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Superadsorbent Ni-Co-S/SDS nanocomposites for Ultrahigh Removal of Cationic, Anionic Organic Dyes and Toxic Metal Ions: Kinetics, Isotherm and Adsorption Mechanism Arif Chowdhury, Afaq Ahmad Khan, Sunita Kumari, and Sahid Hussain ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05775 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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ACS Sustainable Chemistry & Engineering

Superadsorbent Ni-Co-S/SDS nanocomposites for Ultrahigh Removal of Cationic, Anionic Organic Dyes and Toxic Metal Ions: Kinetics, Isotherm and Adsorption Mechanism†

Arif Chowdhury, Afaq Ahmad Khan, Sunita Kumari and Sahid Hussain*

Department of Chemistry, Indian Institute of Technology Patna, Bihta, Patna, Bihar, 801106, India *Corresponding Author Sahid Hussain, E-mail: [email protected]; Fax: +91-612-227-7383; Tel: +91-612-302-8022 Authors: Arif Chowdhury, E-mail: [email protected] Afaq Ahmad Khan, E-mail: [email protected] Sunita Kumari, E-mail: [email protected]

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ABSTRACT Novel adsorbent Ni-Co-S/SDS (nickel cobalt sulfide/sodium dodecyl sulfate) nanocomposites have been synthesized by simple and eco-friendly approach using water as a solvent at low temperature. The prepared nanocomposites structure and morphology were studied by powder XRD, SEM, TEM, XPS, FTIR, TGA, and also the surface area and surface charge of the adsorbents were measured by BET and Zeta potential. These nanocomposites showed excellent adsorption property for both cationic {Crystal violet (CV), Rhodamine B (RhB), Methylene blue (MB), Nile blue A (NB)}, anionic {Methyl orange (MO) and Congo red (CR)} organic dyes and Cr(VI) metal ions. They exhibited maximum adsorption capacity 3598.23 mg g-1 for CR, 3284.08 mg g-1 for MO, 4417.79 mg g-1 for NB, 3598.23 mg g-1 for CV, 1451.64 mg g-1 for MB, 773.47 mg g-1 for RhB and 583.67 mg g-1 for Cr(VI) ions respectively, which are higher adsorption capacities values than reported and exhibited efficient adsorption property for both cationic and anionic dyes. Furthermore, nanocomposites have showed adsorption efficiencies greater than 95% after regenerated and reused upto five times. Moreover, it followed pseudo-second-order kinetics and an isotherm best fit with modified Zhu and Gu model. Based on the adsorption studies, FTIR and Zeta-potential measurements studies confirm that the hydrogen bonding and electrostatic interactions are the predominating factors for first step adsorption process and second step is hydrophobic interaction between adsorbates. Overall remarkable adsorption capacity and reusability of Ni-Co-S/SDS nanocomposites elucidate the scope as environmental remedy for industrial applications.

KEYWORDS: Ni-Co-S/SDS nanocomposites, Ultrahigh adsorption, Organic dyes, Metal Ions, modified Zhu and Gu model, Electrostatic interaction and Hydrogen bonding.

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INTRODUCTION Water pollution has the concerned global environmental issues and has gathered more worldwide attention because of scarcity of drinking water. There are various organic pollutants in our environment and amongst all organic dyes are major contributors to water pollutions as per World Bank report.1-3 Different types of synthetic organic dyes are widely used for dyeing in the industries like textile, printing, paints, rubber, art, paper and biotechnology.4-5 Most of the organic dyes used are highly toxic, mutagenic and carcinogenic.6-7 Untreated dye effluents crop up from these dye industries can worsen water crisis leading to serious hazard on aquatic life and even human beings. In 1974, the Ecological and Toxicological Association of the Dye stuffs Manufacturers (ETAD) were established for monitoring the toxicological impact and decreasing the environmental damages. ETAD survey revealed that 90% dyes used were highly undesirable as they are toxic, mutagenic, and carcinogenic and have LD50 values significantly above 2×103 mg/kg.8 Because of the above concerns, rapid and efficient approach for dyes disposal is becoming a hot topic for research.9 The existence of toxic heavy metal ions in the water can be harmful to human beings and living species even at low concentrations because they are nondegradable in nature.10-11 To deal with dyes and heavy metal ions, varieties of technologies have been developed by scientific research communities. The most efficient and non-destructive way to dye and metal ions removal is adsorption technique because it is flexible, simple and has very high efficiency.12 Several absorbents have been synthesized towards the removal of cationic and anionic dyes from water, but there were no specific adsorbent capable of removing all types of dyes completely with very high efficiency. Therefore, the developments of adsorbents with higher adsorption efficiency for all types of dyes and heavy metal ions are of great interest. Moreover, some adsorbents with good adsorption capacity have been reported, e.g., ZnS nanotubes,13 hierarchical SnS2,14 FeOOH coated Ni-foam,15 mGO/PVA CGs,16 porous hierarchical MgO,17 Cu2O−Ag,18 Iron nanoparticles,19 γ-AlOOH/Fe(OH)3,20 MgO-MgFe2O4 composites.21 However, these composites are suffering from limitations such as high temperature synthetic approaches and aggregation of particles leading to decreased adsorption capacity.22 In addition, all the adsorbents are not capable to adsorb all type of dyes and metal ions. Hence, still it required to develop a new cost effective material at low temperature that provide simple and eco-friendly strategy for adsorption of water pollutants completely. In view of the above, our research group has focused on the metal sulfide with suitable surfactant composites for adsorption of all types of dyes and metal ions.



