A method for detecting the redox capacity of a non-homogeneous aqueous medium under different potential conditions

Abstract

The application provides a method for detecting the redox capacity of a non-homogeneous aqueous medium under different potential conditions, and belongs to the technical field of electrochemical analysis. The method system uses a reticular titanium-based ruthenium iridium oxide coating electrode as a working electrode, improves the upper limit of the sample addition amount of the existing mediated electrochemical analysis system from the mg level to the g level, solves the representativeness problem of the sample in the test of the non-homogeneous aqueous medium, and improves the precision of the redox capacity analysis from mmol e ‑ / kg to μmol e ‑ / kg. By regulating the applied voltage of the working electrode and adding different electronic mediator substances, the test of the redox capacity of the non-homogeneous aqueous medium under different potential conditions can be realized. The method has high sensitivity and good accuracy, in addition, the titanium-based ruthenium iridium oxide coating electrode has strong corrosion resistance, low cost, is convenient to process, and is simple to maintain, and has a good practical application prospect.

Description

Technical Field

[0001] This invention relates to the field of electrochemical analysis, and more particularly to a method suitable for detecting the redox capacity of heterogeneous aqueous media. Background Technology

[0002] The migration, transformation, and remediation of pollutants in underground environments are all related to redox reactions. However, the redox heterogeneity of aquifers makes it difficult to assess the potential for natural pollutant decay, as well as the remediation cycle and reagent dosage. Since over 90% of the redox components in an aquifer are distributed within the aquifer medium, the redox heterogeneity of the aquifer medium can, in most cases, represent the overall redox heterogeneity of the aquifer (see reference 1). The essence of redox heterogeneity in aquifers is the uneven distribution of the content and form of different redox components. Redox capacity, as an indicator, encompasses all redox components in the aquifer medium and can therefore effectively represent its overall redox properties. Furthermore, aquifers contain a wide variety of redox components, each with different redox potentials. Therefore, the redox capacity of aquifers under different potential conditions can accurately characterize the redox heterogeneity of the aquifer medium. However, currently, there is no analytical method to test the redox capacity of aquifers under different potential conditions.

[0003] Electrochemical methods are a method developed in recent years to test the redox capacity of standard redox active components (such as iron minerals and dissolved organic matter) under different potential conditions. The testing principle of this method is to use a working electrode to oxidize or reduce the analyte under certain potential conditions, and then calculate the oxidation or reduction capacity of the analyte based on the current signal on the working electrode. In order to accelerate the electron transfer rate between the working electrode and the analyte during the test, a certain concentration of electron-medium material is often added, as detailed in references 2-4. However, the existing electrochemical methods have the following problems when applied to the redox capacity test of heterogeneous aqueous media under different potential conditions: (1) The upper limit of the sample addition in the existing electrochemical methods is limited by the activity of the glassy carbon crucible of the working electrode, which is only a few milligrams (<10 mg), as detailed in reference 5. However, in reality, heterogeneous aqueous media often require more than 0.5 g of sample to represent its overall redox properties. Therefore, the existing electrochemical methods cannot be applied to the redox capacity test of heterogeneous aqueous media under different potential conditions; (2) The analytical precision of the existing electrochemical methods is usually mmol e - / kg, while the actual redox capacity of heterogeneous aqueous media is low, so it is necessary to improve the test accuracy of dielectric electrochemical method; (3) When gram-level aqueous media is added to the analysis system, it will inevitably wear down the working electrode, and the electrochemical performance will be significantly reduced after the surface of glass carbon crucible is worn down; (4) Glass carbon crucible is expensive and not conducive to widespread application. Summary of the Invention

[0004] The purpose of this invention is to address the aforementioned shortcomings of existing technologies by constructing a method for detecting the redox capacity of heterogeneous aqueous media under different potential conditions. This method increases the upper limit of sample dosage from the mg level to the g level, significantly improves the sensitivity and stability of the working electrode, and simultaneously reduces the cost of the working electrode. Using this method, accurate analysis of the redox capacity of heterogeneous aqueous media at different potentials can be achieved, providing a technical approach for understanding the redox heterogeneity of aquifers.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] The purpose of this invention is to provide a method for detecting the redox capacity of heterogeneous aqueous media under different potential conditions. The method employs a three-electrode system: the working electrode is a ruthenium-iridium-titanium MMO electrode with a high-purity titanium substrate and a ruthenium-iridium-titanium surface coating; the reference electrode is Ag / AgCl; and the counter electrode is a platinum wire, separated from the working electrode chamber by a glass frit core. The method includes the following steps:

[0007] S1, Preparation of dielectric electrochemical test solution

[0008] S11, Preparation of standard solution of mediator

[0009] Weigh out the mediator standard, prepare it with water and remove dissolved oxygen to obtain the mediator standard solution; the mediator includes ABTS and Fe(CN)6. 3- At least one of DCPIP, AQDS, and EV;

[0010] S12, Preparation of background electrolyte

[0011] KCl and MOPS were weighed separately, and water was used to prepare, pH was adjusted and dissolved oxygen was removed to obtain an electrolyte containing KCl and MOPS.

[0012] S2, Balanced Three-Electrode System

[0013] Based on the testing requirements, different mediators are selected, and background electrolyte and mediators are added to the system. A specific constant voltage is applied using an electrochemical workstation to activate free radicals from the ground state of the mediators. When the current signal becomes stable and constant, it indicates that the activation process of the mediators has reached equilibrium.

