ES2585087B1 - Continuous photocatalytic process for the purification of a liquid medium and photocatalytic reactor to carry it out - Google Patents

Abstract

Continuous photocatalytic process for the purification of a liquid medium and photocatalytic reactor to carry it out. # The present invention relates to a continuous photocatalytic process for the purification of a liquid medium characterized in that it comprises: (a) a first reaction stage photocatalytic of the liquid medium in the presence of at least one photocatalyst consisting of TiO nanoparticles {sub, 2}; (b) a step of physical separation of the TiO {sub, 2} nanoparticles, resulting in a purified supernatant; and (c) a stage of extraction of the purified supernatant and separation of the TiO nanoparticles {sub, 2} in the previous stage, which are returned to the reactor for reuse as a photocatalyst. The photocatalytic reactor for carrying out said process is also the object of the invention.

Description

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CONTINUOUS PHOTOCATALYTIC PROCESS FOR THE DEPURATION OF A MEDIA

LIQUID AND PHOTOCATALYTIC REACTOR TO CARRY OUT

DESCRIPTION

Technical sector

The present invention relates to the field of purification or purification and purification of a liquid medium, preferably aqueous. More specifically, it refers to a new continuous process of photocatalysis in suspension with separation (preferably by sedimentation) and reuse of TiO2. The photocatalytic reactor for carrying out said process is also the object of the invention.

Background of the invention

The outlet waters of a WWTP are waters that must comply with the specifications set out in Directive 91/271 / EEC, but in which there are still substances that may be contaminants. In general, the composition of these waters depends fundamentally, on the one hand, on the contributions of industrial wastewater to urban discharge and, on the other, on the type of treatment to which they have been subjected in the corresponding WWTP. Despite this, it can be said that, in general, these waters are characterized by the presence of a wide variety of pathogenic germs, traces of non-biodegradable organic matter by biological processes, suspended solids, turbidity and inorganic contaminants, not eliminated in the treatment of purification.

On the other hand, leachates generated in urban landfills are generally characterized by the presence of high pollution by organic matter, in some cases non-biodegradable, solids, coloration, as well as by the presence of hazardous pollutants, such as heavy metals and other bio compounds -refractories that hinder or prevent its treatment by conventional purification technologies, such as biological treatment. Sometimes its final destination is treatment in urban treatment plants, which can lead to serious operational problems.

Also, some industrial wastewater from sectors such as the pharmaceutical contains dangerous substances, difficult to dispose of by conventional means. In general, biological systems to which adsorption processes by active carbon are added are removed.

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The purification of all these discharges, both urban and industrial, and leachate is key to improving the environmental quality of the receiving flows.

Organic toxic compounds and pathogenic elements can be destroyed using UV photocatalysis, although most commonly contaminated water is disinfected using chemical additives such as chlorine, biocides or other chemicals. There are also adsorption techniques on activated carbon, but these techniques have the disadvantage of producing a large amount of contaminated waste that will require further treatment. They are known drawbacks that such chemical agents are often hindered in their efficiency to destroy non-bacterial species. Other times, they involve the formation of unwanted by-products such as organic halogens subjected to adsorption (AOX) through the interaction of chlorine with the organic matter of water. In addition, chlorine and biocides have a negative impact on the quality of drinking water and, in general, the environment.

The present invention is part of the "Advanced Oxidation Processes (POAs)" in which a set of technologies that are capable of generating reactive oxygen species (ROS), transient species with high oxidant power and low selective attack are grouped , which are highly disinfectant and degrade most of the organic compounds (even the most persistent) until they achieve their total mineralization or the formation of intermediate compounds that are more biodegradable than the former. The main ROS generated by these processes are hydroxyl radicals (OH •), superoxide radicals (O2 • -), hydroperoxide radicals (HO2 •) and hydrogen peroxide molecules (H2O2).

