LYCAON - Chemiluminescence of luminol subjected to hydrodynamic cavitation "on a chip": a fundamental step towards wastewater treatment by green technologies

Cavitation, the growth and sudden collapse of vapor bubbles, is considered for a long time as a promising tool able to contribute to the development of clean technologies. The violent bubbles collapse is responsible for chemical transformations caused by the high temperatures and pressures reached at the end of the collapse and by mass transfer at the vicinity of the bubbles. The energy released by a collapsing bubble transformes H2O vapor molecules into hydroxyl radicals °OH, able to oxidize pollutants present in the aqueous solution.

Cavitation bubbles can be obtained by several ways: ultrasonic acoustic waves in a vessel generate strong pressure fields able to produce cavitation bubbles. Hydrodynamic cavitation, which is generated by the acceleration of a liquid through a physical constriction, also produces cavitation and theoretically allows the processing of larger volumes of fluid, which is interesting as far as industrial applications such as wastewater treatments are concerned. But in the case of hydrodynamic cavitation, the operational time during which a molecule is submitted to the effects of a collapsing bubble is then very short and the yield of °OH radicals is believed to be several orders of magnitude inferior to that obtained in ultrasonic reactors. There is therefore a need to improve our understanding of the mechanisms arising in vaporous clouds in hydrodynamic cavitation.

In this project, we propose a new experimental approach which consists in performing hydrodynamic cavitation of luminol in an aqueous solution inside microchannels. Chemiluminescence of luminol, which reveals the presence of °OH radicals, has been already characterized in ultrasonic reactors. Recently, our team has, for the first time, observed the chemiluminescence of luminol in hydrodynamic reactors thanks to a ‘lab on a chip’ hydrodynamic cavitation reactor. A quantitative determination of the yield of the °OH radicals becomes now possible and the project aims to perform a complete set of experiments in order to get a fundamental knowledge about the mechanisms at the origin of °OH production in hydrocavitating flows, and to compare them to those at the origin of sonochemistry. The results of that study will have a crucial importance for some further applications of hydrodynamic cavitation to wastewater treatment.