Strontium-90 and Carbon-14 are two important radionuclides associated with nuclear waste. Accidental release of these to the environment can cause a hazard which, due to their respective half lives of 28.8 and 5730 years, may persist for hundreds or thousands of years.
The coprecipitation of 90Sr and 14C into calcium carbonate may be applied as a treatment process by the addition of Ca(OH)2 and Na2CO3 to groundwater. This has been demonstrated to be a possible process for the removal of these isotopes from groundwater, achieving over 99.9 % removal for both isotopes under optimal conditions. For 14C these conditions comprised high initial Ca2+:CO32- ratios (10:1). For Sr the optimal conditions comprised low initial Ca2+:CO32- ratios (10:100), both treatments were more favourable alongside high seed crystal surface areas.
Sr was incorporated into the calcite lattice with a gradually increasing lattice strain which resulted in a change in the coordination from 6-fold (calcite like) to 8-fold (aragonite like). The upper limit of this solid solution was determined by the point at which strontianite began to precipitate.
Before this treatment is applied to any site there exist a number of potential complications that should be considered. Since 90Sr and 14C are removed by fundamentally different processes, prolonged Ca2+ and CO32- depletion respectively, achieving simultaneous removal of these radionuclides from the same solution will be less efficient than the values quoted above. Furthermore if 14C bearing carbonate is allowed to equilibrate with a solution containing some 12CO32- (from atmospheric in-gassing), there will be an exchange of isotopes resulting in an accumulation of 14C in solution. Finally, it is believed Sr removal was improved as the proportion of Sr, relative to other divalent cations, increased. This Sr2+ was attracted to the negatively charged calcite surface and generated a localised supersaturation of strontianite. This mechanism however may be inapplicable to Sellafield site conditions as under the higher Sr2+ concentrations used in this study strontianite supersaturation will be reached at lower CO32- concentrations. Groundwater ionic content may also have an impact on this technique; by complexing with Ca2+ and CO32- ions and lowering their activities and thus supersaturations of calcite and strontianite.
Disposal of calcite produced from this process may be achieved through encapsulation and immobilisation in cement grout. Preliminary experiments show that the composition of this grout will control the incorporation environment of the 90Sr and 14C therein. In grouts with high Ground Granulated Blast Furnace Slag:Ordinary Portland Cement (GGBFS:OPC) the calcite dissolved and the contaminants contained within were released into the cement blend. In grouts with low GGBFS:OPC the calcite crystals remained intact and were encapsulated by the cement.
