Weathering of Petroleum Spills
These processes are advection, spreading, evaporation, dissolution, dispersion of oil droplets into the water column, photochemical oxidation, water in oil emulsification, microbial degradation, adsorption onto suspended particulate material, ingestion by organisms, sinking and sedimentation.
The fate of an oil spill in the marine environment is determined by the apparently complex and interrelated weathering processes. The physical and chemical alterations to the spill occurring with time, as well as the rates of these changes, will be influenced by a variety of abiotic environmental parameters, as well as the physical and chemical properties inherent to the oil itself. The weathering processes are described below.
Spreading: Spreading is the first major process which affects the behaviour of crude oil and refined products during the first hours after release at sea. Spreading increases the overall surface area of the slick, this enhancing mass transfer through evaporation and dissolution process. The principal forces influencing the lateral spreading of an oil on a calm sea are gravitational which causes decreasing film thickness, surface tension, inertia forces and frictional forces. The gravitational spreading force is proportional to the film thickness, the thickness gradient, and the density difference between the water and the oil. Spreading is further promoted when the sum of the oil-water and oil-air interfacial tensions is less than that of the water-air interfacial tension. Thus, force is independent of film thickness, and it becomes the dominant process in the final phases of spreading.
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The spreading forces due to gravity and surface tension are retarded by the inertia of the oil body and the oil/water friction. The inertia of a specific oil slick is a function of the thickness and the density, it diminishes rapidly as spreading proceeds. Concurrent with the spreading process, friction between the slick and the surface water increases as the thickness of the oil slick itself decreases and the viscous water oil mixture increases. This frictional retardation force eventually overcomes the inertial forces and it remains the dominant anti spreading force.
Advection(Drift): Advection or drift is a mass transport process. It is characterized by the movement of the centre of mass of a slick. Advection is due to the influence of overlying winds and/or underlying currents. The centre of mass can move at approximately 3% of the wind speed with 20-30 degrees shift to the right, in the northern hemisphere due to coriolis effect in the absence of other perturbations affecting the overall slick travel such as residual currents or land masses. Wind direction and duration can have a dramatic effect on oil spill and trajectory. Drift is also strongly influenced by waves and tidal currents.
However, prediction of oil slick drift due to wind patterns alone is difficult because of wind and wave perturbations. This is compounded by the fact that most surface wave spectra are composed of a number of different wave systems with different periods and directions. Furthermore, most models use wave functions generated by local winds over a specific time period, and in real ocean situations the components of the wave spectrum are swelled reflecting wind generated seas at a time and locality remote from slick. Additional modeling complications result from wave test tank data which demonstrate that, in addition to waves affecting oil slicks, oil slicks also tend to have a concomitant impact upon the resultant wave spectra. It has been observed that oil on water significantly decreased the amplitude of wind waves with increasing wind speeds while wind speeds break the oil slick up into smaller lenses.
Obviously, better determination of wind field vectors is critical for monitoring of slick advection. Obviously, in near shore spills, the effects of spreading as well as advective drift will be important in trying to prevent contamination of shorelines.
Evaporation: Evaporation removes most of the volatile lower molecular weight compounds in an oil spill within a few hours or within a day. Evaporation causes considerable changes in chemical composition and physical properties of the oil. Calculations on evaporation rates is difficult because the rate depends on a number of factors, all of which change with time. Observations of evaporation rate and attempts to predict the rate have been reported.
The rate of evaporative loss from a given volume of oil depends on
1.the area exposed, which tends to increase continuously as the slick spreads.
2. The oil phase component vapour pressures that decrease with the depletion of the lighter components from the slick.
3. The oil-air mass transfer coefficient, which depends primarily on the wind speed but also on the hydrocarbon vapour diffusivity and
4. The possible presence of diffusive barriers such as water in oil emulsion or a skin on the oil surface.
The evaporation rate for specific hydrocarbons is a function of its vapour pressure which in turn is inversely related to the molecular weight. The sea state can also affect the evaporative loss. The sea state may be due to increased wind speed and increased surface area of the oil exposed to the atmosphere. This will enhance molecular exchange. Further, the increase may also be attributed to the formation of aerosols as a result of spray and breaking waves.
