ES2678079B2 - Procedure for obtaining graphenic materials from sludge from wastewater treatment - Google Patents

Procedure for obtaining graphenic materials from sludge from wastewater treatment Download PDF

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ES2678079B2
ES2678079B2 ES201830127A ES201830127A ES2678079B2 ES 2678079 B2 ES2678079 B2 ES 2678079B2 ES 201830127 A ES201830127 A ES 201830127A ES 201830127 A ES201830127 A ES 201830127A ES 2678079 B2 ES2678079 B2 ES 2678079B2
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graphene
sludge
pyrolysis
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Alcaniz Laura Pastor
Hernandez Silvia Donate
Ramirez Javier Eduardo Sanchez
Gomez Hermenegildo Garcia
Sancho Josep Albero
Torres Esther Dominguez
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Depuracion De Aguas Del Mediterraneo S L
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    • CCHEMISTRY; METALLURGY
    • C01INORGANIC CHEMISTRY
    • C01BNON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00Carbon; Compounds thereof
    • C01B32/15Nano-sized carbon materials
    • C01B32/182Graphene
    • C01B32/184Preparation

Description

DESCRIPTION

Procedure for obtaining graphene materials from sludge from wastewater treatment.

Field of the Invention

The present invention belongs to the technical field of new materials and the environment, and more specifically to a process for obtaining graphene materials where their precursors are sludge generated in urban or industrial wastewater treatment processes.

Background of the Invention

Graphene is considered a very promising material in numerous applications ranging from additive in formulations of plastics, paints and other materials, through microelectronics, sensors and biomedicine. This interest in graphene determines that various procedures have been developed to obtain it.

Along with the ideal graphene, a large number of related materials have been characterized and prepared which can be generically indicated as graphene materials. Among these types of materials that have some of the structural characteristics that make them similar to graphene are defective graphenes (F. Banhart, J. Kotakoski and AV Krasheninnikov, Acs Nano, 2011, 5, 25 26-41, LG Caneado, A. Jorio, EHM Ferreira, F. Stavale, CA Achete, RB Capaz, MVO Moutinho, A. Lombardo, TS Kulmala and AC Ferrari, Nano Letters, 2011, 11, 3190-3196 and A. Flashimoto, K. Suenaga, A Gloter, K. Urita and S. lijima, Nature, 2004, 430, 870-873) which are 2D sheets of an atom thick, but where there may be local arrangements other than hexagonal, as well as vacancies of one or more Carbon atoms and holes in the sheet visible by electron microscopy techniques. In addition, there are graphene materials where, together with carbon, other elements (doping heteroatoms) are present in smaller proportions (RT Lv and M. Terrones, Materials Letters, 2012, 78, 209-218 and XW Wang, GZ Sun, P. Routh , DH Kim, W. Huang and P. Chen, Chemical Society Reviews, 2014, 43, 7067-7098), mainly oxygen, but also nitrogen, sulfur, phosphorus and other elements, as well as combinations of them.

The preparation of graphenes can be carried out in multiple ways that can be classified according to whether the precursors are molecules that react to give aggregates with a greater number of carbons that end up forming the graphene sheet (bottom-up synthesis routes) or if, on the contrary, it starts with materials that already contain graphene and that can be subjected to delamination and exfoliation treatments (top-down synthesis routes).

Among the processes in which graphene materials are obtained by exfoliation, in addition to the use of graphites, some are based on the exfoliation of graphite carbonaceous materials that are obtained by pyrolysis of suitable precursors. In these pyrolysis processes, a precursor is subjected to heating at elevated temperatures in the absence of oxygen for a period of time sufficient for the precursor to undergo a profound structural rearrangement resulting from the removal in the gas phase of small molecules containing heteroatoms and that leave a residue with a high carbon content in the solid phase. This carbon residue can, in some cases, contain graphene sheets with an imperfect packaging and, as a consequence, weak that is susceptible to exfoliation to give rise to graphene materials.

Not all precursors are suitable for giving this type of exfoliable graphite carbonaceous materials. Thus, for example, many organic molecules undergo evaporation in the process and are carried by the evacuation system of the pyrolysis equipment or by the flow that maintains the inert atmosphere in the system. Even macromolecules and high molecular weight molecules decompose completely and undergo fragmentation in the pyrolysis process, without giving rise to a significant solid residue that could be graphene. Such is the case, for example, of synthetic polymers such as polystyrene and polyethylene that undergo fragmentation and removal of the resulting molecules in the gas phase without giving rise to graphitic carbonaceous solid residues in appreciable amounts under pyrolysis conditions.

