Sustainable CO₂ Capture and Bio-Fixation Using Functionalized Deep Eutectic Solvents and Microalgae ^†

The urgent need to mitigate anthropogenic CO₂ emissions has spurred innovative approaches for CO₂ capture and utilization. In this study, we explore a novel method involving the capture of CO₂ using a functionalized deep eutectic solvents (DESs) [1,2] and subsequent bio-fixation through microalgae cultivation [3]. The study focuses on the development of an integrated system that efficiently captures CO₂ and harnesses the photosynthetic capabilities of microalgae for sustainable CO₂ reduction.

Two DESs were compared: a binary DES, ChCl:MEA 1:8 (CM) and a ternary DES, ChCl:EG:MEA (CEM) 1:2:1 molar ratio. Choline chloride (ChCl) is a hydrogen bond acceptor, while ethylene glycol (EG) and monoethanolamine (MEA) are hydrogen bond donors. The CO₂ capture took place in a glass bubbler with ceramic diffuser. The desorption process involved immersing the DES in a stainless-steel container, subjecting it to a controlled temperature of 80 °C, and agitating at 12 RCF (Relative Centrifugal Force) within a full vacuum, ensuring effective desorption of CO₂ from the DES. The CO₂-enriched gas was introduced into a microalgae bioreactor series, where it served as a carbon source for photosynthesis [4]. The microalgae culture, Chlorella sp. NIVA-CHL137, was cultivated under controlled environmental conditions, including a day/night photoperiod (13/11 h) and BG-11 cultivation medium. The optical density (OD) and biomass yield were used to evaluate the microalgae development.

The microalgae cultivation in the bioreactor showed CO₂ fixation capabilities, resulting in a significant increase in the microalgal biomass and a corresponding reduction in CO₂ concentration within the system. Using the binary DES (CM) resulted in a significant 26% increase in OD of Chlorella sp. compared to control and the bio-sequestration of 53 mg of carbon per liter, equivalent to an elevation of 194 mg CO₂ per liter of culture. The ternary DES (CEM), on the other hand, exhibited slightly higher CO₂ concentration removal than CM (+1.4%), leading to higher OD and biomass augmentation, compared to the control culture. Additionally, CO₂ desorbed from CEM positively influenced the biomass growth of Chlorella sp. compared to control, with OD surging by 12% during the initial 7 days and a sustained 24.7% increase until day 14, surpassing the performance of the control group (Figure 1).

Our study presents an approach for CO₂ capture and bio-fixation using functionalized DES and microalgae cultivation. The CO₂ desorption process, coupled with the prolific growth of microalgae, highlights the viability of this integrated system as an environmentally friendly and sustainable strategy for mitigating CO₂ emissions. This innovative method has the potential to contribute significantly to the global efforts aimed at addressing climate change and advancing the utilization of captured CO₂ in various applications, including biofuels and bioproducts.