ITMI20090545A1 - COMBINATION OF MATERIALS FOR THE TREATMENT OF CONTAMINATED LIQUIDS THROUGH PHOTOCATALYTIC OXIDATION - Google Patents

Description

Description of the industrial invention entitled:

COMBINATION OF MATERIALS FOR THE TREATMENT OF CONTAMINATED LIQUIDS THROUGH PHOTOCATALYTIC OXIDATION.

The present invention refers to a combination of materials for the removal of contaminants from organic or inorganic liquids, through the photocatalytic oxidation of the contaminants themselves; the invention also refers to a photocatalytic oxidation process of contaminants through said combination of materials, and to a reactor in which the process is conducted.

Photocatalysis is the phenomenon whereby light is able to decrease the activation energy of a chemical reaction. Generally photocatalysis is mediated by a solid material, the photocatalyst, which is usually a semiconductor material. In a semiconductor the energy levels available to the electrons are divided into a valence band (VB) completely filled with electrons and an empty conduction band (CB), separated from each other by a range of prohibited energies (defined in the sector with the wording English "Energy gap", Eg). When a photon with energy hv, higher than the Eg value of a given semiconductor, is absorbed by the latter, there is the "promotion" of an electron from the valence band to the conduction band; in this way a pair of charge carriers is formed, the electron in the conduction band, e-CB, and an empty electronic position within the valence band, known as "vacancy" or with the English definition "hole" and generally indicated with h <+> vB- Through this mechanism, excited states are generated in the material capable of initiating chain processes such as redox reactions and molecular transformations into molecules with which the material comes into contact.

A heterogeneous photocatalytic system consists of particles of the photocatalyst in close contact with a reaction medium, in the liquid or gas phase, in which the reactants are present.

A semiconductor material particularly studied for its photocatalytic properties, in particular in a liquid environment, is titanium dioxide, TiO2. Among the various crystalline (or amorphous) forms of titanium dioxide, those that exhibit photocatalytic activity are rutile and anatase (which are also the most common forms in nature). Both these forms have a high refractive index, n, equal to 2.52 in the case of anatase and 2.72 in the case of rutile, which determines a high degree of "trapping" of light in the structure of the material, and therefore a higher probability of photon absorption. Thanks to the combination of refractive index and high degree of transparency in the visible spectrum region, titanium dioxide is the best semiconductor material for chemical conversion and storage of solar energy, despite absorbing only 5% of the incident radiation.

Activation of TiO2 requires ultraviolet light, which can be that naturally present in solar radiation or can be supplied with lamps. Anatase is also sensitive to the most extreme part of the visible spectrum corresponding to blue-violet. Optical absorption measurements have shown that anatase has a higher absorption threshold than rutile. In particular, for rutile the value of Eg is equal to 3.03 eV, while for anatase it is equal to 3.18 eV. Since the oxidizing capacity of a photo-catalyzed re is greater the higher its Eg is, anatase is the form of TiO2 that exhibits a greater photocatalytic activity; in the rest of the description with the wording "titanium dioxide", or with the formula TiO2, we will mean anatase, rutile or their mixtures; furthermore, since anatase is the most active form, we will mainly refer to this crystalline form.

Summarizing, the main characteristics of anatase are:

- transparency in the visible region;

- high porosity;

- high surface affinity for molecules in the liquid state;

- low costs and easy production in large quantities;

- chemical inertness, non-toxicity and biocompatibility.

The following are the main reactions of the photocatalytic degradation process of contaminants on the surface of titanium dioxide:

Once an electron in the conduction band and a vacancy in the valence band by a photon (reaction I) are generated, these can give rise to unstable or metastable species by contact between TiO2 and oxygen or water (reactions III to XI ), which can then react with contaminants degrading them (e.g., reaction XII); alternatively, the vacancy or the electron present on the surface of the material can react directly with the contaminant (reactions XIII and XIV). In summary, the vacancies and the hydroxyl radicals thus formed, both strongly oxidizing, can be used to oxidize most organic contaminants. After the photocatalysis process, an increase in BOD5 values may occur (BOD is the acronym of the English definition "Biochemical Oxygen Demand", in Italian "biochemical oxygen demand", while the subscript 5 indicates that the quantity of oxygen is measured after 5 days; this measure indicates the quantity of oxygen necessary for the complete oxidation of the organic compounds present in a sample, and is therefore an index of the degree of pollution of the sample liquid by organic oxidizable substances). negative signal, but it is due to the fact that photocatalysis promotes an efficient removal of organic contaminants (which can occur with the further reduction of COD values, "Chemical Oxygen Demand") transforming them into biodegradable species. organic and inorganic pollutants no longer constitute damage to the environment, being, ultimately, soluble salts such as carbonat i, sulphates and nitrates that are deposited on the surfaces where the reactions described above have taken place, as well as to carbon dioxide and water.

