Preparation method of lotus seedpod-based activated carbon with improved low-temperature adsorbability

Abstract

The invention relates to a preparation method of lotus seedpod-based activated carbon with improved low-temperature adsorbability, which comprises the following steps: (S1) crushing lotus seedpod, washing with water, and drying to obtain lotus seedpod powder; (S2) the lotus seedpod powder is added into steeping liquor, steeping activation is conducted at the temperature of 80-100 DEG C in a closed state, and the steeping liquor is a mixed aqueous solution prepared from phosphoric acid, nickel-bipyridine complex salt and a non-ionic surface active agent according to the mass ratio of 100: (15-25): (1-3); after dipping activation is finished, adding a thiol reducing agent, and then performing freeze drying to obtain a precursor; and (S3) carbonizing, washing and drying the precursor in an inert atmosphere to obtain the lotus seedpod-based activated carbon with improved low-temperature adsorbability. The prepared lotus seedpod-based activated carbon overcomes the problems that active sites of a traditional material are easy to agglomerate and sensitive to temperature, shows excellent low-temperature adsorption characteristics, and is suitable for efficient adsorption of cadmium heavy metal ions in water in a low-temperature environment.

Description

Technical Field

[0001] This invention belongs to the field of activated carbon technology, specifically relating to a method for preparing lotus seedpod shell-based activated carbon with improved low-temperature adsorption properties. Background Technology

[0002] Activated carbon, due to its high specific surface area, well-developed pore structure, abundant surface chemical groups, and strong specific adsorption capacity, is widely used in water treatment, solvent recovery, hydrogen storage, catalysis, and other fields. Currently, coal-based granular activated carbon and wood-based activated carbon are commonly used. However, coal is a non-renewable resource, while trees grow very slowly, and logging for charcoal production causes severe ecological damage. Lotus pods, as a biomass plant, possess a unique porous structure and are high in cellulose and lignin. Currently, after removing the lotus seeds, the lotus pod shells are mostly discarded, except for a small portion used in traditional Chinese medicine production, failing to be effectively utilized. Therefore, using lotus pod shells to prepare activated carbon is of great significance in alleviating the conflict between the demand for activated carbon and the protection of ecological resources.

[0003] Heavy metals in water and soil, due to their non-degradable and toxic properties, can cause serious global problems if not treated promptly. Cadmium (Cd) pollution poses the strongest threat to human health, causing toxic reactions even at low concentrations. Improper industrial wastewater discharge and excessive use of agricultural fertilizers can lead to excessive Cd levels in the ecological environment. Activated carbon prepared from lotus seedpods shows promising application prospects for heavy metal pollution remediation. Currently, there are few reports on the preparation of activated carbon from lotus seedpods, and its adsorption performance needs further improvement, especially its temperature sensitivity. At low temperatures (≤15℃), the adsorption performance decreases significantly due to slow molecular / ion diffusion rates and "inertization" of surface active sites, making it unsuitable for low-temperature wastewater treatment. For example, Yang Xiaoran et al. (Agriculture and Technology, 2021, Vol. 41, No. 16) prepared activated carbon from lotus seedpods using zinc chloride as an activator. This activated carbon showed good treatment efficiency in wastewater treatment, but the treatment temperature was 20℃; the adsorption efficiency decreased significantly at lower temperatures. Summary of the Invention

[0004] In view of the above problems, the purpose of this invention is to provide a method for preparing lotus seedpod shell activated carbon, which produces activated carbon with rich pore structure and high specific surface area. This activated carbon has good adsorption performance for cadmium heavy metal ions in water, and overcomes the limitation of traditional activated carbon's poor performance at low temperatures due to temperature sensitivity. It can still maintain high adsorption capacity at low temperatures (≤15℃).

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] A method for preparing lotus seedpod shell-based activated carbon with improved low-temperature adsorption properties includes the following steps:

[0007] (S1) Pretreatment: The lotus seedpod shells are crushed, washed with water, and dried to obtain lotus seedpod shell powder;

[0008] (S2) Impregnation and activation: The lotus seedpod shell powder is added to the impregnation solution and impregnated and activated in a closed state at 80~100℃. The impregnation solution is a mixed aqueous solution of phosphoric acid, nickel-bipyridine complex salt and nonionic surfactant in a mass ratio of 100:(15~25):(1~3). After the impregnation and activation is completed, a thiol reducing agent is added and then freeze-dried to obtain the precursor.

