Decomposition of NO₃^- by lead pellets with wet ball milling

The reduction of an ammonium nitrate solution by lead pellets during wet ball milling was studied, as a method of removing concentrated NO₃^- from wastewater. Metal oxides forming on the surface of lead pellets in a ball mill reactor are continuously removed by contact between the pellets. At temperatures between 25 and 80°C and a ball mill rotation speed, ω, of 80- 180 rpm, initial concentrations of NH₄NO₃ between 0.05 and 0.1 M, NO₃- rapidly reduce to NO₂^-, and then to N₂ and NH₃. Depending on the pH of the solution, the precipitation of basic lead (II) nitrate and basic lead (II) nitrite occurs simultaneously with the reduction of NO₃^- and NO₂^-. As the temperature increases, the rate of reduction of NO₃^- increases and the proportion of NH₃ formed increases, as that of N₂ decreases. The rate of reduction of NO₃^- is governed by the rate of wear of the outer layer of the Pb pellets, which is determined by the speed of rotation of the ball mill. In this system, there are three competitive reactions: the reduction of NO₃^- and NO₂^- by Pb, the reduction of NO₂^- by NH₄⁺ and the precipitation of basic lead (II) salts.