Copper adsorption onto synthesized nitrilotriacetic acid functionalized Fe3O4 nanoparticles: kinetic, equilibrium and thermodynamic studies

Copper adsorption onto synthesized nitrilotriacetic acid functionalized Fe3O4 nanoparticles: kinetic, equilibrium and thermodynamic studies

G Model JECE 723 1–11 Journal of Environmental Chemical Engineering xxx (2015) xxx–xxx Contents lists available at ScienceDirect Journal of Environ...

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G Model

JECE 723 1–11 Journal of Environmental Chemical Engineering xxx (2015) xxx–xxx

Contents lists available at ScienceDirect

Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece

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Copper adsorption onto synthesized nitrilotriacetic acid functionalized Fe3O4 nanoparticles: kinetic, equilibrium and thermodynamic studies

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Dharmveer Singh, Shalini Verma, Ravindra Kumar Gautam, Vijay Krishna*

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Department of Chemistry, University of Allahabad, Allahabad 211002, India

A R T I C L E I N F O

A B S T R A C T

Article history: Received 2 June 2015 Accepted 27 July 2015

The continuous discharge of heavy metals into the near water bodies causes great harm to the human and aquatic ecosystem. Several adsorbent have been developed and applied for effective removal of metal ions from water. In this paper nitrilotriacetic acid (NTA) functionalized Fe3O4 nanoparticles (Fe3O4-NTA) were developed by coprecipitation method with iron salt and NTA, shown to be effective removal of Cu(II) ions from aqueous solution. The functionalized nanoparticles were characterized by Fourier transform infrared spectroscopy (FTIR), X ray diffractometer (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), Brunauer–Emmett–Teller Surface area analyzer (BET), and pHzpc. The various effecting factors of Cu(II) ions adsorption from aqueous solution such as contact time, adsorbate dose, and pH were investigated. The kinetics models such as pseudo-first-order, pseudo-second-order, Elovich, and intra-particles diffusion were applied on experimental Cu(II) adsorption. The kinetic data showed pseudo-second-order model was found to be better agreement with correlation coefficient, R2 = 0.997–0.999 at 60–12 mg L1. The equilibrium adsorption data modeled with Langmuir, Freundlich, and Dubinin–Radushkevich isotherms found to be better fitted with Langmuir reveal the monolayer adsorption of Cu(II) ions on the surface of Fe3O4-NTA with maximum adsorption capacity of 34.63, 38.8 and 40.24 mg L1 at 298, 303 and 308 K, respectively. The positive value of DH (35.127 kJ mol1) and DS (176.031 kJ mol1 K1) indicates the endothermic nature of Cu(II) adsorption onto Fe3O4-NTA. The negative value of DG (7.365–8.005 kJ mol1) indicates spontaneous nature of adsorption. ã 2015 Published by Elsevier Ltd.

Keywords: AFM Equilibrium isotherms Fe3O4 Nitrilotriacetic acid Kinetic

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Introduction The heavy metals discharged into environment through industrial waste have potential to alter the quality of surface and ground water [1,2], and also accumulate in the body of human being and animal through the food chain [3]. Copper (Cu(II)) has considered as one of the most harmful metal due to their toxicity at ultra concentration and non-biodegradability [4]. Discharge of Cu (II) ions into waste water from different industrial process such as electroplating, paint, metal finishing, mining operations, chemical manufacturing, fertilizers and pigment industries have potential health hazard for human and aquatic ecosystem [5]. Although copper is essential micronutrient for animals and takes part as a cofactor in many metalloproteins and as an activator of some enzyme systems [6]. However, ingestion of excess concentration of Cu(II) causes hepatic and renal damage genetic disorder and it may also causes nausea, vomiting, diarrhoea etc. [7]. The excess

* Corresponding author. E-mail address: [email protected] (V. Krishna).

concentration of copper found in plant to be inhibits their growth and impairs important photosynthetic electron transport [8]. Therefore it is necessary to remove Cu(II) ions from wastewater Q2 before their discharge into the aquatic bodies. Several remediation technologies such as coagulation [9], flocculation [10], bioremediation [11], advance oxidation processes [12], membrane separation [13], and adsorption [14] are existed in practice. So for adsorption have been extensively utilized for the adsorptive removal of Cu(II) ions from aqueous solutions. However, metal adsorption on adsorbents creates secondary problems such as disposal of metal loaded adsorbents, high cost of maintenance, energy consumption, regeneration difficulties, and metal seepage in ground water. During the recent year adsorption on magnetic nanoparticles have gain much popularities due to its high surface area to volume ratio, biocompatibility, high physiochemical stability, ease in operation, and recycling in external magnetic field [15–18]. Many magnetic nanoparticles such as magnetite (Fe3O4) [19], hematite (ɑ-Fe2O3) [20], and goethite (ɑ-FeOOH) [14] have been used for the removal of pollutant from aqueous media. However, bare magnetic nanoparticles easily get oxidize in air. Hence several researchers

http://dx.doi.org/10.1016/j.jece.2015.07.020 2213-3437/ ã 2015 Published by Elsevier Ltd.

