Nano-hydroxyapatite loaded biochar composite adsorbent for heavy metal adsorption as well as preparation method and application of nano-hydroxyapatite loaded biochar composite adsorbent

Abstract

The invention discloses a nano-hydroxyapatite loaded biochar composite adsorbent for heavy metal adsorption as well as a preparation method and application thereof, and belongs to the technical field of environmental functional materials. The preparation method comprises the following steps: by taking bean tree peony bark waste and ammonia-alkali residues as raw materials, preparing a charcoal carrier through anaerobic pyrolysis and wet ball milling, then converting a calcium source in an ammonia-alkali residue leaching solution into nano-hydroxyapatite by utilizing a precipitation method, and loading the nano-hydroxyapatite on the surface of charcoal to obtain a final product. The composite adsorbent has the structural advantages of developed mesopores and abundant surface active sites, shows high adsorption capacity and excellent selectivity to Pb (II) and Cd (II) in water, and particularly has strong anti-interference capability in complex water bodies; the preparation process is simple, the environmental compatibility is good, the condition is mild, high-valued utilization of the bean tree peony bark waste and the ammonia alkali residues is achieved, an efficient and environment-friendly novel functional material is provided for heavy metal pollution treatment of water, and remarkable environmental benefits and application and popularization values are achieved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of environmental functional materials, and particularly relates to a nano-hydroxyapatite-loaded biochar composite adsorbent, a preparation method thereof and application thereof in heavy metal adsorption. BACKGROUND

[0002] In recent decades, soil heavy metal pollution has become a global environmental concern, mainly due to the intensification of human activities such as mining and smelting, fertilizer application, and wastewater irrigation. Among them, cadmium (Cd) and lead (Pb) have become the focus of pollution control due to their wide distribution, long environmental residence time, strong toxicity, and easy enrichment in biological bodies. These heavy metals can accumulate in the human body through the food chain, causing multiple organ health problems such as central nervous system damage, cardiovascular dysfunction, reproductive system disorders, and bone lesions, seriously threatening human survival safety. Therefore, developing efficient and feasible heavy metal pollution control technologies has become a key issue that needs to be broken through in the field of environmental science.

[0003] Currently, various remediation technologies have been developed for Cd and Pb contaminated soils, including excavation, soil washing, deep plowing, electrodynamic remediation, phytoremediation, and chemical fixation. Among them, chemical fixation technology has become a research and application hotspot due to its high practicality, significant cost-effectiveness, short implementation period, and stable remediation effect. The selection of the modifier is a key link for the successful implementation of chemical fixation technology. Numerous studies have shown that biochar, as an ideal soil modifier, has shown great potential in heavy metal pollution remediation due to its unique advantages such as stable carbon skeleton structure, good environmental compatibility, easy availability of raw materials, soil property improvement ability, and carbon sequestration promotion.

[0004] However, existing research indicates that the adsorption capacity of biochar is limited, especially for metal ions, with a typical value of only 10-20 mg / g. To achieve the desired remediation effect, the amount of biochar used often needs to be significantly increased, which may cause multiple negative effects, including inhibition of seed germination, disruption of soil microbial community structure, and reduction of soil enzyme activity. These adverse factors not only may weaken the long-term effectiveness of biochar as a soil modifier, but also may restrict its large-scale application in sustainable environmental management. Therefore, improving the heavy metal fixation capacity of biochar through targeted modification methods has become a key strategy to break through its application bottleneck and enhance environmental remediation efficiency.

[0005] Nano-hydroxyapatite (nHAP, Ca 10(PO4)6(OH)2) is considered as an ideal material for remediating heavy metal pollution due to its non-toxicity, excellent biocompatibility, strong stability, and high affinity to metal ions. For example, nHAP can interact with Pb(II) to form a highly stable mineral phase Pb5(PO4)3(OH, Cl, F) with a solubility product constant (K sp ) between 10 -71.6 and 10 -84.4 , exhibiting a strong heavy metal immobilization capacity. However, nHAP nanoparticles are prone to self-aggregation due to their high surface energy, leading to reduced dispersibility and hindered soil migration, which in turn weakens their heavy metal immobilization efficiency. To address this issue, researchers in the field have proposed a composite strategy of loading nHAP on biochar, which not only effectively alleviates the self-aggregation problem of nanoparticles but also functionalizes the biochar, significantly improving its adsorption performance for heavy metals. For example, Liu et al. (Liu et al., 2025. Hydroxyapatite immobilized on peanut shell biochar toward Pb 2+ efficient capture: Performance and mechanism. Colloid Surface A 714, 136570.) successfully prepared nHAP@biochar composite adsorbents using peanut shell-derived biochar as a carrier, which increased the specific surface area by 3.77 times compared to pure nHAP and improved the adsorption capacity for Pb(II) by 18.3 times compared to unmodified biochar, providing an efficient and feasible solution for heavy metal-contaminated soil remediation. However, despite the excellent adsorption performance of the composite adsorbent, the large-scale field application of nHAP still faces a significant cost bottleneck, with an input cost of up to $150,000 to $750,000 per hectare, severely restricting its promotion process.

