KINETICS OF CA-45, CO-60, PB-210, MN-54 AND CD-109 IN THE TISSUE OF THE FRESH-WATER BIVALVE VELESUNIO-ANGASI - FURTHER DEVELOPMENT OF A PREDICTIVE AND MECHANISTIC MODEL OF METAL BIOACCUMULATION

A theoretical and experimental study was performed to determine the ki netics of Ca-45, Co-60, Pb-210, Mn-54 and Cd-109 in the whole soft tis sue of the unionid bivalve Velesunio angasi. This investigation furthe r tested the hypothesis, developed previously for the alkaline-earth m etals, that the biological half-life of a metal in soft tissue is rela ted to its solubility when deposited in the extracellular granules of the bivalve. This hypothesis was tested for the above radionuclides (t racers of the stable metals) by a comparison of (a) a qualitative a pr iori prediction of their biological half-lives in bivalve tissue, base d on critically evaluated log K-sp values for their respective hydroge n phosphate salts, and calibrated to previous experimentally determine d rates of loss for Ca-45 and Ra-226, With (b) their empirical biologi cal half-lives that were investigated experimentally using the radionu clides Ca-45, Co-60, Pb-210, Mn-54 and Cd-109. The results of the expe rimental investigation showed that the mean values calculated for the biological half-lives of Ca-45 and Co-60 in the tissue were 106 and 12 1 days, respectively, but there was no significant (P > 0.05) loss of Pb-210, Mn-54 or Cd-109 from the soft tissue over 160 days, when bival ves were exposed to radionuclide-free water. A chemical model was deve loped from first principles that quantitatively explains the kinetic m echanisms that underlie the differential rates of loss of divalent met als from the extracellular granules of V. angasi. The experimental res ults were consistent with the predictions of the model; however, some investigational limitations were evident, and these are discussed. The uptake of each radionuclide into the bivalve tissue tended to be line ar, but the variation in tissue concentration, between individual biva lves, increased with the period of exposure. Significant (P < 0.001) p ositive correlations were found between tissue concentrations of each radionuclide pairing in both the uptake and loss phases of the experim ent, indicating that individual bivalves were internally consistent in the way that they metabolised these radionuclides.