Potential use of Hydrophobic Aerogels in future oil spill clean-ups

Transcription

1 Potential use of Hydrophobic Aerogels in future oil spill clean-ups Hinna Hafeez, Undergraduate Student, Dalhousie University Abstract For thirty years, oil spill clean up methods did not change until the Deepwater Horizon Spill, the worst marine oil spill in history, occurred in This notion has motivated scientific advancement of more efficient methods than those used in the Deepwater Horizon Spill. To date, the most notable oil spill clean up method that was used is the dispersant Corexit This chemical has been found to negatively affect mammalian cells including inducing oxidative stress and premature cell death. It was also found that the oil dispersed by Corexit 9500 remained in the water and continued to affect marine life one year after the spill. Among the proposals for new clean up methods, hydrophobic, low density solids called aerogels are a serious contender for large scale clean ups in the future. Hydrophobic aerogels act as sponges to absorb oil and not water and can remove several types of oils including dispersed and emulsified oil. The oil can be later removed from the gel to be used commercially. However, aerogels are costly and time-consuming to make and have not yet been tested on a largescale basis. Next steps should include testing aerogels in the ocean and potentially using them to remove remaining dispersed oils caused by the use of Corexit Introduction Science is constantly evolving to come up with better, more efficient methods to accomplish a task, however, it is shocking to find out that methods of oil spill cleanup had not changed for thirty years until the Deepwater Horizon (or BP) Oil Spill in Scientists were pushed to come up with new, more efficient methods of clean up in both efficiency of removal of oil and of cost. One of these methods involves using hydrophobic aerogels, a low density, highly porous material that is extremely lightweight and has one of the highest surface areas per mass known to man (Karatum et al. 2015). They can be created to be hydrophobic, allowing them to selectively absorb oil and not water, making them an ideal candidate for crude oil clean up (Sorour et al. 2016). Aerogels were first introduced in the 1930s and were originally used as aerospace and building insulators. It wasn t until recently, after the Deepwater Horizon Spill, that aerogels were considered to be used to clean crude oil from water (Maleki 2016). An aerogel is created by supercritically drying a conventional gel until no water remains (Dorcheh and Abbasi 2007). Aerogels have many advantages over oil spill clean up methods that were used during the BP oil spill, most notably the use of Corexit 9500, which was used to break down the crude oil (Zheng et al. 2014). This dispersant has since been analyzed to determine its impact on the marine ecosystem and results from these studies have found that Corexit 9500 has negative effects on marine mammals (Zheng et al. 2014). H. Hafeez \ Oceans First, Issue 4, 2017, pgs

2 This paper seeks to synthesize the current knowledge on hydrophobic aerogels in order to evaluate their applicability for use in oil spill clean up. I will outline their advantages and disadvantages, how to maximize their use and why continued research is necessary; as they can be used to improve current marine ecosystems that were damaged by the use of Corexit 9500 during the BP oil spill in Figure 1: A hydrophobic aerogel removes crude oil that has been mixed with water after 4 minutes (40 ml of water/ 5 ml of oil) (Nguyen et al. 2013). 2. Advantages and disadvantages of aerogels The composition of aerogels provide many benefits including their high porosity which allows them to be used for the removal of oil slicks as well as dispersed and emulsified oils (Sorour et al. 2016), making them a better method of clean up than those used to clean the BP oil spill (Maleki 2016). As they can be used to remove several types of oils from water, it becomes unnecessary to come up with different types of removal methods that may be efficient in only removing one type of oil. The high density of aerogels allows a small amount of gel to hold a large amount of oil (Sorour et al. 2016). This high density also causes the gel to float on the surface of water, allowing it to be more easily collected after it has absorbed the oil (Sorour et al. 2016). Crude oil can be extracted from aerogels to be reused, providing another advantage over dispersants such as Corexit 9500 (Karatum et al. 2015). However, they cannot be reused after the extraction of oil, but the benefits of being able to use the collected oil would offset the costs of the gels being a one-time use (Karatum et al. 2015). There are two main ways to recover the oil from the aerogel: chemical extraction and mechanical extraction (Karatum et al. 2015). Chemical extraction works by using solvents to draw out the oil from the gel and mechanical extraction uses a Palmgren drill press to manually press the oil out of the gel (Karatum et al. 2016). While there are many important benefits and advancements associated with aerogels, there are also certain disadvantages. Aerogels have a very long and involved drying process and are currently very expensive to produce. Aerogels are also a very new science in terms of use in ocean sciences and clean up and have yet to be tested on a large scale (Maleki 2016). This means that questions such as how they will be distributed, transported and recollected in the event of an oil spill, have yet to be answered. Scientists are currently 24

