Shewanella exopolymers coated nanometer iron material, preparation method and application thereof

Abstract

The application discloses a shewanella extracellular polymer coated nano-iron material and a preparation method and application thereof, and the method comprises the following steps: after commercially available shewanella is recovered, the shewanella is expanded to a nutrient broth culture medium, and after the shewanella grows to a stable period, the bacterial liquid is washed and centrifuged, and the extracellular polymer (EPS) of the shewanella is extracted by heating. Nano-iron (nZVI) powder prepared by ball milling is added to the EPS solution, and after stirring and mixing, the EPS coated nZVI material (EPS@nZVI) is prepared. The EPS@nZVI is added to a continuous flow denitrification tank or a sequencing batch reactor for treating low carbon-nitrogen ratio sewage and is uniformly mixed and reacted, water samples are taken out, and the concentration changes of nitrate nitrogen and the toxicity of the EPS@nZVI to microorganisms are detected. In order to solve the defects of low electron utilization efficiency of nZVI in treating low carbon-nitrogen ratio wastewater and relieve the toxicity of nZVI to microorganisms, the EPS@nZVI prepared based on nZVI and shewanella EPS not only retains the strong reducibility of nZVI, but also has the potential of EPS promoting extracellular electron transfer, reduces the biological toxicity of nZVI, and has high reduction efficiency of nitrate and is easy to recycle.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of nitrate degradation of low carbon-nitrogen ratio wastewater, and relates to a Shewanella extracellular polymer coated nano-iron material and a preparation method and application thereof. BACKGROUND

[0002] Nitrate is the main form of nitrogen in domestic wastewater, and excessive discharge of nitrate will have a negative impact on the environment. Reducing nitrate in effluent is a problem that needs to be solved. In most municipal wastewater treatment processes, denitrification treatment is mainly carried out by heterotrophic denitrifying bacteria. However, the characteristics of the influent of the municipal wastewater treatment plant are low carbon-nitrogen ratio, and the biodegradability of some refractory organic matter in the wastewater is poor. Insufficient carbon source leads to less available electron donor in the denitrification process, resulting in incomplete denitrification and residual nitrate. Therefore, additional external carbon source is often added to improve the denitrification rate in the wastewater treatment process. However, if the type of external carbon source is not properly selected or the amount of external carbon source is excessive, the operation load of the system will be increased, and there is a risk of COD exceeding the standard.

[0003] Iron and its corrosion products have a wide range of redox potentials, and can be used as direct or indirect electron donors and electron transfer mediators for enhanced microbial water treatment denitrification process. In particular, nano zero-valent iron has a small particle size, a large specific surface area, and a strong reducing property, which has the potential to remove nitrate and directly convert difficult-to-biodegrade organic matter such as nitro compounds and halogenated compounds into easily biodegradable carbon sources. At the same time, it can also be used as an excellent electron donor and can mediate electron transfer in microbial systems, neutralize acid production, optimize microbial community structure, and has many other advantages. Dissolved Fe(II) and Fe(III) can be used as trace elements for microorganisms, which can comprehensively improve the carbon reduction and nitrogen removal efficiency of wastewater. Therefore, adding nano-iron to the microbial water treatment system is an effective strategy to improve the treatment effect of low carbon-nitrogen ratio wastewater. However, the toxicity of nano zero-valent iron to microorganisms cannot be ignored.

[0004] Currently, there have been studies on the use of extracellular polymer coated nano-iron materials in microbial anaerobic digestion process and reduction of pollutants such as Sb(V), but few have been used in activated sludge system to enhance nitrogen reduction process. The differences between extracellular polymers from different sources are often ignored, and the complex structure of extracellular polymers from different bacterial communities has not been clearly understood. The effects of different bacterial communities on the promotion of nano zero-valent iron reactivity and the toxicity mechanism are not clear. SUMMARY

[0005] The present application aims to overcome the shortcomings of the prior art and provide a Shewanella extracellular polymer coated nano-iron (nZVI) material, a preparation method and application thereof, which is efficient and fast in degradation, raw materials are economically available, the method is simple to operate and suitable for industrial application, can reduce nitrate in low carbon-nitrogen ratio wastewater, weaken the microbial toxicity effect of nZVI through EPS, maintain cell integrity while promoting the activity of the microbial system electron transport system, improve the efficiency of reducing nitrate in low carbon-nitrogen ratio wastewater and the maximum acceptable dosage of nZVI by the microbial system.

