Metal partitioning between colloidal and dissolved phases and its relation with bioavailability to American oysters

Kinetics and the extent of metal partitioning between colloidal and dissolved phases and coagulation of metals associated with colloids were examined to determine their effects on the bioavailability of selected metals (Cd, Co, Hg, Ag, Fe, and Zn) to American oysters (Crassostrea virginica) using radiotracer and short term exposure experiments. After dispersion of radiolabeled colloids into low molecular weight (LMW, < 1 kDa) seawater, metal partitioning between dissolved (<1 kDa) and colloidal (1 kDa-0.2 microm) phases resulted in a consistent pattern, with a relatively constant percentage in the colloidal phase for each metal. On average, about 90% of Hg and Fe, approximately 60% of Ag and approximately 40% of Zn, Co, and Cd were measured in the colloidal fraction during a short term exposure experiment, consistent with their partitioning in natural waters. Controlled laboratory experiments carried out in parallel using radioactively tagged colloids showed that coagulation of colloidal species, quantified as the fraction retained by a 0.2 microm filter, was insignificant for most metals under the conditions and time periods of the uptake experiments. The bioavailability of colloidally complexed metals, measured in terms of dry weight concentration factor (DCF, ml g(-1)) and uptake rate constant (ml g(-1) h(-1)), was somewhat depressed compared with their counterpart in the LMW treatment, but could be well predicted from the results of the LMW treatment and metal partitioning. Both DCF values and uptake rate constants were higher in the LMW treatment than in the colloidal treatment. In addition, B-type metals, such as Ag, Hg, and Zn, all had higher values of DCF and uptake rate constants, regardless of treatments, except for Cd which had a lower DCF and uptake rate constant. In contrast, Co and Fe had significantly lower DCF values and uptake rate constants. Most of Hg and Ag (60-80%) were measured in the soft tissue of oysters in both LMW and colloidal treatments. In contrast, 80% of Fe, 75% of Co, and approximately 60% of Cd were observed on the shell, while Zn was found evenly distributed between shell and soft tissue of oysters. These results agree well with the variation pattern of both DCF value and uptake rate constant for these two groups of metals.