Complexation and conformation of lead ion with poly-γ-glutamic acid in soluble state

Complexation of microbial polymer in soluble state could impact the solubility, mobility, and bioavailability of heavy metals in the environment. The complexation of a bacterial exopolymer, poly-γ-glutamic acid (γ-PGA), with Pb2+ was studied using the polarographic method and circular dichroism measurement in soluble state. The number of available binding sites was determined based on the Chau's method and was found to be 0.04, 1.12, 3.56 and 4.51 mmol/(g dry weight of γ-PGA) at pH 3.4, 4.2, 5.0 and 6.2, respectively. Further, the number of binding sites was determined based on the Ruzic's method and was found to be 3.60 and 4.41 mmol/(g dry weight of γ-PGA) for pH 5.0 and 6.2, respectively. The constant (expressed as log K) values were 5.8 and 6.0 at pH 5.0 and 6.2. Compared to biopolymers secreted by other microorganisms, such as extracellular polymeric substances extraction from activated sludge, γ-PGA was a more efficient Pb2+ carrier from pH 5.0 to 6.2. The secondary structure of γ-PGA varied significantly when Pb2+ added. Ca2+ or Mg2+ replace a portion of the adsorbed Pb2+. However, the portion of Pb2+ involved in changing the γ-PGA conformation was not easily replaced by Ca2+ and Mg2+.