Extracellular mercury sequestration by exopolymeric substances produced by Yarrowia spp.: Thermodynamics, equilibria, and kinetics studies

Exopolymeric substances (EPS) produced by highly mercury-resistant strains of the yeast Yarrowia spp. ( Idd1 and Idd2 ) were isolated and studied for their mercury binding potential. Excellent yield (approximately 0.3 g EPS per gram biomass) of soluble EPS in medium with 3% glucose was observed in the Yarrowia cultures 7 day post-inoculation. A gram dry weight of the EPS consists mainly of carbohydrates (0.4 g), protein (0.3–0.4 g), uronic acid (0.02 g), and nucleic acids (0.002 g). Mercury interactions with the biopolymer were measured as uptake kinetics from a simulated aquatic system and modelled with thermodynamics and calculated mass action equilibria. The EPS forms a complex with Hg 2+ in water with small activation energy (≤2 kJ mol −1 ), achieving about 30 mg Hg 2+ adsorption per gram dry weight of EPS. The adsorption models confirmed complexation of Hg 2+ by the EPS via heterogeneous multilayer adsorption that obey second-order kinetics at constant rate of 4.0 and 8.1 mg g −1  min −1 . The EPS used chemisorption as rate-limiting step that controls the uptake of Hg 2+ from aquatic systems during micro-precipitation as bio-removal strategy. The EPS are promising biotechnological tools to design bioreactors for treatment of mercury-rich industrial wastewater.