Hydroxyl radicals are the most powerful oxidizing species that can be used in the treatment of wastewater and industrial effluents. Among the various ways of obtaining hydroxyl radicals, cavitation is a process that meets contemporary environmental requirements. Cavitation is a sustained phenomenon of creation, growth and sudden implosion of steam bubbles. The violence of the implosion of cavitation bubbles breaks some steam molecules. OH- hydroxyl radicals are then obtained from water, a phenomenon reinforced in the presence of hydrogen peroxide (H2O2). In acoustic cavitation, relationships have been established between oscillation frequency, bubble size and radical diffusion. But few articles address wastewater treatment by hydrodynamic cavitation, which is likely to treat larger volumes of liquids than acoustic cavitation. However, the chemiluminescence of luminol allows a direct and in situ study of the production of hydroxyl radicals in cavitation flows. The presence of hydroxyl radicals in ultrasonic tanks was demonstrated in 1994. However, the study of hydrodynamic cavitation of luminol is very recent, with only one German team having qualitatively observed chemiluminescence in hydrodynamic cavitation.
Thus, in collaboration with researchers from the Institut lumière matière de Lyon (CNRS/Claude Bernard University), we recently observed the chemiluminescence of luminol subjected to hydrodynamic cavitation on a chip, and then counted the photons emitted (Podbevsek, 2018). Each microreactor and a photomultiplier positioned opposite each other were inserted into an opaque module, isolated from any external light disturbance. The light emission was correlated with the sound emitted by the implosion of the bubbles, characteristic of the cavitating flow regime. This metrology recorded a rate of about 200 photons/second, corresponding to a total production of 1.6 1010 OH- radicals/minute/litre elapsed downstream of the narrowing that caused cavitation. For this type of shrinkage, radical production depended on the difference between the fluid flow rate and the critical flow rate above which cavitation occurred, regardless of the size of the diaphragm. This preliminary work is continuing to determine whether hydrodynamic cavitation can be seriously considered as a reliable wastewater treatment process.