WO2018078427A1

WO2018078427A1 - Graphene-based filtering element and uses thereof

Info

Publication number: WO2018078427A1
Application number: PCT/IB2016/056551
Authority: WO
Inventors: Ivano Aglietto
Original assignee: Nanosme Se
Priority date: 2016-10-31
Filing date: 2016-10-31
Publication date: 2018-05-03

Abstract

The present invention relates to a filtering element comprising a blend of mesoporous graphene compound having pores with average pore size between 0,4 and 250 nm and at least one spacing material in granular form having an average particle size between 0,1 and 6 mm wherein the contribution of pores having an average pore size from 2 and 50 nm to the total specific surface area of the mesoporous graphene compound is not less than 50% of the total specific surface area and wherein the minimum density of the filtering element is at least the 15% of the bulk density of the spacing material. The invention further relates to a method for the removal or separation of contaminants from a fluid and a method for in-situ groundwater remediation with the filtering element. Furthermore, the invention relates to a filtration mask and a cartridges comprising the filtering element.

Description

GRAPHENE-BASED FILTERING ELEMENT AND USES THEREOF

TECHNICAL FIELD

The present invention relates to graphene-based filtering elements and methods for their preparation .

BACKGROUND ART

In conventional processes for water and gas treatment plants, common adsorbents like granulated activated carbon are widely used to remove different contaminants by adsorption. However, this methods often fail to meet regulatory requirements because of poor removal efficiency at low concentrations of contaminants, slow adsorption rates and low efficiency for several recalcitrant and emergent contaminants .

Conventional membrane filtration processes are efficient and scalable but have several limitations in terms of energy consumption and fouling problems. Thus, their application is delimited within a limited class of contaminants.

For this reason there is a big need to find new materials able to remove recalcitrant and emergent contaminants, in particular at very low concentrations.

In recent years, the literature is focusing on novel nanomaterials that may be used to address major environmental challenges .

Graphene, a one atom thick two-dimensional honeycomb sp2 carbon lattice, is a multifunctional material and possesses a combination of strong mechanical properties, chemical inertness and extremely large surface area. Graphene and graphene-based materials have been investigated to propose a new generation of water treatment membranes, water and gas filtration sorbents, electrodes for contaminants advanced oxidation and sensors for contaminants detection.

Numerous studies have been conducted to demonstrate the removal efficiency of different contaminants using graphene materials, expanded graphite (EG) , graphene nanoplatelets (GNP) , pristine graphene sheets (PGS), graphene flakes (GFL) , graphene oxide (GO) and reduced graphene oxide (RGO) as sorbent materials or for membrane filtration applications.

Despite the fact that it is proven that it is possible to achieve a good efficiency in terms of contaminants removal, these methods reported to use graphene based sorbents are not competitive if compared with traditional sorbent materials already on the market and the improvements achieved till now do not justify the higher cost of these materials. In particular, GO has a limited amount of sorption sites, RGO has a low density of oxygen-containing functional groups and a low colloidal stability. Moreover, the results reported in literature for RGO looks less promising than those obtained with GO.

One of the most popular graphene materials used for water and gas treatment is graphene oxide (GO) due to its lower production costs. GO is an oxidized form of graphene, showing a high density of oxygen functional groups (carboxyl, hydroxyl, carbonyl, and epoxy) and a hydrophilic nature. The high concentration of oxygen-containing functional groups is important for example for the interaction with metal ions.

Many efforts have been oriented till now to use GO and to create graphene oxide membranes. GO membranes are generally made via a layer-by-layer process in which tiny sheets of GO are deposited on each other by either vacuum filtration or evaporation. In terms of graphene-based membranes, in literature are proposed stacked GO barriers and membranes of nanoporous graphene sheets with nano-sized perforations, in particular to remove unwanted ions from water. The control of pore size on a large surface area in a sub-nanometer range can be for example performed with electron beam irradiation. However, this is a not scalable and not cost effective approach. Other techniques, such as for example, low energy ion irradiation, have been proposed, but their use is limited by the presence of defects in the pristine graphene sheet. The ideal and most effective solution would be the use of defect-free single-layer graphene on a porous support, but at the moment this approach it is not scalable, not cost- effective and not available for industrial applications. A further limitation of this procedure consists in that it is common to find, in water and wastewater, several different contaminants with different molecular sizes. Therefore the use of a single graphene sheet with holes of a specific dimension can remove just few of the contaminants of concerns .

Examples of continuous graphene films prepared by chemical vapor deposition and transferred to substrates followed by etching pores on the film are also reported. However, the transfer process limits the scalability of the membrane production. Another method to produce the membrane structure is by restacking GO flakes by filtration of GO dispersions on a filter support but this kind of approach requires large volumes of liquid, significant time and it is not scalable. Other liquid phase processes such as dip- coating or layer-by-layer assembly have similar problems in terms of scalability. Costs, difficulty and complexity in the design and precise control of pore sizes of nanoporous graphene membranes for the moment prevents any real scalable application .

