Phosphoric acid ball-milling synergistically modified reed straw biochar and preparation method thereof

Abstract

The application belongs to the technical field of biochar, and particularly relates to a phosphoric acid ball-milling and synergistic modification reed straw biochar and a preparation method thereof. The preparation method comprises the following steps: (1) raw material pretreatment: washing, drying, crushing and sieving the biomass to obtain biomass powder; (2) phosphoric acid activation: adding the biomass powder into a phosphoric acid solution, stirring and reacting, drying, placing in a muffle furnace, pyrolyzing, cooling to room temperature, and obtaining phosphoric acid activated biochar; (3) washing and drying: washing the phosphoric acid activated biochar to neutral, drying, and grinding to obtain purified activated biochar; (4) ball-milling modification: placing the purified activated biochar in a ball mill for ball-milling treatment to obtain the phosphoric acid ball-milling and synergistic modification reed straw biochar. The biochar prepared by the application has a high specific surface area, a larger pore volume, a large average pore diameter and a strong adsorption capacity.

Description

Technical Field

[0001] This invention belongs to the field of biochar technology, specifically relating to a phosphoric acid ball milling synergistic modification of reed straw biochar and its preparation method. Background Technology

[0002] Biochar, a carbon-rich material produced by the pyrolysis of biomass under oxygen-limited conditions, has become a hot topic in adsorbent research due to its wide availability of raw materials, low cost, and environmental friendliness. However, traditional biochar generally suffers from problems such as low specific surface area, underdeveloped pore structure, and lack of surface active functional groups, resulting in limited adsorption capacity for organic pollutants and making it difficult to meet practical application needs.

[0003] To improve the adsorption performance of biochar, various modification strategies have been proposed, such as chemical activation (e.g., acid and alkali treatment) and physical modification (e.g., ball milling). Phosphoric acid activation introduces phosphoric acid during biomass pyrolysis, promoting pore structure development and introducing phosphorus-containing functional groups. Ball milling modification reduces biochar particle size through mechanical force, increasing specific surface area and exposing more active sites. However, single modification methods often fail to simultaneously optimize both pore structure and surface chemistry.

[0004] Therefore, there is an urgent need for a method for synergistic modification of reed straw biochar by phosphate ball milling and its preparation. Summary of the Invention

[0005] The purpose of this invention is to provide a phosphoric acid ball milling synergistic modification of reed straw biochar and its preparation method. The biochar prepared by this invention has a high specific surface area, large pore volume, large average pore size, and strong adsorption capacity.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A method for preparing phosphoric acid ball milling synergistic modified reed straw biochar includes the following steps: (1) Raw material pretreatment: The biomass is washed, dried, crushed and sieved to obtain biomass powder; (2) Phosphoric acid activation: Biomass powder is added to phosphoric acid solution, stirred and reacted, dried and placed in muffle furnace for pyrolysis, and cooled to room temperature to obtain phosphoric acid activated biochar; (3) Washing and drying: The phosphoric acid activated biochar was washed with deionized water until neutral, dried, and ground to obtain purified activated biochar; (4) Ball milling modification: The purified activated biochar was placed in a ball mill and ball milled to obtain phosphate ball milling synergistic modification of reed straw biochar.

[0008] This invention achieves the regulation of the pore structure and surface chemical properties of biochar through the synergistic effect of phosphoric acid chemical activation and mechanical ball milling. During pyrolysis, phosphoric acid acts as a pore-forming agent, reacting with biomass components to generate gases that escape, etching abundant micropores and mesopores within the carbon framework. Subsequent ball milling breaks up the biochar particles using mechanical shear force, opening closed pores and exposing internal porosity, while simultaneously refining the particles. Through this synergistic effect, both processes ensure a high specific surface area while reducing the mass transfer resistance of large molecular pollutants, achieving a dual improvement in adsorption rate and adsorption capacity.

[0009] Preferably, the concentration of the phosphoric acid solution in step (2) is 1-10 mol / L.

[0010] Preferably, the pyrolysis conditions in step (2) are: a heating rate of 3-10℃ / min, heating to 400-500℃, and holding time of 1-3h.

[0011] Preferably, the mass ratio of purified activated biochar to ball milling media in the ball milling process in step (4) is 1:50-200.

[0012] Preferably, the ball milling speed is 200-500 r / min and the ball milling time is 0.5-2 h.

[0013] This invention has shown through experiments that phosphoric acid concentration and ball milling time have a significant impact on the adsorption performance of biochar. By optimizing phosphoric acid concentration and ball milling time, this invention achieves the optimal match between micropore and mesopore ratios, which retains the advantages of chemically activated functional groups while leveraging the dispersion effect of ball milling, thus comprehensively improving the performance of biochar.

[0014] Preferably, the ball mill in step (4) is a planetary ball mill, and the grinding media is agate balls.

[0015] Preferably, in step (1), the material is pulverized to a size of less than 80-120 mesh.

[0016] Preferably, the biomass includes at least one of wheat straw, pine powder, and reed straw.

[0017] The phosphate ball milling synergistic modification of reed straw biochar was prepared by the aforementioned method.

[0018] Preferably, the specific surface area of ​​the biochar is 297-542 m². 2 / g, pore volume is 0.199-0.237cm³ 3 / g, with an average pore size of 3.1-6.7nm.

