Method for predicting hexavalent chromium reduction capacity of soil

Abstract

The invention relates to a method for predicting the reduction capacity of soil to hexavalent chromium, which comprises the following steps of: uniformly mixing a background electrolyte solution and ABTS (2, 2, 6-trimethyl-1, 3-pentanedione) to obtain a solution A; performing electrochemical oxidation on the solution A until a current signal is stable and unchanged to obtain a solution B; adding a to-be-tested soil sample into the solution B, and electrifying for testing to obtain a time-current curve; carrying out integral calculation on the peak area of the time-current curve to obtain the electron donating capacity EDC of the soil sample; and calculating the reduction capacity of the soil to the hexavalent chromium according to the electron donating capacity EDC. The method disclosed by the invention is simple and rapid, can realize rapid prediction of the hexavalent chromium reduction capacity of the soil, and is high in result accuracy.

Description

Technical Field

[0001] The invention relates to a method for predicting the reduction capacity of soil to hexavalent chromium, and belongs to the field of soil treatment. Background Art

[0002] Hexavalent chromium (Cr(VI)) is a highly toxic heavy metal ion that poses serious hazards to humans and the environment. In recent years, affected by global climate change and intense human activities, underground environmental pollution has become a major challenge facing global sustainable development. In contrast, trivalent chromium (Cr(III)) is less toxic and has a lower solubility, so reducing Cr(VI) to Cr(III) is a common method for remediating chromium pollution in soil. Soil is rich in reducing components such as divalent iron and organic matter. These active components can reduce Cr(VI), thereby intercepting and reducing the concentration of Cr(VI) in the soil, which is the soil self-purification process. Accurately determining the reduction capacity of soil to remove Cr(VI) by itself is of great significance for understanding the natural attenuation law of Cr(VI) and formulating remediation strategies (such as clarifying the type and dosage of remediation agents).

[0003] At present, there have been many studies on the reduction of Cr(VI) by ferrous iron and organic matter in soil. The main understanding is that ferrous iron and Cr(VI) are carried out in a stoichiometric ratio of 3:1. However, the effects of different forms of ferrous iron on the reduction of Cr(VI) vary significantly, and the reaction is difficult to be complete. For example, only a small amount of ferrous iron in pyrite can effectively reduce Cr(VI). Therefore, it is impossible to quantitatively evaluate the reduction capacity of Cr(VI) based on the ferrous iron content in soil. In addition, the composition of organic matter is complex, and the content of reducing components in organic matter of different types and sources varies significantly. Therefore, it is impossible to evaluate the reduction capacity of organic matter in soil for Cr(VI) based on the content of organic matter. In addition, in addition to ferrous iron and organic matter, there may be other reducing components in actual soil that can reduce and remove Cr(VI). Therefore, it is urgent to establish a quantitative evaluation method for the reduction capacity of soil for Cr(VI). Summary of the invention

[0004] In view of the deficiencies of the prior art, the present invention provides a method for predicting the reduction capacity of soil for hexavalent chromium.

[0005] In order to solve the above technical problems, the technical solution of the present invention is as follows:

[0006] A method for predicting the reduction capacity of soil for hexavalent chromium comprises the following steps:

[0007] S1, mixing the background electrolyte solution and 2,2-azino-bis(3-ethyl-benzothiazoline-6-sulfonic acid) (ABTS) to obtain solution A;

[0008] S2. Electrochemically oxidize solution A until the current signal is stable (indicating that the ABTS oxidation process has reached equilibrium), and obtain solution B (oxidation state);

[0009] S3, adding the soil sample to be tested into the solution B, conducting a power-on test, and obtaining a time-current curve; and then integrating and calculating the peak area of ​​the time-current curve to obtain the electron donating capacity (EDC) of the soil sample;

[0010] in, The unit can be selected as mol e - / g; F is the Faraday constant, which represents the charge carried by each mole of electrons, and its value can usually be 96485C / mol; t1 is the starting point of the current peak, and the unit can be selected as s; t2 is the end point of the current peak, and the unit can be selected as s; I(t) is the current value at time t, and the unit can be selected as A; m is the dry weight of the added soil sample, and the unit can be selected as g;

[0011] S4. Calculate the reduction capacity of the soil for hexavalent chromium based on the electron donating capacity EDC.

