A modified biochar, its preparation method and uses

Abstract

This invention discloses a modified biochar, its preparation method, and its uses. The modified biochar of this invention comprises biochar and a calcium-containing compound loaded on the biochar at a loading of 10-30% of the modified biochar mass, wherein the calcium-containing compound is at least one of CaO and Ca(OH)₂. This invention modifies biogas residue with calcium carbonate, resulting in pore filling within the mesopores of the biochar in the 4 nm-10 nm range. The modified biochar promotes the retention of surface O-C=O and C=O oxygen-containing functional groups, and the modified biochar prepared by this invention still maintains good strength. This invention also provides a method and apparatus for treating phosphorus-containing wastewater with modified biochar. Intermittent adsorption reactions of the biochar show that the adsorption performance of the modified biochar of this invention is superior to that of unmodified biochar, with a saturated adsorption capacity between 18 and 20 mg-P / g-BC, making it suitable for use in wastewater treatment plants.

Description

Technical Field

[0001] This invention belongs to the field of resource recycling and wastewater treatment technology, and relates to a modified biochar, its preparation method and uses, specifically a modified biogas residue granular biochar, its preparation method and uses. Background Technology

[0002] Controlling phosphorus pollution is a key factor in preventing eutrophication of water bodies. Meanwhile, as phosphate rock is a non-renewable resource, how to recover and utilize phosphorus from water bodies has become a major research focus. Biochar, a carbon-rich material derived from the pyrolysis of biomass, has been extensively studied as an adsorbent for various pollutants in water. Biochar removes phosphorus from water through different mechanisms such as surface complexation, precipitation, ion exchange, and electrostatic attraction. The phosphorus removal efficiency of biochar depends on its characteristics and environmental factors. Furthermore, biochar can be functionalized or modified to improve its adsorption performance. Phosphorus-adsorbed biochar can be used as a slow-release fertilizer for soil, promoting a healthy phosphorus cycle.

[0003] Biogas residue is a byproduct of anaerobic digestion of kitchen waste. It contains complex components such as high salt content and high organic matter, limiting its direct utilization. However, biogas residue contains 50% to 70% carbon by dry mass, as well as a certain proportion of calcium (Ca) and iron (Fe). This makes the preparation of biochar from pyrolyzed biogas residue a valuable research method for waste reduction and high-value utilization. Biochar, due to its abundant functional groups and porous structure, has been proven to have phosphorus (P) adsorption properties. Considering the non-renewable nature and wide applications of phosphorus, compared to other phosphorus removal methods, biochar prepared from biomass offers advantages in phosphorus adsorption that is simple, effective, and cost-efficient. Furthermore, the adsorbed phosphorus can be reused, meeting certain phosphate rock demand. Current research on biochar phosphorus adsorbents mainly focuses on small-particle powder form. However, powder adsorbents are prone to loss during use, and their diffusion into larger water bodies can cause potential adverse effects. In addition, biogas residue naturally forms aggregates after pyrolysis; therefore, it is necessary to prepare granular biogas residue biochar for adsorption research. Compared to powder, granular form loses some of its adsorption performance. In order to better apply granular biochar to phosphorus adsorption, it is necessary to conduct relevant research to select appropriate modification methods for the preparation process of biochar. Summary of the Invention

[0004] To improve the above-mentioned technical problems, the present invention provides a modified biochar, comprising biochar and a calcium-containing compound supported on the biochar.

[0005] According to an embodiment of the present invention, the loading of the calcium-containing compound is 10 to 30% of the mass of the modified biochar, for example, 13.3%, 19.5%, or 24.8%.

[0006] According to an embodiment of the present invention, the calcium-containing compound is at least one of CaO and Ca(OH)2.

[0007] According to an embodiment of the present invention, the modified biochar is in granular form. For example, the modified biochar has a diameter of approximately 4 mm.

[0008] According to an embodiment of the present invention, the modified biochar has a carbon content of not less than 5%, for example, 8.205% to 11.935%.

[0009] According to an embodiment of the present invention, the modified biochar has an average pore volume of not less than 0.05 cm³. 3 / g, for example, 0.09 cm 3 / g, 0.11 cm 3 / g.

[0010] According to an embodiment of the present invention, the modified biochar has an average specific surface area of ​​not less than 50 m². 2 / g, preferably 50-110 m 2 / g, for example, 57.12 m 2 / g、58.13 m 2 / g or 75.32 m 2 / g.

[0011] According to an embodiment of the present invention, the modified biochar has an average pore size of 5.5-10 nm, preferably 5.5-6.5 nm.

[0012] According to an embodiment of the present invention, the X-ray powder diffraction (XRD) pattern of the modified biochar has a characteristic diffraction peak at a 2θ angle of 31.6°.

[0013] According to an embodiment of the present invention, the calcium-containing compound is filled in the mesopores of biochar, preferably in the mesopores in the 4 nm to 10 nm region.

[0014] According to an embodiment of the present invention, the modified biochar is prepared by mixing biogas residue with calcium carbonate and a thickener solution, followed by granulation and activation treatment.

[0015] The present invention also provides a method for preparing the modified biochar, comprising mixing biogas residue with calcium carbonate and a thickener solution, granulating, and activating to obtain the modified biochar.

[0016] According to an embodiment of the present invention, the mass ratio of biogas residue to calcium carbonate is (1-2):(8-9), for example 1:9, 1.5:8.5 or 2:8.

[0017] According to an embodiment of the present invention, the amount of the thickener solution is 10-80% of the total mass of biogas residue and calcium carbonate, for example, 55 wt%.

[0018] According to an embodiment of the present invention, the concentration of the thickener solution is 5-10 mg / mL, for example, 8.3 mg / mL.

[0019] According to an embodiment of the present invention, the thickener is calcium hydroxymethyl cellulose.

[0020] According to an embodiment of the present invention, the biogas residue is preferably biogas residue from kitchen waste.

[0021] According to an embodiment of the present invention, the holding temperature of the activation treatment is 700-900°C, for example 850°C; and the holding time of the activation treatment is 0.5-2 h, for example 1 h.

