Step-by-step preparation method and application of multi-metal modified biochar

Abstract

The invention discloses a step-by-step preparation method and application of multi-metal modified biochar. According to the method, through an innovative four-stage step-by-step loading process, four metal salt solutions of FeCl3, MnCl2, CaCl2 and MgCl2 are sequentially used for carrying out modification treatment on the biochar, and the Fe-Mn-Ca-Mg quaternary composite modified biochar is successfully prepared. The method is mainly technically characterized by comprising the following steps: (1) pretreating rice straws by adopting a three-stage countercurrent cleaning and gradient drying process; (2) performing programmed temperature control pyrolysis under the protection of nitrogen to prepare basic biochar; and (3) the sequential loading of the four metal ions is realized by accurately controlling reaction conditions. Experimental results show that the adsorption capacity of the prepared quaternary composite modified biochar on As in a water body is increased by 2 times compared with that of original biochar, the removal rates of Cd, Cu and Pb in the water body all exceed 90%, and the removal rates of Ni and Hg in the water body are increased by 10-15%. The material shows excellent synergistic adsorption performance in a multi-metal coexistence system, and an efficient solution is provided for complex heavy metal pollution abatement.

Description

Technical Field

[0001] The present invention belongs to the technical fields of agricultural solid waste resource utilization and environmental pollution control, and particularly relates to a step-by-step preparation method for multi-element metal-modified biochar and its application. The method is particularly suitable for the high-value utilization of biomass raw materials such as rice straw and the remediation of heavy metal-contaminated water bodies. Background Art

[0002] In recent years, the technology of converting rice straw into biochar through pyrolysis has attracted widespread attention. This thermochemical conversion technology has dual advantages: on the one hand, it can significantly reduce the volume of agricultural waste, and on the other hand, it can efficiently convert it into high-value-added products, including bioenergy carriers (such as bio-oil and pyrolysis gas) and carbon-rich solid products (i.e., biochar). Rice straw-derived biochar has the following significant characteristics: (1) high pH value; (2) developed porous structure; (3) rich ash and mineral content; (4) diverse surface functional groups. These characteristics make it show good application prospects in the fields of water purification and soil improvement, specifically: it can effectively adsorb heavy metal pollutants in water, improve soil physical and chemical properties and biological activity, enhance soil fertility, promote the immobilization of heavy metals to achieve soil remediation, and reduce greenhouse gas emissions through carbon sequestration.

[0003] However, existing studies have found that rice straw biochar still has the following technical bottlenecks in practical applications: first, the adsorption capacity for anionic metal elements (such as arsenic, As) is limited; second, it is difficult to achieve synergistic adsorption of multi-metal coexisting systems (such as Ni, Cu, As, Cd, Pb, etc.). These limitations seriously restrict its remediation effect in complex polluted environments (such as water bodies or soils with multi-metal composite pollution). At present, there are still deficiencies in the research on multi-metal modified biochar: (1) Most modification methods use a single metal or simultaneous composite modification, which makes it difficult to accurately control the loading mode of different metals on the biochar surface; (2) There is a lack of systematic modification process optimization, especially the effect of step-by-step modification strategy on the physical and chemical properties and adsorption performance of biochar is still unclear; (3) The synergistic adsorption behavior and actual remediation effect of modified biochar in multi-metal composite pollution systems still need to be further verified.

[0004] To address the above problems, it is urgent to develop an efficient and controllable step-by-step modification method to prepare composite modified biochar with excellent multi-metal adsorption performance and clarify its application potential in water body remediation. Summary of the Invention

[0005] The present invention aims to address the technical bottlenecks of existing biochar materials in heavy metal pollution control, such as poor adsorption selectivity and low multi-metal synergistic removal efficiency, by providing a step-by-step preparation method and application of multi-metal-modified biochar. By precisely controlling the sequential loading of four metal modifiers (Fe, Mn, Ca, and Mg), this method constructs multi-level adsorption sites, significantly enhancing the biochar's synergistic adsorption capacity for both anionic and cationic heavy metals. In particular, it overcomes the key technical challenge of conventional biochar's insufficient adsorption capacity for anionic heavy metals such as As.

