Marta I. Litter

Heterogeneous photocatalysis: Transition metal ions in photocatalytic systems

Abstract The presence of transition metal ions in photocatalytic reactions is reviewed according to two main approaches: (a) the influence of transition metal ions on the rate of photocatalytic reactions (mainly oxidation) and (b) the transformation of the ions to less toxic species or their deposition on the semiconductor catalyst surface for recovery of expensive and useful metals. Most of the proposed mechanisms are discussed, together with experimental physicochemical evidences that support the involved pathways. Practical applications related to environmental protection and industrial processes are described.

Photocatalytic properties of iron-doped titania semiconductors

An exhaustive review on the photochemical properties of iron-doped TiO2 semiconductors is presented. Photocatalytic reactions (reductions and oxidations) using Fe-containing TiO2 on different organic and inorganic substrates are reported. Different aspects relating to structural, surface and photophysical properties of these photocatalysts are extensively discussed. The origin of the photoactivity of this kind of mixed oxides is considered with regards to previously proposed physical and chemical processes and on the role of the iron content.

Photocatalytic bactericidal effect of TiO2 on Enterobacter cloacae: Comparative study with other Gram (−) bacteria

Abstract The bactericidal action of heterogeneous photocatalysis (UV-A/TiO 2 ) has been tested on Enterobacter cloacae , a microorganism very resistant to UV-A irradiation. Results have been compared with other representative strains of Gram (−) bacilli of different photosensitivity like E. coli , Pseudomonas aeruginosa and Salmonella typhimurium . The TiO 2 photocatalytic technology can inactivate bacteria resistant to oxidative membrane damage caused by direct UV irradiation, like E. cloacae , a common soil and aquatic microorganism, which normally is not affected by low UV-A irradiation intensity. In all cases, sublethal UV-A doses provoked an important lethality in the presence of TiO 2 . Inactivation rates of the microorganisms are compared and some clues on the mechanism of bacteria destruction are discussed.

One century of arsenic exposure in Latin America: a review of history and occurrence from 14 countries.

The global impact on public health of elevated arsenic (As) in water supplies is highlighted by an increasing number of countries worldwide reporting high As concentrations in drinking water. In Latin America, the problem of As contamination in water is known in 14 out of 20 countries: Argentina, Bolivia, Brazil, Chile, Colombia, Cuba, Ecuador, El Salvador, Guatemala, Honduras, Mexico, Nicaragua, Peru and Uruguay. Considering the 10 μg/L limit for As in drinking water established by international and several national agencies, the number of exposed people is estimated to be about 14 million. Health effects of As exposure were identified for the first time already in the 1910s in Bellville (Córdoba province, Argentina). Nevertheless, contamination of As in waters has been detected in 10 Latin American countries only within the last 10 to 15 years. Arsenic is mobilized predominantly from young volcanic rocks and their weathering products. In alluvial aquifers, which are water sources frequently used for water supply, desorption of As from metal oxyhydroxides at high pH (>8) is the predominant mobility control; redox conditions are moderate reducing to oxidizing and As(V) is the predominant species. In the Andes, the Middle American cordillera and the Transmexican Volcanic Belt, oxidation of sulfide minerals is the primary As mobilization process. Rivers that originate in the Andean mountains, transport As to more densely populated areas in the lowlands (e.g. Rímac river in Peru, Pilcomayo river in Bolivia/Argentina/Paraguay). In many parts of Latin America, As often occurs together with F and B; in the Chaco-Pampean plain As is found additionally with V, Mo and U whereas in areas with sulfide ore deposits As often occurs together with heavy metals. These co-occurrences and the anthropogenic activities in mining areas that enhance the mobilization of As and other pollutants make more dramatic the environmental problem.

Heterogeneous photocatalytic reactions comparing TiO2 and Pt/TiO2

Abstract Two commercial samples, namely Degussa P-25 and Sachtleben Hombikat UV 100, and platinized modifications of these samples have been tested comparatively as photocatalysts for EDTA oxidation and for Cr(VI) reduction in the absence and in the presence of EDTA. Platinized photocatalysts were prepared by photoimpregnation of H 2 PtCl 6 on TiO 2 by prolonged irradiation in the presence of methanol. Only slight differences in efficiency were found between both pure commercial forms, except for the Cr(VI) reduction in the presence of EDTA, where Hombikat UV 100 samples were more active. Platinized Hombikat UV 100 was found to be less active for Cr(VI) reduction in water. The concentration of dissolved molecular oxygen was crucial for EDTA oxidation and had no influence on Cr(VI) reduction. From kinetic profiles, initial photonic efficiencies ( ζ ) and conversion degrees have been calculated. Platinization did not improve Cr(VI) reduction but increased the initial photonic efficiency for EDTA oxidation. However, after prolonged irradiation, the conversion of EDTA was lower employing platinized samples instead of pure samples. The effect of platinum on titania was analyzed on the basis of proposed mechanisms.

