A biochar modifier for reducing mercury in plants and soil methylmercury, and a preparation method and application thereof

Abstract

The application provides a biochar modifier for reducing plant mercury and soil methylmercury, and a preparation method and application thereof. The biochar modifier provided by the application comprises biochar loaded with selenium and phosphorus. The preparation method of the biochar modifier comprises the following steps: dipping biochar into a solvent containing a selenium modifier and a phosphorus modifier and reacting, and then performing solid-liquid separation on the reaction solution to obtain a solid phase; and the solid phase is dried to obtain the biochar modifier. The biochar modifier provided by the application has a remarkable effect of reducing plant mercury and soil methylmercury, has a simple preparation process, does not cause secondary selenium pollution, and is easy to popularize.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of heavy metal contaminated soil improvement, and particularly relates to a biochar modifier for reducing plant mercury and soil methylmercury, and a preparation method and application thereof. BACKGROUND

[0002] Mercury (Hg) is a global pollutant, which exists in various forms in the environment. Among them, methylmercury (MeHg) is the most toxic form, which has attracted much attention due to its bioaccumulation and significant biomagnification effect in the food chain. Due to human activities or high geological background, the situation of farmland soil Hg pollution is serious, especially in Hg mining areas, where a large amount of Hg-containing substances produced by mining and smelting activities enter the surface water, farmland soil and atmospheric environment. Therefore, exploring corresponding remediation technologies and safe utilization schemes is an important research topic in the field of environment.

[0003] Biochar is a commonly used soil mercury (Hg) pollution solidification / stabilization material, but some documents have reported that biochar often increases soil methylmercury (MeHg), bringing new environmental risks (Shu Rui. Effects of straw biochar application on methylmercury availability and accumulation in paddy soil of mining area[D]. Nanjing University, 2016). In addition, Wang et al. (2021) reported that the addition of biochar promoted the concentration of soil MeHg during rice and wheat rotation, with an increase of 36-303% in the rice season and 52-292% in the wheat season. Therefore, it is necessary to explore a more stable, efficient and safe modified biochar for application in Hg contaminated soil. SUMMARY

[0004] In order to solve one of the above technical problems in the prior art, the present application provides a biochar modifier and a preparation method and application thereof. The biochar modifier of the present application can reduce the risk of plant mercury and soil methylmercury, and selenium and phosphorus show a synergistic mercury reduction effect.

[0005] In a first aspect, the present application provides a biochar modifier for reducing plant mercury and soil methylmercury, which comprises biochar loaded with selenium and phosphorus.

[0006] According to some embodiments of the present application, the biochar comprises straw biochar. The straw of the present application includes but is not limited to rice straw, corn straw, rape straw, etc.

[0007] According to some embodiments of the present application, the biochar comprises a pyrolysis product of straw. In some embodiments, the temperature of pyrolysis is 450-550℃, such as 450℃, 480℃, 500℃, 520℃, 550℃, etc. In some embodiments, the heating rate of pyrolysis is 5-15℃·min-1 for example 5℃·min -1 for example 8℃·min -1 for example 10℃·min -1 for example 12℃·min -1 for example 15℃·min -1 In some embodiments, the pyrolysis time is 3-5 hours, for example 3 hours, 4 hours, 5 hours, etc. In some embodiments, the average particle size of the straw is 10-30 mesh. In some embodiments, the average particle size of the biochar is 80-120 mesh.

[0008] According to some embodiments of the present application, the specific surface area of the biochar is 35-45m 2 ·g -1 According to some embodiments of the present application, the pore volume of the biochar is 5-10cm 3 ·g -1 According to some embodiments of the present application, the average pore diameter of the biochar is 2-5nm.

[0009] According to some embodiments of the present application, the infrared spectrum of the biochar modifier has characteristic peaks at 560±10cm -1 and 605±10cm -1 . The peak at 560±10cm -1 is the bending vibration peak of the phosphorus group (O-P-O) in the biochar modifier, and the peak at 605±10cm -1 is the characteristic peak of the-Se group in the biochar modifier.

[0010] According to some embodiments of the present application, the specific surface area of the biochar modifier is 70-80m 2 ·g -1 . According to some embodiments of the present application, the pore volume of the biochar modifier is 15-20cm 3 ·g -1 . According to some embodiments of the present application, the average pore diameter of the biochar modifier is 2-5nm.

[0011] In a second aspect, the present application provides a preparation method of a biochar modifier for reducing plant mercury and soil methylmercury, comprising the following steps:

[0012] The biochar is immersed in a solvent containing a selenium modifier and a phosphorus modifier and reacted, and the reaction liquid is subjected to solid-liquid separation to obtain a solid phase; the solid phase is dried to obtain a biochar loaded with selenium and phosphorus elements.

[0013] According to some embodiments of the present application, the selenium modifier comprises selenite, such as Na2SeO3. Se, as an essential trace element for living organisms, can have antagonistic effects with heavy metals such as Hg, Cr and Cd. Adding Se in soil can reduce the content of plant mercury and MeHg.

[0014] According to some embodiments of the present application, the phosphorus modifier comprises but is not limited to phosphorus-containing minerals, phosphates, superphosphates, pyrophosphates, hydrogen phosphates, dihydrogen phosphates, etc. Examples of the phosphates, superphosphates, pyrophosphates, hydrogen phosphates, dihydrogen phosphates of the present application include but are not limited to alkali metal (such as sodium, potassium) and alkaline earth metal (such as calcium) phosphates, superphosphates, pyrophosphates, hydrogen phosphates, dihydrogen phosphates, etc.