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The metal sulfides such as NiS, CoS, NiCo2S4,CuCo2S4, SnS, CuS, ZnS and MoS2 have shown the prominent use in supercapacitor application,23-28 dye sensitized solar cells,29-31 electrochemical energy storage32-33 and hydrogen evolution reaction34-37 because of their higher conductivity, thermal stability and diverse redox properties. Among them, NiCo2S4 has a much higher conductivity and significant redox properties compared to binary sulﬁde as electrode materials. Moreover, electrodeposited NiCo2S4 has been used for the hydrogen evolution reaction38 and hollow NiCo2S4 used for oxygen evaluation.39 Besides, there are several reports on metal chalcogenides nanomaterial as photocatalyst for the degradation of organic dye pollutants.40-44 In the photocatalytic process the first and important step is adsorption of target dye pollutants. In view of this, considering the chemical and physical properties of NiCo2S4, we have chosen it as the one of choice material for adsorption studies of organic dyes and metal ions. On the other hand, the properties of nanocomposites could be enhanced through integration of organic part into inorganic material compared to its basic single component.45 The organic moiety can be synthetic polymers, natural polymers or surfactants. Some nanocomposite materials with organic component support has been exhibiting high adsorption capacity such as porous Fe(OH)3@Cellulose,46 titanate layer-naturalpolymers,47 MF@Fe3O4@PDA/PSBMA,48 CaO/gC3N4,49 WOx/C nanowire,50 graphene oxide-chitosan/silica51. In the same way, the surfactants also play crucial role in the nanocomposites, material synthesis as well as their adsorption capacity. SDS is an important surfactant used in research and application among the salts of long primary alkyl chain and it is widely used in nanoparticles synthesis.52-53 It can reduce the interfacial tension between solid and liquid and shows high reproducibility to remove dyes and metal ions from wastewater.22 In the present work, we report a simple and efficient method to synthesize Ni-Co-S/SDS nanocomposites using water as a solvent at low temperature. Importantly, in the synthetic process nanocomposites were formed by reaction with sulfide ions which were obtained by heating of thioacetamide. Hence, the synthesis strategy is eco-friendly without the requirement of any harmful chemicals, inexpensive and no calcination step. To the best of our knowledge, this is the first green approaching strategy for synthesis of Ni-Co-S/SDS nanocomposites at low temperature with high adsorption capacity. Initially, the nanocomposites surface was examined by the variation of surfactant SDS amount at 90 °C. And thereby we studied the importance and role of SDS in the synthesis process. Further, the adsorption isotherms of prepared Ni-Co-S/SDS nanocomposites were examined by removing both anionic and cationic dyes with NH2 groups like nile blue A (NB) and congo red (CR), and without NH2 groups like crystal violet (CV), methylene blue (MB), methyl orange (MO), and rhodamine B (RhB). We found that the maximum 4 ACS Paragon Plus Environment

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adsorption capacities of nanocomposites towards NB, CV, MB, RhB (cationic); CR, MO (anionic) dyes and Cr(VI) ions were determined as 4417.79 mg g-1, 3556.04 mg g-1, 1451.64 mg g-1, 773.47 mg g-1, 3598.23 mg g-1, 3284.08 mg g-1 and 583.67 mg g-1 respectively. To our knowledge, Ni-Co-S/SDS nanocomposites synthesized at low temperature are showing high adsorption capacity than many reported composite adsorption materials. Finally, based on the adsorption properties of adsorbents the plausible mechanism has been proposed and the higher adsorption could be expected due to the presence of hydrogen bonding and electrostatic interactions as predominating factors in the

first step of adsorption and second step is

hydrophobic interaction between adsorbates. Therefore, the synthesized nanocomposites could be employed for the purification of dye and metal ions polluted wastewater due to synthetic simplicity and ultrahigh adsorption capacity.

EXPERIMENTAL SECTION Materials Nickel acetate tetrahydrate [Ni(OAc)2.4H2O, 98.0%], Cobalt acetate tetrahydrate [Co(OAc)2.4H2O, 98.0%], sodium dodecyl sulfate (SDS , 90.0%), thioacetamide (C2H5NS, 99.0%), ethanol (EtOH), Crystal violet (CV), Rhodamine B (RhB), Methylene blue (MB), Nile blue A (NB), Methyl orange (MO) and Congo red (CR) were purchased from Sigma-Aldrich. All chemicals were of analytical grade and were used without further pre-treatment. Throughout the experiments deionized water was used for preparing the solutions. Synthesis of nanocomposites In order to synthesize Ni-Co-S/SDS nanocomposites, cobalt acetate tetrahydrate (1.0 mmol), nickel acetate tetrahydrate (1.0 mmol) and SDS (0 to 2.0mmol) were taken into a round bottom flask and dissolved by adding 10 mL of water. Then the solution was stirred at 90 °C for one hour. In a separate beaker, thioacetamide (3.0 mmol) was dissolved in 5 mL of deionized water. The prepared thioacetamide solution was added dropwise to the first solution under continuous stirring and heated for another two hours at same temperature resulting into the formation of precipitate. Then, the solution is cooled to room temperature (RT) and centrifuged followed by washing with distilled water and ethanol. Finally, black colour solid precipitates were collected by drying under vacuum overnight. The conditions of sample preparations are shown in Table S1†. 5 ACS Paragon Plus Environment


