INTERACTIONS BETWEEN RADIOACTIVELY LABELED COLLOIDS AND NATURAL PARTICLES - EVIDENCE FOR COLLOIDAL PUMPING

It has been hypothesized that colloidal forms of trace metals can be r eactive intermediaries in the scavenging processes leading to the remo val of their particulate forms. A series of radiotracer experiments us ing natural colloidal organic matter from Galveston Bay, USA were carr ied out in order to test this hypothesis. Suspended particle uptake of originally colloidally bound trace metals occurred in a matter of hou rs to days in estuarine waters. After ten days, the majority (>50%) of the colloidal trace metals had been transferred into the particulate phase (greater than or equal to 0.45 mu m), except for Zn-65. Two dist inctively different temporal regions of removal of colloidal trace met als were identified: a faster reaction during the first four hours, fo llowed by a slower reaction after approximately one day. In a separate river water-seawater mixing experiment, the solid/solution partitioni ng of the radiotracers was investigated in the absence of suspended ma tter. About 30% of most of the elements, except Ag and Fe (similar to 60%), were associated with a newly formed particulate phase after eigh t days. There were two major trends: (1) the particulate fraction of F e-59 and Ag-110 increased while the colloidal fraction decreased, sugg esting a colloidal pumping mechanism. (2) The particulate fraction of Mn-54, Ba-133, Zn-65, Cd-109, Sn-113, and Co-60 increased while the LM W (less than or equal to 1 kDa) fraction decreased, suggesting a direc t uptake into the particulate fraction with less involvement of a tran sitory colloidal phase. The values of the particle-water (K-d) and col loid-water partitioning (K-c) coefficients for most trace metals were similar to those observed in Galveston Bay waters, suggesting compleme ntary results to field studies. The results from these experiments sug gested two different pathways for colloidal tracer uptake by particles : (i) colloidal pumping of a major component (e.g., biopolymer) of the colloidal pool and (2) coagulation of trace components (e.g., phytoch elatins) with varying affinities for different trace metals. In suppor t of these two different pathways, a number of correlations of particl e-water (Kd) or colloid-water (K-c) partition coefficients with rate c onstants or reactive fractions were observed. In general, the higher t he K-c values, the higher the reactive fractions, and the faster the t race colloid uptake by particles. Trace metals (e.g., Ag, Sn, Zn, Fe) which were found strongly organically complexed and associated mainly with colloidal matter in estuarine waters had a higher ion-colloid ass ociation rate constant. These experimental results suggest that intera ctions between surface-reactive fractions of the colloidal material an d particles can play a crucial role in the solid-solution partitioning of many trace elements. Copyright (C) 1997 Elsevier Science Ltd.