A coated sustained-release material based on microbial mineralization technology and its preparation method

Abstract

The present invention belongs to the technical field of heavy metal pollution control, and in particular discloses a coated slow-release material based on microbial mineralization technology that can achieve urea slow release and a preparation method thereof. The coated slow-release material uses urea particles as the core, and is coated with urea particles inside by two coating layers of an inner coating layer made of a urease microorganism-polysaccharide polymer complex and an outer coating layer made of calcium carbonate. When it is applied to heavy metal wastewater, urea is slowly released, and the problem of premature hydrolysis of urea caused by directly adding urea to heavy metal wastewater in the existing MICP technology is avoided, thereby avoiding urea waste. At the same time, the outer coating layer also regulates the pH of the heavy metal wastewater applied, thereby protecting the urease microorganism coated therein and preventing adverse effects on activity. It is novel in structure and simple to prepare, and has good application value and use prospects.

Description

Technical Field

[0001] The present invention belongs to the technical field of heavy metal pollution control, and specifically relates to a coated slow-release material based on microbial mineralization technology capable of achieving slow-release of urea and a preparation method thereof. Background Art

[0002] Microbial mineralization is a ubiquitous phenomenon in nature, whereby certain microorganisms utilize their metabolic activities to generate a variety of mineral crystals. Because microbial mineralization technology does not require the addition of large doses of precipitants and involves the metabolism of organisms, cells, and organic matter, it offers advantages over traditional chemical cementation mineralization, such as being environmentally friendly, pollution-free, and ecologically compatible.

[0003] With the rise of microbial mineralization technology, it has been proposed in recent years for treating heavy metal-contaminated wastewater. This technology, known as microbially induced calcium carbonate precipitation (MICP), involves the selective precipitation of heavy metal ions from the environment onto specific organic matter, under the control and induction of biomacromolecules, to form minerals. This process relies on electrostatic interactions between extracellular polymeric substances (EPS) and metal ions to immobilize the metal ions, ultimately transforming them into solid-phase minerals under biological control and influence. For example, a related study has reported the use of a urease-producing bacterium (Sporosarcina pasteurianus) to decompose urea to produce carbonate ions. The resulting carbonate ions then react with heavy metal ions to form insoluble carbonate precipitates, thereby achieving the desired metal removal effect.

[0004] In the past, MICP technology, when used to immobilize heavy metals, involved selecting the microbial strains required for MICP and culturing them for active growth. The bacterial solution was then evenly mixed with a binder (composed of calcium chloride and urea) and applied to the heavy metal-containing wastewater to be treated, where the removal efficiency was measured. However, this current MICP technology typically relies on dissolving excess urea in the wastewater. This results in premature hydrolysis of the urea by urease before the strain-urea mixture is added to the wastewater, reducing the effectiveness of heavy metal immobilization. This, in turn, leads to excessive urea waste and premature hydrolysis in the environment. Summary of the Invention

[0005] To address the current problem of premature decomposition of urea when directly added to wastewater, which occurs when MICP technology is currently used to treat heavy metal pollution in water, the inventors of this invention, building on their extensive research into MICP technology, have improved the structure of this type of material and provided a coated slow-release material. This coated slow-release material releases urea only when applied to heavy metal wastewater, avoiding the premature hydrolysis of urea caused by direct addition, minimizing urea waste and improving treatment efficiency.

[0006] Specifically, the present invention provides the following technical solutions:

[0007] A coated sustained-release material based on microbial mineralization technology comprises a core made of urea granules, an inner coating layer made of a urease microorganism-polysaccharide polymer complex coated on the surface of the core, and an outer coating layer made of calcium carbonate coated on the surface of the inner coating layer.

[0008] Specifically, in the coated sustained-release material, in addition to urea particles, the core material contains a small amount of carbonate ions produced by the decomposition of urea and urease microorganisms due to their contact. However, since this part of the structure is almost an anhydrous environment, the amount of urea decomposed by the contact between the two is extremely small.

