A Study of Rhamnolipid Microbubble Dispersion for Bioremediation Applications

Abstract

This thesis presents a study on the production of microbubble dispersions from rhamnolipid biosurfactant and the mechanism and factors that impact upon the effectiveness of the dispersion for improving bioremediation efficiency, with a specific focus on the use of microbubble dispersion as a carrier for contaminant-degrading bacteria within a bioremediation scenario. Microbubble dispersion is a suspension of a large number of minute spherical gas bubbles encapsulated in a soapy liquid film in an aqueous surfactant solution. Microbubble dispersion has promising potential for enhancing in situ bioremediation owing to its advantages over air sparging, bioventing and surfactant injection. Characterisation studies investigated the stability, size distribution and gas hold-up properties of the rhamnolipid microbubble dispersion. Drainage experiments and contact angle measurements were performed to identify factors that influence P.putida and R.erythropolis adhesion to microbubble dispersion. Fluorescence microscopy was used to investigate the interaction of P.putida and R.erythropolis and hexadecane, a model NAPLcontaminant on the microbubble surface. The LW-AB surface thermodynamic model was applied to quantify the interaction energy to better understand the bacteria/microbubble and bacteria/contaminant/microbubble interactions. The findings of the characterisation studies support that rhamnolipid microbubble dispersion had comparable properties to the synthetic surfactants reported in this study and in the literature. The stability of the rhamnolipid microbubble dispersion, prepared at rhamnolipid concentrations of 500 mg/L, 1,000 mg/L and 4,000 mg/L, was in the range from 385 to 546 seconds. The gas hold-up in the dispersion was fairly constant ranging from 67% to 72%, and majority of the microbubbles were in the size range of 20 μm to 140 μm. The bacterial drainage experiments, coupled with surface free energy calculation by the LWAB model, showed that rhamnolipid microbubble dispersion was more effective in delivering hydrophilic P.putida than hydrophobic R.erythropolis. Bacterial cell surface hydrophobicity and rhamnolipid concentration were demonstrated as two key factors to control when making the microbubble dispersion an effective bacterial carrier. Furthermore, fluorescence microscopic images revealed that rhamnolipid microbubble dispersion might potentially overcome the problem of limited contaminant bioavailability by improving P.putida and R.erythropolis bacterial contact with hexadecane immobilised at the microbubble surface.