Xavier Domènech

Heterogeneous Photocatalysis for Purification, Decontamination and Deodorization of Air

A research review of gas–solid heterogeneous photocatalysis is presented, ranging from details of pioneering works, which dealt with basic phenomena like oxygen and water vapor adsorption, to recent applications to pollutant removal in contaminated atmospheres. Special interest is taken in describing the different reactor configurations studied so far in this emerging and promising field.

Aniline mineralization by AOP's: anodic oxidation, photocatalysis, electro-Fenton and photoelectro-Fenton processes

Abstract The aniline degradation in acidic medium of pH≈3 under photocatalytic and electrochemical conditions has been investigated. The efficiency for substrate mineralization in each process has been comparatively analysed by the decrease in TOC of aniline solutions. Particular emphasis has been made on the role of Fe(II) ions and H 2 O 2 . The electrochemical experiments performed in the presence of both species (electro-Fenton conditions) leads to a fast aniline mineralization, which is notably increased by UVA irradiation (photoelectro-Fenton process). In photocatalysis with TiO 2 suspensions, the presence of H 2 O 2 and Fe(II) ions in solution notably increases the aniline degradation rate at the initial stages of the process, whereas the opposite effect occurs at long irradiation times. Benzoquinone, hydroquinone, nitrobenzene, phenol and 1,2,4-benzenetriol were detected as intermediates by HPLC in both, electrochemical and photocatalytic experiments. Short chain aliphatic acids, such as maleic and fumaric acids, were only found in the electrochemical experiments. Ammonium ions (75–80% of initial nitrogen) were generated in all solutions tested. A general reaction pathway that accounts for aniline mineralization to CO 2 involving those products is proposed.

Fenton and photo-Fenton oxidation of textile effluents

The simultaneous use of Fenton reagent and irradiation for the treatment of textile wastewaters generated during a hydrogen peroxide bleaching process is investigated. The experimental conditions tested during this study provide the simultaneous occurrence of Fenton, Fenton-like and photo-Fenton reactions. The batch experimental results are assessed in terms of total organic carbon reduction. Identification of some of the chemical constituents of the effluent was performed by means of GC-MS. Other pollution related features of the initial effluent-like COD and color were also measured. The main parameters that govern the complex reactive system, i.e., light intensity, temperature, pH, Fe(II) and H2O2 initial concentrations have been studied. Concentrations of Fe(II) between 0 and 400 ppm, and H2O2 between 0 and 10,000 ppm were used. Temperatures above 25 degrees C and up to 70 degrees C show a beneficial effect on organic load reduction. A set of experiments was conducted under different light sources with the aim to ensure the efficiency of using solar light irradiation. The combination of Fenton, Fenton-like and photon-Fenton reactions has been proved to be highly effective for the treatment of such a type of wastewaters, and several advantages for the technique application arise from the study.

Aniline degradation by combined photocatalysis and ozonation

Abstract The combination of TiO 2 -assisted photocatalysis and ozonation in the degradation of aniline in aqueous solution is investigated. From the experimental results obtained it is observed that the ozonation pretreatment followed by photocatalysis strongly increases the yield of TOC removal in comparison to either ozonation or photocatalysis carried out separately. The opposite sequence (photocatalysis pretreatment followed by ozonation) does not enhance the efficiency of aniline degradation. Nevertheless, the highest TOC removal was achieved by simultaneous ozonation and photocatalysis. A mechanism involving the formation of an ozonide anion radical previous to the generation of OH radicals is suggested to explain the synergic effect between ozone and TiO 2 under illumination.

2,4-Dichlorophenoxyacetic acid degradation by catalyzed ozonation: TiO2/UVA/O3 and Fe(II)/UVA/O3 systems

The oxidative degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) by means of two different advanced oxidation technologies based on ozone activation has been studied. Ozonation of 2,4-D in presence of UVA light and TiO2, and in presence of UVA and Fe(II), has been carried out. Both treatment methodologies have been compared in terms of the dependence on several experimental parameters like reaction time, pH, light intensity and catalyst concentration on reaction efficiency. It is shown that the initial solution pH practically has no influence on the efficiency of the processes, due to the fact that the pH rapidly decreases during the first minutes of reaction, always attaining a final value close to pH 3. A linear variation of TOC removal with square root of light intensity has been observed. Some dependence of the amount of catalyst (TiO2 or Fe(II)) in solution on TOC degradation has been detected. The rapidness of substrate dehalogenation has also been examined by following the time course of chloride concentration in solution.

