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Online Gas-Free Electrodialytic KOH Eluent Generator for Ion Chromatography Yifei Lu, Liting Zhou, Bingcheng Yang, Shujun Huang, and Feifang Zhang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b03365 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 4, 2018
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Analytical Chemistry
Online Gas-Free Electrodialytic KOH Eluent Generator for Ion Chromatography Yifei Lu, Liting Zhou, Bingcheng Yang,* Shujun Huang, Feifang Zhang* Shanghai Key Laboratory of New Drug Design, School of Pharmacy, East-China University of Science and Technology, Shanghai 200237, China Corresponding Author *E-mail:
[email protected];
[email protected] ABSTRACT: An online gas-free KOH electrodialytic eluent generator (EDG) with two-membrane configuration is described for ion chromatography (IC). A central eluent channel is separated from two outer regenerant channels bearing KOH solution (or one is water) by stacked cation exchange membranes (sCEM) and a bipolar membrane (BPM) plus stacked CEMs (BPM-sCEM), in which the anion exchange membrane (AEM) of BPM is facing the central channel (the anode direction). Independent fluid input or output ports address all channels. One platinum screen electrode is put in each outer channel and the sCEM side is anode with respect to the cathode of BPM-sCEM side. Under the electric field, enhanced water splitting at the intermediate layer of BPM will occur, offering hydroxide and hydronium, hydroxide electromigrates through AEM into central channel, meanwhile the potassium of KOH feed solution at the anode migrates into the central channel to combine with hydroxide to form a solution of KOH. Since the central eluent channel is spatially isolated from both electrodes, the generated KOH solution is gas-free and no gas removal device is required. More important, AEM side of BPM is contactless with the alkaline solution, nearly avoiding possible AEM degradation when immersed into concentrated alkaline solution. This ensures long-term running stability of the EDG and high purity of the produced KOH solution (as indicated by the typical suppressed background conductance of 0.28 µS/cm). The EDG has a pressure tolerance (at least 21 MPa) and the produced KOH concentration is up to 101 µeq/min with near-unity faradaic efficiency. When operated in both isocratic and gradient modes, the EDG demonstrates excellent reproducibility, as indicated by the retention time of RSD ≤ 0.08% and the peak area of RSD ≤ 0.6%. To the best of our knowledge, this is the first description of gas-free EDG matched conventional IC system as well.
Since the pioneer work of Dasgupta et al. in 1991,1 electrodialytic eluent generator (EDG) has been typical configuration of state-of-the-art ion chromatography (IC) systems. Such device can online generate LiOH, NaOH or KOH eluent simply by adjusting the current, offering great flexibility for performing gradient elution.2 Other kinds of eluents such as MSA acid can also be produced using the concept above. A KOH EDG can produce high-purity KOH eluent and most impurities such as carbonate and other anions always contained in manually prepared KOH solution can be eliminated. This will be helpful to obtain much lower suppressed background, and thereby obtaining lower limit of detection.3 IC applications are dominated by anion analysis and hydroxide is the preferred eluent due to many reasons.4 Among several types of EDGs, KOH EDG is probably the most popular one. Herein we focus on the fabrication of KOH EDG. Three configurations of EDG have been reported, they are (a) single-membrane.2,5 In this configuration, only one cation exchange membrane (CEM) (or stacked CEMs, sCEM) isolates the eluent channel from the regenerant compartment. The cathode is put in the eluent channel while the anode is also immersed in the KOH regenerant compartment. The overwhelming majority of extant KOH EDGs, including all commercial versions, fall in such configuration. An obvious disad-
vantage of such configuration is the produced KOH eluent accompanied by copious amounts of hydrogen gas that must be removed before entering the following IC system.6 There are also possible electrochemical byproducts such as hydrogen peroxide and ozone which are detrimental to the low capacity anion exchange chromatography columns. (b) two-membrane. Such configuration uses two CEMs7 or one CEM and one anion exchange membrane (AEM)1 to isolate the eluent channel from both regenerant compartments, respectively. Both electrodes are put in the regenerant compartment and the electrolysis gas cannot enter the eluent channel, thus generating a gasfree eluent (It should be noted that “gas-free” term only refers to no gas in the eluent channel and the electrolysis gas is definitely produced only in the regenerant compartment). When two CEMs are used to fabricate two-membrane configured KOH EDG, Donnan-breakdown of concentrated KOH feed solution through one CEM (at the side of cathode) occurs.7 Such configuration was reported to have poor faradaic efficiency and very limited KOH concentration range that can be produced.7 Some byproducts can potentially be formed or released into the KOH eluent stream. More important, such configuration is unsuitable for fabricating the EDG that is capable of bearing high-pressure due to ineffective Donnan-breakdown for thicker or multiple layers of CEMs. The other option is to use one CEM and one AEM to fabricate two-membrane con-