GO and, in few cases, RGO have been also used to modify the surface of other sorbent materials, like sand and activated carbons. In literature few examples of sand and other filters media coated with GO and RGO are reported. In these applications GO or RGO creates a coating around the grains of the adsorbent material. In this cases the graphene coated sorbent removes contaminants just as well as the commercially available active carbon or other traditional adsorbents, without any compression of the bed and without any membrane separation effect. Mainly, their efficiency is due to the functionalization and surface modification of GO.

Hierarchically porous materials, defined as materials with pores that extend in different length scales (e.g., micro-, meso-, and macropores), are becoming increasingly important in water, wastewater and gas filtration due to their advantageous diffusion and flux properties and there is a continuous need for novel materials and devices for separation of contaminants from different fluid for various industries . DISCLOSURE OF INVENTION

The object of the present invention is therefore to provide a novel graphene-based filter media which is free from the drawbacks of the filter media and membranes described above. Said object is achieved by the present invention, as it relates to a filtering element according to claim 1, a method for the removal or separation of one or more contaminants from a fluid with the filtering element according to claim 10, a method for in-situ groundwater remediation according to claim 12, a filtration mask according to claim 14 and a cartridge according to claim 15.

In particular, the filtering element of the present invention has a better adsorption rate, molecular sieve and membrane separation effect toward a wide range of contaminants, also when they are present at very low concentrations.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in detail with reference to the Figures of the accompanying drawings, in which :

- Figure 1 shows the conceptual model of the distribution of the spacing material and mesoporous graphene compound inside the filtering element of the present invention;

- Figure 2 shows the pore size distribution of a graphene flake material, measured with the instrument Micrometrics ASAP 2020 - method BJH Adsorption dV/dD Pore Volume;

- Figure 3 shows the isotherm linear plot of the same flake graphene material of Figure 2, measured with the BET method under nitrogen flux, with the instrument Micrometrics ASAP 2020;

- Figure 4 shows different pore size distributions of the graphene material of Figure 3 at different compression rates: 1 (figure 4A) , 5 (figure 4B) and 10 kg-cnr² (figure 4C) ;

- Figure 5 shows a filtering cartridge according to an embodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

According to a first aspect of the invention, it is provided a filtering element for the separation of molecules from different fluids and for different domestic and industrial applications. This filtering element can be used for different kinds of fluids, both in gas and liquid phase, not only for the removal of contaminants but also for the separation of one compound from others in a fluid.

In particular, the filtering element comprises a blend of mesoporous graphene compound having pores with average pore size between 0,4 and 250 nm and at least one spacing material in granular form having an average particle size between 0,1 and 6 mm wherein the contribution of pores having an average pore size from 2 and 50 nm to the total specific surface area of the mesoporous graphene compound is not less than 50% of the total specific surface area, preferably not less than 70%, and wherein the minimum density of the filtering element is at least the 15% of the bulk density of the spacing material.

As used herein, the term "filtering element" refers to a graphene-based hierchically porous monolithic element, with a percolating macroporous network and accessible meso and micropores for contaminants removal from different fluids, that isolates one or more contaminants from a fluid or from a mixture of fluids or separate one or more compounds from a mixture of compounds in a fluid. The filtering element may separate or isolate the selected compounds or contaminants by chemisorption, by absorption, by adsorption on or in the molecular sieve or sequestering the contaminant within a cavity of the molecular sieve, by membrane filtration.

As used herein, the term "bulk density of the spacing material" refers to the ratio of the mass of an untapped powder sample and its volume including the contribution of the interparticulate void volume.

The specific surface area has been measured by Brunauer- Emmett-Teller (BET) nitrogen adsorption method. The presence of macropores (average pore size: 50-250 ran) in the graphene material is important to provide an interconnected framework and acts as a transport system for liquids and gases, thus increasing the accessibility of the mesopores (average pore size: 2-50 nm) and furthermore promoting the capillary effect that is the driven force for the membrane effect of the filtering element of the invention. Figure 2 shows an example of pore size distribution of a mesoporous graphene compound that can be used to produce the filtering element of the invention.

In particular, the mesoporous graphene compound can be selected from the group consisting of non-functionalized or functionalized graphene nanoplatelets (GNP) , non- functionalized or functionalized pristine graphene sheets (PGS), non-functionalized or functionalized graphene flakes (GFL) , non-functionalized or functionalized graphene oxide (GO) , non-functionalized or functionalized reduced graphene oxide (RGO) , non-functionalized or functionalized expanded graphite (EG) , fullerenes, carbon nanotubes and combinations thereof .

As used herein, the term "functionalized" with reference to the mesoporous graphene compound refers to a graphene compound with surface modified with the aim to introduce oxygen or hydrogen functional groups that act as potential sorption sites.

