A porous biochar loaded inorganic oxide composite material, and a preparation method and application thereof
By using porous biochar microspheres activated with KOH and modified with MgO-CaO composite oxides, combined with phosphate precipitation, the problems of insufficient pore structure and easy powder loss in biochar materials were solved, achieving efficient and stable cadmium ion removal and reducing operating costs.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- CHANGZHOU VOCATIONAL INST OF ENG
- Filing Date
- 2026-05-15
- Publication Date
- 2026-06-19
Smart Images
Figure CN122233533A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the fields of porous materials and wastewater treatment technology, specifically to a porous biochar-supported inorganic oxide composite material, its preparation method, and its application. Background Technology
[0002] Cadmium is a highly toxic and bioaccumulative heavy metal pollutant, mainly originating from industrial activities such as mining, electroplating, smelting, fertilizer production, and battery manufacturing. Even at low concentrations, cadmium-containing wastewater can accumulate in organisms through the food chain after entering the environment, causing serious health hazards to humans, including kidney damage, bone softening, and cancer. Therefore, developing efficient, economical, and environmentally friendly cadmium-containing wastewater treatment technologies is of significant practical importance.
[0003] Currently, the main methods for treating cadmium-containing wastewater include chemical precipitation, ion exchange, membrane separation, and adsorption. Chemical precipitation converts cadmium ions into hydroxides or sulfides through the addition of alkali or sulfides, offering simple operation but suffering from problems such as fine, difficult-to-settle precipitates, large sludge volumes, and a high risk of secondary pollution. Ion exchange provides good treatment results, but resin costs are high, and regeneration wastewater treatment is difficult. Membrane separation produces high-quality effluent, but severe membrane fouling and high operating costs limit its large-scale application. Adsorption is widely studied due to its ease of operation, low cost, and high treatment efficiency; the selection of the adsorbent is crucial to its effectiveness.
[0004] Biochar is a carbon-rich material obtained from the pyrolysis of biomass under anaerobic or anoxic conditions. It possesses advantages such as a wide availability of raw materials, high specific surface area, and abundant surface functional groups, exhibiting good affinity for heavy metal ions. However, the pore structure of raw biochar is limited and lacks sufficient active sites, making its adsorption capacity and rate for cadmium ions insufficient for practical applications. To address this issue, researchers have attempted to functionalize biochar through acid-base activation, metal oxide loading, and phosphate modification. Loading with alkaline metal oxides such as magnesium oxide and calcium oxide can promote the formation of hydroxide precipitates of cadmium ions by increasing the solution pH; phosphate modification can permanently immobilize cadmium ions by generating cadmium phosphate precipitates with extremely low solubility. Existing technologies have reported the preparation of phosphate-modified biochar through the co-pyrolysis of potassium dihydrogen phosphate and biomass, which exhibits good adsorption performance for cadmium ions. However, most existing modified biochar exists in powder form, which presents problems such as difficulty in separation and recovery and easy loss in practical water treatment applications. At the same time, the removal mechanism of cadmium ions by a single modification strategy is relatively simple and it is difficult to achieve efficient and stable removal effect. In addition, how to achieve uniform loading and firm anchoring of functional components while maintaining the porous structure of biochar is also a challenge currently facing the technology. Summary of the Invention
[0005] The technical problem to be solved: The purpose of this invention is to provide a porous biochar-supported inorganic oxide composite material, its preparation method and application, which combines the alkaline precipitation effect of MgO-CaO and the chemical precipitation effect of phosphate, and achieves efficient fixation of cadmium ions through a dual precipitation mechanism; at the same time, the millimeter-sized porous microsphere morphology facilitates sedimentation and separation, avoiding the problem of easy loss and difficulty in recycling of powder materials; the freeze-drying process endows the material with a continuous and interconnected pore structure, which is beneficial to mass transfer and exposure of reactive sites.
[0006] Technical solution: A porous biochar-supported inorganic oxide composite material, wherein the material uses KOH-activated cotton stalk porous biochar as a carrier, and MgO-CaO composite oxide nanoparticles are in situ anchored on the inner wall of the pores of the porous biochar. The material is also composited with potassium dihydrogen phosphate and cross-linked with sodium alginate to form freeze-dried porous microspheres.
