High-performance towards removal of toxic hexavalent chromium from aqueous solution using graphene oxide-alpha cyclodextrin-polypyrrole nanocomposites

Journal of Molecular Liquids 211 (2015) 71–77

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

High-performance towards removal of toxic hexavalent chromium from aqueous solution using graphene oxide-alpha cyclodextrin-polypyrrole nanocomposites Vongani P. Chauke a, Arjun Maity b,c,⁎, Avashnee Chetty a a b c

Polymers and Composites, Council for Scientific and Industrial Research (CSIR), Pretoria, South Africa DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria, South Africa Department of Applied Chemistry, University of Johannesburg, South Africa

a r t i c l e

i n f o

Article history: Received 31 March 2015 Received in revised form 8 June 2015 Accepted 11 June 2015 Available online xxxx Keywords: Nanocomposite Polypyrrole Cyclodextrin Graphene oxide Adsorption Cr(VI) removal

a b s t r a c t Graphene oxide (GO) was functionalized with alpha cyclodextrin (αCD) through a covalent bond to form GO-αCD nanocomposites (NC). GO-αCD NC was further modified with polypyrrole (PPY) to afford an advanced GO-αCD-PPY NC for the removal of highly toxic Cr(VI) from water. The prepared GO-αCD-PPY NCs were successfully characterised with AT-FTIR, FE-SEM, HR-TEM, BET and XRD techniques. Adsorption experiments were performed in batch mode to determine optimum conditions that include temperature, pH, concentration of Cr(VI) and contact time. It was deduced from the experiments that the adsorption of Cr(VI) by the GO-αCD-PPY NC is pH and temperature dependent, where optimum adsorption was achieved at pH 2 and it increased with increasing temperature. The adsorption kinetics followed the pseudo-second-order model and the adsorption isotherms fitted well to the Langmuir isotherm model with maximum adsorption capacities ranging from 606.06 to 666.67 mg/g. Effect of co-existing ions studies revealed that cations and anions had no significant effect on the adsorption of Cr(VI). Desorption studies also illustrated that the NC can be re-used up to 3 cycles. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Water resources have been subjected to severe exploitation and damage over several decades by human activities such as mining, manufacturing, chemical and goods production, electricity production, agriculture, and improper sanitation [1]. The main sources of pollution are effluents and solid discharge from industries which include heavy metals from smelting and mining, and nonpoint sources such as soluble salts (natural and artificial), use of insecticides/pesticides, disposal of industrial and municipal wastes, and excessive use of fertilizers [1]. Of particular concern are heavy metals which are becoming one of the most serious environmental problems. Heavy metals are commonly used in leather tanning, metal, nuclear power plants, and metal salt production [2]. Unlike organic contaminants, heavy metals are not biodegradable and have a propensity to accumulate in living organisms [3]. The consequences of heavy metals ingestion and exposure are adverse; they include damage to the central nervous system, skin dermatitis, to mention but few [4]. Several methods including chemical precipitation [5], ion-exchange, membrane filtration [6], and electrochemical methods have been

⁎ Corresponding author at: DST/CSIR National Centre for Nanostructured Materials, Council for Scientific and Industrial Research, Pretoria, South Africa. E-mail addresses: [email protected], [email protected] (A. Maity).

http://dx.doi.org/10.1016/j.molliq.2015.06.044 0167-7322/© 2015 Elsevier B.V. All rights reserved.

proposed, and are currently used [7] for heavy metals. However, these methods have several drawbacks such as fouling of membranes by metal hydroxides and carbonates, low effectiveness in low concentrations of heavy metals, and high costs [7]. Adsorption is renowned as a highly effective wastewater treatment technology, in terms of cost and removal efficiency. It is simple, offers flexibility in design and operation and produces high-quality treated effluent. Moreover, because adsorption is reversible, adsorbents can be re-generated by appropriate desorption processes [7]. In adsorption, choosing and designing smart adsorbents is critical and necessary. Some of the traditional adsorbents that have been used for the removal of heavy metals and Cr(VI) in particular include activated carbon [8], bio adsorbents and low cost adsorbents such as fly ash, clay, and lignin by-products [7]. The drawbacks associated with some of these adsorbents are relatively low adsorption capacities, weak mechanical strength, separation after treatment and low surface areas [9]. Graphene is a monomer source of some carbon allotropes such as fullerenes and carbon nanotubes [10]. Owing to its extraordinary properties that include mechanical strength and flexibility [10], graphene has become the wunderkind in material science and technology since its discovery in 2004 [10]. Graphene, however, has a major drawback of aggregation due to the strong van der Waals interactions and π–π stacking of the graphene sheets [10]. This reduces dispersibility in water and therefore limits applications. Aggregation can be minimized or eliminated by introducing functional groups on the graphene sheets

