Enhanced photocatalytic degradation of dimethyl methylphosphonate in the presence of low-frequency ultrasound

Mechanistic aspects of the role of 20 kHz ultrasonication in photocatalytic oxidation of dimethyl methylphosphonate (DMMP), a simulant for nerve chemical warfare agents, were studied in a batch reactor. We found that DMMP did not undergo mineralization under low frequency (20 kHz) ultrasonic irradiation. The increase of the rate of DMMP photocatalytic mineralization in the presence of ultrasound was not due to deagglomeration of TiO₂, but was associated with enhanced mass transport of reagents. The same intermediate non-volatile products were detected in photocatalytic and sonophotocatalytic degradation. A kinetic model involving all stable intermediate species detected was introduced. Apparent rate constants of all stages of DMMP mineralization increase under sonication. A reaction route of DMMP mineralization without the formation of intermediate products appeared under ultrasonication. Such behaviour was attributed to enabling mass transport of DMMP into micropores and to the surface of TiO₂.