Water assisted synthesis of MWCNTs over natural magnetic rock: An effective magnetic adsorbent with enhanced mercury(II) adsorption property

Chemical Engineering Journal 281 (2015) 468–481

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Water assisted synthesis of MWCNTs over natural magnetic rock: An effective magnetic adsorbent with enhanced mercury(II) adsorption property Hassan Alijani a, Zahra Shariatinia a,⇑, Abdolreza Aroujalian Mashhadi b a b

Department of Chemistry, Amirkabir University of Technology (Tehran Polytechnic), P.O. Box 15875-4413, Tehran, Iran Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), P.O. Box 15875-4413, Tehran, Iran

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Multiwalled carbon nanotubes were

synthesized over a natural magnetic rock catalyst.  The magnetic nanocomposites were employed for mercury removal.  The VSM analysis showed that the CNT nanocomposite have superparamagnetic nature.  The water vapor increased disorder in CNT structure, thus enhanced Hg(II) adsorption.  More than 99% of Hg(II) was removed by CNTs synthesized in the presence of water.

a r t i c l e

i n f o

Article history: Received 15 May 2015 Received in revised form 5 July 2015 Accepted 6 July 2015 Available online 9 July 2015 Keywords: Mercury(II) adsorption Catalyst Nanocomposite Multiwalled carbon nanotubes TEM

a b s t r a c t Herein, multiwalled carbon nanotubes (MWCNTs) were synthesized over natural magnetic rock as an efficient catalyst in the absence or presence of water vapor and the produced magnetic nanocomposites were employed for mercury(II) adsorption. The catalyst and magnetic nanocomposite were characterized using XRD, EDX, VSM, FE-SEM, TEM, FT-IR and Raman techniques. Results of VSM analysis showed that the CNT nanocomposites have superparamagnetic nature which can be applied as magnetic adsorbents. Moreover, Raman study indicated that water vapor increases disorder in CNT structure which simplifies oxidation of raw CNT and as a result enhances mercury adsorption property. Experimental results confirm this hypothesis as maximum adsorptions of 200 and 150 mg g1 were obtained using acid activated CNTs synthesized in the presence and absence of water. Equilibrium time was 90 min in three mercury(II) concentration levels i.e., 10, 20 and 30 mg L1 at ambient temperature and it was revealed that oxidized sorbents followed second order model. It was found that 99.1% of mercury(II) was removed by activated MWCNTs which were synthesized in the presence of water, thus confirmed efficiency of the nanocomposite for mercury(II) uptake. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Heavy metals are a group of metals with an atomic density greater than 6.0 g cm3. Some heavy metals are necessary in life ⇑ Corresponding author. Tel.: +98 2164542766; fax: +98 2164542762. E-mail addresses: [email protected], [email protected] (Z. Shariatinia). http://dx.doi.org/10.1016/j.cej.2015.07.025 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.

cycles, but high concentration of them may represent a serious threat to human health and ecological systems [1,2]. There is a great concern about mercury(II) pollution with respect to other heavy metals, which is due to its high toxicity, persistent character

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in the environment and biomagnifications along the food chain [3]. The maximum allowable level for mercury(II) in surface waters is 0.07 lg L1 [4]. Mercury(II) is released into the environment via textile factories, ores mining and smelting, burning of fossil fuels and chlor-alkali plants [5–7]. Accordingly, extensive studies have been performed to develop mercury(II) removal techniques which include precipitation, membrane filtration, ion exchange, adsorption and bioremediation [8–11]. Among remediation techniques, adsorption with the selection of suitable adsorbents, posses fast rate, large capacity, and high selectivity for hazardous metal ions [12–15]. In recent years, nano-structured carbon-based materials especially carbon nanotubes (CNTs) have been employed in multi-disciplinary studies because of their unique physical, electrical, thermal and chemical properties. CNTs are promising materials for the environmental pollution management due to their highly porous and hollow structure, large specific surface area, light mass density and strong interaction between carbon and target species [16–22]. These potential applications depend on large-scale synthesis of CNTs, hence many methods have been developed to synthesize CNTs such as laser ablation, arc discharge and chemical vapor deposition (CVD) [23]. Comparing with the other methods, CVD technique attracted much attention due to its potential scaling-up possibility. This procedure is based on the catalytic decomposition of hydrocarbons to hydrogen and carbon [24,25]. The roles of catalysts are complete decomposition of hydrocarbons which cause higher yield of CNTs. Two different approaches are employed for CNTs synthesis in CVD method. One rout includes doping catalyst with other metals or doping the metal catalyst in a substrate matrix and other is based on the direct application of iron, cobalt, and nickel as catalysts [26,27]. Mentioned techniques may be suitable for CNTs synthesis however nanotubes yield is limited by the catalytic activity and lifetime. This limitation can be dramatically improved with introduction of small amounts of water vapor to the CVD system which results in high activity and long lifetime for the catalyst [28,29]. In other words, water could remove the amorphous carbon coating of catalyst particles and reactivate it during synthesis process. Moreover, water inhibits the Ostwald ripening of catalyst through offering the oxygen and hydroxyl species. It is known that water vapor can also etch the CNTs and causes more disorder in CNTs structure [30]. This situation simplifies acid oxidation of CNTs as a result increases density of functional groups on CNTs surfaces which exerts positive effect on metal adsorption efficiency. Based on the above noticed cases, we synthesized multiwalled carbon nanotubes (MWCNTs) over natural magnetic rock nanoparticles in the presence of water vapor. To the best of our knowledge, this is the first report about the water assisted synthesis of MWCNTs on a natural magnetic catalyst. Prepared raw and oxidized materials were employed as magnetic adsorbents for efficient adsorption of mercury(II) from water solution. Effective parameters on adsorption process were optimized and the kinetics and isotherms were also investigated. These results revealed some valuable information on the role of water to modify the metal adsorption properties of MWCNTs.

