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Microbial Electrolytic Carbon Capture for Carbon Negative and
Energy Positive Wastewater Treatment
Lu Lu,† Zhe Huang,† Greg H. Rau,‡ and Zhiyong Jason Ren*,†
†
Department of Civil, Environmental, and Architectural Engineering, University of Colorado Boulder, Boulder, Colorado 80309,
United States
‡
Institute of Marine Sciences, University of California, Santa Cruz, California 95064, United States
S Supporting Information
*
ABSTRACT: Energy and carbon neutral wastewater management is a
major goal for environmental sustainability, but current progress has only
reduced emission rather than using wastewater for active CO2 capture and
utilization. We present here a new microbial electrolytic carbon capture
(MECC) approach to potentially transform wastewater treatment to a
carbon negative and energy positive process. Wastewater was used as an
electrolyte for microbially assisted electrolytic production of H2 and OH−
at the cathode and protons at the anode. The acidity dissolved silicate and
liberated metal ions that balanced OH−, producing metal hydroxide, which
transformed CO2 in situ into (bi)carbonate. Results using both artificial
and industrial wastewater show 80−93% of the CO2 was recovered from
both CO2 derived from organic oxidation and additional CO2 injected into
the headspace, making the process carbon-negative. High rates and yields
of H2 were produced with 91−95% recovery efficiency, resulting in a net
energy gain of 57−62 kJ/mol-CO2 captured. The pH remained stable without buffer addition and no toxic chlorine-containing
compounds were detected. The produced (bi)carbonate alkalinity is valuable for wastewater treatment and long-term carbon
storage in the ocean. Preliminary evaluation shows promising economic and environmental benefits for different industries.
■
INTRODUCTION
Wastewater treatment is essential for preventing disease and
protecting the environment, but current treatment processes
are energy intensive and emit greenhouse gas (GHG) due to
fossil fuel use and organic degradation. Every year, approximately 12 trillion gallons of municipal wastewater is treated by
publicly owned treatment works in the United States, which
consumes about 15 GW, or 3% of the total electricity produced
in the U.S.1−3 Moreover, nearly 93 trillion gallons of industrial
wastewater is treated using different processes, and they
generally require high energy due to higher concentration of
contaminants.4 It was estimated that from 2010−2015, the
degradation of organics in municipal and industrial wastewater
contributed 0.6−0.63 × 109 ton (Gt) CO2-equivalent emission
per year, equivalent to nearly 1.5% of the global GHG emission,
and this number will increase to 0.67 Gt by 2020.5,6 Great
progress has been made to increase energy efficiency and
recovering renewable energy from wastewater using technologies such as anaerobic digestion and bioelectrochemical
systems, because the chemical energy content embedded in
wastewater is estimated to be many times that required to treat
the wastewater.7−10 However, these methods only reduce fossil
fuel consumption and its associated carbon emission, and few
have looked at the additional possibility of using wastewater as
both carbon sink and energy source for active and direct CO2
mitigation.
© 2015 American Chemical Society
Point-source CO2 mitigation, in particular carbon capture
and storage (CCS) has been a main strategy for reducing
industrial and energy-related CO2 emissions (e.g., power plants,
cement and petrochemical industries, etc.),11,12 which account
for approximately 60% total CO2 emission due to fossil-fuel
use.13 Conventional CCS consists of energy-intensive and
costly CO2 separation, purification, compression, transportation, and injection underground. A power plant equipped with
CCS incurs at least a 10−40% energy penalty primarily due to
the high-energy-demand solvent/sorbent regeneration process.12−14 Emerging membrane technologies minimize regeneration process and reduce environmental impacts, but system
scale-up has been a challenge.15 Related CCS technologies have
been explored to capture much more dilute CO2 from the
atmosphere, either in combination with biomass combustion/
energy production, BECCS16 or using base solvent absorption17−19 or solid sorbent adsorption.20,21 Generally, these are
even more energy intensive and costly than point-source CO2
mitigation.22 However, a variety of potentially cheaper and high
capacity air CO2 removal strategies have been proposed. For
example, algae were used in bioelectrochemical systems to
Received:
Revised:
Accepted:
Published:
8193
February 17, 2015
May 11, 2015
June 15, 2015
June 15, 2015
DOI: 10.1021/acs.est.5b00875
Environ. Sci. Technol. 2015, 49, 8193−8201
Article
Environmental Science & Technology
one proton for every electron),32 and CO2. Electrons are
accepted by the anode and transferred through an external
circuit to the cathode, where they reduce water to produce H2
and OH−. Abundant and relatively inexpensive silicate minerals
would be exposed to and react with the H+-rich anolyte in situ
to liberate metal ions and silica or other silicon compounds.
