Modified biochar for adsorbing heavy metal ions and preparation method thereof

Abstract

A method for preparing modified biochar for adsorbing heavy metal ions is provided, which uses rice straw as raw material, soaks it in a composite activator solution composed of magnesium chloride and glycolic acid in a mass ratio of 6 to 8:1, then pyrolyzes it at high temperature, and then performs ultrasonic treatment. The present invention uses magnesium chloride and glycolic acid to form a composite activator for soaking rice straw, so that the modified biochar prepared by pyrolysis of rice straw as raw material has an excellent specific surface area and rich pore structure, and a high content of magnesium ions is evenly distributed in the biochar. Ultrasonic treatment is performed after pyrolysis, which further increases the number and stability of functional groups formed in the biochar, thereby improving the modified biochar's adsorption of heavy metal ions Cd. 2+ The adsorption performance is excellent, and the saturated adsorption capacity reaches 143.68 mg / g.

Description

Technical Field

[0001] The present invention relates to the technical field of heavy metal contaminated water treatment, and in particular to a modified biochar capable of adsorbing heavy metal ions and a preparation method thereof. Background Art

[0002] Among various water pollution sources, heavy metal contamination is one of the main sources. Biochar, a carbon-rich material obtained by pyrolysis of biomass under anoxic or micro-oxic conditions, is an environmentally friendly adsorption material with widespread applications in wastewater treatment, soil improvement, and gas purification. However, raw biochar has limited adsorption capacity for cadmium, necessitating modification to improve its adsorption performance.

[0003] Existing technology ("Efficient removal of ammonium in aqueous solution by ultrasonic magnesium-modified biochar," Meitao Tan et al., Chemical Engineering Journal) prepares biochar through pyrolysis, then modifies it by soaking it in magnesium chloride, followed by ultrasonic treatment. Magnesium modification increases the functional groups of metal oxides, and ultrasonic treatment enhances the maximum ammonium adsorption capacity of both raw and magnesium biochar through electron attraction and ion exchange, respectively. This method for heavy metal ion adsorption is rarely reported. Magnesium modification of biochar can enhance its surface activity, anion fixation capacity, and ion exchange capacity, improving its cadmium adsorption performance. However, the cadmium adsorption performance of magnesium-modified biochar is primarily influenced by factors such as the biochar's specific surface area, pore structure, surface functional groups, and magnesium content. Summary of the Invention

[0004] The present invention aims to provide a modified biochar capable of adsorbing heavy metal ions.

[0005] Another object of the present invention is to provide a method for preparing the modified biochar. While increasing the specific surface area and pore structure of the biochar, the content of magnesium ions in the biochar is increased, and the uniform distribution and stability of magnesium ions in the biochar are promoted, more effective functional groups are generated, thereby improving the modified biochar's resistance to heavy metal Cd. 2+ adsorption performance.

[0006] The object of the present invention is achieved through the following technical solutions:

[0007] A modified biochar for adsorbing heavy metal ions is characterized in that: rice straw is used as raw material, soaked in a composite activator solution composed of magnesium chloride and glycolic acid in a mass ratio of 6 to 8:1, then pyrolyzed at high temperature, and then ultrasonically treated to obtain the modified biochar.

[0008] A method for preparing modified biochar for adsorbing heavy metal ions is characterized by using rice straw as raw material, soaking it in a composite activator solution composed of magnesium chloride and glycolic acid in a mass ratio of 6 to 8:1, then pyrolyzing it at high temperature, and then performing ultrasonic treatment.

[0009] Furthermore, in the composite activator solution, the concentration of magnesium chloride is 0.6-1.2 mol / L, and the soaking time is 20-24 hours.

[0010] Further preferably, in the composite activator solution, the concentration of magnesium chloride is 0.8 mol / L, and the soaking time is 24 h.

