The aqueous geochemistry of the rare-earth elements and yttrium: 1. Review of available low-temperature data for inorganic complexes and the inorganic REE speciation of natural waters

Abstract

Literature data on the nature and thermodynamics of inorganic complex species of the REE and Y have been critically reviewed. Theoretical considerations suggest that trivalent REE and Y should exhibit strong, predominantly electrostatic complexing with “hard” ligands such as fluoride, sulfate, phosphate, carbonate and hydroxide and this is borne out by the available experimental data. Complexing of these metals with chloride and nitrate is moderately weak and with ammonia and bisulfide is extremely weak to nonexistent. A considerable amount of concordant thermodynamic data are available at 25°C for the sulfate complexes LnSO + 4 and Ln(SO 4) − 2, the fluoride complexes LnF 2+, LnF + 2 and LnF 0 3, the nitrate complex LnNO 2+ 3 and the chloride complex LnCl 2+, where Ln signifies the REE or Y. There are much fewer reliable data available for the hydroxide, carbonate and phosphate complexes and in fact, the exact stoichiometries of REE and Y complexes with these ligands are still somewhat uncertain. However, it appears that the LnOH 2+, LnCO + 3, Ln(CO 3) − 2, LnHCO 2+ 3 and LnH 2PO 2+ 4 complexes have been identified. Calculation of the speciation of Eu ( βEu = 10 −7 m) in a typical groundwater with βSO 2− 4 = 10 −4 m, βC − = 2·10 −4 m, βF − = 10 −6 m, βCO 2− 3 = 10 −4 m, βNO − 3 = 10 −4 m and βPO 3− 4 = 10 −6 m shows that the simple ion and the sulfate complexes are most important at acidic pH and that the carbonate complexes become predominant at near-neutral to basic pH. Even in relatively saline waters, chloride complexes do not account for a significant fraction of REE in solution and are completely negligible in most surface waters and groundwaters. Nitrate complexes are also negligible even in environments where the concentration of this ligand is artificially high due to pollution. Phosphate and fluoride complexes can attain importance where concentrations of these ligands are somewhat anomalous (i.e. 2–4 orders of magnitude higher than in the model groundwater) due to either natural or anthropogenic factors. Data on the nature and stabilities of REE and Y complexes at low temperature are of critical importance to those concerned with safe nuclear waste disposal, geochemical exploration for REE and Y deposits, and the use of REE and Y as tracers in seawater and fresh water. The information most needed at the present time includes: (1) reliable hydrolysis constants; (2) additional data on phosphate, carbonate and the higher fluoride complexes; (3) data on mixed ligand complexes; and (4) more complete data on the complexation behavior of the +4 and +2 oxidation states of the REE.