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Non-Competitive and Competitive Adsorption of Heavy Metals in Sulfur-functionalized Ordered Mesoporous Carbon Dipendu Saha, Soukaina Barakat, Scott Van Bramer, Karl A. Nelson, Dale K. Hensley, and Jihua Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12190 • Publication Date (Web): 22 Nov 2016 Downloaded from http://pubs.acs.org on November 26, 2016

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ACS Applied Materials & Interfaces

Non-Competitive and Competitive Adsorption of Heavy Metals in Sulfur-Functionalized Ordered Mesoporous Carbon

Dipendu Saha1,*, Soukaina Barakat1, Scott E. Van Bramer2, Karl A. Nelson1, Dale K. Hensley3, Jihua Chen3 1 Department

of Chemical Engineering, of Chemistry, Widener University, One University Place, Chester, PA 19013 3 Center for Nanophase Materials Sciences, Oak Ridge National Laboratory. Oak Ridge, TN 37831, USA 2 Department

*Corresponding author’s E-mail: [email protected] , Phone: +1 610 499 4056, Fax: 610 499 4059 (D. Saha)

ACS Paragon Plus Environment

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Abstract In this work, sulfur-functionalized ordered mesoporous carbons were synthesized by activating the soft-templated mesoporous carbons with sulfur bearing salts that simultaneously enhanced the surface area and introduced sulfur functionalities onto the parent carbon surface. XPS analysis showed that sulfur content within the mesoporous carbons were between 8.2% and 12.9%. The sulfur functionalities include C-S, C=S, -COS and SOx. SEM images confirmed the ordered mesoporosity within the material. The BET surface areas of the sulfur-functionalized ordered mesoporous carbons range from 837 to 2865 m2/g with total pore volume of 0.71 to 2.3 cm3/g. The carbon with highest sulfur functionality was examined for aqueous phase adsorption of mercury (as HgCl2), lead (as Pb(NO3)2), cadmium (as CdCl2) and nickel (as NiCl2) ions in both non-competitive and competitive mode. Under non-competitive mode and at a pH greater than 7.0 the affinity of sulfur-functionalized carbons towards heavy metals were in the order of Hg>Pb>Cd>Ni. At lower pH, the adsorbent switched its affinity between Pb and Cd. In the non-competitive mode, Hg and Pb adsorption showed a strong pH dependency whereas Cd and Ni adsorption did not demonstrate a significant influence of pH. The distribution coefficient for non-competitive adsorption was in the range of 2448-4000 mL/g for Hg, 290-1990 mL/g for Pb, and 550 to 560 mL/g for Cd and 115 to 147 for Ni. The kinetics of adsorption suggested a pseudosecond order model fits better than other models for all the metals. XPS analysis of metal-adsorption carbons suggested that 7-8 % of the adsorbed Hg was converted to HgSO4, 14% and 2 % of Pb was converted to PbSO4 and PbS/PbO, respectively and 5% Cd was converted to CdSO4. Ni was below the detection limit for XPS. Overall results suggested these carbon materials might be useful for the separation of heavy metals. Keywords: Ordered mesoporous carbon, Sulfur functionality, BET surface area, Heavy metal, pH dependency, kinetics.


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1. Introduction

Heteroatom doping on carbon materials dramatically influences the structure and properties of these substances1. Although carbon-based materials often contain oxygen, hydrogen or nitrogen, the presence of sulfur in these carbon materials is quite rare. Usually, variation of synthesis protocol is required to systematically tune the presence of heteroatoms in the carbon skeleton2. Sulfur doping provides several useful characteristics to the host carbon3. Sulfur atoms are much larger than common heteroatoms, like boron or nitrogen, so the sulfur atom protrudes out of the graphene plane. Therefore, it creates an uneven surface with unique properties, such as superconductivity4,5 as revealed in the theoretical studies6,7. The lone pair of electrons in a sulfur atom introduces polarizability and interacts with oxygen. Sulfur is also found in different chemical states in carbon including C-S, S-S, C=S, C-SH, S=O, SOx, and sulfur rings3. Literature reveals that a large number of precursors have been employed in the synthesis protocol. Majority of the precursors are sulfur bearing chemicals or polymers8,9,10,11,12,13, ionic liquids14,15,11 and proteins16,17,18. Few of the reports employed post-synthesis modification using sulfurbearing agents, like 2-thiophenecarboxy acid19. Sulfur-doped mesoporous carbons were synthesized by utilizing sulfur bearing chemicals, like 2-thiophenemethanol20 or furfuryl mercaptans21.The majority of the sulfur-doped carbons contained about ~5 % sulfur. Furthermore, nanocarbons or precursors of nanocarbons were often employed as starting materials22,23. These reagents are expensive, often toxic and difficult to scale-up. Ordered mesoporous carbon is a novel class of carbon-based materials with highly ordered structural integrity and uniform consistency24,25,26. Past work synthesizing of sulfurfunctionalized mesoporous carbons used sulfur containing carbon precursor of 2thiophenemethanol and hard-templating strategy27. The synthesis protocol for softtemplated ordered mesoporous carbons with sulfur functionality has not been reported.

