Preparation method and application of chitosan diatomite composite flocculant

Abstract

The invention discloses a preparation method and application of a chitosan and diatomite composite flocculant, and the preparation method comprises the following steps: preparing a chitosan-diatomite flocculant by using chitosan and diatomite as raw materials, and then adding gamma-aminopropyltriethoxysilane (APTES) to prepare the composite flocculant (APTES-CTS / DTE). Compared with chitosan serving as a flocculating agent, the composite flocculating agent has the advantages that the use amount of the chitosan is reduced; and compared with diatomite serving as a flocculating agent, the harvesting efficiency is remarkably improved. The composite flocculant provided by the invention overcomes the defects of environmental pollution risk and influence on downstream biomass product processing due to adoption of other inorganic / organic flocculants at the present stage, and large dosage and high cost when chitosan is adopted to harvest microalgae.

Description

Technical Field

[0001] The invention belongs to the technical field of composite flocculants and microalgae harvesting, and particularly relates to a preparation method and application of a chitosan diatomaceous earth composite flocculant. Background Art

[0002] Microalgae cells contain high-value nutrients and chemical raw materials such as proteins, lipids, polysaccharides, β-carotene, and various inorganic elements (such as Cu, Fe, Se, Mn, Zn, etc.). The lipids and glycerol rich in algae are good raw materials for preparing liquid fuels. It is a simple autotrophic or heterotrophic photosynthetic organism that can effectively use carbon dioxide and light to synthesize phospholipids, proteins, nucleic acids, and carbon-rich lipids, which are then converted into biodiesel through transesterification reactions, and can also mitigate the greenhouse effect. Due to its fast growth rate and the ability to produce a large number of beneficial biological products and renewable energy, microalgae are widely used in medicine, food, environmental purification, and testing. However, due to the small size (between 2 and 20 μm), low density (average range of 0.3 to 5 g / L), and negatively charged cell surface characteristics of microalgae cells, the process of harvesting microalgae is difficult and the cost is too high, accounting for 20 to 30% of the cost of the entire production process. How to efficiently harvest microalgae and achieve economic benefits is still a huge challenge.

[0003] At present, the methods for harvesting microalgae include centrifugation, filtration, flotation and flocculation. Among them, centrifugation technology can efficiently harvest microalgae, but the cost is too high; although filtration is low-cost, it cannot separate microalgae cells from a large number of cultures, and the biomass harvest rate is low; the use of flotation technology to harvest microalgae, due to the weak adhesion between microalgae cells and bubbles, is easily sheared and desorbed by the water flow environment, thus affecting the harvesting efficiency; while the use of flocculation technology to harvest microalgae is economical and has high harvesting efficiency. Flocculation methods include physical flocculation, chemical flocculation and biological flocculation. Many studies have used different flocculants to harvest microalgae. The flocculation efficiency depends on the choice of flocculants. The flocculants mainly include inorganic flocculants (metal salts, such as FeCl3), inorganic polymers (such as polyacrylamide) and organic polymers (such as cationic starch). Flocculation is a technology with broad prospects in harvesting microalgae.

[0004] In nature, chitosan is the most widely used polysaccharide so far. It is the product of N-deacetylation of chitin, contains free amino groups, and is the only alkaline polysaccharide among natural polysaccharides. It has the characteristics of biodegradability, biocompatibility, and non-toxicity. In dilute solutions, the free amino groups in chitosan molecules are protonated, making their surfaces positively charged, which can attract negatively charged microalgae cells and cause flocculation. Because of its safety, greenness, non-toxicity, and good biocompatibility, chitosan materials are often used as flocculants in the research of microalgae harvesting or synthesized with other materials as composite flocculants for microalgae harvesting.