[0006] In order to achieve the above objectives, the technical solution of the present invention is:

[0007] A step-by-step preparation method of multi-metal modified biochar is carried out according to the following steps:

[0008] (1) Raw material pretreatment: The rice straw is washed in three stages using tap water, deionized water and ultrapure water in sequence. During each stage of washing, the amount of tap water, deionized water and ultrapure water is sufficient to completely immerse the straw and be 4-12 cm above its surface. At the same time, a glass rod is used to gently stir to ensure that impurities on the surface of the straw are fully dissolved or dispersed in the water; the tap water is washed 2-4 times, each time for 12-25 minutes, until the washing liquid is not obviously turbid by naked eye observation; the deionized water is washed 2-3 times, each time for 8-20 minutes; the ultrapure water is washed 1 -2 times, soaking for 4-12 minutes, and the conductivity of the washing liquid is not higher than 1μS / cm to ensure that impurities on the surface of the straw are fully removed; after washing, a three-stage gradient drying is performed, the first stage of gradient drying is natural air drying at 20-30°C and a relative humidity of 35-45% for 46-50 hours, the second stage of gradient drying is low-temperature drying at 38-45°C for 10-14 hours, and the third stage of gradient drying is drying at 70-80°C to constant weight, and finally crushed through a 0.5-2mm sieve to obtain a homogeneous pretreated raw material;

[0009] (2) Biochar preparation: The pretreated raw material was heated to 490-510°C at 8-12°C / min under nitrogen protection, maintained for 1.5-2.5 h, and then cooled to obtain the original biochar BC;

[0010] (3) Multi-component composite modification:

[0011] ①Fe modification: The original biochar BC was mixed with 0.08-0.12 mol / L FeCl3 solution at a solid-liquid ratio of 1 g:90-110 mL, shaken at 23-27 ° C and 180-220 rpm for 1.8-2.2 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 100-110 ° C for 22-26 h to obtain Fe-BC;

[0012] ②Mn modification: Fe-BC was mixed with 0.08-0.12 mol / L MnCl2 solution at a solid-liquid ratio of 1 g:90-110 mL, and the mixture was shaken at 23-27°C and 180-220 rpm for 1.8-2.2 hours. The mixture was filtered and washed until the conductivity of the filtrate was <50 μS / cm, and then dried at 100-110°C for 22-26 hours to obtain Fe-Mn-BC.

[0013] ③Ca modification: Fe-Mn-BC was mixed with 0.08-0.12 mol / L CaCl2 solution at a solid-liquid ratio of 1 g:90-110 mL, and the mixture was shaken at 23-27°C and 180-220 rpm for 1.8-2.2 h. The mixture was filtered and washed until the conductivity of the filtrate was <50 μS / cm, and then dried at 100-110°C for 22-26 h to obtain Fe-Mn-Ca-BC.

[0014] ④Mg modification: Mix Fe-Mn-Ca-BC with 0.08-0.12 mol / L MgCl2 solution at a solid-liquid ratio of 1 g:90-110 mL, oscillate at 23-27 °C and 180-220 rpm for 1.8-2.2 h, filter and wash until the filtrate conductivity is <50 μS / cm, and dry at 100-110 °C for 22-26 h to obtain multi-metal modified biochar Fe-Mn-Ca-Mg-BC.

[0015] In the above step (1), the raw material is pretreated: the rice straw is washed in three stages with tap water, deionized water and ultrapure water in sequence. During each stage of washing, the amount of tap water, deionized water and ultrapure water is enough to completely immerse the straw and be 5-10 cm above its surface. At the same time, a glass rod is used to gently stir to ensure that the impurities on the surface of the straw are fully dissolved or dispersed in the water; the tap water is washed three times, each time for 15-20 minutes, until the washing liquid is not obviously turbid by naked eye observation; the deionized water is washed twice, each time for 10-1 5 minutes; wash once with ultrapure water, soak for 5-10 minutes, and the conductivity of the washing liquid is not higher than 1μS / cm to ensure that impurities on the surface of the straw are fully removed; after washing, carry out three-stage gradient drying, the first stage of gradient drying is natural air drying at 23-27℃ and relative humidity of 40% for 48 hours, the second stage of gradient drying is low-temperature drying at 40℃ for 12 hours, and the third stage of gradient drying is drying at 75℃ to constant weight, and finally crushed through a 0.5-2mm sieve to obtain a homogeneous pretreated raw material.

[0016] In the above step (2), biochar preparation: the pretreated raw material is heated to 495-505°C at 9-11°C / min under nitrogen protection, maintained for pyrolysis for 1.8-2.2h, and cooled to obtain raw biochar BC.