[0009] At the same time, the calcium carbonate used as the material of the outer coating layer is also mainly formed by the carbonate ions produced by the decomposition of part of the urea. The formed calcium carbonate uses the inner coating layer particles coated with the core as the nucleation site, and is deposited on the surface of the inner coating layer to form the outer coating layer.

[0010] In this way, the easily degradable urea granules are encapsulated within a double coating. Even when in contact with the inner coating layer of urease-producing microorganisms, the nearly waterless environment prevents them from prematurely degrading. Simultaneously, the outermost layer is coated with a dense, hard outer calcium carbonate coating, preserving the active ingredients (urea and urease-producing microorganisms) essential for MICP technology in their original state. When this coated sustained-release material is applied to wastewater containing heavy metals, the typically acidic nature of the wastewater disrupts the calcium carbonate structure of the outer coating layer, slowly releasing the urea granules and urease-producing microorganisms within. At this point, the urease-producing microorganisms metabolize and produce urease. Urea, in an aqueous environment, is first hydrolyzed by urease into ammonia and carbamic acid. Carbamic acid then spontaneously hydrolyzes into ammonia and carbonic acid. Heavy metal ions in the wastewater react with carbonate ions produced by urea hydrolysis to form carbonate precipitates, reducing the mobility of the heavy metal ions and completing the heavy metal fixation process.

[0011] Furthermore, the urea particles are spherical with a particle size of 2 mm to 5 mm.

[0012] Furthermore, the urease-producing microorganism is Bacillus pasteurianus with high urease activity. Of course, it can also be any other urease-producing microorganism currently known to be applicable to MICP technology; the polysaccharide polymer is selected from at least one of sodium alginate, nanocellulose, agar, and xanthan gum.

[0013] Furthermore, in the above-mentioned film-coated sustained-release material, the mass proportion of the core is 80% to 95%, while the total mass proportion of the inner film layer and the outer film layer is 5% to 20%.

[0014] The above-mentioned film-coated sustained-release material provided by the present invention is prepared by the following method:

[0015] Step S1, adding a urease microorganism solution to a polysaccharide polymer solution to prepare a urease microorganism-polysaccharide polymer mixed solution;

[0016] Step S2: coating the urea granules and the urease microorganism-polysaccharide polymer mixture and drying the mixture to form a single-layer coating intermediate consisting of a core made of urea granules and an inner coating layer made of the urease microorganism-polysaccharide polymer complex coated on the outer surface of the core;

[0017] Step S3: placing the above-mentioned single-layer coating intermediate in a calcium-containing mineralizing solution to carry out a mineralization reaction, forming an outer coating layer made of calcium carbonate on the outer surface of the inner coating layer, and obtaining a coating-type sustained-release material.

[0018] Specifically, in the above step S1, the volume ratio of the urease microbial solution to the polysaccharide polymer solution is 1:20 to 20:1.

[0019] Furthermore, the urease activity of the urease microbial solution is 2 U / mL to 20 U / mL (1 U represents the amount of amino nitrogen released by urease decomposing urea in the solution within 1 min).

[0020] Specifically, in the above step S2, the coating treatment can be carried out based on a coating machine in the following manner: industrial spherical granular urea is selected and placed in the coating pot of the coating machine, the urease microorganism-polysaccharide polymer mixture is placed in the feeding barrel of the coating machine, and the spray gun is turned on to spray the urease microorganism-polysaccharide polymer mixture. When a layer of sol-like liquid adheres to the surface of the spherical urea, the spraying is stopped until the surface of the spherical urea is dry, and the above-mentioned single-layer coated intermediate is obtained.

[0021] Furthermore, in the above step S2, the usage ratio of urea granules to urease microorganism-polysaccharide polymer mixed solution is 100 mL to 300 mL of urease microorganism-polysaccharide polymer mixed solution per kg of urea granules.

