Petroleum Hydrocarbons:
A Survey of Structures, Weathering, and Remediation
Abstract:
Man’s history with the petroleum industry has produced many geographically distinct, and highly damaging hydrocarbon releases. Among these occurrences, the most influential to the environment include source “point discharges contaminated by urban runoff, refineries, and other coastal effluents”(1). These tend to cause chronic pollution problems primarily within and around the coastal areas where such point sources are identified. More quantifiably extreme spills and discharges have occurred from tankers, while crude or refined petroleum has been in oceanic transit. These latter instances occur with much less frequency, however the shear magnitude of released petroleum invokes immediate threats to the surrounding ecosystems (2). Discharges and spills within the past decade alone have included:
1989: Exxon Valdez - 0.04 megatons into Prince William Sound, AK
1990: Apex Barge - 3,000 cubic meters in Galveston Bay, TX
1990: Mega Borg - 45 cubic meters off Texas coast
1991: Haven - 0.14 megatons off Italian coast
1991: Gulf War - 0.82 megatons deliberately released in Kuwait by Iraq
1993: Braer - 0.08 megatons along the coast of the Shetland Islands, UK
(cited in Swannell et. al.(2))
As a result of such petroleum releases into the environment, emergency measures are required to try to alleviate the impending damage to local ecosystems. Government agencies and scientists have long studied the natural effects of weathering upon petroleum. Results of these studies have provided insight into possible anthropogenic enhancements to natural weathering, while subsequently creating viable support methods to physical cleanup measures. It is the goal of this paper to provide a clear understanding of petroleum as a damaging pollutant to the world’s marine ecosystems. Focus will be placed upon the roles which both natural and anthropogenic influences play in the remediation of petroleum spills, as well as studies aimed at determining the affects of these influences.
Petroleum Structure:
In order to possess a good understanding of petroleum as an environmental pollutant, it is beneficial to first understand the complex chemical identity and structure of this organic material. In fact, petroleum obtained from different geographic locations is likely to possess chemical compositions which are highly specific to those areas (3). Primary to all raw petroleum (known as crude oil) is the hydrocarbon structure, or C-H bonding pair. This backbone component is isolated from centuries of pressure and temperature acting upon sedimentary layers of dead organic matter (1). Multiple C-H bonds will combine to form an array of hydrocarbon compounds, of which three main classes have been identified:
1. Aliphatic hydrocarbons
2. Alicyclic hydrocarbons
3. Aromatic hydrocarbons
Aliphatic hydrocarbons are open-chain compounds in which the linked carbon atoms may contain single (saturated) bonds, or any combination of double and triple (unsaturated) bonds (4). Characteristically, carbon possesses four valence, bonding electrons. Hydrogen is generally an opportunistic bonding atom, and will bind to those valence electrons which have not been utilized by the carbon-carbon chain. Based upon their complexity, aliphatic hydrocarbons may contain as few a two carbons (ethane), or stretch continuously, possessing greater than 78 carbons in a single chain (1).
Examples of aliphatic hydrocarbons:
Alicyclic hydrocarbons will contain ring structures, often comprised of five to six, saturated or unsaturated, carbons. These compounds can appear highly complex in their arrangements of carbon, especially when multi-ringed structures are present (4).
Examples of alicyclic hydrocarbons:
Characteristic of aromatic hydrocarbons is the presence of at least one 6-carbon benzene ring, the structure which separates these compounds from alicyclic hydrocarbons. This benzene may be joined with aliphatic chains, alicyclic structures, or as a combination of benzene rings either linked aliphatically or fused into compounds known as polycyclic aromatic hydrocarbons (4).
Examples of aromatic hydrocarbons:
Aliphatic and alicyclic hydrocarbons are, by themselves, most prevalent in crude oil, and are often quantitated by gas chromatography techniques to evaluate the total concentration of spilled petroleum (5). Aromatic hydrocarbons, although posing the greatest threat to our environment, tend to exist in much lower concentrations. In addition, the more volatile aromatics, such as benzene and toluene, will generally degrade rapidly following a petroleum release (5). This is due to characteristically high vapor pressures and polarity which leaves them susceptible to evaporation and dissolution into the air and surrounding water. Degradation of crude oil will act less favorably upon the decomposition of more complex aromatics, particularly the polycyclic aromatic hydrocarbons (PAH). As a result, PAHs have been utilized extensively in monitoring degradation and toxicity levels, as well as in the source-fingerprinting of oil spills (5). Determinations of the latter can prove valuable in terms of litigation aimed at punishing those responsible for an oil spill or discharge (6).
