A La-Ca layered hydroxide phosphate adsorbent and a two-step synthesis method thereof

Abstract

The invention discloses a La-Ca layered hydroxide phosphate adsorbent and a two-step synthesis method thereof; by changing the metal ions of the synthetic layered hydroxide, La3+ with high selectivity for phosphorus and Ca2+ with low cost and environmental friendliness are selected, a two-step method (coprecipitation and hydrothermal reaction) is adopted, the reaction rate of material synthesis is controlled by a nanofiltration membrane and a diazonium salt is added as an auxiliary agent, and a La-Ca layered hydroxide adsorption material with excellent adsorption effect on phosphorus phosphate in water is synthesized. The invention has the advantages of simple operation, low cost, high phosphorus adsorption capacity of the synthetic material, fast phosphorus adsorption rate, etc.

Description

Technical Field

[0001] The present invention relates to, in particular to, a La-Ca layered hydroxide phosphate adsorbent and a two-step synthesis method thereof. Background Art

[0002] Water is an important resource for human survival. Phosphorus is an abundant non-metallic element on the earth. Its single substance and compounds play an important role in industrial and agricultural production, but the large-scale use of phosphorus compounds causes water pollution.

[0003] Phosphorus in water mainly comes from surface runoff, farm drainage, domestic sewage and industrial wastewater. In sewage treatment plants, microorganisms convert organic polyphosphates into orthophosphates. Therefore, the main form of phosphorus in the effluent of sewage treatment plants is orthophosphate. Phosphorus is an indispensable element for the growth of aquatic plants, but excessive phosphorus content in water bodies will stimulate some algae to overgrow and form algal blooms, which will cause water pollution, affect water quality, and increase water supply costs. The safety threshold of phosphorus in water bodies is 0.05-0.1 mg / l, so controlling the phosphorus content in discharged sewage has become an important link in the process of urban sewage treatment.

[0004] Currently, there are three main methods for removing phosphorus from wastewater: chemical method, biological method and physical and chemical method.

[0005] Chemical methods include chemical precipitation and crystallization, among which phosphorus removal requires the use of a large amount of chemical agents; the cost is high and a large amount of chemical sludge is produced.

[0006] Biological methods include biological phosphorus removal and artificial wetlands. Although biological methods are widely used in phosphorus removal in sewage treatment plants, the temperature, pH value and organic carbon source of sewage have a great influence on the phosphorus removal efficiency.

[0007] The main physical and chemical methods are adsorption. Many adsorbent materials can be recycled, and synthetic adsorbent materials can achieve very high adsorption capacity. Among them, layered hydroxide (LDH) shows very excellent phosphate adsorption performance.

[0008] Layered hydroxide (LDH) is a synthetic material widely used to remove waterborne pollutants such as phosphorus, dyes and bromate. The large surface area and high anion exchange capacity make LDH an ideal material for adsorbing phosphorus from water.

[0009] However, most of the current LDHs have disadvantages such as low adsorption capacity or high synthesis cost. Taking the common Mg-Al layered hydroxide as an example, due to the limitations of the material itself, it is difficult to further increase the adsorption capacity for phosphorus. Commercial lanthanum hydroxide with higher adsorption capacity has a high cost because the raw materials are mainly relatively expensive rare earth elements.

[0010] Diazonium salt is a salt containing a diazo group, which has a strong electron-withdrawing effect and high reactivity. Adding diazonium salt during the synthesis of layered hydroxides can make the diazonium salt insert into the interlayer of the layered hydroxide, regulate the interlayer structure to make the layered structure more regularly arranged, and increase the interlayer spacing to facilitate interlayer ion exchange.

[0011] Nanofiltration membranes can control the diffusion and diffusion rate of different ions. They are used in the synthesis of layered hydroxides to regulate the reaction process by reducing the ion diffusion rate, thereby synthesizing layered hydroxide materials with more complete crystal forms and more regular layered structures. Summary of the invention

[0012] The object of the present invention is to overcome the above-mentioned shortcomings and deficiencies of the prior art and to provide a La-Ca layered hydroxide phosphate adsorbent and a two-step synthesis method thereof.

[0013] The invention has the advantages of simple synthesis process, low synthesis cost, high phosphorus adsorption capacity, fast adsorption rate, good trace phosphorus adsorption effect, and the like.

