The effect of biodegradation and water washing on the occurrence of alkylphenols in crude oils

Biodegraded oils from California and Nigeria were analysed to assess the impact of biodegradation upon alkylphenol occurrence in crude oils. The extent of biodegradation in the 13 Californian samples ranged from very mild (full n-alkane distribution present) to moderate (complete degradation of C15+ n-alkanes and significant reduction of acyclic isoprenoids) corresponding to a range of <1–4 on the biodegradation scale of Peters and Moldowan [Peters, K.E., Moldowan, J.M., 1993. The Biomarker Guide: Interpreting Molecular Fossils in Petroleum and Ancient Sediments. Prentice Hall, Englewood Cliffs, NJ (363 pp)]. Degradation in the four Nigerian oils ranged from very mild/non-degraded (stage 1 or less) to complete removal of n-alkanes and acyclic isoprenoids (stage 5). The data from the naturally-degraded sample suites were compared to the results of an in-vitro aerobic biodegradation experiment conducted with a North Sea oil. Biodegradation resulted in significant reduction in total alkylphenol abundance and preferential depletion of C3 and C2 alkylated compounds in the Nigerian and laboratory degraded samples. These effects were most pronounced in oils in which the methylated naphthalenes had been biodegraded (biodegradation stage 4 or 5). There was no clear relationship between alkylphenol abundance or molecular distributions and the extent of biodegradation in the Californian samples, although alkylphenol distributions in the biodegraded samples were generally dominated by the C3 compounds. Simple oil/water partition calculations showed that these distributions were consistent with the effects of water washing.