Nitrogen and phosphorus synchronous doping biochar, and preparation method and application thereof

Abstract

The application belongs to the technical field of inorganic advanced functional materials, and particularly relates to nitrogen and phosphorus synchronously doped biochar as well as a preparation method and application thereof. In the nitrogen and phosphorus synchronously doped biochar, the phosphorus is loaded in the pore channel of the multi-stage pore biochar, the nitrogen is loaded on the surface of the multi-stage pore biochar, and neither of them has obvious aggregation phenomenon. The nitrogen and phosphorus synchronously doped biochar provided by the application has strong selective adsorption capacity, large adsorption capacity, fast adsorption rate, and is easy to obtain raw materials, low in cost, light in material quality, and convenient to transport and use. The nitrogen and phosphorus synchronously doped biochar provided by the application has excellent adsorption effect and is suitable for water body treatment. The results of examples show that the nitrogen and phosphorus synchronously doped biochar provided by the application can remove more than 96% of chlorine-containing substances in chlorine-containing water bodies and more than 97% of chromium ions in heavy metal contaminated water bodies, and has large adsorption capacity and still maintains the adsorption capacity for chlorine-containing substances and chromium ions after continuous use for 7 days.

Description

Technical Field

[0001] This invention belongs to the field of advanced inorganic functional materials technology, specifically relating to a nitrogen and phosphorus simultaneously doped biochar, its preparation method, and its application. Background Technology

[0002] Industrial wastewater typically contains substances that are difficult to degrade and highly toxic, such as chlorides, fluorides, and heavy metals. How to purify and treat water bodies and restore the ecological environment is a current challenge for development.

[0003] Researchers have conducted extensive studies on this topic, with adsorption for removing harmful substances being the most popular approach. This is because adsorption methods are generally low-cost, do not produce secondary pollution, and use readily available raw materials such as biochar and activated carbon. However, existing adsorption materials generally suffer from poor selective adsorption capacity, which indirectly leads to a decrease in adsorption capacity and efficiency, resulting in less than ideal adsorption effects. Summary of the Invention

[0004] The purpose of this invention is to provide nitrogen and phosphorus simultaneously doped biochar, its preparation method and application. The nitrogen and phosphorus simultaneously doped biochar provided by this invention has strong selective adsorption capacity and good adsorption effect.

[0005] To achieve the above objectives, the present invention provides the following technical solution:

[0006] This invention provides a nitrogen-phosphorus simultaneously doped biochar, comprising hierarchical porous biochar, nitrogen loaded on the surface of the hierarchical porous biochar, and phosphorus loaded within the pores of the hierarchical porous biochar; the nitrogen content is 1.5–4.5 wt%, the phosphorus content is 10.2–15.5 wt%, and the balance is the hierarchical porous biochar; the pore structure of the hierarchical porous biochar includes macropores, mesopores, and micropores; the diameter of the macropores is 30–70 nm; the diameter of the mesopores is 5–20 nm; and the diameter of the micropores is 0.01–1 nm.

[0007] Preferably, the porosity of the nitrogen and phosphorus co-doped biochar is 32-46%.

[0008] Preferably, the particle size of the nitrogen and phosphorus co-doped biochar is 0.5 to 2.5 mm.

[0009] This invention also provides a method for preparing nitrogen and phosphorus simultaneously doped biochar as described above, comprising the following steps:

[0010] (1) Biochar is obtained by crushing biomass and carbonizing it under anaerobic conditions.

[0011] (2) The biochar is activated and then placed in a nitrogen-phosphorus source composite solution for a segmented hydrothermal reaction to obtain nitrogen and phosphorus simultaneously doped biochar; the segmented hydrothermal reaction includes a first hydrothermal reaction, a second hydrothermal reaction and a third hydrothermal reaction performed sequentially; the temperature of the first hydrothermal reaction is 70-90℃; the temperature of the second hydrothermal reaction is 100-140℃; and the temperature of the third hydrothermal reaction is 150-155℃.

[0012] Preferably, the nitrogen-phosphorus source composite solution includes a nitrogen source, a phosphorus source, and water; the nitrogen source includes one or both of ammonia and ammonium nitrate; and the phosphorus source is water-soluble hexametaphosphate.

[0013] Preferably, the segmented hydrothermal reaction includes a first heating, a first hydrothermal reaction, a second heating, a second hydrothermal reaction, a third heating, and a third hydrothermal reaction performed sequentially; the rate of the first heating is 5–20 °C / min; the rate of the second heating is 1–5 °C / min; and the rate of the third heating is 0.5–2 °C / min.

[0014] Preferably, the holding time for the first hydrothermal reaction is 20–90 min; the temperature for the second hydrothermal reaction is 10–15 min; and the temperature for the third hydrothermal reaction is 2–5 min.

[0015] Preferably, the pulverization is grinding; the grinding is ball milling; the ball-to-material ratio of the ball milling is 1:1 to 5, and the time is 25 to 60 minutes.

[0016] The present invention also provides the application of nitrogen and phosphorus simultaneously doped biochar as described in the above scheme or the nitrogen and phosphorus simultaneously doped biochar prepared by the above scheme in the removal treatment of chlorine-containing substances or hexavalent chromium ions.

[0017] Preferably, the application in the removal of chlorine-containing substances or hexavalent chromium ions includes the following steps:

[0018] Nitrogen and phosphorus-doped biochar was added to the water body to be treated for adsorption, followed by solid-liquid separation.

