Adsorption Properties of Pb2+ by Amino Group's ... - ACS Publications

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Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Adsorption Properties of Pb2+ by Amino Group’s Functionalized Montmorillonite from Aqueous Solutions Along Wang, Yuting Chu, Tahir Muhmood, Mingzhu Xia,* Ying Xu, Lei Yang, Wu Lei, and Fengyun Wang* School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, Jiangsu, P.R. China S Supporting Information *

ABSTRACT: The remediation of wastewater containing Pb2+ has attracted much attention due to the harm of Pb2+ to the environment and human health. A promising adsorbent, functionalized montmorillonite modified with a ligand diethylenetriamine (DETA) was prepared under mild reaction conditions. The modified montmorillonite (DETA-MMT) and nature montmorillonite (Na-MMT) were then characterized with the help of XRD, FTIR, TG/DTG, and BET. It was revealed that the modifier DETA was successfully inserted into interlayers of montmorillonite, as demonstrated by the characterizations. Batch adsorption experiments of Pb2+ onto NaMMT and DETA-MMT in solutions were studied as a function of various parameters, such as pH, contact time, initial concentration, and temperature. The maximum adsorption capacity of DETA-MMT was 61.1 mg g−1, which had more than doubled compared with that of NaMMT. The adsorption thermodynamics of Pb2+ by DETA-MMT was studied, and it showed a endothermic process. The adsorption kinetics of two samples both fit well with the pseudo-second-order model. The adsorption isotherms of Na-MMT and DETA-MMT can be well described by the Langmuir isotherm and Freundlich isotherm, respectively. Furthermore, the adsorbed Pb2+ onto DETA-MMT can hardly be dissolved under weak acidic condition (pH >4) according to the desorption experiments of simulated acid rain and Tessier’s sequential extraction procedure.

1. INTRODUCTION With the development of industry and agriculture, aquatic environments have been seriously polluted by heavy metal ions, which have aroused the extensive concern of environmentalists. In addition, these hazardous heavy metals are ordinarily bioaccumulating and nonbiodegradable, causing numerous diseases and disorders in human bodies even at low concentration.1−3 Among these toxic heavy metals, lead is one of the major toxic pollutants diffused into water in various ways, such as lead battery manufacturing, smelters metal, plating, oil refining, paint, and ammunition industries.4,5 Lead cannot be metabolized by organisms, and it is of great harm to the kidneys, nervous system, reproductive system, liver, and brain.6,7 Hence, the elimination of lead from contaminated water before its discharge into aquatic environment is urgent to keep the ecosystem stable and humans healthy.8 To date, many technologies have been employed for the separation of heavy metals from aqueous solutions, such as adsorption, chemical precipitation, redox, membrane separation, ion exchange, as well as biological treatment.9−12 However, in view of economic efficiency, simplicity of design, and environmental protection, some of these methods are infeasible and hard to implement in practice. Among all of the above-mentioned techniques, adsorption has been considered as one of the most feasible, low-cost, easy to operate, and highefficiency methods to remove heavy metals from wastewater.13,14 Many kinds of adsorption materials have been © XXXX American Chemical Society

extensively researched, including activated carbon, carbon nanotubes, zeolites, nonliving biomass, clay minerals, and other aluminosilicates,4,15,16 but most of the adsorbents are hardly applied on large scales because of their low efficiency, high cost, and other natural properties. Therefore, the environmentally friendly, practical, and economic adsorbents should be further studied.17 Clays are abundant and nontoxic natural resources, which have attracted much attention in terms of their highly potential adsorption capacity and relatively low cost. In recent years, montmorillonite has been widely investigated as an outstanding adsorbent in the domain of environmental remediation due to its unique physicochemical properties, such as high CEC (cation exchange capacity), large specific surface area, microlayered structure, and high physical−chemical stability.18−20 The main components of bentonite is mineral montmorillonite with a 2:1-type structure of clay.5,21,22 Because of the isomorphic substitutions on montmorillonite, some exchangeable cations can be adsorbed into layers to offset the unbalanced charge, such as H+, Na+, or Ca2+.23,24 However, the nature montmorillonite often shows some drawbacks in practical application.25 To improve the montmorillonite properties of adsorption capacity, selectivity, Received: March 25, 2018 Accepted: May 31, 2018

