CN102515247B - Zinc-aluminum binary hydrotalcite and application thereof as photocatalytic material used for degrading methyl violet - Google Patents

Abstract

The invention discloses a zinc-aluminum binary hydrotalcite and its application as a photocatalytic material to degrade methyl violet. The general chemical formula of the hydrotalcite is [Zn ²⁺ _1-x Al ³⁺ _x (OH) ₂ ]( CO ₃ ^2- ) _x/2 mH ₂ O], where x is the molar ratio of Al ³⁺ /[Al ³⁺ +Zn ²⁺ ], 0.2≤x≤0.33, m is the amount of crystal water, 2≤m≤ 6. The dihydroxy composite hydroxide of zinc-aluminum binary hydrotalcite in the present invention is used to catalyze the degradation of methyl violet in dye wastewater, the reaction conditions are mild, the dye removal rate is high, and the materials after catalytic degradation are easy to recycle.

Description

Zinc-aluminum binary hydrotalcite and its application as a photocatalytic material for the degradation of methyl violet

(1) Technical field

The invention relates to a hydrotalcite and its application, in particular to a zinc-aluminum binary hydrotalcite and its application as a photocatalytic material for degrading methyl violet.

(2) Background technology

In the current chemical industry, the printing and dyeing industry has been widely concerned due to high pollution and difficult wastewater treatment. Dye wastewater has high chroma, complex chemical composition, poor biochemical properties, and contains a variety of organic substances with biological toxicity or "three-induced" properties, which are difficult to treat by conventional methods. Organic cationic dyes are an important kind of dyes used in practice, also known as salt-based dyes and basic dyes. MethylViolet belongs to organic cationic dyes, its molecular formula is: C ₂₅ H ₃₀ N ₃ Cl, and its structure is shown in formula (I). If the dye wastewater is directly discharged into the environment without treatment, it will cause adverse effects.

Layered anionic clays (Layered Double Hydroxides, LDHs for short) are a class of functional materials with special layered configurations, mainly including hydrotalcites, hydrotalcite-like and intercalated hydrotalcites. Its general formula is: [M ²⁺ _1-x M ³⁺ _x (OH) ₂ ](A ^n- ) _x/n ·mH ₂ O], the structure is very similar to brucite Mg(OH) ₂ . Among them, M ²⁺ and M ³⁺ represent divalent and trivalent metal cations respectively, which are located on the main layer; the subscript x represents the content change of metal elements, and ^An- represents exchangeable anions between layers, such as NO ₃ ^- , CO ₃ ^2- etc. Due to the particularity of the laminate structure, the type and proportion of the laminate cations can be optimized, or various anions can be introduced between the layers, which broadens the application field of hydrotalcite. Due to its special structure, hydrotalcite not only has a certain photocatalytic activity, but also can be well compounded with other photocatalytic materials. It is a new type of photocatalytic material with high efficiency and environmental protection.

(3) Contents of the invention

The purpose of the invention is to provide a zinc-aluminum binary hydrotalcite and its application as a photocatalytic material for degrading methyl violet.

The technical scheme adopted in the present invention is:

The invention provides a zinc-aluminum binary hydrotalcite, the general chemical formula of the hydrotalcite is [Zn ²⁺ _1-x Al ³⁺ _x (OH) ₂ ](CO ₃ ^2- ) _x/2 ·mH ₂ O] , where x is the molar ratio of Al ³⁺ /[Al ³⁺ +Zn ²⁺ ], 0.2≤x≤0.33, m is the amount of crystal water, 2≤m≤6.

The general chemical formula of the hydrotalcite is preferably [Zn ²⁺ _0.75 Al ³⁺ _0.25 (OH) ₂ ](CO ₃ ^2- ) _0.125 ·4H ₂ O].

The hydrotalcite of the present invention is usually prepared as follows: Zn(NO ₃ ) ₂ ·6H ₂ O (0.3mol) and Al(NO ₃ ) ₃ ·9H ₂ O (0.1mol) are dissolved in 300ml of water to prepare For solution A, dissolve 0.8mol NaOH and _{0.05mol Na2CO3} in 300ml of water to prepare solution B, add solution A and solution B to 100ml of deionized water respectively, keep the dropping rate of 1 drop/second, and keep the temperature at 40°C Stir vigorously, keep the pH value between 9 and 10, continue to stir for 60 minutes after the dropwise addition, crystallize at 65°C for 18h, centrifuge, wash the precipitate to neutrality, and dry at 85°C for 12h to obtain the ZnAl-LDHs sample, in which zinc The aluminum ratio is 3:1, that is, x is 0.25, m is 4, and the general chemical formula is

[Zn ²⁺ _0.75 Al ³⁺ _0.25 (OH) ₂ ](CO ₃ ^2- ) _0.125 ·4H ₂ O].

