A method for recovering phosphorus based on wetland plant circulation hydrothermal carbonization

Abstract

The present invention relates to the field of solid waste resource utilization and environmental engineering technology, and specifically to a method for recovering phosphorus based on wetland plant cyclic hydrothermal carbonization, comprising the following steps: (1) washing and drying the harvested wetland plants, crushing and screening; (2) taking biomass powder for hydrothermal carbonization treatment to obtain hydrothermal carbonization products; (3) performing solid-liquid separation on the obtained carbonization products; (4) continuing to utilize the obtained carbonized liquid as a liquid phase medium, mixing biomass raw materials for multi-stage cyclic hydrothermal carbonization, and after the cyclic carbonization is completed, performing solid-liquid separation to obtain phosphorus-rich carbonized liquid and cyclic hydrothermal charcoal; (5) by adjusting the pH of the phosphorus-rich carbonized liquid, phosphorus crystal precipitation is formed and recovered. The present invention realizes effective recovery of phosphorus in wetland plants, and at the same time realizes resource utilization of hydrothermal carbonization products of wetland plants, reduces the impact of hydrothermal carbonization of biomass waste on the environment, and effectively solves the problem of wetland plant residue treatment and disposal.

Description

Technical Field

[0001] The invention relates to the technical field of solid waste resource utilization and environmental engineering, and in particular to a method for recovering phosphorus based on wetland plant circulating hydrothermal carbonization. Background Art

[0002] Phosphorus is one of the macronutrients necessary for plant growth. Applying phosphate fertilizer is an effective way to ensure soil fertility and increase crop yields. However, as the main raw material for phosphate fertilizer production, phosphate rock is a non-renewable strategic resource with prominent resource risks and ecological and environmental risks. In this context, it is necessary to develop alternative phosphorus resources! Eutrophication of water bodies is a serious water quality problem that is prevalent worldwide. Artificial wetland technology is currently widely used for the efficient removal of nitrogen and phosphorus in eutrophic water bodies. Under natural conditions, after the wetland plant residues are decomposed, the nutrients they carry (including N, P and organic matter, etc.) will return to the water body again to participate in the biogeochemical cycle of nutrients, thus causing secondary pollution. How to realize the resource utilization of these wetland plants is crucial to preventing secondary pollution, and it is also the key to realizing phosphorus recovery in water bodies. Compared with crop straw and wood, wetland plants have higher phosphorus recovery potential. Considering the inherent properties of wetland plant waste (e.g., high moisture content, multiple organic components, and multiple phosphorus species), direct extraction of phosphorus from the raw materials is not the best option. In addition, direct application of wetland plant biomass may also lead to the introduction of pathogenic bacteria, rapid leaching of phosphorus, and the threat of heavy metals. Therefore, the recovery and effective reuse of phosphorus from phosphorus-rich wetland plants is an issue worthy of attention.

[0003] Hydrothermal carbonization is a new type of low-temperature technology for preparing hydrothermal charcoal, which can directly treat solid waste with high water content. During the hydrothermal carbonization process of wetland plants, phosphorus in the raw materials will inevitably migrate to the liquid phase product. Most of the current research focuses on the research and modification of hydrothermal charcoal, and there are relatively few analytical studies on carbonized liquid. Since the liquid phase product contains a large amount of organic matter and salts, how to properly treat the liquid phase product is a question worth considering. The water consumption and carbonized liquid production of the hydrothermal carbonization process are huge. If the hydrothermal carbonized liquid is recycled, it is expected to enrich and recover the nutrients in the carbonized liquid, and significantly reduce the output of the carbonized liquid and the subsequent processing cost and difficulty, realizing resource utilization while reducing environmental pollution.

