Effective Removal of Heavy Metal Ions from Industrial Wastes Using

Article pubs.acs.org/IECR

Effective Removal of Heavy Metal Ions from Industrial Wastes Using Thiosalicylhydrazide-Modified Magnetic Nanoparticles Kiomars Zargoosh,* Hamed Abedini, Amir Abdolmaleki, and Mohammad Reza Molavian Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran ABSTRACT: A novel magnetic nanoadsorbent has been synthesized by the covalent immobilization of thiosalicylhydrazide on the surface of Fe3O4 nanoparticles. Size, structure, magnetic property, and porosity of the prepared magnetic nanoparticles (MNPs) were studied by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), Fourier transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD) analysis. The ability of the prepared MNPs for removing heavy metals ions (Pb2+, Cd2+, Cu2+, Zn2+, and Co2+) from industrial wastes was studied, and the effects of different affecting parameters on the adsorption characteristics of the modified MNPs were investigated. The maximum adsorption capacities of Pb2+, Cd2+, Cu2+, Zn2+, and Co2+ were found to be 188.7, 107.5, 76.9, 51.3, and 27.7 mg g−1, respectively. Excellent adsorption capacity of the modified nanoadsorbent together with other advantages such as reusability, easy separation, environmentally friendly composition, and freedom of interferences of alkaline earth metal ions make them suitable adsorbents for removal of heavy metal ions from environmental and industrial wastes.

1. INTRODUCTION Bioaccumulation of the heavy metal ions in the living cells has serious adverse effects on the organ functions in humans and animals. Today, it is well documented that long-term exposure to heavy metal ions can cause different progressive diseases, such as cardiovascular deficiencies, lung problems, bone lesions, neurological damages, hypertension, and cancer.1−6 On the other hand, heavy metals have wide applications in different industrial activities such as chemical, painting, plumbing, and plating; thus, effective removal of heavy metal ions from industrial wastes is of great importance. Various types of materials such as ion exchange resins,7 hydrogels,8 biomimetic compounds,9 biopolymers,10 activated carbon,11 and nanosized adsorbents12−14 have been used for the removal of these toxic metal ions. Iron oxide nanoadsorbents such as hematite (α-Fe2O3), maghemite (γ-Fe2O3), and magnetite (Fe3O4) have been widely used by researchers for removal of different pollutants such as As(V), Cr(VI), Cr2O72−, MnO4−, Cu2+, Pb2+, and Hg2+ from environmental or industrial wastes.15−17 Compared to the traditional adsorbents, nanosized iron oxides have several advantages. First, they are environmentally friendly adsorbents.18 Second, their interactions with adsorbates are fast; thus, contaminants can be adsorbed onto nanosized iron oxides in practically acceptable times.19 Third, magnetic iron oxide nanoparticles can be easily separated from aqueous samples by applying an external magnetic field.20 However, there are technical problems that limited the practical use of iron oxide nanoadsorbents for removal of heavy metal ions from industrial and environmental samples with complex matrixes. First, in aqueous solutions, some of the iron oxide magnetic nanoparticles (MNPs) are unstable and usually tend to aggregate and, thereafter, greatly weaken their efficiency for environmental application.13 Second, the interactions between unfunctionalized iron oxides and metal ions are often irreversible.21 Third, other species such as phosphates also © XXXX American Chemical Society

adsorb well and can out-compete metal ions for sorption sites due to their high concentrations in groundwater.22 Fourth, the most common active sites of the functionalized iron oxide nanoadsorbents are hydroxyl and carboxyl groups; thus, nonheavy metal ions such as Mg2+ and Ca2+ (coexisting ions in real samples) can strongly interact with binding sites and occupy them.23 To prevent the aggregate formation, different processes such as immobilization of the polymer shell on the nanoparticles and dispersion of nanoparticles into conventional porous materials (such as diatomite or granular activated carbon) have been proposed by researchers.23−25 To reduce the interferences of the alkaline earth metal ions on the adsorption of heavy metal ions, the surface of the iron oxide nanoparticles must be modified with suitable ligating agents containing soft donors such as nitrogen and sulfur atoms (to enhance the soft−soft interactions between active sites and heavy metal ions). This paper describes the covalent immobilization of poly (acrylic acid) (PAA) and a ligating agent thiosalicylhydrazide on the surfaces of Fe3O4 MNPs. Immobilization of the polymer shell on the Fe3O4 MNPs can reduce the interactions between nanoparticles and, hence, prevent the aggregate formation as reported previously.23 On the other hand, the presence of soft donor sites such as nitrogen and sulfur atoms in the structure of the immobilized thiosalicylhydrazide can considerably enhance the interactions between adsorbent and heavy metal ions via inner-sphere reaction and, hence, reduce the interferences of the alkaline earth metal ions. Size, structure, and porosity of the modified nanoparticles were studied by scanning electron microscopy (SEM), highReceived: June 22, 2013 Revised: September 20, 2013 Accepted: September 26, 2013

A

dx.doi.org/10.1021/ie401971w | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

Article

cold water. Yield 92%, 0.310 g; FT-IR (KBr): 3262, 3147, 3046, 2940, 2591, 1608, 1583, 1498, 1096 cm−1. Anal. Calcd. (%) for C7H8N2OS: C, 49.98; H, 4.79; N, 16.65; found: C, 49.72; H, 4.83; N, 16.51.

resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), Fourier transform infrared (FT-IR) spectroscopy, Brunauer−Emmett−Teller (BET) method, and X-ray diffraction analysis. Then, the adsorption capacity of the modified nanoparticles for heavy metal ions Pb2+, Co2+, Cd2+, Zn2+, and Cu2+ in different pH solutions was investigated. In addition, the effects of affecting parameters such as metal ion concentration, background electrolytes, and contact time on the heavy metal ion uptake capacity of the prepared nanoparticles were explored. X-ray photoelectron spectroscopy (XPS) data together with adsorption kinetics and isotherms were used to investigate the mechanism of the adsorption of metal ions on the surface of the prepared adsorbent.

