Technical Note pubs.acs.org/ac
Tandem Electrochemical Desalination−Potentiometric Nitrate Sensing for Seawater Analysis Maria Cuartero, Gastón A. Crespo, and Eric Bakker* Department of Inorganic and Analytical Chemistry, University of Geneva, Quai Ernest-Ansermet 30, CH-1211 Geneva, Switzerland S Supporting Information *
ABSTRACT: We report on a methodology for the direct potentiometric determination of nitrate in seawater by in-line coupling to an electrochemical desalination module. A microfluidic custom-fabricated thin layer flat cell allows one to electrochemically reduce the chloride concentration of seawater more than 100-fold, from 600 mM down to ∼2.8 mM. The desalinator operates by the exhaustive electrochemical plating of the halides from the thin layer sample onto a silver element as silver chloride, which is coupled to the transfer of the counter cations across a permselective ionexchange membrane to an outer solution. As a consequence of suppressing the major interference of an ion-exchanger based membrane, the 80 μL desalinated sample plug is passed to a potentiometric flow cell of 13 μL volume. The potentiometric sensor is composed of an all-solid-state nitrate selective electrode based on lipophilic carbon nanotubes (f-MWCNTs) as an ion-to-electron transducer (slope of −58.9 mV dec−1, limit of detection of 5 × 10−7 M, and response time of 5 s in batch mode) and a miniaturized reference electrode. Nitrate is successfully determined in desalinated seawater using ion chromatography as the reference method. It is anticipated that this concept may form an attractive platform for in situ environmental analysis of a variety of ions that normally suffer from interference by the high saline level of seawater.
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third (platinum) or second kind (Ag/AgCl). Indeed, different configurations have recently been explored on the basis of classical electrochemistry in a thin layer sample (99.5% efficiency)15 and by using bipolar electrodes (25% efficiency).16 A suitable anodic potential applied to the electrochemical cell results in chloride removal by oxidation to chlorine gas at the platinum electrode or by deposition as silver chloride at the Ag/ AgCl electrode. Most importantly, the associated electrochemical process for both systems is simple, reversible, and sufficiently selective for chloride and offers the possibility of miniaturization. This is in contrast to established water desalination concepts more focused on the industrial scale with the aim of indiscriminately removing all ions in the water sample.17 It is important to apply these concepts in a thin layer configuration to ensure an exhaustive depletion of chloride in the injected plug and to minimize the required duration of the procedure. Bakker et al. further developed this technology and tested it only in artificial samples.15 A silver wire (OD, 0.3 mm), which acted as working electrode, was inserted in a tubular cation exchanger membrane (Nafion, length, 5.5 cm, and ID, 0.36 mm), and microfluidic connections were added to both extremities of the membrane. The space between the silver wire
he large concentration of sodium chloride in seawater (∼600 mM) makes it difficult to determine the nutrients nitrate, nitrite, and dihydrogen phosphate at the low micromolar level. This is mainly caused by the limited selectivity and sensitivity of the established analytical techniques.1 In consequence, there is an important need for reliable methodologies to detect these species in situ.2 Nitrate is of particular interest because it acts as a marker for water quality and the extent of anthropogenic discharges. It is the main source of nitrogen in marine ecosystems and may have an important influence on microorganism growth rates.3 While traditional analytical techniques such as colorimetry,1,4 UV absorption,5,6 fluorescence,7 chemiluminescence,8,9 and ion chromatography10,11 have been used for measuring nitrate levels in seawater, a pretreatment step is always required. Most of the pretreatments are focused on the analyte itself and involve preconcentration,12 derivatization,6,12 modification of chromatographic columns,13,14 and reverse flow injection analysis.7 While these pretreatments allow one to determine nitrate in seawater as a result of the enhanced sensitivity and selectivity, the complexity of these analytical strategies (e.g., sampling, storage, analysis time, deterioration of the sample, cost per analysis, etc.) are not very attractive for decentralized measurements. In a different approach and inspired by the growing interest in water desalination, the reduction of chloride concentration (halides in general) can be accomplished quite simply by electrochemical transformation or plating on electrodes of the © 2015 American Chemical Society
Received: May 27, 2015 Accepted: July 23, 2015 Published: July 23, 2015 8084
DOI: 10.1021/acs.analchem.5b01973 Anal. Chem. 2015, 87, 8084−8089
Technical Note
Analytical Chemistry
Figure 1. Schematic illustration of both custom-made prototypes: (a) Desalinator unit scale 1:2. (i) Open cell. (1) M4 plastic screws; (2) PEEK inlet/outlet tubes; (3) Copper wires; (4) Plexiglas block; (5) Silver foil; (6) Rubber serpentine, 230 μm thickness, 80 μL; (7) FKL membrane; (8) Rubber serpentine, 600 μm thickness, 200 μL; (9) Silver foil. (ii) Closed cell. (b) Potentiometric flow cell (13 μL). (10) Ag/AgCl wire; (11) PEEK inlet/outlet tube; (12) Custom-made reference electrode; (13) Measurement compartment; (14) All-solid-state nitrate-selective electrode (fMWCNTs as a transducer).
