REE(III) ADSORPTION ONTO MN DIOXIDE (δ-MNO2) AND FE OXYHYDROXIDE: CE(III) OXIDATION BY δ-MNO2

Abstract

The adsorption experiments of rare-earth elements (REE) onto δ-MnO2 and Fe oxyhydroxide precipitates suspended in aqueous NaCl solutions have been conducted. The distribution coefficients, Kd(REE: precipitate/solution) between suspended δ-MnO2 and solutions show extremely large positive Ce anomalies unlike those between Fe oxyhydroxide precipitates and solutions. The iodometric titration of δ-MnO2 precipitates containing different amounts of scavenged Ce reveals that Ce(III) is oxidized to Ce(IV) by δ-MnO2. A convex tetrad effect observed in the series variations of log Kd(REE) between suspending δ-MnO2 and solutions becomes conspicuous with increasing pH in the range of 4.8 < pH < 6.8. This reflects the changing ratio of hydroxyl ion to water molecule coordinating REE(III) adsorbed on δ-MnO2 suspensions with increasing experimental pH in the solution.