Magnetic Rattle-Type Fe3O4@CuS Nanoparticles as Recyclable

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Magnetic Rattle-Type Fe3O4@CuS Nanoparticles as Recyclable Sorbents for Mercury Capture from Coal Combustion Flue Gas Zequn Yang, Hailong Li, Chen Liao, Jiexia Zhao, Shihao Feng, Pu Li, Xi Liu, Jianping Yang, and Kaimin Shih ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00948 • Publication Date (Web): 09 Aug 2018 Downloaded from http://pubs.acs.org on August 9, 2018

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ACS Applied Nano Materials

Magnetic Rattle-Type Fe3O4@CuS Nanoparticles as Recyclable Sorbents for Mercury Capture from Coal Combustion Flue Gas Zequn Yanga, Hailong Lia,b*, Chen Liaob, Jiexia Zhaob, Shihao Fengb, Pu Lia, Xi Liub, Jianping Yangb, Kaimin Shiha** a. Department of Civil Engineering, The University of Hong Kong, Hong Kong, Hong Kong SAR, China b. School of Energy Science and Engineering, Central South University, Changsha, 410083, China

Revision Submitted to ACS Applied Nano Materials

*To whom correspondence should be addressed: E-mail: [email protected]

**To whom correspondence should be addressed: TEL: +852-2859-2973 FAX: +852-2559-5337 Email: [email protected]

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ABSTRACT: Rattle-type Fe3O4@CuS synthesized using a two-step method was applied for elemental mercury (Hg0) adsorption in coal combustion flue gas for the first time. The Fe3O4 with strong magnetization was an ideal candidate as a core to make the sorbent recyclable, while the stabilized ultrathin CuS shell assured the Fe3O4@CuS had a higher Brunauer-Emmett-Teller (BET) surface area with more exposed active sites and stronger magnetization. The optimal operating temperature of 75 ºC allowed for the injection of the sorbent between the wet desulfurization (WFGD) and wet electrostatic precipitator (WESP), which removed the detrimental influence of nitrogen oxides. Simulated flue gas (SFG) in this section showed a slight inhibitive effect on Hg0 adsorption over the Fe3O4@CuS, mainly due to the presence of water vapor (H2O). The inhibition of H2O was proven to be the result of an active site prevention effect instead of the widely recognized competitive adsorption effect. The adsorption capacity and rate of the Fe3O4@CuS for Hg0 capture reached 80.73 mg g-1 and 13.22 µg (g·min)-1, which were the highest values among the magnetic sorbents currently reported for Hg0 removal from coal combustion flue gas. These properties allowed the sorbent to maintain a 100% Hg0 capture efficiency for more than 20 h with only a 50 mg dosage when no regeneration step was applied. Meanwhile, the contacting time between the sorbent and Hg0 was generally less than 5 s in a typical sorbent injection process. Polysulfides dominated the capturing process and primarily contributed to the extremely high adsorption capacity/rate. A multistep reaction mechanism was proposed to explain the Hg0 adsorption over Fe3O4@CuS. At the first stage, polysulfide participated in Hg0 adsorption as the most active component and was consumed rapidly. After that, S-S dimers, sulfides, and even copperterminated sites functioned as the adsorption centers. The as-formed mercury-copper (Hg-Cu) amalgam was transformed into mercury sulfide (HgS), a process that was dependent on the extent of the saturation of sulfide sites. With these advantages, Fe3O4@CuS is a promising, cost-effective, highly recyclable, and efficient alternative to the traditional activated carbon for capturing Hg0 in coal combustion flue gases. KEYWORDS: Copper sulfide; Mercury; Magnetic sorbent; Coal combustion; Flue gas


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1. Introduction According to a newly released mercury emission assessment report in 2018 by the United Nations Environment Programme (UNEP), coal combustion is still a top source of global mercury emissions as it accounts for 22.4% of the total mercury discharged to air.1 Thus, the emitted atmospheric mercury can be converted into toxicant methylmercury under various environmental conditions. Methyl mercury was responsible for the notorious Minamata disease and it is also fatal to humans.2 As of August 2017, the Minamata Convention came into full effect on regulating mercury emission from anthropogenic sources, such as coal combustion power plants, among its 128 signatories.3 Mercury discharged from coal combustion power plants are typically in three different forms: elemental mercury (Hg0), oxidized mercury (Hg2+), and particulate-bound mercury (Hgp).4-5 The Hg2+ and Hgp can be easily removed using a wet desulfurization (WFGD) system and particulate control devices (PCDs) during the flue gas treatment process, respectively.6 In contrast, Hg0 can release into environment from coal combustion power plants since it is difficult to capture due to its high volatility and insolubility in water.7 Among various control techniques attempting to reduce Hg0 emission from coal combustion, activated carbon injection (ACI) is readily available and widely commercialized.8 As a side benefit of plants equipped with PCDs, mercury bounded to activated carbon (AC) will be simultaneously retained with the sorbents and fly ash by the PCDs.9 However, the high carbon content will impede the reusability of fly ash as raw materials for concrete production.10 Fly ash containing certain amounts of carbon is generally dumped into landfills instead of being used in a more environmental friendly way.11 The past evaluation on dumped fly ash showed that a high carbon content actually facilitated the mercury methylation, which goes against ACI’s original purpose.12 The physisorption of mercury on activated carbon also increases the possibility of Hg0 leaching and re-emission into the environment.13-14 More importantly, activated carbon generally suffers the drawback of a limited adsorption rate, often leading to an extremely high operation cost its implementation. The cost could range from 14,400 $/lb Hg to 38,200 $/lb Hg for 99% removal efficiency.15



