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Article

Recyclable MnCl2-Fe2O3@CNT as Sulfur and Water-Resistant Sorbent for Gaseous Elemental Mercury Removal from Coal Combustion Flue Gas

by
Zhuo Liu
1,
Yuchi Chen
2,
Hao Rong
3,
Cui Jie
4,
Xiyan Peng
4,* and
Honghu Li
4,*
1
College of Intelligent Systems Science and Engineering, Hubei Minzu University, Enshi 445000, China
2
Hubei Novel Reactor & Green Chemical Technology Key Laboratory, Key Laboratory for Green Chemical Process of Ministry of Education, School of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan 430074, China
3
Changjiang Institute of Survey, Planning, Design and Research, Wuhan 430014, China
4
Research Center for Environment and Health, School of Information Engineering, Zhongnan University of Economics and Law, Wuhan 430073, China
*
Authors to whom correspondence should be addressed.
Materials 2025, 18(19), 4573; https://doi.org/10.3390/ma18194573
Submission received: 3 July 2025 / Revised: 25 September 2025 / Accepted: 30 September 2025 / Published: 1 October 2025
(This article belongs to the Section Green Materials)
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Abstract

Mercury poses serious hazards to human health. Carbon nanotube (CNT) is a potential material for elemental mercury (Hg0) adsorption removal, however, it shows susceptibility to SO2 and H2O. Herein, CNT is first decorated with Fe2O3 then modified with MnCl2 (MnCl2-Fe2O3@CNT) to enhance SO2 and H2O resistance. The Hg0 removal performance and physical–chemical properties of samples are comprehensively studied. MnCl2(10)FeCNT (10 wt% MnCl2 content) has a high specific surface area (775.76 m2·g−1) and abundant active chlorine (35.01% Cl* content) as well as oxygen species (84.23% Oα content), which endows it with excellent Hg0 adsorption capacity (25.06 mg·g−1) and good SO2 and H2O resistance. Additionally, the superparamagnetic property can enable MnCl2(10)FeCNT to be conveniently recycled. After fifth regeneration, MnCl2(10)FeCNT can still achieve >90% Hg0 removal. The abundant active chlorine and oxygen species over MnCl2(10)FeCNT are responsible for Hg0 removal with HgCl2 as the primary product. This work demonstrates the enhancement of CNT’s resistance to SO2 and H2O by Fe2O3 and MnCl2 modification, which has potential application in flue gas mercury removal.

1. Introduction

Mercury (Hg) is one of the important pollutants recognized by the World Health Organization (WHO) due to its toxicity, atmospheric transport characteristics, and bioaccumulation [1,2,3,4]. As the primary anthropogenic mercury emission source [5,6], coal combustion leads to the decomposition of mercury compounds from coal and causes the emission of gaseous elemental mercury (Hg0) [7]. A part of Hg0 will be oxidized due to its interaction with the flue gas constituents and accumulate in fly ash. The resultant oxidized mercury (Hg2+) and particulate mercury (Hgp) can be efficiently removed by flue gas purification equipment currently installed in coal-fired power plants (e.g., wet scrubbers and dust removers) [8,9]. However, Hg0 with high volatility and low water solubility is difficult to eliminate by the above air purification equipment [10,11]. Therefore, mercury is emitted into the atmospheric environment primarily in the form of Hg0. Worldwide coal combustion produced 533 tons of mercury emissions in 2015 including 229 tons from East and Southeast Asia, 125 tons from South Asia and 46.5 tons from the European Union [12]. In 2015, coal combustion in China produced 73 tons of mercury emissions (54 tons Hg0) [13]. Hg0 can be transported over long distances by air, eventually deposited into natural water bodies and biotically converted into highly toxic dimethyl mercury (MeHg) [3,4]. MeHg is prone to bioaccumulate in the food chain, which can cause diseases in nervous and cardiovascular systems [4]. Aimed at controlling worldwide mercury pollution, the Minamata Convention signed by 128 countries came into effect in 2017 [14]. Hence, developing efficient mercury vapor removal technologies is critical for protecting people′s health and environmental safety.
To achieve mercury emission abatement, various methods have been proposed, such as adsorption [15,16,17], wet oxidation [18,19], catalytic oxidation [20,21,22], photochemical oxidation [23], photocatalytic oxidation [24,25], electrochemical oxidation [26], etc. Among these technologies, wet oxidation, photocatalytic oxidation, and electrochemical oxidation still suffer from the problems of a high construction and operation cost, remaining in the laboratory stage. Removal of Hg0 by adsorption is a more feasible, facile, and cost-effective method. Extensive studies have been conducted on the method of injecting activated carbons upstream of the dust collectors. Although such a technique holds great promise, some challenges still remain for its large-scale application. Activated carbon and other traditional carbon materials such as graphene generally show poor Hg0 removal activities owing to their limited surface active sites. Secondly, the carbon sorbents associated with mercury will be captured by the downstream dust remover together with fly ash. It is a challenging issue to separate the carbon sorbents from the fly ash [27]. Thirdly, SO2 and water vapor are typical components in flue gas, which can exert inhibitory effects on Hg0 removal via the mechanisms of competitive adsorption, active species consumption, etc. [28]. Without good SO2 and H2O resistance, the application value of the sorbent will be significantly reduced. In summary, highly efficient, recyclable, and sulfur as well as water-resistant sorbent for Hg0 capture is urgently needed.
One-dimensional carbon nanotube (CNT) with perfect hexagonal connected structures has attracted widespread attention due to its distinctive physico-chemical properties such as a high specific surface area, developed pore structure, distinct electrical characteristic, and good chemical durability [29,30]. The high surface area and advanced pore structure of CNT can promote the Hg0 mass transfer across the gas–solid interface and provide more anchoring sites for adsorbed Hg0. The excellent electrical conductivity can speed up the charge transfer between Hg0 and active sites over the CNT surface, thus promoting Hg0 oxidation. Additionally, some commonly existing defects or impurities on the CNT surface such as amorphous carbon and oxygen-containing functional group might be conducive to Hg0 adsorption. Despite such excellent performance, there are still the following issues to be addressed when applying carbon nanotubes to Hg0 removal. The primitive carbon nanotube only contains sp2-hybridized carbon and a small number of functional groups, which cannot supply sufficient adsorption sites for Hg0. The hydrophilic groups over the CNT surface such as hydrophilic open ends and oxygen-containing functional groups might boost the water vapor adsorption and exacerbate the competitive adsorption between Hg0 and water vapor [31]. SO2 as the common component in flue gas might also adsorb onto the defects of CNT and compete with Hg0 for the active sites. From this background, it is meaningful to further enhance the Hg0 adsorption on the CNT under complicated flue gas conditions. Manganese has been widely applied as a promoter to the improvement of Hg0 oxidation removal performance due to its multiple valence states and outstanding oxygen storage capacity [32]. For example, MnOX was employed to modify carbon nanotubes for Hg0 oxidation removal [33]. Mn/CNT exhibited a decent Hg0 oxidation ability within the temperature range of 150 to 250 °C. Nevertheless, a 500 ppm SO2 presence would nearly result in the complete loss of the Hg0 removal activity of Mn/CNT due to sulfate formation. Previous studies indicate that halide modified sorbent can achieve good SO2 resistance and improved Hg0 removal capacity [34,35,36]. Therefore, this work is dedicated to the feasibility of modifying CNT by manganese chloride to enhance Hg0 capture performance and strengthen durability under SO2 and H2O presence.
Herein, recyclable MnCl2-Fe2O3@CNT was synthesized and tested for Hg0 capture from simulated flue gas. Fe2O3 decoration could endow CNT with magnetic property, thus facilitating the recycling of deactivated sorbent through magnetic separation [37]. A variety of experiments were applied to determine the physico-chemical properties of MnCl2-Fe2O3@CNT. The influences of flue gas constituents, especially for SO2 and H2O on the Hg0 capture ability, were systematically explored. The correlative mechanisms were further analyzed by the temperature programmed desorption of Hg (Hg-TPD), X-ray photoelectron spectroscopy (XPS), and kinetic analysis.

