Effect of hydrogen peroxide on caffeine oxidation by sono-Fenton technology

Oxidation of waters containing 100.0 mg L-1 of caffeine was conducted by a sono-Fenton treatment employing an ultrasound power of 720W at pH=3.0 and T=25°C. The oxidizing action of hydrogen peroxide was studied in a range between [H2O2]0=0-250.0 mM, using iron ratios of 0.7 mol Fe2+/mol C8H10N4O2. The oxidation of caffeine was fitted to second order kinetics, obtaining removals of 98% when dosing 485 mol H2O2/mol C8H10N4O2. During the oxidation, the water acquired a strong brown colour at the same time as there was a strong increase in turbidity and degree of aromaticity. The interaction of (hydro)peroxo-iron complexes with the byproducts of caffeine degradation (1,3,7-trimethyluric acid, theobromine, paraxanthine, theophylline) generated supramolecular structures responsible for this phenomenon, being 116 mol H2O2/mol C8H10N4O2, the relationship that induced colour and aromaticity, while the formation of turbidity was favoured by using 29 and 116 mol H2O2/mol C8H10N4O2.