Adsorption to goethite of extracellular polymeric substances from Bacillus subtilis

Extracellular polymeric substances (EPS) are heterogeneous biopolymers produced by Gram-negative and Gram-positive bacterial cells. Adsorption of EPS to minerals can alter the substrata physico-chemistry and influence initial bacterial adhesion processes via conditioning film formation, but the effects of solution chemistry on uptake of EPS remain poorly understood. In this study, the adsorption to goethite (α-FeOOH) of EPS isolated from the early stationary growth-phase culture of Bacillus subtilis was investigated as a function of pH and ionic strength (I) in NaCl background electrolyte using batch studies coupled with Fourier transform infrared spectroscopy and size-exclusion high-performance liquid chromatography. Proteins, particularly those of higher molar mass, and phosphorylated macromolecules were adsorbed preferentially. Increasing solution I (1–100 mM NaCl) or pH (3.0–9.0) resulted in a decrease in the mass of EPS adsorbed. Batch studies and diffuse reflectance infrared Fourier transform spectra are consistent with ligand exchange of EPS phosphate groups for surface hydroxyls at Fe metal centers. The data indicate that both electrostatic and chemical bonding interactions contribute to selective fractionation of the EPS solution. Proteins and phosphate groups in phosphodiester bridges of nucleic acids likely play an important role in conditioning film formation at Fe oxide surfaces.