Salt-tolerant modified cationic starch-based flocculant, preparation method thereof and application of the flocculant in flocculation and recovery of microalgae

Abstract

The application discloses a salt-tolerant modified cationic starch-based flocculant and a preparation method and application of the flocculant in flocculation and recovery of microalgae, and belongs to the technical field of flocculation materials. The preparation method comprises the following steps: (1) mixing and stirring an alkali solution and starch, then adding a modifier solution of dodecyl succinic anhydride (DDSA), mixing and reacting, and finally washing and drying to obtain modified starch; and (2) mixing and stirring an alkali solution and the modified starch obtained in the step (1), then adding glycidyltrimethylammonium chloride (GTA), mixing and reacting, and finally washing and drying to obtain the product. The starch is modified by GTA and DDSA to be cationic and hydrophobic, and a salt-tolerant modified cationic starch-based flocculant is successfully prepared. The flocculant has good salt tolerance in microalgae flocculation and recovery, and can still achieve a recovery rate of more than 80% under high salinity, and the flocculant has excellent economic value.

Description

Technical Field

[0001] The present invention relates to the technical field of flocculation materials, and in particular to a salt-tolerant modified cationic starch-based flocculant, a preparation method thereof, and an application thereof in flocculating and harvesting microalgae. Background Art

[0002] Microalgae are single-celled, photosynthetic, autotrophic microorganisms with rapid growth and strong adaptability. They are widely distributed in various water bodies, including ponds, lakes, rivers, and oceans. They are a promising biomass resource for producing food, feed, fuel, and chemical materials. However, one of the current bottlenecks in the large-scale production and application of microalgae is harvesting (i.e., concentrating microalgae cells from the culture medium). Due to their small cell size (3-30 μm) and low biomass concentration (0.5-3 g / L), harvesting microalgae is difficult and costly. For example, in the production of biodiesel from microalgae, the cost of the harvesting process accounts for 10%-20% of the total production cost. Currently, the most commonly used methods for concentrating microalgae are centrifugation and filtration, but these processes are energy-intensive.

[0003] Coagulation / flocculation is the most promising low-cost, low-energy technology for harvesting microalgae. The most commonly used flocculants include inorganic metal salts (such as aluminum and iron salts) and organic polymers (such as polyacrylamide). Inorganic metal salts induce flocculation primarily through ionic interactions (charge neutralization or net-sweeping effects); organic polymer flocculants aggregate individual cells primarily through charge neutralization and bridging adsorption. However, the use of both types of flocculants can lead to contamination of microalgae biomass with metals or potentially toxic acrylamide residues, making them unsuitable for the food or feed industry. Furthermore, most research reports indicate that induced flocculation is more suitable for freshwater microalgae species. Under high salinity conditions, ions in the medium can shield charged sites between the flocculant and the microalgae, weakening charge neutralization. Furthermore, the presence of salt can promote polymer folding and disintegration, affecting the flocculant's conformation and, consequently, impairing bridging adsorption. Therefore, the development of a green, efficient, and salt-tolerant flocculant is crucial for large-scale harvesting of microalgae biomass under diverse culture conditions. Summary of the Invention

[0004] In order to solve the above technical problems, the purpose of the present invention is to provide a salt-tolerant modified cationic starch-based flocculant and its preparation method and application in flocculating and harvesting microalgae, so as to solve the problems of existing flocculation materials in microalgae harvesting, such as easy generation of toxic pollutant residues, low efficiency, and poor salt tolerance.

[0005] The technical solution of the present invention to solve the above technical problems is as follows:

[0006] A method for preparing a salt-tolerant modified cationic starch-based flocculant comprises the following steps:

[0007] (1) first, the alkali solution and starch are mixed and stirred, then the modifier solution is added, mixed and reacted, and finally washed and dried to obtain the modified starch;

[0008] (2) first, the alkali solution and the modified starch obtained in step (1) are mixed and stirred, then the epoxy propyl trimethyl ammonium chloride is added, mixed and reacted, and finally washed and dried to obtain the modified starch.

[0009] The present application has the following advantages: the present application provides a preparation method of a salt-tolerant modified cationic starch-based flocculant, which is obtained by cationic modification and hydrophobic modification of starch by GTA and DDSA, and has the characteristics of high salt tolerance in microalgae harvesting, and can still maintain a high harvesting rate of microalgae under high salinity, and the amount of the flocculant is less. The mechanism of CDCS in flocculating microalgae under high salt environment mainly includes: (1) CDCS has strong salt tolerance, can maintain a relatively stretched molecular chain, large viscosity and high charge density, and thus can play a good charge neutralization effect and bridge adsorption effect. (2) CDCS can form a larger three-dimensional network flocculation under high salt environment, and the flocculation has stronger anti-shearing and re-flocculation ability, and plays a better net capture and sweeping effect in the process of sedimentation, so as to achieve an ideal harvesting rate.

[0010] Further, in step (1), the alkali solution is an ethanol solution of NaOH, and the concentration is 0.05-0.2 g / mL; the mass-volume ratio of starch to alkali solution is 80-120 g:20-40 mL.

