Anomalies detected during hydrodynamic cavitation when using salicylic acid dosimetry to measure radical production

Abstract

Cavitation used to be associated with negative outcomes in hydraulic turbomachinery but nowadays it is often used for water cleaning, microorganism’s destruction and degradation of organic compounds. This study investigated the amount of OH formed during hydrodynamic cavitation using salicylic acid dosimetry. The radical’s amount was evaluated by quantifying the concentration of 2,3-dihydroxybenzoic acid, catechol and 2,5-dihydroxybenzoic acid. Two concentrations of the dosimeter in tap water were investigated, 50 and 300 mg L−1 (pH approx. 2.5). After 90 min of cavitation using a Venturi constriction a sum of the three products was determined at 0.97 µg mL−1 and 1.81 µg mL−1, respectively. However, during the investigation the anomalies were detected in the cavitation development when higher concentration of salicylic acid was used – cavitation appeared more gentle, with less intense collapses, unrelated to the one in pure water. Detailed observations of cavitation and additional bubble dynamics simulations revealed that the decreased surface tension of the acidified salicylic acid solution is the most influential physical characteristic. Further experiments on nucleation and coalescence showed that high concentration of salicylic acid also leads to longer stability of the bubbles and prevents their coalescence due to short-range repulsive forces (steric hindrance), which results in less violent bubble collapse. We also discuss the importance of an appropriate amount of the dosimeter for correct evaluation of OH production in a given cavitation device (50 mg L−1 for the present one). This is essential for further cavitation exploitation studies to avoid false interpretation of the gathered results.