Please cite this article in press as: D. Singh, et al., Copper adsorption onto synthesized nitrilotriacetic acid functionalized Fe3O4 nanoparticles: kinetic, equilibrium and thermodynamic studies, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.07.020

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have functionalized magnetic nanoparticles to enhance the physicochemical stability, biocompatibility, surface functionalities, and adsorption capacities [21–22]. Recently, citric acid coated magnetite have successfully utilized for the adsorption of Cd(II) ions from aqueous solutions [1]. Nano-magnetic cellulose hybrid material with fast kinetics and metallic and bimetallic nanoparticles such as spherical iron oxide nanoparticles have been used towards the adsorption of Hg(II), Cu(II), and Ag(I) [16]. 1,6Hexadiamine functionalized magnetic material was successfully applied for Cu(II) adsorption from aqueous solution [23], Some other adsorbent such as lysine functionalized Fe3O4, L-cysteine functionalized spherical iron oxide metallic and bimetallic nanoparticles with low toxicity and biocompatibility was also used for organic pollution removal [24,25]. Nitrilotriacetic acid (NTA) is a non toxic aminopolycarboxylic acid. It is colorless solid that is used as a chelating agent to control of polyvalent metal ions in aqueous solutions by sequestration. NTA is a biodegradable and tripodal tetradentate trianionic ligand used protein isolation and purification. Due to some deflocculating properties and is stable in both strongly acidic and basic solutions over a wide temperature range NTA also used in acid base titration. Yet, nitrilotriacetic acid (NTA) have not used for the adsorption of environmental pollutant from aquatic media. These properties of NTA motivates us to carry out research on the functionalization of magnetite by NTA and for the utilization as an magnetic nanoadsorbent for the removal of Cu(II) ions from aqueous solutions. In the present work, a novel NTA functionalized Fe3O4 magnetic nanoparticles were synthesized, characterized and used as an nanoadsorbent for the removal of Cu(II) ions from aqueous solution. Batch adsorption experiments were conducted using synthetic aqueous solutions of Cu(II) ions and effect of contact time, adsorbate dose, and pH were investigated. The kinetics, equilibrium isotherms, and thermodynamics data of adsorption processes were evaluated to find out the adsorption mechanism of Cu(II) ions onto the Fe3O4-NTA. An attempt was made to regenerate and reuse the Fe3O4-NTA magnetic nanoparticles.

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Materials and method

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Chemicals

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The entire chemicals were analytical grade and used as received without any further purification. The ferric chloride hexahydrate (FeCl36H2O), ferrous sulfate (FeSO47H2O), ammonium hydroxide (NH4OH 25%), sodium hydroxide (NaOH), sodium chloride (NaCl), and ethanol (C2H5OH) were purchased from the Merck (Mumbai). The nitrilotriacetic acid [N(CH2CO2H)3] and copper nitrate trihydrate (CuNO33H2O) were purchased from Aldrich (USA). The high purity potassium nitrate (KNO3) and nitric acid (HNO3) were purchased from fisher scientific (Mumbai) double distilled deionized water was used for preparing solutions.

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Preparation of Fe3O4-NTA

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The previously reported method was applied for synthesis of bare Fe3O4 nanoparticles [26] and functionalized by nitrilotriacetic acid (NTA). Briefly, 6.1 g of FeCl36H2O and 4.2 g of FeSO47H2O were dissolved in 100 mL distilled water and heated at 90  C, and after that separately two solutions, first,10 mL of NH4OH and second, 0.5 g of NTA dissolved in 50 mL of distilled water were added rapidly and sequentially in the above solution. The mixture was stirred at 90  C for 30 min and then cooled to room temperature. The obtained light brown precipitate was centrifuged at 10,000 rpm and the brown color magnetic nanoparticles were separated by an external magnet. The as prepared magnetic nanoparticle was Fe3O4-NTA. The Fe3O4 nanoparticles were

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prepared by the same method without the addition of NTA. The obtained nanomartials were washed with ethanol and double distilled water. The Fe3O4-NTA particle was oven dried at 100  C for 24 hour. The dried Fe3O4-NTA was crushed to the powder in mortar pestle and the obtained nano material kept in the glass bottle for the characterization and adsorption studies.

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Adsorbate

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Copper nitrate trihydrate (CuNO3.3H2O) was used as a adsorbate to investigate the Cu(II) ions adsorption onto Fe3O4NTA in aqueous solution. Copper nitrate stock solutions of 15, 30, 45, and 60 mg L1 were prepared by dissolving the required amount of CuNO3 in double distilled water, and the working solution was prepared according to requirement. The solution pH measured using a pH meter (Eutech instrument pH 510) and absorption studies were carried out using atomic absorption spectrophotometer (Ruilli 130).

