Catalytic oxidation mechanism of oxy-nitrogen species (NOx) in FeSO4 electrolyte

The effect of small concentrations of nitrite on the oxidation of ferrous ions in sulfuric acid under oxygen pressure at 25 °C was investigated. A mathematical model of the rate of chemical reaction was developed to simulate the concentration versus time trajectories of the prominent species in the system in parallel with previous thermodynamic and kinetics investigations on related systems. The simulations were performed by employing the COMSOL Reaction Engineering Lab Module to find numerical solutions for the model equations. The reaction scheme consists of eight principal reactions: three describing the oxidation of ferrous ions by oxy-nitrogen species, one the formation of the Fe(NO)2+ complex, and four additional reactions that summarize the chemistry of oxy-nitrogen species in acidic solutions. The optimal NaNO2 (HNO2) concentration was determined to be 4.5 mM based on an objective function designed on the basis of two competing considerations: the cost of the nitrite reagent as manifested by the feed concentration employed in the processing scheme, and the processing costs as determined by the time needed to achieve 99.9% oxidation of ferrous ions. The simulations revealed that the concentration of NOx plays a critical role in determining t99.9, the rate of reaction between nitric oxide and nitrate. In a sulfuric acid electrolyte, nitrous acid disproportionates to produce N2O3, NO, and NO2. Ultimately, all the nitrogen species that evolved in the electrolyte from the sodium-nitrite reagent were oxidized to nitrate NO3- as the oxidation of ferrous ions to ferric ions proceeded to completion. The simulation revealed that a ferrous-nitrosyl complex was produced immediately. Thus, researcher postulates that the majority of the NO species were bound in solution, perhaps as Fe(NO)2+.