Although the exact interactions between different types of sulfur functionalities and heavy-metals are not fully understood yet, it has been generally accepted that sulfur functionality has a strong affinity for heavy metals and sulfur-doped carbon materials have been employed extensively in heavy-metal adsorption from aqueous solution28,29,30.


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Recently, Li et al.31 demonstrated 99.9% removal of mercury from aqueous solutions in thiol functionalized porous organic polymers. Besides physisorption in the pores of the parent carbon, different mechanisms have been proposed for affinity-based interactions of heavy metals with sulfur functionalities. Sulfur increases the surface polarity and enhances negative charge on the surface to attract the positively charged heavy metal cation. According to Pearson theory, the affinity of heavy metals towards sulfur is explained by soft acid-soft base interactions3,32,33 where, sulfur functionalities act as soft base and heavy metals as soft acids. Direct chemical interactions also explain the affinity of heavy metals towards sulfur. Mercury reacts with the sulfur functionality forming34 Hg(SH)2, Hg2(SH)2 or even35 HgSO4 or HgO. Strong affinity of heavy metals toward sulfur is also correlated to the availability of many heavy metals sulfides in the earth’s core or the poisoning of precious metal catalyst by sulfur compounds. Despite the known affinity-based heavy metal adsorption in sulfur doped porous carbons, some features on this topic are not clearly understood including qualitative and quantitative identification of the chemical state of adsorbed heavy metals, the degree of affinity of heavy metals towards sulfur functionality and competitive adsorption of heavy metals in aqueous mixtures.

In this article, we report a simple procedure using a soft-templating strategy to synthesize a highly ordered mesoporous carbon material with high sulfur content. In this approach, we use resorcinol and amphiphilic triblock copolymers as a carbon precursor and as pore dictating agents, respectively. These materials are activated with a sulfurbearing compound that simultaneously enhances the surface area and introduces sulfur functionality onto the carbon surface. Adsorption of four toxic heavy metals, including Hg, Pb, Cd and Ni was examined and the chemical states were determined in both noncompetitive and competitive systems to understand heavy metal adsorption in this carbon material.


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2. Experimental 2.1 Synthesis of sulfur-doped mesoporous carbons

The synthesis of sulfur-doped mesoporous carbons involved two steps: synthesis of soft-templated mesoporous carbon and functionalization of the parent carbon by additional activation agent that simultaneously enhanced its surface area and introduced sulfur functionalities. The synthesis of mesoporous carbon followed the similar protocols reported in our previous publications36,37,38. In this research, we have employed resorcinol as carbon precursor and Pluronic F127 as structure dictating agent. In a typical study, 5 g of resorcinol and 4 g of pluronic F127 were dissolved in 30 mL ethanol and 20 mL DI water. After they were dissolved, 0.5 mL 36 N HCl was added and stirred for 30 minutes. Then, 4.8 mL of 36 % formaldehyde solution was added to the mixture as a cross-linking agent and continued to stirring for 3 days until the reaction mixture became cloudy and viscous. The cross-linked polymer was separated from the mixture, put on a petri dish and partially cured overnight at 120 °C overnight to partially cure the polymer. This process converted the color of the polymer from whitish to dark brown. The polymer sample was then carbonized by heating in a porcelain boat with a Lindberg-Blue tube furnace.

The

temperature was increased from room temperature to 100 °C at a ramp rate of 10 °C/min, then to 400 °C at 2°C/min, and finally to 1000 °C at 5°C/min. After that, it is cooled to room temperature. The sample was always kept under nitrogen atmosphere during the carbonization and cooling protocol.

The sulfur functionality was inserted into the mesoporous carbon with sodium thiosulfate (Na2S2O3) as reported by Liu et al39. Solid sodium thiosulfate and mesoporous carbons were mixed in 4:1, 2.5:1 and 1:1 ratios in a lab blender and placed inside the tube furnace in a porcelain boat for further activation and sulfur functionalization. The sample was heated to 800 °C at 10 °C/min and then cooled to room temperature under a nitrogen atmosphere. In this manuscript, the sulfur-doped carbons that synthesized with 4:1, 2.5:1 and 1:1 sodium thiosulfate to carbons ratios are named as MCS-1, MCS-2 and MCS-3, respectively. The pure mesoporous carbon is termed as MC. The scheme of synthesis of ordered mesoporous carbons and sulfur functionalization is shown in figure 1.


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2.1 Materials Characterizations

The sulfur-doped mesoporous carbon materials were characterized for pore textural properties including BET surface area and pore size distribution. Typically, nitrogen adsorption-desorption experiments at 77 K and pressures up to 1 bar were performed using a Quantachrome Autosorb-iQ-any gas instrument. Both BET surface area and pore size distribution by non-local density function theory (NLDFT) were calculated using the instrument’s built-in software. X-ray photoelectron spectroscopy (XPS) analysis was performed using a Thermo-Fisher K-alpha instrument XPS system operating at monochromatic Al-Kα as an x-ray anode. The intensity of X-ray energy and resolution were 1486.6 eV and 0.5 eV, respectively. Scanning electron microscopic (SEM) images were captured using a Carl Zeiss Merlin SEM microscope operating at 20 kV. The energy dispersive X-ray (EDX) results were obtained using a Bruker Nano GmbH with an XFlash detector 5030. TEM images were obtained with a Carl Zeiss Libra 120 TEM operating at 120 kV. Thermogravimetric analysis was performed in TA instruments’ SDT Q600.

