Role of extracellular polymeric substances in biosorption of Pb2+ by a high metal ion tolerant fungal strain Aspergillus niger PTN31

It is widely reported that fungal strains show good heavy metal ions tolerance, but the role of fungal extracellular polymeric substances (EPS) in heavy metal ion biosorption and tolerance is still not clear. In this study, a high metal ion tolerant fungal strain PTN31, identified as Aspergillus niger, was isolated, which could grow in the medium with coexistence of various metal ions. The minimum inhibitory concentration (MIC) for Pb was up to 72 mM. Moreover, biosorption behaviors and mechanisms of Pb2+ by PTN31 and extracted EPS were fully tested. Higher pH promoted biosorption of Pb2+ by the both biosorbents due to deprotonation of the functional groups in microorganisms and EPS. High and low temperature affected the physiological and metabolic activity of the biosorbents, leading to low Pb2+ adsorption. The adsorption kinetic process was well described by pseudosecond order model (R-2 > 0.98) and the adsorption isotherm fit well with Freundlich model (R-2 > 0.95). The maximum monolayer Pb2+ biosorption capacity of extracted EPS form PTN31 was as high as 713.6 mg/g, whereas Pb2+ biosorption capacity of PTN31 was decreased by 40.9%-66.8% after removal of EPS. Therefore, large Pb2+ biosorption capability of EPS is primarily responsible for the high lead tolerance of strain PTN31. EDS and FTIR indicated that EPS with abundant functional groups was highly related to the formation of cell aggregates, which could build a protective barrier for protecting cells from hostile environments.