Article pubs.acs.org/est
CO2 Capture from Simulated Fuel Gas Mixtures Using Semiclathrate Hydrates Formed by Quaternary Ammonium Salts Sungwon Park,† Seungmin Lee,‡ Youngjun Lee,† and Yongwon Seo†,* †
School of Urban and Environmental Engineering, Ulsan National Institute of Science and Technology, Ulsan 689-798, Republic of Korea ‡ Offshore Plant Resources R&D Center, Korea Institute of Industrial Technology, Busan 618-230, Republic of Korea S Supporting Information *
ABSTRACT: In order to investigate the feasibility of semiclathrate hydrate-based precombustion CO2 capture, thermodynamic, kinetic, and spectroscopic studies were undertaken on the semiclathrate hydrates formed from a fuel gas mixture of H2 (60%) + CO2 (40%) in the presence of quaternary ammonium salts (QASs) such as tetra-nbutylammonium bromide (TBAB) and fluoride (TBAF). The inclusion of QASs demonstrated significantly stabilized hydrate dissociation conditions. This effect was greater for TBAF than TBAB. However, due to the presence of dodecahedral cages that are partially filled with water molecules, TBAF showed a relatively lower gas uptake than TBAB. From the stability condition measurements and compositional analyses, it was found that with only one step of semiclathrate hydrate formation with the fuel gas mixture from the IGCC plants, 95% CO2 can be enriched in the semiclathrate hydrate phase at room temperature. The enclathration of both CO2 and H2 in the cages of the QAS semiclathrate hydrates and the structural transition that results from the inclusion of QASs were confirmed through Raman and 1H NMR measurements. The experimental results obtained in this study provide the physicochemical background required for understanding selective partitioning and distributions of guest gases in the QAS semiclathrate hydrates and for investigating the feasibility of a semiclathrate hydrate-based precombustion CO2 capture process.
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INTRODUCTION The inclusion compounds formed by the accommodation of guest molecules into hydrogen-bonded water frameworks are called clathrate hydrates; they can be divided into true clathrate hydrates and semiclathrate hydrates. The well-known gas hydrates belong to true clathrate hydrates and can form three different crystal structures (sI, sII, and sH), which contain differently sized and shaped cages that primarily depend on the molecular sizes of the guest species.1 In gas hydrates, the guest molecules that are captured in the cages interact with the water molecules through van der Waals forces.1 Gas hydrates have many technical and industrial applications in energy and environmental fields, such as the prevention of hydrate plugging in oil and gas pipelines, the exploitation of natural gas hydrates in the deep ocean sediments or permafrost regions, the storage and transportation of natural gas in a solid hydrate form, and CO2 capture and storage.2−6 Quaternary ammonium salts (QASs) such as tetra-nbutylammonium bromide (TBAB) and fluoride (TBAF) form ionic semiclathrate hydrates with water molecules at atmospheric pressure.7−9 In QAS semiclathrate hydrates, the water molecules together with anions such as Br− and F− build a polyhedral host framework of the cages in which the tetra-nbutylammonium cations (TBA+) are incorporated as guest molecules.7−9 Semiclathrate hydrates have many physical and © XXXX American Chemical Society
structural properties in common with gas hydrates because the primary component of both crystal structures is water molecules. However, the primary difference is that, in gas hydrates, the guest molecules are not physically bonded to the host water lattices; thus, there are no chemical interactions between the host and guest molecules. While in QAS semiclathrate hydrates, the guest molecules can both form part of the host lattice and occupy cages of the structure and, thus, have ionic interactions with the host molecules.7−9 Due to the presence of empty cages that can capture smallsized gas molecules at low pressure and high temperature conditions, QAS semiclathrate hydrates have been considered as a new potential gas storage and separation material.10−14 Recently, many studies on semiclathrate hydrates have reported the capture of CO2 from flue and fuel gas mixtures.15−23 The flue gas mixture from conventional coal-fired power plants consists of approximately 10% CO2 and 90% N2 at atmospheric pressure, while the fuel gas mixture from integrated gasification combined cycle (IGCC) power plants is composed of approximately 40% CO2 and 60% H2 with a pressure range Received: March 4, 2013 Revised: May 27, 2013 Accepted: May 29, 2013