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figured KOH EDG.1 This has been demonstrated to work but obvious impurity in the KOH eluent was observed, probably due to the degradation of the AEM contact with the alkaline solution at the cathode.1 The first EDG could not bear high pressure and had to be put on the low-pressure side of the pump.1 Dasgupta et al. pioneered a capillary scale NaOH EDG that can withstand high pressure.8 Small et al. used an ion exchange column for electrically generating KOH eluent.9,10 Such device (also called ion reflux) was capable of withstanding high pressure.10 Later commercial KOH EDGs were introduced by Dionex Corp. and have presently been widely used for anion separa-
tions combined with hydroxide-selective columns.5 The EDGs above are based on single-membrane configuration and the use of degassers to remove the gas is necessary. Presently the raw material (Teflon-AFTM) used to fabricate degasser that can bear typical backpressure in IC is costly and vulnerable. With a degasser, other problems are involved including limited lifetime and more complicated configuration of the sytem. As mentioned above, the two-membrane configured EDG can generate gas-free eluent and do not need to use degasser. By using ion exchange resin bead to replace common membrane sheet, Dasgupta et al. have for the first time described a highpressure capillary scale two-membrane KOH EDG.11
Figure 1. Schematic diagram of two-membrane configured KOH EDG. Note: to indicate clearly, the scale of the BPM is not proportional to its real size. It is also a multiple-function EDG simply by changing the feed electrolyte.6 Such EDG is only useful for capillary scale IC system and is insufficient for conventional IC system due to its limited flow rate range (or very limited KOH concentration range). Recently, an improved ion reflux device has been described for fabricating a recycling IC system, in which the eluent channel was isolated from the cathode via AEMs.12 In such design, KOH EDG and electrodialytic suppression were combined together and it could generate KOH eluent with concentration up to 16.4 µeq/min. More recently, Masunaga et al reported a two-bipolar-boundary device that can generate gas-free NaOH EDG.13 In such a device, two sCEMs are used to isolate the central eluent channel from the regenerant compartment. The central channel is packed by cation exchange resin (CER) and anion exchange resin (AER). It was said that there are two bipolar boundaries for the interfaces of CERAER and AER-CEM, respectively. Water splitting will occur at the bipolar boundary. Such device can generate pure NaOH with concentration up to 100 mM at the flow rate of 1 mL/min (corresponding to 100 µeq/min), which is sufficient to meet the typical requirement of IC system. Such two-membrane EDG is a smart design. A main drawback is limited pressure tolerance range (5.6 MPa) and cannot be applied for most commercial ion exchange columns. Another potential problem is how to evenly pack CER and AER and fix their interface in place. A more recent review has provided more details for the evolution of EDG.14 A breakthrough in membrane science was the introduction of the bipolar membrane (BPM).15 In its simplest form a BPM is a CEM laminated together with an AEM, through an interfacial layer (IL) (or "junction" layer). The IL is the most im-
portant part of the BPM and significantly, enhanced water splitting will occur at the IL under the electric field, then generating hydrogen ions and hydroxide ions.16 Such unique feature of BPM is utilized for production of an acid and a base from a corresponding salt in combination of CEM and AEM, which has been widely used in many industry cases.17,18 It should be noted that these BPM-based electrodialysis devices used in industry cannot bear high pressure encountered in the typical chromatographic system. To the best of our knowledge, there is no report of using BPM to fabricate twomembrane EDG. Herein we described a high-pressure gas-free two-membrane configured KOH EDG (TM-EDG) for conventional IC system by using a BPM plus sCEM and the other sCEM to isolate the central eluent channel from outer reagents compartments.
