Chlorella vulgaris growth from Sodium Bicarbonate Produced Via Carbon Dioxide Absorption Using Sodium Hydroxide

Chlorella vulgaris is a versatile species of algae which can be used for many different applications, including cabin air cleaning for a NASA space camp on Mars, which is of particular interest. For a space camp application in Mars, whose atmosphere contains 95% carbon dioxide, it can be advantageous to grow algae from an easily optimized aqueous carbon solution rather than from the ambient atmosphere. Additionally, by absorbing cabin air (or atmospheric) CO₂ using Sodium Hydroxide, a more efficient removal of gaseous carbon can be achieved than from removal directly using algae. To determine the viability of C. vulgaris growth from inorganic carbon sources in solution rather than from gaseous carbon dioxide, a test was conducted comparing algae growth from 1.9 g/L bicarbonate solution and that from bubbling ambient air (0.07% CO₂) at 1 L/min. The bicarbonate feed solution was prepared by bubbling CO₂-rich air through a sodium hydroxide solution using a bubble column. Considering the effluent CO₂ fraction versus time, the CO₂ effluent in the exiting gas stream was initially very low, indicating a high absorption of CO₂ into the solution. Then, the effluent gas composition rose to approximately one half of the inlet and was stable for a time. By the time that this effluent fraction began to stabilize, the vast majority of the hydroxide ions were consumed to form CO₃^2-. It is during the subsequent stabilization period that the CO₃^2- ions transitioned into HCO₃^- with the addition of more CO₂ gas. Comparing the growth rates of the two algae trials, the algae in the bicarbonate reactor grew more quickly during days 1-7 but slowed behind the ambient air reactor during subsequent days. The bicarbonate reactor peaked during days 5-8 containing 0.6 g/L algae, while the ambient air reactor reached 0.8 g/L algae at day 11. The pH of the bicarbonate-fed algae solution rose to over 11 by day 11, which was a possible cause of slowed growth. Overall, this trial indicates the promise of bicarbonate-fed Chlorella vulgaris for use in this space-camp application and further expands the possibility of a human colony on Mars.