Adsorption Characteristics of Sulfur-Functionalized Silica

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Adsorption Characteristics of Sulfur-Functionalized Silica Microspheres with Respect to the Removal of Hg(II) from Aqueous Solutions Norasikin Saman,† Khairiraihanna Johari,† and Hanapi Mat*,†,‡ †

Advanced Material and Process Engineering Laboratory, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia ‡ Novel Materials Research Group, Nanotechnology Research Alliance, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia S Supporting Information *

ABSTRACT: This paper presents the study of the adsorption characteristics of sulfur-functionalized silica microspheres (SSMs), synthesized through co-condensation of tetraethyl orthosilicate with 3-mercaptopropyl trimethoxysilane (MPTMS) and bis(triethoxysilylpropyl) tetrasulfide (BTESPT) as sulfur ligands, with respect to the removal of Hg(II) from aqueous solutions. The synthesized adsorbents were characterized using a scanning electron microscope, an X-ray diffractometer, a nitrogen adsorption−desorption analyzer, a Fourier transform infrared spectrophotometer, and an energy dispersive X-ray diffractometer. The effects of pH, concentration, temperature, stirring time, and adsorbent reusability were studied via batch adsorption experiments. It was found that the optimal adsorption pH values for all synthesized adsorbents were between 5.8 and 8.2. The adsorption capacity of SMs was 20.0 mg/g and increased to 37.0 and 62.3 mg/g for BTESPT-SMs and MPTMS-SMs, respectively. Hg(II) adsorption was found to be exothermic in nature and followed the chemisorption mechanism. The Langmuir isotherm model was found to be the best fitted model for describing the isotherm data, while the kinetic data obeyed the pseudosecond-order kinetic model, in which film diffusion was found to be the rate-controlling step. The regeneration study using potassium iodide as a regeneration agent showed high reusability, up to five-cycle activity.

1. INTRODUCTION Today, the development of new adsorbents that are capable of removing targeted mercury with high capacity and selectivity has gained a great deal of attention from many researchers. Several adsorptive materials have been found to be capable of adsorbing mercury, such as activated carbon,1,2 modified silica,3−16 zeolites,17 and biomass.18,19 At present, carbonaceous materials, such as activated carbon, are commercially available for industrial applications. The nonpolar surface characteristics of activated carbon somehow make the adsorption of heavy metals from aqueous solutions inefficient. Despite this fact, activated carbon is often used to control emissions from various organic solvents and toxic materials.1 Nonetheless, there is still a need to design new materials that are highly selective toward specific adsorbates. A wide variety of silica-based adsorbents have been synthesized for the removal of mercury from waste streams. Silica is known to have a good mechanical strength, does not swell, and can withstand high temperatures.20 It can be synthesized through various morphologies, including monoliths, fibers, ropes, spheres, gyroids, discoids, hollow tubes, dodecahedra, and thin films. These varieties of morphologies can be made by varying the synthesis conditions, including the synthesis temperature, chemical composition, or different synthesis routes, which are either alkaline or acidic.21−23 The spherical silica can be synthesized by using the classical Stöber method, by manipulating the tetraalkoxysilane/ammonia/ water/alcohol mixture.21 The use of a structure-directing © 2013 American Chemical Society

agent (SDA) has produced various mesoporous silicas such as MCM,3,10,12,14,24,25 SBA,4,6,7,10,16 MSU,5,8,9 and HMS4,8,10,26 family. The presence of a hydroxyl (OH) group on the silica surfaces makes it easy for them to be functionalized by manipulating various functional groups that can influence their adsorption capacity and selectivity. Sulfur atom-containing ligands are commonly used as the functionalization agents to enhance the adsorption of mercury from aqueous solutions (see Table S1 of the Supporting Information). It has been reported that 3mercaptopropyl trimethoxysilane (MPTMS) as a sulfur ligand has been proven to have strong adsorption performance toward mercury,14,24 which is similar to what has been observed for BTESPT that was reported only for SBA.16 An extraordinary high mercury adsorption was reported from BTESPT-functionalized SBA. However, there has been no comparative study reported so far on the effects of thiol (MPTMS) and thioether (BTESPT) on the Hg(II) adsorption process. The use of BTESPT to functionalize SMs, which most likely has the same characteristics as MCM-41, so far has not been reported. The objective of this study is therefore to compare the Hg(II) adsorption characteristics of these two sulfur ligands in functionalizing the silica microspheres (S-SMs) toward the Received: Revised: Accepted: Published: 1225

