Oyster shell modified walnut shell carbon core-shell composite microsphere adsorbent, preparation method and application thereof in high-concentration fluorine-containing wastewater treatment

Abstract

The invention discloses an oyster shell modified walnut shell carbon core-shell composite microsphere adsorbent, a preparation method and application thereof in fluorine-containing wastewater treatment. The preparation method of the core-shell composite microsphere adsorbent comprises the following steps: (1) adding mixed powder of walnut shells and oyster shells in different mass ratios into deionized water for dipping, drying after dipping, pyrolyzing in an inert atmosphere, washing a product to be neutral after pyrolyzing, and drying to prepare oyster shell modified walnut shell carbon; and (2) slowly adding the oyster shell modified walnut shell carbon into a sodium alginate solution, uniformly stirring to obtain a mixture, dropwise adding the mixture into a calcium chloride solution for crosslinking, and washing and drying a product to obtain the oyster shell modified walnut shell carbon core-shell composite microsphere adsorbent. The oyster shell modified walnut shell carbon core-shell composite microspheres prepared by the preparation method disclosed by the invention have proper size, good mechanical strength and excellent adsorption performance.

Description

Technical field:

[0001] The invention relates to the technical field of defluorination of high-concentration fluorine-containing wastewater, and in particular to an oyster shell-modified walnut shell carbon core-shell composite microsphere adsorbent, a preparation method and application thereof in the treatment of high-concentration fluorine-containing wastewater. Background technology:

[0002] At present, high-concentration fluoride-containing wastewater is mainly concentrated in high-end manufacturing industries such as electronics and fluorine chemicals. For example, in the manufacturing process of semiconductor chips, the etching process uses a large amount of fluorine-containing reagents such as hydrofluoric acid and ammonium fluoride, and is accompanied by high-purity water cleaning steps, which will produce wastewater with high fluoride concentration, strong acidity and high environmental toxicity. With the expansion of new energy and semiconductor industries, the discharge of such wastewater has been increasing year by year. Fluoride ions (F-) are difficult to degrade naturally in water bodies. Long-term accumulation leads to deterioration of water quality and will be toxic to humans and aquatic organisms.

[0003] The traditional treatment methods for high-concentration fluoride-containing wastewater mainly include chemical precipitation and adsorption. Chemical precipitation is widely used, but with increasingly stringent fluoride emission standards for industrial wastewater, even multi-stage coagulation and sedimentation cannot meet emission standards, and there are problems such as large flocculant dosage and high sludge production. Adsorption methods represented by activated carbon have the characteristics of fast fluoride removal and simple operation, but activated carbon is relatively expensive for large-scale industrial production, and there is an urgent need to develop new adsorption materials with low cost and high adsorption capacity. Low-cost biochar prepared from biomass has a developed pore structure and a rich variety of surface functional groups, and has great potential as a fluoride removal agent. However, the biochar currently developed is mostly in powder form, which is difficult to separate solids from liquids, easy to clog equipment, and increase water color. In addition, the fluoride removal rate is generally low for high-concentration fluoride-containing wastewater. In response to the above bottleneck problem, the present invention uses oyster shell-modified walnut shell charcoal containing mineral substances and organic matter to construct a composite microsphere adsorbent, and efficiently captures fluoride ions through the synergistic effect of calcium-based chemical precipitation and porous carbon physical adsorption. The microsphere adsorbent has good adsorption-desorption ability and can be recycled, providing an efficient and economical solution for the treatment of fluorine industrial wastewater and meeting the development needs of a green circular economy. Summary of the invention:

[0004] The present invention solves the problems existing in the existing fluorine removal technology and provides an oyster shell modified walnut shell carbon core-shell composite microsphere adsorbent, a preparation method and application thereof in the treatment of high-concentration fluorine-containing wastewater. The core-shell composite microsphere adsorbent proposed by the present invention has good fluorine removal performance, low cost and easy solid-liquid separation, and can efficiently remove fluoride ions in high-concentration fluorine-containing wastewater.

[0005] The first object of the present invention is to provide an oyster shell modified walnut shell carbon core-shell composite microsphere adsorbent, wherein the specific surface area, single point adsorption total pore volume and average pore diameter of the core-shell composite microsphere adsorbent are 4-6 m 2 / g, 0.004 - 0.006 cm 3 / g and 11 - 13 nm.

[0006] The second object of the present invention is to provide a preparation method of the oyster shell modified walnut shell carbon core - shell composite microsphere adsorbent, which comprises the following steps:

[0007] (1) Mixing powders of walnut shell and oyster shell with different mass ratios are added to deionized water for impregnation. After impregnation is completed, it is dried and pyrolyzed in an inert atmosphere. The pyrolysis conditions are: nitrogen atmosphere, pyrolysis temperature is 450°C - 550°C, heating rate is 9 - 12°C / min, pyrolysis time is 1.0 - 3.0 h. After pyrolysis is completed, the product is washed to neutrality and dried to obtain oyster shell modified walnut shell carbon;

[0008] (2) Slowly add the oyster shell modified walnut shell carbon to the sodium alginate solution to obtain a mixture, and then drop the mixture into the calcium chloride solution for cross - linking. The product is washed and dried to obtain the oyster shell modified walnut shell carbon core - shell composite microsphere adsorbent.