[0006] To address this issue, researchers in the field have successfully synthesized high-purity nHAP using low-value by-products (solid waste and distillation waste liquid) from ammonia-alkali plants as the only calcium source, providing a new approach to reducing costs. However, it is worth noting that the leaching solution produced during the pre-washing of ammonia-alkali residue (ASR) has been long neglected and not effectively utilized as an important resource rich in water-soluble calcium salts. Existing research shows that ASR has a large amount of soluble calcium salt components attached to its surface, and if effectively recovered and utilized, it can not only further reduce the production cost of nHAP but also open up new paths for the comprehensive utilization of by-products in the ammonia-alkali industry, bringing significant economic and environmental benefits. Unfortunately, there is no existing technology that publicly discloses a technical solution for synthesizing high-performance nHAP-loaded biochar adsorbent materials based on ammonia-alkali residue (ASR), and further in-depth research is needed on the related preparation process. SUMMARY

[0007] The present application aims to solve the problems existing in the preparation process of the existing load nano-hydroxyapatite biochar composite adsorbent, and innovatively proposes a preparation scheme of the composite adsorbent taking bean bark waste (DDS) as a biochar carrier raw material and taking ammonia-alkali residue (ASR) leaching liquor as a nHAP calcium source. The scheme improves the dispersity of nHAP nanoparticles through the porous structure of the bean bark waste derived biochar, improves the adsorption capacity and selectivity of the composite adsorbent to heavy metals, at the same time, realizes the resource utilization of low-value by-products by means of the ammonia-alkali residue leaching liquor, and greatly reduces the preparation cost of nHAP; in addition, the technology can also realize the high-value utilization of bean bark waste, and finally obtain a load nano-hydroxyapatite biochar composite adsorbent which is environmentally friendly in raw material, simple in preparation, low in cost and excellent in heavy metal adsorption performance, and provides a new functional material with economic and practicality for soil and water heavy metal pollution remediation, which has a wide application prospect.

[0008] The present application is implemented in the following way: a load nano-hydroxyapatite biochar composite adsorbent, which is composed of nano-hydroxyapatite (nHAP) and biochar; the biochar is prepared from bean bark waste (DDS) through anaerobic pyrolysis and wet ball milling process; the nano-hydroxyapatite (nHAP) is prepared by taking ammonia-alkali residue (ASR) leaching liquor as a calcium source and generating through a precipitation reaction with diammonium hydrogen phosphate; the mass ratio of nHAP to biochar is 0.5-3:1.

[0009] Preferably, the mass ratio of nHAP to biochar is 2:1

[0010] The preparation method of the above load nano-hydroxyapatite biochar composite adsorbent includes the following steps:

[0011] S1. Dry and grind the bean bark waste (DDS) and ammonia-alkali residue (ASR) and then sieve them;

[0012] S2. Biochar preparation:

[0013] S21. Anaerobic pyrolysis: pyrolyze the pretreated DDS under an inert atmosphere;

[0014] S22. Wet ball milling: transfer the pyrolysis product to a ball mill tank for ball milling treatment to obtain a biochar suspension;

[0015] S3. Preparation of load nano-hydroxyapatite biochar:

[0016] S31. Mix the pretreated ASR with water, stir, filter, and collect the supernatant rich in calcium ions;

[0017] S32. The ASR leachate is mixed with the biochar suspension obtained in step S22, diluted and stirred;

[0018] S33. A solution of diammonium hydrogen phosphate is added dropwise to the mixed system, stirred, and aged;

[0019] S34. The aged product is filtered, washed, and dried to obtain a biochar composite adsorbent loaded with nano-hydroxyapatite.

[0020] Further, in step S21, the pyrolysis temperature rate is 3-10℃ / min, the pyrolysis temperature is 600-800℃, and the pyrolysis time is 1-5 h.

[0021] Further, the ball mill tank used is a zirconia ball mill tank, the mass ratio of zirconia ball to biochar is 80-150:1, the solid-liquid ratio of biochar to deionized water is 1 g:15-30 mL, the ball milling speed is 200-500 rpm, and the ball milling time is 6-18 h.

[0022] Further, in step S31, the mixing ratio of ASR to water is 1 g:50-150 mL, and the Ca(II) concentration in the supernatant is 1.0-5.0 g / L.

[0023] Further, in step S33, the concentration of the diammonium hydrogen phosphate solution is 0.1-0.3 mol / L, the volume is 50-200 mL, the dropwise addition rate is 0.5-2 mL / min, the stirring is continued for 1-4 h after the dropwise addition is completed, and the aging time is 12-48 h.

[0024] Further, in the obtained biochar composite adsorbent loaded with nano-hydroxyapatite, the mass ratio of nano-hydroxyapatite to biochar is 0.5-3:1, and preferably 2:1.

[0025] Further, it further includes a step of predicting the saturated adsorption amount of heavy metal ions for the composite adsorbent which is calculated by the following formula:

[0026]

[0027] is the adsorption capacity of pure biochar, r is the mass ratio of nHAP to biochar, C1 and k are parameters to be fitted.

[0028] The above-mentioned nano-hydroxyapatite-loaded biochar composite adsorbent can be effectively applied to the adsorption process of heavy metals, such as Pb (II) and / or Cd (II); the adsorption capacity of the optimized composite adsorbent (HMD2) for Pb (II) and Cd (II) reaches 1250 mg / g and 172.4 mg / g respectively, and the adsorption effect exceeds that of most reported adsorbents.

[0029] Further, the pH of the adsorption system is preferably 5.0-7.0, and the adsorption time is 3-8 h.