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Characterization of Fe3O4-NTA

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The infrared spectrum of Fe3O4 and Fe3O4-NTA was obtained through Fourier transform infrared spectroscopy (FTIR) (PerkinElmer—Spectrum RZX-IFTIR) to determine the surface functional groups. A KBr pellet containing the sample was used for the FTIR Spectroscopic measurements and the spectra was recorded from 400 to 4000 cm1. The crystallinity of Fe3O4 and Fe3O4-NTA were determined with the powder X-ray diffractometer (XRD) at ambient environment with CuKa radiation (l = 0.1542 nm) in the range of 20–70 (2u). SEM study was carried out using a scanning electron microscope (JEOL, JSM 6490 LV) at an electron acceleration voltage of 30 kV. The TEM study was carried out with the help of transmission electron microscopy (Tecnai G2 20 TEM, FEI Neitherlands) The surface area of Fe3O4 and Fe3O4-NTA was measured by N2 adsorption–desorption using Brunauer–Emmett–Teller (BET) surface area analyzer at 77 K (Micromeritics ASAP 2020). Atomic force microscopy (AFM) study of Fe3O4 and Fe3O4-NTA were carried out using atomic force microscopy (Thermo microscopy) in ambient environment under the tapping mode. A silicon nitride tip with 20 nm rolling of curvature was used. The AFM samples were prepared by drop-casting magnetic nanoparticles dispersion onto a fresh mica wafer and then dried under room temperature. The pH at pHzpc of Fe3O4-NTA was determined by the solid addition method [27]. The 50 mL solution of 0.1 N KNO3 was taken in beaker. The series of the initial pH (pHi) from 2 to10 by adding either 0.1 N HCl or 0.1 N NaOH solutions were maintained. The adsorbent dose (0.5 g) was added to 50 mL 0.1N KNO3 solution and the suspension was allowed to stir for one hour. The difference between the initial pH (pHi) and final pH (pHf), i.e., pHf–pHi was plotted against pHi to get zero point charge (pHzpc). The point of intersection of the resulting curve was with the abscissa at which pH 0 was taken as pHzpc. The concentration of Cu(II) ions in the aqueous solution was analyzed with the Atomic Absorption Spectrophotometer (Ruilli 130).

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Adsorption experiments

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The batch experiments were performed for investigation of Cu (II) ions adsorption onto the Fe3O4-NTA nanoparticles [19]. The solution of 50 mL of Cu(II) ions concentration (15, 30, 45, and 60 mg L1) was taken in an Erlenmeyer conical flask and 0.1 g Fe3O4-NTA was added and the mixture was shaken at desired equilibrium time in electrically thermostated reciprocating shaker at 120 rpm. The pH of aqueous solution was adjusted with 0.1 M HNO3 or 0.1 M NaOH solutions as desired. Adsorption experiments

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were conducted at 25, 30, 35, and 40  C (0.5). After the equilibrium time, the adsorbent was separated from the aqueous solutions by an external handheld magnet. The concentration of the residual Cu(II) ion was determined by atomic absorption spectrophotometer (Ruilli 130). The Cu(II) removal and amount of Cu(II) adsorbed were determined as follows: Removalð%Þ ¼

qe ¼ 170 169 171

ðC 0  C e Þ  100 C0

(1)

ðC 0  C e Þ V W

(2)

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where C0 and Ce are initial and equilibrium concentrations of Cu(II) ions (mg L1), respectively. The qe is the equilibrium adsorption capacity (mg g1), V is the volume of solution (L) and W is the mass of adsorbent (g).

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Desorption and regeneration studies

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Desorption study was carried out consequently after the batch adsorption studies. Desorption studies may be helpful to explicate the desorption of adsorbate from adsorbent, which would be helpful to make curial environment for disposal problems. For the desorption study, Cu(II) ions loaded Fe3O4-NTA was suspended in various concentration from 0.01 to 0.20 M of HCl solutions and found 0.15 M HCl solution to be effective in adsorbing Cu(II) ions shown in Table 1. The procedure was repeated for many times until the Cu(II) could not be detected in the filtrate. The desorption filtrates show reasonably higher efficiencies for Cu(II) desorption from Fe3O4-NTA. The reusability of Fe3O4-NTA was examined by conducting the adsorption–desorption process for five cycles for Cu(II) ions. Thus the regenerated adsorbent can be use five times in Cu(II) adsorption.

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Replication of batch experiments

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Each batch adsorption experiment above was conducted in triplicate to obtain reproductive results with error <5%. In the case of deviation larger than 5%, more experiments were performed. The experimental data could be reproduced with accuracy greater than 95%. All the data of batch adsorption experiments listed in Section (Results and discussion) are the average values of three tests.