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between 2.5 and 5.0 MPa. 24 Due to the higher CO 2 concentration and higher pressure of the fuel gas mixture that is available at IGCC plants, the fuel gas mixture may be a better target for the QAS semiclathrate hydrate-based CO2 capture process than the flue gas mixture. Furthermore, the hydrate equilibrium pressure difference between CO2 and H2 is significantly larger than that between CO2 and N2 and, thus, the fuel gas mixture is expected to have a higher selectivity for CO2 in the semiclathrate hydrate phase than the flue gas mixture. To design a process to capture CO2 from the fuel gas mixture using semiclathrate hydrates, measurements of phase behavior, gas uptake, guest gas composition, and the crystal structure are needed. Among the QASs, TBAB has been studied extensively as a thermodynamic promoter for clathrate hydrate-based CO2 capture from the fuel gas mixture, while TBAF has rarely been used for the same purpose, even though TBAF exhibits more enhanced thermal stability when involved in forming both pure semiclathrate hydrates with water and double semiclathrate hydrates with guest gases.13−23 In this study, TBAB and TBAF were used as semiclathrate forming materials to capture CO2 from the fuel gas mixture, and the experimental results for the CO2 capture performance of both QASs were compared. The semiclathrate hydrate phase equilibria for the quaternary H2 (60%) + CO2 (40%) + TBAB (or TBAF) + water mixtures at three different concentrations were measured through experiments in order to determine the stability conditions of the double semiclathrate hydrates with guest gases. The CO2 concentrations of both the semiclathrate hydrate phase and vapor phase after the semiclathrate hydrate formation were measured to examine the selective partition of the target gas. The gas uptake and vapor phase composition changes during the semiclathrate hydrate formation were also measured in order to confirm the amount of gas enclathrated, the formation pattern, and the time required to complete the reaction. Lastly, the structural transition and guest gas enclathration were confirmed via Raman and 1H NMR measurements.
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Heise Bourdon tube pressure gauge [CMM-137219, (0 to 10.0) MPa range] having a maximum error of ±0.01 MPa. The experiment for the semiclathrate hydrate-phase equilibrium measurements began by charging the equilibrium cell with approximately 80 cm3 of TBAB (0.6, 3.7, and 7.7 mol %) or TBAF (0.8, 3.3, and 5.3 mol %) solutions. In this study, a step heating and cooling method was adopted to measure the accurate hydrate phase equilibrium points. The temperature was decreased or increased in a specified temperature step and then, was held at that temperature for the remaining time in order to provide sufficient time for equilibrium. After the equilibrium cell was pressurized to the desired pressure using the H2 + CO2 gas, the main system was slowly cooled to a temperature lower than the expected equilibrium temperature. Due to thermal contraction, the cell pressure was slightly decreased by decreasing the temperature in 1.0 K steps with 60 min. Then, an abrupt pressure depression was observed at the semiclathrate hydrate crystal growth after the nucleation stage. When the pressure depression due to the semiclathrate hydrate formation reached a steady-state condition, the temperature was increased in 0.1 K steps with sufficient time of 90 min and accordingly, the cell pressure increased with the semiclathrate hydrate dissociation. After all semiclathrate hydrates were dissociated with the increasing temperature, the cell pressure was again slightly increased due to thermal expansion. The equilibrium pressure and temperature of the three phases [semiclathrate hydrate (H) − water-rich liquid (LW) − vapor (V)] were determined using the intersection between the semiclathrate hydrate dissociation and thermal expansion lines. Gas Composition and Gas Uptake Measurements. The gas composition analyses and gas uptake measurements were conducted in the same apparatus as that used for measuring the semiclathrate hydrate phase equilibria. In order to measure the compositions of the semiclathrate hydrate and vapor phases, a sampling valve (Model 