Sorption of Cd2+ on mercapto and amino functionalized palygorskite

Applied Surface Science 322 (2014) 194–201

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Sorption of Cd2+ on mercapto and amino functionalized palygorskite Xuefeng Liang a,b , Jun Han c , Yingming Xu b,∗ , Lin Wang b , Yuebing Sun b , Xin Tan a a

School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, PR China Key Laboratory of Original Environmental Quality of MOA, Agro-Environmental Protection Institute of Ministry of Agriculture, Tianjin 300191, PR China c Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, PR China b

a r t i c l e

i n f o

Article history: Received 29 June 2014 Received in revised form 13 October 2014 Accepted 16 October 2014 Available online 22 October 2014 Keywords: Palygorskite Mercapto Amino Cadmium Sorption

a b s t r a c t Mercapto and amino functionalized palygorskite samples were prepared by nanotexturization method, respectively, and applied for the sorption of Cd2+ from aqueous solution to investigate the sorption mechanism and assess the application potential for the remediation of Cd polluted water and soils. The samples before and after sorption were characterized through XRD, FTIR, 29 Si CP/MAS NMR and XPS. The sorption thermodynamics, kinetics and mechanisms of Cd2+ sorption on mercapto and amino functionalized palygorskite were studied. Langmuir isotherm was proved to describe the sorption data better and pseudo second order kinetic model could fit the sorption kinetic processes well. Functionalization increased the sorption amounts significantly compared to pristine palygorskite. The positive enthalpy change confirmed that the sorption process was endothermic. The positive entropy changes revealed that sorption of Cd2+ was driven by entropy changes. Combined the results of analyses of the diffraction peaks by XRD, vibration modes of functional groups by FTIR, chemical shifts by 29 Si CP/MAS NMR and binding energies of key elements by XPS, the sorption mechanisms were complexation with mercapto or amino groups, and surface precipitation of CdCO3 . © 2014 Elsevier B.V. All rights reserved.

1. Introduction At present, the remediation of heavy metal polluted soil is a hot topic of environmental science because heavy metal contamination has jeopardized humans and ecosystem health. A piece of news in 2013 about Cd contaminated rice sold in Guangdong province recalled a public concern of Chinese rice safety [1]. At present 19.4% of farm lands in China was heavily contaminated with toxic heavy metals and 82.8% of the contaminants found in farms are inorganic contaminants such as cadmium, mercury, arsenic, copper and lead [2]. A number of remediation techniques for heavy metal contaminated soil have been developed, such as chemical washing [3], phytoremediation [4] and chemical immobilization [5,6]. In situ chemical immobilization relies on the addition of amendments to soils to increase the proportion of heavy metals burden within the soil colloids by sorption, precipitation or solidification in order to reduce the bioavailability or activity of heavy metals. Among all the different immobilization materials, clays have been used widely due to their unique properties such as high surface area, low cost and the ubiquitous occurrence in most soil and sediment environment [7]. For natural clay minerals, there are still

∗ Corresponding author. Tel.: +86 2223618061; fax: +86 22 23618060. E-mail address: [email protected] (Y. Xu). http://dx.doi.org/10.1016/j.apsusc.2014.10.092 0169-4332/© 2014 Elsevier B.V. All rights reserved.

some limitations such as limited adsorption capacity, small metal binding constants and low selectivity. In the recent years, functionalization of clay minerals has been explored in an effort to enhance the heavy metal binding constants and the selectivity of the type of metals [8,9]. The approaches used for the modification of clays include intercalation of functional molecules into the interlayer space, grafting of organic moieties on the surface [10,11]. For example, amino-functionalized MCM-41 and MCM-48 were prepared for the removal of chromate and arsenate [12]. Natural and acid-activated sepiolite were functionalized with mercapto group and applied for the removal of Co2+ , Cu2+ , Mn2+ , Zn2+ , Fe3+ and Cd2+ from aqueous solutions [13]. Ethylenediamine-modified attapulgite was prepared as an effective and recyclable sorbent for the selective removal of Pb2+ from water [14]. Amino functionalized hollow core–mesoporous shell silica spheres with a significant tendency for heavy metal removal were synthesized [15]. Water soluble aminopropyl magnesium functionalized phyllosilicate was prepared and used as a soil-flushing agent for Cd2+ and Pb2+ contaminated soils [16]. Great efforts have been made to study the sorption properties and mechanism on various clay minerals and oxides [17–19]. However there are limited researches on the sorption properties and mechanism of heavy metals on functionalized clays [20]. In this paper, sorption behaviors and mechanism of Cd2+ on mercapto and amino functionalized palygorskite were investigated. It is

X. Liang et al. / Applied Surface Science 322 (2014) 194–201

anticipated that the data generated from this work will be useful in the application of functionalized palygorskite in cleanup of heavy metal from soil environment.

195

2. Experiments

with a monochromatic Al-K␣ X-ray source (hv = 1486.6 eV), hybrid (magnetic/electrostatic) optics, and a multichannel plate and delay line detector. All spectra were recorded using an aperture slot of 300 × 700 ␮m. Survey spectra were recorded with a pass energy of 160 eV, and high-resolution spectra with a pass energy of 40 eV.

