Slow-release calcium nitrate particle composition and method for in-situ remediation of river sediment

Abstract

The invention discloses a slow-release calcium nitrate particle composition and a method for in-situ remediation of black and odorous river sediment by using the composition. The particle composition takes calcium nitrate tetrahydrate, polyvinyl alcohol, sodium alginate and vermiculite powder as raw materials; the preparation method comprises the following steps: mixing calcium nitrate-PVA-SA hydrogel with vermiculite powder, molding, granulating, and coating the surface with the vermiculite powder. The particle composition can realize zero-order dynamic slow release of nitrate, and the release rate is stable. When applied to black and odorous bottom mud polluted by sulfides and ammonia nitrogen according to the ratio of N / (Fe + S) of 1.2-1.5, the acid volatile sulfides in the bottom mud can be stably, continuously and efficiently removed, the oxidation-reduction potential of the bottom mud is improved, anaerobic ammonia oxidation is synergistically excited, ammonia nitrogen in the bottom mud is synchronously reduced through vermiculite adsorption, secondary release of inorganic nitrogen is effectively avoided, and the service life of the bottom mud is prolonged. The in-situ synchronous repair of the black and odorous bottom mud is realized.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of water environment treatment and remediation, and particularly relates to a slow-release calcium nitrate granular composition and a method for in-situ remediation of black and odorous river sediment using the composition. BACKGROUND

[0002] Due to high sulfate concentration, the sediment in water bodies such as bays and tidal rivers has been in an anaerobic state for a long time, resulting in the deposition of excessive pollutants such as ammonia nitrogen and organic matter. Under anaerobic conditions, sulfate is reduced to sulfide, which is the main cause of black and odorous sediment. Existing sediment remediation technologies are mainly divided into ex-situ remediation and in-situ remediation. Ex-situ remediation (such as dredging) is prone to secondary pollution, ecological destruction and other problems, and is also very costly. In contrast, in-situ remediation technology has more application potential due to its small environmental disturbance and lower cost, and is particularly suitable for water bodies that cannot be dredged.

[0003] Among the many in-situ remediation technologies, electron acceptor stimulation method, especially calcium nitrate, has attracted much attention due to its low cost, good remediation effect and small ecological impact. This method eliminates black and odorous substances such as sulfide through the oxidation of nitrate ions and stimulates microbial activity, and has been successfully applied to the treatment of black and odorous sediment in some areas. However, traditional calcium nitrate dosing technology has obvious limitations: first, the risk of secondary pollution is high: direct dosing of calcium nitrate can lead to a dramatic increase in inorganic nitrogen (nitrate nitrogen and secondary ammonia nitrogen) in the sediment and release to the overlying water, posing a risk of exacerbating water eutrophication. And the degree of inorganic nitrogen release is significantly affected by the dosage, depth and method of dosing: excessive dosing can lead to the accumulation of nitrate and may trigger the process of nitrate dissimilatory reduction to ammonium, causing a surge in ammonia nitrogen; insufficient dosing cannot completely eliminate black and odorous or cause its regeneration. In addition, the effective diffusion distance of nitrate in the sediment is limited, affecting the uniformity and depth of remediation, and traditional injection methods and other dosing methods also cause high instantaneous concentration of the agent, leading to rapid release of inorganic nitrogen.

[0004] To overcome the above problems, slow-release calcium nitrate technology emerged as the times require. This technology aims to balance the supply and consumption of nitrate by controlling the release rate, thereby reducing escape, improving utilization, and prolonging the effective action time. Although existing research shows that slow-release technology can significantly reduce the nitrate escape rate from more than 29% in traditional methods to 1-2%, it still has two key defects: first, the release rate is unstable. The existing slow-release nitrate technology (such as granules, tubes, etc.) generally presents a trend of fast then slow, which cannot provide stable and controllable redox conditions throughout the repair period, affecting the durability and controllability of the repair effect. Second, the synchronous and stable reduction ability of ammonia nitrogen is insufficient, and black and odorous sediment is often accompanied by serious ammonia nitrogen pollution. The existing slow-release technology mainly focuses on controlling the release of nitrate, and lacks effective control strategies for simultaneously and stably reducing ammonia nitrogen in sediment, resulting in the risk of ammonia nitrogen release. SUMMARY

[0005] The purpose of the present application is to provide a slow-release calcium nitrate technology with appropriate slow-release period and stable slow-release rate, aiming to solve the problem of synchronous repair of black and odorous sediment and reduction of sediment ammonia nitrogen.

