Method for recycling soybean stalk biochar and application thereof in removing hexavalent chromium in water

Abstract

This invention discloses a method for recycling soybean straw biochar and its application in removing hexavalent chromium from water, belonging to the field of wastewater treatment technology. The soybean straw biochar is produced from agricultural waste soybean straw through a one-step carbonization process. The biochar achieves an adsorption efficiency of over 93.3% for hexavalent chromium in water and exhibits good anti-interference capabilities. The adsorbed biochar is regenerated using a two-step "alkali-acid" desorption process, and the adsorption efficiency remains above 87% after five cycles. This invention has advantages such as wide availability of raw materials, simple preparation process, low cost, no secondary pollution, and excellent recycling performance, making it suitable for the advanced treatment of chromium-containing wastewater.

Description

Technical Field

[0001] This invention relates to the field of wastewater treatment technology, specifically a method for recycling soybean straw biochar and its application in removing hexavalent chromium from water. Background Technology

[0002] Hexavalent chromium is highly toxic and mobile, a typical heavy metal pollutant widely generated in industrial processes such as electroplating, metallurgy, and leather tanning, posing a serious threat to the ecological environment and human health. Traditional treatment methods suffer from low efficiency, high cost, and the potential for secondary pollution. Agricultural and forestry biomass waste is widely available and inexpensive; using it as raw material to prepare adsorbent materials can achieve resource utilization while avoiding secondary pollution. Soybean stalks are naturally rich in cellulose and active functional groups, and can be used for water treatment without complex modification, offering advantages over soybean pellets.

[0003] In existing technologies, such as CN115178235A, which uses ZnCl2 to modify corn stalks to prepare biochar, although it has a good adsorption effect on Cr(VI), there is a risk of zinc ion leaching, which may cause secondary heavy metal pollution. CN105688812A grafts β-cyclodextrin / polyglutamic acid onto the surface of biochar through multi-step organic synthesis, but the process is cumbersome and involves toxic reagents such as acetonitrile. CN109012589A uses urea pre-impregnation modification to introduce nitrogen-containing functional groups, but the introduced exogenous nitrogen-containing groups have the potential to dissolve under acidic conditions, and its nitric acid regeneration method easily damages the carbon skeleton, resulting in a significant decrease in efficiency after 4 cycles. The above methods all fall under the category of exogenous chemical modification, which not only increases the complexity of preparation but also introduces the risk of secondary pollution, deviating from the green intention of "treating waste with waste". Currently, there are no reported technical solutions for directly preparing high-performance biochar adsorbents from native agricultural and forestry waste without adding any modifiers. In general, existing biochar adsorbents are mostly prepared through modification or composite methods, which increases production complexity and environmental risks. Therefore, the method of directly converting single agricultural and forestry waste into water adsorbents without adding chemical reagents has significant environmental value and cost advantages, and has broad application prospects. Summary of the Invention

[0004] The purpose of this invention is to provide a method for recycling soybean straw biochar and its application in removing hexavalent chromium from water, so as to solve the problems mentioned in the background art.

[0005] To achieve the above objectives, the experimental technical solution of this invention is as follows:

[0006] A method for recycling soybean stalk biochar includes the following steps: washing, crushing, and drying soybean stalks, then carbonizing them under a protective gas atmosphere to obtain soybean stalk biochar; using the soybean stalk biochar to adsorb hexavalent chromium in water; and sequentially soaking the soybean stalk biochar after adsorbing hexavalent chromium in alkaline and acidic solutions for desorption, followed by washing and drying to obtain regenerated soybean stalk biochar.

[0007] Furthermore, the carbonization treatment temperature is 600~800 ℃.

[0008] Furthermore, the carbonization treatment temperature is 800 °C.

[0009] Furthermore, the protective gas is nitrogen, with a flow rate of 0.2 L / min, a heating rate of 10 ℃ / min, and a holding time of 2 h.

[0010] Furthermore, the alkaline solution is a 0.1 mol / L NaOH solution, and the acid solution is a 0.1 mol / L HCl solution.

