Novel Regeneration and Utilization Concept Using Rich Chemical

Jun 21, 2019 - MCO2 and MC are the molar mass of CO2 and carbon, respectively. .... Sodium bicarbonate had been commonly used as a carbon source for ...
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Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 11720−11727

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Novel Regeneration and Utilization Concept Using Rich Chemical Absorption Solvent As a Carbon Source for Microalgae Biomass Production Chunfeng Song,† Yiting Qiu,† Meilian Xie,† Jie Liu,† Qingling Liu,† Shuhong Li,*,‡ Luchang Sun,§ Kailiang Wang,§ and Yasuki Kansha∥

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Tianjin Key Laboratory of Indoor Air Environmental Quality Control, School of Environmental Science and Engineering, Tianjin University, 92 Weijin Road, Nankai District, Tianjin 300072, China ‡ State Key Laboratory of Food Nutrition and Safety, College of Food Engineering and Biotechnology, Tianjin University of Science & Technology, No. 29, No. 13 Ave., TEDA, Tianjin 300457, China § China Huadian Engineering Co., Ltd., Beijing 100160, China ∥ Organization for Programs on Environmental Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan ABSTRACT: Integration of chemical absorption and microalgae bioconversion is a promising alternative to overcome the low CO2 fixation efficiency of conventional microalgae CO2 fixation technologies. In this study, two different bicarbonate sources obtained from ammonia and potassium carbonate CO2 absorption processes were used as carbon sources to cultivate Chlorella sp. L38, and different feeding modes were investigated. The experimental results indicated that the biomass concentration could be enhanced (up to 0.6 g/L) by using bicarbonate as carbon source instead of CO2. Particularly, nitrogen and carbon fixation capacity could achieve 48 mg/L and 49 mg/ L/d with a carbon fixation efficiency over 60% when using NH4HCO3 as a carbon and nitrogen source. Additionally, the protein, polysaccharides and lipid yields achieved 298, 12.5, and 59 mg/L, respectively, and were higher than the control condition. It could be observed that using bicarbonate as a linkage to integrate CO2 absorption and bioconversion may be a promising alternative to the conventional CO2 capture technologies.

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fertile land independency, and low energy consumption.9−11 By a photosynthesis process, CO2 in flue gases can be converted into algal biomass using solar energy. In addition, microalgae CO2 fixation can be integrated with wastewater purification through reuse of its rich nutrients.12−14 After cultivation, potential value-added ingredients (such as protein, polysaccharides, lipid, pigment, vitamins and antioxidants, etc.) can be extracted from algal biomass via a biorefinery approach to recycle carbon.15−17 However, one of the existing challenges for microalgae CO2 fixation technology is the low CO2 fixation efficiency due to the low CO2 solubility in the cultivation medium.18 The CO2 diffusion from the gas phase to the aqueous phase is a critical parameter for most photobioreactors. Due to its poor solubility, it is difficult to achieve a very high utilization of CO2. For instance, the carbon utilization efficiency could only achieve up to 38% in Spirulina cultivation and in some improved photobioreactors, CO2 fixation efficiency could

INTRODUCTION Climate change caused by greenhouse gas emissions has attracted much attention. Over 35 Gt CO2 is released annually due to fossil fuel combustion around the world.1,2 CO2 capture, utilization and storage (CCUS) is an effective approach to mitigate the greenhouse effect and climate change. To date, postcombustion CO2 capture has been the most utilized method, and includes absorption, adsorption, membrane, cryogenic and biological, etc.3,4 Among different CO2 capture technologies, chemical absorption has been considered as the most competitive in terms of commercial application. There are several typical solvents can be used as the CO2 absorption medium, such as ammonia, amines (e.g., monoethanolamine/MEA, diethanolamine/DEA, and methyldiethanolamine/MDEA), aqueous solvent (e.g., potassium carbonate), ionic liquid, etc. 5−7 Nevertheless, high energy consumption (approximately 2.2−6 MJ/kg CO2) caused by rich solvent regeneration is a major challenge of the absorption process.8 In addition, the issue of corrosion, toxicity, and secondary pollution needs to also be overcome before largescale application. Microalgae have the advantage of high growth rates, easy genetic tractability, impressive photosynthetic capabilities, © 2019 American Chemical Society