[0022] Specifically, in the process of forming the outer coating layer in the above-mentioned step S3, on the one hand, the above-mentioned single-layer coating intermediate is placed in a calcium-containing mineralizing liquid to undergo a mineralization reaction. The calcium ions contained in the calcium-containing mineralizing liquid undergo a cross-linking reaction with the polysaccharide polymer in the coating (i.e., the inner coating layer) to form a gel, which fixes the urease microorganism in the coating to prevent loss; on the other hand, the urease bacteria with urease activity produced by the urease microorganism contained in the coating hydrolyzes the urea in the core to produce carbonate ions, which react with the calcium ions in the calcium-containing mineralizing liquid to form insoluble calcium carbonate that adheres to the surface of the single-layer film intermediate, thereby forming the outer coating layer.

[0023] Furthermore, the calcium-containing mineralization solution can be any one of water-soluble calcium-containing solutions such as calcium chloride solution, calcium nitrate solution, and calcium acetate solution, or a mixture of at least two thereof; wherein the concentration of calcium ions is 0.1 mol / L to 2 mol / L.

[0024] Furthermore, the volume ratio of the calcium-containing mineralizing liquid to the urea granules is 1:1 to 10:1.

[0025] The present invention realizes the first coating of urea particles through a simple coating technology, and performs mineralization treatment with the carbonate ions generated in the first coating process, that is, realizes the second coating. Only when the obtained coated slow-release material is applied to acidic heavy metal-containing wastewater can the coating layer be destroyed and the urea inside thereof be slowly released, and then a microbial-induced calcium carbonate precipitation reaction occurs, thereby avoiding the waste caused by premature decomposition of urea particles directly added in the prior art.

[0026] Furthermore, the calcium carbonate outer coating formed by the second coating also protects the urease-producing microorganisms within, preventing them from coming into direct contact with acidic, heavy metal-containing wastewater during use and potentially reducing their activity. As the outer coating gradually dissolves, it also adjusts the pH of the heavy metal-containing wastewater, reducing its acidity. This not only slows the release of urea but also protects the urease-producing microorganisms.

[0027] The novel film-coated sustained-release material with the above structure has novel structure and good application performance; and the above preparation method is simple and easy to prepare, which ensures that the film-coated sustained-release material can be prepared at a relatively low cost and has good application value and use prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 is a schematic structural diagram of a film-coated sustained-release material according to Example 1 of the present invention;

[0029] Figure 2 This is a SEM image of the film-coated sustained-release material according to Example 1 of the present invention;

[0030] Figure 3 This is an XRD pattern of the film-coated sustained-release material according to Example 1 of the present invention;

[0031] Figure 4 3 is a comparison chart of fixed heavy metal removal rates of various materials provided according to the embodiments and comparative examples of the present invention. DETAILED DESCRIPTION

[0032] The present invention will be described in detail below with reference to embodiments. It should be noted that the described embodiments are only intended to facilitate understanding of the present invention and do not have any limiting effect on the present invention.

[0033] The urease-producing microorganism used in the following embodiments of the present invention is specifically Bacillus pasteurianus, which is commercially available Bacillus pasteurianus ATCC11859 and is a conventional strain in the art.

[0034] The Bacillus pasteurianus culture liquid is obtained by the following method: Bacillus pasteurianus is inoculated into a culture medium, cultured with shaking at 28°C for 2 days, and after the culture is completed, the absorbance of the culture liquid is measured using a spectrophotometer, and the concentration of the culture liquid is adjusted to OD600=1 by centrifugation or by adding physiological saline.

[0035] The culture medium formula is as follows: 20 g yeast powder, 15 g NH4Cl, and 1 mmol / L NiCl2 are mixed evenly, and water is added to make up to 1 L; the pH is controlled at 9.2~9.3.

[0036] Of course, the above-mentioned urease-producing microorganism can be any other conventional urease-producing microorganism in the art, and is not limited thereto.

[0037] Example 1

[0038] This embodiment provides a film-coated sustained-release material having the following characteristics: Figure 1 The structure shown.

[0039] like Figure 1 As shown, the coated sustained-release material has urea particles with a particle size of about 4 mm as the core 1, which is coated with an inner coating layer 2 made of Bacillus pasteurianus-sodium alginate, and the outermost layer is coated with an outer coating layer 3 made of calcium carbonate.

