CN115228429B

CN115228429B - Magnetic fly ash adsorbent, preparation method and application

Info

Publication number: CN115228429B
Application number: CN202211063807.0A
Authority: CN
Inventors: 陈攀; 伍佳美; 曹建
Original assignee: Central South University
Current assignee: Central South University
Priority date: 2022-08-31
Filing date: 2022-08-31
Publication date: 2023-06-13
Anticipated expiration: 2042-08-31
Also published as: CN115228429A

Abstract

The invention discloses a magnetic fly ash adsorbent and a preparation method thereof, wherein the magnetic fly ash adsorbent comprises the following steps of uniformly stirring fly ash, trivalent ferric salt, aluminum salt and divalent ferric salt; adding alkali liquor; sequentially roasting and grinding to obtain a first modified product; adding a first modified product and 3, 4-dihydroxybenzaldehyde into absolute ethyl alcohol, stirring and heating for reaction, dripping melamine solution into a reaction system for reaction, and sequentially cleaning, suction filtering, drying and grinding after the reaction is finished to obtain a second modified product; adding the second modified product and anhydrous citric acid into water, heating, stirring, reacting, and sequentially cleaning, suction filtering, drying and grinding after the reaction is finished. The magnetic fly ash adsorbent of the invention is used for heavy metal ion Pb ²⁺ 、Cd ²⁺ 、AsO ₃ ^3‑ Has stronger removal rate of Pb ²⁺ The removal rate of the catalyst can reach 99.79% at maximum, and Cd ²⁺ The removal rate of (a) can reach 83.34 percent at maximum, and AsO ₃ ^3‑ The removal rate of the catalyst can reach 91.42 percent at maximum.

Description

Magnetic fly ash adsorbent, preparation method and application

Technical Field

The invention relates to the technical field of heavy metal wastewater treatment, in particular to a magnetic fly ash adsorbent, a preparation method and application thereof.

Background

The nonferrous metal ore mining industry, the smelting and rolling processing industry and the metal product industry are industries with the heavy metal emission amount in the first 3 positions in China. With the increase of the yield of nonferrous metals, a large amount of heavy metal polluted wastewater is discharged into the environment. Through statistics, in 2020, the yield of ten nonferrous metals in China is increased by 5.49% in a same ratio, the total metal (Pb, hg, cd, cr, as) emission amount in the wastewater is 73.129t, and the Pb, cd and As emission amounts account for 56.18%. Therefore, it is necessary to treat Pb, cd and As in the wastewater.

Heavy metal anions such as AsO ₄ ^3- 、AsO ₃ ^3- 、CrO ₄ ^2- 、Cr ₂ O ₇ ^2- The Zeta potential and the pH dependency are opposite to those of heavy metal cations, and different heavy metal ions have different valence and ionic radius, so that the simultaneous removal of anions and cations in a multi-metal wastewater system is a difficult point.

Disclosure of Invention

The technical problem that this application mainly solves is to provide a magnetism fly ash adsorbent, preparation method and application to solve the technical problem that is difficult to realize removing simultaneously of heavy metal anion and heavy metal cation in the multi-metal waste water system that exists among the prior art.

In order to solve the technical problems, one technical scheme adopted by the application is as follows: the preparation method of the magnetic fly ash adsorbent comprises the following steps:

adding fly ash, ferric salt, aluminum salt and ferrous salt into water, and uniformly mixing and stirring to obtain slurry;

adding alkali liquor into the slurry to ensure that the pH value of the slurry is more than or equal to 10, standing for aging, and then sequentially cleaning, suction filtering and drying to obtain an intermediate product;

sequentially roasting and grinding the intermediate product to obtain a first modified product;

adding the first modified product and 3, 4-dihydroxybenzaldehyde into absolute ethyl alcohol, stirring and heating for reaction, dripping melamine solution into a reaction system for reaction after the reaction is finished, and sequentially cleaning, suction filtering, drying and grinding after the reaction is finished to obtain a second modified product;

and adding the second modified product and anhydrous citric acid into water, heating and stirring for reaction, and after the reaction is finished, sequentially cleaning, filtering, drying and grinding to obtain the modified product.

