CN113546952B - Magnetic biochar soil remediation agent for soil heavy metal pollution and application thereof - Google Patents

Magnetic biochar soil remediation agent for soil heavy metal pollution and application thereof Download PDF

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CN113546952B
CN113546952B CN202110616555.9A CN202110616555A CN113546952B CN 113546952 B CN113546952 B CN 113546952B CN 202110616555 A CN202110616555 A CN 202110616555A CN 113546952 B CN113546952 B CN 113546952B
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biochar
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CN113546952A (en
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陈涛
蒋少军
晏波
段连鑫
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South China Normal University
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B09—DISPOSAL OF SOLID WASTE; RECLAMATION OF CONTAMINATED SOIL
    • B09C—RECLAMATION OF CONTAMINATED SOIL
    • B09C1/00—Reclamation of contaminated soil
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
    • B01J20/02—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material
    • B01J20/20—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising inorganic material comprising free carbon; comprising carbon obtained by carbonising processes
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
    • B01J20/28—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof characterised by their form or physical properties
    • B01J20/28002—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof characterised by their form or physical properties characterised by their physical properties
    • B01J20/28009—Magnetic properties
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B09—DISPOSAL OF SOLID WASTE; RECLAMATION OF CONTAMINATED SOIL
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Abstract

The invention belongs to the technical field of soil remediation, and discloses a magnetic biochar soil remediation agent for heavy metal pollution of soil, a preparation method thereof and application thereof in remediation of polluted soil, wherein the magnetic biochar soil remediation agent is prepared by the following steps: crushing the solid waste rich in the magnetic iron/cobalt/nickel compound, and then carrying out chloridizing roasting; magnetic particles are collected through magnetic separation; then mixing the magnetic particles with agricultural wastes or a mixture of the agricultural wastes and urea in proportion, and then carbonizing, ball-milling and magnetically separating the mixture. The invention also discloses a method for restoring soil by using the magnetic biochar, and a method of secondary ploughing and secondary irrigation is adopted, so that the magnetic biochar and the discharged floating biochar are separated by the magnetic rotary cutter, the heavy metal in the soil can be effectively removed, and a new way is provided for the treatment of the heavy metal pollution of the soil.

Description

Magnetic biochar soil remediation agent for soil heavy metal pollution and application thereof
Technical Field
The invention relates to the technical field of soil remediation, in particular to a magnetic biochar soil remediation agent for soil heavy metal pollution and a preparation method and application thereof.
Background
The iron tailings are wastes after mineral separation and are the main components of industrial solid wastes. In the industrial production, a lot of solid wastes rich in magnetic iron/cobalt/nickel compounds such as red mud, steel slag, welding slag, waste iron slag and the like are generated. Iron tailings and a large amount of solid wastes rich in oxides such as iron, cobalt and nickel are left unused and discarded, so that not only is a resource waste, but also serious harm and risk are caused to the environment in the long-term environmental change process. How to reasonably treat and dispose solid waste and improve the added value of the solid waste is crucial to realizing the harmlessness and the recycling of the solid waste. In the same way, a large amount of agricultural wastes (weeds, rice straws, wheat straws, corn straws, fallen leaves, cow dung, pig dung, chicken dung and the like) are generated in the agricultural production and living process, and the biochar is widely applied to the reduction and treatment of the agricultural wastes as an environment-friendly functional material. However, the application of the biochar material obtained by the pyrolysis conversion of agricultural wastes is limited in practical application due to the disadvantages of small particle size and difficult separation.
At present, two paths of harmless treatment and resource treatment are mostly adopted for iron tailings. Firstly, heavy metals are separated from the iron tailings. The other way is to directly carry out resource utilization on the iron tailings. Chinese patent application CN201811427866.5 discloses a roasting treatment method of iron tailings, which comprises roasting the iron tailings in an inert gas atmosphere, recovering volatilized flue gas to obtain metal chloride, and magnetically separating residues to obtain magnetite; according to the invention, through the addition of specific inert gas and chlorinating agent and the control of roasting temperature rise and fall procedures, the magnetic separation of iron tailings and the recovery of valuable metals of lead, copper and zinc are realized, the problems of large pollution and influence on the recovery of metals in flue gas in the iron tailings treatment process are fundamentally reduced, the operation is simple and convenient, the cost is low, and the reduction and recycling benefits of the iron tailings are obvious; the method can reduce and utilize the iron tailings in a resource manner, but the iron tailings are not prepared into high value-added materials.