[0009] Preferably, the mass ratio of oyster shell to walnut shell powder in the mixed powder in step (1) is 1:3 - 3:1. Further preferably, the mass ratio of oyster shell to walnut shell in step (1) is 1:3.

[0010] In step (1), the solid - liquid ratio of the mixed powder to deionized water is 1:9 - 1:12 g / mL, and the impregnation conditions are: impregnation activation at 55°C - 65°C for 2.5 - 3.5 h. Further preferably, the solid - liquid ratio of the mixed powder to deionized water in step (1) is 1:10 g / mL, and the impregnation conditions are: impregnation activation at 60°C for 3 h.

[0011] The pyrolysis conditions in step (1) are: nitrogen atmosphere, pyrolysis temperature is 500°C, heating rate is 10°C / min, pyrolysis time is 2 h.

[0012] Preferably, the mass concentration of the sodium alginate solution in step (2) is 10 - 30 g / L. Further preferably, the mass concentration of the sodium alginate solution in step (2) is 20 g / L.

[0013] Preferably, the embedding amount of oyster shell modified walnut shell carbon (carbon addition amount during the embedding process) in the mixture in step (2) is 80 - 120 g / L, and the mass ratio of oyster shell modified walnut shell carbon to sodium alginate is 4:1 - 8:1. Further preferably, the embedding amount of oyster shell modified walnut shell carbon in the mixture in step (2) is 100 g / L.

[0014] Preferably, the mass concentration of the calcium chloride solution in step (2) is 50 - 70 g / L, the mass ratio of oyster shell modified walnut shell charcoal to calcium chloride is 1.6:1 - 1.7:1, and the crosslinking time is 10 - 14 h. Further preferably, the mass concentration of the calcium chloride solution in step (2) is 60 g / L, the mass ratio of oyster shell modified walnut shell charcoal to calcium chloride is 1.67:1, and the crosslinking time is 12 h.

[0015] The third object of the present invention is to provide the application of the oyster shell modified walnut shell charcoal core - shell composite microsphere adsorbent in the treatment of high - concentration fluoride - containing wastewater, wherein the fluoride ion concentration in the high - concentration fluoride - containing wastewater is 100 - 500 mg / L.

[0016] The oyster shell modified walnut shell charcoal core - shell composite microsphere adsorbent treats the fluoride - containing wastewater through the following steps: adding the oyster shell modified walnut shell charcoal core - shell composite microsphere adsorbent into the fluoride - containing wastewater for adsorption. After the adsorption is completed, the treated wastewater is filtered with a 0.45 - μm filter membrane, the fluoride ion concentration in the wastewater is measured, and the fluoride removal rate is calculated.

[0017] Preferably, 9 - 11 g of the oyster shell modified walnut shell charcoal composite microsphere adsorbent is added per liter of wastewater, the adsorption temperature is 20℃ - 30℃, and the adsorption time is 7 - 9 h.

[0018] The fourth object of the present invention is to provide a regeneration method for the oyster shell modified walnut shell charcoal core - shell composite microsphere adsorbent, which includes the following steps: after the oyster shell modified walnut shell charcoal composite microsphere adsorbent saturated by adsorbing fluoride ions is washed, filtered and dried, it is added to the regeneration solution and regenerated by constant - temperature water - bath oscillation. The regeneration solution is a mixed solution of NaOH and Al(NO3)3 to obtain the regenerated oyster shell modified walnut shell charcoal composite microsphere adsorbent.

[0019] The constant - temperature water - bath oscillation is specifically carried out at a temperature of 24℃ - 26℃ and a rotation speed of 140 - 160 r / min, and the regeneration time is 11 - 13 h. Further preferably, the conditions for the constant - temperature water - bath oscillation are a temperature of 25℃, a rotation speed of 150 r / min, and a regeneration time of 12 h.

[0020] Preferably, the mass concentration of the saturated oyster shell modified walnut shell charcoal core - shell composite microsphere adsorbent is 8 - 12 g / L. Further preferably, the mass concentration of the oyster shell modified walnut shell charcoal core - shell composite microsphere adsorbent saturated by adsorbing fluoride ions is 10 g / L.

[0021] The molar concentration of NaOH in the mixed solution is 0.02 - 0.04 mol / L, and the molar concentration of Al(NO3)3 is 0.05 - 0.15 mol / L. Further preferably, the molar concentration of NaOH is 0.03 mol / L, and the molar concentration of Al(NO3)3 is 0.1 mol / L.

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] (1) Turning waste into treasure. The raw materials of the oyster shell modified walnut shell biochar core-shell composite microsphere adsorbent proposed by the present invention, namely walnut shell and oyster shell, are widely sourced and inexpensive. Calcium chloride, sodium alginate, etc. used are common chemical products. The preparation of the oyster shell modified walnut shell biochar core-shell composite microsphere realizes the synergistic regeneration of agricultural and forestry waste and seafood waste. The process is simple and no toxic reagents are added during the preparation process. The process is green and low-carbon, opening up a new path of "treating pollution with waste".

