Experimental evidence for solubility limitation of the aqueous Ni(II) concentration and isotopic exchange of Ni-63 in cementitious systems

摘要

Ni radioisotopes are present in cementitious repositories for radioactive waste and considered to be safety relevant in performance assessment. The behaviour of non-radioactive nickel and Ni-63 in cement systems has been investigated in batch-type experiments under conditions corresponding to the initial stage of cement degradation. Solubility tests using Ni-63 labelled solutions mixed with an artificial cement pore water (ACW) at pH 13.3 revealed that a Ni-containing precipitate was formed at high Ni concentrations, which limits the concentration of dissolved Ni to (2.9 +/- 0.5) x 10(-7) M. The concentration of dissolved Ni in cement suspensions, however, was controlled by the partitioning of non-radioactive Ni between the hardened cement paste (HCP) and ACW. The concentration of dissolved Ni was found to be independent of the solid-to-liquid (S/L) ratio in the range between 10(-6) kg L-1 and 0.13 kg L-1 ((7.3 +/- 3.9) x 10(-8) M). The concentration of dissolved Ni could not be modelled on the assumption that Ni partitioning is a reversible linear sorption process. The experimental data and the modelling indicate that a solubility-limiting process controls the concentration of dissolved Ni in the cement systems. Measurements of the sorption isotherm showed only a small increase in the concentration of dissolved Ni from about 5 x 10(-8) M to about 8 X 10-7 M while the concentration of added Ni varied over several orders of magnitudes (10(-6) M-5 x 10(-2) M). This finding supports the idea that a solid-solution aqueous-solution system involving Ni may account for the behaviour of Ni in cement systems. The distribution ratio for the partitioning of 13 Ni between HCP and ACW was found to be consistent with literature data obtained under similar experimental conditions (R-d = 0.15 +/- 0.02 m(3) kg(-1)). The Rd value determined on Ni loaded HCP samples (3.9 x 10(-4) mol kg(-1) and 4.3 x 10(-3) mol kg(-1)) increased with increasing Ni concentration in HCP. It is shown that the uptake of Ni-63 can be interpreted in terms of an isotopic exchange process with the non-radioactive Ni of the cement matrix. The distribution coefficient, a, of the exchange process ranges in value between about 0.02 and about 0.06, indicating that only a small portion of the Ni inventory is accessible to isotopic exchange.