[0030] Beneficial effects:

[0031] 1. The nano-hydroxyapatite-loaded biochar composite adsorbent is innovatively developed in the application, the biochar is used as a carrier, the uniform loading of nHAP particles on the surface of the biochar is successfully realized by optimizing the mass ratio of nHAP to biochar, and the adsorption performance and applicability of the material are significantly improved; the surface of the biochar presents an irregular sheet structure, mainly mesoporous (2-20 nm), and the average specific surface area of HMD2 reaches 224 m² / g, which is increased by 21.7% compared with that of pure HAP (184 m² / g). The uniform loading of nHAP effectively solves the problem of easy agglomeration of traditional HAP, improves the dispersibility of nHAP, and increases the available adsorption active sites on the surface of the material, thereby laying a solid structural foundation for efficient adsorption of heavy metals;

[0032] 2. The nano-hydroxyapatite-loaded biochar composite adsorbent HMD2 provided by the application has a maximum adsorption capacity of 1250 mg / g for Pb (II) and 172.4 mg / g for Cd (II), which is significantly better than that of many existing biochar-based adsorbents, and can quickly achieve deep removal of low-concentration heavy metals in water bodies;

[0033] 3. In terms of adsorption kinetics, the composite adsorbent HMD2 performs well, and the adsorption of Pb (II) reaches equilibrium within 3 hours, while the adsorption of Cd (II) reaches equilibrium within 5 hours, and 99% (Pb (II)) and 83% (Cd (II)) of the adsorption capacity can be achieved within the first 2 hours, which effectively solves the problems of long adsorption period and low treatment efficiency of traditional adsorbents, making it an ideal choice for emergency water body remediation scenarios;

[0034] 4. HMD2 performs well in complex water environments, and in the presence of Na + , K + , Ca 2+ , Mg 2+Even in the presence of coexisting cations, its adsorption capacity for Pb(II) remains above 99.5% of the initial value; even in complex water bodies where humic acid (HA) is present, its adsorption efficiency for Pb(II) decreases by only 17.2%, while its adsorption efficiency for Cd(II) increases by 22.1%. This characteristic enables it to effectively address the problem of multi-component interference in actual water bodies and achieve precise adsorption of target heavy metals.

[0035] 5. Regarding pH adaptability, HMD2 maintains stable adsorption capacity for Pb(II) and Cd(II) within the pH range of 5.5-7.5 (Pb(II) ≥ 1187 mg / g, Cd(II) ≥ 131.1 mg / g). By adjusting the pH to 5.5 ± 0.1, precipitation of heavy metal hydroxides can be effectively avoided. This acid-base working environment helps reduce operating costs and secondary pollution risks in practical applications, thereby improving the practicality and environmental friendliness of the material.

[0036] 6. This composite adsorbent can be directly used in industrial wastewater containing Pb(II) and Cd(II) (such as electroplating wastewater and smelting wastewater) as well as polluted surface water. It is flexible in operation and can be used as a static adsorbent for batch treatment of wastewater or made into filter media for dynamic filtration systems. It is suitable for water remediation needs of different scales and shows a wide range of application prospects.

[0037] 7. The composite adsorbent disclosed in this application does not require complex equipment or harsh reaction conditions (such as high temperature and pressure, highly toxic reagents) during its preparation. High-efficiency loading can be achieved simply by adjusting the mass ratio of nHAP to biochar (optimal 2:1). The preparation process is highly stable and low-cost. The raw materials are readily available, making it suitable for industrial mass production, which provides feasibility for the practical application of the material and is expected to promote its widespread use in the field of environmental remediation. Attached Figure Description

[0038] Figure 1 This is a flowchart for preparing nano-hydroxyapatite@biochar composite adsorbent (biochar loaded with nano-hydroxyapatite);

[0039] Figure 2 Neutron plot A shows the X-ray diffraction (XRD) patterns of HMD2, HAP, and MDDS700; subplot B shows the Fourier transform infrared (FTIR) spectra of HMD2, HAP, and MDDS700; and subplot C shows the specific surface area (BET) curves of HMD2, HAP, and MDDS700.

[0040] Figure 3Neutron plot ac is a scanning electron microscope (SEM) image of MDDS700 at different scales, subplot de is a scanning electron microscope (SEM) image of HAP at different scales, and subplot gi is a scanning electron microscope (SEM) image of HMD2 at different scales.

[0041] Figure 4 The distribution map of carbon (C), oxygen (O), phosphorus (P), and calcium (Ca) in HMD2;

[0042] Figure 5 Neutron plot A shows the adsorption performance of different materials for Pb(II) and Cd(II), subplot B shows the effect of initial pH on adsorption efficiency, and subplot C shows the effect of different reaction times on adsorption efficiency.

[0043] Figure 6 This is a comparison between the predicted adsorption capacity of heavy metal ions and the actual experimental values.

[0044] Figure 7 Neutron plot a compares the adsorption capacity of different materials for Pb(II), and subplot b compares the adsorption capacity of different materials for Cd(II).

[0045] Figure 8 Neutron plot a shows the effect of coexisting substances on the adsorption capacity of Cd(II) by HMD2, and subplot b shows the effect of coexisting substances on the adsorption capacity of Pb(II) by HMD2. Detailed Implementation

[0046] The preferred embodiments of the present invention will now be described in detail so that the advantages and features of the present invention can be more easily understood by those skilled in the art, thereby providing a clearer and more explicit definition of the scope of protection of the present invention.