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conditions. Twenty-five milligrams of the adsorbent was added to 25 mL of an aqueous Hg(II) solution. The pH of the Hg(II) solution was then adjusted to the required value by using a HNO3 or NaOH solution. The mixture was shaken for 2 days, which was sufficient to achieve equilibrium conditions. Subsequently, the mixture was filtered using a nylon syringe filter of 0.45 μm to remove the adsorbent particles, and the filtrate was collected to determine the Hg(II) concentration by using a flame atomic absorption spectrophotometer (PerkinElmer Precisely, HGA 900). The quantity of Hg(II) adsorbed [Qe (in milligrams per gram)] was calculated using the mass balance equation (eq 1).

removal of Hg(II) from aqueous solutions. All experimental conditions were kept similar for both adsorbents, allowing better comparison and discussion about the effect of both ligands that could lead to better Hg(II) adsorption performance. Finally, these results would provide a better understanding of the role of both sulfur ligands in the enhancement of the adsorption of Hg(II) from aqueous solutions, and thus, it is generally useful with respect to adsorbent design and applications.

2. METHODS 2.1. Adsorbent Preparation and Functionalization. 2.1.1. Synthesis of the SM Adsorbent. The SM adsorbent was synthesized according to the procedure described by Walcarius and Delacôte14 and Gaslain et al.,24 with some modifications. First, cetyltrimethylammonium bromide (CTAB) as a templating agent was dissolved in 100 mL of deionized water. Then, 14.5 mL of aqueous ammonia (NH4OH, 25%) and 60 g of ethanol (99.5%) were added to form a homogeneous solution. This solution was stirred for 15 min, followed by dropwise addition of 5 mL of tetraethyl orthosilicate (TEOS, 99%) and further stirring for an additional 2 h. The precipitate was filtered, washed, and dried at room temperature (30 ± 1 °C). After drying, the resulting white amorphous silica was ground, and the product was further dried in an oven at 105 ± 0.5 °C. Lastly, the product was calcined at 550 ± 0.5 °C for 5 h to remove the CTAB template. The final product was denoted as SMs. 2.1.2. Synthesis of the S-SM Adsorbent. The in situ functionalization of the SM structural framework was conducted according to the procedure described by Walcarius and Delacôte14 and Gaslain et al.24 Initially, the same procedure for preparing SMs was conducted for acquiring this adsorbent. After the mixture of CTAB, water, and ammonia had been stirred for 15 min, 6 mL of a precursor solution containing 10% (v/v) MPTMS or BTESPT in TEOS was added dropwise. After the mixture had been stirred for 2 h, the product was filtered and washed repeatedly using deionized water. Then, it was dried at room temperature (30 ± 1 °C) and further dried at 105 ± 0.5 °C for 2 days to remove excess water. The template was removed by suspending the solid product in ethanol and then refluxed for 48 h. Finally, the solid products were recovered via filtration, washed using ethanol, dried at room temperature (30 ± 1 °C), and further dried at 105 ± 0.5 °C. The SMs functionalized with MPTMS and BTESPT were denoted as MPTMS-SMs and BTESPT-SMs, respectively. 2.2. Characterization of the Adsorbent. A JEOL JSM6390LV scanning electron microscope (SEM) was used to determine the size and morphology of the synthesized adsorbents. The pore structures were examined using a Bruker D8 Advance X-ray diffractometer (XRD) using Cu Kα (λ = 1.5405 Å) radiation at 40 kV and 40 mA, scanned in the 2θ range between 0.5° and 10° at a scanning speed of 1.3°/min. The N2 adsorption−desorption (NAD) isotherm and thus the surface area and pore volume of the synthesized adsorbents were measured using a Micromeritics ASAP 2000 instrument. The samples were degassed at 393 K overnight, and the NAD isotherm was measured at 77 K. A Fourier transform infrared (FTIR) Perkin-Elmer model 2000 spectrophotometer was used to identify the related functional groups present in the synthesized adsorbents. 2.3. Procedures for the Adsorption and Desorption of Mercury. The adsorption of Hg(II) was assessed under static

