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Mass transport studies at rotating cylinder electrode during zinc removal from dilute solutions
Electrochimica Acta 56 (2011) 1455–1459
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Mass transport studies at rotating cylinder electrode during zinc removal from dilute solutions Alejandro Recéndiz a , Sairi León b , José L. Nava c,∗ , Fernando F. Rivera d a
Dolorey, Centro de Investigación y Desarrollo Tecnológico, Av. Comonfort No. 2050, Torreón, Coahuila, Mexico Universidad Nacional Autónoma de México, Facultad de Estudios Superiores Zaragoza, México D.F., Mexico Universidad de Guanajuato, Departamento de Ingeniería Geomática e Hidráulica, Av. Juárez No. 77, Guanajuato, Gto., Mexico d Universidad Autónoma Metropolitana-Iztapalapa, Departamento de Química, Av. San Rafael Atlixco No. 186, México D.F., Mexico b c
a r t i c l e
i n f o
Article history: Received 25 June 2010 Received in revised form 9 October 2010 Accepted 11 October 2010 Available online 16 October 2010 Keywords: Zinc deposition Mass transport Rotating cylinder electrode Metal ions removal
a b s t r a c t The mass transport in the rotating cylinder electrode (RCE) for the Zn(II) recovery from dilute solutions was investigated. Global mass transport data were obtained by monitoring the (first order) concentration decay of dissolved zinc in sulfate media at pH 2. The electrolyses were performed at holding potential of −1.7 V vs. sat. MSE at Reynolds numbers comprised between 15 470 ≤ Re ≤ 123 680. Based on the analysis of Sh = aReb Sc0.356 correlation, the value of the constant a, associated with shape and cell dimensions, was 0.65; while the constant b, associated with hydrodynamic regime, exhibits a value of 0.48, which obeys to smooth zinc deposits on RCE interface. The mass transport in the Zn(II)/Zn cathode interface process differs with other deposition process, which usually gives roughness deposits. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction The RCE is one of the most common geometries for different types of studies, such as metal ion recovery [1–4], alloy formation [1,3], electrosynthesis [3], corrosion [1,3], effluent treatment [2,5–9] and Hull cell studies [10,11]. RCE’s are particularly well suited for high mass transport studies in the turbulent flow regime [1,12–15]. The mass transport control is imposed by the rotation speed of the inner cylinder and the applied limiting current density. When the fluid flow is generated entirely by an inner RCE, the characteristics of mass transport conditions can be described by a dimensionless group correlation of the following form [1–3,12]: Sh = a Reb Sc0.356
(1)
where the Sherwood number (Sh = km d/D) describes mass transport by forced convection, the Reynolds number (Re = ud/v) is an indication of the fluid flow regime, and the Schmidt number (Sc = v/D) relates the electrolyte transport properties. The average mass transport coefficient is km , d is the diameter of the RCE, D is the diffusion coefficient, u is the peripheral velocity (u = df), v is the kinematic viscosity, and f is the rotational frequency.
∗ Corresponding author. Tel.: +52 4731020100x2289; fax: +52 4731020100x2209. E-mail addresses: [email protected], [email protected] (J.L. Nava). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.10.021
The mass transport correlation is best evaluated through analysis of experimental data [1,3,12,16,17]. There are different correlations that have been experimentally obtained at different types of RCE surfaces (smooth, knurled, longitudinal fins, wrapped with cylindrical wire, expanded metal and metal powder deposit) and broadly described and discussed in two reviews [1,3]. The experimental constant a, is associated with the electrode geometry, shape, cell dimensions [1,3,16] as well as with the diffusion coefficient [17]; whereas b is associated with the hydrodynamic regime [1,3,13,16], on the smooth and roughness RCE interface (given by the nature of reduction reaction) [13,17]. Mass transport studies at RCE in turbulent conditions, Re > 100, have been performed during copper deposition [4,13,16,17], giving powdery deposits and roughness surfaces, resulting in values of b ≈ 0.9. On the other hand, in a previous communication carried out by our group, for smooth RCE in the system I3 − /I− , gave values of b ≈ 0.4 [17], which differs for the obtained analytically, b = 0.66 [12]. These differences of b were discussed in [17], emphasizing that, hydrodynamics in the smooth and rough RCE interface determine this value; while the differences of a values are determined by transport properties of electrolyte. This paper focuses on the mass transport in the RCE for the zinc removal from dilute solutions. Global mass transport data were obtained by monitoring the (first order) concentration decay of zinc ions during electrolysis experiments in the RCE. Complementary rotating disks experiments to determine the diffusion coefficient were performed. The global mass transport properties
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Table 1 RCE reactor parameters and electrolyte properties. Reaction Volume, VR Reactor diameter RCE diameter, d RCE length RCE area, ARCE (in contact with electrolyte) Length and width of the plates used as anodes (attached to the reactor walls) Inter-electrode gap Counter electrode area, ACE (six plates, in contact with the solution) Diffusion coefficient, D Kinematics viscosity,