[0005] Among inorganic materials, natural clay minerals are cheap, readily available, and naturally non-polluting materials. They have large specific surface area and strong cation exchange and adsorption properties, and have excellent harvesting properties for algae. There are reports on the use of acidic chitosan to modify clay minerals for harvesting microalgae, but the performance of chitosan clay mineral composite flocculants is related to their structural characteristics, functional groups, and charge properties. The structure-activity relationship between flocculation performance and structural characteristics needs to be further studied. Summary of the invention

[0006] In order to solve the problems in the prior art of difficulty in harvesting microalgae, high cost of the harvesting process, and high dosage of chitosan as a flocculant, the present invention provides a preparation method and application of a chitosan diatomaceous earth composite flocculant. The composite flocculant has a porous structure and has good flocculation performance for microalgae.

[0007] In order to achieve the above effect, the specific technical scheme adopted by the present invention is: a method for preparing a chitosan diatomite composite flocculant, comprising the following steps:

[0008] Step 1, dissolving chitosan in a 1 mol / L HCl solution, stirring thoroughly with a glass rod to dissolve it, to form a chitosan-HCl solution;

[0009] Step 2: Add diatomaceous earth to the chitosan-HCl solution, place it on a magnetic stirrer and stir the reaction for 24 hours at room temperature to make the diatomaceous earth evenly dispersed in the chitosan-HCl solution to obtain a mixed suspension;

[0010] Step 3, placing the mixed suspension in a centrifuge tube for centrifugation to separate the solid and the liquid, and then drying, grinding and sieving to obtain a chitosan-diatomaceous earth composite material, denoted as CTS / DTE, which is stored in a desiccator for later use;

[0011] Step 4: Take the prepared CTS / DTE, anhydrous ethanol and γ-aminopropyltriethoxysilane (APTES) in a conical flask, ultrasonically treat for 30 minutes, and after the mixture is fully shaken, reflux under magnetic stirring at 85°C for 24 hours. After repeated washing with anhydrous ethanol and centrifugation, place it in an oven at 60°C to dry to obtain APTES-CTS / DTE.

[0012] In the above method, the mass ratio of chitosan to diatomaceous earth is 1:10.

[0013] In step 3 of the above method, the mixed suspension is placed in a centrifuge tube, centrifuged at 4000 rpm for 10 min to separate the solid and liquid, washed with deionized water and placed in an oven at 65° C. to dry for 24 to 48 h. The dried solid is taken out and ground, and passed through a 50-mesh sieve to obtain CTS / DTE.

[0014] At the same time, the present invention also provides an application of APTES-CTS / DTE obtained based on the preparation method of the above-mentioned chitosan diatomaceous earth composite flocculant in the harvesting of microalgae. First, an APTES-CTS / DTE solution with a concentration of 200-800 mg / L is added to a microalgae culture solution at a ratio of 1:4-10, and then the solution is rapidly stirred at 200 rpm for 2 minutes, and then slowly stirred at 50 rpm for 10 minutes. Finally, the solution is allowed to stand for sedimentation, the supernatant is discarded, and the microalgae can be harvested.

[0015] In the above application, the microalgae are cultured in a photobioreactor using BG-11 culture medium at a light-dark ratio of 12:12 and a culture temperature of 25±1°C.

[0016] In the above application, before adding the APTES-CTS / DTE solution, the pH of the microalgae culture solution is adjusted to 3-12 using hydrochloric acid or sodium hydroxide solution.

[0017] In the above application, the static sedimentation time is 5 to 60 minutes.

[0018] Due to the adoption of the above technical scheme, the beneficial effects of the present invention are as follows: the present invention can steadily improve the mechanical strength and chemical stability of chitosan by functionalizing chitosan clay mineral flocculants with organic silane, while achieving directional regulation of the physicochemical properties of the chitosan clay flocculants, and constructing a multifunctional, performance-optimized chitosan-based composite flocculant, which has a good harvesting efficiency for microalgae, which can reach more than 96%. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 a is the SEM image of chitosan;

[0020] Figure 1 b is the SEM image of diatomite;

[0021] Figure 1 c is the SEM image of APTES-CTS / DTE when the volume ratio of APTES-CTS / DTE solution to microalgae culture medium is 1:4;