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through chemical oxidation to afford graphene oxide (GO) [10]. In recent years, there has been growing research interest regarding modification of single graphene/GO sheets with various host molecules such as cylodextrin, metal nanoparticles and polymers to form nanocomposites which show increased surface area and superior properties, compared to the native graphite material. The superior properties are caused by the synergistic effect between the host molecule and graphene nanosheets. Several heavy metals such as arsenate [11], Zn2 +, Cd2 + and Pb2 + [12] have been removed by graphene based adsorbents. Cyclodextrins (CDs), which are significant hosts in supramolecular chemistry [13], are cyclic oligosaccharides composed of six, seven or eight glucose units, which are conical in shape with a hydrophobic inner cavity and a hydrophilic exterior. CDs have the ability to specifically bind to various organic and inorganic pollutants to form inclusion complexes. For this reason, attaching αCDs and polypyrrole (PPY) to GO to afford GO-αCD-PPY nanocomposites (GO-αCD-PPY NC) would be ideal for the removal of heavy metals, in particular, Cr(VI). The GOαCD-PPY NC would also possess increased surface area, good mechanical properties, strong acid resistance and increased adsorption capability. Fan and co-workers [9] demonstrated the fast and efficient removal of Cr(VI) using magnetic αCD-graphene oxide nanocomposites (MCGN), where their adsorption capacity was more than 120 mg/g compared to nanomaterials [14]. The aim of this work is to synthesise GO-αCD NC for the removal of Cr(VI). This nanocomposite will further be modified with PPY to form GO-αCD-PPY NC. Polypyrrole has remarkable properties which are relevant in the removal of heavy metals, which include high chemical stability, ion exchange ability, ease of preparation, and low cost [15]. To the best of our knowledge, there are no reports on the removal of Cr(VI) using GO-αCD-PPY NC, thus we report the synthesis, characterisation and application of the GO-αCD-PPY NC in the removal of Cr(VI) at optimized conditions.

Gilcreas and co-workers [18] was used. Cr(VI) removal efficiency was determined using Eq. (1).  %Cr ðVIÞ removal ¼

C o −C e Co

  100

ð1Þ

where Co and Ce are initial and equilibrium concentration (mg/L) of Cr(VI), respectively. In order to determine the optimum pH for Cr(VI) removal, pH studies were carried out from 2 to 11. The pH of the Cr(VI) solutions was adjusted by using HCl (1.0 M) and NaOH (1.0 M) solutions. Batch adsorption isotherm experiments were carried out at different temperatures, whereby isotherm results were generated by using a mass of 0.025 g of NC in 50 mL Cr(VI) solutions with concentrations ranging between 100 and 700 mg/L at pH 2.0 for all temperatures. The kinetics studies were conducted in a beaker, where a fixed mass of the adsorbent was used at different concentrations of Cr(VI) that was stirred at 200 rpm and operated at ~25 °C using 0.5 g/L dose. The equilibrium sorption capacity and time-dependent capacity were determined using Eqs. (2) and (3): qe ¼



 C o −C e V m

ð2Þ

qt ¼

  C o −C t V m

ð3Þ

where qe and qt are the equilibrium amount and the time-dependent amount of Cr(VI) adsorbed per unit mass (m) of adsorbent (mg/g), respectively, and Ct (mg/L) is the bulk-phase Cr(VI) concentration (mg/L) at any time 't' and V is the sample volume (L). 2.3. Characterisation of the adsorbent

