Multi-parametric evaluation of the chemical ultrasonic irradiation effects in a heterogeneous metal-water system

Abstract

Sonochemical degradation of a metal in aqueous solution with ultrasound has been studied. Chemical effects in the heterogeneous system were evaluated and relationships between formed and consumed species during the sonication process were established statistically, employing factor analysis. Chemical species, such as hydrogen peroxide, nitrate, nitrite and zinc species were generated in a single heterogeneous zinc-water system under ultrasonic irradiation and their concentrations were determined. Different times of irradiation exposure were employed and the composition of the irradiated solution was monitored by ion chromatography (IC), inductively coupled plasma atomic emission spectrometry (ICP-AES) and capillary electrophoresis (CE) techniques. Variations of the hydrogen ion and hydrogen peroxide concentrations were measured by potentiometric and spectrophotometric methods, respectively. The global covariance matrix of the variables was built from experimental data, including irradiation time, nitrite, nitrate, zinc and hydrogen peroxide concentrations, and pH variation. Based on the monitoring of species formed in sonicated solutions and the multi-parametric evaluation obtained from the covariance matrix, some correlation related to the formed and consumed species is shown. Nitrate, nitrite, zinc and hydrogen peroxide formation in aqueous solution is strongly dependent upon the irradiation exposure time. Furthermore, the pH increases slightly as the amount of nitrate and nitrite ions becomes constant. This later observation confirms the concept that hydrogen ions are consumed when nitrate and nitrite ions are formed. The pH increase during sonication of the zinc/water system reveals a rather different behavior when compared to the sonicated single pure water system. At the same sonication stage that nitrate and nitrite concentrations become practically constant, zinc concentration increases as hydrogen peroxide concentration increases. According to these observations, some chemical processes can be proposed. Hydrogen ions generated in sonicated solution are consumed by nitrate and nitrite ions formation. Hydrogen ions and hydrogen peroxide formed during sonication might react with metallic fragments originated eventually from corrosive action of the ultrasound. Zinc hydroxo-complexes are formed predominantly by hydroxyl ions generated during hydrogen peroxide reduction. On a smaller proportion, zinc hydroxo-complexes may be also formed due to the alkaline medium generated during hydrolysis of zinc ions.