An apparatus and method for electrochemically reducing ferric iron to drive iron autotrophic denitrifying microorganisms to reduce nitrate

Abstract

The application discloses a device and method for reducing nitrate by electrochemical reduction of ferric iron to drive iron autotrophic denitrification microorganism to reduce nitrate. The device comprises a cathode pool and an anode pool, which are separated by a cation exchange membrane; the cathode pool is provided with a cathode electrode, a reference electrode and a cathode liquid, the cathode electrode comprises an electrode base and ferrous iron loaded on the electrode base; the anode pool is provided with an anode electrode and an anode liquid; the cathode electrode and the anode electrode form a loop through an external power supply. Iron autotrophic denitrification microorganism is inoculated in the cathode pool, then the wastewater containing nitrate to be treated is introduced, the iron autotrophic denitrification microorganism obtains the electron of ferrous ion to reduce the nitrate into nitrogen, at the same time, ferric ion is produced, the ferric ion obtains the electron from the electrode to be reduced into ferrous ion, and enters a new reaction cycle, so that the iron autotrophic denitrification and the in-situ reduction of ferric ion product can be realized in one reaction device.

Description

Technical Field

[0001] This invention relates to an apparatus and method for reducing nitrates by electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms, belonging to the field of microbial electrochemical technology. Background Technology

[0002] In recent years, human activities have led to the discharge of large amounts of nitrates into natural water bodies, causing a series of environmental problems such as eutrophication. A significant source of nitrates is the effluent from wastewater treatment plants, making it essential to add advanced treatment units for secondary effluent in wastewater treatment plants. Conventional denitrification methods in wastewater treatment plants require the addition of organic carbon sources to the water, resulting in additional carbon emissions and the risk of secondary pollution. Researchers are seeking a method to deeply remove nitrates without consuming organic carbon sources. One strategy is an inorganic electron donor denitrification system, which uses autotrophic microorganisms with ferrous iron as an electron donor to utilize CO2. 2、 HCO3 - and CO3 2- Using a carbon source, nitrates are reduced to nitrogen gas. This achieves deep nitrate removal while eliminating the need for organic carbon sources, reducing resource consumption and greenhouse gas emissions.

[0003] To date, using ferrous iron (Fe2+) as an electron donor for denitrification has been hampered by the need to consume large amounts of iron salts and generate substantial amounts of iron-containing sludge, impacting economic efficiency and hindering practical application. Some studies have employed activated sludge reactors for iron-autotrophic denitrification, but with the continuous addition of ferrous substrate, ferric iron products accumulate and deposit on the surface of the iron-autotrophic denitrifying microbial cells, causing microbial mineralization and severely inhibiting their reactivity. Simultaneously, the increasing amount of iron sludge severely dilutes the system's biomass. This design achieves short-term deep nitrate removal but struggles to maintain stability. Other researchers have maintained the long-term operation of iron-autotrophic denitrification systems by periodically replenishing microorganisms, but this still requires continuous addition of ferrous substrate and generates large amounts of ferric iron sludge requiring subsequent treatment. None of these strategies can achieve stable nitrate removal without the addition of large amounts of chemicals, and they suffer from high resource consumption, complex operation, non-renewable substrates, and the need for further product treatment. Designing an iron-autotrophic denitrifying microbial system for advanced secondary effluent treatment that can regenerate ferric iron in situ into ferrous iron and drive the continuous denitrification reaction is of great significance.

[0004] In some studies, electrochemical reduction systems have been shown to reduce ferric iron to ferrous iron, but these studies all belong to the field of advanced oxidation and have not been applied in microbial systems.

[0005] Patent CN114573102A reports an iron-based autotrophic denitrification device for wastewater denitrification, in which nitrate-containing wastewater is passed through an iron-based carrier and removed through iron-based autotrophic denitrification. However, the iron-based carrier in this method is continuously consumed and cannot operate for long periods, requiring the constant addition of new iron carrier, which leads to significant resource consumption.

[0006] Patent CN115536151A reports a short-cut nitrification / anaerobic ammonium oxidation-ferrotrophic denitrification device for nitrogen and phosphorus removal. In this device, the ferrotrophic denitrification unit continuously receives overflow from the pretreatment unit to replenish microorganisms, reducing the problem of microbial mineralization in ferrotrophic denitrification. However, this method places high demands on the pretreatment unit, requiring continuous replenishment of microorganisms for the ferrotrophic denitrification unit. Furthermore, this method continuously consumes low-cost iron substrates, resulting in significant resource consumption.

[0007] Patent CN215712427U reports a deep denitrification and phosphorus removal coupled disinfection system for wastewater treatment plant effluent, which achieves deep removal of nitrates from wastewater treatment plant effluent through a sulfur-iron autotrophic denitrification filter. This method combines sulfur-autotrophic denitrifying microorganisms and iron-autotrophic denitrifying microorganisms, reducing the consumption of low-valence iron substrates to some extent. However, iron substrates will still be depleted and require continuous addition. Furthermore, the introduction of sulfur-autotrophic denitrification introduces sulfate ions into the effluent, posing a risk of secondary pollution.

[0008] Patent CN116462381A reports a highly efficient electro-Fenton method for treating high-salt organic wastewater, involving electro-Fenton synergy. In this method, ferrous ions are oxidized to ferric ions (Fe3+) and subsequently reduced back to ferrous ions at the cathode. This method achieves in-situ electrochemical reduction of ferric ions. However, this process is only applied to advanced oxidation processes, requiring harsh environmental conditions and potentially unsuitable for microbial environments. Whether similar electroreduction methods are applicable to substrate regeneration processes in ferroautotrophic denitrification has not yet been studied or reported. Summary of the Invention

[0009] The purpose of this invention is to provide an apparatus and method for reducing nitrates by electrochemically reducing ferric iron-driven ferroautotrophic denitrifying microorganisms, which solves the problems of non-renewable ferrous iron substrates and unsustainable systems in existing ferroautotrophic denitrifying microbial systems.