[0040] The above-mentioned film-coated sustained-release material provided in this embodiment is prepared by the following method:

[0041] First, a Bacillus pasteurianus solution with high urease activity is added to a sodium alginate solution to prepare a Bacillus pasteurianus-sodium alginate solution.

[0042] Specifically, in the Bacillus pasteurian-sodium alginate solution, the volume ratio of the Bacillus pasteurian solution to the sodium alginate solution is 1:10; the urease activity of the Bacillus pasteurian solution used is 10 U / mL, and the concentration of the sodium alginate solution is 5 g / L.

[0043] Secondly, industrial spherical granular urea with a particle diameter of 4 mm is selected for spherical urea coating, which is placed in the coating pot of the coating machine, and the prepared Bacillus pasteurianus-sodium alginate solution is placed in the feeding barrel of the coating machine. The spray gun is turned on to spray the Bacillus pasteurianus-sodium alginate solution. When a layer of sol-like liquid adheres to the surface of the spherical urea, the spraying is stopped until the surface of the spherical urea is dry to obtain a single-layer coating intermediate.

[0044] Specifically, 2 mL of the Bacillus pasteurianus-sodium alginate solution and 10 g of urea were used to prepare the coated sustained-release material, that is, the amount of the Bacillus pasteurianus-sodium alginate solution was controlled to be 200 mL / kg of urea.

[0045] Finally, the single-layer coated intermediate was placed in a 1 mol / L calcium chloride solution for a mineralization reaction, and calcium carbonate was deposited on the surface of the single-layer coated intermediate as an outer coating layer to obtain a coated sustained-release material.

[0046] The film-coated sustained-release material obtained in this example was subjected to SEM and XRD tests, and the SEM images and XRD results were shown in Figures 1 and 2. Figure 2 and Figure 3 shown.

[0047] Combine Figure 2 and Figure 3 From the results, it can be seen that the outermost layer of the film-coated sustained-release material is a dense calcium carbonate layer.

[0048] Example 2

[0049] In this embodiment, the structure of the film-coated sustained-release material provided is not described in detail here. It has a structure similar to that of the film-coated sustained-release material in Example 1.

[0050] The following describes the preparation method of the film-coated sustained-release material of this embodiment. The similarities with the preparation method in Example 1 are not repeated here, and only the differences from the preparation method in Example 1 are described.

[0051] The preparation method of this embodiment is different from the preparation method of Example 1 in that, in step 1, the volume ratio of the Bacillus pasteurian solution to the agar solution in the Bacillus pasteurian-agar solution is 1:20; the urease activity of the Bacillus pasteurian solution used is 2 U / mL, and the concentration of the agar solution is 0.1 g / L.

[0052] In step 2, 1 mL of the Bacillus pasteurianus-agar solution and 10 g of urea are used to prepare the coated sustained-release material, that is, the amount of the Bacillus pasteurianus-agar solution is controlled to be 100 mL / kg of urea.

[0053] In step 3, the calcium-containing mineralizing solution is specifically a 0.1 mol / L calcium chloride solution.

[0054] Example 3

[0055] In this embodiment, the structure of the film-coated sustained-release material provided is not described in detail here. It has a structure similar to that of the film-coated sustained-release material in Example 1.

[0056] The following describes the preparation method of the film-coated sustained-release material of this embodiment. The similarities with the preparation method in Example 1 are not repeated here, and only the differences from the preparation method in Example 1 are described.

[0057] The preparation method of this embodiment is different from the preparation method of Example 1 in that, in step 1, the volume ratio of the Bacillus pasteurian solution to the xanthan gum solution in the Bacillus pasteurian-xanthan gum solution is 20:1; the urease activity of the Bacillus pasteurian solution used is 20 U / mL, and the concentration of the xanthan gum solution is 5 g / L.

