DENITRIFICATION OF NITRATE BY COMBINED ULTRASOUND AND ZERO VALENT MAGNESIUM AT pH CONTROLLED CONDITIONS

Abstract

In this study, ultrasound (US), zero-valent magnesium (Mg-0) and their simultaneous combination were tested at different Mg-0 doses (0.5, 1, 1.5, 2 ve 2.5 g/L), pH values (2, 4, 7 ve 9) and ultrasonic powers (30, 60 ve 90 W) for nitrate reduction. Ultrasound alone was found to be ineffective for nitrate removal at different pH values. Effect of ultrasonic power for pH controlled operation, nitrate reduction was increased with increasing dose of magnesium powder. Ultrasound induced profound effects on denitrification capacity of magnesium particles. For example, with controlling pH at 4, 95% of initial nitrate was reduced by 2.5 g/L of magnesium powder within 60 min, while keeping the applying combined US/Mg-0 for 30, 60, and 90 W ultrasonic powers at the same dose, required durations to achieve at the same nitrate removal efficiencies were determined to be approximately 30, 20 and 20 min, respectively. When pH was controlled at 7 and 9, up to 70% of nitrate was removed by 2.5 g Mg-0/L dose after 60 min, but in the same conditions with 90 W ultrasonic power a complete nitrate reduction was attained only within 30 min. Effect of ultrasonic on magnesium surface activation and nitrate removal was arised more clearly at alkaline conditions, when particle surface passivation was increased at increased pH. Nitrogen gas (N-2), nitrite (NO2-) and ammonium/ammonia (NH4+/NH3) were detected as the major denitrification by-products following US/Mg-0 treatment. As more ultrasonic power and magnesium dose were applied, the rate of conversion of nitrate to nitrogen gas increased significantly.