α-MoO3/polyaniline composite for effective scavenging of Rhodamine B, Congo red and textile dye effluent

S. Dhanavela, E. A. K. Nivethaaa, K. Dhanapala, V. K. Guptabc, V. Narayanand and A. Stephen*a
aMaterial Science Centre, Department of Nuclear Physics, University of Madras, Guindy Campus, Chennai-25, India. E-mail: stephen_arum@hotmail.com; Fax: +91-44-22351269; Tel: +91-44-22202802
bDepartment of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247667, India
cDepartment of Applied Chemistry, University of Johannesburg, Johannesburg, South Africa
dDepartment of Inorganic Chemistry, University of Madras, Guindy Campus, Chennai 600 025, India

Received 28th January 2016 , Accepted 9th March 2016

First published on 11th March 2016


Polyaniline modified MoO3 composites were synthesized via a chemical oxidative polymerization method and employed as a novel adsorbent for Rhodamine B (RhB), Congo red (CR) and textile dye effluent. In this preparation, camphor-10-sulphonic acid was used as a dopant and ammonium peroxydisulphate was used as an oxidant for the fabrication of polyaniline. The MoO3/polyaniline composites were characterized by Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), UV-Vis spectroscopy and elemental analysis. The results confirm the successful formation of the MoO3/polyaniline composite. The surface morphology of the samples was characterized by scanning electron microscopy and transmission electron microscopy. The obtained images show the formation of polyaniline-coated MoO3. The adsorption performance of the prepared composites towards RhB and CR was analyzed. The effect of pH on adsorptive removal was investigated. The adsorption isotherm studies were carried out and the results showed that the equilibrium data fitted well to the Langmuir isotherm equation. The MoO3/polyaniline (aniline, 30% w/w) composite shows better scavenging performance with maximum adsorption capacity of 36.36 mg g−1 and 76.22 mg g−1 on RhB and CR dyes from water. In addition, the adsorptive performance on real textile effluent was also tested. The results show that the MoO3/polyaniline composite can be used as a cost-effective alternative adsorbent for the removal of organic pollutants from waste water.


1. Introduction

The contamination of water resources has become a pervasive problem to mankind in the last few decades due to rapid population growth and advanced industrialization in different fields.1 The major causes of water pollution are dye effluents from paper printing, pharmaceuticals and textile dyeing industries. These dyes contaminate the ground water by releasing toxic, carcinogenic and colored waste water.2–4 Color of the dyes affect the nature of water by inhibiting the sunlight irradiation thereby reducing the photosynthetic action. Rhodamine B (RhB) is one such cationic dye widely used in dyeing industries and medicinal applications. Acute exposure to RhB could cause irritation to the skin, eyes, respiratory tract and gastrointestinal tract.5–7 Similarly, Congo red is a benzidine based anionic dye which is capable of dyeing cotton directly. Furthermore, it is used in the rubber, paper and plastic industries. However it is an irritant to eye and skin and can cause an allergic response. It is known to be metabolized to benzidine which is carcinogenic to human.8,9 It is necessary to adopt an effective technique for scavenging of RhB and CR, in order to avoid the adverse effects from these toxic effluents. A lot of conventional treatment methods such as coagulation/flocculation, trickling filter, chemical oxidation, reverse osmosis, adsorption and photocatalysis have been used to remove dye effluents.10–14 Among these, adsorption is an effective method due to the possibility of reusing the spent adsorbent via desorption. Apart from this it is also ecofriendly, simple and highly efficient.2

Conducting polymers such as, polyaniline, polypyrrole, polythiophene, and polyindole and its derivatives are commonly used in scientific as well as industrial studies and various applications such as sensors, diodes, photocatalysts and in microelectronic devices.15,16 Among all the conducting polymers, polyaniline, a p-type material has elicited much attention owing to its unique electrochemical stability, optical and electrical properties, charge separation capability, slow degradation rate, low cost and simple synthesis.17–19 Polyaniline can be doped with protonic acids such as HCl, H2SO4, camphor sulphonic acid and β-naphthalene sulphonic acid and the doping level can be simply controlled by reversible acid/base via doping/dedoping process.20,21 These tailored properties of polyaniline make this material an effective one for use in a wide range of applications. Its unique characteristics such as good stability and adsorption and desorption via doping–dedoping reversibility, make it a suitable material for application as an adsorbent. Moreover, researches have been recently shown that surfactant modified polyaniline can help to promote the adsorptive property.22 Recently, Yan et al., reported phytic acid (anionic) doped polyaniline for the adsorption of methylene blue.23 However, the low mass density of polyaniline inhibits its use as an adsorbent.

In order to overcome this, composites of polyaniline along with metal oxides have been prepared and used. Molybdenum trioxide is a wide band gap (∼3 eV) n-type semiconducting material. MoO3 has several polymorphs, such as the thermodynamically stable orthorhombic α-MoO3, metastable monoclinic β-MoO3 and hexagonal metastable h-MoO3.24 α-MoO3 phase is a layered structure formed by covalent double layers of MoO6 octahedral. This oxide is mainly found in orthorhombic phase, which is an important material for photochromic and electrochromic optical devices, gas sensors, photocatalysts and electrodes in supercapacitor applications.25–27 Various approaches have been followed to synthesize different α-MoO3 morphologies28 such as nanorods, nanofibers, etc., even though α-MoO3 material is a versatile material with application in different fields, the adsorptive removal of pollutants is not fully explored. Recently, Ying Ma et al., has reported the removal of methylene blue using pure α-MoO3 having high adsorption property.29 So, improving the adsorption and recyclability performance of α-MoO3 and its composites has been the major aim of many researchers. To the best of our knowledge, there are no reports on the adsorption study of polyaniline and MoO3 composites.