Q e = [(Co − Ce)V ]/m

(1)

where Co is the initial concentration of the Hg(II) solution, Ce is the final concentration of the Hg(II) solution, V is the Hg(II) solution volume (liters), and m is the mass of adsorbent used (grams). The adsorbent reusability was investigated via an adsorption−desorption experiment. First, the adsorption experiment was conducted by mixing 25 mg of the adsorbent with 25 mL of the Hg(II) solution. After being mixed for 2 days, the mixture was filtered and washed several times using deionized water. Lastly, the adsorbent was dried at 105 ± 0.5 °C overnight. The mercury-loaded adsorbent was leached out via addition of 25 mL of 0.1 M desorption solutions (hydrochloric acid or potassium iodide) and shaken overnight at room temperature (30 ± 1 °C). After that, the adsorbent was separated by filtration and washed repeatedly using deionized water to remove the remaining desorption agent in the adsorbent, followed by drying at 105 ± 0.5 °C overnight, and prepared to be reused for the adsorption experiment. The adsorption and desorption cycles were repeated five times.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of Adsorbents. The synthesis of spherical silica involved the hydrolysis of TEOS in an alcohol/water/ammonia mixture. The addition of a CTAB surfactant template provided a source of micelles for the formation of hexagonal structure.15 BTESPT and MPTMS served as sulfur sources and were functionalized into silicas via a direct assembly pathway (in situ synthesis) using TEOS as a silica precursor. The morphology of the synthesized adsorbents is illustrated in Figure 1. The average particle size of the SMs was 0.47 μm. The particles were still in a spherical shape after the introduction of sulfur ligands. The sizes increased to 0.61 and 0.66 μm after the functionalization using MPTMS and BTESPT, respectively. The presence of sulfur ligands in the framework reduced the XRD pattern intensities (Figure 2), indicating that the adsorbents had a weakly ordered pore arrangement. For SMs, the XRD pattern of the spherical SMs exhibited only a broad single diffraction peak (2θ) between 2.5° and 3.3°. This result indicates that the hexagonal framework of the SMs was not well-arranged. Because the synthesis procedure was based on the procedure for preparing MCM-41, the XRD pattern of the SMs was expected to only have one main correlation reflection at 2θ between 2° and 3°, and the other two weaker reflections at higher 2θ angles, which were indexed as 100, 110, and 200. This indicates a highly ordered hexagonal pore arrangement.14,24 It was reported that the formation of a spherical 1226

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Figure 3. Nitrogen adsorption−desorption isotherms of synthesized adsorbents.