[0024] (2) Dual driving of core and shell, adsorption multiplication. The oyster shell modified walnut shell biochar core-shell composite microsphere adsorbent prepared by the present invention exhibits excellent adsorption performance under the synergistic action of calcium-based chemical precipitation (shell layer) and physical adsorption of porous carbon (core layer). The test results of its adsorption of simulated fluorine-containing wastewater show that under the conditions of an initial fluoride ion concentration of 100 mg / L, an initial pH of 3, a dosage of 10 g / L, a temperature of 25 °C, and an adsorption time of 8 h, the defluorination rate reaches 90.71%, the adsorption capacity is 11.309 mg / g, and the effluent pH is 6.7. While ensuring that the fluoride ion concentration in the effluent meets the "Discharge Standard of Water Pollutants for the Electronic Industry" (GB39731-2020), it also has the effect of adjusting the pH value.

[0025] (3) Microsphere locking fluorine, industrial friendly. Breaking through the industry pain points of easy loss and difficult recovery of powder adsorbents, through the sodium alginate-calcium chloride microsphere embedding technology, the particle size of the adsorbent can be accurately controlled (1-3 mm), the mechanical strength is enhanced, and the solid-liquid separation efficiency is improved. Nanoscale pores are densely distributed on the surface of the microsphere, and the exposure amount of active sites is increased, which is suitable for high-concentration fluorine-containing industrial wastewater scenarios.

[0026] (4) One-key regeneration, circular economy. The oyster shell modified walnut shell biochar core-shell composite microsphere adsorbent prepared by the present invention still has good recycling value after multiple regenerations. In the fluorine-containing wastewater with an initial fluoride ion concentration of 100 mg / L and an initial pH of 3, after adsorption for 8 h at 25 °C, after three regenerations and then adsorption under the same conditions, the fluoride ion removal rate can still reach 71.21%, the adsorption capacity is 9.573 mg / g, and the effluent pH value is 6.2. There is no secondary pollution during the regeneration process, providing a sustainable solution for industrial wastewater treatment. Description of the drawings:

[0027] Figure 1 It is the scanning electron microscope (SEM) result before the adsorption of the oyster shell modified walnut shell biochar core-shell composite microsphere adsorbent in Example 1. Among them, a-1 represents the macroscopic morphology of the microsphere (scale 300 μm), a-2 represents the microscopic morphology of the microsphere (scale 20 μm), and a-3 represents the microscopic morphology of the microsphere (scale 4 μm);

[0028] Figure 2 It is the scanning electron microscope (SEM) result of pure walnut shell carbon microspheres before adsorption in Comparative Example 1. Among them, b-1 represents the macroscopic morphology of the microspheres (scale bar: 200 μm), b-2 represents the microscopic structure of the microspheres (scale bar: 20 μm), and b-3 represents the microscopic structure of the microspheres (scale bar: 5 μm);

[0029] Figure 3 It is the scanning electron microscope (SEM) result of pure oyster shell carbon microspheres before adsorption in Comparative Example 2. Among them, c-1 represents the macroscopic morphology of the microspheres (scale bar: 300 μm), c-2 represents the microscopic structure of the microspheres (scale bar: 10 μm), and c-3 represents the microscopic structure of the microspheres (scale bar: 5 μm);

[0030] Figure 4 It is the scanning electron microscope (SEM) result of oyster shell-modified walnut shell carbon obtained in step (1) of Example 1 before adsorption. Among them, d-1 represents the microscopic morphology of oyster shell-modified walnut shell carbon (scale bar: 20 μm), d-2 represents the microscopic morphology of oyster shell-modified walnut shell carbon (scale bar: 10 μm), and d-3 represents the microscopic morphology of oyster shell-modified walnut shell carbon (scale bar: 5 μm);

[0031] Figure 5 It is the energy dispersive spectroscopy (EDS) of the adsorbents before adsorption in Example 1 and Comparative Examples. Among them, a represents Comparative Example 2, b represents Comparative Example 1, c represents oyster shell-modified walnut shell carbon obtained in step (1) of Example 1, and d represents Example 1;

[0032] Figure 6 It is the X-ray photoelectron spectroscopy (XPS) of the core-shell composite microspheres of oyster shell-modified walnut shell carbon in Example 1 before and after adsorption;

[0033] Figure 7 It is the X-ray diffraction (XRD) of the core-shell composite microspheres of oyster shell-modified walnut shell carbon in Example 1 before and after adsorption;

[0034] Figure 8 It is the Fourier transform infrared spectroscopy (FTIR) of the core-shell composite microspheres of oyster shell-modified walnut shell carbon in Example 1 before and after adsorption. Specific implementation manner:

[0035] The following examples are further illustrations of the present invention rather than limitations thereof.

[0036] Unless otherwise defined, all professional terms used hereinafter have the same meaning as commonly understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the protection scope of the present invention. Unless otherwise specified, the experimental materials and reagents in this article are all conventional commercially available products in this technical field.

