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Fast Potentiometric CO2 Sensor for High-Resolution in Situ Measurements in Fresh Water Systems
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Fast Potentiometric CO2 Sensor for High-Resolution in Situ Measurements in Fresh Water Systems
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  • Rohini Athavale*
    Rohini Athavale
    Eawag—Swiss Federal Institute of Aquatic Science and Technology, Department of Surface Waters Research and Management, Seestrasse 79, CH-6047 Kastanienbaum, Switzerland
    Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Universitätsstrasse 16, CH-8092 Zürich, Switzerland
    *E-mail: [email protected]. Phone Number: +41 587655633.
  • Nadezda Pankratova
    Nadezda Pankratova
    Department of Inorganic and Analytical Chemistry, University of Geneva, Quai E.-Ansermet 30, 1211 Geneva, Switzerland
    Integrated Systems Laboratory (LSI), Swiss Federal Institute of Technology Lausanne (EPFL), CH-1015 Lausanne, Switzerland
  • Christian Dinkel
    Christian Dinkel
    Eawag—Swiss Federal Institute of Aquatic Science and Technology, Department of Surface Waters Research and Management, Seestrasse 79, CH-6047 Kastanienbaum, Switzerland
  • Eric Bakker
    Eric Bakker
    Department of Inorganic and Analytical Chemistry, University of Geneva, Quai E.-Ansermet 30, 1211 Geneva, Switzerland
    More by Eric Bakker
  • Bernhard Wehrli
    Bernhard Wehrli
    Eawag—Swiss Federal Institute of Aquatic Science and Technology, Department of Surface Waters Research and Management, Seestrasse 79, CH-6047 Kastanienbaum, Switzerland
    Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Universitätsstrasse 16, CH-8092 Zürich, Switzerland
  • Andreas Brand
    Andreas Brand
    Eawag—Swiss Federal Institute of Aquatic Science and Technology, Department of Surface Waters Research and Management, Seestrasse 79, CH-6047 Kastanienbaum, Switzerland
    Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Universitätsstrasse 16, CH-8092 Zürich, Switzerland
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Environmental Science & Technology

Cite this: Environ. Sci. Technol. 2018, 52, 19, 11259–11266
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https://doi.org/10.1021/acs.est.8b02969
Published September 3, 2018

Copyright © 2018 American Chemical Society. This publication is licensed under these Terms of Use.

Abstract

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We present a new potentiometric sensor principle and a calibration protocol for in situ profiling of dissolved CO2 with high temporal and spatial resolution in fresh water lakes. The sensor system is based on the measurement of EMF between two solid-contact ion selective electrodes (SC-ISEs), a hydrogen ion selective and a carbonate selective sensor. Since it relies on SC-ISEs, it is insensitive to changes in pressure, thus suitable for in situ studies. Also, as it offers a response time (t95%) of <10 s, it allows for profiling applications at high spatial resolution. The proposed optimum in situ protocol accounts for the continuous drift and change in offset that remains a challenge during profiling in natural waters. The fast response resolves features that are usually missed by standard methods like the classical Severinghaus CO2 probe. In addition, the insensitivity of the presented setup to dissolved sulfide allows also for measurements in anoxic zones of eutrophic systems. Highly resolved CO2 concentration profiles obtained by the novel and robust SC-ISE setup along with the developed optimum in situ protocol allow investigating hotspots of biogeochemical processes, such as mineralization and primary production in the water column and help improving estimates for CO2 turnover in freshwater systems.

