Preparation, Characterization, and Application of Modified Chitosan

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Ind. Eng. Chem. Res. 2009, 48, 4980–4989

Preparation, Characterization, and Application of Modified Chitosan Sorbents for Elemental Mercury Removal Anchao Zhang,† Jun Xiang,*,† Lushi Sun,† Song Hu,† Peisheng Li,‡ Jinming Shi,† Peng Fu,† and Sheng Su† State Key Laboratory of Coal Combustion, Huazhong UniVersity of Science and Technology, Wuhan 430074, China, and School of Power and Mechanical Engineering, Wuhan UniVersity, Wuhan 430074, China

A series of raw, iodine (bromide) or/and sulfuric acid-modified chitosan sorbents were synthesized and comprehensively characterized by N2 isotherm adsorption/desorption method, TGA, FTIR, XRD, and XPS et al. Adsorption experiments of vapor-phase elemental mercury (Hg0) were studied using the sorbents in a laboratory-scale fixed-bed reactor. The results revealed that porosities and specific surface areas of the sorbents decreased after modification. The sorbents operated stably at flue-gas temperature below 140 °C. The chemical reactions of iodine and sulfate ion with the amide of chitosan occurred, and the I2 was found in the sorbents due to the presence of H2SO4. Fixed-bed adsorber tests showed that compared to raw chitosan, the bromide or iodine-modified chitosan could promote the efficiency of Hg0 capture more or less. Mercury removal efficiency could be significantly promoted when an appropriate content of H2SO4 was added, and the iodine and H2SO4 modified sorbents almost had a mercury removal efficiency of 100% for 3 h. The presence of moisture can increase the sorbent’s capacity for mercury uptake due to the existence of active sites, such as sulfonate and amino group. The mercury breakthrough of modified chitosan sorbents decreased with increasing temperature. A reaction scheme that could explain the experimental results was presumed based on the characterizations and adsorption study. 1. Introduction Coal-fired utility boilers and municipal waste incineration plants are major anthropogenic sources of mercury emissions.1–4 Mercury released into the environmental atmosphere can precipitate into lakes, rivers, and estuaries and can be converted through biological processes into an organic form, methyl mercury, which is a neurotoxin that bioaccumulates in fish, animals, and mammals.5 It is well-known that mercury has adverse effects on the central nervous system and causes pulmonary and renal failure, severe respiratory damage, blindness, and chromosome damage.6 On March 15th, 2005, the US Environmental Protection Agency (EPA) proposed to permanently cap and reduce mercury emissions from power plants and when fully implemented in 2018, mercury emissions will be reduced by 69%.7,8 In recent decades, many countries, especially in the developed countries, have taken steps to reduce mercury uses and releases and to protect their citizens from exposure to this toxic heavy metal. Mercury in coal is emitted in an elemental form of mercury (Hg0) through coal combustion, and then a portion of the Hg0 is transformed to oxidized forms (Hg2+X (g)) and particle-bound atoms (Hg (p)).9,10 Most of existing air pollution control technologies, such as electrostatic precipitators and baghouses, can remove effectively the Hg (p), but have a little effect on the Hg0 removal. Additionally, the oxidized forms of mercury, such as HgCl2 and HgO, can be captured effectively in wet scrubbers, since the (Hg2+X (g)) species are generally water-soluble.11 Nevertheless, it is very difficult to remove elemental mercury (Hg0) from the flue gas stream due to its low melting point, high equilibrium pressure, and low solubility in water.5,10,12 * To whom correspondence should be addressed. E-mail: xiangj.hust@ gmail.com. Tel.: 86-27-87542417-8206. Fax: 86-27-87545526. † Huazhong University of Science and Technology. ‡ Wuhan University.

Using solid materials is the most common form of mercury removal from flue gases.13 For example, activated carbons (ACs) are considered as “general purpose” material.9,14,15 The extended surface area and high surface reactivity render them suitable for Hg0 adsorption, but the capacity of raw AC is limited, especially at high temperatures.16For this reason, many investigations about the chemically promoted carbonaceous sorbents have been conducted to evaluate their behaviors on mercury removal.13,17–22 Among all these chemically promoted AC sorbents, the halogenated AC, such as chlorine impregnation,17–19 iodine impregnation,2,19,20 bromide impregnation,21 and sulfur impregnation,22–24 exhibited excellent performance in Hg0 capture, especially the iodine-promoted AC which can achieve 4.8 mg/g mercury absorption capacity.2 However, the use of the carbonaceous sorbent may adversely affect the sales or disposal costs of the captured fly ash.25 Noncarbon sorbents and/or their chemically modified sorbents such as zeolite,4,26 montmorillonite clay,18 sepiolite,27 calcium-based sorbents,28 coal-burned or oilfired fly ash,3,29 and metal oxide,2 were widely evaluated recently. Although some of them display rather higher mercury removal efficiency, for example, sulfurized sepiolite and some modified metal oxides, others are less effective on mercury removal. To achieve the high removal efficiencies required, adsorption-based technologies seem promising.27 In this regard, there is still a strong desire to develop efficient and cost-effective noncarbon sorbents in the field of mercury capture.24 Recently, natural chitin (CT), chitosan (CTS), and their derivatives have been identified as attractive materials due to their unique structure, distinctive properties, safety, and biodegradability.30–36 These polymers have been used as suitable adsorbents for the collection of metal ions, such as Pd2+, Hg2+, Cu2+, As3+, and Cr2+, in aqueous phase.30,35 However, limited reports have been found in the literature on vapor-phase mercury (Hg0) removal using the adsorbent based on chitin, chitosan, and their derivatives. In addition, surface modification has become a popular method for providing a material with desirable

10.1021/ie9000629 CCC: $40.75  2009 American Chemical Society Published on Web 03/30/2009