One of the raw materials for the production of graphene materials is sludge, which is the by-product from the different stages of decontamination of urban wastewater and industrial activities. Its production results from the combination of several phenomena: growth of microorganisms, accumulation of suspended material and accumulation of non-biodegradable organic matter. Sludge is also called those residues from septic tanks and other industrial facilities 5 during the treatment of water and waste.

In sewage treatment urban sludge and industrial sludge are distinguished. Urban sludge is generated during the treatment of domestic wastewater and has an organic matter content of around 70%. Industrial sludge is generated during the treatment of industrial water and waste, its characteristics depend on the industrial activities carried out and some of them may also have a high content of organic matter.

On the other hand, at an industrial level there are other sources of sludge generation with a high content of organic matter, which are formed in the treatment of industrial wastewater or in the treatment of various industrial or agro-food waste. These sludges can be subjected to various processes, with anaerobic digestion and composting being the most common. In the case of anaerobic digestion of waste, biogas and a digested sludge of industrial origin are generated.

There is an interest in the recovery of the sludge produced in the waste water purification processes of both domestic and industrial origin.

Patent CN106348274A describes the preparation of graphene from biomass using agricultural and forest residues as a carbon source.

The method specifically comprises the following steps:

1) add the crushed residues and forest waste biomass to a reaction boiler with water, perform a hydrothermal reaction, cool to room temperature after completion of the reaction, filter, wash and dry to obtain biological solid carbon;

(2) mix the alkali with the biological carbon obtained in step (1), grind sufficiently, mix uniformly, and heat and calcine in the presence of shielding gas; Y

(3) soak a sample obtained after calcination in step (2) with an acidic liquid to remove a by-product, filter, wash the obtained solid with water until the washing liquid is neutral and dry, thus obtaining graphene from a small number of layers.

According to the method, the process is simple, the yield is high, the reaction conditions are mild, the equipment is simple, environmentally friendly, and the biomass, by Coming from agricultural and forest residues, it is available at low cost and in sufficient quantities.

Although graphene can be prepared with the above method using lignocellulosic biomass as raw material, the preparation cycle is long, requires a pretreatment of the biomass and involves the massive use of chemical reagents. Furthermore, in the methodology used, intermediate thermal reactions are necessary in an aqueous medium at temperatures of 180 ° C, causing significant additional energy consumption. On the other hand, the obtained material must be washed and separated using chemical compounds such as sulfuric acid, ethanol, potassium hydroxide, hydrochloric acid and nitric acid that cause the generation of large volumes of liquid waste that require adequate treatment before its discharged, increasing the production costs of graphene.

In summary, the graphene preparation method described in CN106348274A can be expensive, time consuming due to pretreatment of the sample, requires high amounts of thermal energy, requires large volumes of expensive reagents, and is potentially contaminating, all of which hinders its scalability for industrial production graphene.

Description of the Invention

The present invention solves the problems existing in the state of the art by means of a process to obtain graphene materials from by-products of wastewater treatment (Sludge).

The present invention describes the procedure for obtaining graphene materials consisting of defective gratenes containing doping heteroatoms and graphene materials where there are metallic nanoparticles supported on these sheets by pyrolysis in the absence of oxygen from sludge from urban or industrial wastewater treatment plants. with high organic content.

In a first aspect of the invention, the process for obtaining graphene materials from sludge and sludge from wastewater treatment comprises the following steps:

a) Pyrolysis of by-products from wastewater treatment in the absence of oxygen at temperatures between 700 and 1500 ° C.

b) Dispersion of the residue obtained in a solvent and exfoliation of the material.

c) Decantation of the residues that settle spontaneously or after centrifugation at speeds below 8000 RPM.

d) Recovery of the graphene material from the liquid phase.

The absence of oxygen during the pyrolysis process is essential, since its presence at temperatures and under heat treatment conditions causes the combustion of organic matter, not resulting in the formation of any graphitic carbonaceous residue. This absence can be achieved by carrying out the pyrolysis process in an atmosphere that does not contain this chemical element or under vacuum.