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Recently, injection of nanostructured mineral sulfide sorbents was found to be a promising alternative technology for ACI due to the following significant improvements:16 (1) mineral sulfides can convert Hg0 into mercury sulfide (HgS) and therefore minimize the probability of mercury re-emission;17 (2) accumulation of mineral sulfides in fly ash showed no detrimental effect on its reuse;16 (3) mineral sulfides have been well studied for their stability and ability to inhibit mercury methylation;18 and (4) mineral sulfides generally exhibit a high adsorption capacity and rate for Hg0 removal from coal combustion flue gas, due to their “total” coverage by active sites. Despite the aforementioned advantages, the application of mineral sulfide injection is still costly because the contact time between the sorbents and Hg0 in a typical injection process is usually less than 5 s and more than 99% of the “designed” capacity will be left unused in the one-time disposing strategy.19-20 If the used mineral sulfide sorbents can be recycled, it is possible to reduce the operation cost by several orders of magnitude. The development of magnetically recyclable sorbents is scientifically sound and economically feasible. However, to our best knowledge, most of the reported magnetic materials used for Hg0 capture were metal (?) oxides or noble metal based, on which mercury was captured in the form of less stable mercuric compounds such as mercury oxide (HgO) and/or amalgam.21-24 More importantly, the limited magnetization and adsorption capacity/rate of these magnetic sorbents impeded their recyclability and application in industrial Hg0 removal processes6,

23-25

because the addition of the

regeneration step tremendously increased the overall cost. In our preliminary experiments, we found that covellite (CuS) established a much higher adsorption capacity and rate for Hg0 removal from simulated coal combustion than the values reported for mineral sulfides and traditional ACI. Despite the improvements to the Hg0 adsorption efficiency and reductions to the overall cost, it was still a large proportion of the “designed” capacity (>99.99%). Moreover, the aggregation of nanosized CuS prevented the interior active sites from reacting with Hg0, rendering the observed sorption capacity to correspond to only approximately 0.05:1 molar ratio of Hg:CuS. This ratio was far less than the theoretical 1:1 value. Recently, rattle-type materials were proven to be an effective structure to expose as much the active components as possible in nanocomposites by adjusting the core ACS Paragon Plus Environment

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structure and shell thickness.26-27 Due to the oriented aggregation and the Ostwald repining mechanism, hydrothermally synthesized magnetic ferrosoferric oxide (Fe3O4) nanospheres can establish a hollow structure with a relatively high BET surface area.28-29 These facts imply that by covering a spherical magnetic Fe3O4 core with CuS to realize 360 degree active site exposure, a highly efficient, costeffective, and recyclable sorbent for Hg0 removal from coal combustion flue gas can be obtained. In this study, a rattle-type Fe3O4@CuS nanocomposites synthesized using a two-step solvothermal process followed by precipitation methods were used for Hg0 removal from simulated flue gas (SFG) for the first time. The predesigned core-shell structure of Fe3O4@CuS with a less active Fe3O4 core enclosed by a homogeneous, ultrathin, highly active CuS shell grant the sorbent high efficiency, reduced cost, and easily recyclable properties. Both the copper-terminated and multiform sulfide active sites were evidenced to be responsible for the Hg0 adsorption over the Fe3O4@CuS, in which the latter dominated the adsorption. Factoring in the very short contact time between the sorbents and Hg0 in realworld conditions, the Fe3O4@CuS maintaining its 100% Hg0 removal efficiency within 20 h without any regeneration hold large potentials to be applied in future sorbents injection-recycling strategy. 2. Experimental 2.1. Sorbents preparation Synthesis of the rattle-type Fe3O4@CuS magnetic sample was prepared based on a reference method with some modifications.30 In a typical synthesis, 2.7 g of iron chloride (hexahydrate, FeCl3·6H2O, AR 99.0%, Sinopharm), 7.2 g of sodium acetate (dehydrate, CH3COONa, AR 99.0%, Sinopharm), and 2.0 g of polyethylene glycol (6000, Sinopharm) were added into 80 ml of ethylene glycol (C2H6O2, AR 99.0%, Sinopharm). After vigorous sonication and stirring of the solution, the chemicals were completely dissolved and transferred into a 100 ml PTFE-lined stainless-steel autoclave. The autoclave was maintained at 200 ºC for 8 h to obtain a black powder, which was washed with ethanol and water several times and dried at 70 ºC overnight to deliver the uncoated Fe3O4 spherical cores. To prepare the Fe3O4@CuS, 0.5 g of synthesized Fe3O4 spherical cores were suspended in 100 ml of deionized water. Then, 1.7 g of an ammonium sulfide solution [(NH4)2S, AR 20%, Sinopharm] and 0.04 ACS Paragon Plus Environment


