Ultrasonic irradiation of chemical compounds in aqueous solutions

Abstract

The ultrasonic irradiation of para-nitrophenol, S(-II), and parathion is studied in aqueous solutions at 20 kHz and ~ 75 W-cnT2. Para-nitrophenol was degraded primarily by denitration and secondarily by ΌΗ radical attack to yield N 02, NO3, benzoquinone, hydroquinone, 4-nitrocatechol, formate and oxalate. These reaction products and the kinetic observations are consistent with a model involving high-temperature reactions of p-nitrophenol in the interfa.cia.1 region of cavitation bubbles. The average effective temperature of the interfacial region surrounding the cavitation bubbles was estimated to be T ~ 800 K. Ultrasonic irradiation of S(-II) is studied in aqueous solutions over the pH range 7 - 12. The reaction of HS“ with OH is the principal pathway for theoxidation of S(-II) at pH > 10; the oxidation products are SO2“, SO2", and S20 Upon prolonged sonication, SO2" is the only observed product. At pH < 8.5, thermal decomposition of H2S within or near collapsing cavitation bubbles becomes the important pathway and elemental sulfur is found as an additional product of the sonolysis of S(-II). The sonolytic oxidation of H2S at pH > 10 was successfully modeled with an aqueous-phase free-radical chemistry mechanism and assumingcontinuous and uniform ΌΗ input into solution from the imploding cavitation bubbles. Parathion degradation occurred primarily by enhanced hydrolysis and secondarily by direct ΌΗ radical attack.The effect of various physical and chemical parameters on sonolytic yields is examined. The observed effects are in qualitative agreement with the sonolysis mechanisms proposed for the chemicals of interest and the existing hydrodynamic theories of acoustic cavitation. The formation of iodine upon ultrasonic irradiation of potassium iodide solutions and the sonolysis of S(-II) are used as probes to compare the sonochemical efficiency of different experimental set-ups. This work elucidates the mechanisms of the ultrasonic decomposition of typical organic and inorganic pollutants. It is shown that ultrasound has the potential to become a viable alternative for the destruction of chemical contaminants in water and wastewater. The current limitation of sonolysis is its low energy utilization efficiency, but there is room for improvement by optimizing reactor design and physical/chemical operation conditions. This work offers some recommendations and insight in that respect.