EP2035111A2

EP2035111A2 - Metal or metalized fabrics coated with nanostructured titanium dioxide

Info

Publication number: EP2035111A2
Application number: EP07805657A
Authority: EP
Inventors: Massimilano Bestetti; Maria Francesca Brunella; Silvia Franz
Original assignee: Politecnico di Milano
Current assignee: Politecnico di Milano
Priority date: 2006-06-26
Filing date: 2007-06-22
Publication date: 2009-03-18
Also published as: WO2008001405A2; ITMI20061230A1; WO2008001405A3; WO2008001405A8

Abstract

A filtration device comprising at least a metallic or metallized fabric at least partially coated with nanostructured titania in which at least 10% of said titania is anatase, is provided. Owing to titania photocatalytic activity, this device finds application in the decontamination of fluids polluted with organic or inorganic substances. The advantage resulting from the use of such fabrics coated according to the invention is the large active surface area in contact with the fluids to be decontaminated together with the photocatalytic activity of the titania coating, as well as the possibility to suitably reactivate such devices.

Description

"Metal or metallized fabrics coated with nanostructured titanium dioxide"

The present invention relates to a filtration device having photoelectrochemical activity for the decontamination of polluted fluids. Specifically, the invention relates to metallic or metallized fabric suitably coated with nanostructured titania acting as photocatalyst.

The purification of fluids and effluents, mainly industrially generated, from both organic and inorganic contaminants, represents a field of increasing interest for the environmental protection. hi order to remove from water suspended impurities, more or less fine sand or other similar corpuscles, mechanical filters having various morphologies are typically used. For example, cartridge filters in which the filter element has a cylindrical shape and multiple holes, having a size varying according to the required filtration threshold, or sand filters, in which a reservoir contains various quartzite layers having a differentiated particle size and water penetrates the filter bed from top to bottom, resulting in the impurities to deposit on the various quartzite layers, are known. In this second case, the filtration process is similar to the natural one where rain water penetrates the various soil layers to reach the subterranean bed, but the resistance to the penetration of water (pressure loss) gradually increases making the system poorly effective.

Researches for the development of environmentally-compatible processes and technologies leading to the identification of materials active towards harmful organic and inorganic substances and micro-organisms, such as viruses, bacteria, fungi and yeasts, are also known. With reference to such active materials, the filtration on activated carbon, mainly used in the processing of process fluids in some food industries, is known, as it allows, owing to the microscopic extension of the carbon micro-holes, the removal of unpleasant smells and tastes, due to the presence of organic substances (adsorption action). However, although the impurities accumulated during filtration can generally be removed by backwashing the activated carbon, those substances retained, especially by adsorption, are unlikely to be removed. Usually, furthermore, backwashing is carried out only sporadically to prevent the remixing of the bed and the decrease in the absorption and adsorption power. The activated carbon bed can in fact saturate due to the absorption of what wasn't the primary objective of the treatment and, even worse, release upon exiting part of what had been retained, with a concentration higher than the one upon entering. These kinds of active materials are therefore subject to "depletion" with time and poorly reusable.

In the light of what evidenced above, the problem of performing a water purification such as to effectively remove pollutants and contaminants, having either bacterial or viral nature, present therein, has still to be solved. For the removal of polluting compounds, the photocatalysis reaction is known and exploited, which is promoted by a suitable catalyst and is capable of increasing the rate of a chemical reaction by the action of light. Specifically, by exploiting the light energy, the photocatalysts lead to the formation of highly oxidating reagents, which are capable of breaking down organic and inorganic polluting substances present in the environment, first preventing them from being accumulated. It is in fact known that the catalyst exposure to light generates, at the atomic level, excited electronic states from which chain processes such as oxidoreductive reactions and molecular transformations start.

To this purpose, it is known the dispersion of titania powders as photocatalyst agent in contaminated aqueous solutions. However, the use of dispersed particles provides the evident and substantial disadvantage of having to completely remove them once the treatment is terminated. Furthermore, said titania particles dispersed in solution can lead to aggregates, thereby reducing the effectiveness of the treatment.

U.S. Patent No. 7.011.737 discloses an electrical resistive device for use as sensor for hydrogen gas. Specifically, this device comprises a substrate of insulating material upon which a first layer of titanium and a second layer are placed, the latter leaving portions of said first layer uncovered, and titania nanotubes are grown by anodization thereupon. According to the patent, such electrical device, if in the presence of oxygen, can also be used to photocatalytically remove one or more contaminants from the nanotubes, thus providing the electrical resistive device with "self-cleaning" capability. The inventors of the present invention, being aware of the photocatalytic properties of titanium nanotubular substrates, have surprisingly produced a filtration device capable of purifying/decontaminating waters by employing photocatalysis principles. An object of the present invention is therefore to provide a filtration device capable of efficiently removing polluting substances present in contaminated fluids and waters.

A further object of the invention is to provide a method for purifying contaminated fluids and waters. The above objects have been achieved through a filtration device as set forth in claim 1. Further advantages and preferred embodiments of the invention are disclosed in the dependent claims.

The filtration device according to the invention comprises at least a metallic or metallized fabric, at least partially coated with nanostructured titania, of which at least 10% is anatase.

Further characteristics and advantages of the invention will become apparent from the detailed description given hereunder with reference to the exemplary embodiments of the invention given by way of examples and not as a limitation thereof, and to the accompanying drawings in which: Figure 1 represents the ratio of the current concentration of methylene blue to the initial concentration C/C_o (hereinafter referred to as relative concentration) according to the UV irradiation time, both of the fabric anodized at 20 V and heat- treated in air at 400°C for 3 hours (-A-), and the untreated fabric (-■-) as in Example 2; Figure 2 shows the relative concentration C/C_o of methylene blue according to the UV irradiation time, both in the presence of the treated fabric of Figure 1 (-A-), and in absence of the fabric itself (-■-) as in Example 2;

Figure 3 shows the variation in the relative concentration C/C_o of methylene blue according to the UV irradiation time, for four subsequent essays (expl, exp2, exp3, exp4) by using repeatedly the same treated fabric of Figure 1, as in Example 3; Figure 4 shows the relative concentration C/C_o of methylene blue according to the UV irradiation time of Example 4;