350 cm3 8.5 cm 3.8 cm 11 cm 80 cm2 13 cm × 2 cm 2.4 cm 84 cm2 2.92 × 10−6 cm2 s−1 0.01 cm2 s−1
obtained herein were compared with those reported in the literature. 2. Experimental The model solution used in this work was prepared of analytic grade reactants and deionised water Milli-QTM . For the electrolysis test in the RCE the concentration was 0.019 mol dm−3 ZnSO4 ·7H2 O (1270 ppm Zn(II)) in 0.037 mol dm−3 Na2 SO4 at pH = 2. The resulting solution exhibited a conductivity of 97.4 mS cm−1 . It is important to point out that such solution was prepared trying to resemble the amount of zinc ions, conductivity and pH of a typical composition contained in an effluent generated by a plating industry. On the other hand, for rotating disk experiments the same solution, varying only the concentration of sodium sulfate (0.5 mol dm−3 ) to obtain a supporting electrolyte in excess, was prepared. Each electrolytic solution was deoxygenated with nitrogen for about 600 s before each experiment. 2.1. Equipment A potentiostat–galvanostat EG&G model PAR 273 with M270 software was used for the microelectrolysis experiments. For the experiments on RCE, a KepcoTM power supply of 10 A and 20 V capacity was used. The Zn(II) concentrations were determined employing a VarianTM Atomic Absorption Spectrophotometer Model 220 FS. 2.1.1. Microelectrolysis experiments A 0.1 dm3 Pyrex electrochemical cell, with a three-electrode system and nitrogen inlet, was used for the microelectrolysis experiments. The rotating disk electrode (RDE) was a 316 stainless-steel disk with a geometric area of 0.20 cm2 . The electrode surface was polished with a 0.3 m alumina powder/water mixture and rinsed with distilled water, followed by 300 s of ultrasonic bath and a final rinse with distilled water in each experiment. To control the RDE angular velocity, a Radiometer AnalyticalTM velocity controller, model CTV 101, was used. The potentials were measured vs. a saturated mercurous sulfate electrode (sat. MSE), 0.615 vs. SHE/V, Radiometer AnalyticalTM model XR200, and the counter electrode was a graphite rod. 2.1.2. Electrolysis in RCE Fig. 1 shows the diagram of a device fabricated for the laboratory studies, consisting of a 500-cm3 glass reactor with a temperature bath. A 316-type stainless steel cylinder with a 3.8 cm diameter and a length of 11 cm was used as a cathode. As anodes were used six 13 cm long, 2 cm wide and 0.3 cm thick RuO2 /TiO2 DSA which were attached to the reactor walls and connected between each other. The RuO2 /TiO2 DSA were supplied by De NoraTM . CaframoTM model BDC3030 electric motor of variable velocity was used to rotate the inner cylinder. The potentials were measured vs. sat. MSE. Table 1
Fig. 1. Rotating cylinder electrode reactor scheme.
shows the parameters of the RCE cell used in this work and the electrolyte transport properties. 2.2. Methodology 2.2.1. Determination of electrolysis potential for zinc deposition process A series of potential pulses was applied to the RDE, using the cell described in Section 2.1.1, in the potential range of −1.8 ≤ E ≤ −1.5 V vs. sat. MSE during 10 s, with increments of 0.05 V. The corresponding chronoamperograms were obtained at different angular velocities between 16 ≤ ω ≤ 37 s−1 . From these chronoamperograms, sampled current density (at 4.5 s) vs. cathodic potential pulse curves were constructed (j–E). Before every potential pulse, the working electrode was previously coated with zinc in order to simulate the real deposition process; this coating was performed at a constant current density of −10 mA cm−2 , angular velocity of 52 s−1 for 180 s. It is important to mention that the authors decided to construct the polarization curves using sampled current density vs. cathodic potential instead of linear sweep voltammetry technique, because a voltammetric study always involves the competition between the formation rate of deposits at the interface and the polarization rate of the electrode. This last competition is eliminated in the potential pulse technique [16,17]. 2.2.2. Experimental evaluation of the mass transport coefficients in RCE The experimental determination of mass transport coefficients (km ) was achieved by electrolysis experiments at a controlled potential. These experiments were performed in the potential range with a limiting current plateau, which guaranteed the mass transport control of the process. The electrolysis potential was −1.7 V vs. sat. MSE. The studies were made at Reynolds numbers within 15 470 ≤ Re ≤ 123 680 interval, in turbulent flow [1,3,16,17]. It is important to mention that at such turbulent conditions the convection originated from the rotation of the inner cylinder predominates over the convection induced by the gas (oxygen) generated at the counter electrode, which is quickly removed from the counter electrode and electrolyte [16].
A. Recéndiz et al. / Electrochimica Acta 56 (2011) 1455–1459
Fig. 2. Typical sampled current density vs. cathodic potential pulse curves, constructed from typical current transients at sampling time of 4.5 s and at different angular velocities. Electrolyte: 0.019 mol dm−3 Zn(II) in 0.5 mol dm−3 Na2 SO4 at pH = 2. RDE area = 0.20 cm2 . T = 298 K. The inset shows the Levich plot.
Fig. 3. Logarithmic decay of the normalized zinc ions concentration vs. time in the RCE at different Reynolds. Initial composition of electrolyte: 0.019 mol dm−3 Zn(II) in 0.037 mol dm−3 Na2 SO4 at pH = 2. ARCE = 80 cm2 , VR = 350 cm3 , T = 298 K. Electrolysis potential of −1.7 V vs. sat. MSE.
