CO2 Fixation, Lipid Production, and Power Generation by a Novel Air

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CO2 fixation, lipid production and power generation by a novel air-lift-type microbial carbon capture cell system Xia Hu, Baojun Liu, Jiti Zhou, Ruofei Jin, Sen Qiao, and Guangfei Liu Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 13 Aug 2015 Downloaded from http://pubs.acs.org on August 14, 2015

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Environmental Science & Technology

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CO2 fixation, lipid production and power generation by a novel air-lift-type

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microbial carbon capture cell system

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Xia Hu, Baojun Liu, Jiti Zhou*, Ruofei Jin, Sen Qiao and Guangfei Liu

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Key Laboratory of Industrial Ecology and Environmental Engineering, Ministry of

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Education, School of Environmental Science and Technology, Dalian University of

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Technology, Linggong Road 2, Dalian 116024, China.

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* Corresponding author. Tel. /fax: +8641184706252.

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E-mail address: [email protected]

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ACS Paragon Plus Environment


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Abstract

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An air-lift-type microbial carbon capture cell (ALMCC) was constructed for the

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first time by using air-lift-type photobioreactor as the cathode chamber. The

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performance of ALMCC in fixing high concentration of CO2, producing energy

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(power and biodiesel), and removing COD together with nutrients, was investigated

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and compared with the traditional microbial carbon capture cell (MCC) and

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air-lift-type photobioreactor (ALP). The ALMCC system produced a maximum power

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density of 972.5 mW·m-3 and removed 86.69% of COD, 70.52% of ammonium

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nitrogen and 69.24% of phosphorus, which indicate that ALMCC performed better

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than MCC in terms of power generation and wastewater treatment efficiency. Besides,

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ALMCC demonstrated 9.98 and 1.88 fold increase over ALP and MCC in CO2

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fixation rate, respectively. Similarly, the ALMCC significantly presented a higher

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lipid productivity compared to those control reactors. More importantly, the

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preliminary analysis of energy balance suggested that the net energy of the ALMCC

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system was significantly superior to other systems and could theoretically produce

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enough energy to cover its consumption. In this work, the established ALMCC system

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simultaneously achieved the high level of CO2 fixation, energy recycle and municipal

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wastewater treatment effectively and efficiently.

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1. Introduction

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Global warming induced by a rapid increase of CO2 concentration in the

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atmosphere has been identified as one of the major worldwide challenges for

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environmental sustainability1, 2. Microalgae cultivation in municipal wastewater for

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CO2 fixation is considered as one of the most promising and environmental-friendly

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approaches.3,

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ways for biodiesel production and municipal wastewater treatment, and also for

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reducing the need of chemical fertilizers and algae’s life cycle burden.5 However, the

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nitrogen concentration in municipal wastewater is significantly lower than that in the

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medium of microalgae cultivation, which often leads to a relatively low biomass

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productivity, and thus low CO2 fixation efficiency and overall lipid productivity.6

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Fortunately, these concerns can be effectively addressed when microalgae are

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cultivated in the cathode of microbial fuel cells (MFCs), which have been developed

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as an emerging bioelectrochemical technology for wastewater treatment and electrical

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energy generation recently. 7-10 In dual-chamber MFCs, the cation species (e.g., Na+,

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K+, NH4+, Ca2+, and Mg2+) other than protons are primarily responsible for the

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transport of positive charges across Nafion membrane and their concentrations are

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typically 105 times higher than the proton concentration, which resulted in

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accumulation of these cations at the cathode chamber.11, 12 It is most intriguing that the

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species of NH4+ could be substantially accumulated in the cathode chamber. Therefore,

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the sufficient nitrogen would be provided for microalgal growth as long as microalgae

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are cultivated in the cathode of MFCs together with a continuous feeding of municipal

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wastewater into the anode of MFCs.