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properties for practical applications, and many methods of surface modification have been developed, such as copper-doped silica materials silanized with bis-(triethoxy silyl propyl)-tetra sulfide,12 thiol-modified inorganic oxide material,37 and thiolimpregnated carbon.6 All these modified sorbents exhibit higher capacities for Hg0 or Hg2+ capture. The deacetylated amino groups in chitosan can be modified easily,33,35 therefore, the certain functional groups are also expected to have a contribution to the attraction interactions with some special substances, such as vapor phase mercury (Hg0). Literature found that chitosan also demonstrates fairly good adsorption capacities of acid such as hydrochloric acid (HCl), sulfuric acid (H2SO4), and nitric acid (HNO3).31 Moreover, chitosan has significant iodine uptake capacity in the aqueous solution or in a polar organic solvent of iodine (I2) and potassium iodide (KI). Likewise, CTS also display quite good uptake capacity for bromide in the manner with iodine.31 In the case of Hg0 removal, carbon-based and noncarbon sorbents modified by iodine2,19 or sulfuric acid have been studied in detail.1 These studies reported that the capacity of mercury removal can be greatly improved after modification. Due to this feature of chitosan, we attempt to find out whether the chitosan modified by iodine (bromide) or their salt compounds have impact on Hg0 capture. To investigate the performance of modified chitosan in capturing elemental mercury, in this study, a series of raw, iodine (bromide) and/or sulfuric acid-modified sorbents of chitosan were synthesized and the detailed characterization data were reported. The adsorption experiments of vapor-phase elemental mercury (Hg0) were studied using these sorbents in a laboratoryscale fixed-bed reactor. Specifically, the effects of temperature, moisture, and inlet mercury concentration were investigated, and the mechanism of an excellent sorbent for mercury adsorption was also discussed. Although the cost and the actual application of the novel sorbents is not proven yet, compared with the traditional methods, activated carbons sorbents and some material sorbents, it brings us with the new methods and considerations for the increasing pressure of the worldwide environmental pollution. 2. Methodology 2.1. Materials. Chitosan was purchased from the Jinan Haidebei Marine Bioengineering Co. Ltd., China as powdered materials, with a deacetylation degree of 85%. Starch (food grade) was purchased from Wuhan Jinbao Food Co. Ltd., China. Potassium iodide (KI), potassium bromide (KBr), sulfuric acid (H2SO4), and anhydrous ethanol used in this study were all analytical grade and all solutions were prepared with deionized (DI) water. 2.2. Synthesis of Samples. A 4 g portion of CTS was added to a 50 mL bottle containing 30 mL of DI water and 10 mL of anhydrous ethanol under a vigorous stirring condition for 1 h. Then 1 g of potassium bromide was added slowly under constant stirring for 4 h. The bottle was then stored for 16 h at room temperature and shaken periodically during the impregnation time. After soaking, the resultant solid was vacuum filtered, dried at 80 °C in an air environment for 48 h, and named bromide-impregnated CTS (in short, CTS-Br-41, here the number 41 means the mass ratio of chitosan to potassium bromide). The iodide-impregnated CTS sorbent was prepared in the same manner, and named CTS-I-41. In the case of bromide (iodide)-impregnated chitosan containing sulfuric acid, three portions of 4 g chitosan were added to 50 mL bottles containing 40 mL of DI water, 1 g of KBr (KI)

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and three different mass (0.5, 1, and 2 g) of sulfuric acid, respectively. The bottles were then stored for 24 h at room temperature and shaken periodically during the impregnation process. After soaking, the resultant solids were vacuum filtered and then dried at 80 °C in an air environment for 48 h, and the obtained solids were called CTS-Br-S-410, CTS-I-S-410, CTSBr-S-411, CTS-I-S-411, CTS-Br-S-412, and CTS-I-S-412, respectively (Here, S means that H2SO4 was added in the preparation of the samples; 410, 411, and 412 signify the proportion of CTS, modifier, and sulfuric acid based on mass). Similarly, different iodine loading sorbents were also prepared in the same manner as described above, and these samples are named as CTS-S-41, CTS-S-42, CTS-I-S-612, CTS-I-S-812, CTS-I-S-1612 (it containing 16 g CTS), respectively. All samples were ground and sieved to 150 mesh size. 2.3. Characterization. BET Measurement by N2 Sorption. Textural characteristics of the samples were determined by nitrogen (N2) adsorption-desorption method at 77 K on an accelerated surface area and porosimeter (ASAP 2020, Micromeritics Instrument Corporation, USA). The adsorption isotherm was used to calculate the Brunauer-Emmer-Teller (BET) surface area and pore volume. The average pore diameter was calculated from four times of the pore volume over the BET surface area. Elemental Analysis. Elemental analysis was used to estimate the density of the active sites in the sorbent. Carbon, hydrogen, nitrogen, and sulfur wt % (absolute weight) were performed by an elemental analyzer (EL-2, Vario Inc., Germany). To find the quantity of iodine or bromide in the sorbents and their effects on mercury removal, the relative contents of iodine and bromide rwt % (relative weight) in the sorbents were also determined by X-ray fluorescence (XRF) technique using the fluorimeter (EAGLE III, EDAX Inc., USA). Thermal Analysis. The thermal stability of the sample was evaluated using thermal gravimetric analysis (TGA) (STA-409, NETZSCH Corp., Germany). The mass of the sample was about 25 mg and the atmosphere was under nitrogen flow (50 mL/ min). The temperature history involved heating the sample at 8 °C/min from 15 to 100 °C, holding it for 30 min at this temperature, heating further at 8 °C/min to 350 °C.24,40 Fourier Transform Infrared (FTIR) Spectroscopy. The FTIR spectroscopy were recorded in the range 400-4000 cm-1 to study the changes of structure and surface chemistry of the adsorbent (VERTEX 70, Bruker Inc., Germany) using the KBr pressed pellet technique. Moreover, temperature-changing IR spectroscopy technique was also used on a select sorbent to investigate the changes of structure in the same manner with FTIR. X-ray Diffraction (XRD). The XRD patterns for select samples were recorded on an X-ray diffractometer (X’Pert PRO, PANalyticalB.V, Holland) using Cu KR radiation as X-ray source. The scanning range was from 5° to 45° (2θ) with a step size of 0.017° and a step time of 13.065 s. The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. X-ray Photoelectron Spectroscopy (XPS). The spectra were collected by a X-ray photoelectron spectrometer (XSAM800, Kratos Ltd., England), which was evacuated to a pressure of 10-7 Torr, with a pass energy of 20 eV, and the excitation of the spectra was performed by means of monochromatized Al KR radiation. The binding energy (BE) scale was referenced to the energy of the C1s peak of adventitious carbon, BE ) 285.0 eV.

