Investigations of Pb and Ni uptake in cementitious environments

In this work, Pb and Ni uptake in a highly alkaline near-field cementitious environment is investigated. Significant quantities of Pb occur in intermediate-level radioactive wastes (ILW) that will be held in a UK geological disposal facility (GDF). Pb is chemotoxic and a priority for risk assessments as part of the GDF safety-case. Ni will occur in nuclear waste as radioactive isotopes 59Ni and 63Ni. Therefore, it is important to understand their migration and retardation behaviour within the engineered barriers of a GDF. Specifically, in the cementitious material which might be used to backfill the ILW vaults. One such material is the Nirex Reference Vault Backfill (NRVB). In this work, three aspects of NRVB behaviour were investigated: the amount of Pb and Ni ions adsorbed by NRVB, whether that adsorption is reversible, and the phases in NRVB that primarily control the uptake. To reflect the GDF scenario, batch sorption analysis was performed of Pb and Ni uptake onto hydrated NRVB. Pb batch sorption experiments were also carried out on constituent components (CEM-1, Ca(OH)2, CaCO3) and calcium silicate hydrate (C-S-H) gels. The results indicate Langmuir-type sorption onto the NRVB. Desorption experiments suggested that, on these experimental timescales, Pb sorption is reversible, in contrast to previous studies where Pb was introduced during cement curing. Surface analysis techniques found that Pb is homogeneously distributed on the NRVB surface, suggesting that the dominant sorbent phase is C-S-H gel; a hypothesis consistent with the batch sorption measurements. X-ray absorption spectroscopy (results suggest that Pb is linked to the C-S-H gel through Si–O–Pb bonds. In contrast, Ni sorption investigations were complicated by the extremely low solubility-limit of Ni at the pH of NRVB, and trace Ni in the NRVB itself. Precipitation of Ni-based hydroxides and layered double-hydroxides dominate the sequestration mechanisms over any surface adsorption.Open Acces