Comment on: “Origin, timing, and temperature of secondary calcite-silica mineral formation at Yucca Mountain, Nevada” by N. S. F. Wilson, J. S. Cline, and Y. V. Amelin

Yucca Mountain in southern Nevada is being evaluated as a potential site for the geological disposal of high-level nuclear waste. A reliable assessment of the future performance of the repository will require detailed paleohydrogeological information. Hydrogenic secondary minerals from the vadose zone of Yucca Mountain are being studied as paleohydrogeological indicators. A phenomenological model envisaging the deposition of secondary minerals by meteoric fluids infiltrating downward though the vadose zone was proposed in the reviewed paper. Our evaluation reveals that the model is not supported by empiric evidence reported in the paper.     

Comment on “Hydrolysis of neptunium(V) at variable temperatures (10-85 °C)” by L. Rao, T.G. Srinivasan, A.Yu. Garnov, P. Zanonato, P. Di Bernardo, and A. Bismondo

In a recent study [Rao, L., Srinivasan, T.G., Garnov, A.Yu., Zanonato, P., Di Bernardo, P., Bismondo, A., 2004. Hydrolysis of neptunium(V) at variable temperatures (10-85 °C). Geochim. Cosmochim. Acta68, 4821-4830.] the hydrolysis of Np(V) was investigated at 10-85 °C by absorption spectroscopy, potentiometry, and microcalorimetry along the titration of Np(V) solutions with tetramethylammonium hydroxide up to pH 10. However, there is strong evidence that the precautions to avoid competing reactions with carbonate were not sufficient and that the measured effects are not caused by the formation of Np(V) hydroxide complexes but primarily by the formation of Np(V) carbonate complexes. The reported equilibrium constants, enthalpies, entropies, and heat capacities for the complexes NpO₂OH(aq) and are severely in error and must not be used for the geochemical modeling of neptunium. If the hydrolysis constants reported by Rao et al. [Rao, L., Srinivasan, T.G., Garnov, A.Yu., Zanonato, P., Di Bernardo, P., Bismondo, A., 2004. Hydrolysis of neptunium(V) at variable temperatures (10-85 °C). Geochim. Cosmochim. Acta68, 4821-4830] are used to calculate neptunium solubilities in alkaline solutions relevant for nuclear waste repositories, the Np(V) concentrations are overestimated by orders of magnitude.