Journal of Membrane Science 246 (2005) 217–226
Assessing ionic transport during apple juice electro-acidification: influence on system efficiency M. Mondora , A. Lam Quocb , F. Lamarchea,∗ , D. Ippersiela , J. Makhloufb a
Food Research and Development Center, Agriculture and Agri-Food Canada, Saint-Hyacinthe, Que., Canada J2S 8E3 b Department of Food Sciences and Nutrition, Laval University, Sainte-Foy, Que., Canada G1K 7P4 Received 8 May 2003; received in revised form 6 August 2004; accepted 15 August 2004
Abstract The purpose of this work was to assess ionic transport, during electro-acidification of apple juice, and its influence on the acidification rate and energy usage. In order to fulfill this objective, advanced current–voltage characterization of monopolar (cationic and anionic) and bipolar membranes was carried out. Experiments were conducted using two electrodialysis configurations (bipolar-cationic membranes and bipolaranionic membranes) with two different systems (KCl–KCl and KCl–juice). For the bipolar-anionic configuration, the system HCl–juice was also considered. The characteristic values of the transmembrane potential and of the estimated membrane resistance were correlated to the ionic transport through the membranes, and to the energy usage of the systems. Influence of the membrane boundary layers on the transmembrane potential was also investigated by working at different feed flow rates. It was shown that for the present operating conditions, the boundary layers do not affect the transmembrane potential. Furthermore, use of HCl with the bipolar-anionic configuration enables to benefit from the advantages of each configuration: a small anionic membrane resistance due to the transport of H+ , as for the bipolar-cationic configuration, and a high acidification rate of the apple juice due the neutralization of OH− by HCl. It was for this system that the acidification rate was the fastest. © 2004 Elsevier B.V. All rights reserved. Keywords: Electrodialysis; Foods; Ionic transport; Membrane potentials; Membrane resistance
1. Introduction Demand for cloudy or unclarified apple juice has increasing market potential due to its sensory and nutritional qualities. However, the production of high-quality juice is difficult because of its sensory instability. Cloudy apple juice is very sensitive to enzymatic browning because it contains considerable quantities of polyphenols and polyphenol oxidases (PPO). Enzymatic browning reactions are catalyzed by PPO and result in the oxydation of phenolic compounds into o-quinones, which then polymerize into complex darkcolored pigments [1]. ∗ Corresponding author. Tel.: + 1 450 773 1105x222; fax: +1 450 773 8461. E-mail address:
[email protected] (F. Lamarche).
0376-7388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2004.08.018
Acidification of cloudy apple juice to pH 2.0 and readjusting the pH to its initial value was found to irreversibly inhibit PPO activity and stabilize juice color [2]. Tronc et al. [3,4] and Lam Quoc et al. [5] have demonstrated the potential of electrodialysis with bipolar membrane for the inhibition of PPO in cloudy apple juice, without altering juice flavor. Two electrodialysis configurations were used to modify the pH of cloudy apple juice: (A) bipolar and cationic membranes (BP-C); (B) bipolar and anionic membranes (BP-A). In the BP-C configuration, addition of exogenous KCl to the juice was required to obtain a pH of 2.0 [3,4]. However, with the BP-A configuration, a pH of 2.0 was reached without addition of exogenous KCl, moreover acidification rate was increased by a factor of 3, when compared to the BP-C configuration [5]. A yield of 10 L of juice/m2 membrane/min at pH 2.0 was obtained with the BP-A configuration, as compared
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to 3.3 L of juice/m2 membrane/min for the BP-C configuration. Although these previous works have demonstrated the feasibility of using electrodialysis with bipolar membranes for the prevention of enzymatic browning in cloudy apple juice, the fundamental information concerning the dynamic transport in the membranes and in the boundary layers remains limited. Nevertheless, a better understanding of ion transport through monopolar and bipolar membranes is the first step toward the optimization of electrodialysis conditions. In the past, transient-state current or voltage curves were used to study the ionic transport in the diffusion-controlled boundary layers next to ion permeable membranes [6], and to explain overlimiting current behavior [7,8]. Bipolar membrane current–voltage characterization has been investigated before, mainly focusing on the dynamic development of the electrical potential difference [9], but also to investigate the transport in and the energy requirements of different bipolar membranes under water splitting conditions [10,11]. In this contribution, the current–voltage behavior of the monopolar (cationic and anionic) and bipolar membranes was investigated for ideal system of KCl–KCl, and complex systems of KCl–juice and HCl–juice. Characteristics of voltage response curves and of membranes resistances for the different systems are analyzed with focus on electrodialysis cell performance (acidification rate and energy usage).
