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

Area of Science:

• Geochemistry
• Environmental Science
• Mineralogy

Background:

• Rare earth elements (REEs) are vital for clean energy technologies, necessitating research into their environmental occurrence and transport.
• Cerium (Ce) oxidation by manganese oxides (MnO2) creates Ce anomalies, serving as a proxy for biogeochemical processes and Earth's oxygenation history.
• A precise kinetic rate law for Ce(III) oxidation by δ-MnO2 was previously lacking, hindering quantitative applications.

Purpose of the Study:

• To determine the reaction orders and rate constant for the oxidation of Ce(III) by δ-MnO2.
• To develop a kinetic model to differentiate Ce adsorption and oxidation processes.
• To elucidate the multi-step mechanism of Ce(III) oxidation by δ-MnO2.

Main Methods:

• Utilized the initial rate method to determine reaction kinetics.
• Employed kinetic modeling with redox-inert analogues (La, Nd) to distinguish adsorption and oxidation.
• Investigated the multi-step reaction mechanism including adsorption, oxidation, and precipitation.

Main Results:

• The reaction follows a 2.5th order overall: first order for Ce(III) and δ-MnO2, and 0.5th order for OH-.
• The overall rate constant (k) was determined to be 1.4 × 10^6 L^(3/2) mol^(-1/2) g^(-1) h^(-1).
• Ce(III) oxidation by δ-MnO2 involves sequential steps: adsorption, oxidation, and Ce(IV)O2 precipitation.

Conclusions:

• The established rate law provides a quantitative basis for using Ce anomalies in geochemical and paleo-redox studies.
• Understanding the multi-step mechanism refines the interpretation of Ce anomalies as tracers for biogeochemical processes.
• This research enhances the application of Ce anomalies for investigating Earth's past oxygenation events and REE mobility.