[0015] In some specific embodiments, the phosphorus modifier comprises at least one of hydroxyapatite, potassium phosphate, potassium hydrogen phosphate, potassium dihydrogen phosphate, ammonium phosphate, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, calcium hydrogen phosphate, calcium phosphate, potassium pyrophosphate, calcium pyrophosphate, potassium superphosphate, calcium superphosphate.

[0016] In some preferred embodiments, the phosphorus modifier comprises hydroxyapatite. Phosphate plays an important role in heavy metal immobilization. Hydroxyapatite (HAP) has a molecular formula of Ca 10 (PO4)6(OH)2. Due to its large specific surface area, the Ca 2+ in its structure is easily replaced by divalent heavy metal ions, and it is a low-cost, environmentally friendly alternative adsorbent, and is often used to remove and adsorb heavy metals such as cadmium and lead.

[0017] According to some embodiments of the present application, the ratio of the mass of the selenium modifier to the sum of the mass of the biochar and the phosphorus modifier is 1:(1400-1500), such as 1:1410, 1:1.1420, 1:1430, 1:1440, 1:1450, 1:1460, 1:1470, 1:1480, 1:1490, 1:1500, etc.

[0018] According to some embodiments of the present application, the ratio of the mass of the phosphorus modifier to the sum of the mass of the biochar and the selenium modifier is 1:(2-5), such as 1:2, 1:3, 1:4, 1:5, etc.

[0019] According to some embodiments of the present application, the solvent comprises water.

[0020] According to some embodiments of the present application, the temperature of the reaction is 20-30℃, such as 20℃, 22℃, 25℃, 28℃, 30℃, etc.

[0021] According to some embodiments of the present application, the reaction time is 5-8 hours, such as 5 hours, 6 hours, 7 hours, 8 hours, etc.

[0022] According to some embodiments of the present application, the drying temperature is 80-120℃, such as 80℃, 90℃, 100℃, 105℃, 110℃, 120℃, etc.

[0023] According to some embodiments of the present application, the biochar comprises straw biochar.

[0024] According to some embodiments of the present application, the biochar comprises one or more of rice straw biochar, corn straw biochar, and rape straw biochar.

[0025] According to some embodiments of the present application, the biochar is prepared by a method comprising the step of: crushing and pyrolyzing the straw to obtain the biochar. In some embodiments, the pyrolysis temperature is preferably 450-550℃, such as 450℃, 480℃, 500℃, 520℃, 550℃, etc. In some embodiments, the heating rate during the pyrolysis is 5-15℃·min -1 , such as 5℃·min -1 , 8℃·min -1 , 10℃·min -1 , 12℃·min -1 , 15℃·min -1 . In some embodiments, the pyrolysis time is 3-5 hours, such as 3 hours, 4 hours, 5 hours, etc. In some embodiments, the straw is dried before being crushed. In some embodiments, the straw is crushed and passed through a 10-30 mesh sieve. In some embodiments, the pyrolysis is followed by a grinding and sieving process, preferably to pass through an 80-120 mesh sieve.

[0026] In a third aspect, the present application provides a method for reducing mercury and methylmercury in soil, comprising: applying the biochar amendment of the first aspect of the present application or the biochar amendment obtained by the preparation method of the second aspect of the present application to mercury-contaminated soil.

[0027] According to some embodiments of the present application, the amount of the biochar amendment added to the mercury-contaminated soil is 0.3-1wt%, such as 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1wt%, etc.

[0028] In a fourth aspect, the present application provides application of the biochar modifier of the first aspect or the biochar modifier prepared by the preparation method of the second aspect in reducing plant mercury and soil methylmercury.

[0029] Compared with the prior art, the present application has the following beneficial effects:

[0030] 1. The present application provides a biochar loaded with selenium and phosphorus elements as a soil modifier. Compared with the addition of unmodified biochar, the addition of the modifier has a more significant effect on reducing plant total mercury and methylmercury, and significantly reduces soil methylmercury, avoiding the risk of increasing soil methylmercury caused by directly adding unmodified biochar. In addition, the two elements of selenium and phosphorus in the modifier show a synergistic mercury reduction effect.

[0031] 2. The present application prepares a soil modifier by loading selenium and phosphorus elements on biochar by impregnation method, which is simple, green, environmentally friendly and efficient.

[0032] 3. Compared with directly applying a mixture of selenium modifier, phosphorus modifier and biochar to mercury-contaminated soil, the modified biochar modifier loaded with selenium and phosphorus by impregnation method has a more significant effect on reducing plant mercury and soil methylmercury.

[0033] 4. The biochar soil modifier provided by the present application has a low Se addition amount, and the final soil Se addition amount is only 1.51 mg / kg, which can avoid secondary pollution of excessive soil Se and has potential for large-area popularization and application. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 The Fourier transform infrared spectrum (FTIR) of biochar BC and Se-HAP-BC obtained in Example 1 is shown.

[0035] Figure 2 The Langmuir and Freundlich model fitting results of mercury adsorption of biochar BC and Se-HAP-BC obtained in Example 1 are shown.

[0036] Figure 3 The soil pH, DOC, SO4 2- concentration distribution of each treatment group of selenium modification, hydroxyapatite modification and selenium-hydroxyapatite combined modification in the pot experiment (I) is shown.

[0037] Figure 4 The soil water-soluble, simulated gastric acid and organic combined state Hg concentration distribution of each treatment group of selenium modification, hydroxyapatite modification and selenium-hydroxyapatite combined modification in the pot experiment (I) is shown.

[0038] Figure 5The soil MeHg content of each treatment group of selenium modification, hydroxyapatite modification and selenium-hydroxyapatite combined modification in the pot experiment (I) is shown.