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g of Cetyltrimethylammonium bromide (CTAB, C19H42BrN, AR 99.0%, Sinopharm) were added with continual stirring for 1 h. A 50 ml solution of 1.3 g copper sulfate (pentahydrate, CuSO4·5H2O, AR 99.0%, Sinopharm) was then dropwise added to the above suspension. The microparticles were aged for another 3 h under vigorous stirring. After centrifugation followed by ethanol/water washing and 100 ºC drying under vacuum, the final samples were denoted as Fe3O4@CuS. We also prepared bare CuS under identical conditions in deionized water without the addition of Fe3O4 microspheres. 2.2. Sorbents characterizations The crystallinity of the sorbents was analyzed using X-ray diffraction (XRD, D8 Bruker AXS, Germany) with two thetas from 10º to 80º in Cuα (λ = 0.15406 nm) radiation. The morphology of the nanoparticles was examined with transmission electron microscopy (TEM, JEOL 2100F, Japan) at 200 kV. A nitrogen (N2) adsorption and desorption method was used to determine the Brunauer-EmmettTeller (BET) surface area of the sorbents using a BET analyzer (ASAP 2020, Micromeritics, USA). A vibrating sample magnetometer (VSM, SQUID) was employed to characterize the magnetization of the sorbents. Thermogravimetric (TG) tests were conducted with a thermogravimetric analyzer (SDT Q600, TA Instruments, USA) to investigate the thermal stability of the Fe3O4@CuS sorbent under 50 ml min-1 argon (Ar) or air from 100 ºC to 400 ºC with a heating rate of 10 ºC min-1. With a reference of the C 1s binding energy value of 284.8 eV, X-ray photoelectron spectroscopy (XPS) spectra (Thermo ESCALAB 250Xi) were recorded for fresh and spent nano-CuS (sorbent pretreated in the presence of 150 µg/m3 Hg0 carried by N2 overnight). 2.3. Hg0 adsorption activity test The Hg0 adsorption activities over the prepared sorbent were evaluated with a fixed-bed reaction system, as detailed in previous studies.16, 31 Compressed gas cylinders containing N2, oxygen (O2), and sulfur dioxide (SO2) were used to provide the different gas components. Water vapor (H2O) was separately introduced into the gas mixer using pure N2 with a washing bottle filled with water and submerged in an 80 ºC water bath. The total gas flow rate was precisely controlled with a mass flow controller to be 1 liter min-1. Stable vapor-phase Hg0 (90 µg m-3 for experiments and 150 µg m-3 for ACS Paragon Plus Environment

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pretreatment) was provided with a Hg0 permeation device (Dynacal, VICI Metronics) through heating the permeation tube to an constant temperature. A reactor made of borosilicate glass with an inner diameter of 10 mm was placed in a tubular furnace equipped with a controlled temperature variation of less than 2.0 °C. The Hg0 concentration was continuously recorded with a Hg0 analyzer (VM3000, Mercury Instrument, Inc.). In case of interference caused by water and acid gases, such as SO2, the gas flow was washed with a sodium hydroxide solution before entering the Hg0 analyzer. The exhausted gas passed through the chloride impregnated activated carbon trap prior to discharge. Before each test, the gas flow bypassed the reactor loaded with sorbents until the Hg0 concentration fluctuation was lower than 1 µg/m3 for 30 min. The Hg0 concentration in the bypass air was denoted as the inlet Hg0 concentration (Sin). After that, the gas flow passed through the sorbents and the detected Hg0 signal was designated as the outlet Hg0 concentration (Sout). The Hg0 adsorption capacities (St) of the sorbents were calculated using equation (1): St =

1 t2 ( Sin − S out ) × f × dt m ∫t1

(1)

where f is the gas flow rate (m3 h-1), m is the mass of the sorbent (g), and t is the adsorption process duration (h). Six sets of experiments were conducted, and the details are summarized in Table S1. The effects of temperature on the Hg0 adsorption over Fe3O4@CuS were examined in the set I experiment. The Hg0 adsorption activities of Fe3O4@CuS, uncoated Fe3O4, and bare CuS were investigated under the optimal operating temperature of Fe3O4@CuS of 75 °C in the set II experiment. The set III experiment studied the influence of the flue gas components on the Hg0 adsorption over the Fe3O4@CuS sorbent. Injection of the Fe3O4@CuS sorbent was mostly favorable between the WFGD and wet electrostatic precipitator (WESP) systems, where the flue gas temperature is generally in the range of 40-100 ° C, which matches the optimal operating temperature of 75 ºC of the sorbent. To simulate flue gases between WFGD and WESP, the SFG containing 4% O2, 100 ppm SO2, 8% H2O, and balanced in N2 was adopted. In the set IV experiment, specific mechanisms of the inhibitive effect of H2O on Hg0 adsorption were ACS Paragon Plus Environment


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investigated. Two batches of sorbents were pretreated under 150 µg m-3 Hg0 for 30 min. Then, a gas flow containing 20% H2O was injected in the presence and absence of a continuous Hg0 feed. In the set V experiment, half-breakthrough curves depict the adsorption capacity when the outlet Hg0 concentration equaled 50% of the inlet Hg0 concentration and were determined under N2, N2 + 4% O2 and SFG, separately. The Set VI experiment was designed to test the recyclability of the Fe3O4@CuS sorbent. The Fe3O4@CuS sorbent was recycled and reused in the Hg0 removal process under an SFG atmosphere 10 times without any regeneration procedure. For Set I-III experiments, 5 mg sorbents were used and mixed with 0.5 g powdered silicon dioxide to realize uniform dispersion. The corresponding gas hourly space velocity (GHSV) was 5,000,000 h-1. For Set IV experiments, 50 mg sorbents mixed with 0.5 g powdered silicon were applied to simulate the real conditions more properly, for which the as-obtained GHSV equaled to 500,000 h-1. All experiments were repeated at least twice to obtain statistical significance. To identify mercury species adsorbed on the sorbent and clarify the functions of different active sites under different reaction temperatures, three groups of mercury temperature programmed desorption (Hg-TPD) analyses were conducted over the Fe3O4@CuS sorbent pretreated with 150 µg m-3 Hg0 balanced in N2 under various conditions: 75 ºC for 30 min, 75 ºC for 4 h, and 175 ºC for 30 min. After the pretreatment, the sorbents were purged at 1 liter min-1 with pure N2 for 1 h until the outlet Hg0 concentration was stabilized at zero. The Hg-TPD then began under 500 ml min-1 N2 from 50 ºC to 450 º

C with a heating rate of 5 ºC min-1. The outlet Hg0 concentration was continuously recorded by the

mercury analyzer. To further confirm the formation of amalgam over the Fe3O4@CuS surface, a HgTPD analysis was conducted over a commercialized copper powder (Aladdin, 99.9%, metal basis) with identical experimental conditions.