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Material characterizations The morphology and composition of the synthesized nanocomposites were investigated by various instruments. The Fourier transform infrared (FT-IR) spectra of the products were recorded in KBr pellets mode on a Perkin Elmer spectrum 400 FT-IR spectrophotometer. High-resolution scanning electron microscopy (HR-SEM) images were obtained from a FEI-NOVA nano400 microscope with X-ray energy dispersive spectroscopy (EDS) capabilities. The sample was coated with thin film of gold for effectual imaging before being charged. JEOL TEM 200 instrument with the accelerating voltage 200 kV was used to record High resolution transmission electron micrographs (HRTEM). UV-Vis data was obtained by Shimadzo UV-2550 spectrophotometer. Powder X-ray diffraction (P-XRD) patterns were taken from Rigaku X-Ray diffractometer and the scanning rate was 2°/minute within 2θ range of 10-80° at a voltage of 10 kV using Cu Kα radiations (λ=1.5418 Å). The X-ray photoelectron spectroscopy (XPS) spectrum was collected using Omicron ESCA (Oxford instrument Germany). Thermogravimetric analysis (TGA) experiment was performed on SDT Q600 instrument. The samples were allowed to heat from 25 °C to 900 °C at a rate of 10 °C min-1 in presence of nitrogen atmosphere. Surface analysis were performed with liquid N2 (77 K) using a Quantachrome Autosorb iQ2 Analyzer. Before Brunauer-Emmett-Teller (BET) analysis sample was degassed at 100 °C for 1015 h. Zeta potentials were obtained using Malvern Zetasizer nano-Z analyzer. All curve fittings were done with Origin 2018 software. Dye removal application Herein, we firstly calculated the adsorption capacity and adsorption efficiency of the prepared Ni-Co-S nanocomposites with various pollutants like Congo red (CR), methyl orange (MO), Methylene blue (MB), Crystal violet (CV), Nile blue A (NB), Rhodamine B (RhB) and K2Cr2O7 [Cr(VI)] (Table S2†). Typically, 5 mg of Ni-Co-S nanocomposite (0.33 g L-1) was added to 15 mL of each solution (concentration 10 to 5000 mg/L) at neutral pH. Then the mixture was introduced into a 50 mL beaker with constant stirring of medium speed (400-700 rpm) at RT. With significant time of interval, required amount of suspension was taken from mixture and the supernatant was collected by centrifugal separation. The final concentration of the adsorbates was evaluated using a UV-Vis spectrophotometer from the wavelength of maximum absorption. The adsorption capacity (Qe) and adsorption efficiency of adsorbates can be evaluated by using following formula respectively: Qe (mg/g) = [(Co- Ce) ×V]/M 6 ACS Paragon Plus Environment

(1)

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Adsorption efficiency (%) = [(Co- Ce)/Co] ×100

(2)

Where Co and Ce represent initial and equilibrium concentration of adsorbates, M is the weight of adsorbent in gram, V is the volume of solution in litre used in the experiment. The adsorption capacity was obtained from the mass balance of the adsorbent in the system.54

RESULTS AND DISCUSSION

Structure and Morphology of Ni-Co-S/SDS nanocomposites In order to find the structure and morphology of prepared nanocomposites, initially XRD, TEM, SEM, EDX, TGA and XPS measurements were performed. The X-ray diffraction (XRD) patterns of nanocomposites (NCS-1 to NCS-5) are shown in Figure 1. The XRD patterns of nanocomposites NCS-1 to NCS-5 are pretty amorphous and peaks are not well defined. In order to determine the probable phase of nanocomposites NCS-4 were calcined at different temperature (Figure S1†). Calcined nanocomposites showed two phases (hexagonal and cubic). The observed characteristic peaks at 26.73°, 31.46°, 38.17°, 50.46°, and 55.08° were assigned to the (220), (311), (400), (511), and (440) diffraction planes, respectively for cubic phase of CoNi2S4 structure (JCPDS no. 24-0334) with the Fd3̄m space group. The peaks of hexagonal phase of Co0.5Ni1S2 (JCPDS no. 01-070-2848) are 30.14°, 35.01°, 45.76° and 53.54° correspond to the (010), (011), (012) and (110) diffraction planes, respectively. The broad peaks of NCS-1 to NCS-5 were observed at the same peak positions as in NCS-4 (500° C) XRD pattern. The broadening of the peaks may be due to small size and poorly crystalline nature of the nanonocomposites. So, it is concluded that the products NCS-1 to NCS-5 might be composed of the CoNi2S4 cubic phase and Co0.5Ni1S2 hexagonal phase.25 Diffraction pattern showed that Co0.5Ni1S2 hexagonal phase are less prominent compared to CoNi2S4 cubic phase.



NCS 1

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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NCS 2 NCS 3 NCS 4 NCS 5 NCS 4 (500° C) Co1Ni2S4 Co0.5Ni1S4