[0011] Furthermore, the alkali soaking time is 12 hours, and the acid soaking time is 12 hours.

[0012] Furthermore, the adsorption conditions were: pH=2, temperature 25 ℃, and adsorption time 10 h.

[0013] An application of soybean straw biochar in the removal of hexavalent chromium from water involves using regenerated soybean straw biochar, adding it to water containing hexavalent chromium, adjusting the pH to acidic, and then removing the hexavalent chromium through adsorption.

[0014] Furthermore, the dosage of the soybean straw biochar is 1 g / L, the pH is adjusted to 2, the adsorption time is 10 h, and the adsorption temperature is 25 ℃.

[0015] Furthermore, after five adsorption-desorption cycles, the adsorption efficiency of the soybean straw biochar for hexavalent chromium remains above 87%.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] This invention uses agricultural waste soybean stalks as raw material. Without adding chemical modifiers such as ZnCl2 or composite components, biochar with good adsorption properties can be obtained in one step of carbonization. The preparation process is simple, the production cost is low, and the risk of secondary pollution caused by the introduction of chemical reagents is avoided.

[0018] The soybean straw biochar (especially HDC-800) prepared by this invention exhibits excellent adsorption performance for hexavalent chromium in water. Under conditions of pH=2 and an initial concentration of 20 mg / L, the adsorption efficiency can reach over 93.3% after 10 h. Furthermore, it also effectively adsorbs common inorganic ions in water (NO3-). - CO3 2- Cl - SO4 2- K + Mg 2+ Cu 2+ It has good anti-interference ability.

[0019] The "alkali-acid" two-step desorption and regeneration method provided by this invention can more effectively restore oxygen-containing functional groups (such as -COOH) on the surface of biochar compared with single alkali desorption, significantly improving cycle stability. After 5 adsorption-desorption cycles, the adsorption efficiency of HDC-800 for hexavalent chromium can still be maintained above 87%.

[0020] This invention realizes the resource utilization of agricultural waste soybean stalks, which is in line with the environmental protection concept of green, low carbon and circular, and has broad application prospects in the field of deep treatment of chromium-containing wastewater. Attached Figure Description

[0021] Figure 1 shows the FESEM images of (a) HDC-600, (b) HDC-700, and (c) HDC-800.

[0022] Figure 2 The adsorption effect of HDC-600, HDC-700 and HDC-800 on Cr(VI) is shown in the figure.

[0023] Figure 3 This demonstrates the resistance of HDC-800 to common inorganic ions in the adsorption of Cr(VI).

[0024] Figure 4 The adsorption effect of HDC-800 on Cr(VI) in the first cycle is shown.

[0025] Figure 5 The adsorption effect of HDC-800 on Cr(VI) in the second cycle is shown. Detailed Implementation

[0026] Example 1: Preparation of biochar adsorbent HDC-600

[0027] Biochar pretreatment: Wash agricultural and forestry waste soybean stalks, crush them in a pulverizer, pass them through an 80-mesh sieve, and dry them in a vacuum drying oven at 80 ℃ until constant weight.

[0028] Preparation of biochar: 3 g of soybean stalk powder was placed in a ceramic boat and then placed in a tube furnace. The temperature was increased to 600 °C at a rate of 10 °C / min with a flow rate of N2 of 0.2 L / min, and held for 2 h. After cooling to room temperature, the carbonized product was ground and passed through an 80-mesh sieve to obtain the prepared adsorbent, which was named HDC-600.

[0029] Example 2 Preparation of biochar adsorbent HDC-700

[0030] The temperature was raised to 700 ℃ and named HDC-700. The remaining steps were the same as in Example 1.

[0031] Example 3 Preparation of biochar adsorbent HDC-800

[0032] The temperature was raised to 800 ℃ and named HDC-800. The remaining steps were the same as in Example 1.