The first research and publication on these photocatalytic processes to treat contaminants in both the aqueous and gas phases appears in 1976 and was carried out by Carey (Carey et al., Bulletin of Environmental Contamination and Toxicology, 1976, 16 (6), 697-701) . Since then, the number of publications / year has grown exponentially: 30 / year in the 80s, 300 / year in the 90s, up to 2,000 articles published in 2014. There are therefore numerous research papers that They guarantee the effectiveness of photocatalytic processes in the elimination of highly persistent compounds in water. For example, one can cite the work carried out in relation to pesticides used in agriculture (Ormad MP, et al., Ozone Science and Engineering, 2010, 32, 25-32 or Miguel, N., 2010, Doctoral Thesis University of Zaragoza for “Study of the elimination of pesticides usually present in waters of the

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5 Ebro basin through advanced oxidation processes ”); antibiotics and other drugs (Elmolla and Chaudhuri, Desalination, 2010, vol. 252 (1-3), pp. 46-52) or microorganisms present in water (Rincon AG and Pulgarin C., Journal of Solar Energy Engineering, 2007 , vol. 129, No. 1, pp. 100-110).

10 The most studied semiconductor catalyst currently in solar radiation treatments is titanium dioxide (TiO2) due to its characteristics of high photoactivity, low cost, chemical and biological stability and safety. In the last decade, advances in nanotechnology have allowed the development of new products based on TiO2, which has significantly increased its effectiveness. This fact has allowed to reduce the

15 costs associated with its use, positioning it very competitively in the market.

Below is the mechanism of photoinduced activation of titanium dioxide with presence of contaminants (M) (Oppenlander T., 2003, “Photochemical purification of water and air. Advanced oxidation processes (AOPs): main reaction mechanisms, reactor

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20 concepts ”, Editorial WILEY-VCH, Germany):

Although the application of advanced oxidation processes and, in particular, those based on the use of titanium dioxide and ultraviolet light, could be an applicable alternative in the elimination of contaminants present in any type of discharge of contaminated wastewater, this Technology is not used, however, on a real scale. This is mainly due to the associated cost, as well as to the limitations related to its application. In this sense, TiO2 is a compound that, after treatment, cannot be discharged into the environment because it poses an environmental risk. Therefore, the development of technologies that allow permanent fixation or rapid elimination of water are the key to solving the main operational problem currently in existence. Mainly the use of TiO2 has been developed in two different ways:

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•

A line of research has focused on the development of technologies that allow the fixation of TiO2 nanoparticles to a solid material surface that is subjected to continuous washing. This solid surface can be both fixed and mobile. In this line we can mention the Spanish patent ES 2269134, in which the TiO2 nanoparticles adhere to a mobile support that is used fluidized. The main drawback of this technology is the loss of process performance as the specific contact surface between the TiO2 nanoparticles and the liquid medium in which the substances to be treated are significantly reduced. This entails having to use retention times longer than if the nanoparticles were not adhered, with the consequent increase in reactor volume and greater energy expenditure in lighting and mixing. In addition, it is not ensured that the dragging of TiO2 nanoparticles does not occur due to the difficulties encountered for a permanent permanent fixation to a support. This high energy expenditure makes it unprofitable to use this technology on a continuous real scale;

•

The second line of research is based on technologies that allow the rapid removal of TiO2 from water by means of decantation, chemical precipitation or filtration methods for later reuse. In this sense, the methods currently available are based, for example, on the addition of reagents that entail a change in pH (as described in Spanish patent ES 2156094) or modifications on the nanoparticles to vary their composition and thus achieve its decantation (as described in patent ES 2228352). The cost of adding reagents to modify the pH

or the energy needed for nanofiltration makes these designs unfeasible on a real scale. In fact, it is already mentioned in the patent ES 2156094 that there is no method adapted for industrial use that allows the removal of TiO2 particles efficiently and economically.

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It is therefore the object of this invention to present a new process that, working continuously, allows the recovery of TiO2 nanoparticles without the need for external additives and maintaining the efficiency of TiO2 nanoparticles in suspension. In this way, it is possible to reduce the treatment time, as well as the necessary lighting with respect to the existing technologies in the market. The reduction of energy by requiring less lighting and less retention time, not having to add external reagents, together with the reuse of TiO2 nanoparticles, makes this full-scale process for continuous spills viable both economically and environmentally and, at the same time, be competitive against other technologies used today.