In one embodiment, the steps of taking the fly ash, the ferric salt, the aluminum salt and the ferrous salt, adding the fly ash, the ferric salt, the aluminum salt and the ferrous salt into water, mixing and stirring uniformly to obtain slurry are as follows:

every 3-5 kg of fly ash is mixed with 20mol of ferric salt, 5mol of aluminum salt and 11mol of ferrous salt;

the uniform mixing and stirring is specifically carried out by ultrasonic stirring under a closed environment, the stirring temperature is 40-60 ℃, and the stirring speed is 1000-2000 r/min.

In one embodiment, the step of adding alkali liquor into the slurry to enable the pH of the slurry to be more than or equal to 10, reacting, standing and aging, and then sequentially cleaning, suction filtering and drying to obtain an intermediate product comprises the following steps:

the alkali liquor is sodium hydroxide solution or ammonia water, the reaction time is 1-3 h, and the standing and ageing time is 3-12 h.

In one embodiment, the step of sequentially calcining and grinding the intermediate product to obtain a first modified product comprises:

the roasting temperature is 500-600 ℃, and the roasting time is 1-5 h.

In one embodiment, the step of adding the first modified product and 3, 4-dihydroxybenzaldehyde to absolute ethanol, stirring and heating the mixture to react comprises the steps of:

3 to 5mol of 3, 4-dihydroxybenzaldehyde is mixed in each 1kg of the first modified product; the stirring speed of the stirring temperature-rising reaction is 1000-2000 r/min, the reaction temperature is 65-75 ℃, and the reaction time is 3-6 h.

In one embodiment, after the reaction is finished, the melamine solution is dripped into a reaction system to continuously react, and after the reaction is finished, the steps of cleaning, suction filtration, drying and grinding are sequentially carried out to obtain a second modified product are carried out:

3-5 mol melamine solution is added into each 1kg of the first modified product; the melamine solution is a melamine solution dissolved in a dimethyl sulfoxide solution at 70 ℃; the reaction time of the continuous reaction is 10-14 h.

In one embodiment, the second modified product and anhydrous citric acid are added into water, heated and stirred for reaction, and after the reaction is finished, the steps of cleaning, suction filtration, drying and grinding are sequentially carried out, so that the method is characterized in that:

3-5 mol of anhydrous citric acid is mixed in each 1kg of the second modified product, the stirring rate of the heating stirring reaction is 1000-2000 r/min, the reaction temperature is 65-75 ℃, and the reaction time is 1-5 h.

In one embodiment, the cleaning is specifically performed by taking ethanol as a cleaning agent and adopting a magnetic decantation method; the drying is specifically carried out in a vacuum drying oven at 50-70 ℃ for 10-14 h.

The other technical scheme adopted by the application is as follows: there is provided a magnetic fly ash adsorbent prepared by the method of any one of the above embodiments.

The application adopts the following technical scheme: there is provided an application of the magnetic fly ash adsorbent according to any of the above embodiments in heavy metal wastewater treatment.

The beneficial effect of this application is, in contrast to prior art:

according to the preparation method, firstly, nano iron-aluminum oxide is generated on the surface of the fly ash in situ, and the modified fly ash is subjected to grafting reaction, so that on one hand, magnetism is given to the fly ash, and on the other hand, the grafting rate is effectively improved, and therefore the adsorption performance of the magnetic fly ash adsorbent is effectively promoted;

according to the preparation method, 3, 4-dihydroxybenzaldehyde, melamine and citric acid are grafted sequentially in a step-by-step connection mode, so that grafting of various strong adsorption functional groups on the surface of the fly ash is realized, and the adsorption performance of the magnetic fly ash adsorbent is effectively promoted;

the magnetic fly ash adsorbent of the application is used for heavy metal ion Pb ²⁺ 、Cd ²⁺ 、AsO ₃ ^3- Has stronger removal rate of Pb ²⁺ The removal rate of the catalyst can reach 99.79% at maximum, and Cd ²⁺ The removal rate of (a) can reach 83.34 percent at maximum, and AsO ₃ ^3- The removal rate of the catalyst can reach 91.42 percent at maximum.