2. Experimental 2.1. Synthesis of GO-αCD and GO-αCD-PPY NC Graphene oxide (GO) was synthesised using a modified Hummers method [16] through oxidation of graphite flakes (GFs) (Sigma Aldrich, South Africa). The synthesis of GO-αCD was carried as reported in literature [9] with some changes. GO (0.02 g) was dispersed in water through sonication for ~45 min and αCD (0.2 g) (Sigma Aldrich, South Africa) was added to the GO dispersion. The pH of the solution was adjusted between 9 and 10 using different concentrations of NaOH (0.1–7 M) (Sigma Aldrich, South Africa) to find the optimum synthesis conditions, where 5 M gave the highest yield of 1.23 g. To obtain the GO-αCD, the suspension was stirred overnight, centrifuged and dried at 60 °C. The GO-αCDPPY NC were prepared via in situ polymerization of pyrrole (PY) monomer in the presence of FeCl3 (Sigma Aldrich). GO-αCD NC (0.2 g) was sonicated thoroughly and stirred for 24 h. PY monomer (0.8 mL) (Sigma Aldrich, South Africa) was syringed into the reaction flask and stirred for 1 h. FeCl3 (6 g) was then added, and the reaction was carried out for 3 h. After this, acetone was added to stop the polymerization reaction, followed by filtration of GO-αCD-PPY NCs. The NCs were washed with distilled water until the filtrate became colourless and finally the NCs were washed with acetone to remove oligomers [17]. 2.2. Batch adsorption experiment Adsorption studies were carried out in batch mode. Cr(VI) solutions were prepared by diluting the stock solution of 1000 mg/L. These were then placed in a thermostatic shaker (at a speed of 200 rpm) at different temperatures for 24 h. To determine the concentration of the remaining Cr(VI) after adsorption, standard UV–vis spectroscopic method by

FTIR analyses were performed on a Perkin-Elmer Spectrum 100 spectrometer, equipped with an FTIR microscopy accessory and a diamond crystal. The surface morphology of the samples was investigated by an Auriga Field Emission Scanning Electron Microscope (FE-SEM; Carl Zeiss, Germany) and a JEOL JEM-2100 High Resolution Transmission Electron Microscope (HR-TEM; JEOL, Japan). A JEOL JEM-2100 HR-TEM instrument with a LaB6 filament operated at 200 kV was used to obtain TEM images. For the X-ray measurements, a Pan Analytical instrument and X'pert data Collector software was used. The surface area of the nanocomposites was determined by a Micromeritics Tristar II 3020 Surface Area and Porosity BET analyser using nitrogen adsorption. 3. Results and discussion 3.1. Synthesis of GO-αCD and GO-αCD-PPY nanocomposites The FTIR spectra of αCD(a), GO(b), GO-αCD(c), and GO-αCD-PPY NC(d) are presented in Fig. 1. For αCD and GO-αCD, the FTIR spectra show a broad peak at ~ 3375 cm− 1 which is typically assigned to the O–H groups present in αCD and GO sheets. The absorption band at ~1720 cm−1 present in pure GO, due to the carboxyl group which is disappeared after the conjugation of GO with αCD (Fig. 1(c)). This is indicative of the formation of GO-αCD via nucleophile reaction. The somewhat broad vibration peak at 1179 cm−1 in pure GO assigned to the epoxy groups which is also disappeared during the formation of GO-αCD NC. These features indicated that the cyclodextrin molecules could be attached to the GO sheets by nucleophilic reaction between deprotonated hydroxyl groups on the secondary face of the αCD in alkaline solution and the epoxy groups/carboxylic groups on GO [11,19]. Upon polymerizing PY in between the GO-αCD sheets, Fig. 1(d), the O–H, C_O and C–O absorption bands disappeared, whereas, peaks at

V.P. Chauke et al. / Journal of Molecular Liquids 211 (2015) 71–77

features is observed for the GO-αCD composite and the sharp edged GO sheets are covered (Fig. 3(b)). Moreover, the incorporated PPY into and on top of the GO nanosheets, Fig. 3(c and d), appears as dense inconsistent irregular like material.

(c)

(a)

3.2. Batch adsorption studies

Transmittance / a.u.

