Influence of colloids in experimental solubility studies of Ni in natural and synthetic aqueous media

The estimation of nickel solubility values for performance assessment is impaired by the lack of experimentally verified solubility values in conditions relevant to actual nuclear waste disposal sites. For this reason, experimental nickel solubility studies were conducted in conditions relevant to a possible Finnish disposal site, Olkiluoto. Using an initial nickel concentration of 10⁻³ M, nickel solubility was approached from over-saturation in three aqueous media – natural fresh groundwater, synthetic saline groundwater, and cement-conditioned groundwater with added ferrous iron and sulfide. In addition to the nickel solubility values, nickel association with the particle and colloidal phases was measured. Almost 100%of the initial nickel was found associated with the separated particle phase in the natural groundwater and the cement-conditioned groundwater, and in only the natural groundwater about 0.04%or 0.4%with the separated colloidal fraction depending on the initially added amount of ferrous iron and sulfide. In the synthetic saline groundwater, all the nickel was found in the soluble fraction except for a minor fraction sorbed onto the walls of the sample bottle. Colloid-bound nickel did not have any practical influence on the solubility values obtained. The inverse behavior of Ni solubility and sorption in the natural groundwater compared to that in the other two aqueous phases studied was suspected to be caused by the unidentified yellow color of the groundwater for which indications of both possible organic and inorganic nature was obtained. Based on the measured results, the presence of sulfide and ferrous iron would decrease the amount of mobile nickel, both soluble and colloidal, in non-saline natural and cement-conditioned groundwater, whereas in saline groundwater nickel remains in the mobile phases.