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Results and discussion

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Characterization

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The FTIR spectra for Fe3O4, Fe3O4-NTA and Ni(II) ions loaded Fe3O4-NTA nanoparticles were recorded in the range 4000– 400 cm1 depicted in Fig. 1. FTIR spectra of Fe3O4 confirmed through the Fe O vibration with strong characteristic peak at 590 cm1 (Fig. 1(a)). The appearance of characteristic peak at 948 cm1 (O H

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Fig. 1. FT-IR spectra of (a) bare nanoparticles Fe3O4, and (b) functionalized nanoparticles Fe3O4-NTA.

bending), 1126 cm1 (C O stretching), 1637 cm1 (C¼O stretch 1 vibration), 1399 cm (CH2 scissoring) and 3259 cm1 (N H stretching), indicating the existence of NTA and also reveals the successfully modification of Fe3O4 surface (Fig. 1(b)). [17,28]. The presence of CO and CH2 groups comes from the ethanol and ultra pure water adsorbed during the washing of magnetic nanoparticles. The Ni(II) ions loaded spectra of magnetic nanoparticles were also measured (Fig. 1(c)) which reveals slightly change in Fe O peaks. The crystalline structure of Fe3O4 and Fe3O4-NTA were identified with XRD patterns shown in Fig. 2. For Fe3O4, different peaks with 2u at 30.07, 35.22, 43.05, 53.72, 56.93, 62.53 were obtained corresponding to their indices 220, 311, 400, 422, 511, 440 respectively indicated the cubic phase of Fe3O4 nanoparticles (Fig. 2(a)). The similar peaks were obtained for Fe3O4-NTA which indicated that the chemical modification of the magnetic core did not make significant change in the phase properties of Fe3O4 cores (Fig. 2(b)) [29]. The SEM images shown in Fig. 3(a,b) reveal the morphology of unloaded surface of Fe3O4-NTA which is dispersed and fully coated with NTA whereas the Fig. 3(c,d) shown the Cu(II) ion loaded images of Fe3O4-NTA. The sizes of particles were not clearly shown in SEM images so for clear particles observation the TEM analyses were

Table 1 Desorption studies of Cu(II) ions containing Fe3O4-NTA at different concentration of acidic solution. Concentration of HCl (M)

% desorption of Cu(II)

0.01 0.025 0.05 0.075 0.1 0.125 0.15 0.20

18.78 24.87 73.67 95.58 98.16 98.34 98.75 99.04

Fig. 2. X-ray diffraction patterns of (a) bare nanoparticles Fe3O4, and (b) functionalized nanoparticles Fe3O4-NTA.

Please cite this article in press as: D. Singh, et al., Copper adsorption onto synthesized nitrilotriacetic acid functionalized Fe3O4 nanoparticles: kinetic, equilibrium and thermodynamic studies, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.07.020

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Fig. 3. SEM images of (a) bare nanoparticles Fe3O4, (b) functionalized nanoparticles Fe3O4-NTA, and (c,d) Cu(II) ion loaded Fe3O4-NTA. 225 226 227 228

carried out. TEM images depicted in Fig. 4 reveal that the NTA functionalized nanoparticles have polycrystalline structure and partials contained aggregates with no uniform mean size of 15– 34 nm [30].

The BET surface areas of Fe3O4 and Fe3O4-NTA were found to be 84.91 and 46.35 m2 g1. Compared to Fe3O4 nanoparticles, the stability of Fe3O4-NTA nanoparticles were enhanced with the decreased specific surface area, which may be caused by

Fig. 4. TEM image of functionalized nanoparticles Fe3O4-NTA.

Please cite this article in press as: D. Singh, et al., Copper adsorption onto synthesized nitrilotriacetic acid functionalized Fe3O4 nanoparticles: kinetic, equilibrium and thermodynamic studies, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.07.020

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aggregation of NTA molecule on the Fe3O4 surface and increased particles size [31]. Due to AFM probes and the precise positional capability, AFM performed to obtain highly resolved spatial three-dimensional images of the surface ultra structure with molecular resolution in real time with minimal sample preparation [32]. The AFM investigation mainly revealed physically dispersed topographic details of magnetic nanoparticles. The Fig. 5(a,c) shown the 2-D structure of bare Fe3O4 and functionalized Fe3O4 whereas Fig. 5(b, d) shown the 3-D structure of bare Fe3O4 and functionalized Fe3O4NTA. The 3D images of functionalized Fe3O4 have shown the isolated nanoparticles with aggregates having quasi spherical shape. In this study information of surface roughness RA were investigated. The average RA values were found to be 0.612667 nm for bare Fe3O4 before functionalization and increased to 0.696743 nm after functionalization, respectively. Thus it is clear that the surface of functionalized particles have more roughness which has significance for adsorption [33]. The pH is a key factor which regulates the surface charge and protonation degree of adsorbent. The charges on surface have significant role in field of adsorption chemistry. The point of zero charge (pHzpc), means the pH at which the adsorbent surface have no potential difference, was determined through solid addition method. The pHzpc of Fe3O4-NTA was found to be 4.85 shows in Fig. 6. The surface charge of adsorbent changed as the solution pH changed due to functional group on the adsorbent surface. The pH of solution found to be below the pHzpc attributes to produce