With the rapid development of industrialization and urbanization, the increase of toxic elements in environmental media (soil) is caused by industrial activities, and due to the stability and the biological degradability of the toxic elements, the enrichment of crops on toxic metals, the soil remediation and the guarantee of grain safety are widely concerned all over the world. The method for restoring soil by using biochar is widely accepted, and the traditional biochar for restoring soil has certain timeliness, high cost and low restoring efficiency, and the content of heavy metal in soil cannot be reduced.
Chinese patent No. CN104971697B discloses a preparation and application method of a magnetic biochar material for removing arsenic pollution in water body, and the method is implemented by carbonizingModifying the chaff to obtain CRHO, and preparing the load nano Fe3O4Modifying the carbonized chaff particles; the magnetic biochar CRHO-Fe in the invention3O4Not only has higher removal rate to arsenic-containing wastewater, but also has low price, no secondary pollution and convenient magnetic biochar CRHO-Fe3O4The catalyst can be recycled for a plurality of times, and can be widely applied to the field of treatment of middle-low concentration arsenic-containing wastewater; the method is mainly applied to removing As in a water system, and has a single function. Chinese patent No. CN111570494B discloses a heavy metal contaminated soil remediation method, which comprises the steps of adding 0.5-2% of modified magnetic biochar into soil, enabling the modified magnetic biochar to float out of the water surface, and then recovering the magnetic biochar enriched with free Cd and soil granules, so as to achieve the purpose of reducing the total Cd in the soil; the modified magnetic biochar is prepared by crushing biochar raw materials and soaking the biochar raw materials in Fe-containing solution2+In the solution, after oscillation, solid raw materials are obtained by filtration, after drying, magnetic biochar is obtained by pyrolysis treatment, a hydrothermal method is adopted to prepare Ca-Al hydrotalcite adsorbent loaded with magnetic biochar, and then chitosan is adopted to modify the Ca-Al hydrotalcite adsorbent to obtain the magnetic biochar adsorbent; although the method utilizes the characteristic of light weight of the biochar, the magnetization effect of the biochar is not considered, obviously, the floating amount of the mixed soil and the biochar is small, a large amount of biochar cannot be removed, in addition, the soil is usually subjected to combined pollution, the magnetic biochar only has a good effect on Cd in the method, the action of other toxic metals is unknown, and the soil cultivation safety is also unknown.
At present, the method for preparing the magnetic biochar by using solid wastes such as tailings and the like and agricultural wastes and the application of the magnetic biochar in the aspect of soil remediation are not reported.
Disclosure of Invention
The invention aims to solve the problems and provides a magnetic biochar soil remediation agent which has high removal efficiency and stable removal effect on various heavy metals in soil and is easy to separate, and the specific technical scheme is as follows:
a magnetic biochar soil remediation agent for soil heavy metal pollution is magnetic biochar prepared by the following steps:
crushing the solid waste rich in the magnetic iron/cobalt/nickel compound, and then carrying out chloridizing roasting;
magnetic particles are collected through magnetic separation;
then mixing the magnetic particles with agricultural wastes or a mixture of the agricultural wastes and urea in proportion, and then carbonizing, ball-milling and magnetically separating the mixture.
In the technical scheme, the solid waste rich in the magnetic iron/cobalt/nickel compound is one or a mixture of more of iron tailings, steel slag, red mud, welding slag and waste iron slag,
the agricultural waste is plant residue, animal waste or a mixture of the plant residue and the animal waste,
preferably, the plant residues are one or a mixture of weeds, crop straws and fallen leaves,
preferably, the crop straws are rice straws, wheat straws or corn straws,
preferably, the animal waste is livestock and poultry waste.