[0007] The preparation method of the above-mentioned porous biochar-supported inorganic oxide composite material includes the following steps: S1. The washed and dried cotton stalks are crushed and passed through a 100-mesh sieve to obtain cotton stalk powder; the powder is mixed with KOH and then pyrolyzed at high temperature under a nitrogen atmosphere. After natural cooling, it is washed until neutral and dried to obtain porous biochar. S2. The porous biochar obtained in S1 is impregnated in a mixed solution containing magnesium salt and calcium salt under vacuum conditions, then dried and calcined at 500-700℃ for 2-4 hours under a nitrogen atmosphere to obtain porous biochar-supported magnesium-calcium composite oxide composite powder. S3. The porous biochar-supported magnesium-calcium composite oxide powder obtained in S2 is mixed with potassium dihydrogen phosphate powder at a mass ratio of (3-6):1. Sodium alginate aqueous solution is added and stirred to obtain a suspension. The suspension is dropped into calcium chloride solution drop by drop through a dropper. The droplets solidify into spheres. Stirring and crosslinking are continued to obtain wet gel spheres. S4. The wet gel balls obtained in S3 are taken out and transferred to a freeze dryer. After low-temperature freezing, they are vacuum freeze-dried to obtain a porous biochar-supported inorganic oxide composite material.
[0008] Preferably, in step S1, the mass ratio of cotton stalk powder to KOH is 1:(1-3); the high-temperature pyrolysis temperature is 600-700℃, the heating rate is 5-10℃ / min, and the holding time is 1-3h.
[0009] Preferably, in step S2, the solid-liquid ratio of porous biochar to the mixed solution is 1:(5-20) g / mL; the magnesium salt is Mg(NO3)2·6H2O, the calcium salt is Ca(NO3)2·4H2O, and the molar ratio of magnesium to calcium is (1-3):1.
[0010] Preferably, in step S2, the vacuum degree of vacuum-assisted permeation is -0.08 to -0.1 MPa, the pressure holding time is 20-40 min, the calcination temperature is 500-700℃, and the heat holding time is 3 h.
[0011] Preferably, the mass fraction of sodium alginate aqueous solution in step S3 is 2-3%.
[0012] Preferably, the freeze-drying conditions in step S4 are a freezing temperature below -50°C, a vacuum degree of less than 10 Pa, and a freeze-drying time of 24-48 hours.
[0013] Preferably, in the application of the above-mentioned porous biochar-supported inorganic oxide composite material in the treatment of cadmium-containing wastewater, the material is added to the Cd(II)-containing wastewater, and the material is recovered by sedimentation after the reaction is completed.
[0014] Preferably, the amount of the material added to the wastewater is 0.5-5 g / L, the pH of the wastewater is 3-10, and the reaction time is 2-12 h.
[0015] Beneficial effects: The porous biochar-supported inorganic oxide composite material of the present invention has the following advantages: This invention establishes a dual removal mechanism for hydroxide and phosphate precipitation through the synergistic effect of MgO-CaO composite oxide and potassium dihydrogen phosphate. The MgO-CaO composite oxide releases OH- upon contact with water. - , prompting Cd 2+ Cd(OH)2 microprecipitate is formed; at the same time, potassium dihydrogen phosphate releases PO4. 3- with cd 2+ and the Ca released from the material 2+ The reaction produces cadmium phosphate and hydroxyapatite minerals with extremely low solubility, converting cadmium ions from an exchangeable state to a stable mineral state, achieving irreversible fixation and avoiding secondary pollution; This invention utilizes sodium alginate cross-linking molding and freeze-drying technology to prepare millimeter-sized porous microspheres from powdered composite materials. These microspheres exhibit good dispersibility in water, and solid-liquid separation can be achieved through simple sedimentation or filtration after the reaction. This solves the engineering application problems of traditional powdered biochar materials, such as easy loss and difficulty in recycling, thereby reducing material loss and operating costs. This invention employs vacuum-assisted impregnation technology to allow magnesium-calcium salt solution to fully penetrate the micropores, mesopores, and natural macropores of biochar. Then, high-temperature calcination causes MgO-CaO composite oxide nanoparticles to be generated in situ and anchored to the inner wall of the pores, avoiding the aggregation and detachment of functional components on the material surface and significantly improving the structural stability and recyclability of the material. This invention employs a KOH activation combined with a natural multi-level pore retention strategy, enabling the material to simultaneously possess a three-level pore structure of micropores, mesopores, and natural macropores. This provides ample micro-reaction space for ion diffusion and precipitation reactions, accelerates the mass transfer rate, and shortens the processing time. This invention uses agricultural waste cotton stalks as biomass raw material, realizing the resource utilization of waste; the magnesium salt, calcium salt, potassium dihydrogen phosphate and sodium alginate used are all bulk chemical raw materials with low cost; the preparation process does not require the use of toxic organic solvents, and has good environmental friendliness and economic applicability. Attached Figure Description
