Examination of Sulfur-Functionalized, Copper-Doped Iron

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Energy & Fuels 2007, 21, 2688-2697

Examination of Sulfur-Functionalized, Copper-Doped Iron Nanoparticles for Vapor-Phase Mercury Capture in Entrained-Flow and Fixed-Bed Systems D. E. Meyer,† S. K. Sikdar,‡ N. D. Hutson,§ and D. Bhattacharyya*,† Department of Chemical and Materials Engineering, UniVersity of Kentucky, Lexington, Kentucky 40506, U.S. EnVironmental Protection Agency, National Risk Management Research Laboratory, 26 West M. L. King Dr., MS 443, Cincinnati, Ohio 45268, U.S. EnVironmental Protection Agency, National Risk Management Research Laboratory, 109 T. W. Alexander DriVe (E305-01), Research Triangle Park, North Carolina 27711 ReceiVed March 8, 2007. ReVised Manuscript ReceiVed May 31, 2007

The use of copper-doped Fe nanoaggregates silanized with organic sulfur as bis-(triethoxy silyl propyl)tetra sulfide has been investigated for the capture of elemental mercury (Hg0) from the vapor phase for potential power plant applications. Silanization procedures resulted in 70% deposition of the targeted sulfur level, with particles containing approximately 4 wt % S. The addition of copper was found to increase the fixed-bed (total) capacity of this type of sorbent from 170 ( 20 µg Hg‚g sorbent-1 with no copper doping to 2730 ( 80 µg Hg‚g sorbent-1 at 1.2 wt % Cu. When no S is deposited, the capacity of Fe/Cu nanoaggregates was only 180 µg Hg‚g sorbent-1. These findings suggest that a combined Cu-S mechanism is responsible for Hg capture. Moving-bed (injection) testing of the Fe-based sorbents in a simulated flue gas stream showed that the 1.2 wt % Cu sample was able to achieve significant removal of the Hg. At a modest sorbent injection rate of 3.6 × 10-3 g‚L-1‚h-1, this material showed a steady-state removal capacity of 107.5 µg Hg‚g sorbent-1 for an inlet concentration of 17.8 µg‚m-3. On the basis of only 4% usage of the total capacity during single-pass injection, it might be beneficial to develop methods to separate and recycle these materials to reduce power plant operation costs for Hg emissions control.

Introduction Coal-fired power plants in the United States emit roughly 48 tons of mercury annually.1 In 2005, the U.S. Environmental Protection Agency (EPA) promulgated the Clean Air Mercury Rule, calling for a nearly 70% reduction in these Hg emissions by 2018. The reduction will occur in two stages, with a 25-ton cap required in 2010, followed by a 15-ton cap to be implemented by 2018. The deep reductions that are required in the second phase may require mercury-specific control technology for compliance. As a result, there has been considerable work in the development and testing of innovative and cost-effective mercury control technologies.2 Studies have shown that the Hg in the flue gas occurs as either elemental mercury (Hg0), oxidized mercury (Hg2+, usually as HgCl2), or particulate-bound mercury (Hgp).3-5 Particulate bound Hg is easily removed using particulate matter (PM) control equipment such as an electro* Corresponding author. Phone: 859-257-2794. Fax: 859-323-1929. E-mail: [email protected]. † University of Kentucky. ‡ U.S. Environmental Protection Agency, Cincinnati, OH. § U.S. Environmental Protection Agency, Research Triangle Park, NC. (1) EPA: Mercury - Controlling Power Plant Emissions. http:// www.epa.gov/mercury/control_emissions/index.htm (accessed October 27, 2006). (2) Srivastava, R. K.; Hutson, N. D.; Martin, G. B.; Princiotta, F.; Staudt, J. EnViron. Sci. Technol. 2005, 41, 1385-1393. (3) Lu, D. Y.; Granatstein, D. L.; Rose, D. J. Ind. Eng. Chem. Res. 2004, 43, 5400-5404. (4) Pavageau, M.-P.; Pecheyran, C.; Krupp, E. M.; Morin, A.; Donard, O. F. X. EnViron. Sci. Technol. 2002, 36, 1561-1573. (5) Presto, A. A.; Granite, E. J. EnViron. Sci. Technol. 2006, 40, 56015609.

static precipitator (ESP) or fabric filter baghouse. Plants that have a wet flue gas desulfurization scrubber can expect a high level of HgCl2 removal because of its high solubility in water. The elemental mercury vapor (Hg0), however, is much more difficult to remove; it is relatively insoluble in water and is not readily captured simultaneously in other air pollution control equipment.3-5 There are two main approaches that have been studied for Hg0 vapor control. One involves the direct capture of Hg0 by injection of sorbents, usually powdered activated carbons (PACs).6-11 The other approach focuses on the development of oxidation catalysts to transform Hg0 to Hg2+ for removal during scrubbing processes.5,12 Of the two, removal by sorbent injection has received the most focus and is viewed as the more promising. The most widely studied material for injection is powdered activated carbon.8-11 Although it does possess the ability to capture mercury, traditional activated carbon has been found to possess disadvantages, such as a high cost of material and low Hg capacity at elevated temperatures.6 More importantly, (6) Lee, J.-Y.; Ju, Y.; Keener, T. C.; Varma, R. S. EnViron. Sci. Technol. 2006, 40, 2714-2720. (7) Pitoniak, E.; Wu, C.-Y.; Mazyck, D. W.; Powers, K. W.; Sigmund, W. EnViron. Sci. Technol. 2005, 39, 1269-1274. (8) Huggins, F. E.; Huffman, G. P.; Dunham, G. E.; Senior, C. L. Energy Fuels 1999, 13, 114-121. (9) Liu, W.; Vidic, R. D.; Brown, T. D. EnViron. Sci. Technol. 2000, 34, 154-159. (10) Hsi, H.-C.; Rood, M. J.; Rostam-Abadi, M.; Chen, S.; Chang, R. EnViron. Sci. Technol. 2001, 35, 2785-2791. (11) Feng, W.; Borguet, E.; Vidic, R. D. Carbon 2006, 44, 2998-3004. (12) Zhao, Y.; Mann, M. D.; Pavlish, J. H.; Mibeck, B. A. F.; Dunham, G. E.; Olson, E. S. EnViron. Sci. Technol. 2006, 40, 1603-1608.