[0004] Most current research focuses on the recovery of phosphorus from animal manure and sewage sludge using electrochemical and thermal treatment technologies, but there is little focus on the recovery of phosphorus from phosphorus-containing wetland plant residues. In addition, most current research on hydrothermal carbonization products is aimed at the recovery of phosphorus in hydrothermal charcoal, but there is relatively little research on the efficient enrichment and reuse of phosphorus in liquid products. Taking effective measures to efficiently enrich and recover phosphorus from wetland plants is a win-win strategy for alleviating phosphorus resource shortages and treating wetland plant waste, and has practical promotion significance for the industrial application of hydrothermal carbonization treatment of wetland plants. Summary of the invention

[0005] In view of the defect of the prior art in the hydrothermal recovery of phosphorus from wetland plants that lacks the recovery of phosphorus in the liquid phase product, the present invention provides a method for recovering phosphorus based on the circulating hydrothermal carbonization of wetland plants. The specific technical scheme is as follows:

[0006] A method for recovering phosphorus based on wetland plant circulation hydrothermal carbonization, comprising the following steps:

[0007] (1) Wash and dry the harvested wetland plants, then crush and sieve;

[0008] (2) adding wetland plant biomass powder into a liquid medium for hydrothermal carbonization to obtain a hydrothermal carbonization product;

[0009] (3) performing solid-liquid separation on the carbonized product obtained in step (2), and collecting the solid phase product and the liquid phase product;

[0010] (4) using the carbonized liquid obtained in step (3) as a liquid medium, mixing with wetland plant biomass powder to carry out multi-stage cyclic carbonization, and after the multi-stage cyclic carbonization is completed, performing solid-liquid separation to obtain circulating hydrothermal charcoal and phosphorus-rich carbonized liquid;

[0011] (5) By adjusting the pH of the phosphorus-rich carbonized liquid obtained in step (4), phosphorus crystals are precipitated and recovered.

[0012] Furthermore, the wetland plants include common water restoration plants such as duckweed, canna, calamus or lily of the valley.

[0013] Furthermore, in step (2), the liquid medium is one of water and an acid solution.

[0014] Furthermore, the acid solution includes one of sulfuric acid, hydrochloric acid and acetic acid, and the concentration of the acid solution is 0.5 to 2 mol / L.

[0015] Furthermore, the liquid medium is deionized water or 1 mol / L sulfuric acid solution.

[0016] Furthermore, in step (2), the mass ratio of the biomass to the liquid medium is 1:5-20, the carbonization temperature is 200°C-240°C, the insulation time is 1-3h, and the stirring speed is 80-120rpm.

[0017] Furthermore, the mass ratio of biomass to liquid medium was 1:10, the carbonization temperature was 200°C, the insulation time was 2h, and the stirring speed was 100rpm.

[0018] Furthermore, in step (3), the solid phase product is hydrothermal charcoal, and the liquid phase product is carbonized liquid.

[0019] Furthermore, in step (4), the number of cyclic carbonization is 1 to 6 times.

[0020] Furthermore, in step (4), the number of cyclic carbonization is 5 times.

[0021] Furthermore, in step (5), the pH value during the crystallization process is stabilized to 9 to 10, and the mixture is stirred at a speed of 80 to 120 r / min for 0.5 to 1.5 h, and allowed to stand for 0.5 to 1.5 h after the reaction is completed.

[0022] Furthermore, in step (5), the pH of the phosphorus-rich carbonized liquid is adjusted to 9.5, and the mixture is stirred at this pH value at a speed of 100 r / min for 1 hour. After the reaction is completed, the mixture is allowed to stand for 1 hour, and the precipitate is collected, washed with distilled water 2 to 3 times, and dried in a constant temperature drying oven at 50° C. for more than 48 hours until the precipitate has a constant weight.

[0023] Furthermore, the main phosphorus crystallization product of the phosphorus-rich carbonized liquid is hydroxyapatite or struvite.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] (1) The present invention performs cyclic hydrothermal carbonization treatment on wetland plants by recycling liquid products mixed with biomass raw materials, thereby producing a hydrothermal carbonized liquid with a high phosphorus concentration, thereby achieving effective enrichment of phosphorus in the liquid products. The recycling of the hydrothermal carbonized liquid effectively reduces the discharge of carbonization waste liquid.