In the technical scheme, the chloridizing roasting is to roast the solid chloride and the solid waste rich in the magnetic iron/cobalt/nickel compound in a mixing way, or to roast the solid waste rich in the magnetic iron/cobalt/nickel compound after being washed by chloride solution;
preferably, the chloride is hydrochloric acid, calcium chloride or magnesium chloride;
preferably, the chloride solution is a hydrochloric acid solution.
In the technical scheme, the mass percentages of the magnetic particles, the agricultural wastes and the urea are respectively 40-60%, 15-60% and 0-25%,
the preferred mass percentages of the magnetic particles, the agricultural wastes and the urea in the total mass of the three components are respectively
45-60％、15-50％、0-25％；
45-60％、15-40％、0-25％；
45-60%, 15-30% and 5-25%; or
45-60％、15-30％、10-25％；
The preferred mass percentages of the magnetic particles, the agricultural wastes and the urea in the total mass of the three components are respectively
45-55％、20-50％、0-25％；
45-55％、20-45％、0-25％；
45-55％、20-45％、5-25％；
45-55%, 20-40%, 5-25% or
45-55％、20-30％、15-25％.
The preferred mass percentages of the magnetic particles, the agricultural wastes and the urea in the total mass of the magnetic particles, the agricultural wastes and the urea are respectively 50%, 25% and 25%.
The invention also provides a preparation method of any one of the magnetic biochar soil remediation agents, which comprises the following steps:
1. preparation of magnetic particles:
crushing the solid waste rich in the magnetic iron/cobalt/nickel compound, chlorinating the solid waste by chloride to obtain slag, roasting the slag at the temperature of 600-;
2. preparing magnetic biochar:
mixing the prepared magnetic particles with the crushed agricultural wastes and urea according to the mass ratio, carbonizing the mixture, ball-milling the carbonized solid, cooling, taking out the material, separating the grinding medium from the biochar, and magnetically separating to obtain the magnetic biochar material.
As a matter of preference, it is preferred that,
in the chlorination in the step 1, the crushed solid waste is washed by HCl solution, and slag is obtained by filtering after washing, wherein the concentration of the HCl solution is preferably 0.01-0.02M, and the filtering is performed by adopting a 60-100 mesh nylon sieve;
the concrete process parameters of the mixture carbonization in the step 2 are as follows: a temperature programming muffle furnace is adopted, under the condition of oxygen limitation, the temperature rising rate is 5-10 ℃/min, the carbonization time is 30-180min, and the carbonization temperature is 300-; and sieving by using a 60-100 mesh nylon sieve to separate the grinding medium and the biochar.
In the above technical solution, the roasting conditions in step 1 are as follows: adopting a temperature programming muffle furnace, wherein the temperature programming is 10-20 ℃, the temperature balancing time is 20-40min, and the temperature reduction mode is as follows: water quenching;
in the step 2, a planetary ball mill is adopted for ball milling, and the ball-material ratio is 5-10: 1, the rotating speed of the ball mill is 200-.
The invention also provides the application of the magnetic biochar soil remediation agent in remediation of contaminated soil; preferably, the polluted soil is heavy metal moderate and light polluted farmland soil in a mine area.
The invention finally provides a soil remediation method, which comprises the following steps:
(1) applying the magnetic biochar soil remediation agent to soil to be remediated at an application rate of 5-30t/ha, then carrying out first irrigation, and carrying out first plowing after flooding to mix soil and the soil remediation agent;
(2) and (4) irrigating for the second time after the first plowing, performing the second plowing by adopting a magnetic rotary cutter after flooding, naturally settling after plowing until water and soil are separated, draining, and finishing restoration.
The application amount of the soil remediation agent is 10-30t/ha, preferably 10-25t/ha or 10-20t/ha or 10-15t/ha or 10 t/ha;
the first irrigation water is tap water, river water, pond water, underground water or 0.01-0.06M EDTA; the water flooding depth is 2-5 cm;
the second irrigation water is tap water, river water, pond water or underground water, and the water submerging depth is 8-10 cm;
the first turning adopts a rotary plough integrated tractor, ploughs for 10-15min, and rotates for 8-10 min;
and the second plowing adopts rotary tillage for 10-15min by a magnetic rotary cutter, and the plowed materials naturally settle for 10-14 h.