2.1. Preparation of mercapto functionalized sepiolite

3. Results and discussion

The mercapto and amine functionalized palygorskite samples (PAL-SH and PAL-NH2 in short) were prepared by nanotexturization in aqueous gel [21,22]. Pristine palygorskite (PAL) were bought from Jiangsu Jiuchuan Nano-material Technology Co., Ltd. 3-mercaptopropyltrimethoxysilane and 3-aminepropyltrimethoxysilane were purchased from Sigma-Aldrich. All the reagents were used as received without any purification.

3.1. Sorption kinetics

2.2. Sorption experiments Sorption kinetic experiments of Cd2+ on PAL, PAL-SH and PALNH2 were carried out in the following steps. A 1.5 g portion of samples was dispersed in 1500 mL of 50 mg L−1 CdCl2 . The suspension was stirred at a constant speed and maintained at 25.0 ± 0.5 ◦ C. Aliquots (0.5 mL) of the suspension were withdrawn at predetermined time intervals (t) and filtered through a 0.45 ␮m syringe filter. The Cd2+ concentrations in the remaining filtrates were determined by atomic absorption spectrometry (ZEEnit 700P, Analytik Jena, Germany). The sorption amount  t was calculated by Eq. (1): t =

(C0 − Ct ) V m

(1)

where C0 and Ct (mg L−1 ) are the initial and remaining Cd2+ concentrations, respectively; V (mL) is the volume of the suspension; and m (g) is the mass of sample used. After sorption, the supernatants were decanted and the suspensions were dried and ground in ambient temperature 25 ± 0.5 ◦ C for solid phase characterization. Sorption isotherms of Cd2+ on three samples were carried out by a batch equilibration technique. 0.025 g samples were added to 50 mL centrifugal tubes, which were then filled with 25 mL of CdCl2 having different Cd2+ concentrations (0–100 mg L−1 ). The initial pH of CdCl2 solution was about 6.6 and without any adjustment to avoid human intervention. The centrifugal tubes were shaken for 300 min at temperatures of 15.0, 25.0, and 35.0 ◦ C by a rotary shaker. The suspensions were then centrifuged at a speed of 3800 r min−1 for 5 min, and the resulting supernatants were collected to determine the equilibrium Cd2+ concentrations. The equilibrium sorption amounts ( e ) were calculated from the decrease in Cd2+ concentration in the solution phases.

Two different kinetic models were used to fit the experimental data: (i) Pseudo first order kinetic model t = e (1 − e-k1 t )

(2)

where k1 (min−1 ) is the rate constant of first order sorption, and  t and  e (mg g−1 ) are the sorption amounts at a certain time t (min) and at sorption equilibrium, respectively. (ii) Pseudo second order kinetic model 2

t =

e k2 t 1 + e k2 t

(3)

where k2 (g mg−1 min−1 ) is the rate constant of pseudo second order sorption. The two kinetic models were applied to fit the sorption kinetic data as shown in Fig. 1. The sorption processes were observed to be rapid, and the equilibrium was achieved after approximately 60 min for functionalized palygorskite and about 10 min for pristine palygorskite. The first step involved fast sorption of a majority of Cd2+ in solution, followed by slower sorption. Second order kinetic model was observed to fit the kinetic data better than first order model. The sorption kinetic parameters are listed in Table 1.

2.3. Characterization XRD patterns were obtained with an X-ray diffractometer ˚ at a (D/Max 2500, Rigaku) using Cu K␣ radiation ( = 1.542 A) scanning speed of 7◦ min−1 over 2 from 3◦ to 90◦ and operated at 40 kV and 100 mA. FT-IR spectra were recorded on an FT-IR spectrometer (Nicolet 380, Nicolet) in KBr matrix. Raman spectra were taken with a FT-Raman spectrometer (Bruker RFS100/S, Nd YAG Laser, 1064 nm, 50 mW). Solid state 29 Si cross-polarization magic angle spinning nuclear magnetic resonance (CP/MAS NMR) was performed on a Varian Infinity Plus 300 MHz equipped with a 7.5 mm chemagnetics probe. The NMR experiment was conducted under conditions of magic angle spinning at 3.0 kHz and a 4.5 ␮s 90◦ pulse with a repetition delay of 5 s. Chemical shifts have been referenced to the corresponding nuclei in tetramethylsilane. X-ray photoelectron spectra were recorded using an X-ray photoelectron spectrometer (AXIS Ultra DLD, Kratos Analytical)

Fig. 1. Sorption kinetic fit of Cd2+ on PAL, PAL-SH and PAL-NH2 . Table 1 Sorption kinetic parameters of Cd2+ sorption on PAL, PAL-SH and PAL-NH2 . Parameters