[0058] In step 2, 3 mL of the Bacillus pasteurianus-xanthan gum solution and 10 g of urea were used to prepare the coated sustained-release material, i.e., the amount of the Bacillus pasteurianus-xanthan gum solution was controlled to be 300 mL / kg of urea. The particle size of the urea particles used was approximately 5 mm.

[0059] In step 3, the calcium-containing mineralizing solution is specifically a 2 mol / L calcium acetate solution.

[0060] In order to demonstrate the necessity of the structure of the above-mentioned film-coated sustained-release material of the present invention to its performance, the following comparative experiments were conducted.

[0061] Comparative Example 1

[0062] The purpose of this comparative example is to demonstrate the importance of the outer coating layer made of calcium carbonate for the protection and sustained release of urea. To this end, the following first comparative coating-type sustained-release material is provided.

[0063] In the preparation process of the first comparative coated sustained-release material of this comparative example, the similarities with Example 1 are not repeated here, and only the differences from Example 1 are described. The preparation method of this comparative example differs from that of Example 1 in that the third step of conducting a mineralization reaction on the monolayer coated intermediate to form an outer coating layer is omitted; that is, the monolayer film intermediate obtained in the second step is the first comparative coated sustained-release material.

[0064] Comparative Example 2

[0065] The purpose of this comparative example is to reflect the importance of the inner coating layer of urease-containing microorganisms to the application effect. To this end, the following second comparative coating-type sustained-release material is provided.

[0066] In the preparation process of the second comparative coated sustained-release material of this comparative example, the similarities with Example 1 are not repeated here, and only the differences from Example 1 are described. The preparation method of this comparative example differs from that of Example 1 in that the preparation of the Bacillus pasteurianus-sodium alginate solution in the first step is omitted. In the second step, only the sodium alginate solution is added to the feeding bucket of the coating machine and the urea granules are spray-coated. In other words, the inner coating layer of the monolayer film intermediate obtained in the second step lacks the urease-producing microorganism, as compared to Example 1.

[0067] It should be noted that in this comparative example, although the material of the inner coating layer lacks urease microorganisms, the urea particles can still produce some carbonate ions through hydrolysis in a water-poor environment, thereby mineralizing in the third step to form an outer coating layer made of calcium carbonate.

[0068] Comparative Example 3

[0069] This comparative example only provides a gelling material that is the same as the Bacillus pasteurianus-sodium alginate solution in Example 1 as a third comparative material.

[0070] Comparative Example 4

[0071] This comparative example only provides a liquid that is the same as the Bacillus pasteurianus solution in Example 1 as a fourth comparative material.

[0072] In order to verify the application effect of the above materials, each material was applied to 10 L of Cd 2+ In heavy metal wastewater with a concentration of 5 mmol / L, after reaching precipitation equilibrium, the residual Cd was tested by ICP-OES instrument. 2+ Concentration, judge the removal of heavy metal Cd by each material 2+ Ion capability.

[0073] Three groups of parallel samples were set for each material provided in the Examples and Comparative Examples, and the test results were averaged.

[0074] It should be noted that when testing the materials provided in the aforementioned comparative examples, to ensure consistency with the active ingredients involved in the structures of the examples, the missing active components were supplemented with external additives. For example, since the second comparative coated sustained-release material provided in Comparative Example 2 lacked urease-producing microorganisms, an amount of Bacillus pasteurianus equivalent to that prepared in Example 1 was directly added to the heavy metal wastewater. Similarly, an equal amount of urea was directly added to the material tested in Comparative Example 3, while an equal amount of urea and sodium alginate was directly added to the material tested in Comparative Example 4.

[0075] Removal of heavy metal Cd from the materials provided in each embodiment and comparative example 2+ Ionic capacity such as Figure 4 shown.

