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Adsorption enhancement of elemental mercury onto sulphur-functionalized silica gel adsorbents a
a
Khairiraihanna Johari , Norasikin Saman & Hanapi Mat
ab
a
Advanced Materials and Process Engineering Laboratory, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia b
Novel Materials Research Group, Nanotechnology Research Alliance, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia Published online: 01 Oct 2013.
To cite this article: Khairiraihanna Johari, Norasikin Saman & Hanapi Mat , Environmental Technology (2013): Adsorption enhancement of elemental mercury onto sulphur-functionalized silica gel adsorbents, Environmental Technology, DOI: 10.1080/09593330.2013.840321 To link to this article: http://dx.doi.org/10.1080/09593330.2013.840321
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Environmental Technology, 2013 http://dx.doi.org/10.1080/09593330.2013.840321
Adsorption enhancement of elemental mercury onto sulphur-functionalized silica gel adsorbents Khairiraihanna Joharia , Norasikin Samana and Hanapi Mata,b∗ a Advanced
Materials and Process Engineering Laboratory, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia; b Novel Materials Research Group, Nanotechnology Research Alliance, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia
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(Received 11 March 2013; accepted 27 August 2013 ) In this study, elemental mercury (EM) adsorbents were synthesized using tetraethyl orthosilicate (TEOS) and 3mercaptopropyl trimethoxysilane as silica precursors. The synthesized silica gel (SG)-TEOS was further functionalized through impregnation with elemental sulphur and carbon disulphide (CS2 ). The SG adsorbents were then characterized by using scanning electron microscope, Fourier transform infra-red spectrophotometer, nitrogen adsorption/desorption, and energy-dispersive X-ray diffractometer. The EM adsorption of the SG adsorbents was determined using fabricated fixed-bed adsorber. The EM adsorption results showed that the sulphur-functionalized SG adsorbents had a greater Hg◦ breakthrough adsorption capacity, confirming that the presence of sulphur in silica matrices can improve Hg◦ adsorption performance due to their high affinity towards mercury. The highest Hg◦ adsorption capacity was observed for SG-TEOS(CS2 ) (82.62 μg/g), which was approximately 2.9 times higher than SG-TEOS (28.47 μg/g). The rate of Hg◦ adsorption was observed higher for sulphur-impregnated adsorbents, and decreased with the increase in the bed temperatures. Keywords: mercury; silica gel; organosilane; sulphur; adsorption
1. Introduction Mercury is a toxic substance that presents a greatest hazard to human health as well as to environment, even at low concentration. It can accumulate in animals and plants. It can also enter into human body through food cycle,[1] which can cause damage to the central nerve systems.[2] Major anthropogenic sources of mercury emissions into atmosphere are combustion activities such as from the coalfired power plants and incinerators.[3–5] Other sources of mercury in environment include petroleum exploration and processing,[6–8] volcanoes,[9] forest fires,[10,11] and cinnabar (ore).[12] Generally, mercury exists in four different forms, namely elemental (Hg◦ ), monovalent mercury (Hg(I)), divalent mercury (Hg(II)), and monomethyl mercury ((CH3 )Hg),[7] which need to be removed even at low concentrations due to their high toxicity. However, in the case of elemental mercury (EM) (Hg◦ ), it is difficult to remove due to its insolubility in water and cannot be readily captured simply in other air pollution control equipment. Several approaches have been used to reduce mercury emissions from various waste streams, such as adsorption, ion exchange, chemical precipitation, membrane filtration, and photo-reduction.[13–15] Among the conventional chemical and physical methods, adsorption has been widely adopted to reduce mercury emissions using a variety of adsorbent materials.[16] ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
At present, a variety of materials have been used commercially as adsorbents for capturing mercury in liquid and gas streams, which include activated carbon, carbon molecular sieves, zeolites, silica gel (SG), polymer resins, and activated alumina [6,7,17–21] which can act directly as adsorbents or be first modified with reactants/ligands to enhance their sorption capacity. Meyer et al. [22] reported that capturing Hg◦ from the gas phase is more challenging because the mechanism of capture is more complex compared with the simple Hg(II) encountered in the liquid phase. The functionalization of organic and inorganic solid supports with functional groups to enhance adsorbate adsorption capacity and selectivity has been reported in a number of specific applications.[23] The inorganic solid supports, especially SG, are the most widely used solid substrates due to their advantages of having good mechanical strength, do not swell, and can sustain the high temperatures.[24,25] Structurally, they are stable and rigid. The silanol groups on the silica surfaces can serve as point of attachment of chelating agents [26] and can be chemically modified with functional compounds.[24] Meyer et al. [22,27] studied on the use of bis-(triethoxysilyl propyl)tetra sulphide silanized onto the copper-doped mesoporous silica for capturing mercury from the vapour phase with the maximum fixed-bed equilibrium adsorption capacity of 19.789 μg/g. The ordered mesoporous silica grafted
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with 3-mercaptopropyl trimethoxysilane (MPTMS) was reported by Walcarius et al. [28] for the removal of Hg(II) from aqueous solution with an adsorption capacity of 200–400 mg/g was obtained. Several studies on sulphur-impregnated activated carbon for mercury adsorption in vapour phase have also been reported.[19,29–33] The sulphur-impregnated carbon can enhance the mercury removal efficiency over virgin carbon due to the formation of mercuric sulphide on the carbon surfaces. Otani et al.[34] reported the use of carbon disulphide (CS2 ) solution as a sulphurization agent to functionalize the activated carbon, active alumina, and zeolites which can result in an increase in the mercury vapour adsorption capacity of the adsorbents. The removal of mercury vapour from coal combustion flue gas by using activated carbon functionalized with H2 S was reported by Vitolo and Seggiani [33] and Morimoto et al.[35] The increase in the EM removal is due to the formation of HgS as a result of the reaction (adsorption) of Hg◦ with H2 S over the activated carbon surfaces. In the presence of partial oxidation reaction, H2 S may be converted into elemental sulphur (ES) and deposited on the activated carbon surfaces, hence increasing the mercury vapour adsorption capacity. In this study, the sulphur-functionalized SG adsorbents were synthesized and used as adsorbents for Hg◦ adsorption in vapour phase. The effect of various sulphur compounds functionalized SG adsorbents towards the EM (Hg◦ ) adsorption enhancement was studied. The choice of SGs for EM removal may offer better alternative to the existing adsorbents due to their good physical, chemical, and mechanical properties.
2.