[0047] Experimental materials

[0048] The waste from soybean peel (DDS) and ammonia-soda residue (ASR) were taken from a farmers' market and an ammonia-soda plant in northern Jiangsu, China, respectively.

[0049] The bean bud waste mentioned in this patent refers to the outer skin peeled off from the bean bud, which is the larva of the bean hawk moth (Clanis bilineata tsingtauica).

[0050] The ammonia-soda slag waste described in this patent is often referred to as "white mud" or "ammonia-soda waste residue." It is a solid waste generated during the industrial production of soda ash (sodium carbonate) using the upstream process of the ammonia-soda process (also known as the Solvay process). In this upstream production process, limestone and raw salt are the main raw materials. The limestone is calcined at high temperature to produce calcium oxide (quicklime) and carbon dioxide. The calcium oxide is hydrolyzed to produce lime milk (calcium hydroxide). At the same time, the brine produced from the raw salt is subjected to ammonia absorption and carbonization to produce sodium bicarbonate and ammonium chloride mother liquor. The ammonia-soda slag waste is generated in the key ammonia recovery process. This process utilizes the aforementioned lime milk to react with the ammonium chloride mother liquor (2NH4Cl + Ca(OH)2 → 2NH3 + CaCl2 + 2H2O) to recover ammonia for recycling. In this process, the limestone used as raw material typically contains various impurities such as silicon dioxide, magnesium oxide, aluminum oxide, and ferric oxide. Furthermore, the calcination or hydrolysis reaction may be incomplete. These impurities, unreacted calcium hydroxide, and some undecomposed calcium carbonate, along with the slightly soluble substances produced in the reaction (such as calcium sulfate), collectively form this slurry-like ammonia-alkali slag waste. Therefore, the solid phase of ammonia-alkali slag waste typically includes large amounts of calcium carbonate and calcium hydroxide, as well as silicon dioxide, magnesium oxide, and aluminum oxide, along with waste liquid containing high concentrations of calcium chloride and sodium chloride.

[0051] Cadmium nitrate tetrahydrate (Cd(NO3)2·4H2O, purity 99.99%) and magnesium chloride (MgCl2, purity 99.9%) were purchased from Aladdin Company, Shanghai, China. Other reagents, including lead nitrate (Pb(NO3)2), diammonium hydrogen phosphate ((NH4)2HPO4), sodium hydroxide (NaOH), hydrochloric acid (HCl), ammonia (NH3·H2O), and humic acid (HA, biological reagent grade), were all purchased from Sinopharm Group, Shanghai, China.

[0052] Unless otherwise stated, all reagents were of analytical grade and all water used in the experiments was deionized water.

[0053] The metal ion stock solution (concentration 1000 mg / L) was prepared by dissolving 1.5985 g of lead nitrate (Pb(NO3)2) and 2.7445 g of cadmium nitrate tetrahydrate (Cd(NO3)2·4H2O) in deionized water. The required concentration for the experiment was obtained by diluting the above stock solution.

[0054] Example 1

[0055] This embodiment discloses a scheme for preparing a nano-hydroxyapatite@biochar composite adsorbent (biochar loaded with nano-hydroxyapatite) using leaching solution of soybean bark waste (DDS) and ammonia-soda slag (ASR) as precursors. The preparation process is as follows: Figure 1 The specific preparation steps are as follows:

[0056] 1) Both raw materials, namely, Bean Peel Waste (DDS) and Ammonia-Soda Residue (ASR), were dried at 80℃ for 48 h, then ground and passed through a 20-mesh sieve for later use.

[0057] 2) Preparation of biochar carrier: This involves two stages: anaerobic pyrolysis and wet ball milling.

[0058] Phase 1: Anaerobic pyrolysis

[0059] The pulverized DDS was placed in a tube furnace and pyrolyzed under a high-purity nitrogen atmosphere. The pyrolysis temperature was set at 700℃, the holding time was 2 h, and the heating rate was 5℃ / min.

[0060] Second stage: Wet ball milling

[0061] The biochar obtained from pyrolysis was transferred to a zirconia ball mill jar, with the mass ratio of zirconia balls to biochar set at 100:1 and the solid-liquid ratio (biochar to deionized water) maintained at 1:20 (g:mL). Wet ball milling was performed using a planetary ball mill at 300 rpm for a total time of 12 hours, with the mill reversed every 3 hours. After milling, the biochar suspension was collected without further processing.

[0062] Four biochar suspensions were prepared for use, with biochar contents of 1 g, 1.5 g, 3 g and 6 g, respectively.

[0063] 3) Preparation of biochar supported on nano-hydroxyapatite: synthesized by precipitation method.

[0064] Preparation of S1 and ASR leachates and determination of calcium ion concentration

[0065] Ammonia-soda residue (ASR) was mixed with deionized water at a mass-to-volume ratio of 1:100 (g:mL) and stirred at 600 rpm for 24 h. After filtration, the supernatant was collected, and its calcium ion (Ca(II)) concentration was determined to be 2.4 g / L.

[0066] S2, Preparation of Mixed System

[0067] Take 500 mL of the above ASR leachate and mix it with biochar suspensions of different biochar contents, then dilute to 1 L. Stir the mixture at 600 rpm for 4 h.