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the smaller value of the ratio indicates a better mercury uptake capacity and higher mercury removal efficiency.4,29 3. Results and Discussions

Figure 1. Schemetic diagram of the experimental setup for Hg0 capture.

2.4. Experimental Apparatus and Procedures. A laboratory-scale fixed-bed apparatus was constructed, as shown in Figure 1, to explore the characteristic of the sorbents on Hg0 removal. The apparatus consisted mainly of a vapor-phase elemental mercury generator, a feed system, a fixed-bed reactor surrounded by a temperature-controlled oven, an online mercury analyzer, and a data acquisition system. A mercury permeation device (VICI Metronics Inc., Santa Clara, CA) was used as a source of Hg0. The device, designed to produce constant release ratio of Hg0 vapor at a specified temperature, was sealed in a U-type glass tube holder. A high-purity grade of nitrogen gas was supplied as a carrier gas to transport the Hg0 vapor out of permeation tube holder. The nitrogen gas laden with Hg0 was diluted with nitrogen gas of the same grade. Water vapor was generated by an evaporator and carried by nitrogen gas. The content of humidity was measured by FTIR gas analyzer (GASMET DX4000, Temet Corp., Finland) before starting the tests. The total gas flow rate of the simulated gas passing through the fixed-bed reactor was 1.0 L/min, based on the standard temperature and pressure. The fixed-bed reactor, a quartz adsorption column with 0.70 cm i.d. and 120 cm length, was placed in the temperature-controlled electric furnace, which could control the system temperature within (2 °C. These tests were all run at a temperature of 110 °C, and the inlet mercury concentration (Ci) was kept in the range of 30 ( 2 µg/m3 if there was no special specification. To provide enough bed length and to reduce the experimental time, approximately 30 mg of each prepared sorbent and 70 mg of silica having a similar size were well mixed and then were packed into the quartz column as sorbent. This type of silica is known to be inert to mercury during the adsorption time. The Hg0 concentrations were analyzed continuously using an ultraviolet VM3000 (Mercury Instruments Inc., Germany), cold vapor atomic absorption (CVAA) analyzer, which could analyze the inlet or outlet Hg0 concentration of the adsorption bed. During each test, the mercury-laden inlet gas bypassed the fixed-bed reactor and passed the analytical system until the desired inlet elemental mercury concentration was established. Then a given mass of sorbent was packed in the glass column and the adsorption test was initiated by diverting the gas flow through the sorbent column in downflow mode to minimize the potential for fluidization of the bed.38 For all tests, the mercury breakthrough ratio (η) was used to evaluate the performance of the sorbents, and it was defined in the following equation:27,29 η)

C0 × 100 Ci

(1)

where Ci and C0 represent the Hg0 concentrations at the inlet and outlet of the reactor, respectively. It should be noted that

3.1. BET and Element Analysis. BET surface areas, pore structures of modified CTS sorbents are given in Table 1. It was found that KBr (or KI) impregnation actually decreased both the BET surface areas and the pore volumes of the modified samples due to the blockage of internal porosity by incorporated KBr (or KI or H2SO4) molecules. Since the average pore size of the samples also increased after modification, the blocked pores would be micropores, resulting in a decrease of specific surface area and total pore volume.20 Generally, for pure physisorption process, the adsorptive capacity of sorbent increases with increasing the specific surface area.19,27 It was worth noting that the BET surface areas of CTS sorbents were somewhat smaller than that of normal activated carbon sorbents. Therefore, it can be concluded that the physisorption of CTS sorbents for mercury is limited, and the adsorptive mechanisms of modified material sorbents will mainly be the chemisorptions due to the presence of active sites for binding mercury. The data on main element of raw and modified sorbents are also provided in Table 1. It can be found that the Br (I) contents of modified CTS sorbents were somewhat higher than other elements (such as C, H, N, and S) of the sorbents, because the values, which were determined by XRF, were relative values. Apart from I and Br in the CTS sorbents existing as active sites for Hg0 removal, the S element existing in the form of sulfonate39 was also found, which might increase the mercury removal efficiency. 3.2. Thermal Stability. TGA was used on raw and iodine modified chitosan (CTS-I-S-411 and CTS-I-S-412) (Figure 2), as described in section 2.3, to establish the upper temperature limit for the adsorbents.40 Two peaks were observed in the weight loss rate or differential thermogravimetry (DTG) curve corresponding to two desorption processes in different temperature ranges. It was known that the first peaks were observed in the DTG curves corresponding to physically adsorbed water desorption process in the temperature range of 50-100 °C.33 The second peaks exhibited a rapid weight loss at 160-350 °C reaching a maximum at 294 °C for CTS, 225 °C for CTS-I-S411, and 243 °C for CTS-I-S-412. The second degradation stage of modified CTS sorbents took place at lower temperature than the corresponding stage of CTS indicating that CTS-I-S-411 and CTS-I-S-412 are less stable than CTS. This indicates that the modified chitosan is less stable than the chitosan due to the weakening of inter- and extramolecular hydrogen bonding41 and the presence of some active sites for elemental mercury removal. Moreover, the CTS-I-S-412 adsorbent was scanned with temperature-changing IR spectroscopy techniques as shown in Figure 3. It indicated that no significant change was found in the structure of the active layer below 150 °C. On the basis of these results, it is expected that the sorbents will operate stably below 140 °C, which can meet the temperature requirement of mercury removal after the electrostatic precipitator. 3.3. FTIR Analysis. FTIR spectra is a useful tool to identify the presence of certain functional groups in a molecule as each specific chemical bond often has a unique energy absorption band.42 To understand the nature of modification on sorbents and identify the possible sites for mercury binding to the novel modified CTS sorbents, FTIR spectra were obtained for chitosan before and after modification in the region of 400-4000 cm-1. Figure 4 shows the basic characteristics of chitosan (CTS, (a)) at 3436 cm-1 (O-H stretching or N-H stretching), 2828 cm-1


