Abstract
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+.
Highlights
Heavy metals are one of the most serious environmental problems in the world [1,2,3]
The purity and glutamic acid content of γ-PGA were determined by amino acid analysis using thin layer chromatography (TLC) and colorimetric analysis after hydrolysis as described by Wang et al [21]
The purity of γ-PGA was confirmed by the sole detection of glutamic acid in the TLC analysis
Summary
Heavy metals are one of the most serious environmental problems in the world [1,2,3]. The negative effects of heavy metals on the ecosystem vary considerably and have economic and public health significance [4]. Heavy metals do not undergo microbial or chemical degradation and persist for a long time to accumulate in the environment in substantial quantity. Heavy metals can be carried by stormwater run-off and transported to soil, rivers, lakes, seas, and oceans [6], causing a wide range of pollution. Natural organic matter (NOM) dispersed in water systems and soils could bind heavy metals and transport them as a mobile "carrier" [8, 9]. The complexations of metals by NOM affects their speciation, mobility, and bioavailability.
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