Examinando por Autor "Marugan, Javier"

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Comparison of the photocatalytic disinfection of E.COLI suspensions in slurry, wall and fixed-bed reactors
(ELSEVIER, 2009) van Grieken, Rafael; Marugan, Javier; Sordo, Carlos; Pablos, Cristina
The performances of different configurations of photoreactors were compared for the photocatalytic disinfection of E. coli aqueous suspensions and methylene blue photodegradation. Titania was immobilized in an annular reactor in two different ways: on the inner reactor wall and on the packing of a fixed bed. The influence of the increase in the TiO2 layer thickness has been studied, and the results have been compared with those obtained with TiO2 slurries of increasing concentration. Experimental results for methylene blue degradation were in agreement with those expected from the characterization data of the immobilized systems, but they did not fit with the variation of the activity for microorganisms inactivation. The increase in the density of the TiO2 film caused by the heat treatment carried out after every coating cycle reduce the TiO2 surface available for the interaction with bacteria, although it remains accessible for the dye molecules. Although immobilized systems show a lower disinfection activity in deionized water than TiO2 slurries, they show a lower inhibition by the presence of organic matter, leading to comparable irradiation times to reach bacterial concentrations below the detection limit in the treatment of wastewater treatment plant effluents. Moreover, immobilized systems have shown that they are stable and do not present deactivation after several cycles of reuse, being readily applicable for continuous water treatment systems.
Intrinsic kinetic modelling with explicit radiation absorption effects of the photocatalytic oxidation of cyanide with TiO2 and silica-supported TiO2 suspensions
(ELSEVIER, 2008) Marugan, Javier; van Grieken, Rafael; Cassano, Alberto E.; Alfano, Orlando M.
This study is focused on the kinetic modeling of the photocatalytic oxidation of cyanide in slurry reactors. The developed model is based on an accepted reaction mechanism and takes into account explicitly the differences in the local volumetric rate of photon absorption (LVRPA) produced by the unavoidable radiation profiles existing in the photoreactor. The model and its correspondent procedures for the evaluation of the LVRPA distribution and the estimation of the kinetic parameters have been successfully validated with both powder TiO2 and TiO2/SiO2 photocatalysts with improved recovery properties. In both cases, the model reproduces the influence of the catalyst loading, the initial cyanide concentration, and the inlet radiation flux on the reaction rate, with errors below 5 %. The kinetic parameters estimated for the model are independent of the irradiation form, as well as the reactor size and its geometrical configuration, providing the necessary information for scaling-up and designing commercial scale photoreactors.
Kinetics of the photocatalytic disinfection of Escherichia coli suspensions
(ELSEVIER, 2009) Marugan, Javier; van Grieken, Rafael; Sordo, Carlos; Cruz, Cristina
The photocatalytic inactivation of E. coli suspensions has been successfully modelled with kinetic equations based on a simplified reaction mechanism using three parameters: kinetic constant (k), pseudo-adsorption constant (K) and inhibition coefficient (n). This model has been used to fit complex bacterial inactivation curves with shoulder and tails regions in addition to the classical log-linear behaviour. Experiments performed with increasing concentrations of titania, using TiO2/SiO2 photocatalysts and waters with different composition indicates that the most sensitive parameters are the kinetic and the pseudo-adsorption constant, whereas the values of the inhibition coefficient do not seem to be influenced by the experimental conditions in the range studied. The chemical composition of the water strongly influences the efficiency of the disinfection process. However, the effect of different inorganic anions has been found to be produced at very different concentration levels and by different mechanisms, which also affects the values of the kinetic and pseudo-adsorption constants in different ways. Similarly, low concentrations of humic substances inhibit the disinfection process whereas the same concentration of sucrose does not affect at all. Consequently, the macroscopic analysis of real waters based on conductivity and total organic carbon measurements must be carefully considered, as differences in the nature of the inorganic and organic substances present in similar waters could lead to unexpected results.