Certain other elemental compounds can be found within petroleum, most of which are considered to be impurities by the petroleum industry. The presence of sulfur has been demonstrated to be corrosive, malodorous, and poisoning to the daily operations and equipment found in petroleum refineries (7). Nitrogen is also present, often in the form of pyridine and its derivatives. When refined, both of these components tend to become concentrated at high boiling fractions (7). This poses a hazard regarding safe, effective processing and removal of these impurities from petroleum end products. Oxygen may also exist in the form of functional groups attached to hydrocarbons (7). The characteristically dark color of petroleum is produced as a result of these elemental compounds, as well as the presence of metals such as vanadium, nickel, cobalt, and iron. These metals exist in an apparent colloidal suspension within the complex of hydrocarbons, and is likely to be resulting in atmospheric particulate pollution during petroleum refinement (7).
Petroleum Weathering and Anthropogenic Influences:
Hydrocarbons can be found occurring naturally throughout the marine environment, including forms not derived from petroleum. Marine plankton, for instance, produce and release a yearly average four to eight times the amount of hydrocarbons originating from petroleum (1). Yet concern over such naturally occurring hydrocarbons remains low due to their gradual release throughout the year, and their high rates of weathering. It is the accidental and deliberate release of petroleum by man which poses the greatest threat to the environment. When these releases occur, concentrations of hydrocarbons tend to be far too overwhelming for the natural responses of weathering and degradation to be affective. There arises an evident shift in environmental equilibrium towards toxic levels of pollution.
Six primary hydrocarbon weathering patterns occur within the natural environment, as illustrated in Freedman (1):
Among these, four become patterns of distribution. Within the first six to ten hours following an oil spill or discharge, dispersion of the resulting slick will act to gravitate the oil outward to a thickness at or below 0.1mm on the surface of the water (7). Frictional forces have been known to counteract this dispersion. Due to the tremendous amounts of oil released from a spill, this often becomes advantageous in terms of containment for physical clean-up purposes. Aiding in this friction, long, floating oil barriers, known as booms, are often utilized by emergency clean-up crews to prevent a spill from spreading towards shorelines and incurring immediate damage upon local wildlife. Methods for oil containment have also involved the use of high pressure water jets, capable of producing enough force to deflect oncoming oil slicks (8). Simultaneous to these containment efforts, crews will begin physical collection procedures. Utilizing large surface skimmers, they work quickly to gather and contain as much of the floating oil as possible (9). Unfortunately, recovery of large quantities of petroleum by this method is only feasible within the window of containment governed by the oil’s natural distribution (9).
Evaporation will remove the more volatile components of an oil slick with the aid of temperature, wind speed, and water turbulence (1, 7). These hydrocarbons, which can constitute 20 to 50% of most crude oils, are generally lower in molecular weight, and of higher vapor pressure (1, 7). Hydrocarbons which are vaporized, may then undergo degradation in the atmosphere. However, some will enter the atmosphere as aerosols, only to be redeposited in the future (7). Those hydrocarbons which are not volatilized can sometimes be dissolved within the surrounding water or deposited into the local sediment. Dissolution will tend to favor hydrocarbons which possess higher degrees of surface area (such as aromatics), lighter molecular weight, and polar characteristics (1, 7). As with dispersion, the natural processes of evaporation and dissolution can prove detrimental to the environment. Although less toxic hydrocarbons may be evaporated or dissolved, only time will allow the overwhelming concentrations of these hydrocarbons to become distributed below environmentally damaging levels.
The final distribution process of hydrocarbon weathering is emulsion, or the colloidal dispersions of oil into water and water into oil (7). The first formation (oil-in-water) will often reduce the oil to fine droplets, possessing relatively large surfaces areas (7). This can become beneficial to future chemical and biological degradation. However, the residence time for this type of emulsion is often low due to the natural hydrophobic properties of oil. It is the sulfur, nitrogen, and oxygen containing compounds naturally found in petroleum which will often aid in maintaining these emulsions. Such compounds tend to be polar, and hydrophilic by nature. As a result of this positive action, anthropogenic clean-up methods have adopted similar chemical dispersants, or surfactants (7). Treating spilled oil with detergent or fertilizer-based compounds, emergency crews attempt to promote oil-in-water emulsion, and subsequent degradation processes (7).
