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

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.