MODIFICATION OF MULTI-WALL CARBON NANOTUBES FOR THE REMOVAL OF CADMIUM, LEAD AND ARSENIC FROM WASTEWATER
ZLATE S. VELIČKOVIĆa*, ZORAN J. BAJIĆa*, MIRJANA Đ. RISTIĆb, VELJKO R. DJOKIĆb, ALEKSANDAR D. MARINKOVIĆb, PETAR S.
USKOKOVIĆb, MLADEN M. VURUNAa
aMilitary academy, University of Defense, Belgrade, Serbia
bFaculty of Technology and Metallurgy, University of Belgrade, Serbia
Multi-wall carbon nanotubes (MWCNTs) was functionalized with 6-arm amino polyethylene glycol (PEG), and synthesized PEG-MWCNTs was used as adsorbent in order to study adsorption characteristics with respect to Cd(II), Pb(II) and As(V) ions. In batch tests, the influence of contact time, initial metal ion concentration and temperature on the ion adsorption on PEG-MWCNTs was studied. Adsorption of Cd(II), Pb(II) and As(V) on PEG-MWCNTs strongly depends on pH. Time dependent adsorption can be described by intra-particular Weber-Morris kinetic model, and adsorption process was modelled by Koble-Corrigan isotherm, respectively. The maximum adsorption capacities of Cd(II), Pb(II) and As(V) on PEG-MWCNTs, for initial concentration of 10 mg dm−3 and at pH=4, were 77.6, 47.5 and 13 mg g−1 at 25°C, respectively. The competitive adsorption studies showed that the adsorption affinity of ions towards PEG-MWCNTs showed largest adsorption of Cd(II) at pH 8, following by Pb(II) at pH 6, and As(V) at pH 4. Thermodynamic parameters showed that the adsorption of Cd(II), Pb(II) and As(V) ions was spontaneous and endothermic.
(Received January 16, 2013; Accepted March 6, 2013)
Keywords: Cadmium, Lead, Arsenic, Adsorption, Carbon nanotubes, PEG-functionalization.
1. Introduction
Human activities introduce heavy metals and arsenic to the hydrosphere in many ways such as burning of fossil fuels, smelting of ores, municipal sewage, industrial effluent, mining activities, landfill, mineral weathering, underground toxic waste disposal, etc. [1] and [2]. These contaminants, regardless of their sources, are easily dispersed into the aquatic system, and tend to accumulate in living organisms, resulting in various disorders and diseases in the ecosystem [3].
Carbon nanotubes (CNTs), developed in the 1990s, possess potential for the removal of many kinds of pollutants from water because of their ability to establish electrostatic interactions and their large surface areas, CNTs have attracted great attention in analytical chemistry and environmental protection. CNTs have shown exceptional adsorption capabilities and high adsorption efficiencies for various organic and inorganic pollutants [4-6].
Modification of the surface morphology plays an important role in enhancing the sorption capacity of CNTs. Due to their high reactivity with many chemical species, it has been suggested that amino groups together with oxygen groups could serve as coordination and electrostatic interaction sites for transition metal sorption [7-10].
In this study, the possibility of the use of PEG-functionalized multi-walled carbon nanotubes (PEG-MWCNTs) as a sorbent for the removal of Cd(II), Pb(II) and As(V) ions from aqueous solutions was examined.
___________________________________
*Corresponding author: [email protected]
chemic of MW respect (DMF) Millipo prepar South and N 1000 m adsorp
functio
HNO3. groups showe room t water washed for 8 h protect sonicat the dis anhydr Obtain nitroge mixtur filtered times r and fil and m Dried in a ad
2. Exper 2.1. Mater All chemic cal vapour d WCNTs was tively, and th ), conc. H2S ore deionise ration. PEG- Korea) was Na2HAsO4·7H
mg dm−3, wh ption experim
2.2. Prepa Modificati onalization w
Fig. 1
The raw-M . This mixtu s onto the M
d better per temperature,
and vacuum d with DI w h. The oxidiz ted from the ted for 15 m spersion was rous tetrahy ned MWCNT
PEG-NH2 en and prote re, and heate d, dispersed repeated). Af ltered using methanol, foll PEG-MWCN dsorption exp
rimental a rials cals and MW deposition (C
s more than he length bet SO4 (98%) p d (DI) water -6-arm amin used. Analy H2O (Sigma- hich was furth ments.
