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Chronopotentiometric study on the effect of boric acid in the nickel transport properties through a cation-exchange membrane
Desalination 249 (2009) 348–352
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Chronopotentiometric study on the effect of boric acid in the nickel transport properties through a cation-exchange membrane Luciano Marder a,b, Emma M. Ortega Navarro b, Valentín Pérez-Herranz b,⁎, Andréa Moura Bernardes a, Jane Zoppas Ferreira a a b
LACOR, PPGEM, Universidade Federal do Rio Grande do Sul, Campus do Vale, Setor 4, Prédio 74, sala 109, 91501-970, Porto Alegre, RS, Brazil Departamento de Ingeniería Química y Nuclear, Universidad Politécnica de Valencia, P.O. Box 22012, Valencia-46071, Spain
a r t i c l e
i n f o
Article history: Accepted 20 June 2009 Available online 2 October 2009 Keywords: Chronopotentiometry Nickel Boric acid Cation-exchange membrane
a b s t r a c t In this work the effect of boric acid on the nickel transport properties through an IONICS 67-HMR-412 cation-exchange membrane was evaluated by means of chronopotentiometry. Nickel chloride solutions with and without boric acid were investigated. The obtained results showed that the addition of boric acid to a nickel chloride solution does not affect the limiting current density value nor the nickel transport number through the membrane. The obtained results also revealed that, under very high current densities, the secondary transfer phenomenon associated to water splitting occurs when nickel chloride solutions are employed, and a bipolar membrane structure is formed, resulting from a Ni(OH)2 precipitation on the membrane surface. The addition of boric acid to the nickel chloride solution can minimize or avoid this phenomenon depending on the boric acid concentration in the solution. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Electrodialysis, a technique that uses ion-exchange membranes, has been suggested as promising possibilities to remove and recover metal ions from electroplating wastewaters [1–3]. To assure a good performance of this process, it is useful to know the transport number of the ions present in the solutions to be treated through the ionexchange membranes [4]. It is also important to respect some specific conditions of the electrochemical process, such as the limiting current density [5]. These parameters are important to take decisions regarding to which membrane is the most appropriate and which is the maximum current density that can be used for a given application. If in an electrodialysis process the applied current density exceeds the limiting one, operational problems such as the inorganic salts precipitation on the membrane surface, destruction of the membranes and an increase in the energy consumption can occur, decreasing the process efficiency [6]. In a previous work [7], the limiting current density value and the transport number of some transition metal ions through an IONICS 67-HMR-412 cation-exchange membrane were determined by chronopotentiometry. The present work describes a further study carried out using the same electrochemical characterization method. The aim was to investigate the effect of boric acid in the nickel transport properties through an IONICS 67-HMR-412 cation-exchange membrane. Boric
acid is a common additive of nickel electroplating baths and, associated with this metal ion, will be present in the wastewaters generated in the electroplating processes. 2. Experimental 2.1. Solutions The study was carried out with 0.025 M NiCl2 · 6H2O solutions with different H3BO3 concentrations (varying between 0.01 and 0.025 M). Besides, a solution containing only 0.025 M NiCl2 · 6H2O was also evaluated. The solutions were prepared dissolving the respective salts in distilled water. Analytical grade reactives were employed. The pH of all solutions was 4, adjusted with a 0.1 M HCl solution. 2.2. Ion-exchange membrane The ion-exchange membrane used in this work was the IONICS 67-HMR-412 cation-exchange membrane. This is a homogenous acrylic fiber-backed membrane containing sulfonic acid groups as fixed charges [8]. Some of the main properties and characteristics of this membrane are given in Table 1. 2.3. Electrochemical cell and chronopotentiometric measurements
⁎ Corresponding author. Tel.: + 34 96 3877632; fax: + 34 96 3877639. E-mail address: [email protected] (V. Pérez-Herranz). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.06.040
The chronopotentiometric experiments were accomplished using a two-compartment cell. This electrochemical cell was composed of
L. Marder et al. / Desalination 249 (2009) 348–352 Table 1 Properties and characteristics of the IONICS 67-HMR-412 cation-exchange membrane [8]. Properties and characteristics
Value
Thickness (mm) Exchange capacity (meq/g dry resin) Water content (% of wet resin only) Na+ transport number (in 0.01 M NaCl) Effective parameter εa
0.56–0.58 2.10 46 0.99 0.935
a
Our data (determined by chronopotentiometry in 0.025 M NaCl) [7].