[0011] In the preparation of modified biochar using magnesium chloride, the prior art adopts pyrolysis first, then soaking biochar with magnesium chloride, and combining with ultrasonic treatment. In the present invention, rice straw is used as raw material to be pyrolyzed into biochar first, then soaking biochar with magnesium chloride, and finally ultrasonic treatment is performed. It is found that Mg in biochar 2+ The loading content is extremely low, and the number of metal oxide functional groups on the surface of biochar is small. Therefore, the specific surface area and pore structure of biochar are not ideal, resulting in poor adsorption performance of biochar for heavy metal ions. Therefore, we use magnesium chloride to soak the raw straw before pyrolysis. In the subsequent pyrolysis process, magnesium chloride plays the role of an activator, thereby changing the specific surface area and pore structure of biochar. Although the specific surface area and porosity of the biochar finally prepared are improved, the loading rate of metal ions is further reduced, and the adsorption performance for heavy metal ions is still not ideal.

[0012] In the present invention, magnesium chloride and glycolic acid are combined to form a composite activator, and the rice straw is soaked. In the subsequent pyrolysis activation pore-forming process, the composite activator forms pores in the straw raw material, and combines with the special pore structure formed in the straw to effectively retain magnesium chloride in the biochar pores and promote the MgCl2 2+ The formation of uniform distribution makes biochar form a large specific surface area and good pore structure while increasing the Mg 2+ The loading content and Mg 2 + The loading stability of the biochar was further increased during the subsequent ultrasonic treatment, and the broken carbon bonds in the biochar were further combined with the functional groups through ultrasound, which played a role in stabilizing the functional groups.

[0013] Furthermore, the ultrasonic treatment power is 200-300 W, the ultrasonic temperature is 35-45° C., and the ultrasonic time is 1-3 h.

[0014] More preferably, the ultrasonic treatment is performed at 45° C. for 0.5 h, then the ultrasonic temperature is lowered to 35° C. and ultrasonicated again for 0.5 to 2.5 h.

[0015] During the ultrasonic treatment process, the shear force and cavitation effect generated by ultrasound at high temperature promote the breaking of carbon bonds in biochar, and then under the action of ultrasound at a lower temperature, the metal magnesium ions are promoted to combine in the biochar to form more and more stable functional groups.

[0016] Furthermore, the pyrolysis temperature of the high-temperature pyrolysis is 400-700° C., and the pyrolysis time is 0.5-2 h.

[0017] Most specifically, a method for preparing modified biochar for adsorbing heavy metal ions is characterized by comprising the following steps:

[0018] S1. The rice straw was washed 2-3 times with deionized water, dried and cut into 2-3 cm segments, and then further crushed;

[0019] S2. The crushed rice straw was placed in a composite activator solution and soaked for 20-24 hours. After soaking, the rice straw powder was dried at 105 ° C. The composite activator was composed of magnesium chloride and glycolic acid in a mass ratio of 6 to 8:1. The concentration of magnesium chloride in the composite activator solution was 0.6 to 1.2 mol / L;

[0020] S3. Pyrolyze the dried rice straw at 400-700°C for 0.5-2 h. After cooling, wash with deionized water 2-3 times. Filter the resulting precipitate with 0.22 μm filter paper, dry it, and grind it through a 200-mesh sieve to obtain primary modified biochar.

[0021] S4. Add deionized water to the primary modified biochar and stir evenly, ultrasonically treat at 200-300W and a constant temperature of 35-45°C for 1-3 hours, filter and dry the mixture, grind it through a fine sieve to obtain ultrasonically combined magnesium modified biochar. Preferably, the ultrasonic treatment is first performed at 45°C for 0.5 hour, then the ultrasonic temperature is lowered to 35°C and ultrasonicated again for 0.5-2.5 hours.

[0022] The present invention has the following technical effects:

[0023] The present invention uses magnesium chloride and glycolic acid to form a composite activator to soak rice straw, so that the modified biochar prepared by pyrolysis of rice straw has excellent specific surface area and rich pore structure, and a high content of magnesium ions is evenly distributed in the biochar. Ultrasonic treatment is performed after pyrolysis to further increase the number and stability of functional groups formed in the biochar, thereby improving the modified biochar's resistance to heavy metal ions Cd 2+The adsorption performance is excellent, and the saturated adsorption capacity reaches 143.68 mg / g. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 : SEM image of ultrasound combined with magnesium modified biochar prepared by the present invention.

