Biosorption selectivity of rare earth elements onto Euglena mutabilis suspensions and biofilms and the effect of divalent metal ions

Abstract

The increasing demand for electronics has led to a desire to recover rare earth elements (REEs) from non-conventional sources, including mining and liquid waste effluents. Biosorption could be a promising method for adsorbing REEs onto microalgae, but biomass immobilization and light delivery challenges remain. It was recently shown that REEs biosorb 160% more on algal biofilms than suspended biomass due to the extracellular polymeric substance (EPS) matrix that grows abundantly in biofilms. In this work, we present findings on biosorption selectivity for different REEs in sulfate solutions. The maximum adsorption capacities of Euglena mutabilis suspensions and biofilms were determined for a mixed REE sulfate solution at an equimolar initial concentration range of 0.1–1 mol/L of each REE ion. The highest adsorption capacities for the suspension are for Sm and Eu which are 57% and 46% higher, respectively, compared to the average REE adsorption capacity. The biofilms also preferentially adsorb Sm, Eu, Yb and Lu at 0.035, 0.033, 0.033, and 0.031 mmol/g, respectively. The impact of dissolved divalent ions of Ca, Mg, and Fe on REE adsorption was also assessed. When Ca and Mg are added in equimolar amounts to 0.1–1 mmol/L solutions of equimolar La, Eu, and Yb sulfate, the amount of REEs adsorbed onto suspensions increases by 30% while when Fe is added, it decreases by 10%. No change is observed in biofilms except when Fe is added resulting in a reduction of the adsorption capacity by 40%. A possible explanation for the role of Fe is attributed to the formation of stronger bonds at the binding sites compared to Ca and Mg.