[0026] (2) The phosphorus-rich carbonized liquid obtained by the present invention is used as a phosphorus source, and no exogenous metal needs to be added. Hydroxyapatite and struvite crystals as slow-release phosphate fertilizers can be obtained by simply adjusting the pH of the carbonized liquid. The phosphate recovery rate is greater than 99%, thereby realizing the recovery of phosphorus and the resource utilization of the carbonized liquid. This has a positive significance for finding alternative phosphorus sources and alleviating the pressure on phosphate resources.

[0027] (3) The present invention realizes the effective recovery of phosphorus in wetland plants and the resource utilization of the hydrothermal carbonization products of wetland plants, fully utilizes the nutrient elements in the liquid products, reduces the impact of hydrothermal carbonization of biomass waste on the environment, and effectively solves the problem of treatment and disposal of wetland plant residues.

[0028] (4) The crystalline product obtained in the present invention has a low content of heavy metals, and its bioavailability and environmental risks are significantly reduced, making it safer to use as a phosphate fertilizer. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Schematic diagram of the recovery ratio of phosphorus in the cyclic carbonization product in the embodiment of the present invention; wherein the numbers on the vertical axis are the number of cyclic carbonization times.

[0030] Figure 2 It is the XRD spectrum of the phosphorus crystal product in the embodiment of the present invention.

[0031] Figure 3 is a scanning electron microscope image of the phosphorus crystal product in the example of the present invention. DETAILED DESCRIPTION

[0032] In order to make those skilled in the art better understand the scheme of the present invention, the technical scheme of the present invention is clearly and completely described below in conjunction with specific embodiments. It should be noted that the following detailed descriptions are all exemplary and are only embodiments of a part of the present invention, rather than all embodiments.

[0033] Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative work should fall within the scope of protection of the present invention.

[0034] Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those of ordinary skill in the art to which this application belongs. The experimental materials used in the examples of the present invention are all conventional experimental materials in the art and can be purchased through commercial channels. Experimental methods without specifying detailed conditions were performed according to conventional experimental methods or according to the operating instructions recommended by the supplier.

[0035] Example 1

[0036] The gold duckweed harvested from artificial wetlands was selected as the raw material. After washing with water and air-drying for one week, it was placed in an oven at 80°C and dried to constant weight, and then ground with a pulverizer. The ground powder was sieved through a 40-mesh sieve. The gold duckweed powder was taken as the raw material, and the raw material was mixed with deionized water in a fixed ratio of 1:10 and placed in a hydrothermal reactor, and then heated to a preset temperature of 200°C, kept warm for 2 hours and naturally cooled to room temperature, and then the solid and liquid phase products were separated. The solid phase product was dried and crushed to obtain hydrothermal charcoal, and the liquid phase product was carbonized liquid. Carbonized liquid was used instead of deionized water as the liquid phase medium, and the mixed gold duckweed raw material was subjected to 5 cycles of hydrothermal carbonization.

[0037] The fifth-stage carbonized liquid was used as the test material and stirred with a magnetic stirrer at room temperature. The pH value change in the system was monitored online with a pH meter. The pH was adjusted to 9.5 by adding NaOH solution and HCl solution. The pH was maintained at this pH value and stirred for 1 hour at a speed of 100 r / min. After the reaction was completed, it was allowed to stand for 1 hour, and the precipitate was collected by filtration. It was washed with distilled water for 2 to 3 times, and dried in a constant temperature drying oven at 50°C for more than 48 hours until the precipitate had a constant weight to obtain a phosphorus crystal product.

[0038] It was determined that the total phosphorus content in the duckweed biomass raw material was 4.83 mg / g, the inorganic phosphorus content was 3.48 mg / g, and the organic phosphorus content was 1.35 mg / g.

