Uptake of Pu(IV) by hardened cement paste in the presence of gluconate at high and low ionic strengths

The uptake of Pu(IV) by hardened cement paste (HCP) at degradation stage I was investigated in the absence and presence of gluconate (GLU). Furthermore, the influence of the ionic strength was examined by different background electrolytes. For low ionic strength (I = 0.3 M) artificial cement pore water (ACW, pH = 13) and for high ionic strength (I = 2.5 M) cement pore water based on the diluted caprock solution (ACW-VGL, pH = 12.5) were used. Sorption experiments were performed under Ar atmosphere using HCP in the binary system HCP / GLU ([GLU]0/M = 1×10-1 - 1×10-8) and the ternary system HCP / Pu(IV) / GLU ([239Pu(IV)]0/M = 1×10-8, [GLU]0/M = 1×10-2) with S/L of 0.1 – 50 g L-1 within a contact time of 72 h. GLU sorbs strongly on HCP, a saturation of the sorption sites of HCP with GLU was observed at [GLU] ≥ 1×10-4 M at S/L = 5 g L-1. The effects of the order of addition of the components Pu(IV) and GLU showed a low sorption of Pu(IV) to HCP, indicating strong kinetic effects. In the absence of GLU, quantitative uptake (S% = 99%) of Pu(IV) by HCP was observed. GLU had a significant effect on the sorption of Pu(IV) to HCP in both background electrolytes.
Powder samples from batch experiments were investigated by X-ray absorption spectroscopy to determine the structural parameters of the near-neighbour environment of sorbed Pu ([Pu(III)]0/M = 5×10-6) on HCP (S/L = 2.5 g L-1) in dependence of the ionic strength of the background electrolyte (ACW and ACW-VGL) and in presence of GLU ([GLU]0/M = 1×10 2). Pu LIII-edge X-ray absorption near-edge structure spectra confirm the presence of Pu(IV) as the dominant species on HCP. Plutonium was incorporated mainly into the calcium silicate hydrates phases (C-S-H) of HCP. Also, the ionic strength of the background electrolytes, as well as the presence of GLU do not influence the speciation of Pu on HCP during the degradation state I in cement-based repository.