The oxidation product (NO3) of NO pollutant in flue gas used as a nitrogen source to improve microalgal biomass production and CO2 fixation

Jun Cheng*, Yun Huang, Hongxiang Lu, Rui Huang, Junhu Zhou and Kefa Cen
State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China. E-mail: juncheng@zju.edu.cn; Fax: +86 571 87951616; Tel: +86 571 87952889

Received 9th June 2014 , Accepted 29th August 2014

First published on 29th August 2014


Abstract

In order to eliminate the inhibition effect of the toxic nitric oxide (NO) in flue gas on microalgal growth and CO2 fixation, NO was converted by a wet UV/H2O2 method to produce nitrate (NO3), which then be used as a nitrogen source for microalgae to improve its growth. The growth ability and biomass compositions of the microalgae cultivated with the produced NO3 from NO gas were similar to those of the microalgae cultivated with equivalent moles of commercial NaNO3. The NO3 concentration produced from NO increased with UV lamp power, H2O2, and NO concentrations, resulting in an improved microalgal growth. The concentration of NO3 from 500 ppm NO wet-oxidized by 6% (v/v) H2O2 and 55 W UV light was up to 8.8 mM. When the produced nitrate was used as supplementary nitrogen source, the maximum growth productivity of Chlorella PY-ZU1 at 15% (v/v) CO2 reached 1.18 g L−1 per day (0.97 times higher than that cultivated with the standard medium). The peak fixation efficiency of 15% (v/v) CO2 was 69.6% (1.13 times higher than that cultivated with the standard medium).


1. Introduction

Pollutants (including CO2, NOx, SO2, and fine particles) are released into the atmosphere when fossil fuels are burned. As a result, environment and human health are seriously harmed. For example, the greenhouse effect occurs because of excessive CO2 concentrations in the atmosphere, and this condition has caused problems in terms of environmental and energy aspects. Thus, CO2 emissions should be reduced using efficient and economical methods. For microalgae has a higher growth rate (1 to 3-fold increases in biomass per day), and can fix CO2 with efficiency (2–10%) ten times greater than that of terrestrial plants (<1%), one of the efficient CO2 reduction methods involves the cultivation of microalgae in photobioreactors supplied with CO2-enriched gas streams, such as those emitted from coal-fired power plant flue gases.1–4 In addition, the CO2 capture process using microalgae has the following advantages: (i) co-producing high value materials based on biomass, such as biofuel and biogas;5–10 (ii) being an environmental sustainable method that can be connected to urban and industrial sewage cleaning.11

Some high CO2-tolerant microalgae species have been isolated out.12–16 However the inhibitory effects of toxic compounds, such as NOx and SO2, in addition to high CO2 concentrations, on microalgae can be critical.17–21 It was reported that NO in fossil fuel flue gas can be removed and used by the microalgae, Dunaliella tertiolecta.22 However, for almost all of the other microalgal species, the presence of NO will lead to the formation of toxic nitrites or pH decrease in their culture, therefore, it will hinder their growth and CO2 fixation.17–21,23,24

In recent years, some studies have focused on the alleviation of the effect of NO on microalgae growth. These studies have shown that the growth and survival of Synechococcus sp. and Chlorella sp. have improved against exposure to intermittent NO2 by adding growth stimulators, such as triacontanol and sodium bicarbonate.25 The tolerance of Chlorella KR-1 to continuous NO exposure can be enhanced by maintaining the pH of the culture media at an adequate value (∼7), which is achieved by adding an alkaline solution (NaOH).19 However, this condition can be effective for some specific microalgae only. A previous study also showed that the presence of NO may lead to the formation of toxic nitrites in microalgae culture, therefore, its inhibitory effect on microalgae growth was evaluated.24 It must take some techniques making NO dissolve into less NO2 but to more usable substances, such as NO3.

Advanced oxidation process (AOP) can produce free radicals with strong oxidation, such as hydroxyl free radicals (˙OH). By a wet AOP using hydrogen peroxide solution with ultraviolet lamp (UV/H2O2), the toxic NO was completely converted into valuable NO3 without generating any other byproduct.26–29 The wet AOP (UV/H2O2) has been used in coal-fired power plants to simultaneously remove NO, SO2 and Hg pollutants in flue gas. But how to deal with and reutilize the large amount of byproducts (nitrate, sulfate and Hg2+) is a big problem. Whether the oxidation byproduct (NO3) derived from the wet AOP can be consumed and used by microalgae has not been reported in literatures till now. Whether the different oxidation conditions (UV lamp power, H2O2 and NO concentrations) in wet AOP (UV/H2O2) have important effects on microalgae growth has not been clarified. It was first proposed to reutilize the oxidation byproduct (NO3) derived from the wet AOP by microalgae as a supplementary nitrogen source in this paper. This novel process not only eliminated the effect of toxic NO on microalgal growth but also improved microalgal biomass productivity and CO2 fixation. The effects of different UV/H2O2 conditions on microalgal growth and CO2 fixation efficiency were investigated.

