Mineral carbonation for carbon utilization in microalgae culture

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Mineral carbonation for carbon utilization in microalgae culture Zi Ye, Juliana Abraham, Christos Christodoulatos, and Valentina Prigiobbe Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b01232 • Publication Date (Web): 21 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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Energy & Fuels

Mineral carbonation for carbon utilization in microalgae culture Zi Ye,†,‡ Juliana Abraham,†,‡ Christos Christodoulatos,† and 1

Valentina Prigiobbe∗,† †Department of Civil, Environmental, and Ocean Engineering, Stevens Institute of Technology, Hoboken (NJ) U.S.A.. ‡Contributed equally to this work E-mail: [email protected]

2

Abstract

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This paper presents a study on the combination of mineral carbonation with biomass

4

production. Laboratory experiments were performed to investigate the growth of fresh-

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water green microalgae Scenedesmus obliquus in the presence of dissolving mineral

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carbonates, such as nahcolite (NaHCO3 ) and nesquehonite (MgCO3 -3H2 O) at atmo-

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spheric conditions and 25 ◦ C. The cell density of the algae biomass was determined

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using fluorescence measurements. A bio-chemical model was implemented to describe

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the evolution of the biomass and the consumption of carbon substrate. The parameter

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estimates show that the algae growth kinetics in the presence of either NaHCO3 or

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MgCO3 -3H2 O is similar and is comparable to the literature. Moreover, when MgCO3 -

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3H2 O is supplied the algae appear to form clusters, which favor their separation from

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the solution. Overall, this work analyzes the potential to combine two carbon dioxide

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(CO2 ) utilization options, i.e., mineral carbonation and microalgae cultivation, and it

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demonstrates the feasibility of the process. However, control of the pH and the carbon

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dosage is required to attain optimal biomass productivity.

1

ACS Paragon Plus Environment

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Introduction

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Carbon dioxide (CO2 ) concentration in the atmosphere has increased from approximately

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277 ppm at the beginning of the industrial era to 407.80 ppm in 2018 1 . The major contri-

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bution to such an increase is fossil-fuel combustion and industry 2 . Recent work by Le Quéré

21

et al. 2 reports that the total global carbon emission from these anthropogenic activities has

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reached 9.4 ± 0.5 Gt per year. Consequences of carbon accumulation in the atmosphere are

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temperature increase and climate change 3,4 . The Paris Agreement 5 fixes the limit of CO2

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concentration in the atmosphere to keep the global temperature rise within this century well

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below 2 ◦ C above pre-industrial levels. To reach this goal, significant changes in the energy

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production and CO2 management need to be implemented 6 . Carbon Capture Utilization and

27

Storage (CCUS) techniques aim at mitigating CO2 emissions towards the atmosphere and

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include: geological storage, ocean storage, and carbon mineralization 6,7 . Among the most

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promising options, there are geological storage, e.g., in deep saline aquifers, and CO2 utiliza-

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tion in enhanced oil recovery (EOR) with assumed long-term storage. However, the relatively

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high cost of the conventional CCUS systems remains a major barrier to their widespread

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deployment at power plants and other industries. 8,9 Recently, the beneficial reuse or conver-

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sion of CO2 into various products, such as, e.g., chemicals, fuels, and construction materials,

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has been considered 10–15 Among the different options, microalgae cultivation and mineral

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carbonation are able to capture CO2 and convert it into valuable products 16 .

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Microalgae cultivation has the potential to become a major global renewable fuel source,

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utilizing sunlight, CO2 , and nutrients to rapidly produce long chain oils 17–20 . Commer-

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cial scale algal growth reactors, either open-pond systems or enclosed photo-bioreactors,

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require supplemental inorganic carbon sources. Traditionally, this is achieved by supplying

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high-purity gaseous CO2 21 . However, the use of pure gas may have significant impact on

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the overall economics of the process as may require closed reactors and a controlled atmo-

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sphere 22,23 . One of the major barriers to large-scale commercial deployment is indeed the

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need for high-purity CO2 for the otherwise significant potential of algal biofuels 24 . Pure 2


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CO2 sources are rare, and even dilute sources may not be available or not located nearby

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algae production plants 25,26 . To overcome this issue, recent studies have investigated the

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possibility to use directly industrial flue-gases 27,28 . However, raw flue-gases usually con-

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tain other gaseous components such as NOx and SOx that have a negative effect on algae