4

Apart from CO2 fixation, microalgae cultivation provides possible

3



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Recently, microbial carbon capture cells (MCCS) emerged as a new sustainable

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form of MFCs with an algae-assisted cathode, which have attracted much attention in

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the fields of wastewater treatment, clean electric energy production and CO2

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sequestration of discharged from anode in MFCs.13, 14 However, those investigations

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on MCCs generally showed low biomass productivities, which led to low CO2

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fixation efficiency and biodiesel production6, due to the low mixing efficiency and

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photosynthetic rate. To address this inefficiency, the air-lift photobioreactor (ALP)

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may bring a good complement to MCC, owing to its excellent mixing efficiency and

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best volumetric gas transfer through its liquid cylindrical column. In some previous

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reports, the microalgae cultivated in the bioreactors yielded the maximum biomass

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productivity and better specific growth rate.15, 16 In light of the synergetic advantages

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of MCC and ALP, a system could be established by linking the two devices for the

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high concentration of CO2 fixation. Thus, an air-lift-type microbial carbon capture cell

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(ALMCC) could be established by installing the ALP in a MCC as the cathode

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chamber. In the anode compartment of the ALMCC, the organic matters in municipal

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wastewater are oxidized to carbon dioxide, producing protons and electrons.

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Meanwhile, the effluent of the anode containing the residual inorganic nutrients could

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be used as the medium of microalgae cultivated in the cathode chamber of ALMCC.

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This system will achieve high biomass concentration to fix high concentration of CO2

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due to a good mixing in the ALP and constant supplement of nitrogen and minerals

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into cathode of ALMCC through Nafion membrane. In return, the oxygen

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continuously produced from microalgae photosynthesis is used as electron acceptor 4


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for the electricity generation,17 which avoids the use of noble or non-noble catalysts

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for oxygen reduction, resulting in the enhancement of the economic viability and

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environmental sustainability.18 Furthermore, CO2 is added to the cathode chamber for

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CO2 fixation, which also could create a CO2/bicarbonate buffered catholyte system,19,

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20

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Through this combination, the high concentration of CO2 could be fixed and

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bioenergy (bioelectricity and biodiesel) could be produced. At the same time, COD

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and nutrient in municipal wastewater could be removed without extra cost.

thereby diminishing pH imbalances and increasing power density of ALMCC.

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In this study, the ALMCC system was constructed for the first time for

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high-density cultivation of microalgae to fix high level CO2, generate bioenergy and

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treat municipal wastewater. Its performance was investigated in terms of CO2 fixation,

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C, N and P removal, power generation and lipid accumulation. To evaluate the

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advantages of the system, the performance comparison were carried out among the

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ALMCC, MCC and ALP. Moreover, the energy balance of the three reactors was

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analyzed for better understanding of the ALMCC system.

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2. Material and methods

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ALMCC setup

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The ALMCC system was comprised of double-chamber and separated by a

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proton exchange membrane (126 cm2, Nafion 117, Dupont, US). As shown in Figure

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1, it was constructed by assembling two half-cube structures together with rubber

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gaskets. The anode chamber was a simple cube with a liquid volume of 600 mL. The

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cathode chamber of ALMCC was an air-lift-type cubical photobioreactor with a 5



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centric tube and the liquid volume was 500 mL. The sparger at the bottom of the

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cathode chamber of ALMCC received continuous aeration of 10% CO2 with 60

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mL·min-1 during microalgae cultivation to achieve high mass transfer for producing

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more biomass, although the periodic dosing with CO2 rich microbubbles was reported

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to be just as effective in growing microalgae.21 A 10 cm long carbon brush (5 cm in

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diameter) was used as the anode electrode. Prior to usage, the carbon brush was

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pretreated by heating at 450 °C for 30 min.13 The cathode electrode was carbon fiber

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cloth (5 cm × 15 cm =75 cm2). Then the anode and cathode electrodes were connected

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by copper wires and the external resistance was 1000 Ω. Saturated calomel electrode

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(SCEs; Spsic-Rex Instrument Factory, China) was inserted into anode chamber as a

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reference electrode. Both a cubical double-chamber MCC and an independent ALP

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were set up for control experiments.