[0016] Figure 1 Aperture distribution diagram; Figure 2 The adsorption capacity and removal rate of Example 1 at different dosages; Figure 3 The adsorption capacity and removal rate were determined for different ratios and strengths. Figure 4 This is a recycling diagram for an example; Figure 5 The removal rate and corresponding potential changes at different pH values are shown in Example 1. Detailed Implementation
[0017] The present invention will be further described below with reference to embodiments. These embodiments are illustrative of the present invention, but the present invention is not limited to these embodiments: Example 1
[0018] A method for preparing a porous biochar-supported inorganic oxide composite material includes the following steps: S1. The washed and dried cotton stalks are crushed and passed through a 100-mesh sieve to obtain cotton stalk powder; the powder is mixed with KOH at a mass ratio of 1:2, heated to 600℃ at a heating rate of 5℃ / min under a nitrogen atmosphere, pyrolyzed at high temperature for 3h, naturally cooled, washed until neutral, and dried to obtain porous biochar. S2. The porous biochar obtained in S1 was impregnated in a mixed solution containing magnesium salt and calcium salt at a solid-liquid ratio of 1:10 (g / mL). The magnesium salt was Mg(NO3)2·6H2O and the calcium salt was Ca(NO3)2·4H2O. The molar ratio of magnesium to calcium was 2:1. The impregnation was carried out for 30 minutes under a vacuum of -0.1 MPa. After being removed and dried, the mixture was calcined at 600℃ for 2 hours under a nitrogen atmosphere to obtain porous biochar-supported magnesium-calcium composite oxide composite powder. S3. The porous biochar-supported magnesium-calcium composite oxide composite powder obtained in S2 is mixed with potassium dihydrogen phosphate powder at a mass ratio of 5:1. A sodium alginate aqueous solution with a mass fraction of 2% is added and stirred to obtain a suspension. The suspension is dropped dropwise into a calcium chloride solution through a drop ball device. The droplets solidify into balls and are further stirred to crosslink, thus obtaining wet gel balls. S4. The wet gel balls obtained in S3 are taken out and transferred to a freeze dryer. After freezing at -50°C, they are freeze-dried for 48 hours under a vacuum of less than 10 Pa to obtain a porous biochar-supported inorganic oxide composite material.
[0019] Example 2
[0020] A method for preparing a porous biochar-supported inorganic oxide composite material includes the following steps: S1. The washed and dried cotton stalks are crushed and passed through a 100-mesh sieve to obtain cotton stalk powder; the powder is mixed with KOH at a mass ratio of 1:1, heated to 650℃ at a heating rate of 8℃ / min under a nitrogen atmosphere, pyrolyzed at high temperature for 1.5h, naturally cooled, washed until neutral, and dried to obtain porous biochar. S2. The porous biochar obtained in S1 was impregnated in a mixed solution containing magnesium salt and calcium salt at a solid-liquid ratio of 1:5 (g / mL). The magnesium salt was Mg(NO3)2·6H2O and the calcium salt was Ca(NO3)2·4H2O. The molar ratio of magnesium to calcium was 1.5:1. The impregnation was carried out for 20 minutes under a vacuum of -0.08 MPa. After being removed and dried, the mixture was calcined at 550℃ for 2.5 h under a nitrogen atmosphere to obtain porous biochar-supported magnesium-calcium composite oxide powder. S3. The porous biochar-supported magnesium-calcium composite oxide composite powder obtained in S2 is uniformly mixed with potassium dihydrogen phosphate powder at a mass ratio of 3:1. A sodium alginate aqueous solution with a mass fraction of 2.5% is added and stirred to obtain a suspension. The suspension is dropped dropwise into a calcium chloride solution through a drop ball device. The droplets solidify into balls, and the mixture is stirred and crosslinked to obtain wet gel balls. S4. The wet gel balls obtained in S3 are taken out and transferred to a freeze dryer. After freezing at -50°C, they are freeze-dried for 30 hours under a vacuum of less than 10 Pa to obtain a porous biochar-supported inorganic oxide composite material.