The magnetic fly ash adsorption rate of the magnetic fly ash has strong magnetism, and can be separated from waste liquid through the magnet, so that the recovery and the recycling after the wastewater treatment are facilitated.

Drawings

FIG. 1 is a schematic flow chart of one embodiment of a method of preparing a magnetic fly ash adsorbent according to the present application;

FIG. 2 is an XRD diffraction analysis chart of effect example 1 of the present application;

FIG. 3 is a FTIR spectrum of effect example 1 of the present application.

Detailed Description

The present invention will be described in detail with reference to the accompanying drawings.

Referring to fig. 1, fig. 1 is a schematic flow chart of an embodiment of a method for preparing a magnetic fly ash adsorbent according to the present application.

The preparation method comprises the following steps:

s1, adding fly ash, ferric salt, aluminum salt and ferrous salt into water, and uniformly mixing and stirring to obtain slurry.

20mol of ferric salt, 5mol of aluminum salt and 11mol of ferrous salt are mixed with 3-5 kg of fly ash.

The mixing and stirring are carried out in an enclosed environment, the stirring temperature is 40-60 ℃, and the stirring speed is 1000-2000 r/min.

The fly ash, the ferric salt, the aluminum salt and the ferrous salt are uniformly stirred to form slurry, so that a stable reaction system can be provided for subsequent reactions. It is understood that in other embodiments, other stirring methods may be employed, and the effects of this embodiment can be achieved.

S2, adding alkali liquor into the slurry to ensure that the pH value of the slurry is more than or equal to 10, standing for ageing, and then sequentially cleaning, suction filtering and drying to obtain an intermediate product.

Controlling the pH to be more than or equal to 10, wherein iron ions and aluminum ions carry out coprecipitation reaction, and the specific reaction formula is 2Fe ³⁺ +Fe ²⁺ +8OH ^- ＝Fe ₃ O ₄ +4H ₂ O， Al ³⁺ +3OH ^- ＝Al(OH) ₃ At this time, magnetic particles having good magnetic responsiveness can be obtained, which contributes to the next modification treatment.

And S3, sequentially roasting and grinding the intermediate product to obtain a first modified product.

The roasting temperature is 500-600 ℃, the roasting time is 1-5 h, magnetic nano iron and aluminum oxide are generated on the surface of the fly ash in situ after high-temperature roasting, and the nano iron aluminum oxide fly ash composite material is obtained after grinding, namely FA@NMFeAl.

S4, adding the first modified product and 3, 4-dihydroxybenzaldehyde into absolute ethyl alcohol, stirring and heating for reaction, dripping melamine solution into a reaction system for reaction after the reaction is finished, and sequentially cleaning, suction filtering, drying and grinding after the reaction is finished to obtain a second modified product.

Wherein 3 to 5mol of 3, 4-dihydroxybenzaldehyde is mixed in each 1kg of the first modified product; stirring speed of stirring and heating reaction is 1000-2000 r/min, reaction temperature is 65-75 ℃, and reaction time is 3-6 h.

Aiming at the nano iron-aluminum oxide fly ash composite material with the magnetic nano iron and aluminum oxide generated on the surface in situ, firstly, 3, 4-dihydroxybenzaldehyde is grafted on the surface of the fly ash by utilizing the property that catechol-based compound can form a coordination compound with metal ions; then, nitrogen atoms with lone electron pairs on melamine attack carbon atoms with positive charges on carbonyl groups to complete nucleophilic addition reaction to form an intermediate alpha-hydroxylamine compound, and then further dehydrating to form a Schiffbase compound containing imine bonds (C=N), so that melamine is grafted on the surface of the fly ash.