classified as type II.25,27 The type II isotherm is normally found for nonporous materials or for materials with diameters exceeding those of micropores. However, mesoporous materials with an irregular pore system can also exhibit type II isotherms.28 The surface areas (square meters per gram) were analyzed using a Brunauer−Emmet−Teller (BET) model, while the pore diameter (nanometers) and pore volume (cubic centimeters per gram) were calculated using the Barrett−Joyner−Halenda (BJH) equation. The surface area, pore volume, and pore size of the SMs were 960.29 m2/g, 0.64 cm3/g, and 26.56 Å, respectively, which were classified as mesoporous materials. The introduction of the pendant sulfur ligands on the surface and inside the pores partially blocked the adsorption of nitrogen, thus reducing the adsorption of nitrogen into the surface of the materials.3,12 Therefore, the functionalized silica (S-SMs) resulted in smaller surface areas and pores compared to those of the SMs (Table S2 of the Supporting Information). The FTIR spectra of the synthesized adsorbents are shown in Figure 4. For SMs, the recorded major characteristic bands included strong and broad bands between 3600−3200 and 1640 cm−1, which was attributed to O−H stretching from silanol groups and adsorbed water.3,4 Peaks at ∼800 and ∼1100 cm−1 represent -Si-O-Si- symmetrical and antisymmetrical stretching, respectively. The Si−O stretching vibration of the silanol groups was detected at 968 cm−1. After functionalization, FTIR spectra similar to those of the SMs were observed having additional peaks caused by the introduction of a sulfur ligand into the silica frameworks. These could be seen from the characteristic bands of the aliphatic C−H stretching from the C−H, C−H2, and C−H3 stretches at 2850−3000 cm−1 and the C−H2 and C−H3 deformation vibrations at 1350−1480 cm−1. Small peaks at wavenumbers around 500−720 cm−1 were observed in the FTIR spectra of S-SMs, which were attributed to the C−S or S−S vibration bands.15 3.2. Effect of pH on Hg(II) Adsorption. Figure 5 shows the effect of Hg(II) adsorption at different solution pH values. The inset of Figure 5 shows the plot of equilibrium pH versus initial pH. It was observed in the plot that the equilibrium pH (pHe) tended to change from its original pH (pHi). The adsorption at pHi 8) resulted in pHe being less basic, as a result of the increasing H+ concentration. At higher pH values, the Hg(II) ion might be hydrolyzed and thus the proton (H+) released into the medium solution, which would reduce the basicity of the equilibrium solution. It was reported that under basic conditions, mercury in the form of Hg(OH)2 usually dominates and HgOH+ and Hg2+ species will still exist in small amounts.14 As one can see in Figure 5, the adsorption of Hg(II) onto SMs was found to be nearly independent of the solution pH. The introduction of sulfur ligands into the SM adsorbents (BTESPT-SMs and MPTMS-SMs) was found to cause a significant change in the Hg(II) adsorption capacity. At low pH, 1228

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The Langmuir, Freundlich, Dubinin−Radushkevich (DR), and Temkin isotherm models were used to explain the Hg(II) adsorption data. Linear and nonlinear forms of the isotherm models are shown in Table S3 of the Supporting Information. The linear regression coefficient (r2) and χ2 analysis were used to evaluate the best models to describe the Hg(II) adsorption data. χ2 was determined according to eq 2. χ 2 = (Q exp − Q cal)2 /Q cal

(2)

where Qexp and Qcal represent the experimental Q and calculated Q from the isotherm models, respectively. The best models for describing the adsorption isotherm data can be selected if r2 is equal to or near 1 and χ2 is as small as possible.30 The isotherm model parameters and r2 and χ2 values are listed in Table S4 of the Supporting Information. The Langmuir isotherm models were found to be the most suitable models for describing the isotherm for the adsorption of Hg(II) into all adsorbents. The χ2 values show that the DR and Freundlich isotherm models gave the lowest values for BTESPT-SMs and MPTMS-SMs, respectively. However, from the isotherm fitting in Figure 7, the DR and Freundlich lines deviated from the experimental data points. The isotherm fitting was plotted on the basis of the nonlinear equations using the model constant parameters obtained from the linear equation plot analysis. Via comparison of the r2 values, the Langmuir isotherm resulted in very good fitting, with r2 values of >0.990. In addition, the Qm calculated from the Langmuir isotherm was close to the experimental Qmax of both adsorbents. The isotherm model fittings as shown in Figure 7 also show that the theoretical Langmuir lines were closer to the experimental data for both adsorbents. The Langmuir isotherm model implies that Hg(II) adsorbs only on the free surfaces of the adsorbents and forms an adsorbate monolayer. Thus, the existence of sulfur-containing active sites on the adsorbent surfaces was probably the main cause for the increase in the Hg(II) adsorption capacity, even though both adsorbents had smaller surface areas. Attempts were made to compare these equilibrium isotherm results to the data reported in the literature. The adsorption isotherm of Hg(II) using other adsorbents (i.e., biomass and carbonized materials) usually follows the Langmuir and Freundlich isotherm models. However, the isotherms for the adsorption of Hg(II) onto MPTMS- or BTESPT-functionalized silica-based materials have not been discussed to any significant extent. Rostamin and co-workers31 studied the isotherms for adsorption of Hg(II), Pb(II), and Cd(II) onto MPTMSfunctionalized silica hollow nanospheres, in which they found that the Hg(II) adsorption followed the Sips isotherm model. Via comparison of the MPTMS-SMs and BTESPT-SMs with other adsorbents as reported in the literature (see Table S1 of the Supporting Information), the adsorption capacity was found to be lower. This result was probably due to the different characteristics of the silica materials, and the adsorption was performed under different conditions. 3.4. Dependence of Hg(II) Adsorption on Temperature. The effect of temperature on the adsorption of Hg(II) was studied by varying the adsorption temperatures from 30 to 60 °C, as shown in Figure 8. In general, the adsorption of Hg(II) onto synthesized adsorbents decreased with the increase in adsorption temperature. The performance of Hg(II) adsorption could also be determined by calculating the thermodynamic parameters such as changes in free energy

