Toxicity of flue gas components from cement plants in microalgae CO2 mitigation systems

Microalgae-based CO2 capture from flue gas is an attractive mitigation strategy in the cement industry. However, NO (x) and SO (x) components might be harmful to microalgae. We performed toxicity assays, under 2 % (v/v) CO2 and using nitrite, sulfite, or bisulfite salts, on an environmental isolate, identified as Desmodesmus abundans (The University of Texas at Austin (UTEX), no. 2976) and Scenedesmus sp. UTEX1589. Nitrite and sulfite did not inhibit growth at the tested concentrations (0-1,067 ppm (w/v) NO2 (-) and 0-254 ppm (w/v) SO3 (2-)); however, bisulfite was toxic above 39 ppm. Non-toxic concentrations of both sulfur-based compounds stimulated growth, but significantly higher growth rates were only observed for HSO3 (-). Within a narrower range, NO (x) and SO (x) served as a sole nutrient source. Overall, biomass production and growth rates of the environmental isolate were greater. A novel strategy to buffer high concentrations of HSO3 (-) (200 ppm) was developed by adding cement kiln dust (CKD), a byproduct and flue gas component. The results suggest that CKD also provided other beneficial growth components and that sulfur optimization of the culture medium significantly increased carbon assimilation, particularly in D. abundans. In additional simulations of typical flue gas conditions in a modern cement plant (320, 40, and 40 ppm (w/v) of NO2 (-), SO3 (2-), and HSO3 (-), respectively, and 25 % (v/v) CO2), along with the incorporation of 300 ppm CDK, growth of D. abundans was supported. Although further studies are needed, direct utilization of flue gas might be possible with the environmental isolate, where NO (x) , SO (x) , and CKD are all beneficial components of the mitigation system.