Effects of carbon concentration, pH, and bubbling depth on carbon dioxide absorption ratio in microalgae medium.

The microalgae-based CO2 sequestration is considered to be an effective technique with great potential to cope with carbon emission. However, most researches are only focused on microalgae; the effects of physicochemical factors, which are carbon concentration, medium pH, and bubbling depth, on absorption and utilization of supplied CO2 in culture is less known. In order to understand and improve CO2 absorption in microalgae culture, the effects of these three factors were studied with different levels and combinations. Results revealed that when medium carbon concentration increased from 4.76 to 95.24 mmol/L, CO2 absorption ratio increased by about 12%, 10%, 12%, and 11% at medium depths of 10, 20, 40, and 80 cm, with the initial pH 10.6 to 9.7 by bubbling CO2, respectively. As bubbling depth increased from 10 to 80 cm, CO2 absorption ratio increased by about 25%, 22%, and 25% at carbon concentrations of 4.76, 9.52, and 95.24 mmol/L, with the initial pH 10.6 to 9.7 by bubbling CO2, respectively. In range of 10.6-7.0, pH had no significant effect on CO2 absorption ratio (P > 0.05) when carbon concentration is below 9.52 mmol/L, while above 9.52 mmol/L, pH had significant effect on CO2 absorption ratio (P < 0.05). It was found for the first time that the effect of pH on the CO2 absorption ratio was affected by carbon concentration. In addition, equilibrium pH, at which the CO2 partial pressure in the medium equals to that in the air, of medium with different carbon concentrations was also determined. Overall, in microalgae culture for CO2 sequestration, increasing CO2 bubbling depth and keeping higher carbon concentration and higher pH can improve CO2 absorption ratio, which will optimize the biofixation of CO2 by microalgae furthermore.