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Kinetic experiments of electrochemical oxidation of iohexol on BDD electrodes for wastewater treatment
Electrochemistry Communications 23 (2012) 48–51
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Kinetic experiments of electrochemical oxidation of iohexol on BDD electrodes for wastewater treatment G.B. Tissot a, A. Anglada b, P. Dimitriou-Christidis c, d,⁎, L. Rossi c, J. Samuel Arey c, d, Ch. Comninellis a a
Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland Adamant Technologies SA, CH-2300 La Chaux-de-Fonds, Switzerland Environmental Engineering Institute, Ecole Polytechnique Fédérale de Lausanne (EPFL), Station No. 2, CH-1015 Lausanne, Switzerland d Environmental Chemistry, Swiss Federal Institute of Aquatic Science and Technology (Eawag), CH-8600 Dübendorf, Switzerland b c
a r t i c l e
i n f o
Article history: Received 11 May 2012 Received in revised form 3 July 2012 Accepted 4 July 2012 Available online 11 July 2012 Keywords: Boron-doped diamond (BDD) Iohexol X-ray contrast media Electrochemical oxidation
a b s t r a c t Electrolysis of iohexol, an iodinated X-ray contrast medium, on synthetic boron-doped diamond (BDD) thin film electrodes was investigated for the first time. Kinetic experiments of electrochemical oxidation resulted in complete elimination of iohexol. Data from the kinetic experiments were compared against estimates by the kinetic model proposed previously by Comninellis et al. Excellent agreement between the experimental results and the kinetic model was observed. The results suggest that electrolysis on BDD electrodes is a promising method for the elimination of iohexol from wastewater. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Electrochemical oxidation (EO) of organic pollutants was shown in the last decade to be an attractive option for advanced wastewater treatment due to the numerous advantages that it presents over other advanced oxidation processes (AOPs). Its advantages include a low environmental footprint due to no use of chemicals, high removal efficiency for a wide range of chemicals and its amenability to automation due to the simplicity of the process. In addition, innovative electrode materials such as boron-doped diamond (BDD) now make the EO process competitive with other AOPs [1]. However, in order to develop EO methods for wastewater treatment it is not sufficient to demonstrate experimentally their efficiency for removing different pollutants, as it was done previously [2–4], but also to develop theoretical models to predict the kinetics and extent of the oxidation, which would allow for the optimization of the operating conditions. Several kinetic models, including those by Comninellis et al. [5–9], Polcaro et al. [10], Rodrigo et al. [11], and Scialdone et al. [12], were developed previously. A detailed discussion and comparison of different kinetic models in terms of their principles, applicability and limitations can be found in Scialdone and Galia [13]. The model by Comninellis et al. [5–9] is a simple model that describes the evolution of chemical oxygen demand (COD) during
electrolysis in an electrochemical reactor operating in a batch recirculation mode under galvanostatic conditions. In other words this model connects electrochemistry in the reaction layer of the electrode surface to COD, a bulk liquid parameter. It also predicts the current efficiency from the applied and limiting current densities (see Section 3. Theory). The main assumption of this model is that the oxidation of organics is a fast reaction that is controlled by mass transport toward the BDD electrode surface. This has been demonstrated by different studies [5–14]. However, this model is expected to fit experimental data accurately when the current efficiency is close to 1, i.e., in the absence of mass transport limitations [13]. The model has the advantage of simplicity, as it does not use any adjustable parameters. It only requires the determination of the mass transfer coefficient. Finally, the model does not describe separately the concentrations of different organics during electrolysis, as it uses COD to represent the total concentration of organics. The model by Comninellis et al. was found to accurately predict the oxidation kinetics for simple organic molecules [7,8,14–16]; however, its accuracy has not been evaluated for complex molecules. The main objective of this study was to demonstrate that this model is also able to predict the evolution of COD of complex persistent organic compounds such as iohexol, an iodinated X-ray contrast medium (ICM), during their EO on BDD electrodes (Eq. (1)). þ
⁎ Corresponding author at: Environmental Engineering Institute, Ecole Polytechnique Fédérale de Lausanne (EPFL), Station No. 2, CH-1015 Lausanne, Switzerland. Tel.: +41 21 693 80 74; fax: +41 21 693 80 70. E-mail address: petros.dimitriou@epfl.ch (P. Dimitriou-Christidis). 1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2012.07.006
C19 H26 I3 N3 O9 þ 29H2 O→19 CO2 þ 3NH3 þ 3HI þ 72H þ 72e
ð1Þ
ICM are anthropogenic organic substances widely used in medical imaging. They resist biodegradation in engineered and natural systems
G.B. Tissot et al. / Electrochemistry Communications 23 (2012) 48–51
and they are only partially oxidized by strong oxidants like ozone [17]. Therefore, oxidation by EO using BDD and the prediction of its evolution during electrolysis could become an attractive technology for the removal of such compounds from wastewater.
