YREE sorption on hydrous manganese oxide (MnOx) in 0.5 M NaCl

Cerium is the only member of the yttrium and rare earth element (YREE) series that can be oxidized in natural waters from Ce(III) to less soluble Ce(IV), causing anomalous sorption behavior with respect to its strictly trivalent YREE neighbors. Sedimentary Ce anomaly records have been interpreted in terms of episodic shifts in the bottom water oxygenation of the paleo-ocean. However, Ce anomalies also form due to catalytic Ce oxidation on certain sorbent surfaces regardless of ambient redox conditions, thus creating a ‘preformed’ signal that may instead reflect variations in the composition of settling particles. We investigate YREE sorption in 0.5 M NaCl on mixtures of three major components of marine particulate matter: Fe and Mn oxides, and algal debris. Here we report our results for one pure endmember, hydrous manganese oxide (MnOx). Batch experiments with freshly prepared MnOx were conducted under nitrogen atmosphere, to prevent aerobic Ce oxidation and YREE complexation with carbonate, over a range of pH (4-8) at 25.0±0.1°C. After at least 6 hours of equilibration at each pH, solution samples were filtered to 0.22 μm and dissolved YREE concentrations analyzed by ICP-MS to determine their solid/solution distribution coefficient, K. Under the same experimental conditions, log K increases less steeply with pH for MnOx than for hydrous ferric oxide (HFO). This may result from the lower pHzpc of MnOx as well as its higher tendency than HFO to form bidentate edge-sharing YREE surface complexes, as determined by others using X-ray Absorption Fine Structure (XAFS) spectroscopy. A non-electrostatic surface complexation model is being developed to further elucidate these findings. Preferential Ce sorption, implying catalytic Ce oxidation at the mineral-water interface, was observed on MnOx but never on HFO or organic surfaces, in agreement with prior research. This may be related to the generally higher Gibbs free energy gain associated with oxidation reactions in which MnOx is the electron acceptor, although some large organic ligands, such as siderophores, have also been reported to catalyze Ce(III) oxidation. The enhanced Ce removal in anoxic 0.5 M NaCl solutions increases with pH from about 10- to 100-fold relative to the other YREEs, which show comparatively little fractionation among themselves. Sedimentary Ce anomalies are therefore unlikely to provide a reliable and lasting record of bottom water oxygenation when Mn oxides are an important constituent of settling particles and their interpretation as a proxy of paleo-redox conditions should be undertaken with appropriate caution.