[0017] Specifically, in the aforementioned step (2), biochar preparation: the pretreated raw material was heated to 500°C at 10°C / min under nitrogen protection, maintained for pyrolysis for 2 hours, and then cooled to obtain raw biochar BC.

[0018] In the aforementioned step (3), multi-component composite modification:

[0019] ①Fe modification: The original biochar BC was mixed with 0.1 mol / L FeCl3 solution at a solid-liquid ratio of 1 g:100 mL, shaken at 25°C and 200 rpm for 2 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 105°C for 24 h to obtain Fe-BC;

[0020] ②Mn modification: Fe-BC was mixed with 0.1 mol / L MnCl2 solution at a solid-liquid ratio of 1 g:100 mL, shaken at 25°C and 200 rpm for 2 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 105°C for 24 h to obtain Fe-Mn-BC;

[0021] ③Ca modification: Fe-Mn-BC was mixed with 0.1 mol / L CaCl2 solution at a solid-liquid ratio of 1 g:100 mL, shaken at 25°C and 200 rpm for 2 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 105°C for 24 h to obtain Fe-Mn-Ca-BC;

[0022] ④Mg modification: Fe-Mn-Ca-BC was mixed with 0.1 mol / L MgCl2 solution at a solid-liquid ratio of 1 g:100 mL, shaken at 25 °C and 200 rpm for 2 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 105 °C for 24 h to obtain multi-metal modified biochar Fe-Mn-Ca-Mg-BC.

[0023] The aforementioned multi-metal modified biochar is used in the treatment of heavy metal water pollution. The multi-metal modified biochar Fe-Mn-Ca-Mg-BC can adsorb Ni, Cu, As, Cd and / or Pb in water bodies, achieving the purpose of treating heavy metal water pollution.

[0024] The solid-liquid ratio of the aforementioned Fe-Mn-Ca-Mg-BC and water is 1 g:100 mL, and the treatment time is 24 h.

[0025] The concentrations of Ni, Cu, As, Cd and Pb in the aforementioned heavy metal-polluted water bodies reached 100 mg / L.

[0026] The technical solution of the present invention has the following beneficial effects:

[0027] 1. Compared to existing single-stage biochar production technologies, this pioneering "Fe-Mn-Ca-Mg" four-stage step-by-step loading process creates multi-level adsorption sites through the synergistic action of metal ions. A gradient drying-programmed pyrolysis technique was developed to ensure the structural integrity of the raw materials, while a standardized post-processing process was established to guarantee the stability of the modified products. This technology is suitable for deep purification of complex polluted water bodies and for the high-value utilization of agricultural waste. It provides a highly efficient and reliable new adsorption material for heavy metal pollution control and has broad application prospects in environmental remediation.

[0028] 2. The multi-metal modified biochar Fe-Mn-Ca-Mg-BC prepared in steps has an adsorption capacity for As that of the original biochar twice as high, and the removal rate of Cd, Cu, and Pb is >90%, while the removal rate of Ni and Hg is increased by 10-15%. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The adsorption efficiency of multi-metal modified biochar Fe-Mn-Ca-Mg-BC on different metal elements in water. DETAILED DESCRIPTION

[0030] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. The present invention will be further described below with reference to the examples. The examples are only for further supplementation and explanation of the present invention, rather than limitation of the present invention.

[0031] Example 1:

[0032] Preparation of multi-metal modified biochar:

[0033] (1) Raw material pretreatment: The rice straw was washed in three stages using tap water, deionized water, and ultrapure water. During each stage of washing, the amount of tap water, deionized water, and ultrapure water was sufficient to completely immerse the straw and to be 5-10 cm above its surface. A glass rod was used to gently stir the straw to ensure that impurities on the straw surface were fully dissolved or dispersed in the water. The straw was washed with tap water three times, each time for 15-20 minutes, until the wash solution showed no obvious turbidity when observed with the naked eye. The straw was washed with deionized water twice, each time for 10-15 minutes. ; Wash once with ultrapure water, soak for 5-10 minutes, and the conductivity of the washing liquid is not higher than 1μS / cm to ensure that impurities on the surface of the straw are fully removed; after washing, carry out three-stage gradient drying, the first stage of gradient drying is natural air drying at 23-27℃ and relative humidity of 40% for 48 hours, the second stage of gradient drying is low-temperature drying at 40℃ for 12 hours, and the third stage of gradient drying is drying at 75℃ to constant weight, and finally crushed through a 0.5-2mm sieve to obtain homogeneous pretreated raw materials.