[0022] According to an embodiment of the present invention, the activation treatment is carried out in a nitrogen atmosphere.

[0023] According to an embodiment of the present invention, the rate of heating to the holding temperature of the activation treatment is 1 to 15 °C / min, for example 10 °C / min.

[0024] According to an embodiment of the invention, the granulation is carried out in a pelletizing machine. For example, the diameter of the granulated pellet is approximately 4 mm.

[0025] This invention also provides the application of the above-described modified biochar in the treatment of phosphorus-containing wastewater. For example, its application in the adsorption and removal of phosphate-containing wastewater.

[0026] According to an embodiment of the present invention, the modified biochar, upon contact with phosphorus-containing wastewater, forms Ca3H2PO4. 14 Preferably, the X-ray powder diffraction (XRD) pattern of the modified biochar after contact with phosphorus-containing wastewater exhibits a characteristic diffraction peak at a 2θ angle of 26.2 ± 2°. The present invention also provides a method for treating phosphorus-containing wastewater with the above-mentioned modified biochar, comprising contacting the modified biochar with phosphorus-containing wastewater.

[0027] Preferably, the method for treating phosphorus-containing wastewater with the modified biochar includes adding the modified biochar to the phosphorus-containing wastewater and shaking it for 12-48 hours for adsorption.

[0028] According to an embodiment of the present invention, the pH value of the phosphorus-containing wastewater is 6-9 at room temperature.

[0029] The present invention also provides an apparatus for treating phosphorus-containing wastewater, comprising an adsorption column filled with the aforementioned modified biochar.

[0030] The beneficial effects of this invention are:

[0031] (1) This invention modifies biogas residue with calcium carbonate. Biogas residue itself is a raw material containing calcium and phosphorus. After activation treatment, both Ca and P can form stable components. Compared with CaO and calcium hydroxide, calcium carbonate is insoluble in water, stable in properties, and non-corrosive. During the modification process of biogas residue, calcium carbonate is transformed into an active component mainly composed of calcium oxide after heat treatment and fills the mesopores in the 4 nm to 10 nm range of biochar. Calcium hydroxide, which is generated when it comes into contact with water, is slightly soluble in water. Therefore, the modified biochar of this invention has high chemical adsorption potential and is not easily leached. At the same time, calcium carbonate generates carbon dioxide, which reacts with carbon in situ to activate the surface and create pores (equivalent to adding gas during pyrolysis). Biochar modified with calcium carbonate promotes the retention of oxygen-containing functional groups OC=O and C=O on the surface. The strength properties of the modified biochar are weakened, and the modified biochar (sample 2) prepared with a calcium compound loading of 10 wt% still maintains good strength properties. Intermittent adsorption reactions of biochar showed that the modified biochar (sample 2) exhibited superior adsorption performance compared to unmodified biochar, with a saturated adsorption capacity between 18 and 20 mg-P / g-BC. It showed optimal adsorption performance in the pH range of 6 to 9, making it suitable for use in the pH environment of wastewater treatment plants. Furthermore, the modified biochar of this invention is granular, which, compared to powdered biochar, is less prone to caking, clogging, and suspended solids generation during adsorption. Moreover, the preparation process of the modified biochar of this invention does not generate waste liquid or residue, nor does it introduce pollutants (such as calcium chloride, which introduces chlorine and other elements that may generate harmful substances at high temperatures). Therefore, the preparation process is more environmentally friendly and suitable for large-scale treatment of kitchen waste biogas residue.

[0032] (2) After adsorption, SEM observation showed a film coating on the surface of the biochar. EDS scanning showed a high degree of overlap of the three elements Ca, O, and P. Digestion of the biochar also revealed a decrease in the Ca content after adsorption. The change in pore size confirmed that the pore filling in the 4 nm to 6 nm range of the modified biochar was partially retained after washing. This may indicate that CaO and biochar formed a composite structure. Compared with the unmodified biochar, the modified biochar after phosphorus adsorption showed a greater decrease in both the average specific surface area and the average pore volume than the unmodified biochar at the same value. This confirms that the calcium sites formed by the modification enhanced the capture of phosphate, making the pore structure more effectively involved in the adsorption process.

[0033] (3) The FTIR results showed that after adsorption, the Ca-O stretching vibration from CaO was weakened, while that from H2PO4 was weakened. -The -OH stretching vibration was enhanced, and the enhancement effect of modified biochar was more significant compared with that of unmodified biochar. Similarly, XRD analysis showed that the characteristic diffraction peaks of CaMg(CO3)4 or C2H6CaO7 at 31.6° were weakened after phosphate adsorption, while those of Ca3H2PO4 at 26.2° were reduced. 14 The enhanced characteristic diffraction peaks indicate that calcium plays a mediating role in the adsorption process, suggesting that Ca-OP is formed between phosphate and active sites on the calcium-based surface.

[0034] (4) XPS results showed that the P2p peak center of all samples shifted to between 133 eV and 134 eV after adsorption. Different samples carried phosphates of almost the same valence state. Among them, the P2p peaks of unmodified biochar after adsorption of phosphate (sample 1 - after adsorption) and modified biochar prepared with a calcium carbonate loading of 10 wt% after adsorption of phosphate (sample 2 - after adsorption) were significantly different. 3 / 2 PO4 3- / Ca(H2PO4)2、P2p 1 / 2 CaHPO4 / HPO4 2- The increases were 8% and 18% respectively, with modified biochar showing a more significant improvement; after adsorption, CaO in O1s disappeared, and Ca was released. 2+ The reaction with phosphorus produces O=P / / MOP / COP / P-OH and ACP, which, together with the MOP / M-OHP appearing after adsorption at P2p, confirms the formation of coordinate bonds between calcium and phosphorus. The reduction of -OH further confirms the complexation effect of the hydroxyl group and phosphate ligand exchange process. The formed HAP confirms the surface precipitation effect of biochar and phosphate. In Ca2p, after adsorption by modified biochar, Ca-OC forms an inner spherical complex of Ca-OP during the reaction, consistent with the phenomena observed at P2p and O1s. XPS further confirms that modification increases the active adsorption sites of calcium, promoting complexation and surface precipitation. Attached Figure Description

[0035] Figure 1 In the middle (a) and (b), the pyrolysis curves and yields of the modified biochar prepared in Examples 1-3 and Comparative Example 1 are respectively. Figure 1 The upper and lower curves in (a) represent the mass change and mass change rate results of the modified biochar prepared in Examples 1-3 and Comparative Example 1, respectively.

