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Detoxification of waters for their recycling and potabilisation by solar photocatalysis in semi-arid countries

Livrables

UCM: Our research group has prepared two different Ru(II) complexes of anionic and cationic character, respectively, and its potential for photocatalytic oxidation of organic contaminants in water, via singlet oxygen production, has been evaluated by LCEE. CIEMAT: A specially designed titania photocatalyst was prepared by coating Ahlstrom non-woven paper, used as a flexible photocatalytic support, with Millennium PC500 anatase. At the same time, a solar CPC photoreactor with the fixed catalyst inside was used. Several types of reactants were treated: 4-chlorophenol as a model organic pollutant; formetanate, a widely used pesticide in horticulture; a mixture of pesticides used in vineyards; and indigo carmine and Congo red, which are complex multifunctional dye molecules. Besides, other organic pollutants like Orange II and gallic acid were also degraded. Each reaction was performed simultaneously in a solar CPC slurry photoreactor and in the CPC supported catalyst system. Both photoreactors were used under similar solar exposure to better evaluate and validate the results obtained. This new system, in which the final tedious filtration can actually be avoided, constitutes a good alternative to slurries.
Within the framework of this research project, our research group has developed a singlet oxygen photosensitizing material made of porous silicone and an adsorbed polyazaheterocyclic Ru(II) coordination compound [tris(4,7-diphenyl-1,10-phenanthroline) ruthenium(II)]. This photosensitizing material (1.5 m x 4 cm x 2 mm porous silicone strips died with the Ru(II) photosensitizer mentioned above) has been produced at large scale in order to provide our partners with enough material to carry out their water disinfection tests. The key innovative features of this result are the following: - The photosensitizing material can be easily installed in the coaxial or fin-type solar reactor prototypes for water disinfection developed in this project. - The photosensitizing material is very stable when kept in the dark and can be stored for a long time. - The photosensitizing material has an operational life of at least half a year on a daily use basis. - The photosensitizing material installed in the prototypes can disinfect at least 20 L of water per day with an accumulated radiation dose of 0.8 MJ/m2. (100 CFU/mL initial bacteria concentration of E. Coli or E Faecalis). - Comparative disinfection tests carried out with the photosensitizing material prepared in this project and direct solar photolysis or photocatalytic treatment with TiO2 supported on paper show that the photosensitizing material gives the best disinfection results. Availability of photosensitizing materials with high singlet oxygen production efficiencies and long durability are essential requirements for the development of ecologically-friendly water disinfection technologies based on solar light and singlet oxygen photogeneration. Our research group is currently developing new photosensitizers and immobilization procedures on different polymers in order to extend the lifetime of the photosensitizing material and improve the disinfection efficiency.
Recent advances on water disinfection treatments propose singlet oxygen, a reactive oxygen species, as a bactericidal species which can be generated photocatalytically with solar light in aqueous solution and also in heterogeneous phase. Singlet oxygen can therefore be used as an alternative to other typical chemical reagents employed in water disinfection treatment. Its electrophilic character, energy excess (ca. 95 kJ mol 1) and high bimolecular rate constants with biomolecules such as proteins (6 x 107 M 1s 1) and lipids (1 x 105 M 1s 1) makes singlet oxygen a promising bactericidal agent vs. water borne microorganisms. Our research group has developed a new family of singlet oxygen photosensitizers based on polyazaheterocyclic Ru(II) complexes with different structural features. They display singlet oxygen production quantum yields in the 0.2 - 1 range in homogeneous phase and those containing the 4,7-diphenyl-1,10-phenanthroline ligand or its derivatives show very efficient generation of singlet oxygen. We have carried out experiments with aqueous samples containing E. coli or E. faecalis in the presence of RDP2+ [tris(4,7-diphenyl-1,10-phenanthroline)ruthenium(II)] or RSD4 [tris(4,7-diphenylsulfonate-1,10-phenanthrolinyl)ruthenate(II)] photosensitizers, in the dark or under Vis irradiation. No toxic effects were observed for E. coli or E. faecalis after 5 h incubation in the dark with RSD4 10 6 M or with RDP2+ 5 x 10 9 M (due to its very low solybility in water). On the other hand, under Vis irradiation, no disinfection was observed for E. coli or E. faecalis after 4 h irradiation with RSD4 5 x 10 9 M, while a decrease of 5 orders of magnitude in bacteria concentration was observed in the case of RDP2+ photosensitizer. Disinfection assays in homogeneous phase were also performed using suspensions of E. coli in water with RDP2+ or RSD4 at concentrations up to 5.0 x 10 8 M. Significant disinfection (two orders of magnitude decrease in viable bacteria concentration) were obtained after 6 hours irradiation in the presence of the cationic photosensitizer. With the anionic photosensitiser, using the same experimental conditions, low disinfection (ca. 15% decrease in the viable bacteria concentration) has been observed. No efficient disinfection was observed with RSD4 photosensitizer in homogeneous phase while it was clearly observed with RDP2+ (even at nanomolar concentration due to its scarce water solubility). This result can be explained if the anionic character of biological membranes and also of RSD4 photosensitizer is taken into account, which limits the interaction between the microorganism and the sensitizer molecules. On the other hand, the cationic photosensitizer RDP2+ interacts electrostatically with the anionic membrane of the microorganism, leading to efficient inactivation.