Metal ions migrate through a cation exchange membrane
(CEM) to the cathode chamber to form metal hydroxide,
whose subsequent reaction with CO 2 would lead to
spontaneous CO2 capture and transformation into carbonate
or bicarbonate. The process would not only fix the CO2 derived
from wastewater degradation, but it also capture additional
CO2. A small external energy input (0.2−0.8 V) was used to
overcome the thermodynamic barrier of H2 evolution, which is
lower than that typically used to electrolyze water, 1.8−2.0 V,33
with the energy gap supplied by chemical energy released from
the wastewater organics. Because the anode accepts electrons
from microbial organic degradation instead of electrolyte ions,
generation of toxic chlorine-containing compounds such as Cl2,
HCL, ClO−, ClO−3 would be avoided.24−26
We conducted proof-of-concept experiments, characterizing
wastewater organic degradation, ion species transformation, and
H2 production using both artificial wastewater (AW) and real
wastewater−produced water (PW) from shale gas well as
electrolytes and solvents. We then quantified the carbon
balance and energy balance, conducted a preliminary economic
analysis, and discussed the potential of this technology for
various applications.
capture CO2 discharged from the anode in a microbial carbon
capture cell,23 and recent studies found that the acids normally
produced in the anolyte during the electrolysis of saline water
can be reacted with carbonate or silicate minerals to generate
strongly alkaline, CO2-absorptive solutions.24−26 This in effect
greatly accelerates mineral weathering, Nature’s own global
CO2 mitigation mechanism that operates on geologic time
scales.27 The splitting of water in the preceding electrolysis also
produces H2, whose value can help offset the cost of the
process. When powered by nonfossil energy, the system is
strongly CO2-emissions negative. However, the process is
energy consumptive, with nonoptimized, experimental systems
using 426 to 481 kJ per mole of CO2 consumed and stored.26
When seawater or saline groundwater is used, the geographic
location of the operation will thus be limited, and toxic Cl2 and
halogenated organic compounds can be generated during
electrolysis, posing environmental and health risks.
In this study we present a microbial electrolytic carbon
capture (MECC) process, which treats wastewater and uses it
as both an electrolyte and energy source to dissolve base
minerals and capture and transform CO2 in situ, while also
generating H2 as renewable energy. We reasoned that such a
system could exploit via microbial oxidation the chemical
energy contained in wastewater to not only more efficiently
effect mineral dissolution, alkalinity production, CO2 removal
and net carbon-negativity, but to also be net energy-positive
(Figure 1). Here, CO2 is captured and transformed into stable
metal carbonate and bicarbonate, which can be harvested and
used on site as alkalinity compensation for nitrification and
digestion, for improving activated sludge settling properties, or
for environmental uses.26,28,29 Specifically, we hypothesized that
exoelectrogens30,31 on the anode would oxidize organic
substances in wastewater to produce electrons, protons (usually
■
MATERIALS AND METHODS
Wastewater Preparation and Silicate Immobilization.
Two types of wastewaters were used to investigate system
performance. One is artificial wastewater (AW) (4.5 g/L NaCl,
0.9 g/L CH3COONa, 0.31 g/L NH4Cl and 0.13 g/L KCl,
conductivity = 10.65 mS/cm, pH 7.15), which was used as the
anolyte. The catholyte was 4.5 g/L NaCl solution only
(conductivity = 8.93 mS/cm, pH 6.79). No phosphate buffered
content was used in the system. The other wastewater was real
produced water (PW) collected from a hydraulically fractured
shale gas well in Piceance Basin, Colorado. The PW was
pretreated to partially remove solids, oil and volatile
compounds, left the following characteristics: total dissolved
solids (TDS) = 12 442 mg/L, conductivity = 19.6 mS/cm, pH
7.08, COD = 310 mg/L. The ion concentration in the PW can
be found in Table 1. The raw PW was used as both anolyte and
catholyte without any buffer addition, except that the anolyte
was supplemented with 0.5 g/L CH3COONa to match the
COD concentration with AW for quantitative comparison
purpose.
CaSiO3 (wollastonite, 200 mesh, Sigma-Aldrich) was
immobilized on a 100% cotton paper matrix to increase its
contacting surface area with electrolyte. The cotton paper was
first blended with deionized (DI) water in a kitchen blender to
form paper pulp, which then was rinsed using DI water to
remove residual ions and alkalinity (final pH 7). The paper
pulp and CaSiO3 powder were mixed using blender at a weight
ratio of 1:2. The paper carrying 20 mg-CaSiO3/cm2-paper
surface with a thickness of 0.48 mm was then made by adding
the pulp/CaSiO3 mixture to a papermaking mold (Arnold
Grummer). The regenerated paper can be stable in the water
for several weeks without any deterioration.
