Chemical speciation modelling of groundwater in a shallow glacial sand aquifer. Part II, radionuclide speciation and effect of organics

摘要

The aims of the work detailed in this report are to gain understanding of the changes thatudoccur in groundwater chemistry following the addition of a) the radionuclides 58C0 andud63Ni, and b) organic complexants.udThe analytical data used in these modelling studies is as for the first report in this series,udand consists of inductively coupled plasma analyses for total element concentrations ofudthree samples. For reasons that are detailed in the text (section 4.3) only two of the threeudsamples were modelled using the PHREEQE geochemical code in conjunction with theudUWCC thermodynamic database. Acetate and EDTA were used as analogues for theudweakest and strongest binding expected by the natural organic matter present in theudaquifer.udThe results show that at the measured pH, the major form of 58C0 will be the Co2+ ion.udIn the case of 63Ni, the major aqueous phase species is NiC03O. The speciation of cobaltudis unaltered with changes in the pH or Eh, whereas nickel is pH sensitive. The formationudof solid phase cobalt or nickel species is not predicted. The addition of 58C0 to the aquiferudas a CcbH2EDTA is predicted to result in the dissociation of the complex and consequentudformation of the free Co2+ and ZnEDTA2-.udAcetate doesn ot perturb either the natural aqueous speciatiorn the speciationo f the addedudradionuclides. In contrast, using EDTA as an analogue for natural organic matter results inudsignificant perturbations of both the natural speciation and the radionuclide speciation. Itudwas found that 48% of the total iron, 92% of the total cobalt and 99% of the total nickeludwere complexed by EDTA with an EDTA concentration equivalent to the measured totaludorganic carbon concentration of 3.5 and 2.5 mg dm-3 in sample 2 and 3 respectively.