According to the method, the solid waste rich in the magnetic iron/cobalt/nickel compound is subjected to chlorination roasting, so that heavy metals in the solid waste can be effectively separated and removed at a lower roasting temperature, toxic and harmful substances in the waste are effectively reduced, and the waste is subjected to harmless treatment so as to be wider in resource utilization range.
The invention combines agricultural wastes with magnetic substances, is used for soil remediation, is beneficial to the stabilization of soil heavy metals, and improves the remediationThe effect is recovered. In addition, agricultural wastes cannot be directly recycled, but biochar combined with magnetism can be easily recycled from soil-solution by magnetic separation. Conventionally with magnetic substances (e.g. Fe)3O4Nano-particles, nano-zero valent iron) are easily aggregated into large particles, thereby reducing the property of solidifying and stabilizing heavy metals, but the magnetic biochar can reduce Fe3O4And the like, so as to ensure the performance of solidifying and stabilizing the heavy metal.
The invention adopts urea to co-fire to generate the magnetic biochar with lighter weight, the biochar is light in weight and beneficial to floating, part of the magnetic biochar is easy to remove through drainage after soil remediation, meanwhile, the magnetic biochar left in a farmland can promote plant growth due to the fact that the magnetic biochar contains elements such as carbon, nitrogen and the like, and the magnetic biochar left in the farmland can also solidify and stabilize heavy metals in the soil, reduce heavy metal migration and reduce harm and enrichment to crops.
The invention has the beneficial effects that:
1. the method has the advantages of wide raw material source, simple and convenient preparation operation, low cost, obvious reduction and recycling benefits of iron tailings and other solid wastes, and high added value of the prepared magnetic biochar.
2. The method can effectively reduce the risk of transferring heavy metal from the solid waste to the magnetic particles in the roasting and magnetic separation processes, thereby avoiding the problem of secondary pollution caused by high content of heavy metal in the biochar.
3. After the soil is repaired by the method, the soil heavy metal can be effectively removed by separating the magnetic biochar from the discharged floating biochar through the magnetic rotary cutter, and a new way is provided for the treatment of the soil heavy metal pollution.
4. The method provided by the invention realizes the removal of heavy metals in the soil, and simultaneously, the residual biochar passivates the heavy metals in the soil, and meanwhile, the biochar can provide nutrient elements for the soil, so that the method is favorable for the growth of grain crops, and has very high practicability and application value.
Drawings
FIG. 1 shows the results of detecting Cd and Pb in the DTPA extracts in the soil of example 1.
FIG. 2 is a scanning electron microscope image of (a) conventional biochar and (b) magnetic biochar of the present invention.
FIG. 3 is a hysteresis loop diagram of the magnetic biochar of the present invention.
FIG. 4 is a flow chart of the magnetic charcoal preparation and soil remediation application of the present invention.
FIG. 5 shows the results of measuring the contents of heavy metals in soil samples from the paddy soil remediation experiments in example 2.
FIG. 6 shows the results of measuring the content of heavy metals in rice samples for paddy soil remediation experiments in example 2.
FIG. 7 shows the rice yield measured in the rice field soil remediation experiment of example 3.
FIG. 8 is the results of measuring the content of heavy metals in the rice samples for the paddy field soil remediation experiments in example 3.
Detailed Description
The invention is further illustrated by the following examples, which are not intended to be limiting.
The experimental procedures in the following examples are conventional unless otherwise specified.
Main material and reagent sources:
iron tailings: collected from mining company ltd, Dabaoshan, Guangdong.
Steel slag: collected from Shaoguan iron and Steel group, Inc., Guangdong province; steel slag is a by-product in the steel-making process, and is composed of various oxides formed by oxidizing impurities in pig iron, such as silicon, manganese, phosphorus, sulfur, and the like in the smelting process, and salts generated by reacting the oxides with a solvent.
Agricultural wastes are collected in farmlands and orchards in Shaoguan city, Guangdong province.
Urea and other chemicals were purchased from chemical agents limited, chinese medicinal group.
Example 1.