A

DOI: 10.1021/acs.jced.8b00239 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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through a 74 μm size sieve, and kept in a sealed container.30 The schematic diagram of the modification process is shown in Figure S2. 2.3. Characterization. The powder X-ray diffraction (XRD) patterns of the samples were collected using a Bruker D8 X-ray diffractometer with Cu Kα radiation (l = 0.15406 nm, voltage = 40 kV, electrical current = 40 mA, 2θ = 4−50°). Fourier transform infrared spectra (FTIR) were obtained by an FTIR spectrophotometer from 4000 to 550 cm−1 (Thermo Fisher, NicoletiS10, FTIR Spectrometer, USA). The Brunauer−Emmett−Teller (BET) specific surface area of the samples was determined using N2 adsorption−desorption measurements with an Autosorb-iQ-MP special surface area and pore size analyzer (Quantachrome Instruments, USA). Thermogravimetric analysis (TGA) of the samples were performed on an STA 449 F3 Jupiter thermogravimetric analyzer (Netzsch, Germany). The samples were heated at a heating rate of 10 K min−1 from 313 to 1173 K under a high-purity nitrogen (99.999%) flow rate of 40 mL min−1. 2.4. Batch Adsorption Experiments. The adsorption capacity of DETA-MMT to Pb2+ was analyzed using batch adsorption experiments compared with Na-MMT. The primary influence factors in the adsorption experiments, such as initial Pb2+ concentration, pH, contact time, and temperature, were studied comprehensively. The effect of initial Pb2+ concentration was determined by varying concentration from 50 to 500 mg L−1 and adding 0.4 g Na-MMT or DETA-MMT to 100 mL of Pb2+ solution with the conditions of pH 5.5, contact time of 90 min, and at a temperature from 283 to 323 K. The effect of pH value was determined by varying pH from 2 to 5.5 and adding 0.4 g Na-MMT or DETA-MMT to 100 mL of Pb2+ solution with the conditions of Pb2+ concentration at 300 mg L−1, contact time of 90 min, and at a temperature of 293 K. The effect of contact time was determined by varying time from 5 to 150 min and adding 0.4 g Na-MMT or DETA-MMT to 100 mL of Pb2+ solution with the conditions of Pb2+ concentration at 300 mg L−1, pH 5.5, and at a temperature of 293 K. All of the experiments were replicated three times to reduce the systematic error. The concentrations of Pb2+ in solution were measured by AAS (atomic absorption spectrophotometry). The adsorption capacity of Pb2+ was calculated through the following equation31

dispersibility, and thermostability, many special modification technologies have been performed, such as pillared-clay, organoclay, acidulated-clay, as well as polymer/clay composites. In particular, a series of organic modified montmorillonites with particular functional groups (e.g., −SH, −NH2, −COOH, −CS2) have been reported before, which can specifically interact with heavy metals.26 Stathi et al.27 prepared four novel organoclays with four kinds of chelating functionalities (−NH2, −COOH, −SH, or −CS2) pillared into the interlayers of montmorillonite and then investigated their adsorption capacity and selectivity for Pb2+, Cd2+, and Zn2+. de Mello Ferreira Guimarães et al.28 synthesized a bentonite organofunctionalized with a thiol (−SH) group for the adsorption of Ag+. Ö zcan et al.29 prepared an 8-hydroxy qinoline-immobilized bentonite for the removal of Pb2+. The results indicated that the adsorption capacity of Pb2+ increased remarkably compared with the nature montmorillonite. The above research indicates that these organic modifiers with such particular functional groups can efficiently improve the quality of montmorillonite for the removal of heavy metal contaminants. However, little information was reported in the literature about organoclay modified with amino (−NH2) groups. In this work, a cheap and environmentally friendly chelating agent, diethylenetriamine, was employed to modify Na-montmorillonite, and the adsorption properties of Pb2+ in comparison with the nature Na-montmorillonite were studied. The materials were characterized by X-ray powder diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TG), and N2 adsorption−desorption experiments (BET method). Meanwhile, the influence of adsorption factors, such as contact time, pH, initial concentration, and temperature, were investigated, respectively. Lastly, the adsorption mechanism, kinetics, thermodynamics, adsorption stability, and forms of Pb2+ are also discussed in this paper.

