Efficient Removal of Pb(II) from Aqueous Solution by Modified

Article pubs.acs.org/jced

Efficient Removal of Pb(II) from Aqueous Solution by Modified Montmorillonite/Carbon Composite: Equilibrium, Kinetics, and Thermodynamics Kecheng Zhu,†,‡ Hanzhong Jia,*,‡ Fu Wang,‡ Yunqing Zhu,‡ Chuanyi Wang,*,‡ and Chengyu Ma*,† †

School of Environmental Science and Engineering, Donghua University, Shanghai 200051, China Laboratory of Environmental Sciences and Technology, Xinjiang Technical Institute of Physics & Chemistry, Urumqi 830011, China

‡

S Supporting Information *

ABSTRACT: In the present study, a new type of montmorillonite/carbon (MMT/C) adsorbent has been prepared by a one-pot hydrothermal carbonization process. The MMT/C composite was further modified to obtain functionalized MMT/C, i.e., hydroxylated MMT/C, carboxylated MMT/C, and aminated MMT/C. The prepared MMT/C-based sorbents were systematically characterized by X-ray diffraction (XRD), specific surface area, zeta potential, and scanning electron microscopy (SEM). Their adsorption capacity was evaluated by the removal of aqueous Pb(II) ions, following the order carboxylated MMT/C > hydroxylated MMT/C > aminated MMT/C > MMT/C. Moreover, the influential factors for Pb(II) removal by carboxylated MMT/C composites, such as pH, temperature, and initial concentration, were thoroughly explored. The adsorption capacity increases significantly when the pH increases from 2.0 to 5.0 but with minor change beyond. The maximum adsorption capacities of MMT/C−COOH toward Pb(II) are 247.85 mg g−1. The complexation of surface groups such as −COOH on MMT/C−COOH might be responsible for Pb(II) sorption. The adsorption kinetic follows a pseudo-second-order model, and thermodynamic analysis implies an endothermic and spontaneous chemisorption process. The overall results suggest that the obtained composites are promising as an adsorbent for effective removal of Pb(II) from water.

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INTRODUCTION Due to its serious harmful effects on human health, lead (Pb) has been considered as one of the most hazardous heavy metals.1 Pb(II) ions are frequently detected in surface, underground, and residual water because of extensive utilization in the production of car batteries, pigments, paintings, textiles, photographic materials, ammunition, ceramics, and piping material and in welding.2 Generally, the removal of Pb(II) from contaminated water is achieved by hydroxide or sulfide precipitation,3 flocculation,4 membrane separation,5 ion exchange,6 and adsorption techniques.7,8 Among these techniques, adsorption emerges as one of the most effective processes owning to its ecofriendly, efficient, and simple operation features.9 For adsorption technology, it is crucial to prepare a green, cheap, and efficient adsorbent.10,11 Carbonaceous based functional materials have appeared to be a new type of adsorbent for removal of organic and/or inorganic pollutants from water due to their high capacity, simple operation, and easy separation. For example, Fe3O4@C nanocomposites were proven to be effective in eliminating Cr(VI) and dye molecules from aqueous systems.12,13 Carbon− alumina core−shell spheres were also prepared and assessed as adsorbents for dye pollutant removal. 14 However, the complicated preparation procedures of those carbon-based nanosized adsorbents have impeded their practical applications. To meet practical water treatment requirements, fabricating © XXXX American Chemical Society

and preparing highly efficient nanosized adsorbents in a simple and controllable approach still remains challenging. Recently, great attention has been focused on carbon/layered silicate nanocomposites due to their improved properties compared with their counterparts, conventional carbonaceous materials.15 Attapulgite clay@carbon nanocomposite was synthesized and utilized as adsorbent to effectively remove aqueous Cr(VI) and Pb(II).16 Layered double hydroxide@carbon nanospheres were fabricated and display high efficiency for Cu(II) removal.17 In addition, the synthesized smectite@carbon composite also exhibits high adsorption ability for U(VI) ions and dye pollutants compared to raw MMT.18,19 Among natural silicates, montmorillonite (MMT) is wellknown as an ecofriendly and low-cost material that has been widely applied in removing various contaminants in aqueous solution.20 The natural MMT clay can be utilized either as an adsorbent directly for pollutant removal or as a supporting matrix for the preparation of nanosized functional materials owning to its unique properties such as expandible layered structures and large surface areas.21 In the present work, an environmentally benign amorphous carbon nanocomposite is designed and fabricated using montmorillonite clay as template, Received: July 28, 2016 Accepted: November 18, 2016