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Inoculation and operation

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The ALMCC system was operated with a synthetic solution at about 30°C. The

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anode chamber was inoculated with electrochemically active bacteria suspension from

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a MFC using sodium acetate for more than half a year. The synthetic municipal

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wastewater contained sodium acetate, 280 mg·L-1; NH4+-N, 39.04 mg·L-1; PO43--P,

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3.62 mg·L-1; NaHCO3, 600 mg·L-1; NaCl, 12.5 mg·L-1; MgSO4, 6.25 mg·L-1; CaCl2,

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33.34 mg·L-1 and 1 mL·L-1 of trace elements. The synthetic municipal wastewater

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was fed into the anode chamber at a flow rate of 0.8 mL·min-1. The anode chamber

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was covered with tinfoil to avoid illumination and to prevent the growth of algae. The

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anode effluent was autoclaved at 121 °C, followed by cooling to room temperature, 6


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and used as medium of microalgae cultivation in the cathode chamber of the ALMCC.

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The anode effluent was autoclaved to avoid nutrients competition by microbes and

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achieve isolating the role of microalgae growth.

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The cathode chamber was inoculated with the green algae Chlorella vulgaris

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(ESP-6) obtained from National Cheng Kung University previously maintained in

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axenic culture. Before being added into the cathode chamber, the algal suspension

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was centrifuged (8000 rpm for 10 min) and washed 2 times using deionized water

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under sterile conditions to remove residual carbon source for isolating the role of CO2

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fixation, and then re-suspended in sterilized anode effluent. The cathode chamber was

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sterilized with 75% alcohol solution and refilled with sterile anode effluent with

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Chlorella vulgaris (0.1 g·L-1) as inoculum. A continuous illumination of fluorescent

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lamp (light intensity of 8.9 W·m-2) was employed around the cathode. Evaporated

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water from the cathode chamber was replenished with sterilized anode effluent.

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Analysis and calculations

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The cell voltage was recorded by a data acquisition system (PISO-813, ICP DAS

150

Co., Ltd.) every 30 min. Power density and current density were calculated based on

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the effective volume of anode chamber according to a previous report.22 After stable

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voltage outputs, polarization curves were obtained by varying the external resistances

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from 1000 to 50 Ω.13 The concentrations of COD, NH4+-N, NO3--N, NO2--N, and

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PO43--P were measured according to the standard method (APHA, 1999).17 The pH

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value was measured with a pH meter (METTLER TOLEDO FE20K, Switzerland).

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Dissolved oxygen (DO) was measured by a DO meter (YSI 550A, America). The 7



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biomass concentration of Chlorella vulgaris was determined as absorbance23, 24 at 690

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nm by UV-Vis Spectrophotometer (Jasco V-560) and converted to biomass

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concentration using the standard curve shown in Supporting Information (SI) Figure

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S1. The procedures of scanning electron microscopy (SEM) are shown in the SI. CO2

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fixation rate was calculated according to a previous report25 and the equation is shown

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in the SI. The oil-droplet of Chlorella vulgaris stained with Bodipy 505/515 was

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observed by confocal laser scanning microscopy (CLSM, OlympusFV1000).26 The

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lipid content was determined using the method of FTIR spectroscopy (Bruker

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VERTEX 70, Germany) and its accuracy has been validated by traditional methods.27,

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28

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3. Results and discussion

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Power generation of ALMCC

The measurements of lipid content and productivity are described in the SI.

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The ALMCC system was operated for 6 days after the acclimation stage. A MCC

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as control and the operations was identical to the ALMCC. Figure 2A shows the

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electricity productions of ALMCC and MCC with an external resistance of 1000 Ω.

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The maximum voltage output was 0.49 V in the ALMCC, which was higher than that

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of the MCC (0.42 V). Figure 2B exhibits the polarization and power density curves.

