Preparation method of composite flocculant for deep phosphorus removal of low turbidity sewage

Abstract

The present application relates to a kind of preparation method of composite flocculant for low turbidity sewage advanced phosphorus removal, belong to water treatment technical field.Preparation operation steps are as follows: (1) in the residual sludge of sewage treatment plant, add alkali, firing, and prepare biochar;(2) prepare modified biochar with acid washing oscillation;(3) constant temperature oscillation in lanthanum chloride solution is prepared to the biochar of fixed lanthanum hydroxide;(4) the biochar of fixed lanthanum hydroxide and polyaluminum chloride flocculant solution are heated and stirred to react, dry, and obtain the composite flocculant of lanthanum hydroxide fixed biochar modification.The composite flocculant is black powder, and specific surface area is 50~200m 2 / g;Pore type structure is mesoporous, and the total pore volume of the pore of p / p° single point adsorption width less than 187.216nm is 0.990:0.216cm³ / g.The composite flocculant of the present application can be suitable for low turbidity water advanced phosphorus removal treatment.The preparation method of the present application is simple in operation and low in cost.

Description

Technical Field

[0001] This invention relates to the field of water treatment technology, specifically to a method for preparing a composite flocculant for deep phosphorus removal from low-turbidity wastewater. Background Technology

[0002] Phosphorus emissions in nature include agricultural, industrial, and urban emissions. Agriculture is one of the main sources of phosphorus pollution, with excessive fertilizer use and livestock waste directly leading to phosphorus entering water bodies. Industrial and urban emissions are discharged into water bodies through wastewater treatment plants, resulting in large amounts of phosphorus entering aquatic environments. Although nature has a certain capacity to absorb phosphorus, with algae and other plants in aquatic bodies absorbing phosphorus as nutrients, the total amount of phosphorus emitted exceeds the environment's absorption capacity. This leads to eutrophication over time, causing algal blooms, excessive algal growth, and other problems, severely impacting aquatic ecosystems. Effective removal and control of phosphorus levels in water bodies has become paramount; therefore, extreme phosphorus removal is of great significance and necessity for preventing eutrophication, maintaining ecological balance, and improving water quality. Wastewater treatment plants are the main sites for centralized treatment of organic matter, nitrogen, phosphorus, and other pollutants in wastewater. Therefore, phosphorus removal control in wastewater treatment plant effluent is a crucial control point in phosphorus pollution control. If the centralized phosphorus removal effect of wastewater treatment plants is unsatisfactory, and the total phosphorus content in the effluent exceeds the standard, the receiving water body will be affected.

[0003] There is a risk of inducing eutrophication in water bodies. Therefore, wastewater discharge standards in many parts of my country have imposed stricter requirements on phosphorus emissions. Increasingly stringent discharge standards have prompted urban wastewater treatment plants in my country to conduct advanced treatment of low-concentration phosphorus-containing wastewater.

[0004] However, phosphorus in low-turbidity wastewater mainly exists in the form of dissolved phosphates. Dissolved phosphorus can be further divided into soluble PO42-. 3− Soluble polyphosphates and soluble organic phosphates (SOPs) are pollutants. Conventional flocculants are ineffective at removing dissolved phosphates. This means that even with increased flocculant dosage, raising treatment costs and potentially causing secondary pollution, deep phosphorus removal cannot be achieved. Furthermore, conventional flocculants produce fine flocs when treating low-turbidity wastewater, making sedimentation an ineffective method for separating sludge from water. Therefore, a novel flocculant is needed that possesses high phosphorus adsorption capacity, is widely available, simple to prepare, exhibits stable performance, and has significant potential for widespread application. Summary of the Invention

[0005] To address the practical problem of difficulty in removing low-concentration phosphorus from low-turbidity wastewater, this invention provides a method for preparing a composite flocculant for deep phosphorus removal from low-turbidity water.

[0006] The preparation steps of a composite flocculant for extreme phosphorus removal from low-turbidity wastewater are as follows:

[0007] (1) Preparation of biochar

[0008] (1.1) Add alkaline substances to the sludge and stir evenly. The mass ratio of the sludge to the alkaline substances is 1.0:0.2 to 0.5 to obtain alkaline sludge with a pH value of 9.0 to 12.0. Dry it at 75°C or air dry it naturally and crush it into sludge particles.

[0009] (1.2) Sludge particles are burned in a tubular furnace under aeration conditions to produce biochar; the gas used is nitrogen or other inert gas.