[0019] This invention provides a nitrogen-phosphorus co-doped biochar. In the nitrogen-phosphorus co-doped biochar provided by this invention, phosphorus is loaded within the pores of the hierarchical porous biochar, while nitrogen is loaded on the surface of the hierarchical porous biochar, and neither exhibits significant aggregation. The nitrogen-phosphorus co-doped biochar provided by this invention exhibits strong selective adsorption capacity, large adsorption capacity, and fast adsorption rate. Furthermore, the raw materials are readily available, low in cost, and lightweight, facilitating transportation and use. The nitrogen-phosphorus co-doped biochar provided by this invention demonstrates excellent adsorption effect, high adsorption efficiency, and high adsorption capacity, making it suitable for water treatment, especially for treating chlorinated or heavy metal-contaminated water.

[0020] The results of the examples show that the nitrogen-phosphorus co-doped biochar provided by the present invention can remove more than 96% of chlorine-containing substances from chlorinated water and more than 97% of hexavalent chromium ions from heavy metal-polluted water. Furthermore, it exhibits a large adsorption capacity, maintaining its adsorption capacity for chlorine-containing substances and hexavalent chromium ions even after 7 days of continuous use. The nitrogen-phosphorus co-doped biochar provided by the present invention has low cost, good adsorption effect, and significant economic, ecological, and social benefits.

[0021] This invention also provides a method for preparing nitrogen and phosphorus simultaneously doped biochar as described above. The preparation method provided by this invention is simple in steps, convenient to operate, requires low investment, is highly feasible, and has the potential for large-scale industrial application.

[0022] This invention also provides the application of the nitrogen-phosphorus co-doped biochar described in the above-described scheme or the nitrogen-phosphorus co-doped biochar prepared by the above-described scheme in the removal of chlorinated substances or hexavalent chromium ions. The nitrogen-phosphorus co-doped biochar provided by this invention can remove more than 96% of chlorinated substances from chlorinated water bodies and more than 97% of hexavalent chromium ions from heavy metal polluted water bodies. It also exhibits a large adsorption capacity, maintaining its adsorption capacity for chlorinated substances and hexavalent chromium ions even after 7 days of continuous use, resulting in significant economic and ecological benefits. Detailed Implementation

[0023] This invention provides a nitrogen-phosphorus simultaneously doped biochar, comprising hierarchical porous biochar, nitrogen loaded on the surface of the hierarchical porous biochar, and phosphorus loaded within the pores of the hierarchical porous biochar; the nitrogen content is 1.5–4.5 wt%, the phosphorus content is 10.2–15.5 wt%, and the balance is the hierarchical porous biochar; the pore structure of the hierarchical porous biochar includes macropores, mesopores, and micropores; the diameter of the macropores is 30–70 nm; the diameter of the mesopores is 5–20 nm; and the diameter of the micropores is 0.01–1 nm.

[0024] The nitrogen and phosphorus simultaneously doped biochar provided by this invention comprises hierarchical porous biochar; the pore structure of the hierarchical porous biochar includes macropores, mesopores and micropores; the diameter of the macropores is 30-70 nm, preferably 37-62 nm, more preferably 43-54 nm, and even more preferably 48-49 nm; the diameter of the mesopores is 5-20 nm, preferably 7-17.5 nm, more preferably 9-14 nm, and even more preferably 11.5-12 nm; the diameter of the micropores is 0.01-1 nm, preferably 0.08-0.8 nm, more preferably 0.1-0.5 nm, and even more preferably 0.1-0.2 nm.

[0025] The nitrogen-phosphorus co-doped biochar provided by the present invention comprises nitrogen loaded on the surface of the hierarchical porous biochar; the nitrogen content is 1.5-4.5 wt%, preferably 1.9-3.8 wt%, more preferably 2.3-3.2 wt%, and even more preferably 2.7-2.9 wt%.

[0026] The nitrogen and phosphorus simultaneously doped biochar provided by the present invention includes phosphorus loaded in the pores of the hierarchical biochar; the phosphorus content is 10.2-15.5 wt%, preferably 11.1-14.2 wt%, more preferably 12.5-13.7 wt%, and even more preferably 12.9-13.1 wt%.

[0027] In this invention, the porosity of the nitrogen-phosphorus co-doped biochar is preferably 32-46%, more preferably 35-42%, and even more preferably 38-40%.

[0028] In this invention, the particle size of the nitrogen and phosphorus co-doped biochar is preferably 0.5-2.5 mm, more preferably 0.8-1.9 mm, and even more preferably 1-1.3 mm.

[0029] The nitrogen-phosphorus co-doped biochar provided by this invention has nitrogen primarily loaded on the surface of the hierarchical porous biochar and phosphorus primarily loaded within the pores of the hierarchical porous biochar. This separate distribution method enables the nitrogen-phosphorus co-doped biochar to have good selective adsorption capacity for chlorine-containing substances and hexavalent chromium ions.

[0030] This invention also provides a method for preparing nitrogen and phosphorus simultaneously doped biochar as described above, comprising the following steps:

[0031] (1) Biochar is obtained by crushing biomass and carbonizing it under anaerobic conditions.

[0032] (2) The biochar is activated and then placed in a nitrogen-phosphorus source composite solution for a segmented hydrothermal reaction to obtain nitrogen and phosphorus simultaneously doped biochar; the segmented hydrothermal reaction includes a first hydrothermal reaction, a second hydrothermal reaction and a third hydrothermal reaction performed sequentially; the temperature of the first hydrothermal reaction is 70-90℃; the temperature of the second hydrothermal reaction is 100-140℃; and the temperature of the third hydrothermal reaction is 150-155℃.

[0033] This invention involves pulverizing biomass and then carbonizing it under anaerobic conditions to obtain biochar. In this invention, the biomass preferably includes one or more of sawdust, corn silk, and rice straw, with corn silk being more preferred.

