Radiochim. Acta 94, 29–36 (2006) / DOI 10.1524/ract.2006.94.1.29  by Oldenbourg Wissenschaftsverlag, München Experimental evidence for solubility limitation of the aqueous Ni(II) concentration and isotopic exchange of 63 Ni in cementitious systems By Erich Wieland 1 , ∗ , Jan Tits 1 , Andrea Ulrich 2 and Michael H. Bradbury 1 1 Paul Scherrer Institut, Nuclear Energy and Safety Research Department, Laboratory for Waste Management, 5232 Villigen PSI, Switzerland 2 Swiss Laboratories for Materials Testing and Research (Empa), 8600 Dübendorf, Switzerland (Received April 15, 2005; accepted in revised form July 21, 2005) Nickel / Cement / Sorption / Background stable isotope / Isotopic exchange Summary. 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 63 Ni in cement systems has been investigated in batch-type experiments under conditions corresponding to the initial stage of cement degradation. Solubility tests using 63 Ni 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 concen- trations, which limits the concentration of dissolved Ni to (2.9 ± 0.5) × 10 −7 M. The concentration of dissolved Ni in cement suspensions, however, was controlled by the parti- tioning 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) × 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 × 10 −8 M to about 8 × 10 −7 M while the concentra- tion of added Ni varied over several orders of magnitudes (10 −6 M–5 × 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 63 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 R d value determined on Ni loaded HCP samples (3.9 × 10 −4 mol kg −1 and 4.3 × 10 −3 mol kg −1 ) increased with increasing Ni concentration in HCP. It is shown that the uptake of 63 Ni can be interpreted in terms of an isotopic exchange process with the non-radioactive Ni of the cement matrix. The distribution coefficient, α, 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. *Author for correspondence (E-mail: erich.wieland@psi.ch). 1. Introduction Cementitious materials are foreseen to be used in the planned deep underground repositories for long-lived inter- mediate-level (ILW) and low- and intermediate-level (L/ILW) radioactive waste in Switzerland. Similar to the concepts developed worldwide, cement will be employed to condition the waste and to construct the engineered barrier system (components of lining, backfill material). Thus, the near field will consist of about 90 weight percent (wt. %) cementitious materials, of which hardened cement paste (HCP) will be about 20 wt. %. In performance assessment studies it is considered that the source term for radionuclide migration into the host rock is determined by a combina- tion of solubility and sorption constraints in the cementitious near field [1]. The uptake of radionuclides by HCP and ce- ment minerals, thus, plays a decisive role in limiting and retarding their release from the near field. Ni radioisotopes associated with irradiated metallic com- ponents from nuclear power plants are present in cemen- titious repositories for radioactive waste and considered to be safety relevant in performance assessment. Furthermore, non-radioactive Ni, which is present in noticeable quanti- ties in cementitious materials, is an important chemotoxic element. Several studies have reported solubility measure- ments for Ni under the conditions prevailing in cementitious systems [2–9]. The results from the majority of the studies show that the concentration of dissolved Ni is typically be- low 10 −6 M at pH > 12. However, the solubility data could not be interpreted in terms of a consistent thermodynamic model because the uncertainties in the Ni hydrolysis con- stants are large and the solubility-limiting solid phase is not known [e.g. 6, 10]. Until now only few attempts have been undertaken to identify the solid phase limiting the Ni concentration in cementitious systems [5, 7, 11]. The stud- ies of Glasser and co-workers [5, 7] indicated the forma- tion of a Ni-containing solid phase when a NiCl 2 solution was added to a saturated Ca(OH) 2 solution and aged over a time period of 4 weeks. The Ca : Ni ratio of the new phase was determined to be 4 : 1 using analytical electron microscopy. The morphology and bulk composition of the phase, however, is still unknown. Scheidegger et al. [11] used X-ray absorption spectroscopy (XAS) and diffusive re- flectance spectroscopy (DRS) to investigate the nature of