Saccharide-mediated transformation of Ce during the formation of manganese (hydr)oxide

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Ce anomaly is an important indicator for many geochemical processes. Although the role of strong ligand such as siderophore desferrioxamine B (DFOB) in facilitating negative Ce anomaly formation on Mn (hydr)oxide has been well studied, the effect of ubiquitous weak ligands such as polysaccharides on the mobility of Ce have not been considered. Here, we investigated the effects of polysaccharides with different molecular weights on Mn (hydr)oxide formation as well as the distribution coefficients of rare-earth elements (REEs) at the solid–liquid interface. In coprecipitation experiments, REEs distribution coefficients between Mn (hydr)oxide and the solution showed a negative Ce anomaly during coprecipitation of Mn in the presence of dextran with molecular weight of 5000 Dalton (Dextran 5 k). The molecular size of the saccharides affected the crystal growth of Mn (hydr)oxide. Mn (hydr)oxide was deposited on a matrix of polysaccharide molecules to form a metal–organic framework in the presence of dextran. The crystal size decreased with an increase in the length of the saccharide molecular chain. As a control test, when well crystallized δ-MnO2 was used as an adsorbent, the distribution coefficients of the REEs showed a positive Ce anomaly in the presence of dextran 5 k. Based on the results, we suggest that the mechanism by which saccharides affect the sorption behavior of REEs on Mn (hydr)oxide is different from the reported mechanisms of strong metal-binding ligands such as DFOB. Saccharide-mediated electron transfer may occur between Mn (hydr)oxides and Ce(III), resulting in the formation of an organic-Ce(IV) complex in solution. Our results imply that ubiquitous weak ligands such as polysaccharides may significantly affect the mobility of Ce largely due to the association with Mn oxides, which are widespread natural electron acceptors.