[0010] In a first aspect, the present invention provides an apparatus for the electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms to reduce nitrate, comprising a cathode pool and an anode pool separated by a cation exchange membrane;

[0011] The cathode cell is provided with a cathode electrode, a reference electrode and a catholy liquid. The cathode electrode includes an electrode substrate and ferrous iron loaded on the electrode substrate.

[0012] The anode tank is equipped with an anode electrode and an anolyte;

[0013] The cathode electrode and the anode electrode form a circuit through an external power source.

[0014] Based on the above technical solution, the device for reducing nitrate by electrochemical reduction of ferric iron-driven ferrotrophic denitrifying microorganisms involves inoculating ferrotrophic denitrifying microorganisms in the cathode tank during operation, followed by the introduction of nitrate-containing wastewater. The ferrotrophic denitrifying microorganisms obtain electrons from the ferrous iron loaded on the cathode, reducing nitrate ions to nitrogen gas and producing hydroxide ions. Simultaneously, the ferrous iron loses electrons and transforms into ferric iron, while simultaneously obtaining electrons from the cathode electrode surface and being regenerated in situ as ferrous iron, thus entering the next reaction cycle. Electrolysis of water at the anode transfers electrons from the external circuit to the cathode, generating hydrogen ions and oxygen. The hydrogen ions are transferred to the cathode through the cation exchange membrane to neutralize the hydroxide ions generated during the denitrification process, achieving self-stabilization of the system pH. This reduces operational complexity while ensuring continuous and stable removal of nitrates.

[0015] In the above-mentioned device for electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms to reduce nitrate, the electrode substrate can be a carbon felt electrode or a composite electrode containing carbon felt, such as a titanium mesh-carbon rod-carbon felt composite electrode.

[0016] The method for preparing the cathode electrode includes: immersing the electrode substrate in a ferrous salt solution, then adjusting the pH of the solution to 6.0–7.0 (e.g., 7.0), allowing it to stand, and draining the remaining solution; preferably, the concentration of the ferrous salt solution is 3.0–3.5 g / L (e.g., 3.5 g / L); preferably, the standing time is 24 h. Specifically, the ferrous salt solution is a FeSO4·7H2O solution. Specifically, the pH of the solution is adjusted by adding 1 mol / L NaOH solution.

[0017] This invention obtains a cathode electrode by loading ferrous ions onto the surface of a carbon felt electrode (a carbon felt electrode modified with iron deposition). Iron-autotrophic denitrifying microorganisms gain electrons from ferrous ions to reduce nitrates to nitrogen gas, while simultaneously generating ferric ions. The ferric ions gain electrons from the carbon felt electrode and are reduced back to ferrous ions, entering a new reaction cycle. This allows for in-situ regeneration of electron donors under conditions free of organic matter, which is an important factor in ensuring the continuous and stable denitrification of iron-autotrophic denitrification reactors. In one reactor, iron-autotrophic denitrification and in-situ reduction of ferric products are achieved.

[0018] In the above-mentioned device for the electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms to reduce nitrate, the reference electrode is an Ag / AgCl electrode;

[0019] The anode electrode is a carbon electrode.

[0020] In the above-mentioned device for the electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms to reduce nitrate, the cation exchange membrane is a proton exchange membrane, preferably fumasep FKS-PET-130.

[0021] In the aforementioned device for the electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms to reduce nitrate, the composition of the cathode liquid is as follows: 1.0–3.0 g / L NaHCO3, 0.3–0.6 g / L MgSO4·7H2O, 0.01–0.05 g / L CaCl2·2H2O, 0.2–0.4 g / L KH2PO4, 0.08–0.10 g / L NaNO3, 1 mL / L trace element solution, and water as the solvent;

[0022] Preferably, the trace element solution has the following composition: 2.0–4.0 g / L EDTA, 0.05–0.10 g / L ZnCl2, 0.005–0.020 g / L H3BO3, 0.010–0.020 g / L CoCl6·H2O, 0.3–0.6 g / L MnCl2·2H2O, 0.02–0.03 g / L NiCl3·6H2O, 0.02–0.04 g / L Na2MoO4·2H2O, with water as the solvent.

[0023] As an example, the composition of the catholyte is as follows: 2.5 g / L NaHCO3, 0.5 g / L MgSO4·7H2O, 0.01 g / L CaCl2·2H2O, 0.25 g / L KH2PO4, 0.091 g / L NaNO3, and 1 mL / L trace element solution, with water as the solvent; the composition of the trace element solution is as follows: 3.0 g / L EDTA, 0.07 g / L ZnCl2, 0.01 g / L H3BO3, 0.019 g / L CoCl6·H2O, 0.5 g / L LnCl2·2H2O, 0.024 g / L NiCl3·6H2O, and 0.036 g / L Na2MoO4·2H2O, with water as the solvent.

[0024] In the aforementioned device for the electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms to reduce nitrate, the composition of the anolyte is as follows: 1.0–3.0 g / L NaHCO3, 0.3–0.6 g / L MgSO4·7H2O, 0.01–0.05 g / L CaCl2·2H2O, 0.2–0.4 g / L KH2PO4, with water as the solvent, and the pH of the anolyte is adjusted to 4–6 using HCl.