The specific reaction principle is as follows:

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through the reaction, 3, 4-dihydroxybenzaldehyde and melamine are grafted on the surface of the nano iron aluminum oxide fly ash composite material in sequence, and the 3, 4-dihydroxybenzaldehyde and melamine react on the nano modified fly ash to generate an imine bond (C=N) compound with strong binding force on heavy metal ions, so that the product can effectively adsorb the heavy metal ions.

S5, adding the second modified product and anhydrous citric acid into water, heating and stirring for reaction, and after the reaction is finished, sequentially cleaning, suction filtering, drying and grinding to obtain the modified citric acid.

Every 1kg of second modified product is mixed with 3-5 mol of anhydrous citric acid, the stirring rate of heating and stirring reaction is 1000-2000 r/min, the reaction temperature is 65-75 ℃, and the reaction time is 1-5 h.

And (3) grafting anhydrous citric acid onto the surface of the fly ash by utilizing the reaction mechanism, namely carrying out amidation reaction on carboxyl in the citric acid and amino in the melamine to obtain the adsorbent shown in the following formula.

The citric acid contains a plurality of carboxyl groups, so that the citric acid has strong adsorption effect on heavy metal ions, and the adsorption effect of the adsorbent can be further improved.

The cleaning in the steps can be performed by adopting a traditional deionized water cleaning method, or can be performed by adopting an ethanol cleaning agent and a magnetic decantation method, so that the effect can be realized; similarly, the drying in the steps can be performed in a vacuum drying oven at 50-70 ℃ for 10-14 hours, or can be performed in other drying modes, so that the effect can be achieved.

It is worth noting that the purpose of the method is to firstly generate nano iron aluminum oxide on the surface of the fly ash in situ, so that the grafting rate of the subsequent 3, 4-dihydroxybenzaldehyde is improved, and the grafting rate of melamine and citric acid is correspondingly improved.

The traditional aldehyde grafting method is to graft an aminosilane coupling agent on a substrate, and then the amino group at the end part acts with aldehyde group, but the coupling agent has higher reactivity, and early reaction is easy to occur in the use process, so that the expected effect cannot be achieved.

Catechol-based compounds of 3, 4-dihydroxybenzaldehyde have strong complexing ability with metal ions, and have been studiedProved by the irreversible binding affinity of catechol derivatives with ferric oxide and aluminum oxide, the invention generates nano Fe on the surface of the fly ash in situ ₃ O ₄ 、Al ₂ O ₃ Then reacts with 3, 4-dihydroxybenzaldehyde, which not only can endow the fly ash with magnetism, but also can improve the grafting rate of aldehyde.

The following detailed description of the present application is provided in connection with specific embodiments. The following examples are illustrative of the present application, but are not intended to limit the scope of the present application. The apparatus and methods used in the examples, unless otherwise specified, are all common in the art.

The main components of the fly ash used in the following examples and comparative examples are shown in the following table:

example 1:

the embodiment provides a magnetic fly ash adsorbent, which is prepared by the following steps:

(1) Weigh 0.02mol FeCl ₃ ·6H ₂ O、0.011mol FeSO ₄ ·7H ₂ O、0.005mol AlCl ₃ ·6H ₂ Dissolving O and 3g of fly ash FA in 50ml of water, mixing and ultrasonically stirring at a stirring speed of 1000r/min, dropwise adding 1.5mol/L ammonia water solution until the pH of a reaction system is more than or equal to 10, reacting for 1h at 40 ℃, and standing and aging for 3h after the completion;

(2) Washing the product obtained in the step (1) by absolute ethyl alcohol by using a magnetic decantation method, filtering, and then drying in a vacuum drying oven at 60 ℃ for 12 hours, and grinding;

(3) Roasting the product obtained in the step (2) in a roasting furnace at 600 ℃ for 5 hours to obtain a nano iron-aluminum oxide fly ash composite material (FA@NMFeAl);

(4) Adding 0.015mol of 3, 4-dihydroxybenzaldehyde into 50ml of absolute ethyl alcohol, stirring and heating to 70 ℃, stirring at a speed of 2000r/min, adding 3g of FA@NMFeAl, and reacting for 6h;