10

20

30

40

50

60

70

80

90

100

2 (Degree) Figure 1. Powder XRD pattern of Ni-Co-S/SDS nanocomposites prepared at various SDS concentrations. For confirming the elemental composition and chemical state of the synthesized nanocomposites NCS-4, X-ray photoelectron spectroscopy (XPS) measurement was conducted and the results are shown in Figure 2(a-d). The calibration of binding energies was done using the C (1s) peak at 284.6 eV. The survey spectrum (Figure 2a) clearly shows the existence of nickel (Ni), cobalt (Co) and sulfur (S). The detected carbon (C) and oxygen (O) corresponds to the SDS surfactant and surface adsorbed water or hydroxyl groups (Figure S2†). The intensities of the Co 2p and Ni 2p peaks were relatively weak (Figure 2b and 2c). The binding energies at 780.50 and 797.02 eV correspond to Co 2p3/2 and Co 2p1/2 of Co3+ respectively. The binding energies at 783.45 and 798.46eV signify the presence of Co2+. Similarly, the peaks at around 855.15, 873.33 and 857.46, 875.83 eV are assigned to Ni2+ and Ni3+ respectively (Figure 2c). In the S 2p spectrum (Figure 2d), the peaks observed around 168.85 eV is attributed to sulphates due to presence of SDS surfactant in nanocomposite. The binding energies at 163.37 and 162.08 eV in the core level spectrum of S 2p region correspond to S 2p1/2 and S 2p3/2 respectively which are in good agreement with the binding energies of metal sulfides (Ni–S and Co–S bonding).55-57 Therefore, the elemental composition of the NCS-4 nanocomposite surface is consisted of Co2+, Co3+, Ni2+, Ni3+ and S2− with SDS.


600

400

(c)

Ni 2p

sat.

200

0

3+

Ni

3+

Ni

Co 2p

2p3/2

2p1/2 sat.

Co

2+

Co Co

3+

2+

Co

3+

sat.

805 800 795 790 785 780 775

Binding energy (eV)

(d)

2p3/2

2p1/2 Ni

Intensity (a.u.)

S 2p

C 1s

Co LMM

800

Binding Energy (eV)

(b)

S 2p

S

2+

Ni

2p1/2

Intensity (a.u.)

1000

NCS-4

Ni LMM

Ni 2p1/2 Co 2p1/2 Ni 2p3/2 Co 2p3/2

O KLL

Intensity (a.u.)

(a)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


O 1s

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2+

sat.

2-

2p3/2

174 172 170 168 166 164 162 160 158 156

885 880 875 870 865 860 855 850

Binding energy (eV)

Binding energy (eV)

Figure 2. X-ray photoelectron spectroscopies (XPS) of NCS-4 nanocomposites, (a) survey scan; (b) Co 2p; (c) Ni 2p and (d) S 2p

The SEM images of NCS-1 to NCS-5 are shown in Figure 3a-e. It shows sphere like structure and uniform morphologies for all synthesized nanocomposites. The particles appear to be of different size in agglomerated form with change is SDS concentration. Besides that the EDS results of all samples ascertain that the nanocomposites are composed of Co, Ni, S as well as (Figure S3†) oxygen (O) and carbon (C). The Co, Ni, S corresponds to nickel cobalt sulphide and oxygen (O) and carbon (C) corresponds to surfactant (SDS) and adsorbed water. The overall elements percentage from EDS analysis of NCS-4 nanocomposite confirmed that nanocomposite is consisted of CoNi2S4 along with surfactant SDS (Table S3†). Further, TEM study confirmed that NCS-4 has uniform morphologies with size varying in the range of 30 to 60 nm (Figure 3f-h). In the higher magnification of TEM image, lattice fringes were observed and average spacing was found to be 0.284 nm corresponding to the (311) planes of cubic CoNi2S4 phase (Figure 3h). Moreover, the selected area electron diffraction (SAED) pattern of NCS-4 showed broad diffuse rings (Figure 3i) which confirms the poor crystallinity of the material and is in good agreement with the broad peaks of PXRD patterns. The SAED pattern also agrees well with the crystal plane of the cubic phase of CoNi2S4 structure with



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PXRD patterns. Finally, based on PXRD, XPS, TEM and EDS studies, it can be confirmed that NCS-4 nanocomposite is mostly composed of cubic phase of CoNi2S4 and SDS.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

Figure 3. Morphological study of Ni-Co-S nanocomposites prepared at 90°C (samples NCS-1 to NCS-5), (a-e) HR-SEM image at low magnification of NCS-1 to NCS-5 at 5µm respectively; (f) HR-TEM image of NCS-4 at 100 nm; (g) HR-TEM image of NCS-4 at 50 nm; (h) HR-TEM image of NCS-4 at 5 nm; (i) SAED pattern.


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Thermogravimetric analysis (TGA) Thermal stability is the important characteristic property of the materials. Thermogravimetric analysis was performed to study the thermal stability of the Ni-Co-S/SDS nanocomposites (Figure 4). The samples were heated from 25°C to 900 °C with a heating rate of 10 °C min-1 and all TGA curves appears nearly similar. Initially, weight loss for NCS-1 to NCS-5 was observed around 100°C which can be assigned to the loss of sample moisture. In the second step, decay occurred from 100 °C to till 550 °C. The weight loss may have occurred due to the decay of SDS which was bonded to the surface of nanocomposites. The third step decay starts at 650 °C which may be ascribed to sublimation of metal sulfides and remaining 30% of the mass may be metals. TGA curves showed that weight loss of NCS-1 to NCS-5 is nearly similar and hence have similar stabilities. But in case of NCS-0, sublimation of metal sulfides starts at 550 °C, which indicates that the stability of NCS-0 is less than the others. These results show that the synergetic interaction between Ni-Co-S and SDS leads to high thermal stability.

110 100

NCS 0 NCS 1 NCS 2 NCS 3 NCS 4 NCS 5

90 80

Weight(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


70 60 50 40 30 20

200

400

600

o

800

Temperature ( C) Figure 4. Thermogravimetric analyses of Ni-Co-S/SDS nanocomposites. Surface area and porosity The surface area has a particular importance in adsorption of dyes because the adsorption takes place at the surface of nanocomposites. The increase in surface area will increase the active sites and in turn will increase the adsorption efficiency of the nanocomposites. The surface area of Ni-Co-S/SDS nanocomposites was measured from the nitrogen adsorption-desorption isotherms (Figure 5). NCS-0, NCS-3, NCS-4 and NCS-5 show surface area values of 12.07, 16.83, 36.98 and 14.18 m2/g and mean pore size values of 2.64, 2.76, 3.95 and 2.53 nm respectively. From the 11 ACS Paragon Plus Environment


data, it is clear that the surface area of nanocomposites first increases with increase in SDS concentration and a decline was observed after further increase concentration. This is may be due the higher agglomeration of the particles.

