A method for recycling and repairing heavy metal contaminated soil by comprehensive utilization of willow and its derivatives

Abstract

A method for recycling and repairing heavy metal contaminated soil by comprehensively utilizing willow and its derivatives, using willow and organic eluent to synergistically treat heavy metal contaminated sites, improving the efficiency of using willow to repair heavy metal contaminated sites, and shortening the repair time to 2-3 years, while traditional willow repair of heavy metal contaminated sites usually takes 6-10 years, greatly shortening the time for willow to repair heavy metal contaminated sites. At the same time, waste such as trees and leaves generated in the process of plant repair of heavy metal contaminated sites are effectively utilized, and the coarse wood block particles, fine particle compost, biochar and organic eluent obtained by fermentation are reused in site repair, so that they are recycled and reused, avoiding air pollution caused by traditional treatment methods.

Description

Technical Field

[0001] The present invention belongs to the field of soil remediation, and specifically relates to a method for recycling and remediating heavy metal contaminated soil by comprehensively utilizing willow and its derivatives. Background Art

[0002] Among the technologies for remediating contaminated soil, tree restoration is an emerging technology for plant restoration of heavy metal contaminated sites. It mainly uses the tree root system to absorb and enrich pollutants, and transport and store pollutants in the roots and above-ground parts of the plant body. Then, through pruning, felling, and mowing plants, the purpose of removing heavy metals from the soil environment is achieved.

[0003] Willow has the advantages of strong adaptability, rapid growth, large biomass, tolerance to pruning, strong sprouting ability, etc. It can adapt well to polluted sites with low soil nutrients, harsh environmental conditions, and strong biological toxicity, and has become an important material for plant restoration of soil pollution. However, at present, the use of willow to repair heavy metal contaminated sites has a long restoration time, generally requiring 6-10 years of restoration time, and the biomass produced by the restoration is basically incinerated, causing secondary pollution to the atmosphere and other problems.

[0004] Biochar is the pyrolysis residue generated by wood, animal manure, straw, leaves, sludge and other biomass under anaerobic conditions. Studies have shown that biochar has a well-developed pore structure, dense surface negative charges, and is rich in a large number of functional groups such as carboxyl, quinone, and phenolic hydroxyl groups. It is a good adsorbent and has strong adsorption for heavy metals. Biochar fiber can adsorb heavy metal Cu from water. 2+ 、Ni 2+ and Pb 2+ Ions, single component equilibrium adsorption capacity is between 0.080-0.175mmoL / g.

[0005] The biocomposting process is actually the degradation process of organic matter. During this period, a large amount of organic acids will be produced. The fermentation samples contain amino acids, volatile fatty acids and other low-molecular organic acids such as oxalic acid and acetic acid, among which acetic acid is the main component, accounting for 42% to 93%. There are three main modes of leaching of heavy metals in soil by natural organic acids: one is to directly complex with heavy metals to form positively charged complexes; the second is to form complexes with heavy metals after adsorption on the soil surface; the third is to form highly soluble complexes by coordination with heavy metals. Studies have found that 0.1 mol / L oxalic acid can remove 90-95% of Pb, Cd, Zn and 80-85% of Cu in contaminated soil by leaching. The removal effect of acetic acid on Pb is better than that on Cd and Ni, and the removal rate of Pb by ultrasonic leaching with 0.1 mol / L acetic acid can reach 100%, and the highest removal rates of Cd and Ni by 1 mol / L acetic acid leaching are 70.6% and 23.3%, respectively. Natural organic acids have a certain ability to remove heavy metals from contaminated soils, are easily biodegradable, have little damage to the physical and chemical properties of the soil, and have a lower risk of secondary pollution.

[0006] If willow trees are used to repair heavy metal pollution, and the biomass produced by their growth is used to produce biochar and leachate containing organic acids to further leach and enrich heavy metals in the soil, the repair time can be greatly shortened, and problems such as secondary pollution caused by traditional biomass incineration can be avoided. Summary of the invention

[0007] The purpose of the present invention is to provide a method for the comprehensive utilization of willow and its derivatives to cyclically repair heavy metal contaminated soil, thereby improving the repair efficiency of heavy metal contaminated sites and greatly shortening the time for repairing heavy metal contaminated sites using willow. At the same time, waste such as trees and leaves generated during the plant repair of heavy metal contaminated sites can be effectively utilized to avoid air pollution caused by traditional treatment methods.