2. MATERIALS AND METHODS 2.1. Materials. The nature montmorillonite sample used in this study was provided by Wancheng Montmorillonite Co. Ltd. (Heishan, Liaoning, China). The CEC (cation exchange capacity) of this clay measured by an ammonium chloride− ethanol method is 90 mmoL/100g. Diethylenetriamine (DETA), lead nitrate (Pb(NO3)2), sulfuric acid (H2SO4), nitric acid (HNO3), magnesium chloride (MgCl2), acetic acid (HOAc), sodium acetate(NaOAc), hydroxylamine hydrochloride (NH2OH·HCl), ammonium acetate (NH4OAc), hydrogen peroxide (H2O2), hydrochloric acid (HCl), and sodium hydroxide (NaOH) were purchased from Aladdin Chemical Corporation. All of the reagents used in this study were analytical grade and without further purification. Deionized water was used for preparing solutions and washing samples. A brief introduction and structural formula of DETA are shown in Figure S1. 2.2. Preparation of DETA-MMT. The montmorillonite sample (4 g) was suspended in 400 mL of deionized water (1% w/v); then, 1% diethylenetriamine solution, which was equivalent to the amount of CEC, was slowly added to the suspension, and the pH was adjusted to 5 with 0.1 M HCl and 0.1 M NaOH in the case of strong agitation. The mixture was stirred for 24 h in 298 K to ensure the adequate immobilization of diethylenetriamine onto montmorillonite. Afterward, suction filtration was applied to separate the solid and washed twice with deionized water. The composite was subsequently dried at 378 K for one night in the oven and then ground, sieved

qe =

(C0 − Ce) × V m

(1)

where qe (mg g−1) is the adsorption capacity of Pb2+, C0 (mg L−1) and Ce (mg L−1) are the initial Pb2+ concentration and the equilibrium Pb2+ concentration, V (L) is the volume of the solution, and m (g) is the mass of adsorbent used. 2.5. Adsorption Stability and Forms of Pb 2+ . 2.5.1. Simulated Acid Rain Eluent. The adsorption stability experiments were carried out on Na-MMT and DETA-MMT with simulated acid rain eluent in the case of different concentrations of acidic solutions. The simulated acid rain solution was prepared by mixing H2SO4 (98%) and HNO3 (68%) at a ratio of 5:1; then, it was diluted with deionized water to a series of pH values (pH 1−6) and pure water (pH 7).32 First, 1.4 g of Na-MMT or DETA-MMT was added to 700 mL of Pb2+ solution with the concentration of 200 mg L−1 and stirred for 12 h at 293 K. Then, the solid was separated with the help of suction filtration and washed three times with deionized water. After that, the collected solid was divided into seven parts on average and put into 100 mL acidic solutions B



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Na-MMT and DETA-MMT samples, which can reflect basal spacing through the typical characteristic peak of the d001 plane. The typical characteristic peak of Na-MMT in the d001 plane is 7.16° (basal spacing = 1.23 nm), from which it can be determined that the dominant cation between the interlayers of natural montmorillonite is sodium.34 After the intercalation of DETA, the diffraction peak at a 2θ value shifted from 7.16 to 6.65° (basal spacing = 1.33 nm), which meant that the basal spacing of DETA-MMT increased from 1.23 to 1.33 nm. The result that the basal spacing of montmorillonite increased by only 0.1 nm after intercalation can be attributed to the relatively small volume of DETA. The two similar XRD patterns of montmorillonite also demonstrate that its layered structure remains constant after modification. 3.1.2. FTIR Analysis. The FTIR spectra of Na-MMT and DETA-MMT are shown in Figure 2. It is observed that the

with different pH values, respectively. The mixtures were constantly stirred for 3 days at 293 K. The equation of desorption ratio was calculated as follows desorption ratio =

C2 × 100 C0 − C1

(2)

where C0 (mg L−1) is the initial concentration of Pb2+ solution (200 mg L−1), C1 (mg L−1) is the equilibrium concentration of Pb2+, and C2 (mg L−1) is the concentration of desorption solution. 2.5.2. Sequential Extraction Procedure. The forms of Pb2+ adsorbed onto montmorillonite were also studied in this paper according to Tessier’s sequential extraction procedure.33 One g of DETA-MMT was prepared as above, and it was mixed with 500 mL of Pb2+ solution with the initial concentration of 200 mg L−1 and stirred for 3 days at 293 K. The mixture was then allowed to stand and aged for 20 days. Sequential extractions were carried out on the basis of the Tessier procedure on the adsorbed DETA-MMT. Five fractions of the extracted Pb2+ were obtained. They are exchangeable fraction (F-i), carbonatebound fraction (F-ii), Fe−Mn oxide-bound fraction (F-iii), organio-bound fraction (F-iv), and residual fraction (F-v). In addition, residual fraction (F-v) was not discussed here due to the short contact time of Pb2+ and adsorbent. The specific extraction methods are shown in Table 1. Table 1. Sequential Extraction Methods of Pb2+ on DETAMMT forms