[0006] The specific technical solution to achieve the purpose of the present application is:

[0007] A preparation method of a Shewanella extracellular polymer coated nano-iron material, comprising the following steps:

[0008] Step 1: weigh the nutrient broth powder and mix it with distilled water at a mass-volume ratio of 1:20-50 to prepare a nutrient broth culture medium, inoculate the commercially available Shewanella MR-1 pure strain into a serum bottle containing the nutrient broth culture medium in an ultraclean bench, and mix well, wherein the inoculation ratio of the bacterial solution and the culture medium is 1:5-10; seal the serum bottle with a breathable film and place it in a constant temperature shaker, oscillate at a speed of 100-200 rpm at 30-35 ℃ for 24-72 h, then divide the bacterial solution into glycerol tubes, wherein the ratio of glycerol to bacterial solution is 1:1-5, and store in a -80 ℃ refrigerator;

[0009] Step 2: take the expanded and divided bacterial solution in the ultraclean bench, expand it to a conical flask containing the nutrient broth culture medium at an inoculation ratio of 1:100-200, seal it with a breathable film, and oscillate at a speed of 100-200 rpm at 30-35 ℃ for 24-72 h to the stable phase; centrifuge the bacterial solution with deionized water for 2-3 times, then centrifuge at 3000-4000 rpm for 20-30 min; remove the supernatant, transfer the remaining bacterial bodies to a stoppered test tube, add 300-500 mL of 0.9% physiological saline, then place it in a high-pressure sterilization pot at 0.8 MPa and 105 ℃ for 60 min, centrifuge at 3000-4000 rpm for 20-30 min after taking it out, and take the supernatant to extract the Shewanella extracellular polymer, store it at 4 ℃ and detect the protein and polysaccharide contents and the chemical oxygen demand (COD) in the extracellular polymer, the measured protein content is 240-300 mg / L, the polysaccharide content is 30-70 mg / L, and the COD value is 5000-6000 mg COD / L;

[0010] Step 3: weigh the nano-iron powder with a particle size D 50The micron zero-valent iron with a size of 3-4 microns is mixed with ethanol as a ball milling medium in a ratio of 1:5-20, and then added into a pry bar type high-energy ball mill, and ball milled for 4-6 hours to prepare nano zero-valent iron; the nano zero-valent iron is freeze-dried for 24-48 hours, ground into nano iron powder, and then mixed with the Shewanella extracellular polymer in a mass-volume ratio of 1:5-10, and added into a serum bottle for sealing; the serum bottle is placed in a constant-temperature shaker, and oscillated at 100-200 rpm at 25-35 DEG C for 2-4 hours to prepare the extracellular polymer coated nano zero-valent iron material, i.e., the Shewanella extracellular polymer coated nano iron material; wherein the Shewanella is an electroactive bacteria.

[0011] A Shewanella extracellular polymer coated nano iron material prepared by the above method.

[0012] The Shewanella extracellular polymer coated nano iron material is applied in the nitrate reduction of low carbon-nitrogen ratio wastewater, and specifically comprises the following steps: the Shewanella extracellular polymer coated nano iron material EPS@nZVI is added into a continuous flow denitrification tank or a sequencing batch reactor containing low carbon-nitrogen ratio wastewater containing nitrate, and uniformly mixed, water samples are taken from the reactor, and the changes of the concentrations of nitrate nitrogen, nitrite nitrogen and ammonia nitrogen and the toxicity of EPS@nZVI to microorganisms are detected; when treating the low carbon-nitrogen ratio wastewater containing nitrate, the carbon-nitrogen ratio of the influent per liter of wastewater is 1-4, the influent flow rate is 8000-36000 m 3 / d, the mass ratio of nitrate and EPS@nZVI is 1:5-50, and the concentration of nitrate in the effluent of the reactor is at most 4 mg / L; the cell integrity (lactate dehydrogenase release amount) is used as an index for evaluating the toxicity of nZVI to microorganisms, and the maximum acceptable dosage of the modified EPS@nZVI material in the microbial system is at least twice that of the original nZVI.