Figure 7. Isotherm model analysis of Hg(II) adsorption equilibrium data.

Figure 8. Effect of temperature on Hg(II) adsorption capacity.

(ΔG°), enthalpy (ΔH°), and entropy (ΔS°). These parameters were estimated according to the Gibbs expression. 1229

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The negative ΔH° values for the synthesized adsorbents indicate that the adsorption process was exothermic in nature.32 The ΔH° value can be used to measure the force of the interaction between the adsorbent and adsorbate, giving an indication of the bonding strength.33 The ΔH° for physical adsorption ranges from 4 to 40 kJ/mol, compared to that of the chemical adsorption, which ranges from 40 to 800 kJ/mol.33 As shown in Table 1, the ΔH° values suggest that the adsorption

activities. It means that the adsorbents had reached saturation and could not further adsorb any Hg(II) left in the solution. In this study, the kinetics of adsorption of Hg(II) was faster for the SMs than for the sulfur-functionalized adsorbents. The adsorption started to achieve equilibrium after 200 min of contact time. For the BTESPT-SMs and MPTMS-SMs, the adsorption activities were faster for the first 500 min and slowly increased with time until they reached equilibrium adsorption capacity. The minimum time required for achieving equilibrium adsorption conditions for the BTESPT-SMs and MPTMS-SMs was ∼1500 min, which is considered relatively long. The adsorption kinetic data were then further analyzed using the adsorption kinetic models. In this study, four commonly used kinetic models, namely, pseudo-first-order (PFO), pseudo-second-order (PSO), Elovich, and intraparticle diffusion kinetic models, were employed to evaluate the kinetics of adsorption of Hg(II) into the synthesized adsorbents. The adsorption kinetic model equations are given in Table S3 of the Supporting Information. The first three adsorption kinetic models are chemical reaction-based models, while the fourth is the adsorption diffusion-based model. Via comparison of the chemical reaction-based models, the PSO was chosen as the best model for describing the kinetics of adsorption of Hg(II) into the synthesized adsorbents. The nonlinear fitting gave a good fit to the experimental data (Figure 10). In addition, a higher r2 value and the lowest χ2 were observed from all adsorbents (Table S5 of the Supporting Information), thus confirming the applicability of the model for describing the kinetic adsorption data. The Weber−Morris kinetic model is most widely used for describing intraparticle diffusion. kid in the Weber−Morris equation is an intraparticle diffusion constant for the diffusion within the pore space (i.e., pore diffusion, kid1) or along the adsorbent surface within the pore (i.e., surface diffusion, kid2).35 The Weber−Morris plot is shown in Figure 11. The plot has two straight lines for each adsorbent, which indicates more than one-step diffusion existed in the entire adsorption process. For all samples, the kid values for the first straight lines (kid1) were found to be higher than the kid of the second straight lines (kid2). The diffusion of mercury ions was faster during the early stage and became slower during the second stage. The kdi of SMs was always lower than that of S-SMs. With the introduction of sulfur active groups into the SM materials (SSMs), the intraparticle diffusion constants also increased, with the MPTMS-SMs having the highest kid values. The Weber−Morris equation gives only the initial insight into the diffusing mechanism. To confirm the rate-limiting step of the diffusion process, the film diffusion coefficient (Df) and the pore diffusion coefficient (Dp) for the initial adsorption process were calculated. The pore diffusion coefficient (Dp) was obtained from the Boyd plot.36 Bt is a function of F, where F is defined as the ratio of the amount adsorbed at time t to the amount at equilibrium (F = Qt/Qe).