Scheme 1. Synthesis of Thiosalicylhydrazide

2.4. Synthesis of Poly Acrylic Acid (PAA). A previously reported method was used for preparation of poly acrylic acid (PAA).27 Briefly, a 100 g weight of acrylic acid was introduced into a calibrated flask placed in an ice bath and neutralized to pH 7 by NaOH solution. Thereafter, 0.575 g of NaNO2 was added, and the content of the flask was diluted to 250 mL and flushed with a stream of argon. From the prepared mixture, aliquots of 20 mL (8 g of acrylic acid) were taken and transferred into vials which subsequently were installed in a thermostatted bath. The filled vials were thermostatted at 60 °C for 30 min, whereupon (NH4)2S2O8 was added into an amount of 0.253 g. The molar ratio of NaNO2 vs (NH4)2S2O8 was held constant at 1.5. The vials were sealed and slowly heated (during 15 min) until a temperature of 90 °C was reached. Thereafter, the vials were chilled and opened, and the contents precipitated into 7-fold volumes of acetone. Each sediment was brought quantitatively onto a filter, washed, and finally dried to constant weight. 2.5. Preparation of Fe3O4 MNPs. Fe3O4 magnetic nanoparticles (MNPs) were prepared by a chemical coprecipitation method.28 Briefly, FeCl3 (10.8 g) and FeCl2 (4.0 g) were dissolved in degassed hydrochloric acid (2 mol L−1, 50 mL) in a flask at room temperature under sonication. All the solutions were degassed using a vacuum pump and filled with nitrogen gas. The contents of the flask were stirred for 10 min before aqueous ammonia (28%, 80 mL) was injected slowly over 1 h into the mixture under nitrogen while stirring at room temperature. The resulting solid was rinsed with deionized water (3 × 60 mL) and resuspended in deionized water (100 mL). The Fe3O4 MNPs (0.4 g) obtained were rinsed with ethanol several times and dried at 60 °C under vacuum for 3 h. 2.6. Immobilization of Poly Acrylic Acid and Thiosalicylhydrazide on the Fe3O4 MNPs. A previously reported method was used for preparation of PAA-coated Fe3O4 MNPs.29 For covalent immobilization of thiosalicylhydrazide on the PAAcoated Fe3O4 MNPs, the prepared PAA-coated Fe3O4 MNPs were first mixed with 5.0 mL of buffer A (0.003 mol L−1 phosphate, pH 6, 0.1 mol L−1 NaCl) solution and 2.5 mL of 1,1′-carbonyldiimidazole solution (0.03 g mL−1 in buffer A). After being sonicated for 10 min, the reaction mixture was mixed with 50 mL of thiosalicylhydrazide (0.05 mol L−1) and sonicated for another 20 min. Again, 2.5 mL of 1,1′-carbonyldiimidazole solution was added to the reaction mixture, and the resulted mixture was refluxed for 12 h. Finally, the prepared MNPs were magnetically recovered and washed with ethanol and then dried in a vacuum oven. Figure 1 shows the schematic steps for preparation of Fe3O4@PAA@TSH nanoparticles and a possible mechanism for adsorption of heavy metal ions by this system. 2.7. Adsorption Measurement. The adsorption properties of the prepared Fe3O4@PAA@TSH MNPs for heavy metal ions Co2+, Pb2+, Cd2+, Zn2+, and Cu2+ were measured in batch

2. EXPERIMENTAL SECTION 2.1. Reagents and Solutions. All chemicals and reagents were analytical grade. Iron(III) chloride (FeCl3·6H2O), iron(II) chloride (FeCl2·4H2O), cobalt nitrate (Co(NO3)2·6H2O), copper chloride (CuCl2·2H2O), cadmium chloride (CdCl2· 2.5H2O), zinc chloride (ZnCl2), and lead nitrate (Pb(NO3)2) were purchased from Merck. Ammonium persulfate, sodium nitrite, ammonia, acetone, 1,1′-carbonyldiimidazole, acrylic acid, salicylic acid, thionyl chloride, dichloromethane, and hydrazine hydrate were purchased from Sigma-Aldrich. Doubly distilled deionized water was used throughout. 2.2. Apparatus. A Perkin-Elmer 2380-Waltham flame atomic absorption spectrophotometer (FAAS) was used for determination of the metal ions concentration. A Jenway (USA) model 3020 pH meter with a combined glass electrode was used after calibration against standard Merck buffers for pH determinations. A totally glass Fisons (UK) double distiller was used for preparation of doubly distilled water. Field-emission scanning electron microscopy (FE-SEM) S-4160 Hitachi (Japan) and atomic force microscopy (AFM) Nanos Bruker (Germany) were used to investigate the morphology and size distribution of the prepared nanoparticles. Fourier transform infrared (FT-IR) spectra were obtained with a Bruker model Eqvinox 55 LS101 FT-IR spectrophotometer. X-ray diffraction (XRD) measurements were carried using a Siemens D-5000 Xray diffractometer (Germany) with Cu Kα radiation. Magnetite properties of the prepared nanoparticles were determined using a vibrating sample magnetometer (VSM) Kavir Kashan (Iran). Isotherms of the nitrogen adsorption−desorption were recorded on a Micromeritics ASAP 2020 apparatus (USA). A Kratos Axis Ultra DLD spectrometer with a monochromatic Al Kα-X-ray source was used for recording the XPS spectra. 2.3. Synthesis of Thiosalicylhydrazide (TSH). A previously reported method with some modifications was used for preparation of thiosalicylhydrazide.26 The 25 mL round-bottom flask with a magnetic stirring bar was charged with thiosalicylic acid 1 (0.304 g, 2 mmol), thionyl chloride (0.7 mL, 10 mmol), and CH2Cl2 (10 mL) and then refluxed for 2 h. After removal of the solvent under reduced pressure, the resulting thiosalicyl chloride 2 was used in the next reaction. Yield 95%, 0.325 g; FTIR (KBr): 3186, 3046, 2940, 2584, 1772, 1616, 1581, 1495, 1093 cm−1. Anal. Calcd. (%) for C7H5ClOS: C, 48.70; H, 2.92; found: C, 48.42; H, 2.79. Thiosalicyl chloride (0.342 g, 1 mmol), hydrazine hydrate (0.66 mL, 10 mmol), and CH2Cl2 (10 mL) was charged to a 25 mL flask and then refluxed for 14 h at 60 °C. Thiosalicylhydrazide (TSH) 3 was precipitated as a light-lemon-yellow-colored solid by cooling down the solution to 0 °C and washed with 100 mL of B



