The UV-enhanced decomposition of aqueous ammonium nitrite

Abstract

As the decomposition of NH 4NO 2 to N 2 and H 2O is a potential pathway for the loss of N 2 from the aquatic environment, the above reaction has been studied by volumetric gas analysis. Experiments performed with UV irradiation were approximately four times faster than the rate of the dark reaction. However, the increase in the rate of decomposition with light intensity was not strictly linear. The rate of gas evolution increased with [NO 2 −] and, to a lesser extent, with [NH 4 +]. An optimum rate was observed at pH 8–9. A proposed mechanism involves the formation of NO and NO 2 from dark reactions of the excited NO 2 − ions. N 2O 3, formed from NO and NO 2, is thought to react with NH 3, derived from NH 4 +, to produce an intermediate which dissociates to the products in fast steps. The decline in the rate of gas evolution at pH values greater than 9 is ascribed to the hydrolysis of N 2O 3, giving NO 2 − ions. An estimated activation energy was 26 kJ mol −1. In the presence of anatase or colloidal Fe(OH) 3, the rate of gas evolution dropped as the concentration of the semiconductor increased. Quantum yields, estimated using uranyl oxalate actinometry, were φ 240–248=0.058, φ 254=0.041, φ 257–300=0.047 and φ 300–400=0.016.