[0008] In order to achieve the above object, the present invention provides the following technical solutions:

[0009] A method for recycling and repairing heavy metal contaminated soil by comprehensive utilization of willow and its derivatives, comprising the following steps:

[0010] 1) Treatment of the site to be repaired

[0011] The site to be repaired is plowed, and wood particles and compost particles are added during the plowing process, wherein the wood particles have a particle size of 3 to 5 cm, and the compost particles have a particle size of 0.5 to 1.5 cm, and then the site is leveled;

[0012] 2) Arrange the eluent collection system

[0013] Dig a 50-70cm wide drainage ditch every 3-5m at the treatment site, with a digging depth of 20-30cm. Insert a 3-5m long leaching liquid collection pipe every 2-3m in the vertical direction of the drainage ditch, with the insertion depth being the junction of the soil leaching layer and the sedimentary layer. There are collection holes on the leaching liquid collection pipe, the diameter of the collection holes is 0.1-0.2cm, and the distance between every two collection holes is 10-15cm; arrange rotating scattering nozzles in the site, the nozzle arrangement spacing is 45-50% of the nozzle spraying diameter, and the nozzle range is 4.0-15m;

[0014] 3) Forsythia Planting

[0015] Select strong willow branches with a diameter of 0.8-1.5 cm as cuttings, cut them into 20-30 cm cuttings, each with 2-3 axillary buds, and then insert the cuttings into the soil at a density of 4-8 plants / m 2 , the cutting depth is 8 to 10 cm, and then the soil is covered with wood particles with a particle size of 3 to 5 cm;

[0016] 4) Spraying and washing liquid collection

[0017] Before spraying, the eluent collection pipe should be unblocked. The organic eluent should be sprayed once a month with an irrigation intensity of 15-30 mm / h, a working pressure of 20-50 PSI, and a spraying time of 3-5 hours. The organic eluent containing heavy metals is collected through the ditch, which is called the eluent recovery liquid. The eluent recovery liquid is reused as the organic eluent after treatment;

[0018] 5) Harvest management

[0019] The above-ground part of the willow trees is harvested in October and November every year, with the stump height of 3 to 5 cm. The above-ground biomass is concentrated for post-processing, and the obtained wood pellets, pellet compost, biochar and organic leachate are recycled into the willow planting process;

[0020] 6) Add chelating agent

[0021] The heavy metal content in the soil is tested every year. When the heavy metal content in the soil is lower than the agricultural land soil pollution risk screening value, the site remediation is completed.

[0022] Further, in step 3), the leaching recovery liquid treatment method is as follows: the leaching recovery liquid is filtered through a biochar with a thickness of 0.9 to 1 m, and then 0.05-0.1 g of ferrous sulfate is added to each liter of leaching recovery liquid, and then it is electrolyzed for 2 to 3 hours, and then allowed to stand for 2 to 3 hours, and the heavy metal content in the leaching recovery liquid is detected. When the heavy metal content in the filtrate is lower than the emission limit of the "Integrated Sewage Discharge Standard (GB8978-1996)", the treatment is terminated. If the heavy metal content in the filtrate is higher than the emission limit of the "Integrated Sewage Discharge Standard (GB8978-1996)", the electrolysis is continued for 2 to 3 hours and the precipitation is continued for 2 to 3 hours. until the heavy metal content in the filtrate is lower than the emission limit of the Comprehensive Sewage Discharge Standard (GB8978-1996); during the electrolysis process, the cathode material is highly active graphite oxide felt, the anode material is a platinum-plated electrode, the applied voltage is 2-2.5V, the electrode spacing is 1.5-2.0cm, and the oxygen flux is 80-100mL / min; the sludge obtained by electrolysis is collected and recycled.

[0023] Preferably, the wood block particles are obtained by crushing the above-ground trunks of harvested willow trees.

[0024] Furthermore, the specific preparation method of the biochar is: spraying a maltooligosaccharide aqueous solution on the surface of the wood block particles, then placing them in a carbonization furnace, raising the temperature in the carbonization furnace from room temperature to 250-350°C at a rate of 20-23°C / min, and then raising the temperature in the carbonization furnace from 250-350°C to 450-500°C at a rate of 5-10°C / min, maintaining for 10-15 minutes, and carbonizing to obtain biochar.