[0039] In this embodiment, after the biomass is cyclically carbonized, the phosphorus content in the hydrothermal carbonization liquid also increases, and there is a high positive correlation between the two (r=0.902, P<0.01). In this embodiment, the total phosphorus content of the first-level carbonization liquid is 160.79 mg / L, and the total phosphorus content of the fifth-level carbonization liquid is 550.60 mg / L, indicating that the wetland plant duckweed can achieve effective enrichment of phosphorus in the liquid product when the liquid product is cyclically hydrothermally carbonized by recycling the liquid product. After the circulating carbonization liquid in this embodiment is crystallized and precipitated, the inorganic phosphorus content in the supernatant is 1.68 mg / L, and the recovery rate of inorganic phosphorus reaches 99.66%, indicating that almost all phosphates are recovered in the crystallized product in the form of precipitation.

[0040] As shown in Table 1, after the cyclic carbonization of the hornwort biomass in this embodiment, the total phosphorus content in the hydrothermal charcoal generated is 4.85-6.27 mg / g, which is higher than the content in the biomass raw material. After the primary carbonization, the total phosphorus recovery in the hydrothermal charcoal accounts for 54.38%, indicating that the phosphorus in the hornwort is distributed fairly in the solid and liquid phases during the hydrothermal carbonization process. Figure 1As shown in the figure, with the increase of the number of cycles, the recovery ratio of total phosphorus in the hydrothermal carbon decreases with the increase of the number of cycles, and the recovery ratio of total phosphorus in the hydrothermal carbon in the five-stage carbonization is only 18.15%. With the increase of the number of cycles, the recovery ratio of total phosphorus in the carbonization liquid increases from 45.62% in the first-stage carbonization to 81.85%, and phosphorus is enriched in the hydrothermal carbonization liquid. It can be seen that the recycling of the carbonization liquid can promote the migration of phosphorus into the liquid phase product.

[0041] Example 2

[0042] The basic working conditions of Example 2 are the same as those of Example 1, except that the initial liquid phase medium of the hydrothermal carbonization is a 1 mol / L sulfuric acid solution.

[0043] It has been determined that in this embodiment, after the biomass is cyclically carbonized, the phosphorus content in the hydrothermal carbonization liquid also increases, and the total phosphorus content in the carbonization liquid shows a high positive correlation with the number of cycles (r=0.886, P<0.01). In this embodiment, the total phosphorus content of the first-stage carbonization liquid is 278.34 mg / L, and the total phosphorus content of the fifth-stage circulating carbonization liquid is 540.47 mg / L, indicating that the wetland plant gold duckweed can achieve effective enrichment of phosphorus in the liquid phase product when the liquid phase product is cyclically hydrothermally carbonized by recycling. Among them, the inorganic phosphorus content in the five-stage carbonization liquid is 496.89 mg / L, accounting for 91.94%, and its inorganic phosphorus ratio is higher than that of the carbonization liquid obtained under the deionized water environment, indicating that H2SO4 promotes the conversion of organic phosphorus to inorganic phosphorus during the hydrothermal carbonization process. The above results show that from the perspective of crystallization recovery of phosphorus, hydrothermal carbonization under a strong acid medium is more conducive to the crystallization recovery of phosphorus in the carbonization liquid. After the circulating carbonized liquid in this example was crystallized and precipitated, the inorganic phosphorus content in the supernatant was 0.63 mg / L, and the recovery rate of inorganic phosphorus reached 99.87%, indicating that almost all phosphates were recovered in the crystallization product in the form of precipitation.

