Determination of stability constants for the mono- and difluoro-complexes of Y and the REE, using a cation-exchange resin and ICP-MS

Stability constants of the form (F)beta(n) = [MFn3-n][M3+](-1)[F-](-n) (where brackets denote the concentrations of complexes MFn3-n, free metal ions M3+ and free fluoride F-) were determined for the mono- and difluoro-complexes (n = 1, 2) of yttrium and the rare earth elements (Y+REE). A new method was developed, using equilibration with Bio-Rad AG 50W-X2 cation-exchange resin and analysis of the solution phase using inductively-coupled plasma mass spectrometry (ICP-MS). This method enabled us to measure all stability constants under identical conditions and is, therefore, especially sensitive to subtle inter-element variations in (F)beta(1) and (F)beta(2). The pattern of log (F)beta(1) differs distinctly from the majority of those in the literature and contains more structure. The set of (F)beta(1) values that is commonly used for modeling REE speciation in natural waters appears to be in error for REE heavier than Eu. The pattern of log (F)beta(2) is similar in shape to that of log P-F(1), but substantially deviates for Ce. This is reflected in the pattern of stepwise stability constant ratios across the Y + REE series: the average value of K-2/K-1 = (F)beta(2)/((F)beta(1))(2) is 0.07 +/- 0.03 for Y + REE excluding Ce, whereas for Ce it is 0.49. While anomalous behaviour of Ce has been reported for many environments and is known to be caused by its unique redox chemistry, only Ce(III) should be present under the conditions of these experiments. (C) 1999 Elsevier Science Ltd. All rights reserved.