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growth 29 . Other studies show that inorganic carbon, in the form of bicarbonate ions (HCO− 3)

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can be a good alternative source of carbon 29–32 . Photosynthetic microorganisms can uptake

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inorganic carbon only as CO2 gas since the enzyme responsible for the assimilation of the

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carbon (i.e., Ribulose-1,5-bisphosphate carboxylase/oxygenase also commonly known as RU-

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BISCO) can only fix CO2 in gaseous form. However, bicarbonates constitute another form

53

of inorganic carbon that these microorganisms can store depending upon the species and

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the conditions of the environment. Bicarbonate ions can accumulate near the RUBISCO

55

limiting CO2 diffusion through the cell. Then, another enzyme called carbonic anhydrase

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(CA) converts bicarbonate to CO2 before fixation takes place. Specifically the freshwater

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strain used in the current study, Scenedesmus obliquus, can transform bicarbonates into

58

CO2 and, therefore, use both inorganic carbon forms to grow 33 . Moreover, bicarbonate ions

59

are an effective lipid accumulation trigger for a variety of algae, potentially improving the

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commercial use of algae as a biofuel resource 34–36 . Therefore, the dissolution of carbonate

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minerals could be a good alternative option of carbon supply to microalgae, overcoming the

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major impediment towards achieving an economically attractive biomass productivity. Here,

63

we propose a process which combines microalgae cultivation with mineral carbonation. CO2

64

is dissolved into alkaline wastewater or brine to precipitate carbonates, which are then dis-

65

solved for microalgae growth. Conventional mineral carbonation aims at permanently fixing

66

CO2 into stable carbonates, such as, e.g., magnesite and calcite, that can be disposed of

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or reused 7,37 . During the precipitation of the carbonates, hydrated phases may form which

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eventually transform into stable solid phases 38–41 . Mineral carbonation process has been used

69

to produce a large variety of carbonate minerals such as, e.g., calcite (CaCO3 ) 42,43 , nesque-

70

honite (MgCO3 -3H2 O) 44–46 , hydromagnesite Mg5 (CO3 )4 (OH)2 -4H2 O) 47 , magnesite 39,41 , and

3


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sodium bicarbonate (or nahcolite, NaHCO3 ) 48,49 . If carbonates are designed to be reused,

72

e.g., to supply carbon to microalgae, the formation of metastable phases is preferred as they

73

can precipitate quickly and dissolve promptly.

74

This paper presents experimental and modeling efforts to determine the kinetics of mi-

75

croalgae growth in the presence of dissolving carbonate minerals such as, e.g., sodium and

76

magnesium carbonates, and select the best operating conditions to achieve optimal algae

77

growth. Experiments were performed using S. obliquus, NaHCO3 , and MgCO3 -3H2 O to

78

obtain measurements for the development of a bio-chemical model of the process.

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Material and methods

80

Precipitation and characterization of carbonate minerals

81

Sodium bicarbonate was purchased from Fisher Scientific (U.S.A.) and characterized in our

82

laboratory. Nesquehonite was produced following Hänchen et al. 39 . A volume of 400 mL of

83

sodium hydroxide (NaOH, 98.9% purity, Sigma-Aldrich, U.S.A.) solution was prepared using

84

ultra-purified water (Milli-Q, U.S.A.). The final concentration was equal to 1.25 mol/kg. The

85

solution was purged with pure CO2 for 24 hours in a continuously-stirred batch reactor (Series

86

5100 Glass Reactors, Parr Instrument Company, U.S.A). Then, the system was stabilized

87

at a temperature of 25 ◦ C and total pressure of 1 bar of pure CO2 . Once equilibrium was

88

reached, a volume of 30 mL of magnesium chloride (MgCl2 -6H2 O, Sigma-Aldrich, U.S.A.)

89

solution was injected into the reactor at the rate of 10 mL/min using a piston pump (Cole-

90

Parmer, U.S.A.). The initial concentrations of sodium (Na+ ) and magnesium (Mg2+ ) ions

91

were 1.1 and 0.19 mol/kg, respectively, corresponding to an initial supersaturation with

92

respect to MgCO3 -3H2 O as large as 1.69 calculated using the open-source geochemical code

93

PHREEQC 50 . The suspension was mixed for 24 hours, then the solids were collected and

94

washed with ultra-purified water and, finally, dried at room temperature.