[0039] Figure 6 The THg and MeHg content of each treatment group of selenium modification, hydroxyapatite modification and selenium-hydroxyapatite combined modification in the pot experiment (I) is shown.

[0040] Figure 7 The total mercury content of each treatment group of selenium modification, potassium dihydrogen phosphate modification and selenium-potassium dihydrogen phosphate combined modification in the pot experiment (I) is shown.

[0041] Figure 8 The soil methylmercury content of each treatment group of selenium modification, potassium dihydrogen phosphate modification and selenium-potassium dihydrogen phosphate combined modification in the pot experiment (I) is shown.

[0042] Figure 9 The total mercury content of each treatment group of selenium modification, potassium dihydrogen phosphate modification and selenium-potassium dihydrogen phosphate combined modification in the pot experiment (I) is shown.

[0043] Figure 10 The total mercury content of each treatment group of selenium modification, potassium dihydrogen phosphate modification and selenium-potassium dihydrogen phosphate combined modification in the pot experiment (I) is shown.

[0044] Figure 11 The soil methylmercury content of each treatment group of selenium modification, potassium dihydrogen phosphate modification and selenium-potassium dihydrogen phosphate combined modification in the pot experiment (I) is shown.

[0045] Figure 12 The total mercury content of each treatment group of selenium modification, potassium dihydrogen phosphate modification and selenium-potassium dihydrogen phosphate combined modification in the pot experiment (I) is shown. DETAILED DESCRIPTION

[0046] In order to make the objects, technical solutions and advantages of the present application clearer, further detailed description will be made to the present application in combination with embodiments and drawings. The specific embodiments described herein are only used to explain the present application, and shall not be used to constitute any limitation to the present application.

[0047] The experimental methods used in the present application below are all conventional methods unless otherwise specified; the raw materials, reagents, instruments and the like used are all available through commercial channels unless otherwise specified.

[0048] The biochar used in the present application below is rice straw biochar (BC), and the preparation method is as follows:

[0049] After the dry rice straw was ground to pass 20 mesh, it was placed in a sealed stainless steel box and pyrolyzed in a muffle furnace. The pyrolysis temperature was set to 500℃, and the heating rate was 10℃·min -1 and maintained at the target temperature for 4h. The obtained BC after pyrolysis was cooled and ground to pass 100 mesh sieve and placed in a sealed plastic bag for later use.

[0050] The CAS of hydroxyapatite (HAP) used in the present application is 1306-06-5.

[0051] Preparation of Na2SeO3 modified biochar (Se-BC)

[0052] 0.04064g of Na2SeO3 was dissolved in deionized water, and then 60g of biochar BC (mass ratio of Na2SeO3 / BC was 1:1477) was added. After stirring for 6h, the excess water was drained, and then the mixture was placed in an oven at 105℃ to dry to constant weight to obtain Se-BC.

[0053] Preparation of hydroxyapatite modified biochar (HAP-BC)

[0054] 12g of hydroxyapatite (HAP) was placed in deionized water, and stirred until it was completely dispersed. Then, 60g of biochar BC (mass ratio of HAP / BC was 1:5) was added. After stirring for 6h, the excess water was drained, and then the mixture was placed in an oven at 105℃ to dry to constant weight to obtain HAP-BC.

[0055] Preparation of a mixture of Na2SeO3, HAP and biochar

[0056] 0.048767g of Na2SeO3, 12g of HAP and 60g of biochar BC were added to a sealed bag and shaken to mix thoroughly to obtain a mixture of selenium, HAP and biochar.

[0057] Preparation of potassium dihydrogen phosphate modified biochar (MKP-BC)

[0058] 30g of potassium dihydrogen phosphate (MKP) was placed in deionized water, and stirred until it was completely dispersed. Then, 60g of biochar BC (mass ratio of MKP / BC was 1:2) was added. After stirring for 6h, the excess water was drained, and then the mixture was placed in an oven at 105℃ to dry to constant weight to obtain MKP-BC.

[0059] Preparation of Na2SeO3 and HAP combined modified biochar (Se-HAP-BC)

[0060] 0.048767 g of Na2Se03 was dissolved in a beaker, keeping the mass ratio of Na2Se03 / (BC+HAP) as 1:1477; 12 g of HAP was added, keeping the mass ratio of HAP / (BC+Na2Se03) as 1:5; after stirring and dispersing, 60 g of biochar BC was added. After fully stirring for 6 h, the excess water was drained after standing and precipitating, and then it was placed in an oven at 105°C to dry to constant weight, obtaining Se-HAP-BC.

[0061] Example 2: Preparation of biochar modified by Na2Se03 and potassium dihydrogen phosphate (Se-MKP-BC)

[0062] 0.06096 g of Na2Se03 was dissolved in a beaker, keeping the mass ratio of Na2Se03 / (BC+MKP) as 1:1477; 30 g of MKP was added, keeping the mass ratio of MKP / (BC+Na2Se03) as 1:2; after stirring and dispersing, 60 g of biochar BC was added. After fully stirring for 6 h, the excess water was drained after standing and precipitating, and then it was placed in an oven at 105°C to dry to constant weight, obtaining Se-MKP-BC.

[0063] Experiment 1: Fourier infrared spectrum analysis of BC and Se-HAP-BC

[0064] Figure 1 The Fourier infrared spectrum (FTIR) of the prepared BC and Se-HAP-BC is shown in Figure 1. Figure 1 As can be seen, Se-HAP-BC has some new stretching vibration peaks compared with BC. BC contains a bending vibration peak of C=O group at 1420 cm -1 -1, which is enhanced in Se-HAP-BC after modification by Se and HAP, indicating that the aromaticity of Se-HAP-BC increases, and more Π electron donors are available in the heavy metal adsorption process. -1

[0065] Compared with the spectrum of BC, Se-HAP-BC has a new bending vibration peak of O-P-O at 561 cm -1 -1, and a characteristic peak of -Se group at about 605 cm -1 -1, indicating that Se and HAP are successfully loaded on the biochar.

