Titanium dioxide photoinduced degradation of some pesticide/fungicide precursors.
Five-membered nitrogen heterocycles (pyrrole, imidazole and 1,2,4-triazole) have been degraded using titanium dioxide and simulated solar radiation at pH = 8. The degradations followed a simple Langmuir-Hinshelwood mechanism. Accordingly, the adsorption equilibrium constants K of the heterocycles on the titanium dioxide surface and the rate constants k of degradation of the heterocycle-catalyst adduct have been obtained experimentally. While the K values decrease with heterocycle pKa, the k values increase with increasing pKa. Therefore, apparently, the rate constant depends on the availability of the electron pair on nitrogen, but at the same time the electron pair repulsion induced by the negatively charged titanium dioxide surface at pH = 8 causes a reverse effect in the adsorption equilibrium constant. Only in the case of imidazole, where the adsorption equilibrium constant is low enough (K = 0.013 M(-1)), can the rate constant be approximated to a pseudo-first-order rate expression: k(obs) = Kk. In all other cases, k(obs) = Kk/(1 + K(heterocycle)).
Kuehr
Ivana
I
Laboratorio de Química Ambiental, Departamento de Procesos y sistemas, Universidad Simón Bolívar, Apartado Postal 89.000, Caracas, Venezuela.
Núñez
Oswaldo
O
D000327:Adsorption; D004785:Environmental Pollutants / Q000737:chemistry* / Q000528:radiation effects; D052918:Environmental Restoration and Remediation / Q000379:methods; D006863:Hydrogen-Ion Concentration; D007093:Imidazoles / Q000737:chemistry* / Q000528:radiation effects; D007700:Kinetics; D010575:Pesticides / Q000737:chemistry* / Q000528:radiation effects; D011758:Pyrroles / Q000737:chemistry* / Q000528:radiation effects; D014025:Titanium / Q000737:chemistry*; D014230:Triazoles / Q000737:chemistry* / Q000528:radiation effects; D014466:Ultraviolet Rays
D004785:Environmental Pollutants; D007093:Imidazoles; D010575:Pesticides; D011758:Pyrroles; D014230:Triazoles; C009495:titanium dioxide; C045575:1,2,4-triazole; C029899:imidazole; D014025:Titanium