Metal Complexation and Interfacial Behavior of the Microbially Produced Surfactant Monorhamnolipid by Pseudomonas Aeruginosa ATCC 9027

Abstract

The purpose of the research reported in this dissertation is to expand the general knowledge of the chemical properties of monorhamnolipids (produced by P. aeruginosa ATCC 9027) and monorhamnolipid-metal complexes in solution and at interfaces in order to advance the application of these biosurfactants in a variety of applications.The speciation and fragmentation behavior of monorhamnolipids (mRLs) using mass spectrometry at low and high resolution in positive ion mode was investigated as a function of pH, which has yet to be fully discussed to date in the literature. This study laid the groundwork for the speciation and fragmentation behavior of mRLs with two environmentally-relevant heavy metals, Pb²⁺ and UO₂²⁺. It was determined that mRLs form 1:1 and 2:1 mRL-metal complexes with both metal cations across the pH range investigated (pH 4.0, 6.0 and 8.0). mRL-metal complexes were found to fragment differently than free mRLs suggesting coordination of the metal cation in a binding pocket comprised of the mRL carboxylic acid moiety and the rhamnose sugar hydroxyls. This coordination environment was further verified as a function of solution pH using infrared spectroscopy (IR), nuclear magnetic resonance spectrometry (NMR) and hydrogen-deuterium exchange (HDX) mass spectrometry. Adsorption isotherms for mRLs and mRL-metal complexes on two soil components, silica and goethite, were characterized as a function of solution pH using ATR-FTIR and successfully fit to the Frumkin-Fowler-Guggenhiem isotherm to extract relevant thermodynamic adsorption parameters. These studies showed that at low pH, mRLs form bilayers on these surfaces, but the adsorption affinity of the mRLs is dictated by the molecular interactions these species have with the specific oxide surface. At neutral and basic pH values, mRLs were found to adsorb to silica despite the fact that both the surface and the mRLs are negatively charged. The Lewis acid/base interactions of mRLs with goethite at neutral and basic pH values results in multilayer adsorption. Adsorption of mRL-metal complexes on silica and goethite suggests that the interactions between these complexes and soil surfaces may have a direct impact on metal ion remediation efficiencies using mRLs. The adsorption affinity of mRL-Pb²⁺ complexes to silica is greater than that of mRL-UO₂²⁺ complexes; however, mRL-UO₂²⁺ complexes precipitate at the interface. The adsorption affinity of mRL-metal complexes on goethite is not significantly lower than for free mRLs; however, the surface coverage decreases.