[0009] (S3) Carbonization: The precursor is carbonized in an inert atmosphere, washed, and dried to obtain lotus seedpod shell-based activated carbon with improved low-temperature adsorption properties.

[0010] The core mechanism of this invention lies in two aspects. First, by using a mixed aqueous solution composed of phosphoric acid, nickel-bipyridine complex salt, and nonionic surfactant in a specific ratio as the impregnation solution, combined with the subsequent carbonization process, a "synergistic construction of pore structure" is achieved, forming activated carbon with high specific surface area and abundant porosity. Phosphoric acid in the impregnation solution is the main activator; the nickel-bipyridine complex salt is a chloride or bromide salt containing nitrogen heterocycles, which can promote the activation of lotus seedpod shells by phosphoric acid. This may be because the nitrogen heterocycles in the nickel-bipyridine complex salt may generate π-π interactions with the aromatic structure of lignin in the lotus seedpod shell; the nonionic surfactant acts as a penetration enhancer, possessing good solubilizing and wetting effects, thus improving activation efficiency. Second, the stable coordination structure of the nickel-bipyridine complex salt ensures that nickel ions are molecularly dispersed and anchored in the raw material matrix during the impregnation stage. In the subsequent carbonization process, the added thiol reducing agent decomposes upon heating, generating reducing free radicals (such as HS·) or H2S small molecules, which reduce the Ni in the nickel-bipyridine complex salt. 2+ In-situ reduction to nickel nanoparticles; these highly dispersed nickel nanoparticles serve as potent active adsorption sites, enhancing the adsorption of Cd by activated carbon. 2+The invention demonstrates a high capacity for capturing heavy metal ions. This "coordination anchoring-in-situ conversion" mechanism effectively prevents nickel migration and aggregation, achieving highly uniform dispersion of active sites in the carbon matrix, and ensuring strong adhesion to the carbon substrate, preventing detachment or migration. Thirdly, the nickel-bipyridine complex salt simultaneously serves as a "nitrogen source," while the thiol reducing agent acts as a "sulfur source," thus achieving co-doping of the carbon framework during carbonization. This sulfur-nitrogen co-doping optimizes the electron distribution of the carbon material and enhances its surface polarity, further improving the adsorption performance of the activated carbon. Furthermore, the freeze-drying process of this invention avoids component migration and aggregation caused by solvent evaporation during conventional drying, achieving a uniform molecular-level distribution of each material and providing a crucial precursor state for the formation of a uniform pore structure and active sites during the carbonization stage. In summary, this invention, through the precise synergistic effect of its components, prepares an activated carbon with high specific surface area, stable and uniformly dispersed active sites, and a stable doped structure, thereby overcoming the limitations of traditional activated carbon's temperature sensitivity and achieving low-temperature, high-efficiency adsorption of cadmium heavy metal ions in water.

[0011] The improvement of low-temperature adsorption in this invention refers to improving the adsorption performance of activated carbon in low-temperature environments of 15°C and below.

[0012] Further, in step (S1), the crushing is to crush to 100-200 mesh, and the crushing method is airflow crushing and high-speed crushing; the drying is to dry at 60-80℃ for 12-24 hours.

[0013] Preferably, in step (S2), the mass ratio of phosphoric acid, nickel-bipyridine complex salt, and nonionic surfactant in the impregnation solution is 100:(15~20):(1~3). In this invention, high-performance activated carbon is prepared through the synergistic effect of phosphoric acid, nickel-bipyridine complex salt, and nonionic surfactant. However, the three components cannot be combined in any arbitrary proportion. If the content of nickel-bipyridine complex salt is too low, its effect is limited; however, if its content is too high, the phosphoric acid content will be too low, thereby reducing the activation and pore-forming effect.

[0014] Further, in step (S2), the nickel-bipyridine complex salt is a complex formed by nickel ions and 2,2'-bipyridine, and the anion is a halide anion (Cl). - Or Br - Specifically, the active ingredient is selected from at least one of tris(2,2'-bipyridine)nickel chloride ([Ni(bpy)3]Cl2) and tris(2,2'-bipyridine)nickel bromide ([Ni(bpy)3]Br2), preferably tris(2,2'-bipyridine)nickel chloride; the nonionic surfactant is a polyoxyethylene fatty acid ester, preferably at least one of Mize 49 and Mize 52.