[0006] The present application is realized by providing a slow-release calcium nitrate granule composition for in-situ repair of black and odorous sediment and reduction of sediment ammonia nitrogen. The granule composition uses vermiculite mineral powder as aggregate and coating material, and is loaded with calcium nitrate.

[0007] Further, the raw materials of the slow-release calcium nitrate granule composition include calcium nitrate tetrahydrate, polyvinyl alcohol (PVA), sodium alginate (SA), and vermiculite powder.

[0008] Further, the vermiculite powder is raw vermiculite powder or expanded vermiculite powder.

[0009] Further, the particle size of the slow-release calcium nitrate granule composition is 4-15 mm.

[0010] Further, the in-vitro release kinetics of the slow-release calcium nitrate granule composition provided by the present application conforms to the zero-order model, i.e. the release speed is constant, the slow-release half-life is 10-30 days, and the slow-release rate in water is 1-5 mg / (L·d).

[0011] Further, the mass ratio of calcium nitrate tetrahydrate, polyvinyl alcohol, sodium alginate, and vermiculite powder is 1:(0.03-0.05):(0.02-0.03):(1.5-2.5).

[0012] The application further provides a preparation method of the slow-release calcium nitrate particle composition, which comprises the following steps: dissolving calcium nitrate tetrahydrate in water, adding polyvinyl alcohol (PVA) and heating to dissolve, adding sodium alginate (SA) and stirring to form a gelatinous liquid to obtain calcium nitrate-PVA-SA hydrogel, mixing with the first batch of vermiculite powder, completing the shaping and granulation of the particles in the state that the hydrogel is not completely dried, adhering the second batch of vermiculite powder on the surface of the obtained particles, drying, and obtaining the slow-release calcium nitrate particle composition.

[0013] Further, the preparation process of the calcium nitrate-PVA-SA hydrogel satisfies the following ratio relationship: the amount of water is 2-2.5 times the mass of the calcium nitrate tetrahydrate, the amount of the polyvinyl alcohol is 3%-5%, and the amount of the sodium alginate is 2%-3%.

[0014] The third aspect of the application further provides a method for in-situ remediation of river sediment, wherein the river sediment is polluted by sulfide and ammonia nitrogen compounds, and the method comprises the following steps: The slow-release calcium nitrate particle composition is applied to the sediment according to the calculated dose.

[0015] Further, in the method, the slow-release calcium nitrate particle composition is added according to the ratio of N / (Fe+S) being 1.2-1.5, based on the content of acid volatile sulfide and ferrous iron in the black and odorous sediment to be remediated, wherein N is the content of nitrogen in the slow-release calcium nitrate particle composition.

[0016] Compared with the prior art, the application has the following beneficial effects: through the specific formula and process, the application can release the appropriate dose of nitrate at a stable slow-release rate and suitable slow-release period, balance the supply and consumption rate of the sediment nitrate, effectively stimulate the autotrophic denitrification to remove the acid volatile sulfide (AVS) and other blackening and odor-causing substances in the sediment, and avoid the process of producing a large amount of secondary ammonia nitrogen by the nitrate dissimilatory reduction into ammonium caused by high-concentration nitrate; secondly, the slow-release particle composition can maintain a relatively appropriate concentration of nitrate and its secondary nitrogen metabolites, stimulate the background anaerobic ammonia-oxidizing bacteria group (traditional anaerobic ammonia oxidation, sulfate-type anaerobic ammonia oxidation, iron-reducing anaerobic ammonia oxidation, etc.) in the sediment to further reduce the background ammonia nitrogen in the sediment; in addition, the inorganic layered silicate mineral vermiculite is used as the aggregate of the slow-release calcium nitrate particle composition, and the unique lattice structure and porous characteristics of the vermiculite can fix and adsorb ammonia nitrogen, thereby synergistically reducing the background ammonia nitrogen in the sediment; finally, the preparation process of the medicament is simple, the application amount is calculated scientifically, the addition operation is convenient, the construction cost can be reduced by inhibiting the release of inorganic nitrogen, and the application can be widely applied to the gulf and tidal river with serious ammonia nitrogen pollution and high sulfate. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1is a release characteristic and zero-order kinetics fitting graph of the raw vermiculite slow-release calcium nitrate particle composition provided by the embodiment of the present application; Figure 2 is a release characteristic and zero-order kinetics fitting graph of the expanded vermiculite slow-release calcium nitrate particle composition provided by the embodiment of the present application; Figure 3 is a AVS content change graph during the process of repairing black and odorous sediment by the slow-release particle composition; Figure 4 is a ammonia nitrogen content change graph during the process of repairing black and odorous sediment by the slow-release particle composition; Figure 5 is a oxidation reduction potential ORP change graph during the process of repairing black and odorous sediment by the slow-release particle composition. DETAILED DESCRIPTION