2. Materials and methods

2.1 Strains and media

Chlorella PY-ZU1, a highly CO2-tolerant and fast-growing microalgal species, was used in this study. This strain was obtained by γ irradiation and high concentrations of CO2 domesticated from Chlorella pyrenoidosa.15 The cells were maintained in Brostol's solution (also known as soil extract, SE),15,30 containing 0.25 g of NaNO3, 0.075 g of K2HPO4·3H2O, 0.075 g of MgSO4·7H2O, 0.025 g of CaCl2·2H2O, 0.175 g of KH2PO4, 0.025 g of NaCl, 40 mL of soil extract, 0.005 g of FeCl3·6H2O, 1 mL of Fe–EDTA, and 1 mL of A5 solution in 958 mL of de-ionized water.

2.2 System design by which the oxidation product of NO in flue gas with UV/H2O2 is used as a nitrogen source for microalgal growth

Because of its strong oxidation ability and environmentally friendly characteristics, UV/H2O2 AOP has a wide range of studies in the gas purification field. Experimental system in which the NO in flue gas was converted to NO3 as nitrogen source for microalgal growth was performed in a bubble column reactor (Fig. 1). The proposed system comprised the following: (1) 3000 ppm of NO and pure N2 (used as balance gas); (2) mass flow meter; (3) a bubble column reactor (height of 450 mm and inner diameter of 75 mm); (4) cooling water cycle system; (5) sand chip gas distributor (outer diameter of 45 mm, height of 30 mm, and average pore size of 0.105 mm to 0.18 mm); (6) UV lamps (UV lamp powers were changed by replacing and using three sets of UV lamps with different powers (36 W, 55 W, and 75 W, Haining Light Factory). All the lamps were of the same model (L-L) and of the same wavelength of 253.7 nm); and (7) effluent NO scrubber (the residual NO in the mixed gas was further scrubbed using 400 mL mixed solution containing KMnO4 (0.05 mol L−1) and NaOH (0.1 mol L−1; Sinopharm Chemical Reagent, China) to avoid environmental pollution).
image file: c4ra05491a-f1.tif
Fig. 1 Experimental system in which the NO in flue gas was converted to NO3 as nitrogen source for microalgal growth.

The prepared H2O2 solution with the required concentration (1%, 3%, 6%, and 9%) was placed in the bubble column reactor. Temperature was maintained at 25 °C by recycling the cooling water. NO concentration (75, 150, 300, and 500 ppm, balanced with N2) was regulated using a mass flow meter (SevenstarCS200, China). The NO gas passed uniformly across the sand chip gas distributor into the H2O2 solution at a rate of 600 mL min−1. After the UV lamp was turned on, H2O2 was released, forming hydroxyl free radicals (˙OH). These free radicals exhibit an extremely strong oxidation ability that can convert NO into HNO3 without generating any other byproduct via the following reactions (2)(3).26,31

 
H2O2 + hv → 2˙OH (1)
 
NO + ˙OH → HNO2, HNO2 + ˙OH → HNO3 + ˙H (2)
 
NO + ˙OH → ˙H, NO2 + ˙OH → HNO3 (3)

The reaction solution was collected after 6 h and the remaining H2O2 was removed by ultrasonic wave (SK5210HP, China). The solution was then used to make the medium for Chlorella PY-ZU1 by adding the same quantities of nutrients as those present in the SE medium. The initial pH of the medium was adjusted to 6.5 with 0.1 M NaOH. The SE medium was used as the control condition. For the final AOP runs, NO in the reactor was 500 ppm, H2O2 concentration was 6% (v/v), and UV power was 55 W. The medium prepared with the 15 h oxidation solution was used as the CO2 fixation medium and labeled as SE#.