(5) After the reaction of the step (4), dissolving 0.015mol of melamine in 30ml of dimethyl sulfoxide solution at 70 ℃, slowly dripping the solution into a reaction system for overnight reaction, and cleaning, suction filtering, drying and grinding the solution in the next day to obtain Schiff base modified fly ash FA@NMFeAl@CN;

(6) To 50ml of water was added 3g of FA@NMFeAl@CN, stirred at 2000r/min, and warmed to 70℃and 10ml of aqueous solution containing 0.015mol of anhydrous citric acid was added dropwise. And after reacting for 5 hours, cleaning, suction filtering, drying and grinding to obtain the final modified magnetic fly ash FA@NMFeAl@CN@acid.

Example 2:

(1) Weigh 0.02mol FeCl ₃ ·6H ₂ O、0.011mol FeSO ₄ ·7H ₂ O、0.005mol AlCl ₃ ·6H ₂ Dissolving O and 5g of fly ash FA in 50ml of water, mixing and ultrasonically stirring, wherein the stirring speed is 2000r/min, dropwise adding 1.5mol/L ammonia water solution until the pH value of a reaction system is more than or equal to 10, reacting for 3h at 60 ℃, and standing and aging for 12h after the completion;

Example 3:

(1) Weigh 0.02mol FeCl ₃ ·6H ₂ O、0.011mol FeSO ₄ ·7H ₂ O、0.005mol AlCl ₃ ·6H ₂ Dissolving O and 4g of fly ash FA in 50ml of water, mixing and ultrasonically stirring, wherein the stirring speed is 1000r/min, dropwise adding 1.5mol/L NaOH solution until the pH of a reaction system is more than or equal to 10, reacting for 2h at 50 ℃, standing and aging for 7.5h after the completion of the reaction;

(3) Roasting the product obtained in the step (2) in a roasting furnace at 600 ℃ for 3 hours to obtain a nano iron-aluminum oxide fly ash composite material (FA@NMFeAl);

(4) 0.015mol of 3, 4-dihydroxybenzaldehyde is added to 50ml of absolute ethanol, the mixture is stirred and heated to 70 ℃ with a stirring rate of 2000r/min, and 3g of 3-dihydroxybenzaldehyde is addedFA@NMFeAl, reaction 4.5h；

Comparative example 1:

this comparative example provides a magnetic fly ash adsorbent prepared in substantially the same manner as in example 1, except that 8g of fly ash was added in step (1) of this comparative example.

Comparative example 2:

this comparative example provides a magnetic fly ash adsorbent prepared in substantially the same manner as in example 1, except that 0.006mol of 3, 4-dihydroxybenzaldehyde was added in step (4) of this comparative example.

Comparative example 3:

this comparative example provides a magnetic fly ash adsorbent prepared in substantially the same manner as in example 1, except that citric acid was added in step (6) of this comparative example at 0.006mol.

Comparative example 4:

the comparative example provides a magnetic fly ash adsorbent prepared in substantially the same manner as in example 1, except that 3, 4-dihydroxybenzaldehyde was not added in step (4) of the comparative example.

Comparative example 5:

the comparative example provides a magnetic fly ash adsorbent prepared in substantially the same manner as in example 1, except that melamine was not added in step (5) of the comparative example.

Comparative example 6:

the comparative example provides a magnetic fly ash adsorbent prepared in substantially the same manner as in example 1, except that citric acid was not added in step (5) of the comparative example.

Effect example 1: magnetic analysis

XRD diffraction analysis was performed on the raw material fly ash FA of example 1, the magnetic fly ash adsorbent prepared in example 1, and was compared with standard XRD cards of ferroferric oxide, ferric oxide and aluminum oxide, to obtain FIG. 2.

As can be seen from fig. 2, the raw material fly ash FA does not contain ferroferric oxide component, and has no magnetism, and the magnetic fly ash adsorbent prepared after modification has ferroferric oxide component with strong magnetism.