[0036] Figure 2 XPS-C1s fitting diagrams of the modified biochar prepared in Examples 1-3 and Comparative Example 1.

[0037] Figure 3XPS-Ca2p fitting diagrams of the modified biochar prepared in Examples 1-3 and Comparative Example 1.

[0038] Figure 4 In the middle (a) and (b), respectively, the pore size distribution diagram and N2 adsorption-desorption curve of the modified biochar prepared in Examples 1-3 and Comparative Example 1 are shown.

[0039] Figure 5 In the figures (a) and (b), the average pore volume and average specific surface area of ​​the modified biochar prepared in Examples 1-3 and Comparative Example 1 are respectively.

[0040] Figure 6 The average pore size is given by the modified biochar prepared in Examples 1-3 and Comparative Example 1.

[0041] Figure 7 The adsorption capacity of the modified biochar prepared in Example 1 under different pH conditions is shown.

[0042] Figure 8 The adsorption kinetics of the modified biochar prepared in Example 1 and Comparative Example 1 are fitted.

[0043] Figure 9 The figures show the adsorption isotherm reactions of the modified biochar prepared in Example 1 and Comparative Example 1; the insets in the figures show the fitting results of the isotherm models of the modified biochar prepared in Example 1 and Comparative Example 1.

[0044] Figure 10 The changes in surface morphology of the modified biochar prepared in Examples 1-3 and Comparative Example 1 before and after adsorption are shown.

[0045] Figure 11 In the figures (a), (b), (c), and (d), the EDS scan results of P, Fe, O, and Ca on the surface of the modified biochar prepared in Example 1 after adsorption are shown.

[0046] Figure 12 The modified biochar prepared in Examples 1-3 and Comparative Example 1 has a cation composition before and after adsorption.

[0047] Figure 13 Nitrogen adsorption-desorption curves after adsorption by the modified biochar prepared in Examples 1-3 and Comparative Example 1.

[0048] Figure 14 In the figures (a), (b), and (c), the changes in average specific surface area, average pore volume, and average pore size of the modified biochar prepared in Example 1 and Comparative Example 1 after washing (sample 2-washed, sample 1-washed) and after adsorption (sample 2-after adsorption, sample 1-after adsorption), respectively.

[0049] Figure 15The number of pores with different sizes is shown in the modified biochar prepared in Comparative Example 1 (Sample 1 - before adsorption) after washing (Sample 1 - after washing) and after adsorption (Sample 1 - after adsorption).

[0050] Figure 16 The number of pores with different sizes is shown in the modified biochar prepared in Comparative Example 1 after washing (sample 1 - washed) and the modified biochar prepared in Example 1 after washing (sample 2 - washed) and after adsorption (sample 2 - after adsorption).

[0051] Figure 17 FTIR characterization of the modified biochar prepared in Examples 1-3 and Comparative Example 1 before adsorption (Sample 2, Sample 3, Sample 4, Sample 1) and after adsorption (Sample 2 - after adsorption, Sample 3 - after adsorption, Sample 4 - after adsorption, Sample 1 - after adsorption).

[0052] Figure 18 The ratio of phosphorus in the three phases (solution, modified biochar adsorbent, and precipitate) after adsorption by the modified biochar prepared in Examples 1-3 and Comparative Example 1.

[0053] Figure 19 The adsorption curves of the modified biochar prepared in Example 1 are shown under the upper and lower limit experimental conditions.

[0054] Figure 20 In Figure (a), the surface state of the modified biochar prepared in Example 1 after adsorption at 7 g-10 mg / L-1.6 mL / min for 24 hours and 48 hours is shown. Figure 20 (b) shows the adsorption state of the modified biochar prepared in Example 1 after 24 hours and 48 hours of adsorption at a rate of 5 g-30 mg / L-3 mL / min.

[0055] Terminology Definitions and Explanations

[0056] Unless otherwise defined, all scientific and technical terms herein have the same meaning as commonly understood by one of ordinary skill in the art to which the subject matter of the claims pertains. In the specification and claims of this application, unless otherwise stated, scientific and technical terms used herein have the meanings commonly understood by one of ordinary skill in the art. However, for a better understanding of this disclosure, definitions and explanations of some related terms are provided below. Furthermore, in the event of any discrepancy between the definitions and explanations of terms provided herein and the meanings commonly understood by one of ordinary skill in the art, the definitions and explanations provided herein shall prevail.

[0057] The term “at least one” should be understood as “one, two or more”, and “more” should refer to ≥3, such as 3, 4, 5, 6, 7, 8, 9 or 10.

[0058] The term "room temperature" refers to room temperature in the conventional sense in this field, generally 10 to 30°C, preferably 25°C ± 5°C, such as 20°C, 25°C, or 30°C. Detailed Implementation

[0059] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are merely illustrative and explanatory of the present invention, and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0060] Unless otherwise stated, the raw materials and reagents used in the following examples are commercially available products or can be prepared by known methods.

[0061] The main experimental reagents used in this invention are summarized in Table 1.

[0062] Table 1 Summary of Experimental Reagents and Materials

[0063]

[0064] To characterize the effects of modification on biochar in terms of pore size distribution, specific surface area, and functional groups, the modified biochar particles were characterized by nitrogen adsorption-desorption, FTIR, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS).