[0010] (2) Preparation of modified biochar

[0011] (2.1) Wash the biochar with deionized water 3 to 4 times, and dry it under vacuum to obtain washed biochar; grind it to obtain ground biochar.

[0012] (2.2) Add hydrochloric acid solution with a concentration of 0.5 mol / L to 2.5 mol / L to the ground biochar, acid wash and shake, rinse with deionized water, and vacuum dry to obtain modified biochar;

[0013] The mass-to-volume ratio of the above-mentioned ground biochar to hydrochloric acid solution is 1g-2g:10ml;

[0014] (3) Preparation of biochar with immobilized lanthanum hydroxide

[0015] Lanthanum chloride heptahydrate (LaCl3) . Lanthanum chloride (LC4) was dissolved in a hydrochloric acid solution with a concentration of 0.5 mol / L to 5.0 mol / L to obtain a lanthanum chloride solution with a concentration of 5 g / L to 50 g / L. The modified biochar was added to the lanthanum chloride solution with a concentration of 5 g / L to 50 g / L, and the mixture was shaken at a constant temperature. The supernatant was removed, and the mixture was washed three times with deionized water. The mixture was then centrifuged and dried under vacuum to obtain biochar with fixed lanthanum hydroxide.

[0016] The mass-to-volume ratio of modified biochar to lanthanum chloride solution was 0.5–0.8 g: 10 mL;

[0017] (4) Preparation of composite flocculant

[0018] (4.1) Dissolve polyaluminum chloride flocculant in water at a mass ratio of 0.1 to 1.0:10 to obtain polyaluminum chloride flocculant solution;

[0019] (4.2) Add biochar with fixed lanthanum hydroxide to polyaluminum chloride flocculant solution at a mass-volume ratio of 0.5g to 0.8g: 10mL, heat and stir, and dry to obtain composite flocculant modified with lanthanum hydroxide fixed biochar;

[0020] The composite flocculant is a black powder with a specific surface area of ​​50–200 m².2 / g;

[0021] The pore structure is mesoporous. The total pore volume of pores with a single-point adsorption width less than 187.216 nm is 0.990:0.216 cm³ / g; the cumulative pore volume of pores with an adsorption width between 1.7 nm and 300 nm is 0.116-0.336 cm³ / g; the cumulative pore volume of pores with a desorption width between 1.7 nm and 300 nm is 0.125-0.375 cm³ / g; the average pore size is 5 nm to 15 nm; and the iodine adsorption value is 300-500 mg / g.

[0022] The chemical composition of composite flocculants mainly consists of bio-carbon and lanthanum hydroxide, and also includes elements such as oxygen, hydrogen, nitrogen, sulfur, and iron. The content of these elements affects their adsorption performance and chemical reactions.

[0023] The further defined technical solution is as follows:

[0024] In step (1.1), the sludge is the residual sludge from the sewage treatment plant, and the moisture content of the residual sludge is 78-82%.

[0025] In step (1.1), the alkaline substance is sodium hydroxide or urea.

[0026] In step (1.2), the firing conditions are: temperature 600℃ and time 3.0 hours.

[0027] In step (2.1), the product is vacuum dried for 6-7 hours at a temperature of 60-80℃ in a vacuum drying oven; and then ground through a 100-mesh sieve.

[0028] In step (2.2), the pickling and oscillation conditions are: temperature 25-35℃, rotation speed 80-180r / min, and time 12-24h.

[0029] In step (2.2), the vacuum drying is carried out in a vacuum drying oven at a temperature of 60-80℃ for 8-12 hours.

[0030] In step (3), the isothermal oscillation conditions are: temperature 25-35℃, rotation speed 80-180r / min, and time 12-24h.

[0031] In step (3), the centrifugation conditions are: rotation speed 6000-10000 r / min and time 5-10 min.

[0032] In step (4.2), the heating and stirring conditions are: temperature 60-80℃, speed 80-120r / min, and time 12-24h.

[0033] The operation of using the composite flocculant of this invention for deep phosphorus removal in low-turbidity water is as follows:

[0034] (1) Dissolve the composite flocculant and water in a dissolving tank or dissolving vessel to obtain a composite flocculant mixture; use a metering pump to inject the composite flocculant mixture into the dosing port of a tubular mixer or directly into the flocculation zone of the air flotation device; the dosage of the composite flocculant is 20-200 mg / L.

