Chemical Fractionation and Bioavailability of Cobalt-60 to Benthic Deposit-Feeders

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Abstract
Concentrations of radiocobalt (60Co) and stable Co were measured in a series of sequential chemical extractions of freshwater sediment cores and in benthic deposit-feeding organisms. In all sediment samples, a large fraction of the stable Co was measured in the strong-acid (residual) extract. Co concentrations in the exchangeable and carbonate-bound fractions decreased with depth in the core, while Co concentrations in the oxide- and organic-bound or sulfide-bound fractions increased. We hypothesize that the changes in Co partitioning result from the decrease in Eh with depth as FeCO3 dissolves and FeS2 forms. 60Co added to sediments was incorporated quickly by the exchangeable, carbonate-bound, and oxide-bound fractions. Very little 60Co was measured in the organic and residual fractions although these fractions contained more than 50% of the total stable Co. The specific activity of 60Co in organisms (Becquerels 60Co per microgram total Co) that were incubated in sediments spiked with radiocobalt was similar to the specific activity of the exchangeable, carbonate, and oxide fractions. The large percentage of Co in the organic and residual fractions was not accumulated by the benthic deposit-feeders. The concentration factor (ratio of organism concentration to sediment concentration) for radioactive Co was the same as that of stable Co from the first three fractions only, which suggests that there may be a relationship between concentration factors and the bioavailable fraction of a contaminant. Specific activity may provide a simple measure of the bioavailable fraction of substances.