CN112403444A

CN112403444A - Modified biochar for reducing methyl mercury enrichment in rice and method

Info

Publication number: CN112403444A
Application number: CN202011312037.XA
Authority: CN
Inventors: 王定勇; 徐国敏; 杨雪玲; 王明星; 张兵兵; 张�成; 王永敏
Original assignee: Southwest University; Guizhou Material Industrial Technology Research Institute
Current assignee: Southwest University; Guizhou Material Industrial Technology Research Institute
Priority date: 2020-11-20
Filing date: 2020-11-20
Publication date: 2021-02-26
Anticipated expiration: 2040-11-20
Also published as: CN112403444B

Abstract

The invention discloses a modified biochar for reducing methyl mercury enrichment in rice and a method thereof, wherein the modified biochar is prepared by the following method: dissolving chitosan in an acidic aqueous solution with the pH value of 4-5 to obtain a chitosan solution for later use after the chitosan is completely dissolved; sequentially adding the biochar and the chitosan solution into a reactor at a rotating speed of 120-150 r.min^‑1Reacting for 2-3 h under the reaction conditions that the temperature is 40-50 ℃ and the pressure is 20-25 MPa; after the reaction is finished, quickly releasing pressure, taking out a reaction product, repeatedly cleaning the reaction product by using ethanol, drying the reaction product in a vacuum drying oven at the temperature of 80 ℃, and drying the reaction productAnd then obtaining the modified biochar. The modified charcoal can absorb mercury and methyl mercury in soil, fix mercury and methyl mercury in soil, and inhibit the generation of methyl mercury in soil, thereby reducing the enrichment of methyl mercury in rice. The application amount of the traditional biochar can be greatly reduced by adopting the modified biochar, and the cost is reduced.

Description

Modified biochar for reducing methyl mercury enrichment in rice and method

Technical Field

The invention belongs to the technical field of agriculture, and particularly relates to modified biochar for reducing methyl mercury enrichment in rice and a method.

Background

Mercury (Hg) is a global pollutant that can migrate long distances and cause persistent pollution. The occurrence of water was responsible for the recognition of the hazard of mercury, particularly methylmercury, to humans after a water illness event in 1956. Mercury can generate methylation reaction under natural conditions to generate methyl mercury, the biological toxicity of the methyl mercury is far higher than that of inorganic mercury, and the methyl mercury is listed in the blacklist of the Chinese environment priority pollutants in China. In an aquatic organism food chain, due to the biological amplification effect of methyl mercury, high-concentration methyl mercury can be enriched in the bodies of aquatic animals with high nutrition levels, and the main way for exposing methyl mercury is to eat aquatic products of people. However, researches show that the mercury content, particularly the methyl mercury content, of the rice in the mercury mine area is also high, and the methyl mercury content of the rice produced by the polluted soil in partial mercury mine areas in China is as high as 180 mu g/kg^-1Horvat et al, investigated Wanshan Mercury mining areas in Guizhou province, and found that the total mercury content in rice is as high as 569 mug.kg^-1The content of methyl mercury is 145 mug/kg^-1. The research of Feng et al and Zhang et al finds that the rice eating of residents in mercury mining areas is a main way for exposing the methyl mercury in human bodies, the ratio of the intake amount of the rice to the total intake amount is up to more than 90%, and the intake amount of the methyl mercury from the rice of residents in mercury mining areas is far higher than that of other foods.

Rice is one of the most important grain crops in the world, and the rice planting area in the world is about 1.43 multiplied by 10 at present⁹hm²All areApproximately half of the population of the ball is on rice as the primary diet. China is the largest rice producing country in the world, and the rice planting area in 2017 reaches 3 multiplied by 10⁸hm². However, in China, the mercury content of more than 1.6 percent of cultivated land soil exceeds the standard, but in many areas of China, the mercury-polluted soil is still used for planting rice due to the shortage of land resources. Therefore, for the countries using rice as staple food, the eating of mercury-polluted rice is a new health problem, and how to inhibit the formation of methylmercury in rice fields and the enrichment of methylmercury in rice is an important problem to be solved urgently.

