Developing a Rate Law for Ce(III) Oxidation by Manganese Oxides

Rare earth elements (REEs) are critical minerals that are indispensable for clean energy technologies. Understanding REE occurrence and transport in natural environments is important for the prediction and identification of REE resources. Cerium (Ce) is a rare earth element that exhibits multiple oxidation states. The oxidation of dissolved Ce(III) by manganese oxides (MnO₂) and the resulting Ce anomaly is used as an indicator for tracing biogeochemical processes controlling REE transport and mobility, as well as a paleo-redox proxy for understanding Earth's oxygenation events. However, a detailed kinetic rate law for this process is still lacking. This study determines the reaction orders and rate constant for Ce(III) oxidation by δ-MnO₂ using the initial rate method. The overall reaction follows a first order for Ce(III) and δ-MnO₂ and a 0.5th order for OH^-, resulting in an overall 2.5th order. The calculated overall rate constant (k) was 1.4 × 10⁶ L^3/2 mol^-1/2 g^-1 h^-1. Kinetic modeling was employed to distinguish Ce adsorption and oxidation by using redox-inert Ce-analogues La and Nd. Our experimental and kinetic modeling results suggest that Ce(III) oxidation by δ-MnO₂ occurs in multiple steps: the adsorption of Ce(III) on the δ-MnO₂ surface, the oxidation of Ce(III), and surface precipitation of Ce^IVO₂. Our findings provide important insights into the quantitative applications of Ce anomaly as a proxy to investigate various biogeochemical processes.