Conditions for the formation of pure birnessite during the oxidation of Mn(II) cations in aqueous alkaline medium

TL;DR

This study investigates the conditions necessary for synthesizing pure birnessite from Mn(II) in aqueous alkaline solutions, emphasizing the roles of dissolved oxygen and counter-ions, and characterizes the resulting mineral's properties.

Contribution

It provides new insights into the synthesis process of pure birnessite, highlighting the importance of dissolved oxygen and counter-ions in its formation.

Findings

Dissolved oxygen is crucial for birnessite formation.

Pure birnessite can be synthesized under specific conditions.

Counter-ions influence the mineral's properties.

Abstract

Birnessite was synthetized through redox reaction by mixing MnO4-, Mn2+ and OH- solutions. The Mn(VII): Mn(II) ratio of 0.33 was chosen and three methods were used consisting in a quick mixing under vigorous stirring of two of the three reagents and then on the dropwise addition of the third one. The obtained solids were characterized by XRD, FTIR and XPS spectroscopies. Their average oxidation states were determined from ICP and CEC measurements while their surface properties were investigated by XPS. This study provides an increased understanding of the importance of dissolved oxygen in the formation of birnessite and hausmannite and shows the ways to obtain pure birnessite. The role of counter-ion ie. Na+ or K+ was also examined.