[0035] The air flotation device is a dissolved air flotation device or a micro-nano air flotation device.

[0036] The dissolved air flotation device has a dissolved air pressure of 0.2–0.5 MPa and a hydraulic retention time of 15–30 minutes.

[0037] The dissolved gas capacity of the micro-nano bubble generator is 100-300 L / h, and the hydraulic residence time is 15-30 minutes.

[0038] (2) In the tubular mixer, the composite flocculant and the treated low-turbidity wastewater are thoroughly mixed. In the coagulation zone of the flotation device, the composite flocculant and the treated low-turbidity wastewater undergo a flocculation reaction, producing many flocs. In the flotation zone, the flocs fix phosphorus in the wastewater onto the flocs through various mechanisms such as coagulation, adsorption, ion exchange, and chemical reaction, and then remove it with dissolved air bubbles or micro-nano bubbles, thereby achieving deep removal of phosphorus from the water. The total phosphorus concentration in the effluent of low-turbidity wastewater treated by dissolved air flotation or micro-nano flotation can be controlled at 0.10 mg / L (dissolved air flotation) or 0.05 mg / L (micro-nano flotation), which is far lower than the national standards for total phosphorus concentration in effluent.

[0039] Compared with the prior art, the beneficial technical effects of the present invention are reflected in the following aspects:

[0040] 1. The composite flocculant prepared in this invention is a lanthanum-based composite flocculant. The mesoporous structure of the lanthanum-based composite flocculant provides a large specific surface area and high adsorption capacity for flocs. Its main active substance for phosphorus removal is lanthanum hydroxide (La(OH)3). Compared with other substances, lanthanum hydroxide exhibits stronger phosphorus adsorption. Therefore, even at low total phosphorus concentrations, it remains unaffected by interference from other elements, maintaining high selectivity for phosphorus.

[0041] The deep phosphorus removal process of the composite flocculant of this invention is achieved through the interaction of multiple mechanisms, including flocculation, coagulation, chemical reaction, adsorption, ion exchange, and air flotation. Polyaluminum chloride (PAC) is a commonly used coagulant with good coagulation performance. When PAC is added to water, it forms aluminum hydroxide flocs. These flocs trap and sweep up particles and suspended solids in the water, and this coagulation promotes the formation of larger precipitates. However, since the effluent from wastewater treatment plants already meets the comprehensive discharge standards, the water quality is relatively good, and the turbidity is low, generally not exceeding 30 mg / L. Low turbidity water is not conducive to conventional coagulants and hinders the coagulation reaction. Therefore, the biochar in the composite flocculant of this invention can increase the turbidity in the wastewater, which is beneficial for the coagulation reaction of PAC in the effluent, forming larger flocs. On the other hand, taking advantage of the alkali resistance of biochar, lanthanum hydroxide (La(OH)3) is loaded onto the biochar. When polyaluminum chloride (PAC) reacts with biochar to form large flocs, the hydroxyl groups (-OH) on the surface of lanthanum hydroxide (La(OH)3) can react with phosphate ions (PO42-). 3- A chemical reaction occurs, thereby fixing phosphorus as a precipitate onto the flocs. The hydroxyl (-OH) functional groups on the surface of lanthanum hydroxide (La(OH)3) also possess ion exchange capabilities, allowing it to react with phosphate ions in water, thus fixing phosphorus onto the flocs on the lanthanum hydroxide particles. Simultaneously, the presence of surface hydroxyl (-OH) radicals increases the surface activity of the lanthanum hydroxide particles, providing stronger selectivity for phosphate ions in water and facilitating their reaction, thus achieving phosphorus fixation even under low turbidity and low phosphorus concentration conditions. Because the flocs formed under low turbidity conditions are fine and fragmented, conventional sedimentation methods cannot achieve mud-water separation. Therefore, the composite flocculant of this invention can be coupled with air flotation; through air flotation stripping, effective separation of fine flocs can be achieved, thereby realizing deep phosphorus removal from low-turbidity wastewater.

[0042] The lanthanum-based composite flocculant and its application method prepared in this invention address the difficulty of deep phosphorus removal from municipal tailwater. It has the advantages of low dosage, fast flocculation speed, high stability, and easy separation. Verification using laboratory dissolved air flotation and micro-nano flotation devices showed that in low-phosphorus, low-turbidity water with a total phosphorus concentration of 0.7 mg / L and a suspended solids (SS) concentration of 50 mg / L, the effluent total phosphorus concentration reached 0.05 mg / L (micro-nano flotation) or 0.10 mg / L (dissolved air flotation). The composite flocculant of this invention is suitable for deep phosphorus removal treatment of low-turbidity water.