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Fig. 6. Plots of pHf–pHi vs. pHi of functionalized nanoparticles Fe3O4-NTA at adsorbent dose 0.5 g L1 and temperature 303 K.

positive surface charge and favour the anion adsorption. While pH of solution found to be higher than pHzpc produce negative surface and favour cation adsorption. In this system, the adsorbent Fe3O4NTA has one amino group and two carboxylic groups which would

Fig. 5. 2D AFM image of (a) bare Fe3O4, (b) Fe3O4-NTA, and 3D AFM image of (c) bare Fe3O4, (d) Fe3O4-NTA nanoparticles.

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be participate in Cu complex formation. At pH < pHzpc amino group of nitrilotriacetic acid get protonated and produced positive surface under acidic condition. But at pH > pHzpc the carboxylic group of nitrilotriacetic acid get deprotonated and produced negative surface under basic condition [34].

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Effect of contact time and initial Cu(II) concentration

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Contact time is an important factor in adsorption process which leads the rate of adsorption of adsorbate on the surface of adsorbent in per unit time. The adsorption of Cu(II) ion was found rapid initially and then shown down gradually till the attainment of equilibrium as shown in Fig. 7(a). The adsorption of Cu(II) ions get the equilibrium at 35 min. The equilibrium adsorption capacities were found to be 14.2, 13.19, 12.35, and 11.38 mg g1 for 15, 30, 45, and 60 mg L1, respectively. After 35 min of contact time the adsorption becomes constant. The maximum adsorption percentage of Cu(II) ions were found to be 91.20, 87.98, 84.57, and 80.50 for 15, 30, 45, and 60 mg L1, respectively shown in Fig. 7(b).

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Effect of solution pH

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The pH of solution has prominent influence on the state of surface functional groups of adsorbent and phase of contaminant in the solution [35]. Thus it regulates the adsorption behavior of metal ions. At room temperature, the effect of pH on Cu(II) adsorption by Fe3O4-NTA was studied in the pH range from 2 to 8 at the contact time of 45 min with 0.1 g adsorbent dose. Fig. 8 demonstrates the change of Cu(II) ion adsorption percentage with increasing initial solution pH from 2 to 6 and remained almost variable above 6 pH. At low pH more concentration of H+ ions lead to formation of  NH2 to NH3+, which create electrostatic repulsion between Cu(II) ions and the surface of Fe3O4-NTA [47]. However, at higher pH the OH ions in solution lead to conversion of COOH to COO, which provide electrostatic attraction that was favorable for Cu(II) ion adsorption [29]. The Cu(II) ion in aqueous solution present in several forms, such as Cu(II), Cu(OH)+, Cu(OH)2 Cu(OH)3 and Cu(OH)42 depend on the solution pH. The adsorption of Cu(II) ion at the higher pH means pH > 6 decreases, because of precipitation of Cu(OH)2 ions in solution. Thus it is clear that optimized pH 5 is favorable for Cu(II) adsorption on the Fe3O4NTA surface [28].

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Adsorption kinetics modeling

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In order to investigate the adsorption mechanism and potential rate controlling steps, pseudo-first-order, pseudo-second-order, Elovich, and intra-particles diffusion model were applied to fit the experimental data. The Lagergren form of pseudo-first-order model can be written as Eq. (3) [36]:

303

lnðqe  qt Þ ¼ lnqt  k1 t 1

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304 305 306 307 308

(3) 1

where, k1 (min ) is pseudo-first-order rate constant, qe (mg g ) and qt (mg g1) are the metal uptake per unit weight of adsorbent at equilibrium stage and contact time t (min), respectively. The plot of log (qe–qt) vs. t shown in Fig. 9(a) gives a straight line for the pseudo-first-order kinetic model. The value of rate parameters like k1, qe calculated and R2 have been presented in Table 2. The values of metal uptake qe (cal.) from the plot are found to be less than the experimental qe values. The value of R2 are 0.954, 0.916, 0.943, and

Fig. 7. Effect of contact time (a), Effect of Adsorbate dose (b), on the adsorption of Cu (II) at initial concentration: 15 mg L1, 30 mg L1, 45 mg L1, 60 mg L1 onto 0.1 g adsorbent dose.

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Fig. 8. Effect of pH on the adsorption of Cu(II) at initial concentration: 15 mg L1, 30 mg L1, 45 mg L1, 60 mg L1 onto 0.1 g adsorbent dose.