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DOI: 10.1021/acs.jced.6b00676 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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and then functionalized by −COOH, −OH, and −NH2 groups on its surfaces to improve its adsorption capability. The objectives of this study are to (1) evaluate the adsorption capacities of as-prepared MMT/C nanocomposite for Pb(II) removal; (2) explore the influential factors for Pb(II) removal by functionalized MMT/C composites; and (3) further gain understanding of the adsorption kinetics, isotherms, and mechanism of functionalized MMT/C nanocomposite for Pb(II) removal. Such knowledge is significantly crucial for developing ecofriendly and economic MMT@C composites for wastewater treatment.

as an adsorbate. The samples were vacuum-dried at 393 K for 4 h before measurements. The pore size distribution curve was obtained by the desorption branch of the isotherms. Power Xray diffraction (XRD) patterns were recorded by a Bruker D8 Advance diffractometer using Cu Kα radiation (λ = 1.5408 Å). Fourier transform infrared (FTIR) spectra were obtained on a FTS-165 spectrophotometer (BIO-RAD, USA). Scanning electron microscopy (SEM) images were recorded at 15.0 kV on a ZEISS SUPRA 55VP after gold plating. Zeta potential was determined by a ZS90 Malvern instrument. Sorption Experiments. Batch experiments were performed to determine the adsorption capabilities of as-prepared adsorbents at 298−318 K. The adsorption was conducted by adding a certain amount of Pb(II) stock solution into the adsorbent suspension in a 100 mL reaction vial. The loading of adsorbent and initial Pb(II) concentration were 0.4 g L−1 and 100 mg L−1, respectively. Then the suspension was stirred at 150 rpm. At preselected intervals, samples were sacrificed for Pb(II) determination by inductively coupled plasma optical emission spectrometer (VISTA-PRO CCD). For the equilibrium adsorption isotherm tests, 50 mL samples of Pb(II) solution with various concentrations were loaded with 20 mg of adsorbents at pH 5.0 for 24 h. The amount of Pb(II) adsorbed per unit mass of sorbents can be calculated by the following equation:

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EXPERIMENTAL SECTION Materials and Chemicals. D-Glucose, NaCl, Pb(NO3)2, NaOH, HCl, H2O2, ammonium hydroxide, and dextrose monohydrate were supplied from Xinjiang Chemical Reagent Co. (Tianjin, China). The MMT clay was obtained from Lukun Company (Hebei, China) and used without further purification. Sodium saturated MMT (Na-MMT) was prepared by compensating the cation exchange sites on the clay mineral surfaces with Na+. Briefly, a certain amount of the clay was added into 0.1 mol L−1 NaCl solution and stirred for 8 h. After centrifugation at 3295g for 20 min, the supernatant was removed and replenished with another 200 mL of NaCl solution. This process was repeated five times to ensure that the clay mineral surfaces were fully compensated with Na+ ions. The saturated Na-MMT clay was then washed using Milli-Q water until free of Cl−. Preparation of Functionalized Modified MMT/C Nanocomposites. The MMT/C nanocomposites were synthesized by a one-pot hydrothermal carbonization process using Dglucose as precursor of carbon nanoparticles. Specifically, 1.5 g of Na+-MMT was dispersed in 85 mL of ultrapure water with sonication for 1 h, and then the 5.5 g D-glucose monohydrate was slowly mixed with Na+-MMT suspension under magnetic stirring for 1 h to form a homogeneous dispersion. Then, the mixture was transferred to a 100 mL Teflon-lined stainless steel autoclave and heated at 200 °C for 24 h. Then, the products were separated by centrifugation and washed with ultrapure water 4 times to separate residual reagent. The as-prepared nanocomposite was obtained by drying and grinding, and noted as MMT/C. To functionalize the surface of MMT/C by −COOH group, a mixture of MMT/C (2.0 g) and 30 wt % H2O2 solution (50 mL) was vigorously stirred for 4.0 h. The synthesized solid was washed 3 times with Milli-Q water. After drying at 60 °C, a black powder was obtained and named as MMT/C−COOH. For the MMT/C−OH composites, 3.0 g of MMT/C was added into 0.5 M sodium hydroxide solution, then transferred to an autoclave, and heated at 150 °C for 5 h. The mixtures were washed with distilled water repeatedly and dried at 60 °C for 12 h, and then MMT/C−OH was obtained. The MMT/C− NH2 composites were prepared by mixing a given amount of MMT/C with 50 mL of ethane diamine solution and then adding 2.0 g of NaNO2. The suspension was treated with sonication at 50 °C, and the suspension was treated with microwave for 40 min in 90 °C. The products were finally obtained by centrifuging, washing, and drying as described above. Characterization. The Brunauer−Emmett−Teller (BET) specific surface area and pore size were tested using Autosorb iQ-Physisorption Manometric Analyzer (Quantachrome Instruments). Experiments were performed at 77.3 K using nitrogen