[0013] The main innovation of the present application lies in that:

[0014] EPS is a complex compound secreted by microorganisms, and is widely present in natural water bodies and wastewater treatment plants. EPS is mainly composed of proteins and polysaccharides, and contains a small amount of nucleic acids and lipids. The Shewanella used in the present application is an electroactive bacteria, which is a typical model microorganism, and the content of electroactive effective components and the electron supply capacity in the EPS thereof are higher than those of the EPS of activated sludge. The addition of the EPS extracted from Shewanella into low carbon-nitrogen ratio wastewater can greatly improve the efficiency of nZVI in reducing nitrate and weaken the toxicity effect, and promote the extracellular electron transfer process.

[0015] nZVI itself can promote the reduction of nitrate by denitrifying bacteria, but the toxic effect on microorganisms cannot be ignored. The EPS coated nZVI reduces nitrate, which not only further strengthens the reduction of nitrate by denitrifying bacteria, but also weakens the toxicity of nZVI to microorganisms. The EPS coating layer can maintain cell integrity, promote the activity of the microbial system electron transfer system, improve the efficiency of reducing nitrate in low carbon and nitrogen ratio wastewater, and increase the maximum acceptable dosage of nZVI in the microbial system. The specific process is as follows: in the process of EPS@nZVI strengthening microbial denitrification, the phenolic groups and hemiacetal structures rich in EPS have reducing properties and are easy to react with the oxidation layer on the surface of nano-iron to form complexes. In addition, cytochrome C and flavin and other redox characteristic substances can also promote the extracellular electron transfer of the system. The process of EPS coating is beneficial to the aging of nZVI and reduces the direct contact between nZVI and microorganisms, which weakens the biological toxicity of nZVI, promotes the activity of the microbial system electron transfer system, further improves the efficiency of reducing nitrate in low carbon and nitrogen ratio wastewater, and increases the maximum acceptable dosage of nZVI in the microbial system.

[0016] Compared with the prior art, the present application has the advantages that:

[0017] The present application uses Shewanella extracellular polymer to strengthen nano-iron to reduce nitrate in low carbon and nitrogen ratio wastewater, and the EPS reduces the microbial toxicity effect of nZVI, maintains cell integrity, promotes the activity of the microbial system electron transfer system, improves the efficiency of reducing nitrate in low carbon and nitrogen ratio wastewater, and increases the maximum acceptable dosage of nZVI in the microbial system. In the present application, Shewanella EPS as an active component of extracellular electron transfer can further strengthen the reduction of nZVI on nitrate. Compared with activated sludge EPS, Shewanella EPS has higher electrical activity and more redox characteristic components, which can fully play the role of the electron transfer mediator of the microbial system. The high-efficiency and stable EPS@nZVI prepared in the present application can strengthen the microbial reduction of nitrate system, which provides important research ideas and theoretical support for developing low carbon and nitrogen ratio wastewater treatment technology with good application prospect, and the method has the advantages of simple, fast, mild reaction conditions, easy operation and control, and is convenient for industrial application. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 is the SEM diagram of nZVI and EPS@nZVI prepared in Example 1 of the present application;

[0019] Figure 2 is the XRD pattern of nZVI and EPS@nZVI;

[0020] Figure 3is the effect diagram of embodiment 2 of the present application in the continuous flow denitrification tank adding EPS@nZVI on the effects of (a) nitrate nitrogen, (b) nitrite nitrogen, (c) ammonium nitrogen, (d) electron transfer system activity, (e) oxidative stress level and (f) lactate dehydrogenase release amount in the microbial denitrification system. DETAILED DESCRIPTION

[0021] The following detailed description further illustrates the technical solutions of the present application, but is not limited thereto. Any modification or equivalent replacement of the technical solutions of the present application without departing from the spirit and scope of the present application shall be covered in the protection scope of the present application. The following examples use some low carbon-nitrate ratio nitrate-containing wastewater to illustrate the effectiveness of the method of the present application, and the effects on other similar wastewater are similar. The present application uses Shewanella exopolysaccharide-coated nano-iron to reduce nitrate in low carbon-nitrate ratio wastewater, and the steps are as follows: adding the EPS-coated nano-iron material to the low carbon-nitrate ratio wastewater to be treated, the dosage increases with the increase of the concentration of nitrate, the dosage of the iron material is controlled to be less than 1 g / L, and the dosage is adjusted according to the removal effect. When a continuous flow reaction tank or a sequencing batch reactor is used, the iron material can be recycled. The following further gives several comparative examples.

[0022] In the following examples, unless otherwise specified, the raw materials and instruments used are commercially available.

