1. 1
Non-Competitive and Competitive Adsorption of
Heavy-Metals in Sulfur-functionalized Ordered
Mesoporous Carbon
Dipendu Saha1,*, Soukaina Barakat, Scott E. Van Brammer2, Karl A.
Nelson1, Dale K. Hensley3, Jihua Chen3
1 Department of Chemical Engineering,
2 Department 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
*Corresponding author’s E-mail: dsaha@mail.widener.edu , Phone: +1 610 499 4056, Fax:
610 499 4059 (D. Saha)

2. 2
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 revealed that sulfur content within the mesoporous
carbons are within the 8.2 to 12.9 at.% with key sulfur functionalities C-S, C=S, S=O-C and
SOx. SEM images confirmed the ordered mesoporosity within the material. The BET surface
areas of the sulfur-functionalized ordered mesoporous carbons were within 837 to 2865
m2/g with total pore volume 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 PbNO3),
cadmium (as CdCl2) and nickel (as NiCl2) ions in both non-competitive and competitive
mode. In both the modes, the trend of adsorption was Hg>Pb>Cd>Ni. In the non-competitive
mode, Hg and Pb adsorption demonstrated a strong pH dependency whereas Cd and Ni did
not reveal such trend. The distribution coefficient of heavy metal adsorption in non-
competitive mode was in the range of 2400-4000 mL/g for Hg, 280-2400 mL/g for Pb, and
145 to 165 mL/g for Cd and Ni. The kinetics of adsorption suggested that pseudosecond
order model fits better for all the metals. XPS analysis of metal-adsorption carbons suggest
that 7-8 % of the adsorbed Hg was converted to HgSO4, 14% and 2 % of Pb were converted
to PbSO4 and PbS/PbO, respectively and 5% Cd was converted to CdSO4. Ni could not be
detected in XPS owing to low adsorption and insufficient statistics. Overall results suggest
such carbon can be employed for the separation of the majority of heavy metals.
Key words:Orderedmesoporouscarbon,Sulfurfunctionality,BET surfacearea,Heavy metal,
pH dependency, kinetics.
1. Introduction

3. 3
It has been greatly evidenced that heteroatom doping on the carbon materials
dramatically influences its structure and properties1. Although it is true that majority of the
carbon-based materials inherently contain few heteroatoms, most commonly, oxygen,
hydrogenornitrogen,presenceofsulfurin the parentcarbonmaterials is quite rare. Usually,
an additional effort is required to insert such heteroatom in the carbon skeleton and
variation of synthesis protocol can systematically tune the presence of heteroatom
contents2. Sulfur doping is a unique approach of heteroatom doping and provides several
remarkable features to the host carbon3. Unlike previous precursors, a sulfur atom is much
larger compared with boron or nitrogen, so the sulfur atom protrudes out of the graphene
plane, thereby creating 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 induces polarizability and interactions with oxygen. Sulfur stays in different
chemical states in carbon including C-S, S-S, C=S, C-SH, S=O, SOx, or even sulfur rings3.
Literature reveals that a large number of precursors have been employed in the synthesis
protocol; majority of them utilized sulfur bearing chemicals or polymers8,9,10,11,12,13, ionic
liquids14,15,11 and proteins16,17,18. It is also observed that the majority of the sulfur-doped
carbons contained about ~5 % sulfur. Furthermore, as parent carbon materials,
nanocarbons or precursors of nanocarbons were often employed19,20. These fine chemicals
and nanocarbons were expensive, often toxic and difficult to scale-up. Amongst carbon-
based materials, ordered mesoporous carbon is another novel class of materials with highly
ordered structural integrity and uniform consistency. Although past effort revealed the
synthesis of sulfur-functionalized mesoporous carbons, it also employed sulfur containing
carbonprecursor of2-thiophenemethanolandhard-templating strategy21. To the best of our
knowledge, the synthesis protocol of soft-templated ordered mesoporous carbons with
sulfur functionality was not reported.
It has been reported that sulfur functionality has a strong affinity towards heavy
metals and therefore, sulfur-doped carbonshave been employed extensively in heavy-metal
adsorption from aqueous solution22,23,24. Recently, Li et al.25 demonstrated 99.9% of Hg
removal from aqueous solutions in thiol functionalized porous organic polymers. Besides

