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Removal of carcinogenic hexavalent chromium from aqueous solutions using newly synthesized and characterized polypyrrole–titanium(IV)phosphate nanocomposite
Accepted Manuscript Removal of carcinogenic hexavalent chromium from aqueous solutions using newly synthesized and characterized polypyrrole-titanium(IV)phosphate nanocomposite Umair Baig, Rifaqat Ali Khan Rao, Asif Ali Khan, Mohd Marsin Sanagi, Mohammed Ashraf Gondal PII: DOI: Reference:
S1385-8947(15)00870-0 http://dx.doi.org/10.1016/j.cej.2015.06.031 CEJ 13802
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
12 April 2015 4 June 2015 7 June 2015
Please cite this article as: U. Baig, R.A.K. Rao, A.A. Khan, M.M. Sanagi, M.A. Gondal, Removal of carcinogenic hexavalent chromium from aqueous solutions using newly synthesized and characterized polypyrroletitanium(IV)phosphate nanocomposite, Chemical Engineering Journal (2015), doi: http://dx.doi.org/10.1016/j.cej. 2015.06.031
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1 2
Removal of carcinogenic hexavalent chromium from aqueous solutions using newly synthesized and characterized polypyrrole-titanium(IV)phosphate nanocomposite
3 4
Umair Baiga,b, Rifaqat Ali Khan Raoc, Asif Ali Khand,*, Mohd Marsin Sanagie, Mohammed Ashraf Gondalb,a*
5 6 7 8 9 10 11 12 13 14 15 16
a
Center of Excellence for Scientific Research Collaboration with MIT, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia b
c
Laser Research Group, Physics Department & Center of Excellence in Nanotechnology King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
Environmental Research Laboratory, Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh 202002, UP, India d
Analytical and Polymer Research Laboratory, Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh-202002 (UP) INDIA e
Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia
17 18
*Corresponding author’s email: [email protected]; [email protected]
19
Telephone: .:+91-571-2720323 , +9663-8602351/8603274; Fax: +9663-8604281
20 21 22 23 24 25 26 27 28 29 30
1
1
Abstract
2
This paper reports the synthesis of polypyrrole-titanium(IV)phosphate (PPy-TiP) nanocomposite
3
as a synthetic adsorbent for the adsorptive removal of Cr(VI) ions from aqueous solutions. Facile
4
in-situ chemical oxidative polymerization of pyrrole in presence of titanium(IV)phosphate (TiP)
5
formed PPy-TiP nanocomposite. The prepared PPy-TiP nanocomposite was characterized by FT-
6
IR, FE-SEM, TEM, EDAX and BET analysis. A strong interaction between PPy and TiP
7
particles was observed, and the nanocomposite showed remarkable adsorption of Cr(VI) ions
8
from aqueous solutions. In order to optimize the conditions for maximum adsorption of Cr(VI)
9
ions, batch process was used and the effects of contact time, initial concentration, pH, point of
10
zero charge measurement and adsorbent doses were investigated. The equilibrium isotherm data
11
were fitted using Langmuir and Freundlich isotherms. The adsorption process based on the
12
thermodynamic parameters was spontaneous and endothermic. Dubinin-Radushkeuich (D-R)
13
isotherm was used to calculate the mean free energy, which showed that the adsorption process
14
involves chemical forces, hence chemical in nature. The kinetics data were evaluated using the
15
pseudo-first-order and pseudo-second order kinetics model, and it was found that data was best
16
fitted by the pseudo-second-order model for adsorption of Cr(VI) supporting that the adsorption
17
mainly consisted of chemisorption. The desorption studies were also performed by batch process
18
and ~100% desorption of Cr(VI) ions was achieved using 0.5 M NaOH solution as eluent.
19
Keywords: Polymer; Nanocomposite; Synthetic adsorbent; Hexavalent chromium.
20 21 22 23
2
1
1. Introduction
2
Among the heavy metal ions, chromium is on the list of priority pollutants due to its high toxicity
3
as mentioned by the United States Environmental Protection Agency (U.S.E.P.A). The
4
widespread uses of chromium and its compounds in various industries such as mining, tanning,
5
cement, production of steel/metal alloys, electroplating operations, photographic materials,
6
corrosive painting and brass industries have led to its discharge into biosphere in amounts
7
exceeding the safety limits [1-2]. Various epidemiological studies conducted by US, UK, Japan
8
and Europe showed that the workers in the chromate production industry have high risks of lung
9
cancer, respiratory diseases, perforation of the nasal septum, etc. [3]. Chromium generally exists
10
as Cr(VI) or Cr(III) in the aqueous environment, and Cr(VI) is comparatively more toxic and
11
carcinogenic than Cr(III) [4-6]. Thus, its presence in the environment poses a huge risk to the
12
aquatic life and as well as human health [5,7]. As per world standards, the maximum permissible
13
limit of Cr(VI) for discharge to inland surface water and portable water is 0.1 and 0.05 mg L-1,
14
respectively [7,8]. Thus, the removal of Cr(VI) from water using simple and efficient techniques
15
is the need of the hour. Among the Cr(VI) removal techniques [9-11], adsorption is one of the
16
simplest and economical techniques for Cr(VI) removal from waste water as it is easy to operate
17
in comparison to precipitation method, which requires reduction of Cr(VI) to Cr(III) by using
18
reducing agents.
19
Heavy metal ion removal has been previously carried out using different adsorbents such as
20
activated carbon [12], zeolites [13-14], clay [15-16] and agricultural residues [17-18]. However,
21
most of these adsorbents suffer from low adsorption capacities. Thus, the adsorption processes
22
involve multiple usages of the adsorbents, which increases the cost. Nanoparticles due to their
23
large surface area possess high adsorption capacity and hence widely used nowadays as potential
3
1
adsorbents. However, due to difficulty in their separation from the aqueous phase after
2
adsorption, it is not feasible to use nanoparticles for metal removal. In order to overcome these
3
limitations, polymeric organic-inorganic nanocomposites/composites or synthetic adsorbents
4
have been used as matrix to anchor the nanoparticles which can be applied in the field of
5
desalination technology for the removal of toxic metal ions from waste water or aqueous
6
solutions [19-27]. These polymeric nanocomposites/composites or synthetic adsorbents are
7
expected to display new properties and present relatively high metal ion adsorption capacities
8
due to the synergism between the constituents [28].
9
Among the conducting polymers, polypyrrole (PPy) has received significant attention from
10
researchers due to its widespread applications in various fields owing to its high electrical
11
conductivity, environmental stability and facile synthesis at low cost in comparison to other
12
polymers [29-30]. These applications, mainly concentrate on its fascinating electrical
13
conductivity. However, PPy synthesized in solutions; exhibits anion exchanger or cation
14
exchanger behaviour depending on the type of dopant ions [30]. Besides these facts, PPy is also a
15
good prospect for adsorption applications due to positively charged nitrogen atoms in its polymer
16
chains. Thus, due to PPy’s excellent anion-exchange property and positively charged nitrogen
17
atoms in the polymer, it can be used as a potential adsorbent for the removal of anionic species
18
from aqueous solutions [31]. Generally, polymeric materials have low mechanical and thermal
19
stability due to wear and tear. Thus, the nanocomposite of PPy with stable inorganic materials is
20
expected to show higher stability due to synergism between the constituents [32]. Over the past
21
several years, there has been a generous enthusiasm toward the applications of titanium based
22
synthetic adsorbents in the field of water treatment to remove toxic heavy metal ions [33].
23
Titanium phosphates have been known as high performance ion-exchangers, and porous and 4
1
thermally stable materials [34]. Titanium phosphates are amphoteric in nature and can be utilized
2
as cation and anion exchangers [35]. The amphoteric and porous nature of titanium(IV) materials
3
relative to other transition metals makes them highly attractive for removal of high toxicity
4
pollutants [heavy metals like Cr(VI)] from aqueous solutions and other water bodies. Bhaumik et
5
al [35] showed that mesoporous titanium phosphate molecular sieves have high anion exchange
6
capacity. Owing to the beneficial properties of both PPy and TiP towards anion exchange, and
7
the presence of positively charged sites in PPy, the composite of PPy with TiP is expected to
8
prove as an effective material for the removal of Cr(VI) from aqueous solutions. In the present
9
study, polyprrole-titanium(IV)phosphate nanocomposite was prepared by in-situ oxidative
10
polymerization
method
and
the
adsorption
performance
of
the
polypyrrole-
11
titanium(IV)phosphate nanocomosite for the removal of Cr(VI) from aqueous solution was
12
evaluated. To the best of our knowledge, this is the first attempt to synthesize a new polypyrrole-
13
titanium(IV)phosphate nanocomposite for the removal of Cr(VI) from aqueous solutions.
14
2. Experimental
15
2.1. Chemical and reagents
16
Pyrrole monomer (98%) from was purchased from Spectrochem Chemicals (India, Pvt., Ltd.).
17
Anhydrous iron(III)-chloride (FeCl3), methanol, ortho-phosphoric acid (H3PO4), titanium dioxide
18
(TiO2), potassium dichromate (K2Cr2O7), ammonium sulphate and sodium hydroxide were
19
purchased from E. Merck, Chemical Company, Germany. All the chemicals and reagents were of
20
analytical grade and used as received.
