Imaging electrode and method for characterizing reduced phosphorus release of metastable iron ore-containing soil sediment

Abstract

The invention discloses an imaging electrode and method for representing iron ore driven phosphorus release in soil sediment. The imaging electrode comprises a conducting layer, an imaging layer and a diffusion protection layer wrapping the conducting layer and the imaging layer. The conductive layer comprises a conductive base material and a to-be-detected sample covered on the surface of the conductive base material, the to-be-detected sample contains iron ore adsorbed with phosphorus, the imaging layer is a film formed by first gel, a phosphorus adsorbent is dispersed in the first gel, and the first gel is agarose gel or cross-linked gel of polyacrylamide and agarose; and a diffusion protective layer is agarose gel. The imaging electrode can truly and effectively simulate the reduction process of iron ore in soil and sediments, accurately reflect the phosphorus reduction and release behaviors of iron ore in different thermodynamic states in a natural environment, and accurately obtain the actual critical potential of phosphorus release caused by reduction of iron ore in a sample to be detected; and the ratio of the steady-state iron ore to the metastable-state iron ore in the sample can be obtained through the obtained image.

Description

Technical Field

[0001] The present invention belongs to the fields of environmental science and geochemistry, and particularly relates to an imaging electrode and method for characterizing the reduction and phosphorus release of metastable iron ore-containing soil sediments, which can provide technical support for studying the phosphorus cycle in the fields of environmental science and geochemistry. Background Art

[0002] Phosphorus is a key nutrient element that limits the production of global primary producers. Especially in soils and sediments, its bioavailability has an important impact on ecosystem functions. Although dissolved reactive phosphorus can be directly absorbed by organisms and participate in biogeochemical cycles, its content in soils and sediments is usually less than 6% of the total phosphorus. Most phosphorus exists in the form of binding to minerals or organic molecules and is trapped in soils and sediments for a long time, restricting the biological availability of phosphorus.

[0003] Phosphorus bound to iron ore is an important phosphorus form in soils and sediments. The redox reaction of iron ore significantly affects the processes of phosphorus adsorption, fixation, and release. Under oxidizing conditions, iron in soils and sediments exists in the form of solid iron oxides (such as hematite, goethite, etc.), and phosphorus is effectively adsorbed and fixed on the surface of iron ore, resulting in phosphorus being retained in soils or sediments for a long time and being difficult to be utilized by organisms. Under anaerobic conditions, the reduction by microorganisms reduces iron oxides to dissolved ferrous ions (Fe 2⁺ ), thereby promoting the release of phosphorus, converting it into dissolved reactive phosphorus, and improving the biological availability of phosphorus in soils and sediments. The reductive dissolution of iron ore is an important way to improve the biological availability of phosphorus, and this process is closely related to the thermodynamic state of iron ore in soils and sediments.

[0004] The types and forms of iron ore in soils and sediments are complex. There are not only thermodynamically stable standard iron ores (such as goethite, magnetite, etc.), but also metastable iron (a non-standard form of iron ore). In the prior art, the boundary conditions of the redox reaction are judged by the theoretical redox potential of standard iron ore, but this method cannot accurately reflect the actual geochemical process. The same iron ore in different states has different Gibbs free energies, and there are differences in redox potentials. In different regions of the same iron ore, the potential and activity are also heterogeneous. Simply relying on the theoretical potential value of iron ore to judge its boundary of action in the phosphorus cycle cannot accurately reflect the actual situation. Summary of the Invention

[0005] Aiming at the problems in the above prior art, the present invention provides an imaging electrode and method for characterizing the reduction and phosphorus release of metastable iron ore-containing soil sediments, which are applicable to the phosphorus release process driven by iron ores in different thermodynamic states in soil sediments, can truly and effectively simulate the reduction process of iron ore in soils and sediments, and accurately reflect the reduction and phosphorus release behaviors and proportions of iron ores in different thermodynamic states in the natural environment.