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Figure 1. Scheme for the preparation of Fe3O4@PAA@TSH MNPs and a possible mechanism for adsorption of metal ions on them.

Figure 2. SEM image of the prepared MNPs. (a) Fe3O4, (b) Fe3O4@PAA, and (c) Fe3O4@PAA@TSH.

Figure 3. AFM image of the Fe3O4@PAA@TSH MNPs (a). TEM image of the Fe3O4@PAA@TSH MNPs (b).

experiments. We studied the effect of pH (1.0−6.0), kinetics time (0−90 min), and adsorption isotherms (initial concentration 20−450 mg L−1) of the studied metal ions. We also investigated the effects of alkaline/earth metal ion concentrations (0−0.3 mol L−1). Analyzing adsorption behavior of the MNPs involved adding 0.050 g of Fe3O4@PAA@TSH to 50 mL of solution of each metal ion at different concentrations at room temperature. The pH was maintained at a constant value during adsorption. The equilibrium time was less than 45 min. When the adsorption behavior reached equilibrium, the adsorbents were separated by powerful magnets. The concentrations of the studied heavy metal ions in aqueous phase were determined using a Perkin-Elmer 2380-Waltham flame atomic absorption spectrophotometer (FAAS). Metal ion concentrations adsorbed per unit mass of the MNPs (mg metal

ion per g dry MNPs) were calculated by using eq 1. Removal efficiencies (%Re) were calculated by using eq 2. qe =

(C 0 − Ce) × v m × 1000

%Re =

(C 0 − Ce) C0

× 100

(1)

(2)

0

where C and Ce are the concentrations of the metal ions in the aqueous phase before and after the adsorption period, respectively (mg L−1); v is the volume of the aqueous phase (mL), and m is the amount of dry MNPs used (g). 2.8. Recovery and Reuse. Metal ion (0.05 g) loaded MNPs were stirred with (10 mL, 0.1 mol L−1) HCl solution at room temperature for 3 h to desorb the metal ions. The final metal ion C



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ization curves (Figure 6). The saturation magnetization of the naked Fe3O4, Fe3O4@PAA, and Fe3O4@PAA@TSH MNPs was 71, 62, and 53 emu g−1, respectively. The significant reduction in saturation magnetization may be due to the amount of nonmagnetic PAA and PAA-TSH deposited on the Fe3O4 MNPs.23,30 These values are apparently higher than those previously reported for Fe2O3 CAHNs (2.1emu·g−1), γ-Fe2O3 CHNs (22.1 emu·g−1), and amorphous Fe2O3 nanoparticles (0.9 emu·g−1) and comparable with those reported for Fe3O4 CHNs (69.97 emu·g−1).31−33 Magnetic separation of the Fe3O4@PAA@TSH MNPs from the treated heavy metal ion solutions was performed by a magnet with a magnetic field intensity of 0.2 T. It was found that Fe3O4@ PAA@TSH MNPs can be completely separated from solution in less than 20 s. Fast and easy separation of Fe3O4@PAA@TSH MNPs from solutions make them a valuable adsorbent for water treatments. Although several parameters such as shape, size, porosity, and shell have been identified as affecting factors on the magnetic properties of the γ-Fe2O3 and Fe3O4 nanoparticles,31−33 it seems that intrinsic magnetic characteristics of these particles play the most important role in the magnetic separation of these particles from solution. On the basis of the previous reports, it can be concluded that Fe3O4 nanoparticles show better magnetic separation behavior rather than γ-Fe2O3 and amorphous Fe2O3 nanoparticles. For example, Mayo et al. reported that laboratory prepared 12 nm Fe3O4 nanocrystals can be completely retained in columns by applying magnetic field intensity of about 0.1 T.34 Also, effective magnetic separation of 4−12 nm Fe 3 O 4 nanocrystals from solutions has been reported by Yavuz et al.21 On the other hand, problematic magnetic separations have been reported for 10 nm nonporous γ-Fe2O3 and amorphous Fe2O3 nanoparticles from solutions.31,35 3.2. Adsorption Properties of the Modified Fe3O4 MNPs for Metal Ions. 3.2.1. Effect of pH. Figure 7 shows the influence of pH of test solution on the adsorption of heavy metal ions by Fe3O4@PAA@TSH MNPs at pH 1−6. As it is seen, there are significant increases in removal efficiency of the studied metal ions with increasing pH values of the test solution from 1 to about 4.5. While beyond this pH range, the removal efficiency values remain almost constant. The significant enhancement in removal efficiency with increasing pH values from 1 to 4.5 is most probably due to deprotonation of the carboxyl and sulfur groups on the surface of the adsorbent and, hence, increases the binding ability of the active sites toward metal ions. To avoid the risk of the hydroxide precipitation of the studied metal ions, in the subsequent experiments, solutions of pH 5.2 adjusted by the use of HCl or NaOH were used. 3.2.2. Adsorption Kinetics. The rate of the adsorption is one of the most important characteristics of an adsorbent. Figure 8 shows the effects of contact time on the adsorption of the metal ions. As seen, the metal ions all rapidly reached equilibrium at 40 min. In fact, 95% of the metal ions were adsorbed at about 25 min. Compared with other reported adsorbents, the modified Fe3O4 MNPs show fast adsorption.30,36 This fast adsorption could be attributed to the larger surface area of modified MNPs and the higher affinity of the immobilized sulfur and amine groups for adsorbing heavy metal ions. The adsorption kinetics of metal ions with Fe3O4@PAA@TSH MNPs were investigated by Lagergren pseudo-first-order (eq 3) and pseudo-second-order (eq 4) models:37