two 250 mL symmetrical half-cells between which the cationexchange membrane was clamped. The exposed membrane area was 11.95 cm2. Two Ag/AgCl electrodes (obtained by anodic chloride oxidation of silver sheets in 0.1 M HCl solution) were used to impose the current density. Two Ag/AgCl reference electrodes immersed in Luggin capillaries were used to measure the potential drop through the membrane. The membrane potential response to each current density change was recorded (typically 120 s). The constant current density was supplied by a Tacussel Electronique PJT 120-1 potentiostat/galvanostat. The membrane potential drop and the imposed current density were registered using a PC data acquisition system. Current densities between 0.42 and 9.21 mA cm− 2 were applied. All experiments were conducted at room temperature and without stirring. The experiments were accomplished after a membrane equilibration period of at least 24h with a solution with the same characteristics as the ones used in the experiments. 2.3.1. Limiting current density The limiting current density was determined from the current– voltage curves reconstituted from the steady-state membrane potential drop (obtained from the chronopotentiometric curves) corresponding to an applied current density [9–11]. 2.3.2. Nickel transport number through the membrane The nickel transport number through the membrane was determined from the modified Sand's equation (Eq. (1) proposed by Choi et al. [12]:
τ=
2 zj F ε πD 4 tjm tjs
!2 C0 2 i
ð1Þ
where τ is the transition time (s), D is the salt diffusion coefficient in the solution (cm2 s− 1), zj is the charge of the counter-ion, F is the Faraday's constant (96,500 C mol− 1), C0 is the salt concentration in the solution (mol L− 1), i is the current density (A cm− 2), ε is the s fraction of the conducting region of the membrane, and tm j and tj are the counter-ion transport numbers in the membrane and in the solution, respectively. Eq. (1) is used for heterogeneous ion-exchange membranes. However, Eq. (1) can be applied for homogeneous ionexchange membranes, as IONICS 67-HMR-412 cation-exchange membrane, if the coefficient ε is considered as an effective parameter regulating some differences occurring between an ideal system and the real one. Once the value of this coefficient is found for a system where the transport numbers are well known (Table 1 [7]), the Eq. (1) may be applied in other conditions with the same ε for determining the unknown transport numbers. According to Eq. (1), if the salt diffusion coefficient in the solution, the transport number of the counter-ion in the solution and the effective parameter ε are known, the nickel transport number through the membrane can be obtained by the straight line slope between the transitions times (obtained from the chronopotentiometric curves at current densities above the limiting current density) and (C0/i)2. The transition time corresponds to a sharp increase in the membrane potential drop due to the concentration decrease in the depleting solution near the membrane/solution interface and can be deter-
349
mined from the maximum of the derivate of the membrane potential drop regarding to time [13,14]. 3. Results and discussion 3.1. Limiting current density and nickel transport number Fig. 1 shows typical chronopotentiometric curves obtained for a 0.025 M NiCl2 + 0.015 M H3BO3 solution under current densities higher than the limiting current density. As it can be observed, the chronopotentiometric curves are typical for a monopolar membrane. After an instantaneous increase in the membrane potential drop, which is due to the ohmic resistance of the system, the membrane potential drop rises slowly until the inflection point that defines the transition time. Thereafter, the potential growth rate diminishes and the membrane potential drop reaches a certain steady-state value or a quasi-steady-state value. When the current density is switched-off, the membrane potential decreases rapidly to low values due to the relaxation of the concentration profiles in the solution boundary layer [16]. It can also be observed that the transition times decrease with the increase in the current density. This is a consequence of the faster ion depletion in the membrane/solution interface. The effect of boric acid on the chronopotentiometric response of the membrane can be seen in Fig. 2 for a current density of 5.19 mA cm− 2 and a 0.025 M NiCl2 solution at different concentrations of boric acid. As it can be observed in Fig. 2, under the same applied current density, the transition times are closely similar for all the evaluated solutions. However, the difference between the ohmic potential drop over the polarized membrane system (Δφp) and the non-polarized one (Δφnp) varies with the addition of boric acid to the nickel chloride solution. According to Pismenskaya et al. [13] this difference (Δφp − Δφnp) shows the increment in the ohmic potential drop over the membrane system due to the concentration polarization. The main contribution in this increment is given by the diluted diffusion boundary layer (DBL). In the case of the nickel chloride solution without boric acid, the value of Δφnp − Δφp is 0.70 V. In the case of the nickel chloride solutions with boric acid, this difference is 0.46 V, and is independent on the boric acid concentration. Hence the resistance of the polarized DBL near the membrane in contact with nickel chloride without boric acid is slightly higher than when the membrane is in contact with the nickel chloride solutions with boric acid. From the steady-state membrane potential drop, corresponding to an applied current density in the chronopotentiometric curves, the current–voltage curves can be reconstituted and the limiting current density can be determined by the intersection of the two slopes
Fig. 1. Chronopotentiometric curves obtained for the 0.025 M NiCl2 + 0.015 M H3BO3 solution under current densities above the limiting current density.
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Fig. 2. Chronopotentiometric curves obtained for the NiCl2–H3BO3 solutions under a current density of 5.19 mA cm− 2.
belonging to the ohmic and plateau regions, as shown in Fig. 3 for a 0.025 M NiCl2 solution with different concentrations of boric acid. The obtained results show that the addition of boric acid to the nickel chloride solution does not affect the limiting current density value. Limiting current densities of around 3.26 mA cm2 were obtained. On the other hand, the higher resistance of the DBL to the nickel chloride solution without boric acid in comparison to solutions with boric acid, that was previously commented, can be noted in the plateau region of the reconstituted current–voltages curves presented in Fig. 3. The membrane potential drop obtained to an equivalent current density in this current–voltage curve region is relatively higher for the nickel chloride solution without boric acid. In the ohmic region the resistance should be similar for all the evaluated solutions. From the chronopotentiograms of Figs. 1 and 2, the transition times were determined and employed in the τ versus (C0/i)2 plot for the determination of the transport numbers as shown in Fig. 4. These plots were obtained assuming as C0 the nickel chloride concentration in the solution (0.025 M). It can be observed that good linear relationships were obtained between τ and (C0/i)2 for a significant range of current densities above the limiting one for all the evaluated solutions, and that the obtained straight line slopes, which are presented in Table 2, are closely similar. From the straight line slopes of the representation of τ versus (C0/i)2, the nickel transport numbers through the membrane were obtained applying Eq. (1), where the salt diffusion coefficient and counter-ion transport number in the solution, used to calculate the nickel transport number through the
Fig. 4. Transition time as a function of (C0/i)2 for the IONICS 67-HMR-412 cation-exchange membrane in contact with NiCl2–H3BO3 solutions (slope in s(Acm− 2 L)2 mol− 2).