electrode and a reference electrode; see the Supporting Information for details.
and the membrane was selected to achieve a thin layer gap, which was filled with the saline sample. While the approach was conceptually convincing, the desalinated volume of 15 μL was insufficient for reliable downstream analysis. Unpublished efforts to fabricate longer tubular cells to result in an increase of the volume to ∼100 μL were only partly successful. They were met with technical challenges that are due to increased friction between both elements, which made it difficult to assemble the module without damaging the membrane surface. We report here on a series of technical and conceptual innovations that allow one to decrease chloride concentration in seawater to mM levels with a subsequent potentiometric monitoring of nitrate. Nitrate detection is accomplished in an effective volume of ∼13 μL immediately placed after the desalinator unit. An all-solid-state nitrate selective electrode based on lipophilic multiwalled carbon nanotubes (fMWCNTs) and acrylic membrane was developed and characterized for this purpose.
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RESULTS AND DISCUSSION
The goal of this work is the development of a reliable methodology for the determination of nitrate in seawater. An electrochemical pretreatment is performed on the sample with the aim of reducing sodium chloride down to mM levels to eliminate the major interference on the potentiometric nitrate detection. The microfluidic custom-made flat desalination cell is shown in Figure 1a (open and closed cell). Both serpentines were made out of an incompressible rubber that defines the volume of the sample (80 μL) and reference compartment (200 μL) as well as the desirable thin layer conditions for the sample (∼230 μm thickness). The electrochemical process has been described earlier for a tubular system.15 The application of a constant anodic potential between both silver foils provokes the oxidation of the silver foil (Ag/Ag+) with the simultaneous incorporation of chloride (from the sample) to form a AgCl film. It is thus expected that the current must be limited by chloride mass transport to the silver foil. Concurrently, sodium counterions are transported from the sample through the ionexchange membrane to the reference solution to fulfill the charge balance condition.18 The proposed custom-made flat cell provides a larger volume (80 μL), which is easily coupled to a potentiometric sensor or conductometric detector to alternatively examine the ion content of the desalinated plug. Preliminary experiments were focused on the electrochemical characterization of the desalination cell using cyclic voltammetry. Figure 2 shows the recorded cyclic voltammograms in an artificial sample of 0.6 M NaCl at different scan rates from 10 to 100 mV s−1. The results suggest an optimum applied potential range (750−950 mV, less than 2% variation) for achieving desalination. The integrated charges for the oxidized and reduced peaks were equivalent (within 0.8% variation), indicating reversibility of the electrochemical process. In addition, the linearity between peak current and scan rate
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EXPERIMENTAL SECTION Electrochemical Protocol. Figure 1a illustrates the custom-made desalinator cell (open and closed) prepared as described in the Supporting Information. A reference solution (0.1 M NaCl) was introduced into the cell reservoir using a peristaltic pump at 25 μL min−1 (11 min duration) and renewed every 24 h. The sample was introduced into the cell in a similar fashion (4 min). Once the pump was stopped, a constant potential of 800 mV (optimized previously) was applied for 600 s. This perturbation generated a current decay from the chloride electrodeposition on the working electrode. After that, the desalinated sample was replaced by a 10−3 M NaCl solution for regenerating the electrode. This consisted of the application of a negative potential (−900 mV) for 1200 s. The lifetime of silver foils was around 80 cycles and for the membrane 40 cycles following the indicated protocol. For the potentiometric determination of nitrate, the desalinator unit was coupled to the flow cell shown in Figure 1b. This cell contained an all-solid-state nitrate selective 8085
DOI: 10.1021/acs.analchem.5b01973 Anal. Chem. 2015, 87, 8084−8089
Technical Note
Analytical Chemistry
Figure 2. Cyclic voltammograms in 0.6 M NaCl at different scan rates (100, 75, 50, and 25 mV s−1). Outer solution: 0.1 M NaCl. Inset: Peak height current vs scan rate.
