Using polyethylene glycol to promote Nannochloropsis oceanica growth with 15 vol% CO2.

CO2 capture with microalgae has been put forward in response to global concern on greenhouse gas emission. However, the short residence time and slow diffusion of CO2 in water limits the growth of microalgae. In order to improve CO2 transfer from gas phase to liquid phase and utilization by algal cells, polyethylene glycol 200 (PEG 200) was used as CO2 absorbent to promote growth of Nannochloropsis oceanica with the bubbling of 15 vol% CO2. Total inorganic carbon (TIC) absorbed in culture medium remained constant at 5.6 mM when 15 vol% CO2 was bubbled continuously. PEG 200 in the medium provided additional CO2 absorption from 0.6 to 4.8 mM when PEG 200 concentration increased from 0.5 to 4 mM. The specific growth rate of N. oceanica reached the maximum (1.41 d-1) with 1 mM PEG 200 in culture medium, which was 21.5% higher than the specific growth rate without PEG 200. About 79% of the increase in biomass was attributed to the increased TIC with more CO2 dissolution in culture medium because of PEG 200, and about 21% was attributed to PEG 200 itself utilized as an organic carbon source. In conclusion, PEG 200 as a CO2 absorbent can effectively capture flue-gas CO2 for algal growth.