Sonolysis of natural phenolic compounds in aqueous solutions: degradation pathways and biodegradability

The sonochemical degradation in water of p-coumaric acid and p-hydroxybenzaldehyde, compounds that are commonly found in effluents of agricultural origin, was investigated using an 80 kHz horn-type sonicator. Experiments were performed at initial concentrations varying between 10 and 100 mg L−1, an applied power varying between 75 and 150 W and liquid bulk temperatures varying between ambient and 70°C. At the conditions in question, substrate removal was found to increase with increasing power and decreasing initial concentration and temperature. Addition of 1-butanol as a radical scavenger at a concentration of 1000 mg L−1 nearly completely suppressed p-coumaric acid degradation. Conversely, addition of radical promoters (Fe2+ ions or H2O2) had a positive effect on removal, thus implying that degradation proceeds predominantly via hydroxyl radical-induced reactions. Reaction intermediates were determined using liquid and gas chromatography and a reaction network for p-coumaric acid degradation is suggested. Shake flask tests with activated sludge were performed to assess the aerobic biodegradability (in terms of chemical oxygen demand removal) before and after sonochemical treatment. At the conditions under consideration, the use of ultrasound enhanced the aerobic degradability of the substrates in question.