reveals an excellent thin layer behavior of both processes (inset in Figure 2). The wide potential window of 1.9 V for the forward and backward waves indicates a significant ohmic drop in the electrochemical cell19 (200 ohm). The efficiency of the desalinator prototype was further explored in 0.6 M NaCl artificial samples by variation of several parameters that include reference solution concentration, potential, and time period of the excitation pulse. The chloride concentration in the desalinated plug was calculated from conductometric measurements by coupling the ion-chromatography detector to the outlet of the cell. This appears to be more accurate than the calculation of the deposited charge by the mathematical integration of the current decay. This difference is more noticeable when the duration of the step pulse exceeds the optimum time of 600 s. Otherwise, both strategies correspond to each other, with less than 3% variation. For instance, the estimated chloride concentration after desalination of 0.6 M NaCl (800 mV for 1000 s) was 5.5 mM by conductometry, whereas a negative chloride concentration was obtained from the integration of the current decay. Figure S1 shows the observed current decays at a fixed time (1000 s) using different applied potentials. Obviously, the current decay is more pronounced in the optimum potential range (750−950 mV). As observed in Figure 3a, the efficiency of the desalination improved with an increasing applied potential up to 800 mV, remaining almost constant at higher values. A reduction of chloride concentration from 600 to 5.5 mM (99% efficiency) is achieved by applying a constant potential of 800 mV for 1000 s. Afterward, a regeneration step at cathodic potential (−900 mV) of double the time used for desalination in 1 mM NaCl solution was required. This resulted in a 99.6% recovery of the chloride deposited on the working electrode, as calculated from conductivity measurements of the collected samples. Because the regeneration step efficiency (99.6%) was slightly lower than expected from ideal behavior, the lifetimes of the metallic electrodes and the ion-exchange membrane are not infinite and estimated at 80 and 40 cycles, respectively. Indeed, a visible color change of the membrane from yellow to black indicated the deposition of silver halide complexes [AgCln]n−1.20 This was not surprising considering that these
Figure 3. (a) Conductometric chloride detection after sample (0.6 M NaCl) desalination as a function of applied potential (n = 3). (b) Conductometric chloride detection after sample (0.6 M NaCl) desalination as a function of time excitation (n = 3). Reference solution: 0.1 M NaCl.