[0034] (2) Biochar preparation: The pretreated raw material was heated to 500 °C at 10 °C / min under nitrogen protection, maintained for 2 h, and then cooled to obtain the original biochar BC.

[0035] (3) Multi-component composite modification:

[0036] ①Fe modification: The original biochar BC was mixed with 0.1 mol / L FeCl3 solution at a solid-liquid ratio of 1 g:100 mL, shaken at 25°C and 200 rpm for 2 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 105°C for 24 h to obtain Fe-BC;

[0037] ②Mn modification: Fe-BC was mixed with 0.1 mol / L MnCl2 solution at a solid-liquid ratio of 1 g:100 mL, shaken at 25°C and 200 rpm for 2 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 105°C for 24 h to obtain Fe-Mn-BC;

[0038] ③Ca modification: Fe-Mn-BC was mixed with 0.1 mol / L CaCl2 solution at a solid-liquid ratio of 1 g:100 mL, shaken at 25°C and 200 rpm for 2 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 105°C for 24 h to obtain Fe-Mn-Ca-BC;

[0039] ④Mg modification: Fe-Mn-Ca-BC was mixed with 0.1 mol / L MgCl2 solution at a solid-liquid ratio of 1 g:100 mL, shaken at 25 °C and 200 rpm for 2 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 105 °C for 24 h to obtain multi-metal modified biochar Fe-Mn-Ca-Mg-BC.

[0040] Example 2:

[0041] Preparation of multi-metal modified biochar:

[0042] (1) Raw material pretreatment: The rice straw is washed in three stages with tap water, deionized water and ultrapure water in sequence. During each stage of washing, the amount of tap water, deionized water and ultrapure water is enough to completely immerse the straw and be 4-8 cm above its surface. At the same time, it is gently stirred with a glass rod to ensure that impurities on the surface of the straw are fully dissolved or dispersed in the water; the tap water is washed twice, each time for 12-20 minutes, until the washing liquid has no obvious turbidity when observed with the naked eye; the deionized water is washed twice, each time for 15-20 minutes; the ultrapure water is washed twice, each time for 6-12 minutes, and the conductivity of the washing liquid is not higher than 1μS / cm to ensure that impurities on the surface of the straw are fully removed; after washing, three-stage gradient drying is carried out, the first stage of gradient drying is natural air drying at 20℃ and relative humidity 35% for 50h, the second stage of gradient drying is low-temperature drying at 45℃ for 10h, and the third stage of gradient drying is drying at 70℃ to constant weight, and finally crushed through a 0.5-2mm sieve to obtain a homogeneous pretreated raw material;

[0043] (2) Biochar preparation: The pretreated raw material was heated to 510 °C at 12 °C / min under nitrogen protection, maintained for 2.5 h, and then cooled to obtain the original biochar BC;

[0044] (3) Multi-component composite modification:

[0045] ①Fe modification: The original biochar BC was mixed with 0.12 mol / L FeCl3 solution at a solid-liquid ratio of 1 g:90 mL, shaken at 23 °C and 180 rpm for 1.8 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 110 °C for 22 h to obtain Fe-BC;

[0046] ②Mn modification: Fe-BC was mixed with 0.12 mol / L MnCl2 solution at a solid-liquid ratio of 1 g:90 mL, shaken at 23°C and 180 rpm for 1.8 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 110°C for 22 h to obtain Fe-Mn-BC;

[0047] ③Ca modification: Fe-Mn-BC was mixed with 0.12 mol / L CaCl2 solution at a solid-liquid ratio of 1 g:90 mL, shaken at 23°C and 180 rpm for 1.8 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 110°C for 22 h to obtain Fe-Mn-Ca-BC;

[0048] ④Mg modification: Fe-Mn-Ca-BC was mixed with 0.12 mol / L MgCl2 solution at a solid-liquid ratio of 1 g:90 mL, shaken at 23 °C and 180 rpm for 2.2 h, filtered and washed until the filtrate conductivity

[0049] After the temperature dropped below 50 μS / cm, the biochar was dried at 110 °C for 22 h to obtain the multi-metal modified biochar Fe-Mn-Ca-Mg-BC.