Possible treatments for arsenic removal in Latin American waters for human consumption.

Considering the toxic effects of arsenic, the World Health Organization recommends a maximum concentration of 10 microg L(-1) of arsenic in drinking water. Latin American populations present severe health problems due to consumption of waters with high arsenic contents. The physicochemical properties of surface and groundwaters are different from those of other more studied regions of the planet, and the problem is still publicly unknown. Methods for arsenic removal suitable to be applied in Latin American waters are here summarized and commented. Conventional technologies (oxidation, coagulation-coprecipitation, adsorption, reverse osmosis, use of ion exchangers) are described, but emphasis is made in emergent decentralized economical methods as the use of inexpensive natural adsorbents, solar light technologies or biological treatments, as essential to palliate the situation in poor, isolated and dispersed populations of Latin American regions.

Synthesis, characterization and photocatalytic properties of iron-doped titania semiconductors prepared from TiO2 and iron(III) acetylacetonate

Specimens of iron-doped titania containing different amounts of Fe (0.5–5%) were prepared from TiO2 (Degussa P-25) and Fe(III) acetylacetonate by the wet impregnation method. Samples were characterized by X-ray diffraction analysis, specific surface area (BET) measurements, SEM-EDX, atomic absorption and IR and diffuse reflectance spectra. From the structural point of view, the samples were similar to those obtained with Fe(NO3)3 · 9H2O as the precursor, but with a more homogeneous distribution of iron for each mixed oxide sample on the particle surfaces but not between particles. The photocatalytic activity of these samples under near-UV irradiation was better for oxalic acid degradation than for EDTA, and similar for both types of mixed oxide samples. Mixed oxides showed however lower activity than TiO2. Some photodegradation under visible irradiation, not occurring with TiO2, could be observed for oxalic acid when using 5% Fe-containing samples.

Glossary of terms used in photocatalysis and radiation catalysis (IUPAC Recommendations 2011)

This glossary of terms covers phenomena considered under the very wide terms photocatalysis and radiation catalysis. A clear distinction is made between phenomena related to either photochemistry and photocatalysis or radiation chemistry and radiation catalysis. The term “radiation” is used here as embracing electromagnetic radiation of all wavelengths, but in general excluding fast-moving particles. Consistent definitions are given of terms in the areas mentioned above, as well as definitions of the most important parameters used for the quantitative description of the phenomena. Terms related to the up-scaling of photocatalytic processes for industrial applications have been included. This Glossary should be used together with the Glossary of terms used in photochemistry, 3rd edition, IUPAC Recommendations 2006: (doi:10.1351/pac200779030293) as well as with the IUPAC Compendium of Chemical Terminology, 2nd ed. (the “Gold Book”, 2006– doi:10.1351/goldbook) because many terms used in photocatalysis are defined in these documents.

Photocatalytic properties of ZrO2 and Fe/ZrO2 semiconductors prepared by a sol-gel technique

The photocatalytic efficiency of pure and 0.5‐20 wt.% Fe-containing ZrO 2 samples, prepared by a sol‐gel technique, was tested in the transformation of environmentally important substrates, such as nitrite, EDTA and Cr(VI) and compared with that of TiO2 (Degussa P-25). All samples were active, although the efficiency was lower than that of TiO 2. Nevertheless, the presence of iron increased the activity for nitrite photooxidation, with the maximum efficiency at 5 wt.% Fe content. At higher Fe content, the efficiency decreased but it was always higher than that of the undoped sample. For EDTA oxidation and Cr(VI) reduction, the same trend was observed. Remarkably, Cr(VI) reduction in the presence of EDTA was strongly accelerated, all zirconia samples being as active as P-25, although initial rates were lower and somewhat detrimentally affected by the presence of iron. The photocatalytic activity depends on the structural, surface and optical properties of the sample, on the preparation conditions and on the nature of the photocatalytic reaction. ©1999 Elsevier Science S.A. All rights reserved.

Comparison of the photocatalytic efficiency of TiO2, iron oxides and mixed Ti(IV)-Fe(III) oxides : photodegradation of oligocarboxylic acids

Abstract Comparative photo-oxidations of three oligocarboxylic acids, i.e. oxalic acid, EDTA and malonic acid, were performed with different catalysts, i.e. TiO 2 (Degussa P-25), α-Fe 2 O 3 , γ-Fe 3 O 4 and two samples of mixed Fe(III)Ti(IV) oxides containing 0.5 wt.% and 5 wt.% Fe respectively. Different irradiation wavelengths were used. It was found that the photocatalytic efficiency depends mainly on the physicochemical properties of the catalyst, the irradiation wavelength, and the redox potential and adsorption ability of the substrate. The effect of surface complexes formed between the oxide and the organic compound was analysed. TiO 2 was found to be the best catalyst. The relative utility and stability of iron oxides and mixed FeTi oxides is discussed.