Photo-oxidation of biodegraded crude oil and toxicity of the photo-oxidized products

We investigated the physicochemical changes resulting from irradiation by s unlight of biodegraded crude oil. An Arabian light crude oil sample was fir st subjected to microbial degradation. n-Alkanes and aromatic compounds suc h as naphthalenes, fluorenes, dibenzothiophenes and phenanthrenes possessin g short, alkyl side chain(s) were almost completely degraded, while the con tents of the saturated and aromatic fractions were reduced by 70% and 40%, respectively. This biodegraded oil was then suspended in seawater and expos ed to sunlight irradiation for several weeks. The most remarkable change ca used by the irradiation was a substantial decline in the aromatic fraction with a concomitant increase in the resin and asphaltene fractions. A C-13-n uclear magnetic resonance (NMR) spectroscopic analysis showed that the arom aticity of the biodegraded oil was significantly lower in the irradiated sa mple. A field desorption-mass spectrometric (FD-MS) analysis showed that su nlight irradiation reduced the average molecular weight of the oil componen ts and formed oxygenated compounds. Consistent with this observation is tha t the oxygen content in the oil increased as the irradiation was prolonged. The bioavailability of the biodegraded oil was increased by the photo-oxid ation: the growth of seawater microbes was minimal when the non-irradiated biodegraded oil was used as the source of carbon and energy; however, growt h was significant when irradiated biodegraded oil was used. The concentrati on of dissolved organic carbon (DOC) increased linearly during the sunlight irradiation of the biodegraded oil, and this increase was matched by an in crease in ultraviolet-absorptive materials in the seawater. The photochemic ally formed, water-soluble fraction (WSF) showed acute toxicity against the halophilic crustacean, Artemia. (C) 2001 Elsevier Science Ltd. All rights reserved.