CN107716532A - The research method that a kind of charcoal influences on cadmium pollution soil property and Cd fractionation - Google Patents

Abstract

The invention discloses a research method for the influence of biochar on the properties of cadmium-contaminated soil and the form of cadmium. In the research method, three kinds of biochars are prepared from hijiki, rice straw and hickory nut shells, and the three kinds of biochars are compared. The adsorption effect of cadmium in aqueous solution, so as to optimize the biochar with the best adsorption of cadmium. By applying different dosages of optimal biochar in cadmium-contaminated soil, and measuring the changes in the basic physical and chemical properties of the contaminated soil and the chemical form of cadmium, the passivation effect of biochar on the physical and chemical properties of the actual cadmium-contaminated soil and soil cadmium pollution was preliminarily discussed. The effects of biochar on the physical and chemical properties and chemical forms of cadmium in actual cadmium-contaminated soil were studied. Reduce the content of soil available Cd, promote the transformation of soil heavy metal Cd from the exchange state to the carbonate-bound state, iron-manganese oxide-bound state, organic-bound state and residue state, and reduce the bioavailability and ecotoxicity of heavy metal Cd, thereby Greatly reduce the harm of heavy metal cadmium.

Description

A research method for the effect of biochar on cadmium-contaminated soil properties and cadmium speciation

technical field

The invention relates to the research field of biochar on cadmium-contaminated soil, in particular to a research method for the effect of biochar on the properties and forms of cadmium-contaminated soil.

Background technique

Soil is the main natural resource on which human beings depend. However, soil heavy metal pollution is becoming more and more serious. Soil heavy metal pollution has a wide range and long duration, and is highly concealed and cannot be biodegraded. Cadmium is one of the most toxic heavy metal elements, and soil cadmium pollution has become a prominent environmental problem. Soil polluted by cadmium will affect the normal physiological metabolism of crops, and then affect human health through the enrichment of the food chain. In recent years, heavy metal cadmium pollution accidents have occurred frequently in my country. For example, the cadmium pollution incident in Liuyang City, Hunan Province in 2006, and the cadmium pollution incident in Longjiang River, Hechi, Guangxi in 2012. Therefore, for the sustainable development of the environment, reasonable and effective control and treatment of cadmium pollution in soil has become the top priority of environmental governance. At present, the method of treating polluted soil is mainly the in-situ remediation method. Although the in-situ remediation method can only change the existing forms of heavy metals in the soil, it has the advantages of rapidity, cost-effectiveness, and ease of implementation. Agricultural product safety requirements. However, choosing a reasonable passivator to change the form of heavy metals in the soil through adsorption, chelation, precipitation, redox, etc., to reduce the bioavailability of heavy metals is an important idea and method in the in-situ remediation of polluted soil. method. Commonly used inorganic modifiers include lime, bone char, zeolite, phosphate, silicate, clay minerals, etc. Commonly used organic modifiers include green manure, peat, compost and animal manure.

Biochar is a new type of environmental functional material, which has been used in many fields such as agriculture, environmental protection, and medicine. Biochar (biochar, BC) refers to a class of aromatized substances with high carbon content produced by high-temperature pyrolysis of biomass such as agricultural waste under complete or partial anoxic conditions. Biochar has the characteristics of stable properties, remarkable surface porosity, large specific surface area, high surface energy, and strong adsorption. It is an ideal new material for carbon sequestration and emission reduction. Moreover, the surface of biochar has many oxygen-containing functional groups such as carboxyl, phenolic hydroxyl, carbonyl, etc., which constitute its good adsorption characteristics. Therefore, biochar has good application potential in adsorbing and fixing soil heavy metals, reducing the bioavailability of soil heavy metals, etc., and it is feasible as a soil remediation agent. In recent years, more and more attention has been paid to the application of biochar in improving soil fertility, developing soil amendments and treating polluted soil. In addition, the raw materials for the preparation of biochar come from a wide range of sources, mainly including agricultural and forestry wastes such as wood, straw, and fruit shells, and organic wastes such as garbage and sludge generated in industry and urban life. However, different raw materials for biochar preparation, pyrolysis conditions, and preparation processes lead to differences in the structure, specific surface area, pH, and ash content of biochar, so it has different environmental effects and applications. However, at present, the raw materials for the preparation of biochar are mainly agricultural and forestry wastes, and there are few studies on the preparation of biochar from marine biomass as raw materials. Moreover, although the research on the remediation effect and mechanism of biochar on soil cadmium pollution has received great attention, most of the research on soil cadmium pollution is mainly carried out by adding exogenous cadmium to soil. There are few studies on the direct passivation of cadmium-contaminated soil, and the impact of biochar application on soil environmental properties and cadmium form changes is still unclear. It is urgent to strengthen relevant theoretical and applied technology research.