The traditional physical, chemical and biological remediation methods can be used for remedying the mercury pollution of the soil, but the application is less at present due to the defects of poor treatment effect, easy generation of secondary pollution or damage to the soil structure and the like. The biochar serving as a novel adsorption material has the characteristics of large specific surface area, rich surface oxygen-containing functional groups, adsorption sites, a large amount of carbon fiber and the like, so that the biochar is concerned by a plurality of scholars. The biochar can effectively improve the property of the soil, increase the fertility of the soil, change the form of heavy metal in the soil and solidify the activated heavy metal, thereby reducing the bioavailability of the heavy metal in the soil environment. Research shows that after the biochar is added into mercury-polluted soil, mercury can be passivated through chemical adsorption, and the content of effective mercury is reduced; and with the increase of the addition amount, the passivated mercury is more, and the mercury content in the plant body is effectively reduced. These studies all provide preliminary evidence that biochar can fix methylmercury in soil, thereby reducing the mercury content of the edible part of rice. However, most of previous researches are limited to adding common biochar, and a relatively high amount of biochar (the carbon-soil ratio is 3% -10%, which is equivalent to 4500-15000 kg/mu) is usually added to achieve the effect, so that the treatment cost is extremely high, and the popularization and application are difficult. Therefore, it is very important to find a method for inhibiting methylmercury in paddy fields with high efficiency and low cost.

The chitosan is a deacetylated product of chitin which is a natural high polymer material, and the surface of the chitosan contains a large amount of amino (-NH)₂) Hydroxy (-OH) and amido (-CO-NH)₂) And the polymer chain has a flexible structureSo that the heavy metal ion adsorbent has strong adsorption effect on heavy metal ions. At present, the research on applying chitosan to mercury-containing wastewater treatment is more, and the application in soil mercury pollution remediation is relatively lagged. And the natural chitosan has weak mechanical strength and poor stability in water environment, is easy to run off and cannot become a high-performance mercury ion adsorbent.

Disclosure of Invention

In view of the above-mentioned disadvantages of the prior art, the present invention aims to provide a modified biochar and a method for reducing methyl mercury enrichment in rice. The modified charcoal can absorb mercury and methyl mercury in soil, fix mercury and methyl mercury in soil, and inhibit the generation of methyl mercury in soil, thereby reducing the enrichment of methyl mercury in rice. The application amount of the traditional biochar can be greatly reduced by adopting the modified biochar, and the cost is reduced.

The technical scheme of the invention is realized as follows:

a modified biochar for reducing methyl mercury enrichment in rice is prepared by the following steps:

(1) dissolving chitosan in an acidic aqueous solution with the pH value of 4-5 to obtain a chitosan solution for later use after the chitosan is completely dissolved;

(2) sequentially adding the biochar and the chitosan solution into a reactor at a rotating speed of 120-150 r.min^-1Reacting for 2-3 h under the reaction conditions that the temperature is 40-50 ℃ and the pressure is 20-25 MPa;

(3) and after the reaction is finished, quickly relieving the pressure, taking out a reaction product, repeatedly cleaning the reaction product by using ethanol, drying the reaction product in a vacuum drying oven at the temperature of 80 ℃, and drying to obtain the modified biochar.

Further, the mass ratio of the chitosan to the biochar is 1-5: 100.

Further, the mass ratio of chitosan to biochar is 3: 100.

Further, the acidic aqueous solution is an aqueous hydrochloric acid solution or an aqueous sulfuric acid solution.

Further, the reactor is a supercritical carbon dioxide plant.

A method for reducing methyl mercury enrichment in rice comprises the steps of applying biochar to a rice field before rice planting, then turning over soil with the depth of 0-20 cm on the surface layer of the soil in the rice field to enable the biochar to be uniformly mixed with the soil, and then planting the rice; the biochar is the modified biochar for reducing the methyl mercury enrichment in the rice.

Further, the modified biochar is applied at 300 kg/acre.

