Preparation and alkali-metal ion extraction/insertion reactions with nanofibrous manganese oxide having 2 x 4 tunnel structure

Sodium manganese oxide with a 2 x 4 tunnel structure (Na-2 x 4) was hydrothermally synthesized from Na-birnessite material in NaOH solutions at relatively low temperatures. The Na+ extraction and alkali-metal ion insertion reactions were investigated by chemical and X-ray analyses, SEM observation, FT-IR spectroscopy, TG-DTA analyses, pH titration, and K-d measurements. A considerable amount (81%) of Na+ ions in the 2 x 4 tunnel sites were topotatically extracted by acid treatment, accompanied by a slight change of the lattice parameters. Alkali-metal ions could be inserted into the 2 x 4 tunnel sites of the acid-treated sample (Na-2 x 4 (H)), mainly by an ion-exchange mechanism. The pH titration study showed that Na-2 x 4 (H) had simple monobasic acid behavior toward alkali-metal ions with a relatively low exchange capacity below pH 10; the exchange capacity increased in the order Cs+ < Li+ < Na+ < K+ congruent to Rb+ at pH 6. The distribution coefficient (K-d) study on Na-2 x 4 (H) showed the selectivity sequence of Cs+ < Li+ < K+ < Rb+ around pH 4. These results suggest that manganese oxide with a 2 x 4 tunnel structure has a weak ion-sieve property for the adsorption of ions with an effective ionic radius around 1.7 Angstrom.