The action of chlorine dioxide on aquatic organic materials during the disinfection of drinking water

Abstract

Measurements of the Total Organic Halogens (TOX) produced in the National Water Carrier (NWC) of Israel after disinfection with chlorine dioxide revealed that compared to chlorine, ClO₂ forms only a very small amount of chlororganic compounds. Mixtures of Cl₂ and ClO₂ that have been reported to reduce the concentrations of both the chlorinated organics as well as the chlorite Ions produced by ClO₂, were found not to be very useful because of the fact that high bromide concentrations are present In the NWC. Therefore high ratios of ClO₂/Cl₂ are needed to reach a significant reduction in Trihalomethanes (THM's). The amount of THM's can however be satisfactorily reduced when ClO₂ is introduced a few hours before chlorine.

A comparison between Cl₂ and ClO₂ with regard to their reactions with a series of polyeyelic aromatic hydrocarbons (PAH's) showed that the ClO₂ reactions are more unique. While the highly carcinogenic compounds, benz(a)pyrene and 1,2‐benzanthracene react with ClO₂ much more rapidly than with Cl₂, other PAH's, such as naphthalene and fluoranthene, which react quite easily with chlorine, react very slowly or not at all with chlorine dioxide.

The vast differences between the physical factors influencing the reaction rates of Cl₂₂ and ClO₂, and the sequence of their reactions with various PAH's indicate that in contrast to chlorine which reacts by few possible pathways, chlorine dioxide reacts mainly as a one‐electron acceptor. Consequently, the main organic by‐products of ClO₂ seem to be aldehydes, ketones and quinones, beside low molecular weight carboxylic acids. Although aldehydes and carboxylic acids are usually regarded as monitoxic materials, the toxicity of quinones and some keto‐aldehydes are still largely unknown.

Notes

The Environmental Health Laboratory of the Hebrew University Medical School, Jerusalem, Israel

The Central Laboratory of Mekoroth Water Company, Israel