(b)

(d)

4500

4000

3500

3000

2500

2000

1500

1000

500

Wavenumber / cm-1 Fig. 1. FTIR spectra of (a) αCD, (b) GO, (c) GO-αCD, and (d) GO-αCD-PPY NC.

1521, 1468, 1069 and 1004–865 cm− 1 appeared and were assigned to the PY ring stretching, conjugated C–N stretching, C–H stretching and C–H deformation, respectively, thereby signifying the formation of PPY in the NC [19]. The XRD patterns of GO(a) and GO-αCD-PPY NC(b) are shown in Fig. 2. The diffraction peak at 2θ = 11.25° (Fig. 2(a)), is a typical peak for graphene oxide nanosheets [9] and is attributed to the (002) plane. The reduction in intensity of the diffraction peak at 11.25° is observed upon modifying GO with αCD and PPY, Fig. 2(b), which signifies the coating of the GO-αCD nanosheets with PPY. A broad peak is observed at 2θ = 25° which is due to the scattering of PPY chains at interplanar spacing [20]. A shift was observed (Fig. 2(b)) to 2θ = 27.22° after polymerizing PPY into the GO-αCD nanosheets, thus demonstrating the success of PPY polymerization in between the GO-αCD nanosheets. The surface area of GO was not determined due to the physical crisp and rigid nature of GO. The surface area of GO-αCD-PPY NC was 30.639 m2/g. The surface morphologies of the GO, GO-αCD, GO-αCD-PPY NCs were characterised by HR-TEM and FE-SEM as shown in Fig. 3. The HR-TEM image of GO nanosheets (Fig. 3(a)) shows clearly defined and exfoliated structure. On introducing the αCD, a change in morphological

8000 7000

Intensity/cps

6000 5000 4000

2000 1000 0 20

3.2.2. Adsorption isotherms Adsorption isotherms play an important role in predicting the ability of an adsorbent to remove a pollutant down to a specific charge value [22]. The isotherms performed at pH 2, and temperatures of 25, 35, and 45 °C for the removal of Cr(VI) using GO-αCD-PPY NC are presented in Fig. 5. The adsorption capacities increased with increase in temperature which indicates that the adsorption process is endothermic [23]. The increase in adsorption capacity may be due to an increase in thermal energy of the adsorbing species as the temperature was increased [23]. The Langmuir model assumes that the uptake of metal ions occurs on a homogeneous surface by monolayer adsorption without any interaction between adsorbed ions [24]. The linearized form of Langmuir isotherm model can be expressed as follows: ð4Þ

where qm (mg/g) is the maximum amount of Cr(VI) ions adsorbed per unit mass of adsorbent to form a complete monolayer on the adsorbent surface and b (L/mg) is the binding energy constant. According to Kadirvelu and co-workers [25], the crucial characteristics of Langmuir isotherm can be explained in terms of a dimensionless constant separation factor (RL), defined by Eq. (5):

(b)

0

3.2.1. Effect of pH It is well known that Cr(VI) can exist in solution in one of the following ionic forms: chromate (CrO24 −), dichromate (Cr2O27 −) or hydrogen dichromate (HCrO − 4 ), which is governed by solution pH and total chromium concentration [16]. At pH of 2–6, the major ion form of 2− Cr(VI) is HCrO− 4 and above pH 6, the dominating ion form is CrO4 . In this work, pH studies were carried out between 2 and 11 (Fig. 4), where maximum Cr(VI) adsorption was achieved at pH 2. As the pH increased, a drop in adsorption was observed. This shows that the GO-αCD-PPY NC is more favourable for the adsorption of 2− HCrO− 4 rather than CrO4 . This can be explained by examining the surface charge of the GO-αCD-PPY adsorbent and the degree of ionization of the adsorbent [16]. It is known that the initial pH would determine the surface charge of the adsorbent as well as the metal ions [16]. It is indicated that the lower pH 2 results in an increase in H+ ions on the adsorbent surface hence significantly increased the positive charge. The predominant species, HCrO− 4 has relatively smaller size in comparison and Cr2O2− to CrO2− 4 7 . The combined effect of increase in electrostatic force of attraction between positive charge surface of the adsorbent and HCrO− 4 and faster intraparticle diffusion may account for maximum adsorption efficiency at pH 2 [21]. Meanwhile, low pH allows the redox reaction in the aqueous and solid phases as follows: + 3+ + 4H2O [9]. Therefore during adsorption, HCrO− 4 + 7H + 3e ↔ Cr Cr(VI) was partially reduced to Cr3 + by the electron rich polymer on the surface of GO-αCD-PPY NC [16]. As the pH increases, the uptake of Cr(VI) ions decreased which is due to the higher concentration of OH¯ ions present in the solution that competes with Cr(VI) species.