On the other hand, the application of the zinc-aluminum binary hydrotalcite as a photocatalytic material for degrading methyl violet.

Further, the application of the zinc-aluminum binary hydrotalcite as photocatalytic material for degrading methyl violet is as follows: the zinc-aluminum binary hydrotalcite is placed in a methyl violet aqueous solution at 10-50°C and pH 4.0-10.0, The dysprosium lamp is irradiated for 0.5-4.0 hours, and stirred to degrade methyl violet; the initial mass concentration of the methyl violet aqueous solution is 10-200 mg/L, and the mass dosage of the zinc-aluminum binary hydrotalcite is 1-100 mg/mL.

The present invention uses a water bath to control the temperature, and preferably uses NaOH solution and HCl solution to adjust the pH value of the methyl violet aqueous solution.

The higher dosage of zinc-aluminum binary hydrotalcite can effectively adsorb dye (methyl violet) on the surface of the material, so as to facilitate catalytic degradation, the quality dosage of zinc-aluminum binary hydrotalcite of the present invention is preferably 2mg/ml methyl violet aqueous solution, and The mass concentration of the methyl violet aqueous solution is preferably 20 mg/L. If the concentration is too low, the experimental error caused by the degradation of the dye itself will be relatively large.

Hydrotalcites (LDHs), especially zinc-aluminum hydrotalcites (ZnAl-LDHs), have a certain band gap and can generate electron-hole pairs with catalytic activity under suitable light. When organic dyes (methyl violet) are When adsorbed on the surface of hydrotalcite, electron-hole pairs will be captured, and the dye itself will be degraded. The zinc-aluminum binary hydrotalcites (ZnAl-LDHs) of the present invention can be prepared by conventional methods, such as by double-drop co-precipitation, and stored under anhydrous dry conditions for catalytic degradation of methyl The application of violet, the structure of described methyl violet 106 is as shown in formula (I):

Put the synthesized zinc-aluminum binary hydrotalcite in an ultraviolet-visible spectrophotometer, measure its diffuse reflectance spectrum, find the absorption edge in the spectrum, make the absorption edge tangent line to obtain the absorption edge wavelength, and calculate the forbidden energy of the material through the formula. belt width. Select a suitable light source according to the band gap of the material. The main peak energy of the light source is enough to excite the electrons in the valence band of the material to transition to the conduction band to generate electron-hole pairs.

After the light source is determined, the experimental model is selected mainly based on two points: 1. Make full use of the light source to reduce light energy loss; 2. Make the reactor fully irradiated by the light source. There are three options for the reaction model: 1. Parallel reaction model with external illumination; 2. Reaction model with internal illumination; 3. Ring reaction model.

The degradation rate η of zinc-aluminum hydrotalcite material to methyl violet in the reaction system can be calculated by formula (1), namely

η=η _e -η ₁ -η ₂ formula (1)

In the formula (1), η _e is the total removal rate of zinc-aluminum hydrotalcite to methyl violet aqueous solution, η ₁ is the self-degradation rate of methyl violet under light, and η ₂ is zinc-aluminum hydrotalcite to methyl violet under dark state condition Adsorption rate of base violet.

η _e , η ₁ , η ₂ are calculated by formula (2) respectively, namely

η=(C _i -C _e )/C _i *100 formula (2)

In formula (2), C _i (mg L ^-1 ) and C _e (mg L ^-1 ) are the concentration of methyl violet in the reaction system at the beginning of the reaction and at the end of the reaction, respectively, and V(L) is the original added The volume of the methyl violet aqueous solution, m (g), is the dosage of zinc-aluminum hydrotalcite.

The degradation rate of the p-methyl violet aqueous solution of the zinc-aluminum binary hydrotalcite of the present invention is calculated by the above method.

Compared with the prior art, the beneficial effect of the present invention is mainly reflected in that the double hydroxyl compound hydroxide of zinc-aluminum binary hydrotalcite in the present invention is used to catalyze the degradation of methyl violet in dye wastewater, the reaction conditions are mild, and the dye can be removed The efficiency is high, and the materials after catalytic degradation are easy to recycle.