Compared with the prior art, the invention has the following beneficial effects:

1. according to the invention, the biochar is used as a carrier, and chitosan is stably loaded on the surface of the biochar to prepare the modified biochar, so that the modified biochar can provide organic matters required by growth for plants, increase the biomass of rice to a certain extent and promote the growth and development of the rice plants; and after the biological carbon is modified by adopting the chitosan, the adsorption capacity of the biological carbon is increased, mercury and methyl mercury in soil can be adsorbed and fixed in the soil, and meanwhile, the surface of the modified biological carbon contains a large amount of-NH with strong activity₂and-OH, divalent mercury ions in the soil can be effectively chelated, the generation of methyl mercury is inhibited, and the migration of the methyl mercury to plant bodies is inhibited, so that the enrichment of the methyl mercury in the rice is reduced.

2. By using the modified biochar disclosed by the invention, only 300kg (the carbon-soil ratio is 0.2%) of the modified biochar is added in each mu of rice field, so that the generation of methyl mercury in the rice field soil can be effectively inhibited, and the enrichment of methyl mercury in rice can be effectively reduced, so that the consumption of the traditional biochar is greatly reduced, and the cost is reduced.

Drawings

FIG. 1-Electron microscopy scan of modified biochar prepared in example 1.

FIG. 2-Electron microscopy scan of biochar.

FIG. 3-graph of the change in mercury concentration in three treated interstitial water samples, CK1, CK2, and CMBC.

FIG. 4-graph of the change in mercury concentration in three treated soil samples, CK1, CK2, and CMBC.

5-CK1, CK2 and CMBC.

Detailed Description

Modified biochar for reducing methyl mercury enrichment in rice is prepared by the following method:

under acidic conditions, chitosan can only be dissolved, so an acidic aqueous solution is used to dissolve chitosan, and in order to accelerate the dissolution rate, the chitosan can be stirred to accelerate the dissolution.

As the paddy field contains water, mercury in soil can be converted into methyl mercury, the biochar has a certain amount of-OH groups, and has certain adsorption capacity on mercury and methyl mercury, but experiments show that the biochar can increase the content of soluble organic carbon in the soil, provide nutrient substances for microorganisms, and promote the activity of the microorganisms, so that the methylation of the soil mercury is promoted, namely the generation of the methyl mercury is promoted.

Although chitosan has adsorption capacity and can adsorb mercury, chitosan is not easy to dissolve in water, and because the soil of the paddy field contains soil particles, chitosan is difficult to disperse in the paddy field and is mixed with the soil of the paddy field; meanwhile, the rice field contains water, and the chitosan has poor stability, so that the chitosan is easy to lose and cannot exert an adsorption function.

Therefore, the invention adopts the chitosan modified biochar, the modified chitosan can be loaded on the surface of the biochar and uniformly dispersed in a rice field along with the biochar, and the surface of the chitosan contains a large amount of-NH with strong activity₂and-OH group, thereby increasing a large amount of-NH on the surface of the biochar₂and-OH groups, thereby increasing the mercury contribution of biocharAnd the adsorption capacity of methylmercury; meanwhile, chitosan is a polymer and is loaded on the surface of the biochar, like hanging a piece of the chitosan with-NH on the surface of the biochar₂And a network of-OH groups is more favorable for chelating bivalent mercury ions and inhibiting the generation of methyl mercury.

Compared with the traditional method for treating sewage by applying chitosan, the method only utilizes the chitosan to quickly adsorb heavy metals in the sewage so as to achieve the purpose of standard discharge in the sewage treatment process. The invention not only utilizes the adsorption capacity of chitosan, but also utilizes the chelating capacity of chitosan, which can chelate bivalent mercury ions and inhibit the generation of methyl mercury.

In specific implementation, the mass ratio of the chitosan to the biochar is 1-5: 100, and a certain amount of chitosan can be loaded on the biochar through reaction, but researches show that the chitosan loading amount on the biochar reaches the maximum when the mass ratio of the chitosan to the biochar is 3:100, and when the chitosan loading amount is higher than the value, the chitosan cannot be completely loaded on the biochar, so that raw material waste is easily caused.