Please cite this article in press as: D. Singh, et al., Copper adsorption onto synthesized nitrilotriacetic acid functionalized Fe3O4 nanoparticles: kinetic, equilibrium and thermodynamic studies, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.07.020

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Fig. 9. Linear fitting of kinetics (a) pseudo first order, (b) pseudo second, (c) Elovich, (d) inter particles diffusion, for the adsorption of Cu(II) ions at pH 5, adsorbent dose 0.1 g, and contact time 35 min.

Table 2 A comparison of Langmuir, Freundlich equation parameters results for Cu(II) adsorption onto Fe3O4-NTA at different temperature. Cu(II) ion concentration (mg L1)

qe exp (mg g1)

Pseudo-first-order equation

Pseudo-second-order equation 2

k1 (min1)

qe.cal (mg g1)

R

14.4 27.06 37.86 37.80

0.0818 0.07736 0.08199 0.05926

5.122 8.4990 12.924 63.095

0.954 0.916 0.943 0.952

Cu(II) ion concentration (mg L1)

qeexp (mg g1)

Elovich equation

15 30 45 60

14.4 27.06 37.86 37.80

2.79 18.59 8.27 2.99

15 30 45 60

a (mg g1 min2)  104  104  104  104

k2 (gm g1 min1)

qe cal. (mg g1)

R2

0.0345 0.0170 0.0120 0.0075

14.88 28.05 39.70 44.44

0.999 0.998 0.998 0.997

Intra-particles diffusion equation

b (g mg1 min1)

R2

C

k3(mg/g min0.5)

R2

0.8019 0.4811 0.3078 0.2846

0.981 0.954 0.975 0.955

11.64 22.20 30.68 29.32

0.1149 0.2497 0.3786 0.4162

0.993 0.997 0.991 0.998

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0.952 at 15, 30, 45, and 60 mg L1, respectively. The low values of R2 indicate that pseudo-first-order model was not suitable for the adsorption system. The inapplicability of the pseudo-first-order for Cu(II) adsorption were also reported by previous workers [37,38]. The pseudo-second-order model can be expressed as Eq. (4) [39]: t 1 1 ¼ þ t qt k2 q2e qe

(4)

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The rate constant (k2) and equilibrium capacity qe (cal.) were calculated from the linear plot of t/qt vs. t shown in Fig. 9(b). These parameters are listed in Table 2. The values of qe (cal.) agreed perfectly to the experimental values of qe (exp). The high R2 values such as 0.999, 0.998, 0.998, and 0.997 at 15, 30, 45, and 60 mg L1, respectively. The values of R2 indicated suitability of pseudosecond-order model for the Cu(II) ions adsorption onto Fe3O4-NTA. The suitability of pseudo-second-order model reveals the chemisorptions adsorption of Cu(II) ions onto the Fe3O4-NTA for Cu(II) adsorption. The similar observations have been reported in the literature [40,42]. Elovich model [43] used to describe the predominant metal adsorption on the highly heterogeneous surface which can be represented as Eq. (5): 1 1 qt ¼ lnðabÞ þ lnðtÞ

b

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b

365

Adsorption isotherm

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The Langmuir and Freundlich isotherm models were applied to investigate the relationship between the amount of Cu(II) ions adsorption onto the Fe3O4-NTA. The Langmuir isotherm modes can be represented by Eq. (7) [47]:

355 356 357 358 359 360 361 362 363

367 368 369 370

qe ¼ 372 371 373

The value of KL, qm and R2 can be calculated from the slope and intercept of the linear plot of Ce/qe vs. qe demonstrate in Fig. 10(a) and results are listed in Table 3. The qm for Cu(II) ions were obtained 34.63, 38.8 and 40.24 mg g1 at the temperatures 298, 305, and 308 K, respectively. The higher values of R2 were found to be 0.998, 0.998, and 0.999 at 298, 305, and 308 K, respectively. This indicates the applicability of Langmuir isotherm model. The similar results have been reported by [49–52]. The favorability or unfavorability of Langmuir isotherm process for the Cu(II) ion adsorption by Fe3O4-NTA were also determined through the Langmuir separation factor (RL) [53,54], which is defined by the following relationship shows in Eq. (5).

(6)

364

354

qm K L C e 1 þ K L Ce

(7)

where Ce is metal concentration in aqueous phase (mg L1) and qe (mg g1) is the amount of Cu(II) ions adsorbed on per unit weight of

374 375 376 377 378

(8)

(5)

where qt is the quantities of Cu(II) ion adsorbed at time t (mg g1), Kd is the intra particles diffusion constant (mg g1 min1/2). The value of Kd and C were obtained from slope and intercept of plot qt vs. t1/2 shown in Fig. 9(d). The values of the Kd, C, and R2 are given in Table 2. The intercept sign for thickness of boundary layer have higher values shows that the values of Kd is higher than zero that indicates the adsorption process may not be mainly controlled by intra particles diffusion [45]. On the comparison of all four models implied on experimental data of Cu(II) ions adsorption reveals that the correlation coefficient values of pseudo-second-order model best fitted for the experimental kinetics data of Cu(II) ions adsorption onto Fe3O4-NTA, which also suggested that adsorption process is controlled by chemisorptions mechanism [46,28].

