Mercury Removal from Aqueous Solution Using Coke-Derived Sulfur

Feb 10, 2010 - Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto M5S 3E5, Canada. Ind. Eng...
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Mercury Removal from Aqueous Solution Using Coke-Derived Sulfur-Impregnated Activated Carbons Jenny H. Cai and Charles Q. Jia* Department of Chemical Engineering and Applied Chemistry, UniVersity of Toronto, 200 College Street, Toronto M5S 3E5, Canada

Sulfur-impregnated activated carbons (SIACs) produced from oil-sands fluid coke by KOH-SO2 activation were applied to remove mercury ions from aqueous solutions. A pseudo-first-order rate expression can describe the Hg2+ adsorption behavior of SIACs produced in this study. The rate constants were found to be 0.01-0.102 min-1. The Hg2+ adsorption capacities of the examined SIACs ranged from 43 to 72 mg/g, which were comparable to those reported in the literature (35-100 mg/g) and that of a commercial SIAC (41 mg/g). The adsorption capacity increased with increasing specific surface area (SBET) of the SIACs, and a positive correlation was found between the SBET-normalized adsorption capacity and the total sulfur content of the SIACs. Elemental sulfur, disulfide, and thiophene seemed particularly effective in Hg2+ adsorption, as SIACs with these sulfur compounds showed higher adsorption capacities in general. 1. Introduction Mercury is one of the most harmful metals in the environment, because of its toxicity, high volatility, and potential bioaccumulation. It can be released into the aquatic environment from natural processes such as volcanic activity and weathering rocks, as well as from industries such as the chlor-alkali, paint, pulp and paper, oil refinery, electrical, rubber processing, and fertilizer industries.1,2 Commonly used methods to remove mercury ions from industrial wastewater include precipitation, ion exchange, aluminum and iron coagulation, activated carbon adsorption, electrodeposition, and various biological processes.2 Among these methods, activated carbon adsorption is widely used because of its high efficiency and ease of application.1-12 Sulfurimpregnated activated carbon (SIAC) is a type of activated carbon with sulfur impregnated into the carbon matrix, and it has been proven to be more effective in mercury adsorption than virgin activated carbon.1,6,13-15 A few studies have been conducted to investigate the possibility of using different types of activated carbon to remove Hg2+ from aqueous solutions, most of which have focused on investigating the effects of experimental conditions. It has been reported that Hg2+ adsorption is strongly dependent on the agitation time, initial concentration of Hg2+, pH, and activated carbon dosage.1,3-6,8-12 Although the properties of the activated carbon are also important in Hg2+ adsorption, only a little research on this topic has been reported.2,5,6,12 The properties of activated carbon that were investigated include surface area, pore size distribution, particle size, and surface chemistry. Carbon surface chemistry has been of particular interest, as it can affect the performance of activated carbon for mercury removal. Mohan et al. used carbon disulfide to treat air-activated fertilizer waste and observed an increase in the uptake of Hg2+ from wastewater.6 Ranganathan and Balasubramanian prepared sulfide-loaded activated carbon from coconut shells by chemical treatment with sulfide and thermal activation.15 The carbons with and without sulfide treatment were used for Hg2+ adsorption, and the results showed that the sulfide-loaded activated carbon was more effective. Studies of Hg2+ uptake from aqueous * To whom correspondence should be addressed. E-mail: cq.jia@ utoronto.ca.