10.1021/ef070120t CCC: $37.00 © 2007 American Chemical Society Published on Web 06/29/2007

Sulfur-Functionalized, Cu-Doped Fe Nanoparticles

the removal of the PAC occurs in the ESP, contaminating the fly ash that is normally sold for concrete production to help recover plant costs. Therefore, researchers have begun examining a variety of activated carbons to understand how best to improve Hg capacities. Huggins et al. studied activated carbons containing sulfur, chlorine, or iodine using X-ray absorption fine structure to determine the mechanism of Hg capture for each.8 They found that the Hg capture for the sulfur and iodine samples was not dependent on the species of Hg in the gas phase. In addition, they were able to determine that certain gasphase species, such as HCl and H2SO4, had an enhancing effect on Hg capture for the given sorbents, possibly the result of catalyzed oxidation of Hg0. Liu et al. further investigated Hg capture with sulfur-impregnated active carbon under flue-gas conditions and found that, while moisture decreased Hg capacity through competitive sorption, SO2 and NO had no real effect.9 Their work also found that loss of sorption occurs as the temperature is increased beyond 140 °C as the result of suppressed HgS formation. This finding supports the current proposal for sorbent injection just prior to the ESP where temperatures are roughly 140 °C. Hsi et al. examined sulfurimpregnated activated carbon fibers to determine the effects of sulfur speciation and impregnation temperature on Hg capture.10 For an inlet Hg concentration of 50 ppmv, they found that a capacity of 11.3 mg Hg‚g-1 could be achieved at 135 °C when sulfur impregnation occurred at 400 °C. In addition, it was determined that the sulfur deposited as elemental sulfur was most likely responsible for approximately 70% of the Hg sorption, although organic sulfur was not specifically tested. Recent work has appeared in the literature regarding the development of alternative non-carbon materials for Hg capture. Lee et al. have investigated functionalization of silica gel, alumina, zeolites, and montmorillonite with various groups, including amines, amides, thiols, urea, elemental sulfur, sodium sulfide, and sodium polysulfide.6 These groups were selected on the basis of liquid-phase Hg sorption properties. However, most of these materials had little or no affinity for Hg0 in the vapor phase. The best capacity reported was for a supported sodium sulfide, which had a capacity of approximately 283 µg Hg‚g-1 at 140 °C. Pitoniak et al. have presented a more novel approach involving a photocatalytic titania-impregnated silica to simultaneously oxidize and capture Hg0.7 A capacity of 10 mg Hg‚g-1 was obtained for this material, while providing a 90% removal for 500 h of exposure time. Unfortunately, the exposure temperature for Hg capture was not reported. Interestingly, the capacity was increased to nearly 30 mg Hg‚g-1 when the material was doped with HgO prior to exposure. However, such materials will require the installation of UV equipment for successful implementation. The present work focuses on the use of iron nanoaggregates modified with organic sulfur and trace quantities of copper as a candidate material for mercury capture by injection. Fixedbed evaluation of this type of sorbent examining the effects of temperature and sulfur loading have been presented previously by Makkuni and co-workers.13 Preliminary results were encouraging, with a capacity of 5.3 mg‚g-1 reported for a 2.0 wt % Cu/6 wt % S sorbent at 140 °C. Complete removal of Hg0 at an inlet concentration of 3.2 ppb was sustained for >100 min at a rate of 6.2 µg Hg0‚g sorbent-1‚min-1. The choice of Fe as a support platform is beneficial because it is inexpensive and can be functionalized, through both electroplating and surfacechemistry reactions. Previous studies have focused on the use (13) Makkuni, A.; Varma, R. S.; Sikdar, S. K.; Bhattacharyya, D. Ind. Eng. Chem. Res. 2007, 46, 1305-1315.

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of various nanostructured materials containing iron oxides (FexOy) for the oxidation of Hg0 to the more easily captured Hg2+.14,15 Here, the Fe nanoparticles serve only as a support platform and are not intended as active sites for mercury capture. The use of sulfur as the active element for mercury capture is motivated by the work with sulfur-impregnation in activated carbons, which showed an enhanced capacity for elemental mercury capture when compared to traditional activated carbons. For this work, the sulfur is tethered to iron nanoaggregates as organic sulfur via traditional silanization chemistry using a silane material containing a tetrasulfide (S4) center. In an attempt to further increase the capacity of this material, a second metal is added to the iron surface to help activate the sulfur network. Research detailing the removal of mercury from petroleum oil at high temperatures (>200 °C) found that copper sulfide supported on activated carbon was extremely effective in dealing with multiple types of Hg species, both elemental and complexed.16 The chemistry is well-known, with sulfur serving as the active agent during mercury removal.16 Therefore, copper was selected as the doping metal in the hopes that it will promote these same sulfur-mercury interactions. A maximum amount of 1.2 wt % Cu was used because this corresponds to a surface coverage of 1/4 of the total area available, leaving ample area for deposition of S by silanization. The objectives of this work are to (1) examine the impact of Cu loading on Hg0 removal during fixed-bed contact, (2) perform vapor-phase Hg capture during entrained flow in a simulated flue gas stream, and (3) compare the results of fixed-bed and entrained-flow sorbent testing to determine what factors influence dynamic Hg capture. Experimental Methods Ferrous chloride (FeCl2‚4H2O), Fisher Scientific or Merck Co.; cupric chloride (CuCl2‚2H2O), Mallinckrodt; bis-(triethoxy silyl propyl)-tetra sulfide (S4), Gelest; and ethanol and sodium borohydride (NaBH4), Sigma-Aldrich were all used as received unless otherwise stated. All water used was deionized ultrafiltered water (Fisher Scientific) that was deoxygenated prior to use by bubbling ultrahigh purity N2 through it for 2-3 h. For comparison, a commercial powdered activated carbon, Norit Darco HG-LH, was also tested for Hg capture. This material is a brominated form of one of the Department of Energy benchmark sorbents for proposed mercury technologies. Preparation of the Fe/Cu Precursor. An aqueous 0.1 M FeCl2 solution was prepared at a pH of 4.5 to prevent oxidation of the dissolved Fe2+. An aqueous 0.5 M NaBH4 solution was added dropwise to reduce the ferrous ions to the zerovalent form with mixing at 300 RPM. Excess NaBH4 was used (2.5 times) to ensure complete reduction because the borohydride will also react with water during the reduction step. The borohydride reaction was carried out under a N2 atmosphere to prevent the oxidation of freshly formed iron. The reduced Fe was allowed to settle before decanting off the excess solution. The remaining slurry was vacuum-filtered using continuous additions of ethanol to wash the aggregates while minimizing oxidation. The moist cake was recovered for Cu deposition by electroplating. An aqueous CuCl2 solution was prepared in 100 mL of ethanol for copper deposition. The mass of Cu in solution was adjusted to give the desired Cu doping based on weight percent. The actual levels of Cu have been varied to examine the effects of Cu on Hg capture. The moist Fe0 aggregates were added to the copper solution (14) Lee, C. W.; Srivastava, R. K.; Ghorishi, S. B.; Kilgroe, J. D. Proceedings of the 94th Annual Meeting and Exhibition of Air Waste Management Association, Orlando, FL, 2001, Paper No. 156. (15) Borderieux, S.; Wu, C. Y.; Bonzongo, J. C.; Powers, K. Aerosol Air Quality 2004, 4, 7490. (16) Yan, T. Y. Ind. Eng. Chem. Res. 1996, 35, 3697-3701.

