Arsenite âAs(III)â more toxic than arsenate âAs(V)â. 4. ... Degrade arsenic (III) to a less toxic form: arsenate. TiO. 2 ... SHORT-CIRCUIT MEASUREMENTS. 1.
PHOTOELECTROCHEMICAL STUDIES OF THE As(III)/As(V) SYSTEM ON NANOPOROUS TITANIUM DIOXIDE ELECTRODES Damián Monllor-Satoca1, 2, Roberto Gómez1 and Wonyong Choi2
1
Institut Universitari d´Electroquímica i Departament de Química Física, Alacant, Spain 2 School of Environmental Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang, Republic of Korea
INTRODUCTION 1. The As(III) photooxidation
Arsenic 1. Persistent contaminant (continental waters, industry). 2. Human toxicity, even at very low doses. 3. Arsenite –As(III)– more toxic than arsenate –As(V)–. 4. Arsenate is more easily recovered than arsenite.
Hyperpigmentation (black-foot disease), skin, bladder and/or liver cancer, gangrene…
How to reduce arsenic toxicity? 1. Reduce its concentration (adsorption, coagulation, precipitation). 2. Degrade arsenic (III) to a less toxic form: arsenate.
Photooxidation mechanism of As(III)
TiO2 photo(electro)catalysis
VB holes, OH radicals, superoxide radicals.
INTRODUCTION 2. Experimental conditions Photoelectrochemical measurements: Stationary, (photocurrent), open-circuit (photopotential).
Non-stationary
(transients)
/
Short-circuit
Photoelectrochemical cell: a.
3 electrode system: WE, nanocrystalline film (1.5 cm2 exposed area, 5 µm thickness) of P25 deposited over FTO (Doctor Blade method). CE, Pt wire. RE, calomel electrode.
b.
Electrolyte: N2 or O2-purged NaClO4 0.5 M solution, buffered at pH 3.0, with NaAsO2 (arsenite source) or Na2HAsO4 (arsenate source). At pH 3.0, both species are predominantly present as HAsO2 and H2AsO4-.
c.
Light source: 300 W Xe arc lamp, 10-cm water filter, cut-off filter ( > 300 nm), EE illumination. Gas inlet Reference electrode (RE) Gas bubbler Counter electrode (CE) Working electrode (WE) Quartz window
SHORT-CIRCUIT MEASUREMENTS
INTRODUCTION
1. Semiconductor-solution interface
B) Nanocrystalline semiconductors
A) Bulk semiconductors
e h+
e hv
VB
Solution
h+
Conducting substrate
CB
Diffusion
Recombination
hv e h+
D D+ e h+
Oxidation
A
A– Reduction
Charge separation: electric field Charge transport: migration and diffusion
Charge separation: kinetics of the e- and h+ interfacial charge transfer Charge transport: diffusion
SHORT-CIRCUIT MEASUREMENTS
INTRODUCTION
2. Current measurements under illumination
Photocurrent-potential curve
(photoanodes, type n semiconductors)
ncontacto = 0 dn dx 0 x 0
Zone A
dn dx 0 x 0
dn dx , maximum x 0
Zone B
dn j ph FDn contact dx contacto Zone A
Zone C
Iph / A
Zone B Onset
F = c
E/V F
E/V
CB
F
e- CB
h
h
VB
VB
e- CB
F
Zone C
h VB
FTO Semiconductor
Experimental techniques: voltammetry (under illumination), photocurrent transients.
SHORT-CIRCUIT MEASUREMENTS
INTRODUCTION
3. Voltammetric measurements
- Pre-peak (C > 50 µM), at –0.40 V vs SCE.
As(III)
3 0.5
a.
- From 1 to 50 µM: photocurrent lowers (at positive E), increases pre-peak current.
(20 mV/s)
jph / j0
2
- At C > 500 µM: photocurrent increases, saturates at 10-15 mM.
0.1 0 0.05 0.01
1
0.3
0
(2 mV/s)
b.