2. Experimental 2.1. Reagents and solutions The rock sample was obtained from Choghart and Chadormalu Mine–Yazd–I.R., Iran. It was grinded for 72 h till a fine powder was obtained. The working solution of mercury(II), 1000 mg L1, was prepared from mercury(II) chloride salt. The pH adjustments were performed using 0.1 mol L1 of NH3 and HCl solutions. All

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of the glassware were cleaned by soaking in diluted HNO3 or HCl and rinsed with distilled water prior to use.

2.2. Instruments The X-ray diffraction (XRD) patterns were recorded by a Philips-X’pertpro X-ray diffractometer using Ni-filtered Cu Ka radiation. Field-emission scanning electron microscopy (FE-SEM) images were obtained on LEO-1455VP microscope. Micro-Raman spectra were recorded using a Renishaw system 1000 spectrometer, equipped with Leica DMLM microscope, a 25 mW diode laser (782 nm), and a Peltier-cooled CCD detector. Fourier transform infrared spectra (FT-IR) were measured with Equinox 55 Bruker with ATR method over the wavelength range of 400–4000 cm1. Transmission electron microscopy (TEM) was carried out on a LEO 912AB under an accelerated voltage of 120 kV. A digital pH-meter (model 692, Herisau, Metrohm, Switzerland), equipped with a glass-combination electrode was used for the pH adjustment. Separation was assisted using a strong neodymium–iron–b oron (Nd2Fe12B) magnet (1.31 T).

2.3. Synthesis of CNTs Synthesis of MWCNTs was performed in the absence of water vapor (assigned as CNTs) and in the presence of 5% water vapor (assigned as W1CNTs) and 10% water vapor (assigned as WCNTs). For this purpose, a fixed bed flow reactor with an external diameter of 40 mm, an internal diameter of 36 mm and a length of about 880 mm equipped with a thermocouple located in the catalyst bed was employed. About 0.3 g of the catalyst powder was placed in the quartz reactor while a controlled flow of argon was passing over the catalyst at room temperature. The reactor was moved to a furnace and purged with argon till the reactor was reaching the synthesis temperature. After that, the argon flow was switched to methane with a rate of 50 mL min1. The decomposition of methane on the catalyst surface was carried out at 800 °C. After this step, the methane stream was switched back to argon until the temperature was reached to the ambient temperature. Finally, the samples were collected for further analysis.

2.4. Acid activation of composite In order to remove the hemispherical caps on the nanotubes and oxidize CNTs surfaces, a mixture of 2.0 g of prepared composite and 20 mL concentrated nitric acid was placed in an ultrasonic bath for 1 h and then stirred overnight at room temperature [31]. After that, the suspension was filtrated and rinsed with distilled water until the pH of the suspension reached about the neutral value and then the composite powder was dried at 80 °C for 5 h.

2.5. Adsorption experiment The batch process was carried out to investigate mercury(II) adsorption characteristics of magnetic sorbents. In adsorption experiment, four adsorbents i.e., CNTs, oxidized CNTs (CNTs-COOH), WCNTs, and oxidized WCNTs (WCNTs-COOH) were employed. About 15 mg of the sorbents were added to a series of 50 mL sample solutions of mercury(II) ions with the concentration of 50 mg L1. The pH of sample mixture was adjusted to optimum amount by using diluted solutions of HCl and NH3. It was shaken for 90 min to reach the equilibrium. Then, the adsorbent was separated magnetically and residual metal concentrations in the supernatant were determined by ICP-AES.

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3. Results and discussion 3.1. Characterization of catalyst and CNTs

Fig. 1. The FE-SEM image of the grinding magnetic rock prepared from Choghart and Chadormalu Mine–Yazd–I.R., Iran.

The FE-SEM image of natural rock after grinding process is shown in Fig. 1. It is obvious that the catalyst displays semispherical morphology with a non uniform distribution. The EDX analysis was employed to determine the chemical components of the catalyst. Results in Fig. 2a indicates that the iron (70.8%) is a component that has the highest amount in the catalyst. Moreover, catalyst structure is composed of about 22.3% O, 3.4% C, 2.4% Si, and 1.1% Al. As a result, it can be estimated that iron oxide comprises more than 90% of the catalyst structure. The X-ray powder diffraction analysis was also used to verify the crystalline structures of magnetic rock and CNTs. The characteristic peaks due to Fe3O4 crystal structure in Fig. 2b shows scatterings at 2h = 30.36, 35.85, 43.63, 53.52, 57.48 and 63.2° that can be indexed to the (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) planes of a cubic cell [32]. After synthesis of CNTs on the catalyst surface, the change in the XRD pattern is evidenced. The pattern for synthesized CNTs in absence of water includes two peaks near to 2h = 28 and 45° corresponding to CNTs and Fe0 nanoparticles

Fig. 2. (a) The EDX spectrum, (b) XRD patterns, (c) VSM graphs and (d) FT-IR spectra of the catalyst and synthesized composites.