2. Experimental Section

2.1. Preparation of MnCl2-Fe2O3@CNT

Raw carbon nanotubes (single wall, ultra-high purity) were obtained from Zhongke Time Nano Technology Co., Ltd. (Chengdu, China). All the other chemical reagents (analytical grade) involved were acquired from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The preparation method for the recyclable MnCl2-Fe2O3@CNT is typically as follows: first, a stoichiometric amount of Fe(NO3)3·9H2O is dissolved in deionized water, and then with an addition of a desired quantity of carbon nanotubes under vigorous stirring. The resultant mixture is dried at 110 °C overnight and then calcined at 250 °C under N2 flow for 2 h to obtain the Fe2O3 decorated nanotubes with a Fe2O3 loading of 10 wt% (denoted as FeCNT). Second, MnCl2-Fe2O3@CNT samples with a targeting MnCl2 content of 1, 5, 10, 15 wt% are obtained by immersing FeCNT in MnCl2 solution of varying concentrations. The resulting product is put into an ultrasonic bath for 30 min and then dried at 110 °C overnight. The derived sample is denoted as MnCl2(x)FeCNT (x = 1, 5, 10, 15). CNT with a MnCl2 loading of 10 wt% (denoted as MnCl2CNT) was prepared by a similar method.

2.2. Characterization Methods

The characterization details are shown in the Supplementary Material.

2.3. Hg0 Removal Performance Evaluation

A laboratory-scale fix-bed quartz reactor was employed to determine the Hg0 removal activities of the obtained samples. The sketch map of the experimental setup is exhibited in Figure S1. Typically, a sample of around 100 mg is put into the quartz tube reactor, in which the reaction temperature is adjusted by a tubular electric furnace. The simulated coal combustion flue gas is composed of 0/5%vol. O2, 0/500/800 ppm NO, 0/600/1500 ppm SO2, 0/1/3/6%vol. H2O, 270 μg·m−3 Hg0, and N2 as balanced gas. The total flux of simulated gas is maintained at 500 mL·min−1, which corresponded to a gas hourly space velocity (GHSV) of around 72,000 h−1. With N2 carrying, the feed of gaseous Hg0 is supplied by a mercury permeation tube (VICI Metronics, Poulsbo, WA, USA) submersed in an oil bath with temperature regulating. The water vapor is supplied by a steam generator coupled with a constant flow infusion pump (FD-HGPP01, Suzhou Friend Experimental Equipment Co., Ltd., Suzhou, China). The water vapor concentration can be regulated by controlling the injection rate of deionized water with the pump. The gas pipes for Hg0 and water vapor are wrapped with heating tapes and heated to 110 °C to prevent their condensation. An empty bed experiment suggests that the Hg0 deposition on the pipe inner surface can be negligible. The gas constituents such as NO, SO2, O2, and N2 are acquired from the corresponding compressed gas cylinders. Mass flow meters are employed to adjust the gas concentrations. After achieving relatively stable inlet Hg0 concentration during 30 min, the gas flow is switched to pass through the quartz reactor for the Hg0 removal ability evaluation. The detailed experimental conditions for each experiment are shown in Table S1. An online mercury analysis device (QM201H, Suzhou Qing’an Instrument Co., Ltd., Suzhou, China) is utilized to monitor the inlet and outlet Hg0 concentrations. Before entering into the analyzer, the acid gas constituents and moisture are taken out of the gas stream by NaOH, Na2O2, and silica gel. The two equations below are used to calculate the Hg0 removal efficiency (EHg, %) and the Hg0 adsorption capacity ( q t ):
E H g = 1 − H g 0 o u t H g 0 i n × 100 %
q t = 1 m ∫ 0 t ( H g i n 0 − H g o u t 0 ) × F × d t × 10 − 3
where H g i n 0 and H g o u t 0 represent the instantaneous inlet and outlet Hg0 concentrations, respectively, μg·m−3; m is the mass of the sorbent, g; F denotes the total gas flow rate, m3·min−1; t denotes the reaction time, min; and q t   denotes the mercury adsorption capacity, mg·g−1.