Zn(II) decay is faster as the Re increases; this effect is associated with the deposition of zinc over zinc coating. In this paper authors decide to trace the slopes shown in Fig. 3, from the beginning to the end of the entire electrolysis time, and based on these, the values of the apparent first order rate constants (k = km (ARCE /VR )) and mass transport coefficients were obtained using Eq. (3) [7,13,16,19] and the reactor parameters given in Table 1:
3. Results and discussion 3.1. Determination of electrolysis potential for zinc deposition process Fig. 2 shows the J–E curves of the Zn(II)/Zn(0) process performed in the potential range of −1.8 ≤ E ≤ −1.5 V vs. sat. MSE at different angular velocities of RDE showed inside the figure. These curves were constructed from chronoamperograms obtained at different potential pulses. The potential range where the process is controlled by mass transport is −1.8 ≤ E ≤ −1.7 V vs. sat. MSE. Hence, the electrolysis potential was selected at −1.7 V vs. sat. MSE. From the data showed in Fig. 2, limiting current densities (measured at −1.7 V vs. sat. MSE), were plotted vs. the square root of angular velocity, inset of Fig. 2. Through the Levich equation (Eq. (2)) the zinc ions diffusion coefficient gave a value of 2.92 × 10−6 cm2 s−1 , which is similar to that reported by St. Pierre et al. [9] who reported a value of 2.99 × 10−6 cm2 s−1 , in the system 17 mol dm−3 ZnSO4 ·7H2 O + 1 mol dm−1 H2 SO4 at 295 K. jL = 0.62zFD2/3 −1/6 c0 ω1/2
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(2)
jL is the limiting current density (A cm−2 ), z is the number of electrons involved in the overall electrode reaction (2), F is the Faraday constant (96 485 coulomb mol−1 ), v is the kinematic viscosity (value showed in Table 1), c0 is the initial concentration of Zn(II) (1.9 × 10−5 mol cm−3 ), ω is the angular velocity (s−1 ), and D is the diffusion coefficient in cm2 s−1 .
Ln
ct ARCE = −km t c0 VR
(3)
Mass transport coefficients values were comprised between (9.5 × 10−4 < km < 2.8 × 10−3 ) cm s−1 . These values were in the same magnitude order to those obtained previously by St. Pierre et al. during zinc deposition process [9]. Subsequently, dimensionless correlation (Eq. (1)) was evaluated using the km values obtained for each Reynolds number and the transport properties and reactor parameters shown in Table 1. Fig. 4 shows the mass transport characterization. Table 2 exhibits the experimental values of a, and b from Eq. (1) for the Zn(II)/Zn electrochemical system. The values of a and b are 0.65 and 0.48, respectively. It is important to point out that in the case of deposition at potential such that the current is mass transport limited; the development of roughness means an increase in the apparent first order rate constant, slope of Fig. 3, caused by an increase in the value of effective area [13]. Fig. 5 shows the apparent first order rate constants as a function of Reynolds, in order to elucidate if zinc deposits are roughness; these apparent constants are compared
3.2. Mass transport characterization at RCE Fig. 3 shows the normalized logarithmic decay of the Zn(II) concentration as a function of electrolysis time, where ct is the zinc concentration at any time different of zero. All of the electrolyses were performed at −1.7 V vs. sat. MSE, at different Re shown in the figure. In Fig. 3 it is observed that a t < 1500 s there is not an appreciable improvement of the depletion of zinc ions as a function of convection. It is clear that the new phase formation of zinc on stainless steel is random and has a varying degree and distribution of protuberances. This last is in agreement to that previously reported during the electro winning of zinc from concentrated solutions in sulfuric acid medium [18]. However, at t > 2300 s it is observed that
Fig. 4. Mass transport correlations for the Zn(II)/Zn process, evaluated from the electrolyses similar to those shown in Fig. 3.
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Table 2 Mass transport correlations (Sh = aReb Scc ) in RCE reactor. Electrolytic solution
Inter-electrode gap (R2 − R1 )
Number of counter-electrodes
a
b
Zn2+ Cu2+ Cu2+ Cu2+ Cu2+ Cu2+ I3 − /I−
2.4 2.4 2.4 2.4 2.4 – 2.4
6 4 4 6 – – 4
0.65 0.00081 0.012 0.014 0.022 0.079 0.89
0.48 0.91 0.95 0.91 0.91 0.92 0.43
a
This work [17] [16] [16] [16] a [1,3] [17]
This correlation was obtained in RCE with concentric counter electrode.