[0021] Example 3
[0022] A method for preparing a porous biochar-supported inorganic oxide composite material includes the following steps: S1. The washed and dried cotton stalks are crushed and passed through a 100-mesh sieve to obtain cotton stalk powder; the powder is mixed with KOH at a mass ratio of 1:1.5, heated to 680℃ at a heating rate of 10℃ / min under a nitrogen atmosphere, pyrolyzed at high temperature for 1.5h, naturally cooled, washed until neutral, and dried to obtain porous biochar. S2. The porous biochar obtained in S1 was impregnated in a mixed solution containing magnesium salt and calcium salt at a solid-liquid ratio of 1:15 (g / mL). The magnesium salt was Mg(NO3)2·6H2O and the calcium salt was Ca(NO3)2·4H2O. The molar ratio of magnesium to calcium was 2.5:1. The impregnation was carried out for 30 minutes under a vacuum of -0.1 MPa. After being removed and dried, the mixture was calcined at 500℃ for 4 hours under a nitrogen atmosphere to obtain porous biochar-supported magnesium-calcium composite oxide composite powder. S3. The porous biochar-supported magnesium-calcium composite oxide composite powder obtained in S2 is uniformly mixed with potassium dihydrogen phosphate powder at a mass ratio of 4:1. A sodium alginate aqueous solution with a mass fraction of 2.8% is added and stirred to obtain a suspension. The suspension is dropped dropwise into a calcium chloride solution through a drop ball device. The droplets solidify into balls, and the mixture is stirred and crosslinked to obtain wet gel balls. S4. The wet gel balls obtained in S3 are taken out and transferred to a freeze dryer. After freezing at -50°C, they are freeze-dried for 24 hours under a vacuum of less than 10 Pa to obtain a porous biochar-supported inorganic oxide composite material.
[0023] Example 4
[0024] A method for preparing a porous biochar-supported inorganic oxide composite material includes the following steps: S1. The washed and dried cotton stalks are crushed and passed through a 100-mesh sieve to obtain cotton stalk powder; the powder is mixed with KOH at a mass ratio of 1:3, heated to 620℃ at a heating rate of 5℃ / min under a nitrogen atmosphere, pyrolyzed at high temperature for 2h, naturally cooled, washed until neutral, and dried to obtain porous biochar. S2. The porous biochar obtained in S1 was impregnated in a mixed solution containing magnesium salt and calcium salt at a solid-liquid ratio of 1:20 (g / mL). The magnesium salt was Mg(NO3)2·6H2O and the calcium salt was Ca(NO3)2·4H2O. The molar ratio of magnesium to calcium was 3:1. The impregnation was carried out for 30 minutes under a vacuum of -0.1 MPa. After being removed and dried, the mixture was calcined at 600℃ for 2 hours under a nitrogen atmosphere to obtain a porous biochar-supported magnesium-calcium composite oxide powder. S3. The porous biochar-supported magnesium-calcium composite oxide powder obtained in S2 is uniformly mixed with potassium dihydrogen phosphate powder at a mass ratio of 6:1. A sodium alginate aqueous solution with a mass fraction of 3% is added and stirred to obtain a suspension. The suspension is dropped dropwise into a calcium chloride solution through a drop ball device. The droplets solidify into balls and are further stirred to crosslink, thus obtaining wet gel balls. S4. The wet gel balls obtained in S3 are taken out and transferred to a freeze dryer. After freezing at -50°C, they are freeze-dried for 42 hours under a vacuum of less than 10 Pa to obtain a porous biochar-supported inorganic oxide composite material.
[0025] Example 5
[0026] A method for preparing a porous biochar-supported inorganic oxide composite material includes the following steps: S1. The washed and dried cotton stalks are crushed and passed through a 100-mesh sieve to obtain cotton stalk powder; the powder is mixed with KOH at a mass ratio of 1:2.5, heated to 700℃ at a heating rate of 8℃ / min under a nitrogen atmosphere, pyrolyzed at high temperature for 2h, naturally cooled, washed until neutral, and dried to obtain porous biochar. S2. The porous biochar obtained in S1 was impregnated in a mixed solution containing magnesium salt and calcium salt at a solid-liquid ratio of 1:12 (g / mL). The magnesium salt was Mg(NO3)2·6H2O and the calcium salt was Ca(NO3)2·4H2O. The molar ratio of magnesium to calcium was 1:1. The impregnation was carried out for 30 minutes under a vacuum of -0.1 MPa. After being removed and dried, the mixture was calcined at 600℃ for 2 hours under a nitrogen atmosphere to obtain porous biochar-supported magnesium-calcium composite oxide powder. S3. The porous biochar-supported magnesium-calcium composite oxide powder obtained in S2 is uniformly mixed with potassium dihydrogen phosphate powder at a mass ratio of 5.5:1. A sodium alginate aqueous solution with a mass fraction of 2% is added and stirred to obtain a suspension. The suspension is dropped dropwise into a calcium chloride solution through a dropper device. The droplets solidify into spheres. Stirring is continued to crosslink the mixture to obtain wet gel spheres. S4. The wet gel balls obtained in S3 are taken out and transferred to a freeze dryer. After freezing at -50°C, they are freeze-dried for 48 hours under a vacuum of less than 10 Pa to obtain a porous biochar-supported inorganic oxide composite material.
