Characteristics of volatile fatty acids degradation in aqueous solution by the action of ultrasound

Aqueous solutions of volatile fatty acids (VFAs; propionic acid, n-butyric acid and n-valeric acid) with concentrations of about 0.9 mM were exposed to 200 kHz ultrasound with a power of 6.0 W/cm2 per unit volume in a sonochemical reactor under ambient temperature and pressure conditions. The concentration of VFAs decreased with irradiation time, indicating pseudo-first-order kinetics. The rates were in the range 13.6-3.5 (× 10−3) min−1 under argon. Saturated gas affected the degradation rate and the order of the rates was as follows: nitrogen ≤ air < argon. Mixing effects (in the presence of other VFAs) were found to have little influence upon the degradation of individual VFAs.