Manganese oxidizing bacterium granular dry powder and method for passivating Cd in overlying water in rice field by using same

Abstract

The invention belongs to the field of agricultural non-point source pollution control and heavy metal Cd prevention and control, and particularly relates to manganese oxidizing bacterium granular dry powder and a method for passivating Cd in overlying water of a rice field by using the manganese oxidizing bacterium granular dry powder. Manganese oxidizing bacteria granular dry powder containing biomass charcoal and manganese oxidizing bacteria is prepared, the prepared manganese oxidizing bacteria granular dry powder is applied to a rice field, then manganese salt and organic fertilizer are evenly mixed to form manganese fertilizer, the manganese fertilizer is applied to the rice field before irrigation so as to control Cd and manganese oxide minerals in overlying water of the rice field to continuously grow in the biological formation process, and the yield of the rice field is increased. Cd is more easily adsorbed to vacancies of manganese oxide or enters an internal structure, so that the bioavailability of Cd is reduced, Cd is prevented from continuously moving into soil, the absorption of plant root systems on Cd in a soil solution is reduced, and finally, the content of Cd in rice is reduced.

Description

Technical Field

[0001] The invention belongs to the field of agricultural non-point source pollution control and heavy metal Cd prevention and control, and particularly relates to a granular dry powder of manganese oxidizing bacteria and a method for passivating Cd in overlying water of rice fields. Background Art

[0002] Rice is a typical Cd-accumulating plant. If the soil is acidic and the Cd activity is high, the problem of excessive Cd in rice is particularly prominent. If rice with excessive Cd is consumed for a long time, it will have toxic effects on the kidneys, bones, cardiovascular system, etc. and cause a variety of diseases, which seriously endangers human health. Therefore, it is of great significance to ensure the safe production of rice and reduce the risk of human Cd exposure. In recent years, a large number of studies have shown that human activities, including mining, smelting, leather-making wastewater, domestic garbage and industrial waste, have caused irrigation water to gradually become the main source of Cd pollution in rice fields. At the same time, due to atmospheric dry and wet deposition, especially in the southern areas with rich mineral resources or intensive industrial activities, Cd-containing particulate matter also continuously flows into the overlying water system of rice fields. Then, the exogenous input Cd in the overlying water is transported vertically and horizontally into the soil under the action of gravity, water flow, etc., and the rice root cells absorb the effective Cd in the soil solution. 2+ 、CdCl + , loaded into the xylem and transported to the aerial part of rice through the phloem, and finally accumulated in the grains.

[0003] Currently, a large number of experimental studies and applications are underway both domestically and internationally to control heavy metal pollution in soil. These efforts primarily involve adding calcium-silicon-based alkaline materials, clay minerals, organic matter, biochar, and magnetic nanomaterials to contaminated soils to alter the Cd form in the soil, reducing its bioavailability and thereby reducing its accumulation in different rice plant parts. However, given that overlying water is becoming a sink for pollutants in rice paddies, the authors believe that intercepting Cd in overlying water and preventing its migration into the soil warrants greater attention.

[0004] Some researchers have proposed using a combination of constructed wetlands to significantly reduce Cd in irrigation water. However, this method is time-consuming, space-intensive, and complex, making it unsuitable for large-scale deployment. Furthermore, it only addresses Cd in irrigation water and has no effect on controlling Cd entering the overlying water of paddy fields through atmospheric dry and wet deposition. Other researchers have used slaked lime, straw biochar, and artificial zeolite as control materials. In laboratory experiments simulating stagnant ponds and reservoirs, they applied these materials slowly and evenly across the water surface to reduce dissolved Cd ion concentrations in the overlying water. However, these control agents require high application rates, ranging from 5 to 20 tons per hectare of paddy field. This makes transportation difficult and significantly impacts the physical and chemical properties of paddy soil and its microbial ecosystem, potentially leading to secondary pollution.