Copyright © 2018 American Chemical Society

Introduction

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In the face of increasing anthropogenic perturbations to the global carbon cycle, the processes underlying the global carbon budget, including CO2 turnover and its exchange between inland waters and the atmosphere, need to be thoroughly characterized. (1) Since many of these processes, for example, aerobic methane oxidation processes acting as methane filters, often are confined to small spatial scales in lakes, a measurement technique for CO2 with high spatial resolution would be helpful. (2,3) In addition, it is important to understand the occurrence of biogeochemical hotspots of photosynthesis and respiration and their impact on the concentration of dissolved CO2. (4) High-resolution in situ measurements are required to locate and quantify these sources and sinks. (5)
In situ measurements of CO2 can be achieved by several indirect and direct methods. The indirect methods involve measurement of parameters like pH, total alkalinity (TA), dissolved inorganic carbon (DIC) by laboratory-based techniques. (6) Biases and errors associated with calculating CO2 from pH and alkalinity values currently limit the precision of estimates of CO2 emissions from inland waters because they neglect the presence of other organic and inorganic buffer systems. (1,7) Some of the laboratory based techniques have been further developed to be suitable for in situ measurements as reported by several studies in marine environments, for example, in situ spectrophotometric measurements of DIC and pH and TA, (8−10) as well as mass spectrometric approaches for in situ and on-site measurements. (11,12) However, the sampling, reaction, or equilibration time required in these systems spans over several minutes. A response time on the order of minutes does not favor high spatiotemporal measurement demands of inland waters, especially lakes. (13,14) Direct in situ measurements of dissolved CO2 are traditionally obtained by the Severinghaus probe (15) or with infrared spectroscopy (NDIR sensing element). (16) The Severinghaus probe (CO2–SH probe) is inexpensive and easy to handle. It consists of a pH-sensing element in contact with a bicarbonate buffer, which is separated from the sample solution by a gas permeable membrane. Since the equilibrium between bulk and sample phases is established by diffusion, the probe signal needs several minutes to stabilize. This slow response time is a major drawback in applications that require rapid monitoring. (15,17,18) Also, the gas permeable membrane is not only selective for CO2 but also allows diffusion of H2S, which then alters pH of the internal buffer and causes interference in the sulfidic deep waters, typical of stratified, eutrophic lakes. (17,19,20)
Potentiometric sensing offers a promising alternative for in situ high-resolution measurements. (21) The presented CO2 sensing principle is based on the measurement of electromotive force (EMF) of a pH electrode against a carbonate electrode without using an additional reference element. (22) As demonstrated by Xie et al., (22) the following equations illustrate the mechanism of the sensor. Considering the acid dissociation equilibria of dissolved carbon dioxide in water
(1)
activity of dissolved CO2 is linked with that of carbonate and hydrogen ions according to eqs 2 and 3.
(2)
where Ka1 and Ka2 are the two acid dissociation constants for dissolved CO2. The potential response of a cell combining a carbonate- and a hydrogen-ion selective electrode (ISE) gives
(3)
where K and K′ are constants that vary with the inner solution composition of the electrodes. As no gas permeable membrane is involved, no interference from H2S gas is expected, which is in contrast to the Severinghaus probe. A highly selective carbonate-selective electrode based on a new class of carbonate ionophores was reported by the group of Nam and Cha. (23) Selectivity studies on such a carbonate ISE have shown that Cl and HS do not interfere even at high concentrations, which makes the ISE suitable for use in natural waters. (22−26) Xie et al. (22) successfully tested the principle of this approach in the laboratory.
In this study, we present an in situ application and validation of a potentiometric sensing system for dissolved CO2 in the water column of a temperate lake. For such rapid in situ profiling applications, a pressure-insensitive sensor setup is required. We utilized the inherently pressure-proof, solid-contact ion selective electrodes (SC-ISEs) in a double layer design (14) with a carbon nanotube based layer as a solid contact for both H+ and CO32– selective SC-ISEs. Combined with a methacrylic copolymer as membrane matrix for H+, the design improved the insensitivity toward sulfide. (21) We also developed an optimized in situ calibration protocol based on earlier findings (14,21) that identified and corrected for changes in electrode offsets and drifts during profiling in water columns of lakes with strong chemical gradients. Finally, we tested the advantages of our approach over a classical Severinghaus CO2 probe for high resolution depth profiling in fresh water lakes. For the field tests, we selected the eutrophic Lake Rotsee (Switzerland), which has steep redox and solute gradients that are challenging for profiling applications.

