Accepted Manuscript Title: Enhanced sorption of mercury from compact fluorescent bulbs and contaminated water streams using functionalized multiwalled carbon nanotubes Author: Avinash Gupta S.R. Vidyarthi Nalini Sankararamakrishnan PII: DOI: Reference:
S0304-3894(14)00196-4 http://dx.doi.org/doi:10.1016/j.jhazmat.2014.03.020 HAZMAT 15793
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
29-11-2013 10-2-2014 10-3-2014
Please cite this article as: A. Gupta, S.R. Vidyarthi, N. Sankararamakrishnan, Enhanced sorption of mercury from compact fluorescent bulbs and contaminated water streams using functionalized multiwalled carbon nanotubes, Journal of Hazardous Materials (2014), http://dx.doi.org/10.1016/j.jhazmat.2014.03.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced sorption of mercury from compact fluorescent bulbs and contaminated water streams using functionalized multiwalled carbon nanotubes Avinash Gupta#, S.R. Vidyarthi# and Nalini Sankararamakrishnan$,* $
cr
ip t
Centre for Environmental Science and Engineering Indian institute of Technology Kanpur Kanpur, U.P. 208016. # Department of Chemical Engineering Hartcourt Butler Technological Institute, Kanpur, U.P. 208001, India
us
*author for correspondence, Tel: 915122596360, Email: [email protected]
an
Abstract
Three different functionalized multiwalled carbon nanotubes were prepared, namely, oxidized
M
CNTs (CNT-OX), iodide incorporated MWCNT (CNT-I) and sulfur incorporated MWCNT (CNT-S). The as prepared adsorbents were structurally characterized by various spectral
d
techniques like Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDAX),
te
Brunauer, Emmett, and Teller (BET) surface area analyzer, Fourier Transform Infra Red (FTIR)
Ac ce p
and Raman Spectroscopy. Loading of iodide and sulfur was evident from the EDAX graphs. The adsorption properties of Hg2+ as a function of pH, contact time and initial metal concentration were characterized by Cold vapor AAS. The adsorption kinetics fitted the Pseudo Second Order Kinetics and equilibrium was reached within 90 minutes. The experimental data were modeled with Langmuir, Freundlich, Dubinin-Redushkevich and Temkin isotherms and various isotherm parameters were evaluated. It was found that the mercury adsorption capacity for the prepared adsorbents were in the order of CNT-S > CNT-I > CNT-OX > CNT. Studies have been conducted to demonstrate the applicability of the sorbent towards the removal of Hg(0) from broken compact fluorescent light (CFL) bulbs and Hg(II) from contaminated water streams. Key words: Mercury, Contamination, MWCNTs, CFL, adsorption
Page 1 of 47
Ce Equilibrium Concentration (mg L-1) qe Amount of metal ion adsorbed at equilibrium (mg g-1) Amount of metal ion adsorbed at a given time (mg g-1)
qm Maximum adsorption Capacity (mg g-1) Pseudo second order rate constant (g-1mg-1min)
an
k2 ’
us
qt
cr
ip t
Nomenclature
kint Weber-Morris diffusion rate constant (mg-1g-1min1/2) Langmuir isotherm Constant (L mg-1)
M
b
RL Separation factor
d
KF Freundlich constant (L g-1)
T Absolute temperature (K)
te
nF Heterogenity of the sorption surface
Ac ce p
R Universal gas constant (8.314 J mol-1.K)
KT Temkin’s equilibrium binding constant (L mg-1) bT Temkin isotherm constant (KJ mol-1) BT Variation of adsorption energy (KJ mol-1) β
Coefficient of activity related mean sorption energy (mol2 KJ-1)
ε
Polanyi Potential
EDR Mean free energy of sorption (KJ mol-1)
Page 2 of 47
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1. Introduction Mercury is a highly toxic heavy metals and various uses of mercury include production of chlor-
cr
alkali, fossil fuels, in various switches, wiring devices, measuring and control devices, lighting,
us
and dental work. The highly reactive inorganic form of Hg(II) is most toxic and its toxicity arises due to its binding ability to sulfur containing proteins. Due to inherent toxicity of mercury EPA
an
recommends a limit of 2.0 µg L-1 [1] in water. Bureau of Indian Standards [2] recommends the
M
permissible concentration of Mercury in drinking water to be 1.0 μg L-1. Conventional techniques for Hg removal from aqueous solutions include sulphate or hydrazine
d
precipitation, ion-exchange, liquid–liquid extraction, adsorption and solid phase extraction via
te
activated carbon adsorption [3]. In recent years, carbon nanotubes (CNTs) have received
Ac ce p
considerable attention due to their unique one-dimensional nanostructure and range of fascinating mechanical, physical, chemical and electronic properties. High sorption capability of chemical pollutants by CNT is attributed to its hollow and layered nanostructure with a characteristically large surface area [4,5]. Ren and coworkers [6] reviewed the application of CNTs towards environmental pollution management. It has been found useful in the removal of various heavy metals like Pb(II) [7], Pb(II), Cu(II) and Cd(II) [8], Ni(II) [9] etc. CNTs have been found useful for Hg(II) remediation as well. For instance, Nassereldeen et al.[10] reported the use of CNTs grown on Granular activated carbon for the removal of Hg(II).