3. Results and discussions 3.1. Sorbents characterizations 3.1.1. Textural properties. The XRD patterns of the uncoated Fe3O4 microspheres and bare CuS matched their standard patterns (PDF#85-1436 and PDF#06-0464), as shown in Figure S1. No ACS Paragon Plus Environment

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impurities, such as the precursors Cu9S8, Cu2S, or Fe2O3, was recorded. The weakened, but overlapped, (103) and (006) peaks and intensified (101) peak in the bare CuS pattern indicated its preferential growth and the planes exposure along the direction.32 In this direction, the S-S dimers (S22-) bonded by Van der Waals forces exist and are easily decomposed and reconstructed into polysulfide (Sn2-).32-33 For the Fe3O4@CuS pattern, both peaks indexed to Fe3O4 and CuS were observed, which demonstrated that the synthesis of the Fe3O4/CuS nanocomposite was successful. In addition, the existence of Cu2S (PDF#72-1701) in Fe3O4@CuS indicated the interaction between Fe3O4 and CuS contributed to the CuS reduction. The broadened peak widths resulted from the nanoscale particle size.34 The reduced peak intensities was attributed to the CuS in the Fe3O4@CuS pattern and indicate its low content and/or poor crystallinity.30 The TEM images shown in Figures 1(a-d) illustrate the nanostructure of the Fe3O4@CuS sorbent. Compared with the interior of the microspheres, the exterior with lower contrast implies that the interior and exterior parts were comprised of different chemical compositions. The diameters of the Fe3O4@CuS microspheres were between 150-300 nm with a 130-280 nm core and a generally 10 nm homogeneous coated CuS shell (as marked by the red bars in Figure 1), indicating that the morphology of the sorbent was truncated. An energy-dispersion X-ray spectroscopy (EDS) analysis to assess the elemental distribution of the Fe3O4@CuS sorbent was conducted to further prove the presence of a rattle-type structure. As presented in Figures 1(e-h), the Fe and O were centrally distributed. The Cu and S were scattered at the exterior of the nanoparticles at a much lower content than that of Fe and O. These observations evidenced that the Fe3O4 cores were enclosed by CuS shells to form an intrinsic rattle-type structure of Fe3O4@CuS. The positively charged CTAB adsorbed on the Fe3O4 surface enabled the stabilization of S2-, which then reacted with CuSO4 to precipitate the immobilized CuS shells (shown in Scheme 1).35 Its stable immobilization guarantees the efficient recycling of the main active components (sulfides and copper-terminated sites). The ultrathin CuS shell not only maintained the magnetization of the Fe3O4 to a large extent but also warranted their full contact with Hg0.



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As listed in Table S2, the uncoated Fe3O4 and bare CuS exhibited high BET surface areas of 44.23 and 30.43 m2/g, respectively. The BET surface area of bare CuS was much higher than the commercialized CuS of 0.48 m2/g, which was contributed by the addition of CTAB as a surfactant and particle size adjuster.36 After immobilizing the CuS on the Fe3O4 surface, the BET surface area of Fe3O4 was decreased, likely due to the pore blockage. However, the Fe3O4 supporter elevated the BET surface area of the Fe3O4@CuS sorbent to 36.57 m2 g-1, even higher than nanosized CuS and assured the maximum surface exposure of CuS.

3.1.2. Magnetization and thermal stability. As shown in Figure 2 and summarized in Table S2, the uncoated Fe3O4 and bare CuS showed a very strong saturation magnetization of 93.26 emu g-1 and negligible magnetization, respectively. After coating with the CuS, the saturation magnetization of the rattle-type Fe3O4@CuS decreased to 49.67 emu g-1. However, the magnetization of Fe3O4@CuS was still stronger than those for previously reported sulfide-based magnetic sorbents.6, 25, 37 In addition, the Fe3O4@CuS exhibited a magnetization hysteresis with a coercivity of more than 15,000 A m-1. The magnetize properties warranted a large amount of the sorbents to be easily recycled from fly ash, and hence reduce the overall recycling and reuse cost. As shown in Figure S2(a), the Fe3O4@CuS sorbent exhibited two decomposition spikes under a pure Ar atmosphere, peaking at 186.3 and 243.9 ° C. The first spike was attributed to the removal of physically adsorbed surface hydroxyl groups and water, and the second spike was due to the thermal crystal transformation of Fe3O4 into γ-Fe2O3.38-39 The decomposition peaks of CuS were either negligible or incorporated into the Fe3O4 peaks because of the low CuS content. The decomposition peaks demonstrated that the Fe3O4 core was relatively stable when the temperature was below 200.0 ºC under an inert atmosphere. The thermal stability of the Fe3O4@CuS heated under air is shown in Figure S2(b). The TG curve pattern under air showed the same tendency as that under Ar, but with negligible difference in peaking decomposition temperature below 250.0 ºC. However, a strong mass increase that began from 250.0 ºC and centered at 294.8 ºC was recorded, which was assigned to the oxidation of ACS Paragon Plus Environment

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Fe3O4.40 The TG curve of CuS under air is presented in Figure S2(c). The weight losses centered at 161.6 and 307.4 °C were attributed to the decomposition of CuS into Cu1+xS,41 while the oxidation of the CuS was unobserved when the temperature was below 350 °C. This finding suggests that the Fe3O4@CuS could be stably stored under air as well as injected in the presence of O2 when the temperature is below 150.0 °C. All these facts suggest that the Fe3O4@CuS could be stably stored under air as well as injected in the presence of O2 when the temperature is below 250.0 ºC.