[0005] Manganese (Mn) is the second most abundant metallic element in the Earth's crust after iron. Manganese oxide minerals are also commonly found in soil environments. They possess low zero-point charge, strong surface activity, and high redox potential, resulting in strong adsorption and co-precipitation of heavy metals. They serve as a key precipitation reservoir for Cd and have demonstrated significant potential for controlling heavy metals in rice fields. Numerous studies indicate that the presence of manganese oxide minerals in soil is primarily attributed to the biooxidation of manganese-oxidizing bacteria. Our research group has isolated an acid-tolerant strain of manganese-oxidizing bacteria from manganese-rich soil environments. Klebsiella sp. M3, deposited with the China Center for Type Culture Collection on March 24, 2021, under the accession number CCTCC NO: M 2021261, is capable of growing under acidic, neutral, and weakly alkaline conditions and efficiently oxidizing Mn ions. Furthermore, during the formation of manganese oxide minerals, the strain continuously grows, absorbing other toxic and hazardous metals into the vacancies of the manganese oxide or into its internal structure. In summary, under natural conditions, the passivation of Cd by biogenic manganese oxides by native manganese-oxidizing bacteria in paddy soils is relatively slow, resulting in exogenous Cd in the overlying water still migrating into the soil, being absorbed by rice roots, and concentrating in the rice grains. Therefore, enhancing the passivation ability of manganese-oxidizing bacteria in overlying water for Cd and effectively regulating the formation of biogenic manganese oxide minerals to maximize Cd interception in the paddy soil surface are key to preventing and controlling Cd contamination in rice.

[0006] In conclusion, seeking an economically feasible and environmentally friendly in situ control measure for Cd in paddy field overlying water is of great significance to improving the safety and quality of rice. Summary of the Invention

[0007] The present invention aims to solve the above technical problems and provide a granular dry powder of manganese oxidizing bacteria and a method for passivating Cd in overlying water in rice fields. The preparation method of the granular dry powder of manganese oxidizing bacteria comprises: (1) obtaining biochar, and pretreating the biochar to obtain pretreated biochar; (2) obtaining manganese oxidizing bacteria and activating and culturing them, and centrifuging the activated cultured bacterial solution to collect the bacterial cells; (3) mixing the collected bacteria with a protective agent to form a bacterial suspension, wherein the protective agent is a mixture of trehalose, peptone, yeast extract, and glutathione; (4) adding the pretreated biochar to the bacterial suspension, and oscillating and adsorbing for 2-4 hours to allow the bacteria to fully adhere to the biochar material to obtain a bacteria-charcoal complex; (5) Mixing the bacteria-charcoal complex with sodium carboxymethyl cellulose solution to form 1-3 mm particles; (6) The particles are hot-air dried until the moisture content is less than 10%; the particles are pre-frozen at -15 to -20°C, and then vacuum-dried for 36 to 48 hours to obtain a granular dry powder of manganese oxidizing bacteria.