Silver
impregnated CNT [11] have been found useful for the caputre of elemental Hg from flue gases. Afshar et al. [12], developed aminated CNTs for the quanittiative solid phase extraction of
Page 3 of 47
Hg(II). An uptake capacity of 78.1 mg g-1 was reported for Hg(II) using virgin multiwalled carbon nano tubes [13]. Thus to improve the selectivity and adsorption capacity of the sorbent, functionalization with suitbale ligands has been one of the most resorted method. The high
ip t
affinity between Hg(II) and sulfur groups was put to advantage to develop various functionalized adsorbents using sulfur based ligands such as dithiocarbamate [14], 1-furoyl thiourea urea [15],
cr
thiol [16, 17], and benzoythiourea [18] etc. It is also well known that powdered KI [19] or KI
us
modified adsorbent [20] have been found useful for the capture of mercury. Thus, keeping in mind the interesting properties of MWCNT, and higher stability constants for Hg-S complexes
an
and Hg-I complexes, functionalization of MWCNTs were performed using Sulfur and Iodide containing ligands. Various functional groups like hydroxyl, carbonyl and carboxyl are
M
introduced on MWCNT by oxidation technique [21]. Thus three diffent kinds of MWCNTs
d
namely, oxidised (CNT-OX), sulfur incorporated (CNT-S) and iodide incoporated (CNT-I) were
te
sysnthesized and evaluated for Hg(II) removal. Systematic structural characterization were performed using various techniques and optimization of various paramters including reaction
Ac ce p
time, pH and dose rate were carried out. Kinetic and isotherm models were evaluated and a suitable mechanism for the adsorption has also been postulated. It is well known that CFL bulbs contain low pressure mercury vapor and release of toxic vapors arises when there is breakage and its concenration ranges bewtween 0.1 to 3.6 mg [22]. There are very few reports in the literature to treat the Hg(0) vapor emerging from CFL bulbs [23, 24]. Thus the prepared adsorbent was applied to the treatment of both Hg(0) from the CFL bulbs and Hg(II) from coal wash water streams. 2. Material and Methods 2.1 Instrumentation
Page 4 of 47
Mercury was analyzed by Cold Vapor Atomic Absorption Spectrometry (CVAAS) (MA-5840 analyzer ECIL). Experimental samples were filtered using Whatman 0.45 mm filter paper and the filtrates after suitable dilutions, were analyzed. Control experiments showed that no sorption
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occurred on either glassware or filtration systems. All assays were carried out in triplicate and only mean values are presented. Scanning Electron Microscopy (SEM) was done using FEI
cr
Quanta 200 machine. The samples were gold coated to improve their conductivity to obtain good
us
images. Elemental analysis was done by EDAX with a field emission scanning electron microscope operated at an accelerating voltage of 10 kV. The adsorbent was analyzed for the
an
specific surface area, pore volume, and PSD by N2-physisorption using Autosorb-1C instrument (Quantachrome, USA). Chemisorption analysis was also carried out to measure the active metal
M
surface area. The multipoint Brunauer, Emmett, and Teller (BET) surface area was measured
d
from the nitrogen adsorption/desorption isotherm. Infra-red measurements were made with KBr
Ge crystal.
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2.2 Materials
te
pelts using Tensor 27 (Bruker, Germany) in the attenuated total reflectance (ATR) mode using
Multi-walled carbon nanotubes (MWCNTs) of 10–20 nm diameters were purchased from Nanoshell USA. All other reagents were analytical grade and were used as received. Aqueous solutions at various concentrations were prepared from HgCl2 and used as a source for Hg(II). 2.3 Preparation of functionalized MWCNT: Oxidized form of CNT (CNT-OX) was prepared by heating the acquired plain MWCNTs with nitric acid at 70˚C for 4h followed by thorough washing with distilled water. Sulfur functionalized CNTs (CNT-S) was prepared by heating 1 g of CNT-OX and 2.5 mL of CS2 in 10 ml of methanol at 70˚C for 4h followed by washing with distilled water and air drying it.
Page 5 of 47
Iodide was functionalized on MWCNTs by heating 1 g of CNT-OX in 10 ml of 0.1 M of KI at 60˚C followed by repeated washing with distilled water and dried in the oven. The schematic representations of the preparation of various CNTs are shown in Fig.1.
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2.4 Adsorption Experiments
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Batch experiments were carried out with synthetic solutions of Hg(II) in 100 ml conical flasks at 110 rpm of orbital stirring in an incubator shaker, at room temperature and 2 h of contact time
us
with the adsorbent. Samples were then centrifuged for 10 min filtered with Whatman No. 42
an
filter paper, and filtrates were analyzed for Hg(II) concentration by CVAAS. Unless otherwise stated the parameters with synthetic water were: sample volume 20 ml, sorbent dose 0.02 g, pH
M
6, initial Hg(II) concentration 10 ppm, equilibration time 4 h. The effect of pH on adsorption of Hg(II) was studied in a pH range of 3 to 10 by agitating adsorbent (0.02g) with Hg(II) (20 ml, 10
d
ppm) for 2 h at 110 rpm. The pH was adjusted by adding aqueous solutions of 0.1M HCl or 0.1M
te
NaOH. Isothermal studies were carried out by adding 0.02 g adsorbent into 20 ml of Hg(II)
Ac ce p
solution of varying concentrations from 10 to 120 mg L-1. The amount of Hg(II) adsorbed and percentage removal of Hg(II) were calculated using the following equation:
qe =
(C0 − Ce ) × V W
% Removal of Hg(II) =
(C 0 − C e ) × 100 C0
(1)
(2)
Where qe is the amount of mercury adsorbed (mgg-1), C0 and Ce are the initial and equilibrium concentrations (mg L-1), V is the volume of the aqueous solution and W is the mass of the adsorbent (g) used in adsorption experiments. Kinetic studies were conducted by equilibrating 20
Page 6 of 47
ml of 100 mg/l of Hg(II) at a dose rate of 1.0 g/l and the amount of Hg(II) adsorbed were monitored at regular intervals of time.