2.3 NO3 produced from NO oxidation used as supplement nitrogen source to improve Chlorella PY-ZU1 growth and CO2 fixation

All of the cultivation experiments were performed in an artificial greenhouse at 27 °C. Approximately 270 mL SE medium was inoculated with 30 mL of Chlorella PY-ZU1 pre-culture in the bioreactor (BR, 160 mm × Φ56 mm, 300 mL of working volume). For the verification experiments of using NO3 (derived from NO oxidation by UV/H2O2) as a nitrogen source for Chlorella PY-ZU1, continuous light of 52 μmol m−2 s−1 at the surface of BR was supplied by four cool white lights combined with two plant lights (Philips, TLD 36W) that were fixed above the BR. For the other experiments in this study, 68 μmol m−2 s−1 of light was supplied by six cool white lights (Philips, TLD 36W) at the surface of BR. The mixed gas of 15% (v/v) CO2 containing different NO concentrations was bubbled at a rate of 30 mL min−1 through a long steel pipe (180 mm × Φ3 mm). The NO concentrations were controlled at 0, 75, 150, 300, and 500 ppm by a mass flow meter (Sevenstar CS200, China).

Chlorella PY-ZU1 was cultured in SE# and aerated continuously with 15% (v/v) CO2 in nine-stage sequential bioreactors30 to investigate the effect of NO3 produced from NO on CO2 fixation. For comparison, Chlorella PY-ZU1 was cultured with SE medium and aerated continuously with 15% (v/v) CO2 or with 15% (v/v) CO2 gas containing 500 ppm NO. The influent and effluent CO2 concentrations were monitored online by a CO2 analyzer (Servomex4100, UK). CO2 fixation efficiency was calculated according to the carbon dioxide difference between influent and effluent as described in a previous study.30

 
image file: c4ra05491a-t1.tif(4)
where the total input CO2 = influent CO2 concentration × influent flow rate, and the total output CO2 = effluent CO2 concentration × effluent flow rate.

2.4 Analysis of microalgal productivity and biomass compositions

During cultivation, 10 mL of the samples was dewatered by centrifugation (Beckman Avanti J26-XP, USA) at 8500 rpm for 10 min and dried at 70 °C for 24 h to obtain the weight of the dried biomass. Biomass concentration (g L−1) was calculated from the microalgal dry weight produced per liter. Growth productivity (AGP, g L−1 per day) was calculated using eqn (5):
 
image file: c4ra05491a-t2.tif(5)
where M1 is the biomass concentration at time t1 and M2 is the biomass concentration at time t2. Total carbohydrate quantity was determined using the anthrone method (with glucose as the standard).8 The lipid of the biomass was extracted as described in a previous study.6 Fatty acid compositions were determined by gas chromatography (Agilent 7890A, USA).

2.5 Calculation of NO oxidation efficiency and residual NO concentration

The NO3 concentrations in the collected solution as prepared in Section 2.2 were analyzed with ion chromatography (MagIC, Metrohm, Switzerland). The NO oxidation efficiency (mean value) was calculated according to NO3 in the solution using eqn (6):
 
image file: c4ra05491a-t3.tif(6)
where image file: c4ra05491a-t4.tif is the molar concentration of NO3 in volume V (L) of the oxidized solution and image file: c4ra05491a-t5.tif is the total number of moles of NO flowing into the oxidation reactor. In this study, NO3 was the only product of NO oxidation; thus, NO oxidation efficiency also corresponded to NO3 production efficiency. The remaining NO concentration (mean value) was calculated using eqn (7):
 
CNOout = CNOin × (1 − NO oxidation efficiency (7)

3. Results and discussion

3.1 Effects of NO on the growth of Chlorella PY-ZU1

The effects of NO concentrations on the growth of Chlorella PY-ZU1 and the pH of the culture were examined in the BR (Fig. 2). Chlorella PY-ZU1 showed a higher tolerance to NO than other NO-tolerant algal strains, which could not grow under 150 ppm NO.20 When aerated with 15% CO2 gas containing 150 ppm NO, biomass concentration of Chlorella PY-ZU1 decreased after 5 days of cultivation, and the pH of culture decreased to 6.27. The maximum biomass concentration was 2.03 g L−1 and decreased by 24.3% to that of microalgae cultivated without NO aeration (2.68 g L−1). When NO concentration was further increased to 500 ppm, microalgae could grow but with a 50.7% decrease in the maximum biomass concentration to that of microalgae cultivated without NO. The decrease in biomass yield was due to pH decrease in the culture caused by NO aeration.19,20 The pH of the culture decreased with the increasing cultivation time. Once the pH of the culture decreased beyond the adequate range (6.5–7.5 for Chlorella), the microalgae growth was inhibited. This was why the biomass concentration of Chlorella PY-ZU1 decreased after 5 days cultivation with >150 ppm of NO. However, Chlorella PY-ZU1 showed a higher tolerance to NO than Chlorella KR-1,20 whose growth was completely suppressed when aerated with 15% CO2 gas containing 300 ppm NO. This verified that microalgae tolerance to NO depends on the microalgae species but with a decrease in biomass productivity.19
image file: c4ra05491a-f2.tif
Fig. 2 Effects of NO on Chlorella PY-ZU1 growth and pH of the cultures.