95

The solids, namely, the commercial NaHCO3 and the produced MgCO3 -3H2 O, were char4


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acterized with a Scanning Electron Microscope (SEM, Auriga 40, Zeiss, U.S.A) and X-ray

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diffraction analysis (XRD, Ultima IV, Rigaku, U.S.A.). The results of these characteriza-

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tions are shown in Figure 1. As illustrated in the figure, both measured spectra compare

99

very well with the reference 51 and the SEM image in part b of the figure shows the typical

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needle-like shape of MgCO3 -3H2 O.

Figure 1: Characterization results of the solids used for algae growth. X-ray diffraction analysis of the solid products and SEM images in the insets for: (a) commercial NaHCO3 and (b) produced MgCO3 -3H2 O. The reference for the XRD spectra is the Jade database 51 .

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Microalgae growth medium

102

The growth medium selected to provide nutrients to microalgae was prepared following the

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recipe of commercial BG-11 (BG-11 Freshwater Solution, Sigma-Aldrich, U.S.A.). Within

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this medium, the sources of carbon, magnesium, and nitrogen were modified on the basis of

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the type of experiment. Table 1 specifies these sources. Media BG-111 and BG-112 were used

5


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in the microcosm tests, as described in section Microcosm bottles, below. Whereas, media

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BG-111 through BG-114 were employed in batch tests, as described in section Continuously-

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stirred batch reactor, below. In particular, in media BG-113 and BG-114 the source of

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+ nitrogen was changed from NO− 3 to NH4 to better control the pH during the experiments.

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Oxidized nitrogen sources like nitrate must be reduced before algae uptake generating alka-

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linity, whereas the nitrogen consumed as ammonium produces acidity 52 . Table 1: Sources of carbon, magnesium, and nitrogen used to modify the original BG-11 medium

BG-111 . BG-112 BG-113 BG-114

Carbon

Magnesium

Nitrogen

NaHCO3

MgSO4 -7H2 O

NaNO3

MgCO3 -3H2 O MgCO3 -3H2 O

NaNO3

NaHCO3

MgSO4 -7H2 O

NH4 Cl

MgCO3 -3H2 O MgCO3 -3H2 O

NH4 Cl

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Toxicity assessment

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R Toxicity assessments of the freshwater microalgae S. obliquus (ATCC 11477TM ) to Mg2+

114

were performed. Different concentrations of the cation were used to identify the maximum

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concentration of MgCO3 -3H2 O that could be used in the algae cultures without any toxicity

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effect on S. obliquus. Therefore, different concentrations of Mg2+ as MgCO3 -3H2 O were

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tested in a simple and cost-effective 24-well microplate test, following the protocol described

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by Abraham et al. 53 . The objective of this assessment was to evaluate the toxicity, if any,

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of the high concentrations of Mg2+ in solution. Therefore, the rest of the nutrients were

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provided in excess, including C, to avoid inhibition of growth due to lack of nutrients. The

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pH of the prepared solutions was adjusted by adding sulfuric acid (H2 SO4 , 98 % purity,

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Sigma-Aldrich, U.S.A.) up to complete dissolution of the solids.

6


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Growth of microalgae

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Experiments of microalgae growth were carried out in closed systems using microcosm air-

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tight bottles and a continuously-stirred batch reactor. Details of the set-ups are provided,

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herein.

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Microcosm bottles

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Closed systems used to grow microalgae consisted of microcosm airtight 250 mL bottles with

129

200 mL working volume. Experimental conditions were selected to test a wide range of carbon

130

concentrations. Preliminary tests to identify the optimal concentration of carbon provided

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as HCO− 3 were also carried out. Here, NaHCO3 was used and the total carbon concentration

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was varied from 0 and 1,000 mg/L (Results not shown). Based on the outcomes of these

133

preliminary tests, a narrower range was chosen, namely 12-400 mg/L, for further tests where

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carbon was supplied as either NaHCO3 or MgCO3 -3H2 O.

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In all experiments, S. obliquus was initially grown in sterile medium BG-111 or BG-113

136

and used as . The pH of the media prior to inoculation was adjusted between 7 and 8 by

137

adding H2 SO4 . All tests were performed at least in duplicate and were incubated for 7 days

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at 25 ◦ C in a growth chamber (Caron, U.S.A.), under continuous shaking at 120 rpm using

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a 14:10 hours light:dark photoperiod and 68 µmol m−2 s−1 of light intensity. Samples were

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taken at the beginning and at the end of each test and analyzed.

