Self-supporting porous composite filter ball in water treatment and preparation method thereof

Abstract

The application discloses a self-supporting porous composite filter ball in water treatment and a preparation method thereof, and belongs to the technical field of water treatment functional materials. The composite filter ball is prepared from agricultural and forestry biomass and iron tailings as main raw materials, supplemented with clay and ammonium phytate, through mechanical force chemical auxiliary low-temperature modification and inert atmosphere carbonization. The obtained filter ball can maintain an integral structure without external support, has high mechanical strength and developed pores, can simultaneously remove various pollutants such as organic pollutants and nitrogen and phosphorus nutrients in water, is easy to fill and recycle, is especially suitable for farmland water retreat surface source pollution treatment, and has good environmental benefits and application prospect.

Description

Technical Field

[0001] This invention relates to the field of functional materials for water treatment, and in particular to a self-supporting porous composite filter ball for water treatment and its preparation method. Background Technology

[0002] With the intensive development of agriculture, agricultural non-point source pollution caused by farmland runoff has become increasingly prominent. Fertilizers, pesticides, and livestock manure applied in farmland enter water bodies through surface runoff and lateral seepage under the influence of rainfall and irrigation, becoming a significant source of nitrogen, phosphorus, and pesticide pollutants. Farmland runoff is characterized by its high dispersion, large fluctuations, and high difficulty in treatment, making it a key target for water environment management. Furthermore, emerging pesticide pollutants, such as atrazine, pose potential risks to aquatic ecosystems and human health due to their high mobility, recalcitrant nature, and stable presence in water bodies. Therefore, there is an urgent need to develop efficient, stable, and economical pollutant removal materials and technologies.

[0003] Biochar is a porous carbon material formed by the pyrolysis of agricultural and forestry biomass under anaerobic conditions. It features a large specific surface area, well-developed pore structure, and abundant oxygen-containing functional groups on its surface. It can effectively remove organic pollutants and heavy metals from water bodies through mechanisms such as pore filling, π-π interactions, surface complexation, and electrostatic adsorption. However, existing biochar is mostly in powder or granular form, which presents problems such as easy loss, difficulty in recycling, and insufficient mechanical strength in practical applications, limiting its engineering applications.

[0004] To improve material performance, existing research has explored combining biochar with metals or minerals. Iron tailings, a typical industrial solid waste, contain abundant iron oxides and silica-alumina mineral components, exhibiting certain adsorption activity and structural stability. Introducing them into biochar systems not only enables the resource utilization of solid waste but also enhances the adsorption and catalytic capacity for organic pollutants through iron-based active sites. Current research indicates that metal modification or mineral composites can significantly improve the pore structure and surface properties of biochar, enhancing its removal performance for pesticide pollutants. However, these studies primarily focus on powdered or filled materials, lacking a molded material system that combines structural stability, adsorption performance, and ease of engineering application.

[0005] In engineering applications, filter balls are a common type of packing material, offering advantages such as easy filling and recycling. However, existing filter balls mostly rely on the outer shell structure filled with functional materials, making it difficult to achieve integrated molding of the material body. This results in low material utilization efficiency and limited structural stability. Furthermore, during the molding and carbonization process of biochar, uneven release of internal moisture and volatiles can easily cause the material to crack or break, making it difficult to obtain structurally stable spherical materials.

[0006] Therefore, how to achieve stable molding of composite materials and construct self-supporting filter balls with good structural stability and adsorption performance has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0007] The purpose of this invention is to address the shortcomings of existing technologies by proposing a self-supporting porous composite filter ball for water treatment and its preparation method.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: a self-supporting porous composite filter ball for water treatment, comprising the following components by weight: biomass powder: 70-82 parts, iron tailings powder: 15-22 parts, clay: 3-8 parts, ammonium phytate: 2.5-5.5 parts, and deionized water: 20-40 parts.

[0009] Preferably, the composite filter ball is prepared by carbonization under an inert atmosphere after modification with ammonium phytate. During the carbonization process, the biomass powder forms a porous carbon-based skeleton structure, and the iron tailings powder is dispersed in the carbon-based skeleton and provides iron-based active sites. Clay serves as a binder and structural support phase, thereby improving the mechanical strength and molding stability of the filter ball.

[0010] Preferably, the composite filter ball can maintain its complete spherical structure without external support.

[0011] Preferably, the particle size of the self-supporting porous composite filter ball is 2-3 cm.

[0012] Preferably, the biomass powder is selected from one or more of coconut shell powder, corn stalk powder, and wheat stalk powder, with a particle size of 80-200 mesh.

