Preparation method of pesticide pollution remediation agent for underground water of karst landform

Abstract

The invention discloses a preparation method of a pesticide pollution remediation agent for groundwater of karst landforms, and relates to the technical field of groundwater pollution remediation. The method has the effect of enhancing pesticide degradation, pesticide pollutants can be efficiently degraded in a complex environment of karst underground water by compounding hard water-resistant fungi, dehalogenation bacteria and denitrifying bacteria, and a better remediation effect is achieved; the medicament can improve the stability and the adsorption capacity of the carrier, the bionic calcium phosphate nano carrier is adopted, the specific surface area is high, the adsorption capacity is excellent, the adsorption capacity on pollutants can be effectively enhanced, and the remediation effect is improved; the agent has an efficient repairing mechanism, the load capacity of the flora is enhanced through biological induction mineralization and physical adsorption enhancement of a bacterium-carrier complex, the release rate of the flora is controlled through an intelligent coating technology, and precise repairing is achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of groundwater pollution remediation, and in particular to a method for preparing a medicament for remediating pesticide pollution in karst groundwater. Background Art

[0002] Due to its high permeability, high calcium and sodium ion background, and rapid dynamic migration characteristics, karst groundwater causes the widespread spread of pesticide pollutants in the groundwater, increasing the difficulty of pesticide pollution remediation.

[0003] Traditional remediation methods are often restricted by the complex environmental conditions of groundwater and are difficult to effectively degrade or remove pesticide pollution.

[0004] Therefore, developing repair agents with strong adaptability is an urgent problem to be solved;

[0005] The combination of bionic carriers and composite bacterial agent systems, based on the degradation ability of natural microorganisms and by designing carriers with high specific surface area and adsorption properties, can effectively improve the degradation efficiency of pesticides and provide new ideas and technologies for the pollution remediation of karst groundwater. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for preparing a pesticide contamination remediation agent for karst groundwater, and to 2+ / Mg 2+ Based on the background of the virus and the characteristics of fast dynamic migration, a "bionic carrier-composite bacterial agent" pharmaceutical system is constructed to solve the technical problems existing in the existing technology.

[0007] To achieve the above object, the present invention provides the following technical solution: a method for preparing a pesticide-contaminated groundwater remediation agent in karst landforms, comprising at least the following steps:

[0008] S1: Prepare a hard water resistant fungus-denitrifying bacteria composite system to finally obtain a composite bacterial agent;

[0009] S2: Construction of biomimetic calcium phosphate nanocarriers;

[0010] S3: Construction of bacteria-vector complex;

[0011] S4: Complete the preparation of pesticide pollution remediation agent based on S1-S3.

[0012] Furthermore, the S1 at least includes the following steps:

[0013] S1.1: Carrying out strain selection and morphology, the strains including white rot fungi, dehalogenating bacteria and denitrifying bacteria, the white rot fungi at least including Trametes versicolor spore powder, the dehalogenating bacteria being in a freeze-dried powder state, and the denitrifying bacteria being in a freeze-dried powder state;

[0014] S1.2: Pre-treating the strains;

[0015] S1.3: Compounding the functional bacterial flora, mixing the Trametes versicolor activated spore powder, the dehalogenating bacteria freeze-dried powder and the denitrifying bacteria freeze-dried powder according to a ratio of 6:2:2, and then adding a metabolic activator, the metabolic activator including a laccase activator, a dehalogenation auxiliary factor and a denitrification promoter;

[0016] S1.4: Forming the compound microbial agent, uniformly dispersing the mixed bacterial flora and the metabolic activator in a sterile protective agent, and vacuum freeze-drying to obtain a compound microbial agent freeze-dried powder.

[0017] Further, the S1.2 at least includes the following steps:

[0018] Trametes versicolor spore activation, inoculating the spore powder into an acidic culture medium containing 1 mM CuSO4, and culturing at 25℃ for 48 hours, to induce the expression of calcium-binding protein, the acidic culture medium having a pH of 4.5, and the acidic culture medium containing 2% calcium carbonate,

[0019] Dehalogenating bacteria resuscitation, placing the dehalogenating bacteria freeze-dried powder in an anaerobic bottle, and standing at 37℃ for 24 hours, the anaerobic bottle containing 0.1 mg / L vitamin B 12 , 10 mM sodium acetate;

[0020] Denitrifying bacteria activation, inoculating the freeze-dried powder of the denitrifying bacteria into a nitrate culture medium, and aerating and culturing at 30℃ and 150 rpm until the OD 600 =1.0, the nitrate culture medium being 50 mg / L NO3 - , and the pH being 7.2.