[0027] Comparative Example 1 The main difference between Comparative Example 1 and Example 1 is that: without KOH activation, the washed and dried cotton stalks were crushed, passed through a 100-mesh sieve to obtain cotton stalk powder, heated to 600°C at a heating rate of 5°C / min under a nitrogen atmosphere, kept at the temperature for 2 hours for pyrolysis, naturally cooled, washed until neutral, and dried to obtain biochar; the subsequent steps were the same as in Example 1.
[0028] Comparative Example 2 The main difference between Comparative Example 2 and Example 1 is that there is no MgO-CaO loading, and S1 is the same as in Example 1; step S2 is omitted, and the porous biochar obtained in S1 is directly subjected to steps S3 and S4.
[0029] Comparative Example 3 The main difference between Comparative Example 3 and Example 1 is that: no potassium dihydrogen phosphate composite was used, and S1 and S2 were the same as in Example 1; the step of adding potassium dihydrogen phosphate in S3 was omitted, and the composite powder obtained in S2 was directly added to sodium alginate aqueous solution for drop ball crosslinking and freeze drying.
[0030] Comparative Example 4 The main difference between Comparative Example 4 and Example 1 is that S1, S2, and S3 are the same as in Example 1, but the drop ball crosslinking and freeze-drying steps of S4 are omitted, and the composite powder obtained in S3 is directly dried and used.
[0031] Comparative Example 5 The main difference between Comparative Example 5 and Example 1 is that S1, S2, and S3 are the same as in Example 1; in S4, the wet gel balls are taken out, rinsed, and then placed in a 60°C oven to dry directly for 24 hours, instead of freeze-drying.
[0032] Comparative Example 6 The main difference between Comparative Example 6 and Example 1 is that S1 and S2 are the same as in Example 1; after the composite powder obtained in S2 is uniformly mixed with potassium dihydrogen phosphate powder at a mass ratio of 5:1, it is directly pressed into sheet-like particles without adding sodium alginate or performing calcium chloride crosslinking.
[0033] Comparative Example 7 The main difference between Comparative Example 7 and Example 1 is that: S1 is the same as in Example 1; in S2, magnesium salt and calcium salt are mixed in a molar ratio of 6:1 to prepare a mixed solution, and the rest is the same as in Example 1; S3 and S4 are the same as in Example 1.
[0034] Comparative Example 8 The main difference between Comparative Example 8 and Example 1 is as follows: S1 is the same as Example 1; in S2, calcium salt is prepared separately (without magnesium salt), and the porous biochar obtained in S1 is impregnated in a solution containing Ca(NO3)2·4H2O, the rest is the same as Example 1; S3 and S4 are the same as Example 1.
[0035] Comparative Example 9 The main difference between Comparative Example 9 and Example 1 is that the calcination temperature in step S2 is 800°C.
[0036] Comparative Example 10 The main difference between Comparative Example 10 and Example 1 is as follows: S1 is the same as Example 1; in S2, aluminum salt (Al(NO3)3·9H2O) and calcium salt (Ca(NO3)2·4H2O) are mixed in an Al / Ca molar ratio of 1:2, and the porous biochar obtained in S1 is impregnated in the mixed solution, and the rest is the same as Example 1; S3 and S4 are the same as Example 1.
[0037] Performance testing: Adsorption test This invention simulates acidic mine wastewater by preparing a Cd(II)-containing solution using deionized water. 50 mL of 10 mg·L⁻¹ solution was used. -1 A Cd(II) solution was added to a 100 mL centrifuge tube and centrifuged using a 0.1 mol·L⁻¹ centrifuge tube. -1 Nitric acid and sodium hydroxide were used to adjust the pH of the Cd(II)-containing solution to 5.0. Then, 1 g·L⁻¹ was added to the solution. －1 The adsorbent was used, and the mixed solution was transferred to 25°C at 180 r·min. -1 The reaction was carried out in a constant temperature shaker for 1440 min. The removal effect of the examples and comparative examples was investigated, and the dosage of Example 1 (0.5 g / L~5 g·L⁻¹) was compared. －1 The effect of initial pH (3.0~10.0) on the removal efficiency of Cd(II) by the adsorbent was investigated. Each experiment was repeated three times, and the average value was taken as the final experimental result. After the reaction was completed, the solution was filtered through filter paper, and the residual concentration of Cd(II) in the filtrate was determined by atomic absorption spectrophotometry. The removal rate and adsorption capacity of Cd(II) in aqueous solution of the present invention were calculated according to equations (1) and (2).
