Preparation method and application of chitosan diatomite composite flocculant

Abstract

The application discloses a preparation method and application of a chitosan diatomite composite flocculant, and the preparation method is to prepare the chitosan diatomite flocculant by taking chitosan and diatomite as raw materials, and then adding gamma-aminopropyl triethoxysilane (APTES) to prepare the composite flocculant (APTES-CTS / DTE). Compared with chitosan as the flocculant, the composite flocculant reduces the dosage of chitosan; compared with diatomite as the flocculant, the composite flocculant significantly improves the recovery efficiency. The composite flocculant overcomes the pollution risk of other inorganic / organic flocculants to the environment, the influence on downstream biomass product processing, and the insufficient of large dosage and high cost of chitosan in the recovery of microalgae at the present stage.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of composite flocculants and microalgae harvesting, and particularly relates to a preparation method and application of a chitosan diatomite composite flocculant. BACKGROUND

[0002] Microalgae cells contain high-value nutrients such as proteins, lipids, polysaccharides, beta-carotene, and various inorganic elements (such as Cu, Fe, Se, Mn, Zn, etc.), and chemical raw materials, and 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 utilize carbon dioxide and light to synthesize phospholipids, proteins, nucleic acids, and carbon-rich lipids, thereby converting them into biodiesel through ester exchange reactions, while also mitigating the greenhouse effect. Due to its rapid growth and ability to produce large amounts of beneficial biological products and renewable energy, microalgae are widely used in the fields of medicine, food, environmental purification, and detection. However, due to the small size (2-20 μm) and low density (average range of 0.3-5 g / L) of microalgae cells, and the negative charge on the cell surface, the process of harvesting microalgae is difficult, the cost is too high, accounting for 20-30% of the total production process cost, and how to efficiently harvest microalgae and achieve economic benefits remains a significant challenge.

[0003] Currently, the methods for harvesting microalgae include centrifugation, filtration, air flotation, and flocculation. Centrifugation can efficiently harvest microalgae, but the cost is too high. Filtration has a low cost, but it cannot separate microalgae cells from a large amount of culture, and the biomass harvesting rate is low. The use of air flotation technology to harvest microalgae is not reliable due to the weak adhesion between microalgae cells and bubbles, which is easily detached by water flow shear, thereby affecting the harvesting efficiency. The use of flocculation technology to harvest microalgae has economic benefits and high harvesting efficiency. Flocculation methods include physical flocculation, chemical flocculation, and biological flocculation. Many studies have used different flocculants to harvest microalgae, and the flocculation efficiency depends on the selection of flocculants. 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 promising technology for harvesting microalgae.

[0004] In nature, chitosan is a polysaccharide that has been widely used so far, which is the product of N-deacetylation of chitin, contains free amino groups, is the only alkaline polysaccharide among natural polysaccharides, and has characteristics such as biodegradability, biocompatibility, and non-toxicity. In a dilute solution, the free amino groups in the chitosan molecule are protonated, making the surface positively charged, which can attract negatively charged microalgae cells, resulting in flocculation. Due to its safety, greenness, non-toxicity, and good biocompatibility, chitosan materials are often used as flocculants or combined with other materials to synthesize composite flocculants for microalgae harvesting in research.

[0005] Among inorganic materials, natural clay minerals have excellent harvesting performance on algae due to their large specific surface area and strong cation exchange performance and adsorption characteristics, as a cheap and readily available, naturally non-polluting material. At present, there have been reports on the modification of clay minerals with acid chitosan for harvesting microalgae, but the performance of chitosan clay mineral composite flocculant is related to its structural characteristics, functional groups and charge properties, and the structure-activity relationship between flocculation performance and structural characteristics needs to be further studied. SUMMARY

[0006] In order to solve the problems of difficult harvesting of microalgae, high cost of harvesting process and high dosage of chitosan as flocculant in the prior art, the application provides a preparation method and application of a chitosan diatomite composite flocculant, which has a porous structure and good flocculation performance on microalgae.

[0007] To achieve the above effects, the application adopts the following specific technical scheme: a preparation method of a chitosan diatomite composite flocculant, comprising the following steps:

[0008] Step one, dissolve chitosan in 1mol / L HCl solution, stir with a glass rod to dissolve, and form a chitosan-HCl solution;

[0009] Step two, add diatomite to the chitosan-HCl solution, place it on a magnetic stirrer and stir at room temperature for 24h, so that the diatomite is uniformly dispersed in the chitosan-HCl solution to obtain a mixed suspension;

[0010] Step three, centrifuge the mixed suspension to separate the solid and liquid, then dry, grind and sieve to prepare a chitosan-diatomite composite material, denoted as CTS / DTE, which is stored in a desiccator for standby;

[0011] Step four, take the prepared CTS / DTE, anhydrous ethanol and gamma-aminopropyl triethoxysilane (APTES) in an Erlenmeyer flask, ultrasonic treat for 30min, then fully shake the mixture, and magnetically stir and condense at 85℃ for 24h, then repeatedly wash and centrifuge with anhydrous ethanol, and place it in an oven at 60℃ for drying to obtain APTES-CTS / DTE.

