Deleterious effect of ammonium extraction from soil assisted by ultrasound

Ultrasound has been used to assist extraction procedures from solid samples based on the decrease of gradient concentration in the vicinity of particulate matter and the microjet produced during cavitation. In the present work, the harmful effect yield by ultrasound-assisted ammonium extraction from soil samples was observed by comparing with an extraction process assisted by magnetic stirring. Three soil samples from agricultural areas of Bahia state in Brazil were submitted to both extraction processes and ammonium concentrations for conventional stirring procedure were 25% higher than those obtained by the ultrasound-assisted extraction. This same effect was also observed by irradiating ammonium solutions with low frequency ultrasonic waves. After 5 min of ultrasonic irradiation, more than 25% of the ammonium was lost probably as N-2. Possible conversions of N-NH4(+) to hydrazine, hydroxylamine, nitrite and nitrate were evaluated. In addition, an increasing in nitrate concentration by irradiating solution with high saline concentration independent on the initial ammonium concentration was observed. (C) 2004 Elsevier B.V. All rights reserved.