Radiolysis of aqueous nitrate solutions

Abstract

The radiolysis of aqueous nitrate ion solutions was studied as a function of the concentration in the presence of air, oxygen and hydrogen at different concentrations. In dilute solutions, NO^–₃ is reduced to NO₂ by hydrated electrons and H atoms. OH radicals oxidize the NO^–₂ formed. In the presence of oxygen at relatively low concentrations of NO^–₃, NO₂ reacts with HO₂. Primary yields of e^–_aq, H, OH and H₂ O₂ were determined for the range of 10^–3–10^–1 M NO^–₃, G_R=G_e^–aq+G_H increased from 3.21 to 3.46 accompanied by a corresponding decrease of G_H2. The observed yields of G_OH= 2.62 ± 0.03 and of G_H2O2= 0.73 ± 0.02 were independent of NO^–₃ concentration. For higher NO^–₃ concentrations a linear increase of the total yield G_R+ 2G_H2 was observed. This phenomena may be interpreted as the formation of NO^–₂ and OH radicals by subexcitation electrons, which otherwise would form molecular hydrogen or hydrated electrons. Above 1 M, nitrate decomposes by “direct action” to give nitrite. The yield of “molecular” hydrogen decreases with increasing nitrate concentration, whereas the yields of hydrogen peroxide remains constant with a G(H₂O₂)= 0.75 ± 0.02 over the whole concentration range up to 5 M. Oxygen 18-enriched nitrate ions in H₂O¹⁶ and H₂O¹⁸ used to determine the origin of the hydrogen peroxide at high NO^–₃ concentration, show that the part of H₂O₂ which originates from the decomposition nitrate ion by direct action is directly proportional to the fraction of energy absorbed by these ions.