aration of PE ion of the with SOCl2 [4
1. Reaction pa
MWCNTs we ure was then
MWCNT sur rformance th
the oxidized m-filtered thro
ater until the zed nanotube e moisture.
minutes, trans s heated at 7 drofuran (T T-COCl was
(1.2 g) was ected agains ed at 35 °C, in a 5% NaH fter removal
a 0.05 μm p lowing by d NTs showed periments.
approach
WCNTs were CVD) method 95% and t tween 5 and p.a., conc. HN
r (18 MΩ cm no polyethyle ytical-grade s
-Aldrich) we her diluted w
EG-MWCN e raw ma 4], and amin
athways appli of PEG-6-a
ere first treat sonicated fo rface 11. O han ones oxi
d MWCNTs ough a 0.05 e pH was neu es (90 mg) w
Thionyl chl sferred on a m 70 °C for 24
HF) (Baker) dried in a va dispersed i st moisture.
under magn HCO3 by so
of supernata pore size PT
rying in a v d significant p
obtained fro d were used the outer an 200 μm. All HNO3 (65%) m resistivity) ene glycol ( standards: ca
ere employe with DI water
NTs
aterial was ation with PE
ied to obtain P arm modificati
ted with a (v or 3 h at 40°C
Oxidation w idized with
(o-MWCNT µm pore si utral. The sa were disperse loride (20 cm
magnetic stir 4 h. Obtained ) using a 0 acuum oven in anhydrous
MWCNT-C netic stirring onication, and
ant, precipita TFE membra
vacuum oven polymerizati
om Sigma-Al as received w nd inner diam l other reage
p.a., and me ) was used fo (PEG-NH2, admium nitra ed to prepar r to the requi
conducted EG-NH2 (Fig
PEG-MWCNT ion agent (b).
v/v 3:1) mixt C in an ultra with sulphuri KMnO4 or H Ts) were adde
ze PTFE me ample was dr d in anhydro m3, Fluka) w rrer and heat d product w .05 μm por at 60 °C for s DMF (40 COCl (50 m g, for 72 h.
d resulting d ate was dispe ane filter. Ex
n at 60 °C fo ion degree so
ldrich. MWC without purif meters were ents such as d
ethanol were or sample wa Mr ≈15000 ate (Baker), l e a stock so ired ionic con
via oxid g. 1).
Ts (a), and stru
ture of conce asonic bath to c acid and n H2O2 [11,12 ed slowly to embrane filte
ried in a vac ous DMF (1 was added, t ted at 50 °C as vacuum f e size PTFE
3 h.
cm3) in a v mg) was add
Obtained pr dispersion wa
ersed in DI w xtensive wash for 3 h gave
o it had to be
CNTs prepar fication. The e 20–30, 5–
dimethylform e used as re ashing and s g mol-1) (S lead nitrate ( olutions con oncentrations
dation, subs
ucture
entrated H2S to introduce nitric acid m 2]. After coo
300 cm3 of c er. The filtra cuum oven a
cm3) in a ap the dispersio
for 3 h, and filtered with E membrane vials flushed ded into a r
roduct was v as centrifuge water by son
hing with D a PEG-MW e ground for
red by a e purity 10 nm, mamide eceived.
solution Sunbio, (Baker) ntaining for the
sequent
SO4 and oxygen mixture oling to
cold DI ate was t 80 °C pparatus on was d finally excess e filter.
with a reaction vacuum ed (two ication, I water WCNTs.
r its use
2.3. Adsorption experiments
Cd(II), Pb(II) and As(V) adsorption capacities of PEG-MWCNTs was determined in a batch reactors. Batch sorption experiments were performed using 100 cm3 vial with addition of 10 mg PEG-MWCNTs and 100 cm3 (m/V=100 mg dm-3) of Cd(II), Pb(II), As(V) solution of initial concentrations (C0) 0.1, 0.5, 1, 2, 5 and 10 mg dm−3. The bottles were placed in an ultrasonic bath.