[0015] Further, in step (S2), the total concentration of the impregnation solution is 30-40 wt%; the impregnation temperature is 80-100℃, and the impregnation time is 2-4 hours. There is no particular limitation on the solid-liquid ratio of the lotus seedpod powder to the impregnation solution; the impregnation solution only needs to completely submerge the lotus seedpod powder, for example, a solid-liquid ratio of 1 kg: 4.0-8.0 L.

[0016] Further, in step (S2), the thiol reducing agent is selected from at least one of 2-mercaptoethanol and thioglycolic acid; the amount of the thiol reducing agent is 4 to 8 wt% of the nickel-bipyridine complex salt.

[0017] Further, in step (S2), the freeze-drying conditions are as follows: the impregnated mixture is first frozen in liquid nitrogen for 1 to 3 hours, and then vacuum-dried in a cold trap at -40 to -30°C for 8 to 12 hours with a vacuum degree of 10 to 40 Pa.

[0018] Further, in step (S3), the inert atmosphere is nitrogen and / or argon; the carbonization conditions are: holding at 400~600℃ for 2~5h; the washing is boiling in water for 30~60min to remove residual phosphoric acid in the carbonized material; the drying is drying at 100~150℃ for 12~24h.

[0019] Secondly, the present invention provides a lotus seedpod shell-based activated carbon with improved low-temperature adsorption properties, which is prepared by the above-mentioned preparation method.

[0020] Furthermore, the lotus seedpod shell-based activated carbon with improved low-temperature adsorption properties has an iodine adsorption value of over 1200 mg / g at 25°C and over 1000 mg / g at 5°C; a cadmium adsorption value of over 70 mg / g at 25°C and over 55 mg / g at 5°C; and the adsorption value at 5°C is not less than 80% of that at 25°C.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] This invention employs a highly active impregnation solution, formulated with phosphoric acid, a nickel-bipyridine complex salt, and a nonionic surfactant in a specific mass ratio, to impregnate and activate lotus seedpod shell powder. Combined with freeze-drying and subsequent carbonization processes, a carbon framework with high specific surface area and well-developed porosity is synergistically constructed. Simultaneously, through a "coordination anchoring-in-situ transformation" mechanism, highly dispersed and stable nickel nano-active sites are formed within the carbon matrix, achieving stable co-doping of the carbon framework with sulfur and nitrogen elements. The resulting activated carbon overcomes the problems of easy aggregation of active sites and temperature sensitivity found in traditional materials, exhibiting excellent "low-temperature adsorption" characteristics, making it suitable for the efficient adsorption of cadmium heavy metal ions in water under low-temperature environments. Attached Figure Description

[0023] Figure 1 This is a SEM image of the lotus seedpod shell-based activated carbon with improved low-temperature adsorption properties prepared in Example 1. Detailed Implementation

[0024] The present invention will be further described below with reference to specific embodiments, but the present invention is not limited to the following embodiments.

[0025] Unless otherwise specified, the experimental methods described in the following examples are conventional methods; the reagents and materials described are commercially available unless otherwise specified.

[0026] Example 1

[0027] (S1) Pretreatment: The lotus seedpod shells are air-jet broken down to 200 mesh, washed twice with water, and then dried in an oven at 90°C for 24 hours to obtain lotus seedpod shell powder;

[0028] (S2) Impregnation and activation: Phosphoric acid, nickel tris(2,2'-bipyridine)chloride ([Ni(bpy)3]Cl2) and Mize 49 were prepared into a mixed aqueous solution with a total concentration of 40wt% at a mass ratio of 100:15:1, which is the impregnation solution; then, lotus shell powder was added to the impregnation solution at a solid-liquid ratio of 1kg:6.0L, and impregnated and activated at 90℃ for 3h in a closed state; after the impregnation and activation were completed, 2-mercaptoethanol (6% of the mass of [Ni(bpy)3]Cl2) was added to form a mixture, and then the mixture was first frozen in liquid nitrogen for 1.5h, and then vacuum dried in a cold trap at -30℃ for 10h (vacuum degree 10~40Pa) to obtain the precursor;

[0029] (S3) Carbonization: The material was placed in a rotary kiln and heated to 500℃ at a rate of 5℃ / min under a nitrogen atmosphere, and held at that temperature for 3 hours. The rotation speed of the rotary kiln was 20 rpm. After carbonization, the material was naturally cooled to room temperature and then boiled in water for 40 minutes to remove residual phosphoric acid. Subsequently, it was dried in an oven at 120℃ for 24 hours to obtain lotus seedpod shell-based activated carbon with improved low-temperature adsorption properties (its SEM image is shown below). Figure 1 (As shown).