2.2 Adsorption of heavy-metals

The four heavy metals were chosen for this study are mercury (Hg), lead (Pb), cadmium (Cd) and nickel (Ni). Mercury (II) chloride (HgCl2), cadmium (II) chloride (CdCl2), nickel (II) chloride (NiCl2) and lead (II) nitrate (Pb(NO3)2) were chosen as the salts for the corresponding metal source. For Pb, the nitrate salt was selected due to the low solubility of PbCl2. All the solutions were made in high purity molecular biology grade water in the highest concentration of 100 ppm and the lower concentration were made by subsequent dilution. The lower pH solutions were made by adequate mixing with HCl, whereas the higher pH solutions were generated with NaOH adjustment.

The carbon material with highest sulfur content (MCS-1) was used to study heavy metal adsorption. For non-competitive (pure metal basis) adsorption, both pH and kinetic studies were conducted using 100 ppm metal solutions. For pH studies, the pH was varied from 3 to 9 unless the pH was limited by precipitations of the metal hydroxides. For each


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metal, the pH with the highest adsorption was then used for the kinetic studies. The competitive adsorption studies were performed at pH of the initial solution as any attempts to modify the pH of the solution resulted in precipitation of the metal salts or hydroxides.

For all the pH and kinetic runs, 25 mL of 100 ppm metal salt solutions were combined with 0.025 g of carbon material in a 100 mL round-bottom flask and stirred. Those combinations for pH studies were stirred for 4 hr and those for the kinetic studies were stirred for 2 mins to 4 hrs. Upon completion of stirring, the solution was filtered to remove the carbon and the solution was analyzed to determine the metal concentrations. The carbon materials were later analyzed by XPS to determine the chemical nature of the adsorbed metal salts.

The concentration of all the metals was measured using a Perkin Elmer 5000 Flame Atomic Absorption Spectrophotometer. Calibration standards were run prior to each analysis. The linear working range for mercury and lead was up to 100 ppm, for nickel and cadmium the linear working range was up to 25 ppm and solutions were diluted as needed.

3. Results and Discussions 3.1. Materials Characteristics 3.1.1. Pore Textural Properties

The degree of burn-off (amount of mass lost due to activation) in MCS-1, -2 and -3 are 24.5, 35 and 55%, respectively. The pore textural properties of the sulfur-doped mesoporous carbons are provided in table-1.


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Table 1. Pore textural properties of sulfur-doped mesoporous carbons Carbon type

MC

BET SSA

Micropore

Total pore

(m2/g)

volume (cm3/g)

volume (cm3/g)

0.18

0.6

605

MCS-1

837

0.23

0.71

MCS-2

1228

0.38

1.1

MCS-3

2865

0.82

2.3

Nitrogen adsorption-desorption plots of all the carbon materials are shown in figure S1 of supporting information. The isotherms are of type IV according to IUPAC nomenclature and the large hysteresis loops suggest the presence of mesoporosity. The BET specific surface areas were calculated within P/P0 values of 0.05 to 0.3 and these values for MC, MCS-1, -2 and -3 are 605, 837, 1228 and 2865 m2/g, respectively. Increase in surface area due to reaction with sodium thiosulfate suggested that sodium sulfate acted as a chemical activation agent. The pore size distribution is obtaining by combining the non-local density function theory (NLDFT) plots for N2 adsorption at 77 K and CO2 adsorption at 273 K (fig 2). The pore size distribution suggests that these carbons have a median mesopore width of 40-45 Å and micropore widths of 15, 8.2, 4.7 and 3.5 Å. According to table 1, both total pore volume and the micropore volume increase with the increase in BET surface area. Although the BET surface area and pore volume of all the sodium thiosulfate treated mesoporous carbons increased compared to the pure mesoporous carbon (MC), the trend in BET surface area suggests that a higher ratio of sodium thiosulfate does not increase the surface area and likely a 1:1 ratio of carbon to sodium thiosulfate produces the optimum porosity. This trend contradicts the previous results reported by Liu et al39, who employed glucose as carbon source to synthesize sulfur doped porous carbons by reacting with sodium thiosulfate.


