Pb Biosorption by Leclercia adecarboxylata: Protective and Immobilized Mechanisms of Extracellular Polymeric Substances

Abstract Immobilization of lead by phosphate solubilizing bacteria (PSBs) has much advantages compared to conventional bioremediation methods due to the high efficiencies of immobilization particularly in the formation of insoluble Pb-phosphate compounds. The growth characteristics of Leclercia adecarboxylata , a lead resistance PSB, with the presence of Pb(NO 3 ) 2 was observed, and the role of different layers of extracellular polymeric substance (EPS) in Pb 2+ adsorption were also explored. Results showed that high concentration of Pb 2+ could inhibit the growth and metabolism of PSBs, especially in inhibiting the formation of intermediate products of the tricarboxylic acid cycle, such as succinic acid and citric acid. As the first reactive barrier to resist lead stress, soluble EPS (S-EPS) showed maximum adsorption ability of Pb 2+ , reached to 0.59 mmol·g -1 , while the maximum sorption capacities of loosely bound EPS (LB-EPS) and tightly bound EPS (TB-EPS) were 0.011 mmol·g -1 and 0.004 mmol·g -1 for Pb 2+ , respectively. Three-dimensional fluorescence spectrum demonstrated that tyrosine protein-like substances and tryptophan protein-like substances were likely to play a significant role in resistance of lead stress in three layers of EPS, while carboxyl (-COOH), hydroxyl (-OH) and amino functional groups on EPS provided the bounding sites for immobilizing Pb 2+ . X-ray diffraction (XRD) investigation revealed that Pb 2+ loaded on EPS did not form new crystalline phases, indicated that complexation was the dominant mechanism to remove lead ions in EPS. In addition, both with hydrophobic and hydrophilic groups in EPS were found, which could also capture Pb 2+ through electrostatic attraction and ion-exchange. All the results obtained shown that the secretion of EPS by Leclercia adecarboxylata was one of the mechanisms to resist heavy metal stress and also could reduce the bioavailability of lead.