CN112316916B - Mesoporous core-shell structure nano zero-valent iron gel microsphere for stably loading citrate organic ligand and preparation method thereof - Google Patents
Mesoporous core-shell structure nano zero-valent iron gel microsphere for stably loading citrate organic ligand and preparation method thereof Download PDFInfo
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Abstract
The invention provides a mesoporous core-shell structure nano zero-valent iron gel microsphere for stably loading citrate organic ligands, which comprises a citric acid carboxylated chitosan shell and nano zero-valent iron particles coated in the citric acid carboxylated chitosan shell, wherein the surface of the nano zero-valent iron particles is loaded with the citrate organic ligands. The mesoporous core-shell structure nano zero-valent iron gel microsphere provided by the invention combines the directional adsorption capturing effect of citric acid carboxylation chitosan shells on trace POPs in natural water bodies with more strong oxidative free radicals which are generated by nano zero-valent iron under the assistance of citrate and can degrade the POPs, and can realize efficient degradation and removal of the trace POPs remained in the water bodies. The gel microsphere is environment-friendly, is easy to separate and recover from the water body after the water body is restored, and is a material extremely suitable for water body restoration.
Description
Technical Field
The invention relates to the field of water body repair materials, in particular to a mesoporous core-shell structure nano zero-valent iron gel microsphere for stably loading citrate organic ligands and a preparation method thereof.
Background
Persistent Organic Pollutants (POPs) comprise low-concentration and high-hazard chemical substances such as steroid estrogen, pesticides, antibiotics and the like discharged in a point source or non-point source pollution mode in an intensive dairy farm and a modern high-density plantation. In recent years, as demands for agricultural products are increasing, the agricultural industry featuring intensive and high density is rapidly developed. In the agricultural product production process, a large amount of POPs directly or indirectly enter the environment through animal excretion, fertilization, wastewater discharge and other modes, and are further diffused in the environment through rainfall leaching, rainfall runoff, migration and transformation and other modes. POPs entering the environment in a non-point source pollution mode are difficult to effectively control, and POPs in the cultivation wastewater are difficult to completely remove by the traditional sewage treatment process, so that POPs pollutants with different degrees are detected in main rivers including Yangtze river, zhujiang river, pinghuajiang river and the like at present, the detection rate is even up to 100%, and the existence of the POPs is detected in underground water and drinking water. The existence of these POPs severely threatens the safety of the aquatic ecosystem and the physical health of humans.
The research on POPs pollutant degradation in China is mostly focused on biodegradation of sewage plants and improvement of advanced treatment processes, and the main treatment technology can be summarized as follows: biological processes, membrane treatment techniques, advanced oxidation techniques, and adsorption techniques. However, in natural bodies of water, most methods have significant limitations, such as: in the biological method, the microorganisms in the activated sludge environment are very little difficult to domesticate in natural water; the membrane treatment technology has high cost and is only suitable for sewage treatment plants; microwaves, photocatalysis and ultrasound cannot be applied to deep water bodies and underground water bodies; nano gold, nano zinc oxide, nano silver and the like, has high manufacturing cost and is easy to cause secondary pollution; the adsorption technology is not really used for removing POPs, has poor pollutant adsorption and removal stability, is easy to desorb and secondary pollutes water bodies, and the like. In addition, because the concentration of POPs in the natural water body is low, the POPs are basically calculated by ng/L, and most treatment methods are difficult to achieve good treatment effects and difficult to realize the restoration of the natural water body.
Therefore, a method and a material which are convenient, efficient and environment-friendly, can be applied to the in-situ removal of POPs in natural water and underground water, and realize the directional adsorption and in-situ degradation of the POPs in the natural water and the underground water are urgently needed.
Disclosure of Invention
In view of the above, the invention provides a mesoporous core-shell structure nano zero-valent iron gel microsphere for stably loading citrate organic ligands and a preparation method of the gel microsphere, and the gel microsphere can be used for natural water body restoration, is environment-friendly and easy to recycle, is easy to biodegrade under the condition of difficult recycling, and can be used for efficiently degrading and removing low-concentration trace persistent organic pollutants in natural water bodies.
