KR20180045130A - Green production method of magnetic psuedo graphene oxide using sucrose - Google Patents

Abstract

The present invention relates to a manufacturing method of a pseudo graphene oxide magnetic absorbent which can be magnetically separated in water after radioactive materials such as uranium and thorium contained in groundwater are adsorbed and removed. The manufacturing method comprises the following steps of: manufacturing a pseudo graphene similar to graphene (S10); manufacturing a pseudo graphene oxide (S20); and manufacturing a pseudo graphene oxide magnetic material (S30).

Description

TECHNICAL FIELD [0001] The present invention relates to a method for producing an environmentally similar graphene oxide magnetic adsorbent using a disaccharide,

The present invention relates to a method for producing a magnetic graphene oxide magnetic adsorbent capable of adsorbing or removing radioactive materials such as uranium and thorium contained in ground water or by self-separating in water after adsorption,

Unlike the traditional Hummer's method, which uses graphite and a number of chemicals, it dissolves the pyrolysis of the disaccharide sugar and the ozone oxidation process, simplifying the economical and eco- , ≪ / RTI >

The magnetic grains of the graphene oxide are mixed with a magnetic material to form a magnetic adsorbent, thereby being able to overcome separation limitations and to be applied to various water treatments.

In connection with the present invention, Korean Patent Laid-Open Publication No. 10-2015-0117869 (published on Oct. 21, 2015) discloses a technique for producing a fine powder of sugar-graphite and a method for producing sugar graphene.

The present invention is directed to the development of a fiber having excellent performance as compared with the prior art, and it is an object of the present invention to produce a magnetic graphene oxite magnetic substance using disaccharide sugar, It is a different invention in its constitution.

Further, the present invention can remove uranium, which is a natural radioactive material, by using ozone oxidation which does not require a chemical substance, and then can be separated for reuse.

Paper 'Graphene from Sugar and its Application in Water Purification, ACS Appl. Mater. Interfaces 2012, 4, 4156-4163 discloses sand coated with sugar graphene as the main material, and the material to be removed from water is an organic material which is an insecticide.

In the present invention, the particles of the magnetic grains of the similar graphene oxide using the disaccharide sugar are nano-sized, have a high specific surface area and a high treatment efficiency, and form an oxygen functional group through ozone oxidation to adsorb and remove natural radioactive materials such as uranium . In addition, since magnetic separation can be performed by synthesizing with a magnetic material, it is possible to overcome the limitation of application of nanomaterials.

As described above, the present invention is different from the technology disclosed in the above-mentioned papers for purposes and technical constructions.

Korean Patent Laid-Open No. 10-2015-0117869 (Publication date October 21, 2015)

Graphene from Sugar and its Application in Water Purification, ACS Appl. Mater. Interfaces 2012, 4, 4156-4163, Soujit Sen Gupta and 4 others

Disclosure of the Invention It is an object of the present invention to provide a method for producing a magnetic graphene oxide magnetic adsorbent capable of adsorbing or removing radioactive substances such as uranium and thorium contained in ground water or self-separating in water after adsorption .

In order to achieve the above object,

In the present invention, the disaccharide sugar is gradually heated from room temperature to 200 ° C for 1 hour in an electric furnace under a nitrogen atmosphere,

Maintained at 200 < 0 > C for 1 hour,

Heated rapidly from 200 ° C to 750 ° C for 1 hour,

Maintaining at 750 DEG C for 3 hours to produce a graphene-like graphene through pyrolysis (S10);

Dispersing the simulated graphene in distilled water, and then ozone-oxidizing the simulated graphene for 25 to 35 minutes with an ozone generator (S20);

For magnetic separation, ferromagnetic Fe ²⁺ and Fe ³⁺ were added to distilled water in which the similar graphene oxide was dispersed in a nitrogen atmosphere, and the pH was adjusted to 10 with an ammonia solution, followed by stirring at 75 to 85 ° C to obtain a similar graphene oxide magnetic substance (S30) of preparing a similar graphene oxide magnetic adsorbent.