[0012] Furthermore, in S1, in the background electrolyte solution, the concentration of the inorganic salt is 0.08-0.12 M, and the concentration of MOPS is 0.008-0.012 M, wherein the inorganic salt includes one or more of KCl and NaCl.

[0013] Furthermore, in S1, the pH value of the background electrolyte solution is 7.0.

[0014] Furthermore, in S1, the background electrolyte solution and the ABTS solution are evenly mixed in a volume ratio of 35-45:1; wherein the concentration of ABTS in the ABTS solution is 8-12 mM.

[0015] Furthermore, in S2, solution A is electrochemically oxidized using chronoamperometry.

[0016] Furthermore, when conducting electrochemical oxidation and power-on tests, a glassy carbon electrode was used as a working electrode, a platinum wire was used as an auxiliary electrode, and Ag / AgCl was used as a reference electrode. The working electrode, the auxiliary electrode, and the reference electrode were electrically connected to the electrochemical workstation, respectively, to form a three-electrode system.

[0017] Furthermore, when conducting electrochemical oxidation and power-on tests, the voltage was set to +0.4-+0.6V.

[0018] Furthermore, electrochemical oxidation and current-on tests were performed under anaerobic conditions.

[0019] Furthermore, in S1, electrochemical oxidation and power-on tests were performed under a nitrogen atmosphere.

[0020] Optionally, in S3, the amount of the soil sample to be tested added is 0.1-1.5 g, preferably 0.2-1 g.

[0021] The method of the present invention overcomes the shortcomings of the existing analytical methods for evaluating the reduction capacity of soil to remove Cr(VI), and provides an analytical method with a simple method and high accuracy. The redox potential of Cr(VI) under neutral conditions is about 0.4V. The present invention can effectively measure the redox capacity of soil at different potentials by using a mediated electrochemical method. Therefore, the electron donating capacity measured by the mediated electrochemical method can be used to evaluate the reduction capacity of soil for Cr(VI).

[0022] The method of the invention is simple and quick, can realize the rapid prediction of the hexavalent chromium reduction capacity of the soil, and the result has high accuracy.

[0023] The method for predicting the reduction capacity of soil for hexavalent chromium of the present invention requires low instrument cost, short analysis time, high detection efficiency, good repeatability and convenience, and low use cost; and the method has high precision and accuracy. The present invention achieves low cost, high efficiency, convenience, and evaluation of the reduction capacity of soil for Cr(VI), and has good application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a curve showing the change of dissolved Cr(VI) over time during the reaction of the soil sample with low concentration Cr(VI) in the present invention.

[0025] Figure 2 This is a curve showing the change of EDC of a soil sample over time during the reaction of the soil sample with low concentration Cr(VI) in the present invention.

[0026] Figure 3 It is a linear relationship fitting curve between the change in dissolved Cr(VI) and the change in EDC of the soil sample during the reaction of the soil sample with low concentration Cr(VI) in the present invention.

[0027] Figure 4 This is a curve showing the change of dissolved Cr(VI) over time during the reaction of the soil sample with high concentration Cr(VI) in the present invention.

[0028] Figure 5 This is a curve showing the change of EDC of a soil sample over time during the reaction of the soil sample with high concentration of Cr(VI) in the present invention.

[0029] Figure 6It is a linear relationship fitting curve between the change in dissolved Cr(VI) and the change in EDC of the soil sample during the reaction of the soil sample with high concentration Cr(VI) in the present invention.

[0030] Figure 7 This is a curve showing the change of dissolved Cr(VI) over time during the reaction of the fully oxidized soil sample with high concentration Cr(VI) in the present invention.

[0031] Figure 8 It is a linear relationship fitting curve between the change in dissolved Cr(VI)-Cr(VI) adsorption amount and the change in EDC of the soil sample during the reaction between the soil sample and high concentration Cr(VI) in the present invention.

[0032] Fig. 9 This is the it curve of EDC of 1g soil sample tested by mediated electrochemical method in the present invention. DETAILED DESCRIPTION

[0033] The present invention will be described in detail below in conjunction with the embodiments. It should be noted that the embodiments and features in the embodiments of the present invention can be combined with each other without conflict.

[0034] Example 1

[0035] This example is about the establishment of an analytical method for Cr(VI).