Changes in the physical properties of oil resulting in the loss of volatile hydrocarbons include increase in density and the kinematic viscosity. These alterations inhibit spreading and molecular diffusion of the remaining oil components and in this regard, the evaporation of petroleum hydrocarbons eventually becomes self limiting.
There are two general approaches to calculating evaporation rates. First is the pseudo component approach in which the oil is postulated to consist of a number of components or pseudo components ,of defined volatility characteristics similar to that of the oil. As evaporation proceeds, the change in oil composition is computed and the falling vapour pressure is calculated from Raoult’s Law at the desired temperature. The second approach is to postulate an analytical expression for the amount evaporated as a function of time and composition. In the latter case, a method was proposed by which oil distillation data could be used to predict vapour pressure and hence evaporation rates.
Evaporation rates and composition changes can be measured by simple pan evaporation experiments, either outdoors or in wind tunnels, with an attempt to extrapolate the results to oceanic conditions. There remains a need to improve the prediction of oil evaporation rates and to characterize oil volatility characteristics more accurately by means of information obtained from pan evaporation experiments, distillation temperature data and evaporation by a controlled air flow bubbled through the oil. These would give estimates of oil fractions evaporated under various defined conditions and enable calculation of the fractional retention of specific hydrocarbons at various times. Such capability would be invaluable as a means of calculating changes in density or viscosity, assessing changing toxicity, and improving identification of slick samples for legal purposes.
Dissolution : Dissolved hydrocarbon concentrations in water are particularly important because of their potentiality for exerting a toxic effect on biological systems. Concentrations of dissolved hydrocarbons rarely exceed one part per million and dissolution does not make a significant contribution to the removal of oil from the sea surface.
Dissolution takes place directly from the slick to the water column. The extent of dissolution is obviously influenced by the aqueous solubility of the oil. Rates of dissolution for the various components of a petroleum slick depend on the interactions between the inherent properties of the oil that is molecular structure of compounds and relative abundance of these components and the physio chemical properties of the environment that is salinity, temperature, vapour pressure, etc.
Solubility is a function of molecular structure and depends on the molecular volume and presence of active groups. Other factors which affect solubility of petroleum hydrocarbons are :
1. The dissolved organic matter (DOM) in the marine environment due to its surface active nature.
2. The presence of co-solutes (hydrocarbon mixtures).
3. Type of substituents present in the molecular structure. Rig formation also enhances solubility for a given carbon number or molar volume.
Dispersion: The rate of natural dispersion is largely dependent upon the nature of the oil and the sea state and is enhanced significantly in the presence of breaking waves. The quantity of oil spilled and the degree of spread are related to slick thickness. The slick thickness is an important factor in the rate of dispersion since smaller droplets are produced form thin films. Furthermore, oil that is fluid and can spread by other weathering processes may disperse completely in moderate sea conditions within a few days. Conversely, viscous oils or those that form stable water in oil emulsions tend to form thick lenses and will show little tendency to disperse. Such oils can persist for several weeks.
Dispersion or oil in water emulsion results from the incorporation of small particles or globules of oil into the water column. Oil begins to be dispersed immediately after it is discharged and the dispersion process is most significant during the first ten hours. Once formed, the particles can then continue to break down and/or disperse throughout the lifetime of the spill. By one hundred hours, dispersion has usually overtaken spreading as the primary mechanism for distributing the spilled oil about its centre of mass.
Dispersion results in exposure of sub-surface marine organisms to particulates and dissolved oil. These organisms, in turn, may mediate the sedimentation of some of the oil through incorporation into fecal pellets.
The nature of the fluid mechanics of the event resulting in natural vertical dispersion is not well understood and is undoubtedly complex. Breaking or surface turbulence waves probably rise and coalesce with the slick, while the smaller droplets are conveyed with water eddies downward to become permanently incorporated into the water column.