[0050] Example 3:

[0051] Preparation of multi-metal modified biochar:

[0052] (1) Raw material pretreatment: The rice straw is washed in three stages with tap water, deionized water and ultrapure water in sequence. During each stage of washing, the amount of tap water, deionized water and ultrapure water is enough to completely immerse the straw and be 8-12 cm above its surface. At the same time, it is gently stirred with a glass rod to ensure that impurities on the surface of the straw are fully dissolved or dispersed in the water; the tap water is washed 4 times, each time for 18-25 minutes, until the washing liquid has no obvious turbidity when observed with the naked eye; the deionized water is washed 3 times, each time for 8-16 minutes; the ultrapure water is washed once, each time for 4-8 minutes, and the conductivity of the washing liquid is not higher than 1μS / cm, to ensure that impurities on the surface of the straw are fully removed; after washing, the three-stage gradient drying is carried out. The first stage of gradient drying is natural air drying at 30℃ and relative humidity of 45% for 46 hours, the second stage of gradient drying is low-temperature drying at 38℃ for 14 hours, and the third stage of gradient drying is drying at 80℃ to constant weight. Finally, it is crushed and passed through a 0.5-2mm sieve to obtain a homogeneous pretreated raw material;

[0053] (2) Biochar preparation: The pretreated raw material was heated to 490 °C at 8 °C / min under nitrogen protection, maintained for 1.5 h, and then cooled to obtain the original biochar BC;

[0054] (3) Multi-component composite modification:

[0055] ①Fe modification: The original biochar BC was mixed with 0.08 mol / L FeCl3 solution at a solid-liquid ratio of 1 g:110 mL, shaken at 27 °C and 220 rpm for 2.2 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 100 °C for 26 h to obtain Fe-BC;

[0056] ②Mn modification: Fe-BC was mixed with 0.08 mol / L MnCl2 solution at a solid-liquid ratio of 1 g:110 mL, shaken at 27°C and 220 rpm for 2.2 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 100°C for 26 h to obtain Fe-Mn-BC;

[0057] ③Ca modification: Fe-Mn-BC was mixed with 0.08 mol / L CaCl2 solution at a solid-liquid ratio of 1 g:110 mL, shaken at 27°C and 220 rpm for 2.2 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 100°C for 26 h to obtain Fe-Mn-Ca-BC;

[0058] ④Mg modification: Fe-Mn-Ca-BC was mixed with 0.08 mol / L MgCl2 solution at a solid-liquid ratio of 1 g:110 mL, shaken at 27 ° C and 220 rpm for 2.2 h, filtered and washed until the filtrate conductivity

[0059] After the carbon dioxide concentration was less than 50 μS / cm, the biochar was dried at 100 °C for 26 h to obtain multi-metal modified biochar Fe-Mn-Ca-Mg-BC.

[0060] Example 4:

[0061] Preparation of multi-metal modified biochar:

[0062] (1) Raw material pretreatment: The rice straw is washed in three stages with tap water, deionized water and ultrapure water in sequence. During each stage of washing, the amount of tap water, deionized water and ultrapure water is enough to completely immerse the straw and be 5-10 cm above its surface. At the same time, it is gently stirred with a glass rod to ensure that impurities on the surface of the straw are fully dissolved or dispersed in the water; the tap water is washed three times, each time for 15-20 minutes, until the washing liquid has no obvious turbidity when observed with the naked eye; the deionized water is washed twice, each time for 10-15 minutes; the ultrapure water is washed once, each time for 5-10 minutes, and the conductivity of the washing liquid is not higher than 1μS / cm to ensure that impurities on the surface of the straw are fully removed; after washing, the three-stage gradient drying is carried out. The first stage of gradient drying is natural air drying at 25℃ and relative humidity 38% for 48 hours, the second stage of gradient drying is low-temperature drying at 40℃ for 11 hours, and the third stage of gradient drying is drying at 75℃ to constant weight. Finally, it is crushed and passed through a 0.5-2mm sieve to obtain a homogeneous pretreated raw material;

[0063] (2) Biochar preparation: The pretreated raw material was heated to 500 °C at 9 °C / min under nitrogen protection, maintained for 1.8 h, and then cooled to obtain the original biochar BC;

[0064] (3) Multi-component composite modification:

[0065] ①Fe modification: The original biochar BC was mixed with 0.09 mol / L FeCl3 solution at a solid-liquid ratio of 1 g:100 mL, shaken at 26°C and 200 rpm for 1.9 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 100°C for 25 h to obtain Fe-BC;

[0066] ②Mn modification: Fe-BC was mixed with 0.09 MnCl2 solution at a solid-liquid ratio of 1 g:110 mL, shaken at 24°C and 190 rpm for 2 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 105°C for 23 h to obtain Fe-Mn-BC;

[0067] ③Ca modification: Fe-Mn-BC was mixed with 0.11 CaCl2 solution at a solid-liquid ratio of 1 g:95 mL, shaken at 24°C and 190 rpm for 2 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 105°C for 22 h to obtain Fe-Mn-Ca-BC;

[0068] ④Mg modification: Fe-Mn-Ca-BC was mixed with 0.10 MgCl2 solution at a solid-liquid ratio of 1 g:100 mL, shaken at 27 °C and 210 rpm for 2.1 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 100 °C for 24 h to obtain multi-metal modified biochar Fe-Mn-Ca-Mg-BC.