Qe =

(C0 − Ce)V m

(1)

where C0 and Ce are the initial concentration and equilibrium concentration of Pb(II) (mg L−1), respectively; V is the solution volume (L); and m is the mass of sample adsorbent (g). To explore the role of pH, the initial reaction pH was adjusted to different values, i.e., 2.0, 3.0, 4.0, and 5.0, with HCl or NaOH solution. Meanwhile, the adsorption experiments were also conducted under different temperatures for 24 h.

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RESULTS AND DISCUSSION Characterization of Nanocomposites. XRD. The XRD patterns of Na+-MMT and MMT@C composites are presented in Figure 1. The Bragg equation (eq 2) was used to calculate the basal spacing of clay minerals: d(001) =

1.54178 2 sin(2θ /2)

(2)

Figure 1. XRD patterns of the Na+-MMT and as-prepared MMT/C. B



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where d(001) denotes the basal spacing of clay minerals, and θ is the incident angle. As presented in Figure 1, the basal spacing of Na+-MMT is ∼12.3, which is consistent with a previous report.22 For the synthesized MMT/C composite, the basal spacing increases to ∼15.06 (Figure 1), suggesting that the clay interlayer distance is around 5.1 Å after subtracting 9.6 Å of one smectite layer thickness. The results indicate that some carbon clusters might be inserted into the clay interlayers with size ∼5 Å.23 SEM. The SEM images of Na+-MMT clay and MMT/C composite are shown in Figure 2. The pristine MMT presents a

Table 1. Surface and Pore Properties of MMT and MMT/C sample

BET surf area (m2 g−1)

total pore vol (cm3 g−1)

av pore diam (nm)

MMT MMT/C

41.796 69.447

0.512 0.928

3.623 3.948

micropores in the MMT/C composites, and the remarkable hysteresis loops corresponding to the capillary condensation at relative pressures ranging from 0.4 to 0.8 signify the presence of mesoporous structure.21,25 Meanwhile, the peak of pore size of Na+-MMT is 3.9 nm, while the MMT/C composites partially inherit the pore distribution around 4 nm of MMT (Figure 3b). It is noted that a new peak of pore size of 7.8 nm appears for MMT/C, which is associated with the as-prepared carbon particles. The results indicate that the carbonaceous nanomaterials are really integral parts of the MMT/C composites which have a significant influence on their porosity and specific surface area. ATR-FTIR. Infrared spectroscopy is used for structural characterization of MMT and MMT/C composites (Figure 4a). The characteristic bands of MMT at 1029 and 530 cm−1 correspond to Si−O stretching and Al−O−Si deformations, respectively.26 The band at 3635 cm−1 is due to the vibrational stretching of structural −OH groups. A broad band centered at 3422 and 1654 cm−1, corresponding to the hydrogen bonded OH stretching, can be attributed to the hydrated water on the clay surface.27 After the HTC process, some new peaks appear. The peak at 2941 cm−1 is obviously observed, which is assigned to C−H vibrations. The peaks at 1707 and 1623 cm−1 are assigned to the stretching vibration of CO and CC, respectively.28 The obtained results suggest that MMT clay is partially modified by the functional carbonaceous species. ATR-FTIR spectra of three functionalized MMT/C composites, i.e., carboxylated MMT/C, hydroxylated MMT/ C, and aminated MMT/C, are compared with that of MMT/C. As shown in Figure 4b, MMT/C−COOH gives strong absorption peaks at 3400 and 1714 cm−1, corresponding to O−H stretching vibrations and CO vibrations, respectively. Compared to MMT/C, MMT/C−OH exhibits higher intensities for peaks at 1585, 1402, and 3400 cm−1. For MMT/C−NH2, the peaks appearing at 3307, 1638, and 1448 cm−1 are assigned to N−H stretching vibrations, N−H bend vibrations, and C−N vibrations, respectively.29 These results suggest that MMT/C composites are successfully modified by designated functional groups.