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Table 1. Pore Structure Parameters and Elemental Analysis of CTS Sorbents element (wt/%)

element (rwt/%)

sorbents

BET (m2/g)

total pore volume (cm3/g)

mean pore diameter (nm)

C

H

N

S

Br

I

CTS CTS-Br-41 CTS-Br-S-410 CTS-Br-S-411 CTS-Br-S-412 CTS-I-41 CTS-I-S-410 CTS-I-S-411 CTS-I-S-412

2.7894 0.6862 0.4872 0.4622 0.3938 0.6141 0.5397 0.3682 0.2642

0.01430 0.00241 0.00221 0.00225 0.00212 0.00219 0.00340 0.00274 0.00203

10.5055 14.0209 18.1558 19.4637 21.5661 14.2575 25.1602 29.7472 30.6816

38.73 36.20 29.38 23.95 22.92 37.07 27.55 25.45 24.49

7.78 7.12 6.31 5.99 5.84 7.37 5.76 6.22 5.99

6.99 6.59 5.37 4.39 4.22 6.73 5.02 4.66 4.55

sa s 2.15 5.99 7.49 s 2.08 5.24 7.53

s 79.81 69.02 50.37 47.37 s s s s

s s s s s 83.07 74.71 57.37 39.19

a

The “s” means no element was detected in the sorbent.

Figure 2. TG analysis for CTS, CTS-I-S-411, and CTS-I-S-412.

Figure 4. FTIR spectra of the sorbents: (a) CTS, (b) CTS-I-41, (c) CTSS-41, (d) CTS-S-42, (e) CTS-I-S-410, and (f) CTS-I-S-412.

Figure 3. FTIR spectra of CTS-I-S-412 sorbent at different temperature.

(C-H stretching), 1658 cm-1 (CdO stretching, amide I), 1598 cm-1 (N-H bending, amide II), 1154 cm-1 (bridge-O-stretching), and 1085 cm-1 (C-O stretching).30–33 For iodine impregnated chitosan (CTS-I-41) the wide absorption band at 3445 cm-1, corresponding to the stretching vibration of -NH2 group and -OH group, shifted to the higher wavenumber. The absorption band at 1092 cm-1, assigned to C-O stretching vibrations,31,42 also showed a smaller shift to higher wavenumber. These changes indicate that the -NH2, -OH and C-O groups of CTS are involved in the iodine adsorption process. Unlike CTS-I-41 (b), due to the presence of H2SO4 in the synthesis process of CTS-I sorbent, for CTS-I-S-410 (e) and CTS-I-S412 (f), the intensities of the N-H and O-H stretching vibrations in the region of 3150-3500 cm-1 decreased, and the characteristic peaks of amide Ι and amide Π shifted to lower wavenumber (CTS-I-S-410: from 1658, 1598 to 1643 and 1543 cm-1; CTS-I-S-412: from 1658, 1598 to 1635 and 1529 cm-1). The significant decrease of transmittance and changes of N-H stretching (3100-3500 cm-1) and N-H bending (1400-1660

cm-1) after modification indicate that the N-H vibration is affected due to the modification.34 Moreover, to investigate the effect of H2SO4 on sorbents preparation, the FTIR spectra of CTS-S-41 (c) and CTS-S-42 (d) are also given in Figure 4. It can be seen that the significant differences between CTS-I-S412 and CTS-S-42 were the bands at about 3400 and 800-1600 cm-1. For example, the intensities of N-H stretching at 3436 cm-1 and C-N stretching at 1109 cm-1 was decreased and the new band at 1189 cm-1 appeared due to the presence of iodine. These bands are closely related to the N-H bending, C-N stretching, and N-H rocking bonds. In other words, modification affects all of the bonds with N atoms, indicating that nitrogen atom is the main adsorption site for modification.34,42 This implies that the nitrogen atoms should be the main adsorption sites not only for H2SO4, but also for iodine on chitosan. Similarly, Sun et al. also found that the SO42- (at about 619 cm-1) participates in the chelation rather than existing as an ion in the complex.34 Clearly, all of the evidence support the chemical reaction among CTS, iodine, and sulfuric acid, and these functional groups are expected to have significant effect on mercury removal. 3.4. X-ray Diffraction Analysis. X-ray diffraction patterns of chitosan and its modified sorbents are shown in Figure 5. The chitosan showed two characteristic peaks around 2θ ) 10.6° and 20.1°, indicating the particular crystalline structure of the chitosan related to those peaks.30,33,35 The reflection at 2θ ) 10.6° was assigned to crystal forms I and the strongest reflection

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Figure 5. XRD patterns of the sorbents: (a) CTS, (b) CTS-I-41, (c) CTSS-42, (d) CTS-I-S-410, (e) CTS-I-S-411, (f) CTS-I-S-412.

appeared at 2θ ) 20.1° corresponding to crystal forms II.33 Except for the new peaks around 2θ ) 27.2° and 38.8° assigned to KI, the XRD pattern of iodine modified chitosan (CTS-I-41) was almost the same as that of chitosan at 2θ ) 10.6° and 20.1°, which indicated that the crystallinity of chitosan was not destroyed. While regarding the sorbent of CTS-S-42 as is depicted in Figure 5c, it was found that the crystal forms I moved toward higher diffraction angle, and crystal forms II still stayed at 2θ ) 20.1°. Meanwhile, it can be seen clearly that the area under amorphous peak of 2θ ) 20.1° decreased after modification. This is caused by the fact that inter- and extramolecular hydrogen bonds are destroyed due to the presence of H2SO4, implying an interaction between H2SO4 and chitosan. However, in the case of CTS-I-S-410 sorbent, the crystallinity of chitosan (including the inter- and extramolecular hydrogen bonds) was absolutely destroyed when a small amount of sulfuric acid was added as is shown in Figure 5d. The new crystal phase of K2SO4 can be observed at 2θ ) 17.6°, 21.3°, 25.4°, 29.7°, 30.7°, 35.9°, and 43.4° (JCPDS 00-005-0613). This implies that the reaction between KI and H2SO4 occurred, and K2SO4 and I2 are produced. However, when the added amount of H2SO4 was increased to 1 and 2 g (as seen in Figure 5 curves e and f), the crystal phase of K2SO4 can be seen clearly, and the crystallinity of CTS-I-S-412 was increased. Moreover, the crystal phase of I2 also be seen clearly at 2θ ) 23.9°, 28.7° (JCPDS 00-005-0558). The presence of iodine and sulfonate functional groups in chitosan will contribute greatly to the mercury removal of modified sorbents according to the mechanism of iodine-impregnated actived carbon for mercury removal.19 Introduction of these certain functional groups into polysaccharide structures should disrupt the crystalline structure of chitosan as shown in Figure 5, especially by the loss of hydrogen bonding.33 Some literature reported that the crystallinity of chitosan polymer could play a restrictive role on metal sorption capacity. The crystallinity parameter of chitosan is a keyparameter in the accessibility to internal sites for water, metal ions, and other materials. From this point of view, we can reasonably understand that more active sites, such as an amino group, would be obtained after the destruction of the crystallinity or the loss of the hydrogen bonding. This change caused by the presence of H2SO4 may be helpful for some functional group, such as iodine, to graft onto chitosan. 3.5. XPS Analysis. XPS spectra are widely used to distinguish the different forms of the same element and to identify

