The partition of fluoranthene and pyrene between suspended particles and dissolved phase in the Humber Estuary: a study of the controlling factors

Particle]water interactions are one of the most important mechanisms controlling the distribution and movement of hydrophobic organic chemicals such as polycyclic aromatic hydrocarbons PAHs. in aquatic environments e.g. estuaries and oceans.. To accurately predict the transport and fates of hydrophobic contaminants in estuarine and coastal marine environments, the partition coefficient Kp. and organic carbon normalised partition coefficient Koc . are widely used in various biogeochemical models. Such partition coefficients may be calculated from the so-called linear free energy relationships between Kp, Koc and more easily measured parameters such as the octanol-water partition coefficient Kow.. However Kp and Koc values measured for real environmental samples of water and particles obtained from seasonal field surveys in the Humber Estuary, UK deviated from such ideals. For example, KP values showed no correlation with the fraction organic carbon content of particles foc . and instead of being constant, Koc , values varied with foc. Both Kp and Koc were 1]3 orders of magnitude higher than those predicted from the simple equilibrium]partitioning model, but were in good agreement with several other recently published field studies. To improve our understanding of PAH partitioning, the soot carbon SC. content of particulate samples was measured so that the simple partition model can be extended to incorporate SC. The partition coefficients derived from the extended partition model are very close to the field Kps. The results suggest that PAHs associated with particles are in fact present in the form of soot and soot-like particles that are not subject to particle]water equilibrations. In other words, the PAHs on soot-like particles are extremely strongly bound and not influenced by further partitioning between the particles and water. However, there are still limitations with the extended partition model as it failed to simulate the field Koc values. No correlation was found between the partition coefficients and salinity, which again we attribute to the non-equilibrium nature of the PAHs. However both Kp and Koc generally decreased with increasing suspended solids concentrations SSC. in the estuary. Such a relationship is probably due to the mixing of soot-like permanently suspended particles with resuspended estuarine sedimentary particles at the higher SSC values, and to the increased concentrations of dissolved organic carbon DOC. associated with increase in SSC. However, it has been shown that DOC concentrations are not a suitable tool for correcting the SSC effect. There is therefore an urgent need to harmonise the laboratory and field approaches for Kp measurements.