3.2. Hg0 adsorption activity test 3.2.1. Influence of temperatures and sorbents. The adsorption capacities of the Fe3O4@CuS sorbent were calculated at 25, 75, 125, and 175 ºC based on their 4-h breakthrough curves (shown in Figure S3(a)) and equation (1) to be 4.14, 4.23, 3.97 and 2.75 mg g-1, respectively. The adsorption capacities suggest that the Hg over the Fe3O4@CuS surface was primarily chemisorbed, since the reactants could attain more kinetic energy when the temperature was raised from 25 to 75 ºC.42 Further increases of the reaction temperature from 75 ºC to 175 ºC limited Hg0 adsorption. The decreased adsorption capacity at higher temperatures was probably due to the mercury desorption above 100 ºC.43 The optimal temperature of 75 ºC indicates that the regenerable Fe3O4@CuS could be injected in the tailed section between the WFGD and WESP systems, where the flue gas temperature is generally between 40 ºC and 100 ºC.6 In this scenario, the detrimental effect on Hg0 adsorption over the mineral sulfides caused by NOX9 will be fully removed by the selective catalytic reduction (SCR) and WFGD systems. This advantage gives a potentially high Hg0 removal efficiency and a large number of recycle times for the Fe3O4@CuS sorbent. Moreover, the WESP was designed to collect small amounts of fine particles from the flue gas, which makes the recycling of the sorbent much easier than that of the ESP-collected fly ash. Figure S3(b) presents the Hg0 adsorption performance over the uncoated Fe3O4, the bare CuS, and the rattle-type Fe3O4@CuS at 75 ºC. The breakthrough was reached within the 4-h test for the Fe3O4 microspheres. The adsorption capacity of the nanoscale bare CuS was 4.28 mg g-1, which was slightly higher than that of the Fe3O4@CuS sorbent. Considering that in the Fe3O4@CuS, Fe3O4 accounted for ACS Paragon Plus Environment


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its primary weight, the core-shell structure enhanced the efficiency of CuS for Hg0 adsorption since the CuS was fully exposed.

3.2.2. Influence of flue gas components. As shown in Figure 3, 100 ppm of SO2 in the presence of 4% O2 exhibited a negligible effect on the Hg0 adsorption over the Fe3O4@CuS sorbent at 75 ºC. The H2O slightly inhibited Hg0 adsorption by approximately 7% due to competitive adsorption and/or covering the active adsorption sites for Hg0. To further evidence the specific roles of these two effects, the interaction between H2O and Hg0 over Fe3O4@CuS are shown in Figure 4. When 20% H2O was fed into a continuous gas flow containing Hg0, the outlet Hg0 concentration increased to 46.2 µg m-3 within 30 min and would continue to increase if the H2O was kept on. In contrast, when the inlet Hg0 was cut off, the 20% H2O repelled much less Hg0 from the system with a peaking concentration of 10.6 µg m-3 and decreased to zero within 30 min. The amount of Hg0 repelled out from the sorbents surface was much less than its theoretical amount derived based on its 30-min adsorption capacity. Therefore, both the competitive adsorption and the site prevention effects accounted for the adsorption capacity decline in the presence of H2O. The site prevention effect was the dominant one instead of the widely recognized competitive adsorption.6 Due to these two effects, the adsorption performance of the Fe3O4@CuS for Hg0 could be recovered when the H2O is cut off, and the detrimental effects will not accumulate with the exposure time.

3.2.3. Adsorption capacities and rates comparison of different magnetic sorbents. The halfbreakthrough curves of Fe3O4@CuS for Hg0 adsorption under pure N2, N2 + 4% O2, and SFG at 75 ºC are presented in Figure 5(a). The outlet Hg0 concentration reached the half-breakthrough point at 57, 52, and 45 h, respectively under pure N2, N2 + 4% O2, and SFG. The O2 slightly shortened the time for Fe3O4@CuS to reach the half-breakthrough point, likely because the CuS was partially oxidized by O2. Under SFG, the adsorption capacity of Fe3O4@CuS was further suppressed, mainly due to the existence of H2O. The detrimental effect of H2O was maintained at the same level for a relatively long experiment, which was consistent with our assumption that the inhibitive influence of H2O on Hg0 adsorption will not accumulate with the exposure time. The Elovich model was adopted to estimate the breakthrough ACS Paragon Plus Environment

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Hg0 adsorption capacities of Fe3O4@CuS under different atmospheres based on the half-breakthrough curves44 with the assumption that the residual Hg0 was only adsorbed on the unoccupied surface sites at time t over Fe3O4@CuS: C = Cm × (1 − e −δt )

(2)

In equation (2), the Cm (mg g-1) represents the Hg0 adsorption capacities at the breakthrough points, C is the amount of adsorbed Hg0 at time t, and δ is the fraction of unoccupied surface at that time. The calculated breakthrough curves are plotted in Figure 5(b). The extremely high correlation coefficients (R2> 99.99%) evidenced that the Elovich model is applicable for predicting the adsorption capacities of Hg0 over the Fe3O4@CuS sorbent. The equilibrated Hg0 adsorption capacities were 88.71, 85.39, and 80.73 mg g-1 under pure N2, N2 + 4% O2, and SFG, respectively. The average Hg0 adsorption rates were obtained based on the half-breakthrough curves to be 13.25, 13.70, and 13.22 µg (g·min)-1. To obtain a better comparison, the Hg0 adsorption capacities and rates of different magnetic sorbents are listed in Table S3.6, 21-23, 25, 37, 45-46 The comparison clearly shows the rattle-type Fe3O4@CuS magnitude-levelly had a superior performance than all the reported magnetic sorbents in both the adsorption capacity and the rate at their optimal operating temperature.

