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Atmospheric CO2 Capture and Biofuel Production of Microalgae

Purpose, Identity of Microalgae

The increasing amount of carbon dioxide produced from burning fossil fuels has led to a climate change that could have serious and catastrophic consequences. Therefore, any method that can capture the carbon dioxide from flue gas and prevent it from entering the atmosphere is of the utmost importance. Microalgae can be used to not only capture CO2 from flue gas, but the captured CO2 can be utilized as biofuels.[endnoteRef:2] Because of their ability to fix CO2 as either dissolved CO2 or CO2 salts such as Na2CO3, NaHCO3, or H2CO3, microalgae have higher photosynthetic efficiency than terrestrial plants.[endnoteRef:3] This fact leads many researchers to believe that microalgae are the best candidates for carbon capture. Like all plants, microalgae get their energy from a process known as the Calvin Cycle (scheme 1). Three molecules of Ribulose 1,3 bisphosphate react with three molecules of CO2 catalyzed by the enzyme rubisco to generate six molecules of 3-phosphoglycerate (the carbons that come from carbon dioxide are in red). These six molecules are reduced using six molecules of ATP and six molecules of NADPH (the ATP and NADPH are generated during the light dependent stage of photosynthesis) to produce six molecules of glyceraldehyde 3-phosphate. Five of these molecules are used to regenerate the three molecules of ribulose 1,3 bisphosphate and one is used to make half of glucose (2 cycles are necessary to produce one full glucose molecule). Under certain conditions, the glucose is broken down and stored as triglycerides (TGs). [2: Saifuddin, N.; Aisswarya, K.; Juan, Y. P; Priatharsini, P. Sequestration of High Carbon Dioxide Concentration for Induction of Lipids in Microalgae for Biodiesel Production. Journal of Applied Sciences 2015. 15(8), 1045-1058.] [3: Moreira, D.; Pires, J.C.M. Atmospheric CO2 capture by algae: Negative carbon dioxide emission path. Bioresource Technology 2016. 215, 371-379. ]

Philip Mannino

Tingyu Wen

Kaixuan Xu

A few groups have proposed that these TGs, which are carbon dense, would be best stored underground so as to lower the atmospheric CO2 concentration.[endnoteRef:4] Many other groups, however, have proposed utilizing these TGs as biofuels. The most widely accepted method for converting these TGs into biofuel is through a process known as transesterification1 (scheme 2). In this process, the TGs are reacted with three equivalents of an alcohol, typically methanol, producing three long chain esters and glycerol. The three long chain esters would then be isolated and used as biofuels. [4: R. Sayre. Microalgae: The Potential for Carbon Capture. BioScience 2010, 60, 722-726.]

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Scheme 1: The Calvin Cycle

Scheme 2: Transesterification

Microalgae lipid through transesterification reaction with microwave irradiation:

The suitability of Nannochloropsis sp. Lipid was determined by Fourier Transform Infra-Red Spectrometry (FT-IR). In the FT-IR spectra, every peak is assigned to a functional group. There are two shape peaks at 2839 and 2949 cm-1, showing the vibration of C-H bond on carbohydrates and lipid. The peak at 1012 cm-1 indicates C-O-C bond of polysaccharides. The bending of methyl lipids was shown at 1450 cm-1. A broad peak at 3251 cm-1 shows the O-H stretching from water and N-H stretching from proteins of Nannochloropsis sp. Another functional group from the proteins is at 1643 cm-1, indicating amide 1 (C=O) stretching. These results show the presence of methyl and methylene groups, indicating the methyl ester formation. FT-IR spectra also shows that the most suitable time for microwave treatment for biodiesel production is 30 min. (Figure 1) 1

Figure 1: FT-IR spectra at three different duration of microwave treatment

Growth and Lipid Productivity of Nannochloropsis sp. With different concentrations of CO2 in Gaseous and Solution Form:

Microalgae grow at higher CO2 concentration (1–10%) present higher biomass productivity compared to those grow at atmospheric CO2 concentration. Table 1 summarizes the experiments done on microalgae for atmospheric CO2 capture. CO2 removal rates and biomass productivity when growing autotrophically were recorded. A pre-processing of gaseous stream may be performed to concentrate CO2 to increase the efficiency of CO2 capture by microalgae. Besides the CO2 concentration, temperature and light intensity can also affect microalgae growth and, consequently, the CO2 fixation rates and biomass productivity. The photosynthetic efficiency of microalgae and CO2 solubility drop at high temperatures. Intense light penetrates better into high-density cultures.2