[0069] In order to obtain the solution of the present invention and verify the technical effect of the present invention, the inventors conducted a large number of experimental studies, some of which are recorded as follows:

[0070] The rice straw is sequentially washed in three stages using tap water, deionized water, and ultrapure water. During each stage of washing, the amount of tap water, deionized water, and ultrapure water used is sufficient to completely immerse the straw and to be 5-10 cm above its surface. The straw is gently stirred with a glass rod to ensure that impurities on the surface of the straw are fully dissolved or dispersed in the water. The rice straw is washed with tap water three times, each time for 15-20 minutes, until the washing liquid is not obviously turbid when observed with the naked eye. The rice straw is washed with deionized water twice, each time for 10-15 minutes. The rice straw is washed with ultrapure water once, each time for 5-10 minutes, and the conductivity of the washing liquid is not higher than 1 μS / cm, so as to ensure that impurities on the surface of the straw are fully removed. After washing, the rice straw is subjected to three-stage gradient drying. The first stage of gradient drying is natural air drying at 23-27° C. and relative humidity of 40% for 48 hours. The second stage of gradient drying is low-temperature drying at 40° C. for 12 hours. The third stage of gradient drying is drying at 75° C. to constant weight. The rice straw is finally crushed and passed through a 0.5-2 mm sieve to obtain a homogeneous pretreated raw material.

[0071] The pretreated raw material was placed in a tubular pyrolysis furnace and heated to 500°C at a rate of 10°C / min under nitrogen protection for two hours for slow pyrolysis. After the pyrolysis was completed, the furnace temperature was naturally cooled to below room temperature and the product was removed to obtain raw biochar (BC).

[0072] The four metal elements were loaded sequentially using a step-by-step impregnation method: 10g of raw biochar was mixed with 1L of a 0.1mol / L FeCl3 solution and shaken at 25°C and 200rpm for 2h. The mixture was filtered, washed with deionized water multiple times to remove excess salt, and dried at 105°C for 24h to obtain Fe-loaded biochar (Fe-BC). The Fe-loaded biochar was then placed in a 0.1mol / L manganese chloride solution, and the impregnation, filtration, washing, and drying steps were repeated to produce the modified biochar loaded with Mn (Fe-Mn-BC). The Fe- and Mn-loaded biochar was then placed in a 0.1mol / L calcium chloride solution, and the process was repeated again to produce the Ca-loaded modified biochar (Fe-Mn-Ca-BC). Finally, the Fe-, Mn-, and Ca-loaded biochar was placed in a 0.1mol / L magnesium chloride solution to complete the Mg-loaded modified biochar (Fe-Mn-Ca-Mg-BC).

[0073] To evaluate the modification effect, a multi-metal adsorption comparison experiment was designed: simulated contaminated water (pH = 6.5 ± 0.2) containing 100 mg / L each of Ni, Cu, As, Cd, Pb, and Hg was prepared. One gram of original biochar (BC) and each modified biochar (Fe-BC, Fe-Mn-BC, Fe-Mn-Ca-BC, and Fe-Mn-Ca-Mg-BC) was mixed with 100 mL of the contaminated water sample. The mixture was shaken at 200 rpm for 24 hours in a dark room at 25°C. After filtration through a 0.45 μm microporous membrane, the residual metal concentrations in the filtrate were determined by ICP-MS.

[0074] The experimental results are shown in Table 1. The quaternary composite modified biochar (Fe-Mn-Ca-Mg-BC) exhibits the best adsorption performance: the As removal rate reaches 68.7±0.57%, which is nearly twice that of the original biochar (36.72±0.15%); the removal rates of Cd, Cu, and Pb are all over 90%, among which the Pb removal rate is as high as 95.58±0.08%; the removal rates of Ni and Hg reach 82.15±0.21% and 76.59±2.25%, respectively, which are 10-15% higher than those of the original biochar.