In this work camphor-10-sulphonic acid doped polyaniline modified α-MoO3 composite was prepared using chemical oxidative polymerization method. The dye removal efficiency of the composites on Rhodamine B, Congo red and effluents from the textile industries (real sample analysis) were analyzed.

2. Materials and methods

2.1 Materials

Aniline (99%, Rankem) and ammonium heptamolybdatetetrahydrate (98%, Rankem) used in the present study were of analytical reagent grade. Camphor-10-sulphonic acid (98% pure) and ammonium peroxydisulphate (98% pure) were purchased from Sigma Aldrich chemicals. Ethyl alcohol (99% pure) was from SRL chemicals. RhB and CR dyes were obtained from Qualigens. All experiments were carried out using double distilled water.

2.2 Preparation of MoO3 platelets

MoO3 powder was synthesized by simple solid state decomposition method. 2.43 g of ammonium heptamolybdate tetrahydrate was taken and grounded in a mortar for one hour and annealed in an alumina crucible at 500 °C for three hour under air. The obtained product was washed with distilled water and dried in an air oven.

2.3 Preparation of MoO3/polyaniline

MoO3/polyaniline was synthesized via chemical oxidative polymerization method.30,31 0.1 g of aniline monomer was dissolved in 50 mL of distilled water. To this camphor-10-sulphonic acid (dopant) solution (aniline[thin space (1/6-em)]:[thin space (1/6-em)]CSA = 1[thin space (1/6-em)]:[thin space (1/6-em)]0.5, molar ratio) was added. Subsequently, 1 g of the prepared MoO3 power was added to the previously prepared acidified monomer solution. The polymerization was initiated by the dropwise addition of ammonium peroxydisulphate (aniline[thin space (1/6-em)]:[thin space (1/6-em)]APS = 1[thin space (1/6-em)]:[thin space (1/6-em)]1) to the above prepared solution under constant stirring at 0–4 °C. After the complete addition of oxidant, the reaction was kept under stirring for 24 h. The obtained light green precipitate was washed with water several times followed by ethanol and dried in an air oven. Polyaniline was synthesized by the same procedure without the addition of the MoO3 powder. In the similar way, various concentrations of polyaniline (aniline[thin space (1/6-em)]:[thin space (1/6-em)]CSA[thin space (1/6-em)]:[thin space (1/6-em)]APS = 1[thin space (1/6-em)]:[thin space (1/6-em)]0.5[thin space (1/6-em)]:[thin space (1/6-em)]1) modified MoO3 composites were prepared using different monomer concentrations 0.1, 0.3, 0.5, 0.75 and 1 g with 1 g of MoO3 powder and labeled as PM-1, PM-2, PM-3, PM-4 and PM-5.

2.4 Characterization details

X-ray diffraction patterns of the prepared samples were obtained from GE X-RAY DIFFRACTION SYSTEM-XRD 3003 TT with CuKα1 radiation (λ = 1.5406 Å) for 2θ = 10–70°. The morphology of the MoO3 powder and composite were examined using HITACHI SU-6600 FESEM and TEM was carried out using TECHNAI INSTRUMENT operating at operating voltage of 200 kV, equipped with EDAX and SAED facilities. FTIR spectra were obtained using Perkin-Elmer FTIR system. DAR400-XM 1000(OMICRON Nanotechnologies, Germany) equipped with dual Al/Mg anodes as the X-ray source was used for the X-ray photoelectron spectroscopy (XPS) measurements. The Al anode was used to obtain the survey and elemental spectra. All spectra were calibrated using C 1s peak at 284.5 eV to exclude the charging effect on the sample. Raman spectrum was recorded using laser Raman microscope, RAMAN-11 NANOPHOTON CORPORATION, Japan. Surface area and pore structure was analyzed by nitrogen gas adsorption and desorption method. The nitrogen adsorption and desorption curves were measured at the temperature of liquid nitrogen (−196 °C) using Quantachrome Nova Win (QUANTACHROME INSTRUMENTS V 11.02). The surface area and pore size was obtained with Brunauer–Emmett–Teller and Barret–Joyner–Halenda (BJH) methods. The UV-Vis studies and absorbance of dye in the solution was measured using UV-Visible spectrophotometer ANALYTIK JENA SPECORD 200 PLUS.

3. Results and discussion

3.1 Structural investigation

Fig. 1 displays the XRD patterns of polyaniline (PANI), MoO3 and as prepared MoO3/PANI composite samples (PM-1 to 5). The XRD pattern of MoO3 can be indexed to the orthorhombic phase. The obtained pattern was in good agreement with the JCPDS (Joint Committee on Powder Diffraction Standards) card no. 05-0508.32,33 The average crystallite size calculated using Scherrer's formula was ∼43 nm. The lattice parameters were determined using XRDA software and found to be a = 3.961, b = 13.855 and c = 3.697. The main peaks observed at 12.76°, 23.33°, 25.70°, 27.34° and 38.98°correspond to the (0 2 0), (1 1 0), (0 4 0), (0 2 1) and (0 6 0) planes, respectively. The peaks observed at 2θ ∼ 14.08°, 2θ ∼ 19.9° and 2θ ∼ 25.8° correspond to (010), (100) and (110) diffraction planes of PANI. The crystallinity of PANI is due to the periodicity of the partially reduced and oxidized chains which are parallel and perpendicular to aniline chains.34 The XRD pattern of all the composites (PM-1, PM-2, PM-3, PM-4 and PM-5) are similar and the patterns clearly show the maintenance of MoO3 crystal structure even after the addition of different concentrations of PANI. This indicates that the polymerization of aniline monomers on the surface of MoO3. Increasing the concentration of PANI in the composites leads to an increase in the amorphous nature of composites. The obtained results confirm the formation of MoO3/PANI composite.
image file: c6ra02576e-f1.tif
Fig. 1 XRD patterns of MoO3, PANI and its composites.