[0011] Further, in step (1), the modifier is dodecyl succinic anhydride, the concentration of the modifier solution is 0.3-1 g / mL, and the solvent is ethanol; the volume-mass ratio of the modifier solution to starch is 8-12 mL:80-120 g.

[0012] Further, in step (1), the temperature of the mixed reaction is 80-100℃, and the time is 50-100 min; the drying temperature is 40-50℃.

[0013] The beneficial effects of the above further technical solutions are: the present application modifies starch by dodecyl succinic anhydride, successfully prepares dodecyl succinic anhydride modified starch, reduces the crystallinity of starch, improves the hydrophobic property of starch, and promotes the molecular association in the molecular chain of the prepared salt-tolerant modified cationic starch-based flocculant through the hydrophobic microzones formed by the mutual aggregation of hydrophobic groups, so that the flocculant has the characteristics of hydrophobic associating polymer.

[0014] Furthermore, in step (2), the alkaline solution is an ethanol solution of NaOH with a concentration of 0.01-0.02 g / mL; the volume mass ratio of the alkaline solution, modified starch and epoxypropyltrimethylammonium chloride is 20-40 mL:80-120 g:10-30 g.

[0015] Furthermore, in step (2), the temperature of the mixing reaction is 60-80°C, and the time is 4-8 h; and the drying temperature is 40-50°C.

[0016] The beneficial effect of adopting the above-mentioned further technical solution is as follows: the present invention successfully prepares modified cationic starch by cationically modifying starch with epoxypropyltrimethylammonium chloride, and obtains salt-resistant properties through the ionizable positive and negative charge groups contained in the molecule and the anti-polyelectrolyte behavior of the amphoteric polyelectrolyte.

[0017] A salt-tolerant modified cationic starch-based flocculant is prepared by adopting the above preparation method.

[0018] Application of the salt-tolerant cationic starch-based flocculant in microalgae harvesting.

[0019] The present invention has the following beneficial effects:

[0020] The present invention successfully prepared a salt-tolerant modified cationic starch-based flocculant by cationizing and hydrophobizing starch with GTA and DDSA. The flocculant has good salt tolerance in microalgae flocculation harvesting and can still achieve a harvest rate of more than 80% under high salinity. The flocculant dosage is relatively low, and it has excellent economic value. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 SEM and XRD results of CS, CCS, DCS and CDCS in Experimental Example 1, where (A) is SEM and (B) is XRD;

[0022] Figure 2 is the CS, CCS, DCS and CDCS in Test Example 1 1 H-NMR and zeta potential results, where (A) is 1 H-NMR, (B) is zeta potential;

[0023] Figure 3 The harvest rates of CCS and CDCS under different salinities in Experiment 2 vary with the dosage, where (A) is 3 ppt, (B) is 10 ppt, (C) is 15 ppt, and (D) is 20 ppt.

[0024] Figure 4Photos of the sedimentation effects of CCS and CDCS flocculating microalgae at different salinities in Experiment 2, where (A) is 3 ppt, (B) is 10 ppt, (C) is 15 ppt, and (D) is 20 ppt.

[0025] Figure 5 The hydrodynamic diameter, viscosity, and zeta potential of CS, CCS, DCS, and CDCS at different salinities in Experimental Example 3, where (A) is the hydrodynamic diameter, (B) is the viscosity, and (C) is the zeta potential;

[0026] Figure 6 The zeta potential changes of the system when CCS and CDCS flocculated microalgae at different salinities in Experiment 3, where (A) is 3 ppt, (B) is 10 ppt, (C) is 15 ppt, and (D) is 20 ppt.

[0027] Figure 7 The microscopic morphology of flocs formed after CCS and CDCS flocculated microalgae at different salinities in Experiment 3, where (A) is 3 ppt, (B) is 10 ppt, (C) is 15 ppt, and (D) is 20 ppt.

[0028] Figure 8 The real-time particle size changes of CCS and CDCS flocculated microalgae under different salinities in Experiment 3, where (A) is 3 ppt, (B) is 10 ppt, (C) is 15 ppt, and (D) is 20 ppt. DETAILED DESCRIPTION

[0029] The principles and features of the present invention are described below in conjunction with the accompanying drawings. The examples are only used to explain the present invention and are not intended to limit the scope of the invention. In the embodiments, if specific conditions are not specified, they are carried out according to conventional conditions or conditions recommended by the manufacturer. If the manufacturer of the reagents or instruments is not specified, they are all conventional products that can be purchased commercially.

[0030] Example 1:

[0031] A method for preparing a salt-tolerant dodecenylsuccinic anhydride-modified cationic starch-based flocculant comprises the following steps:

[0032] (1) Preparation of dodecenylsuccinic anhydride modified starch DCS

[0033] First, 3.0 g of NaOH was dissolved in 30 mL of anhydrous ethanol and evenly sprayed onto 100.0 g of corn starch while stirring continuously for alkalization. Then, 6.0 g of dodecenylsuccinic anhydride (DDSA) was dissolved in 10.0 mL of anhydrous ethanol and slowly added dropwise to the alkalized starch while stirring continuously. After mixing evenly, the mixture was reacted at 90°C for 80 min. Finally, the mixture was washed with anhydrous ethanol and dried in a vacuum at 45°C to obtain the product.

