CN110668546A

CN110668546A - Method for catalytic reduction of uranyl ions in uranium-containing wastewater

Info

Publication number: CN110668546A
Application number: CN201911026122.7A
Authority: CN
Inventors: 阳鹏飞; 徐源合; 柯国军
Original assignee: University of South China
Current assignee: University of South China
Priority date: 2019-10-25
Filing date: 2019-10-25
Publication date: 2020-01-10
Anticipated expiration: 2039-10-25
Also published as: CN110668546B

Abstract

The invention provides a method for catalytically reducing uranyl ions in uranium-containing wastewater, which comprises the steps of using nano-scale magnetic cobalt-manganese spinel as a catalyst, using thiourea as a reducing agent, and catalytically reducing the uranyl ions in the uranium-containing wastewater into tetravalent uranium ions to precipitate in the wastewater under the conditions that the reaction temperature is 80-130 ℃ and the pH value of the wastewater is 4.5-8. When the nano-scale magnetic cobalt-manganese spinel is used as a catalyst of the catalytic reduction reaction, divalent and trivalent cobalt-manganese ions coexist in the catalyst simultaneously, a self-oxidation-reduction system can be formed, and experiments prove that the nano-scale magnetic cobalt-manganese spinel has better oxidation-reduction performance on hexavalent uranyl ions in wastewater; under the condition of existence of thiourea as a reducing agent, the catalytic reduction capability of the thiourea on hexavalent uranyl ions in the wastewater can reach 6500 mg/g.

Description

Method for catalytic reduction of uranyl ions in uranium-containing wastewater

Technical Field

The invention belongs to the field of catalytic reduction, and particularly relates to a method for catalytically reducing uranyl ions in uranium-containing wastewater.

Background

In recent years, with the continuous development of nuclear energy, the quantity of uranium-containing wastewater generated by the nuclear industry is increasing, and uranium is a natural radioactive element which enters into organisms to influence the normal growth of the organisms and even can induce various pathological changes, thus bringing about increasingly severe threats to the environment and the ecology. Therefore, how to properly treat the radioactive uranium-containing wastewater becomes a global environmental protection problem. At present, methods for removing or enriching radioactive uranium element in water environment include: chemical precipitation, ion exchange, solvent extraction, adsorption, reduction, and the like.

The uranium-containing wastewater mainly comprises hexavalent uranium which migrates in an aqueous solution in the form of a soluble uranyl carbonate/uranyl fluoride complex. The traditional single inorganic material as an adsorbent has low adsorption capacity. Hexavalent uranium can be reduced into tetravalent uranium through catalytic reduction reaction, uranyl ions are enriched and precipitated in the forms of uranium ore, uranite and the like, and the uranyl ions are removed from an aqueous solution, so that the design and preparation of a novel catalyst for catalytic reduction of the hexavalent uranyl ions become a research hotspot.

For example, patent application CN201810765731.3 discloses a method for recovering uranium from uranium-containing wastewater and groundwater by efficient electrochemical reduction and enrichment. The method uses a metal electrode as a cathode and an anode to construct an electrochemical system, utilizes the property that hexavalent uranyl ions can obtain electrons to be reduced into tetravalent insoluble uranium dioxide, and reduces the hexavalent uranium into the uranium dioxide through the electrode and enriches the uranium dioxide on the surface of the electrode under the condition of no exogenous additive. The uranium plating layer can further catalyze and reduce the hexavalent uranium in the wastewater to form a new uranium plating layer. After the electrochemical reduction enrichment is completed, the electrode enriched with uranium dioxide is taken out of the solution, and the efficient reduction removal of uranium in wastewater and underground water can be realized. By further putting the electrode into dilute nitric acid solution for oxidation, efficient recovery can be further realized. The method has wide application range, and can realize high-efficiency removal and recovery of uranyl carbonate in low-concentration and high-concentration uranium-containing wastewater and carbonate-containing underground water.

However, the electrode treatment method is difficult to be applied industrially in a large scale, so that a new method for catalytically reducing uranyl ions in uranium-containing wastewater is needed in the field.