Ce 1 Ce þ ¼ qe qm b qm

(a)

3000

73

40

60

80

100

RL ¼

1 1 þ bC 0

ð5Þ

2θ/(degree) Fig. 2. XRD patterns of (a) GO and (b) GO-αCD-PPY NC.

where b is the Langmuir constant and C0 is the initial concentration of metal ion. RL value indicates the type of Langmuir isotherm, where if

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V.P. Chauke et al. / Journal of Molecular Liquids 211 (2015) 71–77

Fig. 3. HR-TEM images of (a) GO, (b) GO-αCD, (c) GO-αCD-PPY NC and (d) SEM image of GO-αCD-PPY NC.

RL = 0 it is indicative of an irreversible adsorption, 0 b RL b 1 indicates a favourable adsorption, RL = 1 indicates a linear adsorption and finally if RL N 1, it is indicative of unfavourable adsorption [25]. The RL values obtained in the present study (Table 1), were all 0 b RL b 1 which indicates a favourable adsorption at all the temperatures (25 °C, 35 °C and 45 °C), and it also demonstrates the efficiency of the GO-αCD-PPY adsorbent.

Freundlich isotherm assumes that the uptake metal ions occur on a heterogeneous surface by multilayer adsorption and that the amount of adsorbate adsorbed increases infinitely with an increase in concentration [26]. The Freundlich isotherm is described as: qe ¼ K f  C 1=n e

ð6Þ

where Kf and n are constants of Freundlich isotherm incorporating adsorption capacity (mg/g) and intensity, while Ce and qe are the remaining concentration of the adsorbate at equilibrium (mg/L) and

100

700

80 600

500

q (mg/g)

40

400

e

% Removal

60

20

25 300

O

C

O

35 C O

45 C

0 0

2

4

6

8

10

12

pH

200 0

50

100

150

200

250

300

350

C (mg/L) e

Fig. 4. pH effect on the removal efficiency of Cr(VI) using GO-αCD-PPY NC (Mass of GO-αCD-PPY = 0.025 g, [Cr(VI)] = 200 ppm, contact time = 24 h).

Fig. 5. Adsorption isotherms for GO-αCD-PPY at different temperatures.

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V.P. Chauke et al. / Journal of Molecular Liquids 211 (2015) 71–77

0.7

Table 1 Langmuir and Freundlich adsorption parameters for Cr(VI) removal using GO-α-CD-PPY NC. Freundlich model

qm (mg/g)

b(L/mg)

RL

R2

KF (mg/g)

1/n

R2

606.06 625.42 666.67

1.142 0.888 1.071

0.0033 0.0042 0.0035

0.999 0.999 0.999

514.80 503.86 515.48

0.0317 0.0450 0.0512

0.9265 0.9330 0.9222

0.5

the amount adsorbed at equilibrium (mg/g), respectively. Taking the logarithm from Eq. (6), a linearized form of Freundlich isotherm can be described as Eq. (7):

0.4

e

298 308 318

Langmuir model

(a) 0.6

C (mg/L)

Temp. (K)