On the comparison of photocatalysts activity: A novel procedure for the measurement of titania surface in TiO2/SiO2 materials
(ELSEVIER, 2007) Marugan, Javier; Lopez-Muñoz, Maria Jose; Aguado, José; van Grieken, Rafael
The distribution of two different phases in a mixed oxide material could be investigated through several physicochemical characterization techniques. However, the estimation of the fraction of the total surface area corresponding to each oxide is a very difficult task. In this work we present a novel procedure for the determination of the titanium dioxide surface in titania-silica materials. This new method is based on the measurement of the phosphorus content of the mixed oxide after reaction with phenylphosphonic acid. The quantification of the TiO2 surface has permitted the comparison of the catalytic activity of different materials in processes in which titanium dioxide is the only catalytically active phase and silica behaves as an inert support, as for instance in photocatalytic reactions. The activity of several TiO2/SiO2 photocatalysts for cyanide and methanol photooxidation have been analysed and compared with pure TiO2 materials in terms of equal mass of semiconductor, photonic efficiency and active surface area. The results suggest the possibility of achieving surface activity rates even higher than the material Degussa P25 when using nanocrystalline titania supported on silica.
Photocatalytic degradation of iron cyanocomplexes by TiO2 based catalysts
(ELSEVIER, 2005) van Grieken, Rafael; Aguado, José; Lopez-Muñoz, Maria-José; Marugan, Javier
The removal of iron cyanocomplexes in industrial effluents is a difficult process, due to the resistance of these compounds to conventional treatments for cyanide wastewater detoxification. The mechanism of both the homogeneous photolysis of these compounds and their heterogeneous photocatalytic oxidation with Degussa P25 TiO2 and silica-supported TiO2 photocatalysts have been investigated. The activities of the tested catalysts for complexed cyanide degradation were found to be different from those observed for free cyanide photo-oxidation. The best activity was found for the photocatalyst synthesized by supporting 20 wt% of TiO2 on SBA-15 silica as compared with the commercial catalyst Degussa P25 and the other supported catalysts tested. On the basis of detected intermediate species, a mechanism for iron cyanocomplexes photodegradation is suggested. The influence of the textural properties of the support and titania loading on the process is discussed. The results point out that the high activity observed when SBA-15 is used as support of TiO2 seems to be related to the microporosity of the material acting as molecular sieve which avoids the deactivation of the semiconductor. The porous structure of the SBA-15 material limits the access of the iron cyanocomplexes to the TiO2 particles whereas the free cyanides homogeneously released can reach the semiconductor surface, being subsequently oxidized to cyanate.
Photocatalytic inactivation of bacteria in water using suspended and immobilized silver TiO2
(ELSEVIER, 2009) van Grieken, Rafael; Marugan, Javier; Sordo, Carlos; Martinez, Patricia; Pablos, Cristina
Incorporation of silver to titanium dioxide is of great interest for photocatalytic disinfection applications since in addition to the enhancement of the electron-hole separation and interfacial charge transfer and the increase in the visible light response, silver compounds present a strong bactericidal effect. Ag/TiO2 materials used in suspension and immobilized in two different configurations (catalytic wall and fixed-bed reactors) have been prepared, characterized and tested using Escherichia coli as model microorganism. Although the incorporation of silver to powdered Degussa P25 TiO2 increases the activity, the thermal treatment required for the stabilization of the supported metal particles reduces the global efficiency. The comparison with experiments of dye photodegradation indicates that the activity of Ag/TiO2 is mainly due to the bactericidal role of silver and not to the enhancement of the photocatalytic mechanism. The best tested system has been proved to be the Ag/TiO2 catalytic wall reactor with a 0.6 wt% of Ag loading, showing a high activity both in relative (per gram of TiO2) and absolute terms, an optimal use of the radiation source, and a good stability of the film with negligible silver lixiviation, allowing the continuous treatment of water.
Photonic efficiency for methanol photooxidation and hydroxyl radical generation on silica-supported TiO2 photocatalysts
(ELSEVIER, 2006) Marugan, Javier; Hufschmidt, Dirk; Lopez-Muñoz, Maria-Jose; Selzer, Volker; Bahnemann, Detlef
Hydroxyl radical species are considered to be responsible for many oxidation pathways of chemical compounds initiated by advanced oxidation technologies and particularly in heterogeneous photocatalytic processes. However, not many attempts have been made to quantify the generation rate of these species for newly developed photocatalysts, especially for the large number of supported photocatalysts synthesized during the last years. This work focuses on the evaluation of the photonic efficiency for the hydroxyl radical generation on several silica supported TiO2 photocatalysts recently developed. The study has been carried out by using methanol as hydroxyl radical scavenger. The influence of the mesoporous structure of the support and of its titania loading on the photonic efficiency of the materials has been analyzed. The importance of diffusional restrictions within the porous structure of the support has also been investigated by using a larger hydroxyl radical scavenger molecule, namely n-butanol.
