Impact of humate complexation on the adsorption of REE onto Fe oxyhydroxide

Adsorption experiments of rare-earth elements (REE) onto hydrous ferric oxide (HFO) were performed to evaluate the impact of organic complexation on both REE(III) adsorption and the Ce(III) oxidation rate. Scavenging experiments were performed at pH 5.2 with NaCl and NaNO3 solutions containing either free REE (III) or REE(III)-humate complexes. The log K(d)(REE) patterns obtained from HFO suspensions exhibit a slight positive Ce anomaly and an M-type lanthanide tetrad effect, in contrast with the partitioning between REE(III)-humate complexes and HFO, which yields completely flat distribution patterns. The "organic" partitioning runs yield log K(d)(REEorganic)/log K(d)(DOC) ratios (DOC = dissolved organic carbon) close to 1.0, implying that the REE(III) and humate remain bound to each other during the adsorption experiment. The lack of any positive Ce anomaly or M-type lanthanide tetrad effect in the organic experiments seems to reflect an anionic adsorption of the REE-humate complex. Adsorption onto HFO takes place via the humate side of the REE(III)-humate complexes. The oxidation of Ce(III) by Fe(III) and the proportion of surface hydroxyl groups coordinated to REE(III) at the HFO surface are the two most commonly invoked processes for explaining the development of positive Ce anomalies and the M-type tetrad lanthanide effect. However, such processes cannot proceed since the REE are not in direct contact with the HFO suspensions, the latter being shielded by PHA. The present results further complicate the use of Ce anomalies as reliable paleoredox proxies in natural precipitates. They are also further demonstration that organic matter may inhibit the lanthanide tetrad effect in geological samples.