3. Results and Discussion

3.1. Physical and Chemical Properties of MnCl2-Fe2O3@CNT

The microstructure of the as-prepared sorbents was studied by TEM analysis as exhibited in Figure 1. In comparison with the original CNT (Figure S2), FeCNT still retains the one-dimensional (1-D) tubular structure and three-dimensional (3-D) entangled framework of CNT as shown in Figure 1a. Moreover, irregular-shaped nanoparticles with particle sizes ranging from 5 to 20 nm can be observed on the CNT surface. From the HRTEM image in Figure 1b, the interplanar lattice fringes of 0.275 and 0.372 nm can be clearly observed, which are indexed to the (104) plane and (012) plane of Fe2O3, respectively. The continuous diffraction rings in the SAED pattern as displayed in Figure 1c show the polycrystalline structure of Fe2O3 nanoparticles with relatively low crystallinity. After MnCl2 modification, very small particles with a size around 2.5 nm can be found on the surface of MnCl2(10)FeCNT as shown in Figure 1d and Figure S3. As revealed in Figure 1f, the HRTEM image exhibits a lattice spacing of 0.231 nm which belongs to the (015) plane of MnCl2. HAADF-STEM and EDS-mapping were further employed to explore the element distribution on the MnCl2(10)FeCNT surface. As shown in Figure 1g–l, the elements including C, O, Fe, Mn, and Cl are distributed on MnCl2(10)FeCNT in the nanoscale. The above results substantiate the existence of nano-crystalline Fe2O3 and MnCl2 on the tubular skeleton of the carbon nanotube, which can be further confirmed by the XRD analysis as later discussed.
The pore structure parameters of CNT, FeCNT, and MnCl2(10)FeCNT are shown in Figure 2 and Table 1. The N2 adsorption/desorption curves for all the samples displayed in Figure 2a are consistent with the type-IV isotherm [38]. The adsorption capacity of N2 shows an increasing trend with the increase in relative pressure (P/P0). All isotherms of the samples exhibit H3 hysteresis loops at high relative pressures, suggestive of their mesoporous properties. The pore size distribution curves as displayed in Figure 2b further demonstrate the co-existence of micropores and mesopores with a pore diameter mainly smaller than 20 nm. Moreover, the BET surface area of MnCl2(10)FeCNT is lower than that of CNT, 775.76 m2·g−1 for MnCl2(10)FeCNT versus 1307.01 m2·g−1 for CNT. This is possibly owing to the fact that the introduced Mn and Fe species are deposited onto the CNT surface and partially enter into the inner pores as shown in Table 1. The lower BET surface area of MnCl2(10)FeCNT exhibits no correlation with its Hg0 removal performance, implying that the pore characteristics might not be the decisive factor for the Hg0 removal.
Figure 2c displays the XRD profiles of CNT, FeCNT, and MnCl2(10)FeCNT. For the CNT sample, the two broad peaks associated with carbon nanotube can be observed at 24° and 43.8°. For FeCNT, the diffraction peaks centered at 33.3°, 35.7°, 49.5°, and 54.2° attributed to maghemite Fe2O3 appearing in the curve [39]. This indicates that Fe2O3 is successfully decorated onto the CNT surface. No visible diffraction peaks associated with MnCl2 can be found in MnCl2(10)FeCNT, suggesting that the MnCl2 species is mainly amorphous or has a very small grain size.
The functional groups on the samples were characterized by the FT-IR technique. As shown in Figure 2d and Figure S4, the two broad peaks centered at 3442 cm−1 and 1625 cm−1 are attributed to symmetric/asymmetric stretching vibration and bending vibration of the -OH group, respectively [40,41,42]. The existence of C=O can be verified by the peak located at 1752 cm−1 [43]. Three peaks at approximately 756 cm−1 (aromatic hydrogen bending), 1407 cm−1 (C=C stretching), and 1478 cm−1 (-CH2- bending) correspond to the aromatic skeleton of CNT [38,44]. These functional groups, especially oxygen-containing functional groups, might play a certain role in Hg0 adsorption and oxidation. Additionally, all the samples show a similar distribution of functional groups, indicating that MnCl2 modification and Fe2O3 decoration will not impair the basic structure of the CNT.
The chemical states of Mn, Cl, Fe, and O on the surface of fresh MnCl2(10)FeCNT were examined by XPS. For the Mn 2p3/2 spectra displayed in Figure 3a, three characteristic peaks located at 642.8, 641.8, and 640.7 eV are ascribed to the presence of Mn4+, Mn3+, and Mn2+, respectively [45,46]. It is revealed that Mn4+, Mn3+, and Mn2+ coexist on the surface of MnCl2(10)FeCNT, which might be attributed to the electronic interaction of MnCl2 with Fe2O3 as well as CNT support. The multiple valence states of Mn are beneficial to the electron transfer from adsorbed Hg0 to MnCl2(10)FeCNT sorbent. As shown in Figure 3b, the XPS spectra pertaining to Cl 2p are divided into two overlapping peaks located at 199.6 and 197.8 eV, which can be attributed to active chlorine species (Cl*) and ionic chlorine (Cl−), respectively [47,48]. Active chlorine species is considered to be active for bonding with Hg0 due to its high affinity to mercury. The O 1s spectra as shown in Figure 3c are resolved into two peaks centered at 529.6 and 531.5 eV, which correspond to lattice oxygen (Oβ) and chemisorbed oxygen (Oα), respectively [49]. It can be seen that MnCl2(10)FeCNT possesses abundant chemisorbed oxygen species on its surface, which might be conducive to Hg0 adsorption. The fitted Fe 2p spectra are exhibited in Figure 3d. The binding energy peaks at 713.6 and 711.3 eV are assigned to the presence of Fe3+ [50,51]. The binding energy peak at around 710 eV is attributed to Fe2+ presence. The Fe3+ species is dominant on the MnCl2(10)FeCNT surface, which is in accordance with the XRD analysis. The EPR technique was employed to identify the vacancies over MnCl2(10)FeCNT. As shown in Figure S5, a signal at g = 2.003 is detected over MnCl2(10)FeCNT, which can be attributed to the oxygen vacancies [36].
The magnetic hysteresis curve for as-prepared MnCl2(10)FeCNT is displayed in Figure 4. The fresh MnCl2(10)FeCNT exhibits a saturation magnetization of 0.77 emu·g−1. In addition, the magnetization hysteresis and coercivity for MnCl2(10)FeCNT are indiscernible, suggestive of its superparamagnetic property [52]. This characteristics of MnCl2(10)FeCNT can make it readily magnetized when it is exposed to an external magnetic field. While in the absence of the magnetic field, MnCl2(10)FeCNT will become unmagnetized. MnCl2(10)FeCNT is successfully separated from the mixture of fly ash and sorbent by a hand magnet. Thereby, the magnetically responsive MnCl2(10)FeCNT might be conveniently reclaimed from the fly ash by employing high gradient magnetic separators (HGMS), which increase its potential for application. The thermogravimetric (TG) analysis results as shown in Figure S6 indicate that MnCl2(10)FeCNT has a goo stability below 500 °C. The weight loss below 150 °C can be attributed to surface water desorption. The weight loss above 500 °C is due to the decomposition of MnCl2 and the carbon skeleton.