[0030] Example 2

[0031] The rest is the same as in Example 1, except that the mass ratio of phosphoric acid, tris(2,2'-bipyridine)nickel chloride ([Ni(bpy)3]Cl2) to Mize 49 in the impregnation solution in step (S2) is 100:20:2.

[0032] Example 3

[0033] The rest is the same as in Example 1, except that the mass ratio of phosphoric acid, nickel tris(2,2'-bipyridine) chloride ([Ni(bpy)3]Cl2) to Mize 49 in the impregnation solution in step (S2) is 100:25:3.

[0034] Example 4

[0035] The rest is the same as in Example 1, except that in step (S2), nickel tris(2,2'-bipyridine)bromine ([Ni(bpy)3]Br2) is used in place of nickel tris(2,2'-bipyridine)chloride ([Ni(bpy)3]Cl2) by mass, and the amount of 2-mercaptoethanol used is 8% of the mass of [Ni(bpy)3]Br2.

[0036] Example 5

[0037] The rest is the same as in Example 1, except that in step (S2), 2-mercaptoethanol is replaced with mercaptoacetic acid or the like, and its amount is 4% of the mass of [Ni(bpy)3]Cl2, and the like is replaced with the like by mass of Mize 52 or the like by mass of Mize 49.

[0038] Comparative Example 1

[0039] The rest is the same as in Example 1, except that the impregnation solution in step (S2) is an aqueous solution of phosphoric acid and methyl methacrylate 49 in a mass ratio of 100:1, that is, the impregnation solution does not contain [Ni(bpy)3]Cl2.

[0040] Comparative Example 2

[0041] The rest is the same as in Example 1, except that the mass ratio of phosphoric acid, tris(2,2'-bipyridine)nickel chloride ([Ni(bpy)3]Cl2) to Mize 49 in the impregnation solution in step (S2) is 100:5:1.

[0042] Comparative Example 3

[0043] The rest is the same as in Example 1, except that the mass ratio of phosphoric acid, tris(2,2'-bipyridine)nickel chloride ([Ni(bpy)3]Cl2) to Mize 49 in the impregnation solution in step (S2) is 100:35:3.

[0044] Comparative Example 4

[0045] The rest is the same as in Example 1, except that: in step (S2), 2-mercaptoethanol is not added after the impregnation and activation are completed, that is, the material after the impregnation and activation are directly freeze-dried.

[0046] Testing and Analysis

[0047] 1) Specific surface area measurement

[0048] The specific surface area of ​​activated carbon prepared in the examples and comparative examples was determined by low-temperature nitrogen adsorption experiments using a Tristar II 3020 fully automated specific surface area analyzer manufactured by Micromeritics Instrument Corporation, USA. The specific test results are shown in Table 1.

[0049] 2) Adsorption performance test

[0050] Iodine adsorption value test: According to GB / T 12496.8-1999, the iodine adsorption value of the activated carbon prepared in the examples and comparative examples was determined at low temperature (5℃) and room temperature (25℃) to evaluate its micropore development and physical adsorption capacity.

[0051] Cadmium adsorption test: The test was conducted at a simulated low temperature environment of 5℃ and a normal temperature of 25℃. 0.06g of activated carbon prepared in different examples and comparative examples was weighed and placed in conical flasks. 100mg / L of Cd was added to each flask. 2+ 100 mL of solution was shaken at 150 rpm for 360 min at 5℃ and 25℃, centrifuged, filtered, and the Cd of the filtrate was measured. 2+ Concentration. Adsorption capacity is calculated using the following formula I:

[0052] q=(c0-c) / m*V (Formula I)

[0053] In Formula I, c0 represents the concentration of ions before adsorption (mg / L), c represents the concentration of ions in the solution after adsorption (mg / L), q represents the adsorption amount (mg / g), V represents the volume of the solution (L), and m represents the mass of activated carbon.

[0054] The test data for the above adsorption performance are shown in Table 1.