75

0.3

0.2 O

25 C

1 ln ðqe Þ ¼ lnK f þ ðC e Þ n

ð7Þ

0.1

where Kf (mg/g) and 1/n constants are related to the adsorption capacity and intensity of adsorption, respectively. Fig. 6(a) and (b) show the linearized Langmuir and Freundlich isotherm plots for the three different temperatures. All the Freundlich and Langmuir isotherm parameters are shown in Table 1 where the correlation coefficient values (R2) for the Langmuir models (0.999) were greater than correlation coefficient values for the Freundlich model (0.927). Therefore, the Langmuir models fitted the isotherm data better than the Freundlich model. The maximum adsorption capacities were 606 mg/g at 25 °C, 625 mg/g at 35 °C and 666.67 mg/g at 45 °C. These values are significantly higher than what has been reported in literature where values, as shown in Table 2, were 20.05 mg/g for PEI modified activated carbon [27], 65.23 mg/g for graphene oxide [8], 69.91 mg/g for magnetic cyclodextrin [8], 120.19 mg/g for magnetic αCD/graphene oxide nanocomposite [14], 227 mg/g for polypyrrole-polyaniline nanofibers [16] and 497.1 mg/g for graphene oxide-polypyrrole [28]. The higher adsorption capacity could be attributed to increased surface area, the abundant hydroxyl groups and cavities, and the presence of PPY moiety on the GO surface, thus making GO-αCD-PPY a promising nanoadsorbent for Cr(VI) removal.

0

35 O C 45 O C

0

50

100

150

200

250

300

350

400

C e (mg/L) 6.55

(b) 6.5

Ln q

e

6.45

6.4 25 O C 35 O C

6.35

O

45 C

3.2.3. Adsorption kinetics In order to analyse the nature of kinetics and the rate of adsorption of total Cr(VI) by GO-αCD-PPY NC, pseudo-first-order and pseudosecond-kinetic order models were used to test experimental results. Fig. 7(a) shows the uptake of Cr(VI) onto GO-αCD-PPY NC at varying concentrations with respect to time, where a rapid uptake is observed during the first few minutes. This is because of the larger surface area and easily accessible adsorption sites on GO-αCD-PPY NC. The uptake of Cr(VI) eventually reaches equilibrium between the solid and liquid phases in the adsorption system. The linearized forms of the pseudofirst-order (Eq. (8)) and pseudo-second order (Eq. (9)) models are: logðqe −qt Þ ¼ log qe − t 1 t ¼ þ qt k2 q2e qe

K1 t 2:303

ð8Þ ð9Þ

where k1 (1/min) and k2 (g/mg/min) are the pseudo-first-order and pseudo-second-order rate constants, respectively, qt is the amount of Cr(VI) adsorbed at any time 't' (mg/g) and qe is equilibrium adsorption capacity (mg/g). The rate constants and adsorption capacities were calculated from the plot log (qe–qt) vs t for pseudo-first-order (figure not shown here) and t/qt vs t for pseudo-second order (Fig. 7(b)). The adsorption kinetics of Cr(VI) removal by GO-αCD-PPY NC was best described by the pseudo-secondorder model (R2 ~ 0.996–0.982) compared to pseudo-first-order model (R2 ~ 0.971–0.882). The K2 values for removal of Cr(VI) at 50 ppm, 100 ppm and 200 ppm were 0.0080, 0.0007 and 0.0001, respectively

6.3 2.5

3

3.5

4

4.5

Ln C

5

5.5

6

e

Fig. 6. Plot of equilibrium data for (a) Langmuir and (b) Freundlich isotherm models at different temperatures, 25°, 35° and 45 °C.

where K2 at 50 ppm was the highest. This shows that the removal of Cr(VI) at 50 ppm was faster than at 100 ppm and 200 ppm possibly due to the greater degree of freedom of distribution of adsorbate over the surface of the adsorbent for the lower concentration Cr(VI) solution [28]. The experimental qe values were almost equal to the calculated qe values for the pseudo-second-order model. 3.2.4. Effect of co-existing ions and desorption studies In this work, the metal ions that were studied along with Cr(VI) include cations and anions such as Co2+, Cu2+, Ni2+, Pb2+, Zn2+, SO2− 4 , and NO− 3 (Fig. 8). From these experiments, it is observed that neither the cations nor anions interfered with the adsorption of Cr(VI) on the GO-αCD-PPY NC. This is due to the fact that at lower pH the GO-αCDPPY NC is positively charged, which means that there was repulsion from positively charged cations thus making it easier for Cr(VI) to be and NO− adsorbed. Furthermore, SO2− 4 3 are low affinity ligands which are known to form outer-sphere complexes with binding surfaces. Their low affinity nature implies weaker adsorption thus negligible competition with Cr(VI) [29]. Thus, the adsorption of Cr(VI) by GO-αCD-PPY NC would be highly efficient using industrial wastewater.