2. Theory 2.1. Monopolar membrane and ions transport Monopolar membranes used in electrodialysis act as ionexchange membranes, and are considered a system of two or several phases. In literature, ion-exchange membranes are described as a combination of a gel phase being an aqueous solution of fixed and mobile ions with the polymer matrix included, and an equilibrium electroneutral solution filling the intergel spaces [12,13]. There is likewise a third inert phase formed from hydrophobic parts of the polymer matrix and/or from the inert binder introduced during the synthesis stage. Monopolar membranes are in theory permeable to either anions or cations. However, in practice, anionic membranes shows a high permeability for protons, while they are nearly impermeable for other cations, and the permeability of hydroxyl ions in a cationic membrane is much higher than that of other anions. This behavior is due to the exceptionally high mobility of H+ and OH− ions, which have different and more rapid transport mechanism (tunneling mechanism) [14]. 2.2. Transmembrane potential through monopolar and bipolar membranes Ion and water transport in the membranes will determine predominately the process performances. For this reason,
ionic transport in electrodialysis system is often described at the membrane scale [13]. However, in some circumstances, the boundary diffusion layers may also significantly influence the ionic transport and must be considered in the treatment [15]. There are three contributions to the transmembrane potential in absence of an external electrical field, that is to say Donnan, diffusion and streaming potentials. Donnan potentials result from the difference of ion concentration between membrane and adjacent bulk solutions, which leads to a potential difference at the interfaces [16]. In addition, if the bulk solutions have different concentrations, a transfer of ions will take place from the concentrated solution to the diluted solution. Since ions are moving at different velocities in the membrane, a separation of charges takes place setting up an electrical field [16,17]. Thus even though no external electrical field is imposed on the system, an electrical potential exists through the monopolar membrane (diffusion potential). When osmotic transfer is present, another potential (streaming) is observed. This potential arises from the influence of water transport on the permeation of counter-ions in the membrane [18]. In presence of an external electrical field, there is a fourth contribution to the transmembrane potential, since ions transfer will not only take place by diffusion but also by electromigration. This potential is due to the ohmic resistance of the membrane (ohmic potential) [18]. Concerning the bipolar membranes, when they are placed into an external electrical field, they will split water molecules, into protons and hydroxyl ions [19]. Since bipolar membrane consists of a cation and an anion selective layers, the aforementioned potentials presented for the monopolar membrane still applied. 2.3. Membrane resistance Due to membranes microheterogeneousity, there are at least two different contributions to the membrane resistance: a “gel phase resistance” who is function of the aqueous solution of fixed and mobile ions including the polymer matrix, and an “intergel phase resistance” who is function of the electroneutral solution filling the intergel spaces such as fissures or inner electroneutral parts. It is possible using structure-kinetics models to describe the influence of each fraction (conducting and non-conducting) on the membrane resistance and ionic transport through the membrane [13,20]. However, as pointed out by Wilhelm et al. [10,11], it is also possible from the potential measurement with and without an external electrical field, to estimate the membrane resistance: R = Rgel + Rintergel =
φE − φ I
(1)
where R is the membrane resistance, that is to say the summation of the gel phase resistance, and of the intergel phase resistance, φE is the transmembrane
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Fig. 1. Electrodialysis experimental systems: (A) bipolar-cationic membranes configuration; (B) bipolar-anionic membranes configuration. The Ag/AgCl electrodes are denoted as: ( ), ( ), ( ). Bipolar membrane: BP; cationic membrane: C; and anionic membrane: A.
potential in presence of an external electrical field (Donnan + diffusion + streaming + ohmic), φ is the instantaneous transmembrane potential, when the current is switched off (Donnan + diffusion + streaming), and I is the current. For the situation where the boundary diffusion layers significantly affect the ionic transport, their resistances will also contribute to the membrane resistance (Eq. (1)).