[0034] (2) Preparation of salt-tolerant dodecenylsuccinic anhydride-modified cationic starch-based flocculant CDCS

[0035] First, 0.5 g of NaOH was dissolved in 30 mL of anhydrous ethanol, and the mixture was evenly sprayed into 100.0 g of DCS obtained in step (1) and stirred continuously for alkalization. Then, 20.0 g of epoxypropyltrimethylammonium chloride was added to the alkalized DCS and mixed evenly, and the mixture was reacted at 70°C for 6 h. Finally, the mixture was washed and purified with anhydrous ethanol until no chloride ions were detected in the washing solution, and vacuum dried at 45°C to obtain the product.

[0036] Example 2:

[0037] A method for preparing a salt-tolerant dodecenylsuccinic anhydride-modified cationic starch-based flocculant comprises the following steps:

[0038] (1) Preparation of dodecenylsuccinic anhydride modified starch DCS

[0039] First, 2.0 g of NaOH was dissolved in 30 mL of anhydrous ethanol and evenly sprayed onto 100.0 g of corn starch (CS) with continuous stirring for alkalization. Then, 5.0 g of dodecenylsuccinic anhydride (DDSA) was dissolved in 10.0 mL of anhydrous ethanol and slowly added dropwise to the alkalized starch with continuous stirring. After mixing evenly, the mixture was reacted at 90°C for 70 min. Finally, the mixture was washed with anhydrous ethanol and dried in a vacuum at 45°C to obtain the product.

[0040] (2) Preparation of salt-tolerant dodecenylsuccinic anhydride-modified cationic starch-based flocculant

[0041] First, 0.4 g of NaOH was dissolved in 30 mL of anhydrous ethanol, and the mixture was evenly sprayed into 100.0 g of DCS obtained in step (1) and stirred continuously for alkalization. Then, 18.0 g of epoxypropyltrimethylammonium chloride was added to the alkalized DCS and mixed evenly. The mixture was reacted at 70°C for 5 h. Finally, the mixture was washed and purified with anhydrous ethanol until no chloride ions were detected in the washing solution. The mixture was then vacuum dried at 45°C to obtain the product.

[0042] Example 3:

[0043] A method for preparing a salt-tolerant dodecenylsuccinic anhydride-modified cationic starch-based flocculant comprises the following steps:

[0044] (1) Preparation of dodecenylsuccinic anhydride modified starch DCS

[0045] First, 4.0 g of NaOH was dissolved in 30 mL of anhydrous ethanol and evenly sprayed onto 100.0 g of corn starch (CS) with continuous stirring for alkalization. Then, 7.0 g of dodecenylsuccinic anhydride (DDSA) was dissolved in 10.0 mL of anhydrous ethanol and slowly added dropwise to the alkalized starch with continuous stirring. After mixing evenly, the mixture was reacted at 90°C for 90 min. Finally, the mixture was washed with anhydrous ethanol and dried in a vacuum at 45°C to obtain the product.

[0046] (2) Preparation of salt-tolerant dodecenylsuccinic anhydride-modified cationic starch-based flocculant

[0047] First, 0.6 g of NaOH was dissolved in 30 mL of anhydrous ethanol, and the mixture was evenly sprayed into 100.0 g of DCS obtained in step (1) and stirred continuously for alkalization. Then, 22.0 g of epoxypropyltrimethylammonium chloride was added to the alkalized DCS and mixed evenly, and the mixture was reacted at 70°C for 7 h. Finally, the mixture was washed and purified with anhydrous ethanol until no chloride ions were detected in the washing solution, and vacuum dried at 45°C to obtain the product.

[0048] Comparative Example 1:

[0049] A method for preparing a modified cationic starch-based flocculant comprises the following steps:

[0050] First, 0.5 g of NaOH was dissolved in 30 mL of anhydrous ethanol and evenly sprayed into corn starch (CS) with continuous stirring for alkalization. Then, 20.0 g of epoxypropyltrimethylammonium chloride was added to the alkalized corn starch and mixed evenly. The mixture was reacted at 70°C for 6 h. Finally, the modified cationic starch-based flocculant CCS was prepared by washing with anhydrous ethanol until no chloride ions were detected in the washing solution. The solution was then vacuum-dried at 45°C.

[0051] Test Example 1: Characterization of Physicochemical Properties of Flocculants

[0052] 1. Characterization methods

[0053] (1) SEM characterization: The DCS and CDCS prepared in Example 1, the CCS prepared in Comparative Example 1, and the raw material CS samples were fixed on plastic films, respectively, and gold was sprayed on their surfaces. SEM tests were performed using a scanning electron microscope (S-4800, Hitachi, Japan) with a voltage of 5 kV and a magnification of 1000-2000 times.

[0054] (2) XRD characterization: The XRD spectra of the DCS and CDCS prepared in Example 1, the CCS prepared in Comparative Example 1, and the raw material CS sample were measured using an X-ray diffractometer (DX-2700, Haoyuan, China) in the diffraction angle range of 10°-70°, and the crystallinity was calculated using Origin 2021 software.

