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

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 Hg²⁺ in water with small activation energy (≤2 kJ mol^-1), achieving about 30 mg Hg²⁺ adsorption per gram dry weight of EPS. The adsorption models confirmed complexation of Hg²⁺ 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 Hg²⁺ 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.