Contents of the invention

The technical problem to be solved by the present invention is to provide a research method on the impact of biochar on the properties of cadmium-contaminated soil and the form of cadmium in view of the deficiencies in the prior art. By this research method, different biochars have been studied to reduce the effective state of Cd in soil. It promotes the transformation of soil heavy metal Cd from the exchange state to the carbonate-bound state, iron-manganese oxide-bound state, organic-bound state and residue state, reducing the bioavailability and ecotoxicity of heavy metal Cd, thereby greatly reducing the harm of heavy metal cadmium .

The invention discloses the following technical scheme: a research method for the influence of biochar on the properties of cadmium-contaminated soil and the form of cadmium, and the research method is as follows:

(1) Experimental materials

The soil for testing was collected from the Xiasiling Tungsten Mine, which is located in Xuechuan Village, Heqiao Town, Lin’an City, Zhejiang Province. The sampling depth was 0-20cm. After the soil was collected, it was air-dried, and plant roots, stones and other sundries were removed. Pass through a 10-mesh sieve and mix for later use. The physical and chemical properties of the tested soil are: pH value 5.77, organic matter 32.40mg·kg ^-1 , available phosphorus 23.19mg·kg ^-1 , total nitrogen 1.54g·kg ^-1 , available potassium 147.8mg · kg ^-1 ;

The biochar raw material Hijiki was taken from Dongtou, Wenzhou City, Zhejiang Province. Pecan shells and rice straw were provided by Zhejiang Academy of Agricultural Sciences. The raw materials were washed with tap water, dried naturally, and then placed in ziplock bags for use;

(2) Preparation of biochar

Biomass carbonization adopts oxygen-limited and temperature-controlled carbonization method. The raw material hijiki/hickory shell/rice straw is loosely loaded into an open-type programmable tube furnace and carbonized at 500°C without oxygen, and the heating rate is 25°C·min ^-1 After reaching the final temperature, continue to carbonize for 3 hours. After the tube furnace is naturally cooled to room temperature, take it out and weigh it, and calculate the carbonization yield; put the carbonized product in an oven at 80°C for 24 hours, grind and pass through 10 and 100 mesh sieves respectively, and place Store them in ziplock bags for later use. The obtained biochars are marked as: Y500-Sargassum charcoal, H500-Hickory nut charcoal, S500-Rice straw charcoal;

(3) Preparation of cadmium stock solution

Accurately weigh 2.0314g of CdCl ₂ 2.5H ₂ O into a beaker, add an appropriate amount of distilled water, stir until completely dissolved, transfer to a 1000mL volumetric flask, and constant volume to obtain 1000mg L ^-1 Cd ²⁺ stock solution, room temperature Save and dilute according to experimental needs when ready to use;

(4) Adsorption experiment

Dilute the Cd ²⁺ stock solution to 25, 50, 100, 300, and 500 mg·L ^-1 respectively, adjust the pH value of the Cd ²⁺ solution at different concentrations to 7, and accurately weigh 0.05 g of Y500, H500 and S500 respectively. In a 100mL conical flask, add 50mL of adjusted Cd ²⁺ solution respectively, shake well, put it in a shaker, shake at 25°C, 150r ^· min ^-1 for 24h, then filter, and measure the concentration of Cd ²⁺ in the filtrate , and calculated the removal rate of biochar to Cd ²⁺ ;

The experiment was set up in three parallels, and the results were averaged. The Cd2 ⁺ concentration in the filtrate is measured by plasma atomic emission spectrometry (ICP-AES, Prodigy), and the adsorption amount and removal rate are calculated, and the calculation method is as follows:

In the formula: q _e is the adsorption capacity (mg·g ^-1 ); C ₀ is the mass concentration of cadmium solution before adsorption (mg·L ^-1 ); C _e is the mass concentration of cadmium solution after adsorption (mg·L ^-1 ); V is the volume of cadmium solution (mL); M is the biochar quality (mg) taken by weighing; U is the cadmium removal rate (%);

(5) Pot experiment

Put the air-dried test soil of 10 mesh sieves into pots, 500g in each pot, then add the biochar with the maximum cadmium adsorption capacity into the pots according to the mass percentage of 1%, 2%, and 5%, fully mix, Adjust the water content to 60% of the maximum water holding capacity in the field; at the same time, set the polluted soil without adding biochar as the blank control CK, a total of 4 treatments, and each treatment has 3 repetitions; the soil is supplemented with deionized water every 1d, Keep the water holding capacity at about 60% in the field, and after cultivating at room temperature for 60 days, take samples to analyze the physical and chemical properties of the soil after cultivating and the chemical form of heavy metal Cd in it.

