Petroleum: facts, composition, fate in the environment at GeoChemBio

Fate of petroleum in the environment

Introduction

Era of catastrophic oil spills began on March 16, 1978, when the grounding of the supertanker Amoco Cadiz off the coast of Brittany, France resulted in record release of 1,604,500 barrels (219,797 tons) of light crude oil and 4,000 tons of fuel oil into the sea.

A year and a half later, on 3 June 1979, the blowout of oil well Ixtoc I in the Gulf of Mexico produced the biggest discharge of petroleum estimated in 3.3 million barrels. For comparison, Deepwater Horizon well blowout that happened on April 20, 2010 dumped almost 5 million barrels of oil into the Gulf waters.

According to International Tankers Owners Pollution Federation statistics, in the last five decades, approximately 38 accidents involving supertankers have taken place, affecting the coasts of different countries. The major oil spills have occurred in western and Mediterranean Europe, North Africa and North America.

Stages of oil spill in marine environment

The physical and chemical changes that spilled oil undergoes are collectively known as weathering.

Spreading, evaporation, dispersion
The first stage takes from a few hours to several days after discharge. During this stage, the oil spreads over the surface with speed that depends on viscosity of the oil. Liquid oils initially spread as a coherent slick. At temperatures below their pour point, oils rapidly solidify and may form a thick layer on the surface of the water. Winds and waves tend to break the layer into narrow bands or windrows parallel to the wind direction.
In the process of spreading, the oil loses its more volatile components by evaporation. The quantity of oil which evaporates depends on the oil grade and ranges from 10% for heavy oils to up to 75% for light oils. In general, oil components with a boiling point below 200°C will evaporate within 24 hours in temperate conditions. The evaporation is enhanced in high energy environment (winds, waves, currents, high temperatures).
Spreading and water turbulence cause oil dispersion; the initial slick begins to break up. Simultaneously the oil is separated into two layers: the surface layer that contains the lightest components and the sub-surface layer. The surface film may slow down evaporation and modify mechanical properties of the waves. A number of chemical pollutants (organic chlorides, pesticides, detergents, etc.) are getting concentrated in the film. Heavy metals as well as bacteria and viruses are picked by polar absorption and are also retained in the layer.
Some of the oil does not float on the surface but gets submerged and forms underwater plumes that can stretch for miles. Concentration of oil hydrocarbons in plume that originated from Deepwater Horizon spill was found to exceed 50 micrograms per liter. The plumes are quite stable at 3,000 feet underwater surface. Some of heaviest fractions of the oil can sink to the sea bottom, where it can get buried into an anoxygenic zone persist for decades.
Emulsification
Wave action on the sea surface and near shore whip floating oil into emulsion. There are two forms of emulsification. Oil-in-water emulsion is formed on the surface and then dispersed by currents and waves. Water-in-oil (mousse) emulsion can contain up to 80% water depending on the type of oil. It is often semi-solid and has a strong red/brown, orange or yellow color. Mousses are highly stable and may remain emulsified indefinitely. Musse has volume many times bigger than the original oil.
Sedimentation
A significant portion of the oil that has not been evaporated, emulsified, or solubilized in the water may end up in the bottom sediment. Most oil pollutants are readily sedimented by natural sorptive agents. These natural sorbers may be classified as follows: organic debris, such as leaves, twigs, and microscopic plant and animal remains; inorganics, such as dusts, sands, clay minerals suspended within body of water. On exposed, high energy beaches, large amounts of sediment can be incorporated and the oil can form dense tar mats. Water turbulence causes mixing of the oil with sediments on beaches and in shallow water-logged areas of the shores. Once oil has been mixed with fine organic and inorganic particles, it will sink to the bottom if washed back out to sea by storms, tides or currents. In sheltered places, in marshes and estuaries, incorporation of oil in sediments greatly reduces its bioavailability and ensures its persistence in the environment for a long time.

Chemical and biological degradation

The major portion of the hydrocarbons remaining in water and sediments after manual cleaning operations (application of dispersants, skimming, in situ burning, etc.) is degraded by chemical and biological processes over periods stretching from a few weeks to several years. Chemical degradation, mainly photo-oxidation, is initiated by primary photolysis that gives rise to free, highly reactive radicals which rapidly recombine into a large number of chemical compounds: polymers, oxidation products (alcohols, aldehydes, ketons), etc.
Microorganisms (bacteria and fungi) play a dominant role in the biodegradation of oil.

Learn more about petroleum spill bioremediation at MetaMicrobe:

Introduction
Monitoring oil components
Monitoring oil degraders
Factors of oil biodegradation
Approaches to oil bioremediation
Oil bioremediation bacteria
Oil bioremediation fungi

Polymerization and formation of tar lumps
This stage stretches over a long period of time. Very heavy hydrocarbons (up to C 40) often containing other elements such as oxygen, nitrogen and chloride as well as some mineral compounds (especially iron oxide) form stable agglomerates which extremely recalcitrant to degradation and persist in environment for decades.