[0019] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows: 1. This invention achieves the regulation of the pore structure and surface chemical properties of biochar through the synergistic effect of phosphoric acid chemical activation and mechanical ball milling. Through the synergistic effect of the two, a high specific surface area is ensured and the mass transfer resistance of macromolecular pollutants is reduced, thereby achieving a dual improvement in adsorption rate and adsorption capacity.

[0020] 2. Through experiments, this invention has found that phosphoric acid concentration and ball milling time have a significant impact on the adsorption performance of biochar. By optimizing phosphoric acid concentration and ball milling time, this invention achieves the optimal matching of micropore-mesopore ratio, which not only retains the advantages of chemically activated functional groups, but also leverages the dispersion effect of ball milling, thereby achieving a comprehensive improvement in the performance of biochar. Attached Figure Description

[0021] Figure 1 Scanning electron microscope (SEM) images of biochar prepared in Examples 1, 2, 3, and 8, wherein (a, b) are SEM images of BC prepared in Example 1, (c, d) are SEM images of Q-BC prepared in Example 2, (e, f) are SEM images of PBC-3 prepared in Example 3, and (g, h) are SEM images of Q-PBC prepared in Example 8. Figure 2 The images shown are scanning electron microscope (SEM) images of activated biochar prepared in Examples 3-7, where (a, b, c) are SEM images of PBC prepared in Examples 3-5, (a) is an SEM image of PBC-1; (b) is an SEM image of PBC-2; (c) is an SEM image of PBC-3; (d) is an SEM image of KBC-3 prepared in Example 6; and (e) is an SEM image of CBC-3 prepared in Example 7. Figure 3 (a) N2 adsorption-desorption isotherms of Examples 1 and 8; (b) Q-PBC N2 adsorption-desorption isotherms of Example 1.

[0022] Figure 4 The XRD patterns and FT-IR images of BC, Q-BC, PBC and Q-PBC obtained in Examples 1, 2, 3 and 8 are shown.

[0023] Figure 5 XPS spectra of BC, Q-BC, PBC and Q-PBC obtained in Examples 1, 2, 3 and 8 respectively, (a) is the full spectrum, (b) is C1s, (c) is O1s and (d) is P2p.

[0024] Figure 6The figures are related to the activated biochar prepared in Examples 3-7, where (a) is the adsorption efficiency spectrum of activated biochar for RhB, and (b) is the adsorption amount and removal rate spectrum of activated biochar.

[0025] Figure 7 The graph shows the effect of different ball milling times on the adsorption capacity and removal rate of Q-PBC prepared in Examples 8-11.

[0026] Figure 8 The graphs show the adsorption efficiency of Q-PBC activated with different H3PO4 concentrations for 100 mg / L RhB in Examples 8, 13-15.

[0027] Figure 9 The graphs show the adsorption capacity and removal rate of 100 mg / L RhB by Q-PBC activated with different H3PO4 concentrations in Examples 8, 12-15.

[0028] Figure 10 The figure shows the effect of RhB solutions with different pH values ​​on the adsorption efficiency of Q-PBC; (a) and (b) are the adsorption capacity and removal rate spectra at different pH values; (c) is the effect of the number of repeated regeneration cycles on the Q-PBC removal rate. The experimental conditions in the figure are: adsorbent dosage: 20 mg, pH: 5.0, RhB solution concentration: 100 mg / L. Detailed Implementation

[0029] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] The raw materials used in the following embodiments of the present invention are all commercially available products, as detailed in Table 1: Table 1. Reagents required for the experiment

[0031] The preparation methods for wheat straw powder, pine wood powder and reed straw powder are as follows: wheat straw, pine wood powder and reed straw are washed with water, dried and pulverized to less than 100 mesh.

[0032] All phosphoric acid solutions are aqueous solutions of phosphoric acid.

[0033] Example 1 This embodiment provides a primary biochar (BC) preparation method comprising the following steps: Step 1: Preparation of raw biochar (BC): Carbon materials were prepared by high-temperature pyrolysis. 15g of washed and dried reed stalks were weighed and placed in a crucible, covered with pure aluminum foil, and then pyrolyzed in a muffle furnace. Under oxygen-limited conditions (physically isolating air by wrapping with aluminum foil), the temperature was increased to 450℃ at a rate of 5℃ / min, held at that temperature for 2 hours, and then cooled to room temperature. The material was thoroughly washed with deionized water until the pH was neutral, and then dried in a forced-air drying oven at 60℃ for 12 hours. Finally, the material was ground in an agate mortar and passed through a 100-mesh sieve.

[0034] Example 2 This embodiment provides a ball-milled biochar (Q-BC), the preparation method of which includes the following steps: Step 1: Preparation of raw biochar (BC): Carbon materials were prepared by high-temperature pyrolysis. 15g of washed and dried reed stalks were weighed and placed in a crucible, covered with pure aluminum foil, and then pyrolyzed in a muffle furnace. Under oxygen-limited conditions, the temperature was increased to 450℃ at a rate of 5℃ / min, held at that temperature for 2 hours, and then cooled to room temperature. The material was thoroughly washed with deionized water until the pH was neutral, and then dried in a forced-air drying oven at 60℃ for 12 hours. Finally, the material was ground in an agate mortar and passed through a 100-mesh sieve.

