New insights into cerium anomalies in organic-rich alkaline waters

Cerium anomaly development in natural waters is commonly related to the mechanism of oxidative scavenging of tetravalent cerium by iron and/or manganese oxides. In this study, a new mechanism for the development of Ce anomalies is described, which combines the oxidation of Ce at high pH by carbonate and the preferential adsorption of Ce(IV) to humic acids. This new mechanism was experimentally elucidated by studying the competition between carbonate and humic acids for complexing rare earth elements (REE). These experiments showed that above pH 8.2, 8.6 or 8.7 (with decreasing alkalinity from 10− 2 to 10− 3 mol L− 1), Ce(III) is readily oxidized into Ce(IV), which is then preferentially adsorbed onto humic acids. This preferential uptake of Ce results in the development of a negative Ce anomaly (as low as 0.05) in the “truly” dissolved part of the solution (i.e., < 5 kDa), and a complementary positive anomaly (up to 1.22) occurs in the organic colloidal fraction. The positive and negative Ce anomalies remained hidden until the organic and inorganic fractions of the solution were separated. Therefore, Ce anomalies became apparent only after ultrafiltration of the waters and the subsequent isolation of the two fractions. The Ce anomaly is thus more likely to be a proxy of redox conditions in ultrafiltered waters than in unfiltered waters or in waters filtered to < 0.2 µm. The removal (e.g., by coagulation and/or flocculation) of organic molecules in organic-rich alkaline waters might lead to the development of a negative Ce anomaly in the resulting organic-poor waters. In contrast, some organic-poor alkaline waters may develop positive Ce anomalies due to preferential complexation of Ce(IV) by dissolved carbonate.