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3.1.2. X-ray Photoelectron Spectroscopy (XPS)

The surface functionalities of the sulfur-doped mesoporous carbons were identified by X-ray photoelectron spectroscopy (XPS). The XPS spectrum in Figure 3(a) shows the presence of C(1s), O(1s) and S (2s, 2p) peaks. The quantitative analysis of the surface functionalities is shown in table 2. The total carbon content in MCS-1, -2 and 3 are 67, 76.5 and 82.9 %, respectively. The low carbon content in MCS-1 has been compensated by oxygen and sulfur atoms, which are 20.1 and 12.9%, respectively, are are highest amongst all the sulfur-doped porous carbons synthesized in this work. The total sulfur content of MCS-1 is 12.9%. With the exception of Liu et al.39, 12.9% of sulfur is one of the highest sulfur levels reported in literature3. The key sulfur functionalities identified by XPS include C=S, C-S, -COS, and SOx. Other possible sulfur functionalities include S-O, C-O, C-S-C and C-SH, which are not resolved by XPS because they have overlapping energy levels. In all the carbons, the most abundant sulfur functionalities are C-S and SOx, as observed in table 2. This table also shows the S-mass %, S-mass/surface area (g/m2) and S-mass/volume (g/m3). To calculate S-mass/surface area, BET surface area of each carbon was used standard, whereas to calculate S-mass/volume, the approximate density of carbons used as 1.8 gm.cm3. In addition, all the samples contain about 1% sodium that might be from the sodium thiosulfate. Based upon the qualitative analysis of the sulfur-functionalities of the mesoporous carbons, it is clear that a higher ratio of sodium thiosulfate during the activation of pure mesoporous carbons enhanced both sulfur and oxygen content. Such trend opposes the pore textural properties as it was observed that higher ratio of sodium thiosulfate lowered the pore textural properties. Figure 3(b) shows the peak deconvolution of MCS-1 for S(2p) and C(1s) (inset). Based on the sulfur contents, MCS-1 was selected for further studies and heavy metal adsorption testing. Although MCS-3 has the highest BET surface area and may have higher adsorption, the higher level of sulfur functionalities in MCS-1 will provide better insight into the role of sulfur on the binding of heavy metals on the carbon surface.


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Table 2. XPS analysis of surface functionality

Content/functionality

MCS-1

MCS-2

MCS-3

Total carbon content

67

76.5

82.9

Total oxygen content

20.1

12.8

8.9

Total sulfur (excluding S-O, 12.9

10.5

8.2

(at.%)

C-O, C-S-C, C-SH) C=S

0.3

0.22

0.2

C-S

7.4

6.7

5.1

-COS

0.9

0.8

0.9

SOx

4.3

2.7

1.9

S-O/C-O

8.6

6.3

5.4

S-mass%

26.83

23.03

18.74

S-mass/surface area

3.21X10-4

1.88 X10-4

6.54 X10-4

4.83X10-4

4.25 X10-4

3.37 X10-4

(g/m2) S-mass/volume (g/m3)

3.1.3. Electron Microscopy and EDX spectra

The SEM and TEM images for MCS-1 are shown in Fig 4(a)-(c) and (d), respectively, at different angles and magnifications. Figure 4(a) shows the side-view of the mesopores and clearly reveals the channel-like structures of the mesopores. Fig 4(b) and 4(c) reveal the top-view of the mesopores, and shows pore mouths that are clearly visible as black dots or circles. These mesopores are highly ordered and regularly spaced. Close observation of these pores shows that they are about 60 Å wide with a wall thickness of 25-30 Å. The energy dispersive X-ray (EDX) spectra and mappings for MCS-1 are shown in figure 5(a)(d). In addition to carbon, oxygen and sulfur, the EDX spectrum also shows the presence of small amount of sodium and aluminium. The sodium is from sodium thiosulfate and


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aluminium may be an impurity from the porcelain boat used for carbonization and activation.

3.1.4. Thermogravimetric Analysis

The results of thermogravimetric analysis of MCS-1 are shown in figure 6. The sample is fairly stable up to 300 °C in both nitrogen and air. In nitrogen, the weight loss is less than 20% up to 1000 °C. In air, MCS-1 loses over 75 % of the mass between 400 °C and 580 °C with a large derivative peak at 530 °C. The inset figure that illustrates the TGA under nitrogen reveals several peaks, with three prominent peaks at 245 to 675 °C regions. These peaks may be associated with partial decomposition of sulfur and/or oxygen functionalities.

3.2 Non-competitive heavy-metal adsorption

The influence of pH on non-competitive adsorption of heavy metals is shown in figure 7 and the more detailed views are shown in figure 8(a)-(d). The dependence of adsorption of mercury and lead on the pH of the incipient solution is strong but nickel and cadmium did not demonstrate such dependence. The highest adsorption for mercury is at a pH of 5, with a significant reduction at both the lower and higher pH levels. The adsorption of mercury at pH 5 is similar to previous reports40,41. For lead, adsorption was low from pH 2 to 5 but increased monotonically to pH 9. The influence of pH on cadmium adsorption is very small, with a range of only 35.5 to 35.92 mg/g, with only a slight increase between pH 3 and pH 5. For nickel, adsorption remains almost constant with a slight decrease at the pH of 3.6; any attempt to increase the pH results in precipitation of hydroxides. It is clear that, at a pH higher than 7.0 (alkaline medium), our adsorbent material has the strongest affinity for mercury followed by lead, cadmium and nickel. However, at the lower pH, (acidic medium) the adsorbent switches affinity between lead and cadmium and order of affinity is mercury>cadmium>lead>nickel. Its affinity towards nickel is very small in all conditions.


