1. J. D. Jorgenson, U.S. Department of the Interior, U.S. Geological Survey, U.S. (2000).
2. B. Depuydt, M. De Jonghe, W. De Baets, I. Romandic, A. Theuwis, C. Quaeyhaegens, C. Deguet, T. Akatsu and F. Letertre, Chapter 1-Germanium Materials, Germanium-Based Technologies, Elsevier, Oxford, pp. 11-I (2007).
3. S. Nusen, Z. Zhu, T. Chairuangsri and C.Y. Cheng, Hydrometallurgy, 151, 122 (2015).
4. F. Arroyo and C. Fernández-Pereira, Ind. Eng. Chem. Res., 47, 3186 (2008).
5. D. D. Harbuck, J.C. Judd and D.V. Behunin, Solvent Extraction and Ion Exchange, 9, 383 (1991).