solutions by steam-activated carbon and sulfurized steamactivated carbons revealed that the order of their adsorption capacity is steam-activated carbon in the presence of SO2 and H2S > steam-activated carbon in the presence of SO2 > steamactivated carbon in the presence of H2S > steam-only activated carbon. It was suggested that the Hg2+ adsorption capacity of activated carbons is associated with their sulfur contents.1 Nabais et al. modified the surface of activated carbon fibers with powdered elemental sulfur and H2S gas.7 They found that the most effective parameter for mercury uptake is the type of sulfur introduced rather than total sulfur amount. They suggested that H2S treatment leads to the formation of functional groups in which the sulfur is more accessible to mercury than the functional groups formed during the reaction with powdered sulfur. However, the types of functional groups formed during the modification processes were not identified. Although surface modification of activated carbon is promising for improving Hg2+ adsorption, more work is needed to fully understand what type of functional groups on the surface of activated carbon is beneficial for Hg2+ adsorption and how it works. SIACs produced from oil-sands fluid coke in this study showed high surface areas, controllable sulfur contents, and controllable sulfur species.16 In this study, the application of this type of SIAC was explored. The purpose of this study was to evaluate the performance of SIACs produced from KOH-SO2 activation in mercury ion adsorption and to investigate the effects of SIAC properties on mercury adsorption, especially the effect of sulfur content and species. 2. Experimental Section 2.1. Adsorbents. The adsorbents used to adsorb mercury ions from aqueous solutions were produced from oil-sands fluid coke using KOH and SO2 activation. In this process, KOH and SO2 were used to activate two types of oil-sands fluid cokes (FC-S and FC-I) and to add sulfur to the coke in one step. FC-S and FC-I were produced from petroleum cokes that were obtained from two different companies. FC stands for the specific type of coke (i.e., fluid coke), which is a byproduct of the fluid coker. Details of the activation process were documented by Cai.16 Therefore, the activated carbons produced from this process are sulfur-impregnated activated carbons that have high sulfur

10.1021/ie901194r  2010 American Chemical Society Published on Web 02/10/2010

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Table 1. Summary of the Properties of the SIAC Samples sample no.

sample ID

SBET (m2/g)

total S content (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

S-KS-1 S-KS-2 S-KS-4 S-KS-5 S-KS-12 S-KS-14 S-KS-15 S-KS-16 I-KS-1 I-KS-2 I-KS-3 I-KS-12 I-KS-15 I-KS-16 S-K-2 S-K-6 S-K-7 S-K-10 S-K-12 S-S-1 I-S-1 SIAC-BG

1451 1222 2108 2505 1174 1753 431 1183 1958 1299 2281 1567 410 1183 2501 732 1695 2088 1498 19 13 664

8.10 7.31 4.84 3.50 5.14 2.70 5.31 4.52 2.46 0.96 2.20 6.10 7.41 3.48 0.09 0.61 0.17 0.12 0.27 9.34 8.04 8.11

contents. Activations were conducted at 600-900 °C, for 15-120 min, with KOH/coke ratios of 1:1-3:1.16 Five KOHactivated carbons (nos. 15-19 in Table 1) that were activated without the presence of SO2, two SO2-activated carbons (nos. 20 and 21) without the presence of KOH, and one commercial SIAC (SIAC-BG) were used for comparison purposes. Table 1 lists the properties of these samples. The BET surface area (SBET) and pore size distribution of these samples were determined using a surface area and pore size analyzer (SA3100, Coulter). Total sulfur content was measured with an elemental analyzer (Vario EL III). Sulfur forms in SIAC samples before and after mercury adsorption were identified and quantified using X-ray absorption near-edge structure (XANES) spectroscopy. XANES analyses were conducted at the Canadian Synchrotron Radiation Facility (CSRF) on the 1 GeV electron storage ring, at Aladdin, University of Wisconsin (Madison, WI). Detailed description of XANES analysis can be found elsewhere.17 2.2. Mercury Ion Adsorption. The procedure for the determination of Hg2+ adsorption capacity was designed based on ASTM method D3860-98. HgCl2 (from Aldrich) was used to prepare mercury solution with an initial Hg2+ concentration of 100 mg/L, and 0.1 g of activated carbon was added to 100 mL of the HgCl solution to adsorb mercury. The initial pH of the solution was 4.8-5.0, and pH was monitored during the Hg2+ adsorption by measuring it at certain time intervals. The suspension was shaken at 130 rpm and 25 °C. A 10-mL plastic syringe was used to sample 1 mL of the solution at certain time intervals for 930 min or longer. The sample solution was then filtered with a syringe filter (0.22 µm), diluted with deionized water, and analyzed by inductively coupled plasma (ICP, Optima 7300, PerkinElmer). A plot of the Hg2+ concentration in each sample as a function of time gave the adsorption curve. The adsorption capacity was expressed as the amount of Hg2+ adsorbed per unit weight of carbon. The adsorption isotherms of Hg2+ on activated carbon were determined by adsorbing Hg2+ from a series of HgCl2 solutions with different SIAC dosages. The suspension was shaken at 130 rpm and 25 °C over 20 h. Samples were taken from the suspension afterward and analyzed by ICP to determine the equilibrium concentration of Hg2+.

