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- Resumen es exacto "The aim of the thesis was to evaluate the effectiveness of heterogeneous photocatalysis with TiO2 for the treatment of the pollutant Cr(VI) in aqueous phase, through its transformation into the less toxic species Cr(III). The effectiveness of heterogeneous photocatalysis with TiO2 under UV light for the reduction of Cr(VI) with the addition of citric acid (Cit) as electron donor agent was studied. The effect of the initial Cit/Cr(VI) molar ratio (RM) on the Cr(VI) reduction rate was studied (Cr(VI) = 0,8 mM, 0 ) RM ) 500); it was found that the addition of Cit facilitates the photocatalytic reduction, and that the rate of Cr(VI) conversion is a function of RM, with an optimal value at 1,25, although higher reduction rates at RM = 500 were found.
The effect of pH on the Cr(VI) reduction was studied at RM = 2,5; it was observed that the reduction rate decreases as pH increases, with a change on the kinetics of Cr(VI) reduction from a first-order behavior (pH 2) to a biexponential rate (3 ) pH ) 6). It was
noted that once Cr(VI) is completely reduced, Cit and NPOC degradation are arrested, which could be due to the inactivation of the catalyst by deposition of Cr(III) species on its surface. Electronic paramagnetic resonance (EPR) measurements carried out on the TiO2/Cr(VI)/Cit system allowed to explain the initial stages of the photocatalytic Cr(VI) reduction. It was confirmed that the reduction is initiated by an one-electron process forming Cr(V), which combines with Cit forming the Cr(V)-Cit2 complex. The quantitative measurements indicate that approximately 10% of reduced Cr(VI) is transformed into Cr(V)-Cit2, and that an increase in the Cit/Cr(VI) molar ratio increases the rate of Cr(V)-Cit2 formation up to a maximum at RM = 2,5, while the rate of disappearance of the complex decreases with increasing RM, due to its higher stability at higher Cit concentrations. The fraction of Cr(VI) transformed into Cr(V)-Cit2 is independent of the initial Cr(VI) concentration, but it decreases with increasing pH. The temporal Cr(V)-Cit2 evolution was adjusted to a first order kinetic model of formation and subsequent disappearance. We also studied the effectiveness of the heterogeneous photocatalysis with TiO2 modified with hydroxoaluminiumtricarboxymonoamide phthalocyanine (AlTCPc), an organic dye, for Cr(VI) reduction under visible irradiation (; > 500 nm), using 4- chlorophenol (4-CP) as electron donor agent. We evaluated the importance of TiO2 modified as active support of the dye by comparing the photocatalytic activity with an
XVI equivalent sample where the dye was adsorbed on alumina (Al2O3), an inert support. It was observed that, under visible irradiation, the photocatalytic Cr(VI) reduction only took place when the dye-modified TiO2 (TiO2-AlTCPc) sample was used, regardless the AlTCPc concentration on the semiconductor surface, at least in the studied range of values. We also studied the effect of the modified TiO2 concentration on the Cr(VI) reduction rate, noting an increase with the increasing photocatalyst concentration.
Experiments were conducted with varying initial 4-CP concentrations (0 ) [4-CP] ) 4
mM), concluding that the presence of 4-CP was essential for the Cr(VI) reduction reaction, but the initial rate was independent of the used 4-CP concentration. The kinetics of Cr(VI) reduction was adjusted in all cases to a first order kinetics, noting that
4-CP degradation stopped after Cr(VI) reduction was complete. Benzoquinone (BQ) was the only detected intermediate compound for the degradation of 4-CP. We compared the effectiveness of 4-CP as a donor, carrying out comparative experiments with other phenolic compounds; it was found that phenol has a similar effectiveness to that of 4-CP, while BQ is not effective because it cannot be oxidized by the AlTCPc radical cation, while hydroquinone and catechol reduced directly Cr(VI) before irradiation. The temporal evolution of the intermediate Cr(V) species in the TiO2- AlTCPc/Cr(VI)/4-CP system under visible irradiation through its stabilization with Cit was also studied; the results showed the same kinetic behavior observed for the TiO2/Cr(VI)/Cit system under UV irradiation, although the reduced Cr(VI) fraction that is transformed into Cr(V) is significantly higher (50% vs. 10% for the system under UV light). Finally, we evaluated the effect of the irradiation wavelength on the efficiency of utilization of photons by the system for Cr(VI) reduction, founding a higher efficiency under UV irradiation or UV + visible than under UV light.
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