The mesoporous core-shell structured nano zero-valent iron gel microsphere for stably loading citrate organic ligands provided by the invention comprises a citric acid carboxylated chitosan shell and nano zero-valent iron particles coated in the citric acid carboxylated chitosan shell, wherein the surface of the nano zero-valent iron particles is loaded with the citrate organic ligands. The shell formed by carboxylating chitosan with citric acid has extremely high specific surface area and developed pore structure, and active groups (hydroxyl and carboxyl) on the surface for directionally adsorbing and capturing POPs (such as estrogen and other pollutants) are four times that of the traditional chitosan gel material, so that the POPs with extremely low concentration in natural water can be adsorbed on the surface and in pore channels of the material. By loading the citrate organic ligand on the surface of the nano zero-valent iron particles, the phenomena of oxidation passivation, agglomeration, inactivation and the like of the nano zero-valent iron particles can be effectively avoided, the nano zero-valent iron particles can be highly dispersed in the citric acid chitosan, and further, the nano zero-valent iron active sites in a highly dispersed state are ensured to be fully utilized. And the citrate in the surface loaded citrate organic ligand and the citrate carboxylated chitosan shell can be utilized to extract a large amount of hydrogen ions in water, provide the hydrogen ions for the internal nanometer zero-valent iron and generate hydroxyl free radicals under the aerobic condition. The gel microsphere material can continuously give ferrous ions in the core shell, activate oxygen and generated hydrogen peroxide to generate hydroxyl free radicals and superoxide free radicals to attack adsorbed pollutants, generate ferrite free radicals, directly excite free electrons to directly attack specific positions of POPs, and form dimers or polymers with lower toxicity through deprotonation. The gel microsphere provided by the invention combines the directional adsorption and capture effects of citric acid carboxylation chitosan shells on trace POPs in natural water bodies with more strong oxidative free radicals (including hydroxyl, superoxide free radicals, ferrite free radicals and the like) which are generated by nano zero-valent iron under the assistance of citrate, and also has deprotonation generated by free electrons, so that the efficient degradation and removal of trace POPs remained in the water bodies can be realized. Meanwhile, the gel microsphere is made of materials harmless to the environment, and can generate a large number of strong oxidative free radicals directly through the actions of water, dissolved oxygen and the like in a water body, and the gel microsphere does not depend on the manual addition of extra industrial chemicals to promote degradation, so that the gel microsphere is environment-friendly. And after repairing the water body, the nano zero-valent iron is easy to separate and recover from the water body, the problem of difficult recovery after putting the nano zero-valent iron is solved, secondary pollution to the water body is avoided, even under the condition that materials are difficult to recover individually, the rapid oxidation of the iron core after biological decomposition and shell decomposition of the nano zero-valent iron can not influence the environment, the nano zero-valent iron is a material extremely suitable for repairing the water body, and the nano zero-valent iron has a good prospect in the field of in-situ repair of polluted water bodies. In addition, the gel microsphere can continuously give ferrous ions under the neutral condition, is matched with excitants such as hydrogen peroxide, is more beneficial to generating a large number of hydroxyl free radicals with strong oxidability, is beneficial to effectively degrading POPs under the neutral condition, and overcomes the defect that the degradation capacity of the existing Fenton-like reagent under the neutral condition is low.