[0065] Scanning electron microscopy and X-ray energy dispersive spectroscopy (SEM-EDS): A scanning electron microscope (SUPRA 55 SAPPHIRE, Germany) was used to characterize the microstructure of the sample surface. Four samples—sample 1, sample 2, sample 3, sample 4, and their adsorbed samples (sample 1 after adsorption, sample 2 after adsorption, sample 3 after adsorption, and sample 3 after adsorption)—were subjected to a 30-second gold sputtering treatment to improve their conductivity. They were then fixed to the instrument's sample stage with adhesive tape and observed using a SEM lens at 100x and 1200x magnification, respectively. In parallel, an EDS (energy dispersive spectroscopy) device was used to analyze the apparent elemental distribution of C, O, P, Ca, Fe, Mg, and Al in the samples.

[0066] Thermogravimetric analysis (TG): A thermogravimetric analyzer (TGA-50, Shimadzu, Japan) was used to characterize the weight loss of the sample in different temperature ranges and to analyze the thermal decomposition characteristics of the sample. 15 mg–20 mg of sample powder was placed in a sample pan. The heating range was 30 °C–900 °C at a heating rate of 10 °C / min, and the N2 flow rate was 10 mL / min. After the reaction, the TG curve showing the mass change of the sample with temperature, and the first derivative (DTG) curve of the thermogravimetric curve with respect to time were plotted.

[0067] Material hardness testing: The hardness of the biochar was tested using a universal testing machine (AG-X plus 100KN Shimadzu, Japan).

[0068] X-ray photoelectron spectroscopy (XPS): This method uses an X-ray photoelectron analyzer (Escalab Xi + Thermo Fisher, USA) to study the molecular structure and atomic valence states of samples by measuring the binding energy of photoelectrons excited by X-ray radiation. The basic testing parameters of this invention are as follows: Al-Kα is used as the radiation source (…). hν =1486.6 eV), pressure range in the range of 1×10⁻⁶ eV. -9 mbar to 7×10 -9 mbar. Peaks of C1s, O1s, Ca2p, and P2p were obtained before and after adsorption of the modified biochar. Energy dispersive spectroscopy (EDS) correction was performed using the C1s peak at a binding energy of 284.8 eV, and peak fitting was performed using Avantage software (Thermo Fisher, USA).

[0069] FT-IR Spectroscopy: This invention uses a Fourier transform infrared spectrometer (IRTRACER-100, Japan) to determine the surface functional groups of modified biochar. By receiving vibrational information of molecular bonds, the chemical bonds and functional groups contained in the sample are determined. Before testing, KBr and biochar powder samples were dried at 105°C. 1 mg of biochar sample and 100 mg of KBr sample were mixed and ground in a mortar. First, pure KBr was used as a blank sample for noise (background) subtraction. Then, the disc containing the sample and KBr was pressed at 10 MPa for 1 to 2 minutes and placed in the infrared spectrometer for FTIR analysis. The scanning range of the sample was 600 cm⁻¹. -1 ~4000 cm -1 The resolution is 2.00 cm. -1 .

[0070] Elemental analysis: The content of C, H, O, N, S and other elements in biochar was determined using an elemental analyzer (PerkinElme 2400, USA). The biochar was dried in an oven at 105℃, and 1.5 mg to 3 mg of the dried biochar powder was weighed, tightly wrapped in aluminum foil, and placed into the sample cell of the instrument for analysis.

[0071] Raman spectroscopy: A Raman spectrometer (Renishaw 287Q00, UK) was used to determine the graphitization structure and defects of biochar by excitation light waves to characterize the carbon structure information of the samples. Specific parameters were as follows: laser source set to 532 nm, resolution 500 nm, laser exposure time 10 s, and focus on the central region at 1100 cm⁻¹. -1After performing a surface scan, removing noise, and normalizing the data, spectral information at that wavelength position is collected.

[0072] Example 1

[0073] The method for preparing modified biochar includes mixing 1g of calcium carbonate with 9g of biogas residue powder, adding 5.5g of calcium carboxymethyl cellulose thickener solution (prepared by adding 1g of calcium carboxymethyl cellulose to 120 mL of ultrapure water), forming a mud cake, and then using a pelletizing machine to manufacture uniformly sized granular biogas residue particles. The temperature is then raised to 850℃ in a N2 atmosphere at a heating rate of 10℃ / min, held for 1 hour, and then allowed to cool naturally to room temperature in a nitrogen atmosphere. After pyrolysis, modified biochar is obtained with a mass of approximately 28mg~35mg and a diameter of approximately 4mm, named Sample 2 (also known as Sample 2 - before adsorption).

[0074] Example 2

[0075] The only difference from Example 1 is the mixing mass ratio of calcium carbonate and biogas residue powder: 1.5g of calcium carbonate and 8.5g of biogas residue powder are mixed to obtain modified biochar with a mass of about 28 mg to 35 mg and a diameter of about 4 mm, which is named Sample 3 (also known as Sample 3 - before adsorption).

[0076] Example 3

[0077] The only difference from Example 1 is the mixing mass ratio of calcium carbonate and biogas residue powder: 2g of calcium carbonate and 8g of biogas residue powder are mixed to obtain modified biochar with a mass of about 28 mg to 35 mg and a diameter of about 4 mm, which is named Sample 4 (also known as Sample 4 - before adsorption).

[0078] Comparative Example 1

[0079] The only difference from Example 1 is the mixing mass ratio of calcium carbonate and biogas residue powder: 0g of calcium carbonate and 10g of biogas residue powder are mixed to obtain pre-adsorption biochar without calcium compounds, with a mass of about 28 mg to 35 mg and a diameter of about 4 mm, named Sample 1 (also known as Sample 1 - pre-adsorption).

[0080] The pyrolysis curves and yields of the modified biochar prepared in Examples 1-3 and Comparative Example 1 are as follows: Figure 1 As shown in (a) and (b), the results indicate that the yield of the modified biochar is higher than the theoretical yield. Figure 1 The line graph in (b) shows that the addition of calcium carbonate promoted the retention of biochar after pyrolysis. This may be because calcium reacts with some organic compounds during pyrolysis, promoting the retention of oxygen-containing functional groups.