extraction methods

F-i F-ii

8 mL of MgCl2 solution (1 M, pH 7) at 298 K for 1 h 8 mL of NaOAc solution (1 M) adjusted to pH 5 with HOAc at 298 K for 5 h 20 mL of 0.04 M NH2OH·HCl in 25% HOAc (v/v) at 369 K for 6 h 3 mL of 0.02 M HNO3 and 5 mL of 30% H2O2 at 358 K for 2 h, and another 3 mL of 30% H2O2 was added into the mixture for further 3h then cooling to 298 K, 5 mL of 3.2 M NH4OAc in 20% HNO3 (v/v) was mixed with the sample for 30 min

F-iii F-iv

Figure 2. FTIR spectra of DETA-MMT and Na-MMT.

absorption peak of Na-MMT at 3617 cm−1 is a stretching vibration of the −OH group bound to an aluminum (Al−OH). The peak at 1112 cm−1 is ascribed to the Si−O outside-of-plane stretching vibration, and the most intensive peak at 979 cm−1 is ascribed to the Si−O in-plane stretching vibration, which is a typical peak of the materials with silicate structure. The peak at 1438 cm−1 is due to the stretching vibration of CO32− related to dolomite (an impurity of montmorillonite).35 Similarly, the absorption peak of DETA-MMT at 3617 cm−1 is also a stretching vibration of the −OH group; The peaks at 1112 and 979 cm−1 are attributed to the Si−O outside-of-plane and inplane stretching vibrations, respectively. Besides, several new peaks arouse on DETA-MMT. The new absorption band between 3000 and 3500 cm−1 represents the O−H stretching vibration, and the peak at 1632 cm−1 represents the bending vibrations of the N−H group, which may be overlapped by the bending vibrations of O−H group.36 The weak peak at 1350 cm−1 is attributed to the stretching vibration of the C−N group, and that at 1471 cm−1 is attributed to the bending vibrations of the C−H group. These results indicate that the modifier DETA has grafted onto montmorillonite successfully, and they is also consistent with the results of XRD patterns. 3.1.3. TG/DTG Analysis. The TG/DTG curves of samples are illustrated in Figure 3. The thermal stability and the mass loss of DETA grafted on montmorillonite can be shown according to the TG/DTG tests. In TG curves (solid lines) of Na-MMT, two main stages of mass loss can be clearly seen, taking place in the range of 308−503 K and 673−1013 K. The mass loss of 1.86% in the first 503 K is due to the evaporation

3. RESULTS AND DISCUSSION 3.1. Characterization. 3.1.1. XRD Analysis. XRD is one of the most important techniques used to illustrate the changes of layer spacing and the structural characterization of natural and modified montmorillonite. Figure 1 shows the XRD patterns of

Figure 1. XRD patterns of DETA-MMT and Na-MMT. C



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Table 2. Textural Properties of DETA-MMT and Na-MMT sample

SBET (m2 g−1)

average pore radius (nm)

total pore volume (cm3 g−1)

Na-MMT DETA-MMT

113.016 194.760

3.24677 2.09583

0.1835 0.2041

DETA and the increases in basal spacing. On the contrary, in the case of the increases in specific surface area and total pore volume, many new available adsorption sites were simultaneously produced,40 so that a much better adsorption capacity of DETA-MMT for Pb2+ can be presented in the following adsorption experiments. 3.2. Pb2+ Adsorption by Na-MMT and DETA-MMT. 3.2.1. Effect of Contact Time. Figure 5 shows the effect of Figure 3. TG/DTG curves of DETA-MMT and Na-MMT. The solid lines are TG curves and the dotted lines are DTG curves.

of physical adsorbed water and structural water.37 Another stage at 673−1013 K of mass loss of ∼6.09% can be attributed to the dehydroxylation of the layer structure.38 As for DETAMMT, the total quantity of mass loss is obviously larger than that of Na-MMT. The similar curve about 2.50% mass loss is due to the evaporation of water below 523 K. After that, a continuous mass loss of ∼9.80% totally happens at the temperature of 523−1013 K. In terms of the DTG curves (dotted lines) of the DETA-MMT, it can be inferred that the 3.15% mass loss is due to the desorption of DETA at 523−673 K and the mass loss of 6.65% at 673−1013 K results from the dehydroxylation of the aluminosilicates. 3.1.4. N2 Adsorption−Desorption Isotherm Analysis. The N2 adsorption−desorption isotherms of Na-MMT and DETAMMT are shown in Figure 4. The isotherm features are fit well