Figure 5 shows the relative concentration C/Q of methylene blue according to the UV irradiation time of Example 4 and Example 5 as compared;

Figure 6 shows the relative concentration C/C_o of methylene blue according to the UV irradiation time of Example 4 and of Example 6 as compared;

Figure 7 shows the relative concentration C/C_o of methylene blue according to the UV irradiation time of Example 7, for a system with alternatively Wood source and germicidal source;

Figure 8 shows the relative concentration C/C_o of methylene blue according to the UV irradiation time using the germicidal source with and without the treated fabric, as in Example 8;

Figure 9 shows the relative concentration C/C_o of methylene blue according to the UV irradiation time by using two different initial concentrations of methylene blue, as in Example 9. In the present invention, the term "fabric" refers to both non- woven fabric and woven fabric. The term "non- woven fabrics" refers to fabrics made of threads oriented either directionally or at random and led together e.g. by friction and/or cohesion and/or adhesion; in the case of the present invention, they can be non-woven fabrics obtained e.g. by sintering. The term "woven fabrics" refers to fabrics obtained by braiding threads by weaving.

The adjective "metallic" as referred to threads and/or fabrics, means titanium or a titanium alloy threads and/or fabrics.

The adjective "metallized" as referred to threads and/or fabrics, means threads and/or fabrics which can be coated with titanium or a titanium alloy. The term "contaminant" or "pollutant" in the present invention refers to both inorganic substances, such as e.g. nitrogen oxides, and organic substances, such as e.g. hexavalent chromium compounds or organic colorants, and micro-organisms, such as for example viruses, bacteria, fungi and yeasts.

Therefore, the invention concerns a filtration device comprising at least a metallic or metallized fabric at least partially coated with nanostructured titania, in which at least 10% of said titania is anatase. According to the present invention the threads can be braided to form for example:

- a led fabric with square or rectangular openings, in which the warp threads are parallel to the length of the fabric while the weft threads are parallel to the height or width, and they cross with each other alternating one above and one below, thus forming 90° angles between them;

- a led fabric, whose warp and weft threads cross with each other alternatively two above and two below and tend to form angles different from 90°;

- a herringbone fabric, obtained by crossing the warp threads two above and two below, then inverting their crossings at a predetermined distance;

- a rep fabric, which is produced by braiding the warp threads leaving some space between each other and having a thread diameter greater than that of the weft threads which are strongly woven one against the other;

- an inverse rep fabric, which is obtained by reversing the rep fabric's positions with a great number of warp threads as closed as possible and weft threads having a greater diameter and being spaced out;

- a Touraille fabric, which is obtained by braiding the warp threads two above and two below, being spaced out, with a diameter of the warp thread greater than the one of the weft threads which are woven strongly one against the other; - an inverse Touraille fabric having a warping similar to the Touraille with the exception of the arrangement of the spaced-out weft threads, but with a Touraille weave so as to not solicit too much the warp threads.

Next to the most common kinds of weaving indicated hereupon, all the other types of known weaves are also included. According to one embodiment of the invention, metallized fabrics, i.e. fabrics which according to the present invention can conveniently be coated with thin layers of titanium or titanium alloy, are for example stainless steels (Aisi 304, 304L, 316, 316L, 310, 314, 410, 430); special steels (Cromax, Incoloy, Ni-Cr Alloys, Inconel, Nikel, Monel, Duplex); carbon steels; zinc-coated iron; aluminium alloys; brass; bronze; glass fibre.

Preferably in the present invention, metallic threads are used for the formation of fabrics according to the invention.

According to alternative embodiments of the present invention, the fabric can have various shapes, such as e.g. flat, cylindrical, corrugated or pleated.

Titania (TiO₂) is the coating of the metallic or metallized fabric according to the invention.

It is known that titania exists in three different crystalline structures, i.e. rutile, anatase e brookite, and at the amorphous state. Rutile and anatase are the most diffused forms in nature. Rutile is the most thermodynamically stable and the most industrially used crystalline structure, whereas anatase is metastable. According to the present invention, titania is used in a nanostructured form, i.e. having fundamental geometric units characterized by sizes in the order of nanometres (10^"9 m). Such a nanostructured form offers the advantage of allowing the coating, placed on the fabric surface, to have a large surface area in contact with the fluids to be treated, which adds to the large surface area offered by the supporting fabric itself, on which TiO₂ is placed.

In order to achieve the objects of the invention, nanostructured titania is at least 10% in the form of anatase. Below such value in fact, the titania photocatalytic activity during filtration is to be considered negligible for the present invention's purpose of decontaminating fluids. Above 10% anatase, on the contrary, a photocatalytic activity sufficient to solve the technical problem has been observed, and it has also been observed that the higher said percentage of anatase the greater the effectiveness of the filtration device according to the invention. Preferably, therefore, nanostructured titania is 50% in the form of anatase. More preferably, the nanostructured titania coating is at least 90% in the form of anatase, and most preferably it is essentially anatase. In fact, it has been observed that the photoelectrochemical action of the crystalline anatase-type titania is more effective than the one of the rutile/anatase crystalline mixtures, and it is notably more effective than the one of the amorphous titania, therefore, in the presence of a anatase percentage as high as possible, the photocatalytic activity of titania during filtration is to be considered optimal for the present invention' s purpose of decontaminating fluids.

Without wishing to be bound by any theory, it is believed that titania shows a filled valence band and an empty conduction band, the difference between which is called energy gap, which is the minimal energy required to make the material conductive; in particular, this gap is about 3.10 eV for rutile and about 3.23 eV for anatase. In the case where the promotion of the electron to the conduction band occurs by absorption of a light photon, i.e. by photo-excitation, the electron leaves a void on the valence band and 'photo-excited electron- void' pairs generate in the material. Said pairs are responsible for the photocatalytic activity of titania, since they are located on the surface and can react with the molecules of water absorbed on it to form highly reactive hydroxyl radicals. Both electronic voids and hydroxyl radicals are highly oxidating and as such they can thus be used to oxidate most of the contaminants present in water.

According to a preferred embodiment of the invention, the fabric is titanium woven fabric at least partially coated with nanostructured titania in a nanotubular form.