Moreover, activated mesoporous graphene compounds may be used. It is well known and already reported that graphene materials can be also activated with standard physical or chemical processes, increasing their specific surface area. Also activated graphene materials can be used and more specifically for the purpose of the present disclosure activation processes that increase the mesoporores concentration instead of the number of micropores can be used. Examples of methods to produce activated mesoporous graphene compounds are illustrated in Zhu, Y. et al. 2011. "Carbon-based supercapacitors produced by activation of graphene". Science, 332, 1537-1541 and Kim TY, Jung G, Yoo S, Suh KS, Ruoff RS (2013) "Activated graphene-based carbons as supercapacitor electrodes with macro- and mesopores". ACS Nano 7 : 6899-6905) .

The surface of the mesoporous graphene compound is more accessible for ions or molecules and has a typical nitrogen adsorption isotherm with hysteresis loop due to capillary condensation in mesopores. Furthermore mesopores provide easier access to both large and small fractions of contaminant compounds on the adsorbent surface.

In particular, according to an embodiment of the present invention, the mesoporous graphene compound of the invention is a graphene compound with a isotherm classified like (IV) Type according to the IUPAC classification where a well- defined hysteresis is observed, which is evidence of a substantial volume of mesopores, in which irreversible capillary condensation occurs. Figure 3 shows the isotherm distribution of a graphene flake material that can be used for the purpose of this invention.

According to an embodiment of the invention, the mesoporous graphene compound has an average lateral size of the graphene sheets from 3 nm to 40 um, preferably of from 2 to 20 μm.

As used herein, the term "average lateral size" refers to the average length taken along the major axis of the sheets of the graphene compound, measured by Transmission Electron Microscopy (TEM) or other standard methods reported in literature (for example "Measuring the lateral size of liquid-exfoliated nanosheets with dynamic light scattering" - Mustafa Lotya, Aliaksandra Rakovich, John F Donegan and Jonathan N Coleman - Nanotechnology, Volume 24, Number 26) .

The spacing material can be selected from the group consisting of sand, activated carbons, zeolites, bentonite, alumina, aluminum silicate, chitosan, clay, bentonite, diatomite, montmorillonite, iron oxides, titanium oxide, quartzite, silver, hematite, iron or a mixture thereof.

The presence of the spacing material allows to avoid the complete adhesion of the graphene sheets that can promote the formation of channels inside the filtering element and therefore of undesired preferential ways for the flow without the possibility for the contaminants to be sorbed or separated by membrane filtration. In fact, in use, the hydraulic pressure of the fluid during separation or filtration tends to compress the graphene sheets creating local impermeable barriers. When this phenomena occurs, the fluid flows around the macropores with a "wall effect" phenomena, therefore promoting preferential ways and reducing the efficiency of contaminants removal.

To build a compact and stable monolithic material it is needed to have a homogeneous distribution of the spacing material in granular form, that is with an average particle size of between 0,1 and 6 mm to give stability to the structure.

Moreover, according to an embodiment of the present invention, the filtering element may further comprise spacing material in fine powder, that is with an average particle size of between 5 nm and 90 μm, that can intercalate into the mesoporous graphene compound, keeping a minimum distance between two graphene sheets and avoiding that they completely stack together. This assures, during the filtration process, the stable geometry of nanochannels .

Moreover, the use of a specific spacing material in fine powder form can confer to the filtering element specific properties. For instance, it is possible to add hematite nanoparticles to increase the efficiency in terms of arsenic removal or silver nanoparticles to improve the efficiency in terms of antimicrobial agent or nanoparticles of titanium dioxide to improve the oxidation of some organic compounds.

Figure 1, illustrates a schematic representation of the filtering element 10 of the present invention that includes granules 1 and fine powder particles (or nanoparticles) 2 of the spacing material distributed between sheets of the mesoporous graphene compound 4 and, in some cases, depending on their dimension, intercalated inside the macropores and mesopores of the mesoporous graphene compound 4. Some contaminants 3 will be adsorbed and absorbed into micro, and mainly into meso and macropores of the mesoporous graphene compound 4, some others will be blocked by the nanochannels formed between two or more layers of the mesoporous graphene compound 4. The granules 1 and the powder particles 2 of the spacing material secure a regular flow of the fluid.

Considering the density, in general terms, a minimum density is needed to have a mechanical stability of the final monolithic structure and to avoid the creation of internal channels where the fluid could flow without be subject to sorption processes and membrane filtration separation. The minimum density of the filtering element of the present invention depends from the density of mesoporous graphene compound and of the spacing material used to prepare the blend. However, the minimum density of the filtering element is at least the 15% of the bulk density of the spacing material .

For example in case the mesoporous graphene compound is a graphene flake material with a bulk density of 0.004 g-cnr³ and the spacing material is a granular activated carbon with a density of 0.51 g-cnr³, the minimum density of the final filtering element should be not less than 0.15 g-cnr³.

