Preparation method and application of a biochar soil remediation agent

Abstract

The present invention discloses a preparation method of a biochar soil remediation agent, comprising the following steps: 1) using rice husk as raw material, drying and crushing it, and then performing anaerobic carbonization to obtain biochar; 2) adding the biochar to a magnesium salt solution and stirring it, then heating it in a water bath to 70-90°C and adding a carbonate solution and continuing to stir it, rinsing and drying it to obtain silicon-activated biochar; 3) adding the silicon-activated biochar to a hydroxyl-modified solution and stirring the reaction for 2-5 hours to obtain the biochar soil remediation agent. The present invention achieves efficient fixation of soil heavy metals by silicon activation and combined hydroxyl modification of the biochar, and can effectively reduce the bioavailability of passivated soil heavy metals through complexation, precipitation and other effects.

Description

Technical Field

[0001] The invention belongs to the field of metal-contaminated soil remediation and relates to a method for preparing silicon-activated combined hydroxyl-modified biochar. Background Art

[0002] Industrial and municipal waste discharge, mining activities, and improper use of fertilizers and pesticides have resulted in the release of large amounts of toxic and hazardous heavy metals into the soil environment, causing serious pollution. According to the 2014 National Soil Pollution Survey Bulletin, heavy metal concentrations in my country's soils exceed standards, particularly for elements such as copper (Cu), chromium (Cr), cadmium (Cd), and lead (Pb), which have a more serious impact. Heavy metal pollution in soils poses a serious threat to human health and food security. Compared with numerous methods for mitigating heavy metals in soils, such as electrokinetic remediation and thermal desorption, in situ fixation is considered a cost-effective solution. The development of highly efficient and long-term stable passivating agents is key to improving remediation effectiveness.

[0003] Biochar, a common soil remediation agent, refers to a highly aromatic solid product produced by the pyrolysis of biomass under anoxic conditions. Its rich pore structure allows biochar to immobilize heavy metals through pore adsorption and electrostatic attraction, thereby reducing their bioavailability and mobility in soil. However, unmodified biochar has few active functional groups and weak heavy metal immobilization, making it difficult to achieve efficient and stable remediation of heavy metal contaminated soils. Therefore, appropriate modification of biochar to improve its passivation properties and rate is crucial.

[0004] my country's annual rice husk production rate exceeds 40 million tons, and the unreasonable use of rice husks causes serious waste of resources. Therefore, using waste rice husks to prepare biochar can improve the utilization rate of waste resources, which is in line with the concept of green environmental protection and sustainable development. In addition to carbon, rice husk biochar also contains a large amount of unused inert silicon, and its content is generally in the range of 16%-33% (Environ. Sci. Technol. 2019, 53, 23, 13570–13582). Silicon may also be activated in the common strong alkali modification of biochar, but in this process silicon exists in the form of silicate, and silicate is easily lost during the material cleaning process. The double hydrolysis of magnesium salts and carbonates can form a weakly alkaline environment. After silicon is activated to silicate, it can react with Mg 2+The formation of magnesium silicate precipitation prevents the loss of active silicon, and the introduced carbonate ions can also form carbonate precipitates with heavy metals, improving the heavy metal fixation performance of biochar. In addition, functional group grafting modification can effectively increase the active sites on the biochar surface. β-cyclodextrin is non-toxic and contains a large number of active hydroxyl groups, which can provide binding sites for heavy metals, further improving the biochar's fixation performance and efficiency for soil heavy metals. Therefore, the double hydrolysis of magnesium salts and carbonates for silicon activation combined with β-cyclodextrin for functional group modification can effectively improve the passivation performance and stability of biochar for soil heavy metals. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for preparing a biochar soil remediation agent, which solves the current problems of low efficiency and poor heavy metal fixation performance of biochar by silicon activation and combined hydroxyl modification of biochar.

[0006] A method for preparing a biochar soil remediation agent comprises the following steps:

[0007] 1) Using rice husk as raw material, drying and crushing it and then performing anaerobic carbonization to obtain biochar;

[0008] 2) adding the biochar to the magnesium salt solution and stirring, then heating in a water bath to 70-90° C. and adding the carbonate solution and continuing to stir, rinsing and drying to obtain silicon-activated biochar;

[0009] 3) Add silicon-activated biochar to the hydroxyl-modified solution and stir for 2-5 hours to obtain a biochar soil remediation agent.