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Figure 1. Experimental setup for fixed-bed testing of Hg sorbents.

and allowed to mix for electroplating to occur. The particles were again vacuum-filtered under ethanol and dried overnight under a vacuum. Deposition of Bis-(Triethoxy Silyl Propyl)-Tetra Sulfide (S4). A known mass of Fe/Cu nanoaggregates was added to ethanol containing 3% bis-(triethoxy silyl propyl)-tetra sulfide (S4) by volume. The mass of sulfur in solution corresponds to a theoretical sulfur content of 6 wt %. The solvent was removed by evaporation under a nitrogen stream with mixing at room temperature, and the particles were vacuum-dried at 100 °C to facilitate silanization of the metal surface (∼12 h). Surface Area Measurements by N2 Sorption. For a selected case, a 100 mg sample of the silanized Fe/Cu aggregates was prepared by outgassing for 6 h at 120 °C under N2 refluxing. The N2 adsorption isotherm at 77.3 K was then measured using a Micromeritics Tristar 3000 pore volume analyzer. The total surface area was then calculated using the Brunauer-Emmett-Teller (BET) method. Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray (EDX) Analysis. Aggregate size was examined using a Hitachi S900 field emission SEM for high magnification imaging. The solid-phase composition was determined using a Hitachi SEM 3200 electron microscope equipped with an EDX detector. Samples were mounted on aluminum grids with carbon adhesive and coated in Au/Pd prior to analysis. Fixed-Bed Vapor-Phase Hg Capture. The total capacity of all synthesized sorbents was determined using fixed-bed testing. The experimental setup is shown in Figure 1. The packed-bed consisted of 10 mg of sorbent blended with 1 g of sand (Aldrich, 50-70 mesh). The sand was found to have a negligible capacity for Hg during blank tests. The bed was placed in a 0.9 cm i.d. glass U-tube. The bed was suspended in one arm of the U-tube on 0.1 g of glass wool to prevent loss of the bed under a gas flow. The U-tube was maintained at 140 °C using an oil bath. Mercury vapor was generated using a mercury permeation tube (VICI Metronics) that is capable of producing 1596 ng‚min-1 of Hg0 at 100 °C. The permeation tube was supported on glass beads packed in a glass U-tube and maintained at 100 °C using an oil bath. Ultrahigh-purity nitrogen, N2 (99.999%, Scott Gross), was used as the carrier gas and was fed into the system at 60 mL‚min-1 using a flow meter and pressure regulator at the tank. At these conditions, the concentration of Hg0 in the inlet to the packed bed was 26.6 ppb. The empty-bed residence time based on a bed height of approximately 1 cm was 0.65 s. The concentration of Hg used was 1 order of magnitude larger than the concentration of Hg in most flue gases (2 ppb) but allows for the rapid attainment of equilibrium capacities. The system was equipped with four three-way valves that allow for bypass of the Hg source, the packed bed, or both. Qualitative

Meyer et al. real-time monitoring of Hg was achieved using a Buck Scientific Cold Vapor Mercury Analyzer 400. The detector supplied an output signal in terms of the millivolt response to the change in percent transmittance of the 253.6 nm wavelength detector to a coupled computer with a sampling interval of 0.5 s. Typical tests began by first bypassing the Hg source and feeding nitrogen through the packed bed to establish a zero baseline for the detector. Next, the packed bed was bypassed and nitrogen was fed to the emitter to determine the signal for the Hg concentration in the feed. This number is important because it determines when breakthrough has occurred. Finally, the Hg-enriched nitrogen was fed through the packed bed until a minimum of 90% exhaustion was achieved. To ensure that this level was reached, the fixed bed was bypassed after each run to verify that the feed baseline had not drifted. The contents of the bed were digested overnight using 10 mL of 4 M nitric acid. This procedure is a modified form of the one reported previously by Makkuni and co-workers, which had an associated error of only 10%.13 The amount of Hg was quantified using inductively coupled plasma (ICP) elemental analysis. Hg Analysis Using Inductively-Coupled Plasma Analysis. Digested samples were analyzed for Hg using a Varian Vista-Pro CCD Simultaneous ICP-OES (optical emission spectrometer) under the supervision of the University of Kentucky’s Environmental Research and Training Laboratory. The analysis of Hg was performed at wavelengths of 194.164 and 253.652. These wavelengths were selected because of their lack of susceptibility to Cu interference. The instrument was programmed to obtain readings in triplicate for each sample. An acidic rinse cycle was used between samples to avoid carryover. The detector was calibrated to a lower limit of 0.25 ppm Hg, with a linear trend existing over the entire calibration range. The upper calibration limit was selected on the basis of the anticipated amount of Hg captured for a given sorbent and ranged from 2 to 6 ppm Hg. Standards were prepared by digesting fresh sorbent (no Hg exposure) in 4 M nitric acid (1 mg‚mL-1) and spiking it with appropriate quantities of a known Hg standard (Fisher) to produce the desired concentrations. Prior to analysis of the digested samples, known samples were first prepared and analyzed to verify the accuracy of the machine. Both a 1.5 ppm and 0.5 ppm known sample routinely gave errors of less than 1%. Digested samples of exhausted beds were split in half, with a 0.5 ppm Hg spike added to one sample to verify the analysis by the method of additions. This method was checked using a 0.5 ppm known sample, which yielded a recovery error of 9%. Values reported are for sorbent samples that had less than 15% error by the method of additions. Moving-Bed (Injected) Vapor-Phase Hg Capture in Simulated Flue Gas. In-flight mercury capture experiments were conducted in the U.S. EPA’s entrained flow reactor (EFR; Research Triangle Park, NC). The EFR, shown in Figure 2, consists of a 4 cm i.d. Pyrex reactor tube that is 332 cm in length. The reactor is heated by three electric tube furnaces (Lindberg, USA) to maintain a constant temperature. The temperature is measured at several locations using type K thermocouples inserted into the gas stream at a 90° offset from each gas sample port. A water-cooled methane gas burner provides combustion flue gases (CO, CO2, H2O, and O2) while other flue gas components (N2, SO2, NO, HCl) are introduced into the reactor at constant concentrations using compressed gases and mass flow controllers. Elemental mercury vapor (Hg0) is generated using a permeation device (Dynacalibrator, VICI Metronics) with a N2 carrier and is subsequently mixed with the simulated flue gas before entering the EFR. For these experiments, the sorbent was entrained into the reactor with a nitrogen purge stream. The sorbent feeding assembly consisted of a gas supply manifold, a feed tube, and a syringe pump mounted to a vibrating plate. Nitrogen was fed through the manifold into the top of the test tube and exhausted through the feed tube mounted in the center of the reactor. The plate vibrated in order to fluidize the top layer of the sorbent as the syringe pump advanced the test tube with respect to the feed tube at a constant rate. The sorbent was entrained into the nitrogen as the sorbent level