Description of the invention

It is a first object of the invention a continuous photocatalytic process for the purification of a liquid medium characterized by comprising:

(to)

a first stage of photocatalytic reaction of the liquid medium in the presence of at least one photocatalyst consisting of TiO2 nanoparticles, where said photocatalytic reaction is carried out in at least one fully agitated photocatalytic reactor and with adequate illumination for the Photocatalysis process In this way, the liquid medium will come into direct contact with the TiO2 nanoparticles activated by lighting the reactor, which oxidize and break down the contaminating elements (organic matter) present in the liquid medium until they are converted into H2O and CO2. In general, the lighting conditions and the minimum exposure and reaction times will depend on the liquid medium to be treated and, therefore, on the substances included in the medium that can be oxidized. These conditions are known to any person skilled in the art who is going to implement the photocatalysis reaction. Similarly, both the lighting and agitation means are not limiting to the process. Thus, for example, as lighting means, solar or artificial lighting means can be used, by means of solar or UVC, UVA or VIS lighting lamps. This lighting can be exterior or interior to the liquid medium. With regard to the stirring means, any suitable means can be used to achieve complete mixing conditions in the digester, such as paddle stirring. However, as described, the agitation method will not affect the outcome of the process, as long as it is

get a good homogeneous distribution or suspension of the particles in the reactor. In relation to TiO2 nanoparticles, these may be contained in the reactor in a concentration preferably between 0.5 and 2.5 g / L, and more preferably between 1 and 2 g / l;

(b)

once the photocatalytic reaction is finished, a step of physical separation of the TiO2 nanoparticles is carried out, preferably by gravity decantation without agitation. This step will be carried out for a time preferably between 15 minutes and 100 minutes, resulting in a purified supernatant. At this stage, in addition to the TiO2 nanoparticles, another type of residual inorganic particles will decant, if present;

(C)

then, the purified supernatant is extracted and the TiO2 nanoparticles (preferably separated by decantation) are returned to the reactor for reuse, that is, to be able to continue being used as a photocatalyst. The supernatant will be removed slowly enough not to create turbulence that will disperse the nanoparticles again.

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In a particular embodiment of the invention the process can be used not only to purify, but also to purify the liquid medium to be treated.

Although the main object of the invention relates to a photocatalytic process operating continuously, in particular embodiments of the invention the process may also be carried out in batch.

Thanks to the process described, object of the invention, it is possible to achieve a recovery of more than 95% of the TiO2 nanoparticles for reuse.

For the purposes of this patent, nanoparticle means a particle with a dimension less than 40 nm. Preferably, as a photocatalyst product, TiO2 nanoparticles immobilized in nanoagglomerations may be used by means of a binder additive as described in the international application WO 2009/074120. This product does not lose efficiency with respect to the commercial TiO2 most commonly used in suspension and has the advantage of being able to be reused by physical separation in the liquid or aqueous medium, as is the case with products that are fixed to both fixed and mobile external physical supports. Therefore, the use of this photocatalytic product for this application, or any other that meets the same characteristics, allows to work with the performance of the products in suspension, but with the advantages of the supported materials, without the need to provide other additives as flocculants, coagulants, or pH correctors.

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In a particular embodiment of the invention, the process will comprise an additional stage of purging the TiO2 nanoparticle stream that is returned to the reactor, said reactor being fed with new TiO2 nanoparticles. The purge flow will be a function of the liquid medium to be treated.

Additionally, the extracted purified supernatant may be sent to an additional filtration stage, if required by the quality of the receiving medium, in order to eliminate any residual nanoparticles that may remain in the supernatant. The type of filtration will not be limiting, and any method currently available in the state of the art can be used.