[0018] In order to make the purpose, technical scheme and advantages of the present application clearer, the present application is further described in detail below in combination with the drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and do not limit the present application.

[0019] The embodiment of the present application aims at the pollution of black and odorous sediment in river, especially the pollution of sulfide and ammonia nitrogen compounds, and provides a slow-release calcium nitrate particle composition. The particle composition uses vermiculite mineral powder as aggregate and coating material and is loaded with calcium nitrate. Specifically, the raw materials of the slow-release calcium nitrate composition include calcium nitrate tetrahydrate, polyvinyl alcohol (PVA), sodium alginate (SA) and vermiculite powder, wherein the vermiculite powder can be raw vermiculite powder or expanded vermiculite powder; the polyvinyl alcohol and sodium alginate are used to form a slow-release hydrogel and together serve as a binder of the particles, so as to achieve the loading and slow-release control of calcium nitrate.

[0020] The particle size of the prepared slow-release calcium nitrate composition is preferably 4-15 mm, and the in vitro release kinetics of the particles conforms to the zero-order model, that is, the release speed is constant, the slow-release half-life is 10-30 days, and the slow-release rate in water is 1-5 mg / (L·d).

[0021] The embodiment of the present application also provides a preparation method of the above slow-release calcium nitrate composition, which comprises: dissolving calcium nitrate tetrahydrate in water, adding polyvinyl alcohol (PVA) and heating to dissolve, then adding sodium alginate (SA) and stirring to form a gelatinous liquid to obtain a calcium nitrate-PVA-SA hydrogel; adding the first batch of vermiculite powder and mixing, completing the molding and granulation of the particles in the state that the hydrogel is not completely dried, and adhering the second batch of vermiculite powder to the surface of the obtained particles, and drying to prepare the slow-release calcium nitrate particles.

[0022] Specifically, according to 100 g of calcium nitrate tetrahydrate, the corresponding amount of polyvinyl alcohol is 2-5 g; Preferably, a defoaming agent is also contained, which can inhibit the generation of foam during stirring and heating, thereby avoiding the interference of air bubbles in the subsequent mixing, granulation and tableting of vermiculite powder, and ensuring the slow-release performance of the final slow-release granular composition.

[0023] The third aspect of the present application also provides a method for in-situ remediation of river sediment, which comprises: applying the above slow-release calcium nitrate composition to the sediment according to the calculated dose, specifically, in the method, based on the content of acid volatile sulfide and ferrous in the black and odorous sediment to be remediated, the slow-release calcium nitrate composition is added according to the ratio of N / (Fe+S) being 1.2-1.5, wherein N is the content of nitrogen in the slow-release calcium nitrate composition.

[0024] The present application is further explained and illustrated by the following examples Example 1: Preparation of slow-release calcium nitrate granular composition (1) Take 300 g of calcium nitrate tetrahydrate, dissolve in 700 mL of water, and stir until completely dissolved. Add 15 g of polyvinyl alcohol PVA to the solution, heat to boiling and stir until the PVA is completely dissolved. Then add 8 g of sodium alginate SA, continue to stir until a uniform gel liquid is formed, add 0.2 mL of n-octanol or tributyl phosphate as a defoaming agent, continue to heat and concentrate, take out and cool to room temperature to obtain a calcium nitrate-PVA-SA hydrogel solution.