Reactor Construction and Operation. Each MECC
reactor contains two cylindrical chambers (4 cm long × 3 cm
Figure 1. Schematic of the direct microbial electrolytic carbon capture
(MECC) process for in situ carbon sequestration and H2 production
during wastewater treatment. Wastewater was used as an electrolyte
for microbially assisted electrolytic production of H2 and OH− at the
cathode and protons (acidity) at the anode. This acidity dissolved
silicate and liberated metal ions that balanced the OH−, producing
metal hydroxide, which spontaneously transformed CO2 in situ into
metal (bi)carbonate.
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Table 1. Changes in Ion Concentration and Conductivity of the Anolyte and Catholyte in MECC Processes Using Either
Artificial Wastewater (AW) or Produced Water (PW) Electrolytesa
ions (mg/L)
electrolyte
samples
status
conductivity (mS/cm)
Ba2+
Sr2+
Ca2+
Mg2+
K+
Na+
Cl−
Br−
PO43−
Si
AW
anolyte
initial
final
control
10.65
10.81
10.68
−
−
−
−
−
−
−
497.0
97.7
−
4.2
6.4
77.9
9.0
76.0
2104
1313
2147
2943
3005
3025
−
−
−
−
−
−
−
112.5
21.9
catholyte
initial
finalb
finalc
8.93
11.58
11.56
−
−
−
−
−
−
−
4.8
121.2
−
2.5
2.3
−
69.6
68.7
1824
2601
2610
2727
2794
2795
−
−
−
−
−
−
−
1.4
1.7
anolyte
initial
final
control
19.68
20.35
21.35
28.2
6.4
18.3
11.08
4.16
9.49
98.5
400.3
29.8
8.32
6.39
6.92
99.3
12.9
98.9
4854
4239
4887
6273
6311
6305
43.2
45.4
45.4
1.08
0.45
0.99
23.3
129.2
24.6
catholyte
initial
finalb
finalc
19.60
22.71
22.70
28.9
5.2
8.2
11.20
3.28
4.10
97.8
6.4
84.3
8.45
4.42
4.57
42.4
122.0
123.2
4660
5315
5387
6277
6223
6244
43.7
43.8
43.5
1.79
0.12
0.18
26.9
41.6
42.0
PW
a
Abiotic MECC without applied voltage was conducted as control. Symbol “−“ indicates value is not detectable. bCatholyte that has been filtered
through 0.22 um membrane to remove the suspended solid in solution for DIC testing. cAcidified catholyte with all suspended solid in solution has
been completely dissolved for TIC testing.
volume in the gas bag and reservoir headspace. Chemical
oxygen demand (COD) was measured using a standard
method (HACH Company) and acetate consumption was
calculated based on 0.78 g-COD/g-sodium acetate assuming
complete oxidation. Then sodium acetate was converted to
initial total organic carbon input. The concentrations of ion
species were analyzed by inductively coupled plasma optical
emission spectroscopy (ARL Fisons 3410+) and ion
chromatography (Dionex 4500). Total inorganic carbon
(TIC) was measured based on established chemical gas
extraction-absorption method.35 Briefly, 10% (w/w) phosphoric acid was added extract CO2, which was carried by CO2 free
high-purity N2 (99.999%) to a three-stage alkali absorption cell.
The absorption solution was first titrated to pH 8.3
(phenolphthalein end point) by 0.1 M HCl to neutralize
−
surplus OH− and transfer CO2−
3 to HCO3 , then to pH 4.4
(methyl orange end point) using 0.01 M HCl to neutralize
HCO−3 . The HCl consumption in the second titration step was
used for calculation TIC. The carbon recovery was more than
99% ± 1 based on statistical analysis of inorganic carbon in a
standard sodium carbonate solution. Dissolved inorganic
carbon (DIC) was obtained using the same method after
0.22 um membrane filtration. The difference between TIC and
DIC was the inorganic carbon embedded in suspended solid in
solution. This method was also used for determination of
inorganic carbon on the paper matrix by soaking it into CO2free DI water for acid extraction of CO2. All TIC and DIC
results were reported by deducting background values of raw
electrolytes and paper. X-ray diffraction (XRD) was carried out
on a Panalytical X’Pert diffractometer using Cu radiation at
45KV/40 mA. Scans were run over the range of 10−90° with a
step size of 0.0131 and a counting time of 250 s per step.
Electrical current was determined by measuring the voltage
over a high-precision resistor using a data acquisition system
(model 2700, Keithley) at 10 min intervals. H2 production rate
(m3-H2/m3 reactor/day) and volumetric current density (A/
m3) was normalized to effective anode chamber volume.