The adjective "nanotubular" and/or the term "nanotubule" refer to structures in the form of little tubes or tubules, i.e. hollow cylindrical structures, having sizes in the order of nanometres. In particular, according to the present invention, the TiO₂ nanotubule pore diameter shows to be in the range of tens to hundreds nanometres, whereas the thickness of the nanotubule walls is in the range of about 10 to about 100 nanometres. According to the present invention, nanotubules are preferred, as this form provides an advantageously much larger active surface area in contact with the fluid to be decontaminated, thus contributing to the increase of the photocatalytic activity of the fabric coating.

Advantageously according to an alternative embodiment, metallic or metallized woven fabrics can be also made with combinations of threads of different nature, i.e. alternating for example metallic or metallized threads, and threads made of non- electricity-conductive materials, such as for example polymeric and/or glassy materials. This allows to make fabrics with at least two electrically unconnected zones. This combination allows to produce structured filtrating devices made of regions electrically unconnected but structurally belonging to the same fabric fragment. Such configuration allows to polarize the fabric during operation without needing an additional electronic element.

The titania-coated fabric can be used to degrade pollutants under irradiation with radiations having a spectral intensity ranging from the visible to the ultraviolet field. The photocatalytic action can be modified if the fabric is electrically polarized, via an external circuit, for example a current generator. In this case, the degradation action is photoelectrochemical. The external polarization circuit provides or removes electrons from the fabric coated according to the invention, modifying the behaviour of the oxidoreductive reactions occurring on the titania surface.

The at least partially titania-coated metallic fabric can be woven so as to have sectors of the same fabric which are electrically isolated, even though constitutionally belonging to the same fabric. The electrically separated zones of the same fabric, woven in this way, can be polarized, via an external current generator, having an opposite polarity and adjustable intensities. So the same fabric can act as poli-electrode system, i.e. as an electrochemical cell, and in the presence of visible UV-irradiation as photoelectrochemical cell.

According to still another embodiment of the invention, the nanostructured titania coating which is at least 10% anatase, is added with other nanostructured materials improving the photocatalytic activity of the filtration device of the invention. "Other nanostructured materials" means titanium oxides, titanium carbides, titanium nitrides, titanium oxynitrides, transition metals, such as for example gold, copper, silver particles, or carbon nanotubules.

Titanium oxides, titanium carbides, titanium nitrides, but preferably titanium oxynitrides having the general formula Tiθ_2-xN_y, can be deposited for example with the magnetron sputtering PVD technique on metallic or metallized fabric. The final fabric coated at least partially with preferably nanostructured titanium oxynitrides, in which at least 10% is anatase, shows a photocatalytic activity advantageously improved in the visible spectral region.

As for transition metals, without wishing to be bound by any specific theory, it is believed that transition metal nanoparticles reduce the probability of recombination of the electron-gap pairs due to the fact that the gaps remain on the semiconductor surface (TiO₂) and the electrons accumulate on the transition metal nanoparticles. This allows to advantageously increase the photocatalytic activity of the nanostructured ΗO₂ of which at least 10% is anatase. hi particular, the photocatalytic effectiveness of titania, obtained by anodic oxidation of titanium or its alloys, for example with aluminium and vanadium, can be improved by depositing transition metal nanoparticles, e.g. noble metals, on the surface of the nanostructured titania film. The probability of recombination of the electron-gap pairs is thereby reduced, and the photocatalytic effectiveness is thus increased. The deposition of noble metal nanoparticles, e.g. either gold or silver or copper, on the nanostructured titania surface by anodic oxidation improves the catalytic effectiveness.

In one embodiment of the invention, gold nanoparticles are used, and their deposition on titania can be achieved by chemical reduction of gold salts dissolved in solution. The reduction action can be carried out chemically, i.e. with an agent which, by oxidating, reduces to metallic gold, or photochemically, through the UV-vis irradiation action generating gaps-electrons pairs hi titania. The resulting electrons reduce cationic gold to metallic gold. Either by controlling the concentration of the gold salt and reducing agent, e.g. sodium borohydride, in the solution in which the fabric at least partially coated with nanostructured titania which is at least 10% anatase is also immersed, or by controlling the intensity of the employed irradiation, it is possible to control the sizes and the amounts of the metallic gold particles depositing on the nanostructured titania which is at least 10% anatase. Alternatively, the particles can be deposited by cathodically polarizing the fabric in a solution containing dissolved gold salts.

The action of degradating the pollutants by nanostructured titania which is at least 10% anatase, in the presence of UV-VIS irradiation, can be made more effective by supplementing the titania with carbon nanotubes (CNT). Therefore, the titania present on the fabric surface can be supplemented with carbon nanotubes, e.g. grown by chemical vapour deposition (CVD) technique using, for example, acetylene as precursor gas of the carbon deposition. Furthermore, according to a still another embodiment of the invention, it is possible to combine in the same filtration device two or more fabrics of the invention, of equal or different kind, having e.g. equal or different mesh, where "mesh" of the fabric means each passing opening delimited by at least three metallic or metallized threads in contact with each other. Preferably, such fabrics have different mesh in a descending order considering the flow direction of the fluid to be decontaminated, so as to progressively and advantageously increase both the device filtration capability and the active surface thereof in contact with the fluid.

According to another aspect, the invention relates to a process for preparing the filtration device described above comprising the steps of: a) providing at least a metallic or metallized fabric; b) forming nanostructured titania on the metallic or metallized fabric of step a), so that at least 10% of said nanostructured titania is nanostructured anatase.

As for step a), a fabric, e.g. a woven fabric of the kind abovementioned, is provided according to the present invention by suitably braiding metallic or metallized threads. According to a preferred embodiment, the threads are metallic threads, preferably titanium wires. More preferably, a led woven titanium fabric with square openings is provided.

According to another embodiment of the invention, the threads are metallized threads, i.e. made of such a material as for example steel or aluminium alloys or glass fibre, as described above, which are at least partially coated with a layer of titanium and then braided to form a metallized woven fabric. Such coating can be made via e.g. physical vapour deposition (PVD) techniques on each filament which is then woven.

