Influence of bacterial extracellular polymeric substances on the sorption of Zn on γ-alumina: A combination of FTIR and EXAFS studies

Abstract

Extracellular polymeric substances (EPS) isolated from bacteria, are abound of functional groups which can react with metals and consequently influence the immobilization of metals. In this study, we combined with Zn K-edge Extended X-ray Absorption Fine Structure (EXAFS), Fourier Transform Infrared (FTIR) spectroscopy, and High-Resolution Transmission Electron Microscopy (HRTEM) techniques to study the effects of EPS isolated from Bacillus subtilis and Pseudomonas putida on Zn sorption on γ-alumina. The results revealed that Zn sorption on aluminum oxide was pH-dependent and significantly influenced by bacterial EPS. At pH 7.5, Zn sorbed on γ-alumina was in the form of Zn-Al layered doubled hydroxide (LDH) precipitates, whereas at pH 5.5, Zn sorbed on γ-alumina was as a Zn-Al bidentate mononuclear surface complex. The amount of sorbed Zn at pH 7.5 was 1.3–3.7 times higher than that at pH 5.5. However, in the presence of 2 g L−1 EPS, regardless of pH conditions and EPS source, Zn + EPS + γ-alumina ternary complex was formed on the surface of γ-alumina, which resulted in decreased Zn sorption (reduced by 8.4–67.8%) at pH 7.5 and enhanced Zn sorption (increased by 10.0–124.7%) at pH 5.5. The FTIR and EXAFS spectra demonstrated that both the carboxyl and phosphoryl moieties of EPS were crucial in this process. These findings highlight EPS effects on Zn interacts with γ-alumina.