membrane, correspond to the nickel chloride data. In this way, the NiCl2 diffusion coefficient in the solution used was 1.2 × 10− 5 cm2 s− 1 and the nickel transport number in the solution was 0.394. The NiCl2 diffusion coefficient data was obtained from the equivalent conductivity data at infinite dilution using the Nernst–Einstein equation. The nickel transport number in the solution was also calculated by employing the equivalent conductivity data [15]. The values of the calculated nickel transport number through the IONICS 67-HMR-412 cation-exchange membrane are presented in Table 2, and it can be concluded that the addition of boric acid to the solution does not affect the nickel transport number through membrane and values of approximately 0.9 were obtained. The deviation of nickel transport number from 1 can be due to chloride transport through the membrane since it is a small anion with a high mobility and it can penetrate into the membrane causing the Donnan's exclusion failure. The non influence of boric acid on the limiting current density value and on the nickel transport number through the membrane should be associated to the boric acid characteristics in aqueous solutions, showed in Table 3 [17]. As it can be observed in Fig. 5, drawn from Table 3 data, in diluted aqueous solutions containing nickel chloride and boric acid, besides the nickel species, the other predominant species are B(OH)3 and H2BO− 3 in acid and alkaline regions, respectively [17]. As at pH 4 boric acid should be under B (OH)3 form, no other positively charged ionic species should be transported with nickel ions through the membrane. In this way the limiting current density value and the nickel transport number through the membrane presented similar values, independently of the presence or not of boric acid in the nickel chloride solution. These properties of boric acid can be clearly observed when chronopotentiometric curves are obtained under a same current density. As it can be observed in Fig. 2, the chronopotentiometric curves, obtained for the nickel chloride solutions with different boric acid concentrations, show equivalent transitions times as the nickel chloride solution without boric acid, when a same current density is applied. Since the addition of boric acid to the nickel chloride solution does not cause an increase in the concentration of positively charged species in the Table 2 Nickel transport numbers through the IONICS 67-HMR-412 cation-exchange membrane in contact with NiCl2–H3BO3 solutions. Solution
Fig. 3. Current–voltage curves of the IONICS 67-HMR-412 cation-exchange membrane in contact with NiCl2–H3BO3 solutions obtained from the steady-state membrane potential drop of the chronopotentiometric curves.
0.025 M NiCl2 0.025 M NiCl2 + 0.010 M 0.025 M NiCl2 + 0.015 M 0.025 M NiCl2 + 0.020 M 0.025 M NiCl2 + 0.025 M
H3BO3 H3BO3 H3BO3 H3BO3
pH
Straight line slope (s(Acm− 2 L)2 mol− 2)
tm j
4 4 4 4 4
1.217 1.225 1.222 1.219 1.216
0.896 0.895 0.895 0.896 0.896
L. Marder et al. / Desalination 249 (2009) 348–352
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Table 3 Thermodynamic constants the NiCl2–H3BO3 systems [17]. Equilibrium
Constant
B(OH)3 ↔ (H2BO3)− + H+ Ni2+ + 2 H2O ↔ Ni(OH)2 + 2 H+ Ni2+ + Cl− ↔ NiCl+ NiCl2 ↔ Ni2+ + 2 Cl−
5.8 × 10− 10 3.2 × 10− 11 2.5 2.4 × 103
solution, the required time to produce the ion depletion in the membrane/solution interface, should be independent of the presence or not of boric acid in the solution. 3.2. Transport phenomena at higher currents densities In a previous work [7] it could be observed that when nickel and cobalt chloride solutions were employed, the chronopotentiometric curves obtained at over-limiting currents showed an increment in the membrane potential drop after an apparent stabilization period, as it can be observed schematically in Fig. 6 (full line). This behavior coincided with the formation of a Ni(OH)2 or Co(OH)2 precipitate on the anodic side of the membrane surface. The formed precipitate increased the membrane resistance and caused the increase in the membrane potential drop. This effect should be a consequence of the water splitting phenomenon in the anodic membrane/solution interface (H2 O ↔ H+ + OH−). The generated H+ ions were transported through the cation-exchange membrane toward the cathodic cell compartment, meanwhile the OH− ions combine with the nickel or cobalt ions, forming the observed metal hydroxide precipitate (Mez+ + zOH− ↔ Me(OH)z↓). The last section of these chronopotentiometric curves (full line), when the current was switched-off, also showed a different pattern of a classical monopolar chronopotentiometric curve (dashed line). Instead of the fast decay of the membrane potential to low values (close to zero), common for monopolar membranes, when nickel and cobalt chloride solutions were employed, immediately after switching-off the over-limiting current density, the membrane potential dropped to a plateau-value that extended over time until reaching values close to zero. This behavior was similar to that found by Wilhelm et al. [18] for bipolar membranes and is related to the hindered recombination of hydroxide ions and protons, resulted from water splitting in the bipolar membrane junction. Hence it could be concluded in this previous work that under high over-limiting currents a water splitting phenomenon occurred when nickel and cobalt chloride solutions were employed and a bipolar layer was formed by the metal hydroxide and the membrane. A similar behavior was found by Kang et al. [19] studying the electrochemical behavior of ion-exchange membranes coated with iron hydroxide, by
Fig. 5. Speciation of the ionic species in a 0.025 M NiCl2 + 0.015 M H3BO3 solution [17].
Fig. 6. Schematic chronopotentiometric curve obtained at high over-limiting current densities for nickel and cobalt chloride solutions in contact with the IONICS 67-HMR412 cation-exchange membrane (full line). Dashed line corresponds to a classical shape of a chronopotentiometric curve for a monopolar membrane [7].