[0014] S3, Sample Measurement

[0015] After the system in step S2 is in equilibrium, the sediment sample to be tested is added. The active mediator causes the sediment in the aqueous medium to be tested to be oxidized or reduced, generating an oxidation / reduction current. The time-current curve of the sample gaining and losing electrons is then tested using an electrochemical workstation.

[0016] The current response of the sample under test is obtained from the time-current curve. The amount of electron transfer in the sample can be obtained by integrating the peak area. The calculation formula is shown in Equation 1.

[0017]

[0018] Where EDC and EAC represent the reducing capacity or oxidizing capacity of the sample to be tested, in units of mol e. - / g;

[0019] F is the Faraday constant, representing the charge carried by each mole of electrons, with a value of 96485 C / mol;

[0020] t1 is the starting point of the current peak, in seconds;

[0021] t2 is the end point of the current peak, in seconds;

[0022] I(t) is a function of current changing with time, and the unit of current I is A;

[0023] m represents the dry weight of the sample added, in grams.

[0024] Furthermore, the concentration of the mediator standard solution is 2–10 mM, and the background electrolyte contains 50–100 mM KCl and 5–20 mM MOPS, with a pH of 7.0 ± 0.1.

[0025] Furthermore, the volume-to-mass ratio of the background electrolyte to the sample to be tested is 40 mL: (0.5–2) g.

[0026] Furthermore, in step S3, a constant voltage range of -0.45V to +0.7V is applied to the electrochemical workstation.

[0027] Furthermore, during the redox capacity determination process, the amount (moles) of the mediator added is determined based on the redox capacity of the sediment sample, and the number of moles of the mediator added is approximately 2 to 4 times the amount of electrons donated by the sample.

[0028] Furthermore, when the mediator is ABTS, the electrochemical workstation applies a constant voltage of +0.7V.

[0029] Furthermore, when the mediator is Fe(CN)6 3-The electrochemical workstation applies a constant voltage of +0.45V.

[0030] Furthermore, when the mediator is DCPIP, the electrochemical workstation applies a constant voltage of +0.25V.

[0031] Furthermore, when the mediator is AQDS, the electrochemical workstation applies a constant voltage of -0.22V.

[0032] Furthermore, when the mediator is EV, the electrochemical workstation applies a constant voltage of -0.45V.

[0033] Compared with the prior art, the beneficial effects of the present invention are:

[0034] (1) An improved dielectric electrochemical analysis system was adopted, replacing the glassy carbon electrode, the core component of the dielectric electrochemical method, with a ruthenium-iridium oxide coated electrode, which significantly improved the electrocatalytic activity of the working electrode and increased the sample dosage to several grams, so as to realize the testing of the redox capacity of aqueous media.

[0035] (2) The proposed method system, suitable for testing the redox capacity of heterogeneous aqueous media, can accurately test the redox capacity of sediments at different redox potentials. Compared with the glassy carbon electrode dielectric electrochemical analysis system, the ruthenium-iridium-titanium MMO electrode dielectric electrochemical analysis system improves the detection limits of standard samples (FeSO4 and Fe(OH)3) from 1.68 to 21.63 μmol e - / L decreased to 0.95–2.16 μmol e - / L, while the testing accuracy ranges from 0.25 to 3.43 μmol e - / L increased to 0.08~0.26μmol e - / L. The ruthenium-iridium-titanium MMO electrode dielectric electrochemical analysis system achieved an accuracy of 0.08–2.74 mmol e for EDC and EAC of sediments with different lithologies / compositions. - / kg, the spiked recoveries were all between 95% and 105%.

[0036] (3) The improved dielectric electrochemical testing system has simple electrode maintenance, low cost, and easy-to-process electrode shape and size, and has good practical application prospects. Attached Figure Description

[0037] Figure 1 A schematic diagram of a method system for detecting the redox capacity of a heterogeneous aqueous medium provided by the present invention;

[0038] Figure 2 The present invention provides a test to assess the effect of the amount of sediment sample in aquatic media on the Fe content and DOC content of the sediment.

[0039] Figure 3 Potential windows (af) and cyclic voltammetry curves (gl) of each mediating material for glassy carbon electrode, ruthenium-iridium-titanium MMO electrode, titanium mesh electrode, platinum electrode, graphite electrode and carbon felt electrode provided by the present invention.

[0040] Figure 4 The present invention provides an it curve for measuring the redox capacity of aqueous media deposits using ruthenium-iridium-titanium MMO electrodes, titanium mesh electrodes, platinum electrodes, graphite electrodes, and carbon felt electrodes as working electrodes.

[0041] Figure 5 The figure shows the it curves of dielectric electrochemical test standards using the ruthenium-iridium-titanium MMO electrode system. The inset shows the linear relationship between the amount of standard sample added and the measured redox capacity.

[0042] Figure 6 The figure shows the redox capacity and spiked recovery of aqueous media deposits (at +0.7V or -0.45V) using a ruthenium-iridium-titanium MMO electrode system.

[0043] Figure 7 The figure shows the redox capacity and spiked recovery of aqueous media deposits (at +0.45V, +0.25V, or -0.22V) using a ruthenium-iridium-titanium MMO electrode system. Detailed Implementation

[0044] To make the objectives, technical solutions, and advantages of the present invention clearer, embodiments of the present invention are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0045] Terminology Explanation:

[0046] Heterogeneous aquifer: refers to an aquifer in which the permeability coefficients are not exactly the same at different spatial points in the seepage field;

[0047] Redox capacity: The content of redox active components, or the total amount of electrons that can be transferred (released or stored).