(1) hydrolysis of TEOS; (2) gelation and ageing; and (3) drying. First, the homogeneous solution of ethanol, water, and aqueous ammonia (base catalyst) was prepared. Six millilitres of TEOS was added drop-wise into the ethanol/water/ammonia solution while being stirred at 600 rpm for 10 min, and the stirring was then continued for another 2 h. The hydrolysis and ageing of gels were carried out at 30◦ C and 50◦ C, respectively. After two days of ageing, the produced precipitate was filtered, washed using deionized water, and dried at ambient temperature (30 ± 1◦ C). The final product was then ground to produce a fine powder before it underwent characterization and adsorption studies. This SG sample was denoted as SG-TEOS. The same procedure was also carried out to synthesize SG using MPTMS as a precursor, and the synthesized product was denoted as SG-MPTMS. 2.2.2. Preparation of sulphur-impregnated SG adsorbents The solid–solid impregnation procedure used in this study was similar to the method previously discussed in the literature.[30,31,37] The ES was ground into fine powder and mixed thoroughly with SG, having a sulphur to SG ratio of 6:1. An inert atmosphere was created within the tube furnace by flowing the nitrogen gas for about 10min. The impregnation temperature was set at 200◦ C for 6 h, then raised to 600◦ C and held at that level for 3 h, and then left to cool to the room temperature (30 ± 10◦ C). This sample was denoted as SG-TEOS(ES). In the case of the wet impregnation process, the SGs were soaked with CS2 for 24 h, followed by evaporating the solvent in nitrogen gas flow.[34] This sample was denoted as SG-TEOS(CS2 ).
Experimental
2.1. Materials The following chemicals were used in the synthesis and functionalized procedures: tetraethyl orthosilicate (TEOS) (99%, Merck, Germany), MPTMS (ACROS, Belgium), and CS2 (Merck, Germany). These chemicals were used as received. Other chemicals such as ethanol [EtOH] (99%), ammonia solution [NH3 ] (25%), and ES (size particle < 40 μm) were obtained from Merck (Germany) and J.T. Baker (Europe). Deionized water used throughout the experiments was obtained from a Purite Water System model Select Analyst HP40 (UK) that is available in our laboratory. The gas mercury samples were generated by using Valco Instruments Co. Inc. (VICI) Metronics mercury permeation tube using N2 as a diluent gas. 2.2. Adsorbent preparation 2.2.1. Preparation of SG adsorbents The preparation of SG adsorbents using sol–gel process was carried out according to the modified method as stated by Stober et al.[36] It was prepared through three steps:
2.3.
Characterizations of adsorbents
The morphology of the synthesized SG adsorbents was analysed by using the scanning electron microscope (SEM) model JEOL JSM-6390LV. The surface area and pore volume of the adsorbents were determined by using the nitrogen adsorption/desorption method using Micromeritics ASAP 2000. The Fourier transform infra-red (FTIR) spectra of SG adsorbents were obtained using the FTIR spectrophotometer (model Perkin Elmer Model 2000) over 4000–400 cm−1 , using the KBr disc method to determine the existence of the functional groups on the adsorbent surfaces. 2.4. Elemental mercury (Hg◦ ) adsorption experiment A schematic diagram of the experimental set-up for Hg◦ adsorption is shown in Figure 1, which is similar to the one previously employed by Granite and co-workers to screen sorbents for mercury capture from nitrogen and flue gas.[38] Elemental mercury was generated from a mercury permeation device (VICI Metronics Inc., USA) and loaded in a
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Environmental Technology
Figure 1.
A schematic diagram of experimental set-up for EM (Hg◦ ) adsorption study.
U-tube, supported with glass beads, and was maintained at 50 ± 1.0◦ C in a water bath. Nitrogen (99.999%) was flowed through the permeation tube, yielded the constant EM concentration (200–240 μg/m3 ), which was controlled by the mass flow controller with a feed of 0.1 dm3 /min. The gas containing EM passed through the blank tube and then bypassed the stream to determine the baseline. After the steady-state concentration was achieved, the gas was diverted to adsorbent bed (AB) stream and the mercury concentration in downstream (after packed bed) was determined. The mercury concentration was detected using mercury analyser (JEROME, Arizona Instrument LLC) at selected time intervals. This process was allowed to proceed until the Hg concentration as measured by the mercury analyser reached about 90% exhaustion. The EM adsorption experiment was carried at bed temperatures of 50◦ C and 60◦ C. In practice, the bed temperature is similarly used in a number of gas-processing plants, for instances, 61◦ C at Salam gas plant,[39] 18◦ C at PTT GSP-5 gas plant, Rayong, Thailand, and 30–40◦ C at Enterprise Meeker, Colorado.[15] 3.
3
Results and discussion
3.1. Adsorbent characterizations In this study, the synthesis of SGs was prepared according to the Stöber method by using ammonia as a catalyst in basic condition.[36] With the presence of co-solvent (e.g. ethanol) in basic condition, the morphology of SGs tended to be spherical.[40] Figure 2(a) shows the SEM image of SG synthesized using TEOS as a precursor (i.e. denoted as SG-TEOS). This image clearly illustrates no significant variation in spherical particles sizes (0.27–0.29 μm). The SEM image of SG-MPTMS (Figure 2(b)) demonstrates a
fractured and irregular shape with particle sizes in the range of 1.37–3.95 μm. Figure 2(c) and (d) exhibits the morphology and particle sizes of sulphur-functionalized SGs, synthesized via wet and dry impregnation with CS2 and ES, respectively. The morphology of both adsorbents also did not significantly differ from the SG-TEOS (Figure 2(a)) after the introduction of sulphur by using impregnation methods. The FTIR spectra of the SG adsorbents are shown in Figure 3. The basic characteristics of synthesized SG-TEOS in the range of 4000–2800 cm−1 showed a large and broad band near 3400 cm−1 , which was attributed to the OH bond stretching from silanol group (Si–OH) and some adsorbed waters. In general, this strong peak, related to the hydrolysis and polycondensation of TEOS with water, could be described with the existence of hydroxyl group on the surface of silica. This band was often accompanied by another adsorption peak at 1634 cm−1 which belonged to the –OH vibrations of adsorbed water. The observed peaks around ∼1030–1200 and ∼800 cm−1 were attributed to Si–O–Si asymmetric stretching and the bands around ∼950 cm−1 belonged to Si–O vibrations, indicating the occurrence of silica matrices. These results are similar to those reported by previous researchers.[41,42] In the FTIR spectrum of SG-MPTMS, it was observed that the bands for the –CH2 and –CH3 appeared around ∼2929 cm−1 , and Si–CH2 vibration for the MPTMS was observed at 1411 cm−1 . In addition, the characteristic feature of S–H stretching band was observed around ∼2557 cm−1 , indicating that the SG adsorbent which had the S–H functional groups had been successfully synthesized by using MPTMS as a precursor. The FTIR spectra of the SG-TEOS(CS2 ) and SG-TEOS(ES) were similar to the synthesized SG-TEOS. Both spectra revealed that there
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Figure 2.
SEM images of SG adsorbents: (a) SG-TEOS; (b) SG-MPTMS; (c) SG-TEOS(CS2 ); and (d) SG-TEOS(ES).
Figure 3.
