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

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 60 min for all lanthanides, followed by slow attainment of equilibrium in around 3 h. Desorption was successful with 0.05 M HCl and the algal biomass was reused for three cycles without significant loss in original lanthanide uptake capacity.