[0081] XPS-C1s fitting results of the modified biochar prepared in Examples 1-3 and Comparative Example 1 are as follows: Figure 2 As shown in the figure, the results indicate that the proportion of C=O first increases with the degree of modification, reaching a maximum of 22% in sample 2. Then, with further increases in modification, the proportion of C=O begins to decrease, reaching 9% and 5% in samples 3 and 4, respectively. The calcium carbonate used for modification undergoes a decomposition reaction during pyrolysis, generating carbon dioxide. This carbon dioxide may react with the biochar skeleton in a redox reaction, potentially eroding the carbon skeleton and promoting the formation of more oxygen-containing functional groups. Therefore, the proportion of C=O increases with the degree of modification. However, C=O is less stable, and when carbon dioxide levels increase further (in samples 3 and 4), it readily reacts with C=O, leading to a decreasing trend in C=O proportion.

[0082] The XPS-Ca2p fitting results of the modified biochar prepared in Examples 1-3 and Comparative Example 1 are as follows: Figure 3 As shown in the figure. The results show that the modification process causes calcium species to shift to positions with lower binding energies. Therefore, the CaO / Ca(OH)2 ratio of all modified biochars is higher than that of unmodified biochar (sample 1). Among them, sample 2 shows Ca-OC / Ca-OP with binding energies of 348.5 / 351.9 eV, while samples 3 and 2 show other calcium species with binding energies of 345.9 eV / 349.5 eV. The results of C1s show that as the proportion of calcium carbonate increases, the overmodified biochar will undergo different changes, such as the formation of phosphates that are bound to calcium in different forms.

[0083] The pore size distribution and nitrogen adsorption-desorption curves of the modified biochar prepared in Examples 1-3 and Comparative Example 1 are shown below. Figure 4 As shown in (a) and (b), the results indicate that the nitrogen adsorption capacity decreases with increasing calcium carbonate content after modification.

[0084] The average pore volume, average specific surface area, and average pore size of the modified biochar prepared in Examples 1-3 and Comparative Example 1 are as follows: Figure 5 (a), (b) and Figure 6As shown in the figure. The results indicate that the average pore volume, average specific surface area, and average pore size decreased after biochar modification. These results suggest that pore filling occurred during the modification process, and the filling occurred within smaller pore sizes. Combined with the pore size distribution diagram, it can be seen that the number and volume of all types of pores decreased. The number of mesopores in the unmodified biochar of Comparative Example 1 was greater than that in the modified biochar of Examples 1-3, and the difference in the number of mesopores between the modified biochar of Sample 3 and Sample 4 further decreased, indicating that the CaO formed by the pyrolysis after modification mainly filled the mesopores, especially the 4 nm to 6 nm mesopore region. Compared with Sample 2, the average BET specific surface area of ​​Sample 1 decreased by approximately 34%, from 111.18 nm. 2 / g decreased to 73.52 m of sample 2. 2 / g, the average pore volume decreased significantly by 20%, from 0.15 cm³ in sample 1. 3 / g decreased to 0.11 cm of sample 2. 3 / g. The average pore size increased by 12.9%, from 5.314 nm in sample 1 to 5.999 nm in sample 2. This indicates that the pores were filled during the modification process. From Figure 6 The pore size distribution diagram shows that the number of pores in the entire 2 nm to 10 nm region of the modified biochar, especially in the 4 nm to 10 nm region, is significantly reduced, thus leading to an increase in the average pore size after modification. This may be because calcium carbonate decomposes at the preparation temperature to form calcium oxide, and the resulting modified material fills the mesopores in the 2 nm to 10 nm region, thereby significantly reducing the BET specific surface area and pore volume of the biochar.

[0085] Application examples

[0086] 1. Adsorption experimental methods

[0087] The phosphorus solution required for the experiment was prepared using potassium dihydrogen phosphate. 50 mL of the reaction solution was added to a 150 mL conical flask and the reaction was carried out on a shaker at 140 rpm for 24 hours. The concentration of the phosphorus solution was then measured. The specific experimental parameters are as follows: (1) Adsorption isotherm: Phosphorus solutions with concentrations of 10 mg / L, 20 mg / L, 30 mg / L, 40 mg / L, 50 mg / L, 80 mg / L, 100 mg / L, and 200 mg / L were prepared and the pH was adjusted to between 6.5 and 7. 50 mL of the above concentrations of solution were taken into a 150 mL conical flask. 90 ± 5 mg of biochar adsorbent was weighed using an analytical balance and added to the conical flask. The flask was placed in a constant temperature shaking oven and shaken at 140 r / min for 24 hours. The sample was then taken out and filtered through a 0.2 μm filter membrane. The phosphorus concentration was then determined by ion chromatography. (2) Adsorption kinetics: Prepare a 50 mg / L phosphorus solution. Take 50 mL of the above concentration solution into a 150 mL Erlenmeyer flask. Weigh 90 ± 5 mg of biochar adsorbent using an analytical balance and add it to the Erlenmeyer flask. Place the flask in a constant temperature shaking incubator and shake at 140 r / min for 24 hours. Then, take out the sample and filter it through a 0.2 μm filter membrane. Use ion chromatography to determine the phosphorus concentration. (3) Effect of adsorption pH: Prepare a 50 mg / L phosphorus solution. Use 1 mol / L HCl and NaOH solutions to adjust the pH of the phosphorus solution to 3, 4, 5, 6, 7, 8, 9, 10, and 11, respectively. Weigh 90 ± 5 mg of biochar adsorbent using an analytical balance and add it to the Erlenmeyer flask. Place the flask in a constant temperature shaking incubator and shake at 140 r / min for 24 hours. Then, take out the sample and filter it through a 0.2 μm filter membrane. Use ion chromatography to determine the phosphorus concentration.

[0088] 2. Data Analysis Methods

[0089] Adsorption capacity calculation:

[0090]

[0091] In the formula, q (mg / g) is the adsorption amount, V (L) is the solution volume, C0 (mg / L) is the initial concentration of the phosphorus solution, and C e (mg / L) is the equilibrium concentration of the phosphorus solution, and m(g) is the dosage of biochar adsorbent.