Materials and Methods

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Sensor Design

The sensor system consisted of an H+-selective SC-ISE measured against a CO32–-selective SC-ISE in a potentiometric set up without use of any reference element. Functionalized multiwalled carbon nanotubes (f-MWCNTs) were used as a transducing material in a double layer (DL) design to fabricate both SC-ISEs. Carboxylic-acid groups were created on the surface of the MWCNTs by oxidation followed by amide formation with octadecylamine to yield f-MWCNTs. (27) Nanotubes were dispersed in tetrahydrofuran by slight sonication to form a homogeneous dispersion which was then drop cast on a polished mini glassy carbon electrode (GCE, Metrohm AG, Switzerland) surface to form a uniform transducing layer. Separate SC-ISEs for CO32– and H+ were fabricated by casting an ion-selective membrane on top of the f-MWCNT layer. A methacrylic copolymer (MMA-DMA) based membrane, was cast for H+-selective (21) and a polyvinyl chloride (PVC) based membrane matrix for the CO32-selective SC-ISE (22) (details in SI). The sensor quality was assessed by checking for Nernstian response (Figure 1), insensitivity toward redox, long-term stability, water-layer formation and sulfide insensitivity (see SI).

Figure 1

Figure 1. Response (with respective slopes in mV/decade reported in parentheses) of (a) carbonate and hydrogen ion selective SC-ISEs in the laboratory (b) CO2–ISE couples on shipboard (c) CO2–SH probe before the deployment on change in dissolved CO2 concentrations.

Field Site

Lake Rotsee, an eutrophic lake in Central Switzerland was chosen as the study site. It has a surface area of 0.48 km2, and average and maximum depths of 9 and 16 m, respectively. A strong chemocline is located between 8 and 11 m during the stratification period, which starts in spring and holds until late autumn. This eutrophic lake exhibits steep redox and nutrient gradients across the oxycline during the stratification period. (28) The profiling tests were performed in November 2017, that is, at the end of the summer stratification when accumulation of CO2 and other mineralization products reached maximum concentrations in the hypolimnion. Convective mixing in the surface waters maintained the steep gradients near the thermocline. These physicochemical conditions in the lake allowed for testing the sensors within a range of sharp changes in both analyte concentration and redox conditions.

Set up for in Situ Profiling

The potentiometric CO2 sensing system was integrated into our custom-built profiling ion analyzer (PIA) (Figure S5). (14) PIA consisted of a cubic aluminum frame with dimensions 50 × 50 × 60 cm. On top of this frame, a data-recording unit (a Z-Brain data logger from Schmid Engineering Systems AG, Switzerland) with its power supply was placed. During the calibration on shipboard and in situ deployment for recording profiles, the SC-ISEs were connected to waterproof, galvanically isolated potentiometric-sensing units in order to preclude cross talk. For signal stability and noise rejection an instrumentation amplifier (Burr-Brown, Model INA116, from Texas Instruments, Dallas, TX, USA) with ultralow input bias current (3 fA) and a high common-mode rejection (84 dB) was utilized. The EMF for each indicator H+-selective SC-ISE (mini glassy carbon electrode, Metrohm AG, Switzerland) was measured against CO32–-selective SC-ISE (mini glassy carbon electrode, Metrohm AG, Switzerland) with a platinum wire as common solution ground (INA116 datasheet, BURR-BROWN). (29) PIA was equipped with additional sensors for characterization of the water column, specifically a CTD probe for measuring conductivity, temperature and depth (XR-420, RBR Ltd., Canada) and needle type optodes for recording dissolved oxygen concentrations (PreSens - Precision Sensing GmbH, Germany). A syringe sampler integrated into the PIA system (KC Denmark) at the center of the cubic frame allowed retrieval of 12 water samples at different depths with a volume of 60 mL along with simultaneous recording of EMF by ISEs and all other integrated measurements. PIA was deployed with an electric winch in the lake with a constant profiling speed of 5 mm s–1, and syringe samples were taken simultaneously during the profile recording at different depth intervals. A Severingshaus gas sensing probe for CO2 was also accommodated in the whole set up (Idronaut S.r.l, I-20047 Brugherio, Italy).