3.2.4. Recyclability of Fe3O4@CuS. Figure 6 presents the Hg0 removal efficiency over Fe3O4@CuS recycled from previous Hg0 removal processes with no regeneration. Each Hg0 removal process lasted for 4 h. In the first 6 cycles, the Hg0 removal efficiency under SFG was maintained at 100%. From the 7th cycle to the 10th cycle, the removal efficiency gradually declined from 98.6% to 93.7%. After operating for 10 cycles, the mass of the Fe3O4@CuS recycled from the mixture of powdered silica and Fe3O4@CuS was 49 mg, accounting for 98% of the originally used sorbents. Moreover, the Fe3O4@CuS pretreated by Hg0 to 20% penetrated was characterized using XRD and VSM (shown in Figure S1 and Figure 2, respectively). No obvious difference was observed in the crystallinity and magnetization after pretreated by Hg0 until the Hg0 removal efficiency equaled to 80%. These results prove that the intrinsic structure, compositions and magnetic properties of the sorbents after Hg0 adsorption remained almost



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unchanged compared to the fresh samples. The Fe3O4@CuS maintained 100% Hg0 capture efficiency for over 5 h without any efficiency decline when no regeneration steps were applied, while the contact time between the sorbents and Hg0 is generally less than 5 s in a real-world injection strategy. Therefore, the Fe3O4@CuS sorbent holds substantial promise in the future injection-recycling strategy for capturing Hg0 in a coal combustion flue gas.

3.3. Mechanisms involved in Hg0 adsorption over Fe3O4@CuS 3.3.1. Hg-TPD. As shown in Figure 7(a), three characteristic desorption peaks were observed over the Fe3O4@CuS after pretreated with 150 µg m-3 Hg0 for 30 min. The peaks centered at 205, 300, and 405 ºC were due to the desorption and/or decomposition of metacinnabar (β-HgS), cinnabar (α-HgS), and HgO, respectively.6,

47-48

The sulfide active sites predominantly contributed to Hg0 adsorption,

accounting for 90.75% of the adsorbed mercury (listed in Table S4), which evidenced a much higher affinity of CuS towards Hg0 compared with that of Fe3O4. A 4-hour pretreatment even increased the contents of HgS to 93.24%. Interestingly, a relatively small peak centered at 130 ºC occurred when the pretreatment time was prolonged to 4 h (shown in Figure 7b). The peaking temperature was in accordance with the desorption temperature of the Hg-Ag amalgam.49 Considering the Fe3O4 was encapsulated by the CuS, it is reasonable to assume that the spike that peaked at 130 ºC was primarily due to the decomposition of the Hg-Cu amalgam. To further confirm this, a Hg-TPD experiment over the copper powder after Hg0 adsorption was conducted. The centered desorption temperature was 125 ºC (shown in Figure 7d), which matched the unnamed 130 ºC decomposition peak in Figure 7(b). Therefore, the peak occurring at 130 ºC over the 4-h pretreated Fe3O4@CuS likely resulted from the decomposition of the Hg-Cu amalgam. During a shorter time pretreatment process, the sulfur sites would preferentially react with Hg0 and/or the adsorbed mercury on the copper-terminated sites and would transform into HgS immediately when the sulfide sites were abundant.50 Raising the pretreatment temperature to 175 ºC partially disabled the roles of the sulfide active sites, especially in the formation of metacinnabar; thus resulting in much fewer Hg0 desorbed from the used sorbent (shown in Figure 7c). In addition, role of the copper-terminated site was impaired due to the ACS Paragon Plus Environment

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decomposition of the Hg-Cu amalgam at temperatures ranging from 105 to 150 ºC. Therefore, the Hg0 removal efficiency declined stepwise when the reaction temperature was raised from 75 ºC to 175 ºC.

3.3.2. XPS. As shown in Figures S4(a) and (b), the Fe 2p spectra peaks located at 710.85-710.88 eV and 723.65-723.88 eV were assigned to the Fe 2p 3/2 and Fe 2p 1/2 of Fe2+ due to the spin-orbit splitting. The satellite shakeup that appeared at 716.77-717.27 eV further confirmed the existence of Fe2+. The Fe 2p 3/2 and Fe 2p 1/2 of Fe3+ increased to 713.67-713.84 eV and 726.31-726.89 eV with a satellite shakeup at 719.83-720.30 eV.51 After the Hg0 adsorption, 2.7% of the Fe3+ in the Fe3O4@CuS was reduced into Fe2+ (listed in Table 1). The reduction of Fe3+ was confirmed by the O 1s XPS patterns shown in Figures S4(c) and (d), in which a downshift separated the peak of O 1 s in the spent Fe3O4@CuS, which appeared at 529.90 eV indicating the reduction of oxygen-contained groups in Fe3O4.52 As shown in Figures S4(e) and (f), the peaks with binding energies at 931.90-931.95 eV and 951.73951.74 eV belonged to the Cu 2p 3/2 and Cu 2p 1/2 of Cu+, respectively. The Cu2+ characterized its Cu 2p 3/2 and Cu 2p 1/2 spikes at 934.67-934.80 eV and 954.65-955.02 eV with a satellite shakeup at 942.60-943.93 eV.53 For the spent Fe3O4@CuS sorbent, the ratio of Cu+ increased by 12.1% compared with that of the fresh sorbent, which also indicates that the CuS participated in the reaction with Hg0 more actively than Fe3O4. The electron transfers were likely due to the direct interactions between the copper-terminated sites with gas phase Hg0. This may also be a result of the indirect interactions between the sulfides and adsorbed mercury. The S 2p spectra analyses of the fresh and spent Fe3O4@CuS are shown in Figures 8(a) and (b), respectively. The spikes that peaked at 161.84-161.92 eV, 163.24-163.33 eV, 163.83-165.20 eV, and 168.87-169.14 eV were attributed to the S2-, S22-, Sn2-, and S6+, respectively.6 The existence of the Sn2resonated our speculation from the XRD pattern of Fe3O4@CuS concerning the existence of Sn2- due to surface reconstruction. The Sn2- is renowned for its extremely high affinity with Hg0.54 The high concentration of Sn2- in CuS compared with other mineral sulfides primarily led to the extraordinary Hg0