[0022] Figure 1 d is the SEM image of APTES-CTS / DTE when the volume ratio of APTES-CTS / DTE solution to microalgae culture medium is 1:10;

[0023] Figure 2 Fourier transform infrared spectra of chitosan, diatomaceous earth and APTES-CTS / DTE;

[0024] Figure 3 It is the full XPS spectrum of chitosan, diatomaceous earth and APTES-CTS / DTE;

[0025] Figure 4 It is the flocculation efficiency diagram of APTES-CTS / DTE on microalgae at different ratios and dosages;

[0026] Figure 5 It is the flocculation efficiency diagram of APTES-CTS / DTE on microalgae at different pH;

[0027] Figure 6 is the flocculation efficiency of APTES-CTS / DTE on microalgae at different sedimentation times;

[0028] Figure 7 The SEM image of the flocs of APTES-CTS / DTE and microalgae;

[0029] Figure 8 Schematic representation of the protein, carbohydrate, and lipid contents of C. sorokiniana after harvesting by centrifugation and flocculation. DETAILED DESCRIPTION

[0030] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is further described in detail below with reference to the accompanying drawings and embodiments.

[0031] Example 1

[0032] The preparation process and characterization results of the composite flocculant of the present invention are as follows: 1g of chitosan is accurately weighed, dissolved in 100mL, 1mol / L HCl solution, and fully stirred with a glass rod to dissolve it to obtain a chitosan-HCl solution. According to the mass ratio of chitosan to diatomaceous earth of 1:10, 10g of ground and sieved diatomaceous earth is weighed, added to the chitosan solution, placed on a magnetic stirrer and stirred for reaction at room temperature for 24h, so that the diatomaceous earth is evenly dispersed in the chitosan-HCl solution to obtain a mixed suspension. The mixed suspension is placed in a centrifuge tube, centrifuged at 4000rpm for 10min to separate the solid and liquid, washed with deionized water and placed in an oven at 65°C for 24 to 48h, the dried solid is taken out and ground, passed through a 50-mesh sieve, and a chitosan-diatomaceous earth composite material is obtained, which is recorded as CTS / DTE.

[0033] Take 1g of the prepared CTS / DTE, 100mL of anhydrous ethanol and 0.75mL of APTES in a conical flask, ultrasonicate for 30min to fully shake the mixture, then reflux under magnetic stirring at 85°C for 24h, wash repeatedly with anhydrous ethanol, centrifuge and place in an oven at 60°C to dry to obtain the composite flocculant APTES-CTS / DTE. The scanning electron microscope image, Fourier transform infrared spectrum image and XPS full spectrum image of the obtained composite flocculant, chitosan and diatomaceous earth are shown in Figures 1 to 3 shown.

[0034] in, Figure 1 a is a SEM image of chitosan at a magnification of 1000 times, from which it can be observed that its surface is rough and not smooth, and has no pores. Figure 1 b is a SEM image of diatomite magnified 2000 times. It can be observed that the diatomite is disc-shaped with many micropores on the disc. Figure 1 c is the SEM image of APTES-CTS / DTE when the volume ratio of APTES-CTS / DTE solution to microalgae culture medium is 1:4. It can be seen from the figure that the APTES-CTS / DTE material is still in the shape of a diatomaceous earth disc, but blocky substances are attached to the disc, and the surface of the disc is smooth and still has pores. Figure 1 d is the SEM image of APTES-CTS / DTE when the volume ratio of APTES-CTS / DTE solution to microalgae culture solution is 1:10. It can be seen from the figure that the disc in the APTES-CTS / DTE material is slightly deformed, and more blocky substances are attached to the micropores, which are piled up but unevenly distributed. This shows that chitosan is attached to diatomaceous earth and piled up because APTES can couple organic molecules with inorganic fillers, thereby enhancing the adhesion between chitosan and diatomaceous earth.

