The 20 kHz sonochemical degradation of trace cyanide and dye stuffs in aqueous media

Abstract

Cyanide in alkaline aqueous solution is reported to be destroyed by a facile sonochemical process. In the presence of power ultrasound emitted from a horn probe with a frequency of 20 kHz and up to 60 W cm-2 intensity a decay in the concentration with first order kinetics is monitored by ion selective electrode techniques. Characteristic features of the process are (i) a rate constant inversely proportional to the solution volume, (ii) a decrease in rate upon changing the gas phase in the reactor from air to argon, and (iii) an increase in rate in the presence of higher intensity ultrasound. With these characteristic features the process is interpreted in terms of a sonochemical degradation initiated by a high energy intermediate such as OH produced in violent cavitation events. Further evidence for this mechanistic conclusion comes from complementary experiments employing low concentrations of dyes (alizarin and Procion Blue) in aqueous solutions. The sonochemical degradation of these dye solutions with 20 kHz power ultrasound is monitored by following the UV–VIS absorption. Characteristic features for the dye degradation process are in agreement with those observed for the cyanide sonolysis and therefore attributed to the same kind of sonochemical mechanism. The sonochemical degradation of cyanide proceeds with a rate an order of magnitude faster than that of the dye bleaching, possibly due in part to the faster rate of diffusion of cyanide compared to the larger organic dye molecules.