Our research group has developed a singlet oxygen photosensitizing material made of porous silicone and an adsorbed polyazaheterocyclic Ru(II) coordination compound [tris(4,7-diphenyl-1,10-phenanthroline) ruthenium(II)]. Singlet oxygen is the lowest excited state of molecular oxygen. As a reactive oxygen species it has an electrophilic character, an energy excess of ca. 95 kJ mol 1 over its ground state and a long lifetime in condensed phase. It is known to inactivate bacteria efficiently since it is able to react with the proteins and lipids present in biological membranes. The key innovative features of this result are the following: - Singlet oxygen is efficiently produced by the Ru(II) photosensitizer immobilized on the polymer support. - The reactive oxygen species has a long lifetime, which is an essential feature in order to allow singlet oxygen diffusion from the silicone material towards the external aqueous phase. - The use of porous silicone displays the following advantages: high oxygen permeability (high oxygen solubility and large oxygen diffusion coefficient), the porosity of the polymer allows easy interaction with microorganisms to promote disinfection, the polymer shows good mechanical, thermal and chemical stabilities, and good optical properties as well. - The use of Ru(II) photosensitizers shows the following advantages: strong light absorption in the visible range (400-550 nm), long excited state lifetimes (in the microsecond range) to allow collisional deactivation by molecular oxygen, and high probability of excited state quenching by oxygen leading to singlet oxygen. The photosensitizing material can be produced at large scale and tests that demonstrate the viability of this material for water disinfection with solar light have been performed with two solar photoreactors with different configurations. Efficient disinfection of gram and gram(+) bacteria has been achieved with solar collectors containing less than 1 m2 of polymer illuminated area, allowing a daily production of 20-40 L of disinfected potable water. On site tests in developing countries such as Morocco, Tunicia and Egypt are currently underway to demonstrate the applicability of this technology to water disinfection in rural communities. Dissemination of these results among public institutions will facilitate the use of this technology and will maximize the benefits in terms of public health and life quality of population in developing countries. UCM: The photosensitizing material (1.5 m x 4 cm x 2 mm died porous silicone strips has been produced at large scale in order to provide our partners with enough stripes to carry out their water disinfection tests. - The photosensitizing material can be easily installed in the coaxial or fin-type solar reactor prototypes for water disinfection developed in this project. - The photosensitizing material is very stable when kept in the dark and can be stored for a long time. - The photosensitizing material has an operational life of at least half a year on a daily use basis (about 1MJ/m2 day, in the 360-700 nm region). - The photosensitizing material installed in the prototypes can disinfect at least 20 L of water per day with an accumulated radiation dose of 0.8 MJ/m2. (100 CFU/mL initial bacteria concentration of E Coli or E Faecalis). PHOTEC: Solar Photocatalytic Decontamination and Disinfection of Water using New Photoreactor (Compound Parabolic Collector. The photocatalytic treatment of water contaminated with E.Coli have been carried out. Tests have been done using: supported titanium dioxide (TiO2 PC500) on non-woven paper sheet; polymer supported Ru(II)-complexes. We can conclude that the use of UV-VIS sunlight to disinfect contaminated drinking water in a full-scale continuous flow solar reactor is promising. CIEMAT: The main objectives of this study are: - Assessment of the disinfection ability of Ru(II) [RuL3] coordination compounds immobilized on polymer strips on E. Coli in two CPC collectors under natural solar irradiation. - Comparison of main reaction parameters: initial concentration of E.Coli, performance of different catalysts, reactor configuration, flow rate. - Study of the disinfection capability of Ru-photosensitizer and TiO2-photocatalyst coupling system. The bacterial strain used is E. Coli K-12. Experimental series: initial conc. 1exp4 CFU/mL and > 1exp4 CFU/mL. 10L/min of flow rate. Results obtained for the supported photosensitizer material are compared with those obtained with other photocatalyst material KN47, developed by Ahlstrom (France). The total irradiation time was 90 minutes. Disinfection at E. Coli concentrations of Co -1exp4 CFU/mL In the range of bacterial concentration under study the Ru(II) [RuL3] catalyst shows a higher disinfection capability than the KN47 Ahlstrom paper.
