Studies on the photolytic behavior of dibenzothiophene in crude oil/water systems.

Abstract

The fate of crude oil and its components in the environment is influenced by transport and transformation processes such as partitioning, sorption and photolysis. This study investigates the behavior of dibenzothiophene (DBT), a large component of the sulfur aromatics present in crude oil, when it is photolyzed under simulated light conditions in various media including saline waters and suspended (dust) particles. To carry out the studies, analytical techniques were developed for the fractionation, extraction and analysis of the oil components including the DBT, in oil/water samples. Aqueous samples of DBT were exposed to light and photolysis rates were measured under varying oil concentrations, saline conditions and amount of suspended particles. Photolysis transformed DBT into more water-soluble products and a predominant photoproduct formed in the aqueous systems was dibenzofuran, a highly toxic compound. Other oxygenated products were formed in the crude oil/water systems. Saline solutions showed twofold enhancement of the photolysis rates from a 30$\perthous$ to a 40$\perthous$ salinity. Outdoor photolysis indicated the formation of an unresolved complex mixture different from that produced under simulated light conditions. The effect of suspended particles in the outdoor exposure showed reduced photolysis rates from 0.644 day$\sp{-1}$, without dust, to 0.333 day$\sp{-1}$, with dust. The partitioning of DBT in aqueous crude oil solutions were also enhanced in 40$\perthous$ salinity (K$\sp\prime\sb{\rm D}$, 0.1444 $\times$ 10$\sp{-6}$) compared to distilled water (K$\sp\prime\sb{\rm D}$, 0.21163 $\times$ 10$\sp{-6}$). Increasing oil concentrations in the system from 40-2000 mg/L increased the K$\sp\prime\sb{\rm D}$ values in both distilled water and saline waters. The enhancement was greater in the saline solutions (log K$\sp\prime\sb{\rm D}$, 4.973 to 5.455) compared to the values in distilled water (log K$\sp\prime\sb{\rm D}$, 5.230 to 5.669), indicating that the DBT preferentially distributed itself in the oil phase of the system. A significant decrease in the K$\sp\prime\sb{\rm D}$ of DBT was also observed with increased concentration of suspended particles in the aqueous crude oil systems. The adsorption isotherms of DBT in both distilled water and saline water showed a linear adsorption relationship between water and adsorbent at sufficiently low concentrations of DBT (1 mg/L) described by an adsorption isotherm typical for hydrophobic compounds. This study describes the role of photolysis in the fate of dibenzothiophene in a simulated environment depicting an oil spill scenario. The partitioning of DBT in the water/oil is influenced by photolysis and other factors which are characteristics of its surrounding environment, such as salt concentration and amount of suspended particles.