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.