Coulombic efficiency (CE, %), cathodic hydrogen recovery
(%), and overall H2 recovery (%) were calculated based on
diameter) made of cubic polycarbonate and separated by a
cation exchange membrane (CEM) (CMI-7000, Membrane
International). An Ag/AgCl reference electrode (0.198 V vs
SHE; RE-5B, BASi) was placed into the anode chamber for
potential measurements. Graphite-fiber brush (2.5 × 2.5 cm,
0.23 m2) and carbon cloth (7 cm2, 30% wet proofing; Fuel Cell
Earth, Woburn, MA) containing a 0.5 mg/cm2 of Pt catalyst
were used as the anode and cathode, respectively.34
The microbial anodes were acclimated first in single-chamber
microbial fuel cells using the same AW or PW with the sludge
inocula obtained from an anaerobic digester in Boulder Water
Resource Recovery Facility (Boulder, CO). After stable voltage
generation for more than 30 days, the anodes were transferred
to the MECC reactors. Each anode or cathode chamber was
connected with an external reservoir, and a volume of 230 mL
electrolyte was recycled at a rate of 3 mL/min between each
chamber and the respective reservoir (Figure 1). The paper
carrying silicate was placed in the anolyte reservoir. A gas bag
(500 mL, Calibrated Instruments Inc.) was connected to the
catholyte reservoir headspace for gas collection at atmospheric
pressure. Pure CO2 was intermittently injected into the
headspace to maintain a CO2 concentration of 5−15% in
order to simulate typical flue gas condition. A fixed voltage of
0.8 V was applied from an external power source in between
the electrodes across a 10 Ω resistor. Both reactors and
reservoirs were tightly sealed with nonair permeable tubing to
prevent atmospheric CO2 intrusion, and all headspaces were
sparged with CO2-free, ultra high purity N2 (99.999%) before
experiments. Two abiotic controls with and without external
applied voltage were operated in parallel with active reactors to
investigate potential carbon capture and H2 production under
abiotic ± electrochemical conditions. The same concentration
of CO2 CaSiO3 were applied to the controls as the active
reactors.
Analyses and Calculations. The concentration of H2 or
CO2 was analyzed by a gas chromatograph (Model 8610C, SRI
Instruments) equipped with a thermal conductivity detector
with nitrogen and helium as the carrier gas, respectively. The
cumulative volume of each gas was calculated based on gas
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previously reported methods.36 The amount of electric energy
consumption (WE) over a period of time (t) was calculated by
equation WE = ∫ t0 IEapdt, where I and Eap are current and the
applied voltage, respectively. The produced H2 energy was
calculated based on its upper heating value of combustion
(285.83 kJ/mol).
Bacterial Community Structure Analysis. The bacterial
community structures of anode biofilms under different
conditions were analyzed using high-throughput 454 GS-FLX
pyrosequencing of the 16S rRNA. Another biofilm sample
obtained from the anode of an active PBS-buffered microbial
electrolysis reactor without silicate dissolution and CO2 capture
was used as a control for community analysis. The
pyrosequencing and bioinformatics analysis were carried out
according to previously described methods.37 Raw sequencing
data were deposited to the NCBI Sequence Read Archive
(SRA) with accession No. SRP051647.
■
RESULTS AND DISCUSSION
Current Production and Buffer Capacity Formation.
The MECC reactors were operated in continuous-flow mode
using both AW and PW. Despite a similar COD provided in
both reactors, the maximum current generated from PW
reached to 130 A/m3, which was 58% higher than the peak
current from AW (Figure 2). This should be mainly due to the
more than doubled conductivity of PW than the synthetic
media.38 However, the current gradually dropped in the PW
reactor, suggesting recalcitrant substrates in raw produced water
inhibited the continued biodegradation. This is supported by
the COD removal results. Compared with nearly 100% COD
removal in the AW, the COD removal in the PW was around
56%. Because a similar COD concentration was needed to
perform the study, no produced water only control was
performed. However, the same raw produced water was treated
using a microbial capacitive desalination cell without any
acetate amendment, and a COD reduction of 50−80% was
observed in the anode chamber during batch operation.39 Near
zero current was produced in abiotic control reactors even with
the same applied voltage and substrate (SI Figure S1).
The pH in the AW electrolyte fluctuated at the beginning,
while the pH of PW electrolyte was more stable due to it higher
alkalinity. The pH of both wastewaters stabilized over time in
spite of the addition of silicate in the anode chamber and CO2
in the cathode chamber, suggesting sufficient buffer capacity
formed via CO2 reaction with OH− and formation of metal
(bi)carbonates. For both wastewaters, an apparent and
sustainable pH cycle was observed between the anolyte influent
(higher pH) and the effluent (lower pH) in continuous-flow
operation (Figure 2). This indicates that protons were
generated by microbial oxidation in the anode chamber,
which were then effectively consumed during silicate
dissolution to release mineral ions. This will help maintain
neutral anolyte pH, favoring the growth of effective microbial
communities.32 The pH in both catholytes remained very stable
at around 9.2, because the capture and reduction of CO2 in the
catholytes effectively consumed the accumulated OH− and
40,41
generated (bi)carbonate buffer (HCO−3 /CO2−
preventing
3 ),
pH increase as observed in normal water electrolysis. All the
results above were consistently found in replicate experiments
(SI Figure S5). In contrast, anolyte pH increased from neutral
to 9.6−10.0 due to spontaneous wollastonite dissolution in
abiotic controls with or without applied voltage for both
wastewaters (SI Figure S1, S2), because no protons were
Figure 2. Current generation and pH profile of the anolyte and
catholyte during active MECC operation using (A) artificial
wastewater and (B) produced water electrolytes. The wastewaters
inflow and outflow of each chamber were sampled and measured with
time. The arrows indicate silicate dosing to anolyte reservoir and
external CO2 injection to the headspace of catholyte reservoir. Abiotic
control results on the same parameters can be found in SI Figure S1.
produced in abiotic reactors to neutralize the pH increase.
