How does ozone react with organic matter?

The reactions between ozone and organic matter mainly include two types:

① Oxygenation reaction, where ozone molecules add to carbon-carbon double bonds to form epoxy adducts, turning the double bonds into single bonds, followed by the breaking of carbon-carbon bonds to generate corresponding carbonyl and hydroxyl oxides, such as the formation of 1,3-cycloaddition products (Criegee mechanism) when ozone reacts with alkenes;

② Hydrogen abstraction reaction, where ozone removes hydrogen atoms from organic hydrocarbons, disrupting their structural stability, such as the dehydrogenation of formic acid radicals, causing them to mineralize into carbon dioxide.


Ozone is unstable in aqueous solutions and prone to decomposition reactions that produce more reactive hydroxyl radicals (·OH, E=+2.33V). Hydroxyl radicals are very reactive, with reaction rate constants with organic compounds generally ranging from 10ⁿ (n=8) to 10ⁿ (n=9) L/(mol▪8), allowing for rapid, thorough, and non-selective degradation of organic pollutants in solution. Compared to direct oxidation reactions between ozone molecules and organic matter, the oxidation reactions between hydroxyl radicals and organic matter are called indirect oxidation reactions of ozone.

The reactions between hydroxyl radicals and organic matter mainly include two modes: ① Addition reaction, where hydroxyl radicals add to carbon-carbon double bonds, carbon-nitrogen double bonds, and sulfur-oxygen double bonds in organic compounds to form hydroxylated products or oxygen-rich intermediates, which is a common reaction mode with very fast reaction rates close to diffusion control; ② Hydrogen abstraction reaction, where hydroxyl radicals react with weaker carbon-hydrogen single bonds and sulfur-hydrogen single bonds to remove hydrogen atoms.

In Staehelin and Hoigne's pioneering work, the concepts of initiators, promoters, and terminators of ozone decomposition reactions were proposed early on. Hydroxide ions (OH¯) are one of the important initiators, making ozone very unstable in alkaline solutions. The process of OH¯ initiating ozone decomposition can be represented by the following reaction equations:

The rate of reaction between O3 and OH¯ (Eq. 1-2-1) is not fast, but once this reaction is initiated and produces HOO¯， a series of subsequent free radical generation and transformation reactions occur rapidly, propagating step by step, so ozone decomposition reactions are also called chain decomposition reactions. In chain reactions, the mutual transformation among ▪O2¯， ▪O3¯， ▪HO₃， ▪HO₂， and ▪OH promotes the continuous progress of the chain reaction.

In addition to OH¯， there are other substances in solution that can generate HOO¯ and ▪O₃¯ and initiate ozone chain decomposition reactions, such as H₂O₂， humic acids, and inorganic metal ions (Fe²⁺， Co²+, Mn²+), etc. The relevant reaction equations are as follows:

H₂O₂→HO₂¯+H+ (Eq. 1-2-9)
HS+O₃→HS+ + ▪O₃¯ (Eq. 1-2-10)
Fe²+ +O₃→Fe³+ +▪O₃- (Eq. 1-2-11)

If certain substances in aqueous solutions can react with O₃ or ·OH and generate ·O₂-, then the free radicals in the above chain reactions are continuously regenerated, allowing the chain reactions to continue. These substances are called promoters of ozone decomposition reactions and include fatty alcohols, aromatic hydrocarbons, formate salts, humic acids, etc. Methanol is a typical promoter, and its promotion process can be represented by the following reaction equations:

CH₃OH+O₃→·OOCH₂OH+·OH (Eq. 1-2-12)
.OOCH₂OH→CH₂O+·HO₂ (Eq. 1-2-13)
.OOCH₂OH+OH→CH₂O+H₂O+·O₂- (Eq. 1-2-14)

On the other hand, if some substances in solution react with free radicals and no longer generate free radicals afterward, then the chain reaction will stop. These substances are called terminators of ozone decomposition reactions and include t-butanol, CO₃²¯， HCO₃¯， NO₂¯， etc.