A lanthanum modified biochar for ammonium chloride wastewater treatment, a preparation method and an application method thereof

Abstract

The present application relates to a lanthanum modified biochar for ammonium chloride wastewater treatment, a preparation method and application thereof. The biochar contains hydroxyl and carboxyl oxygen-containing functional groups and has a mesoporous structure, and La2O3 nanoparticles are uniformly loaded on the surface and pore inner wall of the biochar, the nanoparticles have unsaturated coordination lanthanum active sites, and selected area electron diffraction shows a diffraction ring corresponding to the (101) crystal face. The biochar has selective adsorption capacity for NH4 + and simultaneously inhibits Cl ‑ co-adsorption. The preparation method comprises: obtaining biochar by first pyrolysis of biomass raw materials, then immersing in a lanthanum nitrate solution for ultrasonic assisted treatment, drying, second pyrolysis in an activation gas atmosphere, and finally rapid cooling to lock the crystal phase. The biochar is applied to ammonium chloride wastewater treatment and can efficiently and selectively recover ammonium nitrogen. The nitrogen-loaded biochar after adsorption saturation can be used as a slow-release nitrogen fertilizer for agriculture, realizing the integration of wastewater nitrogen resource recovery and agricultural slow-release fertilization, and effectively alleviating the risk of soil salinization.

Description

Technical Field

[0001] This invention relates to the field of environmental functional materials and water treatment technology, specifically to a lanthanum-modified biochar for ammonium chloride wastewater treatment, its preparation method, and its application method. Background Technology

[0002] Wastewater containing ammonium chloride (NH4Cl) is widely generated from the excessive application of ammonium fertilizers, leachate from livestock and poultry manure, pharmaceutical production, and municipal wastewater treatment processes. Improper discharge of NH4Cl-rich wastewater significantly increases the nitrogen load on receiving water bodies, thereby accelerating eutrophication of surface waters, causing groundwater pollution, and wasting recoverable nitrogen resources. Excessive application of ammonium chloride (NH4Cl) fertilizer introduces large amounts of chloride ions (Cl-) into the soil. - ), with Cl - Increased concentrations significantly increase soil solution conductivity and osmotic pressure, inhibiting crop roots from absorbing water and nutrients. They also accelerate secondary soil salinization, damage soil aggregate structure, reduce soil aeration and water retention capacity, inhibit the activity of some soil microorganisms, and interfere with nitrogen cycling and organic matter decomposition processes, thereby weakening soil fertility and ecological stability, leading to limited crop growth or even reduced yields.

[0003] Currently, various treatment technologies have been developed for wastewater rich in NH4Cl, including air stripping, adsorption, ion exchange, chemical precipitation, membrane separation, and advanced oxidation processes. The key question is how to recover and reuse NH4Cl from wastewater and prevent Cl from escaping. - The hazards of nitrogen oxides are a current research focus. Adsorption is considered highly promising due to its simple operation, cost-effectiveness, wide availability of raw materials, and minimal environmental impact. Common adsorbents include biochar, bentonite, adsorption resins, and zeolites. Among them, biochar has attracted widespread attention as a highly efficient adsorbent due to its well-developed porous structure, high specific surface area, and abundant surface functional groups. However, raw biochar is generally less effective against ammonium nitrogen (NH4+). + The limited adsorption capacity of (-N) is mainly attributed to its underdeveloped microporous structure, insufficient active sites, and the presence of Cl- - In the presence of competing ions, NH4 + -N has weaker selectivity. Summary of the Invention

[0004] To overcome these limitations, this invention provides a lanthanum-modified biochar for ammonium chloride wastewater treatment, its preparation method, and its application method, as detailed below: A lanthanum-modified biochar for treating ammonium chloride wastewater, wherein the lanthanum-modified biochar contains hydroxyl and carboxyl oxygen-containing functional groups and has a mesoporous structure, and the surface and inner walls of the pores of the lanthanum-modified biochar are uniformly loaded with La2O3 nanoparticles, wherein the La2O3 nanoparticles have unsaturated coordinated lanthanum active sites, and the selected area electron diffraction of the La2O3 nanoparticles shows diffraction rings corresponding to the La2O3 (101) crystal plane. The lanthanum-modified biochar has a positive effect on NH4. + It has selective adsorption capacity and selectively inhibits Cl. - Co-adsorption.

[0005] Preferably, the selected area electron diffraction of the La2O3 nanoparticles also exhibits diffraction rings corresponding to the La2O3 (100) crystal plane, the La2O3 (001) crystal plane and the La2O3 (102) crystal plane, and the selected area electron diffraction of the La2O3 nanoparticles exhibits polycrystalline concentric diffraction rings corresponding to the (101), (100), (001) and (102) crystal planes.

[0006] Preferably, the interplanar spacing of the La2O3 nanoparticles is 0.312 nm.

[0007] Secondly, the present invention provides a method for preparing lanthanum-modified biochar for ammonium chloride wastewater treatment, comprising the following steps: S1. The biomass raw material is subjected to a first pyrolysis at 400-700℃ to obtain biochar; S2. The biochar is impregnated in a lanthanum nitrate solution and subjected to ultrasonic-assisted impregnation treatment to load lanthanum ions onto the surface and inside the pores of the biochar. S3. After solid-liquid separation and drying of the impregnated biochar, a second pyrolysis is carried out at 650-750℃ in an atmosphere containing activating gas to convert lanthanum ions into La2O3 nanoparticles. At the same time, the activating gas creates secondary pores in the biochar to obtain the lanthanum-modified biochar. S4. The product obtained in step S3 is rapidly cooled to room temperature at a cooling rate of not less than 50℃ / min to lock the metastable crystalline phase structure of the La2O3 nanoparticles.

[0008] Preferably, in step S3, the activating gas is one or more of water vapor, carbon dioxide, or ammonia, and the flow rate of the activating gas is 50-300 mL / min.

[0009] Preferably, in step S1, the first pyrolysis is carried out under an inert atmosphere for 2-4 hours; the biomass raw material is corn straw.

[0010] Preferably, in step S2, the amount of lanthanum nitrate used is 3%-5% of the biochar mass based on lanthanum element; the concentration of the lanthanum nitrate solution is 0.1-0.2 mol / L; the ultrasonic-assisted impregnation treatment time is 10-60 min; the water bath heating temperature is 60-90℃; and the stirring time is 4-6 h.

[0011] Preferably, the first and second pyrolysis processes are performed at a temperature of 700°C for 1-3 hours.

[0012] Thirdly, this invention provides the application of lanthanum-modified biochar in the treatment of ammonium chloride wastewater. The lanthanum-modified biochar is added to the wastewater containing ammonium chloride for adsorption treatment. The biochar selectively adsorbs NH4+ through the unsaturated lanthanum-coordinated active sites of La2O3 and oxygen-containing functional groups. + At the same time, inhibiting the effect of Cl - Co-adsorption.

