Cement flue gas as a potential source of nutrients during CO2 mitigation by microalgae

Abstract

Microalgae flue gas mitigation is an attractive strategy to reduce greenhouse gases. To enhance economic feasibility and sustainability, the gas is being contemplated as a source of nutrients for byproduct generation. As an unexploited resource, all gas components should be considered to assess its potential. In this study, a cultivation strategy for a CO2-tolerant isolate, Desmodesmus abundans RSM, was implemented in a 3-L customized photobioreactor under continuous light (190μmolPAR-photonsm−2s−1) considering cement kiln dust (CKD) and major gases at concentrations close to maximums (25% CO2, 800ppm NO, 200ppm SO2). Two gas supply schemes were tested (continuous and 24h aeration cycles of flue gas followed by 24h no flue gas) in complete (BG11) and incomplete (BG11-N-S) medium using CKD to control pH >5.0. Continuous aeration inhibited growth after 60h. At low pH, uninterrupted changes in culture pH might compromise the ability of microalgae to adapt. Conversely, 24h aeration cycles with CKD allowed microalgae growth in BG11 and BG11-N-S (0.227±0.012 and 0.164±0.009gd.w.L−1d−1, respectively). Growth parameters were significantly higher during flue gas aeration in BG11, compared to BG11-N-S and a CO2 control. Maximum specific growth rate was reduced by 17% in BG11-N-S, while biomass depended parameters showed reductions around 28%. Biomass protein content also decreased from 28.5 to 22.7% w/w in BG11 and BG11-N-S, respectively. Compare to BG11, BG11-N-S showed less SOx and NOx in solution that evidence oxidation to sulfate and nitrite, respectively, and assimilation as a nutrient resource. CKD incorporated low concentrations of Pb and As, below regulatory limits, and without an adverse effect on growth. Further studies with real flue gas are needed for biomass valorization and large-scale implementation of the cultivation strategy, where the gas and particulates serve as a nutrient resource.