Table 1. Thermodynamic Parameters of Hg(II) Adsorption with Synthesized Adsorbents

enthalpy, ΔH° (kJ/mol) entropy, ΔS° (J mol−1 K−1) Gibbs free energy, ΔG° (kJ/mol)

SMs

BTESPTSMs

MPTMSSMs

−9.412 0.090 −9.440

−47.465 1.731 −47.990

−47.768 1.766 −48.303

process could be considered as chemisorption for MPTMSSMs and BTESPT-SMs, while it could be considered as physisorption for SMs. These results were slightly in contrast to those obtained from the Temkin and Dubinin−Radushkevich (DR) isotherm model analyses. The values of EDR for all adsorbents were found to be ∼13 kJ/mol, which was in the range expected for chemisorption,31 while the values of bT fell into the range of the physisorption mechanism. The positive values of ΔS° suggest that the randomness of the solid− solution interface increased during the Hg(II) sorption process. The negative sign of ΔG° indicates that the adsorption of Hg(II) onto synthesized adsorbents was spontaneous. The larger magnitude of the negative ΔG° indicates a more favorable adsorption process.33,34 3.5. Dependence of Hg(II) Adsorption on Stirring Time. The dependence of Hg(II) adsorption on stirring time is shown in Figure 9. The adsorption rate increased rapidly at the

Figure 9. Effect of contact time on Hg(II) adsorption of synthesized adsorbents.

Bt = − 0.4977 − ln(1 − F ), F > 0.85

initial contact time, and thereafter, the adsorption rate increased gradually until it achieved an equilibrium adsorption capacity. As in the early stage, a high adsorption rate was observed, which might be due to more vacant sites being available for adsorption. As time increased, the available surfaces for adsorption became limited, thus reducing the adsorption rate. After that, the increased contact time between the adsorbents and solution did not show any adsorption

Bt =

{

π −

[π − (π 2F /3)]

(3)

2

} , F < 0.85

(4)

The slope (s) of the plot was used to calculate the pore diffusion coefficient (Dp) using eq 5: s = π 2(Dp /R2) 1230

(5)

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By assuming that the adsorbent was spherical in shape, the film diffusion coefficient (Df) was calculated using eq 6. Q t /Q e = 6(Df /πR2)t 0.5

(6)

where R is the radius of adsorbents. The diffusion coefficients for all adsorbents are listed in Table S5 of the Supporting Information. The results show that the values of Dp were larger than those of Df; therefore, pore diffusion was faster, thus confirming that film diffusion is the rate-limiting step for all adsorbents. In addition, via a comparison between adsorbents, the MPTMS-SMs exhibited the largest film diffusion coefficient (Df = 2.281 × 10−15 m2/min), which was still smaller than its pore diffusion coefficient (Dp = 3.447 × 10−15 m2/min), which was smaller than those obtained for other adsorbents. 3.6. Adsorbent Regeneration. A regeneration study was conducted to demonstrate the reusability of the adsorbent. It was conducted for up to five cycles, which was enough to demonstrate the reusability performance of adsorbents as reported in the literature.37−39 The study was conducted with only MPTMS-SMs, which had the highest adsorption capacity among the three synthesized adsorbents. Figure 12 shows the

Figure 12. Regenerability of MPTMS-SMs using 0.1 M HCl and 0.1 M KI solutions as desorption agents.