[0025] As an example, the composition of the anolyte is as follows: 2.5 g / L NaHCO3, 0.5 g / L MgSO4·7H2O, 0.01 g / L CaCl2·2H2O, 0.25 g / L KH2PO4, with water as the solvent; the pH of the anolyte is adjusted to 6 using 1 mol / L HCl.

[0026] In the above-mentioned device for the electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms to reduce nitrate, the device is an anaerobic microbial reaction device;

[0027] A cathode gas collection pipe is provided above the cathode pool. The cathode gas collection pipe is equipped with a one-way gas valve to discharge the gas generated during the denitrification process and prevent the backflow of external oxygen, thus ensuring an anaerobic environment.

[0028] An anode gas collection pipe is provided above the anode pool. The anode gas collection pipe is equipped with a one-way gas valve to discharge the gas generated by water electrolysis and prevent the backflow of external oxygen, thus ensuring an anaerobic environment.

[0029] In the aforementioned electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms for nitrate reduction, the cathode tank is equipped with an inlet pipe and an outlet pipe. The inlet is located at the bottom of the cathode tank, and the outlet is located at the top of the cathode tank to achieve continuous water intake. During operation, water in the anode tank loses electrons and is oxidized into oxygen and hydrogen ions. The lost electrons are conducted to the cathode through an external circuit, while hydrogen ions enter the cathode tank from the anode tank through the cation exchange membrane. Nitrate wastewater enters the cathode tank from the bottom through the inlet and flows upward. As it flows over the cathode surface, it reacts with the ferroautotrophic denitrifying microorganisms on the cathode surface. The ferroautotrophic denitrifying microorganisms gain electrons from the ferrous iron loaded on the cathode and denitrify nitrate ions into nitrogen gas. The effluent, after deep nitrate removal, is discharged from the outlet at the top of the cathode tank.

[0030] Secondly, the present invention provides a method for the electrochemical reduction of nitrate by ferric-driven ferroautotrophic denitrifying microorganisms, employing the apparatus described in any of the above-mentioned embodiments, comprising the following steps:

[0031] S1. Inoculate iron-autotrophic denitrifying bacteria into the cathode pool;

[0032] S2. Connect the cathode electrode, the reference electrode, and the anode electrode to the electrochemical workstation respectively;

[0033] S3. Introduce the nitrate-containing wastewater to be treated into the cathode cell and turn on the electrochemical workstation;

[0034] In the cathode pool, the iron-autotrophic denitrifying bacteria reduce nitrate to produce nitrogen gas, consume the ferrous iron on the surface of the cathode electrode to produce ferric iron, and at the same time, the ferric iron obtains electrons from the surface of the cathode electrode and is regenerated in situ as ferrous iron.

[0035] In the anode pool, water undergoes electrolysis, transferring electrons from the external circuit to the cathode electrode and generating hydrogen ions and oxygen. The hydrogen ions neutralize the hydroxide ions generated by denitrification in the cathode pool through the cation membrane.

[0036] In the above-mentioned method for reducing nitrate by electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms, the ferroautotrophic denitrifying bacteria are anaerobic sludge from a wastewater treatment plant that has been domesticated with ferrous salts.

[0037] Inoculate 150-200 mL of the iron-autotrophic denitrifying bacteria into each 1 L of the catholy solution, such as inoculating 190 mL of the iron-autotrophic denitrifying bacteria into each 1 L of the catholy solution.

[0038] In the above-mentioned method for reducing nitrate by electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms, in step S3, the working potential of the electrochemical workstation is set to -0.6 to -0.8V relative to the standard hydrogen electrode, such as -0.7V.

[0039] The nitrate-containing wastewater is fed continuously with a hydraulic retention time of 48–36 hours (e.g., 41.7 hours).

[0040] The anolyte does not need to be replaced; pH adjustment is only required during the initial setup.

[0041] In step S3, the temperature of the cathode cell is controlled to be 33℃~37℃, such as 35℃.

[0042] The present invention has the following beneficial effects:

[0043] Compared with the prior art, the present invention has the following beneficial technical effects:

[0044] (1) Microorganisms can reduce nitrates by obtaining electrons from ferrous iron loaded on the cathode, and the product is nitrogen gas, which can effectively remove total nitrogen from wastewater;

[0045] (2) By adding a cation exchange membrane between the cathode tank and the anode tank, the hydrogen ions generated in the anode tank can enter the cathode tank through the cation exchange membrane and neutralize the hydroxide ions generated in the denitrification process of the cathode tank, thus achieving self-stabilization of the system pH without the need for manual adjustment and simplifying the operation process.

[0046] (3) The presence of the cation exchange membrane prevents oxygen from entering the cathode from the anode, maintains the anaerobic environment of the cathode, and ensures the continuous and stable progress of the iron autotrophic denitrification process.

[0047] (4) An iron autotrophic denitrification reaction occurs on the cathode. The autotrophic microorganisms do not require organic carbon sources, thereby reducing carbon emissions during the nitrate removal process and eliminating the risk of secondary pollution caused by organic matter.

[0048] (5) The ferrous iron loaded on the cathode can directly obtain electrons from the cathode and be reduced to ferrous iron after being utilized by iron autotrophic denitrifying microorganisms. This realizes the in-situ recycling and regeneration of the substrate, without the need to continuously add ferrous salts to the system, and without generating a large amount of residual iron-containing sludge, thus reducing resource consumption. Attached Figure Description

[0049] Figure 1 This is a schematic diagram of the device for reducing nitrates by electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms according to the present invention.