3. Theory
2. Materials and methods
η¼
Iohexol (C19H26I3N3O9) and sodium perchlorate (NaClO4) were of analytic grade and were acquired from Sigma–Aldrich. BDD film electrodes were supplied by Adamant Technologies. The BDD film was synthesized by the hot filament chemical vapor deposition technique (HF-CVD) on conducting p-Si substrate. Oxidation of iohexol (3.6 mM of iohexol in 0.05 M NaClO4) was performed in a single compartment electrolytic flow cell under galvanostatic conditions using a Diacell-PS® (Adamant Technologies). The initial chemical oxygen demand (COD) was 82 mmol O2/L. Perchlorate was used as an inactive electrolyte for the fundamental understanding of the kinetics of iohexol elimination, avoiding electrochemical interferences by oxidants formed from oxidation of the electrolyte. BDD electrodes were used as the anode and cathode. Each electrode was a disc with an area of 67 cm2. The interelectrode gap was 1 mm. The electrolyte was stored in a 1-dm3 thermoregulated glass tank and circulated through the electrolytic cell by a centrifugal pump. The flow rate of the electrolyte in the cell was 300 L/h. The mass transfer coefficient was estimated using the ferro/ ferricyanide redox couple with the limiting current technique [18], and was corrected using the diffusion coefficient of iohexol estimated from the Wilke–Chang expression (Eq. (2)) [19] (DIohexol = 4.0 · 10 − 10 m 2/s) and by assuming the thickness of the diffusion layer at the anode surface as constant. The estimated value of the mass transfer coefficient was 2.06· 10−5 m/s. DAB ¼
1:17⋅10−16 ⋅ðT Þ ðϕ⋅MB Þ1=2 μ B ⋅V 0:6 A
ð2Þ
where MB is the molecular weight of the solvent, T the temperature (K), VA the molar volume of the solute (m3/kmol) and ϕ the Wilke–Chang association parameter. The integral current efficiency (ACE) for the anodic oxidation was calculated from the values of the COD using Eq. (3). ACE ¼ 4 F V
ðCOD0 −CODt Þ I t
ð3Þ
where COD0 and CODt are the chemical oxygen demand (in mol O2/L) at times 0 and t, respectively, I the current (A), V the volume of the electrolyte (L) and F the Faraday constant (96487 C/mol). Knowing the integral current efficiency is important in the evaluation of the treatment cost of this technology, e.g., for 99% elimination of iohexol from water. Iohexol concentration during anodic oxidation was monitored by Hewlett Packard 1100 HPLC-UV system. The UV detector was set at 254 nm. Iohexol was separated with a Vydac C18 column (25 cm, 4.6 mm, 5 μm particle size) at a constant temperature of 40 °C. The mobile phase consisted of a mixture of 5% methanol:95% de-ionized water (0.1% formic acid, pH 2.7). The separation was performed isocratically with a mobile phase flow rate of 1.0 mL/min. The COD concentration was measured during electrolysis using a Dr. Lange LASA50 system. Electrolysis of iohexol in 0.8 L of electrolyte (3.65 mM in 0.05 M NaClO4) was performed at 20 °C under galvanostatic conditions using different current densities corresponding to the half of the initial limiting current density (i = 33 mA/cm 2) and to the initial limiting current density (i = 66 mA/cm 2). The initial limiting current density was calculated from Eq. (5) using km,Iohexol = 2.06 ∙ 10 −5 m/s and COD0 = 82 mmol O2/L.