In order to evaluate the effect of pH on adsorption, the initial pH values of the solutions were set at 4.0, 5.0, 6.0, 7.0 and 8.0 by adjustment with 0.01 and 0.1 mol dm−3 NaOH, and 0.01 and 0.1 mol dm−3 HNO3, at 25 ºC. The optimal pH was found to be 8 for Cd2+, 6 for Pb2+ and 4 for As(V), and these pH was used throughout all the adsorption experiments. Time-dependent sorbate concentration changes were examined in the range 1–240 minutes, and it was found that the optimal time (60 min) was sufficient to achieve equilibration of the system. The mixtures of PEG- MWCNTs and ionic solutions, after sonication was filtered through a 0.2 μm PTFE membrane filter, acidified and analyzed.
2.4. Adsorbent characterization
Fourier-transform infrared (FTIR) spectra were recorded in transmission mode using a BOMEM (Hartmann&Braun) spectrometer. FTIR spectra were recorded before and after adsorption at initial sorbate concentrations of 10 mg dm-3. Samples for FTIR determination were ground with spectral grade KBr in an agate mortar. All FTIR measurements were carried out at room temperature.
JEOL JSM-6390LV scanning electron microscope. Sputtering (with gold) has been performed on BALTEC SCD 005 sputter coater.
Thermogravimetric analysis (TGA) was performed using a TA Instruments SDT Q600 from 20 to 800 °C at a heating rate of 20 °C min−1 and a air (200 mL min−1). Elemental analyses were performed using a VARIO EL III Elemental analyser.
The BET specific surface area, pore specific volume and pore diameter were measured by nitrogen adsorption/desorption at 77.4 K using a Micromeritics ASAP 2020MP gas sorption analyzer. The pH values at the point of zero charge (pHPZC) of the samples were measured using the pH drift method [9]. The zeta potential measurements of the PEG-MWCNTs samples were performed using a Zeta-sizer Nano-ZS equipped with a 633 nm He-Ne laser (Malvern). The coordination number (CN) can be obtained from the relationship between the concentration of amine groups (DAKaiser – degree of amination obtained by Kaiser test), and maximum adsorption capacity. Coordination number refers to the number of ligand atoms surrounding the central atom [12].
Cd(II), Pb(II) and As(V) were analyzed by the inductively coupled plasma mass spectrometry (ICP-MS) according to the literature method using an Agilent 7500ce ICP-MS system (Waldbronn, Germany) equipped with an octopole collision/reaction cell, Agilent 7500 ICP-MS ChemStation software, a MicroMist nebulizer and a Peltier cooled (2 °C) quartz Scott- type double pass spray chamber. Standard optimization procedures and criteria specified in the manufacturer’s manual were followed.
3. Results and discussion 3.1. PEG-MWCNTs characterization
The FTIR spectra of PEG-MWCNTs, PEG-MWCNTs/Cd(II), PEG-MWCNTs/Pb(II) and PEG-MWCNTs/As(V) are given in Fig. 2. An analysis of FTIR spectra of PEG-MWCNTs shows presence of a weak band at ≈1640 cm-1 assigned to stretching of the carbonyl (C=O) overlapping with OH bending vibration. In addition, the bands at ≈1480 and 1150 cm-1, correspond to N–H in- plane and C–N bond stretching vibration, respectively. The broad peaks at 3300–3600 cm-1 were due to the NH2 stretch of the amine group overlapped with OH stretching vibration. A band at
≈800 cm-1 was attributed to the out-of-plane NH2 bending vibration mode [7]. The most intensive peak, after functionalization of MWCNTs with PEG-NH2, appear at ≈1100 cm-1 corresponding to C–O–C stretching vibration from PEG moieties which indicate surface functionality change [8].
Adsorption capabilities of PEG-MWCNTs surface functional groups, as potential binding sites for divalent cations [9,12], depend on the adsorption condition, primarily on solution pH.
Divalent cations may form complexes with amino and residual carboxylic and phenolic groups,
more favourable interaction could be expected with former at pH higher than 6 (pKa 3–6), as ionized form could play significant role in uptake of divalent cations. Adsorption capabilities of PEG-MWCNTs with respect to As(V) is higher at lower pH, found pH 4, due to protonation of amino group and more favourable electrostatic interaction with arsenate anion. Characteristics bands of PEG-MWCNTs are slightly shifted, their intensities changed or disappeared after sorption experiments, and this properties depends on sorbate concentrations [9,12].