When operating in an atmosphere that does not contain oxygen, the pyrolysis system may be subjected to a slight overpressure with respect to atmospheric pressure in order to prevent atmospheric oxygen from entering the system.

Pyrolysis temperature is another important variable that influences the content of heteroatoms that remain in the residue, the density of defects and the packaging of the graphene sheets that are formed. The deep structural reorganization process that leads to the graphitization of the precursor organic matter begins to be observed at temperatures of around 700 ° C, being already notable in many cases at temperatures of 900 ° C. As the temperature increases, as a general rule, materials with a higher percentage of carbon and a lower density of defects result. This favors the packaging of the graphene sheets, which determines that the exfoliation process can be less efficient. It is known that, due to its high crystallinity, the direct exfoliation of graphite to single layer graphene by ultrasound treatment does not take place with appreciable performance in most solvents.

In another aspect of the present invention, the precursor materials of graphene materials are organic compounds of diverse nature in the form of a complex mixture that are found in wastewater and accumulate in the sludge generated in the sedimentation process. These organic compounds of natural origin can be accompanied by synthetic polymers that are used as coagulants, as well as metallic compounds that can be added to promote coagulation or that may be present in waters of industrial origin. These sludges can be used as graphene precursors, either after undergoing a drying process or containing a percentage of moisture.

Sludge generated in the purification process and containing organic matter can also be used as graphene precursors and may have been subjected to anaerobic digestion processes for the fermentation and decomposition of a certain percentage of organic matter to biogas or other biological transformations. They can also undergo pyrolysis to form graphene sludge and sludge from wastewater treatment from farms and livestock farms, agri-food industries and agroforestry industries.

Another aspect of the invention is the possibility of carrying out the process under the flow of a gas that does not contain oxygen, so that a certain flow rate of the inert gas circulates through the pyrolysis system, entraining and separating from the system any molecule that may the gas phase. Nitrogen and argon, among others, can be used as inert gases, and this stream may or may not contain a certain proportion of hydrogen, which contributes to the reducing environment of the process and reduces the content of oxygen and heteroatoms that remain in the carbonaceous residue. At pyrolysis temperatures, particularly when certain transition metals such as Co, Ni and Cu are present, hydrogen reacts with the heteroatoms present in the sample undergoing pyrolysis and forms hydrogenated molecules such as H2O, NH3, SH2, etc. they migrate to the gas phase and reduce the heteroatom content of the resulting carbonaceous residue. This pyrolysis is carried out with a flow of H2 and an inert gas selected from N2, Ar, He, C02 or combinations of two or more of the above gases in any proportion.

In this way, the presence of hydrogen or another reducing reagent can affect the resulting graphene material, its structure, heteroatom content and properties.

Another aspect of the invention refers to the fact that when pyrolysis is carried out under vacuum, it is necessary to work at very low pressures, less than 10-3 Torr, which ensure that during the pyrolysis period the oxygen exposure of the carbonaceous residue is very limited so that combustion does not occur.

Another aspect of the invention refers to the exfoliation of material, which is carried out in step b) of the process by means of ultrasound treatment or by mechanical agitation with high shear power. After pyrolysis, the carbonaceous material can generate graphene with high efficiency by treatment, among other described forms, with ultrasound of the carbonaceous residue to give rise to its exfoliation and separation into graphene sheets.

In this way, the graphene material can be separated from other amorphous forms of carbon or from the inorganic material that may form part of the sludge. Thus, when treating a dispersion of the carbonaceous residue with ultrasounds in water or organic solvents known in the state of the art such as alcohols, amides, aromatic compounds and others, a persistent dispersion of graphene is generated that does not sediment by gravity or by centrifugation at low revolutions. .

The material that settles after the dispersion of the graphene in the liquid medium can be separated from the graphene material that remains in the suspension. This sedimentation can be promoted and accelerated by centrifugation or other means.

Subsequent to the purification of the graphene material, it can be recovered by filtration, centrifugation at high revolutions above 8000 rpm or by evaporation of the liquid phase. The cycle of i) redispersion by sonication, ii) separation by sedimentation and iii) recovery of the graphene can be carried out several times to achieve a better quality of the graphene material.