[0092] Figure 7The adsorption performance of the modified biochar prepared in Example 1 as an adsorbent at different pH values ​​is shown. The adsorption capacity of the modified biochar under different pH conditions shows that the adsorbent has the best adsorption capacity in the initial pH range of 6–9, approaching 20 mg-P / g-BC, with an equilibrium pH of around 11. The pH of phosphorus-containing wastewater is typically between 5 and 8, which is basically consistent with the pH range of wastewater treatment plants, demonstrating the application potential of the adsorbent in water treatment plants. The equilibrium pH being higher than the initial pH value may be related to ligand exchange between the surface hydroxyl groups and phosphate groups, or the partial release of Ca from the modified calcium component during adsorption. 2+ When introduced into water, it causes the water to become more alkaline. The adsorption performance of the adsorbent decreases at pH levels greater than 8 because, in a highly alkaline environment, hydroxyl (-OH) ions compete with negatively charged phosphate ions for positively charged adsorption sites. Furthermore, higher pH levels are unfavorable for protonation of the adsorbent surface, hindering electrostatic attraction to negatively charged phosphate ions. A further decrease in adsorption performance was also observed in the lower pH range (pH = 3–6).

[0093] Figure 8 The adsorption kinetics fitting results of the modified biochar prepared in Example 1 and Comparative Example 1 as adsorbents are shown. Pseudo-first-order and pseudo-second-order models were used for fitting. For the sample after adsorption by the adsorbent prepared in Example 1 (named Sample 2 - after adsorption), R... 2 The values ​​were 0.78 and 0.90 respectively, indicating that sample 2 showed a higher fit to the pseudo-second-order kinetic model after adsorption; for sample 1 after adsorption, the R values ​​for pseudo-first-order and pseudo-second-order kinetic models were... 2 The values ​​are 0.36 and 0.60, respectively. Among them, the R1 of the kinetic model after adsorption for sample 1 is... 2 The lower adsorption performance of sample 2 is mainly attributed to the lower adsorption performance of unmodified biochar, which reaches saturation in a short time and cannot fit well with the kinetic model.

[0094] Figure 9 The adsorption isotherms of the modified biochar prepared in Example 1 and Comparative Example 1 as adsorbents are shown. The results indicate that the modified biochar obtained in Example 1 (Sample 2) has significantly improved adsorption performance compared to the unmodified biochar in Comparative Example 1 (Sample 1). In the equilibrium concentration range of 0-50 mg-P / L, the adsorption capacity increases with increasing equilibrium concentration. However, in the equilibrium concentration range of 50-120 mg-P / L, the adsorption capacity tends to weaken due to surface precipitation caused by high concentrations on the adsorbent surface, which is significantly different from powdered adsorbents. The goodness of fit (R0) of Sample 2 to the Langmuir model is shown. 2 A score of 0.994 is better, while Sample 1 does not show a high fit for either model.

[0095] Prepare a phosphorus solution with a concentration of 50 mg / L. Take 50 mL of the above-concentration solution into a 150 mL Erlenmeyer flask. Weigh 90±5 mg of the modified biochar prepared in Examples 1-3 and Comparative Example 1 as an adsorbent using an analytical balance. Add the biochar into the Erlenmeyer flask and place it in a constant temperature shaking oven. Shake at 140 r / min for 24 hours, and then take out the sample. Figure 10 The surface morphology changes of the biochar prepared in Examples 1-3 and Comparative Example 1 before and after adsorption, characterized by SEM, are shown. It was observed that the surface of Sample 1 prepared in Comparative Example 1 did not change significantly before and after adsorption. However, for the modified biochar prepared in Examples 1-3 (Samples 2, 3, and 4), the surface changed from rough to smooth, and a distinct coating appeared on the surface. The degree of coating increased with the increase of the amount of calcium carbonate modifier. Therefore, this coating may be the leaching of calcium-containing compounds generated during modification, forming compounds with phosphate on the adsorbent surface. During the actual experiment, a white coating continuously appeared on the surface of the modified biochar during the adsorption reaction.

[0096] The modified biochar prepared in Example 1 was characterized by EDS scanning after adsorbing phosphorus solution. The results are as follows: Figure 11 As shown, on the surface of the adsorbed sample (sample 2 - after adsorption), EDS scanning revealed a high degree of overlap between calcium, phosphorus, and oxygen in the biochar surface layer. In contrast, due to the presence of iron-based flocculants in the biochar residue, iron constituted a significant portion of the biochar surface; however, iron and phosphorus did not show a high degree of overlap. This suggests that phosphorus adsorption may be related to the presence of oxygen and calcium.

[0097] After digestion of biochar, the elemental content of cations was determined, and the results are as follows: Figure 12 As shown in the figure, the results illustrate the changes in the proportions of the top four elements (calcium, iron, aluminum, and magnesium) before and after adsorption. Consistent with the EDS scan results, Fe had the largest proportion, followed by Ca. Furthermore, the proportion of calcium in the modified biochar decreased significantly after adsorption, indicating that calcium ions may have dissolved during adsorption, participating in the reaction with phosphate ions and undergoing surface precipitation adsorption, thus being consumed.

[0098] In summary, during the adsorption process, calcium ions in the modified biochar of this invention leach out from the biochar and undergo an adsorption reaction with phosphate ions on the biochar surface, generating a calcium-phosphorus complex surface precipitate, thereby forming a coating material on the biochar surface mainly composed of calcium, oxygen, and phosphorus.

[0099] Figure 13The nitrogen adsorption-desorption curves of biochar prepared in Examples 1-3 and Comparative Example 1 are shown. The nitrogen adsorption capacity in the low-pressure range (P / P0 < 0.05) and the medium-pressure range (0.05 < P / P0 < 0.80) reflects the number of micropores and mesopores, respectively. It can be seen that, compared to the unmodified sample 1 after adsorption, the modified biochar after adsorption has fewer micropores and mesopores. Comparing the nitrogen adsorption-desorption curves before adsorption, it was found that the nitrogen adsorption capacity of sample 1 was enhanced after adsorption, while the nitrogen adsorption capacity of sample 2 did not change significantly before and after adsorption. The nitrogen adsorption capacity of sample 3 after adsorption was slightly improved compared to before adsorption. This further confirms the dissolution of soluble substances during adsorption and the pore occupancy caused by adsorption.