[0037] A preparation method of an adsorbent of oyster shell modified walnut shell carbon core-shell composite microspheres, comprising the following steps:

[0038] (1) Add a mixed powder of oyster shell and walnut shell powder with a mass ratio of 1:3 - 3:1 to deionized water, keep it at 600 - 700 rpm and 55°C - 65°C on a constant temperature magnetic stirrer for impregnation activation for 2.5 - 3.5 h. After the activation is completed, take it out, centrifuge and dry it, then dry it at 105°C for 8 h. Crush the dried oyster shell and walnut shell powder through a 100-mesh sieve and collect it. Place it in a square quartz boat and push it into the furnace chamber of a tube furnace. Introduce nitrogen (flow rate is about 250 - 350 mL / min). After exhausting the air, turn on the tube furnace switch, set the heating rate at 9 - 12°C / min, heat up to 450°C - 550°C and keep it for pyrolysis for 1 - 3 h. After the program ends, turn off the tube furnace and nitrogen, take out the pyrolysis product and immediately put it into deionized water, wash it repeatedly until the pH value of the supernatant is 7, dry it in an oven at 105°C for 24 h, and collect the dried carbon through a 100-mesh sieve to obtain oyster shell modified walnut shell carbon.

[0039] (2) Add sodium alginate powder to deionized water, heat it to 60°C by water bath and stir to dissolve it to obtain a sodium alginate solution with a mass concentration of 10 - 30 g / L. Slowly add oyster shell modified walnut shell carbon to this solution according to an embedding amount of 80 - 120 g / L, continuously stir evenly, and inject the mixture drop by drop into a CaCl2 solution with a mass concentration of 50 - 70 g / L using a syringe. The mixture undergoes solidification cross-linking in the CaCl2 solution for 10 - 14 h, then wash it with deionized water 4 - 5 times, and dry it in an oven at 60°C for 12 h to obtain an adsorbent of oyster shell modified walnut shell core-shell composite microspheres.

[0040] In the following embodiments, preferably, the pyrolysis conditions in step (1) are: nitrogen atmosphere, pyrolysis temperature of 500°C, heating rate of 10°C / min, and pyrolysis time of 2 h.

[0041] In the following embodiments, preferably, the mass concentration of the sodium alginate solution in step (2) is 20 g / L; the embedding amount of oyster shell modified walnut shell carbon in the mixture is 100 g / L; the mass concentration of the calcium chloride solution is 60 g / L, the mass ratio of oyster shell modified walnut shell carbon to calcium chloride is 1.67:1, and the cross-linking time is 12 h.

[0042] The oyster shell modified walnut shell carbon core-shell composite microsphere adsorbent treats fluorine-containing wastewater through the following steps: Add the oyster shell modified walnut shell carbon core-shell composite microsphere adsorbent to the fluorine-containing wastewater for adsorption. After the adsorption is completed, filter the treated wastewater with a 0.45 μm filter membrane, measure the fluoride ion concentration in the wastewater, and calculate the defluorination rate.

[0043] In the following examples, preferably, the fluoride ion concentration in the fluorine-containing wastewater is 100-500 mg / L.

[0044] In the following examples, preferably, 9-11 g of oyster shell-modified walnut shell carbon core-shell composite microsphere adsorbent is added to each liter of wastewater, the adsorption temperature is 20°C to 30°C, and the adsorption time is 7-9 h.

[0045] A regeneration method for the oyster shell-modified walnut shell carbon core-shell composite microsphere adsorbent includes the following steps: After the oyster shell-modified walnut shell carbon core-shell composite microsphere adsorbent saturated after adsorbing fluoride ions is washed, filtered, and dried, it is added to a regeneration solution, and constant temperature water bath oscillation is carried out for regeneration. The regeneration solution is a mixed solution of NaOH and Al(NO3)3 to obtain the regenerated oyster shell-modified walnut shell carbon core-shell composite microsphere adsorbent.

[0046] In the following examples, preferably, the constant temperature water bath oscillation is specifically carried out at a temperature of 24°C - 26°C and a rotation speed of 140 - 160 r / min, and the regeneration time is 11 - 13 h. Further preferably, the conditions for the constant temperature water bath oscillation are a temperature of 25°C, a rotation speed of 150 r / min, and a regeneration time of 12 h.

[0047] In the following examples, preferably, the molar concentration of NaOH in the mixed solution is 0.02 - 0.04 mol / L, and the molar concentration of Al(NO3)3 is 0.05 - 0.15 mol / L. Further preferably, the molar concentration of NaOH is 0.03 mol / L, and the molar concentration of Al(NO3)3 is 0.1 mol / L.