If a mesoporous graphene material with a bulk density around 0.02 g-cnr³ and a spacing material with a density around 0.55 g-cnr³ are used, the optimum density of the filtering element to optimize the membrane filtration process should be no less than 0.4 g-cnr³ and more preferably in the range 0.4- 1.7 g-cnr³. Higher density values will always provide a better membrane filtration process but increasing the density will increase the pressure drop of the filtering element and for this reason it is better to avoid higher density value when there is not a significant improvement of the filtration process.

Moreover, the final density of the filtering element has to be selected on the basis of the target contaminants to be removed. When contaminants have to be removed mainly by membrane filtration processes, it is important to create a filtering element with a high pressure and therefore a higher density of the bed, to create a homogeneous stack of graphene sheets and therefore a stable distribution of nanochannels .

When the target molecules of the filtration process are contaminants to be removed mainly by processes of adsorption into mesopores, it is important to build the filtering element to assure a capillary flux that is the ideal condition for the mesopore filtration. In case the contaminants are mainly removed by absorption into macropores, it is important to use a mesoporous graphene compound where the contribution of macropores to the specific surface area of the mesoporous graphene is not less than 8% of the total specific surface .

However, it is important to note that, as showed in Figure 4, high compression rates of the bed of the filtering element do not modify the pore distribution of the mesoporous graphene compound inside the bed.

Figure 4A, Figure 4B and Figure 4C show the pore size distribution of the graphene flake material compressed with different compression rate, respectively 1, 5 and 10 kg-cnr ². The comparison of these three pore size distribution between themselves and with the pore size distribution reported in Figure 2, where the graphene flake material is not compressed at all, show that the pressed graphene sheets maintain their pore size distribution, allowing a homogeneous capillary flow and create a membrane system combined with adsorption process.

On the contrary, a too low compression of the filtering element cannot be sufficient to create the desired compact monolithic structure with the risk of the creation of undesired internal channels where the contaminants bypass the pores of the filtering element.

As a general consideration, high density of the filtering element assures the best operative conditions in particular for fluids with multi-contaminants.

The filtering element of the invention has a very high efficiency for water and gas treatment and more generically for separation of different molecules from fluids, thanks to the combination of two complementary, combined and synergistic effects: sorption (adsorption and absorption) and membrane filtration. Although the main description of this invention is focused on water, wastewater and gas treatment, the same concepts and techniques may be more generally applied to separate compounds or particles or molecules from any kind of fluid, no matter the viscosity of the fluid and its physicochemical properties. Just for example the filtering element can be used to remove contaminants or toxic substances from oil, to extract sugar from juice fruits or for example to remove additives or other chemical species in the synthesis process of pharmaceutical products, biological processes, lubricants, glucose from bloodstream or other applications. The target fluid of application of the filtering element of the present invention can also be one among blood, hydrocarbons with or without water content, combustible fuels and any kind of fluid in gas phase. For water, drinking water and wastewater treatment applications, the contaminants that can be removed are non-polar compounds, organochlorine compound, heavy metals, hazardous isotopes like cesium-137, strontium-90, technetium-99 and iodine-131, emergent contaminants, like for example antibiotics, endocrinal disruptors compounds, steroids and hormones, flame retardants, pesticides, chelating agents, fluorinated compounds, disinfection by-products and antimicrobials. In gas phase, the filtering element of the present invention can remove with efficiency higher than 99, 9% sulphur compounds, like for example hydrogen sulfide, mercaptans and siloxanes, but also hydrocarbons and chlorinated solvent, dioxins and furanes, styrene, vapors of different solvents and many other recalcitrant compounds.

The filtering element of the present invention can be prepared with a simple physical mixing process of the mesoporous graphene compound with the spacing material. In particular, the spacing material is mixed with the mesoporous graphene compound. The mixing process can be performed by any kind of standard mixing machine but preferably a ploughshare mixer or a blender mixer. Other methods can be used such as vacuum mixing processes, vacuum filtration or wet deposition or other processes for the combination of the mesoporous graphene compound with the spacing material.

In another embodiment of the present invention the mesoporous graphene compound and the spacing material are mixed and homogeneously dispersed in liquid phase. The use of liquids can be useful in case it is appropriate to disperse nanoparticles of other compounds inside the graphene sheets, as for example silver nanoparticles, titanium dioxide, iron oxides nanoparticles. The filtering element of the invention works in terms of sorption (absorption and adsorption) in macro, meso and micropores but also like an ideal size-exclusion filtration system thanks to the membrane effect. In case contaminants are mainly removed by the membrane separation process it is possible to optimize the efficiency of the process using mesoporous graphene compounds with an oxygen content lower than 2% in weight, creating a structure with more hydrophobic channels. It is well known in fact that the presence of oxygen confers hydrophilic properties to the filtering element, while the n-bond aromatic rings of its structure make it hydrophobic conversely. With less oxygen content the membrane structure will be more stable increasing the n-n interactions between the graphene sheets.

By adjusting the C/O ratio content of the mesoporous graphene compound it is possible to change its hydrophilicity and hydrophobicity . Therefore, in terms of membrane selectivity, it is preferable to use a mesoporous graphene compound with a C/O ratio at least above 40.