Firstly, the magnetic biochar soil remediation agent is prepared
The method comprises the following steps:
1. preparation of magnetic particles:
100g of iron tailings with impurities and sediment removed are taken, crushed by a crusher, washed by 200ml of 0.01M HCl solution, filtered by a 60-mesh nylon sieve to obtain slag, roasted in a high-temperature furnace, cooled and then subjected to magnetic separation by a strong magnet to collect magnetic particles. Wherein, the roasting condition is as follows: adopting a temperature programming muffle furnace, wherein the temperature programming is 10 ℃, the temperature balancing time is 30min, and the temperature reduction mode is as follows: water quenching and roasting at 600 deg.c.
2. Preparing magnetic biochar:
collecting mango leaves, air-drying and crushing into pieces to prepare powder which is sieved by a 10-mesh sieve.
Fully mixing the prepared magnetic particles, mango leaf powder and urea according to a mass ratio;
carbonizing the mixture, performing ball milling on the carbonized solid, cooling the ball milling tank after the ball milling is finished, taking out the material, and separating grinding medium steel balls and biochar with different particle sizes by a 60-mesh nylon sieve; magnetic separation is carried out by a strong magnet to obtain the magnetic biochar material.
Wherein, the mixture carbonization condition is an oxygen-limited condition, and the oxygen limitation adopts nitrogen.
The specific technological parameters of carbonization are as follows: a temperature programming muffle furnace is adopted, under the condition of oxygen limitation, the temperature rising rate is 5 ℃/min, the carbonization time is 120min, and the carbonization temperature is 500 ℃.
The ball milling adopts a planetary ball mill, the ball material ratio is 6: 1, the rotating speed of the ball mill is 300r/min, and the ball milling time is 10 h.
Setting different mixture ratios of mango leaf powder and urea, setting the mass ratio of the urea to the mango leaf powder to be 0, 20, 40, 50, 60, 80 and 100 percent to be 7, mixing the mixture of the urea and the mango leaf powder with magnetic particles in equal mass ratio, and preparing 7 magnetic biochar samples according to the method, wherein the samples are numbered as T-BC1, T-BC2, T-BC3, T-BC4, T-BC5, T-BC6 and T-BC7, and sequentially represent the mass ratios of the urea to the mango leaf powder to be 0, 20, 40, 50, 60, 80 and 100 percent.
Second, soil experiment
Experimental topsoil (0-20 cm) was collected from a field near the mine area in the great hills, guangdong, and after air-drying, soil samples were screened (<2mm and 0.15mm) and stored for analysis. The soil is contaminated with a variety of heavy metals, which may be due to mining activities near the field. Table 1 summarizes the physicochemical properties of the soil samples taken.
TABLE 1
Figure GDA0003657090680000071
Preparation of experimental control MB: loading the obtained mango leaf powder into a ceramic crucible, heating in a muffle furnace, and pyrolyzing at 10 deg.C for min-1Is increased in temperature and is in N2Keeping the temperature constant at 600 deg.C for 2h, and cooling the obtained material to obtain control product, named as MB.
The soil samples were repaired with MB, T-BC1, T-BC2, T-BC3, T-BC4, T-BC5, T-BC6 and T-BC7, respectively, and the repairing effects of the different repairing agents were evaluated. The blank sample was not treated with a repair agent. Adding 2g of the remediation agent into each 100g of soil sample, keeping the water holding capacity of the soil at 70% after mixing, detecting Cd and Pb in the soil by adopting a DTPA extraction method after incubating for 14 days at 25 ℃, and evaluating the solidification and stabilization effects of different remediation agents.
Experimental results; in addition to the influence of soil characteristics on the bioavailability of Cd and Pb, direct adsorption and stabilization of Cd and Pb by T-BC is also an important process. The capacity of T-BC to fix Cd and Pb gradually increased with the increase of the content of mango leaf powder and reached a maximum after T-BC1 (FIG. 1). In addition, as shown in fig. 1, the application of T-BC and MB significantly reduced the content of DTPA extracts Cd and Pb in both soils compared to the control soil sample (soil not treated with remediation agent), which may be attributed to the alkaline species and iron oxide in BC. The data also show that DTPA extracts Cd and Pb were higher at T-BC5, T-BC6 and T-BC7, which is probably due to the low biochar in T-BC resulting from the lower amount of mango leaf powder mixed in the co-pyrolysis stage. Therefore, agricultural wastes are added in a proper amount in the process of preparing the magnetic biochar.