[0010] Every 100 mL of the hydroxyl-modified solution contains 2-5 g of sodium hydroxide, 2-5 g of beta-cyclodextrin and 0.5-3 mL of epichlorohydrin.

[0011] In step 1), the rice husk is dried and crushed to 60-120 mesh, and then subjected to oxygen-free carbonization at 400-500° C. Specifically, the oxygen-free carbonization is carried out under a N 2 atmosphere.

[0012] Wherein, the concentration of the magnesium salt solution and the carbonate solution is 0.1-1.5 mol / L.

[0013] Wherein, the solid-liquid ratio of the biochar to the magnesium salt solution is 1g:20mL.

[0014] Wherein, the solid-liquid ratio of the biochar to the carbonate solution is 1 g:10 mL.

[0015] Preferably, every 100 mL of the hydroxyl-modified solution contains 3 g of sodium hydroxide, 4 g of β-cyclodextrin and 1 mL of epichlorohydrin.

[0016] The prepared biochar soil remediation agent can adsorb and fix soil heavy metals and promote their transformation from exchangeable state to carbonate-bound state and residual state, so it can be used for soil heavy metal pollution remediation.

[0017] The beneficial effects of the present invention are:

[0018] This method achieves efficient fixation of soil heavy metals through silicon activation and hydroxyl modification of biochar. It also effectively reduces the bioavailability of passivated soil heavy metals through complexation and precipitation. It also addresses the problem of rice husk waste disposal, promoting the coordinated development of resource utilization and ecological environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a scanning electron microscope image of the silicon-activated combined with hydroxyl-modified biochar in Example 1;

[0020] Figure 2 Schematic diagram of silicon morphology classification of silicon-activated biochar in Example 1;

[0021] Figure 3 This is a diagram showing the functional group content of silicon-activated and hydroxyl-modified biochar in Example 2. DETAILED DESCRIPTION

[0022] The present invention will be described in detail below by way of examples.

[0023] Example 1 Preparation of Silicon-activated Combined Hydroxyl-modified Biochar

[0024] 1) Wash the rice husks with clean water and dry them in a forced air drying oven at 60°C. Then, grind the rice husks into powder using a grinder and pass through a 60-mesh standard sieve for later use;

[0025] 2) The rice husk powder was placed in a tube furnace, heated to 400°C at a rate of 5°C / min under a nitrogen atmosphere, kept at this temperature for 2 hours, cooled naturally to room temperature, and ground to obtain biochar;

[0026] 3) The prepared 1.0 mol / L magnesium sulfate solution and biochar were mixed at a solid-liquid ratio of 1:20 (g / ml), magnetically stirred at 400 r / min for 5 h, heated in a water bath to 80°C, and then 1.0 mol / L sodium carbonate solution was added at a solid-liquid ratio of 1:5 (g / ml), magnetically stirred for 2 h, and then filtered with ultrapure water. The solid residue was placed in a forced air drying oven and dried at 60°C. Finally, it was ground through a 60-mesh standard sieve to obtain silicon-activated biochar.

[0027] 4) Add 3 g of potassium hydroxide and 4 g of β-cyclodextrin to 100 mL of water, add 1 mL of epichlorohydrin, and shake at room temperature for 6 hours to prepare the modified solution. Mix 1.0 g of silicon-activated biochar with 100 mL of the modified solution and stir for 4 hours. Wash, filter, and dry the product to obtain the hydroxyl-modified biochar.

[0028] The main form of silicon in unmodified biochar is amorphous silicon dioxide, while the amorphous silicon in biochar modified by silicon activation and hydroxyl groups has been transformed into MgSiO3. Figure 1 It can be seen that the surface of biochar activated by silicon and modified by hydroxyl groups presents a nano-flower structure, which is the characteristic structure of magnesium silicate. Figure 2 It can be seen that the unmodified biochar is mainly composed of amorphous silicon, with a content of up to 97.6%. The contents of dissolved silicon, active silicon and effective silicon in silicon-activated biochar and silicon-activated combined hydroxyl-modified biochar have been significantly increased, and the proportion of amorphous silicon has decreased significantly.