[0048] The transformation process of ABTS (Eh=+0.7V, vs. SHE) and ABTS radical:

[0049]

[0050] [Fe(CN)6] 3-(Eh=+0.45V, vs.SHE) and [Fe(CN)6] 4- Transformation process:

[0051]

[0052] The conversion process between DCPIP (Eh = +0.25V, vs. SHE) and DCPIPH2:

[0053]

[0054] The conversion process between AQDS (Eh = -0.22V, vs. SHE) and AH2QDS:

[0055]

[0056] The conversion process between EV (Eh = -0.45V, vs. SHE) and EV radicals:

[0057]

[0058] Reference Appendix Figure 1 This diagram illustrates the system constructed for detecting the redox capacity of heterogeneous aqueous media under different potential conditions. The system is a ternary electrode system. The peak current area of ​​the sample is measured using an electrochemical workstation and then converted into the redox capacity of the aqueous medium at the corresponding redox potential. The ruthenium-iridium-titanium MMO electrode used in this invention is a commonly available commercially available electrode. It consists of a titanium substrate coated with an oxide layer containing ruthenium and iridium.

[0059] In this invention, in order to obtain higher detection sensitivity, the applicant conducted a screening study on the reference working electrode, as detailed below:

[0060] Glassy carbon electrode, ruthenium-iridium-titanium MMO electrode, titanium mesh electrode, platinum electrode, graphite electrode, and carbon felt electrode were selected as research objects, and their electrochemical performance was tested on a single-channel constant electrochemical workstation (Shanghai Chenhua Instrument Co., Ltd. CHI760E).

[0061] (1) Electrode Potential Window Testing. A three-electrode system was constructed using the research object as the working electrode, an Ag / AgCl electrode as the reference electrode (0.2V relative to the hydrogen standard electrode at room temperature), and a platinum sheet as the counter electrode. A solution containing 0.1M KCl and 0.01M MOPS with a pH of 7 was used as the electrolyte, and the solution temperature was maintained at 25℃. First, the open-circuit potential of each electrode was tested. Then, using linear sweep voltammetry, the potential windows of each electrode from the open-circuit potential to 2V (vs. SHE) were obtained by scanning from the open-circuit potential to -2V (vs. SHE) at a scan rate of 5mV / s. The test results showed that the potential windows of each electrode were within the range required for electrochemical testing (-0.45V to +0.7V, vs. SHE).

[0062] See appendix Figure 3 The potential windows (ae) for the glassy carbon electrode, ruthenium-iridium-titanium MMO electrode, titanium mesh electrode, platinum electrode, graphite electrode, and carbon felt electrode used in this embodiment are shown. The results show that the potential windows of each electrode are within the range required for electrochemical testing (-0.45V to +0.7V, vs. SHE).

[0063] (2) Cyclic voltammetry of each electrode against the mediating substance. Cyclic voltammetry curves of different media were then tested using different working electrodes. A three-electrode system was constructed by using the research object as the working electrode, the Ag / AgCl electrode as the reference electrode (0.2V relative to the hydrogen standard electrode at room temperature), and a platinum sheet as the counter electrode.

[0064] A solution containing 0.1 M KCl, 0.01 M MOPS, and 10 mM ABTS, with a pH of 7, was used as the electrolyte, and the solution temperature was maintained at 25 °C. The scan rate was set to 10 mV / s, and the scan range was +0.2 V to +1 V (vs. SHE), with a forward scan performed for two cycles. The electrode area used for each working electrode was measured to obtain the cyclic voltammetry curves of each electrode against ABTS.

[0065] Replace 10 mM ABTS with 10 mM Fe(CN)6 3- The scanning ranges were changed sequentially to +0.8V~+0V (vs. SHE), +0.6V~-0.2V (vs. SHE), -0.6V~+0.2V (vs. SHE), and -0.6V~+0.2V (vs. SHE), using 10mM DCPIP, 10mM AQDS, and 10mM EV, with negative scanning performed for two cycles. The electrode area used for each working electrode during testing was measured to obtain the electrode pair for Fe(CN)6. 3- Cyclic voltammetry curves for DCPIP, AQDS, and EV.

[0066] See appendix Figure 3 To illustrate the use of glassy carbon electrodes, ruthenium-iridium-titanium MMO electrodes, titanium mesh electrodes, platinum electrodes, graphite electrodes, and carbon felt electrodes as working electrodes in this embodiment, the cyclic voltammetry (fk) curves of each mediator were tested. The figure shows that, among the cyclic voltammetry curves of each working electrode for each mediator, the ruthenium-iridium-titanium MMO electrode exhibits a larger current density response.

[0067] (3) It curve testing of the deposits using each electrode. The research object was used as the working electrode, the Ag / AgCl electrode as the reference electrode (0.2V relative to the hydrogen standard electrode at room temperature), and a platinum wire as the counter electrode. The counter electrode was separated from the working electrode chamber by a glass frit core. Each electrode was connected to the electrochemical workstation to construct a three-electrode system. Each working electrode used the same working area (approximately 20 cm²). 2 Except for the platinum electrode, which is 4cm 2 To avoid the influence of atmospheric oxygen, the experiment was conducted in an oxygen-free glove box. 40 mL of oxygen-free 0.1 M KCl (containing 0.01 M MOPS, pH = 7.0) electrolyte solution was added to the reactor, and the electrolyte level was kept consistent with the electrode. An electrochemical workstation was connected, and current-time measurements were performed at a constant voltage of +0.70 V (vs. SHE). 1 mL of ABTS mediator was added, and the oxidation current was recorded. Subsequently, a clay sediment suspension was injected, and the electrochemical behavior of the sample was analyzed by monitoring current changes. The working electrode suitable for efficient and rapid redox capacity testing of aqueous media sediments was selected based on testing time and efficiency. The testing efficiency was as follows: MMO electrode > carbon felt electrode > graphite electrode > titanium mesh electrode > platinum electrode.