[0048] Example 1:

[0049] An oyster shell-modified walnut shell core-shell composite microsphere adsorbent for treating high-concentration fluorine-containing wastewater, and its preparation method includes the following steps:

[0050] (1) Add 10 g of a mixed powder of oyster shell powder and walnut shell powder with a mass ratio of 1:3 to 100 mL of deionized water, keep it at 650 rpm and 60°C on a constant temperature magnetic stirrer for impregnation activation for 3 h. After the activation is completed, take it out, centrifuge, and dry. Then dry it at 105°C for 8 h. Crush the dried walnut shell powder and oyster shell and pass through a 100-mesh sieve to collect the mixed powder. Place it in a square quartz boat and push it into the furnace chamber of a tube furnace. Pass in nitrogen (flow rate about 300 mL / min). After exhausting the air, turn on the tube furnace switch, set a heating rate of 10°C / min, heat up to 500°C, and keep it for pyrolysis for 2 h. After the program ends, turn off the tube furnace and nitrogen, take out the pyrolysis product and immediately put it into deionized water, wash it repeatedly until the pH value of the supernatant is 7, put it into an oven and dry it at 105°C for 8 h. Pass the dried carbon through a 100-mesh sieve to collect, and obtain the oyster shell-modified walnut shell carbon.

[0051] (2) Add sodium alginate powder to deionized water. After stirring and dissolving it by heating in a water bath at 60 °C, a sodium alginate solution with a mass concentration of 20 g / L is prepared. Slowly add 10 g of oyster shell-modified walnut shell charcoal. After continuously stirring evenly, use a 10 mL syringe to dropwise inject the mixture into 100 ml of a CaCl2 solution with a mass concentration of 60 g / L for solidification and crosslinking for 12 h. Then wash it 4 - 5 times with deionized water and place it in an oven to dry at 60 °C for 12 h to obtain the oyster shell-modified walnut shell core-shell composite microsphere adsorbent. The specific surface area, single-point adsorption total pore volume, and average pore diameter of this adsorbent are 4.456 m 2 / g, 0.0047 cm 3 / g and 11.733 nm respectively.

[0052] In the fluorine-containing wastewater with an initial fluorine ion concentration of 100 mg / L and an initial pH of 3, add the above-prepared oyster shell-modified walnut shell core-shell composite microsphere adsorbent according to the amount of adding 10 g of the composite microsphere adsorbent per liter of fluorine-containing wastewater. Conduct a shaking flask test at 150 rpm and 25 °C. After 8 h of adsorption, the defluorination rate reaches 90.71%, the adsorption capacity is 11.309 mg / g, and the pH of the effluent is 6.7.

[0053] Characterize the oyster shell-modified walnut shell core-shell composite microsphere adsorbent obtained in Example 1, as shown in Figure 1 、 5 -8.

[0054] Comparative Example 1

[0055] It is the same as Example 1, except that: in step (1), 10 g of the mixed powder is changed to 10 g of walnut shell powder.

[0056] Treat the fluorine-containing wastewater with an initial fluorine ion concentration of 100 mg / L and an initial pH of 3 by the same steps as in Example 1. The defluorination rate is 59.94%, the adsorption capacity is 8.180 mg / g, and the pH value of the effluent is 3.9.

[0057] Comparative Example 2

[0058] It is the same as Example 1, except that: in step (1), 10 g of the mixed powder is changed to 10 g of oyster shell powder.

[0059] Treat the fluorine-containing wastewater with an initial fluorine ion concentration of 100 mg / L and an initial pH of 3 by the same steps as in Example 1. The defluorination rate is 61.06%, the adsorption capacity is 6.106 mg / g, and the pH value of the effluent is 7.5.

[0060] Compared with Comparative Examples 1-2, the core-shell composite microsphere adsorbent prepared by using oyster shell powder (Comparative Example 2) or walnut shell powder (Comparative Example 1) alone has a fluorine removal rate and adsorption amount for treating fluorine-containing wastewater that are much lower than those in Example 1. The composite microsphere adsorbent obtained by the synergistic effect of oyster shell powder and walnut shell powder in the present invention has a fluorine removal rate of 90.71%, an adsorption amount of 11.309 mg / g, and an effluent pH of 6.7, which can ensure that the fluorine in the wastewater is discharged in compliance with the standard while also having the effect of adjusting the pH value.

[0061] The SEM results of pure oyster shell carbon microspheres before adsorption (Comparative Example 2), pure walnut shell carbon microspheres (Comparative Example 1), and oyster shell modified walnut shell carbon obtained in step (1) of Example 1 were compared with oyster shell modified walnut shell carbon core-shell composite microspheres (Example 1), and the results are shown in Table 1.

[0062] Table 1 Surface characteristics of oyster shell modified walnut shell carbon core-shell composite microsphere adsorbent and related adsorbents

[0063]

[0064] The EDS results of pure oyster shell carbon microspheres before adsorption (Comparative Example 2), pure walnut shell carbon microspheres (Comparative Example 1), and oyster shell modified walnut shell carbon obtained in step (1) of Example 1 were compared with the oyster shell modified walnut shell carbon core-shell composite microspheres (Example 1), respectively, and combined with SEM, Table 2 was obtained.