[0029] In an alkaline environment, epichlorohydrin decyclization can form epoxy groups with the hydroxyl groups of β-cyclodextrin. This process helps to introduce hydroxyl-rich β-cyclodextrin into biochar to enhance its heavy metal complexing ability. Figure 3 It can be seen that the functional group content in the unmodified biochar is 0.42 mmol / g, and the surface functional group content of the modified silicon-activated combined with hydroxyl-modified biochar reaches 1.46 mmol / g, which is about 3.5 times that of the unmodified biochar.

[0030] Example 2 Preparation of Silicon-activated Combined Hydroxyl-modified Biochar

[0031] 1) Wash the rice husks with clean water and dry them in a forced air drying oven at 80°C. Then, grind the rice husks into powder using a grinder and pass through a 100-mesh standard sieve for later use;

[0032] 2) The rice husk powder was placed in a tube furnace, heated to 450°C at a rate of 5°C / min under a nitrogen atmosphere, kept at this temperature for 2 hours, cooled naturally to room temperature, and ground to obtain biochar;

[0033] 3) The prepared 0.5 mol / L magnesium chloride solution and biochar were mixed at a solid-liquid ratio of 1:20 (g / mL), magnetically stirred at 500 r / min for 4 h, heated in a water bath to 75°C, and then 0.5 mol / L potassium carbonate solution was added at a solid-liquid ratio of 1:10 (g / mL), magnetically stirred for 2 h, and then filtered with ultrapure water. The solid residue was placed in a forced air drying oven and dried at 60°C. Finally, it was ground through a 100-mesh standard sieve to obtain silicon-activated biochar.

[0034] 4) Add 2 g of potassium hydroxide and 5 g of β-cyclodextrin to 100 mL of water, add 2 mL of epichlorohydrin, and shake at room temperature for 6 hours to prepare the modified solution. Mix 1.0 g of silicon-activated biochar with 100 mL of the modified solution and stir for 5 hours. Wash, filter, and dry the product to obtain silicon-activated and hydroxyl-modified biochar.

[0035] Example 3 Preparation of Silicon-activated Combined Hydroxyl-modified Biochar

[0036] 1) Wash the rice husks with clean water and dry them in a forced air drying oven at 100°C. Then, grind the rice husks into powder using a grinder and pass through a 120-mesh standard sieve for later use;

[0037] 2) The rice husk powder was placed in a tube furnace, heated to 450°C at a rate of 5°C / min under a nitrogen atmosphere, kept at this temperature for 2 hours, cooled naturally to room temperature, and ground to obtain biochar;

[0038] 3) The prepared 0.1 mol / L magnesium chloride solution and biochar were mixed at a solid-liquid ratio of 1:20 (g / mL), magnetically stirred at 500 r / min for 4 h, heated in a water bath to 90°C, and then 0.1 mol / L potassium carbonate solution was added at a solid-liquid ratio of 1:10 (g / mL), magnetically stirred for 2 h, filtered with ultrapure water, and the solid residue was placed in a forced air drying oven at 60°C and finally ground through a 120-mesh standard sieve to obtain silicon-activated biochar.

[0039] 4) Add 5 g of sodium hydroxide and 3 g of β-cyclodextrin to 100 mL of water, add 0.5 mL of epichlorohydrin, and shake at room temperature for 5 hours to prepare the modified solution. Mix 1.0 g of silicon-activated biochar with 100 mL of the modified solution and stir for 3 hours. Wash, filter, and dry the product to obtain silicon-activated and hydroxyl-modified biochar.

[0040] Example 4 Preparation of Silicon-activated Combined Hydroxyl-modified Biochar

[0041] 1) Wash the rice husks with clean water and dry them in a forced air drying oven at 100°C. Then, grind the rice husks into powder using a grinder and pass through a 100-mesh standard sieve for later use;

[0042] 2) The rice husk powder was placed in a tube furnace, heated to 400°C at a rate of 5°C / min under a nitrogen atmosphere, kept at this temperature for 2 hours, cooled naturally to room temperature, and ground to obtain biochar;

[0043] 3) The prepared 1.5 mol / L magnesium sulfate solution and biochar were mixed at a solid-liquid ratio of 1:20 (g / mL), magnetically stirred at 500 r / min for 4 h, heated in a water bath to 70°C, and then 1.5 mol / L sodium carbonate solution was added at a solid-liquid ratio of 1:1 (g / mL). The mixture was magnetically stirred for 2 h, filtered with ultrapure water, and the solid residue was placed in a forced air drying oven and dried at 60°C. Finally, it was ground through a 100-mesh standard sieve to obtain silicon-activated biochar.