3.2. Hg0 Removal Performance of MnCl2-Fe2O3@CNT

3.2.1. Hg0 Removal Performance of Different MnCl2-Fe2O3@CNT Samples

The Hg0 removal abilities of different MnCl2-Fe2O3@CNT samples were examined under N2 atmosphere at 100 °C and the results are displayed in Figure 5. CNT can achieve more than 71% Hg0 removal during the testing period. CNT has a large specific surface area (1307.01 m2·g−1) and developed pore structure. It can facilitate the Hg0 diffusion across the gas–solid interface over CNT and can provide storage space for the physically adsorbed Hg0. Moreover, some oxygen-containing functional groups naturally existing (including C=O and COOH, etc.) on the CNT surface can function as adsorption sites for Hg0 and thus endow CNT with a certain chemical adsorption capacity for Hg0. Even so, the Hg0 removal efficiency of CNT substantially exhibits a decreasing trend, owing to the limited surface adsorption sites. Fe2O3 decoration slightly enhances the Hg0 removal efficiency over FeCNT. The introduction of Fe2O3 with certain oxidation capacity might promote the Hg0 oxidation over CNT [53]. For MnCl2(10)FeCNT, it can stably achieve almost 100% Hg0 removal during the testing period. The excellent Hg0 removal performance of MnCl2(10)FeCNT might be attributed to the important roles of introduced active Mn and Cl species in Hg0 adsorption and oxidation [54,55]. The effect of MnCl2 loading on Hg0 removal over MnCl2-Fe2O3@CNT was further investigated and the results are shown in Figure 5b. The 1 wt% MnCl2 loading results in a neglectable increase of Hg0 removal efficiency over MnCl2(1)FeCNT. For the cases of 10 wt% and 15 wt% MnCl2 loading, the Hg0 removal efficiency is increased to nearly 100%. A higher loading amount of MnCl2 is expected to enrich the Mn and Cl adsorption sites and thereby elevates the Hg0 removal performances of MnCl2-Fe2O3@CNT samples. If not otherwise specified, MnCl2(10)FeCNT is employed as the sample for further investigations in the following experiments.

3.2.2. Effect of Reaction Temperature

As exhibited in Figure 6a, the Hg0 removal performance of MnCl2(10)FeCNT in the temperature range of 50–200 °C is illustrated. It is revealed that the most suitable temperature for Hg0 capture over MnCl2(10)FeCNT is 100 °C. The Hg0 removal efficiency increases from 91.3% to 100% when the temperature is elevated from 50 °C to 100 °C. It is suggested that chemisorption is an important path for Hg0 capture over MnCl2(10)FeCNT. A higher temperature can help the reactants to gain greater kinetic energy, thus leading to a promotive effect on the reaction of adsorbed Hg0 with the surface active species over MnCl2(10)FeCNT [56]. However, the Hg0 removal efficiency displays a decreased tendency with a further increase in temperature to 150 °C and 200 °C. The reason for the dropping trend of Hg0 removal efficiency may lie in the decomposition/desorption of the adsorbed mercury species at a higher temperature. This can be illustrated by the Hg-TPD analysis in the following discussion. Additionally, the diffusion of Hg0 from the gas phase onto the MnCl2(10)FeCNT surface will be inhibited at a higher temperature, thus resulting in a detrimental effect on the subsequent chemisorption. As for CNT, its Hg0 removal efficiency apparently shows a downward trend and decreases significantly in the temperature range of 50 °C to 150 °C. This result can be interpreted on the basis that most of Hg0 is physically adsorbed onto the CNT surface. Increasing the temperature will speed up the desorption of such weakly bonded mercury species. It can be expected that the adsorption of Hg0 over CNT might be severely disturbed by the commonly existing flue gas components such as SO2 and H2O due to the weak bonding of Hg0 to CNT.

3.2.3. Effect of SO2

As a general acidic gas component in flue gas, SO2 might influence the Hg0 removal over MnCl2(10)FeCNT. The effect of SO2 on Hg0 removal was examined at 100 °C as shown in Figure 6b and Figure S7. It can be seen that SO2 apparently exhibits an inhibitory effect on Hg0 removal over CNT. When 1500 ppm SO2 is present in the feed gas, the Hg0 removal efficiency of CNT eventually decreases to 47.8% during the testing period. This observation may be attributed to SO2 being able to compete with Hg0 for the adsorption sites over CNT [28]. By way of contrast, MnCl2(10)FeCNT can still achieve more than 93% Hg0 removal with 1500 ppm SO2 presence. The Hg0 removal efficiency of MnCl2(10)FeCNT only decreased by around 7% with the increase of SO2 concentration from 0 to 1500 ppm, though MnCl2(10)FeCNT also faces the SO2 poisoning problem. The above results manifest that MnCl2 modification and Fe2O3 decoration can relieve the inhibitory effect from SO2 and greatly enhance the SO2 resistance of the CNT composite. Based on the previous studies [57], Cl can efficiently adsorb the Hg atom and capture its electrons due to the high electronegativity of Cl. The affinity of Cl to Hg0 might promote the adsorption and oxidation of Hg0 through the HgCl2 route over MnCl2(10)FeCNT, which is less influenced by SO2. Additionally, Fe species might function as a SO2 trap agent to protect the active species for Hg0 over MnCl2(10)FeCNT, thus enhancing its ability to resist SO2 [58].
To elucidate the role of SO2 on Hg0 removal and clarify the mechanism of MnCl2 modification and Fe2O3 decoration on the enhancement of SO2 resistance, the surface chemical properties of MnCl2(10)FeCNT after being used under SO2 were further analyzed by XPS. For the Mn 2p spectra shown in Figure 7a, the intensity of the peak at about 640.7 eV attributed to Mn2+ is strengthened, which indicates that part of the Mn species with high valence states is reduced to Mn2+ due to its interaction with SO2. The presence of SO2 can cause the conversion of active Mn4+ and Mn3+ into Mn2+ bonded with SO42− that is thermally stable and chemically inert for Hg0 adsorption and oxidation. This will inevitably impact the Hg0 removal performance of the sorbent. In the Fe 2p spectra as shown in Figure 7b, the chemical state of Fe over MnCl2(10)FeCNT is changed due to the SO2 exposure. The intensity of the fitting peak at around 710 eV associated with Fe2+ is increased, implying that Fe3+ has participated in the reaction with SO2 and been reduced to Fe2+. In line with the previous studies [35,59], Fe3+ can compete with Mn4+ and Mn3+ for capturing SO2. Thereby, the Mn sites over MnCl2(10)FeCNT would be somewhat less influenced by SO2 due to the protecting effect of the Fe presence. For the O 1s spectra shown in Figure 7c, the new fitting peak appearing at 532 eV can be attributed to the presence of SO42− [27]. Moreover, the content of Oα as well as Oβ are decreased significantly. It is confirmed that sulfate species is produced and accumulated on the MnCl2(10)FeCNT surface as a result of SO2 exposure. The formed sulfate species can also be verified by the presence of two fitting peaks at 169.9 and 168.6 eV in the S 2p spectra displayed in Figure 7d, which can be assigned to HSO4− and SO42−, respectively. The formation of sulfate aside, SO2 exposure might change the chemical state of chlorine over MnCl2(10)FeCNT, thus impacting its Hg0 removal performance. As exhibited in Figure S8, the binding energy of the shoulder peak in the Cl 2p spectra of the SO2-exposed MnCl2(10)FeCNT is decreased to 198.5 eV, lower than that of fresh MnCl2(10)FeCNT (199.6 eV), which indicates that the valence state of chlorine over MnCl2(10)FeCNT is somewhat decreased due to the SO2 exposure. In other words, the active chlorine species over MnCl2(10)FeCNT is partially consumed by SO2, in accordance with the previous studies [28,60]. Hence, SO2 can consume the active oxygen as well as chlorine species for Hg0 removal and cause the sulfation of the sorbent, thereby showing a negative effect on Hg0 removal. Thanks to the multiple active sites over MnCl2(10)FeCNT, it can achieve the ability to resist SO2 to a certain extent.

