Incorporation of initiation, promotion and inhibition in the Rct concept and its application in determining the initiation and inhibition capacities of natural water in ozonation

Ozonation is widely employed in water treatment to purify water. The R_ct concept, which is defined as the ratio of OH exposure to ozone exposure, has been commonly used to quantify the OH concentration generating from ozone decomposition and model the removal of organic pollutant. Due to its empirical nature, however, the R_ct concept is limited in quantitatively elucidating how initiator, promoter and inhibitor involved in the OH chain reactions affect its value. A new R_ct model was developed by integrating the R_ct concept and the transient steady-state OH concentration to evaluate the influences of these modes of reactions on the R_ct value. It was found that the R_ct value is not only the ratio of OH exposure to ozone exposure but also the ratio of the total initiation capacity to the total inhibition capacity in a system. The presence of promoter, however, does not affect the R_ct value although it does accelerate ozone decomposition leading to lower ozone and OH exposures and result in hindered removal of target pollutant. The hindered removal of ibuprofen by ozonation in the presence of methanol (promoter) can be quantitatively described by the new R_ct model. The model can also be used to quantify the initiation and inhibition capacities of an ozonation system via the addition of an external inhibitor. Its application in determining the initiation and inhibition capacities of natural water was demonstrated.