Adsorption of REE(III)-humate complexes onto MnO2: Experimental evidence for cerium anomaly and lanthanide tetrad effect inhibition

International audienceAdsorption experiments of rare-earths elements (REE) onto MnO2 have been conducted to evaluate the effects of REE organic complexation on both REE(III) adsorption kinetics and Ce(III) oxidation rates. Two types of aqueous solutions-NaCl and NaNO3-were tested at pH 5 and 7.5. REE(III) adsorption kinetics and Ce(III) oxidation rates is evidenced to depend strongly on REE speciation. Time-series experiments indicate that a stationary exchange equilibrium is reached within less than 10 h when dissolved REE(III) occurs as free species whereas steady state is not reached before 10 d when REE occur as REE-humate complexes. Humate complexation also results in an inhibition of positive Ce anomalies and M-type lanthanide tetrad effect development in REE patterns. Monitoring of dissolved organic carbon (DOC) concentrations showed that ratios of DOC d REE d K log K log organic ratios were close to 1.0, implying that the REE(III) and humate remained bound to each other. The lack of Ce anomaly development when REE occurred as REE-humate complexes seems to arise from the humate shielding of the MnO2 surface and Ce(IV) preferential adsorption inhibition. The suppression of the lanthanide tetrad effect suggests that REE(III)-humate complexes is bound to MnO2 by the humate side (anionic adsorption). Thus, Ce cannot be used as a reliable proxy of redox conditions, in either organic-rich waters, or precipitates formed at equilibrium with organic-rich waters. Furthermore, they explain why despite the Author to whom correspondence should be addressed (melanie.davranche@univ-rennes1.fr) 2 development of strongly oxidizing conditions and the presence of MnO2 in the aquifer, no (or insignificant) negative Ce anomalies are observed in organic-rich waters