49
According to the Comninellis et al. model, the current efficiency, η, can be estimated from the following equations, ilim ðt Þ iappl
ð4Þ
ilim ðt Þ ¼ 4⋅F ⋅km ⋅CODt
ð5Þ
where ilim(t) is the limiting current density (A/m 2) at time t, iappl the applied current density (A/m 2), 4 the number of exchanged electrons per mol of O2, and km the average mass transfer coefficient in the electrochemical reactor (m/s). In our experiments ilim(0) was calculated to be 66 mA/cm 2. The theoretical model defines different operating regimes, depending on the applied current density. i) Electrolysis under current control (α b 1) Under this regime the COD decreases linearly with time until a critical time (tcr, Eq. (6)). At this time, or at the corresponding critical COD (CODcr , Eq. (7)), the applied current density is equal to the limiting current density. t cr ¼
1−α V α ⋅ A⋅km
CODcr ¼
with
α¼
i 4Fkm
i
. ilim
ð6Þ
ð7Þ
ii) Electrolysis under mass-transport control (α ≥ 1) Under this regime organic side reactions (such as oxygen evolution) occur, resulting in a decrease in the current efficiency. Under these conditions, the COD removal follows an exponential decay trend with respect to time. The equations that describe the temporal evolution of COD are summarized in Table 1. 4. Results and discussion Fig. 1 shows that the experimental COD values obtained as a function of the charge applied (Q) relative to the theoretical charge for iohexol mineralization (Eq. (1) Qth = 72 F) for both α= 0.5 and α= 1 are in excellent agreement with the theoretical model. Furthermore, Fig. 1A shows that the critical time, at which there is a change of the kinetic regime from current-control-limited to mass-transport-limited, is very close to that predicted by Eq. (6) (tcr = 90 min). Fig. 1B shows that for COD values lower than CODcr there is an exponential decay of COD during electrolysis, as expected from the model. Table 2 reports comparison of the experimental and theoretical ACE (Eq. (3)) at different times during electrolysis. Again, there is an excellent agreement between the experimental and theoretical values. The decrease of ACE with time after the critical time (α = 0.5) or when working at the initial limiting current density (α = 1) is due to the side reaction of oxygen evolution (Eq. (8)). þ
2H2 O→O2 þ 4H þ 4e
−
ð8Þ
Table 1 Chemical oxygen demand (COD) evolution equations during electrochemical oxidation according to the Comninellis et al. model [7–9]. Operating regime
Current condition
Current control
αb1
Mixed control
αb1
Mass-transport control
α≥1
α≥1
COD evolution expression CODt ¼ COD0 1−α A Vkm t CODt ¼ COD0 1−α A Vkm t CODt ¼ α ⋅COD0 exp − A Vkm t þ 1−α α CODt ¼ COD0 exp − A Vkm t
50
G.B. Tissot et al. / Electrochemistry Communications 23 (2012) 48–51
A
A
Q/Q th
COD 1.0 COD0 0.9
0
0.2
0.4
0.6
0.8
1
Q/Q th 0
1.2
COD COD0
0.8
CODIoh COD0
0.7 0.6
CODInter COD0
0.5 0.4
0.4
0.6
0.8
1
1.2
1.0 OH
0.9
H N
O
0.8
OH
I
I
O
OH
0.7
H N
OH
N
0.6
I
lohexol
O
HO
0.5
OH
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0.0 0
30
60
90
120
150
180
210
0.0 0
Time [min]
B 0
0.5
1
30
1.5
2
COD COD0
0.8 0.7 0.6 0.5 0.4
0.8
180
210
CODInter COD0
0.6
2.5
OH H N
OH
I
O
OH H N
OH
N I
0.5
0.2 180
2
I
0.3
150
1.5
0.7
0.1 120
1
O
CODIoh COD0
0.0
0.5
0.9
0.2
90
150
1.0
0.4
60
120
Q/Q th
0.3
30
90
B
2.5
0
0
60
Time [min]
Q/Q th
COD 1.0 COD 0 0.9
0.2
O
HO
lohexol OH
0.1
210
0.0
Time [min]
0
Fig. 1. Evolution of COD (relative to the initial value) as a function of time and electrical charge during iohexol electrolysis. Electrolyte: 0.05 M NaClO4 +3.65 mM iohexol, COD0 =82 mM, T=20 °C. (x) Experimental COD; Continuous line: Theoretical (Table 1). (A): Electrolysis under mixed control regime α=0.5 (i=33 mA/cm2). (B): Electrolysis under mass transport control regime α=1 (i=66 mA/cm2). Qth: Theoretical charge for complete iohexol mineralization (Eq. (1)).
