCl concentration on the current/potential on the current/potential characteristics, attained at the Pt electrodecharacteristics, attained at the Pt electrode
supporting electrolyte: 0.25M NaOH + 0.5M Na2SO4
A. D
e B
att
isti
et
al., Ele
ctro
chem
. S
olid
-Sta
te L
ett
., 8
(200
5)
D3
5
3.3 Electrochemical3.3 Electrochemical Oxidation Oxidation – – The electrode The electrode materialmaterial
Effect of NaBr concentration Effect of NaBr concentration on the current/potential on the current/potential characteristics, attained at the Pt electrodecharacteristics, attained at the Pt electrode
supporting electrolyte: 0.25M NaOH + 0.5M Na2SO4
A. D
e B
att
isti
et
al., Ele
ctro
chem
. S
olid
-Sta
te L
ett
., 8
(200
5)
D3
5
3.3 Electrochemical3.3 Electrochemical Oxidation Oxidation – – The electrode The electrode materialmaterial
Effect of NaF concentration Effect of NaF concentration on the current/potential on the current/potential characteristics, attained at the Pt electrodecharacteristics, attained at the Pt electrode
supporting electrolyte: 0.25M NaOH + 0.5M Na2SO4
A. D
e B
att
isti
et
al., Ele
ctro
chem
. S
olid
-Sta
te L
ett
., 8
(200
5)
D3
5
3.3 Electrochemical3.3 Electrochemical Oxidation Oxidation – – The electrode The electrode materialmaterial
Concomitant with oxygen evolution reaction; Good faradaic yields at high-oxygen overvoltage anodes; Weakly adsorbed hydroxyl radicals are the main factor leading to electrochemical incineration; As an extreme case, hydroxyl radicals may act within a reaction cage nearby the electrode surface.
A. De Battisti et al., Electrochem. Solid-State Lett., 8 (2005) D35
objectivesobjectives
sterilization of solutions sterilization of solutions for medical purposesfor medical purposes
low capacity municipal low capacity municipal plants plants
final treatmentfinal treatment
a real approach… for the potabilization of watera real approach… for the potabilization of water
Generation of fresh water anolyte and catholyte
Some chemical reactions that may take place in the electrochemical treatment of potable water
Anode reactions Cathode reactions
2H2O 4e 4H O2 2H2O 2e H2 2OH
2H2O 2e 2H H2O2 О2 е О2
O2 + Н2О 2e O3 2 Н О2 Н2О 2е НО2 ОН
OH e HO НО2 Н2О е HO 2ОН
3H2O 6e O3 6H О2 2 Н 2е Н2О2
O2 + 2OH 3e O3 H2O ecathode + Н2О еaq
Н2О е HO Н Н еaq H
Н2О2 е НО2 Н Н2О еaq H ОН
3OH 2e НО2 Н2О CO32 6Н 4е HCHO + Н2О
H2O 2e 2H O CO32 8Н 6е CH3OH + 2Н2О
H2O e H OH 2CO32 4Н 2е C2O42 + 2Н2О
3OH 2e HO2 H2O 2CO2 2Н 2е Н2C2О4
2Cl 2e Cl2 CO2 2Н 2е НCОOH
Cl H2O 2e HClO H CO32 2 Н2О 2е HCO2 + 3ОН
Cl 2 Н2О 4e HClO2 3Н 2SO42 5H2O8e S2O32 10OH
HCl 2H2O 5e ClO2 5H 2SO42 4Н 2е S2O62 2Н2О
Cl 4OH 4e ClO3 2 Н2О SO42 4H2O 2e SO32 2OH
Cl 4OH 5e ClO2 2 Н2О SO42 4 Н 2e H2SO3 Н2О
Cl 2OH 2e ClO Н2О NO3 5H2O6e NH2OH + 7ОН
Cl 2H2O 5e ClO2 4 Н 2NO3 2H2O4e N2O42 + 4ОН
2SO42 2e S2O82 N2 + 5H + 4e N2H5
2H2CO32 2e C2O62 + 4H Fe2+ + 2e Fe
23 cm
29 cm CharacteristicsCharacteristics
produced water: 1 liter/minuteproduced water: 1 liter/minute