[0050] Figure 2 A schematic diagram of a secondary effluent advanced treatment process in a wastewater treatment plant using an electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms to reduce nitrates.

[0051] Figure 1 and Figure 2 The markings in the text are as follows:

[0052] 100-Cathode cell; 110-Cathode electrode; 120-Reference electrode; 130-Cathode gas collection pipe; 140-Cathode cell one-way gas valve; 150-Cathode cell inlet; 160-Cathode cell outlet;

[0053] 111-Titanium mesh; 112-Carbon rod; 113-Carbon felt;

[0054] 200 - Anode tank; 210 - Anode electrode; 220 - Anode tank exhaust pipe; 230 - Anode tank liquid replacement pipe;

[0055] 300-cation exchange membrane;

[0056] 400 - Reference electrode terminal, 410 - Working electrode terminal, 420 - Auxiliary electrode terminal.

[0057] Figure 3 This is a diagram showing the nitrate concentrations of the influent and effluent of the cathode pool during operation of the device of the present invention, when the iron-autotrophic denitrifying microorganisms act alone, when the electrochemical device acts alone, and during the operation of the device for reducing nitrates by iron-autotrophic denitrifying microorganisms driven by electrochemical reduction of ferric iron. Detailed Implementation

[0058] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0059] In the description of this invention, it should be noted that the terms "upper," "lower," etc., indicating the orientation or positional relationship are based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the system or component referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, the use of terms such as "first," "second," etc., to define components is merely for the convenience of distinguishing the aforementioned components; unless otherwise stated, these terms have no special meaning and should not be construed as indicating or implying relative importance.

[0060] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "assembly," "setup," and "connection" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0061] The following is a detailed description of the electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms for nitrate reduction provided in the embodiments of the present invention, with reference to the accompanying drawings.

[0062] like Figure 1 As shown, the device for reducing nitrate by electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms according to the present invention includes a cathode tank 100 and an anode tank 200, separated by a cation exchange membrane 300.

[0063] The cathode tank 100 is equipped with a cathode electrode 110, a reference electrode 120, and a catholyte (not shown in the figure). The cathode electrode 110 includes an electrode substrate and ferrous iron loaded on the electrode substrate. On the one hand, microorganisms can obtain electrons from the ferrous iron loaded on the cathode electrode 110 to reduce nitrates, and the product is nitrogen gas, which can effectively remove total nitrogen from wastewater. On the other hand, the ferrous iron loaded on the cathode electrode 110, after being utilized by iron-autotrophic denitrifying microorganisms, can generate ferric iron, which can directly obtain electrons from the cathode in situ and be reduced back to ferrous iron. This method achieves in-situ recycling of the substrate, eliminating the need for continuous addition of ferrous salts to the system and preventing the generation of large amounts of residual iron-containing sludge, thus reducing resource consumption. The cathode electrode 110 has a carbon felt electrode or a composite electrode containing carbon felt as its electrode substrate. On one hand, the carbon felt electrode has good conductivity and abundant porosity, resulting in a large specific surface area that provides numerous attachment sites for ferrous iron. On the other hand, the porous structure facilitates the subsequent bonding of the electrode with iron-autotrophic denitrifying microorganisms, and the loose and porous carbon felt can serve as a growth substrate for microorganisms, accommodating more biomass. As an example, the carbon felt electrode is a titanium mesh-carbon rod-carbon felt composite electrode, where the titanium mesh 111 and carbon rod 112 support the carbon felt 113. The titanium mesh 111 can also be made of other materials with good conductivity, corrosion resistance, acid and alkali resistance, and oxidation resistance. For example, the titanium mesh-carbon rod-carbon felt composite electrode includes a titanium mesh 111 on the upper surface and a carbon felt 113 on the lower surface. Multiple carbon rods 112 are vertically arranged between the titanium mesh 111 and the carbon felt 113, and several layers of parallel vertically arranged carbon felt are arranged between the titanium mesh 111 and the carbon felt 113. The size of the titanium mesh-carbon rod-carbon felt composite electrode is not limited and can be adjusted according to the size of the deep denitrification unit in actual engineering. The cathode electrode 110 can be obtained by immersing the electrode substrate in a 3.0–3.5 g / L ferrous salt solution, then adding 1 mol / L NaOH solution to adjust the pH to 6.0–7.0, allowing it to stand for 24 hours, and then draining the remaining solution. In this process, ferrous iron (Fe2+) undergoes a dissolution-precipitation equilibrium in water. At lower pH levels, a large amount of solid ferrous iron compounds dissolve to form ferrous ions. At higher pH levels, it remains stably present in solid form on the surface of the carbon felt electrode. This invention ensures a stable loading of ferrous iron by adjusting the solution pH to 6.0–7.0. For example, the ferrous salt solution is FeSO4·7H2O solution. The reference electrode 120 is an Ag / AgCl electrode.The catholy solution comprises: 1.0–3.0 g / L NaHCO3, 0.3–0.6 g / L MgSO4·7H2O, 0.01–0.05 g / L CaCl2·2H2O, 0.2–0.4 g / L KH2PO4, 0.08–0.10 g / L NaNO3, and 1 mL / L trace element solution in water. The trace element solution comprises: 2.0–4.0 g / L EDTA, 0.05–0.10 g / L ZnCl2, 0.005–0.020 g / L H3BO3, 0.010–0.020 g / L CoCl6·H2O, 0.3–0.6 g / L MnCl2·2H2O, 0.02–0.03 g / L NiCl3·6H2O, and 0.02–0.04 g / L... Na₂MoO₄·2H₂O, with water as the solvent. The composition of the catholyte can be rationally adjusted according to the nitrate concentration in the nitrate-containing wastewater to be treated and the elements required for the growth of ferroautotrophic denitrifying bacteria. A cathode gas collection pipe 130 is installed above the cathode tank, and a one-way gas valve 140 is installed on the cathode gas collection pipe 130 to discharge the gas generated during the denitrification process and prevent backflow of external oxygen, ensuring an anaerobic environment. Ferroautotrophic denitrification occurs at the cathode. The autotrophic microorganisms do not require organic carbon sources, thereby reducing carbon emissions during the nitrate removal process and eliminating the risk of secondary pollution caused by organic matter. In addition, the cathode tank 100 is equipped with a cathode tank inlet 150 and a cathode tank outlet 160. The cathode tank inlet 150 is at the bottom of the cathode tank 100, and the cathode tank outlet 160 is at the top of the cathode tank 100 to achieve continuous water intake. During operation, nitrate wastewater enters the cathode tank 100 from the bottom of the cathode tank through the cathode tank inlet 150 and flows upward. When it flows over the surface of the cathode electrode 110, it reacts with the iron autotrophic denitrifying microorganisms on the cathode surface. The iron autotrophic denitrifying microorganisms obtain electrons from the ferrous iron loaded on the cathode and denitrify nitrate into nitrogen gas. The effluent after deep nitrate removal is discharged from the cathode tank outlet 160 at the top of the cathode tank.

