A method and system for in-situ remediation of self-sustaining shale gas flowback fluid

Abstract

The present application relates to sewage resource utilization and environmental protection technical field, disclose a kind of self-sustaining type shale gas flow-back fluid in-situ remediation method and system, comprising the following steps: active carbon fiber felt and Ag-g-C3N4 / TiO2 / Ti cathode electrode material are placed in parallel, edge is surrounded by foam and forms cavity, and cavity is filled with photosynthetic biological algae to form cathode electrode;Preparation carbon-coated iron mesh anode electrode;Anode electrode is installed in bottom mud layer, and cathode electrode is installed in water layer;Anode electrode and cathode electrode are connected with battery through wire, for collecting electric energy.The scheme couples photoelectrocatalysis with sediment-type microbial fuel cell, transfers the electrons generated by anode to cathode, promotes microbial metabolism, enhances the remediation performance of bottom mud, and overcomes the shortcomings of cathode photoelectrocatalytic material, such as easy recombination of photo-generated charges and low catalytic efficiency, improves the degradation performance of overlying water pollutants, fully utilizes the advantages of both and avoids their shortcomings, to achieve the purpose of overlying water and bottom mud collaborative purification.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of sewage resource utilization and environmental protection technology, and particularly relates to a method and system for in-situ remediation of shale gas flowback fluid of self-sustaining type. BACKGROUND

[0002] Shale gas is an important unconventional natural gas resource, and its exploitation is continuously promoted worldwide. Hydraulic fracturing is a key technology for shale gas exploitation. During the fracturing process, a large amount of fracturing fluid is injected into the deep underground. After the fracturing operation is completed, part of the fracturing fluid and the formation water mixture gradually flow back to the ground from the formation under the action of pressure difference, forming flowback fluid. The flowback process usually takes place within a few days to weeks after fracturing, but the generation of flowback fluid can last for a long time. The flowback fluid contains fracturing fluid components, formation water, and some new substances that may be generated. At the same time, the properties of formation water are relatively complex, containing various inorganic salt ions, and possibly containing some naturally occurring hydrocarbon compounds, organic acids and other organic substances. Therefore, flowback fluid has the characteristics of complex composition, high salt, and high organic matter content, and improper treatment can cause serious pollution to groundwater, soil and other environments. At the same time, with the increasingly stringent environmental regulations, efficient treatment of shale gas development flowback fluid has become a key problem.

[0003] At present, the treatment of shale gas flowback fluid mainly uses a combination of physical and chemical treatment technologies such as filtration, flotation, centrifugal separation, coagulation and sedimentation, oxidation treatment, and acid-base neutralization. However, these technologies involve equipment investment, reagent consumption, energy consumption, and subsequent sludge treatment, resulting in complex process, difficult operation and maintenance, and high treatment cost, which limits their promotion and application. Microbial method has unique advantages in treating shale gas flowback fluid, as it can degrade organic pollutants in the flowback fluid into harmless substances, and is one of the current research hotspots. However, the microbial method also faces many challenges, such as the high salinity and complex composition of the flowback fluid, which may inhibit microbial activity. Therefore, further research and development and innovation are needed to develop efficient, economical and environmentally friendly treatment technologies for shale gas flowback fluid.

[0004] Sediment microbial fuel cells use indigenous microorganisms in the sediment to directly degrade organic pollutants, and also achieve energy recovery, which has great application prospects in pollutant treatment and environmental remediation. However, there are still many problems to be solved in this technology. First, its efficiency is still relatively low, and the cycle is too long. Second, the overlying water body also contains a large amount of pollutants, and the pollutants in the sediment continue to release into the overlying water, aggravating the deterioration of the water quality of the overlying water body. Third, the low dissolved oxygen in the overlying water has become one of the technical problems restricting its large-scale application. Mechanical aeration has the problems of high cost and difficulty in maintaining the effect. In addition, the cathode is commonly used for oxygen reduction reaction, and the utilization efficiency of the cathode is not high.

[0005] In view of this, the present application provides a self-sustaining shale gas flowback fluid in-situ repair method and system, which effectively makes up for the deficiencies of the prior art. SUMMARY

[0006] The present application aims to provide a self-sustaining shale gas flowback fluid in-situ repair method and system to solve the technical problem of low utilization efficiency of the cathode when the existing sediment microbial fuel cell processes flowback fluid.

[0007] To achieve the above-mentioned purpose, the present application adopts the following technical scheme: a self-sustaining shale gas flowback fluid in-situ repair method, comprising the following steps:

[0008] Step one, preparation of Ag-g-C3N4 / TiO2 / Ti cathode electrode

[0009] S1-1: configuration of solution: disperse cetyltrimethylammonium chloride and sodium hydroxide in water to obtain mixed solution A; disperse silver nitrate and dicyanediamine in water to obtain mixed solution B;

[0010] S1-2: preparation of cathode electrode material: first, place the titanium mesh in mixed solution A for hydrothermal reaction, then immerse it in hydrochloric acid solution after cooling, and clean it with deionized water; then immerse it in mixed solution B after calcination and cooling, freeze-dry; finally, wrap it with tin foil paper and calcine to obtain Ag-g-C3N4 / TiO2 / Ti cathode electrode material;

[0011] S1-3: preparation of cathode electrode: place the activated carbon fiber felt and Ag-g-C3N4 / TiO2 / Ti cathode electrode material in parallel, surround the edges with polyethylene plastic foam to form an internal cavity, and fill the cavity with photosynthetic biological algae to obtain the cathode electrode;

[0012] Step two, preparation of carbon-coated iron wire mesh anode electrode

[0013] S2-1, pretreatment of iron wire mesh;