The reaction of the gel microsphere in the aerobic water body comprises the following steps:
Fe 0 (s) +O 2(aq) +2H + →Fe 2+ +H 2 O 2(aq) ；
Fe 2+ +H 2 O 2(aq) →Fe 3+ +OH·+OH - (aq) ；
Fe 2+ +O 2(aq) →Fe 3+ +O 2 - ·；
Fe 2+ +O 2 - ·+2H + →Fe 3+ +H 2 O 2(aq) ；
the invention also provides a preparation method of the mesoporous core-shell structure nano zero-valent iron gel microsphere for stably loading the citrate organic ligand, which comprises the following steps:
(1) Preparing nano zero-valent iron particles:
placing the ferrous sulfate heptahydrate solution in a reactor filled with high-purity nitrogen, then adding a reducing agent solution into the ferrous sulfate heptahydrate solution under the ultrasonic condition for reaction, collecting solids after the reaction is finished, washing the solids in sequence, and drying the solids in vacuum to obtain regular spherical nano zero-valent iron particles; the ferrous sulfate heptahydrate solution is prepared by mixing ultrapure water and ferrous sulfate heptahydrate (FeSO4.7H2O), a high-strength rubidium ferroboron magnet is adopted to collect solids, repeated washing is needed during water washing, cleaning is ensured, and vacuum drying is used for avoiding the oxidation of generated nano zero-valent iron particles;
(2) Loading citrate organic ligand:
washing the nano zero-valent iron granular acid obtained in the step (1), adding the washed nano zero-valent iron granular acid into a deoxidized trisodium citrate solution, uniformly mixing by vortex, then injecting the obtained suspension into a reaction kettle with the pressure being more than atmospheric pressure, performing oxygen-isolation heating reaction under the assistance of ultrasound, collecting solids after the reaction is finished, and washing by adopting methanol to obtain nano zero-valent iron granules with citrate organic ligands loaded on the surfaces; the method comprises the steps of carrying out acid washing on nano zero-valent iron particles to remove oxide films possibly existing on the surfaces of the particles, collecting the prepared nano zero-valent iron particles with citrate organic ligands loaded on the surfaces by adopting a high-strength rubidium boron iron magnet, wherein methanol used for cleaning is anaerobic methanol, and repeatedly washing for a plurality of times by adopting anaerobic methanol;
(3) Preparation of gel microspheres:
adding the nano zero-valent iron particles with the surfaces loaded with the citrate organic ligands obtained in the step (2) into a citric acid carboxylation chitosan solution, uniformly vortex mixing and ultrasonic dispersing to prepare a viscous suspension, then dropwise adding the obtained suspension into a sodium hydroxide solution under the anaerobic condition to crosslink the gel microspheres, keeping the gel microspheres stable in the sodium hydroxide solution for a certain time, and then taking out the gel microspheres and washing the gel microspheres with anaerobic methanol to obtain the stable nano zero-valent iron gel microspheres with mesoporous core-shell structures. Other solutions that provide hydroxide ions may be used in place of the sodium hydroxide solution.
According to the preparation method provided by the invention, firstly, regular spherical nano zero-valent iron particles can be prepared by an ultrasonic-assisted liquid phase reduction method, then, citrate organic ligands are loaded on the surfaces of the nano zero-valent iron particles by a hydrothermal method to form nano zero-valent iron particles which are difficult to agglomerate and can be highly dispersed and are loaded with the citrate organic ligands on the surfaces, and then, the nano zero-valent iron particles are coated with citric acid carboxylated chitosan under the assistance of ultrasonic waves to form citric acid carboxylated chitosan shells; has the characteristics of simple operation, low cost, easy mass production, strong environmental friendliness and the like.
Further, in the step (3), the preparation method of the citric acid carboxylated chitosan solution comprises the following steps: a. adding chitosan into a citric acid ethanol solution, stirring at constant temperature for reaction for 0.5-1 h, performing ultrasonic treatment for 15-30 min, performing high-speed centrifugal treatment for 10-20 min, repeatedly washing and centrifuging the obtained solid with absolute ethanol, and performing vacuum drying to obtain citric acid carboxylated chitosan powder;
the principle of citric acid carboxylated chitosan is as follows:
b. mixing the powder obtained in the step a with acetic acid, stirring until the powder is completely dissolved, and then performing ultrasonic degassing to obtain the citric acid carboxylated chitosan solution.