[0064] The anode tank 200 contains an anode electrode 210 and an anolyte (not shown in the figure); the anode electrode 210 is a carbon electrode. An anode tank exhaust pipe 220 is located above the anode tank, and a one-way valve (not shown in the figure) is installed on the exhaust pipe to discharge the gas generated during water electrolysis and prevent backflow of external oxygen, ensuring an anaerobic environment. The composition of the anolyte 202 is as follows: 1.0–3.0 g / L NaHCO3, 0.3–0.6 g / L MgSO4·7H2O, 0.01–0.05 g / L CaCl2·2H2O, 0.2–0.4 g / L KH2PO4, with water as the solvent. The pH of the anolyte is adjusted to 4–6 using HCl. The composition of the anolyte can be adaptively adjusted according to the composition (ionic strength) of the catholyte to maintain osmotic pressure balance between the cathode and anode chambers on both sides of the proton exchange membrane. For example, to simplify the system construction process, the anolyte uses a composition essentially the same as the catholyte, eliminating the need for additional anolyte composition design while maintaining basic osmotic pressure balance. The cathode electrode 110 and anode electrode 210 form a circuit via an external power supply. During operation, water in the anode pool 200 loses electrons and is oxidized into oxygen and hydrogen ions. The lost electrons are conducted to the cathode through the external circuit, while the hydrogen ions enter the cathode pool 100 from the anode pool 200 through the cation exchange membrane 300. Optionally, an anode pool exchange pipe 230 is provided on the side wall of the anode pool 200 to handle unexpected situations such as anolyte leakage and contamination that may occur during long-term system operation.

[0065] The cation exchange membrane 300 is a proton exchange membrane, preferably Fumasep FKS-PET-130. By placing the cation exchange membrane between the cathode and anode tanks, hydrogen ions generated in the anode tank can pass through the cation exchange membrane into the cathode tank and neutralize the hydroxide ions generated during the denitrification process in the cathode tank. This achieves self-stabilization of the system pH without the need for manual adjustment, simplifying the operation process. At the same time, the presence of the cation exchange membrane prevents oxygen from the anode tank from entering the cathode tank, maintaining the anaerobic environment in the cathode tank and ensuring the continuous and stable operation of the iron autotrophic denitrification process.

[0066] During operation, the reference electrode 120 is placed in the cathode cell 100 and connected to the reference electrode terminal 400 in the electrochemical workstation, the cathode electrode 110 is placed in the cathode cell 100 and connected to the working electrode terminal 410 in the electrochemical workstation, and the anode electrode 210 is placed in the anode cell 200 and connected to the auxiliary electrode terminal 420 in the electrochemical workstation.

[0067] The present invention discloses a method for reducing nitrates by electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms, using the apparatus described in any of the above-mentioned embodiments, comprising the following steps: S1, inoculating ferroautotrophic denitrifying bacteria in the cathode tank; S2, connecting the cathode electrode, the reference electrode, and the anode electrode to an electrochemical workstation respectively; S3, introducing nitrate-containing wastewater to be treated into the cathode tank and turning on the electrochemical workstation; in the cathode tank, the ferroautotrophic denitrifying bacteria reduce nitrates to produce nitrogen gas, consuming ferrous iron on the surface of the cathode electrode to produce ferric iron, while ferric iron obtains electrons from the surface of the cathode electrode and is regenerated in situ as ferrous iron; in the anode tank, water undergoes electrolysis, transferring electrons from the external circuit to the cathode electrode and generating hydrogen ions and oxygen, the hydrogen ions neutralizing hydroxide ions generated by denitrification in the cathode tank through the cation exchange membrane.

[0068] According to the present invention, the nitrate-containing wastewater to be treated can be nitrate wastewater that has undergone primary treatment (physical treatment) and secondary treatment (biological treatment). For example... Figure 2 As shown, the primary treatment facility consists of a coarse screen, an equalization tank, an aerated grit chamber, and a fine screen arranged in sequence; the secondary treatment facility consists of an anaerobic tank, an anoxic tank, and an aerobic tank arranged in sequence. Advanced treated effluent can be obtained through the method of this invention.