The above research method used marine biomass (hijiki) and agricultural and forestry waste (rice straw, hickory shell) as raw materials to prepare three kinds of biochars, and compared the adsorption effects of three kinds of biochars on cadmium in aqueous solution, so as to optimize The best biochar for cadmium adsorption. By applying different dosages of optimal biochar in cadmium-contaminated soil, and measuring the changes in the basic physical and chemical properties of the contaminated soil and the chemical form of cadmium, the passivation effect of biochar on the physical and chemical properties of the actual cadmium-contaminated soil and soil cadmium pollution was preliminarily discussed. The effects of biochar on the physical and chemical properties and chemical forms of cadmium in actual cadmium-contaminated soil were studied. Reduce the content of soil available Cd, promote the transformation of soil heavy metal Cd from the exchange state to the carbonate-bound state, iron-manganese oxide-bound state, organic-bound state and residue state, and reduce the bioavailability and ecotoxicity of heavy metal Cd, thereby Greatly reduce the harm of heavy metal cadmium.

Description of drawings

Fig. 1 Effect of initial concentration of Cd on the adsorption of Cd by different biochar materials (H500, S500 and Y500);

Fig. 2 Scanning photos of hijiki biochar samples (a × 2000 times, b × 15000 times);

The infrared spectrogram of Fig. 3 Sargassum charcoal sample;

Fig. 4 Sargassum charcoal thermal gravimetric diagram;

Figure 5 Sargassum charcoal nitrogen adsorption analytical isotherm (a) and pore size distribution (b);

Fig. 6 The effect of hijiki charcoal addition on the pH of polluted soil;

Fig. 7 The effect of Sargassum charcoal addition on available phosphorus in polluted soil;

Fig. 8 Effect of Sargassum charcoal addition on available potassium in polluted soil;

Fig. 9 The effect of Sargassum charcoal addition on the total nitrogen of polluted soil;

Figure 10 The impact of hijiki charcoal addition on polluted soil organic matter;

Fig. 11 Effect of Sargassum charcoal on the speciation of heavy metal Cd in polluted soil.

detailed description

Embodiment of the present invention is described in further detail below:

A research method on the influence of biochar on cadmium-contaminated soil properties and cadmium forms, the research method is as follows:

(1) Experimental materials

The soil for testing was collected from the Xiasiling Tungsten Mine, which is located in Xuechuan Village, Heqiao Town, Lin’an City, Zhejiang Province. The sampling depth was 0-20cm. After the soil was collected, it was air-dried, and plant roots, stones and other sundries were removed. Pass through a 10-mesh sieve and mix for later use. The physical and chemical properties of the tested soil are: pH value 5.77, organic matter 32.40mg·kg ^-1 , available phosphorus 23.19mg·kg ^-1 , total nitrogen 1.54g·kg ^-1 , available potassium 147.8mg · kg ^-1 ;

The biochar raw material Hijiki was taken from Dongtou, Wenzhou City, Zhejiang Province. Pecan shells and rice straw were provided by Zhejiang Academy of Agricultural Sciences. The raw materials were washed with tap water, dried naturally, and then placed in ziplock bags for use;

(2) Preparation of biochar

Biomass carbonization adopts oxygen-limited and temperature-controlled carbonization method. The raw material hijiki/hickory shell/rice straw is loosely loaded into an open-type programmable tube furnace and carbonized at 500°C without oxygen, and the heating rate is 25°C·min ^-1 After reaching the final temperature, continue to carbonize for 3 hours. After the tube furnace is naturally cooled to room temperature, take it out and weigh it, and calculate the carbonization yield; put the carbonized product in an oven at 80°C for 24 hours, grind and pass through 10 and 100 mesh sieves respectively, and place Store them in ziplock bags for later use. The obtained biochars are marked as: Y500-Sargassum charcoal, H500-Hickory nut charcoal, S500-Rice straw charcoal;

(3) Preparation of cadmium stock solution

Accurately weigh 2.0314g of CdCl ₂ 2.5H ₂ O into a beaker, add an appropriate amount of distilled water, stir until completely dissolved, transfer to a 1000mL volumetric flask, and constant volume to obtain 1000mg L ^-1 Cd ²⁺ stock solution, room temperature Save and dilute according to experimental needs when ready to use;

(4) Adsorption experiment

Dilute the Cd ²⁺ stock solution to 25, 50, 100, 300, and 500 mg·L ^-1 respectively, adjust the pH value of the Cd ²⁺ solution at different concentrations to 7, and accurately weigh 0.05 g of Y500, H500 and S500 respectively. In a 100mL conical flask, add 50mL of adjusted Cd ²⁺ solution respectively, shake well, put it in a shaker, shake at 25°C, 150r ^· min ^-1 for 24h, then filter, and measure the concentration of Cd ²⁺ in the filtrate , and calculated the removal rate of biochar to Cd ²⁺ ;