Bands structure in a spectrum of PEG-MWCNTs treated with Cd and Pb are similar. It could be observed peak shift from ≈1642 cm−1 to 1630 cm−1 for Cd(II), and 1638 cm−1 with Pb(II) indicting influence of cation size and charge density on extent of peak shifting. Peak observed at
≈1383 cm−1 is significantly increased, assigned to overlapped stretching vibration of SO2 and symmetric of COO-, which reflects to the bond strength increase of these groups after cation adsorption. Also, from Figs. 2 a and b it could be observed that peak at ≈1088 cm−1 has shifted and significantly increased. Small intensity increase and shifting are also noted for a peaks at ≈818 and
≈812 cm−1 for Cd and Pb, respectively 9,11. Broad band at ≈3429 cm-1 (Fig. 2 a and b), ascribed to OH and NH2 stretching vibrations, asymmetric and symmetric, is significantly affected by adsorbed divalent cations. New peaks observed in a spectrum of PEG-MWCNTs/Cd(II), appear as a splitting of the band at ≈3429 cm−1 giving a slightly resolved peaks at ≈3370 cm-1, ≈3529 cm-1 and ≈3631 cm-1. For a lead newly emerged peaks are at ≈3432 cm-1 and ≈3450 cm-1. This indicates changes of stretching vibrations of amino and hydroxyl groups due to cadmium and lead ions interaction with electron densities at those groups.
Fig. 2. FTIR spectrum of PEG-MWCNTs, and after treatment in an aqueous solutions (m/V=100 mg dm-3, T=25 oC) of (a) Cd(II) (C[Cd2+]0=10 mg dm-3) at pH 8, (b) Pb(II)
(C[Pb2+]0=10 mg dm-3) at pH 6, and (c) As(V) (C[As(V)]0=10 mg dm-3) at pH 4.
Analysis of the spectra before and after As(V) adsorption at PEG-MWCNTs could be noted a substantial changes of the stretching vibration frequency of the uncomplexed/unprotonated arsenate anion reflected as a more intense peak at ≈860 cm-1 15, while the frequency of the bonded arsenate has emerged at ≈712 cm-1 13, 14. Broad band at ≈3429 cm-1 (Fig. 2c), ascribed to OH and NH2 stretching vibrations, asymmetric and symmetric, is not significantly affected by adsorbed arsenic. As a consequence of the amino group protonation, and probably due to interaction with arsenic species, new bands at ≈3050 cm-1, ≈2020 cm-1 and 1450 cm-1 appeared, indicating that stretching and bending vibrations of ammonium group are influenced by arsenic adsorption.
SEM images show that untreated MWCNTs (Fig 3a) form large aggregates with medium diameter around 5µm due to Van der Waals interactions, while o-MWCNTs (Fig 3b) are not agglomerated because functionalities are mutually repelled by electrostatic forces. Figure 3c and d show that PEG-MWCNTs have polymerized beehive like structure.
Fig. 3.
BET s zeta po
Adso raw-M o-MW e-MW PEG- MWCN given i
Fig. 4 some i
Comparative
specific surfa otential are g Table 1
orbent MWCNT WCNT WCNT
NTs
Results of el in Table 2.
Table 2. E Sample raw-MWCN o-MWCNT e-MWCNT PEG-MWC displays TG information r
e overview of S
ace area and given in Tabl . BET specific Specific
surface area (m2 g-1)
187.6 78.5 101.2
22.5 lemental ana
Elemental ana
e C (%
NT 97.4
T 82.1
T 80.0
CNTs 78.2 GA weight-lo
related to the
SEM images:
d porosity of le 1.
c surface area Average
pore volume (cm3 g-1)
0.755 0.328 0.538 0.226 alysis, DAKais
alysis, DAKaiser
%) H (%) 6 0.32 3 1.18 8 1.76 2 1.98 oss curves ob ermal stabilit
(a) raw-MWC MWCNTs.