3 working on ways to maximize the efficiency and costs of production of aerogels as well as the best methods of extracting the oil so that they can be later used commercially (Sorour et al. 2016). 3. Maximizing aerogel efficiency Recollecting the oil from oil spills saves resources originally lost in widespread disasters. Seven different recipes of hydrophobic aerogels were compared based on their specific surface areas, yield, oil removal efficiency as well as their hydrophobicity (Sorour et al. 2016). Yield was measured by percentage of the mass of the final product to the mass of the aerogel precursors and varied between the recipes at % yield (Sorour et al. 2016). Oil removal efficiency was measured by the percent of vegetable oil removed in both fresh and 3% saline water and hydrophobicity was measured by the contact angle of a droplet of water on the gel; as the contact angle increased, the gel was determined to be more hydrophobic (Sorour et al. 2016). The most efficient recipe had the highest oil removal at 96% in fresh water and 89% in saline water and a relatively high yield at 36% (Sorour et al. 2016). Sorour et al. conducted their experiment to not only compare the efficiency of oil removal of different gels, but also to test their feasibility, in terms of their synthesis and cost of being used as a method of clean up in the event of a spill. They were able to reduce the synthesis time from several days to 5 hours and by minimizing the costly precursors, they were able to create a low-cost gel that would be ideal for large scale production (Sorour et al. 2016). The preliminary analysis of their sample was found to be in the range of $ dollars per kilogram (Sorour et al. 2016). Karatum et al. determined the best way to extract the crude oil from the aerogel. They compared chemical and mechanical extraction methods using identical gels saturated with oil and found that chemical extraction resulted in a 70-75% recovery, while mechanical extraction recovery was highly variable in terms of percent recovery (Karatum et al. 2015). However, they determined mechanical extraction as the better method of extraction even taking into account its inconsistency. This is because it takes much less time to extract the oil mechanically rather than chemically. It takes 3 minutes for mechanical extraction while it takes 24 hours for chemical extraction. Second, chemical extraction is more difficult to maintain and requires more careful monitoring as too strong of a solvent will erode the gel to a point where it cannot be separated from the oil. The oil is also less likely to be contaminated when extracted mechanically rather than chemically, allowing it to be used without having to worry about what effects the solvents will have on the processing of the crude oil (Karatum et al. 2015). 4. Continued impacts of the BP oil spill on marine ecosystems Aerogels could be used to remove emulsified oils, and as a result, improve the marine ecosystem in the Gulf of Mexico by reducing the amount of contaminated prey. After the BP oil spill, alkane levels, an indicator of oil levels, were measured along the shores of the Gulf of Mexico to measure the remnants of the oil spill immediately after the spill (May 2010), a few months after the spill (August 2010) and a year after the spill (May 2011) (Liu et al. 2014). It was found that the amount of suspended alkane particles went up after the clean 25