[0065] Table 2 SEM-EDS analysis of oyster shell modified walnut shell carbon core-shell composite microsphere adsorbent and related adsorbents

[0066]

[0067]

[0068] Figure 6 It shows that the Ca2p peak before adsorption (binding energy 348-351 eV) corresponds to Ca 2+ , indicating that the surface calcium is enriched in the form of calcium-based shells; the C1s peak (284.8eV) shows that graphite carbon (CC) comes from the walnut shell carbon matrix; the O 1s peak (531eV) mainly comes from the lattice oxygen in the calcium-based shell, which is consistent with the calcium-based shell. The F1s peak (684-688eV) shows that a significant F signal appears after adsorption, which is speculated that fluoride ions react with surface calcium to form CaF2 precipitation. The chemically active sites are clear, which verifies the chemical activity of the shell.

[0069] Figure 7 The results show that there are new obvious diffraction peaks of CaF2, with peak positions at about 28.3°, 47.1°, and 56.4°, confirming that fluoride ions (F - ) and calcium-based active sites (Ca 2+A precipitation reaction occurs to form CaF₂. The formation of CaF₂ after adsorption indicates that the calcium-based component is enriched on the surface (shell layer), preferentially reacting with fluoride ions, while the internal carbon matrix (core) remains stable. The amorphous broad peak of the carbon matrix shows no obvious change before and after adsorption, indicating that it provides physical adsorption sites through the pore structure and synergistically enhances the chemical adsorption of the shell layer.

[0070] Figure 8 shows that before adsorption, the broad peak at 3400 - 3200 cm -1 corresponds to the stretching vibration of free hydroxyl groups or hydroxyl groups (-OH) of adsorbed water on the surface of biochar, indicating that the material is hydrophilic and may participate in physical adsorption; the peak at 1700 - 1650 cm -1 may originate from the C=O stretching vibration of carboxylic acids, suggesting the presence of acidic functional groups on the surface of the carbon matrix; the absorption peak at 1600 - 1500 cm -1 corresponds to the C=C skeletal vibration of the aromatic ring, reflecting the graphitized structure of the carbon matrix. After adsorption, the peak intensities at 1420 - 1450 cm -1 and 713 cm -1 decrease or disappear, indicating that Ca 2+ reacts with F - to form CaF₂. The stability of the functional groups of the carbon matrix (such as the unchanged C=C peak) indicates that it provides a physical adsorption path through the pore structure and is not significantly damaged during the adsorption process.

[0071] In summary, SEM shows that the surface of the microspheres is covered with a uniform bright white thin layer (scale bar 4 μm), the inside is a dark gray porous carbon matrix, the carbon particles are stacked in layers, there are local microcracks, and there are alternating bright and dark texture differences at the interface between the carbon matrix and the shell layer. The morphology comparison implies the formation of a core-shell structure (calcium-based shell + carbon-based core). The carbon matrix presents dense nanoscale pores (10 - 50 nm), and the pores are interconnected (three-dimensional network), and the interface is tightly combined; EDS / XPS confirms that calcium is enriched on the surface (Ca = 16.63 wt%), the proportion of the internal C1s peak (284.8 eV) increases significantly, the Ca signal decreases with depth, and the carbon-based core dominates the inside (C = 28.80 wt%), directly supporting the core-shell stratification; FTIR / XRD jointly prove that the chemical adsorption of the calcium-based shell layer plays a dominant role. The changes in hydroxyl and carboxyl groups show that the shell layer may also assist adsorption through the coordination of surface hydroxyl groups. The high specific surface area (66.68 m 2 / g) is contributed by the hierarchical pores of the carbon matrix, and the compactness of the shell layer does not significantly hinder mass transfer, indicating that the shell-core interface is optimized.

[0072] Conclusion: The oyster shell modified walnut shell carbon core-shell composite microspheres adsorb fluoride ions through chemical adsorption of the shell layer (releasing Ca 2+ , reacting with F -High-efficiency fluoride removal is achieved through the formation of CaF2 precipitate, physical adsorption on the core layer (the micro-mesoporous hierarchical pores of the carbon matrix provide a high specific surface area to accelerate the diffusion of fluoride ions to the active sites), and the synergistic protection effect (the shell layer prevents the direct contact of the carbon matrix with the acidic environment to reduce corrosion, and the carbon matrix pores buffer the stress of the shell layer to avoid crack propagation and improve stability).

[0073] Comparative Example 3

[0074] Same as Example 1, except that: in step (1), oyster shell powder is replaced with calcium carbonate powder.

[0075] The fluorine-containing wastewater with an initial fluoride ion concentration of 100 mg / L and an initial pH of 3 was treated in the same steps as in Example 1. The fluoride removal rate was 82.37%, the adsorption capacity was 8.503 mg / g, and the pH value of the effluent was 7.1.

[0076] Comparing Example 1 with Comparative Example 3, in Comparative Example 3, oyster shell powder was replaced with calcium carbonate, and the fluoride removal rate and adsorption capacity of the prepared composite microsphere adsorbent were both lower than those of Example 1. Oyster shell powder not only contains calcium carbonate, but also contains minerals such as iron oxide and magnesium oxide, as well as organic matter, which synergistically modify the walnut shell. In the present invention, oyster shell powder is used to modify the walnut shell, and the core-shell composite microspheres obtained by the combined action of the two have excellent adsorption performance.