Figure 1. Langmuir isotherms for Hg2+ adsorption onto different samples produced from FC-S at 25 °C. Table 2. Parameters of Langmuir and Freundlich Models for the Adsorption of Hg2+ on Activated Carbons Langmuir model sample S-KS-2 S-K-6

2

R

0.93 0.97

Q0 (mg/g) 112 54

Freundlich model b

R -3

9.6 × 10 2.6 × 10-2

2

0.96 0.91

KF (mg/g)

1/n

5.7 12.3

0.48 0.24

3. Results and Discussion 3.1. Adsorption Isotherms. The adsorption isotherms of Hg2+ on activated carbon were studied at 25 °C (298 K). The data were fitted to both the Langmuir isotherm model and the Freundlich isotherm model. The Langmuir equation has the linearized form Ce/qe ) 1/(Q0b) + (1/Q0)Ce

(1)

where Ce is the equilibrium concentration of the solution (mg/L) and qe is the amount of adsorbate in the activated carbon (mg/g). By calculating the intercept and the slope, Q0 (the adsorption capacity) and b (the Langmuir constant related to the energy of adsorption) can be obtained.5,15 Figure 1 shows a linear plot of the specific sorption (Ce/qe) against the equilibrium concentration (Ce) for Hg2+ adsorption using two samples: S-KS-2 and S-K-6. Both samples were produced at 600 °C with a KOH/coke ratio of 3:1. S-KS-2 was produced from KOH-SO2 activation for 1 h, and S-K-6 was produced by KOH activation for 15 min. It should be noted that S-KS-2 and S-K-6 were prepared under very different conditions and had very different sulfur contents. They were chosen to determine the effect of sulfur content. The values of Q0, b, and the correlation coefficient (R2) of Hg2+ adsorption are reported in Table 2. The higher Q0 value of S-KS-2 indicates that this sample has a higher adsorption capacity of Hg2+ than S-K-6. Comparing the properties of the two samples, it can be seen that S-K-6 has a much lower SBET and sulfur content, which results in its low Q0 value. The constant b in eq 1 is related to the free energy of adsorption (b ∝ e-∆G/RT) and can be used to calculate the dimensionless separation factor RL, which is defined as RL ) 1/(1 + bC0), where C0 is the initial concentration (mg/L). According to Hall et al., RL is an indicator of the favorability of an isotherm.18 RL > 1 indicates unfavorable adsorption; RL ) 1 is for a linear adsorption; 0 < RL < 1 indicates favorable adsorption; and RL ) 0 indicates an irreversible adsorption. For both S-KS-2 and S-K-6, the RL values are between 0 and 1 (0.51 and 0.28, respectively), suggesting favorable adsorption of Hg2+ by these activated carbon. The Freundlich adsorption isotherm model often fits the data on rough surfaces better than the Langmuir equations, because it was modified from the Langmuir equation to account for surface roughness and heterogeneity of the adsorbent. The Freundlich isotherm model describes the relationship between

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Figure 2. Freundlich isotherms for Hg2+ uptake onto different samples produced from FC-S at 25 °C.