Several expressions for natural dispersion rates (vertical transport of small particles of oil) have been assembled. The simplest approach for including dispersion in an oil spill model is that used in the SLICKTRAC model who tabulated estimated vertical dispersion rates expressed as percentage of the oil per day as a function of sea state and duration of the spill.
Mathematical treatment proposed equations for transport rates as a function of the oil slick thickness, the oil-water interfacial tension, the sea state and in particular, the fraction of the sea covered by breaking waves. Although there are some data on this fraction, it is only for seas in the absence of oil. Oil reduces the incidence of breaking waves. When the surface layer of water is well mixed, vertical eddy diffusion presumably cause further transport downward and langmuir circulation cells may even be more important.
Emulsification : Most oils tend to absorb water to form water in oil emulsions which can increase the volume of pollutant three or four fold. Such emulsions are often extremely viscous thereby inhibiting other processes which would otherwise dissipate the oil. This accounts for the persistence of light and medium oils on the sea surface. In moderate to rough sea conditions, most oils rapidly form emulsions. Emulsions may separate out into oil and water again if heated by sunlight under calm conditions or when stranded on shorelines.
The tendency of certain oils to form stable water in oil emulsified mixture or mousse is extremely oil dependent and appears to rely heavily on the presence of wax and asphaltic materials in the whole crude. Refined petroleum products which do not contain asphalts and waxes do not form stable water in oil emulsions. In addition to the presence of wax and asphalt, the formation of oxygenated surface active materials, due to photochemical and microbial degradation processes, also appear to be important in stabilizing water in oil emulsion. Oils with asphaltene contests below 0.5% tend to form stable emulsions ( chocolate mousse) whilst those containing less tend to disperse. Other indigenous surface active agents such as metalloporphyrins, nitrogen, sulphur and oxygen compounds may also be important. Surface active compounds surround the water droplets preventing water-water droplet coalescence and ultimate oil-water phase separation.
Significant changes in the theological properties of the slick or oil mass can be observed during the formation of water in oil emulsion. The formation of stable water in oil emulsion is important as it affects cleanup, combustibility, and control effectiveness and severely limits additional weathering and removal of components from the emulsified mass. Emulsification significance reduce evaporative and dissolution weathering and penetration of nutrients and dissolved oxygen into the interior of the emulsified mass can inhibit degradation processes.
In the area of modeling the formation of water in oil emulsion, significant progress has been made in predictions and input turbulent energy. However, much understanding is still empirical at best, and most data used for model verification are extremely oil specific. Additional effort is required towards identifying the surface active materials responsible for stabilisation of the water in oil emulsion.
Absorption of water usually results in black oil changing colour to brown, orange or yellow. As the emulsion develops, the movement of the oil in the waves causes droplets of water taken up in the oil to become smaller and smaller making the emulsion to become progressively more viscous. As amounts of water absorbed increases, the density of the emulsion approaches that of sea water.
Sedimentation: In actual oil spills in the marine environment there are several mechanism by which petroleum can reach the sediments. These are:
1. Sorption of oil by suspended particles.
2. Ingestion of oil by zooplankton and incorporation into fecal pellets
3. Weathering of oil by physical/chemical processes.
4. Direct mixing of oil and sediments.
Particulate-oil interactions are however a dominant process in the ultimate deposition of petroleum. The most probable mechanism for the rapid sedimentation of oil is through :
1. Adsorption of detritus or clay particles
2. Sorption of oil droplets on settling particles. The nature of this sedimentation is, of course, a function of the oil composition as well as the type of particulates. In addition to sorption reaction in the water column, hydrocarbons can associate with particles before entering estuarine and coastal waters. The nature of the particulate material is also critical in controlling the extent of oil particulate adsorption and sinking.
