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Abstract
The ozonation of esculetin (6,7-dihydroxycoumarin), a major pollutant present in the wastewater generated in the cork industry, was accelerated at high pH, with apparent second-order rate constants in the range from 3.3 × 104 L/(mol·s) at pH=2 to 8.4 × 107 L/(mol·s) at pH=9. The acid-base equilibrium of esculetin was studied, resulting in a pKa value of 7.37. Taking into account this pKa, the rate constants for the reaction between ozone and the un dissociated and dissociated forms of esculetin were 3.0 × 104 and L/(mol·s) 6.67 × 108 L/(mol·s), respectively. Apparent first-order rate constants for the photolysis by UV irradiation were also evaluated, with values between 0.12 × 10−2 min−1 at pH=2 and 1.15 × 10−2 min−1 at pH=9, while the quantum yields for this photo-degradation reaction varied from 0.99 × 10−2 mol/Eins to 11.1 × 10−2 mol/Eins at these pHs. The Fenton's reagent system was used for the generation of hydroxyl radicals, and the rate constant for the reaction between esculetin and these radicals was determined to be 1.06 × 1010 L/(mol·s). Finally, several chemical oxidation systems were used in the degradation of this pollutant: single oxidants (ozone, UV irradiation) and advanced oxidation processes (Fenton's reagent, UV/H2O2, O3/H2O2, O3/UV, O3/H2O2 /UV, and photo-Fenton system). The results revealed that the most efficient methods in terms of esculetin removal were ozonation among the single oxidants, and the photo-Fenton system among the combined processes.
Acknowledgments
Financial support from the “Comision Interministerial de Ciencia y Tecnologia” (CICYT of Spain), under Project PPQ2001-0744 is acknowledged by the authors.