[0068] See appendix Figure 3 The it curves (it-time curves) of the redox capacity of aqueous media deposits were obtained by using MMO electrodes, titanium mesh electrodes, platinum electrodes, graphite electrodes, and carbon felt electrodes as working electrodes for dielectric electrochemical oxidation testing. The testing efficiency was as follows: MMO electrode > carbon felt electrode > graphite electrode > titanium mesh electrode > platinum electrode. The testing efficiency of each electrode's it curve was consistent with the current density of the ABTS response of each electrode to the mediating substance. Finally, the MMO electrode was selected as the working electrode for electrochemical testing of redox capacity under different potential conditions in heterogeneous aqueous media.

[0069] Based on the above research, the three-electrode system used for detection was determined to be as follows: the working electrode is an MMO electrode with high-purity titanium as the base material and ruthenium-iridium oxide as the surface coating material; the reference electrode is Ag / AgCl; and the counter electrode is a platinum wire, which is separated from the working electrode chamber by a glass frit core.

[0070] The method for detecting the redox capacity of heterogeneous aqueous media under different potential conditions provided by this invention is as follows:

[0071] Step S1: Preparation of dielectric electrochemical test solution

[0072] ①Preparation of standard solutions for mediating substances

[0073] Five mediator standards with different redox potentials were selected and prepared into multiple mediator standard solutions. High-purity nitrogen (99.999%) was introduced into the solution for at least 30 minutes to eliminate the interference of oxygen in the solution, and the solutions were placed in an anaerobic glove box at 25±2℃ for later use.

[0074] ② Electrochemical Testing Background: Preparation of Electrolyte Solution

[0075] Potassium chloride (KCl) and 3-(N-morpholino)propanesulfonic acid (MOPS) were selected as the electrolyte and pH buffer salt, respectively. The standard was placed in water and the pH of the solution was adjusted to 7 using sodium hydroxide (NaOH). High-purity nitrogen gas (99.999%) was introduced into the solution for at least 30 minutes to eliminate the interference of oxygen in the solution, and the solution was placed in an anaerobic glove box at 25±2℃ for later use.

[0076] Step S2: Test System Setup

[0077] Under anaerobic conditions, an MMO electrode with high-purity titanium as the matrix material and ruthenium-iridium oxide as the surface coating material was used as the working electrode, Ag / AgCl as the reference electrode, and platinum wire as the counter electrode. The counter electrode was separated from the working electrode chamber by a glass frit core. All electrodes were connected to an electrochemical workstation to construct a three-electrode system.

[0078] Step S3, Sample Measurement

[0079] The reaction vessel was placed on a stirrer, and background electrolyte and mediating material were added. A constant voltage was applied using an electrochemical workstation, and the current response as a function of time was measured using the chronoamperometry method. When the current signal reached a stable and essentially constant state, the sediment sample to be tested was added, causing the aqueous sediment to be oxidized or reduced, generating an oxidation / reduction current. The time-current curve of the sample gaining and losing electrons during this process could then be obtained using the electrochemical workstation.

[0080] The amount of electrons transferred in the sample can be obtained by integrating the peak areas of the current response generated by the sample (EDC is the oxidation current peak, and EAC is the reduction current peak), as shown in Equation 1.

[0081]

[0082] Where EDC and EAC represent the measured reduction or oxidation capacity of the sample, in units of mol e. - / g;

[0083] F is the Faraday constant, representing the charge carried by each mole of electrons, and its value is generally considered to be 96485 C / mol;

[0084] t1 is the starting point of the current peak, in seconds;

[0085] t2 is the end point of the current peak, in seconds;

[0086] I(t) is a function of current changing with time, and the unit of current I is A;

[0087] m represents the dry weight of the sample added, in grams.

[0088] In some specific embodiments, dielectric electrochemical analysis is performed using a CHI1000C multichannel potentiostat, with a three-electrode system. A piece of material with an area of ​​approximately 100 cm² is used. 2 The mesh-like MMO electrode is closed in a ring and fixed in an acrylic jar with a bottom diameter of approximately 6 cm and a height of approximately 6 cm. The top is secured and connected with a stainless steel electrode clamp, serving as the working electrode for the test. An Ag / AgCl electrode filled with saturated KCl is used as the reference electrode, and a platinum wire is used as the counter electrode, separated from the main chamber by a glass frit core. The container is wrapped with tin foil to protect it from light, and a magnetic stir bar is placed at the bottom for continuous stirring during the test. To eliminate interference from atmospheric oxygen, the test must be conducted in an oxygen-free environment.

[0089] To verify the accuracy and reliability of the detection method provided by this law, the applicant conducted the following research:

[0090] 1. To investigate the required dosage of representative samples for water-containing media with different textures.