The method for producing a similar graphene oxide magnetic adsorbent according to the present invention has the following effects.

first. Unlike the traditional Hummers method, which is manufactured using existing graphite and many chemicals, it is a simple, chemical-free, economical and environmentally friendly process through the pyrolysis of disaccharide sugar and ozone oxidation. It produces similar graphene oxide, And thus can be applied to various water treatments.

second. In order to produce similar graphene, sugar is used as a representative material of disaccharides, and compared with the case of using graphite obtained through mining by Hummer's method, eco-friendly materials are used by using sugar obtained from plants .

third. Ozone is oxidized by ozone oxidation by a corona discharge method that generates ozone by supplying oxygen as a source of oxygen, and then an anionic oxygen functional group is formed on the surface by pyrolysis at 750 ° C. in a nitrogen atmosphere to produce a similar graphene having a large nano- Like graphene oxide, it is advantageous in that it is simpler than the conventional Hummer's method and does not require a chemical agent in the oxidation process, and provides a method suitable for large scale production economically and environmentally friendly.

fourth. It is difficult to apply the existing graphene-based adsorbent to the water treatment process by rapidly and effectively removing contaminants in the water and then recovering and reusing it through magnetic separation by synthesizing a graphene oxide powder similar to Fe ₃ O ₄ , which is a ferromagnetic material, Limitations can be overcome.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a chemical structure of a disaccharide sugar according to the present invention, which is converted into a similar graphene through a temperature control program and a chemical mechanism; FIG.
2 is a view showing a chemical structure of a similar graphene oxide and a process of producing a similar graphene oxide by oxidizing ozone by using an ozone generator using similar graphenes according to the present invention.
FIG. 3 is a diagram showing the process of producing a quasi-graphene oxide magnetic material by synthesizing a quasi-graphene oxide with Fe ³⁺ / Fe ²⁺ according to the present invention, and a chemical structure of the resultant product.
FIG. 4 is a graph comparing graphite oxide prepared by the Hummer's method, in which graphite is chemically oxidized and peeled, and graphite oxide is prepared according to the present invention.
FIG. 5 is a TEM analysis (a) of a similar graphene oxide magnetic material according to the present invention and FIG. 5 (b) showing a ring pattern analysis by TEM diffraction of a similar graphene oxide.
FIG. 6 is a graph showing the relationship between the functional groups of pseudo graphene (PG), pseudo graphene oxide (PGO) and similar pseudo graphene oxide (MPGO) FT-IR analysis result graph.
FIG. 7 is a graph showing the crystal structure analysis of XRD-like graphene oxide and similar graphene oxide magnetic material according to the present invention. FIG.
8 is a graph of an XPS analysis showing the morphology of carbon of a similar graphene oxide magnet according to the present invention.
FIG. 9 is a graph showing the characteristics of carbon of similar graphene oxide and similar graphene oxide magnetic material manufactured by Raman spectroscopy according to the present invention. FIG.
FIG. 10 is a graph showing the analysis results of MPMS performed to confirm the magnetic properties of the similar graphene oxide and the graphene oxide magnetic material prepared according to the present invention. FIG.
FIG. 11 is a view (a, b) showing the dispersion state of the magnetic grains of similar graphene oxide prepared according to the present invention in the presence of external magnetic force,
12 is a graph showing an optimum condition (pH 4, amount of adsorbent used 1.2 g) of an artificial contaminated water by spiking Uranyl Acetate (UO ₂ (CH ₃ COO) ₂ ) on groundwater used as actual drinking water according to the present invention. / L) graph (a, b) showing the adsorption capacity test of U (VI) in water for a similar graphene oxide magnetic substance.
13 is a view showing an example of use for adsorption removal of U (VI) material in water using a magnetic grains similar to the graphene oxide according to the present invention.

Hereinafter, a method for producing an environmentally similar graphene oxide magnetic adsorbent using the disaccharide according to the present invention will be described in detail.

As described above, the method for producing the magnetic graphene oxide magnetic adsorbent according to the present invention is characterized in that in the electric furnace under nitrogen atmosphere, the disaccharide sugar is gradually heated from room temperature to 200 ° C for 1 hour,

Maintained at 200 < 0 > C for 1 hour,

Heated rapidly from 200 ° C to 750 ° C for 1 hour,

Maintaining at 750 DEG C for 3 hours to produce a graphene-like graphene through pyrolysis (S10);

Dispersing the simulated graphene in distilled water, and then ozone-oxidizing the simulated graphene for 25 to 35 minutes with an ozone generator (S20);

For magnetic separation, ferromagnetic Fe ²⁺ and Fe ³⁺ were added to distilled water in which the similar graphene oxide was dispersed in a nitrogen atmosphere, and the pH was adjusted to 10 with an ammonia solution, followed by stirring at 75 to 85 ° C to obtain a similar graphene oxide magnetic substance (S30). ≪ / RTI >

≪ Similar graphene fabrication step (S10) >

The temperature at which sugar is melted and caramelized is 186 ° C. In step (S10), the sugar is gradually heated from room temperature to 200 deg. C for 1 hour.