[0075] Table 1 Removal effect of biochar on Ni, Cu, As, Cd and Pb co-polluted water

[0076]

[0077]

[0078] Figure 1The results provide a more intuitive demonstration of the adsorption efficiency of various metal elements by the multi-metal modified biochar, Fe-Mn-Ca-Mg-BC. Notably, the biochar's adsorption capacity for each metal exhibits a step-wise increase with the addition of additional modified metals, confirming the effectiveness of synergistic multi-metal modification. In particular, the quaternary modified biochar significantly outperformed biochar modified with a single or small number of metals in terms of removal efficiency for all tested metals.

[0079] The Fe-Mn-Ca-Mg-BC composite modified biochar of this invention forms abundant adsorption sites on the biochar surface through the synergistic action of four metals: the Fe / Mn oxides primarily capture anions such as As, the Ca / Mg components preferentially bind cations such as Cd / Pb, and the porous structure of the carbon matrix itself provides diffusion channels for metal ions. This multi-mechanism synergy makes it an ideal material for treating complex heavy metal-contaminated water bodies.

[0080] According to the disclosure and guiding principles of the foregoing specification, those skilled in the art to which the present invention belongs can freely adjust and adapt the above-mentioned embodiments to optimize and improve their effects. Therefore, the scope of application of the present invention is not limited to the specific embodiments clearly shown, and any reasonable improvements and changes should also be included in the scope of rights of the present invention. In addition, although certain specific terms are used in the description of this specification, these words are only used to facilitate explanation and communication and do not constitute any form of limitation on the practical application of the present invention. The present invention is intended to provide an open and flexible solution for the relevant technical field, so that practitioners can make appropriate adjustments and optimizations based on specific needs and actual conditions.

Claims

1. A step-by-step preparation method for multi-element metal modified biochar, characterized by: The step-by-step preparation method is carried out as follows: (1) Raw material pretreatment: The rice straw is washed in three stages using tap water, deionized water and ultrapure water in sequence. During each stage of washing, the amount of tap water, deionized water and ultrapure water is sufficient to completely immerse the straw and be 4-12 cm above its surface. At the same time, a glass rod is used to gently stir to ensure that impurities on the surface of the straw are fully dissolved or dispersed in the water; the tap water is washed 2-4 times, each time for 12-25 minutes, until the washing liquid is not obviously turbid by naked eye observation; the deionized water is washed 2-3 times, each time for 8-20 minutes; the ultrapure water is washed 1 -2 times, soaking for 4-12 minutes, and the conductivity of the washing liquid is not higher than 1μS / cm to ensure that impurities on the surface of the straw are fully removed; after washing, a three-stage gradient drying is performed, the first stage of gradient drying is natural air drying at 20-30°C and a relative humidity of 35-45% for 46-50 hours, the second stage of gradient drying is low-temperature drying at 38-45°C for 10-14 hours, and the third stage of gradient drying is drying at 70-80°C to constant weight, and finally crushed through a 0.5-2mm sieve to obtain a homogeneous pretreated raw material; (2) Biochar preparation: The pretreated raw material was heated to 490-510°C at 8-12°C / min under nitrogen protection, maintained for 1.5-2.5 h, and then cooled to obtain the original biochar BC; (3) Multi-component composite modification: ①Fe modification: The original biochar BC was mixed with 0.08-0.12 mol / L FeCl3 solution at a solid-liquid ratio of 1 g:90-110 mL, shaken at 23-27 ° C and 180-220 rpm for 1.8-2.2 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 100-110 ° C for 22-26 h to obtain Fe-BC; ②Mn modification: Fe-BC was mixed with 0.08-0.12 mol / L MnCl2 solution at a solid-liquid ratio of 1 g:90-110 mL, and the mixture was shaken at 23-27°C and 180-220 rpm for 1.8-2.2 hours. The mixture was filtered and washed until the conductivity of the filtrate was <50 μS / cm, and then dried at 100-110°C for 22-26 hours to obtain Fe-Mn-BC. ③Ca modification: Fe-Mn-BC was mixed with 0.08-0.12 mol / L CaCl2 solution at a solid-liquid ratio of 1 g:90-110 mL, and the mixture was shaken at 23-27°C and 180-220 rpm for 1.8-2.2 h. The mixture was filtered and washed until the conductivity of the filtrate was <50 μS / cm, and then dried at 100-110°C for 22-26 h to obtain Fe-Mn-Ca-BC. ④Mg modification: Mix Fe-Mn-Ca-BC with 0.08-0.12 mol / L MgCl2 solution at a solid-liquid ratio of 1 g:90-110 mL, oscillate at 23-27 °C and 180-220 rpm for 1.8-2.2 h, filter and wash until the filtrate conductivity is <50 μS / cm, and dry at 100-110 °C for 22-26 h to obtain multi-metal modified biochar Fe-Mn-Ca-Mg-BC.