The titanium dioxide maintains its characteristics unaltered: the photocatalysis process takes place without the support being degraded, and a continuous and constant efficacy is guaranteed over time. The TiO2 molecule in fact participates as a catalyst, and not directly in the chemical transformation processes.

A problem with these photocatalytic systems is the recombination of electrons and vacancies within the material (reaction II): when this recombination occurs, the catalyzation by TiO2 of the reactions that ultimately lead to the removal of contaminants does not take place. and the only observed effect is the degradation of light energy to heat.

To avoid recombination, it is possible to use TiO2 particles of small dimensions, and in particular in the range of 10-25 nm. It is in fact known that the distance that the holiday takes in a region free from fields in this material before recombining with an electron is about 0.1 μm; consequently, by using particles with dimensions smaller than this distance, the recombination phenomenon is partially reduced. The use of very fine particles in suspension in liquid phases, however, has the disadvantage of requiring very long times for the removal of the particles themselves from the purified liquid at the end of the process and the removal efficiency is however not high.

Alternatively, it is possible to use TiO2 layers deposited on supports. The use of these layers (generally referred to in the industry as films) eliminates the problem of removal at the end of the process, but entails problems in the efficiency of the photochemical process; in fact, too thin layers have a low absorption coefficient for energies close to those necessary for the band gap, while thick layers re-propose the problem of recombination between electrons and vacancies.

The literature describes attempts to avoid the recombination of the e-CB / h <+> VB pairs, based on the doping of titanium dioxide with noble metals (eg Ag, Pt, Au, etc.) and non-noble metals (eg. Fe <3+>); the process yields are however limited as the correct dosage of the metal is very difficult. Furthermore, there are often problems of interaction between the contaminants and the metals used which lead to competitive effects on the yield of the process and a gradual decrease in the efficiency of photocatalysis. Some proposed couplings, for example titanium dioxide with CdS, lead to environmental problems with the release of cadmium in solution. Similarly, couplings of titanium dioxide with non-metallic elements, such as fluorine, nitrogen, sulfur and other compounds, were in fact inapplicable or in any case not effective.

These problems have up to now been the major limiting factor for the application of this technology.

The need to find a way to increase the yield of photocatalysis with semiconductor-based systems, and in particular TiO2, is therefore still open in the sector. The object of the present invention is to provide a combination of materials which leads to improved photocatalysis yields with respect to known systems, as well as to provide a process and a reactor which use said combination of materials. These objects are achieved with the present invention, which in its first aspect relates to a photocatalyst consisting of a combination of titanium dioxide and carbon nanotubes in contact with each other.

The invention will be described below with reference to the single figure (Figure 1), which schematically shows a possible reactor for carrying out the process of the invention.

Titanium dioxide is preferably anatase, but rutile (or a mixture of the two forms) can also be used. Titanium dioxide can be used both in the form of powders, granules or bodies obtained by compression of powders (known in the sector as "pellets"), and in the form of film of the material, mainly in the anatase form, deposited with carbon nanotubes on different types of surfaces, or directly on surfaces containing a fair concentration of said nanotubes.

If titanium dioxide is used in the form of pellets, the reaction kinetics are slower and therefore longer treatment times are required. On the other hand, the problem of removing titanium dioxide from the purified liquid downstream of the process is almost eliminated.

Carbon nanotubes are best known in the industry with the English definition "Carbon Nanotubes" or with the abbreviation CNTs, which will be used in the rest of the text. CNTs are hollow structures, formed by carbon atoms that are arranged on cylindrical surfaces (a single surface in the case of single-walled nanotubes, forming nanotubes known in the industry with the abbreviation "SWNTs", or more concentric walls, forming nanotubes known as' <,> MWNTs <,,>). structures can vary between about 0.7 and 10 nm, while their length can reach values between 10 <4> and 10 <5> times their diameter. These cylindrical nanometer-sized molecules can conduct electricity at room temperature with a almost zero resistance This phenomenon is known as the ballistic effect, whereby electrons move freely through the structure.

The coupling of CNTs to titanium dioxide can be done directly using CNTs or mixtures of carbon and graphite materials (expanded graphite, expanded temographite, graphene or other nanostructured materials) which contain a concentration of CNTs not less than 10% by weight.