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For heavy metal adsorption, it is also a common practice to calculate the distribution coefficient ( K d ), defined as

Kd =

Co − C f  V    ….(1) Cf  M 

where, Co is the initial concentration of metal, C f is final concentration after adsorption, V is total volume of solution in mL and M is the mass of adsorbent in g. The values of distribution coefficients corresponding to each pH values of the solutions are shown in right-side axes of figure 8(a)-(d). It is obvious that higher adsorption amount will result in higher K d values and therefore, the mercury possess the highest K d values of 2400 to 4000 mL/g within the pH range of 2 to 9. Lead demonstrated the lower values of 280 to 2000 mL/g. The distribution coefficients for cadmium is even lower, from 550 to 560 mL/g. Obviously, nickel demonstrates lowest distribution coefficients of 120-150 mL/g. Although few literature suggested that distribution coefficient for mercury in some adsorbents could be one order of magnitude higher than that of ours27,31 our values were higher (often order of magnitude) than several other carbonaceous and non-carbonaceous adsorbents reported in literature40,35 ,42,43,44,45. The distribution coefficient values are shown in table 3. It is worth mentioning in this context is that no effort was made to measure the change in anion concentration in the solution after adsorption. Although rarely reported, anions may have a competitive adsorption in the mesoporous carbons that influence the overall uptake of heavy metal ions.

Table 3. Values of distribution coefficients

pH 2 3.5 5 7 9

Kd, Hg 2448 3347 4000 3000 2125

Kd, Pb 290 311 280 720 1990

Kd, Cd 550 550 560 560 560

Kd, Ni 147 115 146 142 139

The role of surface functional groups along with the influence of pH on heavy-metal adsorption is a very complex phenomenon and several factors can simultaneously


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influence the adsorption and chemical complexation processes. In addition to sulfur functionality, oxygen functionality may also influence the heavy-metal adsorption46. Mercury and mercuric ion (Hg2+) can react with sulfur functionality on carbon surface forming different complexes or dimers 47,48,49,50 like,

  →(HgCl2 )2 2HgCl2 ←  …..(1)   → −S − Hg + + H + −SH + Hg 2+ ←  …..(2)   → −S − HgOH + H + −SH + HgOH + ←  …..(3)   → S − HgOH + H 2O −SH + Hg(OH )2 ←  ….(4)   → S − Hg − O − Hg − OH + 2H 2O −SH + 2Hg(OH )2 ←  ….(5)   → HgO + H + HgOH + ←  …..(6)   → Hg2 2+ + SO4 2− + H 2O 2Hg + SO32− + 2OH − ←  …(7) At low pH, the carbon surface is slightly positively charged41, so that the positively charged mercury complex (like S-Hg+) or Hg2+ may act as electrostatic barrier and prohibit further adsorption47. This may explain the lower adsorption of mercury at low pH. When the pH of the solution increases sufficiently the positively charged mercury species become neutral47, like S-HgOH, Hg(OH)Cl or Hg(OH)2. These neutral species overcomes the electrostatic barrier and increase the adsorption. When the pH of the solution increases more, different negatively charged species are generated41,51 by the high concentration of hydroxides like Hg(OH)3-. The negatively charged species will undergo electrostatic repulsion from the slightly negatively carbon surface at elevated pH and reduce adsorption. At higher pH, the competitive adsorption of OH- may also lower the overall uptake of mercury41. It was suggested that the partial blockage of mercury ion as OH(HgClOH) may also hinder its adsorption at elevated pH levels52. It is worth mentioning in this context is that the key sulfur functionality that plays the significant role in the capturing mercury is thiol group (C-SH). In our carbon materials, C-SH could not be explicitly quantified and its presence could be reduced relative to other sulfur functionalities, like C-S or SOx. Higher thiol levels in this carbon material might have enhanced the overall adsorption or distribution


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coefficient for mercury. In these materials no white agglomerate of Hg(OH)2 or red precipitate of HgO was observed, probably because of

the very low initial mercury

concentration.

The low adsorption of other metals compared to mercury may be related to several complex phenomena including poor affinity, soft acid-soft base interactions or lower micropore volumes. Although studies on interactions between lead and sulfur functionality were not as detailed as those for mercury, it is suggested that lead has a lower affinity than mercury towards sulfur functionality. The dependence of cadmium adsorption on solution pH is very small. A previous report53 suggested that surface oxygen functionality, mostly carboxylic group and hydroxyl group may coordinate with cadmium ions and form complexes that enhance overall adsorption. It was also suggested that the pH dependency of cadmium might be attributed to complex electrostatic interactions and chemical complexations54,55. At lower pH, the adsorption is reduced by the competition with protons and Cd2+ ions in slightly positively charged carbon surface. At higher pH, different complexes may form, including Cd(OH)2, Cd(OH)+ or Cd(OH)3- that facilitate adsorption54. Although the overall nature of pH dependency in our result is similar to previous reports55,56 the actual of adsorption changes only slightly with pH (35.5 to 35.92 mg/g) and suggests that these phenomena have negligible influence in our experiments. The literature reports both higher53 and lower56,54 adsorption of cadmium. It is important to emphasize that we have direct evidence that cadmium chemically interacts with sulfur functionality on carbon surface to produce CdSO4, which is described later.