FTIR spectra of SG adsorbents.
were no additional sulphur-related bands observed which might be due to the total sulphur content in the SG, which was too little and did not chemically bind onto silica surfaces. In addition, it might have not been well dispersed on the silica surfaces; as a result, the sulphur-based functional groups could not be detected by the FTIR analysis. The total Brunauer, Emmett and Teller (BET) surface area and porosity of all adsorbents are summarized in Table 1. The surface area for SG-TEOS obtained was 9.6 m2 /g. The adsorbents exhibited the physical properties
of low surface area and large pore diameter. Without sulphur impregnation, the SG-TEOS had a higher surface area compared with SG-TEOS(ES) and SG-TEOS(CS2 ). Similar observation was reported by Li et al. [43] on the development of silica/vanadia/titania catalyst for the removal of EM. The pure SG had a higher surface area before inclusion and doping with TiO2 and V2 O5 . Hence, the present study found that the deposition of sulphur compounds onto the SG slightly reduced the surface area as evidenced by the SG-TEOS(ES) and SG-TEOS(CS2 ). This might be due
Environmental Technology Table 1.
5
Structural characteristics, Hg◦ adsorption capacity, and rate of adsorption of SG adsorbents.
Adsorbents
SG-TEOS
SG-MPTMS
Structural characteristics Morphology Spherical Spherical Particle size (μm) 0.27–0.29 1.47–2.11 9.60 18.30 SBET (m2 /g) 0.03 0.02 Vp (cm3 /g) 94.21 11.56 Rm (Å) Hg ◦ adsorption capacity and rate of adsorption 50 50 AB temperature (◦ C) Adsorption capacity (μg/g) 28.47 40.80 Rate of adsorption (ng/g min) 42.00 47.43
SG-TEOS(CS2 )
SG-TEOS(ES)
Spherical 0.18–0.33 5.05 0.126 20.73
Spherical 0.26–0.39 5.60 0.011 77.23
50 82.62 64.26
60 71.36 51.88
50 78.59 62.64
60 61.01 32.23
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Note: SBET , BET, Brunauer, Emmett and Teller surface area; Vp , BJH, Barrett-Joyner-Halenda, pore volume; Rm , average pore diameter.
Figure 4. (a) Elemental mercury (Hg◦ ) adsorption capacity of SG adsorbents and (b) adsorption rate of synthesized adsorbents. Experimental conditions: inlet Hg◦ concentration =∼200 μg/m3 and T = 50◦ C.
to the blockage of internal porosity by the incorporation of sulphur compounds (e.g. ES and CS2 ).[32,34] According to Hsi et al.,[32] sulphur molecules which are smaller than the adsorbent pore widths could penetrate into the adsorbent pores, and adsorb into the pore walls or be deposited on the outer surface of the adsorbents for the smaller pore width. Therefore, the reductions in surface area and pore volume were observed for SG-TEOS(ES) and SGTEOS(CS2 ) adsorbents. The use of MPTMS as a precursor produced the SG (SG-MPTMS) with the highest surface area (18.30 m2 /g), but had the lowest pore diameter than other adsorbents. 3.2. Elemental mercury (Hg◦ ) adsorption capacity The dimensionless adsorption breakthrough curves for the adsorbents are shown in Figure 4(a). The concentration (C) at given time (t) had been normalized using the inlet concentration (C0 ). In this study, a value of 90% breakthrough of the Hg inlet concentration was used. It was because the larger adsorbent loadings would adsorb greater amounts of Hg◦ and therefore it would require longer time to generate a complete breakthrough curve.[44] The total Hg◦ breakthrough adsorption capacities for all adsorbents are listed in Table 1. The adsorption capacity for each adsorbent was calculated by integrating the area
above the breakthrough curve. As shown in Figure 4(a), the adsorption capacity obtained for SG-TEOS was found to be 28.47 μg/g. This might be due to the contribution of large pore diameter while having a small surface area (9.60 m2 /g). Meanwhile, the large surface area and small pore size of SG-MPTMS gave higher adsorption capacity (40.80 μg/g) than SG-TEOS. This might be due to the existence of the thiol functional group in SG-MPTMS that had greater affinity towards EM. Vidic and Siler [45] reported that the thiol-impregnated carbon performed significantly better than the other chelating agent-impregnated carbon for vapour-phase EM adsorption. Generally, the adsorbent containing sulphur active sites should be capable of adsorbing more mercury compared with those samples containing no sulphur active sites. As can be seen from Figure 4(a), the breakthrough curve of SG-TEOS(ES) and SG-TEOS(CS2 ) adsorbents was larger than SG-TEOS, therefore the sulphur-impregnated SGs exhibited the enhancement in Hg◦ adsorption efficiency. As expected, the adsorption capacity of SG-TEOS(ES) was higher than of SG-TEOS. Due to higher impregnated temperature (e.g. 600◦ C), the sulphur was suspected to be distributed in the pore structure of SG. As reported by Liu et al. [37] and Feng et al.,[46] samples impregnated with ES at 250–600◦ C would result in larger EM adsorption capacity. They stated that, at impregnation temperature less than
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250◦ C, a major fraction of the ES would be in the forms of S6 –S8 , while about 75% of the ES would be in the forms of S2 –S6 at 600–650◦ C. The S2 and S6 were found more reactive than S8 due to the smaller chain, which could easily migrate into the narrower pores of adsorbent matrices. The strongly bonded sulphur to the adsorbents surface in the microporous region required the mercury to diffuse a longer distance for chemisorptions to occur. Among all the adsorbents, the highest adsorption capacity of 82.62 μg/g was obtained by the SG-TEOS(CS2 ), even though it had low surface area and pore volume compared with other adsorbents. Zhao et al. [47] reported a similar trend, where the adsorbent with poor fine pore distribution exhibited the most effective Hg◦ capture. Besides that, according to Jurng et al.,[48] the BET surface area cannot be directly related to mercury adsorption due to the difference in pore size distributions between the adsorbents. The potential use of sulphuring agent was reported by Mohan et al.,[49] in which an increase in Hg(II) uptake was observed for the carbon pre-soaked in the CS2 solution. Otani et al.[34] reported that adsorbents (active alumina, zeolite, and activated carbon) treated with CS2 solution can increase the adsorption capacity of mercury vapour. They found that the adsorbed mass of mercury increased with the increase in mass fraction of sulphur impregnated on activated carbon. Rate of Hg◦ adsorption The initial adsorption rates (i.e. up to 60 min) of all adsorbents are shown in Figure 4(b), which was obtained from the cumulative data of the Hg breakthrough curves. The rate of adsorption was determined from the slope of the graph, and the amount of mercury adsorbed at time t (mt ) was calculated using Equation (1) [44]:
Figure 5. Effect of temperatures on adsorption rate of SG adsorbents. Experimental conditions: inlet Hg◦ concentration = ∼ 200 μg/m3 .