[0044] As can be seen from Table 1, the total phosphorus content of the hydrothermal carbon obtained by the first two carbonizations in this embodiment is 3.33 mg / g and 4.21 mg / g, respectively, which are lower than the original biomass. After the first carbonization in this embodiment, the recovery rate of total phosphorus in the hydrothermal carbon is 38.47%, which is significantly lower than the first carbonization with deionized water as the liquid medium, which shows that the acidic liquid medium is conducive to the migration of phosphorus into the liquid product. With the increase of the number of cycles, the total phosphorus content in the hydrothermal carbon obtained by the first four-stage cyclic carbonization increases to 8.55 mg / g with the increase of the number of cycles, and the total phosphorus content in the hydrothermal carbon decreases after the five-stage cyclic carbonization. There is a positive correlation between the total phosphorus content of the hydrothermal carbon and the number of cycles (r=0.865, P<0.01). More than 99.19% of the phosphorus in the first-stage hydrothermal carbon with sulfuric acid as the liquid medium exists in the form of inorganic phosphorus. With the increase of the number of cycles, the proportion of organic phosphorus gradually increases, but it is basically lower than the same-level circulating hydrothermal carbon with deionized water as the liquid medium. The presence of sulfuric acid makes most of the phosphorus in the raw materials exist in the carbonization liquid in the form of inorganic phosphorus. The strong acidic liquid phase environment is more conducive to the dissolution of metal ions. As the number of cycles increases, a large amount of metal ions and phosphorus accumulate in the liquid phase. The metal ions will complex with phosphorus and remain in the hydrothermal carbon through adsorption or precipitation. Figure 1 It can be seen that the total phosphorus recovery ratio in the circulating carbonized liquid phase product in this embodiment is higher than 60%, and phosphorus is enriched in the carbonized liquid.

[0045] The XRD patterns of the phosphorus crystal products in Examples 1 and 2 are as follows: Figure 2 As shown, in the crystallized product of Example 1, the main crystals are CaC2O4·nH2O and Ca 10 (PO4)6(OH)2(HAP). In the crystallized product of Example 2, the main crystals are Mg(NH4)PO4·6H2O(MAP) and CaC2O4·nH2O. Compared with Example 1, the content of CaC2O4·nH2O in the crystals obtained in Example 2 is significantly reduced. This is due to the presence of excessive SO4 in the sulfuric acid phase. 2- , during the hydrothermal carbonization process, 2+ The reaction generates CaSO4 precipitation, making the Ca in the carbonized liquid 2+ The reduction of Ca content is beneficial to the formation of MAP. The scanning electron microscope images of the phosphorus crystal products in Examples 1 and 2 are observed. Figure 3 As shown, it can be seen that the two crystalline products have obvious differences in their apparent morphology. The phosphorus crystalline product obtained in Example 1 is mainly composed of irregularly shaped particle clusters and some spherical particles, and the phosphorus crystalline product obtained in Example 2 shows lamellar and needle-like structures of different sizes and shapes.

[0046] Table 1 Phosphorus content in the carbonization products of Examples 1 and 2

[0047]

[0048] Example 3 Detection of heavy metal content in circulating carbonized liquid and crystallized product

[0049] The heavy metal contents in the carbonized liquid and the crystallized product obtained in Examples 1 and 2 were detected. The results are shown in Tables 2 and 3.

[0050] The presence of heavy metals is the main potential obstacle to the application of hydrothermal carbonization process. Generally speaking, Hg and As contained in solid waste tend to form aerosol particles during heat treatment due to their high volatility, while less volatile elements such as Zn, Pb, Cr and Ni will remain in the hydrothermal carbonization liquid and the carbonization liquid. Arsenic, zinc, cadmium and nickel were detected in the hydrothermal carbonization liquid, but cadmium and arsenic were almost not detected in the produced crystalline products. Cr, Cu and As were detected in the raw material of golden duckweed, with contents of 25.00 mg / kg, 6.75 mg / kg and 43.75 mg / kg, respectively, but not in the crystalline products, because the heavy metals were fixed in the hydrothermal carbonization due to adsorption or precipitation. The tested heavy metal concentrations all meet the limit values ​​specified in the "Limit Requirements for Toxic and Hazardous Substances in Fertilizers" (GB 38400-2019). This shows that the crystalline products formed by hydrothermal carbonization liquid are safe and feasible for use as fertilizers. Compared with directly using duckweed raw materials, the bioavailability and environmental risks of heavy metals in phosphorus recovery products are significantly reduced, and it is safer to use the crystallized product prepared from phosphorus-rich hydrothermal carbonization liquid as a phosphate fertilizer.