Example 2
Firstly, preparing the magnetic biochar soil remediation agent
The method comprises the following steps:
1. preparation of magnetic particles:
weighing 100g of iron tailings without impurity and sediment, crushing the iron tailings by a crusher, adding 200ml of 0.01M HCl to wash the crushed iron tailings, filtering the iron tailings by a 100-mesh nylon sieve to obtain slag, putting the washed slag into a temperature programming muffle furnace to roast at 1000 ℃ for 30min, programming the temperature to 10 ℃, balancing the temperature for 20min, and cooling: water quenching, cooling slag, and separating the roasted slag by strong magnetism to obtain magnetic particles.
2. Preparing magnetic biochar:
weighing 10g of prepared magnetic particles, 5g of rice straw powder and 5g of urea, grinding and mixing, then transferring the magnetic particles into a covered ceramic crucible, carbonizing the magnetic particles for 1h at 600 ℃ in a muffle furnace (including an oxygen-limited condition and at a temperature rise rate of 5 ℃/min), cooling, taking out the carbonized material, transferring the carbonized material into a ball milling tank, carrying out ball milling for 6h according to a ball-to-material ratio of 10: 1, cooling the ball milling tank, separating a mixture, sieving a 100-mesh nylon sieve to separate biochar, carrying out magnetic separation on the obtained biochar by using a strong magnet, and carrying out magnetic separation on the magnetic biochar.
And (3) detecting the performance characteristics:
(1) according to the magnetic biochar prepared in the embodiment, magnetic particles are embedded into pores of the biochar and are not separated under the action of external force of ball milling, so that the biochar is endowed with magnetism, the structure of the biochar is observed by using a Scanning Electron Microscope (SEM), the structural diagrams of the traditional biochar and the magnetic biochar provided by the invention are respectively shown in figures 2a and 2b, the traditional biochar has a porous structure, and meanwhile, some ash is formed on the surface of the traditional biochar.
(2) The magnetic biochar prepared in this example was subjected to a high-precision hysteresis loop test (vibration sample magnetometer (VSM), USA, LakeShore,7404), and the obtained hysteresis loop graph is shown in fig. 3, which further confirms that the tailing-based biochar prepared in this example has strong magnetism and can be subjected to magnetic separation.
Second, the soil remediation experiment in the rice field
According to the flow shown in fig. 4, the preparation method of the magnetic biochar in this embodiment is used for mass production, soil remediation plot experiments are performed on farmland around a certain mining area in south China, and different groups of experiments are set for different usage amounts of the magnetic biochar, wherein the different usage amounts of the magnetic biochar are set to be 0, 5, 10, 20 and 30 tons/hectare (t/ha).
The soil remediation method comprises the following steps:
(1) after the magnetic biochar soil restoration agent is applied to a farmland, irrigating river water for the first time, wherein the water flooding depth is 3cm, and then ploughing by using a rotary plough integrated tractor for the first time, ploughing for 12min and ploughing for 9min so as to uniformly mix the magnetic biochar with soil;
(2) irrigating river water for the second time after the first ploughing, wherein the water flooding depth is 9cm, then ploughing for the second time by using a magnetic rotary cutter, carrying out rotary tillage for 12min, separating magnetic biochar in soil by using the magnetic rotary cutter (namely the magnetic rotary cutter is a rotary cutter on a rotary cultivator and is made of magnetized steel so as to have iron absorption), and removing 20-30% of the magnetic biochar.
(3) Naturally settling for 12 hours after the second plowing, separating water and soil, then draining water, and discharging the floating magnetic biochar along with the water to remove part of the magnetic biochar.