In the case of using titanium dioxide in granules or pellets, the semiconductor particles are coupled with CNTs that adhere to the surface of the titanium dioxide, forming a unique structure. In this way, maximum TiO2 exposure is obtained, optimizing process yields.

The possibility of coexisting and intimately mixing aqueous solutions (the most interesting to be treated, because by far the most abundant form of liquid residues to be decontaminated) with CNTs, notoriously hydrophobic, is due to a phenomenon that occurs on the surface of TiO2in following the absorption of light radiation (mainly UV, but also visible at low wavelength in the case of anatase). Different reactions occur by irradiation of TiO2 with UV light compared to those of photocatalysis. The electrons, in fact, reduce the cation Ti <4+> to Ti <3+>, while the vacancies oxidize the anions O <2->. In this process, an oxygen atom is expelled and a so-called “oxygen holiday” is created. The sequence of reactions can be summarized as follows:

The oxygen vacancies are replaced by water molecules, resulting in a completely hydrophilic surface. In the anatase exposed to UV light, very low contact angles with water are obtained (<1 °); this results in the rare property of attracting rather than repelling water, a characteristic defined as super-hydrophilicity. The greater the exposure of the surface to UV radiation, the smaller the contact angle between the water and the surface itself becomes. After about thirty minutes under a moderate intensity UV light source, the contact angle tends to zero: the water remains flat on the surface instead of forming droplets. If the lighting is interrupted, the superhydrophilic behavior is maintained for about two days. The phenomenon does not uniformly affect the entire surface of TiO2: rather, an ordered checkerboard of very small hydrophilic areas is formed on this surface, alternating with analogous hydrophobic areas. In this way, TiO2 acts by allowing the coexistence of hydrophilic and hydrophobic phases, thus giving rise to an excellent coupling from the point of view of stability in solution, of titanium dioxide with hydrophobic materials that contain a high percentage of nanotubes.

The inventor observed that the coupling of TiO2 and CNTs greatly improves the properties of the first material, because the electrons that are released as a result of activation by lighting are easily transferred and transported in the CNTs. It follows that the possibility of recombination of the e-CB / h <+> VB pairs in the titanium dioxide structure is greatly reduced, increasing the yield of the process.

In addition, the combination of carbon nanotubes with TiO2ne increases the absorption threshold which goes from 3.18 (in the case of anatase) to 3.54 eV, with an increase in the oxidizing capacity of the photocatalyst.

The percentage by weight of the nanotubes that must be present in the mixture depends on the form in which it is present in titanium dioxide. In the case of using titanium dioxide in the form of nano particles, the weight ratio between titanium dioxide and carbon nanotubes can vary between about 10: 1 and 1: 1, and is preferably about 5: 1; in the case of titanium dioxide in pellets this ratio can vary between about 20: 1 and 10: 1, and is preferably about 15: 1; finally, if the titanium dioxide is in the form of a thin film, then it is preferable to deposit the titania film on a surface with a carbon nanotube content of not less than 30% with respect to the weight of the titanium dioxide.

In a second aspect, the invention relates to a process of treating contaminated liquids which includes the photocatalytic oxidation of the latter, with improved yields compared to known photocatalytic processes, made possible by the use of the combination of materials previously described.

The reactions of the photocatalytic mechanism underlying the process are as follows:

When an electron is transferred from TiO2 to CNTs, reactions from (XVIII) to (XX) take place.

The radiation used to activate TiO2 can have a wavelength in the ultraviolet (UV-A, UV-B or UV-C) or even in the visible spectrum; in fact, while the TiO2 used alone has a maximum absorption efficiency at a wavelength of 364 nm (UV-A region), the coupling with CNTs broadens the spectrum of absorbable radiation, so that the process is also activated in the visible region at a wavelength of about 410 nm.

For the purposes of the process, the TiO2 / CNTs combination can be brought into contact with the liquid to be treated in various ways. The simplest application is the use of TiO2 in the form of powders, on whose surface there are CNTs, which are added to the water to form a suspension.

