Rhamnolipid biosurfactant complexation of rare earth elements.

Rare earth elements (REE) are vital for modern technologies and considered critical materials. This study investigated monorhamnolipid biosurfactant interactions with REE as the basis for REE recovery technology. Conditional stability constants (log β), measured using a resin-based ion exchange method, are reported for 16 REE and metals. These results were combined with existing data for 10 other metals to assess comparative strength and determinants of binding. The stability constants could be divided into three groups: weakly, moderately, and strongly bound. The REE were all in the strongly bound group (UO22+, Eu3+, Nd3+, Tb3+, Dy3+, La3+, Cu2+, Al3+, Pb2+, Y3+, Pr3+, and Lu3+) with log β ranging from 9.82 to 8.20. The elements Cd2+, In3+, Zn2+, Fe3+, Hg2+, and Ca2+ were moderately bound with log β=7.17-4.10. Finally, Sr2+, Co2+, Ni2+, UO22+, Ba2+, Mn2+, Mg2+, Rb+, and K+ were weakly bound with log β=3.95-0.96. Two log β values are reported for the uranyl ion due to two distinct binding regions. A mixed metals study and associated selectivity coefficients confirmed monorhamnolipids preferentially remove metals with large log β values over those with smaller values. Preferential complexation by monorhamnolipids may constitute a green pathway for recovery of REE from alternative, non-traditional sources.