[0013] Preferably, the iron tailings powder is a solid waste generated from iron ore beneficiation, mainly composed of Fe2O3, SiO2 and Al2O3, with a particle size of 200-300 mesh.

[0014] Preferably, the clay is selected from one or both of kaolin or bentonite, with a particle size of 200-300 mesh.

[0015] Furthermore, the present invention also provides a method for preparing a self-supporting porous composite filter ball for water treatment, comprising the following steps: S1. Preparation of ammonium phytate solution: Add ammonium phytate to deionized water and stir until completely dissolved to obtain ammonium phytate solution; S2. Powder mixing and integrated modification into slurry: Biomass powder, iron tailings powder and clay are added to a planetary ball mill, zirconia grinding balls are added, and dry grinding is carried out for 10-15 min to obtain mixed powder. Then ammonium phytate solution is added to the mixed powder, and ball milling is continued at 50-55℃ for 1.0-1.5 h to make the mixed powder reach a plastic state that can be formed into a ball by hand and dispersed by light pressure. Then the material is allowed to stand and rest to obtain a plastic intermediate. S3. Granulation and pelletizing: The plastic intermediate is transferred to a disc granulator and rolled into pellets at a speed of 20-30 r / min to obtain the composite filter ball precursor; S4. Preparing and staged drying: The composite filter ball precursor is placed in a forced-air drying oven and dried in a gradient manner to obtain the dried composite filter ball; S5. Inert atmosphere carbonization: The dried composite filter balls are placed in a tube furnace, nitrogen is introduced, the temperature is raised to 650-750℃, the temperature is held for 1.5-2.5 hours, and then naturally cooled to room temperature to obtain self-supporting porous composite filter balls for water treatment.

[0016] Preferably, in S2, the zirconia grinding balls are composed of 10mm large balls and 5mm small balls in a weight ratio of 3:7, the ball-to-material ratio is 5:1, and the grinding speed is 300-400 r / min.

[0017] Preferably, gradient drying in S4 refers to pre-drying at 40-80℃ for 1.5 h, and then heating to 80-120℃ for 3 h.

[0018] Preferably, the flow rate of nitrogen gas introduced in step S5 is 100-150 ml / min.

[0019] Preferably, the heating rate of the tubular furnace in S5 is 8-12℃ / min.

[0020] Preferably, the preparation mechanism of the self-supporting porous composite filter ball for water treatment according to the present invention is explained as follows: This invention utilizes the unique hexaphosphate-ammonium bifunctional molecular structure of ammonium phytate to achieve interfacial covalent bonding of ternary components and construction of highly active sites at the molecular level through two continuous and interconnected reaction stages: low-temperature chemical modification and high-temperature pyrolysis conversion. This solves the technical contradictions of weak interfacial bonding, low utilization rate of iron active sites, and irreconcilable mechanical strength and adsorption performance in existing biochar-iron tailings composite materials.

[0021] The molecular structure of ammonium phytate is as follows:

[0022] During the low-temperature modification stage at 50-55℃, ammonium phytate molecules are first dispersed at the molecular level by dissolving in deionized water. Subsequently, under the mechanical force of a planetary ball mill, they undergo sufficient interfacial contact and specific chemical reactions with the three raw material components. The impact, shearing, and friction generated during ball milling not only refine the powder particles to the submicron level, significantly increasing the interfacial contact area, but also generate a large number of lattice defects and unsaturated sites on the particle surface. At the same time, a localized instantaneous high-temperature and high-pressure environment is formed, providing the necessary activation energy for the dehydration condensation reaction. This allows the COP and Si-OP covalent bond formation reactions to proceed efficiently under the mild conditions of 50-55℃. The phosphate groups of ammonium phytate undergo a dehydration condensation reaction with the abundant hydroxyl and carboxyl groups on the surface of biochar, forming COP covalent bonds. This grafting of phytic acid molecules onto the biochar surface at single points not only prevents the aggregation of phytic acid molecules due to intermolecular hydrogen bonding during subsequent heating, ensuring their monomolecular dispersion throughout the system, but also significantly improves the hydrophilicity of the biochar surface, allowing water to penetrate uniformly into the biochar particles, improving the plasticity of the mixture, and providing a good technological foundation for subsequent granulation. Simultaneously, the phosphate groups of ammonium phytate undergo a similar dehydration condensation reaction with the silanol groups on the clay surface, forming Si-OP covalent bonds. This molecular-level chemical bonding is far stronger than the physical van der Waals forces between traditional clay particles, providing sufficient bonding strength for the material even in the wet stage, making the granulated spheres less prone to breakage during handling and drying. More importantly, the phosphate groups of ammonium phytate react with the Fe2+ on the surface of iron tailings... 3+ It has extremely strong selective chelating ability and can bind with Fe. 3+ The formation of thermodynamically stable hexadecimal chelates occurs only on the outermost surface of the iron tailings particles. This chelation does not dissolve the iron tailings themselves and destroy their structural integrity. At the same time, it fixes the iron element at the molecular level, providing a uniform and highly dispersed reaction precursor for the generation of highly active Fe-NP sites in the subsequent high-temperature stage.