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3.0 Results and Discussion 3.1 Physical Characterization
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3.1.1 Scanning Electron Microscopy
us
The surface morphology of plain CNT, CNT-OX, CNT-S and CNT-I were studied with the help of SEM fitted with EDX (Fig.2 ). Original plain CNT had diameter in the range of 20 – 80 nm
an
while the length varies from 1 – 10 μm (Fig. 1a). Loaded catalyst particles are seen throughout the plain CNT. After oxidation (Fig.1b) and functionalization with iodide (Fig.1c) and sulfur
M
ligands (Fig.1d), the metal particles were removed and there was no detectable change in the
d
surface morphology implying minimal damage during oxidation and functionalization
te
procedures. EDAX spectrum of CNT-I (Fig.3a) and CNT-S (Fig 4a) revealed weight % of iodide and sulfur to be 0.24 and 13.07% respectively. Additionally in functionalized CNTs elemental
Ac ce p
mapping images (Fig.3b-d, Figs.4b-d) revealed the uniform deposition of sulfur and iodide ligands in the respective CNTs. 3.1.2 FTIR spectra
FTIR spectra from the CNTs show a broad peak at ~3430Ԝcm−1 which is a characteristic of the
O-H stretch of hydroxyl group (Fig. 5) arising from the oscillation of carboxyl groups. Carboxyl
Page 7 of 47
group on the surface of the plain CNTs (Fig. 5a) could be attributed to the partial oxidation of the surface of CNTs during purification by the manufacturer. Further, broad peak at 3430 cm-1 are
ip t
also observed in CNT-OX (Fig. 5b), CNT-S (Fig.5c) and CNT-I (Fig.5d) . The IR spectra of
an
us
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oxidized CNTs show three major peaks, located at, 3425, 2293, and 1575Ԝcm−1. The peak at
te
d
M
3425Ԝcm−1 can be assigned to the O–H stretch from carboxyl groups (O=C−OH and C−OH)
Ac ce p
while the peak at 2293Ԝcm−1 can be associated with the O−H stretch from strongly hydrogen-
bonded −COOH. The peak at 1575Ԝcm−1 is related to the carboxylate anion stretch mode. In the
sulfur functionalized CNTs (Fig. 5c), characteristic peaks at 2295 cm-1 and 833 cm-1 which
Page 8 of 47
could be assigned to polarizable -SH stretching and C-S vibration respectively. There is also a large peak at 1066 cm-1 which is representative of S=O stretching vibration. Furthermore iodide functionalized CNTs (Fig. 5d) exhibits notably sharp peak at 450 cm-1 arising from the C-I bond
ip t
[25].
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3.1.3 Raman spectra
The Raman spectra of CNTs and Functionalized CNTs (Fig.6) showed two characteristic peaks
us
around 1344 cm-1 (D band) and 1569cm-1 (G band). The in-plane tangential stretching of the
an
carbon-carbon bonds in graphene sheets gives rise to G band, whereas the D band arises either due to the defective sites in the hexagonal framework CNTs or the presence of amorphous
M
carbon [26]. The ratio between the areas of these two bands (ID/IG) has been used to quantify the degree of purification [27-30] of CNTs. Presence of amorphous carbon is indicated by the
d
higher intensity of D band compared to G band. For the present material, the ratio (Table 1)
te
ranged from 0.971 to 1.037, indicating the presence of disordered graphite components in both
Ac ce p
the plain and functionalized CNTs.
3.1.4 Surface area measurements
The specific surface area of the sorbents was obtained over the relative pressure range from 0.05 to 0.35 using the standard BET method. The total pore volume was calculated from the amount
adsorbed at a relative pressure close to unity (i.e., 0.994). Barrett–Joyner–Halenda (BJH) and
density functional theory (DFT) methods were used for calculating the meso- and micropore
Page 9 of 47
volumes. Both these methods were applied using the instrument's software supplied by Quantachrome. Table 2 summarizes the data for BET surface area and pore volumes of various CNTs. It is evident that the surface area of the CNTs increased from 85.1 m2 g-1 to 152.5 m2 g-1
ip t
upon oxidation and further functionalization does not affect the surface area of the sorbent.
During oxidation the catalyst metal particles are removed and more pores are opened for sorption
cr
which accounts for the increased surface area. Decreased pore volumes in functionalized CNTs
us
could be attributed to the blockage of inter-bundle galleries and intra-bundle interstitial channels
an
by various functional groups. 3.2 Effect of initial pH and mechanism of interaction
M
Variation of initial pH in the range of 3 – 9 was examined using CNT, CNT-OX, CNT-S and CNT-I . Due to precipitation of Hg(II) at higher pH values, experiments were limited up to pH
d
9.0 only. Initial concentration of Hg(II) was maintained at 100 mg/l. Efforts were made not to
te
maintain the pH throughout the sorption experiments. The results obtained are shown in Fig. 7a.
