Electrochemical determination method for redox activity of actual soil sample

Abstract

The invention discloses an electrochemical determination method for the redox activity of an actual soil sample. According to the method, the oxidation-reduction capacity of an actual soil sample is measured mainly through specific electrochemical equipment according to a timing-amperometry. The specific technical principle is as follows: in the electrochemical determination process, firstly, a specific mediating substance is selected to promote electron transfer between a substance with redox activity in a sample and a working electrode, and then the electron transfer quantity between the soil sample and the working electrode is determined under different voltages; the determination of the redox activity of the soil sample is realized. The method is simple to operate, not only can quickly and accurately evaluate the redox capacity of the actual soil sample on the basis of not destroying the soil sample, but also has important guiding significance for a natural attenuation monitoring technology of an actual polluted site.

Description

Technical Field

[0001] The invention discloses an electrochemical determination method for the redox activity of an actual soil sample, relates to the field of physical and chemical property analysis of soil samples, and specifically relates to a quantitative test method for the redox capacity of an actual soil sample. Background Art

[0002] Soil is a complex system composed of minerals, organic matter, microorganisms, air, water and other substances, and the redox substances in it give the soil redox activity. Redox activity drives the element cycle in the soil, enabling the soil to connect the atmosphere, biosphere and lithosphere, and at the same time affects the morphology, migration and utilization of redox-sensitive pollutants in the soil system. In particular, soil samples located in underground aquifers provide "self-purification ability" for the aquifers, that is, the natural repair ability of the aquifers. Studies have shown that the redox activity of soil samples in aquifers is significantly correlated with the abiotic decay rate of organic pollution in groundwater. Therefore, characterizing the redox capacity of soil samples can not only deeply understand the physical and chemical properties of the soil, but also help to evaluate and predict the abiotic natural decay capacity of organic pollution driven by soil redox activity. On this basis, the natural decay potential of organic pollutants in groundwater can be evaluated, providing theoretical guidance and technical methods for the risk control of groundwater organic pollution.

[0003] Important chemical parameters that indicate the redox activity of a substance include: redox potential (Eh), electron donating capacity (EDC), and electron accepting capacity (EAC). Electrochemical testing methods are widely used to test the redox capacity of samples. Usually, the electrochemical method uses a platinum electrode as the indicator electrode and an Ag / AgCl or saturated calomel electrode as the reference electrode. The electrochemical method is inserted into the system to be tested to form a primary cell for measurement, which can directly measure the kinetic and thermodynamic parameters of the sample to be tested. Electrochemical methods are divided into potentiometric and amperometric methods. The potentiometric method can characterize the redox potential (Eh) of the sample to be tested. The redox potential is a comprehensive indicator of soil environmental conditions, which characterizes the relative degree of soil oxidation and reduction. In the amperometric method, by applying a constant potential to the working electrode and measuring the response of the current over time, the EDC and EAC of the sample to be tested can be obtained. According to the magnitude of the applied potential, the amperometric method can be divided into electrochemical oxidation method and electrochemical reduction method. Through these two methods, we can measure the reduction capacity and oxidation capacity of the sample to be tested respectively.

[0004] The "electrochemical determination method of the redox activity of actual soil samples" involved in the present invention, in order to solve such problems, replaces the working electrode with a glassy carbon crucible electrode on the basis of ordinary electrochemical equipment, greatly increasing the contact area between the working electrode and the sample to be measured. At the same time, a soluble electron transfer mediator is added on the basis of the electrochemical determination method to accelerate the transfer of electrons between the redox pair to be measured and the working electrode, so as to quickly reach equilibrium. Therefore, the present invention takes the current method as the basis, selects a suitable mediator, prepares the soil sample into a suspension, and screens the appropriate suspension concentration, optimizes parameters such as voltage and current measurement time, and realizes the determination of the redox activity of the actual soil sample. The entire operation process is simple, the equipment cost is low, and the measurement result accuracy is high. Summary of the invention

[0005] The present invention provides a medium current method capable of measuring the redox capacity of soil samples. The soil sample is configured into a suspension of appropriate concentration, and through the electrochemical determination method of the three-electrode system, a suitable mediating substance is added and the determination parameters are optimized to promote the electron transfer rate between the sample to be tested and the working electrode, obtain the number of electron transfers between the substance to be tested and the working electrode at a certain voltage, and realize rapid and accurate testing of the redox activity of the soil sample.

