Mechanisms of Sonochemical Transformation of Nitrate and Nitrite under Different Conditions: Influence of Additives and pH

Abstract

The sonochemical NO3-reduction (2 mM) under Ar at 850 kHz was evaluated at pH 1, 3, and 7, and with the addition of formic, ethylenediaminetetraacetic, and citric acids. In all cases, the NO3-removal was lower than 15% after 180 min under ultrasound. The low NO3-transformation was attributed to the rapid NO2-reoxidation in the sonochemical system. In contrast, the ultrasonic NO2-(2 mM) oxidation under Ar at the same frequency was effective at pH 3 (with HClO4) after 240 min of reaction. Different experimental conditions were also tested: under O2atmosphere, using HCl as the acidifier, adding t-BuOH as HO•scavenger, KIO4as an oxidizing agent, and changing the pH to 1. NO3-in solution was the main product in all cases, and all the experimental data for NO2-decay could be adjusted to a first-order kinetics. At pH 1, under Ar, an almost complete NO2-oxidation took place with an increased rate compared with the experiment at pH 3. Mechanisms under the different conditions are discussed.