Publication

Molecular-Level Transformation of Dissolved Organic Matter during Oxidation by Ozone and Hydroxyl Radical

Urs von Gunten
2020
Journal paper
Abstract

Ozonation of drinking and wastewater relies on ozone (O-3) and hydroxyl radical ((OH)-O-center dot) as oxidants. Both oxidants react with dissolved organic matter (DOM) and alter its composition, but the selectivity of the two oxidants and mechanisms of reactivity with DOM moieties are largely unknown. The reactions of O-3 and (OH)-O-center dot with two DOM isolates were studied by varying specific ozone doses (0.1-1.3 mg-O-3/mg-C) at pH( )7. Additionally, conditions that favor 0 3 (i.e., addition of an (OH)-O-center dot scavenger) or (OH)-O-center dot (i.e., pH 11) were investigated. Ozonation decreases aromaticity, apparent molecular weight, and electron donating capacity (EDC) of DOM with large changes observed when O-3 is the main oxidant (e.g., EDC decreases 63-77% for 1.3 mg-O-3/mg-C). Both O-3 and (OH)-O-center dot react with highly aromatic, reduced formulas detected using high-resolution mass spectrometry (O:C = 0.48 +/- 0.12; H:C = 1.06 +/- 0.23), while (OH)-O-center dot also oxidizes more saturated formulas (H:C = 1.64 +/- 0.26). Established reactions between model compounds and O-3 (e.g., addition of one to two oxygen atoms) or (OH)-O-center dot (e.g., addition of one oxygen atom and decarboxylation) are observed and produce highly oxidized DOM (O:C > 1.0). This study provides molecular-level evidence for the selectivity of O-3 as an oxidant within DOM.

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Related concepts (34)
Ozone
Ozone (ˈoʊzoʊn) (or trioxygen) is an inorganic molecule with the chemical formula O3. It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope O2, breaking down in the lower atmosphere to O2 (dioxygen). Ozone is formed from dioxygen by the action of ultraviolet (UV) light and electrical discharges within the Earth's atmosphere. It is present in very low concentrations throughout the latter, with its highest concentration high in the ozone layer of the stratosphere, which absorbs most of the Sun's ultraviolet (UV) radiation.
Oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as well as with other compounds. Oxygen is Earth's most abundant element, and after hydrogen and helium, it is the third-most abundant element in the universe. At standard temperature and pressure, two atoms of the element bind to form dioxygen, a colorless and odorless diatomic gas with the formula O2.
Ozone–oxygen cycle
The ozone–oxygen cycle is the process by which ozone is continually regenerated in Earth's stratosphere, converting ultraviolet radiation (UV) into heat. In 1930 Sydney Chapman resolved the chemistry involved. The process is commonly called the Chapman cycle by atmospheric scientists. Most of the ozone production occurs in the tropical upper stratosphere and mesosphere. The total mass of ozone produced per day over the globe is about 400 million metric tons.
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