The filtration process is based on the movement of molecules between the stacked graphene sheets, where graphene sheets are blocked in a monolithic compact structure thanks to the presence of the spacing material. The global distribution of the graphene sheets in the filtering element is characterized by a high disorder that assure a continuous and regular flow. The particular geometrical distribution created by the graphene sheets in the filtering element of the invention improves the capillary flow inside the filtering element, optimizing the adsorption process inside the mesopores.

The flow rate is fast into the filtering element, due to capillary-like high pressure created in mesopores and into nanochannels , and works at relatively low pressure, while producing high-quality effluent.

The weight ratio between mesoporous graphene compounds and spacing material depends from the average dimension of spacing material and its density. As a general consideration, it is important to take into account that the efficiency of the filtering element in terms of contaminants removal or molecular separation is attributable to the presence of mesoporous graphene compound and only in part to the spacing material. The spacing material has the principal role of creating a physical support for the graphene sheets to assure a regular flux without creating wall effect through the graphene sheets. For this reason the right proportion between the mesoporous graphene compound and the spacing material is the one that assure a regular flow without wall effect and undesired channelings.

For example, in case the spacing material is an activated carbon with an average particle size of 1 mm the percentage by weight of mesoporous graphene compound into the final mix can be in the range from 10 to 80% by weight on the total weight of the filtering element and preferably in the range from 30 to 50%. As a general rule, if the average particle size of the spacing material increases, the mesoporous graphene compound content in the mix can increase. For this reason, it is better to use the spacing material mainly in granular form, but also, where possible, to add the spacing material in form of fine powder to create a more uniform structure .

In general terms, the granules are enough to create a compact and stable structure and the presence of the spacing material in fine powder can be optional. If present it can be usually limited to a 1-50% by weight of the weight of the spacing material in form of granules. A higher concentration of fine powder can create a higher pressure drop.

In one embodiment of the present invention, the fine powder can be the same type of the spacing material of the granules or a different one. In the first case it is possible to create the powder directly from the granules during the mixing process, using a ploughshare mixer with high velocity or as well using a mortar. The filtering element can be of any dimension and geometrical shape. The thickness of the filtering element can be of any size but it is suggested to have a thickness of not less than 1000 nm and more preferably in the range 10÷200 mm. The minimum thickness cannot be less of the average particle size of the powder or granule of the spacing material added into the blend. That is, the minimum thickness is the thickness that allow to have at least one layer of the blend mesoporous graphene compound/spacing material and so far it depends from the dimension of the particles of the spacing material .

In terms of maximum thickness, it is important to take into account that in case the membrane filtration process is relevant in the application, there will be an important phenomena obstruction of the nanochannels in the first millimeters of the filtering element and, as a consequence, an increase of the pressure drop.

To overcome this physical barrier, the pressure of the inlet fluid will increase and, as a result, also the chance of formation of channelings inside the bed will increase, decreasing the efficiency of the filtering element.

In terms of sorption (adsorption in meso and micropores and absorption in macropores) several sorption phenomena can contribute, depending on the type of mesoporous graphene compound used to prepare the filtering element: electrostatic interaction, hydrogen bonding, hydrophobic effect, n-n bonding and covalent bonding. Furthermore, the presence or absence of functional groups in the graphene structure will determine the mechanism of the adsorption process. Electrostatic interactions and hydrogen bonding are the predominant adsorption mechanisms when it is relevant the presence of functional groups. In case of graphene materials with low content of functional groups, like for example RGO, van der Waals interactions between aromatic rings of both adsorbate and adsorbent are the main adsorption mechanisms. Hydrophobic effect, predominant in graphene materials with low oxygen content, is effective for the removal of hydrophobic organic compounds, non-polar hydrocarbons .

In sorbent materials of the prior art working in the range of micropores and ultra-micropores , like for example activated carbons, the molecular sieving effect may occur, due to the shape of pores that does not allow the molecules to penetrate into micropores or because the pore width is narrower than the molecular size of the adsorbate. Also in other sorbent with the presence of some functional group in the microporosity it can be experienced the pore blocking effect that forces the contaminant molecule to be adsorbed only in pores with size greater than the size of the molecule. It is clear that this kind of molecules, like for example dyes, cannot be adsorbed.

On the contrary, the filtering element of the present invention can separate large molecules by adsorption in mesopores and by membrane filtration.

The membrane filtration phenomena is of particular interest for the contaminants that are usually not removed, or are removed with low efficiency, from sorption processes in macro and mesopores. MTBE (methyl tert-butyl ether) with a molecular size around 0.65 nm, can be removed completely by the filtering element of the present invention. MTBE is a very soluble contaminant, used in gasoline at low levels to replace lead as an octane enhancer causing, in several cases, groundwater contamination. There is a big need of new effective materials for MTBE filtration since all sorbents available on the market are not effective at low concentrations. Other molecules, not easily removed with known sorption processes and that can be easily separated by the filtering element of the present invention are for example: humic substances, glucose (average size -0.90 nm) , fructose (-0.98), sucrose (-1.06 nm) , phenol (-0.50 nm) , polyethylene glycol (-0.70 nm) , methyl mercaptan (-0.43 nm) , hemoglobin (-3.25 nm) and serum albumin (-3.55 nm) .