141

Continuously-stirred batch reactor

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Open systems used to grow microalgae comprised a continuously-stirred batch reactor equipped

143

with temperature and pressure sensors. A solution of 520 mL containing the medium (i.e.,

144

BG-111 through BG-114 depending on the experiment performed) was prepared and placed

145

into the 600 mL double-jacked glass vessel at atmospheric conditions of CO2 and 25 ± 2 ◦ C.

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In the experiments, where MgCO3 -3H2 O was used, the solids were completely dissolved and

147

the pH was adjusted between 7 and 8. Then, the solution was inoculated with an initial 7


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cell density of ∼ 2.5·105 cells/mL of S. obliquus. During the experiment, the suspension was

149

stirred at 120 rpm, illuminated with 14:10 hours light:dark photoperiod at 98 µmol/(m2 s)

150

of LED light intensity, achieved by 4 LED lights (40 W, 36 LED, 10 inches in length and

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0.75 inches wide each) equally distributed around the reactor. The pH was adjusted with

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hydrochloric acid (HCl, Fisher Scientific, U.S.A.). Samples were taken twice a day and

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characterized offline.

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Characterization of the solution

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Samples, collected during the tests, were analyzed for pH using a digital sensor (Oakton PC

156

700 pH meter, U.S.A.) with measurement error of ± 0.01. Upon filtration using a 0.45 µm

157

pore size filter, the total carbon (TC) was determined by means of a UV-Persulfate TOC

158

Analyzer (Phoenix 8000 instrument, Teledyne Tekmar, U.S.A.) and Mg2+ concentration

159

was determined using inductively coupled plasma-optical emission spectrometry (ICP-OES,

160

Model 5100, Agilent Technologies, U.S.A).

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Characterization of the microalgae suspension

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Microalgal density was determined periodically on unfiltered samples by measuring their

163

R fluorescence on a microplate reader (Cytation 3, Biotek ) utilizing 445/685 nm excita-

164

tion/emission filters and the results were expressed as a mean of cell density (CD, cells/mL).

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Moreover, a standard curve to obtain a relationship between cell density and dry weight

166

(DW, g/L) was generated. Glass microfiber filters (GE Bio-Sciences, part no. 1821-055)

167

were initially dried in an oven at 105 ◦ C for two hours. Separately, microalgae suspensions

168

were filtered and dried at the same temperature for two hours, as well. The dried filters

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were weighted at room temperature before and after filtering. Microalgae cells were counted

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using the hemocytometer protocol 54 . The relationship between fluorescence measurements-

171

cell counting and dry weight were found to be: DW = CD/(4·107 ) with R2 = 0.996 and

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DW = CD/(3·107 ) with R2 = 0.996 for NaHCO3 and MgCO3 -3H2 O, respectively. Images of 8


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the green color developed due to the microalgae growth were recorded and microscopy cell

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R analysis was performed using an image reader (Cytation 3, Biotek ).

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Modeling

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Geochemical model

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A geochemical model was developed to describe the chemical composition of the investigated

178

system and implemented in the open-source software PHREEQC 50 . The dominant chemical

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reactions for the dissolution/precipitation of the carbonate minerals,

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NaHCO3 + H+ ←→ Na+ + HCO− 3,

(1)

MgCO3 · 3H2 O + H+ ←→ Mg2+ + HCO− 3 + 3H2 O,

(2)

Mg2 (CO3 )(H2 O) − 3H2 O + H+ ←→ 2Mg2+ + HCO− 3 + 5H2 O,

(3)

and the aqueous speciation,

CO2,gas ←→ CO2,sol ,

(4)

CO2,sol + H2 O ←→ H2 CO3 ,

(5)

+ H2 CO3 ←→ HCO− 3 +H ,

(6)

+ 2− HCO− 3 ←→ CO3 + H .

(7)

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reactions 1 through 7 were assumed at local equilibrium during the growth of the microalgae.

