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Alpha-ketoglutaric acid production using electrodialysis with bipolar membrane
Journal of Membrane Science 536 (2017) 37–43
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Alpha-ketoglutaric acid production using electrodialysis with bipolar membrane Mateusz Szczygiełda, Krystyna Prochaska
MARK
⁎
Institute of Chemical Technology and Engineering, Poznan University of Technology, Berdychowo str. 4, 60-965 Poznan, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords: Alpha-ketoglutaric acid Bipolar membrane Electrodialysis Current efficiency
Environmentally friendly process of production of alpha-ketoglutaric acid (AKG) from model water solutions by bipolar membrane electrodialysis (EDBM) was carried out. Two-chamber EDBM stack with the anion exchange membrane-bipolar membrane (AM-BM) configuration was used to transport AKG ions through AM from the diluate to concentrate chamber and convert them to AKG acidic form. The influence of initial pH of diluate solution, initial concentration of AKG salts in the diluate and concentrate chambers and current efficiency was evaluated by considering such factors as voltage drop, alpha-ketoglutaric acid concentration, current efficiency and energy consumption for 1 kg of AKG production. Under optimum conditions the process EDBM allows achievement of a high concentration of AKG, high current efficiency and low energy consumption, equal to 4.83 g/L, 71.8%, 3.72 kW h/kg, respectively, while the corresponding results obtained after 180 min EDBM process without optimization were 1.6 g/L, 24% and 15.26 kW h/kg, respectively. In addition, the obtained results indicate that bipolar membrane electrodialysis may be used for the production of AKG from actual post fermentation broth in the future.
1. Introduction Alpha-ketoglutaric acid (AKG) is a low molecular organic compound which is classified as a keto acid or oxoacid having in its structure both carboxyl groups as well as ketone groups [1]. It should be noted that keto acids having a carbonyl group in the alpha position are key intermediates in the amino acid metabolism and tricarboxylic acid (TCA) cycle [2]. In addition, a wide range of applications of this compound, e.g. in food industry, chemical industry, agriculture and especially in medicine and pharmacy (used as dietary supplement, component of infusion solutions, an agent that improves nitrogen balance in patients with burns etc.) is the reason for increasing demand for this metabolite expected in the coming years [3–5]. As well known, the majority of alpha-ketoglutaric acid is produced by chemical synthesis from diethyl succinate and diethyl oxalate [6]. However, this synthesis process is multi-step, low yielding and needs dangerous substrates such as cyanides, toluene and sodium metal, which can generate toxic waste [7,8]. Therefore, increased interest in biotechnological methods for conversion of carbon sources (glycerol, ethanol, nparaffins) into organic compounds using microorganisms can be an alternative eco-friendly way of obtaining valuable raw materials [9]. Due to the multitude of by-products present in actual post-fermentation
broth, the process of AKG production requires multiple purification steps (filtration, precipitation and acidification) [7]. Furthermore, it is important that the AKG present in actual fermentation broth occurs mainly in the form of AKG salt. Therefore, the acidification allows conversion of the salt to the acidic form. However, on the one hand, in the acidification step it is necessary to use large amounts of mineral acids (which may have a negative impact on the environment). On the other hand, this process can generate a considerable amount of wastes [10]. It appears that the use of membrane separation techniques including bipolar membrane electrodialysis (EDBM) can allow the efficient, wasteless and environmentally safe obtaining of valuable raw material [11,12]. Moreover, membrane-based processes show high potential for scaling up from laboratory to industrial level. Unfortunately, the main disadvantage of using the electrodialysis process are the operation cost (connected with high electric energy consumption) which are greater than that of the precipitation process. Bipolar membrane (BM) consisting of cation and anion exchange layer can split water into OH- and H+ ions at the intermediate layer under reverse bias conditions and directly convert organic salts into organic acids [13,14]. According to literature, various organic acids and bases have been produced by EDBM. For example in 2002 Xu et al. [15] have presented the process of production of citric acid from
Abbreviations: AKG, alpha-ketoglutaric acid; EDBM, bipolar membrane electrodialysis; AM, anion exchange membrane; BM, bipolar membrane; TCA, tricarboxylic acid cycle ⁎ Corresponding author. E-mail address: [email protected] (K. Prochaska). http://dx.doi.org/10.1016/j.memsci.2017.04.059 Received 20 January 2017; Received in revised form 24 April 2017; Accepted 26 April 2017 Available online 27 April 2017 0376-7388/ © 2017 Elsevier B.V. All rights reserved.
Journal of Membrane Science 536 (2017) 37–43
M. Szczygiełda, K. Prochaska
sodium citrate by bipolar membrane electrodialysis. The same authors have reported that the highest citric acid concentration obtained after 200 min of EDBM process was equal to 30 g/L. In 2011 Wang et al. [12] have suggested that EDBM can be used for the production of monoprotic, diprotic and triprotic organic acids such as acetic acid, oxalic acid and citric acid, from aqueous solutions containing salts of the corresponding acids. Similarly Fu et al. [16] have used bipolar membrane electrodialysis to convert sodium succinate into succinic acid. In addition, many literature reports indicate that EDBM can be effectively used for the production of other organic acids, especially lactic acid [17], fumaric acid [18], tartaric acid [19], salicylic acid [20] and amino acids [21]. In this study bipolar membrane electrodialysis process was used to produce alpha-ketoglutaric acid from model solutions with sodium alpha-ketoglutarate and to evaluate the influence of initial pH of diluate solutions, initial concentration of AKG in the concentrate and diluate chamber and current density, on the performance of EDBM process. Moreover, the optimized bipolar membrane electrodialysis was carried out.
Fig. 1. Schematic of two-chamber laboratory EDBM configuration (AM-BM) (modeled after [12]).
concentrate samples were collected at regular time intervals and analyzed. The voltage changes as a function of time were recorded to evaluate electrical resistance of the stack. All EDBM experiments were conducted for 180 min. Fig. 1 shows the electrodialysis membrane stack with AM-BM configuration consisting of 1 anion exchange membranes (AM) and 1 bipolar membranes (BM), which could be used to convert the sodium alpha-ketoglutarate into alpha-ketoglutaric acid. The electrolyte solution (0.3 M Na2SO4) was placed between the bipolar membrane and the anode. During the EDBM processes, when the constant electric field was applied, alpha-ketoglutarate ions present in diluate solution were transported across the anion exchange membranes to the concentrate chamber. At the same time, alpha-ketoglutarate salt was converted into alpha-ketoglutaric acid in the concentrate chamber with the H+ ions produced from the water split by the bipolar membrane.