PAL

PAL-SH

PAL-NH2

Pseudo first order kinetics  m1 (mg g−1 ) 5.78 ± 0.42 31.03 ± 0.93 16.34 ± 0.56 0.27 ± 0.11 0.089 ± 0.0097 0.037 ± 0.0034 k1 (min−1 ) 0.65175 0.95961 0.98101 R2 Pseudo second order kinetics −1 6.49 ± 0.51 35.361 ± 1.221 20.90 ± 0.94  m2 (mg g ) 0.046 ± 0.023 0.0032 ± 5.3E-4 0.0017 ± 2.6E-4 k2 (g mg−1 min−1 ) 0.75469 0.97063 0.98536 R2

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maximum sorption amount of Cd2+ on PAL, PAL-SH and PAL-NH2 were approximately 11.66, 28.01 and 23.80 mg g−1 , respectively. The thermodynamic parameters (H◦ , S◦ , and G◦ ) for Cd2+ sorption on three samples can be determined from the temperature dependence [25]. The sorption equilibrium constants (K0 ) were obtained following methods [26,27]. The sorption isotherm data were plotted as ln( e /Ce ) versus  e , and linear regression was performed by a least-squares analysis to determine the value of ln K0 based on the y-axis intercept at  e = 0 [28,29]. Based on the ln K0 values for the three temperatures, G◦ was calculated from the relationship

3.2. Sorption thermodynamics The commonly used isotherms for quantitative descriptions of sorption data are the Langmuir and Freundlich isotherms: (i) Langmuir isotherm e =

m KL Ce 1 + KL Ce

(4)

where  m is the maximum sorption amount and KL is Langmuir constant. (ii) Freundlich isotherm e =

Go = −RT ln K0

m KL Ce 1 + KL Ce

(5)

(6)

where K0 is the sorption equilibrium constant, R is the gas constant, and T is the absolute temperature. The increased negative values of G◦ with increasing temperature demonstrated that the sorption reactions were thermodynamically more favorable at higher temperatures. The corresponding enthalpy change (H◦ ) and entropy change (S◦ ) can be obtained from the slope and intercept from the Van’t Hoff plot [30]:

where KF and n are Freundlich constants. Unlike the Langmuir isotherm, the Freundlich isotherm does not predict the maximum sorption amount on the sorbent surface. The isotherms of Cd2+ sorption on PAL-SH and PAL-NH2 at different temperatures shown in Fig. 2 were typical H-type isotherms characteristic of chemical sorption according to the Giles classification [23], while they were labeled as type I isotherms according to the IUPAC classification [24]. The isotherms with a marked initial slope indicated that intercalated materials might have acted as high-efficacy adsorbents at low concentrations. The Langmuir isotherm could fit the sorption data of Cd2+ on three samples better than the Freundlich isotherm as shown in Table 2, and the

ln (K0 ) =

H o S o − R RT

(7)

The observed positive enthalpy change confirmed that the sorption process was endothermic. The much greater and positive S◦ value revealed that the sorption of Cd2+ on three samples was

Table 2 Isotherm parameters for Cd2+ sorption on PAL, PAL-SH and PAL-NH2 . 288.15 K

Samples PAL

PAL-SH

PAL-NH2

Langmuir isotherm  m (mg g−1 ) KL (mg−1 ) R2 Freundlich isotherm KF (mg−1 ) n R2 Thermodynamic parameters lnK0 G◦ (kJ mol−1 ) S◦ (J mol−1 K−1 ) H◦ (kJ mol−1 ) Langmuir isotherm  m (mg g−1 ) KL (mg−1 ) R2 Freundlich isotherm KF (mg−1 ) N R2 Thermodynamic parameters lnK0 G◦ (kJ mol−1 ) S◦ (J mol−1 K−1 ) H◦ (kJ mol−1 ) Langmuir isotherm  m (mg g−1 ) KL (mg−1 ) R2 Freundlich isotherm KF (mg−1 ) N R2 Thermodynamic parameters lnK0 G◦ (kJ mol−1 ) S◦ (J mol−1 K−1 ) H◦ (kJ mol−1 )

9.20 ± 1.41 0.016 ± 0.0037 0.99459 0.25 ± 0.039 0.73 ± 0.047 0.99339 −1.89 4.55 264.01 8.05

298.15 K

308.15 K

10.36 ± 0.81 0.049 ± 0.0082 0.99110

11.66 ± 0.75 0.089 ± 0.015 0.99335

0.98 ± 0.22 0.54 ± 0.068 0.97921

1.99 ± 0.40 0.43 ± 0.065 0.97380

−0.68 1.68

0.28 −0.72

27.42 ± 0.83 3.99 ± 0.68 0.98884

27.46 ± 0.716 5.98 ± 0.91 0.99162

28.01 ± 1.10 10.87 ± 2.18 0.97761

18.38 ± 1.57 0.17 ± 0.041 0.92494

19.02 ± 1.58 0.17 ± 0.039 0.92499

20.64 ± 1.78 0.15 ± 0.039 0.89711

6.47 −15.51 129.33 21.70

6.86 −17.00.

7.06 −18.08.