[0034] In this invention, the pulverization is preferably grinding; the grinding is preferably ball milling; the ball-to-material ratio of the ball milling is preferably 1:1 to 5, more preferably 1:2 to 4, the time is preferably 25 to 60 min, more preferably 30 to 40 min; the pulverization equipment is preferably a mortar and pestle.

[0035] In this invention, the target particle size of the pulverized material is preferably 0.55-2.55 mm, more preferably 0.85-1.95 mm, and even more preferably 1.05-1.35 mm.

[0036] In this invention, the oxygen-free condition is preferably an inert gas; the inert gas preferably includes one or more of nitrogen and argon.

[0037] In this invention, the carbonization temperature is preferably 450-750℃, more preferably 550-700℃, and even more preferably 600-650℃; the holding time is preferably 40-180 min, more preferably 60-120 min, and even more preferably 80-90 min; the carbonization equipment is preferably a carbonization furnace.

[0038] After obtaining biochar, the present invention activates the biochar and then places it in a nitrogen-phosphorus source composite solution for a staged hydrothermal reaction to obtain nitrogen and phosphorus simultaneously doped biochar. In the present invention, the activation is preferably steam activation.

[0039] In this invention, the nitrogen-phosphorus source composite solution preferably includes a nitrogen source, a phosphorus source, and water; the nitrogen source preferably includes one or two of ammonia and ammonium nitrate; and the phosphorus source is preferably water-soluble hexametaphosphate.

[0040] In this invention, the concentration of nitrogen source in the nitrogen-phosphorus source composite solution is preferably 1-10 mol / L, more preferably 3-7 mol / L, and even more preferably 5-6 mol / L, and the concentration of phosphorus source is preferably 5-15 mol / L, more preferably 7-13 mol / L, and even more preferably 9-11 mol / L.

[0041] In this invention, the segmented hydrothermal reaction preferably includes a first heating, a first hydrothermal reaction, a second heating, a second hydrothermal reaction, a third heating, and a third hydrothermal reaction performed sequentially; the equipment for the segmented hydrothermal reaction is preferably a three-necked flask.

[0042] In this invention, the first heating rate is preferably 5 to 20 °C / min, more preferably 8 to 16 °C / min, and even more preferably 10 to 13 °C / min.

[0043] In this invention, the heat preservation time of the first hydrothermal reaction is preferably 20-90 min, more preferably 30-60 min, and even more preferably 40-50 min.

[0044] In this invention, the second heating rate is preferably 1 to 5 °C / min, more preferably 2 to 4 °C / min, and even more preferably 3 °C / min.

[0045] In this invention, the holding time for the second hydrothermal reaction is preferably 10 to 15 minutes, more preferably 12 to 13 minutes.

[0046] In this invention, the third heating rate is preferably 0.5 to 2 °C / min, more preferably 0.7 to 1.5 °C / min, and even more preferably 1 to 1.2 °C / min.

[0047] In this invention, the holding time for the third hydrothermal reaction is preferably 2-5 minutes, more preferably 3-4 minutes, and even more preferably 3 minutes. This invention controls the nitrogen and phosphorus content of the simultaneously doped biochar by adjusting the temperature and holding time at each stage of the hydrothermal reaction, as well as the concentration of the nitrogen-phosphorus source composite solution, thereby obtaining a product within the target range.

[0048] The present invention also provides the application of nitrogen and phosphorus simultaneously doped biochar as described in the above scheme or the nitrogen and phosphorus simultaneously doped biochar prepared by the above scheme in the removal treatment of chlorine-containing substances or hexavalent chromium ions.

[0049] In this invention, the application in the removal of chlorine-containing substances or hexavalent chromium ions preferably includes the following steps: adding nitrogen and phosphorus-doped biochar into the water to be treated for adsorption followed by solid-liquid separation.

[0050] In this invention, the mass ratio of the nitrogen-phosphorus co-doped biochar to the water to be treated is preferably 15-150:1000, more preferably 20-100:1000, and even more preferably 30-60:1000.

[0051] In this invention, the adsorption time is preferably 0.1 to 170 h, more preferably 1 to 100 h, and even more preferably 7 to 24 h.

[0052] In this invention, the solid-liquid separation is preferably filtration; the pore size of the filter is preferably less than three-quarters of the particle size of the nitrogen and phosphorus co-doped biochar.

[0053] To further illustrate the present invention, the following detailed description of the embodiments is provided in conjunction with the present invention, but these descriptions should not be construed as limiting the scope of protection of the present invention.

[0054] Example 1

[0055] This embodiment prepares a nitrogen and phosphorus simultaneously doped biochar, including the following steps:

[0056] (1) The corn silk was ball-milled at a ball-to-material ratio of 1:1 for 25 minutes. The particle size of the corn silk reached 0.55-2.55 mm. After ball milling, it was carbonized at 750℃ for 40 minutes in a nitrogen atmosphere to obtain biochar.

[0057] (2) A nitrogen-phosphorus source composite solution with a nitrogen source concentration of 10 mol / L and a phosphorus source concentration of 5 mol / L was prepared. The biochar was first activated with water vapor and then placed in the prepared ammonia-water-soluble hexametaphosphate composite solution. A staged hydrothermal reaction was carried out, namely, a first heating, a first hydrothermal reaction, a second heating, a second hydrothermal reaction, a third heating, and a third hydrothermal reaction were performed sequentially. The first heating rate was 5 °C / min, and the first hydrothermal reaction was carried out at 90 °C for 20 min. The second heating rate was 4 °C / min, and the second hydrothermal reaction was carried out at 140 °C for 10 min. The third heating rate was 2 °C / min, and the third hydrothermal reaction was carried out at 150 °C for 4 min. Finally, nitrogen and phosphorus simultaneously doped biochar was successfully prepared. In this embodiment, the nitrogen and phosphorus simultaneously doped biochar had a particle size of 0.5–2.5 mm, a porosity of 45.25%, a nitrogen content of 4.50 wt%, and a phosphorus content of 10.20 wt%.