3. Materials and methods 3.1. Experimental systems A six-compartment membrane module, as presented in Fig. 1, was used for these experiments. The module used was an ED-1-BP electrodialysis cell (100 cm2 of effective electrode surface) from Electrosynthesis Co. Inc. (Lancaster, PA). The anode, a dimensionally stable electrode (DSA), and the cathode, a 316 stainless steel electrode, were supplied with the cell. The experiments were performed with two membrane configurations: bipolar-cationic (BP-C) and bipolaranionic (BP-A). A total of five membranes are used: three bipolar membranes (BP-1 from Tokuyama Soda Ltd.) and two monopolar membranes (Neosepta CMX cationic membranes or Neosepta AMX anionic membranes from Tokuyama Soda Ltd.). The intermembrane gap is 0.75 mm, the membrane thicknesses are 0.17 and 0.16 mm for the CMX and AMX membranes, respectively. The membranes are equilibrated in a 0.2 M KCl or HCl solution for at least 24 h outside the membrane module. After then, the membranes are placed in alternance in the electrodialysis cell. Each membrane has an effective surface area of 100 cm2 . The acidification with the BP-C configuration was performed for two systems: KCl–KCl and KCl–juice. For the BP-A configuration, the
electro-acidification was studied with three different systems: KCl–KCl, KCl–juice and HCl–juice. This arrangement defines three closed loops containing the solution to alkalinize (0.2 M KCl solution or 0.2 M HCl solution), the solution to acidify (0.2 M KCl or the apple juice) and a 0.2 M Na2 SO4 solution used as rinsing solution for the electrodes of the electrodialysis cell. The apple juice (Oasis) was given by Lassonde Inc. (Rougemont, Que.). Each closed loop was connected to a separate external reservoir, allowing for continuous recycling. The electro-acidification was carried out with electrolytes volumes of 5 L for the solution to alkalinize (KCl or HCl), 5 L for the KCl solution or 3 L for the apple juice to acidify and 5 L for the Na2 SO4 solution. The flow rate was controlled at 0.75 or 1.50 L/min, using panel-mount flow meters, and the temperature of the electrolytes was maintained at 15 ◦ C. Electro-acidification process was monitored with a YSI conductivity meter (model 35, Yellow Springs, OH) and a pH meter (model AP61, Fisher Scientific, Montreal, Que.). After each experiment, the membranes are restored by rinsing the compartments with water (three wash cycles of 10 min each) and immersion in the appropriate salt or acid solution. The energy required for the electro-acidification was estimated as described by Lam Quoc et al. [5]. Experiments were conducted randomly and each experiment was conducted in duplicate. 3.2. Transmembrane potential measurement The transmembrane potential was measured with spiral Ag/AgCl electrodes placed on each side of the monopolar or bipolar membranes. The electrodes are inserted into the separators/turbulence promoters, such that they do not
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disturb the flow in the electrodialysis cell. The transmembrane potential was measured with two RadioShack multimeters model 22-178 (RadioShack, Inter Tan Ltd., Canada). Measurements through the monopolar and bipolar membranes are performed simultaneously, as a function of time, under a constant cell potential of 10 V. In a first set of experiments, the residual potential (potential free from IR effects: Donnan + diffusion + streaming potentials) was measured every 5 min, as the instantaneous potential value obtained upon interruption of the electrical field. At the end of the experiment, the evolution of the residual potential was followed until the system reaches the steady state (approximately 10 min). Furthermore, in order to determine if the boundary layers present at the membranes have a significant influence on the transmembrane potential and on the value of the membrane resistance, a second set of experiments was performed. Every 5 min, the flow rate of the different solutions was increased from 0.75 to 1.50 L/min, until transmembrane potential reached steady state. The variation of the transmembrane potential resulting from this increase was recorded. 3.3. Electrodes preparation The Ag/AgCl electrodes were prepared by chlorination of silver wires having a diameter of 0.65 mm, using a method derived from Shoemaker et al. [21]. Such electrodes allow measurement of the transmembrane potential without the influence of polarization voltages or over-potentials. In the first step, the silver wires are rinsed with methanol, acetone and water to remove the impurities. Then, they are immersed in ammoniac solution to remove any oxide traces. The wire was then placed in an electrolysis bath and was connected to an anode of a continuous current source. The cathode was a platinum wire and the electrolyte was a 0.25 M KCl solution. The treatment was performed for 30 min with a current intensity of 10–12 mA/cm2 . Scanning electron microscopy measurements were done to verify the homogeneity and uniformity of the AgCl deposit.