Filtration of asbestos fibers during asbestos remediation and the treatment of water contaminated by perfluorinated compounds (PFCs), classified like persistent organic pollutants with a large molecular size are other examples of effective applications of the membrane filtration phenomena of the filtering element of the present invention.

Moreover, the filtering element of the present invention can separate proteins, antibiotics and dyes both by adsorption in mesopores and membrane filtration.

According to a further embodiment of the present invention, the filtering element can be packed in a cartridge.

With reference to Figure 5 the cartridge comprises a filtering chamber 4 and at least one pair of openings 6 for the inlet and outlet respectively of the fluid to be filtered into/out of the filtering chamber 4. The filtering chamber 4 and the openings 6 are in fluidic connection with each other. The filtering element is arranged in the filtering chamber 4.

The cartridge can be also designed to work with a radial flux. Moreover, it is also possible to design filtering systems comprising two or more filtering elements that work in series or in parallel. Yet in another aspect of the present invention the filtering element can be used for in-situ groundwater remediation permeable reactive barriers in place of iron zero valent, zeolites, activated carbons or other materials.

The filtering element can be filled into the continuous permeable barrier or into the reactor gate just by gravity and then compacted. The main advantages to use the filtering element of the present invention for this application are the higher efficiency in terms of contaminants removal and the possibility of tailoring the global porosity of the filter bed just changing the ratio mesoporous graphene compound/spacing material and its density. In this way it is possible to create the desired filtering speed in the filtering element without creating any modification to hydrodynamic conditions of the aquifer, a typical problem in this kind of in-situ applications.

Conversely, it is possible to increase the filtering speed inside the filtering element and therefore creating a booster zone without the need of the funnel barriers typically used in a "funnel and gate" configuration.

For this application, the spacing material in form of fine powder or granular powder can be iron zero valent.

In another embodiment of the present invention, it is possible to use the filtering element of the present invention to produce cartridges for filtration masks with interesting and efficient applications in particular for most recalcitrant compounds like radioactive substances, chlorinated solvents, phosgene, other organic compounds and particulates. Air pollution is one of most critical problems in several countries for adverse health effect and there is a high need of new products and solutions to create protection devices. The membrane filtration effect of the filtering element of the present invention can be used to remove all particulates, PM1 included.

Other applications of the filtering element of the present invention are: purification of nucleic acids (DNA, RNA) , removal of microorganisms from liquids with particular interest to processes of sterilization and to prevent biodeterioration of fuels, removal of water from fuels based on the membrane filtration effect.

The invention will be now described with reference to the following specific examples, it being understood that these examples are given for the purpose of illustration only and are not to be taken as in any way limiting the invention beyond the scope of the appended claims.

EXAMPLE 1

PREPARATION OF THE FILTERING ELEMENT AND EVALUATION OF ITS EFFICIENCY

Graphene flakes (GF) , characterized by large flakes size (lateral size ~ 15÷25 μm) , a C/O around 140, a bulk density around 0,036 g-cnr³ and a large pore size distribution with a wide range of meso and macropores was purchased by the company NANOSME SE . The pore size distribution of graphene flakes is reported in Figure 2. In this case the surface area contribution of the mesopores is of 85, 25% and 6,53% for the macropores.

Water washed granular activated carbon (GAC) with an average particle size around 1÷2 mm, an bulk density of 0,51 g- and an ash content less than 5% was purchased by Shanxi Xinhua Chemical Co. Ltd..

50 grams of GAC were first crushed into a fine powder (with an average diameter between 0.05 and 0.1 mm) using a manual laboratory mortar

Then 70 grams of GF, 50 grams of the fine powder of GAC and 160 grams of granular GAC were mixed together in a ploughshare mixer for about 8 minutes to create a uniform mixture (GCMIX1) . A cartridge with height of 300 mm and a radius of 20 mm was filled with 280 grams of GCMIX1 and pressed with a hydraulic press to have a final density inside the filtration chamber of 0.74 g-cnr³.

30 liters of distilled water was contaminated with these chemicals: methyl tert-butyl ether, iodine, arsenic, cadmium and toluene. The solution was flushed through the cartridge with a flow rate of 30 ml-min^-1. After 3 liters of water passed through the cartridge, pressure was stabilized around 2,1 bar and a sample was taken from the outlet of the cartridge and analyzed by an external certified laboratory. The results are illustrated in Table 1. Table 1

EXAMPLE 2

COMPARISON OF DIFFERENT FILTRATION CHAMBERS WITH DIFFERENT GEOMETRY

The filtration test of example 1 was repeated but with a cartridge of different dimensions: 20 mm height and 7 . 75 of radius (same filtration volume) and with the same contact time. The results are illustrated in table 2 . TABLE 2

The efficiency of the two cartridges is more or less the same but with the advantage that in this second case the same flow rate, with the same quantity of GCMIX1 requires a lower pressure, around 0.55 bar.