[0013] Preferably, the adsorption treatment temperature is 20-40℃, the adsorption time is 440-720 min, the dosage of lanthanum-modified biochar is 0.2 g / L, and the removal rate of ammonium nitrogen is not less than 90%.

[0014] Preferably, after adsorption treatment, solid-liquid separation is performed to obtain lanthanum-modified biochar loaded with ammonium nitrogen. The lanthanum-modified biochar loaded with ammonium nitrogen is used as a slow-release nitrogen fertilizer in agricultural planting. After being applied to the soil for 30 days, the effective ammonium nitrogen content in the soil is not less than 40 mg / Kg, and the soil electrical conductivity is reduced by at least 25% compared with the application of the same amount of ammonium chloride fertilizer.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. The lanthanum-modified biochar prepared by this invention through a specific process is effective in reducing NH4 in ammonium chloride wastewater. + It exhibits excellent adsorption capacity, with a maximum adsorption capacity of approximately 47 mg / g. Crucially, BC-La shows excellent adsorption capacity for NH4+. + It exhibits high selectivity, while being sensitive to coexisting Cl... - Its co-adsorption capacity is extremely low, thus enabling it to efficiently and selectively separate and enrich ammonium nitrogen in high-salt ammonium chloride wastewater rich in competing ions. This solves the problems of poor selectivity and susceptibility to Cl- ions in traditional adsorbents. - The industry's pain points are severely disrupted.

[0016] 2. This invention prepares lanthanum-modified biochar through a synergistic technique of secondary pyrolysis, activation pore formation, and rapid quenching. Experimental results show that it exhibits a typical type IV isotherm and possesses a well-developed mesoporous structure, and is NH4+. +The rapid diffusion and mass transfer provided ample channels; through ultrasonic-assisted impregnation and high-temperature conversion, La2O3 nanoparticles with unsaturated lanthanum coordination active sites (interface spacing 0.312 nm, corresponding to the (101) crystal plane) were uniformly loaded onto the surface of biochar and the inner wall of the pores; XPS analysis confirmed that these unsaturated coordination sites selectively chemisorb NH4. + The core of this process involves surface complexation and ion exchange to achieve the control of NH4. + The strong immobilization of lanthanum significantly activated oxygen-containing functional groups such as hydroxyl and carboxyl groups on the surface of biochar, forming a synergistic adsorption system with the active sites of La, further enhancing the adsorption capacity.

[0017] 3. In this invention, the nitrogen-loaded biochar after adsorption saturation can be directly used as a slow-release nitrogen fertilizer in agricultural production. Pot experiments have confirmed that this material can continuously and stably release ammonium nitrogen in the soil, and the effective ammonium nitrogen content in the soil is maintained at 35-50 mg / Kg within 30 days, effectively avoiding the drawbacks of traditional ammonium chloride quick-acting fertilizers, such as explosive release in the early stage and leaching loss in the later stage. More importantly, due to the high selective adsorption and fixation capacity of this material for ammonium ions, it significantly inhibits the co-adsorption and release of chloride ions. After application, the soil conductivity and salinity are reduced by at least 25% compared with the soil treated with the same amount of fertilizer, effectively alleviating the problem of secondary soil salinization caused by chloride ion accumulation. At the same time, the biochar matrix itself can also significantly increase the soil organic matter content, improve the soil aggregate structure, and promote the healthy development of crop roots, thereby achieving multiple goals of chloride ion risk control, soil quality improvement, and increased crop yield and efficiency while purifying wastewater.

[0018] 4. In this invention, NH4 + Stable bound to the surface of La2O3 crystals by chemical bonds, it is slowly released into the soil, avoiding the short-term burst release and subsequent leaching loss of fast-acting nitrogen fertilizer, thus achieving long-term and stable nitrogen supply. Pot experiments show that alfalfa with nitrogen-loaded biochar has robust root development, stable plant growth, and a total biomass comparable to that of the chemical fertilizer group. Moreover, soil electrical conductivity and salinity are significantly reduced, and organic matter content is increased, effectively mitigating the salinization risk caused by ammonium chloride application. At the same time, it improves soil aggregate structure and water and fertilizer retention capacity. Attached Figure Description

[0019] Figure 1 The graph shows the effect of different lanthanum loadings on the removal rate of ammonium nitrogen in ammonium chloride wastewater.

[0020] Figure 2 This is a comparison chart of the adsorption rates of ammonium nitrogen by lanthanum-modified biochar prepared at different pyrolysis temperatures.

[0021] Figure 3The images show scanning electron microscopy (SEM) characterizations of the lanthanum-modified biochar and the unmodified biochar of this invention. In the images, a is the SEM image of the unmodified corn straw biochar, b is the SEM image of BC-La, and c is the surface scan map of La.

[0022] Figure 4 The images show the TEM, HRTEM, and SAED characterizations of BC-La from this invention, where a is a low-magnification TEM image, b is a high-magnification TEM image, c is a high-resolution HRTEM lattice fringe pattern, and d is a SAED selected area electron diffraction pattern.

[0023] Figure 5 This is a comparison of FTIR images of unmodified biochar and lanthanum-modified biochar.

[0024] Figure 6 The figures show the nitrogen adsorption-desorption isotherms and pore size distribution curves of unmodified biochar and lanthanum-modified biochar. In the figure, a represents the nitrogen adsorption-desorption isotherm and pore size distribution curve of unmodified biochar, and b represents the nitrogen adsorption-desorption isotherm and pore size distribution curve of lanthanum-modified biochar.

[0025] Figure 7 The images show the XPS comparison before and after lanthanum-modified biochar adsorption of ammonium chloride wastewater. In the images, a is the full XPS spectrum, b is the La3d high-resolution fine spectrum, c is the N 1s high-resolution fine spectrum, and d is the O 1s high-resolution fine spectrum.

[0026] Figure 8 Single-factor conditions for the adsorption of NH4 by lanthanum-modified biochar + -N and Cl - The experimental results of the effect on performance are shown in the figure, where a represents the effect of adsorption time on NH4. + The curve showing the effect of adsorption capacity, where b represents the coexistence of Cl. - Adsorption kinetics control curves, c is the curve showing the effect of solution pH on the adsorption capacity of ammonium nitrogen, and d is the curve showing the initial NH4+. + The effect curve of -N concentration on adsorption capacity.

[0027] Figure 9 Lanthanum-modified biochar for NH4 adsorption + Comparison of fitting curves between the pseudo-first-order and pseudo-second-order dynamic models of -N.

[0028] Figure 10 Lanthanum-modified biochar for NH4 adsorption + Comparison of fitting curves of Langmuir and Freundlich isothermal adsorption models for -N.

