Decomposition of NO3- by lead pellets with wet ball milling

The reduction of an ammonium nitrate solution by lead pellets during wet ba ll milling was studied, as a method of removing concentrated NO3- from wast ewater. Metal oxides forming on the surface of lead pellets in a ball mill reactor are continuously removed by contact between the pellets. At tempera tures between 25 and 80 degrees C and a ball mill rotation speed, omega, of 80-180 rpm, initial concentrations of NH4NO3 between 0.05 and 0.1 M, NO3- rapidly reduce to NO2-, and then to N-2 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 prop ortion of NH3 formed increases, as that of N-2 decreases. The rate of reduc tion of NO3- is governed by the rate of wear of the outer layer of the Pb p ellets, which is determined by the speed of rotation of the ball mill. In t his system, there are three competitive reactions: the reduction of NO3- an d NO2- by Pb, the reduction of NO2- by NH4+ and the precipitation of basic lead (II) salts. (C) 1999 Elsevier Science Ltd. All rights reserved.