[0055] (3) 1 H-NMR characterization: DCS and CDCS prepared in Example 1, CCS prepared in Comparative Example 1, and raw material CS samples were dissolved in D2O solvent, transferred to NMR sample tubes, and tested using an NMR spectrometer (DRX-500, Bruker, Germany). 1 H-NMR. The data were solvent corrected using MestReNova 15.0 software, and the chemical shifts were interpreted based on the chemical environment of the hydrogen atoms in the reactants, taking into account inductive effects, conjugated effects, and local paramagnetic effects.

[0056] (4) Determination of cationic substitution degree (DSC): The total nitrogen content in the DCS and CDCS prepared in Example 1, the CCS prepared in Comparative Example 1, and the raw material CS samples was determined by Kjeldahl nitrogen determination, and DSC calculation was performed according to formula (1).

[0057] Formula (1);

[0058] Wherein, 162 is the molar mass of anhydroglucose residue (g / mol); ω is the difference in total nitrogen content of the sample before and after cationization modification (%); and 151.6 is the molar mass of GTA (g / mol).

[0059] (5) Determination of degree of anhydride substitution (DSA): Disperse 1.5 g of the DCS or CDCS sample prepared in Example 1 in 20 mL of ethanol, add 15 mL of a 2.5 mol / L hydrochloric acid isopropanol solution, and stir for 30 min. Filter the solution with filter paper and rinse with ethanol until no chloride ions are detected in the filtrate. Finally, completely dissolve the sample on the filter paper in 100 mL of distilled water and titrate with a 0.1 mol / L NaOH solution. Use phenolphthalein as an indicator and record the volume of NaOH consumed. Calculate DSA according to formula (2).

[0060] Formula (2);

[0061] Wherein, 162 is the molar mass of anhydroglucose residue (g / mol); V is the titration volume of NaOH solution (mL); M is the molar concentration of NaOH solution (mol / L); 268 is the molar mass of DDSA (g / mol); and W is the dry weight of the sample to be tested (g).

[0062] (6) Contact angle measurement: DCS and CDCS prepared in Example 1, CCS prepared in Comparative Example 1, and the raw material CS sample were pressed into thin sheets on a tablet press. The thin sheets were then placed on a glass slide, and the contact angle of the droplet was measured on a contact angle tester (SDC-350, Dongguan Shengding, China).

[0063] (7) Zeta potential characterization: Sample solutions of 0.1 g / L were prepared at pH 4, 7, and 10, respectively, and added to the omega electrode of a zeta potential meter (Litesizer 500, Anton Paar, Austria). The ambient temperature was set at 25 °C and the voltage was set at 20 V.

[0064] 2. Experimental Results

[0065] Figure 1 Figure (A) shows the microscopic morphology of CS, DCS, CCS, and CDCS. CS particles are spherical or elliptical, with a smooth surface and regular arrangement. DCS particles maintain an elliptical shape but a relatively rough surface. CCS and CDCS particles have uneven surfaces with sharp corners, a certain degree of adhesion between particles, and disordered arrangement. Compared with CS and DCS, the morphology of CCS and CDCS has changed significantly, indicating that the etherification reaction and esterification reaction have changed the microscopic morphology of starch granules to a certain extent. Figure 1 As shown in Figure (B), CS and the modified starches CCS, DCS, and CDCS all exhibit typical A-type diffraction peaks, with strong peaks at 15°, 17°, 18°, and 23°, respectively. This indicates that modification with GTA and DDSA does not alter the starch crystal structure. Calculated crystallinity values ​​for CS, CCS, DCS, and CDCS are 34.40%, 26.30%, 31.70%, and 28.50%, respectively. This indicates that etherification and esterification reactions primarily occur in the amorphous regions of the starch, resulting in a decrease in starch crystallinity.

[0066] Figure 2 The middle (A) figure shows 1 The H-NMR results showed that the chemical shifts of 3.66 ppm, 3.85 ppm and 3.98 ppm were attributed to the H of CH2- in the starch skeleton, 5.39 ppm was attributed to the H of the hydroxyl group in starch, and 4.79 ppm was attributed to D2O. The chemical shift of 3.24 ppm was attributed to the +The three directly connected methyl H groups, with a chemical shift of 3.33 ppm, are attributed to the H groups of CH2 attached to the epoxy group in GTA. Both shifts appear in CCS and CDCS. The chemical shift of 1.20 ppm is attributed to the methyl H groups at the end of the long carbon chain of DDSA, and the chemical shift of 1.92 ppm is attributed to the methylene H groups bound to -CH=CH- in the carbon chain of DDSA. Both of these cases appear in DCS and CDCS. Taken together, these results indicate that CCS, DCS, and CDCS were all successfully prepared.

[0067] Measurements and calculations revealed that the cationic substitution of CCS and CDCS was 14.1% and 11.8%, respectively, while the anhydride substitution of DCS and CDCS was 1.44% and 1.28%, respectively. Since CDCS has a greater molar mass than CCS and DCS of the same mass, the cationic and anhydride substitution of CDCS are slightly lower than those of the same mass. The substitution results also indicate that CCS, DCS, and CDCS were successfully prepared.