concentration in the aqueous phase was determined by using a FAAS. Thereafter, the MNPs were neutralized with dilute NaOH, washed with deionized water, and again subjected to adsorption processes to determine the reusability of the MNPs. Adsorption−desorption cycles were repeated four times by using the same MNPs.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Prepared MNPs. Figure 2 shows the SEM images of the obtained Fe3O4 nanoparticles before and after immobilization of PAA and TSH on them. As is clear from SEM images, the diameters of the Fe3O4 nanoparticles are about 25−35 nm and Fe3O4 nanoparticles have homogeneous size. An AFM image of the synthesized Fe3O4@PAA@ TSH MNPs together with a TEM image have been depicted in Figure 3. As is clear from Figure 3, the diameters of the prepared nanoparticles are in the range of 25−35 nm and have homogeneous size. It is immediately obvious that there is a satisfactory agreement between the results obtained by SEM, AFM, and TEM. The surface modification of the Fe3O4 MNPs was confirmed by FT-IR. The FT-IR spectra of the Fe3O4, Fe3O4@PAA, TSH, and Fe3O4@PAA@TSH have been depicted in Figure 4a−d,

Figure 4. FT-IR spectra for (a) Fe3O4; (b) Fe3O4@PAA; (c) TSH; (d) Fe3O4@PAA@TSH.

respectively. As is clear from Figure 4a, peaks of 500−750 cm−1 belong to the Fe3O4.23 As can be seen from Figure 4b, the introduction of PAA to the surface of MNPs can be confirmed by bands at 1730 cm−1 (stretching vibration of carbonyl group), 1460 cm−1 (bending vibration of CH2), and 1320 cm−1 (stretching vibration of C−O group). The similarity between Figure 4c,d spectra clearly confirms that TSH has been successfully bonded to the surface. In Figure 4c,d, three strong bands at 1470, 2569, and 3307 cm−1 are ascribed to the benzene (stretching vibration), S−H (stretching vibration), and N−H (stretching vibration) groups, respectively. The XRD patterns of the Fe3O4, Fe3O4@PAA, and Fe3O4@ PAA@TSH have been depicted in Figure 5. The reflection peak positions and relative intensities of the MNPs agree well with XRD patterns for MNPs in the literature.23,29 The 3 types of MNPs possessed superparamagnetic properties, which could be seen from the room-temperature magnet-

ln(qe − qt ) = ln(qe) − k1t D

(3)



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Figure 5. The XRD patterns of the synthesized MNPs. (a) Fe3O4, (b) Fe3O4@PAA, and (c) Fe3O4@PAA@TSH.

Figure 6. Room-temperature magnetization curves of the prepared MNPs. (a) Fe3O4, (b) Fe3O4@PAA, and (c) Fe3O4@PAA@TSH.

⎛ ⎞ ⎛1⎞ t 1 ⎟ = ⎜⎜ ⎟⎟t + ⎜⎜ 2⎟ qt ⎝ qe ⎠ ⎝ k 2 × qe ⎠ −1

with this fact that chelating functional groups on the surface of the Fe3O4@PAA@TSH MNPs can adsorb the metal ions via complexation reaction. 3.2.3. Adsorption Isotherms of Fe3O4@PAA@TSH MNPs for the Metal Ions. The effects of the solution concentration on the adsorption capacities of the metal ions were studied under batch condition at pH 5.2 and temperature of 298 K. In these experiments, the Fe3O4@PAA@TSH MNPs (1.0 g L−1) were equilibrated with metal ion solutions at different concentrations (20−450 mg L −1 ) for 40 min. Then, the equilibrium concentrations of the metal ions in the solution were determined by using FAAS. Finally, the equilibrium isotherms for the adsorption of the studied metal ions on the Fe3O4@PAA@TSH MNPs were analyzed using the Langmuir model (eq 5) and Freundlich model (eq 6).