4. 4
physisorption in the pores of the parent carbon, different types of mechanisms have been
suggested towards affinity-based interactions of heavy metals with sulfur functionalities.
Sulfur increases the surface polarity and enhances negative charge on the surface thereby
attracting the positively charged heavy metal cation. According to Pearson theory, the
affinity of heavy metals towards sulfur may also be explained by soft acid-soft base
interactions3,26,27, where,sulfurfunctionalities act assoft baseandheavymetals assoftacids.
Direct chemical interactions are also suggested towards affinity of heavy metals towards
sulfur. Mercury may react with the sulfur functionality forming 28 Hg(SH)2, Hg2(SH)2 or
even29 HgSO4 or HgO. Strong affinity of heavy metals toward sulfur may also be correlated to
the availability of many heavy metals in sulfide form in earth’s core or the poisoning of
precious metal catalyst in presence of sulfur compounds. Despite the known facts about
affinity-based heavy metal adsorption in sulfur doped porous carbons, few features on this
topic was not clearly revealed, like qualitative and quantitative identifications of chemical
state of heavy metals upon adsorption, degree of affinity of heavy metals towards sulfur
functionality and competitive adsorption of heavy metals where they simultaneously
present in the aqueous mixture.
In this article, we have a reported a simple procedure to synthesize a highly ordered
mesoporous carbon with high sulfur content by using soft-templating strategy. In this
approach, we have employed resorcinol and amphiphilic triblock copolymers as carbon
precursor and pore dictating agents, respectively, followed by the activation with a sulfur-
bearing compound that simultaneously enhanced the surface area and introduced sulfur
functionality onto the carbon surface. Four most important and toxic heavy-metals, Hg, Pb,
Cd and Ni adsorption was examined in both non-competitive and competitive basis to
understand the heavy metal adsorption in this carbon followed by their chemical states in
carbon upon adsorption.
2. Experimental
2.1 Synthesis of sulfur-doped mesoporous carbons

5. 5
The synthesis of sulfur-doped mesoporous carbon involved two steps; synthesis of
soft-templated mesoporous carbon, and then functionalization of the parent carbon by
additional activation agent that simultaneously enhanced its surface area and introduced
sulfur functionalities. The synthesis of mesoporous carbon employed similar protocols
reported in our previous publications30,31,32. 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 mins. Then, 4.8 mL
36 % formaldehyde solution was added to the mixture as cross-linking agent and continued
to stirring for 3 days till the reaction mixture became turbid and viscous. The cross-linked
polymer was separated from the mixture, put on a perti dish and at 120 °C overnight to
partially cure the polymer. This process converted the color of the polymer from turbid to
dark brown. In carbonization, the polymer sample is put in a porcelain boat and heated to
100 °C at a ramp rate of 10 °C/min, then up to 400 °C at 2°C/min, and finally up to 1000 °C
at 5°C/min in a Lindberg-Blue tube furnace. The sample was always kept under nitrogen
atmosphere during the carbonization protocol.
In order to insert sulfur functionality to the mesoporous carbon, we have employed
sodium thiosulfate (Na2S2O3) as the key sulfur-bearing compound as mentioned by Liu et
al33. Solid sodium thiosulfate and mesoporous carbonswere mixed in 4:1, 2.5:1 and 1:1 ratio
in a lab blender and put inside the tube furnace in a porcelain boat for further activation and
sulfur functionalization. It is heated upto 800 °C at 10 °C/min and then cooled down to room
temperature in nitrogenatmosphere.In this manuscript, the sulfur-dopedcarbonsthat were
synthesized with 4:1, 2.5:1and 1:1ratios ofsodiumthiosulfate to carbonsare namedasMCS-
1, MCS-2 and MCS-3, respectively. The pure mesoporous carbon is termed as MC
2.1 Materials Characterizations
All the sulfur-doped mesoporous carbons were characterized with pore textural
properties including BET surface area and pore size distribution. Typically, nitrogen
adsorption-desorption experiments at 77 K and pressure upto 1 bar was performed in