[0023] Example 1

[0024] In this embodiment, Shewanella exopolysaccharide-coated nano-iron material is prepared, and the specific steps are as follows:

[0025] 2.0 g of nutrient broth powder was weighed and dissolved in 100 mL of ultrapure water to prepare a Shewanella culture medium, 1 mL of commercially available Shewanella MR-1 pure bacterial strain was recovered in a serum bottle containing the above-mentioned nutrient broth medium, and the serum bottle was sealed with a gas permeable membrane. The serum bottle was placed in a constant temperature shaker, and cultured at 35 ℃ with a rotation speed of 150 rpm for 48 h. The bacterial solution was then divided into glycerol tubes, with a glycerol to bacterial solution ratio of 1:1-5, and stored in a -80 ℃ refrigerator.

[0026] The 1 mL of Shewanella liquid after expansion and dispensing was expanded in a clean bench to a conical flask containing 100 mL of culture medium, and was oscillated at 100 rpm at 30℃ after being sealed with a breathable film for 48 h to the stable period. The bacterial liquid was washed 3 times by centrifugation with deionized water, and was centrifuged at 3200 rpm for 30 min. The supernatant was removed, and the remaining bacterial body was transferred to a stoppered test tube, 300 mL of 0.9% physiological saline was added, and then it was placed in a high-pressure sterilization pot at 0.8 MPa and 105℃ for 60 min. After being taken out, it was centrifuged at 3200 rpm for 30 min, and the supernatant was extracted to the Shewanella exopolysaccharide, which was stored at 4℃ and detected for protein and polysaccharide content and COD in the exopolysaccharide. The polysaccharide content was 52 mg / L, the protein content was 294 mg / L, and the COD was 5750 mg COD / L.

[0027] The micrometer-sized zero-valent iron with a particle size D 50 of 3-4 μm was 50 g, 1 L of anhydrous ethanol was used as a ball milling medium, the micrometer-sized iron was uniformly mixed with the ethanol, and then was added to a pry bar type high-energy ball mill for ball milling for 4 h to prepare nano-sized zero-valent iron. Figure 1 The SEM image of the nano iron in (a) can find that the ball-milled nZVI is mainly in the form of fragmented particles. Then, after being freeze-dried for 48 h, 10 g of the nano iron powder was weighed, uniformly mixed with 100 mL of the Shewanella exopolysaccharide, and was added to a serum bottle for sealing. The serum bottle was placed in a constant-temperature shaker, and was oscillated at 150 rpm at 35℃ for 3 h to prepare the exopolysaccharide-coated nano iron material. The SEM image of the exopolysaccharide-coated nano iron in (b) can find that the prepared EPS@nZVI is mainly in the form of curled nano-sized flakes, and there is obviously EPS macromolecular substance attached therebetween. Figure 1 The SEM image of the nano iron in (a) can find that the ball-milled nZVI is mainly in the form of fragmented particles. Then, after being freeze-dried for 48 h, 10 g of the nano iron powder was weighed, uniformly mixed with 100 mL of the Shewanella exopolysaccharide, and was added to a serum bottle for sealing. The serum bottle was placed in a constant-temperature shaker, and was oscillated at 150 rpm at 35℃ for 3 h to prepare the exopolysaccharide-coated nano iron material. The SEM image of the exopolysaccharide-coated nano iron in (b) can find that the prepared EPS@nZVI is mainly in the form of curled nano-sized flakes, and there is obviously EPS macromolecular substance attached therebetween. Figure 2 The XRD patterns of the nZVI and the EPS@nZVI in (c) and (d) can find that the zero-valent iron peak intensity of the EPS@nZVI pattern is lower than the standard peak intensity, and the diffraction peak of iron oxide appears, which indicates that the EPS may passivate the nZVI in the mixing process of the EPS and the nZVI, so that an iron oxide layer is generated on the surface of the nZVI, and then the toxicity of the nZVI to the microbial system is reduced.

