Cm complexation with aqueous phosphates at elevated temperatures


Cm complexation with aqueous phosphates at elevated temperatures

Huittinen, N.; Jordan, N.; Demnitz, M.; Lösch, H.; Starke, S.; Brendler, V.

Orthophosphate ions (H2PO4-, HPO42-, and PO43-) are ubiquitous in the environment and may originate from the natural decomposition of rocks and minerals (e.g. monazite or apatite), agricultural runoff, or from wastewater treatment plants. Furthermore, the potential use of monazite (LnPO4) ceramics for the immobilization of specific actinide-containing waste streams may become an important source of phosphates in the future [1–2]. Among the inorganic ligands, phosphates are strong complexants and can be expected to influence the speciation of dissolved radioactive contaminants when present in solution. However, very little data is available on the complexation of especially actinides with aqueous phosphates, even though these complexation reactions precede any aqueous synthesis of monazite ceramics and are expected to occur in natural waters as well as in the proximity of monazite-containing high-level waste repositories. The existing data also suffers from an almost systematic absence of independent spectroscopic validation of the stoichiometry of the proposed complexes.
In the present work, time-resolved laser fluorescence spectroscopy (TRLFS) has been employed to study the complexation of the actinide Cm3+ (5×10-7 M) as a function of total phosphate concentration (0–0.5 M Σ(PO4)) in the temperature regime 25–80°C, using NaClO4 as a background electrolyte (0.5–2.1 M). The studies have been conducted in the acidic pH-range ( log[H+] = 1–2.5) to avoid precipitation of solid Cm rhabdophane (CmPO4×nH2O). Under these experimental conditions, the trivalent actinide cation was found to form a complex with the anionic H2PO4- species, i.e. CmH2PO42+ and Cm(H2PO4)2+, depending on the solution pH and the total phosphate concentration, Figure 1.
The complexation reaction occurs at lower total phosphate concentration when increasing the ionic strength or the temperature. Using specific ion interaction theory (SIT) and the Van’t Hoff equation, obtained conditional constants at varying ionic strengths and temperatures have been extrapolated to infinite dilution (logβ0) and values for the enthalpy ΔRH° (assumed constant between 25 to 80 °C) and entropy ΔRS° of reaction have been acquired. The results of the extrapolations are shown exemplarily for the CmH2PO42+ species in Figure 2.
The new thermodynamic data derived in this fundamental study will contribute to a fundamental process understanding necessary to critically assess the environmental fate of actinides in the environment.

  • Lecture (Conference)
    Radiation in the environment – scientific achievements and challenges for the society, 16.-17.04.2018, Helsinki, Finland

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Publ.-Id: 28197