[0025] Preferably, the mass percentage of the maltooligosaccharide aqueous solution is 2-5%.

[0026] Furthermore, the specific preparation steps of the granular compost are: crushing the collected branches and leaves of the above-ground part of the willow into fine granular fragments with a particle size of 0.5 to 1.5 cm, subjecting the fine granular fragments to strip composting fermentation, supplying oxygen through mechanical composting, watering once every three days, and the amount of watering does not exceed 5% of the volume of the compost, measuring the pH of the leachate every day from the 5th day of fermentation, starting from the 5th day of fermentation, collecting the leachate when the pH of the leachate is lower than 6.5, stopping collecting when the pH of the leachate is greater than 6.5, fermenting for more than 30 days to complete the decomposition, and obtaining fine granular compost; the collected leachate is stored below 4°C for future use.

[0027] Further, saponin is added to the collected filtrate and mixed evenly to obtain an organic eluent, wherein the amount of saponin added is 0.06-0.1 g / L.

[0028] Preferably, the compost is turned and oxygen is supplied according to the fermentation core temperature. When the fermentation core temperature is lower than 30°C, the compost is not turned. When the fermentation core temperature is between 30 and 50°C, the compost is turned once every 8 to 10 days. When the fermentation core temperature is between 50 and 70°C, the compost is turned once every 3 to 4 days. When the fermentation core temperature exceeds 70°C, the compost is turned every day.

[0029] Preferably, the species of the willow are bamboo willow, golden willow and / or weeping willow.

[0030] Willow has high biomass yield, fast growth, extensive root structure, low impact on the food chain, strong adaptability to various environments, strong tolerance to heavy metals, and the ability to accumulate high levels of heavy metals in above-ground tissues. Therefore, willow is a preferred tree species for phytoremediation of heavy metal contaminated sites.

[0031] When the willow is used to repair a heavy metal contaminated site, the present invention uses an organic leaching solution for spraying during the planting process, which is beneficial to the leaching and enrichment of heavy metals in the soil and improves the heavy metal soil repair efficiency.

[0032] Before planting willows, mix wood particles into the soil, then plant willow cuttings, and then cover the site with wood particles, which can effectively inhibit the growth of weeds, maintain soil temperature and humidity, improve soil fertility, and promote the germination and seedling growth of willows. At the same time, because the soil surface is covered with a layer of wood particles, on the one hand, it inhibits the growth of weeds by shading; on the other hand, even if weeds sprout, due to the large diameter of the wood particles, it is difficult for the buds to drill out of the gaps, thereby inhibiting the growth of weeds and reducing the cost of field maintenance and management.

[0033] In the present invention, willow biochar is prepared by using harvested willow branches. The obtained biochar has a well-developed pore structure, dense surface negative charges, and is rich in a large number of functional groups such as carboxyl, quinone, and phenolic hydroxyl groups. It is a good adsorbent and has strong adsorption for heavy metals. The biochar is then used to filter the eluent to effectively remove the heavy metals in the eluent.

[0034] The present invention utilizes willow branches and leaves to prepare fine-particle compost, collects organic leachate during the compost production process to prepare an organic leaching solution, sprays the organic leaching solution during the growth of the willow, and the organic leaching solution can promote the desorption of heavy metal ions from the surface of soil particles. The organic acid in the organic leaching solution forms a soluble chelate with the heavy metal ions within a wide acidity range, increases the effectiveness and mobility of the heavy metal ions, and helps to improve the repair effect of heavy metal-contaminated soil. In addition, the organic leaching solution can also improve the soil physical and chemical properties and the community structure of soil microorganisms when entering the soil. It avoids the use of traditional inorganic leaching agents to repair heavy metal-contaminated soil, and the loss of soil nutrients caused by the destruction of the aggregate structure in the soil and the change of the soil physical and chemical properties.

[0035] In addition, saponin in the organic eluent is a biosurfactant that can be adsorbed on the soil surface and occupy soil adsorption sites to weaken the adsorption of heavy metals by soil particles, and can complex heavy metals adsorbed by the soil, thereby eluting heavy metals into the solution, which is conducive to the enrichment of heavy metals in plants, thereby improving the remediation efficiency. Moreover, it has a large and complex molecular structure, is easily degradable, has low biological toxicity, and will not cause secondary pollution to the environment.