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[33,34]. These two peaks are observable in the XRD pattern of WCNTs along with a third peak at 2h = 38° which shows a higher intensity than that of its related peak in the XRD pattern of Fe3O4. This peak can be attributed to the phase change of Fe nanoparticles in the presence of water vapor. In other words, formation of Fe nanoparticles is because of the iron reduction by hydrogen which is generated during the thermal cracking of methane (Eqs. (1) and (2)).

CH4 ! 2H2 þ C Fe3 O4 þ 4H2 ! 3Fe þ 4H2 O

ð1Þ ð2Þ

In fact, methane is decomposed to carbon and hydrogen in the presence of metal based catalysts and in subsequent step, iron oxide is converted to H2O and iron nanoparticles. Water vapor causes formation of oxide layer on the Fe surface hence, the XRD pattern of WCNTs is composed of excess peaks with respect to the pattern in absence of water (CNTs). The magnetic hysteresis loops of the catalyst, CNTs, WCNTs and oxidized compounds are shown in Fig. 2c. The saturated magnetization of the magnetic rock was 51.08 emu g1. After synthesis of CNTs on the catalyst surface, the quench in the magnetic moment was observed. Magnetization was decreased to 28 and 41 emu g1 for WCNTs and CNTs, respectively, which is due to the existence of CNTs layers on the surfaces of the nanoparticles. Lower magnetization of WCNTs with respect to raw CNTs can be due to the presence of FeO (FeII) or Fe2O3 (FeIII) phases in WCNTs structure however, raw CNTs structure contain Fe0 nanoparticles, as a result the latest composite shows higher saturated magnetization relative to WCNTs. Owing to dissolving of metal fragments in concentrated

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acid, the CNTs-COOH and WCNTs-COOH exhibited saturation values of 3.5 and 6.3 emu g1, respectively. According to the results, the CNTs-COOH has a lower saturation value in comparison with WCNTs-COOH. This situation may be owing to the lower stability of zero valence iron with respect to oxide species in the presence of concentrated acid and conversion of Fe species to iron oxide which has a lower magnetic saturation. The remanence values of Fe3O4, CNTs, WCNTs, CNTs-COOH and WCNTs-COOH were 0.068, 0.054, 0.032, 0.008 and 0.001 emu g1, respectively, indicating the materials are superparamagnetic particles with a single magnetic domain. The FT-IR spectra of natural magnetic rock, WCNTs and WCNTs-COOH are demonstrated in Fig. 2d. Several characteristic bands are observed in the spectra of rock sample corresponding to Fe–O (500–650 cm1) [35], Si–O–Si stretching vibration (1050 cm1), –CH and –OH vibrations (2900–3500 cm1). After the synthetic process, some distinct peaks are appeared in this spectrum which can be assigned to C@C stretching bands in the CNTs structure (1100–1500 cm1). Moreover, the intensity of –CH vibration (2940 cm1) is increased owing to high density of CNTs on catalyst surface. In the spectrum of acid activated WCNTs, intensities of iron oxide peaks are decreased owing to dissolution of metal components in concentrated acid. Furthermore, the peaks due to AC@O and –COOH stretching vibrations (1630 and 2500–3500 cm1) are appeared in this spectrum. Fig. 3a–c illustrates the FE-SEM images of MWCNTs on the surface of the catalyst. According to the image in Fig. 3a, CNTs form bundles and it is difficult to identify the individual tubes because of the overlapping between them as a result of strong interaction. Moreover, the images of the W1CNTs (Fig. 3b) and WCNTs (Fig. 3c)

Fig. 3. The FE-SEM images of synthesized CNTs in the presence of water vapor, (a) 0% H2O, (b) 5% H2O, (c) 10% H2O and (d) TEM image of synthesized CNTs in the presence of 10% water vapor.

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reveal that with increasing the water amount, the CNTs posses higher diameters. This situation can be explained with this fact that formation of amorphous carbon on catalyst surface may inhibit the iron particles from sintering but water vapor can remove the amorphous layer and encourage the sintering of the iron particles, consequently the large iron particles will initiate the growth of CNTs with larger-diameters [36]. Other reason is etching of small-diameter CNTs with excess water which leaves large diameter CNTs. Fig. 3d displays the TEM image of WCNTs synthesized using 10% water vapor in the methane stream. The image indicates that the CNTs are 20–50 nm tubes which have disorders on their structures. Moreover, there are several particles in the inner tubes that may be the iron catalysts initiating the CNT growth. Raman spectroscopy as a nondestructive technique is considered to be a very good candidate for CNTs characterization. The Raman spectrum of MWCNTs in Fig. 4a consists of D mode at around 1350 cm1 as a result of in-plane breathing mode of A1g symmetry due to the presence of six fold aromatic rings and G mode at around 1580 cm1 due to the in-plane bond stretching of sp2 hybridized carbon [37,38]. These peaks appear at approximately the same position for all samples. The intensity ratio of the G-band to D-band (IG/ID) is directly proportional to the in-plane crystallite size and is often regarded as an indication of the purity of as-prepared carbon nanotubes samples. According to the spectra, with increasing the water amount, the IG/ID values

are decreased revealing the degree of disorder is increased. This result is due to the fact that as-grown CNTs can also be etched by water vapor along with amorphous carbon which leads in increasing the degree of disorder; moreover, later etching by water vapor as a weak oxidizer causes high activity and long catalyst lifetime. Disorder in CNTs structure offers an appropriate situation for its oxidation and simplifies acid treatment of CNTs thus increases the basic centers on CNTs surfaces which can improve metal adsorption properties of as-synthesized CNTs. 3.2. Determination of surface acidic groups A typical Boehm titration was used to determine distribution of the acidic groups on CNTs surface [39]. 20 mL of 0.01 mol L1 NaOH solution in 0.1 mol L1 NaCl was slowly added into a vessel containing oxidized MWCNTs and shacked for 24 h. The solid mass was then separated and 5 mL aliquots were titrated with 0.01 mol L1 HCl in 0.1 mol L1 NaCl solution and the pH was monitored. Structures of raw and oxidized CNTs contain phenolic and carboxylic groups. The raw CNT had total acidic sites of 0.6 mmol g1 while the value was 1.02, 4.1, and 5.01 mmol g1 for WCNT, CNT-COOH and WCNT-COOH respectively. According to the results, the number of acidic sites of WCNT-COOH is 8 times greater than that of the raw CNT hence it has a good potential to be employed as adsorbent.