Fig. 2 shows the evolution of CODt (relative to COD0) during electrolysis for both α = 0.5 and α = 1. The same figure shows the evolution of iohexol and intermediates (CODinter) (calculated from Eq. (9)), all expressed relative to COD0. ½Iohexolt COD0 CODinter ¼ CODt − ½Iohexol0
ð9Þ
where [Iohexol]t and [Iohexol]0 correspond to the iohexol concentration in the electrolyte (mol/L) at time t and 0, respectively. Fig. 2 shows that the amount of formed intermediates decreases by working at the initial limiting current density (α = 1). This is in agreement with
30
60
90
120
150
180
210
Time [min] Fig. 2. Evolution of COD (relative to the initial value) as a function of time and electrical charge during iohexol electrolysis. Electrolyte: 0.05 M NaClO4 + 3.65 mM iohexol, COD0 = 82 mM, T = 20 °C. (x) COD; (○) CODIoh, corresponding to iohexol; (◊) CODInter, corresponding to intermediate (Eq. (9)). (A): Electrolysis under mixed control regime α= 0.5 (i = 33 mA/cm2). (B): Electrolysis under mass transport control regime α = 1 (i = 66 mA/cm2). Qth: Theoretical charge for complete iohexol mineralization (Eq. (1)).
previous results [14,15] and it is explained by the high local concentration of hydroxyl radicals at the electrode surface relative to the iohexol concentration. In fact the high concentration of hydroxyl radicals favors the complete mineralization of iohexol (Eq. (1)) rather than the diffusion of the transformation products to the electrolyte bulk. 5. Conclusions This study investigated the electrochemical oxidation of iohexol on BDD electrodes. Both current efficiency for COD elimination and
Table 2 Comparison between experimental and theoretical current efficiencies (Eq. (3)) obtained at different times during electrolysis in 0.05 M NaClO4 + 3.65 mM iohexol, Velectrolyte = 0.8 L, T = 20 °C. α
I
Integral current efficiency (ACE) [%]
[mA/cm2]
Initial Exp.
0.5 1 a b c d e
33 66
tcr = 90 min calculated from Eq. (5). After 120 min of electrolysis. After 210 min of electrolysis. Experimental value. Theoretical value.
100 100
d
Intermediateb
tcra Theor. 100 100
e
Finalc
Exp.
Theor.
Exp.
Theor.
Exp.
Theor.
100 –
100 –
90 49
94 50
76 37
80 40
G.B. Tissot et al. / Electrochemistry Communications 23 (2012) 48–51
the evolution of COD were in agreement with the theoretical model developed by Comninellis et al. assuming mass-transport control [7–9]. Furthermore, as predicted by the model, the amount of transformation products is lower when working at the limiting current of iohexol oxidation. These findings allow the possibility to modulate the applied current density at the limiting current in order to obtain 100% of current efficiency during electrolysis, as it was reported previously [16]. Finally, the findings demonstrate the effectiveness of EO on BDD electrodes for the elimination of environmentally persistent molecules, like iohexol, from wastewater. Acknowledgments We acknowledge Dominique Grandjean and Caroline Miles for the analytical support. References [1] M.E.H. Bergmann, In: in: Ch. Comninellis, G. Chen (Eds.), Electrochemistry for the Environment, Springer, New York, 2010, (Ch. 7). [2] D. Gandini, E. Mahè, P.A. Michaud, W. Haenni, A. Perret, Ch. Comninellis, Journal of Applied Electrochemistry 30 (2000) 1345–1350. [3] G. Foti, D. Gandini, Ch. Comninellis, A. Perret, W. Haenni, Electrochemical and Solid-State Letters 2 (1999) 228–230.