[0036] Unless otherwise specified, only reagents confirmed to be analytically pure and distilled or deionized water or water of equivalent purity were used in the analysis, and all analyses and sample treatments were carried out in air at 25±2°C. Weigh 147 mg of potassium dichromate (K2Cr2O7) and dissolve it in 100 mL of ultrapure water to obtain a Cr(VI) standard solution with a Cr(VI) concentration of 1 mM. Take 0 μL, 20 μL, 40 μL, 60 μL, 80 μL and 100 μL of the Cr (VI) standard solution in 6 5 mL centrifuge tubes, add 0.25 mL 1 + 1 sulfuric acid solution (98 wt% sulfuric acid is slowly added to the same volume of water and mixed), 0.25 mL 1 + 1 phosphoric acid solution (pure phosphoric acid and water are mixed in equal volumes) and 1 mL DPC color developer (0.2 g of diphenylcarbohydrazide is weighed and dissolved in 50 ml of acetonitrile and then ultrapure water is added to 100 mL to obtain DPC color developer), and ultrapure water is used to make up to 4 mL. After 10 minutes of color development, the absorbance values ​​of dissolved Cr (VI) of different concentrations are measured at λ = 540 nm, and the linear relationship (standard curve) between the millimolar concentration of dissolved Cr (VI) and the absorbance value is established:

[0037] A=ε×C×L

[0038] Wherein, A is the absorbance of dissolved Cr(VI) under certain concentration conditions; C is the millimolar concentration of dissolved Cr(VI), in mmol / L; ε is the millimolar extinction coefficient of dissolved Cr(VI), in mM -1 cm -1 The millimolar extinction coefficient of dissolved Cr(VI) is 43.5 mM at wavelength λ = 540 nm. -1 cm -1 ; L is the optical path of the cuvette, in cm.

[0039] When testing the sample, take about 1 mL of sample solution in a 5 mL centrifuge tube, add 0.25 mL of 1+1 sulfuric acid solution, 0.25 mL of 1+1 phosphoric acid solution and 1 mL of DPC colorimetric agent in sequence, and use ultrapure water to make the volume 4 mL. After 10 minutes of color development, measure the absorbance at λ=540 nm, and convert the dissolved Cr(VI) concentration according to the standard curve.

[0040] Example 2

[0041] This example is about the determination of the electron donating capacity of the soil sample and the reduction capacity of the soil sample for hexavalent chromium.

[0042] Unless otherwise specified, all analyses and sample handling were performed in an anaerobic glove bag at 25±2°C and 100 vol% N2. Before placing the solution in the glove bag, high-purity nitrogen (99.999%) was introduced for at least 30 min to eliminate the interference of oxygen in the solution. The electrochemical analysis used a CHI1000C electrochemical workstation, a glassy carbon crucible as the working electrode and reaction vessel, an Ag / AgCl electrode filled with saturated KCl as the reference electrode, and a platinum wire electrode as the auxiliary electrode, which was separated from the working electrode chamber by a glass sand core. Each electrode was connected to the electrochemical workstation to construct a three-electrode system. The reaction vessel was placed on a stirrer and stirred continuously at 700 rpm with a 1 cm polytetrafluoroethylene-coated magnetic stirring bar.

[0043] First, add 40mL of 0.1M KCl (containing 0.01M MOPS, pH=7.0) to the reactor to remove dissolved oxygen as the background electrolyte solution, and add about 2mL of the same background electrolyte solution to the counter electrode glass tube so that the liquid level in the counter electrode glass tube is flush with the liquid level in the working electrode reactor. Connect the reactor to the electrochemical workstation, set the constant voltage to +0.5V (vs.Ag / AgCl), and perform the it curve test under the potential conditions given by the workstation. Stir in the dark during the test. Start the electrochemical test program, measure the oxidation current continuously every 5s, and obtain the current-time curve. After the current reaches stability, 1mL ABTS (concentration of 10mM) is injected into the reactor from the feed port, and then an oxidation current response curve is generated. Thereafter, the current gradually decreases with time. When the reaction current returns to the baseline and reaches stability, it indicates that the system ABTS oxidation has reached equilibrium. At this time, add the soil sample to be tested (0.2-1g). Since the oxidized ABTS can mediate the loss of electrons in the added sample, it will continue to generate oxidation current. Further, the time-current curve of the test sample during the electron loss process can be obtained through the electrochemical workstation. EDC is calculated by determining the peak area of ​​the current response, and the formula is as follows:

[0044]

[0045] Wherein, the EDC is the electron donating capacity of the soil sample measured, in mol e - / g; F is the Faraday constant, representing the charge carried by each mole of electrons, and its value is 96485C / mol; t1 is the starting point of the current peak, in seconds; t2 is the ending point of the current peak, in seconds; I(t) is a function of the change of current over time, and the current I is in A; m is the dry weight of the soil sample added during the test, in g.