[0014] The present invention is achieved by 3+ and Ca 2+ The metal ions used as the synthetic layered hydroxides are first co-precipitated at a certain temperature, and then subjected to high-pressure hydrothermal reaction, so that the synthesized material has a more complete crystal structure; the reaction rate is controlled by a nanofiltration membrane, so that the reaction is more fully, slowly and controllably carried out, thereby optimizing the material structure and size, and improving physical properties such as specific surface area and interlayer spacing;

[0015] The present invention adds diazonium salt as an auxiliary agent during the synthesis process to regulate the layered spatial structure of the material, and the synthesized material has an appropriate interlayer spacing, which is conducive to the exchange of interlayer ions. In summary, through these methods, an adsorption material with high selectivity and high capacity for phosphorus is synthesized, which can achieve the removal of trace phosphorus (0.5mgP / L to 1.0mgP / L) in water. Compared with other layered hydroxides used for phosphorus removal, the adsorption capacity and adsorption rate are improved while the cost is reduced.

[0016] The present invention is achieved through the following technical solutions:

[0017] A two-step method for synthesizing La-Ca layered hydroxide phosphate adsorbent:

[0018] The reactor is divided into two spaces, inner and outer, by a nanofiltration membrane 1, i.e., an inner space 2 and an outer space 3;

[0019] The inner space 2 is filled with deionized water containing diazonium salt; the outer space 3 is filled with deionized water;

[0020] Adding a complex cationic salt solution 4 and an interlayer anionic salt solution 5 dropwise into the inner layer space 2;

[0021] Alkaline solution 6 is added dropwise into the outer space 3 to generate a precipitate in the inner space 2. The temperature is maintained for reaction for 4-6 hours to obtain a mixed solution. The mixed solution is placed in a high-pressure reactor for heating and reaction, and then the mixed solution is taken out and allowed to stand;

[0022] After standing, the supernatant was discarded and deionized water was added for shaking and washing, and then the mixture was allowed to stand again and the supernatant was discarded. This process was repeated, and finally the precipitate was freeze-dried to obtain a white powdery La-Ca layered hydroxide phosphate adsorbent.

[0023] The composite cationic salt solution 4 in the inner space 2 is a La-Ca composite ion solution containing diazonium salt; the hydroxide in the alkali solution 6 in the outer space 3 slowly passes through the nanofiltration membrane 1 into the inner space 2, causing the pH of the solution in the inner space 2 to rise, inducing a layered hydroxide precipitation crystallization reaction; at the same time, the nanofiltration membrane 2 prevents the Ca in the inner space 2 from 2+ ,La 3+ The multivalent ions are reversed into outer space3.

[0024] The above-mentioned composite cationic salt solution 4 is 0.7~0.8mol / L La 3+ With 0.2~0.3mol / L Ca 2+ A composite solution;

[0025] The above-mentioned interlayer anion salt solution 5 is a 0.45-0.55 mol / L Na2CO3 solution;

[0026] The above-mentioned alkali solution 6 is 1-2 mol / L NaOH solution.

[0027] During the dropwise addition of the composite cationic salt solution 4, the interlayer anionic salt solution 5 and the alkali solution 6, the rate is 1.0 to 3.0 ml / min, the water bath temperature is 65° C. to 90° C., and the stirring rate is 300 to 600 r / min.

[0028] The inner space 2 contains deionized water containing diazonium salt, specifically, deionized water containing 0.5 wt % to 2.0 wt % of diazonium salt.

[0029] In the above-mentioned composite cation salt solution 4, La 3+ and Ca 2+ The molar ratio of the concentration is 1:3.

[0030] The generation of precipitate in the inner space 2 specifically means that during the reaction of the precipitate, the pH value of the solution in the inner space 2 is 9-11.

[0031] The mixed solution is placed in a high pressure reactor for heating and reacting, specifically the reaction temperature is 200° C.-400° C., and the reaction time is 2-3 hours.

[0032] The molecular weight cutoff of the nanofiltration membrane 1 is 200-1000 Daltons (Da).

[0033] By using the above method, a La-Ca layered hydroxide phosphate adsorbent can be obtained.

[0034] Compared with the prior art, the present invention has the following advantages and effects:

[0035] Currently, the most commonly used layered hydroxide for phosphorus removal is Mg-Al LDH, which has the disadvantages of low adsorption capacity and slow adsorption rate. The reasons for these disadvantages are that the material has irregular crystal structure, defects in spatial structure, and unstable reaction rate, which affect physical properties such as specific surface area and interlayer spacing. Layered hydroxides containing La and other divalent metals have the disadvantage of high cost.