Fig. 4. (a) The Raman spectra of prepared CNTs, (b) effect of pH (time = 120 min, adsorbent = 15 mg, mercury concentration = 50 mg L1), (c) zero charge potential curve of the adsorbents, and (d) effect of adsorbent dosage on mercury adsorption (pH = 5, time = 120 min, mercury concentration = 50 mg L1).

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efficiency reach a plateau hence, 0.25 g L1 was selected as the optimum adsorbent dose for the experiments.

3.3. Effect of pH In order to evaluate the influence of pH parameter on mercury(II) adsorption, the experiments were carried out in the pH range of 2.0–10.0 keeping other variables constant. About 0.015 g of the adsorbent was suspended in 50 mL of mercury(II) solution (10 mg L1) at several pH values. These samples were shacked for 120 min. Then, the sorbent was separated and removal percent was calculated using following equation, where C0 and Ce are the initial and equilibrium concentrations (mg L1) of mercury(II) ions in the solution.

R% ¼ ðC 0  C e Þ  100=C 0

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ð3Þ

The results in Fig. 4b showed that maximum removals of mercury(II) using four types of adsorbents were obtained within the pH range of 5.0–9.0. The dependence of sorption on the pH is related to the metal species in the solution. By changing pH of the solution, mercury is predicted to exist predominantly as a monomeric species, Hg2+, and multinuclear oligomeric hydroxide complexes such as [Hg(OH)]+ and Hg(OH)2 [40]. Another effective parameter on mercury adsorption is the electrical properties of sorbent functional groups which could be modified by pH of solution. The change of the surface properties of MWCNTs can be clearly seen by inspecting their zeta potential curves, which are shown in Fig. 4c. The isoelectrical point of the CNTs and WCNT is about pH  3.8. Additionally, the CNTs become much more negatively charged after oxidation and the isoelectrical point is about pH  2. The zeta potential of CNT and WCNT decreased from 17 and 12 mV to 9.6 and 17 mV after the acid treatment. This change can be explained by the introduction of acidic and hydroxyl groups at the tip and on the outer shell of the tubes by the acid treatment. According to the results at acidic solution (pH = 2), the CNT surface is positively charged but oxidized CNT has a negative charge while with increasing the pH value up to 5.0, the negatively charged sites on the surface of CNTs are increased as the zeta potential of CNT, WCNT, CNTCOOH and WCNT-COOH reached to 25, 28, 31, and 36 respectively. Based on the above-mentioned issues, the adsorption of mercury(II) species onto hydrated CNTs could be described as a chemical coordination and electrostatic process. The increase in the mercury removal with pH increase is based on the increase in surface negative charge followed by a decrease in competition between H3O+ and target ions hence, the removal efficiency of target ions increases at higher pH. To evaluate the role of precipitation on mercury(II) removal, a control study in the absence of adsorbent (blank) was also performed using WCNTs-COOH as the sorbent. Result demonstrated that at pH’s greater than 5, about 3–12% of mercury(II) was removed as hydroxide from solution. Maximum removal efficiency at pH 5.0 (50 mg L1) was 32%, 37%, 56% and 80% for CNTs, WCNTs, oxidized CNTs and oxidized WCNTs, respectively. These values were reached to 41%, 50%, 72% and 99% at the pH = 9.0, but the adsorptions were conducted at pH 5.0 to avoid the contribution from precipitation. 3.4. Effect of adsorbent dosage In order to maximize the interactions between metal ions and adsorption sites of adsorbent, it is necessary to optimize the adsorbent dose. The effect of this parameter on the removal efficiency of mercury(II) is shown in Fig. 4d. It is observed that the removal efficiencies increase greatly as the dosages of sorbents increase from 0.1 to 0.25 g L1, and then reach a steady state. However, the adsorption capacities of all sorbents have no significant changes over the mentioned dosage rang. This situation can be justified with the fact that the increase in the adsorbent dose may multiply the number of available adsorption sites. Moreover, high dosage may result in aggregation of the adsorbent which causes removal

