(125d) Borate-Promoted CO2 Capture for Microalgae Cultivation

A major barrier to economic viability of microalgae biofuels is the high cost of concentrated CO₂ supply cost. Furthermore, global algae biofuel generation would be seriously restricted if micro-algae farms are restricted to the vicinity of CO₂ sources along with the additional limitations of low slope lands and favorable climate. One potential solution to decrease CO₂-related costs is to cultivate microalgae cultures in high pH media (pH >10) that can uptake ambient/atmospheric CO₂ at high rates. In addition, adding high concentrations of carbonate/bicarbonate to the media (i.e. media with high carbonate alkalinity) provides high dissolved inorganic carbon (DIC) for photosynthesis. The presence of rate promotors like boric acid further aids in CO₂ hydration and absorption and can lower the requirements for physical mass transfer and mixing costs in high pH media. In this study, we report results from studies that compare rates of DIC uptake in borate-promoted and promoter-free alkaline solutions exposed to the atmosphere. The experiments were set up by maintaining constant system parameters including the initial pH and total alkalinity in both the systems. Along with the initial and final DIC content, pH was continuously measured until both the system reached equilibrium with the atmosphere. A mathematical model, developed from chemical equilibria first principles, was fit to the measured pH values to determine the rate of DIC increase, mass transfer coefficients and the enhancement factor. Our results showed that the rate of DIC increase in the borate promoted systems was at least 50% higher than the non-promoted carbonate/bicarbonate solutions. Our ongoing studies are investigating the cultivation of the alkaliphilic Chlorella sorokiniana str SLA-04 in borate promoted carbonate/bicarbonate systems to achieve high biomass productivity without concentrated CO₂ inputs.