REE(III) adsorption onto Mn dioxide (δ-MnO2) and Fe oxyhydroxide:: Ce(III) oxidation by δ-MnO2

The adsorption experiments of rare-earth elements (REE) onto delta -MnO2 and Fe oxyhydroxide precipitates suspended in aqueous NaCl solutions have been conducted. The distribution coefficients, K-d(REE: precipitate/solution) between suspended delta -MnO2 and solutions show extremely large positive Ce anomalies unlike those between Fe oxyhydroxide precipitates and solutions. The iodometric titration of delta -MnO2 precipitates containing different amounts of scavenged Ce reveals that Ce(III) is oxidized to Ce(IV) by delta -MnO2. A convex tetrad effect observed in the series variations of log K-d(REE) between suspending delta -MnO2 and solutions becomes conspicuous with increasing pH in the ran ge of 4.8 < pH < 6.8. This reflects the changing ratio of hydroxyl ion to water molecule coordinating REE(III) adsorbed on delta -MnO2 suspensions with increasing experimental pH in the solution. Copyright (C) 2001 Elsevier Science Ltd.