Figure 2. SEM images of (a) Na+-MMT and (b) MMT/C.

well-developed stacked layer structure with smooth surface and sharp edge, which is in agreement with previous study (Figure 2a).24 For MMT/C composite, the clay layers become a little disordered and less stacked compared with MMT clay, and the surface appears to be rough and the edge becomes blunt. This visualized morphological change can be attributed to the presence of flaky and granular carbon particles (in the red circles of Figure 2b), which is in line with the observation of the lower and broader peak in the XRD spectrum due to the deficiency of structural constancy (Figure 1). The spherical carbonaceous particles with smooth surfaces are adhered to the MMT flakes firmly (Figure 2b). The results based on both XRD results and SEM images confirm that the carbon is not only intercalated into the clay interlayer but also decorated at the external surface of MMT clay. BET. Figure 3 shows the N2 adsorption−desorption isotherms and pore size distributions of MMT and MMT/C composites. The BET specific surface areas of Na+-MMT and MMT/C are 41.796 and 69.447 m2 g−1, respectively, suggesting that as-prepared carbonaceous nanoparticles contribute to the surface area (Table 1). The increase in adsorbed volume at low relative pressure indicates the presence of a small number of

Figure 3. (a) N2 adsorption−desorption isotherm and (b) pore size distribution of Na+-MMT and MMT/C. C



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Figure 4. FTIR spectra of (a) Na+-MMT, MMT/C, and (b) functionalized MMT/C.

Figure 5. (a) Effect of solution pH on the Pb(II) adsorption by MMT, MMT/C, and functionalized MMT/C−COOH, C0 = 100 mg L−1, T = 308 K, and t = 24 h. (b) Effect of solution pH on the zeta potential of MMT, MMT/C, and functionalized MMT/C.

Batch Sorption Experiments. Effect of Functional Group and Solution pH. To probe the role of solution pH on Pb(II) removal by MMT, MMT/C, and functionalized MMT/C, adsorption experiments were conducted under various pH conditions, such as 2.0, 3.0, 4.0, and 5.0, respectively. The relationship between the Pb(II) adsorption capacities on adsorbents and the solution pH is displayed in Figure 5a. Noted that the adsorption capacities are significantly improved after the modification of MMT/C composites, which can be attributed to the provision of more functional groups and adsorption sites for the functionalized MMT/C.18 However, for the studied various functionalized MMT/C composites, their Pb(II) adsorption capacity follows the order carboxylated MMT/C > hydroxylated MMT/C > aminated MMT/C > MMT/C. The maximum adsorption capacity is observed for MMT/C−COOH, i.e., 199.63 mg g−1, among the as-prepared MMT/C composites. Depending on solution pH, the surface charge of MMT/C composites and metal ions exists in different states. Thus, the Pb(II) adsorption capacity of MMT/C composites varies with pH-related conditions. In this study, the adsorption capacity of various MMT/C composites, i.e., MMT, MMT/C, and functionalized MMT/C associated with various modification methods, was evaluated by the amount of Pb(II) removal under different pH conditions (Figure 5a). As a trend, the adsorption capacity increases with increasing pH over the range of 2−5 for all of the tested systems. In those reaction systems, Pb(II)