[0029] Figure 11The images show the cyclical cultivation of alfalfa in pot experiments under different nitrogen supply treatments; where a is the blank control group, b is the ammonium chloride fertilizer group, and c is the nitrogen-loaded BC-La slow-release group. Detailed Implementation

[0030] Example 1: Preparation of Lanthanum-Modified Biochar This embodiment provides a method for preparing lanthanum-modified biochar for ammonium chloride wastewater treatment, including the following steps: S1, First pyrolysis Corn stalks are washed to remove surface mud and impurities, dried at 105℃ to constant weight, mechanically pulverized, and passed through an 80-mesh sieve to obtain corn stalk powder.

[0031] Take 20g of corn stalk powder and place it in a tube furnace. Heat the temperature to 700℃ at a rate of 10℃ / min and pyrolyze it for 2h under a nitrogen protective atmosphere (flow rate 100mL / min) to obtain biochar (denoted as BC).

[0032] S2, Ultrasonic-assisted impregnation with lanthanum loading Lanthanum nitrate (La(NO3)3·6H2O) was dissolved in deionized water to prepare a 0.1 mol / L lanthanum nitrate solution.

[0033] Take 5g of BC prepared in step S1 and add it to 100mL of lanthanum nitrate solution (solid-liquid ratio 1:20). Place it in an ultrasonic cleaner at room temperature and perform ultrasonic-assisted impregnation treatment at a frequency of 40kHz for 30min to fully load lanthanum ions onto the surface and pores of the biochar.

[0034] S3. Solid-liquid separation and drying The ultrasonically impregnated mixture was separated by vacuum filtration, and the solid product was collected. It was washed three times with deionized water to remove free lanthanum ions on the surface. The washed solid product was then dried in a 105℃ forced-air drying oven for 12 hours to obtain lanthanum-loaded biochar.

[0035] S4, Second pyrolysis The dried lanthanum-loaded biochar was placed in a tube furnace and heated to 700°C at a rate of 5°C / min. During the heating and holding process, steam was introduced into the tube furnace as an activating gas at a flow rate of 100 mL / min, and the pyrolysis was carried out at 700°C for 2 hours.

[0036] During the second pyrolysis process, water vapor reactivates the biochar and creates pores, further increasing the specific surface area and mesoporous ratio of the biochar. At the same time, lanthanum ions loaded on the surface of the biochar are converted into La2O3 nanoparticles at a high temperature of 700℃, which are uniformly anchored on the surface and inner wall of the pores of the biochar.

[0037] S5, rapid cooling to lock the crystal phase After pyrolysis, the heating power is turned off, and a large amount of nitrogen gas (flow rate 500 mL / min) is immediately introduced into the tube furnace. The product is rapidly cooled to room temperature at a cooling rate of not less than 50 °C / min to lock the metastable crystalline phase structure of the La2O3 nanoparticles and obtain the lanthanum-modified biochar.

[0038] Example 2 Effect of different lanthanum loading on the adsorption performance of ammonium nitrogen This example is used to investigate the effect of different lanthanum nitrate concentrations on the adsorption performance of the product in order to determine the optimal range of lanthanum loading.

[0039] Following the preparation method of Example 1, only the concentration of the lanthanum nitrate solution in step S2 was changed, while all other conditions remained exactly the same as in Example 1, to prepare lanthanum-modified biochar with different lanthanum loadings. The concentrations of the lanthanum nitrate solution were set to 0.01 mol / L, 0.02 mol / L, 0.04 mol / L, 0.06 mol / L, 0.08 mol / L, 0.10 mol / L, and 0.15 mol / L, respectively, corresponding to theoretical lanthanum loadings (as a percentage of lanthanum mass to biochar mass) of 1%, 2%, 3%, 5%, 8%, 10%, and 15%, respectively.

[0040] 0.2 g of each of the lanthanum-modified biochars with different lanthanum loadings was added to 100 mL of wastewater containing 100 mg / L ammonium chloride. Adsorption was carried out at 25 °C and 200 rpm for 12 h with shaking. After adsorption, solid-liquid separation was performed, and the residual ammonium nitrogen concentration in the filtrate was measured. The ammonium nitrogen removal rate of each material was calculated. The results are as follows: Figure 1 As shown.

[0041] Experimental results show that as the lanthanum loading increases from 1% to 3%, the ammonium nitrogen removal rate significantly improves. The ammonium nitrogen removal rate peaks at 3% lanthanum loading, and the effect is also significant at 5%. Further increasing the lanthanum loading to 7%, 10%, and 15% does not significantly increase the ammonium nitrogen removal rate; instead, it slightly decreases. This indicates that a lanthanum loading of 3%-5% is effective, with 3% being the optimal value. The number of lanthanum active sites directly determines the ammonium enrichment capacity; too low a lanthanum content results in insufficient active sites and low adsorption capacity, while excessive lanthanum will clog pores and reduce the adsorption effect.

[0042] Example 3: Effect of different pyrolysis temperatures on the adsorption performance of ammonium nitrogen This embodiment is used to investigate the effect of the second pyrolysis temperature on the adsorption performance of the product in order to determine the optimal pyrolysis temperature range.

[0043] Following the preparation method of Example 1, only the second pyrolysis temperature in step S4 was changed, while the other conditions were exactly the same as in Example 1 (lanthanum loading was 5%), lanthanum-modified biochar at different pyrolysis temperatures was prepared. The second pyrolysis temperatures were set to 400℃, 500℃, 600℃, and 700℃, respectively.

[0044] 0.2 g of each of the lanthanum-modified biochars prepared at different pyrolysis temperatures was added to 100 mL of wastewater containing 100 mg / L ammonium chloride. The adsorption was carried out at 25 °C and 200 rpm for 12 h with shaking. After adsorption, solid-liquid separation was performed, and the residual ammonium nitrogen concentration in the filtrate was measured. The ammonium nitrogen removal rate of each material was calculated. The results are as follows: Figure 2 As shown.

[0045] Experimental results show that the removal rate of ammonium nitrogen continuously increases with the pyrolysis temperature from 400℃ to 700℃. The materials prepared under pyrolysis conditions of 400℃ and 500℃ exhibit lower adsorption performance, mainly because the biochar at lower temperatures is insufficiently carbonized, resulting in incomplete pore structure development, smaller specific surface area, and incomplete conversion of lanthanum ions to La₂O₃, limiting the number of effective active sites. When the pyrolysis temperature increases to 600℃, the adsorption performance significantly improves, and the pore structure and carbon framework order of the material are markedly enhanced. The optimal adsorption performance and the highest ammonium nitrogen removal rate are achieved at a pyrolysis temperature of 700℃.