Although some previous

reports suggested a pH dependency for nickel adsorption57,58 we barely observe any dependence. Additionally, a very low uptake of nickel in our carbon material probably confirms that the chemical interaction is negligible. The lower uptake of nickel may also be caused by the low surface area and micropore volume of the adsorbent itself.

The lower adsorption of lead as well as cadmium compared to mercury observed in sulfur-doped microporous carbons is also reported by Gomez-Serrano et al50. They suggest that HgCl2 does not dissociate completely; in low concentration, about 2% undergoes


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  →[HgCl]+ + Cl − ). primary dissociation50, 59 ( HgCl2 ← 

The secondary dissociation (

  → Hg 2+ + Cl − ) is even smaller. Somewhat different scenario can be observed for [HgCl]+ ←  lead. Although, not directly observed for Pb(NO3)2, it can be suggested that very large amount of Pb(NO3)2 can undergo primary and secondary dissociation50,59 similar to that of PbCl2. The dissociation of CdCl2 also involves a complex equilibrium system50,59 (

  → CdCl2 + Cl − ←   →[CdCl3 ]− ). In absence of excess chloride ions, it may Cd 2+ + 3Cl − ←     → Cd 2+ + 2Cl − ). undergo secondary dissociation ( CdCl2 ← 

Therefore, Based upon these

equilibria systems, the key adsorbate species could be50 HgCl2 or (HgCl2)2, for mercury, Pb2+ or [PbCl]+ for lead and Cd2+ or [CdCl3]- for cadmium. According to Pearson hard soft acid based theory (HSAB), hard acids favor to coordination with hard bases and soft acids to soft bases. The sulfur-functionalized carbon acts as a soft base. As a general rule, the neutral species are softer acids compared to metal ions and this supports the higher adsorption of mercury as it was predominately in undissociated form. In addition, the soft (or hard) acids can be quantified through absolute hardness (η). The η values of mercury, lead and cadmium are 7.7, 8.46 and 10.29 eV, respectively50. The degree of softness of acidity decreases in the same order as the adsorption to the carbon materials, Hg>Pb>Cd.

The non-competitive kinetic data of heavy-metal adsorption is shown in figure 9. The kinetic experiments were performed at the pH with the maximum adsorption. The mercury system reached equilibrium in about 150 mins and the lead system reached equilibrium in less than 100 mins. This is slower than the literature reports for mercury adsorption on carbon-based materials in as low as 60 minutes45. With quaternary ammonium-functionalized magnetic mesoporous silica, kinetics as fast as 4 mins have been reported40. The kinetics of cadmium was faster than most of the reports published elsewhere.

The mechanism for metal adsorption includes four steps, (i) migration of metal ion from bulk of the solution to the carbon particle (bulk diffusion), (ii) diffusion of ion through the boundary layer of the material (film diffusion), (iii) transport of the ion from the


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surface of the material to the pores (intraparticle diffusion) and (iv) adsorption or chemical complexation at the active sites.

The interparticle diffusion equation is47

qt = K id t 1/2 + C ……..(8) where, qt is the adsorption amount at time t and K id is the intraparticle diffusion rate constant (mg g-1 min-1/2). The interparticulate rate limits the rate of adsorption only if the linear regression of qt versus t 1/2 passes through the origin47. The results for mercury show three distinct linear regions and are consistent with an adsorption mechanism that includes three of the four possible steps, (i) or (ii), (iii) and (iv)47. For the lead, there are two linear regions and for cadmium and nickel, there is one linear region as a result of the fast kinetics for these systems. The overall results suggest that none of the metal adsorption has intraparticle diffusion as the sole-rate limiting step. The adsorption kinetics was also fit to pseudofirst and pseudosecond order rate equations. The pseudofirst order rate equation is given as

 k  log(qe − qt ) = log qe −  1  t ……(9)  2.303  where, qe is equilibrium adsorption amount, qt is adsorption amount at any time t, and k1 is pseudofirst order rate constant. k1 (min-1) is calculated from slope of log(qe − qt ) versus t. The pseudosecond order rate equation is,

 1 t 1 = +   t ……(10) 2 qt k2 qe  qe  where k2 (g mg-1min-1) is the pseudosecond order rate constant. It can be calculated from the slope and intercept of

t versus t . The experimental results for mercury and lead qt

adsorption kinetics fit a pseudofirst order rate equation with regression (R2) values of 0.95 and 0.76, respectively. Cadmium and nickel adsorption kinetics did not fit a pseudofirst order rate equation. A pseudosecond order rate equation fits all the metal adsorption kinetic data with R2 values of 0.95 to 0.99. In addition, the equilibrium adsorption amount ( qe ) calculated by the pseudosecond order model is very close to the experimental values.


