Dissolved Transfer And Photosynthetic Conversion Of Flue-gas-derived CO₂ To Promote Carbon Fixation And Lipid Enrichment In Microalgae

Microalgae biological method to reduce CO₂ emisssions from coal-fired flue gas is of great significance to alleviate the greenhouse effect and develop low-carbon economy.However,the carbon sequestration industry of microalgae is still in its infancy at present.Some technical bottlenecks,such as low solubility of flue-gasderived CO₂ in water and limited photosynthetic efficiency of microalgae,need to be solved.In this paper,polyethylene glycol 200?PEG 200?was used to enhance the dissolved mass transfer of flue-gas-derived CO₂,thus improving carbon sequestration rate of microalgae.Ammonium ferric citrate?AFC?was used to promote photosynthetic conversion of flue-gas-derived CO₂ and enhance carbon fixation and lipid accumulation of microalgae.A pilot system of circulating flow type photobioreactor was established and optimized to increase the carbon fixation rate of microalgae.PEG 200 was used to enhance the dissolved mass transfer of flue-gas-derived CO₂ from gas phase to liquid phase to promote the growth and carbon fixation rate of microalgae cells.Total inorganic carbon absorbed in the cell free culture medium remained constant at 5.6 m M.PEG 200 provided additional CO₂ absorption from 0.6 to 4.8 m M when the PEG 200 concentration increased from 0.5 to 4 m M.The specific growth rate and carbon fixation rate of N.oceanica both first increased with increasing PEG 200 concentration and then decreased.1 m M of PEG 200 absorbent resulted in the maximum specific growth rate?1.41 d-1?and carbon fixation rate?0.31 g/L/d?,which were 21.5% and 18.2% higher than 0 m M PEG 200,respectively.Microscopic test showed that the addition of 1 m M PEG 200 increased the microalgal cell wall thickness by 25.5%,the starch granule content by 34.6%,and the lipid droplet content by 199.7%.The lipid content of algal biomass and lipid yield reached peaks of 38.3% and 0.44 g/L,respectively.AFC was used to promote photosynthetic conversion of flue-gas-derived CO₂ and enhance carbon fixation and lipid accumulation of microalgae.The biomass dry weight and pigment content of Haematococcus pluvialis at green growth stage both first increased with increasing AFC concentration and then decreased.5 ?M of AFC increased the biomass dry weight by 13% to 1.1 g/L,chlorophyll by 23% to 98 mg/L and carotenoid by 10% to 12 mg/L,respectively.The biomass dry weight of Haematococcus pluvialis at red lipid production stage also first increased with increasing AFC concentration and then decreased.0.5 ?M of AFC increased the biomass dry weight by 19% to 3.14 g/L and starch granule content by 36% to 16%.Photosynthetic test showed that Fe3+ and NH4+ increased the number of photosynthetic reaction centers and light capture efficiency of PS?,thus improving the ability of Haematococcus pluvialis to fix CO₂.Lipid and astaxanthin content were positively correlated with AFC concentration,which increased to 41.1% and 19.7 mg/g respectively when adding 5 ?M of AFC.The reason was that the appropriate amount of Fe3+ catalyzed the intracellular Habers-Weiss reaction to convert the low reactive oxygen species into the high reactive oxygen species,thus inducing more fatty acids and astaxanthin to accumulate in Haematococcus pluvialis.A pilot scale system of circulating flow type photobioreactor was established.Carbon fixation rate and astaxanthin content of Haematococcus pluvialis were increased by optimizing and regulating the CO₂ concentration.Under the condition of 15% CO₂,the peak carbon fixation rate of Haematococcus pluvialis at the green growth stage was 29.8% higher than that of 9% CO₂,reaching 0.64 g/L/d;the biomass dry weight at the red lipid production stage was 36.6% higher than that of 9% CO₂,reaching 1.38 g/L.Circulating flow type photobioreactor can effectively improve the mixing uniformity of microalgae in culture medium and reduce the deposition of microalgae cells in column photobioreactor.Therefore,it is beneficial to industrial engineering application.