with that obtained on a smooth RCE interface developed during the reduction of I3 − [17]. It is important mention that both set of experiments were performed in the same RCE with inter electrode gap of 2.4 cm. From the analyses of apparent rate constants it is clearly observed that this kinetics values are slightly less for zinc deposition than that for triiodide reduction. This slight difference is on the contrary to that expected, for example Gabe and Walsh [13] informed higher values of this parameter (in two magnitude orders) for copper powdery deposits whit regard to that obtained herein. The apparent first order rate constants values obtained here, in connection with the coefficient b = 0.48, confirms that zinc deposits are smooth. These smooth deposits are formed by the moderate leaching of zinc electrodeposits at pH = 2. Current efficiency values, at 80% of zinc recovery, were between 35 and 60%, evidencing the above. On the other hand, the higher values of k for the system I3 − /I− are determined by the higher value of the diffusion coefficient D = 1.12 × 10−5 cm2 s−1 [17], than that of zinc, D = 2.92 × 10−6 cm2 s−1 , and for the moderate leaching of zinc. The coefficient a and exponent b, obtained in this investigation, were also compared in Table 2 with those reported by Nava and coworkers [16,17] (copper deposition and reduction of I3 − on RCE), Walsh and coworkers [1,3] (copper deposition on RCE). The values of a for zinc deposition were less than the obtained in the soluble system, (I3 − /I), and higher than that obtained for copper deposits, these variations are mainly determined by the transport properties of the electrochemical system [17]. The value of b = 0.48, for zinc deposits, was similar than the presented for the triiodide reduction [17], b = 0.43, characteristic of smooth RCE interface as previously discussed, on the contrary to the powdery copper
deposits, which gives values of b ≈ 0.9 [1,3,16,17]. It is important to remark that mass transport correlations for the process Zn(II)/Zn in a RCE are rather limited in the literature. From the analysis of the correlation obtained here in the system Zn(II)/Zn on RCE, it is clearly observed that at electrolysis potential of −1.7 V vs. sat. MSE and Reynolds values comprised between 15 470 ≤ Re ≤ 123 680, in turbulent flow, smooth RCE deposits were obtained. This last put on evidence that mass transport correlation is best evaluated through analysis of experimental data [1,3,12,16], because hydrodynamics on the smooth and roughness RCE interface, among other geometrical parameters and transport properties modify the mass transport. 4. Conclusions Measurements of mass transport on the RCE were performed using the electrolysis technique, employing the zinc deposition. The Zn(II) concentration was 0.019 mol dm−3 ZnSO4 ·7H2 O (1270 ppm Zn(II)) in 0.037 mol dm−3 Na2 SO4 at pH = 2. Based on the analysis of Sh = aReb Sc0.356 correlation, the value of the constant a, associated with shape and cell dimensions, was 0.65; while the value of b, associated with the hydrodynamic regime, was 0.48. The value of a, obtained herein, was higher, in one magnitude order, than those reported in the literature for copper deposition; while for the reduction of triiodide, this value trend tends to invert; these variations are attributed to differences in geometrical parameters of the RCE and diffusion coefficient of electro active species. The value of b = 0.48, for zinc deposits, was similar than the presented for the triiodide reduction, b = 0.43, confirming that zinc deposits are smooth, on the contrary to the powdery copper deposits, which gives values of b ≈ 0.9. These smooth deposits are formed by the moderate leaching of zinc electrodeposits in the acidic medium, pH = 2. For the design purposes of RCE, it is recommendable that the electrochemical engineer should obtain experimentally the corresponding mass transport correlation, because it depends on geometrical parameters, hydrodynamics on the smooth and roughness RCE interface, diffusion coefficient and morphology of the electrodeposits. References
Fig. 5. Apparent first order rate constant as a function of Reynolds for zinc deposits, evaluated from the electrolyses similar to those shown in Fig. 3; these are compared with that obtained on a smooth RCE interface developed during the reduction of I3 − [17].
[1] J. Low, C. Ponce de León, F.C. Walsh, Aust. J. Chem. 58 (2005) 246. [2] F. Rivera, I. González, J.L. Nava, Environ. Technol. 29 (2008) 817. [3] D.R. Gabe, G.D. Wilcox, J. González-Garcia, F.C. Walsh, J. Appl. Electrochem. 28 (1998) 759. [4] F.S. Holland, U.S. Patent 4028199 (1977). [5] F.C. Walsh, Pure Appl. Chem. 73 (2001) 1819. [6] J.M. Grau, J.M. Bisang, J. Chem. Technol. Biotechnol. 77 (2002) 465. [7] F.C. Walsh, in: J.D. Genders, N.L. Weinberg (Eds.), Electrochemistry for a Cleaner Environment. Electrosynthesis, New York, NY, 1992. [8] G. Chen, Sep. Purif. Technol. 38 (2004) 11. [9] J. St. Pierre, N. Masse, E. Frechette, M. Bergeron, J. Appl. Electrochem. 26 (1996) 369. [10] C. Madore, M. Matlosz, D. Landolt, J. Appl. Electrochem. 22 (1992) 1155.
A. Recéndiz et al. / Electrochimica Acta 56 (2011) 1455–1459 [11] [12] [13] [14] [15]
N. Zech, E.J. Podlaha, D. Landolt, J. Appl. Electrochem. 28 (1998) 1251. D.R. Gabe, J. Appl. Electrochem. 4 (1974) 91. D.R. Gabe, F.C. Walsh, J. Appl. Electrochem. 14 (1984) 565. J.M. Grau, J.M. Bisang, J. Appl. Electrochem. 35 (2005) 285. A.H. Nahlé, G.W. Reade, F.C. Walsh, J. Appl. Electrochem. 25 (1995) 450.
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[16] F.F. Rivera, J.L. Nava, Electrochim. Acta 52 (2007) 5868. ˜ Electrochim. Acta [17] F.F. Rivera, J.L. Nava, M.T. Oropeza, A. Recéndiz, G. Carreno, 55 (2010) 3275. [18] A. Recéndiz, I. González, J.L. Nava, Electrochim. Acta 52 (2007) 6880. [19] J.L. Nava, E. Sosa, C. Ponce de León, M.T. Oropeza, Chem. Eng. Sci. 56 (2001) 2695.
