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

The adsorption experiments of rare-earth elements (REE) onto delta -MnO2 an d Fe oxyhydroxide precipitates suspended in aqueous NaCl solutions have bee n conducted. The distribution coefficients, K-d(REE: precipitate/solution) between suspended delta -MnO2 and solutions show extremely large positive C e 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 delt a -MnO2. A convex tetrad effect observed in the series variations of log K- d(REE) between suspending delta -MnO2 and solutions becomes conspicuous wit h 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. Co pyright (C) 2001 Elsevier Science Ltd.