Nannochloropsis sp. strain was cultivated within five different concentrations of CO2 (1, 10, 15, 20, and 25%) (v/v). According to figure 2, CO2 concentration at 15% gave the most rapid growth of Nannochloropsis sp., producing the highest dry biomass weight of 0.441 g/L at day 8. The second highest dry weight is 0.392 g/L on day 8 at 10% of CO2 concentration. Throughout the cultivation period, the Nannochloropsis sp. strain indicated an increasing trend at the first 8 days at 10%, 15%, and 20% concentrations of CO2. At 1% CO2 as a control group, the strain grew much slower than other concentrations. The concentration of CO2 in the atmosphere is about 0.0387% (v/v)[endnoteRef:5], which is not as high as the best growth condition for microalgae. However, from combustion processes, wastes gases contains more than 15% CO2, showing a possibility as a source for industrial microalgae production. From other researchers’ study, there is a specific growth rate of Nannochloropsis sp., increasing from 0.33-0.52/day cultivated with atmosphere air with 15% CO2.[endnoteRef:6] All the results show that high concentration of CO2 boosts photosynthetic efficiency. Thus, microalgae can produce a higher quantity of biomass within a shorter time. On the other hand, the Nannochloropsis sp. strain produced maximum amount of lipid with 15% CO2 showing in Figure 3. The total lipid productivity was determined after eight days of cultivation under five different CO2 gas concentrations. The highest yield of lipid productivity is 18.93mg/L per day with 15% CO2. [5: Kumar, A.; S. Ergas, X.; Yuan, A.; Sahu; Zhang, Q. et al., Enhanced CO2 fixation and biofuel production via microalgae: Recent developments and future directions. Trends Biotechnical. 2010, 28, 371-380.] [6: Jiang, L.; Luo, S.; Fan, X.; Yang, Z.; Guo, R. Biomass and lipid production of marine microalgae using municipal wastewater and high concentration of CO2. Applied Energy 2011, 88, 3336-3341.]

Solute carbonates or dissolved CO2 can be absorbed directly by microalgae. In this study, the ability of microalgae growth with carbonate salts from fuel gases was measured by a series solutions containing HCO3- and CO32- in five concentrations (2,5,10,15,20% (v/v)). The results show an increasing rate on the dry biomass weight as the carbonate concentration increased. (Figure 4) The highest dry biomass of 0.55g was found at the 20% carbonate on day 10, corresponding to the highest dry biomass of 0.44g at the 15% CO2 gas on day 8. These observations indicate that growth rate of microalgae in solute carbonate condition is better and faster than in CO2 gas conditions. Figure 5 shows the effect of series fuel gas solution on lipid productivity. Nannochloropsis sp. produce more in a more concentrated fuel gas condition. The maximum yield of lipid productivity was found at 20% carbonate solution with 23 mg/L per day. It was also 27% higher than that produced by the 2% carbonate solution.

Table 1: Atmosphere carbon capture by microalgae.

Figure 2: Comparison of dry biomass vs times of Nannochloropsis sp. strain cultivated in five different flasks with CO2 concentrations of 1, 10, 15, 20 and 25% (v/v).

Figure 3: Comparison of lipid productivity vs CO2 level of Nannochloropsis sp. strain cultivated in five different flasks with CO2 concentrations of 1, 10, 15, 20 and 25% (v/v).

Figure 4: Comparison of dry biomass vs times of Nannochloropsis sp. strain cultivated in sodium bicarbonate/sodium carbonate series solutions with concentration of 2, 5, 10, 15, 20% (v/v).

Figure 5: Comparison of lipid productivity vs flue gas solution (%) of Nannochloropsis sp. strain cultivated in sodium bicarbonate/sodium carbonate series solutions with concentration of 2, 5, 10, 15, 20% (v/v).

Merits and Deficiencies

Compared to terrestrial plants, microalgae have higher efficiency in converting solar energy into biomass (ten times higher than terrestrial plants). These microorganisms present high growth rate with some species double their cell count in few hours. High growth rate ensures high CO2 fixation rate. 1.83 kg of CO2 can be fixed via cultivating one kilogram of microalgae. Species such as Anabaena sp. and Chlorella vulgaris presents high CO2 fixation rates at 1.45 g L-1 d-1 and 6.24 g L-1 d-1, respectively.[endnoteRef:7] Compared to other plants, arable land not required with microalgae production. They are also easier to grow. Even lower grade water (wastewater) could be used as culture medium. Anaerobic digestion of microalgae, which includes processes such as: hydrolysis, acidogenesis, acetogenesis and methanogenesis, converts organic biomass into volatile fatty acids and methane under little or without oxygen conditions.[endnoteRef:8] In addition to converting lipid to ester as biofuel (mentioned in the beginning), some species are able to produce a considerable amount of methane. Saccharina latissima and Ulva lactuca can produce 68.2 and 95.6 ml of methane per gram of algae, in 36 and 42 days respectively.[endnoteRef:9] If the purity of recovered methane is higher than 95%, the