Kinetics of activated carbon promoted ozonation of polyphenol mixtures in water

The kinetics of the activated carbon ozonation of mixtures of three polyphenol compounds (gallic acid, tyrosol, and syringic acid) in water has been studied from the application of a double mechanism of reactions to experimental data. These mechanisms involve direct ozone reactions with starting polyphenols and hydroxyl free radical reactions with intermediates and end reaction products, mainly saturated carboxylic acids (the concentration being represented with the chemical oxygen demand, COD). The direct ozone reaction mechanism develops during the initial period (15-60 min) where polyphenols are removed. The kinetic regime of the process was fast of pseudo first order, and at 25 degrees C and pH 5 the direct rate constants of the reactions between ozone and gallic acid, syringic acid, and tyrosol were determined from different kinetic methods. Once polyphenols disappear and saturated carboxylic acids (end reaction products) are present, the removal of these compounds goes through hydroxyl free radical oxidation. This second mechanism involves the reaction of ozone and the organic matter, through hydrogen peroxide formation, both in bulk water and on the carbon surface, as the initiating step of hydroxyl radical formation. In the absence of activated carbon, the activation energy of the bulk water ozone-organic matter reaction was found to be 36.3 +/- 4.5 kcal center dot mol(-1). In the presence of activated carbon, at the conditions here applied in addition to bulk water chemical reaction, the process depends on mass-transfer resistances, and the values of the volumetric liquid-side and individual liquid-solid external mass-transfer coefficients were found to be 0.71 +/- 0.05 min(-1) and 1.19 +/- 0.10 x 10(-3) m center dot min(-1), respectively.