The Influence of Redox Reactions on the Uptake of Dissolved Ce by Suspended Fe and Mn Oxide Particles

Laboratory experiments were conducted to evaluate the partitioning of rare earth elements (REE) between solution and suspended particles. Because of their strong tendency to complex, the REE can be used to study a variety of marine processes and in particular particle scavenging. In this study, an emphasis was placed on examining abiotic redox processes that influence the uptake of dissolved Ce by particles. Batch sorption experiments were conducted with REE and synthetic mineral phases over the range of pH 4-9. The solutions varied in ionic strength between 0 and 0.7 M and consisted of individual solutes (NaNO3, NaCl, and Na2SO4), ionic mixtures that duplicate the seawater composition, and natural seawater. The uptake of REE from solution was also studied at a Pt electrode coated with delta-MnO2 using cyclic voltametry. Experimental results are consistent with uptake of dissolved Ce onto delta-MnO2 occurring by a combination of oxidative scavenging and surface complexation. The contribution of oxidative scavenging to the removal of Ce from solution is most pronounced at acidic pH, where the strictly trivalent REE exhibit little propensity for sorption onto delta-MnO2. Sorption of dissolved Ce onto FeOOH occurs in a manner analogous to that of the other strictly trivalent REE and no contribution from oxidative scavenging is observed on this mineral phase at low pH. Our work also substantiates the hypothesis that anions in solution, particularly SO42- and Cl-, as well as those adsorbed on the surface of the particles, influence the extent of Ce uptake by delta-MnO2.