Extraction of PCBs from Sediments: Towards Bioavailability Assessment Based on Supercritical Fluid Extraction
Sammanfattning: Persistent organic pollutants (POPs) pose serious threats to our global environment. These compounds have high chemical and biological stability and they are also very lipophilic. These characteristics not only render POPs the ability to spread widely and pollute remote areas, they also make them prone to accumulate in adipose tissues of living organisms. Consequently, it is important to monitor POP concentrations in the environment, and to evaluate the risks associated with polluted sites. Sediments and soils act as major sinks for POPs in the environment, so the extraction and analysis of POPs in those matrices are of great interest. However, total POP concentrations are not always indicative of the toxicity of a sediment or soil. As discussed in the thesis, POPs tend to become sequestered in sediments and soils over time, which causes their bioavailability to decrease. Therefore it is essential to know not only the total POP concentration, but also the bioavailable POP concentration. The techniques for analyte determination have improved tremendously since the POP problem became evident in the 1950's, and detection limits have decreased drastically. The technology for sample extraction, however, has largely remained the same. Only in the last 10–15 years have significant advances been made. The first part of this thesis describes two recent extraction techniques, pressurized liquid extraction (PLE) and supercritical fluid extraction (SFE), and discusses the improvements they represent in relation to Soxhlet, the most widely used classical extraction technique. Experimental work demonstrates that the recoveries of POPs from environmental matrices can be improved by the use of recent extraction techniques, and also that a higher selectivity of the extraction can be realized. Whereas sample preparation previously was accomplished by a series of discrete steps, of which one was sample extraction, it is now possible to develop approaches where extraction, clean-up, and concentration are accomplished on-line with the final analysis equipment. The second part of the thesis reports work where the inherent selectivity of SFE is utilized with the aim to develop selective extractions that extract only the fraction of PCBs that is associated loosely enough with the sediment to be bioavailable. Accumulation experiments with midge larvae and with eels indicate that it is possible to remove, and hence quantify, the bioavailable fraction of PCBs from a sediment, but more research is needed to further refine and validate the methods developed in this thesis.
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