Scavenging of dissolved yttrium and rare earths by precipitating iron oxyhydroxide: experimental evidence for Ce oxidation, Y-Ho fractionation, and lanthanide tetrad effect

Abstract

Scavenging experiments were performed at pH 3.6 to 6.2 with synthetic solutions containing dissolved Fe (≈7 mg/L), Rare Earths and Yttrium (ΣREY: ≈61 μg/L) in a matrix of 0.01 M HCl, and with natural water from Nishiki-numa spring, Japan, with the aim to study the fractionation that results from the interaction of dissolved REY with precipitating Fe oxyhydroxide. All patterns of apparent REY distribution coefficients between Fe oxyhydroxide and solution, appD REY, show negative anomalies at Y, La, and Gd, and the M-type lanthanide tetrad effect. These features become more pronounced with increasing pH. At pH ≤ 4.6, positive anomalies of appD Ce give evidence for oxidative scavenging of Ce on the Fe oxyhydroxide. A time-series experiment at pH 3.5 suggests that a stationary exchange equilibrium for the REY(III) is reached within less than 6.5 min, whereas the Ce(IV)/Ce(III) redox-equilibrium is not attained before 120 min. Oxidation rates of Ce(III) were found to decrease significantly during the first minutes after Fe oxyhydroxide formation, indicating that the capacity for Ce(III) oxidation is drastically higher in systems in which fresh Fe oxyhydroxides precipitate than in systems in which dissolved REY interact with pre-formed Fe oxyhydroxides. This additionally complicates the use of Ce anomalies of natural precipitates as quantitative paleo-redox-proxies. Radius-independent fractionation of REY(III) is very similar in experiments using synthetic solutions and natural water, despite the additional precipitation of hydrous Al oxides from the latter. Because there is no change of solution-complexation (speciation) along the REY series, radius-independent fractionation of REY(III) is likely due to differences between the stabilities of surface-complexes of the individual members of the REY series. The results presented here are an experimental verification of a natural process that may produce the lanthanide tetrad effect in geological samples.