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Mass transport studies at rotating cylinder electrode during zinc removal from dilute solutions Alejandro Recéndiz a , Sairi León b , José L. Nava c,∗ , Fernando F. Rivera d a
Dolorey, Centro de Investigación y Desarrollo Tecnológico, Av. Comonfort No. 2050, Torreón, Coahuila, Mexico Universidad Nacional Autónoma de México, Facultad de Estudios Superiores Zaragoza, México D.F., Mexico Universidad de Guanajuato, Departamento de Ingeniería Geomática e Hidráulica, Av. Juárez No. 77, Guanajuato, Gto., Mexico d Universidad Autónoma Metropolitana-Iztapalapa, Departamento de Química, Av. San Rafael Atlixco No. 186, México D.F., Mexico b c
a r t i c l e
i n f o
Article history: Received 25 June 2010 Received in revised form 9 October 2010 Accepted 11 October 2010 Available online 16 October 2010 Keywords: Zinc deposition Mass transport Rotating cylinder electrode Metal ions removal
a b s t r a c t The mass transport in the rotating cylinder electrode (RCE) for the Zn(II) recovery from dilute solutions was investigated. Global mass transport data were obtained by monitoring the (first order) concentration decay of dissolved zinc in sulfate media at pH 2. The electrolyses were performed at holding potential of −1.7 V vs. sat. MSE at Reynolds numbers comprised between 15 470 ≤ Re ≤ 123 680. Based on the analysis of Sh = aReb Sc0.356 correlation, the value of the constant a, associated with shape and cell dimensions, was 0.65; while the constant b, associated with hydrodynamic regime, exhibits a value of 0.48, which obeys to smooth zinc deposits on RCE interface. The mass transport in the Zn(II)/Zn cathode interface process differs with other deposition process, which usually gives roughness deposits. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction The RCE is one of the most common geometries for different types of studies, such as metal ion recovery [1–4], alloy formation [1,3], electrosynthesis [3], corrosion [1,3], effluent treatment [2,5–9] and Hull cell studies [10,11]. RCE’s are particularly well suited for high mass transport studies in the turbulent flow regime [1,12–15]. The mass transport control is imposed by the rotation speed of the inner cylinder and the applied limiting current density. When the fluid flow is generated entirely by an inner RCE, the characteristics of mass transport conditions can be described by a dimensionless group correlation of the following form [1–3,12]: Sh = a Reb Sc0.356
(1)
where the Sherwood number (Sh = km d/D) describes mass transport by forced convection, the Reynolds number (Re = ud/v) is an indication of the fluid flow regime, and the Schmidt number (Sc = v/D) relates the electrolyte transport properties. The average mass transport coefficient is km , d is the diameter of the RCE, D is the diffusion coefficient, u is the peripheral velocity (u = df), v is the kinematic viscosity, and f is the rotational frequency.
∗ Corresponding author. Tel.: +52 4731020100x2289; fax: +52 4731020100x2209. E-mail addresses: [email protected], [email protected] (J.L. Nava). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.10.021
The mass transport correlation is best evaluated through analysis of experimental data [1,3,12,16,17]. There are different correlations that have been experimentally obtained at different types of RCE surfaces (smooth, knurled, longitudinal fins, wrapped with cylindrical wire, expanded metal and metal powder deposit) and broadly described and discussed in two reviews [1,3]. The experimental constant a, is associated with the electrode geometry, shape, cell dimensions [1,3,16] as well as with the diffusion coefficient [17]; whereas b is associated with the hydrodynamic regime [1,3,13,16], on the smooth and roughness RCE interface (given by the nature of reduction reaction) [13,17]. Mass transport studies at RCE in turbulent conditions, Re > 100, have been performed during copper deposition [4,13,16,17], giving powdery deposits and roughness surfaces, resulting in values of b ≈ 0.9. On the other hand, in a previous communication carried out by our group, for smooth RCE in the system I3 − /I− , gave values of b ≈ 0.4 [17], which differs for the obtained analytically, b = 0.66 [12]. These differences of b were discussed in [17], emphasizing that, hydrodynamics in the smooth and rough RCE interface determine this value; while the differences of a values are determined by transport properties of electrolyte. This paper focuses on the mass transport in the RCE for the zinc removal from dilute solutions. Global mass transport data were obtained by monitoring the (first order) concentration decay of zinc ions during electrolysis experiments in the RCE. Complementary rotating disks experiments to determine the diffusion coefficient were performed. The global mass transport properties
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Table 1 RCE reactor parameters and electrolyte properties. Reaction Volume, VR Reactor diameter RCE diameter, d RCE length RCE area, ARCE (in contact with electrolyte) Length and width of the plates used as anodes (attached to the reactor walls) Inter-electrode gap Counter electrode area, ACE (six plates, in contact with the solution) Diffusion coefficient, D Kinematics viscosity,
350 cm3 8.5 cm 3.8 cm 11 cm 80 cm2 13 cm × 2 cm 2.4 cm 84 cm2 2.92 × 10−6 cm2 s−1 0.01 cm2 s−1