[0091] Clay, fine sand, medium sand, and coarse sand were selected as samples of aquatic media sediments with different textures. Weigh 0.01g (10mg), 0.05g (50mg), 0.1g (100mg), 0.5g (500mg), 1g, 2g, and 5g of aquatic media sediments into centrifuge tubes for chemical extraction.

[0092] ① Total iron test of sediments. Weigh the aqueous medium and place it in a 50 mL centrifuge tube. Add 3.6 M sulfuric acid and 40% hydrofluoric acid, with 24 mL of 3.6 M sulfuric acid and 3 mL of 40% hydrofluoric acid added per gram of sediment (wet weight). Then, tightly cap the centrifuge tube, seal it with sealing film, and place it on a shaker (25℃, 220 rpm) for 24 hours.

[0093] After shaking for 24 hours, the sediment suspension was removed and centrifuged at 10,000 rpm for 10 minutes. The supernatant was then used for testing. In a 5 mL centrifuge tube, 200 μL of 0.5% phenanthroline, 1 mL of 10% hydroxylamine hydrochloride, 20 μL of 0.1 M boric acid, 20 μL of supernatant, and 1 mL of ammonium acetate-acetic acid buffer were added sequentially. The volume was then adjusted to 4 mL using a wash bottle. After 10 minutes of color development, the absorbance was measured at λ = 510 nm using a UV-Vis spectrophotometer. The iron content in the sediment was calculated based on the standard curve.

[0094] ② Decomposed Organic Matter (DOC) Test: Weigh the aqueous medium and place it in a 50 mL centrifuge tube, adding an appropriate amount of ultrapure water. The mass ratio of sediment to water is 5:1. Shake at 2000 rpm for 1 hour using a high-frequency shaker to fully disperse the aqueous medium and ensure complete dissolution of dissolved organic matter in the sediment. Centrifuge at 10000 rpm for 10 minutes, and filter the supernatant through a 0.45 μm organic filter. Test the total organic carbon in the filtrate using a TOC instrument (TOC-L, Shimadzu).

[0095] See appendix Figure 2 This embodiment tests the effect of aquifer sediment sample quantity on sediment Fe content and DOC. A sample quantity greater than 0.5g is required to accurately represent the average properties of aquifer sediments.

[0096] 2. To investigate the accuracy of redox capacity tests for representative redox-active species (iron oxides, iron-containing clay minerals, manganese minerals, and organic matter) at different potentials.

[0097] Compare two electrochemical testing systems: (1) glassy carbon electrode as working electrode and (2) MMO electrode as working electrode.

[0098] First, the redox capacity of representative redox active species (iron oxides, iron-containing clay minerals, manganese minerals, and organic matter) was tested using a glassy carbon electrode. The same background electrolyte solution was first added to the reactor, and an electrochemical workstation was connected. A constant voltage was set to the redox potential corresponding to each mediator. It-line curves were measured under the given potential conditions of the workstation, with stirring in the dark during the test. The electrochemical test was then started, and the corresponding mediator was added. After the mediator was activated and stabilized, the sample was added. Based on the peak area of ​​the redox current peak of the sample, the EDC / EAC at the corresponding potential of the sample was calculated.

[0099] The MMO electrode was then used as the working electrode to test the redox capacity of representative redox active species (iron oxides, iron-containing clay minerals, manganese minerals, and organic matter). The testing procedure was the same as that for the glassy carbon electrode.

[0100] As shown in Table 1, the two test systems used in this embodiment were used to test ABTS and Fe(CN)6 respectively. 3- DCPIP was used as a mediator to perform dielectric electrochemical oxidation to determine the electron supply capacity of reduced samples.

[0101] As shown in Table 2, the electron accepting capacity of the oxidized sample was measured by using two test systems according to this embodiment, with AQDS and EV as the mediators, respectively, for the dielectric electrochemical reduction determination.

[0102] As shown in Tables 1 and 2, the EDC / EAC test results of the electrochemical testing system provided by this invention and the existing electrochemical method for testing standard samples at different potentials are all within the statistically permissible error range, indicating that the detection method provided by this invention has high accuracy.

[0103] Table 1.

[0104]

[0105] Table 2.

[0106]

[0107]

[0108] 3. To investigate the detection limit of a method for detecting the redox capacity of heterogeneous aqueous media under different potential conditions.

[0109] The standard sample for EDC testing is FeSO4, and the standard sample for EAC testing is Fe(OH)3.

[0110] Detection limit test method: The linear curve of the standard concentration is tested, and the lowest concentration within the 99% confidence interval of the linear fit is taken. The test is performed 8 times, and the detection limit is 2.998 times the standard deviation of the 8 results (detection limit = 2.998 × STDEV).

[0111] Precision % test method: The results of N repeated tests deviate from each other. The sample is tested 3 times at a certain suitable concentration. Precision = STDEV / AVE × 100%.

[0112] Accuracy testing method: Test the sample at a suitable concentration 3 times, accuracy = STDEV

[0113] refer to Figure 5 The figure shows the it curve of the dielectric electrochemical test standard using the MMO electrode system according to this embodiment. The inset shows the linear relationship between the amount of standard sample added and the measured redox capacity.

[0114] The results are shown in Table 3. It can be seen that the detection method provided by the present invention has a lower detection limit, higher stability, and shorter detection time than the existing traditional dielectric electrochemical method.

[0115] Table 3. Comparison of detection limit results.

[0116]

[0117] 4. To examine the accuracy of the method for detecting the redox capacity of heterogeneous aqueous media under different potential conditions.

