Kinetics of 45Ca, 60Co, 210Pb, 54Mn and 109Cd in the tissue of the freshwater bivalve Velesunio angasi: further development of a predictive and mechanistic model of metal bioaccumulation

A theoretical and experimental study was performed to determine the kinetics of 45Ca, 60Co, 210Pb, 54Mn and 109Cd in the whole soft tissue of the unionid bivalve Velesunio angasi. This investigation further tested the hypothesis, developed previously for the alkaline-earth metals, that the biological half-life of a metal in soft tissue is related to its solubility when deposited in the extracellular granules of the bivalve. This hypothesis was tested for the above radionuclides (tracers of the stable metals) by a comparison of (a) a qualitative a priori prediction of their biological half-lives in bivalve tissue, based on critically evaluated log Ksp values for their respective hydrogen phosphate salts, and calibrated to previous experimentally determined rates of loss for 45Ca and 226Ra, with (b) their empirical biological half-lives that were investigated experimentally using the radionuclides 45Ca, 60Co, 210Pb, 54Mn and 109Cd. The results of the experimental investigation showed that the mean values calculated for the biological half-lives of 45Ca and 60Co in the tissue were 106 and 121 days, respectively, but there was no significant (P > 0.05) loss of 210Pb, 54Mn or 109Cd from the soft tissue over 160 days, when bivalves were exposed to radionuclide-free water. A chemical model was developed from first principles that quantitatively explains the kinetic mechanisms that underlie the differential rates of loss of divalent metals from the extracellular granules of V. angasi. The experimental results 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 linear, but the variation in tissue concentration, between individual bivalves, increased with the period of exposure. Significant (P < 0.001) positive correlations were found between tissue concentrations of each radionuclide pairing in both the uptake and loss phases of the experiment, indicating that individual bivalves were internally consistent in the way that they metabolised these radionuclides.