the existence of a particular element in a material.34,36,42 Figure 6 shows the resultant survey spectra of the XPS analysis of CTS-I-S-410 (curve a) and CTS-I-S-412 (curve b). The relative intensities and binding energies (BEs) of the C1s spectrum and O1s spectrum of curve b were almost the same as those of curve a, while the relative intensity of the I3d spectrum of curve b was evidently smaller than that of curve a. However, for the S2p spectrum, they exhibited the opposite tendency because of the addition of sulfuric acid. This indicates that the net content of iodine in CTS-I-S-412 is obviously less than that of CTSI-S-410, while the content of S is exactly the opposite. These results were consistent with that of elemental analysis. Table 2 shows the XPS spectra data of C1s, O1s, N1s, I3d and S2p of CTS-I-S-410(a) and CTS-I-S-412(b). Interestingly, although the reactions among chitosan, KI and H2SO4 were occurred as is discussed in FTIR and XRD analysis, the BEs of C1s, O1s, I3d and S2p were almost the same for CTS-I-S410 and CTS-I-S-412. Nevertheless, the BE of S2p decreased from 169.6 eV, that of H2SO4, to 169.1 eV,43 that of CTS-IS-410, and to 169.3 eV, that of CTS-I-S-412. While the BE of N1s increased from 399.4 eV to 401.8 eV. This indicates that the -NH2 is involved during the modification. Evidently, nitrogen atom provides electron pair, and sulfur atom inclines to accept the electron. Notice that iodine was detected by its 3d5/2, 3d3/2 doublet at 618.6 and 630.3 eV, respectively, which are the characteristic peaks of KI. This provides an evidence for the presence of unreacted iodine atoms and no XPS peak of I2 is found.44,45 Furthermore, to validate the presence of I2 or the result of XRD analysis, a more accurate chemical method was used, that is the reaction of iodine and starch. A 0.5 g portion of starch was added to a 50 mL narrow-mouthed bottle containing 30 mL of DI water. After the starch was dissolved completely, the same amount of KI and the prepared samples were added and stirred. The changes of these samples with starch are shown in Figure 7. The color of bottles a, b, and c has little change, but the color of bottles d and e become deep blue. These discoveries indicate that elemental iodine (I2) existed in sorbents of CTSI-S-411 and CTS-I-S-412, which means that the reaction of KI and H2SO4 occurred and the I2 is generated, which then can be adsorbed by chitosan. The amount of I2 may be under the lower limit of detection of XPS, so the XPS peak of I2 is not obtained. 3.6. Performance of Mercury Adsorption by Modified Sorbents. The key parameters affecting Hg0 adsorption by modified CTS sorbents may include the sulfuric acid (H2SO4) content, iodine loading, adsorption temperature,3 moisture content,23 and the inlet vapor-phase Hg0 concentration.3,6 The effects of these parameters are discussed in the sections that followsa case study by CTS-I-S-410 sorbent. We have known that the initial mercury removal efficiency is a crucial factor in sorbent injection technology because of the short contact time between the sorbents and the flue gases in the commercial plants.29 In this work, our particular interest is focused on the initial minimum mercury breakthrough and the performance of mercury capture when the samples appear in the first few hours of the adsorption reaction, not the first seconds or minutes for the sake of cyclic utilization. The adsorption time was varied from 30 to 225 min according to the outlet mercury concentration and the types of sorbents. 3.6.1. Effect of H2SO4 Content. The results of the Hg0 breakthrough of CTS sorbents that were chemically modified by KBr and KI at different H2SO4 addition are shown in Figure 8 and 9, respectively. As shown in Figure 8, the breakthrough curve of H2SO4 impregnated CTS (CTS-S-42) sorbent was in


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Figure 6. XPS spectra of C, O, N, I, and S on CTS-I-S-410 (a) and CTS-I-S-412 (b) sorbents. Table 2. The XPS spectra data of C1s, O1s, N1s, S2p, and I3d binding energy (eV) sample

C1s

O1s

N1s

S2p

I(3d5/2, 3d3/2)

KI H2SO4 CTS-I-S-410 CTS-I-S-412

s s 285.1 285.2

s s 532.6 532.6

s s 399.4 401.8

s 169.6 169.1 169.3

618.9, 630.4 s 618.8, 630.3 618.8, 630.3

top left corner and demonstrated a little effect on mercury removal. CTS-Br-41 sorbent revealed the same performance, while the initial removal efficiencies of CTS-Br-S-411 and CTSBr-S-412 significantly increased with increasing sulfuric acid content. However, compared with CTS modified by KBr and Figure 8. Hg0 breakthrough curves of Br-impregnated CTS at different H2SO4 additions.

Figure 7. Interaction between starch and samples: (a) KI, (b) CTS-I-41, (c) CTS-I-S-410, (d) CTS-I-S-411, and (e) CTS-I-S-412 and starch.

