Contaminant salts as enhancers of sonocatalytic degradation of organic water pollutants: Effect of concentration, reaction time and adsorption on the efficiency of enhancement and the fate of concurrently formed H2O2

Abstract

The effect of salts/anions on ZnO-mediated sonocatalytic degradation of phenol in water is investigated. Anions such as CH3COO−, SO42−, CO32−, HCO3−, NO3−, F−, Cl− and PO43− enhance the degradation. However, the effect is dependent on concentration of the anion and duration of reaction. The enhancing effect in sonocatalysis is different from and contradictory to the inhibiting effect of anions in photo- and sonophoto- catalysis. The enhancement is maximum in the presence of F−, SO42− and CH3COO− and the least in the presence of HCO3− and PO43− though minor exceptions are there depending on the experimental conditions. The basic mechanism of enhancement involves the formation of reactive radical anions by interaction of the anions with the highly reactive .OH radicals and their reaction with phenol. The lower reactivity of the radical anions compared to OH is compensated by the relatively higher concentration. Moreover, they interact exclusively with the substrate whereas the .OH participates in many reactions simultaneously including recombination and self- deactivation. Other effects of dissolved salts/anions such as increasing the ionic strength of the medium, modification in the partition coefficient, decreasing vapor pressure, increase in hydrophilicity etc. also favor sonocatalytic degradation of phenol. The concentration of concurrently formed H2O2 does not increase corresponding to the degradation of phenol and varies in a wave-like fashion (oscillation) due to the simultaneous formation and decomposition with either of them dominating periodically. The anions also influence the nature of the oscillation curve. Possible reasons for the anion effect and potential practical application are discussed.