7010, Rheodyne, U.S.) with a loop volume of 20 μL was installed and connected to a gas chromatograph (HP 5890II, U.S.) through a high-pressure metering pump (Eldex, U.S.). A thermal conductivity detector (TCD) and a Porapak Q column (Supelco, U.S.) were used as the detector and the column, respectively. Argon was used as the carrier gas. The reactor was initially pressurized using a H2 + CO2 gas mixture supplied from a gas cylinder and then maintained at a constant pressure using a microflow syringe pump (Model 500D, ISCO, U.S.) operated in a constant pressure mode. Under these isothermal and isobaric conditions, the experiment was conducted in a batch manner with a fixed amount of water. The reaction time was counted right after the nucleation of hydrate crystals. During the hydrate formation, the compositions of the vapor phase were analyzed via an online gas chromatograph at a time interval of 10 min, after the vapor phase was circulated through a sampling line with a highpressure metering pump to equilibrate both compositions of the cell and loop for more than 5 min. It was confirmed that 5 min pumping is long enough to equilibrate both compositions of the cell and loop. After it was confirmed that the CO2 composition in the vapor phase had stabilized, the corresponding composition of the semiclathrate hydrate phase was also measured using gas chromatograph after the vapor phase had been evacuated at 253.15 K, and the entire hydrate phase was dissociated at 298.15 K. Uncertainty for all concentrations of solutions and CO2 compositions is ±0.1%. For the gas uptake measurement, the volumetric gas consumption was measured during the semiclathrate hydrate
EXPERIMENTAL SECTION
Stability Condition Measurements. The gas mixture of H2 (60%) + CO2 (40%) used for this study was supplied by PSG Gas Co. (Republic of Korea). TBAB with a purity of 99% and TBAF solution with concentration of 75% in water were purchased from Sigma-Aldrich (U.S.). Doubly distilled deionized water was used. In particular, the samples for the 1 H NMR measurements were prepared from deuterated water (D2O, 99.9 atom % D, Cambridge Isotope Laboratories, U.S.). All materials were used without further purification. The experimental apparatus for the semiclathrate hydrate phase equilibria was specifically designed to accurately measure the semiclathrate hydrate dissociation pressures and temperatures. The equilibrium cell was made of 316 stainless steel and had an internal volume of approximately 250 cm3. The cell content was vigorously agitated using an impeller-type stirrer. A thermocouple with an accuracy of ±0.1% of reading [(73.15 to 1273.15) K range] was inserted into the cell to measure the temperature of the inner content. This thermocouple was calibrated using an ASTM 63C mercury thermometer (Ever Ready Thermometer, U.S.) with a resolution of ±0.1 K. A pressure transducer (S-10, Wika, Germany) with an accuracy of ±0.25% (0 to 10.0 MPa range) was used to measure cell pressure. The pressure transducer was also calibrated using a B
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formation. The volume of the gas supplemented to the reactor was recorded at regular time intervals, and finally the moles of the gas consumed during the gas hydrate formation were calculated after the compressibility factor and hence the density was calculated by Pitzer correlations.25 Raman and 1H NMR Analyses. For the Raman measurements, the semiclathrate hydrates formed were finely powdered in a liquid N2 vessel and pelletized into cylinders (1 cm diameter and 0.5 cm height). Under atmospheric pressure conditions, the Raman spectra were obtained using a JASCO NRS-3100 Raman spectrometer (Japan) with a thermoelectrically cooled CCD detector and a 1800 grooves/mm holographic grating. The excitation source was a diode-pumped solid-state laser emitting a 532 nm line, and the laser intensity was 7 mW. The temperature of the sample was maintained at approximately 170 K during the measurement by controlling the flow rate of the liquid N2 vapor. The 1H magic angle spinning (MAS) NMR spectra were collected using a Varian INOVA600 spectrometer with a spin rate of 10 kHz, a pulse length of 5 μs, and a repetition time of 15 s. The samples were packed into a 4 mm o.d. zirconia rotor and analyzed at 183 K. More details of the experimental apparatus and procedure have been provided in previous papers.13,14,26,27
Figure 1. Semiclathrate hydrate phase equilibria for the H2 + CO2 + TBAB + water mixtures.