Laboratory Analysis

Dissolved inorganic carbon (DIC) was sampled in 12 mL pre-evacuated DIC exetainers (VWR International GmbH) after filtering through 0.2 μm cellulose acetate syringe filter (VWR International GmbH) on shipboard and measured by TOC analyzer (SHIMADZU, Switzerland). Total sulfide was determined by spectrophotometric analysis following Cline et al. (30) (SI), and concentrations of major cations (NH4+, K+, Ca2+, Mg2+, Na+) and anions (Cl, SO42-, NO3-) in the water column were measured by ion chromatography (Metrohm AG, Switzerland) (Table S1).

Calculation of Dissolved CO2 Activity

The DIC as well as dissolved ion concentrations were measured on syringe samples. A CTD probe recorded pH and temperature in situ in parallel to syringe sampling. These results were compiled as an input to the PHREEQC program for speciation calculations. (31) The activity of dissolved CO2 was obtained as output and was further used for the in situ calibrations.

Optimization of Calibration Protocol

To have an optimum calibration protocol with minimum sampling efforts we evaluated different calibration schemes. Table 1 (Figure S2) summarizes four strategies for calibration and corresponding values of calibration parameters obtained via these strategies. Specifically in Table 1a, the first scheme is based on shipboard calibration before the deployment, carried out in order to check the performance of already mounted sensors on PIA in terms of slope and intercept. The following in situ strategies involved in situ sampling with the syringe sampler (integrated in PIA) during ongoing profiling to compensate for drift and changes in temperature experienced during profiling in the water column. For optimization of the calibration protocol (Scheme 3, Table 1a) both shipboard and in situ calibration efforts were blended together to consider drift, interferences from other ions and temperature changes as additional factors to the standard calibration parameters namely slope and intercept.
Table 1. (a) Different Calibration Strategies, (b) Calibration Schemes with Corresponding Parameters for CO2–ISE Couple 1, (c) Calibration Schemes with Corresponding Parameters for CO2–ISE Couple 2, and (d) Calibration Types and Parameters for CO2–ISE SH Probe
(a) different calibration strategiesa
calibration schemedescription
1shipboard calibration
2in situ calibration; slope value from shipboard calibration; drift and intercept fitted from all sampling points
3in situ calibration; slope value from shipboard calibration; drift and intercept obtained from two (1st and last) sampling points
4in situ calibration, slope value, drift, and intercept obtained from fit to all sampling points
(b) calibration schemes with corresponding parameters for CO2–ISE couple 1a
calibration typeslope (mV/decade)intercept (mV)drift (mV/s)
131.06–65.36
231.06–45.940.005
331.06–45.690.004
432.54–38.760.003
(c) calibration schemes with corresponding parameters for CO2–ISE couple 2a
calibration typeslope (mV/decade)intercept (mV)drift (mV/s)
127.30–83.24
227.30–70.230.010
327.30–68.850.009
432.82–43.460.003
(d) calibration types and parameters for CO2–ISE SH probe
calibration typeslope (mV/decade)intercept (mV)drift (mV/s)
156.8570.86
2
356.8598.310
448.3563.810
a

Optimal scheme in bold.