[0118] The accuracy was evaluated by testing the spiking recovery rate of aqueous media using added standard components.

[0119] The detailed operating conditions and processing results are as follows:

[0120] Four different sediment textures were selected: sand, loam, silt, silty loam, and silty clay. The clay particle content increased sequentially. Sediments at different burial depths were selected from each texture to represent different redox states. An improved dielectric electrochemical testing system was used to test the electron-donating capacity (EDC) of the sediments at different potentials. Using ABTS (+0.7V) and EV (-0.45V) as mediators, the EDC and EAC of 11 sediments were tested. Sediments with higher EDC and EAC at +0.45V, +0.25V, and -0.22V were selected for further analysis. The sediment sample dosage was 1–2 g. Based on the test results of each sediment, standard substances were added according to a molar electron ratio of 0.5–1.5 between the sample and the standard substance. Eh = +0.7V (ABTS as mediator) and Eh = +0.45V (Fe(CN)6) 3- Siderite was used as the standard sample for EDC testing at Eh = +0.25V (DCPIP as the mediator), and reduced chlorite was used as the standard sample for EDC testing at Eh = -0.22V (AQDS as the mediator) and Eh = -0.45V (EV as the mediator). Fe(OH)3 was used as the standard sample for EAC testing at Eh = -0.22V (AQDS as the mediator) and Eh = -0.45V (EV as the mediator). Based on the test results of the redox capacity of sediments in each aquifer, standard samples were added to the sediment samples at a ratio of 0.5 to 1.5 of the molar electrons of the actual sediment sample to the standard sample. For EAC or EDC < 0.5 μmol e -For aquifer media samples of / g, the dosage of standard samples was consistent with that used in individual standard sample testing. The electron capacity values ​​of the "sediment sample" and "sediment sample + standard sample" were subtracted from the values ​​obtained from the individual tests to obtain the electron capacity of the added standard sample. The electron capacity of the sediment containing the standard sample was then tested again using MMO electrode dielectric electrochemical analysis. The difference between the two test results was used to obtain the electron capacity of the standard sample, which was compared with the electron capacity of the standard sample tested alone to obtain the spiked recovery rate. Since siderite has aging issues that significantly affect its EDC, the EDC of siderite was tested simultaneously with each test of the sediment containing the standard sample. All siderite used in the experiment was used within three days of preparation. Each sample was analyzed at least three times.

[0121] See appendix Figure 6 To demonstrate the dielectric electrochemical testing of aqueous media deposits (at +0.7V or -0.45V) and spiked recoveries using the MMO electrode system in this embodiment, the spiked recoveries were 99.21 ± 4.22% for ABTS testing and 100.05 ± 3.19% for EV testing.

[0122] See appendix Figure 7 To perform dielectric electrochemical testing of aqueous media deposits (at +0.45V, +0.25V, or -0.22V) and spiked recoveries using the MMO electrode system according to this embodiment, Fe(CN)6 was used. 3- The spiked recoveries of the test sediments were 102.79±6.97%, the spiked recoveries of the DCPIP test sediments were 99.20±9.11%, and the spiked recoveries of the AQDS test sediments were 100.47±5.31%.

[0123] Example 1

[0124] This embodiment provides an EDC for testing aqueous media under +0.7V conditions.

[0125] Three types of sediments with different textures were selected as samples for testing: clay loam, sandy soil, and peat soil.

[0126] First, 40 mL of 0.1 M KCl (containing 0.01 M MOPS, pH = 7.0) was added to the reactor as a background electrolyte solution to remove dissolved oxygen. Approximately 2 mL of the same background electrolyte solution was added to the counter electrode glass tube, ensuring the liquid level in the counter electrode glass tube was level with the liquid level in the working electrode reactor. The reactor was connected to an electrochemical workstation, and a constant voltage of +0.5 V (vs. Ag / AgCl) was set. An it-time curve was tested under the given potential conditions of the workstation, with stirring in the dark during the test. The electrochemical test program was started, and the oxidation current was continuously measured every 5 seconds to obtain a current-time curve. After the current stabilized, 1 mL of ABTS (10 mM concentration) was injected into the reactor through the feed port, subsequently generating an oxidation current response curve. Thereafter, the current gradually decreased over time. When the reaction current returned to the baseline and stabilized, it indicated that the ABTS oxidation in the system had reached equilibrium. When the sample to be tested is added at this point, since the oxidized ABTS can mediate the reduction of the added sample by losing electrons, an oxidation current will continue to be generated. The time-current curve of the sample losing electrons can then be obtained using an electrochemical workstation. The EDC is calculated by determining the peak area of ​​the current response, using the following formula:

[0127]

[0128] Where EDC represents the measured reduction capacity of the sample, in mol e - / g;

[0129] F is the Faraday constant, representing the charge carried by each mole of electrons, and its value is generally considered to be 96485 C / mol;

[0130] t1 is the starting point of the current peak, in seconds;

[0131] t2 is the end point of the current peak, in seconds;

[0132] I(t) is a function of current changing with time, and the unit of current I is A;

[0133] m represents the dry weight of the sample added, in grams.

[0134] Example 2

[0135] This embodiment provides an EDC for testing aqueous media under +0.45V conditions.

[0136] Two types of sediments with different textures, clay loam and peat soil, were selected as the samples to be tested.