[0023] During the high-temperature carbonization stage at 650-750℃ in a nitrogen atmosphere, the initial structure formed in the low-temperature stage undergoes selective pyrolysis transformation and reconstruction, ultimately forming a stable functionalized composite system. In this process, the COP bonds, due to their relatively low thermal stability, gradually decompose upon heating, generating highly reactive phosphorus-containing free radicals (such as ·PO3 and ·PO2). These free radicals immediately undergo addition reactions with the unsaturated carbon sites formed during biochar pyrolysis, generating CP covalent bonds with higher thermal stability. These bonds have better thermal stability and become the main connection method between the carbonized biochar and phosphorus species, ensuring that phosphorus species are not lost during long-term use. Simultaneously, the CO2 and H2O gases released during the COP bond decomposition process etch the biochar framework in situ, forming numerous interconnected mesopores and macropores, significantly optimizing the pore structure of the filter balls and improving the specific surface area and pollutant mass transfer efficiency. Unlike COP bonds, Si-OP bonds exhibit excellent thermal stability. At the carbonization temperature of this invention, most remain intact, continuing to serve as the primary bonding bridge between clay and biochar / iron tailings. A small amount of amorphous SiO2 and metaphosphate generated during decomposition fills interfacial defects, forming a continuous glassy phase bonding layer, further enhancing the mechanical strength of the material. Simultaneously, the phytic acid-iron hexadecanthate chelate formed at low temperatures undergoes pyrolysis. The ammonium ions and phosphate groups contained in the ammonium phytate molecule act as nitrogen and phosphorus sources, respectively, coordinating with the chelated Fe atoms to generate atomically dispersed Fe-N3P1 highly active sites in situ. These sites possess significantly higher catalytic activity than traditional iron oxides, effectively activating dissolved oxygen in water and exhibiting significant catalytic degradation of recalcitrant pesticide pollutants such as atrazine. This represents a functional upgrade from simple physical adsorption to a synergistic adsorption-catalytic degradation process.

[0024] Ultimately, through the synergistic effect of the aforementioned mechanochemical-assisted low-temperature modification and high-temperature pyrolysis transformation, biochar forms a continuous porous carbon-based framework. Clay is tightly bonded to the carbon-based framework through Si-OP and CP covalent bonds to form a stable structural support. Iron elements in iron tailings are transformed into atomically dispersed Fe-NP highly active sites uniformly loaded on the carbon-based framework. The three elements form an organic whole through a continuous chemical bonding network, maintaining a complete spherical structure without any external support, while simultaneously improving mechanical strength, pore structure, and catalytic activity.

[0025] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention modifies phytate to construct a continuous chemical bonding network composed of COP, Si-OP, and CP covalent bonds, replacing the physical bonding effect of traditional clay. This significantly improves the mechanical strength and structural stability of the material while reducing the amount of clay used. It effectively avoids the pore blockage caused by clay hydration expansion, achieving optimization of mechanical strength and pore structure, and resolving the inherent contradiction of "increased strength - decreased performance" in existing materials.

[0026] 2. This invention utilizes the specific hexagonal chelation effect of ammonium phytate on iron ions to fix iron elements in iron tailings, and then generates Fe-NP highly active catalytic sites through high-temperature pyrolysis. These sites have higher catalytic activity and resistance to leaching than traditional iron oxides, and can effectively activate dissolved oxygen in water to degrade recalcitrant organic pollutants, achieving a functional upgrade from simple physical adsorption to synergistic adsorption-catalytic degradation.

[0027] 3. This invention uses ammonium phytate as a single modifier, simultaneously functioning as an interfacial crosslinking agent, a nitrogen-phosphorus co-doping source, a pore-forming agent, and an iron chelating agent. No additional additives are required, simplifying the process and reducing production costs. The process retains the basic framework of traditional granulation and carbonization, adding only one step of mechanochemical-assisted modification, making it easy for large-scale industrial production and possessing promising engineering application prospects.