Received: Revised: Accepted: Published: 11720

April 18, 2019 June 8, 2019 June 21, 2019 June 21, 2019 DOI: 10.1021/acs.iecr.9b02134 Ind. Eng. Chem. Res. 2019, 58, 11720−11727

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Industrial & Engineering Chemistry Research achieve 33−58% via Chlorella sp.19−21 Increasing gas−liquid contact time or the interfacial area might be an effective way but energy-intensive. In order to improve carbon fixation efficiency, optimization of existing microalgae cultivation processes should be carried out. Typically, CO2 molecules enter microalgal cells in two pathways: (1) dissolved CO2 molecules reach the nucleoid structure by crossing the thylakoid membrane via diffusion of carbonic anhydrase; and 2) HCO3− is initially produced via extracellular reaction between CO2 and H2O, and then transported to the nucleoid structure where it is catalyzed to CO2 and fixed in the Calvin cycle.22,23 Until now, most of the attention related to microalgal-based CO2 fixation is put on the first pathway. Apart from intensifying CO2 transport through the microalgal cell, using HCO3− as a medium to enhance CO2 fixation might also be a potential pathway. The objective of this work is to investigate the possibility of combining CO2 absorption and microalgae utilization via bicarbonate (which can be obtained at the CO2 chemical absorption column). Specifically, two typical CO2 absorption solvents, ammonia and K2CO3, are selected to absorb CO2 and produce bicarbonate (NH4HCO3 and KHCO3). Subsequently, the obtained bicarbonate can be used as carbon source for microalgae growth to achieve the target of simultaneous CO2 capture and utilization in the proposed hybrid processes. To verify the feasibility of absorption-microalgae hybrid concept, the influence of rich solvent (i.e., NH4HCO3 and KHCO3) on the microalgae growth performance under different cultivation conditions was investigated. Biomass accumulation, nitrogen and carbon fixation efficiency, and the production of potential value-added ingredients were also evaluated.

concentration of 20.0 mmol/L, and tested three ratios of NH4HCO3 to KHCO3, including 1:5, 1:3, and 1:1. In addition, the batch feeding point were set at Day 0, 3, 6, 12. For each feeding time, the amount of bicarbonate supplement (in solution form) was the same and the total concentrations of bicarbonate added were 2.5, 10.0, and 20.0 mmol/L, respectively. Analysis Methods. After harvest, the microalgae were filtrated through a 0.45 μm membrane and then dried at 105 °C for 48 h for the dry weight measurements. Standard curve of biomass concentration (Xbiomass, mg/L) and the absorbance values were obtained by optical density (OD) measurements at the wavelength of 680 nm by an ultraviolet−visible (UV−vis) spectrophotometer (752N, INESA Analytical Instrument Co Ltd., China) via the following equation:24 Xbiomass = 0.243·OD680 + 0.002, R2 = 0.995

(1)

The OD680, controlled between 0.1 and 1.0, was measured using 3 mL of microalgae suspension of each sample. The biomass concentration was measured every 3 days. Biomass productivity (Pbiomass, mg/(L/day)) was measured according to the following equation: Pbiomass(mg/(L/day)) =

ΔX Δt

(2)

Where ΔX (mg/L) is the variation of biomass concentration at the corresponding cultivation period Δt (days). The ammonium-N concentration and nitrate-N concentration in the medium were determined by colorimetric methods.25 The liquid samples were collected from the Erlenmeyer flasks and filtered through a 0.45 μm membrane. For nitrate-N analysis, the absorbance of supernatant was measured by optical density (OD) measurements at the wavelength of 220 nm (OD220) and 275 nm (OD275) using the UV/vis spectrophotometer after appropriate dilution with deionized water. For ammonium-N analysis, after adding Nessler’s reagent, the absorbance of the mixture was measured by optical density (OD) measurements at the wavelength of 420 nm (OD420) using the UV/vis spectrophotometer. On the basis of the nitrogen concentration, the nitrogen fixation fraction (φN) and nitrogen recycle fraction (φN‑recycle) could be calculated by the following equations:

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MATERIALS AND METHODS Microalgae Strains and Cultivation Medium. Chlorella sp. L38 (obtained from the Algal Collection of Applied Microalgae Biology Laboratory of the Ocean University of China) was selected as a target algae for the absorptionmicroalgae hybrid CO2 capture system. The inoculum for the experiments was prepared by cultivate L38 with BG11 medium with a flow rate of 20 mL/min of 5% CO2. Microalgae cultivation was undertaken in a 250 mL Erlenmeyer flask filled with 200 mL culture medium (BG11). The illumination intensity was maintained at approximately 6000 Lux using a 24-h fluorescent lamp. The microalgae were precultured to an exponential phase and then inoculated into Erlenmeyer flasks with an inoculum size of the OD680 (the optical density measurements at the wavelength of 680 nm) at 0.2. The cultivation temperature was controlled at 30 ± 1 °C, for 21 days. Each condition was carried out in triplicate. Experimental Conditions. Ammonium bicarbonate (NH4HCO3) and potassium bicarbonate (KHCO3) were attained individually and mixed in a modified BG11 medium (without NaNO3). The two bicarbonates were assumed to be obtained at the bottom of an absorption column and added into the microalgae cultivation medium by a batch feeding mode. The initial pH was adjusted to around 8.0. When KHCO3 was used individually, it was supplemented with NaNO3 as a nitrogen source, and the amount of nitrogen was the same as that in the NH4HCO3. The concentration of each bicarbonate was investigated at 2.5, 10.0, and 20.0 mmol/L, respectively. The blend absorption solution (mixture of NH4HCO3 and KHCO3) was controlled with a total

φN(%) =

XbiomassNalgae Ninitial

φN − recycle(%) =

100%

(3)

Ninitial − Nresidual − XbiomassNalgae Ninitial

100% (4)

Where Ninitial (mg/L) is the nitrogen added to the culture medium, Nresidual (mg/L) is the nitrogen left in the culture medium. Nalgae (wt %) is the nitrogen content of the biomass determined by an elemental analyzer (EA3000, LEEMAN China Ltd.). The CO2 sequestration capacity (PCO2, mg/(L/day)) was calculated by the work of Xie et al.,26 as follows: ji MCO2 zyz PCO2 in biomass = PbiomassCalgaejjj z j MC zz k {

11721

(5)

DOI: 10.1021/acs.iecr.9b02134 Ind. Eng. Chem. Res. 2019, 58, 11720−11727

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Figure 1. (a) Algal biomass and (b) pH variation under the different CO2 absorption solution and mixture ratio.

where Calgae is the carbon content of the microalgae biomass determined by element analysis. MCO2 and MC are the molar mass of CO2 and carbon, respectively. After being dried and ground, all the algae powder samples were analyzed for carbon (C) and nitrogen (N) contents using an elemental analyzer (EA3000, LEEMAN China Ltd.). Measured values of nitrogen were multiplied by 6.25 (nitrogen-to-protein conversion factor) to estimate the protein content, as follows:27 X protein = ΔXNalgae6.25

the mean based on parallel experiments and were considered at 95% confidence intervals.

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RESULTS AND DISCUSSION Biomass Variation. On the basis of the algal biomass variation curves, as shown in Figure 1a, it could be observed that biomass accumulation increased with the initial concentration (from 2.5 to 20 mmol/L) of NH4HCO3 or KHCO3. Chi et al.30 also verified that high concentrations of bicarbonate could facilitate microalgal biomass accumulation, with Na2CO3 used as the CO2 absorption solvent to provide NaHCO3 as nutrient for microalgae. Low concentration bicarbonate solution led to insufficient nutrition and limited microalgae growth. Because of the batch-feeding mode adopted in the experiment, when the total bicarbonate concentration was low (such as 2.5 mmol/L), the supply of carbon and nitrogen in the early stage was very limited. Although the feeding was completed in the later stage, the overall growth trend of microalgae would become slow. For NH4HCO3, the biomass concentration varied from 0.34 to 0.55 g/L, and increased by 61%. By contrast, the biomass concentrations only increased by 18% (from 0.40 to 0.47 g/L) when the initial concentration of KHCO3 varied from 2.5 to 20 mmol/L. One possible reason for this situation was that the ammonium from the NH4HCO3 solution was more preferable for microalgae, while the nitrate added in the KHCO3 solution would be further transformed in cells, resulting in more ATP consumption.31 Another difference between the two bicarbonates was that the KHCO3 solution introduced more K+ in the medium and the mixed BG-11 medium already contained a certain amount of potassium (such as K2HPO4). It was reported that K+ on the envelope of chloroplast cells can regulate the stomata to maintain osmotic pressure but excess K+ can also disrupt the photosynthesis process, blocking the passage for toxic substances and gases exchange.32 Therefore, along with the increase of KHCO3 concentration, though more carbon was available, the accumulation of biomass was still not obvious, which was inferior to that in NH4HCO3 solution. When using the blend solution of NH4HCO3 and KHCO3, the biomass concentration tended to increase with the proportion of NH4HCO3 in the mixture, and achieved 0.61 g/L, whereas the ratio of NH4HCO3 to KHCO3 was controlled at 1:1. This was due to the ammonium nitrogen source being insufficient when the NH4HCO3 concentration was low and the K+ content was still relatively high. With the ratio of NH4HCO3