The experiment was set up in three parallels, and the results were averaged. The Cd2 ⁺ concentration in the filtrate is measured by plasma atomic emission spectrometry (ICP-AES, Prodigy), and the adsorption amount and removal rate are calculated, and the calculation method is as follows:

In the formula: q _e is the adsorption capacity (mg·g ^-1 ); C ₀ is the mass concentration of cadmium solution before adsorption (mg·L ^-1 ); C _e is the mass concentration of cadmium solution after adsorption (mg·L ^-1 ); V is the volume of cadmium solution (mL); M is the biochar quality (mg) taken by weighing; U is the cadmium removal rate (%);

(5) Pot experiment

Put the air-dried test soil of 10 mesh sieves into pots, 500g in each pot, then add the biochar with the maximum cadmium adsorption capacity into the pots according to the mass percentage of 1%, 2%, and 5%, fully mix, Adjust the water content to 60% of the maximum water holding capacity in the field; at the same time, set the polluted soil without adding biochar as the blank control CK, a total of 4 treatments, and each treatment has 3 repetitions; the soil is supplemented with deionized water every 1d, Keep the water holding capacity at about 60% in the field, and after cultivating at room temperature for 60 days, take samples to analyze the physical and chemical properties of the soil after cultivating and the chemical form of heavy metal Cd in it.

Experimental results:

1. There are obvious differences in the composition and properties of biochar prepared from different biomass raw materials. The mass composition, yield and ash content of hijiki charcoal, pecan shell charcoal and rice straw charcoal are listed in Table 1.

Table 1 Basic physicochemical properties of biochar

It can be seen from Table 1 that the charcoal yield and ash content were hijiki charcoal > straw charcoal > hickory charcoal. It shows that hijiki charcoal may contain large minerals. In addition, elemental composition analysis is one of the simplest and most important methods to judge the structure and properties of biochar. People usually use the atomic ratio of various elements to characterize the physicochemical properties of adsorbents. Among them, the atomic ratios of O/C and (N+O)/C represent the polarity of the adsorbent, and the larger the value, the greater the polarity. The H/C atomic ratio characterizes the aromaticity of the adsorbent, and the smaller the value, the higher the aromaticity ^[8] . It can be seen from Table 1 that both H/C and O/C are hijiki charcoal > straw charcoal > hickory charcoal, indicating that the polarity of hijiki charcoal is greater than that of straw charcoal Straw charcoal is larger than hijiki charcoal. These properties will affect the adsorption performance of these three biochars to heavy metal cadmium.

2. Research on the adsorption performance of different types of biochar on heavy metal cadmium

The influence and changing law of different types of biochar on the removal rate of Cd ²⁺ at different initial concentrations are shown in Fig. 1. It can be seen from Figure 1 that when the initial concentration of Cd is 10-200 mg·L ^-1 , the adsorption capacity of three carbons on Cd increases with the increase of the initial concentration of Cd. When the initial concentration of Cd was greater than 200 mg·L ^-1 , the adsorption capacity of hickory shell carbon on Cd ²⁺ tended to be stable. However, the adsorption capacity of rice straw charcoal and Hijiki charcoal tended to be stable when the initial concentration of Cd was greater than 300 mg·L ^-1 . This is because the adsorption sites on the surface of biochar are limited, when the adsorption sites are not fully occupied, the adsorption capacity will continue to increase, and when the adsorption sites on the biochar surface are saturated, the adsorption capacity for Cd tends to be Stablize. Moreover, the adsorption performance of Sargassum charcoal on Cd was significantly better than that of rice straw charcoal and pecan charcoal. Therefore, hijiki charcoal was selected in this experiment for detailed characterization and for subsequent pot experiments. ·

3. Characterization of physical and chemical properties of Sargassum charcoal

Figure 2 is a scanning electron micrograph of hijiki charcoal obtained at 500°C. It can be seen from Figure 2 that the surface of hijiki charcoal is rough, and there are certain disordered pores on the surface and section, which further aggravates the roughness of hijiki charcoal surface. This is because during the pyrolysis process, as the ambient temperature rises, a large amount of energy is released from the inside of the hijiki raw material after being heated, which flushes the internal pores of the raw material, making the pore distribution of the biochar disorder and increasing the biochar. The surface roughness is increased, so hijiki charcoal has better heavy metal adsorption performance.