Planting rice and basic fertilizer (NH) in a direct seeding mode after draining4NO3、120mg N kg-1Soil and K2HPO4、30mg P kg-1Soil and 75.7mg Kkg-1Soil), the nutrient supply level can meet the normal growth demand of rice; and topdressing is carried out in the heading period, and other management is carried out according to local farm work management. Harvesting the rice after 120 d.
After the magnetic charcoal is applied, soil samples are respectively taken for analysis after the magnetic rotary knife is ploughed and after water is drained.
With HCl-HNO3-HF-HClO4Method pair for digesting soil by acid systemSoil samples were analyzed and the concentrations of metals in the extracts were measured using atomic absorption spectrometry (AAS, ice3300 Thermo Scientific, usa) and inductively coupled plasma mass spectrometry (ICP-MS, 7900, agilent, usa) to obtain the Cd, As, Pb content of the soil.
The contents of Cd, As and Pb in the soil are shown in FIG. 5, the total amount of heavy metals in the soil can be reduced by secondary plowing and draining, and the heavy metals in the soil can be reduced by magnetic separation of magnetic biochar during secondary plowing. The drainage is to remove the floating biochar in the overlying water, and the drained reduction heavy metal mainly comprises a soil solution and a part of heavy metal adsorbed on the biochar. When the application amount is more than 20t/ha, the removal amount of the heavy metal content is not large along with the increase of the biochar amount, and further proves that the removal of the magnetic biochar from the soil in a certain repair stage mainly shows that the reduction of the heavy metal content of the soil is in a part which is biologically effective in reducing the heavy metals in the soil.
By HNO3-H2O2Rice samples were digested and the concentrations of heavy metals in rice were measured using inductively coupled plasma mass spectrometry (ICP-MS, 7900, Agilent, USA).
The heavy metal content in the rice is measured after the rice is harvested, and the result is shown in fig. 6, after the soil is repaired, the heavy metal content in the brown rice is reduced along with the increase of the application amount of the magnetic biochar, and when the application amount is higher than 10t/ha, the heavy metal content in the brown rice can be guaranteed to be lower than the limit value of 0.2mg/kg in GB2762-2017 national food safety standard pollutant limit.
Example 3
Firstly, the magnetic biochar soil remediation agent is prepared
The method comprises the following steps:
1. preparation of magnetic particles:
weighing 100g of steel slag without impurities, crushing the steel slag by a crusher, adding 200ml of 0.01M HCl, washing the crushed steel slag, filtering the steel slag by a 100-mesh nylon sieve to obtain slag, putting the washed slag into a temperature programming muffle furnace to roast at 600 ℃ for 30min, programming the temperature to 20 ℃, balancing the temperature for 40min, and cooling: water quenching, cooling slag, and separating the roasted slag by strong magnetism to obtain magnetic particles.
2. Preparing magnetic biochar:
weighing 10g of prepared magnetic particles, 6g of rice straw powder and 4g of urea, grinding and mixing, then transferring the mixture into a ceramic crucible with a cover to be carbonized for 1h under the condition of 600 ℃ in a muffle furnace (including the condition of oxygen limitation and the temperature rise rate of 10 ℃/min), cooling, taking out the carbonized material, transferring the carbonized material into a ball-milling tank to be ball-milled for 6h according to the ball-material ratio of 10: 1, separating the mixture after the ball-milling tank is cooled, sieving 100-mesh nylon sieve to separate grinding medium steel balls and biochar with different particle sizes, carrying out magnetic separation on the obtained biochar by using a strong magnet, and magnetically separating the magnetic biochar.
Second, soil remediation experiment
The magnetic charcoal prepared by the invention is used for soil remediation experiments, the experiment for soil remediation of the paddy field is carried out according to the experiment method of the example 2, and the difference from the example 2 is that the rice is planted by a transplanter.
After the rice is harvested, the yield of different treatment groups and the heavy metal content in the rice are measured, the yield result is shown in figure 7, and the heavy metal content result in the rice is shown in figure 8. The results show that the application of different steel slag-based magnetic biochar increases the rice yield by 5%, 11%, 13% and 12% respectively, and that the application of the magnetic biochar can improve the soil environment quality, promote the plant production and increase the rice yield (fig. 7). In addition, the steel slag-based magnetic biochar also has a remarkable effect on reducing heavy metals in soil, so that the heavy metal enrichment in rice is reduced (figure 8), and when the application amount is higher than 10t/ha, the content of the heavy metals in brown rice can be guaranteed to be lower than the limit value of 0.2mg/kg in GB2762-2017 'pollutant limit in national food safety standards'.