f raw, o-, e-
a and porosity Average
pore diameter
(nm) 16.09 16.72 21.25 17.40
ser and CN va
r and CN value N (%) S (%
0 0
0.49 0.6 4.08 0.5 5.64 0.1 btained upon ty of function
CNTs [7], (b)
and PEG-M
y of raw, o-, e- e
r pHPZC 4.98 2.43 5.91 5.64 alues of raw-
es of raw-, o-,
%) DA
(mm 0
64
58 0
12 3
n heating o- nalities prese
o-MWCNTs [
MWCNTs, as
- and PEG-MW
Zeta potential (m
-13.7 (pH 5.30) -50.0 (pH
3.98) -26.9 (pH
6.60) -28.4 (pH
6.50) -, o-, e- and P
e- and PEG-
AKaiser
ol g–1) CN
- - - - .65 3.36 .25 4.72 and PEG-M ent on MWC
[7], (c) and (d
s well as pHP
WCNTs .
mV) Re
H [9
H [9
H [9
H Th
stu PEG-MWCN
-MWCNTs.
N Ref.
[9]
[9]
6 [9]
2 This stud MWCNTs, an
CNTs surface d) PEG-
PZC and
ef.
9]
9]
9]
his dy NTs are
dy d gives e.
Fig. 4. TGA curves of o- and PEG-MWCNTs.
Peak degradation temperature of o-CNT occurs at 700 °C, and PEG-MWCNTs at 696 °C indicating that amino functionalities are of lower stability (significant loos in the region 300-600
°C), and they could participate in some thermal reactions at temperature >600 °C. Based on the TGA results of PEG-MWCNTs it provides an estimation of 40 wt% attached PEG-NH2.
3.2. Adsorption kinetics
In order to investigate the kinetics of adsorption of Cd, Pb and As, adsorption kinetic models (pseudo-first order or Lagergren model, pseudo-second order or Ho-McKay model, Roginsky-Zeldovich-Elovich equation and second-order rate equation), and adsorption diffusion models (liquid film linear driving force rate equation, liquid film diffusion mass transfer rate equation, homogeneous solid diffusion model, parabolic or Weber-Morris model, Dunwald- Wagner model and double exponential model) were used 16.
Non-linear regression of experimental data, using Origin 8.0, showed that the best fitting kinetic model is parabolic or Weber-Morris model giving the highest values of correlation coefficients than the other investigated models (Fig. 5). Therefore, this model could be used for the description of the adsorption kinetic of Cd, Pb and As on the PEG-MWCNTs. Weber-Morris found that in many adsorption cases, solute uptake varies proportionally with t1/2 rather than with the contact time t 17, according to the equation:
qt = kp • t0.5 + C (1)
where kint (g mg-1 min-0.5) is the intra-particle diffusion rate constant and C is parameter with value other than zero when intra-particle diffusion is not sole rate controlling step and when the film diffusion is simultaneously involved 16.
The curves of Cd(II), Pb(II) and As(V) adsorption exhibited two distinct phases. The first phase (initial steep slope) indicates the instantaneous adsorption of the ions for approximately 30 min. At 30 min., the removal of Cd, Pb and As ions were reached 93.8%, 75.6% and 74.5%, respectively, with the increase in ion uptake less than 0.1% per minute after that period, and equilibrated at 97% for Cd, 78% for Pb, and 76% for As. The steep slope may be attributed to the diffusion of the ions from the aqueous phase to the outer-surface of the PEG-MWCNTs. The initial adsorption has reached equilibrium gradually, exhibiting a classical physisorption process.
The second phase exhibits a gradual attainment of equilibrium due to the intra-particle diffusion and low porosity of PEG-MWCNTs (Table 1).
to calc Jovano Peterso fitting Koble–
and F heterog
experim for org
heterog below adsorp proces
Fig. 5. Kine phase for
3.4. Adsor Many adso culate variou ovic-Freundl on, Temkin, data were ob –Corrigan is Freundlich
geneous adso
The isothe mental data.
ganic compou
The slope geneity, bec unity impli ption [20]. La ss.
etic plot of exp the Weber-Mo
rption isothe orption isoth us adsorption lich, Dubini
Koble-Corri btained using sotherm [18]
isotherm m orption surfa
erm constant Freundlich i unds or high
e ranges bet coming more es chemisorp angmuir mod
perimental da orris intra-par
P
erms herm models
n parameters in–Radushke igan (K-C) a g Koble-Corr is a three-p models for
aces.