[0046] For the same lanthanum loading, the adsorption performance was optimal under high-temperature pyrolysis conditions at 700℃, and the resulting product was designated BC-La. The pore structure of the material was further optimized, and the specific surface area and mass transfer channels were significantly improved. Figure 6 At the same time, the degree of ordering of the carbon skeleton is improved, making NH4 + The diffusion and enrichment processes are more efficient, and promote the uniform exposure and stable dispersion of La active sites.

[0047] The above results indicate that pyrolysis temperature has a significant impact on the pore structure, carbon skeleton order, and the formation and dispersion of La2O3 nanoparticles. Too low a temperature results in insufficient carbonization and an underdeveloped pore structure in the biochar, while too high a temperature may lead to collapse of the biochar pore structure and aggregation of active sites. 700℃ is the optimal pyrolysis temperature, which ensures both a well-developed pore structure and a high specific surface area in the biochar, as well as complete conversion and uniform dispersion of La2O3 nanoparticles.

[0048] Example 4: Microstructure characterization test of biochar before and after modification The morphology of unmodified corn straw biochar (BC, prepared by step S1 of Example 1) and BC-La prepared in Example 1 were characterized by scanning electron microscopy (SEM). The samples were attached to the surface of conductive adhesive, sputtered with gold, and then scanned by SEM. The observation scale was uniformly set to 10 μm, and the surface morphology images of the two sets of samples were taken.

[0049] Figure 3 (a) is a SEM image of unmodified corn straw biochar. The carbon skeleton structure is dense and compact, with a smooth and flat surface, few pores and small pore size. No nanoparticles are attached to the carbon surface, resulting in a lack of mass transfer channels for ion diffusion. The surface can be used to bind NH4. + The number of effective active sites is extremely small, which is the core reason why the adsorption capacity of ammonium nitrogen in the original biochar is low.

[0050] Figure 3 (b) is a SEM image of BC-La of the present invention. As can be seen from the image, the BC-La matrix forms a well-developed and interconnected multi-level pore structure with rich pore size and good pore connectivity. Nanoscale La2O3 particles are uniformly distributed on the outer surface and inner wall of the pores of the carbon material. The particles are small in size and there is no large-area agglomeration of lanthanum oxide. The lanthanum active component is uniformly coated in the entire carbon framework pore system.

[0051] Figure 3 (c) is Figure 3 (b) shows the surface scan map of La element. The fluorescence signal is uniformly distributed and completely covered, which proves that the La element is uniformly dispersed on the nanoparticles on the surface of biochar. The lanthanum component is successfully loaded onto the carbon framework, with good dispersion, forming a large number of unsaturated coordinated lanthanum active sites.

[0052] Further characterization of BC-La was performed using TEM, high-resolution TEM (HRTEM), and selected area electron diffraction (SAED). Results are shown below. Figure 4 .

[0053] Figure 4 (a) is a low-magnification TEM image with a scale bar of 0.5 μm. The sheet-like carbon skeleton matrix of corn straw-based biochar can be observed under low-magnification transmission electron microscopy. Multiple dark clusters are distributed on the surface of the carbon matrix, which are the aggregation sites of lanthanum oxide nanoparticles loaded on the surface of biochar. Lanthanum species are successfully composite and attached to the surface of the carbon skeleton without large-area blocky agglomeration. The nanoparticles are uniformly embedded in the surface and shallow pores of the carbon material, proving that the citric acid complexation-assisted sol-gel process of the present invention can effectively load lanthanum precursors onto the biomass carbon matrix.

[0054] Figure 4 (b) is a high-magnification TEM image with a scale bar of 100 nm. Figure 4(a) Magnified observation reveals that a large number of near-spherical nanoparticles are uniformly dispersed on the carbon substrate surface, with particle sizes concentrated in the 30–80 nm range. The particles are well dispersed and there is no large-area stacking or agglomeration blocking the pores. This indicates that the ultrasonic dispersion step effectively inhibits lanthanum ion aggregation, allowing the nano-La2O3 particles to be fully exposed for NH4 adsorption. + Provides ample touch interfaces.

[0055] Figure 4 (c) is a high-resolution lattice fringe pattern obtained by HRTEM with a scale bar of 5 nm. A high-resolution lattice image was taken of a region of a single nanoparticle, showing continuous and regular alternating bright and dark lattice fringes. The lattice fringe spacing measured by magnification of the local inset is 0.312 nm. This interplanar spacing is completely matched with the characteristic crystal plane (101) in the La2O3 standard PDF card, directly confirming that the nanoparticle crystal phase is lanthanum oxide crystal. There are a large number of lattice defects on the crystal surface, forming unsaturated coordinated lanthanum active sites. These sites can specifically bind NH4 in water through surface complexation. + Ions are the core active centers for the selective enrichment of ammonium nitrogen in materials.

[0056] Figure 4 (d) is the SAED selected area electron diffraction pattern with a scale bar of 51 nm. Selected area electron diffraction patterns were collected on the La2O3 nanoparticle region. The image shows multiple sets of clear, concentric ring diffraction rings. The four diffraction rings from the inside to the outside correspond to the (101), (100), (001), and (102) crystal planes of the La2O3 crystal, respectively. The continuous polycrystalline diffraction rings prove that the La2O3 in the sample is a polycrystalline nanoparticle, which is consistent with the complete lattice fringes observed by HRTEM. The polycrystalline structure brings abundant crystal edges and defect sites, further increasing the number of unsaturated lanthanum coordination sites and strengthening the NH4+ ionization. + Its complexing and immobilization capabilities, while also enabling NH4 + It is chemically bonded to the surface of La2O3 crystals to achieve long-term slow-release of ammonium nitrogen in the soil environment.

[0057] In summary, the preparation process of this invention can uniformly load polycrystalline nano-La2O3 particles onto the surface of biochar. The crystals contain a large number of unsaturated coordinated lanthanum active sites, which selectively capture NH4 through surface complexation and ion exchange mechanisms. + Furthermore, its strong binding stability provides a microcrystalline basis for the material's high adsorption capacity, ion selectivity, and slow-release nitrogen supply performance.

[0058] Example 5: Characterization Experiments of Surface Functional Groups and Pore Structure Before and After Biochar Modification Dry and ground BC and BC-La powders were compressed with KBr to prepare tablets, and the wavenumber range was measured in the range of 4000-1000 cm⁻¹. -1 Infrared absorption spectrum curves were collected, and the results are shown in [the table below]. Figure 5 .