[0051] Table 2 Heavy metal content in the circulating carbonized liquid of Examples 1 and 2

[0052]

[0053] Table 3 Heavy metal content in the crystallized products of Examples 1 and 2

[0054]

[0055] Example 4

[0056] Canna harvested from artificial wetlands was selected as raw material, washed with water and air-dried for one week, placed in an oven at 80°C and dried to constant weight, then ground with a pulverizer, and the ground powder was sieved through a 40-mesh sieve. Canna powder was used as raw material, the raw material was mixed with deionized water at a fixed ratio of 1:10 and placed in a hydrothermal reactor, then heated to a preset temperature of 200°C, kept warm for 2 hours and naturally cooled to room temperature, and then the solid and liquid phase products were separated. The solid phase product was dried and crushed to obtain hydrothermal charcoal, and the liquid phase product was carbonized liquid.

[0057] In this example, the total phosphorus content in the canna biomass raw material is 2.70 mg / g. After measurement, the recovery rate of phosphorus in the solid phase product is 4.87%, indicating that phosphorus migrates to the liquid phase product during the hydrothermal carbonization process of wetland plant-based biomass, but the degree of migration varies depending on the type of raw material.

[0058] Example 5

[0059] Canna harvested from artificial wetlands was selected as raw material, washed with water and air-dried for one week, placed in an oven at 80°C and dried to constant weight, then ground with a pulverizer, and the ground powder was sieved through a 40-mesh sieve. Canna powder was used as raw material, the raw material was mixed with deionized water at a fixed ratio of 1:10 and placed in a hydrothermal reactor, then heated to a preset temperature of 220°C, kept warm for 2 hours, and then naturally cooled to room temperature, and then the solid and liquid phase products were separated. The solid phase product was dried and crushed to obtain hydrothermal charcoal, and the liquid phase product was carbonized liquid.

[0060] The recovery rate of phosphorus in the solid phase product in this example was determined to be 10.65%, indicating that phosphorus migrated to the liquid phase product during the hydrothermal carbonization process of wetland plant-based biomass, but the degree of migration varied depending on the hydrothermal carbonization temperature.

[0061] Example 6

[0062] Canna harvested from artificial wetlands was selected as raw material, washed with water and air-dried for one week, placed in an oven at 80°C and dried to constant weight, then ground with a pulverizer, and the ground powder was sieved through a 40-mesh sieve. Canna powder was taken as raw material, the raw material was mixed with deionized water at a fixed ratio of 1:10 and placed in a hydrothermal reactor, then heated to a preset temperature of 240°C, kept warm for 2 hours and naturally cooled to room temperature, and then the solid and liquid phase products were separated. The solid phase product was dried and crushed to obtain hydrothermal charcoal, and the liquid phase product was carbonized liquid.

[0063] The recovery rate of phosphorus in the solid phase product in this example was determined to be 34.80%, indicating that phosphorus migrated to the liquid phase product during the hydrothermal carbonization process of wetland plant-based biomass, but the degree of migration varied depending on the hydrothermal carbonization temperature.

[0064] Example 7

[0065] Canna harvested from artificial wetlands was selected as raw material, washed with water and air-dried for one week, placed in an oven at 80°C and dried to constant weight, then ground with a pulverizer, and the ground powder was sieved through a 40-mesh sieve. Canna powder was taken as raw material, the raw material was mixed with deionized water at a fixed ratio of 1:5 and placed in a hydrothermal reactor, then heated to a preset temperature of 200°C, kept warm for 2 hours and naturally cooled to room temperature, and then the solid and liquid phase products were separated. The solid phase product was dried and crushed to obtain hydrothermal charcoal, and the liquid phase product was carbonized liquid.