[0028] 4. This invention uses agricultural and forestry biomass and iron tailings, two major solid wastes, as the main raw materials, realizing the high-value utilization of solid waste. The prepared self-supporting porous filter balls can maintain their complete structure without external support, are easy to fill and recycle, and can simultaneously remove organic pollutants, heavy metals, and nitrogen and phosphorus nutrients from water bodies, making them particularly suitable for the treatment of complex non-point source pollution such as farmland runoff. Detailed Implementation

[0029] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with existing known technologies. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0030] Example 1: A specific preparation method for a self-supporting porous composite filter ball for water treatment, comprising the following steps: S1. Preparation of ammonium phytate solution: Add 2.5g of ammonium phytate to 20g of deionized water and stir until completely dissolved to obtain ammonium phytate solution; S2. Powder mixing and integrated modification into slurry: 70g coconut shell powder (80 mesh), 15g iron tailings powder (200 mesh), and 3g kaolin (200 mesh) were added to a planetary ball mill. Zirconia grinding balls (10mm large balls and 5mm small balls in a weight ratio of 3:7) were added at a ball-to-powder ratio of 5:1. The mixture was dry-milled at 300r / min for 10min to obtain a mixed powder. Then, ammonium phytate solution was added to the mixed powder. The mixture was ball-milled at 50℃ for 1h until it reached a plastic state that could be formed into a ball by hand and dispersed when lightly pressed. The mixture was then allowed to stand for 10min to obtain a plastic intermediate. S3. Granulation and pelletizing: The plastic intermediate is transferred to a disc granulator and rolled into pellets at a speed of 20 r / min to obtain the composite filter ball precursor; S4. Preparing and staged drying: The composite filter ball precursor is placed in a forced-air drying oven, pre-dried at 40℃ for 1.5 h, and then heated to 80℃ for 3 h to obtain the dried composite filter ball. S5. Inert atmosphere carbonization: The dried composite filter balls are placed in a tube furnace, nitrogen is introduced at a flow rate of 100 ml / min, the temperature is raised to 650℃ at a heating rate of 8℃ / min, held at this temperature for 2.5 h, and then naturally cooled to room temperature to obtain self-supporting porous composite filter balls for water treatment with a particle size of 2.1-2.4 cm and an average particle size of 2.2 cm.

[0031] Example 2: A specific preparation method for a self-supporting porous composite filter ball for water treatment, comprising the following steps: S1. Preparation of ammonium phytate solution: Add 40g of ammonium phytate to 300g of deionized water and stir until completely dissolved to obtain ammonium phytate solution; S2. Powder mixing and integrated modification into slurry: 760g of corn straw powder (120 mesh), 180g of iron tailings powder (250 mesh), and 50g of kaolin (240 mesh) were added to a planetary ball mill. Zirconia grinding balls (10mm large balls and 5mm small balls in a weight ratio of 3:7) were added at a ball-to-material ratio of 5:1. The mixture was dry-milled at 350r / min for 12min to obtain a mixed powder. Then, ammonium phytate solution was added to the mixed powder, and the mixture was ball-milled at 52℃ for 1.2h until it reached a plastic state that could be formed into a ball by hand and dispersed when lightly pressed. The mixture was then allowed to stand for 30min to obtain a plastic intermediate. S3. Granulation and pelletizing: The plastic intermediate is transferred to a disc granulator and rolled into pellets at a speed of 25 r / min to obtain the composite filter ball precursor; S4. Preparing and staged drying: The composite filter ball precursor is placed in a forced-air drying oven, pre-dried at 60℃ for 1.5 h, and then heated to 100℃ for 3 h to obtain the dried composite filter ball. S5. Inert atmosphere carbonization: The dried composite filter balls are placed in a tube furnace, nitrogen is introduced at a flow rate of 120 ml / min, the temperature is raised to 700℃ at a heating rate of 10℃ / min, held at this temperature for 2 hours, and then naturally cooled to room temperature to obtain self-supporting porous composite filter balls for water treatment with a particle size of 2.4-2.7 cm and an average particle size of 2.5 cm.