[0036] The present invention uses an electrolytic method to treat the leaching recovery liquid. The O2 dissolved in the water undergoes a two-electron redox reaction on the surface of the cathode highly active graphite oxide felt to generate H2O2. H2O2 generates highly oxidizing OH under electrocatalysis or catalysis of other metal ions, which precipitates heavy metal ions while oxidizing organic pollutants, decomposing organic nitrogen, phosphorus and other nutrients that are difficult for plants to use into effective nitrogen, phosphorus and potassium. The treated leaching recovery liquid is re-sprayed on the willow trees, and the nutrients in it are easily absorbed by the plants. This process generates a chain reaction, generates H2O2 in situ, and recycles Fe 2+ , greatly reducing the dosage of H2O2 and iron ion catalyst in traditional electrochemical treatment. And the cathode high-activity graphite oxide felt has high electron transfer efficiency, which increases the conversion efficiency.

[0037] The role of adding a small amount of ferrous sulfate to the elution recovery solution:

[0038] First, ferrous sulfate decomposes into Fe in the leachate. 2+ , Fe 2+ It has strong redox properties and can convert some heavy metals such as Cu 2+ Cr 6+ etc. are reduced to insoluble substances, while Fe 2+ Fe(OH)3 precipitate is formed.

[0039] Secondly, a part of the ferrous ions will react with the dissolved oxygen in the aqueous solution and be oxidized into trivalent ferrous ions. The trivalent ferrous ions will form multi-nuclear complexes after hydrolysis, and react with heavy metal pollutants in the water to form flocculation and precipitate hydroxides for precipitation, which can effectively remove heavy metals in the leachate.

[0040] Again, the added ferrous sulfate can provide Fe for the electrolysis reaction. 2+ catalyst.

[0041] Compared with the prior art, the present invention has the following beneficial effects:

[0042] 1). The present invention utilizes willow and organic eluent to collaboratively treat heavy metal contaminated sites, effectively solving the air pollution caused by wastes such as trees generated by plant restoration during the treatment process. At the same time, it improves the efficiency of using willow to repair heavy metal contaminated sites, and the repair time can be shortened to 2-3 years. Traditional willow repair of heavy metal contaminated sites usually takes 6-10 years, which greatly shortens the time for willow to repair heavy metal contaminated sites.

[0043] 2) In the process of willow restoration, the wastes such as trees generated by the present invention are fermented to obtain willow coarse wood particles, fine particle compost, biochar and organic leaching liquid, which are then reused in site restoration and recycled.

[0044] 3) The present invention sprays organic eluent during the willow planting process, which can improve the efficiency and effect of heavy metal remediation while also improving the physical and chemical properties of the soil and the community structure of soil microorganisms. This avoids the use of traditional inorganic eluents to repair heavy metal-contaminated soil, the destruction of soil aggregate structure and the loss of soil nutrients caused by changes in soil physical and chemical properties.

[0045] 4). The present invention adds ferrous sulfate to the elution recovery liquid and then performs electrolytic treatment, which effectively removes heavy metal ions in the elution recovery liquid while retaining other effective components therein, and can be repeatedly used as an organic elution liquid. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 This is a graph showing the change in heavy metal content in the leachate at different elution times according to Example 2 of the present invention. DETAILED DESCRIPTION

[0047] The present invention is further described below in conjunction with specific implementation modes.

[0048] A method for recycling and repairing heavy metal contaminated soil by comprehensive utilization of willow and its derivatives, comprising the following steps:

[0049] 1) Treatment of the site to be repaired

[0050] The site to be repaired is plowed, and wood particles and compost particles are added during the plowing process, wherein the wood particles have a particle size of 3 to 5 cm, and the compost particles have a particle size of 0.5 to 1.5 cm, and then the site is leveled;

