Biosorption of lanthanum by acidophile Euglena mutabilis biofilms and the role of extracellular polymeric substances

Abstract

Enhancing the recovery of rare earth elements from wastewaters is of increasing interest given their growing economic and strategic importance as critical materials for a range of technologies including those required for renewable energy. We are particularly interested in the potential for using microalgal biofilms as sorbents given that they can grow as concentrated structures in the absence of organic carbon. In this study we assess and contrast biofilm and suspensions of Euglena mutabilis with respect to the kinetics and isotherms for the biosorption of lanthanum. The impact of extracellular polymeric substances (EPS) on biofilm biosorption was also examined. Suspension and biofilm adsorption kinetics were found to be similar with equilibrium being after approximately 2 to 3 h. Biofilms adsorbed significantly greater amounts of lanthanum compared to the suspension with a maximum adsorption capacity of 65 mg/g and 25 mg/g respectively for an increase of approximately 160 %. This difference in maximum adsorption capacities was attributed to the biofilm's increased EPS content which had an estimated adsorption capacity of 360 mg/g EPS. The proposed mechanism for EPS adsorption of lanthanum is via an ion exchange mechanism where calcium and magnesium were released into solution after lanthanum uptake. FTIR results also showed no change between the suspended Euglena and biofilm in terms of the available functional groups for adsorption.