(4) −1

where qt (mg g ) is the adsorption at time t (min); qe (mg g ) is the adsorption capacity at adsorption equilibrium; and k1 (min−1) and k2 (g mg−1 min−1) are the kinetic rate constants for the pseudo-first-order and the pseudo-second-order models, respectively. The kinetic adsorption data were fitted to eqs 3 and 4, and the calculated results were depicted in Table 1. As it is clear from Table 1, the correction coefficients (R2) for the secondorder kinetic model are greater than 0.99 which suggests the applicability of this kinetic equation and confirms the secondorder nature of the adsorption phenomenon of metal ions on the Fe3O4@PAA@TSH MNPs. The second-order nature of the adsorption processes confirms that the metal ions have been adsorbed via chemical reactions.7 This observation is consistent E



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Figure 7. Effect of pH on the adsorption of metal ions; adsorbent: 0.05 g, concentration of initial metal ions: 100 mg L−1; volume of metal ions solution: 50 mL; time: 1 h, at 298 K.

qe = KF × Ce(1/ n)

In these equations, qe, Ce, qm, KL, KF, and n are the equilibrium adsorption capacity of ions on the adsorbent (mg g−1), the equilibrium ions concentration in solution (mg L−1), the maximum capacity of the adsorbent (mg g−1), the Langmuir adsorption constant (L mg−1), the Freundlich constant (L mg−1), and the heterogeneity factor, respectively.38 The results are shown in Figure 9 and Table 2. As is clear from Figure 9 and Table 2, the equilibrium capacities (qe) of the metal ions were increased with increasing concentrations until reaching equilibrium. In addition, correlation coefficient (R2) values for the Langmuir model are higher than those of the Freundlich model. This observation clarifies that adsorption isotherm data for the metal ions are consistently better with Langmuir than Freundlich isotherms. As can be seen from Table 2, the maximum capacity (qm) for the studied metal ions calculated by the Langmuir equation decreases in the order Pb2+ > Cd2+ > Cu2+ > Zn2+ > Co2+. The observed differences in the maximum capacities of the studied metal ions are most probably due to their different affinity to interact with sulfur and amine groups of the MNPs surface. Pb2+, Cd2+, and Cu2+ are known as soft acids; thus, they can strongly interact with sulfur and amine groups to form a complex. Zn2+ and Co2+ are on the borderline of the soft and hard

Figure 8. Effect of time on the adsorption of metal ions; adsorbent: 0.05 g; concentration of metal ions: 100 mg L−1; volume of test solution: 50 mL; pH: 5.2; temperature: 298 K.

⎞ ⎛ 1 ⎞ ⎛ Ce 1 ⎟⎟ ⎟⎟Ce + ⎜⎜ = ⎜⎜ qe ⎝ qmax ⎠ ⎝ KL × qmax ⎠

(6)

(5)

Table 1. Characteristics of the Applied Kinetics Models for Fitting of the Experimental Results in Figure 8 first-order

a

second-order

metal ions

q (mg g−1)a

R2

k1

qe (mg g−1)

R2

k2

qe (mg g−1)

Pb2+ Cd2+ Cu2+ Zn2+ Co2+

99.4 79.7 73.2 58.1 52.6

0.9570 0.8979 0.9359 0.8910 0.8847

0.0782 0.0698 0.0978 0.0667 0.0763

42.5 14.1 51.6 15.4 25.2

0.9999 0.9999 0.9997 0.9997 0.9998

0.0055 0.0103 0.0044 0.0081 0.0061

101.0 80.6 75.7 59.5 54.3

Experimental values of the adsorption capacities. F



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3.2.4. Effect of Temperature on the Adsorption of Metal Ions. The effects of the temperature on the adsorption capacities of the metal ions were studied under batch conditions at pH 5.2 and equilibrium time of 40 min. These experiments were carried out in five different temperatures (298, 308, 318, 328, and 338 K). In each temperature, the Fe3O4@PAA@TSH MNPs (1.0 g L−1) were equilibrated with metal ion solutions at different concentrations (20−450 mg L−1) for 40 min. Then, the equilibrium concentrations of the metal ions in the solution were determined by using FAAS. The obtained results were analyzed using the Langmuir and Freundlich models. In all cases, the values of correlation coefficients (R2) for the Langmuir model were higher than those found for the Freundlich model, and this shows that the Langmuir model fitted the experimental data well. The R2 values for the Langmuir model were greater than 0.99 for studied metal ions over the temperature range of 298−338 K. The qm values for each metal ion at different temperatures were calculated using the Langmuir model. The results are depicted in Figure 10. As is apparent, the qm values for studied metal ions smoothly increased with increasing temperature until a temperature of about 328 K was reached, While further increase in temperature revealed no measurable effect on the qm values, at 328 K, the maximum adsorption capacities of Pb2+, Cd2+, Cu2+, Zn2+, and Co2+ were found to be 196.2, 122.4, 91.8, 65.3, and 43.4 mg g−1, respectively. 3.2.5. Determination of Pore Size, Pore Volume, and Surface Characteristics of Fe3O4@PAA@TSH MNPs. Pore size, pore volume, and surface characteristics of the prepared Fe3O4@ PAA@TSH nanoparticles were studied using the nitrogen adsorption−desorption isotherms. The surface area was determined by the Brunauer−Emmett−Teller (BET) method, and the pore size distribution was calculated using the Barret− Joyner−Halenda (BJH) model. A typical isotherm for nitrogen adsorption−desorption on the surface of the Fe3O4@PAA@ TSH nanoparticles is shown in Figure 11 a. As is clear from Figure 11a, the isotherms of the Fe3O4@PAA@TSH nanoparticles are consistent with the characteristic of a type IV isotherm with a type H3 hysteresis loop.51 Type IV isotherm is characteristic of mesopore structures in the size range of 2−50 nm.51 The observed hysteresis extended to P/P0 ≈ 1 indicates the presence of macropores (>50 nm in size). These observations could be further confirmed by wide pore size distribution of the Fe3O4@PAA@TSH nanoparticles (Figure 11b). As shown in Figure 11b, the pore size distribution curves of the Fe3O4@ PAA@TSH shows a sharp peak in the range of 3−6 nm and a broad peak ranging from 12 to 60 nm. BET surface area (SBET) and the total pore volume of the synthesized MNPs are 78.2 m2 g−1 and 0.321 m3 g−1, respectively. Compared to the previously reported iron-oxide nanoadsorbents, Fe3O4@PAA@TSH nanoparticles show significant enhancement in the BET surface area. The BET surface area of the Fe3O4@PAA@TSH MNPs are bigger than those of the flowerlike Fe3O4 (34 m2 g−1),52