[0066] Experiment 2: Adsorption isotherm analysis of BC and Se-HAP-BC on Hg in solution

[0067] Adsorption isotherm model plays an important role in studying the migration rate or retention rate of Hg(II) on the solid surface of biochar. The adsorption isotherm of BC and Se-HAP-BC on Hg(II) is shown in Figure 2. Figure 2 ​The experimental data were fitted by Langmuir and Freundlich equations, respectively, as shown in Figure 2. The results were calculated by the following equations: Figure 2 It can be seen that the equilibrium adsorption capacity of BC and Se-HAP-BC both increased with the increase of initial concentration of Hg(II), and the adsorption capacity of Hg(II) basically reached equilibrium when its concentration was 60 mg / L. Under the same initial concentration, the maximum equilibrium adsorption capacity of Se-HAP-BC was nearly twice that of BC, indicating that the removal capacity of Hg(II) by biochar was significantly enhanced after Se and P combined modification.

[0068] The Langmuir model fitting results of BC and Se-HAP-BC were better than the Freundlich model, and the correlation coefficients R 2 were 1 and 0.894, respectively, indicating that the adsorption of BC and Se-HAP-BC to Hg was mainly monolayer adsorption, and the adsorption sites on the surface were uniformly distributed, mainly chemical adsorption.

[0069] Experiment three: specific surface area and pore structure characterization of BC and Se-HAP-BC

[0070] The specific surface area and pore structure of BC and Se-HAP-BC were analyzed by N2 adsorption method at 77 K using a specific surface area analyzer (ASAP 2460, Micromeritics, USA), and the results are shown in Table 1.

[0071] From the data analysis in Table 1, it can be seen that the specific surface area and pore volume of Se-HAP-BC were about twice that of rice straw biochar. The average pore diameter of BC was 2.97 nm, and the average pore diameter of Se-HAP-BC increased by 24% compared with BC. The results showed that after modification of original rice straw biochar by sodium selenite and hydroxyapatite, the specific surface area, pore volume and pore diameter were greatly increased.

[0072] Table 1 Specific surface area and pore structure of biochar

[0073] Biochar species Specific surface area (m 2 ·g -1 )]]> Pore volume (cm 3 ·g -1 )]]> Average pore diameter (nm) BC 38.03 8.74 2.97 Se-HAP-BC 76.02 17.47 3.67

[0074] Experiment four: pot experiment (I)

[0075] Pot experiment (I) was carried out in a greenhouse on the roof of the College of Resources and Environment, Guizhou University in winter 2022. The test soil for pot experiment was collected from the surface soil (0-20 cm) of Hg-polluted paddy field (N27°33′51″, E109°11′35″) in Wanshan mining area, Guizhou Province, China. The contents of Hg, Se and P in the soil were 3.8 mg / kg, 1.25 mg / kg and 746 mg / kg, respectively. After removing roots and gravel from the soil, the soil was dried, crushed and sieved through a 6.0 mm sieve for use.

[0076] As shown in Table 2, 6 treatment groups (Nos. 2-7) were set up, each of which was set up in parallel with 3 groups, and a control group (CK group) without the addition of biochar was set up. 1.5 kg of soil was loaded into a plastic pot with a volume of 3 L, different soil amendments were added according to a mass ratio of 0.5% (amendment / soil), and were thoroughly mixed. The final Se concentration in the soil treated with Se-HAP-BC was 1.93 mg / kg (measured during the middle of planting), which did not exceed the excessive value of soil Se content (3 mg / kg) in the Specification for Land Quality Geochemical Evaluation (DZ / T 0295-2016). Then a small amount of Hg-free tap water was added to the pot, and it was aged for 3-4 days for standby. The potted crop was duckweed, and the planting period was 60 days. After the purchased seedlings were transplanted into the soil, they were irrigated with tap water, and nitrogen fertilizer was applied once during the middle of planting.

[0077] Table 2

[0078]

[0079] At the 60th day of the pot experiment, soil and plant samples were collected. After removing the surface soil, rhizosphere soil samples were collected in 50 mL centrifuge tubes and immediately stored in a refrigerator for frozen preservation. After freeze-drying, grinding, and passing through a 100-mesh sieve, the soil samples were stored in polyethylene sealed bags for determination of soil Hg forms, available Se, and physical and chemical indicators such as pH, dissolved organic carbon (DOC), and sulfate. Since the stems of this variety of duckweed are well developed, the leaves are small and few, so only stem samples were collected into polyethylene self-sealing bags, washed with tap water and deionized water, then dried at low temperature (40°C), ground and stored in polyethylene sealed bags for determination of total mercury (THg) and methylmercury (MeHg) in plants.

[0080] The following method for determining the total mercury content of plants was used: Milestone DMA-80 direct mercury analyzer according to USEPA Method 7473 was used for determination. About 0.02-0.05 g (accurate to 0.0001 g) of plant sample was weighed and placed directly on the sample injector for total mercury determination of the plant sample.