s, a, b and n isotherm [19 hly interactive
e f
e k C
q tween 0 and e heterogene
ptions proce del is used f
ata and calcul rticle model fo PEG-MWCNT
were used i s according t evich, Dubi and Fritz-Sch rrigan and Fr parameter eq
representing
e
q a
1 n are calcula 9] is widely a
e species on
n
e , and linea d 1 is a m eous as its v ess, while n for calculatio
ated paramete for Cd(II), Pb(
Ts.
in order to d to: Langmui inin-Astakho hlünder isoth eundlich isot quation, whic g the equi
n e n e
bC aC
ated from the applied in he activated ca arized form l measure of a value gets clo
above one on thermodyn
ers of the first (II), As(V) ads
escribe adso ir, Freundlic ov, Radke-P
herm. It was therms (Figs ch incorpora ilibrium ad
e nonlinear r eterogeneous rbon and mo
qe log log adsorption in
oser to zero is an indicat namic param
t and second sorption on
orption proce ch, Sips, Jov Prausnitz, R found that t s. 6 and 7).
ated both La dsorption da
regression fit s systems esp olecular sieve
f n
k log
g
ntensity or o. Whereas, a
tion of coop meters of ads
ess, and vanovic, Redlich-
the best angmuir ata for
(2)
tting of pecially
es.
Ce (3) surface a value perative sorption
nonlin
Ta
Isot Koble- a ((dm b((dm R Freu
K ((mg g
mg R Result Koble- was gi
Fig. 6. F As(V) at pH
Fig. 7. (a As(V) at p 10.0 mg d As(V)
The results near method,
able 3. Adsorp
therm Corrigan m3 mg-1)n) m3 mg-1)n)
n R2 undlich
KF
g-1) (dm3 g-1)1/n)
n R2
ts suggest tha -Corrigan iso iven in Table
Freundlich ads H 4 (c) on the 2.0,
a) Koble-Corr pH 4 on the PE dm-3, m/V = 10
) on the PEG-
s of experim are given in
ption isotherm Cd 25°C 3 19.07 4 -0.478 -0 0.399 0 0.999 0
40.44 4 1.288 1 0.990 0 at adsorption otherm. The e 4.
sorption isothe e PEG-MWCN 5.0, and 10.0
rigan isotherm EG-MWCNTs 00 mg dm-3), ( MWCNTs (C0
ental data fit Table 3.
ms parameters f d(II)
35°C 45°
40.58 40.9 0.112 -0.3 0.584 0.50 0.996 0.99
46.49 60.9 1.523 1.49 0.996 0.99 n of Cd(II), P maximum a
erms of Cd(II) NTs at 25, 35 a 0 mg dm-3, m/V
ms for adsorpt s at 25 °C. (C0
(b) Effect of pH
0 = 5 mg dm-3,
tting to Kobl
for Cd(II), Pb
°C 25°C 96 44.58
03 0.829 01 1.362 93 0.996
92 20.31 98 1.842 94 0.956 Pb(II), As(V) adsorption o
I) at pH 8 (a), and 45 °C. (C V = 100 mg dm
ion of Cd(II) a
0 = 0.10, 0.20 H on adsorpti , m/V = 100 m
le-Corrigan a
b(II) and As(V Pb(II)
35°C 45 47.31 49.
0.855 0.7 1.344 1.1 0.994 0.9
21.21 23.
1.857 1.8 0.955 0.9 ) on PEG-M f investigate
Pb(II) at pH 6 C0 = 0.10, 0.20
m-3).
and Pb(II) at p 0, 0.50, 1.0, 2.0 ion of Cd(II), P mg dm-3, T = 2
and Freundli
V) removal on
°C 25°C .56 0.8543 787 -0.818 85 0.056 998 0.977
.25 5.371 827 2.784 965 0.921 WCNTs is b ed ions obtai
6 (b), and 0, 0.50, 1.0,
pH 6, and 0, 5.0, and
Pb(II) and 25 oC).
ich isotherms
PEG-MWCN As(V) 35°C 0.9894
-0.815 0.059 0.989
6.033 2.523 0.938 best modelle ined experim
s, using
NTs.
45°C 3.568 -0.478
0.162 0.987
7.289 2.960 0.974 d using mentally
Values of n > 1 indicate that adsorption processes slightly decreased at lower sorbate concentration, and also indicates on presence of different active centres where the highest energies ones is of higher activity, e.g. participate in a initial adsorption step. Also, values higher than 1 indicates that cooperative mechanism, i.e. physisorption and chemisorption, are operative having different contribution at different phase of system equilibration.