In particular, the reactor is a supercritical carbon dioxide unit. Supercritical carbon dioxide is a fluid with a low density like a liquid and a low viscosity like a gas, having solubility for many solid substances. By utilizing the excellent solubility of the supercritical carbon dioxide, the chitosan in the acidic aqueous solution permeates into each part of the biochar, so that partial hydroxyl on the surface of the chitosan reacts with carboxyl on the surface of the biochar, and the chitosan is stably loaded on the surface of the biochar.

The modified biochar is applied to reduction of methyl mercury enrichment in rice, before rice is planted, the modified biochar is applied to a rice field, then soil with the depth of 0-20 cm on the surface layer of the rice field soil is ploughed, the modified biochar is uniformly mixed with the soil, and then the rice is planted. And the modified biochar is applied at 300 kg/mu of modified biochar.

The present invention will be described in further detail with reference to the accompanying drawings and specific embodiments.

Firstly, preparing modified biochar

Example 1

Dissolving chitosan in an acidic aqueous solution with the pH value of 4-5 according to the amount of 3% of the biochar, stirring for 2 hours, and after completely dissolving, respectively adding the biochar and the solution into supercritical carbon dioxide (scCO)₂) In the device, the rotating speed is 120 r.min^-1And reacting for 2 hours under the reaction conditions of the temperature of 40 ℃ and the pressure of 20MPa, quickly decompressing, taking out a reaction product, repeatedly cleaning with ethanol, and drying in a vacuum drying oven at the temperature of 80 ℃ for 12 hours to obtain the chitosan modified biochar.

The scanning electron microscope of the modified biochar prepared in this example is shown in fig. 1, and the scanning electron microscope of the biochar is shown in fig. 2, which shows that: the structure of the modified biochar is loose relative to the biochar, and small particles are obviously increased, which shows that chitosan is loaded on the biochar, and amino (-NH) is increased₂) Hydroxy (-OH) and amido (-CO-NH)₂) Etc. are favorable for increasing the Hg²⁺The adsorption capacity of the composite material can inhibit the generation of the methyl mercury, is beneficial to enhancing the adsorption of the methyl mercury and inhibiting the migration of the methyl mercury to plants.

The same procedures as in example 1 were carried out for example 2 and example 3, except that the amount of chitosan in example 2 was 1% of the biochar, the modified biochar obtained was designated as BC-chitosan (1%), the amount of chitosan in example 3 was 5% of the biochar, the modified biochar obtained was designated as BC-chitosan (5%), and the modified biochar obtained in example 1 was designated as BC-chitosan (3%). The modified biochar and Biochar (BC) obtained in examples 1 to 3 were analyzed.

1. The elemental composition and the results of the ratio analysis are shown in Table 1.

TABLE 1 elemental composition and proportion table

As can be seen from Table 1: examples 1 and 3, chitosan is loaded on the surface of the biochar, so that the O/C atomic ratio of the biochar surface is obviously increased, and the polar functional groups on the modified biochar surface are increased, and the sites for combining with mercury ions are increased.

2. The results of the adsorption-desorption isotherm analysis of N2 are shown in Table 2.

TABLE 2 Components N₂Structural parameters of adsorption-desorption

As can be seen from Table 2: the specific surface area and the pore volume of the chitosan-loaded biochar are both significantly lower than those of unmodified biochar, which indicates that chitosan is successfully grafted on the inner hole and the surface of the biochar under the action of a supercritical carbon dioxide fluid medium.

Second, experiment was conducted using the modified charcoal prepared in example 1

1. Experimental Material

The rice variety to be tested is 'Fengyou 210', which is provided by Longpingbreeder Co., Ltd, and rice seedlings with basically consistent sizes are selected for testing after field seedling raising.

The soil to be tested is collected in 0-20 cm of soil on the surface layer of a test farm cultivated land of southwest university, the soil is neutral purple soil developed by purple sandstone of the Jurashi group of Jurassic period, and the basic properties of the soil to be tested are shown in Table 3. Removing large granule sandstone and plant residue from soil to be tested, air drying, grinding with 2mm sieve, and adding exogenous mercury until the mercury content in soil is about 5mg/kg^-1And then aging for later use.