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adsorption activity. A total of 28.9% of Sn2- was reduced during the Hg0 adsorption process, again confirmed that the Sn2- was the main sulfide form involved in the Hg0 adsorption. The Hg 4f 5/2 and 7/2 peaks are shown in Figure 8(c). The deconvoluted peaks located at 98.95 eV and 100.50 eV were assigned to the Hg 4f 5/2 peaks belonging to Hg0 and Hg2+, respectively.55 Generally, the relatively weak interactions between the metal-terminated sites and mercury only resulted in 0.19 e electron transfer between mercury and the iron site.56 The presence of Hg0 further proved the interaction between Hg0 and the copper-terminated site formed the Hg-Cu amalgam, since the interaction between Hg0 and the sulfide/oxide sites led to electron transfers and the transformation of Hg0 to Hg2+. The ratio of the Hg-Cu amalgam increased in the XPS patterns for the sorbent pretreated for 12 h compared with that pretreated for 4 h. This result suggests that when the sulfides in the Fe3O4@CuS were consumed as the adsorption proceeded. The combination between Hg0 and sulfides slowed, and more Hg-Cu accumulated on the sorbent surface.

3.3.3. Mechanisms discussions. Based on the Hg-TPD and XPS results above, a multistep reaction process was proposed to explain the Hg0 adsorption over the Fe3O4@CuS as listed in reactions 3-(N+4). Hg0(g) + Cu-Sn →Cu-Sn-1 + HgS (ad)

(3)

Hg0(g) + Cu-Sn-1 → Cu-Sn-2 + HgS (ad)

(4)

… Hg0(g) + Cu-S2 → CuS + HgS (ad)

(N+1)

Hg0(g) + CuS → [Hg·Cu]-S

(N+2)

[Hg·Cu]-S → Cu-[ ] + HgS (ad)

(N+3)

Hg0(g) + Cu-[ ] → Hg-Cu(ad)

(N+4)

where Sn represents polysulfides due to surface reconstruction (n ≥ 3), S-S dimers along the directions (n = 2) and sulfides (n = 1). The [Hg·Cu] is the mercury combined on the Cu-terminated sites and [ ] denotes the coordinate unsaturated sites. At the first stage, Hg0 preferred being adsorbed on polysulfide active sites and consumed the polysulfide quickly (reactions 3 and 4). Afterwards, the S-S dimers and sulfide functionalized as adsorption centers as polysulfides were consumed (reactions N+1 ACS Paragon Plus Environment

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and N+3). Meanwhile, Hg0 adsorbed on copper-terminated sites form the Hg-Cu amalgam. When the sulfides sites are relatively abundant, the Hg-Cu amalgam was transformed into HgS immediately (reaction N+2). However, when the sulfide sites were reduced (reaction N+2) or depleted (reaction N+4), the transformation rate of Hg-Cu decreased, and more Hg-Cu accumulated on the Fe3O4@CuS surface.

4. Conclusions In this study, Fe3O4@CuS prepared using a two-step method was applied to capture Hg0 in coal simulated combustion flue gas. The rattle-type structure granted Fe3O4@CuS a relatively high BET surface area and strong magnetization. The optimal operating temperature of 75 ºC allowed for the injection of the sorbent between the WFGD and WESP systems, preventing the detrimental effects of NOX. Although the content of CuS in the Fe3O4@CuS sorbent was relatively low, the full exposure of the main active sites induced its extremely high availability. The adsorption capacity and rate can be as high as 80.73 mg g-1 and 13.22 µg (g·min)-1. These capacities were at least two orders of magnitude higher than any magnetic sorbents previously reported. The superior adsorption capacity led to its exceptional recyclability for maintaining 100% Hg0 removal efficiency for more than 5 h with only a 50 mg dosage. Oxides, sulfides, and metal-terminated sites contributed to the Hg0 adsorption over the Fe3O4@CuS sorbent, whereas the sulfides, especially the polysulfides active sites, dominated the adsorption process. A multistep reaction process was proposed based on the Hg-TPD and XPS studies, which suggests that the polysulfide was primarily involved in Hg0 adsorption at the first stage and was consumed rapidly. Then, S-S dimers, sulfides even copper-terminated sites functioned as adsorption centers. Meanwhile, the Hg-Cu amalgam formed in the process and was partially transformed to HgS depending on the extent of sulfide sites saturation. This work provides a new insight that illustrates the simplification of polysulfide-containing magnetic materials design for Hg0 adsorption in coal combustion flue gas. This new sorbent with cost-effective, highly recyclable, and superiorly efficient properties makes it a promising alternative strategy to tradition ACs.

AUTHOR INFORMATION ACS Paragon Plus Environment


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Corresponding authors: *H. Li. Email: [email protected], Tel: 86-18670016725, 526 Fax: 86-731-88879863. **K. Shih. Email: [email protected], Tel: 852-2859-2973, Fax: 852-2559-5337 ORCID: Hailong Li: 0000-0003-0652-6655 Kaimin Shih: 0000-0002-6461-3207

ACKNOWLEDGEMENTS This project was supported by the National Natural Science Foundation of China (No. 51476189, 51776227) and the Research Council of Hong Kong (17257616, C7044-14G, and T21-771/16R).

Supporting Information Experimental conditions; BET surface areas and saturation magnetizations of the sorbents; Adsorption capacities/rates comparison of different magnetic sorbents; Mercury species analysis over Fe3O4@CuS; XRD patterns of as prepared sorbents; TG-DSC curves of Fe3O4@CuS microspheres under Ar and air; Hg0 adsorption over Fe3O4@CuS at different temperatures and different sorbents at 75 oC; XPS patterns of Fe 2p, O 1s and Cu 2p of fresh and spent Fe3O4@CuS sorbents.

