| Summary: | 碩士 === 國立成功大學 === 化學工程學系碩博士班 === 101 === Global climate change has become a critical issue due to the influence of the green house effect. How to conserve energy and reduce carbon dioxide emissions has been one of the most urgent global issues. Recently, biofixation of CO2 by Spirulina has attracted much attention due to its efficient CO2 fixation ability and the resulting biomass of Spirulina platensis is rich in C-phycocyanin (C-PC), which is widely used as colorants, diagnosis reagent, nutritious supplements, and pharmaceuticals. Therefore, using the Spirulina platensis to mitigate CO2 emissions and simultaneously produces C-PC has a great potential.
This work firstly made efforts on developing the C-phycocyanin extraction process. For the optimization of the cell disruption process, direct utilization of the osmotic pressure from different phosphate buffer concentration exhibited the highest extraction efficiency, which could reach to nearly 100% recovery after 12 hour extraction time. Reducing the phosphate buffer concentration from 0.2 M to 0.15 M, the extraction efficiency still remained the same. Different biomass-solvent ratio did not significantly affect the extraction efficiency of C-phycocyanin. The purification process of C-phycocyanin was also developed in this study. Fractional precipitation and adsorption of protein with activated carbon were applied. The results show that fraction precipitation by 40% saturation of ammonium sulfate could give better purification results (1.86 purity and 90% recovery). In addition, the anion exchange chromatography (via fast protein liquid chromatography) was applied for C-PC purification, achieving a C-PC purity of 3.62 (44% recovery). Further combining fractional precipitation and anion exchange chromatography, the purity of C-PC was further increased to 4.33 with 33% recovery.
This work was also undertaken to optimize the microalgae cultivation processes to obtain higher biomass productivity, CO2 removal rate and C-phycocyanin productivity. First, a flat-type photobioreactor was developed to reduce light shading effect. The biomass production, overall biomass productivity and CO2 removal rate by using flat-type increased by 2.1 fold when compared with the flask photobioreactor. Next, the effect of irradiation conditions on the performance of cell growth, CO2 fixation rate and C-PC production of Spirulina sp. was further investigated using the flat-type reactor. As the light intensity increased from 100 to 700 µmol/m2/s, the biomass productivity sharply increased from 0.14 to 0.74 g/L/d along with approximately three-fold increase in CO2 removal efficiency. In addition, the maximum C-PC productivity also increased from 0.02 to 0.11 g/L/d. After determining the suitable light intensity, the concentration of key components in the culture medium was adjusted to further enhance the performance of CO2 fixation and C-PC production and reduce the cost. First, to reduce the medium cost, CO2 was used as the main carbon source, while the initial concentration of NaHCO3 was decreased. The results show that reducing NaHCO3 concentration to 25% of the original one did not significantly affect the cell growth and C-phycocyanin production, but the medium cost could be lowered by 55%. Second, since C-PC content of the Spirulina platensis strain markedly decreased under nitrogen-depleting conditions, the initial NaNO3 concentration was adjusted to extend the cultivation period of the Spirulina platensis culture before reaching nitrogen depletion. This strategy further elevated the maximum C-PC productivity from 0.11 g/L/d to 0.13 g/L/d. As for the temperature effect on the cultivation of Spirulina platensis, it was found that controlling the temperature at 32oC gave the best cell growth, CO2 removal rate and C-PC production, while cell growth rate decreased when the temperature was higher than 35oC or lower than 30oC. Finally, while using the suitable photobioreactor, light intensity, medium composition and temperature, the Spirulina platensis was cultivated with an innovated pH control system, in which the culture pH was controlled via the feeding of 2.5% CO2, instead of acid/alkaline titration. The results show that controlling pH at 9.0 and 9.5 was suitable for cell growth, but further increased in pH slightly inhibited the cell growth. In addition, the CO2 removal efficiency sharply increased from 13.6% to 65.1% by applying the CO2-mediated pH control strategy. Moreover, the C-PC content and productivity were also increased to 16.8% and 0.17 g/L/d, respectively, with the CO2-mediated pH control. Therefore, the proposed pH control system could avoid the problem of excessive addition of acid or alkaline, resulting in better CO2 fixation efficiency and higher C-phycocyanin productivity. Moreover, the feasibility of outdoor cultivation of Spirulina platensis was also examined. The results show that Spirulina could tolerate high temperature and high light intensity encountered during outdoor cultivation, giving a biomass productivity, CO2 consumption rate and C-PC productivity of 0.13 g/L/d, 0.21 g/L/d and 0.02 g/L/d, respectively. This performance is lower than that obtained from indoor cultivation.
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