[0058] Example 2

[0059] This embodiment prepares a nitrogen and phosphorus simultaneously doped biochar, including the following steps:

[0060] (1) The corn silk was ball-milled at a ball-to-material ratio of 1:5 for 60 minutes. The particle size of the corn silk reached 0.55-2.55 mm. After ball milling, the mixture was carbonized at 450℃ for 120 minutes in a nitrogen atmosphere to obtain biochar.

[0061] (2) A nitrogen-phosphorus source composite solution with a nitrogen source concentration of 5 mol / L and a phosphorus source concentration of 10 mol / L was prepared. The biochar was first activated with water vapor and then placed in the prepared ammonia-water-soluble hexametaphosphate composite solution. A staged hydrothermal reaction was carried out, namely, a first heating, a first hydrothermal reaction, a second heating, a second hydrothermal reaction, a third heating, and a third hydrothermal reaction were performed sequentially. The first heating rate was 20 °C / min, and the first hydrothermal reaction was carried out at 70 °C for 90 min. The second heating rate was 5 °C / min, and the second hydrothermal reaction was carried out at 100 °C for 15 min. The third heating rate was 0.5 °C / min, and the third hydrothermal reaction was carried out at 155 °C for 2 min. Finally, nitrogen and phosphorus simultaneously doped biochar was successfully prepared. In this embodiment, the nitrogen and phosphorus simultaneously doped biochar had a particle size of 0.5–2.5 mm, a porosity of 37.01%, a nitrogen content of 3.22 wt%, and a phosphorus content of 10.9 wt%.

[0062] Example 3

[0063] This embodiment prepares a nitrogen and phosphorus simultaneously doped biochar, including the following steps:

[0064] (1) The corn silk was ball-milled at a ball-to-material ratio of 1:3 for 40 minutes. The particle size of the corn silk reached 0.55-2.55 mm. After ball milling, it was carbonized at 650℃ for 70 minutes in a nitrogen atmosphere to obtain biochar.

[0065] (2) A nitrogen-phosphorus source composite solution with a nitrogen source concentration of 10 mol / L and a phosphorus source concentration of 15 mol / L was prepared. The biochar was first activated with water vapor and then placed in the prepared ammonia-water-soluble hexametaphosphate composite solution. A staged hydrothermal reaction was carried out, namely, a first heating, a first hydrothermal reaction, a second heating, a second hydrothermal reaction, a third heating, and a third hydrothermal reaction were performed sequentially. The first heating rate was 15 °C / min, and the first hydrothermal reaction was carried out at 80 °C for 50 min. The second heating rate was 1 °C / min, and the second hydrothermal reaction was carried out at 120 °C for 15 min. The third heating rate was 1 °C / min, and the third hydrothermal reaction was carried out at 152 °C for 2 min. Finally, nitrogen and phosphorus simultaneously doped biochar was successfully prepared. In this embodiment, the nitrogen and phosphorus simultaneously doped biochar had a particle size of 0.5–2.5 mm, a porosity of 39.57%, a nitrogen content of 4.22 wt%, and a phosphorus content of 14.99 wt%.

[0066] Example 4

[0067] This embodiment prepares a nitrogen and phosphorus simultaneously doped biochar, including the following steps:

[0068] (1) The corn silk was ball-milled at a ball-to-material ratio of 1:2 for 50 minutes. The particle size of the corn silk reached 0.55-2.55 mm. After ball milling, the mixture was carbonized at 550℃ for 100 minutes in a nitrogen atmosphere to obtain biochar.

[0069] (2) A nitrogen-phosphorus source composite solution with a nitrogen source concentration of 8 mol / L and a phosphorus source concentration of 12 mol / L was prepared. The biochar was first activated with water vapor and then placed in the prepared ammonia-water-soluble hexametaphosphate composite solution. A staged hydrothermal reaction was carried out, namely, a first heating, a first hydrothermal reaction, a second heating, a second hydrothermal reaction, a third heating, and a third hydrothermal reaction were performed sequentially. The first heating rate was 10 °C / min, and the first hydrothermal reaction was carried out at 90 °C for 90 min. The second heating rate was 5 °C / min, and the second hydrothermal reaction was carried out at 140 °C for 15 min. The third heating rate was 2 °C / min, and the third hydrothermal reaction was carried out at 153 °C for 4 min. Finally, nitrogen and phosphorus simultaneously doped biochar was successfully prepared. In this embodiment, the nitrogen and phosphorus simultaneously doped biochar had a particle size of 0.5–2.5 mm, a porosity of 42.01%, a nitrogen content of 3.54 wt%, and a phosphorus content of 13.05 wt%.

[0070] Example 5

[0071] This embodiment prepares a nitrogen and phosphorus simultaneously doped biochar, including the following steps:

[0072] (1) The corn silk was ball-milled at a ball-to-material ratio of 1:4 for 30 minutes. The particle size of the corn silk reached 0.55-2.55 mm. After ball milling, the mixture was carbonized at 500℃ for 160 minutes in a nitrogen atmosphere to obtain biochar.

