Birnessites with different average manganese oxidation states synthesized, characterized, and transformed to todorokite at atmospheric pressure

Todorokite is a common manganese oxide mineral, with a tunnel structure, found in Earth surface environments, and is easily synthesized from layered birnessite. The aim of the current study was to prepare birnessites with different average manganese oxidation states (AOS) by controlling the MnO /Mn ratio in concentrated NaOH or KOH. A series of (Na,K)-birnessites, Na-birnessites, and K-birnessites with different AOS was synthesized successfully in strongly alkaline media. The (Na,K)-birnessites and Na-birnessites prepared in NaOH clearly contained both large (500-1000 nm) and small (40-400 nm), plate-shaped crystallites. The K-birnessites prepared in KOH media consisted mostly of irregular (100-200 nm), plate-shaped crystallites. The degree of transformation of birnessite to todorokite at atmospheric pressure decreased as the AOS values of (Na,K)-birnessites and Na-birnessites increased from 3.51 to 3.80. No todorokite was present when a Na-birnessite with an AOS value of 3.87 was used as the precursor. Pyrophosphate, which is known to form strong complexes with Mn at a pH range of 1-8, was added to a suspension of (Na,K)-birnessites in order to sequester the available Mn in (Na,K)-birnessites. Removal of Mn from birnessite MnO layers by pyrophosphate restricted transformation to todorokite no-(Na,K)-birnessite transformed to todorokite after pyrophosphate treatment. The interlayer K initially within (Na,K)-birnessites could not be completely ion-exchanged with Mg to form todorokite at atmospheric pressure. No todorokite was forthcoming from K-birnessites even from those with small AOS values (3.50)