[0014] S2-2, configuration of ligand solution: disperse o-phenanthroline and terephthalic acid in water, adjust the pH, and obtain the ligand solution;

[0015] S2-3, preparation of anode electrode: hydrothermal reaction of the pretreated iron wire mesh sample in the ligand solution, washing of the removed iron wire mesh, calcination in the presence of protective gas, and cooling to obtain the anode electrode;

[0016] Step three, system construction and operation

[0017] S3-1, installation system: in the waste liquid system to be treated with bottom mud layer and water layer after standing, the anode electrode is installed in the bottom mud layer, and the cathode electrode is installed in the water layer. When the cathode electrode is installed, the Ag-g-C3N4 / TiO2 / Ti electrode faces upward, and the activated carbon fiber felt faces downward;

[0018] S3-2, system operation: the anode electrode and the cathode electrode are connected through wires and connected with a storage battery; samples are collected from the overlying water and the bottom mud regularly, and the remediation efficiency of the water body and the bottom mud is determined.

[0019] The principle of the scheme is:

[0020] The scheme uses clean and inexpensive sunlight and indigenous microorganisms in the bottom mud as driving force, adds photosynthetic algae into the internal cavity composed of the Ag-g-C3N4 / TiO2 / Ti electrode and the carbon felt, suspends in the overlying water body as the cathode electrode, uses hydrothermal combined high-temperature carbonization method to prepare carbon-coated iron mesh which is buried in the bottom mud as the anode electrode, connects the anode electrode and the cathode electrode through wires and connects with a storage battery for collecting electric energy. The system couples photoelectrocatalysis and sediment-type microbial fuel cell, fully gives play to the advantages of both and avoids the shortcomings of both, so as to achieve the purpose of the overlying water and the bottom mud being purified cooperatively. In addition, the scheme not only realizes the purification of the wastewater return liquid, but also collects electric energy, effectively realizes the resource utilization of the wastewater, and improves the environmental protection benefit.

[0021] The advantages of the scheme are:

[0022] 1. Compared with the utilization efficiency of the existing technology cathode, the scheme realizes the simultaneous degradation of pollutants by the anode and the cathode, improves the utilization rate of the cathode, promotes the diffusion rate of the pollutants in the sediment, improves the performance of the water body purification, and has low cost and is easy to control and manage.

[0023] 2. The scheme couples photoelectrocatalysis and sediment-type microbial fuel cell, can transfer the electrons generated by the anode of the sediment-type microbial fuel cell to the cathode, promotes the metabolism of the microorganisms, strengthens the remediation performance of the bottom mud, overcomes the shortcomings of the photo-generated charges of the cathode photoelectrocatalytic material being easy to recombine and the catalytic efficiency being low, improves the degradation performance of the overlying water pollutants, fully gives play to the advantages of both and avoids the shortcomings of both, so as to achieve the purpose of the overlying water and the bottom mud being purified cooperatively.

[0024] 3. The scheme uses the carbon felt as the fixed bed of the photosynthetic algae, uses the oxygen production characteristics of the biological algae photosynthesis, improves the reaction efficiency of the oxygen and the cathode photoelectrode, and produces more hydroxyl radicals (·OH), peroxide radicals (·O2 -) and oxidizing species such as hydrogen peroxide (H2O2), which promotes the purification rate of the overlying water and also solves the technical problems that restrict the large-scale application of microbial fuel cells.

[0025] 4、The Ag-g-C3N4 / TiO2 / Ti cathode electrode material developed by the scheme has antibacterial performance, can effectively inhibit the growth of microorganisms on the surface of the cathode electrode, maximizes the use of clean and inexpensive sunlight, and ensures the overlying water body repair performance and long-term operation stability.

[0026] 5、The scheme uses clean and inexpensive sunlight and indigenous microorganisms in the sediment as driving force, is a green and environmentally friendly repair technology, and has the advantages of simple electrode production, good stability, long service life, simple system structure, no additional maintenance, small investment, etc. It can realize self-sustaining, persistent and large-scale in-situ repair of shale gas flowback fluid, black and odorous water bodies and other polluted water bodies, and has broad application space.

[0027] Preferably, as an improvement, in S1-1, the mass ratio of cetyltrimethylammonium chloride, sodium hydroxide and water in the mixed solution A is 16-64:240:1000; the mass ratio of dicyandiamide, silver nitrate and water in the mixed solution B is 16.81-37.62g:0.1699-0.8494:100.

[0028] Beneficial effects: The scheme adopts the above settings, which is convenient for production to obtain excellent Ag-g-C3N4 / TiO2 / Ti cathode electrode material. The applicant found through long-term experiments that if the amount of cetyltrimethylammonium chloride in the mixed solution A is too small, TiO2 crystal nucleus will easily grow too much, forming larger particles and smaller specific surface area, which will not be conducive to the immobilization of Ag-g-C3N4; if the amount of cetyltrimethylammonium chloride is too much, the crystal growth direction will change, resulting in irregular shape; if the amount of sodium hydroxide in the mixed solution A is too small, the alkalinity will be insufficient, resulting in that TiO2 cannot completely cover the surface of the titanium sheet; if the amount of sodium hydroxide is too much, TiO2 will easily fall off. If the amount of dicyandiamide in the mixed solution B is too small, the amount of carbon precursor will be small, resulting in low content of g-C3N4; if the amount of dicyandiamide is too much, the g-C3N4 covering layer will be too thick, reducing the effective electron transfer; if the amount of silver nitrate in the mixed solution B is too small, the amount of Ag component will be too small; if the amount of silver nitrate is too much, the particle size of Ag will be large and the dispersibility will be poor.

