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Article

Research on On-Line Detection Method and System for Nitrate in Groundwater

by
Rong Xiong
1,2,
Yang Li
3 and
Zhenxing Ren
2,4,*
1
School of Architecture and Civil Engineering, Chengdu University, Chengdu 610106, China
2
State Key Laboratory of Transducer Technology, Beijing 100080, China
3
Aerospace Information Research Institute, Chinese Academy of Sciences, Beijing 100080, China
4
School of Mechanical Engineering, Chengdu University, Chengdu 610106, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2023, 13(17), 9743; https://doi.org/10.3390/app13179743
Submission received: 2 August 2023 / Revised: 25 August 2023 / Accepted: 26 August 2023 / Published: 28 August 2023
Browse Figures
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Abstract

:
In order to monitor the concentration of nitrate in groundwater and evaluate water quality, a specialized sensor has been developed to solve the problems of the passivation effect, short life and poor repeatability of electrodes in conventional electrochemical detection, a novel preparation method of copper-sensitive film was proposed, and the method was combined with a microelectrochemical sensing electrode to detect nitrate in water. Microfabrication technology was used to prepare the micro sensor electrode. A total of 0.05 mol/L CuSO4 was added into the supporting electrolyte, and the dissolution and deposition of copper on the surface of the working electrode and the catalytic reduction reaction of nitrate ions were performed. On-line cleaning of microelectrode, in situ deposition of copper-sensitive film and detection of nitrate concentration in water were realized, respectively. An on-line modified electrode was used to detect the nitrate standard sample, achieving a high detection sensitivity (0.031 μA/[μmol L−1]) in the concentration range of 0–3500 μmol/L. The relative standard deviation of the same nitrate standard sample repeated for 15 times was only 2.48%, which improved the long-term repeatability of nitrate detection, and, at the same time, an on-line system was adopted that included an on-line detection system (DS) and a wireless transmission system (WS).

1. Introduce

With the rapid development of industry and agriculture and the expansion of urban population, nitrate (NO3−) has become one of the most common pollutants in surface water and groundwater worldwide. The application of large amounts of nitrogen fertilizers in agricultural production and the direct discharge of domestic sewage and nitrogen-containing industrial wastewater all lead to an overall upward trend in nitrate concentration in natural water bodies, which has become an important environmental problem [1,2]. At the same time, nitrate has strong solubility; its migration rate in water or soil is very fast, so once it enters the natural environment, it is widely dispersed through various circulation activities in nature and thus nitrate widely exists in the natural environment and ecosystem.
Many studies have proved that excessive nitrate content in water can cause serious environmental and physiological problems [3,4,5]. When the nitrate content in water exceeds the standard, it will accelerate the eutrophication of water and poison aquatic animals and plants. Long-term consumption of drinking water with excessive nitrate will also significantly increase the risk of muscle cramps, methemoglobin disease and stomach cancer [6]. Excessive nitrate in the water will seriously affect drinking water safety and fishery production. Therefore, the detection of nitrate concentration in water is of great practical significance.
At present, nitrate detection methods mainly include spectrophotometry, chromatography, luminescence analysis, capillary electrophoresis, electrochemical detection, etc. [7,8,9,10]. Among them, gas-phase molecular absorption spectrometry, ultraviolet spectrophotometry and phenol disulfonic acid spectrophotometry are standard methods for nitrate detection. Such detection methods based on optical principles have the advantages of low detection limit, good accuracy and high sensitivity [11,12,13,14,15,16], but expensive optical analytical instruments are required for detection. The commercial testing equipment developed based on this principle usually has the problems of large volume, stringent water sample pretreatment requirements and high price, which cannot fully meet the practical needs of on-site, rapid and multi-point testing in China’s vast waters. The nitrate detection method based on electrochemical principle has the advantages of simple operation, fast response speed, high sensitivity and less required reagents, which is conducive to the realization of on-site, rapid and portable detection. Therefore, in recent years, researchers have paid attention to this issue, and a variety of electrochemical electrodes for nitrate detection have been developed [17,18,19,20,21,22,23,24]. Most of these studies focused on the optimization of modification methods and the selection of sensitive materials to improve the detection sensitivity of the electrode to nitrate, but rarely considered the service life and maintenance cycle of the electrode [25,26]. When the electrochemical electrode is applied in the aqueous environment, the analyte to be measured will have an electrochemical reaction on the surface of the sensitive material, and the concentration of the analyte can be measured by examining the response signal. This detection method is efficient: it requires simple equipment, is low cost and has high sensitivity. However, in specific applications, for the same electrode, with the increase in detection times, the reaction product will gradually adhere to the surface of the sensing material so that its effective area is gradually reduced and the response signal of the modified electrode is also reduced, which is called the electrode passivation effect. Due to the existence of the passivation effect, when the electrochemical sensor is used for detection, the sensor probe must be calibrated regularly or a disposable probe must be used, and this frequent manual maintenance work greatly limits the promotion and application of electrochemical sensors [27,28,29].
In this paper, the electrochemical deposition and dissolution of copper ions in the voltage-controlled deposition solution on the surface of the sensitive electrode are utilized to achieve the in situ deposition of copper-sensitive materials on the surface of the working electrode and the dissolution and elimination of copper. In addition, a continuously updated sensitive detection interface is constructed for the on-line detection of nitrate. In order to avoid the passivation effect of the electrode surface, the electrocatalytic activity of the copper modified layer is maintained and the repeatability and sensitivity of nitrate detection in water are greatly improved. On this basis, an electrochemical detection method for nitrate concentration in water was developed.

