On the degradation of petroleum residues in the marine environment

Chemosphere Vol. 9, PP 539 - 545 ©Pergamon Press Ltd. 1980. Printed in Great B~italn

ON T H E

DEGRADATION

OF P E T R O L E U M

RESIDUES

0045-6535/80/0901-055~02.00/0

IN THE

MARINE

ENVIRONMENT

$. Albaig~s and M.R. Cuberes Institute of Bio-Organic Chemistry (C.S.I.C.) $orge Girona Salgado. Barcelona-34. Spain.

Introduction Petroleum released in bulk at sea spreads rapidly on the surface in the Form of slicks, which are Further dispersed and degraded by physical, chemical and biological processes (I). As a result of these processes the remainder appears in the marine environment mainly in two Forms: as agregates o£ different types and sizes (e.g., tar balls , hydrocarbons adsorbed onto organic and inorganic particles, etc . . . . ), and dissolved in the water, the Former being more widely distributed, probably due to preferential Formation From the slicks and lower degradation rates. The investigation o£ the biogeochemistry of these residues deserves much interest From the standpoint of the assessment of the ultimate Fate of petroleum at sea and its subsequent impact on the marine environment. Apart from studies on alkane oxidation there is little evidence available to explain what happens to the remaining oil components. This paper attempts to present some evidence, acquired either From laboratory or Field studies, on the degradation of the heavier and more r e f r a c t o r y tarry residues, inasmuch as it can contribute to the removal of the latter From the sea. Experimental The oil weathering test was performed with an Arabian light residue ( > 2 1 0 ° C ) The oil (10 grs.) was placed in a circular glass container (30 cm. i.d.) Filled with 4 liters of sea water and about 500 grs. of soil. Hydrocarbon-Free air was bubbled 8 hours per day throughout the mixture at the same time as this was irradiated with a UV lamp (365 nm.). The water was renewed every Four days. Four c o ~ tainers were arranged in this manner and the oil recovered after 15, 30, 60 and 120 days respectively, by extraction with chloroform. Tar ball samples were collected during a survey cruise (july 1977) between Cartagena (Spain) and C i v i t t a v e c h i a (Italy), with a neuston net. Samples were dissolved in toluene to eliminate extraneous materials and stored at -4oc until analysis. Hydrocarbon profiles were obtained in a GC apparatus (Perkin-Elmer 990) operated with 2 m. x 0,2 mm glass c o l u m n s ( 3 % Dexsil on Gas-Chrom0 1OO-120) From 150 to 3OO°C at 6°C/min. Oil extracts were d e a s p h a l t e n e d in ~ - p e n t a n e (40 volumes in excess) ther fractionated in saturates, mono-, di-, polyaromatics and resins by tography in a mixed a l u m i n a - s i l i c a column (I: 1 vol.), according to the described by Hirsch et al (2). The resolution of the last two Fractions

CHEM 9/9 - C

and Furchromamethod was

540

accomplished by elution with pentane-benzene nol (1:1:3), respectively.

(1:1) and benzene-ethyl ether-metha-

Alternatively, deasphaltened oils were distilled at 0,1 mm Hg in a Dixon rings packed micro-column (5 x 0,5 cm.), collecting £ractions at 300-350, 350-403 and 400-450oc. These £ractions were also resolved by hydrocarbon types as reported above and analyzed by high resolution mass spectrometry (AEI MS9025) at 70 eV (3) and 12 eV (4). Structural indices o£ resins and asphaltenes were determined £rom their NMR and IR spectra (5,6). Molecular weights were determined in a 301A Mechrolab VPO using dioxane-chloro£orm (1:1) as solvent. Results and discussion Degradative processes o£ petroleum at sea are poorly understood compared with physicochemical dispersion. However, it has been recognized that they are generally long-term processes and oxidative, both chemical and biological in nature. This is also in£erred £rom £igure I, which shows the results o£ a £our month oil weathering test which has served as the basis £or the present study.

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Figure 1.- Variation o£ the chemical composition o£ an Arabian light residue (>210oc) during a weathering simulated test. Numbers in brackets indicate the content o£ n + isopara££ins in saturates and t r ~ a r o m a t i c s in aromatics, o£ £ractions 300-350°C (upper) and 400-500°C (lower). Curves correspond to IR spectra (1600-1700 cm -1 range).