Ac ce p
It is evident from the figure that except plain MWCNT, all the other sorbents tested exhibited maximum adsorption in the pH range of 6 to 8. In the case of plain MWCNT, adsorption was found to be maximum in the pH range of 5 – 8. Thus, in further experiments the initial pH was maintained at 6.0. To study the surface charge of the sorbent, pHZPC of the sorbents were found out by plotting the initial pH Vs change of initial and final pH (Δ pH) and the results obtained are depicted in Fig 7b. It is evident from the plot that pHZPC of the plain and functionalized CNTs are in the range of 5 to 6. Thus surface of the sorbents < pH 5 is acidic in nature and will have a net positive charge owing to the protonation of active sites. It is well known that in the regions of high capacity (pH 5 – 8), dominant species of Hg(II) are Hg2+ and Hg(OH)+ ions. Thus, in the case of plain MWCNTs and CNT-OX, surface functional groups like hydroxyl and carboxyl
Page 10 of 47
groups cause a rise in negative charge on the surface of the carbon at pH values greater than 5 and the oxygen atoms donate a single pair of electrons to cationic mercury ions as depicted in Fig.8. A similar mechanism has been suggested for the adsorption of heavy metal ions over
ip t
CNTs by Rao et al. [6]. In the case of CNT-OX more hydroxyl and carboxylic functional groups are introduced on their surface which accounts for their increased cation exchange capacity.
cr
Increased adsorption of CNT-OX over plain CNTs is discussed in the section below.
us
Mechanism of interaction of CNT-S and CNT-I with Hg(II) is also depicted in Fig.8 . In the case of sulfur functionalized CNTs, additional functional groups like S=O, -SH, S-S are introduced in
an
the CNT surface which is evident from FTIR spectra (Fig. 9a). After Hg(II) sorption, the changes in the spectra are shown Fig. 9b. It is clear that additional peak is observed at 492cm-1
M
confirming the Hg-S interaction. Further after Hg(II) loading slight upfield shift of
–SH
d
vibration from 2295 to 2310 cm-1 and C-S vibration from 833 to 842 cm-1 confirms the
te
complexation between Hg(II) and sulfur group. Earlier works on surface modification by sulfurization have shown that thiols (-SH) are the most dominant binding group for Hg(II) and
Ac ce p
disulfur bonding is formed between Hg(II) and the thiol groups [31, 32] It was found that using virgin MWCNTs, CNT-OX and CNT-S as sorbents the value of initial pH changed from 7.0 to 6.8. This could be attributed to the release of protons during sorption of Hg(II) ions.
FTIR spectra of CNT-I and CNT-I-Hg are shown in Fig. 9b. It is evident from the figure that a slight upfield shift of carbon halide vibration from 440 to 448 cm-1 was observed. Further, after Hg(II) loading, a sharp band at 480cm-1 was observed which could be attributed to Hg-I vibration. As both Hg(II) and iodide being soft ligands there exists an soft-soft interaction and
Page 11 of 47
results in the formation of iodated mercury compounds [19, 20] as shown in the reaction scheme (Fig.8).. 3.3 Effect of Adsorbent dose
ip t
Dose rate experiments were conducted by equilibrating 10 mg/l concentration of Hg(II) set at pH 6 and the dose rate of the sorbents were increased from 0.5 to 2.5 gL-1. The equilibrations were
cr
carried out for 2 h and the concentration of Hg(II) adsorbed was monitored and the results
us
obtained are shown in Table 3. It is evident that both CNT-S and CNT-I were efficient in the removal of Hg(II) even at very low dose rate.
an
3.4 Adsorption isotherms:
The Hg(II) adsorption isotherms for CNT, CNT-OX, CNT-I and CNT-S are presented in Fig.
M
10a. Adsorption data of Hg(II) over various functionalized CNTs were modeled using various
d
isotherms. Langmuir model is the most commonly used model to describe the formation of
te
homogeneous monolayer on the adsorption surface. The adsorption of Hg(II) ions from the bulk to functionalized CNT surface could be expressed by linearized Langmuir expression [33] as
Ac ce p
1 1 1 (3) = + qe q0 C e K L q m
Where, qe is the amount of Hg(II) adsorbed (mg g-1) at equilibrium and Ce is the equilibrium concentration (mg L-1). The empirical constants qm and b denote the monolayer capacity and energy of adsorption, respectively, and were calculated from the slope and intercept of plot between 1/Ce and 1/qe (Fig.10b). The constant ‘ b’ is attributed to the affinity between the sorbent and sorbate in the given system. The values obtained for the various constants are given in Table 4. It is evident from the data that the maximum adsorption capacity of the four sorbents towards Hg(II) were in the order of CNT-S > CNT-I > CNT-OX > CNT-I. It is evident that the sorption capacity of CNT-S is 27.5 times higher than plain CNT. High sorption capacity of
Page 12 of 47
CNT-S towards Hg(II) could be attributed to the formation of strong Hg-S bond. Among the four sorbents it is also clear that the affinity constant is high for Hg(II) and CNT-S system and regression coefficient was >0.97. Adsorption capacity of CNT-S is significantly higher than the
ip t
adsorption capacity of various carbon adsorbents, such as, plain activated carbon (25.8 mg g-1) [34], Carbon aerogel (34.9 mg g-1) [35], MWCNTs (78.1 mg g-1) [13], MWCNT-amine (1.27 mg
cr
g-1) [12], thiol modified-activated carbon (121.6 mg g-1) [32] and recently reported sulfur
us
incorporated SWCNTs (131 mg g-1) [17]. The higher adsorption capacity CNT-S could be attributed to the strong soft acid–soft base interactions between Hg(II) ions and the thiol groups
an
on the nano tube surface. The marginal increase in the capacity of CNT-S over SWCNT-S [17] could be attributed to the surface morphology. Comparing the intensity of D band to G band
M
obtained from Raman spectra, it was found that ID/IG of CNT-S (1.0272) was 10 times higher
d
than that obtained from SWCNT-S (0.121) [17]. An increase in the ratio indicated the defect
te
sites arising due to the presence of graphite particles or layer mismatch. It is reported that edges of these graphite particles and layer mismatch have high adsorption potentials [36]. This explains
Ac ce p
the increased uptake capacity of CNT-S over SWCNT-S [17] reported elsewhere. Further analysis of Langmuir model could be arrived based on a dimensionless equilibrium parameter called separation factor (RL) [37].