(6)

Polysaccharide concentration in the microalgae was measured using the modified phenol-sulfuric acid method.28 In detail, the algae samples were dried at 105 °C for 12 h and ground to powder for analysis. 3.0 mL of deionized water was added to 20 mg of microalgae powder, and stored in the water bath at 100 °C for 2 h. Subsequently, the supernatant was filtered after centrifugation, and was fixed with ethyl alcohol of 1:4 dilution at 4 °C for 12 h, centrifuged again and filter residue was added with deionized water. After blending, the polysaccharide concentration was measured using the phenol-sulfuric acid method. The polysaccharide yield (Xpolysaccharide, mg/L) was calculated using the following equation: X polysacccharide =

VH2Oxpolysaccharidef M microalgae

Xbiomass

(7)

Where xpolysaccharide (mg/L) is the concentration of polysaccharide according to the standard curve, f is the dilution ratio in the measurement. Mmicroalgae (mg) is the mass of microalgae powder sample. VH2O (mL) is the volume of deionized water added to the microalgae powder. Lipids concentration was measured by the standard curve method (the excitation and emission spectra were set at 465 and 580 nm, respectively) using a spectrofluorophotometer F96s (Shanghai Lengguang Technology Co., Ltd.,). The neutral lipids concentration in the sample, were expressed as triolein equivalents.29 The standard curve of lipid concentration (Xlipid, mg/L) and absorbance values were obtained by adding different concentrations of triolein solution to replace microalgae suspension, as in the following equation: Xlipid = 1.98OD580 − 7.10, R2 = 0.990

(8)

One-way analysis of variance (ANOVA) was used for statistical analysis. Results were expressed as means ± standard error of 11722

