Interactions between radioactively labeled colloids and natural particles: Evidence for colloidal pumping


1997 Jul


Wen LS
Santschi PH
Tang DG

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It has been hypothesized that colloidal forms of trace metals can be reactive intermediaries in the scavenging processes leading to the removal of their particulate forms. A series of radiotracer experiments using natural colloidal organic matter from Galveston Bay, USA were carried out in order to test this hypothesis. Suspended particle uptake of originally colloidally bound trace metals occurred in a matter of hours 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 distinctively different temporal regions of removal of colloidal trace metals were identified: a faster reaction during the first four hours, followed by a slower reaction after approximately one day. In a separate river water-seawater mixing experiment, the solid/solution partitioning of the radiotracers was investigated in the absence of suspended matter. About 30% of most of the elements, except Ag and Fe (similar to 60%), were associated with a newly formed particulate phase after eight days. There were two major trends: (1) the particulate fraction of Fe-59 and Ag-110 increased while the colloidal fraction decreased, suggesting 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 LMW (less than or equal to 1 kDa) fraction decreased, suggesting a direct uptake into the particulate fraction with less involvement of a transitory colloidal phase. The values of the particle-water (K-d) and colloid-water partitioning (K-c) coefficients for most trace metals were similar to those observed in Galveston Bay waters, suggesting complementary results to field studies. The results from these experiments suggested 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., phytochelatins) with varying affinities for different trace metals. In support of these two different pathways, a number of correlations of particle-water (Kd) or colloid-water (K-c) partition coefficients with rate constants or reactive fractions were observed. In general, the higher the K-c values, the higher the reactive fractions, and the faster the trace 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 association rate constant. These experimental results suggest that interactions between surface-reactive fractions of the colloidal material and particles can play a crucial role in the solid-solution partitioning of many trace elements. Copyright (C) 1997 Elsevier Science Ltd