In order to achieve the objects of the present invention, to have the nanostructured titania on the metallic or metallized fabric in step b), it has been chosen to form titania by anodization, also referred to as anodic oxidation. Specifically, titania anodization is carried out in an electrolytic cell, in which the fabric is anodically polarized in suitable electrolytic solutions. According to the invention, the anodization is performed by applying a cell tension in the range of 1 to 50 V, for a time ranging from tens of minutes to tens of hours, in a pH range preferably poorly acid, such as to lead to the rapid growth of a nanostructured titania layer on the fabric surface. Without wishing to be bound by any theory it is in fact believed that, when an electric current is circulated through an electrolytic cell in which titanium acts as anode, anions O^2" migrate toward the anode, to which they transfer the electric charges, they are carrying, and deposit thereon, while combining with the titanium itself.

In a preferred embodiment of the invention, the anodization of the titanium fabric is performed at room temperature, by setting a cell tension ranging from 1 to 30 V. Advantageously, in fact, in said preferred embodiment, the nanostructured titania is obtained in an amorphous nanotubular form, being nanotubules preferred, since, as abovementioned, such a form increases the active surface area in contact with the fluid to be decontaminated. The properties of the oxide layer also depend on the composition of the electrolytic solution in the cell and the operating conditions. In order to obtain nanostructured TiO₂ according to the invention, different electrolytic solutions can be employed for the anodic oxidation of titanium.

Preferably, electrolytic solutions containing mineral acids such as, for example, sulphuric and hydrofluoric acid, or neutral solutions of sulphate salts and fluoride salts, at temperatures in the range of 20 to 40°C, are used. More preferably according to the present invention, aqueous solutions of sulphuric acid (H₂SO₄, 0.1 - 1.5 mol/1, preferably 1 mol/1) with varying amounts of hydrofluoric acid (HF in the range of 0.01% and 2% in weight, preferably 0.05% to 1%), are used. Still more preferably, aqueous solutions of sulphate salts, preferably Na₂SO₄, for example about 1 mol/1, and fluoride salts, preferably NaF, for example 0.01 - 2% in weight, are used.

In the preferred and advantageous embodiment in step b), the fabric is immersed in the Na₂SO₄/NaF-containing electrolytic solution and connected with the positive pole of a direct current generator while the negative pole is connected to a counterelectrode. The metal to be oxided, i.e. titanium, thus acts as anode. The resulting nanostructured TiO₂ coating, whose thickness can range from a few nanometres to tens of micrometers, has good toughness, adherence, compactness and abrasion resistance characteristics.

Through an appropriate selection of the electrolytic solution and process parameters, such as for example the applied electrolytic cell voltage and the immersion time in the electrolytic solution, TiO₂ coatings having the desired characteristics can thus be obtained. For example, in 1 mol/1 sulphuric acid containing 0.15% in weight hydrofluoric acid, thin and dense coatings can be obtained by setting the applied potentials higher than 30 V, or thick and porous by setting potentials lower than 30 V, as broader explained hereinafter; the porosity level especially affects the abrasion resistance of the final filtration device.

Furthermore, advantageously the cell voltage has been observed to be the parameter which most affects the average diameter of the nanotubule pores. In fact, the pore average diameter proves to increase as the cell voltage increases. For example, in 1 mol/1 sulphuric acid containing 0.15% in weight hydrofluoric acid, the pore average diameter increases from 25 nm to 100 nm as the applied voltage increases from 10 V to 25 VrTherefόre, in a still more preferred embodiment of the present invention, the titanium fabric anodization is carried out in 1 mol/1 sulphuric acid and 0.15% in weight hydrofluoric acid at room temperature, setting the cell voltage of about 20 V. Nanostructured titania in a nanotubular substantially amorphous form is thereby obtained, having a tubule average diameter of about 100 nanometres and a tubule average thickness of about 20 nanometres.

Step b) of the process according to the invention includes a subsequent nanostructured titania conversion treatment such that at least 10% of nanostructured titania is in the form of anatase. Said conversion is performed according to the invention by heat treatment at temperatures in the range of 200 to 600⁰C for a time in the range of about 10 minutes to about 10 hours.

In a preferred embodiment, the heat treatment following the low-voltage anodization according to the invention allows to advantageously convert amorphous titania nanotubules into anatase/rutile nanotubules, in which the percentage of anatase, compared to the rutile one, decreases as the heat-treatment temperature increases and increases as the heat-treatment time increases. In a preferred embodiment, therefore, the fabric treated by anodization in 1 mol/1 sulphuric acid and 0.15% in weight hydrofluoric acid at room temperature, setting a cell voltage of about 20 V, is then heat-treated in air in a furnace at temperatures around 400°C for about 3 hours. A titanium fabric coated with nanotubular titania which is at least 10% in the form of anatase is thereby obtained.

Analogously and alternatively, the heat treatment can be carried out in controlled atmospheres, e.g. argon, nitrogen or oxygen.

In one embodiment, the amount of titania can be increased by vacuum deposition (PVD) of titania.

According to another preferred embodiment, the process for preparing the filtration device of the invention further comprises a step of adding other nanostructured materials on the nanostructured titania surface.

Such other nanostructured materials can be, as described above, titanium oxides, titanium carbides, titanium nitrides, titanium oxynitrides, transition metals, e.g. gold, copper or silver particles, or carbon tubules.

In case of oxides, carbides, nitrides and oxynitrides, a vapour deposition is performed in which titania and the mixed oxides, carbides, nitrides, and the like are deposited by physical vapour deposition (PVD) techniques, e.g. such as reactive magnetron sputtering, in which a titanium source (target) and a gaseous mixture made of argon (sputtering gas) and of the reactive gas (oxygen, nitrogen, acetylene, etc.) are used.

In case of transition metal nanoparticles, they can be deposited on the nanostructured titania surface by chemical, photochemical or electrochemical reduction, as described above. The deposition of transition metal nanoparticles, preferably noble metals, e.g. gold and silver, or copper, on the nanostructured titania surface by anodic oxidation improves the photocatalytic effectiveness.

According to another aspect, the invention relates to the reactivation of the nanostructured titania surface, achieved by repeating the same step of the formation of the same dioxide, i.e. by anodic oxidation and optionally subsequent heat treatment.

According to a still another aspect, the present invention relates to a method of purifying contaminated fluids and waters by using the filtration device of the invention.