[0029] Preferably, as an improvement, in S1-2, the hydrothermal reaction is a hydrothermal reaction at 140-160℃ for 24h; the immersion in hydrochloric acid solution is an immersion in 1-2M hydrochloric acid solution for 24h; and the calcination is a calcination at a rate of 2-4℃ / min to 500-550℃, and holding for 2.5-4h.

[0030] Advantages: The above settings facilitate the production of the Ag-g-C3N4 / TiO2 / Ti cathode electrode material with excellent performance. The applicant has found through long-term experiments that if the hydrothermal reaction temperature is too high, the TiO2 crystal nucleus grows too fast, and large particles are easily formed, which leads to a small contact interface with Ag-g-C3N4; if the hydrothermal reaction temperature is too low, the corrosion is insufficient, and it is difficult to form TiO2 crystal nucleus. If hydrochloric acid immersion is not used after hydrothermal reaction, sodium ions will adhere to the surface; if the concentration of hydrochloric acid immersion after hydrothermal reaction is too high, the TiO2 crystal nucleus is easily separated; if the concentration of hydrochloric acid immersion after hydrothermal reaction is too low, TiO2 cannot be effectively formed. If the calcination temperature after hydrochloric acid immersion is too high or the heating rate is too fast, the TiO2 layers are easily separated; if the calcination temperature after hydrochloric acid immersion is too low or the heating rate is too slow, the TiO2 crystal nucleus does not grow completely, affecting its crystal structure; if the calcination time after hydrochloric acid immersion is too long, the TiO2 crystal nucleus is easily separated; if the calcination time after hydrochloric acid immersion is too short, the TiO2 crystal nucleus does not grow completely, affecting its crystal structure.

[0031] Preferably, as an improvement, in S1-2, the immersion time in the mixed solution B is 24-36h; and the calcination is a calcination at 520-540℃ for 4h.

[0032] Advantages: The above settings facilitate the production of the Ag-g-C3N4 / TiO2 / Ti cathode electrode material with excellent performance. The applicant has found through long-term experiments that if the immersion time in the mixed solution B is too short, the ion exchange is not complete, the Ag particles are large and easily agglomerated, affecting the performance of the electrode; if the immersion time in the mixed solution B is too long, the TiO2 crystal nucleus is easily separated due to excessive ion exchange reaction, affecting the long-term stability of the electrode. If the calcination temperature is too high, the g-C3N4 formed is too little due to excessive pyrolysis of the carbon precursor; if the calcination temperature is too low, the crystal type of the material is poor and the catalytic performance is poor due to incomplete pyrolysis of the carbon precursor; if the calcination time is too long, the Ag particles are large due to excessive growth of the Ag crystal nucleus; if the calcination time is too short, the crystal type of the material is poor and the catalytic performance is not good due to incomplete pyrolysis of the carbon precursor.

[0033] Preferably, as an improvement, in S1-3, the distance between the activated carbon fiber felt and the Ag-g-C3N4 / TiO2 / Ti cathode electrode is 2-3 cm.

[0034] Beneficial effects: The above settings facilitate the production of the Ag-g-C3N4 / TiO2 / Ti cathode electrode with excellent performance. The applicant has found through long-term experiments that if the distance is too long, the oxygen produced cannot fully contact the cathode electrode due to oxygen diffusion, reducing the effective use of the oxygen produced; if the distance is too short, the photosynthetic biological algae directly contact the cathode electrode, reducing the biological activity of the photosynthetic biological algae and the utilization efficiency of the cathode electrode to sunlight.

[0035] Preferably, as an improvement, in S2-1, the pretreated iron wire mesh is obtained by sequentially immersing the iron wire mesh in 0.5M hydrochloric acid solution, 1M sodium hydroxide solution, and water for 2h; then placing the iron wire mesh in a muffle furnace for heat treatment at 300-350℃ for 2h.

[0036] Beneficial effects: The above settings facilitate the pretreatment of the iron wire mesh to improve the performance of the prepared anode electrode. The applicant has found through long-term experiments that if the calcination temperature in the muffle furnace after immersion is too low, the surface of the iron wire mesh will not be clean, affecting the uniformity of the subsequent carbon coating and the long-term stability of the electrode; if the calcination temperature in the muffle furnace after immersion is too high, the iron wire mesh will deform, making it difficult to control the pore size and surface properties.

[0037] Preferably, as an improvement, in S2-2, the mass ratio of o-phenanthroline, terephthalic acid, and water in the ligand solution is 0.18-0.36:0.66:50.

[0038] Beneficial effects: The above settings facilitate the preparation of the anode electrode with excellent performance. The applicant has found through long-term experiments that if the amount of o-phenanthroline in the ligand solution is too small, iron elements will overflow, affecting the uniformity of the subsequent carbon coating; if the amount of o-phenanthroline is too large, the carbon layer will be too thick, resulting in low graphitization; if the amount of terephthalic acid in the ligand solution is too small, it cannot effectively coordinate, affecting the uniformity of the subsequent carbon coating; if the amount of terephthalic acid is too large, terephthalic acid will be lost seriously, increasing the cost.

[0039] Preferably, as an improvement, in S2-3, the hydrothermal reaction is carried out at 120-140℃ for 24-48h; the iron wire mesh is taken out, washed clean, and then placed in a muffle furnace, with argon or nitrogen as the protective gas, and heated to 850-900℃ at a rate of 3-5℃ / min, and held for 2-3h; after cooling to room temperature, the anode electrode material is obtained.