EXPERIMENT SECTION Regents and materials. Analyte solutions were prepared in the form of either sodium or potassium salts. Typically, these were analytical grade chemicals, used as received from the manufacturer. Ultra-pure water (Milli-Q, USA) was used throughout with a specific resistivity at 18.2 MΩ·cm. CEMs and BPMs were purchased from Fumatch Corp. (Germany) and Jinqiu Corp. (Jinhua, China), respectively. The mineral water was from local market and the tap water was collected in the lab. Chromatographic System. IC equipment (Thermo Scientific DionexTM, ICS-2000) was used to evaluate the performance of KOH EDG. A PEEK chromatographic pump was used to drive pure water to pass through a self-made water purifier (fabri-
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Analytical Chemistry cated by connecting a short anion exchange trap and a short cation exchange trap in series), then went to the EDG cartridge. The injection volume was 25 µL. An analytical column (IonPac AS20, 4 mm i.d. × 250 mm length) housed in a column oven (35 °C) was used for separation of model inorganic anions. A chromatographic flow rate of 1 mL/min was used throughout unless otherwise stated. A self-made electrodialytic membrane suppressor operated at the recycle mode, and a DS6 conductivity detector equipped by ICS-2000. In addition, a conventional KOH EDG (DionexTM RFC-30) was used for comparison. Fabrication of KOH EDG. The schematic diagram of KOH TM-EDG was illustrated in Figure 1. Briefly, the device consists of three compartments, one central eluent and two regenerant at both sides. A sCEM and a BPM plus sCEM separating three channels are in the sandwich configuration. The AEM side of BPM is facing the eluent channel and the CEM side of BPM is contact with sCEM. Both porous platinum screen electrodes are placed on the two regenerant channels, which are in contact with CEMs via a sulfonate group functionalized gasket screen. Both cartridges housing membranes made of PEEK are assembled together by metal screws. Unless otherwise stated, 2 M KOH was used for feeding both regenerant channels. The KOH feeding solution (2 liter) is driven by a peristaltic pump (BT100, Lead Fluid Technology Corp., Baoding, China) to pass through both regenerant channels in one direction at the flow rate of 1mL/min. The effluent directly goes to the reservoir and is recycled. Pure water driven by a high-pressure pump flows through a water purifier mentioned above and then feeds into the eluent channel in the opposite direction with the regenerant. Calibration of KOH concentration generated. The concentration of the KOH eluent generated was measured on line by conductivity measurement using a homemade high pressure conductivity cell (cell constant measured to be ~26.9 cm, with conductivity measured in the conventional manner with a Dionex CDM-1 conductivity detector) as described previously.6 The detector output was used to measure the generated KOH concentration via a pre-calibrated equation. For higher KOH concentration, it is difficult to get accurate readout by conductivity detector above, thus titration method was used to measure higher KOH concentration generated by collecting KOH effluent with a given volume.
RESULTS AND DISCUSSION Operation principle. It is well known that enhanced water splitting in the IL of BPM will occur under a direct current, then generating H+ and OH-.15 In such process, no hydrogen and oxygen gas is generated, which is unlike conventional water electrolysis. For the TM-EDG described here, pure water is pumped through the central eluent channel while concentrated KOH feed solution flows through outer regenerant channels. Under the electric field, the water electrolysis occurs at the anode and cathode of the device, and the hydronium ions generated at the anode displace K+ ions in the feed solution. The displaced K+ ions migrate across sCEM into the central channel and stays there due to the Donnan exclusion from BPM. In the meanwhile, the OH- ions from water splitting in the IL of BPM migrate across ABM of BPM into the central channel, and combine with K+ ions to produce KOH solution. The KOH concentration is determined by the applied current and the carrier flow rate through the central channel.