3.5. Effect of contact time and kinetic study Mercury(II) adsorption using four adsorbents in three concentration levels (10, 20 and 30 mg L1) was performed within 5–120 min. Due to the availability of a large number of adsorption sites, the value of Qe was increased rapidly within the first 60 min and then, it was slowed down as the sites were gradually filled up. In this step, the kinetics will be more dependent on the rate at which the analyte is transported from the liquid phase to the adsorption sites. The increase in Qe was not significant after 90 min hence this time was selected for further work. According to results in Fig. 5a–c, all sorbents showed the same pattern for mercury uptake but at the same experimental conditions, WCNTs-COOH exhibited the highest efficiency that reveals the synthetic rout has positively modified adsorption properties of MWCNTs. An appropriate kinetic model can quantify the changes in analyte adsorption with time hence, two main kinetic models, i.e., the pseudo first-order and four types of pseudo second-order models were applied to explain mercury(II) adsorption behavior by all adsorbents. The data of first order model for three mercury concentration levels is given in Table 1 as k1, Qe and Qt are the pseudo-first-order adsorption rate constant (min1), the amount adsorbed per unit mass at equilibrium and at any time t, respectively. The equations for pseudo first order and types 1, 2, 3, 4 of pseudo second-order expression [41,42] are given in Table 2 and the results for Qe and K2 at three different mercury concentration levels based on type 1 of second order model (as most applied model) are depicted in Table 3. Moreover, the parameter based on types 2, 3, 4 of second-order model at concentration level of 10 mg L1 are listed at Table 4. It was observed that all second order types showed high linearity with the same R2 value. The K2, the second-order rate constant (g mg1 min1), and Qe values obtained from the five linear forms of pseudo second-order expressions were approximately different. The very high R2 values (>0.99) for all types of pseudo second-order expression suggests that it is appropriate to use pseudo second-order model. According to results in Table 4, type 2 pseudo second-order model was found to well represent the experimental Qe of mercury onto CNTCOOH and WCNT-COOH. However, the Qe value of CNT and WCNT based on type 1 expression showed better fitting with the experimental value. The plots of first-order model and type 1 expression of second-order kinetic models for all adsorbents at three concentration levels are displayed in Fig. 6a–c and Fig. 7a–c, respectively, indicating the plots have good linearity for mercury adsorption but according to the data in Tables 1, 3 and 4, the sorbents showed different behaviors. The results for non-oxidized CNTs revealed that the Qe obtained based on the second-order model has a high deviation from experimental value, consequently first-order model can be accepted as the kinetic mechanism for mercury adsorption using CNTs and WCNTs. However, results for oxidized CNTs confirmed that the second-order model can better describe the mercury adsorption kinetics. The available kinetic equations were also compared based on the root-mean-square error (RMSE) as following equation [43], where Qexp and Qc are the experimental and calculated data from nonlinear models.

RMSE ¼

X 2 ðQ exp  Q c Þ=Q exp

ð4Þ

According to the results (Tables 1, 3 and 4), an error analysis revealed that the first-order model for mercury adsorption using CNT and WCNT possessed lower RMSE values with respect to the

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Fig. 5. The effect of shacking time on mercury adsorption at three mercury concentration levels: (a) 10 mg L1, (b) 20 mg L1 and (c) 30 mg L1 (pH = 5, adsorbent = 0.25 g L1).

Table 1 Data of first-order kinetic model for adsorption of mercury onto the adsorbents (Time = 90 min, concentration = 10, 20 and 30 mg L1, adsorbent = 0.25 g L1).

Table 2 The equations and plots related to the pseudo first order and five types of pseudo second-order kinetics model.

Ci (mg L1)

First order

CNT

W-CNT

CNT-COOH

W-CNT-COOH

Type

10

R2 Q K1 Qexp Deviation (%) RMSE

0.999 22.6 0.052 23.2 2.58 0.19

0.999 24.0 0.051 25.2 4.7 0.51

0.994 36.5 0.05 32.41 12.6 0.97

0.996 35.02 0.05 39.6 11.5 0.9

First order Type 1 second Type 2 second Type 3 second Type 4 second

R2 Q K1 Qexp Deviation (%) RMSE

0.999 38.86 0.051 39.52 1.6 0.32

0.999 44.25 0.051 45.51 2.7 0.58

0.982 55.14 0.049 63.26 12.8 2.78

0.995 69.296 0.05 78.87 12.1 1.8

R2 Q K1 Qexp Deviation (%) RMSE

0.999 58.5 0.063 57.14 2.3 0.71

0.999 56.31 0.052 47.61 18.2 0.25

0.90 58.49 0.038 81.3 28.0 2.89

0.992 100.48 0.048 117.09 14.1 3.2

20

30

second-order model. Hence, first-order model could better describe the adsorption behavior of the mercury ions on these sorbents. CNT-COOH and WCNT-COOH showed different behaviors

order order order order

Linear form

Plot

ln(Qe  Qt) = ln Qe  k1t t/Qt = 1/(K2 Q2e ) + (1/Qe) t 1/Qt = 1/Qe + (1/K2 Q2e ) 1/t 1/t = k2 Q2e /Qt  k2Q2e /Qe Qt/t = k2 Qe2  K2Q2e Qt/Qe

ln(Qe  Qt) vs. t t/Qt vs. t 1/Qt vs. 1/t 1/t vs. 1/Qt Qt/t vs. Qt

relative to non-oxidized CNTs as second-order model indicated lower RMSE values and confirmed that this model better fits with experimental results for mercury adsorption by the two latest adsorbents. 3.6. Isotherm study and error analysis Results of the equilibrium adsorption isotherms for mercury(II) in the concentration range 0.5–80 mg L1 at optimum conditions are presented in Fig. 8. The sorption by oxidized WCNT is approximately complete (99.8% and Qe = 1.66 mg g1) at 0.5 mg L1 of sorbate concentration but it is decreased at higher concentrations. Removal efficiency of mercury(II) with initial concentration of 0.5 mg L1 is equal to 87% (Qe = 1.45 mg g1),

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Table 3 Data of type 1 expression of second-order kinetic model for adsorption of mercury onto the synthesized adsorbents (Time = 90 min, concentration = 10, 20 and 30 mg L1, adsorbent = 0.25 g L1). Adsorbent