353

Ce 1 Ce ¼ þ qe K L qm qm

where a is initial adsorption rate (mg g1 min2) and b is the desorption constant rate (mg1 min1) related to the extent of surface coverage and activation energy for chemisorptions (g mg1). The obtained values of a and b from the intercept and slope of plot of qt vs. ln t shows in Fig. 9(c). The values of rate constant have been presented in Table 2. The R2 were found to be 0.981, 0.954, 0.975, and 0.955 at 15, 30, 45, and 60 mg L1, respectively, are lower than the pseudo-second-order model. The Intra-particles diffusion model is generally used to investigate the rate limiting of reaction. which can be represented by the Eq. (6) [44]: qt ¼ K d t1=2 þ C

351 350 352

Fe3O4-NTA at equilibrium. The qm is the maximum adsorption capacity (mg g1) reflected on a complete monolayer adsorption and KL is the Langmuir constant (L mg1) related to adsorption energy. The Langmuir isotherm parameters can be obtained from its linearized form as represented in Eq. (4) [48]:

Fig. 10. Linear fitting of adsorption isotherm models (a) Langmuir isotherm, and (b) Freundlich isotherm for the adsorption of Cu(II) ions at pH 5, adsorbent dose 0.1 g, and contact time 35 min.

Please cite this article in press as: D. Singh, et al., Copper adsorption onto synthesized nitrilotriacetic acid functionalized Fe3O4 nanoparticles: kinetic, equilibrium and thermodynamic studies, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.07.020

380 379 381 382 383 384 385 386 387 388 389 390

Q3 391

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9

Table 3 A comparisons of pseudo-first-order, pseudo-second-order, Elovich and Intra-particles parameters of Cu(II) adsorpton onto Fe3O4-NTA at different initial concentration. Temp. (K)

298 303 308

RL ¼ 393 392 394 395 396

397 398 399 400 401 402

Langmuir

Freundlich

qm

KL(L mg

34.63 38.80 40.24

1.087 1.00 0.991

1

)

RL Range

R

0.0577-0.0153 0.0625-0.0163 0.0672-0.0012

0.998 0.998 0.999

1 1 þ K L C0

(9)

where C0 is the initial concentration (mg L1) and KL is the Langmuir constant (L g1). The parameter RL indicates the shape of isotherm accordingly: RL > 1 means adsorption is unfavorable, R L = 1 means linear adsorption, 0 < RL < 1 means adsorption is favorable and RL = 0 adsorption is irreversible. The calculated RL value for different initial Cu(II) ions concentration were found to be 0 < RL < 1, listed in Table 3 indicated the favorability of Cu(II) ion adsorption onto Fe3O4-NTA. The linear form of Freundlich isotherm model is generally express as follows (Eq. (10)) [55]: 1 logqe ¼ logK F þ logC e n

404 403 405 406 407 408 409 410 411 412 413 414 415 416

(10)

where KF is the Freundlich constant (mg g1) and n the adsorption intensity. The value of KF and n were calculated through the intercept and slope of plot ln qe vs. ln Ce (Fig. 10(b). The obtained n values for Cu(II) ions adsorption are 3.55, 3.56, and 3.57 mg g1 at 298, 305, and 308 K, respectively. These values are in the range 1 < n > 10 which is favorable for the adsorption process. The R2 values found to be 0.940, 0.934, and 0.998 at 298, 305, and 308 K, respectively. Dubinin–Radushkevich isotherm is an empirical model generally applied to express the nature of adsorption mechanism i.e. physical or chemical with a Gaussian energy distribution onto a heterogeneous surface. The model is represented by the following Eqs. ((11), (12)) [56,57]:   (11) lnqe ¼ lnQ D  BD e2

420 421 422 423 424 425

Dubinin–Radushkevich

n

KF

R

QD

E

R2

3.556 3.568 3.572

16.943 17.758 18.36

0.940 0.934 0.942

28.21 31.32 37.32

9.213 9.876 10.118

0.942 0.963 0.983

2

The obtained QD values for Cu(II) ions adsorption are 28.21, 31.32, and 37.32 mg g1 at 298, 305, and 308 K, respectively (Table 3). The magnitude of E is useful for estimating the type of sorption reaction, If E < 8 kJ mol1, the adsorption process was physical in nature and in the 8–16 kJ mol1 range and it was chemical in nature [58]. The calculated values of E are found to be 9.213, 9.876 and 10.118 kJ mol1 at 298, 305, and 308 K, respectively, which suggested that the adsorption process was chemical in nature. For comparison, the resulting correlation coefficients (R2) of the applied three models at three temperatures were lower than those of Langmuir model. The better fitting of Langmuir isotherm model indicated the homogeneous adsorption surface with all the adsorption sites having equal adsorption affinity [59].