Disclosure of Invention

Oxygen ions in the spinel crystal structure are arranged according to cubic close packing, divalent cations are filled in one-eighth tetrahedral voids, and trivalent cations are filled in one-half octahedral voids to form a cube. The material has wide application in the fields of electricity, magnetism, catalysis, energy storage and conversion and the like. Co ions and Mn ions belong to transition metal elements, are easy to gain and lose electrons and have the potential of serving as a catalyst, and meanwhile, the catalyst with the coexistence of divalent and trivalent Co ions and Mn ions has the auto-oxidation-reduction performance, namely, the catalyst has the oxidation property and the reduction property at the same time. Considering the combination of all the above excellent performances, the product prepared by a certain technology should have good catalytic reduction, and is an excellent catalyst, which can be applied to the reduction and precipitation of high valence metal ions.

Therefore, there is a need in the art for a new method for preparing nanoscale magnetic cobalt manganese spinel and applications thereof.

Therefore, the invention firstly provides a preparation method of the nanoscale magnetic cobalt-manganese spinel, which comprises the steps of dissolving a cobalt source and a manganese source in a mixed solution containing ammonia water and ammonium chloride, adding sodium hypochlorite as an oxidant, oxidizing part of bivalent manganese into trivalent manganese to form a homogeneous system with coexisting bivalent and trivalent manganese ions, reacting for more than 2 hours at 120-180 ℃ by taking urea as a precipitating agent, crystallizing to form a precursor, and calcining the precursor sample at 400-600 ℃ for more than 2 hours to obtain the nanoscale magnetic cobalt-manganese spinel.

In a specific embodiment, the cobalt source and the manganese source are cobalt chloride hexahydrate and manganese chloride tetrahydrate, and the preparation method is carried out in an oxygen-isolated environment.

In one specific embodiment, the ratio of the amounts of cobalt to manganese species in the cobalt and manganese sources is 1: 1 to 6, wherein the ratio of the amount of cobalt element to the amount of ammonia water is 1: 2 to 10, the ratio of the amount of cobalt element to the amount of ammonium chloride is 1: 2-12, the mass ratio of cobalt element to sodium hypochlorite is 1: 0.1 to 100, the ratio of the amount of cobalt to the amount of urea is 1: 2 to 100.

In one specific embodiment, the ratio of the amounts of cobalt to manganese species in the cobalt and manganese sources is 1: 2-3, preferably 1: 2.5, the mass ratio of the cobalt element to the ammonia water is 1: 4-8, preferably 1: 6, the mass ratio of cobalt element to ammonium chloride is 1: 4-8, preferably 1: 6, the mass ratio of cobalt element to sodium hypochlorite is 1: 0.5-10, preferably 1: 1, the ratio of the amount of cobalt element to the amount of urea substance is 1: 5-50, preferably 1: 35.

in a specific implementation mode, dissolving urea and ammonium chloride in water to prepare a solution A, dissolving a cobalt source and a manganese source in water to prepare a solution B, uniformly mixing the solution A and the solution B, adding ammonia water of the solution C, adding a sodium hypochlorite solution of the solution D, oxidizing for 10-120 min by using sodium hypochlorite under an oxygen-free condition, and then crystallizing and calcining the solution to obtain the nano-scale magnetic cobalt-manganese spinel.

In a specific embodiment, the crystallization temperature is 140-160 ℃, the crystallization time is 4-20 hours, the calcination temperature is 450-550 ℃, and the calcination time is 4-8 hours.

In a specific embodiment, a step of cleaning and drying the precursor is further included between the step of crystallizing and the step of calcining, the cleaning includes alternately cleaning with ethanol and deoxidized deionized water, and the drying is normal pressure drying or vacuum drying at 40-150 ℃.

The invention also provides the nanoscale magnetic cobalt-manganese spinel prepared by the method.

The invention also provides application of the nano-scale magnetic cobalt manganese spinel prepared by the method in removing uranyl ions in uranium-containing wastewater.

In a specific embodiment, the nanometer magnetic cobalt manganese spinel is used as a catalyst and is added into the uranium-containing wastewater together with a reducing agent, and is used for catalytically reducing uranyl ions in the uranium-containing wastewater into tetravalent uranium ions for precipitating and removing in the wastewater; or the nanometer magnetic cobalt manganese spinel is used as an adsorbent and added into uranium-containing wastewater to adsorb uranyl ions in the uranium-containing wastewater onto the nanometer magnetic cobalt manganese spinel so as to be separated and removed from the wastewater.

The invention also provides a method for catalytically reducing uranyl ions in uranium-containing wastewater, which comprises the step of catalytically reducing the uranyl ions in the uranium-containing wastewater into tetravalent uranium ions by using nano-scale magnetic cobalt-manganese spinel as a catalyst and thiourea as a reducing agent under the conditions that the reaction temperature is 80-130 ℃ and the pH value of the wastewater is 4.5-8, and precipitating the tetravalent uranium ions in the wastewater.