Meanwhile, the magnet is used for magnetic examination, and the adsorbent and the simulated wastewater can be subjected to solid-liquid separation through the magnet, so that the magnetism of the magnetic fly ash adsorbent prepared in the embodiment 1 is further verified.

FTIR spectrum analysis was performed on the raw material fly ash FA of example 1 and the magnetic fly ash adsorbent prepared in example 1, to obtain fig. 3. As can be seen from fig. 3, the magnetic fly ash adsorbent has functional groups such as hydroxyl groups, amino groups, carbon-nitrogen double bonds, and the like, which have strong binding force on heavy metal ions.

Effect example 2: adsorption effect analysis

Pb was carried out on the magnetic fly ash adsorbents prepared in examples 1 to 3 and comparative examples 1 to 6 described above ²⁺ 、Cd ²⁺ 、AsO ₃ ^3- And simulating an adsorption experiment of wastewater.

Pb was used at 1000mg/L ²⁺ 、Cd ²⁺ 、AsO ₃ ^3- 9 parts of 100mg/L heavy metal simulated wastewater are prepared in stock solutions, and 10ml of each part is prepared. Preparing Pb in one portion ²⁺ 、Cd ²⁺ 、AsO ₃ ^3- The mixed simulated wastewater was subjected to adsorption experiments using the magnetic fly ash adsorbent prepared in example 1—example 1 (mixed). The fly ash adsorbents obtained in examples 1 to 3 and comparative examples 1 to 6 were each dosed at 0.01g, and were subjected to room temperature shaking on a thermostatic water bath shaker at a rotational speed of 200r/min for 24 hours. Taking supernatant, detecting residual heavy metal concentration by ICP-OES (inductively coupled plasma emission spectrometer) and calculating Pb by adsorbent ²⁺ 、Cd ²⁺ 、AsO ₃ ^3- The removal rate of (2) was measured to obtain the following table data.

From the above table data, it can be seen that:

magnetic fly ash adsorbents prepared in examples 1 to 3 against Pb ²⁺ 、Cd ²⁺ 、AsO ₃ ^3- Has stronger removal rate of Pb ²⁺ The removal rate of the catalyst can reach 99.79% at maximum, and Cd ²⁺ The removal rate of (a) can reach 83.34 percent at maximum, and AsO ₃ ^3- The removal rate of the catalyst can reach 91.42 percent at maximum.

As is clear from comparison of example 1 with comparative examples 1, 2 and 3, the adsorption effect of the magnetic fly ash adsorbent was inhibited by too much added fly ash, too little added 3, 4-dihydroxybenzaldehyde and too little added citric acid.

The reason for this is mainly: fe (Fe) ₃ O ₄ At high contents, the magnetic properties of the adsorbent are increased, and Fe is increased because the catechol-based compound can coordinate with metal ions ₃ O ₄ The content can improve the grafting rate of the 3, 4-dihydroxybenzaldehyde.

Aldehyde (-CHO) and amino (-NH) ₂ ) The reaction molar ratio is 1:1, so that the grafting ratio of 3, 4-dihydroxybenzaldehyde influences the grafting ratio of melamine. Similarly, the grafting ratio of melamine also affects the grafting ratio of citric acid.

Therefore, when the fly ash is excessively added, fe is caused ₃ O ₄ The relatively low content results in a decrease in the grafting rate of the subsequent three chemicals and ultimately affects the number of functional groups contained in the adsorbent, resulting in a decrease in the removal rate.

When the addition amount of 3, 4-dihydroxybenzaldehyde is reduced, the grafting ratio of the subsequent melamine and citric acid is affected, so that the grafting ratio of the subsequent three chemical agents is reduced, and finally the number of functional groups contained in the adsorbent is affected, so that the removal rate is reduced.

When the amount of citric acid added is reduced, the amount of functional groups contained in the adsorbent is affected, resulting in a reduction in the removal rate.