Type 1 10 mg L1 R

CNT W-CNT CNT-COOH W-CNT-COOH

2

0.997 0.998 0.998 0.999

20 mg L1 2

30 mg L1

Q

K2

Qexp

RMSE

R

Q

K2

Qexp

RMSE

R2

Q

K2

Qexp

RMSE

27.1 29.0 36.7 45.4

0.0022 0.0024 0.002 0.002

23.2 25.2 32.4 39.6

0.60 0.43 0.56 0.66

0.997 0.997 0.998 0.999

46.5 51.8 70.2 88.5

0.0012 0.0014 0.0012 0.0009

39.52 45.51 63.26 78.87

0.98 1.32 1.83 1.31

0.997 0.996 0.999 0.999

67.1 64.9 88.5 130.5

0.0009 0.0011 0.0011 0.0006

57.14 47.61 81.3 117.0

1.62 2.01 1.82 1.29

Table 4 Data of types 2, 3 and 4 expressions of second-order kinetic model for adsorption of mercury onto the adsorbents (Time = 90 min, concentration = 10 mg L1, adsorbent = 0.25 g L1). Adsorbent

CNT W-CNT CNT-COOH W-CNT-COOH

Type 2

Type 3

Type 4

R2

Q

K2

Qexp

R2

Q

K2

Qexp

R2

Q

K2

Qexp

0.998 0.998 0.998 0.999

30.3 30.3 31.25 38.46

0.0016 0.0016 0.0019 0.0018

23.2 25.2 32.41 39.6

0.998 0.998 0.999 0.997

30.02 30.55 38.59 46.57

0.0016 0.0020 0.0018 0.0015

23.2 25.2 32.41 39.6

0.984 0.995 0.987 0.997

29.8 30.8 38.25 46.72

0.0016 0.0019 0.0018 0.0015

23.2 25.2 32.41 39.6

Fig. 6. The Lagergren pseudo-first order plots at three mercury concentration levels: (a) 10 mg L1, (b) 20 mg L1 and (c) 30 mg L1 (pH = 5, adsorbent = 0.25 g L1).

69% (Qe = 1.15 mg g1) and 65% (Qe = 1.08 mg g1) for CNT-COOH, WCNT and raw CNT, respectively. These results reflected the efficiency of the sorbents for the removal of mercury(II) from an aqueous solution in a wide range of concentrations. Moreover, it is

confirmed that WCNTs-COOH has the highest efficiency among all sorbents. The effect of mercury(II) concentration on the sorbent was also analyzed in terms of Langmuir and Freundlich models. The Langmuir model verifies that the sorbent is structurally

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Fig. 7. The pseudo-second order plots at three mercury concentration levels: (a) 10 mg L1, (b) 20 mg L1 and (c) 30 mg L1 (pH = 5, adsorbent = 0.25 g L1).

Table 5 Freundlich and Langmuir Isotherms and their linear form equations and plots.

Fig. 8. The effect of initial mercury concentration on the removal efficiency (pH = 5, adsorbent 0.25 = g L1, time = 90 min).

homogeneous as all the sorption sites are energetically the same. This model can be expressed as four linear type expressions as can be seen in Table 5 [44], where Qe is the amount of metal ions adsorbed per unit mass of the sorbent (mg g1) and Ce is the amount of metal ions in the liquid phase at equilibrium (mg L1).

Type

Linear form

Plot

Freundlich Type 1 Langmuir Type 2 Langmuir Type 3 Langmuir Type 4 Langmuir

Log Qe = log Kf + 1/n log Ce Ce/Qe = Ce/Qm + 1/b Qm 1/Qe = 1/Ce (Qmb) + 1/Qm Qe = Qm  (1/b) Qe/Ce Qe/Ce = b Qm – b Qe

Log(Qe) vs. log Ce Ce/Qe vs. Ce 1/Qe vs. 1/Ce Qe vs. Qe/Ce Qe/Ce vs. Qe

The Qm is the maximum adsorption capacity and b is the Langmuir coefficient [45]. The curves of type 1 expression (as most applied model) for Langmuir model are depicted in Fig. 9a–d and results for all applied equations are exhibited in Table 6. According to the results, type 1 and type 2 equations showed the best fits relative to other types since they indicated relatively greater R2 values. It was observed from the results that the determined isotherm parameters (Table 6) are approximately the same with low differences in the four diverse types of linearized forms of Langmuir isotherm expressions. Different outcomes imply that estimating isotherm parameters with the linear method is difficult [46]. Moreover, the results revealed that the maximum adsorption capacities of WCNTs and WCNTs-COOH are about 10% and 16% greater than those of the raw and oxidized CNTs. The linear form of Freundlich model is presented in Table 7, where n and Kf are the Freundlich coefficients which are evaluated from the slopes and intercepts of linear plot. According to graphs in

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Fig. 9. The Freundlich adsorption isotherm models for mercury adsorption on (a) CNTs, (b) WCNTs, (c) CNTs-COOH and (d) WCNTs-COOH (pH = 5, adsorbent = 0.25 g L1, time = 90 min).

