Kinetics and mechanisms of formation of earthy and musty odor compounds: Chloroanisoles during water chlorination

- Anisole can be chlorinated by HOCl to form chloroanisoles and para-products are more easily produced than ortho-products.
- Hydrogen ion can promote the reaction during the formation of chloroanisoles.
- Relative importance of factors in chloroanisole formation is: chlorine dose > initial anisole concentration > temperature.

Chloroanisoles are often reported as off-flavor compounds which produce an earthy and musty flavors and odors in drinking water. To improve understanding and ultimately minimize the formation of 2,4-dichloroanisole (2,4-DCA), 2,6-dichloroanisole (2,6-DCA) and 2,4,6-trichloroanisole (2,4,6-TCA), which have low odor threshold concentrations (OTC: 0.03-4 ng L−1), a kinetic database for the chlorination of anisole was established by kinetic measurements. The results showed that HOCl reacted with anisole in acidic solution, with the hydrogen ion as an important catalyst. Quantification of product distribution of the produced chloroanisoles demonstrated that a chlorine attack in the para-position was favored over the ortho-position. A kinetic model was formulated, which permitted investigation of the relative importance of the chlorine dose and other water quality parameters including the concentrations of anisole and several metal ions, as well as temperature, on the product distribution of chloroanisoles. In general, high chlorine doses led to low concentrations of intermediates. The presence of ions such as Fe3+ and Al3+ facilitated the formation of chloroanisoles, but Zn2+ and Mn2+ did not. The kinetic model can be applied to optimize water chlorination and minimize earthy and musty odors.