Photochemical weathering study of Brazilian petroleum by EPR spectroscopy

The EPR spectrum of the petroleum of the Campos Basin, Rio de Janeiro, Brazil shows signals characteristic of free radical, vanadyl (VO2+) and iron III ion (Fe3+). The value of g = 2.0045 +/- 0.0001 corresponds to free radical species with a peak-to-peak line width DeltaH=4.64 +/- 0.03 G. The g spectroscopic factor suggests the presence of free radicals in aromatic systems containing carbon, nitrogen and oxygen. Deltag(o) (chemical shift) = 21.6 +/- 0.1 determined for the VO2+ species characterizes porphyrin and non-porphyrin (with sulfur) complexes in asphaltenes of the crude oil. The species observed with g = 2.16 +/- 0.01 corresponds to Fe3+ in Fe2O3, and therefore is not part of the molecular structure of the oil under study. The photochemical weathering of Brazilian oil as a film over seawater was monitored by EPR. After 100 h of solar irradiation the line width of the radical decreased by 10.6%, indicating rearrangement among radicals. The Deltag(o) value for VO2+ in the irradiated oil decreased to 19.5 +/- 0.1, characterizing degradation of porphyrin systems. Solar irradiation promotes the partial destruction of the asphaltenic fraction of the oil. (C) 2003 Elsevier B.V. All rights reserved.