chemisorption is prevalent at higher temperature (140◦ C). In their findings, the reduction of Hg◦ capture capacity for the heat-treated adsorbent was due to the decomposition at certain surface functional group of the adsorbent. In this study, the effect of bed temperature (50◦ C and 60◦ C) gave less significance in the Hg◦ sorption capacity reduction. We assumed that some moisture remained bonded on the adsorbent surface, which might have played an important role in the Hg◦ adsorption.[53]
3.3.
mt = mt−dt + (Cin − Cout )F,
(1)
where mt−dt (ng Hg/g adsorbent) is the amount of Hg adsorbed until time t (minute), Cin and Cout (μg/m3 ) are the concentrations of Hg inlet and outlet, respectively. The highest adsorption rate was 64.26 ng/g min obtained for SG–CS2 followed by SG-TEOS(ES) (62.64 ng/g min), SG-MPTMS (47.43 ng/g min), and SGTEOS (42.00 ng/g min). These results indicate that the existence of sulphur compound in the silica matrices resulted in higher initial Hg◦ adsorption rate. In order to elucidate the effect of temperature, the adsorption performances of both SG-TEOS(CS2 ) and SG-TEOS(ES) at 50◦ C and 60◦ C were studied. As can be seen in Figure 5, the Hg◦ adsorption capacity and adsorption rate at bed temperature of 50◦ C were higher than at 60◦ C due to the fact that the Hg◦ adsorption was an exothermic process, and thus increased the temperature, leading to unfavourable Hg◦ adsorption. Similar observations were also reported by several researchers.[50–52] As reported by Krishnan et al.,[52]
4. Conclusions The sulphur-functionalized SGs were successfully synthesized and used as adsorbents for the removal of EM in vapour phase. The SG-MPTMS gave better result of Hg◦ breakthrough adsorption capacity (40.80 μg/g) compared with SG-TEOS. The impregnation of SG-TEOS with CS2 and ES enhanced the Hg◦ breakthrough adsorption capacity of SG-TEOS, yielding from 28.47 to 82.62 μg/g and 78.59 μg/g, respectively. This indicated that the sulphur compounds had a greater affinity towards EM. The Hg◦ adsorption for all adsorbents was an exothermic process, and thus the Hg◦ adsorption performances deteriorated with the increase in AB temperatures. In this study, simple gas compositions were employed, consisting of mercury in nitrogen. To our knowledge, coal-derived flue gas is a complex mixture containing many acid gases, moisture, fly ash particles, and carbon monoxide. A typical untreated flue gas, which contains other gases such as O2 , H2 O, SO2 , SO3 , HCl, NO, and NO2 , [54,55] may give an impact upon the adsorbent, with regard to capture mercury, in which this could be a potential area for future study and research. Acknowledgement We would like to thank Mrs Samsina and Mr Zaimi at the Petronas Research and Scientific Services (PRSS) for their kind assistance in BET measurement.
Environmental Technology Funding The financial supports from MOSTI under the eScience Research Program [Project Vot. 79410]; MOHE under the Fundamental Research Grant Scheme [Project Vot. 78602]; the 2009 ExxonMobil Research Grant, ExxonMobil Exploration and Production Malaysia Inc. [Project Vot. 73339]; and the University Research Grant [Project Vot. GUP 00H63] are gratefully acknowledged.
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[19] Karatza D, Lancia A, Musmarra DD, Zucchini C. Study of mercury absorption and desorption on sulfur impregnated carbon. Exp Therm Fluid Sci. 2000;21:150–155. [20] Yang RT. Nanostructured adsorbents. Adv Chem Eng. 2001;27:77–124. [21] Wankat PC. Separation process engineering. 2nd ed. New York: Prentice Hall; 2007. [22] Meyer DE, Meeks N, Sikdar SK, Hutson ND, Hua D, Bhattacharyya D. Copper-doped silica materials silanized with bis-(triethoxy silyl propyl)-tetra sulfide for mercury vapor capture. Energy Fuels. 2008;22:2290–2298. [23] Khan A, Mahmood F, Khokhar MY, Ahmed S. Functionalized sol–gel material for extraction of mercury (II). React Funct Polym. 2006;66:1014–1020. [24] Perez-Quintanilla D, Del Hierro I, Carillo-Hermosilla F, Fajardo M, Sierra I. Adsorption of mercury ions by mercapto-functionalized amorphous silica. Anal Bioanal Chem. 2006;384:827–838. [25] El-Nahhal M, El-Ashgar NM. A review on polysiloxaneimmobilized ligand systems: synthesis, characterization and applications. J Organomet Chem. 2007;692: 2861–2886. [26] Alcantara EFC, Faria EA, Rodrigues DV, Evangelista SM, DeOliveira E, Zara LF, Rabelo D, Prado AGS. Modification of silica gel by attachment of 2-mercaptobenzimidazole for use in removing Hg(II) from aqueous media: a thermodynamic approach. J Colloid Interf Sci. 2007;311:1–7. [27] Meyer DE, Sikdar SK, Hutson ND, Bhattacharyya D. Examination of sulfur-functionalized, copper-doped iron particles for vapor phase mercury capture in entrained flow and fixed bed systems. Energy Fuels. 2007;21:2688–2697. [28] Walcarius A, Etienne M, Lebeau B. Rate of access to the binding sites in organically modified silicates. 2. Ordered mesoporous silicas grafted with amine or thiol groups. Chem Mater. 2003;15:2161–2173. [29] McLaughlin JB, Activated carbon adsorption for the removal of mercury from flue gas emissions [M.S. thesis]. Department of Civil and Environmental Engineering, School of Engineering. Pittsburgh (PA): University of Pittsburgh; 1995. [30] Korpiel JA, Vidic RD. Effect of sulfur impregnation method on activated carbon uptake of gas-phase mercury. Environ Sci Technol. 1997;31(8):2319–2325. [31] Liu W, Vidic RD, Brown TD. Optimization of high temperature sulfur impregnation on activated carbon for permanent sequestration of elemental mercury vapors. Environ Sci Technol. 2000;34:483–488. [32] Hsi HC, Rood MJ, Rostam-Abadi M, Chen S, Chang R. Mercury adsorption properties of sulfur-impregnated adsorbents. J Environ Eng. 2002;128(11):1080–1089. [33] Vitolo S, Seggiani M. Mercury removal from geothermal exhaust gas by sulfur impregnated and virgin activated carbons. Geothermics. 2002;31:431–442. [34] Otani Y, Emi H, Kanaoka C, Uchijima I, Nishino H. Removal of mercury vapor from air with sulfur-impregnated adsorbents. Environ Sci Technol. 1988;22:708–711. [35] Morimoto T, Wu S, Uddin MA, Sasaoka E. Characteristics of the mercury vapor removal from coal combustion flue gas by activated carbon using H2 S. Fuel. 2005;84: 1968–1974. [36] Stober W, Frink A, Bohn E. Controlled growth of monodisperse silica spheres in the micron size range. J Colloid Interf Sci. 1968;26:62–69. [37] Liu W, Vidic RD, Brown TD. Optimization of sulfur impregnation protocol for fixed-bed application of activated carbonbased sorbents for gas-phase mercury removal. Environ Sci Technol. 1998;32(4):531–538.