[0137] First, the same background electrolyte solution was added to the reactor, and an electrochemical workstation was connected. A constant voltage of +0.25V (vs. Ag / AgCl) was set, and an it-time curve was tested under the given potential conditions of the workstation, with stirring in the dark during the test. The electrochemical test program was started, and the oxidation current was continuously measured every 5 seconds to obtain the current-time curve. After the current reached stability, 1 mL of Fe(CN)6 was added... 3- (Concentration 10 mM) was injected into the reactor through the feed port, subsequently generating a reduction current response curve. Thereafter, the current gradually decreased over time, and when the reaction current returned to the baseline and stabilized, it indicated that the system Fe(CN)6... 3- The reduction reaches equilibrium. At this point, the sample to be tested is added, because the reduced Fe(CN)6... 3- It can mediate the reduction of the added sample by losing electrons, thereby generating an oxidation current. The time-current curve of the sample during electron loss can then be obtained using an electrochemical workstation. EDC is calculated by determining the peak area of ​​the current response, using the same formula as in Example 1.

[0138] Example 3

[0139] This embodiment provides an EDC for testing aqueous media under +0.25V conditions.

[0140] Two types of sediments with different textures, clay loam and peat soil, were selected as the samples to be tested.

[0141] First, the same background electrolyte solution was added to the reactor, and an electrochemical workstation was connected. A constant voltage of +0.05V (vs. Ag / AgCl) was set, and an it-time curve was tested under the given potential conditions of the workstation, with stirring in the dark during the test. The electrochemical test program was started, and the oxidation current was continuously measured every 5 seconds to obtain the current-time curve. After the current stabilized, 1 mL of DCPIP (10 mM concentration) was injected into the reactor through the feed port, subsequently generating a reduction current response curve. Thereafter, the current gradually decreased over time. When the reaction current returned to the baseline and stabilized, it indicated that the DCPIP reduction in the system had reached equilibrium. At this point, the test sample was added. Since the reduced DCPIP (DCPIPH2) can mediate the reduction of the added sample by losing electrons, an oxidation current was generated. The time-current curve of the test sample during electron loss could be obtained using the electrochemical workstation. The EDC was calculated by determining the peak area of ​​the current response, using the same formula as in Example 1.

[0142] Example 4

[0143] This embodiment provides an EDC for testing aqueous media at -0.22V.

[0144] Two types of sediments with different textures, clay loam and peat soil, were selected as the samples to be tested.

[0145] First, the same background electrolyte solution was added to the reactor, and an electrochemical workstation was connected. A constant voltage of -0.42V (vs. Ag / AgCl) was set, and the time-current curve (IT) was tested under the given potential conditions of the workstation, with stirring in the dark during the test. The electrochemical test program was started, and the oxidation current was continuously measured every 5 seconds to obtain the current-time curve. After the current stabilized, 1 mL of AQDS (10 mM concentration) was injected into the reactor through the feed port, subsequently generating a reduction current response curve. Thereafter, the current gradually decreased over time. When the reaction current returned to the baseline and stabilized, it indicated that the AQDS reduction in the system had reached equilibrium. At this point, the sample to be tested was added. Since the reduced AQDS (AH₂QDS) can mediate the reduction of the added sample by gaining electrons, a reduction current will continue to be generated. The time-current curve of the sample gaining electrons can be obtained through the electrochemical workstation. The EAC is calculated by determining the peak area of ​​the current response, using the following formula:

[0146]

[0147] Where EAC is the measured reduction capacity of the sample, in mol e - / g;

[0148] F is the Faraday constant, representing the charge carried by each mole of electrons, and its value is generally considered to be 96485 C / mol;

[0149] t1 is the starting point of the current peak, in seconds;

[0150] t2 is the end point of the current peak, in seconds;

[0151] I(t) is a function of current changing with time, and the unit of current I is A;

[0152] m represents the dry weight of the sample added, in grams.

[0153] Example 5

[0154] This embodiment provides an EDC for testing aqueous media at -0.45V.

[0155] Three types of sediments with different textures were selected as samples for testing: clay loam, sandy soil, and peat soil.

[0156] First, the same background electrolyte solution was added to the reactor, and an electrochemical workstation was connected. A constant voltage of -0.65V (vs. Ag / AgCl) was set, and an it-time curve was tested under the given potential conditions of the workstation, with stirring in the dark during the test. The electrochemical test program was started, and the oxidation current was continuously measured every 5 seconds to obtain the current-time curve. After the current stabilized, 1 mL of EV (10 mM concentration) was injected into the reactor through the feed port, subsequently generating a reduction current response curve. Thereafter, the current gradually decreased with time, and when the reaction current returned to the baseline and stabilized, it indicated that the EV reduction in the system had reached equilibrium. At this point, the sample to be tested was added. Since the reduced EV can mediate the electron gain reduction of the added sample, a reduction current will continue to be generated. The time-current curve of the sample gaining electrons can be obtained through the electrochemical workstation. The EAC is calculated by determining the peak area of ​​the current response, using the same formula as in Example 4.

[0157] Each sample in Examples 1-5 was tested three times, and the test results are shown in Table 4.

[0158] Table 4. Results of redox capacity testing for Examples 1-5.

[0159]

[0160] Precision % test method: The results of N repeated tests deviate from each other. The sample is tested 3 times at a certain appropriate concentration. Precision = STDEV / AVE × 100%. The glassy carbon electrode is limited by the amount of sample added. For sediment samples, only the MMO electrode system is used to test the precision.