Figure 5. Effect of contact time on the removal of Pb2+ under the condition of C0 = 300 mg L−1, pH 5.5, T = 293 K.

contact time on the adsorption properties of Pb2+ onto NaMMT and DETA-MMT. It can be seen that the adsorption of Pb2+ onto both increases rapidly in the first 30 min. After that, the adsorption equilibrium is reached within further 120 min, where it is up to the maximum adsorption capacity of Pb2+ under certain conditions. In the beginning, the adsorption processes are rapid mainly due to the plentiful exposed adsorption sites on the external surface and interlayers of NaMMT and DETA-MMT. Afterward, the limited adsorption sites are quickly occupied by Pb2+ in solution until saturated adsorption is reached.8 Obviously, the adsorption capacity of Pb2+ is much higher than Na-MMT. It may be due to the increasing basal spacing and adsorption sites of DETA-MMT mentioned above. 3.2.2. Effect of pH. The pH value in solution is the most significant factor in the adsorption process. It can not only change the surface charge of montmorillonite but also affect the species of Pb2+ in solution.41 It has been reported that the dominant species of lead in solution is Pb2+ at pH 8.42 The species changes are shown as follows

Figure 4. N2 adsorption−desorption isotherms of DETA-MMT and Na-MMT.

with type IV for both of them, and the hysteresis loops accord with type H4 on the basis of the IUPAC classification, which are characteristics of layered materials.39 In addition, the values of the specific surface area, average pore radius, and total pore volume of the two samples are listed in Table 2. It can be seen that the specific surface area and total pore volume of DETAMMT are larger than Na-MMT. On the contrary, the average pore radius become smaller. The results suggest that a large number of small pores are created on DETA-MMT after modification, which should be attributed to the intercalation of

OH−

OH−

H

H

H 2O + Pb2 + Hooo+oI Pb(OH)+ Hooo+oI Pb(OH)2 ↓

In this paper, the pH value range we studied is 2−5.5 to prevent the formation of hydroxide in high-pH solution. The results are shown in Figure 6. It can be seen that the adsorption capacity of Pb2+ increases rapidly onto both Na-MMT and D



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ion-exchange and chelation reaction happening in further inner layers that is caused by the more violent movements of molecules at higher temperature. It also may be due to the enhanced penetration of Pb2+ from the bulk solution to the adsorbent surface.44 It is obvious that the percent removal decreases with the increase in Pb2+ initial concentration. The percent removal can reach >99.6% of Pb2+ onto DETA-MMT at relatively low Pb2+ concentration ( 0, and ΔS0 > 0 means the adsorption process of Pb2+ can increases the confusion of the contact surface of the solid and liquid. The

Figure 7. Effect of initial concentration and temperature on the removal of Pb2+ under the condition of pH 5.5, t = 90 min.

the adsorption of Pb2+ in a range of 50−500 mg L−1 at different temperatures (283−343 K). It is observed that the adsorption capacity of Pb2+ increases rapidly with the increase in Pb2+ initial concentration at relatively low concentration (50−300 mg L−1) on both Na-MMT and DETA-MMT. In the case of higher concentration (300−500 mg L−1), the adsorption capacity of Pb2+ increases slightly due to the limited quantity of adsorption sites, and it almost approaches saturation at the concentration of >300 mg L−1. The maximum adsorption capacity of Na-MMT and DETA-MMT is about 30.5 and 61.1 mg g−1 at 293 K, respectively. In addition, it can be seen that the adsorption capacity increases slowly with the increase in temperature. This phenomenon may be attributed in part to the

Figure 8. Thermodynamic fitting of adsorption process of Pb2+ by DETA-MMT. E



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Table 3. Adsorption Thermodynamics Parameter Values of DETA-MMT T (K)

ΔG0 (kJ mol−1)

ΔH0 (kJ mol−1)

283 293 303 313 323

−0.306 −0.0779 −1.209 −1.528 −2.003

11.305

ΔS0 (J (mol·K) 41.120

−1

R2

)

0.9918

ΔG0 < 0 shows that the adsorption process is a spontaneous reaction. Moreover, with the increase in temperature, ΔG0 is gradually decreasing, which shows a favorable adsorption process at higher temperature. 3.4. Kinetic Studies of Adsorption. The pseudo-firstorder kinetic and pseudo-second-order kinetic models are investigated with sorption kinetic data to describe the kinetics of Pb2+ removal, which can well clarify the equilibrium adsorption capacity and adsorption rate. The pseudo-firstorder kinetic model is on account of the assumption that the adsorption rate is proportional to the only primary factor, such as the free adsorption sites in the adsorption system. The rate equation is ln(qe − qt ) = ln qe − K1t

Figure 9. Pseudo-second-order kinetic models for the adsorption of Pb2+ onto DETA-MMT and Na-MMT.