[0059] As can be seen from the figure, 3400cm -1 In the vicinity of the BC-La region, both BC and BC-La exhibit characteristic absorption peaks due to the stretching vibration of hydroxyl groups (-OH). Compared to the original BC, the absorption intensity of this peak in BC-La is significantly increased, and the peak position shifts slightly, indicating that the lanthanum ion undergoes complexation coordination with the hydroxyl group, activating the surface hydroxyl functional groups. (1500 cm⁻¹) -1 In the vicinity of the lanthanum band, both groups of samples exhibited C=O stretching vibration peaks corresponding to carboxyl groups (-COOH). The C=O absorption peak intensity of BC-La was much higher than that of the original biochar, indicating that lanthanum modification increased the carboxyl content on the carbon surface. Carboxyl groups can fix NH4 through ion exchange and hydrogen bonding. + 1000-1300cm -1 Range: Both groups of samples showed CO single bond vibration absorption peaks, with increased BC-La peak intensity, indicating an increase in the total amount of carbon-oxygen-containing functional groups.

[0060] La 3+ After introduction, it can undergo complexation reactions with the hydroxyl and carboxyl groups on the surface of biochar, activating and enriching oxygen-containing functional groups; the hydroxyl and carboxyl groups can react with NH4 in water. + The synergistic effect of electrostatic attraction and ion exchange significantly increases the number of ammonium ion binding sites, thereby enhancing the material's ammonium nitrogen adsorption capacity.

[0061] Example 6 Characterization experiments of surface functional groups and pore structure before and after biochar modification After vacuum degassing pretreatment, BC-La powder was used to test the nitrogen adsorption-desorption curve at 77K using a fully automated surface area analyzer. The pore size distribution differential curve was also acquired simultaneously. The results are shown in [Figure number missing]. Figure 6 In the figure, a represents the nitrogen adsorption-desorption isotherm and pore size distribution curve of BC, and b represents the nitrogen adsorption-desorption isotherm and pore size distribution curve of BC-La. The figure shows that both materials exhibit typical Type IV isotherms with hysteresis loops, indicating that the materials are predominantly mesoporous. Compared to BC, BC-La shows an overall increase in adsorption capacity across the entire relative pressure range, indicating an increase in both specific surface area and pore volume. The pore size distribution in the figure further shows that the modified material has a more concentrated pore size distribution and an optimized micro / mesoporous ratio, which is beneficial for the diffusion and enrichment of adsorbates in the pores. In summary, La modification not only modulates the functional group structure of biochar surface but also significantly improves the pore structure, thereby synergistically enhancing its adsorption performance.

[0062] KOH pretreatment etches the carbon framework, constructing a continuous, interconnected micro-mesoporous multi-level mass transfer channel; the uniformly developed pore structure is NH4 +Diffusion provides ample pathways and significantly increases the specific surface area of ​​the carbon matrix, allowing a large number of La2O3 nano-active sites to be fully exposed and form a synergistic adsorption system with oxygen-containing functional groups.

[0063] Example 7 Characterization of the XPS interfacial interaction mechanism of lanthanum-modified biochar before and after adsorption BC and BC-La solid samples after reaching equilibrium with adsorbed ammonium chloride wastewater were taken, thoroughly dried and ground, compressed into tablets, and then collected on an XPS spectrometer. The binding energy range was measured from 0 to 900 eV, and full spectrum and fine peak-separated spectrum of each element were collected.

[0064] Figure 7 (a) is the XPS full spectrum. Both the pre- and post-adsorption full spectra show obvious characteristic peaks for La 3d, C 1s, and O 1s, with a newly added clear N 1s characteristic peak after adsorption. The elemental composition remains unchanged, indicating that the lanthanum component is stably loaded onto the biochar matrix, and the ammonium adsorption reaction occurs only on the material surface, without damaging the basic carbon skeleton structure of the biochar. The significant shifts in the intensity of characteristic peaks and baselines before and after adsorption suggest that the NH4+ in the water... + It undergoes interfacial chemical reactions with the La active sites and oxygen-containing functional groups on the material surface, altering the electronic distribution and chemical environment of the material surface elements.

[0065] Figure 7 (b) is the high-resolution fine spectrum of La 3d, showing that La exhibits spin-split double peaks. 5 / 2 The characteristic peak is located around 835 eV, La 3d 3 / 2 The characteristic peaks are located around 851 eV, and both sets of peaks can be separated and fitted to represent two lanthanum components with different coordination states. Comparing the spectra before and after adsorption, the adsorption of NH4... + The overall binding energy of the La 3d double peaks shifted slightly, and the relative proportions of the fitted peaks of the two components changed significantly; this proves that the unsaturated coordinated La atoms on the lanthanum oxide surface react with NH4+. + Coordination complexation and ion exchange occur, and the outer electron cloud of the La atom is transferred, confirming that the unsaturated coordinated lanthanum active site in La2O3 is responsible for capturing and fixing NH4+. + The core reaction site.

[0066] Figure 7 (c) shows the high-resolution fine spectrum of N 1s. The N 1s spectrum was fitted to three nitrogen configurations: pyridinic nitrogen (398.5 eV), pyrrolic nitrogen (400 eV), and graphitic nitrogen (402 eV). Before adsorption, the material contained only trace amounts of background nitrogen functional groups, and the pyrrolic nitrogen signal intensity was low; adsorbed NH4... +The fitted peak area of ​​pyrrole nitrogen increased significantly, and the relative proportions of pyridine nitrogen and graphitic nitrogen changed simultaneously. Pyrrole nitrogen is rich in lone pairs of electrons and can bind to cation NH4 through electrostatic attraction and hydrogen bonding. + This demonstrates that nitrogen-containing functional groups on the material surface synergistically participate in the adsorption and fixation process of ammonium ions.

[0067] Figure 7 (d) is the high-resolution fine spectrum of O 1s. The O 1s spectrum can be fitted to characteristic peaks of two types of oxygen-containing functional groups: C–O single bond (533.5 eV) and C=O carbonyl group (532 eV). Adsorption of NH4 + Subsequently, the peak areas and binding energies of both the C–O and C=O components showed significant changes; the C–O and C=O groups in the hydroxyl and carboxyl groups can bind with NH4 through hydrogen bonding and ion exchange. + In combination, the lanthanum modification process increases the content of oxygen-containing functional groups, which form a synergistic adsorption system with the La active sites, further enhancing the enrichment capacity of ammonium nitrogen.

[0068] In summary, BC-La affects the NH4+ content of water bodies. + The enrichment process is not a simple physical adsorption, but a multi-mechanism synergistic chemisorption process: the unsaturated lanthanum coordination sites of La2O3 on the material surface and NH4 + Surface complexation and ion exchange occur, providing core sites for selective adsorption; oxygen-containing functional groups such as hydroxyl and carboxyl groups on the surface of biochar assist in the fixation of NH4 through electrostatic attraction and hydrogen bonding. + The pyrrole-type nitrogen-containing functional groups generated during the adsorption process further enhance the stability of ammonium ion binding; multiple synergistic effects enable the coexistence of high chloride ions in ammonium chloride wastewater containing NH4. + Highly efficient and stable enrichment, and NH4 + By binding the adsorbed nitrogen-loaded biochar to the material surface in a chemical bond form, a microscopic theoretical support is provided for the long-term slow release of ammonium nitrogen in the soil.