6. 6
Quantachrome’s Autosorb-iQ-any gas instrument. Both BET surface area and pore size
distribution by non-local density function theory (NLDFT) were calculated in the built-in
softwareofthe instrument. X-ray photoelectronspectroscopy(XPS)analysis was performed
in 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 in Carl Zeiss
Merlin SEM microscope operating at 30 kV. The energy dispersive X-ray (EDX) results as a
part of the SEM were obtained with a system from Bruker Nano GmbH using an XFlash
detector 5030. TEM images were taken in Carl Zeiss Libra 120 TEM operating at 120 kV.
Thermogravietric analysis was performed in TA instruments’ SDT Q600 instrument.
2.2 Adsorption of heavy-metals
Four types of heavy-metals that were chosen for this study are mercury (Hg), lead
(Pb), cadmium (Cd) and nickel (Ni). HgCl2, CdCl2 and NiCl2 and Pb(NO3)2 were chosen as the
salts for the corresponding metal source. For Pb, the nitrate salt was not selected owing to
the low solubility of PbCl2. All the solutions were made in high purity molecular biology
gradewater in thehighest concentrationof100 ppm andthe lowerconcentrationweremade
by subsequent dilution. The lower pH solutions were made by adequate mixing with HCl,
whereas the higher pH solutions were generated by NaOH adjustment.
In the courseofstudyof heavy-metal adsorption,onlyonetype ofcarbonwasselected
with highest sulfurcontent (MCS-1). Fornon-competitive (puremetal basis) adsorption, two
types of studies were performed for each metal, a pH run and a kinetic run with a constant
initial metal concentration of 100 ppm. The pH of the solutions was varied with about 3 to
11 unless precipitations of metals were observed owing to the hydrolysis. Additionally, no
attempt was made to produce the solutions of pH higher 11 due to the limiting accuracy of
the pH meter in reading the high pH value. The pH run was used to determine the pH that
corresponds to the highest adsorption, and the kinetic runs were performed in that
particular pH. For competitive adsorption, pH adjustment was not performed owing to
precipitations of few metals when the base (NaOH) was added.

7. 7
For all the pH and kinetic runs, solutions of 100 ppm of the metal salts were made in
25 mL of pure water and mixed with 0.025 g of carbon in a 100 mL round-bottom flask and
stirred for required interval of time. For all the pH runs, the mixture stirred for4 hr, whereas
for the kinetic runs, they were stirred for different time of intervals of 2 mins to 4 hrs. Upon
completion of stirring, the solution was filtered to separate the carbon and filtrate solution
was used to analyze the remaining metal contents. Some of the carbons separated by
filtration were employed for further XPS studies to analyze the chemical nature of the
adsorbed metal salts.
The concentration of all the metals was measured in xxx atomic absorption
spectroscope (AAS) with an appropriate hollow cathode lamp for a particular element. The
calibration plot of each of the metals was made prior to the experiment in each time of the
analysis. For Hg and Pb, a linear calibration profiles were achieved upto 100 ppm but for Ni
and Cd, linearity was restricted to only 25 ppm, and hence appropriate dilution was
performed for those metals.
3. Results and Discussions
3.1. Materials Characteristics
3.1.1. Pore Textural Properties
The degree of burn-off 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.
Nitrogen adsorption-desorption plots of all the carbons (inset of fig. 1) at 77 K revealed a
type IV adsorptionisothermaccordingto IUPAC nomenclatureand thelarge hysteresisloops
suggest the presence of mesoporosity. The BET specific surface area of MC, MCS-1, -2 and -3
are 605, 837, 1228 and 2865 m2/g, respectively. The pore size distribution is obtaining by
combining the non-local density function theory(NLDFT) plotsforN2 adsorptionat 77 K and
CO2 adsorption at 273 K (fig 1). The pore size distribution suggests that the carbons have a
median mesopore width at about 40-45 Å and several micropore widths at 15, 8.2, 4.7 and
3.5 Å. Accordingto table 1, it is also observedthat total and microporevolumeincreases with
the increase in BET surface area. The highest pore volume belonged to MCS-3 (2.3 cm3/g),

8. 8
but the total pore micropore volume belonged to 0.82 cm3/g, respectively. Although the BET
surface area and pore volume of all the sodium thiosulfate treated mesoporous carbons
enhanced compared to the pure mesoporous carbon (MC), the trend in BET surface area
suggests that a higher ratio of sodium thiosulfate is not good towards enhancing the surface
area and probably a 1:1 ratio of carbon to sodium thiosulfate could be the most optimal in
getting the best porosity. Such trend contradicts the past result reported by Liu and
Antoinette33, who employed glucose as carbon source to synthesize sulfur doped porous carbons
by reacting with sodium thiosulfate.
3.1.2. X-ray Photoelectron Spectroscopy (XPS)
The surface functionalities of the sulfur-doped mesoporous carbons were obtained
by X-rayphotoelectronspectroscopy(XPS). Figure2(a)show theoverall spectrumofthe XPS
and it shows the presence of C(1s), O(1s) and S (2s, 2p) peaks. The overall quantitative
analyses of the surface functionalities are shown in table 2. 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
compensatedby oxygenand sulfuratoms, which are 20.1 and 12.9%, respectively, which are
highest amongst all the sulfur-doped porous carbons synthesized in this work. The sulfur
functionalities of12.9% is oneofthehighest sulfurcontents oncarbonreportedinliterature3
except by Liu and Antoinette33. The key sulfur functionalities that were distinctly identified
through XPS are C=S, C-S, S=O-C, and SOx. The other sulfur functionalities that may also be
present in these carbons are S-O, C-O, C-S-C and C-SH and could not be separated in XPS
owing to their overlapping energy levels. In all the carbons, the highest sulfur functionality
belongs to the C-S and SOx, as observed in the table. Besides such functionality, all the
samples have about 1% of sodium that might have originated from sodium thiosulfate. From
the trend in qualitative analysis of the sulfur-functionalities of the mesoporous carbons, it
has been evident that a higher ratio of sodium thiosulfate during the activation of pure
mesoporouscarbonsenhancedbothsulfurandoxygencontentoftheresultantcarbons. Such
trend is in the opposite side that of pore textural properties as it was observed that higher
ratio of sodium thiosulfate lowered the pore textural properties. Figure 2(b) shows an
example of the peak deconvolution of MCS-1 for S-2p and C-1s (inset). Based on the sulfur