2. Experimental Part

2.1. Reagents and Instruments

S-4800 Scanning electron microscope (FE-SEM, Hitachi, Tokyo, Japan); Camry Reference-600 Electrochemical analyzer (Gamry, Warminster, PA, USA); AUW Electronic Balance (Shimadzu Corporation, Kyoto, Japan); BioSpec-nano UV Landscape Photometer (Shimadzu, Japan); Direct-Q3UV high purity water machine (Millipore Corporation, Burlington, MA, USA); Digital pH meter (Shanghai Zhiguang Instrument Company, pHS-25 type, Shanghai, China); CHI111 Ag/AgCl reference electrode (Chenhua, Shanghai, China).
CuSO4·5H2O, Na2SO4, NaNO3 and 98%H2SO4 (Beijing Chemical Reagent Company, Beijing, China); nitrate standard sample 50 mg/L (Standardization Institute, Ministry of Environmental Protection, Beijing, China). The chemicals and solvents were analytically pure, and the experimental water was 30 MΩ·cm deionized water. Without special instructions, all solutions were obtained by dissolving quantitative solid reagents in deionized water at room temperature (25 °C). All experiments were performed in a three-electrode system, with reference to Ag/AgCl electrodes, working electrodes and laboratory-made microsensing electrodes.

2.2. Experimental Process

The microsensor electrode chip prepared by MEMS technology is used as the input end of excitation voltage and the receiver end of the response current of the electrochemical sensor electrode in the indoor testing room (State Key Laboratory of Sensors, China). The sensor chip integrates the working electrode (WE) made of platinum, the counter electrode (CE) made of platinum and the insulating layer on the same glass substrate, overcoming the shortcomings of the traditional discrete electrode system with its complex preparation process and poor electrode consistency. The WE is circular and its geometric area is 1 mm2. The shape of CE is a 320° arc of a ring with an inner diameter of 2.2 mm and an outer diameter of 3.8 mm, as shown in Figure 1. The electrode structure is concentrically symmetrical, with uniform electric field distribution, which is consistent with the diffusion direction of the ions in the electrochemical reaction system. It can better suppress current noise and eliminate interference and improve the electrochemical performance of the sensor. All electrodes are connected to the PCB through gold wire. The detailed fabrication process of the microelectrode has been reported in our previous published paper [30,31].
For NO3− detection, Na2SO4 solution containing a certain concentration of Cu2+ ions is used as the testing base solution (50 mL, 0.1 mol/L Na2SO4 + 0.05 mol/L CuSO4, pH = 2.0). Each detection cycle includes three stages:
(a)
In the cleaning stage, at the dissolution potential E1, the microelectrode is cleaned, which removes the copper-sensitive layer deposited on the electrode surface.
(b)
In the modification stage, at the deposition potential E2 of the Cu2+ ions, electrodeposition is added to modify the microelectrode so that a layer of copper-sensitive material is deposited onto the electrode surface.
(c)
In the detection stage, the response current is detected at the detection potential E3 of the reduction peak of the NO3− ions.
During detection, the voltage on the working electrode is changed according to the requirements of each of the three stages so that the electrochemical catalytic reaction occurring during each test is carried out on a newly prepared sensitive surface, thus eliminating the influence of the electrode passivation effect on NO3− detection. Based on this test principle, the change curve of the potential on the working electrode over time during one detection cycle is shown in Figure 2:
In order to comprehensively investigate the relevant potential parameters, in 50 mL mixed solution containing 0.1 mol/L Na2SO4, 0.05 mol/L CuSO4 and 3.5 mol/L NaNO3 (in the subsequent optimization experiment, if there is no special explanation, the test solution used is all mixed solution with this ratio), cyclic voltammetry was performed in the range of +0.5 V to −0.8 V to determine the peak potential values of relevant parameters in the three stages. The scanning results are shown in Figure 3:
As can be seen from the figure, two peaks appeared in the process of voltage scanning from +500 mV to −800 mV. Combined with the chemical composition of the solution system, it can be seen that the reduction peak at −209 mV is related to the reduction–deposition reaction of Cu2+ ions, and a large number of Cu2+ ions are reduced and electrodeposited on the surface of the working electrode. The curve peak at −597 mV is the reduction peak caused by the electrocatalytic reduction in NO3− ions on the surface of the copper-sensitive film, and the electrode has the largest response current value at this time. When the voltage is scanned to the forward potential, the copper on the surface of the working electrode begins to dissolve. When the dissolution current reaches the maximum value at +217 mV, the copper-sensitive layer begins to dissolve in large quantities.