[0008] Furthermore, the biomass charcoal is rice husk charcoal or straw charcoal, and has a pH of 8 to 10; Furthermore, the pretreatment includes washing with deionized water and drying; Furthermore, pretreated biochar was added to the bacterial suspension, wherein the bacterial dry weight: biochar weight = 1:5; Furthermore, the temperature of the hot air drying is controlled at 40-60°C; Furthermore, the dried biochar is sieved; Furthermore, the protective agent comprises, by mass percentage, an aqueous solution of 4%-5% trehalose, 2% peptone, 1%-2% yeast extract, and 0.05% glutathione; Furthermore, the manganese oxidizing bacteria is Klebsiella sp. M3, deposit number CCTCC NO: M 2021261; Furthermore, the pretreated biochar was added to the manganese oxidizing bacteria collected by centrifugation at a bacteria:charcoal mass ratio of 1:5, and adsorbed under shaking at 25°C for 2-4 hours; Furthermore, the bacteria-charcoal complex is mixed with a 4%-5% sodium carboxymethylcellulose solution and formed into 1-3 mm particles by an extruder; Furthermore, the manganese oxidizing bacteria were deposited in the China Center for Type Culture Collection, Wuhan, China, on March 24, 2021; The present application also provides a method for passivating Cd in overlying water of rice fields using granular dry powder of manganese oxidizing bacteria, comprising the following steps: (1) Apply the prepared manganese oxidizing bacteria granular dry powder to the rice field at a rate of 0.35-0.6 t / ha; (2) Manganese fertilizer application: manganese salt is mixed evenly with organic fertilizer to form manganese fertilizer, and the manganese fertilizer is applied to the rice field before irrigation; Furthermore, the manganese oxidizing bacteria granular dry powder is applied to the paddy field in three times: during the paddy field soaking period, 0.15-0.2 t / ha of granular dry powder is applied to the paddy field once; after the rice enters the tillering stage, 0.1-0.15 t / ha of granular dry powder is applied to the paddy field once; after the rice enters the jointing stage, 0.1-0.15 t / ha of granular dry powder is applied to the paddy field once; Furthermore, the manganese salt used is manganese sulfate; Furthermore, the manganese fertilizer is applied 5 to 8 times in total. During the rice transplanting and tillering stages, the application rate of manganese sulfate per hectare of paddy field is 3.75 to 4.5 kg each time; during the rice jointing and booting stages, the application rate of manganese sulfate per hectare of paddy field is 7.5 to 10.5 kg each time; The beneficial effects of the present invention are: The preparation method of the granular dry powder particles of manganese oxidizing bacteria provided by the present invention is simple, and the dry powder particles and manganese fertilizer can be directly applied to rice fields in multiple times, and the operation and application are simple, flexible, and non-toxic. After application, the manganese oxidizing bacteria have strong adaptability to the rice field environment, can grow rapidly, and exert oxidizing activity. More importantly, the manganese oxide minerals continue to grow during the biological formation process, making it easier for Cd to be adsorbed into the vacancies of manganese oxides or enter the internal structure, thereby reducing the bioavailability of Cd, preventing Cd from continuing to migrate into the soil, reducing the absorption of Cd in the soil solution by plant roots, and ultimately reducing the Cd content in rice.

[0009] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application so that it can be implemented in accordance with the contents of the specification, and to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the following is a detailed description of the preferred embodiment of the present application in conjunction with the accompanying drawings.

[0010] Based on the detailed description of the specific embodiments of the present application in conjunction with the accompanying drawings below, those skilled in the art will become more aware of the above and other objects, advantages and features of the present application. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings required for the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present application. For those of ordinary skill in the art, other drawings can also be obtained based on these drawings without inventive work. In all drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn according to the actual scale.

[0012] Figure 1 for Klebsiella sp. HRTEM image of Cd passivation product during M3 manganese oxide process; Figure 2 This is a comparison chart of the content of different forms of Mn in the surface soil of the experimental field and the control field; Figure 3 This is a comparison chart of the available Cd content in the surface soil of the experimental field and the control field. Specific embodiments In order to make the purpose, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all of the embodiments. In the following description, specific details such as specific configurations and components are provided only to help fully understand the embodiments of the present application. Therefore, it should be clear to those skilled in the art that various changes and modifications can be made to the embodiments described herein without departing from the scope and spirit of the present application. In addition, for clarity and brevity, the description of known functions and structures has been omitted in the embodiments.

[0014] It should be understood that references throughout this specification to "one embodiment" or "this embodiment" mean that a particular feature, structure, or characteristic associated with the embodiment is included in at least one embodiment of the present application. Therefore, the appearance of "one embodiment" or "this embodiment" throughout this specification does not necessarily refer to the same embodiment. Furthermore, these particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0015] In addition, the present application may repeat reference numerals and / or letters in different examples. This repetition is for the purpose of simplicity and clarity and does not in itself indicate the relationship between the various embodiments and / or settings discussed.