Scaling-up slurry reactors for the photocatalytic oxidation of CYANIDE with TiO2 and SILICA-SUPPORTED TiO2 suspensions
(ELSEVIER, 2009) Marugan, Javier; van Grieken, Rafael; Cassano, Alberto E.; Alfano, Orlando M.
A scaling-up methodology for the design of large scale slurry reactors for the photocatalytic oxidation of cyanide is proposed. The only experimental information required to be determined at laboratory scale is the intrinsic kinetics that describes the explicit dependence of the reaction rate with the local volumetric rate of photon absorption (LVRPA). Based on the kinetic model and the information about the geometry, irradiation source and the cyanide and catalyst concentrations as operation conditions, the performance of a larger scale reactor has been simulated following a predictive procedure with no adjustable parameters. The validation of the method has been carried out in a bench-scale reactor with ten times higher irradiated volume and a different geometry and irradiation source, in order to ensure that the conclusions about the applicability of the scaling-up model are independent of these parameters. The proposed scaling-up methodology and their correspondent procedures for the evaluation of the LVRPA distribution on the photoreactors has been successfully validated both with commercial TiO2 and a silica-supported TiO2 synthesized in our laboratory. The normalized root mean square error in the verification of the conversions predicted by the model for the larger scale reactor when compared with the experimental data are 7.7% and 6.2 % for TiO2 and TiO2/SiO2, respectively.
Solar photocatalytic degradation of dichloroacetic acid with silica-supported titania at pilot plant scale
(ELSEVIER, 2007) Marugan, Javier; Aguado, José; Gernjak, Wolfgang; Malato, Sixto
A multivariate analysis using experimental design techniques was performed to determine the effect of iron, hydrogen peroxide and titanium dioxide in the solar photodegradation of dichloroacetic acid in a combined TiO2/photo-Fenton process. The study was carried out at pilot-plant scale, using TiO2 supported on silica as the heterogeneous photocatalyst to facilitate separation of the solids after the reaction, and iron concentrations of less than 3 mg·L-1 to avoid removal of iron from the effluent. The results show that iron is the most important factor influencing the reaction rate, which suggests that in strongly acidic solutions, the Fenton mechanism controls the process even at such low iron concentrations. Under these conditions, the expected synergism between TiO2 and iron degradation pathways seems to be negligible compared to the antagonistic effect between hydrogen peroxide and TiO2, which reduces the activity of the combined system. However, in the absence of hydrogen peroxide, the activity achieved by the combined Fe/TiO2 system is similar to that of the photo-Fenton process, with the advantage that H2O2 is replaced by a reusable TiO2-based catalytic material. Consequently, the analysis of pilot-plant operation economics took not only the degradation rate, but also the cost of chemicals into account.
Synthesis, characterization and activity of photocatalytic SOL-GEL TiO2 powders and electrodes
(ELSEVIER, 2009) Marugan, Javier; Christensen, Paul; Egerton, Terry; Purnama, Herry
The efficiency of photoelectrocatalytic processes is strongly influenced by the electrode type and synthesis procedure. This work reports the sol-gel synthesis and characterization of TiO2 and Fe-doped TiO2 powders and electrodes as a function of pH and preparation temperatures and the correlation of their photoelectrochemical properties with their activity for azo dye decolourization. pH is shown to be the variable that most influences the formation of the TiO2 crystalline phases, the photocurrents of the electrodes, and consequently their photocatalytic activity. Electrodes synthesized at low pH and heated at temperatures below ~ 600 oC show a significant increase in the photocurrent recorded in the presence of methanol. This is attributed to suppression of the charge recombination rather than to a conventional current-doubling mechanism. The photocatalytic activities of the electrodes show a good correlation with the photocurrent measured in the presence of methanol. The mechanistic implications of this correlation are discussed. Iron-doping reduces the activity of the sol-gel electrodes for dye decolourisation. This contrasts with previous results for bacteria disinfection, which is believed to proceed by a hydroxyl radical mechanism. Consequently, it may be that the behaviour of iron as recombination centres or as trapping sites that improve the separation charges may depend on whether the photoelectrocatalytic reaction proceeds by a hydroxyl radical mechanism, as was shown for deactivation of E. coli or by direct hole-transfer, as is suggested by the parallels between dye decolouration and the electrode response to methanol that are reported in this paper.