[0043] 2. The preparation method of the lanthanum-based composite flocculant of the present invention involves stably loading lanthanum hydroxide onto modified biochar using methods such as isothermal oscillation. The lanthanum hydroxide-fixed biochar is then added to a polyaluminum flocculant solution, stirred until homogeneous, and dried in an oven at 55°C for 3 hours. After cooling to room temperature, a lanthanum hydroxide-fixed biochar-modified composite flocculant is obtained. Compared with many adsorption substrate materials, biochar has advantages such as simple preparation process, wide availability, low price, and strong environmental friendliness. Furthermore, this preparation method combines the high selectivity of metallic lanthanum for phosphorus with the environmental friendliness of biochar, offering advantages such as simple operation, low cost, high removal efficiency, and low effluent concentration. Attached Figure Description

[0044] Figure 1 This is a graph showing the total phosphorus concentration in the effluent from the micro-nano air flotation device.

[0045] Figure 2 This is a graph showing the total phosphorus concentration in the effluent from a dissolved air flotation unit. Detailed Implementation

[0046] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0047] Example 1

[0048] The preparation steps of a composite flocculant for extreme phosphorus removal from low-turbidity wastewater are as follows:

[0049] (1) Preparation of biochar

[0050] (1.1) Take 100g of residual sludge from the sewage treatment plant (with a water content of about 80%), add 20g of sodium hydroxide and stir evenly to obtain alkaline sludge with a pH value of 10.0. Dry it naturally, grind it and pass it through a 100-mesh sieve.

[0051] (1.2) The sludge particles were heated by a tube furnace and burned at high temperature under nitrogen gas. The burning temperature was 600℃ and the burning time was 3 hours to obtain biochar. The biochar was then cooled in an oxygen-free environment for 24 hours and stored at room temperature.

[0052] (2) Preparation of modified biochar

[0053] (2.1) Wash the biochar with deionized water three times, and dry it in a vacuum drying oven at 80°C for 8 hours to obtain washed biochar. Grind it and pass it through a 100-mesh sieve to obtain ground biochar.

[0054] (2.2) 10g of ground biochar was completely immersed in 200mL of 1mol / L hydrochloric acid solution, and then placed in a constant temperature shaking oven at (25℃, 120r / min) for 24h of acid washing and shaking. After rinsing with deionized water, it was vacuum dried in a vacuum drying oven at 80℃ for 8h to obtain modified biochar.

[0055] (3) Preparation of biochar with immobilized lanthanum hydroxide

[0056] 5.0g of lanthanum chloride heptahydrate (LaCl3) . Lanthanum chloride (LaCl3) was dissolved in 1000 mL of 1.0 mol / L hydrochloric acid solution to obtain a 5.0 g / L lanthanum chloride solution. 10 g of modified biochar was added to 200 mL of the 5.0 g / L lanthanum chloride solution and shaken at 25 °C and 120 rpm for 12 h. The supernatant was removed, and the mixture was washed three times with deionized water. The mixture was centrifuged at 6000 rpm for 10 min, the supernatant was removed, and the mixture was vacuum dried at 80 °C for 8 h to obtain biochar with fixed lanthanum hydroxide.

[0057] (4) Preparation of composite flocculant

[0058] (4.1) Dissolve 50g of flocculant in 500mL of deionized water to obtain a polyaluminum chloride flocculant solution with a concentration of 100g / L.

[0059] (4.2) Add 25g of biochar with fixed lanthanum hydroxide to 500mL of polyaluminum chloride flocculant solution with a concentration of 100g / L, and shake for 12h in a constant temperature shaking box at a temperature of 60℃ and a rotation speed of 120r / min.

[0060] Centrifuge at 6000 r / min for 10 min, remove the supernatant, and dry to obtain a composite flocculant of lanthanum hydroxide-fixed biochar.