Figure 2. Diagram of the entrained flow reactor (EFR) used for moving-bed (injection) testing. Table 1. Experimental Conditions for Entrained-Flow Mercury Capture Experiments target

unit

wet basis

dry basis

remark

HCl SO2 NO CO2 CO O2 H2O N2 Hg0 (g) total flow temperature

ppm ppm ppm vol % ppm vol % vol % vol % µg/Nm3 L/min °C

10 500 200 3.5 ∼5 6.8 6.8 balance ∼21.5 14 140

10.8 538 215 3.8 ∼5 7.3 NA balance ∼20 13 140

from gas cylinders from gas cylinders from gas cylinders from methane burner from methane burner from methane burner from methane burner from cylinders/burner Hg0 permeation tube std cond, 1 atm, 20 °C 139-144 °C

approached the end of the feed tube and was subsequently injected down the centerline of the EFR. The EFR is capable of simulating a variety of conditions over a wide temperature range. In these experiments, the gases were mixed to simulate the flue gas that would be expected from the combustion of a low-sulfur Western sub-bituminous coal (e.g., such as that from Wyoming’s Powder River Basin). The experimental conditions are given in Table 1. Experiments were initiated by heating the reactor to the desired temperature and performing the necessary calibration checks for continuous emissions monitoring. Baseline measurements were collected before beginning the injection of the sorbent(s). In all of these tests, the sorbents were diluted with an inert material (diatomaceous earth (DE) flour) at varying mixtures depending upon the desired sorbent injection rate. This dilution allowed the syringe pump to perform in the middle of its operating range and helped to maintain a steady sorbent addition rate. A “blank” (DE-only) experiment was performed to ensure that the DE had no Hg sorption capacity. The sorbent-DE mixture was

added continuously for at least 20 min before stopping the injection and diverting the gas stream to bypass the on-line Hg analysis. The system was cooled, and the reactor walls and tubing were thoroughly cleaned between experiments to prevent “memory effects” due to any accumulated sorbent on the reactor or tubing walls. Sampling and Measurement of Hg during Injection. An advanced Hg continuous emission monitor (CEM) Nippon DM6B (NIC, Japan) was used for measuring the concentration and speciation of Hg in the simulated coal combustion flue gas. The CEM has an internal sample conditioning system where the sample passes through a dust filter and is then separated into two sectionss the total Hg (HgT) section and the Hg0 section. The HgT section contains a dry reduction catalyst to reduce oxidized Hg to Hg0; the Hg0 section does not contain a reduction catalyst. Liquids and other interfering substances were removed by an internal sample conditioning apparatus, and additional moisture was removed by chilling the sample to approximately 5 °C using an internal piezoelectric chiller. The Hg CEM then measures Hg in the conditioned samples using cold vapor atomic absorption. The CEM automatically performs zero and span calibration checks with an internal Hg vapor generator. The samples for mercury analysis were extracted from the EFR through a glass tube inserted into the reactor tube after the third electric furnace. The extraction tube had a blind end and a circular 1/4′′ aperture which was turned in the opposite direction of the bulk flow in order to exclude as much PM as possible. The sample was transported through 1/4′′ perfluoroalkoxy tubing and through a 0.45 µm polytetrafluoroethylene filter to remove PM close to the sample extraction point. The filters are disposable and were replaced after each experiment. The sample was heated until reaching the Hg CEM conditioner. All data (Hg0 and HgT) were recorded to a data acquisition system every 10 s.


Meyer et al.

Figure 3. SEM micrographs of silanized Fe/Cu nanoaggregates used to determine base-particle size. The aggregates are formed as new particles grow on the surface of larger particles, producing a chain structure.

Results and Discussion The discussion of results will first address characterization of the sorbents used for testing. This will be followed by separate summaries of the data for fixed-bed and moving-bed (injection) testing. The results of both contact modes will then be compared and a suitable criterion identified for predicting moving-bed behavior on the basis of the performance of fixed-bed removal. Sorbent Characterization. The base particle size for the typical silanized nanoaggregates was determined using SEM micrographs of the particles at 60 000× magnification (Figure 3). It is clear in the images that the aggregates grow in a chainlike structure, with the nucleation of new base particles occurring at the surface of larger particles. When a population size of n ) 100 particles is used, the average base-particle diameter is 80 ( 30 nm. The BET surface area determined for the sulfurcoated sorbent is 24.3 m2‚g-1. The BET surface area of the Darco HG-LH activated carbon based on the stated sample preparation is 305.9 m2‚g-1. Sorbent composition was determined using EDX analysis. The results for all tested sorbents are summarized in Table 2. These values represent an average of 3-5 readings per sample from various locations on the specimen. The particles show a low level of oxidation using these synthesis techniques. A total of five samples were examined. Three of the sorbents are Fe0 containing variable levels of Cu (0.1, 0.8, and 1.2 wt %) and approximately 3.5-3.9 wt % S as silanized S4. The S data suggests that only 70% of the S4 was silanized during deposition. A fourth sorbent was prepared in the absence of Cu and twice the amount of S (7.1 wt %) to determine the impact of S on Hg capture. The last sorbent included with this work is Fe0 nanoaggregates containing 2.0 wt % Cu and no S to determine the role of Cu during exposure to Hg. Some residual chloride from the metal salts was also found during EDX analysis and is listed in Table 2. The Darco HG-LH activated carbon is a brominated lignite-derived sorbent that was found to have the following composition by weight using EDX analysis: 70.0% C, 18.6% O, 3.5% Ca, 2.6% Br, 2.0% Si, 1.5% Na, 1.0% S, 0.5% Mg, and 0.4% Fe. Fixed-Bed Hg0 Capture. The real-time mercury removal during fixed-bed testing of the Fe-based sorbents is shown in Figure 4, along with the data for the activated carbon Darco HG-LH. The preliminary trace curve data obtained were misrepresentative because initial data were masked by the lag time of Hg in the detector tube for the concentrations used.