[0006] The above object of the present invention is achieved as follows: An imaging electrode for characterizing the reduction and phosphorus release of metastable iron ore-containing soil sediments, comprising a conductive layer, an imaging layer, and a diffusion protection layer wrapped outside the conductive layer and the imaging layer; The conductive layer includes a conductive substrate and a test sample covered on the surface of the conductive substrate, and the surface where the test sample is located is in close contact with the imaging layer, The test sample contains iron ore adsorbed with phosphorus, The imaging layer is a film formed by a first gel, and a phosphorus adsorbent is dispersed in the first gel. The first gel is an agarose gel or a cross-linked gel of polyacrylamide and agarose; The diffusion protection layer is agarose gel.

[0007] The diffusion protection layer wraps the conductive layer and the imaging layer, and plays a role in fixing the conductive layer and the imaging layer and ion exchange at the electrode interface.

[0008] Preferably, the iron ore includes thermodynamically stable or metastable iron ore.

[0009] Preferably, the iron ore is derived from one or more of goethite, hematite or magnetite. Thermodynamically stable iron ore is high-crystalline goethite, hematite or magnetite, and thermodynamically metastable iron ore is non-standard iron ore produced by goethite, hematite or magnetite through the redox environment generated by microorganisms in a complex natural environment.

[0010] Preferably, the conductive substrate is conductive carbon paper.

[0011] Preferably, the conductive layer is prepared by the following method: Disperse the test sample in a solvent, and then coat the obtained dispersion on the surface of the conductive substrate, and obtain it after drying.

[0012] Preferably, the solvent is a mixed solution of ethanol and Nafion.

[0013] Preferably, the dispersion is coated on the hydrophilic surface of the conductive carbon paper.

[0014] Preferably, the coating thickness of the test sample is 0.5 - 5 mm.

[0015] Preferably, the phosphorus adsorbent is selected from nanoparticles of La(OH)3, ZrO2 or CaCO3.

[0016] Preferably, the imaging layer is prepared by the following method: Add a phosphorus adsorbent to a mixed solution containing acrylamide, N,N-methylenebisacrylamide, and agarose, disperse it evenly to obtain a gel solution, then add an initiator and a catalyst to the gel solution, and inject the coagulation solution into a mold and cure it to obtain the fixed film.

[0017] Preferably, in the gel solution, the mass fraction of the phosphorus adsorbent is 1-10%.

[0018] Preferably, the initiator is ammonium persulfate.

[0019] Preferably, the catalyst is TEMED.

[0020] Preferably, the solvent of the gel solution is water.

[0021] Preferably, the agarose gel is prepared from an agarose solution with a mass fraction of 0.5-5%.

[0022] The present invention also provides a method for characterizing the reduction and phosphorus release of metastable iron ore-containing soil sediments, including the following steps: (1) Set up a three-electrode system, use the imaging electrode containing the sample to be measured as the working electrode, place the working electrode and the reference electrode in an airtight first electrolyte, place the counter electrode in an airtight second electrolyte, and separate the first electrolyte and the second electrolyte only by a proton exchange membrane; the first electrolyte and the second electrolyte are deoxygenated. (2) Under anaerobic conditions, adjust the reduction potential of the three-electrode system and react for more than 12 h to obtain the imaging electrodes treated at different reduction potentials. (3) Contact each of the imaging electrodes obtained in step (2) with a color reagent, and the minimum reduction potential corresponding to the imaging electrode that can form an image is the critical reduction potential of the sample to be measured.

[0023] Preferably, the method further includes: (4) Analyze the image formed by the imaging electrode obtained at a reduction potential of -0.21 V using image J software, and the area ratio of the area where the phosphorus release amount exceeds 3 ug / cm 2 and the area where it is lower than 3 ug / cm 2 is the content ratio of stable iron ore and metastable iron ore. The imaging obtained using the standard potential (-0.21 V, SHE) of Shewanella to reduce iron ore in soil and sediments can effectively simulate the reduction situation of soil or sediments in the natural environment, thereby obtaining the relative content of various thermodynamic forms of iron ore in the soil or sediments, and thus obtaining the true phosphorus release situation.

[0024] Preferably, the reference electrode is an Ag / AgCl electrode, and the counter electrode is a platinum mesh electrode.

[0025] Preferably, the first electrolyte and the second electrolyte are KCl solutions containing MOPS buffer. The concentration of the MOPS buffer is 10 mM, and the pH is 7.0. The concentration of the KCl solution is 0.1 M.