And maintained at 200 ° C for 1 hour.

At this time, the temperature is gradually increased from room temperature to 200 ° C and then maintained at 200 ° C for 1 hour, in order to prevent the sugar from being burned when the temperature is raised rapidly, and to prevent boiling of the melt outside the container.

The reason for raising the temperature up to 750 ° C is that the disaccharide becomes similar graphene in the vicinity of 750 ° C and when considering the yield to obtain the resultant from the material, .

The reason for rapidly raising the temperature from 200 ° C to 750 ° C for 1 hour is to increase the efficiency of graphenization, which converts the disaccharide into pure carbon and converts the carbon into a small number of layers.

After completion of the pyrolysis process, allow to cool to room temperature without any action.

The resultant product obtained at room temperature is pulverized into a powder form.

At this time, it is a similar graphene having a micro or nanometer size and a small specific surface area which is similar to graphene.

≪ Similar graphene oxide production step (S20) >

The similar graphene produced in the similar graphene producing step (S10) is oxidized by ozone by an ozone generating apparatus which generates ozone gas by the injected oxygen to produce a similar graphene oxide.

At this time, the ozone oxidation treatment is performed by adding similar graphene of 2.0 to 3.0 g based on 1 L of distilled water to distilled water and dispersing,

As was flown ozone into oxygen (O ²⁾ is injected into the 3 ~ 7L / min from the oxygen tank (O ³⁾ of ozone from the ozone generator (Ozone Generator) for generating (O ³⁾ is made.

More specifically, ozone is oxidized while ozone (O ³ ) is supplied at a rate of 8 to 12 g / hr and a pressure of 1 to 2 kg / cm 2 while stirring the distilled water dispersed in the graphene with a magnetic stirrer.

At this time, the optimum time of ozone oxidation is determined by the adsorption performance evaluation according to the ozone oxidation time.

The adsorption performance for uranium increased until 30 minutes of ozone oxidation and the adsorption performance did not show any significant difference after 30 minutes. If it is less than 25 minutes, formation of oxygen functional groups may be insufficient, and if it exceeds 30 minutes, it is not appropriate from a cost point of view.

Therefore, the proper ozone oxidation time is 25 minutes to 30 minutes, more preferably 30 minutes.

FIG. 2 is a specific example of the ozone oxidation process.

≪ Similar graphene oxide magnetic material manufacturing step (S30) >

For magnetic separation, the similar graphene oxide is first dispersed in distilled water in a nitrogen atmosphere.

Next, Fe ²⁺ and Fe ³⁺ , which are ferromagnetic materials, are added to distilled water in which the similar graphene oxide is dispersed.

Next, the pH of the distilled water containing Fe ²⁺ and Fe ³⁺ is adjusted to 10 using an ammonia solution, and then the mixture is stirred at 75 to 85 ° C to produce a similar graphene oxide magnetic material.

This is because Fe ²⁺ and Fe ³⁺ precipitate from the vicinity of pH 10, which controls the pH to 10, on the surface of the similar graphene oxide.

If the stirring temperature is lower than 75 캜, Fe ²⁺ and Fe ³⁺ may not be completely synthesized with Fe ₃ O _4. If the stirring temperature exceeds 85 ° C, it is not preferable from a cost standpoint. Therefore, the stirring temperature is preferably maintained in the range of 75 to 85 ° C. More preferably at 80 占 폚.

The above-described technical construction will be described in more detail through examples.

In general, 100 g of sugar is put into an electric furnace and inputted into a temperature control program so as to be heated according to the following steps (a to e). Then, nitrogen is injected into the electric furnace to form a nitrogen atmosphere and then pyrolysis is started.

a) Slowly heat from room temperature to 100 ° C for 30 minutes.

b) Heat slowly from 100 ° C to 200 ° C for 30 minutes.

c) Since the melting point of the disaccharide is 186 ° C, it is kept at 200 ° C for 1 hour.

d) The temperature is ramped up rapidly from 200 ° C to 750 ° C for 1 hour.

e) Keep at 750 ° C for 3 hours.