[0068] The contact angle reflects the hydrophobicity of a substance. Therefore, measuring the contact angle of starch can be used to assess the success of hydrophobic modification. The contact angle of DCS, obtained by modifying CS with DDSA, increased significantly, from 17.78° (CS) to 120.67°. This indicates that the esterification reaction between CS and DDSA introduces the long carbon chains of DDSA, transforming the surface properties from hydrophilic to hydrophobic. The contact angle of CCS, obtained by etherifying CS with GTA, remained almost unchanged (approximately 18°), indicating that GTA modification has little effect on the hydrophilicity and hydrophobicity of starch. In contrast, the contact angle of CDCS, obtained by etherifying DCS with GTA, was 46.208°. While smaller than that of DCS, it was significantly greater than that of CS. This may be due to the slightly lower degree of anhydride substitution of CDCS. Overall, compared with CS and CCS, CDCS exhibits a stronger hydrophobicity, attributed to the long 12 C chains of DDSA. When CDCS is dissolved in water, due to the different polarities of hydrophobic groups and water molecules, the hydrophobic microregions formed by the mutual aggregation of hydrophobic groups can promote intramolecular association within the CDCS molecular chain. Therefore, CDCS has the characteristics of a hydrophobic associating polymer.

[0069] like Figure 2As shown in FIG. B, the zeta potentials of CS, CCS, DCS and CDCS were -0.7 ~ -11.8 mV, +35.1 ~ 23.6 mV, -1.2 ~ -15.6 mV and +33.2 ~ +18.4 mV, respectively, in the pH range of 4 to 10. Due to the influence of the charge state of the particle surface on the pH of the solution, the zeta potential values of the four showed the rule of acidic conditions greater than neutral conditions greater than alkaline conditions. CS and DCS showed negative charge properties, while CCS and CDCS were positively charged; and the zeta potential values of DCS and CDCS were slightly lower than those of CS and CCS, respectively. The reasons are: (1) natural starch is negatively charged, and CCS and CDCS introduce positively charged quaternary ammonium salt groups after etherification with GTA, thus showing positive charge characteristics; (2) DCS and CDCS introduce negatively charged carboxyl groups after esterification modification with DDSA, so the negative charge of DCS is enhanced compared to CS, and the positive charge of CDCS is weakened compared to CCS. In summary, the CDCS molecule contains groups that can be ionized into positive and negative charges, and can obtain salt-resistant properties through the counterion behavior of amphoteric polyelectrolytes.

[0070] Test Example 2: Flocculation performance of microalgae

[0071] I. Experimental method

[0072] The experiment of flocculation recovery of microalgae was carried out at room temperature using a six-coagulation test machine (ZR4-6, China Shenzhen Zhongrun Company). 200 mL of microalgae suspension was added to a 250 mL beaker, stirred at a speed of 200 rpm for 1 minute, then a predetermined dose of flocculant was added and stirred for 2 minutes to mix thoroughly. Then, the stirring speed was reduced to 50 rpm for 15 minutes to make the microalgae flocculate into flocs. Finally, stop stirring, after 15 minutes of sedimentation, the liquid at the liquid surface of 2 centimeters was sucked, and the absorbance at 685 nm wavelength was measured by ultraviolet spectrophotometer (UV-7504C, China Shanghai Xinmao). The flocculation performance was characterized by the harvest efficiency (HE), and the calculation formula was: HE (%) = (A1-A2) / A1x100%. Wherein, A1 and A2 are the OD 685 values of microalgae samples before and after flocculation, respectively.

[0073] The microalgae sample was purchased from Nanjing Qiheng Aquaculture Co., Ltd. (Jiangsu, China), and the culture conditions were natural temperature, natural light and natural ventilation. BG-11 medium was used to culture in an open runway water tank, and the culture period was 10-15 days to the optical density (OD 685) reached about 1.50. The basic characteristics of the microalgae sample are as follows: salinity of 3.00±0.02ppt, pH of 7.25±0.02, and dry weight of microalgae biomass of 0.91±0.04 g / L. Adding NaCl to the above microalgae samples can obtain microalgae samples with different salinities while ensuring pH and OD 685 and the dry weight of microalgae biomass remained unchanged.