[0038] R=C0-C e / C0×100% (1) q e =(C0-C e )*V / m (2) In the formula: R is the removal rate, %; C0 and C e These are the initial and equilibrium Cd(II) concentrations, in mg·L⁻¹. -1 ;q e Adsorption capacity, mg·g -1 V is the solution volume, mL; m is the adsorbent mass, mg. The surface charge of the adsorbent was analyzed by investigating the Zeta potential values of the adsorbent in aqueous solutions at different pH values using a Zetasizer 3000HS Zeta potential meter; the pore structure of the adsorbent was investigated by a pore structure analyzer (Mc, ASAP2020, USA).
[0039] from Figure 1As can be seen, the peak pore size distribution of the unmodified cotton stalk biochar is around 12 nm, with a low pore volume and narrow distribution. The material in Example 1, prepared by KOH activation, MgO-CaO / KH2PO4 loading, and sodium alginate cross-linking to form spheres, shows a shift in the peak pore size distribution to around 13 nm, a significant increase in overall pore volume, and a more uniform distribution of mesopores in the 10-20 nm range. This indicates that the activation process successfully introduced more mesoporous structures, providing sufficient channels for Cd(II) diffusion, while the loading and spheroidization processes did not cause severe pore blockage, preserving the porous characteristics of the material and facilitating the exposure of active sites and the adsorption reaction.
[0040] from Figure 2 As can be seen, the removal rate of Cd(II) in the porous biochar-supported inorganic oxide composite material first increases and then decreases with increasing dosage, while the unit adsorption capacity continuously decreases with increasing dosage. This phenomenon can be systematically analyzed by combining the material's structural characteristics and adsorption mechanism: when the dosage increases from 0.5 g / L to 3.0 g / L, the removal rate rapidly increases from about 25% to its peak. The core reason is that with the increase in material dosage, the available physical adsorption sites (the well-developed pores of KOH-activated biochar), chemical active sites (the alkaline precipitation sites of MgO-CaO composite oxides and the coordination precipitation sites of potassium dihydrogen phosphate), and pH buffering capacity within the system are simultaneously enhanced. The contact probability and efficiency between Cd(II) and the material are significantly improved. At the same time, the three-dimensional porous structure of sodium alginate cross-linked microspheres provides sufficient channels for the diffusion and reaction of Cd(II), synergistically achieving efficient removal. When the dosage exceeds 3.0 g / L, the removal rate gradually decreases. The decrease in adsorption capacity is due to the following reasons: excessive addition leads to agglomeration of microspheres, which encapsulates and shields some active sites. Simultaneously, the continuous release of alkaline components from MgO-CaO at high dosages causes the solution pH to become too high, easily forming a dense hydroxide precipitate layer on the material surface, hindering the diffusion of Cd(II) into the internal pores. Furthermore, the increased mass transfer resistance caused by excessive material further reduces the effective adsorption efficiency. The continuous decrease in adsorption capacity per unit volume is essentially because the total amount of Cd(II) in the solution is fixed. At lower dosages, the active sites of the material can fully contact Cd(II), resulting in high site utilization per unit mass of material, with an adsorption capacity of approximately 27 mg / g. As the dosage increases, Cd(II) in the solution is rapidly consumed, and the newly added material's active sites cannot fully contact Cd(II), significantly reducing site utilization. Simultaneously, the agglomeration effect further exacerbates the decrease in adsorption efficiency per unit volume, ultimately reducing the adsorption capacity per unit volume to approximately 2 mg / g at a dosage of 5.0 g / L. Considering both removal rate and material utilization rate, the optimal dosage of this composite material is 3.0 g / L.