[0051] 2) Arrange the eluent collection system

[0052] Dig a 50-70cm wide drainage ditch every 3-5m at the treatment site, with a digging depth of 20-30cm. Insert a 3-5m long leaching liquid collection pipe every 2-3m in the vertical direction of the drainage ditch, with the insertion depth being the junction of the soil leaching layer and the sedimentary layer. There are collection holes on the leaching liquid collection pipe, the diameter of the collection holes is 0.1-0.2cm, and the distance between every two collection holes is 10-15cm; arrange rotating scattering nozzles in the site, the nozzle arrangement spacing is 45-50% of the nozzle spraying diameter, and the nozzle range is 4.0-15m;

[0053] 3) Forsythia Planting

[0054] Select strong willow branches with a diameter of 0.8-1.5 cm as cuttings, cut them into 20-30 cm cuttings, each with 2-3 axillary buds, and then insert the cuttings into the soil at a density of 4-8 plants / m 2 , the cutting depth is 8 to 10 cm, and then the soil is covered with wood particles with a particle size of 3 to 5 cm;

[0055] 4) Spraying and washing liquid collection

[0056] Before spraying, the eluent collection pipe should be unblocked. The organic eluent should be sprayed once a month with an irrigation intensity of 15-30 mm / h, a working pressure of 20-50 PSI, and a spraying time of 3-5 hours. The organic eluent containing heavy metals is collected through the ditch, which is called the eluent recovery liquid. The eluent recovery liquid is reused as the organic eluent after treatment;

[0057] 5) Harvest management

[0058] The above-ground part of the willow trees is harvested in October and November every year, with the stump height of 3 to 5 cm. The above-ground biomass is concentrated for post-processing, and the obtained wood pellets, pellet compost, biochar and organic leachate are recycled into the willow planting process;

[0059] 6) Add chelating agent

[0060] The heavy metal content in the soil is tested every year. When the heavy metal content in the soil is lower than the agricultural land soil pollution risk screening value, the site remediation is completed.

[0061] Preferably, the wood block particles are obtained by crushing the above-ground trunks of harvested willow trees.

[0062] Furthermore, the specific preparation method of the biochar is: spraying a maltooligosaccharide aqueous solution on the surface of the wood block particles, then placing them in a carbonization furnace, raising the temperature in the carbonization furnace from room temperature to 250-350°C at a rate of 20-23°C / min, and then raising the temperature in the carbonization furnace from 250-350°C to 450-500°C at a rate of 5-10°C / min, maintaining for 10-15 minutes, and carbonizing to obtain biochar.

[0063] Preferably, the mass percentage of the maltooligosaccharide aqueous solution is 2-5%.

[0064] Furthermore, the specific preparation steps of the granular compost are: crushing the collected branches and leaves of the above-ground part of the willow into fine granular fragments with a particle size of 0.5 to 1.5 cm, subjecting the fine granular fragments to strip composting fermentation, supplying oxygen through mechanical composting, watering once every three days, and the amount of watering does not exceed 5% of the volume of the compost, measuring the pH of the leachate every day from the 5th day of fermentation, starting from the 5th day of fermentation, collecting the leachate when the pH of the leachate is lower than 6.5, stopping collecting when the pH of the leachate is greater than 6.5, fermenting for more than 30 days to complete the decomposition, and obtaining fine granular compost; the collected leachate is stored below 4°C for future use.

[0065] Further, saponin is added to the collected filtrate and mixed evenly to obtain an organic eluent, wherein the amount of saponin added is 0.06-0.1 g / L.

[0066] Preferably, the compost is turned and oxygen is supplied according to the fermentation core temperature. When the fermentation core temperature is lower than 30°C, the compost is not turned. When the fermentation core temperature is between 30 and 50°C, the compost is turned once every 8 to 10 days. When the fermentation core temperature is between 50 and 70°C, the compost is turned once every 3 to 4 days. When the fermentation core temperature exceeds 70°C, the compost is turned every day.

[0067] Preferably, the species of the willow are bamboo willow, golden willow and / or weeping willow.

[0068] The experiment was carried out according to the above implementation steps. The specific experimental control conditions are shown in Table 1. Table 2 shows the changes in heavy metal content in the soil under different treatment conditions.

[0069] It can be seen from Table 2 that the heavy metal content in the soil gradually decreases with the extension of the willow planting time. Using the method described in the present invention, the average content of Cu, Pb, and Zn in the soil in the third year is lower than the agricultural land soil pollution risk screening value. In Example 3, the average content of Cu and Pb in the soil in the second year is already lower than the agricultural land soil pollution risk screening value, indicating that the method can effectively reduce the heavy metal content in the soil in a relatively short period of time.