(4) Description of drawings

Fig. 1 is the X-ray diffraction pattern of zinc-aluminum binary hydrotalcite in embodiment 1;

Fig. 2 is the diffuse reflectance spectrogram of zinc-aluminum binary hydrotalcite in embodiment 2;

Fig. 3 is the reaction model diagram in embodiment 3: 3-1 is the external illumination parallel reaction model, and wherein 1 is reactor; 2 is light source; 3 is optical filter; 3-2 is internal illumination reaction model, and wherein 1 is reaction 2 is a light source; 3-3 is a circular reaction model, wherein 1 is a reactor; 2 is a light source;

Fig. 4 is the methyl violet aqueous solution absorption curve and the light time relation figure among the embodiment 4;

Fig. 5 is the total removal rate, degradation rate and adsorption rate figure of methyl violet aqueous solution in embodiment 4;

Fig. 6 is the standard curve of embodiment 5 methyl violet.

(5) Specific implementation methods

The present invention is further described below in conjunction with specific embodiment, but protection scope of the present invention is not limited thereto:

The preparation of embodiment 1 zinc-aluminum binary hydrotalcite

Dissolve Zn(NO ₃ ) ₂ ·6H ₂ O (0.3mol) and Al(NO ₃ ) ₃ ·9H ₂ O (0.1mol) in 300ml _of water to prepare solution A, and dissolve 0.8molNaOH and _{0.05molNa2CO3} in Prepare solution B in 300ml water, add solution A and solution B dropwise to 100ml deionized water respectively, keep the dropping rate of 1 drop/second, stir vigorously at constant temperature of 40°C, keep the pH value between 9 and 10, drop Continue to stir for 60min after the addition, crystallize at 65°C for 18h, centrifuge, wash the precipitate with deionized water until neutral, dry at 85°C for 12h to obtain a zinc-aluminum binary hydrotalcite (ZnAl-LDHs) sample, X-ray diffraction pattern ( XRD) as shown in Figure 1, wherein the zinc-aluminum ratio is 3: 1, that is, x is 0.25, m is 4, and the general chemical formula is

[Zn ²⁺ _0.75 Al ³⁺ _0.25 (OH) ₂ ](CO ₃ ^2- ) _0.125 ·4H ₂ O].

Example 2

Get 2000mg of zinc-aluminum binary hydrotalcites with a zinc-aluminum ratio of 3:1 prepared by the method in Example 1, scan the 200-800nm wavelength range in a UV-visible spectrophotometer (type 2550, Shimadzu), and measure the zinc-aluminum binary water. The diffuse reflectance spectrum of talc is shown in Figure 2. According to the absorption edge of the measured spectrum, the absorption edge wavelength is obtained, and then the band gap is calculated according to the formula Eg=1240/λg (Eg is the band gap energy, and λg is the absorption edge wavelength). able.

Conclusion: The bandgap energy of zinc-aluminum binary hydrotalcite is 3.05eV, and a dysprosium lamp can be chosen as the excitation light source.

Example 3

Prepare 3 parts of 50ml 20mg L ^-1 methyl violet aqueous solution, weigh three parts of the zinc-aluminum binary hydrotalcite prepared by the method of Example 1, each part of 100mg, add respectively in the above-mentioned methyl violet aqueous solution to be treated (form 3 group), the control temperature is 25°C, pH=7, and they are respectively placed in three different reaction models (3-1 in Fig. 3 is an externally illuminated parallel reaction model; reaction model), control the irradiation time of the dysprosium lamp to be 1h, after stirring, centrifuge the three groups of solutions at 2000rpm for 10min, take the supernatant respectively to measure the absorbance of methyl violet at 590nm, and calculate the methyl violet according to the methyl violet standard curve. concentration, according to formulas (1) and (2), the degradation rate of zinc-aluminum hydrotalcite to methyl violet aqueous solution in the external light parallel reaction model is 35%.

Conclusion: The external illumination parallel reaction model has a simple reactor, low requirements for experimental conditions, and the light utilization rate is not lower than the other two models. Therefore, the external illumination parallel reaction model is selected. Under this model, the degradation of hydrotalcite to methyl violet aqueous solution rate can reach 35%.

Example 4

Prepare 3 parts of 50ml 20mg·L ^-1 methyl violet aqueous solution and divide them into 3 groups. Group 1 weighs 100mg of zinc-aluminum binary hydrotalcite prepared by the method in Example 1, controls the temperature at 25°C and pH=7, and places it outside In the parallel reaction model, irradiate with a dysprosium light source, stir, and take samples at 0, 30, 60, 90, and 120 minutes of irradiation respectively. The samples are centrifuged at 2000 rpm for 10 minutes, and the supernatant is taken to measure the concentration of methyl violet in the range of 200 to 800 nm. Absorbance (Shimadzu Model 2550 UV-Vis spectrophotometer), with the wavelength as the abscissa and the absorbance of methyl violet as the ordinate, draw the absorption curves at different time points as shown in Figure 4.