[0033] The soybean stalks used in this invention were collected from conventional farmland and used after impurity removal. The tube furnace is a programmed temperature rise type, with a quartz tube diameter of 40 mm and a heating zone length of 300 mm; standard specifications are sufficient to achieve the technical effects of this invention. Vacuum drying was performed at 80 ℃ and a vacuum degree of -0.1 MPa. Vacuum filtration was performed using a 0.45 μm aqueous filter membrane. Washing to neutrality refers to repeated rinsing with ultrapure water 3-5 times until the wash solution tested neutral (pH=6.5-7.5) on pH paper. The regenerated biochar, after vacuum drying at 80 ℃ for 12 h, was directly used in the next adsorption experiment without further grinding and sieving.

[0034] Example 4 Characterization

[0035] FESEM

[0036] Figure 1a It's HDC-600. Figure 1b It's HDC-700. Figure 1c The image shows an FESEM image of HDC-800. As the carbonization temperature increases, the morphology of soybean stalk biochar gradually changes from the blocky structure of HDC-600 to the lamellar structure of HDC-800. The higher the carbonization temperature, the easier it is to create defects, which is beneficial for the reactants to come into contact with more active sites.

[0037] N2- adsorption and desorption

[0038] As shown in Table 1, HDC-600, HDC-700, and HDC-800 all have mesoporous structures, and the specific surface area of ​​HDC-800 is 26.17 m². 2 ·g -1 It is significantly greater than HDC-600 and HDC-700.

[0039] Table 1 Pore structure parameters of HDC-600, HDC-700, and HDC-800

[0040]

[0041] Example 5 Adsorption Test

[0042] Weigh 0.05 g of adsorbents HDC-600, HDC-700, and HDC-800 into three-necked flasks, add 50 mL of 20 mg / L K2Cr2O7 solution, adjust the pH of the solution to 2 with 1 mol / L HCl and 1 mol / L NaOH solution, and set the reaction temperature to 25 ℃. Place the resulting solution in a 25 ℃ constant temperature water bath and rotate it at a speed of 300 r / min to ensure that the adsorbent and solution are in full contact.

[0043] A portion of the supernatant was collected at 60, 90, 120, 150, 180, 210, 240, 360, 480, and 600 min, respectively. The supernatant was filtered through a 0.22 μm filter membrane, and the absorbance of Cr(VI) was measured at 540 nm using a UV-Vis spectrophotometer via the diphenylcarbazide colorimetric method. The concentration of Cr(VI) was calculated based on the change in absorbance. The reduction rate of Cr(VI) was calculated using the following formula:

[0044]

[0045] In the formula, η represents the removal rate of Cr(VI), C0 represents the initial concentration of Cr(VI), and C t It indicates the concentration at a certain moment in the reaction process.

[0046] Depend on Figure 2 It can be seen that HDC-800 has the best adsorption effect, reaching an adsorption efficiency of 93.3% after 10 h, which is better than HDC-600 and HDC-700. The adsorption effects of HDC-600 and HDC-700 on Cr(VI) are not significantly different.

[0047] Example 6: Anti-interference test.

[0048] Weigh 50 mg of adsorbent HDC-800 into a three-necked flask, add 50 mL of 20 mg / L Cr(VI) solution, adjust to pH=2, and then add 5 mmol of different inorganic anions and cations (no ions added to the control group, NO3- and NO2- added to the experimental groups respectively). - CO3 2- Cl - SO4 2- K + Mg 2+ Cu 2+The resulting solution was placed in a 25 ℃ constant temperature water bath and rotated at a speed of 300 r / min for 10 h. Compared with the control group, the addition of inorganic ions had little effect on the removal of Cr(VI) by the adsorbent HDC-800, indicating that the adsorbent HDC-800 has a relatively small effect on the removal of common inorganic ions (NO3) in the adsorption of Cr(VI). - CO3 2- Cl - SO4 2- K + Mg 2+ Cu 2+ It has good anti-interference capabilities, see Figure 3 .

[0049] Example 7: Recycling of HDC-800

[0050] To verify the cyclic stability of different regeneration methods, two parallel experiments were set up. The adsorbent used was HDC-800 prepared in the same batch, and the adsorption conditions were the same as in Example 5 (pH=2, 25 ℃, 10 h, 50 mg adsorbent, 50 mL 20 mg / L Cr(VI) solution). After each adsorption cycle, the adsorbed HDC-800 was recovered by vacuum filtration, desorbed and regenerated according to the corresponding method, washed with ultrapure water until neutral, and vacuum dried at 80 ℃ for 12 h before being used for the next adsorption cycle.