[0073] (2) A nitrogen-phosphorus source composite solution with a nitrogen source concentration of 3 mol / L and a phosphorus source concentration of 7 mol / L was prepared. The biochar was first activated with water vapor and then placed in the prepared ammonia-water-soluble hexametaphosphate composite solution. A staged hydrothermal reaction was carried out, namely, a first heating, a first hydrothermal reaction, a second heating, a second hydrothermal reaction, a third heating, and a third hydrothermal reaction were performed sequentially. The first heating rate was 15 °C / min, and the first hydrothermal reaction was carried out at 80 °C for 40 min. The second heating rate was 3 °C / min, and the second hydrothermal reaction was carried out at 130 °C for 13 min. The third heating rate was 0.5 °C / min, and the third hydrothermal reaction was carried out at 150 °C for 5 min. Finally, nitrogen and phosphorus simultaneously doped biochar was successfully prepared. In this embodiment, the nitrogen and phosphorus simultaneously doped biochar had a particle size of 0.5–2.5 mm, a porosity of 32.44%, a nitrogen content of 2.77 wt%, and a phosphorus content of 11.87 wt%.

[0074] Example 6

[0075] This embodiment prepares a nitrogen and phosphorus simultaneously doped biochar, including the following steps:

[0076] (1) The corn silk was ball-milled at a ball-to-material ratio of 1:2 for 60 minutes. The particle size of the corn silk reached 0.55-2.55 mm. After ball milling, it was carbonized at 700℃ for 80 minutes in a nitrogen atmosphere to obtain biochar.

[0077] (2) A nitrogen-phosphorus source composite solution with a nitrogen source concentration of 1 mol / L and a phosphorus source concentration of 5 mol / L was prepared. The biochar was first activated with water vapor and then placed in the prepared ammonia-water-soluble hexametaphosphate composite solution. A staged hydrothermal reaction was carried out, namely, a first heating, a first hydrothermal reaction, a second heating, a second hydrothermal reaction, a third heating, and a third hydrothermal reaction were performed sequentially. The first heating rate was 5 °C / min, and the first hydrothermal reaction was carried out at 70 °C for 90 min. The second heating rate was 1 °C / min, and the second hydrothermal reaction was carried out at 100 °C for 10 min. The third heating rate was 0.5 °C / min, and the third hydrothermal reaction was carried out at 150 °C for 5 min. Finally, nitrogen and phosphorus simultaneously doped biochar was successfully prepared. In this embodiment, the nitrogen and phosphorus simultaneously doped biochar had a particle size of 0.5–2.5 mm, a porosity of 39.32%, a nitrogen content of 3.67 wt%, and a phosphorus content of 12.92 wt%.

[0078] Example 7

[0079] This embodiment prepares a nitrogen and phosphorus simultaneously doped biochar, including the following steps:

[0080] (1) The corn silk was ball-milled at a ball-to-material ratio of 1:5 for 25 minutes. The particle size of the corn silk reached 0.55-2.55 mm. After ball milling, it was carbonized at 500℃ for 140 minutes in a nitrogen atmosphere to obtain biochar.

[0081] (2) A nitrogen-phosphorus source composite solution with a nitrogen source concentration of 10 mol / L and a phosphorus source concentration of 15 mol / L was prepared. The biochar was first activated with water vapor and then placed in the prepared ammonia-water-soluble hexametaphosphate composite solution. A staged hydrothermal reaction was carried out, namely, a first heating, a first hydrothermal reaction, a second heating, a second hydrothermal reaction, a third heating, and a third hydrothermal reaction were performed sequentially. The first heating rate was 5 °C / min, and the first hydrothermal reaction was carried out at 70 °C for 90 min. The second heating rate was 1 °C / min, and the second hydrothermal reaction was carried out at 100 °C for 10 min. The third heating rate was 0.5 °C / min, and the third hydrothermal reaction was carried out at 151 °C for 2 min. Finally, nitrogen and phosphorus simultaneously doped biochar was successfully prepared. In this embodiment, the nitrogen and phosphorus simultaneously doped biochar had a particle size of 0.5–2.5 mm, a porosity of 36.77%, a nitrogen content of 3.51 wt%, and a phosphorus content of 13.54 wt%.

[0082] Example 8

[0083] This embodiment prepares a nitrogen and phosphorus simultaneously doped biochar, including the following steps:

[0084] (1) The corn silk was ball-milled at a ball-to-material ratio of 1:5 for 60 minutes. The particle size of the corn silk reached 0.55-2.55 mm. After ball milling, the mixture was carbonized at 750℃ for 50 minutes in a nitrogen atmosphere to obtain biochar.

[0085] (2) A nitrogen-phosphorus source composite solution with a nitrogen source concentration of 10 mol / L and a phosphorus source concentration of 15 mol / L was prepared. The biochar was first activated with water vapor and then placed in the prepared ammonia-water-soluble hexametaphosphate composite solution. A staged hydrothermal reaction was carried out, namely, a first heating, a first hydrothermal reaction, a second heating, a second hydrothermal reaction, a third heating, and a third hydrothermal reaction were performed sequentially. The first heating rate was 20 °C / min, and the first hydrothermal reaction was carried out at 80 °C for 20 min. The second heating rate was 5 °C / min, and the second hydrothermal reaction was carried out at 140 °C for 10 min. The third heating rate was 2 °C / min, and the third hydrothermal reaction was carried out at 154 °C for 5 min. Finally, nitrogen and phosphorus simultaneously doped biochar was successfully prepared. In this embodiment, the nitrogen and phosphorus simultaneously doped biochar had a particle size of 0.5–2.5 mm, a porosity of 41.37%, a nitrogen content of 4.42 wt%, and a phosphorus content of 15.1 wt%.

[0086] Example 9

[0087] This embodiment prepares a nitrogen and phosphorus simultaneously doped biochar, including the following steps:

[0088] (1) The corn silk was ball-milled with a ball-to-material ratio of 1:1 for 45 minutes. The particle size of the corn silk reached 0.55-2.55 mm. After ball milling, it was carbonized at 650℃ for 110 minutes in a nitrogen atmosphere to obtain biochar.