The structure of functional groups on the surface of hijiki charcoal was further characterized. Figure 3 is the infrared spectrum of hijiki charcoal samples. The broad peak at 3425cm-1 in the figure is the characteristic absorption peak of hydroxyl-OH, and these hydroxyl groups may originate from carbohydrates in organic matter. The absorption peaks at 2932 and 2847cm ^-1 are the CH antisymmetric stretching vibration of -CH ₂ and the CH symmetric stretching vibration of -CH ₃ and -CH ₂ respectively. These groups mainly come from carbohydrates and aliphatic compounds in organic matter and alicyclic compounds, etc. The absorption peak at 1604cm ^-1 is the stretching vibration of C=C, C=O in the aromatic ring and the antisymmetric stretching vibration of -COO-. The absorption peak at 1446cm ^-1 is the shear deformation vibration of -CH ₂ group in carbohydrates and aliphatic compounds and the asymmetric deformation vibration of CH in -CH ₃ in aliphatic and lignin. 1108cm ^-1 is the stretching vibration of CO in carbohydrates. Therefore, it can be seen from the infrared spectrum that the obtained Sargassum charcoal is rich in organic functional groups, which play a vital role in the process of adsorbing heavy metals.

Figure 4 is the thermogravimetric analysis diagram of Hijiki charcoal samples. It can be seen from this figure that there are three weight losses in the thermogravimetric (TG) curve, and the mass loss from room temperature to 200 °C is about 4wt%, which can be considered as the evaporation of a small amount of adsorbed water on the sample surface. The second obvious weight loss step (about 15wt%) on the TG curve can be considered as the removal of organic matter such as cellulose. The thermal weight loss from 600-1000 °C corresponds to the thermal decomposition process of lignin. This is consistent with the results of the content of organic functional groups in hijiki charcoal in the infrared spectrum.

A typical nitrogen adsorption-desorption isotherm is shown in Fig. 5a, and the BJH pore size distribution curve is shown in Fig. 5b. The adsorption/desorption hysteresis cycle of Sargassum charcoal mainly appeared at 0.5-1.0P/P _o , and the isotherm showed Type IV with H3 hysteresis loop, which indicated that Sargassum charcoal sample had mesoporous structure. In addition, the pore size distribution curve of the sample can be obtained from the adsorption branch of the isotherm (Fig. 5b). The pore size distribution peaked at 20nm, which further indicated that Hijiki charcoal had certain pores, which was consistent with the results of scanning electron microscopy. The specific surface area of hijiki charcoal sample obtained at 500℃ is 2.73m ² ·g ^-1 , and the average pore diameter is 13.23nm.

4. Analysis of soil properties and soil Cd chemical speciation after adding Sargassum charcoal

(1) Effect of adding Sargassum charcoal on the pH value of polluted soil

The pH value of the soil is the main factor controlling the adsorption-desorption and precipitation-dissolution balance of heavy metals. The pH value of the soil has a great influence on the migration and availability of heavy metals in the soil and the harm of heavy metals to plants. The pH of the original soil sampled was weakly acidic (pH=5.28), and hijiki charcoal was added to the soil. As shown in Figure 6, different hijiki charcoal additions could significantly increase the soil pH value, and with the addition of hijiki charcoal The increase of sargassum charcoal addition showed an increasing trend, and the more sargassum charcoal was added, the closer the soil pH was to neutrality. Adding 1%, 2%, and 5% Sargassum charcoal increased the soil pH by 0.30, 0.78, and 1.61, respectively, compared with the control. This is because there are different concentrations of alkaline substances in the biochar ash, such as K, Ca, Na, Mg oxides, hydroxides, carbonates, etc., and applying them to the soil can increase the soil base saturation and reduce the exchangeable aluminum level. , thereby increasing soil pH. Therefore, adding hijiki charcoal can significantly increase the soil pH value, which has a certain effect on the prevention and control of soil acidification. Moreover, since the increase of soil pH value will affect the hydrolysis balance of heavy metal Cd, Cd will be fixed by complexation and precipitation.

(2) Effect of adding Sargassum charcoal on available phosphorus in polluted soil

Figure 7 shows the effect of adding different amounts of Sargassum charcoal on available phosphorus in polluted soil. It can be seen from the figure that different additions of Sargassum charcoal have a greater impact on the available phosphorus content in the polluted soil. Compared with the control, the available phosphorus in the soil increased significantly, increasing by 100.1%, 327.4% and 652.7%, respectively. %. Therefore, the addition of Sargassum charcoal can significantly increase the content of available phosphorus in soil. On the one hand, this is related to the high available phosphorus content of hijiki charcoal itself, and on the other hand, hijiki charcoal has a certain absorption effect on phosphorus nutrients. It has been reported that biochar can not only generate negative charges, but also generate positive charges, so it can absorb phosphorus nutrients that organic matter does not. On the one hand, biochar avoids its loss by reducing the dissolution of phosphorus nutrients. On the other hand, biochar can be used as a slow-release carrier of phosphorus nutrients, so that phosphorus can be released continuously and slowly in the soil, so as to achieve the effect of maintaining fertility. In addition, biochar can provide a good environment for microorganisms, especially bacteria, to mineralize and dissolve organic and inorganic phosphorus, so that these P can be used and absorbed by plants. Therefore, adding hijiki charcoal can significantly increase soil available phosphorus content.