[0006] The actual soil sample redox capacity determination method provided by the present invention is as follows:

[0007] (1) A 0.05-0.2 mol / L potassium chloride solution background solution is prepared, to which 0.01-0.05 mol / L 3-(N-morpholine)propanesulfonic acid is added as a buffer, and the pH is adjusted to 7.0-7.5 with potassium hydroxide to obtain a background solution.

[0008] (2) After soil samples are collected and transported, they are vacuum sealed and stored at low temperature. When used, they are transferred to an anaerobic incubator. In the anaerobic glove box, 0.5-2 g of soil sample is dispensed into a 20 mL small brown bottle, and 5-15 mL of potassium chloride background solution is added to prepare a soil suspension.

[0009] (3) Using CHI660E electrochemical workstation as control unit, a 9 mL glassy carbon crucible electrode was selected as the working electrode, a Φ4 mm*45 mm Ag / AgCl electrode was selected as the reference electrode, and a 0.5*150 mm spring platinum wire electrode was selected as the counter electrode. The three electrodes formed a three-electrode electrochemical system. The platinum wire electrode was separated from the other two electrodes by a salt bridge, and the chronoamperometry was used to determine the redox capacity of the soil samples.

[0010] (4) 2,2′-hydrazine-bis(3-ethylbenzothiazoline-6-sulfonic acid) diamine salt was selected as the mediating substance in the electrochemical oxidation current test, and the soil sample mediated electrochemical oxidation current (I ox ) test. Add 4-8 mL of background solution to the working electrode of the three-electrode electrochemical system, insert the reference electrode and the counter electrode 1-2 cm below the liquid surface, and ensure that the reference electrode does not contact the working electrode and the counter electrode. Then apply a voltage of +0.3-0.5 V, and measure the current response at an interval of 1 s. Record the current change until the absolute value of the current is less than 2.0×10 -6 A~4.0×10 -6 A. After the current reaches equilibrium, add 50-150 μL of 10 mmol / L 2,2′-hydrazine-bis(3-ethylbenzothiazoline-6-sulfonic acid) diamine salt. After the current reaches stable equilibrium again, add 250-1000 μL of soil sample suspension and record the change in current.

[0011] (5) Ethyl viologen dibromide was selected as the mediating substance in the electrochemical reduction current test, and the soil sample mediated electrochemical reduction current (I red ) test. Add 4-8 mL of background solution to the working electrode of the three-electrode electrochemical system, insert the reference electrode and the counter electrode 1-2 cm below the liquid surface, and ensure that the reference electrode does not contact the working electrode and the counter electrode. Then apply a voltage of -0.5-0.7 V, and measure the current response at an interval of 1 s. Record the current change until the absolute value of the current is less than 2.0×10 -6 A~4.0×10 -6 A. After the current is balanced, add 50-150 μL of 10 mmol / L ethyl viologen dibromide stock solution. After the current is stabilized and balanced again, add 250-1000 μL of soil sample suspension and record the change in current.

[0012] (6) The current change curve over time from the time when the oxidation current or reduction current changes when the soil sample is added to the time when the current returns to the initial value is taken as the oxidation or reduction current curve, and the integral baseline is the initial value of the current. The obtained oxidation or reduction current curve is imported into the origin software for integral analysis. The oxidation current (I ox ) or reduction current (I red ) integral values, and the EDC and EAC of the soil samples were calculated using formulas (1) and (2), respectively.

[0013]

[0014] Where: F is Faraday constant (96485C / mol), I oxis the current (A) during electrochemical oxidation, I red is the current (A) during electrochemical reduction, m A is the mass of sample added to the system (g), t0, t end They are the start time and end time (s) respectively.