The method for purifying contaminated fluids and waters according to the invention comprises the steps of: i. placing at least one filtration device according to the present invention along the flow direction of the fluid to be decontaminated, said device being oriented so that the fluid passing through said device is forced; ii. irradiating the device with a radiation having a spectral intensity ranging from the visible to the ultraviolet field; • iii. having the fluid to be decontaminated flow through the irradiated filtration device.

Step i. of the method according to the invention thus implies that the device is placed such as to have the flow of the fluids to be treated through it, so as to make sure both the filtration and the contact between the fluid and the titania active surface. The one or more fabrics of the device of the invention will conveniently and advantageously be in the forms and arranged according to the characteristics of the fluid to be treated. In fact, in case of a fluid containing an amount of corpuscles greater than the contaminants dissolved therein, a substantially flat form of the at least one fabric could be convenient, for example substantially arranged perpendicularly to the flow direction, thus facilitating the mechanical filtration; on the contrary, in case of an amount of contaminants greater than the corpuscles, a form e.g. corrugated or pleated of the at least one fabric could be convenient, for example inclined considering the flow direction, so as to increase the active surface in contact with the fluid itself.

Step ii. implies irradiation with a radiation having a spectral intensity ranging from the visible to the ultraviolet field, preferably in the ultraviolet. Due to this irradiation, oxidoreduction reactions of the chemical species present in the fluid in contact with the fabric take place at the TiO₂ coating of the metallic or metallized fabric. The fabric can have an oxidoreductive catalytic action also by electric polarization of the net. In the present invention in fact, the metallic or metallized- thread fabric can act as electrode and therefore the photochemical action can be controlled by electric polarization of the conductive fabric itself in the fluid to be decontaminated.

In one embodiment of the invention, if the fabric is coated with nanostructured titania supplemented with other nanostructured materials, particularly with titanium oxynitrides, step ii. of the method implies an irradiation with a radiation advantageously having a spectral intensity in the visible field. In step iii., the fluid to be decontaminated is flown through the device according to the invention. According to a preferred embodiment of the method of the present invention, the fluids to be treated are clear so that the irradiation doesn't undergo any shielding by the suspended corpuscles, which would diminish its intensity thus decreasing the photocatalytic activity of the TiO₂-coated fabric. If, however, the fluids to be treated were not clear, it would be possible to facilitate and conveniently imply a mechanical filtration step of the fluids themselves prior to their being decontaminated by the fabric coated according to the invention. Alternatively to the use of a traditional mechanical filter, as an integration of what described for step i. in case of a great amount of corpuscles, the mechanical filtration can conveniently be achieved by combining in the device of the invention two or more fabrics of the same or different kind, having different mesh arranged in a descending order considering the flow direction of the fluid to be decontaminated, so as to progressively and advantageously increase both the device filtration capability and the active surface thereof in contact with the fluid.

According to alternative applications of the present invention, the metallic or metallized fabric can be used in the filtration of polluted atmospheres, for example, in the filtration of air contaminated by the presence of nitrogen oxides (NO e NO₂).

The invention will be now described in detail by way of examples, having an illustrating and not limiting purpose, of the production of the filtration device, preparation process thereof, and method of purifying contaminated fluids using said device.

EXAMPLES

Example 1. Preparation of a metallic woven fabric partially coated with nanostructured TiO₂

Step I: Provision of the led-type woven titanium fabric with square openings.

Step II: Electrochemical treatment of the titanium woven fabric.

The anodization treatment of the titanium fabric included an initial step of pre- treatment of the sample. The titanium fabric having mesh 18, 200 micrometers thread diameter, and 13 x 2.5 cm² in size of Step I was scoured in acetone and ultrasounded for 10 minutes, deoxidized in a concentrated solution of 1 mol/1 H₂SO₄ and 20% in weight HF for 5 seconds and finally rinsed with plenty of distilled water. The sample showed to be of a light grey colour. The same titanium fabric was then immersed for an area of 7 x 2.5 cm² in 100 ml of a 1 mol/1 H₂SO₄ and 0.2% HF in weight solution and the end not immersed in the solution was connected via a terminal to the positive pole of an electric generator. A graphite bar of about 10 x 3 cm² size was partially immersed in the same solution, and its end not immersed in the solution was connected via a terminal to the negative pole of the same electric generator. Through the generator, the fabric was anodically polarized at 15 V, while the graphite bar served as anode. An initial current of about 120 mA was registered, decreasing to 90 mA after about 4 hours from the beginning of the essay. The anodization reaction was carried out at room temperature. The electrochemical anodization treatment lasted 20 hours and 20 minutes. At the end of the electrochemical anodization treatment, a nanostructured titania, amorphous tubular coating was obtained, in a percentage of at least 90%. Step III: Heat treatment of the titanium fabric

The anodized fabric sample as obtained in Step II was subsequently subjected to heat treatment in air at 400°C for 3 hours. The furnace had been pre-heated to the treatment temperature; the sample was then put into it and, after 3 hours, the sample was extracted and cooled at room temperature in air. A woven fabric coated with tubular nanostructured titania of which at least

90% is in the form of anatase was obtained.

Example 2. Effectiveness test of the partially nanostructured-TiO₂-coated metallic woven fabric of Example 1 in the photodegradation of methylene blue

From the sample of the titanium woven fabric of Example 1 , a fragment of 3 x 3 cm² size was cut off. This fragment was placed on the bottom of a 200 ml Pirex® beaker containing 20 ml of a solution of methylene blue 5 x 10^" mol/1. The fabric fragment was therefore at a distance of about 0.5 mm from the solution/air interphase. A 1 cm long magnetic small anchor was inserted in the beaker and the beaker was placed on a magnetic stirrer with which the small anchor was rotated at about 300 rpm. The anodized titanium fabric fragment was then irradiated with UV irradiation via the use of a 250 W UV-A, UV-B and UV-C-emitting lamp at a distance of 20 cm from the fabric itself. The lamp was placed on the Pirex® beaker at a distance of about 10 cm from the methylene blue solution. Then, two specimens of about 4 ml volume were collected, after 30 minutes and after 60 minutes from the beginning of the irradiation, and they were compared with a sample of methylene blue solution extracted from the beaker before the beginning of the irradiation.

