Chlorination of natural organic matter: kinetics of chlorination and of THM formation

The kinetics of the formation of trihalomethanes (THMs) and of chlorine consumption for the chlorination of natural organic matter with an excess of chlorine (50 μM > [Cl2]o>210 μM) was investigated. THM precursors could be divided into a fast and a slowly reacting fraction. Long term chlorine demand and the formation of THM could be described by second order kinetics. Rate constants were between 0.01 and 0.03 M−1 s−1 in the pH range 7–9 for surface waters and humic materials extracted from surface waters. A groundwater gave a higher rate constant of 0.124 M−1 s−1. Resorcinol-type structures were tested with respect to kinetics and yield of THM formation. They could possibly be responsible for the fast reacting THM precursors, which represent 15–30% of the THM precursors of natural waters. Additional classes of compounds that might contribute to the initial THM formation include readily enolizable compounds such as β-diketones and β-ketoacids. Experiments with phenol showed that slowly reacting THM precursors may consist of phenolic compounds. The influence of pretreatments (UV/visible irradiation, ozone and chlorine dioxide) on chlorine demand and THM formation from NOM was also studied: UV/visible irradiation does not alter THM formation but leads to a higher chlorine demand. Preoxidation with ozone leads to a lower THM formation with an unaltered chlorine demand and preoxidation with chlorine dioxide reduces THM formation and the chlorine demand.