Whenever the process is carried out as claimed, the photocatalytic process will be made viable both economically and environmentally on an industrial scale (without flow limitation) working continuously. However, as indicated above, the process would also be applicable to the treatment of effluents in batch. In this way, an existing problem in water purification is solved, by achieving a tertiary treatment of purification and purification continuously at a cost usually assumed in a water purification facility. Until now, all photocatalysis treatments with TiO2 had prices that made them unfeasible. Up to 100 times reuse of nanoparticles reduces operating costs and ensures that they are not poured into the receiving environment, eliminating the environmental hazards that this would entail. Similarly, by not requiring chemical reagents potentially hazardous to the environment, costs are further reduced. It is therefore a process that does not generate waste (such as those generated by active carbon, commonly used), or hazardous by-products such as chlorination.

As described, compared to existing technologies, the process of the invention has, among others, the following advantages:



On the one hand, it allows to obtain a better efficiency than the technologies that are based on fixing the TiO2 nanoparticles to a solid material surface that is subjected to continuous washing. In this way, it is possible to retain them in the rector as if they were fixed, which allows them to be retained without losing performance and without requiring extra costs for their separation. Specifically, the dispersed, separable and reusable TiO2 guarantees an effectiveness up to 10 times greater than the products usually supported;

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

On the other hand, it allows the separation of nanoparticles by decantation without the need to add any type of reagents (such as flocculants, coagulants or pH correctors), or complicated nanofiltration.

The process described and claimed can be used in conventional water treatment stations, whether urban wastewater, industrial wastewater, drinking water or cooling water, in order to eliminate contaminants present in them, not removable or very expensively removable by other existing means. Among these substances, dangerous for human health and the environment, it is worth highlighting:



Organic or inorganic oxidizable matter such as ammonium and COD not biodegradable by conventional biological means in urban or industrial wastewater or waste leaching products;



Pathogenic microorganisms in sewage, drinking or process water, which are usually eliminated by chlorination or exposure to UV ultraviolet radiation, such as Escherichia coli, Salmonella or Legionella;



Hazardous or preferred substances (understood as such the substances selected for presenting a significant risk as described in Directive 2008/105 / EC, transposed by Spanish legislation according to Royal Decree 60/2011) in both urban and industrial wastewater, such as for example, waters from pharmaceutical industries;



AOX from cooling towers.

The object of the invention is also the photocatalytic reactor for carrying out said process, which is characterized in that it comprises at least one reaction chamber, a physical separation medium (preferably by decantation) and a suitable means for the recirculation of the nanoparticles. of TiO2 separated, in order to be reused in the reactor. In case of working in discontinuous or semicontinuous, the same reaction chamber could function as a means of separating the TiO2 nanoparticles (decanter), also having means for extracting the supernatant. In this case, however, the means for the recirculation of the TiO2 nanoparticles would not be necessary, since these would be retained in the chamber itself as a decanted product, being able to be used for the next cycle. In other embodiments, instead of a single equipment, different units could be arranged for the different phases of the process, located one after the other.

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Brief description of the figures

Figure 1 shows the process diagram divided into 4 phases, where:



(1) corresponds to Phase 1 of advanced oxidation or photocatalytic reaction;



(2) corresponds to Phase 2 of separation of the TiO2 nanoparticles, preferably by decantation;



(3) corresponds to Phase 3 emptying of the supernatant; Y



(4) corresponds to Phase 4 or filling phase, during which it feeds on

again the reactor with the liquid medium to be treated; Figure 2 shows the results of the decantation of the TiO2 nanoparticles used. It shows both the reduction of TiO2 nanoparticles in the medium, and the decrease in turbidity in the same against time during a phase of gravity decantation; Figure 3 shows the results of the elimination of organic matter versus the time measured with two organic substances. The first graphs show the elimination of caffeine and the following of 2,4-dichlorophenol; Figure 4 shows the recovery procedure and the reuse results of the photocatalytic product (TiO2) used after recovery; Figure 5 shows the elimination of pathogenic microorganisms measured as removal of Enterococci and Escherichia coli against time.

Detailed description of the invention

The previous figures show the results of the tests performed to demonstrate the effectiveness of the claimed photocatalytic process.