[0021] Further, the laccase activator in the S1.3 is 2,5-dimethoxybenzoic acid (1 mM) added in the Trametes versicolor activation culture medium;

[0022] The dehalogenation auxiliary factor is cobalamin (0.05 mg / L) added in the dehalogenating bacteria resuscitation system;

[0023] The denitrification promoter is sodium molybdate (0.01 mM) supplemented in the denitrifying bacteria culture medium.

[0024] Further, the skimmed milk sugar-fucrose complex at least includes a skimmed milk sugar-fucrose complex.

[0025] Further, the S2 at least includes the following steps:

[0026] S2.1: Prepare the raw materials, using calcium chloride (CaCl2) and disodium hydrogen phosphate (Na2HPO4) as raw materials, according to Ca 2+ :PO4 3- =1.67:1 molar ratio, adjust the solution pH to 9.0-10.0, obtain NH3·H2O, and add sodium bicarbonate (NaHCO3) to make CO3 2- The concentration is 20-50 mM to form carbonated hydroxyapatite (CHAP) precursor solution;

[0027] S2.2: Template-induced mineralization: 5-10 wt% of amphiphilic block copolymer Pluronic F127 was added to the precursor solution as a mesoporous template, and 0.1-0.5 wt% of polyaspartic acid (PASP) was used to regulate crystal nucleation. The solution was hydrothermally reacted at 150°C for 24 hours to generate CO3. 2- doped hydroxyapatite cores;

[0028] S2.3: Pore formation and purification: The product was immersed in a mixture of ethanol and hydrochloric acid (volume ratio 9:1, containing 0.1M HCl) and ultrasonically treated for 2 hours to remove the template. After centrifugal washing, it was calcined at 400℃ in a nitrogen atmosphere for 2 hours to obtain a pore size of 20±2nm and a specific surface area of ​​>200m 2 / g porous hydroxyapatite carrier;

[0029] S2.4: Surface functional modification: the calcined support was immersed in 5 wt% polyaspartic acid (PASP) solution (pH 6.0), stirred at 60 ° C for 6 hours, washed with deionized water and dried to form a functional support with enriched carboxylic acid groups on the surface. 2+ / Cd 2+ The adsorption capacity was increased to >200 mg / g.

[0030] Furthermore, the step S3 at least includes the following steps:

[0031] The freeze-dried powder of the composite bacterial agent was dispersed in sterile saline containing 0.1% Tween 80 at a ratio of 1:10 (w / v) and allowed to stand at room temperature for 30 minutes;

[0032] Synchronous lytic metabolic activator;

[0033] Biomimetic calcium phosphate nanocarriers modified with polyaspartic acid (PASP) were immersed in a saturated CaCO3 solution (pH 8.0) and sonicated for 30 min to activate the carboxylic acid groups.

[0034] The pretreated bacterial solution was mixed with the activated carboxylic acid groups at a mass ratio of 1:5 and reacted in a constant temperature shaker (25°C, 100 rpm) for 6 h, maintaining the pH at 7.5-8.0 (automatically regulated by adding 0.1 M NaHCO3).

[0035] Calcium binding protein secreted by bacteria and Ca 2+ Combination induces secondary mineralization of hydroxyapatite to form a bonding layer, and the carboxylic acid groups of PASP are cross-linked with bacterial extracellular polysaccharides (EPS) through hydrogen bonds and coordination, with a loading rate of >90%;

[0036] Gradient adsorption process: The initially loaded complex was transferred to a vacuum adsorption tank (initial pressure -0.08 MPa), and the pressure was restored to normal pressure in stages (increased by 0.02 MPa every 0.5 hours), so that the bacteria were embedded in the carrier mesopores (pore size 20 ± 2 nm) to form a complex;

[0037] For washing and purification, the complex was washed three times with a protective solution containing 5% trehalose (pH 7.0) to remove unbound bacteria;