[0035] Figure 2 The infrared spectra of APTES-CTS / DTE, chitosan and diatomaceous earth. In the infrared spectrum of APTES-CTS / DTE, the characteristic peaks are related to the stretching vibration of -OH in the silanol group (3430 cm -1 -3784cm -1 ), Si-O asymmetric vibration (1623cm -1 , 1630cm -1 ), asymmetric vibration of Si-O-Si (1087cm -1 -1090cm -1 ) and Si-O stretching vibration, the characteristic peaks of chitosan in the composite flocculant, such as -NH2 and -CN, disappeared, chitosan interacted with diatomite, and the -NH3 on chitosan was hydrogen-bonded with the surface of diatomite, which weakened the vibration peak, indicating that the composite flocculant was successfully prepared.

[0036] Figure 3 The full XPS spectra of APTES-CTS / DTE, chitosan and diatomite. It can be seen from the full spectrum that the modified composite flocculant has the characteristic peaks of N in chitosan and Si in diatomite, but the content of N is relatively low. The characteristic peaks of C, O, N and Si appear at 284.8eV, 532.8eV, 399.8eV and 102.8eV respectively. The materials in each proportion contain the same groups, which proves that the modified composite flocculant is successfully prepared.

[0037] Example 2

[0038] To investigate the flocculation effect of APTES-CTS / DTE at different ratios and concentrations on Chlorella: At pH = 3, different ratios (1:4, 1:6, 1:8, 1:10) and different doses (200-800 mg / L) of composite flocculant solutions were added to the microalgae culture solution, and the mixture was stirred rapidly at 200 rpm for 2 min, and then slowly stirred at 50 rpm for 10 min. The absorbance of the supernatant at 680 nm was measured and the flocculation efficiency was calculated. The flocculation efficiency results are shown in Figure 2. Figure 4 As shown, when the dosage of composite flocculant APTES-CTS / DTE (1:10) is 700 mg / L, the best flocculation efficiency (96.26%) can be achieved.

[0039] Example 3

[0040] To investigate the flocculation effect of APTES-CTS / DTE on Chlorella at different pH values: hydrochloric acid and sodium hydroxide solution were used to adjust the pH value of the microalgae culture solution (3-12), and the composite flocculant solution with the best ratio and concentration obtained in Example 2 was added to carry out the flocculation experiment. 700 mg / L APTES-CTS / DTE (1:10) was added to the microalgae culture solution, and the mixture was stirred rapidly at 200 rpm for 2 min, then slowly stirred at 50 rpm for 10 min, and then allowed to settle for 20 min. The absorbance of the supernatant at 680 nm was measured and the flocculation efficiency was calculated. The flocculation results are shown in Figure 2. Figure 5 As shown, the results show that at pH = 3, the flocculation efficiency is the highest, reaching 96.26%. Compared with the flocculation efficiency of self-flocculation at pH = 12, the addition of flocculants at pH = 12 is about 30% higher than the self-flocculation efficiency, proving that the composite flocculant has a good flocculation performance on Chlorella.

[0041] Example 4

[0042] Test the flocculation effect of different settling times on Chlorella: Flocculation experiments were carried out under the optimal conditions of Examples 2 and 3 above. 700 mg / LAPTES-CTS / DTE (1:10) was added to the microalgae culture solution at pH = 3, and the mixture was stirred rapidly at 200 rpm for 2 min, then slowly stirred at 50 rpm for 10 min, and then allowed to settle for 5 to 60 min. The absorbance of the supernatant at 680 nm was measured and the flocculation efficiency was calculated. The flocculation results are shown in Figure 2. Figure 6 As shown, under the optimal conditions, the flocculation efficiency can reach 98.64% after 60 minutes of sedimentation.

[0043] Example 5

[0044] from Figure 7 From the scanning electron microscope image of the flocs, it can be observed that the microalgae are aggregated on the surface of the flocculant in a piled shape, indicating that the flocculant has good flocculation properties and can effectively aggregate microalgae cells.