[0066] It was determined that the recovery rate of phosphorus in the solid phase product in this example was 8.50%, indicating that phosphorus migrated to the liquid phase product during the hydrothermal carbonization process of wetland plant-based biomass, but the degree of migration varied depending on the solid-liquid ratio of the hydrothermal carbonization raw material and the liquid medium.

[0067] Example 8

[0068] Canna harvested from artificial wetlands was selected as raw material, washed with water and air-dried for one week, placed in an oven at 80°C and dried to constant weight, then ground with a pulverizer, and the ground powder was sieved through a 40-mesh sieve. Canna powder was taken as raw material, the raw material was mixed with deionized water at a fixed ratio of 1:20 and placed in a hydrothermal reactor, then heated to a preset temperature of 200°C, kept warm for 2 hours and naturally cooled to room temperature, and then the solid and liquid phase products were separated. The solid phase product was dried and crushed to obtain hydrothermal charcoal, and the liquid phase product was carbonized liquid.

[0069] It was determined that the recovery rate of phosphorus in the solid phase product in this example was 2.18%, indicating that phosphorus migrated to the liquid phase product during the hydrothermal carbonization process of wetland plant-based biomass, but the degree of migration varied depending on the solid-liquid ratio of the hydrothermal carbonization raw material and the liquid medium.

[0070] Comparative Example 1

[0071] The basic working conditions of Comparative Example 1 are the same as those of Example 1, except that the initial liquid phase medium of the hydrothermal carbonization is 1 mol / L sodium hydroxide solution, and the number of cycles is 3 times.

[0072] It was determined that the total phosphorus content of the primary carbonized liquid in this comparative example was 28.77 mg / L, the total phosphorus content in the hydrothermal charcoal was 9.54 mg / g, and the recovery rate of phosphorus in the solid phase product was 80.78%, indicating that a higher pH value would promote the precipitation of phosphorus and cations, thereby enriching phosphorus in the hydrothermal biochar and inhibiting the migration of phosphorus to the liquid phase product. In this comparative example, after three-stage cyclic carbonization, the total phosphorus content in the liquid phase product was only 57.18 mg / L, which was much lower than the total phosphorus content in the liquid phase product in Example 1 and Example 2.

[0073] Comparative Example 2

[0074] The goldfish algae harvested from artificial wetlands were selected as raw materials, washed with water and air-dried for one week, placed in an oven at 80°C and dried to constant weight, and then ground with a pulverizer. The ground powder was sieved through a 40-mesh sieve. Canna powder was taken as raw material, the raw material was mixed with deionized water at a fixed ratio of 1:10 and placed in a hydrothermal reactor, then heated to a preset temperature of 260°C, kept warm for 2 hours, and then naturally cooled to room temperature, and then the solid and liquid phase products were separated. The solid phase product was dried and crushed to obtain hydrothermal charcoal, and the liquid phase product was carbonized liquid.

[0075] It was determined that the recovery rate of phosphorus in the solid phase product in this comparative example was 91.64%, indicating that increasing the hydrothermal carbonization temperature would inhibit the migration of phosphorus into the liquid phase product.

[0076] Comparative Example 3

[0077] The basic operating conditions of Comparative Example 3 are the same as those of Example 4, except that the hydrothermal carbonization temperature is 260°C.

[0078] It was determined that the recovery rate of phosphorus in the solid phase product in this comparative example was 70.20%, indicating that increasing the hydrothermal carbonization temperature would inhibit the migration of phosphorus into the liquid phase product.