[0041] from Figure 3As can be seen from the data, under the condition of 3.0 g / L added to the water, comparing Example 1 and Example 3, it is evident that when the Mg / Ca molar ratio is adjusted from 2:1 to 3:1, the removal rate decreases from 98.5% to 97.2%, and the adsorption capacity decreases from 9.85 mg / g to 9.72 mg / g. This is because CaO reacts with water to generate Ca(OH)2, releasing OH-. - Increase pH and provide Ca 2+ MgO participates in phosphate precipitation reactions, while its hydration rate is relatively slow, primarily acting as a pH buffer and surface complexing agent. When the Mg / Ca ratio is too high, Ca... 2+ The relative deficiency led to a decrease in the precipitation of cadmium phosphate and hydroxyapatite, while the slightly slower rate of pH increase affected Cd. 2+ The initial precipitation rate is the primary factor, therefore a 2:1 molar ratio is more effective in achieving optimal synergy between MgO and CaO. Secondly, Comparative Example 1, without KOH activation, had a low specific surface area and pore development of biochar, resulting in a significant decrease in the subsequent MgO-CaO loading and distribution uniformity. This reduced the attachment sites for potassium dihydrogen phosphate and significantly weakened the contribution of physical adsorption, resulting in a removal rate of only 72.3%. Comparative Example 2, omitting MgO-CaO loading and relying solely on KH2PO4 precipitation and biochar adsorption, achieved a removal rate of only 68.7%. Comparative Example 3, omitting KH2PO4 compounding and relying solely on alkaline precipitation of MgO-CaO and biochar adsorption, achieved a removal rate of 75.1%. Both were lower than in Example 1, indicating a synergistic effect between the alkaline precipitation of MgO-CaO and the chemical precipitation of KH2PO4, neither of which can be omitted. Furthermore, Comparative Example 4, without spheroidization, was in powder form, exhibiting some dispersibility but difficult to separate and prone to loss, resulting in a removal rate of 81.2%. Comparative Example 5 used oven drying instead of freeze-drying; the surface tension of moisture caused pore shrinkage and collapse, reducing the specific surface area, resulting in a removal rate of 83.5%. Comparative Example 6, without sodium alginate crosslinking, had poor particle mechanical strength and was easily disintegrated in water, resulting in a removal rate of 79.4%. This demonstrates that millimeter-scale microsphere forming, freeze-drying to preserve pores, and sodium alginate crosslinking to enhance structural stability significantly contribute to material performance. In addition, Comparative Example 7 reversed the Mg / Ca molar ratio to 1:3, reducing the removal rate to 70.6%; Comparative Example 8 only loaded CaO without adding MgO, resulting in a removal rate of only 65.8%. The former suffered from excessive CaO leading to a rapid increase in pH and Cd. 2+ Soluble Cd(OH)3 is generated under extremely alkaline conditions. - and Cd(OH)4 2- The complex reduced precipitation efficiency; the latter, lacking the pH buffer and surface complexation of MgO, also performed poorly. In Comparative Example 9, the calcination temperature was raised to 800℃, exceeding the planned range, leading to phosphate decomposition or solid-phase reaction with metal oxides to form insoluble phosphate complexes, PO4. 3-The release capacity was reduced, with a removal rate of only 77.9%. The performance of Comparative Example 10 was not only significantly lower than that of Example 1, but even slightly lower than that of Comparative Example 8, demonstrating that the advantage of the MgO-CaO composite oxide does not simply stem from the combination of the two oxides, but rather from the combined effect of MgO's unique alkaline buffering, surface complexation, ion exchange capacity, and pH synergistic regulation mechanism with CaO. Al2O3 cannot replace MgO in forming similar multiple synergistic effects with CaO, further highlighting the inventiveness and irreplaceability of this invention's choice of MgO-CaO as the composite oxide.
[0042] from Figure 4 As can be seen from the adsorption kinetic data of the five cycles, the C / C ratio of the porous biochar-supported inorganic oxide composite material prepared in Example 1 continued to decrease with the extension of adsorption time during the cycle. Moreover, the kinetic characteristics of rapid adsorption and gradual equilibrium were maintained in all five cycles, and the overall adsorption rate and removal effect only showed a slight decrease. This is because the material uses KOH-activated cotton stalk biochar as a carrier, which has a rich and stable hierarchical pore structure, providing ample channels for the diffusion and adsorption of Cd(II). At the same time, the in-situ anchored MgO-CaO composite oxide and the composite potassium dihydrogen phosphate can achieve efficient fixation of Cd(II) through the synergistic effect of alkaline precipitation and phosphate precipitation. Combined with the porous microsphere structure formed by sodium alginate cross-linking and freeze-drying, it can not only avoid the loss of active components during the cycle, but also maintain the integrity of the material structure, reduce pore collapse and particle agglomeration, so that the material retains a large number of effective active sites after repeated use, thus exhibiting excellent cycle stability and continuous removal capacity. Only a small number of active sites are occupied or slight structural damage causes a slight decrease in the later cycle adsorption rate, and the overall material still has good engineering application potential.
