To further a quantitative understanding of the effects of weathering on oil spilled at sea, a series of
laboratory experiments were conducted to investigate the dissolution and photodegradation kinetics of
Kuwait crude oil in seawater. A simple and practical method was developed for the preparation of
stable homogeneous samples of sea water-soluble oil (SWSF) at 25 "C. The dissolution was monitored
by two independent but complementary methods; total dissolved oil was measured by a photochemical
dissolved organic carbon (DOC) technique whilst the dominant soluble constituents, aromatic
hydrocarbons, were measured by ultraviolet-fluorescence (UVF) spectrophotometry. Maximum
dissolution was observed after slowly stirring oil on seawater for 5 days when concentrations were,
respectively, 3.2-3.6 fig C mL * seawater (DOC) and 2.4 /xg diesel equivalents mL ' seawater (UVF).
C^ntification of the photodegradation of individual SWSF constituents required development and
optimisation of a more specific analytical method. The final method involved addition of deuterated
internal standards, extraction with n-pentane, controlled evaporation (micro Kudema-Danish
concentration) and gas chromatography-mass spectrometry (GC-MS). Particular care was taken to
eliminate sample contamination and storage conditions were also optimised. The recovery of
deuterated internal standards through the isolation steps ranged from 30% for benzene-d^ to 100% for
phenanihrene-diQ. The method allowed more than fifty compounds, mainly low molecular weight
aromatic hydrocarbons, to be identified and quantified in the SWSF. Alkylated benzenes ( Q ^ were
the most abundant (98% of the dissolved oil) followed by alkylated naphthalenes (C0.3) (= 2%). The
oil to seawater partition coefficient of individual hydrocarbons (K^^) showed that hydrocarbons of
high aqueous solubility (e.g C0.2 alkylbenzenes) also possessed the lowest partition values, whereas
compounds with high partition coefficients (e.g. alkylnaphthalenes) remained mainly in the oil phase.
A reproducible method of simulating solar irradiation was established under controlled environmental
conditions using a calibrated 1.8 kW xenon lamp. The photodegradation of a model SWSF compound
(phenanthrene) followed first-order kinetics (kp= 0.317+0.029) with a half life (tj^J of 2.2 hr at 25
"^C when irradiated at 194 Wm'-. The extrapolation of to latitude 30 *'N (Florida summer sunlight)
showed that phenantiirene would degrade in = 9 hr. Some photoproducts including fluorene,
fluorenone, 2,2'-diformylbiphenyl, 2,3:4,5-dibenzoxepin, 3,4-benzocoumarin, and 9,10-
phenanthrenequinone were identified by GC-MS, although these accounted for only 11% of the initial
mass of phenanthrene. UVF and GC-MS methods for examining the photodegradation of phenanthrene
were compared.
The photochemical fate of the characterised SWSF was investigated using the optimised irradiation
system. Benzene and C^s alkylated benzenes showed no evidence of photodegradation after periods
of up to 48 hour under the lamp, however, some C4 alkylbenzenes (jec-butylbenzene, 1,2-
dimethylbenzene, 1,2,4,5- + 1,2,3,4-tetramethylbenzene and 1,2,3,4-tetrametiiylbenzene) appeared
to follow first-order kinetics with respect to irradiation time. The photodegradation rate constants (kp)
of the alkylated benzene and alkylated naphthalene isomers were statistically different demonstrating
that certain isomers will phoiodegrade faster tiian others under the same conditions. Half lives ranged
from 11.8 to 33.6 hr at 25 ''C. The majority of naphthalene (i.e. C^^^) identified in the SWSF of
Kuwait crude oil exhibited first-order photodegradation kinetics with differing rate constants. Half
lives were: naphtiialene. 15.7 hr; 1-methyl and 2-methyl naphthalene, 14.9 and 12 hr, respectively;
l-l-2-ethylnaphthalene, 15.9; 1,6-dimethylnaphthalene, 10.4hr; 1,3,7-and 1,2,4-trimethylnaphthalene,
11.95 and 12 hr, respectively. The low initial concentration of other naphthalene isomers did not allow
for accurate photodegradation kinetics to be determined. Extrapolating the data to latitude 30 "N
showed that die photodegradation ti;^ expected for tiie determined SWSF compounds were between
4.2 to 7 days. This would make photodegradation an effective weathering process for the removal of
spilled oil, particularly in high solar radiation environments.
| Date of Award | 1994 |
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| Original language | English |
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| Awarding Institution | |
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