In this way, the graphene materials result from subjecting the sludges that constitute waste from the treatment of residual waters of different origins and with a different degree of drying to pyrolysis in the absence of oxygen at temperatures above 700 ° C and typically in the range between 900 and 1200 ° C, subsequent exfoliation by ultrasound or sufficient agitation and purification, eliminating residues that do not disperse and settle in the process.

Another aspect of the invention relates to that the dispersions obtained in step b) contain grate monolayers, multilayer grate particles with stacking or combinations thereof.

Another aspect of the invention refers to that the steps b) and c) of the process can be carried out repeatedly depending on the purity of the material that is desired. The resulting graphene materials can be kept in suspension, for example, in water or can be separated from the liquid phase by high speed centrifugation (greater than 8000 rpm), by filtration or by evaporation of the liquid phase.

The dry graphene materials can be redispersed almost completely or completely again by sonication in the appropriate solvent.

Another aspect of the invention refers to the fact that the graphene material obtained is doped with oxygen, nitrogen, sulfur, phosphorus, boron or combinations thereof in a content of less than 10%.

Another aspect of the invention refers to the fact that the graphene material obtained contains on its surface nanoparticles of iron, aluminum or silicon and their oxides. Graphene materials obtained in sludge pyrolysis may contain small nano-sized particles adsorbed from another non-carbonaceous material, which is also formed in the pyrolysis process. This is due to the adsorption capacity that characterizes graphene and the possible presence in the mud of inorganic material, typically metal ions, that do not evaporate in the pyrolysis process.

In this way the inorganic material segregates in a different phase and remains adsorbed on the graphene surface as it is formed in the process. pyrolytic. A typical case is the formation of iron nanoparticles on graphene, due to the possible iron content of graphene precursor sludge.

Once the material is formed, these metallic nanoparticles can be spontaneously oxidized by exposure to the environment, forming metallic oxide nanoparticles or they can be converted into other metallic compounds, such as sulfides, by suitable treatments.

These metals can also react at the pyrolysis temperatures with the carbon present in the medium, forming metal carbide nanoparticles on the surface of the graphene. These types of graphene materials with metallic nanoparticles are typically formed in sludges that contain a high content of metals, such as certain sludges from industrial sources or sludges generated in the treatment of urban wastewater that have undergone coagulation and flocculation processes with metallic salts.

The graphene materials obtained in sludge pyrolysis can be characterized by the usual techniques in the state of the art that demonstrate 2D morphology (electron microscopy), the nature of the mono- or few layers of the material (atomic force microscopy), the presence of oxygen and other heteroatoms (elemental analysis and X-ray photoelectron spectroscopy, XPS), the sp2 nature of carbon atoms (13C NMR in solid state, XPS and Raman spectroscopy), the presence of defects in the network (Raman spectroscopy), the existence of supported metal nanoparticles and their size distribution (transmission electron microscopy, XPS, X-ray diffraction, elemental analysis), among many other techniques that may include UV-Vis absorption spectroscopy (transparency and light absorption) ), thermogravimetry in oxygen (inorganic residue), gas adsorption at constant temperature (measurement of specific area cific), electrical conductivity measurements (specific and surface electrical resistance), among others possible.

The graphene materials described here can have different applications as additives for plastic materials, paints, asphalts, ceramics and in other fields. In particular, those materials containing Fe nanoparticles have been tested as catalysts in processes of relevance for the purification of residual waters, presenting many of the characteristics that are expected for graphenes supporting nanoparticles.

Brief description of the figures:

Figure 1. Raman spectra recorded for the solid sample of the graphene material resulting from the pyrolysis of the primary sediment sludge in three different regions using a 532nm laser as the excitation source and measuring the spectrum in 3x3mm regions.

Figure 2. Transmission electron microscopy images of the particles present in a methanolic suspension that results after the dispersion of the graphene material obtained in the pyrolysis of the primary sludge from the treatment of urban wastewater in the city of Murcia.

Figure 3. Raman spectra recorded for the solid sample of the graphene material resulting from the pyrolysis of sludge from the digester in two different regions using a 532 nm laser as the excitation source and measuring the spectrum in 3 * 3 pm regions.

Figure 4. Electron microscopy images of the graphene material sample obtained in the pyrolysis of sludge from a biodigester. The presence of metallic nanoparticles is clearly shown in the image at higher magnification.

