Overview of the potential of microalgae for CO2 sequestration
Int. J. Environ. Sci. Technol. (2014) 11:2103–2118 | DOI 10.1007/s13762-013-0487-6
V. Bhola • F. Swalaha • R. Ranjith Kumar • M. Singh • F. Bux
Institute for Water and Wastewater Technology, Durban
University of Technology, Durban, South Africa
College of Engineering, University of Georgia, Athens, USA
An economic and environmentally friendly approach of overcoming the problem of fossil CO2 emissions would be to reuse it through fixation into biomass. Carbon dioxide (CO2), which is the basis for the formation of complex sugars by green plants and microalgae through photosynthesis, has been shown to significantly increase the growth rates of certain microalgal species. Microalgae possess a greater capacity to fix CO2 compared to C4 plants.
Selection of appropriate microalgal strains is based on the CO2 fixation and tolerance capability together with lipid potential, both of which are a function of biomass productivity. Microalgae can be propagated in open raceway ponds or closed photobioreactors. Biological CO2 fixation also depends on the tolerance of selected strains to high temperatures and the amount of CO2 present in flue gas, together with SOx and NOx.
Potential uses of microalgal biomass after sequestration could include biodiesel production, fodder for livestock, production of colorants and vitamins. This review summarizes commonly employed microalgal species as well as the physiological pathway involved in the biochemistry of CO2 fixation. It also presents an outlook on microalgal propagation systems for CO2 sequestration as well as a summary on the life cycle analysis of the process.
Keywords: CO2 sequestration, Flue gas, Life cycle analysis, Microalgae, Photosynthesis
- Threat of global climate change has been mainly attributed to greenhouse gas (GHG) emissions of which carbon dioxide (CO2) contributes up to 68 % of total emissions (Brennan and Owende 2010; Ho et al. 2011; Kumar et al. 2011).
- This is partly due to the high-energy usage and the dependence on coal for electricity generation (Kumar et al. 2011).
- Biological CO2 fixation appears to be the only economical and environmentally viable technology of the future (Ho et al. 2011; Kumar et al. 2011).
- When compared to terrestrial plants, microalgae and cyanobacteria have faster growth rates, and their CO2-fixation efficiency is also between 10 and 50 times higher (Costa et al. 2000; Langley et al. 2012).
- Although terrestrial plants are responsible for fixing around 500 billion tones of CO2 per annum, they are expected to play a minor role (3–6 %) in the overall reduction in atmospheric CO2 (Skjanes et al. 2007).
- During the mid-1970s, the US Department of Energy (DOE) began encouraging research pertaining to microalgal wastewater treatment (Benemann et al. 1977).
- The use of microalgae can be classified as a direct CO2 mitigation technology.
- Microalgae which are primitive, unicellular, microscopic (2–200 lm) organisms that are the principal producers of O2 on earth (Khan et al. 2009).
- Microalgae are able to endure high concentrations of CO2, and this inherent ability makes them very advantageous in utilizing CO2 from flue gases of power plants.
- They are fast growers with biomass volumes that double within 24 h.
- At a flow rate of 0.3 L/min of air with 4 % CO2 concentration, most microalgal strains are able to achieve a carbon-fixation rate of roughly 14.6 gcm-2/day (Farrelly et al. 2013).
- Brackish aquatic environments appear to be ideal areas to sample for superior carbon sequesters as they are rich in dissolved CO2, O2 and dissolved salts.
- The drop in specific growth rate could be attributed to shear stress (Kumar et al. 2011).
- In 2002, Zhang et al. concluded that the gas transfer coefficient increases with a decrease in the CO2 concentration from the inlet gas stream.
- Henry’s law states that CO2 dissolves in water to an extent determined by its partial pressure (PCO2), temperature, as well as the interaction of dissolved CO2 with other solutes in the water Carroll and Mather 1992).
- Solubility of CO2 in freshwater is also significantly higher as opposed to solubility in salt water (Carroll and Mather 1992).
- Microalgae are able to fix CO2 from a range of sources, such as the atmosphere, industrial exhaust gases (flue and flaring gas) as well as in the form of soluble carbonates (NaHCO3 and Na2CO3) (Wang et al. 2008).
- In recent years, there has been talk of genetically manipulating microalgal strains to enhance their properties pertaining to carbon mitigation and mass culture (Farrelly et al. 2013).
- Microalgae and cyanobacterial species routinely used for CO2 mitigation include Anabaena sp., Botryococcus braunii, Chlamydomonas reinhardtii, Chlorella sp., Chlorocuccum littorale, Scenedesmus sp., and Spirulina sp. (de Morais and Costa 2007; Ota et al. 2009; Packer 2009; Chen et al. 2010; Chiang et al. 2011; Ho et al. 2011).
- Green microalgae that are effective carbon sequesters generally belong to the genera Chlorococcum, Chlorella, Scenedesmus and Euglena.
- Specific growth rate and CO2-fixation rate during closed cultivation were observed to be 1.78 and 5.39 times higher that of open cultivation, respectively.
- Under the proper cultivation mode, Chlorella sp. exhibit much potential as effective carbon sequesters.
- This essentially means that 1 g of CO2 consumed corresponds to the release of 0.74 g of O2.
- Low culture densities exposed to high light intensities would lead to photoinhibition of cells.
- Studies have also suggested that growth under red light (600–700 nm) enhanced PSII relative to PSI, whereas blue light (400–500 nm) could induce PSI. These findings suggest that blue and red lights are more suitable than others for both microalgal cell growth and CO2 mitigation (You and Barnett 2004; Ravelonandro et al. 2008).
- When assessing the CO2 balance of a system, it is necessary to take into account the total discharge from fossil energy versus the CO2 uptake of the microalgae during cultivation (Khoo et al. 2011).
- Microalgae are promising candidates for CO2 mitigation, which aids in combating GHG-related environmental impacts.
- In comparison with terrestrial plants, microalgae are capable of fixing CO2 at a rate several times higher than plants owing to their high photosynthetic efficiencies.