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The peak power density in the ALMCC reached to 972.5 mW·m-3, which was 46.47%

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higher than that (520.6 mW·m-3) produced from the MCC. The difference between

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two systems was mainly ascribed to the performance of cathodes (the anode chambers

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of ALMCC and MCC were identical), whereas the cathode reaction relied on the

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dissolved oxygen (DO) produced by algae in the cathode compartment.23 As shown in 8


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Figure 2C, the DO concentration was 9.74 ± 0.16 mg·L-1, which was 27.31% higher

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than that in the MCC (7.08 ± 0.19 mg·L-1) at the same operation. The DO levels

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approached saturation in the current study, which is an inhibitor of higher microalgal

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metabolic activity. However, such high DO level was then counterproductive. It might

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be due to that O2 was effectively stripped by dosing with CO2 rich microbubbles.21 It

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also was demonstrated that the dosing of CO2 and the stripping of O2 were sufficiently

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fast because CO2 was always in excess (saturated) in the culture medium and O2 was

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removed as fast as it was produced.29 Actually, the DO generation in the two reactors

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mainly relied on the photosynthesis reaction of algae in their cathode chambers. The

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algae concentration was 3.04 ± 0.16 g·L-1 in the ALMCC on the 6th day, and it was

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about twice higher compared to that in the MCC (Figure 3A), which indicates that the

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power output is limited by algal photosynthesis.

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In addition, the difference of electricity production between the ALMCC and

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MCC was also due to the fact that the internal resistance (152.02 Ω) in the ALMCC is

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lower than that of MCC (204.05 Ω). The internal resistance was proved to be a key

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factor on the electricity production of MFCs.22, 30 In the ALMCC, the cultivation of

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algae in the air-lift-type photobioreactor enhanced the efficient mixing of algal cells,15

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which reduced the liquid-phase mass transfer resistance. Unlike the ALMCC cell, the

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substantial amount of algal cells grown on the wall and bottom of the cathode in the

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MCC increased both mass transfer resistance and internal resistance. Even, a part of

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algal cells adhering to the carbon cloth may lead to higher mass transfer resistance of

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proton and oxygen. It is worth noting that, in the ALMCC, the carbon cloth could be 9



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immobilized by centric-tube in the ALP (cathode chamber) close to proton exchange

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membrane, which led to the electrode spacing of 1 cm. Such immobilization could not

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be achieved in the cathode chamber of the MCC, hence the electrode spacing of about

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2 cm was adopted. It was confirmed again that the power generation of MFC can be

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enhanced by reducing the electrode spacing.22 The above results indicate that the

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ALMCC can increase the electricity generation.

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The pH of the catholyte (the medium of microalgae culture) decreased from 7.72

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± 0.13 to 6.84 ± 0.02 in the ALMCC and from 7.58 ± 0.09 to 6.79 ± 0.06 in the MCC

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(Figure 2D) till a stable pH of catholyte (nearly 7) were obtained, which might be due

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to the combined effect of oxygen reduction and CO2 buffering in cathode.23, 31 It was

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reported that CO2 can be utilized as pH buffer to replace the expensive and

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non-sustainable buffer solution which is not practical for large scale application.19, 20

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In general, the pH can be increased from 6.7-7.4 to 8.0-9.0 during algal

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photosynthesis because of the consumption of carbon dioxide.32-34 To counteract such

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change, CO2 is injected into the culture medium as a pH controller. In the present

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study, 10% CO2 was aerated into the cathode compartments of the ALMCC in

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response to pH controller and carbon source of algal cultivation. Because the biomass

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concentration of Chlorella vulgaris was the highest at the 10% CO2, which was

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23.68%, 29.27% and 48.68% higher than that under CO2 levels of 5%, 15% and 20%,

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respectively (SI Figure S2). It was also demonstrated that the best algal growth

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potential could be obtained under 10% CO2.35, 36 Too high concentration of CO2 (>

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20%) would induce low pH, which may cause the decrease of the activity of carbonic 10


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extracellular anhydrase37,

38

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cathode chamber. Therefore, 10% CO2 was continuously added, not only for algal

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growth as carbon source, but also as the catholyte buffer system and pH regulator of

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ALMCC.