Some methods were used to alleviate microalgae growth inhibition caused by NO, such as controlling culture pH and adding some growth stimulators to culture.25 Although Dunaliella tertiolecta could use NO dissolved in microalgae culture as a nitrogen source, NO absorbed in the medium could be converted to NO2 and then oxidized to NO3.22 This oxidation process was extremely slow. The improvement effect of little NO3 produced from NO on Chlorella PY-ZU1 did not overcome the toxic effect of NO. Thus, a much faster NO oxidation method will be needed.

3.2 Confirmation of using NO3 (derived from NO oxidation by UV/H2O2) as a nitrogen source for Chlorella PY-ZU1

During UV/H2O2 AOP process, the remaining H2O2 concentration in the solution was decreased with the oxidation time, resulting in a decrease in NO3 production efficiency.26 In the process of 500 ppm NO oxidized by 55 W UV/6% H2O2, the NO3 production rate was stabilized at 0.427 mM h−1 and 53% of NO was converted into NO3 in the first 6 h [Fig. 3(a)]. In the next 6 h, the NO3 production rate gradually decreased to 10.65% with H2O2 digestion. After 15 h, NO3 concentration in the solution reached to 8.8 mM. The total NO3 concentration in the medium prepared with this oxidation solution was 11.8 mM, which could satisfy the NO3 requirement of Chlorella PY-ZU1 under 15% CO2.30 Chlorella PY-ZU1 cultivated in the SE# medium under 52 μmol m−2 s−1 of continuous light and 15% CO2 for 11 d exhibited a peak growth productivity and maximum biomass concentration of 0.76 g L−1 per day and 5.48 g L−1, respectively. These values were almost equal to those of Chlorella PY-ZU1 (0.73 g L−1 per day and 5.31 g L−1, respectively) cultivated in the SE medium with 11.8 mM commercial NaNO3. In addition, the growth curve of Chlorella PY-ZU1 cultivated with NO3 produced from NO is consistent with that of the Chlorella PY-ZU1 cultivated with commercial NaNO3 [Fig. 3(b)].
image file: c4ra05491a-f3.tif
Fig. 3 Microalgal growth with NO3 derived from NO oxidation and commercial NaNO3.

The total carbohydrate quantity of the dried biomass of Chlorella PY-ZU1 cultivated with NO3 produced from NO (41.57%, w/w biomass) was almost equal to that of the Chlorella PY-ZU1 cultivated with commercial NaNO3 (43.57%; data not shown). The lipid contents in the two biomasses were 18.11% and 17.92%, respectively. The biodiesel compositions from these two kinds of biomasses were analyzed (Table 1). The fatty acid profiles indicated the presence of C16: 0, C16: 1, C16: 2, C16: 3, C18: 0, C18: 1, C18: 2, and C18: 3. Palmitic acid, oleic acid, linoleic acid, and linolenic acid were considered as the main components, which ranged from 12% to 24% of the total fatty acids. These results indicated that oxidation product of NO (derived from NO in flue gas by UV/H2O2) can be used as a nitrogen source for Chlorella PY-ZU1 instead of the commercial NaNO3.

Table 1 Compositions of lipids in microalgae cultivated with commercial NaNO3 and NO3 derived from NO oxidation
Conditions Commercial NaNO3 NO3 derived from NO oxidation
Lipid content (% of dry biomass) 17.92 18.11
Lipids composition (% of total lipid) C16: 0 23.85 ± 0.29 22.37 ± 0.10
C16: 3 7.02 ± 0.34 6.80 ± 0.29
C18: 0 3.15 ± 0.26 3.17 ± 0.01
C18: 1 15.88 ± 0.75 14.82 ± 0.76
C18: 2 15.52 ± 0.83 14.76 ± 0.57
C18: 3 12.77 ± 0.34 12.65 ± 0.46
Others (C16–C24) 21.8 ± 0.63 25.4 ± 0.45
Total 100 100


