Vacuum-UV photolysis of aqueous solutions of nitrate: effect of organic matter I. Phenol

Abstract

Water homolyses on vacuum-UV excitation ( λ < 200 nm) to give HO . radicals, H . atoms and, with lower efficiency, hydrated electrons. These primary species induce a series of reactions partially depleting nitrate and quantitatively mineralizing organic compounds, e.g. phenol, in aqueous solutions. The rates of oxidative degradation of phenol and its oxidation products strongly depend on the dissolved oxygen concentration. The formation of dihydroxybenzenes, trihydroxybenzene and oxalic acid as oxidation intermediates was observed in irradiation experiments with oxygen- and air-saturated solutions, but no significant concentrations of these compounds were observed in experiments with argon-saturated solutions. The depletion rates of NO 3 − have been reported previously to be slow yielding mainly nitrite and N 2O as reaction products. Vacuum-UV irradiation of aqueous solutions containing NO 3 − and phenols results in the simultaneous mineralization of phenol and depletion of NO 3 −, yielding mainly NH 4 + and, in much lower yields, NO 2 −. The experimental observations indicate that the nitrogen-containing inorganic ions formed during NO 3 − depletion promote the oxidation of the dissolved organic matter independent of the presence of oxygen. A possible reaction mechanism is discussed, in which the interaction of O 2NOOH, ONOOH, NO . and NO . with organic matter is proposed as being mainly responsible for the overall observed behaviour. The interaction between organic substrates and NO . seems to favour further reduction to NH 4 +, whereas a one-electron reduction yielding N 2O is observed in the absence of organic substrates. The effect of CO 3 2− on these reactions is also discussed. Under continuous irradiation, NH 4 + is subsequently re-oxidized. Complete oxidation to NO 3 − is observed only in experiments with oxygen-saturated solutions. Experiments with air- or argon-saturated solutions show only 30% and 10% NO 3 − formation respectively due to the simulataneous formation of N 2. The reduction of NO 3 − to NH 4 + and the oxidation of NH 4 + to NO 3 − seem to involve a series of common intermediates interrelated by many redox reactions and reaction equilibria where the pH, availability of electrons and the presence of protons or hydrogen donors and molecular oxygen determine their importance.