Adding additional CaSiO3 did not further elevate anolyte pH,
indicating equilibrium was reached between the wollastonite
and ions liberation therefrom. The catholyte pH in abiotic
controls stayed relative stable (PW) or decreased to 4.8−6.5
(AW), indicating no or limited OH− production (SI Figure S1,
S2).
The capability of keeping pH stable in both anode and
cathode chambers is novel, sustainable, and important, because
pH fluctuation is a major challenge in both abiotic electrolysis
and bioelectrochemical systems.32,40,41 In such systems, the H+
produced at the anode or OH− produced at cathode during
electrolysis cannot efficiently transfer between the chambers to
maintain electroneutrality, which causes significant pH drop in
the anode and pH increase in the cathode unless high buffer
capacity is provided. The MECC process resolves this problem
by integrating the electrolysis with mineral dissolution,
alkalinity production, CO2 uptake, and (bi)carbonate formation, allowing such in situ reactions to alleviate pH
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fluctuation, eliminate expensive buffer addition, and maintain
effective microbial, and electrochemical reactions.
Carbon Capture Mechanism and Carbon Balance.
Compared with the abiotic control without applied voltage, a
significant increase in Ca2+ concentration (from nondetectable
to 497.0 mg/L) and Si content (from nondetectable to 112.5
mg/L) was observed in the anolyte of active MECC reactors,
indicating microbially facilitated silicate dissolution from the
paper support liberated cations for electrolytic reactions (Table
1). Meanwhile, the turbidity of catholytes increased due to the
formation of white suspended solid in the liquid, and white
precipitate was found accumulated on the wall and membrane
surface facing the cathode (SI Figure S7), both were identified
as polymorphous CaCO3 by XRD quantitative crystalline phase
analyses (SI Table S1, Figure S8). These results confirm that
CO2 was captured and stored in MECCs in part by forming
insoluble carbonate through the reaction with the cations
derived from the silicate. Such direct precipitation of carbonate
was not observed in a previous abiotic electrochemical
system,26 which could have been due to the low CO2
concentration and lower pH of final solutions (7.6−8.0, soluble
bicarbonate dominant).
Carbon balance analysis shows a total of 80−93% of the CO2
generated in the anode chamber or artificially injected in
cathode chamber was captured in the MECC systems instead of
being released to the atmosphere (Figure 3). Specifically,
NaHCO3 and Na2CO3, that is, the metal cation directly came
from the electrolytes, a feature seen previously in abiotic silicate
electrolysis.26 This is further supported by the observation of
Na+ transport from the anode chamber to the cathode chamber
for charge balance in both AW and PW electrolytes (Table 1).
Regarding the capture of CO2 derived from organic
degradation in the anode chamber, for both electrolytes, the
majority of the carbon (73−81%) was fixed on the silicate
paper matrix (Figure 3). This carbon was apparently captured
as insoluble CaCO3 through direct reaction of anolyte CO2
with the CaSiO3, because the DIC in the anolyte only
accounted for 13−20% of total carbon captured. The results
above were consistently found in replicate experiments (SI
Figure S6, Table S2). Relatively little CO2 (0.216−0.252
mmol) was observed in abiotic control anode chambers,
presumably derived from the natural decomposition of organics
in nonsterile wastewaters. These concentrations were about an
order of magnitude lower than in the bioactive reactors (2.41−
3.52 mmol). Furthermore, this small amount of carbon was
then nearly all captured on the silicate paper matrix (SI Figures
S3 and S4). For the abiotic control cathode CO2 capture, only
1.5−6.3% of the headspace CO2 was transferred into the
catholyte even in the presence of applied voltage. This transfer
was apparently solely due to gas-electrolyte equilibrium
reactions. No CaCO3 precipitation was found in abiotic
controls.
Based on the experimental results and ion analysis, we
reasoned that facilitation of Ca2+ transport from the anode to
the cathode chamber converted most CO2 to CaCO3 in the
cathode chamber, in which pH was consistently higher than 9.