4 up while the amount of dissolved alkanes went down after one year, meaning that the dissolved oil separated from water over time. The increase in alkane particle suspension provides evidence that suspended oil contaminants can be transported to the shoreline through surface waters and oil slicks do not have to be present for marine life to be affected (Liu et al. 2014). The largest of these toxic effects is on plankton, fish eggs and larvae (US Fish & Wildlife Services 2010). Through biomagnification, the concentration of oil increases in higher trophic levels as larger fish consume many contaminated preys and subsequently become contaminated through direct ingestion (US Fish & Wildlife Services 2010). Reduced growth, changes in respiration and heart rates and impairment of reproduction are all side effects of this biomagnification in the higher trophic levels of marine animals (US Fish & Wildlife Services 2010). Corexit 9500, used to clean up the BP oil spill, continues to have ongoing impacts on marine life. In order to clean up the spill, it was sprayed on the ocean surface and released underwater near the well site (Zheng et al. 2014). At the time of the spill, had BP not used this dispersant to diffuse the oil, the impacts on the shoreline would have been much more severe (Zheng et al. 2014). Prior to the use of Corexit 9500, it was tested by the Environmental Protection Agency with seven other dispersants before being assessed as a suitable choice (Zheng et al. 2014). Even though all the tests were conducted with standard regulations and Corexit 9500 was found to be the best fit, its toxicity was measured in a limited number of cell types and mainly focused on the effects of the dispersant on the endocrine system (Zheng et al. 2014). Zheng et al. performed tests to better understand the effects of Corexit 9500 s toxicity on different mammalian cells to expand our knowledge on its effects across multiple species and cell types (Zheng et al. 2014). They used five different cell types: skin cancer cells, neuronal cells, astrocytoma cells and two different types of kidney cells to measure cytotoxicity effects (Zheng et al. 2014). They found that Corexit 9500 s toxicity is directly proportional to its concentration. It exhibits this toxicity by inducing oxidative stress, affecting mitochondrial function and by inducing premature cell death (Zheng et al. 2014). They predicted that the toxic effects of Corexit 9500 would be magnified if residual crude oil is also present in the marine environment (Zheng et al. 2014). It is also important to note that aerogels are not only useful in the event of an oil spill and can have applications when there is no crisis present. Their high porosity allows them to remove oils that have been emulsified, which before now, was thought that they could not be completely removed. This means that aerogels could be used in the Gulf of Mexico where Corexit 9500 was used to try and remove this left over crude oil, improving ecosystems and minimizing toxicity of Corexit 9500 on marine mammals. 5. Conclusion Though aerogels are a new concept in terms of their use for oil clean up, they have great potential to be a better method than those used in the Deepwater Horizon Spill. Their properties, such as high porosity and high density, allow them to remove multiple types of oils, including surface oil as well as dispersed and emulsified oil. This eventually may result in only needing one type of removal method of oil rather than several that remove different types of oil (Sorour et al. 2016). Hydrophobic aerogels also have the added benefit that the 26

5 oil that is collected can later be extracted and used (Karatum et al. 2015). However, aerogels continue to have some kinks that need to be worked out before they can be used on a large-scale basis, such as in the event of a crisis like an oil spill. They continue to take a long time to make and are expensive, though scientists have already found ways to greatly reduce these disadvantages (Sorour et al. 2016). It is critical to research aerogels because they can also be used to remove left over dispersed oils caused by the use of Corexit 9500 in the clean up of the Deepwater Horizon Spill. In order to be able to be a viable resource in the event of an oil spill, more large-scale research must be done in the ocean. Testing their ability to remove of emulsified oils could be a viable experiment that would not involve adding oil to the ocean. Further experiments must also be conducted to see the best way to synthesize large quantities of aerogels quickly if it is necessary and to see the best way to send out and retrieve aerogels in the ocean so that there are not any that have been left behind after they have completed their job so that we are well prepared in the event of an emergency. References Dorcheh AS, Abbasi MH Silica aerogel; synthesis, properties and characterization. J Mater Process Tech. 199 (2008): Karatum O, Steiner SA, Griffin JS, Shi W, Plata DL Flexible, Mechanically Durable Aerogel Composites for Oil Capture and Recovery. ACS Appl Mater Interfaces. 8 (1): Liu Z, Liu J, Gardner WS, Shank GC, Ostrom NE The impact of Deepwater Horizon oil spill on petroleum hydrocarbons in surface waters of the northern Gulf of Mexico. Deep-Sea Res Pt II. 129: Maleki H Recent advances in aerogels for environmental remediation applications: A review. Chem Eng J. 300: Nguyen ST, Feng J, Le NH, Le ATT, Hoang N, Tan VBC, Duong HM Cellulose Aerogel from Paper Waste for Crude Oil Spill Cleaning. Ind Eng Chem. 52: Sorour MH, Hani HA, Al-Bazedi GA, EL-Rafei AM Hydrophobic silica aerogels for oil spills clean-up, synthesis, characterization and preliminary performance evaluation. J Porous Mat. 23 (5): US Fish & Wildlife Service Effects of Oil on Wildlife and Habitat [internet]. [Unknown place of publication]: US Fish & Wildlife Service; [cited Apr 2]. Available from: Zheng M, Ahuja M, Bhattacharya D, Clement TP, Hayworth JS, Dhanasekaran M Evaluation of differential cytotoxic effects of the oil dispersant Corexit Life Sci. 95 (2):