[0035] Step 2: Preparation of ball-milled biochar (Q-BC): 1.0g of the prepared raw biochar BC was weighed and placed into a planetary ball mill. The material was placed into an agate sealed jar at a mass ratio of 1:100. After grinding at a rate of 300r / min for 8 hours, the material was taken out and marked as ball milled char Q-BC.

[0036] Example 3 This embodiment provides H3PO4 activated carbon (PBC), the preparation method of which includes the following steps: Weigh 15.0g of reed straw powder and add it to 1000mL of 1mol / L H3PO4 solution. Stir and react for 12h. After drying, heat the mixture in a muffle furnace to 450℃ at a heating rate of 5℃ / min and maintain the temperature for 2h. After cooling to room temperature, wash, dry, and grind to 100 mesh. The resulting H3PO4 activated carbon is labeled as PBC-3.

[0037] Example 4 This embodiment provides H3PO4 activated carbon (PBC), the preparation method of which includes the following steps: Weigh 15.0g of wheat straw powder and add it to 1000mL of 1mol / L H3PO4 solution. Stir and react for 12h. After drying, heat the mixture in a muffle furnace to 450℃ at a heating rate of 5℃ / min and maintain the temperature for 2h. After cooling to room temperature, wash, dry, and grind to 100 mesh. The resulting H3PO4 activated carbon is labeled as PBC-1.

[0038] Example 5 This embodiment provides H3PO4 activated carbon (PBC), the preparation method of which includes the following steps: Weigh 15.0g of pine wood powder and add it to 1000mL of 1mol / L H3PO4 solution. Stir and react for 12h. After drying, heat the mixture in a muffle furnace to 450℃ at a heating rate of 5℃ / min and maintain the temperature for 2h. After cooling to room temperature, wash, dry, and grind to 100 mesh. The resulting H3PO4 activated carbon is labeled as PBC-2.

[0039] Example 6 This embodiment provides a KOH-activated carbon (KBC) preparation method including the following steps: Weigh out 15.0g each of wheat straw powder, pine wood powder, and reed straw powder, and add them to 1000mL of 1mol / L KOH solution. Stir and react for 12h. After drying, heat the mixture in a muffle furnace to 450℃ at a heating rate of 5℃ / min and maintain the temperature for 2h. After cooling to room temperature, wash, dry, and grind to 100 mesh. The KOH activated carbons prepared using wheat straw powder, pine wood powder, and reed straw are labeled as KBC-1, KBC-2, and KBC-3, respectively.

[0040] Example 7 This embodiment provides H2C2O4 activated carbon (CBC), the preparation method of which includes the following steps: Weigh out 15.0g each of wheat straw powder, pine powder, and reed straw powder, and add them to 1000mL of 1mol / L H2C2O4 solution. Stir and react for 12h. After drying, heat the mixture in a muffle furnace to 450℃ at a heating rate of 5℃ / min and maintain the temperature for 2h. After cooling to room temperature, wash, dry, and grind to 100 mesh. The H2C2O4 activated carbons prepared from wheat straw powder, pine powder, and reed straw are designated as CBC-1, CBC-2, and CBC-3, respectively.

[0041] Example 8 This embodiment provides a ball-milled phosphate carbon (Q-PBC), the preparation method of which includes the following steps: Weigh 15.0g of reed straw powder and add it to 1000mL of 7.5mol / L phosphoric acid solution. Stir and react for 12h. After drying, heat the mixture in a muffle furnace to 450℃ at a heating rate of 5℃ / min and maintain the temperature for 2h. After cooling to room temperature, wash and dry the mixture, grind it to 100 mesh, weigh 1.0g of the powder and put it into a planetary ball mill. Place the powder into an agate sealed container at a mass ratio of material:ball = 1:100 and grind it at a rate of 300r / min for 0.5h. Remove the powder and label it as ball mill carbon Q-PBC.

[0042] Performance testing Performance tests were performed on the biochar prepared in Examples 1-8. 1. Characterization of biochar: SEM: The morphology of the material surface was observed at different magnifications using a German ZEISS Sigma360 scanning electron microscope; BET: The experiment used a specific surface area analyzer (ASAP2460 model) from Micromeritics, USA, to degas the biochar material at a degassing temperature of 120℃ for 12 hours. The specific surface area and pore size distribution of the biochar material were then measured and analyzed under a N2 atmosphere.

[0043] XRD: The crystal structure of the biochar material was analyzed using a Bruker D8 Advance X-ray diffractometer (Germany). The tests were performed using CuKα radiation, with a scan range of 5-90°, a scan rate of 2°C / min, an operating voltage of 40kV, and a current of 40mA.

[0044] XPS: Biochar materials are tested using a Thermo Scientific K-Alpha X-ray photoelectron spectrometer from the United States. By analyzing the photoelectron energy distribution on the material surface, the valence state and functional groups of different samples can be determined.

[0045] FTIR: Experiments were conducted using a Thermo Fisher Scientific Nicoleti S20 infrared spectrometer (USA) at 400-4000 cm⁻¹. -1 The biochar sample was scanned within the scanning range to analyze the chemical bonds and functional groups present in the sample.

[0046] 2. Rhodamine B adsorption experiment: First, take 100 mL of 100 mg / L RhB solution and place it in a beaker. Weigh the required amount of materials for the experiment and stir until adsorption equilibrium is reached under light-protected conditions. Then, take the solution at adsorption equilibrium and centrifuge it at 8000 r / min. Collect the supernatant and measure the absorbance of the solution using a spectrophotometer. Calculate the adsorption amount and pollutant removal rate using formulas 1 and 2.