2. Material and methods 2.1. Materials The study of alpha-ketoglutaric acid production from model solutions was performed with sodium alpha-ketoglutarate as a substrate. All sodium alpha-ketoglutarate solutions were prepared by adding NaOH to water solutions of alpha-ketoglutaric acid. Moreover model solutions were prepared with deionized water of conductivity not exceeding 3 µS/cm, their pH were adjusted in range 3–10 by the addition of sodium hydroxide. In addition, the conductivity of initial solutions of alpha-ketoglutaric acid is shown in Table 1. All components of model solutions were purchased from Sigma-Aldrich. 2.2. Bipolar membrane electrodialysis equipment and methods
2.3. Analytical methods
In this study a two-chamber laboratory EDBM setup with a stack consisting of 1 bipolar (PC 200bip) and 1 anion-exchange (PC 200D) membrane (produced by PCCell GmbH, Germany) was used. The spacing between the membranes was 10 mm in thickness and the effective surface area of each membrane was equal to 64 cm2. Additionally, the two-chamber laboratory EDBM setup was connected to a peristaltic pump (Verder, Poland) DC power supply (NDN) and a multifunction meter (Elmetron, Poland) measuring the pH, temperature and conductivity of working solutions. The cathode was made of steel 314 and the anode was made of titanium plated with iridium. In each compartment, solutions were circulated at the flow rate of 5.6 L/h. The process was carried out at 25 ± 2 °C and under constant current density from the range 65–125 A/m2. During the EDBM processes, the pHvalue, temperatures of diluate and concentrate solutions were controlled. In each experiment the model solution was fed to the diluate chamber while the solution of sodium alpha-ketoglutarate at the concentration from the range 0.3–5 g/L and pH of 8.5 was fed into the concentrate chamber, which allowed a decrease in the cell voltage at the beginning of the experiment. In EDBM processes the diluate and
The contents of alpha-ketoglutaric acid and its salts in the starting solution and the product obtained in EDBM processes were determined using high performance liquid chromatography HP Agilent 1100 Series (Germany) equipped with an autosampler, interface (HP 35900), RI Detector (HP 1047 A), pump (HP1050), and separating column Rezex ROA-Organic Acid H+(8%), Phenomenex®. The eluent of 2.5 mM H2SO4 solution was constantly supplied at the rate of 0.9 ml/min. The column temperature and that at the input to the detector was 40 °C, P=0.56 MPa. All samples were acidified to pH≤2 by addition of 0.1 ml 25% H2SO4 to 1 ml of sample before analysis. 3. Calculations The average value of energy consumed for 1 kg of alpha-ketoglutaric acid production was determined using the equation:
E=
Conductivity, mS/cm
3 5 7 10
3.42 4.69 5.74 6.96
(1)
where: E – energy consumption needed to produce 1 kg of AKG, kW h/ kg; U – voltage, V; I – current, A; m – mass of the final product, g; t – time, h; The average value of the current efficiency was calculated on the basis of the following equation:
Table 1 The change in conductivity of AKG solution with different initial concentration. Concentration, g/L
U ·I ·t m
CE =
F·z·V·ΔCdil ·100% n·I·Δt
(2)
where: CE – current efficiency, %; F – Faraday's constant (96,485), C/ mol; I – current, A; z – valence of ions; V – diluate volume, L; ΔCdil – change of AKG concentration in diluate chamber, mol/L; n – number of 38
Journal of Membrane Science 536 (2017) 37–43
M. Szczygiełda, K. Prochaska
cells; Δt – time, s; The degree of dissociation of monoprotic acid was calculated as follows:
α1 =
[A− ] [A−] = [HA][A−] CHA
nificantly with decreasing initial pH of the solution in the diluate chamber. The highest concentration of alpha-ketoglutaric acid in the concentrate chamber after 180 min of EDBM process (equal to 2.18 g/ L) was obtained when the initial pH of starting solution was the lowest and equal to 3. On the one hand, a higher degree of dissociation of salts present in the initial solution can facilitate the transport of AKG anions through the anion exchange membrane, while on the other hand, to obtain a higher pH value of working solutions it is necessary to add a greater amount of OH- ions, which can compete with AKG anions in transfer across the anion exchange membrane and also decrease EDBM efficiency [24]. Moreover, analysis of Fig. 1, (which illustrates the EDBM stack configuration) implies that during EDBM processes, additional hydroxyl ions are generated (as products of water reduction which occurs at the cathode). This phenomenon leads to the alkalization of diluate solutions in the time of EDBM processes, as shown in Fig. 3b. It should also be noted that, in all systems considered, the alkalization of diluate solutions rapidly increases to pH of about 11 after exceeding the equivalence point. However, for the solutions of initial pH values lower or equal to 5, the effect of alkalization is observed after a much longer time (Fig. 3b). A similar relationship has been observed by other authors on titration of weak organic acids [25]. Fig. 4 compares the changes in concentration of OH- ions in the diluate chamber (determined by titration of AKG), during EDBM processes of 1-component model solutions of initial pH 4 and 10, respectively. In both cases the concentration of OH- in the diluate chamber increases during EDBM. However, in bipolar membrane electrodialysis of a solution with a low initial pH value, the hydroxyl ions produced before reaching the equivalence point can be neutralized by hydrogen ions present in the starting solution or generated by the bipolar membrane. The migration of hydrogen ions across the anion exchange membrane to the diluate chamber can be explained by H+ ions leakage due to their small diameter and high mobility [18]. In addition, as shown in Fig. 4, the increased competition between OHand AKG anions is observed in about 80 min after reaching the equivalence point. On the other hand, during bipolar membrane electrodialysis of a solution with a high initial pH value, the presence of a large number of OH- ions, initially added to the starting solution and generated by the cathode during the EDBM process, is an obstacle for the transport of AKG ions in the process of EDBM. Therefore, the use of solutions of low initial pH is recommended.
(3)
where: CHA = [HA]+[A ] – total concentration of weak acid The fraction of the acid that is not dissociated was calculated as follows: -
α0 =
[HA] [HA] = [HA][A−] CHA
(4)
and moreover the acid dissociation constant was calculated on the basis of the following equation:
Ka = [H +]
log
α1 α0
α1 = pH – pKa α0
(5)
(6)
where: Ka – dissociation constant. 4. Results and discussion 4.1. Effect of pH value for the AKG solutions As well known, alpha-ketoglutaric is a weak organic acid which, depending on the pH of solutions, dissociates in two stages: a) H2AKG + H2O ↔ HAKG− + H+ Ka1=3.4·10−3 b) HAKG− + H2O ↔ AKG2− + H+ Ka2=2.1·10−5 where Ka is the acid dissociation constant, which is a measure of acid strength. In addition, using Eqs. (3)–(6) [22] one can find the relation between the degree of dissociation and pH of AKG acid, presented in Fig. 2. The results shown in Fig. 2. indicate that the degree of dissociation of alpha-ketoglutaric acid increases with increasing pH of solution, similarly as for other organic acids [23]. Moreover, it should be noted that AKG acid reaches a maximum level of dissociation in solutions with pH higher than 7. The state of dissociation of the compounds present in operating solution determines the EDBM process. Thus in our study the effect of pH value of the initial solution on the production of AKG from 1component model solution (with different initial pH value from the range 3–10), in the time of EDBM processes was checked. All EDBM processes were carried out at the current density of 65 A/m2. Fig. 3a illustrates the change in the concentration of alphaketoglutaric acid in the concentrate chamber during the EDBM processes in which the initial solutions of pH equal to 3, 4, 6, 8 and 10 were used. The concentration of alpha-ketoglutaric acid in the concentrate chamber in the time of EDBM processes, increased sig-
4.2. Effect of the initial concentration of AKG salts in concentrate chamber In the next part of this study the effect of initial concentration of AKG salts in the concentrate chamber on the production of alphaketoglutaric acid in the EDBM processes (at different initial concentrations ranging from 0.3 to 5 g/L) was analyzed. All EDBM processes were carried out at the current density of 65 A/m2. Fig. 5a shows the effect of the initial concentration of AKG salts in the concentrate chamber on the voltage drop of the stack in the time of EDBM. The initial voltage drop of the stack increases rapidly with decreasing starting concentration of AKG salts in the concentrate chamber and decreases in each EDBM processes until reaching plateau. A similar relationship has been observed by Zhou et al. [26] during EDBM production of water insoluble organic acid. It is also worth noting that the greatest decrease in voltage (from 35.1 to 18.4 V) after 180 min of EDBM process was noted when the initial concentration of AKG salts in the concentrate chamber was the lowest, of 0.3 g/L. A low initial concentration of salts in the concentrate chamber is responsible for a relatively high resistance and, as consequence, high voltage at the beginning of the process. Furthermore, the observed voltage drop in the following minutes of the EDBM process is due to large amounts of H+ and OH- ions which are generated by bipolar membrane and AKG ions transported through the anion exchange membrane from the diluate to the concentrate chamber, which leads to an increase in the conductivity of solutions in the concentrate compartment [20]. As shown in Fig. 5a,
Fig. 2. Degree of dissociation as a function of pH-value of AKG.