Table 4. Experimental values of Cd(II), Pb(II) and As(V) uptake on PEG-MWCNTs at 25°C.
Initial concentration
(mg dm-3)
Cd(II), pH 8 Pb(II), pH 6 As(V), pH 4 qe
(mg g-1)
% of removal
qe (mg g-1)
% of removal
qe (mg g-1)
% of removal 0.1 0.991 99.1 0.840 84.0 0.979 97.9 0.2 1.96 98.0 1.66 83.1 1.87 93.6 0.5 4.72 94.4 3.78 75.7 3.71 74.3
1 9.08 90.8 7.20 72.0 4.61 46.1
2 17.1 85.6 14.08 70.4 4.94 24.7
5 39.0 78.0 33.9 67.8 7.56 15.1
10 77.6 77.6 47.5 47.5 13.0 13.0
3.5. Error functions
In order to measure the quality of fitting the validation of adsorption isotherms were accomplished using different error functions alongside with the correlation coefficient R2. Using nonlinear regression instead of linear incorporates the minimization or maximization of error distribution between the experimental data and the predicted isotherms based on its convergence criteria [21]. The data analysis was accomplished using Marquardt’s percent standard deviation (MPSD); hybrid fractional error function (HYBRID); average relative error (ARE); average relative standard error (ARS); sum squares error (ERRSQ/SSE); normalized standard deviation (NSD); standard deviation of relative errors (sRE); spearman’s correlation coefficient (rs) and nonlinear chi-square test (χ2) (Table 5).
Table 5. Values of correlation coefficients and error functions for Cd(II), Pb(II) and As(V) adsorption on PEG-MWCNTs for K-C and Freundlich isotherms.
Cd(II) Pb(II) As(V)
K-C F K-C F K-C F
R2 0.973 0.971 0.995 0.955 0.989 0.939 MPSD 45.76 50.22 57.72 90.16 25.47 40.37 HYBRID 84.45 84.73 59.33 179.5 11.55 38.54 ARE 23.63 28.77 31.44 57.61 12.02 26.98 ARS 0.3736 0.4584 0.471 0.823 0.2079 0.3686 ERRSQ 19.34 20.19 11.30 93.40 1.581 8.857
NSD 37.36 45.84 47.13 82.31 20.79 36.86 sRE 25.05 30.38 33.53 63.09 12.96 28.90 rs 0.9232 0.9199 0.955 0.629 0.9937 0.9649 χ2 3.378 4.237 2.373 8.976 0.4619 1.927
3.3. Adsorption thermodynamics
The Gibbs free energy (ΔG0), enthalpy (ΔH0) and entropy (ΔS0) of adsorption were calculated using the Van’t Hoff thermodynamic equations:
)
0
RT ln( b G
(4)
RT HR S
b) 0 0
ln( (5)
where b is nondimensional Langmuir constant, T is the absolute temperature in K and R is the universal gas constant (8.314 J mol-1 K-1). ΔH0 (kJ mol-1) and ΔS0 (J mol-1 K-1) can be obtained from the slope and intercept of ln(b) versus 1/T plot, assuming the sorption kinetics to be under steady-state conditions. The calculated thermodynamic values (Table 6) provide information on the adsorption mechanism.
Table 6. Calculated Gibbs free energy of adsorption, enthalpy and entropy for Cd(II), Pb(II) and As(V) adsorption on PEG-MWCNTs at 298, 308 and 318 K.
ΔG0
ΔH0 ΔS0 298 K 308 K 318 K
Cd(II) -39.52 -41.66 -43.71 21.98 209.8 Pb(II) -42.11 -44.24 -46.15 17.04 201.9 As(V) -45.02 -47.62 -49.83 25.50 240.8
The negative adsorption standard free energy changes (ΔG0) (kJ mol-1) and positive standard entropy changes (ΔS0) at all temperatures indicate that the adsorption reactions are spontaneous. For both sorbents the adsorption is endothermic (ΔH0 is positive). The decrease of Gibbs free energy (ΔG0) with increasing temperature indicates that spontaneity of the reaction increases.