[0047] ; ; In the formula: C0 is the initial concentration of pollutants, mg / L; C e V is the equilibrium concentration of pollutant adsorption, mg / L; V is the solution volume, mL; m is the adsorbent mass, mg; Q e R represents the amount of pollutant adsorbed, in mg / L; R represents the removal rate, in percentages.

[0048] pass Figure 1As can be seen, Figures (a) and (b) show BC produced by direct high-temperature calcination of reed straw, which retains the natural skeletal structure of the straw, with honeycomb-like pores and voids on the material surface. Figures (c) and (d) show Q-BC after ball milling. Ball milling causes the original skeletal structure of the material to collapse, the particle size to decrease and the distribution to become more uniform, new fracture surfaces to appear on the surface, and the specific surface area to increase. The surface of PBC biochar activated by H3PO4 shows obvious corrosion marks, and the surface roughness of the material is increased compared to BC. Because H3PO4 reacts with carbon under high temperature conditions to produce a large amount of CO2 and water vapor, the gas escapes and forms pores in the material. Therefore, although the overall skeletal structure is similar to that of the original carbon material, the porosity is significantly increased. Q-PBC modified by H3PO4 and ball milling has the advantages of both small particle size and high porosity, and the surface exhibits a multi-level pore coexistence feature, with the most complex morphology compared to the other three materials. This not only further exposes the internal pore structure and increases the specific surface area of ​​the material, but also provides more oxygen-containing and phosphorus-containing functional groups, providing more adsorption sites.

[0049] according to Figure 2 It can be seen that the surface roughness of PBC-1, PBC-2, and PBC-3 all increased, but the overall framework remained structurally intact. In particular, the straw fiber structure of PBC-3 was not damaged. However, the activation of KBC-3 and CBC-3 caused damage to the amorphous portion of the surface cellulose and fracture of the carbon skeleton, resulting in increased pores but limited pore connectivity. Table 2 shows that the specific surface area of ​​PBC-3 was slightly lower than that of PBC-2, but its pore volume and average pore size were significantly higher than the other samples. Combined with SEM images, it was found that the pore structure of PBC-3 was mainly mesoporous, with a hierarchical structure of micropores and mesopores, which is more conducive to pollutant adsorption. The high specific surface area of ​​PBC-2 is due to the dense woody structure of pine powder etching to generate a large number of micropores during phosphoric acid activation. The narrow channels of these micropores restrict the entry of large molecular pollutants, thus affecting the adsorption effect. Analysis shows that the cellulose-lignin composite structure of reed straw is more likely to form a stable mesoporous structure under the activation of H3PO4, while the vigorous reaction of KOH and H2C2O4 on reed straw disrupts the balance of pore development. This multi-factor coupling effect allows PBC-3 to achieve an optimal combination between pore size distribution and structural stability, resulting in good adsorption performance.

[0050] Table 2. Specific surface area and pore volume / pore diameter of PBC-1, PBC-2, PBC-3, KBC-3 and CBC-3

[0051] Analysis of nitrogen adsorption-desorption characterization tests and BET results for BC and Q-PBC revealed that the specific surface area of ​​BC is only 6.145 m². 2 / g, pore volume is 0.008cm 3 / g, with a pore size of 16.764nm, while Q-PBC, after phosphoric acid impregnation activation and ball milling treatment, showed a significant increase in nitrogen adsorption capacity and a specific surface area as high as 316.348m². 2 / g, pore volume is 0.331cm 3 / g, with a pore size of 7.380nm, demonstrating that phosphoric acid activation treatment has a significant impact on the regulation of pore size and pore structure in carbon materials. Figure 3 (a) It can be observed that the adsorption-desorption curve of BC is not completely closed and the curve is not smooth. This may be due to the small specific surface area and few pores of the original biochar material, resulting in low nitrogen adsorption capacity. Table 3 shows that the specific surface area and pore volume of BC are low, indicating that the internal pore structure of biochar is underdeveloped, the pore size distribution is uneven, and the adsorption performance in practical applications is poor. The large average pore size is due to the fact that the material itself is mainly mesoporous and macroporous, lacking microporous structure. In contrast, Figure (b) shows a typical H3-type hysteresis loop under higher relative pressure, proving that there are layered and slit-like pores in the sample. The specific surface area and pore volume of Q-PBC are significantly increased by tens of times, and the average pore size is reduced to 7.38 nm. This is because H3PO4, as a pore-forming agent, reacts fully with the biomass components under high temperature conditions, producing rich microporous and mesoporous structures. The ball milling further pulverizes and refines the biochar, increasing the surface roughness of the material, exposing more active sites, and the pore structure also becomes mainly microporous and mesoporous.

[0052] Table 3. Specific surface area and pore volume / pore diameter of BC and Q-PBC

[0053] The crystal structures of four biochar materials were analyzed by XRD, such as... Figure 4 As shown in (a), all samples exhibit a characteristic diffraction peak at approximately 26.5° (2θ), corresponding to the (002) crystal plane of graphitic carbon. Diffraction peaks attributable to CaCO3 (23.1°, 29.4°) were observed in BC and Q-BC, while new diffraction peaks corresponding to SiP2O7 (PDF#04-011-2952) appeared in PBC and Q-PBC, confirming the successful introduction of phosphorus through H3PO4 activation.