TiO2 deactivation during gas-phase photocatalytic oxidation of ethanol

The deactivation of TiO2 Degussa P25 during the gas-phase photocatalytic oxidation of ethanol has been studied. Water vapor plays a clear competitive role for surface sites adsorption, thus hampering the ethanol photo-oxidation. Dark adsorption of ethanol on a fresh catalyst shows a Langmuirian behavior with the formation of a monolayer of adsorbate. Dark adsorption in a TiO2 surface that has been used in consecutive photocatalytic experiments of ethanol degradation gives non-Langmuirian isotherms, indicating the existence of noticeable changes of the catalyst surface structure. After several irradiations the catalyst activity decreases. Such deactivation has been investigated, observing that the rate constant of ethanol and acetaldehyde (its main degradation product) oxidation decreases with irradiation time. Several surface treatments have been studied in order to find suitable procedures for catalytic activity recovery, but regular decay of activity is always observed after every treatment.

Photocatalytic degradation of short-chain organic diacids

Abstract The heterogeneous photocatalytic oxidation of fumaric, maleic and oxalic acids over TiO 2 has been investigated. For aqueous suspensions at pH lower than the point of zero charge (pzc) of TiO 2 , the photocatalytic degradation of the three studied diacids follows the Langmuir–Hinshelwood kinetic model, with the rate constant of the process decreasing in the order oxalic acid > maleic acid ≅ fumaric acid. At these low pH media, the adsorption of the organic diacids onto TiO 2 particles is a key feature for their degradation, which is initiated by a photo-Kolbe process. For fumaric and maleic acids, a cis – trans isomerisation induced by the interaction between adsorbed molecule and semiconductor surface occurs. At pH’s higher than the pzc of TiO 2 the rate of oxalic acid oxidation decreases noticeably, while fumaric and maleic acids are both efficiently degraded in homogeneous phase by reacting with OH radicals photochemically generated on the TiO 2 surface, giving rise to a significant increment of both isomers degradation rate with increasing pH. At these pH’s higher than the pzc of the TiO 2 , the three studied diacids show a very low degree of adsorption onto the semiconductor surface and no evidence of cis – trans isomerisation for both maleic and fumaric acids is detected. In accordance with the observed pH effects on degradation rate and over detected intermediates, a different mineralisation pathway is proposed as function of initial pH.

Replacement of H2O2 by O2 in Fenton and photo-Fenton reactions

The consumption of oxygen during the degradation of aniline by Fenton and photo-Fenton reactions is studied. The effect that parameters like aniline, Fe(II) and H2O2 initial concentration, pH, temperature and O2 flow rate have on the ratio O2 consumed/H2O2 consumed is examined. The determination of those combinations of experimental conditions for which an effective partial replacement of H2O2 by O2 as electron acceptor takes place is investigated. The results show that this replacement takes place in a variable extent, but the presence of H2O2 is necessary along the reaction, and the maximum consumption of O2 only takes place when the ratio amount of aniline mineralized vs. initial aniline concentration is minimal.

Redox photodegradation of 2,4-dichlorophenoxyacetic acid over TiO2

The photodegradation of 2,4-dichlorophenoxyacetic acid using titanium dioxide as photocatalyst was investigated. The yield of 2,4-D photodegradation depends on the mass of the semiconductor, temperature and light intensity. The highest conversion rates were obtained at pH = 3.5, which is close to the pKa of the acid and lower than the point of zero charge of TiO2. The experimental data fit a Langmuir-Hinshelwood kinetic model. The rate and equilibrium adsorption constants were obtained from those data at different temperatures. Some aromatic intermediate products of 2,4-D photodegradation were detected (2,4-dichlorophenol, hydrohydroquinone, chlorohidroquinone). An analysis of total organic carbon (TOC) showed that a complete mineralization of 2,4-D can be easily achieved. On the other hand, the results obtained in experiments using platinized TiO2 lead us to assume that both oxidation and reduction steps should be taken into account if an explanation of the experimental data is to be made.

Photocatalyzed Degradation of Phenol, 2,4‐Dichlorophenol, Phenoxyacetic Acid and 2,4‐Dichlorophenoxyacetic Acid over SupportedTiO2 in a Flow System

The photodegradation of phenol, 2,4-dichlorophenol, phenoxyacetic acid and 2,4-dichlorophenoxyacetic acid using TiO 2 as photocatalyst is investigated. The photodegradations of these compounds have been conducted in a continuous mode by means of a flow system, in which TiO 2 remains fixed onto glass pearls. The use of this system gives high yields of degradation for the chemicals tested, except for 2,4-dichlorophenol for which a slow dechlorination process is observed. The rate of photodegradation depends on the pH of the solution, the point of zero charge of TiO 2 and the pK a of the chemicals being the key parameters. The main aromatic intermediates detected have been hydroquinone, paraquinone and hydrohydroquinone during phenol degradation ; phenol and hydroquinone during phenoxyacetic acid degradation ; chlorohydroquinone and chlorophenol during 2,4-dichlorophenol degradation ; and dichlorophenol during 2,4-dichlorophenoxyacetic acid degradation. Finally, some long term irradiations with phenol as model compound have been performed, showing high degrees of photodegradation. It has been observed that only a periodic evacuation of the effluent out of the reactor is needed to sustain high percentages of photodegradation.