[0040] Beneficial effects: the scheme adopts the above settings, which is convenient for preparing anode electrode with excellent performance. The applicant finds through long-term experiments that if the hydrothermal reaction temperature is too high, the carbon wrapping will be uneven due to the fast growth of crystal nucleus, which affects the long-term stability of the electrode; if the hydrothermal reaction temperature is too low, it is difficult to form carbon materials due to the inability to stimulate coordination reaction; if the hydrothermal reaction time is too short, the carbon wrapping will be uneven due to incomplete coordination reaction, which affects the long-term stability of the electrode; if the hydrothermal reaction time is too long, the carbon layer is easy to separate. If the roasting in the muffle furnace after the hydrothermal reaction does not use protective gas, the carbon precursor will burn, and the carbon material cannot be formed; if the roasting temperature is too high or the temperature rises too fast, the graphitization of the carbon material is not high, and the wrapping is poor; if the roasting temperature is too low or the temperature rises too slowly, the crystal type of the carbon material is not high, and the production cost is increased; if the roasting holding time in the muffle furnace is too long, the carbon material loss is large, and the energy consumption is increased; if the roasting holding time is too short, the graphitization of the carbon material is not high due to insufficient crystal nucleus growth.

[0041] Preferably, as an improvement, the scheme also provides an Ag-g-C3N4 / TiO2 / Ti cathode electrode material, which is prepared by the above method.

[0042] Preferably, as an improvement, the scheme also provides a self-sustaining shale gas flow-back fluid in-situ repair system, which comprises a to-be-treated waste liquid system having a bottom mud layer and a water layer after standing, a cathode electrode is arranged in the water layer, an anode electrode is arranged in the bottom mud layer, the distance between the anode electrode and the contact interface of the water layer and the bottom mud layer is 2-10 cm; the anode electrode is the carbon-wrapped iron mesh prepared above; the cathode electrode is the Ag-g-C3N4 / TiO2 / Ti cathode electrode prepared above, when the cathode electrode is installed, one side of the Ag-g-C3N4 / TiO2 / Ti cathode electrode material faces upward, one side of the activated carbon fiber felt faces downward, and the cathode electrode is horizontally suspended 2-10 cm below the water surface; the anode electrode and the cathode electrode are connected through wires and connected with a storage battery for collecting electric energy. BRIEF DESCRIPTION OF DRAWINGS

[0043] Figure 1 It is a structural schematic diagram of the Ag-g-C3N4 / TiO2 / Ti cathode electrode in the embodiment of the application.

[0044] Figure 2 It is a structural schematic diagram of the self-sustaining shale gas flow-back fluid in-situ repair system in the embodiment of the application.

[0045] Figure 3 It is a curve graph of the removal rate of organic matter in the bottom mud under different experimental conditions

[0046] Figure 4 Figure 1 is a graph of TOC removal rate in overlying water under different experimental conditions.

[0047] Figure 5 Figure 2 is a graph of ammonia nitrogen removal rate in overlying water under different experimental conditions.

[0048] Figure 6 Figure 3 is a graph of ammonia nitrogen removal rate in overlying water under the operating conditions of Example 1, Example 2, Comparative Example 1, Comparative Example 4 and Comparative Example 5. DETAILED DESCRIPTION

[0049] The application will be further described in conjunction with the following examples, but the embodiments of the application are not limited thereto. If not specifically indicated, the technical means used in the following examples and experimental examples are the conventional means known to those skilled in the art, and the materials, reagents, etc. used can be obtained from commercial channels.

[0050] The reference signs in the drawings of the specification include: Ag-g-C3N4 / TiO2 / Ti cathode electrode material 1, cavity 2, polyethylene plastic foam 3, photosynthetic algal 4, activated carbon fiber felt 5, cathode electrode 6, water layer 7, water surface 8, wire 9, storage battery 10, interface 11, anode electrode 12, bottom sediment layer 13, sunlight 14.

[0051] General description of the scheme

[0052] The scheme provides a self-sustaining shale gas flowback fluid in-situ remediation method, which comprises the following steps:

[0053] Step one, the preparation method of Ag-g-C3N4 / TiO2 / Ti cathode electrode material, the steps are as follows:

[0054] S1-1: Preparation of solution: dissolve cetyltrimethylammonium chloride (CTAC) and NaOH in H2O, control the mass ratio of CTAC:NaOH:H2O to be (16-64):240:1000, and obtain a uniform mixed solution A; weigh 16.81-37.62 g of dicyanediamine and 0.1699-0.8494 g of AgNO3, add silver nitrate and dicyanediamine to 100 ml of H2O, and stir to obtain a mixed solution B.

[0055] S1-2: Preparation of cathode electrode material: Transfer mixed solution A to a polytetrafluoroethylene reactor and control the filling volume to 60-80%. Then, place the titanium mesh vertically in the middle of the reactor and hydrothermally react at 140-160°C for 24 hours. After cooling to room temperature, immerse it in a 1-2M HCl solution for 24 hours and rinse with deionized water. Then, place it in a muffle furnace and heat it to 500-550°C at a rate of 2-4°C / min, keep it warm for 2.5-4 hours, cool it to room temperature, and immerse the obtained sample in mixed solution B for 24-36 hours. Then, freeze-dry it. Then, wrap it with tin foil and calcine it at 520-540°C for 4 hours. By adjusting the composition of mixed solution B, Ag-g-C3N4 / TiO2 / Ti cathode electrode materials with different components were obtained.

[0056] This solution also provides an Ag-g-C3N4 / TiO2 / Ti cathode electrode material, which is prepared by the above method.

