| Popis: |
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 phosphate fertilizer plants. Solid phosphate monazites are one of the most important ore bodies for the recovery of REE[1], while future monazite applications may involve their use as immobilization matrices for specific HLW streams.[2] Among the inorganic ligands, phosphates are strong complexants and can be expected to influence the speciation of dissolved contaminants when present in solution. However, very little data is available on the complexation of actinides and lanthanides with aqueous phosphates, even though they precede any aqueous synthesis of monazite ceramics and are expected to occur in natural waters, in the proximity of monazite-containing high-level waste repositories as well as in the processing of rare earth elements. The existing data also suffers from an almost complete absence of independent spectroscopic validation of the stoichiometry of the proposed complexes. Furthermore, there are no studies dealing with the impact of elevated temperatures on An/Ln complexation with aqueous phosphates, despite the relevance of high temperatures both in the proximity of heat-generating HLW repositories and in REE leaching from monazites minerals. In this study, the complexation of Cm(III) (5×10−7 M) and Eu(III) (5×10−6 M) was investigated by means of laser-induced luminescence spectroscopy as a function of the total phosphate concentration (0–0.5 M Σ(PO4)) using NaClO4 as a background electrolyte (I = 0.5–3.1 M), in the temperature range from 25 to 80 °C. Experiments were conducted in the acidic pH range (−log[H+] = 1 and 2.5) to avoid the precipitation of solid Cm/Eu rhabdophane (MePO4×nH2O). The formation of MeH2PO42+, Me(H2PO4)2+, and Me(HPO4)+ was observed, depending on the solution pH and the total phosphate concentration. Upon increasing both the ionic strength and temperature, complexation was found to be promoted.[3] By applying the specific ion interaction theory (SIT), the obtained conditional constants at varying ionic strengths and temperatures were extrapolated to infinite dilution (log β0). The molal standard enthalpy of reaction ΔRHm° (assumed constant between 25 to 80 °C) and molal standard entropy of reaction ΔRSm° were derived by using the Van’t Hoff equation. The new thermodynamic data derived in this fundamental study will support the optimization of technological strategies applied to access raw materials and contribute to a fundamental process understanding necessary to critically assess the environmental fate of actinides and lanthanides. |