[0026] Preferably, the proton exchange membrane is Nafion-117. It can prevent the generated ferrous ions from diffusing and migrating to the counter electrode and being oxidized and precipitated.

[0027] Preferably, the anaerobic environment in step (2) is an environment with an oxygen concentration below 0.1 ppm.

[0028] Preferably, the color reagent in step (3) is a mixed solution of molybdate and ascorbic acid.

[0029] The imaging electrode and method for characterizing the reduction and phosphorus release of metastable iron ore-containing soil sediments provided by the present invention can truly simulate the reduction process of trivalent iron in soil and sediments. For iron ores in different thermodynamic states, their reduction potentials are different. At the same reduction potential, the more stable the thermodynamic state of the iron ore, the more difficult it is to be reduced, and the lower the phosphorus release efficiency. The imaging electrode and characterization method of the present invention can accurately obtain the actual critical potential at which the iron ore in the sample to be measured undergoes reduction and releases phosphorus, and can know the proportion of stable iron ore and metastable iron ore in the sample through the obtained imaging, thereby providing data support for researchers to know the phosphorus release ability and release amount in the soil to be measured.

[0030] During the reduction reaction process, the released phosphate can be quickly adsorbed and fixed by the phosphorus-locking components in the imaging layer. Through the color imaging method, researchers can effectively characterize the reduction and phosphorus release process of iron ores in different thermodynamic states in soil and sediments and know the critical potential and the proportion of iron ores in different thermodynamic states.

[0031] The present invention provides a method for effectively characterizing and improving the efficiency of iron ore reduction and phosphorus release in soil and sediments, provides a scientific basis for optimizing the management of soil and sediment phosphorus, and provides a new technology for research and application in related fields.

[0032] The present invention can truly and effectively simulate the reduction process of iron ore in soil and sediments, and accurately reflect the reduction and phosphorus release behavior of iron ores in different thermodynamic states in the natural environment.

[0033] The present invention adopts a color imaging technique to quickly characterize the release and fixation of phosphate during the reduction reaction process, can intuitively reflect the phosphorus release under different thermodynamic states of iron ore, and provides a convenient and efficient analysis means for related research and application.

[0034] The present invention can obtain the accurate critical reduction potential for the release of phosphorus from iron ore in soil, and know the phosphorus release capacity and release amount in soil, providing a scientific basis and technical support for optimizing phosphorus management in soil and sediment. Description of the Drawings

[0035] Figure 1 It is a schematic diagram of the assembly structure of the imaging electrode in Example 1.

[0036] Figure 2 It is a schematic diagram of the assembly structure of the three - electrode system in Example 1, where 1 is the working electrode, 2 is the reference electrode, 3 is the counter electrode, and 4 is the proton exchange membrane.

[0037] Figure 3 It is the imaging diagram on the imaging electrode in Example 1. From left to right are the blank background diagram, the imaging diagram of phosphorus released from iron ore without redox - activated (thermodynamically stable iron ore in the experimental group), and the imaging diagram of phosphorus released from metastable iron after redox - fluctuation activation (metastable iron ore in the experimental group).

[0038] Figure 4 It is the imaging result of the imaging electrode of the experimental group at each reduction potential in Example 1.

[0039] Figure 5 It is a comparison diagram of the redox potentials of metastable iron ore and stable iron ore. Detailed Embodiments

[0040] The technical solution of the present invention will be further elaborated below in combination with the description of the drawings and detailed embodiments. The protection scope of the present invention is not limited by the detailed embodiments, but is defined by the claims. Examples

[0041] (1) 1.0 g / L goethite was placed under redox - alternating conditions for activation to prepare metastable iron. Under anaerobic conditions, Shewanella bacteria fully reduced goethite, and then air was introduced for full oxidation. After 30 consecutive redox - alternations, it was washed clean with pure water and stored by freeze - drying as the obtained metastable goethite. The goethite without redox treatment was stable goethite.

[0042] 2.0 g / L of stable and metastable goethite were respectively mixed and shaken with 12 mM potassium dihydrogen phosphate solution. After adsorption, the two types of goethite containing phosphate were washed clean with pure water and stored by freeze - drying for use. The obtained metastable goethite containing phosphorus was the experimental group, and the obtained goethite containing phosphorus was the control group, serving as the simulated soil for the experimental group and the control group.