RL =
1 1 + bC 0
(4)
where C0 is the initial concentration of Hg(II) and ‘b’ is the Langmuir adsorption equilibrium constant (ml mg-1). The value of RL indicates the isotherm shapes to be favorable (0
S0304-3894(14)00196-4 http://dx.doi.org/doi:10.1016/j.jhazmat.2014.03.020 HAZMAT 15793
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
29-11-2013 10-2-2014 10-3-2014
Please cite this article as: A. Gupta, S.R. Vidyarthi, N. Sankararamakrishnan, Enhanced sorption of mercury from compact fluorescent bulbs and contaminated water streams using functionalized multiwalled carbon nanotubes, Journal of Hazardous Materials (2014), http://dx.doi.org/10.1016/j.jhazmat.2014.03.020 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced sorption of mercury from compact fluorescent bulbs and contaminated water streams using functionalized multiwalled carbon nanotubes Avinash Gupta#, S.R. Vidyarthi# and Nalini Sankararamakrishnan$,* $
cr
ip t
Centre for Environmental Science and Engineering Indian institute of Technology Kanpur Kanpur, U.P. 208016. # Department of Chemical Engineering Hartcourt Butler Technological Institute, Kanpur, U.P. 208001, India
us
*author for correspondence, Tel: 915122596360, Email: [email protected]
an
Abstract
Three different functionalized multiwalled carbon nanotubes were prepared, namely, oxidized
M
CNTs (CNT-OX), iodide incorporated MWCNT (CNT-I) and sulfur incorporated MWCNT (CNT-S). The as prepared adsorbents were structurally characterized by various spectral
d
techniques like Scanning Electron Microscopy (SEM), Energy Dispersive X-ray (EDAX),
te
Brunauer, Emmett, and Teller (BET) surface area analyzer, Fourier Transform Infra Red (FTIR)
Ac ce p
and Raman Spectroscopy. Loading of iodide and sulfur was evident from the EDAX graphs. The adsorption properties of Hg2+ as a function of pH, contact time and initial metal concentration were characterized by Cold vapor AAS. The adsorption kinetics fitted the Pseudo Second Order Kinetics and equilibrium was reached within 90 minutes. The experimental data were modeled with Langmuir, Freundlich, Dubinin-Redushkevich and Temkin isotherms and various isotherm parameters were evaluated. It was found that the mercury adsorption capacity for the prepared adsorbents were in the order of CNT-S > CNT-I > CNT-OX > CNT. Studies have been conducted to demonstrate the applicability of the sorbent towards the removal of Hg(0) from broken compact fluorescent light (CFL) bulbs and Hg(II) from contaminated water streams. Key words: Mercury, Contamination, MWCNTs, CFL, adsorption
Page 1 of 47
Ce Equilibrium Concentration (mg L-1) qe Amount of metal ion adsorbed at equilibrium (mg g-1) Amount of metal ion adsorbed at a given time (mg g-1)
qm Maximum adsorption Capacity (mg g-1) Pseudo second order rate constant (g-1mg-1min)
an
k2 ’
us
qt
cr
ip t
Nomenclature
kint Weber-Morris diffusion rate constant (mg-1g-1min1/2) Langmuir isotherm Constant (L mg-1)
M
b
RL Separation factor
d
KF Freundlich constant (L g-1)
T Absolute temperature (K)
te
nF Heterogenity of the sorption surface
Ac ce p
R Universal gas constant (8.314 J mol-1.K)
KT Temkin’s equilibrium binding constant (L mg-1) bT Temkin isotherm constant (KJ mol-1) BT Variation of adsorption energy (KJ mol-1) β
Coefficient of activity related mean sorption energy (mol2 KJ-1)
ε
Polanyi Potential
EDR Mean free energy of sorption (KJ mol-1)
Page 2 of 47
ip t
1. Introduction Mercury is a highly toxic heavy metals and various uses of mercury include production of chlor-
cr
alkali, fossil fuels, in various switches, wiring devices, measuring and control devices, lighting,
us
and dental work. The highly reactive inorganic form of Hg(II) is most toxic and its toxicity arises due to its binding ability to sulfur containing proteins. Due to inherent toxicity of mercury EPA
an
recommends a limit of 2.0 µg L-1 [1] in water. Bureau of Indian Standards [2] recommends the
M
permissible concentration of Mercury in drinking water to be 1.0 μg L-1. Conventional techniques for Hg removal from aqueous solutions include sulphate or hydrazine
d
precipitation, ion-exchange, liquid–liquid extraction, adsorption and solid phase extraction via
te
activated carbon adsorption [3]. In recent years, carbon nanotubes (CNTs) have received
Ac ce p
considerable attention due to their unique one-dimensional nanostructure and range of fascinating mechanical, physical, chemical and electronic properties. High sorption capability of chemical pollutants by CNT is attributed to its hollow and layered nanostructure with a characteristically large surface area [4,5]. Ren and coworkers [6] reviewed the application of CNTs towards environmental pollution management. It has been found useful in the removal of various heavy metals like Pb(II) [7], Pb(II), Cu(II) and Cd(II) [8], Ni(II) [9] etc. CNTs have been found useful for Hg(II) remediation as well. For instance, Nassereldeen et al.[10] reported the use of CNTs grown on Granular activated carbon for the removal of Hg(II).