Table 6 Langmuir isotherm models of mercury adsorption onto various synthesized MWCNTs (Time = 90 min, adsorbent = 0.25 g L1, sample volume = 50 mL). Adsorbent

Type 1

a

Type 3

Type 4

v2

R2

Q

b

R2

Q

b

R2

Q

b

R2

Q

b

0.991 0.995 0.990 0.995

1.0 0.52 1.92 7.29

0.994 0.996 0.994 0.994

100 111.1 166.6 200

0.075 0.079 0.12 1.0

0.991 0.995 0.990 0.995

91 104 150 200

0.08 0.08 0.14 0.35

0.936 0.973 0.949 0.938

90.16 102.8 147.2 180.7

0.084 0.089 0.15 2.74

0.936 0.973 0.949 0.938

92.43 104.3 150 183.4

0.079 0.087 0.15 2.57

R

CNT W-CNT CNT-COOH WCNT-COOH

Type 2

a 2

Non linear.

Table 7 Data of Freundlich isotherms for mercury adsorption by the synthesized MWCNT adsorbents (Time = 90 min, adsorbent = 0.25 g L1, sample volume = 50 mL). Constants

CNT

W-CNT

CNT-COOH

W-CNT-COOH

R2 n Kf R2(Non

0.938 2.32 14.25 0.943 3.93

0.960 2.29 16.29 0.973 2.09

0.951 2.37 29.56 0.969 4.54

0.898 4.11 94.77 0.946 15.87

v2

linear)

Fig. 10a–d and the results in Table 7, this model shows approximately a good linearity but the linearity is lower than that of the Langmuir model hence it may be concluded that the adsorption process does not follow this model. The accuracy of the isotherm models were further evaluated by chi-square test (v2). This error

function is given as Eq. (5), where Qexp and Qc are the experimental data and the calculated from nonlinear models [47].

v2 ¼

X ðQ exp  Q c Þ2 Qc

ð5Þ

Based on the results given in Tables 6 and 7 and Fig. 11, the R2 values for nonlinear models are superior to 0.94 indicating low variances about the mean values but the Langmuir model has a lower v2 value and reveals that the Langmuir model can better describe adsorption behavior of mercury by the sorbents. 3.7. Thermodynamic study The adsorption capacity of mercury(II) with initial concentration of 50 mg L1 and adsorbent dosage of 0.25 g L1 increases with

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Fig. 10. The type 1 Langmuir adsorption isotherm model for mercury adsorption on (a) CNTs, (b) WCNTs, (c) CNTs-COOH and (d) WCNTs-COOH (pH = 5, adsorbent = 0.25 g L1, time = 90 min).

increase in temperature from 298 to 318 K. This result indicates that mercury(II) adsorption on all adsorbents obeys an endothermic path. The thermodynamic parameters for the adsorption process, DH (kJ mol1), DS (J K1 mol1) and DG (kJ mol1) could be evaluated using the Eqs. (6–8) [48], where R and T are the gas constant (8.314  103 kJ K1 mol1) and absolute temperature (K), respectively.

DG ¼ RT ln ðqe =ce Þ DG ¼ DH  T DS ln ðqe =ce Þ ¼ DS=R  DH=RT

ð6Þ ð7Þ ð8Þ

The results in Table 8 indicates that mercury(II)–CNT interactions were accompanied by a decrease in Gibbs free energy which made the interactions spontaneous and suggests that the process is feasible at higher temperatures. The value of DH was estimated within the range of 24.74–28.75 kJ mol1 from CNT to WCNT-COOH. The heat evolved during physical sorption and condensation falls into 2.1–20.9 kJ mol1, while the heat of chemisorption generally is in a range of 80–200 kJ mol1 [14]. As a result, it seems that mercury(II) sorption is along with a physicochemical sorption process. The positive enhanced values of DS from CNT to WCNT-COOH show increase in randomness at the solid–solution interface and also indicate an affinity of the sorbent towards the mercury(II).

3.8. Desorption and reusability From practical point of view, regeneration or desorption of the target analyte from the sorbent material makes the sorption process more economical. According to the results for the effect of pH, the adsorption was not efficient in acidic medium. Therefore, elution with acidic solution may be favorable. As a result, different concentrations of HCl solution (0.1, 0.3 and 0.5 mol L1) were employed to regenerate the adsorbents. It was observed that with increase in HCl concentration from 0.1 to 0.5 mol L1 the release of mercury(II) ions was increased from 76% to 97%. Hence, HCl solution with concentration of 0.5 mol L1 was employed for regeneration of the adsorbents. Moreover, in order to evaluate the reusability of the sorbents, they were subjected to several loadings with the sample solution and subsequent elution. It was found that after 6 cycles of sorption and desorption (Fig. 12) the removals were 89.5%, 71.5%, 51% and 45% for WCNT-COOH, CNT-COOH, WCNT and raw CNT which confirm their efficiency and good stability as regenerable adsorbents. The stability of magnetic WCNTCOOH composite in acidic solution was also investigated. After performing the elution process and separation of the adsorbent, amount of dissolved iron ions in the eluted solution was determined by FAAS. The result indicates that after 6 cycles only 3.5– 4% of iron was dissolved which shows good stability for the WCNT-COOH magnetic composite.

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479

Fig. 11. The theoretical adsorption model fitted with experimental data using (a) CNTs, (b) WCNTs, (c) CNTs-COOH and (d) WCNTs-COOH.