[0054] To quantitatively assess the structural evolution, the Pseudo-Voigt function was used for peak fitting after background subtraction to determine the full width at half maximum (FWHM) of the (002) peak. Since the (002) diffraction peak represents a graphite stacked structure, the crystallite size along the c-axis was subsequently calculated using the Scherrer formula based on this peak: D = Kλ / βcosθ (Formula 3); Where K = 0.89, λ = 0.15406 nm, β is the full width at half maximum (FWHM) in radians, and θ is the Bragg angle.

[0055] As shown in Table 4, the full width at half maximum (FWHM) of the original BC was 0.167°, corresponding to a crystallite size of 48.213 nm. After ball milling (Q-BC), the FWHM decreased to a minimum of 0.138°, and the crystallite size increased to 58.409 nm. This peak sharpening may be attributed to the removal of the amorphous carbon layer during mechanical crushing. Although the particle size decreased, this may have exposed more ordered graphite domains, thereby enhancing diffraction coherence.

[0056] Table 4. Crystal structure parameters of biochar samples

[0057] Conversely, among all samples, those activated with H3PO4 (PBC) exhibited the largest full width at half maximum (FWHM) (0.201°) and the smallest crystallite size (40.092 nm). This peak broadening may be related to the chemical etching effect, which introduces micropores and structural defects, thereby reducing the size of the relevant diffraction peaks. This explanation is consistent with the significant increase in specific surface area observed in BET analysis.

[0058] When the activated biochar underwent further ball milling (Q-PBC), the full width at half maximum (FWHM) decreased to 0.160°, and the crystallite size increased to 50.279 nm, values ​​falling between those of BC and PBC. This partial restoration of structural order suggests that mechanical treatment alleviated the partial disorder introduced by chemical activation, likely through the removal of a highly defective surface layer and the promotion of more uniform dispersion of phosphorus-containing crystalline phases such as SiP₂O₇. Therefore, Q-PBC appears to achieve a structure combining the porosity resulting from H₃PO₄ activation with the more ordered carbon framework refined by ball milling, a structure potentially advantageous for adsorption applications.

[0059] The effects of changes in chemical bonds and functional groups on the adsorption performance of biochar samples were investigated using FT-IR, and the data were plotted as follows: Figure 4 As shown in (b), the FT-IR spectra of the four adsorbent materials were similar. At 3425 cm⁻¹ -1 Strong stretching vibration peaks were observed in the vicinity, mainly originating from hydroxyl groups in the material or adsorbed water. However, after modification, the intensity of the OH peak gradually weakened due to the dehydration of some hydroxyl groups or their reaction with phosphoric acid. But Q-PBC showed a peak intensity at 3709 cm⁻¹. -1 and 3780cm -1 The appearance of a new vibrational peak at 1605 cm⁻¹ indicates the formation of new active sites on the carbon surface. -1The peak observed corresponds to the C=C vibration peak of the aromatic ring. The increased peak intensity after ball milling and modification indicates that phosphoric acid activation and ball milling have a good synergistic effect on promoting the formation of the aromatic ring. PBC and Q-PBC peaks at 1175 cm⁻¹ -1 The peak appearing at [location] corresponds to the P=O stretching vibration peak, indicating that phosphoric acid modification successfully introduced phosphate groups into the biochar material. The higher dispersion of the P=O peak in Q-PBC also proves that ball milling can further optimize the distribution of phosphate groups, providing more active sites for adsorption. [The peaks at 1086 cm⁻¹ correspond to the peaks of the P=O stretching vibration in Q-PBC, indicating that the modification successfully introduced phosphate groups into the biochar material.] -1 and 1095cm -1 A CO stretching vibration peak appears at the point, indicating the presence of oxygen-containing functional groups such as phenol, ether, or ester groups in the sample.

[0060] XPS characterization analysis. For example... Figure 5 As shown, the content of P atoms increased significantly after impregnation and activation with H3PO4, indicating that using H3PO4 as an activator can successfully introduce P-containing groups into biochar. Figure 5 (bd) shows that the peak values ​​of the C1s spectrum of the samples at binding energies of 284.80 eV, 286.27 eV, and 289.01 eV correspond to the presence of CC, CO, and OC=O, respectively. The peak intensities indicate that the proportion of oxygen-free carbon is much higher than that of oxygen-containing carbon, suggesting that the main framework structure of carbon in the material is still composed of CC bonds. The proportion of the CC peak area in H3PO4 and the three samples after ball milling is higher than 76.5% of that in BC, indicating that phosphoric acid activation and ball milling can promote the transformation of amorphous carbon into ordered graphite microcrystals.

[0061] O1s spectra show characteristic peaks for P=O and PO near 531.71 eV and 533.51 eV, respectively. Furthermore, the P=O peak position of Q-PBC shifted after ball milling, which is related to the optimization of phosphate group distribution during the ball milling process. The hydroxyl oxygen ratio decreased from 24.9% in BC to 17.7% in Q-PBC because phosphate reacts with hydroxyl groups to form POC bonds. Figure 5 (d) In the XPS plot of P2p, PBC and Q-PBC showed obvious peaks compared to the unactivated material, confirming that the phosphorus-oxygen components were successfully loaded onto the biochar surface.