Example 1 was compared with comparative examples 4 to 6, respectively, and it was found that the effect of the adsorbent prepared by adding only two modified agents of three agents of 3, 4-dihydroxybenzaldehyde, melamine and citric acid was far less than that of examples 1 to 3, indicating that the three agents had a synergistic effect. The 3, 4-dihydroxybenzaldehyde contains aldehyde groups, the melamine contains a plurality of amino groups, the citric acid contains carboxyl and hydroxyl, and the three medicaments are added step by step. 3, 4-dihydroxybenzaldehyde and melamine react on nano modified fly ash to generate an imine bond (C=N) compound with strong binding force to heavy metal ions, and then amino reacts with carboxyl of citric acid to finally prepare the heavy metal adsorbent containing nano metal oxide, C=N, polyamino and carboxyl, so that three medicaments are indispensable.

The foregoing description is only of embodiments of the present invention, and is not intended to limit the scope of the invention, and all equivalent structures or equivalent processes using the descriptions and the drawings of the present invention or directly or indirectly applied to other related technical fields are included in the scope of the present invention.

Claims

1. The preparation method of the magnetic fly ash adsorbent is characterized by comprising the following steps:

adding the second modified product and anhydrous citric acid into water, heating and stirring for reaction, and after the reaction is finished, sequentially cleaning, suction filtering, drying and grinding to obtain the modified product;

wherein, every 3-5 kg of fly ash is mixed with 20mol of ferric salt, 5mol of aluminum salt and 11mol of ferrous salt;

3 to 5mol of 3, 4-dihydroxybenzaldehyde is mixed in each 1kg of the first modified product;

3-5 mol melamine solution is added into each 1kg of the first modified product;

3 to 5mol of anhydrous citric acid is mixed with every 1kg of the second modified product.

2. The preparation method of claim 1, wherein the steps of taking the fly ash, the ferric salt, the aluminum salt and the ferrous salt, adding the fly ash, the ferric salt, the aluminum salt and the ferrous salt into water, mixing and stirring the fly ash, the ferric salt, the aluminum salt and the ferrous salt uniformly to obtain the slurry are as follows:

3. The preparation method according to claim 1, wherein the step of adding alkali liquor into the slurry to make the pH of the slurry be equal to or more than 10, reacting, standing and aging, and then sequentially cleaning, suction filtering and drying to obtain an intermediate product comprises the following steps:

4. The method according to claim 1, wherein in the step of sequentially calcining and grinding the intermediate product to obtain the first modified product:

the roasting temperature is 500-600 ℃, and the roasting time is 1-5 h.

5. The method according to claim 1, wherein the step of adding the first modified product and 3, 4-dihydroxybenzaldehyde to absolute ethanol, stirring and heating the mixture to react:

the stirring speed of the stirring temperature-rising reaction is 1000-2000 r/min, the reaction temperature is 65-75 ℃, and the reaction time is 3-6 h.

6. The preparation method according to claim 1, wherein after the reaction, the melamine solution is dripped into the reaction system to continuously react, and after the reaction, the steps of washing, suction filtration, drying and grinding are sequentially carried out to obtain a second modified product are carried out:

the melamine solution is a melamine solution dissolved in a dimethyl sulfoxide solution at 70 ℃; the reaction time of the continuous reaction is 10-14 h.

7. The preparation method according to claim 1, wherein the steps of adding the second modified product and anhydrous citric acid into water, heating and stirring for reaction, and sequentially cleaning, suction filtering, drying and grinding after the reaction is finished are carried out, so as to obtain the modified citric acid:

the stirring speed of the heating stirring reaction is 1000-2000 r/min, the reaction temperature is 65-75 ℃, and the reaction time is 1-5 h.

8. The preparation method according to claim 1, wherein the cleaning is specifically performed by using ethanol as a cleaning agent and adopting a magnetic decantation method; the drying is specifically carried out in a vacuum drying oven at 50-70 ℃ for 10-14 h.

9. A magnetic fly ash adsorbent prepared by the method of any one of claims 1 to 8.

10. Use of the magnetic fly ash adsorbent according to claim 9 in heavy metal wastewater treatment.