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To design the experiments, the chemical composition of the liquid phase at the oper-

183

ating conditions for microalgae growth, i.e., atmospheric pressure of CO2 and 25 ◦ C, were

184

determined. To this aim, the supersaturation ratios with respect to NaHCO3 (Snah ), MgCO3 -

185

3H2 O (Snes ), and artinite (Mg2 (CO3 )(H2 O)-3H2 O, Sart ) were calculated as a function of ion

9


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concentration. Namely, Snah , Sart and Snes , are expressed by 55 : Snah = Sart =

Snes =

aN a+ aHCO3−

1/2

Ksp,nah

,

a2M g2+ aHCO3− a5H2 O

(8)

!1/8

a3H + Ksp,art aM g2+ aHCO3− a3H2 O

,

(9)

,

(10)

!1/5

aH + Ksp,nes

187

where a and Ksp are the activity and the solubility product of the subscript species and solid

188

phase, respectively. At 25 ◦ C, log10 Ksp,nah , log10 Ksp,art , and log10 Ksp,nes are, correspondingly,

189

-0.11, 19.66, and 4.99.

190

Figure 2 reports the composition diagrams with the supersaturation ratio as contours

191

and on the axes either Na+ (parts a and b) or Mg2+ (parts c and d) concentrations and

192

pH. The corresponding total carbon concentration is also provided as a reference. The gray

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shaded area indicates the region, where microalgae growth is feasible (i.e., within pH 6 and

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9), and the system is undersaturated with respect to any solid phase (nahcolite, artinite,

195

and nesquehonite). Namely, in the case of the system where nesquehonite was added as a

196

carbon source the maximum allowable pH was set equal to 8.5 (parts c and d of Figure 2).

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The initial conditions in each test were selected within this area, i.e., the pH was initially

198

always adjusted to 7 and the concentration of carbon lower than 400 mg/L.

10


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Energy & Fuels

Figure 2: Contours of Snah (solid blue), Sart (dashed gray) and Snes (solid black), calculated as in eqs. 8, 9, and 10, respectively. Parts (a) and (b) were calculated considering media BG-111 and BG-113 . Parts (c) and (d) were calculated considering media BG-112 and BG114 .

11


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Algae growth model

200

To describe the evolution of the algal growth in the batch reactor system, the Monod kinetics

201

model was implemented, XS dX = µmax − Kd X, dt Ks + X dS XS = −µmax , dt Y (Ks + X)

(11) (12)

202

where t is time (d), X is the concentration of microalgae (mg/L), S is the concentration of

203

substrate (i.e., carbon) (mg/L), and Ks , Kd , µmax , and Y are model parameters, namely,

204

Monod half-saturation constant (mg/L), endogenous decay coefficient (1/d), maximum spe-

205

cific growth rate of microalgae (1/d), and yield (-), respectively. The Monod model assumes

206

that the only limiting growth factor is the availability of carbon and that the macro- and

207

micro-nutrients are present in excess. Eqs. 11 and 12 were solved in MATLAB 56 and used

208

to fit the experimental data. The value of Kd was considered negligible since during the

209

experimental time period of 7 days no cell decay was observed.

210

For the tests carried out in the microcosm bottles, the growth rate was calculated us-

211

ing the initial and the final algae concentrations, i.e., X0 and Xf , respectively, assuming

212

exponential growth and using the following equation 57 ,

µ=

lnXf −lnX0 , tf −t0

(13)

213

where t0 and tf refer to the initial and final time (d). The carbon utilization efficiency (Ec ,

214

%) was also determined as Ec =

Ng C a , S0

(14)

215

where S0 is the initial concentration of the substrate, Ca is the carbon content within the

216

microalgal cell, determined using a CHNS elemental analyzer, and was found to be equal to

217

43.39 and 46.89 % of dry weight for NaHCO3 and MgCO3 -3H2 O, respectively,. Finally Ng 12


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Energy & Fuels

(cell/mL or mg/L) is the net algal growth,

Ng = X f − X 0 .

(15)

219

Results

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Microalgal toxicity assessment

221

Magnesium is an essential nutrient for microalgal development since it is part of the chloro-

222

phyll structure, the main molecule responsible for photosynthesis in green microalgae and

223

plants. Earlier work conducted by 58 shows that the biomass and the lipid content of S.

224

obliquus increases with magnesium concentration up to 150 mg/L. However, alkaline-earth

225

elements could be toxic above certain concentrations.

Figure 3: Results of the toxicity assessment given as cell density (CD) of S. obliquus vs. time measured for different concentrations of Mg2+ supplied as MgCO3 -3H2 O. The control test was run using Mg2+ concentration present in the regular BG-11 medium, i.e., 7.5 mg/L.

