Photolytic ozonation mechanism

Peyton G R, Glaze W H (1987) Mechanism of Photolytic Ozonation - Photochemistry of Environmental Aquatic Systems, R G Zirka and W J Cooper, American Chemical Society Symposium-Series, 327 76-88, Washington DC. 

Photolytic Ozonation has been known for over a decade as a powerful water treatment process for the destruction of organic compounds, but its mode of action has not been understood. It is shown using kinetic arguments and data from scavenging experiments that photolysis of aqueous ozone yields hydrogen peroxide, which then reacts with further ozone in a complex series of reactions to yield hydroxyl radical. The proposed mechanism is shown to explain pH behavior of model compound destruction in earlier data. 

In the present study, the mechanism of photolytic ozonation was investigated in the laboratory by measuring ozone, hydrogen peroxide, and total oxidant levels in purified water as a function of time in a two-liter stirred tank reactor in which concurrent ozone sparging and UV irradiation were taking place. Experiments were run in the presence and absence of various radical scavengers and the results Interpreted in terms of reaction rate constants taken from the literature. This paper will discuss the mechanism of photolytic ozonation, show its relationship to other ozonation processes, and... 

Photolysis Abiotic oxidation occurring in surface water is often light mediated. Both direct oxidative photolysis and indirect light-induced oxidation via a photolytic mechanism may introduce reactive species able to enhance the redox process in the system. These species include singlet molecular O, hydroxyl-free radicals, super oxide radical anions, and hydrogen peroxide. In addition to the photolytic pathway, induced oxidation may include direct oxidation by ozone (Spencer et al. 1980) autooxidation enhanced by metals (Stone and Morgan 1987) and peroxides (Mill et al. 1980). 

Ozone is formed by the photolytic decomposition of NO2 yielding oxygen radicals and by the reaction sequence NO2 — HNO3 — NO3 O3. In particular, the reaction of NO2 with hydroxyl radicals to form HNO3 increases ozone concentration because two radicals, NO2- and -OH, which catalyze ozone decomposition, are removed. Other radicals are also important for ozone destruction in the stratosphere, especially chlorine oxides see Chlorine, Bromine, Iodine, Astatine Inorganic Chemistif). The mechanism of ozone destruction is complicated as there many compounds involved. Chlorine nitrate and dinitrogen pentoxide can act as reservoir species for CIO-, NO2-, and NO3- radicals. 

Halite. Recent interest on the dissolution reaction of halite is due to the significance of NaCl in atmospheric chemistry. Oum et al. (1998) has recently shown that airborne hydrated sea-salt microparticles are involved in the photolytic formation of chlorine by reaction with ozone. Little is actually known about the mechanisms of the dissolution process of the familiar table salt. In general, it is assumed from casual observation that the dissolution occurs stoichiometrically, with a decrease in free energy, and with a low activation energy. 

Chapman (1930a, b) showed successfully that the characteristics of the height of the ozone layer and ozone density in the earth s atmosphere can be described by assuming only oxygen is present as a reactive species in the atmosphere, and this reaction scheme is called pure oxygen theory or Chapman mechanism. Photolysis of O2 occurs by the solar radiation of wavelength shorter than 242 nm (see Sect. 4.3.1). Only the photolytic process. 

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