soluble complexes may diffuse away from the electrode to the membrane surface. This type of contamination was equally observed for tubular Nafion membranes.19 Regardless of the NaCl level in the reference solution, this factor did not have a significant effect on the efficiency of the desalination process (see Figure S2). Note that the use of a highly concentrated solution (0.6 M NaCl) may cause a contamination of the desalinated sample during the time of contact. Because no potential is applied during the 4 min period that the sample is moved out of the cell, cotransport of NaCl from the reference to the sample solution across the membrane is believed to be the cause of such contamination.15 Figure 3b shows the chloride concentration after desalination for several time periods using a defined applied potential (800 mV). The duration of the electrolysis had less influence on the desalination performance than the applied potential (see Figure 3a). Consequently, an optimum time interval of 550−650 s was adequate to achieve an efficiency of 99.5%, which corresponds to 3 mM NaCl. To validate the optimized protocol (800 mV, 600 s), a seawater sample was desalinated. The observed current profiles of both desalination and regeneration processes (at −900 mV, 1200 s, 1 mM NaCl) are shown in Figure S3. The integrated charge from the current decay (4.611 C) indicated that 597 nm chloride was removed from the solution and deposited on the 8086
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of practical aspects during flow conditions such as excellent adhesion to the PEEK body electrode. The all-solid-state nitrate electrode was then incorporated into the flow cell together with the miniaturized reference electrode. Figure S5 illustrates a calibration curve obtained with the potentiometric flow cell, continuously injecting solutions of 80 μL (equal to the volume of desalinated samples) with different nitrate concentrations. In principle, the response time of the electrode (less than 50 s in the whole concentration range) may allow nitrate detection in desalinated samples. The slope was Nernstian (−58.3 ± 0.1 mV dec−1) and the limit of detection was slightly higher ((4.3 ± 0.2) × 10−7 M) than that obtained in batch mode (0.9 × 10−7 M). The difference in the limit of detection was probably related to the higher surface to volume ratio of the cell, but the use of a custom-made miniaturized reference electrode may also have contributed to the difference. Other analytical characteristics relevant to nitrate determination were also evaluated. The electrode showed a potential drift of less than 50 μV h−1 for 5 h (medium-term stability) in the whole range of nitrate concentrations (from 10−8 to 10−3 M). The recovery of the membrane was evaluated by alternating the measurement of 10−8 M NaNO3 and several more concentrated nitrate solutions. As observed in Figure 5a, steady state signals were observed after 100 s (from 10−8 to 10−4 M) and after 15 min to reach the baseline (from 10−3 to 10−8 M). Figure 5b shows multiple calibration curves performed by successive additions of nitrate in different background solutions of increasing complexity. Specifically, water, 1 mM NaCl, 0.6 M NaCl, and artificial seawater were tested as matrices. The presence of 1 mM chloride did not affect the slope of the calibration graph (−58.2 ± 0.2 mV dec−1) but increased the limit of detection to (1.1 ± 0.1) × 10−6 M compared to a pure water background (5.0 × 10−7 M). Detection of nitrate in untreated seawater is not conceivable by potentiometry (log KNO3−,Cl− = −2.5). In contrast, desalinated 0.6 M NaCl (final concentration of 3 mM NaCl) and 1 mM NaCl solution behaved the same in terms of slope and limit of detection, as expected. Desalinated synthetic seawater showed a significant deterioration of the limit of detection (3.8 ± 0.1) × 10−6 M, likely owing to the high sulfate amount in the sample (ca., 40 mM) that evidently is not removed by desalination. Selectivity coefficients over nitrite (−2.6) and hydrogen phosphate (−3.7) are listed in the Supporting Information. These values indicate that the presence of either 5 mM HPO42− (485 ppm) or 0.4 mM NO2− (18 ppm) would set the limit of detection of the proposed methodology to 1 μM (0.06 ppm) for nitrate. Therefore, even in severely contaminated seawater or estuarine samples, the detection of nitrate should be possible. Submicromolar nitrate levels (uncontaminated water) cannot currently be detected by this approach and require a further decrease of the detection limit. Electrode exposure to high saline solutions was explored to evaluate adverse effects in the potentiometric response. After repetitive exposures to 0.6 M NaCl and desalinated sample with spiked amounts of nitrate (from 10−7 to 10−3 M), both baseline recovery and nitrate detection exhibited suitable reproducibility (%RSD ∼ 0.8) and no sign of electrode poisoning (Figure S6). This points to an excellent robustness of the system for an application in the field.