[0081] The following method for determining the methylmercury content of plants was used: 0.1-0.2 g (accurate to 0.0001 g) of plant sample was weighed and placed in a 50 mL polyethylene centrifuge tube, first treated with KOH-methanol solution (250 mg·L -1)digestion 3h, 30min shaking once. After cooling, 3mL of concentrated HC1 was added slowly, and 10mL of dichloromethane was sucked into a centrifuge tube after cooling, then shaken for 30min, centrifuged, filtered, and the remaining dichloromethane was transferred to a new 50mL centrifuge tube and diluted to 45mL with deionized water. The centrifuge tube was placed in a water bath at 45°C and heated to evaporate most of the dichloromethane, then warmed to 80°C, and blown with nitrogen at a flow rate of 200-300mL·min -1 for 8min to remove residual dichloromethane, and finally diluted to 50mL with deionized water. The water sample after back extraction was determined by gas chromatography combined with cold vapor atomic fluorescence mercury detector (CVAFS) after ethylation.

[0082] The following method was used to measure the pH of the soil: a certain amount of soil sample was weighed into a 50mL polyethylene centrifuge tube according to the ratio of soil to water 1:2.5, shaken for 30min, and then measured using a pH meter (SX731, SANXIN, China).

[0083] The following method was used to measure the DOC of the soil: 1g of soil sample was weighed into a 50mL polyethylene centrifuge tube, 20mL of deionized water was added, and shaken for 24h. Then filtered with a 0.45μm microporous filter, and the supernatant was stored in a brown bottle and adjusted to pH 1-2 by adding phosphoric acid, and determined using a fully automatic total organic carbon analyzer (Vario TOC, Elementar, Germany).

[0084] The following method was used to measure the SO4 2- of the soil: 1g of soil sample was weighed into a 50mL polyethylene centrifuge tube, 20mL of deionized water was added, and shaken for 24h. Then filtered with a 0.22μm microporous filter, and the supernatant was stored in a brown bottle and determined using an ion chromatograph (Dionex ICS-1100).

[0085] The following method was used to measure the available selenium content of the soil: 1g of soil sample was accurately weighed into a 50mL polyethylene centrifuge tube, 15mL of 0.5mol·L -1 potassium dihydrogen phosphate (adjusted to pH about 8) was added, and the centrifuge tube was placed in a shaker at room temperature for 210r·min -1 for 90min, then centrifuged at 3000r·min -1 for 15min, 5mL of supernatant was taken into a 25mL colorimetric tube, and 0.5mL of 4.5mol·L -1HCl and 1 mL of 5% potassium persulfate, after shaking, placed in a water bath for 1 h at 90 ℃, after cooling to room temperature, add 1 mL of 3% oxalic acid, continue to heat for 30 min, after cooling, add 7.5 mL of high-purity concentrated HCl, heat for another 15 min, cool to 25 mL, and determine using atomic fluorescence spectrometer (AFS-9700).

[0086] (1)Effects of selenium and hydroxyapatite combined modified biochar on soil pH, DOC, SO4 2- and available Se concentration

[0087] Soil pH, DOC, SO4 2- concentration in different treatment groups are shown in Figure 3 Compared with the control, the pH of the BC treatment group increased slightly, while the rest of the modified biochar treatments decreased, with the largest decrease of 10% in the Se-HAP-BC treatment group. Compared with the control, the DOC content of the BC treatment group increased by 10%, while the modified biochar treatments all decreased the DOC content, with the highest reduction rate of 38% in the HAP-BC treatment group. Compared with the control, the sulfate concentration of the BC treatment group did not change much, while the Se-BC treatment group significantly reduced the soil sulfate concentration (48%), and the two P-containing modified treatment groups increased the soil sulfate content, with the largest increase of 225% in the Se-HAP-BC treatment group.

[0088] From Figure 3 The distribution of available Se concentration in soil showed that the Se-BC treatment group had slightly higher available Se content than the non-Se-containing treatment group, while the Se-HAP-BC treatment group had 71% higher available Se content than the Se-BC treatment group, indicating that Se-HAP-BC treatment promoted the release of soil Se.

[0089] (2)Effects of selenium and hydroxyapatite combined modified biochar on the reduction of available Hg and MeHg in soil

[0090] The distribution of water-soluble, simulated gastric acid and organic-bound Hg concentration in soil corresponding to the Se-BC treatment group, HAP-BC treatment group and Se-HAP-BC treatment group is shown in Figure 4 and Table 3.

[0091] Table 3

[0092]

[0093] The above results indicate that, compared with the control group, there was no significant difference in the water-soluble Hg content in the unmodified BC group. However, after modification with Se and P, the water-soluble Hg content in the Se-BC, HAP-BC, and Se-HAP-BC treatment groups all decreased to varying degrees. Among them, the Se-BC and HAP-BC treatment groups decreased by 17% and 30% respectively compared with the control group; the Se-HAP-BC treatment group showed the largest decrease, at 41%.

[0094] Simulated gastric acid-extractable hemoglobin (Hg) is also one of the forms that is easily absorbed by plants. Compared with the control, both the BC treatment group and the Se-BC treatment group showed a slight increase in simulated gastric acid-extractable Hg. Conversely, the HAP-BC treatment group and the Se-HAP-BC treatment group showed a decrease of 15% and 37% respectively compared with the control group.

[0095] Organically bound Hg refers to the form bound to organic matter such as humic substances, and studies have shown that this form is most strongly correlated with Hg methylation potential. Compared with the control, except for the BC treatment which increased organically bound Hg by 12%, all other treatments showed a decrease. Among them, the Se-BC treatment group and the HAP-BC treatment group decreased by 24% and 20% respectively compared with the control group; the Se-HAP-BC treatment showed the largest decrease, at 29%.