[0061] The composite flocculant prepared in Example 1 is a black powder with a specific surface area of ​​85.092 m² / g and a mesoporous structure (2 nm to 100 nm). The total pore volume of pores with a single-point adsorption width less than 187.216 nm is 0.990:0.216 cm³ / g. The cumulative pore volume of pores with an adsorption width between 1.7 nm and 300 nm in the 2 nm to 100 nm range is 0.216 cm³ / g. The cumulative pore volume of pores with a desorption width between 1.7 nm and 300 nm in the 2 nm to 100 nm range is 0.225 cm³ / g. The average adsorption pore diameter (4V / A) of the 2 nm to 100 nm mesopores is 10.156 nm. The average adsorption pore size (4V / A) of the mesopores in the 2nm–100nm range is 9.594nm, and the average desorption pore size (4V / A) of the mesopores in the 2nm–100nm range is 8.464nm. The iodine adsorption value is 352.44±13.41mg / g. Its chemical composition mainly consists of bio-carbon and lanthanum hydroxide, and it also contains elements such as oxygen, hydrogen, nitrogen, sulfur, and iron. The content of these elements affects its adsorption performance and chemical reactions.

[0062] Example 2

[0063] The preparation steps of a composite flocculant for extreme phosphorus removal from low-turbidity wastewater are as follows:

[0064] (1) Preparation of biochar

[0065] (1.1) Take 200g of residual sludge from the sewage treatment plant (with a moisture content of about 80%), add 50g of urea, mix evenly to obtain alkaline sludge with a pH value of 10.0, dry it naturally, grind it, and pass it through a 100-mesh sieve.

[0066] (1.2) The sludge particles were heated by a tubular furnace and burned at high temperature under nitrogen gas. The burning temperature was 600℃ and the burning time was 3.0 hours to obtain biochar. The biochar was then cooled in an oxygen-free environment for 24 hours and stored at room temperature.

[0067] (2) Preparation of modified biochar

[0068] (2.1) Wash the biochar with deionized water three times, and dry it in a vacuum drying oven at 80°C for 8 hours to obtain washed biochar. Grind it and pass it through a 100-mesh sieve to obtain ground biochar.

[0069] (2.2) 10g of ground biochar was completely immersed in 200mL of 2.5mol / L hydrochloric acid solution, and then placed in a constant temperature shaking oven at (25℃, 120r / min) for 12h of acid washing and shaking. After rinsing with deionized water 3 times, it was vacuum dried in a vacuum drying oven at 80℃ for 8h to obtain modified biochar.

[0070] (3) Preparation of biochar with immobilized lanthanum hydroxide

[0071] 5.0g of lanthanum chloride heptahydrate (LaCl3) . Lanthanum chloride (LaCl3) was dissolved in 1000 mL of 2.0 mol / L hydrochloric acid solution to obtain a 5.0 g / L lanthanum chloride solution. 10.0 g of modified biochar was added to 200 mL of the 5.0 g / L lanthanum chloride solution and shaken at 25 °C and 120 rpm for 12 h. The supernatant was removed, and the mixture was washed three times with deionized water. The mixture was centrifuged at 6000 rpm for 10 min, the supernatant was removed, and the mixture was vacuum dried at 80 °C for 8 h to obtain biochar with fixed lanthanum hydroxide.

[0072] (4) Preparation of flocculants

[0073] (4.1) Add 25g of polyaluminum sulfate to 500ml of deionized water and dissolve it completely to obtain a polyaluminum sulfate solution with a concentration of 100g / L.

[0074] (4.2) Add 25g of lanthanum hydroxide-fixed biochar to 500ml of 100g / L polyaluminum sulfate solution, mix thoroughly, and shake at 60℃ and 120r / min for 12h in a constant temperature shaker. Centrifuge at 6000r / min for 10min, remove the supernatant, and dry to obtain the composite flocculant of lanthanum hydroxide-fixed biochar.

[0075] Example 3

[0076] Using actual effluent from a wastewater treatment plant in Hefei City, two flotation devices were used: a micro-nano flotation device and a dissolved air flotation device. The dosage of composite flocculant in both reactors was 100 mg / L, the hydraulic retention time was 25 minutes, the dissolved air pressure was 0.4 MPa (dissolved air flotation), and the dissolved air flow rate was 200 L / h (micro-nano flotation). The total phosphorus concentration in the wastewater effluent was 0.747 mg / L. After operation of both the dissolved air flotation and micro-nano flotation devices, effluent samples were collected at 1 hour, 2 hours, 3 hours, and 4 hours after operation, and the total phosphorus concentration was measured. Analysis showed that the effluent concentration from the micro-nano flotation device was controlled at 0.05 mg / L, achieving the ultimate phosphorus removal effect (see details). Figure 1 The total phosphorus concentration in the effluent from the dissolved air flotation unit was controlled at 0.10 mg / L, achieving a good deep phosphorus removal effect (see details). Figure 2 ).

