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

Abstract

The uptake of Pu(IV) by hardened cement paste (HCP) at degradation state I was investigated in the absence and presence of gluconate (GLU). Furthermore, the influence of the ionic strength was examined in different background electrolytes. Artificial cement pore water (ACW, pH = 13) was used for low ionic strength (I = 0.3 M), and cement pore water based on the diluted caprock solution (ACW-VGL, pH = 12.5) was used for high ionic strength (I = 2.5 M). Sorption experiments were performed under an Ar atmosphere using HCP in the HCP/GLU binary system ([GLU]0 = 1 × 10−1–1 × 10−8 M) and the HCP/Pu(IV)/GLU ternary system ([239Pu(IV)]0 = 1 × 10−8 M, [GLU]0 = 1 × 10−2 M) with solid-to-liquid (S/L) ratios of 0.5–50 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 on the sorption of Pu(IV) on HCP were investigated. In the absence of GLU, a quantitative uptake (S% ≥ 99%) of Pu(IV) by HCP was observed, independent of the ionic strength of the background electrolytes. In the presence of 1 × 10−2 M GLU, the sorption of Pu(IV) on HCP was significantly lower. For X-ray absorption fine structure (XAFS) measurements, powder samples with Pu ([239Pu(III)]0 = 5 × 10−6 M) sorbed on HCP (S/L = 2.5 g L–1) were prepared at pH ≈ 13 in ACW and ACW-VGL, respectively. One additional sample was prepared in the presence of GLU ([GLU]0 = 1 × 10−2 M) with ACW-VGL as the electrolyte for comparison. Pu LIII-edge X-ray absorption near-edge structure (XANES) spectra show that Pu is in the tetravalent oxidation state after being taken up by the HCP. The structural parameters obtained from extended X-ray absorption fine structure (EXAFS) analysis and comparison with literature indicate incorporation of Pu(IV) into the calcium-silicate-hydrate (C-S-H) phases of HCP. The different ionic strengths and the presence of GLU had no influence on the near-neighbor environment of Pu in HCP.