5. Conclusions
The applicability of the isotherm equation to describe the adsorption process was judged by the correlation coefficients and error functions. Thus, the adsorption isotherm models fitted the data better by the use of Koble–Corrigan isotherm. Adsorption kinetic studies showed that the best fitting kinetic model is parabolic or Weber-Morris model giving the highest values of correlation coefficients than the other investigated models. Therefore, this model could be used for the prediction of the kinetics of adsorption of Cd, Pb and As on the PEG-MWCNTs. The adsorption properties of raw-MWCNTs were significantly improved by amino-functionalization, and modification of nanotubes using PEG generated novel material suitable for removal of heavy metal and metalloid species from water. Chemical modification of MWCNTs with PEG-NH2 offers an alternative to produce of filtration membranes for the removal of heavy metals from industrial waters at higher temperatures, and for preconcentration of heavy metals in analytical chemistry and environmental protection.
Acknowledgment
Financial support through the Ministry of Education, Science and Technological development of the Republic of Serbia, Project No. 172013, is gratefully acknowledged.
References
[1] W. Salomons, U. Förstner, P. Mader, (1995). Heavy Metals Problems and Solutions, Springer, New York .
[2] D. Mohan, C. U. Pittman Jr., J. Hazard. Mater., 142, 1–53 (2007).
[3] F. Miculescu, M. Miculescu, L. T. Ciocan, A. Ernuteanu, I. Antoniac, I. Pencea, E. Matei, Digest J. Nanom. Biostruct. 6 (3), 1117-1127 (2011).
[4] M. O’Connell, (2006). Carbon nanotubes: properties and applications, Ch. 8, CRC Press Taylor & Francis Group, Boca Raton, FL, USA.
[5] S. Iijima, Nature 354, 56-58 (1991).
[6] L. M. Dai, A.W.H. Mau, Adv. Mater. 13, 899-913 (2001).
[7] G. Vuković, A. Marinković, M. Obradović, V. Radmilović, M. Čolić, R. Aleksić, P.S. Uskoković, Appl. Surf. Sci. 255, 8067-8075 (2009).
[8] M.J. Kim, J. Lee, D. Jung, S.E. Shim, Syntetic metals, 160, 1410-1414 (2010).
[9] G.D. Vuković, A.D. Marinković, M. Čolić, M.Ð. Ristić, R.A. Aleksić, A. Perić-Grujić, P.S. Uskoković, Chem. Eng. J. 157, 238–248 (2010).
[10] M. A. Tofighy, T. Mohammadi, J. of Haz. Mat. 185, 140–147 (2011).
[11] N. A. Buang, F. Fadil, Z. A. Majid, S. Shahir, Digest J. Nanom. Biostruct. 7(1), 33-39 (2012) [12] G. D. Vuković, A. D. Marinković, S. D. Škapin, M. Đ. Ristić, R. Aleksić, A. A. Perić-Grujić, P. S. Uskoković, Chem. Eng. J. 173(2011) 855– 865.
[13] S. Myneni, S. Traina, G. Waychunas, T. Logan, Geochim. Cosmochim.
Acta., 62, 3285-3300 (1998).
[14] Z. Veličković, G. D. Vuković, A. D. Marinković, M. S. Moldovan, A. A. Perić- Grujić, P.S.
Uskoković, M. Đ. Ristić, Chem. Eng. J., 181-182, 174- 181 (2012).
[15] S. Goldberg, C. T. Johnston, J. Colloid. Interface Sci., 234, 204–216 (2001).
[16] H. Qiu, L. LV, B. Pan, Q. Zhang, W. Zhang, Q. Zhang, J. Zhejiang Univ Sci A 10(5), 716-724 (2009)
[1] W. J. Weber, J. C. Morris, Sanit. Eng. Div., ASCE 89 (SA2), 31–59 (1963).
[2] R. A. Koble, T. E. Corrigan, Ind. Eng. Chem. 44, 383–387 (1952).
[3] H. M. F. Freundlich, J. Phys. Chem. 57, 385–471 (1906).
[4] F. Haghseresht, G. Lu, Energy Fuels 12, 1100–1107 (1998).
[5] K.Y. Foo, B.H. Hameed, Chem. Eng. J. 156, 2–10 (2010).