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[38] Granite EJ, Pennline HW, Hargis RA, Hargis A. Novel sorbents for mercury removal from flue gas. Ind Eng Chem Res. 2000;39(4):1020–1029. [39] El Ela MA, Mahgoub IS, Nabawi MH, Abdel Azim M. Mercury monitoring and removal at gas-processing facilities: case study of Salam gas plant. SPE Proj Fac Const. 2008;3(1):1–9. [40] Saman N, Johari K, Mat H. Effects of synthesis parameters towards morphological properties of silica xerogels. Asian J Appl Sci. 2012;5:247–251. [41] Manohar DM, Krishnan KA, Anirudhan TS. Removal of mercury (II) from aqueous solutions and chlor-alkali industry wastewater using 2-mercaptobenzimidazole-clay. Water Res. 2002;36:1609–1619. [42] Wang Z, Wu G, He C. Ion-imprinted thiol-functionalized silica gel sorbent for selective separation of mercury ions. Microchim Acta. 2009;165:151–157. [43] Li Y, Murphy PD, Wu CY, Powers KW, Bonzongo JCJ. Development of silica/vanadia/titania catalysts for removal of elemental mercury from coal-combustion flue gas. Environ Sci Technol. 2008;42:5304–5309. [44] Eswaran S, Stenger HG. Gas-phase mercury adsorption rate studies. Energy Fuel. 2007;21:852–857. [45] Vidic RD, Siler DP. Vapor-phase elemental mercury adsorption by activated carbon impregnated with chloride and chelating agents. Carbon. 2001;39:3–14. [46] Feng W, Borguet E, Vidic RD. Sulfurization of a carbon surface for vapor phase mercury removal-II: sulfur forms and mercury uptake. Carbon. 2006;44:2998–3004.
[47] Zhao P, Guo X, Zheng C. Removal of elemental mercury by iodine-modified rice husk ash sorbents. J Environ Sci. 2010;22(10):1629–1636. [48] Jurng J, Lee TG, Lee GW, Lee SJ, Kim BH, Seier J. Mercury removal from incineration flue gas by organic and inorganic adsorbents. Chemosphere. 2002;47: 907–913. [49] Mohan D, Gupta VK, Srivastava SK, Chander S. Kinetics of mercury adsorption from wastewater using activated carbon derived from fertilizer waste. Colloids Surf A: Physicochem Eng Aspects. 2001;177:169–181. [50] Li YH, Lee CW, Gullet BK. The effect of activated carbon surface moisture on low temperature mercury adsorption. Carbon. 2002;40:65–72. [51] Yan R, Liang DT, Tsen L, Wong YP, Lee YK. Bench scale experimental evaluation of carbon performance on mercury vapour adsorption. Fuel. 2004;83:2401–2409. [52] Krishnan SV, Gullett BK, Jorewlczt W. Sorption of elemental mercury by activated carbons. Environ Sci Technol. 1994;28:1506–1512. [53] Bacon R, Tang MM. Carbonization of cellulose fibers-II. Physical property study. Carbon. 1964;2:221–225. [54] Granite EJ, King WP, Stanko DC, Pennline HW. The implications of mercury interactions with band-gap semiconductor oxides. Main Group Chem. 2008;7(3): 227–237. [55] Granite EJ, Pennline HW. Photochemical removal of mercury from flue gas. Ind Eng Chem Res. 2002;41: 5470–5476.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Adsorption enhancement of elemental mercury onto sulphur-functionalized silica gel adsorbents a
a
Khairiraihanna Johari , Norasikin Saman & Hanapi Mat
ab
a
Advanced Materials and Process Engineering Laboratory, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia b
Novel Materials Research Group, Nanotechnology Research Alliance, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia Published online: 01 Oct 2013.
To cite this article: Khairiraihanna Johari, Norasikin Saman & Hanapi Mat , Environmental Technology (2013): Adsorption enhancement of elemental mercury onto sulphur-functionalized silica gel adsorbents, Environmental Technology, DOI: 10.1080/09593330.2013.840321 To link to this article: http://dx.doi.org/10.1080/09593330.2013.840321
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Environmental Technology, 2013 http://dx.doi.org/10.1080/09593330.2013.840321
Adsorption enhancement of elemental mercury onto sulphur-functionalized silica gel adsorbents Khairiraihanna Joharia , Norasikin Samana and Hanapi Mata,b∗ a Advanced
Materials and Process Engineering Laboratory, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia; b Novel Materials Research Group, Nanotechnology Research Alliance, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia
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(Received 11 March 2013; accepted 27 August 2013 ) In this study, elemental mercury (EM) adsorbents were synthesized using tetraethyl orthosilicate (TEOS) and 3mercaptopropyl trimethoxysilane as silica precursors. The synthesized silica gel (SG)-TEOS was further functionalized through impregnation with elemental sulphur and carbon disulphide (CS2 ). The SG adsorbents were then characterized by using scanning electron microscope, Fourier transform infra-red spectrophotometer, nitrogen adsorption/desorption, and energy-dispersive X-ray diffractometer. The EM adsorption of the SG adsorbents was determined using fabricated fixed-bed adsorber. The EM adsorption results showed that the sulphur-functionalized SG adsorbents had a greater Hg◦ breakthrough adsorption capacity, confirming that the presence of sulphur in silica matrices can improve Hg◦ adsorption performance due to their high affinity towards mercury. The highest Hg◦ adsorption capacity was observed for SG-TEOS(CS2 ) (82.62 μg/g), which was approximately 2.9 times higher than SG-TEOS (28.47 μg/g). The rate of Hg◦ adsorption was observed higher for sulphur-impregnated adsorbents, and decreased with the increase in the bed temperatures. Keywords: mercury; silica gel; organosilane; sulphur; adsorption
1. Introduction Mercury is a toxic substance that presents a greatest hazard to human health as well as to environment, even at low concentration. It can accumulate in animals and plants. It can also enter into human body through food cycle,[1] which can cause damage to the central nerve systems.[2] Major anthropogenic sources of mercury emissions into atmosphere are combustion activities such as from the coalfired power plants and incinerators.[3–5] Other sources of mercury in environment include petroleum exploration and processing,[6–8] volcanoes,[9] forest fires,[10,11] and cinnabar (ore).[12] Generally, mercury exists in four different forms, namely elemental (Hg◦ ), monovalent mercury (Hg(I)), divalent mercury (Hg(II)), and monomethyl mercury ((CH3 )Hg),[7] which need to be removed even at low concentrations due to their high toxicity. However, in the case of elemental mercury (EM) (Hg◦ ), it is difficult to remove due to its insolubility in water and cannot be readily captured simply in other air pollution control equipment. Several approaches have been used to reduce mercury emissions from various waste streams, such as adsorption, ion exchange, chemical precipitation, membrane filtration, and photo-reduction.[13–15] Among the conventional chemical and physical methods, adsorption has been widely adopted to reduce mercury emissions using a variety of adsorbent materials.[16] ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