might be adsorbed on the adsorbent surface by complexation with −COOH, −OH, and −NH2 groups, respectively. The relative adsorption capacities among the functionalized MMT/ C associate well with the surface charge of the functionalized MMT/C. As shown in Figure 5b, the zeta potential of functionalized MMT/C composites decreases with the increasing of pH value. Noted that the zeta potential values of functionalized MMT/C follow the reverse order with their adsorption ability, i.e., carboxylated MMT/C < hydroxylated MMT/C < aminated MMT/C in the pH range from 2.0 to 7.0. The low zeta potential of carboxylated MMT/C indicates that the surfaces of the carboxylated MMT/C are more negatively charged compared with those of hydroxylated MMT/C and aminated MMT/C.29 Especially, positively charged surface of MMT/C−NH2 was observed when the pH value was lower than 3, which can be attributed to the protonation of the amidogen.30 On the other hand, the Pb(II) adsorption site might be occupied by a large amount of hydrogen ions when the solution pH is ∼2, making the adsorption quantity close to zero. In addition, the MMT/C and functionalized MMT/C show stronger adsorption ability toward Pb(II) at acidic condition compared with the raw MMT, which might be due to the functional groups on MMT/C composites, such as −COOH, −OH, and −NH2, having stronger complexation ability than that of Si−OH and Al−OH on the surface of MMT.18 D



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Figure 6. (a) Adsorption isotherm models for Pb(II) onto MMT/C−COOH. (b) EDXS spectrum of MMT/C−COOH with Pb(II).

Table 2. Date Fitting Results of Two Sorption Isotherm Models Langmuir −1

Freundlich −1

2

T (K)

pH

Qmax (mg g )

KL (L mg )

R

298 308 318

2.0 ± 0.1

212.70 ± 5.71 235.35 ± 9.36 247.85 ± 10.51

0.219 ± 0.03 0.285 ± 0.07 0.609 ± 0.12

0.992 0.983 0.977

KF (mg

1−n

n

−1

L g )

63.75 ± 13.93 82.52 ± 13.09 102.56 ± 16.03

n

R2

3.720 ± 0.21 4.151 ± 0.25 4.596 ± 0.28

0.896 0.932 0.910

Figure 7. Evolution of Pb(II) adsorption capacity as a function of contact time. Experimental conditions: pH = 5, sample dose is 50 mg/50 mL, initial Pb(II) concentration is 100 mg L−1, temperature is 308 K.

Sorption Isotherms. The interaction between adsorbate and adsorbent is always described by adsorption isotherms, such as Langmuir and Freundlich isotherm models, which were tested for the Pb(II) adsorption equilibrium in the present study. The Langmuir isotherm model, considered as single molecule adsorption which assumes uniform adsorption onto the solid surfaces, follows the equation31 Qe =

ln Q e = ln KF +

(4)

where KF and 1/n are indicative constants for adsorption capacity of the sorbent and adsorption intensity, respectively. The values of n indicate favorable adsorption falling in the range of 1 < n < 10. Figure 6a presents the Pb(II) adsorption isotherms on MMT/C−COOH under various temperatures, i.e., 298, 308, and 318 K. The adsorption capacities of Pb(II) by MMT/C− COOH increase significantly with the initial concentration increase in the range of 0−100 mg L−1. After the Pb(II) concentration is beyond 100 mg L−1, the adsorption capacity is little changed. The fitting results using Langmuir and Freundlich models are listed in Table 2. The correlation coefficient (R2) values demonstrate that the adsorption process can be well described by the Langmuir model rather than the Freundlich model. This indicates that the adsorption site energy is constant and there is a maximum monolayer surface

Q maxKLCe 1 + KLCe

1 ln Ce n

(3)

where Qe (mg g−1) is the equilibrium adsorption capacity, Qmax (mg g−1) is the maximum adsorption capacity, KL (L mg−1) is the equilibrium constant of the sorption process, and Ce (mg L−1) is the equilibrium concentration of Pb(II) in solution. On the other hand, the Freundlich equation derived from heterogeneous sorption on surface is expressed as follows:32 E



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Table 3. Kinetic Fitting Parameters for the Pseudo-First-Order Equation and Pseudo-Second-Order Equation pseudo-first-order equation

pseudo-second-order equation

adsorbate

Qe (mg g−1)

Qe,c (mg g−1)

k1 (min−1)

R2

Qe,c (mg g−1)

k2 (g mg−1 min−1)

R2

Pb(II)