[0032] Example 3: A specific preparation method for a self-supporting porous composite filter ball for water treatment, comprising the following steps: S1. Preparation of ammonium phytate solution: Add 55g of ammonium phytate to 400g of deionized water and stir until completely dissolved to obtain ammonium phytate solution; S2. Powder mixing and integrated modification into slurry: 820g of wheat straw powder (200 mesh), 220g of iron tailings powder (300 mesh), and 80g of bentonite (300 mesh) were added to a planetary ball mill. Zirconia grinding balls (10mm large balls and 5mm small balls in a weight ratio of 3:7) were added at a ball-to-powder ratio of 5:1. The mixture was dry-milled at 400r / min for 15 min to obtain a mixed powder. Then, ammonium phytate solution was added to the mixed powder, and the mixture was ball-milled at 55℃ for 1.5 h until it reached a plastic state that could be formed into a ball by hand and dispersed when lightly pressed. The mixture was then allowed to stand for 60 min to obtain a plastic intermediate. S3. Granulation and pelletizing: The plastic intermediate is transferred to a disc granulator and rolled into pellets at a speed of 30 r / min to obtain the composite filter ball precursor; S4. Preparing and staged drying: The composite filter ball precursor is placed in a forced-air drying oven, pre-dried at 80℃ for 1.5 h, and then heated to 120℃ for 3 h to obtain the dried composite filter ball. S5. Inert atmosphere carbonization: The dried composite filter balls are placed in a tube furnace, nitrogen is introduced at a flow rate of 150 ml / min, the temperature is raised to 750℃ at a heating rate of 12℃ / min, held at this temperature for 1.5 h, and then naturally cooled to room temperature to obtain self-supporting porous composite filter balls for water treatment with a particle size of 2.1-2.4 cm and an average particle size of 2.2 cm, a particle size of 2.8-3.0 cm and an average particle size of 2.9 cm.

[0033] Comparative Example 1: The difference between Comparative Example 1 and Example 2 is that ammonium phytate is not added.

[0034] Comparative Example 2: The difference between Comparative Example 2 and Example 2 is that ammonium phytate is replaced with phytic acid.

[0035] Comparative Example 3: The difference between Comparative Example 3 and Example 2 is that the 30-minute resting time for the material was omitted in step S2, and the resulting plastic intermediate was directly granulated and pelletized.

[0036] Comparative Example 4: The difference between Comparative Example 4 and Example 2 is that in step S2, corn stalk powder, iron ore tailings powder and clay are not added to the planetary ball mill, but are mixed in a high-speed mixer at a speed of 350 r / min for 12 min to obtain a mixed powder. Then, ammonium phytate solution is added to the mixed powder, and stirring is continued at 52°C for 1.2 h to make the mixed powder reach a plastic state that can be formed into a ball by hand and dispersed by light pressure. Then, the mixture is allowed to stand for 30 min to obtain a plastic intermediate.

[0037] Comparative Example 5: The difference between Comparative Example 5 and Example 2 is that the pre-drying process is omitted in step S4, and the composite filter ball precursor is placed in a forced-air drying oven and dried directly at 100°C for 3 h.

[0038] Performance testing: The composite filter balls prepared in Examples 1-3 and 1-5 were subjected to system testing. The specific test items and test standards are as follows: 1. Single particle compressive strength: Referring to the method of QB / T 4383-2012, 10 intact filter balls were randomly selected, and a universal testing machine was used to pressurize them at a constant rate until the particles broke. The maximum pressure value was recorded and the average value was taken. 2. Bulk density: Referring to the method of GB / T 30202, filter balls are naturally filled into a container of known volume, and after compaction, the total mass is weighed and the mass per unit volume is calculated. 3. Breakage rate: Add the filter balls to a shaker containing 500 ml of deionized water and shake at 200 r / min for 100 h. After removing them, sieve them with a 2 mm standard sieve, weigh the mass of the broken material under the sieve, and calculate the mass loss rate, which is the breakage rate. 4. Crack rate: Visually inspect the carbonized filter balls of each embodiment and comparative example and count the number of cracked particles, and calculate the ratio of the number of cracked particles to the total number of particles. 5. Specific surface area: Referring to GB / T 19587-2017 "Determination of specific surface area of ​​solid substances by gas adsorption BET method", a fully automatic specific surface area and pore size analyzer was used. The filter ball samples prepared in each example and comparative example were degassed under vacuum at 120℃ for 4 h. Nitrogen was used as the adsorbate, and adsorption-desorption tests were carried out at 77 K. The specific surface area was calculated by BET method. 6. Mesopore ratio: Test method: BJH pore size distribution method; Calculation method: Calculate the proportion of the pore volume of 2-50nm mesopores to the total pore volume from the pore size distribution data; 7. Iron leaching rate: Referring to GB 5085.3-2007 "Identification Standard for Hazardous Waste - Leaching Toxicity Identification", the filter ball was soaked in deionized water, and water samples were taken at different time points. The total iron concentration in the leachate was determined by inductively coupled plasma atomic emission spectrometry, and the iron leaching rate was calculated. 8. Static adsorption capacity of atrazine: Referring to the general method in the field of environmental materials, the filter ball was contacted with the atrazine solution until adsorption equilibrium was reached under constant temperature and shaking conditions. The residual concentration of atrazine in the filtered solution was determined by high performance liquid chromatography, and the adsorption capacity was calculated. 9. Dynamic removal rate of atrazine: Referring to relevant research by AWWA and the dynamic adsorption experimental method of water treatment filter media, filter balls were packed into a filter column with a diameter of 5 cm and a length of 25 cm. Simulated farmland runoff containing atrazine was introduced at a flow rate of 2 L / h. The concentration of atrazine was detected by liquid-liquid extraction-gas chromatography-mass spectrometry. Samples were taken immediately when liquid began to flow out of the outlet (i.e., when the empty bed volume multiple BV=0) to determine the initial breakthrough removal rate. After the system had been running continuously for 24 h, samples were taken again to determine the stable operation removal rate. The average of the two measurements, the initial breakthrough point and the 24-hour stable point, was taken as the dynamic removal rate. 10. COD removal rate: Refer to GB 11914-1989, determine the chemical oxygen demand of the water sample by potassium dichromate method, and calculate the total removal capacity of the composite filter ball for organic pollutants; 11. Total phosphorus removal rate: Referring to GB 11893-1989, the change in total phosphorus content in the water was determined by the ammonium molybdate spectrophotometric method, and the removal capacity of the filter ball for phosphorus pollutants was calculated. 12. Microbial attachment capacity: Referring to the evaluation method of the biofilm formation capacity of filter media in WQTC 59007, after the filter balls were placed in a simulated water treatment system containing activated sludge and operated for a fixed period of time, the biomass of microorganisms attached to the surface of the filter balls was measured by the biomass determination method (lipophosphorus method or dry weight method) to evaluate the film-forming performance of the filter balls as a carrier for microbial attachment. 13. Recycling and regeneration performance: The filter balls were reused in the atrazine removal experiment. After each adsorption cycle, desorption or low-temperature thermal regeneration was performed, and the adsorption capacity decay rate was measured after 15 cycles. The experimental results are shown in Table 1.