Figure 3. Effect of SBET on Hg2+ adsorption capacity.

the adsorbed amount per unit weight of adsorbent and the equilibrium concentration of adsorbate. Its linear form is log qe ) log KF + (1/n) log Ce

(2)

In eq 2, qe is the amount of adsorbate adsorbed by activated carbon (mg/g), and Ce is the equilibrium concentration of the adsorbate in solution (mg/L). KF is a constant indicative of the relative adsorption capacity of the adsorbent (mg/g), and 1/n is a constant indicative of the intensity of the adsorption.19 A logarithmic graph of the Freundlich isotherm model of Hg2+ adsorption on S-KS-2 and S-K-6 is shown in Figure 2. The Freundlich constants 1/n and KF were calculated using the slopes and intercepts of the lines (Table 2). A small 1/n and large KF of Hg2+ adsorption onto an activated carbon usually indicate a high affinity to Hg2+ and a high adsorption capacity.20 However, because S-K-6 was better fitted to the Langmuir model, given its higher correlation coefficient for the Langmuir model than for the Freundlich model, it cannot be concluded that S-K-6 has a higher adsorption capacity. In fact, S-K-6 has a lower adsorption capacity (32.7 mg/g) than S-KS-2 (53.2 mg/g). The values of 1/n for these samples are between 0 and 1, indicating that the surfaces of these samples are heterogeneous in nature.12 The corresponding Freundlich and Langmuir parameters along with correlation coefficients are listed in Table 2. The correlation coefficients indicate that the data for Hg2+ adsorption by S-KS-2 fit the Freundlich model better than the Langmuir equation, whereas for S-K-6, the adsorption data fit the Langmuir model better. The mercury adsorption process takes place at the liquid-solid boundary, and the diffusion process occurs in a complex matrix. The reaction is thus expected to be heterogeneous. The complex adsorption phenomenon could involve chemical interactions between the solute and the chemical groups on the activated carbon surface, in addition to other driving forces, such as electrostatic, van der Waals, and hydrophobic interactions.21 Because sample S-KS-2 has a high sulfur content, it is quite possible that a chemical reaction takes place between the mercury and sulfur functional groups on the SIAC surface. However, for sample S-K-6, the sulfur content is only 0.6%, which probably does not react with mercury. Therefore, the better fitting of the Freundlich model is in good agreement with the fact that the surface of S-KS-2 is more heterogeneous because of the loading of sulfur, whereas the surface of S-K-6 is more homogeneous. 3.2. Adsorption Capacity. The SIACs produced in this study exhibit high Hg2+ adsorption capacities, ranging from 42.6 to 71.5 mg/g. To evaluate the performance of the SIACs produced in this study, a commercial SIAC-BG (provided by Barrick Gold

Figure 4. Data fitting of SBET versus Hg2+ adsorption capacity using an exponential function.

Corporation) was used to adsorb Hg2+ from HgCl2 solution under the same adsorption conditions. The Hg2+ adsorption capacity of SIAC-BG was 40.8 mg/g, which is lower than the values for all of the SIACs produced in this study. The known Hg2+ adsorption capacities reported in the literature for adsorption conditions similar to those applied in this study are about 35-100 mg/g.1,10,15 Effect of SBET. The SBET values of SIACs can significantly affect the adsorption of mercury. Ekinci et al. used three types of activated carbon from apricot stones, furfural, and coals to remove Hg2+ from aqueous solution.5 Their results suggested that an increase in SBET resulted in an increase in the adsorption of Hg2+. This finding is also supported by the study of Zhang et al.,12 who used organic sewage sludge as the starting material. Although a correlation between the SBET value of activated carbon and its Hg2+ adsorption capacity was suggested, the correlation between adsorption capacity and SBET was difficult to establish because of the interference of other properties of SIACs. Figure 3 reveals a positive linear correlation between SBET and adsorption capacity. However, the straight line does not go through the origin, although theoretically, the adsorption capacity should be zero if there is no surface area. Therefore, the plot of SBET versus adsorption capacity was also fitted using a nonlinear function, namely, an exponential function (Figure 4). It was found that the enhancement effect of SBET on adsorption capacity diminished when SBET was higher than about 1000 m2/g. This is an important finding, which reveals that extremely high

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Figure 5. Hg2+ adsorption capacities of SIACs with similar sulfur contents (0.1-0.6%) but different surface areas.