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

[0013] In step three of the above method, the mixed suspension is placed in a centrifuge tube and 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 for drying for 24-48 h, the dried solid is taken out and ground, and passed through a 50 mesh sieve to obtain CTS / DTE.

[0014] Meanwhile, the application also provides application of APTES-CTS / DTE in microalgae harvesting, which is obtained by the preparation method of the above chitosan diatomite composite flocculant. First, a 200-800 mg / L APTES-CTS / DTE solution is added to the microalgae culture solution at a ratio of 1:4-10, then stirred at 200 rpm for 2 min, stirred slowly at 50 rpm for 10 min, and finally the supernatant is discarded after standing and settling, and the microalgae is harvested.

[0015] In the above application, the microalgae is cultured in a photobioreactor using a BG-11 medium under the condition of a light-dark ratio of 12:12, and the culture temperature is 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 a hydrochloric acid or sodium hydroxide solution.

[0017] In the above application, the standing and settling time is 5-60 min.

[0018] Due to the above technical solutions, the application has the following beneficial effects: the chitosan clay mineral flocculant is modified by organic silane functionalization, which can steadily improve the mechanical strength and chemical stability of chitosan, and realize directional regulation of the physicochemical properties of the chitosan clay flocculant, thereby constructing a multifunctional and performance-optimized chitosan-based composite flocculant. The composite flocculant has good harvesting efficiency for microalgae, which can reach more than 96%. BRIEF DESCRIPTION OF DRAWINGS

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

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

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

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

[0023] Figure 2 The Fourier infrared spectrogram of chitosan, diatomite and APTES-CTS / DTE;

[0024] Figure 3 The XPS full spectrogram of chitosan, diatomite and APTES-CTS / DTE;

[0025] Figure 4 The flocculation efficiency diagram of APTES-CTS / DTE on microalgae under different proportions and dosages;

[0026] Figure 5 The flocculation efficiency diagram of APTES-CTS / DTE on microalgae under different pH values;

[0027] Figure 6 The flocculation efficiency of APTES-CTS / DTE on microalgae under different settling times;

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

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

[0030] In order to make the purpose, technical scheme and advantages of the present application more clear, the present application will be further described in detail below in combination with the drawings and examples.

[0031] Example 1

[0032] The preparation process and characterization results of the composite flocculant of the present application: 1g of chitosan was accurately weighed and dissolved in 100mL of 1mol / L HCl solution, and a glass rod was used to fully stir to dissolve it to obtain a chitosan-HCl solution. According to the mass ratio of chitosan to diatomite of 1:10, 10g of ground and sieved diatomite was weighed and added to the chitosan solution, and placed on a magnetic stirrer for stirring at room temperature for 24h, so that the diatomite was uniformly dispersed in the chitosan-HCl solution to obtain a mixed suspension. The mixed suspension was placed in a centrifuge tube and centrifuged at 4000rpm for 10min to separate the solid and liquid, washed with deionized water and placed in an oven for drying at 65℃ for 24-48h. The dried solid was taken out and ground, and sieved through a 50 mesh sieve to prepare a chitosan-diatomite composite material, which was denoted as CTS / DTE.

[0033] The prepared CTS / DTE 1 g, 100 mL of anhydrous ethanol and 0.75 mL of APTES were taken in a conical flask, the mixture was fully shaken by ultrasonic for 30 min, then magnetically stirred at 85°C under condensation reflux for 24 h, washed repeatedly with anhydrous ethanol and centrifuged, then placed in an oven at 60°C to dry to obtain the composite flocculant APTES-CTS / DTE. The scanning electron microscope images, Fourier transform infrared spectra and XPS full spectra of the obtained composite flocculant and chitosan, diatomite are shown in Figures 1 to 3 .

[0034] Among them, Figure 1 a is the SEM image of chitosan at 1000 times magnification, and the surface of chitosan is rough and not smooth, and there is no pore. Figure 1 b is the SEM image of diatomite at 2000 times magnification, and the diatomite is in the form of a disc, and there are 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 solution is 1:4, and it can be seen from the figure that the disc of diatomite in the APTES-CTS / DTE material is still present, but the disc is attached to a blocky substance, and the disc surface 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, and 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, showing a stacked shape but uneven distribution. It shows that chitosan is attached to diatomite, and the stacked shape is due to the coupling of APTES to organic molecules and inorganic fillers, which enhances the adhesion between chitosan and diatomite.

[0035] Figure 2 is the infrared spectrum of APTES-CTS / DTE, chitosan and diatomite. In the infrared spectrum of APTES-CTS / DTE, the characteristic peaks are related to the stretching vibration of -OH in silanol group (3430 cm -1 -3784cm -1 ), the asymmetric vibration of Si-O (1623 cm -1 , 1630 cm -1 ), the asymmetric vibration of Si-O-Si (1087 cm -1 -1090cm -1 ) and the stretching vibration of Si-O. The characteristic peaks of -NH2 and -CN related to chitosan in the composite flocculant disappear, chitosan and diatomite interact, the hydrogen bond between -NH3 on chitosan and the surface of diatomite weakens the vibration peak, indicating that the preparation of the composite flocculant is successful.