Further, in the step a, the mass concentration of the citric acid ethanol solution is 2%, and the mass volume ratio of the chitosan to the citric acid ethanol solution is 1g:100ml;
in the step a, the temperature of constant-temperature stirring reaction is 50-80 ℃, the centrifugal treatment rotating speed is 3000-4500 r/min, and the vacuum drying temperature is 40-60 ℃;
in the step b, the volume concentration of the acetic acid is 1%, and the mass volume ratio of the powder to the acetic acid is 0.5-1.5 g: 50-150 ml; the ultrasonic degassing time is 15-30 min. In 100ml of acetic acid, when the powder is less than 0.5g, the gel strength is too weak, the molding is poor, and when the powder is more than 2.0g, the molding is too compact, the gaps are less, and the adsorption and reaction effects are affected.
Further, in the step (3), the mass-volume ratio of the nano zero-valent iron particles loaded with the citrate organic ligand on the surface to the citric acid carboxylated chitosan solution is 1g:40ml, and the concentration of the sodium hydroxide solution is 0.5-2.0 mol/L.
Further, in the step (3), the vortex mixing time is 3-5 min, the ultrasonic time is 15-30 min, the dropping speed of the suspension is 4-8 drops/second, the suspension drops are adhered together too quickly, gel microspheres are difficult to form, the optimal dropping height is 10cm away from the liquid level, and the gel microspheres are kept stable in sodium hydroxide solution for 24-48 h.
Further, in the step (1), the reducing agent solution isSodium borohydride solution or potassium borohydride solution, BH in reducing agent solution 4 - And Fe in ferrous sulfate heptahydrate solution 2+ Is 4:1 (n (BH 4 - )：n(Fe 2+ ) =4:1); the purity of the nitrogen is 99.999%; the vacuum drying temperature is 40-60 ℃.
Further, in the step (2), the concentration of the trisodium citrate solution is 0.2291mol/L, and the mass-volume ratio of the nano zero-valent iron particles to the trisodium citrate solution is 1g:20ml.
In the step (2), acid washing is carried out by adopting strong acid with the pH value of 1-3, the acid washing time is 10-30 s, the vortex mixing time is 3-5 min, the heating temperature is 50-70 ℃ during the oxygen-isolation heating reaction, and the reaction time is 8-9 h. At the temperature and time, nanometer zero-valent iron particles with optimal reactivity and surfaces loaded with citrate organic ligands can be synthesized.
The invention has the beneficial effects that:
the gel microsphere provided by the invention combines the directional adsorption and capture effects of citric acid carboxylation chitosan shells on trace POPs in natural water bodies with more strong oxidative free radicals (including hydroxyl, superoxide free radicals, ferrite free radicals and the like) which are generated by nano zero-valent iron under the assistance of citrate, and also has deprotonation generated by free electrons, so that the efficient degradation and removal of trace POPs remained in the water bodies can be realized. Meanwhile, the gel microsphere is made of materials harmless to the environment, and can generate a large number of strong oxidative free radicals directly through the actions of water, dissolved oxygen and the like in a water body, and the gel microsphere does not depend on the manual addition of extra industrial chemicals to promote degradation, so that the gel microsphere is environment-friendly. And after repairing the water body, the nano zero-valent iron is easy to separate and recover from the water body, the problem of difficult recovery after putting the nano zero-valent iron is solved, secondary pollution to the water body is avoided, even under the condition that materials are difficult to recover individually, the rapid oxidation of the iron core after biological decomposition and shell decomposition of the nano zero-valent iron can not influence the environment, the nano zero-valent iron is a material extremely suitable for repairing the water body, and the nano zero-valent iron has a good prospect in the field of in-situ repair of polluted water bodies. In addition, the gel microsphere can continuously give ferrous ions under the neutral condition, is matched with excitants such as hydrogen peroxide, is more beneficial to generating a large number of hydroxyl free radicals with strong oxidability, is beneficial to effectively degrading POPs under the neutral condition, and overcomes the defect that the degradation capacity of the existing Fenton-like reagent under the neutral condition is low.