H2SO4, CTS sorbents modified by KI and H2SO4 showed excellent mercury removal efficiencies, especially when the mass proportion of CTS, KI, and H2SO4 was 4:1:2. The value η of mercury breakthrough for CTS-I-S-411 reached 0.15 in 200 min, and the value η for CTS-I-S-412 stayed near zero for about 175 min, which revealed the best performance among all the tested sorbents. However, the mercury breakthrough η curve of CTS-Br-S-411 quickly decreased with the reaction time, resulting in a smaller mercury capture capacity. Because of the smaller BET surface areas of these sorbents as shown in Table 1 and the performances of mercury capture by the sorbents, it is clear that the mercury removal process of CTS-S-42 is

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Figure 9. Hg0 breakthrough curves of I-impregnated CTS at different H2SO4 additions.

physisorption,20 while the processes of other samples are a combination of physisorption and chemisorption. It should be pointed out that the mercury capacity adsorbed by the physisorption process on CTS sorbents is very limited, and it is the chemisorption/chemical reactions, where mercuric bromide, mercuric iodine, and other formations enable the modified CTS to remove more Hg0.2,19 Interestingly, the amount of H2SO4 in CTS-Br and CTS-I sorbents should be suitable, otherwise the filtered solid sorbents are not obtained, they change to a black liquid in the process of drying in an electric oven. This also indicates that the chemical reactions of iodine and sulfate ion with the amide of CTS occurred,31 which is consistent with that of FTIR and XRD analysis. Notice that the significant difference between CTS-I41 and CTS-I-S-412 is the content of H2SO4 and speciation of iodine. We could suppose that the I2 and some active sulfur sites contribute most to Hg0 removal. In addition, another phenomenon to be noted is the effect of H2SO4 on vapor-phase mercury removal. Li et al.46 have reported that a H2SO4-treated activated carbon revealed an excellent Hg0 capture capacity compared to the nontreated activated carbon. It was suggested that Hg0 adsorption capacity of H2SO4-treated activated carbon can be explained by the enhanced adsorption potential and enthalpy of adsorption and the enhancement is a result of the narrow microporosity and increased surface polarity of the carbon due to H2SO4 impregnation.1 Similarly, Uddin et al.1 also confirmed that the added H2SO4 enhanced the Hg0 removal performance of the AC sample. It was assumed that the reactions between SO2, O2, and H2O occurred and produced H2SO4, which promoted significantly the Hg0 removal. Unfortunately, the negative influence of H2SO4 on Hg0 removal still existed. Presto et al.47 tested an extreme case of sulfur uptake with H2SO4-FGD, an activated carbon sample prepared by soaking FGD in 95% H2SO4, which had a sulfur content of 10.6% and had a mercury content after exposure to H2SO4-FGD similar to that of the raw FGD. This test suggested that high concentrations of surface-bound sulfur inhibit mercury. In this work, the CTS-S-42 also exhibited similar results with that of H2SO4-FGD. This may be caused by the fact that in the process of mercury capture, the existence of H2SO4 impregnated in sorbents is not predominant, which is also related to the physicochemical properties of actived carbon and the preparation method of H2SO4-treatment. 3.6.2. Effect of the Iodine Loading. The effect of iodide additive is shown in Figure 10. It can be seen that the breakthrough ratio for I-modified CTS sorbents was different with iodine content. Among all the four sorbents, CTS-I-S-412 exhibited the most excellent mercury removal ability. However,

Figure 10. Hg0 breakthrough curves of I-impregnated CTS at different iodine contents.

Figure 11. Effect of temperature on Hg0 breakthrough by CTS-I-S-410.

it should be pointed out that the proportion (25%) of iodine potassium and chitosan is very high, making these modified CTS sorbents unacceptable for commercial application. Fortunately, the lower iodine loading sorbent (CTS-I-S-1612) also revealed initial maximum mercury removal efficiency of 100%. Moreover, as was analyzed above, some unreacted KI in CTS-I-S412 sorbent still existed. Therefore, a more comprehensive preparation optimization of CTS-I-S sorbent with higher CTS content and lower iodine loading is possible and necessary for further practical use on mercury capture from coal-fired flue gas. 3.6.3. Effect of the Temperature. To identify the temperature effects on the Hg0 breakthrough, the adsorption bed temperature was varied from 40 to 135 °C with CTS-I-S-410 sorbent and the results are presented in Figure 11. The curves demonstrated an obvious decrease in Hg0 breakthrough ratio as the adsorption temperature was increased. At 40 °C the initial value η of mercury breakthrough reached 0.3, but it increased rapidly with the reaction time, compared to 0.1 and the slow increase at 135 °C. The increase in mercury removal efficiency with an increase in temperature is typical of a chemisorption mechanism. The mechanism is that Hg0 vapor reacts with impregnated iodine or some other active sites on the sorbents surface; that is, chemisorption occur on the surface of Iimpregnated CTS-I-S-410 that, being a chemical process, requires activation energy and proceeds at a limited rate which increases with rise in temperature.19,48 The conclusions reached in this study for the sorption mechanism in CTS-I-S-410 were found to agree with the findings of Krishnan,49 whom studied the effect of temperature on mercury removal when using S-impregnated HGR as a sorbent. They found that for runs at 25 °C the presence of S was not required for mercury sorption and the addition of sulfur caused decreased mercury sorption,


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Figure 12. Effect of moisture content on Hg0 breakthrough by CTS-I-S410.