21.80 ± 1.14 0.76 ± 0.18 0.97262

22.55 ± 1.04 0.96 ± 0.20 0.97709

23.80 ± 1.33 1.26 ± 0.33 0.96354

10.42 ± 1.35 0.24 ± 0.053 0.94402

11.61 ± 1.22 0.22 ± 0.044 0.95393

12.73 ± 1.45 0.22 ± 0.048 0.93805

3.63 −8.70. 129.73 28.55

4.20 −10.41

4.40 −11.28

X. Liang et al. / Applied Surface Science 322 (2014) 194–201

A 10

PAL

B

8

PAL-SH

30 25 20

Γe (mg kg-1)

6

Γe (mg kg-1)

197

4

15 10

2 288.15 K 298.15 K 308.15 K Langmuir fit Freundlich fit

0

0

10

20

30

288.15 K 298.15 K 308.15 K Langmuir fit Freundlich fit

5 0

40

0

2

4

6

C

PAL-NH 25

8

10

12

14

16

C e (mg L -1)

-1

C e (mg L ) 2

Γe (mg kg-1)

20

15

10

5

288.15 K 298.15 K 308.15 K Langmuirfit Freundlich fit

0 0

5

10

15

20

25

C e (mg L -1) Fig. 2. Sorption isotherm fit for Cd2+ sorption on PAL, PAL-SH and PAL-NH2 .

3.3. XRD analyses The XRD patterns of PAL, PAL-SH, PAL-NH2 and the sorption products are shown in Fig. 3. Palygorskite (Mg5 (Si,Al)8 O20 (OH)2 ·8H2 O, JCPDS card No. 31-0783) was detected by Jade 6.5. All the samples showed an orthorhombic crystalline system of palygorskite with Pn space group [31]. The XRD indexes are listed in Table 3. The characteristic peak positions and dspacing of PAL-Cd did not change obviously after the sorption of Cd2+ , indicating that the structure and crystallinity of palygorskite were maintained. In PAL-NH2 -Cd sample, new diffraction peaks were detected at approximately 2 = 23.3◦ , 30.2◦ and 36.1◦ , and these corresponded to cadmium carbonate (Otavite CdCO3 , JCPDS card No. 42-1342) [32]. The results showed that cadmium carbonate precipitation was one of the main sorption mechanisms.

Another broad band at 1654.73 cm−1 was attributed to the bending vibration of H O H. A weak absorption band at approximately 1437.75 cm−1 was assigned to the v3 mode asymmetric stretching of CO3 2− in the interlayer[34], indicating that some CO3 2− existed in PAL samples. In the spectrum of PAL-Cd, the bands of v(OH), ␦(H O H), v(H2 O), vs (Al–OH) and v(M–O) did not change much compared to the spectrum of PAL.

F E D C B A (110)

3.4. FT-IR analyses The FT-IR spectrum of PAL shown in Fig. 4 exhibited an intense broad band at 3552.29 cm−1 , which was attributed to the stretching vibration of structural O H groups, hydrogen bonded with interlamellar water, or O H groups in adjacent layers[33].

∗

∗

Intensity

driven by entropy changes. The results of the temperature dependence experiment demonstrated that greater sorption occurred at higher temperatures, and thus, the application of three samples in the summer season would be preferred for the immobilization of Cd in soils.

5

(200)

10

(130)

15

(040) (121)

(221)

20

25

(231) (400)

(331)

30

35

40

2θ (°) Fig. 3. XRD patterns of PAL, PAL-SH, PAL-NH2 and sorption products (A: PAL, B: PAL-Cd, C: PAL-SH, D: PAL-SH-Cd, E: PAL-NH2 , F: PAL-NH2 -Cd).

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X. Liang et al. / Applied Surface Science 322 (2014) 194–201

Table 3 XRD indexes of PAL, PAL-SH, PAL-NH2 and sorption products. hkl

(1 1 0) (2 0 0) (1 3 0) (0 4 0) (1 2 1) (3 1 0) a* b c *

PAL

PAL-Cd

PAL-SH

PAL-SH-Cd

PAL-NH2

PAL-NH2 -Cd

2

d(Å)

2

d(Å)

2

d(Å)

2

d(Å)

2

d(Å)

2

d(Å)

8.42 13.73 16.34 19.83 20.89 21.35

10.49 6.45 5.42 4.47 4.25 4.16 12.89 18.04 5.19

8.24 13.59 16.21 17.63 19.68 20.73

10.72 6.51 5.46 5.03 4.51 4.28 13.37 17.97 5.24

8.34 13.65 16.30 19.74 20.73 21.23

10.59 6.48 5.43 4.49 4.28 4.18 12.98 18.26 5.22

8.34 13.66 16.27 19.72 20.68 21.36

10.59 6.48 5.44 4.50 4.29 4.16 12.96 18.34 5.23

8.37 13.65 16.30 19.78 20.64 21.37

10.56 6.48 5.44 4.48 4.30 4.15 12.95 18.19 5.26

8.38 13.74 16.34 19.79 20.79 21.33

10.54 6.44 5.42 4.48 4.30 4.16 12.95 18.19 5.12

The lattice constants a, b and c were calculated from peak locations and miller indices by Jade 6.5.