[0057] S1-3: Preparation of cathode electrode: Activated carbon fiber felt and Ag-g-C3N4 / TiO2 / Ti cathode electrode were placed in parallel, and polyethylene plastic foam was placed around the edge to form an internal cavity, which was filled with photosynthetic algae to obtain a cathode electrode;

[0058] Specifically, this solution also provides a Ag-g-C3N4 / TiO2 / Ti cathode electrode, such as Figure 1 As shown, it includes parallel placed activated carbon fiber felt 5 and Ag-g-C3N4 / TiO2 / Ti cathode electrode material 1 (prepared by the method in step 1 above), with a spacing of 2 to 3 cm to form a sandwich, and polyethylene plastic foam 3 is laid around the sandwich to form an internal cavity 2, and the cavity 2 is filled with photosynthetic algae 4.

[0059] This solution also provides a method for installing an Ag-g-C3N4 / TiO2 / Ti cathode electrode, comprising the following steps: When installing the cathode electrode, the Ag-g-C3N4 / TiO2 / Ti cathode electrode material 1 is positioned upward, and the activated carbon fiber felt 5 is positioned downward. By adjusting the width and thickness of the polyethylene foam 3, the cathode electrode is horizontally suspended at different depths below the water surface 8.

[0060] As a reference, when treating the flowback fluid in this scheme, the cathode electrode is horizontally suspended 2 to 10 cm below the water surface by adjusting the width and thickness of the interlayer polyethylene plastic foam to form a cathode electrode.

[0061] Step 2: Preparation of carbon-wrapped wire mesh anode electrode, the steps are as follows:

[0062] S2-1, pretreatment of iron wire mesh: the iron wire mesh was immersed in 0.5M HCl, 1M NaOH, H2O for 2h in turn. The iron wire mesh was placed in a muffle furnace and heat treated at 300-350℃ for 2h to obtain the pretreated iron wire mesh sample.

[0063] S2-2, preparation of ligand solution: o-phenanthroline and terephthalic acid were used as ligands, which were dissolved in 50mL H2O, 0.5mo / L HNO3 was added dropwise to adjust the pH to 5, and the mass ratio of o-phenanthroline: terephthalic acid: H2O was controlled at (0.18-0.36): 0.66: 50 to obtain the ligand solution.

[0064] S2-3, preparation of anode electrode: the ligand solution was transferred to a hydrothermal reactor, and the filling amount was controlled at 60-80%. Then, the pretreated iron wire mesh sample was placed in the reactor, and hydrothermal reaction was carried out at 120-140℃ for 24-48h. After the iron wire mesh was taken out and washed clean, it was placed in a muffle furnace, argon or nitrogen was used as protective gas, and the temperature was raised to 850-900℃ at a rate of 3-5℃ / min, and the temperature was kept for 2-3h. After cooling to room temperature, the anode electrode material was obtained.

[0065] The scheme also provides a carbon-coated iron wire mesh anode electrode, which is prepared by the above method.

[0066] Step three, system construction and operation

[0067] S3-1, installation of system: the scheme also provides a self-sustaining type shale gas flowback fluid in-situ remediation system, as shown in Figure 2 which includes a waste liquid system to be treated with a bottom mud layer 13 and a water layer 7 after standing, a cathode electrode 6 is arranged in the water layer 7, and an anode electrode 12 is arranged in the bottom mud layer 13. The anode electrode 12 is 2-10cm away from the interface 11 where the water layer 7 and the bottom mud layer 13 contact.

[0068] As a reference, the anode electrode 12 in the scheme is a carbon-coated iron wire mesh (prepared by the method in step two above). The cathode electrode 6 includes parallelly placed active carbon fiber felt 5 and Ag-g-C3N4 / TiO2 / Ti cathode electrode material (prepared by the method in step one above), with a spacing of 2-3cm to form a sandwich, and the periphery of the sandwich is paved with polyethylene plastic foam 3 to form an internal cavity 2, and the cavity 2 is filled with photosynthetic algal 4. When the cathode electrode 6 is installed, the Ag-g-C3N4 / TiO2 / Ti cathode electrode material is upward, and the active carbon fiber felt 5 is downward. In the scheme, the width and thickness of the sandwich polyethylene plastic foam 3 are adjusted to horizontally suspend the cathode electrode 6 at a position 2-10cm below the water surface 8 to form the cathode electrode 6.

[0069] S3-2, System running: the anode electrode 12 and the cathode electrode 6 are connected through the wire 9 and connected with the battery 10 for collecting electric energy. At this time, it is considered that the system has been successfully started, and samples can be collected from the overlying water and the bottom mud regularly to determine the repair efficiency of the water body and the bottom mud.

[0070] Example 1

[0071] Step one, preparation of Ag-g-C3N4 / TiO2 / Ti cathode electrode

[0072] 1.6 g of cetyltrimethylammonium chloride (CTAC) and 24 g of NaOH were dissolved in 100 ml of H2O to obtain a uniform mixture A; 16.81 g of dicyanediamine and 0.1699 g of AgNO3 were added to 100 ml of H2O, stirred to obtain mixture B. Mixture A was transferred to a polytetrafluoroethylene reaction kettle, and the filling amount was controlled to be 60%, then a 5 cm*5 cm titanium mesh was vertically placed in the middle position of the reaction kettle, and hydrothermal reaction was carried out at 160°C for 24 h. After cooling to room temperature, it was immersed in 1M HCl solution for 24 h and washed with deionized water. Then, it was placed in a muffle furnace and heated to 500°C at a rate of 3°C / min, and held for 2.5 h. After cooling to room temperature, the obtained sample was immersed in mixture B for 24 h, and then freeze-dried. Then, it was wrapped with tin foil paper and calcined at 520°C for 4 h to obtain the Ag-g-C3N4 / TiO2 / Ti cathode electrode material.