[0068] S3. Precipitation reaction and subsequent treatment

[0069] Using a syringe pump, 100 mL of a 0.18 mol / L diammonium hydrogen phosphate ((NH4)2HPO4) solution was added dropwise to the above mixture at a rate of 1 mL / min, while maintaining a stirring speed of 600 rpm during the addition. After the addition was complete, stirring was continued for 2 h, followed by standing and aging for 24 h.

[0070] The reaction product was filtered and repeatedly rinsed with deionized water until it was nearly neutral. Then it was placed in an oven and dried at 80°C for 24 hours.

[0071] S4. Naming and Characterization of Composite Adsorbents

[0072] The resulting composite adsorbents were labeled as HAP, HMD3, HMD2, HMD1 and HMD0.5, respectively, with corresponding mass ratios of nHAP (nano-hydroxyapatite) to biochar of 1:0, 3:1, 2:1, 1:1 and 0.5:1.

[0073] In addition, a control sample without nHAP was prepared. Specifically, 500 mL of ASR leachate was mixed with 1 g of biochar suspension, and then diluted to 1 L. No diammonium hydrogen phosphate solution was added to the mixture to synthesize nHAP. The resulting product was denoted as MDDS700, and the mass ratio of nHAP to biochar in the product was 0:1.

[0074] MDDS700 is a pure biochar material (without nHAP), which is compared with other samples containing nHAP.

[0075] Related performance tests

[0076] 1) Structural characterization

[0077] ① Figure 2 Neutron diagram A shows the X-ray diffraction (XRD) patterns of HMD2, HAP, and MDDS700. For MDDS700, a broad diffraction peak appears at the 2θ angle in the range of 20° to 30°, indicating the presence of an amorphous carbon structure in the sample, consistent with existing reports.

[0078] The XRD pattern of HAP shows distinct characteristic diffraction peaks at 2θ angles of 25.9°, 32.1°, 39.6°, 44.4°, 46.7°, 49.7°, and 53.2°, corresponding to hydroxyapatite (CaO) and its constituent elements. 10 The (002), (211), (310), (222), (400), (213) and (004) crystal planes of (PO4)6(OH)2. This result confirms that hydroxyapatite can be successfully synthesized using ammonia-soda slag (ASR) leachate as a calcium source.

[0079] The XRD pattern of HMD2 showed characteristic peaks similar to those of HAP, indicating that hydroxyapatite (HAP) had been successfully loaded onto biochar.

[0080] ② The Fourier transform infrared (FTIR) spectra of HMD2, HAP, and MDDS700 are shown in subplot B of Figure 2: For MDDS700, 3375 cm⁻¹ -1 1581 cm -1 and 1265 cm -1 The peaks at these locations correspond to stretching vibrations of −OH, aromatic C=C, and aromatic C−O or phenol −OH, respectively. The FTIR spectrum of HAP at 1034 cm⁻¹... -1 963 cm -1 602 cm -1 and 564 cm -1 Characteristic peaks appear at [location], which are attributed to the asymmetric stretching vibrations of P-OH or P=O in phosphate groups. Additionally, at 3450 cm⁻¹... -1 and 1643 cm -1 The broad peak at 1420 cm⁻¹ indicates the vibration of −OH in the adsorbed water; while the peak at 1420 cm⁻¹... -1 and 873 cm -1 The peak at that location corresponds to carbonate (CO3) 2- The vibration of HAP may be due to the partial replacement of phosphate ions with carbonate ions when HAP is exposed to air.

[0081] The FTIR spectrum of HMD2 exhibits characteristics of both MDDS700 and HAP, including the 1603 cm⁻¹ spectrum. -1 Aromatic C=C vibrations at 1034 cm⁻¹, and 1034 cm⁻¹ -1 602 cm -1 and 564 cm -1 The presence of characteristic peaks related to phosphate provides further evidence for the successful combination of HAP and biochar.

[0082] ③ The specific surface area (BET) of MDDS700, HAP, and HMD2 was tested. The BET test results for HMD2 are shown below. Figure 2 Neutron diagram C. Tests revealed that the N2 adsorption-desorption isotherms of all three materials exhibited type IV characteristics, accompanied by H3 type hysteresis loops, indicating that their pore structure is mainly mesoporous (2-50 nm).

[0083] Among them, MDDS700 showed a significant increase in N2 adsorption at a relative pressure of 1.0, indicating the presence of macropores (>50 nm). In contrast, the pores of HMD2 were mainly distributed in the 2-20 nm range (average 10.6 nm), a range larger than that of HAP (3-15 nm, average 7.41 nm) but smaller than that of MDDS700 (10-120 nm, average 45.0 nm). This difference is because HAP nanoparticles in HMD2 occupy the channels of MDDS700, while MDDS700 simultaneously promotes the dispersion of these nanoparticles.

[0084] Specific surface area data supports the corresponding conclusion: MDDS700 (235 m²) 2 / g)> HMD2 (224 m 2 / g)> HAP (184m) 2 / g). Compared with pure HAP, HMD2 has a 21.7% increase in specific surface area and a 12.3% increase in total pore volume, which helps to improve its adsorption capacity for heavy metals.

[0085] ④ The surface morphology of MDDS700, HAP and HMD2 is as follows Figure 3 As shown in the figure, MDDS700 exhibits an irregular sheet-like structure with obvious cracks (>50 nm) and pores (2-50 nm) on its surface. HAP is composed of highly aggregated nanoparticles with an average particle size of approximately 20 nm. In contrast, HMD2 nanoparticles are uniformly distributed on the surface of the biochar sheet-like structure.