[0076] from Figure 4 It can be seen that each of the film-coated sustained-release materials provided in the examples has good heavy metal Cd 2+ Ion removal ability, while the comparative materials provided in the comparative examples all performed poorly. Compared with Example 1, Comparative Example 1 lacks the protection of the outer membrane layer made of calcium carbonate, which not only lacks the slow-release effect of the urea inside it, but also fails to regulate the pH of the heavy metal wastewater, causing Bacillus pasteurianus to directly contact the acidic heavy metal wastewater, reducing its activity and ultimately showing a worse removal effect. Compared with Example 1, the Bacillus pasteurianus in Comparative Examples 2 and 4 were also directly exposed to the heavy metal wastewater, resulting in direct contact, which adversely affected the activity and thus showed an extremely poor removal effect. Compared with Example 1, Comparative Example 3 also exhibited a poor removal effect because, on the one hand, the directly added urea would be hydrolyzed prematurely, and on the other hand, the activity of Bacillus pasteurianus was affected by direct contact with acidic heavy metal wastewater. However, the Bacillus pasteurianus in Comparative Example 3 showed a slightly better removal effect than the directly exposed Bacillus pasteurianus in Comparative Examples 2 and 4 due to the complexing effect of sodium alginate. It can be seen that the polysaccharide polymer also shows a protective effect on the urease microorganisms complexed with it to a certain extent.

Claims

1. A coated sustained-release material based on microbial mineralization technology, characterized in that: The invention comprises a core made of urea particles, an inner coating layer made of a urease microorganism-polysaccharide polymer complex and coated on the outer surface of the core, and an outer coating layer made of calcium carbonate and coated on the outer surface of the inner coating layer. Wherein, the polysaccharide polymer is selected from at least one of sodium alginate, nanocellulose, agar, and xanthan gum; The calcium carbonate is formed by placing a single-layer coating intermediate composed of the core and the inner coating layer in a calcium-containing mineralization liquid to undergo a mineralization reaction, wherein the urease microorganism hydrolyzes the urea particles to produce carbonate ions, which then react with calcium ions in the calcium-containing mineralization liquid; The calcium-containing mineralizing liquid is selected from at least one of calcium chloride solution, calcium nitrate solution, and calcium acetate solution.

2. The film-coated sustained-release material according to claim 1, wherein The urea particles are spherical with a particle size of 2 mm to 5 mm.

3. The film-coated sustained-release material according to claim 1, characterized in that: The urease-producing microorganism is Bacillus pasteurianus.

4. The film-coated sustained-release material according to any one of claims 1 to 3, characterized in that In the film-coated sustained-release material, the mass proportion of the core is 80% to 95%, and the total mass proportion of the inner film layer and the outer film layer is 5% to 20%.

5. The method for preparing the film-coated sustained-release material according to any one of claims 1 to 4, wherein: Including steps: S1, adding the urease microorganism solution to the polysaccharide polymer solution to prepare a urease microorganism-polysaccharide polymer mixed solution; S2. Coating urea particles with the urease microorganism-polysaccharide polymer mixture as a coating material to form a single-layer coating intermediate; wherein the single-layer coating intermediate comprises a core portion made of urea particles and an inner coating layer made of the urease microorganism-polysaccharide polymer complex coated on the outer surface of the core portion; S3. Placing the single-layer coating intermediate in a calcium-containing mineralizing solution to carry out a mineralization reaction, forming an outer coating layer made of calcium carbonate on the outer surface of the inner coating layer, and obtaining the coating-type sustained-release material.

6. The preparation method according to claim 5, wherein In step S1, the volume ratio of the urease microbial solution to the polysaccharide polymer solution is 1:20 to 20:1; and the urease activity of the urease microbial solution is 2 U / mL to 20 U / mL.

7. The preparation method according to claim 5, wherein In step S2, the usage ratio of the urea granules to the urease microorganism-polysaccharide polymer mixture is 1 kg urea granules: 100 mL~300 mL urease microorganism-polysaccharide polymer mixture.

8. The preparation method according to claim 5, wherein In step S3, the calcium-containing mineralizing solution is selected from at least one of a calcium chloride solution, a calcium nitrate solution, and a calcium acetate solution; wherein the concentration of calcium ions is 0.1 mol / L to 2 mol / L.

9. The preparation method according to claim 8, characterized in that The volume ratio of the calcium-containing mineralizing liquid to the urea granules is 1:1 to 10:1.

Images