13


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Therefore, toxicity assessment of magnesium was carried out changing its concentration

227

from 7.5 (control) to 800 mg/L provided as MgCO3 -3H2 O. The mineral carbonate was com-

228

pletely dissolved prior to toxicity assessment. The results are shown in Figure 3 as algae

229

cell density over time. Here, it is possible to see that the algae growth in the presence of

230

MgCO3 -3H2 O is initially slower than that of the control system. To gain an insight into this

231

behaviour, speciation diagrams were calculated at the conditions applied in these tests. The

232

graphs are reported in Figure 2 of the supporting information (SI) document. Here, we can

233

see that, in the range of 6–9 of the operating pH, the carbon in solution partitions within

234

various aqueous complexes, which may reduce the bio-availability of the carbon to the algae

235

for growth. However, after the third day, the growth profile is similar in all tested cases and,

236

2 eventually, the concentration is the same within the uncertainty of the measurements (σm,800

237

2 2 2 2 2 = 5.74·105 , σm,ctrl = 3.22·105 , σm,100 = 3.92·105 , σm,200 = 3.64·105 , σm,400 = 1.70·105 , σm,600

238

2 = 3.14·105 and σd2 = 2.13·106 , where σm and σd2 are the uncertainty of the measurements at

239

given Mg2+ concentrations and the variability of the data). This suggests that magnesium

240

is not toxic to S. obliquus within the investigated concentration range.

241

Growth of microalgae in microcosm bottles

242

Experiments in a closed system using microcosm bottles were performed to study the feasi-

243

bility of microalgae growth under controlled carbon sources, namely, NaHCO3 , and MgCO3 -

244

3H2 O. The conditions applied are listed in Table 2 together with the results, which are also

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shown in Figure 4. In these tests, the concentration of carbon was varied between 0 and

246

400 mg/L. In the table, the symbol td (d) indicates the doubling time, td = ln2/µ, and Ng∗

247

is the net growth subtracted by the net growth measured in the test where no carbon was

248

supplied. This operation was performed in order to remove the effect of CO2 contained in

249

the headspace of the microcosm bottles.

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As seen in this table and in parts a and d of Figure 4, the largest net growth is obtained for

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200 mg/L of total carbon supplied, with the largest value when NaHCO3 was used as carbon 14


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Table 2: Experimental conditions and growth parameters for the experiments run using microcosm bottles. Experiments 1 through 6 and 7 through 12 were run using NaHCO3 and MgCO3 -3H2 O, respectively. Exp C, # mg/L Ctrl 1 5 vol.% CO2 1 12 2 20 3 50 4 100 5 200 6 400 7 12 8 20 9 50 10 100 11 200 12 400

Ng∗ , µ, cells/mL 1/day 7.58·106 0.39 5 2.92·10 0.13 7.35·105 0.17 6 2.59·10 0.27 3.99·106 0.31 6 5.98·10 0.35 6 5.53·10 0.34 3.62·105 0.14 5 0.16 6.37·10 6 2.56·10 0.26 2.95·106 0.29 6 4.30·10 0.33 3.42·106 0.29

td , Ec , day % 1.77 5.14 26 4.00 40 2.58 56 2.25 43 2.01 32 2.03 15 5.05 47 4.31 50 2.68 80 2.36 62 2.12 34 2.36 13

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source. Overall the growth rate increases with carbon concentration. However, the carbon

253

utilization efficiency (Ec ) increases until 50 mg/L of carbon concentration, indicating that

254

the supplied carbon is not all uptaken by the algae. In all tests pH increases, but the change

255

decreases with carbon concentration. This is because the cation concentration increases as

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well with increasing carbon content and becomes the dominant species controlling the pH of

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the system.

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Figure 5 shows the color development in each microcosm bottle as well as microscopy

259

images of the microalgae at the end of the experiment. As illustrated by these images the

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intensity of the green color of the suspension increases with carbon concentration. The bottles

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of tests 1 through 6 express a more pronounced green color than those of tests 7 through 12.

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This is because in the latter set of experiments, the microalgae settle towards the bottom of

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the containers as the shaking was interrupted. As a matter of fact, in this case, the microalgae

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acquire a cluster structure (images in part b of Figure 5) making them heavier and, therefore,

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favoring settling. This type of behavior could be due to the presence of Mg2+ cations, which

266

interact with the negative charges of the microalgae membrane, compress the double layer 15


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4: Microalgae tests in microcosm bottles. (a) through (c) carbon supplied by NaHCO3 ; (d) through (f) carbon supplied by MgCO3 -3H2 O.

