Three ultrasonic reactors of different frequencies were used: 20 kHz (Bandelin SonoPlus, with a tip diameter of 12 mm and submersed into the liquid from the top of an 100 mL cylindrical glass reactor with actual volume of solution for degradation studies being 80 mL; total supplied power of 180 W) and 300 and 520 kHz ultrasonic reactors (UNDATIM Ultrasonics).