21
2.2. Synthesis
22
2.2.1. Titanium(IV)phosphate
5
1
TiP
was
prepared
by
simple
precipitation
technique
from aqueous
solutions
of
2
titanium(IV)sulfate and H3PO4. Titanium(IV)sulphate solution was prepared as previously
3
described [33-34]. With vigorous stirring, aqueous solution of H3PO4 was added dropwise into
4
titanium(IV)sulfate solution. The mixture was stirred for 24 h at 25 ºC to form an absolute
5
precipitate. After 24 h, resulting precipitate was filtered off, washed with demineralized water
6
(DMW) and dried at 100 oC for 24 h. Finally, the material was ground by pastel mortar to yield a
7
fine TiP powder.
8
2.2.3. Polypyrrole-titanium(IV)phosphate nanocomposite
9
In-situ oxidative chemical polymerization [38-40] of pyrrole with FeCl3 as the oxidizing agent,
10
in the presence of TiP particles, was used in the preparation of the nanocomposite (PPy-TiP). A
11
definite amount of TiP was dispersed in 100 mL of DMW under ultrasonic vibrations at 25 oC
12
for 1 h. The dispersed TiP solution was then poured into a 500 mL round-bottom flask equipped
13
with a magnetic stirrer and a definite amount of pyrrole (monomer) was added. For proper
14
adsorption of pyrrole monomers on the TiP particles, vigorous stirring of the solutions for 4 h
15
was ensured. 2 g of FeCl3 was dissolved in demineralized water (100 mL) and poured into the
16
pyrrole adsorbed TiP dispersion. The reaction mixture was stirred for 24 h at 25 ºC. After 24 h,
17
synthesized PPy-TiP nanocomposite was filtered and washed with DMW and methanol to
18
remove unreacted oxidant. The obtained powder was dried completely at 70 ºC for further
19
analysis. Pure PPy was synthesized by a similar method as the preparation of PPy-TiP
20
nanocomposite without using the TiP nanoparticles. A graphical demonstration of the
21
preparation of PPy-TiP nanocomposite is indicated in Scheme. 1.
22
2.3. Characterization
6
1
Fourier transform infra-red (FTIR) spectra were recorded on Perkin Elmer 1725 FTIR
2
instrument. Field emission scanning electron microscopy (FE-SEM) was carried out by LEO
3
435-VF scanning electron microscope to study the surface morphology. Elemental analysis was
4
carried out by using energy dispersive analyser unit (EDAX) attached with FE-SEM.
5
Transmission electron microscopy (TEM) were achieved by JEOL TEM (JEM 2100F)
6
instrument. Specific surface area and pore-size distribution were measured by Brunauer–
7
Emmett–Teller (BET) nitrogen adsorption-desorption method using a Micromeritics Model
8
3Flex Surface Characterisation Analyzer (Atlanta, USA). Metal ion concentrations in solutions
9
were determined by using atomic absorption spectrometer (AAS) Model: GBC 902, Australia.
10
2.4. Adsorption studies
11
Adsorption studies were carried out by batch process. 0.2 g adsorbent was placed in a conical
12
flask. 20 mL solution of metal ions of desired concentration was added and the mixture was
13
shaken in incubator for 24 h. The mixture was filtered and then final concentration of metal ions
14
was determined in the filtrate by atomic absorption spectrometry. The amount of metal ions
15
adsorbed were calculated by subtracting the final concentration from initial concentration [41]
16
using the following relation: = − ×
1
17
where qe is the amount of metal ions adsorbed per unit weight of adsorbent (mg g-1). Co and Ce
18
are the initial and equilibrium concentrations of metal ion in solution. V is the volume (L) and M
19
is the mass of adsorbent (g).
20
The % adsorption was calculated as
7
% =
− × 100 2
1
2.5. Effect of pH
2
The effect of pH on the adsorption of Cr(VI) ions was analyzed using standard protocol [41].
3
50mL of Cr (VI) solution was poured into a series of conical flasks. The required pH of the
4
solution was regulated by adding 0.1N HCl or NaOH solution in each conical flask. The initial
5
concentration of Cr(VI) in this solution was then determined as follows. 20 mL of the adjusted
6
pH solution of Cr(VI) ions was taken in a series of conical flasks and treated with 0.2 g of
7
adsorbent for 24 h in shaker incubator. The mixture was then filtered and final concentration of
8
Cr(VI) in the filtrate was determined as described above.
9
2.6. Point of zero charge
10
The points of zero charge (pHpzc) of TiP and PPy-TiP nanocomposite were ascertained by the
11
well-known solid addition method [42]. 40 mL solution of 0.1 M KNO3 was transferred into
12
series of conical flasks. The variation in initial pH (pHi) of the solutions was maintained at about
13
1 to 10 by adding either acid or base solution of the known concentration (0.1 M HCl or 0.1 M
14
NaOH). The total volume of the solution in each flask was adjusted precisely to 50 mL by adding
15
KNO3 of the same strength and the initial pH (pHi) of the solutions was then observed accurately
16
using pH meter. Later 0.2 g of adsorbent was placed into each flask and allowed to equilibrate
17
for 24 h with occasional manual shaking and the final pH (pHf) of the supernatant liquid was
18
recorded. The difference between the initial pH (pHi) and final pH (pHf) values was calculated
19
according to equation (∆pH = pHi–pHf). A ∆pH plot was then constructed against pHi, and the
20
point of intersection of the resulting curve with abscissa, at which ∆pH = 0, gave the pHpzc. The
21
same procedure was applied for 0.05 M KNO3 solutions and DDW.
22
2.7. Effect of time 8
1
A series of 100 mL conical flasks, each having 0.2 g adsorbent and 20 mL solution of known
2
Cr(VI) concentration (200, 400, 600, 800 and 1000 mg L−1; pH = 7) were placed in a shaker
3
incubator and at the predetermined intervals, the solutions of the specified flasks was taken out
4
and filtered. The concentration of Cr(VI) in the filtrate was determined by AAS and the amount
5
of Cr(VI) adsorbed in each case was then determined as described above.
6
2.8. Breakthrough capacity
7
0.2 g of adsorbent was loaded into a glass column of 0.6 cm internal diameter with glass wool
8
support at the end. 1 L of Cr(VI) solution containing 200 mg L−1 (pH = 7) initial concentration
9
(Co) was prepared and then passed through the column. The flow rate was adjusted to 1 mL
10
min−1. The effluent was collected in 10 mL fractions in 0 to 100 mL effluent range, and after 100
11
mL range, the effluent was collected in 50 mL fractions in 100-350 mL range. The amount of
12
Cr(VI) was determined in each fraction (C) with the help of AAS. The breakthrough curve was
13
obtained by plotting C/Co based on the volume of the effluent.
14
2.9. Desorption of Cr(VI) by batch process
15
The study of desorption of chromium(VI) was carried out by using a batch process. 0.2 g
16
adsorbent was treated with 50 mL of Cr(VI) solution (300 mg L−1). The adsorbent was washed
17
thoroughly with distilled water to remove excess of Cr(VI) ions. The exhausted adsorbent was
18
then treated with 20 mL 0.5 M NaOH solution and after 24 h; the amount of Cr(VI) desorbed
19
was determined. The experiment was repeated several times to ensure reproducibility.
20
3. Results and discussion
21
3.1. FTIR spectroscopic studies
22
The FT-IR spectra of PPy, PPy-TiP and Cr(VI) adsorbed PPy-TiP nanocomposite are shown in
23
Fig. 1. The FT-IR spectra of PPy shows absorption peak at 902 cm−1 in its finger print region,
9
1
which is characteristic of C–H out-of-plane deformation vibration, confirming the formation of
2
PPy by the monomer. The band at 1300 cm−1 is attributed to the C–N in-plane vibration,
3
and the bands at 1130 to 1190 cm−1 are related to the C–N stretch modes. The peak around
4
1310 cm-1 is related to C–N in-plane stretching vibration and the strong absorption bands at
5
1539 cm−1 and 1449 cm−1 correspond to the C–C stretching vibration of the pyrrole ring. The
6
PPy-TiP nanocomposite showed similar peaks as in PPy except for an intense peak at 1190 cm-1
7
overlapping with the C-N peak (1130 – 1190 cm-1), which may be attributed to the -OH group of
8
phosphate. After binding of Cr(VI) to the PPy-TiP nanocomposite (Fig. S1), the peak at 1190
9
cm-1 further intensified indicating the interaction of Cr(VI) with phosphate of TiP and C-N
10
groups of polymer backbone (PPy).
11
3.2. FE-SEM and TEM studies
12
The FE-SEM images of PPy, TiP and PPy-TiP nanocomposite before adsorption and PPy-TiP
13
nanocomposite after adsorption are shown in Fig. 2 (a-d) respectively. Fig. 2a and 2b shows the
14
granular morphology of PPy and TiP, however, the morphology of the composite (Fig. 2c) is
15
quite different from that of the parental components. PPy deposits are clearly visible at the
16
surface of TiP, which has porous and globular structure; providing a good platform for the
17
adsorption of Cr(VI). After adsorption of Cr(VI) on PPy-TiP nanocomposite (Fig. 2d), the
18
surface morphology of PPy-TiP nanocomposite has been changed significantly, showing that
19
Cr(VI) has been adsorbed on the surface of PPy-TiP nanocomposite. Fig. 3 shows the TEM
20
micrographs of PPy-TiP nanocomposite with granular morphology having an average particle
21
size of ~35-45 nm. The TiP nanoparticles can be seen as dark spots encapsulated in PPy matrix
22
having an average particle size of ~20-25 nm.
10
1
3.3. Energy dispersive x-ray analysis (EDAX)
2
To confirm the adsorption of Cr(VI) on PPy-TiP nanocomposite, EDAX analyses were
3
performed before and after adsorption of Cr(VI). The percentage compositions of PPy-TiP
4
nanocomposite before and after adsorption of Cr(VI) are presented in Table 1. The percentage
5
composition of chromium after adsorption confirmed that Cr(VI) has been successfully adsorbed
6
on PPy-TiP nanocomposite.