Through the above steps (a) to (e), the sugar is converted into the similar graphene. FIG. 1 shows a process of converting a disaccharide sugar into a similar graphene through a temperature control program and a chemical mechanism together with a chemical structure.

Add 5 g of the similar graphene obtained in the above process (a to e) into 2 L of distilled water. Connect the oxygen cylinder to the ozone generator and adjust the flow rate so that 5 liters per minute is supplied to the ozone generator.

At this time, about 10 g of ozone is generated per hour, and a disk diffuser (gas diffusion device) is installed so that the ozone gas can effectively contact with the similar graphene in the water at the end of the tube to be discharged. So as to diffuse upward from the floor (Figure 2).

FIG. 2 shows the process of making similar graphene as a similar graphene oxide by an ozone generator, and also shows the chemical structure of the similar graphene oxide.

Activate the ozone generator and perform ozone oxidation for 30 minutes. At this time, stirring is carried out so that oxidation can be actively performed.

Ozone oxidation forms anionic functional groups of anions such as carboxyl (-COOH), carbonyl (-C═O), and hydroxyl (-OH) on the surface of the like graphene and is converted to a similar graphene oxide.

The similar graphene oxide obtained through such a conversion process is filtered and stored in a dry oven at 105 ° C for drying.

1 g of dried similar graphene oxide is dispersed in 1 L of distilled water. Fe ³⁺ / Fe ²⁺ mixed solution is prepared by dissolving 3g of Fe (NO ₃ ) ₃ · 9H ₂ O and 1.105g (2: 1 molar ratio) of Fe (NO ₃ ) ₂ · 6H ₂ O in 50mL of distilled water.

The Fe3 + / Fe2 + mixed solution is slowly added to the distilled water with the similar graphene oxide dispersed therein at room temperature and under a nitrogen atmosphere. Thereafter, the pH of the solution is adjusted to 12 by addition of 1M NaOH solution, followed by stirring at 80 ° C for 1 hour.

FIG. 3 shows the process of preparing a similar graphene oxide as a similar graphene oxide magnetic material by the Fe ³⁺ / Fe ²⁺ mixed solution and the chemical structure of the resultant process.

Similar graphene oxides are synthesized with Fe ³⁺ / Fe ²⁺ and become similar graphene oxide magnetic bodies. The resultant is cooled to room temperature, filtered and dried several times with distilled water to remove impurities and dried in a 65 ° C dry oven.

The prepared graphene oxide magnetic adsorbent was characterized by the following technique.

The presence of functional groups is confirmed by FT-IR, and X-ray photoelectron spectroscopy (XPS) analysis is used to analyze the bond type of carbon and oxygen.

In addition, BET surface area analysis is performed to measure the specific surface area and X-ray diffraction (XRD) analysis is performed to confirm the crystal structure. To confirm the sp2 hybridization degree, Raman spectroscopy analysis was performed. Magnetic strength measurement was performed using MPMS (Magnetic Property Measurement System) to check whether magnetic separation was possible.

Batch experiments were carried out to evaluate the adsorption capacity of the prepared graphene oxide magnetic adsorbents on radioactive materials. Uranyl Acetate (UO ₂ (CH ₃ COO) ₂ ) was spiked into uncontaminated groundwater for adsorption experiments on U (VI) to prepare artificially contaminated water.

50 mL of this solution was put into a polypropylene tube with 0.06 g (1.2 g / L) of a similar graphene oxide magnetic substance, and the pH was adjusted to 4. After the adsorption for a certain period of time, the concentration of the filtrate was measured by ICP-OES, .

The results of the above embodiment will be described below.

Materials BET surface area (m 2 / g)  Similar graphene 674  Similar graphene oxide 655  Similar graphene oxide magnetic material 422

Table 1 above shows the results of BET specific surface area analysis of the products prepared through the present invention. Hummer's method-based graphene material was compared with several other studies.

And the specific surface area after the synthesis with the ferromagnet was decreased. It is considered that the iron component is coated on the surface of the similar graphene to block some pores.

