Showing papers by "Marta I. Litter published in 2007"

Heterogeneous photocatalysis of Cr(VI) in the presence of citric acid over TiO2 particles: Relevance of Cr(V)–citrate complexes

Abstract: TiO2-photocatalytic reduction experiments of Cr(VI) (0.8 mM) under near UV (366 nm) irradiation in the presence of citric acid (0 ≤ [citric acid] (mM) ≤ 40) were performed at pH 2 under air bubbling. Addition of citric acid facilitates Cr(VI) reduction, hindering the electron-shuttle mechanism taking place in pure water. TOC monotonously decreases until all Cr(VI) was reduced. The maximum rate of Cr(VI) reduction was attained for an initial citric acid/Cr(VI) molar ratio, R, equal to 1.25, a further increment in R being detrimental; however, Cr(VI) decay in the presence of citric acid was always faster than in its absence. Cr(VI) reduction takes place through Cr(V) species, readily complexed by citrate and detected by EPR spectroscopy. Quantitative EPR determinations indicate that an important fraction (nearly 15%) of the reduced Cr(VI) is transformed to Cr(V)–Cit, which also undergoes a photocatalytic transformation. The detrimental effect taking place at high conversions for R > 1.25 can be ascribed to secondary steps, i.e., the competition between Cr(VI) and Cr(V) complexes for conduction band electrons or to the competition of Cr(V)–Cit and Cit for holes.

Photocatalytic degradation of citric acid under different conditions: TiO2 heterogeneous photocatalysis against homogeneous photolytic processes promoted by Fe(III) and H2O2

Abstract: Degradation of citric acid (Cit) at relatively high concentrations by photocatalysis over TiO2 under air bubbling and near UV-light was tested under different conditions. The reaction was improved by addition of H2O2, Fe(III) or both. Minor amounts of H2O2 were effective to enhance the oxidation and the mineralization. H2O2 addition impacts more on the initial rate, while the effect of Fe(III) is more important after prolonged irradiation. The kinetic profiles were analyzed and compared, and the best ratio of reagents was found (1:1:1 H2O2/Cit/Fe molar ratio). However, a lower amount of Fe (1:1:0.2 H2O2/Cit/Fe ratio) was also effective. Similar experiments under the optimal conditions in the absence of TiO2 were performed for comparison. When both H2O2 and Fe(III) were present, the behavior with and without TiO2 was very similar. So far, 3-oxoglutaric acid was detected as a reaction intermediate. A mechanistic analysis is initiated to explain some features of these complex systems.

Low-cost TiO2 photocatalytic technology for water potabilization in plastic bottles for isolated regions. Photocatalyst fixation

Abstract: Experiments to evaluate the photocatalytic activity of supported TiO 2 to potabilize water in common plastic PET bottles under solar irradiation were performed. Commercial titanium dioxide (Degussa P-25) was applied to different cheap materials-glass rings, glass rods and porcelain beads-by dip coating, or directly to the plastic wall of the bottles. The adherence and stability of TiO 2 on the supports and the photocatalytic activity in bottles under solar irradiation was evaluated using model compounds as 4-chlorophenol and 2,4-dichlorophenoxyacetic acid. Rings were found to be the best glass supports, but PET bottles were superior for this specific application, as no fragile fillings are used, and the materials can be easily fabricated on site.

Field Tests of the Solar Water Detoxification SOLWATER Reactor in Los Pereyra, Tucumán, Argentina

Abstract: The SOLWATER reactor prototype is composed of two tubes containing a supported heterogeneous photocatalyst (Ahlstrom© paper impregnated with titanium dioxide), and two tubes containing a supported photosensitizer (designed and provided by G. Orellana, Universidad Complutense, Madrid, Spain). The tubes are placed on a CPC collector and run in series. Electricity is provided by a solar panel, and the recirculation rate is ca 13 L min -1 . Total volume in the feed tank plus tubes is 20 L. The reactor was designed and constructed by the consortium of a European research project whose objective is on the development of a fully autonomous solar reactor system to purify drinking water in remote locations of developing countries. The prototype was placed in the yard of a shanty house in Los Pereyra, Tucumdn, Argentina. Water to feed the reactor is taken from the shallow aquifer through an open well. This water is contaminated with high counts of coliforms and Enterococcus faecalis. It also contains widely variable levels ofPseudomonas aeruginosa. The chemical composition of the water shows high levels of natural organic matter and of various inorganic pollutants. The reactor has been running since February 22, 2005. This paper presents the results collected in three months of operation. Around 4 hr operation on a sunny day, and 5-6 hr on a cloudy day are required to totally destroy fecal coliforms and Ent. faecalis. Even 24 h after the experiment is concluded, no cultivable bacteria are seen by the membrane filtration method (measured colony forming units after 24 hr=0). On the other hand, a small number of total coliforms remain (a few percent or less of the original count) at the end of some of the latest experiments. Possible explanations for this result are the drop in ambient temperature, the decrease in solar irradiance, and the exhaustion of the catalyst and sensitizer. P. aeruginosa is much more resistant, and only partial destruction is observed in those time intervals. The evolution of chemical parameters is also presented and discussed.