[0069] Example 8 Single-factor conditions on the adsorption of NH4 by BC-La + -N and Cl - Performance impact Using BC-La as the experimental material, single-factor batch adsorption experiments were conducted to investigate the effects of various environmental conditions on the ammonium enrichment capacity and ion selectivity of the material. The basic experimental conditions were uniform: adsorption temperature 25℃, material dosage 0.2 g / L, ammonium chloride simulating wastewater system, and shaking speed 150 rpm. Three parallel samples were set for each experiment. After adsorption, the samples were filtered and separated, and the NH4+ content in the filtrate was measured. + -N、Cl - The remaining concentration was used to calculate the corresponding adsorption capacity (qt), and the results are shown in [the table]. Figure 8 .

[0070] Figure 8 (a) Results of the adsorption time gradient experiment, with adsorption time gradients set at 60 min, 200 min, 320 min, 440 min, 600 min, and 720 min. In the initial adsorption phase (0-440 min), BC-La adsorbed NH4... + The adsorption capacity increases rapidly over time, resulting in a large amount of free NH4 in the solution. + It can rapidly bind to the exposed unsaturated coordinated La active sites and hydroxyl / carboxyl oxygen-containing functional groups on the material surface; when the adsorption time reaches 600 min, the adsorption capacity reaches its peak. Further extending the adsorption time to 720 min results in a slight decrease in adsorption capacity, indicating that the system has reached adsorption equilibrium. This demonstrates that the BC-La of this invention can rapidly capture NH4+ in water. + It can complete the enrichment of most ammonium nitrogen in a short time. The adsorption time is best at 440-720 min, and the optimal equilibrium adsorption time is controlled at around 600 min.

[0071] Figure 8 (b) for coexisting Cl - Adsorption kinetics comparison test results, using the above time gradient, only detected Cl in the system - Adsorption capacity. BC-La adsorption capacity for Cl over the entire time period. - The adsorption capacity remained stable in the range of 0.43-0.51 mg / g, which is extremely low and far lower than that of NH4 under the same conditions. + Saturated adsorption capacity (approximately 47 mg / g); the curve is flat with no obvious upward trend. This demonstrates that BC-La possesses excellent cation selectivity, and the unsaturated coordination sites of La2O3 specifically recognize and bind only NH4+. + It hardly adsorbs and coexists with Cl. - It can achieve efficient separation and enrichment of ammonium nitrogen in high chloride ion ammonium chloride wastewater systems, avoiding interference from chloride ion competitive adsorption.

[0072] Figure 8 (c) The results of the experiment on the effect of solution pH gradient were shown. The pH gradient of the wastewater was adjusted to 4, 5, 6, 7, 8, and 9, while the other adsorption conditions were kept constant. The NH4 content of each group was measured. + Adsorption capacity. In the pH range of 4-8, the adsorption capacity of ammonium nitrogen continuously increases with increasing pH, reaching its peak at pH=8 (neutral to slightly alkaline); the adsorption capacity decreases slightly after pH>8. Under acidic conditions, a large amount of H₂... + Will with NH4 + Competition for La active sites occurs, and acidic environments can cause slight dissolution of La2O3, reducing the number of effective active sites. In the neutral to weakly alkaline range, the Lewis acid sites of La2O3 exhibit the best activity and can be stably coupled with NH4+. +Surface complexation occurs; NH4 in a strongly alkaline environment + When converted into free NH3 molecules, the content of adsorbable ionic ammonium ions decreases, and the adsorption capacity decreases.

[0073] Figure 8 (d) is the initial NH4 + The initial ammonium nitrogen (C0) concentration gradient experiment was conducted, setting initial concentration gradients of 25 mg / L, 50 mg / L, 100 mg / L, 200 mg / L, and 300 mg / L in the wastewater. With increasing initial nitrogen concentration, the adsorption capacity of BC-La ammonium ions continuously increased. However, the growth rate slowed down after the concentration exceeded 200 mg / L, and the curve gradually flattened, exhibiting typical monolayer saturated adsorption characteristics. This indicates that the material surface can be used to bind NH4+. + The total number of La active sites is fixed, and the saturated adsorption capacity is directly determined by the lanthanum loading and the number of active sites; high-concentration ammonium chloride wastewater can still achieve efficient ammonium nitrogen enrichment, and the material is suitable for treating different concentrations of ammonium-containing wastewater.

[0074] In summary, BC-La affects NH4 + The material exhibits rapid adsorption rate and optimal adsorption performance under neutral to weakly alkaline conditions; it also shows good adsorption properties for NH4. + It exhibits high selectivity and almost no co-adsorption of Cl. - It is suitable for treating ammonium chloride wastewater. It should be noted that the initial NH4 in this gradient experiment... + The -N concentration was only 300 mg / L at most, which did not reach the complete adsorption saturation plateau of the material. Figure 8 (d) shows a continuous upward trend in the adsorption capacity curve without a clear plateau; while the isothermal adsorption model fitting experiment ( Figure 10 According to the Langmuir equation, a theoretical saturation adsorption capacity exists at higher equilibrium concentrations. The adsorption follows the monolayer saturation adsorption law, and the saturation adsorption capacity is controlled by the total number of lanthanum active sites on the surface. NH4 + Stable fixed on the surface of La2O3 by chemical bonding, it is not easily leached and desorbed by coexisting chloride ions. It not only meets the needs of ammonium nitrogen resource recovery from wastewater, but also provides macroscopic data support for the long-term slow-release nitrogen supply of nitrogen-loaded biochar applied to the soil.

[0075] Example 9: BC-La adsorption of NH4 + -N kinetics and isothermal adsorption model fitting experiment I. Dynamic Model Fitting Experiment Preparation of initial NH4 +Ammonium chloride with a concentration of 100 mg / L was used to simulate wastewater. The dosage of BC-La was 0.2 g / L. The temperature was kept constant at 25℃ and the oscillation speed was 150 rpm. After filtration, the concentration of residual ammonium nitrogen in the filtrate was measured, and the adsorption capacity (qt) at each time point was calculated. The pseudo-first-order kinetic and pseudo-second-order kinetic models were fitted using the experimental data to obtain two fitting curves.