[0077] Example 2

[0078] Same as Example 1, except that: the mass ratios of oyster shell powder and walnut shell powder are 1:1 and 3:1 respectively.

[0079] When the mass ratio of oyster shell powder and walnut shell powder is 1:1, the fluoride removal rate is 75.24%, the adsorption capacity is 8.065 mg / g, and the pH value of the effluent is 7.0; when the mass ratio of oyster shell powder and walnut shell powder is 3:1, the fluoride removal rate is 84.25%, the adsorption capacity is 8.698 mg / g, and the pH value of the effluent is 7.3.

[0080] Comparative Example 4

[0081] Same as Example 2, except that: the oyster shell-modified walnut shell carbon prepared with oyster shell powder and walnut shell powder with a mass ratio of 3:1 was directly used for fluoride removal treatment. The fluoride removal rate was 28.79%, the adsorption capacity was 2.862 mg / g, and the pH of the effluent was 7.3.

[0082] Example 3

[0083] Same as Example 1, except that: the initial fluoride ion concentration is 300 mg / L. The fluoride removal rate of the core-shell composite microsphere adsorbent is 80.25%, the adsorption capacity is 24.505 mg / g, and the pH value of the effluent is 6.5.

[0084] Example 4

[0085] Same as Example 1, except that: the initial fluoride ion concentration is 500 mg / L. The fluoride removal rate of the core-shell composite microsphere adsorbent is 72.27%, the adsorption capacity is 37.421 mg / g, and the pH value of the effluent is 6.4.

[0086] Example 5

[0087] Same as Example 1, except that: the adsorption time of the core-shell composite microsphere adsorbent is 2 h. The fluoride removal rate of the core-shell composite microsphere adsorbent is 79.50%, the adsorption capacity is 10.687 mg / g, and the pH value of the effluent is 6.4.

[0088] Example 6

[0089] Same as Example 1, except that: the adsorption time of the core-shell composite microsphere adsorbent is 7 h, and the core-shell composite microsphere adsorbent is added according to the amount of 11 g of the composite microsphere adsorbent per liter of fluoride-containing wastewater, and the shaking flask test is carried out at 30 °C. The fluoride removal rate of the core-shell composite microsphere adsorbent is 88.22%, the adsorption capacity is 9.99 mg / g, and the pH value of the effluent is 6.6.

[0090] Example 7

[0091] Same as Example 1, except that: the adsorption time of the core-shell composite microsphere adsorbent is 9 h respectively, and the core-shell composite microsphere adsorbent is added according to the amount of 9 g of the composite microsphere adsorbent per liter of fluoride-containing wastewater, and the shaking flask test is carried out at 20 °C. The fluoride removal rate of the core-shell composite microsphere adsorbent is 87.51%, the adsorption capacity is 12.12 mg / g, and the pH value of the effluent is 6.8.

[0092] Example 8

[0093] Same as Example 1, except that:

[0094] In step (1), the solid-liquid ratio of the mixed powder to deionized water is 1:8 g / mL, and the impregnation conditions are: impregnation activation at 55 °C for 4 h; the pyrolysis conditions are: the pyrolysis temperature is 450 °C, the heating rate is 9 °C / min, and the pyrolysis time is 3 h.

[0095] In step (2), the mass concentration of the sodium alginate solution is 10 g / L; the embedding amount of the oyster shell-modified walnut shell charcoal in the mixture is 80 g / L; the mass concentration of the calcium chloride solution is 50 g / L, the mass ratio of the oyster shell-modified walnut shell charcoal to calcium chloride is 1.6:1, and the crosslinking time is 10 h.

[0096] The fluoride removal rate of the core-shell composite microsphere adsorbent is 72.27%, and the adsorption capacity is 9.716 mg / g.

[0097] Example 9

[0098] Same as Example 1, except that:

[0099] In step (1), the solid-liquid ratio of the mixed powder to deionized water is 1:12 g / mL, and the impregnation conditions are: impregnation activation at 65 °C for 2 h; the pyrolysis conditions are: pyrolysis temperature of 550 °C, heating rate of 12 °C / min, and pyrolysis time of 1 h.

[0100] In step (2), the mass concentration of the sodium alginate solution is 30 g / L; the embedding amount of oyster shell modified walnut shell charcoal in the mixture is 120 g / L; the mass concentration of the calcium chloride solution is 70 g / L, the mass ratio of oyster shell modified walnut shell charcoal to calcium chloride is 1.7:1, and the cross-linking time is 14 h.

[0101] The defluorination rate of the core-shell composite microsphere adsorbent is 63.89%, and the adsorption capacity is 8.589 mg / g.

[0102] Example 10

[0103] Dissolve 0.5 g of the oyster shell modified walnut shell core-shell composite microspheres obtained in Example 1 in 50 mL of a mixed regeneration solution of NaOH and Al(NO3)3. The molar concentrations of NaOH and Al(NO3)3 are 0.03 mol / L and 0.1 mol / L respectively. Perform regeneration by shaking in a water bath at 150 rpm and 25 °C. After 12 h, wash, filter, and dry the regenerated oyster shell modified walnut shell composite microspheres. Then add the regenerated core-shell composite microspheres to the wastewater with an initial fluoride ion concentration of 100 mg / L and an initial pH of 3. Add the regenerated core-shell composite microspheres according to the amount of 10 g of the composite microsphere adsorbent per liter of wastewater. Conduct a shake flask test at 150 rpm and 25 °C. After 8 h of adsorption, the defluorination rate of the core-shell composite microsphere adsorbent is 81.69%, the adsorption capacity is 10.982 mg / g, and the effluent pH is 6.4.