3.2 FTIR analysis

Fig. 2 shows the FTIR spectra of MoO3 and MoO3/PANI composites in the range of 4000–400 cm−1. The peaks of MoO3 and PANI match well with the previous reports.35,36 FTIR spectrum of MoO3 shows absorption band in the region of 3519, 1629, 984, 881, 678 and 608 cm−1. FTIR spectra of PM-1, 2, 3 & 5 shows peaks at 3235, 1569, 1488, 1300, 1242 and 1146 cm−1. The peak at around 3519 cm−1 corresponds to the interstitial water and hydroxyl group, δ(OH). The peaks at 984 cm−1 and 881 cm−1 are assigned to the Mo[double bond, length as m-dash]O stretching mode and Mo–O–Mo vibrations of Mo6+ which indicates the layered orthorhombic phase of MoO3. The asymmetric band appearing at 1629 cm−1 is due to the vibration of δ(H2O) which is attributed to the bending mode of hydroxyl group of the absorbed water in the sample. The peaks appearing at 678 cm−1 and 608 cm−1 are due to the bending mode vibration of the Mo–O–Mo entity where each O2− ion is shared by three Mo6+ ions. Peak observed nearly at 3235 cm−1 corresponds to the N–H stretching vibrations and that at 1242 cm−1 could be assigned to the aromatic C–N+ stretching vibration mode. The peaks at 1569 cm−1 and 1488 cm−1 correspond to the quinoid ring and benzoid ring modes respectively. The quinoid ring stretching indicates the existence of CSA in PANI chains.37 The peak at 1300 cm−1 is due to the C–N stretching of an aromatic amine. The C–H in plane bending mode appears at 1146 cm−1. In Fig. 2, a shift in the peak of Mo–O–Mo towards lower wavenumber (635, 630, 626 and 610 cm−1) is observed for PM-1, 2, 3 & 5 which can be attributed to the interaction of MoO3 with PANI. FTIR spectrum of PANI is presented in ESI 1.
image file: c6ra02576e-f2.tif
Fig. 2 FTIR spectra of MoO3 and MoO3/PANI composites.

3.3 Raman analysis

Raman spectroscopy is the technique which is very sensitive to the surface phase. Raman spectra of MoO3 and MoO3/PANI composites are shown in Fig. 3. The increase in intensity of Raman peaks of MoO3 and PANI is observed with concentration of PANI. Raman scattering bands of MoO3 shown in Fig. 3a, are in good agreement with the previous reports.38 The peak at 992 cm−1 indicates the terminal oxygen stretching mode of Mo[double bond, length as m-dash]O. The Raman band observed at 663 and 818 cm−1 in the MoO3 spectrum reveals the metal–oxygen stretching modes of edge sharing O–Mo–O and corner sharing O–Mo–O. The 243 and 287 cm−1 peaks represent the wagging vibrations of the double bond O[double bond, length as m-dash]Mo[double bond, length as m-dash]O and twist modes of O[double bond, length as m-dash]Mo[double bond, length as m-dash]O. The peaks at 335 and 374 cm−1 belong to the O–Mo–O bending and scissoring vibrations. The peak observed at 483 cm−1 is attributed to the deformation mode of MoO3. It can be found that besides the peaks of MoO3, peaks observed at 584, 610, 1382, 1449 and 1548 cm−1 in the composites confirm the existence of PANI vibrations. In MoO3/PANI composites (Fig. 3), the peak observed at 1548 cm−1 corresponds to the C–C stretching vibration in the quinoid ring and peak at 1449 cm−1 is assigned to the C[double bond, length as m-dash]C stretching vibration in the benzoid ring. The peak at 1382 cm−1 is assigned to the C–N stretching vibration of the cation radical species indicating an intermediate level of doping. Bipolaronic benzene ring deformations and bipolaronic amine deformations were observed at 610 and 584 cm−1 respectively.39 Raman spectrum of PANI is presented in ESI 2. Mo[double bond, length as m-dash]O stretching modes of MoO3 in the composites are shifted to lower wavenumber (Fig. 3b). This progressive shift to lower wavenumber is mainly due to the interaction between MoO3 and PANI.
image file: c6ra02576e-f3.tif
Fig. 3 Raman spectra of (a) MoO3 and (b) MoO3/PANI composites.