TABLE 3 basic Properties of the soil tested

Item	pH	SO₄ ^2-/mg·kg^-1	NH₄ ⁺-N/mg·kg^-1	Organic matter/g.kg^-1	THg/μg·kg^-1	Mercury methyl/ng.kg^-1
							Soil(s)	7.5	22.4	43.3	9.1	169.1	26.8

According to GB15618-2018 'agricultural land soil pollution risk control standard', when the mercury content in soil is higher than 3mg/kg, the soil can not be used for planting rice, other dry land crops need to be improved, if the mercury content is higher, edible agricultural products can not be planted, and the mercury content in the experimental soil is 5 mg/kg.

2. Design of experiments

The test was carried out with 3 treatments, namely a control treatment without biochar (CK1), a control treatment with unmodified biochar (CK2) and a treatment with chitosan-modified biochar (CMBC), each treatment consisting of 6 parallel pots.

A polyethylene flowerpot with an upper opening with the diameter of 245mm, a lower opening with the diameter of 210mm and the height of 245mm is selected as a pot culture container. Weighing 18 parts of test soil aged by 7kg of exogenous mercury in 2019, 5 months and 8 days, wherein N, P elements and K elements added in each part of soil are respectively 150 mg/kg, 100 mg/kg and 85 mg/kg^-1A base fertilizer ofUniformly mixing the middle 6 parts, and then potting to process CK 1; 6 portions of unmodified biochar 14g (the content of the biochar is 2g kg)^-1Namely the carbon-soil ratio is 0.2 percent), uniformly mixing, potting and processing for CK 2; the other 6 parts are respectively added with 14g of chitosan modified biochar (the content of the biochar is 2 g.kg)^-1I.e. the carbon-soil ratio is 0.2%), uniformly mixed and potted, and subjected to CMBC treatment. Adding water into each pot, submerging the pot for 2cm in soil, stabilizing the pot for one week, transplanting the seedlings in 2019, 5 months and 15 days, planting two rice seedlings in each pot, and culturing the seedlings in a glass net room. The rice is irrigated with tap water regularly during the whole growth process, and the water level is kept about 2cm above the soil.

The control group only added with chitosan is not arranged, firstly, the chitosan is required to be dissolved under an acidic condition, and the soil is generally neutral, so that the chitosan cannot be dissolved in the soil, and the soil can be damaged after the chitosan is added into the soil after the chitosan is dissolved by adding acid; secondly, the addition amount of the chitosan is less, and if the chitosan is added in the experiment, the amount of the chitosan is about 0.41g, so the influence is very small.

3. Sample collection and processing

Collecting interstitial water samples and soil samples in the growth process of rice and collecting plant samples in the mature period.

(1) Interstitial water sample: water samples were collected every other week after transplanting, and interstitial water samples of rice were collected with an intermittent water sampler (purchased from SHANGHAI SAFE BIOTECH, a sampler auto-filtration 10 μm filter membrane), and stored in a refrigerator at 4 ℃.

(2) Soil sample: the soil sample is collected once every two weeks, the rice soil sample is collected by a multi-point collection method (the middle point of two plants and 4 points in the peripheral cross direction), and then is freeze-dried by a freeze dryer (ZX-LGJ-18, Shanghai know-Xin laboratory instruments & ltSUB & gt) after collection, ground, sieved by a 100-mesh sieve, sealed by a self-sealing bag, and refrigerated and stored by a refrigerator.

(3) Mature plant sample: collecting roots, rice hulls and rice of rice, washing with tap water, then washing with pure water, freeze-drying, weighing, crushing, sieving with a 100-mesh sieve, sealing with a self-sealing bag, and refrigerating and storing in a refrigerator.

4. Sample measuring method

(1) Method for measuring methyl mercury (MeHg)

Interstitial water sample: the interstitial water sample is acidified and distilled at 150 ℃, when 80-85% of the interstitial water sample is distilled, the distillate is poured into a bubble bottle, and the sample is measured by gas chromatography-cold atomic fluorescence spectrometry (CVAFS, Brooks Rand III, USA).

Soil sample: adding CuSO into soil sample₄The solution (GR) was leached with 25% nitric acid (GR), extracted with dichloromethane, followed by counter-extraction with water, and determined by GC-CVAFS.