[0070] In the control group, although the content of heavy metals in the soil decreased with the increase of planting time, the rate was slow. It took 6 to 8 years for the content of heavy metals in the soil to be lower than the screening value of soil pollution risk for agricultural land.

[0071] Treatment of elution recovery liquid

[0072] Treatment method of leaching recovery liquid: the leaching recovery liquid is filtered through a biochar with a thickness of 0.9 to 1 m, and then 0.05 to 0.1 g of ferrous sulfate is added to each liter of leaching recovery liquid, and then it is electrolyzed for 2 to 3 hours and left to stand for 2 to 3 hours, and the heavy metal content in the leaching recovery liquid is detected. When the heavy metal content in the filtrate is lower than the discharge limit of the "Comprehensive Sewage Discharge Standard (GB8978-1996)", the treatment is completed. If the heavy metal content in the filtrate is higher than the discharge limit of the "Comprehensive Sewage Discharge Standard (GB8978-1996)", the heavy metal content in the filtrate is lower than the discharge limit of the "Comprehensive Sewage Discharge Standard (GB8978-1996)". The electrolysis step is continued for 2 to 3 hours and the precipitation step is continued for 2 to 3 hours until the heavy metal content in the filtrate is lower than the emission limit of the Integrated Sewage Discharge Standard (GB8978-1996); during the electrolysis process, the cathode material is highly active oxidized graphite felt, the anode material is a platinum-plated electrode, the applied voltage is 2 to 2.5 V, the electrode spacing is 1.5 to 2.0 cm, and the oxygen flux is 80 to 100 mL / min; the sludge obtained by electrolysis is collected and recycled.

[0073] Repeat 3 times and test the elution recovery solution each time. The test results are as follows Figure 1 .

[0074] from Figure 1 It can be seen that as the treatment steps proceed, the heavy metal content in the leachate gradually decreases. The removal rates of Cu, Pb, and Zn in the leachate by biochar reached 73.1%, 50.9%, and 82.9%, respectively. After the first electrolysis, the content of Cu, Pb, and Zn basically reached the third-level standard of the emission limit of the "Shanghai Local Standard for Integrated Wastewater Discharge Standards (DB31 / 199-2018)". The second and third electrolysis changes were small, so that the content of Cu, Pb, and Zn reached the first-level standard of the emission limit of the "Shanghai Local Standard for Integrated Wastewater Discharge Standards (DB31 / 199-2018)".

[0075]

[0076]