DOI: 10.1021/acs.iecr.9b02134 Ind. Eng. Chem. Res. 2019, 58, 11720−11727

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Industrial & Engineering Chemistry Research to KHCO3 increased from 1:5 to 1:1, the ratio of carbon and nitrogen was approximately 1.71, which was beneficial for the accumulation of microalgae biomass. However, some studies have shown the opposite results which presented maximum biomass with a relatively high C/N ratio when using nitrate as the nitrogen source. Nayak et al.33 selected different concentrations of NaHCO 3 (0, 2.5, 5, 7.5, 10 g/L) supplemented with nitrate from BG-11 medium to cultivate Chlorella sp. HS2, observing that addition of 7.5 g/L NaHCO3led to the highest biomass productivity and under this condition, the optimum C/N ratio was 4.32. Another study has also claimed that a C/N ratio of 4.84 offered by NaHCO3 and NaNO3 was more suitable for Scenedesmus sp. to obtain the highest biomass.34 In this study, when cultivated in the mixture of NH4HCO3and KHCO3 with a 1:5 ratio (C/N ratio was around 5.14), Chlorella sp. L38 did not grow well and with the increasing ammonium content, improved growth condition were obtained at the C/N ratio of 1.71. It is proposed that the different microalgae species preferred different C/N ratios under specific culture conditions, and the composition of the nitrogen source showed great significance. Undoubtedly, aside from the carbon and nitrogen element, phosphorus was also important to microalgae growth, though the demand for it was not high. In conventional BG-11 medium, the concentration of phosphorus was about 7.1 mg/L provided by K2HPO4. Therefore, a better C/N/P ratio obtained in the blend solution was 34:20:1. In addition, it could be also observed that the microalgae growth rate would increase after batch feeding at the third, sixth and 12th day, which indicated that the batch feeding mode could supply nutrition in time. Meanwhile, pH adjustment at batch-feeding point (the third, sixth, and 12th day) could also avoid highly alkaline conditions and promote microalgae growth, as shown in Figure 1b. The use of bicarbonate instead of CO2 for microalgae cultivation has attracted more attention in recent years. However, most research has focused on potassium/ sodium bicarbonate, and less attention has been paid to ammonium bicarbonate, which may trigger ammonia escape and ammonia toxicity. Sodium bicarbonate had been commonly used as a carbon source for the study of growth and biochemical composition in a range of microalgae species.35−37 White et al.38 also found addition of sodium bicarbonate (1 g/L) resulted in significantly higher final mean cell abundances for both T. suecica and N. salina species. The results from Pancha et al.39 clearly indicated that 0.6 g/L sodium bicarbonate was the optimum concentration resulting in 20.91% total lipid and 25.56% carbohydrate along with 23% increase in biomass production and bicarbonate addition could show stress ameliorating effects in nitrate and phosphate starvation conditions. The above studies showed bicarbonate played a significant role in biomass accumulation but concentrations needed to be controlled in a low range. However, in our study, high concentration of NH4HCO3 was optimum because the batch-feeding mode could not only effectively decrease ammonia stress for microalgae but also control bicarbonate addition to avoid high nutrition load. In addition, mixing two or more kinds of bicarbonate, which provided both carbon and nitrogen sources to replace BG-11 elements, was a promising idea for balancing microalgae nutrition and realizing economic viability. pH variation of the microalgae solution under different cultivation modes is shown in Figure 1b. When using inorganic carbon and nitrate as nutrients for microalgae cultivation, OH−

would be generated associated with photosynthesis process and released to the cultivation medium, and result in pH increase.40,41 By contrast, NH4+ escape or utilization via microalgae would lead to increased H+ concentration and pH drop.42 From the results in Figure 1b, it was found that the pH varied in the range of 8−10 for NH4HCO3 cultivation, and 8− 12 for KHCO3. With the increase in cultivation time, for both absorption solvents, the pH of medium presented increasing trends, and the increasing rate was obvious under higher bicarbonate concentration. The pH increase was due to substantial carbon utilization via efficient photosynthesis. Meanwhile, with the difference in nitrogen source (ammonia-nitrogen in NH4HCO3 and nitric nitrogen in KHCO3), the pH of medium was found to be lower for NH4HCO3 than that for KHCO3. This can be explained by the fact that the cell transport mechanism for the two nitrogen sources was different. In later cultivation periods, the pH of the medium decreased when using NH4HCO3 as an absorption solvent. The NH4HCO3 may have a buffer function which can mitigate sharp pH increases along with microalgae growth. By contrast, the pH would increase to 10−12 when using KHCO3 as a carbon source; this adversely affected microalgae growth. When using the mixture of NH4HCO3 and KHCO3 for microalgae cultivation, pH increases could be effectively controlled by increasing the NH4HCO3 proportion. For the blend solvent cultivation mode, the pH for the whole cultivation period varied in the range of 9−11. This is likely due to the ammonia nitrogen utilization via microalgae, escape, or buffering with bicarbonate, and further effort should be paid to the investigation of this mechanism. Nitrogen Migration. The variation in nitrogen concentration in the cultivation medium under different absorption solutions is shown in Figure 2a. It was found that the nitrogen concentration would decrease along with microalgae growth, and the rate of decrease became slower. Based on the results of biomass accumulation, this may be due to high nitrogen absorption rate in the initial cultivation. Compared with KHCO3 (193 mg/L), residual nitrogen concentration in the microalgae solution was 169 mg/L when NH4HCO3 was used as the absorption solution for microalgae cultivation. Roopnarain et al.43 also indicated that ammonium is the preferred nitrogen source for microalgae compared to nitrate. The reason might be that ammonia nitrogen is easier to be utilized via algae cell than nitrate nitrogen because of the much more reduced state of ammonium in comparison to nitrate. It could also be explained that for Chlorella sp. L38, the energy consumption for ammonia nitrogen assimilation might be lower than nitrate nitrogen.40 In comparison, more energy would be consumed to transform nitrate into microalgae protein or amino acids in the nitrate assimilation process. Meanwhile, part of ammonia nitrogen might escape as free NH3 in the highly alkaline conditions. Compared to NH4HCO3 alone (approximately 78.7 mg/L), residual nitrogen concentration in cultivation medium under the NH4HCO3 and KHCO3 mixed solution was only 43.0 mg/L. This was due to the pH of the mixture solvent being higher than NH4HCO3, and led to a higher NH3 escape. In addition, more nitrogen could be converted into biomass due to a better growth performance under the NH4HCO3 and KHCO3 mixed cultivation mode. The nitrogen migration and fixation fraction under different absorption solution cultivation modes are shown in Figure 2b, c. From Figure 2b, it can be observed that more nitrogen was 11723