[0038] Stabilization treatment was carried out. First, freeze fixation was performed, and the complex was immersed in a cryoprotectant (10% glycerol + 5% defatted lactose), cooled to -80°C at 1°C / min and pre-frozen for 12 hours, followed by vacuum freeze drying (cold trap -50°C, vacuum degree 10Pa) for 48 hours to form polyporous bacteria-carrier composite particles (particle size 1-3mm, porosity >40%). Secondly, intelligent coating was carried out. Fluidized bed spraying technology was used to coat the surface of the polyporous bacteria-carrier composite particles with a pH-sensitive ethyl cellulose membrane (thickness 50-100 μm). The pH-sensitive ethyl cellulose membrane slowly released the bacteria at pH 6.5-7.5 (release rate ≤5% / day) and rapidly disintegrated within 30 minutes at pH <6.0 (release rate >90%).

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] 1. The present invention has the ability to enhance the degradation effect of pesticides. By combining hard water tolerant fungi, dehalogenating bacteria and denitrifying bacteria, it can efficiently degrade pesticide pollutants in the complex environment of karst groundwater, achieving better remediation effects.

[0041] 2. The agent of the present invention can improve the stability and adsorption capacity of the carrier. It uses biomimetic calcium phosphate nanocarriers with high specific surface area and excellent adsorption capacity, which can effectively enhance the adsorption capacity of pollutants and improve the repair effect;

[0042] 3. The agent of the present invention has a highly efficient repair mechanism. It enhances the loading capacity of the bacterial community through the bio-induced mineralization and physical adsorption of the bacteria-carrier complex, and controls the release rate of the bacterial community through intelligent coating technology to achieve precise repair.

[0043] 4. The agent of the present invention has strong adaptability. The agent system is designed for the high calcium background and dynamic migration characteristics of karst groundwater. It has strong adaptability and can maintain a high repair efficiency under complex water quality conditions. DETAILED DESCRIPTION

[0044] The technical solutions in the embodiments of the present invention will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0045] A method for preparing a pesticide-contaminated groundwater remediation agent for karst landforms comprises at least the following steps:

[0046] S1: Prepare a hard water resistant fungus-denitrifying bacteria composite system to finally obtain a composite bacterial agent;

[0047] S2: Construction of biomimetic calcium phosphate nanocarriers;

[0048] S3: Construction of bacteria-vector complex;

[0049] S4: Complete the preparation of pesticide pollution remediation agent based on S1-S3.

[0050] S1 includes at least the following steps:

[0051] S1.1: Select strains and determine their morphology. The strains include white rot fungi, dehalogenating bacteria, and denitrifying bacteria. The white rot fungi include at least spore powder of Coriolus versicolor. The dehalogenating bacteria are in the form of freeze-dried powder, and the denitrifying bacteria are in the form of freeze-dried powder.

[0052] S1.2: Pre-treat the strain;

[0053] In this example, the strain sources and morphologies are:

[0054] White rot fungi: Trametes versicolor (CGMCC No. 5.776) spore powder, laccase activity ≥ 300 U / g;

[0055] Dehalogenating bacteria: Dehalococcoides mccartyi strain 195 (ATCC BAA-2100) lyophilized powder;

[0056] Denitrifying bacteria: Pseudomonas stutzeri ATCC 17588 freeze-dried powder.

[0057] S1.3: Compound the functional bacteria by mixing the activated spore powder of Coriolus versicolor, freeze-dried powder of dehalogenating bacteria, and freeze-dried powder of denitrifying bacteria in a ratio of 6:2:2, and then adding a metabolic activator, which includes a laccase activator, a dehalogenation cofactor, and a denitrification promoter;

[0058] S1.4: Forming the composite bacterial agent: evenly disperse the mixed bacterial flora and metabolic activator in S1.3 in a sterile protective agent, and obtain freeze-dried powder of the composite bacterial agent after vacuum freeze-drying.

[0059] S1.2 includes at least the following steps:

[0060] To activate the spores of Coriolus versicolor, the spore powder was inoculated into an acidic medium containing 1 mM CuSO4 and cultured with shaking at 25°C for 48 hours to induce the expression of calcium binding protein. The pH of the acidic medium was 4.5 and the acidic medium contained 2% calcium carbonate.