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M. Szczygiełda, K. Prochaska
Fig. 3. Change in the concentration of AKG in concentrate chamber (ΔC) (a) and pH–value in diluate chamber (pH) (b) during EDBM processes, C0dil=3 g/L, C0con=0.3 g/L, i=65 A/m2, T=25 ± 2 °C.
Fig. 5b shows the effect of the initial concentration of AKG salts in the concentrate chamber on the AKG ions transport through the anion exchange membrane during EDBM processes. The acid concentration increased rapidly at first, when a high initial concentration of salts in acid compartment was used. These differences in the AKG concentration increase are due to significant differences in the initial conductivities of concentrate solutions, moreover they decrease in the time. Is therefore clear that the higher the resistance, the lower the number of AKG ions transported. The initial concentration of AKG salts in the concentrate chamber affects the current efficiency in EDBM processes (Fig. 5c). In all systems studied, the highest current efficiency was reached after 30 min of EDBM process. Furthermore, the higher initial concentration of AKG salts in the concentrate chamber leads to an increase in the current efficiency at the beginning of the EDBM process, which is related to increased transport of AKG ions through the anion exchange membrane in the first period of EDBM (Fig. 5b). However, current efficiency decreases with time especially dramatically in the systems with a high initial concentration in the concentrate chamber. On the one hand, the trend of growing concentration is not linear and weakens with time [17]. On the other hand, as reported in [12], the pass of OH− through
Fig. 4. Change in the concentration of OH- in diluate chamber during EDBM processes of 1-component model solutions, C0dil=3 g/L, C0con=0.3 g/L, i=65 A/m2, T=25 ± 2 °C.
higher initial concentrations of AKG salts ( 3 g/L and 5 g/L) in the concentrate chamber imply a higher conductivity of solutions in 180 min of EDBM. However, it should be stressed that a further increase in the initial concentration of AKG salts can lead to a decrease in EDBM efficiency (as a result of back-diffusion) [17].
Fig. 5. Changes in the voltage drop (U) (a), concentration of AKG in concentrate chamber (ΔC) (b), current efficiency (CE) (c) and energy consumption (EC) (d) during EDBM processes, C0dil=5 g/L, i=65 A/m2, T=25 ± 2 °C.
40
Journal of Membrane Science 536 (2017) 37–43
M. Szczygiełda, K. Prochaska
concentrate compartments. As reported in [28] the diffusion transport has a significant influence on the ions transported across the ionexchange membranes and cannot be neglected in the EDBM system. The effect of the initial concentration of AKG salts in the diluate chamber on the current efficiency is illustrated in Fig. 6c. The increase in AKG ion transport through the anion-exchange membrane caused by the extra driving force leads to a rapid increase in the current efficiency for the initial salts concentration varying in the range of 1–10 g/L. A similar relationship has been observed by other authors studying EDBM/ED production of other compounds such as: salicylic and γamino butyric acid [20,29]. Fig. 6d illustrates the effect of the initial concentration of AKG salts in the diluate chamber on the energy consumption for 1 kg of alphaketoglutaric produced. It is obvious that higher initial concentration of AKG salts in the diluate chamber was conducive to decreasing energy consumption needed for production of 1 kg of AKG acid in EDBM. Firstly, higher AKG salts concentrations reduce the electrical resistance of the stack and secondly a greater amount of the acid was produced in 180 min of the EDBM process.
the anion exchange membrane can reduce the current efficiency of EDBM process in BM-AM configuration. The results obtained in this part of the study indicate that the initial concentration of AKG salts in the concentrate chamber has a significant impact on the energy consumption for production of 1 kg of alphaketoglutaric acid. As shown in Fig. 5d, the energy consumption for 1 kg of alpha-ketoglutaric acid production rapidly decreases (from 10 to 5.6 kW h/kg after 180 min of EDBM), with increasing initial salts concentration in the concentrate chamber (form 0.3–5 g/L, respectively). This effect can be explained by an decrease in the area resistance of the solutions with their increasing concentration [27]. Therefore, much more energy was consumed to overcome electrical resistance, when the initial concentration of salts in concentrate chamber was too low [26]. 4.3. Effect of the initial concentration of AKG salts in diluate chamber Another objective of our study was to assess the impact of the initial concentration of AKG salts in the diluate chamber on the production of alpha-ketoglutaric acid in the process of EDBM performed for AKG solutions of different initial concentrations varying in the range from 3 to 10 g/L. All EDBM processes were carried out at the current density of 65 A/m2. Fig. 6a shows the effect of the initial concentration of AKG salts in the diluate chamber on the voltage drop of the stack during the EDBM process. The results indicate that the voltage value observed at the beginning of the EDBM process decreases with increasing initial salts concentration in the dilute chamber, which is related to the lower resistance. (similarly as described in Section 4.2). Fig. 6b illustrates the effect of the initial concentration of AKG salts in the diluate chamber on the alpha-ketoglutaric acid concentration. Is easy to see that the AKG acid concentration increases with increasing initial concentration of AKG salts. As shown in Fig. 6b, the highest AKG concentration after 180 min of EDBM process, equal to 3.1 g/L, was obtained when the starting solution concentration was equal to 10 g/L AKG salts. This effect can be explained by the presence of additional driving force due to the concentration gradient between the diluate and
4.4. Effect of the current density The current density is the next parameter which significantly affected the production of alpha-ketoglutaric acid in the process of EDBM. Fig. 7a presents the effect of the current density of 65, 85 and 125 A/m2, on the voltage drop at the stack in the time of EDBM. The voltage drop across the EDBM stack significantly increases with increasing current density. The increase in current density implies an increase in the electrical resistance. The same effect has been reported earlier in [30,31]. Additionally, the authors of these papers have concluded that more electrical energy is consumed to overcome the electrical resistance at a higher current density. Furthermore, it can be noticed that the voltage drop at the EDBM stack decreases with time as a result of water dissociation by bipolar membrane as well as the effect of migration of AKG ions. However, as reported by Wang et al. [32], the contribution of water dissociation by bipolar membrane to the voltage drop is more significant.