[0025] (2) Take 700 g of raw vermiculite powder, add the above prepared calcium nitrate-PVA-SA hydrogel solution, mix thoroughly by kneading, form uniform lumps, use a plodder to press the lumps into a strip, then use a traditional Chinese medicine pill making machine to make the granular semi-finished product.

[0026] (3) Transfer the granular semi-finished product to a preheated coating machine, and uniformly sprinkle 100 g of raw vermiculite powder in the process of continuous rolling, so that the surface of the granules is wrapped with a layer of vermiculite powder. After drying, raw vermiculite calcium nitrate slow-release granules with a particle size of 8 mm are obtained.

[0027] Example 2: Preparation of slow-release calcium nitrate granular composition (1) Take 300 g of calcium nitrate tetrahydrate, dissolve in 700 mL of water, and stir until completely dissolved. Add 15 g of polyvinyl alcohol PVA to the solution, heat to boiling and stir until the PVA is completely dissolved. Then add 8 g of sodium alginate SA, continue to stir until a uniform gel liquid is formed, add 0.2 mL of n-octanol or tributyl phosphate as a defoaming agent, continue to heat and concentrate, take out and cool to room temperature to obtain a calcium nitrate-PVA-SA hydrogel solution.

[0028] (2) Weigh 500 g of expanded vermiculite powder, add the calcium nitrate-PVA-SA hydrogel solution prepared above, knead and mix thoroughly to form a uniform mass, use a strip press to press the mass into strips, and then use a traditional Chinese medicine pill making machine to make it into granular semi-finished products.

[0029] (3) Transfer the semi-finished granules to a preheated coating machine. During the continuous rolling process, 100 g of expanded vermiculite powder is evenly sprinkled in so that the surface of the granules is coated with a layer of vermiculite powder. After heating and drying, expanded vermiculite calcium nitrate slow-release granules with a particle size of 6 mm are obtained.

[0030] Example 3: Determination of the release performance of the sustained-release granule composition Accurately weigh 100 mg of the slow-release calcium nitrate granule composition prepared in Example 1 above into three 1 L stemmed beakers, add 1 L of ultrapure water, and seal with tissue culture sealing film to slow down water evaporation. Take approximately 2 mL of water sample daily to determine the nitrate nitrogen concentration, and perform kinetic fitting on the obtained data to obtain the following results: Figure 1 and Figure 2 The fitting equation and R shown 2 Value, calcium nitrate release half-life and reaction rate constant.

[0031] The results are as follows: The sustained-release calcium nitrate granule composition prepared in Example 1 has a suitable sustained-release period and a stable controlled-release rate. The sustained-release curves all conform to zero-order kinetics, and R0... 2 The values ​​are 0.959 and 0.971 respectively, and the reaction rate constant k is... A The dosages were 2.90 and 2.69 mg / (L·d), respectively, and the sustained-release half-life t0 was... 1 / 2 The values ​​were 16.32 and 16.65 days, respectively. This indicates that the particulate composition prepared in this invention achieves stable and controllable sustained release with a essentially constant release rate.

[0032] Example 4: The sustained-release calcium nitrate granule composition of Example 2 was tested, and the results were the same as those of Example 3.

[0033] Example 5: Remediation Experiment of Black and Odorous Sediment (1) Sample preparation: The surface black and smelly bay bottom sediment at a depth of about 0-30 cm was collected from a tidal river channel. Large branches, sand and garbage and other debris were removed, and the sediment was stirred evenly with an electric stirrer, sealed and stored for later use. The content of acid volatile sulfide (AVS) and ferrous iron in the initial sediment was determined.

[0034] (2) Dosage calculation: The empirical parameter N / (Fe+S) = 1.2 is used to calculate the redox equivalent of removing AVS and ferrous iron from the sediment by the biochemical process, where N is the nitrogen content in the slow-release calcium nitrate granule composition. Then, the nitrate equivalent required to remove AVS and ferrous iron from the sediment is calculated based on this empirical parameter.