Drawings
The invention is further described below with reference to the accompanying drawings and examples:
FIG. 1 is a graph comparing the effect of gel microspheres (C-nZVI@Chitosan) with ordinary nano zero-valent iron particles (nZVI) to degrade 17β -estradiol;
FIG. 2 is a schematic illustration of gel microspheres (C-nZVI@Chitosan) in pure water;
FIG. 3 is a 10000-fold scanning electron microscope image of gel microspheres (C-nZVI@Chitosan);
FIG. 4 is a view of a 30000-fold scanning electron microscope of the gel microsphere (C-nZVI@Chitosan);
FIG. 5 is a view of a 100000-time scanning electron microscope of the nano zero-valent iron particles (nZVI) prepared by the invention;
FIG. 6 is an XRD comparison of gel microspheres (C-nZVI@Chitosan) after reaction with ordinary nano zero-valent iron particles (nZVI).
Detailed Description
Example 1
The mesoporous core-shell structured nano zero-valent iron gel microsphere for stably loading citrate organic ligands provided by the embodiment comprises a citric acid carboxylated chitosan shell and nano zero-valent iron particles coated in the citric acid carboxylated chitosan shell, wherein the surface of the nano zero-valent iron particles is loaded with the citrate organic ligands.
The preparation method of the mesoporous core-shell structure nano zero-valent iron gel microsphere for stably loading citrate organic ligand provided by the embodiment comprises the following steps:
(1) Preparing nano zero-valent iron particles:
mixing ultrapure water and ferrous sulfate heptahydrate (FeSO4.7H2O) to prepare ferrous sulfate heptahydrate solution;
in an ice bath state, fully deoxidized ultrapure water and sodium borohydride (NaBH 4) are mixed to prepare sodium borohydride solution;
ferrous sulfate heptahydrateThe solution was placed in a reactor filled with high purity nitrogen (99.999% pure nitrogen) and then placed in an ultrasonic generator with a power of 360W under ultrasonic conditions according to BH in sodium borohydride solution 4 - And Fe in ferrous sulfate heptahydrate solution 2+ Is 4:1 (n (BH 4 - )：n(Fe 2+ ) =4:1), slowly injecting sodium borohydride solution into the reactor through a peristaltic pump to react until black suspension does not appear in the reactor, after the reaction is finished for 20min, moving the reactor into a nitrogen glove box, collecting solids by adopting a high-strength rubidium ferroboron magnet, repeatedly washing for three times by adopting deoxidized ultrapure water, and then carrying out vacuum drying at the temperature of 40-60 ℃ to obtain regular spherical nano zero-valent iron particles (nZVI); placing the dried nano zero-valent iron particles into a sealed brown bottle filled with nitrogen gas for storage for later use; in this example, the average particle size of the obtained nano zero-valent iron particles was 50nm, see fig. 5;
(2) Loading citrate organic ligand:
0.3g of nano zero-valent iron particles obtained in the step (1) are put into hydrochloric acid with pH value of 3 to be washed for 30s (nitric acid can be used for replacing hydrochloric acid to wash acid), then the nano zero-valent iron particles are added into 6ml of deoxidized trisodium Citrate solution with concentration of 0.2291mol/L and subjected to ultrasonic deoxidization, vortex mixing is carried out for 3min, then the obtained suspension is injected into a reaction kettle with pressure being higher than atmospheric pressure, under the assistance of ultrasonic, simethicone is used as a heating medium to be heated to 53.7 ℃ in an oxygen-isolation way, the reaction is carried out for 8.11h, after the reaction is finished, high-strength rubidium ferroboron magnet is used for collecting solid, and anaerobic methanol is used for repeatedly washing for three times, thus obtaining nano zero-valent iron particles (Citrite-nZVI) with surfaces loaded with Citrate organic ligands; the prepared nano zero-valent iron particles with the surfaces loaded with citrate organic ligands are placed in a water bath nitrogen blowing instrument at 30 ℃ for constant temperature drying, and then are placed in a sealed brown bottle filled with nitrogen for storage, and the operations are all carried out in a glove box filled with nitrogen; the hydrochloric acid, the trisodium citrate solution and the methanol adopted in the step are subjected to ultrasonic degassing for 30min, then nitrogen is continuously filled for 15min for sealing and preservation, the anaerobic liquid is ensured, and the pressure in the reaction kettle is slightly higher than the atmospheric pressure by properly filling the nitrogen;