[0028] Example 2

[0029] This example was carried out in a continuous flow reaction tank, and the specific steps were as follows:

[0030] The influent flow rate of the continuous flow reaction tank was 36000 m 3d, the MLSS of the denitrification sludge was 3400 mg / L, the HRT was 24 h, and the influent of the reaction tank was domestic wastewater containing 48 mg / L of nitrate. The low C / N ratio domestic wastewater (C / N ratio of 2) was treated by coagulation and sedimentation, and then entered the biochemical reaction section, and then entered the secondary sedimentation tank for sedimentation. 1000 mg / L of EPS@nZVI (wherein the EPS content was 5750 mg COD / L) was added to the denitrification tank of the biochemical reaction section and mixed uniformly to improve the nitrate reduction efficiency. The mass ratio of nitrate and EPS@nZVI in each liter of wastewater was 6:125. Part of the zero-valent iron was corroded into iron ions, which directly reacted with phosphate to achieve simultaneous removal of phosphate. The supernatant discharged from the secondary sedimentation tank was the treated water, and the change of the concentration of nitrate with time was determined by Figure 3 (a) From the change of the effluent nitrate concentration, it can be seen that the addition of 1000 mg / L of EPS@nZVI can improve the nitrate reduction efficiency of the microbial system. From the kinetic curve, it can be seen that the nitrate reduction rate of the EPS@nZVI group is slightly higher than that of the nZVI group, and is much higher than that of the EPS group and the pure microbial group. Especially in the 4th hour of the reaction, the nitrate reduction efficiency of the EPS@nZVI group is increased by 13.44% compared with the original nZVI group, and the concentration of nitrate in the effluent is reduced to below 1 mg / L after 24 h of reaction. From Figure 3 (b) From the change of the concentration of nitrite nitrogen, it can be found that the concentration of nitrite nitrogen first increases and then decreases. From Figure 3 (c) From the change of the concentration of ammonia nitrogen, it can be seen that the ammonia nitrogen accumulation of the microbial system after adding EPS is higher than that of the nZVI group, which indicates that the regulation of EPS@nZVI on the nitrogen reduction process also promotes the process of nitrate dissimilatory reduction to ammonium (DNRA), which reduces part of the nitrate and nitrite to ammonia, and the ammonium can be recycled as a resource. In addition, from Figure 3 (d) From the ETSA value of the electron transfer system, it can be seen that the electron transfer system activity of the EPS@nZVI group is also higher than that of the nano-iron group, which indicates that EPS@nZVI promotes the extracellular electron transfer process of the microbial system. Regarding the cytotoxicity, from Figure 3 (e) From the ROS value of the intracellular oxidative stress level, it can be found that the oxidative stress level of the microbial system after adding EPS@nZVI is slightly higher than that of the nano-iron group, and since the oxidative stress level is related to the denitrification activity of the microbial system, it indicates that EPS@nZVI may further promote the microbial denitrification process. From Figure 3(f) The release amount of lactate dehydrogenase LDH value can be found that nZVI caused much higher cell damage to the microbial system than EPS@nZVI. Compared with the pure microbial system, EPS@nZVI almost showed no damage to the cell integrity, while the release amount of lactate dehydrogenase LDH value of the microbial system after adding nZVI increased significantly, indicating that the EPS@nZVI modified material can reduce the biological toxicity of nano-iron. Part of the activated sludge containing nano-iron in the secondary sedimentation tank is backflowed, and the remaining activated sludge enters the high-gradient magnetic separator for magnetic separation to recover nano-iron. The sludge after magnetic separation and the physicochemical sludge after coagulation and sedimentation enter the thickener together, and after concentration, they are dewatered by a plate and frame filter press, and the sludge is transported for disposal. The supernatant and dewatering liquid are discharged into the adjusting tank. The nano-iron obtained by magnetic separation is re-injected into the biochemical reaction section, and new nano-iron is supplemented to ensure the effective concentration of iron in the biochemical reaction section.

[0031] Example 3

[0032] This example is carried out in a sequencing batch reactor, and the specific steps are as follows:

[0033] The sequencing batch wastewater treatment process consists of five stages: influent, stirring (the stirring speed is 300 rpm during operation), sedimentation (settling), decanting (effluent), and idling (standing or standby). Each operation is repeated and completed in the same tank. The influent flow rate of the sequencing batch reactor is 15000 m 3 / d, provided with two groups, alternating operation, the influent C / N ratio of each reactor is 1, and the influent nitrate concentration is 35 mg / L. The above prepared EPS@nZVI is added to the sequencing batch reactor and mixed uniformly, and the dosage is 2000 mg / L, that is, the mass ratio of nitrate and EPS@nZVI in each liter of wastewater is 7:300. After 36 h of operation, the concentration of nitrate in the effluent is reduced to below 1 mg / L, part of the nitrate is reduced to nitrogen gas through the denitrification process or is reduced to ammonium through the nitrate dissimilation process, and the ammonium can be reused as a resource. This process does not need to set up a special secondary sedimentation tank and sludge backflow system, but will periodically discharge part of the activated sludge containing nano-iron, and the remaining activated sludge enters the high-gradient magnetic separator for magnetic separation to recover nano-iron. The sludge after magnetic separation and the physicochemical sludge after coagulation and sedimentation enter the thickener together, and after concentration, they are dewatered by a plate and frame filter press, and the sludge is transported for disposal. The supernatant and dewatering liquid are discharged into the adjusting tank. The nano-iron obtained by magnetic separation is re-injected into the sequencing batch reactor, and new nano-iron is supplemented to ensure the effective concentration of iron in the biochemical reaction section.