[0073] The prepared Ag-g-C3N4 / TiO2 / Ti electrode and the 5 cm*5 cm carbon felt were placed in parallel form with a spacing of 3 cm, and a polyethylene plastic foam with a width of 0.5 cm was laid around the sandwich to form an internal cavity. The photosynthetic algae were added to the internal cavity to prepare the cathode electrode.

[0074] Step two, preparation of carbon-coated iron mesh anode electrode

[0075] A 5 cm*5 cm iron mesh was sequentially immersed in 0.5M HCl, 1M NaOH, and H2O for 2 h. The iron mesh was placed in a muffle furnace and heat treated at 300°C for 2 h to obtain a pretreated iron mesh sample. 0.3412 g of o-phenanthroline and 0.66 g of terephthalic acid were dissolved in 50 mL of H2O, and 0.5 mo / L of HNO3 was added dropwise to adjust the pH to 5 to obtain a ligand solution. The ligand solution was transferred to a hydrothermal reaction kettle, and the filling amount was controlled to be 80%, then the above-mentioned iron mesh sample was placed in the reaction kettle, and hydrothermal reaction was carried out at 120°C for 24 h. After the iron mesh was taken out and washed clean, it was placed in a muffle furnace, argon or nitrogen was used as the protective gas, and the temperature was raised to 900°C at a rate of 5°C / min, and held for 2 h. After cooling to room temperature, the anode electrode was obtained.

[0076] Step three, system construction and operation

[0077] The prepared carbon-coated iron mesh was buried 5 cm below the interface between the sediment and water to form the anode electrode; the prepared Ag-g-C3N4 / TiO2 / Ti surface of the cathode electrode faced upward, the active carbon fiber felt faced downward, the width of the interlayer polyethylene plastic foam was adjusted, the cathode was suspended horizontally 2 cm below the water surface to form the cathode electrode; the anode electrode and the cathode electrode were connected to the storage battery through wires, that is, the system has been successfully started.

[0078] Example 2

[0079] Step one, preparation of Ag-g-C3N4 / TiO2 / Ti cathode electrode

[0080] 3.2 g of cetyltrimethylammonium chloride (CTAC) and 24 g of NaOH were dissolved in 100 ml of H2O to obtain a uniform mixture A; 36.62 g of dicyandiamide and 0.4696 g of AgNO3 were added to 100 ml of H2O, stirred to obtain mixture B. Mixture A was transferred to a polytetrafluoroethylene reaction kettle, and the filling amount was controlled to be 60%, then a 5 cm*5 cm titanium mesh was vertically placed in the middle position of the reaction kettle, and hydrothermal reaction was carried out at 160°C for 24 h. After cooling to room temperature, it was immersed in 1M HCl solution for 24 h and washed with deionized water. Then, it was placed in a muffle furnace, heated to 500°C at a rate of 3°C / min, and kept for 4 h. After cooling to room temperature, the obtained sample was immersed in mixture B for 24 h, and then freeze-dried. Then, it was wrapped with tin foil paper and calcined at 520°C for 4 h to obtain the Ag-g-C3N4 / TiO2 / Ti cathode electrode material.

[0081] The prepared Ag-g-C3N4 / TiO2 / Ti electrode and the 5 cm*5 cm carbon felt were placed in parallel form with a spacing of 3 cm, and the width of the polyethylene plastic foam around the interlayer was 0.5 cm, forming an internal cavity. The photosynthetic algae were added to the internal cavity to prepare the cathode electrode.

[0082] Step two, preparation of carbon-coated iron mesh anode electrode

[0083] The 5cm*5cm iron wire mesh was sequentially immersed in 0.5M HCl, 1M NaOH, H2O for 2h. The iron wire mesh was placed in a muffle furnace and heat treated at 300℃ for 2h to obtain a pretreated iron wire mesh sample. 0.2121g of o-phenanthroline and 0.66g of terephthalic acid were dissolved in 50mL of H2O, 0.5mo / L of HNO3 was added dropwise, and the pH was adjusted to 5 to obtain a ligand solution. The ligand solution was transferred to a hydrothermal reactor, and the filling amount was controlled to be 80%. Then, the above-mentioned iron wire mesh sample was placed in the reactor, and hydrothermal reaction was carried out at 120℃ for 24h. After the iron wire mesh was taken out and washed clean, it was placed in a muffle furnace, argon or nitrogen was used as a protective gas, and the temperature was raised to 900℃ at a rate of 5℃ / min, and the temperature was kept for 2h. After cooling to room temperature, an anode electrode was obtained.

[0084] Step three, system construction and operation

[0085] The prepared carbon-coated iron wire mesh was embedded horizontally at a depth of 5cm below the interface between the sediment and water to form an anode. The Ag-g-C3N4 / TiO2 / Ti surface of the prepared cathode electrode faced upwards, the active carbon fiber felt faced downwards, the width of the interlayer polyethylene plastic foam was adjusted, and the cathode was suspended horizontally at a depth of 2cm below the water surface to form a cathode. The anode electrode and the cathode electrode were connected to the storage battery through wires, and the system was successfully started.

[0086] Comparative Example 1

[0087] The preparation process of this comparative example 1 was similar to that of Example 1, except that in step one, carbon felt material was used as the cathode electrode material to construct the cathode electrode.

[0088] Comparative Example 2

[0089] The preparation process of this comparative example 2 was similar to that of Example 1, except that in step three, the cathode and the anode were not connected by wires and were in an open circuit state.

[0090] Comparative Example 3

[0091] The preparation process of this comparative example 3 was similar to that of Example 1, except that in step one, no photosynthetic algae was added to the cathode electrode.