Fig. 6. Changes in the voltage drop (U) (a), concentration of AKG in concentrate chamber (ΔC) (b), current efficiency (CE) (c) and energy consumption (EC) (d) during EDBM processes, C0con=1 g/L, i=65 A/m2, T=25 ± 2 °C.
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Journal of Membrane Science 536 (2017) 37–43
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Fig. 7. Changes in the voltage drop (U) (a), concentration of AKG in concentrate chamber (ΔC) (b), current efficiency (CE) (c) and energy consumption (EC) (d) during EDBM processes, C0dil=5 g/L, C0con=1 g/L, T=25 ± 2 °C.
4.5. Optimization of EDBM processes for the production of AKG
Fig. 7b presents the effect of current density on the concentration of alpha- ketoglutaric acid. It is easy to see that the concentration of AKG acid increases with increasing current density, which is consistent with literature and results of our own previous investigation indicating that it is typical trend of electrodialysis processes [18,20,33]. Obviously, the driving force of the process increases with increasing current density, which leads to higher transport of AKG ions through the anion exchange membrane and simultaneously enhances the water dissociation by bipolar membrane according to the second Wien effect [11]. Analyzing the effect of the current density on the current efficiency of EDBM process (Fig. 7c) one can conclude that in all systems considered, the current efficiency is the highest at the beginning of EDBM process and thus the values of current efficiency dramatically decrease with the progress of the process. This effect could mainly be caused by the hydroxide ions competition [16]. In EDBM process, when a high current density was applied, greater amounts of hydroxide ions were generated by the cathode reaction in the EDBM stack, which consequently led to enhancement of the competition with AKG ions to go across the anion exchange membrane. Moreover these hydroxyl ions can react with H+ ions generated by BM [12]. Similar effects have been described in [32]. Moreover, as reported by Li [34], a high current density can lead to strengthened dissociation of water to H+ and OHions and water transport caused by the electro-osmosis effect [26,34]. Fig. 7d illustrating the effect of the current density on energy consumption for 1 kg of alpha-ketoglutaric production indicates that the energy consumption increases with increasing value of applied current density. As mentioned previously, with increasing current density a greater part of the total electrical energy had to be consumed to overcome the electrical resistance. Furthermore, the energy consumption rapidly grows during the EDBM processes. As shown in Fig. 7b the increment in the concentration of the acid decreases during EDBM process, which consequently leads to increase in the energy consumption for 1 kg of alpha-ketoglutaric production in the later time of processes (Fig. 7d). Therefore, EDBM process at high current density and run for a long time is uneconomical and/or impractical from the point of view of getting a high concentration of AKG.
At the final part of this study the EDBM process was carried out in the optimized experimental conditions (i.e. in the conditions which allowed to obtained the best results, shown above), so at C0dil=10 g/L, C0con=3 g/L, i=65 A/m2 and pHinit.=4. As follows from the above discussion, the current density of 65 A/m2 is optimal for the production of AKG from practical and economical point of view. This current density results in decreasing concentration of AKG acid after 180 min of EDBM, at the same time allowing high current efficiency and low energy consumption. As shown in Fig. 8, a very low starting pH value of the diluted solution (equal to 4), allowed maintaining a low pH value from the range of 4–5.6 during the entire EDBM process. A significant part of OH- ions (generated at the cathode) was used for neutralization of diluate solution and could not compete with AKG ions in the transport through the anion exchange membrane. The AKG salt concentrations in the diluate and concentrate chamber were equal to 10 and 3 g/L, respectively. This allowed obtaining high conductivity of solutions (the voltage drop in the range of 15.8–14.6 V, respectively) as well as high concentration gradient, which led to diffusion transport supporting the AKG ions transfer by AM.
Fig. 8. Changes in the pH value in diluate chamber during EDBM process, C0dil=10 g/L, C0con=3 g/L, i=65 A/m2, pHinit=4, T=25 ± 2 °C.
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Finally in the optimized EDBM process, a high concentration of AKG, current efficiency and low energy consumption equal to 4.83 g/L, 71.8%, 3.72 kW h/kg, respectively, were obtained, while the corresponding values in the EDBM process without optimization were equal to 1.6 g/L, 24% and 15.26 kW h/kg, respectively, after 180 min of EDBM. Thus, the obtained results are much better, however the future studies should be focused on the possibilities of further lowering the energy consumption, as high values of this parameter are the greatest disadvantage of the electrodialysis processes.
[5]
[6]
[7]
[8]
5. Conclusion [9]
A novel, environmental friendly bipolar membrane electrodialysis method, was used for the production of alpha-ketoglutaric acid from model water solutions. The effects of operating conditions such as: i) initial pH value in diluate chamber, ii) initial concentration of sodium alpha-ketoglutarate in working solutions and iii) current efficiency, on the EDBM processes were investigated. In addition, the optimized bipolar membrane electrodialysis was carried out. The performance of EDBM processes was evaluated from the voltage drop, alphaketoglutaric acid concentration, current efficiency and energy consumption for production of 1 kg of alpha-ketoglutaric acid. According to the results, the highest efficiency of EDBM processes was obtained when the initial pH value in the diluate chamber was equal to 3 (it reduced a competition between OH- and AKG anions), the initial concentration of AKG salts in the concentrate chamber was equal to 3 g/L (decreased resistance of the EDBM stack), the initial concentration of AKG salts in the diluate chamber was equal to 10 g/L (increased additional driving force due to the concentration gradient) and current density was equal to 65 A/m2 (reduced energy consumption). Moreover carrying out the process EDBM in the optimal conditions, allowed getting high concentration of AKG, high current efficiency and low energy consumption equal to 4.83 g/L, 71.8%, 3.72 kW h/kg, respectively, while the corresponding data obtained in the EDBM process without optimization were equal to 1.6 g/L, 24% and 15.26 kW h/kg, respectively after 180 min of EDBM. Furthermore, bipolar membrane electrodialysis may be used for production of AKG acid from the actual post fermentation broth. However, further studies are needed because the present work describes only a preliminary research. Thus, it is too early to talk about using EDBM for an industrial production of alphaketoglutaric acid. On the other hand, the results obtained may provide an optimistic basis for further studies. There is much more work to do before bringing the process coupling to industrialization. The most important will be to scale-up the experiment from currently one repeating unit and further purification of the final product.
[10]
[11] [12]
[13]
[14]
[15] [16] [17]
[18] [19]
[20] [21] [22] [23] [24]
[25] [26]
[27]
Acknowledgements [28]
The authors wish to acknowledge the financial support from the Polish Ministry of Science and Higher Education (Grant No. DS-PB/ 0701). We would like to thank Dr hab. Daria SzymanowskaPowałowska from Poznan University of Life Sciences for her inspiration to study AKG acid.
