Characterization and Degradation of Fatty Acid Methyl Esters and Biodiesel in Artificial Seawater Under UV Exposure Using Raman Spectroscopy
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As global concerns over the environmental impact of petroleum fuels increase, alternatives such as biofuels are becoming more prevalent. Most of the world’s commodities are shipped over the oceans by large cargo vessels and the carriage of potentially hazardous cargo has safety and environmental risks. The carriage of biofuels is becoming more commonplace and an environmental discharge of such fuels is a potential given international allowances for cargo tank cleaning. The fate of biodiesel in the marine environment is a newer focus of research as the industry expands. This research characterizes the vibrational modes of the building blocks of biodiesel (fatty acid methyl esters (FAMEs)) and a commercial biodiesel sample through Raman spectroscopy. The concentration sensitivity of the Raman spectroscope in concentrations of ultra-pure and artificial seawater was found to be low; concentrations of 20% oil in water were necessary for good spectral resolution. Commercial biodiesel in artificial seawater was exposed under a UV lamp for 23 days to evaluate the degradation mechanisms. Isomerization was evidenced by the decline of the cis configurational vibration modes of the =CH bends. Photo-oxidation was apparent as the samples’ double bonds broke down over time. It was shown from comparison with a lab-made FAME mix that the antioxidants in commercial biodiesel slowed this process considerably. Hydrolysis occurred breaking down the biodiesel’s FAMEs into their relative free fatty acid and methanol. Methanol evaporated significantly from the sample; its presence was not measured directly through Raman spectroscopy but through a literature review and evaluation of vapor pressures, it was shown that methanol formed. The toxicity of methanol is a concern to marine organisms; however if most of the methanol formed through this degradation evaporates and exists the marine environment through natural physical oceanographic and transport processes, it is unlikely that biodiesel residue into the ocean will cause measurable harm to aquatic organisms.
- Chemical engineering