EVIDENCE OF CARBOXY ESTER MECHANISMS IN NITRIFICATION FROM THE DYNAMIC REACTION KINETICS AT NATURAL CONCENTRATIONS. Part III. The Oxidation of Ammonia in Nitrification Follows Pseudo Half Order Kinetics Compatible with a Carboxy Ester Mechanism where Hydroxamic Acid is an Intermediate not Hydroxylamine.

Ammonia pulse test experiments at different pHs were conducted on a submerged biofilter with fast recirculation acting as a batch reactor. The steady-state ammonia concentrations in an organic carbon substrate-free environment were similar to natural water courses and the introduced pulses were less than 1 mg / l of total ammonia nitrogen. At pH levels of 7.2 to 9.0 ammonia concentrations returned to the steady state values within 30 minutes, while a significant lag with a slower reaction was noted at a pH of 6.2. Integrated half-order plots have better correlations than the integrated Michaelis-Menten plots. Half-order reaction kinetics differs from Michaelis-Menten type kinetics in a subtle way, with a second molecule leaving in the equilibrium-binding step. Half-order kinetics is compatible with the theory that the reactions proceed via a carboxylic ester route, using ATP in a cyclic fashion, with iron-stabilized hydroxamic acid as the true intermediate in ammonia oxidation. The intermediate undergoes a hydrolysis reaction to nitrite and the ester is recycled. The mechanisms include features which allow efficient dynamic control and optimisation as demanded by evolution without extremely toxic hydroxylamine as an intermediate. Design equations are provided for the optimisation of nitrification filters at low and medium TAN concentrations, based upon half-order kinetics and liquid film diffusion. Similar experiments at very high ammonia concentrations, such as those in wastewater treatment, described in the literature indicate that the reaction kinetics, with pre-oxygenated water and favourable temperature conditions, then follows second-order kinetics compatible with a dimer reaction as given in the carboxy ester mechanisms. Data from the literature that examined nitrification in soils using high concentrations of urea indicate that under these conditions, the toxicity of nitrite and ammonia forces the production of nitrous oxides or given time to optimise the diffusion resistance is increased in the form of an intercellular gel.