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

## TL;DR

This study determines the rate law for the oxidation of Ce(III) by manganese oxides, offering insights into how cerium anomalies can track biogeochemical processes and Earth's oxygenation history.

## Contribution

The study provides a detailed kinetic rate law for Ce(III) oxidation by δ-MnO2, including reaction orders and a rate constant.

## Key findings

- The reaction follows first order for Ce(III) and δ-MnO2, and 0.5th order for OH–, with an overall 2.5th order.
- The rate constant was calculated as 1.4 × 10⁶ L³/² mol⁻¹/² g⁻¹ h⁻¹.
- Ce(III) oxidation involves adsorption, oxidation, and surface precipitation steps.

## Abstract

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 L3/2 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 CeIVO2. Our findings provide important insights into the quantitative
applications of Ce anomaly as a proxy to investigate various biogeochemical
processes.