16


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favoring coagulation, as previously reported by Vandamme et al. 59 , Das et al. 60 , Xia et al. 61 .

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Such as phenomenon is important to the practicality of the implementation of the overall

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process, as fast settling can reduce significantly the cost of separation of the algae from the

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solution.

Figure 5: Pictures of the microcosm bottles taken at the end of the tests and corresponding microscopy images of the microalgae. Microscopy pictures are a merge between the brightfield and the fluorescence image.

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Growth of microalgae in a continuously-stirred batch reactor

272

Experiments in an open continuously-stirred batch reactor were conducted using different

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sources of carbon, namely, atmospheric CO2 , NaHCO3 , and MgCO3 -3H2 O. The carbon

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concentrations provided through the carbonate minerals were varied between 50 and 200

275

mg/L, which were selected on the basis of the best Ng∗ calculated from the experiments run 17


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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in the closed microcosm bottles.

277

The applied experimental conditions are listed in Table 3 in conjunction with the es-

278

timates of the model parameters, namely, µmax , Ks , and Y , and the carbon utilization

279

efficiency (i.e., Ec ). The measurements of cell density, total dissolved carbon, and pH as

280

a function of culturing time are shown in Figure 6 and 7. The error bars for cell density

281

measurements shown in the corresponding panels were determined over four replicates. In

282

these figures, the optimized model is also shown. It is evident that the model catches very

283

well the evolution of the microalgae and the consumption of the substrate. The bottom

284

images were taken at end of the experimental time period. Table 3: Experimental conditions applied in experiments run in the continuously-stirred batch reactor and model parameter estimates. Experiments 13 through 17 and 18 through 24 were run using NaHCO3 and MgCO3 -3H2 O, respectively. Exp C, # mg/L 13 50 14 50 15 100 16 100 17 200 18 50 19 50 20 100 21 100 22 100 23 200 24 200

N source N-NH+ 4 N-NH+ 4 N-NH+ 4 N-NO− 3 N-NO− 3 N-NH+ 4 N-NH+ 4 N-NH+ 4 N-NH+ 4 N-NH+ 4 N-NO− 3 N-NO− 3

µmax , Ks , Y, Ec , 1/day mg/L (-) % 0.95 1.93 1.61 100 0.79 0.90 1.36 86 1.20 0.13 0.98 66 1.20 0.13 0.98 56 0.94 2.88 1.42 65 0.90 0.04 2.01 88 0.88 0.07 1.83 82 0.77 0.97 0.89 45 1.49 0.09 1.63 77 1.84 1.18 1.06 43 0.59 0.48 0.55 26 0.91 1.00 0.94 31

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+ In these tests, two different sources of nitrogen were provided, either NO− 3 or NH4 . This

286

is because when NO− 3 was used, the pH increased up to 12, which is higher than the physi-

287

ological pH for algae, thus inhibiting their growth. Moreover, such an alkaline pH triggered

288

the precipitation of solid phases, such as, e.g., artinite, nesquehonite, and struvite 57,62 , which

289

limited nutrient availability to the algae. Algae uptake N as NH3 , transforming it either from

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+ NO− 3 while consuming acidity or from NH4 while producing protons. In the former case, the

291

reaction favors the increase of the pH of the solution whereas in the latter case, the reaction 18


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helps the system to balance the solution pH. As evident from the results reported in Table

293

3 and Figures 6 and 7, the concentration profiles in experiments 15 and 16, which were run

294

with the same initial amount of carbon but different source of nitrogen, are reproducible. As

295

expected, in exp. 15, a smaller variation of pH was recorded and lower pH adjustments were

296

required by using HCl, whereas in exp. 16, much larger pH oscillations occurred and more

297

frequent pH adjustments were necessary. This is because in the latter case more alkalinity

298

was present in the system and, therefore, the addition of an acidic solution was required to

299

control pH. Similar observations can be made for exp. 17, 23, and 24, where N was sup-

300

52 plied in the form of NO− showed that when growing S. obliquus on 3 . A previous study

301

ammonium using both atmospheric conditions and 5 vol.% CO2 as C sources, a significant

302

decrease in pH is observed resulting in the inhibition of growth unless the pH is adjusted.

303

Figures 2 and 3 in the SI document show the carbonate species in solution as a function of

304

pH during these experiments.

