Oil spills into water include release of crude oil during extraction and transport, as well as spills of refined petroleum products by end users. The resulting environmental pollution can have severe impacts on ecosystems, contaminate drinking water, have a dangerous impact on air quality, and create major economic impact. Clean-up costs for major spills are massive. For example, in relation to the Deepwater Horizon spill, BP reported clean-up costs of $42 billion, private compensation costs of $14 billion with a further $20 billion set aside as a victims’ compensation fund, and $0.5 billion for environmental health initiatives and research.
Clean-up methods include physical containment and removal, controlled burning, chemical dispersal, chemical solidification, and bioremediation. In many cases, a combination of these is required. Inflatable booms are often used at an early stage, to contain the spill, close off narrow entrances and shield sensitive habitats. Initial physical removal can be carried out using skimming vessels or vacuums combined centrifuges to separate oil from water. Where spills are too extensive for direct physical removal, dispersants, solidifiers or biological agents may be used. This article will focus on these chemical and biological approaches.
Dispersants cause oil to break up and become water-soluble, spreading through the volume of water rather than remaining on the surface as an oil slick. Use of dispersants is the primary method used to deal with major oil spills. This has two positive impacts on the spill. Firstly, it reduces the immediate and obvious environmental impact, preventing the slick washing up on beaches and marshes, and from harming marine animals. Secondly, it may enable the oil to be more readily broken down by bacteria. However, there is also significant evidence that dispersants greatly increase the toxicity of the oil, causing significant environmental damage as the mixture of substances enters the food chain.
One common form of dispersant is a surface-active agent, more commonly contracted to surfactant. A surfactant consists of molecules that spontaneously bond to form enclosed bubbles. They do this by reducing the surface tension at the boundary where a liquid meets another liquid, gas or solid. Most surfactants are organic compounds that are amphiphilic – with a hydrophobic insoluble tail and an hydrophilic soluble head (see graphic, lower right). The tails may also be lipophilic, interacting more strongly with oil than with water. When a surfactant interacts with oil and water, the hydrophobic/lipophilic tails interact with the oil, trapping it, while the hydrophilic heads interact strongly with the water, forming the outer layer. This can result is spherical droplets known as micelles (see graphic, upper right), hollow spheres known as liposomes, or bilayer sheets. This process is known as emulsification, and causes the oil that would otherwise float on the surface to sink to the bottom or remain suspended in the water.
The most widely-used dispersant is Corexit, which contains the surfactant 2-Butoxyethanol, as well as other agents. That has been shown to cause cancer in animals and may also be carcinogenic in humans, as well as possibly causing damage to developing foetuses and negatively impacting both male and female fertility. Research shows that when the substances within Corexit are combined with oil, there are synergistic effects that massively increase toxicity, with one study finding the overall increase in toxicity was 52-fold. The Deep Water Horizon spill involved almost 4.9 million barrels of oil with two million gallons of Corexit added to it. There were widespread health impacts reported, including acute respiratory and cardiovascular problems, skin rashes, cancer, liver damage, and kidney damage. Corexit has also been shown to bioaccumulate, as smaller fish consume the toxin and spread it to larger predators.
Solidifiers cause oil to form into a solidified rubbery mass that floats on the surface and can be more easily removed from the water. Solidifiers are normally scattered over the surface of the oil slick as insoluble pellets. These pellets then absorb the oil, or cause it to coalesce into a gel, rendering it relatively harmless until the solid mass can be collected. This approach greatly reduces the toxins entering the water column, prevents harm to shoreline environments or adhesion to other bodies in the water, and suppresses the release of harmful vapours that would otherwise be released by the oil.
Solidifiers have no precise definition and include sorbents, gelling agents, cross-linking agents and products which combine these effects. Ideal sorbents are lipophilic and hydrophobic, so that they attract oil and repel water. Sorbents may absorb oil into the bulk of the material, or adsorb the oil only at the surface. Natural sorbents have been used for a very long time to clean up minor oil spills. Natural sorbents include peat moss, wood fibre, cotton fibre, straw, sawdust, and ground corncobs. They are non-toxic but have relatively low oil recovery capability. They are generally considered as a physical rather than a chemical removal method.
Gelling agents increase the viscosity of oil to the extent that it becomes an inert semi-solid mass. They achieve this by forming long-chain molecules. Cross-linking solidifiers also work in a similar way.
In many ways solidifiers can be considered the best available technology, since they are the only way of rapidly rendering the oil inert while also facilitating oil recovery. However, solidifiers are currently only proven for relatively small and contained spills. Further research is required to ensure that this approach can be effectively deployed to deal with large spills.
Bioremediation is the use of microorganisms or biological agents to break down or remove oil. Bacteria feed on hydrocarbon based oil, converting it to CO2 and water. This natural process can be accelerated by seeding the oil spill with specific microorganisms, or by applying a bioremediation accelerator which assists the action of pre-existing local microorganisms. A combination of these two approaches is also possible.
The activity of Alcanivorax borkumensis, a bacterium that naturally occurs in oil-contaminated seawater, can be accelerated through the application of phosphorus and nitrogen fertilisers. This method has been used for many years and was employed as part of the clean-up from the Exxon Valdez disaster. Typically, this achieves a doubling of the natural rate of oil breakdown, but this is insufficient to prevent major environmental damage.
A proprietary bioremediation accelerator, Oil Spill Eater II (OSE II) has been in use since 1989. It contains an enzyme, nutrients and biological surfactants. The enzyme helps to convert oil into an available food source for native bacteria, allowing them to break down the oil much more rapidly. The surfactants disperse the oil, forming micelles, which increases the surface area over which the enzymes can act on the oil, accelerating the process further. The nutrients attract indigenous bacteria, causing them to rapidly colonise the micelles. The OSE II does not contain any bacteria; it purely enables the action of indigenous local microorganisms. It is normally applied in a 1:50 concentration with water, which can be sprayed over an oil spill by systems ranging in size from hand-held sprayers to ships with spraying booms.
Bioremediation is not limited to oil spills; it can be an effective way to deal with the other great hydrocarbon based polluter – plastic waste. For example, an enzyme with an enhanced ability to break down plastic has been created by linking two different enzymes found in plastic-eating bacteria discovered in a Japanese landfill site.