7
3.4. Surface area and pore size analysis
8
The surface area was determined by the Brunauer, Emmett and Teller (BET) method while pore
9
size distribution measured from N2 adsorption-desorption isotherm at 77 K by the Barret, Joyner
10
and Halenda (BJH) shown in Fig. 5. Specific surface area and pore size distribution parameters
11
are listed in Table S1. From Fig. 4 and Table S1, PPy-TiP nanocomposite showed BET specific
12
surface area of 59.3956 m²/g. The BJH adsorption/desorption average pore diameters were
13
calculated and found to be 160.932 and 139.512 Å, while the adsorption/desorption average pore
14
diameters associated with BET were found to be 103.0875 and 112.6687 Å, respectively. On the
15
basis of the obtained values (Table S1), the sample; PPy-TiP nanocomposite is consistent with
16
the pore diameter classification of International Union of Pure and Applied Chemistry (IUPAC).
17
According to IUPAC notation, microporous materials have pore diameters of less than 2 nm and
18
macroporous materials have pore diameters of greater than 50 nm; the mesoporous category thus,
19
lies in the middle.
20
3.5. Effect of contact time and initial concentration
21
The adsorbent showed excellent capacity towards Cr(VI) ions. The adsorption of Cr(VI) at
22
different initial Cr(VI) concentrations (200-1000 mg L-1) as a function of contact time is shown 11
1
in Fig. 5. The adsorption capacity increased with time and reached a constant value
2
(equilibrium). The contact time needed for the initial Cr(VI) concentrations (200-600 mg L-1) to
3
reach equilibrium was 120 to 240 min. While for 800-1000 mg L-1 initial Cr(VI) concentration,
4
equilibrium time of 300 min was required.
5
3.6. Effect of pH and surface charge measurement
6
Solution pH stands out as one of the crucial parameters affecting the adsorption of metal ions. The
7
effect of pH of the solution on the adsorption of Cr(VI) on TiP and PPy-TiP nanocomposite was
8
studied in the pH range 1-10 with 200 mg L-1 initial Cr(VI) concentration. It can be seen from
9
Fig. 6, that the adsorption of Cr(VI) on both TiP and PPy-TiP is maximum at acidic pH (pH = 2.0),
10
with adsorption percentage close to 99% with PPy-TiP and almost 48% with TiP. Further, the
11
adsorption capacity of TiP decreases on pH values greater than 2; and for PPy-TiP nanocomposite,
12
it almost remained the same.
13
The various species of Cr(VI) existing in aqueous solution at different pH values are
14
H2CrO4 (pH<1); HCrO4- and Cr2O7-2 (pH 2-6) and CrO4-2 (pH >6) [43]. The increased adsorption
15
potential of the adsorbents at pH 2 may be assumed due to strong electrostatic attractions
16
between adsorbent surfaces and adsorbate species. At pH 2, the presence of high concentrations
17
of H+ ions within the aqueous medium makes the surface of the adsorbents (both TiP and PPy-
18
TiP) positively charged, which enhances the adsorption of the chromate anions (HCrO4- and
19
Cr2O7-2) due to strong electrostatic forces of attraction [44-45]. Increase in pH of the solution
20
above pH 2 decreases the positive charge of the TiP surface due to decreased protonation, which
21
consequently decreases the adsorption of the chromate anions at pH values greater than 2. At pH
22
values lower than 2 the decreased adsorption on both the adsorbents is due to the competition
23
between the protons and the active chromium species to bind with the adsorbents [46]. Increase 12
1
in pH of the solution above pH 2 did not affect the adsorption of chromate ions in case of PPy-
2
TiP composite, which may be attributed to the structural modification of PPy due to the
3
incorporation of TiP. The surface of the PPy contains several –NH functionalities, which might
4
assist the ion-dipole fashioned binding of the dissolved metal ions from the solution irrespective
5
of the increasing pH.
6
The pHpzc of an adsorbent is a measure of the pH at which its surface has net electrical
7
neutrality. An adsorbant surface is negatively charged at pH > pHpzc, neutral at pH = pHpzc and
8
has a net positive charge at pH < pHpzc [47]. It can be seen from the Fig. 7 (a) and (b) that both
9
TiP and PPy-TiP have positive surfaces at pH 2 and below it. With increase in pH, the surface
10
loses positive charge and becomes negative. Thus, the nature of chromium species present in the
11
solution and the surface charges of the adsorbents at different pH values fully support the
12
maximum adsorption at pH 2.
13
3.7. Adsorption Isotherms
14
The Langmuir and Freundlich isotherm models was used to analyze the adsorption mechanism of
15
PPy-TiP with Cr(VI). This explained the chemical interaction between homogeneous and
16
heterogeneous adsorbent surface, respectively as evident from the following equations [41]: 1 1 1 1 = × + 3 × 1 log = log#$ + log 4 n
17
The details of the equation parameters can be seen in the supplementary section under the
18
heading adsorption Isotherm. The values of b and qm were calculated from the straight line plot
19
drawn between 1/qe and 1/Ce (Fig. 8a). The Langmuir parameters reported in Table 2 indicates 13
1
high values of correlation coefficient (R2= 0.9934) at normal temperature (30 oC) indicating that
2
Cr(VI) concentration ranges from 300-700 mg g-1. The exceptionally high adsorption capacity
3
(qm = 31.64 mg g-1) in comparison to other reported adsorbents (Table S2) [29, 49-52] indicated
4
the efficacy of PPy-TiP nanocomposite towards the removal of Cr(VI) from aqueous solution.
5
The separation factor (RL) in Langmuir isotherm can be calculated from the well-known relation
6
[53]. The value of RL indicates the type of isotherm; if RL>1, the isotherm is unfavourable, if RL
7
= 1 its linear; favorable if 0
8
range of 0-1 in all the experimental concentrations of Cr(VI) indicated that the adsorption is
9
favourable in nature (Table 2). The adsorption isotherm analysis (Fig. 8a , 8b) indicates that the
10
correlation coefficient of Langmuir model is higher than that of Freundlich model, hence it can
11
be concluded that Langmuir model is the best fit model for the adsorption of Cr(VI) onto PPy-
12
TiP nanocomposite.
13
3.8. Dubinin–Radushkevich (D-R) isotherm
14
D-R isotherm associated with the heterogeneity of the surface energies was also used to evaluate
15
the chemical or physical nature of the adsorption and the mean free energy of adsorption as
16
described by the following equations 5, 6 and 7 [54]. ' = ' – )* + 5 * = -.' 1 +
17
0=
1
23+4
1 6 (7)
14
1
The details of the equation parameters can be seen in the supplementary section under the
2
heading Dubinin–Radushkevich (D-R) isotherm. The obtained results are given in respective
3
Table 2 and Fig. S3. The linear plot of lnqe versus ɛ2 (Fig. S3) with high correlation coefficient
4
(R2 = 0.9963) showed that D-R isotherm best fitted for this system. The magnitude of mean free
5
energy E<8 kJ mol-1 represents physical nature of the adsorption while E in between 8-16 kJ mol-
6
1
7
exchange. The value of E obtained in our case was 15.800 kJ mol-1 which confirmed that
8
adsorption of Cr(VI) proceeded by ion-exchange process.
9
3.9. Adsorption kinetics
indicates that adsorption process is chemical in nature and most probably proceeds via ion-
10
Lagergren pseudo-first order model [55] and pseudo-second order model [56] were used to
11
explain the kinetics of Cr(IV) adsorption, as described by the following equations 8 and 9: log − 5 = log −
#1 × t 8 2.303
1 1 = + × 9 + 5 #+ . 12
The details of the equation parameters can be seen in the supplementary section under the
13
heading adsorption kinetics. The obtained results are given in respective Table 3 and Fig. 9. It
14
can be inferred from the data that although values of R2 are reasonably high but adsorption
15
capacity determined experimentally (qe(exp)) differed appreciably when compared to the
16
adsorption capacity calculated from the model (qe(cal)). This indicated that adsorption process
17
does not follow pseudo-first-order adsorption kinetics model. From Table 3 and Fig. 9, the
18
values of R2 at all experimental concentrations were very close to 1 and qe(cal) values calculated
19
from the model were also found to be very close to experimental values qe(exp) confirming the
20
applicability of pseudo-second-order equation. The pseudo-second-order model proposed by Ho
15
1
and Mekay [54] is based on the assumption that adsorption follows chemisorption, therefore it
2
can be concluded that adsorption of Cr(VI) was chemical in nature and most probably proceeded
3
via ion-exchange.
4
3.10. Thermodynamic studies
5
The effect of temperature on the adsorption of Cr(VI) was studied at temperature ranging from
6
30, 40 and 50 ºC. Thermodynamic parameters such as standard free energy change (∆Gº),
7
standard enthalpy change (∆Hº) and standard entropy change (∆Sº) were calculated using the
8
following equations: [57] ΔG = −-.' #< 10
9 10
where R is universal gas constant (8.314 mol-1 K-1), T is temperature in kelvin and Kc is the equilibrium constant. The value of Kc can be calculated as: #< =
=> 11
11
where, Kc is the adsorption equilibrium constant, CAC and Ce are equilibrium concentrations of
12
Cr(VI) on the adsorbent (mg g-1) and in the solution (mg L-1), respectively.
13
The standard enthalpy (∆Hº) and entropy (∆Sº) were determined by using the following
14
equation: ΔS ΔH B 12 '#< = ? − -.
