Interaction of rare earth elements with a brown marine alga in multi-component solutions

Abstract

The present study elucidates the underlying mechanisms involved in the interaction of lanthanides (lanthanum, cerium, europium and ytterbium) with a brown marine alga, Turbinaria conoides in quaternary mixtures. Competition between the lanthanides in occupying the binding sites of T. conoides was clearly identified, which was due to competitive ion-exchange mechanism. This competition affected the affinity of T. conoides towards each lanthanide. In comparison to single-component lanthanide system, uptake capacities in quaternary systems decreased by 75.8, 72.5, 65.0 and 70.2% for La, Ce, Eu and Yb, respectively. Biosorption mechanism was confirmed through SEM/EDX and pH-edge data, which implies that when virgin T. conoides are exposed to lanthanide solutions, La3+ cations may replace some of the alkali and alkaline earth metals naturally present in the cell wall through ion-exchange mechanism; and this phenomenon was favored at pH 5, under examined conditions. Quaternary biosorption isotherms were successfully modeled using the extended Langmuir model with a constant interaction factor. The model predictions agreed well with the experimental data. The magnitude of Eu competition over lanthanides was significant, yielding a lowest interaction factor (η=0.91) for Eu over other lanthanides. Biosorption kinetics revealed that more than 91% of sorption process was completed within 60min for all lanthanides, followed by slow attainment of equilibrium in around 3h. Desorption was successful with 0.05M HCl and the algal biomass was reused for three cycles without significant loss in original lanthanide uptake capacity. © 2010 Elsevier B.V.