Claims

1. A method for recycling and repairing heavy metal contaminated soil by comprehensive utilization of willow and its derivatives, comprising the following steps: 1) Treatment of the site to be restored The site to be repaired is plowed, and wood particles and compost particles are added during the plowing process. The wood particles have a particle size of 3 to 5 cm, and the compost particles have a particle size of 0.5 to 1.5 cm. Then the site is leveled; 2) Arrange the eluent collection system Dig a 50-70cm wide drainage ditch every 3-5m at the treatment site, with a digging depth of 20-30cm. Insert a 3-5m long leaching liquid collection pipe every 2-3m in the vertical direction of the drainage ditch, with the insertion depth being the junction of the soil leaching layer and the sedimentary layer. There are collection holes on the leaching liquid collection pipe, with a diameter of 0.1-0.2cm, and a spacing of 10-15cm between every two collection holes. Arrange rotating scattering nozzles in the site, with the nozzle arrangement spacing being 45-50% of the nozzle spraying diameter, and the nozzle range being 4.0-15m. 3) Willow Planting Select strong willow branches with a diameter of 0.8-1.5 cm as cuttings, cut them into 20-30 cm cuttings, each with 2-3 axillary buds, and then insert the cuttings into the soil at a density of 4-8 plants / m 2 , the cutting depth is 8~10cm, and then the soil is covered with wood particles with a particle size of 3~5cm; 4) Spray and rinse liquid collection Before spraying, the eluent collection pipe should be unblocked. The organic eluent should be sprayed once a month with an irrigation intensity of 15-30 mm / h, a working pressure of 20-50 PSI, and a spraying time of 3-5 hours. The organic eluent containing heavy metals is collected through the ditch, which is called the eluent recovery liquid. The eluent recovery liquid is reused as the organic eluent after treatment. The leaching recovery liquid treatment method comprises: the leaching recovery liquid is filtered through a biochar with a thickness of 0.9 to 1 m, and then 0.05 to 0.1 g of ferrous sulfate is added to each liter of the leaching recovery liquid, and then the leaching recovery liquid is subjected to electrolysis for 2 to 3 hours and allowed to stand for 2 to 3 hours, and the heavy metal content in the leaching recovery liquid is detected. When the heavy metal content in the filtrate is lower than the discharge limit of the "Comprehensive Sewage Discharge Standard (GB8978-1996)", the treatment is terminated; if the heavy metal content in the filtrate is higher than the discharge limit of the "Comprehensive Sewage Discharge Standard (GB8978-1996)", the treatment is terminated. The electrolysis step is continued for 2 hours and the precipitation step is continued for 2 hours until the heavy metal content in the filtrate is lower than the emission limit of the "Comprehensive Sewage Discharge Standard (GB8978-1996)". During the electrolysis process, the cathode material is highly active oxidized graphite felt, the anode material is a platinum-plated electrode, the applied voltage is 2-2.5V, the electrode spacing is 1.5-2.0cm, and the oxygen flux is 80-100mL / min. The bottom sludge obtained by electrolysis is collected and recycled. 5) Harvest management The above-ground part of the willow trees is harvested in October and November every year, with the stump height of 3 to 5 cm. The above-ground biomass is concentrated for post-processing, and the obtained wood pellets, pellet compost, biochar and organic leachate are recycled into the willow planting process; The specific preparation method of the biochar is as follows: spraying a maltooligosaccharide aqueous solution on the surface of the wood block particles, then placing the wood block particles in a carbonization furnace, raising the temperature in the carbonization furnace from room temperature to 250-350°C at a rate of 20-23°C / min, and then raising the temperature in the carbonization furnace from 250-350°C to 450-500°C at a rate of 5-10°C / min, maintaining the temperature for 10-15 minutes, and carbonizing the wood block particles to obtain the biochar. The specific preparation steps of the granular compost are: crushing the collected branches and leaves of the above-ground part of the willow into fine granular fragments with a particle size of 0.5-1.5 cm, composting the fine granular fragments in a strip composting manner, turning the compost to supply oxygen, watering once every three days, and the amount of watering does not exceed 5% of the volume of the compost, measuring the pH of the leachate every day from the 4th day of fermentation, collecting the leachate when the pH of the leachate is lower than 6.5, and stopping collecting when the pH of the leachate is greater than 6.5, fermenting for more than 30 days to complete the decomposition, and obtaining fine granular compost; the collected leachate is stored below 4° C. for future use; Add saponin to the collected filtrate and mix well to obtain an organic eluent, wherein the amount of saponin added is 0.06-0.1 g / L; 6) Add chelating agent The heavy metal content in the soil is tested every year. When the heavy metal content in the soil is lower than the agricultural land soil pollution risk screening value, the site remediation is completed.

2. The method for recycling and repairing heavy metal contaminated soil by comprehensive utilization of willow and its derivatives according to claim 1 is characterized in that: The wood block particles are obtained by crushing the above-ground trunks of harvested willow trees.

3. The method for recycling and repairing heavy metal contaminated soil by comprehensive utilization of willow and its derivatives according to claim 1 is characterized in that: The mass percentage of the maltooligosaccharide aqueous solution is 2-5%.

4. The method for recycling and repairing heavy metal contaminated soil by comprehensive utilization of willow and its derivatives according to claim 1 is characterized in that: The compost is turned and oxygen is supplied according to the temperature of the fermentation core. When the temperature of the fermentation core is lower than 30°C, the compost is not turned. When the temperature of the fermentation core is between 30 and 50°C, the compost is turned once every 8 to 10 days. When the temperature of the fermentation core is between 50 and 70°C, the compost is turned once every 3 to 4 days. When the temperature of the fermentation core exceeds 70°C, the compost is turned every day.

5. The method for recycling and repairing heavy metal contaminated soil by comprehensive utilization of willow and its derivatives according to claim 1 is characterized in that: The species of the willow are bamboo willow, golden willow and / or weeping willow.

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