51
[4] P. Canizares, J. Lobato, R. Paz, M.A. Rodrigo, C. Saez, Chemosphere 67 (2007) 832–838. [5] Ch. Comninellis, Electrochimica Acta 39 (1994) 1857–1862. [6] O. Simond, V. Schaller, Ch. Comninellis, Electrochimica Acta 42 (1997) 2009–2012. [7] M. Panizza, P.A. Michaud, G. Cerisola, Ch. Comninellis, Journal of Electroanalytical Chemistry 507 (2001) 206–214. [8] M. Panizza, P.A. Michaud, G. Cerisola, Ch. Comninellis, Electrochemistry Communications 3 (2001) 336–339. [9] A. Kapalka, et al., In: in: Ch. Comninellis, G. Chen (Eds.), Electrochemistry for the Environment, Springer, New York, 2010, (Ch. 1). [10] A.M. Polcaro, A. Vaccas, S. Palmas, M. Mascia, Journal of Applied Electrochemistry 33 (2003) 885–892. [11] P. Canizares, M. Diaz, J.A. Dominguez, J. Garcia-Gomez, M.A. Rodrigo, Industrial and Engineering Chemistry Research 43 (2004) 1915–1922. [12] O. Scialdone, Electrochimica Acta 54 (2009) 6140–6147. [13] O. Scialdone, A. Galia, In: in: Brillas, Martinez-Huitle (Eds.), Synthetic Diamond Films, Wiley, 2011, (Ch. 11). [14] M. Panizza, P.A. Michaud, J. Iniesta, Ch. Comninellis, G, Cerisola, Annali di Chimica (Rome) 92 (2002) 995–1006. [15] J. Iniesta, P.A. Michaud, M. Panizza, Ch. Comninellis, Electrochemistry Communications 3 (2001) 346–351. [16] M. Panizza, A. Kapalka, Ch. Comninellis, Electrochimica Acta 53 (2008) 2289–2295. [17] M.M. Huber, A. Gobel, C.S. McArdell, U. von Gunten, Environmental Science and Technology 39 (2005) 4290–4299. [18] R. Greef, R. Peat, L.M. Peter, D. Pletcher, J. Robinson, Instrumental Methods in Electrochemistry, Ellis Horwood Ltd, Chichester UK, 1985. [19] R.S. Brodkey, H.C. Hershey, Transport Phenomena: A Unified Approach, Brodkay Publishing, 1988. (787).
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Kinetic experiments of electrochemical oxidation of iohexol on BDD electrodes for wastewater treatment G.B. Tissot a, A. Anglada b, P. Dimitriou-Christidis c, d,⁎, L. Rossi c, J. Samuel Arey c, d, Ch. Comninellis a a
Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland Adamant Technologies SA, CH-2300 La Chaux-de-Fonds, Switzerland Environmental Engineering Institute, Ecole Polytechnique Fédérale de Lausanne (EPFL), Station No. 2, CH-1015 Lausanne, Switzerland d Environmental Chemistry, Swiss Federal Institute of Aquatic Science and Technology (Eawag), CH-8600 Dübendorf, Switzerland b c
a r t i c l e
i n f o
Article history: Received 11 May 2012 Received in revised form 3 July 2012 Accepted 4 July 2012 Available online 11 July 2012 Keywords: Boron-doped diamond (BDD) Iohexol X-ray contrast media Electrochemical oxidation
a b s t r a c t Electrolysis of iohexol, an iodinated X-ray contrast medium, on synthetic boron-doped diamond (BDD) thin film electrodes was investigated for the first time. Kinetic experiments of electrochemical oxidation resulted in complete elimination of iohexol. Data from the kinetic experiments were compared against estimates by the kinetic model proposed previously by Comninellis et al. Excellent agreement between the experimental results and the kinetic model was observed. The results suggest that electrolysis on BDD electrodes is a promising method for the elimination of iohexol from wastewater. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Electrochemical oxidation (EO) of organic pollutants was shown in the last decade to be an attractive option for advanced wastewater treatment due to the numerous advantages that it presents over other advanced oxidation processes (AOPs). Its advantages include a low environmental footprint due to no use of chemicals, high removal efficiency for a wide range of chemicals and its amenability to automation due to the simplicity of the process. In addition, innovative electrode materials such as boron-doped diamond (BDD) now make the EO process competitive with other AOPs [1]. However, in order to develop EO methods for wastewater treatment it is not sufficient to demonstrate experimentally their efficiency for removing different pollutants, as it was done previously [2–4], but also to develop theoretical models to predict the kinetics and extent of the oxidation, which would allow for the optimization of the operating conditions. Several kinetic models, including those by Comninellis et al. [5–9], Polcaro et al. [10], Rodrigo et al. [11], and Scialdone et al. [12], were developed previously. A detailed discussion and comparison of different kinetic models in terms of their principles, applicability and limitations can be found in Scialdone and Galia [13]. The model by Comninellis et al. [5–9] is a simple model that describes the evolution of chemical oxygen demand (COD) during