3 -1

Quantity adsorbed (cm g STP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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160

NCS 0 NCS 3 NCS 4 NCS 5

140 120 100 80 60 40 20 0 0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/Po) Figure 5. The nitrogen adsorption-desorption isotherm of Ni-Co-S/SDS nanocomposites. Adsorption Isotherms We first studied the mechanism of interaction between the adsorbent and the adsorbate by examining the different adsorption isotherms using NCS-4 with varied concentration of adsorbates as shown in Figure 6. UV-Vis absorption spectra (Figure S4†) confirm that adsorption capacity of NCS-4 for all dyes is greater than all prepared nanocomposites so it was selected to evaluate adsorption isotherm. Typically, 5 mg Ni-Co-S nanocomposites were added into 15 mL of solution (500-5000 mg/L for CR, MO, NB, CV; 100-1000 mg/L for MB, RhB and 30-1000 mg/L for Cr(VI) ) and agitated at RT until the equilibrium was reached. The obtained equilibrium adsorption data can be used to evaluate adsorption isotherms by fitting against the Langmuir and Freundlich isotherm models as per equation 3 and 4 respectively.58-59 𝑄𝑒 =

𝑄𝑚𝐾𝐿𝐶𝑒

(3)

1 + 𝐶𝑒𝐾𝐿

𝑄𝑒 = 𝐾𝐹 + 𝐶1/n e

(4)

Where, Ce (mg L−1) and Qe (mg g-1) denotes as the equilibrium concentration of adsorbates after adsorption and the equilibrium adsorption capacity respectively. Qm (mg g-1) refers to maximum adsorption capacity and KL (L mg-1) is the Langmuir constant that indicates the binding 12 ACS Paragon Plus Environment

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energy of adsorption. In Freundlich isotherm, KF and n are the constants which indicate the adsorption capacity and the adsorption intensity respectively. The value of Qm and KL can be evaluated from non-linear curve fitting. According to the Freundlich isotherm, adsorption is supposed to be multilayer, non-ideal and non-uniform distribution of sorption sites with different affinities60 whereas in the Langmuir isotherm adsorption is based on an ideal monolayer and homogenous distribution of identical sorption sites with equivalent energy on the surface of adsorbent.61-62

4500 4000 3500

Qe (mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3000 2500

MO MB CR CV NB RhB Cr(VI)

2000 1500 1000 500 0

0

500 1000 1500 2000 2500 3000 3500 4000

Ce (mg/L) Figure 6. The equilibrium adsorption capacity of different adsorbates with NCS-4. The fitting using the Langmuir adsorption and the Freundlich adsorption model of adsorbates are shown in Figure S5†. The correlation coefficients for Langmuir and Freundlich isotherm model were found to be RL2: 0.90168 for NB, 0.97193 for CR, 0.90736 for CV, 0.98750 for MO, 0.97331 for MB, 0.98602 for RhB, 0.86283 for Cr(VI) and RF2: 0.93483 for NB, 0.85652 for CR, 0.94990 for CV, 0.80867 for MO, 0.95003 for MB, 0.81105 for RhB, 0.95660 for Cr(VI) respectively (Table 1). Thus, we can see that most of the adsorption isotherms are better fitted with the Langmuir model. Moreover, it was found from the Langmuir isotherm results that the maximum adsorption capacity of NCS-4 for NB, CR, CV, MO, MB, RhB and Cr(VI) are 4607.20 mg g-1, 3567.41 mg g-1, 3159.69 mg g-1, 3284.20 mg g-1, 1662.49 mg g-1, 855.38 mg g-1, 597.70 mg g-1, respectively which are somewhat different from the experimental Qe values i.e. 4455.44 mg g-1, 3687 mg g-1, 3459 mg g-1, 3339.4 mg g-1, 1412.92 mg g-1, 766.11 mg g-1, 585.88 mg g-1 respectively. 13 ACS Paragon Plus Environment


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Table 1. Isotherm parameters of the Langmuir and Freundlich model for the adsorption of dyes and Cr(VI) metal ions. Langmuir model Adsorbate

Nature

Qm (mg

KL (L

g-1)

mg-1)

Freundlich model RL2

KF (L mg-1)

n

RF2

NB

Cationic

4607.20

0.00763

0.90168

857.37

4.74

0.93483

CR

Anionic

3567.41

0.02188

0.97193

838.26

5.39

0.85652

CV

Cationic

3159.69

0.06947

0.90736

844.29

5.68

0.94990

MO

Anionic

3284.20

0.02085

0.98750

793.77

5.53

0.80867

MB

Cationic

1662.49

0.01301

0.97331

163.73

2.79

0.95003

RhB

Cationic

855.38

0.02311

0.98602

151.33

3.72

0.81105

Cr(VI)

Ions

597.70

0.02047

0.86283

99.22

3.63

0.95660

The adsorption capacity is very high for NCS-4 with low surface area of 36.98 m2/g. As the Langmuir model is only monolayer adsorption, it may not get such large adsorption capacities. Hence, the Langmuir adsorption model may not be suitable to explain the adsorption mechanism. It was therefore thought worthwhile to fit the adsorption with Zhu and Gu model. The adsorption phenomenon with this model occur in two steps:

63-64

In the first step, interaction between adsorbates and the solid surface of the

adsorbent takes place, whereas the second step includes the formation of hemimicelles at the adsorbed site leading to increase in adsorption. This increase is attributed to the hydrophobic interaction between the adsorbed species. The adsorption isotherm for this type of systems can explained by the following equation (5):63-64 𝑘1𝐶𝑒

𝑘3 𝑛

(

𝑄𝑒 = 𝑄𝑚𝑎𝑥1 + 𝑘 𝐶

1 𝑒

)

+ 𝑘2𝐶𝑛𝑒 - 1

(1 +

)

𝑘2𝐶𝑛𝑒 - 1

(5)

Where Ce (mg L−1) refers as equilibrium concentration of adsorbates after adsorption, Qe (mg g-1) denotes as the equilibrium adsorption capacity, Qmax (mg g-1) is the limiting adsorption capacity at high concentration, k1, k2 and k3 are the equilibrium adsorption constants and n is the average aggregation number of the hemimicelles. It is observed that this model fits better for all adsorbates with the experimental results (R2: 0.97155 for NB, 0.97612 for CR, 0.98100 for CV, 0.98958 for MO, 0.97837 for MB, 0.99682 for RhB, 0.95969 for Cr(VI) ) compared to Langmuir and Freundlich isotherm model (Figure S5-6†) for the optimized parameters in Table 2. As per Zhu and Gu model, it was suggested that k3=1;63 but, in our case it is giving best fit with k3=3, which indicates that one active site of the 14 ACS Paragon Plus Environment

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nanocomposites can adsorb more than one adsorbate molecule.65 The fitting were done by trial and error with nonlinear curve fitting. The maximum adsorption capacities (Qmax) of Zhu and Gu model isotherms of NCS-4 for NB, CR, CV, MO, MB, RhB and Cr(VI) are 4417.79, 3598.23, 3556.04, 3284.08, 1451.64, 773.47 and 583.67 mg g-1 (Figure 7). The adsorption capacity for NB is significantly higher than all other dyes which are primarily because of the higher aggregation number and more active site of the adsorbate which facilitate the hemimicelle formation. Though the specific surface area of Ni-Co-S/SDS nanocomposites is low, our results show extremely higher adsorption capacity than most of the high surface area adsorbents in the recently reported literature (Table S4†). 6000 5000

Qm(mg g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4000 3000 2000

Cr(VI) Rhodamine B Methylene blue Methyl orange Crystal violet Congo red Nile blue

1000 0

Figure 7. The maximum adsorption capacity of various cationic, anionic organic dyes and Cr(VI) ions for the synthesized NCS-4. Table 2. Adsorption isotherm parameters of Zhu and Gu model for different adsorbates. Zhu and Gu model Adsorbate

Qmax(mg g-1)

n

k1

k2

k3

R2

NB

4417.79

6

0.07630

4.24 × 10-14

3

0.97155

CR

3598.23

3

0.02188

3.05 × 10-9

3

0.97612

CV

3556.04

4

0.06947

2.90 × 10-10

3

0.98100

MO

3284.08

3

0.02085

4.67 × 10-10

3

0.98958

MB

1451.64

3

0.01301

3.08 × 10-5

3

0.97837

RhB

773.47

3

0.02311

7.35 × 10-5

3

0.99682

Cr(VI)

583.67

5

0.10611

3.89 × 10-10

3

0.95969



Adsorption Kinetics Since the highest Qm was observed for NB, it was further used to evaluate adsorption kinetics at fixed concentration of 100 mg L-1 with NCS-4 loading of 0.3 g L-1 at RT. To investigate the effect of contact time, the time dependent absorption spectra of NB solutions were taken as shown in Figure S7†. The adsorption rate was initially rapid in the first 10 min and the uptake rate gradually decreases with the increasing time and equilibrium adsorption was attained within 40 min (Figure 8). The types of adsorption kinetics can be explained by using either pseudo-second-order kinetic model or pseudo-first-order kinetic model66-68 and the linearized form of equations can be represented respectively as: 𝑡 𝑞𝑡

=

1

𝑡

𝑘2 𝑞2𝑒

(6)

+ 𝑞𝑒

ln (𝑞𝑒 ― 𝑞𝑡) = ln 𝑞𝑒 ― 𝑘1𝑡

(7)

Where qe (mg g-1) and qt (mg g-1) are the equilibrium adsorption capacity and the equilibrium adsorption capacity at any time t (min) of adsorbate respectively, k1 (min-1) is the rate constant for pseudo-first-order kinetics and k2 (g mg-1 min-1) is rate constant for pseudo-second-order kinetics. The value of qe, k2 and k1 can be calculated from a linear plot of t/qt versus t and ln(qe-qt) versus t respectively.

