Microbial Electrolytic Carbon Capture for Carbon Negative and Energy

Jun 15, 2015 - Enhanced Electricity Generation and Pollutant Degradation by Hybrid Photoelectrochemical and Microbial Fuel Cells. Yue Du , Xiangtong Z...
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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.

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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. © XXXX 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: February 17, 2015 Revised: May 11, 2015 Accepted: June 15, 2015

A

DOI: 10.1021/acs.est.5b00875 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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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

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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. B

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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) K+

Na+

Cl−

Br−

PO43−

Si

− 4.2 6.4

77.9 9.0 76.0

2104 1313 2147

2943 3005 3025

− − −

− − −

− 112.5 21.9

− 4.8 121.2

− 2.5 2.3

− 69.6 68.7

1824 2601 2610

2727 2794 2795

− − −

− − −

− 1.4 1.7

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

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

2+

Sr

Ca

2+

electrolyte

samples

status

conductivity (mS/cm)

AW

anolyte

initial final control

10.65 10.81 10.68

− − −

− − −

− 497.0 97.7

catholyte

initial finalb finalc

8.93 11.58 11.56

− − −

− − −

anolyte

initial final control

19.68 20.35 21.35

28.2 6.4 18.3

catholyte

initial finalb finalc

19.60 22.71 22.70

28.9 5.2 8.2

PW

Ba

2+

Mg

2+

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. a

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 C

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Environmental Science & Technology 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.

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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 preventing generated (bi)carbonate buffer (HCO−3 /CO2− 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 D

DOI: 10.1021/acs.est.5b00875 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology 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: E

DOI: 10.1021/acs.est.5b00875 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology 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 F

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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 G

DOI: 10.1021/acs.est.5b00875 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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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.

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AUTHOR INFORMATION

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*Phone: (303) 492-4137; fax: (303) 492-7217; e-mail: jason. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. National Science Foundation under Awards CBET-1235848 and IIP-1445084. REFERENCES

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DOI: 10.1021/acs.est.5b00875 Environ. Sci. Technol. XXXX, XXX, XXX−XXX