Figure 5. Elemental composition determined by electron microscopy of the sample whose image corresponds to Figure 4. Note that the sample support is a carbon coated Cu grid and that the Cu detected in the measurement must be due to the grid.

Description of embodiments

Having described the present invention, it is further illustrated by the following examples.

Example 1. Graphene doped with N and with defects obtained by pyrolysis of primary sedimentation sludge in urban wastewater treatment.

10 grams of sludge formed in the decantation-flocculation process of urban wastewater (specifically from an urban wastewater treatment plant) were used, which were previously dried in an oven at 60 ° C for 8 h with residual humidity in around 10%, which were placed in a 50 ml porcelain crucible, forming a uniform bed with the smallest possible thickness. The crucible containing the sludge was placed in the central part of a 20 cm diameter horizontal tubular electric quartz furnace whose ends protrude from the furnace about 10 cm so that these ends do not get to heat up to the temperatures of the central part. The furnace was closed with tight fitting Teflon gaskets and a nitrogen stream of 10 ml * min-1 was established. The furnace heating was programmed so that the temperature rose to ° C * min-1 up to 900 ° C (about 3 h), keeping the temperature at 900 ° C for another 3 h.

At the end of this time, the furnace heating stopped, allowing it to cool spontaneously below 150 ° C (approximately 2 hours), while maintaining the flow of inert argon gas. The resulting grayish residue was dispersed in a 200 ml capacity beaker containing 100 ml of water at room temperature and an ultrasound treatment (750 W, 60 min) was carried out using an ultrasound generator whose tip is inserted into the center of the aqueous phase. At the end of this time, the suspension was decanted, eliminating the large particles present in the sludge residue. The suspension was then placed in a suitable container for use in an ultracentrifuge, and the dispersion was centrifuged at increasing speeds. Rotation speeds below 2000 rpm cause a sediment of unwanted particles that are removed, remaining with the liquid phase that contains the graphene material.

The weight obtained of graphene was 1.4 g. The characterization results of the graphene material obtained in the present example are shown in Figures 1 and 2.

The suspension can be centrifuged in a second cycle at higher speeds up to 8000 rpm, separating the decanted material. The liquid phase can be subjected to lyophilization or another procedure to obtain dry graphene material that is capable of elemental analysis.

Analytical data shows that, alongside carbon, other heteroatoms are also present in the graphene material sample. Among them, the most abundant are oxygen and nitrogen, but other elements are also detected in smaller amounts such as sulfur and phosphorous.

An analogous procedure can be carried out at temperatures in the range of 700 to 1200 ° C, with a decrease in the proportion of N being observed with increasing temperature. Table 1 summarizes the data obtained.

Table 1. Main analytical data of the graphene material resulting from the pyrolysis of primary sludge from an urban wastewater treatment plant.

Figure imgf000009_0001

Example 2: Qraphene materials obtained by pyrolysis of sludge from digesters used in biogas generation processes.

A procedure similar to that described in Example 1 is followed, using the same equipment, but it started with a sludge from a biodigester that produces anaerobic fermentation of part of the organic matter, generating methane and carbon dioxide, among other gases.

In this way, a graphene material was obtained in a yield of 7%, lower than that obtained in Example 1. This lower yield is in accordance with the lower organic matter content of this sludge. Figures 3 to 5 present characterization data of this graphene material.

The electron microscopy images in Figure 4 show the presence of metallic nanoparticles supported on the graphene sheets. These nanoparticles have iron in their composition, as established by the energy dispersion scanning analysis technique coupled to the scanning microscopy equipment as indicated in Figure 5.

Example 3: Gratenos obtained by pyrolysis of secondary sludge from urban wastewater.

10 g of secondary sludge generated in urban wastewater treatment was subjected to pyrolysis using the same equipment described in Example 1 and carrying out the treatment at 900 ° C, obtaining gratenes with properties similar to those shown in Example 1, but with a yield of 8%.

Example 4: Gratenos obtained by pyrolysis of mixed sludge from mixing primary and secondary sludge in the purification of urban wastewater.

Pyrolysis was carried out under the conditions of Example 3 using a mixed primary and secondary sludge that was mixed prior to the graphene generation process, obtaining a graphene material as shown in Figure 2 in a yield of 9% by weight.

Example 5: Gratenos obtained by pyrolysis of thickened sludge generated in the purification of urban wastewater.