3. Results and Discussion

3.1. Parameter Optimization

(1)
Dissolution potential E1
To ensure repeatability of the test, the copper already present on the electrode surface must be completely dissolved before each remodification. In order to obtain the most efficient surface cleaning solution, the microelectrode was placed in the test solution and the optimized solution potential was investigated by examining the time–current curve. The results are shown in Figure 4; that is, E2 = −300 mV and E3 = −600 mV were kept unchanged while the time–current scan curve was changed in which E1= (+0.1 V, +0.2 V, +0.3 V, +0.4 V, +0.5 V). It can be seen from the figure that when E1 = +0.1 V, the dissolution current gradually decreases from 30 μA in the initial stage to about 100 nA in the second stage, which indicates that the copper film prepared in the previous deposition operation on the electrode surface is being effectively dissolved. When E1 varies between +0.2 V and +0.5 V, the corresponding dissolution current is always about 100 nA. At the same time, the current values corresponding to the dissolution stage are also very different under different dissolution potentials, which indicates that the dissolution potentials within this range cannot effectively remove the deposited copper on the electrode surface and thus each subsequent electrodeposition process is carried out on the electrode surface in different states.
In order to explore the optimal value, in the same test environment, one of the values of E1 = (+0.11 V, +0.12 V, +0.13 V, +0.14 V, +0.15 V, +0.16 V) was changed again, and the test results were shown in Figure 3b. It can be seen that the deposition current value decreases obviously under different dissolution potentials. This indicates that the dissolution potential in this range can effectively dissolve the copper on the electrode surface so that the subsequent deposition process is carried out on the electrode surface environment in a similar state. When E1 = 0.14 V, the process of decreasing the dissolution current from the maximum value to 100 nA takes the shortest time.
(2)
Deposition potential E2
In order to select and optimize deposition potential E2, when one of the values of E2 = (−0.1 V, −0.2 V, −0.3 V, −0.4 V) is changed under the above optimized parameter settings, a series of samples with different concentrations are tested and the optimal value is determined by comparing the response sensitivity under different E2 values. When E2 = −0.3 V is taken as an example, the optimization experiment is illustrated.
When E2 = −0.3 V, the electrochemical response curve of the sensing electrode is shown in Figure 5. It can be seen that in the detection stage (T = 4.0–5.0 s), the response current value of the electrode increases with the increase in NaNO3 concentration in the test solution.
When E2 = one of (−0.1 V, −0.2 V, −0.3 V, −0.4 V), the above experiments are repeated and the linear fitting lines at T = 4.04 s are obtained, respectively. The comparison results under the four circumstances are shown in Figure 6. In order to eliminate the influence caused by background noise, the longitudinal coordinate of the curve is the difference between the response current of the test liquid and that of the blank solution. It is found that when E2 = −0.1 V, the background noise of the electrode is too large, resulting in no linear relationship between the response current value and the concentration of the test liquid. When E2 = −0.3 V or −0.4 V, there is a good linear relationship between the response current and the concentration of the test solution, which indicates that the modified microelectrode can be used for the detection of nitrate ions.
When E2 = −0.4 V, the electrode response sensitivity is the highest, so the deposition potential value is selected as E2 = −0.4 V in subsequent experiments.
(3)
Deposition potential E3
As can be seen from Figure 3, under the detection system selected in this study, the reduction peak potential of nitrate is −597 mV. In order to obtain the maximum response current signal, the detection potential E3 is selected to be −597 mV.
In summary, through systematic optimization experiments, the optimal parameter settings of modified microelectrodes for nitrate ion detection were determined and are shown in Table 1:

3.2. Performance Test

The fundamental purpose of using newer modified microelectrodes to detect nitrate ions is to eliminate the electrode passivation effect and improve the long-term repeatability of sensor detection. Therefore, the reproducibility and consistency of the proposed renewable microelectrode for nitric acid detection were examined in detail.
(1)
Repeatability
The same microelectrode was used to perform nine repeated tests in the test solution (a mixture of 0.1 mol/L Na2SO4, 0.05 mol/L CuSO4 and 1000 µmol/L NaNO3) with the new modification method, and the results are shown in Figure 7.
The time–current curve obtained by nine tests basically coincides with the response curve of the detection stage, and the response current value of the electrode is −120.20 ± 1.51 µA with a maximum relative deviation of 1.26%. This indicates that the catalytic activity of the sensitive film on the surface of the modified electrode is relatively stable and not affected by the passivation effect.
(2)
Consistency
Five different microelectrodes were used in the test solution (a mixture of 0.1 mol/L Na2SO4, 0.05 mol/L CuSO4 and 500 µmol/L NaNO3) to perform repeated tests three times each, and the results are shown in Figure 8.
The response curve of the detection stage is also basically coincident with the time–current curve obtained from the 15 tests. The response current value of the electrode is −105.11 ± 2.61 µA, and the maximum relative deviation is 2.48%. This indicates that the new modification method can also show better repeatability and consistency when applied to different microelectrodes.
(3)
Detection performance of nitrate
Under the optimal parameter settings, the electrode pair concentrations are 0, 5, 10, 40, 100, 200, 400, 700, 1000, 1500, 2000, 2500, 3000, 3500 µmol/L, and all 14 nitrate samples were detected. The experimental results showed that there was a good linear relationship between the electrode response current value y and NO3− concentration x. The linear fitting between the two results in the functional relationship as follows:
y [µA/µmol L−1] = −0.031x − 2.757, R2 = 0.9879
These results indicate that the on-line modified copper-sensitive film modified electrode can be used for the detection of nitrate concentration in water.