[29] [30]
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Alpha-ketoglutaric acid production using electrodialysis with bipolar membrane Mateusz Szczygiełda, Krystyna Prochaska
MARK
⁎
Institute of Chemical Technology and Engineering, Poznan University of Technology, Berdychowo str. 4, 60-965 Poznan, Poland
A R T I C L E I N F O
A B S T R A C T
Keywords: Alpha-ketoglutaric acid Bipolar membrane Electrodialysis Current efficiency
Environmentally friendly process of production of alpha-ketoglutaric acid (AKG) from model water solutions by bipolar membrane electrodialysis (EDBM) was carried out. Two-chamber EDBM stack with the anion exchange membrane-bipolar membrane (AM-BM) configuration was used to transport AKG ions through AM from the diluate to concentrate chamber and convert them to AKG acidic form. The influence of initial pH of diluate solution, initial concentration of AKG salts in the diluate and concentrate chambers and current efficiency was evaluated by considering such factors as voltage drop, alpha-ketoglutaric acid concentration, current efficiency and energy consumption for 1 kg of AKG production. Under optimum conditions the process EDBM allows achievement of a high concentration of AKG, high current efficiency and low energy consumption, equal to 4.83 g/L, 71.8%, 3.72 kW h/kg, respectively, while the corresponding results obtained after 180 min EDBM process without optimization were 1.6 g/L, 24% and 15.26 kW h/kg, respectively. In addition, the obtained results indicate that bipolar membrane electrodialysis may be used for the production of AKG from actual post fermentation broth in the future.
1. Introduction Alpha-ketoglutaric acid (AKG) is a low molecular organic compound which is classified as a keto acid or oxoacid having in its structure both carboxyl groups as well as ketone groups [1]. It should be noted that keto acids having a carbonyl group in the alpha position are key intermediates in the amino acid metabolism and tricarboxylic acid (TCA) cycle [2]. In addition, a wide range of applications of this compound, e.g. in food industry, chemical industry, agriculture and especially in medicine and pharmacy (used as dietary supplement, component of infusion solutions, an agent that improves nitrogen balance in patients with burns etc.) is the reason for increasing demand for this metabolite expected in the coming years [3–5]. As well known, the majority of alpha-ketoglutaric acid is produced by chemical synthesis from diethyl succinate and diethyl oxalate [6]. However, this synthesis process is multi-step, low yielding and needs dangerous substrates such as cyanides, toluene and sodium metal, which can generate toxic waste [7,8]. Therefore, increased interest in biotechnological methods for conversion of carbon sources (glycerol, ethanol, nparaffins) into organic compounds using microorganisms can be an alternative eco-friendly way of obtaining valuable raw materials [9]. Due to the multitude of by-products present in actual post-fermentation
broth, the process of AKG production requires multiple purification steps (filtration, precipitation and acidification) [7]. Furthermore, it is important that the AKG present in actual fermentation broth occurs mainly in the form of AKG salt. Therefore, the acidification allows conversion of the salt to the acidic form. However, on the one hand, in the acidification step it is necessary to use large amounts of mineral acids (which may have a negative impact on the environment). On the other hand, this process can generate a considerable amount of wastes [10]. It appears that the use of membrane separation techniques including bipolar membrane electrodialysis (EDBM) can allow the efficient, wasteless and environmentally safe obtaining of valuable raw material [11,12]. Moreover, membrane-based processes show high potential for scaling up from laboratory to industrial level. Unfortunately, the main disadvantage of using the electrodialysis process are the operation cost (connected with high electric energy consumption) which are greater than that of the precipitation process. Bipolar membrane (BM) consisting of cation and anion exchange layer can split water into OH- and H+ ions at the intermediate layer under reverse bias conditions and directly convert organic salts into organic acids [13,14]. According to literature, various organic acids and bases have been produced by EDBM. For example in 2002 Xu et al. [15] have presented the process of production of citric acid from
Abbreviations: AKG, alpha-ketoglutaric acid; EDBM, bipolar membrane electrodialysis; AM, anion exchange membrane; BM, bipolar membrane; TCA, tricarboxylic acid cycle ⁎ Corresponding author. E-mail address: [email protected] (K. Prochaska). http://dx.doi.org/10.1016/j.memsci.2017.04.059 Received 20 January 2017; Received in revised form 24 April 2017; Accepted 26 April 2017 Available online 27 April 2017 0376-7388/ © 2017 Elsevier B.V. All rights reserved.
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sodium citrate by bipolar membrane electrodialysis. The same authors have reported that the highest citric acid concentration obtained after 200 min of EDBM process was equal to 30 g/L. In 2011 Wang et al. [12] have suggested that EDBM can be used for the production of monoprotic, diprotic and triprotic organic acids such as acetic acid, oxalic acid and citric acid, from aqueous solutions containing salts of the corresponding acids. Similarly Fu et al. [16] have used bipolar membrane electrodialysis to convert sodium succinate into succinic acid. In addition, many literature reports indicate that EDBM can be effectively used for the production of other organic acids, especially lactic acid [17], fumaric acid [18], tartaric acid [19], salicylic acid [20] and amino acids [21]. In this study bipolar membrane electrodialysis process was used to produce alpha-ketoglutaric acid from model solutions with sodium alpha-ketoglutarate and to evaluate the influence of initial pH of diluate solutions, initial concentration of AKG in the concentrate and diluate chamber and current density, on the performance of EDBM process. Moreover, the optimized bipolar membrane electrodialysis was carried out.
Fig. 1. Schematic of two-chamber laboratory EDBM configuration (AM-BM) (modeled after [12]).
concentrate samples were collected at regular time intervals and analyzed. The voltage changes as a function of time were recorded to evaluate electrical resistance of the stack. All EDBM experiments were conducted for 180 min. Fig. 1 shows the electrodialysis membrane stack with AM-BM configuration consisting of 1 anion exchange membranes (AM) and 1 bipolar membranes (BM), which could be used to convert the sodium alpha-ketoglutarate into alpha-ketoglutaric acid. The electrolyte solution (0.3 M Na2SO4) was placed between the bipolar membrane and the anode. During the EDBM processes, when the constant electric field was applied, alpha-ketoglutarate ions present in diluate solution were transported across the anion exchange membranes to the concentrate chamber. At the same time, alpha-ketoglutarate salt was converted into alpha-ketoglutaric acid in the concentrate chamber with the H+ ions produced from the water split by the bipolar membrane.