[0092] Comparative Example 4

[0093] The preparation process of this comparative example 4 was similar to that of Example 1, except that in step one, the obtained sample was not treated in the mixed solution B when preparing the cathode electrode material, and the obtained electrode material was a TiO2 / Ti cathode electrode material.

[0094] Comparative Example 5

[0095] The preparation process of the comparative example 5 refers to the example 1, except that in step one, when preparing the cathode electrode material, the mixed solution B is not added with AgNO3, and the obtained electrode material is g-C3N4 / TiO2 / Ti cathode electrode material.

[0096] After a running time, samples are periodically collected from the overlying water and the sediment to determine the remediation efficiency of the water body and the sediment, and the experimental results are shown in Figure 3 、 Figure 4 and Figure 5 .

[0097] The experiment of the comparative example 1 shows that there are a large number of indigenous microorganisms in the sediment, which can spontaneously degrade the organic matter in the sediment, but due to the limitation of the cathode reaction, it is difficult to supply a large number of generated electrons and grow in a poor environment, resulting in low removal performance of the organic matter in the sediment. After 60 days of operation, the removal rate is only 3.8%. The experiment of the comparative example 2 shows that the photoelectrocatalysis is placed in the overlying water body, and the removal rate of the organic matter in the sediment is only 2.1%. It is confirmed that relying on microbial metabolism alone cannot achieve efficient removal of organic matter in the sediment. In the example 1 and the example 2 of the present scheme, the removal rates of the organic matter in the sediment are as high as 28.7 and 25.9% respectively, and the treatment efficiency is much higher than that of the traditional microbial fuel cell and photocatalysis, which shows that the system couples the photoelectrocatalysis with the sediment microbial fuel cell, can transfer the electrons generated by the anode of the sediment microbial fuel cell to the cathode, promote the metabolism of the microorganisms, and strengthen the remediation performance of the sediment. In addition, when no photosynthetic algae is added to the cathode electrode (such as the comparative example 3), the removal rate of the organic matter in the sediment is 16.8%, which shows that the photosynthesis of the algae produces oxygen, increases the oxygen content of the overlying water body, promotes the photoelectrocatalysis to generate more active groups, improves the efficiency of the cathode reaction, and thus effectively improves the treatment capacity of the anode for the organic pollutants.

[0098] The experimental results confirm that after a running time, the water quality of the overlying water body is obviously improved. The experimental results of the removal of ammonia nitrogen in the overlying water body are shown in Figure 4The experimental results of Comparative Example 1 show that the cathode reduction reaction of the sediment-type microbial fuel cell has a weak effect on ammonia nitrogen removal in the overlying water body, and the removal rate is only 26.3% after 60 days of operation. The experimental results of Comparative Example 2 show that the active free radicals generated by photocatalysis have the function of removing ammonia nitrogen, but the performance is not high, and the removal rate is only 20.1%. In Examples 1 and 2, the ammonia nitrogen removal rate in the overlying water body is as high as 95.6% or more, and the treatment efficiency is much higher than that of traditional microbial fuel cells and photocatalysis. Similarly, when no photosynthetic algae are added to the cathode electrode (Comparative Example 3), the ammonia nitrogen removal rate in the overlying water body is only 78.8%, which confirms that the photosynthesis of algae increases the oxygen content in the overlying water body, promotes the generation of more active groups by photoelectrocatalysis, and thus effectively improves the processing capacity of the cathode photoelectrocatalysis. In addition, Figure 5 The experimental results of the overlying water TOC removal. The results further confirm that the coupling of photoelectrocatalysis and sediment-type microbial fuel cells in the system can transfer the electrons generated by the anode of the sediment-type microbial fuel cell to the cathode, overcome the shortcomings of easy recombination of photo-generated charges and low catalytic efficiency of the cathode photoelectrocatalytic material, improve the degradation performance of the overlying water pollutants, and at the same time, the oxygen production characteristics of biological algae photosynthesis can effectively promote the reaction efficiency of the cathode photoelectrode, generate more free radicals, and promote the purification speed of the overlying water.

[0099] In addition, Figure 6The ammonia nitrogen removal rate curve of the overlying water body under the operating conditions of Example 1, Example 2, Comparative Example 1, Comparative Example 4 and Comparative Example 5 is drawn. The experimental results of Comparative Example 1 show that when the carbon felt material without photocatalytic function is used as the cathode electrode material, the ammonia nitrogen in the overlying water body has a weak removal effect, which should be attributed to the cathode reduction reaction of the microbial fuel cell. The results prove that the photoelectrocatalysis plays an important role in the coupling system. The experimental results of Comparative Example 4 show that when the TiO2 / Ti electrode material is used as the cathode electrode material, the ammonia nitrogen removal performance of the overlying water body is obviously improved, but the removal rate is only 28.5%. This is due to the wide band gap of TiO2, and the utilization efficiency of sunlight is limited. When the g-C3N4 / TiO2 / Ti electrode material is used as the cathode electrode material (Comparative Example 5), the ammonia nitrogen removal performance of the overlying water body is further enhanced, and the removal rate reaches 76.3%. This is because the composite of g-C3N4 and TiO2 can significantly improve the charge transfer and the absorption capacity of visible light, and improve the photoelectrocatalytic performance. In addition, it can be obviously found that with the prolongation of the running time, the ammonia nitrogen removal performance of the overlying water body in Comparative Example 4 and Comparative Example 5 shows a trend of first increasing and then decreasing, which is due to a large number of microorganisms parasitizing on the electrode surface, reducing the utilization efficiency of sunlight. In the present scheme, the ammonia nitrogen removal performance of the overlying water body in Example 1 and Example 2 remains high, and there is no obvious decreasing trend. Therefore, the Ag-g-C3N4 / TiO2 / Ti cathode electrode material developed in the present application has antibacterial performance, can effectively inhibit the growth of microorganisms on the surface of the cathode electrode, makes it maximize the utilization of clean and cheap sunlight, and guarantees the overlying water body repair performance and the long-term running stability.