[0034] Example 4

[0035] This example is carried out in a membrane bioreactor, and the specific steps are as follows:

[0036] The MBR membrane assembly with hollow structure is placed in the aeration tank, and the water treated by aerobic aeration and biological treatment is pumped out through the filter membrane. The activated sludge and macromolecular organic matter in the biochemical reaction tank are intercepted by using the membrane separation equipment, and the secondary sedimentation tank is omitted. The concentration of activated sludge is greatly improved, and the hydraulic retention time (HRT) and sludge retention time (SRT) can be controlled respectively, and the refractory substances are continuously reacted and degraded in the reactor. The influent of the membrane bioreactor is landfill leachate wastewater, and the flow rate is 8000 m 3 / d, and four groups are provided and alternately operated. The C / N ratio of the influent of each reactor is 4, and the nitrate concentration of the influent is 85 mg / L. The EPS@nZVI material with a dosage of 500 mg / L is added to the membrane bioreactor and uniformly mixed, that is, the mass ratio of nitrate and EPS@nZVI in each liter of wastewater is 17:100. The concentration of nitrate in the effluent is reduced to below 4 mg / L, part of the nitrate is reduced to nitrogen gas through the denitrification process or is reduced to ammonium through the nitrate dissimilation process, and the ammonium can be reused as a resource. This process does not need to set a special secondary sedimentation tank and sludge return system, but part of the activated sludge containing nano-iron will be discharged periodically, the remaining activated sludge enters the high-gradient magnetic separator for magnetic separation, the nano-iron is recovered, the sludge after magnetic separation and the physicochemical sludge after coagulation and sedimentation enter the thickening tank together, and after thickening, the sludge is dewatered by a plate and frame filter press, and the sludge is transported and disposed, and the supernatant and dewatering liquid are discharged into the adjusting tank. The nano-iron obtained by magnetic separation is re-added to the sequencing batch reactor, and new nano-iron is supplemented to ensure the effective concentration of iron in the biochemical reaction section.

[0037] Example 5

[0038] This embodiment is carried out in an anaerobic granular sludge reactor, and the specific steps are as follows:

[0039] The anaerobic granular sludge reactor is an expanded granular sludge bed (EGSB) in a cylindrical tower shape, which is divided into a water inlet and distribution system, a reaction zone, a three-phase separation zone and a water outlet channel system. The EGSB reactor is provided with a special water outlet reflux system. The EGSB reactor has a large height-diameter ratio of 4.5, and the height of the reactor of the production device is up to 15 meters. The expanded bed of granular sludge improves the contact between organic matter in wastewater and microorganisms, strengthens the mass transfer effect, and improves the biochemical reaction speed of the reactor, thereby greatly improving the treatment efficiency of the reactor. The influent flow rate of the anaerobic granular sludge reactor is 12000 m 3 / d, with four groups, alternating operation, each group of reactor water C / N ratio of 4, water nitrate concentration of 70 mg / L, the dosage of EPS@nZVI is 1000 mg / L. That is, the mass ratio of nitrate and EPS@nZVI in per liter of wastewater is 7:100. Add EPS@nZVI to the EGSB reactor and mix well. In the effluent after 36 hours of reaction, the concentration of nitrate is reduced to below 1 mg / L. Part of the nitrate is reduced to nitrogen gas through the denitrification process or reduced to ammonium through the nitrate dissimilation process. Ammonium can be reused as a resource. Part of the activated sludge containing nano iron is recirculated, and the remaining activated sludge enters the high gradient magnetic separator for magnetic separation to recover nano iron. The sludge after magnetic separation and the physicochemical sludge after coagulation and sedimentation enter the thickening tank together. After thickening, it is dewatered by plate and frame filter press. The sludge is transported and disposed. The supernatant and dewatering liquid are discharged into the adjusting tank. The nano iron obtained by magnetic separation is re-injected into the biochemical reaction section, and new nano iron is added to ensure the effective concentration of iron in the biochemical reaction section.