Fig. 2. Electrodialysis with bipolar-cationic membranes (system KCl–KCl). Temperature: 15 ◦ C; cell potential: 10 V: (A) (♦) pH–KCl acid; (): V − C with current; ( ) V − C without current; (B) (♦) pH–KCl acid; (䊉) V − BP with current; ( ) V − BP without current; (C) () resistance BP; and () resistance C.
4. Results and discussion 4.1. Acidification rate and energy usage during electro-acidification 4.1.1. System KCl–KCl In the BP-C configuration, the H+ generated by the bipolar membrane gradually replace the K+ which migrate through the cationic membrane and form KOH with the OH− generated by the bipolar membrane. The overall result is the formation of HCl and KOH in compartment A and B, respectively (Fig. 1A). In the BP-A configuration, the OH− generated by the bipolar membrane gradually replace the Cl− which migrate through the anionic membrane and form HCl with the H+ generated by the bipolar membrane. As for the BP-C con-
figuration, the overall result is the formation of HCl and KOH (Fig. 1B). From Fig. 2 (BP-C) and 3 (BP-A), it was observed that pH evolution in the acidic compartment was similar for both configurations. The acidification rate was in the order of 6 × 10−4 mol of H+ accumulated in the solution to acidify by minute. However, the BP-C configuration required 80% more energy to obtain a pH of 1.25 (9.20 kW h/m3 of KCl to acidify versus 5.10 kW h/m3 ). This is certainly due to the loss of H+ through the cationic membrane, which is more significant than the loss of OH− through the anionic membrane in the BP-A configuration. In order to confirm this hypothesis, the K+ ionic flux for the BP-C configuration was estimated from the acid
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accumulation in the KCl solution to acidify, knowing that the K+ permeation through the cationic membrane was similar to the rate of H+ accumulated in the acidic compartment. In the same manner, for the BP-A configuration, the accumulation of base into the compartment of KCl to alkalinize was used to estimate the ionic flux of Cl− through the anionic membrane. These ionic fluxes were compared to the total ionic flux in order to establish the “useful flux” for each species: J=
nI Fz
(2)
where J is the total ionic flux (mol/s), n is the number of primary stack (n = 2), I is the current (A), F is the Faraday constant (96,486 A s/mol) and z is the valence absolute value (z = 1). Based on the results, the permselectivity of the Cl− in the BP-A configuration reached 90.2%, as compared to 58.9% for the K+ in the BP-C configuration. This explains the higher energy consumption for the BP-C configuration. 4.1.2. Systems KCl–juice and HCl–juice The acidification performance is function of the nature and of the ionic concentration of the solution to acidify. From an electro-chemical point of view, apple juice is mainly constituted of potassium which is the predominant cation (764 ± 45 mg/L), while magnesium and calcium are present but in significantly lower concentration (35 ± 5 and 27 ± 3 mg/L, respectively) [5]. Malic acid is the predominant anion, it forms a potassium salt resulting in a buffering system. For both configurations, replacing the KCl solution in the acidic compartment by apple juice resulted in a decrease of the acidification rate which was controlled by the buffering capacity of malic acid (Fig. 4 (BP-C) and 5 (BP-A)). In addition for the BP-C configuration, the acidification of the juice was complicated by the loss of H+ through the cationic membrane. Both phenomena together explain the pseudo-steady state pH value of 2.25 reached after 90 min of treatment [5]. For the system KCl–juice, the BP-A configuration was more performant than the BP-C configuration, due to the permeation of Cl− anions through the anionic membrane resulting in the retention of H+ in the acidic compartment. The target pH 2.0 was easily reached and the energy usage was more than two times less than with the BP-C configuration (5.35 kW h/m3 of juice versus 12.40 kW h/m3 ). Furthermore, the use of HCl instead of KCl, as the solution to alkalinize, enables the neutralization of the OH− generated by the bipolar membrane and would eliminate the competition between both anions Cl− and OH− for permeation through the anionic membrane. The acidification rate was increased by 25%, when KCl solution was replaced by HCl solution, while the energy requirement remains similar for both systems.