In a specific embodiment, the reaction temperature of the catalytic reduction is 95-115 ℃, preferably 100 ℃, and the pH value of the wastewater is 5-7, preferably 6.

In a specific embodiment, the time for catalytic reduction is 1 hour or more, preferably 2 hours or more, more preferably 12 hours or more, and more preferably 16 to 30 hours.

The invention also provides a method for adsorbing uranyl ions in uranium-containing wastewater, which comprises the step of adsorbing the uranyl ions in the uranium-containing wastewater to the solid nano-scale magnetic cobalt manganese spinel by using the nano-scale magnetic cobalt manganese spinel as an adsorbent under the conditions that the reaction temperature is 80-130 ℃ and the pH value of the wastewater is 5-9, so that the uranyl ions are separated from the wastewater.

In a specific embodiment, the adsorption temperature is 85-120 ℃, preferably 100 ℃, and the pH value of the wastewater is 5.5-8, preferably the pH value of the wastewater is 6.

In a specific embodiment, the adsorption time is 6 hours or more, preferably 8 to 30 hours.

The invention has at least the following beneficial effects:

1. the preparation method of the invention has the following characteristics: (1) a certain mass of NH was used in the experiment₄Preparing a complexing solution from Cl and ammonia water, wherein the solution has a complexing effect on cobalt ions and manganese ions; the complex is also used to immobilize unstable Mn produced by oxidation of oxidant NaClO³⁺Ions, so that the spinel obtained by preparation can form an autocatalytic reduction system. And NH₄The buffer of the complex solution formed by Cl and ammonia contributes to the final formation of spinel with uniform particle size. (2) In the method, sodium hypochlorite is used as oxygenThe oxidant can artificially control the oxidation amount of cobalt and manganese, namely the proportion of trivalent metal ions and divalent metal ions in the product, thereby being beneficial to preparing the spinel product with optimal performance. (3) Simple process, convenient operation and easy control of the process. (4) The reaction speed is high, and the method is suitable for large-scale industrial production. (5) The preparation of the common spinel needs the temperature of over 800 ℃, and the ideal spinel can be obtained only under the condition of about 500 ℃ by adopting the preparation method of the invention.

2. The nanoscale magnetic cobalt manganese spinel prepared by the method has high crystal form integrity degree, uniform particles and particle size of about 50 nm; has strong magnetism and is easy to separate under the condition of an external magnetic field. When the nanometer magnetic cobalt-manganese spinel is used as a catalyst for catalytic reduction reaction, divalent and trivalent cobalt-manganese ions coexist in the catalyst simultaneously, an auto-oxidation-reduction system can be formed, and experiments prove that the nanometer magnetic cobalt-manganese spinel has better oxidation-reduction performance on hexavalent uranyl ions in wastewater; under the condition of existence of thiourea as a reducing agent, the catalytic reduction capability of the thiourea on hexavalent uranyl ions in the wastewater can reach 6500 mg/g. In addition, the nanometer magnetic cobalt-manganese spinel has good adsorption performance on hexavalent uranyl ions, the maximum adsorption capacity can reach 3200mg/g, and the adsorption effect of the adsorbent is more than 10 times that of the traditional inorganic adsorbent. In conclusion, the nanoscale magnetic cobalt manganese spinel material has a good application prospect in uranium-containing wastewater treatment.

Drawings

FIG. 1 is an EDS picture of a nanoscale magnetic Co/Mn spinel obtained in example 1 of the present invention.

FIG. 2 is an SEM photograph of the nanoscale magnetic Co/Mn spinel obtained in example 1 of the present invention.

FIG. 3 is an XRD spectrum of the nanoscale magnetic Co/Mn spinel obtained in example 1 of the present invention.

FIG. 4 shows the VSM spectrum of the nanoscale magnetic Co/Mn spinel obtained in example 1 of the present invention.

FIG. 5 is an XPS spectrum of a nanoscale magnetic Co/Mn spinel obtained in example 1 of the present invention.

FIG. 6 is a graph showing the effect of the initial volume of uranium-containing wastewater on the performance of the nano-scale magnetic Co/Mn spinel catalytic reduction U (VI) in example 2 of the present invention.

FIG. 7 is a graph showing the effect of reaction time on the performance of the nano-scale magnetic Co/Mn spinel catalytic reduction U (VI) in example 2 of the present invention.

