Decomposition of NO 3− by lead pellets with wet ball milling

Abstract

The reduction of an ammonium nitrate solution by lead pellets during wet ball milling was studied, as a method of removing concentrated NO 3 − 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 NH 4NO 3 between 0.05 and 0.1 M, NO 3 − rapidly reduce to NO 2 −, and then to N 2 and NH 3. 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 NO 3 − and NO 2 −. As the temperature increases, the rate of reduction of NO 3 − increases and the proportion of NH 3 formed increases, as that of N 2 decreases. The rate of reduction of NO 3 − 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 NO 3 − and NO 2 − by Pb, the reduction of NO 2 − by NH 4 + and the precipitation of basic lead (II) salts.