Hollow-fibre liquid-phase microextraction of polychlorinated biphenyls: dynamic aspects and analytical challenges associated with their speciation

Abstract

In the present article, the hollow-fibre liquid-phase microextraction of poly-chlorinated biphenyls (PCBs) was conducted under non-equilibrium conditions, to investigate the bioavailability of PCBs in natural waters. The study was conducted for 12 PCB congeners (log K _ow ranging from 5.2 to 8.2) in the ng L⁻¹ range. This appeared as a major challenge since aqueous solutions in this concentration range tend to evolve rapidly due to adsorption of PCBs on glass walls. The average aqueous diffusion layer was measured to be 43 ± 2 µm at 500 rpm. Aqueous diffusion coefficients of PCBs estimated from experimental data were found to be about two times lower than those predicted by the Hayduk-Laurie equation, possibly due to the underestimation of the molar volume of PCBs, the aggregation of PCBs in the aqueous phase, or a decrease of the actual aqueous concentration during the time of extraction. The presence of Aldrich humic acid (AHA) in the solution decreased, as expected, the mass transfer of PCBs to the fibre, but the flux was not linked either to the total or to the free PCB concentration. This suggests a semi-labile behaviour for the AHA–PCB complex, which was confirmed by the effect of stirring speed on the amount of PCBs extracted in the presence and in the absence of AHA. The whole of these observations suggests that diffusion in solution is not only one of the limiting process for the extraction of PCBs but also supports the need for more experimental data to understand in detail the mechanism of extraction of hydrophobic compounds, and their bioavailability in the presence of aquatic complexants.

Keywords:

• bioavailability
• diffusion coefficient
• polychlorinated biphenyl
• hollow-fibre liquid-phase microextraction
• extraction kinetics
• glassware adsorption
• PCB–humic interaction

Acknowledgements

This work was supported by the ECODIS project (European Commission's 6th framework program, subpriority 6.3 ‘Global Change and Ecosystems’, contract 518043) for funding contributing to this work. The authors also thank Joop Hermens, Thomas ter Laak and Gareth Thomas for their critical comments on this work.