[0055] Table 1 Specific surface area and adsorption performance test

[0056]

[0057] As shown in Table 1, the lotus seedpod shell-based activated carbon with improved low-temperature adsorption prepared in the embodiments of the present invention has a high specific surface area and excellent adsorption performance at both low and normal temperatures. Specifically, the iodine adsorption value reaches over 1200 mg / g at 25℃ and remains above 1000 mg / g at 5℃; the cadmium adsorption capacity is as high as over 70 mg / g at 25℃ and remains above 55 mg / g at 5℃; and the adsorption capacity at 5℃ is not less than 80% of that at 25℃. In Comparative Example 1, a nickel-bipyridine complex was not used during impregnation and activation, resulting in a lower specific surface area of ​​the prepared activated carbon, especially with poor low-temperature adsorption performance. The component ratios of the impregnation solutions in Comparative Examples 2 and 3 are not within the scope of the present invention, resulting in a lower specific surface area and poorer adsorption performance of the prepared activated carbon. In Comparative Example 4, no thiol reducing agent was used, resulting in a higher specific surface area of ​​the prepared activated carbon, but its adsorption performance was lower.

Claims

1. A method for preparing lotus seedpod shell-based activated carbon with improved low-temperature adsorption properties, characterized in that, Includes the following steps: (S1) Pretreatment: Crush the lotus seedpod shells, wash them with water, and dry them to obtain lotus seedpod shell powder; (S2) Impregnation and activation: The lotus seedpod shell powder is added to the impregnation solution and impregnated and activated in a closed state. The impregnation solution is a mixed aqueous solution of phosphoric acid, nickel-bipyridine complex salt and nonionic surfactant in a mass ratio of 100:(15~25):(1~3). After the impregnation and activation is completed, a thiol reducing agent is added and then freeze-dried to obtain the precursor. (S3) Carbonization: The precursor is carbonized in an inert atmosphere, washed, and dried to obtain lotus seedpod shell-based activated carbon with improved low-temperature adsorption properties.

2. The preparation method according to claim 1, characterized in that, Step (S1) involves crushing the material to 100-200 mesh using airflow crushing or high-speed crushing. The drying process involves drying the material at 60-80°C for 12-24 hours.

3. The preparation method according to claim 1, characterized in that, In step (S2), the mass ratio of phosphate, nickel-bipyridine complex salt and nonionic surfactant in the impregnation solution is 100:(15~20):(1~3).

4. The preparation method according to claim 1, characterized in that, In step (S2), the nickel-bipyridine complex salt is a complex formed by nickel ions and 2,2'-bipyridine, and the anion is a halide anion, specifically selected from at least one of tris(2,2'-bipyridine)nickel chloride and tris(2,2'-bipyridine)nickel bromide, preferably tris(2,2'-bipyridine)nickel chloride; the nonionic surfactant is a polyoxyethylene fatty acid ester, preferably at least one of Mize 49 and Mize 52.

5. The preparation method according to claim 1, characterized in that, In step (S2), the total concentration of the impregnation solution is 30-40 wt%; the impregnation temperature is 80-100℃; and the impregnation time is 2-4 h.

6. The preparation method according to claim 1, characterized in that, In step (S2), the thiol reducing agent is selected from at least one of 2-mercaptoethanol and thioglycolic acid; the amount of the thiol reducing agent is 4 to 8 wt% of the nickel-bipyridine complex salt.

7. The preparation method according to claim 1, characterized in that, In step (S2), the freeze-drying conditions are as follows: the impregnated mixture is first frozen in liquid nitrogen for 1 to 3 hours, and then vacuum dried in a cold trap at -40 to -30°C for 8 to 12 hours with a vacuum degree of 10 to 40 Pa.

8. The preparation method according to claim 1, characterized in that, In step (S3), the inert atmosphere is nitrogen and / or argon; the carbonization conditions are: holding at 400~600℃ for 2~5h; the washing is boiling in water for 30~60min to remove residual phosphoric acid in the carbonized material; the drying is drying at 100~150℃ for 12~24h.

9. A lotus seedpod shell-based activated carbon with improved low-temperature adsorption properties, characterized in that, It is prepared by the method described in any one of claims 1-8.

10. The lotus seedpod shell-based activated carbon with improved low-temperature adsorption properties according to claim 9, characterized in that, The lotus seedpod shell-based activated carbon with improved low-temperature adsorption properties has an iodine adsorption value of over 1200 mg / g at 25°C and over 1000 mg / g at 5°C; a cadmium adsorption value of over 70 mg / g at 25°C and over 55 mg / g at 5°C; and the adsorption value at 5°C is not less than 80% of that at 25°C.

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