[0039] Table 1 Performance Test Results

[0040] Data Analysis: As can be seen from the performance test data in Table 1, Examples 1-3 adopted the technical solution of the present invention, and through the ammonium phytate chelation and mechanochemical assisted modification process, the interface structure and active site distribution of the ternary system of biochar, iron tailings and clay were reconstructed, and the mechanical strength, pore structure, pollutant removal performance and long-term stability of the composite filter ball were comprehensively improved. Among them, Example 2 had the best comprehensive performance.

[0041] Regarding the compressive strength, bulk density, breakage rate, and cracking rate of individual particles, Example 2 used a combination of corn stalk powder and kaolin. Under ball milling conditions of 350 r / min, ammonium phytate achieved sufficient chemical bonding (COP and Si-OP covalent bonds) at the interfaces of each component. The restoring process ensured uniform distribution of moisture and binder. Carbonization at 700℃ ensured sufficient consolidation of the biochar skeleton while avoiding excessive shrinkage microcracks that might be caused by high temperatures. The resulting filter balls have a dense structure with few defects and tight packing between particles, exhibiting excellent compressive strength and breakage resistance in mechanical tests, with an extremely low cracking rate.

[0042] Regarding specific surface area and mesopore ratio, carbonization at 700℃ falls within the active temperature range of COP bond thermal decomposition. At this temperature, the phosphorus species introduced by ammonium phytate decompose, releasing gases such as CO2 and H2O, which uniformly and effectively etch the carbon skeleton in situ, forming numerous interconnected mesopores. This process avoids incomplete pore-forming reactions at excessively low temperatures or sintering and collapse of the pore structure at excessively high temperatures. Therefore, the specific surface area and mesopore ratio in Example 2 are both at ideal levels, providing excellent mass transfer channels for pollutant diffusion and adsorption.

[0043] Regarding iron leaching rate, the hexapeptide effect of ammonium phytate will remove Fe from the surface of iron tailings. 3+ Firmly fixed at the organic-inorganic interface, some Fe was carbonized at 700℃. 3+ It is transformed into Fe-NP coordination structure or iron phosphate compounds. These phases have extremely low water solubility and can effectively inhibit the migration and dissolution of iron ions into water, so that the filter ball loses very little iron during use and ensures the long-term stability of the active sites.

[0044] Regarding atrazine adsorption capacity and dynamic removal rate, the Fe-NP active sites not only efficiently adsorb atrazine molecules through coordination and π-π stacking, but also continuously activate dissolved oxygen in the water to generate reactive oxygen species, thereby oxidizing and degrading the adsorbed pollutants in situ, achieving a synergistic effect of adsorption-catalytic degradation. In dynamic testing, the filter balls exhibited high removal efficiency upon initial penetration, and maintained an excellent removal rate even after 24 hours of continuous operation, indicating strong resistance to penetration and good regeneration performance.