Group 2 under the same conditions and operations as Group 1, measured the adsorption of zinc-aluminum binary hydrotalcite to methyl violet in a dark state, that is, without light source irradiation, and other conditions were the same, and measured the absorbance of methyl violet at 590nm after 120 minutes of reaction , calculate the methyl violet concentration according to the methyl violet standard curve, and calculate the adsorption rate of zinc-aluminum binary hydrotalcite to methyl violet under the dark state according to formula (2);

Group 3 is under the same conditions and operations as Group 1, without adding zinc-aluminum binary hydrotalcite, measuring the degradation of methyl violet under light source irradiation, measuring the absorbance of methyl violet at 590nm after 120 minutes of reaction, and calculating according to the methyl violet standard curve For the concentration of methyl violet, the self-degradation rate of methyl violet under light is calculated according to the formula (2).

Get the reaction solution after 120min treatment in group 1, measure the absorbance of methyl violet at 590nm, calculate the methyl violet concentration according to the methyl violet standard curve, calculate the zinc aluminum hydrotalcite to the methyl violet aqueous solution according to formula (2) total removal rate.

The degradation rate η of zinc aluminum hydrotalcite material to methyl violet can be calculated by formula (1), namely

η=η _e -η ₁ -η ₂ formula (1)

In the formula (1), η _e is the total removal rate of zinc-aluminum talcite to methyl violet, η ₁ is the self-degradation rate of methyl violet under light, and η ₂ is zinc-aluminum hydrotalcite to methyl violet under dark state conditions the adsorption rate.

η _e , η ₁ , η ₂ are calculated by formula (2) respectively, namely

η=(C _i -C _e )/C _i *100 formula (2)

In the formula (2), C _i (mg·L ^-1 ) and _Ce (mg·L ^-1 ) are the concentrations of methyl violet in the reaction system at 0 min and 120 min respectively, and V(L) is the originally added formazan The volume of base violet aqueous solution, m (g) is the dosage of zinc aluminum hydrotalcite.

Calculate the total removal rate (60%), methyl violet self-degradation rate (20%) and adsorption rate (5%) of the methyl violet aqueous solution according to formula (1) and formula (2). The results are shown in Figure 5.

Conclusion: Under the irradiation of dysprosium light source, zinc-aluminum binary hydrotalcite with a zinc-aluminum ratio of 3:1 can degrade methyl violet, and the degradation rate reaches 35%.

The preparation of embodiment 5 methyl violet standard curve

Based on the initial solution of methyl violet (1mg/L), prepare solutions with concentrations of 0.04mg/L, 0.09mg/L, 0.18mg/L, 0.35mg/L, and 0.07mg/L, and use deionized water as a blank For reference, measure the absorbance of each solution at 590nm with a Shimadzu 2550 UV-Vis spectrophotometer, and perform linear fitting to obtain a standard curve of methyl violet solution absorbance A-concentration C, as shown in Figure 6.

Claims (2)

1. The application of a zinc-aluminum binary hydrotalcite as a photocatalytic material for degrading methyl violet, characterized in that the application is as follows: placing the zinc-aluminum binary hydrotalcite in a methyl violet aqueous solution at 10-50°C, pH Under the condition of 4.0 to 10.0, the dysprosium lamp is irradiated for 0.5 to 4.0 hours and stirred to degrade methyl violet; the initial mass concentration of the methyl violet aqueous solution is 10 to 200 mg/L, and the mass dosage of the zinc-aluminum binary hydrotalcite 1-100 mg/mL methyl violet aqueous solution; the general chemical formula of the hydrotalcite is [Zn ²⁺ _1-x Al ³⁺ _x (OH) ₂ ] ^x+ (CO ₃ ^2- ) _x/2 mH ₂ O, Where x is the molar ratio of Al ³⁺ to [Al ³⁺ +Zn ²⁺ ], 0.2≤x≤0.33, m is the amount of crystal water, 2≤m≤6. 2. The application according to claim 1, characterized in that the general chemical formula of the hydrotalcite is [Zn ²⁺ _0.75 Al ³⁺ _0.25 (OH) ₂ ] ^x+ (CO ₃ ²⁻ ) _0.125 ·4H ₂ O.

Images