3.2.4. Effect of NO

The effect of NO on Hg0 removal over MnCl2(10)FeCNT was further investigated at 150 °C as exhibited in Figure 6c. With the introduction of 500 ppm NO into the feed gas, the Hg0 removal efficiency of MnCl2(10)FeCNT is increased from 91.2% to 97.5%. A further increase in NO concentration to 800 ppm leads to a slight enhancement of Hg0 removal. This observation indicates that NO can facilitate the Hg0 removal over MnCl2(10)FeCNT. It has been reported that the adsorbed NO can be converted into active N-containing species such as NO2 and NO+ over metal cations with the assistance of surface active oxygen species [61,62]. This newly formed nitrogenous species might act as the new active site to oxidize Hg0 into Hg2+. This is not easy for the direct reaction of the gas-phase NO with Hg0, while Hg0 can be heterogeneously oxidized by the above discussed active nitrogenous species effectively. With regard to CNT, the introduction of 800 ppm NO into the feed gas results in a slight decrease in the Hg0 removal efficiency. This is possibly caused by the competitive adsorption between NO and Hg0. The limited active oxygen species over CNT which plays a role in Hg0 adsorption cannot maintain the production of active N-containing species from adsorbed NO.
A further clarification of the promotion effect of NO on Hg0 removal over MnCl2(10)FeCNT arises from the experiments of XPS. The XPS spectra pertaining to Mn 2p, Fe 2p, O 1s, and N 1s for MnCl2(10)FeCNT used under NO are shown in Figure 8. For Mn 2p spectra as exhibited in Figure 8a, the peak intensity for Mn2+ of MnCl2(10)FeCNT is strengthened after being used under NO. This observation might be owing to the conversion of Mn4+ and Mn3+ into Mn2+ to produce surface nitrogenous species. As shown in Figure 8b, the content of Fe2+ of the sample is also increased, indicating that the Fe3+/Fe2+ redox pair participates in the reaction with NO. For the O 1s spectra as exhibited in Figure 8c, a broad shoulder peak located at around 533 eV attributed to surface adsorbed oxygen species appears, while the peak intensity of Oα and Oβ both decrease significantly. This result indicates that Oα and Oβ have been consumed by NO to generate the aforementioned surface adsorbed oxygen species, which can be further verified by the results from the N 1s spectra as shown in Figure 8d. It can be seen that three characteristic peaks centered at 407.4, 404.9, and 400 eV associated with nitrate, surface adsorbed NO2 on metal sites (nitrite), and surface adsorbed NO appear in the N 1s spectra of the sample after being used under NO [63,64]. The above results confirm the formation of active N-containing species over MnCl2(10)FeCNT, which can act as new active sites for Hg0 removal. Therefore, NO exhibits a promotion effect on Hg0 removal over MnCl2(10)FeCNT.

3.2.5. Effect of H2O

As a crucial component in the coal combustion flue gas, water vapor may strongly influence the Hg0 removal over MnCl2(10)FeCNT. The Hg0 removal efficiencies under different water vapor concentrations at 150 °C over the samples are displayed in Figure 6d. With the introduction of 3%vol. H2O into the feed gas, the Hg0 removal efficiency of MnCl2(10)FeCNT seems to decrease a little but it still maintains around 92.1%. A further increase of H2O content to 6%vol., and a decrease of Hg0 removal efficiency to 75.2% can be observed. The slight inhibitory effect of H2O on Hg0 removal over MnCl2(10)FeCNT can be due to the competitive adsorption of Hg0 and H2O onto the surface actives [65]. For CNT, the presence of 3%vol. H2O has a significant negative effect on Hg0 removal and decreases its efficiency to almost 0%. There are many defects on carbon nanotubes, which may result in hydrophilic groups such as oxygen-containing functional groups and hydrophilic open ends. Water molecules can easily adsorb onto the hydrophilic sites on the CNT which may be beneficial for Hg0 adsorption. In comparison, the decrement of Hg0 removal efficiency for MnCl2(10)FeCNT can be neglected in the presence of 3%vol. H2O, though MnCl2(10)FeCNT cannot survive the negative effect of H2O. Overall, MnCl2(10)FeCNT can achieve satisfactory H2O resistance. Abundant active sites are generated and exposed on the MnCl2(10)FeCNT surface. MnCl2(10)FeCNT can still afford accessible Hg0 adsorption sites, though part of the active sites are occupied by H2O. The effects of other flue gas components like NO2/HCl and GHSV on Hg0 removal have also been studied. As shown in Figure S9, both NO2 and HCl are beneficial for Hg0 removal over MnCl2(10)FeCNT. High GHSV will decrease the residence time for Hg0 removal, thus decreasing the Hg0 removal efficiency. In addition, the Hg0 removal performance of MnCl2(10)FeCNT under the simulated flue gas conditions was further verified aimed at its potential application. As displayed in Figure S10, MnCl2(10)FeCNT can achieve 91.6% Hg0 removal in the presence of 5%vol. O2, 500 ppm NO, 1500 ppm SO2, and 3%vol. H2O during the testing period. The promotional effect of O2 and NO might somewhat offset the impact of SO2 and H2O on Hg0 removal. Therefore, MnCl2(10)FeCNT might be a potential candidate mercury sorbent with application prospect, which can be applied in flue gas treatment due to its excellent performance and tolerance to SO2 and H2O.