Jialin et al. [20] when a layer of Mg(OH)2 is formed on the surface of a cation-exchange membrane and by Choi et al. [10] when an Al(OH)3 layer is formed on the surface of a cation-exchange membrane. The results obtained in the present work show that the secondary transfer phenomenon of water splitting, which leads up to the precipitation of Ni(OH)2 on the membrane surface and the bipolar behavior, can be minimized or avoided depending on the boric acid concentration in the solution. As shown in Fig. 7, for the 0.025 M NiCl2 solution, the increase in the membrane potential drop, as well as the bipolar behavior, starts to appear around a current density of 5.52 mA cm− 2, approximately 1.7 times the limiting current density. Adding 0.01 M H3BO3 to the nickel chloride solution, this behavior appears at higher current densities, around 7.87 mA cm− 2, approximately 2.4 times the limiting current density, as it can be observed in Fig. 8. For more concentrated solutions in boric acid (0.015–0.025 M) this behavior is not observed until a current density of 9.21 mA cm− 2, approximately 2.8 times the limiting current density, as it can be observed in Fig. 9. The increase in the membrane resistance, resulted from the formed Ni(OH)2 precipitate on the membrane surface when 0.025 M NiCl2 and 0.025 M NiCl2 + 0.01 M H3BO3 solutions are employed, can be observed in the reconstituted current–voltage curves presented previously in Fig. 3. When these solutions are employed a second
Fig. 7. Chronopotentiometric curves obtained for the 0.025 M NiCl2 solution under high current densities.
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Under higher current densities a water splitting phenomenon occurs, when nickel chloride solutions are employed, and a bipolar structure is formed, resulted from a nickel hydroxide precipitation on membrane surface. The boric acid addition to the nickel chloride solution can minimize or avoid this phenomenon, depending on the boric acid concentration in the solution. This behavior should be associated to the buffer properties of boric acid in solution. Acknowledgements The authors wish to express our gratitude for the support of this work by the CAPES, Brazil (Ref. no. 080/04) and the Ministerio de Ciencia y Tecnología, Spain (Ref. no. PHB2003-0093-PC). Luciano Marder is grateful to the CAPES (BEX 0547/04-02) and to CNPq (140417/2003-06) for a postgraduate grant. Fig. 8. Chronopotentiometric curves obtained for the 0.025 M NiCl2 + 0.01 M H3BO3 solution under high current densities.
plateau is observed in the over-limiting current region, just when the nickel hydroxide precipitate should be formed. For the other investigated solutions, where the nickel hydroxide precipitate is not observed, the current–voltage curves show a classical pattern of a monopolar ion-exchange membrane. The influence of boric acid on the water splitting and membrane scaling phenomena should be associated to the buffer properties of boric acid [21,22]. Once in solution, boric acid should be dissociated + according the following reaction: H3BO3 ↔ H2BO− [23]. This 3 +H behavior leads up to a supplying of protons in the diffusion boundary layer at current densities where the water splitting becomes possible. This fact delays the pH increase in the membrane/solution interface associated with the onset of water splitting. Because of the supplying of additional protons from the boric acid dissociation, the current densities for the water splitting should be extended to higher current density values. 4. Conclusions Chronopotentiometry is a suitable technique to obtain information regarding the boric acid influence on nickel transport properties through the IONICS 67-HMR-412 cation-exchange membrane. The addition of boric acid to a nickel chloride solution does not affect the limiting current density value nor the nickel transport number through the membrane. This behavior should be associated to the boric acid properties in aqueous solutions.
References [1] N. Tzanetakis, W.M. Taama, K. Scott, R.J.J. Jachuck, R.S. Slade, J. Varcoe, Sep. Purif. Technol. 30 (2003) 113–127. [2] L. Marder, A.M. Bernardes, J. Zoppas Ferreira, Sep. Purif. Technol. 37 (2004) 247–255. [3] R.K. Nagarale, G.S. Gohil, V.K. Shahi, Adv. Colloid Interface Sci. 119 (2006) 97–130. [4] R. Lteif, L. Dammak, C. Larchet, B. Auclair, Eur. Polym. J. 37 (2001) 627–639. [5] V. Markovac, H.C. Heller, Plating Surf. Finish. 68 (1981) 66–69. [6] R.D. Noble, S.A. Stern, Membrane Separations Technology — Principles and Applications, Elsevier, Amsterdam, 1995. [7] L. Marder, E.M. Ortega Navarro, V. Pérez-Herranz, A.M. Bernardes, J. Zoppas Ferreira, J. Membr. Sci. 284 (2006) 267–275. [8] Ionics Bulletin No. CR 67.1 July, 1990. [9] M. Taky, G. Pourcelly, C. Gavach, A. Elmidaoui, Desalination 105 (1996) 219–228. [10] J.-H. Choi, H.-J. Lee, S.-H. Moon, J. Colloid, Interf. Sci 238 (2001) 188–195. [11] R. Ibanez, D.F. Stamatialis, M. Wessling, J. Membr. Sci. 239 (2004) 119–128. [12] J.-H. Choi, S.-H. Kim, S.-H. Moon, J. Colloid Interf. Sci. 241 (2001) 120–126. [13] N. Pismenskaya, P. Sistat, P. Huguet, V. Nikonenko, G. Pourcelly, J. Membr. Sci. 228 (2004) 65–76. [14] E. Volodina, N. Pismenskaya, V. Nikonenko, C. Larchet, G. Pourcelly, J. Colloid Interf. Sci. 285 (2005) 247–258. [15] D.R. Lide, Handbook of Chemistry and Physics, CRC Press, New York, 1997. [16] N.D. Pismenskaya, V.V. Nikonenko, E.I. Belova, G. Yu, P.Sistat Lopatkova, G. Pourcelly, K. Larshe, Russ. J. Electrochem. 43 (2007) 307–327. [17] I. Puigdomench, Hidra Medusa — Make Equilibrium Diagrams using Sophisticated Algorithms, Royal Institute of Technology, Stockholm, 2001. [18] F.G. Wilhelm, N.F.A. van der Vegt, M. Wessling, H. Strathmann, J. Electroanal. Chem. 502 (2001) 152–166. [19] M.S. Kang, Y.J. Choi, H.J. Lee, S.H. Moon, J. Colloid Interf. Sci. 273 (2004) 523–532. [20] L. Jialin, W. Yazhen, Y. Changying, L. Guangdou, S. Hong, J. Membr. Sci. 147 (1998) 247–256. [21] K.-E. Bouhidel, M. Rumeau, Desalination 167 (2004) 301–310. [22] N. Zech, D. Landolt, Electrochimica Acta 45 (2000) 3461–3471. [23] J.J. Qin, M.-N. Wai, M.-H. Oo, F.-S. Wong, J. Membr. Sci. 208 (2002) 213–221.