At present, a variety of materials have been used commercially as adsorbents for capturing mercury in liquid and gas streams, which include activated carbon, carbon molecular sieves, zeolites, silica gel (SG), polymer resins, and activated alumina [6,7,17–21] which can act directly as adsorbents or be first modified with reactants/ligands to enhance their sorption capacity. Meyer et al. [22] reported that capturing Hg◦ from the gas phase is more challenging because the mechanism of capture is more complex compared with the simple Hg(II) encountered in the liquid phase. The functionalization of organic and inorganic solid supports with functional groups to enhance adsorbate adsorption capacity and selectivity has been reported in a number of specific applications.[23] The inorganic solid supports, especially SG, are the most widely used solid substrates due to their advantages of having good mechanical strength, do not swell, and can sustain the high temperatures.[24,25] Structurally, they are stable and rigid. The silanol groups on the silica surfaces can serve as point of attachment of chelating agents [26] and can be chemically modified with functional compounds.[24] Meyer et al. [22,27] studied on the use of bis-(triethoxysilyl propyl)tetra sulphide silanized onto the copper-doped mesoporous silica for capturing mercury from the vapour phase with the maximum fixed-bed equilibrium adsorption capacity of 19.789 μg/g. The ordered mesoporous silica grafted
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with 3-mercaptopropyl trimethoxysilane (MPTMS) was reported by Walcarius et al. [28] for the removal of Hg(II) from aqueous solution with an adsorption capacity of 200–400 mg/g was obtained. Several studies on sulphur-impregnated activated carbon for mercury adsorption in vapour phase have also been reported.[19,29–33] The sulphur-impregnated carbon can enhance the mercury removal efficiency over virgin carbon due to the formation of mercuric sulphide on the carbon surfaces. Otani et al.[34] reported the use of carbon disulphide (CS2 ) solution as a sulphurization agent to functionalize the activated carbon, active alumina, and zeolites which can result in an increase in the mercury vapour adsorption capacity of the adsorbents. The removal of mercury vapour from coal combustion flue gas by using activated carbon functionalized with H2 S was reported by Vitolo and Seggiani [33] and Morimoto et al.[35] The increase in the EM removal is due to the formation of HgS as a result of the reaction (adsorption) of Hg◦ with H2 S over the activated carbon surfaces. In the presence of partial oxidation reaction, H2 S may be converted into elemental sulphur (ES) and deposited on the activated carbon surfaces, hence increasing the mercury vapour adsorption capacity. In this study, the sulphur-functionalized SG adsorbents were synthesized and used as adsorbents for Hg◦ adsorption in vapour phase. The effect of various sulphur compounds functionalized SG adsorbents towards the EM (Hg◦ ) adsorption enhancement was studied. The choice of SGs for EM removal may offer better alternative to the existing adsorbents due to their good physical, chemical, and mechanical properties.
2.
(1) hydrolysis of TEOS; (2) gelation and ageing; and (3) drying. First, the homogeneous solution of ethanol, water, and aqueous ammonia (base catalyst) was prepared. Six millilitres of TEOS was added drop-wise into the ethanol/water/ammonia solution while being stirred at 600 rpm for 10 min, and the stirring was then continued for another 2 h. The hydrolysis and ageing of gels were carried out at 30◦ C and 50◦ C, respectively. After two days of ageing, the produced precipitate was filtered, washed using deionized water, and dried at ambient temperature (30 ± 1◦ C). The final product was then ground to produce a fine powder before it underwent characterization and adsorption studies. This SG sample was denoted as SG-TEOS. The same procedure was also carried out to synthesize SG using MPTMS as a precursor, and the synthesized product was denoted as SG-MPTMS. 2.2.2. Preparation of sulphur-impregnated SG adsorbents The solid–solid impregnation procedure used in this study was similar to the method previously discussed in the literature.[30,31,37] The ES was ground into fine powder and mixed thoroughly with SG, having a sulphur to SG ratio of 6:1. An inert atmosphere was created within the tube furnace by flowing the nitrogen gas for about 10min. The impregnation temperature was set at 200◦ C for 6 h, then raised to 600◦ C and held at that level for 3 h, and then left to cool to the room temperature (30 ± 10◦ C). This sample was denoted as SG-TEOS(ES). In the case of the wet impregnation process, the SGs were soaked with CS2 for 24 h, followed by evaporating the solvent in nitrogen gas flow.[34] This sample was denoted as SG-TEOS(CS2 ).
Experimental
2.1. Materials The following chemicals were used in the synthesis and functionalized procedures: tetraethyl orthosilicate (TEOS) (99%, Merck, Germany), MPTMS (ACROS, Belgium), and CS2 (Merck, Germany). These chemicals were used as received. Other chemicals such as ethanol [EtOH] (99%), ammonia solution [NH3 ] (25%), and ES (size particle < 40 μm) were obtained from Merck (Germany) and J.T. Baker (Europe). Deionized water used throughout the experiments was obtained from a Purite Water System model Select Analyst HP40 (UK) that is available in our laboratory. The gas mercury samples were generated by using Valco Instruments Co. Inc. (VICI) Metronics mercury permeation tube using N2 as a diluent gas. 2.2. Adsorbent preparation 2.2.1. Preparation of SG adsorbents The preparation of SG adsorbents using sol–gel process was carried out according to the modified method as stated by Stober et al.[36] It was prepared through three steps:
2.3.
Characterizations of adsorbents
The morphology of the synthesized SG adsorbents was analysed by using the scanning electron microscope (SEM) model JEOL JSM-6390LV. The surface area and pore volume of the adsorbents were determined by using the nitrogen adsorption/desorption method using Micromeritics ASAP 2000. The Fourier transform infra-red (FTIR) spectra of SG adsorbents were obtained using the FTIR spectrophotometer (model Perkin Elmer Model 2000) over 4000–400 cm−1 , using the KBr disc method to determine the existence of the functional groups on the adsorbent surfaces. 2.4. Elemental mercury (Hg◦ ) adsorption experiment A schematic diagram of the experimental set-up for Hg◦ adsorption is shown in Figure 1, which is similar to the one previously employed by Granite and co-workers to screen sorbents for mercury capture from nitrogen and flue gas.[38] Elemental mercury was generated from a mercury permeation device (VICI Metronics Inc., USA) and loaded in a
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Environmental Technology
Figure 1.
A schematic diagram of experimental set-up for EM (Hg◦ ) adsorption study.