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4.2. Evolution of the transmembrane potential for monopolar and bipolar membranes, with and without an external electrical field In this study, the variation with time of the transmembrane potential, in presence of an external electrical field, was small. Only the systems where the transport of H+ leads to a significant decrease of the membrane resistance (BPC configuration (Figs. 2A and 4A) and BP-A configuration with the system HCl–juice (Fig. 6A)) have resulted in a monopolar transmembrane potential decreasing with time. It was also observed that there is no clear difference between the potential curves for the flow rate at 0.75 L/min versus 1.50 L/min (Figs. 4–6), which means that the contribution of the boundary layers resistance to the membrane resistance is small. Thus it is expected that they do not limit the ion transport and will not be considered in this treatment. In the absence of an external electrical field, the measured transmembrane potential corresponds to the summation of the Donnan, diffusion and streaming potentials through the membrane. Based on the reported results (Figs. 2A–6A), the transmembrane potential was always small, a few millivolts, through the monopolar membranes. This residual potential is not function of the solutions under treatment (KCl, juice, HCl) since similar results were reported for the different systems under consideration. However, for the bipolar membrane, the potential in absence of an electrical field (Figs. 2B–6B) represents a significant fraction (around 60%) of transmembrane potential in presence of an external electrical field. During electrodialysis, the thin transitory region between the cationic and anionic layers of the bipolar membrane contains essentially water having a concentration of H+ and OH− in the order of 1 × 10−7 M. In absence of an external electrical field, this corresponds to a theoretical potential through the bipolar membrane of 0.72 V [19]. When the external electrical field is switch-off at the end of the treatment, ions diffuse inside the bipolar membrane due to the concentration gradient. During that time, the potential decreases while the concentration of ions in the transitory region increases. For all systems considered in this study, except for the system HCl–juice, at the end of the treatment, we have an acid solution and an alkaline solution separated by the bipolar membrane. The H+ and other cations in presence (mainly K+ ) will diffuse through the cationic layer of the bipolar membrane, while the OH− and other anions (mainly Cl− ) will diffuse through the anionic layer. This results in the formation of water and KCl in the transitory region of the bipolar membrane. Since the ionic concentration inside the membrane was always different than outside the membrane, the bipolar membrane potential differs from 0 (Figs. 2B–5B). This behavior of the steady state bipolar transmembrane potential, which was different from zero for bipolar membrane placed between an acid and a base, in absence of an external electrical field was also reported by Simons
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Fig. 3. Electrodialysis with bipolar-anionic membranes (system KCl–KCl). Temperature: 15 ◦ C; cell potential: 10 V: (A) (♦) pH–KCl acid; () V − A with current; ( ) V − A without current; (B) (♦) pH–KCl acid; (䊉) V − BP with current; ( ) V − BP without current; (C) () resistance BP; and () resistance A.