FIG. 8 is a graph showing the effect of pH on the performance of the nano-magnetic Co/Mn spinel catalytic reduction U (VI) in example 2 of the present invention.

FIG. 9 is a graph showing the effect of the reaction temperature on the performance of the nano-magnetic Co/Mn spinel catalytic reduction U (VI) in example 2 of the present invention.

FIG. 10 is a graph showing the effect of catalyst dosage on the performance of the nano-magnetic Co/Mn spinel catalytic reduction U (VI) in example 2 of the present invention.

FIG. 11 is an SEM photograph of the nano-scale magnetic Co/Mn spinel obtained in example 3 of the present invention after adsorbing uranium in wastewater, for comparison with the SEM photograph before not participating in the adsorption reaction, i.e., FIG. 2.

FIG. 12 is the EDS spectrum of the nano-scale magnetic Co/Mn spinel obtained in example 3 of the present invention after adsorbing uranium in wastewater, for comparison with the EDS spectrum before not participating in the adsorption reaction, i.e., FIG. 1.

FIG. 13 is a graph showing the effect of the initial volume of uranium-containing wastewater on the adsorption of U (VI) onto nanoscale magnetic Co/Mn spinel in example 3 of the present invention.

FIG. 14 is a graph showing the effect of adsorption time on the U (VI) adsorption performance of nanoscale magnetic Co/Mn spinel in example 3 of the present invention.

FIG. 15 is a graph showing the effect of pH on the U (VI) adsorption performance of nanoscale magnetic Co/Mn spinel in example 3 of the present invention.

FIG. 16 is a graph showing the effect of adsorption temperature on the U (VI) adsorption performance of nanoscale magnetic Co/Mn spinel in example 3 of the present invention.

FIG. 17 is a graph showing the effect of the amount of the added adsorbent on the U (VI) adsorption performance of the nanoscale magnetic Co/Mn spinel in example 3 of the present invention.

Detailed Description

The present invention is specifically illustrated by the following examples, but the scope of the present invention is not limited to the following examples.

Example 1

According to n [ CO (NH)₂)₂]：n(Co²⁺+Mn²⁺)＝10：1，n(Mn²⁺)：n(Co²⁺) 2.5: 1, 21g of CO (NH) was weighed out separately₂)₂Dissolved in 50mL of deoxygenated deionized water and 4.81g of NH₄Dissolving Cl in 50mL of deoxidized deionized water, and mixing the Cl and the deionized water to prepare a solution A; 5.946g MnCl are weighed₂·4H₂O and 1.978g CoCl₂·6H₂Dissolving O in 100mL of deoxidized deionized water to prepare a solution B. The two solutions were added simultaneously to a three-necked flask with a stirrer, followed by addition of 13mL of concentrated ammonia water, followed by stirring at 60 ℃ for 60 min. Transferring the prepared solution into a polytetrafluoroethylene autoclave for crystallization, operating a liquid transfer gun to quickly and dropwise add 2.8mL of NaClO solution, quickly covering the cover of the autoclave, putting the autoclave into an oven, and reacting for 12 hours at a constant temperature of 150 ℃. And after the reaction is finished, cooling the reaction kettle to room temperature, collecting a precipitate product obtained by the reaction, alternately cleaning the precipitate product with ethanol and deoxidized deionized water for several times, and drying the precipitate product at 60 ℃ for 8 hours to obtain a cobalt-manganese oxide product, namely a precursor. And (3) putting the cobalt-manganese oxide prepared in the previous step into a muffle furnace, heating to 500 ℃ in the air atmosphere, preserving the heat for 6 hours, taking out the cobalt-manganese oxide after calcination, and cooling to room temperature to obtain the nanoscale magnetic Co/Mn spinel.

FIG. 1 is an EDS picture of a nano-scale magnetic Co/Mn spinel prepared by the present invention, and it can be seen that the elements of the nano-scale magnetic cobalt manganese spinel are cobalt, manganese and oxygen.

FIG. 2 is an SEM photograph of a nanoscale magnetic Co/Mn spinel, and it can be seen from the photograph that the generated crystal is smooth in appearance, complete in structure and cubic in shape.

FIG. 3 is an XRD pattern of a nanoscale magnetic Co/Mn spinel. By comparison with a standard card (18-0408), this was found to be a cubic spinel structure, consistent with SEM observations. And the XRD spectral line shows that the product crystal formed by the method has high crystal forming rate.