3.4 X-ray photoelectron spectroscopy analysis

The chemical state analysis and elemental composition of the as-synthesized MoO3/PANI composite was analyzed by XPS. The survey XPS spectrum (Fig. 4a) shows that the composite contains C, Mo, O, N and S. In the spectrum of Mo-3d, the double binding energy pattern (i.e.) 3d5/2 and 3d3/2 with an integrated peak area of 3[thin space (1/6-em)]:[thin space (1/6-em)]2 are observed (Fig. 4b). The 3d5/2 peak (A) at 232.45 eV and the Mo 3d3/2 peak (C) at 235.59 eV corresponds to the Mo5+ state. The Mo 3d5/2 peak (B) at 233.49 eV and Mo 3d3/2 peak (D) at 236.62 eV belong to Mo6+.40 Fig. 4c shows the deconvoluted spectrum of N-1s. The peak appearing at 398.66 eV is associated with C[double bond, length as m-dash]N. The binding energy at 402.7 eV belongs to C–N+ which is due to the interaction between nitrogen (N+ cation radical) and protons introduced by the camphor-10-sulphonic acid.41 The O 1s core level spectrum (Fig. 4d) of PM-2 consists of three components (A, B & C) at 530.42, 531.77 and 533.30 eV. The first component is attributed to the O2− ions in the orthorhombic structure of MoO3. The second component is assigned to the O2− ions in the oxygen deficient regions within the matrix of MoO3 as well as O[double bond, length as m-dash]S of SO3 groups in the composites. The third component at 533.30 eV is attributed to the chemically absorbed oxygen site (–OH of water molecules).42,43 In Fig. 4e, C 1s spectrum can be deconvoluted into three distinct peaks at 284.74, 286.12 and 289.09 eV, which are associated with the C–C/C–H, C–N/C[double bond, length as m-dash]N/C–S and π–π* environments respectively.44,45 Sulphonic S-2p peak was observed at 168.27 eV in Fig. 4f which confirms the presence of one sulfur atom for every two nitrogen atoms.46 This shows the existence of the camphor-10-sulphonic acid in the composite.
image file: c6ra02576e-f4.tif
Fig. 4 XPS Spectra of MoO3/PANI composite (PM-2) (a) survey spectrum, (b) Mo 3d spectrum (c) C 1s spectrum (d) N 1s spectrum (e) O 1s spectrum (f) S 2p spectrum.

3.5 Morphology, elemental analysis and SAED pattern

Fig. 5 shows the typical FESEM images of PANI (a), MoO3 (b) and composite (PM-2) taken at different resolutions (c & d). TEM images of MoO3, TEM of the composite (PM-2) taken at low and high magnification of the composite are shown in Fig. 5e–g respectively. The micrograph of pristine PANI, Fig. 5a, shows highly agglomerated granular particles. The agglomeration of the particles implies that they are well interconnected with each other and it suggests that they have sufficient binding energy to combine with neighboring molecules. Fig. 5b shows the platelet – like morphology of MoO3 with polygonal grain shapes, having a particle size of several nanometers. The image obtained for MoO3 particles with PANI is shown in Fig. 5c. The high magnification image (Fig. 5d) clearly shows that the layer of PANI (grey area) on the outer surface of MoO3. Fig. 5e shows the irregular polygonal grain shaped morphology in the range of 90–400 nm. In the case of PM-2 (Fig. 5f), it can be clearly observed that crystalline MoO3 particles have been wrapped by the thin amorphous layer. The coating of PANI on MoO3 is also clearly evident from Fig. 5g.
image file: c6ra02576e-f5.tif
Fig. 5 FESEM images of PANI (a), MoO3 (b), low (c) and high magnification (d) of its composite (PM-2) & TEM images of MoO3 (e), low (f) and high magnification (g) of its composite (PM-2).

The selected area diffraction (Fig. 6) of PM-2 shows the high degree of crystallinity of MoO3 in the composite. The d-spacing values were calculated and the planes observed were indexed to the diffraction from (200) and (141) planes respectively. The obtained planes correlate well to the planes observed for orthorhombic MoO3 (JCPDS no. 05-0508). The obtained results are also in good agreement with the XRD report.


image file: c6ra02576e-f6.tif
Fig. 6 SAED pattern of MoO3/PANI composite (PM-2).

Fig. 7 shows the EDAX spectrum of MoO3/PANI binary system (PM-2), which reveals that the composite is mainly composed of C, O, Mo, N and S. The appearance of sulfur confirms the presence of sulphonic acid doped PANI in the composite. It implies that no other impurity is present in the system.


image file: c6ra02576e-f7.tif
Fig. 7 EDX spectra of MoO3/PANI composite (PM-2).

3.6 Mechanism for the formation of composites

Schemes for the formation of composites (CSA doped PANI with MoO3) and resultant composite structures are shown in Fig. 8. The anilinium MoO3 complex is formed by the addition of camphor sulphonic acid and MoO3 platelets to the aniline monomer solution. Here, aniline monomer gets protonated and becomes anilinium cation. When MoO3 particles are added to the monomer solution, MoO3 particles could be placed at the core of that complex. Polymerization reaction occurs strictly on the surfaces of MoO3 in the complex with the addition of ammonium persulphate. The formed anilinium cation absorbed on the negatively charged MoO3 nanoparticles due to the surface hydroxyl and strong electrostatic force attraction.35
image file: c6ra02576e-f8.tif
Fig. 8 Mechanism of MoO3/PANI composite formation.

3.7 UV-Vis studies

Fig. 9 shows the UV-Vis absorbance spectra of MoO3, PANI and PM-2. The absorption edge of MoO3 was observed at 350 nm.47 Absorption spectrum of PANI shows three characteristic bands at around 378, 465 and 850 nm respectively. The characteristic band at 378 nm is due to the π–π* transition of benzoid ring. The shoulder peak at 465 nm is due to the polaron–π* band. The broad band in the range of 850 nm is due to the π–polaron band transitions.48 This reveals the doping of PANI with CSA. The absorption spectra of PM-2 shifts to higher wavelength compared to that of PANI, which implies the strong interaction between PANI and MoO3.
image file: c6ra02576e-f9.tif
Fig. 9 UV-Vis spectra of MoO3, PANI and PM-2.

3.8 N2 adsorption–desorption study

The specific surface area and pore structure of MoO3 and PM-2 at 77 K was investigated using the N2 adsorption and desorption measurements. The surface area and pore volume of MoO3 and PM-2 are 2.277 m2 g−1, 0.00864 cm3 g−1 and 27.317 m2 g−1, 0.00805 cm3 g−1 respectively. The pore radius of MoO3, PM-2 are found to be 10.94 nm and 10.85 nm respectively. It exhibits mesoporous structure. PM-2 possesses large surface area when compared to MoO3, which can be beneficial to the enhanced contact between the adsorbate and adsorbent molecules.