[22]. Clearly, the value of the transmembrane potential at steady state will be function of the ionic environment and concentration. However, for the system HCl–juice (Fig. 6B), this is the H+ and Cl− that diffuse inside the bipolar membrane and will form an HCl solution in equilibrium with the bulk solutions. It was observed that for this system the bipolar residual potential decreased to 0 in two steps: a slow first phase having a length of 10 min and a second quicker phase having a length of 5 min (Fig. 6B). When the external electrical field is switch-off, the anionic layer of the bipolar membrane contains principally OH− ions. Similarly, the cationic layer contains principally H+ ions. The first diffusion step results
Fig. 4. Electrodialysis with bipolar-cationic membranes (system KCl–juice). Temperature: 15 ◦ C; cell potential: 10 V: (A) (♦) pH–juice; () V − C with current: 0.75 L/min; () V − C with current: 1.50 L/min; ( ) V − C without current; (B) (♦) pH–juice; (䊉) V − BP with current: 0.75 L/min; () V − BP with current: 1.50 L/min; ( ) V − BP without current; (C) () resistance BP: 0.75 L/min; (䊉) resistance BP: 1.50 L/min; () resistance C: 0.75 L/min; and (): resistance C: 1.50 L/min.
in the formation of water through the reaction of OH− and H+ , during this step the difference of concentration between the inside and the outside of the bipolar membrane decreases slowly, as for the transmembrane potential. Then during the second step, that is to say when all the OH− ions have reacted with the H+ ions, it is mainly the Cl− and H+ ions that will diffuse to form HCl in the transitory region. At this moment the difference of concentration between the inside and outside of the bipolar membrane becomes less significant and ultimately the system reaches the equilibrium and the transmembrane potential becomes 0. Thus the evolution of the transmembrane potential is a quick and simple method
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Fig. 5. Electrodialysis with bipolar-anionic membranes (system KCl–juice). Temperature: 15 ◦ C; cell potential: 10 V: (A) (♦) pH–juice; () V − A with current: 0.75 L/min; () V − A with current: 1.50 L/min; ( ) V − A without current; (B) (♦) pH–juice; (䊉) V − BP with current: 0.75 L/min; () V − BP with current: 1.50 L/min; ( ) V − BP without current; (C) () resistance BP: 0.75 L/min; (䊉) resistance BP: 1.50 L/min; () resistance A: 0.75 L/min; and () resistance A: 1.50 L/min.
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Fig. 6. Electrodialysis with bipolar-anionic membranes (system HCl–juice). Temperature: 15 ◦ C; cell potential: 10 V: (A) (♦) pH–juice; () V − A with current: 0.75 L/min; () V − A with current: 1.50 L/min; ( ) V − A without current; (B) (♦) pH–juice; (䊉) V − BP with current: 0.75 L/min; () V − BP with current: 1.50 L/min; ( ) V − BP without current; (C) () resistance BP: 0.75 L/min; (䊉) resistance BP: 1.50 L/min; () resistance A: 0.75 L/min; and () resistance A: 1.50 L/min.
to characterize the exchange properties and ionic diffusion of monopolar and bipolar membranes.
this parameter can be estimated only from the transmembrane potentials (Eq. (1)).