[0044] 4) Add 3 g of sodium hydroxide and 2 g of β-cyclodextrin to 100 mL of water, add 3 mL of epichlorohydrin, and shake at room temperature for 5 hours to prepare the modified solution. Mix 1.0 g of silicon-activated biochar with 100 mL of the modified solution and stir for 2 hours. Wash, filter, and dry the product to obtain silicon-activated and hydroxyl-modified biochar.

[0045] Test example:

[0046] Test method:

[0047] 1. Soil treatment

[0048] Place 50g of air-dried, sieved soil sample in a glass bottle. Weigh 0.2g, 0.35g, and 0.50g of the material at 0.4%, 0.7%, and 1.0% addition rates, respectively, and mix evenly with the soil sample (three replicates per treatment). Place the bottle in a constant-temperature incubator and regularly add water to maintain a moisture content of 50%. Additionally, sample the soil on day 35, air-dry it, grind it through a 0.25mm standard sieve, and store it in a bag.

[0049] 2. Determination method

[0050] Determination of CaCl2-extractable Cd concentration: The soil samples were extracted with CaCl2 solution at a solid-liquid ratio of 1:10 (m:v) for 2 h, and then the Cd concentration in the solution was determined by flame atomic absorption spectrometry.

[0051] Tessier continuous extraction fractionation exchangeable cadmium (EXC-Cd) concentration determination: 1.0 g of soil sample was added to a 50 mL plastic centrifuge tube, and 8 mL of 1 mol / L magnesium chloride solution was added to the tube. The tube was shaken at room temperature for 1 h (200 r / min), centrifuged for 10 min (4000 r / min), the supernatant was removed, the removed solution was filtered, and the volume was made up with a 50 mL volumetric flask. The Cd concentration in the solution was then determined by flame atomic absorption spectrometry.

[0052] 3. Test results

[0053] Table 1 Results of soil CaCl2-extractable Cd content at different passivator addition amounts

[0054]

[0055] Table 2 Results of exchangeable cadmium content in soil according to Tessier classification at different passivator addition amounts

[0056]

[0057]

[0058] Results indicate that the conversion of inert silicon in biochar to silicate and the increase in active hydroxyl groups by β-cyclodextrin are the primary factors enhancing the remediation performance of silicon-activated combined with hydroxyl-modified biochar. With increasing additions of silicon-activated combined with hydroxyl-modified biochar, the fixation efficiency gradually increased. Simultaneously, carbonate minerals, functional groups, and silicate precipitation promoted the conversion of EXC-Cd to carbonate-bound (CAB-Cd) and residual (RES-Cd) forms.

Claims

1. A method for preparing a biochar soil remediation agent, characterized in that The following steps are involved: 1) Wash the rice husks with clean water and dry them in a forced air drying oven at 60°C. Then, use a grinder to grind the rice husks into powder and pass it through a 60-mesh standard sieve for later use; 2) The rice husk powder was placed in a tube furnace, heated to 400°C at a rate of 5°C / min under a nitrogen atmosphere, kept at this temperature for 2 hours, cooled naturally to room temperature, and ground to obtain biochar; 3) The prepared 1.0 mol / L magnesium sulfate solution and biochar were mixed at a solid-liquid ratio of 1:20 g / ml, and magnetically stirred at 400 rpm for 5 h. After heating to 80°C in a water bath, 1.0 mol / L sodium carbonate solution was added at a solid-liquid ratio of 1:5 g / ml, and magnetically stirred for 2 h. Subsequently, ultrapure water was used for filtration, and the solid residue was dried in a forced air drying oven at 60°C. Finally, it was ground through a 60-mesh standard sieve to obtain silicon-activated biochar; 4) Add 3 g of potassium hydroxide and 4 g of β-cyclodextrin to 100 mL of water, add 1 mL of epichlorohydrin, and shake at room temperature for 6 h to prepare the modified solution. Take 1.0 g of silicon-activated biochar and mix it with 100 mL of the modified solution and stir for 4 h. The product is washed, filtered, and dried to obtain hydroxyl-modified biochar.

2. Application of a biochar soil remediation agent in the remediation of soil heavy metal cadmium pollution, wherein the biochar soil remediation agent is prepared according to the method of claim 1.

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