2. Material and methods 2.1. Materials The study of alpha-ketoglutaric acid production from model solutions was performed with sodium alpha-ketoglutarate as a substrate. All sodium alpha-ketoglutarate solutions were prepared by adding NaOH to water solutions of alpha-ketoglutaric acid. Moreover model solutions were prepared with deionized water of conductivity not exceeding 3 µS/cm, their pH were adjusted in range 3–10 by the addition of sodium hydroxide. In addition, the conductivity of initial solutions of alpha-ketoglutaric acid is shown in Table 1. All components of model solutions were purchased from Sigma-Aldrich. 2.2. Bipolar membrane electrodialysis equipment and methods
2.3. Analytical methods
In this study a two-chamber laboratory EDBM setup with a stack consisting of 1 bipolar (PC 200bip) and 1 anion-exchange (PC 200D) membrane (produced by PCCell GmbH, Germany) was used. The spacing between the membranes was 10 mm in thickness and the effective surface area of each membrane was equal to 64 cm2. Additionally, the two-chamber laboratory EDBM setup was connected to a peristaltic pump (Verder, Poland) DC power supply (NDN) and a multifunction meter (Elmetron, Poland) measuring the pH, temperature and conductivity of working solutions. The cathode was made of steel 314 and the anode was made of titanium plated with iridium. In each compartment, solutions were circulated at the flow rate of 5.6 L/h. The process was carried out at 25 ± 2 °C and under constant current density from the range 65–125 A/m2. During the EDBM processes, the pHvalue, temperatures of diluate and concentrate solutions were controlled. In each experiment the model solution was fed to the diluate chamber while the solution of sodium alpha-ketoglutarate at the concentration from the range 0.3–5 g/L and pH of 8.5 was fed into the concentrate chamber, which allowed a decrease in the cell voltage at the beginning of the experiment. In EDBM processes the diluate and
The contents of alpha-ketoglutaric acid and its salts in the starting solution and the product obtained in EDBM processes were determined using high performance liquid chromatography HP Agilent 1100 Series (Germany) equipped with an autosampler, interface (HP 35900), RI Detector (HP 1047 A), pump (HP1050), and separating column Rezex ROA-Organic Acid H+(8%), Phenomenex®. The eluent of 2.5 mM H2SO4 solution was constantly supplied at the rate of 0.9 ml/min. The column temperature and that at the input to the detector was 40 °C, P=0.56 MPa. All samples were acidified to pH≤2 by addition of 0.1 ml 25% H2SO4 to 1 ml of sample before analysis. 3. Calculations The average value of energy consumed for 1 kg of alpha-ketoglutaric acid production was determined using the equation:
E=
Conductivity, mS/cm
3 5 7 10
3.42 4.69 5.74 6.96
(1)
where: E – energy consumption needed to produce 1 kg of AKG, kW h/ kg; U – voltage, V; I – current, A; m – mass of the final product, g; t – time, h; The average value of the current efficiency was calculated on the basis of the following equation:
Table 1 The change in conductivity of AKG solution with different initial concentration. Concentration, g/L
U ·I ·t m
CE =
F·z·V·ΔCdil ·100% n·I·Δt
(2)
where: CE – current efficiency, %; F – Faraday's constant (96,485), C/ mol; I – current, A; z – valence of ions; V – diluate volume, L; ΔCdil – change of AKG concentration in diluate chamber, mol/L; n – number of 38
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cells; Δt – time, s; The degree of dissociation of monoprotic acid was calculated as follows:
α1 =
[A− ] [A−] = [HA][A−] CHA
nificantly with decreasing initial pH of the solution in the diluate chamber. The highest concentration of alpha-ketoglutaric acid in the concentrate chamber after 180 min of EDBM process (equal to 2.18 g/ L) was obtained when the initial pH of starting solution was the lowest and equal to 3. On the one hand, a higher degree of dissociation of salts present in the initial solution can facilitate the transport of AKG anions through the anion exchange membrane, while on the other hand, to obtain a higher pH value of working solutions it is necessary to add a greater amount of OH- ions, which can compete with AKG anions in transfer across the anion exchange membrane and also decrease EDBM efficiency [24]. Moreover, analysis of Fig. 1, (which illustrates the EDBM stack configuration) implies that during EDBM processes, additional hydroxyl ions are generated (as products of water reduction which occurs at the cathode). This phenomenon leads to the alkalization of diluate solutions in the time of EDBM processes, as shown in Fig. 3b. It should also be noted that, in all systems considered, the alkalization of diluate solutions rapidly increases to pH of about 11 after exceeding the equivalence point. However, for the solutions of initial pH values lower or equal to 5, the effect of alkalization is observed after a much longer time (Fig. 3b). A similar relationship has been observed by other authors on titration of weak organic acids [25]. Fig. 4 compares the changes in concentration of OH- ions in the diluate chamber (determined by titration of AKG), during EDBM processes of 1-component model solutions of initial pH 4 and 10, respectively. In both cases the concentration of OH- in the diluate chamber increases during EDBM. However, in bipolar membrane electrodialysis of a solution with a low initial pH value, the hydroxyl ions produced before reaching the equivalence point can be neutralized by hydrogen ions present in the starting solution or generated by the bipolar membrane. The migration of hydrogen ions across the anion exchange membrane to the diluate chamber can be explained by H+ ions leakage due to their small diameter and high mobility [18]. In addition, as shown in Fig. 4, the increased competition between OHand AKG anions is observed in about 80 min after reaching the equivalence point. On the other hand, during bipolar membrane electrodialysis of a solution with a high initial pH value, the presence of a large number of OH- ions, initially added to the starting solution and generated by the cathode during the EDBM process, is an obstacle for the transport of AKG ions in the process of EDBM. Therefore, the use of solutions of low initial pH is recommended.
(3)
where: CHA = [HA]+[A ] – total concentration of weak acid The fraction of the acid that is not dissociated was calculated as follows: -
α0 =
[HA] [HA] = [HA][A−] CHA
(4)
and moreover the acid dissociation constant was calculated on the basis of the following equation:
Ka = [H +]
log
α1 α0
α1 = pH – pKa α0
(5)
(6)
where: Ka – dissociation constant. 4. Results and discussion 4.1. Effect of pH value for the AKG solutions As well known, alpha-ketoglutaric is a weak organic acid which, depending on the pH of solutions, dissociates in two stages: a) H2AKG + H2O ↔ HAKG− + H+ Ka1=3.4·10−3 b) HAKG− + H2O ↔ AKG2− + H+ Ka2=2.1·10−5 where Ka is the acid dissociation constant, which is a measure of acid strength. In addition, using Eqs. (3)–(6) [22] one can find the relation between the degree of dissociation and pH of AKG acid, presented in Fig. 2. The results shown in Fig. 2. indicate that the degree of dissociation of alpha-ketoglutaric acid increases with increasing pH of solution, similarly as for other organic acids [23]. Moreover, it should be noted that AKG acid reaches a maximum level of dissociation in solutions with pH higher than 7. The state of dissociation of the compounds present in operating solution determines the EDBM process. Thus in our study the effect of pH value of the initial solution on the production of AKG from 1component model solution (with different initial pH value from the range 3–10), in the time of EDBM processes was checked. All EDBM processes were carried out at the current density of 65 A/m2. Fig. 3a illustrates the change in the concentration of alphaketoglutaric acid in the concentrate chamber during the EDBM processes in which the initial solutions of pH equal to 3, 4, 6, 8 and 10 were used. The concentration of alpha-ketoglutaric acid in the concentrate chamber in the time of EDBM processes, increased sig-
4.2. Effect of the initial concentration of AKG salts in concentrate chamber In the next part of this study the effect of initial concentration of AKG salts in the concentrate chamber on the production of alphaketoglutaric acid in the EDBM processes (at different initial concentrations ranging from 0.3 to 5 g/L) was analyzed. All EDBM processes were carried out at the current density of 65 A/m2. Fig. 5a shows the effect of the initial concentration of AKG salts in the concentrate chamber on the voltage drop of the stack in the time of EDBM. The initial voltage drop of the stack increases rapidly with decreasing starting concentration of AKG salts in the concentrate chamber and decreases in each EDBM processes until reaching plateau. A similar relationship has been observed by Zhou et al. [26] during EDBM production of water insoluble organic acid. It is also worth noting that the greatest decrease in voltage (from 35.1 to 18.4 V) after 180 min of EDBM process was noted when the initial concentration of AKG salts in the concentrate chamber was the lowest, of 0.3 g/L. A low initial concentration of salts in the concentrate chamber is responsible for a relatively high resistance and, as consequence, high voltage at the beginning of the process. Furthermore, the observed voltage drop in the following minutes of the EDBM process is due to large amounts of H+ and OH- ions which are generated by bipolar membrane and AKG ions transported through the anion exchange membrane from the diluate to the concentrate chamber, which leads to an increase in the conductivity of solutions in the concentrate compartment [20]. As shown in Fig. 5a,
Fig. 2. Degree of dissociation as a function of pH-value of AKG.