[0096] Soil MeHg content in different treatment groups is shown in the figure. Figure 5 As shown in Table 3, the soil MeHg concentration in the control group was 8.96 ± 1.31 ng / g. Compared with the control, the soil MeHg concentration was slightly increased in the BC treatment and decreased by only 5% in the Se-BC treatment; however, both P-modified biochars significantly reduced soil MeHg, with the HAP-BC treatment reducing it by 39% and the Se-HAP-BC treatment showing the most significant reduction at 63%. This indicates that the combined modification of biochar with Se and HAP enhanced the reduction effect on soil MeHg concentration and produced a synergistic effect in reducing methylmercury in the soil.

[0097] (3) The effect of selenium and hydroxyapatite combined modified biochar on reducing THg and MeHg in water spinach

[0098] Figure 6 Table 4 shows the THg and MeHg contents of water spinach in the Se-BC treatment group, HAP-BC treatment group, and Se-HAP-BC treatment group.

[0099] Table 4

[0100]

[0101] The THg and MeHg concentrations in the control group were 40.11 ± 4.64 ng / g and 15.20 ± 2.46 ng / g, respectively. Compared with the control group, the THg and MeHg concentrations in the BC treatment group were both reduced, with a reduction rate of 21% and 45%, respectively. The THg and MeHg concentrations in the Se-BC treatment group were not significantly different from those in the BC treatment group, while the THg and MeHg concentrations in the two P-modified biochar treatment groups were significantly reduced, especially in the Se-HAP-BC treatment group. Compared with the control group and the BC treatment group, the Se-HAP-BC treatment reduced the THg concentration in the water spinach by 71% and 63%, respectively, and reduced the MeHg concentration by 70% and 46%, respectively.

[0102] The results showed that all biochar treatments could reduce the enrichment of THg and MeHg in water spinach. Compared with unmodified biochar, Se and HAP modified biochar could further reduce the Hg content in water spinach, and the joint modified biochar Se-HAP-BC had the most significant reduction effect, better than the sum of the reduction effect of selenium and phosphorus single modification, showing a synergistic reduction effect of selenium and phosphorus joint modification.

[0103] (4) Mercury reduction effect of selenium and potassium dihydrogen phosphate combined modified biochar

[0104] The test results of soil methylmercury content, total mercury content in water spinach, and methylmercury content in water spinach after using potassium dihydrogen phosphate modified biochar and selenium and potassium dihydrogen phosphate combined modified biochar are shown in Table 5, Figure 7 to Figure 9 .

[0105] Table 5

[0106]

[0107]

[0108] The results showed that compared with Se modification and MKP modification alone, Se and MKP combined modified biochar had a more obvious effect on reducing soil methylmercury and plant methylmercury content, and Se and MKP showed a certain synergistic effect on reducing methylmercury.

[0109] Experiment five pot experiment (II)

[0110] Pot experiment (II) was conducted in a greenhouse on the roof of the College of Resources and Environmental Engineering of Guizhou University in winter 2023. The test soil was collected from the surface soil of 0-20 cm in a Hg contaminated paddy field in Wanshan mining area, Guizhou Province, China. The soil in this paddy field is different from the soil in the paddy field of pot experiment (I), and the Hg content in the soil is 7.8 mg·kg -1After removing roots and gravel in the soil, the soil was air-dried, crushed and passed through a 6.0 mm sieve for use. The experiment was set up with 3 treatment groups (see Table 6), and each treatment group was set up in triplicate. A volume of 3 L of plastic pots was filled with 1.5 kg of soil, and different soil amendments were added at a mass ratio of 0.5% (amendment / soil) and mixed thoroughly. Then a small amount of Hg-free tap water was added to the pots, and the pots were aged for 3-4 days for use. The pot experiment crop was duckweed, and the planting period was 30 days. After the purchased seedlings were transplanted into the soil, they were irrigated with tap water, and nitrogen fertilizer was applied once during the middle of the planting period. The sample collection, treatment and analysis processes were the same as those of the pot experiment (I).

[0111] Table 6

[0112]

[0113] (1) Comparison of total mercury of duckweed between mixed application and modified application

[0114] The comparison of total mercury of duckweed between the control group and the Se-HAP-BC treatment group using the mixed application method (mixing the sodium selenite, hydroxyapatite and biochar mixture prepared in Comparative Example 3 into the soil at an addition amount of 0.5%) is shown in Table 7. Figure 10

[0115] The total mercury content of duckweed in the control group was 77.38 ± 8.50 ng / g, the total mercury content of duckweed in the mixed application group was 60.32 ± 12.45 ng / g, and the total mercury content of duckweed in the Se-HAP-BC treatment group was 38.72 ± 1.33 ng / g. The total mercury content of the mixed application treatment group was reduced by 22% compared to the control, and the total mercury content of the Se-HAP-BC treatment group was reduced by 50% compared to the control, which was reduced by 36% compared to the mixed application. It can be seen that, compared to simply physically mixing selenium modifiers, phosphorus modifiers and biochar, the present application can significantly reduce the total mercury content of plants by modifying biochar through the impregnation method and loading Se and P onto the biochar.

[0116] (2) Comparison of soil methylmercury between mixed application and modified application

[0117] The comparison of soil methylmercury between the control group and the Se-HAP-BC treatment group using the mixed application method (mixing the sodium selenite, hydroxyapatite and biochar mixture prepared in Comparative Example 3 into the soil at an addition amount of 0.5%) is shown in Table 8. Figure 11 ​The soil methylmercury content of the control group was 5.12±0.84 ng / g, the soil methylmercury content of the mixed application group was 5.33±0.52 ng / g, and the soil methylmercury content of the Se-HAP-BC treatment group was 4.40±0.32 ng / g. The soil methylmercury content of the mixed application group was slightly higher than that of the control group, and there was no significant difference. The soil methylmercury content of the Se-HAP-BC treatment group was reduced by 14% compared with the control group, and was reduced by 17% compared with the mixed application.