[0061] To revive dehalogenating bacteria, place the freeze-dried powder of dehalogenating bacteria in an anaerobic bottle and let it stand at 37℃ for 24 hours. The anaerobic bottle contains 0.1mg / L vitamin B 12 , 10 mM sodium acetate;

[0062] Denitrifying bacteria were activated by inoculating the freeze-dried powder of denitrifying bacteria into nitrate culture medium and incubating at 30°C and 150rpm with aeration until OD 600 =1.0, nitrate medium is 50mg / L NO3 - , pH 7.2.

[0063] The laccase activator in S1.3 was 2,5-dimethoxybenzoic acid (1 mM) added to the activation medium of Coriolus versicolor;

[0064] The dehalogenation cofactor was cobalamin (0.05 mg / L) added to the dehalogenating bacteria recovery system;

[0065] The denitrification promoter is sodium molybdate (0.01 mM) added to the denitrifying bacteria culture medium.

[0066] The skimmed lactose-trehalose complex includes at least the skimmed lactose-trehalose complex.

[0067] S2 includes at least the following steps:

[0068] S2.1: Prepare the raw materials, using calcium chloride (CaCl2) and disodium hydrogen phosphate (Na2HPO4) as raw materials, according to Ca 2+ :PO4 3- =1.67:1 molar ratio, adjust the solution pH to 9.0-10.0, obtain NH3·H2O, and add sodium bicarbonate (NaHCO3) to make CO3 2- The concentration is 20-50 mM to form carbonated hydroxyapatite (CHAP) precursor solution;

[0069] S2.2: Template-induced mineralization: 5-10 wt% of amphiphilic block copolymer Pluronic F127 was added to the precursor solution as a mesoporous template, and 0.1-0.5 wt% of polyaspartic acid (PASP) was used to regulate crystal nucleation. The solution was hydrothermally reacted at 150°C for 24 hours to generate CO3. 2- doped hydroxyapatite cores;

[0070] S2.3: Pore formation and purification: The product was immersed in a mixture of ethanol and hydrochloric acid (volume ratio 9:1, containing 0.1M HCl) and ultrasonically treated for 2 hours to remove the template. After centrifugal washing, it was calcined at 400℃ in a nitrogen atmosphere for 2 hours to obtain a pore size of 20±2nm and a specific surface area of ​​>200m 2 / g porous hydroxyapatite carrier;

[0071] S2.4: Surface functional modification: the calcined support was immersed in 5 wt% polyaspartic acid (PASP) solution (pH 6.0), stirred at 60 ° C for 6 hours, washed with deionized water and dried to form a functional support with enriched carboxylic acid groups on the surface. 2 + / Cd 2 +The adsorption capacity is increased to >200mg / g;

[0072] Performance characterization, using XRD patterns (JCPDS 09-0432) and EDS analysis (Ca / P = 1.67) to verify the composition, BET test showed the uniformity of the mesoporous structure (pore size distribution ± 2nm), XPS spectrum confirmed the successful grafting of PASP (N1s peak binding energy 399.8eV), the carrier on the Ca 2+ = 200mg / L simulated groundwater immersion for 30 days without structural collapse (Ca 2+ Adsorption capacity <5mg / g), and for Pb 2+ The Langmuir adsorption capacity reaches 235 mg / g.

[0073] S3 includes at least the following steps:

[0074] The freeze-dried powder of the composite bacterial agent was dispersed in sterile saline containing 0.1% Tween 80 at a ratio of 1:10 (w / v) and allowed to stand at room temperature for 30 minutes;

[0075] Synchronous lytic metabolic activator;

[0076] Biomimetic calcium phosphate nanocarriers modified with polyaspartic acid (PASP) were immersed in a saturated CaCO3 solution (pH 8.0) and sonicated for 30 min to activate the carboxylic acid groups.

[0077] The pretreated bacterial solution was mixed with the activated carboxylic acid groups at a mass ratio of 1:5 and reacted in a constant temperature shaker (25°C, 100 rpm) for 6 h, maintaining the pH at 7.5-8.0 (automatically regulated by adding 0.1 M NaHCO3).