Silver
impregnated CNT [11] have been found useful for the caputre of elemental Hg from flue gases. Afshar et al. [12], developed aminated CNTs for the quanittiative solid phase extraction of
Page 3 of 47
Hg(II). An uptake capacity of 78.1 mg g-1 was reported for Hg(II) using virgin multiwalled carbon nano tubes [13]. Thus to improve the selectivity and adsorption capacity of the sorbent, functionalization with suitbale ligands has been one of the most resorted method. The high
ip t
affinity between Hg(II) and sulfur groups was put to advantage to develop various functionalized adsorbents using sulfur based ligands such as dithiocarbamate [14], 1-furoyl thiourea urea [15],
cr
thiol [16, 17], and benzoythiourea [18] etc. It is also well known that powdered KI [19] or KI
us
modified adsorbent [20] have been found useful for the capture of mercury. Thus, keeping in mind the interesting properties of MWCNT, and higher stability constants for Hg-S complexes
an
and Hg-I complexes, functionalization of MWCNTs were performed using Sulfur and Iodide containing ligands. Various functional groups like hydroxyl, carbonyl and carboxyl are
M
introduced on MWCNT by oxidation technique [21]. Thus three diffent kinds of MWCNTs
d
namely, oxidised (CNT-OX), sulfur incorporated (CNT-S) and iodide incoporated (CNT-I) were
te
sysnthesized and evaluated for Hg(II) removal. Systematic structural characterization were performed using various techniques and optimization of various paramters including reaction
Ac ce p
time, pH and dose rate were carried out. Kinetic and isotherm models were evaluated and a suitable mechanism for the adsorption has also been postulated. It is well known that CFL bulbs contain low pressure mercury vapor and release of toxic vapors arises when there is breakage and its concenration ranges bewtween 0.1 to 3.6 mg [22]. There are very few reports in the literature to treat the Hg(0) vapor emerging from CFL bulbs [23, 24]. Thus the prepared adsorbent was applied to the treatment of both Hg(0) from the CFL bulbs and Hg(II) from coal wash water streams. 2. Material and Methods 2.1 Instrumentation
Page 4 of 47
Mercury was analyzed by Cold Vapor Atomic Absorption Spectrometry (CVAAS) (MA-5840 analyzer ECIL). Experimental samples were filtered using Whatman 0.45 mm filter paper and the filtrates after suitable dilutions, were analyzed. Control experiments showed that no sorption
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occurred on either glassware or filtration systems. All assays were carried out in triplicate and only mean values are presented. Scanning Electron Microscopy (SEM) was done using FEI
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Quanta 200 machine. The samples were gold coated to improve their conductivity to obtain good
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images. Elemental analysis was done by EDAX with a field emission scanning electron microscope operated at an accelerating voltage of 10 kV. The adsorbent was analyzed for the
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specific surface area, pore volume, and PSD by N2-physisorption using Autosorb-1C instrument (Quantachrome, USA). Chemisorption analysis was also carried out to measure the active metal
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surface area. The multipoint Brunauer, Emmett, and Teller (BET) surface area was measured
d
from the nitrogen adsorption/desorption isotherm. Infra-red measurements were made with KBr
Ge crystal.
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2.2 Materials
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pelts using Tensor 27 (Bruker, Germany) in the attenuated total reflectance (ATR) mode using
Multi-walled carbon nanotubes (MWCNTs) of 10–20 nm diameters were purchased from Nanoshell USA. All other reagents were analytical grade and were used as received. Aqueous solutions at various concentrations were prepared from HgCl2 and used as a source for Hg(II). 2.3 Preparation of functionalized MWCNT: Oxidized form of CNT (CNT-OX) was prepared by heating the acquired plain MWCNTs with nitric acid at 70˚C for 4h followed by thorough washing with distilled water. Sulfur functionalized CNTs (CNT-S) was prepared by heating 1 g of CNT-OX and 2.5 mL of CS2 in 10 ml of methanol at 70˚C for 4h followed by washing with distilled water and air drying it.
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Iodide was functionalized on MWCNTs by heating 1 g of CNT-OX in 10 ml of 0.1 M of KI at 60˚C followed by repeated washing with distilled water and dried in the oven. The schematic representations of the preparation of various CNTs are shown in Fig.1.
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2.4 Adsorption Experiments
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Batch experiments were carried out with synthetic solutions of Hg(II) in 100 ml conical flasks at 110 rpm of orbital stirring in an incubator shaker, at room temperature and 2 h of contact time
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with the adsorbent. Samples were then centrifuged for 10 min filtered with Whatman No. 42
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filter paper, and filtrates were analyzed for Hg(II) concentration by CVAAS. Unless otherwise stated the parameters with synthetic water were: sample volume 20 ml, sorbent dose 0.02 g, pH
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6, initial Hg(II) concentration 10 ppm, equilibration time 4 h. The effect of pH on adsorption of Hg(II) was studied in a pH range of 3 to 10 by agitating adsorbent (0.02g) with Hg(II) (20 ml, 10
d
ppm) for 2 h at 110 rpm. The pH was adjusted by adding aqueous solutions of 0.1M HCl or 0.1M
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NaOH. Isothermal studies were carried out by adding 0.02 g adsorbent into 20 ml of Hg(II)
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solution of varying concentrations from 10 to 120 mg L-1. The amount of Hg(II) adsorbed and percentage removal of Hg(II) were calculated using the following equation:
qe =
(C0 − Ce ) × V W
% Removal of Hg(II) =
(C 0 − C e ) × 100 C0
(1)
(2)
Where qe is the amount of mercury adsorbed (mgg-1), C0 and Ce are the initial and equilibrium concentrations (mg L-1), V is the volume of the aqueous solution and W is the mass of the adsorbent (g) used in adsorption experiments. Kinetic studies were conducted by equilibrating 20
Page 6 of 47
ml of 100 mg/l of Hg(II) at a dose rate of 1.0 g/l and the amount of Hg(II) adsorbed were monitored at regular intervals of time.