(3) Preparation of gel microspheres:
adding 0.25g of nano zero-valent iron particles with the surfaces loaded with citrate organic ligands obtained in the step (2) into 10ml of citric acid carboxylation chitosan solution, vortex mixing for 3min, then performing ultrasonic dispersion for 20min to prepare viscous suspension, then dropwise adding the obtained suspension into sodium hydroxide solution with the concentration of 0.5mol/L at the dropping speed of 5 drops/second by adopting an automatic sampler under the anaerobic condition, wherein the dropping height is 10cm away from the liquid level, so as to crosslink and form gel microspheres, keeping the gel microspheres stable in the sodium hydroxide solution for 24h, taking out the gel microspheres, washing with anaerobic methanol for three times, thus preparing stable nano zero-valent iron gel microspheres (C-nZVI@Chitosan) with mesoporous core-shell structures, soaking the prepared gel microspheres in anaerobic ultrapure water, and sealing and preserving, wherein the sealing and preserving are shown in figure 2.
In this embodiment, the preparation method of the citric acid carboxylated chitosan solution includes the following steps:
a. adding 1g of chitosan into 100ml of 2% citric acid ethanol solution, heating to 55 ℃, stirring at constant temperature for reacting for 0.5h, performing ultrasonic treatment for 20min, performing high-speed centrifugal treatment at the rotating speed of 4500r/min for 10min, repeatedly washing the solid obtained by centrifugal treatment with absolute ethyl alcohol for three times, and performing vacuum drying at the temperature of 40-60 ℃ to obtain citric acid carboxylated chitosan powder;
b. mixing 1g of the powder obtained in the step a with 100ml of acetic acid with volume concentration of 1%, stirring until the powder is completely dissolved, and then performing ultrasonic degassing for 30min to obtain the citric acid carboxylated chitosan solution.
The mesoporous core-shell structure nano zero-valent iron gel microsphere with stable loading citrate organic ligand prepared by adopting the embodiment is used for treating a water sample containing trace POPs:
the 17 beta-estradiol is used as a target, water samples to be treated with different pH values are prepared with the concentration of the 17 beta-estradiol being 400 mu g/L, and the mass volume ratio is 0.05g:100ml of a treatment material is added into a water sample to be treated, and the water sample is subjected to light-proof constant-temperature shaking for 90 minutes at the temperature of 25 ℃ and the speed of 200 revolutions per minute, so that the degradation and removal rate of 17 beta-estradiol by nano zero-valent iron particles (nZVI) and mesoporous core-shell structure nano zero-valent iron gel microspheres (C-nZVI@Chitosan) with stable citrate organic ligands under different pH conditions is tested; determination of removal Rate: taking out 0.8ml degradation liquid at 0, 2, 5, 10, 15, 30, 45, 60 and 90min, immediately adding 0.8ml methanol to terminate reaction, filtering with 0.22 μm filter membrane, and measuring 17 beta-estradiol concentration by high performance liquid chromatography; the results are shown in FIG. 1.
As can be seen from FIG. 1, the degradation removal rate of the gel microsphere (C-nZVI@Chitosan) prepared by the embodiment is far better than that of the common nano zero-valent iron particles (nZVI), the degradation effect of the gel microsphere (C-nZVI@Chitosan) prepared by the embodiment under the acidic condition is best, the degradation rate can reach 99.9%, and the degradation rate is faster.
Finally, it is noted that the above embodiments are only for illustrating the technical solution of the present invention and not for limiting the same, and although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that modifications and equivalents may be made thereto without departing from the spirit and scope of the technical solution of the present invention, which is intended to be covered by the scope of the claims of the present invention.