[0016] The term "and / or" in this article is only a description of the association relationship of associated objects, indicating that three relationships can exist. For example, A and / or B can mean: A exists alone, B exists alone, and A and B exist at the same time. The term " / and" in this article describes another type of association object relationship, indicating that two relationships can exist. For example, A / and B can mean: A exists alone, and A and B exist alone. In addition, the character " / " in this article generally indicates that the previous and subsequent associated objects are in an "or" relationship.

[0017] The term "at least one" in this article is merely a description of the association relationship between associated objects, indicating that three relationships may exist. For example, at least one of A and B can mean: A exists alone, A and B exist at the same time, and B exists alone.

[0018] It should also be noted that, in this document, relational terms such as first and second are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "include," "comprises," or any other variations thereof are intended to cover non-exclusive inclusion.

[0019] Examples 1 to 4 are based on experiments in rice fields near a tungsten mine in Lin'an District, Hangzhou. The region has a subtropical monsoon climate along the middle and lower reaches of the Yangtze River, with an average annual temperature of 16°C and approximately 1500 mm of annual precipitation. Two plots of equal size (8 m × 5 m in length and width) were selected near a tungsten mine. The soil pH was 5.5, and the total Cd content was 0.8 mg / kg. Both plots were planted with the rice variety "Huazhongyou 9326." Rice seedlings were transplanted in mid-June. Fertilizer and water management were implemented according to local planting practices, with irrigation primarily coming from nearby rivers and natural precipitation. One plot served as an experimental field, with 0.16 t / ha of granular dry powder applied during the flooding phase, 0.15 t / ha after the rice entered the tillering stage, and 0.1 t / ha after the rice entered the jointing stage. Manganese sulfate was mixed evenly with organic fertilizer and applied to the paddy field before irrigation, without tilling. Four applications of 4.0 kg / ha of manganese sulfate were applied during the transplanting and tillering stages, and two applications of 8.0 kg / ha were applied during the jointing and booting stages. A separate plot served as a control field, where no manganese oxidizing bacteria granules or manganese salts were applied throughout the rice cultivation period.

[0020] Example 1 Comparison of Cd concentrations in overlying water samples between the experimental and control fields.

[0021] In the third year of the experiment, Cd concentrations in the overlying water of the experimental and control fields were collected and measured. The results showed that Cd concentrations in the overlying water of the experimental rice fields during the tillering, jointing, heading, and maturity stages were 20.2, 15.5, 10.3, and 8.2 μg / L, respectively; while Cd concentrations in the overlying water of the control rice fields during the tillering, jointing, heading, and maturity stages were 70.5, 60.4, 70.8, and 57.5 μg / L, respectively. This suggests that Cd is entering the overlying water of the rice fields surrounding the tungsten mining area through river irrigation water and atmospheric deposition. Although Cd concentrations in the overlying water of the control fields did not exceed the Cd limit of 0.1 mg / L specified in the "Standard for Agricultural Irrigation Water Quality" (GB 5084-2021), rice is a high Cd accumulator, easily absorbing Cd through its roots and transferring it to its grains, posing a potential health risk to long-term consumption. However, with the repeated application of manganese-oxidizing bacteria granular dry powder and manganese salts in the experimental field, the Cd concentration in the overlying water of the experimental field was much lower than that in the control field during the same period. It is speculated that this is because the experimental field produces manganese oxides, which have a strong adsorption and passivation ability for Cd, thereby reducing the Cd concentration in the overlying water.

[0022] To further prove this conjecture, we will Klebsiella sp. M3 was cultured in a liquid medium rich in Mn and Cd ions. After 3 days, the solid product was recovered and characterized by HRTEM-EDS. Figure 1As shown in the figure, the main elements in the solid product are C, O, Mn, Fe, and Cd, among which C mainly comes from the bacteria and Fe comes from the culture medium. The Mn and O elements indicate that a large amount of manganese oxides are generated in the product. The most important thing is that the Cd element is evenly distributed among the elements, and the EDS results show that the mass proportion of Cd is 2.17%, which proves that the biogenic manganese oxides in the water body do adsorb and fix Cd, thereby removing it from the solution.

