David A. Reckhow
Low molecular weight aldehydes such as formaldehyde may be formed as disinfection byproducts (DBPs) of ozonation when low molecular weight, aliphatic, alpha olefins are present as contaminants in water. Such alkenes form a small but significant part of crude oil, and would therefore be present in water contaminated with an oil spill. This project studied the kinetics of the reaction between ozone and 1-hexene in water as part of a study to predict the effects of an oil spill on the water quality in the Wachusett reservoir in Massachusetts. The data for the reaction rate constant for the aqueous ozonolysis of low molecular linear alpha olefins that are liquid under ambient conditions (such as 1-hexene) is sparse in existing literature, and such data would be essential to any attempt to model the formation of DBPs due to ozonation of water contaminated with crude oil. Two methods were used to measure the rate constant for the reaction between ozone and 1-hexene in water. The first method used direct spectrophotometry, and measured the concentration of ozone over time as it was reacted with an excess of 1-hexene. The data was then fit to a pseudo first order kinetic model and the reaction rate constant was calculated. The rate constant calculated using the above method was found to be many orders of magnitude smaller than those for ethene and propene calculated previously, and it was theorized that the decay of ozone here was limited by the rate of dissolution of 1-hexene in water instead of the reaction itself. Therefore, a second method, involving competition kinetics was employed where 1-hexene was made to react with ozone in competition with styrene (whose rate constant for aqueous ozonolysis has been calculated previously), and the yields of the products of each olefin were quantified using gas chromatography. These yields were then used to calculate the rate constant for the reaction between 1-hexene and ozone using equations derived through the course of this project. The rate v constant for 1-hexene calculated using this method was found to be of a similar order of magnitude to those for ethene and propene. Since the electron density around the double bond does not change significantly with small changes in alkene chain length, it was theorized that the rate constant of the reaction between ozone and other linear alpha olefins such as 1-pentene and 1-heptene would be very similar to that of 1-hexene. Finally, the formation of aldehydes due to ozonation of a 1 mg/L DOC aqueous solution of West Texas Intermediate crude oil was modeled.