[0076] Depend on Figure 9 The kinetic curves show that during the initial adsorption phase (0–60 min), BC-La adsorbs NH4+. + The adsorption capacity increases rapidly; during this stage, a large number of unsaturated lanthanum coordination sites, hydroxyl and carboxyl oxygen-containing functional groups on the material surface are not occupied, and free NH4 in the solution... + The adsorption process rapidly binds to the material surface through electrostatic attraction and ion exchange. After adsorption time exceeds 60 minutes, the rate of increase in the adsorption curve slows down until adsorption equilibrium is reached at approximately 600 minutes. At this point, ion diffusion within the pores becomes the rate-controlling step of adsorption.

[0077] Comparing the fitting curves of the two models, the pseudo-second-order kinetic model curve showed a higher fit and better correlation coefficient with the experimental data. The pseudo-second-order kinetics, with electron exchange and coordination chemistry reactions as the rate-determining steps, demonstrates the effect of BC-La on NH4+. + The adsorption is mainly chemical adsorption, rather than physical adsorption relying solely on intermolecular forces, which corroborates the interfacial mechanism of coordination reaction between lanthanum sites and ammonium ions as characterized by XPS.

[0078] II. Isothermal Adsorption Model Fitting Experiment Preparation of gradient initial NH4 + -N concentration ammonium chloride wastewater was treated with a uniform addition of 0.2 g / L BC-La, and the solution was shaken at 25°C until adsorption equilibrium was reached. The concentration of ammonium nitrogen in the equilibrium liquid phase was then measured after filtration. e ), calculate the equilibrium adsorption capacity (q) e The experimental data were nonlinearly fitted using the Langmuir monolayer adsorption model and the Freundlich multilayer adsorption model, respectively, to obtain two isothermal fitting curves.

[0079] Figure 10 The isotherm curves show that as C... e The q of BC-La continues to rise. e The adsorption capacity increases continuously; when the initial ammonium nitrogen concentration exceeds 200 mg / L, the increase in adsorption capacity decreases significantly, and the curve approaches a plateau region, proving that NH4 can bind to the material surface. +The total number of active sites is limited, resulting in a fixed saturation adsorption capacity. The Langmuir model fitting curve shows better overlap with all measured data than the Freundlich model, conforming to the Langmuir monolayer uniform adsorption theory. This indicates that the sol-gel combined with ultrasonic process of this invention can uniformly disperse La2O3 nanoparticles inside and outside the pores of biochar, resulting in uniform physicochemical properties of the surface adsorption sites, with each lanthanum active site binding only one NH4+. + There is no mutual interference between the sites.

[0080] In summary, BC-La affects NH4 + It possesses rapid kinetic response characteristics, enabling the enrichment of most ammonium nitrogen in a short time, resulting in short treatment time and high treatment efficiency in actual wastewater treatment. The adsorption process is dominated by chemical adsorption, relying on the complexation and ion exchange reaction between lanthanum unsaturated coordination sites and ammonium ions to achieve fixation, with physical adsorption playing only an auxiliary role. Ammonium ions undergo uniform monolayer adsorption on the BC-La surface, and the saturated adsorption capacity of the material is determined by the total number of lanthanum active sites on the surface. The kinetic and isothermal adsorption fitting conclusions are completely consistent with the microscopic characterization results of SEM, TEM, FTIR, and XPS.

[0081] Example 10 Pot Experiment In this embodiment, the ammonium nitrogen-loaded lanthanum modified biochar (BC-La) obtained after adsorption saturation in Example 8 was used as the test slow-release nitrogen fertilizer. Alfalfa was used as the test crop. Three groups of control pot experiments were set up, and cultivation observations were carried out in three cultivation cycles: short-term, medium-term and long-term. After harvest, plant biomass and soil physicochemical indicators were measured to verify the effects of nitrogen-loaded BC-La on long-term slow-release ammonium nitrogen, soil improvement and reduction of nitrogen leaching.

[0082] Three parallel potted plant treatments were set up, with identical soil basic properties, seeding rate, and field water holding capacity management across all groups, differing only in nitrogen source treatment: Group A (blank control group): Soil without any nitrogen fertilizer or modified biochar, only blank soil; Group B (Ammonium chloride fertilizer group): An equal amount of pure ammonium chloride quick-acting nitrogen fertilizer was applied to the soil; Group C (Nitrogen-loaded BC-La slow-release group): Lanthanum-modified biochar with adsorption of saturated ammonium nitrogen was added to the soil, and the total nitrogen application rate was exactly the same as that of Group B (ammonium chloride).

[0083] Alfalfa seeds were sown uniformly, and water was replenished regularly throughout the process to maintain suitable soil moisture content. The plants were continuously cultivated in three stages: short-term, medium-term, and long-term, and their growth status was photographed at each stage. After the entire cultivation cycle was completed, the plants were harvested whole, and the roots, stems, and leaves were separated, dried, and weighed. Soil samples were collected from each group simultaneously to test soil pH, electrical conductivity, salinity, organic matter, and available ammonium nitrogen content.

[0084] Figure 11The images show potted alfalfa plants cultivated in three cycles under different nitrogen supply treatments. The horizontal groups a, b, and c correspond to the blank control group, the ammonium chloride nitrogen fertilizer group, and the nitrogen-loaded BC-La slow-release group, respectively. The vertical three rows represent the short-term, medium-term, and long-term cultivation stages, respectively. During short-term cultivation, the growth of the three groups of plants was relatively similar. In the mid-term stage, group B experienced a large release of nitrogen from the fast-acting fertilizer in the early stage, resulting in a temporary advantage in plant growth, but signs of yellowing and premature aging of leaves were already observed. Group C relied on ammonium ions bound and fixed by lanthanum active sites to slowly release nitrogen, resulting in stable plant growth and fewer yellow leaves. After long-term cultivation, group B experienced rapid leaching and loss of fast-acting nitrogen, leading to nitrogen deficiency in the soil. This resulted in large-scale wilting of alfalfa, severe browning of leaves, and the worst growth. In contrast, group C, with its BC-La slow-release material, continuously and stably supplied ammonium nitrogen, and the number of surviving plants and the amount of green leaves were significantly better than the other two groups. This directly demonstrates that lanthanum-modified biochar can stably bind the enriched ammonium ions to the lanthanum active sites and achieve long-term slow release, avoiding the defects of short-term explosive release and later deficiency and loss of nitrogen from fast-acting fertilizers. The slow-release performance of the material for ammonium nitrogen was directly verified from the morphological observation of crop growth.

[0085] After the entire cultivation cycle was completed, all plants in each group were harvested, and the roots, stems, and leaves were separated and dried to constant weight. The dry weight data are shown in Table 1 below.

[0086] Table 1. Results of alfalfa plant biomass and soil physicochemical properties.