3.3 Effects of different NO conversion conditions on the growth of Chlorella PY-ZU1

The NO3 concentration produced from NO increased with increase of lamp power, H2O2, and NO concentration. As a result, microalgae growth was improved. Under UV light irradiation, H2O2 can release ˙OH free radicals. ˙OH free radicals exhibit strong oxidation ability to convert NO to NO3.26,29 A high concentration of produced NO3 in AOPs results in a high biomass yield during microalgae cultivation.30,32

NO3, the oxidation product derived from 300 ppm NO with 6% H2O2 for 6 h, could increase the biomass productivity of Chlorella PY-ZU1 under 15% CO2 as UV lamp power was increased (Fig. 4). The maximum biomass concentration of microalgae was evidently increased from 3.45 g L−1 to 3.85 g L−1 [Fig. 4(b)] as UV lamp power increased from 36 W to 55 W. However, with further increasing the UV lamp power from 55 to 75 W, the growth rate of maximum biomass concentration gradually decreased. Two main reasons could explain the results. On one hand, under UV light irradiation, H2O2 can release ˙OH free radicals by eqn (1) reaction.26 The ˙OH free radicals have extremely strong oxidation ability to convert NO into NO3 according to eqn (2) and (3). Therefore, compared with the reaction system without UV light, addition of UV light can greatly enhance NO conversion into NO3. Furthermore, increasing UV lamp power can improve the energy density per unit in solution, thus produce more effective photons and ˙OH free radicals. Therefore, the NO3 produced rate increased with an increase in UV lamp power.26,31 Consequently, the maximum biomass concentration of Chlorella PY-ZU1 was increased. On the other hand, once the power of UV lamp exceeds a certain value, some side reactions, such as eqn (8) and (9), may occur in the solution, leading to a great loss of ˙OH free radicals.27 Therefore, a further increase in UV lamp power only has a little impact on NO3 production and thus a little effect on the growth of Chlorella PY-ZU1.

 
H2O2 + ˙OH → HO2˙ + H2O (8)
 
˙OH + ˙OH → H2O2 (9)


image file: c4ra05491a-f4.tif
Fig. 4 Effects of UV lamp power and H2O2 concentration on NO3 production and microalgal growth.

Similarly, the NO3 production efficiency derived from NO (300 ppm) by UV/H2O2 (55 W of UV for 6 h) increased from 56.60% to 79.33% and the derived NO3 concentration increased from 2.70 mM to 3.79 mM [Fig. 4(c)] when H2O2 concentration increased from 3% to 6%. This finding resulted in an evident increase in the maximum biomass concentration of microalgae from 3.43 g L−1 to 3.85 g L−1 [Fig. 4(d)]. However, a further increase in H2O2 concentration from 6% to 9% did not increase the maximum biomass concentration (stabilized at 3.91 g L−1). This is mainly because appropriate H2O2 concentration may cause a reaction such as eqn (1) in the solution. Therefore, within a certain range, the increase in H2O2 concentration can improve the yield of NO3,26 and then increased the biomass growth of Chlorella PY-ZU1.25 Once H2O2 concentration exceeding a certain value, any further increase may cause side reactions as eqn (8) and (9) which lead to a decrease in the oxidation ability of free radicals.27 Therefore, further increase in H2O2 concentration only had little effect on the yield of NO3 and a slight impact on biomass production of Chlorella PY-ZU1.

NO3 production efficiency decreased from 91.26% to 53.00% [Fig. 5(a)] as NO concentration increased from 75 ppm to 500 ppm because of the limitation of NO residence time and ˙OH free radicals.26,31 However, the derived NO3 concentration from NO increased from 1.09 mM to 4.22 mM; thus, the maximum biomass concentration of Chlorella PY-ZU1 increased from 3.05 g L−1 to 4.15 g L−1 [Fig. 5(b)].


image file: c4ra05491a-f5.tif
Fig. 5 Effects of NO concentration on NO3 production and microalgal growth.

3.4 CO2 fixation by Chlorella PY-ZU1 cultivated with NO3 derived from NO oxidation

When 500 ppm NO was directly aerated into microalgal culture, biomass production was decreased by 50.7% to that of 2.68 g L−1 of microalgae cultivated without aerated NO (Fig. 2(a)). By contrast, biomass production increased when 500 ppm NO was converted into nitrate by UV/H2O2 as a supplement nitrogen source for microalgae under continuous light of 68 μmol m−2 s−1. Overall, the maximum biomass concentration and peak growth productivity of Chlorella PY-ZU1 were 5.40 g L−1 and 1.18 g L−1 per day. These dependent parameters increased by 107.7% and 96.7%, respectively, compared with those of the microalgae cultured in the SE medium (2.68 g L−1 and 0.60 g L−1 per day, respectively) (Fig. 6).
image file: c4ra05491a-f6.tif
Fig. 6 CO2 fixation and biomass growth of Chlorella PY-ZU1 cultivated with NO3 derived from NO oxidation.