The process works with TiO2 / CNTs combination quantities varying within extremely wide ranges; in principle, any quantity of said combination in a liquid to be treated gives rise to the desired effects. To have an efficient process, however, said quantity must be included in an optimal range. In fact, if the TiO2 / CNTs combination is present in too low quantity, the efficiency of the process is clearly reduced, while if said quantity is too high there is a very turbid liquid, which limits the passage of radiation with a diffusion effect of the beam. bright, scarce irradiation of the particles farthest from the surface of the liquid, which also in this case leads to reduced efficiency. As part of the process in question, the optimal quantities of titanium dioxide and nanotubes that are used for the photo catalytic process are related to the type of contaminants to be treated. On the basis of the pilot scale treatments carried out, it has been determined that the concentration in titanium dioxide granules can vary in most cases in a range between 0.1 and 2.3 g / liter, depending on the type of contaminants to be remove in the contaminated liquid and by the chemical characteristics the liquid itself, while the quantity of CNTs is derived from that of TiO2. Said values can certainly be higher in the case of using titanium dioxide in the form of pellets. Furthermore, in the case of use of titanium dioxide in terms of thin film, the reference data is not the quantity of titanium dioxide in the liquid to be treated, but the surface of the film to radiation.

Another parameter that greatly influences the yield of the photocatalytic process is the pH of the treated liquid.

In an acidic environment, the TiO2 surfaces are positively charged and the absorption of negatively charged contaminants is favored, conversely in a basic environment the absorption of positive ions is favored. Furthermore, in an acidic environment the risk of carbonate formation is reduced following the formation of carbon dioxide during the process. This aspect has some relevance as carbonates exert a negative action against titanium dioxide, therefore their formation should be avoided.

Titanium dioxide has a zero charge at pH 6.5, so slightly acidic or near neutral conditions are the least convenient for treating most contaminated liquids.

The temperature during the process increases as an effect of the oxidation-reduction reactions and, in the case of the use of UV lamps, especially due to the heating induced by their irradiation. The rise in temperature values that are recorded during the treatment is of the order of 5-25 ° C, and does not affect the efficiency of the treatment; in fact, titanium dioxide maintains its photocatalytic properties almost unaltered up to about 900 ° C.

The irradiation time during the process varies from a minimum of 10 minutes to a maximum of three hours, depending on the type of contaminants to be removed and the quality objectives of the waste water.

Hydrogen peroxide can be added to the liquid to be treated, which has been shown to be able to increase the degradation of contaminants and speed up the reaction kinetics. Hydrogen peroxide can react with superoxide radicals according to the following reactions:

When H2O2 is present at high concentrations, it can act as follows:

Excessive concentrations of hydrogen peroxide can hinder the photocatalysis process mainly for two reasons. The first is related to the competition in terms of light absorption between H2O2 and TiO2, the second is that part of the hydroxy radicals are destroyed by the presence of hydrogen peroxide and converted into hydroperoxyl radicals HO2 <.> Which are eliminated by other hydroxyl radicals producing H2O O2. During the photocatalysis process, the hydrogen peroxide is consumed almost entirely, thus not causing any problems for any subsequent treatment stages.

In the process of the invention, oxygen is also preferably added to the liquid containing the contaminants during the treatment, preferably in the form of micro bubbles in order to increase the specific contact surface. The presence of oxygen strongly increases the efficiency of photo catalytic reactions. The addition of oxygen is of some importance because, when absorbed on the TiO2 surface, it reacts with the e-CBs forming the superoxide radical O2 <.->:

Furthermore, oxygen, as an accepting electron, partially contains the recombination of the e-CB / h <+> VB pairs; a similar function is also performed by hydrogen peroxide.

The process is able to remove most of the organic and inorganic contaminants. In particular, removal yields of over 98% are achieved for COD, BOD5, petroleum hydrocarbons, chlorinated hydrocarbons, some heavy metals, hexavalent chromium, phenols and other recalcitrant compounds.

Of particular importance is the ability of the process to remove MTBE (Methyl-Tertiary-Butyl-Ether) with the achievement of the most restrictive quality standards required by international regulations (<5 μg / l) with no input concentration limits. In the case of MTBE treatment, it has been found that the treatment can also be conducted at neutral or basic pH and often a lowering of pH values is detected during the reaction as a result of the formation of formic acid and subsequently acetic acid. The first phase of destruction of the MTBE molecule is the extraction of an α-hydrogen by the hydroxyl radical to form an organic radical that reacts with dissolved oxygen in solution. Note that the most important oxidation of ethers is the conversion into esters. Therefore, the unstable intermediate compound is quickly convented to TBF (t-butyl formate). Although hydroxyl radicals behave as strong oxidants, other active species such as electrons and superoxide anions can react with organic compounds. As a consequence, TBF can be degraded in two ways. Esters are subject to both acidic and basic hydrolysis producing carboxylic acid and alcohol. In the first case the TBF will produce TBA (tbutyl alcohol) and formic acid. The latter is oxidized by oxygen by photocatalysis and produces carbon dioxide and water on the surface of the titanium dioxide-carbon nanotube particle. Alternatively, TBF is reduced with electrons to form 2-methyl-1-propene and formic acid. Under acidic conditions and in the presence of hydroxyl radicals, TBA is dehydrated to alkenes, which are strongly oxidizing and produce formic acid and acetone. Formic acid, like other by-products, is degraded to acetic acid and formaldehyde. When the reaction is complete, all organic compounds are degraded to carbon dioxide and water. It should be noted that the reaction of TBA is much slower than that of MTBE and TBF, while TBF is degraded faster than MTBE.