9. 9
contents, we have selected MCS-1 for further studies and heavy metal adsorption. Although
MCS-3 has the highest BET surface area and apparently may have a higher adsorption
amount, higher sulfur functionalities in MCS-1 will provide a better insight into the role of
sulfur in influencing the binding of heavy-metals on the carbon surface.
3.1.3. Electron Microscopy and EDX spectra
The SEM images of MCS-1 are shown in Fig 3(a)-(c) in different angles and
magnifications. Figure 3(a) shows the side-view of the mesopores that clearly reveals the
channel-like structures of the mesopores. Fig 3(b) and 3(c) reveal the top-view of the
mesopores,wherethe pore mouthswereclearly visible asblack dotsorcircles. It is observed
that these mesopores are highly ordered and in equal distance from each other. A close
observation of these pores suggests that the width of these pores is about 60 Å, and wall
thickness is about 25-30 Å. The mesopore width obtained in the visual observation is very
close to that of obtained adsorption measurements (46 Å). The TEM image is shown in fig
3(d) but it couldnot reveal the orderedstructure. The energydispersive X-ray (EDX) spectra
and mappings are shown in figure 4(a)-(d). Besides the peaks for carbon, oxygen and sulfur,
it also shows the presence of small amount of sodium and aluminium. The sodium is
originated from sodium thiosulfate and aluminium might have originated from porcelain
boat during carbonization or activation. Sulfur contents calculated from EDX are MCS-1:
12.5-13.9%, MCS-2: 9.8-11.1%, and MCS-3: 7.55-9.53%, which are close to that of obtained
from XPS analysis.
3.1.4. Thermogravimetric Analysis
The results ofthermogravimetric analysis ofMCS-1 areshownin figure5. The sample
is fairly stable up to 150-300 °C in nitrogen and air. In nitrogen, it looses less than 20% of its
weight upto 1000 °C. In air, it suffers a sharp loss of over 75 % from 400 °C to 580 °C and
because of such trend a large derivative peak appears at about 530 °C. The inset figure that
demonstrates the TGA under nitrogen reveals several peaks, with three prominent peaks at
245 to 675 °C regions. Most likely these peaks are associated with partial decomposition of
sulfur and/or oxygen functionalities.

10. 10
3.2 Non-competitive heavy-metal adsorption
The influence of pH of the solution on the equilibrium non-competitive adsorption of
heavy metals is shown in figure 6(a)-(d). The adsorption of mercury and lead demonstrated
a strong pH dependence, whereas such effect for cadmium and nickel is poor. The highest
adsorption of mercury took place at about pH=5, the lower or upper pH demonstrated
inferior adsorption equilibrium. The optimum mercury adsorption in the pH level of 5 to 6
is similar to those of previous reports34,35. For lead, adsorption was low within pH=2 to 5
and started to rise monotonically upto the pH of 9. For cadmium, the influence of solution
pH on equilibrium adsorption is quite narrow, within 35.5 to 35.92 mg/g, respectively. It
showed only a slight but sharp increase from pH=3 to 5 and remained same upto pH of9. For
nickel, except a narrow drop in the pH of 4, the influence of pH on equilibrium uptake is
completely negligible. For these data, it is clear that our adsorbent material has strongest
affinity for mercury followed by lead, cadmium and nickel. Its affinity towards nickel is very
small. For heavy metal adsorption, it is also a common practice to calculate the distribution
coefficient ( Kd
), defined as
Kd =
Co - Cf
Cf
V
M
æ
èç
ö
ø÷ ….(1)
where, Cois the initial concentration of metal, Cf 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 correspondingto each pHvalues ofthe solutions areshown in right-
side axes of figure 6(a)-(d). It is obvious that higher adsorption amount will result in higher
Kd values and therefore, the mercury possess the highest Kd values of 2400 to 4000 mL/g
within the pH range of 2 to 9. Lead demonstrated the following lower values of 280 to 2400
mL/g. The values of distribution coefficients of cadmium and nickel were much smaller,
within about 145 to 165 mL/g. Although few literature suggested that distribution
coefficient formercuryin someadsorbents couldbeone orderofmagnitude higher than that
of ours21,25 our values were higher (often order of magnitude) than several other
carbonaceous and non-carbonaceous adsorbents reported in literature34,29 ,36,37,38,39.