4.3. Evolution of the membrane resistance
4.3.1. Cationic membrane resistance For the BP-C configuration with the system KCl–KCl, an increase of 50% of the conductivity for the KCl solution to acidify and approximately 30% for the KCl solution to alkalinize was observed, while the cationic membrane shows a decrease of the membrane resistance of only 33% (Fig. 2C)). It was also conceivable to observe a significant decrease of the cationic membrane resistance due to the increase of the presence of H+ in the ionic flux with a decrease of pH. In this case, it is conceivable that another phenomenon occured
In Fig. 7, the evolution of the cell resistance is presented for the different systems, under a cell potential of 10 V and a flow rate of 0.75 L/min. Despite the fact that important variation of solutions conductivities is observed during the treatment, the cell resistance remains relatively stable. It is only for the BP-A configuration with the system HCl–juice that a significant decrease (35%) was observed. Clearly, the membrane resistance is the main factor affecting the cell resistance and
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Fig. 7. Cell resistance. Temperature: 15 ◦ C; cell potential: 10 V; flow rate: 0.75 L/min: (♦) BP-C KCl–KCl; () BP-A KCl–KCl; () BP-C KCl–juice; (䊉) BP-A KCl–juice; and () BP-A HCl–juice.
inside the cationic membrane and limited the decrease of the membrane resistance. For the BP-C configuration with the system KCl–juice (Fig. 4C), a decrease of the cationic membrane resistance was observed at the beginning of the treatment. This decrease was attributable to the fact that the juice is poor in cations ([K+ ] = 0.02 M) and the H+ have very quickly replaced the K+ in the ionic flux through the cationic membrane [4,5]. For the juice, which is less conductive than the KCl solution, the acidification resulted in a larger cationic membrane resistance. 4.3.2. Anionic membrane resistance For the BP-A configuration with the system KCl–KCl (Fig. 3C), the resistance of the anionic membrane was approximately seven times larger than the resistance of the cationic membrane with the BP-C configuration (a resistance of 0.53 as compared to 0.075 ) (Fig. 2C). Based on the data from the membrane manufacturer, the resistance of both membrane types is similar when operated with KCl solution. Furthermore, membrane thickness is similar for both membrane types, suggesting that this parameter should not have a significant influence on the membrane resistance. For the BP-A configuration, an increase of the anionic membrane resistance from 0.53 to 0.9 was observed, when the KCl solution was replaced by the juice (Figs. 3C and 5C). This is certainly attributable to the fact that the juice is less conductive than the KCl solution. For the system HCl–juice with the BP-A configuration (Fig. 6C), by comparing the conductivity of both solutions permeating the membrane, we can predict an increase of the resistance of the anionic membrane when the Cl− are the only ions transported through the membranes, as compared to the situation where both Cl− and OH− are transported. From another point of view, the larger conductivity of the
HCl solution, when compared to the KCl solution, for a given concentration of 0.2 M, would decrease the membrane resistance. Both phenomena have inverse effects on the membrane resistance and it was expected that they would compensate each other. In practice, a significant decrease of the anionic membrane resistance was observed for the system HCl–juice, when compared to the system KCl–juice (membrane resistance of 0.31 as compared to 0.9 ). Furthermore, although the conductivity of the HCl solution decreases during the treatment due to a decrease of the acid concentration, a decrease of the anionic membrane resistance from 0.4 to 0.2
was observed (Fig. 6C). 4.3.3. Neutralization interface For the situation where the pH of the bulk solutions are significantly different, as it was the case for the present experiments, it was not a negligible counter-current transport of H+ and OH− that was observed, but a significant flux [14]. For example, in the production of acid and base by electrodialysis, some authors have reported a transport of H+ through the anionic membrane in the order of 66% [23]. The water concentration inside the membrane is also a factor that will influence the membrane resistance [24]. In order to explain the results obtained for the resistances, we propose the notion of “neutralization interface”. It is a zone inside the membrane, where water molecules are formed due to the reaction between H+ and OH− . This phenomenon will induce an effect of ionic dilution attributable to the water formation and this will result in an increase of the membrane resistance. For the BP-C configuration with the system KCl–KCl, water formation may explain the fact that the cationic membrane resistance did not decrease as expected following the 40% increase in KCl solutions conductivities. For the BPA configuration with the system KCl–KCl, this resulted in