In the correct conduct of the process it is therefore necessary from time to time, depending on the initial concentrations of the MTBE in the liquid to be treated, to correctly determine the times of the photocatalysis process to ensure not only the complete removal of the MTBE but also of the by-products of reaction. The effectiveness of the treatment as well as the simplicity of the process make this type of treatment an extremely interesting and innovative solution for the removal of MTBE.

In one of its last aspects, the invention relates to a photocatalytic reactor for carrying out the process described above, shown schematically in figure 1.

The process of the invention could, in principle, also work with natural radiation. However, this condition is not very useful from an industrial point of view, as the process could only be conducted during the day and with favorable climatic conditions, and also only outdoors. It is clear that in this case the treatment times would be very high and not compatible with the needs of an industrial treatment process. For industrial purposes it is therefore necessary to free oneself from these conditions, and to be able to operate the process at any time and at any time. For this purpose, the reactor can be equipped with UV radiation sources (lamps); given the importance of this embodiment, the reactor will be described only in this mode.

The liquid to be treated enters the reactor, 10, from the inlet pipe on the left in the figure, and leaves the reactor after being treated through the pipe on the right in the figure, according to the flow direction of the liquid indicated by the arrows.

The walls 11 of the reactor 10 can be made of different materials, which in any case have the characteristic of being compatible with the degree of chemical aggressiveness of the contaminants to be treated and resistant to the wavelength spectrum emitted by the lamps present in the reactor. The internal surface of the walls 11 can also be coated with material or in any case by a film reflecting the wavelength emitted by the lamps in order to optimize the yield of the process.

the reactor 10 is equipped at least with the following components. A series of low or high pressure lamps 12, for the emission of photons useful for activating the particles of titanium dioxide and carbon nanotubes. The lamps can have emissions with different wavelengths, it is however important to cover the range around the peak wavelength in the UV-A range and, if desired, also in the visible. A range between 300 and 420 nm can generally be assumed to be valid. The number, geometry, center distance, orientation and construction type of the lamps may vary according to the geometry assumed by the reactor 10. The lamps are protected by pipes in quartz or other transparent material at the aforementioned wavelength. All the lamps are also connected to a power supply, ignition and intensity regulator system, 13.

The reactor 10 is also equipped with a venting system 14 with possibly an attached filter for the treatment of the outgoing gases.

Titanium dioxide with carbon nanotubes is preferably used inside the reactor in the form of powders, also in the form of nanoparticles, or pellets, which are dosed in the mixture to be treated with a special dispenser 15. The choice of the morphology of the photocatalytic material depends mainly from the flow rates of liquid to be treated and from the type and concentrations of contaminants present in them.

Alternatively, the present invention has the same validity, but it is believed that the yield of the process is lower, if the titanium dioxide, mainly in the anatase form coupled with CNTs, is used in the form of a thin film. In this case, TiO2 can be used by depositing it mixed with CNTs on special surfaces that are placed inside the reactor 10. It is also possible to provide for a deposition of TiO2 on films of nanostructured materials with sufficient percentages of CNTs, arranged inside the reactor.

Since, as mentioned in the description of the process, this has a higher efficiency if the liquid is oxygenated during the treatment, continuous oxygenation is provided in the reactor within which the process takes place. For this purpose, oxygenation systems usually available on the market can be used. A system with membrane diffusers, 16, with the formation of microbubbles, whose number, geometry and correct arrangement depend on the geometry assumed by the reactor, proved to be particularly effective for the treatment in question. Finally, if the TiO2 / CNTs combination is used in the form of powders and / or pellets, it may be useful to have a constant dispersion of said powders in the liquid to be treated. For this purpose, the reactor 10 can be equipped with any continuous mixing system available on the market. Figure 1 illustrates the case of Venturi ejectors, 17, the number and correct arrangement of which depends on the geometry of the reactor 10, according to principles well known to those skilled in the art.