Shipboard calibration was performed on the ship just before lowering the PIA system in the lake for in situ measurements. Three buffer solutions with different dissolved CO2 concentrations coinciding with the typical concentrations in the lake were used for shipboard calibration: 10 mM Tris buffer solutions with 5 mM bicarbonate and 150 μM chloride was tuned with sulfuric acid to different pH values of 7.52, 8.04, and 8.74 to attain three buffer solutions with aCO2 = 302, 95, and 19 μM respectively. The CO2–SH probe was checked for the Nernstian response (Figure 1c) using similar buffer solutions in the laboratory prior to the field deployment.
In situ calibration, where the obtained raw EMF profile can be corrected for drift and interference from other ions, has been proven essential to obtain accurate high-resolution concentration profiles with ISEs. (14,21) This published in situ calibration protocol (eq 4) considers the value of EMF and the corresponding concentration of analyte at all sampling points to derive the values of the slope S [mV/decade], drift d [mV/s], and intercept E0 [mV] using least-squares optimization:
(4)
where EMF is expressed in mV, t is the time passed since the start of profiling [s], aa [mol/L] is the activity of the ion of interest, and ab [mol/L] is the activity of an interfering ion. Owing to the use of highly selective ionophores for preparation of ion selective membrane (ISM) for both for H+ and CO32– (23,25−27,32) and significantly low concentrations of relevant interfering ions (Table S1) in the lake water column, there was no necessity to correct for the interference by the factor cb × Ka,bpot. The temperature affected the slope (∼10%) of the divalent CO32– when tested over a range of temperatures typically found in the lake. Therefore, the temperature dependence of S was explicitly included in solving for aa. T is temperature in K, and T0 is temperature at the beginning of profiling in eq 5
(5)
The optimized eq 5 with respect to drift, interferences and temperature compensation was used for calibrating the CO2 sensor setup. The calibration parameters allowed converting the high-resolution EMF profiles to activity of dissolved CO2.

Response Time

To assess how fast the CO2–ISE couple responds to the changes in activity of CO2 under in situ conditions, an in situ response time test was carried out. We chose two water depths at 6.5 and 8.5 m, based on the profiles available on board. The whole PIA set up with ISE couples and SH type probe was then taken to 6.5 m and rested there for ∼10 min to observe the stable response and was driven to the next point, that is, 8.5 m with the speed 5 cm/s, which is 10 times the normal profiling speed.