[0079] Comparative Example 4

[0080] Canna harvested from artificial wetlands was selected as raw material, washed with water and air-dried for one week, placed in an oven at 80°C and dried to constant weight, then ground with a pulverizer, and the ground powder was sieved through a 40-mesh sieve. Canna powder was taken as raw material, the raw material was mixed with 0.015 mol / L calcium chloride solution at a fixed ratio of 1:10, placed in a hydrothermal reactor, and then heated to a preset temperature of 200°C, kept warm for 2 hours, and then naturally cooled to room temperature, and then the solid and liquid phase products were separated. The solid phase product was dried and crushed to obtain hydrothermal charcoal, and the liquid phase product was carbonized liquid.

[0081] It was determined that the recovery rate of phosphorus in the solid phase product in this comparative example was 47.22%, indicating that the addition of calcium chloride caused most of the free phosphorus in the liquid medium to be fixed in the hydrothermal carbon. Calcium ions can enhance the enrichment of phosphorus in the hydrothermal carbon, increase the recovery rate of phosphorus in the solid phase product, and thus inhibit the migration of phosphorus to the liquid phase product.

[0082] Comparative Example 5

[0083] Water hyacinth harvested from artificial wetlands was selected as raw material, washed with water and air-dried for one week, placed in an oven at 80°C and dried to constant weight, then ground with a pulverizer, and the ground powder was sieved through a 40-mesh sieve. Water hyacinth powder was used as raw material, the raw material was mixed with deionized water at a fixed ratio of 1:10 and placed in a hydrothermal reactor, then heated to a preset temperature of 200°C, kept warm for 2 hours and naturally cooled to room temperature, and then the solid and liquid phase products were separated. The solid phase product was dried and crushed to obtain hydrothermal charcoal, and the liquid phase product was carbonized liquid.

[0084] The total phosphorus content in the water hyacinth biomass raw material in this comparative example is 2.20 mg / g. The recovery rate of phosphorus in the solid phase product was determined to be 79.85%, indicating that the degree of phosphorus migration to the liquid phase product during the hydrothermal carbonization of wetland plant-based biomass varies depending on the type of raw material.

Claims

1. A method for recovering phosphorus based on wetland plant circulation hydrothermal carbonization, characterized in that: The following steps are involved: (1) Wash and dry the harvested wetland plants, then crush and sieve; (2) adding wetland plant biomass powder into a liquid medium for hydrothermal carbonization to obtain a hydrothermal carbonization product; (3) performing solid-liquid separation on the carbonized product obtained in step (2), and collecting the solid phase product and the liquid phase product; (4) using the carbonized liquid obtained in step (3) as a liquid medium, mixing with wetland plant biomass powder to carry out multi-stage cyclic carbonization, and after the multi-stage cyclic carbonization is completed, performing solid-liquid separation to obtain circulating hydrothermal charcoal and phosphorus-rich carbonized liquid; (5) adjusting the pH of the phosphorus-rich carbonized liquid obtained in step (4) to form a phosphorus crystal precipitate and recover the phosphorus; The wetland plants are one or more of hornwort, canna, calamus, and lily of the valley; In step (2), the mass ratio of the biomass to the liquid medium is 1:5-20, the carbonization temperature is 200°C-240°C, the insulation time is 1-3h, and the stirring speed is 80-120rpm; In step (2), the liquid medium is one of water and 0.5-2 mol / L sulfuric acid solution; In step (3), the solid phase product is hydrothermal charcoal, and the liquid phase product is carbonized liquid; In step (4), the number of carbonization cycles is 1 to 6 times; In step (5), the pH of the phosphorus-rich carbonized liquid is adjusted to 9.5, and the mixture is stirred at this pH value at a speed of 100 r / min for 1 hour. After the reaction is completed, the mixture is allowed to stand for 1 hour, and the precipitate is collected, washed with distilled water 2 to 3 times, and dried in a constant temperature drying oven at 50° C. for more than 48 hours until the precipitate has a constant weight.

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