[0118] It can be seen that, compared with directly mixing selenium modifiers and phosphorus modifiers with biochar, the present application can significantly reduce the soil methylmercury content by modifying biochar through the impregnation method and loading Se and P onto biochar.

[0119] (3) Comparison of Ipomoea nil methylmercury between mixed application and modified application

[0120] The mixed application method (the mixture of sodium selenite, hydroxyapatite and biochar prepared in Comparative Example 3 was applied to the soil at an addition amount of 0.5%) was used to compare the Ipomoea nil methylmercury with the control group and the Se-HAP-BC treatment group as shown in Figure 12 The Ipomoea nil methylmercury content of the control group was 8.70±0.63 ng / g, the Ipomoea nil methylmercury content of the mixed application group was 7.09±0.64 ng / g, and the Ipomoea nil methylmercury content of the Se-HAP-BC treatment group was 4.32±0.26 ng / g. The Ipomoea nil methylmercury content of the mixed application and Se-HAP-BC treatment groups was reduced to different degrees compared with the control group, wherein the mixed application was reduced by 19% compared with the control group, and the Se-HAP-BC treatment group was reduced by 50% compared with the control group.

[0121] It can be seen that, compared with directly mixing selenium modifiers and phosphorus modifiers with biochar, the present application can significantly reduce the plant methylmercury content by modifying biochar through the impregnation method and loading Se and P onto biochar.

[0122] Mechanism discussion

[0123] Compared with the BC, Se-BC, HAP-BC and Se+HAP+BC mixture treatment groups, the Se-HAP-BC treatment group had a better reduction effect on soil methylmercury and plant mercury. The mechanism can be explained from the following aspects:

[0124] The Se-HAP-BC has better adsorption and fixation performance for Hg, and reduces the methylatable and bioavailable Hg forms. The specific surface area and pore size of the biochar are significantly increased after the modification by sodium selenite and HAP (see Table 1), thereby increasing the adsorption and fixation performance for Hg. The adsorption experiment also shows that the adsorption performance of Se-HAP-BC for Hg is increased by nearly 2 times compared with BC, indicating that the biochar modified by sodium selenite and HAP has strong adsorption and fixation performance. The distribution of available Hg in the soil also shows that only HAP-BC and Se-HAP-BC reduce the water-soluble and simulated gastric acid forms that are most easily utilized by organisms. HAP is a six-hexagonal crystal structure formed by tetrahedral stacking, and its special crystal structure has strong binding force for divalent heavy metal ions. On the one hand, the Ca 3- and PO4 2+ in the surface structure can exchange ions to combine with heavy metal ions, and on the other hand, the dissolved PO4 3- in HAP can form coprecipitation with heavy metal ions. The hydrolysis and precipitation of HAP in the soil can also cause the production of some alkaline substances such as carbonates and oxides, so as to increase the pH of the soil and increase the complexation of Hg 2+ . However, the two P-containing treatment groups in the present application slightly reduce the pH, which may be due to the hydrolysis of [Ca8H2(PO4)6·5H2O] in the HAP dissolution process, releasing H + , thereby reducing the pH of the soil.

[0125] ②Se-HAP-BC makes more soil Hg into a more difficult to use mercury form than organic bound mercury, making the mercury in the soil more stable. The results of the distribution of soil Hg forms show that BC treatment significantly increases organic bound mercury, while other modified biochar significantly reduces organic bound mercury, especially Se-HAP-BC treatment. This shows that the mercury adsorbed in Se-HAP-BC does not exist in the form of organic bound mercury, but is combined with Se-HAP-BC into the following more difficult to use residual state. The extraction method of mercury forms in this application comes from the research of Bloom et al (Bloom, N. S., Preus, E., Katon, J., Hiltner, M., 2003. Selective extractions to assess the biogeochemically relevant fractionation of inorganic mercury in sediments and soils. Analytica Chimica Acta 479, 233-248), which is more suitable for the separation of mercury forms. The organic bound mercury in this study mainly refers to the form combined with organic matter such as humus. Although the organic bound state is more difficult to be used by organisms than the water-soluble state and the simulated gastric acid state, the study found that the organic bound state has the strongest correlation with the methylation potential of Hg, indicating that this form is not stable. While BC significantly increases organic bound mercury, it increases the methylation potential of mercury, and also makes the fixing effect of BC on mercury in some soil environments unstable, and even may increase the Hg and MeHg content of crops. Se-HAP-BC significantly reduces organic bound mercury, converts soil mercury into more difficult to use forms, thereby reducing soil methylmercury, and also makes its remediation effect more stable.

[0126] ③Se and P in Se-HAP-BC have a synergistic effect on reducing soil methylmercury. The reduction effect of Se-HAP-BC on soil MeHg, plant total mercury and methylmercury is better than the sum of the reduction effects of single modified biochar (HAP-BC and Se-BC), indicating that the combined modification of Se and P in Se-HAP-BC has a synergistic effect on reducing soil methylmercury and plant mercury. The distribution of available Se in soil shows that the available Se in soil in Se-HAP-BC treatment is significantly higher than that in other treatment groups. This may be due to the fact that P and Se have similar chemical properties, and Se and P compete for adsorption complexation sites in soil, so the addition of P in soil promotes the release of Se in soil particles. Compared with Se-BC, the available Se in Se-HAP-BC increases significantly, so the antagonistic effect of Se and Hg is enhanced, thereby further reducing the methylation and bioavailability of soil Hg.