Results and Discussion

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The CO2–ISE couple utilizes an all-solid-state design inherently insensitive to pressure, which is a prerequisite for in situ measurements in the water column. Both CO32– and H+ selective SC-ISEs show close to Nernstian response with values of slopes −27.2 mV/decade (limit of detection, LOD ∼2 μM) and 56.3 mV/decade (LOD ∼1 nM), respectively in the laboratory (Figure 1a). With alkalinity values during the stratification period ranging from around 1.5 to 3.5 mmol L–1 and pH between 7 and 9, the bicarbonate ion predominates in the carbonate equilibria in the water column. The lower detection limits of our CO32– ISEs were in the micromolar range and allowed measurements in the typical pH range in Lake Rotsee with lowest carbonate concentrations above the LOD of carbonate sensor. Although the functional range can extend to pH values lower than the pKa1 of the CO2 system (22) it will depend on the alkalinity of the system which should be high enough to keep carbonate concentrations above the limit of 1 μM. A sulfide sensitivity test up to 100 μM dissolved total sulfide in a buffered solution (Tris buffer pH 8) (Figure S1) did not show any drift due to sulfide interference in the laboratory, which can be attributed to choice of a DL design employing f-MWCNTs as a transducer. The use of carbon nanotubes also makes the CO2–ISE couple light-insensitive against variable light conditions in the field. (21) The water layer test and redox sensitivity test (33−35) did not show any significant changes in EMF (Table S2). The response time (t95%) of both the SC-ISEs and the CO2 ISE-couple was estimated according to IUPAC conventions (36) and was found to be <10 s. Hence, the fast response, stability, and insensitivity to sulfide and the light conditions with LODs low enough for measurements in natural waters puts forward the CO2–ISE couple as a suitable candidate for in situ profiling in lakes.
We also observed a Nernstian response for both CO2 ISE-couples during shipboard calibration on PIA and for CO2–SH probe in the laboratory prior to the deployment (Figures 1b and 1c). The interaction of the lake matrix with the membrane surface and leaching of membrane components may cause a drift in the signal and can impart slow changes in the intercept (E0). However, drift experienced by the sensors (Figure S3 and Table 1) and the changes in the intercept (14) could not be controlled by calibration with buffer solution on shipboard only. Sampling during ongoing EMF profiling followed by chemical analysis was necessary for empirical correction. (14) Single values for a parallel drift (34) and intercept were obtained for each EMF profile by fitting log aa to eq 5 using least-squares optimization. The slopes obtained from shipboard calibration for CO2–ISE couple and from the laboratory calibration for CO2–SH (Table 1) were taken over for the different in situ calibration strategies, as the in situ interference correction was not needed for the slope. Calibration scheme 2 as described in Table 1a utilized the value of slope obtained by shipboard calibration and values for drift and intercept for both sensor sets were obtained by fitting log aa to eq 5 by least-squares optimization only for drift and intercept (Table 1b and 1c). To further reduce the effort of sampling at as many depth points as possible (10 points in the case of calibration scheme 2) to derive the values of drift and intercept, calibration scheme 3 used only two depth points at the beginning and end of the profile yielded a better fit of the activity profile to the syringe concentrations for dissolved CO2 compared to calibration scheme 2 (Figure 2). Because of the reduced sampling effort, calibration scheme 3 was considered as the optimal strategy. Calibration scheme 4 corresponded to the published strategy: (12,13) in this case all the calibration parameters were derived by fitting log aa to eq 5 using least-squares optimization by varying d, E0, and S considering all sampling points. The resulting values of slightly super-Nernstian slopes (close to calibration scheme 3) for both couples confirm that the use of slope values from shipboard calibration without interference correction was suitable.

Figure 2

Figure 2. Depth profiles of activity of dissolved CO2 obtained by CO2 ISE couple calculated with different calibration parameters corresponding to the three in situ calibration schemes as described in Table 1. Please note that the dark blue line for calibration scheme 2 is almost overlapped with the light blue line of calibration scheme 4.

Similar calibration protocols were carried out also for the CO2–SH probe (Table 1d, Figure S4). Except that the probe needed to rest at two sampling points for ∼10 min to record its stable response considered to calculate drift and intercept. Water depths of 6.5 and 8.5 m were chosen for this purpose. The value of slope was carried forward from the laboratory analysis with the Tris buffer solutions (SI). The failure of the CO2–SH probe to reproduce the syringe concentrations (Figure S4) is a result of its slow response to changes in the concentration. Also, the gas permeable membrane in CO2–SH probe, is not selective to CO2 but allows contamination with other gases such as H2S that alters the pH of the buffer solution and leads to measurements that overestimate CO2 (Figure S1).

Temporal Response

An experiment across the oxycline with two stops compares the performance of both CO2–ISE couple and SH–CO2 probe (Figure 3). At 6.5 m water depth CO2 concentrations were low due to photosynthetic activity and convective mixing. Stable response was observed at this depth for both sensors with a slight drift for SH–CO2 probe. In a next step, the whole PIA setup was lowered to the depth point at 8.5 m with 10 times the normal speed (4 cm/s). The travel time between the two depths was ∼50 s. In this short time interval, a sharp jump in ΔEMF/slope for CO2–ISE couple indicates that the sensing system responded rapidly to an activity change of log aCO2 of 0.78 units which matches well with the immediate jump of 0.8 in ΔEMF/slope for the CO2–ISE couple. The fluctuations in the response of CO2–ISE couple seen at 8.5 m could be explained by the location of this sampling point just at the end of the steep oxycline where any depth fluctuations cause significant changes in dissolved CO2 activity. In contrast, the response of CO2–SH probe shows a very smooth response curve which extends for >5 min until it is stable. Thus, the steep concentration changes are missed by CO2–SH probe owing to its slow response.