Table 2. Summary of Sorbent Mass Composition Data Acquired Using EDX Analysis sorbent

silicon

oxygen

sulfur

carbon

iron

copper

chlorine

Fe-1 Fe-2 Fe-3 Fe-4 Fe-5

9.2% 5.0% 2.1% 4.6% 0.0%

7.5% 8.1% 16.3% 18.7% 20.9%

7.1% 3.8% 3.9% 3.5% 0.0%

12.8% 6.4% 6.6% 5.8% 0.0%

62.9% 75.7% 70.3% 62.9% 75.6%

0.0% 0.1% 0.8% 1.2% 2.0%

0.6% 0.7% 3.4% 1.5%

Therefore, the linear slope of the upper portion of the breakthrough curve was used to project concentration data back to the x axis to determine the onset of breakthrough. This assumption implies a linear rate of adsorption over the entire range of breakthrough. A value of 90% breakthrough will be used for the comparison of bed exhaustion because the rate of exhaustion tended to slow dramatically around this point for the Fe-based sorbents. Of the Fe-based sorbents, the 1.2 wt % Cu and 3.5 wt % S sorbent had the highest capture, with only a 60% breakthrough after 10 min. In fact, a breakthrough of 90% was not achieved until 17 min (not shown). When the copper doping is reduced by a factor of 10, 90% breakthrough is achieved in 3 min. When no copper is added and the S wt % is doubled, the time to achieve 90% breakthrough is 4.5 min. The last Fe-based sorbent shown contains 2 wt % copper (double copper loading) with no S. Again, a rapid breakthrough is achieved on the basis of the 90% breakthrough time of 2 min. The activated carbon demonstrated the greatest levels of Hg removal with no breakthrough occurring after 10 min and a 90% breakthrough at 28 min. These results suggest that the brominated activated carbon is a better material for Hg capture. However, this is not necessarily the case if one considers the amount of surface area tested. Using the BET surface areas reported above, the surface area of activated carbon is 13 times more than the area of the Fe-based sorbents. Therefore, the activated carbon should be able to sustain Hg removal longer than the Fe-based sorbents when using identical masses of each. In fact, the surface-area normalized capacity for the Darco sorbent is 9.6 µg Hg‚m-2, which is much smaller if compared to the value of 112.2 µg Hg‚m-2 for the 1.3 wt % Cu and 3.5 wt % S sorbent. The performances of both the non-sulfur and non-copper sorbents are important for identifying the means by which Hg removal occurs. If Cu alone is the active component during mercury uptake, then the non-sulfur material should be capable of sustaining a greater removal rate of mercury for longer periods of time when compared to the 1.2 wt % Cu sorbent



Figure 4. Hg vapor breakthrough during fixed-bed contact with sulfur-functionalized Fe/Cu nanoparticles at 140 °C indicating the importance of both S and Cu in the removal process. CFeed ) 0.022-0.040 µg Hg‚cm-3

Figure 5. Effects of copper doping on total Hg capture at 140 °C using sulfur-functionalized Fe nanoaggregates. The impact of copper becomes evident between 0.8 and 1.3 wt % as the capacity increases from microgram quantities to 2.8 mg‚g-1.

because there is approximately twice the amount of copper present. In a similar manner, if S is the primary means of Hg removal, then the non-copper/high-sulfur sorbent should lead to better results. For both cases, it is evident that the 1.2 wt % Cu/3.5 wt % S sorbent sustained much greater levels of mercury removal given the longer period of total capture before breakthrough of the trace curve. On the basis of this observation, the interaction of S and Cu must be necessary for successful Hg removal using this type of platform. Total Hg Capacity of Sorbents. The total capacities of all tested sorbents based on ICP digestion analysis are shown in Figure 5 as a function of the wt % Cu doping. As one would expect, the total capacity of the Fe-based sorbents increases with increasing copper from 170 ( 20 µg Hg‚g sorbent-1, with no copper doping, to 2730 ( 80 µg Hg‚g sorbent-1 at 1.2 wt % Cu. However, the increase is not directly proportional on the basis of the sharp increase in capacity around 1 wt % Cu. This increase might reflect a shift to the Cu-S combined mechanism. Further proof of this combined Cu-S effect can be found by examining the role of S. When the breakthrough curve data was used, it was determined that the non-sulfur sorbent was much less effective than the silanized Fe/Cu nanoaggregates for sustained capture. This is further supported by the total capacity of only 180 µg Hg‚g sorbent-1 obtained for the non-sulfur sorbent. This capacity is nearly the same as that of the noncopper sorbent with 7.1 wt % S. Therefore, the combination of both elements as a means of vapor-phase mercury removal

improves the capacity by more than an order of magnitude when compared to the individual elemental systems. The results also indicate that residual chloride, which can interact with Hg, is not a predominant mechanism for mercury removal with this material on the basis of the low capacity of the non-sulfur sorbent, which contained 1.5 wt % Cl. Interestingly, the total capacity of the Darco HG-LH activated carbon is 2942 µg Hg‚g sorbent-1. This value is only 8% larger than the capacity of the 1.2 wt % Cu/3.5 wt % S Fe-based sorbent. The difference in performance of the activated carbon and Fe-based sorbents during fixed-bed testing can be attributed to surface-area/accessibility effects. When the capacities are normalized using the amount of surface area used, the 1.2 wt % Cu-Fe sorbent has a capacity of 112.2 µg Hg‚m-2 as compared to only 9.6 µg Hg‚m-2 for the Darco HG-LH. Without a full understanding of the capture mechanism for both Br and S, a definitive quantitative comparison of the two mechanisms is not possible. However, a general evaluation of the two with regard to the molar ratio of Hg to each might provide some insight into the relative affinity of Hg for each. Therefore, the total Hg molar capacities for the Darco and 1.2 wt % Cu-Fe sorbents were normalized by the number of moles of the active elemental center (AEC ) Br or S). The resulting values are 4.5 × 10-2 mol Hg‚mol AEC-1 for the brominated Darco HG-LH sorbent and 1.1 × 10-2 mol Hg‚mol AEC-1 for the silanized sulfur, copper-doped sorbent. This would suggest that Hg strongly interacts with Br when compared to S. Verification of


Meyer et al.