[0025] Figure 2 : Comparison of the adsorption performance of different biochars on cadmium ions.

[0026] Figure 3 : Adsorption effect of four biochars on cadmium ions by different modified biochars at different pH.

[0027] Figure 4 :The modified biochar UDMBC in the present invention has a great influence on Cd 2+ Adsorption fitting curve.

[0028] Figure 5 : UDMBC isothermal adsorption curve of modified biochar in the present invention. DETAILED DESCRIPTION

[0029] The present invention is described in detail below through examples. It is necessary to point out that the following examples are only used to further illustrate the present invention and are not to be construed as limiting the scope of protection of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention based on the above-mentioned contents of the present invention.

[0030] Example 1

[0031] A method for preparing modified biochar for adsorbing heavy metal cadmium comprises the following steps:

[0032] S1. The rice straw was washed three times with deionized water, dried, cut into 2-3 cm segments, and then further crushed;

[0033] S2. The crushed rice straw was placed in a composite activator solution and soaked for 24 hours. After soaking, the rice straw powder was dried at 105 ° C. The composite activator was composed of magnesium chloride and glycolic acid in a mass ratio of 7:1. The concentration of magnesium chloride in the composite activator solution was 0.8 mol / L;

[0034] S3. Pyrolyze the dried rice straw at 500°C for 1 h. After cooling, wash twice with deionized water. The resulting precipitate is filtered through 0.22 μm filter paper, dried, and ground through a 200-mesh sieve to obtain primary modified biochar.

[0035] S4. Add deionized water to the primary modified biochar and stir evenly. Ultrasonicate the mixture at a constant temperature of 35°C for 1 h at an ultrasonic power of 250 W. After the ultrasonication, filter and dry the mixture, grind it through a fine sieve, and obtain ultrasonically combined magnesium-modified biochar.

[0036] Control experiment:

[0037] In order to conduct an effective comparison of experimental effects, the modified biochar prepared in Example 1 was recorded as UDMBC. On the basis of Example 1, original biochar was prepared (without using an activator and ultrasonic means, the crushed rice straw was directly pyrolyzed), which was recorded as BC; compared with Example 1, no activator treatment was used, and the biochar prepared by only ultrasonic treatment after pyrolysis was recorded as UBC. Compared with Example 1, in the activator soaking treatment step, only magnesium chloride of equal concentration was used instead of a composite activator for soaking, and no ultrasonic treatment was performed after subsequent pyrolysis. The prepared biochar was recorded as MBC; compared with Example 1, the soaking was performed with a composite activator, and no ultrasonic treatment was performed after pyrolysis. The prepared biochar was recorded as MBC1. On the basis of the same activator treatment as MBC, ultrasonic treatment was performed after pyrolysis, and the prepared biochar was recorded as UMBC.

[0038] Mg in biochar 2+ Assay:

[0039] Determination of Mg in biochar using inductively coupled plasma optical emission spectrometer (ICP-OES: Agilent 5110) 2+ The results are shown in Table 1.

[0040] Table 1:

[0041]

[0042] From the above table, we can know that the Mg content in MBC treated with single magnesium chloride is 2+ The content of Mg is 12.36%. After further ultrasound, the 2+ Instead, it decreased, which indicates that ultrasound caused the attachment of unstable Mg 2+ In MBC1 obtained by the composite treatment of magnesium chloride and glycolic acid, Mg 2+ The content increased significantly, indicating that glycolic acid promoted the 2+ Adhesion in biochar, secondly, further ultrasound was performed on this basis, and it was found that Mg 2+ The content did not change, that is, glycolic acid not only increased Mg 2+ The adhesion rate of Mg 2+ The adsorption stability in biochar makes the subsequent adsorption of heavy metal ions Mg 2+It is stable and does not fall off, which improves the adsorption stability of biochar.