Figure 13. Effect of inlet Hg0 concentration on Hg0 breakthrough and uptake by CTS-I-S-410.

suggesting that S-free sites were the centers of mercury capture at the lower temperature. Similarly, this explanation also can be used for the iodine-modified CTS sorbents. 3.6.4. Effect of Moisture. Chitosan has been widely investigated as a very promising material for the recovery of metal ion from the contaminated wastewater.31,32 Therefore, it is necessary to research the performances of modified CTS sorbents in Hg0 removal when moisture is presented. Figure 12 shows the impact of carrier moisture content on mercury breakthrough by CTS-I-S-410 sorbent. Obviously, it indicated that mercury removal efficiency increased with the moisture content in the simple simulated flue gas (N2 + Hg0). The research related to the effect of moisture on carbonaceous sorbents was explained in detail by Liu and Vidic.23 They found that the total mercury uptake capacity did not change significantly when 5% moisture was introduced in the carrier gas. However, carbon adsorptive capacity decreased as much as 25% when the moisture content increased to 10%. The reason is that for a small mount of moisture (below 5%) in the carrier gas, the capillary condensation might be the dominant process. While as the water vapor content increased to 10%, water molecules were able to fill the microspores so that isolated water zones merged to block the access to some active sites on the sorbent surface, therefore, creating additional mass transfer resistance for the adsorption of elemental mercury.23 The conclusion from the comparison of the physicochemical properties of S-impregnated AC and I-modified CTS is that the mechanism of moisture filling the microspores is not suitable for CTS sorbents due to their smaller BET surface areas. In addition, the effect of moisture on Hg0 removal also had been studied extensively by Li50 and Laumb.51 Li et al. reported that the moisture on activated carbon surfaces has a significant effect on Hg0 adsorption at room temperature.50 The adsorbed H2O is closely associated with surface oxygen complexes, and the removal of the H2O from the carbon surface by lowtemperature heat treatment reduces the number of active sites that can chemically bond Hg0 or eliminates the reactive surface conditions that favor Hg0 adsorption. Laumb et al. studied the effect of SO2, NO2, HCl, and H2O on mercury removal by carbon-based sorbents. They observed that the absence of water in the gas composition has a similar lowering effect on S (VI) concentration.51 It is assumed that water is required for sulfuric acid formation via either hydration of sulfur trioxide or sulfur dioxide to sulfurous acid and subsequent oxidation. We noted that besides having the active site, iodine, the CTS-I-S-410 also contained sulfonate and some unreacted amino group. The acidic property or acidic species on surface of CTS-I-S-410 will be strengthened when moisture is presented. This may be the main

reason that they have opposite tendency of mercury removal compared with activated carbon sorbent when 10% moisture was added. 3.6.5. Effect of Inlet Mercury Concentration. To identify the Hg0 concentration effect on the Hg0 adsorptivity, the initial inlet Hg0 concentration was varied from 30 µg/m3 to 120 µg/ m3 with CTS-I-S-410. The breakthrough and mercury uptake capacity are depicted in Figure 13. The CTS-I-S-410 sorbent showed a decrease in mercury capture efficiency with increasing Hg0 concentrations. This can be attributed to the relative decrease in effective adsorption sites bonding mercury with increasing initial Hg0 concentration, that is to say, for a certain amount of sorbent and a fixed adsorber, the effective sites for Hg0 removal and the contact time of mercury and sorbent are fixed. Therefore, with the increasing of initial Hg0 concentration, the outlet Hg0 concentration would be increased naturally. Interestingly, the Hg0 adsorption capacity per gram of sorbent significantly increased with Hg0 concentration as seen from the right part of Figure 12. In addition, it is noted that at high influent Hg0 concentrations of 120 µg/m3, the adsorption capacity of CTS-I-S-410 was approximately 69 µg/g at 70 min, which was almost 2.3 times higher than that of the same sorbent at a lower influent Hg0 concentration. Clearly, the amount of Hg0 adsorbed increased nonlinearly with the vapor-phase mercury concentration.3 This is consistent with reports that the mercury removal rate increases with the rise of the initial concentration of mercury.3,8 The capacity is lowered when the concentration is low because the entire adsorption rate depends more on the diffusion of mercury than on the surface adsorption rate.52 3.7. Possible Mechanism of Mercury Removal for CTS-I-S-412 Sorbent. On the basis of the above results and analyses, we present a schematic diagram of mercury removal route for CTS-I-S-412 sorbent, which shows an excellent efficiency in capturing Hg0 among all the prepared sorbents. As shown in Figure 14, KI reacts with H2SO4 (route 1) and produces the I2, and certainly some unreacted I- exists in the aqueous. The amine group of chitosan reacts with I2 and I(route 2), then Hg0 reacts with these active sites (such as I2 and I-) and produces HgI2 and some other compounds of iodine and mercury19 (route 3). In addition, H2SO4 can also react with CTS and this produces a sulfonate-containing chitosan complex (route 4). This complex reacts with Hg2+ and forms HgSO4 (route 5), which can more easily occur because of the combined action of the presence of moisture and the formed iodinecontaining chitosan complex. The reaction of route 1, which was verified by the reaction experiment of CTS-I-S-412 and starch, has been proven to be

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capture efficiency with increasing Hg0 concentration, while the Hg0 adsorption capacity significantly increased with Hg0 concentrations. The mechanisms of adsorption on iodinemodified CTS sorbents were also proposed. The synthetic optimization of CTS-I-S-412 sorbent and desorption or regeneration experiment using a chemical method will be further evaluated for getting a type of promising and low-cost sorbent. Acknowledgment

Figure 14. Mechanism of Hg0 removal by iodine and sulfuric acid modified chitosan.