[0078] Calcium binding protein secreted by bacteria and Ca 2+ Combination induces secondary mineralization of hydroxyapatite to form a bonding layer, and the carboxylic acid groups of PASP are cross-linked with bacterial extracellular polysaccharides (EPS) through hydrogen bonds and coordination, with a loading rate of >90%;

[0079] Gradient adsorption process: The initially loaded complex was transferred to a vacuum adsorption tank (initial pressure -0.08 MPa), and the pressure was restored to normal pressure in stages (increased by 0.02 MPa every 0.5 hours), so that the bacteria were embedded in the carrier mesopores (pore size 20 ± 2 nm) to form a complex;

[0080] For washing and purification, the complex was washed three times with a protective solution containing 5% trehalose (pH 7.0) to remove unbound bacteria;

[0081] Stabilization treatment was carried out. First, the complex was frozen and fixed. The complex was immersed in a cryoprotectant (10% glycerol + 5% defatted lactose), cooled to -80°C at 1°C / min and pre-frozen for 12 hours. Then, it was vacuum freeze-dried (cold trap -50°C, vacuum degree 10Pa) for 48 hours to form polyporous bacteria-carrier composite particles (particle size 1-3mm, porosity >40%). Secondly, intelligent coating was carried out. Fluidized bed spraying technology was used to coat the surface of the polyporous bacteria-carrier composite particles with a pH-sensitive ethyl cellulose membrane (thickness 50-100μm). The pH-sensitive ethyl cellulose membrane slowly released the bacteria at pH 6.5-7.5 (release rate ≤5% / day) and rapidly disintegrated within 30 minutes at pH <6.0 (release rate >90%).

[0082] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims rather than the foregoing description, and it is intended that all variations that come within the meaning and range of equivalents of the claims be embraced therein.

Claims

1. A method for preparing a pesticide-contaminated groundwater remediation agent in karst landforms, characterized by: At least the following steps are included: S1: Prepare a hard water resistant fungus-denitrifying bacteria composite system to finally obtain a composite bacterial agent; S2: Construction of biomimetic calcium phosphate nanocarriers; S3: Construction of bacteria-vector complex; S4: Complete the preparation of pesticide pollution remediation agent based on S1-S3.

2. The method for preparing a pesticide-contaminated remediation agent for karst groundwater according to claim 1, characterized in that: Said S1 at least comprises the following steps: S1.1: Select and morphologically analyze the strains, wherein the strains include white rot fungi, dehalogenating bacteria, and denitrifying bacteria. The white rot fungi include at least spore powder of Coriolus versicolor. The dehalogenating bacteria are in the form of freeze-dried powder, and the denitrifying bacteria are in the form of freeze-dried powder. S1.2: Pre-treat the strain; S1.3: Compounding the functional bacteria: mixing activated spore powder of Coriolus versicolor, freeze-dried powder of dehalogenating bacteria, and freeze-dried powder of denitrifying bacteria in a ratio of 6:2:2, and then adding a metabolic activator, wherein the metabolic activator includes a laccase activator, a dehalogenation cofactor, and a denitrification promoter; S1.4: Forming the composite bacterial agent: evenly disperse the mixed bacterial flora and metabolic activator in S1.3 in a sterile protective agent, and obtain freeze-dried powder of the composite bacterial agent after vacuum freeze-drying.

3. The method for preparing a pesticide-contaminated remediation agent for karst landform groundwater according to claim 2, characterized in that: Said S1.2 at least comprises the following steps: The spores of Versicolor versicolor were activated by inoculating the spore powder into an acidic medium containing 1 mM CuSO4 and shaking cultured at 25°C for 48 hours to induce the expression of calcium binding protein. The pH of the acidic medium was 4.5 and the acidic medium contained 2% calcium carbonate. To revive the dehalogenating bacteria, place the freeze-dried powder of the dehalogenating bacteria in an anaerobic bottle containing 0.1 mg / L vitamin B and place it at 37°C for 24 hours. 12 , 10 mM sodium acetate; Denitrifying bacteria were activated by inoculating the freeze-dried powder of denitrifying bacteria into nitrate culture medium and incubating at 30°C and 150rpm with aeration until OD 600 =1.0, the nitrate culture medium is 50mg / L NO3 - , pH 7.

