A New Type of Manganese Oxide (MnO2·0.5H2O) Derived from Li1.6Mn1.6O4 and Its Lithium Ion-Sieve Properties

A new type of adsorbent, MnO2·0.5H2O, was synthesized from Li1.6Mn1.6O4 followed by acid treatment. The hydrothermal reaction of monoclinic type γ-MnOOH with LiOH solution at 120 °C resulted in a H+/Li+ ion exchange, giving orthorhombic LiMnO2. The orthorhombic LiMnO2 was transformed to cubic Li1.6Mn1.6O4 (Li2Mn2O5) at a temperature higher than 400 °C. The conversion of Li1.6Mn1.6O4 to MnO2·0.5H2O by acid treatment resulted in contraction of the lattice constant a0 from 8.14 to 8.05 Å with little dissolution of manganese. The new adsorbent showed a markedly high ion-exchange capacity (7.5 mmol/g) for lithium ions due to its ion-sieve property. The pH titration study suggested the formation of large numbers of uniform sorption sites having relatively high acidity. The uptake of Li+ ions from lithium-enriched seawater was found to be 5.3 mmol/g (37 mg/g), which is the maximum among the adsorbents studied to date.