Figure 6. An examination of the intial rate of adsorption (q) for Fe/Cu-S nanoaggregates, Fe/S nanoaggregates, and brominated activated carbon (A). After longer exposure times, the brominated Darco sorbent overtakes the Fe/Cu-S sorbent (B). Table 3. The Corrected Inlet Hg Concentration and Resulting Initial Rate of Adsorption for Fixed-Bed Sorbent Testing at 140 °C and a N2 Flowrate of 60 cm3 min-1

material

actual inlet Hg concentration (ppb)

maximum rate of adsoprtion (µg Hg‚g sorbent-1 min-1)

linear rate of adsorption (µg Hg‚g sorbent-1 min-1)

Fe-1; 0.0 wt % Cu, 7.1 wt % S Fe-2; 0.1 wt % Cu, 3.8 wt % S Fe-4; 1.2 wt % Cu, 3.5 wt % S Fe-5; 2.0 wt % Cu, 0 wt % S Darco HG-LH

22 33 40 23 28

132 198 242 138 169

62 146 242 94 169

this observation will require knowledge of the number of S or Br atoms that define a site for Hg capture. Rate of Adsorption. The rate of adsorption can be determined from the slope of the graph of the capacity (q, µg Hg‚g sorbent-1) as a function of time. The capacity at each sample time was obtained by integrating the area above the breakthrough curve obtained for each sorbent using rectangular approximation. The final capacities (q∞) calculated by integration were lower than the actual capacities determined by ICP analysis. However, the on-line detector was not calibrated for exact concentration measurements and was intended to provide the relative trend of Hg capture. Therefore, the data for q(t) was multiplied by the ratio of the digested q∞ to the integrated q∞ to normalize the data to the actual value. The resulting plot for the initial uptake (2 min) of all sorbents tested is shown in Figure 6A. The slope of each line represents the rate of adsorption, which is nearly linear for much of the initial contact. The linear rate was calculated for each sorbent to allow comparison of the three types of interactions (S, Cu-S, and Br) represented.

The highest rate of adsorption was obtained using the 1.2 wt % Cu and 3.5 wt % S sorbent, which has a rate of 242 µg Hg‚g sorbent-1‚min-1. However, this rate is more than the theoretical maximum rate of 160 µg Hg‚g sorbent-1‚min-1 calculated using the intended generation rate of 1.596 µg Hg‚min-1 and a catalyst mass of 0.01 g. This means the emitter rate was actually not what was anticipated and explains the difference in capacities obtained by ICP analysis and integration of the trace curve. Therefore, the actual Hg generation rate was determined for each sorbent by recalculating the integrated capacity to match the digested capacity using a varying Hg generation rate. Variations in the emitter rate can be attributed to a build of Hg in the emitter housing tube when the tube was bypassed under heat between runs. The resulting inlet concentration of Hg is shown in Table 3 along with the rate of adsorption for each sorbent. When the copper content is reduced to 0.1 wt %, the capacity is only 146 µg Hg‚g sorbent-1‚min-1 for a similar inlet Hg concentration and is less than the maximum possible. When no copper is present, the rate of adsorption for high-sulfur loading (7.1 wt



%) is only 62 µg Hg‚g sorbent-1‚min-1, which is the smallest rate for all sorbents tested. However, when no S is deposited, the rate of adsorption for high-level copper doping (2.0 wt %) is only increased to 94 µg Hg‚g sorbent-1‚min-1 for approximately the same inlet concentration. These results further support a combined Cu and S mechanism for Hg capture. The Darco HG-LH sorbent has a rate of 169 µg Hg‚g sorbent-1‚min-1, which also reflects the maximum rate of adsorption based on the inlet concentration of Hg. Therefore, both the Cu-S and brominated platforms have the potential for high rates of Hg adsorption during short contact times. To further compare the performance of the two platforms, the q(t) curves for the 1.2 wt % Cu/3.5 wt % S Fe sorbent and Darco HG-LH sorbent are shown in Figure 6B for a 30 min exposure time. It is evident that, while the Fe/Cu-S sorbent performs better during the initial uptake, the brominated activated carbon is able to sustain larger rates of adsorption as it becomes exhausted. This should be expected though given the surface-area effects discussed previously. Fixed-Bed Mass Transfer. The removal of a chemical species by adsorption involves the transport of the species from the fluid phase to an active site on the surface of the sorbent, with subsequent interaction and entrapment. This process is quantified by obtaining the lumped fixed-bed mass-transfer coefficient (kDS, cm3‚g-1‚s-1, where S is the specific surface area per unit volume of the sorbent) using generated breakthrough data. This is most often accomplished by visualizing the adsorption zone as a moving-wave migrating down the length of the bed, which assumes total exhaustion upstream of the wave and fresh sorbent downstream. Using these assumptions, the boundary conditions at the edges of the control volume remain unchanged as the wave moves down the length of the bed. This allows the derivation of the following solution presented by Klotz for the equation describing mass-transfer within the active zone of the bed:1717