In this case, the source of OH- is from water splitting in the IL of BPM, not from water electrolysis at the cathode. Thus, the solution feeding the cathode channel does not have to be KOH; it can be pure water or any aqueous solution that is electrochemically inactive at the cathode. In fact, the device has been proved to work well for using pure water to feed the cathode channel and at least 30 mM KOH could be generated. Due to high operation electric resistance at such condition, it was not easy to generate KOH eluent with high concentration, thus the use of a dilute KOH or use the same KOH solution with the anode channel was recommended. For convenience, the latter was used in the present study. Current-voltage behavior. The current-voltage behavior of the TM-EDG under the forward bias was explored, as shown in Figure 2. Such range is the working range of KOH EDG. In the range of 0 to 2.55 V, the produced current of the EDG approached zero, which probably resulted from that no water splitting occurred in such range. When further increasing the voltage, a rapid increase of the current was observed. It can be explained by the electromigration of cations and anions, in which hydroxide from water splitting in the IL of BPM migrated toward the anode while hydronium migrated toward the cathode, potassium in the anode channel migrated to central channel. In such voltage range, the fitted line between the applied voltage and the corresponding current showed linear correlation, and the intersection point of the extension line of the fitted line with x-axis was ∼2.55 V. The slope of the fitted line, which could be approximately regarded as the resistor of the TM-EDG, could be computed to be ∼61 Ω. Low resistor of the TM-EDG guarantees the use of high applied current and then higher KOH concentration. Dependence of KOH concentration generated on the applied current and the flow rate. The basic function of an EDG is online to generate eluent with the concentration correlated with the applied current. Here the dependence of the concentration of KOH eluent generated by the TM-EDG on the current and the flow rate was tested. As illustrated in Figure 3, at the typical flow rate (1 mL/min) used in IC, good linear correlation (correlation coefficient, R2=0.9999) of the KOH concentration generated with the current was observed. The slope (k) of the fitted plot was 0.613±0.008 mM/mA, close to the theoretical value of 0.621 mM/mA, showing nearideal Faradaic efficiency. Owing to large fluctuation of conductivity detector output when monitoring high KOH concentration, it is difficult to accurately measure KOH concentration by such way, thus the titration method was chosen and the result was shown in Figure 3. As can be seen, the computed KOH concentration also showed good linearity with the current, as indicated by the R2=0.9999. Up to 101.5 mM, the TMEDG still demonstrated near-ideal Faradaic efficiency (k=0.614±0.012 mM/mA). Such concentration range is comparable to that of commercial single-membrane KOH EDG (100 mM), which can satisfy the majority of IC applications.
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40
30
20 I (mA) =16.3*V (V)-41.6 10
0 0
2
4
6
Voltage, V
Figure 2. Current-voltage behavior of the TM-EDG
Figure 3. Correlation of the applied current and the generated KOH concentration at the flow rate of 1mL/min. According to Faradaic law, the produced KOH concentration can be speculated to be inversely proportional to the flow rate of the pure water when keeping the applied current constant. As shown in Figure S1, the produced KOH concentration is highly dependent on the flow rate. At the fixed current, the higher the flow rate, the smaller the KOH concentration. The slope of the plot between the current and the produced KOH concentration is much different for different flow rate (inset in Figure S1). Thus, the produced KOH can also be manipulated by varying the flow rate. Open circuit penetration. In the case of the present KOH TM-EDG, the central channel being essentially pure water, the osmotic potential of high concentration KOH feed solution at regenerant channels may overcome the Donnan potential, thus causing undesired penetration of KOH into central channel even when no current flows.11 In our previous work, the penetration under open circuit was named as open circuit penetration (OCP), which can unambiguously characterize the pene-
tration.11 Obviously, such a penetration is undesirable for performing gradient operation and commonly a penetration value of
