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 contro lling the distribution and movement of hydrophobic organic chemicals such a s polycyclic aromatic hydrocarbons (PAHs) in aquatic environments (e.g. est uaries and oceans). To accurately predict the transport and fates of hydrop hobic contaminants in estuarine and coastal marine environments, the partit ion coefficient (K-p) and organic carbon normalised partition coefficient ( K-oc) are widely used in various biogeochemical models. Such partition coef ficients may be calculated from the so-called linear free energy relationsh ips between K-p, K-oc and more easily measured parameters such as the octan ol-water partition coefficient (K-ow). However K-p and K-oc values measured for real environmental samples of water and particles obtained from season al field surveys in the Humber Estuary, UK deviated from such ideals. For e xample, K-p values showed no correlation with the fraction organic carbon c ontent of particles (f(oc)) and instead of being constant, K-oc, values var ied with f(oc). Both K-p and K-oc 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 imp rove 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 ex tended partition model are very close to the field K(p)s. The results sugge st that PAHs associated with particles are in fact present in the form of s oot and soot-like particles that are not subject to particle-water equilibr ations. In other words, the PAHs on soot-like particles are extremely stron gly 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 K-oc values. No correlation was f ound between the partition coefficients and salinity, which again we attrib ute to the non-equilibrium nature of the PAHs. However both K-p and K-oc ge nerally 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 par ticles at the higher SSC values, and to the increased concentrations of dis solved organic carbon (DOC) associated with increase in SSC. However, it ha s been shown that DOC concentrations are not a suitable tool for correcting the SSC effect. There is therefore an urgent need to harmonise the laborat ory and field approaches for K-p measurements. (C) 1999 Elsevier Science B. V. All rights reserved.