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

[0037] Example 2

[0038] The flocculation effect of APTES-CTS / DTE with different proportions and concentrations on Chlorella vulgaris was explored. Different proportions (1:4, 1:6, 1:8, 1:10) and different doses (200-800 mg / L) of composite flocculant solution were added to the microalgae culture solution at pH=3, and the absorbance at 680 nm of the supernatant was measured and the flocculation efficiency was calculated. The flocculation efficiency results are shown in Table 1. Figure 4 As shown in Table 1, when the dose 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] The flocculation effect of APTES-CTS / DTE on Chlorella vulgaris at different pH was explored. The pH of the microalgae culture solution was adjusted to 3-12 using hydrochloric acid and sodium hydroxide solution, and the best proportion and concentration of composite flocculant solution obtained in Example 2 was added for flocculation experiment. 700 mg / L of APTES-CTS / DTE (1:10) was added to the microalgae culture solution, stirred at 200 rpm for 2 min, then stirred slowly at 50 rpm for 10 min, and then settled for 20 min. The absorbance at 680 nm of the supernatant was measured and the flocculation efficiency was calculated. The flocculation results are shown in Table 2. Figure 5 As shown in Table 2, the results show that at pH=3, the flocculation efficiency is the highest, which can reach 96.26%. Compared with the self-flocculation efficiency at pH=12, the flocculation efficiency after adding the flocculant at pH=12 is about 30% higher than the self-flocculation efficiency, which proves that the composite flocculant has good flocculation performance on Chlorella vulgaris.

[0041] Example 4

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

[0043] Example 5

[0044] from Figure 7 The scanning electron microscope images of the flocs show that the microalgae aggregate on the surface of the flocculant in a stacked manner, indicating that the flocculant has good flocculation performance and can effectively aggregate microalgae cells.

[0045] Example 6

[0046] After centrifugation harvesting of *C. sorokinina*, the contents of protein, carbohydrates, and lipids were 22.79%, 23.30%, and 22.80%, respectively. After flocculation harvesting using APTES-CTS / DTE, the contents were 29.89%, 25.91%, and 36.20%, respectively. Figure 8 As shown, compared with flocculation harvesting, centrifugation resulted in the loss of certain biomass components, with losses of 7.10%, 2.61%, and 13.40%, respectively. Harvesting using APTES-CTS / DTE does not interfere with the extraction of microalgal biochemical components, and APTES-CTS / DTE is biocompatible and suitable for microalgal harvesting.

[0047] Example 7

[0048] To explore the economic benefits, a simple cost analysis was conducted on the products prepared using the composite flocculant used in this invention, as shown in the table below:

[0049]

[0050] The above-described embodiments and accompanying drawings are merely preferred embodiments of the present invention and are not limited to the specific methods described above. Without departing from the spirit and scope of the claims of the present invention, those skilled in the art can make various 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-diatomic clay composite flocculant, characterized in that It comprises the following steps: Step one, dissolve chitosan in 1 mol / L HCl solution, stir with a glass rod to dissolve, form chitosan-HCl solution; Step two, add diatomite into chitosan-HCl solution, place on a magnetic stirrer and stir at room temperature for 24 h, so that diatomite is uniformly dispersed in chitosan-HCl solution, to obtain a mixed suspension; Step three, place the mixed suspension in a centrifuge tube for centrifugal treatment, separate solid and liquid, then dry, grind and sieve, to obtain chitosan-diatomite composite material, recorded as CTS / DTE, stored in a desiccator for standby; Step four, take prepared CTS / DTE, anhydrous ethanol and gamma-aminopropyl triethoxysilane (APTES) in a conical flask, ultrasonic treat for 30 min, after fully shaking the mixture, magnetically stir and condense reflux at 85℃ for 24 h, repeatedly wash and centrifuge with anhydrous ethanol, then place in an oven and dry at 60℃ to obtain APTES-CTS / DTE.

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

10.

3. The method for preparing chitosan diatomite composite flocculant according to claim 1, characterized in that: In step three, place the mixed suspension in a centrifuge tube, centrifuge at 4000 rpm for 10 min, separate solid and liquid, wash with deionized water and dry in an oven at 65℃ for 24-48 h, take out the dried solid, grind and sieve through a 50 mesh sieve, to obtain CTS / DTE.

4. The use of APTES-CTS / DTE prepared according to the method of claim 1-3 in the harvesting of microalgae, characterized in that: First, add APTES-CTS / DTE solution with a concentration of 200-800 mg / L into microalgae culture solution at a ratio of 1:4-10, then stir rapidly at 200 rpm for 2 min, slowly stir at 50 rpm for 10 min, finally discard supernatant after standing and settling, and collect microalgae.

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

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

7. Use of APTES-CTS / DTE according to claim 4 in the harvesting of microalgae, characterized in that: The standing and settling time is 5-60 min.

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