[0074] 2. Experimental Results

[0075] Figure 3 The effects of CCS and CDCS dosage on the HE of microalgae samples at different salinities were shown. The results showed that at a natural salinity of 3 ppt, as the flocculant dosage increased, the adsorption rates of CCS and CDCS on microalgae initially increased and then stabilized, reaching over 90%. The minimum flocculant dosage required to achieve 90% HE was defined as the optimal dosage. At a salinity of 3 ppt, the optimal dosages for CCS and CDCS were 10 mg / L and 3 mg / L, respectively. Figure 3 Figure (A). However, when the salinity of the microalgae sample increased to 100 ppt and 15 ppt, despite a CCS dosage of up to 100 mg / L, the harvest efficiency only reached 57.9% and 44.2%, respectively, indicating low harvest efficiency. The CDCS prepared in this application achieved a harvest efficiency exceeding 90% at optimal dosages of only 10 mg / L and 25 mg / L at these two salinities, demonstrating that the flocculant prepared in this invention can significantly improve salt tolerance and flocculation performance, thereby saving flocculant dosage. When the salinity of the microalgae sample was further increased to 20 ppt, the harvest efficiency remained unsatisfactory, less than 40%, even at a CCS dosage of 200 mg / L. Furthermore, increasing the dosage did not significantly improve the harvest efficiency. This is primarily due to the shielding of the positive charge of CCS in a high-salinity environment, preventing it from effectively neutralizing the charge. However, the CDCS prepared in this invention still exhibited good flocculation efficiency at high salinity, achieving a HE of over 80% at 100 mg / L, and the HE continued to increase with increasing dosage.

[0076] Figure 4The settling effect of the flocculation of CCS and CDCS at the optimal or maximum dosage is shown. When the salinity is 3 ppt, both CCS and CDCS can effectively settle the microalgae cells to the bottom of the beaker, and the supernatant is clear and transparent. When the salinity is 10 ppt, it is observed that the liquid on the upper quarter of the surface after flocculation by CCS is light green and relatively clear, indicating that the microalgae flocculation body has been partially settled. When the salinity is 15 ppt and 20 ppt, there are still a large number of microalgae cells in the supernatant after flocculation by CCS. However, the supernatant treated by the CDCS prepared by the present application is relatively clear, but the transparency of the supernatant decreases with the increase of the salinity. The HE of CDCS decreases at a higher salinity. One reason is that the high salinity increases the density of the microalgae culture medium, and the microalgae flocculation is suspended in the culture medium, which is difficult to settle by gravity. Another reason is that the high ionic strength of the high salinity solution also shields the cationic charge of the CDCS to some extent, which is not conducive to charge neutralization. However, at the same time, the high polarity of the solution can enhance the hydrophobic interaction between the CDCS molecules, thereby offsetting the negative effects of charge shielding and maintaining the flocculation ability at high salinity.

[0077] Test Example 3: Flocculation mechanism

[0078] I. Experimental method

[0079] (1) Salt tolerance analysis of flocculant

[0080] In order to study the salt tolerance of the flocculant, the present application determines the hydrodynamic diameter, viscosity and zeta potential of the DCS, CDCS prepared in Example 1, CCS prepared in Comparative Example 1 and raw material CS sample at the flocculation salinity (3 ppt, 10 ppt, 15 ppt and 20 ppt). Prepare 0.1 g / L flocculation sample solution of different salinity, and use the nanoparticle size zeta potential instrument (Litesizer500, Anton-Paar Company, Austria) to determine the hydrodynamic diameter. Prepare 2.5 g / L sample solution of different salinity, and use a rotary viscometer (NDJ-5S, Rion, China) to determine the viscosity. Set the rotation speed to 60 r / min, select the appropriate rotor to ensure the included angle between 15% and 90%, and record the data. Prepare 0.1 g / L natural pH sample solution of different salinity, and use the palladium electrode of the zeta potential instrument to determine the zeta potential value.

[0081] (2) Zeta charge analysis of flocculation system

[0082] The microalgae samples before and after flocculation are filtered with a 0.45 μm filter membrane, and the filtrate is diluted 20 times and then added to the Omega electrode. The zeta potential of the flocculation system is determined by the zeta potential analyzer.

[0083] (3) Analysis of microalgae flocculation morphology

[0084] The initial microalgae suspension and the flocculated microalgae flocs were aspirated and dropped onto a glass slide to prepare specimen slides. The morphology of the microalgae (microalgae cells or microalgae flocs) was then observed using a microscope (XSP-63B, Shanghai Optics, China) at a magnification of 10 × 20 times.

[0085] (4) Real-time particle size analysis of microalgae flocs

[0086] In order to study the process of floc formation, fragmentation and reflocculation, the present invention monitors the floc particle size in real time. First, the microalgae cells coagulate to form large flocs after adding flocculants, which is the growth stage. When the flocs grow to the maximum particle size and remain stable, they enter the initial stabilization stage. After the initial stabilization stage, the stirring speed is increased to 350 rpm for 5 minutes, so that the flocs are broken under the shear force generated by high-speed stirring. This is the fragmentation stage. Subsequently, the stirring speed is reduced to 50 rpm for 15 minutes to allow the broken flocs to re-flocculate. This is the reflocculation stage. When the flocs grow to the maximum particle size again and remain stable, they enter the final stabilization stage. The floc particle size distribution is monitored in real time using a laser particle size analyzer (Betterize 2000, Dandong Better, China), and D is recorded every 30 seconds. 50 The floc strength factor (SF) and recovery factor (RF) are calculated as follows: SF (%) = (d2 / d1) × 100%; RF (%) = (d3-d2) / (d1-d2) × 100%. Where d1, d2, and d3 are the average particle sizes (μm) of the flocs at the initial stabilization stage, the breakup stage, and the final stabilization stage, respectively.