[0086] Compared to pure HAP, the nanoparticles in HMD2 exhibit better dispersion and a more developed porous structure on the surface. Element mappings ( Figure 4 The results showed that the four elements C, O, P and Ca were evenly distributed in HMD2, which further confirmed the successful preparation of nHAP@biochar composite adsorbent.

[0087] 2) Adsorption performance study

[0088] Figure 5 Neutron plot A illustrates the adsorption behavior of composite adsorbents prepared by combining nHAP and biochar at different mass ratios for metal ions. As shown in the figure, pure MDDS700 exhibits the lowest adsorption capacity, with an adsorption capacity of 68.80 mg / g for Pb(II) and 33.24 mg / g for Cd(II). This limited adsorption performance is related to the limited number of available adsorption sites on the MDDS700 surface.

[0089] As the amount of nHAP incorporated increased from 0.5 (HMD0.5) to 2 (HMD2), the adsorption capacity of the composite adsorbent was significantly improved, with the adsorption capacity for Pb(II) increasing from 646.3 mg / g to 1187 mg / g and the adsorption capacity for Cd(II) increasing from 97.58 mg / g to 131.1 mg / g.

[0090] Increasing the nHAP ratio to 3 (HMD3) further reduced the adsorption capacity significantly, with the adsorption capacity for Pb(II) stabilizing at 1196 mg / g and the adsorption capacity for Cd(II) stabilizing at 137.7 mg / g.

[0091] Based on the above results, it can be inferred that the optimal composite ratio of nHAP to biochar is 2:1, which promotes the effective dispersion of HAP nanoparticles on the biochar surface. Further increasing the nHAP ratio may cover available adsorption sites. Therefore, HMD2 was selected as the optimal material for subsequent studies.

[0092] 3) Effect of initial pH value on the adsorption of metal ions by HMD2

[0093] See results Figure 5 Neutron diagram B: When the pH value increases from 2.5 to 5.5, the adsorption capacity of HMD2 increases significantly, with the adsorption capacity for Pb(II) increasing from 358.8 mg / g to 1187 mg / g and the adsorption capacity for Cd(II) increasing from 83.54 mg / g to 131.1 mg / g. When the pH value increases from 5.5 to 7.5, the adsorption capacity increases only slightly; at pH 7.5, the adsorption capacity for Pb(II) tends to be 1199 mg / g and the adsorption capacity for Cd(II) tends to be 137.9 mg / g.

[0094] The above trend can be explained as follows: when pH < 3.5, HAP will partially dissolve, resulting in a significant reduction in the available adsorption sites on the HMD2 surface; in an acidic environment, the high concentration of protons in the solution will fiercely compete with Pb(II) and Cd(II) for the limited adsorption sites on the HMD2 surface.

[0095] Although the best adsorption performance was obtained at pH 7.5, subsequent experiments were conducted at pH 5.5 ± 0.1 to avoid the formation of hydroxide precipitation.

[0096] 4) Effect of reaction time on HMD2 adsorption behavior

[0097] See Figure 5Neutron diagram C: Within the initial 2 hours, both Pb(II) and Cd(II) metal ions exhibited rapid adsorption on HMD2, reaching over 99% and 83% of their respective equilibrium adsorption capacities. This rapid adsorption stemmed from the abundant available adsorption sites on the HMD2 surface during the initial stage of the reaction, coupled with a strong concentration gradient, which jointly propelled the adsorption process.

[0098] As the reaction proceeds, these adsorption sites gradually become saturated, leading to a decrease in the adsorption rates of both ions. Specifically, Pb(II) reaches an adsorption stable state at approximately 3 h, while Cd(II) reaches stability at approximately 5 h.

[0099] It is noteworthy that the adsorption rate and adsorption capacity of Pb(II) are consistently higher than those of Cd(II), indicating that HMD2 has a stronger affinity for Pb(II).

[0100] 5) Effect of initial metal ion concentration on HMD2 adsorption behavior

[0101] The study found that when the initial concentration of Cd(II) increased from 10 mg / L to 200 mg / L and the initial concentration of Pb(II) increased from 300 mg / L to 700 mg / L, the adsorption capacity of HMD2 significantly improved: the adsorption capacity for Cd(II) increased from 19.83 mg / g to 166.4 mg / g, and the adsorption capacity for Pb(II) increased from 599.9 mg / g to 1218 mg / g. This improvement stems from the abundant available adsorption sites on the HMD2 surface, while the higher initial concentration helps reduce mass transfer resistance, thereby promoting more efficient adsorption.

[0102] However, when the initial concentration of Cd(II) was further increased to 250 mg / L and the initial concentration of Pb(II) was further increased to 800 mg / L, the adsorption capacity only showed a slight increase. This plateauing trend indicates that the number of active sites on the HMD2 surface is close to saturation, and the adsorption process gradually reaches equilibrium.

[0103] 6) To further elucidate the adsorption mechanism of HMD2 for the two metal ions and quantify its adsorption capacity, the Langmuir model, Freundlich model, and Dubining-Radushkevich (DR) model were used to analyze the obtained data. Among them, the Langmuir model showed the best fit: the coefficient of determination (R²) for Pb(II) was [value missing]. 2 The value of R for Cd(II) is 0.9999. 2 The value is 0.9972. This result indicates that the adsorption of Pb(II) and Cd(II) on HMD2 mainly follows a monolayer chemisorption process on a uniform surface.