Figure 6. Dependence of Hg2+ adsorption capacities on mesopore and micropore surface area.

surface areas of SIACs might not be necessarily provide high Hg2+ adsorption. This is also supported by the calculation result that the coverage of Hg2+ ions on the SIAC surface is very small (∼1.5 m2 for 100 mg of Hg2+ in 1 g of SIAC). In fact, using SIACs with moderate specific surface areas for Hg2+ adsorption can be more economically efficient, because a large amount of KOH is usually needed to produce SIACs with high surface areas. To confirm this finding, SIACs with similar sulfur contents (0.1-0.6%) but various SBET values were chosen to show the relationship between SBET and adsorption capacity. It is clearly seen that the Hg2+ adsorption capacity increases with increasing SBET (Figure 5) and that the increase slows when SBET is higher. This behavior might be associated with the pore structure in the SIAC. Because high porosity is usually contributed by micropores, these small sizes might not be accessible to Hg2+, especially when other mercury species such as HgCl2 and HgOHCl are present.8 Figure 6 illustrates the dependence of Hg2+ adsorption on the surface area of mesopores and micropores. A better linear correlation with higher slope was found between adsorption capacity and mesopore surface area, which supports the conclusion that Hg2+ adsorption is more dependent on mesopores than on micropores. Effect of Total Sulfur Content. To show the real correlation between total sulfur content and Hg2+ adsorption capacities, it is ideal to use SIACs with similar SBET values. However, it is difficult to prepare SIAC samples with same SBET value but different sulfur contents. To resolve this issue, the sulfur content and Hg2+ adsorption capacity of each sample was normalized by its SBET value. In doing this, the reasonable assumption that

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Figure 7. Relationship between normalized total S content and Hg2+ adsorption capacity.

the sulfur distribution in SIAC is uniform was applied. Figure 7 clearly shows that the specific Hg2+ adsorption capacity increased with increasing specific sulfur content. An F test confirmed that the regression of total sulfur versus adsorption capacity was significant. These results provide more evidence to support a previous study that suggested that the Hg2+ adsorption capacity increases with increasing sulfur content.1 Effect of Sulfur Form. The adsorption of HgCl2 on activated carbon in aqueous solutions can occur by two mechanisms: (i) adsorption of HgCl2 and/or (ii) reduction of Hg2+ by the surface functional group.22 During mercury adsorption, interactions between Hg2+ and sulfur functional groups on the SIAC surface could occur. According to Pearson theory, during an acid-base reaction, hard acids prefer to coordinate with hard bases, and soft acids prefer to coordinate with soft bases. Hg2+ is a Lewis acid, and thus, the interaction of Hg2+ species such as HgCl2, (HgCl2)2, Hg(OH)2, and HgOHCl with certain surface sulfur groups (soft bases) is likely to be favored in the pH range of 4-6.23,24 Little work has been done on the effective sulfur forms in mercury adsorption. Mohan et al. observed an increase in the uptake of Hg2+ with the adsorbent presoaked in carbon disulfide,6 and Feng et al. found that elemental sulfur was probably most effective for capturing vapor-phase mercury, whereas thiophene and sulfate also showed positive correlations with mercury uptake.13 Samples S-KS-12, S-KS-16, and I-KS-16 have high Hg2+ adsorption capacities (48.2, 54.7, and 57.4 mg/g, respectively), because of their high surface areas. However, it is interesting that, although they have similar SBET values, their adsorption capacities decreased with increasing sulfur content. Therefore, the effect of sulfur forms in SIAC was investigated with the consideration that sulfur forms can influence mercury adsorption, as suggested by previous studies.6,13 It was difficult to determine how sulfur forms affect the adsorption. Therefore, the changes in sulfur forms after adsorption were detected with the hope that these could provide information on the type of sulfur species that might have been involved in the reaction between sulfur and mercury. To this end, a 0.1-g sample S-KS-12 was added to 100 mL of deionized water and shaken overnight, as in the Hg2+ adsorption experiment. By this means, the effect of water on sulfur form changes was investigated. After Hg2+ adsorption, the sulfur forms in the S-KS-12 Hgadsorbed sample and the S-KS-12 control sample did not change (Figure 8), yet the amounts of each form changed. This suggests that, for S-KS-12 adsorption, the sulfur form did not change much during the adsorption or the change was too small to be detected.