Figure 9. Equilibrium isotherms of metal ions by Fe3O4@PAA@TSH MNPs, performed in batch mode; adsorbent: 1.0 g L−1; initial concentration of metal ions: 20−450 mg L−1; temperature: 298 K; pH: 5.2; time: 40 min.

acids; therefore, they have lower affinity to interact with sulfur and amine groups. To investigate the competitive adsorptions for mixtures of Pb2+, Cd2+, Cu2+, Zn2+, and Co2+ ions, 0.050 g of Fe3O4@PAA@ TSH was added to a solution (50 mL) containing equal initial concentrations (30 mg·L−1) of each metal ion. The pH was maintained at 5.2, and the equilibrium time was 40 min. After 40 min, the adsorbents were separated by powerful magnets and the removal efficiencies (%Re) were calculated by using eq 2. The obtained removal efficiencies for the studied metal ions decrease in the order Pb2+ (85.7%) > Cd2+ (72.6%) > Cu2+ (61.3%) > Zn2+ (43.8%) > Co2+ (38.9%). These results are in agreement with those obtained for single-metal systems. Table 3 compares the maximum capacity of the prepared Fe3O4@PAA@TSH MNPs with those of the previously reported adsorbents. As is clear from Table 3, Fe3O4@PAA@TSH MNPs possess significant improvements over the existing adsorbents for removal of heavy metal ions. High adsorption capacity of the prepared Fe3O4@PAA@TSH MNPs together with their appropriate characteristics such as reusability, easy synthesis, easy separation, and environmentally friendly composition make them suitable alternatives to the well-known and widely used adsorbents for removal of heavy metal ions from aqueous samples.

Table 2. Langmuir and Freundlich Adsorption Isotherm Constants, Correlation Coefficients, and Adsorption Capacities Langmuir metal ions 2+

Pb Cd2+ Cu2+ Zn2+ Co2+

−1

−1

Freundlich −1

2

KL (L mg )

qm (mg g / mmol g )

R

0.2611 0.1407 0.2895 0.0994 0.1418

188.7/0.911 107.5/0.956 76.9/1.211 51.3/0.784 27.7/0.470

0.9998 0.9991 0.9998 0.9996 0.9997 G

KF (mg

1−(1/n)

L

46.18 30.69 31.49 13.49 10.93

1/n

−1

g )

n

R2

3.271 4.125 5.583 4.012 5.862

0.8089 0.9301 0.8741 0.8262 0.8320



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Table 3. Comparison of Adsorption Capacities of Different Adsorbents for Removal of Heavy Metal Ions adsorption capacities (mg g−1) type of adsorbent hematite (α-Fe2O3) nanoparticles polymer-modified Fe3O4 nanoparticles amino-functionalized Fe3O4 nanoadsorbent Fe/Sn mixed-oxides silica-supported dithiocarbamate TiO2 nanoparticles DNPH modified γ-Al2O3 amino-functionalized Fe3O4@SiO2 MNPs salicylic acid type chelate adsorbent polyaspartyl polymer and chitosan semi-interpenetrating polymer network soy protein hollow microspheres mesoporous Fe3O4 MNPs surfactant modified titanate nanotubes marine algal biomass Sargassum sp Fe3O4@PAA@TSH MNPs

Pb

2+

Cd

166.1 5.06 70.4 100 76.6 86.9 22.5

2+

29.6 4.07 40.32 7.9 100

169.2

57.1 3.25 19.27 106.4

145.8 240.3 188.7

85.43 107.5

Cu2+ 84.46 126.9 12.43

Zn2+

Co2+

43.4

20.36 15.3 41.66 30.8 36.9 40.72 92.43 16.47 41.7 62.9 76.9

31.2 7.48 175

51.3

27.7

reference 15 23 29 39 40 41 42 43 44 45 46 47 48 49 50 this work

γ-Fe2O3 (56 m2 g−1),52 and flowerlike Fe3O4 (34 m2 g−1)52 but less than that of the chestnutlike hierarchical Fe2O3 (143.12 m2 g−1).32 It must be noted, that high SBET values cannot be considered as the only criterion for the strong adsorption of heavy metal ions on the surface of Fe3O4@PAA@TSH MNPs, because SBET values demonstrate the ability of the adsorbent for physisorption of the N2 molecules, where adsorption of the metal ions on the surface of Fe3O4@PAA@TSH MNPs includes complex reaction (chemisorption) and thus obeys the Langmuir isotherm. 3.2.6. XPS Study. XPS data were used to investigate the nature of the adsorption of metal ions on the surface of the prepared adsorbent. On the basis of the Pearson theory (hard and soft acids and bases), adsorption of heavy metal ions on the surface of the Fe3O4@PAA@TSH particles may include complexation reaction between metal ions and nitrogen and sulfur atoms of TSH. To examine this suggestion, the XPS spectra of the Pb2+ loaded Fe3O4@PAA@TSH adsorbent were recorded and binding energies of Pb 4f7/2, Pb 4f5/2, S 2p3/2, and 2p1/2 were compared with reference values. Pb 4f7/2 and Pb 4f5/2 binding energies were shown in Figure 12a. As is clear from Figure 12a, Pb 4f7/2 and Pb 4f5/2 binding energies are in agreement with those obtained for bound lead atoms and chemically adsorbed lead atoms on sulfhydryl containing adsorbents.54−56 On the basis of the

