An extracellular polymeric substance quickly chelates mercury(II) with N-heterocyclic groups

•A strain of Klebsiella oxytoca tolerates the Hg(II) toxicity by producing a specific EPS.•The EPS contains a specific exopolysaccharide and outer membrane proteins.•The protein fraction of EPS is rich of histidine which binds quickly Hg2+.•Cyclic Voltammetry analysis indicates the Hg2+ complexing properties of EPS.•The EXAFS model indicates imidazole rings as the major ligands for Hg(II).

A strain of Klebsiella oxytoca DSM 29614 is grown on sodium citrate in the presence of 50 mg l−1 of Hg as Hg(NO3)2. During growth, the strain produces an extracellular polymeric substance (EPS), constituted by a mixture of proteins and a specific exopolysaccharide. The protein components, derived from the outer membrane of cells, are co-extracted with the extracellular exopolysaccharide using ethanol. The extracted EPS contains 7.5% of Hg (total amount). This indicates that EPS is an excellent material for the biosorption of Hg2+, through chemical complexation with the EPS components. The binding capacity of these species towards Hg2+ is studied by cyclic voltammetry, and Hg L3-edge XANES and EXAFS spectroscopy. The results found indicate that Hg2+ is mainly bound to the nitrogen of the imidazole ring or other N-heterocycle compounds. The hydroxyl moities of sugars and/or the carboxyl groups of two glucuronic acids in the polysaccharide can also play an important role in sequestring Hg2+ ions. However, N-heterocyclic groups of proteins bind Hg2+ faster than hydroxyl and carboxyl groups of the polysaccharide.

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