[0051] First cycle (single alkali solution regeneration control group):

[0052] After each adsorption cycle, the adsorbed HDC-800 was soaked in 10 mL of 0.1 mol / L NaOH solution for 12 h for desorption, followed by filtration. The above adsorption-desorption process was repeated a total of 5 times.

[0053] The results showed that the adsorption efficiency of HDC-800 for Cr(VI) was 93%, 82%, 74%, 69%, and 64% respectively, indicating that the adsorption performance continued to decline during regeneration with a single alkali solution.

[0054] Second cycle ("base-acid" two-step desorption experimental group):

[0055] The first two cycles used a single alkaline solution for desorption (the method was the same as in the first cycle). From the third cycle onwards, a two-step "alkali-acid" desorption method was used, that is, the solution was soaked in 10 mL of 0.1 mol / L NaOH solution and 10 mL of 0.1 mol / L HCl solution for 12 h each for desorption.

[0056] The results showed that the adsorption efficiencies in the 3rd, 4th, and 5th cycles were 91%, 88%, and 87%, respectively. Compared with the first cycle, the two-step alkali-acid desorption stabilized the adsorption efficiency from 64% (5th cycle in the first cycle) to above 87%, indicating that the two-step alkali-acid desorption method can effectively restore the adsorption performance of HDC-800 and is significantly better than single alkali desorption.

[0057] The "alkali-acid" two-step desorption and regeneration method provided by this invention can significantly improve the cycle stability of biochar compared with single alkali desorption, and effectively solve the problem that the adsorption performance decreases with the number of cycles.

[0058] Based on the above-described preferred embodiments of the present invention, and through the foregoing description, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification.

Claims

1. A method for recycling soybean straw biochar, characterized in that, Includes the following: Soybean stalks are washed, crushed, and dried, and then carbonized under a protective gas atmosphere to obtain soybean stalk biochar. The soybean stalk biochar is used to adsorb hexavalent chromium in water. The soybean stalk biochar after adsorbing hexavalent chromium is successively soaked in alkaline solution and acid solution for desorption, washed, and dried to obtain regenerated soybean stalk biochar.

2. The method for recycling soybean straw biochar according to claim 1, characterized in that, The carbonization process is carried out at a temperature of 600~800 ℃.

3. The method for recycling soybean stalk biochar according to claim 2, characterized in that, The carbonization process is carried out at a temperature of 800 °C.

4. The method for recycling soybean straw biochar according to claim 1, characterized in that, The protective gas is nitrogen, with a flow rate of 0.2 L / min, a heating rate of 10 ℃ / min, and a holding time of 2 h.

5. The method for recycling soybean straw biochar according to claim 1, characterized in that, The alkaline solution is a 0.1 mol / L NaOH solution, and the acid solution is a 0.1 mol / L HCl solution.

6. The method for recycling soybean straw biochar according to claim 1, characterized in that, The alkali solution soaking time is 12 hours, and the acid solution soaking time is 12 hours.

7. The method for recycling soybean straw biochar according to claim 1, characterized in that, The adsorption conditions were: pH=2, temperature 25 ℃, and adsorption time 10 h.

8. An application of soybean straw biochar in the removal of hexavalent chromium from water, characterized in that, Soybean straw biochar prepared by any one of claims 1 to 7 is added to water containing hexavalent chromium, and the pH is adjusted to acidic. After adsorption, hexavalent chromium is removed.

9. The application according to claim 8, characterized in that, The dosage of soybean straw biochar was 1 g / L, the pH was adjusted to 2, the adsorption time was 10 h, and the adsorption temperature was 25 ℃.

10. The application according to claim 8, characterized in that, After five adsorption-desorption cycles, the adsorption efficiency of the soybean straw biochar for hexavalent chromium remained above 87%.

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