15
By plotting a graph, lnkc versus 1/T (Fig. 8c); the value of ∆Hº and entropy ∆Sº can be estimated
16
from the slope and intercept [58]. Table 4 shows the negative values of ∆Gº and positive ∆Hº
17
indicating that the Cr(VI) adsorption process is spontaneous and endothermic [59]. The ∆Gº
18
values decreased with increase in temperature, which suggested an increased spontaneity with
19
temperature. The positive value of ∆Sº suggested an increased randomness at the solid-solute
16
1
interface during adsorption. The Cr(VI) ions replace some water molecules previously adsorbed
2
on the surface of the adsorbent and these displaced water molecules gain more randomness than
3
lost by the adsorbed Cr(VI) ions [60].
4
3.11. Break through capacity
5
The break through curve (Fig. 10) indicated that 30 mL solution containing 200 mg L-1 initial
6
Cr(VI) concentration can be passed through the column containing 0.2 g of adsorbent without
7
detecting Cr(VI) in the effluent. The break through and exhaustive capacities were found to be
8
30 and 175 mg g-1 respectively.
9
3.12. Desorption Studies
10
In order to make the process more economical it is significant to desorb the adsorbed metal ions
11
for the reuse of the adsorbent. Batch adsorption studies were carried out using 0.5 M NaOH
12
solution as the desorbing agent. Fig. 11a showed that adsorption of Cr(VI) was 82 % at 300 mg
13
L-1 initial Cr(VI) concentration and desorption was 80 %. But, above 300 mg L-1 initial Cr(VI)
14
concentration of Cr(VI),
15
indicated that adsorption was maximum (88 %) at 0.2 g adsorbent dose and 100 % desorption
16
was achieved with 0.5 M NaOH. Therefore it can be concluded that 0.2 g of adsorbent dose is
17
sufficient for the optimum adsorption and desorption of Cr(VI) ions from a solution containing
18
300 mg L-1 initial Cr(VI) concentration.
19
The PPy-TiP nanocomposite conveys dynamic destinations (active sites), which gives a decent
20
plat form or adsorption locales for adsorption of Cr(VI). A schematic illustration of adsorbtion
21
and desorption of Cr(VI) onto PPy-TiP nanocomposite is reprensted in Sheme 2. Basically, the
22
TiP serves as a template for composite formation via in situ chemical oxidation polymerization
23
process. PPy chains adhere onto TiP surfaces and form thermally stable nanocomposite PPy-TiP
the % adsorption and desorption decreased. Similarly Fig. 11b
17
1
as shown in Scheme 1. The adsorption of Cr(VI) ions from the solution is governed by the
2
solution pH, electrical neutrality of the adsorbent surfaces, and the speciation of chromium ions
3
at different pH values. The PPy coating greatly enhances the adsorption capacity of TiP, which
4
may be partly by providing more surface area with different chemical functionalities for the
5
interaction with different species of chromium ions.
6 7
4. Conclusion
8
In the present work, the PPy-TiP nanocomposite showing significant potential for the removal of
9
Cr(VI) from aqueous solution; was successfully synthesized by simple in-situ chemical oxidative
10
polymerization method. The results of FTIR and EDAX studies reveal that Cr(IV) has been
11
successfully absorbed on PPy-TiP nanocomposite. The ideal conditions for adsorption were
12
found to be: contact time of 19 min, Cr(VI) initial concentration of 200 mg L-1 and an adsorbent
13
dose of 0.2 g. The results shows that Langmuir and D-R isotherm models are well fitted in all the
14
experimental conditions. Thermodynamic parameters indicated spontaneous and endothermic
15
nature of the adsorption. The mechanism for the removal of Cr(VI) by PPy-TiP nanocomposite
16
indicated that adsorption process is chemical in nature and most probably proceeds via ion-
17
exchange. The economic viability of the material has been demonstrated by desorbing the
18
adsorbent with 0.5 M NaOH solution and 100% desorption was achieved using 0.2 g adsorbent
19
dose with 300 mg L-1 initial Cr(VI) concentration. These studies suggested that PPy-TiP
20
nanocomposite can be used as potential adsorbent for the removal of Cr(VI) from aqueous
21
solutions.
22
Supporting Information
18
1
Supporting information includes FTIR spectra of PPy-TiP nanocomposite before adsorption and
2
PPy-TiP nanocomposite after adsorption of Cr(VI), D-R isotherm model, percent adsorption of
3
Cr(VI) onto PPy-TiP nanocomposite with different concentrations, surface area, pore volume
4
and pore size analysis details of PPy-TiP nanocomposite and Comparison of adsorption capacity
5
of the PPy-TiP nanocomposite with other adsorbents for Cr(VI) removal at 25 oC.
6 7 8
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25
Table 1. Percent composition of carbon, nitrogen, oxygen, titanium, phosphorus and chromium in PPy-TiP nanocomposite by EDAX analysis. Element C N O Ti P Cr
Weight (%) PPy
PPy-TiP (Before Adsorption)
PPy-TiP (After Adsorption)
64.73 19.04 16.23 Nil Nil Nil
44.36 10.84 34.16 4.90 5.75 Nil
47.83 11.41 33.72 2.20 2.36 2.47
Table 2. Langmuir, Freundlish and D-R isotherm parameters for Cr(VI) adsorption on to PPyTiP nanocomposite. Langmuir, Freundlish and D-R isotherm parameters Langmuir constants qm (mg/g) 31.64 Kf (mg/g) 13.316
R2 0.993
b (L/mg) RL 0.048 0.094 Freundlich constants R2 0.989
1/n 0.1482 D-R constants
qm (mg/g) 42.7
β -2x10-9
E 15.800
R2 0.996
Table 3. Pseudo-first order and pseudo-second order rate parameters for the adsorption of Cr(VI) at different concentrations. Concentrations
Pseudo-first order
Pseudo-second order
(mg L-1)
qe (exp)
qe (cal)
K1
R2
qe (cal)
K2
h
R2
200
19.20
19.742
0.0070
0.9133
19.34
0.0081
3.041
0.9996
400
27.6
22.014
0.0063
0.9602
25.45
0.0058
3.618
0.9995
600
29.8
29.518
0.0035
0.8915
30.48
0.0034
3.234
0.9986
800
40.4
44.463
0.0060
0.9526
35.65
0.0024
3.056
0.9965
1000
50.2
54.460
0.0081
0.8915
49.75
0.0016
3.987
0.9943
Table 4.Thermodynamic parameters at different temperatures for the adsorption of Cr(VI) on polypyrrole-titanium(IV)phosphate nanocomposite. Temperature (oC)
ln Kc
ΔGo (kJ mol-1)
30
0.9627
-2.425
40
1.098
-2.859
50
1.166
-3.136
ΔHo (kJ mol-1) ΔSo (kJ mol-1 K-1)
18.619
0.080
R2
0.9742
Fig. 1. FTIR spectra of (a) TiP, (b) PPy and (c) PPy-TiP nanocomposite.
(a)
(b)
(c)
(d)
Fig. 2. FE-SEM images of (a) PPy (b) TiP, (c) PPy-TiP nanocomposite before adsorption and (d) PPy-TiP nanocomposite after adsorption of Cr(VI).
(a)
(b)
Fig. 3. TEM micrograph of (a) PPy-TiP nanocomposite without particle size measurement and (b) PPy-TiP nanocomposite with particle size measurement.
0.0006
dV/dD (cm3 g-1 Å̊)
0.0005
0.0004
0.0003
0.0002
0.0001
0 0
200
400
600
800
1000
1200
Pore diameter (Å̊)
(c)
0.18
Pore Volume (cm3 g-1)
0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 0
200
400
600
800
1000
Pore diameter (Å)
Fig. 4. (a) N2 adsorption-desorption isotherms of PPy-TiP nanocomposite, (b) pore size distribution and (c) pore volume curves of PPy-TiP nanocpmosite.
60
200 ppm 400 ppm 600 ppm 800 ppm 1000 ppm
50
qt (mg/g)
40
30
20
10
0 0
100
200
300
400
500
Time (min)
Fig. 5. The effect of contact time on the Cr(VI) adsorption onto PPy-TiP nanocomposite.
Fig. 6. Effect of pH on the removal of Cr(VI) by TiP and PPy-TiP nanocomposite.
(a)
(b)
Fig. 7. Point of zero charge curves for (a) TiP and (b) PPy-TiP nanocomposite.
Fig. 8. (a) Fit of equilibrium data to Langmuir isotherm model (b) Fit of equilibrium data to Freundlich isotherm model and (c) plot of lnKc versus 1/T for Cr(VI) adsorption onto PPy-TiP nanocomposite.
Fig. 9. Pseudo-second-order kinetic model for adsorption of Cr(VI) by the PPy-TiP nanocomposite.
Fig. 10. Breakthrough curve of obtained for removal of Cr(VI) by PPy-TiP phosphate nanocomposite.
Fig. 11. Adsorption and desorption capacities of PPy-TiP nanocomposite (a) with varying concentration of Cr(VI) and (b) with loading of PPy-TiP nanocomposite for removal of Cr(VI).
(a)
FeCl3 H2 O
Pyrrole
Polypyrrole
(b)
Scheme 1. Schematic diagram of the formation mechanism of (a) PPy and (b) PPy-TiP nanocomposite.
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr Cr
Cr Cr
Cr
Cr
Scheme 2. The mechanistic representation of adsorption and desorption of Cr(VI) on PPy-TiP nanocomposite.
1 2
Research Highlights:
3
Synthetic nano-adsorbent were synthesized for the removal of Cr(VI).
4
Equilibrium isotherm data were analyzed using Langmuir and Freundlich isotherms.