3.2.6. Kinetic Analysis

The previous studies have indicated the interpretation of adsorption dynamics of Hg0 by the pseudo-second order model, in which chemical adsorption is considered as the rate-determining step [42]. With the employment of this model, the equilibrium Hg0 adsorption capacity can be obtained by the experimental data. The model equation is displayed in Equation (3):
q t = t 1 k 2 q e 2 + t q e
where q t and q e (mg·g−1) represent the amount of captured Hg0 at time t and at the equilibrium time, respectively; k 2 represents the rate constant for the pseudo-second order model, g·mg−1·min−1); and t (min) denotes the adsorption time.
The obtained adsorption capacity curves (according to the data displayed at the bottom right of Figure 9a) and the simulated results are shown in Figure 9a. It is shown that the pseudo-second order model is competent for describing the Hg0 adsorption kinetics over MnCl2(10)FeCNT, showing a correlation coefficient (R2) of 0.9994. Additionally, the adsorption capacity at equilibrium based on the kinetic fitting is 25.06 mg·g−1, which is one or two orders of magnitude higher than that of traditional carbon-based sorbents. It also has a Hg0 adsorption capacity comparable to that of the sulfide composite. The comparison of MnCl2(10)FeCNT with other reported sorbents is exhibited in Figure 9b (see Table S2 for the details). As can be seen, MnCl2(10)FeCNT outperforms most of the reported carbon-based sorbents. Moreover, the magnetically responsive MnCl2(10)FeCNT can be effectively separated and recycled from fly ash as shown in Figure 4. Considering its good resistance to SO2 and H2O, MnCl2(10)FeCNT might hold promise for future application.

3.2.7. Regeneration Capability

The regeneration capability of the sorbent is critical for the real application. In this work, the mercury laden sorbent is regenerated by heat treatment at 400 °C under N2 flow for 2 h. As shown in Figure S11, the Hg0 removal efficiency of MnCl2(10)FeCNT does not decrease significantly after the fifth regeneration (from 100% to 92.8%). The above results reveal that MnCl2(10)FeCNT shows good reusability in the laboratory tests. The Mn 2p, Cl 2p, O1s, and Fe 2p spectra of MnCl2(10)FeCNT after fifth regeneration have been displayed in Figure S12. It can be seen that the regenerated MnCl2(10)FeCNT exhibits a lower content of Mn4+and Mn3+ while a larger content of Mn2+ than fresh MnCl2(10)FeCNT. The chemisorbed oxygen (Oα) content of regenerated MnCl2(10)FeCNT decreases from 84.23% to 74.72% compared to the fresh MnCl2(10)FeCNT. The Cl* content for regenerated MnCl2(10)FeCNT also shows a decreasing trend. Moreover, as shown in Figure S12, the BET surface area of regenerated MnCl2(10)FeCNT just shows a certain decrease. This can to some extent explain the reason for the performance decline after the fifth regeneration.

3.3. Hg0 Removal Mechanism

3.3.1. Hg-TPD Analysis

To identify the formed mercury species on MnCl2(10)FeCNT, the temperature programmed desorption of Hg (Hg-TPD) from the spent MnCl2(10)FeCNT was carried out in comparison with the CNT. As shown in Figure 10a, the mercury desorption from MnCl2(10)FeCNT exhibits a strong peak at 200 °C and a weak shoulder peak at 280 °C. Previous studies indicate that the desorption temperatures for different Hg compounds can be arranged in such ascending order: HgBr2 < HgCl2 < HgS < HgO < HgSO4 [66,67]. Considering the chemical characteristic of MnCl2(10)FeCNT, the first peak can be attributed to the thermal decomposition of HgCl2 on the MnCl2(10)FeCNT surface and the latter peak can be ascribed to surface HgO. The above results manifest that the adsorbed Hg0 is oxidized by the active chlorine and oxygen species over the MnCl2(10)FeCNT surface, mainly present in the form of HgCl2, which is consistent with the following XPS results. For CNT, two desorption peaks located at 160 °C and 300 °C can be observed as shown in Figure 10b. The two peaks are associated with physically adsorbed or weakly bonded Hg0 and HgO, respectively. Physically adsorbed Hg0 covers a large proportion of mercury species on CNT, thus resulting in a significant decrease in its Hg0 removal performance with temperature increasing.