[0045] Example 6

[0046] After centrifugation of C. sorokiniana, the contents of protein, carbohydrates, and lipids were 22.79%, 23.30%, and 22.80%, respectively. The contents of C. sorokiniana after flocculation with APTES-CTS / DTE were 29.89%, 25.91%, and 36.20%, respectively. Figure 8 Compared with flocculation harvesting, the biomass components obtained by centrifugation all had certain losses, which were 7.10%, 2.61% and 13.40% respectively. The use of APTES-CTS / DTE for harvesting microalgae will not interfere with the extraction of biochemical components. APTES-CTS / DTE is biocompatible and applicable for microalgae harvesting.

[0047] Example 7

[0048] In order to explore the economic benefits, a simple cost analysis was conducted on the articles prepared by the composite flocculant used in the present invention, as shown in the following table:

[0049]

[0050] The above-described embodiments and drawings are only preferred embodiments of the present invention and are not limited to the specific methods of the above-described embodiments. Without departing from the spirit of the present invention and the scope of protection of the claims, technical researchers in this field may perform various forms of equivalent changes or chemical modifications, all of which should be within the scope of protection of the present invention.

Claims

1. A method for preparing a chitosan diatomite composite flocculant, characterized in that The following steps are involved: Step 1, dissolving chitosan in a 1 mol / L HCl solution, stirring thoroughly with a glass rod to dissolve it, to form a chitosan-HCl solution; Step 2: Add diatomaceous earth to the chitosan-HCl solution, place it on a magnetic stirrer and stir the reaction for 24 hours at room temperature to make the diatomaceous earth evenly dispersed in the chitosan-HCl solution to obtain a mixed suspension; Step 3, placing the mixed suspension in a centrifuge tube for centrifugation to separate the solid and the liquid, and then drying, grinding and sieving to obtain a chitosan-diatomaceous earth composite material, denoted as CTS / DTE, which is stored in a desiccator for later use; Step 4: Take the prepared CTS / DTE, anhydrous ethanol and γ-aminopropyltriethoxysilane (APTES) in a conical flask, ultrasonically treat for 30 minutes, and after the mixture is fully shaken, reflux under magnetic stirring at 85°C for 24 hours. After repeated washing with anhydrous ethanol and centrifugation, place it in an oven at 60°C to dry to obtain APTES-CTS / DTE.

2. The method for preparing the chitosan diatomite composite flocculant according to claim 1, characterized in that: The mass ratio of chitosan to diatomaceous earth is 1:

10.

3. The method for preparing the chitosan diatomite composite flocculant according to claim 1, characterized in that: In the step 3, the mixed suspension is placed in a centrifuge tube, centrifuged at 4000 rpm for 10 minutes to separate the solid and liquid, washed with deionized water and placed in an oven at 65° C. to dry for 24 to 48 hours, and the dried solid is taken out, ground, and passed through a 50-mesh sieve to obtain CTS / DTE.

4. An application of APTES-CTS / DTE obtained by the preparation method of chitosan diatomaceous earth composite flocculant according to any one of claims 1 to 3 in microalgae harvesting, characterized in that: First, add an APTES-CTS / DTE solution with a concentration of 200 to 800 mg / L into the microalgae culture solution at a ratio of 1:4 to 10, then stir rapidly at 200 rpm for 2 minutes, then stir slowly at 50 rpm for 10 minutes, and finally discard the supernatant after standing and settling, and the microalgae can be harvested.

5. The use of APTES-CTS / DTE in microalgae harvesting according to claim 4, characterized in that: The microalgae are cultured in a photobioreactor using BG-11 culture medium under the condition of a light-dark ratio of 12:12 and a culture temperature of 25±1°C.

6. The use of APTES-CTS / DTE in microalgae harvesting according to claim 4, characterized in that: Before adding the APTES-CTS / DTE solution, the pH of the microalgae culture solution is adjusted to 3-12 using hydrochloric acid or sodium hydroxide solution.

7. The use of APTES-CTS / DTE in microalgae harvesting according to claim 4, characterized in that: The static sedimentation time is 5 to 60 minutes.

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