[0043] Adsorption performance test:

[0044] In 50mL of 160 mg / L Cd 2+ The solution was added to a 100 mL centrifuge tube and 50 mg of adsorbent was added for batch adsorption test. Figure 2 It can be seen that the modified biochar prepared from rice straw has a significant effect on Cd 2+ The order of adsorption capacity is as follows: in Example 1, ultrasound combined with composite activator ultrasound modified biochar UDMBC (143.68 mg / g) > composite activator soaked modified biochar MBC1 (118.32 mg / g) > ultrasound combined with magnesium chloride soaked modified biochar UMBC (102.71 mg / g) > magnesium chloride soaked modified biochar MBC (89.50 mg / g) > pyrolysis ultrasonic modified biochar UBC (52.12 mg / g) > original biochar BC (37.80 mg / g). In this process, the biochar prepared by soaking the raw material with equal concentration of glycolic acid and ultrasonic treatment after pyrolysis had a higher adsorption capacity for Cd. 2+ The adsorption capacity is only 39.83 mg / g, which is not much different from the original biochar.

[0045] It can be seen that the biochar prepared by magnesium chloride modification alone has a lower Cd 2+ The adsorption performance of biochar was significantly improved. When ultrasound was applied on this basis, the adsorption performance of biochar was further improved. 2+ The adsorption performance of biochar was significantly improved compared with that of biochar modified with magnesium chloride alone and biochar modified with magnesium chloride and ultrasonic treatment. 2+ The adsorption performance was further improved, and the degree of improvement was more obvious than that of the ultrasound combined with single magnesium chloride modification group (UMBC).

[0046] (1) Adsorption tests of four biochars, MBC, MBC1, UMBC and UDMBC, at different pH values:

[0047] 50 mg of adsorbent was mixed with 50 mL of 160 mg / L Cd 2+ The solution was added to a 100 mL centrifuge tube for batch adsorption experiments, and the pH of the solution was adjusted to 3, 4, 5, 6, 7, and 8. The solution was shaken on a shaker at 160 rpm and 25°C for 1440 min. The solution was filtered through a 0.45 μm filter membrane, and the Cd content in the filtrate was measured by flame atomic absorption spectroscopy (FAAS) at 277.7 nm. 2+ concentration.

[0048] Effects of four biochars, MBC, MBC1, UMBC and UDMBC, on Cd at different pH values 2+ The adsorption efficiency is shown in the attached Figure 3 As shown in the figure, at pH 3, the adsorption capacities of MBC, MBC1, UMBC, and UDMBC are relatively low. When the pH value is between 4 and 7, the adsorption efficiency and removal rate of UDMBC significantly increase, with the maximum removal rate reaching 89.27%. At pH 7, the maximum adsorption capacity of UDMBC reaches 142.83 mg / g.

[0049] (2) Adsorption kinetics model

[0050] 50 mg of adsorbent was mixed with 50 mL of 50 mg / L Cd 2+ The solution was added to a 100 mL centrifuge tube for batch adsorption experiments, and the pH of the solution was adjusted to 7. The mixture was shaken at 160 rpm and 25°C for 0-1440 min. The filtrate was filtered through a 0.45 μm filter membrane, and the Cd content in the filtrate was measured by flame atomic absorption spectroscopy (FAAS) at 277.7 nm. 2+ concentration.

[0051] The adsorption process was fitted using pseudo-first-order kinetic model and pseudo-second-order kinetic model. Figure 4 The pseudo-second-order kinetic model of UDMBC has a higher fitting degree for the adsorption process, and the results show that the adsorption of Cd 2+ The mechanism of Cd is mainly controlled by the binding force between the adsorbate and the adsorbent when sharing or exchanging ions / electrons. 2+ Adsorption onto biochar is likely controlled by chemisorption.

[0052] (3) Adsorption isotherm:

[0053] 50 mg of adsorbent was mixed with 50 mL of Cd solution with a concentration of 10–200 mg / L. 2+ The solution was added to a 100 mL centrifuge tube for batch adsorption experiments. The mixture was shaken at 160 rpm and 25°C for 1440 min. The filtrate was filtered through a 0.45 μm filter membrane and the Cd content in the filtrate was measured by flame atomic absorption spectroscopy (FAAS) at 277.7 nm. 2+ concentration.