U-tube, supported with glass beads, and was maintained at 50 ± 1.0◦ C in a water bath. Nitrogen (99.999%) was flowed through the permeation tube, yielded the constant EM concentration (200–240 μg/m3 ), which was controlled by the mass flow controller with a feed of 0.1 dm3 /min. The gas containing EM passed through the blank tube and then bypassed the stream to determine the baseline. After the steady-state concentration was achieved, the gas was diverted to adsorbent bed (AB) stream and the mercury concentration in downstream (after packed bed) was determined. The mercury concentration was detected using mercury analyser (JEROME, Arizona Instrument LLC) at selected time intervals. This process was allowed to proceed until the Hg concentration as measured by the mercury analyser reached about 90% exhaustion. The EM adsorption experiment was carried at bed temperatures of 50◦ C and 60◦ C. In practice, the bed temperature is similarly used in a number of gas-processing plants, for instances, 61◦ C at Salam gas plant,[39] 18◦ C at PTT GSP-5 gas plant, Rayong, Thailand, and 30–40◦ C at Enterprise Meeker, Colorado.[15] 3.
3
Results and discussion
3.1. Adsorbent characterizations In this study, the synthesis of SGs was prepared according to the Stöber method by using ammonia as a catalyst in basic condition.[36] With the presence of co-solvent (e.g. ethanol) in basic condition, the morphology of SGs tended to be spherical.[40] Figure 2(a) shows the SEM image of SG synthesized using TEOS as a precursor (i.e. denoted as SG-TEOS). This image clearly illustrates no significant variation in spherical particles sizes (0.27–0.29 μm). The SEM image of SG-MPTMS (Figure 2(b)) demonstrates a
fractured and irregular shape with particle sizes in the range of 1.37–3.95 μm. Figure 2(c) and (d) exhibits the morphology and particle sizes of sulphur-functionalized SGs, synthesized via wet and dry impregnation with CS2 and ES, respectively. The morphology of both adsorbents also did not significantly differ from the SG-TEOS (Figure 2(a)) after the introduction of sulphur by using impregnation methods. The FTIR spectra of the SG adsorbents are shown in Figure 3. The basic characteristics of synthesized SG-TEOS in the range of 4000–2800 cm−1 showed a large and broad band near 3400 cm−1 , which was attributed to the OH bond stretching from silanol group (Si–OH) and some adsorbed waters. In general, this strong peak, related to the hydrolysis and polycondensation of TEOS with water, could be described with the existence of hydroxyl group on the surface of silica. This band was often accompanied by another adsorption peak at 1634 cm−1 which belonged to the –OH vibrations of adsorbed water. The observed peaks around ∼1030–1200 and ∼800 cm−1 were attributed to Si–O–Si asymmetric stretching and the bands around ∼950 cm−1 belonged to Si–O vibrations, indicating the occurrence of silica matrices. These results are similar to those reported by previous researchers.[41,42] In the FTIR spectrum of SG-MPTMS, it was observed that the bands for the –CH2 and –CH3 appeared around ∼2929 cm−1 , and Si–CH2 vibration for the MPTMS was observed at 1411 cm−1 . In addition, the characteristic feature of S–H stretching band was observed around ∼2557 cm−1 , indicating that the SG adsorbent which had the S–H functional groups had been successfully synthesized by using MPTMS as a precursor. The FTIR spectra of the SG-TEOS(CS2 ) and SG-TEOS(ES) were similar to the synthesized SG-TEOS. Both spectra revealed that there
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Figure 2.
SEM images of SG adsorbents: (a) SG-TEOS; (b) SG-MPTMS; (c) SG-TEOS(CS2 ); and (d) SG-TEOS(ES).
Figure 3.
FTIR spectra of SG adsorbents.
were no additional sulphur-related bands observed which might be due to the total sulphur content in the SG, which was too little and did not chemically bind onto silica surfaces. In addition, it might have not been well dispersed on the silica surfaces; as a result, the sulphur-based functional groups could not be detected by the FTIR analysis. The total Brunauer, Emmett and Teller (BET) surface area and porosity of all adsorbents are summarized in Table 1. The surface area for SG-TEOS obtained was 9.6 m2 /g. The adsorbents exhibited the physical properties
of low surface area and large pore diameter. Without sulphur impregnation, the SG-TEOS had a higher surface area compared with SG-TEOS(ES) and SG-TEOS(CS2 ). Similar observation was reported by Li et al. [43] on the development of silica/vanadia/titania catalyst for the removal of EM. The pure SG had a higher surface area before inclusion and doping with TiO2 and V2 O5 . Hence, the present study found that the deposition of sulphur compounds onto the SG slightly reduced the surface area as evidenced by the SG-TEOS(ES) and SG-TEOS(CS2 ). This might be due
Environmental Technology Table 1.
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Structural characteristics, Hg◦ adsorption capacity, and rate of adsorption of SG adsorbents.
Adsorbents
SG-TEOS
SG-MPTMS
Structural characteristics Morphology Spherical Spherical Particle size (μm) 0.27–0.29 1.47–2.11 9.60 18.30 SBET (m2 /g) 0.03 0.02 Vp (cm3 /g) 94.21 11.56 Rm (Å) Hg ◦ adsorption capacity and rate of adsorption 50 50 AB temperature (◦ C) Adsorption capacity (μg/g) 28.47 40.80 Rate of adsorption (ng/g min) 42.00 47.43
SG-TEOS(CS2 )
SG-TEOS(ES)
Spherical 0.18–0.33 5.05 0.126 20.73
Spherical 0.26–0.39 5.60 0.011 77.23
50 82.62 64.26
60 71.36 51.88
50 78.59 62.64
60 61.01 32.23
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Note: SBET , BET, Brunauer, Emmett and Teller surface area; Vp , BJH, Barrett-Joyner-Halenda, pore volume; Rm , average pore diameter.
Figure 4. (a) Elemental mercury (Hg◦ ) adsorption capacity of SG adsorbents and (b) adsorption rate of synthesized adsorbents. Experimental conditions: inlet Hg◦ concentration =∼200 μg/m3 and T = 50◦ C.