[0023] Example 2 Comparison of Mn contents in different forms in the surface soil of paddy fields between the experimental group and the control group.

[0024] In the third year, the Mn occurrence forms and contents in the topsoil (about 5 cm) of the experimental and control fields were collected and measured. Figure 2 It can be seen that the Mn forms in the topsoil of the control fields remained largely unchanged at different rice growth stages, with acid-soluble Mn accounting for approximately 40%, organically bound Mn for approximately 3%, reducible Mn for approximately 15%, and residual Mn for approximately 42%. Compared to the control fields, the proportions of residual and organically bound Mn in the experimental fields after application of manganese salts and manganese-oxidizing bacteria granular powder remained largely unchanged, but the proportion of acid-soluble Mn decreased by 9%-17%, while the proportion of reducible Mn increased by 6%-15%. This suggests that the granular manganese-oxidizing bacteria powder has a strong adaptability to the environment. As Mn ions in the overlying water above the added manganese salts are oxidized by the manganese-oxidizing bacteria, high-valent manganese oxides are produced, which are then deposited in the topsoil.

[0025] Example 3 Comparison of available Cd content in the surface soil of paddy fields between the experimental group and the control group.

[0026] In the third year of the experiment, the DTPA-extractable Cd content in the surface soil (about 5 cm) of the rice planted in the experimental field and the control field at the tillering, jointing, heading and maturity stages was collected and measured. Figure 3 It can be seen that the DTPA-extractable Cd content in the surface soil of the control field is 0.16~0.20 mg / kg, while the DTPA-extractable Cd content in the surface soil of the experimental field is about 0.075~0.10 mg / kg. It can be seen that after applying the granular dry powder of manganese oxidizing bacteria and manganese salts, the content of available Cd in the surface soil is significantly reduced. Figure 2 The proportion of acid-soluble Mn in the surface soil decreased and the proportion of reducible Mn increased, indicating that biogenic manganese oxides not only adsorbed Cd in the overlying water, but also changed the occurrence form of Cd and reduced its bioavailability.

[0027] Example 4 Comparison of Cd content in the roots of rice plants in the experimental group and the control group.

[0028] In the third year of the experiment, rice root samples were collected simultaneously from the experimental field and the control field during the rice maturity period. After digestion and determination, it was found that the Cd content in the rice roots of the experimental field was 0.9 mg / kg, significantly lower than the 2.3 mg / kg in the control field, a decrease of 60.9%. This result shows that under the same rice variety and planting conditions, long-term influence of river irrigation near the mining area and atmospheric dry and wet deposition will cause significant Cd enrichment in the rice roots. However, through the intervention of the combined application technology of manganese oxidizing bacteria dry powder particles and manganese salts, the Cd content in the roots of rice planted in the experimental field was significantly reduced. The technical method provided by the present invention can effectively intercept Cd in the overlying water, reduce its bioavailability, and thus block the migration of Cd to the rice roots, providing a reliable path for ensuring food security.

[0029] The steps for passivating Cd in the overlying water of the test field with granular dry powder of manganese oxidizing bacteria in Examples 5 to 7 are the same as those in Examples 1 to 4, except that the amount of granular dry powder of manganese oxidizing bacteria and the amount of manganese salt used are different, as shown in Table 1. Table 1 Test results of Examples 5-6 , As can be seen from Table 1, compared with the control field not using the technology of the present invention, the method provided by the present invention can passivate Cd in the overlying water of the rice field, reduce the bioavailability of Cd, and reduce the absorption of Cd by the rice roots, so that the Cd content in the rice roots is reduced by 55.65%-64.3%, thereby ultimately reducing the Cd content in the rice.