[0089] (2) A nitrogen-phosphorus source composite solution with a nitrogen source concentration of 3 mol / L and a phosphorus source concentration of 5 mol / L was prepared. The biochar was first activated with water vapor and then placed in the prepared ammonia-water-soluble hexametaphosphate composite solution. A staged hydrothermal reaction was carried out, namely, a first heating, a first hydrothermal reaction, a second heating, a second hydrothermal reaction, a third heating, and a third hydrothermal reaction were performed sequentially. The first heating rate was 12 °C / min, and the first hydrothermal reaction was carried out at 78 °C for 40 min. The second heating rate was 3 °C / min, and the second hydrothermal reaction was carried out at 120 °C for 15 min. The third heating rate was 2 °C / min, and the third hydrothermal reaction was carried out at 150 °C for 2 min. Finally, nitrogen and phosphorus simultaneously doped biochar was successfully prepared. In this embodiment, the nitrogen and phosphorus simultaneously doped biochar had a particle size of 0.5–2.5 mm, a porosity of 32.03%, a nitrogen content of 1.72 wt%, and a phosphorus content of 10.23 wt%.

[0090] Example 10

[0091] This embodiment prepares a nitrogen and phosphorus simultaneously doped biochar, including the following steps:

[0092] (1) The corn silk was ball-milled with a ball-to-material ratio of 1:1 for 55 minutes. The particle size of the corn silk reached 0.55-2.55 mm. After ball milling, it was carbonized at 520℃ for 150 minutes in a nitrogen atmosphere to obtain biochar.

[0093] (2) A nitrogen-phosphorus source composite solution with a nitrogen source concentration of 9 mol / L and a phosphorus source concentration of 14 mol / L was prepared. The biochar was first activated with water vapor and then placed in the prepared ammonia-water-soluble hexametaphosphate composite solution. A staged hydrothermal reaction was carried out, namely, a first heating, a first hydrothermal reaction, a second heating, a second hydrothermal reaction, a third heating, and a third hydrothermal reaction were performed sequentially. The first heating rate was 16 °C / min, and the first hydrothermal reaction was carried out at 83 °C for 60 min. The second heating rate was 5 °C / min, and the second hydrothermal reaction was carried out at 135 °C for 12 min. The third heating rate was 1.5 °C / min, and the third hydrothermal reaction was carried out at 153 °C for 3 min. Finally, nitrogen and phosphorus simultaneously doped biochar was successfully prepared. In this embodiment, the nitrogen and phosphorus simultaneously doped biochar had a particle size of 0.5–2.5 mm, a porosity of 40.24%, a nitrogen content of 3.87 wt%, and a phosphorus content of 12.94 wt%.

[0094] Comparative Example 1

[0095] This comparative example prepared a nitrogen and phosphorus simultaneously doped biochar, including the following steps:

[0096] (1) The corn silk was ball-milled at a ball-to-material ratio of 1:1 for 25 minutes. The particle size of the corn silk reached 0.55-2.55 mm. After ball milling, it was carbonized at 750℃ for 40 minutes in a nitrogen atmosphere to obtain biochar.

[0097] (2) A nitrogen-phosphorus source composite solution with a nitrogen source concentration of 10 mol / L and a phosphorus source concentration of 0 mol / L was prepared. The biochar was first activated with water vapor and then placed in the prepared ammonia-water-soluble hexametaphosphate composite solution. A staged hydrothermal reaction was carried out, namely, a first heating, a first hydrothermal reaction, a second heating, a second hydrothermal reaction, a third heating, and a third hydrothermal reaction were performed sequentially. The first heating rate was 5 °C / min, and the first hydrothermal reaction was carried out at 90 °C for 20 min. The second heating rate was 4 °C / min, and the second hydrothermal reaction was carried out at 140 °C for 10 min. The third heating rate was 2 °C / min, and the third hydrothermal reaction was carried out at 150 °C for 4 min. Finally, nitrogen and phosphorus simultaneously doped biochar was successfully prepared. In this embodiment, the nitrogen and phosphorus simultaneously doped biochar had a particle size of 0.5–2.5 mm, a porosity of 45.97%, a nitrogen content of 6.23 wt%, and a phosphorus content of 0 wt%.

[0098] Comparative Example 2

[0099] This comparative example prepared a nitrogen and phosphorus simultaneously doped biochar, including the following steps:

[0100] (1) The corn silk was ball-milled at a ball-to-material ratio of 1:1 for 25 minutes. The particle size of the corn silk reached 0.55-2.55 mm. After ball milling, it was carbonized at 750℃ for 40 minutes in a nitrogen atmosphere to obtain biochar.

[0101] (2) A nitrogen-phosphorus source composite solution with a nitrogen source concentration of 0 mol / L and a phosphorus source concentration of 5 mol / L was prepared. The biochar was first activated with water vapor and then placed in the prepared ammonia-water-soluble hexametaphosphate composite solution. A staged hydrothermal reaction was carried out, namely, a first heating, a first hydrothermal reaction, a second heating, a second hydrothermal reaction, a third heating, and a third hydrothermal reaction were performed sequentially. The first heating rate was 5 °C / min, and the first hydrothermal reaction was carried out at 90 °C for 20 min. The second heating rate was 4 °C / min, and the second hydrothermal reaction was carried out at 140 °C for 10 min. The third heating rate was 2 °C / min, and the third hydrothermal reaction was carried out at 150 °C for 4 min. Finally, nitrogen and phosphorus simultaneously doped biochar was successfully prepared. In this embodiment, the nitrogen and phosphorus simultaneously doped biochar had a particle size of 0.5–2.5 mm, a porosity of 45.88%, a nitrogen content of 0 wt%, and a phosphorus content of 11.66 wt%.