(3) Effect of adding Sargassum charcoal on available potassium in polluted soil

Soil available potassium refers to potassium that is easily absorbed and utilized by crops, and has a direct impact on the potassium nutritional status of crops. Its content is an important indicator for judging the abundance of potassium in the soil. It can be seen from Figure 7 that the application of Sargassum charcoal had a significant impact on soil available potassium content. Compared with the control treatment CK, the addition of different amounts of Sargassum charcoal all increased the soil available potassium content, and the increase was 269.5-271.2% compared with the control treatment. There was a significant difference between adding hijiki charcoal and the control treatment, but there was no significant difference between the treatments with different hijiki charcoal additions. It shows that hijiki charcoal has a good effect on improving the level of available potassium in polluted soil. This is because hijiki charcoal has a large specific surface area and strong adsorption performance, so potassium can be adsorbed on its surface and is not easy to lose. However, the amount of hijiki charcoal did not significantly increase the content of available potassium in the polluted soil, so other factors can be considered to determine the amount of hijiki charcoal added.

(4) Effect of adding Sargassum charcoal on total nitrogen in polluted soil

The effect of adding Sargassum charcoal on total nitrogen in polluted soil is shown in Figure 9. It can be seen from the figure that compared with the control treatment CK, the soil total nitrogen content of different Sargassum charcoal additions increased by 8.76%, 21.90%, and 28.47%, respectively. The statistical results showed that there was no significant difference between adding 1% hijiki charcoal and the control CK, which indicated that the application of a small amount of hijiki charcoal had no significant effect on the content of total nitrogen in the soil. When the amount of Sargassum charcoal added reached more than 2%, the soil total nitrogen content increased with the increase of biochar addition, and the difference was significant compared with the control. This is because the application of biochar can reduce nitrogen leaching, and the microorganisms in the soil can improve soil aeration, inhibit the denitrification of nitrogen microorganisms, reduce the formation and emission of NOx, and increase the total nitrogen storage in the soil. Moreover, the addition of a small amount of biochar did not lead to a significant reduction in nitrogen loss and brought more nitrogen. Therefore, in agricultural production, the amount of biochar added should be at least 2% to achieve the purpose of increasing soil total nitrogen.

(5) Effect of adding Sargassum charcoal on organic matter in polluted soil

Soil organic matter can improve soil structure, improve soil water and fertilizer retention capacity, improve soil aeration and water permeability, support microbial activities and provide nutrients for plants, etc. It is one of the important indicators of soil fertility. The effect of adding Sargassum charcoal on the organic matter of polluted soil is shown in Figure 10. Compared with the control (CK) without hijiki charcoal, adding different levels of hijiki charcoal could significantly increase the organic matter content of polluted soil. However, there were great differences in the effects of different hijiki charcoal additions on soil organic matter. With the increase of hijiki charcoal addition, the soil organic matter content increased. Adding 1%, 2%, and 5% hijiki charcoal to the soil increased 51.55%, 55.01%, and 78.07% compared with the control group, respectively, and the difference reached a significant level. Adding 1% and 2% of hijiki charcoal had no significant difference before the treatment of soil organic matter, when the addition of hijiki charcoal reached 5%, the content of soil organic matter was higher than that of adding 1% and 2% hijiki charcoal Significantly increased. Previous studies have shown that biochar can increase the level of soil organic matter, and the increase depends on the amount and stability of biochar. Sargassum biochar can improve the level of soil organic matter. On the one hand, Sargassum charcoal itself has a high organic matter content. On the other hand, it may be because biochar can promote the formation of soil organic-mineral complexes and improve the stability of aggregates. and reduce organic matter leaching.

(6) Analysis of Cd chemical speciation in polluted soil after adding Sargassum charcoal

