Coupled acidification and ultrasound with iron enhances nitrate reduction

Contaminated soils, especially when pollutant concentrations are high, pose potentially serious threats to surface and groundwater quality, when there are spills, discharges, or leaking storage tanks. For in situ remediation of nitrate in groundwater, the use of zero-valent iron (Fe(0)) is suggested. The formation of passivating scales on Fe(0) over time may limit the long-term reduction potential of Fe(0). The aim of this study was to investigate the effect of ultrasound and pH on the destruction of passive oxide film. Batch tests were conducted in a specially designed protocol using ultrasound, and changing iron (commercial iron powder of micro-scale grain size) loading and pH. The results showed deactivation of the degradation process by Fe(0) with combined ultrasound/iron system and with ultrasound alone. However, the degradation rate increases with decrease in pH. The degradation rate was 45% for pH 2 and 20% for pH 4. The combination of iron, acidification, and ultrasound showed excellent degradation efficiency, and the degradation rate was 99%. Acidification could destroy passive oxide film and activate iron, thus, hastening the reaction between Fe(0) and nitrate. Ultrasound was helpful in destroying or preventing the formation of passive oxide film under acidification.