to the blockage of internal porosity by the incorporation of sulphur compounds (e.g. ES and CS2 ).[32,34] According to Hsi et al.,[32] sulphur molecules which are smaller than the adsorbent pore widths could penetrate into the adsorbent pores, and adsorb into the pore walls or be deposited on the outer surface of the adsorbents for the smaller pore width. Therefore, the reductions in surface area and pore volume were observed for SG-TEOS(ES) and SGTEOS(CS2 ) adsorbents. The use of MPTMS as a precursor produced the SG (SG-MPTMS) with the highest surface area (18.30 m2 /g), but had the lowest pore diameter than other adsorbents. 3.2. Elemental mercury (Hg◦ ) adsorption capacity The dimensionless adsorption breakthrough curves for the adsorbents are shown in Figure 4(a). The concentration (C) at given time (t) had been normalized using the inlet concentration (C0 ). In this study, a value of 90% breakthrough of the Hg inlet concentration was used. It was because the larger adsorbent loadings would adsorb greater amounts of Hg◦ and therefore it would require longer time to generate a complete breakthrough curve.[44] The total Hg◦ breakthrough adsorption capacities for all adsorbents are listed in Table 1. The adsorption capacity for each adsorbent was calculated by integrating the area
above the breakthrough curve. As shown in Figure 4(a), the adsorption capacity obtained for SG-TEOS was found to be 28.47 μg/g. This might be due to the contribution of large pore diameter while having a small surface area (9.60 m2 /g). Meanwhile, the large surface area and small pore size of SG-MPTMS gave higher adsorption capacity (40.80 μg/g) than SG-TEOS. This might be due to the existence of the thiol functional group in SG-MPTMS that had greater affinity towards EM. Vidic and Siler [45] reported that the thiol-impregnated carbon performed significantly better than the other chelating agent-impregnated carbon for vapour-phase EM adsorption. Generally, the adsorbent containing sulphur active sites should be capable of adsorbing more mercury compared with those samples containing no sulphur active sites. As can be seen from Figure 4(a), the breakthrough curve of SG-TEOS(ES) and SG-TEOS(CS2 ) adsorbents was larger than SG-TEOS, therefore the sulphur-impregnated SGs exhibited the enhancement in Hg◦ adsorption efficiency. As expected, the adsorption capacity of SG-TEOS(ES) was higher than of SG-TEOS. Due to higher impregnated temperature (e.g. 600◦ C), the sulphur was suspected to be distributed in the pore structure of SG. As reported by Liu et al. [37] and Feng et al.,[46] samples impregnated with ES at 250–600◦ C would result in larger EM adsorption capacity. They stated that, at impregnation temperature less than
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250◦ C, a major fraction of the ES would be in the forms of S6 –S8 , while about 75% of the ES would be in the forms of S2 –S6 at 600–650◦ C. The S2 and S6 were found more reactive than S8 due to the smaller chain, which could easily migrate into the narrower pores of adsorbent matrices. The strongly bonded sulphur to the adsorbents surface in the microporous region required the mercury to diffuse a longer distance for chemisorptions to occur. Among all the adsorbents, the highest adsorption capacity of 82.62 μg/g was obtained by the SG-TEOS(CS2 ), even though it had low surface area and pore volume compared with other adsorbents. Zhao et al. [47] reported a similar trend, where the adsorbent with poor fine pore distribution exhibited the most effective Hg◦ capture. Besides that, according to Jurng et al.,[48] the BET surface area cannot be directly related to mercury adsorption due to the difference in pore size distributions between the adsorbents. The potential use of sulphuring agent was reported by Mohan et al.,[49] in which an increase in Hg(II) uptake was observed for the carbon pre-soaked in the CS2 solution. Otani et al.[34] reported that adsorbents (active alumina, zeolite, and activated carbon) treated with CS2 solution can increase the adsorption capacity of mercury vapour. They found that the adsorbed mass of mercury increased with the increase in mass fraction of sulphur impregnated on activated carbon. Rate of Hg◦ adsorption The initial adsorption rates (i.e. up to 60 min) of all adsorbents are shown in Figure 4(b), which was obtained from the cumulative data of the Hg breakthrough curves. The rate of adsorption was determined from the slope of the graph, and the amount of mercury adsorbed at time t (mt ) was calculated using Equation (1) [44]:
Figure 5. Effect of temperatures on adsorption rate of SG adsorbents. Experimental conditions: inlet Hg◦ concentration = ∼ 200 μg/m3 .
chemisorption is prevalent at higher temperature (140◦ C). In their findings, the reduction of Hg◦ capture capacity for the heat-treated adsorbent was due to the decomposition at certain surface functional group of the adsorbent. In this study, the effect of bed temperature (50◦ C and 60◦ C) gave less significance in the Hg◦ sorption capacity reduction. We assumed that some moisture remained bonded on the adsorbent surface, which might have played an important role in the Hg◦ adsorption.[53]
3.3.
mt = mt−dt + (Cin − Cout )F,
(1)
where mt−dt (ng Hg/g adsorbent) is the amount of Hg adsorbed until time t (minute), Cin and Cout (μg/m3 ) are the concentrations of Hg inlet and outlet, respectively. The highest adsorption rate was 64.26 ng/g min obtained for SG–CS2 followed by SG-TEOS(ES) (62.64 ng/g min), SG-MPTMS (47.43 ng/g min), and SGTEOS (42.00 ng/g min). These results indicate that the existence of sulphur compound in the silica matrices resulted in higher initial Hg◦ adsorption rate. In order to elucidate the effect of temperature, the adsorption performances of both SG-TEOS(CS2 ) and SG-TEOS(ES) at 50◦ C and 60◦ C were studied. As can be seen in Figure 5, the Hg◦ adsorption capacity and adsorption rate at bed temperature of 50◦ C were higher than at 60◦ C due to the fact that the Hg◦ adsorption was an exothermic process, and thus increased the temperature, leading to unfavourable Hg◦ adsorption. Similar observations were also reported by several researchers.[50–52] As reported by Krishnan et al.,[52]
4. Conclusions The sulphur-functionalized SGs were successfully synthesized and used as adsorbents for the removal of EM in vapour phase. The SG-MPTMS gave better result of Hg◦ breakthrough adsorption capacity (40.80 μg/g) compared with SG-TEOS. The impregnation of SG-TEOS with CS2 and ES enhanced the Hg◦ breakthrough adsorption capacity of SG-TEOS, yielding from 28.47 to 82.62 μg/g and 78.59 μg/g, respectively. This indicated that the sulphur compounds had a greater affinity towards EM. The Hg◦ adsorption for all adsorbents was an exothermic process, and thus the Hg◦ adsorption performances deteriorated with the increase in AB temperatures. In this study, simple gas compositions were employed, consisting of mercury in nitrogen. To our knowledge, coal-derived flue gas is a complex mixture containing many acid gases, moisture, fly ash particles, and carbon monoxide. A typical untreated flue gas, which contains other gases such as O2 , H2 O, SO2 , SO3 , HCl, NO, and NO2 , [54,55] may give an impact upon the adsorbent, with regard to capture mercury, in which this could be a potential area for future study and research. Acknowledgement We would like to thank Mrs Samsina and Mr Zaimi at the Petronas Research and Scientific Services (PRSS) for their kind assistance in BET measurement.
Environmental Technology Funding The financial supports from MOSTI under the eScience Research Program [Project Vot. 79410]; MOHE under the Fundamental Research Grant Scheme [Project Vot. 78602]; the 2009 ExxonMobil Research Grant, ExxonMobil Exploration and Production Malaysia Inc. [Project Vot. 73339]; and the University Research Grant [Project Vot. GUP 00H63] are gratefully acknowledged.
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