The reactor of the invention can be used in combination with liquid pre or post-treatment units, to form a liquid treatment plant, according to methods known in the field.

For example, upstream of the reactor 10 there may be a pre-treatment and conditioning unit, in which the liquid to be treated in the reactor is subjected to filtration to remove coarse particles and any unwanted substances that may affect the photocatalytic process, or in which it is pretreated to decrease its turbidity, which would negatively affect the performance of reactor 10. The pretreatment unit can also be used to perform the dosage of adjuvant chemical compounds of the photocatalytic process, such as hydrogen peroxide, and to carry out any pH correction.

Downstream of the reactor 10, a post-treatment unit can be provided, for example comprising microfiltration, nanofiltration or ultrafiltration systems, or even centrifugation or other decantation and sedimentation systems; this unit is especially useful in the case of the presence of the TiO2 / CNTS combination in the form of granules and / or nano particles, for its recovery and reuse and to prevent these powders from being discharged downstream of the plant. If the granules or pellets (0 nano particles) of the combination are effectively recovered, recirculation can be carried out inside the plant for a significant number of treatment cycles.

The pre- and post-treatment units are not necessary for the operation of the reactor 10. For example, if the liquid entering the reactor is not turbid and does not contain particulates, it is possible to avoid the use of the pre-treatment unit ; on the other hand, in the case in which the liquid leaving the reactor 10 does not contain particulates, as for example when titanium dioxide is used in the form of pellets (whose separation from the liquid is immediate and already takes place inside the reactor 10), the post-treatment unit can be avoided.

Pre- and post-treatment units are known in the art, and need no further description here.

Claims (14)

Claims 1. Photocatalyst material consisting of a combination of rutile and / or anatase and carbon nanotubes in contact with each other. 2. Photocatalyst material according to claim 1, wherein said rutile and / or anatase is in the form of powders, granules or pellets, or of a thin layer deposited on a support. 3. Photocatalyst material according to claim 1, wherein said carbon nanotubes are used alone or in a mixture with other carbon and graphite based materials, wherein said mixture contains a concentration of carbon nanotubes not lower than 10% by weight . 4. Photocatalyst material according to claim 1, wherein the weight ratio between rutile and / or anatase and carbon nanotubes is between 10: 1 and 1: 1 when rutile and / or anatase are in the form of powders or nanoparticles, between 20: 1 and 10: 1 when rutile and / or anatase are in the form of granules or pellets, and in which the content of carbon nanotubes is not less than 30% with respect to the weight of rutile and / or anatase when the latter are below form of a layer deposited on a support. 5. Photocatalyst material according to claim 4, wherein said weight ratio is about 5: 1 when rutile and / or anatase are in the form of powders or nanoparticles, and about 15: 1 when rutile and / or anatase are in the form of granules or pellets. 6. Process for treating a liquid containing at least one oxidizable contaminant by photocatalytic oxidation of said contaminant in the presence of the material of claim 1. 7. Process according to claim 6 wherein the photocatalyst material is irradiated with UV-A light. 8. Process according to claim 6 in which, when anatase is used, the material is irradiated with wavelength light in the blue or violet range of the visible spectrum. 9. Process according to claim 6 wherein the photocatalyst material is added to said liquid in an amount ranging from 0.1 to 2.3 g of rutile and / or anatase per liter of liquid. 10. Process according to claim 6 wherein hydrogen peroxide is added to said liquid. 11. Process according to claim 6 wherein gaseous oxygen is added to said liquid, preferably in the form of microscopic bubbles. 12. Process according to claim 6 wherein said contaminant is Methyl-Tertiar-Butyl-Ether. 13. Reactor (10) for carrying out the process of claim 6, having walls (11) made or covered with materials compatible with the contaminants to be treated, with any chemical reagents added in the process, possibly reflecting UV-A rays and resistant to spectrum of UV or visible wavelength emitted by lamps (12) present in the reactor and connected to a power supply, ignition and intensity regulator system (13), and comprising a vent system (14) and a dispenser (15) for the addition to the liquid to be treated of photo-catalysed material consisting of a combination of rutile and / or anatase and carbon nanotubes in contact with each other. 14. Reactor according to claim 13, further comprising a filter for the treatment of the outgoing gases on said vent system, means (16) for oxygenating the liquid to be treated, and a continuous mixing system (17) of the liquid in treatment.