『Abstract
Rare earth elements (REE) release rates were measured during
the open-system dissolution of natural apatite at 25℃, 2.8<pH<9.2,
and at ionic strengths ranging from 0.001 to 0.02 mol/kg. All
inlet solutions were Ca, REE, and P free. In agreement with previous
studies, apatite dissolution rates determined from outlet solution
Ca concentrations decrease monotonically with increasing pH. Outlet
solution molar REE/Ca ratios differ from that of the dissolving
apatite during the experiments, this behaviour is interpreted
to stem from dissolution or precipitation of minor quantities
of rhabdophane (REE(PO4)・nH2O).
This interpretation is supported by solute speciation calculations,
which indicate that REE are retained in the solid phase when the
outlet fluids are supersaturated with respect to Nd-rhabdophane
(Ksp≒1024.5±0.5), but preferentially
released when the outlet fluids are undersaturated with respect
to Nd-rhabdophane. The relative concentrations of the REE in the
outlet solutions suggest that the secondary rhabdophane is slightly
enriched in light REE (Ce to Eu). The results of this study suggest
that rhabdophane dissolution and precipitation during apatite-fluid
interaction plays a major role in controlling surface and ground
water REE signatures.
Keywords: Apatite dissolution; REE signatures; Rhabdophane precipitation』
1. Introduction
2. Theoretical background
3. Materials and methods
4. Results
5. Discussion
5.1. The mechanism of REE release from apatite
5.2. Implications for REE content of natural waters
5.3. Implications for the storage of radioactive actinides
6. Conclusions
Acknowledgements
References