The influence of size distribution on the particle concentration effect and trace metal partitioning in rivers

The particle concentration effect (p.c.e.) is an unexpected decline in part ition coefficients (K-d) as suspended particulate matter (SPM) increases. T his anomaly has been attributed to a variety of causes, but most often to t he existence of colloidal forms of the adsorbate, which are included, in er ror, in the dissolved fraction when calculating K-d. To test this hypothesi s we have directly measured colloidal, macroparticulate, and truly dissolve d 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 chara cteristics. These include factors that are all likely to influence partitio ning between dissolved and solid phases, such as DOG, pH, and competing cat ions. The p.c.e. is clearly evident in these rivers, and explicit considera tion of colloidal metals eliminates or reduces this anomaly in nearly all c ases. Furthermore, a substantial portion of metals occurs in the colloidal fraction, and the amount of colloidal metals increases with SPM. Both of th ese conditions are necessary for the colloidal model to explain the p.c.e.. Important differences are observed among rivers and metals, and in some ca ses systematic decreases in K-d continue even when colloidal metals are tak en into account. This additional decrease can be eliminated by excluding la rge particles having negligible surface complexation sites. When correction s are applied for both colloids and large particles, K-d values become trul y constant. Copyright (C) 1999 Elsevier Science Ltd.