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COD and nutrients removal in the ALMCC system

and lead to the imbalance of pH between anode and

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The organics and nutrients removal was also investigated during electricity

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generation. The COD decreased to 24.16 ± 7.72 mg·L-1 from 181.45 ± 14.68 mg·L-1

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in the anode of ALMCC and got the removal efficiency of 86.69% (Table 1), which

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was higher than that in MCC (82.45%). At the end of operation, no COD was detected

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in the cathodes of both ALMCC and MCC, which might be assimilated by Chlorella

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vulgaris because it can grow mixotrophically with high biomass concentration.26 As

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shown in Table 1, NH4+-N concentrations were reduced to less than half of the initial

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concentration in both the anode chambers of ALMCC and MCC. It could be ascribed

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to NH4+ transport into the cathode chamber through the Nafion membrane.11 NO3--N

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and NO2--N were not observed in the cathode chamber because of the absence of

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aerobic bacteria and the presence of the DO. The constant supply of NH4+ from anode

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into cathode increased the removal efficiencies of total nitrogen to 70.52% in the

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ALMCC and 60.29% in the MCC. The removal rates of PO43--P were 69.24% and

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65.39% in the ALMCC and the MCC systems, respectively. PO43--P concentrations

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were decreased due to the bacterial uptake in anode compartment and the algal uptake

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in cathode compartment. The above results indicate that the removal of carbon and

244

nutrients in the ALMCC was higher than that in the MCC system. 11



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CO2 fixation and lipid accumulation in the ALMCC

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To explore the advantages of ALMCC on CO2 fixation and lipid accumulation,

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the cells growth, CO2 fixation rate and lipid accumulation of Chlorella vulgaris in the

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ALMCC were investigated and compared with the other two control reactors. As

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indicated in Figure 3A, Chlorella vulgaris grew better in the ALMCC though the

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same concentrations (0.1 g·L-1) were inoculated. The biomass concentration reached

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3.04 ± 0.16 g·L-1 on the 6th day, resulting in the biomass productivity of 482.50 ±

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20.03 mg·L-1·d-1 and the CO2 fixation rate of 887.81 ± 36.86 mg·L-1·d-1. These results

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clarify that the CO2 fixation rate was 1.88 and 9.98 fold higher than those of in the

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MCC and ALP, respectively (Figure 3B). On the one hand, Chlorella vulgaris cultured

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in air-lift-type photobioreactor as the cathode of ALMCC have the higher mixing

256

efficiency, mass transfer and photosynthetic rate compared with the MCC. These

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advantages of ALMCC are attributed to the regular circulation of algal cultivation in

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air-lift-type photobioreactor through the air rising from the centric tube, which makes

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the circulatory liquid flow out on top of the centric tube and gravitationally forced

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downward.39 However, the gas rising in the cathode of MCC, was the only driving

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force for the culture mixing of microalgae. It has been well studied that the regular

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circulation of the culture could lead to a more effective mixing for microalgae growth,

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which enhances the biomass productivity of microalgae.16,40 Furthermore, the CO2

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fixation rate of Chlorella vulgaris cultivated in an independent ALP was significantly

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lower than that in the ALMCC, which could be due to the cation species (Na+, K+,

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NH4+, Ca2+, and Mg2+) could be transported to cathode chamber in MFCs by Nafion 12


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membrane and their concentrations in cathode chamber were several orders of

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magnitude larger than proton concentration.11, 12 These cation species are essential

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nutrients and micronutrients for the algae growth and metabolism,41 and especially the

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nitrogen source as a key factor. Because the synthetic municipal wastewater was fed

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continuously in the anode of ALMCC, nitrogen and minerals were supplied

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continuously from anode to cathode of ALMCC, which could provide sufficient

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nutrients for the algae growth. Comparatively, nitrogen concentration in the ALP from

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the anode effluent of ALMCC was only (17.06 ± 0.69 mg·L-1) and consumed

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gradually with algal growth. In addition, no additional nitrogen source was supplied

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(meaning that Chlorella vulgaris grew under nitrogen deficiency), which led to the

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low biomass productivity, and moreover the cells were severely broken. Unlikely, the

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algal cells nearly kept the morphologies (with little variation) in the ALMCC and

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MCC (Figure 3C). It was demonstrated that nitrogen deficiency or starvation pressure

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could lead to the increase of the cell morphological complexity.26 Thus, the excellent

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CO2 fixation ability makes the ALMCC as an efficient and potential candidate for

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CO2 mitigation.