NB

280 240

qt (mg g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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200 160 120 80 40 0 0

20 40 60

80 100 120 140 160 180 200

t (min) 16 ACS Paragon Plus Environment

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Figure 8. Effect of contact time on the adsorption of Nile blue A using NCS-4. The parameters of pseudo-first and pseudo-second order kinetic models were calculated using equation 6 and 7 and are shown in Table 3. The correlation coefficient (R2) values with the fitted plot of the pseudo-first-order and pseudo-second-order model were found to be 0.96143 and 0.99984 respectively (Figure S8a-b†). On comparison, an excellent fit was obtained with pseudosecond-order model where the theoretical and experimental results are in good agreement with each other. It is therefore concluded that the pseudo-second-order model is more appropriate to explain the adsorption mechanism for NCS-4.69-70 Table 3. Kinetic parameters for the adsorption of Nile blue A by NCS-4. Model Pseudo-first order

Pseudo-second order

Parameters

Values

qe,exp (mg g-1 )

277.08

qe,cal (mg g-1 )

116.6

k1 (min-1)

0.0499

R2

0.9614

qe,cal (mg g-1 )

283.28

k2 (g mg-1 min-1)

0.0011

R2

0.9998

One of the most important aspects of dye adsorption is the performance of the adsorbent towards a dye with a very low concentration in solution. It is believed that most of the adsorbents cannot achieve an effective adsorption at very low concentration of organic dyes in aqueous solutions. To evaluate the removal efficiency at a low concentration, 2 mg nanocomposites are exposed to 15 mL of 10 ppm aqueous solution of dyes. It was found that the adsorptions of most of the dyes (NB, CV, MB and CR) are almost 100%, whereas for RhB and MO, only 60% and 33% removal was observed (Figure S9†). It was further investigated with higher loading of adsorbent. RhB adsorption was nearly 100% but MO could not be removed completely (Figure S10†). Furthermore, the adsorption efficiency at very low concentration (1 ppm and 0.1 ppm solution of NB) was evaluated (Figure S11†). It was found that the adsorbent works well even at very low concentration. The adsorption effects of mixed dyes (anionic and cationic) were also investigated. Both dyes were adsorbed simultaneously (Table S5†) as observed for single dyes and no appreciable effect were seen (Figure S12†).



Recyclability Since NB consists of both R3N+ group and NH2 group, it was chosen for recyclability. The recyclability of NCS-4 was investigated by adsorption–desorption of NB. After adsorption, the dye was desorbed with 5 mL of ethanol and the adsorbent was collected by centrifugation, washed with ethanol and water until dye colour is washed off. Finally, the adsorbent was dried under vacuum and recycled for the next experimentation. The recyclability of NCS-4 was done for 5 times with an adsorption efficiency of greater than 95% (Figure 9). The excellent adsorption capacity, cost effective synthesis and good reusability of the nanocomposites makes the protocol potentially viable for waste water treatment. The PXRD result also shows that there was no change in the peak pattern after five cycles of reuse compared with the fresh one (Figure S13†).

100

Adsorption efficiency(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 31

80 60 40 20 0

1

2

3

4

5

6

Cycle number Figure 9. Recyclability of the NCS-4 nanocomposite for the adsorption of NB. Effect of pH on adsorption The effect of pH on the adsorption capacity of NCS-4 towards MB is shown in Figure 10(a). The adsorption capacity increases strongly with increase in pH and this phenomenon could be understood by the increased negative zeta potential value of NCS-4 with pH (Figure 10b). Under acidic condition, the negative charge density on the surface is relatively decreased therefore the resultant electrostatic interaction decreases between the adsorbents and MB. Similarly, under basic environment the negative charge density on the surface is enhanced causing an increase in the adsorption capacity. The zeta potentials value at neutral pH was -26.8 mV, indicates stability of NCS-4. 18 ACS Paragon Plus Environment

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(a)

(b)

MB

280

0 NCS 4 -10

Zeta potential (mV)

240

Qe(mg/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200 160 120 80 2

4

6

8

10

12

-20

-30

-40

-50

14

2

pH

4

6

8

10

12

pH

Figure 10. (a) The effect of pH on the adsorption capacity of NCS-4 nanocomposite toward MB. (b) Zeta potential at various pH for NCS-4 nanocomposite. Adsorption Mechanisms In general electrostatic interaction, ionic exchange, hydrogen bonding, surface area and coordination with metal are the major contributing interactions in the dye adsorption mechanism. Based on the experimental results, the ultrahigh adsorption capacity of the NCS-4 may be explained by considering more than one interactions mentioned above. As observed from the zeta potential values (Figure 10b), the adsorbent is having the negative surface charge in the pH range from 2 to 12. This negative surface charge of the adsorbent may initiate the electrostatic interactions and play a primary role in the adsorption mechanism for positive dyes, while the other interactions are responsible for the adsorption of negative dyes.71-72 Since the surface area value of the adsorbent is low, its effect is insignificant on adsorption mechanism. Despite having low surface area value (36.98 m2 g-1) of NCS-4, it has ultrahigh adsorption capacity. Since, it has high surface charge which clearly indicates that it is not only the physical adsorption but also the electrostatic and other adsorptions. The NCS-4 exhibits high adsorption capacity towards the cationic dyes NB, MB, CV and RhB. These dyes have quaternary ammonium (R3N+) groups which is responsible for the electrostatic interaction with negative surface charge of NCS-4. Apart from R3N+ groups, some of the dyes contain –NH2, -O- groups which may be responsible for hydrogen bonding with the surface caped water on adsorbent (Scheme 1). The presence of surface water has already been confirmed by XPS, TGA and FTIR (Figure 11a). On the other hand NCS-4 also exhibits high adsorption capacity for a negative dye CR. As CR contains –NH2 groups which can also form hydrogen bonding with the surface caped water. Besides hydrogen bonding, 19 ACS Paragon Plus Environment


Page 20 of 31

an azo group (-N=N-) and –NH2 group are also capable of forming coordinate bond with metals. Since MO consists of only a negative group and an azo group (-N=N-) so its mechanism will only involve metal coordination.73 The adsorption mechanism is similar to isotherm reported by Zhu and Gu which involves four region: (1) initial increase in the rate of adsorption because of the monolayer electro-static interactions, hydrogen bonding and metal coordination between the solid charged surface and the adsorbate molecules as found in the Langmuir adsorption isotherm; (2) in this region, adsorption process continue till isoelectric points appears and the interaction initiated between adsorbate molecules at high concentration of dyes; (3) at higher concentration of adsorbate, second layer adsorption begins where individual adsorbed adsorbate molecules behave as an active site leading to the hemimicelles formation and the adsorption rate increases dramatically; (4) above the critical micelle concentration the surface is largely covered with hemimicelles leading to no further measurable change in adsorption capacity.