Plant sample: digesting a plant sample by using KOH-methanol (GR) in a water bath at 75-80 ℃ for 3h, adjusting the pH value to be acidic by using HCl (GR), extracting by using dichloromethane, carrying out water phase back extraction, and determining by using a GC-CVAFS method.

(2) Method for measuring total mercury (THg)

Interstitial water sample: after water sample is acidified, 200 mul BrCl (GR) is added for oxidation for at least 12h, and Hg in various forms in water is oxidized into Hg ²⁺200 μ L of hydroxylamine hydrochloride (GR) was added 30min before the assay to reduce excess BrCl, 2mL of SnCl was added to the bubbler₂(GR), taking a proper amount of sample, adding Hg into a bubbling bottle²⁺All reduced to Hg⁰And then measured by cold atomic fluorescence spectrometry (Model 2500, tekran, USA).

Soil sample: adding 5mL of aqua regia prepared in situ into soil sample, heating in water bath at 95 deg.C for 5min, adding 1mL of BrCl, continuing water bath for 30min, fixing volume, standing for 24h, adding 200 μ L of hydroxylamine hydrochloride 30min before measurement, and adding SnCl₂Reduction and determination with a cold atomic absorption mercury porosimeter (model F732-VJ, Shanghai Huaguang instruments and Meteorology).

Plant sample: adding the HNO prepared in situ into the plant sample₃(GR)/H₂SO₄(GR) (volume ratio 4:1), digesting in water bath at 95 ℃ for 3h, adding 500 mu LBrCl for oxidation, fixing volume, standing for 24h, adding 200 mu L of hydroxylamine hydrochloride 30min before measurement, and adding 2mL of SnCl₂Reducing and measuring by a cold atomic absorption mercury detector.

5. Quality control

Glassware used in this test was soaked in nitric acid (25% by volume) for 24h, washed with ultrapure water (Milli-Q), and then burned at 500 deg.C in a muffle furnaceBurning for 30min, and cooling in mercury-free environment. The analysis process adopts a blank test and a parallel sample test, and GSB04-1729-2004(Hg (NO)₃)₂standard solution) for recovery. GSS-7(GBW07407, institute of geophysical and chemical investigation) and GBW10016(GSB-7) are respectively used for quality control in the measurement of methyl mercury and THg of soil and rice plants, and the standard addition recovery rates are 89-105% and 83-106%. The detection limit of the measurement method is 0.002 mug.kg^-1And 0.01. mu.g.kg^-1The Relative Standard Deviation (RSD) of the sample repeat measurements is not more than 5% and 8%, respectively.

6. Results of the experiment

1.1 characteristics of mercury concentration variation in interstitial water samples

As shown in FIG. 3, the THg mass concentrations of interstitial water in 3 treatments, namely CK1, CK2 and CMBC, in the whole growth process of rice are respectively 0.29-5.01, 0.41-4.63 and 0.35-3.48 mug.L^-1Within the range, there is significant correlation (P < 0.05); the THg mass concentrations of 3 treatments at 5 months and 22 days are respectively 5.01, 1.88 and 1.27 mug.L^-1Significantly correlated (P < 0.05), it is evident that the CMBC treatment has the lowest mass concentration of THg, probably due to the addition of Hg already at 5 months and 8 days of aging²⁺And biochar which is used for treating Hg in the aging process²⁺There is a certain amount of adsorption; the THg mass concentration fluctuation is small and the whole concentration is small after the flowering period, and the THg mass concentration is respectively 1.05, 1.21 and 0.67 mu g.L after the 110d and 3 kinds of treatment of rice growth by 9 months and 2 days^-1。

The mass concentration of the methyl mercury in each treatment reaches the maximum at the 42 th day of the growth of rice when the methyl mercury is in 26 days after 6 months; the mass concentration of the methyl mercury is higher before the jointing stage of 7 months and 17 days, but the fluctuation is larger; the mass concentration of the methyl mercury is obviously reduced and tends to be stable before and after the flowering period of 7 months and 30 days. In the whole growth process of the rice, the mass concentration of the 3 types of treated methyl mercury is respectively 0.054-1.09, 0.035-1.03 and 0.028-0.61 mu g.L^-1Wherein the CMBC has a methylmercury mass concentration significantly lower than CK1(P < 0.05) and CK2(P < 0.05), indicating that the chitosan-modified biochar is capable of inhibiting the production of methylmercury in interstitial water.