[0015] (7) Each sample was tested 2 to 4 times in parallel. The electrodes were cleaned and background solution was added before each measurement.

[0016] The present invention also provides verification of the feasibility and stability of the method for determining soil samples by mediated electrochemical oxidation and reduction method, the method is as follows:

[0017] (1) The CHI660E electrochemical workstation was selected as the control unit, a 9 mL glassy carbon crucible electrode was selected as the working electrode, a Φ4 mm*45 mm Ag / AgCl electrode was selected as the reference electrode, and a 0.5*150 mm spring platinum wire electrode was selected as the counter electrode. The three electrodes formed a three-electrode system. The platinum wire electrode was separated from the two outer electrodes by a salt bridge, and the chronoamperometry was used to verify the feasibility and stability of the method.

[0018] (2) Magnetite was selected as the standard sample for the determination of sample reducing capacity (EDC) by mediated electrochemical oxidation method, and 2,2′-hydrazine-bis(3-ethylbenzothiazoline-6-sulfonic acid) diamine salt was selected as the mediating substance in the mediated electrochemical oxidation current test. The mediated electrochemical oxidation current (I ox ) test. Specifically, it includes: adding 4-8 mL of background solution to the working electrode of the three-electrode electrochemical system, inserting the reference electrode and the counter electrode 1-2 cm below the liquid surface, and ensuring that the reference electrode does not contact the working electrode and the counter electrode. Then apply a voltage of +0.3-0.5 V, the current response measurement interval is 1 s, and the current change is recorded until the absolute value of the current is less than 2.0×10 -6 A~4.0×10 -6 A. After waiting for the current to be balanced, add 50-150 μL of 10 mmol / L 2,2′-hydrazine-bis(3-ethylbenzothiazoline-6-sulfonic acid) diamine salt. After the current is stabilized and balanced again, add different volumes (20-100 μL) of 5-10 g / L magnetite suspension. After the determination is completed, integrate the results and establish a linear relationship between the calculated EDC and the mass of magnetite added.

[0019] (3) Ferrihydrite was selected as the standard sample for the electrochemical reduction method to determine the sample oxidation capacity (EAC), and ethyl viologen dibromide was selected as the mediating substance in the electrochemical reduction current test. The electrochemical reduction current (I red) test. Specifically, it includes: adding 4-8 mL of background solution to the working electrode of the three-electrode electrochemical system, inserting the reference electrode and the counter electrode 1-2 cm below the liquid surface, and ensuring that the reference electrode does not contact the working electrode and the counter electrode. Then apply a voltage of -0.5-0.7 V, the current response measurement interval is 1 s, and the current change is recorded until the absolute value of the current is less than 2.0×10 -6 A~4.0×10 -6 A. After waiting for the current to be balanced, add 50-150 μL of 10 mmol / L ethyl viologen dibromide. After the current is stabilized and balanced again, add different volumes (20-100 μL) of 0.2-0.8 g / L ferrihydrite suspension. After the measurement, integrate the results and establish a linear relationship between the calculated EAC and the mass of ferrihydrite added.

[0020] The present invention has the following advantages:

[0021] 1. This method changes the working electrode on the basis of the traditional electrochemical system and adds a mediating substance to promote the electron transfer between the actual soil sample and the working electrode, avoid the phenomenon of no electron transfer between the soil sample and the working electrode, and can significantly shorten the electrochemical measurement time.

[0022] 2. The sample preservation and measurement environment of this method can greatly reduce the damage of the environment to the redox activity of soil samples, so the measurement accuracy is higher.

[0023] 3. This method can characterize the reduction activity and oxidation activity of soil samples separately, so the measurement results are more intuitive. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a conceptual diagram of the dielectric current method equipment and principle.

[0025] Figure 2 This is a graph showing the reduction capacity measurement results of actual soil samples provided in Example 1 of the present invention.

[0026] Figure 3 This is a graph showing the results of measuring the oxidation capacity of actual soil samples provided in Example 2 of the present invention.