[0104] After recycling and regenerating three times and then adsorbing under the same conditions, the defluorination rate of the core-shell composite microsphere adsorbent is 71.21%, the adsorption capacity is 9.573 mg / g, and the effluent pH value is 6.2.

[0105] Comparative Example 1

[0106] Same as Example 10, except that the regeneration solvent is 0.1 mol / L NaOH solution. The defluorination rate of the core-shell composite microsphere adsorbent is 49.29%, the adsorption capacity is 6.627 mg / g, and the effluent pH is 5.8.

[0107] Comparative Example 2

[0108] Same as Example 10, except that the regenerating solvent is 0.1 mol / L HCl solution, the defluorination rate of the core-shell composite microsphere adsorbent is 36.41%, the adsorption capacity is 4.895 mg / g, and the pH of the effluent is 3.2.

[0109] The description of the above embodiments is only used to help understand the technical solution and its core idea of the present invention. It should be pointed out that for those skilled in the art of this technology, without departing from the principle of the present invention, several improvements and modifications can be made to the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. An oyster shell modified walnut shell charcoal core-shell composite microsphere adsorbent, characterized in that, The specific surface area, single-point adsorption total pore volume, and average pore diameter of the core-shell composite microsphere adsorbent are 4-6 m 2 / g, 0.004-0.006 cm 3 / g, and 11-13 nm, respectively.

2. The preparation method of the oyster shell modified walnut shell carbon core-shell composite microsphere adsorbent according to claim 1, characterized in that, It includes the following steps: (1) Add the mixed powder of walnut shell and oyster shell with different mass ratios into deionized water for impregnation. After the impregnation is completed, dry it and pyrolyze it in an inert atmosphere. The pyrolysis conditions are as follows: nitrogen atmosphere, pyrolysis temperature is 450°C - 550°C, heating rate is 9 - 12°C / min, pyrolysis time is 1.0 - 3.0 h. After the pyrolysis is completed, wash the product to neutrality and dry it to obtain oyster shell modified walnut shell carbon; (2) Slowly add the oyster shell modified walnut shell carbon into the sodium alginate solution to obtain a mixture, and then drop the mixture into the calcium chloride solution for cross-linking. The product is washed and dried to obtain the oyster shell modified walnut shell carbon core-shell composite microsphere adsorbent.

3. The preparation method according to claim 2, characterized in that, In the mixed powder described in step (1), the mass ratio of oyster shell powder to walnut shell powder is 1:3 - 3:

1.

4. The preparation method according to claim 2, characterized in that, The mass concentration of the sodium alginate solution described in step (2) is 10 - 30 g / L.

5. The preparation method according to claim 2 or 4, characterized in that, In the mixture described in step (2), the embedding amount of oyster shell modified walnut shell carbon is 80 - 120 g / L, and the mass ratio of oyster shell modified walnut shell carbon to sodium alginate is 4:1 - 8:

1.

6. The preparation method according to claim 2 or 4, characterized in that, The mass concentration of the calcium chloride solution described in step (2) is 50 - 70 g / L, the mass ratio of oyster shell modified walnut shell carbon to calcium chloride is 1.6:1 - 1.7:1, and the cross-linking time is 10 - 14 h.

7. Use of the oyster shell-modified walnut shell carbon core-shell composite microsphere adsorbent described in claim 1 in the treatment of high-concentration fluorine-containing wastewater, characterized in that, The fluoride ion concentration in the high-concentration fluoride-containing wastewater is 100 - 500 mg / L.

8. The application according to claim 7, wherein: Add 9 - 11 g of oyster shell modified walnut shell carbon composite microsphere adsorbent per liter of wastewater, the adsorption temperature is 20°C - 30°C, and the adsorption time is 7 - 9 h.

9. The regeneration method of the oyster shell modified walnut shell carbon core-shell composite microsphere adsorbent according to claim 1, characterized in that, (9) It includes the following steps: After the oyster shell modified walnut shell carbon core-shell composite microsphere adsorbent saturated after adsorbing fluoride ions is washed, filtered and dried, add it to the regeneration solution and carry out regeneration through constant temperature water bath oscillation. The regeneration solution is a mixed solution of NaOH and Al(NO3)3 to obtain the regenerated oyster shell modified walnut shell carbon composite microsphere adsorbent.

10. The regeneration method according to claim 9, wherein, The concentration of the oyster shell modified walnut shell carbon core-shell composite microsphere adsorbent is 8 - 12 g / L, the molar concentration of NaOH in the mixed solution is 0.02 - 0.04 mol / L, and the molar concentration of Al(NO3)3 is 0.05 - 0.15 mol / L.

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