[0046] Therefore, the reduction capacity of the soil sample for hexavalent chromium can be further calculated based on the electron donating capacity EDC.

[0047] Example 3

[0048] This example is used to verify the applicability and accuracy of the method of using soil sample EDC to predict its reduction capacity for Cr(VI). The specific process is as follows:

[0049] Under anaerobic conditions, 10 g of soil powder was weighed and placed in a 120 mL anaerobic glass bottle, and 100 mL of pH buffer containing 5 mM PIPES (piperazine-1,4-diethanesulfonic acid) was added thereto, and pH was adjusted to 7 with 1 M NaOH. Potassium dichromate was added to the anaerobic glass bottles respectively, so that the Cr (VI) in the reaction system was 0.03, 0.06, 0.09, 0.15 mM, 0.3 mM and 0.6 mM, respectively. The glass bottles were wrapped with tin foil to avoid light, and the containers were sealed with butyl rubber. The glass anaerobic bottles were placed on a shaker at a speed of 220 rpm and the temperature was controlled at 25 °C.

[0050] For the groups with Cr(VI) concentration of 0.03, 0.06, 0.09, and 0.15 mM, 2 mL of suspension was taken from the anaerobic bottle using a syringe at 0, 1, 2, 5, 10, 20, 40, and 60 min of the constant temperature oscillation reaction, and then centrifuged at 8000 rpm for 5 min to separate the solid and liquid, and obtain the supernatant and the solid part; similarly, for the groups with Cr(VI) concentration of 0.15 mM, 0.3 mM, and 0.6 mM, 2 mL of suspension was taken from the anaerobic bottle using a syringe at 0, 5, 10, 20, 40, 60, and 120 min of the constant temperature oscillation reaction, and then centrifuged at 8000 rpm for 5 min to separate the solid and liquid, and obtain the supernatant and the solid part. The supernatant was tested for the concentration of dissolved Cr(VI) using the method described in Example 1, and the solid part was tested for the EDC of the soil sample using the method described in Example 2. The specific results of each concentration group are shown in Tables 1 and 2:

[0051] Table 1

[0052]

[0053] Table 2

[0054]

[0055]

[0056] The results are as follows Figure 1-6 As shown (the EDC data in the figure is obtained by converting the EDC data of the solid part in Tables 1 and 2). Figure 3 It can be seen from the above that when the concentration of Cr(VI) in the soil is ≤ the initial EDC of the soil, the amount of Cr(VI) removed is approximately equal to 1 / 3 of the change in the reduction capacity of the soil sample, that is, △Cr(VI)=1 / 3△EDC. Figure 6 It can be seen that when the concentration of Cr(VI) in the soil is greater than the initial EDC of the soil, the amount of Cr(VI) removed is greater than 1 / 3 of the change in the reduction capacity of the soil sample, that is, △Cr(VI)>1 / 3△EDC.

[0057] Example 4

[0058] This example is an experiment on the interference of soil adsorption on the method of estimating Cr(VI) reduction capacity using soil sample EDC in Example 3.

[0059] The soil sample used in Example 3 was exposed to oxygen to completely oxidize it (i.e., the EDC of the soil after oxidation was approximately 0 μmole - / g). Under anaerobic conditions, 10 g of completely oxidized soil powder was weighed and placed in a 120 mL anaerobic glass bottle, and 100 mL of pH buffer containing 5 mM PIPES was added thereto, and pH was adjusted to 7 with 1 M NaOH. Potassium dichromate was added to the anaerobic glass bottles respectively, so that the Cr(VI) in the reaction system was 0.15 mM, 0.3 mM and 0.6 mM, respectively. The glass bottles were wrapped with tin foil to avoid light, and the containers were sealed with butyl rubber. The glass anaerobic bottles were placed on a shaker at a speed of 220 rpm and the temperature was controlled at 25 °C.

[0060] At 0, 1, 2, 5, 10, 20, 40, 60, and 120 minutes of constant temperature oscillation reaction, 2 mL of suspension was taken from the anaerobic bottle using a syringe, and then centrifuged at 8000 rpm for 5 minutes to separate the solid and liquid. The supernatant was tested for dissolved Cr(VI) concentration using the DPC method, and the specific results are shown in Table 3.