Glossary φm: membrane potential (V) t: time (s) τ: transition time (s) D: salt diffusion coefficient in the solution (cm2 s−1), zj: charge of the counter-ion j F: Faraday's constant (96,500 C mol−1) C0: salt concentration in the solution (mol L−1) i: current density (A cm−2) ε: effective parameter regulating differences between an ideal system and the real one tm j : counter-ion transport number in the membrane tsj : counter-ion transport number in the solution Δφp: ohmic potential drop over the polarized membrane system Δφnp: ohmic potential drop over the non-polarized membrane system DBL: diffusion boundary layer
Fig. 9. Chronopotentiometric curves obtained under a current density of 9.21 mA cm− 2 for 0.025 M NiCl2 solutions with different boric acids concentrations (0.015–0.025 M).
Contents lists available at ScienceDirect
Desalination j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / d e s a l
Chronopotentiometric study on the effect of boric acid in the nickel transport properties through a cation-exchange membrane Luciano Marder a,b, Emma M. Ortega Navarro b, Valentín Pérez-Herranz b,⁎, Andréa Moura Bernardes a, Jane Zoppas Ferreira a a b
LACOR, PPGEM, Universidade Federal do Rio Grande do Sul, Campus do Vale, Setor 4, Prédio 74, sala 109, 91501-970, Porto Alegre, RS, Brazil Departamento de Ingeniería Química y Nuclear, Universidad Politécnica de Valencia, P.O. Box 22012, Valencia-46071, Spain
a r t i c l e
i n f o
Article history: Accepted 20 June 2009 Available online 2 October 2009 Keywords: Chronopotentiometry Nickel Boric acid Cation-exchange membrane
a b s t r a c t In this work the effect of boric acid on the nickel transport properties through an IONICS 67-HMR-412 cation-exchange membrane was evaluated by means of chronopotentiometry. Nickel chloride solutions with and without boric acid were investigated. The obtained results showed that the addition of boric acid to a nickel chloride solution does not affect the limiting current density value nor the nickel transport number through the membrane. The obtained results also revealed that, under very high current densities, the secondary transfer phenomenon associated to water splitting occurs when nickel chloride solutions are employed, and a bipolar membrane structure is formed, resulting from a Ni(OH)2 precipitation on the membrane surface. The addition of boric acid to the nickel chloride solution can minimize or avoid this phenomenon depending on the boric acid concentration in the solution. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Electrodialysis, a technique that uses ion-exchange membranes, has been suggested as promising possibilities to remove and recover metal ions from electroplating wastewaters [1–3]. To assure a good performance of this process, it is useful to know the transport number of the ions present in the solutions to be treated through the ionexchange membranes [4]. It is also important to respect some specific conditions of the electrochemical process, such as the limiting current density [5]. These parameters are important to take decisions regarding to which membrane is the most appropriate and which is the maximum current density that can be used for a given application. If in an electrodialysis process the applied current density exceeds the limiting one, operational problems such as the inorganic salts precipitation on the membrane surface, destruction of the membranes and an increase in the energy consumption can occur, decreasing the process efficiency [6]. In a previous work [7], the limiting current density value and the transport number of some transition metal ions through an IONICS 67-HMR-412 cation-exchange membrane were determined by chronopotentiometry. The present work describes a further study carried out using the same electrochemical characterization method. The aim was to investigate the effect of boric acid in the nickel transport properties through an IONICS 67-HMR-412 cation-exchange membrane. Boric
acid is a common additive of nickel electroplating baths and, associated with this metal ion, will be present in the wastewaters generated in the electroplating processes. 2. Experimental 2.1. Solutions The study was carried out with 0.025 M NiCl2 · 6H2O solutions with different H3BO3 concentrations (varying between 0.01 and 0.025 M). Besides, a solution containing only 0.025 M NiCl2 · 6H2O was also evaluated. The solutions were prepared dissolving the respective salts in distilled water. Analytical grade reactives were employed. The pH of all solutions was 4, adjusted with a 0.1 M HCl solution. 2.2. Ion-exchange membrane The ion-exchange membrane used in this work was the IONICS 67-HMR-412 cation-exchange membrane. This is a homogenous acrylic fiber-backed membrane containing sulfonic acid groups as fixed charges [8]. Some of the main properties and characteristics of this membrane are given in Table 1. 2.3. Electrochemical cell and chronopotentiometric measurements
⁎ Corresponding author. Tel.: + 34 96 3877632; fax: + 34 96 3877639. E-mail address: [email protected] (V. Pérez-Herranz). 0011-9164/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2009.06.040
The chronopotentiometric experiments were accomplished using a two-compartment cell. This electrochemical cell was composed of
L. Marder et al. / Desalination 249 (2009) 348–352 Table 1 Properties and characteristics of the IONICS 67-HMR-412 cation-exchange membrane [8]. Properties and characteristics
Value
Thickness (mm) Exchange capacity (meq/g dry resin) Water content (% of wet resin only) Na+ transport number (in 0.01 M NaCl) Effective parameter εa
0.56–0.58 2.10 46 0.99 0.935
a
Our data (determined by chronopotentiometry in 0.025 M NaCl) [7].