electrolysis in an electrochemical reactor operating in a batch recirculation mode under galvanostatic conditions. In other words this model connects electrochemistry in the reaction layer of the electrode surface to COD, a bulk liquid parameter. It also predicts the current efficiency from the applied and limiting current densities (see Section 3. Theory). The main assumption of this model is that the oxidation of organics is a fast reaction that is controlled by mass transport toward the BDD electrode surface. This has been demonstrated by different studies [5–14]. However, this model is expected to fit experimental data accurately when the current efficiency is close to 1, i.e., in the absence of mass transport limitations [13]. The model has the advantage of simplicity, as it does not use any adjustable parameters. It only requires the determination of the mass transfer coefficient. Finally, the model does not describe separately the concentrations of different organics during electrolysis, as it uses COD to represent the total concentration of organics. The model by Comninellis et al. was found to accurately predict the oxidation kinetics for simple organic molecules [7,8,14–16]; however, its accuracy has not been evaluated for complex molecules. The main objective of this study was to demonstrate that this model is also able to predict the evolution of COD of complex persistent organic compounds such as iohexol, an iodinated X-ray contrast medium (ICM), during their EO on BDD electrodes (Eq. (1)). þ
⁎ Corresponding author at: Environmental Engineering Institute, Ecole Polytechnique Fédérale de Lausanne (EPFL), Station No. 2, CH-1015 Lausanne, Switzerland. Tel.: +41 21 693 80 74; fax: +41 21 693 80 70. E-mail address: petros.dimitriou@epfl.ch (P. Dimitriou-Christidis). 1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2012.07.006
C19 H26 I3 N3 O9 þ 29H2 O→19 CO2 þ 3NH3 þ 3HI þ 72H þ 72e
ð1Þ
ICM are anthropogenic organic substances widely used in medical imaging. They resist biodegradation in engineered and natural systems
G.B. Tissot et al. / Electrochemistry Communications 23 (2012) 48–51
and they are only partially oxidized by strong oxidants like ozone [17]. Therefore, oxidation by EO using BDD and the prediction of its evolution during electrolysis could become an attractive technology for the removal of such compounds from wastewater.
3. Theory
2. Materials and methods
η¼
Iohexol (C19H26I3N3O9) and sodium perchlorate (NaClO4) were of analytic grade and were acquired from Sigma–Aldrich. BDD film electrodes were supplied by Adamant Technologies. The BDD film was synthesized by the hot filament chemical vapor deposition technique (HF-CVD) on conducting p-Si substrate. Oxidation of iohexol (3.6 mM of iohexol in 0.05 M NaClO4) was performed in a single compartment electrolytic flow cell under galvanostatic conditions using a Diacell-PS® (Adamant Technologies). The initial chemical oxygen demand (COD) was 82 mmol O2/L. Perchlorate was used as an inactive electrolyte for the fundamental understanding of the kinetics of iohexol elimination, avoiding electrochemical interferences by oxidants formed from oxidation of the electrolyte. BDD electrodes were used as the anode and cathode. Each electrode was a disc with an area of 67 cm2. The interelectrode gap was 1 mm. The electrolyte was stored in a 1-dm3 thermoregulated glass tank and circulated through the electrolytic cell by a centrifugal pump. The flow rate of the electrolyte in the cell was 300 L/h. The mass transfer coefficient was estimated using the ferro/ ferricyanide redox couple with the limiting current technique [18], and was corrected using the diffusion coefficient of iohexol estimated from the Wilke–Chang expression (Eq. (2)) [19] (DIohexol = 4.0 · 10 − 10 m 2/s) and by assuming the thickness of the diffusion layer at the anode surface as constant. The estimated value of the mass transfer coefficient was 2.06· 10−5 m/s. DAB ¼
1:17⋅10−16 ⋅ðT Þ ðϕ⋅MB Þ1=2 μ B ⋅V 0:6 A
ð2Þ
where MB is the molecular weight of the solvent, T the temperature (K), VA the molar volume of the solute (m3/kmol) and ϕ the Wilke–Chang association parameter. The integral current efficiency (ACE) for the anodic oxidation was calculated from the values of the COD using Eq. (3). ACE ¼ 4 F V
ðCOD0 −CODt Þ I t
ð3Þ
where COD0 and CODt are the chemical oxygen demand (in mol O2/L) at times 0 and t, respectively, I the current (A), V the volume of the electrolyte (L) and F the Faraday constant (96487 C/mol). Knowing the integral current efficiency is important in the evaluation of the treatment cost of this technology, e.g., for 99% elimination of iohexol from water. Iohexol concentration during anodic oxidation was monitored by Hewlett Packard 1100 HPLC-UV system. The UV detector was set at 254 nm. Iohexol was separated with a Vydac C18 column (25 cm, 4.6 mm, 5 μm particle size) at a constant temperature of 40 °C. The mobile phase consisted of a mixture of 5% methanol:95% de-ionized water (0.1% formic acid, pH 2.7). The separation was performed isocratically with a mobile phase flow rate of 1.0 mL/min. The COD concentration was measured during electrolysis using a Dr. Lange LASA50 system. Electrolysis of iohexol in 0.8 L of electrolyte (3.65 mM in 0.05 M NaClO4) was performed at 20 °C under galvanostatic conditions using different current densities corresponding to the half of the initial limiting current density (i = 33 mA/cm 2) and to the initial limiting current density (i = 66 mA/cm 2). The initial limiting current density was calculated from Eq. (5) using km,Iohexol = 2.06 ∙ 10 −5 m/s and COD0 = 82 mmol O2/L.