[0045] Regarding COD and total phosphorus removal rates, the high specific surface area and well-developed mesoporous structure facilitate the diffusion and interfacial contact of various organic pollutants. The catalytic oxidation of Fe-NP sites further mineralizes and decomposes the adsorbed organic matter, thus resulting in a significant removal effect on the comprehensive organic pollution index COD. Iron oxides in iron tailings have a specific affinity for phosphate ions. The uniform dispersion of iron components by ammonium phytate fully exposes these active sites. Simultaneously, the stable pores constructed by Si-OP bonds promote the transport and adsorption of phosphate ions within the filter balls, resulting in equally excellent total phosphorus removal rates.

[0046] Regarding microbial attachment, the abundant mesoporous and microporous structure and good hydrophilicity of the filter ball surface (derived from the phosphorus- and nitrogen-containing polar groups introduced by ammonium phytate) provide ideal fixation sites for microorganisms. The pore structure of Example 2 is neither too dense to restrict microbial permeation nor too sparse to lack anchoring, thus forming a microenvironment conducive to biofilm development, resulting in the largest amount of attached biomass.

[0047] Finally, regarding the cyclic adsorption capacity decay rate, Example 2 exhibited the highest adsorption capacity retention rate after 15 adsorption-desorption cycles. This is attributed to three factors: First, the CP covalent bonds and Si-OP bonds impart chemical stability between the carbon framework and the inorganic phase, making the structure less prone to damage during cyclic use; second, the Fe-NP sites possess catalytic regeneration characteristics, continuously degrading adsorbed pollutants during operation rather than simply accumulating them, thus mitigating site saturation; and third, the iron element is chemically anchored, resulting in minimal loss of active components during long-term use.

[0048] In comparison: Comparative Example 1: Due to the lack of ammonium phytate, the entire system relies on the physical mixing of biomass, iron tailings, and clay. In the wet state, the binding force comes solely from the van der Waals forces formed between the clay and water, resulting in poor plasticity and loose granules. During drying and carbonization, the lack of chemical bridging leads to uneven internal stress, causing extensive cracking and breakage. After carbonization, the specific surface area and mesoporous ratio originate solely from the pyrolysis of the biomass itself, far lower than the in-situ pore-forming effect of the ammonium phytate system. Iron is dispersed as loose iron oxide particles, lacking anchoring effect, resulting in severe leaching. There are no Fe-NP active sites; removal of atrazine relies solely on physical adsorption, which is inefficient and rapidly deactivated after cycling.

[0049] Comparative Example 2: Although phytic acid can chelate iron ions and form COP bonds, the lack of ammonium ions leads to two key problems: First, Fe-NP coordination structures cannot be formed during carbonization, and phases with lower catalytic activity, such as iron phosphate or iron pyrophosphate, are instead produced, resulting in significantly insufficient catalytic degradation ability of atrazine. Second, the strong acidity of phytic acid may partially dissolve mineral components during ball milling, damaging the integrity of the carbon skeleton. Simultaneously, the lack of alkaline NH3 release to promote Si-OP bond stability results in lower interfacial bonding strength compared to the ammonium phytate system. Therefore, although its various indicators are superior to the phosphorus-free system, there is still a significant gap compared to Example 2.

[0050] Comparative Example 3: The absence of the pre-fermentation step meant that the ammonium phytate solution, moisture, and various powder components did not have sufficient time to achieve spontaneous and uniform distribution and complete reaction before granulation. Some areas were moisture-rich with excessive chemical bonding, resulting in severe shrinkage during drying; other areas were insufficiently moist with weak adhesion. This inhomogeneity led to numerous microscopic defects and localized stress concentrations within the spheres, manifesting as increased cracking rate and decreased compressive strength during drying and carbonization. Simultaneously, uneven pore development slightly affected the specific surface area and mesopore ratio. The incomplete chemical bonding reaction also resulted in a slightly lower iron anchoring effect and active site density compared to the fully pre-fermented example.

[0051] Comparative Example 4: High-speed stirring cannot provide the impact, shearing, and friction effects of ball mill grinding balls on powder particles, resulting in insufficient particle refinement and a significantly lower specific surface area and interfacial contact area compared to the ball milling system. Simultaneously, the lack of mechanochemical activation effects hinders the efficient dehydration condensation reaction of COP and Si-OP bonds at 50-55℃, occurring only to a limited extent on the outer surface of the particles, with almost no chemical bonding within. This leads to weak overall interfacial bonding of the filter balls, insufficient compressive strength, poor pore structure development, partial iron fixation only in the outer layer, and easy leaching of iron from the internal iron tailings. Furthermore, the active sites are unevenly distributed and have low density, resulting in performance inferior to the ball milling-modified examples.