[0043] (2) Add 10 mg of the simulated soil from the experimental group and the control group to 950 μL of ethanol and 50 μL of Nafion (5%) solution respectively, and then ultrasonically treat for 10 min. Brush the ultrasonically treated solution onto the hydrophilic layer of the conductive carbon paper (Sigracet 29BC, Germany) with a brush 1 cm wide. The area of the conductive carbon paper is 1×2 cm 2 , and then dry it in a fume hood at room temperature to obtain the conductive layer.

[0044] (3) Prepare a uniform mixed solution of 28.5% acrylamide, 1.5% N,N-methylenebisacrylamide and 0.3% agarose, then add the activated ZrO2 nanoparticles and mix them in a mass ratio of 1 - 10%, and disperse them in an ultrasonic crusher to obtain a coagulation solution containing ZrO2 nanoparticles; at this time, quickly transfer the solution evenly to a preheated glass plate. After standing at room temperature for 60 min, cut it into strips of 1×2 cm with a mold to obtain a number of imaging layers.

[0045] (4) Press and fix the conductive layer and the imaging layer on the simulated soil loading surface, and prepare a gel solution containing 15% acrylamide and 0.3% agarose. After cooling to 40 - 60 °C, ultrasonically mix it evenly, then add ammonium persulfate initiator (70 μL) and TEMED catalyst (25 μL), mix well, and pour it around the pressed conductive layer and imaging layer, and continue to cool and solidify for 1 hour. Then soak the gel in deionized water for at least 24 hours, and cut off the excess diffusion protection layer to obtain the imaging electrode. The diffusion protection layer formed by the gel contains ion transport channels, allowing the electrolyte ions and the released iron ions and phosphate ions to shuttle freely, constituting a conductive circuit.

[0046] (5) Use an electrochemical workstation to control the electrode potential and monitor the electrolysis reaction. Take the prepared imaging electrodes of the experimental group and the control group as the working electrodes to construct a three-electrode system. Use an Ag / AgCl electrode as the reference electrode and a platinum mesh electrode as the counter electrode. Use an H-type electrolytic cell separated by a proton exchange membrane (Nafion-117) to prevent the migration of ferrous ions to the counter electrode for oxidation. Add MOPS buffer solution (10 mM, pH 7.0) to 0.1 M KCl solution to ensure the stability of the electrolyte.

[0047] (6) Set the electrochemistry reaction environment, adjust the reduction potential to -0.21 V (relative to the standard hydrogen electrode SHE), and carry out the reaction in an anaerobic environment for 12 hours. Keep the oxygen concentration below 0.1 ppm to simulate the reduction reaction conditions in soil and sediment. After the experiment, take out the treated imaging electrodes and the control group electrodes, react them with a color reagent (a mixed solution of molybdate and ascorbic acid) respectively, and then use a scanner for imaging, using the imaging electrode without simulated soil as the blank group.

[0048] (7) The results are as Figure 3 shown. The highest reduction potential of Shewanella in the soil is -0.21 V. Set this potential on the imaging electrode to reduce different types of soil. Compared with the background value of phosphorus release from the stable iron ore in the control group (3 μg / cm 2 ), the phosphorus release amount in the experimental group exceeds 3 μg / cm 2 . This indicates the existence of metastable iron, which can release bioavailable phosphorus under microbial reduction. According to the bar scale information in the figure, the area of the numerical region greater than 3 is 1.84 cm 2 , and the total area of the imaging region is 2 cm 2 . Therefore, in this sample, the area ratio of the metastable iron region (greater than 3) = 1.84 / 2 = 92%, that is, the content ratio of metastable iron ore in the sample is 92%.

[0049] (8) Similar to (6), set up the electrochemical reaction environment, use the imaging electrode of the experimental group as the working electrode, adjust the reduction potential to -0.1 V, -0.15 V, -0.21 V, and react for 12 hours in an anaerobic environment with the oxygen concentration maintained below 0.1 ppm. After the experiment, take out the treated imaging electrode, react it with a color reagent (a mixed solution of molybdate and ascorbic acid), and then use a scanner for imaging.