In the test of the results shown in Figure 2 the photocatalytic process was carried out by keeping the TiO2 nanoparticles in suspension at a water concentration of 1g / l. Once the process was finished, the TiO2 nanoparticles were allowed to decant. In said figure 2 the evolution of turbidity and the percentage reduction of TiO2 nanoparticles in the aqueous medium over time can be observed.

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Figure 3 shows the results of the caffeine and 2,4-dichlorophenol elimination tests as a function of time at various concentrations of TiO2 (from 0 to 2 g / l). The tests were carried out in a solar chamber (500 W / m2). Figure 3 shows the results obtained with two commercial TiO2 products treating 45 mg / l of caffeine in distilled water and 100 mg / l of 2,4-dichlorophenol in ultrapure water.

Figure 4 shows the elimination of caffeine in the different reuses of TiO2 separated by decantation treating 45 mg / l of caffeine. The reuse is carried out according to the procedure shown in the figure.

Figure 5 shows the removal of Enterococci and E. Coli performed in a bacterial suspension in 0.9% NaCl in synthetic wastewater. The figures show the percentage of elimination of these parameters with and without TiO2 particles in suspension.

The tests demonstrate that, as described, the claimed process is applicable to the use of any photocatalyst product that meets a series of conditions, such as those derived from the results of the tests as shown in the accompanying figures. This description and which are detailed below:

(to)

an activity assured as a photocatalyst with an elimination of organic matter measured as elimination of caffeine and dichlorophenol with an elimination yield of 80% in 60 minutes for caffeine and 60% in 200 minutes for 1,2-dichlorophenol, as shown in the figure 3;

(b)

an activity secured as a photocatalyst with a removal of at least 6 logarithmic units in 15 minutes for Escherichia coli, as shown in Figure 5;

(C)

a minimum reuse capacity, understanding as such that less than 10% efficiency of the nanoparticles is lost after reusing the same catalyst 100 times;

(d)

a minimum settling capacity of 95% in 15 minutes, as shown in Figure 2.

Claims (9)

image 1 1. Continuous photocatalytic process for the purification of a liquid medium characterized by comprising:

(to)

a first stage of photocatalytic reaction of the liquid medium in the presence of at least one photocatalyst consisting of TiO2 nanoparticles, where said photocatalytic reaction is carried out in at least one fully agitated photocatalytic reactor;

(b)

once the photocatalytic reaction is finished, a step of physical separation of the TiO2 nanoparticles is carried out, resulting in a purified supernatant;

(C)

finally, the purified supernatant is extracted and the TiO2 nanoparticles separated in the previous stage are returned to the reactor for reuse as a photocatalyst.

2.

Process according to claim 1, wherein the TiO2 nanoparticles are contained in the photocatalytic reactor in a concentration between 0.5 and 2.5 g / l.

3.

Process according to claim 1 or 2, wherein the TiO2 nanoparticles are immobilized in nanoagglomerations by means of a binder additive.

Four.

Process according to any one of claims 1 to 3, wherein the physical separation is carried out by decantation.

5.

Process according to any one of the preceding claims, characterized in that it comprises an additional stage of purging the TiO2 nanoparticle stream that is returned to the reactor.

6.

Process according to any one of the preceding claims, characterized in that it comprises an additional stage of filtration of the purified supernatant.

7.

Process according to any one of claims 1 to 6 wherein the liquid medium contains at least one hazardous or preferred substance as described in RD 60/2011.

8.

Process according to any one of claims 1 to 6 wherein the liquid medium is wastewater, potable or process water and contains at least one pathogenic microorganism for human health.

12 image2 9. Photocatalytic reactor for carrying out a process according to any one of claims 1 to 8, characterized in that it comprises at least one reaction chamber, a decanter and a suitable means for the recirculation of the TiO2 nanoparticles separated in the decanter 10. Reactor according to claim 9, wherein the reaction chamber and the decanter are located in series, one after the other. 11. Reactor according to claim 9, wherein the reaction chamber and the decanter They form the same team. fifteen 13