3.3.2. XPS Analysis

To reveal the mechanism of Hg0 capture by MnCl2(10)FeCNT, the chemical states of surface elements over spent MnCl2(10)FeCNT after capturing Hg0 were further investigated by XPS analysis and the results are shown in Figure 11. As evident in Figure 11a,f, Hg-laden MnCl2(10)FeCNT exhibits a lower content of Mn4+and Mn3+, and a larger content of Mn2+ than fresh MnCl2(10)FeCNT, which suggests that Mn4+ and Mn3+ participate in the Hg0 removal process and are thereby converted into Mn2+. It is known that Hg0 is prone to adsorb onto Mn cations with high valence states, which will lead to the reduction in Mn by electron donation from adsorbed Hg0. Meanwhile, as shown in Figure 11b, the chemisorbed oxygen (Oα) content of Hg-laden MnCl2(10)FeCNT decreases from 84.23% to 79.69% compared to the fresh MnCl2(10)FeCNT, accompanied with a slight increase in lattice oxygen (Oβ) content. This indicates that Oα is involved in the Hg0 capture process over the sorbent, in agreement with the previous studies [68]. Chemisorbed oxygen species including O2− are beneficial for Hg0 removal and can interact with adsorbed Hg0 to produce HgO, thus resulting in the conversion of Oα to Oβ over MnCl2(10)FeCNT. Figure 11c shows Cl 2p spectra for the Hg-laden MnCl2(10)FeCNT. It can be seen that the composition ratio of Cl* in the Cl 2p spectra obviously decreases over spent MnCl2(10)FeCNT accompanied with an increase in Cl− content, suggestive of a conversion of Cl* to Cl− involved in the Hg0 capture process. The consumption of Cl* and increase in Cl− ratio suggests the generation of HgCl2 over spent MnCl2(10)FeCNT. Chen et al. studied the Hg0 removal by MnCl2 doped iron-carbon sorbent and found that MnCl2 modification enhanced the SO2 resistance of the sorbent [54]. The manganese sites played important roles in the Hg0 removal. Density functional theory (DFT) calculations in the previous studies by Ji et al. indicate that Hg0 can strongly interact with Cl atoms that are capable of capturing electrons on the MnCl2 (110) surface [55]. Both Mn and Cl are responsible for the stable Hg0-MnCl2 adsorption system with the electron transfer among Mn (d-orbit), Cl (p-, d-orbitals), and Hg (s-, d-orbitals). Xu et al. investigated the Hg0 removal over CuCl2-modified FeOCl [57]. The DFT results also indicate that the electronegativity of Cl will be enhanced due to the acceleration of electron transfer by the adjacent metal atom like Cu, thereby strengthening the electronic interaction of adsorbed Hg0 with Cl. By quantitative calculation and analysis as displayed in Figure 11d,f, it is revealed that the conversion of Fe3+ to Fe2+ is also involved in the Hg0 removal process. Fe3+ can capture electrons from adsorbed Hg0 which somewhat contribute to Hg0 removal over MnCl2(10)FeCNT. The possible mercury product over spent MnCl2(10)FeCNT was further examined by XPS as shown in Figure 11e. The two binding energy peaks at 104.2 and 100.7 eV in the Hg 4f spectra can be ascribed to Hg2+. The above results manifest that Hg0 is chemisorbed onto MnCl2(10)FeCNT and oxidized into Hg2+ by active chlorine (Cl*) as well as oxygen species (O*) over a sorbent surface. According to the previous studies, the abundant active chlorine and oxygen species over MnCl2(10)FeCNT have played important roles in the Hg0 removal. The involved reactions are shown as follows:
Hg0(g) → Hg0(ad)
Hg0(ad) + O* → HgO
Hg0(ad) + Cl* → HgCl2

4. Conclusions

In this work, recyclable MnCl2-Fe2O3@CNT was synthesized and tested for Hg0 capture from flue gas. MnCl2(10)FeCNT sorbent with 10 wt% MnCl2 content can achieve excellent Hg0 adsorption capacity (25.06 mg·g−1), which outperforms most of the reported carbon-based sorbents. SO2 can consume active oxygen as well as chlorine species for Hg0 removal and cause the sulfation of the sorbent, thereby slightly suppressing the Hg0 removal over MnCl2(10)FeCNT. In the presence of NO, the active N-containing species including adsorbed NO2 can be generated over MnCl2(10)FeCNT, which can act as new active sites for Hg0 removal. Therefore, NO exhibits a promotion effect on Hg0 removal over MnCl2(10)FeCNT. The presence of H2O slightly inhibits the Hg0 removal over MnCl2(10)FeCNT due to the competitive adsorption of H2O with Hg0 onto the active sites. In the presence of 5%vol. O2, 500 ppm NO, 1500 ppm SO2, and 3%vol. H2O, around 91.6% Hg0 removal efficiency can be achieved by MnCl2(10)FeCNT. Overall, Fe2O3 decoration and MnCl2 modification greatly promote the Hg0 removal performance and SO2 as well as H2O resistance of the CNT sample. Additionally, the superparamagnetic property of MnCl2(10)FeCNT can enable it to be conveniently separated from fly ash by magnetic separation. The Hg0 removal efficiency of MnCl2(10)FeCNT does not decrease significantly after the fifth regeneration, suggesting its good reusability. According to the Hg-TPD and XPS analysis, Hg0 is chemisorbed onto MnCl2(10)FeCNT and oxidized into Hg2+ mainly as HgCl2 by active chlorine as well as oxygen species over the sorbent surface. The abundant active chlorine and oxygen species over MnCl2(10)FeCNT have played important roles in the Hg0 removal. Although this study demonstrates the high potential of MnCl2(10)FeCNT for flue gas mercury treatment, the pathway to commercialization still presents exciting challenges. The next critical steps will involve engineering solutions to ensure long-term stability, conducting techno-economic analyses to evaluate cost-competitiveness, and developing scalable protocols for industrial-scale production. The effectiveness, regeneration capability, operation cost and secondary pollution risk of the proposed sorbent requires further examination under real flue gas conditions in future work.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/ma18194573/s1, Figure S1: The schematic diagram of the experimental system; Figure S2: The TEM images of original CNT; Figure S3: The TEM images of MnCl2(10)FeCNT; Figure S4: The enlarged FTIR spectra for the samples; Figure S5: The EPR results of MnCl2(10)FeCNT; Figure S6: The TG results of MnCl2(10)FeCNT; Figure S7: The effects of SO2 on Hg0 removal over CNT, MnCl2CNT, FeCNT and MnCl2(10)FeCNT; Figure S8: The Cl 2p spectra of the SO2-exposed MnCl2(10)FeCNT; Figure S9: The effects of HCl and NO2 on Hg0 removal over MnCl2(10)FeCNT (a); The effects of GHSV and sorbent mass on Hg0 removal over MnCl2(10)FeCNT (b); Figure S10: The Hg0 removal performance of MnCl2(10)FeCNT under simulated flue gas; Figure S11: The regeneration performance of MnCl2(10)FeCNT; Figure S12: The XPS spectra pertaining to Mn 2p (a), Cl 2p (b), O 1s (c) and Fe 2p (d) of MnCl2(10)FeCNT after fifth regeneration; The N2 adsorption/desorption curves (e) and pore size distribution curves (f) of MnCl2(10)FeCNT after fifth regeneration; Table S1: The detailed experimental conditions for each experiment; Table S2: The comparison of MnCl2(10)FeCNT with other reported sorbents. Refs. [69,70,71,72,73,74,75,76,77,78,79,80] can be found in Supplementary Materials.

Author Contributions

Z.L. was involved in writing the original draft, in conceptualization, and data curation. Y.C. was involved in formal analysis, software, and methodology. H.R. was involved in resources, review, and editing. C.J. was involved in data curation and formal analysis. X.P. was involved in validation, review, and editing. H.L. was involved in review, editing, and supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by Enshi State Science and Technology Plan Project (D20230089) and the National Natural Science Foundation of China (52100134 and 52200197).

