Preparation and Alkali-Metal Ion Extraction/Insertion Reactions with Nanofibrous Manganese Oxide Having 2 × 4 Tunnel Structure

Sodium manganese oxide with a 2 × 4 tunnel structure (Na-2 × 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 × 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 × 4 tunnel sites of the acid-treated sample (Na-2 × 4 (H)), mainly by an ion-exchange mechanism. The pH titration study showed that Na-2 × 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 + ≅ Rb + at pH 6. The distribution coefficient ( K d ) study on Na-2 × 4 (H) showed the selectivity sequence of Cs + < Li + < K + < Rb + around pH 4. These results suggest that manganese oxide with a 2 × 4 tunnel structure has a weak ion-sieve property for the adsorption of ions with an effective ionic radius around 1.7 Å.