5
The pseudo-second-order model described adequately the adsorption kinetics data.
6
The adsorption and desorption studies suggested 100% desorption of Cr(VI) ions.
7 8
26
1 2 3 4
Graphical Abstract
5
Cr
Cr
Cr Cr Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
6 7 8
27
S1385-8947(15)00870-0 http://dx.doi.org/10.1016/j.cej.2015.06.031 CEJ 13802
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
12 April 2015 4 June 2015 7 June 2015
Please cite this article as: U. Baig, R.A.K. Rao, A.A. Khan, M.M. Sanagi, M.A. Gondal, Removal of carcinogenic hexavalent chromium from aqueous solutions using newly synthesized and characterized polypyrroletitanium(IV)phosphate nanocomposite, Chemical Engineering Journal (2015), doi: http://dx.doi.org/10.1016/j.cej. 2015.06.031
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1 2
Removal of carcinogenic hexavalent chromium from aqueous solutions using newly synthesized and characterized polypyrrole-titanium(IV)phosphate nanocomposite
3 4
Umair Baiga,b, Rifaqat Ali Khan Raoc, Asif Ali Khand,*, Mohd Marsin Sanagie, Mohammed Ashraf Gondalb,a*
5 6 7 8 9 10 11 12 13 14 15 16
a
Center of Excellence for Scientific Research Collaboration with MIT, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia b
c
Laser Research Group, Physics Department & Center of Excellence in Nanotechnology King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
Environmental Research Laboratory, Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh 202002, UP, India d
Analytical and Polymer Research Laboratory, Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh-202002 (UP) INDIA e
Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia
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*Corresponding author’s email: [email protected]; [email protected]
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Telephone: .:+91-571-2720323 , +9663-8602351/8603274; Fax: +9663-8604281
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1
1
Abstract
2
This paper reports the synthesis of polypyrrole-titanium(IV)phosphate (PPy-TiP) nanocomposite
3
as a synthetic adsorbent for the adsorptive removal of Cr(VI) ions from aqueous solutions. Facile
4
in-situ chemical oxidative polymerization of pyrrole in presence of titanium(IV)phosphate (TiP)
5
formed PPy-TiP nanocomposite. The prepared PPy-TiP nanocomposite was characterized by FT-
6
IR, FE-SEM, TEM, EDAX and BET analysis. A strong interaction between PPy and TiP
7
particles was observed, and the nanocomposite showed remarkable adsorption of Cr(VI) ions
8
from aqueous solutions. In order to optimize the conditions for maximum adsorption of Cr(VI)
9
ions, batch process was used and the effects of contact time, initial concentration, pH, point of
10
zero charge measurement and adsorbent doses were investigated. The equilibrium isotherm data
11
were fitted using Langmuir and Freundlich isotherms. The adsorption process based on the
12
thermodynamic parameters was spontaneous and endothermic. Dubinin-Radushkeuich (D-R)
13
isotherm was used to calculate the mean free energy, which showed that the adsorption process
14
involves chemical forces, hence chemical in nature. The kinetics data were evaluated using the
15
pseudo-first-order and pseudo-second order kinetics model, and it was found that data was best
16
fitted by the pseudo-second-order model for adsorption of Cr(VI) supporting that the adsorption
17
mainly consisted of chemisorption. The desorption studies were also performed by batch process
18
and ~100% desorption of Cr(VI) ions was achieved using 0.5 M NaOH solution as eluent.
19
Keywords: Polymer; Nanocomposite; Synthetic adsorbent; Hexavalent chromium.
20 21 22 23
2
1
1. Introduction
2
Among the heavy metal ions, chromium is on the list of priority pollutants due to its high toxicity
3
as mentioned by the United States Environmental Protection Agency (U.S.E.P.A). The
4
widespread uses of chromium and its compounds in various industries such as mining, tanning,
5
cement, production of steel/metal alloys, electroplating operations, photographic materials,
6
corrosive painting and brass industries have led to its discharge into biosphere in amounts
7
exceeding the safety limits [1-2]. Various epidemiological studies conducted by US, UK, Japan
8
and Europe showed that the workers in the chromate production industry have high risks of lung
9
cancer, respiratory diseases, perforation of the nasal septum, etc. [3]. Chromium generally exists
10
as Cr(VI) or Cr(III) in the aqueous environment, and Cr(VI) is comparatively more toxic and
11
carcinogenic than Cr(III) [4-6]. Thus, its presence in the environment poses a huge risk to the
12
aquatic life and as well as human health [5,7]. As per world standards, the maximum permissible
13
limit of Cr(VI) for discharge to inland surface water and portable water is 0.1 and 0.05 mg L-1,
14
respectively [7,8]. Thus, the removal of Cr(VI) from water using simple and efficient techniques
15
is the need of the hour. Among the Cr(VI) removal techniques [9-11], adsorption is one of the
16
simplest and economical techniques for Cr(VI) removal from waste water as it is easy to operate
17
in comparison to precipitation method, which requires reduction of Cr(VI) to Cr(III) by using
18
reducing agents.
19
Heavy metal ion removal has been previously carried out using different adsorbents such as
20
activated carbon [12], zeolites [13-14], clay [15-16] and agricultural residues [17-18]. However,
21
most of these adsorbents suffer from low adsorption capacities. Thus, the adsorption processes
22
involve multiple usages of the adsorbents, which increases the cost. Nanoparticles due to their
23
large surface area possess high adsorption capacity and hence widely used nowadays as potential
3
1
adsorbents. However, due to difficulty in their separation from the aqueous phase after
2
adsorption, it is not feasible to use nanoparticles for metal removal. In order to overcome these
3
limitations, polymeric organic-inorganic nanocomposites/composites or synthetic adsorbents
4
have been used as matrix to anchor the nanoparticles which can be applied in the field of
5
desalination technology for the removal of toxic metal ions from waste water or aqueous
6
solutions [19-27]. These polymeric nanocomposites/composites or synthetic adsorbents are
7
expected to display new properties and present relatively high metal ion adsorption capacities
8
due to the synergism between the constituents [28].
9
Among the conducting polymers, polypyrrole (PPy) has received significant attention from
10
researchers due to its widespread applications in various fields owing to its high electrical
11
conductivity, environmental stability and facile synthesis at low cost in comparison to other
12
polymers [29-30]. These applications, mainly concentrate on its fascinating electrical
13
conductivity. However, PPy synthesized in solutions; exhibits anion exchanger or cation
14
exchanger behaviour depending on the type of dopant ions [30]. Besides these facts, PPy is also a
15
good prospect for adsorption applications due to positively charged nitrogen atoms in its polymer
16
chains. Thus, due to PPy’s excellent anion-exchange property and positively charged nitrogen
17
atoms in the polymer, it can be used as a potential adsorbent for the removal of anionic species
18
from aqueous solutions [31]. Generally, polymeric materials have low mechanical and thermal
19
stability due to wear and tear. Thus, the nanocomposite of PPy with stable inorganic materials is
20
expected to show higher stability due to synergism between the constituents [32]. Over the past
21
several years, there has been a generous enthusiasm toward the applications of titanium based
22
synthetic adsorbents in the field of water treatment to remove toxic heavy metal ions [33].
23
Titanium phosphates have been known as high performance ion-exchangers, and porous and 4
1
thermally stable materials [34]. Titanium phosphates are amphoteric in nature and can be utilized
2
as cation and anion exchangers [35]. The amphoteric and porous nature of titanium(IV) materials
3
relative to other transition metals makes them highly attractive for removal of high toxicity
4
pollutants [heavy metals like Cr(VI)] from aqueous solutions and other water bodies. Bhaumik et
5
al [35] showed that mesoporous titanium phosphate molecular sieves have high anion exchange
6
capacity. Owing to the beneficial properties of both PPy and TiP towards anion exchange, and
7
the presence of positively charged sites in PPy, the composite of PPy with TiP is expected to
8
prove as an effective material for the removal of Cr(VI) from aqueous solutions. In the present
9
study, polyprrole-titanium(IV)phosphate nanocomposite was prepared by in-situ oxidative
10
polymerization
method
and
the
adsorption
performance
of
the
polypyrrole-
11
titanium(IV)phosphate nanocomosite for the removal of Cr(VI) from aqueous solution was
12
evaluated. To the best of our knowledge, this is the first attempt to synthesize a new polypyrrole-
13
titanium(IV)phosphate nanocomposite for the removal of Cr(VI) from aqueous solutions.
14
2. Experimental
15
2.1. Chemical and reagents
16
Pyrrole monomer (98%) from was purchased from Spectrochem Chemicals (India, Pvt., Ltd.).
17
Anhydrous iron(III)-chloride (FeCl3), methanol, ortho-phosphoric acid (H3PO4), titanium dioxide
18
(TiO2), potassium dichromate (K2Cr2O7), ammonium sulphate and sodium hydroxide were
19
purchased from E. Merck, Chemical Company, Germany. All the chemicals and reagents were of
20
analytical grade and used as received.
21
2.2. Synthesis
22
2.2.1. Titanium(IV)phosphate
5
1
TiP
was
prepared
by
simple
precipitation
technique
from aqueous
solutions
of
2
titanium(IV)sulfate and H3PO4. Titanium(IV)sulphate solution was prepared as previously
3
described [33-34]. With vigorous stirring, aqueous solution of H3PO4 was added dropwise into
4
titanium(IV)sulfate solution. The mixture was stirred for 24 h at 25 ºC to form an absolute
5
precipitate. After 24 h, resulting precipitate was filtered off, washed with demineralized water
6
(DMW) and dried at 100 oC for 24 h. Finally, the material was ground by pastel mortar to yield a
7
fine TiP powder.