[0100] To verify whether the change in nitrogen adsorption-desorption capacity after adsorption is due to the dissolution of soluble substances during adsorption and the adhesion of phosphate to pores during adsorption, this invention compares the biochar sample 1 prepared in Comparative Example 1, the biochar sample 2 prepared in Example 1, and their washed samples (denoted as Sample 1-washed and Sample 2-washed, respectively) with the corresponding adsorbed samples (denoted as Sample 1-adsorbed and Sample 2-adsorbed, respectively). The specific operation process is as follows: Biochar sample 1 and Sample 2 formed after pyrolysis are placed in conical flasks of ultrapure water and shaken at 140 rpm for 24 hours. The washed samples are named Sample 1-washed and Sample 2-washed. Sample 1 and Sample 2 are placed in a 50 mg / L phosphorus solution and shaken for 24 hours. The samples obtained after adsorbing the phosphorus solution are named Sample 1-adsorbed and Sample 2-adsorbed, respectively.

[0101] Figure 14 This variation in pore size characteristics is illustrated in Table 2, which shows the results of data comparison between samples. The results indicate that, after washing, the average BET specific surface area of ​​biochar sample 1 (Comparative Example 1) increased by 32%, the average pore volume increased by 5%, and the average pore size decreased by 12% compared to before washing. (Observations) Figure 15 The pore size distribution diagram shows that after washing, the pore size in the 4-10 nm region of sample 1 decreased compared to before washing, while the pore size in the 2-4 nm region increased. This increase in the number of small pores is what led to the decrease in the average pore size (from 5.937 nm in sample 1 to 5.236 nm). This means that during the washing process, inorganic salts and some soluble substances inside the biochar were washed away, forming new microporous cavities and a more uneven internal pore size, resulting in an increase in specific surface area and pore volume. This process explains why some inorganic salts, soluble substances, and ash remain after the pyrolysis of unmodified biochar. A redistribution of pore size may occur during the adsorption process.

[0102] Table 2 Comparison of pore size characteristics of biochar samples 1 and 2 after washing and adsorption.

[0103]

[0104] Observing the average BET of all samples, we can see that the average BET specific surface area of ​​sample 2 is 82.25 m². 2 / g is significantly less than 128.54 m of sample 1. 2 / g, which reflects the difference in pore size distribution between the modified and unmodified samples. To determine whether the pore filling caused by modification is a complete and tight composite structure formed with the biochar, or whether there are components washed out during adsorption, the present invention performed pore size analysis on sample 2-wash, which was obtained by washing biochar sample 2 prepared in Example 1. It can be seen that the specific surface area and pore volume of sample 2 after washing increased by 34% and 13%, respectively, compared with those before washing. Moreover, the specific surface area and pore volume of sample 2 after washing are lower than those of sample 1 and sample 1-wash, that is, the pore filling generated by biochar modification has a substantial impact on the pore size structure of biochar, and this structure still exists after washing. In addition, the average pore size of sample 2 after washing decreased by 9% compared with those before washing, which is also lower than the average pore size of sample 1.

[0105] Figure 16 The pore size distributions of samples 1-washed, 2-washed, and 2-washed were compared. A comparison of samples 1 and 2 after washing revealed that the pore size distribution in the 2 nm–4 nm region of sample 2 did not change significantly before and after washing. However, the changes in pore size distribution in this region differed between samples 1-washed and 2-washed. This may be because the modification process restricted the outflow of this material, or the excess calcium oxide produced during modification dissolved and formed new Ca-P compounds with phosphorus in the biogas residue, thus clogging the pores. Furthermore, after washing, the number of pores in the mesoporous region beyond 3 nm increased compared to before washing; however, the number of mesopores in this region remained lower in sample 2-washed than in sample 1-washed. This indicates that the modification did indeed improve the phosphorus adsorption of biochar, and the active sites of the modified calcium structure enhanced the capture of phosphate ions, allowing the pore structure to participate more effectively in the adsorption process.

[0106] The present invention further uses Fourier transform infrared spectroscopy to scan the surface of the biochar prepared in Examples 1-3 and Comparative Example 1. The results are as follows: Figure 17 As shown, the FTIR results indicate that, compared to biochar sample 1 prepared in Comparative Example 1, the modification improved the biochar's viscosity at 500 cm⁻¹. -1 Up to 800 cm -1 The vibration peak in this section was significantly enhanced, especially at 875 cm⁻¹. -1 1420 cm -1 Characteristic peaks, which were enhanced with the amount of calcium modification added, were observed at all positions, especially at 530 cm⁻¹. -1The vibrational peak in this segment is mainly caused by the MO or M-OH groups of metal oxides, such as complex oxides like magnesium oxide, aluminum oxide, and calcium oxide. This vibrational peak weakens after adsorption, possibly because the oxides participate in the adsorption process, thus reducing the intensity of this characteristic peak. At 875 cm⁻¹ -1 and 1449 cm -1 The vibrational peak at this location originates from the Ca-O stretching vibration of CaO, which is weakened after adsorption, at 1420 cm⁻¹. -1 The sites primarily originate from the Ca-O stretching vibration / OH vibration of Ca(OH)₂, which also weakens after adsorption, indicating the formation of Ca-OP between the phosphate and the active sites on the calcium-based surface. These results are also confirmed by XPS. After adsorption, a 1626 cm⁻¹ site can be observed. -1 The characteristic peak at the location is significantly enhanced, and this characteristic peak mainly originates from H2PO4. - The -OH stretching vibration indicates that the modified biochar interacts with H2PO4 during adsorption. - Specific binding occurred, resulting in an enhancement of the vibrational peak of dihydrogen phosphate ions on the surface. Furthermore, at 1023 cm⁻¹... −1 Attributable to H2PO4 - or HPO4 2- The PO symmetric stretching vibration was present in all samples, consistent with XPS results, confirming the presence of phosphorus in the biogas residue. In summary, these results demonstrate that biochar successfully binds to phosphate, and that the calcium oxide-based composite oxide plays a crucial role in the adsorption process, combining with phosphate to form Ca-OP.