RESULTS AND DISCUSSION The fuel gas mixture from the IGCC plant is available at pressures between 2.5 and 5.0 MPa, which indicates that additional pressurization might not be required for the clathrate hydrate-based CO2 capture process, especially in low temperature regions. However, the clathrate hydrate formation conditions of the fuel gas mixture should be more stabilized for the clathrate hydrate-based process in order to become more economically competitive than the conventional CO2 capture methods. The stabilization of clathrate hydrates can be achieved through the addition of thermodynamic promoters, which can significantly reduce the clathrate hydrate equilibrium pressures at a given temperature or raise the clathrate hydrate equilibrium temperature at a given pressure. In this study, TBAB and TBAF, which are two representative QASs, were tested as both effective thermodynamic promoters and semiclathrate forming materials for CO2 capture from the fuel gas mixture. The three-phase equilibria (H − LW − V) for the quaternary H2 (60%) + CO2 (40%) + TBAB + water systems were measured in order to determine the stability conditions of the TBAB semiclathrate hydrates at three different concentrations of TBAB (0.6, 3.7, and 7.7 mol %) and are presented in Figure 1. The experimental results clearly demonstrate that the presence of TBAB significantly reduces the formation pressure of the H2 + CO2 + TBAB semiclathrate hydrates at a given temperature. TBAB forms semiclathrate hydrates with water under atmospheric pressure conditions and the dissociation temperature for the type A crystal structure (TBAB·26H2O) is reported to be 285.25 K.7,13,28,29 TBAB semiclathrate hydrates are known to have large, partially broken cages that are occupied by TBA+ and also small dodecahedral (512) cages that are initially vacant and, thus, can capture both H2 and CO2.8,12,13 As observed in the CH4 + TBAB semiclathrate hydrate systems, increasing the TBAB concentration to 3.7 mol % resulted in increased thermal stability. However, the stabilization effect of TBAB at 7.7 mol % was less than that at 3.7 mol %, which corresponds to the stoichiometric concentration of type A TBAB semiclathrate hydrate. At
TBAB concentrations higher than 3.7 mol %, the excess amount of TBAB molecules, which cannot be incorporated into the semiclathrate cages, remains as free ions of TBA+ and Br−, functioning as an inhibitor. TBAF also forms semiclathrate hydrates with water under atmospheric pressure conditions, and the dissociation temperature of the cubic structure with a hydration number of TBAF·29·7H2O is 300.85 K.7,14,29,30 TBAF semiclathrate hydrates also have small dodecahedral cages that may be vacant or partially occupied with water molecules.14,30 The three-phase (H − LW − V) equilibria for the H2 + CO2 + TBAF + water systems were also experimentally determined at three different concentrations of TBAF (0.8, 3.3, and 5.3 mol %) and are shown in Figure 2. The H − LW − V equilibrium conditions of the H2 + CO2 + TBAF semiclathrate hydrates shifted significantly to the stabilized regions represented by higher temperature and lower pressure conditions when compared with the equilibrium conditions of the H2 + CO2 hydrate system. The maximum stabilization effect was observed at TBAF 3.3 mol %, which corresponds to the stoichiometric concentration of TBAF·29·7H2O. For the fuel gas mixture used in this study, TBAF generally exhibited a greater equilibrium pressure reduction at a specified temperature or equilibrium temperature increase at a specified pressure than TBAB. The experimental results imply that TBAF can form semiclathrate hydrates with the actual fuel gas mixture from the IGCC plants at approximately 300 K without additional compression. The CO2 compositions were analyzed after completing the semiclathrate hydrate formation at each condition in order to examine the selective partitioning of CO2 in the semiclathrate hydrate and vapor phases. For all cases, ΔT, which is defined as the temperature difference between the equilibrium and experimental temperatures and is considered the driving force for semiclathrate hydrate formation, was fixed at 4.0 K at 8.0 MPa. In this study, all of the experiments were conducted at 8.0 MPa and a driving force of ΔT = 4.0 K in order to compare the experimental results from QAS systems with those from pure
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CO2 in the hydrate or semiclathrate phase. As can be seen in Figure 3, the final CO2 compositions of the semiclathrate hydrate phase are independent of the initial TBAF concentration of the solution. However, in the TBAB 3.7 mol % system, a slightly lower CO2 composition of the semiclathrate hydrate phase was observed when compared with the TBAB 0.6 mol % system. The slight difference in the CO2 compositions of the semiclathrate hydrate phase for the TBAB semiclathrate hydrates might be attributed to the different semiclathrate structures that are induced separately depending on the initially charged TBAB concentrations.8,28 However, more details on the relation between the cage occupancies of guest gases and the TBAB semiclathrate structures should be investigated through more sophisticated and advanced analysis methods, such as X-ray diffraction and neutron diffraction. The experimental results clearly demonstrate that at room temperature, only one step of semiclathrate hydrate formation from the fuel gas mixture can give highly enriched 95% CO2 that could be ultimately used in industry or in sequestration. Figure 4 shows the gas uptake curves for the pure H2 + CO2 hydrate, H2 + CO2 + TBAB semiclathrate hydrates, and H2 + Figure 2. Semiclathrate hydrate phase equilibria for the H2 + CO2 + TBAF + water mixtures.