[0087] 2. Experimental Results

[0088] (1) Salt tolerance of flocculants

[0089] The experimental results are as follows Figure 5 As shown. Figure 5 Figure (A) shows that the hydrodynamic diameters of CCS, DCS, and CDCS are similar in low-salinity water, with CDCS slightly larger than both CCS and DCS. All three are smaller than CS. This is primarily due to a certain degree of starch degradation during the etherification and esterification reactions. The hydrodynamic diameters of CS and CCS are significantly affected by salinity, decreasing from 18.83 μm and 8.40 μm at 3 ppt salinity to 0.95 μm and 0.98 μm at 20 ppt salinity, respectively. In comparison, DCS and CDCS are less affected by salinity, decreasing from 8.5 μm and 9.33 μm at 3 ppt salinity to 4.05 μm and 4.60 μm, respectively. This is primarily due to the incorporation of long hydrophobic 12 carbon chains in DCS and CDCS. In a highly polar, high-salinity environment, this enhances intramolecular hydrophobic association, maintaining a relatively stretched molecule.

[0090] like Figure 5 As shown in Figure (B), when salinity increases from 3 ppt to 20 ppt, the viscosities of CS, CCS, DCS, and CDCS all decrease: from 2.1 MPa·s to 1.7 MPa·s, from 8.25 MPa·s to 1.6 MPa·s, from 2.4 MPa·s to 2.0 MPa·s, and from 2.5 MPa·s to 1.8 MPa·s, respectively. The viscosity of CCS is extremely sensitive to changes in salinity. Due to the curling of CCS molecular chains and the weakening of intermolecular crosslinking, the apparent viscosity decreases, which is corroborated by the changes in hydrodynamic diameter. In contrast, DCS and CDCS exhibit relatively constant viscosity and strong salt tolerance due to the enhanced hydrophobic association and increased physical crosslinking points between molecules.

[0091] like Figure 5 As shown in Figure (C), high salt environments generally shield charged particles (both positive and negative) in solution: the zeta potential of negatively charged CS and DCS decreases from 4.3 mV and 5.4 mV at 3 ppt to 1.6 mV and 2.3 mV at 20 ppt; while the zeta potential of positively charged CCS and CDCS decreases from 29.6 mV and 28.1 mV at 3 ppt to 2.3 mV and 12.8 mV at 20 ppt. These experimental results indicate that DCS and CDCS modified with DDSA exhibit enhanced salt tolerance. This is primarily due to the fact that in high salt environments, the hydrophobic groups in DCS and CDCS result in larger particle diameters, more stretched molecular chains, and more exposed charge sites.

[0092] (2) Changes in zeta potential of the flocculation system

[0093] The variation trend of zeta potential in the supernatant after flocculation with flocculant dosage is basically consistent with that of HE, and both increase with the increase of flocculant dosage ( Figure 6 At salinities of 3, 10, 15, and 20 ppt, the zeta potentials of the flocculation systems were -17.4, -20.4, -29.7, and -31.5 mV, respectively, when the CCS dosages were 10, 100, 100, and 200 mg / L. The zeta potentials of the flocculation systems were -30.2, -30.4, -27.1, and -34.5 mV, respectively, when the CDCS dosages were 3, 10, 25, and 100 mg / L.

[0094] At 3 ppt salinity, which can achieve good harvest effect, the zeta potential of CCS and CDCS at the best dose is negative, indicating that both can play a bridging and adsorption role in flocculating microalgae cells, making the system charge reach the ideal harvest rate before reaching the unstable interval (-10 mV~+10 mV). Combined with the harvest rate of microalgae at the corresponding salinity and dose, it can be found that for CCS, the zeta potential increases significantly with the increase of the dose at 3 ppt salinity, indicating that the charge neutralization effect is good. At 10, 15 and 20 ppt salinity, even if the dosage of flocculant is large, the zeta potential of the system still does not increase significantly, indicating that the charge shielding effect is strong at this time, and it is difficult to play a strong charge neutralization effect.

[0095] For the CDCS prepared by the application, the zeta potential value at the best dose under the four salinity gradients is significantly lower than that of CCS at the best dose, indicating that CDCS has the effect of charge neutralization and bridging and adsorption mechanism. The possible ways are the hydrophobic force of CDCS, the interaction with algal EOM, etc., which makes CDCS have stronger affinity to algal cells and the formed flocs are more hydrophobic, so as to better separate from water.

[0096] (3) Microscopic morphology of flocs

[0097] Figure 7 The optical microscope morphology of microalgae cells and flocs formed under the best / most dose is shown. After adjusting the salinity of the microalgae sample, the algal cells in the microalgae sample without adding flocculant are regular spherical, and the cells are green due to the enrichment of chlorophyll. The distance between the cells is relatively uniform, and part of the cells have signs of movement during observation, indicating that the microalgae cells can be uniformly suspended in the culture medium, and the life activity is strong. Figure 7 As shown in the middle (A) of the figure, under the natural salinity (3 ppt), the microalgae cells flocculated by CCS and CDCS gather together to form larger flocs. The difference is that the flocs formed by CCS have clear boundaries, and the formation process of the flocs is that a few microalgae cells are first aggregated together to form a floc core under the action of the flocculant, and then the floc core attracts surrounding algal cells to grow and form large flocs under the action of charge neutralization and bridging and adsorption. The flocs formed by CDCS are often connected by loose small flocs to form a three-dimensional network structure, and CDCS can adsorb microalgae fines or small flocs through the network trapping and sweeping effect, so as to promote the sedimentation of the flocs. At higher salinity, CCS can only form smaller flocs, which are difficult to separate from water due to the larger buoyancy effect during sedimentation, and show poor harvest rate. CDCS can still form larger three-dimensional network flocs to promote sedimentation.