[0027] Figure 4 This is a diagram showing the feasibility and stability test results of the mediated electrochemical oxidation method provided in Example 3 of the present invention.

[0028] Figure 5 This is a diagram showing the feasibility and stability test results of the mediated electrochemical reduction method provided in Example 4 of the present invention. DETAILED DESCRIPTION

[0029] Implementation case 1: Rapid determination of the reduction capacity of actual soil samples

[0030] (1) preparing a background solution of 0.1 mol / L potassium chloride solution, adding 0.01 mol / L 3-(N-morpholine)propanesulfonic acid as a buffer, and adjusting the pH to 7.3 with potassium hydroxide;

[0031] (2) Weigh 54.868 mg of 2,2′-hydrazine-bis(3-ethylbenzothiazoline-6-sulfonic acid) diamine salt and dissolve it in 10 mL of background solution to prepare a mediator stock solution with a concentration of 10 mmol / L;

[0032] (3) After collection, the samples were kept under vacuum and low temperature during storage and transportation. Weigh 0.1 g of soil sample and dissolve it in 10 mL of background solution to prepare a soil sample suspension to be tested;

[0033] (4) The experiment was conducted using an electrochemical workstation CHI660E as the control unit. A 9 mL glassy carbon crucible electrode was selected as the working electrode, a Φ4 mm*45 mm Ag / AgCl electrode was used as the reference electrode, and a 0.5*150 mm spring platinum wire electrode was used as the counter electrode. The three electrodes formed a three-electrode system. The platinum wire electrode was separated from the two outer electrodes by a salt bridge, and the chronoamperometry method was used for the measurement.

[0034] (5) Add 6 ml of background solution to the working electrode, apply a voltage of +0.41 V, and wait until the absolute value of the current is less than 3.0 × 10 -6 A and after equilibrium, add 100 μL-10mmol / L 2,2′-hydrazine-bis(3-ethylbenzothiazoline-6-sulfonic acid) diamine salt stock solution, and wait until the absolute value of the current is less than 3.0×10 -6 After A and equilibrium, add 500 μL of the soil sample suspension to be tested, and wait for the measured current to be close to the current value before adding the suspension to complete the measurement, and obtain the mediated electrochemical oxidation current I of the soil sample ox Each sample was measured three times in parallel, and the electrode was cleaned and background solution was added again after each measurement.

[0035] (6) After the measurement, the curve obtained by the mediated electrochemical oxidation method was integrated using the origin software, and the EDC of the soil sample was calculated using formula (1). The results are shown in the attached Figure 2 .

[0036]

[0037] Where: F is Faraday constant (96485C / mol), I ox is the current (A) during electrochemical oxidation, m A is the mass of sample added to the system (g), t0, t end They are the start time and end time (s) respectively.

[0038] Implementation Case 2: Rapid Determination of Oxidation Capacity of Actual Soil Samples

[0039] The steps are the same as those in implementation case 1, except that:

[0040] Step (2) is to weigh 37.411 mg of ethyl viologen dibromide and dissolve it in 10 mL of background solution to prepare a mediator stock solution with a concentration of 10 mmol / L.

[0041] Step (5) The applied voltage is -0.61 V, and the mediating substance added is ethyl viologen dibromide, and the mediated electrochemical reduction current I of the soil sample is obtained. red .

[0042] In step (6), the EAC of the soil sample is calculated according to formula (2). The results are shown in the attached Figure 3 .

[0043]

[0044] Where: F is Faraday constant (96485C / mol), I red is the current (A) during electrochemical reduction, m A is the mass of sample added to the system (g), t0, t end They are the start time and end time (s) respectively.

