Coprecipitation of radionuclides with calcite: estimation of partition coefficients based on a review of laboratory investigations and geochemical data

Coprecipitation of radionuclides with secondary solids is currently neglected in safety assessments for radioactive waste repositories, although this process is thought to be important in limiting radionuclide solution concentrations. This paper provides a systematic review of laboratory data on metal coprecipitation with calcite, presented in the form of phenomenological partition coefficients. The aim of this investigation is to provide a consistent set of parameter values for the quantitative modelling of radionuclide coprecipitation with calcite, which will be the dominant alteration product in cementitious repositories accessed by carbonate-rich groundwater. From the data reviewed, empirical correlations have been derived that relate experimentally determined partition coefficients (lambda(Me)) to measurable chemical properties of the coprecipitated metals (ionic radii and sorption parameters of the incorporated trace metals, solubility products of the pure metal carbonates). These correlations have then been used to predict the partition coefficients of radionuclides for which no laboratory data exist. Such predictions indicate that the actinides will partition strongly into calcite under reducing conditions (lambda(Me) similar to 200-1000 for trivalent, lambda(Me) similar to 20-200 for tetravalent actinides). Nickel(II) incorporation will be moderate (lambda(Me) similar to 1-10), while incorporation of ions like U(VI), Cs(I), Sr(II) and Ra(II) in calcite will be weak (lambda(Me) < 1). In spite of substantial uncertainties, the estimated partition coefficients are sufficiently accurate to allow a semiquantitative evaluation of the effect of radionuclide coprecipitation with calcite in limiting radionuclide solution concentrations in well characterised repository environments. (C) 1999 Elsevier Science Ltd. All rights reserved.