39
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M. Szczygiełda, K. Prochaska
Fig. 3. Change in the concentration of AKG in concentrate chamber (ΔC) (a) and pH–value in diluate chamber (pH) (b) during EDBM processes, C0dil=3 g/L, C0con=0.3 g/L, i=65 A/m2, T=25 ± 2 °C.
Fig. 5b shows the effect of the initial concentration of AKG salts in the concentrate chamber on the AKG ions transport through the anion exchange membrane during EDBM processes. The acid concentration increased rapidly at first, when a high initial concentration of salts in acid compartment was used. These differences in the AKG concentration increase are due to significant differences in the initial conductivities of concentrate solutions, moreover they decrease in the time. Is therefore clear that the higher the resistance, the lower the number of AKG ions transported. The initial concentration of AKG salts in the concentrate chamber affects the current efficiency in EDBM processes (Fig. 5c). In all systems studied, the highest current efficiency was reached after 30 min of EDBM process. Furthermore, the higher initial concentration of AKG salts in the concentrate chamber leads to an increase in the current efficiency at the beginning of the EDBM process, which is related to increased transport of AKG ions through the anion exchange membrane in the first period of EDBM (Fig. 5b). However, current efficiency decreases with time especially dramatically in the systems with a high initial concentration in the concentrate chamber. On the one hand, the trend of growing concentration is not linear and weakens with time [17]. On the other hand, as reported in [12], the pass of OH− through
Fig. 4. Change in the concentration of OH- in diluate chamber during EDBM processes of 1-component model solutions, C0dil=3 g/L, C0con=0.3 g/L, i=65 A/m2, T=25 ± 2 °C.
higher initial concentrations of AKG salts ( 3 g/L and 5 g/L) in the concentrate chamber imply a higher conductivity of solutions in 180 min of EDBM. However, it should be stressed that a further increase in the initial concentration of AKG salts can lead to a decrease in EDBM efficiency (as a result of back-diffusion) [17].
Fig. 5. Changes in the voltage drop (U) (a), concentration of AKG in concentrate chamber (ΔC) (b), current efficiency (CE) (c) and energy consumption (EC) (d) during EDBM processes, C0dil=5 g/L, i=65 A/m2, T=25 ± 2 °C.
40
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concentrate compartments. As reported in [28] the diffusion transport has a significant influence on the ions transported across the ionexchange membranes and cannot be neglected in the EDBM system. The effect of the initial concentration of AKG salts in the diluate chamber on the current efficiency is illustrated in Fig. 6c. The increase in AKG ion transport through the anion-exchange membrane caused by the extra driving force leads to a rapid increase in the current efficiency for the initial salts concentration varying in the range of 1–10 g/L. A similar relationship has been observed by other authors studying EDBM/ED production of other compounds such as: salicylic and γamino butyric acid [20,29]. Fig. 6d illustrates the effect of the initial concentration of AKG salts in the diluate chamber on the energy consumption for 1 kg of alphaketoglutaric produced. It is obvious that higher initial concentration of AKG salts in the diluate chamber was conducive to decreasing energy consumption needed for production of 1 kg of AKG acid in EDBM. Firstly, higher AKG salts concentrations reduce the electrical resistance of the stack and secondly a greater amount of the acid was produced in 180 min of the EDBM process.
the anion exchange membrane can reduce the current efficiency of EDBM process in BM-AM configuration. The results obtained in this part of the study indicate that the initial concentration of AKG salts in the concentrate chamber has a significant impact on the energy consumption for production of 1 kg of alphaketoglutaric acid. As shown in Fig. 5d, the energy consumption for 1 kg of alpha-ketoglutaric acid production rapidly decreases (from 10 to 5.6 kW h/kg after 180 min of EDBM), with increasing initial salts concentration in the concentrate chamber (form 0.3–5 g/L, respectively). This effect can be explained by an decrease in the area resistance of the solutions with their increasing concentration [27]. Therefore, much more energy was consumed to overcome electrical resistance, when the initial concentration of salts in concentrate chamber was too low [26]. 4.3. Effect of the initial concentration of AKG salts in diluate chamber Another objective of our study was to assess the impact of the initial concentration of AKG salts in the diluate chamber on the production of alpha-ketoglutaric acid in the process of EDBM performed for AKG solutions of different initial concentrations varying in the range from 3 to 10 g/L. All EDBM processes were carried out at the current density of 65 A/m2. Fig. 6a shows the effect of the initial concentration of AKG salts in the diluate chamber on the voltage drop of the stack during the EDBM process. The results indicate that the voltage value observed at the beginning of the EDBM process decreases with increasing initial salts concentration in the dilute chamber, which is related to the lower resistance. (similarly as described in Section 4.2). Fig. 6b illustrates the effect of the initial concentration of AKG salts in the diluate chamber on the alpha-ketoglutaric acid concentration. Is easy to see that the AKG acid concentration increases with increasing initial concentration of AKG salts. As shown in Fig. 6b, the highest AKG concentration after 180 min of EDBM process, equal to 3.1 g/L, was obtained when the starting solution concentration was equal to 10 g/L AKG salts. This effect can be explained by the presence of additional driving force due to the concentration gradient between the diluate and
4.4. Effect of the current density The current density is the next parameter which significantly affected the production of alpha-ketoglutaric acid in the process of EDBM. Fig. 7a presents the effect of the current density of 65, 85 and 125 A/m2, on the voltage drop at the stack in the time of EDBM. The voltage drop across the EDBM stack significantly increases with increasing current density. The increase in current density implies an increase in the electrical resistance. The same effect has been reported earlier in [30,31]. Additionally, the authors of these papers have concluded that more electrical energy is consumed to overcome the electrical resistance at a higher current density. Furthermore, it can be noticed that the voltage drop at the EDBM stack decreases with time as a result of water dissociation by bipolar membrane as well as the effect of migration of AKG ions. However, as reported by Wang et al. [32], the contribution of water dissociation by bipolar membrane to the voltage drop is more significant.