3.9 Dye adsorption studies and mechanism

Removal of carcinogenic, toxic dye from aqueous solution is the major scope of the present work. Besides the conventional techniques, various adsorbents have been used for this purpose. MoO3 platelets have active surface containing functional hydroxyl groups which can interact with RhB through electrostatic interactions as RhB is a cationic dye.29,49,50 The efficiency of this material can be increased by tailoring the properties of PANI. PANI has received tremendous interest owing to its active site (positively charged nitrogen) present in the backbone of polymeric chain. This reactive center is also responsible for enhancing dye adsorptive property via dopants (cationic or anionic). That is, opposite charged states can be generated, which permit the adsorption of dyes based on the positive or negative charge species via weak interactions.51,52

CR is an acidic dye and in the aqueous solution, the sulphonate groups (dye-SO3Na) of the dye are dissociated and converted to anionic dye ions. Camphor-10-sulphonic acid doped PANI modified MoO3 for improving adsorptive property for the removal of RhB and CR have been demonstrated in this section.

Adsorption of RhB and CR onto MoO3/polyaniline samples was carried out in the dark at ambient temperature. Reaction suspensions were prepared by adding 50 mg of the adsorbent into 100 mL of RhB solution with an initial concentration of 2.1 × 10−5 mol L−1 and CR solution with an initial concentration of 1.5 × 10−5 mol L−1, respectively. This reaction mixture was agitated at a rate of 150 rpm with a stirrer. 4 mL of samples were collected from this mixture at regular intervals of time. Collected sample was restored into the mixture to attain constant evaluation after UV-Vis measurement. The concentration of the RhB and CR in the supernatant solution was analyzed using a double beam spectrophotometer at the wavelength of maximum absorbance at 555 nm and 498 nm, respectively. The removal of dyes from the initial concentration was calculated using the following equation,

 
image file: c6ra02576e-t1.tif(1)
where C0 and Ct are the initial concentration of dye in solution and dye concentration in solution after adsorption, respectively.

The uptake was evaluated as (2),

 
image file: c6ra02576e-t2.tif(2)
where, Qt is the amount of dye adsorbed on the samples at given time, t (mg g−1). Ct is the dye concentration in the solution after adsorption (mg L−1). W is the mass of the sample taken for the experiments (mg). V is the volume of the solution in liters.

Fig. 10a shows the time dependent UV-Vis Spectra for the adsorption of RhB by PM-2. Adsorption performance of as prepared MoO3, PANI and its composites are presented in ESI 4. From Fig. 10a, it is evident that the peak intensity of RhB at 555 nm decreased significantly after the addition of PM-2. It exhibits the highest adsorptive capacity to the removal of RhB upto 15.04 mg g−1 and removal efficiency of 75.2%. This value is four times higher than the value of bare MoO3. The obtained capacities of MoO3, PANI and PM-1, 3, 4 and 5 are 3.88, 2.05, 12.07, 9.01, 8.29 and 6.43 mg g−1 respectively (Fig. 10b). The variation in the adsorption capacity values obtained for the composites with different aniline monomer concentrations is related to the synergistic effect of achieving high adsorptive property. This is due to the combinatory effect of electrostatic and π–π stacking interactions from composites. In detail, the interactions between RhB and adsorbent can be explained by the binding mechanism, illustrated in Fig. 11a.


image file: c6ra02576e-f10.tif
Fig. 10 (a) UV-Vis spectra of the time dependent adsorption of RhB aqueous solution by PM-2 & (b) adsorption capacity curves of RhB on to MoO3, PANI and its composites.

image file: c6ra02576e-f11.tif
Fig. 11 Proposed binding mechanism of RhB (a) and CR (b) onto MoO3/PANI composites.

There are many factors that may influence the adsorption of dye molecule to the adsorbent. They are the charge and structure of dye molecule, surface properties of the adsorbent, electrostatic interaction, hydrogen bonding, van der Waal's forces, hydrophobic and hydrophilic interactions.53 In the case of MoO3 particles into the RhB dye solution, 3.88 mg g−1 of the dye was adsorbed on the surface of MoO3. It is due to the migration of RhB molecules on the surface of MoO3 through van der Waal's and electrostatic interactions35 between cationic dye and negatively charged metal oxide surface (3).

 
(MoO3)OH + RhB+(dye) → (MoO3)OH–RhB+(dye) (3)

For the case of PANI, 2.05 mg g−1 of the dye was adsorbed on the surface of PANI due to the presence of negatively charged sulphonic group present in the system as well as by the presence of π–π stacking interactions between aromatic ring of PANI and RhB.54 The presence of the protonated state is evident from the N-1s spectra of XPS. The possibility of π–π interaction can be confirmed from the UV-Vis spectrum of PM-2.

 
((PANI)NH+·SO3 + RhB+(dye) → (PANI)NH+·SO3–RhB+(dye)) (4)

Similarly, adsorption of CR onto MoO3/polyaniline samples was carried out in the same condition as mentioned above. Fig. 12 shows the time dependent UV-Vis spectra for the adsorption of CR by PM-2 and adsorption performance of the as prepared MoO3, PANI and its composites. From Fig. 12a, it is evident that the peak intensity of CR at 498 nm decreased significantly after the addition of PM-2. The obtained adsorption capacities of PM-1 to 5, MoO3 and PANI are 13.90, 16.44, 15.60, 15.01, 14.01, 1.15 and 13.14 mg g−1 respectively (Fig. 12b). It can be observed that MoO3 exhibited extremely low capacity when compared to PANI. The obtained adsorption rate of composites, suggest that the composition of MoO3 significantly increases the dye removal capacity of PANI. Actually the difference in the capacity of bare PANI and MoO3/PANI was mainly caused by the low mass density of polymer which is hard to be settled. MoO3 in the composite act as the support medium for their adsorption characteristics where the ability to settle of the adsorbent increased. The obtained synergistic effect of the adsorption behavior can be described through the following binding mechanism given in Fig. 11b. The trend of CR removal capacity was observed as MoO3/PANI > PANI > MoO3. MoO3 showed lowest adsorptive capacity which does not have any functional group for strong interaction except the possibility of hydrogen bonding between surface hydroxyl group of MoO3 and amine group of CR and whereas PANI shows better performance due to the electrostatic interaction between the negatively charged sulphonate goups of CR dye and positively charged amine group of PANI (6). On the other hand, π–π interaction and hydrogen bonding between the CR and PANI also influences high adsorption capacity (7).