correct. Literature31 reported that elemental iodine (or ionic iodine) and sulfuric acid can be adsorbed by chitosan in polar solvents. Mercuric iodide can be formed because of the presence of I2 and I-.19 This implies that the reactions of route 2, route 3, and route 4 are also correct. Fortunately, the last hypothesis has been confirmed by Huggins et al.45 From the S, Cl, and I, as well as from Hg XANES spectra, their study reveals evidence of the sorption mechanism involving the activating compounds (S- and I-) as well as acidic species of sulfur and chlorine in the flue gas. Nevertheless, the study only gives the evidence of mercury binding in the oxidized state. A similar experiment investigated by Hutson et al.53 has shown the binding of oxidized mercury species on prehalogenated powdered activated carbons. In addition, it is reported that that HgCl2 will be the major oxidized species at temperatures of > 700 K in the actual flue gases, with HgSO4(s) becoming the major species at temperatures of < 590 K.54 Therefore, at the temperatures of this study, the balance of Hg2+ would be transitioning from HgI2(s) to HgSO4(s). As noted, the Frandsen model predicts HgSO4(s) to be the most stable form of mercury at temperatures less than 320 °C. Furthermore, sulfur is known to produce an acid gas that interferes with the trapping of mercury by activated carbon.54 Form these literature reports, therefore, it is reasonable to conclude that the presence of H2SO4 is significant in the preparation of the CTS-I-S-412 sorbent for Hg0 capture. 4. Conclusions Vapor-phase elemental mercury adsorption experiments were carried out with the novel modified chitosan samples using a laboratory-scale fixed-bed reactor. To clearly understand the mechanism of the sorbents in capturing mercury, some samples were also characterized. The following are the conclusions: The amide of chitosan is the main adsorption site for iodine and sulfuric acid. The I2 is found in the sorbents of CTS-I-S411 and CTS-I-S-412 due to the addition of H2SO4. The adsorbents were suited for use in the low temperature at the end of the flue gas treatment train, with a maximum operational temperature of 140 °C. To obtain higher mercury removal efficiency, the addition content of sulfuric acid in the preparation process of CTS-Br and CTS-I sorbents should be suitable. Generally, the iodine-modified CTS sorbents demonstrated higher Hg0 capture efficiency than that of bromide-modified CTS sorbents. Among all the sorbents used, the CTS-I-S-412 sorbent shows the best mercury removal efficiency, which almost has the mercury capture efficiency of 100% for 3 h. Moisture presence can increase modified chitosan sorbent’s capacity for mercury uptake due to its unique physicochemical properties. The sorbent of CTS-I-S-410 shows a decrease in mercury

This work was supported by the National Natural Science Foundation of China (NSFC) (No. 50776037, No. 50721005), Program for New Century Excellent Talents in University (No. NCEF-07-0335) and the National High Technology Research and Development of China (No. 2007AA05Z308). These supports are gratefully acknowledged. The authors also express their thanks to Huazhong University of Science and Technology Analytical and Testing Center for their help with the measurements of the samples. Literature Cited (1) Uddin, M. A.; Yamada, T.; Ochiai, R. Role of SO2 for Elemental Mercury Removal from Coal Combustion Flue Gas by Activated Carbon. Energy Fuels 2008, 22, 2284–2289. (2) Granite, E. J.; Pennline, H. W.; Hargis, R. A. Novel Sorbents for Mercury Removal from Flue Gas. Ind. Eng. Chem. Res. 2000, 39, 1020– 1029. (3) Serre, S. D.; Silcox, G. D. Adsorption of Elemental Mercury on the Residual Carbon in Coal Fly Ash. Ind. Eng. Chem. Res. 2000, 39, 1723– 1730. (4) Jurng, J.; Lee, T. G.; Lee, G. W. Mercury Removal from Incineration Flue Gas by Organic and Inorganic Adsorbents. Chemosphere 2002, 47, 907–913. (5) Galbreath, K. C.; Zygarlicke, C. J.; Tibbetts, J. E. Effects of NOx, a-Fe2O3, g-Fe2O3, and HCl on Mercury Transformations in a 7-kW Coal Combustion System. Fuel Process. Technol. 2004, 86, 429–448. (6) Padak, B.; Brunetti, M.; Lewis, A.; Wilcox, J. Mercury Binding on Activated Carbon. EnViron. Prog. 2006, 25, 319–326. (7) US EPA, Clean Air Mercury Rule, www.epa.gov/mercury (accessed March 15, 2005).. (8) Eswaran, S.; Stenger, H. G.; Fan, Z. Gas-Phase Mercury Adsorption Rate Studies. Energy Fuels 2007, 21, 852–857. (9) Skodras, G.; Diamantopoulou, I.; Pantoleontos, G. Sakellaropoulos. Kinetic Studies of Elemental Mercury Adsorption in Activated Carbon Fixed Bed Reactor. J. Hazard. Mater. 2008, 158, 1–13. (10) Lu, D. Y.; Granatstein, D. L.; Rose, D. J. Study of Mercury Speciation from Simulated Coal Gasification. Ind. Eng. Chem. Res. 2004, 43, 5400–5404. (11) Galbreath, K. C.; Zygarlicke, C. J. Mercury Speciation in Coal Combustion and Gasification Flue Gases. EnViron. Sci. Technol. 1996, 30, 2421–2426. (12) Makkuni, A.; Varma, R. S.; Sikdar, S. K. Vapor Phase Mercury Sorption by Organic Sulfide Modified Bimetallic Iron-Copper Nanoparticle Aggregates. Ind. Eng. Chem. Res. 2007, 46, 1305–1315. (13) Yang, H. Q.; Xu, Z. H.; Fan, M. H. Adsorbents for Capturing Mercury in Coal-Fired Boiler Flue Gas. J. Hazard. Mater. 2007, 146, 1– 11. (14) Yan, R.; Liang, D. T.; Tay, J. H. Control of Mercury Vapor Emissions from Combustion Flue Gas. EnViron. Sci. Pollut. Res. 2003, 10, 399–407. (15) Lee, S. H.; Park, Y. O. Gas-Phase Mercury Removal by CarbonBased Sorbents. Fuel Process. Technol. 2003, 84, 197–206. (16) Mei, Z. J.; Shen, Z. M.; Yuan, T.; Wang, W. H. Removal of VaporPhase Elemental Mercury by N-doped CuCoO4 Loaded on Activated Carbon. Fuel Process. Technol. 2007, 88, 623–629. (17) Vidic, R. D.; Siler, D. P. Vapor-phase Elemental Mercury Adsorption by Activated Carbon Impregnated with Chloride and Chelating Agents. Carbon 2001, 39, 3–14. (18) Lee, S. S.; Lee, J. Y.; Keener, T. C. Novel Sorbents for Mercury Emissions Control from Coal-Fired Power Plants. J. Chin. Inst. Chem. Eng. 2008, 39, 137–142. (19) Lee, S. Y.; Seo, Y. C.; Jurng, J. S. Removal of Gas-Phase Elemental Mercury by Iodine- and Choride-Impregnated Activated Carbon. Atmos. EnViron. 2004, 38, 4887–4893.

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ReceiVed for reView January 15, 2009 ReVised manuscript receiVed March 10, 2009 Accepted March 10, 2009 IE9000629

Preparation, Characterization, and Application of Modified Chitosan

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