two 250 mL symmetrical half-cells between which the cationexchange membrane was clamped. The exposed membrane area was 11.95 cm2. Two Ag/AgCl electrodes (obtained by anodic chloride oxidation of silver sheets in 0.1 M HCl solution) were used to impose the current density. Two Ag/AgCl reference electrodes immersed in Luggin capillaries were used to measure the potential drop through the membrane. The membrane potential response to each current density change was recorded (typically 120 s). The constant current density was supplied by a Tacussel Electronique PJT 120-1 potentiostat/galvanostat. The membrane potential drop and the imposed current density were registered using a PC data acquisition system. Current densities between 0.42 and 9.21 mA cm− 2 were applied. All experiments were conducted at room temperature and without stirring. The experiments were accomplished after a membrane equilibration period of at least 24h with a solution with the same characteristics as the ones used in the experiments. 2.3.1. Limiting current density The limiting current density was determined from the current– voltage curves reconstituted from the steady-state membrane potential drop (obtained from the chronopotentiometric curves) corresponding to an applied current density [9–11]. 2.3.2. Nickel transport number through the membrane The nickel transport number through the membrane was determined from the modified Sand's equation (Eq. (1) proposed by Choi et al. [12]:
τ=
2 zj F ε πD 4 tjm tjs
!2 C0 2 i
ð1Þ
where τ is the transition time (s), D is the salt diffusion coefficient in the solution (cm2 s− 1), zj is the charge of the counter-ion, F is the Faraday's constant (96,500 C mol− 1), C0 is the salt concentration in the solution (mol L− 1), i is the current density (A cm− 2), ε is the s fraction of the conducting region of the membrane, and tm j and tj are the counter-ion transport numbers in the membrane and in the solution, respectively. Eq. (1) is used for heterogeneous ion-exchange membranes. However, Eq. (1) can be applied for homogeneous ionexchange membranes, as IONICS 67-HMR-412 cation-exchange membrane, if the coefficient ε is considered as an effective parameter regulating some differences occurring between an ideal system and the real one. Once the value of this coefficient is found for a system where the transport numbers are well known (Table 1 [7]), the Eq. (1) may be applied in other conditions with the same ε for determining the unknown transport numbers. According to Eq. (1), if the salt diffusion coefficient in the solution, the transport number of the counter-ion in the solution and the effective parameter ε are known, the nickel transport number through the membrane can be obtained by the straight line slope between the transitions times (obtained from the chronopotentiometric curves at current densities above the limiting current density) and (C0/i)2. The transition time corresponds to a sharp increase in the membrane potential drop due to the concentration decrease in the depleting solution near the membrane/solution interface and can be deter-
349
mined from the maximum of the derivate of the membrane potential drop regarding to time [13,14]. 3. Results and discussion 3.1. Limiting current density and nickel transport number Fig. 1 shows typical chronopotentiometric curves obtained for a 0.025 M NiCl2 + 0.015 M H3BO3 solution under current densities higher than the limiting current density. As it can be observed, the chronopotentiometric curves are typical for a monopolar membrane. After an instantaneous increase in the membrane potential drop, which is due to the ohmic resistance of the system, the membrane potential drop rises slowly until the inflection point that defines the transition time. Thereafter, the potential growth rate diminishes and the membrane potential drop reaches a certain steady-state value or a quasi-steady-state value. When the current density is switched-off, the membrane potential decreases rapidly to low values due to the relaxation of the concentration profiles in the solution boundary layer [16]. It can also be observed that the transition times decrease with the increase in the current density. This is a consequence of the faster ion depletion in the membrane/solution interface. The effect of boric acid on the chronopotentiometric response of the membrane can be seen in Fig. 2 for a current density of 5.19 mA cm− 2 and a 0.025 M NiCl2 solution at different concentrations of boric acid. As it can be observed in Fig. 2, under the same applied current density, the transition times are closely similar for all the evaluated solutions. However, the difference between the ohmic potential drop over the polarized membrane system (Δφp) and the non-polarized one (Δφnp) varies with the addition of boric acid to the nickel chloride solution. According to Pismenskaya et al. [13] this difference (Δφp − Δφnp) shows the increment in the ohmic potential drop over the membrane system due to the concentration polarization. The main contribution in this increment is given by the diluted diffusion boundary layer (DBL). In the case of the nickel chloride solution without boric acid, the value of Δφnp − Δφp is 0.70 V. In the case of the nickel chloride solutions with boric acid, this difference is 0.46 V, and is independent on the boric acid concentration. Hence the resistance of the polarized DBL near the membrane in contact with nickel chloride without boric acid is slightly higher than when the membrane is in contact with the nickel chloride solutions with boric acid. From the steady-state membrane potential drop, corresponding to an applied current density in the chronopotentiometric curves, the current–voltage curves can be reconstituted and the limiting current density can be determined by the intersection of the two slopes
Fig. 1. Chronopotentiometric curves obtained for the 0.025 M NiCl2 + 0.015 M H3BO3 solution under current densities above the limiting current density.