15
8
10 8 6 4 2 1
4
0 -0.8
-0.4
0.0 E / VSCE
0.4
0.8
j / mA·cm
-2
0.2
12
jph / j0
0 M 500 M
0.1
0.0 -0.6
-0.3
0.0
0.3
0.6
E / VSCE
No As(III): one wave. With As(III): two waves.
With As(III): onset more negative (recombination reduced). With As(III): transients decay: adsorbed species
SHORT-CIRCUIT MEASUREMENTS
INTRODUCTION
4. Transient measurements
As(III)
3 concentration regions:
8
6
1.2
4
1.0
j / j0
j / j0
Na. 2
Transients: 1 min EE illumination, +0.6 V vs SCE.
a. 0 M to 25 µM: photocurrent lowering. b. 25 µM to 10 mM: linear increase of photocurrent. c. 10 mM to 15 mM: photocurrent saturation.
0.8
2
0
0
0
4
8
250 500 [NaAsO2] / M
12
16
Multiplication of photocurrent: As(III) effectively oxidizes, in the absence of molecular oxygen.
20
[NaAsO2] / mM
Ob. 2
Photocurrent multiplication factor: lower than with N2. Photocurrent saturation: [As(III)] > 15 mM.
1.2
0.8 j / j0
j / j0
0.20
0.4
Lowering of photocurrent: molecular oxygen acts as an efficient CB electron scavenger:
0.16
0
0.0
250
500
[NaAsO2] / M
0
4
8
12
[NaAsO2] / mM
16
20
O2 eBC H HO2
SHORT-CIRCUIT MEASUREMENTS
INTRODUCTION
4. Transient measurements
Transients: 1 min EE illumination, +0.6 V vs SCE.
As(V) 1.0 1.0
Photocurrent lowering with increasing [As(V)] 0.8
[As(V)] = 500 µM 60% initial photocurrent.
j / j0
j / j0
0.8
0.6
0.6
[As(V)] = 15 mM 40% initial photocurrent.
0 250 500 [Na2HAsO4] / M
0.4 0
4
8
12
16
[Na2HAsO4] / mM
-Low concentration regime: blockage of surface active sites (by adsorption). -High concentration regime: As(V) acting as an electron scavenger (oxidant).
SHORT-CIRCUIT MEASUREMENTS
INTRODUCTION
5. Oxygen production measurements
Oxygen production: Clark-type electrode 24
Calibration: with the N2-purged solution. TiO2 electrode polarized at +0.6 V vs SCE.
As(III) 1 mM As(III) 0 mM
20
After 1 h illumination:
C - C0 / M
16
[O2] (no As) = 11 µM [O2] (with As) = 6 µM
12
11 µM Less O2 produced with As(III):
8
6 µM As(III) photooxidation predominates over H2O photooxidation.
4 0
On 0
20
40 t / min
60
(captures holes more efficiently than water)
INTRODUCTION
SHORT CIRCUIT
OPEN-CIRCUIT MEASUREMENTS 1. Potential-time transients
Reproduce the real behavior of the photocatalyst. Don´t allow to individually study the anodic and cathodic processes. Zone A
Zone B
Zone C
e e e
CB
e e e
F Vph
E /V
F VB
h+ h+ h+
h+ h+ h+
Open-circuit potential (OCP)-time curve (photoanodes, type n semiconductors)
FTO Semiconductor
E0
E/V
Zone A
Zone B
Zone C
Zone D
Zone E
Darkness (Stationary state)
Photopotential development (Non-stationary state)
Ilumination (Stationary state)
Photopotential relaxation (Non-stationary state)
Darkness (Stationary state)
Photopotential definition
Ess t Light On
Light Off
Experimental technique: chronopotentiometry (in the dark and under illumination).
SHORT CIRCUIT
INTRODUCTION
OPEN-CIRCUIT MEASUREMENTS 2. Photopotential measurements 0.0
0.0
E0
As(III)
As(V)
Ess
E / VSCE
E / VSCE
-0.2
-0.4
-0.2 E0 Ess
-0.4 -0.6
-0.8
0
4
8
12
[NaAsO2] / mM
16
-0.6
0
4
8
12
16
[Na2HAsO4] / mM
Saturation at 25 µM.