11. 11
The role of surface functionality on heavy-metal adsorption along with the influence
of pH is a very complex phenomenon and several factors can simultaneously dictate the
adsorption and chemical complexation processes. Besides sulfur functionality, it has been
suggested oxygen functionality may also influence the heavy-metal adsorption40. Mercury
and mercuric ion (Hg2+) can react with sulfur functionality on carbon surface forming
different types of complexes or dimers 41,42,43,44 like,
2HgCl2
¾ ®¾¬ ¾¾ (HgCl2 )2
-SH + Hg2+ ¾ ®¾¬ ¾¾ -S - Hg+
+ H+
-SH + HgOH+ ¾ ®¾¬ ¾¾ -S - HgOH + H+
-SH + Hg(OH)2
¾ ®¾¬ ¾¾ S - HgOH + H2O
-SH + 2Hg(OH)2
¾ ®¾¬ ¾¾ S - Hg -O - Hg -OH + 2H2O
HgOH+ ¾ ®¾¬ ¾¾ HgO + H+
2Hg + SO3
2-
+ 2OH- ¾ ®¾¬ ¾¾ Hg2
2+
+ SO4
2-
+ H2O
At low pH, the carbon surface is slightly positively charged35, and hence the positively
charged mercury complex (like S-Hg+) or Hg2+ may act as electrostatic barrier and prohibit
further adsorption of mercury41. This may explain the lower adsorption of mercury in lower
pH values. When the pH of the solution gradually increases, at certain point, the positively
charged mercury species became neutral41, like S-HgOH, Hg(OH)Cl or Hg(OH)2. Such neutral
species overcame the electrostatic barrier and hence adsorption reached its peak value.
When the pH of the solution is further increased, different types of negatively charged
species may generate35,45 in presence of plentiful of OH-, like Hg(OH)3-. This negatively
charged species may again suffer from electrostatic repulsion from the slightly negatively
carbon surface at elevated pH. At higher pH, the competitive adsorption of OH- may also
lower the overall uptake mercury35. It was suggested that the partial blockage of mercury
ion as OH(HgClOH) may also hinder its adsorption at elevated pH levels 46 . It is worth
mentioning in context that the key sulfur functional 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 lower compared to other sulfur

12. 12
functionalities, like C-S or SOx. Higher thiol functionality in this carbon might have enhanced
the overall adsorption or distribution coefficient of mercury. In the process of adsorption of
mercury with sulfur-functionalized carbons, no visual observation of white agglomerate of
Hg(OH)2 or red precipitate of HgO was observed, probably owing to the very low initial
concentration of mercury.
The lower adsorption of other metals compared to mercury might be related to
several other complex phenomena including poor affinity, soft acid-soft base interactions or
lower micropore volumes. Although the studies on interactions between lead and sulfur
functionality were not as details as that of mercury, generally it is suggested that lead has a
lower affinity towards sulfur functionality compared to that of mercury. The dependence of
cadmium adsorption on solution pH is very small. In a previous report47, it was suggested
that surface oxygen functionality, mostly carboxylic group and hydroxyl group may
coordinatewith cadmium ionforming few complexesand enhance overall adsorption.It was
also suggested that pH dependency on the cadmium might be attributed to the complex
electrostatic interactions and chemical complexations48,49. At lower pH, the adsorption is
lower owing to the competition with protons and Cd2+ ions in slightly positively charged
carbon surface. At higher pH, different types of complex may form, including Cd(OH)2,
Cd(OH)+ or Cd(OH)3- that facilitate the adsorption48. Although the overall nature of pH
dependency in our result appears to be similar to that of previous reports49,50 the actual of
adsorption of amount only slightly changes with the pH (35.5 to 35.92 mg/g) and hence it
suggests that such phenomena have negligible influence in our case. In the literature, the
equilibrium adsorption of cadmium were reported to be both higher47 and lower50,48
compared to that of ours. It is also important to emphasize that we have found a direct
evidence that cadmium chemically interacts with sulfur functionality on carbon surface to
produce CdSO4, which has been described later. Although some other previous reports
suggested the pH dependency of nickel adsorption51,52 we did not reveal such dependency.
Additionally, a very low uptake of nickel in our carbon material probably confirms that the
chemical interaction was negligible. The lower uptake of nickel may also be caused by the
low surface area and micropore volume of the adsorbent itself.