water system. The final CO2 concentrations in the vapor and semiclathrate hydrate phases for the pure H2 + CO2 hydrate, H2 + CO2 + TBAB semiclathrate hydrates, and H2 + CO2 + TBAF semiclathrate hydrates are presented in Figure 3. After
Figure 4. Gas uptake curves during the formation of the pure H2 + CO2 hydrate, H2 + CO2 + TBAB semiclathrates, and H2 + CO2 + TBAF semiclathrates at 8.0 MPa and ΔT = 4.0 K.
CO2 + TBAF semiclathrate hydrates at 8.0 MPa and ΔT = 4.0 K. The gas consumption resulting from the hydrate or semiclathrate formation was monitored from the beginning of the agitation and the actual reaction time was measured right after the nucleation of the hydrate or semiclathrate crystals after some induction time. The accumulated amount of gas consumed during the gas hydrate or semiclathrate hydrate formation indicates the total amount of gas enclathrated in the cages of the gas hydrate or semiclathrate hydrates. The gas uptakes for each hydrate or semiclathrate system are expressed as the ratio of the moles of the consumed gas to the moles of the initially charged water and, as a result, are related to the conversion of water into gas hydrates or semiclathrate hydrates. As shown in Figure 4, the gas uptake of the pure H2 + CO2 hydrate system was much higher than the other systems because the H2 + CO2 hydrate, known to form sI hydrate,31 has both small (512) and large (51262) cages for capturing the H2 and CO2 molecules. It should be noted that, when compared with the TBAB semiclathrate hydrate systems, the TBAF semiclathrate hydrate systems exhibited a much lower gas
Figure 3. CO2 concentrations in the vapor and semiclathrate phases at 8.0 MPa and ΔT = 4.0 K.
confirming the completion of the hydrate or semiclathrate formation, the CO2 composition in the vapor phase was measured and then the CO2 composition of the retrieved gas from the hydrate or semiclathrate phase was measured. The CO2 compositions in the hydrate or semiclathrate phase are higher than 90%, which indicates the significant enrichment of D
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uptake despite their structural similarity. Because the TBAF semiclathrate hydrates have dodecahedral (512) cages that are partially filled with the water molecules,30 they provide fewer dodecahedral cages for capturing CO2 and H2 molecules than the TBAB semiclathrate hydrates. The experimental results indicate that TBAB is a better QAS in terms of the amount of gas enclathrated, whereas TBAF is a better QAS for thermodynamic stability. Figure 5 shows the CO2 composition change in the vapor phase during the hydrate or semiclathrate hydrate formation at
Figure 5. CO2 concentration change behavior in the vapor phase during hydrate formation at 8.0 MPa and ΔT = 4.0 K.
Figure 6. Raman spectra of the enclathrated CO2 molecules in the H2 + CO2 hydrate, H2 + CO2 + TBAB (3.7 mol %) semiclathrate, and H2 + CO2 + TBAF (3.3 mol %) semiclathrate.