97.64 ± 1.61

12.72 ± 0.21

0.007 ± 0.001

0.696

98.04 ± 0.85

0.003 ± 0.0001

0.999

enthalpy variation (ΔH°), entropy variation (ΔS°), and Gibbs free energy (ΔG°) are calculated by the following equations.41

coverage. The theoretical Pb(II) adsorption capacity of MMT/ C−COOH was 247.85 mg g−1, which is greater than that of most carbon-based nanocomposites, including modified activated carbon and carbon nanotube.33−37 In addition, energy-dispersive X-ray spectroscopy (EDXS) was utilized to confirm the adsorption process through the analysis of the content of surface elements. The presence of Pb, C, Si, O, and Al on MMT/C−COOH surfaces is confirmed by the observed signals of these elements (Figure 6b). It is noted that a strong signal of Pb element can be observed on MMT/ C−COOH surface, suggesting that Pb(II) has been significantly adsorbed onto MMT/C−COOH surfaces. In a typical experiment, the surface elements on MMT/C−COOH are obtained as Pb 20.74%, which is consistent with the adsorption result (Figure 6b and Table 2). Adsorption Kinetics. Adsorption rate is another crucial factor for designing adsorption contactors in practical applications. From this prospect, the correlation of Pb(II) adsorption capacity with contact time was studied. As shown in Figure 7, the adsorption amount of Pb(II) increases rapidly to 93.64 mg g−1 in 50 min and then levels off at 97.64 mg g−1. The high removal rate at the beginning of the Pb(II) adsorption can be attributed to the abundant active groups available on the adsorbent surface. On the other hand, the slow removal rate after 50 min of contact time is due to the exhaustion of the adsorption sites, and a charge balance might be attained between adsorbent and Pb(II).38 Two models, i.e., pseudo-first-order equation and pseudosecond-order equation, always utilized to describe the sorption kinetics of carbon materials,39 are expressed as follows:

ln(K °) =

ΔS° ΔH ° − R RT

ΔG° = T ΔS° − ΔH °

(7) (8)

(6)

where T is the temperature in kelvins, R is the universal gas constant (8.314 J mol−1 K−1), and K° is the equilibrium constant of sorption reaction (i.e., KL for eq 3). By plotting ln(K°) versus 1/T, ΔH° and ΔS° are obtained from the slope and intercept, respectively (Figure S2). The obtained thermodynamic parameters for Pb(II) adsorption on MMT/ C composites are listed in Table S1. In all cases, the negative value of ΔG° reveals that the Pb(II) adsorption on the MMT/ C−COOH is feasible and spontaneous. With increasing temperature, the value of ΔG° decreases, suggesting that the higher temperature is in favor of the adsorption process.42 The positive value of ΔH° (38.40 kJ mol−1) confirms that the Pb(II) adsorption on the MMT/C−COOH is an endothermic process. Therefore, the increase in temperature enhances the adsorption force and weakens the dissociation of Pb(II) from active sites.43 A positive value of ΔH° suggests some structural changes in adsorbate and adsorbent during the adsorption.44 Meanwhile, ΔS > 0 indicates that Pb(II) adsorption on MMT/ C in aqueous solution is a process of entropy increasing. These results demonstrate that the Pb(II) sorption on MMT/C− COOH is an endothermic and spontaneous process. Mechanisms of Pb(II) Removal by MMT/C−COOH. As discussed above, the electrostatic attraction might be the main driving force to bind Pb(II) ions onto the MMT/C−COOH surface. To explore the removal mechanism, the MMT/C− COOH samples with and without Pb(II) adsorption were characterized by ATR-FTIR (Figure 8a). For Pb(II) solution, three peaks appear at 3377, 1653, and 1377 cm−1, which are

where Qe (mg g ) and Qt (mg g ) are the sorption capacities at equilibrium and time t (min), respectively. k1 (min−1) is the rate constant of pseudo-first-order adsorption, and k2 (g mg−1 min−1) refers to the equilibrium rate constant of pseudosecond-order adsorption. The related linear relationships are depicted in Figure S1, and the corresponding parameters are shown in Table 3. The pseudo-second-order model with higher correlation coefficient is more suitable to describe the adsorption process than that of the pseudo-first-order kinetic model. In addition, the values of Qe (from experiment) approximate to Qe,c, calculated by pseudo-second-order equation (Table 3). Thus, the sorption process can be attributed to chemical adsorption, through covalent forces, or sharing of electrons between adsorbent and adsorbate.40 Similar results were obtained in previous work, such as Cr(III) on chitosan/attapulgite composites40 and U(VI) on montmorillonite@carbon composite.18 Adsorption Thermodynamics. Figure 6a shows the sorption isotherms of Pb(II) onto the MMT/C−COOH under different temperatures. The adsorption capacity of Pb(II) increases with the temperature increase, demonstrating that the adsorption process is endothermic. Thermodynamic parameters including