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Fig. 2. Chronopotentiometric curves obtained for the NiCl2–H3BO3 solutions under a current density of 5.19 mA cm− 2.
belonging to the ohmic and plateau regions, as shown in Fig. 3 for a 0.025 M NiCl2 solution with different concentrations of boric acid. The obtained results show that the addition of boric acid to the nickel chloride solution does not affect the limiting current density value. Limiting current densities of around 3.26 mA cm2 were obtained. On the other hand, the higher resistance of the DBL to the nickel chloride solution without boric acid in comparison to solutions with boric acid, that was previously commented, can be noted in the plateau region of the reconstituted current–voltages curves presented in Fig. 3. The membrane potential drop obtained to an equivalent current density in this current–voltage curve region is relatively higher for the nickel chloride solution without boric acid. In the ohmic region the resistance should be similar for all the evaluated solutions. From the chronopotentiograms of Figs. 1 and 2, the transition times were determined and employed in the τ versus (C0/i)2 plot for the determination of the transport numbers as shown in Fig. 4. These plots were obtained assuming as C0 the nickel chloride concentration in the solution (0.025 M). It can be observed that good linear relationships were obtained between τ and (C0/i)2 for a significant range of current densities above the limiting one for all the evaluated solutions, and that the obtained straight line slopes, which are presented in Table 2, are closely similar. From the straight line slopes of the representation of τ versus (C0/i)2, the nickel transport numbers through the membrane were obtained applying Eq. (1), where the salt diffusion coefficient and counter-ion transport number in the solution, used to calculate the nickel transport number through the
Fig. 4. Transition time as a function of (C0/i)2 for the IONICS 67-HMR-412 cation-exchange membrane in contact with NiCl2–H3BO3 solutions (slope in s(Acm− 2 L)2 mol− 2).
membrane, correspond to the nickel chloride data. In this way, the NiCl2 diffusion coefficient in the solution used was 1.2 × 10− 5 cm2 s− 1 and the nickel transport number in the solution was 0.394. The NiCl2 diffusion coefficient data was obtained from the equivalent conductivity data at infinite dilution using the Nernst–Einstein equation. The nickel transport number in the solution was also calculated by employing the equivalent conductivity data [15]. The values of the calculated nickel transport number through the IONICS 67-HMR-412 cation-exchange membrane are presented in Table 2, and it can be concluded that the addition of boric acid to the solution does not affect the nickel transport number through membrane and values of approximately 0.9 were obtained. The deviation of nickel transport number from 1 can be due to chloride transport through the membrane since it is a small anion with a high mobility and it can penetrate into the membrane causing the Donnan's exclusion failure. The non influence of boric acid on the limiting current density value and on the nickel transport number through the membrane should be associated to the boric acid characteristics in aqueous solutions, showed in Table 3 [17]. As it can be observed in Fig. 5, drawn from Table 3 data, in diluted aqueous solutions containing nickel chloride and boric acid, besides the nickel species, the other predominant species are B(OH)3 and H2BO− 3 in acid and alkaline regions, respectively [17]. As at pH 4 boric acid should be under B (OH)3 form, no other positively charged ionic species should be transported with nickel ions through the membrane. In this way the limiting current density value and the nickel transport number through the membrane presented similar values, independently of the presence or not of boric acid in the nickel chloride solution. These properties of boric acid can be clearly observed when chronopotentiometric curves are obtained under a same current density. As it can be observed in Fig. 2, the chronopotentiometric curves, obtained for the nickel chloride solutions with different boric acid concentrations, show equivalent transitions times as the nickel chloride solution without boric acid, when a same current density is applied. Since the addition of boric acid to the nickel chloride solution does not cause an increase in the concentration of positively charged species in the Table 2 Nickel transport numbers through the IONICS 67-HMR-412 cation-exchange membrane in contact with NiCl2–H3BO3 solutions. Solution
Fig. 3. Current–voltage curves of the IONICS 67-HMR-412 cation-exchange membrane in contact with NiCl2–H3BO3 solutions obtained from the steady-state membrane potential drop of the chronopotentiometric curves.
0.025 M NiCl2 0.025 M NiCl2 + 0.010 M 0.025 M NiCl2 + 0.015 M 0.025 M NiCl2 + 0.020 M 0.025 M NiCl2 + 0.025 M
H3BO3 H3BO3 H3BO3 H3BO3
pH
Straight line slope (s(Acm− 2 L)2 mol− 2)
tm j
4 4 4 4 4
1.217 1.225 1.222 1.219 1.216
0.896 0.895 0.895 0.896 0.896
L. Marder et al. / Desalination 249 (2009) 348–352
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Table 3 Thermodynamic constants the NiCl2–H3BO3 systems [17]. Equilibrium
Constant
B(OH)3 ↔ (H2BO3)− + H+ Ni2+ + 2 H2O ↔ Ni(OH)2 + 2 H+ Ni2+ + Cl− ↔ NiCl+ NiCl2 ↔ Ni2+ + 2 Cl−
5.8 × 10− 10 3.2 × 10− 11 2.5 2.4 × 103
solution, the required time to produce the ion depletion in the membrane/solution interface, should be independent of the presence or not of boric acid in the solution. 3.2. Transport phenomena at higher currents densities In a previous work [7] it could be observed that when nickel and cobalt chloride solutions were employed, the chronopotentiometric curves obtained at over-limiting currents showed an increment in the membrane potential drop after an apparent stabilization period, as it can be observed schematically in Fig. 6 (full line). This behavior coincided with the formation of a Ni(OH)2 or Co(OH)2 precipitate on the anodic side of the membrane surface. The formed precipitate increased the membrane resistance and caused the increase in the membrane potential drop. This effect should be a consequence of the water splitting phenomenon in the anodic membrane/solution interface (H2 O ↔ H+ + OH−). The generated H+ ions were transported through the cation-exchange membrane toward the cathodic cell compartment, meanwhile the OH− ions combine with the nickel or cobalt ions, forming the observed metal hydroxide precipitate (Mez+ + zOH− ↔ Me(OH)z↓). The last section of these chronopotentiometric curves (full line), when the current was switched-off, also showed a different pattern of a classical monopolar chronopotentiometric curve (dashed line). Instead of the fast decay of the membrane potential to low values (close to zero), common for monopolar membranes, when nickel and cobalt chloride solutions were employed, immediately after switching-off the over-limiting current density, the membrane potential dropped to a plateau-value that extended over time until reaching values close to zero. This behavior was similar to that found by Wilhelm et al. [18] for bipolar membranes and is related to the hindered recombination of hydroxide ions and protons, resulted from water splitting in the bipolar membrane junction. Hence it could be concluded in this previous work that under high over-limiting currents a water splitting phenomenon occurred when nickel and cobalt chloride solutions were employed and a bipolar layer was formed by the metal hydroxide and the membrane. A similar behavior was found by Kang et al. [19] studying the electrochemical behavior of ion-exchange membranes coated with iron hydroxide, by
Fig. 5. Speciation of the ionic species in a 0.025 M NiCl2 + 0.015 M H3BO3 solution [17].