FIG. 4 is a VSM spectrum of a nanoscale magnetic Co/Mn spinel. By map analysis, we can see that the prepared nano product has certain magnetism. The nano product is convenient to recover and separate in practical use.

FIG. 5 is an XPS spectrum of a nanoscale magnetic Co/Mn spinel. As can be seen by the graph, Co in the spinel is mainly divalent, while Mn in the spinel has two valence states of divalent and trivalent, which indicates that the spinel can be used for the autocatalytic reduction reaction when the spinel is used together with a reducing agent.

Example 2

Putting a certain mass of the nano-scale magnetic Co/Mn spinel prepared by the method and a certain mass of thiourea into uranium (uranyl ion) containing wastewater with a certain volume and a certain concentration, then putting the wastewater into a pressure reaction kettle, adjusting the temperature to a certain value, and carrying out catalytic reduction reaction. After the catalytic reduction reaction is finished, taking supernatant liquid, filtering the supernatant liquid by using a 0.45 mu m microporous filter membrane, and detecting the uranium ion concentration by using an ultraviolet spectrophotometer.

After the catalytic reduction reaction, hexavalent uranyl ions are catalytically reduced into tetravalent uranium ions to be precipitated at the bottom of a container for containing wastewater, and then the cobalt-manganese spinel catalyst is separated from the tetravalent uranium ions through precipitation by a magnet.

6-10 are graphs of experimental data corresponding to example 2, wherein the abscissa of FIG. 7 is time in hours, and the left ordinate of FIGS. 6-10 represents the catalytic capacity of the catalyst, i.e., how many mg of uranyl ions can be catalytically reduced per gram of catalyst.

Experiments show that under the conditions that the reaction temperature is 80-130 ℃, the preferable reaction temperature is 95-115 ℃, the pH value of the wastewater is 4.5-8, and the preferable pH value of the wastewater is 5-7, and the time of catalytic reduction is more than 1 hour, preferably more than 2 hours, and more preferably more than 12 hours, uranyl ions in the uranium-containing wastewater are catalytically reduced into tetravalent uranium ions to be precipitated in the wastewater.

Specifically, it can be seen from experimental data that: the adsorbent of 3mg and the reducing agent thiourea of 500mg have good catalytic reduction effect on the uranium (VI) solution under the conditions that the initial uranium concentration of the uranium solution is 50mg/L, the volume is 400mL, the temperature is 100 ℃ and the pH value is 6, and the reduction effect reaches 6500 mg/g. This is the amount of uranyl ion treatment that is not at all achievable with typical adsorbents or catalysts. This provides a new solution for uranium waste water treatment.

Example 3

Putting a certain mass of the prepared nano-scale magnetic Co/Mn spinel into uranium (uranyl ion) containing wastewater with a certain volume and a certain concentration, then putting the wastewater into a pressure reaction kettle, adjusting the temperature to a certain value, and carrying out adsorption reaction. After adsorption is finished, taking supernatant, filtering the supernatant by using a 0.45 mu m microporous filter membrane, and detecting the uranium ion concentration by using an ultraviolet spectrophotometer.

After complete adsorption, hexavalent uranyl ions are adsorbed on the solid cobalt manganese spinel, so that the uranyl ions can be separated from the wastewater.

Fig. 11 is a scanned picture of cobalt-manganese spinel after adsorbing uranium, and it can be seen from the picture that uranyl ions are adsorbed on the surface of cobalt-manganese spinel to form a rough spherical shape. As can be seen from comparison with the SEM photograph before the adsorption reaction, that is, fig. 2, the nano-scale cobalt manganese spinel after adsorbing uranium is agglomerated.

Fig. 12 is an EDS picture of cobalt manganese spinel after uranium adsorption. By comparing fig. 12 with the EDS spectrum of the nano cobalt manganese spinel before participating in the adsorption reaction, i.e., fig. 1, we can clearly see that the uranium element newly appears after adsorption. This phenomenon indicates that uranyl ions are adsorbed by the nanoscale cobalt manganese spinel adsorbent.

Fig. 13-17 are graphs of experimental data corresponding to example 3, where the abscissa of fig. 14 is time in hours and the left ordinates of fig. 13-17 each represent the adsorption capacity of the adsorbent, i.e., how many mg of uranyl ions can be adsorbed per gram of adsorbent.

Experiments show that under the conditions that the adsorption temperature is 80-130 ℃, the preferred adsorption temperature is 85-120 ℃, the pH value of the wastewater is 5-9, and the preferred pH value of the wastewater is 5.5-8, and the adsorption time is more than 6 hours, preferably 8-30 hours, uranyl ions in the uranium-containing wastewater are adsorbed on the nano magnetic cobalt-manganese adsorbent.