4. On-Line Monitoring System

In order to use sensors to monitor the nitrate concentration of groundwater on-line, an on-line monitoring system is adopted. This system consists of an on-line detection system (DS) and a wireless transmission system (WS), as shown in Figure 9. The on-line detection system consists of a MPS430 microcontroller, sensors, signal processing circuits and AD conversion circuits. The AD is a 16-bit high-precision converter AD7705 (Analog Devices, Inc., Wilmington, MA, USA), which collect nitrate concentration signals in groundwater through sensors. Each on-line detection system is fixed on anchor rods and is inserted into the groundwater to detect quality.
The main function of wireless transmission systems is to transmit nitrate concentration signals over long distances. The power supply of each system is powered by batteries. The wireless transmission system is connected to the on-line detection system through wired means.
The wireless transmission module is composed of CC1101 (TI Company in Dallas, TX, USA) and a microcontroller; the signal is transmitted through the chip and antenna.
The receiving end is also composed of CC1101 (TI Company in USA) and a microcontroller. After processing has been performed by the microcontroller, it is connected to the upper computer through a serial port circuit and displayed on the upper computer.
In order to reduce energy consumption in the network and extend the entire monitoring network lifecycle, the LEACH (low-energy adaptive clustering hierarchy) protocol is adopted for wireless transmission [32]. This protocol has the characteristics of simple implementation, autonomous cluster head election and convenient expansion.
In order to collect nitrate concentration from multiple points, the whole system sets up multiple on-line monitoring points, each of which is assigned an identity code. The data transmission format of each monitoring point includes both identity code and nitrate concentration information, and the data are received by terminal system (TS).

5. Conclusions

In view of the problems such as contamination of the sensor electrode by the test solution, short life, poor repeatability and inability to implement on-line monitoring in order to process conventional electrochemical detection, an on-line modification method of copper-sensitive film was proposed in this paper, and the method was applied to the detection of nitrate ions. By using copper dissolution and deposition, the self-made microsensing electrode can be cleaned on site and the sensitive film can be redeposited in situ to eliminate the passivation effect of the electrode. At the same time, based on the electrocatalytic reduction process of the nitrate ion by using copper-sensitive film in an acidic solution environment, continuous detection of nitrate ion concentration in water samples is realized, which greatly improves the long-term repeatability of the detection results. The maximum relative deviation of repeated experiments with the developed method is only 2.48%, which provides a feasible technical path for real-time on-line monitoring of nitrate concentration in water.

Author Contributions

Methodology, R.X.; Investigation, Z.R.; Data curation, Y.L.; Writing—original draft, R.X. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by State Key Laboratory of Transducer Technology grant number SKT1505. And The APC was funded by Research on Electrochemical Microsensors and Control Systems for Nitrate Detection.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data supporting the findings of this study are available within this article.

Conflicts of Interest

The authors declare no conflict of interest.