Zooplankton ingest small particles of oil that are dispersed throughout the water column. Subsequently, elimination of petroleum in the feces result in hydrocarbon incorporation in bottom sediments as fecal pellets in addition to the particulate oil which are transported by hydrodynamic processes. Generally, extensive modification of oil occurs before it is sedimented. Weathering and fractionated of oil before incorporation onto suspended particulate materials appear to be common. Weathering processes first form a cracked, scaly surface on oil pancakes, which then flake in turbulent seas. This process exposes fresh oil within the pancakes and this exposure produces a surface sheen until a new skin forms. After weathering, the skin flakes off on agitation and the process is repeated. The flaking of these particles seems to be a significant intermediate in the dispersion of oil spill, and the increased density of the chemically weathered flakes relative to the pancakes suggests that this process may ultimately result in the sedimentation of the oil.
Direct mixing of oil and sediments also introduce petroleum into bottom sediments. Oiling of various shoreline habitats and redistribution of oiled intertidal substrates into the near shore environment during storm events also results. Oil is incorporated into the sediments under a water depth of at least 10m due to intense mixing of oil and water by gale force winds. Some of the oil globules carried to the bottom by the turbulence are mixed with sandy and gravelly sediments and remained near the bottom.
In regard to oil stranded on coastlines, the behaviour of spilled oil in different environment is primarily dependent on the porosity of sediments and the energy of waves acting on the coastline. Rocky shore tends to “self clean” within a matter of months, whereas soft sediment lagoons or mangrove swamps act as long term petroleum sinks. On cobble and Sandy beaches, oil can sink deeply into the sediments and remain longer than on bare rocks, protected by a skin of weathered oil and may remain essentially unchanged for a long time. Tidal pumping is the active factor causing penetration into the sediments. Sediment grain size and composition control the rate of penetration. In muddy sediments, penetration is minimal, and only the upper few centimeters are affected.
Photo-Oxidation: In the presence of oxygen, natural sunlight has sufficient energy to transform many petroleum hydrocarbons into compounds possessing significant chemical and biological activities. The mechanism of this photo involved process can be described as free radical chain reaction in the presence of oxygen which results in the formation of hydroxyl compounds, aldehydes, ketones and ultimately, low molecular weight carboxylic acids. Higher molecular weight intermediates can be formed concomitantly through recombination (polymerization) or condensation reactions of aldehydes and ketones by phenols, or by esterification between alcohols and carboxylic acids. Sunlight may also cause copolymerizations by way of thermally induced oxidations, such as the copolymerization with oxygen by methylstyrenes and indene.
A kinetic study on photochemical weathering attempted to fix spectrophotomeric (UV) data with various kinetic models and suggested the most probable as being a second order autocatalytic process. The proposed mechanism involved reaction of oxygen with reactant to produce an intermediate radical which in turn results in the formation of an oxidation product capable of forming additional free radicals by itself. These radicals further react with the reactant and product molecules to produce more oxygenated products and so on. The rates of photo oxidation are considered to depend on the wavelength; they are also affected, to some extent by turbidity levels and SPM concentrations (particularly for higher molecular weight aromatics). Photosensitizing compounds such as xanthone, 1-nath-thol, naphthalene derivatives have been shown to increase photo oxidization rates for petroleum hydrocarbons. Compounds suitable as sensitizers must have strong absorption properties in the visible (or near UV) region which results in a formation of a singlet or triplet free-radical chain reactions capable of proceeding at low temperatures. Obviously, this compound must also be lipophilic and stable to oxidative processes within the oil-water system.
The presence of inhibitors, such as sulphur sulphur compounds such as thiocyclanes can restrict the formation of radical species or inhibit singlet oxygen mediated peroxide formation. Humic substances can reduce the photolysis rate of UV-sensitive compounds, but they also photosensitize transformations of other organic compounds through an intermediate transfer of energy to molecular oxygen.
The composition of various petroleums and petroleum products will influence both rates and extents of photo induced oxidative processes both enhancing dissolution of products and by increasing the general toxicity of water soluble fractions. Dissolution of petroleum components is facilitated by irradiation accelerated formation of polar organics moieties in the film, as well as by surface active products generated from photo oxidation. Oil-water emulsions are enhanced by surfactants resulting from chain breaking reactions in sulphoxide formation and it has been suggested that this process contributed more towards degradation of slicks than the formation of water soluble oxidation products.