Table 4. Kinetic Parameter Values of Na-MMT and DETAMMT pseudo-second-order adsorbent

(7)

n−1

× 100

31.55 56.91

K2 (mg (g·min)−1) −3

6.42 × 10 5.31 × 10−3

R2

Δq (%)

0.997 0.999

0.25 0.077

Langmuir isotherm model is the most popular adsorption isotherm model, which can well describe the experimental data within a wide range of concentrations. The Langmuir isotherm model is based on the assumptions that (1) the surface properties of adsorbent are homogeneous; (2) there is no interaction among molecules adsorbed on the adsorbent surface; and (3) the surface of the adsorbent is monolayer adsorption.48 The equation of the Langmuir isotherm model is

Ce C 1 = + e qe KLqmax qmax −1

(10) 2+

where Ce (mg L ) is the equilibrium concentration of Pb , qe (mg g−1) is the equilibrium adsorption capacity, qmax (mg g−1) is the maximum adsorption capacity, and KL (L mg−1) is the equilibrium adsorption constant related to the adsorption capacity. The plots of Ce/qe to Ce are shown in Figure 10a. The parameter values of qmax and KL can be calculated though the slope and intercept, respectively. In addition, the separation factor (RL, also called equilibrium parameter) was also studied to estimate whether the adsorption process is favorable. The equation is 1 RL = 1 + KLC0 (11)

where qe (mg g−1) is the equilibrium adsorption capacity, qt (mg g−1) is the amount of Pb2+ adsorbed onto montmorillonite at time t (min), and K2 (mg (g·min)−1) is the adsorption rate constant of the pseudo-second-order kinetic model. The plots of t/qt to t are shown in Figure 9, from which the parameter values of qe and K2 can be calculated though the slope and intercept, respectively. The kinetic parameter values of Na-MMT and DETA-MMT are listed in Table 4. The results indicate a higher correlation coefficient of the pseudo-second-order kinetic model compared with the pseudo-first-order model (shown in Table S1) for both samples. The standard deviation Δq (%) was also calculated for the further proof of the validity of the pseudo-second-order kinetic model.46,47 The equation is Δq (%) =

qe (mg·g )

Na-MMT DETA-MMT

where qe (mg g−1) is the equilibrium adsorption capacity, qt (mg g−1) is the amount of Pb2+ adsorbed onto montmorillonite at time t (min), and K1 (min−1) is the adsorption rate constant of pseudo-first-order kinetic model. The plots of ln(qe − qt) to t are shown in Figure S4, from which the parameter values of qe and K1 can be calculated though the intercept and slope, respectively. The pseudo-second-order kinetic model is on account of the assumption that the adsorption rate is proportional to the square of free adsorption sites. The rate equation is t 1 t = + 2 qt qe K 2qe (8)

∑ [(qt ,exp − qt ,cal)/qt ,exp]2

−1

where KL (L mg−1) is the equilibrium adsorption constant and C0 (mg L−1) is the initial concentration of Pb2+. The value of RL reflects the shape of the isotherms to be unfavorable (RL > 1), favorable (0 < RL < 1), or irreversible (RL = 0).49 The Freundlich isotherm model is an empirical equation applied to multilayer adsorption of heterogeneous systems.45 The equation is 1 ln qe = ln Ce + ln K f (12) n

(9) 2

Considering the values of the correlation coefficients R and the standard deviation Δq (%) together, it quite affirming that the adsorption process of both Na-MMT and DETA-MMT can be well depicted by the pseudo-second-order kinetic model. 3.5. Isotherm Studies of Adsorption. The Langmuir and Freundlich isotherm models are two common and practical adsorption isotherm models used in adsorption studies. The

where Ce (mg L−1) is the equilibrium concentration of Pb2+, qe (mg g−1) is the equilibrium adsorption capacity, n is the F



