Nickel uptake by calcite

Full text of publication follows: For confidence in risk assessment, parameters that describe the behaviour of radioactive compounds with soil or rock minerals are imperative. For designing safe disposal facilities for radioactive waste, reactive barriers, where dissolved components can react with reactive surfaces, offer some insurance against escape. Radioactive nickel can be produced during radiation of stainless steel. Nickel is soluble as a cation, Ni2+, and is known to associate in particular with manganese oxides/hydroxides and pyrite, substituting for the metal in the solid. Recent studies have shown that Ni2+ is also effectively removed from water by calcite, with uptake rates on the order of 90% in the pH range of about 7.5 to 8.5 and Ni concentrations ranging from a few to several hundred ppb. The purpose of this work was to investigate the Ni-CO2-H2O-CaCO3 system at a pH range typical for natural groundwater. In solutions under-saturated with respect to Ni(CO3)2(s) and Ni(OH)2(s), adsorption occurs through exchange with Ca2+ in a surface-adsorbed layer on calcite. The validity of this exchange reaction is supported by adsorption isotherms and experiments where Ca2+(aq) was varied by changing pH and pCO2(g). Greater adsorption of Ni2+ occurred in higher pH solutions, where Ca2+ activities were lowest. The stoichiometric composition and corresponding equilibrium constants for the reactions at the calcite-solution interface were evaluated using FITEQL. The best fit to the experimental data was obtained with a single exchangeable surface complex, >CO3Ni+, that describes Ni sorption over a wide range of experimental conditions. Nickel sorption was apparently independent of surface charge, which suggested that the surface complex had covalent character and did not remain hydrated in spite of high hydration energy of Ni2+. On the other hand, desorption experiments indicated that Ni2+ is not rapidly exchangeable with Ca2+ and is strongly retained by the solid phase. Results were also consistent with incorporation of Ni2+ into the calcite structure as a solid solution. Experiments underway now are designed to determine the distribution coefficient of nickel in calcite and the influence of solution composition and precipitation rate on its magnitude. (authors)