[0062] Using a 100 mg / L RhB solution as the pollutant, the best combination of biomass material and modified material with the best adsorption effect was selected from three different biomass materials and three different modified materials. Phosphoric acid-modified reed straw biochar showed the best adsorption performance for RhB. Further analysis was conducted to determine the optimal concentration and milling time for phosphoric acid-modified ball-milled biochar (Q-PBC) to adsorb the pollutant.

[0063] Example 9 The difference between this comparative example and Example 8 is that the ball milling time is 0 hours.

[0064] Example 10 The difference between this comparative example and Example 8 is that the ball milling time is 1 hour.

[0065] Example 11 The difference between this comparative example and Example 8 is that the ball milling time is 2 hours.

[0066] Example 12 The difference between this comparative example and Example 8 is that it uses a 1.0 mol / L phosphoric acid solution.

[0067] Example 13 The difference between this comparative example and Example 8 is that it uses a 2.5 mol / L phosphoric acid solution.

[0068] Example 14 The difference between this comparative example and Example 8 is that it uses a 5 mol / L phosphoric acid solution.

[0069] Example 15 The difference between this comparative example and Example 8 is that it uses a 10 mol / L phosphoric acid solution.

[0070] The adsorption performance of the biochar prepared in Examples 9-15 was tested: 1. Screening of biomass and activators according to Figure 6 It can be seen that, compared with KOH and H2C2O4, H3PO4 has the best activation effect on reed straw. Combined with characterization analysis, SEM analysis shows that the H3PO4-activated sample maintains the integrity of the carbon skeleton while improving the surface roughness, and the fibrous structure characteristics of reed straw-based PBC-3 are preserved. However, KOH and H2C2O4-activated KBC-3 and CBC-3 show carbon skeleton breakage and significantly reduced pore connectivity due to the violent activation reaction. BET characterization data shows that although the specific surface area of ​​PBC-3 (310.872 m²) is... 2 / g) is lower than PBC-2 (542.624m 2 / g), but the hierarchical pore structure dominated by mesopores in PBC-3 is more conducive to pollutant diffusion and adsorption, while the narrow micropore channels of PBC-2 are easily restricted by macromolecular mass transfer. The cellulose-lignin structure of reed straw forms a stable mesoporous network under the activation of H3PO4, but the strong corrosiveness of KOH and oxalic acid destroys pore development, resulting in a significant decrease in the specific surface area and pore volume of KBC-3 and CBC-3. Phosphoric acid modification can also introduce more phosphate groups, hydroxyl groups and other oxygen-containing functional groups on the surface of biochar. These functional groups can enhance the adsorption capacity for dyes with polar groups such as RhB. At the same time, it may remove non-carbon components in biomass, and the formed phosphates can also serve as templates for pore formation, further enhancing the pore structure of the carbon.

[0071] 2. Optimized process of phosphate ball milling Experimental studies have shown that, at a pyrolysis temperature of 450℃, the modified ball-milled biochar with a concentration of 7.5 mol / L H3PO4 and a ball-milling time of 0.5 h exhibits the best RhB adsorption performance.

[0072] The adsorption performance of H3PO4-activated reed straw charcoal for RhB changes with ball milling time as follows: Figure 7 As shown, the adsorption capacity initially increases and then decreases. The adsorption capacity before ball milling was 341.88 mg / g, with a pollutant removal rate of only 85%. After 0.5 hours of ball milling, the adsorption capacity reached 401.7 mg / g, and the removal rate reached 100%. However, as the ball milling time continued to increase, both the adsorption capacity and removal rate showed a significant downward trend compared to the 0.5-hour time. After 2 hours of ball milling, the adsorption capacity dropped to 389 mg / g. This is because the initial ball milling mechanically broke down the original particle structure of the biochar, increasing the specific surface area and porosity, and exposing more oxygen-containing functional groups formed by phosphate activation, thus enhancing the adsorption capacity for RhB. However, excessive ball milling, to some extent, damaged the original pore structure of the material. Excessively small particle sizes can also lead to adsorption on the container wall due to electrostatic adsorption or secondary agglomeration, reducing the number of active functional groups and weakening the material's adsorption capacity. Therefore, 0.5 hours will be used for subsequent ball milling to prepare the ball-milled char, ensuring maximum efficiency in pollutant adsorption.

[0073] The reed straw charcoal exhibited the fastest adsorption efficiency and the highest pollutant removal rate at an H3PO4 concentration of 7.5 mol / L, while the adsorption effect of the charcoal modified with 10 mol / L H3PO4 decreased. This may be because when the H3PO4 concentration exceeds a certain threshold, excessive corrosion occurs on the biochar surface, reducing the number of effective adsorption sites. The internal structure is also damaged; the biochar skeleton collapses during pyrolysis, making it difficult to form pores conducive to adsorption, thus leading to a decline in adsorption performance. Figure 9It can be seen that when the H3PO4 concentration is 7.5 mol / L, the adsorption capacity of pollutants reaches 400 mg / g, and the removal rate of RhB reaches 100%.