The collected specimens show a blue dye of descending intensity as the treatment time of the methylene blue solution increases. The analysis of the concentration variation of methylene blue in the collected specimens was carried out by UV spectroscopy. Figure 1 shows the variation of the ratio of the actual methylene blue concentration (C) to the initial concentration (C₀) according to the duration of the treatment. The results are compared with the ones obtained by using a titanium fabric fragment not subjected to electrochemical anodization treatment. After one hour of irradiation, the ratio C/C_o had decreased from 1 to 0.77 using the same non-anodized fabric and from 1 to 0.13 using the anodized fabric.

In order to differentiate the effect of the sole UV irradiation on the methylene blue photodegradation from the effect of the anodized and irradiated fabric, the photodegradation experiment was repeated in absence of fabric an the relative results were compared with the ones obtained in the presence of the anodized fabric. As shown in Figure 2, the sole UV irradiation leads to a decrease in C/C_o from 1 to 0.77, whereas the presence of the anodized fabric produces a decrease in C/C_o to 0.13. Consequently, the methylene blue degradation detected in Example 1 is ascribable to the degradation action of the UV irradiation and not to the presence of non-anodized titanium fabric. Example 3. Evaluation of the lifetime of the partially nanostructured-

TiO₂-coated metallic woven fabric of Example 1 hi order to evaluate the fabric lifetime, the photodegradation test was repeated 4 times (expl, exp2, exp3, exp4 with a total duration of 4 hours) by repeatedly using the same fabric fragment and recovering the 5 x 10^'5 mol/1 methylene blue solution without washing the fabric fragment between each test. The results obtained are shown in Figure 3. Despite the dispersion of the data of the 4 tests, probably ascribable to non systematic experimental errors in the sampling and analysis process, the repeated use of the same fabric fragment for the degradation of solutions of 500 ml volume of methylene blue of 5 x 10^"5 mol/1 concentration does not cause a decrease in the photocatalytic activity of the fabric itself, hi other words, the same fabric fragment results to be still active and not less effective after being used for 4 hours. The dispersion of points is linked to the experimental uncertainty.

Example 4. Preparation of a partially nanostructured-Ti0₂-coated metallic woven fabric and effectiveness test of the partially nanostructured-Tiθ₂-coated metallic woven fabric in the photodegradation of methylene blue. Step I: Anodization of the titanium woven fabric and subsequent heat treatment

A titanium woven fabric, of the led-fabric type, having a size of 53.25 cm²

(35.5 cm x 1.5 cm) was anodized by setting a voltage of 20 V in a 1 mol/1 sulphuric acid and 0.2% hydrofluoric acid solution for 20 hours at room temperature, by using as cathode a graphite layer having the same size. After the electrochemical treatment, the woven fabric was subjected to heat treatment at 400°C for 3 hours.

A woven fabric coated with tubular nanostructured titania of which at least 75% is in the form of anatase was obtained.

Step II: Photodegradation of methylene blue A photoreactor consisting of 4 Wood lamps (black light), with spectral emission in the range of about 350 to 400 nm, a power of 8 W, mounted on a circular vertical support made of electropolished aluminium, was provided. Thereby, the radiation emitted by the lamps is reflected inside the aluminium tube. A Pirex® glass vial was inserted at the centre of such photoreactor, where a previously prepared methylene blue solution (initially 6.25 x 10^"5 mol/1) of 500 ml volume was circulated. The solution was flown at a constant speed by the use of an adjustable-flow peristaltic pump. In the present example, the pump rate was of 1.25 ml/s. The titanium net anodized and heat treated as described in Step I was then inserted in the vial.

During the essay, solution specimens of a volume of about 5 ml were collected at controlled times. The total duration of the experiment was 74 hours. The specimens collected during the test were analysed by UV spectroscopy, which allowed, upon calibration, to quantitatively determine the actual concentration of methylene blue. Figure 4 shows the relative concentration of methylene blue expressed as the C/C_o ratio, where C is the concentration at the time t and C₀ is the initial concentration, according to the duration of the treatment. After about 10 hours of treatment, the initial methylene blue concentration was observed to decrease by 50%. The C/C_o ratio showed an exponential decrease according to the duration of the treatment.

Example 5. Evaluation of the lifetime of the partially nanostructured- Tiθ₂-coated metallic woven fabric of Example 4

The same woven fabric as Example 4 was used, subjected to no cleaning and contaminant-removal treatment so as to verify the effectiveness of the same fabric by long exposure to the substances to be degradated. A methylene blue solution (C₀ =

6.25 x 10^"5 mol/1) having a volume of 500 ml was flown inside the photodegradation cell described in Example 4.

The total duration of the treatment was 72 hours. As in the preceding Examples, specimens of treated solution were collected at predetermined times, which were and are analysed by UV spectroscopy. The results were reported in a graph in terms of C/C_o according to the duration of the treatment and compared with the ones of

Example 4. This graph, represented in Figure 5, shows that the use of the non-cleaned woven fabric does not imply a decrease in its functionality. In fact, the curves of the relative concentration of methylene blue according to the duration of the treatment relative to the test of Example 4 and to the present test do not show substantial differences.

Example 6. Preparation of a partially nanostructured-TiCh-coated metallic woven fabric and effectiveness test of the partially nanostructured-Ti0₂-coated metallic woven fabric in the photodegradation of methylene blue as the surface area of the fabric changes

Step I: Anodization of the titanium woven fabric and subsequent heat treatment

A titanium woven fabric having a size of 63.9 cm² (35.5 cm x 1.8 cm) was anodized by setting a voltage of 25 V in a 1 mol/1 sulphuric acid and 0.2% in weight hydrofluoric acid solution for 20 hours at room temperature, using as cathode a graphite fragment having the same size. Then, the woven fabric was subjected to heat treatment at 400°C for 3 hours.

A woven fabric coated with tubular nanostructured titania of which at least 85% is in the form of anatase was obtained.