obtained herein were compared with those reported in the literature. 2. Experimental The model solution used in this work was prepared of analytic grade reactants and deionised water Milli-QTM . For the electrolysis test in the RCE the concentration was 0.019 mol dm−3 ZnSO4 ·7H2 O (1270 ppm Zn(II)) in 0.037 mol dm−3 Na2 SO4 at pH = 2. The resulting solution exhibited a conductivity of 97.4 mS cm−1 . It is important to point out that such solution was prepared trying to resemble the amount of zinc ions, conductivity and pH of a typical composition contained in an effluent generated by a plating industry. On the other hand, for rotating disk experiments the same solution, varying only the concentration of sodium sulfate (0.5 mol dm−3 ) to obtain a supporting electrolyte in excess, was prepared. Each electrolytic solution was deoxygenated with nitrogen for about 600 s before each experiment. 2.1. Equipment A potentiostat–galvanostat EG&G model PAR 273 with M270 software was used for the microelectrolysis experiments. For the experiments on RCE, a KepcoTM power supply of 10 A and 20 V capacity was used. The Zn(II) concentrations were determined employing a VarianTM Atomic Absorption Spectrophotometer Model 220 FS. 2.1.1. Microelectrolysis experiments A 0.1 dm3 Pyrex electrochemical cell, with a three-electrode system and nitrogen inlet, was used for the microelectrolysis experiments. The rotating disk electrode (RDE) was a 316 stainless-steel disk with a geometric area of 0.20 cm2 . The electrode surface was polished with a 0.3 m alumina powder/water mixture and rinsed with distilled water, followed by 300 s of ultrasonic bath and a final rinse with distilled water in each experiment. To control the RDE angular velocity, a Radiometer AnalyticalTM velocity controller, model CTV 101, was used. The potentials were measured vs. a saturated mercurous sulfate electrode (sat. MSE), 0.615 vs. SHE/V, Radiometer AnalyticalTM model XR200, and the counter electrode was a graphite rod. 2.1.2. Electrolysis in RCE Fig. 1 shows the diagram of a device fabricated for the laboratory studies, consisting of a 500-cm3 glass reactor with a temperature bath. A 316-type stainless steel cylinder with a 3.8 cm diameter and a length of 11 cm was used as a cathode. As anodes were used six 13 cm long, 2 cm wide and 0.3 cm thick RuO2 /TiO2 DSA which were attached to the reactor walls and connected between each other. The RuO2 /TiO2 DSA were supplied by De NoraTM . CaframoTM model BDC3030 electric motor of variable velocity was used to rotate the inner cylinder. The potentials were measured vs. sat. MSE. Table 1
Fig. 1. Rotating cylinder electrode reactor scheme.
shows the parameters of the RCE cell used in this work and the electrolyte transport properties. 2.2. Methodology 2.2.1. Determination of electrolysis potential for zinc deposition process A series of potential pulses was applied to the RDE, using the cell described in Section 2.1.1, in the potential range of −1.8 ≤ E ≤ −1.5 V vs. sat. MSE during 10 s, with increments of 0.05 V. The corresponding chronoamperograms were obtained at different angular velocities between 16 ≤ ω ≤ 37 s−1 . From these chronoamperograms, sampled current density (at 4.5 s) vs. cathodic potential pulse curves were constructed (j–E). Before every potential pulse, the working electrode was previously coated with zinc in order to simulate the real deposition process; this coating was performed at a constant current density of −10 mA cm−2 , angular velocity of 52 s−1 for 180 s. It is important to mention that the authors decided to construct the polarization curves using sampled current density vs. cathodic potential instead of linear sweep voltammetry technique, because a voltammetric study always involves the competition between the formation rate of deposits at the interface and the polarization rate of the electrode. This last competition is eliminated in the potential pulse technique [16,17]. 2.2.2. Experimental evaluation of the mass transport coefficients in RCE The experimental determination of mass transport coefficients (km ) was achieved by electrolysis experiments at a controlled potential. These experiments were performed in the potential range with a limiting current plateau, which guaranteed the mass transport control of the process. The electrolysis potential was −1.7 V vs. sat. MSE. The studies were made at Reynolds numbers within 15 470 ≤ Re ≤ 123 680 interval, in turbulent flow [1,3,16,17]. It is important to mention that at such turbulent conditions the convection originated from the rotation of the inner cylinder predominates over the convection induced by the gas (oxygen) generated at the counter electrode, which is quickly removed from the counter electrode and electrolyte [16].
A. Recéndiz et al. / Electrochimica Acta 56 (2011) 1455–1459
Fig. 2. Typical sampled current density vs. cathodic potential pulse curves, constructed from typical current transients at sampling time of 4.5 s and at different angular velocities. Electrolyte: 0.019 mol dm−3 Zn(II) in 0.5 mol dm−3 Na2 SO4 at pH = 2. RDE area = 0.20 cm2 . T = 298 K. The inset shows the Levich plot.
Fig. 3. Logarithmic decay of the normalized zinc ions concentration vs. time in the RCE at different Reynolds. Initial composition of electrolyte: 0.019 mol dm−3 Zn(II) in 0.037 mol dm−3 Na2 SO4 at pH = 2. ARCE = 80 cm2 , VR = 350 cm3 , T = 298 K. Electrolysis potential of −1.7 V vs. sat. MSE.
