Biotransformation of low sulfur fuel oil HDME 50 in seawater at sub-arctic temperatures

Current oil spill response technologies include mechanical recovery, in situ burning and the use of oil dispersant. The use of dispersant enhances the dissolution of oil into the water by breaking it into smaller droplets, decreasing the oil accumulation at the water surface and reducing the amount of oil which may be drifted to shore. Biodegradation of dispersed oil can be fast and extensive if the oil is present at the ppm levels following a successful application of dispersants. Some studies however, also found that biodegradation might be inhibited by dispersants. Since using dispersant can be an effective method to enhance removal of spilled oil even in cold marine condition, biodegradation of dispersed oil in seawater at low temperature has gained much focus internationally . Biodegradation of dispersed oils has been demonstrated at temperature from 5 to -6 °C, which usually completed within several months. The sulphur emission control areas (SECA) have been introduced in Europe since January 2015, to reduce harmful sulphur oxide (SOx) emissions from ship leading to the limit of sulfur content in ship fuel oil to 0.1%. In addition, the global regulations (MARPOL2020) came into effect from January 2020, which set the sulfur limit for fuel oil to 0.5% for sea areas outside SECA. To meet those requirements, the ships can switch to higher quality fuel oil with lower sulfur content. A new generation of marine fuel oil so called ultra-low sulfur fuel oil (ULSFO) or hybrid fuel oil, have been developed and used increasingly within and outside SECA. The hybrid fuel oils (LSFO) which has less than 0.1% sulfur tend to be rich in waxes, resulting in high pour point (). High waxes content and high pour point affect the dispersibility of the oil at low temperature presenting challenge for their removing from marine environment at low temperatures. Since LSFO has been used only recently, very little is known about the biodegradation of LSFO in general and of the dispersed LSFO in particularly. Knowledge about the biodegradation potential of LSFO is important knowledge in oil spill preparedness. In this study, we used Hybrid Wide Range Gas Oil (HDME 50), one of the few LSFOs which are available on the market. HDME 50 has been described as poorly dispersed naturally and after treatment with dispersants at temperatures below 5 °C. The oil however showed dispersibility at 13 °C. We therefore investigate the biotransformation of the chemically dispersed HDME 50 at 13 °C and non-dispersed oil film at 5 and 13 °C, a typical Norwegian summer and winter temperature in seawater.

当前溢油响应技术主要涵盖机械回收、原位焚烧以及石油分散剂的应用。分散剂可将原油破碎为微小液滴，以此提升原油在水体中的溶解效率，降低水面原油聚集量，同时减少可能被冲至岸线的溢油量。若分散剂施用成功，使水体中石油浓度处于ppm级别，分散态原油的生物降解过程可快速且广泛地进行。不过，部分研究也发现，分散剂可能会抑制生物降解作用。鉴于即便在寒冷海洋环境中，分散剂仍是提升溢油移除效率的有效手段，低温海水中分散态原油的生物降解已成为国际学界的关注热点。已有研究证实，在5℃至-6℃的温度范围内，分散态原油可完成生物降解，该过程通常耗时数月。自2015年1月起，欧洲开始实施硫排放控制区（Sulphur Emission Control Areas, SECA）制度，旨在减少船舶排放的有害硫氧化物（Sulphur Oxide, SOx），将船舶燃料油的硫含量限值降至0.1%。此外，全球海事法规《MARPOL2020》于2020年1月正式生效，该法规将硫排放控制区外海域的燃料油硫含量限值设定为0.5%。为满足上述要求，船舶可切换使用低硫含量的高品质燃料油。目前，新一代船用燃料油——超低硫燃料油（Ultra-Low Sulfur Fuel Oil, ULSFO）或称混合燃料油（Hybrid Fuel Oil）已被开发出来，并在硫排放控制区内外得到日益广泛的应用。硫含量低于0.1%的混合燃料油（Low Sulfur Fuel Oil, LSFO）往往富含蜡质，进而表现出较高的倾点（）。高蜡质含量与高倾点特性会在低温环境下影响该类燃油的分散性，为低温海洋环境下的溢油移除工作带来挑战。由于LSFO仅在近年投入使用，目前学界对其生物降解特性，尤其是分散态LSFO的生物降解特性的了解仍十分有限。而掌握LSFO的生物降解潜力，对于溢油应急准备工作具有重要意义。本研究选用了市场上为数不多的LSFO产品之一——混合宽馏分瓦斯油（Hybrid Wide Range Gas Oil, HDME 50）。已有研究表明，在5℃以下的环境中，HDME 50无论自然状态还是经分散剂处理后，其分散性均较差，但在13℃环境下则具备可分散性。因此，本研究针对两种工况开展实验：一是经化学分散剂处理后的HDME 50在13℃海水中的生物转化过程，二是未分散的HDME 50油膜分别在5℃与13℃（对应挪威海域典型的夏季与冬季海水温度）下的生物转化过程。