The pyrolysis was carried out using an oven like the one described in Example 1 at 900 ° C, avoiding the presence of oxygen by means of a vacuum pump that maintained the system at a pressure of 10-3 Torr and using as a precursor a sludge formed in the thickener and characterized by a high iron content. Under these conditions, a graphene similar to that shown in Figure 4 was obtained and whose main characteristic is the presence of iron nanoparticles deposited on the graphene. The yield of the graphene material was 15% by weight.

Example 6: Graphenes obtained by pyrolysis of digested dehydrated sludge.

The pyrolysis process by heating in an electric oven at 900 ° C under vacuum was carried out in this example with sludge from a digester and which had been dehydrated before being subjected to heat treatment. The graphene material is similar to that obtained in Example 2, but with a yield of 12% with respect to the weight of the precursor and where the presence of nanoparticles consisting of iron and its oxides are also observed.

Example 7: Qraphene materials obtained by pyrolysis of sludge from the purification of wastewater from the non-food industry of fruit juices.

The agri-food industry and especially those of fruit juices produce a large amount of residues, these residues once treated (dehydrated) are suitable for the preparation of graphene materials by pyrolysis of them at temperatures between 800-1000 ° C under a flow of argon to generate a residue that can be exfoliated in graphene sheets with a yield of 5 of 10%.

Example 8: Qraphene materials obtained by pyrolysis of sludge from septic tanks.

There are a large number of rural homes and industries that do not have a direct connection to the sewage network, and the use of septic tanks is necessary for the treatment of the generated wastewater. Biological sludge is produced in septic tanks and must be managed before final disposal. Once treated, these sludges are suitable for the preparation of graphene materials by pyrolysis thereof at temperatures between 800-1000 ° C under an inert gas stream (argon) to generate a residue capable of being exfoliated in graphene sheets with a 11% yield.

Example 9: Qraphene materials obtained by pyrolysis of the organic fraction of urban solid waste (MSW).

The procedure described is similar to the previous examples, the material is dried at 60 ° C, homogenized and subsequently subjected to a pyrolysis process at temperatures between 800-1000 ° C under a stream of inert gas (argon) to generate a susceptible residue of being exfoliated in graphene sheets with a yield of 8%.

Example 10: Graphene materials obtained by pyrolysis of the sludge submitted to the composting process.

The procedure described is similar to the previous examples, material 5 is oven dried, homogenized and subsequently subjected to a pyrolysis process at temperatures between 800-1000 ° C under a stream of inert gas (argon) to generate a residue susceptible to be exfoliated in graphene sheets with a yield of 10%.

Example 11: Graphene materials obtained by pyrolysis of starch residues.

The described procedure is similar to the previous examples, the material is oven dried, homogenized and subsequently subjected to a pyrolysis process at temperatures between 800 1000 ° C under a stream of inert gas (argon) to generate a residue that can be exfoliated in graphene sheets with a yield of 7%.

Claims (5)

1. Procedure for obtaining graphene materials from sludge from wastewater treatment, characterized by comprising the following 5 stages: a. Pyrolysis of by-products from the treatment of wastewater in the absence of oxygen at temperatures between 700 and 1500 ° C.
b. Dispersion of the residue obtained in water or other solvents and exfoliation of the material.
c. Decantation of the residues that settle spontaneously or after centrifugation at speeds below 8000 RPM.
d. Recovery of the graphene material from the liquid phase.
and. Pyrolysis is carried out under vacuum at a pressure less than 10-3 Torr, with a flow of H2 and an inert gas selected from N2, Ar, He, CO2 or combinations of two or more of the above gases in any proportion .
F. The exfoliation is carried out by ultrasound exfoliation or mechanical agitation with high shear power.
2. Process according to claim 1, characterized in that the graphene material obtained is doped with oxygen, nitrogen, sulfur, phosphorus, boron or combinations thereof in a content of less than 10%.
3. Method according to claims 1 to 2, characterized in that the dispersions obtained in step b) contain gratene monolayers, multilayer gratene particles with stacking or combinations thereof.
Method according to claims 1 to 3, characterized in that the graphene material obtained contains nanoparticles of other components on its surface.
5. Process according to claim 4, where the nanoparticles that are deposited on the graphene sheets are made of iron, aluminum or silicon or their oxides or carbides.
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