electrode surface. This amount corresponds to the one calculated from conductometric measurements (99.3% efficiency, a reduction from 607 ± 1 to 4 ± 1 mM chloride). To evaluate any eventual loss of nitrate during the desalination process, a recovery experiment was performed by injecting desalinated and seawater plugs into a commercial ion chromatograph. The observed nitrate concentration in seawater was 93 ± 3 μM against 92 ± 1 μM in desalinated seawater. The desalination process preserves the initial nitrate concentration, indicating the absence of cross-contamination or other parasitic processes (Figure S4). Having established a reliable chloride reduction in artificial and real samples, the next step was to couple the desalination module to a potentiometric nitrate determination. This process first involved the development of an appropriate nitrateselective electrode. Solid-state technology was selected due to its intrinsic possibility of miniaturization and implementation in flow cells with a small cell volume of a few μL. Lipophilic multiwalled carbon nanotubes (labeled as fMWCNTs) were utilized as an ion-to-electron transducer because of their excellent properties that have been extensively reported.21,22 The functionalized f-MWCNTs were easily solubilized in THF, allowing for a facile and rapid deposition by drop casting onto the sensing area of a commercial glassy carbon electrode. Figure 4 shows the observed calibration
Figure 4. Potentiometric calibration curve (batch conditions) for nitrate detection for three different membrane compositions (%RSD ∼ 0.5, n = 4).
curves for an acrylic membrane (∼D = 10−11 cm2s−1)23 and two PVC membranes (∼D = 10−8 cm2s−1)24 with bis(2-ethylhexyl) phthalate (DEHP) and o-2-nitrophenyl octyl ether (o-NPOE) as plasticizers. Very reproducible Nernstian response slopes were obtained for nitrate in all cases (−58.9 ± 0.2 mV dec−1 for the acrylic membrane, −59.2 ± 0.2 mV dec−1 for DEHP, and −59.3 ± 0.1 mV dec−1 for o-NPOE). While a lower limit of detection was expected with the use of acrylic membrane due to its lower ion diffusion coefficient, only a slight difference was found among these membranes (0.9 ± 0.1) × 10−7 M for the acrylic membrane, (2.2 ± 0.2) × 10−7 M for DEHP, and (3.3 ± 0.1) × 10−7 M for o-NPOE. Furthermore, insignificant differences were obtained for the selectivity coefficients of these membranes (see the Supporting Information). The acrylic membrane was selected not only due to its appropriate analytical performance (e.g., lowest LOD and proper discrimination toward chloride and sulfate) but also because 8087
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desalination volume and the incorporation of all-solid-state potentiometric electrodes reported here will form the basis for this future work.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b01973. Materials, reagents, and equipment. Preparation of the flat desalination cell. Potentiometric cell for nitrate determination. Membranes selectivity. (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Swiss National Science Foundation and the European Union (FP7-GA 614002-SCHeMA project) for supporting this research. The authors additionally thank Guy LeCoultre for the fabrication of the desalination cell, Thomas Cherubini for the fabrication of the potentiometric flow cell, and Majid Ghahraman Afshar for the copolymer preparation.
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REFERENCES