[0030] Comparative Example 1 A separate plot (8 m × 5 m in length × width) was selected near the experimental and control fields and similarly planted with "Huazhongyou 9326" rice. Rice seedlings were transplanted in mid-June. Fertilizer and water management was implemented according to local planting practices, with irrigation primarily coming from nearby rivers and natural precipitation. A 0.16 t / ha granular dry powder was applied during the paddy-soaking period, followed by a further 0.16 t / ha of granular dry powder after the rice entered the tillering stage, and again at 0.1 t / ha after the rice entered the jointing stage. No manganese salts were applied throughout the planting process. In the third year, Cd concentrations in overlying water, the content of different manganese forms in the surface soil, DTPA-extractable Cd content, and Cd content in plant roots were measured using the same sampling methods as in Examples 1-4. The results showed that the Cd concentrations in overlying water during the tillering, jointing, heading and maturity stages of rice were 60.1, 48.2, 56.9 and 49.6 μg / L, respectively; the acid-soluble Mn in the surface soil accounted for about 27%, the organically bound Mn accounted for about 4%, the reducible Mn accounted for about 26%, and the residual Mn accounted for about 43%; the DTPA-extractable Cd content in the surface soil was 0.15-0.18 mg / kg; and the Cd content in the rice roots during maturity was 2.1 mg / kg. It can be seen that compared with the control field, the application of manganese oxidizing bacteria dry powder particles changed the manganese storage form in the surface soil, reducing the proportion of acid-soluble Mn and increasing the proportion of reducible Mn, indicating that the added manganese oxidizing bacteria can oxidize and precipitate the inherent dissolved Mn in irrigation water or soil, and at the same time passivate Cd, resulting in a decrease in the Cd concentration in the overlying water, the effective Cd content in the surface soil, and the Cd content in the rice roots. However, due to the low concentration of dissolved manganese, the amount of manganese oxides produced is insufficient, and the passivation efficiency of Cd is far lower than that of the experimental field treatment.

[0031] Comparative Example 2 A separate plot (8 m × 5 m in length × width) was selected near the experimental and control fields and similarly planted with "Huazhongyou 9326" rice. Rice seedlings were transplanted in mid-June. Fertilizer and water management was implemented according to local planting practices, with irrigation water primarily coming from nearby rivers and natural precipitation. Manganese sulfate was mixed evenly with organic fertilizer and applied to the paddy fields before irrigation, without plowing. Four applications of 4.0 kg / ha of manganese sulfate were applied during the transplanting and tillering stages, and two applications of 8.0 kg / ha were applied during the jointing and booting stages. Granular dry powder of manganese-oxidizing bacteria was not applied during the entire planting process. In the third year, Cd concentrations in overlying water, the content of different Mn forms in the surface soil, DTPA-extractable Cd content, and Cd content in plant roots were measured using the same sampling methods as in Examples 1-5. The results showed that Cd concentrations in overlying water ranged from 50.2 to 61.3 μg / L; acid-soluble Mn accounted for approximately 45% of surface soil, organically bound Mn for approximately 3%, reducible Mn for approximately 12%, and residual Mn for approximately 40%; DTPA-extractable Cd content in surface soil ranged from 0.13 to 0.17 mg / kg; and Cd content in rice roots at maturity was 1.8 mg / kg. Compared to the control field, three consecutive years of applying appropriate amounts of manganese salt reduced Cd concentrations in overlying water, available Cd content in surface soil, and Cd content in rice roots. However, the Cd passivation efficiency was far lower than that observed in the experimental field. This is because the inherent manganese oxidizing bacteria in the soil can oxidize dissolved Mn to produce manganese oxides, thereby passivating Cd. However, due to the wide variety of soil microorganisms and the fact that the inherent manganese oxidizing bacteria are not the dominant species, the manganese ions produced by the dissolution of added manganese salts cannot be effectively oxidized. The amount of manganese oxides produced is low and the passivation effect on Cd is weak. Instead, they migrate into the soil solution, causing the acid-soluble manganese content to increase.

[0032] The foregoing description is merely a preferred embodiment of the present invention and does not limit the scope of protection of the present invention. Those skilled in the art will readily appreciate that the present invention is susceptible to various modifications and variations. Any variation, modification, substitution, integration, or parameter change to these embodiments, while remaining within the spirit and principles of the present invention and achieving the same functionality, through conventional substitution or by otherwise achieving the same functionality, falls within the scope of protection defined by the claims of the present invention.

