Publication

The particle concentration effect (p.c.e.) is an unexpected decline in partition coefficients (K-d) as suspended particulate matter (SPM) increases. This anomaly has been attributed to a variety of causes, but most often to the existence of colloidal forms of the adsorbate, which are included, in error, in the dissolved fraction when calculating K-d. To test this hypothesis we have directly measured colloidal, macroparticulate, and truly dissolved metals (Cd, Cu, Pb, Zn, Fe, Al, Mn, Ag) monthly for one year (6/96-7/97) in four Connecticut rivers having a range of ancillary biogeochemical characteristics. These include factors that are all likely to influence partitioning between dissolved and solid phases, such as DOG, pH, and competing cations. The p.c.e. is clearly evident in these rivers, and explicit consideration of colloidal metals eliminates or reduces this anomaly in nearly all cases. Furthermore, a substantial portion of metals occurs in the colloidal fraction, and the amount of colloidal metals increases with SPM. Both of these conditions are necessary for the colloidal model to explain the p.c.e.. Important differences are observed among rivers and metals, and in some cases systematic decreases in K-d continue even when colloidal metals are taken into account. This additional decrease can be eliminated by excluding large particles having negligible surface complexation sites. When corrections are applied for both colloids and large particles, K-d values become truly constant. Copyright (C) 1999 Elsevier Science Ltd.