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With the efficient microalgae cultivation, the lipid accumulation could be

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achieved besides the power generation and CO2 fixation in this study. CLSM and

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FTIR were combined to investigate the lipid accumulation of Chlorella vulgaris.

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Oil-droplet formation of algal cells stained with Bodipy 505/515 was observed by

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CLSM. As shown in Figure 4A, the green fluorescence (oil-droplet) intensities of

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microalgae in the ALP were stronger than those cultivated in the ALMCC and MCC, 13



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which indicated that the lipid content was higher in the ALP. However, there was little

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difference of green fluorescence intensities in microalgae between the ALMCC and

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MCC. To further determine accurately the lipid content of Chlorella vulgaris, FTIR

292

spectroscopic method was chosen for the quantitative analysis of the lipid content,

293

and the peaks in the range of 2800-3000 cm-1 can well characterize the lipid content

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changes of Chlorella vulgaris (Figure 4B). The lipid contents and productivities were

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calculated by using the equation in the SI. As shown in Figure 4C, the lipid contents

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of Chlorella vulgaris were 22.47 ± 2.86 and 20.86 ± 1.93% in the ALMCC and MCC,

297

which were about the half of the results from the previous studies,42,

298

because the microalgae were cultivated in the systems with nitrogen sufficiency.

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However, we observed the different result when Chlorella vulgaris grew in the ALP,

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the lipid content of Chlorella vulgaris reached 56.75 ± 0.92%, which was 2.53 fold

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higher than that in the ALMCC. The recent evidence indicates that the lipid content in

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Chlorella vulgaris could be doubled or even tripled under nitrogen starvation.44

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However, the lipid productivity (109.44 ± 5.71 mg·L-1·d-1) in the ALMCC was still

304

2.87 and 1.85 fold higher than those in the ALP and MCC, because the biomass

305

productivity of microalgae in the ALMCC was 9.78 and 1.84 fold higher than that in

306

the ALP and MCC, respectively. The above results again demonstrate that the

307

ALMCC can be used to cultivate microalgae as biodiesel feedstock.

308

The energy balance analysis

43

possibly

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It is necessary to analyze the energy balance of three different systems since the

310

pumping and aerating consumed additional electric energy. The preliminary analysis 14


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of energy balance included the processes of energy production and consumption but

312

excluded the processes of upstream. It was demonstrated that the municipal

313

wastewater used as culture medium of microalgae could offset most of the energy

314

consumption derived from upstream processing of the algae cultivation.5 The energy

315

production and consumption of three different systems were evaluated as shown in

316

Table 2. Energy production was comprised of electric energy and biodiesel. Electric

317

energy was derived from the power production of the MFC, while biodiesel came

318

from the lipid accumulation of microalgae with the conversion efficiency of 30%

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from diesel to electricity.23 Energy consumption consisted of the pumping system

320

(anolyte feeding), aeration system (10% CO2 aeration) and biodiesel producing

321

(dewater, oil extraction and fuel conversion). The energy consumption of the pumping

322

system was estimated according to a previous report.45 The aeration system was

323

calculated based on the aeration efficiency of 1.2 kg O2·kWh-1.46 The biodiesel

324

producing includes dewater, oil extraction and fuel conversion processing, while the

325

energy consumption was 0.17, 0.21 and 0.17 kWh per unit of kWh of biodiesel

326

produced, respectively.47 As shown in Table 2, the energy recovery in the ALMCC for

327

6 days reached 1.034 kWh·m-3 and was significantly superior compared to the other

328

two control systems. The highest energy production in the ALMCC could be ascribed

329

to the significantly apparent energy recovery. With regard to the ALP system, the

330

energy production was the least among the three systems, because the biodiesel

331

production was considered as the only energy source, and the biodiesel productivity

332

was still the lowest compared with the other two systems. In practical, the ALMCC 15



333

system could produce more energy. The energy production can be increased by

334

improving the biodiesel and electric energy production. The lipid productivity of the

335

Chlorella vulgaris can be obviously increased through two-stage cultivation process

336

(First stage: Chlorella vulgaris is cultivated under nutrient sufficiency to improve the

