Kinetics and initial photocatalytic pathway of tryptophan, important constituent of microorganisms
Résumé
The adsorption equilibrium in the dark and under UV light, the kinetic of photocatalytic oxidation and the main initial pathways of tryptophan photocatalytic degradation has been investigated in aerated TiO2 Degussa P-25 aqueous suspensions illuminated at lambda > 340 nm. Tryptophan has been chosen as a model of organic compounds of amino acids, elemental constituents of DNA, microorganisms, e.g. bacteria, fungi, virus among others to better understand their photocatalytic degradation. Langmuir and Langmuir-Hinshelwood models have been used to determine the adsorption, the rate constants and the surface coverage of TiO2. Despite a low coverage of the TiO2 surface in the dark a high photocatalytic degradability and initial mineralization occurred. However, the surface coverage of TiO2 was about doubled under UV light suggesting the formation of active sites under irradiation. LC-MS, LC-UV with or without derivatization analyses allowed us to identify, mono-, di- and tri-hydroxylated compounds, deamination of heterocycle, the formation of six organic acid, oxalic, oxamique, lactic, formic, acetic and propanoic acids and of three linear amino acids, serine, aspartic acid and glycine. A degradation mechanism is proposed to the formation of oxamic acid. The study of the mineralization of carbon and nitrogen showed the presence of trace of organic molecules mineralizable with difficulty. Nitrogen atoms were predominantly photoconverted into NH4+. After more than 95% of carbon mineralization, NH4+/NO3- ratio was 4. The identification of the photocatalytic products, their evolution and the TOC evolution indicated three initial competitive pathways: about 50% of hydroxylation with 6-7% of the hydroxylation of carbon bearing NH2 and COO-, about 30% of decarboxylation and less than 20% of nitrogen-carbon (N-C) cleavage. (C) 2009 Elsevier B.V. All rights reserved.