The dissolution and photodegradation of Kuwaiti crude oil in sea water. Part 1: Quantitative dissolution and analysis of the seawater-soluble fraction

Abstract

The present report describes a reliable method for the preparation of a solution of Kuwaiti crude oil in seawater, and for determining the concentrations of its components. The dissolution rate was monitored by two complementary methods: total dissolved oil was measured by a photochemical dissolved organic carbon (DOC) technique, whilst fluorescent aromatic hydrocarbons, the dominant soluble constituents, were determined by ultraviolet fluorescence (UVF) spectrophotometry. Maximum dissolution was observed after five days stirring of oil on water when concentrations were, respectively, 3.2–3.6 pg C ml − 1 seawater (DOC) and 2.4 μg diesel equivalents ml − 1 seawater (UVF). However, whilst DOC and UVF measurements are suitable for monitoring dissolution, neither provides an accurate estimate of true oil concentrations. The DOC method underestimates volatile constituents (which dominate the water soluble fraction) and UVF detects only fluorescent components and is dependent on a proper match of calibrant and analyte compositions. A more accurate measurement of the concentrations of dissolved hydrocarbons in the seawater was made for samples taken from the dissolution apparatus after five days stirring, by addition of deuterated internal standards, extraction with n-pentane, controlled evaporation (micro-Kuderna-Danish concentrator) and gas chromatography-mass spectrometry (GC-MS). In a series of related experiments, the recovery of the deuterated internal standards through the isolation steps was found to range from 21–32% for deuterated benzene, to 51–102% for deuterated phenanthrene. The result of these experiments is a practical method for the preparation and quantitative analysis of oil in seawater, which should also apply to any light crude oil. The method is particularly applicable to studies of the kinetics of oil degradation; the results of these studies will be reported in subsequent publications.