A novel system integrating electrolysis and ionic membranes (EIMs) enables artificial carbon concentration and alleviation of metal cation stress in microalgae cultivation

Abstract

Microalgae-based carbon sequestration is emerging as a green and sustainable way to achieve negative carbon while recycling CO₂ into biomass used for the production of bioenergy and value-added products. However, its successful implementation is still to be realized due to the low solubility of CO₂ and ion accumulation with the addition of bicarbonate in the culture medium. In this study, we proposed, developed and verified a novel system integrating electrolysis and ionic membranes (EIMs) that enables the artificial recycling of CO₂ utilization and alleviation of metal cation stress in microalgae cultivation. HCO₃⁻ was selected to transfer from the cathode chamber to the culture pond with sodium bicarbonate as the catholyte, while Na⁺ cations were blocked with the anionic membrane in EIMs, accompanied by a gradually decreasing pH value, which facilitates microalgae growth. The reliability and universality of EIMs was further verified with both a cation-tolerant marine strain, Dunaliella salina HTBS, and cation-sensitive freshwater strains, Chlamydomonas and Chlorella. In particular, the cell densities of cation-sensitive strains in EIMs were much higher than those in the NaHCO₃ group in both 800 mL- and 150 L-scale applications, demonstrating their great potential. Moreover, the intracellular metabolites were not affected when microalgae were cultured in EIMs, implying their feasibility for commercial cultivation. Therefore, we established robust EIMs that facilitate both the efficient utilization of CO₂ and commercial application, which will shed light on the development of green technology for microalgae-based carbon sequestration in the future.