A large number of experiments have proved that the environmental behavior, bioavailability and toxicity of heavy metals are not only related to the total amount of heavy metals, but also closely related to the chemical forms of heavy metals. The occurrence forms of heavy metals are important indicators for judging the toxicity and ecological risks of heavy metals in soil. Cadmium in soil mainly exists in five forms: exchange state, carbonate binding state, iron-manganese oxidation binding state, organic binding state and residue state. The bioavailability of different forms of cadmium is quite different. Among the chemical forms of heavy metal Cd, the exchangeable state has strong mobility and is easy to be directly used by organisms; plants mainly absorb the exchanged state in the soil, while the carbonate-bound state and iron-manganese oxide-bound state are potentially available states. The combined state and the residue state are unavailable states, which cannot be used by plants and have low mobility. When the total cadmium content is the same, the lower the exchange state cadmium content, the lower the bioavailability; on the contrary, when the total cadmium content is the same, the carbonate bound state, iron manganese oxide bound state, organic bound state and residue state The higher the content of cadmium in each form, the lower its bioavailability. It can be seen from Figure 11 that, compared with the control treatment CK without adding biochar, the application of Sargassum charcoal led to a significant decrease in the content of soil exchanged cadmium, carbonate bound state, iron manganese oxide bound state, organic bound state and residue state. Cadmium content was increased (Figure 11). After 30 days of cultivation in the polluted soil, compared with the control treatment, the content of exchangeable cadmium in the 1%, 2%, 5% Sargassum charcoal treatment was reduced from 5.20mg/kg to 2.92, 1.94, 3.05mg/kg respectively . Therefore, each treatment significantly reduced the content of soil exchangeable Cd. The content of carbonate-bound cadmium increased from 0.30mg/kg to 0.83, 1.05, 0.91mg/kg; the content of iron-manganese oxide-bound cadmium increased from 2.59mg/kg to 3.86, 4.71, 3.82mg/kg; The content of bound cadmium increased from 0.76mg/kg to 0.95, 1.33, 0.94mg/kg respectively; the content of residual cadmium increased from 1.20mg/kg to 1.96, 2.47, 1.94mg/kg respectively. Different Sargassum charcoal addition treatments had different effects on soil Cd content, the exchanged Cd content decreased significantly compared with the control, and the carbonate-bound, iron-manganese oxide-bound, organic-bound and residual Cd contents were significantly different from those of the control. significantly increased in comparison. This is due to the high specific surface area and porosity of Sargassum charcoal, which can fix heavy metal Cd through surface adsorption after it is applied to the soil. In addition, pH value is an important factor affecting soil dissolution-precipitation, adsorption-desorption and other reactions, thus affecting the bioavailability of heavy metals. Sargassum charcoal itself has a high pH value. After being added to the soil, the pH of the soil will increase, and the negative charge of the colloid on the soil surface will increase, which will promote the reaction of Cd ²⁺ with the carbonate in the soil to form carbonate precipitation. However, iron and manganese oxides in soil are amphoteric colloids, and the adsorption of heavy metals mainly depends on the negative surface charge, while the addition of Sargassum charcoal increases the soil pH value, making H ⁺ , Fe ³⁺ , Al ³⁺ , Mn As the concentration of ²⁺ decreased, the competitive adsorption with heavy metal Cd weakened, and the formed iron-manganese oxide further enhanced the adsorption of Cd. Therefore, the pH of the soil added with Sargassum charcoal increased accordingly, resulting in the increase of carbonate-bound Cd and iron-manganese oxide-bound Cd.

In addition, from the infrared spectrogram (Figure 3), it can be seen that hijiki charcoal is rich in a large number of oxygen-containing functional groups, such as carboxyl and hydroxyl groups, and the carboxyl and hydroxyl groups react with Cd ²⁺ in the soil solution through complexation or chelation. Insoluble complexes, thereby changing the adsorption capacity of soil for heavy metal ions. Therefore, the organic matter in the soil is more tightly bound to Cd, which further leads to a decrease in the content of exchangeable Cd. In summary, the addition of Sargassum charcoal can transform soil heavy metal Cd from an exchange state to a carbonate-bound state, an iron-manganese oxide-bound state, an organic-bound state, and a residue state, thereby reducing the bioavailability and ecological quality of heavy metal Cd. Toxicity, thereby greatly reducing the harm of heavy metal cadmium. Moreover, different amounts of Sargassum charcoal had different effects on soil Cd forms. When the application amount of Sargassum charcoal was 2%, the content of exchanged cadmium decreased the most, and the content of carbonate-bound, iron-manganese oxidation-bound, and organic-bound Cadmium forms the most and works best. Therefore, in the process of using Sargassum charcoal to improve heavy metal Cd-contaminated soil, the best improvement effect can be obtained by choosing the amount of carbon addition reasonably according to the characteristics of the influence of different biochar additions on the soil Cd form.

5. Results

(1) Sargassum charcoal has the best adsorption effect on heavy metal cadmium among the three biochars;

(2) The pH, available phosphorus, available potassium, total nitrogen and organic matter content of polluted soil can be significantly increased after adding Sargassum charcoal to polluted soil. And it increased with the addition of Sargassum charcoal;

(3) The application of different amounts of Sargassum charcoal significantly reduced the content of soil available Cd, and promoted the soil heavy metal Cd from the exchange state to the carbonate-bound state, iron-manganese oxide-bound state, organic-bound state and residue state transform. When the application rate of Sargassum charcoal was 2%, the content of exchanged cadmium decreased the most, and the formation of carbonate-bound, iron-manganese oxidation-bound and organic-bound cadmium was the most, and the effect was the best.

