Reduction rate of sodium nitrate by lead drops with wet-ballmilling

Reduction of sodium nitrate solution by lead drops with wet-ballmilling is studied in order to remove concentrated NO₃^- from wastewater. Metal oxides forming on the lead drops surfaces in a ballmill reactor was continuously removed by inter-ball contacting In the 5-80°C temperature range, initial concentration of NaNO₃ 0.05-0.1 M, and ballmill rotation speed, ω , 80-180 rpm, NO₃^- is rapidly reduced to NO₂^-, then slowly to N₂ and NH₃. NO₃^- is reduced to NO₂^- 32-100% in 10 h, but the reduction rate of NO₂^- is very slow, so the maximum percentages of N² and NHs from NO₂^- are only 6 and 10 %, respectively. Above 40°C, NO₃^- is reduced under zero-order reaction and NO₂^- is formed corresponding to the reduction stoichiometry. The reduction rate is proportional to ω^1/2 [rpm^1/2], and the apparent activation energy is 27.5 kJ·mor^-1, but independent of the NO₃^- concentration. These results suggest that the rate determining step is the peeling of PbO layers on the particle surface. Below 25°C, the reduction rate decreases with reaction time as the surface is not covered completely with PbO due to the lowering of the rate.