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Table 2 Comparison of adsorption capacity values of GO-αCD-PPY NC with other adsorbents for Cr(VI) removal at 25 °C. Adsorbent

qm (mg/g)

Equilibrium time (min)

Optimum pH

References

PEI modified activated carbon Graphene oxide Magnetic cyclodextrin Magneti-α-cyclodextrin/graphene oxide Polypyrrole-polyaniline Graphene oxide-polypyrrole Graphene oxide-αCD-polypyrrole

20.05 65.23 69.91 120.19 227–294 491.1 606–666

– – – 60 30–180 1440 30–200

3–4 2 2 2 2 3 2

[27] [8] [8] [8] [16] [28] This work

The re-usability of GO-αCD-PPY NC was determined by performing adsorption–desorption studies five times, consecutively using 200 ppm Cr(VI) solutions. These were done by the adsorption of Cr(VI) on

500

(a)

50 mg/L 100 mg/L 200 mg/L 400

GO-αCD-PPY NC (0.25 g) and desorbing Cr(VI) at different concentrations of NaOH (0.1–1.0 M), where 0.5 M was found to be the optimum NaOH concentration for desorbing Cr(VI) (figure omitted). The GO-αCD-PPY NC effectively adsorbed-desorbed Cr(VI) in the first three consecutive cycles where the regeneration of the GO-αCD-PPY NC was achieved by treating it with 2 M HCl. A decrease in efficiency was however observed on the 4th to 5th cycles. This may be attributed to the oxidation of the polymer through repeated oxidation reactions which occur when using highly concentrated Cr(VI) solutions due to the presence of the highly oxidizing Cr(VI) species. It is expected that the life-span of the adsorbents will be significantly increased when treating lower Cr(VI) concentration solutions, such as for polishing applications.

q (mg/g)

300

t

4. Conclusions

200

100

0 0

100

200

300

400

500

600

t (min) 6 (b)

50 mg/L 100 mg/L 200 mg/L

5

The synthesis of a novel GO-αCD-PPY NC was successfully achieved and used for the removal of Cr(VI) from aqueous solution. Experimental results demonstrated that GO-αCD-PPY NC was very effective in removing Cr(VI) with fast kinetics and the rate of Cr(VI) sorption followed the pseudo-second-order kinetic model. The adsorption isotherms fitted well to the Langmuir isotherm model, where the maximum adsorption capacities were 606 mg/g at 25 °C, 625 mg/g at 35 °C and 666.67 mg/g at 45 °C. The presence of co-existing ions (cations and anions) had no significant influence on the efficiency of GO-αCD-PPY NC for Cr(VI) removal. Moreover, regeneration studies revealed that the adsorbent could be reused up to 3 cycles. These results suggested that GO-αCD-PPY NC could be potential adsorbent for Cr(VI) remediation from water. Further studies i.e. column dynamic studies are ongoing to assess potential applicability of the media in removing Cr(VI) from real water samples.

120

4 100

Pb2+ 2+

Zn

3

NO

t/q

t

80

3

2+

% Removal

Ni

2

Co

60

Cu So

40

1 20

0 0

100

200

300

400

500

600

t (min) Fig. 7. (a) Effect of contact time and initial concentration and (b) Pseudo-second-order kinetic model for adsorption of Cr(VI) using GO-αCD-PPY NC.

2+

2+

0 0 50 100 150 Initial Concentration of Co-existing ions (mg/L)

200

Fig. 8. Effect of co-existing ions on Cr(VI) removal by the GO-αCD-PPY NC.

24

V.P. Chauke et al. / Journal of Molecular Liquids 211 (2015) 71–77

Acknowledgements This work is financially supported by a grant from the NRF (PDP) and DST/CSIR, SA. We would like to thank National Centre for Nanostructured Materials for providing instrumental facilities.

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