Metastable Hydrated Carbonates from a Brine to Supply Carbon to Microalgae Harvested for Biofuel Production

Abstract

In this paper, a study to develop a new process which combines mineral carbonation - or carbon dioxide (CO2) mineralization - with microalgae growth is presented. Carbon dioxide is used to precipitate nesquehonite (MgCO3-3H2O). The magnesium carbonate crystals are then dissolved in the presence of freshwater green microalgae Scenedesmus obliquus (S. obliquus) providing them carbon dioxide for cell growth as an alternative to either enriched CO2-gas or commercial sodium bicarbonate (NaHCO3). The biomass of S. obliquus is eventually used to produce biogas by anaerobic digestion or bio-oil through fatty acid extraction. Systematic experiments of MgCO3-3H2O crystallization were carried out using MgCl2-NaOH-CO2 aqueous system. The tests were run at 25 oC and 1 bar of CO2. Dissolution experiments of the produced MgCO3-3H2O crystals were performed by incubating S. obliquus, at 25oC and atmospheric conditions, and for a photoperiod consisting of 14 hours of light and 10 hours of dark. Results show that the precipitation of MgCO3-3H2O reaches steady-state within a few minutes, producing needle-like crystals that can dissolve during algae growth. The values of the microalgae cell density after 7 days of growth in the test with MgCO3-3H2O and NaHCO3 are of the same order of magnitude (i.e., approximately 107 cells/mL). This suggests the use of MgCO3-3H2O is a promising alternative to commercial/captured gaseous CO2 and carbonates for microalgae growth.