 
CR–SO3Na + H2O → CR–SO3 + Na+ (5)
 
CR–SO3 + NH+ (PANI) → CR–SO3–NH+ (PANI) (6)
 
(PANI)NH+·SO3 + (–N[double bond, length as m-dash]N–)–CR → (PANI)NH+·SO3–(–N[double bond, length as m-dash]N–)–CR (7)


image file: c6ra02576e-f12.tif
Fig. 12 (a) UV-Vis spectra of the time dependent adsorption of CR aqueous solution by PM-2 & (b) adsorption capacity curves of CR on to MoO3, PANI and its composites.

From the obtained results, a combination of both the above mentioned interactions enhances the activity of composites as adsorbent. It is clear from the XRD pattern of composites that the amorphous nature of the system increases with the addition of different monomer concentrations. This amorphous nature leads to more defects in the composites. This is because of the presence of covalent like bonding between the OH group of MoO3 to the NH2+ group of PANI as observed for the case of adsorption of methylene blue(cationic dye) on the polypyrrole and TiO2 composites55 and acid red G (anionic dye) on the polyaniline/TiO2 composites. As shown in Fig. 10 & 12, for the case of composites, removal performance increased up to PM-2 and then it decreases for the rest of the composites (PM-1, 3, 4 & 5). This could be due to the lower concentration of PANI (PM-1) leading to the lower adsorptive capacity due to the lack of the number of binding sites. On the other hand, the higher concentration of PANI, causes agglomeration, leading to the overlapping of binding sites, which in turn increases the diffusion path length thereby decreasing the sorption sites.22 From these results, the optimized PM-2 sample was subjected to further studies.

3.9.1 Effects of contact time, temperature on adsorption. The effect of contact time (at ambient temperature) on the removal of RhB and CR on PM-2 is shown in Fig. 13a. The adsorption of RhB and CR increases with increase in contact time and the equilibrium was attained within 60 min. Initially, increase in the dye adsorption is due to the adsorption of dye molecules on to the vacant surface sites and thereafter the adsorption reaches equilibrium due to the saturation of sorption sites. The influence of solution temperature on adsorption of RhB and CR at 15 °C, 25 °C and 35 °C is shown in Fig. 13b. It can be observed that there is no significant change on the adsorption behavior while changing the temperature. This implies that PM-2 is temperature independent in the range of 15 to 35 °C.
image file: c6ra02576e-f13.tif
Fig. 13 The effect of contact time (a) and temperature (b) for RhB and CR adsorption on PM-2 (RhB solution with an initial concentration of 2.1 × 10−5 mol L−1 and CR solution with an initial concentration of 1.5 × 10−5 mol L−1, respectively & 50 mg of the adsorbent into 100 mL of dye solutions).
3.9.2 Effect of pH on adsorption. The adsorption capacity strongly depends on pH of the solution which influences the stability and structure of dye molecules, and properties as well as functionality of adsorbent. The influence of the solution pH on adsorption capacities was evaluated. This was done by adjusting the pH of the solution between 3 to 10 using 0.1 M of HCl or NaOH at room temperature (concentration of dye is 2.1 × 10−5 mol L−1). Time dependent absorption spectra for the adsorption of RhB by PM-2 at pH 3 are shown in Fig. 14a. Effect of pH on dye removal efficiency and adsorption capacity of PM-2 is shown in Fig. 14b and c. It can be observed that, maximum adsorptive removal efficiency (91%) and adsorption rate (18.18 mg g−1) are obtained at pH 3. The uptake of RhB decreases to 16.08, 15.02 and 9.98 mg g−1 on increasing the pH of solution to 5, 7 and 10 respectively. It can be observed that low capacity of dye removal is obtained at higher pH. This indicates that the pH of the solution is remarkable in our study. Similar behavior of pH influence on cationic dye was observed by Rajeev kumar et al.22 The phenomenon for this behavior can be explained using the surface functional group of the composite. Hamdi M. H. Gad and Ashraf A. El-Sayed56 reported that at pH lower than 4, RhB ions are cationic and monomeric but on increasing the pH of the solution RhB ions become zwitterionic (Fig. 14d). This is because of the dissociation acid group (pKa ∼ 3) in the dye at higher pH.57,58 When the pH of the solution is higher than 4 (pH > 4), zwitterionic form of RhB in water may increase the aggregation of RhB due to the electrostatic interaction between carboxyl and xanthene groups of monomers, which leads to the formation of larger molecules. This aggregation of dye molecules reduces the adsorption capacity and hinders its entry into the pores. On increasing the pH of the solution, deprotonation of the imine group of polyaniline leads to the lower adsorptive property where the possibility of electrostatic interaction between sulphonic acid group in the composite and nitrogen group of RhB decreases.
image file: c6ra02576e-f14.tif
Fig. 14 (a) UV-Vis spectra of the time dependent adsorption of RhB aqueous solution by MoO3/PANI composite (PM-2) at pH 3 & (b) dye removal efficiency and adsorption capacity curves of RhB on to PM-2 (c) effect of pH on adsorption capacity of PM-2 (d) molecular form of RhB (cationic and zwitterionic form).