The efficiency of solar disinfection by heterogeneous photocatalysis was measured. The disinfection was performed with sol-gel immobilized TiO2 films over glass rings. Spring water naturally polluted with coliform bacteria was exposed to sunlight in plastic bottles in solar collectors and the disinfection effectiveness was measured. Total and fecal coliforms quantification was performed by means of the definite substrate method in order to obtain the efficiency of each technique. An important part of this study has been to determine the bacterial regrowth in water after disinfection by heterogeneous photocatalysis. The disinfection with TiO2 turned out to be more efficient than the current method of SODIS for total coliforms as well as for fecal ones. In a sunny day (more than 1000 W/m 2 irradiance), the disinfection with immobilized TiO2 needed fifteen minutes of irradiation to reduce the fecal coliforms content to zero and thirty minutes for total coliforms, less than half times required using SODIS. Bacterial regrowth of total coliforms was observed for SODIS disinfection, more frequently than for fecal coliforms. In contrast, when using the catalyst TiO2 no regrowth was detected, neither for total nor fecal coliforms. The disinfection process using TiO2 keeps water free of coliforms at least for seven days. The demonstration opens the possibility of application of this simple method in rural areas of developing countries. Other possible use is in disinfection of water in short periodes of time in zones where the distribution systems are collapsed by extreme weather phenomena.
To avoid the final tedious filtration of titania photocatalyst when used in slurry and which makes the whole system unrealistic and absoluty not adaptable and transferable to developing countries, we had to deposit the solid on a flexible, photo-inert support: the AHLSTROM paper prepared by J. DUSSAUD (partner n°8) which could be inserted on both types of supports (fin and coaxial) inside the final AQUACAT prototype. The photocatalytic activity of the resulting solid was tested in different reactions of degradation of pollutants: chemicals (phenol), pesticide (diuron, imazapyr), dyes (Congored, Remazol Black 5 , Reactive Red 2). All these pollutants could be successfully totally degraded. In parallel, it was checked that the AQUACAT prototype was efficiently designed: proportionality of the reaction rate with (i) the radiant flux; (ii)the initial concentration and (iii) the mass of catalyst employed directly linked to the number of tubings used. The initial paper underwent some titania leakage. This was remediated by a new preparation of paper-supported titania with less photocatalyst and an increase in the silica binder. The conclusions are that the AQUACAT prototype using supported titania is working in optimal conditions for a given solar radiant flux. The reaction rate was found to be independent of the flow rate. This makes possible the use of a low flow rate (2 L/min). The positive consequences of using low flow rates are the following: (1) preservation of the deposited catalyst from abrasion and leaching of titania from it support. (2) use of a low electrical power provided by the photovoltaic panels for the recirculating pump. (3) conservation of purified (drinkable) water in clean conditions (no recontamination). (4) conservation of the pump in constant steady state working conditions. (5) maintenance of the photocatalytic bed and of the tubings and reservoir of the whole prototype in clean conditions between two successive runs (prevention of the development of biofilms, of fungi, algae, ... detected during the storage of the prototype on the catalyst in contact with humid air).
This result shows the capability for the drinking water disinfection of the solar photocatalysis with TiO2 and Ruthenium complexes -Ru(II)- in a solar photoreactor. Both systems are proven to be sucessfull if used separated. Nevertheless, the combined effect of both disposed one after the other (the order is not important, as it has been proven) yields not significative efficiency improvement, since no synergetic effects could be detected in the experimental series performed. Immobilized Ru(II)-complexes disinfection results Ru(II)-complexes within this project are specifically designated for the drinking disinfection experiments. In the experiments with Ru(II)-complexes immobilized on polymer strips the E. Coli concentration already decreased 4 decimal powers within 60 min. Comparing the Ru(II)-complexes performance as a photosensitizer for bacteria disinfection with a blank experiment (only light, no catalyst), it becomes clear that Ru(II)-complexes involved processes have a strong impact in bacteria deactivation. From achieved results, has been observed that the bacteria concentration in the disinfection experiment with Ru(II) decreases fast, while the concentration of the blank experiment almost keeps being the same. Ru(II) versus immobilized TiO2-P25 on Ahlstrom paper When comparing disinfection capacities of Ru(II)-complexes immobilized on polymer strips and TiO2 P25 immobilized on Ahlstrom (KN47-type) paper matrixes, both in the plane support reactor, the Ru(II) photosensitized disinfection starts faster. That means that the reaction constant is higher and the disinfection performance is better. Effects of TiO2 with Ru(II) in series This part analyses the possible synergetic effect of Ru(II) together with TiO2. To examine this point, the two different catalysts were connected in series to assess the performance of hybrid catalysed systems.It can be observed that the experiment with only Ru(II)-complex has the best disinfection activities, followed by the experiment with KN47 with P25 TiO2 coating. Up to date, a small difference exists to the disinfection results of the experiment with Ru(II) and KN47 with P25 TiO2 coating in series. Therefore, no synergetic effects could be detected in the experimental series performed.