Table 8 Thermodynamic data for mercury adsorption by the synthesized MWCNT adsorbents (Time = 90 min, adsorbent = 0.25 g L1, mercury concentration = 10 mg L1, sample volume = 50 mL). Adsorbent

CNT WCNT CNT-COOH WCNT-COOH

DG (kJ mol1) 298 K

308 K

318 K

1.57 2.11 4.08 6.89

2.38 3.11 5.02 8.15

3.34 3.97 6.21 9.28

DH (kJ mol1)

DS (J K1 mol1)

24.74 25.49 27.58 28.75

88.21 92.70 106.06 119.63

3.9. Adsorption mechanism Mercury adsorption onto CNT surface may take place through physical or chemical adsorption. The results of kinetic, isotherm and thermodynamic studies can exhibit a standpoint about the adsorption mechanism. First-order kinetic model represents physical adsorption or ion exchange mechanism however, second-order model is equal to chemisorption. Moreover, Freundlich isotherm corresponds to physical adsorption while the Langmuir model shows monolayer chemisorption process. It is known that thermodynamic data also can predict adsorption mechanism. If the heat of sorption was in the range of 2.1–20.9 kJ mol1, the process is physical, whereas the heat of chemisorption is in a range of 80–200 kJ mol1. Based on above description and according to

Fig. 12. Reusability of (a)WCNT-COOH, (b) CNT-COOH, (c) WCNT and (d) CNT, after 6 cycles of sorption and desorption (pH = 5, adsorbent = 0.25 g L1, time = 90 min, mercury concentration = 10 mg L1).

the experimental results, mercury(II) sorption is along with a physicochemical sorption process. In other words, the adsorption process is not a net physical or chemical interaction because the

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adsorption followed second-order kinetic model and Langmuir adsorption isotherm but the heat of sorption is greater than 20 kJ mol1 and less than 80 kJ mol1. This result indicates that the mechanism of mercury adsorption is complex and both the chemical and physical adsorptions exist at the same time in this adsorption process.

mercury(II) ions was reduced to 0.18 lg L1. Result showed that 99.1% of mercury(II) ions has been removed and confirmed that the proposed method has a substantial ability for mercury(II) solidification from water solutions.

3.10. Comparison of the CNTs performances

This study demonstrated water assisted synthesis of MWCNTs on natural magnetic rock as an efficient catalyst. Results revealed that increasing water vapor causes more disorder on CNTs structures which simplifies oxidation of the CNTs and creation of functional groups compared with absence of water vapor. The Hg(II) ion adsorption experiments showed that an efficient magnetic adsorbent was successfully generated by this method. The water vapor improved the adsorption properties of CNTs, making the surface more suitable for Hg(II) ion adsorption because the water assisted oxidized CNTs exhibit 33% higher adsorption capacity for Hg(II) ions compared to oxidized CNTs synthesized in absence of water vapor. The kinetics of Hg(II) adsorption on the four CNTs powders revealed different behaviors so that the non-oxidized CNTs followed first-order kinetics but second-order model well described adsorption properties of the oxidized sorbents. In consequence, water assisted CNTs are promising materials with potential applications as adsorbents in water treatment.

In this research, four adsorbents, i.e., raw CNTs, WCNTs, CNTs-COOH and WCNTs-COOH, have been used for removal of mercury(II) from aqueous media. The results indicated that WCNTs-COOH has the best performance among all of the four adsorbents examined. The removal efficiencies at optimum conditions and initial concentration of 10 mg L1 were 58, 63, 81 and 99.1% for raw CNTs, WCNTs, CNTs-COOH and WCNTs-COOH, respectively. Moreover, the maximum Langmuir monolayer capacities were 91, 104, 150 and 200 mg g1 for the adsorbents according to the mentioned layout. In the synthetic rout, WCNTs-COOH was obtained in the presence of 10% water vapor. It was observed that water vapor caused more disorder in the CNT structure in consequence, the CNT can be simpler attacked by HNO3 which leads in creation of more functional groups on the CNT surface hence, it can capture higher amounts of mercury ions. Moreover, the VSM analysis indicated that WCNTs-COOH has a stronger magnetic property with respect to CNTs-COOH which can be due to the presence of more amounts of magnetic nanoparticles. The nanoparticles can also participate in adsorption process therefore, the adsorbent showed more efficiency for the mercury removal. 3.11. Comparison with literature

4. Conclusions

Acknowledgments The financial support of this work by the Research Council of Amirkabir University of Technology (Tehran Polytechnic), Tehran, Iran is gratefully acknowledged. References

A comparative study for mercury adsorption using CNTs tested in the present study and other sorbents, which have been reported in the literature, is presented in Table 9. It is obvious that the performances of the prepared sorbents are appropriate with respect to adsorption time so that they demonstrate satisfactory sorption capacities which are compatible with the values found in the literature. Synthesized CNTs appear to be promising sorbents for the removal of mercury from aqueous systems in order to solidify it and reduce its hazardous impact. 3.12. Treatment of real sample In order to investigate practical application of the proposed method, one real sample has been treated with WCNT-COOH at optimum experimental condition. The wastewater has been selected from Babolsar cannery factory. The initial concentration of mercury(II) ions in the wastewater sample which has been determined by ICP method was 20.6 lg L1. After treatment of this solution at optimum conditions, the final concentration of

Table 9 Comparison of the mercury adsorption property between the synthesized MWCNT materials and some other sorbents reported in literature. Adsorbent

Qm (mg g1)

Time (min)

Working pH

Ref.

Porous phosphate-thiol Zinc oxide-CMK-3 Malt spent rootlets Aluminosilicate sieve MWCNTs Eucalyptus bark WCNTs WCNT-COOH

581 526 50.4 20.65 13.16 33.11 104 200

24 h 120 24 h 5h 120 10 90 90

7 6 5–6 6 5 5 5 5

[2] [3] [4] [7] [9] [14] This work This work

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