Delivery of hydrophobic substrates to degrading organisms in two-phase partitioning bioreactors
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This thesis examined the use of two-phase partitioning bioreactors (TPPBs) for the biodegradation of poorly water-soluble compounds. TPPBs are stirred tank bioreactors composed of a biocatalyst-containing aqueous phase and an immiscible second phase containing large amounts of poorly water-soluble or toxic substrates. Degradation of the bioavailable substrate in the aqueous phase will result in equilibrium-driven partitioning of additional substrate from the immiscible phase into the aqueous phase, theoretically allowing for complete substrate degradation. Fundamental work was undertaken with the PCB-degrading organisms Burkholderia xenovorans LB400 in liquid-liquid and solid-liquid TPPBs. Initially biphenyl was used as the sole carbon source due to its hydrophobic nature and structural similarity to the environmentally relevant PCBs. The critical LogKO/W (octanol/water partitioning coefficient) of the organism was determined to be 5.5 and its growth kinetics on biphenyl were determined in a liquid-liquid TPPB. A polymer selection strategy for solid-liquid TPPBs was developed in the next chapter, and it was shown in the following chapter that biphenyl degradation in solid-liquid TPPBs was mass transfer limited, as described mathematically utilising the previously estimated microbial kinetics. The fundamental knowledge gained in the early chapters was then applied to the degradation of PCBs by the same organism. It was shown that the aqueous phase availability of PCBs is the rate-limiting step in biphasic bioreactors, and not the mass transfer rate. The low specific microbial degradation rates, resulting from substrate-limited growth were addressed with increased biomass concentrations; however, it was also found that an additional carbon source was required to maintain microbial activity over an extended period of time. Pyruvic acid was selected as a carbon source which, once added to actively PCB-degrading cells, maintained the cells’ activity towards PCBs and up to 85 % of 100 mg l-1 was degraded in 15 h. It was shown as the final contribution in this thesis that TPPBs can be combined with a PCB soil extraction step as a potential remediation scheme for PCB contaminated soil. PCBs were extracted from soil with polymer beads (up to 75 % removal), followed by biodegradation of the PCBs in a solid-liquid TPPB in which PCBs were delivered to the degrading organism from the same polymer.