8
2.2.3. Polypyrrole-titanium(IV)phosphate nanocomposite
9
In-situ oxidative chemical polymerization [38-40] of pyrrole with FeCl3 as the oxidizing agent,
10
in the presence of TiP particles, was used in the preparation of the nanocomposite (PPy-TiP). A
11
definite amount of TiP was dispersed in 100 mL of DMW under ultrasonic vibrations at 25 oC
12
for 1 h. The dispersed TiP solution was then poured into a 500 mL round-bottom flask equipped
13
with a magnetic stirrer and a definite amount of pyrrole (monomer) was added. For proper
14
adsorption of pyrrole monomers on the TiP particles, vigorous stirring of the solutions for 4 h
15
was ensured. 2 g of FeCl3 was dissolved in demineralized water (100 mL) and poured into the
16
pyrrole adsorbed TiP dispersion. The reaction mixture was stirred for 24 h at 25 ºC. After 24 h,
17
synthesized PPy-TiP nanocomposite was filtered and washed with DMW and methanol to
18
remove unreacted oxidant. The obtained powder was dried completely at 70 ºC for further
19
analysis. Pure PPy was synthesized by a similar method as the preparation of PPy-TiP
20
nanocomposite without using the TiP nanoparticles. A graphical demonstration of the
21
preparation of PPy-TiP nanocomposite is indicated in Scheme. 1.
22
2.3. Characterization
6
1
Fourier transform infra-red (FTIR) spectra were recorded on Perkin Elmer 1725 FTIR
2
instrument. Field emission scanning electron microscopy (FE-SEM) was carried out by LEO
3
435-VF scanning electron microscope to study the surface morphology. Elemental analysis was
4
carried out by using energy dispersive analyser unit (EDAX) attached with FE-SEM.
5
Transmission electron microscopy (TEM) were achieved by JEOL TEM (JEM 2100F)
6
instrument. Specific surface area and pore-size distribution were measured by Brunauer–
7
Emmett–Teller (BET) nitrogen adsorption-desorption method using a Micromeritics Model
8
3Flex Surface Characterisation Analyzer (Atlanta, USA). Metal ion concentrations in solutions
9
were determined by using atomic absorption spectrometer (AAS) Model: GBC 902, Australia.
10
2.4. Adsorption studies
11
Adsorption studies were carried out by batch process. 0.2 g adsorbent was placed in a conical
12
flask. 20 mL solution of metal ions of desired concentration was added and the mixture was
13
shaken in incubator for 24 h. The mixture was filtered and then final concentration of metal ions
14
was determined in the filtrate by atomic absorption spectrometry. The amount of metal ions
15
adsorbed were calculated by subtracting the final concentration from initial concentration [41]
16
using the following relation: = − ×
1
17
where qe is the amount of metal ions adsorbed per unit weight of adsorbent (mg g-1). Co and Ce
18
are the initial and equilibrium concentrations of metal ion in solution. V is the volume (L) and M
19
is the mass of adsorbent (g).
20
The % adsorption was calculated as
7
% =
− × 100 2
1
2.5. Effect of pH
2
The effect of pH on the adsorption of Cr(VI) ions was analyzed using standard protocol [41].
3
50mL of Cr (VI) solution was poured into a series of conical flasks. The required pH of the
4
solution was regulated by adding 0.1N HCl or NaOH solution in each conical flask. The initial
5
concentration of Cr(VI) in this solution was then determined as follows. 20 mL of the adjusted
6
pH solution of Cr(VI) ions was taken in a series of conical flasks and treated with 0.2 g of
7
adsorbent for 24 h in shaker incubator. The mixture was then filtered and final concentration of
8
Cr(VI) in the filtrate was determined as described above.
9
2.6. Point of zero charge
10
The points of zero charge (pHpzc) of TiP and PPy-TiP nanocomposite were ascertained by the
11
well-known solid addition method [42]. 40 mL solution of 0.1 M KNO3 was transferred into
12
series of conical flasks. The variation in initial pH (pHi) of the solutions was maintained at about
13
1 to 10 by adding either acid or base solution of the known concentration (0.1 M HCl or 0.1 M
14
NaOH). The total volume of the solution in each flask was adjusted precisely to 50 mL by adding
15
KNO3 of the same strength and the initial pH (pHi) of the solutions was then observed accurately
16
using pH meter. Later 0.2 g of adsorbent was placed into each flask and allowed to equilibrate
17
for 24 h with occasional manual shaking and the final pH (pHf) of the supernatant liquid was
18
recorded. The difference between the initial pH (pHi) and final pH (pHf) values was calculated
19
according to equation (∆pH = pHi–pHf). A ∆pH plot was then constructed against pHi, and the
20
point of intersection of the resulting curve with abscissa, at which ∆pH = 0, gave the pHpzc. The
21
same procedure was applied for 0.05 M KNO3 solutions and DDW.
22
2.7. Effect of time 8
1
A series of 100 mL conical flasks, each having 0.2 g adsorbent and 20 mL solution of known
2
Cr(VI) concentration (200, 400, 600, 800 and 1000 mg L−1; pH = 7) were placed in a shaker
3
incubator and at the predetermined intervals, the solutions of the specified flasks was taken out
4
and filtered. The concentration of Cr(VI) in the filtrate was determined by AAS and the amount
5
of Cr(VI) adsorbed in each case was then determined as described above.
6
2.8. Breakthrough capacity
7
0.2 g of adsorbent was loaded into a glass column of 0.6 cm internal diameter with glass wool
8
support at the end. 1 L of Cr(VI) solution containing 200 mg L−1 (pH = 7) initial concentration
9
(Co) was prepared and then passed through the column. The flow rate was adjusted to 1 mL
10
min−1. The effluent was collected in 10 mL fractions in 0 to 100 mL effluent range, and after 100
11
mL range, the effluent was collected in 50 mL fractions in 100-350 mL range. The amount of
12
Cr(VI) was determined in each fraction (C) with the help of AAS. The breakthrough curve was
13
obtained by plotting C/Co based on the volume of the effluent.
14
2.9. Desorption of Cr(VI) by batch process
15
The study of desorption of chromium(VI) was carried out by using a batch process. 0.2 g
16
adsorbent was treated with 50 mL of Cr(VI) solution (300 mg L−1). The adsorbent was washed
17
thoroughly with distilled water to remove excess of Cr(VI) ions. The exhausted adsorbent was
18
then treated with 20 mL 0.5 M NaOH solution and after 24 h; the amount of Cr(VI) desorbed
19
was determined. The experiment was repeated several times to ensure reproducibility.
20
3. Results and discussion
21
3.1. FTIR spectroscopic studies
22
The FT-IR spectra of PPy, PPy-TiP and Cr(VI) adsorbed PPy-TiP nanocomposite are shown in
23
Fig. 1. The FT-IR spectra of PPy shows absorption peak at 902 cm−1 in its finger print region,
9
1
which is characteristic of C–H out-of-plane deformation vibration, confirming the formation of
2
PPy by the monomer. The band at 1300 cm−1 is attributed to the C–N in-plane vibration,
3
and the bands at 1130 to 1190 cm−1 are related to the C–N stretch modes. The peak around
4
1310 cm-1 is related to C–N in-plane stretching vibration and the strong absorption bands at
5
1539 cm−1 and 1449 cm−1 correspond to the C–C stretching vibration of the pyrrole ring. The
6
PPy-TiP nanocomposite showed similar peaks as in PPy except for an intense peak at 1190 cm-1
7
overlapping with the C-N peak (1130 – 1190 cm-1), which may be attributed to the -OH group of
8
phosphate. After binding of Cr(VI) to the PPy-TiP nanocomposite (Fig. S1), the peak at 1190
9
cm-1 further intensified indicating the interaction of Cr(VI) with phosphate of TiP and C-N
10
groups of polymer backbone (PPy).
11
3.2. FE-SEM and TEM studies
12
The FE-SEM images of PPy, TiP and PPy-TiP nanocomposite before adsorption and PPy-TiP
13
nanocomposite after adsorption are shown in Fig. 2 (a-d) respectively. Fig. 2a and 2b shows the
14
granular morphology of PPy and TiP, however, the morphology of the composite (Fig. 2c) is
15
quite different from that of the parental components. PPy deposits are clearly visible at the
16
surface of TiP, which has porous and globular structure; providing a good platform for the
17
adsorption of Cr(VI). After adsorption of Cr(VI) on PPy-TiP nanocomposite (Fig. 2d), the
18
surface morphology of PPy-TiP nanocomposite has been changed significantly, showing that
19
Cr(VI) has been adsorbed on the surface of PPy-TiP nanocomposite. Fig. 3 shows the TEM
20
micrographs of PPy-TiP nanocomposite with granular morphology having an average particle
21
size of ~35-45 nm. The TiP nanoparticles can be seen as dark spots encapsulated in PPy matrix
22
having an average particle size of ~20-25 nm.
10
1
3.3. Energy dispersive x-ray analysis (EDAX)
2
To confirm the adsorption of Cr(VI) on PPy-TiP nanocomposite, EDAX analyses were
3
performed before and after adsorption of Cr(VI). The percentage compositions of PPy-TiP
4
nanocomposite before and after adsorption of Cr(VI) are presented in Table 1. The percentage
5
composition of chromium after adsorption confirmed that Cr(VI) has been successfully adsorbed
6
on PPy-TiP nanocomposite.