Zn(II) decay is faster as the Re increases; this effect is associated with the deposition of zinc over zinc coating. In this paper authors decide to trace the slopes shown in Fig. 3, from the beginning to the end of the entire electrolysis time, and based on these, the values of the apparent first order rate constants (k = km (ARCE /VR )) and mass transport coefficients were obtained using Eq. (3) [7,13,16,19] and the reactor parameters given in Table 1:
3. Results and discussion 3.1. Determination of electrolysis potential for zinc deposition process Fig. 2 shows the J–E curves of the Zn(II)/Zn(0) process performed in the potential range of −1.8 ≤ E ≤ −1.5 V vs. sat. MSE at different angular velocities of RDE showed inside the figure. These curves were constructed from chronoamperograms obtained at different potential pulses. The potential range where the process is controlled by mass transport is −1.8 ≤ E ≤ −1.7 V vs. sat. MSE. Hence, the electrolysis potential was selected at −1.7 V vs. sat. MSE. From the data showed in Fig. 2, limiting current densities (measured at −1.7 V vs. sat. MSE), were plotted vs. the square root of angular velocity, inset of Fig. 2. Through the Levich equation (Eq. (2)) the zinc ions diffusion coefficient gave a value of 2.92 × 10−6 cm2 s−1 , which is similar to that reported by St. Pierre et al. [9] who reported a value of 2.99 × 10−6 cm2 s−1 , in the system 17 mol dm−3 ZnSO4 ·7H2 O + 1 mol dm−1 H2 SO4 at 295 K. jL = 0.62zFD2/3 −1/6 c0 ω1/2
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(2)
jL is the limiting current density (A cm−2 ), z is the number of electrons involved in the overall electrode reaction (2), F is the Faraday constant (96 485 coulomb mol−1 ), v is the kinematic viscosity (value showed in Table 1), c0 is the initial concentration of Zn(II) (1.9 × 10−5 mol cm−3 ), ω is the angular velocity (s−1 ), and D is the diffusion coefficient in cm2 s−1 .
Ln
ct ARCE = −km t c0 VR
(3)
Mass transport coefficients values were comprised between (9.5 × 10−4 < km < 2.8 × 10−3 ) cm s−1 . These values were in the same magnitude order to those obtained previously by St. Pierre et al. during zinc deposition process [9]. Subsequently, dimensionless correlation (Eq. (1)) was evaluated using the km values obtained for each Reynolds number and the transport properties and reactor parameters shown in Table 1. Fig. 4 shows the mass transport characterization. Table 2 exhibits the experimental values of a, and b from Eq. (1) for the Zn(II)/Zn electrochemical system. The values of a and b are 0.65 and 0.48, respectively. It is important to point out that in the case of deposition at potential such that the current is mass transport limited; the development of roughness means an increase in the apparent first order rate constant, slope of Fig. 3, caused by an increase in the value of effective area [13]. Fig. 5 shows the apparent first order rate constants as a function of Reynolds, in order to elucidate if zinc deposits are roughness; these apparent constants are compared
3.2. Mass transport characterization at RCE Fig. 3 shows the normalized logarithmic decay of the Zn(II) concentration as a function of electrolysis time, where ct is the zinc concentration at any time different of zero. All of the electrolyses were performed at −1.7 V vs. sat. MSE, at different Re shown in the figure. In Fig. 3 it is observed that a t < 1500 s there is not an appreciable improvement of the depletion of zinc ions as a function of convection. It is clear that the new phase formation of zinc on stainless steel is random and has a varying degree and distribution of protuberances. This last is in agreement to that previously reported during the electro winning of zinc from concentrated solutions in sulfuric acid medium [18]. However, at t > 2300 s it is observed that
Fig. 4. Mass transport correlations for the Zn(II)/Zn process, evaluated from the electrolyses similar to those shown in Fig. 3.
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Table 2 Mass transport correlations (Sh = aReb Scc ) in RCE reactor. Electrolytic solution
Inter-electrode gap (R2 − R1 )
Number of counter-electrodes
a
b
Zn2+ Cu2+ Cu2+ Cu2+ Cu2+ Cu2+ I3 − /I−
2.4 2.4 2.4 2.4 2.4 – 2.4
6 4 4 6 – – 4
0.65 0.00081 0.012 0.014 0.022 0.079 0.89
0.48 0.91 0.95 0.91 0.91 0.92 0.43
a
This work [17] [16] [16] [16] a [1,3] [17]
This correlation was obtained in RCE with concentric counter electrode.