427

Thermodynamics studies

441

The thermodynamics parameter were applied to investigate the nature of Cu(II) adsorption on the surface of Fe3O4-NTA. The thermodynamics parameter viz. change in Gibbs free energy (DG ) (kJ mol1), standard enthalpy change (DH ) (kJ mol1) and the standard entropy change (DS ) (J k1 mol1) were investigated at different temperatures 298, 305, and 308 K to determining the feasibility, spontaneity and heat change of the adsorption process. These parameters have been calculated using the following equations Eqs. (14–17) [60,61]:

442

DG ¼ RTlnK d Kd ¼

  1 e ¼ RTln 1 þ Ce 418 417 419

2

428 429 430 431 432 433 434 435 436 437 438 439 440

443 444 445 446 447 448 449 450

(14)

qe Ce

(15)

(12)

where qe and QD are the amount of adsorbate in the adsorption at equilibrium (mg g1) and theoretical isotherm saturation capacity (mg g1), BD is the Dubinin–Radushkevich isotherm constant (mol2 KJ2), e is the Polanyi potential, and R, T and Ce are represent the gas constant (8.314 J mol1 K1), adsorbate temperature (K), and adsorbate equilibrium constant (mg L1), respectively. The mean energy of adsorption, E (kJ mol1), is calculated by the following equation " # 1 (13) E ¼ pffiffiffiffiffiffiffiffiffi 2BD

426

Table 4 Thermodynamics parameter for Cu(II) adsorption onto Fe3O4-NTA at different temperature. Temperature (K)

DG (kJ mol1)

DH (kJ mol1)

DS (kJ mol1 K1)

298 303 308

7.3653 7.7156 8.0058

35.127

176.031

DG ¼ DH  T DS

lnK d ¼

DS  R



(16)

DH 

(17)

RT 1

where R is the universal gas constant (8.314 J mol K), T is the absolute temperature in Kelvin, Kd is distribution coefficient for Cu (II) ions distribution between the solid and liquid phases at equilibrium. The values of DH and DS were calculated from the slopes and intercepts of the plot log Kc versus 1/T. The various thermodynamic parameters at the three temperatures studied are given in Table 4. The positive value of DH (35.127 kJ mol1) and DS (176.031 kJ mol1 K1) indicates the endothermic nature of Cu (II) adsorption onto Fe3O4-NTA. The negative value of DG (7.365– 8.005 kJ mol-1) indicates spontaneous nature of adsorption [62].

452 451 453

Conclusions

461

In this study novel NTA functionalized Fe3O4 was successfully synthesized and characterized through various adsorbent techniques such as FTIR, XRD, SEM, TEM, BET, pHzpc and AFM analysis. The functionalized adsorbent then used for Cu(II) adsorption from

462

Please cite this article in press as: D. Singh, et al., Copper adsorption onto synthesized nitrilotriacetic acid functionalized Fe3O4 nanoparticles: kinetic, equilibrium and thermodynamic studies, J. Environ. Chem. Eng. (2015), http://dx.doi.org/10.1016/j.jece.2015.07.020

454 455 456 457 458

459 460

463 464 465

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481

aqueous solution. The result demonstrated that removal efficiency was highly pH dependent and maximum adsorption obtained at pH 5. The adsorption of Cu(II) reached equilibrium rapidly with 35 min and high adsorption affinity adsorption achieved by electrostatic attraction between Cu(II) and COOH group of Fe3O4-NTA surface. The adsorption data followed the Langmuir isotherm model which is best fitted for linearity of different concentration solution of metal and assumed a monolayer coverage and uniform activity distribution on the adsorbent surface. The experimental data is giving best linear fitted with pseudo-second-order describe chemisorptions of Cu(II) ions onto Fe3O4-NTA. Thermodynamic studies shown that the adsorption process of Cu(II) was feasible spontaneous and endothermic in nature. The Cu(II) ion desorption by 0.15 M HCl solution suggested that Fe3O4-NTA should be promising and renewable adsorbent for the Cu(II) removal aqueous solution.

482 Q4

Uncited reference

467 468 469 470 471 472 473 474 475 476 477 478 479 480

483

[41].

484

Acknowledgements

485

492

D. Singh and R.K.G are thankful to the University Grants Commission; New Delhi for the award of Senior Research Fellowship (SRF). The authors are thankful to the Head, Department of Chemistry, University of Allahabad for providing the lab facility. The authors are also grateful to Nano Application Centre; University of Allahabad for SEM and XRD analysis, SAIF; Punjab University Chandigarh for FTIR analysis and Institute Instrumentation Centre; IIT Roorkee for TEM and AFM analysis.

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