305

In all investigated conditions, the microalgae were able to grow at similar rates and the

306

maximum cell density reached at the end of the experimental time increases with the initial

307

carbon supplied. This is also qualitatively evident from the microscopy images reported at

308

the bottom of these figures, where it is possible to notice a more concentrated suspension

309

as the carbon increases. The model captures well the observed trend of both the growth of

310

the cells and the consumption of the substrate (parts a through j of Figures 6 and 7) and

311

the estimated values of the parameters are within those reported in the literature, 63 with

312

the values of µmax , Ks , and Y changing within 0.6–1.8 1/day, 0.13–3 mg/L, and 0.55–2.01,

313

respectively. The average value for µmax , Ks , and Y for NaHCO3 and MgCO3 -3H2 O are 1.02

314

1/day, 1.19 mg/L, 1.27 and 1.05 1/day, 0.55 mg/L, 1.27, respectively. Moreover, the carbon

315

efficiency reaches values up to 80–100 %. Such a high values of Ec was calculated in the tests

316

(namely, exp. 13, 14, 18, and 19) where the lowest carbon concentration was supplied, i.e.,

317

50 mg/L, indicating a deficiency of carbon provided to the system, in contrast to the other

318

experiments, where larger carbon concentration was supplied. Here, the value of Ec varies

19


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Figure 6: Cell density, total dissolved carbon, and pH during the culturing time of microalgae using NaHCO3 as carbon source. Dots are the measurements and the lines refer to the bestfit with the model. The microscopy images were taken at the end of the tests, i.e., after 7 days.

20


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Figure 7: Cell density, total dissolved carbon, and pH during the culturing time of microalgae using MgCO3 -3H2 O as carbon source. Dots are the measurements and the lines refer to the best-fit with the model. The microscopy images were taken at the end of the tests, i.e., after 7 days.

21


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within a large range of 26–77 %. In future work, we will study how to optimize the carbon

320

supply to obtain the best growth while meeting the requirement of carbon fixation.

Figure 8: Comparison of the net growth as a function of time measured in our work and in the literature where 5 vol.% of CO2 64 and direct flue-gas with 0.5 vol.% 64 and 20 vol.% 65 were supplied 64,65 .

321

Finally, Figure 8 shows a comparison between the results from this work, where the

322

highest growth was recorded, and literature data where carbon was provided to Scenedesmus

323

sp. as gaseous CO2 64,65 . Details about the tests from the literature are reported in Table 1

324

of the SI document. As evident from this diagram, our data compare well with the literature

325

in an early stage of algae growth reaching a plateau after 5 days of culture. This could be

326

due to the complete consumption of the supplied carbon to the system. Supply of mineral in

327

a series of steps coupled with the removal of cation might be therefore required to increase

328

the net growth.

22


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Conclusions

330

In this paper, a process combining mineral carbonation and microalgae (i.e., Scenedesmus

331

obliquus) growth for biomass production was presented. The process consists of a carbon

332

capture step and subsequent carbon reuse for algae growth through its release by dissolution

333

of the carbonate minerals, e.g., nahcolite (NaHCO3 ) and nesquehonite (MgCO3 -3H2 O) at

334

atmospheric conditions. The cell density within the algae biomass was determined using

335

fluorescence measurements. Batch tests show that microalgae grow well under the applied

336

conditions, with a better yield when NaHCO3 was supplied to the system. The measure-

337

ments were described adequately by a bio-chemical model, implemented in this work and

338

the estimated growth kinetics compares well with those in the the literature.

339

Overall, this paper demonstrates that carbonate species can be provided for algae growth

340

as an alternative option to the conventional one of direct gaseous CO2 injection. This method

341

can be particularly useful for algae production sites where gaseous carbon dioxide is not

342

available from nearby sources. To the best of the authors knowledge, this is the first study

343

where carbon utilization for biomass production, mediated through mineral carbonation, has

344

been investigated.

345

Acknowledgement

346

This work was supported by the Consortium for Energy, Environment and Demilitarization

347

(CEED) contract number SINIT-15-0013. The authors would like to thank the undergrad-

348

uate student Sarah Chan for helping in part of the laboratory work reported here. The

349

authors would like to thank also two anonymous reviewers that with their comments helped

350

improving the manuscript.

23


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Supporting Information Available

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A supporting information document is included and contains additional information regard-

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ing this work.

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Mineral carbonation for carbon utilization in microalgae culture

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