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Materials 18 04573 g001
Figure 1. The TEM images (a,b) and SAED pattern of FeCNT (c); TEM images (d–f) of MnCl2(10)FeCNT; HAADF STEM and EDS-mapping images (g–l) of MnCl2(10)FeCNT.
Figure 1. The TEM images (a,b) and SAED pattern of FeCNT (c); TEM images (d–f) of MnCl2(10)FeCNT; HAADF STEM and EDS-mapping images (g–l) of MnCl2(10)FeCNT.
Materials 18 04573 g001
Materials 18 04573 g002
Figure 2. The N2 adsorption/desorption curves (a), pore size distribution curves (b), XRD profiles (c), and FTIR profiles (d) of the samples.
Figure 2. The N2 adsorption/desorption curves (a), pore size distribution curves (b), XRD profiles (c), and FTIR profiles (d) of the samples.
Materials 18 04573 g002
Materials 18 04573 g003
Figure 3. The XPS spectra pertaining to Mn 2p (a), Cl 2p (b), O 1s (c), and Fe 2p (d) of fresh MnCl2(10)FeCNT.
Figure 3. The XPS spectra pertaining to Mn 2p (a), Cl 2p (b), O 1s (c), and Fe 2p (d) of fresh MnCl2(10)FeCNT.
Materials 18 04573 g003
Materials 18 04573 g004
Figure 4. The magnetic hysteresis curve for MnCl2(10)FeCNT.
Figure 4. The magnetic hysteresis curve for MnCl2(10)FeCNT.
Materials 18 04573 g004
Materials 18 04573 g005
Figure 5. The Hg0 removal performances of CNT, FeCNT, and MnCl2(10)FeCNT (a); the effect of MnCl2 loading on Hg0 removal over MnCl2-Fe2O3@CNT (b).
Figure 5. The Hg0 removal performances of CNT, FeCNT, and MnCl2(10)FeCNT (a); the effect of MnCl2 loading on Hg0 removal over MnCl2-Fe2O3@CNT (b).
Materials 18 04573 g005
Materials 18 04573 g006
Figure 6. The effects of temperature (a), SO2 (b), NO (c), and H2O (d) on Hg0 removal over CNT and MnCl2(10)FeCNT.
Figure 6. The effects of temperature (a), SO2 (b), NO (c), and H2O (d) on Hg0 removal over CNT and MnCl2(10)FeCNT.
Materials 18 04573 g006
Materials 18 04573 g007
Figure 7. The XPS spectra pertaining to Mn 2p (a), Fe 2p (b), O 1s (c), and S 2p (d) of MnCl2(10)FeCNT after being used under SO2.
Figure 7. The XPS spectra pertaining to Mn 2p (a), Fe 2p (b), O 1s (c), and S 2p (d) of MnCl2(10)FeCNT after being used under SO2.
Materials 18 04573 g007
Materials 18 04573 g008
Figure 8. The XPS spectra pertaining to Mn 2p (a), Fe 2p (b), O1s (c), and N1s (d) of MnCl2(10)FeCNT used under NO.
Figure 8. The XPS spectra pertaining to Mn 2p (a), Fe 2p (b), O1s (c), and N1s (d) of MnCl2(10)FeCNT used under NO.
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Materials 18 04573 g009
Figure 9. The obtained and simulated adsorption capacity curves for MnCl2(10)FeCNT (a) and the comparison of MnCl2(10)FeCNT with other reported sorbents (b).
Figure 9. The obtained and simulated adsorption capacity curves for MnCl2(10)FeCNT (a) and the comparison of MnCl2(10)FeCNT with other reported sorbents (b).
Materials 18 04573 g009
Materials 18 04573 g010
Figure 10. The Hg-TPD curves for MnCl2(10)FeCNT (a) and CNT (b).
Figure 10. The Hg-TPD curves for MnCl2(10)FeCNT (a) and CNT (b).
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Materials 18 04573 g011
Figure 11. The XPS spectra pertaining to Mn 2p (a), O1s (b), Cl 2p (c), Fe 2p (d), and Hg 4f (e) of spent MnCl2(10)FeCNT after capturing Hg0; the comparison of chemical states between fresh and spent MnCl2(10)FeCNT (f). Cl* represents active chlorine.
Figure 11. The XPS spectra pertaining to Mn 2p (a), O1s (b), Cl 2p (c), Fe 2p (d), and Hg 4f (e) of spent MnCl2(10)FeCNT after capturing Hg0; the comparison of chemical states between fresh and spent MnCl2(10)FeCNT (f). Cl* represents active chlorine.
Materials 18 04573 g011
Table 1. Pore parameters of the samples.
Table 1. Pore parameters of the samples.
SamplesBET Surface AreaPore VolumePore SizeMicropore AreaExternal Surface Area
(m2·g−1)(cm3·g−1)(nm)(m2·g−1)(m2·g−1)
CNT1307.011.278.29971.45335.56
FeCNT1065.140.787.31790.13275.01
MnCl2(10)FeCNT775.760.708.27560.49215.27
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Liu, Z.; Chen, Y.; Rong, H.; Jie, C.; Peng, X.; Li, H. Recyclable MnCl2-Fe2O3@CNT as Sulfur and Water-Resistant Sorbent for Gaseous Elemental Mercury Removal from Coal Combustion Flue Gas. Materials 2025, 18, 4573. https://doi.org/10.3390/ma18194573

AMA Style

Liu Z, Chen Y, Rong H, Jie C, Peng X, Li H. Recyclable MnCl2-Fe2O3@CNT as Sulfur and Water-Resistant Sorbent for Gaseous Elemental Mercury Removal from Coal Combustion Flue Gas. Materials. 2025; 18(19):4573. https://doi.org/10.3390/ma18194573

Chicago/Turabian Style

Liu, Zhuo, Yuchi Chen, Hao Rong, Cui Jie, Xiyan Peng, and Honghu Li. 2025. "Recyclable MnCl2-Fe2O3@CNT as Sulfur and Water-Resistant Sorbent for Gaseous Elemental Mercury Removal from Coal Combustion Flue Gas" Materials 18, no. 19: 4573. https://doi.org/10.3390/ma18194573

APA Style

Liu, Z., Chen, Y., Rong, H., Jie, C., Peng, X., & Li, H. (2025). Recyclable MnCl2-Fe2O3@CNT as Sulfur and Water-Resistant Sorbent for Gaseous Elemental Mercury Removal from Coal Combustion Flue Gas. Materials, 18(19), 4573. https://doi.org/10.3390/ma18194573

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