[0161] Sample accuracy test method: Test the sample at a suitable concentration 3 times, accuracy = STDEV.

[0162] References:

[0163] 1Lau MP,Sander M,Gelbrecht J,et al.Solid phases as importantelectronacceptors in freshwater organic sediments[J].Biogeochemistry,2015,123(1-2):49-61.

[0164] 2 Aeschbacher M, Sander M, Schwarzenbach RP. Novel Electrochemical Approach to Assess the Redox Properties of Humic Substances[J]. Environmental Science & Technology, 2010, 44(1): 87 - 93.

[0165] 3 Gorski CA, Sander M, Aeschbacher M, et al. Assessing the redox properties of iron-bearing clay minerals using homogeneous electrocatalysis[J].

[0166] Applied Geochemistry, 2011, 26: S191 - S193.

[0167] 4 Sander M, Hofstetter TB, Gorski CA. Electrochemical Analyses of Redox-Active Iron Minerals: A Review of Nonmediated and Mediated Approaches[J]. Environmental Science & Technology, 2015, 49(10): 5862 - 5878.

[0168] 5 Walpen N, Schroth MH, Sander M. Quantification of Phenolic Antioxidant Moieties in Dissolved Organic Matter by Flow-Injection Analysis with Electrochemical Detection[J]. Environmental Science & Technology, 2016,

[0169] 50(12): 6423 - 6432.

[0170] For the parts not covered above, the prior art shall apply.

[0171] Although specific embodiments of the present invention have been described in detail by way of examples, those skilled in the art should understand that the above examples are for illustrative purposes only and are not intended to limit the scope of the invention. Those skilled in the art can make various modifications or additions to the described specific embodiments or use similar methods to replace them, without departing from the direction of the invention or exceeding the scope defined by the appended claims. Those skilled in the art should understand that any modifications, equivalent substitutions, improvements, etc., made to the above embodiments based on the technical essence of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for testing the redox capacity of heterogeneous aqueous media under different potential conditions, characterized in that, A three-electrode system is employed, with the working electrode being a ruthenium-iridium-titanium MMO electrode with a high-purity titanium substrate and a ruthenium-iridium-titanium coating; the reference electrode being Ag / AgCl; and the counter electrode being a platinum wire separated from the working electrode chamber by a glass frit core. The method includes the following steps: S1, Preparation of dielectric electrochemical test solution S11, Preparation of standard solution of mediator Weigh out the mediator standard, prepare it with water and remove dissolved oxygen to obtain the mediator standard solution; the mediator includes ABTS and Fe(CN)6. 3- At least one of DCPIP, AQDS and N,N'-diethyl-4,4'-bipyridinium (EV); S12, Preparation of background electrolyte KCl and MOPS were weighed separately, and water was used to prepare, pH was adjusted and dissolved oxygen was removed to obtain an electrolyte containing KCl and MOPS. S2, Balanced Three-Electrode System According to the test requirements, different mediators are selected, and background electrolyte and mediators are added to the system. A specific constant voltage is applied using an electrochemical workstation to activate free radicals from the ground state of the mediators. When the current signal becomes stable and constant, it indicates that the activation process of the mediators has reached equilibrium. S3, Sample Measurement After the system in step S2 is in equilibrium, the sediment sample to be tested is added. The active mediator causes the sediment in the aqueous medium to be tested to be oxidized or reduced, generating an oxidation / reduction current. The time-current curve of the sample gaining and losing electrons is then tested using an electrochemical workstation. The current response of the sample under test is obtained from the time-current curve. The amount of electron transfer in the sample can be obtained by integrating the peak area. The calculation formula is shown in Equation 1. Formula 1, in, and The reducing or oxidizing capacity of the sample is expressed in mol e. - / g; is the Faraday constant, representing the charge carried by each mole of electrons, with a value of 96485 C / mol; This is the starting point of the current peak, measured in seconds (s). This is the end point of the current peak, measured in seconds (s). I is a function of current as a function of time, and the unit of current I is A; The dry weight of the sample added is in grams.

2. The method as described in claim 1, characterized in that, The concentration of the mediator standard solution is 2~10 mM, and the background electrolyte contains 50~100 mM KCl and 5~20 mM MOPS, with pH=7.0±0.

1.

3. The method as described in claim 1, characterized in that, The volume-to-mass ratio of the background electrolyte to the sample to be tested is 40 mL: (0.5~2) g.

4. The method as described in claim 1, characterized in that, In step S3, a constant voltage range of -0.45V to +0.7V is applied to the electrochemical workstation.

5. The method as described in claim 4, characterized in that, During the redox capacity determination process, the amount of mediator added is determined based on the redox capacity of the sediment sample, and the number of moles of mediator added is 2 to 4 times the amount of electrons donated by the sample.

6. The method as described in claim 5, characterized in that, When the mediator is ABTS, the electrochemical workstation applies a constant voltage of +0.7 V.

7. The method as described in claim 5, characterized in that, When the mediating substance is Fe(CN)6 3- The electrochemical workstation applies a constant voltage of +0.45 V.

8. The method as described in claim 5, characterized in that, When the mediator is DCPIP, the electrochemical workstation applies a constant voltage of +0.25 V.

9. The method as described in claim 5, characterized in that, When the mediator is AQDS, the electrochemical workstation applies a constant voltage of -0.22 V.

10. The method as described in claim 5, characterized in that, When the mediator is EV, the electrochemical workstation applies a constant voltage of -0.45 V.

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