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an increase of the anionic membrane resistance followed by a stabilization of this resistance, instead of a decrease with the increase of the conductivity of both solutions during the treatment (Fig. 3C). By developing in more details the notion of “neutralization interface”, it is clear that the exact position of this interface will also influence the effect of the dilution. The closer the interface is from the downstream of the membrane, the larger the zone of dilution in the membrane. If the zone of dilution is close to the membrane upstream, its effect will be less significant. The water formed inside the membrane by the reaction of H+ and OH− will become part of the main ionic flux, which leads to the conclusion that the membrane resistance is function of the position of the neutralization interface. For the BP-A configuration with the system KCl–juice, it was believe that the accumulation of OH− in the KCl solution, which was more significant than the accumulation of H+ in the juice at the half time of the treatment, has contributed to move the neutralization interface from the membrane downstream to the membrane upstream, which decreased the effect of dilution. This resulted in a decrease of the anionic membrane resistance after the half time of the treatment (Fig. 5C). For the HCl–juice system, the resistance of the anionic membrane is smaller than for the KCl–KCl and KCl–juice systems due to the absence of water formation. Furthermore, the formation of HCl inside the membrane decreases significantly the anionic membrane resistance as a function of treatment evolution (Fig. 6C). This phenomenon was observable even when the concentration and the conductivity of the downstream HCl solution are decreasing. 4.3.4. Bipolar membrane resistance The variation of the bipolar membrane resistance, with the nature of the bulk solutions, was similar but less important than for monopolar membranes (Figs. 2C–6C). This is certainly attributable to the fact that in presence of an external electrical field, it is mainly the H+ and OH− ions that will migrate out of the transition layer and the water molecules that will diffuse inside the membrane [19].
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low electro-acidification performance and to a high-energy consumption. For the case of a biological solution that is poor in ions, especially the cations, such as the apple juice, this configuration is not appropriate when a large pH variation is desired. A limit pH is imposed by the low concentration in organic acid of the juice [5]. The cell resistances of the BP-A configuration were larger than the resistances of the BP-C configuration (Fig. 7) due to the water formation, which was more significant for the anionic membrane. However, the migration of anions (Cl− ), which are responsible of the solution acidification, represents a larger fraction of the ionic flux than the K+ in the BPC configuration. This resulted in a more efficient treatment when compared to the BP-C configuration. The use of HCl instead of KCl, for the BP-A configuration enables to benefit from the advantages of each configuration: a small anionic membrane resistance due to the transport of H+ , as for the BP-C configuration, and a high acidification rate due the neutralization of OH− by the HCl. It was for this system that the acidification rate was the highest. The measurement of the transmembrane potential in absence of an external electrical field, enabled to establish the qualitative contribution of Donnan, diffusion and streaming potentials to the total transmembrane potential. In absence of an external electrical field, only the bipolar transmembrane potential is significant. The evolution of the potential after the external electrical field is switched-off also enabled to improve our understanding of the ionic diffusion into the transition layer of the bipolar membrane. Furthermore, influence of the membrane boundary layers on the ionic transport was also investigated by working at different feed flow rates. It was shown that for the present operating conditions, the boundary layers do not significantly affect the ionic transport through the membranes. However, further results including the measurements of transport number of anion (Cl− ) and cations (K+ , H+ ) and a complete mass balance on electrodialysis system would be required to fully demonstrate the assumptions proposed in this paper.
Acknowledgements 5. Conclusion In this study, the performance of two electro-acidification configurations (BP-C and BP-A) was compared using inorganic solutions and apple juice under different conditions. The measurement of the transmembrane potential and the estimation of the membrane resistance during the treatments enabled to assess ion transport and its influence on system performance. The cell resistances of the BP-C configuration (Fig. 7) were lower than for the BP-A counterpart configuration, and this was principally attributable to the small resistance of the cationic membrane. However, the H+ who are not effectively retained in the acidic compartment, have contributed to a
The financial support of the “Conseil des Recherches en Pˆeche et en Agroalimentaire du Qu´ebec”, A. Lassonde inc., Agriculture and Agri-Food Canada and of the Natural Sciences and Engineering Research Council of Canada is acknowledged.
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