Behaviour of natural organic mater during ultrasonic irradiation

Ultrasonic irradiation has been one of the advanced oxidation methods for natural organic matter (NOM) removal. Its application simplicity and no production of toxic by-products make the system attractive. It has been shown that high power ultrasound produces strong cavitation in aqueous solution causing shock wave and reactive free radicals by the violent collapse of the cavitation bubble. These effects should contribute to the physical disruption and inactivation of microbial structures as well as to the decomposition of toxic chemicals in the water. The objective of this research was to investigate the effectiveness of low frequency – high power sonication for NOM removal (measured by both means of UV254 absorbance and TOC) as well as controlling DBPs formation by varying irradiation time, light intensity, NOM concentration. Temperature, pH, conductivity, redox potential, and turbidity were monitored to evaluate the experimental study. The results indicate a strong capacity of sonication for NOM removal. The preliminary experiments showed that NOM removal is influenced by the intensity and time of sonication. Ultrasound, applied under such conditions, may be a realistic possibility for large-scale treatment; however, further studies should be performed to evaluate its effectiveness on different water sources.

The potential of ultrasound for the intensification of chemical and biological processes in water technology is very broad. Fig. 7. TOC removal at 42 W/cm2 in function of sonication time and humic acid concentration with (right side) or without (left side) 15 mg/L of nitrate. Some incongruence was measured in respect to absorbance removal and TOC removal. The effects of ultrasonication on the humic acids could be explained by two mechanisms. One is the chemical degradation of the humic acids in the cavitation bubbles by oxidation. The other is the physical aggregation of humic acid fragments. The main physical effect of powerful implosion of cavitation bubbles in the natural water is the fragmentation of the suspended solids. This effect is easily measurable by turbidity growth. At the same time, high turbidity value influences positively the absorbance value, in contrast to TOC values, which are independent of turbidity. The removal of natural organic matter from solution is influenced by parameters such as intensity and time of sonication. 20 min of sonication at 42 W/cm2 provided the best performances with respect to TOC removal.