Figure 10. Effects of temperature on the maximum adsorption capacity (qm) of the Fe3O4@PAA@TSH MNPs for adsorption of studied metal ions. Experiments were performed in batch mode; adsorbent: 1.0 g L−1; initial concentration of metal ions: 20−450 mg L−1; temperature: 298− 338 K; pH: 5.2; time: 40 min.

chestnutlike hierarchical Fe3O4 (14.9 m2 g−1),32 hierarchically hollow spheres α-Fe2O3 (12.2 m2 g−1),53 α-Fe2O3 (40 m2 g−1),52

Figure 11. Nitrogen adsorption−desorption isotherms (a) and pore-size distribution curve (b) of the Fe3O4@PAA@TSH MNPs. H



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Figure 12. XPS spectra of Fe3O4@PAA@TSH MNPs after adsorption of Pb2+. (a) Pb 4f of Fe3O4@PAA@TSH MNPs after adsorption of Pb2+; (b) S 2p of Fe3O4@PAA@TSH MNPs after adsorption of Pb2+.

and amine sites of the modified MNPs for complex formation with softer ions. 3.3. Desorption and Repeated Use. From an economic point of view, regeneration of advanced adsorbents is an important feature to evaluate their repeatability in use and the possibility of removing toxic metals from the environmental samples; thus, the reusability of the prepared Fe3O4@PAA@ TSH MNPs was studied using the procedure described in Section 2.8, and the results are depicted in Figure 14. As is

previous reports, typical S 2p3/2 binding energies for unbound thiols are between 163 and164 eV and for bound thiols are lower than 163 eV.57 As depicted in Figure 12b, the binding energy of the S 2p3/2 peak is 161.8 eV, consistent with the sulfur atoms bound to the metal atoms.57,58 The above results show that heavy metal ions can be adsorbed on the surface of the Fe3O4@PAA@ TSH particles via complexation reactions (chemisorption). 3.2.7. Effect of Background Electrolytes. Commonly, real samples of the heavy metal ions have complex matrixes, for example, groundwater and environmental samples often contain significant amounts of alkaline/earth metal ions.43 Thus, a suitable adsorbent must be able to remove heavy metal ions from samples in the presence of an excess of other coexisting cationic species. To examine the applicability of the prepared Fe3O4@ PAA@TSH MNPs for removal of heavy metal ions from real samples, the effects of the Na+, K+, and Mg2+ ions on adsorption capacity of Pb2+ were studied. As depicted in Figure 13, the

Figure 14. Performance of Fe3O4@PAA@TSH MNPs by multiple regeneration cycles.

obvious from Figure 14, the metal ion adsorption capacity of Fe3O4@PAA@TSH MNPs remained almost constant for the 4 cycles, which indicates that the interactions between donor sites on the surface of Fe3O4@PAA@TSH MNPs and metal ions are reversible. The above results indicated that the proposed Fe3O4@PAA@TSH MNPs can be practically used for removal of heavy metal ions from real samples. 3.4. Removal of Heavy Metal Ions from Industrial Wastes. The practical utility of the prepared nanoadsorbent for removal of heavy metal ions from real samples was evaluated using different industrial wastes. Industrial wastes were collected (on April 2013) from Moham industrial complex, Raad plating company, and Iran Aircraft Manufacturing Industrial Company

Figure 13. Effect of the alkaline/earth ions on the adsorption capacity of Fe3O4@PAA@TSH MNPs for Pb2+.

adsorption capacity of Pb2+ slightly decreased (about 1%) with increasing coexisting ions in the region of 0−0.05 mol L−1. These data indicated that the synthesized adsorbent can sufficiently remove the heavy metal ions from aqueous samples in the presence of other coexisting ions. This freedom of the alkaline/ earth ions interferences may be due to higher affinity of the sulfur I



Article

Table 4. Performance Characteristics of the Prepared Nanoadsorbent for Removal of Heavy Metal Ions from Industrial Wastes Raad plating Co.