Figure 8. (a) ATR-FTIR spectra of MMT/C−COOH, MMT/C− COOH-Pb(II), and Pb(II) solution. (b) Effect of ionic strength on adsorption capacities of Pb(II). Experimental conditions: sample dose is 20 mg/50 mL, pH = 5, initial Pb(II) concentration is 60 mg L−1, temperature is 298 K.

ln(Q e − Q t) = ln Q e − k1t

(5)

t 1 t = + 2 Qt Qe k 2Q e −1

−1

F



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assigned to the stretching vibration of −OH (H2O), bending vibration of −OH (H2O), and Pb−OH. After Pb(II) adsorption, the peak at 1377 cm−1 shows a slight red shift to 1400 cm −1 , which corresponds to carboxylate COO − symmetric bending vibration. However, the intensities of peaks at 3400 and 1714 cm−1 normalized to those of MMT/ C−COOH are decreased. The results suggest that COOH reacts with Pb(II) and turns into COO− during the Pb(II) adsorption.45 This ligand exchange phenomenon suggests that inner-sphere complexes might be formed onto MMT/C− COOH surface. As reported previously, the outer-sphere complexes show strong ionic strength dependence due to counteranion competition in the background electrolytes, while the influence of ionic strength is limited for inner-sphere.46 To distinguish the type of surface complexes, the influence of ionic strength on Pb(II) removal was investigated (Figure 8b). Note that the adsorption capacities of MMT/C−COOH are independent of ionic strength, further confirming that the ligand exchange occurs between surface carboxy groups and Pb(II) through inner-sphere complexes on the MMT/C− COOH surface. The proposed dominant mechanism for Pb(II) removal can be illustrated as follows:

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*E-mail: [email protected]. Phone: +86-911-3835879. Fax: +86-911-3838957. *E-mail: [email protected]. Phone: +86-911-3835879. Fax: +86-911-3838957. *E-mail: [email protected]. Phone: +86-911-3835879. Fax: +86-911-3838957. ORCID

Hanzhong Jia: 0000-0003-1842-4218 Funding

Financial support by the NSFC (Grant No. U1403295 and 41301543), the CAS West Light Foundation (2015-XBQN-A03), the High-Tech R&D Project of Xinjiang (201415110), the National Key R&D Program (2016YFC0400501), the CAS Youth Innovation Promotion Association (2016380), and the Xinjiang High-Level Talents Introducing Program is greatly appreciated. Notes

(9)

The authors declare no competing financial interest.

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(MMT/C−COOH)2 + Pb2 + → (MMT/C−COO−)2 −Pb2 + + 2H+

(10)

(11)

The above reaction between Pb(II) and −COOH might induce the releasing of H+ in solution, which is also supported by the decreased pH value during the adsorption of Pb(II) by MMT/ C−COOH (Figure S3).

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CONCLUSIONS In this study, MMT/C and functionalized MMT/C nanocomposites were synthesized by a one-pot hydrothermal process. The spectroscopic characteristic results confirmed that the MMT were decorated by functional carbonaceous species successfully. Their adsorption properties were evaluated by the Pb(II) removal. Consequently, Pb(II) adsorption onto m-MMT/C was strongly affected by the surface functional groups. The Pb(II) adsorption capacity follows the order carboxylated MMT/C > hydroxylated MMT/C > aminated MMT/C > MMT/C. Mechanistic study implies that the complexation of Pb(II) with functional groups on the surface of adsorbents plays an important role in its removal. Relatively, high temperature and low pH value favor the Pb(II) adsorption on MMT/C−COOH. All of those results indicate that the functionalized MMT/C composites have excellent adsorption properties for removing Pb(II) from contaminated water. Further experiments for the treatment of industrial effluents are in progress.

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MMT/C−COOH + Pb(OH)+ → MMT/C−COO−−Pb2 + + H 2O

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MMT/C−COOH + Pb2 + → MMT/C−COO−−Pb2 + + H+

Kinetic models for Pb(II), temperature dependence of equilibrium constants, pH change with added MMT/C− COOH, and thermodynamic parameters (PDF)

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.6b00676. G



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Efficient Removal of Pb(II) from Aqueous Solution by Modified

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