[0036] The synthesis method of the present invention is simple and easy to implement. 3+ and Ca 2+ The material synthesized by the two-step method has a regular crystal structure, the addition of diazonium salt optimizes the spatial structure of the material, and the use of nanofiltration membrane to control the reaction rate improves the material's specific surface area, interlayer spacing and other physical properties.

[0037] The synthesis method of the present invention can greatly improve the phosphorus adsorption capacity of the product material while reducing the cost.

[0038] In addition, the product material obtained by the present invention can quickly reduce the phosphate concentration in water in a short time (taking a 1 mgP / L phosphate solution as an example, it can be reduced to about 0.2 mgP / L in about 5 minutes, and the adsorption equilibrium is reached after half an hour, and the phosphate concentration is below 0.05 mgP / L). BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 It is the reactor used in the examples of the present invention.

[0040] Figure 2 This is the adsorption isotherm of an embodiment of the present invention when 0.02 g of adsorption material is added to 100 ml of a solution with a gradient phosphorus concentration at 25°C.

[0041] Figure 3 In the examples of the present invention, 0.02 g of the adsorbent material was added to 100 ml of a 1 mg / l solution at 25°C and the phosphorus concentration in the solution was measured at different times. DETAILED DESCRIPTION

[0042] The present invention is further described in detail below in conjunction with specific embodiments.

[0043] Example:

[0044] like Figure 1-3 As shown, the present invention discloses a two-step method for synthesizing La-Ca layered hydroxide phosphate adsorbent, which can be achieved by the following steps:

[0045] The reactor is divided into two spaces, inner and outer, by a nanofiltration membrane 1, i.e., an inner space 2 and an outer space 3;

[0046] The inner space 2 is filled with deionized water containing diazonium salt; the outer space 3 is filled with deionized water;

[0047] Adding a complex cationic salt solution 4 and an interlayer anionic salt solution 5 dropwise into the inner layer space 2;

[0048] Alkaline solution 6 is added dropwise into the outer space 3 to generate a precipitate in the inner space 2. The temperature is maintained for reaction for 4-6 hours (preferably 5 hours) to obtain a mixed solution. The mixed solution is placed in a high-pressure reactor for heating and reaction, and then the mixed solution is taken out and allowed to stand;

[0049] After standing, the supernatant is discarded and deionized water is added for shaking and washing, and then the mixture is allowed to stand again and the supernatant is discarded. This process is repeated multiple times (preferably 3-5 times), and finally the precipitate is freeze-dried to obtain a white powdery La-Ca layered hydroxide phosphate adsorbent.

[0050] The composite cationic salt solution 4 in the inner space 2 is a La-Ca composite ion solution containing diazonium salt; the hydroxide in the alkali solution 6 in the outer space 3 slowly passes through the nanofiltration membrane 1 into the inner space 2, causing the pH of the solution in the inner space 2 to rise, inducing a layered hydroxide precipitation crystallization reaction; at the same time, the nanofiltration membrane 2 prevents the Ca in the inner space 2 from 2+ ,La 3+ The multivalent ions are reversed and enter outer space3.

[0051] The above-mentioned composite cationic salt solution 4 is 0.7~0.8mol / L La 3+ With 0.2~0.3mol / L Ca 2+ A composite solution;

[0052] The above-mentioned interlayer anion salt solution 5 is a 0.45-0.55 mol / L Na2CO3 solution (preferably 0.45-0.5 mol / L Na2CO3).

[0053] The alkali solution 6 is a 1-2 mol / L NaOH solution (preferably 1-1.2 mol / L NaOH).

[0054] During the dropwise addition of the composite cation solution 4, interlayer anion salt solution 5 and alkali solution 6, the rate is 1.0-3.0 ml / min, the water bath temperature is 65° C.-90° C. (preferably 80° C.), and the stirring rate is 300-600 r / min.

[0055] The inner space 2 contains deionized water containing diazonium salt, specifically, deionized water containing 0.5wt% to 2.0wt% (preferably 0.5wt% to 1.5wt% or 1w.t.% to 1.3wt%) of diazonium salt.

[0056] The general formula of diazonium salt is R-N2 + X - : R refers to an organic group, such as an alkyl or aryl group, and X refers to any anion, usually a halogen ion.