[0069] According to the present invention, the iron-autotrophic denitrifying bacteria can be anaerobic sludge bacteria from wastewater treatment plants, including Denitrosoma, Diaphorobacter, and Commonas, which have been domesticated with ferrous salts (FeSO4·7H2O). For example, anaerobic sludge bacteria from wastewater treatment plants with an MLVSS of about 2 g / L can be mixed with iron-autotrophic denitrification culture medium at a 1:1 (volume ratio) and cultured in an anaerobic reactor for 20-30 days to obtain the anaerobic sludge bacteria.

[0070] According to the present invention, the operating potential of the electrochemical workstation is set to -0.6 to -0.8V relative to the standard hydrogen electrode to drive the reduction of nitrate by ferroautotrophic denitrifying microorganisms through electrochemical reduction of ferric iron. The nitrate-containing wastewater is continuously fed in, and the hydraulic retention time can be 48 to 36 hours. The temperature of the cathode pool is controlled at 33°C to 37°C to provide a suitable reaction temperature for the cathode pool.

[0071] The present invention will now be described in further detail with reference to specific embodiments. The given embodiments are merely illustrative of the invention and not intended to limit its scope. The embodiments provided below can serve as a guide for further improvements by those skilled in the art and do not constitute a limitation on the invention in any way.

[0072] Unless otherwise specified, the methods used in the following embodiments are conventional methods, performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Unless otherwise specified, the materials and reagents used in the following embodiments are commercially available.

[0073] In the following embodiments, the cathode electrode substrate is a cuboid with a length of 10cm, a width of 10cm, and a height of 20cm. A 10cm×10cm titanium mesh and carbon felt are provided on its upper and lower surfaces. Carbon rods are provided along the four vertical edges. Vertical carbon felts of 10cm×20cm are provided on the front and rear surfaces respectively. Four layers of 10cm×20cm vertical carbon felts are provided between these two layers of carbon felts. The six layers of vertical carbon felts are spaced 2cm apart.

[0074] The carbon felt can have a unit area weight of 0.1 g / cm³. 2 The resistance value through the plane is <5mΩ·cm. 2 ;

[0075] The carbon rod can have a diameter of 3 mm and a resistivity of 0.05 Ω / cm.

[0076] The diameter of the titanium mesh wire can be 1 mm, and the mesh size can be 1 to 2 mm.

[0077] The reference electrode used in the following examples is an Ag / AgCl electrode.

[0078] The cation exchange membrane used in the following examples is model fumasep FKS-PET-130.

[0079] The catholy solution in the following examples has the following composition: 2.5 g / L NaHCO3, 0.5 g / L MgSO4·7H2O, 0.01 g / L CaCl2·2H2O, 0.25 g / L KH2PO4, 0.091 g / L NaNO3, and 1 mL / L trace element solution, with water as the solvent; the trace element solution has the following composition: 3.0 g / L EDTA, 0.07 g / L ZnCl2, 0.01 g / L H3BO3, 0.019 g / L CoCl6·H2O, 0.5 g / L MnCl2·2H2O, 0.024 g / L NiCl3·6H2O, and 0.036 g / L Na2MoO4·2H2O, with water as the solvent;

[0080] The composition of the anolyte is as follows: 2.5 g / L NaHCO3, 0.5 g / L MgSO4·7H2O, 0.01 g / L CaCl2·2H2O, 0.25 g / L KH2PO4, with water as the solvent; the pH of the anolyte is adjusted to 6.0 using 1 mol / L HCl.

[0081] The anaerobic sludge bacteria communities Denitrosoma, Diaphorobacter, and Commonas acclimated with ferrous salts (FeSO4·7H2O) in the following examples were obtained through the following steps: Anaerobic sludge from a wastewater treatment plant with an MLVSS of about 2 g / L was mixed with an iron autotrophic denitrification culture medium at a 1:1 (volume ratio) (the culture medium is a ferrous suspension, and it should be ensured that the sludge and the culture medium are fully mixed), and cultured in an anaerobic reactor for 20-30 days. The composition of the iron autotrophic denitrification culture medium is as follows: 27.8 g / L FeSO4·(H2O)7, 2.5 g / L NaHCO3, 0.5 g / L MgSO4·7H2O, 0.01 g / L CaCl2·2H2O, 0.25 g / L KH2PO4, 0.67 g / L NaNO3, and 1 mL / L of trace element solution in water. The pH is adjusted to 7.0 using 1 mol / L NaOH solution. The composition of the trace element solution is as follows: 3.0 g / L EDTA, 0.07 g / L ZnCl2, 0.01 g / L H3BO3, 0.019 g / L CoCl6·H2O, 0.5 g / L MnCl2·2H2O, 0.024 g / L NiCl3·6H2O, 0.036 g / L Na2MoO4·2H2O in water.

[0082] Example 1: Electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms to reduce nitrates

[0083] I. Preparation of Cathode Electrode - Iron Deposition Surface Modified Carbon Felt Electrode

[0084] The iron-deposited surface-modified carbon felt electrode is obtained through the following steps:

[0085] 1) Cut two pieces of titanium mesh, each 10cm x 10cm, soak them in deionized water for 24 hours and then dry them for later use; soak the titanium mesh in 1mol / L HCl for 12 hours; rinse it with deionized water and then soak it in 1mol / L NaOH for 12 hours, rinse it with water and then dry it for later use.

[0086] 2) Cut two 10cm×10cm pieces of carbon felt and six 10cm×20cm pieces of carbon felt, soak them in deionized water for 24 hours and then dry them for later use; soak the titanium mesh in 1mol / L HCl for 12 hours; rinse it with deionized water and then soak it in 1mol / L NaOH for 12 hours, rinse it with water and then dry it for later use.