[0098] (4) Dynamic particle size of flocs

[0099] In order to compare the size of flocs formed by the two flocculants at different salinities, as well as the shear resistance and reflocculation ability of the flocs, the particle size of the flocs was monitored in real time during slow stirring, fast stirring and resuming slow stirring. The results are as follows: Figure 8 shown.

[0100] At natural salinity (3 ppt), both 10 mg / L CCS and 3 mg / L CDCS achieved greater than 90% HE. At these doses, the average particle size of flocs formed by CDCS (389.8 μm) was slightly larger than that of CCS (372.3 μm). Under high external shear forces, CDCS-formed flocs maintained a relatively large size, with an average particle size of 249.1 μm, while the particle size of flocs formed by CCS decreased by more than half (160.3 μm). Upon resuming slow stirring, the broken flocs reaggregated and recovered into larger flocs. The reaggregated flocs by CDCS reached an average particle size of 249.1 μm, while those formed by CCS only recovered to 198.2 μm. Calculations showed that the SF and RF of flocs formed by CDCS were 63.0% and 64.2%, respectively, which were significantly greater than those formed by CCS (43.1% and 17.9%). This indicates that the flocs formed by CDCS have stronger shear resistance and recovery ability, and their flocculation performance is more stable in practical applications.

[0101] As the salinity of the microalgae increased, the size of the flocs formed by both CCS and CDCS decreased. When the salinity increased from 10 ppt to 20 ppt, the size of the flocs formed by CCS decreased from approximately 90 μm to 20 μm; that of CDCS decreased from approximately 330 μm to 290 μm. The average particle size of the flocs during the initial stabilization phase was generally consistent with the floc morphology observed under a microscope. After fragmentation and reflocculation, the average particle size of the flocs formed by CDCS also reached approximately 200 μm. At 10 ppt, 15 ppt, and 20 ppt, the SF of the flocs formed by CDCS were 50.1%, 49.5%, and 50.6, respectively; and the RF were 57.7%, 49.3%, and 31.8%, respectively. Furthermore, after floc fragmentation, reflocculation, and sedimentation, the final HEs of CDCS at salinities of 3 ppt, 10 ppt, 15 ppt, and 20 ppt were measured to be 72.0%, 63.6%, 52.8%, and 42.2%, respectively. These values ​​were significantly higher than those achieved by CCS (45.9%, 29.6%, 16.4%, and 12.4%). This indicates that CDCS exhibits strong salt tolerance and resistance in high-salinity environments, allowing the formed flocs to maintain a relatively large size, facilitating their sedimentation in high-density culture media and achieving a relatively ideal HE.

[0102] The above description is only the preferred embodiment of the present application, and is not intended to limit the present application. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. Application of a salt-tolerant cationic starch-based flocculant in flocculating and harvesting microalgae, characterized in that: The salt-tolerant cationic starch-based flocculant is prepared by the following method: (1) First, the alkali solution and starch are mixed and stirred, then the modifier solution is added, mixed and reacted, and finally washed and dried to obtain modified starch; (2) firstly mixing the alkaline solution with the modified starch obtained in step (1) and stirring, then adding epoxypropyltrimethylammonium chloride, mixing and reacting, and finally washing and drying to obtain; In step (1), the modifier is dodecenylsuccinic anhydride, the concentration of the modifier solution is 0.3-1 g / mL, and the solvent is ethanol; the volume mass ratio of the modifier solution to the starch is 8-12 mL: 80-120 g; In step (2), the alkaline solution is an ethanol solution of NaOH with a concentration of 0.01-0.02 g / mL; the volume mass ratio of the alkaline solution, modified starch and epoxypropyltrimethylammonium chloride is 20-40 mL:80-120 g:10-30 g.

2. The use of the salt-tolerant cationic starch-based flocculant according to claim 1 in flocculating and harvesting microalgae, characterized in that: In step (1), the alkaline solution is an ethanol solution of NaOH with a concentration of 0.05-0.2 g / mL; the mass volume ratio of starch to alkaline solution is 80-120 g:20-40 mL.

3. The use of the salt-tolerant cationic starch-based flocculant according to claim 1 in flocculating and harvesting microalgae, characterized in that: In the step (1), the temperature of the mixing reaction is 80-100°C and the time is 50-100 min; the drying temperature is 40-50°C.

4. The use of the salt-tolerant cationic starch-based flocculant according to claim 1 in flocculating and harvesting microalgae, characterized in that: The temperature of the mixing reaction in step (2) is 60-80°C and the time is 4-8 hours; the drying temperature is 40-50°C.

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