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exists (ion exchange). In the case of DETA-MMT, the adsorption isotherm can be better depicted by the Freundlich isotherm model according to the correlation coefficients. This phenomenon can be explained by the fact that the adsorption process is a heterogeneous system and the dominant adsorption mechanisms of Pb2+ onto DETA-MMT are chelation with DETA together with ion exchange.3 Moreover, the high value of n of DETA-MMT also reflects the positive adsorption of Pb2+. The maximum adsorption capacities of Na-MMT and DETA-MMT calculated though the isotherm model are 31.78 and 64.14 mg g−1 respectively, which are very close to the experimental values. The larger KL of DETA-MMT also represents the better adsorption capacity of Pb2+ because of its positive correlation. The parameters RL of Na-MMT and DETA-MMT are both between 0 and 1, which reflects a favorable adsorption process of Pb2+.51 3.6. Adsorption Stability and Extraction Studies. 3.6.1. Simulated Acid Rain Eluent. The adsorption stability of adsorbents is a significant factor, which can determine the application prospect and domain of adsorbents. In this study, the adsorbed samples of DETA-MMT and Na-MMT were subjected to simulated acid rain eluent with different pH (1−7) to evaluate their adsorption stability for Pb2+. The experiments were conducted for 3 days at 293 K constantly. Then, the solutions were separated through centrifugation and the concentrations of Pb2+ were tested with the help of AAS. Figure 11 shows the desorption performance of Pb2+ on NaMMT and DETA-MMT under the condition of different pH

Figure 10. Langmuir (a) and Freundlich (b) isotherm models for the adsorption of Pb2+ onto DETA-MMT and Na-MMT with the conditions of Pb2+ concentration at 300 mg L−1..

dimensionless coefficient that can evaluate the intensity of adsorption, and Kf is the equilibrium adsorption constant related to the adsorption energy of the Freundlich isotherm model. The plots of ln qe to ln Ce are shown in Figure 10b, from which the parameter values of n and Kf can be calculated though the slope and intercept, respectively. Furthermore, the higher the value of n in the Freundlich isotherm, the easier the adsorption of Pb2+ onto montmorillonite.50 The isotherm parameter data of Na-MMT and DETA-MMT are listed in Table 5. It can be seen that the correlation coefficients of Na-MMT are fit well with the Langmuir isotherm model, which infers that the adsorption process is a monolayer adsorption and only one kind of adsorption site

Figure 11. Desorption ratio of DETA-MMT and Na-MMT at different pH values.

solutions, and the data are listed in Table 6. It can be seen that both samples perform poor desorption in the case of weaker acidic solutions (pH >4), and it is unfavorable with the increase in pH value. In particular, the desorption ratio of DETA-MMT is only 1.94% at pH 4 (lower than Na-MMT), and it is evident that the desorption ratio of DETA-MMT is lower than NaMMT at pH 3. However, in a strong acid solution (pH ≤2), both showed an exceedingly high desorption ratio. The phenomenon can be explained by the fact that the layer structure of montmorillonite might be collapsed or the Pb2+ was replaced by H+ due to the strong acid, and thus the adsorbed Pb2+ can be mostly released. Normally, there is no such extreme strong acid rain condition in environment, so that the adsorbed heavy metals can be locked onto the DETA-

Table 5. Isotherm Parameter Data of Na-MMT and DETAMMT model

parameter

Na-MMT

DETA-MMT

Langmuir

qmax (mg g−1) KL (L mg−1) R2 RL n Kf R2

31.78 0.0676 0.999 0.029−0.228 4.1 8.14 0.885

64.14 1.304 0.988 0.0015−0.015 5.3 22.67 0.997

Freundlich

G



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Table 6. Desorption Ratio Data at Different pH Values of Na-MMT and DETA-MMT pH

7

6

5

4

3

2

1

Na-MMT DETA-MMT

0.81 0.14

1.07 0.51

2.81 1.32

6.17 1.94

58.31 12.23

94.15 59.70

95.55 75.26

fraction in this system. On account of the modification by organic chelating agent, the Pb2+ was combined onto DETAMMT mostly as a result of chelation. The organio-bound fraction (F-iv) of Pb2+ is quite stable in the environment, and it can effectively mitigate the threat of Pb2+ to human health. 3.7. Adsorption Mechanism. The possible mechanisms for the protonation of DETA and intercalation montmorillonite are shown in Figure S5. The ion exchange of Na-MMT is responsible for the uptake of Pb2+ so that the desorption ratio is closely related to the concentration of H+ in solution. The batch adsorption process, adsorption stability, and extraction experiments of Pb2+ by DETA-MMT were carried out systematically in aqueous solution. The results consistently indicated that Pb2+ ions could be adsorbed and firmly immobilized by DETA-MMT, especially the combination through chelation. A possible mechanism of chelating with Pb2+ can be seen from Figure 13.