[0100] The above is only an embodiment of the present application, and the specific technical solutions and / or common knowledge of the scheme are not described in detail. It should be noted that for those skilled in the art, without departing from the technical solutions of the present application, a number of modifications and improvements can be made, which should also be considered as the protection scope of the present application, and these will not affect the effect and practicality of the patent. The protection scope claimed in the present application should be subject to the content of its claims, and the specific embodiments in the specification can be used to explain the content of the claims.

Claims

1. A self-sustaining in-situ remediation method for shale gas flowback fluid, characterized by: The steps include: Step 1: Preparation of Ag-g-C3N4 / TiO2 / Ti cathode electrode S1-1: Prepare a solution: Disperse cetyltrimethylammonium chloride and sodium hydroxide in water to obtain a mixed solution A; the mass ratio of cetyltrimethylammonium chloride, sodium hydroxide, and water in the mixed solution A is 16-64:240:1000; disperse silver nitrate and dicyandiamide in water to obtain a mixed solution B; the mass ratio of dicyandiamide, silver nitrate, and water in the mixed solution B is 16.81-37.62:0.1699-0.8494:100; S1-2: Preparation of cathode electrode material: first, place the titanium mesh in mixed solution A, hydrothermally react at 140-160°C for 24 hours, cool, immerse in 1-2 M hydrochloric acid solution for 24 hours, and rinse with deionized water; then, calcine, cool, immerse in mixed solution B for 24-36 hours, and freeze-dry; finally, wrap with tin foil and calcine to obtain Ag-g-C3N4 / TiO2 / Ti cathode electrode material; the calcination is to increase the temperature to 500-550°C at a rate of 2-4°C / min and keep warm for 2.5-4 hours; the calcination is to calcine at 520-540°C for 4 hours; S1-3: Preparation of cathode electrode: Activated carbon fiber felt and Ag-g-C3N4 / TiO2 / Ti cathode electrode material were placed in parallel with a spacing of 2-3 cm, and polyethylene plastic foam was placed around the edge to form an internal cavity, which was filled with photosynthetic algae to obtain a cathode electrode; Step 2: Preparation of carbon-wrapped wire mesh anode electrode S2-1, pre-treatment of wire mesh; S2-2. Prepare a ligand solution: disperse o-phenanthroline and terephthalic acid in water, and adjust the pH to obtain a ligand solution; The mass ratio of o-phenanthroline, terephthalic acid and water in the ligand solution is 0.18-0.36:0.66:50; S2-3. Preparation of anode electrode: subjecting the pretreated wire mesh sample to a hydrothermal reaction in a ligand solution, removing the wire mesh, rinsing it, and then calcining it under a protective gas atmosphere. After cooling, the anode electrode is obtained. Step 3: System construction and operation S3-1. Installation system: In a waste liquid system to be treated having a bottom mud layer and a water layer after standing, the anode electrode is installed in the bottom mud layer, and the cathode electrode is installed in the water layer. When the cathode electrode is installed, the Ag-g-C3N4 / TiO2 / Ti electrode side faces upward and the activated carbon fiber felt side faces downward; S3-2. System operation: The anode electrode and the cathode electrode are connected to each other through a wire and connected to a battery; samples are collected from the overlying water and bottom sediment regularly to measure the restoration efficiency of the water body and bottom sediment.

2. The method for in-situ remediation of self-sustaining shale gas flowback fluid according to claim 1, characterized in that: In S2-1, the pretreatment of the wire mesh includes immersing the wire mesh in 0.5M hydrochloric acid solution, 1M sodium hydroxide solution, and water for 2 hours in sequence; then placing the wire mesh in a muffle furnace and heat treating it at 300-350°C for 2 hours to obtain a pretreated wire mesh sample.

3. The method for in-situ remediation of self-sustaining shale gas flowback fluid according to claim 1, characterized in that: In S2-3, the hydrothermal reaction is carried out at 120-140 ° C for 24-48 hours. The wire mesh is removed and rinsed clean, and then placed in a muffle furnace. Argon or nitrogen is used as a protective gas, and the temperature is increased to 850-900 ° C at a rate of 3-5 ° C / min. The temperature is kept at this temperature for 2-3 hours, and the anode electrode material is obtained after cooling to room temperature.

4. A self-sustaining shale gas flowback fluid in-situ remediation system, characterized by: The invention comprises a waste liquid system to be treated having a bottom mud layer and a water layer after standing, wherein a cathode electrode is provided in the water layer, an anode electrode is provided in the bottom mud layer, and the anode electrode is 2 to 10 cm away from the contact interface between the water layer and the bottom mud layer; the anode electrode is a carbon-coated wire mesh prepared by the method according to any one of claims 1 to 3; and the cathode electrode is an Ag-g-C3N4 / TiO2 / Ti cathode electrode prepared by the method according to claim 1; When the cathode electrode is installed, the Ag-g-C3N4 / TiO2 / Ti cathode electrode material side faces upward, the activated carbon fiber felt side faces downward, and the cathode electrode is horizontally suspended 2 to 10 cm below the water surface; the anode electrode and the cathode electrode are connected by a wire and connected to a battery for collecting electrical energy.

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