Table 4 Summary of FT-IR assignments of PAL, PAL-SH, PAL-NH2 and sorption products. Assignment

PAL

PAL-Cd

PAL-SH

PAL-SH-Cd

PAL-NH2

PAL-NH2 -Cd

vs (Al OH)

3614.49 3552.29 3415.94 – – 1654.73 1437.75 1197.87 1027.52 982.23 476.62

3615.28 3552.9 3405.8 – – 1655.45 1449.59 1198.48 1028.42 982.03 473.23

3614.51 3551.81 3415.18 2931.09 2549.43 1654.62 1443.21 1195.42 1027.98 984.50 477.72

3614.36 3551.94 3414.86 2928.56 – 1654.94 1406.29 1194.73 1028.12 984.96 478.33

3614.16 3552.07 3421.06 2934.38 – 1654.47 1458.19 1196.78 1028.22 982.68 478.11

3614.11 3552.35 3422.10 – – 1654.85 1437.66 1197.03 1030.78 982.88 478.44

v(OH) v(H2 O) v(C H) v(S H) ı(H O H) v3 (CO3 2− ) v(Si O) v(Si–O–Si) v(Si O) v(M O)

The FT-IR spectra of PAL-SH and PAL-SH-Cd were generally similar to that of pristine palygorskite as shown in Table 4. For PAL-SH the peaks due to C H stretching vibration of chain methylene ( CH2 ) groups at 2931.09 cm−1 and very weak stretching vibration of mercapto group at 2549.43 cm−1 were found. The weak absorption peaks for the stretch of C H in FTIR were clearly detected in Raman spectra as shown in Fig. 5. The peaks at about 2916 cm−1 were corresponding to C H stretching of the functional groups in PAL-SH, PAL-NH2 and sorption products. For PAL-SHCd, there were no new peaks appeared in FTIR spectra. However, the mercapto group stretching vibration disappeared. It indicated that Cd2+ reacted with mercapto groups on the surface of PAL-SH. For PAL-NH2 -Cd, the peak corresponding to C H stretching also disappeared compared to PAL-NH2 , indicating that amino group

Fig. 4. FT-IR spectra of PAL, PAL-SH, PAL-NH2 and sorption products (A: PAL, B: PAL-Cd, C: PAL-SH, D: PAL-SH-Cd, E: PAL-NH2 , F: PAL-NH2 -Cd).

reacted with Cd2+ . Meanwhile v3 mode of CO3 2− was strengthened after sorption, which was consistent with the XRD analyses of the formation of CdCO3 . 3.5.

29 Si

CP/MAS NMR analyses

The 29 Si CP/MAS NMR spectra of sorption products PAL-SH-Cd and PAL-NH2 -Cd are shown in Fig. 6. For PAL-SH-Cd and PAL-NH2 Cd, the spectra consisted of four well-resolved resonances, which were similar with the samples before sorption. However, the chemical shifts for T2 [RSi(OSi)2 (OH)] and T3 [RSi(OSi)3 ] resonances in sorption products were different from the samples before sorption obviously, indicating that great changes took place in the chemical surrounding of organosiloxanes. That is to say, the grafted organosiloxanes played an important role in the sorption process. The changes in signal intensity can be attributed to the complexation of Cd2+ with mercapto or amino site in the form of T2 and T3 species and, to a lesser extent, on the Q3 species.

Fig. 5. Raman spectra of PAL-SH, PAL-NH2 and sorption products.

X. Liang et al. / Applied Surface Science 322 (2014) 194–201

199

The Cd3d binding energies in PAL-Cd as shown in Fig. 7A were 406.5 eV (Cd3d5/2 ) and 413.2 eV (Cd3d3/2 ), which were obviously different from that in CdCl2. The values of binding energies were similar to the that in Cd2+ -exchanged montmorillonite [35]. Cd2+ can react with surface hydroxyl to form surface complexation. The S2p spectra in PAL-SH are shown in Fig. 7B. The sulfur has two different chemical shifts. The binding energy at 163.5 eV was corresponding to mercapto group, and the other one at 164.1 eV was due to disulfide bond [36,37]. The mercapto groups can be transform into disulfide bond. 2Sur–SH + 0.5O2 → Sur–S–S–Sur + H2 O

Fig. 6.

29

Si CP/MAS NMR spectra of sorption products.

3.6. XPS analyses The binding energy of main elements of the six samples were listed in Table 5. The different peak binding energy indicated different chemical surrounding before and after sorption. The binding energy of Cd3d3/2 in PAL-Cd, PAL-SH-Cd and PAL-NH2 -Cd were different from each others, which revealed various sorption mechanisms. The shifts of peak binding energies in Al2p and Si2p demonstrated the changes of functional groups at the surface.