Claims (9)
1. A mesoporous core-shell structure nanometer zero-valent iron gel microsphere for stably loading citrate organic ligand is characterized in that: the citric acid carboxylation chitosan composite material comprises a citric acid carboxylation chitosan shell and nano zero-valent iron particles coated in the citric acid carboxylation chitosan shell, wherein the surface of the nano zero-valent iron particles is loaded with citrate organic ligands, and the citric acid carboxylation chitosan shell has a pore structure; the gel microsphere is prepared by the following steps: (1) preparing nano zero-valent iron particles:
placing the ferrous sulfate heptahydrate solution in a reactor filled with high-purity nitrogen, then adding a reducing agent solution into the ferrous sulfate heptahydrate solution under the ultrasonic condition for reaction, collecting solids after the reaction is finished, washing the solids in sequence, and drying the solids in vacuum to obtain regular spherical nano zero-valent iron particles;
(2) Loading citrate organic ligand:
washing the nano zero-valent iron granular acid obtained in the step (1), adding the washed nano zero-valent iron granular acid into a deoxidized trisodium citrate solution, uniformly mixing by vortex, then injecting the obtained suspension into a reaction kettle with the pressure being more than atmospheric pressure, performing oxygen-isolation heating reaction under the assistance of ultrasound, collecting solids after the reaction is finished, and washing by adopting methanol to obtain nano zero-valent iron granules with citrate organic ligands loaded on the surfaces;
(3) Preparation of gel microspheres:
adding the nano zero-valent iron particles with the surfaces loaded with the citrate organic ligands obtained in the step (2) into a citric acid carboxylation chitosan solution, uniformly vortex mixing and ultrasonic dispersing to prepare a viscous suspension, then dropwise adding the obtained suspension into a sodium hydroxide solution under the anaerobic condition to crosslink the gel microspheres, keeping the gel microspheres stable in the sodium hydroxide solution for a certain time, and then taking out the gel microspheres and washing the gel microspheres with anaerobic methanol to prepare the stable nano zero-valent iron gel microspheres with mesoporous core-shell structures;
in the step (3), the preparation method of the citric acid carboxylated chitosan solution comprises the following steps:
a. adding chitosan into a citric acid ethanol solution, stirring at constant temperature for reaction for 0.5-1 h, performing ultrasonic treatment for 15-30 min, performing high-speed centrifugal treatment for 10-20 min, repeatedly washing and centrifuging the obtained solid with absolute ethanol, and performing vacuum drying to obtain citric acid carboxylated chitosan powder;
b. mixing the powder obtained in the step a with acetic acid, stirring until the powder is completely dissolved, and then performing ultrasonic degassing to obtain the citric acid carboxylated chitosan solution.
2. A method for preparing the mesoporous core-shell structure nano zero-valent iron gel microsphere stably loaded with citrate organic ligand according to claim 1, which is characterized in that: the method comprises the following steps:
(1) Preparing nano zero-valent iron particles:
placing the ferrous sulfate heptahydrate solution in a reactor filled with high-purity nitrogen, then adding a reducing agent solution into the ferrous sulfate heptahydrate solution under the ultrasonic condition for reaction, collecting solids after the reaction is finished, washing the solids in sequence, and drying the solids in vacuum to obtain regular spherical nano zero-valent iron particles;
(2) Loading citrate organic ligand:
washing the nano zero-valent iron granular acid obtained in the step (1), adding the washed nano zero-valent iron granular acid into a deoxidized trisodium citrate solution, uniformly mixing by vortex, then injecting the obtained suspension into a reaction kettle with the pressure being more than atmospheric pressure, performing oxygen-isolation heating reaction under the assistance of ultrasound, collecting solids after the reaction is finished, and washing by adopting methanol to obtain nano zero-valent iron granules with citrate organic ligands loaded on the surfaces;
(3) Preparation of gel microspheres:
adding the nano zero-valent iron particles with the surfaces loaded with the citrate organic ligands obtained in the step (2) into a citric acid carboxylation chitosan solution, uniformly vortex mixing and ultrasonic dispersing to prepare a viscous suspension, then dropwise adding the obtained suspension into a sodium hydroxide solution under the anaerobic condition to crosslink the gel microspheres, keeping the gel microspheres stable in the sodium hydroxide solution for a certain time, and then taking out the gel microspheres and washing the gel microspheres with anaerobic methanol to obtain the stable nano zero-valent iron gel microspheres with mesoporous core-shell structures.