[0107] like Figure 18 The study demonstrated the proportion of phosphorus in the three phases after a batch adsorption reaction at a phosphorus concentration of 200 mg / L, revealing that sample 2- showed the highest phosphorus adsorption on its surface. This further confirms the influence of calcium as an active site in phosphorus adsorption. Furthermore, the adsorption process of phosphate is a complex process involving electrostatic attraction, ligand exchange to form inner spheroidal complexes, and finally, surface precipitation.

[0108] As shown in Table 3, the present invention first utilizes the cross-sectional area of ​​the continuous flow reaction column (approximately 3.14 cm²). 2 The column height (approximately 12 cm) and the sample bulk density (2.05 ± 0.1 cm³) were also considered. 3The bulk density of the sample was determined by [a method / g], and the adsorption dosage in the reaction column was determined to be between 5 g and 7 g. To observe the effect of different influent concentrations on long-term adsorption, the reaction concentration of the continuous flow was set to 10 mg / L, 20 mg / L, and 30 mg / L. To determine whether the saturation time and the changes in the adsorption curves of different experimental combinations were within acceptable ranges, three sets of experiments were designed: one representing the upper limit of the low adsorbent dosage-high concentration-high flow rate condition: "5 g-30 mg / L-3 mL / min"; another representing the lower limit of the high adsorbent dosage-low concentration-low flow rate condition: "7 g-10 mg / L-1.6 mL / min"; and the third representing the combination between these two conditions: "6 g-10 mg / L-1.6 mL / min".

[0109] Table 3 Bulk density of samples

[0110]

[0111] Adsorption curves of different experimental combinations are as follows Figure 19 As shown, this reflects that the saturation time of all experimental combinations will vary between approximately 1000 and 7000 minutes, the experimental combination with low dosage will change rapidly in the early stage of the reaction, and the breakthrough time (C) t The prediction (for the time point where / C0 is greater than 0.1) will occur between 0 and 3000 minutes.

[0112] The actual adsorption states under extreme conditions after 24 hours and 48 hours are as follows: Figure 20 (The green object in the image is the adsorption column clamp.) As shown, due to the leaching of calcium from the calcium-modified biochar, a large amount of white surface precipitate (mainly calcium phosphate salts including hydroxyapatite) forms on the surface of the biochar. It can be seen that in the experimental combination of "7 g-10 mg / L-1.6 mL / min", after 24 hours of reaction, the surface precipitate decreases from the bottom of the column to the top, indicating that there are still adsorption sites to be adsorbed. Figure 19 As shown, C at this time tThe CO / C ratio is approximately 0.1, meaning that about 90% of the influent phosphate is removed per unit time. Over the total reaction time of 1440 minutes, approximately 2.304 L (1440 min × 1.6 mL / min) of phosphate is bound by biochar (2.304 L × 10 mg / L × 90%). The binding capacity of each gram of biochar is approximately 2.96 mg-P / g-BC (calculated by dividing 20.7 mg by 7 g). After 48 hours, the surface precipitate increases overall and tends to move towards the top of the column. For the experimental combination of "5 g - 30 mg / L - 3 mL / min", a large amount of surface precipitate forms after 24 hours of reaction, with almost no change after 48 hours, indicating that a 5 g adsorbent dosage cannot maintain long-term adsorption performance in this scenario. Furthermore, the adsorption curve shows that when the adsorbent is exhausted, the CO / C ratio decreases. t After C0 reaches a stable value, the C content of the experimental combination is 5 g - 30 ppm - 3 mL. t The CO value is approximately 0.8, showing a significant difference from 0.6 for 6 g-10 ppm-1.6 mL / min. This difference is likely due to the varying degrees of occupancy of the adsorbent pores at different concentrations. Furthermore, the CO value after depletion of the powdered adsorbent... t The CO value is typically above 0.9, which indicates that the particulate adsorbent of the present invention has a longer service life.

[0113] The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiments. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method of producing a modified biochar, characterized by, The preparation method comprises mixing biogas residue and calcium carbonate and a thickening agent solution, granulating, and performing activation treatment to obtain the modified biochar; The mass ratio of the calcium carbonate to the biogas residue is 1:9; The amount of the thickening agent solution accounts for 10-80% of the total mass of the calcium carbonate and the biogas residue; and the thickening agent is calcium hydroxymethyl cellulose; The holding temperature of the activation treatment is 700-900 ℃; and the holding time of the activation treatment is 0.5-2 h; The activation treatment is performed in a nitrogen atmosphere.

2. The production method according to claim 1, wherein The modified biochar comprises biochar and a calcium-containing compound loaded on the biochar; The loading amount of the calcium-containing compound is 10-30% of the mass of the modified biochar; The calcium-containing compound is at least one of CaO and Ca(OH)2.

3. The production method according to claim 2, wherein The carbon content of the modified biochar is not less than 5%; The average pore volume of the modified biochar is not less than 0.05 cm 3 / g; The average specific surface area of the modified biochar is not less than 50 m 2 / g; The average pore size of the modified biochar is 5.5-10 nm; The calcium-containing compound is filled in mesopores in the range of 4 nm-10 nm of the biochar.

4. Application of the modified biochar prepared by the preparation method of any one of claims 1-3 to treatment of phosphorus-containing wastewater.

5. A method for treating phosphorus-containing wastewater using the modified biochar produced by the production method according to any one of claims 1 to 3, characterized in that, The method comprises contacting the modified biochar prepared by the preparation method of any one of claims 1-3 with the phosphorus-containing wastewater.

6. An apparatus for treating phosphorus-containing wastewater, characterized by comprising: The device comprises an adsorption column filled with the modified biochar prepared by the preparation method of any one of claims 1-3.

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