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efficiency would decrease due to NH3 escape. Compared to the standalone absorption solution, using the NH4HCO3 and KHCO3 mixture to cultivate microalgae could improve nitrogen fixation capacity (up to 48 mg/L). With the ratio of NH4HCO3 to KHCO3 increased from 1:5 to 1:1, the nitrogen fixation fraction varied in the range of 20% ∼ 40%. Furthermore, the nitrogen recycle fraction also increased to 35%, which was higher than standalone absorption solution (approximately 22% at 10 mmol/L NH4HCO3 dosage). This may be due to the alkaline pH condition in the NH4HCO3 and KHCO3 blend solution. Carbon Fixation and Conversion Efficiency. The carbon sequestration capacity of microalgae under different absorption solution is shown in Figure. 3. When using

Figure 3. Carbon sequestration capacity under different CO2 absorption solvent and mixture ratio.

NH4 HCO3 as the absorption solution for microalgae cultivation, the carbon sequestration capacity was 24, 30, and 43 mg/(L/day) at the initial dosages of 2.5, 10, and 20 mmol/ L, respectively. By contrast, the carbon sequestration capacity was lower for the case of KHCO3, at 34 mg/(L/day) even under an initial dosage of 20 mmol/L. The carbon fixation performance could be enhanced using a blended absorption solution. When the ratio of NH4HCO3 to KHCO3 was set at 1:1, the carbon sequestration capacity could achieve 49 mg/ (L/day), which indicated that using a blended absorption solution might be beneficial to intensify carbon fixation and conversion efficiency. It should be noted that the CO2 fixation efficiency for conventional microalgae processes is only in the range of 10−40% because of the low CO2 solubility cultivation medium.44,45 Therefore, more effort is required to overcome this challenge. In the current process, instead of directly pumping CO2 into cultivation medium, CO2 was first converted into bicarbonate via an alkaline absorption solution, and then used as alternative carbon source for microalgae. Based on this innovative approach, carbon fixation efficiency could be improved to 60−80%,; 3−8 times higher than the conventional microalgae CO2 fixation processes.46 Value-Added Ingredient Production. Further to nitrogen and carbon fixation efficiencies, the potential value-added ingredients were also investigated, as shown in Figure 4. As presented in Figure 4a, the absorption solution had a

Figure 2. Nitrogen concentration (a) variation, (b) migration, and (c) fixation fraction in microalgae solution under different CO 2 absorption solvent and mixture ratios.

fixed via microalgae when feeding NH4HCO3 into the cultivation medium, and achieved 41 mg/L when the initial NH4HCO3 feeding concentration was set at 20 mmol/L, which was around twice as high than KHCO3 (only 24 mg/L). In addition, the nitrogen fixation capacity increased with the initial bicarbonate concentration, but the fixation fraction decreased. For example, the nitrogen fixation fraction dropped from 70 to 15% when initial NH4HCO3 concentrations varied from 2.5 to 20 mmol/L, as depicted in Figure 2c. This indicated that although increasing the dosage of absorption solution could facilitate microalgae growth, nitrogen utilization 11724

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Figure 4. Value-added ingredients yield under different CO2 absorption solvent and mixture ratio. (a) Protein, (b) polysaccharides, and (c) lipid.