[0035] (3) Experimental setup: Take equal amounts of bottom mud into multiple PET wide-mouth bottles and divide them into three groups: blank control group, raw vermiculite slow-release granule group of Example 1, and expanded vermiculite slow-release granule group of Example 2.

[0036] (4) Indicator monitoring: Samples were taken on days 0, 1, 4, and 9 to measure the AVS, ammonia nitrogen content, and oxidation-reduction potential (ORP) in the sediment. The results are as follows: Figures 3-5 As shown.

[0037] Results analysis: such as Figures 3-5 As shown, both the raw vermiculite slow-release granule group and the expanded vermiculite slow-release granule group showed significant effects starting from day 4. By day 9, the removal rates of AVS and ammonia nitrogen in the sediment reached 90-91% and 75-77%, respectively, and the sediment ORP increased by 62-65%. In contrast, the blank control group, under natural anaerobic conditions without intervention, showed even increased AVS and ammonia nitrogen content, and the ORP remained in a very low negative range without any upward trend. This indicates that both types of slow-release calcium nitrate granules can rapidly and effectively remove black and odorous substances from sediment, increase sediment ORP, and reduce sediment ammonia nitrogen load.

[0038] In summary, this invention provides a method for simultaneously and efficiently remediating black and odorous sediment and reducing ammonia nitrogen pollution in sediment by designing vermiculite calcium nitrate particles with specific stable slow-release properties, thus solving key problems in the prior art.

[0039] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A slow-release calcium nitrate granule composition, characterized in that, The raw materials of the slow-release calcium nitrate granule composition include: calcium nitrate tetrahydrate, polyvinyl alcohol, sodium alginate, and vermiculite powder.

2. The particulate composition according to claim 1, characterized in that, The vermiculite powder is either raw vermiculite powder or expanded vermiculite powder.

3. The particulate composition according to claim 1, characterized in that, The particle size of the slow-release calcium nitrate granule composition is 4-15 mm.

4. The particulate composition according to claim 1, characterized in that, The in vitro release kinetics of the sustained-release calcium nitrate granule composition conforms to a zero-order model, with a sustained-release half-life of 10-30 days and a sustained-release rate of 1-5 mg / (L·d) in water.

5. The particulate composition according to any one of claims 1-4, characterized in that, The mass ratio of calcium nitrate tetrahydrate, polyvinyl alcohol, sodium alginate, and vermiculite powder is 1: (0.03-0.05): (0.02-0.03): (1.5-2.5).

6. A method for preparing a sustained-release calcium nitrate granule composition as described in any one of claims 1-5, comprising: Calcium nitrate tetrahydrate was dissolved in water, polyvinyl alcohol was added and heated to dissolve, and sodium alginate was added and stirred to form a gel-like liquid, thus obtaining calcium nitrate-PVA-SA hydrogel. The first batch of vermiculite powder was added and mixed. The particles were formed and granulated while the hydrogel was not completely dry. The second batch of vermiculite powder was then adhered to the surface of the resulting particles. After drying, a slow-release calcium nitrate particle composition was obtained.

7. The method according to claim 6, characterized in that, The preparation process of the calcium nitrate-PVA-SA hydrogel satisfies the following ratio: based on the mass of the calcium nitrate tetrahydrate, the amount of water is 2-2.5 times, the amount of polyvinyl alcohol is 3%-5%, and the amount of sodium alginate is 2%-3%.

8. The method according to claim 6, characterized in that, After sodium alginate is added and a gel-like liquid is formed, an antifoaming agent is added, which is n-octanol or tributyl phosphate.

9. A method for in-situ remediation of riverbed sediment, wherein the riverbed sediment includes sulfide and ammonia nitrogen compound pollution, the method comprising the following steps: The above-mentioned slow-release calcium nitrate granule composition was applied to the sediment at the calculated dosage.

10. The method according to claim 9, characterized in that, Based on the content of volatile acidic sulfides and ferrous iron in the black and odorous sediment to be remediated, a slow-release calcium nitrate granular composition is added according to the N / (Fe+S) ratio of 1.2-1.5, where N is the nitrogen content in the slow-release calcium nitrate granular composition.

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