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Figure 8. Sulfur form changes in sample S-KS-12 before and after Hg2+ adsorption.

Figure 10. Relationship between sulfur forms in SIAC samples (S-KS-2, S-KS-16, and I-KS-16) and Hg2+ adsorption capacity.

Figure 9. Sulfur forms in SIAC samples used for Hg2+ adsorption.

The large reduction in sulfate amount can be attributed to the high solubility of sulfate in water and the prolonged shaking time. As a result, the proportion of other sulfur species increased. Figure 9 shows that S-KS-16 and I-KS-16 with high Hg2+ adsorption capacities both have high contents of disulfide and certain amounts of elemental sulfur that might enhance the adsorption. Samples S-S-1 and I-S-1 have extremely low SBET values. However, their adsorption capacities are not very low (22.34 and 23.26 mg/g, respectively), which is due partially to their high sulfur contents and might also be associated with the large amounts of elemental sulfur and thiophene present in these two samples. These findings provide a piece of evidence to support previous studies in which disulfide, elemental sulfur, and thiophene in SIACs were suggested to enhance mercury adsorption.6,13 The plot in Figure 10 shows a strong dependence of Hg2+ adsorption on the total amount of sulfur in the forms of elemental sulfur and disulfide in the samples S-KS-12, S-KS16, and I-KS-16. This provides further evidence for the effect of these forms of sulfur on mercury adsorption. Most of the SIAC samples produced in this study had various specific surface areas. Therefore, the contents of reduced sulfur and oxidized sulfur in the SIACs were normalized by the SBET values to eliminate the effect of surface area and to investigate whether there was a correlation between the amount of sulfur in different oxidation states and the Hg2+ adsorption capacity. The normalized sulfur content in reduced/oxidized form versus the normalized Hg2+ adsorption capacity is plotted in Figure 11, and a strong linear relationship between the normalized reduced sulfur and the normalized Hg2+ adsorption capacity was found. An F test was carried out to examine whether the regression was significant. The results indicated that, with 95% confidence, the regression between reduced sulfur and adsorption capacity was

Figure 11. Relationship between sulfur forms in SIAC samples (S-KS-2, S-KS-16, and I-KS-16) and Hg2+ adsorption.

significant. On the other hand, the regression between oxidized sulfur and adsorption capacity was insignificant. Therefore, a high content of reduced sulfur in SIACs can enhance Hg2+ adsorption. 3.3. Adsorption Kinetics. It is generally accepted that the process of adsorption itself is very rapid and not the rate-limiting step in the uptake of organic/inorganic compounds, whereas the adsorption rate is controlled by film diffusion or particle diffusion.6 Hg2+ adsorption on SIACs in this study is more likely film-diffusion-controlled, because the system had fairly poor mixing, a dilute concentration of adsorbate, a small particle size, and/or a high affinity of adsorbate for adsorbent. A pseudo-first-order rate expression was applied to calculate the rate of mercury adsorption by SIAC (eq 3). This equation has commonly been used to describe the behavior of mercury adsorption from aqueous solutions6,9,12 qt ) qe(1 - e-kt)