Unfortunately, the anthropogenic treatment of oil with surfactants is currently being regarded as having dangerous repercussions. Fatalities in marine bird populations, as a result of petroleum exposure, has also been linked to these surfactant. An examination was performed, which considered the effects of hydrocarbon ingestion on marine bird immune systems (3). First order, or direct ingestion of petroleum was specifically linked to the oiling of feathers which tended to cause a degradation in the feather’s natural capacities: water repellency, insulation, and plumage (3). It is believed that emulsion-promoting surfactants added to a spill will also bind to the waxy, hydrophobic feathers of marine birds, producing a greater affinity for the feathers to accept water. As an instinctive function of these losses, birds will begin preening their feathers, and subsequently ingest high levels of hydrocarbons. Aside from the large numbers of fatalities, hydrocarbon contamination of marine birds has resulted in many different physiological problems including hypothermia, dehydration, infection, arthritis, eye irritation, and gastrointestinal disorders (3).
Water-in-oil emulsions produce the reverse effects of oil-in-water emulsions. They tend to exist with comparatively lower surface area and a high residency rate. Degradation of such emulsions is reduced, and the risks of this mousse-like substance affecting shoreline habitats is greatly increased (1, 7). In coastal regions, these emulsions will eventually touch land, coating the shoreline in thick oily residue, and mixing with the local sediments (1). Cleanup efforts for this shoreline damage will usually begin with the physical collection and removal of petroleum. According to Prince, contaminated sand is often dug into a network of trenches (9). The surrounding oil is then scraped into these trenches, vacuumed into large collection trucks, and hauled away from the spill site. Rocky coastlines utilize a separate technique in which the oil is “wash[ed] back into the sea and collect[ed] with skimmers”(9). Throughout these mechanical processes, workers will also walk the exposed shorelines, manually raking and removing oil-laden sediment and rocks (10). Based upon the extent of remaining oil residue, decisions will then be made whether to allow only natural weathering processes to continue the cleanup, or to incorporate anthropogenic methods of hydrocarbon degradation, such as bioremediation.
Photochemical-oxidation and biodegradation are two natural processes which chemically transform hydrocarbons. Photochemical-oxidation, as its name implies, is the chemical restructuring of hydrocarbons in the presence of light and oxygen (7). Commonly, oxygen functional groups will form, as oxygen from the surrounding air and water continually bombard hydrocarbons. As this continues, polarity of these new compounds will increase, making them more water soluble and, subsequently, more susceptible to biodegradation (7). Unfortunately, photo-oxidation of the water-in-oil emulsions will often result in floating tar balls which, when mixed with sediment, will take on asphalt-like properties (3). This can develop into oceanic pollution promoted by circulating current systems. A study performed in 1973 in the Sargasso Sea estimated the accumulation of nearly 65 thousand tons of this tar (1).
Biodegradation is currently the most widely studied process by which hydrocarbons are chemically transformed into more useful compounds. The primary agents for these transformations are microorganisms such as bacteria and fungi (11). Bacteria will usually become the most prominent hydrocarbon-degrading organisms during an oil spill. This is due to the largely numbered and diverse species capable of residing and proliferating throughout the earth’s water systems. Indeed, it is this water which is required to transport bacteria to a spill site as well as maintaining a medium in which they may survive. Hydrocarbons, although a prime carbon and energy source, cannot provide a viable substrate for bacterial growth (7). When an oil spill reaches local shorelines, it is the sediment indigenous to the area which tends to become a breeding ground for bacteria. Sufficient levels of bacterial nutrients, such as nitrogen and phosphorous, must also be available in and around an oil spill site to accommodate continual bacterial growth and enrichment (7, 11).
As the natural weathering processes of dispersion, dissolution, and oil-in-water emulsion begin to distribute hydrocarbons within the water column, indigenous strains of bacteria will simultaneous move to oxidize and utilize these organic chemicals. The choice and speed of these transformations tend to be based upon the complexity of the hydrocarbon. As mentioned previously, polycyclic aromatic compounds will degrade much slower than lesser aromatics or straight-chain aliphatics, making them a good source for monitoring overall degradation levels (5). During microbial transformation, oxygen from the environment and the respiratory agent NADH, which is found in bacteria, will collectively contribute an [OH] group to the hydrocarbon structure, thereby displacing a hydrogen (7). Further addition of NADH and enzymes, along with oxygen or water depending upon the hydrocarbon, will continue to transform the molecule until a viable or less harmful product is created (7). Carbon dioxide is most often produced, as well as Acetyl CoA and Succinyl CoA both of which contribute to the Krebs cycle of respiration (7).