The partitioning of uranium and neptunium onto hydrothermally altered concrete

Abstract

SummaryPartition coefficients (K d ) of U(VI) and Np(V) on untreated and hydrothermally altered concrete were measured in 0.01 M NaCl and 0.01 M NaHCO 3 solutions as functions of concentration of the radionuclides, pH, and time. The partition coefficients for both U(VI) and Np(V) on hydrothermally altered concrete are significantly lower than those on untreated concrete. The partition of both U(VI) and Np(V) are pH dependent, although the pH dependence does not necessarily reflect precipitation of U and Np-bearing phases. Both sorption and precipitation appear to control partitioning of U to concrete; sorption alone appears to control partition of Np to concrete.The presence of 0.01 M carbonate species in solution decreases K d of U(VI) for both treated and untreated concrete to a similar value of ~500 mL/g indicating a significant impact on U(VI) sorption. However, the presence of carbonate only reduced the K d of Np(V) by one order of magnitude or less. Our results also show that significant amounts of uranium and neptunium in aqueous phase are associated with suspended colloids. In most cases, the colloid-associated uranium and neptunium are the predominant species in the liquid phase. X-ray absorption spectroscopy * Authors to whom correspondence should be addressed.analysis of U/concrete mixtures at different pHs and times indicate that uranyl ions are partitioned as monomeric species on untreated concrete, but oligomeric species on hydrothermally altered concrete. Similar analysis of Np/concrete mixtures shows that about half of the partitioned Np(V) is reduced to Np(IV) over a period of 6 months.

EXAFS investigation on U(VI) immobilization in hardened cement paste: influence of experimental conditions on speciation

“…EXAFS studies on U(VI)-loaded untreated and hydrothermally treated HCP samples further revealed preservation of the UO 2 2+ moiety in all samples, indicating that the +VI redox state was stable [5,6]. Samples with U(VI)-sorbed onto hydrothermally altered concrete also suggested cluster formation or surface precipitation as indicated by the presence of U−U backscattering contributions in the EXAFS spectra [6].…”

“…Samples with U(VI)-sorbed onto hydrothermally altered concrete also suggested cluster formation or surface precipitation as indicated by the presence of U−U backscattering contributions in the EXAFS spectra [6]. The chemical nature of the U(VI) species immobilized in HCP has not yet been identified.…”

Characterization of Plutonium, Neptunium, Strontium on Manganese Solids from Permanganate Reduction

“…65 These researchers used XANES absorption edge determinations and the degree of "axial" O bonding from the EXAFS to estimate the amount of Np(V). They reported that most of the Np(V) reduced over time in the solution or concrete slurries that had pH values of 9.3 through 12.3.…”

EXAFS study of U(VI) uptake by calcium silicate hydrates

“…Although X-ray absorption fine structure (XAFS) spectroscopy has become an increasingly important tool for investigating immobilization processes in cementitious systems (e.g., [14][15][16][17][18][19][20][21][22][23][24][25][26]), very few studies have used XAFS to develop a molecular-level understanding of the U(VI) uptake by cementitious materials [27,28]. In these studies the coordination environment of U(VI)-doped untreated and hydrothermally treated concrete was probed in the alkaline pH range (pH 9-12).…”