-ln

() ( ) ( )

kDSC0 kDS C )G+ W+1 C0 q∞ V V

(1)

where C is the outlet concentration of Hg, µg Hg‚cm-3; C0 is the inlet concentration of Hg, µg Hg‚cm-3; G is the equivalent bed volume of gas passed through the bed, cm3; W is the mass of the sorbent bed, g; and V is the volumetric gas flow rate, cm3‚s-1. The value of kDS can be calculated from the slope of a linear fit of the LHS of eq 1 as a function of G. In general, data over a range of C/C0 from 0.1 to 0.5 are the most reliable for this analysis because of deviations from assumed behavior as the bed is exhausted. Because only the linear portion of the breakthrough curve is fit, it should be noted that the y intercept is much lower than the actual value that represents the onset of breakthrough (C/C0 > 0). Therefore, calculation of kDS using the value of the y intercept obtained from the fit yields an inaccurate result and will not be discussed. The fits of the data for the sorbents tested are shown in Figure 7, with the resulting values for the mass transfer coefficient shown in Table 4. For the benefit of the reader, the data have been split into two plots to make it easier to see the fits. As expected, the 1.2 wt % Cu/ 3.5 wt % S Fe-based sorbent and Darco HG-LH sorbent have the largest mass transfer coefficients, 7.7 and 7.2 × 102 cm3‚g-1‚s-1, respectively. Surprisingly, the non-S material yielded the next largest value of 6.4 × 102 cm3‚g-1‚s-1, followed by the 0.1 wt % Cu/3.8 wt % S sorbent with a value of 5.9 × (17) Klotz, I. M. Chem. ReV. 1946, 39, 241.

Figure 7. A plot of -ln(C/C0) as a function of the effective bed volumes of gas (G) passed through the fixed-bed after the onset of breakthrough for (A) low-capacity materials and (B) high-capacity materials. The mass transfer coefficient can be obtained from the slope of the linear fit using eq 1. Table 4. Fixed-Bed Mass Transfer Coefficients (kDS) Obtained Using Eq 1 material

kDS × 10-2 (cm3 g-1 s-1)

Fe-1; 0.0 wt % Cu, 7.1 wt % S Fe-2; 0.1 wt % Cu, 3.8 wt % S Fe-4; 1.2 wt % Cu, 3.5 wt % S Fe-5; 2.0 wt % Cu, 0 wt % S Darco HG-LH

4.1 5.9 7.7 6.4 7.2

102 cm3‚g-1‚s-1. The non-Cu material produced the smallest mass transfer coefficient, 4.1 × 102 cm3‚g-1‚s-1. These results suggest that, of the two materials deposited on the Fe nanoaggregates, the Cu interaction is more significant than the S on the basis of the larger mass transfer coefficient when compared to the case of non-Cu. Therefore, when combined, the Cu-S is most likely enhancing the initial interaction of the sorbent with Hg, with S acting as the point of immobilization. Entrained-Flow (Injection) Testing for Hg0 Removal. The results for a 20 min injection of the 0.0, 0.1, 0.8, and 1.2 wt % Cu sorbents at 3.6 × 10-3 g‚L-1‚h-1 into the simulated flue gas system are shown in Figure 8. The performance of the commercial sorbent Darco HG-LH is included for comparison. The total Hg removal (Hg0 + Hg2+) is presented because this value is approximately the same as that for the elemental mercury. For the given conditions in Table 1, the residence time in the contactor tube is 17.9 s. At the start of sorbent injection, the process is in an unsteady state based on the continuous change in total Hg at the outlet. By the end of testing, the curves approach a limiting value for total Hg, which signifies the onset of steady-state behavior. The discussion of results for sorbents during injection will be based on their steady-state performance. Therefore, the outlet Hg concentration at the end of testing will be used for all calculations. The 0.0 and 0.1 wt % Cu sorbents achieved approximately the same level of removal, 4%. The 1.2 wt % Cu/3.5 wt % S sorbent was able to obtain a larger removal level of 36%. On the basis of the inlet Hg concentration


Figure 8. Results for mercury removal from simulated flue gas during injection using silanized Fe nanoaggregates with variable weight fractions of Cu at an injection rate of 3.6 × 10-3 g‚L-1‚h-1.

of 17.8 µg‚m-3, the steady-state capacity during injection for this sorbent is 107.5 µg Hg‚g sorbent-1. It should be noted that this capacity is not the total (exhausted) capacity, but merely the removal capacity of the sorbent for the given residence time within the flue gas during a single pass. As expected on the basis of the percent removal, the capacities for the non-copper and 0.1 wt % Cu sorbents are nearly identical at 13.4 and 11.8 µg Hg‚g sorbent-1, respectively. The Darco HG-LH activated carbon had a steady-state removal capacity of 269 µg Hg‚g sorbent-1, removing 94% of the inlet Hg. An additional set of tests was performed on the 1.2 wt % Cu/3.5 wt % S sorbent to determine the effects of injection rate on Hg removal using injection rates of 7.1 × 10-3 and 1.4 × 10-2 g‚L-1‚h-1. The results of these tests are compared to the

Meyer et al.

data for 3.6 × 10-3 g‚L-1‚h-1 and shown in Figure 9 in terms of the steady-state percentage of mercury removal as a function of the mass injection rate. When the injection rate is doubled from 3.6 × 10-3 to 7.1 × 10-3 g‚L-1‚h-1, the percent removal doubled as well from 36 to 72%, the targeted reduction in Hg emissions mandated by the EPA. When the injection rate is again doubled from 7.1 × 10-3 to 1.4 × 10-2 g‚L-1‚h-1, the percent removal is only increased an additional 24%. On the basis of the nonlinear nature of this data, it is logical to assume that the Hg undergoes resistance to mass transfer both during transport through the bulk phase to the particle and at the particle surface for lower injection rates. However, the effects of mass transfer/ adsorption at the particle surface become dominant at higher injection rates, causing the percent removal to approach a limiting value of less than 100%. The removal capacities calculated for the 7.1 × 10-3 and 1.4 × 10-2 g‚L-1‚h-1 injection rates are 107.5 and 68 µg Hg‚g sorbent-1, respectively. The decrease in removal capacity at 1.4 × 10-2 g‚L-1‚h-1 is expected because the actual adsorption kinetics are ratecontrolling at this injection rate and the addition of extra sorbent mass does not lead to increased Hg removal. Therefore, less saturation of the adsorption sites per mass of material will occur, resulting in a lower removal capacity. Comparison of Fixed-Bed and Injection Testing Results. The capacities achieved for both fixed- and moving-bed contactors have been discussed for both the Fe-based and activated carbon materials. These data are summarized in Table 5 for injection rates of 3.6 × 10-3 g‚L-1‚h-1. The fraction of the total capacity utilized during a single-pass injection has been calculated and included as a means of comparison. In all cases,

Figure 9. Effects of injection rate on Hg removal from simulated flue-gas using silanized Fe/Cu nanoaggregates (1.2 wt % Cu, 3.5 wt % S).