Although Chlorella can tolerate up to 50% concentration of CO2, the biomass concentration does not reach a higher value (almost <1 g L−1).33 That makes CO2 mitigation by microalgae difficult. The appropriate concentration of CO2 for microalgae growth is always below 10%. Anjos et al. optimized CO2-mitigation by Chlorella vulgaris P12 under different CO2 concentrations (ranging from 2% to 10%). Results showed that 6.5% was the most appropriate CO2 concentration for Chlorella P12.34 When Chlorella pyrenoidosa was cultivated with SE medium, experiments also showed that 6% was the most appropriate CO2 concentration.15 In order to increase the ability of Chlorella to grow under higher CO2 concentrations, Chlorella pyrenoidosa was mutated by nuclear irradiation and domesticated with high concentrations of CO2 in our previous study. The most appropriate CO2 concentration for the mutant Chlorella PY-ZU1 was up to 12% (v/v).15,30

CO2 fixation experiments were performed in a nine-stage sequential bioreactor described in the previous studies.15,30 The sequential bioreactor was filled with SE# medium and operated for 2 days without microalgae to determine the abiotic removal of CO2. Hence, the abiotic removal of CO2 should be eliminated in the calculation of CO2 fixation efficiency by microalgae.

In the nine-stage sequential bioreactor, the CO2 fixation efficiency of the microalgae cultivated at 500 ppm NO was lower than that of the microalgae cultivated without NO (Fig. 6). The peak CO2 fixation efficiency of 26.2% was decreased by 19.9%, whereas the mean CO2 fixation efficiency of 17.3% was decreased by 33.2%. However, when 500 ppm NO was converted into NO3 by UV/H2O2 as a supplement nitrogen source for Chlorella PY-ZU1, CO2 fixation efficiency was higher than that of microalgae cultured in the SE medium without NO. The peak and mean CO2 fixation efficiency were 69.6% and 52.3%, respectively, increased by 112.8% and 101.9% compared with those of the microalgae cultivated in the SE medium without aerated NO (32.7% of the peak CO2 fixation efficiency and 25.9% of the mean CO2 fixation efficiency).

Ramanan et al. has demonstrated an increase in CO2 fixation efficiency by maneuvering chemically aided biological sequestration of CO2. Chlorella sp. showed the peak CO2 fixation efficiency of 46% at input CO2 concentration of 10%.35 Chiu et al. replaced a half of the culture broth with fresh medium every day to enhance growth rate of Chlorella sp. and CO2 reduction. The CO2 fixation efficiency of Chlorella sp. was 16% at input CO2 concentration of 15%.36 In this study, the produced NO3 from the oxidation of 500 ppm NO was used as supplementary nitrogen source. The peak CO2 fixation efficiency of Chlorella PY-ZU1 was 69.6% at input CO2 concentration of 15%. These results indicated that NO3 derived from NO oxidation as a nitrogen source for microalgae growth can overcome the toxic effect of NO and improve microalgal biomass production and CO2 fixation.

4. Conclusions

NO pollutant in flue gas could be converted into useful NO3 by UV/H2O2 oxidation. The NO3 product can be used as a nitrogen source to improve microalgal growth and CO2 fixation ability. When NO3 derived from 500 ppm NO oxidation was used as a nitrogen source, the peak growth productivity and CO2 fixation efficiency of Chlorella PY-ZU1 were increased by 96.67% (1.18 g L−1 per day) and 112.8% (69.6%), respectively. This finding provided information regarding environmental and economical benefits to culture microalgae with waste carbon and nitrogen sources (exhaust CO2 gas and NO oxidation products) in flue gas.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (51176163, 51476141), National High Technology R&D Program of China (2012AA050101), International Sci. & Tech. Cooperation Program of China (2012DFG61770), Zhejiang Provincial Natural Science Foundation of China (LR14E060002), Program for New Century Excellent Talents in University (NCET-11-0446), Specialized Research Fund for the Doctoral Program of Higher Education (20110101110021), Science and Technology Project of Guangxi Province (1346011-1).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra05491a

This journal is © The Royal Society of Chemistry 2014
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