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3.0 Results and Discussion 3.1 Physical Characterization
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3.1.1 Scanning Electron Microscopy
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The surface morphology of plain CNT, CNT-OX, CNT-S and CNT-I were studied with the help of SEM fitted with EDX (Fig.2 ). Original plain CNT had diameter in the range of 20 – 80 nm
an
while the length varies from 1 – 10 μm (Fig. 1a). Loaded catalyst particles are seen throughout the plain CNT. After oxidation (Fig.1b) and functionalization with iodide (Fig.1c) and sulfur
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ligands (Fig.1d), the metal particles were removed and there was no detectable change in the
d
surface morphology implying minimal damage during oxidation and functionalization
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procedures. EDAX spectrum of CNT-I (Fig.3a) and CNT-S (Fig 4a) revealed weight % of iodide and sulfur to be 0.24 and 13.07% respectively. Additionally in functionalized CNTs elemental
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mapping images (Fig.3b-d, Figs.4b-d) revealed the uniform deposition of sulfur and iodide ligands in the respective CNTs. 3.1.2 FTIR spectra
FTIR spectra from the CNTs show a broad peak at ~3430Ԝcm−1 which is a characteristic of the
O-H stretch of hydroxyl group (Fig. 5) arising from the oscillation of carboxyl groups. Carboxyl
Page 7 of 47
group on the surface of the plain CNTs (Fig. 5a) could be attributed to the partial oxidation of the surface of CNTs during purification by the manufacturer. Further, broad peak at 3430 cm-1 are
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also observed in CNT-OX (Fig. 5b), CNT-S (Fig.5c) and CNT-I (Fig.5d) . The IR spectra of
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us
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oxidized CNTs show three major peaks, located at, 3425, 2293, and 1575Ԝcm−1. The peak at
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d
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3425Ԝcm−1 can be assigned to the O–H stretch from carboxyl groups (O=C−OH and C−OH)
Ac ce p
while the peak at 2293Ԝcm−1 can be associated with the O−H stretch from strongly hydrogen-
bonded −COOH. The peak at 1575Ԝcm−1 is related to the carboxylate anion stretch mode. In the
sulfur functionalized CNTs (Fig. 5c), characteristic peaks at 2295 cm-1 and 833 cm-1 which
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could be assigned to polarizable -SH stretching and C-S vibration respectively. There is also a large peak at 1066 cm-1 which is representative of S=O stretching vibration. Furthermore iodide functionalized CNTs (Fig. 5d) exhibits notably sharp peak at 450 cm-1 arising from the C-I bond
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[25].
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3.1.3 Raman spectra
The Raman spectra of CNTs and Functionalized CNTs (Fig.6) showed two characteristic peaks
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around 1344 cm-1 (D band) and 1569cm-1 (G band). The in-plane tangential stretching of the
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carbon-carbon bonds in graphene sheets gives rise to G band, whereas the D band arises either due to the defective sites in the hexagonal framework CNTs or the presence of amorphous
M
carbon [26]. The ratio between the areas of these two bands (ID/IG) has been used to quantify the degree of purification [27-30] of CNTs. Presence of amorphous carbon is indicated by the
d
higher intensity of D band compared to G band. For the present material, the ratio (Table 1)
te
ranged from 0.971 to 1.037, indicating the presence of disordered graphite components in both
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the plain and functionalized CNTs.
3.1.4 Surface area measurements
The specific surface area of the sorbents was obtained over the relative pressure range from 0.05 to 0.35 using the standard BET method. The total pore volume was calculated from the amount
adsorbed at a relative pressure close to unity (i.e., 0.994). Barrett–Joyner–Halenda (BJH) and
density functional theory (DFT) methods were used for calculating the meso- and micropore
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volumes. Both these methods were applied using the instrument's software supplied by Quantachrome. Table 2 summarizes the data for BET surface area and pore volumes of various CNTs. It is evident that the surface area of the CNTs increased from 85.1 m2 g-1 to 152.5 m2 g-1
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upon oxidation and further functionalization does not affect the surface area of the sorbent.
During oxidation the catalyst metal particles are removed and more pores are opened for sorption
cr
which accounts for the increased surface area. Decreased pore volumes in functionalized CNTs
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could be attributed to the blockage of inter-bundle galleries and intra-bundle interstitial channels
an
by various functional groups. 3.2 Effect of initial pH and mechanism of interaction
M
Variation of initial pH in the range of 3 – 9 was examined using CNT, CNT-OX, CNT-S and CNT-I . Due to precipitation of Hg(II) at higher pH values, experiments were limited up to pH
d
9.0 only. Initial concentration of Hg(II) was maintained at 100 mg/l. Efforts were made not to
te
maintain the pH throughout the sorption experiments. The results obtained are shown in Fig. 7a.
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It is evident from the figure that except plain MWCNT, all the other sorbents tested exhibited maximum adsorption in the pH range of 6 to 8. In the case of plain MWCNT, adsorption was found to be maximum in the pH range of 5 – 8. Thus, in further experiments the initial pH was maintained at 6.0. To study the surface charge of the sorbent, pHZPC of the sorbents were found out by plotting the initial pH Vs change of initial and final pH (Δ pH) and the results obtained are depicted in Fig 7b. It is evident from the plot that pHZPC of the plain and functionalized CNTs are in the range of 5 to 6. Thus surface of the sorbents < pH 5 is acidic in nature and will have a net positive charge owing to the protonation of active sites. It is well known that in the regions of high capacity (pH 5 – 8), dominant species of Hg(II) are Hg2+ and Hg(OH)+ ions. Thus, in the case of plain MWCNTs and CNT-OX, surface functional groups like hydroxyl and carboxyl
Page 10 of 47
groups cause a rise in negative charge on the surface of the carbon at pH values greater than 5 and the oxygen atoms donate a single pair of electrons to cationic mercury ions as depicted in Fig.8. A similar mechanism has been suggested for the adsorption of heavy metal ions over
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CNTs by Rao et al. [6]. In the case of CNT-OX more hydroxyl and carboxylic functional groups are introduced on their surface which accounts for their increased cation exchange capacity.