Claims

1. A method for preparing Shewanella Exopolymers coated nanometer iron material, characterized in that, The method comprises the following steps: Step 1: The nutrient broth powder is weighed and mixed with distilled water at a mass-volume ratio of 1:20-50 to prepare a nutrient broth culture medium; the pure Shewanella MR-1 strain is inoculated into a serum bottle containing the nutrient broth culture medium in an ultraclean bench, and mixed uniformly, wherein the inoculation ratio of the bacterial solution and the culture medium is 1:5-10; the serum bottle is sealed with a breathable film and placed in a constant-temperature shaker, and is cultured at 30-35 DEG C and a rotation speed of 100-200 rpm for 24-72 h; then the bacterial solution is divided into glycerol tubes, wherein the ratio of glycerol and bacterial solution is 1:1-5, and the glycerol tubes are stored in a-80 DEG C refrigerator; Step 2: The expanded bacterial solution is taken out in an ultraclean bench, inoculated into a conical flask containing the nutrient broth culture medium at a ratio of 1:100-200, sealed with a breathable film, and cultured at 30-35 DEG C and a rotation speed of 100-200 rpm for 24-72 h until the stable period; the bacterial solution is centrifuged with deionized water for 2-3 times, and centrifuged at 3000-4000 rpm for 20-30 min; the supernatant is removed, the remaining bacterial body is transferred into a stoppered test tube, 300-500 mL of 0.9% physiological saline is added, and then the test tube is placed in a high-pressure sterilization pot at 0.8 MPa and 105 DEG C for 60 min; after being taken out, the test tube is centrifuged at 3000-4000 rpm for 20-30 min, and the supernatant is obtained, that is, the Shewanella exopolysaccharide is extracted, and is stored at 4 DEG C and detected; the protein content is 240-300 mg / L, the polysaccharide content is 30-70 mg / L, and the COD value is 5000-6000 mg COD / L; Step 3: micron-sized zero-valent iron with a particle size D 50 The micron-sized zero-valent iron with a particle size D of 3-4 μm is mixed with ethanol in a ratio of 1:5-20, and then added to a pry bar high-energy ball mill. The mixture is ball-milled for 4-6 h to prepare nano-sized zero-valent iron. The nano-sized zero-valent iron is freeze-dried for 24-48 h, ground into nano-sized iron powder, and then mixed with the Shewanella exopolysaccharide in a mass-volume ratio of 1:5-10. The mixture is added to a serum bottle and sealed. The serum bottle is placed in a constant-temperature shaker, and oscillated at 100-200 rpm at a temperature of 25-35 °C for 2-4 h to obtain the exopolysaccharide-coated nano-sized zero-valent iron material, i.e., the Shewanella exopolysaccharide-coated nano-sized iron material. The Shewanella is an electroactive bacteria.

2. A Shewanella exopolysaccharide-coated nano-iron material prepared by the method of claim 1.

3. Application of the Shewanella exopolysaccharide-coated nano-iron material of claim 2 in strengthening nitrate reduction of low-carbon-nitrogen-ratio wastewater.

4. Use according to claim 3, characterized in that, Specifically includes: The Shewanella extracellular polymer coated nano-iron material is added to a continuous flow denitrification tank or a sequencing batch reactor containing low carbon-nitrogen ratio wastewater with nitrate and mixed uniformly, water samples are taken from the reactor effluent and the changes of nitrate, nitrite and ammonia nitrogen concentrations and the toxicity of the Shewanella extracellular polymer coated nano-iron material to microorganisms are detected; when treating the low carbon-nitrogen ratio wastewater with nitrate, the carbon-nitrogen ratio of the influent per liter of wastewater is 1-4, the influent flow rate is 8000-36000 m 3 / d, wherein the mass ratio of nitrate and the Shewanella extracellular polymer coated nano-iron material is 1:5-50, and the nitrate concentration in the reactor effluent is at most 4 mg / L; the cell integrity, i.e., the amount of lactate dehydrogenase released, is used as an index for evaluating the toxicity of nano zero-valent iron to microorganisms, and the maximum acceptable dosage of the modified Shewanella extracellular polymer coated nano-iron material in the microbial system is at least doubled compared with the original nano zero-valent iron.

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