Article: Daria Boglaienko, Berrin Tansel. Instantaneous stabilization of floating oils by surface application of natural granular materials (beach sand and limestone)Marine Pollution Bulletin. 91(2015)107-112. http://dx.doi.org/10.1016/j.marpolbul.2014.12.020
This may be the first time that you have heard the phrase “granular material”, but you are undoubtedly familiar with sand, snow, nuts, rice, coffee and corn flakes. They are all granular materials and are ubiquitous in our daily lives.
What is special about the members of this group? One important feature is that they have the properties of both solids and liquids. Take sand as an example. Sand castles at the beach have certain frames and shape. This is a special character that only solids have. However, when you fill a container with sand, it takes the shape of the container as liquids do. Therefore, granular materials are suggested to be an additional state of matter on top of solid, liquid and gas.
Adding small amount of water to sand significantly changes its properties. The reason is that water adds bridges between the sand particles and therefore wet sands hold better than dry ones. Imagining how hard it will be building a sand arch by dry sand (fig 1)? Reversely, if we add granular material into liquid, what would happen?
(More on granular materials, check on http://www.physicscentral.com/explore/action/granular.cfm)
Scientists found that granular material introduced into hydrophobic liquid (e.g. oil) significantly changes the characteristic of the liquid. Liquid adhere to particles and forms a “raft”, which sinks once it gets too dense (too many particles). This mechanism can be extremely useful in separating floating oil layer after oil spills. A group of scientists were interested in seeing how efficient this approach works.
Granular material used in this study consisted of limestone sand and quartz sand. Three sizes were chosen to study the effect from particle size on removal efficiency of oil. For every one-gram of particulate, there are 2900 particles of the largest group of sand and 1,700,000 particles of the finest sand (fig 2 top). 1 mL of crude oil was added into 100 mL saltwater in 125 mL Erlenmeyer flask. 0.5~3.5 g of sands were added in while the flasks began to shake. Once the oil-particle aggregates formed, they were filtering through fiberglass filter to calculate the capture efficiency of granular materials (fig 2 bottom). Because the aggregates were captured by the filter, the weight difference between before and after the experiment indicates the weight of sand plus oil. This weight difference was subtracted of the weight of particles to get the weight of oil.
Results and Implications
Results show significant reduction of floating oil with more sand added (fig 3). The oil capture efficiency increases most dramatically between 1g and 2g of sand per gram of floating oil (fig 4). As to the difference between granular particle sizes, it was found that smallest particles showed largest oil content.
This experiment demonstrated strong evidence of using natural granular material (lime sand and quartz sand) to aggregate floating crude oil. The aggregation settled very quickly and was stable. This reveals the potential of using natural granular materials to control the transport of floating oil. Because particle-oil aggregates sink, less oil will be spread by surface currents as well as entering the atmosphere by air-sea exchange.
Currently, spreading dispersants is the most common way to stabilize surface oil after oil spill. These dispersants divide oil into small droplets which are more available to bacteria. However, this application of dispersants is not applicable near the coast and the toxicity of dispersants remains to be studied. Natural granular material could be a more eco-friendly tool in fixing coastal floating oil.
Interested in how the experiment was set up? Watch this video clip:
Caoxin is a graduate student in the Graduate School of Oceanography at the University of Rhode Island. Her research interest lies in persistent organic pollutants in the environment. When she is not doing research she likes to create new cuisines.