The S2p peaks S2p1/2 and S2p3/2 at 164.7 and 163.5 eV indicated that sulfur existed in a chemical state as a free mercapto group throughout the organic monolayer [38,39]. The area ratio of mercapto group to total sulfur species was about 48%, indicating that disulfide bond was the main chemical species. The S2p spectra in PAL-SH-Cd are shown in Fig. 7C. The binding energy at 163.2 eV was corresponding to mercapto group, and the second one at 162.4 eV was due to Cd S bond. And the third one at 164.0 eV corresponding to disulfide bond was also found. As shown in Fig. 7D, Cd3d spectra in PAL-SH-Cd had three different chemical shift. The binding energy at 406.1 eV was similar to that in PAL-Cd. It indicated that the surface hydroxyl group on the surface still had effect in the sorption process. The Cd3d spectrum had two sharp peaks at 405.5 eV (Cd3d5/2 ) and 412.3 eV (Cd3d3/2 ), which were in good agreement with the reported values for CdS [40]. The third peak at 404.9 eV was due to the hydroxide, oxide or carbonate precipitates [41]. The sorption mechanisms can be described as follows:

Fig. 7. XPS spectra of PAL, PAL-SH, PAL-NH2 and sorption products.

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X. Liang et al. / Applied Surface Science 322 (2014) 194–201

Table 5 XPS parameters of PAL, PAL-SH and PAL-NH2 and sorption products. Element

Index

PAL

PAL-Cd

PAL-SH

PAL-SH-Cd

PAL-NH2

PAL-NH2 -Cd

Al2p

Peak B.E. At.% Peak B.E. At.% Peak B.E. At.% Peak B.E. At.% Peak B.E. At.% Peak B.E. At.%

74.33 5.21 102.63 23.01 531.69 56.38 – – – – – –

75.07 4.37 103.24 22.63 532.36 56.73 – – – – 413.24 0.33

74.98 3.47 103.07 22.68 532.33 47.48 163.99 4.54 – – – –

74.67 3.70 102.78 22.28 531.96 48.30 163.49 3.80 – – 412.30 0.83

74.63 3.65 102.80 23.26 531.98 50.93 – – 399.79 2.55 – –

74.24 4.41 102.42 25.34 531.54 59.75 – – 399.54 0.65 411.87 0.50

Si2p O1s S2p N1s Cd3d3

401.75 0.58

Fig. 8. Schematic of sorption mechanisms of Cd2+ on PAL, PAL-SH and PAL-NH2 .

For natural palygorskite, the surface complexation was the main mechanism. Sur–OH + Cd2+ → Sur–O–Cd+ + H+ 2Sur–OH + Cd2+ → Sur–O–Cd–O–Sur + 2H+ For mercapto functionalized Sur–SH + Cd2+ → Sur–S–Cd+ + H+

palygorskite,

2Sur–SH + Cd2+ → Sur-S–Cd–S–Sur + 2H+ As shown in Fig. 7E, N1s spectra of PAL-NH2 can be divided into two chemical states. The peak at 399.7 eV was due to the free amino group [42] and the peak at 401.8 eV was protonated NH3 + [37]. Sur–NH2 + H2 O → Sur–NH3 + + OHThe Cd3d and N1s spectra in PAL-NH2 -Cd were shown in Fig. 7F. The N1s chemical shift was similar with PAL-NH2 , without any obvious changes. And the peak at 405.5 eV for Cd3d5/2 was due to the coordination effect of amino and Cd2+ and the peak at 404.8 eV was due to CdCO3 . Cd

2+

−

+ 2OH + CO2 → CdCO3 + H2 O

Sur–NH2 + Cd2+ → Sur–NH2 –Cd2+

In all, sorption of Cd2+ on PAL, PAL-SH and PAL-NH2 involved different mechanisms as shown in Fig. 8. For pristine palygorskite, Cd2+ reacted with surface hydroxyl groups to form surface complexation; for mercapto functionalized palygorskite, the complexation of Cd2+ with mercapto groups existed in addition to the complexation with surface hydroxyl groups; for amino functionalized palygorskite, surface precipitation of Cd2+ with OH− and CO2 to form CdCO3 played an important role in the sorption process except complexation with amino and hydroxyl groups. The introduction of mercapto and amino group can increase the sorption amounts for heavy metals such as Pb2+ , Cd2+ and Cu2+ , and the sorption amounts were reported in our former publication [22]. In the current paper, the sorption behavior and mechanism of Cd2+ on PAL-SH and PAL-NH2 were focused on and in the future sorption of other heavy metals on the functionalized palygorskite should be emphasized. 4. Conclusion The mercapto and amino functionalized palygorskite has application potential for treatment of heavy metal polluted water and soil. And in the next step, the sorption behaviors in actual soil environment or soil extractable solutions should be conducted and further investigations will focus on elucidating the sorption