with that obtained on a smooth RCE interface developed during the reduction of I3 − [17]. It is important mention that both set of experiments were performed in the same RCE with inter electrode gap of 2.4 cm. From the analyses of apparent rate constants it is clearly observed that this kinetics values are slightly less for zinc deposition than that for triiodide reduction. This slight difference is on the contrary to that expected, for example Gabe and Walsh [13] informed higher values of this parameter (in two magnitude orders) for copper powdery deposits whit regard to that obtained herein. The apparent first order rate constants values obtained here, in connection with the coefficient b = 0.48, confirms that zinc deposits are smooth. These smooth deposits are formed by the moderate leaching of zinc electrodeposits at pH = 2. Current efficiency values, at 80% of zinc recovery, were between 35 and 60%, evidencing the above. On the other hand, the higher values of k for the system I3 − /I− are determined by the higher value of the diffusion coefficient D = 1.12 × 10−5 cm2 s−1 [17], than that of zinc, D = 2.92 × 10−6 cm2 s−1 , and for the moderate leaching of zinc. The coefficient a and exponent b, obtained in this investigation, were also compared in Table 2 with those reported by Nava and coworkers [16,17] (copper deposition and reduction of I3 − on RCE), Walsh and coworkers [1,3] (copper deposition on RCE). The values of a for zinc deposition were less than the obtained in the soluble system, (I3 − /I), and higher than that obtained for copper deposits, these variations are mainly determined by the transport properties of the electrochemical system [17]. The value of b = 0.48, for zinc deposits, was similar than the presented for the triiodide reduction [17], b = 0.43, characteristic of smooth RCE interface as previously discussed, on the contrary to the powdery copper
deposits, which gives values of b ≈ 0.9 [1,3,16,17]. It is important to remark that mass transport correlations for the process Zn(II)/Zn in a RCE are rather limited in the literature. From the analysis of the correlation obtained here in the system Zn(II)/Zn on RCE, it is clearly observed that at electrolysis potential of −1.7 V vs. sat. MSE and Reynolds values comprised between 15 470 ≤ Re ≤ 123 680, in turbulent flow, smooth RCE deposits were obtained. This last put on evidence that mass transport correlation is best evaluated through analysis of experimental data [1,3,12,16], because hydrodynamics on the smooth and roughness RCE interface, among other geometrical parameters and transport properties modify the mass transport. 4. Conclusions Measurements of mass transport on the RCE were performed using the electrolysis technique, employing the zinc deposition. The Zn(II) concentration was 0.019 mol dm−3 ZnSO4 ·7H2 O (1270 ppm Zn(II)) in 0.037 mol dm−3 Na2 SO4 at pH = 2. Based on the analysis of Sh = aReb Sc0.356 correlation, the value of the constant a, associated with shape and cell dimensions, was 0.65; while the value of b, associated with the hydrodynamic regime, was 0.48. The value of a, obtained herein, was higher, in one magnitude order, than those reported in the literature for copper deposition; while for the reduction of triiodide, this value trend tends to invert; these variations are attributed to differences in geometrical parameters of the RCE and diffusion coefficient of electro active species. The value of b = 0.48, for zinc deposits, was similar than the presented for the triiodide reduction, b = 0.43, confirming that zinc deposits are smooth, on the contrary to the powdery copper deposits, which gives values of b ≈ 0.9. These smooth deposits are formed by the moderate leaching of zinc electrodeposits in the acidic medium, pH = 2. For the design purposes of RCE, it is recommendable that the electrochemical engineer should obtain experimentally the corresponding mass transport correlation, because it depends on geometrical parameters, hydrodynamics on the smooth and roughness RCE interface, diffusion coefficient and morphology of the electrodeposits. References
Fig. 5. Apparent first order rate constant as a function of Reynolds for zinc deposits, evaluated from the electrolyses similar to those shown in Fig. 3; these are compared with that obtained on a smooth RCE interface developed during the reduction of I3 − [17].
[1] J. Low, C. Ponce de León, F.C. Walsh, Aust. J. Chem. 58 (2005) 246. [2] F. Rivera, I. González, J.L. Nava, Environ. Technol. 29 (2008) 817. [3] D.R. Gabe, G.D. Wilcox, J. González-Garcia, F.C. Walsh, J. Appl. Electrochem. 28 (1998) 759. [4] F.S. Holland, U.S. Patent 4028199 (1977). [5] F.C. Walsh, Pure Appl. Chem. 73 (2001) 1819. [6] J.M. Grau, J.M. Bisang, J. Chem. Technol. Biotechnol. 77 (2002) 465. [7] F.C. Walsh, in: J.D. Genders, N.L. Weinberg (Eds.), Electrochemistry for a Cleaner Environment. Electrosynthesis, New York, NY, 1992. [8] G. Chen, Sep. Purif. Technol. 38 (2004) 11. [9] J. St. Pierre, N. Masse, E. Frechette, M. Bergeron, J. Appl. Electrochem. 26 (1996) 369. [10] C. Madore, M. Matlosz, D. Landolt, J. Appl. Electrochem. 22 (1992) 1155.
A. Recéndiz et al. / Electrochimica Acta 56 (2011) 1455–1459 [11] [12] [13] [14] [15]
N. Zech, E.J. Podlaha, D. Landolt, J. Appl. Electrochem. 28 (1998) 1251. D.R. Gabe, J. Appl. Electrochem. 4 (1974) 91. D.R. Gabe, F.C. Walsh, J. Appl. Electrochem. 14 (1984) 565. J.M. Grau, J.M. Bisang, J. Appl. Electrochem. 35 (2005) 285. A.H. Nahlé, G.W. Reade, F.C. Walsh, J. Appl. Electrochem. 25 (1995) 450.
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[16] F.F. Rivera, J.L. Nava, Electrochim. Acta 52 (2007) 5868. ˜ Electrochim. Acta [17] F.F. Rivera, J.L. Nava, M.T. Oropeza, A. Recéndiz, G. Carreno, 55 (2010) 3275. [18] A. Recéndiz, I. González, J.L. Nava, Electrochim. Acta 52 (2007) 6880. [19] J.L. Nava, E. Sosa, C. Ponce de León, M.T. Oropeza, Chem. Eng. Sci. 56 (2001) 2695.