MMT for a long period under certain conditions to decrease the threat to human health. 3.6.2. Sequential Extraction Procedure. The stabilities of five forms of Pb2+ are increasing successively in nature. Exchangeable fraction (F-i) is easy to migrate, transform, and be absorbed by plants, which indicates a great harm to human beings. Carbonate-bound fraction (F-ii) shows pH-sensitive values in the environment. Fe−Mn oxide-bound fraction (F-iii) is closely related to pH values and oxidation−reduction potential. Organio-bound fraction (F-iv) means that Pb2+ is firmly combined with organic chelating agents or sulfide. Residual fraction (F-v) is the result of the natural geological weathering process, which can stably exist in sediments for a long time and is hard to be absorbed by organisms. Figure 12 shows four forms of Pb2+ adsorbed onto DETAMMT and the proportions according to Tessier’s sequential

4. CONCLUSIONS The functionalized adsorbent DETA-MMT with high adsorption capacity was synthesized successfully under mild reaction conditions. The samples were also characterized by the analysis methods including XRD, FTIR, TG/DTG, as well as BET, and all of the characterizations can confirm each other that the modifier DETA was inserted into interlayers of montmorillonite. The specific surface area of DETA-MMT doubled compared with nature montmorillonite despite the slight increases in layer spacing. The modified montmorillonite was then investigated for the adsorption of Pb2+ from aqueous solutions, and the results showed that the maximum adsorption capacity of Pb2+ increased by more than two times. The batch adsorption experiments illustrated that the removal capacity of Pb 2+ was crucially dependent on pH and the initial concentration of solution and it was favorable with an increase in pH in acidic solution. The adsorption thermodynamics of Pb2+ by DETA-MMT showed that the adsorption process was a endothermic reaction. The adsorption kinetics of both Na-MMT and DETA-MMT showed a better fit with the pseudo-secondorder model. The adsorption isotherm data of Na-MMT could be well described by the Langmuir isotherm. However, the adsorption isotherm data of DETA-MMT followed the Freundlich isotherm better, which inferred that more than one kind of adsorption site happened on DETA-MMT (ion

Figure 12. Proportions of Pb2+ forms adsorbed onto DETA-MMT.

extraction procedure. The exchangeable fraction (F-i) of Pb2+ is only 0.27%, which means only a little Pb2+ adsorbed onto DETA-MMT can be adsorbed by plants and released into the environment. The 0.74% carbonate-bound fraction (F-ii) may be due to the conversion of CaCO3 (an impurity of montmorillonite) into PbCO3, and the result is also consistent with the simulated acid rain experiments above. The phenomenon can be explained by the fact that the Ksp of PbCO3 (7.4 × 10−14) is smaller than that of CaCO3 (3.36 × 10−9). The third part, the Fe−Mn oxide-bound fraction (F-iii), happens owing to the existence of so much Fe2O3 and MnO in montmorillonite, particularly at relatively high pH value (pH 6).52 The organio-bound fraction (F-iv) of Pb2+ is the major

Figure 13. Possible mechanism of modified montmorillonite and Pb2+ adsorption process. H



Article

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exchange and chelation). The adsorption stability experiments of simulated acid rain showed that the release of Pb2+ could hardly happen in the case of weak acidic condition (pH >4), reflecting a firm immobilization of Pb2+ by DETA-MMT under certain conditions. Besides, Tessier’s sequential extraction procedure experiments showed the proportion of four different forms of Pb2+ adsorbed onto DETA-MMT. The active part exchangeable fraction was only 0.27%, inferring a passivation of Pb2+ after adsorption. For the most part, organio-bound fraction was stable, and Pb2+ was firmly locked into the interlayers of DETA-MMT in the form of chelation to prevent harm to human beings. In summary, the modified montmorillonite showed great potential and promising advantages for the removal of heavy metals.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00239. Brief introduction of DETA, schematic diagram of modification, SEM images Na-MMT and DETA-MMT, pseudo-first-order kinetic model, and possible mechanism of modification. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*M.X.: E-mail: [email protected]. *F.W.: E-mail: [email protected]; wangfy@njust. edu.cn. ORCID

Tahir Muhmood: 0000-0002-7441-1617 Fengyun Wang: 0000-0002-2359-9875 Funding

We acknowledge the National Natural Science Foundation of China (51572130, 51242001, 51572127, and 5122351). Notes

The authors declare no competing financial interest.

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