Extraction and Characterization of Polyhydroxyalkanoates from Purple Non-Sulfur Bacteria Treating Fuel Synthesis Process Water

The development of plastics has been one of the major materials achievements of the past century, providing many advantages to reduce the cost of products with desirable characteristics such as lightweight, ease of molding, high strength to weight ratio, and good barrier properties. However, their use has also resulted in a major environmental problem due to their highly resistant nature to biodegradation, resulting in major pollution of both terrestrial and aquatic environments and significant impacts on aquatic- and birdlife. New, biodegradable plastics are being developed to realize the benefits of plastics while mitigating their major drawback. Polyhydroxyalkanoates (PHAs) are a type of plastics that are produced by bacteria intracellularly using a broad range of feedstocks. Extraction of the PHA from the bacterial cells is a major component of the processing cost and typically uses hazardous solvents such as chloroform or acetone. In this study, four solvents were tested for their PHA extraction efficiency from a new PHA production process using purple non-sulfur bacteria enriched mixed cultures grown on fuel synthesis wastewater, as well as a comparative culture grown on acetate. Two solvents were commercially used with high environmental risk: chloroform, and acetone; and two greener solvents, diethyl carbonate and dimethyl carbonate. Two operational factors, solvent ratio and reaction time, were assessed on PHA yield using a response surface experimental methodology. Extracted polymers were quantified and identified by gas chromatography equipped with flame ionization detection (GC-FID) and characterized using differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and X-ray diffractometer (XRD). Results from this study showed the highest extraction yield was obtained from acetone as an extraction solvent. Both DEC and DMC achieved less than half the extraction efficiency as either of the two commercially used solvents. However, further optimizations of experimentation could improve extraction efficiency. The PHA extracted from the microbial biomass conformed with the characteristics expected for short chain polyhydroxyalkanoates, showing similarity to the polyhydroxy-valerate-co-butyrate standard with the exception of PHA extracted from FSW using acetone. GC-FID analysis indicated the PHV monomer was present in the samples, but that PHB was not. Both samples extracted with DMC showed a semi-crystalline nature with relatively high purity, while those for chloroform-methanol and acetone showed variation in purity and/or crystallinity as well as thermal properties depending on the feedstock fed to the culture.