Fig. 6. Schematic chronopotentiometric curve obtained at high over-limiting current densities for nickel and cobalt chloride solutions in contact with the IONICS 67-HMR412 cation-exchange membrane (full line). Dashed line corresponds to a classical shape of a chronopotentiometric curve for a monopolar membrane [7].
Jialin et al. [20] when a layer of Mg(OH)2 is formed on the surface of a cation-exchange membrane and by Choi et al. [10] when an Al(OH)3 layer is formed on the surface of a cation-exchange membrane. The results obtained in the present work show that the secondary transfer phenomenon of water splitting, which leads up to the precipitation of Ni(OH)2 on the membrane surface and the bipolar behavior, can be minimized or avoided depending on the boric acid concentration in the solution. As shown in Fig. 7, for the 0.025 M NiCl2 solution, the increase in the membrane potential drop, as well as the bipolar behavior, starts to appear around a current density of 5.52 mA cm− 2, approximately 1.7 times the limiting current density. Adding 0.01 M H3BO3 to the nickel chloride solution, this behavior appears at higher current densities, around 7.87 mA cm− 2, approximately 2.4 times the limiting current density, as it can be observed in Fig. 8. For more concentrated solutions in boric acid (0.015–0.025 M) this behavior is not observed until a current density of 9.21 mA cm− 2, approximately 2.8 times the limiting current density, as it can be observed in Fig. 9. The increase in the membrane resistance, resulted from the formed Ni(OH)2 precipitate on the membrane surface when 0.025 M NiCl2 and 0.025 M NiCl2 + 0.01 M H3BO3 solutions are employed, can be observed in the reconstituted current–voltage curves presented previously in Fig. 3. When these solutions are employed a second
Fig. 7. Chronopotentiometric curves obtained for the 0.025 M NiCl2 solution under high current densities.
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Under higher current densities a water splitting phenomenon occurs, when nickel chloride solutions are employed, and a bipolar structure is formed, resulted from a nickel hydroxide precipitation on membrane surface. The boric acid addition to the nickel chloride solution can minimize or avoid this phenomenon, depending on the boric acid concentration in the solution. This behavior should be associated to the buffer properties of boric acid in solution. Acknowledgements The authors wish to express our gratitude for the support of this work by the CAPES, Brazil (Ref. no. 080/04) and the Ministerio de Ciencia y Tecnología, Spain (Ref. no. PHB2003-0093-PC). Luciano Marder is grateful to the CAPES (BEX 0547/04-02) and to CNPq (140417/2003-06) for a postgraduate grant. Fig. 8. Chronopotentiometric curves obtained for the 0.025 M NiCl2 + 0.01 M H3BO3 solution under high current densities.
plateau is observed in the over-limiting current region, just when the nickel hydroxide precipitate should be formed. For the other investigated solutions, where the nickel hydroxide precipitate is not observed, the current–voltage curves show a classical pattern of a monopolar ion-exchange membrane. The influence of boric acid on the water splitting and membrane scaling phenomena should be associated to the buffer properties of boric acid [21,22]. Once in solution, boric acid should be dissociated + according the following reaction: H3BO3 ↔ H2BO− [23]. This 3 +H behavior leads up to a supplying of protons in the diffusion boundary layer at current densities where the water splitting becomes possible. This fact delays the pH increase in the membrane/solution interface associated with the onset of water splitting. Because of the supplying of additional protons from the boric acid dissociation, the current densities for the water splitting should be extended to higher current density values. 4. Conclusions Chronopotentiometry is a suitable technique to obtain information regarding the boric acid influence on nickel transport properties through the IONICS 67-HMR-412 cation-exchange membrane. The addition of boric acid to a nickel chloride solution does not affect the limiting current density value nor the nickel transport number through the membrane. This behavior should be associated to the boric acid properties in aqueous solutions.
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Glossary φm: membrane potential (V) t: time (s) τ: transition time (s) D: salt diffusion coefficient in the solution (cm2 s−1), zj: charge of the counter-ion j F: Faraday's constant (96,500 C mol−1) C0: salt concentration in the solution (mol L−1) i: current density (A cm−2) ε: effective parameter regulating differences between an ideal system and the real one tm j : counter-ion transport number in the membrane tsj : counter-ion transport number in the solution Δφp: ohmic potential drop over the polarized membrane system Δφnp: ohmic potential drop over the non-polarized membrane system DBL: diffusion boundary layer
Fig. 9. Chronopotentiometric curves obtained under a current density of 9.21 mA cm− 2 for 0.025 M NiCl2 solutions with different boric acids concentrations (0.015–0.025 M).