[0087] Table 1 shows that the blank control group, without exogenous nitrogen supplementation, had the lowest dry weight of roots, stems, and leaves, as well as the lowest total biomass among the three groups, indicating that nitrogen deficiency significantly inhibited alfalfa growth. The ammonium chloride fertilizer group had the highest total dry weight, but significantly higher stem biomass and lower root biomass, suggesting that short-term, large-scale release of readily available nitrogen easily leads to excessive vegetative growth, weak root development, and poor crop resistance. The nitrogen-loaded BC-La slow-release group had a total biomass similar to the fertilizer group, and its root dry weight reached 56.1 mg, the highest among the three groups. The steadily released ammonium nitrogen continuously supplies nutrients to the roots, promoting robust root development, facilitating long-term water and nutrient absorption, and enhancing the plant's tolerance to poor soil and stress.

[0088] Applying ammonium chloride fertilizer significantly increases soil electrical conductivity and salinity, exacerbating secondary soil salinization caused by chloride ion accumulation. The nitrogen-loaded BC-La slow-release group exhibited a soil electrical conductivity of only 550 mg / cm and a soil salinity as low as 0.79 g / kg, significantly lower than the fertilizer group, effectively mitigating the negative impacts of chloride ion damage on soil and crops. Furthermore, biochar itself is an organic carbon-based material; the slow-release group achieved a soil organic matter content of 84.23 g / kg, higher than both the control group and the fertilizer group, demonstrating its long-term ability to improve soil aggregate structure and enhance soil water and fertilizer retention capacity.

[0089] The blank control group had no exogenous nitrogen input and the lowest available ammonium nitrogen content in the soil; the ammonium chloride fertilizer group experienced significant nitrogen leaching loss in the early stages, and the available ammonium nitrogen in the soil was only 38.4 mg / Kg after cultivation; the nitrogen-loaded BC-La group relied on the chemical bonding of unsaturated lanthanum sites on La2O3 to fix NH4+. + It slowly decomposes and releases nitrogen in the soil. After the experiment, the available ammonium nitrogen in the soil can reach 43.8 mg / Kg, which can maintain the stable nitrogen pool in the soil for a long time and significantly reduce the risk of nitrogen leaching and loss.

[0090] Based on comprehensive analysis of potted plant appearance, plant biomass, and multiple soil testing indicators, it can be confirmed that the nitrogen-loaded lanthanum-modified biochar of this invention can stably bind ammonium ions, achieving long-term and stable nitrogen supply. This not only solves the drawbacks of traditional ammonium chloride quick-acting nitrogen fertilizer, such as short-term explosive release and subsequent nitrogen deficiency and leaching, but also improves soil physicochemical properties, alleviates soil salinization caused by chloride ions, and simultaneously realizes the integrated recycling of ammonium nitrogen from wastewater and slow-release fertilization for agricultural use.

Claims

1. A lanthanum-modified biochar for treating ammonium chloride wastewater, characterized in that, The lanthanum-modified biochar contains hydroxyl and carboxyl oxygen-containing functional groups and has a mesoporous structure. The surface and inner walls of the pores of the lanthanum-modified biochar are uniformly loaded with La2O3 nanoparticles. The La2O3 nanoparticles have unsaturated coordinated lanthanum active sites, and the selected area electron diffraction of the La2O3 nanoparticles shows diffraction rings corresponding to the La2O3 (101) crystal plane. The lanthanum-modified biochar has a positive effect on NH4. + It has selective adsorption capacity and selectively inhibits Cl. - Co-adsorption.

2. The lanthanum-modified biochar for ammonium chloride wastewater treatment according to claim 1, characterized in that, Selected area electron diffraction of the La2O3 nanoparticles also shows diffraction rings corresponding to the La2O3 (100) crystal plane, La2O3 (001) crystal plane and La2O3 (102) crystal plane, and the selected area electron diffraction of the La2O3 nanoparticles shows polycrystalline concentric diffraction rings corresponding to the (101), (100), (001) and (102) crystal planes.

3. The lanthanum-modified biochar for treating ammonium chloride wastewater according to claim 2, characterized in that, The interplanar spacing of the La2O3 nanoparticles is 0.312 nm.

4. The method for preparing lanthanum-modified biochar for ammonium chloride wastewater treatment according to any one of claims 1-3, characterized in that, Includes the following steps: S1. The biomass raw material is subjected to a first pyrolysis at 400-700℃ to obtain biochar; S2. The biochar is impregnated in a lanthanum nitrate solution and subjected to ultrasonic-assisted impregnation treatment to load lanthanum ions onto the surface and inside the pores of the biochar. S3. After solid-liquid separation and drying of the impregnated biochar, a second pyrolysis is carried out at 650-750℃ in an atmosphere containing activating gas to convert lanthanum ions into La2O3 nanoparticles. At the same time, the activating gas creates secondary pores in the biochar to obtain the lanthanum-modified biochar. S4. The product obtained in step S3 is rapidly cooled to room temperature at a cooling rate of not less than 50℃ / min to lock the metastable crystalline phase structure of the La2O3 nanoparticles.

5. The preparation method according to claim 4, characterized in that, In step S1, the first pyrolysis is carried out under an inert atmosphere for 2-4 hours; the biomass raw material is corn straw. The first and second pyrolysis processes are carried out at a temperature of 700℃ for 1-3 hours.

6. The preparation method according to claim 4, characterized in that, In step S2, the amount of lanthanum nitrate used is 3%-5% of the biochar mass based on lanthanum element; the concentration of the lanthanum nitrate solution is 0.1-0.2 mol / L; the ultrasonic-assisted impregnation treatment time is 10-60 min; the water bath heating temperature is 60-90℃; and the stirring time is 4-6 h.

7. The preparation method according to claim 4, characterized in that, The first and second pyrolysis processes are carried out at a temperature of 700℃ for 1-3 hours.

8. The application of lanthanum-modified biochar according to claim 1 or 2 in the treatment of ammonium chloride wastewater, characterized in that, The lanthanum-modified biochar was added to wastewater containing ammonium chloride for adsorption treatment. The biochar selectively adsorbed NH4 through the unsaturated lanthanum-coordinated active sites and oxygen-containing functional groups of La2O3. + At the same time, inhibiting the effect of Cl - Co-adsorption.

9. The application according to claim 8, characterized in that, The adsorption treatment temperature is 20-40℃, and the adsorption time is 440-720 min; the dosage of the lanthanum-modified biochar is 0.2 g / L. The removal rate of ammonium nitrogen is not less than 90%.

10. The application according to claim 9, characterized in that, After adsorption treatment, solid-liquid separation is performed to obtain lanthanum-modified biochar loaded with ammonium nitrogen. The lanthanum-modified biochar loaded with ammonium nitrogen is used as a slow-release nitrogen fertilizer in agricultural planting. After 30 days of application to the soil, the effective ammonium nitrogen content in the soil is not less than 40 mg / Kg, and the soil electrical conductivity is reduced by at least 25% compared with the application of the same amount of ammonium chloride fertilizer.

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