Table 2 below shows the results of the analysis of the main characteristics of the drinking groundwater used to test the U (VI) adsorption removal ability of the similar graphene oxide magnetic body.

pH ORP
(mV) DO
(ppm) EC
(ms / cm) U (VI)
(mg / L) Ca ²⁺
(mg / L) Na ²⁺
(mg / L) 6.58 161.3 5.98 0.701 ND 28.96 13.31 K ²⁺
(mg / L) Mg ²⁺
(mg / L) Cl ^-
(mg / L) NO ^3-
(mg / L) SO ₄ ^2-
(mg / L) F ^-
(mg / L) 1.716 3.62 29.29 36.05 26.13 ND

FIG. 4 is a graph illustrating the process of preparing graphene oxide according to the present invention and the process of producing graphene oxide through a Hummer's method of peeling off graphite after chemical oxidation.

4, it can be seen that the method of oxidizing the disaccharide according to the present invention through pyrolysis and oxidation by ozone is a simpler, more economical and eco-friendly method than the method of peeling using the existing strong oxidizing agent.

FIG. 5 (a) is a TEM analysis result of a similar graphene oxide magnetic material manufactured according to the present invention. Similar graphene oxide magnetic bodies were found to have a similar graphene oxide size of about 200 nm surrounded by Fe ₃ O ₄ of less than 10 nm.

FIG. 5 (b) shows a ring pattern analysis by TEM diffraction of a similar graphene oxide according to the present invention. Many points appeared to form a circle.

This phenomenon is a ring pattern observed when many layers of graphite are peeled off through Hummer's Method to form multiple layers of graphene oxide, ie, a multi-layer graphene oxide.

Therefore, it has been confirmed that the graphenized sugar through pyrolysis has a layer structure similar to that of graphene produced by a chemical method.

FIG. 6 is a graph showing the relationship between the number of functional groups of Pseudo Graphene (PG), Pseudo Graphene Oxide (PGO) and the like of Magnetic Pseudo Graphene Oxide (MPGO) -IR analysis results.

It was confirmed that the similar graphene converted by thermal decomposition in the disaccharide was converted into a pure carbon material free of oxygen functional groups and that the similar graphene oxide formed a functional group containing oxygen on the surface. It was also confirmed that this was not lost even when synthesized with the ferromagnetic material as in the analysis results.

FIG. 7 shows the results of analysis of crystal structure confirmed by XRD of the similar graphene oxide and the graphene oxide magnetic material according to the present invention. Pseudo-Graphene Oxide (PGO) showed peaks at broad amorphous materials near 2-Theta 24.7 ° and 43.1 °.

This is a common phenomenon in XRD analysis of reduced graphene oxide prepared by Hummer's method, which means that both materials have a similar structure.

Similar graphene oxide magnetic body (Magnetic Pseudo Graphene Oxide, MPGO) was confirmed as having a crystal structure pattern of the Fe ₃ O ₄ because of the resulting composite with the Fe ₃ O ₄ as compared to JCPDS card after XRD analysis.

FIG. 8 shows the results of XPS analysis showing the carbon type of the produced magnetic grains of the similar graphene oxide.

As a result, a peak indicating CC or C = C was formed near 284.47 eV, and peaks indicating CO, C═O and OC = O were observed at 285.94 eV, 288.06 eV and 289.15 eV, respectively, Means that the carbon of the pin is oxidized through ozone oxidation and bonded to oxygen.

These peaks are also similar to those observed in oxidized graphene materials in the Hummer's method. Therefore, it is confirmed that the similar graphene oxide magnetic adsorbent produced through the process of the present invention has an oxidation form very similar to that of graphene oxide produced by Hummer's method.

FIG. 9 is an analysis of the carbon characteristics of a similar graphene oxide produced by Raman spectroscopy (Raman spectroscopy) and a similar graphene oxide magnetic material according to the present invention.

As a result of analysis, it was found that G band, which means sp2 bond, D band, which means sp2 bond defect due to oxidation, and 2D band, which means two dimensional planar structure, are at 1398 cm ^-1 and 1356 cm ^-1 and 2550 cm ^-1 to 2700 cm ^-1 , respectively.

This is typically observed peaks in graphene oxide or reduced graphene oxide via the Hummer's method and this is one of the evidences that the material produced according to the present invention has properties very similar to those produced by the Hummer's method.

