| The global warming and energy crisis as two significant issues in twenty-first century have already caused great impacts on human lives and development. The carbon dioxide fixation by microalgae, owing to the ability of microalgae to convert CO2into cellular lipid product, the better environmental adaptability, the higher photosynthetic efficiency and the higher reproduction rate, has become one of the effective solutions to greenhouse effect and energy crisis. However, the low efficiency of CO2fixation and conversion into cellular lipid product has always been the bottleneck in utilization of CO2from flue gas by microalgae. In this study, the effect of nitrogen depression and CO2concentration on microalgal biomass and lipid accumulation is first compared, followed by the process intensification of microalgal CO2fixation from flue gas and its conversion into lipid product through a method of pH regulation controlled by CO2switch and the coupled membrane technology. The main results obtained in this study are listed as the following:(1) The biomass growth is negatively correlated with lipid accumulation of Chlorella vulgaris under the condition of nitrogen depression. With the initial nitrate concentration of1mM, the maximum lipid content and productivity achieved are43.28%and68.08mg L-1d-1respectively while the biomass productivity obtained is minimum and reached110mg L-1d-1. With the initial nitrate concentration of7mM, by contrast, the biomass productivity is increased to234mg L-1d-1while the corresponding lipid content and productivity are only5.50%and15.9mg L-1d-1respectively.(2) An appropriate increase in the inlet CO2concentration can promote microalgae biomass growth and intracellular lipid accumulation. Nevertheless, the higher concentration of CO2will cause an obviously inhibition on biomass growth and lipid synthesis. At1%CO2, Chlorella vulgaris can achieve the maximum biomass productivity and lipid productivity of220.86mg L-1d-1and21.28mg L-1d-1, respectively. When the inlet CO2concentration is increased to15%, the biomass and lipid productivity reduced to120.64mg L-1d-1and8.35mg L-1d-1respectively, showing clearly the inhibition of biomass growth with the increasing of inlet CO2concentration.(3) The pH regulation controlled by CO2was proposed to realize the enhancement of biomass density and lipid content for Chlorella vulgaris cultivated under high CO2concentration. At10%CO2, the medium pH can rise from6.3to10.0sharply after CO2concentration is switched from10%to0.03%. Simultaneously, the biomass concentration of Chlorella vulgaris increases from0.82g L-1to0.98g L-1, the intracellular lipid content is elevated from a low level to23.4%and the lipid productivity reached76.88mg L-1d-1within3days. It is worth noting that the nitrate concentration of culture medium is maintained over50mg L-1, indicating that the rapid lipid accumulation caused by pH regulation is independent of nitrogen depression effect. The results reveal the pH regulation by feed CO2switch is a new method to improve microalgae lipid accumulation at high CO2like flue gas.(4) The aeration of microporous membrane can promote CO2removal ratio of Chlorella vulgaris at high CO2. The comparison of airlift sparger and microporous membrane aeration shows that the maximum rate and ratio of CO2reduction in membrane photobioreactor can increase by3.4-fold than that in airlift photobioreactor, reaching483.66mg L-1d-1and14.96%respectively. The much smaller bubble size generated by membrane photobioreactor enhanced the gas-liquid specific surface area and mixing effect of culture medium extensively, resulting that the the mass transfer efficiency and the uptake CO2of Chlorella vulgaris at high CO2are significantly improved.To our best knowledge, the new strategy of pH regulation and membrane technology proposed in this study has enhanced the CO2fixation and lipid conversion of Chlorella vulgaris at high CO2,, allowing the full utilization of flue gas by microalgae. |
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