Mechanistic Features of the TiO2 Heterogeneous Photocatalysis of Arsenic and Uranyl Nitrate in Aqueous Suspensions Studied by the Stopped-Flow Technique

• Published in: Chemphyschem Vol. 17; no. 6; pp. 885 - 892
• Main Authors: Meichtry, Jorge M., Levy, Ivana K., Mohamed, Hanan H., Dillert, Ralf, Bahnemann, Detlef W., Litter, Marta I.
• Format: Journal Article
• Language: English
• Published: Germany Blackwell Publishing Ltd 16.03.2016; Wiley Subscription Services, Inc
• Subjects: arsenic; heterogeneous catalysis; Nanoparticles; Nitrates; photocatalysis; stopped flow; uranium; uranium; heterogeneous catalysis; arsenic; photocatalysis; stopped flow
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.201500949

The dynamics of the transfer of electrons stored in TiO2 nanoparticles to AsIII, AsV, and uranyl nitrate in water was investigated by using the stopped‐flow technique. Suspensions of TiO2 nanoparticles with stored trapped electrons (etrap−) were mixed with solutions of acceptor species to evaluate the reactivity by following the temporal evolution of etrap− by the decrease in the absorbance at λ=600 nm. The results indicate that AsV and AsIII cannot be reduced by etrap− under the reaction conditions. In addition, it was observed that the presence of AsV and AsIII strongly modified the reaction rate between O2 and etrap−: an increase in the rate was observed if AsV was present and a decrease in the rate was observed in the presence of AsIII. In contrast with the As system, UVI was observed to react easily with etrap− and UIV formation was observed spectroscopically at λ=650 nm. The possible competence of UVI and NO3− for their reduction by etrap− was analyzed. The inhibition of the UVI photocatalytic reduction by O2 could be attributed to the fast oxidation of UV and/or UIV. Bait and trap: Stopped flow shows that AsV cannot be reduced by the trapped electrons (etrap−) of TiO2, but increases the rate of the electron‐transfer reaction between O2 and etrap−. AsIII cannot be reduced by etrap− and inhibits the rate of the reaction between O2 and etrap−. UVI reacts easily with etrap− with UIV formation and is reduced in preference to NO3−.