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Applsci 13 09743 g001
Figure 1. Photo of the microelectrode fabricated by MEMS technology and the configuration of the electrochemical experimental system.
Figure 1. Photo of the microelectrode fabricated by MEMS technology and the configuration of the electrochemical experimental system.
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Figure 2. Schematic representation of potential protocol in one detection cycle: relation curve of the operating voltage on the working electrode with time including: (a) cleaning stage, (b) modification stage and (c) detection stage.
Figure 2. Schematic representation of potential protocol in one detection cycle: relation curve of the operating voltage on the working electrode with time including: (a) cleaning stage, (b) modification stage and (c) detection stage.
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Figure 3. Cyclic voltammetry scanning curve in 50 mL mixed solution containing 0.1 mol/L Na2SO4, 0.05 mol/L CuSO4 and 3.5 mol/L NaNO3: (a) NO3− reduction peak; (b) Cu2+ reduction deposition peak; (c) Cu dissolution peak.
Figure 3. Cyclic voltammetry scanning curve in 50 mL mixed solution containing 0.1 mol/L Na2SO4, 0.05 mol/L CuSO4 and 3.5 mol/L NaNO3: (a) NO3− reduction peak; (b) Cu2+ reduction deposition peak; (c) Cu dissolution peak.
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Figure 4. Chronoamperometry scan voltammograms of calibration samples: time–current curve of the microelectrode in the testing solution when E2 = −0.3 V, E3 = −0.6 V: (a) E1 from curve a to curve e is 0.1, 0.2, 0.3, 0.4, 0.5 V, respectively; (b) E1 from curve f to curve l is 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16 V, respectively.
Figure 4. Chronoamperometry scan voltammograms of calibration samples: time–current curve of the microelectrode in the testing solution when E2 = −0.3 V, E3 = −0.6 V: (a) E1 from curve a to curve e is 0.1, 0.2, 0.3, 0.4, 0.5 V, respectively; (b) E1 from curve f to curve l is 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16 V, respectively.
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Figure 5. Chronoamperometry scan voltammograms of calibration samples: time–current curve of the microelectrode in the test solution. NaNO3 concentrations in the test solution are as follows: 0, 5, 10, 40, 100, 200, 400, 700, 1000, 1500, 2000, 2500, 3000, 3500 µmol/L: (a) complete time–current curve; (b) amplification of the part of the figure with time of 4 s to 5 s.
Figure 5. Chronoamperometry scan voltammograms of calibration samples: time–current curve of the microelectrode in the test solution. NaNO3 concentrations in the test solution are as follows: 0, 5, 10, 40, 100, 200, 400, 700, 1000, 1500, 2000, 2500, 3000, 3500 µmol/L: (a) complete time–current curve; (b) amplification of the part of the figure with time of 4 s to 5 s.
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Figure 6. Corresponding calibration plots of current values: fitting lines between the electrode response current value and the NO3− concentration under different values of deposition potential E2; NaNO3 concentrations in testing solutions are as follows: 0, 5, 10, 40, 100, 200, 400, 700, 1000, 1500, 2000, 2500, 3000, 3500 µmol/L.
Figure 6. Corresponding calibration plots of current values: fitting lines between the electrode response current value and the NO3− concentration under different values of deposition potential E2; NaNO3 concentrations in testing solutions are as follows: 0, 5, 10, 40, 100, 200, 400, 700, 1000, 1500, 2000, 2500, 3000, 3500 µmol/L.
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Figure 7. Repeated testing of samples with the same concentration nine times in a mixture of 0.1 mol/L Na2SO4, 0.05 mol/L CuSO4 and 1000 µmol/L NaNO3; the resulting chronocurrent scanning curve and the corresponding response current value change the curve with the test times (tested once every 5 min and repeated nine times).
Figure 7. Repeated testing of samples with the same concentration nine times in a mixture of 0.1 mol/L Na2SO4, 0.05 mol/L CuSO4 and 1000 µmol/L NaNO3; the resulting chronocurrent scanning curve and the corresponding response current value change the curve with the test times (tested once every 5 min and repeated nine times).
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Figure 8. Five different microelectrodes were tested with the same concentration of samples three times each in a mixture of 0.1 mol/L Na2SO4, 0.05 mol/L CuSO4 and 500µmol/L NaNO3; the resulting chronocurrent scanning curve and the corresponding response current value changed with the test times (tested once every 5 min and repeated three times).
Figure 8. Five different microelectrodes were tested with the same concentration of samples three times each in a mixture of 0.1 mol/L Na2SO4, 0.05 mol/L CuSO4 and 500µmol/L NaNO3; the resulting chronocurrent scanning curve and the corresponding response current value changed with the test times (tested once every 5 min and repeated three times).
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Figure 9. Block diagram of the on-line monitoring system.
Figure 9. Block diagram of the on-line monitoring system.
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Table 1. Optimization parameter settings (with the current at T = 4.04 s as the electrode response signal).
Table 1. Optimization parameter settings (with the current at T = 4.04 s as the electrode response signal).
Detection PhaseWorking Potential (mV)Duration Time (s)
Cleaning +14010
Modification −4004
Detection −5971
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Xiong, R.; Li, Y.; Ren, Z. Research on On-Line Detection Method and System for Nitrate in Groundwater. Appl. Sci. 2023, 13, 9743. https://doi.org/10.3390/app13179743

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Xiong R, Li Y, Ren Z. Research on On-Line Detection Method and System for Nitrate in Groundwater. Applied Sciences. 2023; 13(17):9743. https://doi.org/10.3390/app13179743

Chicago/Turabian Style

Xiong, Rong, Yang Li, and Zhenxing Ren. 2023. "Research on On-Line Detection Method and System for Nitrate in Groundwater" Applied Sciences 13, no. 17: 9743. https://doi.org/10.3390/app13179743

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