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List of Scheme:

Scheme 1. The schematic illustration of the preparation of rattle-type Fe3O4@CuS.



Scheme 1. The schematic illustration of the preparation of rattle-type Fe3O4@CuS.


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List of Table:

Table 1. Elements analysis of fresh and spent Fe3O4@CuS sorbents.



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Table 1. Elements analysis of fresh and spent Fe3O4@CuS sorbents.

Fe3+/Fet

Fe2+/Fet

Cu2+/Cut

Cu+/Cut

S2-/St

S22-/St

Sn2-/St

Fresh

0.38

0.62

0.55

0.45

0.49

0.14

0.37

Spent

0.35

0.65

0.43

0.57

0.54

0.38

0.08

Fet: Fe3+ + Fe2+; Cut: Cu2+ + Cu+; St: S2- + S22- + Sn2-; S22-: S-S dimers; Sn2-: polysulfide.


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List of figures:

Figure 1. (a-d) TEM images and (e-h) EDS mapping of the Fe3O4@CuS microspheres. Figure 2. Magnetization of as prepared sorbents. Figure 3. Influence of different flue gas components on Hg0 removal over Fe3O4@CuS. Figure 4. Inhibition mechanism of H2O on Hg0 adsorption over Fe3O4@CuS. Figure 5. (a) Half-breakthrough curves of Fe3O4@CuS; and (b) calculated half-breakthrough capacities (scattered dots) and simulated breakthrough capacities (solid lines) of Fe3O4@CuS under different atmospheres.

Figure 6. Recyclability of Fe3O4@CuS for Hg0 capture. Figure 7. Hg-TPD patterns of Fe3O4@CuS pretreated at (a) 75 oC for 30 min; (b) 75 oC for 4 h; and (d) 175 oC for 30 min; and (c) Hg-TPD pattern of copper powder pretreated at 75 oC for 30 min.

Figure 8. XPS patterns of (a-b) S 2p; and (c) Hg 4f of fresh and spent Fe3O4@CuS sorbents.



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Figure 1. (a-d) TEM images and (e-h) EDS mapping of the Fe3O4@CuS microspheres.

(e) Fe Kα1

(a)

(b)

(c)

(d)

(f) O Kα1


(g) Cu Kα1

(h) S Kα1

Page 27 of 35

Figure 2. Magnetization of as prepared sorbents. 100

50 M (emu/g)

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


0 Fe3O4 CuSx

-50

Fresh Fe3O4@CuS Spent Fe3O4@CuS -100 -20000

-10000

0 H (Oe)

10000

20000



Figure 3. Influence of different flue gas components on Hg0 removal over Fe3O4@CuS. 1.0 Normalized outlet Hg0 concentration

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

Pure N2 N2 + 4% O2

0.8

N2 + 4% O2 + 100 ppm SO2 SFG

0.6 0.4 0.2 0.0 0

40

80

120 160 Time (min)

200

240

280


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Page 29 of 35

ACS Applied Nano Materials 0

Figure 4. Inhibition mechanism of H2O on Hg adsorption over Fe3O4@CuS. 60 H2O off

Outlet Hg0 concentration (µg/m3)

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

H2O off

H2O on

50 With Hg0 feed Without Hg0 feed

40 30 20 10 0

0

20

40

60 Time (min)

80

100

120



Figure 5. (a) Half-breakthrough curves of Fe3O4@CuS and (b) calculated half-breakthrough capacities (scattered dots) and simulated breakthrough capacities (solid lines) of Fe3O4@CuS under different atmospheres.

Normalized outlet Hg0 concentration

1.0 (a)

0.9 0.8

Pure N2

0.7

N2 + 4% O2 SFG

45 h

0.6

52 h 57 h

0.5 0.4 0.3 0.2 0.1 0.0

0

5

10

15

20

25 30 35 Time (h)

40

45

50

55

60

90 (b) 80 Adsorption capacity (mg/g)

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

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70 60 50 40

Pure N2

30

N2 + 4% O2 SFGx

20 10 0

0

100

200

300 Time (h)

400

500

600


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Figure 6. Recyclability of Fe3O4@CuS for Hg0 capture. 100 Hg0 removal efficiency (%)

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


100

100

100

100

1

2

3

4

100

99.8

98.6

96.7

94.5

93.7

7

8

9

10

80

60

40

20

0

5 6 Rounds



Figure 7. Hg-TPD patterns of Fe3O4@CuS pretreated at (a) 75 oC for 30 min; (b) 75 oC for 4 h; and (c) 175 oC for 30 min; and (d) Hg-TPD pattern of copper powder pretreated at 75 oC for 30 min. 120


(a) 100 80 60 40 20 0 50

150

250 350 Temperature (oC)

450

550

800 (b) Outlet Hg0 concentration (µg/m3)

700 600 500 400 300 200 100 0 50

150


450

550

70 (c) Outlet Hg0 concentration (µg/m3)

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

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60 50 40 30 20 10 0 50

150


450

550


Page 33 of 35

ACS Applied Nano Materials 50 (d)


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

40

30

20

10

0 50

150


450

550



Figure 8. XPS patterns of (a-b) S 2p; and (c) Hg 4f of fresh and spent Fe3O4@CuS sorbents. Fresh S 2p

(a)

Intensity (a.u.) 174

172

170

168 166 164 162 Binding energy (eV)

160

158

161.84

163.33

Intensity (a.u.) 174

156

(b)

Spent S 2p 169.14

165.20

172

170

168

166 164 162 Bind energy (eV)

160

158

Spent Hg 4f

156

(c)

100.50 Intensity (a.u.)

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

112

104.10

110

108

106 104 102 Binding energy (eV)

98.95

100

98

96


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


Graphical Abstract


Magnetic Rattle-Type Fe3O4@CuS Nanoparticles as Recyclable

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