[0102] Comparative Example 3

[0103] This comparative example prepared a nitrogen and phosphorus simultaneously doped biochar, including the following steps:

[0104] (1) The corn silk was ball-milled at a ball-to-material ratio of 1:1 for 25 minutes. The particle size of the corn silk reached 0.55-2.55 mm. After ball milling, it was carbonized at 750℃ for 40 minutes in a nitrogen atmosphere to obtain biochar.

[0105] (2) A nitrogen-phosphorus source composite solution with a nitrogen source concentration of 30 mol / L and a phosphorus source concentration of 10 mol / L was prepared. The biochar was first activated with water vapor and then placed in the prepared ammonia-water-soluble hexametaphosphate composite solution. A staged hydrothermal reaction was carried out, namely, a first heating, a first hydrothermal reaction, a second heating, a second hydrothermal reaction, a third heating, and a third hydrothermal reaction were performed sequentially. The first heating rate was 5 °C / min, and the first hydrothermal reaction was carried out at 90 °C for 20 min. The second heating rate was 4 °C / min, and the second hydrothermal reaction was carried out at 140 °C for 10 min. The third heating rate was 2 °C / min, and the third hydrothermal reaction was carried out at 150 °C for 4 min. Finally, nitrogen and phosphorus simultaneously doped biochar was successfully prepared. In this embodiment, the nitrogen and phosphorus simultaneously doped biochar had a particle size of 0.5–2.5 mm, a porosity of 44.74%, a nitrogen content of 9.78 wt%, and a phosphorus content of 13.01 wt%.

[0106] Comparative Example 4

[0107] This comparative example prepared a nitrogen and phosphorus simultaneously doped biochar, including the following steps:

[0108] (1) The corn silk was ball-milled at a ball-to-material ratio of 1:1 for 25 minutes. The particle size of the corn silk reached 0.55-2.55 mm. After ball milling, it was carbonized at 750℃ for 40 minutes in a nitrogen atmosphere to obtain biochar.

[0109] (2) A nitrogen-phosphorus source composite solution with a nitrogen source concentration of 10 mol / L and a phosphorus source concentration of 40 mol / L was prepared. The biochar was first activated with water vapor and then placed in the prepared ammonia-water-soluble hexametaphosphate composite solution. A staged hydrothermal reaction was carried out, namely, a first heating, a first hydrothermal reaction, a second heating, a second hydrothermal reaction, a third heating, and a third hydrothermal reaction were performed sequentially. The first heating rate was 5 °C / min, and the first hydrothermal reaction was carried out at 90 °C for 20 min. The second heating rate was 4 °C / min, and the second hydrothermal reaction was carried out at 140 °C for 10 min. The third heating rate was 2 °C / min, and the third hydrothermal reaction was carried out at 150 °C for 4 min. Finally, nitrogen and phosphorus simultaneously doped biochar was successfully prepared. In this embodiment, the nitrogen and phosphorus simultaneously doped biochar had a particle size of 0.5–2.5 mm, a porosity of 44.79%, a nitrogen content of 4.39 wt%, and a phosphorus content of 17.13 wt%.

[0110] Comparative Example 5

[0111] This comparative example prepared a nitrogen and phosphorus simultaneously doped biochar, including the following steps:

[0112] (1) The corn silk was ball-milled at a ball-to-material ratio of 1:1 for 25 minutes. The particle size of the corn silk reached 0.55-2.55 mm. After ball milling, it was carbonized at 750℃ for 40 minutes in a nitrogen atmosphere to obtain biochar.

[0113] (2) A nitrogen-phosphorus source composite solution with a nitrogen source concentration of 10 mol / L and a phosphorus source concentration of 5 mol / L was prepared. The biochar was first activated with water vapor and then placed in the prepared ammonia-water-soluble hexametaphosphate composite solution. A staged hydrothermal reaction was carried out, namely, a first heating, a first hydrothermal reaction, a second heating, and a second hydrothermal reaction were performed sequentially. The first heating rate was 5 °C / min, and the first hydrothermal reaction was carried out at 90 °C for 20 min. The second heating rate was 4 °C / min, and the second hydrothermal reaction was carried out at 140 °C for 10 min. Finally, nitrogen and phosphorus simultaneously doped biochar was successfully prepared. In this embodiment, the nitrogen and phosphorus simultaneously doped biochar had a particle size of 0.5–2.5 mm, a porosity of 37.03%, a nitrogen content of 4.97 wt%, and a phosphorus content of 10.49 wt%.

[0114] Comparative Example 6

[0115] This comparative example prepared a nitrogen and phosphorus simultaneously doped biochar, including the following steps:

[0116] (1) The corn silk was ball-milled at a ball-to-material ratio of 1:1 for 25 minutes. The particle size of the corn silk reached 0.55-2.55 mm. After ball milling, it was carbonized at 750℃ for 40 minutes in a nitrogen atmosphere to obtain biochar.

[0117] (2) A nitrogen-phosphorus source composite solution with a nitrogen source concentration of 10 mol / L and a phosphorus source concentration of 5 mol / L was prepared. The biochar was first activated with water vapor and then placed in the prepared ammonia-water-soluble hexametaphosphate composite solution. The hydrothermal reaction was carried out in stages, that is, the first heating and the first hydrothermal reaction were carried out sequentially. The first heating rate was 5℃ / min, and the first hydrothermal reaction was carried out at 90℃ for 20 min. Finally, nitrogen and phosphorus simultaneously doped biochar was successfully prepared. In this embodiment, the particle size of the nitrogen and phosphorus simultaneously doped biochar was 0.5-2.5 mm, with only micropore structure and no mesopore or macropore structure.