Fig. 15a shows the effect of pH of the solution on CR (concentration of dye is 1.5 × 10−5 mol L−1) sorption onto MoO3/PANI (PM-2). The adsorption of CR decreases on increasing the solution pH from 5 to 10 whereas at lower pH (pH 3) it shows low adsorptive capacity. Specifically the maximum removal efficiency (94.6%) was observed at pH 5 whereas the removal efficiencies were 61.3%, 82.2% and 32.5% at pH 3, 7 & 10 respectively. The observed dependence of solution pH on CR removal could be explained interms of surface functional group of composite and CR molecule. Several reports revealed the same behavior that adsorptive capacity decreases as the solution pH increases (>5).8,59–61 This is because the surface of the composite becomes highly positively charged at lower pH and an electrostatic interaction between negatively charged dye anions and positively charged imine groups of polyaniline occurs, resulting in higher adsorption of CR dye. On the other hand, protonated tautomeric species of CR zwitterions influences lower adsorptive capacity at lower pH (<5). The zwitterionic form of CR is given in Fig. 15b. Sushanta Debnath et al., suggested that at pH lower than 5, CR ions are cationic.62 This is due to the formation of two protonated tautomeric species where the protons are attached to the ammonium nitrogen and the azonium group of CR molecule. This is the cationic form of CR and hinders its adsorption on PM-2. As the solution pH increases, the number of positively charged sites decrease and negatively charged sites increase. These negatively charged sites do not favor the adsorption of dye anions due to the electrostatic repulsion.


image file: c6ra02576e-f15.tif
Fig. 15 (a) Effect of pH on adsorption capacity of PM-2 (b) molecular form of CR (anionic and zwitterionic form).
3.9.3 Isotherm studies. Sorption capacity studies for the removal of RhB and CR are shown in Fig. 16. This study was performed by mixing 50 mg of adsorbent with 100 mL of RhB and CR solution at various dye concentrations (10–50 mg) at room temperature. Various models have been applied to describe the behavior of adsorbent materials. We considered two adsorption isotherm, Langmuir63 and Freundlich64 for the adsorption equilibrium data. The models can be expressed by the following relation,
 
image file: c6ra02576e-t3.tif(8)
 
image file: c6ra02576e-t4.tif(9)

image file: c6ra02576e-f16.tif
Fig. 16 Langmuir isotherm and Freundlich plot for RhB (a & b) and CR (c & d).

Langmuir isotherm (8) describes that, the adsorption occurs at particular homogeneous sites within the adsorbent and sorption of each sorbate molecule on to the surface of the adsorbent has uniform adsorption activation energy. Where, Ceq is the equilibrium concentration of dye (mg L−1), Qeq is the amount of dye adsorbed (mg g−1). Qmax is the maximum adsorption capacity of the adsorbent (mg L−1). KL is the Langmuir adsorption constant (L mg−1) and related to free energy of adsorption. The value of Qmax and KL can be calculated from the intercept and the slope of the linear plot of the Ceq./Qeq. verses Ceq.. Freundlich isotherm (9) adsorption model assumes that reversible and multilayer adsorption occurs on a heterogeneous surface. The value or the constants KF and 1/n are obtained from the linear plots of log[thin space (1/6-em)]Qeq. versus log[thin space (1/6-em)]Ceq. The measured values of the Langmuir and Freundlich isotherm parameters summarized in Table 1. It can be seen that the correlation coefficient (R2) values in Table 1, for Langmuir isotherm model is better than Freundlich isotherm model which indicates adsorption equilibrium data better fitted to the Langmuir isotherm model. These results suggest that the adsorption of RhB and CR onto the PM-2 is monolayer adsorption process and all sites are identical. The applicability of Langmuir isotherm can be expressed by a dimensionless separation factor (RL), called equilibrium parameter (10)

 
image file: c6ra02576e-t5.tif(10)
where C0 is the highest initial dye concentration (mg L−1) and b is the Langmuir constant. The value of RL shows the type of isotherm. The adsorption process is favorable when 0 < RL < 1, linear when RL = 1, unfavorable when RL > 1 and irreversible when RL = 0.22 The RL value obtained for the adsorption of RhB and CR, in the range of 0–1 as shown in Table 1, indicates the process is favorable. The maximum adsorption capacity of composite was found to be 36.36 mg g−1 for RhB and 76.22 mg g−1 for CR.

Table 1 Adsorption isotherm parameters for RhB and CR adsorption onto MoO3/PANI composite (PM-2)
Dye Langmuir model Freundlich model
Qmax (mg g−1) KL (L mg−1) RL R2 KF (L mg−1) n R2
RhB 36.36 0.027 0.270 0.9996 5.96 0.782 0.6095
CR 76.22 0.013 0.434 0.9865 45.38 3.2 0.9724


The obtained adsorptive capacity is higher than most of the other adsorbents reported earlier (Table 2) for RhB and CR adsorption. The high adsorption capacity compared to other adsorbents may be attributed to the following reasons. The adsorbent can remove both the organic dye ions via electrostatic interactions. This may be due to the π–π overlapping of camphor-10-sulphonic acid doped PANI with the aromatic rings of the dye molecules. Hydrogen bonding interaction of the CR as well as RhB molecule with the PM-2 also leads to the high adsorption capacity.