Saturation at 4 µM.
OCP in the dark: –500 mV. OCP under illumination: –200 mV.
OCP in the dark: –200 mV. OCP under illumination: +150 mV.
As(III)
In the dark, OCP lowering Adsorption of anionic species (AsO2-). Under illumination, OCP lowering Electron injection from adsorbed As(III).
As(V)
In the dark, OCP lowering Adsorption of anionic species (H2AsO4-). Under illumination, OCP increasing Electron withdrawal from adsorbed As(V).
INTRODUCTION
SHORT CIRCUIT
OPEN-CIRCUIT MEASUREMENTS 3. OCP relaxation measurements
As(III) 0 0.004 0.004 0.02 0.5 2 10 15
0.8 0.6 0.4 0.2
1.0 1.0
[NaAsO2] / mM 2
(E0---EE Ess)) (E---EEEss)))///(E (E (E (E ss ss 00 ss ss
(E - Ess) / (E0 - Ess)
1.0
As(V)
Light off
0.0
0.8 0.8 0.6 0.6 0.4 0.4
[Na ] / mM [Na HAsO [Na2HAsO HAsO mM 4 ]] // mM 2 2
0.2 0.2
Light Light Light off off off
0.0 0.0 0
200
400
600
800
00
200 200
4 4
00 0 0.003 0.003 0.003 0.003 0.015 0.015 0.015 0.2 0.2 0.2
600 600
0.5 0.5 0.5 0.5 22 2 10 10 10
t/s
400 400 tt/ /ss
800 800
[As(III)] < 20 µM: recombination. [As(III)] > 20 µM: recombination.
[As(V)] < 3 µM: recombination. [As(V)] > 3 µM: recombination.
High [As(III)], relaxation slower than without.
High [As(V)], relaxation quicker than without.
Relaxation with O2: quicker than with N2. No significant changes with [As]. a. Low concentration regime, recombination lowers Surface active sites blockage. b. High [As(V)], recombination accelerates As(V) could act as an efficient electron acceptor.
SHORT CIRCUIT
INTRODUCTION
OPEN-CIRCUIT MEASUREMENTS 3. OCP relaxation measurements
Modelling the relaxation curves Pseudo-first order recombination constants, at zero-time
E Ess E0 Ess
1 exp kr t
a. Recombination constant values: kr (t = 0) / s
-1
one order of magnitude higher for As(V).
kr (t = 0) / s
-1
0.01
1E-3
b. Recombination constant variation: 0
20 40 [As] / M
1E-3
Increase with [As]
As(V) As(III) 0
4
8 [As] / mM
12
16
c. Low concentration regime (0-5 µM): Arsenite and arsenate reduce recombination.
INTRODUCTION
SHORT CIRCUIT
OPEN CIRCUIT
MECHANISTIC ASPECTS
General mechanism: As(IV) species play a central role
h TiO2 eBC hBV
III Asads
Os hBV Os Os eBC Os IV h / Os Asads IV V Asads Asads eBC
IV III V 2Asads Asads Asads V III Asads 2eBC Asads
In the presence of O2:
IV ads
As
H
V H HO2 O2 Asads HO2 O2 eBC
INTRODUCTION
SHORT CIRCUIT
OPEN CIRCUIT
MECHANISM
CONCLUSIONS
1. Stationary and non-stationary electrochemical measurements (photocurrent and photovoltage) have proven to be useful in the (partial) ellucidation of the arsenic (III) photooxidation mechanism.
2. It has been proven that both the As(III) and As(V) species are adsorbed at the TiO2 surface, attaining saturation at very low concentrations (around 25 M and 4 M, respectively).
3. The As(III) photoxidation proceeds in the absence of O2, even at potential values where water is not undergoing photooxidation, pointing to the valence band free holes as the main photoactive species.
4. As(V) could also act, once adsorbed, as an electron acceptor species, although not effectively, provided significant photocurrents are registered when photooxidizing As(III).
INTRODUCTION
SHORT CIRCUIT
OPEN CIRCUIT
MECHANISM CONCLUSIONS
Thank you all for your attention
감사합니다 !