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mechanisms at molecular level and practical application in field scale demonstration. Acknowledgements This research was supported by Central Public Research Institutes Basic Funds for Research and Development (AgroEnvironmental Protection Institute, Ministry of Agriculture, 2014-szjj-lxf-01), National Natural Science Foundation of China (no. 21177068), Program for Distinguishing Scholar and Innovative Team of MOA (2012-jcrc-xym), and Special Fund for Agro-scientific Research in the Public Interest (no. 201203045). References [1] R. Bian, D. Chen, X. Liu, L. Cui, L. Li, G. Pan, D. Xie, J. Zheng, X. Zhang, J. Zheng, A. Chang, Biochar soil amendment as a solution to prevent Cd-tainted rice from China: results from a cross-site field experiment, Ecol. Eng. 58 (2013) 378–383. [2] M.o.E. Protection, M.o.L.a.R.o. China, National Survey of Soil Pollution Bulletin, China Environ. Protect. Ind., 2014, pp. 10–14. [3] T. Makino, K. Sugahara, Y. Sakurai, H. Takano, T. Kamiya, K. Sasaki, T. Itou, N. Sekiya, Remediation of cadmium contamination in paddy soils by washing with chemicals: selection of washing chemicals, Environ. Pollut. 144 (2006) 2–10. [4] Poonam R. Bhardwaj, R. Sharma, N. Handa, H. Kaur, R. Kaur, G. Sirhindi, A.K. Thukral, Prospects of field crops for phytoremediation of contaminants, in: P. Ahmad, S. Rasool (Eds.), Emerging Technologies and Management of Crop Stress Tolerance, Academic Press, San Diego, CA, 2014, pp. 449–470. [5] Y.-T. Chang, H.-C. Hsi, Z.-Y. Hseu, S.-L. Jheng, Chemical stabilization of cadmium in acidic soil using alkaline agronomic and industrial by-products, J. Environ. Sci. Health, A: Environ. Sci. Eng. Toxic Hazard. Subst. Control 48 (2013) 1748–1756. [6] A. Zanuzzi, A. Faz, J.A. Acosta, Chemical stabilization of metals in the environment: a feasible alternative for remediation of mine soils, Environ. Earth Sci. 70 (2013) 2623–2632. [7] H. Bradl, Adsorption of heavy metal ions on clays, in: P. Somasundaran, A. Hubbard (Eds.), Encyclopedia of Surface and Colloid Science, Taylor and Francis, New York, NY, 2006, pp. 471–483. [8] M. Cruz-Guzmán, R. Celis, M.C. Hermosín, W.C. Koskinen, E.A. Nater, J. Cornejo, Heavy metal adsorption by montmorillonites modified with natural organic cations, Soil Sci. Soc. Am. J. 70 (2006) 215–221. [9] L. Mercier, T.J. Pinnavaia, Heavy metal ion adsorbents formed by the grafting of a thiol functionality to mesoporous silica molecular sieves: factors affecting Hg(II) uptake, Environ. Sci. Technol. 32 (1998) 2749–2754. [10] M. Jaber, J. Miehé-Brendlé, Organoclays: preparation properties and applications, in: V. Valentin, M. Svetlana, T. Michael (Eds.), Ordered Porous Solids, Elsevier, Amsterdam, 2009, pp. 31–49. [11] F. Wypych, Chemical modification of clay surfaces, in: P. Somasundaran, A. Hubbard (Eds.), Encyclopedia of Surface and Colloid Science, Taylor and Francis, New York, NY, 2006, pp. 1256–1272. [12] A. Benhamou, J.P. Basly, M. Baudu, Z. Derriche, R. Hamacha, Aminofunctionalized MCM-41 and MCM-48 for the removal of chromate and arsenate, J. Colloid Interface Sci. 404 (2013) 135–139. [13] M. Doˇgan, Y. Turhan, M. Alkan, H. Namli, P. Turan, Ö. Demirbas¸, Functionalized sepiolite for heavy metal ions adsorption, Desalination 230 (2008) 248–268. [14] Y. Deng, Z. Gao, B. Liu, X. Hu, Z. Wei, C. Sun, Selective removal of lead from aqueous solutions by ethylenediamine-modified attapulgite, Chem. Eng. J. 223 (2013) 91–98. [15] A.M. El-Toni, M.A. Habila, M.A. Ibrahim, J.P. Labis, Z.A. Alothman, Simple and facile synthesis of amino functionalized hollow core–mesoporous shell silica spheres using anionic surfactant for Pb(II), Cd(II), and Zn(II) adsorption and recovery, Chem. Eng. J. 251 (2014) 441–451. [16] Y.-C. Lee, E.J. Kim, D.A. Ko, J.-W. Yang, Water-soluble organo-building blocks of aminoclay as a soil-flushing agent for heavy metal contaminated soil, J. Hazard. Mater. 196 (2011) 101–108. [17] H.B. Bradl, Adsorption of heavy metal ions on soils and soils constituents, J. Colloid Interface Sci. 277 (2004) 1–18.