EXAMPLE 3

EFFICIENCY OF FILTRATION WITH DIFFERENT DENSITY OF THE FILTERING ELEMENT

GF and GAC of example 1 were used to prepare different filtering elements, changing the GAC/GF weight ratio and also the density of the mixture (different compression rates) inside the filtering chamber like the one illustrated in example 2. In all the prepared mixtures, GAC was added like a mix of granular and fine powder with the same dimensions reported in Example 1. The fine powder was about 33% in weight of the granules. A sample of 120 liters of distilled water was contaminated with MTBE, phenol, boron, chloroform and sodium.

GAC/GF weight proportion and density obtained inside the cartridge are illustrated in table 3. The results of the filtration tests are illustrated in table 4 .

Table 3

Table 4

Based on the results of table 4, it is possible to draw the following conclusions. The cartridge filled with GCMIX-D is the one that shows the best results removing completely all contaminants with an efficiency over 99, 99% except for sodium. In fact, MTBE, phenol and boron are removed completely by the membrane filtration process; they are blocked inside the nanochannels created by the stacked graphene sheet. Sodium is not removed by this phenomena as sodium ions are too small (~ 0,19 nm) . Chloroform is removed by absorption into macropores.

The removal of phenol, boron and MTBE in GCMIX-A and GCMIX- B is lower than GCMIX-D because there are less graphene sheets. The efficiency of the cartridge filled with GCMIX- C is good but lower than GCMIX-D because the material is less compressed and therefore there is lower concentration of nanochannels. The filter GF-B is filled only with GF but with no compression, so in this case the contaminants are removed only by the sorption processes (absorption and adsorption) and in fact the behavior is quite similar to the results illustrated for the test in batch mode (GF-BATCH, graphene sheets in suspension) .

EXAMPLE 4

REMOVAL OF PAHs FROM POMACE OIL

Polycyclic aromatic hydrocarbons (PAHs) are aromatic compounds containing from two to eight conjugated ring systems that have various toxicological effects. Few of them are carcinogenic, like for example benzo (a) pyrene .

There is a particular attention in food industry for these compounds because they can also be produced by processing some oils of plant origin and in particular olive pomace oil.

A sample of pomace oil from Italy was tested with a cartridge filled with GCMIX-C of example 3. Table 5 illustrates the results of the filtration test, wherein it is clear that all PAHs have been removed with a very high efficiency. The pomace oil was filtered as received, without any further modification and without heating, despite the high viscosity. Probably in this case an important role is played by the high concentration of macropores inside the graphene material used that facilitates the transport inside the mesopores and furthermore improves the capillary flow.

Table 5

EXAMPLE 5

LANDFILL GAS TREATMENT FOR ENERGY RECOVERY APPLICATIONS Siloxanes are silicon-containing organic volatile compounds that are found quite often into waste landfill sites and in digesters, due to their presence in many domestic products, like for example deodorants. They migrate out with the landfill gas and when they burn into dedicated energy recovery systems they produce a fine, solid crystalline silica that sticks on the internal combustion system and can severely increase the cost of maintenance of the internal combustion equipment.

A cartridge was filled with a mix prepared starting from the same graphene compound of example 1 and a GAC with mesh size of 12x40 (0.42 to 1.70mm) .

The cartridge has been filled with a mixture of GAC and GF with a weight ratio GAC/GF = 0,5 and a final density of 0.68 g-cnr³. In case of gas filtration the phenomena of wall effect inside the filtering element is much more limited and for this reason is possible to increase the content of GF thus increasing the efficiency.

The inlet of the cartridge has been connected to a derivation pipe of an HDPE pipeline of landfill gas extracted in a waste disposal site in north of Italy. The flow rate of the landfill gas was around 80 liters -hour^-1. The gas collected at the outlet of the filter was sampled in a Tedlar gas sampling bag and sent to a certified laboratory for analysis. Although the purpose of this test was mainly to evaluate the filtration efficiency of the filtering element of the present invention in terms of siloxane removal, also other contaminant were examined. The results of the filtration test are listed in table 6.

The results of table 6 show that the filtering element of the present invention can remove completely from the landfill gas not only all siloxanes but also aliphatic chlorinated solvents and organic aromatic solvents.

Siloxanes have quite a large molecular size (as opposed to silanes) and it is well know that they are not easily removed by sorption processes, so it is possible to assume that they are removed by the membrane filtration process in the filtering element. Organic aromatic solvents and chlorinated solvents are removed mainly by sorption process and for the latter probably the main contribution is due to the adsorption into mesopores but also macropores. Carbon dioxide, nitrogen and methane are not adsorbed with standard sorption processes and due to their low molecular size (CO₂ -0,23 nm, CH₄ -0,30 nm and N₂ -0,31 nm) neither they are removed by the membrane effect created by the compressed graphene nanosheets. In case of methane this is an advantage for this specific application because it is the combustible material needed for the landfill gas energy recovery production .