Figure 10. Chemical reaction kinetics-based model analysis of Hg(II) adsorption kinetic data.

amount of Hg(II) adsorbed with respect to the number of regeneration cycles. Cycle 1 was the adsorption of fresh adsorbent. The amount of Hg(II) adsorbed was 45.57 mg/g. After adsorbent regeneration using HCl as a regeneration agent, the amount of Hg(II) adsorbed for the second and third cycles decreased by ∼10% (≈40 mg/g). The amount of Hg(II) adsorbed was reduced an additional 9% (≈36 mg/g) for the fourth cycle and remained at ≈36 mg/g during the fifth cycle. It was found that as shown in Figure 12, the use of KI as a regeneration agent showed greater reusability compared to that of HCl. During the desorption process, the anion and cation charges of KI and HCl disrupt and loosen the S−Hg coordination complex. The halide ions of KI and HCl might form complexes with Hg in desorption solutions. For instance, mercury might form complexes, HgXnn+2− for n = 1−4, depending on the concentration of halide ions in solution.40 However, via comparison of the affinity of Cl, I, and S for soft acid Hg, I has the greatest affinity, followed by S and Cl. I is a softer Lewis base then S, while Cl is a borderline base; from the HSAB principle, softer acids tend to combine with softer bases,

Figure 11. Intraparticle/Weber−Morris plots of Hg(II) adsorption kinetic data.

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because softer acids prefer softer bases.41 Anion I tends to form a complex with Hg in solution compared to the S−Hg complex on adsorbent surfaces. The affinity of Cl for Hg is lower than that of S, and the S−Hg complex on adsorbent surfaces is relatively more stable. This might be one of the reasons for low adsorption after regeneration using HCl as a desorption agent. In addition, low adsorption might be due to the partial destruction of the silica adsorbent under acidic conditions.42 Silica materials are easy to hydrolyze under acidic conditions; thereafter, the physical and chemical characteristics of the silica might be changed and thus reduce the adsorption capacity. The increase in regeneration activity could be achieved by using a higher regeneration agent concentration. It was previously reported that the use of 12 M HCl as a regeneration agent is effective for recovering more than 90% of mercury, thus improving the performance of the recycled adsorbents.9 However, applying a high concentration of regeneration agent might cause adsorbent particle destruction.11

Higher Education (MOHE) under the Fundamental Research Grant Scheme (FRGS) (Grant Vote 78602), and the University Research Grant (Grant Vote GUP 00H63) is gratefully acknowledged.

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4. CONCLUSIONS The sulfur-functionalized silica microspheres (S-SMs) were prepared by using in situ synthesis of TEOS and sulfur ligands (BTESPT and MPTMS). The presence of sulfur ligands decreased the properties of SMs; however, it contributed to a higher Hg(II) adsorption. The Hg(II) capacity was found to be dependent on various adsorption conditions, such as the pH of the Hg(II) solution, the initial concentration, the temperature, and the agitation time. The highest Hg(II) adsorption capacity obtained under the experimental conditions presented here was 62.3 mg/g for the MPTMS-SM adsorbent. The isotherm data fit well to the Langmuir isotherm model, while the kinetic data followed the pseudo-second-order (PSO) kinetic model, with film diffusion found to be a rate-limiting step. The kinetic modeling results show that chemisorption was the main factor for determining the rate of adsorption of Hg(II) onto the synthesized adsorbents, which was consistent with the thermodynamic analysis. The regenerability results show that the adsorbent was indeed reusable, and a higher adsorption capacity was observed for KI as a regeneration than for HCl.

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ASSOCIATED CONTENT

S Supporting Information *

Hg(II) adsorption capacity of sulfur-functionalized silica adsorbents (Table S1), the surface area and pore characteristics of the synthesized adsorbents (Table S2), adsorption isotherm and kinetic model equations (Table S3) and constant parameters calculated from the adsorption isotherm model and kinetic model analyses (Tables S4 and S5, respectively). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +607-5581463. Telephone: +607-5535590. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Malaysian Ministry of Science, Technology, and Innovation (MOSTI) under the eScience Research Fund (Grant Vote 79281), the Malaysian Ministry of 1232

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