7
3.4. Surface area and pore size analysis
8
The surface area was determined by the Brunauer, Emmett and Teller (BET) method while pore
9
size distribution measured from N2 adsorption-desorption isotherm at 77 K by the Barret, Joyner
10
and Halenda (BJH) shown in Fig. 5. Specific surface area and pore size distribution parameters
11
are listed in Table S1. From Fig. 4 and Table S1, PPy-TiP nanocomposite showed BET specific
12
surface area of 59.3956 m²/g. The BJH adsorption/desorption average pore diameters were
13
calculated and found to be 160.932 and 139.512 Å, while the adsorption/desorption average pore
14
diameters associated with BET were found to be 103.0875 and 112.6687 Å, respectively. On the
15
basis of the obtained values (Table S1), the sample; PPy-TiP nanocomposite is consistent with
16
the pore diameter classification of International Union of Pure and Applied Chemistry (IUPAC).
17
According to IUPAC notation, microporous materials have pore diameters of less than 2 nm and
18
macroporous materials have pore diameters of greater than 50 nm; the mesoporous category thus,
19
lies in the middle.
20
3.5. Effect of contact time and initial concentration
21
The adsorbent showed excellent capacity towards Cr(VI) ions. The adsorption of Cr(VI) at
22
different initial Cr(VI) concentrations (200-1000 mg L-1) as a function of contact time is shown 11
1
in Fig. 5. The adsorption capacity increased with time and reached a constant value
2
(equilibrium). The contact time needed for the initial Cr(VI) concentrations (200-600 mg L-1) to
3
reach equilibrium was 120 to 240 min. While for 800-1000 mg L-1 initial Cr(VI) concentration,
4
equilibrium time of 300 min was required.
5
3.6. Effect of pH and surface charge measurement
6
Solution pH stands out as one of the crucial parameters affecting the adsorption of metal ions. The
7
effect of pH of the solution on the adsorption of Cr(VI) on TiP and PPy-TiP nanocomposite was
8
studied in the pH range 1-10 with 200 mg L-1 initial Cr(VI) concentration. It can be seen from
9
Fig. 6, that the adsorption of Cr(VI) on both TiP and PPy-TiP is maximum at acidic pH (pH = 2.0),
10
with adsorption percentage close to 99% with PPy-TiP and almost 48% with TiP. Further, the
11
adsorption capacity of TiP decreases on pH values greater than 2; and for PPy-TiP nanocomposite,
12
it almost remained the same.
13
The various species of Cr(VI) existing in aqueous solution at different pH values are
14
H2CrO4 (pH<1); HCrO4- and Cr2O7-2 (pH 2-6) and CrO4-2 (pH >6) [43]. The increased adsorption
15
potential of the adsorbents at pH 2 may be assumed due to strong electrostatic attractions
16
between adsorbent surfaces and adsorbate species. At pH 2, the presence of high concentrations
17
of H+ ions within the aqueous medium makes the surface of the adsorbents (both TiP and PPy-
18
TiP) positively charged, which enhances the adsorption of the chromate anions (HCrO4- and
19
Cr2O7-2) due to strong electrostatic forces of attraction [44-45]. Increase in pH of the solution
20
above pH 2 decreases the positive charge of the TiP surface due to decreased protonation, which
21
consequently decreases the adsorption of the chromate anions at pH values greater than 2. At pH
22
values lower than 2 the decreased adsorption on both the adsorbents is due to the competition
23
between the protons and the active chromium species to bind with the adsorbents [46]. Increase 12
1
in pH of the solution above pH 2 did not affect the adsorption of chromate ions in case of PPy-
2
TiP composite, which may be attributed to the structural modification of PPy due to the
3
incorporation of TiP. The surface of the PPy contains several –NH functionalities, which might
4
assist the ion-dipole fashioned binding of the dissolved metal ions from the solution irrespective
5
of the increasing pH.
6
The pHpzc of an adsorbent is a measure of the pH at which its surface has net electrical
7
neutrality. An adsorbant surface is negatively charged at pH > pHpzc, neutral at pH = pHpzc and
8
has a net positive charge at pH < pHpzc [47]. It can be seen from the Fig. 7 (a) and (b) that both
9
TiP and PPy-TiP have positive surfaces at pH 2 and below it. With increase in pH, the surface
10
loses positive charge and becomes negative. Thus, the nature of chromium species present in the
11
solution and the surface charges of the adsorbents at different pH values fully support the
12
maximum adsorption at pH 2.
13
3.7. Adsorption Isotherms
14
The Langmuir and Freundlich isotherm models was used to analyze the adsorption mechanism of
15
PPy-TiP with Cr(VI). This explained the chemical interaction between homogeneous and
16
heterogeneous adsorbent surface, respectively as evident from the following equations [41]: 1 1 1 1 = × + 3 × 1 log = log#$ + log 4 n
17
The details of the equation parameters can be seen in the supplementary section under the
18
heading adsorption Isotherm. The values of b and qm were calculated from the straight line plot
19
drawn between 1/qe and 1/Ce (Fig. 8a). The Langmuir parameters reported in Table 2 indicates 13
1
high values of correlation coefficient (R2= 0.9934) at normal temperature (30 oC) indicating that
2
Cr(VI) concentration ranges from 300-700 mg g-1. The exceptionally high adsorption capacity
3
(qm = 31.64 mg g-1) in comparison to other reported adsorbents (Table S2) [29, 49-52] indicated
4
the efficacy of PPy-TiP nanocomposite towards the removal of Cr(VI) from aqueous solution.
5
The separation factor (RL) in Langmuir isotherm can be calculated from the well-known relation
6
[53]. The value of RL indicates the type of isotherm; if RL>1, the isotherm is unfavourable, if RL
7
= 1 its linear; favorable if 0
8
range of 0-1 in all the experimental concentrations of Cr(VI) indicated that the adsorption is
9
favourable in nature (Table 2). The adsorption isotherm analysis (Fig. 8a , 8b) indicates that the
10
correlation coefficient of Langmuir model is higher than that of Freundlich model, hence it can
11
be concluded that Langmuir model is the best fit model for the adsorption of Cr(VI) onto PPy-
12
TiP nanocomposite.
13
3.8. Dubinin–Radushkevich (D-R) isotherm
14
D-R isotherm associated with the heterogeneity of the surface energies was also used to evaluate
15
the chemical or physical nature of the adsorption and the mean free energy of adsorption as
16
described by the following equations 5, 6 and 7 [54]. ' = ' – )* + 5 * = -.' 1 +
17
0=
1
23+4
1 6 (7)
14
1
The details of the equation parameters can be seen in the supplementary section under the
2
heading Dubinin–Radushkevich (D-R) isotherm. The obtained results are given in respective
3
Table 2 and Fig. S3. The linear plot of lnqe versus ɛ2 (Fig. S3) with high correlation coefficient
4
(R2 = 0.9963) showed that D-R isotherm best fitted for this system. The magnitude of mean free
5
energy E<8 kJ mol-1 represents physical nature of the adsorption while E in between 8-16 kJ mol-
6
1
7
exchange. The value of E obtained in our case was 15.800 kJ mol-1 which confirmed that
8
adsorption of Cr(VI) proceeded by ion-exchange process.
9
3.9. Adsorption kinetics
indicates that adsorption process is chemical in nature and most probably proceeds via ion-
10
Lagergren pseudo-first order model [55] and pseudo-second order model [56] were used to
11
explain the kinetics of Cr(IV) adsorption, as described by the following equations 8 and 9: log − 5 = log −
#1 × t 8 2.303
1 1 = + × 9 + 5 #+ . 12
The details of the equation parameters can be seen in the supplementary section under the
13
heading adsorption kinetics. The obtained results are given in respective Table 3 and Fig. 9. It
14
can be inferred from the data that although values of R2 are reasonably high but adsorption
15
capacity determined experimentally (qe(exp)) differed appreciably when compared to the
16
adsorption capacity calculated from the model (qe(cal)). This indicated that adsorption process
17
does not follow pseudo-first-order adsorption kinetics model. From Table 3 and Fig. 9, the
18
values of R2 at all experimental concentrations were very close to 1 and qe(cal) values calculated
19
from the model were also found to be very close to experimental values qe(exp) confirming the
20
applicability of pseudo-second-order equation. The pseudo-second-order model proposed by Ho
15
1
and Mekay [54] is based on the assumption that adsorption follows chemisorption, therefore it
2
can be concluded that adsorption of Cr(VI) was chemical in nature and most probably proceeded
3
via ion-exchange.
4
3.10. Thermodynamic studies
5
The effect of temperature on the adsorption of Cr(VI) was studied at temperature ranging from
6
30, 40 and 50 ºC. Thermodynamic parameters such as standard free energy change (∆Gº),
7
standard enthalpy change (∆Hº) and standard entropy change (∆Sº) were calculated using the
8
following equations: [57] ΔG = −-.' #< 10
9 10
where R is universal gas constant (8.314 mol-1 K-1), T is temperature in kelvin and Kc is the equilibrium constant. The value of Kc can be calculated as: #< =
=> 11
11
where, Kc is the adsorption equilibrium constant, CAC and Ce are equilibrium concentrations of
12
Cr(VI) on the adsorbent (mg g-1) and in the solution (mg L-1), respectively.
13
The standard enthalpy (∆Hº) and entropy (∆Sº) were determined by using the following
14
equation: ΔS ΔH B 12 '#< = ? − -.