EXAMPLE 6

GAS TREATMENT IN AN INDUSTRIAL FACILITY

Using the same mixture of example 5, a filtering system containing 6 cartridges was prepared in parallel configuration. Each chamber has a diameter of 450 mm and height of 40 mm. The filtering system has been tested to remove acetone and styrene from gas stream produced from an industrial facility working with resins compounds. The results of the filtration test are illustrated in table 7.

EXAMPLE 7

GROUNDWATER REMEDIATION

The filtering element of the present invention has been tested for groundwater remediation of an aquifer contaminated by spills of solvents in a chemical industry (Table 8) and groundwater contaminated by PFAs in a large area as a consequence of the release of industrial activities using perfluorinated (PFC) hydrocarbons (Table 9) . The materials GF and GAC of example 1 were used and mixed together, but in this case the activated carbon was used only in granular form. Fine sand in granular form, with particle dimension in the range 150-250 μm, was added in a percentage of 5% by weight. The weight ratio GAC/GF used was 2,1. The mixed materials were pressed in a cartridge with height 100 mm and diameter 80 mm. The density of the filtering element was 1,15 g-cnr³.

The results reported in table 8 and in table 9 are an evidence of the efficiency of the filtering element of the present invention in the removal of recalcitrant and emergent contaminants also when they are present at very low concentration, a scenario that is critical for all the commercial filter media proposed till today for commercial applications .

Claims

1.- A filtering element comprising a blend of mesoporous graphene compound having pores with average pore size between 0,4 and 250 nm and at least one spacing material in granular form having an average particle size between 0,1 and 6 mm wherein the contribution of pores having an average pore size from 2 and 50 nm to the total specific surface area of the mesoporous graphene compound is not less than 50% of the total specific surface area and wherein the minimum density of the filtering element is at least the 15% of the bulk density of the spacing material.

2. - The filtering element according to claim 1, characterized in that the mesoporous graphene compound is selected from the group consisting of non-functionalized or functionalized graphene nanoplatelets (GNP) , non- functionalized or functionalized pristine graphene sheets (PGS), non-functionalized or functionalized graphene flakes (GFL) , non-functionalized or functionalized graphene oxide (GO) , non-functionalized or functionalized reduced graphene oxide (RGO) , non-functionalized or functionalized expanded graphite (EG) , fullerenes, carbon nanotubes and combinations thereof .

3. - The filtering element according to claim 1, characterized in that the spacing material can be selected from the group consisting of sand, activated carbons, zeolites, bentonite, alumina, aluminum silicate, chitosan, clay, bentonite, diatomite, montmorillonite, iron oxides, titanium oxide, quartzite, silver, hematite, iron or a mixture thereof.

4.- The filtering element according to claim 3, characterized in that the spacing material is activated carbon .

5.- The filtering element according to claim 1, characterized in that the contribution of mesopores to the total specific surface area of the mesoporous graphene compound is not less than 70% of the total specific surface area .

6. - The filtering element according to claim 1, characterized in that the mesoporous graphene compound has graphene sheets with an average lateral size of from 3 nm to 40 μm.

7. - The filtering element according to claim 6, characterized in that the mesoporous graphene compound has graphene sheets with an average lateral size of from 5 to 20 μm.

8. - The filtering element according to claim 1, characterized in that it further comprises a spacing material in the form of fine powder having an average particle size of between 5 nm and 90 μm.

9. - The filtering element according to claim 8, characterized in that said spacing material in the form of fine powder is present in an amount of 1 to 50% by weight of the weight of the spacing material in form of granules.

10.- Method for the removal or separation of one or more contaminants from a fluid comprising the step of contacting a filtering element according to any of claims 1 to 9 with said fluid.

11. - Method according to claim 10, characterized in that said fluid is selected from the group consisting of water, wastewater, gases, blood, oils, fruit juice, hydrocarbons with or without water content and combustible fuels .

12. - Method for in-situ groundwater remediation comprising the step of contacting the groundwater with a permeable reactive barrier comprising the filtering element according to any of claims 1 to 9.

13. - Method for in-situ groundwater remediation according to claim 12, characterized in that the spacing material is iron zero valent.

14. - Filtration mask comprising a filtering element according to any claims 1 to 9.

15. - Cartridge for the removal or separation of one or more contaminants from a fluid comprising a filtering chamber

(104) and at least one pair of openings (106) for the inlet and outlet respectively of the fluid to be filtered into/out of the filtering chamber (104), the filtering chamber (104) and the openings (106) being in fluidic connection with each other and the filtering chamber (104) containing a filtering element according to any of claims 1 to 9.