Claims (1)

1. A research method on the impact of biochar on cadmium-contaminated soil properties and cadmium forms, characterized in that, its research method is as follows: (1) Experimental materials The soil for testing was collected from the Xiasiling Tungsten Mine, which is located in Xuechuan Village, Heqiao Town, Lin’an City, Zhejiang Province. The sampling depth was 0-20cm. After the soil was collected, it was air-dried, and plant roots, stones and other sundries were removed. Pass through a 10-mesh sieve and mix for later use. The physical and chemical properties of the tested soil are: pH value 5.77, organic matter 32.40mg·kg ^-1 , available phosphorus 23.19mg·kg ^-1 , total nitrogen 1.54g·kg ^-1 , available potassium 147.8mg · kg ^-1 ; The biochar raw material Hijiki was taken from Dongtou, Wenzhou City, Zhejiang Province. Pecan shells and rice straw were provided by Zhejiang Academy of Agricultural Sciences. The raw materials were washed with tap water, dried naturally, and then placed in ziplock bags for use; (2) Preparation of biochar Biomass carbonization adopts oxygen-limited and temperature-controlled carbonization method. The raw material hijiki/hickory shell/rice straw is loosely loaded into an open-type programmable tube furnace and carbonized at 500°C without oxygen, and the heating rate is 25°C·min ^-1 After reaching the final temperature, continue to carbonize for 3 hours. After the tube furnace is naturally cooled to room temperature, take it out and weigh it, and calculate the carbonization yield; put the carbonized product in an oven at 80°C for 24 hours, grind and pass through 10 and 100 mesh sieves respectively, and place Store them in ziplock bags for later use. The obtained biochars are marked as: Y500-Sargassum charcoal, H500-Hickory nut charcoal, S500-Rice straw charcoal; (3) Preparation of cadmium stock solution Accurately weigh 2.0314g of CdCl ₂ 2.5H ₂ O into a beaker, add an appropriate amount of distilled water, stir until completely dissolved, transfer to a 1000mL volumetric flask, and constant volume to obtain 1000mg L ^-1 Cd ²⁺ stock solution, room temperature Save and dilute according to experimental needs when ready to use; (4) Adsorption experiment Dilute the Cd ²⁺ stock solution to 25, 50, 100, 300, and 500 mg·L ^-1 respectively, adjust the pH value of the Cd ²⁺ solution at different concentrations to 7, and accurately weigh 0.05 g of Y500, H500 and S500 respectively. In a 100mL conical flask, add 50mL of adjusted Cd ²⁺ solution respectively, shake well, put it in a shaker, shake at 25°C, 150r·min ^-1 for 24h, then filter, and measure the concentration of Cd ²⁺ in the filtrate , and calculated the removal rate of biochar to Cd ²⁺ ; The experiment was set up in three parallels, and the results were averaged. The Cd2 ⁺ concentration in the filtrate is measured by plasma atomic emission spectrometry (ICP-AES, Prodigy), and the adsorption amount and removal rate are calculated, and the calculation method is as follows: <mrow><msub><mi>q</mi><mi>e</mi></msub><mo>=</mo><mfrac><mrow><mo>(</mo><msub><mi>C</mi><mi>o</mi></msub><mo>-</mo><msub><mi>C</mi><mi>e</mi></msub><mo>)</mo><mi>V</mi></mrow><mi>M</mi></mfrac><mo>-</mo><mo>-</mo><mo>-</mo><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow></mrow> <mrow><mi>U</mi><mi>%</mi><mo>=</mo><mfrac><mrow><mo>(</mo><msub><mi>C</mo>mi><mi>o</mi></msub><mo>-</mo><msub><mi>C</mi><mi>e</mi></msub><mo>)</mo><mn>100</mn><mi>%</mi></mrow><msub><mi>C</mi><mi>o</mi></msub></mfrac><mo>-</mo><mo>-</mo><mo>-</mo><mrow><mo>(</mo><mn>2</mn><mo>)</mo></mrow></mrow> In the formula: q _e is the adsorption capacity (mg·g ^-1 ); C ₀ is the mass concentration of cadmium solution before adsorption (mg·L ^-1 ); C _e is the mass concentration of cadmium solution after adsorption (mg·L ^-1 ); V is the volume of cadmium solution (mL); M is the biochar quality (mg) taken by weighing; U is the cadmium removal rate (%); (5) Pot experiment Put the air-dried test soil of 10 mesh sieves into pots, 500g in each pot, then add the biochar with the maximum cadmium adsorption capacity into the pots according to the mass percentage of 1%, 2%, and 5%, fully mix, Adjust the water content to 60% of the maximum water holding capacity in the field; at the same time, set the polluted soil without adding biochar as the blank control CK, a total of 4 treatments, and each treatment has 3 repetitions; the soil is supplemented with deionized water every 1d, Keep the water holding capacity at about 60% in the field, and after cultivating at room temperature for 60 days, take samples to analyze the physical and chemical properties of the soil after cultivating and the chemical form of heavy metal Cd in it.