[0054] Draw the isothermal adsorption curve, see attached Figure 5 , according to the R of the two adsorption isotherm models 2 The Langmuir adsorption isotherm model for Cd 2+ The adsorption has a higher R 2 This indicates that Cd 2+The main adsorption onto these biochars was due to the uniform energy surface, which was a monolayer adsorption rather than a multi-layer heterogeneous adsorption mode. The Langmuir isotherm model estimated the maximum adsorption capacity of UMBC to be 180.5 mg / g, and the Langmuir isotherm model estimated the maximum adsorption capacity of UDMBC to be 314.05 mg / g.

[0055] The above examples and comparative experiments show that the present invention combines ultrasound with magnesium-modified biochar to remove Cd 2+ The adsorption capacity of the biochar was higher than that of the original biochar and the single modified biochar. Therefore, ultrasound combined with magnesium modification can be used as an effective material to remove Cd from aqueous solution. 2+ , and has broad application prospects in the field of heavy metal polluted water purification.

[0056] Example 2

[0057] A method for preparing modified biochar for adsorbing heavy metal cadmium comprises the following steps:

[0058] S1. The rice straw was washed twice with deionized water, dried and cut into 2-3 cm segments, and then further crushed;

[0059] S2. The crushed rice straw was placed in a composite activator solution and soaked for 20 hours. After soaking, the rice straw powder was dried at 105 ° C. The composite activator was composed of magnesium chloride and glycolic acid in a mass ratio of 8:1. The concentration of magnesium chloride in the composite activator solution was 0.6 mol / L;

[0060] S3. Pyrolyze the dried rice straw at 700°C for 0.5 h. After cooling, wash twice with deionized water. The resulting precipitate is filtered through 0.22 μm filter paper, dried, and ground through a 200-mesh sieve to obtain primary modified biochar.

[0061] S4. Deionized water was added to the primary modified biochar and stirred evenly. The mixture was ultrasonically treated at 200 W and a constant temperature of 45°C for 1.5 h. After the ultrasonic treatment, the mixture was filtered, dried, and ground through a fine sieve to obtain ultrasonically combined magnesium-modified biochar.

[0062] The modified biochar prepared in this example was heated to 50 mL and 160 mg / L Cd 2+ In ionic solution, Cd 2+ The saturated adsorption capacity reached 138.7 mg / g.

[0063] Comparative Example 1

[0064] Compared with Example 2, the composite activator used was equimolar hydrochloric acid instead of glycolic acid, and the remaining steps were the same as in Example 2.

[0065] Mg prepared in Comparative Example 1 2+ The loading content is 13.09%. 2+ The saturated adsorption capacity of Mg is 108.39 mg / g. At the same time, other acids such as phosphoric acid and acetic acid were also tried, but they did not significantly increase the adsorption capacity of Mg. 2+ Loading content in biochar.

[0066] Example 3

[0067] A method for preparing modified biochar for adsorbing heavy metal cadmium comprises the following steps:

[0068] S1. The rice straw was washed three times with deionized water, dried, cut into 2-3 cm segments, and then further crushed;

[0069] S2. The crushed rice straw was placed in a composite activator solution and soaked for 22 hours. After soaking, the rice straw powder was dried at 105 ° C. The composite activator was composed of magnesium chloride and glycolic acid in a mass ratio of 6:1. The concentration of magnesium chloride in the composite activator solution was 1.2 mol / L;

[0070] S3. Pyrolyze the dried rice straw at 400°C for 2 h. After cooling, wash the resulting precipitate three times with deionized water. Filter the precipitate with 0.22 μm filter paper, dry it, and grind it through a 200-mesh sieve to obtain primary modified biochar.

[0071] S4. Add deionized water to the primary modified biochar and stir evenly. Ultrasonicate at 300W and 40°C for 3 h. After the ultrasonication, filter and dry the mixture, grind it through a fine sieve to obtain ultrasonically combined magnesium-modified biochar.

[0072] The modified biochar prepared in this example was heated to 50 mL and 160 mg / L Cd 2+ In ionic solution, Cd 2+ The saturated adsorption capacity reached 140.27 mg / g.