[0057] In the above composite cation solution 4, La 3+ and Ca 2+ The molar ratio of the concentration is 1:3.

[0058] The generation of precipitate in the inner space 2 specifically means that during the reaction of the precipitate, the pH value of the solution in the inner space 2 is 9 to 11 (preferably 10, ±0.5).

[0059] The mixed solution is placed in a high pressure reactor for heating and reacting, specifically the reaction temperature is 200° C.-400° C., and the reaction time is 2-3 hours.

[0060] The molecular weight cutoff of the nanofiltration membrane 1 is 200-1000 Daltons (Da).

[0061] By using the above method, a La-Ca layered hydroxide phosphate adsorbent can be obtained.

[0062] The obtained products were subjected to adsorption isotherm and adsorption kinetics experiments respectively. Figure 1 , Figure 2 The test results show that:

[0063] This material has a high adsorption capacity for phosphorus and a fast adsorption rate, and can quickly reduce the phosphorus content in the solution within 5 minutes.

[0064] The La-Ca layered hydroxide phosphate adsorbent of the present invention can be synthesized through the above two-step method.

[0065] The present invention changes the metal ions of the synthetic layered hydroxide, selects La3+ with high selectivity for phosphorus and Ca2+ with low cost and environmental friendliness, adopts a two-step method (coprecipitation and hydrothermal reaction), controls the reaction rate of material synthesis through a nanofiltration membrane and adds diazonium salt as an auxiliary agent, thereby synthesizing a La-Ca layered hydroxide adsorption material with excellent adsorption effect on phosphorus phosphate in water.

[0066] The implementation methods of the present invention are not limited to the above-mentioned embodiments, and any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principles of the present invention shall be equivalent replacement methods and shall be included in the protection scope of the present invention.

Claims

1. A two-step method for synthesizing La-Ca layered hydroxide phosphate adsorbent, characterized in that: The reactor is divided into two spaces, inner and outer, by a nanofiltration membrane (1), namely, an inner space (2) and an outer space (3); The inner space (2) is filled with deionized water containing diazonium salt; the outer space (3) is filled with deionized water; Adding a complex cationic salt solution (4) and an interlayer anionic salt solution (5) dropwise into the inner layer space (2); Adding an alkali solution (6) dropwise into the outer space (3), a precipitate is generated in the inner space (2), and the temperature is maintained for 4-6 hours to obtain a mixed solution, the mixed solution is placed in a high pressure reactor for heating and reaction, and then the mixed solution is taken out and allowed to stand; After standing, the supernatant was discarded and deionized water was added for shaking and washing, and then the supernatant was discarded again, and this process was repeated. Finally, the precipitate was freeze-dried to obtain a white powdery La-Ca layered hydroxide phosphate adsorbent; The composite cationic salt solution (4) is 0.7-0.8 mol / L La 3+ With 0.2~0.3 mol / L Ca 2+ A composite solution; The interlayer anion salt solution (5) is a 0.45-0.55 mol / L Na2CO3 solution; The alkali solution (6) is a 1-2 mol / L NaOH solution; During the dropwise addition of the composite cationic salt solution (4), the interlayer anionic salt solution (5) and the alkali solution (6), the rate is 1.0-3.0 ml / min, the water bath temperature is 65°C-90°C, and the stirring rate is 300-600 r / min; The deionized water containing diazonium salt is deionized water containing 0.5wt% to 2.0wt% of diazonium salt; The mixed solution is placed in a high pressure reactor for heating and reaction, specifically the reaction temperature is 200°C -400°C, and the reaction time is 2-3 hours.

2. The two-step method for synthesizing La-Ca layered hydroxide phosphate adsorbent according to claim 1, characterized in that: In the complex cation salt solution (4), La 3+ and Ca 2+ The molar ratio of the concentration is 1:

3.

3. The two-step method for synthesizing La-Ca layered hydroxide phosphate adsorbent according to claim 1, characterized in that: The generation of a precipitate in the inner space (2) specifically means that during the reaction of the precipitate, the pH value of the solution in the inner space (2) is 9-11.

4. The two-step method for synthesizing La-Ca layered hydroxide phosphate adsorbent according to claim 1, characterized in that: The molecular weight cut-off of the nanofiltration membrane (1) is 200-1000 Daltons.

5. A La-Ca layered hydroxide phosphate adsorbent, characterized in that Obtained by the method according to any one of claims 1 to 4.

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