13. 13
The lower adsorption of lead as well as cadmium compared to lead is also observed
in sulfurized microporous carbons reported by Gomez-Serrano et al44. It was also suggested
that HgCl2 does not dissociate into consecutive ions; in low concentration, about 2%
undergoes primary dissociation44, 53 ( HgCl2
¾ ®¾¬ ¾¾ [HgCl]+
+Cl-
). The secondary
dissociation ( [HgCl]+ ¾ ®¾¬ ¾¾ Hg2+
+Cl-
) is even smaller. Slightly different scenario can be
observedfor lead.Although, not directly observed forPb(NO3)2, it can besuggestedthat very
large amount of Pb(NO3)2 can undergo primary and secondary dissociation44,53 similar to
that of PbCl2. CdCl2 may also be dissociated and some complex equilibrium may be
reached44,53 (Cd2+
+ 3Cl- ¾ ®¾¬ ¾¾ CdCl2 +Cl- ¾ ®¾¬ ¾¾ [CdCl3]-
). In absenceofexcesschlorideions,
it may undergo secondary dissociation ( CdCl2
¾ ®¾¬ ¾¾ Cd2+
+ 2Cl-
). Therefore, it can be
assumed that in the solution, the key adsorbate species could be44 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 coordinate 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
adsorptionofmercuryas it wasmostly in undissociated form.Additionally, the soft (orhard)
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, respectively44. It is interesting to observe that the
degreeofsoftnessofacidity decreasesin the orderofHg>Pb>Cd,which is exactly in the same
order of their adsorption in carbon.
The non-competitive kinetic data of heavy-metal adsorption is shown in figure 8. The
kinetic experiments were performed in the corresponding pH, where the metal adsorption
was highest according to the pH studies. It took about 150 mins to reach the equilibrium for
mercury whereas lead reached equilibrium in less than 100 mins. For carbon-based
materials, the kinetics of mercury adsorption was as low as 60 mins as observed in
literature39. For quaternary ammonium-functionalized magnetic mesoporous silica, the
kinetics was as fast as 4 mins only34. The kinetics of cadmium was faster than most of the
reports published elsewhere.

14. 14
The mechanism by which the metal adsorption takes place may consist of 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 layeroroutside film of the particle (film diffusion),
(iii) transport of the ion from outside surface to the pores of the carbon (intraparticle
diffusion) and (iv) adsorption or chemical complexation at the active sites of the carbon. The
interparticle diffusion equation can be given as41
qt = Kidt1/2
+C……..(2)
where, is the adsorption amount at time t and Kid
is the intraparticle diffusion rate
constant (mg g-1 min-1/2). It is suggested that the entire adsorption mechanism is
intraparticle rate limiting only if the linear regression of qt
versus t1/2
plot passes through
the origin41. Formercury kinetics, we foundthat therewere three distinct linear regionsthat
may be attributed to step (i) or (ii), (iii) and (iv)41. For the lead, there were only two linear
regions and for cadmium and nickel, there were only one linear region because of their very
fast kinetics. The overall results suggest that none of the metal adsorption can be related to
intraparticle diffusion as the sole-rate limiting step.
It is also a common practice to fit the kinetics of adsorption by pseudofirst and
pseudosecond order rate equations. Pseudofirst order rate equation is given by
log(qe - qt ) = logqe -
k1
2.303
æ
èç
ö
ø÷ t ……(3)
where, qe is equilibrium adsorption amount, qt is adsorption amount at any time t, and k1is
pseudofirst order rate constant. k1(min-1) can be calculated from slope of the log(qe - qt )
versus t.
Pseudosecond order rate equation can be given by,
t
qt
=
1
k2qe
2
+
1
qe
æ
èç
ö
ø÷ t ……(4)
where k2 (g mg-1min-1) is the pseudosecondorderrateconstant.It can becalculated from the
slope and intercept of
t
qt
versus t . In our calculation, we found that mercury and lead
qt