Although Na+ was observed to participate carbonate formation,
it will not change the stoichiometry in terms of carbon capture
and H2 production. We write the representative reaction
equations as below assuming acetate is the organic substrate,
calcium is the main cation, and CaCO3(s) is the sole carbon
end product (ignores Ca(HCO3)2 (aq) production):
Anode reactions:
(organic oxidation)CH3COOH(aq) + 2H 2O
→ 8H+ + 2CO2(g) + 8e−
(1)
(anode cation liberation)4CaSiO3(s) + 8H+
→ 4Ca 2 + + 4SiO2(s) + 4H 2O
(2)
(anode CO2 conversion)2CaSiO3(s) + 2CO2(g)
→ 2CaCO3(s) + 2SiO2(s)
(3)
Cathode reactions:
Figure 3. Carbon balance and recovery in MECC processes using (A)
artificial wastewater electrolyte and (B) produced water electrolyte.
Organic carbon derived from wastewater organic consumption was the
only carbon input for anode reaction, and externally injected CO2 was
the only carbon source for cathode reaction. DIC represents total
dissolved inorganic carbon.
(cathode H 2production)8H 2O + 8e− → 8OH− + 4H 2(g)
(4)
(cathode cation conversion)4Ca 2 + + 8OH−
→ 4Ca(OH)2(s)
approximately 47% (AW) or 30% (PW) of the headspace CO2
in the catholyte reservoir was converted to insoluble CaCO3
(Figure 3). In addition, more than half of the CO2 captured
existed in the form of dissolved inorganic carbon (DIC).
Catholyte filtrate contained a small amount of Ca2+ (Table 1),
suggesting that the DIC was primarily composed of soluble
(5)
(cathode CO2 conversion)4Ca(OH)2(s) + 4CO2(g)
→ 4CaCO3(s) + 4H 2O
(6)
Net reaction:
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Table 2. Energy Expenditure and Energy Production (as H2) Indicates MECC Can Be an Energy-Positive Processa
energy production as H2
coulombic efficiency (%)
cathodic H2 recovery (%)
overall H2 recovery (%)
current density (A/m3)
H2 production rate (m3-H2/m3/d)
produced H2 energy (kJ/mol-total C/CO2 captured)
a
energy expenditure and carbon capture
AW
82
91
75
81
0.81
146
PW
80
95
76
123
1.29
129
captured carbon from organics (%)
AW
93
PW
91
captured carbon from CO2 (%)
86
80
energy expenditure (kJ/mol-total C/CO2 captured)
84
72
Net energy gain (kJ/mol-total C/CO2 captured) 62 (AW) and 57 (PW).
under different electrolytes, high throughput sequencing was
conducted for the anode biofilm samples obtained from three
reactors containing AW, PW, and acetate+phosphate buffer,
respectively. All three reactors were inoculated with the same
sludge. Results show only a slight difference existed among the
three bacterial community structures despite different electrolyte conditions in terms of salinity and ion composition (SI
Figure S9). Geobacter spp. was found dominant in all samples
and accounted for 80−89% of total composition. Geobacter spp.
has been well studied in bioelectrochemical systems30 and can
live in saline environment.42 Because Geobacter spp. primarily
metabolize simple organics such as acetate, the degradation of
complex organic matters exist in wastewater may rely on the
synergy of a consortium of microorganisms, with polymerdegrading bacteria breaking down complex substrates into
simple organic products, so the majority of the exoelectrogens
can further convert these products to electrical current.43
Technology Potentials and Challenges. Wastewater As
Electrolyte for Carbon Mitigation and Energy Production. As
demonstrated in the study, saline wastewater can be an ideal
candidate for the technology due to its high conductivity and
buffer capacity. For example, it is estimated that each year
nearly 880 billion gallons (3.3 billion m3) of flowback and
produced water are generated from oil and natural gas industry
in the U.S.,44 and the treatment and reuse of such water
becomes an emerging need, because the current practice of
long-distance transportation and deep well injection poses
multiple health and safety concerns.39,44,45 Produced water
contains high concentrations of salt and biodegradable
hydrocarbon, which is an ideal source for this technology.
High concentrations of metal cations, such as Ca2+, Mg2+, Na+,
Sr2+, and Ba2+, are already present in these solutions,46−48
which may facilitate carbonate precipitation. The process also
improves the removal of TDS, which has been a huge challenge
for traditional filtration based technologies for produced water
reuse. Seawater can also be used, especially when it is associated
wastewater treatment and desalination processes. In Hong
Kong, more than 270 million m3 of seawater is used for toilet
flushing every year for 6 million people, and more than 21 cities
are considering similar practice to address fresh water
shortage.49 Currently, the treatment of this kind of water is
expensive (US$0.16/m3) by mixing it with wastewater from
freshwater uses in existing municipal wastewater facilities.49
Treatment of these saline wastewaters using the MECC will
receive renewable energy and carbon credit, which can also
benefit advanced treatment and reuse processes such as
desalination.50 The potentials and challenges of using these
waste streams in MECC however do require better understanding of the water characteristics and experimental
investigations.