8.0 MPa and ΔT = 4.0 K. The CO2 composition change behavior in the vapor phase is closely related to both the final vapor phase compositions depicted in Figure 3 and the gas uptake results shown in Figure 4. The H2 + CO2 hydrate system, which exhibited the lowest CO2 composition in the vapor phase in Figure 3 and the highest gas uptake in Figure 4, demonstrated a drastic drop in the CO2 concentration of the vapor phase as the hydrate formation reaction proceeded, while the TBAB and TBAF semiclathrate hydrate systems exhibited a slight decrease. After a long time, the final values of the CO2 concentrations for each system shown in Figure 5 converge to the values for each system presented in Figure 3. It was confirmed from both Figures 3 and 5 that the hydrate or semiclathrate formation was complete within 60 min because no more effective changes in both the gas uptake and the CO2 concentration were observed after 60 min. The final CO2 composition in the vapor phase is primarily influenced by the gas uptake, which is closely related to the conversion of water to gas hydrates or semiclathrate hydrates, because the CO2 compositions in the hydrate phase are almost identical for each system. As seen in Figure 3, both CO2 and H2 molecules are captured in the QAS semiclathrate hydrates formed from the fuel gas mixture even though the CO2 molecules are predominantly enriched in the semiclathrate structure. The macroscopic compositional analyses shown in Figure 3 can offer basic inclusion patterns and concentrations of the two guests in semiclathrate hydrate phase. However, experimental results from the Raman and 1H NMR measurements can provide more accurate information about the structure and guest distribution in the semiclathrate hydrates cages. Figure 6 presents the Raman spectra of the H2 + CO2 hydrate, H2 + CO2 + TBAB
semiclathrate hydrate, and H2 + CO2 + TBAF semiclathrate hydrate. The H2 + CO2 hydrate shows two major bands appearing at 1276 and 1380 cm−1, which are the same as those from the pure CO2 hydrate and, thus, indicate the sI hydrate formation. Unlike the pure CH4 hydrate, the Raman spectrum of the pure CO2 hydrate does not exhibit peak splitting for the guests in two different cages even though the CO2 molecules can be captured in both the small and large cages of the sI hydrate.32 In both H2 + CO2 + TBAB and H2 + CO2 + TBAF semiclathrate hydrates, the Raman peaks from the enclathrated CO2 molecules were demonstrated at 1274 and 1380 cm−1, which are the same peak positions obtained from both CO2 + TBAB and CO2 + TBAF semiclathrate hydrates.13,14 The slight peak shift (1276 cm−1 → 1274 cm−1) can be attributed to the structural transition of the sI to the semiclathrate. From the Raman spectra collected in this study, the CO2 enclathration in the gas hydrate or semiclathrate hydrate phase and structural transition due to the QAS inclusion were clearly confirmed, but information on the cage occupancy of each guest could not obtained because there is not a one-to-one match between each Raman peak and each hydrate cage. The 1H NMR measurements were used to confirm the enclathration of the H2 molecules in the H2 + CO2 + TBAB semiclathrate hydrates formed from deuterium oxide (D2O). Figure 7 represents 1H NMR spectra of the H2 + CO2 hydrate, pure TBAB semiclathrate hydrate, and H2 + CO2 + TBAB semiclathrate hydrate. In the 1H NMR spectrum of the H2 + CO2 hydrate, the broad peak at 6.5 ppm can be assigned to a residual HDO impurity, and the sharp peak at 4.3 ppm can be assigned to H2 molecules captured in cages of the sI hydrate. Controversy remains about the single or double H2 occupancy in the 512 cages of hydrate.31,33−38 However, judging from the E
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ACKNOWLEDGMENTS
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REFERENCES
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This research was supported by the Future Creativity and Innovation project (2012) of the UNIST (1.130015.01).
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Figure 7. 1H NMR spectra of the H2 + CO2 hydrate, pure TBAB (3.7 mol %) semiclathrate, and H2 + CO2 + TBAB (3.7 mol %) semiclathrate.
present experimental results as well as previous literature reports obtained from NMR and Raman spectroscopy36−38 and considering the relatively low pressure conditions for preparing the samples in this study, the peak at 4.3 ppm is attributable to single H2 occupancy in the 512 cages. The peak at 4.3 ppm can be found again in the H2 + CO2 + TBAB semiclathrate hydrate, which again indicates the H2 enclathration in the 512 cages of the semiclathrate hydrate. The very broad peak at 0.8 ppm resulted from the TBA+ captured in the partially broken large cages of the semiclathrate hydrate. TBAF is a better QAS in terms of thermodynamic stability, whereas TBAB is a better QAS in terms of gas uptake. One of the future challenges for semiclathrate hydrate-based precombustion CO2 capture process is a thorough investigation or synthesis of new QASs that can provide both better gas uptake and thermodynamic stability. For QAS added systems, with only one step of semiclathrate hydrate formation, approximately 95% CO2 in the semiclathrate hydrate phase can be achieved at much higher temperatures or much lower pressure conditions when compared with the pure water system.
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ASSOCIATED CONTENT
S Supporting Information *
Tables of semiclathrate phase equilibrium data for the H2 (60%) + CO2 (40%) + TBAB + water and H2 (60%) + CO2 (40%) + TBAF + water mixtures. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +82-52-217-2821; fax: +82-52-217-2819; e-mail: ywseo@ unist.ac.kr. Notes
The authors declare no competing financial interest. F
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dx.doi.org/10.1021/es400966x | Environ. Sci. Technol. XXXX, XXX, XXX−XXX
