Characterisation and Environmental Effects of Unresolved Complex Mixtures of Hydrocarbons

Kevin V. Thomas

PhD – March 1995

Department of Environmental Sciences, University of Plymouth

The gas chromatograms of crude oil hydrocarbons reveal both resolved and unresolved components. The unresolved feature is commonly referred to as the unresolved complex mixture (UCM). UCMs are thought to result from the co-elution of complex mixtures of hydrocarbons with similar chemical properties and become more obvious as resolved components are removed by processes such as weathering and refining. Consequently UCMs are a prominent feature in oil-polluted sediments, biodegraded crudes and refinery products. The characterisation of both aliphatic and aromatic unresolved complex mixtures (UCMs) of hydrocarbons, as well as their possible effects on the environment, is described.

An aliphatic hydrocarbon UCM isolated from the base oil of Silkolene 150 lubricating oil was characterised by a combination of micro vacuum-distillation and oxidative degradation. Vacuum-distillation produced six distillate cuts and a residue which were all highly unresolved by GC (ca. 95%). The average molecular weight of each cut was determined by probe CIMS (310 - 440 Daltons), and varied by ~20 Daltons. CrO3 oxidation of each fraction yielded similar distributions of n-monocarboxylic acids, ketones and lactones as well as CO2 (ca. 6%). The resolved products of oxidation suggest that the aliphatic UCM is a rather homogeneous mixture of highly branched alkanes. However a significant amount of the products remain unresolved (UCMox.; ca. 70-95%).

A retro-structural analysis approach, using an aromatic UCM oxidant (RuO4), combined with a mass balance approach, was used to characterise aromatic UCMs. Following reproducibility studies and the analysis of authentic aromatic compounds, the method was applied to the characterisation of unresolved aromatic refinery oil fractions and a suite of aromatic UCM distillate fractions. Selected refinery oils were separated into mono-, di-, tri- and tetraaromatics by HPLC and shown to be mainly unresolved by GC (ca. 80%). RuO4 oxidation of these fractions yielded DCM soluble products (24 - 74%), water soluble products (0 - 10%) and CO2 (12 -78%). The principal resolved products in each oxidation were monocarboxylic acids and dicarboxylic acids which were used to reconstruct precursor compounds. Vacuum-distillation of Tia Juana Pesado crude (Venezuela) gave six cuts and a residue which were analysed by GC, 1H NMR, UV and probe CIMS to obtain molecular weight (171 - 301 Daltons) and broad structural information whilst RuO4 oxidation was used to obtain molecular information via the retro-structural analysis approach. This showed that the aromatic UCM was in fact highly aliphatic and contained alkyl and cycloalkyl tetralins.

A significant advancement in the quantitative characterisation of UCMox. and subsequently the characterisation of aromatic UCMs was made. Ion cyclotron resonance spectrometry (ICR) was used to characterise the RuO4 oxidation products of selected refinery fractions. Analysis of the oxidation products of a monoaromatic refinery fraction indicated the presence of monocarboxylic acids (C1 - C21; 58%) and alicyclic carboxylic acids (C7 - C19; 16%), a hydrogenated monoaromatic sample contained monocarboxylic acids (C1 - C20; 30%), dicarboxylic acids (C2 - C11, 7%) and alicyclic carboxylic acids (C7 - C18; 11%) , whilst a diaromatic fraction contained monocarboxylic acids (C10 - C19; 7%), alkyl phthalic acids (C8 - C17; 17%) and cycloalkyl phthalic acids (C11 - C15; 3%). Retro-structural analysis suggests that the non-hydrotreated monoaromatic UCM is mainly comprised of alicyclic and alkyl substituted benzenes, the monoaromatic UCM isolated from the hydrotreated oil of alkyl and cycloalkyl substituted tetralins and the diaromatic fraction of alkyl and cycloalkyl naphthalenes. This was supported by FIMS analysis of the fractions prior to oxidation.

As an investigation of the environmental toxicity of UCMs, the effect of a saturated aliphatic UCM, and its chemical oxidation products, on the feeding rate of mussels (Mytilus edulis), was investigated. The UCM had little effect, whilst oxidation resulted in an increase in toxicity. The non-toxic nature of the hydrocarbons was attributed to their low aqueous solubility, whilst oxidation resulted in the formation of products with a greater solubility, which were sufficiently hydrophobic to be narcotic toxicants.

Parts of this work have been published [Thomas et al., (1993) Organic Geochemistry, Falch Hurtigtrykk, Norway (Abstract), 717-719; Thomas et al., (1995) Water Research 29, 379-382 [doi: 10.1016/0043-1354(94)E0111-I]].

Copyright 1995 K.V. Thomas. All rights reserved.

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