2. The step-by-step preparation method of multi-element metal modified biochar according to claim 1, characterized in that: In the step (1), the rice straw is washed in three stages using tap water, deionized water, and ultrapure water in sequence. During each stage of washing, the amount of tap water, deionized water, and ultrapure water is sufficient to completely immerse the straw and to be 5-10 cm above its surface. The straw is gently stirred with a glass rod to ensure that impurities on the surface of the straw are fully dissolved or dispersed in the water. The rice straw is washed with tap water three times, each time for 15-20 minutes, until the washing liquid is not obviously turbid when observed with the naked eye; and the rice straw is washed with deionized water twice, each time for 10-15 minutes. ; Wash once with ultrapure water, soak for 5-10 minutes, and the conductivity of the washing liquid is not higher than 1μS / cm to ensure that impurities on the surface of the straw are fully removed; after washing, carry out three-stage gradient drying, the first stage of gradient drying is natural air drying at 23-27℃ and relative humidity of 40% for 48 hours, the second stage of gradient drying is low-temperature drying at 40℃ for 12 hours, and the third stage of gradient drying is drying at 75℃ to constant weight, and finally crushed through a 0.5-2mm sieve to obtain homogeneous pretreated raw materials.

3. The step-by-step preparation method of multi-element metal modified biochar according to claim 1, characterized in that: In the step (2), biochar is prepared by heating the pretreated raw material to 495-505° C. at a rate of 9-11° C. / min under nitrogen protection, maintaining pyrolysis for 1.8-2.2 h, and cooling to obtain raw biochar BC.

4. The step-by-step preparation method of multi-element metal modified biochar according to claim 3, characterized in that: In the step (2), biochar is prepared by heating the pretreated raw material to 500° C. at a rate of 10° C. / min under nitrogen protection, maintaining pyrolysis for 2 h, and cooling to obtain raw biochar BC.

5. The step-by-step preparation method of multi-element metal modified biochar according to claim 1, characterized in that: In the step (3), multi-component composite modification: ①Fe modification: The original biochar BC was mixed with 0.1 mol / L FeCl3 solution at a solid-liquid ratio of 1 g:100 mL, shaken at 25°C and 200 rpm for 2 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 105°C for 24 h to obtain Fe-BC; ②Mn modification: Fe-BC was mixed with 0.1 mol / L MnCl2 solution at a solid-liquid ratio of 1 g:100 mL, shaken at 25°C and 200 rpm for 2 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 105°C for 24 h to obtain Fe-Mn-BC; ③Ca modification: Fe-Mn-BC was mixed with 0.1 mol / L CaCl2 solution at a solid-liquid ratio of 1 g:100 mL, shaken at 25°C and 200 rpm for 2 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 105°C for 24 h to obtain Fe-Mn-Ca-BC; ④Mg modification: Fe-Mn-Ca-BC was mixed with 0.1 mol / L MgCl2 solution at a solid-liquid ratio of 1 g:100 mL, shaken at 25 °C and 200 rpm for 2 h, filtered and washed until the filtrate conductivity was <50 μS / cm, and dried at 105 °C for 24 h to obtain multi-metal modified biochar Fe-Mn-Ca-Mg-BC.

6. The use of multi-metal modified biochar according to claims 1-5 in the treatment of heavy metal water pollution, characterized in that: Multi-metal modified biochar Fe-Mn-Ca-Mg-BC can adsorb Ni, Cu, As, Cd and / or Pb in heavy metal-contaminated water bodies, achieving the purpose of treating heavy metal water pollution.

7. The use of multi-element modified biochar in heavy metal water pollution control according to claim 6, characterized in that: The solid-liquid ratio of the aforementioned Fe-Mn-Ca-Mg-BC and heavy metal-contaminated water is 1g:100mL, and the treatment time is 24h.

8. The use of multi-element modified biochar in heavy metal water pollution control according to claim 6, characterized in that: The concentrations of Ni, Cu, As, Cd and Pb in the heavy metal-polluted water body reach 100 mg / L.

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