2.

4. The method for preparing a pesticide-contaminated remediation agent for karst landform groundwater according to claim 2, characterized in that: The laccase activator in S1.3 is 2,5-dimethoxybenzoic acid added to the activation medium of Coriolus versicolor; The dehalogenation cofactor is cobalamin added to the dehalogenating bacteria recovery system; The denitrification promoter is sodium molybdate added to the denitrifying bacteria culture medium.

5. The method for preparing a pesticide-contaminated remediation agent for karst groundwater according to claim 2, characterized in that: The skimmed lactose-trehalose complex at least includes the skimmed lactose-trehalose complex.

6. The method for preparing a pesticide-contaminated remediation agent for karst landform groundwater according to claim 2, characterized in that: Said S2 at least comprises the following steps: S2.1: Prepare raw materials, using calcium chloride and disodium hydrogen phosphate as raw materials, according to Ca 2+ :PO4 3- =1.67:1 molar ratio, adjust the solution pH to 9.0-10.0, obtain NH3·H2O, and add sodium bicarbonate to make CO3 2- The concentration is 20-50 mM to form a carbonated hydroxyapatite precursor solution; S2.2: Template-induced mineralization: 5-10 wt% of amphiphilic block copolymer Pluronic F127 was added to the precursor solution as a mesoporous template, and 0.1-0.5 wt% of polyaspartic acid was used to regulate crystal nucleation. The solution was hydrothermally reacted at 150 °C for 24 h to generate CO3. 2- doped hydroxyapatite cores; S2.3: Pore formation and purification: The product was immersed in an ethanol-hydrochloric acid mixture and ultrasonically treated for 2 hours to remove the template. After centrifugal washing, it was calcined at 400℃ in a nitrogen atmosphere for 2 hours to obtain a pore size of 20±2nm and a specific surface area of ​​>200m 2 / g porous hydroxyapatite carrier; S2.4: Surface functional modification: the calcined support was immersed in a 5 wt% polyaspartic acid solution, stirred at 60 ° C for 6 hours, washed with deionized water and dried to form a functional support with enriched carboxylic acid groups on the surface. 2 + / Cd 2 +The adsorption capacity is increased to >200mg / g.

7. The method for preparing a pesticide-contaminated remediation agent for karst landform groundwater according to claim 6, characterized in that: The S3 at least includes the following steps: The freeze-dried powder of the composite bacterial agent was dispersed in sterile saline containing 0.1% Tween 80 at a ratio of 1:10 and allowed to stand at room temperature for 30 minutes; Synchronous lytic metabolic activator; The biomimetic calcium phosphate nanocarriers with surface modified polyaspartic acid were immersed in saturated CaCO3 solution and sonicated for 30 min to activate the carboxylic acid groups; The pretreated bacterial solution was mixed with the activated carboxylic acid groups at a mass ratio of 1:5 and reacted in a constant temperature oscillator for 6 hours, maintaining the pH at 7.5-8.0; Calcium binding protein secreted by bacteria and Ca 2+ Combined with hydroxyapatite, it induces secondary mineralization to form a bonding layer. The carboxylic acid groups of PASP cross-link with bacterial extracellular polysaccharides through hydrogen bonds and coordination, with a loading rate of >90%; Gradient adsorption process: the initially loaded complex is transferred to a vacuum adsorption tank and gradually restored to normal pressure, which allows the bacteria to embed into the carrier mesopores to form a complex. After washing and purification, the complex was rinsed three times with a protective solution containing 5% trehalose to remove unbound bacteria; Stabilization treatment is carried out. First, freeze fixation is performed. The complex is immersed in a cryoprotectant and pre-frozen to -80°C at 1°C / min for 12 hours. It is then vacuum-freeze-dried for 48 hours to form polyporous bacteria-carrier composite particles. Secondly, intelligent coating is carried out. Fluidized bed spraying technology is used to coat the surface of the polyporous bacteria-carrier composite particles with a pH-sensitive ethyl cellulose membrane. The pH-sensitive ethyl cellulose membrane slowly releases the bacteria at a pH of 6.5-7.5 and rapidly disintegrates within 30 minutes when the pH is <6.0.