Specifically, the 5mg adsorbent has a good adsorption effect on the uranium (VI) solution under the conditions that the initial uranium concentration of the uranium solution is 50mg/L, the volume of wastewater is 320mL, the temperature is 100 ℃ and the pH value is 6, and the adsorption effect reaches 3200 mg/g.

The foregoing is a more detailed description of the invention in connection with specific preferred embodiments and it is not intended that the invention be limited to these specific details. For those skilled in the art to which the invention pertains, several simple deductions and substitutions can be made without departing from the spirit of the invention, and all shall be considered as belonging to the protection scope of the invention.

Claims

1. A method for catalytically reducing uranyl ions in uranium-containing wastewater comprises the steps of using a nano-scale magnetic cobalt-manganese spinel as a catalyst, using thiourea as a reducing agent, and catalytically reducing the uranyl ions in the uranium-containing wastewater into tetravalent uranium ions to precipitate in the wastewater under the conditions that the reaction temperature is 80-130 ℃ and the pH value of the wastewater is 4.5-8.

2. The method of claim 1, wherein the reaction temperature of the catalytic reduction is 95-115 ℃, preferably 100 ℃, and the pH value of the wastewater is 5-7, preferably the pH value of the wastewater is 6.

3. The method according to claim 1, wherein the time for the catalytic reduction is 1 hour or more, preferably 2 hours or more, more preferably 12 hours or more, and still more preferably 16 to 30 hours.

4. The method as claimed in claim 1, wherein the nanoscale magnetic cobalt manganese spinel is prepared by a method comprising: dissolving a cobalt source and a manganese source in a mixed solution containing ammonia water and ammonium chloride, adding sodium hypochlorite as an oxidant, oxidizing part of bivalent manganese into trivalent manganese to form a homogeneous system with coexisting bivalent and trivalent manganese ions, reacting for more than 2 hours at 120-180 ℃ by taking urea as a precipitator to crystallize to form a precursor, and calcining the precursor sample at 400-600 ℃ for more than 2 hours to obtain the nanoscale magnetic cobalt-manganese spinel.

5. The method according to claim 4, wherein the cobalt source and the manganese source are cobalt chloride hexahydrate and manganese chloride tetrahydrate, and the oxidation step in the preparation method is performed in an oxygen-free environment.

6. The method of claim 4, wherein the ratio of the amounts of the cobalt to manganese species in the cobalt and manganese sources is 1: 1 to 6, wherein the ratio of the amount of cobalt element to the amount of ammonia water is 1: 2 to 10, the ratio of the amount of cobalt element to the amount of ammonium chloride is 1: 2-12, the mass ratio of cobalt element to sodium hypochlorite is 1: 0.1 to 100, the ratio of the amount of cobalt to the amount of urea is 1: 2 to 100.

7. The method of claim 6, wherein the ratio of the amounts of the cobalt to manganese species in the cobalt and manganese sources is 1: 2-3, preferably 1: 2.5, the mass ratio of the cobalt element to the ammonia water is 1: 4-8, preferably 1: 6, the mass ratio of cobalt element to ammonium chloride is 1: 4-8, preferably 1: 6, the mass ratio of cobalt element to sodium hypochlorite is 1: 0.5-10, preferably 1: 1, the ratio of the amount of cobalt element to the amount of urea substance is 1: 5-50, preferably 1: 35.

8. the method as claimed in claim 4, wherein the nano-scale magnetic cobalt manganese spinel is obtained by dissolving urea and ammonium chloride in water to obtain a solution A, dissolving a cobalt source and a manganese source in water to obtain a solution B, uniformly mixing the solutions A and B, adding an ammonia water solution C, adding a sodium hypochlorite solution D, oxidizing for 10-120 min with sodium hypochlorite under an oxygen-free condition, and then crystallizing and calcining the solution.

9. The method according to claim 4, wherein the crystallization temperature is 140 to 160 ℃, the crystallization time is 4 to 20 hours, the calcination temperature is 450 to 550 ℃, and the calcination time is 4 to 8 hours.

10. The method according to claim 4, further comprising a step of washing and drying the precursor between the step of crystallizing and the step of calcining, wherein the washing comprises alternately washing with ethanol and deoxidized deionized water, and the drying is atmospheric drying or vacuum drying at 40-150 ℃.