CH3COOH(aq) + 2H 2O + 6CaSiO3(s) + 4CO2(g)
VDC
⎯⎯→ 6CaCO3(s) + 6SiO2(s) + 4H 2(g)
(7)
Based on this reaction, for every 6 mol of organic and
inorganic carbon captured, there will be 4 mol H2 produced.
Based on carbon balance, this study shows that up to 93% of
the carbon otherwise internally generated and emitted to the
atmosphere was captured in the MECC system. VDC is the
applied voltage (0.8 V) used to overcome thermodynamic
barrier of the reaction, which is much lower than that used in
abiotic water electrolysis (1.8−2.0 V).33 The voltage difference
is made up by the microbially facilitated release of chemical
energy stored in the wastewater organics.
H2 Production and Energy Balance. Continuous H2 was
produced in the active reactors using either AW or PW
electrolyte. High Coulombic efficiencies (80−82%) for both
electrolytes indicated that the majority of organic substrates
were converted to current, which then was used to reduce H+
for H2 production with extremely high efficiencies with the
cathodic H2 recovery ranged from 91−95% (Table 2). All of
these lead to a high recovery of H2 yield (75−76%) of the
theoretical maximum value. A higher H2 production rate was
observed from the PW MECC due to the higher current
density obtained from the reactor. No H2 was found in abiotic
controls even with a same applied voltage of 0.8 V.
Table 2 shows the preliminary energy balance of the MECC
systems. The reactors consumed between 72 and 84 kJ of
electrical energy for each mole CO2 captured (excluding
pumping energy), which is a significant reduction compared
with CCS-type systems.22 The energy savings was due to
energy supplemented from in situ microbial conversion of
chemical energy to current. As a result, the H2 produced from
the MECCs represent an energy output of 129−146 kJ/mol-C/
CO2 converted, which means a net energy gain of 57−62 kJ/
mol-C/CO2 converted (Table 2, SI Table S3). The energy
benefits can be even bigger if the calculation includes the
energy savings for wastewater treatment. In comparison, the H2
energy produced in abiotic electrochemical systems can only
compensate for 30−79% of electric energy input.25,26 Electrochemical CO2 capture through silicate dissolution consumed
426−481 kJ/mol-CO2 energy,26 and energy consumption was
>266 kJ/mol-CO2 when using carbonate.25 The state-of-the-art
solid sorbents adsorption or solvents absorption for postcombustion CO2 capture consume ∼90 kJ/mol-CO213 and 41−
185 kJ/mol-CO2,14 respectively, and more energy is needed for
compression, transport and storage of collected CO2. Moreover, the sorbents/solvents are very costly compared with
globally abundant and inexpensive silicates and wastewater.
Adaptability of Anode Microbial Community. To
reveal the impacts of MECC process on microbial communities
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pH based on carbonic acid pKa as well as the cation species
present in the electrolyte. This makes operation and product
generation likely controllable. For example, solid carbonate
products can be separated from the solution to reduce TDS and
reused onsite for wastewater alkalinity compensation and
improving activated sludge settling properties.28,29 They may
also be collected as feed stock materials for cement production
and other commercial uses. However, efficient separation of
such solid products needs further investigation, and more
characterizations are needed to understand the potential fouling
problem in the system. The deposition of CaCO3 on
membrane surface may hinder further ion transport and
decrease system performance. To solve this problem,
membrane maybe removed or replaced with higher throughput
separators, and operation needs to be optimized for solid
separation. To replace high-cost Pt-based electrodes, studies
have demonstrated that stainless steel, Ni alloys, or MoS2 can
be cheaper alternatives for electrolytic H2 production.56 Most
common rocks have chemical compositions that when
solublized can provide alkalinity needed for CO2 capture and
conversion. Indeed, the slow, spontaneous reaction of excess
CO2 with alkaline minerals contained in the Earth’s crust is the
primary way that atmospheric CO2 is naturally modulated over
geologic time scales.27 If cost-effective mechanisms can be
found to speed up such reactions, then global alkaline mineral
deposits, in particular silicate minerals, could provide a virtually
unlimited reagent supply for CO2 mitigation. For example, the
Columbia River basalts in the U.S. alone is estimated at over
500 million Gt, which is 10 000 times that needed to remove all
the cumulative anthropogenic emissions of 20 000 Gt of CO2.26
Further research is needed to explored in the use of globally
abundant, mafic and ultramafic silicates in the MECC process
and to determine the optimized cost, safety, impacts, and
benefits of doing so.
Preliminary Analysis Demonstrates Economic Benefits.