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Increased adsorption of CNT-OX over plain CNTs is discussed in the section below.
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Mechanism of interaction of CNT-S and CNT-I with Hg(II) is also depicted in Fig.8 . In the case of sulfur functionalized CNTs, additional functional groups like S=O, -SH, S-S are introduced in
an
the CNT surface which is evident from FTIR spectra (Fig. 9a). After Hg(II) sorption, the changes in the spectra are shown Fig. 9b. It is clear that additional peak is observed at 492cm-1
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confirming the Hg-S interaction. Further after Hg(II) loading slight upfield shift of
–SH
d
vibration from 2295 to 2310 cm-1 and C-S vibration from 833 to 842 cm-1 confirms the
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complexation between Hg(II) and sulfur group. Earlier works on surface modification by sulfurization have shown that thiols (-SH) are the most dominant binding group for Hg(II) and
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disulfur bonding is formed between Hg(II) and the thiol groups [31, 32] It was found that using virgin MWCNTs, CNT-OX and CNT-S as sorbents the value of initial pH changed from 7.0 to 6.8. This could be attributed to the release of protons during sorption of Hg(II) ions.
FTIR spectra of CNT-I and CNT-I-Hg are shown in Fig. 9b. It is evident from the figure that a slight upfield shift of carbon halide vibration from 440 to 448 cm-1 was observed. Further, after Hg(II) loading, a sharp band at 480cm-1 was observed which could be attributed to Hg-I vibration. As both Hg(II) and iodide being soft ligands there exists an soft-soft interaction and
Page 11 of 47
results in the formation of iodated mercury compounds [19, 20] as shown in the reaction scheme (Fig.8).. 3.3 Effect of Adsorbent dose
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Dose rate experiments were conducted by equilibrating 10 mg/l concentration of Hg(II) set at pH 6 and the dose rate of the sorbents were increased from 0.5 to 2.5 gL-1. The equilibrations were
cr
carried out for 2 h and the concentration of Hg(II) adsorbed was monitored and the results
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obtained are shown in Table 3. It is evident that both CNT-S and CNT-I were efficient in the removal of Hg(II) even at very low dose rate.
an
3.4 Adsorption isotherms:
The Hg(II) adsorption isotherms for CNT, CNT-OX, CNT-I and CNT-S are presented in Fig.
M
10a. Adsorption data of Hg(II) over various functionalized CNTs were modeled using various
d
isotherms. Langmuir model is the most commonly used model to describe the formation of
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homogeneous monolayer on the adsorption surface. The adsorption of Hg(II) ions from the bulk to functionalized CNT surface could be expressed by linearized Langmuir expression [33] as
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1 1 1 (3) = + qe q0 C e K L q m
Where, qe is the amount of Hg(II) adsorbed (mg g-1) at equilibrium and Ce is the equilibrium concentration (mg L-1). The empirical constants qm and b denote the monolayer capacity and energy of adsorption, respectively, and were calculated from the slope and intercept of plot between 1/Ce and 1/qe (Fig.10b). The constant ‘ b’ is attributed to the affinity between the sorbent and sorbate in the given system. The values obtained for the various constants are given in Table 4. It is evident from the data that the maximum adsorption capacity of the four sorbents towards Hg(II) were in the order of CNT-S > CNT-I > CNT-OX > CNT-I. It is evident that the sorption capacity of CNT-S is 27.5 times higher than plain CNT. High sorption capacity of
Page 12 of 47
CNT-S towards Hg(II) could be attributed to the formation of strong Hg-S bond. Among the four sorbents it is also clear that the affinity constant is high for Hg(II) and CNT-S system and regression coefficient was >0.97. Adsorption capacity of CNT-S is significantly higher than the
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adsorption capacity of various carbon adsorbents, such as, plain activated carbon (25.8 mg g-1) [34], Carbon aerogel (34.9 mg g-1) [35], MWCNTs (78.1 mg g-1) [13], MWCNT-amine (1.27 mg
cr
g-1) [12], thiol modified-activated carbon (121.6 mg g-1) [32] and recently reported sulfur
us
incorporated SWCNTs (131 mg g-1) [17]. The higher adsorption capacity CNT-S could be attributed to the strong soft acid–soft base interactions between Hg(II) ions and the thiol groups
an
on the nano tube surface. The marginal increase in the capacity of CNT-S over SWCNT-S [17] could be attributed to the surface morphology. Comparing the intensity of D band to G band
M
obtained from Raman spectra, it was found that ID/IG of CNT-S (1.0272) was 10 times higher
d
than that obtained from SWCNT-S (0.121) [17]. An increase in the ratio indicated the defect
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sites arising due to the presence of graphite particles or layer mismatch. It is reported that edges of these graphite particles and layer mismatch have high adsorption potentials [36]. This explains
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the increased uptake capacity of CNT-S over SWCNT-S [17] reported elsewhere. Further analysis of Langmuir model could be arrived based on a dimensionless equilibrium parameter called separation factor (RL) [37].
RL =
1 1 + bC 0
(4)
where C0 is the initial concentration of Hg(II) and ‘b’ is the Langmuir adsorption equilibrium constant (ml mg-1). The value of RL indicates the isotherm shapes to be favorable (0