[0050] (9) As Figure 4 shown, at the reduction potential of -0.1 V, there is no significant imaging on the corresponding imaging electrode, while the imaging electrode reacting at -0.15 V shows imaging due to phosphorus capture. Therefore, it can be judged that -0.15 V is the critical reduction potential of the experimental group.

[0051] The standard redox electrode potentials of goethite and magnetite reported in the literature are -0.27 V and -0.31 V respectively.

[0052] The redox electrode potential of metastable magnetite measured by the method in Example 1 is -0.15 V.

[0053] Figure 5 shows the comparison of the reduction potentials of stable and metastable goethite and magnetite reported in the prior art.

[0054] The above results indicate that it is difficult to accurately reflect the actual geochemical process by relying on the standard redox potential of iron ore to judge its reaction boundary. The same iron ore in different forms has different Gibbs free energies, resulting in different redox potentials. The imaging electrode developed in the present invention more accurately reflects the redox boundary conditions under the actual thermodynamic state, providing more accurate data for determining the adsorption and desorption of iron and phosphorus.

Claims

1. An imaging electrode for characterizing the reduction and release of phosphorus in soil sediments containing metastable iron ore, characterized in that: It comprises a conductive layer, an imaging layer and a diffusion protection layer wrapped outside the conductive layer and the imaging layer; The conductive layer includes a conductive substrate and a sample to be tested covered on the surface of the conductive substrate, and the surface where the sample to be tested is located is in close contact with the imaging layer. The sample to be tested contains iron ore adsorbed with phosphorus, The imaging layer is a thin film formed by a first gel, a phosphorus adsorbent is dispersed in the first gel, and the first gel is agarose gel or a cross-linked gel of polyacrylamide and agarose; The diffusion protection layer is agarose gel.

2. The imaging electrode according to claim 1, characterized in that: The iron ore includes thermodynamically stable or thermodynamically metastable iron ore, and the iron ore is derived from one or more of goethite, hematite or magnetite.

3. The imaging electrode according to claim 1, characterized in that: The conductive layer is prepared by the following method: The sample to be tested is dispersed in a solvent, and then the obtained dispersion is coated on the surface of the conductive substrate and dried to obtain the result.

4. The imaging electrode according to claim 1, characterized in that: The phosphorus adsorbent is selected from nanoparticles of La(OH)3, ZrO2 or CaCO3.

5. The imaging electrode according to claim 1, characterized in that: The imaging layer is prepared by the following method: The phosphorus adsorbent is added to a mixed solution containing acrylamide, N,N-bisacrylamide and agarose, and dispersed uniformly to obtain a gel solution. Then, an initiator and a catalyst are added to the gel solution, and the coagulation solution is injected into a mold and solidified to obtain the fixed film.

6. A method for characterizing phosphorus release from soil sediment containing metastable iron ore using the imaging electrode described in any one of claims 1 to 5, characterized in that: The steps include: (1) Setting up a three-electrode system, using the imaging electrode containing the sample to be tested as a working electrode, placing the working electrode and the reference electrode in a gas-tight first electrolyte, placing the counter electrode in a gas-tight second electrolyte, the first electrolyte and the second electrolyte being separated only by a proton exchange membrane; the first electrolyte and the second electrolyte being deoxygenated; (2) Under anaerobic conditions, adjusting the reduction potential of the three-electrode system and reacting for more than 12 hours to obtain the imaging electrodes treated at different reduction potentials; (3) Each of the imaging electrodes obtained in step (2) is contacted with a developer, and the minimum reduction potential corresponding to the imaging electrode capable of forming an image is the critical reduction potential of the sample to be tested.

7. The method according to claim 6, characterized in that The method further comprises: (4) The image formed by the imaging electrode obtained at a reduction potential of -0.21 V was analyzed using image J software, and the phosphorus release amount exceeded 3 ug / cm 2 Areas with a concentration below 3 ug / cm 2 The area ratio of the region is the content ratio of stable iron ore and metastable iron ore.

8. The method according to claim 6, characterized in that The reference electrode is an Ag / AgCl electrode, and the counter electrode is a platinum mesh electrode.

9. The method according to claim 6, characterized in that The proton exchange membrane is Nafion-117.

10. The method according to claim 6, characterized in that The developer described in step (3) is a mixed solution of molybdate and ascorbic acid.

Images