Other products of photo-oxidation processes include water soluble metal salts, such as vanadium salts formed by the reaction of vanadyl porphyrins with peroxide. Crudes of medium sulphur content have been shown to be photo degraded with respect to sulphur compounds, even though sulphur is inhibiting to propagation of the free radical chain process. Toxic compounds may be both formed and degraded by photo induced processes. Photo toxicity effects on the primary productivity of a micro algae and marine phytoplankton communities is a function of illumination and chemical dispersants. Photo oxidation can also influence the spreading properties of crude petroleums and petroleum products and these effects must be taken into consideration with regard to the use of photo sensitizing agents.
Auto-oxidation: Auto-oxidation is a free chain process in the absence of ionic catalysts, with rate of propagation controlled by the rate of hydrogen atom removal by an album proxy radical. The mechanisms involved are multi step, free radical reactions that are very complex and detailed in behaviour. They are subject to induction periods and rates are sensitive to both inhibitory and acceleratory efforts of trace elements and to the physio-chemical environment.
Constituents of oil that are active in these reactions include branched hydrocarbons, benzylic C-H bonds, aromatic rings, etc. Potential inhibitors in petroleum include amine, metastable radicals and sulphur compounds while the accelerators are the transition metal complexes and selected organics. The issue is therefore entirely one of the rate and the basic theory cannot be of much help in assessing environmental rates. However, it can predict the probable products and help to rationalize those actually found.
There are a few relevant observation on dark auto oxidation of petroleum in the environment. The chemical mechanisms involved in oil auto-oxidation and photo-oxidation are probably similar. The formation of all the oxygenated compounds can be rationalized by known types of photo-reactions and auto-oxidazation.
Microbial Biodegradation: The fate of petroleum spill in a marine/ estuarine environment, as related to biodegradative process, encompass degradation, through microbial metabolism, ingestion by zooplankton, uptake and possible retention by marine invertebrates and vertebrates, as well as bioturbative effects. All serve to partition the petroleum hydrocarbons into the water column, biomass and sediment regime of the ecosystem.
The pathways used for biodegradation of petroleum tend to fall into two distinct approaches: that used by bacteria and that of eukaryotic invertebrate and vertebrate systems. Microorganisms such as bacteria, yeasts and fungi are important in the degradation of petroleum in surface films, slicks in the water column and sediments. Zooplankton aid in the sedimentation of oil droplets and oil associated with particulate matter through their ingestion of micro particulate oil from the water column, followed by excretion of oil in the feces. Benthic invertebrates such as polychaetes, are important in the oxidation and recycling of sediment organic matter and play significant role in the degradation of sediment bound oil. Various types of micro organisms capable of oxidizing petroleum hydrocarbons and related compounds directly or by co-oxidation are widespread in nature. Over 200 species of bacteria, yeasts and filamentous fungi have been shown to metabolize one or more hydrocarbon compounds ranging in complexity from methane to compounds of over 40 carbon atoms.
From overall oceanic temperature distribution, the marine bacteria are predominantly psychrophilic that is obligate and facultative and show wide distribution even in the extremely cold Arctic and Antarctic environments. Psychrophilic pseudomonads are both ubiquitous and generally dominant species in marine environment. Other representative groups in the water column and sediments are Arthrobacter, cornybacterium, Vibrio, Enterobacter, Anchromobacter, Brevibacterium, Aeromomas and Acinetobacter sp.
Yeasts and fungi also have wide distributions in marine environments, including polar regions. Cladosporium resinae represents one of the most ubiquitous and prominent hydrocarbon-utilizing filamentous fungi.
The petroleum degrading potential for any mixed microbial populations seems to be a function of both total numbers and types of the environment as well as seasonal population fluxes. Inputs of petroleum into the marine environment result in selection towards hydrocabonaclastic organisms, causing a shift within the general population towards a relative increase in hydrocarbon utilizing microbes. Areas suffering chronic oil pollution tend to have higher heterotopic population relative to less or non-pollutant areas. Within chronically contaminated area, the hydrocarbonoclastic microbial population in the water column may show greater capacity to utilize petroleum components, and be present in greater numbers than the sediment population. This may be in part due to greater and/or more consistent exposure to petroleum pollutants, as compared to the fluxes of these pollutants into the sediment environment through the water sediment interface.