1.2 characteristics of the change in Mercury concentration in soil samples

The characteristics of the THg, methyl mercury content and methylation rate of the soil treated in different ways during the growth period of the rice are shown in figure 4.

As can be seen from FIG. 4(a), the THg content of soil treated with CK1 was not much changed, and was 2.79-3.79 mg/kg^-1Within the range; besides two abnormal points of 17 days and 30 days of 7 months, the fluctuation of the THg content L of the CK2 treated soil is small and is 2.59-3.55 mg-kg^-1Within the range; the THg content of CMBC treated is 2.76-4.35 mg/kg^-1Within the range.

As can be seen from FIG. 4(b), the difference of the methyl mercury content in the soil in the 3 treatments is large, and the difference is respectively 5.78-161.15, 14.63-203.45 and 1.42-82.62 mug-kg^-1The content of the methylmercury of the CMBC is obviously lower than that of CK1(P < 0.05) and CK2(P < 0.01), but the change trend of the content of the methylmercury of the soil is basically consistent, and the content of the methylmercury in the soil gradually increases along with the growth time. The content of methyl mercury in the soil treated by 3 kinds of soil in the mature period is 161.15, 203.45 and 59.59 mug-kg^-1The content of the methylmercury of CK2 is very much higher than that of CK1(P < 0.01) because biochar can increase the content of soluble organic carbon in soil, can provide nutrients required by microorganisms, promote the activity of the microorganisms and promote the methylation of mercury, so that the content of the methylmercury of the soil is higher than that of a control group without biochar after the biochar is added, and the content of the methylmercury of the soil in CMBC treatment is very much lower than that of CK1(P < 0.01) and CK2(P < 0.01) because the surface of chitosan contains a large amount of NH with strong activity₂and-OH, which can chelate divalent mercury ions, thereby inhibiting the formation of methylmercury.

FIG. 4(c) shows the methylation rates of mercury in the soils in different periods of 3 treatments, and it can be seen that the methylation rates of mercury in all 3 treatments are gradually increased, and the methylation rates are respectively in the ranges of 0.16% -4.41%, 0.48% -6.74% and 0.034% -2.16%, and the methylation rates of mercury in the soils treated by CMBC are lower than CK1(P < 0.01) and CK2(P < 0.01). Compared with CK1, the CMBC soil mercury methylation rate is reduced by 51.1% -79.1%, which shows that the anaerobic methylation of mercury can be effectively inhibited by adding chitosan modified biochar in soil.

1.3 characteristics of variation of biomass and mercury content of each part of rice in mature period

(1) Biomass of each part

As can be seen from table 4, both treatments with added biochar had plant dry weights greater than the blank treatment. The dry weight of the root, the stem, the leaf and the kernel processed by CK2 is respectively 3.3, 1.3, 1.1 and 2.1 times of that of the root, the stem, the leaf and the kernel processed by CK1, and the dry weight of the root, the stem, the leaf and the kernel processed by CMBC is respectively 2.5, 1.2, 1.4 and 2.2 times of that of the root, the stem, the leaf and the kernel processed by CK 1. The addition of the biochar can provide organic matters required by plant growth, can increase the biomass of rice to a certain extent, and promotes the growth and development of rice plants, which is similar to the research results of Shu R and the like.

TABLE 4 weight/g of each part of 3 treated rice plants in the maturation stage

Treatment of	Root of herbaceous plant	Stem of a tree	Leaf of Chinese character	Grain	Total amount of
						CK1	7.2	27.1	17.2	15.5	67.0
CK2	23.4	35.9	18.6	33.2	111.1
						CMBC	17.8	32.1	23.7	34.4	108.0

(2) Methyl mercury concentration in rice roots, rice hulls and rice in mature period

The methyl mercury content in the rice roots, rice and rice hulls treated in different maturation periods is shown in fig. 5.