[0127] (4) Se-HAP-BC significantly reduced soil DOC concentration while significantly increased SO4 2- As mentioned above, soil amendment with biochar will cause changes in soil physicochemical properties, and both the increase of sulfate and DOC in soil can promote soil Hg methylation. In this study, both HAP-BC and Se-HAP-BC significantly reduced soil DOC concentration compared to the control, which reduced the effective carbon source for Hg-methylating microorganisms and thus reduced soil MeHg. This might be due to the HAP loaded on biochar, which reduced the surface area of biochar and thus reduced the decomposition of organic molecules on the surface of biochar. On the other hand, the PO4 3- and other functional groups on the surface of HAP adsorbed and complexed DOC, which reduced soil DOC concentration. In this study, the SO4 2- concentration in BC treatment did not differ significantly from the control, on the contrary, HAP-BC and Se-HAP-BC significantly promoted the increase of soil SO4 2- concentration. S and P have great similarity in chemical properties, and will compete for adsorption in soil particles. Exogenous addition of P will inevitably cause a large amount of sulfur to be dissolved, thus significantly increasing the SO4 2- concentration. SO4 2- has a double-edged effect on Hg methylation. On the one hand, sulfate can promote the growth of SRB and thus promote Hg methylation; but when SO4 2- exceeds a certain concentration, such as 30 mg / L, a large amount of sulfur ions will form HgS precipitate with Hg 2+ , thus reducing the amount of methylatable Hg.

[0128] The technical scheme of the present application is not limited to the above specific embodiments, and any technical modification made according to the technical scheme of the present application falls within the protection scope of the present application.

Claims

1. A biochar amendment for reducing plant mercury and soil methylmercury, comprising biochar loaded with both selenium and phosphorus; The preparation method of the biochar amendment for reducing plant mercury and soil methylmercury includes the following steps: Biochar was impregnated in a solvent containing selenium and phosphorus modifiers and reacted. The reaction solution was separated into a solid phase. The solid phase was dried to obtain biochar loaded with both selenium and phosphorus. The selenium modifier includes selenite; The phosphorus modifier includes at least one of phosphorus-containing minerals, phosphates, superphosphates, pyrophosphates, hydrogen phosphates, and dihydrogen phosphates. The mass ratio of the selenium modifier to the sum of the masses of the biochar and the phosphorus modifier is 1:(1400~1500); the mass ratio of the phosphorus modifier to the sum of the masses of the biochar and the selenium modifier is 1:(2~5).

2. The biochar modifier according to claim 1, characterized in that, The biochar comprises the pyrolysis products of straw.

3. The biochar modifier according to claim 2, characterized in that, The straw includes one or more of rice straw, corn straw, and rapeseed straw.

4. The biochar modifier according to claim 2, characterized in that, The pyrolysis temperature is 450~550℃.

5. The biochar modifier according to claim 2, characterized in that, The pyrolysis time is 3 to 5 hours.

6. The biochar modifier according to claim 2, characterized in that, The average particle size of the straw is 10-30 mesh.

7. The biochar modifier according to claim 2, characterized in that, The average particle size of the biochar is 80-120 mesh.

8. The biochar modifier according to claim 1, characterized in that, The infrared spectrum of the biochar modifier is at 560±10 cm⁻¹. -1 605±10cm -1 It has characteristic peaks.

9. The biochar modifier according to claim 1, characterized in that, The selenium modifier includes Na2SeO3.

10. The biochar modifier according to claim 1, characterized in that, The phosphorus modifier includes at least one of hydroxyapatite, potassium phosphate, potassium hydrogen phosphate, potassium dihydrogen phosphate, ammonium phosphate, diammonium hydrogen phosphate, diammonium dihydrogen phosphate, calcium hydrogen phosphate, calcium phosphate, potassium pyrophosphate, calcium pyrophosphate, potassium superphosphate, and calcium superphosphate.

11. The biochar modifier according to claim 1, characterized in that, The solvent includes water.

12. The biochar modifier according to claim 1, characterized in that, The reaction temperature is 20~30℃; and / or the reaction time is 5~8 hours; and / or the drying temperature is 80~120℃.

13. The biochar modifier according to claim 1, characterized in that, The biochar includes straw biochar.

14. The biochar modifier according to claim 1, characterized in that, The biochar includes one or more of rice straw biochar, corn straw biochar, and rapeseed straw biochar.

15. The biochar modifier according to claim 2, characterized in that, The biochar is prepared by a method comprising the following steps: crushing straw and pyrolyzing it to obtain the biochar.

16. The biochar modifier according to claim 15, characterized in that, The pyrolysis temperature is 450~550℃.

17. The biochar modifier according to claim 15, characterized in that, The heating rate during pyrolysis is 5~15℃·min. -1 .

18. The biochar modifier according to claim 15, characterized in that, The pyrolysis time is 3 to 5 hours.

19. The biochar modifier according to claim 15, characterized in that, The straw is first dried and then crushed.

20. The biochar modifier according to claim 15, characterized in that, After pyrolysis, the material is further ground and sieved.

21. The biochar modifier according to claim 15, characterized in that, Grind until it passes through an 80-120 mesh sieve.

22. A method for reducing phytomercury and soil methylmercury, comprising: The biochar amendment according to any one of claims 1 to 21 is applied to mercury-contaminated soil.

23. The method according to claim 22, characterized in that, The biochar amendment is added to the mercury-contaminated soil at a rate of 0.3-1 wt%.

24. The method according to claim 22, characterized in that, The biochar amendment is added at a rate of 0.4 to 0.6 wt% in the mercury-contaminated soil.

25. The use of the biochar amendment as described in any one of claims 1 to 21 in reducing plant mercury and soil methylmercury.

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