Scheme 1. Plausible Mechanism for the Adsorption of NB dye on the surface of adsorbent (n =2 and 3). In order to confirm the interactions, FTIR spectral analysis of NCS-4 before and after dye adsorption was made (Figure 11). It has been observed that abundant H2O was adsorbed on the NCS-4 surface. A peak shift was observed corresponding to stretching frequency of –NH2 group and –OH group 20 ACS Paragon Plus Environment

Page 21 of 31

from 3419 cm-1 to 3380 cm-1 and 3240 to 3230 cm-1 respectively after NB adsorption and from 3440 to 3411 cm-1 for –OH group after MB adsorption. This suggests the existence of H-bonding interactions between –OH groups on the nanocomposites and –NH2 or R3N groups of dyes.74 A peak shift was observed from 1587 to 1581cm-1 for NB and from 1601 to 1596 cm-1 for MB corresponding to stretching frequency aromatic C=C groups which suggest - interactions. NB and CR show extremely high adsorption capacity because of presence of –NH2 groups that responsible for hydrogen bonding formation which enhance the active sites. The other IR bands of MB and MO also swing to smaller wave number than that of pure dyes which may be for interaction between dyes.75 In case of MO (negative dye), the peak at 1120 cm−1 and 1040 cm−1 arise from the vibration of sulfonic group also shifted to 1117 cm−1 and 1034 cm−1 (Figure S14†) respectively after adsorption. It may be indicative of complexation to metal present on nanocomposites surface via unidentate bonding with metal through oxygen.76 High surface charge of NCS-4 nanocomposite implies that the electrostatic coagulation along with adsorption is present at the adsorbent surface.

1624

Transmittance(a.u.)

Transmittance (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3240 1624 3230

1581 1327

3419 3380

4000

3500

(a) NCS 4 (Before) (b) NCS 4 (After) (c) Pure NB

3000

2500

2000

1000

500

885

1361

3440

-1

1100

1627 1488 1326 1596 1384

3411

1587 1336

1500

632

1632 1145

4000

3500

(a) NCS 4 (Before) 1492 1397 889 (d) NCS 4 (After) 1601 (e) Pure MB 3000

2500

2000

1500

1000

500

-1

Wavenumber (cm )

Wavenumber (cm )

Figure 11. FT-IR spectra of (a) NCS-4 before adsorption; (b) NCS-4 after NB adsorption; (c) Pure NB; (d), NCS-4 after MB adsorption and (e) Pure MB.

CONCLUSIONS In summary, we have demonstrated the ultra-high adsorption efficiency of newly developed Ni-Co-S/SDS nanocomposites synthesized at low temperature through a facile and simple approach. The structure and morphology of Ni-Co-S nanocomposites has well described by the support of characterisation techniques. The maximum adsorption capacities of nanocomposites towards NB, CV, MB, RhB (cationic), CR, MO (anionic) dyes and Cr(VI) ions were determined as 4417.79 mg g-1, 3556.04 mg g-1, 1451.64 mg g-1, 773.47 mg g-1, 3598.23 mg 21 ACS Paragon Plus Environment


Page 22 of 31

g-1, 3284.08 mg g-1 and 583.67 mg g-1 respectively. The adsorption of dyes onto nanocomposites followed pseudo-second-order kinetics and an isotherm model similar to the Zhu and Gu model with some modification. Interestingly, the mechanism of adsorption is elucidated with the help of zeta potential, FTIR analysis and pH effect studies. Based on these studies, it is concluded that the hydrogen bonding and electrostatic interactions are the predominating factors for the first step adsorption properties of nanocomposites towards dyes. In the second step, - and other hydrophobic interactions between dyes leads to hemimicelles formation on the nanocomposites surface. The synthetic simplicity, excellent adsorption capacity and reusability open up remarkable possibility for researchers in the field of environmental remediation.

ASSOCIATED CONTENT Supporting Information Synthesis table of nanocomposite, molecular structure of dyes, PXRD pattern at different temperature, XPS analysis, EDS analysis, adsorption kinetics with different adsorbent, Langmuir, Freundlich and Zhu and Gu isotherm model, adsorption kinetic, kinetic model, UV-Vis spectra of several studies, adsorption effect of anionic and cationic mixed organic dyes, PXRD of fresh and reused adsorbent, FTIR spectra NCS-4 before and after adsorption of MO dye and comparison table of adsorption capacity with reported adsorbent.

AUTHOR INFORMATION Corresponding Author *Sahid Hussain, E-mail: [email protected]; Tel: +91-612-302-8022

ACKNOWLEDGEMENTS The authors A. Chowdhury and S. Kumari are thankful to IIT Patna and A. A. Khan is thankful to UGC for their research fellowships. The authors are thankful to Dr. M. Ravichandra for his valuable suggestions. The authors would like to acknowledge the Indian Institute of Technology Patna for financial support. 22 ACS Paragon Plus Environment

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