Study of the oxidation kinetics of nitrite ions by potassium ferrate(VI)

Abstract

The oxidation kinetics of nitrite to nitrate ions by potassium ferrate(VI) in a wide range of pH, from neutral to alkaline medium has been studied. Two series of kinetic experiments based on the different technologies for obtaining potassium ferrate(VI) were conducted. In the first series of experiments, crystalline potassium ferrate(VI) was prepared by the chemical synthesis. In the pH range 6.5–11, solid K 2 FeO 4 was added to solutions of nitrite ions at known concentration and pH. Kinetic studies were performed under pH and ionic strength controlled conditions. Half times for these experiments ranged from milliseconds to a few minutes. In the second series of experiments, solutions of K 2 FeO 4 were generated by the electrochemical synthesis. Alkaline solutions of potassium ferrate(VI) in 8.0 M KOH were obtained at t=20 o C by electrochemical dissolution of steel plates (S=10 cm 2 ) containing: C – 0.16 %; Mn – 0.43 %; Si – 0.04 %; S – 0.03 %; P – 0.04 %; the rest is Fe. The experiments were performed with fresh solutions of potassium ferrate(VI) and solutions, aged after the synthesis for several days. Kinetic studies were performed under nitrite concentrations controlled conditions. Half times in this case were in the range of several minutes to hours. It is found out that the maximum reaction rate is achieved by aging solution K 2 FeO 4 after the synthesis for 8 days. Both methods show that the oxidation rate is controlled by the concentration of protonated ferrate, . The dependence of the observed rate constants on pH, nitrite concentrations and ionic strength has been investigated. The true rate constants of reaction are found. Particular orders of the reaction by nitrite and ferrate ions and the overall order of the reaction are calculated. The oxidation rate constant of water by Fe(VI) is found. The studies presented in this paper are important because the oxidation kinetics of harmful inorganic and organic substances by ferrates(VI) of alkali metals is widely used to determine the optimal parameters of technological processes for the purification of surface water, industrial wastewater and air.