[0074] 3. Decoupling analysis of the effects of chemical activation and ball milling on RhB adsorption To elucidate the performance of Q-PBC compared to different activation processes and preparation routes, Table 5 summarizes and compares the adsorption capacity and key process parameters of Rhodamine B, highlighting the advantages of Q-PBC over traditional biochar modification strategies. Specifically, to decouple the effects of ball milling and chemical activation, a normalized comparison was performed on PBC and Q-PBC under the same phosphoric acid impregnation conditions (1.0 mol / L). As shown in Table 5, the adsorption capacity of PBC was 74 mg / g, while Q-PBC, after an additional 0.5 h of ball milling, exhibited a significantly increased adsorption capacity, reaching 280 mg / g. Given that the activator and impregnation concentrations remained constant, the approximately fourfold increase in adsorption capacity is mainly attributed to the ball milling effect, rather than a cumulative chemical effect.

[0075] Furthermore, compared with biochar derived from pyrolysis and one-step chemical activation, Q-PBC exhibited a significantly higher adsorption capacity, confirming the effectiveness of combining chemical activation with mechanical treatment. Moreover, the adsorption performance of Q-PBC for RhB was significantly affected by both ball milling time and impregnation concentration.

[0076] Table 5. Comparison of RhB adsorption performance of biochar prepared by different activation strategies

[0077] The adsorption performance of biochar samples over a wide initial solution pH range was investigated, and their adsorption stability under actual working conditions was systematically evaluated through repeated adsorption-desorption cycle tests. The results are as follows: Figure 10 As shown in the figure. Specifically, RhB solutions with different initial pH values ​​were used to reflect the pH-dependent adsorption behavior of biochar; at the same time, under optimal pH conditions, using a fixed adsorbent dosage and RhB concentration, the reusability of Q-PBC was further evaluated through five consecutive adsorption-desorption cycles.

[0078] like Figure 10 As shown in a, BC exhibits extremely limited adsorption capacity across the entire pH range, with a maximum adsorption capacity of only 38.49 mg / g at pH 5. This poor performance can be attributed to insufficient pore structure development and a lack of oxygen-containing functional groups on the surface, resulting in a low density of effective adsorption sites.

[0079] After ball milling, the adsorption capacity of Q-BC was significantly enhanced, reaching 271.35 mg / g at pH 5. This enhancement effect may be related to the reduced particle size and the increased abundance of -OH and other oxygen-containing groups, which contribute to the formation of hydrogen bonds and π-π interactions with RhB. However, as... Figure 10 a and Figure 10 As shown in b, the adsorption performance of Q-BC significantly decreases under alkaline conditions (pH 9–11). This decrease is generally attributed to the pH-dependent changes in RhB speciation: at higher pH levels, RhB tends to exist as a neutral dipole or closed-ring lactone, rather than the cationic form predominant under acidic conditions. This structural shift is expected to weaken electrostatic interactions and reduce the overall interaction strength between RhB and the porous surface. Both the driving force for adsorption and the effective binding affinity decrease, leading to reduced adsorption performance. It should be noted that surface charge properties were not directly measured in this study. Therefore, the proposed explanation is based on indirect evidence of known RhB speciation behavior and adsorption trends, and the combined effect of surface chemistry and electrostatic interactions needs further confirmation through Zeta potential or related analyses.

[0080] In contrast, phosphoric acid activation significantly enhances the adsorption stability and pH adaptability of the material. For example... Figure 10 As shown in a and 10b, both PBC and Q-PBC maintained consistently high adsorption capacity and removal rates over a wide pH range of 3 to 11. This improved pH stability stems from the introduction of phosphorus-containing functional groups. P=O, P-OH, and phosphate groups maintain a partially protonated state in weakly acidic or neutral environments; although partial deprotonation occurs under alkaline conditions, they retain high polarity and continue to facilitate effective adsorption interactions. These functional groups provide abundant active sites for electrostatic attraction, hydrogen bonding, and Lewis acid-base interactions, enabling the materials to effectively adsorb RhB independently of pH fluctuations. Among the tested materials, Q-PBC exhibited the highest adsorption capacity, reaching 479.50 mg / g at pH 5, with a removal rate of 95.90%. Figure 10 As shown in b, the excellent pH-independent properties of Q-PBC stem from the synergistic effect of phosphoric acid activation and ball milling, which together construct a hierarchical porous structure, increase the degree of carbon aromatization, and introduce uniformly dispersed phosphorus-containing functional groups.

[0081] like Figure 10As shown in Figure c, the reusability of Q-PBC was evaluated through five consecutive adsorption-desorption cycles. After each adsorption experiment, the discarded Q-PBC was regenerated by a thorough ultrasonic cleaning process to remove adsorbed contaminants, followed by repeated washing with deionized water and drying at 60°C for subsequent reuse. Although a gradual decrease in removal rate was observed with increasing cycle number (possibly due to pore blockage, partial loss of active sites, or slight structural changes), the material maintained stable performance, with a removal rate remaining above 70% after the fifth cycle. Similar regeneration behavior has been reported in biochar-based adsorbents for RhB removal, where adsorption capacity retention typically ranges from 50% to 70% after 3–6 cycles, depending on the regeneration method and material structure. In this context, the stability observed in this study is comparable to levels reported in previous studies, indicating that Q-PBC possesses acceptable reusability under the test conditions. These results collectively demonstrate that Q-PBC possesses good structural stability and reusability, enabling effective regeneration for continuous applications without significant loss of function. They confirm its adsorption stability under reusable conditions and highlight its potential for continuous and long-term environmental remediation applications.