[0045] Implementation case 3: Establishment of feasibility and stability verification method of mediated electrochemical oxidation method

[0046] The steps are the same as those in implementation case 1, except that:

[0047] In step (3), 0.07 g of magnetite is weighed and dissolved in 10 mL of background solution to prepare a magnetite suspension to be tested;

[0048] In step (5), the soil sample suspension was replaced with a magnetite suspension, and 20, 40, 60, 80, and 100 μL of the magnetite suspension were added in sequence to measure the electrochemical oxidation current I of the magnetite suspensions of different concentrations. ox ;

[0049] After the determination in step (6), the results were analyzed using origin software to obtain the EDC of magnetite suspensions of different concentrations, and a linear relationship between EDC and the mass of added magnetite was established. The results are shown in the attached Figure 4 .

[0050] Implementation Case 4: Establishment of feasibility and stability verification method of mediated electrochemical reduction

[0051] The operation steps are the same as those in implementation case 1, except that:

[0052] Step (2) is to weigh 37.411 mg of ethyl viologen dibromide and dissolve it in 10 mL of background solution to prepare a mediator stock solution with a concentration of 10 mmol / L.

[0053] Step (3) is to weigh 0.0056 g of ferrihydrite and dissolve it in 10 mL of background liquid to prepare a ferrihydrite suspension to be tested.

[0054] In step (5), the applied voltage is -0.61 V, the mediating substance added is ethyl viologen dibromide, the soil sample suspension is replaced with a ferrihydrite suspension, 20, 40, 60, 80, and 100 μL of the ferrihydrite suspension are added in sequence, and the mediating electrochemical reduction current I of the ferrihydrite suspensions of different concentrations is measured. red ;

[0055] In step (6), the EAC of ferrihydrite suspensions of different concentrations was obtained according to formula (2), and a linear relationship between EAC and the mass of ferrihydrite added was established. The results are shown in the attached Figure 5 .

[0056] Although the present invention has been described in detail above by general description and specific embodiments, it is obvious to those skilled in the art that some modifications or improvements can be made to the present invention. Therefore, these modifications or improvements made without departing from the spirit of the present invention all belong to the scope of protection claimed by the present invention.

Claims

1. An electrochemical determination method for the redox activity of actual soil samples, characterized in that: The following steps are involved: (1) preparing a 0.05-0.2 mol / L potassium chloride solution background solution, adding 0.01-0.05 mol / L 3-(N-morpholine)propanesulfonic acid as a buffer, and adjusting the pH to 7.0-7.5 with potassium hydroxide to obtain a background solution; (2) After soil samples were collected and transported, they were stored in a vacuum seal at low temperature. When used, they were transferred to an anaerobic incubator. In the anaerobic glove box, 0.5 to 2 g of soil samples were dispensed into 20 mL small brown bottles, and 5 to 15 mL of potassium chloride background solution was added to prepare a soil suspension. (3) According to the method for preparing the suspension of the soil sample to be tested described in step (2), a CHI660E electrochemical workstation is used as the control unit, a glassy carbon crucible electrode with a volume of 9 mL is selected as the working electrode, a Φ4mm*45mm Ag / AgCl electrode is selected as the reference electrode, and a 0.5*150mm spring platinum wire electrode is selected as the counter electrode. The three electrodes form a three-electrode system, and a salt bridge is used to separate the platinum wire electrode from the other two electrodes. The chronoamperometry is used to determine the redox capacity of the soil sample.

2. According to the electrochemical determination method of the redox activity of an actual soil sample as described in claim 1, a mediating substance needs to be added before the redox capacity of the soil sample is determined, so as to promote the electron transfer process between the sample to be tested and the working electrode.

3. According to the role of the mediating substance in the chronoamperometry of claim 2, 2'-hydrazine-bis(3-ethylbenzothiazoline-6-sulfonic acid) diamine salt is selected as the mediating substance in the electrochemical oxidation current test, characterized in that: The following steps are involved: (1) Add 4-8 mL of background solution to the working electrode, insert the reference electrode and the counter electrode 1-2 cm below the liquid surface, and ensure that the reference electrode does not contact the working electrode and the counter electrode, to build a three-electrode electrochemical test system; (2) dissolving 2,2′-hydrazine-bis(3-ethylbenzothiazoline-6-sulfonic acid) diamine salt in the background solution to prepare a stock solution with a concentration of 5 to 15 mmol / L; (3) The applied voltage is +0.3~0.5V, the current response measurement interval is 1s, and the current sensitivity must be greater than the actual current response (10 -3 ~10 ～5 A), record the current change until the absolute value of the current is less than 2.0×10 -6 A~4.0×10 -6 A and reach balance; (4) Add 50-150 μL of 10 mmol / L 2,2′-hydrazine-bis(3-ethylbenzothiazoline-6-sulfonic acid) diamine salt stock solution and record the current change until the absolute value of the current is less than 2.0×10 -6 A~4.0×10 -6 A and reach equilibrium.