Step II: Photodegradation of methylene blue The test was carried out as in Example 4, with the only exception that two anodized, heat-treated titanium woven fabrics were used: the titanium woven fabric of Example 4 and the titanium woven fabric prepared in Step I of the present Example 6. They were inserted together in the vial at the centre of the structure supporting the UV lamps. The total duration of the treatment was 27 hours. The results are shown in

Figure 6, which reports the comparison between the woven fabric of Example 4 (-•-) and the woven fabrics of both Example 4 and Example 6 (-A-). It was observed that the concurrent presence of two fabrics according to the invention significantly increases the photodegradation process of methylene blue, owing to the greater active surface of nanostructured titania in contact with the solution compared to the one of the single fabric of Example 4. In particular, it was observed that by increasing the fabric surface in contact with the methylene blue solution from 53.25 cm² to 117.15 cm² the irradiation time necessary to photodegradate 50% of the initially present methylene blue diminished from 10 to 8 hours. Example 7. Effectiveness test of the partially nanostructured-Ti0₂-coated metallic woven fabric of Example 6 in the photodegradation of methylene blue as the emission spectrum of the UV source changes

A single lamp system is used, alternating two different sources in order to detect the effect of the UV source emission on the effectiveness of the methylene blue photodegradation process. In both cases it is a 500 ml newly prepared solution of methylene blue 6.25 x 10^'5 mol/1.

Step I: a Wood lamp (black light) with spectral emission between about 350 and 400 run and a power of 8 W, and the woven fabric of Example 6 were used. The experiment has a duration of 25.5 hours. Step II: A germicidal source with a spectral emission between 250 and 260 nm,

37.8 cm long, 2.6 cm in diameter and with a UV irradiation power of 4.6 W, and the titanium net prepared during Example 6 were used. The experiment has a duration of 23 hours.

Figure 7 shows the variation of the C/C_o relative concentration of methylene blue according to the irradiation time by using the woven fabric of Example 6 and alternatively the Wood source (-A-) or the germicidal source (-■-). The irradiation time necessary to reduce the initial amount of methylene blue by 50% was of 16 hours by using the Wood source and of 3 hours by using the germicidal source.

Example 8. Effectiveness test of the partially nanostructured-TiC^-coated metallic woven fabric in the photodegradation of methylene blue, comparison with the use of the sole germicidal UV source

This experiment aims at observing the effect of the sole germicidal source in absence of the titanium woven fabric, on the photodegradation of 500 ml of a 6.25 x 10^"5 mol/1 methylene blue solution. The experiment has a duration of 24 hours.

As shown in Figure 8, where the results of Experiment 7 (-■-) are compared with the ones of the present Example (- ▲ -), the exposure of the methylene blue solution to the sole germicidal source does not lead to a significant degradation of the examined compound in the considered times.

Example 9. Effectiveness evaluation of the partially nanostructured-Ti0₂- coated metallic woven fabric of Example 6 in the photodegradation of methylene blue as the concentration of methylene blue changes

The experiment is the same as the one described in Example 7. The only difference is the decrease in the concentration of methylene blue from 6.25 x 10^"5 mol/1 to 6.25 x 10^"6 mol/1. The sampling is performed frequently throughout the first hour from the beginning of the treatment. The experiment has a duration of 180 minutes. The results obtained are shown in Figure 9 (-■-), where they are compared to the ones of Example 7 (-A-). By using an initial concentration of methylene blue 10 times lower than the one of Example 7, it was observed a decrease in the irradiation time necessary to diminish by 50% the initial concentration of methylene blue from 3 hours to 39 minutes. Example 10. Modification of titania by adding noble metal nanoparticles

A sample of metallic fabric, obtained by braiding titanium threads of 0.2 micrometers in diameter so as to make a led fabric, was anodized by setting a voltage of 22 V in a 0.2% in weight hydrofluoric acid and 1 mol/1 sulphuric acid solution at 25°C for 20 hours. The anodized metallic fabric was then heat treated in air at 380°C for 3 hours. A woven fabric coated with tubular nanostructured titania of which at least

90% is in the form of anatase was obtained.

The fabric was immersed in a chloroauric acid solution (0.01 mol/1) and lighted up with a UV source (Wood lamp). The immersion lasted a few minutes.

The coated fabric treated in chloroauric acid solution and UV source was used for essays of pollutants photodegradation.

Example 11. Modification of titania by adding carbon nanotubes

A sample of titanium fabric was anodized by setting a voltage of 25 V according to the processes described in the preceding examples, so as to have the surface covered with nanotubular titania with at least 85% anatase. Such coated fabric was used as carbon nanotubes growth substrate. At this purpose, a H₂-C₂H₂ mixture for the carbon nanotubes growth was used. The carbon nanotubes growth was performed by the CVD deposition technique. The growth temperatures on the catalysed surface were lower than 550°C. The growth time at that temperature was a few tens of minutes. Example 12. Use of the coated fabric as electrode

A sample of titanium alloy (Ti₆Al₄V) fabric was anodized by setting a voltage of 20 V according to the processes described in the preceding examples, so as to have the surface covered with titania. The fabric was then subjected to heat treatment at 380⁰C in air for 3 hours. A woven fabric coated with tubular nanostructured titania of which at least

70% is in the form of anatase was obtained.

The coated fabric was immersed in a methylene blue solution (C₀ = 10^"6 mol/1). A non-coated titanium fabric was also immersed in the solution. The two fabrics were connected to the opposite poles of an adjustable-intensity current generator. At the same time, the titania-coated fabric was lighted up with UV source (Wood light).

Example 13. Use of the filtration system for the degradation of pollutants present in the atmosphere

A 2 x 2 cm² sample of titanium woven fabric was anodized by setting a voltage of 28 V according to the processes described in the preceding examples, so as to have the surface covered with titania. The woven fabric was subjected to heat treatment at 380°C in air for 3 hours.

A woven fabric coated with tubular nanostructured titania of which at least 80% is in the form of anatase was obtained.

The coated fabric was immersed in an Erlenmeyer flask. 0.4 ml of dichloromethane and a pH indicator (litmus paper) were placed in the flask. The flask was sealed and lighted up with UV source (Wood lamp). Under UV lightening a variation of the litmus paper colour was observed, which indicated the completed conversion of dichloromethane in the presence of TiO₂ and UV irradiation.