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All the calculated rate constants are listed in table 4, which shows that the pseudosecond order rate constants increase with decreasing equilibrium uptake.

Table 4. Rate constants of heavy-metal adsorption Rate constants

Hg

Pb

Cd

Ni

Pseudofirst order (k1) (min-1) Pseudosecond order (k2) (g mg-1min-1)

0.019

0.030

--

--

9.39x10-4

5.96x10-3

3.59 x10-3

4.41x10-2

3.3 Competitive heavy-metal adsorption

Competitive heavy-metal adsorption studies were performed with a pH 5.5 mixture of all four metal ions at 100 ppm concentration. Increasing or decreasing the pH with HCl or NaOH caused formation of a white precipitate of metal hydroxides or salts. The order of heavy-metal adsorption almost matched the non-competitive adsorption results (Hg>Pb>Cd>Ni). The adsorption amounts were, Hg: 70.75 mg/g, Pb: 29.98 mg/g, Cd: 4.96 mg/g, Ni: 1.2 mg/g (figure 10). Interestingly, this result suggests that the carbon surface has very high affinity for mercury followed by lead and virtually no affinity towards cadmium or nickel when they are simultaneously present in the system. Under these conditions, it is likely the mercury is preferentially adsorbed and that it occupies most of the active sites on the surface. The mercury adsorption dropped only about 11 % compared non-competitive adsorption, whereas lead, cadmium and nickel suffered 56, 86, and 90 % drop, respectively, compared to non-competitive adsorption. It is also worth mentioning that the total adsorption of all the metals together is about 106.9 mg/g, which is higher than any of the individual non-competitive heavy-metal adsorption, reported earlier. Owing to the absence of literature data, we could not compare our competitive adsorption with any previously published works.


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3.4 Chemical analysis of heavy-metal loaded carbons

Upon heavy-metal adsorption at the optimum pH level, the carbons were filtered, dried and analyzed by XPS to determine the chemical nature of the adsorbed heavy metals. Figure 11 shows the XPS spectra of mesoporous carbons loaded with mercury under noncompetitive mode and clearly shows the presence of mercury. The doublet mercury peaks for Hg4f7/2 and Hg4f5/2 appeared at 101.18 eV and 105.09 eV, respectively. In the complex formation, these peak positions were similar to previous literature reports40 and only slightly lower than the pure state (101.58 eV and 106.68, respectively40). Additional peak fitting and quantification suggest that 8% of the adsorbed mercury was in the form of HgSO4 and the remaining mercury (92 %) was found as either HgCl2 or a HgO complex. XPS does not resolve these chemical states because the binding energy levels overlap in the region of ~101.5 eV. Chemical interactions with sulfur functionalities were also shown for lead and cadmium. The peaks of Pb4f5/2 and Pb4f7/2 at 144.08 and 139.18 eV were observed in the carbons after adsorption. Further analysis did not reveal PbSO4, however 2% PbS or PbO was observed on the carbon surface. XPS cannot resolve the lead sulfide and lead oxide since these binding energy levels overlap at 137 eV. The remaining lead was in the form of Pb(NO3)2. For cadmium, the peaks for Cd3d3/2 and Cd3d5/2 peaks appeared at 413 and 405 eV, respectively. Quantitative analysis suggests that about 5% cadmium is in the form of CdSO4 and the remaining cadmium was either CdCl2 or CdO (these binding energies overlap at 405.5 eV). XPS analysis could not detect nickel from nickel-adsorbed carbon. XPS results suggest that affinity towards sulfur in the order of Hg>Cd>Pb. The higher adsorption of lead relative to cadmium may also be caused by the porosity of the materials.

Analysis of carbon by XPS after competitive adsorption clearly detected mercury, lead and cadmium, however nickel was not detected. Quantitative analysis suggests that 7 % of the adsorbed mercury is converted to HgSO4. This is only 1 % less than the 8% HgSO4 observed for non-competitive adsorption. The remaining mercury is either HgCl2 or HgO. No CdSO4 formation was detected, which is consistent with the very low adsorption observed in the competitive adsorption experiment. The results for lead were surprising


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with large amount of PbSO4 (14 %) detected and no PbS/PbO was observed. It is evident that the lead interaction is very different under competitive adsorption since no PbSO4 and only 2% PbS/PbO was detected in the non-competitive experiments. The very high concentration of PbSO4 may contribute to the increased adsorption of heavy metals (106.9 mg/g) in the competitive adsorption experiment. The current study could not identify the mechanism for PbSO4 formation under competitive mode and a more controlled series of experiments, beyond the scope of this study, is required to determine the interactions between lead and carbon surface in the complex environment of competitive adsorption.