Table 2 Comparison to efficiency of various adsorbents used for RhB and CR removal from aqueous solutions
Adsorbent Adsorption capacity (mg g−1) Reference
Rhodamine B Congo red
SiO2@MgxSyOz 52.71 65
Red mud 5.5 66
Kaolinite 46.08 67
Fly ash 10 68
Hypercross-linked polymeric adsorbent 2.1 69
PVC@graphene–polyaniline 40.0 54
Eucalyptus wood saw dust 31.25 70
Cattail root 34.59 71
Hollow Zn–Fe2O4 nanospheres 16.58 72
Macauba palm cake 32 73
MoO3/PANI 36.36 76.22 This work


3.9.4 Mechanism analysis. Fig. 17 shows the FTIR spectra of RhB, CR, PM-2 after adsorption of CR and RhB. After adsorption of RhB and CR, the bands at 1569 cm−1 and 1488 cm−1 (in Fig. 2) in the PM-2 belonging to the quinoid and benzoid rings of PANI slightly shifted to the lower wavenumbers which is due to the π–π interaction between the aromatic rings of PANI and dye molecules.22 The characteristic peak of C–N stretching mode also shifted from 1300 cm−1 to 1296 cm−1 (for CR adsorption) and 1298 cm−1 (for RhB adsorption), indicating the interaction between the C–N of PM-2 and dyes. A shift in the peak of Mo–O–Mo towards lower wavenumber (601 cm−1 for CR adsorption and 596 cm−1 for RhB adsorption) is also observed, indicating the interaction between PM-2 with adsorbate. Moreover, the presence of characteristic peaks of CR and RhB on PM-2 after adsorption indicates the adsorption of dyes onto the adsorbent. The obtained results show that the functional groups of PM-2 might be involved in the adsorption process and also confirms the adsorption through the π–π interaction, electrostatic interaction and hydrogen bonding.
image file: c6ra02576e-f17.tif
Fig. 17 FTIR spectra of CR, PM-2 after CR adsorption (PM-2 +CR), RhB and PM-2 after RhB adsorption (PM-2 + RhB).
3.9.5 Textile effluent test. The optimized adsorbent which is having maximum adsorption capacity was further used to study the removal of dye from industrial effluent (real sample analysis) which was collected from the textile industry (Tirupur, Tamilnadu, India). To reduce the turbidity property of the real textile effluent, 100 mL of the effluent was diluted with 400 mL of the water (1[thin space (1/6-em)]:[thin space (1/6-em)]4 ratio). Reaction suspensions were prepared by adding 50 mg of the adsorbent into 100 mL of dye solution. This reaction mixture was agitated at a rate of 150 rpm with a stirrer. The reaction was carried out in a dark and room temperature condition. 4 mL of the samples were collected from this mixture at regular intervals of time. The initial and adsorbent treated real effluent concentrations were examined by UV-Vis spectrophotometer at a wavelength of 585 nm (given at inset of Fig. 18). Time coarse adsorption curves for industrial effluent using PM-2 is given in Fig. 18. It shows that composite exhibited 53.8% adsorption capability. The obtained capability is due to the possibility of the presence of electrostatic interactions, hydrogen bonding and π–π interactions between the textile dye molecules with PM-2.
image file: c6ra02576e-f18.tif
Fig. 18 Time dependent dye removal performance on textile dye.
3.9.6 Regeneration and reusability. The regeneration of adsorbent and removal efficiency of dyes are important in environmental pollution control. After dye adsorption, the residual dye in the supernatant was discarded and the adsorbent alone was separated. The recyclic ability of the PM-2 sample was achieved by using the ethanol solution as desorption agent as shown in Fig. 19a. Four cycles of adsorption and desorption studies were carried out on RhB, CR and textile dye. The adsorptive removal efficiency of the sample was 91% after first cycle (Fig. 19b). After first cycle, the removal efficiency of the sample on RhB decreases to 89.7, 87.3 and 82.1 for the next three consecutive cycles. Similarly, for the CR and textile dye, more than 80% and 51% could be recovered after fourth cycle, which indicates the high regeneration capacity of PM-2. The reduction in adsorption capacity after each cycle is due to the incomplete desorption of dye molecules.
image file: c6ra02576e-f19.tif
Fig. 19 Schematic illustration of recyclic process of prepared samples (a) and adsorption efficiency of MoO3/PANI composite (PM-2) on RhB, CR and textile dye (b).

4. Conclusion

MoO3/PANI composites were prepared using chemical oxidative polymerization method. The formation of composites by the binding of MoO3 to the imine and amine nitrogen atoms of PANI via complexation is evident from the FTIR, Raman and XPS analysis. Semi-crystalline nature of PANI and orthorhombic structure of MoO3 is confirmed from XRD and SAED analysis. Further, surface morphology and elemental analysis were performed and explained in detail using FESEM, TEM and EDX analysis. The adsorptive performance of MoO3/PANI composites was demonstrated and evaluated on RhB and CR. The maximum adsorptive capacity of the PM-2 with RhB and CR are determined to be 36.36 mg g−1 and 76.22 mg g−1. Further, the composite effectively adsorbs the industrial textile effluent with efficiency of more than 51%. The obtained results show synergistic performance on dye removal due to the various interactions. From the present study it is evident that MoO3/PANI composite is a cost effective and promising material for solving environmental problems.

Acknowledgements

One of the authors S. D. acknowledges UGC-UPE-Phase II for its financial assistance in the form of fellowship. The National Center for Nanoscience and Nanotechnology, University of Madras is acknowledged for the FESEM, TEM, RAMAN and XPS facilities. SAIF, IIT Madras is acknowledged for FTIR measurements. Author would like to thank Dr J. Senthilselvan, Asst. Professor, Department of Nuclear Physics, University of Madras for the UV-Vis Measurements.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02576e

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