a

HESA

Moham

metal ions

%Re

qe

cyclea

%Re

qe

cycle

%Re

qe

cycle

Pb2+ Cd2+ Zn2+ Cu2+ Co2+

97.0 94.7 99.2 92.2 78.5

1.94 1.42 278.7 15.7 2.37

1 2 6 2 4

99.0 96.0 98.5 98.0 76.4

0.99 0.96 274.4 14.7 1.56

1 2 5 3 3

98.3 93.1 98.6 95.2 45.2

1.76 0.67 284.0 9.58 0.51

2 2 6 3 2

To use minimum amounts of nanoadsorbent, the adsorption−desorption cycles were repeated. (6) Su, C. C.; Tsai, K. Y.; Hsu, Y. Y.; Lin, Y. Y.; Lian, I. B. Chronic exposure to heavy metals and risk of oral cancer in Taiwanese males. Oral Oncol. 2010, 46, 586. (7) Nabi, S. A.; Shahadat, M.; Shalla, A. H.; Khan, A. M. T. Removal of heavy metals from synthetic mixture as well as pharmaceutical sample via cation exchange resin modified with Rhodamine B: Its thermodynamic and kinetic studies. Clean−Soil Air Water 2011, 39, 1120. (8) El-Hag Ali, A. Removal of heavy metals from model wastewater by using carboxymehyl cellulose/2-acrylamido-2-methyl propane sulfonic acid hydrogels. J. Appl. Polym. Sci. 2012, 123, 763. (9) Sun, B.; Tian, H. Y.; Zhang, C. X.; An, G. Preparation of biomimetic-bone materials and their application to the removal of heavy metals. AIChE J. 2013, 59, 229. (10) Wu, N.; Wei, H.; Zhang, L. Efficient removal of heavy metal ions with biopolymer template synthesized mesoporous titania beads of hundreds of micrometers size. Environ. Sci. Technol. 2012, 46, 419. (11) Zaini, M. A. A.; Amano, Y.; Machida, M. Adsorption of heavy metals onto activated carbons derived from polyacrylonitrile fiber. J. Hazard. Mater. 2010, 180, 552. (12) Hasanzadeh, R.; Najafi Moghadam, P.; Samadi, N. Synthesis and application of modified poly (styrene-alt-maleic anhydride) networks as a nano chelating resin for uptake of heavy metal ions. Polym. Adv. Technol. 2013, 24, 34. (13) Zhang, Q.; Pan, B.; Zhang, S.; Wang, J.; Zhang, W.; Lv, L. New insights into nanocomposite adsorbents for water treatment: A case study of polystyrene-supported zirconium phosphate nanoparticles for lead removal. J. Nanopart. Res. 2011, 13, 5355. (14) Bonato, M.; Ragnarsdottir, K. V.; Allen, G. C. Removal of uranium (VI), lead (II) at the surface of TiO2 nanotubes studied by X-ray photoelectron spectroscopy. Water Air Soil Pollut. 2012, 223, 3845. (15) Chen, Y. H.; Li, F. A. Kinetic study on removal of copper (II) using goethite and hematite nano-photocatalysts. J. Colloid Interface Sci. 2010, 347, 277. (16) Hu, J.; Chen, G. H.; Lo, I. M. C. Removal and recovery of Cr (VI) from wastewater by maghemite nanoparticles. Water Res. 2005, 39, 4528. (17) Badruddoza, A. Z. M.; Tay, A. S. H.; Tan, P. Y.; Hidajat, K.; Uddin, M. S. Carboxymethyl-beta-cyclodextrin conjugated magnetic nanoparticles as nano-adsorbents for removal of copper ions: Synthesis and adsorption studies. J. Hazard. Mater. 2011, 185, 1177. (18) Hua, M.; Zhang, S.; Pan, B.; Zhang, W.; Lv, L.; Zhang, Q. Heavy metal removal from water/wastewater by nanosized metal oxides: A review. J. Hazard. Mater. 2012, 211−212, 317. (19) El-Sayed, M. A. Some interesting properties of metals confined in time and nanometer space of different shapes. Acc. Chem. Res. 2001, 34, 257. (20) Zhao, X.; Lv, L.; Pan, B.; Zhang, W.; Zhang, S.; Zhang, Q. Polymer-supported nanocomposites for environmental application − A review. Chem. Eng. J. 2011, 170, 381. (21) Yavuz, C. T.; Mayo, J. T.; Yu, W. W.; Prakash, A.; Falkner, J. C.; Yean, S.; Cong, L.; Shipley, H. J.; Kan, A.; Tomson, M.; Natelson, D.; Colvin, V. L. Low-field magnetic separation of monodisperse Fe3O4 nanocrystals. Science 2006, 314, 964. (22) White, B. R.; Stackhouse, B. T.; Holcombe, J. A. Magnetic γ-Fe2O3 nanoparticles coated with poly-l-cysteine for of As (III), Cu (II), Cd (II), Ni (II), Pb (II) and Zn (II). J. Hazard. Mater. 2009, 161, 848.

(HESA). AAS was used to determine the concentrations of the heavy metal ions in the samples before and after treatment with nanoadsorbent. The results were depicted in Table 4. These results indicate that the proposed nanoadsorbent can be used for effective removal of heavy metal ions from complex industrial wastes.

4. CONCLUSIONS A novel magnetic nanoadsorbent for removal of Co2+, Pb2+, Cd2+, Zn2+, and Cu2+ ions from aqueous samples has been developed by immobilization of PAA and TSH on the Fe3O4 nanoparticles. The maximum adsorption capacities of Pb2+, Cd2+, Cu2+, Zn2+, and Co2+ were found to be 188.7, 107.5, 76.9, 51.3, and 27.7 mg g−1, respectively. In addition, it was found that the presence of amine and sulfur binding sites on the surface of the prepared MNPs not only can significantly improve the adsorption capacity of the adsorbent but also can notably reduce the interferences of the alkaline/earth metal ions. These improvements may be due to higher affinity of the sulfur and amine sites of the modified MNPs for complex formation with heavy metal ions via soft−soft interactions. Appropriate characteristics of the modified Fe3O4 MNPs such as high adsorption capacity, reusability, easy synthesis, easy separation, and environmentally friendly composition make them suitable alternatives to the well-known and widely used adsorbents for removal of heavy metal ions from aqueous samples. The proposed nanoadsorbent was successfully applied for removal of heavy metal ions from complex industrial wastes.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +98 3113913287. Fax: +98 3113912352. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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