Example 14. Effectiveness test of the partially anatase-type nanostructured-TiC>₂-coated metallic woven fabric in the photodegradation of methylene blue

A 3 x 3 cm² sample of titanium fabric was subjected to anodization in 1 mol/1 sulphuric acid and 0.15% in weight hydrofluoric acid solution, at room temperature, at 24 V for 22 hours. The anodized fabric was then subjected to heat treatment in air for 2 hours at 200°C. A woven fabric coated with tubular nanostructured titania of which at least

15% is in the form of anatase was obtained.

The woven sample was placed on the bottom of a 200 ml Pirex® beaker containing 20 ml of a 5 x 10^"5 mol/1 methylene blue solution, under the same experimental irradiation conditions as Example 2. After one hour of irradiation with UV radiation, the C/C_o ratio (current concentration of methylene blue to initial concentration) decreased from 1 to 0.70, this result having to be compared with the values of Example 2 both relative to the device according to the invention, decreasing from a C/C_o ratio of 1 to 0.13, and relative to the non-anodized device, decreasing from 1 to 0.77. This allows to observe that for values not much above a 10% content of anatase, the device according to the present invention removes amounts of contaminants greater than the non-anodised device, although lower than the preferred embodiment of the invention.

The surprising results achieved by the device according to the invention are evident from the examples illustrated above and the accompanying drawings, in which the choice of metallic woven fabrics is extremely advantageous in terms of contact area between the fluid to be treated and the active material. In fact, a metallic woven fabric coated with nanostructured titania, as it is or modified by activation of other oxides and/or other elements, e.g. metallic elements, resulted to be a very effective filtration device.

Claims

I . A filtration device comprising at least a metallic or metallized fabric at least partially coated with nanostructured titania, wherein at least 10% of said nanostructured titania is anatase.

2. The device according to claim 1, wherein the metallized fabric is made of a material selected from the group consisting of steel, carbon steel, zinc-coated iron, aluminium alloy, brass, bronze or glass fibre, and has a metallic coating.

3. The device according to claim 1 or 2, wherein the metallic or metallized fabric is a woven fabric.

4. The device according to claim 3, wherein the woven fabric is selected from the group consisting of led woven fabric, herringbone woven fabric, rep woven fabric, inverse rep woven fabric, Touraille woven fabric and inverse Touraille woven fabric.

5. The device according to any one of claims 1-4, wherein the nanostructured titania is at least 50% anatase.

6. The device according to any one of claims 1-5, wherein the nanostructured titania is at least 90% anatase.

7. The device according to any one of claims 1-6, wherein the nanostructured titania is essentially anatase.

8. The device according to any one of claims 1-7, wherein the nanostructured titania is in a tubular form.

9. The device according to any one of claims 1-8, wherein the nanostructured titania is added with at least a nanostructured material.

10. The device according to claim 9, wherein the nanostructured material is selected form the group consisting of titanium oxides, titanium carbides, titanium nitrides, titanium oxynitrides, transition metals and carbon tubules.

I I. The device according to claim 10, wherein the nanostructured materials are titanium oxynitrides.

12. The device according to any one of claims 1-11, wherein the metallic or metallized fabric has a flat, cylindrical, corrugated or pleated form.

13. The device according to any one of claims 1-12, comprising at least two metallic or metallized fabrics having equal or different mesh.

14. The device according to claim 13, wherein said at least two metallic or metallized fabrics are serially placed along the filtration direction of the device, with respect to which the mesh of the at least two metallic or metallized fabrics are in a descending order.

15. A process for the production of the device according to any one of claims

1-14, comprising the steps of: a) providing at least a metallic or metallized fabric; b) forming nanostructured titania on the metallic or metallized fabric of step a), such that at least 10% of said nanostructured titania is anatase.

16. The process according to claim 15, wherein step b) comprises a first anodization treatment of the fabric of step a) by applying a voltage in the range of 1 to 50 V for about 10 minutes to about 10 hours and using an electrolytic solution at a temperature in the range of 5 to 80°C; and a subsequent conversion treatment of the resulting nanostructured titania into nanostructured titania with at least 10% anatase.

17. The process according to claim 16, wherein the applied voltage is in the range of 1 to 30 V.

18. The process according to any one of claims 15-17, wherein the electrolytic solution of the treatment by anodization is an aqueous solution of sulphuric acid and hydrofluoric acid or an aqueous solution of sodium sulphate and sodium fluoride.

19. The process according to claim- 18, wherein the aqueous solution comprises

0.1 to 1.5 mol/1 of sulphuric acid or sodium sulphate and 0.01 to 2% in weight of hydrofluoric acid or sodium fluoride.

20. The process according to claim 19, wherein the electrolytic solution of the treatment by anodization is an aqueous solution at room temperature comprising 1 mol/1 of sulphuric acid or sodium sulphate and 0.15% in weight of hydrofluoric acid or sodium fluoride and the voltage applied is about 20 V.

21. The process according to any one of claims 15-20, wherein the conversion treatment of the nanostructured titania comprises a heat treatment at temperatures in the range of 200 to 600°C, for a time in the range of about 10 minutes to about 10 hours.

22. The process according to claim 21, wherein the heat treatment is carried out at a temperature of about 400 °C for about 3 hours.

23. The process according to any one of claims 15-22, further comprising a step of adding nanostructured materials to the nanostructured titania.

24. The process according to claim 23, wherein the nanostructured materials are anodized by physical or chemical vapour deposition techniques.

25. The process according to any one of claims 15-24, further comprising a step of reactivating the nanostructured titania.

26. The process according to claim 25, wherein the reactivation step comprises the replication of step b).

27. A method for degrading and decomposing inorganic and organic pollutants and micro-organisms in contaminated fluids and waters comprising the steps of: i. placing at least one device according to any one of claims 1-14 along the flow direction of the fluid to be decontaminated, said device being oriented so that the fluid passing through said device is forced; ii. irradiating the device with a radiation having a spectral intensity ranging from the visible to the ultraviolet field; iii. having the fluid to be decontaminated flow through said irradiated device.

28. The method according to claim 27, further comprising the step of: iv. placing a mechanical filter along the flow direction of the fluid to be decontaminated, said mechanical filter being oriented so that the fluid passing through said filter is forced, and being upstream of the device according to any one of claims 1- 14, being said step iv to be performed before step ii.