Figure 3

Figure 3. Temporal response of CO2–SH probe and CO2–ISE couple on fast profiling with 5 cm/s (10-fold normal speed). Slopes for both type of sensors as per calibration scheme 3. The blue points on the dissolved oxygen profile and on the depth profile indicate the syringe sampling locations.

Embedded Details in Highly Resolved CO2 Profiles

In situ high-resolution profiles of the activity of dissolved CO2 from 2 to 12 m in Lake Rotsee taken at a profiling speed of 0.5 cm/s show distinct features which can be linked to other physicochemical parameters to infer the governing biogeochemical processes (Figure 4). During profiling, the epilimnion had rather uniform O2 concentrations (250 μM) down to 8 m. The photosynthetic utilization in the epilimnion tends to reduce the CO2 content (26 μM) above 8 m. The CO2 profiles obtained by the ISE couples and the SH probe agreed with the syringe concentrations down to the oxycline. Below the oxycline a strong increase in CO2 concentration was observed due to its accumulation in the anoxic hypolimnion fed by inputs from the sediments and mineralization processes in the water column. In general, the CO2 profiles obtained with the ISE-couple showed detailed features between the sampling points, which indicates distinct biogeochemical activity at the cm scale. For instance, at depth 8.5 m, a small hump extending over ∼20 cm in the CO2 profile was observed, which was totally missed by the CO2–SH probe because of its slow response. This small feature coincided with a peak in the turbidity profile, which typically indicates a layer with high cell numbers that accumulated at the upper end of the thermocline and performed oxic respiration processes and photosynthesis at low-light conditions. The occurrence of this phenomenon has been described for Lake Rotsee by Brand et al. (28) and Oswald et al. (37)

Figure 4

Figure 4. In situ high-resolution profiles of physicochemical characteristics of water column, activity of dissolved CO2 by CO2–ISE couples and a comparison with simultaneous response of CO2–SH probe and syringe samples at profiling speed 0.5 cm/s in Lake Rotsee.

In summary, we were able to develop an in situ setup for dissolved CO2 measurements at high temporal and spatial resolution. We recommend careful conditioning of the SC-ISEs to achieve reliable response close to LOD as determined in the laboratory. The in situ sampling and calibration steps could not be avoided in the field and were needed for the determination of drift and optimum offset potentials. As only two sampling points were necessary for compensating for the drift and changes in offset potentials according to the optimized calibration protocol, users may also employ other sampling devices such as conventional Niskin bottles that can be triggered at a specific depth for calibration of EMF data. The optimized in situ calibration protocol allows reducing sampling efforts for reliable results. The response time of few seconds offered by the CO2–ISE setup opens the perspective to capture the fast dynamic of biogeochemical processes and to map hot spots of CO2 production. The sulfide insensitivity observed in this field application with up to 100 μM total sulfides facilitates using the CO2–ISE in anoxic waters without interference. This simple yet elegant potentiometric sensing system for CO2 could serve as an effective tool for studying biogeochemical processes occurring at small scales in freshwater systems.

Supporting Information

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.8b02969.

  • Experimental section with materials and chemicals, sensor fabrication, laboratory tests, sulfide sensitivity test (Figure S1), schemes in optimization of in situ calibration protocol (Figure S2), depth profile of raw and drift corrected ΔEMF (Figure S3), depth profiles of activity of dissolved CO2 obtained by CO2–SH probe with different calibration schemes (Figure S4), in situ profiling set up (Figure S5), selectivity coefficients for ion selective membranes and mean concentrations of relevant ions in the lake water column (Table S1), and performance of SC-ISEs during laboratory tests (Table S2) (PDF)