(1) Patey, M. D.; Rijkenberg, M. J. A.; Statham, P. J.; Stinchcombe, M. C.; Achterberg, E. P.; Mowlem, M. TrAC, Trends Anal. Chem. 2008, 27, 169−182. (2) Hanrahan, G.; Ussher, S.; Gledhill, M.; Achterberg, E. P.; Worsfold, P. J. TrAC, Trends Anal. Chem. 2002, 21, 233−239. (3) Chen, C.-T. A. Nutrient Cycling in the Oceans. In Oceanography, Encyclopedia of Life Support Systems (EOLSS); Eolss Publishers: Oxford, UK, 2007; Vol. 1, pp 331−343. (4) Yao, W. S.; Byrne, R. H.; Waterbury, R. D. Environ. Sci. Technol. 1998, 32, 2646−2649. (5) Maruo, M.; Doi, T.; Obata, H. Anal. Sci. 2006, 22, 1175−1178. (6) Kieber, R. J.; Seaton, P. J. Anal. Chem. 1995, 67, 3261−3264. (7) Masserini, R. T.; Fanning, K. A. Mar. Chem. 2000, 68, 323−333. (8) Cox, R. D. Anal. Chem. 1980, 52, 332−335. (9) Aoki, T.; Fukuda, S.; Hosoi, Y.; Mukai, H. Anal. Chim. Acta 1997, 349, 11−16. (10) Wang, R.; Wang, N.; Ye, M.; Zhu, Y. J. Chromatogr. A 2012, 1265, 186−190. (11) Jiang, X.-L.; Lim, L. W.; Takeuchi, T. Anal. Bioanal. Chem. 2009, 393, 387−391. (12) Wada, E.; Hattori, A. Anal. Chim. Acta 1971, 56, 233−240. (13) Ito, K.; Nomura, R.; Fujii, T.; Tanaka, M.; Tsumura, T.; Shibata, H.; Hirokawa, T. Anal. Bioanal. Chem. 2012, 404, 2513−2517. (14) Zhang, M.; Yuan, D. X.; Chen, G. H.; Li, Q. L.; Zhang, Z.; Liang, Y. Microchim. Acta 2009, 165, 427−435. (15) Grygolowicz-Pawlak, E.; Sohail, M.; Pawlak, M.; Neel, B.; Shvarev, A.; de Marco, R.; Bakker, E. Anal. Chem. 2012, 84, 6158− 6165. (16) Knust, K. N.; Hlushkou, D.; Anand, R. K.; Tallarek, U.; Crooks, R. M. Angew. Chem., Int. Ed. 2013, 52, 8107−8110. (17) Knust, K. N.; Hlushkou, D.; Tallarek, U.; Crooks, R. M. ChemElectroChem 2014, 1, 850−857. (18) Cuartero, M.; Crespo, G. A.; Bakker, E. Anal. Chem. 2015, 87, 1981−1990. (19) Cuartero, M.; Crespo, G. A.; Afshar, M. G.; Bakker, E. Anal. Chem. 2014, 86, 11387−11395.
Figure 5. (a) Potentiometric time trace obtained for incremental concentrations of nitrate after exposure to 10−8 M of NaNO3. (b) Calibration curves for nitrate in the indicated samples.
To validate the traceability of the proposed methodology, a seawater sample from “Costa Calida”, Murcia, Spain (37°35′54″ N 1°18′50″ O) was analyzed. The concentration of nitrate in the desalinated seawater (i.e., after applying the electrochemical protocol) was determined by potentiometric readout (96 ± 3 μM) and ion chromatography (92 ± 1 μM) as a reference method.
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CONCLUSIONS We demonstrate here the important necessity of desalinating water samples for a reliable determination of nitrate by potentiometry with ion-selective electrodes. To pursue the decentralization of such analytical determinations in diverse ecosystems (e.g., nitrate concentration profile as a function of depth in the ocean), a flat desalinator prototype was fabricated, characterized, and optimized. This unit allows one to effectively reduce chloride concentration by about 3 orders of magnitude by applying 800 mV for 600 s. After removing the major interference for detection by ionophore based membranes, potentiometric nitrate determination was accomplished in artificial and natural samples and compared to the values obtained by ion chromatography. The analysis time was 2 samples per hour, which is primarily limited by the speed of the desalination process. For performing multianalyte detection of nitrate, nitrite, dihydrogen phosphate, ammonium, etc., a series of flow cells need to be consecutively coupled after the desalinator unit. The key technical innovations of the 80 μL 8088
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Analytical Chemistry (20) Liu, J.; Wang, Z.; Liu, F. D.; Kane, A. B.; Hurt, R. H. ACS Nano 2012, 6, 9887−9899. (21) Rius-Ruiz, F. X.; Crespo, G. A.; Bejarano-Nosas, D.; Blondeau, P.; Riu, J.; Xavier Rius, F. Anal. Chem. 2011, 83, 8810−8815. (22) Crespo, G. A.; Macho, S.; Rius, F. X. Anal. Chem. 2008, 80, 1316−1322. (23) Heng, L. Y.; Hall, E. A. H. Anal. Chim. Acta 1996, 324, 47−56. (24) Wang, H.; Sun, L. L.; Armstrong, R. D. Electrochim. Acta 1996, 41, 1491−1493.
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