[0118] The nitrogen and phosphorus co-doped biochar prepared in Examples 1-10 and Comparative Examples 1-6 were subjected to adsorption tests. A mixed solution of ferric chloride, hypochlorite (total chloride ion concentration 5 mmol / L) and hexavalent chromium ion (concentration 5 mmol / L) was used as the treatment object. The results are shown in Tables 1 and 2.

[0119] Table 1. Adsorption results of nitrogen and phosphorus co-doped biochar in Examples 1-10 and Comparative Examples 1-6

[0120]

[0121] As shown in Table 1, the nitrogen and phosphorus co-doped biochar provided by the present invention has good selective adsorption of chlorine-containing substances and hexavalent chromium ions, and the removal rate is high. The removal rate of chlorine-containing substances is above 96%, and the removal rate of hexavalent chromium ions is above 97%.

[0122] Table 2. Adsorption processes of nitrogen and phosphorus co-doped biochar in Examples 1-10 and Comparative Examples 1-6 (calculated with Example 1 as 100%)

[0123]

[0124]

[0125] Note: Adsorption rate = (initial mass of adsorbed substance - final mass of adsorbed substance) / initial mass of adsorbed substance; effective adsorption time refers to the time from the start of adsorption until the concentration of the adsorbed substance does not change within 20 minutes.

[0126] As shown in Table 2, the nitrogen and phosphorus co-doped biochar provided by this invention has a large adsorption capacity, a high adsorption rate, and achieves long-term adsorption.

[0127] As can be seen from the above embodiments, the nitrogen and phosphorus co-doped biochar provided by the present invention has strong selective adsorption capacity, large adsorption capacity, and fast adsorption rate, and is suitable for the removal of chlorine-containing substances and hexavalent chromium ions.

[0128] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, and not all embodiments. Other embodiments can be obtained based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A nitrogen-phosphorus co-doped biochar, comprising hierarchical porous biochar, nitrogen loaded on the surface of the hierarchical porous biochar, and phosphorus loaded within the pores of the hierarchical porous biochar; wherein the nitrogen content is 1.5-4.5 wt%, the phosphorus content is 10.2-15.5 wt%, and the balance is the hierarchical porous biochar; the pore structure of the hierarchical porous biochar includes macropores, mesopores, and micropores; the diameter of the macropores is 54-70 nm; the diameter of the mesopores is 5-20 nm; and the diameter of the micropores is 0.01-1 nm. The method for preparing the nitrogen and phosphorus simultaneously doped biochar includes the following steps: (1) Biochar is obtained by crushing biomass and then carbonizing it under anaerobic conditions; (2) The biochar is activated and then placed in a nitrogen-phosphorus source composite solution for a segmented hydrothermal reaction to obtain nitrogen and phosphorus simultaneously doped biochar; the segmented hydrothermal reaction includes a first hydrothermal reaction, a second hydrothermal reaction and a third hydrothermal reaction performed sequentially; the temperature of the first hydrothermal reaction is 70~90 ℃; the temperature of the second hydrothermal reaction is 100~140 ℃; the temperature of the third hydrothermal reaction is 150~155 ℃.

2. The nitrogen and phosphorus simultaneously doped biochar according to claim 1, characterized in that, The porosity of the nitrogen and phosphorus simultaneously doped biochar is 32-46%.

3. The nitrogen and phosphorus simultaneously doped biochar according to claim 1 or 2, characterized in that, The nitrogen and phosphorus co-doped biochar has a particle size of 0.5~2.5 mm.

4. The method for preparing nitrogen and phosphorus simultaneously doped biochar according to any one of claims 1 to 3, comprising the following steps: (1) Biochar is obtained by crushing biomass and then carbonizing it under anaerobic conditions; (2) The biochar is activated and then placed in a nitrogen-phosphorus source composite solution for a segmented hydrothermal reaction to obtain nitrogen and phosphorus simultaneously doped biochar; the segmented hydrothermal reaction includes a first hydrothermal reaction, a second hydrothermal reaction and a third hydrothermal reaction performed sequentially; the temperature of the first hydrothermal reaction is 70~90 ℃; the temperature of the second hydrothermal reaction is 100~140 ℃; the temperature of the third hydrothermal reaction is 150~155 ℃.

5. The preparation method according to claim 4, characterized in that, The nitrogen-phosphorus source composite solution includes a nitrogen source, a phosphorus source, and water; the nitrogen source includes one or both of ammonia and ammonium nitrate; and the phosphorus source is water-soluble hexametaphosphate.

6. The preparation method according to claim 4, characterized in that, The segmented hydrothermal reaction includes a first heating, a first hydrothermal reaction, a second heating, a second hydrothermal reaction, a third heating, and a third hydrothermal reaction performed sequentially; the rate of the first heating is 5~20 ℃ / min; the rate of the second heating is 1~5 ℃ / min; and the rate of the third heating is 0.5~2 ℃ / min.

7. The preparation method according to claim 4 or 6, characterized in that, The holding time for the first hydrothermal reaction is 20-90 min; the holding time for the second hydrothermal reaction is 10-15 min; and the holding time for the third hydrothermal reaction is 2-5 min.

8. The preparation method according to claim 4, characterized in that, The pulverization is grinding; the grinding is ball milling; the ball-to-material ratio of the ball milling is 1:1~5, and the time is 25~60 min.

9. The application of the nitrogen-phosphorus co-doped biochar according to any one of claims 1 to 3 or the nitrogen-phosphorus co-doped biochar obtained by the preparation method according to any one of claims 4 to 8 in the removal treatment of chlorine-containing substances or hexavalent chromium ions.

10. The application according to claim 9, wherein the application in the removal of chlorine-containing substances or hexavalent chromium ions comprises the following steps: Nitrogen and phosphorus-doped biochar was added to the water body to be treated for adsorption, followed by solid-liquid separation.

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