4. The test method of mediated electrochemical oxidation current according to claim 3, wherein the reducing capacity (EDC) of an actual soil sample is determined by mediated electrochemical oxidation, characterized in that: The following steps are involved: (1) On the basis of the mediated electrochemical oxidation current test described in claim 3, 250 to 1000 μL of soil sample suspension is added, the change of current is recorded, and the measured current returns to the current value before the suspension is added, and the soil sample oxidation current test is completed; (2) using the mediated electrochemical oxidation method for the reduction capacity of the soil sample described in step (1), taking the current change curve over time from the time when the sample current changes to the time when the current returns to the initial value as the oxidation current curve, and taking the integral baseline as the initial current value, the obtained oxidation current curve is imported into the origin software for integral analysis; (3) The oxidation current (I ox ) integral value, and use formula (1) to calculate EDC, Where: F is Faraday constant (96485C / mol), I ox is the current (A) during electrochemical oxidation, m A is the mass of sample added to the system (g), t0, t end are the start time and end time (s) respectively; (4) Each soil sample was tested 2 to 4 times in parallel. The electrodes were cleaned and background solution was added before each measurement.

5. According to the principle of selecting the mediating substance in the chronoamperometry of claim 2, ethyl viologen dibromide is selected as the mediating substance in the electrochemical reduction current test, and the test of the electrochemical reduction current is carried out, characterized in that: The following steps are involved: (1) Add 4-8 mL of background solution to the working electrode, insert the reference electrode and the counter electrode 1-2 cm below the liquid surface, and ensure that the reference electrode does not contact the working electrode and the counter electrode, to build a three-electrode electrochemical test system; (2) dissolving ethyl viologen dibromide in a background solution to prepare a stock solution with a concentration of 5 to 15 mmol / L; (3) The applied voltage is -0.5 to 0.7 V, the current response measurement interval is 1 s, and the current sensitivity must be greater than the actual current response (10 -3 ~10 ～5 A), record the current change until the absolute value of the current is less than 2.0×10 -6 A~4.0×10 -6 A and reach balance; (4) Add 50-150 μL of 10 mmol / L ethyl viologen dibromide stock solution and record the current change until the absolute value of the current is less than 2.0 × 10 -6 A~4.0×10 -6 A and reach equilibrium.

6. The method for testing the mediated electrochemical reduction current according to claim 5, wherein the oxidizing capacity (EAC) of an actual soil sample is determined by the mediated electrochemical reduction method, characterized in that: The following steps are involved: (1) On the basis of the mediated electrochemical reduction current test described in claim 5, 250 to 1000 μL of soil sample suspension is added, the change of current is recorded, and the reduction current test is completed after the measured current returns to the current value before the suspension is added; (2) according to the mediated electrochemical reduction method of the soil sample oxidation capacity described in step (1), the current change curve over time from the time when the sample current changes to the time when the current returns to the initial value is used as the reduction current curve, the integral baseline is the initial current value, and the obtained reduction current curve is imported into the origin software for integral analysis; (3) The oxidation current (I red ) integral value, and the oxidation capacity of the soil sample to be tested, i.e., EAC, is calculated by formula (2). Where: F is Faraday constant (96485C / mol), I red is the current (A) during electrochemical reduction, m A is the mass of sample added to the system (g), t0, t end are the start time and end time (s) respectively; (4) Each sample was tested 2 to 4 times in parallel. The electrodes were cleaned and background solution was added before each measurement.

Images