[0104] Using the Langmuir constant (b), through formula R L =1 / (1+bC0) Calculate the dimensionless separation factor (R) L ), of which 0 <R L <1 indicates that the adsorption process is favorable, R L A value >1 indicates an unfavorable adsorption process. The calculated Ri for Pb(II) is... L The value range is 3.1 × 10. -4 ~8.3×10 -4 The RL values ​​for Cd(II) ranged from 0.017 to 0.303, confirming that HMD2 has favorable adsorption performance for both ions. The RL value for Pb(II) was significantly lower, indicating that it has a stronger affinity for HMD2 and a better adsorption effect than Cd(II).

[0105] The maximum adsorption capacity (q) obtained from the Langmuir model max The adsorption capacity of HMD2 for Pb(II) was 1250 mg / g and that for Cd(II) was 172.4 mg / g, highlighting the superior adsorption performance of HMD2 for Pb(II).

[0106] Based on the above construction method of biochar composite adsorbent coated and loaded with nano-carboxylated apatite, and the formation and control of adsorption sites on its surface, this patent proposes a method for predicting the adsorption capacity of heavy metal ions (such as Pb(II)) based on the characteristics of this composite adsorbent. This model is based on the following technical concept: the total adsorption capacity of the composite adsorbent is its two main adsorption components, biochar and nHAP(H), with the independent variable being the mass ratio of nHAP to biochar, defined as... The model assumes that biochar (BC) has inherent adsorption sites and adsorption capacity. When nHAP is loaded, it physically covers some of the adsorption sites of biochar, rendering them ineffective. nHAP itself has a high adsorption capacity and plays a major role. According to the mixture rule, the total adsorption capacity of the composite adsorbent is... Calculated using the following formula:

[0107] and These represent the actual effective adsorption capacities contributed by the biochar and nHAP components in the composite adsorbent, respectively. and These are the mass fractions of biochar and nHAP in the composite adsorbent, respectively. Therefore, the independent variable can be... To represent the quality score and We can obtain:

[0108]

[0109]

[0110] After substituting, we get:

[0111]

[0112] Assuming the intrinsic adsorption capacity of the nHAP component, i.e., the amount of nHAP adsorbed for a fixed weight, is a constant and unaffected by biochar, or the effect of biochar on it is negligible, this intrinsic adsorption capacity is defined as the parameter to be fitted. ,but The adsorption capacity of biochar components The adsorption capacity is reduced due to the coverage of nHAP, while the adsorption capacity of pure biochar (MDDS700) is a known constant. This is biochar in This refers to the intrinsic adsorption capacity when the site is uncovered. Assuming that nHAP coverage of biochar sites follows a simple site competition, the proportion of uncovered sites on the biochar... The amount of nHAP Inversely proportional. ; This is a coverage factor, representing the efficiency of nHAP coverage of biochar sites. The effective adsorption capacity of the biochar. equal to its inherent capacity Multiply by the proportion of uncovered sites Then we get: ; It is a known constant ( ),and It is the second parameter to be fitted.

[0113] Will and After substituting, we get:

[0114]

[0115] After merging and rearranging, we get:

[0116]

[0117] Based on the experimental results of this invention, after fitting the above prediction formula, the fitted parameters C1 = 1737.6374 mg / g and k = 0.0010 are obtained. The calculation results are as follows:

[0118]

[0119] Comparison of experimental and predicted values ​​of the effect of the mass ratio of nHAP to biochar on adsorption capacity: Figure 6 As shown, the prediction method in this patent can simulate the actual adsorption amount well, with the average percentage error controlled within 10%.

[0120] The above prediction method was validated using test results under the conditions of r=1.5, 2.5, and 4.0. The comparison between the model prediction values ​​and experimental values ​​is shown below. It can be seen that this method also has good prediction effect when validated by samples.

[0121]

[0122] like Figure 7 As shown in neutron diagrams a and b, HMD2 exhibits an adsorption capacity for Pb(II) and Cd(II) that exceeds that of many previously reported biochar-based adsorbents.

[0123] 7) Effect of coexisting substances on the adsorption performance of HMD2

[0124] Coexisting components: Potassium ions (K⁺), Sodium ions (Na⁺), Calcium ions (Ca⁺) 2 ⁺), magnesium ions (Mg) 2 ⁺), lead ions (Pb) 2 ⁺), cadmium ions (Cd) 2 The concentration of each component was 100 mg / L, consisting of ⁺ and humic acid (HA).

[0125] Depend on Figure 8 As can be seen from neutron diagram a, Na + K + The effect of monovalent cations on the adsorption of Cd(II) by HMD2 is negligible, with only a slight decrease in adsorption capacity, ranging from 2.18% to 4.15%.

[0126] Conversely, Ca 2+ Mg 2+ Pb 2+ The presence of divalent cations leads to a more significant decrease in the adsorption capacity of HMD2 for Cd(II), with reductions of 22.9%, 11.0%, and 12.6%, respectively. This is because these cations have similar hydrated ionic radii and the same charge state as Cd(II), resulting in them competing fiercely with Cd(II) for active adsorption sites (such as phosphate groups) on the HMD2 surface.