(3)

where qt is the adsorption capacity (mg/g) at any time t (min) and qe is the equilibrium capacity (mg/g). k is the pseudo-firstorder rate constant (min-1). Because qt and qe can be calculated, the slope of a logarithmic plot of ln(qe - qt) - ln qe versus t gives the adsorption rate constant, k, according to eq 4. A linear straight-line plot in all cases was observed with correlation coefficients higher than 0.95, which indicates that the adsorption reaction can be approximated as pseudo-first-order. ln(qe - qt) - ln qe ) -kt

(4)

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Table 3. Adsorption Constants for Hg

Uptake by SIACs

sample

rate constant, k (min-1)

correlation coefficient, R2

S-KS-1 S-KS-2 S-KS-4 S-KS-5 S-KS-12 S-KS-14 S-KS-15 S-KS-16 I-KS-1 I-KS-2 I-KS-3 I-KS-12 I-KS-15 I-KS-16

0.024 0.035 0.044 0.057 0.048 0.017 0.032 0.060 0.030 0.026 0.102 0.010 0.051 0.049

0.9814 0.9947 0.9950 0.9961 0.9944 0.9795 0.9810 0.9975 0.9873 0.9542 0.9641 0.9917 0.9519 0.9917

The k values for the SIACs produced in this study were in the range of 0.01-0.102 min-1 (Table 3). Under similar adsorption conditions (25-29 °C, pH of 5-5.5, and initial mercury concentration around 100 mg/L), it was found that the k values obtained in this study were similar to those from the commercial activated carbon SIAC-BG (0.11 min-1), a H2SO4activated furfural (0.097 min-1), and a H2SO4- and (NH4)2S2O8activated coirpith (0.075 min-1).3,9 They were much higher than the values obtained from activated carbon produced from organic sewage sludge using H2SO4, H3PO4, and ZnCl2 (0.008-0.018 min-1) and from an air-activated fertilizer waste (0.0102 min-1).6,12 The k values obtained in this study were lower than that from a ZnCl2-activated carbon produced from walnut shell (0.15 min-1).10 However, the initial concentration of mercury solutions was not mentioned in that study, and normally, a lower initial concentration results in a higher rate constant.3,9 4. Conclusions The data on Hg2+ adsorption on KOH-SO2-activated carbon fit the Freundlich model, suggesting that the SIAC surface is heterogeneous and that the process might be dominated by chemical adsorption. The Hg2+ adsorption capacity of the SIACs examined in this work ranged from 43 to 72 mg/g, which is comparable to those reported in the literature (35-100 mg/g) and that of a commercial SIAC (41 mg/g). It was found that the Hg2+ adsorption capacity increased with increasing SBET of the SIAC; however the enhancement of SBET diminished when the SBET value was greater than about 1000 m2/g. A positive correlation was found between the SBET-normalized Hg2+ adsorption capacity and the total sulfur content of SIACs. Elemental sulfur, disulfide, and thiophene can enhance Hg2+ adsorption, because SIACs with these sulfur forms showed higher adsorption capacities than others. A pseudo-first-order rate expression can describe the Hg2+ adsorption behavior of the SIACs in this study with correlation coefficients higher than 0.95. Under similar adsorption conditions, the rate constants (0.01-0.102 min-1) of mercury adsorption by KOH-SO2-activated carbon were comparable to those reported in previous studies (0.008-0.15 min-1) and that of a commercial SIAC (0.11 min-1). Acknowledgment The XANES analysis was conducted at the Canadian Synchrotron Radiation Facility (CSRF), University of Wisconsin, WI, U.S.A. We thank Astrid Jurgensen at the CSRF for her assistance. Literature Cited (1) Anoop Krishnan, K.; Anirudhan, T. S. Removal of mercury (II) from aqueous solutions and chlor-alkali industry effluent by steam activated and

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ReceiVed for reView July 28, 2009 ReVised manuscript receiVed January 28, 2010 Accepted January 29, 2010 IE901194R