[0061] The results are as follows Figure 7-8 As shown (the EDC data in the figure is obtained by converting the EDC data of the solid part in Table 2). Figure 8 It can be seen that when the concentration of Cr(VI) in the soil is greater than the initial EDC of the soil, the amount of Cr(VI) removed minus the amount of Cr(VI) adsorbed by the soil sample is approximately equal to 1 / 3 of the change in the reduction capacity of the soil sample, that is, △Cr(VI)-△Cr(VI) 吸附 =1 / 3△EDC.

[0062] The method for measuring the adsorption of Cr(VI) by soil samples is as follows: the soil samples are exposed to oxygen to completely oxidize them (i.e., the EDC of the soil after oxidation is approximately 0 μmole - / g). Potassium dichromate was added thereto respectively to adjust the Cr(VI) in the reaction system to 0.15mM, 0.3mM and 0.6mM. The reaction was carried out under constant temperature oscillation. The concentration of dissolved Cr(VI) in the supernatant was measured at the predetermined sampling time, and the adsorption amount of Cr(VI) by the adsorbed soil sample was calculated.

[0063] Table 3

[0064]

[0065] Through the above experiments, it can be predicted that the reduction capacity of the soil samples used in Examples 3 and 4 for hexavalent chromium is about 0.0026 mmol / g.

[0066] In summary, the method of the present invention can more accurately predict the reduction capacity of soil for hexavalent chromium. In various cases, the reduction capacity of soil for hexavalent chromium can be further more accurately predicted by directly measuring the EDC of the soil.

[0067] The contents explained in the above embodiments should be understood as these embodiments are only used to more clearly illustrate the present invention, and are not used to limit the scope of the present invention. After reading the present invention, various equivalent forms of modifications to the present invention by those skilled in the art all fall within the scope defined by the claims attached to this application.

Claims

1. A method for predicting the reduction capacity of soil for hexavalent chromium, characterized in that: The steps include: S1, mixing the background electrolyte solution and 2,2-azino-bis(3-ethyl-benzothiazoline-6-sulfonic acid) uniformly to obtain solution A; S2, electrochemically oxidizing solution A until the current signal is stable to obtain solution B; S3, adding the soil sample to be tested into the solution B, conducting a test by applying power, and obtaining a time-current curve; Then integrate and calculate the peak area of ​​the time-current curve to obtain the electron donating capacity (EDC) of the soil sample; in, F is the Faraday constant, t1 is the starting point of the current peak, t2 is the ending point of the current peak, I(t) is the current value at time t, and m is the dry weight of the added soil sample; S4. Calculate the reduction capacity of the soil for hexavalent chromium based on the electron donating capacity EDC.

2. The method according to claim 1, characterized in that In S1, in the background electrolyte solution, the concentration of the inorganic salt is 0.08-0.12 M, and the concentration of MOPS is 0.008-0.012 M, wherein the inorganic salt includes one or more of KCl and NaCl.

3. The method according to claim 1, characterized in that In S1, the pH value of the background electrolyte solution is 7.

0.

4. The method according to claim 1, characterized in that: In S1, the background electrolyte solution and the ABTS solution are evenly mixed in a volume ratio of 35-45:1; wherein the concentration of ABTS in the ABTS solution is 8-12 mM.

5. The method according to claim 1, characterized in that In S2, solution A is electrochemically oxidized using chronoamperometry.

6. The method according to any one of claims 1 to 5, characterized in that: When conducting electrochemical oxidation and power-on tests, a glassy carbon electrode is used as the working electrode, a platinum wire is used as the auxiliary electrode, and Ag / AgCl is used as the reference electrode. The working electrode, auxiliary electrode, and reference electrode are electrically connected to the electrochemical workstation, respectively, to form a three-electrode system.

7. The method according to any one of claims 1 to 5, characterized in that: When conducting electrochemical oxidation and power-on tests, set the voltage to +0.4-+0.6V.

8. The method according to any one of claims 1 to 5, characterized in that: Electrochemical oxidation and galvanic tests were performed under anaerobic conditions.

9. The method according to any one of claims 1 to 5, characterized in that: In S1, electrochemical oxidation and current-on tests were performed under a nitrogen atmosphere.

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