[0082] Biochar was produced by pyrolysis of reed straw, pine powder, and wheat straw. H3PO4, H2C2O4, and KOH were used as modifiers. Through experimental comparison, it was determined that the reed straw biochar modified with phosphoric acid had the highest adsorption efficiency for RhB.

[0083] (1) Wheat straw, pine powder, and reed straw were impregnated and activated using three activators: H3PO4, KOH, and H2C2O4. SEM analysis showed that the H3PO4-activated samples maintained the integrity of the carbon skeleton while improving the surface roughness, and the fibrous structure characteristics of reed straw-based PBC-3 were preserved. However, the KOH and H2C2O4-activated KBC-3 and CBC-3 showed carbon skeleton breakage and significantly reduced pore connectivity due to the violent activation reaction. The cellulose-lignin structure of reed straw formed a stable mesoporous network under H3PO4 activation, while the strong corrosiveness of KOH and H2C2O4 destroyed pore development, resulting in a significant decrease in the specific surface area and pore volume of KBC-3 and CBC-3. It can be seen that H3PO4-activated reed straw (PBC-3) achieved the best balance between pore size and structural strength through pore structure optimization, and its adsorption performance was significantly better than other combinations.

[0084] (2) At a pyrolysis temperature of 450℃, the optimal preparation conditions for the modified biochar material were a phosphoric acid concentration of 7.5 mol / L and a ball milling time of 0.5 h. Under these conditions, the material exhibited a high specific surface area and an optimized microporous-mesoporous hierarchical pore structure. The surface was rich in phosphate ester groups (POC) and oxygen-containing functional groups, which significantly enhanced the adsorption capacity for RhB. After ball milling, a large number of adsorption sites inside the material were exposed, the particle size was reduced to the micron level, and the adsorption capacity for RhB was 401.7 mg / g, with a removal rate of 100%.

[0085] (3) The prepared BC, Q-BC, PBC and Q-PBC were characterized by SEM, XRD, BET, XPS and FT-IR, and batch adsorption experiments were carried out under different pH, adsorbent dosage, initial concentration and temperature conditions. Characterization analysis showed that the composite-treated Q-PBC material exhibited a significant small particle size and multi-level pore structure. The characteristic diffraction peaks corresponding to SiP2O7 appeared in XRD, proving that phosphate groups were successfully introduced, and the peak intensity was significantly improved compared with the unmodified state. The specific surface area and pore volume increased by tens of times compared with the unmodified state, because the ball milling treatment can refine the particles and eliminate stress defects, thereby optimizing the crystal structure. FT-IR showed a pore size of 1175 cm⁻¹. -1 The enhancement and increased dispersion of the P=O characteristic peak confirm that ball milling effectively optimizes the distribution of phosphate groups on the material surface. After five cycles of repeated use, the removal rate of RhB remains above 70%, demonstrating good regeneration capability.

[0086] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing phosphate ball milling synergistic modification of reed straw biochar, characterized in that, Includes the following steps: (1) Raw material pretreatment: The biomass is washed, dried, crushed and sieved to obtain biomass powder; (2) Phosphoric acid activation: Biomass powder is added to phosphoric acid solution, stirred and reacted, dried and placed in muffle furnace for pyrolysis, and cooled to room temperature to obtain phosphoric acid activated biochar; (3) Washing and drying: The phosphoric acid activated biochar was washed until neutral, dried, and ground to obtain purified activated biochar; (4) Ball milling modification: The purified activated biochar was placed in a ball mill and ball milling was performed using ball milling media to obtain phosphate ball milling synergistic modified reed straw biochar.

2. The method for preparing phosphoric acid ball milling synergistic modification of reed straw biochar according to claim 1, characterized in that: The concentration of the phosphoric acid solution in step (2) is 1-10 mol / L.

3. The method for preparing phosphoric acid ball milling synergistic modification of reed straw biochar according to claim 1, characterized in that: The pyrolysis conditions in step (2) are: heating rate of 3-10℃ / min, heating to 400-500℃, and holding time of 1-3h.

4. The method for preparing phosphoric acid ball milling synergistic modification of reed straw biochar according to claim 1, characterized in that: In step (4), the mass ratio of purified activated biochar to ball milling media is 1:50-200.

5. The method for preparing phosphoric acid ball milling synergistic modification of reed straw biochar according to claim 1, characterized in that: The ball milling speed is 200-500 r / min, and the ball milling time is 0.5-2 h.

6. The method for preparing phosphoric acid ball milling synergistic modification of reed straw biochar according to claim 1, characterized in that: The ball mill mentioned in step (4) is a planetary ball mill, and the grinding media is agate balls.

7. The method for preparing phosphoric acid ball milling synergistic modification of reed straw biochar according to claim 1, characterized in that: In step (1), the material is pulverized to a size of less than 80-120 mesh.

8. The method for preparing phosphoric acid ball milling synergistic modification of reed straw biochar according to claim 1, characterized in that: The biomass includes at least one of wheat straw, pine powder, and reed straw.

9. A phosphoric acid ball milling synergistic modified reed straw biochar prepared by the preparation method according to any one of claims 1-8.

10. The phosphoric acid ball milling synergistic modification of reed straw biochar according to claim 9, characterized in that: The specific surface area of ​​the biochar is 297-542 m². 2 / g, pore volume is 0.199-0.237cm³ 3 / g, with an average pore size of 3.1-6.7nm.

Images