1. Sources and Significance
Ozone reactions during water treatment are complex and often produce a wide range of unstable oxidation byproducts, usually oxygenated and polar. Low-molecular-weight byproducts such as aldehydes are among the intermediate products formed when ozone attacks the organic matter present in raw waters. If oxidized further, these aldehydes can produce aldo-acids and carboxylic acids. Formaldehyde, a ubiquitous component of the environment, may be introduced into drinking water by ozone treatment, natural metabolism, and commercial processes.
There are two postulated mechanisms for aldehyde formation during ozone treatment. The first involves a 2-step Criegee attack at unsaturated C–C bonds by molecular ozone with ozonides or epoxides formed as intermediates.1 The second involves an indirect reaction of OH radicals.2 Although the levels of aldehyde formation are usually a function of ozone dose, their concentrations are often controlled in water treatment by increasing the pH and thus the alkalinity of the water.
Aldehydes are unlikely to pose a serious health hazard to the consumer at the microgram-per-liter concentrations usually encountered in drinking water treatment. However, they react with nucleophiles even at these low levels and can therefore be a potential threat.3 For example, formaldehyde, acetaldehyde, and crotonaldehyde are known animal carcinogens. Formaldehyde is a known human carcinogen.4 Aldehydes also may serve as important components of assimilable organic carbon in promoting undesirable bioactivity.