337

biomass productivity; Second stage: after the algae have grown to stationary phase,

338

the culture is transferred to nutrient-deficient condition to increase the lipid content).36,

339

43

340

improved through the optimization of cathode catalyst and operation.49, 50

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Supporting Information

Importantly, the electric energy production in the MFC can be substantially

342

The methods of calculating CO2 fixation rate, lipid content and productivity,

343

energy consumption of pumping system, and the procedures for SEM of microalgae

344

cells are described in the Supporting Information (SI). The standard curve for

345

calculation of algal biomass concentration and the biomass concentrations of

346

microalgae under different CO2 levels are shown in Figure S 1-2. This information is

347

available free of charge via the Internet at http://pubs.acs.org/.

348

Acknowledgements

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We wish to thank Professor Jo-Shu Chang in Department of Chemical

350

Engineering, National Cheng Kung University, and Professor Xin-Qing Zhao in the

351

school of life science and biotechnology, Dalian University of Technology for the

352

supply of Chlorella vulgaris, ESP-6 for this study. We wish to extend appreciation to

353

Dr. Qidong Zhao and Dr. Pancras Ndokoye in the school of Environmental Science

354

and Technology, Dalian University of Technology for grammar errors and polishing of 16


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23



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503 504

Table 1. Characteristics of the synthetic solution in the ALMCC and MCC (mg·L-1)

Anode influent of ALMCC Anode effluent of ALMCC Cathode effluent of ALMCC Anode influent of MCC Anode effluent of MCC Cathode effluent of MCC

COD

NH4+-N

PO43--P

181.45 ± 14.68 24.16 ± 7.72 N/D 181.45 ± 14.68 32.03 ± 3.76 N/D

39.04 ± 1.23 17.06 ± 0.69 11.51 ± 0.99 39.04 ± 1.23 17.89 ± 0.74 15.50 ± 0.70

3.62 ± 0.19 2.14 ± 0.15 1.11 ± 0.08 3.62 ± 0.19 2.31 ± 0.2 1.25 ± 0.12

505

24


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506 507

Table 2. Energy production and consumption in the ALMCC, MCC and ALP ALMCC

Input

Output

Net

MCC

ALP

-3

-3

(kWh·m )

(kWh·m )

(kWh·m-3)

pump

-0.000018

- 0.000018

N/A

aeration

-0.082

-0.082

-0.082

biodiesel production

-1.192

-0.619

-0.382

electricity

0.140

0.075

N/A

biodiesel

2.168

1.126

0.695

total

1.034

0.501

0.231

508

25



509 510

A

511 512

B

513 514

C

515 516 517

Figure 1. Structure (A), photo (B), and schematic illustration of the functional principles (C) of ALMCC. 26


Page 26 of 33

Page 27 of 33


518 519

A

520 521

B

522 523 524

C

525 27



526 527

D

528 529

Figure 2. Performance of the ALMCC and MCC: (A) Voltage outputs with time; (B)

530

Polarization and power density curves; (C) Changes of dissolved oxygen with time;

531

(D) pH changes with time. Control: MCC.

532

28


Page 28 of 33

Page 29 of 33


533 534

A

535 536

B

537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 29



553 554

C

555 556

Figure 3. The performance of cell growth and CO2 fixation efficiency: (A) Biomass

557

concentration with time; (B) CO2 fixation rate; (C) SEM images and photo of algae in

558

three reactors. a: ALMCC, b: MCC, c: ALP d: the microalgae suspension in three

559

reactors. Control 1: MCC and control 2: ALP.

560

30


Page 30 of 33

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561

A

562 563

B

564 565

C

566 567

Figure 4. Lipid accumulation: (A) CLSM images of algae labeled in vivo with

568

Bodipy 505/515. a: ALMCC, b: MCC, c: ALP; (B) Expanded part of infrared spectra 31



569

showing the band around 3000-2800 cm-1 of algae. (C) Lipid content and productivity

570

of algae in three reactors. Control 1: MCC and control 2: ALP.

32


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84x47mm (300 x 300 DPI)


CO2 Fixation, Lipid Production, and Power Generation by a Novel Air

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