The UV-Enhenced Decoposition of Aqueous Ammonium Nitrite

As the decomposition of NH4NO2 to N-2 and H2O 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 [NO2-] and, to a lesser extent, with [NH2+]. An optimum rate was observed at pH 8-9. A proposed mechanism involves the formation of NO and NO2 from dark reactions of the excited NO2- ions. N2O3, formed from NO and NO2, is thought to react with NH3, derived from NH4+, 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 N2O3, giving NO2- 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 phi(240-248) = 0.058, phi(254) = 0.041, phi 257-300 = 0.047 and phi(300-400) = 0.016.