4. Conclusion In this work, we reported the synthesis of sulfur functionalized ordered mesoporous carbon by soft-templating. Sulfur functionality was inserted onto the carbon by synthesizing the mesoporous carbon with sodium thiosulfate to enhance the surface area and introduced the sulfur functionality on the carbon surface. The BET surface area of the carbon materials ranged from of 837 to 2865 m2/g and total sulfur content ranged from 8.9 to 20.1 % with C-S and SOx as the primary sulfur functionality. One carbon material demonstrated highly ordered mesoporosity observed by SEM imaging. The sulfur functionalized mesoporous carbons were examined to study the aqueous phase adsorption of mercury, lead, cadmium and nickel in both competitive and non-competitive mode. In both modes, mercury demonstrated the maximum adsorption whereas nickel the minimum. Mercury and lead adsorptions were strongly influenced by the pH of the incipient solution, whereas cadmium and nickel did not demonstrate significant pH dependence. Under the competitive mode, the order of adsorption of heavy-metals followed the similar trend as that of non-competitive adsorption above neutral pH, but the overall adsorption amount was higher. The heavy metal adsorption was controlled by both sulfur functionality and pore textural properties. XPS analysis of the mesoporous carbon after metal adsorption revealed that part of the metals were converted to metallic sulfate or sulfides on the carbon surface.


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Acknowledgement D. S. acknowledges Faculty development award from Widener University. S. B. acknowledges the financial support from Ivanhoe Foundation for Masters thesis (Faculty advisor: D.S.) TEM (J.C.) and SEM (D.K.H.) experiments were conducted at the Center for Nanophase Materials Sciences of ORNL, which is a DOE Office of Science User Facility.


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Fig 1. Schematic of soft-templated mesoporous carbon synthesis and simultaneous sulfur doping and activation


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Fig. 2. Pore size distribution of pure and sulfur-doped mesoporous carbons as obtained by CO2 adsorption at 273 K and N2 adsorption at 77 K.


22

C-SH C-S-C

C-C

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Intensity (a.u.)

Page 23 of 37

(b)

O-C=O C-O-C

294 289 284 Binding Eneregy (eV)

279

SO4 SO3 SO2

-COS C=S

174

172

170

168

166

164

162

160

158

Binding Energy (eV) Fig. 3. Overall XPS scan of the sulfur-doped mesoporous carbons (a), detailed analysis of the S-2p peak of MCS-1 (inset: C-1s peak analysis of MCS-1) (b)


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20 nm

10 nm

(a)

20 nm

(c)

Page 24 of 37

(b)

(d)

Fig 4. SEM image of MCS-1((a)-(c)). The parallel mesopore channels are observed in (a) and ordered mesopore openings in (b) and (c). TEM image of MCS-1 (d)


24

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cps/eV

2.2

C

2.0

S

1.8

CPS (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.6

1.4

1.2

1.0

S

C

O Na Al O

Na

Al

S

0.8

0.6

(a)

0.4

0.2

0.0

0.5 0.5

1.0 1.0

1.5 1.5

2.0 2.0

2.5 2.5

keV

3.0 3.0

3.5 3.5

4.0 4.0

(keV)

(c)

(b)

(d)

Fig 5. Energy dispersive x-ray (EDX) pattern of MCS-1 (a), EDX mapping of sulfur (b), carbon (c) and oxygen (d). The bottom SEM image shows the area on which the EDX was performed. The scale bar in the SEM image is 20 nm.


25


0.7

Air Nitrogen

100

0.6 0.5

80 Deriva' ve wt.% (wt.%/°C)

0.04

0.4

0.035

60

0.03

0.025

0.3

0.02

0.015

40

0.01

0.005

0.2

0 0

200

400

600 800 1000

Temperature (°C)

20

Deriva' ve wt.% (wt.%/ ͦC)

120

Wt.%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 37

0.1

0 0

200

400

600

800

0 1000

Temperature ( ͦC) Fig. 6. Thermogravimetric analysis of MCS-1 in nitrogen and air. The dotted lines represent the derivative of the thermogram. The inset figure shows the derivative of thermogram for nitrogen


26

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Fig 7. Effect of pH in the adsorption of mercury (Hg), lead (Pb), cadmium (Cd) and nickel (Ni).


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Fig 8. Effect of solution pH on heavy-metal adsorption and the corresponding distribution coefficient (a: Hg, b: Pb, c: Cd and d: Ni) The red curves (left-axis) correspond to the metal adsorption, and dotted blue curves (right-axis) correspond to the distribution coefficient.


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Fig 9. Adsorption kinetics of Hg, Pb, Cd and Ni in MCS-1 at the optimal pH level (Hg=5.0, Pb=9.0, Cd, Ni-7.0)


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Fig 10. Competitive adsorption data for Hg, Pb, Cd and Ni in MCS-1, The pH of mixture was 5.5 and not adjusted.


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Hg-4f

compe

ve

compe

Noncompe ve

Cd-3d

Pb-4f ve

Noncompe ve

compe

ve

Noncompe ve

Fig 11. XPS analysis of Hg, Pb and Cd adsorbed-mesoporous carbon (MCS-1) in both noncompetitive and competitive mode


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