Fig. 6. Changes in the voltage drop (U) (a), concentration of AKG in concentrate chamber (ΔC) (b), current efficiency (CE) (c) and energy consumption (EC) (d) during EDBM processes, C0con=1 g/L, i=65 A/m2, T=25 ± 2 °C.
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Fig. 7. Changes in the voltage drop (U) (a), concentration of AKG in concentrate chamber (ΔC) (b), current efficiency (CE) (c) and energy consumption (EC) (d) during EDBM processes, C0dil=5 g/L, C0con=1 g/L, T=25 ± 2 °C.
4.5. Optimization of EDBM processes for the production of AKG
Fig. 7b presents the effect of current density on the concentration of alpha- ketoglutaric acid. It is easy to see that the concentration of AKG acid increases with increasing current density, which is consistent with literature and results of our own previous investigation indicating that it is typical trend of electrodialysis processes [18,20,33]. Obviously, the driving force of the process increases with increasing current density, which leads to higher transport of AKG ions through the anion exchange membrane and simultaneously enhances the water dissociation by bipolar membrane according to the second Wien effect [11]. Analyzing the effect of the current density on the current efficiency of EDBM process (Fig. 7c) one can conclude that in all systems considered, the current efficiency is the highest at the beginning of EDBM process and thus the values of current efficiency dramatically decrease with the progress of the process. This effect could mainly be caused by the hydroxide ions competition [16]. In EDBM process, when a high current density was applied, greater amounts of hydroxide ions were generated by the cathode reaction in the EDBM stack, which consequently led to enhancement of the competition with AKG ions to go across the anion exchange membrane. Moreover these hydroxyl ions can react with H+ ions generated by BM [12]. Similar effects have been described in [32]. Moreover, as reported by Li [34], a high current density can lead to strengthened dissociation of water to H+ and OHions and water transport caused by the electro-osmosis effect [26,34]. Fig. 7d illustrating the effect of the current density on energy consumption for 1 kg of alpha-ketoglutaric production indicates that the energy consumption increases with increasing value of applied current density. As mentioned previously, with increasing current density a greater part of the total electrical energy had to be consumed to overcome the electrical resistance. Furthermore, the energy consumption rapidly grows during the EDBM processes. As shown in Fig. 7b the increment in the concentration of the acid decreases during EDBM process, which consequently leads to increase in the energy consumption for 1 kg of alpha-ketoglutaric production in the later time of processes (Fig. 7d). Therefore, EDBM process at high current density and run for a long time is uneconomical and/or impractical from the point of view of getting a high concentration of AKG.
At the final part of this study the EDBM process was carried out in the optimized experimental conditions (i.e. in the conditions which allowed to obtained the best results, shown above), so at C0dil=10 g/L, C0con=3 g/L, i=65 A/m2 and pHinit.=4. As follows from the above discussion, the current density of 65 A/m2 is optimal for the production of AKG from practical and economical point of view. This current density results in decreasing concentration of AKG acid after 180 min of EDBM, at the same time allowing high current efficiency and low energy consumption. As shown in Fig. 8, a very low starting pH value of the diluted solution (equal to 4), allowed maintaining a low pH value from the range of 4–5.6 during the entire EDBM process. A significant part of OH- ions (generated at the cathode) was used for neutralization of diluate solution and could not compete with AKG ions in the transport through the anion exchange membrane. The AKG salt concentrations in the diluate and concentrate chamber were equal to 10 and 3 g/L, respectively. This allowed obtaining high conductivity of solutions (the voltage drop in the range of 15.8–14.6 V, respectively) as well as high concentration gradient, which led to diffusion transport supporting the AKG ions transfer by AM.
Fig. 8. Changes in the pH value in diluate chamber during EDBM process, C0dil=10 g/L, C0con=3 g/L, i=65 A/m2, pHinit=4, T=25 ± 2 °C.
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Journal of Membrane Science 536 (2017) 37–43
M. Szczygiełda, K. Prochaska
Finally in the optimized EDBM process, a high concentration of AKG, current efficiency and low energy consumption equal to 4.83 g/L, 71.8%, 3.72 kW h/kg, respectively, were obtained, while the corresponding values in the EDBM process without optimization were equal to 1.6 g/L, 24% and 15.26 kW h/kg, respectively, after 180 min of EDBM. Thus, the obtained results are much better, however the future studies should be focused on the possibilities of further lowering the energy consumption, as high values of this parameter are the greatest disadvantage of the electrodialysis processes.
[5]
[6]
[7]
[8]
5. Conclusion [9]
A novel, environmental friendly bipolar membrane electrodialysis method, was used for the production of alpha-ketoglutaric acid from model water solutions. The effects of operating conditions such as: i) initial pH value in diluate chamber, ii) initial concentration of sodium alpha-ketoglutarate in working solutions and iii) current efficiency, on the EDBM processes were investigated. In addition, the optimized bipolar membrane electrodialysis was carried out. The performance of EDBM processes was evaluated from the voltage drop, alphaketoglutaric acid concentration, current efficiency and energy consumption for production of 1 kg of alpha-ketoglutaric acid. According to the results, the highest efficiency of EDBM processes was obtained when the initial pH value in the diluate chamber was equal to 3 (it reduced a competition between OH- and AKG anions), the initial concentration of AKG salts in the concentrate chamber was equal to 3 g/L (decreased resistance of the EDBM stack), the initial concentration of AKG salts in the diluate chamber was equal to 10 g/L (increased additional driving force due to the concentration gradient) and current density was equal to 65 A/m2 (reduced energy consumption). Moreover carrying out the process EDBM in the optimal conditions, allowed getting high concentration of AKG, high current efficiency and low energy consumption equal to 4.83 g/L, 71.8%, 3.72 kW h/kg, respectively, while the corresponding data obtained in the EDBM process without optimization were equal to 1.6 g/L, 24% and 15.26 kW h/kg, respectively after 180 min of EDBM. Furthermore, bipolar membrane electrodialysis may be used for production of AKG acid from the actual post fermentation broth. However, further studies are needed because the present work describes only a preliminary research. Thus, it is too early to talk about using EDBM for an industrial production of alphaketoglutaric acid. On the other hand, the results obtained may provide an optimistic basis for further studies. There is much more work to do before bringing the process coupling to industrialization. The most important will be to scale-up the experiment from currently one repeating unit and further purification of the final product.
[10]
[11] [12]
[13]
[14]
[15] [16] [17]
[18] [19]
[20] [21] [22] [23] [24]
[25] [26]
[27]
Acknowledgements [28]
The authors wish to acknowledge the financial support from the Polish Ministry of Science and Higher Education (Grant No. DS-PB/ 0701). We would like to thank Dr hab. Daria SzymanowskaPowałowska from Poznan University of Life Sciences for her inspiration to study AKG acid.
[29] [30]
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