541

The major chemical compositional changes observed during the experiment were in accordance with previous £ield observations (7) consisting o£ an increase in the resin and asphaltene contents and a decrease in the saturate to aromatic hydrocarbon ratio. Oxidative processes, induced by the photochemical action o£ sunlight, are apparently responsible For the new£ormation o£ resins and asphaltenes (8) which appears to progress di££erently, initially £aster £or the asphaltenes and more steadily £or the resins. In this regard it must be noted that the carbonylic band (1710 cm "I) in the in£rared spectra increases more strongly in the resin £ractJon~ probably because they originate £rom the aromatics by photo-oxidation, whilst asphaltenes are primarily £ormed £rom heterocompounds by £ree radical condensations. The nature o£ the oxidation products seems to be rather varied, bearing in mind the appearence o£ bands at 1150 (sulphones), 1670 (2-quinolones), 1730 (carboxylic acids and anhydrides) and 1690 cm "1 (alkylarylketones), although the latter is largely predominant in the £inal products. Sulphones have been recently identi£ied in the weathered residue £rom the Amoco Cadiz oil spill (9). In both asphaltenes and resins the major uptake o£ oxygen occurred during the £irst weeks o£ the test (see table 1). Simultaneously, a slight increase o£ the C/H ratio was observed, pointing to an increase in the aromatic character o£ the samples. Accordingly, the structural indexes calculated £or these £ractions (see table 2) showed increases in the aromaticity £actor (£a) and the naphthenic carbon content (% CN). An interesting £eature observed as weathering proceeded was the dramatic decrease o£ the molecular weight o£ the asphaltenes. This £act can be explained by the degradation o£ the corresponding molecular backbones, or by a simple dilution e££ect produced by the lighter newly-£ormed molecules. Because asphaltenes consist o£ a cluster o£ polycondensed naphteno-aromatic rings linked by saturated carbon chains or loose nets o£ naphtenic rings (10) we may assume that biodegradation accounts reasonably £or that observation. This seems to contradict the £indings o£ Rubinstein et al (11) on £ield biodegraded oils according to which biodegradation does not a££ect the composition o£ petroleum asphaltenes. However, in the present case, it must be considered that biodegradation should be £acilitated by the easy photo-oxidation o£ the alkylchains in benzylic positions. The variations o£ the branchiness inde~(B.I.) and o£ the relative length o£ the alkyl chains (L~)(see table 2) aF£ord additional support to this assumption. A similar but less pronounced trend was also displayed by the resin £raction. In addition, the analysis o£ asphaltenes isolated £rom authentic sea-weathered samples (tar balls), showed £air agreement with those results previously reported £or laboratory samples (see table I), thus validating these conclusions. Biodegradation has been also suggested as an explanation £or the decrease o£ the saturated to aromatic hydrocarbons ratio observed during weathering (see Fig. I). In this respect, it is widely accepted that n-alkanes are metabolized more rapidly than either naphthenes or aromatics (12, 13). However, these hydrocarbon £amilies do not exhibit a uni£orm reactivity throughout their complete molecular range. Degradation o£ alkanes seems to be slower with increasing chain length, as shown in table 3. This causes a relative enrichment o£ the weathered residue in higher para££ins, which could explain the chromatographic pro£iles generally displayed by highly degraded tar balls (particles o£ about 1 mm in diameter) collected from the sea (£igure 2).

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544

DiFFerent reactivity is also exhibited by aromatic hydrocarbons depending on the type oF alkyl substituents and condensations (14). Accordingly, the aromatic Fraction undergoes several changes in composition, the most significant being the decrease oF the alkylbenzene series in the monoaromatic Fraction, and the initial increase and later depletion o£ the polyaromatic hydrocarbons. In turn, diaromatics seem to be less aFFected. The Final result is a slight decrease o£ the aromatics, especially aFFecting the easily oxydizable Fractions o£ higher aromaticity and molecular weight (see Figure I), probably by their transForma~on into resins.

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Figure 2.- Gas chromatograms oF two types o£ pelagic tar balls collected at the Western Mediterranean.

weathered residues accumulate components more resistant to However, degradation likely pursues on the surface oF these tarry residues Facilitating that Fine particles Flake oFF. This process, together with simple physical break-up, continually reduces the size oF pelagic tar balls that, owing to their increased density, are incorporated into the water column, as recent measurements have shown (15). In this manner,

Further degradation.

In conclusion, it seems plausible to accept, From the present knowledge, that biodegradation and sedimentation could significantly contribute to the ultimate Fate o£ petroleum residues at sea, although the relative rates o£ both processes remain to be determined.

545

Acknowled@ment The £inancial support was provided by the Osborne Company (Puerto de Santa Maria, Spain) as part o£ the Award on Environmental Protection granted to one o£ us (J.A.). Re£erences I. National Academy o£ Sciences. Wash.D.C., 107 pp. 1975. 2. D. E. Hirsch, R. L.Hopkins, Anal.Chem., 44, 915 (1972).

"Petroleum in the Marine Environment".

H.J. Coleman,

F. O. Cotton,

3. A. Hood, M.J. O'Neal, Adv. in Mass Spectrometry, (1959). 4. H. E. Lumpkin,

T. Aczel, Anal.Chem., 36, 181

5. G. A. Haley, Anal.Chem., 6. 3. W. Ramsay, Dev., 6, 231 (1967)

Env. Prot. A@ency,

E. V. Overton,

10. T. F. Yen, Pp. Div. Petrol. 11. C. Spyckerelle,

R. R. Colwell,

Ind. En@. Chem. Prod. Pes

o£ petroleum on Arctic and Subar ~ Ed. D.C. Malins, Academic Press,

Ecol. Res. Serv., Publ. 660/3-73-013

J. L. Laseter,

Chemosphere,

Chem.~ Amer. Chem.

O. S. Strausz,

12. N.J.L. Bailey, A. M. Jobson, 13. J. D. Walker, (1976)

(1964)

J. C. Petersen,

7. R. C. Clarck, W. D. Mc.Leod in "E££ects tic marine environments and organisms", Vol.I, (1977).

9. J. R. Patel,

p. 179. Pergamon Press,

44, 580 (1972)

F. R. Mc.Donald,

8. M. H. Feldman, (1 973) •

C. J. Thompson,

Geochim.

8, 539 (1979).

Soc., I_~7, FIO2 (1972)

Cosmochim Acta, 43, I (1979)

M. A. Rogers, Chem. Geol, 11, 203 (1973) L. Petrakis,

Con. J. Microbiol.,

14. D. T. Gibson in "Dahlem Workshop on the Nature o£ Seawater", Goldberg, 667 (Berlin). 15. B. F. Morris, J. N. Butter, T.D. Sleeter, J. Cadwallader, Const. Int. Explor. Mer., 171, 107 (1977)

(Received in The Netherlands 1 August 1980)

2_~2, 1209 Ed. E.D.

Rapp. P. Reun.