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significant influence on the protein yield. Gorl et al.47 previously claimed that when the cultivation medium is lacking in nitrogen, phycocyanin and chlorophyll would be degraded within 1−10 days. If microalgae are deprived of nitrogen, the synthesis of biomolecules rich in nitrogen (such as proteins and chlorophylls) would be decreased and biomolecules rich in carbon (such as carbohydrates and/or lipids) are accumulated.48,49 When the initial feeding dosages of NH4HCO3 and KHCO3 were both increased to 20 mmol/L, microalgal protein yield was 254 and 153 mg/L, respectively. This was due to ammonia nitrogen being easier to convert into protein compared to nitrate. Therefore, for the blend absorption solution, increasing NH4HCO3 proportion was beneficial to improve protein yield. When the ratio of NH4HCO3 to KHCO3 was altered from 1:5 to 1:1, the protein yield increased to 298 mg/L. Figure 4b illustrates the polysaccharide yields under different absorption solution cultivation. Within contrast to protein, high polysaccharide yields were achieved at low initial absorption solution dosages. When the feeding concentration of NH4HCO3 or KHCO3 was set at 2.5 mmol/L, the polysaccharide yields were found to be highest. Previous literature has also indicated that the carbohydrate generating process happened in the Calvin cycle of photosynthesis and the nitrogen starvation is one of the most effective ways to promote carbohydrate accumulation.50,51 Particularly for KHCO3, the polysaccharides yield could achieve 12.5 mg/L. For the blend solution, the highest polysaccharides yield was 11.8 mg/L under the ratio of NH4HCO3 to KHCO3 at 1:5. The potential lipids yield for different conditions is depicted in Figure 4c. It was observed that although algal biomass concentration increased with the increased dosages of absorption solution, the increase in lipids yield was not significant. When KHCO3 was used as absorption solution and the initial dosage was set at 10 mmol/L, approximately 42.2 mg/L of lipids could be extracted from the microalgae biomass. In the blend absorption solution, increasing the NH4HCO3 proportion adversely affected the lipids accumulation, which was different to the biomass accumulation trends. Nayak et al.52 reported that nitrogen starvation has been regarded as a convenient strategy to stimulate lipid accumulation in the autotrophic cultivation of microalgae, because of its high efficiency and ease of execution. The detailed mechanism is that nitrogen depletion leads to changes of the cellular carbon flux from protein to lipid biosynthesis in microalgae.53 The highest lipid yield (59 mg/L) was obtained at the ratio of NH4HCO3 to KHCO3 set at 1:5. As a result, nitrate nitrogen might be more suitable for algal lipid production.

CONCLUSIONS Bicarbonate as a carbon source for microalgae could be an effective approach to integrate CO2 absorption and bioconversion. The experimental results indicated that microalgae CO2 fixation efficiency could be enhanced by 3−8 fold by using bicarbonate as alternative carbon source to CO2. When using NH4HCO3 (rich solvent of ammonia CO2 absorption) to cultivate Chlorella L38., nitrogen and carbon fixation capacity could achieve 48 mg/L and 49 mg/L/d, respectively. Furthermore, by producing potential value-added compounds (e.g., 298 mg/L protein, 12.5 mg/L/d polysaccharides and 59 mg/L/d lipid), the techno-economic feasibility of the proposed absorption-microalgae integrated CO2 capture and conversion process can be increased.

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]. Tel: +86-022-8740-1255. ORCID

Chunfeng Song: 0000-0002-9617-8297 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by the National Key Research and Development Program-China (2017YFE0127200), National Natural Science Foundation of China (21878228 and 31701526), Natural Science Foundation of Tianjin City (17JCQNJC08500), Young Elite Scientists Sponsorship Program by Tianjin City (TJSQNTJ-2017-03), and International Cooperation Research Centre of Carbon Capture in Ultralow Energy-consumption (Tianjin).

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DOI: 10.1021/acs.iecr.9b02134 Ind. Eng. Chem. Res. 2019, 58, 11720−11727

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DOI: 10.1021/acs.iecr.9b02134 Ind. Eng. Chem. Res. 2019, 58, 11720−11727