3. The method for preparing the mesoporous core-shell structure nano zero-valent iron gel microsphere for stably supporting citrate organic ligands according to claim 2, which is characterized in that: in the step (3), the preparation method of the citric acid carboxylated chitosan solution comprises the following steps:
a. adding chitosan into a citric acid ethanol solution, stirring at constant temperature for reaction for 0.5-1 h, performing ultrasonic treatment for 15-30 min, performing high-speed centrifugal treatment for 10-20 min, repeatedly washing and centrifuging the obtained solid with absolute ethanol, and performing vacuum drying to obtain citric acid carboxylated chitosan powder;
b. mixing the powder obtained in the step a with acetic acid, stirring until the powder is completely dissolved, and then performing ultrasonic degassing to obtain the citric acid carboxylated chitosan solution.
4. The method for preparing the mesoporous core-shell structured nano zero-valent iron gel microsphere stably loaded with the citrate organic ligand according to claim 3, which is characterized in that:
in the step a, the mass concentration of the citric acid ethanol solution is 2%, and the mass volume ratio of the chitosan to the citric acid ethanol solution is 1g:100ml; in the step a, the temperature of constant-temperature stirring reaction is 50-80 ℃, the centrifugal treatment rotating speed is 3000-4500 r/min, and the vacuum drying temperature is 40-60 ℃;
in the step b, the volume concentration of the acetic acid is 1%, and the mass volume ratio of the powder to the acetic acid is 0.5-1.5 g: 50-150 ml; the ultrasonic degassing time is 15-30 min.
5. The method for preparing the mesoporous core-shell structure nano zero-valent iron gel microsphere stably loaded with the citrate organic ligand according to claim 4, which is characterized in that:
in the step (3), the mass volume ratio of the nano zero-valent iron particles loaded with the citrate organic ligand on the surface to the citric acid carboxylation chitosan solution is 1g:40ml, and the concentration of the sodium hydroxide solution is 0.5-2.0 mol/L.
6. The method for preparing the mesoporous core-shell structure nano zero-valent iron gel microsphere for stably supporting citrate organic ligands according to claim 2, which is characterized in that:
in the step (3), vortex mixing time is 3-5 min, ultrasonic time is 15-30 min, dropping speed of suspension is 4-8 drops/second, and gel microspheres are kept stable in sodium hydroxide solution for 24-48 h.
7. The method for preparing the mesoporous core-shell structure nano zero-valent iron gel microsphere for stably supporting citrate organic ligands according to claim 2, which is characterized in that:
in the step (1), the reducing agent solution is sodium borohydride solution or potassium borohydride solution, and BH is contained in the reducing agent solution 4 - With ferrous sulfate heptahydrate solution Fe 2+ The molar ratio of (2) is 4:1; the purity of the nitrogen is 99.999%; the vacuum drying temperature is 40-60 ℃.
8. The method for preparing the mesoporous core-shell structure nano zero-valent iron gel microsphere for stably supporting citrate organic ligands according to claim 2, which is characterized in that:
in the step (2), the concentration of the trisodium citrate solution is 0.2291mol/L, and the mass volume ratio of the nanometer zero-valent iron particles to the trisodium citrate solution is 1g:20ml.
9. The method for preparing the mesoporous core-shell structure nano zero-valent iron gel microsphere for stably supporting citrate organic ligands according to claim 2, which is characterized in that:
in the step (2), acid washing is carried out by adopting strong acid with the pH value of 1-3, the acid washing time is 10-30 s, the vortex mixing time is 3-5 min, the heating temperature is 50-70 ℃ during the oxygen-isolated heating reaction, and the reaction time is 8-9 h.
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