15. 15
adsorption kinetics can be fit to pseudofirst order rate equation with regression (R2) values
of 0.95 and 0.76, respectively, whereas cadmium and nickel adsorption could not be fit.
Pseudosecondorderrateconstantcan befit to all the metal adsorptionwith R2 valuesof0.95
to 0.99. It was also observed that the equilibrium adsorption amount ( qe
) calculated by
model fitting is very close to the experimental values. All the calculated rate constants were
provided in table 3. It was observed that pseudosecond orderrate constants values increase
with the decreasing amount of equilibrium uptake.
3.3 Competitive heavy-metal adsorption
Competitive heavy-metal adsorption was performed for one equilibrium point only
in an aqueous solution with respect to 100 ppm of each metal concentration. The pH the
resultant mixture was measured to be 5.5. Any attempt to change the pH of the solution
either by HCl or NaOH adjustment caused white precipitate formation of constituent metals
(mostly in the order of Ni>Cd>Pb) and hence the adsorption experiments were performed
without any pH adjustment. The order of heavy-metal adsorption was in the same fashion as
that of non-competitive adsorption (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 8). 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. 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.
3.4 Chemical analysis of heavy-metal loaded carbons
Upon heavy-metal adsorption at the optimum pH level, the carbons were separated,
dried and analyzed under XPS to detect the chemical nature of heavy-metals. In the XPS of
mesoporous carbons loaded with mercury under non-competitive mode, the presence of

16. 16
mercury is clearly detected (fig 9). The doublet of mercury peak of Hg4f7/2 and Hg4f5/2
appearedat about101.18eV and105.09 eV, respectively. In the complex formation,the peak
positions aresimilar to that of previouslyreportedinliterature34 and slightly lowerthan that
of pure state (101.58 eV and 106.68, respectively34). Further peak fitting and quantification
suggested that about 8% of mercury adsorbed in the carbon are in the form of HgSO4. Rest
of the mercury (92 %) are in the chemical state of either HgCl2 or other converted complex
of HgO. XPS could not distinctly identify those chemical states as the binding energy levels
overlap in the region of ~101.5 eV. Chemical interactions with sulfur functionalities were
also revealed for lead and cadmium. The peaks of Pb4f5/2 and Pb4f7/2 at 144.08 and 139.18
eV were evident in the carbons after adsorption. Further analysis did not reveal any PbSO4,
however 2% PbS or PbO was found on the carbon surface. XPS could not distinguish sulfide
and oxide of lead as their binding energy level overlap at around ~137 eV. Rest of the lead
was in the form of PbNO3. For cadmium, the peaks for Cd3d3/2 and Cd3d5/2 peaks appeared
at about ~413 and 405 eV, respectively. Quantitative analysis suggested that about 5%
cadmium was in the form of CdSO4. Remaining cadmium was either in the form of either
CdCl2 or CdO (both binding energies overlap at about 405.5 eV). XPS analysis could not
identify nickel from nickel-adsorbed carbon owing to insufficient statistics. XPS results
suggest that affinity towards sulfur may be in the order of Hg>Cd>Pb. The higher adsorption
of lead compared to cadmium may also be attributed to the influence of porosity.
Upon analyzing the carbonin XPS aftercompetitive adsorption,mercuryand lead and
cadmium were clearly detected, but not nickel, which is similar to that of previous
observation. Quantitative analysis suggested that 7 % of adsorbed mercury converted to
HgSO4 that is only 1 % lower compared to non-competitive adsorption. Rest of the mercury
remained as HgCl2 or HgO form. For cadmium, no CdSO4 formation was detected that also
supports very low adsorption under competition. Surprisingly, very large amount of PbSO4
formation (14 %) was detected and no PbS/PbO was observed. It is quite evident that the
lead interacted quite differently under competition as no PbSO4 was detected under non-
competitive mode and only2% PbS/PbO was observed.VerylargeamountofPbSO4 may also
contribute to the overall high adsorption of combined heavy-metals (106.9 mg/g) compared
to anyindividual non-competitive adsorption.The currentstudy couldnot identify the actual