When petroleum hydrocarbon becomes available to a microbial community, biodegradation of most petroleum compounds occurs simultaneously, but at widely differing rates. Generally, the biodegradation of the n-alkanes is most rapid, followed closely by simple aromatic components. The isoalkanes, cycloakanes and condensed aromatics are degraded more slowly. Various hydrocarbon components may also influence each other’s degradation indirectly through the phenomena of co-metabolism or “diauxie”. In the first process, a normally refractory hydrocarbon may be degraded in the presence of a second readily degradable hydrocarbon. In the case of diauxie, the presence of a more easily utilised hydrocarbon represses enzyme induction necessary for metabolism of the second hydrocarbon. The latter is degraded only after the first is exhausted.
Factors Affecting Petroleum Biodegradable Rates.
Inherent chemistry of polluting oil and environmental variables affect biodegradation rates. These various factors are:
1. Composition and Weathering of Petroleum : Low percentage of biodegradation can result from high proportions of condensed polyaromatic, condensed cycloparaffinic and asphaltic petroleum components because these compounds are biodegraded at extremely slow rates if at all. Toxicity of certain petroleum components can also delay or prevent the biodegradation of susceptible ones. Toxic and lipophilic substances such as pesticides, polychlorinated biphenyls and mercury can be concentrated in the oil slick above ambient concentration in the water and may inhibit biodegradation. Photo-oxidation may remove methyl branches that block biodegradation but in high concentrations photo oxidation products may become toxic to microorganisms. Formation of mousse reduces the surface area and the availability of mineral nutrients and oxygen, thus hindering biodegration.
2. Temperature : The environmental temperature can affect degradation rates by acting upon the microbial populations in several ways. The nature of the marine environment restricts petroleum degradation to the mesophilic and pschrophilic organisms. Generally, the rate and extent of hydrocarbon biodegradation are severely restricted S6 low water temperatures. Low temperatures generally suppress degradation rates by suppressing growth rates and metabolic rates of the microbes involved, and /or by actually inhibiting growth due toxic volatile compounds that evaporate more slowly at low temperature, or to the increased solubilities of potentially toxic petroleum compounds at higher temperatures may occur.
3. Oxygen : Both free and dissolved oxygen levels in the immediate environment are important to microbial degradation, as dictated by both microbial growth and the oxygen requirements for complete oxidation of petroleum hydrocarbons.
4. Nutrients : Petroleum degrading microorganisms need to obtain mineral nutrients from seawater. Nitrogen and phosphorus have been shown to be limiting factors to both rates and extents of petroleum compound degradation, as well as have stimulating effect by addition of nutrient supplements for example ammonium sulphate and potassium hydrogen phosphate, to the immediate experimental environment.
The effect of iron on petroleum degradation in sea water has been studied and results suggested that it may become limiting when precipitated out of the environment as iron hydroxide, under alkaline conditions. However, both the natural abundance of Iron in the lithosphere and marine pH ranges would probably prevent this limitation from occurring.
Rates of Petroleum Biodegradation
Petroleum degradation rates have been estimated in four types of experimental systems in ways that might be applied to spill conditions in temperate waters viz: fermentation studies, seawater enrichments under optimized conditions in situ enrichments and in situ petroleum degradation potentials. High density monoculture in fermenters provide estimate of optimal rates, while using nutrient stimulation biodegradation in situ rates may be approached. Under prolonged exposure of a marine environment to petroleum pollutants, conditions equal to in situ marine enrichment may be approached whereas in situ petroleum degradation potentials measure the potential of a marine environment for petroleum biodegradation before microbial population has an opportunity to shift in response to the petroleum exposure. This potential is greatly influenced not only by the prevailing environmental conditions but also by previous history of exposure of the site to petroleum pollutants.