[0127] Unlike this, such as Figure 8As shown in neutron diagram b, the adsorption capacity of HMD2 for Pb(II) remains largely unaffected regardless of the presence of monovalent or divalent cations, consistently maintaining over 99.5% of its initial adsorption capacity. This phenomenon indicates that HMD2 exhibits superior selectivity for Pb(II) compared to Cd(II).

[0128] Furthermore, the presence of humic acid (HA) had opposite effects on the adsorption of Cd(II) and Pb(II): specifically, the adsorption capacity of Cd(II) increased by 22.1%, while the adsorption capacity of Pb(II) decreased significantly by 17.2%. The increase in Cd(II) adsorption capacity may be due to the strong binding affinity between HA and HMD2, which introduces additional functional groups (such as carboxyl and phenolic hydroxyl groups), which can serve as supplementary adsorption sites, thereby promoting the adsorption of more Cd(II) from aqueous solution. However, for Pb(II), which already has a very high adsorption capacity (>1100 mg / g), the binding of HA to HMD2 may block the effective Pb(II) adsorption sites on the HMD2 surface. In addition, the formation of soluble Pb(II)-HA complexes may hinder the precipitation of Pb(II) in the form of lead phosphate mineral phases (such as hydroxyapatite), thus reducing the overall adsorption efficiency.

[0129] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made based on the content of the present invention specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the protection scope of the present invention.

Claims

1. A biochar composite adsorbent supported on nano-hydroxyapatite for heavy metal adsorption, characterized in that, It is composed of nano-hydroxyapatite and biochar; The biochar is prepared from waste soybean bark through anaerobic pyrolysis and wet ball milling processes. The nano-hydroxyapatite is generated by reacting diammonium hydrogen phosphate with ammonia-alkali residue leachate as a calcium source through a precipitation reaction. The mass ratio of nano-hydroxyapatite to biochar is 0.5~3:

1.

2. The biochar composite adsorbent supported on nano-hydroxyapatite as described in claim 1, characterized in that, The mass ratio of nano-hydroxyapatite to biochar is 2:

1.

3. The method for preparing the biochar composite adsorbent supported on nano-hydroxyapatite as described in any one of claims 1-2, characterized in that, Includes the following steps: S1. Dry, grind, and sieve the waste bean curd skin and ammonia-alkali residue; S2. Biochar preparation: S21. Anaerobic pyrolysis: Pyrolysis of pretreated soybean peel waste under an inert atmosphere; S22. Wet ball milling: The pyrolysis products are transferred to a ball milling jar for ball milling to obtain a biochar suspension; S3. Preparation of biochar supported on nano-hydroxyapatite: S31. Mix the pretreated ammonia-alkali residue with water, stir, filter, and collect the supernatant rich in calcium ions; S32. Mix the ammonia-soda residue leachate with the biochar suspension obtained in step S22, dilute and stir; S33. Add diammonium hydrogen phosphate solution dropwise to the mixed system, stir, and let it stand for aging; S34. The aged product is filtered, washed and dried to obtain a biochar composite adsorbent loaded with nano-hydroxyapatite.

4. The method for preparing the biochar composite adsorbent supported on nano-hydroxyapatite as described in claim 3, characterized in that, In step S21, the pyrolysis heating rate is 3~10℃ / min, the pyrolysis temperature is 600~800℃, and the pyrolysis time is 1~5 h.

5. The method for preparing the biochar composite adsorbent supported on nano-hydroxyapatite as described in claim 3, characterized in that, The ball milling jar used was a zirconia ball milling jar. The mass ratio of zirconia balls to biochar was 80~150:1, the solid-liquid ratio of biochar to deionized water was 1 g:15~30 mL, the ball milling speed was 200~500 rpm, and the ball milling time was 6~18 h.

6. The method for preparing the biochar composite adsorbent supported on nano-hydroxyapatite as described in claim 3, characterized in that, In step S31, the mixing ratio of ammonia-alkali residue and water is 1g:50~150mL, and the Ca(II) concentration in the supernatant is 1.0~5.0g / L; in step S33, the concentration of diammonium hydrogen phosphate solution is 0.1~0.3 mol / L, the volume is 50~200 mL, the dropping rate is 0.5~2 mL / min, and stirring is continued for 1~4 h after the addition is completed, and the standing aging time is 12~48 h.

7. The method for preparing the biochar composite adsorbent supported on nano-hydroxyapatite as described in claim 3, characterized in that, This also includes the saturated adsorption capacity of heavy metal ions for the composite adsorbent. The prediction steps are calculated using the following formula: ； is the adsorption capacity of pure biochar, r is the mass ratio of nHAP to biochar, and C1 and k are the parameters to be fitted.

8. The method for preparing the biochar composite adsorbent supported on nano-hydroxyapatite as described in claim 3, characterized in that, In the obtained biochar composite adsorbent loaded with nano-hydroxyapatite, the mass ratio of nano-hydroxyapatite to biochar is 0.5~3:

1.

9. The application of the biochar composite adsorbent supported on nano-hydroxyapatite as described in claim 1 in the adsorption of heavy metals, characterized in that, The heavy metals are Pb(II) and / or Cd(II).

10. The application as described in claim 9, characterized in that, The pH of the adsorption system was 5.0–7.0, and the adsorption time was 3–8 h.

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