[0087] 3) Cut out 4 carbon rods with a diameter of 3mm and a length of 11cm, and 1 carbon rod with a diameter of 3mm and a length of 5cm. Soak them in deionized water for 24 hours and then dry them for later use. Soak the titanium mesh in 1mol / L HCl for 12 hours. Rinse it with deionized water and then soak it in 1mol / L NaOH for 12 hours. Rinse it with water and then dry it for later use.

[0088] 4) Construct a rectangular composite electrode with a length of 10cm, a width of 10cm, and a height of 20cm using the materials from steps 1 to 3. Set 10cm×10cm titanium mesh and carbon felt on the upper and lower surfaces. Set carbon rods along the four vertical edges. Set 10cm×20cm vertical carbon felts on the front and back surfaces respectively. Set 4 layers of 10cm×20cm vertical carbon felts between these 2 layers of carbon felts. The 6 layers of vertical carbon felts are spaced 2cm apart. Set a 5cm carbon rod at the center of the upper surface. Bind and fix the materials together with titanium wire with a diameter of 0.1mm at the connection points. The composite electrode is now constructed.

[0089] 5) Place the composite electrode in the cathode cell, add 3.5 g / L FeSO4·7H2O solution to submerge the composite electrode, then add 1 mol / L NaOH solution to the cathode cell to adjust the pH to 7.0. After standing for 24 h, drain the remaining FeSO4·7H2O solution to complete the ferrous iron loading of the cathode.

[0090] II. Electrochemical reduction of ferric iron driving ferroautotrophic denitrifying microorganisms to reduce nitrates

[0091] The above-mentioned cathode electrode-iron-deposited surface modified carbon felt electrode was assembled into a device for the electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms to reduce nitrate. The cathode liquid was used to simulate the nitrate wastewater to be treated. The specific steps are as follows:

[0092] 1) The cathode electrode 110 is placed in the cathode cell 100 and connected to the working electrode terminal 410 of the electrochemical workstation. The reference electrode 120 (Ag / AgCl electrode) is placed in the cathode cell 100 and connected to the reference electrode terminal 400 of the electrochemical workstation. The anode electrode 210 (carbon rod) is placed in the anode cell and connected to the auxiliary electrode terminal 420 of the electrochemical workstation.

[0093] 2) Inoculate 400 mL of anaerobic sludge bacteria from a wastewater treatment plant, including Denitrosoma, Diaphorobacter, and Commonas, which have been acclimated with ferrous salts (FeSO4·7H2O), into the cathode tank 100, and add 2100 mL of cathodic solution. Add anodic solution to the anode tank so that the liquid level is level with the liquid level in the cathode chamber.

[0094] 4) Cathode liquid is continuously introduced into cathode tank 100 at a flow rate of 60 ml / h, and the cathode tank temperature is controlled at about 35℃. The cathode effluent is collected, and no additional operation is performed on the anode tank. The nitrate content in the cathode tank influent and effluent is measured every 24 hours.

[0095] 5) Turn on the electrochemical workstation and set the working potential to -0.7V (relative to the standard hydrogen electrode).

[0096] Detection of nitrate concentration in the influent and effluent of the cathode tank: After passing the collected water sample through a 0.45 μm filter membrane, the nitrate content was determined by ultraviolet spectrophotometry (HJ / T346 2007).

[0097] In addition to the electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms for nitrate reduction, the following two controls were set up:

[0098] A. Iron autotrophic denitrification reduces nitrate;

[0099] Nitrate reduction was achieved using an iron-autotrophic denitrification unit, which had the same structure as the electrochemical reduction of ferric iron-driven iron-autotrophic denitrification microbial nitrate reduction unit, but was not connected to the electrochemical workstation. The iron-autotrophic denitrification unit was operated with the same influent and effluent conditions as the electrochemical reduction of ferric iron-driven iron-autotrophic denitrification microbial nitrate reduction unit, and the nitrate concentrations in the influent and effluent were monitored.

[0100] B. Electrochemical reduction of nitrates;

[0101] An electrochemical device was used for nitrate reduction. Its structure was identical to that of an electrochemical device for reducing nitrates using ferric iron-driven ferroautotrophic denitrifying microorganisms, but without inoculation with ferroautotrophic denitrifying microorganisms. The electrochemical device was operated with the same influent and effluent conditions as the device for reducing nitrates using ferric iron-driven ferroautotrophic denitrifying microorganisms, and the nitrate concentrations in the influent and effluent were monitored.

[0102] Figure 3 The curves showing the nitrate concentration changes in the influent and effluent of the cathode tank in each treatment group (hydraulic retention time was 41.7 hours) are shown. Figure 3The nitrate concentration curve of the effluent from the electrochemical iron autotrophic denitrification system shows that after the nitrate-containing wastewater is treated as cathodic liquid in the cathode tank, the nitrate concentration in the effluent is significantly reduced. Furthermore, the system exhibits stable denitrification performance over a prolonged period. During 30 days of operation, the effluent nitrate concentration ranged from a minimum of 1.5 mg / L to a maximum of 5 mg / L, with a nitrate removal efficiency maintained between 70% and 90%. This demonstrates that, through the method of this invention, autotrophic denitrifying bacteria acquire electrons from ferrous iron loaded on the cathode surface, reducing nitrate to nitrogen gas and simultaneously generating ferric ions. These ferric ions then acquire electrons from the carbon felt electrode and are reduced back to ferrous iron, entering a new reaction cycle. This achieves in-situ regeneration of the electron donor, ensuring the continuous progress of iron autotrophic denitrification. The key features of this invention are that it requires no organic matter addition, no continuous addition of ferrous iron, and does not produce iron-containing sludge.