17. 17
reasonofhighamount ofPbSO4 formationundercompetitive mode. A morecontrolledseries
of experiments were needed to reveal the interactions between lead and carbon surface in
the complex environmentofcompetitive adsorptionandthosewere beyondthescopeof this
study.
4. Conclusion
In this work, we have reported synthesis of sulfur functionalized ordered
mesoporous carbon by soft-templating. Sulfur functionality was inserted onto the carbon by
reacting the mesoporous carbon with sodium thiosulfate that simultaneously enhanced the
surface area and introduced sulfur functionality on the carbon surface. The BET surface area
of the carbon materials were in the range of 837 to 2865 m2/g and total sulfur content of 8.9
to 20.1 % with highest sulfur functionality belonged to C-S and SOx. One carbon material
demonstrated highly ordered mesoporosity as observed in the SEM images. The sulfur
functionalized mesoporous carbon was examined with aqueous phase adsorption of
mercury, lead, cadmium and nickel in both competitive and non-competitive mode. In non-
competitive mode, mercury demonstrated highest adsorption amount followed by lead and
cadmium. Nickel demonstrated negligible adsorption. Mercury and lead adsorption were
strongly influenced by the pH of the solution, where cadmium demonstrated only minimum
influence of pH. Under competitive mode, order of adsorption of heavy-metals followed the
similar trend that of non-competitive adsorption, but the total adsorption amount was
higher than that of non-competitive adsorption. 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.
Acknowledgement
D. S. acknowledgesFaculty development awardfromWidener University. S. B. acknowledges
the financial supportfromIvanhoeFoundationforMastersthesis(Faculty advisor: D.S.) TEM

18. 18
(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.
Table 1. Pore textural properties of sulfur-doped mesoporous carbons

19. 19
Carbon type BET SSA
(m2/g)
Micropore
volume (cm3/g)
Total pore
volume (cm3/g)
MC
605
0.18 0.6
MCS-1 837 0.23 0.71
MCS-2 1228 0.38 1.1
MCS-3 2865 0.82 2.3
Table 2. Analysis of surface functionality by XPS analysis
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,
C-O, C-S-C, C-SH)
12.9 10.5 8.2
C=S 0.3 0.22 0.2
C-S 7.4 6.7 5.1
S=O-C 0.9 0.8 0.9
SOx 4.3 2.7 1.9
S-O/C-O 8.6 6.3 5.4
Table 3. Rate constants of heavy-metal adsorption

20. 20
Rate constants Hg Pb Cd Ni
Pseudofirst
order (k1)
(min-1)
0.019 0.030 -- --
Pseudosecond
order (k2)
(g mg-1min-1)
9.39x10-4 5.96x10-3 3.81 x10-2 9.56x10-2

21. 21
Fig. 1. Pore size distribution of pure and sulfur-doped mesoporous carbons. Inset figure
shows N2 adsorption-desorption plot at 77 K.

22. 22
Fig. 2. 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)
100200300400500600700800900
Intensity(a.u.)
Binding Energy (eV)
MCS-1
MCS-2
MCS-3
S
(2p)
C
O
S
(2s)
(a)
158160162164166168170172174
Intensity(a.u.)
Binding Energy (eV)
SO4
SO3
SO2 S=O-C
C-SH
C-S-C
C=S
(b)

23. 23
Fig 3. SEM image of MCS-1, where the parallel mesopore channels can be visualized (a),
ordered mesopore openings ((b) and (c)), TEM image of MCS-1 (d)
20 nm 20 nm
10 nm
(a) (b)
(c) (d)

24. 24
Fig 4. Energy dispersive x-ray (EDX) pattern of MCS-1 (a), EDX mapping of sulfur (b), carbon
(c) and oxygen (d)
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
keV
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
cps/eV
C O SS Na Al
AlNaO
C S
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
(keV)
CPS(a.u.)
(a)
(b)
(c) (d)

25. 25
Fig. 5. 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
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0
20
40
60
80
100
120
0 200 400 600 800 1000
Deriva'vewt.%(wt.%/ͦC)
Wt.%
Temperature ( ͦC)
Air
Nitrogen
0
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0 200 400 600 800 1000
Deriva'vewt.%
(wt.%/°C)
Temperature (°C)

26. 26
Fig 6. Effect of solution pH on metal adsorbed 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.

27. 27
Fig 7. Adsorption kinetics of Hg, Pb, Cd and Ni in MCS-1
0
10
20
30
40
50
60
70
80
0 50 100 150 200 250
MetalAdsorbed,non-compeve
(mg/g)
Time (min)
Hg Pb Cd Ni

28. 28
Fig 8. Competitive adsorption data for Hg, Pb, Cd and Ni in MCS-1

29. 29
Fig 9. XPS analysis of Hg, Pb and Cd adsorbed-mesoporous carbon (MCS-1) in both non-
competitive and competitive mode
Hg-4f
compe ve
Non-
compe ve
Non-
compe ve
Pb-4f
compe ve
Non-
compe ve
Cd-3d
compe ve

30. 30
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TOC Graphic
Hg-4f
compe&&ve
Non-
compe&&ve
10 nm
158160162164166168170172174
Intensity(a.u.)
Binding Energy (eV)
SO4
SO3
SO2 S=O-C
C-SH
C-S-C
C=S