Figure 10. Comparison of fixed-bed and steady-state (SS) injection capacities for the removal of vapor-phase Hg0 showing the impact of both surface area and copper loading.



Table 5. Summary of Sorbent Performace Data for Fixed and Moving-Bed Contact fixed-bed total capacity

steady-state moving-bed capacity

material

(µg Hg g-1)

(µg Hg m-2)

(µg Hg g-1)

(µg Hg m-2)

fraction of total capacity used during injection (%)

Fe-1; 0.0 wt % Cu, 7.1 wt % S Fe-2; 0.1 wt % Cu, 3.8 wt % S Fe-3; 0.8 wt % Cu, 3.9 wt % S Fe-4; 1.2 wt % Cu, 3.5 wt % S Darco HG-LH

171 356 837 2730 2942

7.0 14.7 34.4 112.2 9.6

13 12 30 108 269

0.6 0.5 1.2 4.4 0.9

7.9 3.3 3.6 3.9 9.1

the amount of Hg removed during injection is significantly less than the total capacity achieved for a given material in its exhausted state. This finding is significant because it suggests that sorbents can be recaptured and recycled in power plant operations to help reduce material costs. For the silanized copper-doped Fe sorbents, the fraction of capacity used is only 3-4% in all cases. The non-copper Fe sorbent utilized twice this fraction, exhausting 7.9% of its capacity. However, this number can be misleading in that it is a reflection of the smaller total capacity of the non-copper material and not more efficient Hg capture. The AEC-normalized moving-bed capacities (AEC ) S) were calculated for all Fe sorbents to check this assumption. With a capacity of only 169 µg Hg‚g AEC-1, the non-copper sorbent is indeed less efficient than the copper-doped sorbents, which had capacities ranging from 286 µg Hg‚g AEC-1 to 2.8 mg Hg‚g AEC-1. This means that the addition of just 0.1 wt % Cu can enhance the interaction of Hg with S sites by nearly double during injection. For the activated carbon, 9% of the total capacity was exhausted during injection. The difference in fractional use between the silanized copper-doped Fe sorbents and the activated carbon is again a reflection of surface-area effects. A stated goal of this work is to identify a simple method for evaluating sorbents for Hg capture using fixed-bed testing and predict its performance during injection. This will reduce the cost of material development because fixed-bed testing can be done on a smaller scale and is much simpler to operate. While there are numerous ways to correlate the data, the most straightforward approach involves a graphical comparison of the surface-area normalized total capacities of the sorbents obtained during both modes of contact. The resulting plot is shown in Figure 10. The general trend of the data shows that the Hg capture per unit of surface area during entrained contact will increase as the fixed-bed capacity increases. Therefore, injecting greater quantities of surface area for low-capacity materials should result in greater Hg removal. This explains why the high surface area Darco HG-LH sorbent had the highest removal during injection when compared to the nanoaggregates. This finding suggests that Hg capture can be enhanced for a given sorbent by increasing the mass-normalized surface area. Also shown in Figure 10 is the apparent trend in Hg capture to increase linearly with Cu doping, which confirms the enhancing capability of Cu for Hg capture. Ultimately, the goal is to obtain a design correlation to predict sorbent performance during injection. However, this will require the acquisition of more data for key variables such as surface area, and more importantly, the mechanism of Hg capture. Conclusions The use of copper-doped Fe nanoaggregates silanized with bis-(triethoxy silyl propyl)-tetra sulfide has been investigated as a possible platform for Hg removal using moving-bed (injection) technology. The silanization procedures employed

were moderately efficient, resulting in 70% deposition of the targeted organic S level. The addition of Cu enhances the interaction of Hg with S on the basis of the increase of the fixedbed (total) capacity from 170 ( 20 µg Hg‚g sorbent-1 with no copper doping to 2730 ( 80 µg Hg‚g sorbent-1 at 1.2 wt % Cu. However, Cu doping by itself is not sufficient for Hg removal and therefore suggests that a combined Cu-S mechanism involving enhancement of the Hg-S complex by Cu is responsible for Hg capture. This is further supported through examination of the mass transfer coefficients for the various materials, which showed larger values for kDS (decreased mass transfer resistance) when sorbents are doped with Cu. Further studies into the mechanism for mercury capture with this type of sorbent are necessary to understand this effect. The total capacity of the 1.2 wt % Cu material is comparable to commercially available activated carbon, the brominated Darco HG-LH, which had a total capacity of 2942 µg Hg‚g sorbent-1. Entrained flow (moving bed) testing of the Fe-based sorbents in simulated flue gas found that only samples with larger amounts of copper doping (1.2 wt % Cu) were able to achieve a significant removal of Hg. This material had a removal rate of 36% at an injection rate of 3.6 × 10-3 g‚L-1‚h-1 and was found to remove 90% at 1.4 × 10-2 g‚L-1‚h-1. At lower injection rates, it is apparent that mass transfer resistance between the bulk phase and particle is significant and needs to be studied in detail to optimize the performance of the sorbent. These findings also suggest that kinetic limitations are ratecontrolling during Hg adsorption at increased injection rates because less than 100% removal is achieved. It is possible to predict injection performance on the basis of fixed-bed capacities if the surface area of the sorbent is taken into account. However, a general correlation will require a robust data set for multiple sorbents representing a wide range of chemical compositions, geometries, and internal structures. Ultimately, the EPA-targeted 70% reduction in Hg emissions could be achieved using this type of sorbent if an injection ratio of 1.2 × 10-7 kg sorbent‚L flue gas treated is employed. On the basis of only 4% usage of the total capacity during single-pass injection, it might be beneficial to develop methods to separate and recycle these materials to reduce power plant operation costs for Hg emissions control. The larger utilized fraction of the total capacity (10%) for the Darco HG-LH sorbent is the result of increased exposed surface area during injection and illustrates the importance of surface area for assessing the performance of sorbents. Acknowledgment. This work is made possible with funding provided by the U.S. Environmental Protection Agency in cooperation with the University of Cincinnati. Mercury analysis by ICP for fixed-bed testing was carried out at the University of Kentucky’s Environmental Research and Training Laboratory. Dr. Yongxin Zhao of Arcadis, an on-site contractor to NRMRL/RTP, performed the tests on the EPA’s entrained flow reactor. EF070120T

Examination of Sulfur-Functionalized, Copper-Doped Iron

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