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

Scavenging experiments were performed at pH 3.6 to 6.2 with synthetic solut ions containing dissolved Fe (approximate to 7 mg/L), Rare Earths and Yttri um (Sigma REY: approximate to 61 mu g/L) in a matrix of 0.01 M HCl, and wit h natural water from Nishiki-numa spring, Japan, with the aim to study the fractionation that results from the interaction of dissolved REY with preci pitating Fe oxyhydroxide. All patterns of apparent REY distribution coeffic ients between Fe oxyhydroxide and solution, D-app(REY), show negative anoma lies at Y, La, and Gd, and the M-type lanthanide tetrad effect. These featu res become more pronounced with increasing pH. At pH less than or equal to 4.6, positive anomalies of D-app(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 l ess than 6.5 min, whereas the Ce(IV)/Ce(III) redox-equilibrium is not attai ned before 120 min. Oxidation rates of Ce(III) were found to decrease signi ficantly during the first minutes after Fe oxyhydroxide formation, indicati ng that the capacity for Ce(III) oxidation is drastically higher in systems in which fresh Fe oxyhydroxides precipitate than in systems in which disso lved REY interact with pre-formed Fe oxyhydroxides. This additionally compl icates the use of Ce anomalies of natural precipitates as quantitative pale o-redoxproxies. Radius-independent fractionation of REY(III) is very simila r in experiments using synthetic solutions and natural water, despite the a dditional precipitation of hydrous Al oxides from the latter. Because there is no change of solution-complexation (speciation) along the REY series, r adius-independent fractionation of REY(III) is likely due to differences be tween 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 geologi cal samples. Copyright (C) 1999 Elsevier Science Ltd.