Structural Determination of Zn and Pb Binding Sites in Penicillium chrysogenum Cell Walls by EXAFS Spectroscopy

Fungal cell walls possess strong complexing properties, which make them valuable biosorbents to remove heavy metals from wastewaters. The binding mechanisms of Zn and Pb to Penicillium chrysogenum cell walls have been studied by solution chemistry and extended X-ray absorption fine structure (EXAFS) spectroscopy as a function of the complexation rate. It is shown that Zn and Pb bind to the predominant phosphoryl (95%) and minor carboxyl groups (5%) with a reversed affinity. Zn is predominantly complexed to four PO 4 groups in a tetrahedral configuration at low (7.6 x 10 -3 mmol/g) to high (0.15 mmol/g) Zn concentration and additionally to COOH groups at total saturation of reactive sites (0.22 mmol/g). In contrast, carboxyl complexes of Pb (≡(COO) n -Pb) are formed at low Pb concentration (5.6 10 -3 mmol/g), and their formation is followed by ≡(PO 4 ) n -Pb complexes at higher complexation rate. The difference in complexation affinity by reactive PO 4 and COOH groups observed by EXAFS provides a molecular level explanation for the differences in Pb and Zn isotherms. The Pb isotherm exhibits two plateaus, which correspond to the successive saturation of COOH and PO 4 sites, whereas the Zn isotherm has a single-site Langmuir shape because low affinity minor ≡ (COO) n -Zn complexes formed at high metal concentration are masked by more abundant ≡(PO 4 ) 4 -Zn complexes, which readily form.