15
By plotting a graph, lnkc versus 1/T (Fig. 8c); the value of ∆Hº and entropy ∆Sº can be estimated
16
from the slope and intercept [58]. Table 4 shows the negative values of ∆Gº and positive ∆Hº
17
indicating that the Cr(VI) adsorption process is spontaneous and endothermic [59]. The ∆Gº
18
values decreased with increase in temperature, which suggested an increased spontaneity with
19
temperature. The positive value of ∆Sº suggested an increased randomness at the solid-solute
16
1
interface during adsorption. The Cr(VI) ions replace some water molecules previously adsorbed
2
on the surface of the adsorbent and these displaced water molecules gain more randomness than
3
lost by the adsorbed Cr(VI) ions [60].
4
3.11. Break through capacity
5
The break through curve (Fig. 10) indicated that 30 mL solution containing 200 mg L-1 initial
6
Cr(VI) concentration can be passed through the column containing 0.2 g of adsorbent without
7
detecting Cr(VI) in the effluent. The break through and exhaustive capacities were found to be
8
30 and 175 mg g-1 respectively.
9
3.12. Desorption Studies
10
In order to make the process more economical it is significant to desorb the adsorbed metal ions
11
for the reuse of the adsorbent. Batch adsorption studies were carried out using 0.5 M NaOH
12
solution as the desorbing agent. Fig. 11a showed that adsorption of Cr(VI) was 82 % at 300 mg
13
L-1 initial Cr(VI) concentration and desorption was 80 %. But, above 300 mg L-1 initial Cr(VI)
14
concentration of Cr(VI),
15
indicated that adsorption was maximum (88 %) at 0.2 g adsorbent dose and 100 % desorption
16
was achieved with 0.5 M NaOH. Therefore it can be concluded that 0.2 g of adsorbent dose is
17
sufficient for the optimum adsorption and desorption of Cr(VI) ions from a solution containing
18
300 mg L-1 initial Cr(VI) concentration.
19
The PPy-TiP nanocomposite conveys dynamic destinations (active sites), which gives a decent
20
plat form or adsorption locales for adsorption of Cr(VI). A schematic illustration of adsorbtion
21
and desorption of Cr(VI) onto PPy-TiP nanocomposite is reprensted in Sheme 2. Basically, the
22
TiP serves as a template for composite formation via in situ chemical oxidation polymerization
23
process. PPy chains adhere onto TiP surfaces and form thermally stable nanocomposite PPy-TiP
the % adsorption and desorption decreased. Similarly Fig. 11b
17
1
as shown in Scheme 1. The adsorption of Cr(VI) ions from the solution is governed by the
2
solution pH, electrical neutrality of the adsorbent surfaces, and the speciation of chromium ions
3
at different pH values. The PPy coating greatly enhances the adsorption capacity of TiP, which
4
may be partly by providing more surface area with different chemical functionalities for the
5
interaction with different species of chromium ions.
6 7
4. Conclusion
8
In the present work, the PPy-TiP nanocomposite showing significant potential for the removal of
9
Cr(VI) from aqueous solution; was successfully synthesized by simple in-situ chemical oxidative
10
polymerization method. The results of FTIR and EDAX studies reveal that Cr(IV) has been
11
successfully absorbed on PPy-TiP nanocomposite. The ideal conditions for adsorption were
12
found to be: contact time of 19 min, Cr(VI) initial concentration of 200 mg L-1 and an adsorbent
13
dose of 0.2 g. The results shows that Langmuir and D-R isotherm models are well fitted in all the
14
experimental conditions. Thermodynamic parameters indicated spontaneous and endothermic
15
nature of the adsorption. The mechanism for the removal of Cr(VI) by PPy-TiP nanocomposite
16
indicated that adsorption process is chemical in nature and most probably proceeds via ion-
17
exchange. The economic viability of the material has been demonstrated by desorbing the
18
adsorbent with 0.5 M NaOH solution and 100% desorption was achieved using 0.2 g adsorbent
19
dose with 300 mg L-1 initial Cr(VI) concentration. These studies suggested that PPy-TiP
20
nanocomposite can be used as potential adsorbent for the removal of Cr(VI) from aqueous
21
solutions.
22
Supporting Information
18
1
Supporting information includes FTIR spectra of PPy-TiP nanocomposite before adsorption and
2
PPy-TiP nanocomposite after adsorption of Cr(VI), D-R isotherm model, percent adsorption of
3
Cr(VI) onto PPy-TiP nanocomposite with different concentrations, surface area, pore volume
4
and pore size analysis details of PPy-TiP nanocomposite and Comparison of adsorption capacity
5
of the PPy-TiP nanocomposite with other adsorbents for Cr(VI) removal at 25 oC.
6 7 8
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25
Table 1. Percent composition of carbon, nitrogen, oxygen, titanium, phosphorus and chromium in PPy-TiP nanocomposite by EDAX analysis. Element C N O Ti P Cr
Weight (%) PPy
PPy-TiP (Before Adsorption)
PPy-TiP (After Adsorption)
64.73 19.04 16.23 Nil Nil Nil
44.36 10.84 34.16 4.90 5.75 Nil
47.83 11.41 33.72 2.20 2.36 2.47
Table 2. Langmuir, Freundlish and D-R isotherm parameters for Cr(VI) adsorption on to PPyTiP nanocomposite. Langmuir, Freundlish and D-R isotherm parameters Langmuir constants qm (mg/g) 31.64 Kf (mg/g) 13.316
R2 0.993
b (L/mg) RL 0.048 0.094 Freundlich constants R2 0.989
1/n 0.1482 D-R constants
qm (mg/g) 42.7
β -2x10-9
E 15.800
R2 0.996
Table 3. Pseudo-first order and pseudo-second order rate parameters for the adsorption of Cr(VI) at different concentrations. Concentrations
Pseudo-first order
Pseudo-second order
(mg L-1)
qe (exp)
qe (cal)
K1
R2
qe (cal)
K2
h
R2
200
19.20
19.742
0.0070
0.9133
19.34
0.0081
3.041
0.9996
400
27.6
22.014
0.0063
0.9602
25.45
0.0058
3.618
0.9995
600
29.8
29.518
0.0035
0.8915
30.48
0.0034
3.234
0.9986
800
40.4
44.463
0.0060
0.9526
35.65
0.0024
3.056
0.9965
1000
50.2
54.460
0.0081
0.8915
49.75
0.0016
3.987
0.9943
Table 4.Thermodynamic parameters at different temperatures for the adsorption of Cr(VI) on polypyrrole-titanium(IV)phosphate nanocomposite. Temperature (oC)
ln Kc
ΔGo (kJ mol-1)
30
0.9627
-2.425
40
1.098
-2.859
50
1.166
-3.136
ΔHo (kJ mol-1) ΔSo (kJ mol-1 K-1)
18.619
0.080
R2
0.9742
Fig. 1. FTIR spectra of (a) TiP, (b) PPy and (c) PPy-TiP nanocomposite.
(a)
(b)
(c)
(d)
Fig. 2. FE-SEM images of (a) PPy (b) TiP, (c) PPy-TiP nanocomposite before adsorption and (d) PPy-TiP nanocomposite after adsorption of Cr(VI).
(a)
(b)
Fig. 3. TEM micrograph of (a) PPy-TiP nanocomposite without particle size measurement and (b) PPy-TiP nanocomposite with particle size measurement.
0.0006
dV/dD (cm3 g-1 Å̊)
0.0005
0.0004
0.0003
0.0002
0.0001
0 0
200
400
600
800
1000
1200
Pore diameter (Å̊)
(c)
0.18
Pore Volume (cm3 g-1)
0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 0
200
400
600
800
1000
Pore diameter (Å)
Fig. 4. (a) N2 adsorption-desorption isotherms of PPy-TiP nanocomposite, (b) pore size distribution and (c) pore volume curves of PPy-TiP nanocpmosite.
60
200 ppm 400 ppm 600 ppm 800 ppm 1000 ppm
50
qt (mg/g)
40
30
20
10
0 0
100
200
300
400
500
Time (min)
Fig. 5. The effect of contact time on the Cr(VI) adsorption onto PPy-TiP nanocomposite.
Fig. 6. Effect of pH on the removal of Cr(VI) by TiP and PPy-TiP nanocomposite.
(a)
(b)
Fig. 7. Point of zero charge curves for (a) TiP and (b) PPy-TiP nanocomposite.
Fig. 8. (a) Fit of equilibrium data to Langmuir isotherm model (b) Fit of equilibrium data to Freundlich isotherm model and (c) plot of lnKc versus 1/T for Cr(VI) adsorption onto PPy-TiP nanocomposite.
Fig. 9. Pseudo-second-order kinetic model for adsorption of Cr(VI) by the PPy-TiP nanocomposite.
Fig. 10. Breakthrough curve of obtained for removal of Cr(VI) by PPy-TiP phosphate nanocomposite.
Fig. 11. Adsorption and desorption capacities of PPy-TiP nanocomposite (a) with varying concentration of Cr(VI) and (b) with loading of PPy-TiP nanocomposite for removal of Cr(VI).
(a)
FeCl3 H2 O
Pyrrole
Polypyrrole
(b)
Scheme 1. Schematic diagram of the formation mechanism of (a) PPy and (b) PPy-TiP nanocomposite.
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr Cr
Cr Cr
Cr
Cr
Scheme 2. The mechanistic representation of adsorption and desorption of Cr(VI) on PPy-TiP nanocomposite.
1 2
Research Highlights:
3
Synthetic nano-adsorbent were synthesized for the removal of Cr(VI).
4
Equilibrium isotherm data were analyzed using Langmuir and Freundlich isotherms.
5
The pseudo-second-order model described adequately the adsorption kinetics data.
6
The adsorption and desorption studies suggested 100% desorption of Cr(VI) ions.
7 8
26
1 2 3 4
Graphical Abstract
5
Cr
Cr
Cr Cr Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
6 7 8
27