As can be seen from FIG. 5(a), the content of methylmercury in the roots treated with CK1 was (3.46. + -. 0.34) mg/kg^-1The content of methylmercury in the CK2 treated roots is (2.45 +/-0.065) mg.kg^-1The reason why the content of the methyl mercury is significantly lower than that of CK1 treatment (P is less than 0.05) is that after the biochar is added, the soluble organic carbon is increased, the binding force of the soluble organic carbon and mercury is strong, methyl mercury can be adsorbed and fixed in soil, and therefore the absorption amount of the methyl mercury of plant roots is reduced, so that the content of the methyl mercury of the rice roots after the unmodified biochar is added is lower than that of the control treatment without the biochar. The content of methyl mercury in roots of the rice treated by the CMBC is (0.93 +/-0.046) mg/kg^-173.1 percent lower than CK1(P < 0.05) and 62.0 percent lower than CK2(P < 0.05), because the amino and hydroxyl on the surface of the chitosan biochar can react with Hg in soil after being added²⁺Fixing and inhibiting the methylation of mercury in soil, thereby reducing the content of methyl mercury in the roots of rice.

As shown in FIG. 5(b), the methylmercury contents of the 3 treated rice hulls, CK1, CK2 and CMBC, were (3.98. + -. 0.32, 1.42. + -. 0.035 and 0.89. + -. 0.11) mg/kg^-1The content of methyl mercury in the CMBC treated rice hulls is obviously lower than that of CK1(P is less than 0.05) and CK2(P is less than 0.05), and the content of methyl mercury in the CK1 treated rice hulls is obviously lower than that of the CK1 treated rice hulls and that of the CK2 treated rice hullsThe mercury content was 4.47 times that of the CMBC treatment with the addition of CMBC, and the methyl mercury content of the CK2 treatment was 1.60 times that of the CMBC treatment.

As is clear from FIG. 5(c), the methylmercury contents of the 3 treated rice, CK1, CK2 and CMBC, were (11.69. + -. 1.34, 10.54. + -. 0.050 and 2.86. + -. 0.10) mg/kg^-1The methylmercury in the CMBC treated rice is significantly lower than CK1(P < 0.01) and CK2(P < 0.05), 75.8% lower than CK1 and 72.9% lower than CK 2. Therefore, the CMBC treatment can obviously reduce the content of methyl mercury in the rice hulls and rice.

Therefore, the enrichment of the edible part of the methyl mercury of the rice can be inhibited by adding the biochar, and the inhibition capability of the biochar modified by adding the chitosan on the enrichment of the methyl mercury is stronger.

Finally, it should be noted that the above-mentioned examples of the present invention are only examples for illustrating the present invention, and are not intended to limit the embodiments of the present invention. Variations and modifications in other variations will occur to those skilled in the art upon reading the foregoing description. Not all embodiments are exhaustive. All obvious changes and modifications of the present invention are within the scope of the present invention.

Claims

1. The modified biochar for reducing methyl mercury enrichment in rice is characterized by being prepared by the following method:

2. The modified biochar for reducing methyl mercury enrichment in rice as claimed in claim 1, wherein the mass ratio of chitosan to biochar is 1-5: 100.

3. The modified biochar for reducing methyl mercury enrichment in rice as claimed in claim 2, wherein the mass ratio of chitosan to biochar is 3: 100.

4. The modified biochar for reducing methyl mercury enrichment in rice as claimed in claim 1, wherein the acidic aqueous solution is hydrochloric acid aqueous solution or sulfuric acid aqueous solution.

5. The modified biochar for reducing methyl mercury enrichment in rice as claimed in claim 1, wherein the reactor is a supercritical carbon dioxide device.

6. A method for reducing methyl mercury enrichment in rice comprises the steps of applying biochar to a rice field before rice planting, then turning over soil with the depth of 0-20 cm on the surface layer of the soil in the rice field to enable the biochar to be uniformly mixed with the soil, and then planting the rice; the method is characterized in that the biochar is the modified biochar for reducing the methyl mercury enrichment in rice according to any one of claims 1-4.

7. The method of claim 6, wherein the modified biochar is applied at 300 kg/acre.