[0052] Comparative Example 5: When wet filter balls are directly exposed to a 100°C high-temperature environment, the surface moisture evaporates rapidly, causing rapid shrinkage, while the internal moisture diffuses slowly, creating a huge thermo-hygroscopic stress gradient. This leads to numerous penetrating microcracks and even macroscopic cracking in the filter balls. Although the chemical bonding and pore-forming mechanism of ammonium phytate still plays a role, and therefore indicators related to chemical composition such as specific surface area, iron leaching rate, and atrazine removal capacity remain at moderate levels, cracking and internal defects severely damage the structural integrity, manifested as a significant decrease in compressive strength and an increased breakage rate. In practical engineering applications, cracked filter balls are prone to further pulverization and loss during use, failing to meet the reliability requirements for long-term operation.

[0053] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A self-supporting porous composite filter ball for water treatment, characterized in that, It includes the following components by weight: biomass powder: 70-82 parts, iron tailings powder: 15-22 parts, clay: 3-8 parts, ammonium phytate: 2.5-5.5 parts, deionized water: 20-40 parts; The composite filter ball is prepared by carbonization in an inert atmosphere after modification with ammonium phytate. During the carbonization process, the biomass powder forms a porous carbon-based skeleton structure, and the iron tailings powder is dispersed in the carbon-based skeleton and provides iron-based active sites. Clay is used as a binder and structural support phase to improve the mechanical strength and molding stability of the filter ball. The composite filter ball can maintain its complete spherical structure without external support.

2. The self-supporting porous composite filter ball for water treatment according to claim 1, characterized in that, The particle size of the self-supporting porous composite filter ball is 2-3 cm.

3. The self-supporting porous composite filter ball for water treatment according to claim 1, characterized in that, The biomass powder is selected from one or more of coconut shell powder, corn stalk powder, and wheat stalk powder, with a particle size of 80-200 mesh.

4. The self-supporting porous composite filter ball for water treatment according to claim 1, characterized in that, The iron tailings powder is a solid waste generated from iron ore beneficiation, mainly composed of Fe2O3, SiO2 and Al2O3, with a particle size of 200-300 mesh.

5. The self-supporting porous composite filter ball for water treatment according to claim 1, characterized in that, The clay is selected from one or both of kaolin or bentonite, with a particle size of 200-300 mesh.

6. The method for preparing self-supporting porous composite filter balls for water treatment according to any one of claims 1-5, characterized in that, Includes the following steps: S1. Preparation of ammonium phytate solution: Add ammonium phytate to deionized water and stir until completely dissolved to obtain ammonium phytate solution; S2. Powder mixing and integrated modification into slurry: Biomass powder, iron tailings powder and clay are added to a planetary ball mill, zirconia grinding balls are added, and dry grinding is carried out for 10-15 min to obtain mixed powder. Then ammonium phytate solution is added to the mixed powder, and ball milling is continued at 50-55℃ for 1.0-1.5 h to make the mixed powder reach a plastic state that can be formed into a ball by hand and dispersed by light pressure. Then the material is allowed to stand and rest to obtain a plastic intermediate. S3. Granulation and pelletizing: The plastic intermediate is transferred to a disc granulator and rolled into pellets at a speed of 20-30 r / min to obtain the composite filter ball precursor; S4. Preparing and staged drying: The composite filter ball precursor is placed in a forced-air drying oven and dried in a gradient manner to obtain the dried composite filter ball; S5. Inert atmosphere carbonization: The dried composite filter balls are placed in a tube furnace, nitrogen is introduced, the temperature is raised to 650-750℃, the temperature is held for 1.5-2.5 hours, and then naturally cooled to room temperature to obtain self-supporting porous composite filter balls for water treatment.

7. The method for preparing self-supporting porous composite filter balls for water treatment according to claim 6, characterized in that, The zirconia grinding balls in S2 are composed of 10mm large balls and 5mm small balls in a weight ratio of 3:7, with a ball-to-material ratio of 5:

1. The grinding speed is 300-400 r / min, and the settling time is 10-60 min.

8. The method for preparing self-supporting porous composite filter balls for water treatment according to claim 6, characterized in that, The gradient drying in S4 refers to pre-drying at 40-80℃ for 1.5 h, and then raising the temperature to 80-120℃ for 3 h.

9. The method for preparing self-supporting porous composite filter balls for water treatment according to claim 6, characterized in that, The flow rate of nitrogen gas introduced in S5 is 100-150 ml / min.

10. The method for preparing self-supporting porous composite filter balls for water treatment according to claim 6, characterized in that, The heating rate of the tube furnace in the S5 is 8-12℃ / min.

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