[0043] from Figure 5 As can be seen, the dosage of 3.0 g / L in Example 1 varies with different pH values and the corresponding potential changes; the removal rate is significantly lower at pH 3-4, and the absolute value of the Zeta potential is also smaller. This is because high concentrations of H2... + with cd 2+ It competes with active sites on the material surface, while inhibiting the hydrolysis of MgO-CaO, making it difficult to form hydroxide precipitates, and exhibiting weak electrostatic attraction; the removal rate rises rapidly and reaches its peak at pH 5-7, and the Zeta potential becomes significantly more negative. At this point, H +Competition decreases, the negative charge on the material surface increases, and electrostatic attraction strengthens. Simultaneously, the precipitation reaction of MgO-CaO and phosphate ions proceeds fully, exhibiting the strongest synergistic effect. At pH 8-10, the removal rate decreases slowly but remains at a relatively high level overall. The Zeta potential continues to become more negative, but the rate of increase slows. This is because under strongly alkaline conditions, Cd(II) forms negatively charged hydroxyl complexes, which electrostatically repel the similarly negatively charged material surface. Simultaneously, excess OH... - It will form a deposit layer on the material surface, blocking some of the pores and causing a decrease in the removal rate.
[0044] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.
Claims
1. A porous biochar-supported inorganic oxide composite material, characterized in that: The material uses KOH-activated porous biochar from cotton stalks as a carrier. MgO-CaO composite oxide nanoparticles are in situ anchored on the inner wall of the pores of the porous biochar. The material is also composited with potassium dihydrogen phosphate and cross-linked with sodium alginate to form freeze-dried porous microspheres.
2. The method for preparing porous biochar-supported inorganic oxide composite material according to claim 1, characterized in that: Includes the following steps: S1. The washed and dried cotton stalks are crushed and passed through a 100-mesh sieve to obtain cotton stalk powder; the powder is mixed with KOH and then pyrolyzed at high temperature under a nitrogen atmosphere. After natural cooling, it is washed until neutral and dried to obtain porous biochar. S2. The porous biochar obtained in S1 is impregnated in a mixed solution containing magnesium salt and calcium salt under vacuum conditions, then dried and calcined at 500-700℃ for 2-4 hours under a nitrogen atmosphere to obtain porous biochar-supported magnesium-calcium composite oxide composite powder. S3. Mix the porous biochar-supported magnesium-calcium composite oxide powder obtained in S2 with potassium dihydrogen phosphate powder at a mass ratio of (3-6):1, add sodium alginate aqueous solution, and stir to obtain a suspension. The suspension was dropped dropwise into a calcium chloride solution using a dropper device. The droplets solidified into spheres, and the mixture was stirred and cross-linked to obtain wet gel spheres. S4. The wet gel balls obtained in S3 are retrieved and transferred to a freeze dryer for vacuum freeze drying to obtain a porous biochar-supported inorganic oxide composite material.
3. The method for preparing porous biochar-supported inorganic oxide composite materials according to claim 2, characterized in that: In step S1, the mass ratio of cotton stalk powder to KOH is 1:(1-3); the high-temperature pyrolysis temperature is 600-700℃, the heating rate is 5-10℃ / min, and the holding time is 1-3h.
4. The method for preparing porous biochar-supported inorganic oxide composite material according to claim 2, characterized in that: In step S2, the solid-liquid ratio of porous biochar to the mixed solution is 1:(5-20); the magnesium salt is Mg(NO3)2·6H2O, the calcium salt is Ca(NO3)2·4H2O, and the molar ratio of magnesium to calcium is (1-3):
1.
5. The method for preparing the porous biochar-supported inorganic oxide composite material according to claim 2, characterized in that: In step S2, the vacuum degree of vacuum-assisted permeation is -0.08 to -0.1 MPa, the pressure holding time is 20-40 min, the calcination temperature is 500-700℃, and the heat holding time is 3 h.
6. The method for preparing porous biochar-supported inorganic oxide composite material according to claim 2, characterized in that: The mass fraction of sodium alginate aqueous solution in step S3 is 2-3%.
7. The method for preparing the porous biochar-supported inorganic oxide composite material according to claim 2, characterized in that: The freeze-drying conditions in step S4 are: freezing temperature below -50℃, vacuum degree less than 10 Pa, and freeze-drying time of 24-48h.
8. The application of the porous biochar-supported inorganic oxide composite material according to claim 1 in the treatment of cadmium-containing wastewater, characterized in that: The material is added to wastewater containing Cd(II), and the material is recovered by sedimentation after the reaction is completed.
9. The application according to claim 8, characterized in that: The material is added to the wastewater at a rate of 0.5-5 g / L, the wastewater pH is 3-10, and the reaction time is 2-12 h.