Claims

1. A method for preparing a granular dry powder of manganese oxidizing bacteria, characterized in that: include: (1) obtaining biochar, and pretreating the biochar to obtain pretreated biochar; (2) obtaining manganese oxidizing bacteria and activating and culturing them, and centrifuging the activated cultured bacterial solution to collect the bacterial cells; (3) mixing the collected bacteria with a protective agent to form a bacterial suspension, wherein the protective agent is an aqueous solution including trehalose, peptone, yeast extract, and glutathione; (4) adding the pretreated biochar to the bacterial suspension, and oscillating and adsorbing for 2-4 hours to allow the bacteria to fully adhere to the biochar material to obtain a bacteria-charcoal complex; (5) Mixing the bacteria-charcoal complex with sodium carboxymethyl cellulose solution to form 1-3 mm particles; (6) The particles are hot-air dried until the moisture content is less than 10%; the particles are pre-frozen at -15 to -20°C, and then vacuum-dried for 36 to 48 hours to obtain a granular dry powder of manganese oxidizing bacteria.

2. The method for preparing a granular dry powder of manganese oxidizing bacteria according to claim 1, characterized in that: The biomass charcoal is rice husk charcoal or straw charcoal, and has a pH of 8-10.

3. The method for preparing a granular dry powder of manganese oxidizing bacteria according to claim 1, characterized in that: Pretreated biochar was added to the bacterial suspension, with the dry weight of bacteria: weight of biochar being 1:

5.

4. The method for preparing a granular dry powder of manganese oxidizing bacteria according to claim 1, characterized in that: The protective agent comprises, by mass percentage, an aqueous solution of 4%-5% trehalose, 2% peptone, 1%-2% yeast extract, and 0.05% glutathione.

5. The method for preparing a granular dry powder of manganese oxidizing bacteria according to claim 1, characterized in that: The pretreated biochar was added to the manganese oxidizing bacteria collected by centrifugation at a bacteria:charcoal mass ratio of 1:5, and adsorbed under shaking at 25°C for 2-4 hours.

6. Manganese oxidizing bacteria granular dry powder prepared according to the method for preparing manganese oxidizing bacteria granular dry powder according to any one of claims 1 to 5.

7. The method for passivating Cd in overlying water of paddy fields using granular dry powder of manganese oxidizing bacteria according to claim 6, characterized in that: (1) Apply the prepared manganese oxidizing bacteria granular dry powder to the rice field at a rate of 0.35-0.6 t / ha; (2) Application of manganese fertilizer: Manganese salt is mixed evenly with organic fertilizer to form manganese fertilizer, and the manganese fertilizer is applied to the rice field before irrigation.

8. The method for passivating Cd in overlying water of paddy fields using granular dry powder of manganese oxidizing bacteria according to claim 7, characterized in that: The manganese oxidizing bacteria granular dry powder is applied to the paddy field three times: during the paddy field soaking period, 0.15-0.2 t / ha of granular dry powder is applied to the paddy field once; after the rice enters the tillering period, 0.1-0.15 t / ha of granular dry powder is applied to the paddy field once; after the rice enters the jointing period, 0.1-0.15 t / ha of granular dry powder is applied to the paddy field once.

9. The method for passivating Cd in overlying water of paddy fields using granular dry powder of manganese oxidizing bacteria according to claim 7, characterized in that: The manganese salt used is manganese sulfate.

10. The method for passivating Cd in overlying water of paddy fields using granular dry powder of manganese oxidizing bacteria according to claim 7, characterized in that: The manganese fertilizer is applied 5 to 8 times in total. During the transplanting and tillering stages, the application rate of manganese sulfate per hectare of paddy field is 3.75 to 4.5 kg each time; during the jointing and booting stages of rice, the application rate of manganese sulfate per hectare of paddy field is 7.5 to 10.5 kg each time.

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