[0103] Depend on Figure 3 The nitrate concentration curves in the influent and effluent of the ferroautotrophic denitrification unit show that, without an electrochemical driving system, the ferroautotrophic denitrification process can only remove a certain amount of nitrates during the first 5 days of operation. Subsequently, the nitrate concentration in the effluent rises to around 15 mg / L, at which point it has almost no nitrate removal capacity. This is because the initial ferrous substrate is rapidly depleted, and without the continuous addition of ferrous iron, there is a lack of electron donors, making it impossible to maintain stable denitrification capacity.

[0104] Depend on Figure 3 The curves showing the changes in nitrate concentration in the influent and effluent of the electrochemical device indicate that, without the action of iron-autotrophic denitrifying microorganisms, the nitrate concentrations in the influent and effluent are basically the same, suggesting that electrochemical action alone cannot achieve nitrate reduction.

[0105] The comparison results above show that the electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms for nitrate reduction of the present invention can effectively reduce ferric iron to ferrous iron and continuously provide electron donors for ferroautotrophic denitrifying microorganisms, thereby driving the continuous removal of nitrates.

[0106] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for the electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms to reduce nitrate, characterized in that, The apparatus used includes a cathode cell and an anode cell, separated by a cation exchange membrane; The cathode cell is provided with a cathode electrode, a reference electrode and a catholy liquid. The cathode electrode includes an electrode substrate and ferrous iron loaded on the electrode substrate. The electrode substrate is a carbon felt electrode or a composite electrode containing carbon felt; The method for preparing the cathode electrode includes: immersing the electrode substrate in a ferrous salt solution, then adjusting the pH of the solution to 6.0-7.0, allowing it to stand, and draining the remaining solution; The anode tank is equipped with an anode electrode and an anolyte; The anode electrode is a carbon electrode; The cathode electrode and the anode electrode form a circuit through an external power source; The cation exchange membrane is a proton exchange membrane; The composition of the catholy solution is as follows: 1.0~3.0 g / L NaHCO3, 0.3~0.6 g / L MgSO4·7H2O, 0.01~0.05 g / L CaCl2·2H2O, 0.2~0.4 g / L KH2PO4, 0.08~0.10 g / L NaNO3, 1 mL / L trace element solution, and water as the solvent; The composition of the anolyte is as follows: 1.0~3.0 g / L NaHCO3, 0.3~0.6 g / L MgSO4·7H2O, 0.01~0.05 g / L CaCl2·2H2O, 0.2~0.4 g / L KH2PO4, with water as the solvent, and the pH of the anolyte is adjusted to 4~6 using HCl; Includes the following steps: S1. Inoculate iron-autotrophic denitrifying bacteria into the cathode pool; S2. Connect the cathode electrode, the reference electrode, and the anode electrode to the electrochemical workstation respectively; S3. Introduce the nitrate-containing wastewater to be treated into the cathode cell and turn on the electrochemical workstation; In the cathode cell, the iron-autotrophic denitrifying bacteria reduce nitrate to produce nitrogen gas, consume the ferrous iron on the surface of the cathode electrode to produce ferric iron, and at the same time, the ferric iron obtains electrons from the surface of the cathode electrode and is regenerated in situ as ferrous iron. In the anode pool, water undergoes electrolysis, transferring electrons from the external circuit to the cathode electrode and generating hydrogen ions and oxygen. The hydrogen ions neutralize the hydroxide ions generated by denitrification in the cathode pool through the proton exchange membrane. The iron-autotrophic denitrifying bacteria are anaerobic sludge from wastewater treatment plants that have been domesticated with ferrous salts.

2. The method for reducing nitrates by electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms according to claim 1, characterized in that: The concentration of the ferrous salt solution is 3.0~3.5 g / L; The settling time is 24 hours.

3. The method for reducing nitrates by electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms according to any one of claims 1-2, characterized in that: The reference electrode is an Ag / AgCl electrode.

4. The method for reducing nitrates by electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms according to any one of claims 1-2, characterized in that: The cation exchange membrane is fumasep FKS-PET-130.

5. The method for reducing nitrate by electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms according to any one of claims 1-2, characterized in that: The trace element solution has the following composition: 2.0~4.0 g / L EDTA, 0.05~0.10 g / L ZnCl2, 0.005~0.020 g / L H3BO3, 0.010~0.020 g / L CoCl6·H2O, 0.3~0.6 g / L MnCl2·2H2O, 0.02~0.03 g / L NiCl3·6H2O, 0.02~0.04 g / L Na2MoO4·2H2O, with water as the solvent.

6. The method for reducing nitrate by electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms according to any one of claims 1-2, characterized in that: A cathode gas collection pipe is provided above the cathode pool, and a one-way gas valve is provided on the cathode gas collection pipe; An anode gas collection pipe is provided above the anode pool, and a one-way gas valve is provided on the anode gas collection pipe.

7. The method for reducing nitrate by electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms according to claim 1, characterized in that: Inoculate 150-200 mL of the iron-autotrophic denitrifying bacteria into each 1 L of the cathode solution.

8. The method for reducing nitrate by electrochemical reduction of ferric iron-driven ferroautotrophic denitrifying microorganisms according to claim 1 or 7, characterized in that: In step S3, the operating potential of the electrochemical workstation is set to -0.6 to -0.8 V relative to the standard hydrogen electrode; The nitrate-containing wastewater is fed continuously with a hydraulic retention time of 48-36 hours. The temperature of the cathode cell is controlled at 33℃~37℃.

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