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Author Information

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  • Corresponding Author
    • Rohini Athavale - Eawag—Swiss Federal Institute of Aquatic Science and Technology, Department of Surface Waters Research and Management, Seestrasse 79, CH-6047 Kastanienbaum, SwitzerlandInstitute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Universitätsstrasse 16, CH-8092 Zürich, SwitzerlandOrcidhttp://orcid.org/0000-0001-8132-7571 Email: [email protected]
  • Authors
    • Nadezda Pankratova - Department of Inorganic and Analytical Chemistry, University of Geneva, Quai E.-Ansermet 30, 1211 Geneva, SwitzerlandIntegrated Systems Laboratory (LSI), Swiss Federal Institute of Technology Lausanne (EPFL), CH-1015 Lausanne, Switzerland
    • Christian Dinkel - Eawag—Swiss Federal Institute of Aquatic Science and Technology, Department of Surface Waters Research and Management, Seestrasse 79, CH-6047 Kastanienbaum, Switzerland
    • Eric Bakker - Department of Inorganic and Analytical Chemistry, University of Geneva, Quai E.-Ansermet 30, 1211 Geneva, SwitzerlandOrcidhttp://orcid.org/0000-0001-8970-4343
    • Bernhard Wehrli - Eawag—Swiss Federal Institute of Aquatic Science and Technology, Department of Surface Waters Research and Management, Seestrasse 79, CH-6047 Kastanienbaum, SwitzerlandInstitute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Universitätsstrasse 16, CH-8092 Zürich, Switzerland
    • Andreas Brand - Eawag—Swiss Federal Institute of Aquatic Science and Technology, Department of Surface Waters Research and Management, Seestrasse 79, CH-6047 Kastanienbaum, SwitzerlandInstitute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Universitätsstrasse 16, CH-8092 Zürich, Switzerland
  • Author Contributions

    R.A. and N.P. contributed equally.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors are grateful for the financial support from the Swiss National Science Foundation (SNF Grant 147654). We would like to thank Dajing Yuan for his help in synthesis of f-MWCNTs in the laboratory.

References

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  • Abstract

    Figure 1

    Figure 1. Response (with respective slopes in mV/decade reported in parentheses) of (a) carbonate and hydrogen ion selective SC-ISEs in the laboratory (b) CO2–ISE couples on shipboard (c) CO2–SH probe before the deployment on change in dissolved CO2 concentrations.

    Figure 2

    Figure 2. Depth profiles of activity of dissolved CO2 obtained by CO2 ISE couple calculated with different calibration parameters corresponding to the three in situ calibration schemes as described in Table 1. Please note that the dark blue line for calibration scheme 2 is almost overlapped with the light blue line of calibration scheme 4.

    Figure 3

    Figure 3. Temporal response of CO2–SH probe and CO2–ISE couple on fast profiling with 5 cm/s (10-fold normal speed). Slopes for both type of sensors as per calibration scheme 3. The blue points on the dissolved oxygen profile and on the depth profile indicate the syringe sampling locations.

    Figure 4

    Figure 4. In situ high-resolution profiles of physicochemical characteristics of water column, activity of dissolved CO2 by CO2–ISE couples and a comparison with simultaneous response of CO2–SH probe and syringe samples at profiling speed 0.5 cm/s in Lake Rotsee.

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  • Supporting Information

    Supporting Information


    The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.8b02969.

    • Experimental section with materials and chemicals, sensor fabrication, laboratory tests, sulfide sensitivity test (Figure S1), schemes in optimization of in situ calibration protocol (Figure S2), depth profile of raw and drift corrected ΔEMF (Figure S3), depth profiles of activity of dissolved CO2 obtained by CO2–SH probe with different calibration schemes (Figure S4), in situ profiling set up (Figure S5), selectivity coefficients for ion selective membranes and mean concentrations of relevant ions in the lake water column (Table S1), and performance of SC-ISEs during laboratory tests (Table S2) (PDF)


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