10 shows the results of the MPMS analysis conducted to confirm the magnetic properties of the similar graphene oxide and the graphene oxide magnetic material according to the present invention.

As shown in FIG. 9 (a), the similar graphene oxide magnetic material exhibited a magnetic strength of up to 41.41 emu / g at room temperature, and was confirmed to be sufficiently separable by magnetic separation using an electromagnet or the like.

Also, as shown in FIG. 9 (b), when the ZFC (Field Coefficient Coefficient) / FC curve of 5K to 300K is observed, similar graphene oxide magnetic materials show superparamagnetism at room temperature conditions such as Fe ₃ O ₄ appear.

FIG. 11 shows the dispersion state of the magnetic grains of the similar graphene oxide according to the present invention in water under the presence or absence of external magnetic force. And it is possible to recover in water by magnetic separation.

Fig. 12 shows the results of simulated artificial polluted water by spiking Uranyl Acetate (UO ₂ (CH ₃ COO) ₂ ) on the groundwater used as actual drinking water. (VI) adsorption capacity test of graphene oxide magnetic substance.

The initial concentration of 2 mg / L, which is close to the reported maximum concentration of U (VI) present as a natural radioactive substance, was paralleled after 3 hours and showed a high adsorption rate of 96%.

In addition, the adsorption isotherm at the initial concentration of 2 ~ 50 mg / L showed the maximum adsorption capacity of 31.74 mg / g.

Therefore, as a result of testing groundwater contaminated with radioactive materials artificially, similar graphene oxide magnetic bodies can be applied to actual field groundwater.

Figure 13 is an illustration of use for adsorption removal of U (VI) material in water using a similiar graphene oxide magnetic material in accordance with the present invention.

The similar graphene oxide magnetic adsorbent prepared according to the present invention is different from the conventional Hummer's method, which is produced by using existing graphite and a number of chemicals, in which the pyrolysis of disaccharide sugar and the simple, It is an economical and environmentally friendly process and it has the advantage of being able to apply similar graphene oxide to a variety of water treatments by overcoming the limitation of separation by combining magnetic materials and thus being industrially applicable.

Claims (3)

In an electric furnace in a nitrogen atmosphere, disaccharide sugar was gradually heated from room temperature to 200 ° C for 1 hour,
Maintained at 200 < 0 > C for 1 hour,
Heated rapidly from 200 ° C to 750 ° C for 1 hour,
Maintaining at 750 DEG C for 3 hours to produce a graphene-like graphene through pyrolysis (S10);

Dispersing the simulated graphene in distilled water, and then ozone-oxidizing the simmered graphene to an ozone generator for 25 to 30 minutes to produce a similar graphene oxide (S20);

For the magnetic separation, a ferromagnetic Fe ²⁺ / Fe ³⁺ mixed solution was added to distilled water in which the similar graphene oxide was dispersed in a nitrogen atmosphere, the pH was adjusted to 10-12 with an ammonia solution, and the mixture was stirred at 75-85 ° C And a step (S30) of producing a graphene oxide magnetic substance similar to that of the graphene oxide magnetic adsorbent.
The method according to claim 1,
Ozone oxidation was performed by adding similar graphene of 2.0 to 3.0 g based on 1 L of distilled water to distilled water,
Ozone (O ³ ) is flowed from an ozone generator (Ozone Generator) which generates ozone (O ³ ) from oxygen (O ² ) injected at 3 to 7 L / min from an oxygen tank,
The ozone (O ³ ) was subjected to ozone oxidation treatment for 25 minutes to 30 minutes while flowing ozone (O ³ ) at a rate of 8 to 12 g / hr and a pressure of 1 to 2 kg / cm 2 while stirring the distilled water with the similar graphene dispersed therein with a magnetic stirrer ≪ / RTI > characterized in that the graphene oxide magnetic adsorbent is a graphen oxide.
The method according to claim 1,
The Fe ²⁺ / Fe ³⁺ mixed solution was prepared by dissolving Fe (NO ₃ ) ₃ · 9H ₂ O 3g and Fe (NO ₃ ) ₂ · 6H ₂ O 1.105 (2: 1 molar ratio) in 50 mL distilled water Of the graphene oxide magnetic adsorbent.

Images