[0077] Those skilled in the art will readily understand that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a composite flocculant for extreme phosphorus removal from low-turbidity wastewater, characterized in that, The operation steps are as follows: (1) Preparation of biochar (1.1) Add alkaline substances to the sludge and stir evenly. The mass ratio of the sludge to the alkaline substances is 1.0:0.2-0.5 to obtain alkaline sludge with a pH value of 9.0-12.

0. Dry it at 75°C or air dry it naturally and crush it into sludge particles. The sludge is the residual sludge from a wastewater treatment plant, and the moisture content of the residual sludge is 78-82%. The alkaline substance is sodium hydroxide; (1.2) Sludge particles are burned in a tubular furnace under aeration to produce biochar; the gas is nitrogen or other inert gas. (2) Preparation of modified biochar (2.1) Wash the biochar with deionized water 3 to 4 times and dry it under vacuum to obtain water-washed biochar; After grinding, ground biochar is obtained; The grinding process passes through a 100-mesh sieve; (2.2) Add hydrochloric acid solution with a concentration of 0.5 mol / L to 2.5 mol / L to the ground biochar, acid wash and shake, rinse with deionized water, and vacuum dry to obtain modified biochar; The mass-to-volume ratio of the above-mentioned ground biochar to hydrochloric acid solution is 1g-2g:10ml; (3) Preparation of biochar with immobilized lanthanum hydroxide Lanthanum chloride heptahydrate (LaCl3) . Lanthanum chloride (7H2O) is dissolved in hydrochloric acid solution with a concentration of 0.5 mol / L to 5.0 mol / L to obtain a lanthanum chloride solution with a concentration of 5 g / L to 50 g / L. The modified biochar was added to a lanthanum chloride solution with a concentration of 5 g / L to 50 g / L, and shaken at a constant temperature. The supernatant was removed, and the biochar was washed three times with deionized water. It was then centrifuged and vacuum dried to obtain biochar with fixed lanthanum hydroxide. The mass-to-volume ratio of modified biochar to lanthanum chloride solution was 0.5–0.8 g: 10 mL; (4) Preparation of composite flocculant (4.1) Dissolve polyaluminum chloride flocculant in water at a mass ratio of 0.1 to 1.0:10 to obtain polyaluminum chloride flocculant solution; (4.2) Add biochar with fixed lanthanum hydroxide to polyaluminum chloride flocculant solution at a mass-volume ratio of 0.5g to 0.8g: 10mL, heat and stir, and dry to obtain composite flocculant modified with lanthanum hydroxide fixed biochar; The composite flocculant is a black powder with a specific surface area of ​​50-200 m². 2 / g; The pore structure is mesoporous. The total pore volume of pores with a single-point adsorption width of less than 187.216 nm is 0.990:0.216 cm³ / g; the cumulative pore volume of pores with an adsorption width between 1.7 nm and 300 nm is 0.116-0.336 cm³ / g; the cumulative pore volume of pores with a desorption width between 1.7 nm and 300 nm is 0.125-0.375 cm³ / g; the average pore size is 5 nm to 15 nm; and the iodine adsorption value is 300-500 mg / g.

2. The preparation method according to claim 1, characterized in that: In step (1.2), the firing conditions are: temperature 600℃ and time 3.0 hours.

3. The preparation method according to claim 1, characterized in that: In step (2.1), the vacuum drying is carried out in a vacuum drying oven at a temperature of 60-80℃ for 6-7 hours.

4. The preparation method according to claim 1, characterized in that: In step (2.2), the pickling and oscillation conditions are: temperature 25-35℃, rotation speed 80-180r / min, and time 12-24h.

5. The preparation method according to claim 1, characterized in that: In step (2.2), the vacuum drying is carried out in a vacuum drying oven at a temperature of 60-80℃ for 8-12 hours.

6. The preparation method according to claim 1, characterized in that: In step (3), the isothermal oscillation conditions are: temperature 25-35℃, rotation speed 80-180r / min, and time 12-24h.

7. The preparation method according to claim 1, characterized in that: In step (3), the centrifugation conditions are: rotation speed 6000-10000 r / min and time 5-10 min.

8. The preparation method according to claim 1, characterized in that: In step (4.2), the heating and stirring conditions are: temperature 60-80℃, speed 80-120r / min, and time 12-24h.

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