49
According to the Comninellis et al. model, the current efficiency, η, can be estimated from the following equations, ilim ðt Þ iappl
ð4Þ
ilim ðt Þ ¼ 4⋅F ⋅km ⋅CODt
ð5Þ
where ilim(t) is the limiting current density (A/m 2) at time t, iappl the applied current density (A/m 2), 4 the number of exchanged electrons per mol of O2, and km the average mass transfer coefficient in the electrochemical reactor (m/s). In our experiments ilim(0) was calculated to be 66 mA/cm 2. The theoretical model defines different operating regimes, depending on the applied current density. i) Electrolysis under current control (α b 1) Under this regime the COD decreases linearly with time until a critical time (tcr, Eq. (6)). At this time, or at the corresponding critical COD (CODcr , Eq. (7)), the applied current density is equal to the limiting current density. t cr ¼
1−α V α ⋅ A⋅km
CODcr ¼
with
α¼
i 4Fkm
i
. ilim
ð6Þ
ð7Þ
ii) Electrolysis under mass-transport control (α ≥ 1) Under this regime organic side reactions (such as oxygen evolution) occur, resulting in a decrease in the current efficiency. Under these conditions, the COD removal follows an exponential decay trend with respect to time. The equations that describe the temporal evolution of COD are summarized in Table 1. 4. Results and discussion Fig. 1 shows that the experimental COD values obtained as a function of the charge applied (Q) relative to the theoretical charge for iohexol mineralization (Eq. (1) Qth = 72 F) for both α= 0.5 and α= 1 are in excellent agreement with the theoretical model. Furthermore, Fig. 1A shows that the critical time, at which there is a change of the kinetic regime from current-control-limited to mass-transport-limited, is very close to that predicted by Eq. (6) (tcr = 90 min). Fig. 1B shows that for COD values lower than CODcr there is an exponential decay of COD during electrolysis, as expected from the model. Table 2 reports comparison of the experimental and theoretical ACE (Eq. (3)) at different times during electrolysis. Again, there is an excellent agreement between the experimental and theoretical values. The decrease of ACE with time after the critical time (α = 0.5) or when working at the initial limiting current density (α = 1) is due to the side reaction of oxygen evolution (Eq. (8)). þ
2H2 O→O2 þ 4H þ 4e
−
ð8Þ
Table 1 Chemical oxygen demand (COD) evolution equations during electrochemical oxidation according to the Comninellis et al. model [7–9]. Operating regime
Current condition
Current control
αb1
Mixed control
αb1
Mass-transport control
α≥1
α≥1
COD evolution expression CODt ¼ COD0 1−α A Vkm t CODt ¼ COD0 1−α A Vkm t CODt ¼ α ⋅COD0 exp − A Vkm t þ 1−α α CODt ¼ COD0 exp − A Vkm t
50
G.B. Tissot et al. / Electrochemistry Communications 23 (2012) 48–51
A
A
Q/Q th
COD 1.0 COD0 0.9
0
0.2
0.4
0.6
0.8
1
Q/Q th 0
1.2
COD COD0
0.8
CODIoh COD0
0.7 0.6
CODInter COD0
0.5 0.4
0.4
0.6
0.8
1
1.2
1.0 OH
0.9
H N
O
0.8
OH
I
I
O
OH
0.7
H N
OH
N
0.6
I
lohexol
O
HO
0.5
OH
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0.0 0
30
60
90
120
150
180
210
0.0 0
Time [min]
B 0
0.5
1
30
1.5
2
COD COD0
0.8 0.7 0.6 0.5 0.4
0.8
180
210
CODInter COD0
0.6
2.5
OH H N
OH
I
O
OH H N
OH
N I
0.5
0.2 180
2
I
0.3
150
1.5
0.7
0.1 120
1
O
CODIoh COD0
0.0
0.5
0.9
0.2
90
150
1.0
0.4
60
120
Q/Q th
0.3
30
90
B
2.5
0
0
60
Time [min]
Q/Q th
COD 1.0 COD 0 0.9
0.2
O
HO
lohexol OH
0.1
210
0.0
Time [min]
0
Fig. 1. Evolution of COD (relative to the initial value) as a function of time and electrical charge during iohexol electrolysis. Electrolyte: 0.05 M NaClO4 +3.65 mM iohexol, COD0 =82 mM, T=20 °C. (x) Experimental COD; Continuous line: Theoretical (Table 1). (A): Electrolysis under mixed control regime α=0.5 (i=33 mA/cm2). (B): Electrolysis under mass transport control regime α=1 (i=66 mA/cm2). Qth: Theoretical charge for complete iohexol mineralization (Eq. (1)).