[0073] During the ultrasonic treatment process, it was also found that in the ultrasonic system of the present invention, the modified biochar prepared at an ultrasonic power of 200-300 W showed a trend of gradually increasing with the ultrasonic power. After exceeding 300 W, the biochar obtained by the treatment showed a trend of gradually increasing with the ultrasonic power. 2+ The adsorption performance showed a slow downward trend.

[0074] Example 4

[0075] A method for preparing modified biochar for adsorbing heavy metal cadmium comprises the following steps:

[0076] S1. The rice straw was washed three times with deionized water, dried, cut into 2-3 cm segments, and then further crushed;

[0077] S2. The crushed rice straw was placed in a composite activator solution and soaked for 24 hours. After soaking, the rice straw powder was dried at 105 ° C. The composite activator was composed of magnesium chloride and glycolic acid in a mass ratio of 7:1. The concentration of magnesium chloride in the composite activator solution was 0.8 mol / L;

[0078] S3. Pyrolyze the dried rice straw at 500°C for 1 h. After cooling, wash twice with deionized water. The resulting precipitate is filtered through 0.22 μm filter paper, dried, and ground through a 200-mesh sieve to obtain primary modified biochar.

[0079] S4. Add deionized water to the primary modified biochar and stir evenly. First, ultrasonicate at 45°C for 0.5 h, then reduce the ultrasonic temperature to 35°C and ultrasonicate again for 0.5-2.5 h. The ultrasonic power is constant at 250 W. After the ultrasonication, filter and dry the mixture, grind it through a fine sieve to obtain ultrasonically combined magnesium-modified biochar.

[0080] The modified biochar prepared in this example was heated to 50 mL and 160 mg / L Cd 2+ In ionic solution, Cd 2+ The saturated adsorption capacity reached 141.28 mg / g.

Claims

1. A method for preparing modified biochar for adsorbing heavy metal ions, characterized by: The method uses rice straw as raw material, soaks it in a composite activator solution composed of magnesium chloride and glycolic acid, then performs high-temperature pyrolysis, and then performs ultrasonic treatment. In the composite activator solution, the mass ratio of magnesium chloride to glycolic acid is 6-8:1, the concentration of magnesium chloride is 0.6-1.2 mol / L, the soaking time is 20-24 hours, the ultrasonic treatment power is 200-300W, the ultrasonic temperature is 35-45°C, the ultrasonic time is 1-3 hours, and the high-temperature pyrolysis temperature is 400-700°C, and the pyrolysis time is 0.5-2 hours.

2. The method for preparing modified biochar for adsorbing heavy metal ions according to claim 1, wherein: The ultrasonic treatment is performed at 45° C. for 0.5 h, then the ultrasonic temperature is lowered to 35° C. and ultrasonicated again for 0.5 to 2.5 h.

3. A method for preparing modified biochar for adsorbing heavy metal ions, characterized in that: The steps include: S1. Wash the rice straw with deionized water 2 to 3 times, dry it, cut it into 2 to 3 cm segments, and then further crush it; S2. The crushed rice straw was placed in a composite activator solution and soaked for 20 to 24 hours. After soaking, the rice straw powder was dried at 105°C. The composite activator was composed of magnesium chloride and glycolic acid in a mass ratio of 6 to 8:

1. The concentration of magnesium chloride in the composite activator solution was 0.6 to 1.2 mol / L. S3. Pyrolyze the dried rice straw at 400-700°C for 0.5-2 h. After cooling, wash with deionized water 2-3 times. Filter the resulting precipitate with 0.22 μm filter paper, dry it, and grind it through a 200-mesh sieve to obtain primary modified biochar. S4. Add deionized water to the primary modified biochar and stir evenly. Heat at 200-300W and 35-45℃. Ultrasonic treatment was performed at a constant temperature for 1 to 3 h, and the mixture was filtered, dried, and ground through a fine sieve to obtain ultrasound-combined magnesium-modified biochar.

4. The method for preparing modified biochar for adsorbing heavy metal ions according to claim 3, wherein: The ultrasonic treatment is first performed at 45° C. for 0.5 h, then the ultrasonic temperature is lowered to 35° C. and ultrasonicated again for 0.5 to 2.5 h.

Images