Claims (9)

1. The magnetic biochar soil remediation agent for heavy metal pollution of soil is characterized in that: the magnetic biochar is prepared by the following steps:
crushing the solid waste rich in the magnetic iron/cobalt/nickel compound, and then carrying out chloridizing roasting;
magnetic particles are collected through magnetic separation;
then mixing the magnetic particles with a mixture of agricultural wastes and urea according to a certain proportion, and then carrying out carbonization, ball milling and magnetic separation, wherein the mass percentages of the magnetic particles, the agricultural wastes and the urea are respectively 45-60%, 15-30% and 5-25%;
the solid waste rich in the magnetic iron/cobalt/nickel compounds is one or a mixture of iron tailings, red mud, steel slag, welding slag and waste iron slag, and the agricultural waste is plant residues or animal wastes or a mixture of the plant residues and the animal wastes;
the chloridizing roasting is to adopt chloride solid and the solid waste rich in the magnetic iron/cobalt/nickel compound to be mixed and roasted, or the solid waste rich in the magnetic iron/cobalt/nickel compound is roasted after being washed by chloride solution, and the roasting temperature is 600-1000 ℃; the chloride solid is hydrochloric acid, calcium chloride or magnesium chloride; the chloride solution is hydrochloric acid solution;
the concrete technological parameters of the carbonization are as follows: a temperature programmed muffle furnace is adopted, under the condition of limited oxygen, the temperature rise rate is 5-10 ℃/min, the carbonization time is 30-180min, and the carbonization temperature is 300-700 ℃; sieving with 60-100 mesh nylon sieve to separate grinding medium and biochar;
the ball milling adopts a planetary ball mill, the ball material ratio is 5-10: 1, the rotating speed of the ball mill is 200-.
2. The magnetic biochar soil remediation agent of claim 1, wherein:
the plant residue is one or a mixture of weeds, crop straws and fallen leaves.
3. The magnetic biochar soil remediation agent of claim 2, wherein:
the crop straws are rice straws, wheat straws or corn straws, and the animal wastes are animal wastes.
4. The magnetic biochar soil remediation agent of claim 1, wherein:
the mass percentages of the magnetic particles, the agricultural wastes and the urea in the total mass of the magnetic particles, the agricultural wastes and the urea are respectively 50%, 25% and 25%.
5. The use of a magnetic biochar soil remediation agent as claimed in any one of claims 1 to 4 for remediation of contaminated soil.
6. The use of claim 5, wherein: the polluted soil is heavy metal moderate and mild polluted farmland soil in a mine area.
7. A soil remediation method, comprising the steps of:
(1) applying the magnetic biochar soil remediation agent of any one of claims 1 to 4 to the soil to be remediated at an application rate of 5-30t/ha, followed by a first irrigation, a first turn-over after flooding, mixing the soil with the soil remediation agent;
(2) and (4) irrigating for the second time after the first plowing, performing the second plowing by adopting a magnetic rotary cutter after flooding, naturally settling after plowing until water and soil are separated, draining, and finishing restoration.
8. The method of claim 7, wherein:
the application amount of the soil remediation agent is 10-30 t/ha;
the first irrigation water is tap water, river water, pond water, underground water or 0.01-0.06M EDTA; the water flooding depth is 2-5 cm;
the second irrigation water is tap water, river water, pond water or underground water, and the water submerging depth is 8-10 cm;
the first turning adopts a rotary plough integrated tractor, ploughs for 10-15min, and rotates for 8-10 min;
and the second plowing adopts magnetic rotary blade rotary tillage for 10-15min, and natural settlement is carried out for 10-14h after plowing.
9. The method of claim 8, wherein: the application amount of the soil remediation agent is 10-25t/ha, or 10-20t/ha, or 10-15t/ha, or 10 t/ha.
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