Singlet molecular oxygen is a reactive oxygen species which can be efficiently generated via quenching of the excited state of a sensitizer by ground state molecular oxygen in the so-called photosensitization process. Singlet oxygen can be used for different applications, such as organic synthesis, dye bleaching, disinfection processes and photodynamic therapy. Production of singlet oxygen by solid-supported sensitizers is often employed because the photosensitizer can be readily removed at the end of the process. In order to design the most adequate system for each application, it is essential to carry out a precise quantification of the singlet oxygen production by the supported photosensitizers. In practice, this involves measurement of the singlet oxygen production quantum yields (PHIdelta). While this issue is well solved in homogeneous media due to availability of various standards, it is far from established in heterogeneous systems. Our research group has proposed methylene blue (MB) dyed Nafion films as a convenient reference system to quantify singlet oxygen production in solid samples. We have thoroughly characterized the production of singlet oxygen generated by MB photosensitization in films of Nafion ionomer and we have compared our results with those of two well known standards: MB in methanol and tris(2,2-bipyridyl)ruthenium(II) in acetonitrile. We have determined singlet oxygen production quantum yields of MB in dry Nafion (PHIdelta = 0.24) and in methanol-swollen Nafion (PHIdelta = 0.49). After the characterization of the MB/Nafion system, we have concluded that Nafion films loaded with MB are homogeneous, reproducible and stable systems suitable to be used as a reference for PHIdelta determination in solid phase.
The disinfection of waters intended to human consumption consists above all in eliminating all pathogenic organisms that can affect public health. At the same time, formation of disinfection by-products (DBP) dangerous for health must be avoided. The aim of this work was to evaluate the potential formation of DBP during the dual photocatalysis-photosensitizing treatment. Taking into account principles of oxidation process, only bromate and organo-brominated compounds could be formed among the known DBP. In each studied cases (bromide in pure water, river water with naturally present bromide or river water spiked with bromide) never bromate and brominated organic compound were found in water after 3 to 7 hours of treatment on prototype (3 tubes of TiO2 supported on 1049 or 1048 Ahlstrom papers + 2 tubes of Ru strips), under artifical irradiation (> 350 nm) or sun light irradiation. However, it was observed sometimes a slight decreasing of bromide level in studied waters.
During the AQUACAT project recent advances in solar water treatment (disinfection & decontamination) were confirmed and improved by using TiO2 and Ru(II) complexes as fixed catalyst. Ru(II) complexes are used as an alternative to chlorination or pasteurisation for potable water conditioning, meanwhile TiO2 photocatalyst is used as an unique way for oxidation of organic matter pollution dissolved in drinking water. Our firm has designed a new solar system for water treatment by coupling the contribution and research work of all partners (catalyst supporting, optics, disinfection and decontamination tests, handling recommendations, etc.) and the final result was a simple and robust equipment completely driven by solar energy. This equipment is able to produce potable water from polluted surface waters or wells in remote regions where no tap water and/or electricity are available. Also its design makes it an useful equipment for Scientists and Researchers working on water treatment issues. As a result of all these works our firm has got an important experience and skilled personnel in the engineering design of solar reactors for the use in different fields as disinfection, decontamination, wastewater treatment, fine chemical reactions and also in the design of new equipment for other solar topics as water heating. UCM: Two models of CPC solar photocatalytic collectors have been received from AOSOL (coaxial and fin-type). Both collectors included the TiO2 paper, that was exchanged for the photosensitizing material prepared at UCM to test the effect of singlet oxygen on bacterial disinfection). The supporting units and all the necessary equipment were constructed and assembled by UCM, in order to set up the solar demonstrator reactors. The layout of the units is according to the plans provided by ECOSYSTEM, with slight modifications. From the engineering point of view, both systems operate in closed circulating batch mode, for the batch treatment of up to 30 L of water. Each unit consists of (a) a storage tank (with vent hole), (b) CPC collector unit (coaxial: 4 out of 5 tubes with photosensitizing material; fin-type: 5 out of 7 tubes containing photosensitizing material), (c) pump, (d) radiometer, (e) 2 thermocouples (water inlet & outlet) and datalog, (f) PVC piping and valves and (g) stainless steel frame. After their construction and assembling, the prototypes had been tested by UCM for bacterial disinfection of potable water (Escherichia coli and Enterococcus faecalis at 100 and 10000 CFU/mL initial concentrations, respectively). Bacterial inactivation (reduction of the initial concentration by 2-3 orders of magnitude) has been achieved with an accumulated energy of 0.8 MJ/m2L in the visible region (350-700 nm) with both collector models and types of bacteria, although the fin-type collector seems to be more efficient.