201

[18] J.H. Potgieter, S.S. Potgieter-Vermaak, P.D. Kalibantonga, Heavy metals removal from solution by palygorskite clay, Miner. Eng. 19 (2006) 463–470. [19] G.E. Brown, G.A. Parks, Sorption of trace elements on mineral surfaces: modern perspectives from spectroscopic studies, and comments on sorption in the marine environment, Int. Geol. Rev. 43 (2001) 963–1073. [20] R. Celis, M. Carmen Hermosín, J. Cornejo, Heavy metal adsorption by functionalized clays, Environ. Sci. Technol. 34 (2000) 4593–4599. [21] X. Liang, Y. Xu, G. Sun, L. Wang, Y. Sun, Y. Sun, X. Qin, Preparation and characterization of mercapto functionalized sepiolite and their application for sorption of lead and cadmium, Chem. Eng. J. 174 (2011) 436–444. [22] X. Liang, Y. Xu, X. Tan, L. Wang, Y. Sun, D. Lin, Y. Sun, X. Qin, Q. Wang, Heavy metal adsorbents mercapto and amino functionalized palygorskite: preparation and characterization, Colloids Surf., A 426 (2013) 98–105. [23] C. Giles, T. MacEwan, S. Nakhwa, D. Smith, Studies in adsorption. Part XI. A system of classification of solution adsorption isotherms, and its use in diagnosis of adsorption mechanisms and in measurement of specific surface areas of solids, J. Chem. Soc. 3 (1960) 3973–3993. [24] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquérol, T. Siemieniewska, Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity, Pure Appl. Chem. 57 (1985) 603–619. [25] W. Li, G. Pan, M. Zhang, D. Zhao, Y. Yang, H. Chen, G. He, EXAFS studies on adsorption irreversibility of Zn(II) on TiO2 : temperature dependence, J. Colloid Interface Sci. 319 (2008) 385–391. [26] D. Singh, R.G. McLaren, K.C. Cameron, Zinc sorption–desorption by soils: effect of concentration and length of contact period, Geoderma 137 (2006) 117–125. [27] A.A. Khan, R.P. Singh, Adsorption thermodynamics of carbofuran on Sn (IV) arsenosilicate in H+ , Na+ and Ca2+ forms, Colloids Surf. 24 (1987) 33–42. [28] H.K. Boparai, M. Joseph, D.M. O’Carroll, Kinetics and thermodynamics of cadmium ion removal by adsorption onto nano zerovalent iron particles, J. Hazard. Mater. 186 (2011) 458–465. [29] X. Tan, J. Hu, G. Montavon, X. Wang, Sorption speciation of Nickel(II) onto Ca-montmorillonite: batch, EXAFS techniques and modeling, Dalton Trans. 40 (2011) 10953–10960. [30] S.I. Lyubchik, A.I. Lyubchik, O.L. Galushko, L.P. Tikhonova, J. Vital, I.M. Fonseca, S.B. Lyubchik, Kinetics and thermodynamics of the Cr(III) adsorption on the activated carbon from co-mingled wastes, Colloids Surf., A: Physicochem. Eng. Aspects 242 (2004) 151–158. [31] S. Zhou, L. Wang, S. Shen, Fabrication of Ni–P/palygorskite core–shell linear powder via electroless deposition, Appl. Surf. Sci. 257 (2011) 10211–10217. [32] G.-B. Cai, G.-X. Zhao, X.-K. Wang, S.-H. Yu, Synthesis of polyacrylic acid stabilized amorphous calcium carbonate nanoparticles and their application for removal of toxic heavy metal ions in water, J. Phys. Chem. C 114 (2010) 12948–12954. [33] C. Serna, G. VanScoyoc, Infrared study of sepiolite and palygorskite surfaces, Dev. Sedimentol. 27 (1979) 197–206. [34] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, sixth ed., John Wiley & Sons, Inc., Hoboken, NJ, 2009. [35] S.H. Kim, N.H. Heo, G.H. Kim, S.B. Hong, K. Seff, Preparation, crystal structure, and thermal stability of the cadmium sulfide nanoclusters Cd6 S4 4+ and Cd2 Na2 S4+ in the sodalite cavities of zeolite A (LTA), J. Phys. Chem. B 110 (2006) 25964–25974. [36] P. Tingaut, R. Hauert, T. Zimmermann, Highly efficient and straightforward functionalization of cellulose films with thiol-ene click chemistry, J. Mater. Chem. 21 (2011) 16066–16076. [37] I. Brand, M. Nullmeier, Thin films on titania, and their applications in the life sciences, in: C.S.S.R. Kumar (Ed.), Nanostructured Thin Films and Surfaces, Wiley, Weinheim, 2010, pp. 203–250. [38] J. Zuo, E. Torres, Comparison of adsorption of mercaptopropyltrimethoxysilane on amphiphilic TiO2 and hydroxylated SiO2 , Langmuir 26 (2010) 15161–15168. [39] T. Ishida, N. Choi, W. Mizutani, H. Tokumoto, I. Kojima, H. Azehara, H. Hokari, U. Akiba, M. Fujihira, High-resolution X-ray photoelectron spectra of organosulfur monolayers on Au(111): S(2p) spectral dependence on molecular species, Langmuir 15 (1999) 6799–6806. [40] Y. Liu, H. Zhou, B. Zhou, J. Li, H. Chen, J. Wang, J. Bai, W. Shangguan, W. Cai, Highly stable CdS-modified short TiO2 nanotube array electrode for efficient visible-light hydrogen generation, Int. J. Hydrogen Energy 36 (2011) 167–174. [41] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X ray photoelectron spectroscopy, Physical Electronics, 1995, Chanhassen, Minnesota. [42] F. Truica-Marasescu, M.R. Wertheimer, Nitrogen-rich plasma-polymer films for biomedical applications, Plasma Process Polym. 5 (2008) 44–57.