Fig. 2 shows the evolution of CODt (relative to COD0) during electrolysis for both α = 0.5 and α = 1. The same figure shows the evolution of iohexol and intermediates (CODinter) (calculated from Eq. (9)), all expressed relative to COD0. ½Iohexolt COD0 CODinter ¼ CODt − ½Iohexol0
ð9Þ
where [Iohexol]t and [Iohexol]0 correspond to the iohexol concentration in the electrolyte (mol/L) at time t and 0, respectively. Fig. 2 shows that the amount of formed intermediates decreases by working at the initial limiting current density (α = 1). This is in agreement with
30
60
90
120
150
180
210
Time [min] Fig. 2. Evolution of COD (relative to the initial value) as a function of time and electrical charge during iohexol electrolysis. Electrolyte: 0.05 M NaClO4 + 3.65 mM iohexol, COD0 = 82 mM, T = 20 °C. (x) COD; (○) CODIoh, corresponding to iohexol; (◊) CODInter, corresponding to intermediate (Eq. (9)). (A): Electrolysis under mixed control regime α= 0.5 (i = 33 mA/cm2). (B): Electrolysis under mass transport control regime α = 1 (i = 66 mA/cm2). Qth: Theoretical charge for complete iohexol mineralization (Eq. (1)).
previous results [14,15] and it is explained by the high local concentration of hydroxyl radicals at the electrode surface relative to the iohexol concentration. In fact the high concentration of hydroxyl radicals favors the complete mineralization of iohexol (Eq. (1)) rather than the diffusion of the transformation products to the electrolyte bulk. 5. Conclusions This study investigated the electrochemical oxidation of iohexol on BDD electrodes. Both current efficiency for COD elimination and
Table 2 Comparison between experimental and theoretical current efficiencies (Eq. (3)) obtained at different times during electrolysis in 0.05 M NaClO4 + 3.65 mM iohexol, Velectrolyte = 0.8 L, T = 20 °C. α
I
Integral current efficiency (ACE) [%]
[mA/cm2]
Initial Exp.
0.5 1 a b c d e
33 66
tcr = 90 min calculated from Eq. (5). After 120 min of electrolysis. After 210 min of electrolysis. Experimental value. Theoretical value.
100 100
d
Intermediateb
tcra Theor. 100 100
e
Finalc
Exp.
Theor.
Exp.
Theor.
Exp.
Theor.
100 –
100 –
90 49
94 50
76 37
80 40
G.B. Tissot et al. / Electrochemistry Communications 23 (2012) 48–51
the evolution of COD were in agreement with the theoretical model developed by Comninellis et al. assuming mass-transport control [7–9]. Furthermore, as predicted by the model, the amount of transformation products is lower when working at the limiting current of iohexol oxidation. These findings allow the possibility to modulate the applied current density at the limiting current in order to obtain 100% of current efficiency during electrolysis, as it was reported previously [16]. Finally, the findings demonstrate the effectiveness of EO on BDD electrodes for the elimination of environmentally persistent molecules, like iohexol, from wastewater. Acknowledgments We acknowledge Dominique Grandjean and Caroline Miles for the analytical support. References [1] M.E.H. Bergmann, In: in: Ch. Comninellis, G. Chen (Eds.), Electrochemistry for the Environment, Springer, New York, 2010, (Ch. 7). [2] D. Gandini, E. Mahè, P.A. Michaud, W. Haenni, A. Perret, Ch. Comninellis, Journal of Applied Electrochemistry 30 (2000) 1345–1350. [3] G. Foti, D. Gandini, Ch. Comninellis, A. Perret, W. Haenni, Electrochemical and Solid-State Letters 2 (1999) 228–230.
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