VERTIMAR-2005
Symposium on Marine Accidental Oil Spills
Photochemistry of the Prestige fuel oil in heterogeneous phase
JosepMBAYONA, Joan MORAN and JoanALBAIGES
Environmental Chemistry Department / IIQAB-CSIC, JordiGirona, 18, E-08034 Barcelona, SPAIN
ABSTRACTThis presentation reports on the photolysis of the major chemical classes of compounds, which constitute the Prestige fuel oil, namely maltenes (aliphatic and aromatic hydrocarbons, and resins) and asphalthenes, when the fuel oil deposited onto sand was irradiated with simulated solar light (Xenon lamp). Kinetics and the formation of intermediates during the photooxidation of individual fractions or the whole fuel oil were investigated by TLC-FID, GC-MS and FTIR spectroscopy. Direct and indirect photolysis were evidenced by comparing the results of the two sets of experiments, which may contribute to understand the possible transformation of fuel oil emulsions that reached the beaches.
1. INTRODUCTION
Photolysis is one of the most important processes affecting to the oil chemical composition once it is released into the environment, in a time span between few weeks to several months. Photolysis involves a large number of chemical oxidation reactions leading to the carbon oxidation. It can occur directly when molecules can absorb the solar irradiation or indirectly when a photosensitizer is needed because the target molecule does not contain any chromophore. The intermediate products of photooxidation can be more toxic than the parent compounds and, therefore, be of higher ecotoxicological concern (Lee 2001).
A lot of information exists on the heterogeneous photolysis of water-oil in the water surface (Payne and Phillips, 1985, Garret et al. 1998). However, few data of crude oils or oil products deposited on sand beach is available.
In this work, a Suntest apparatus equipped with a Xe lamp (507 W/m2) was used to irradiate either single fractions (aliphatics, aromatics, resins and asphaltenes) or the maltenic and asphaltene fractions of the Prestige fuel oil. Maltenes were isolated by precipitation of asphaltenes with n-pentane and the obtained maltenes were fractionated by column chromatography with silica-alumina as adsorbents. Irradiation experiments were performed at ca. 40 ºC, from 2 to18 h, in closed quartz vials containing precleaned sand and fuel oil. Dark control samples were also obtained during the photooxidation process. The formation of volatile species has been monitored by headspace solid-phase microextraction (HS-SPME) followed by GC-MS. On the other hand, semivolatiles have been extracted with ethyl acetate and analysed by GC-MS.
2. RESULTS AND DISCUSSION
Whole fuel irradiation. The change in composition of the 4 major chemical classes, namely aliphatics, aromatics, resins and asphaltenes were monitored by TLC-FID. As shown in Figure 1, a decline in the aromatic bands was observed from 41 to 28.7%, whereas resins and asphaltenes increased from 28.6 to 41.9% and 12.1 to 15.8%, respectively. This could be accounted for the oxidation of PAHs leading to their incorporation into the most polar fractions. Aliphatics show also a decline in concentrations but less remarkable than PAHs.
Figure 1. Compositional changes in the Prestige fuel oil following an irradiation experiment with a Xenon lamp at different times.
Maltenes irradiation. Two different irradiation tests were performed: a) on the discrete fractions (aliphatics, aromatics, carbazols and resins) and b) on the whole maltene fraction. Generally, photooxidation rates were faster when discrete fractions were evaluated in comparison with the whole maltene fraction. This could be accounted for the quenching effects of the resins present in the maltenic fraction. Moreover, the aliphatic fraction was not photooxidised when it was irradiated as a discrete fraction in comparison to the maltenic fraction where photosensitizers can induce an indirect photooxidation process. The aromatic fraction was photooxidised in both procedures but a selectivity depending on the number of alkyl substituents and the shape of the molecule was observed. In fact, the photooxidation rate increased with the alkylation degree (parent<mono<di<trialkyl), attributable to the inductive effect of the alkyl substituents. Moreover, the photodegradation rate also increased with the number of aromatic rings from naphthalene to pyrene and the geometry of the fused rings (phenanthrene vs anthracene). A variety of photooxidation intermediates were identified from the irradiation of the PAH fraction, such as ketones, aldehydes, quinones, hydroxy-PAHs, PAH anhydrides and linear carboxylic acids (n-C6-C10). However, their abundance was also declining according with the irradiation time. Carbazole and its alkylated derivatives are rapidly photooxidised being almost degraded after 6 h of irradiation. The irradiation of the resin fraction did not show the formation of volatile photoproducts but the FTIR spectra (Figure 2) shows an increase in the absorption bands of sulfoxide (1034 cm-1) and sulfone (1100-1300 cm-1). On the other hand, the oxidation of carbonyl groups to acids is evidenced by a shift from 1698 to 1723 cm-1. Finally, an increase in the 3400 cm-1 is attributable to the OH group.
Figure 2: FTIR spectra of the resin fraction following its irradiation at 6, 12 and 18 h.
Asphaltenes irradiation The asphaltenic fraction did not show any formation of volatile compounds but as in the case of resins the oxidation was monitored by FTIR. The following transformations were identified: a) decline of the C=C band (1603 cm-1), b) increase of the OH band (3400 cm-1), c) increase of the C=O band (1703 cm-1) and d) increase of a broad sulfone band (1290-1030 cm-1).
REFERENCES
Garret R.M., Pickering I. J., HaithC.E. and Prince R.C. (1998) Photooxidation of crude oils. Environ. Sci.Technol. 32: 3719-3723.
LeeR.F. (2003). Phtooxidation and phototoxicity of crude and refined oils. Spill Sci. & Technol.Bull.8: 157-162.
Payne J. R. and Phillips C. R. (1985) Photochemistry of petroleum in water. Environ. Sci.Technol 19: 569-579.