Preliminary economic analysis for lab-scale MECC systems
showed that the net cost for mitigating one ton of CO2 could
be $48/t- CO2 mitigated, which was calculated based on a
combination of CO2 capture cost (capital plus operation cost),
potential cost offsets (revenue of H2 and wastewater treatment), and avoided CO2 emission through reduced fossil fuel
consumption for wastewater treatment and commercial H2
production (nature gas reforming). The detailed calculation can
be found in SI. This net cost is well below the $70−270/t- CO2
estimated for coal power plant with geological storage CCS,12
and also below the cost for direct air CO2 capture using
chemical/thermal methods (on the order of $1000/t- CO222)
or abiotic electrolytic dissolution of silicate ($86/t- CO2).26
Furthermore, as reported by many microbial electrochemical
technology studies, the current high capital cost of MECC can
be decreased if low cost materials and reactor configuration can
be developed. As previously mentioned, the CO2 mitigation
capacity and efficiency of MECC can be further improved
through the selective formation of bicarbonate, which per mol
of cations captures and stores twice the amount of CO2 than
does carbonate. Although this study demonstrates that MECC
has potential for significant energy savings and carbon benefits
for the wastewater industry, further work is needed to better
understand the technology barriers and to optimize system
designs, operational protocols, and applications.
The process may also be applicable to municipal wastewater
with lower salt concentration. Unlike traditional bioelectrochemical processes whose performance are limited by the low
conductivity and alkalinity in municipal wastewater, results in
this study show that MECC increases the conductivity and
alkalinity during the dissolution of silicate and CO2 (Table 1).
Additionally, the process can create a benign cycle as carbonate
precipitation reduces TDS buildup in the effluent. The TDS
may even decrease after reactions if the wastewater has initial
hardness. This cycle also avoids the need of external buffer
solution and alleviates a major challenge in system scale up.
Previous studies have demonstrated that microbial electrolytic
process can be used in real municipal wastewater for H2
production.51 If the process can be used in municipal
wastewater, the potential carbon and energy benefits can be
remarkable for utilities,3 as a potential of 50.3 million tons of
CO2 can be sequestered and 1.5 million tons of H2 (60.5
terawatt hour energy) can be produced per year in the US alone
based on eq 7, assuming wastewater with moderate organics
concentration of 200 mg/L COD. While this study focuses on
proof-of-concept, further studies are needed to characterize and
improve system compatibility for municipal wastewater.
Ideal Locations for MECC Carbon Capture, Energy
Production, And Wastewater Treatment. The MECC process
captures the CO2 generated during the wastewater treatment,
and it captures more CO2 from other sources such as flue gas or
even potentially from air. This is significant for the wastewater
industry as well as other industries with significant carbon
emissions. Interestingly, we found wastewater plants commonly
colocated with big carbon emitters such as power plants,
cement plants, and refineries. This presents a good opportunity
for both industries, as the wastewater facility can help capture
and sequester the CO2 emitted from the nearby point source
and generate carbon credits, and the emitter can save costs by
avoiding the use of expensive and energy intensive CCS
systems yet still meet EPA mandates on carbon pollution
reduction, such as the recently implemented Clean Power Plan
for existing power plants in 2014.52 One the other hand, the
waste heat generated from neighoring plants maybe used for
acceleration of microbial reaction process in MECC to save
more applied energy.53 However, this effort should not
compromise the needs of meeting effluent regulatory requirements, so more studies are needed to investigate the treatment
efficiency using different wastewater streams.
In addition, many wastewater facilities are located on the
coast and discharge billions of gallons of treated effluent to the
ocean. If the MECC process is integrated into these facilities
and discharges dissolved metal hydroxide- or carbonate-rich
effluent to the ocean, excess CO2 could be subsequently
captured and stored by using >70% surface area of the earth as
a natural, passive CO2 absorber, an alternative that should be
cheaper and simpler than forcing air through CO2 absorbing
device using high-energy machinery.17,19,22 In any case, such
alkalinity addition to the ocean would be helpful in maintaining
the CaCO3 saturation state of seawater, thus helping mitigate
the effects of ongoing ocean acidification on marine shellfish
and corals.54,55 Furthermore, the conversion of CO2 to ocean
alkalinity provides effective, long-term carbon storage; the
residence time of Ca2+ and Mg2+ (and by inference, that of
balancing HCO3− and CO32− ions) in seawater is ≥1Myrs.
System Optimization toward Practical Application. The
formation of insoluble metal carbonate such as CaCO3 or
soluble bicarbonate in the MECC is highly influenced by the
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ASSOCIATED CONTENT
S Supporting Information
*
Additional tables and figures are included.The Supporting
Information is available free of charge on the ACS Publications
website at DOI: 10.1021/acs.est.5b00875.
■
AUTHOR INFORMATION
Corresponding Author
*Phone: (303) 492-4137; fax: (303) 492-7217; e-mail: jason.
[email protected].
Notes
The authors declare no competing financial interest.
■
■
ACKNOWLEDGMENTS
This work was supported by the U.S. National Science
Foundation under Awards CBET-1235848 and IIP-1445084.
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