Mesoporous Zirconium Phosphonate Hybrid Material as Adsorbent to

Article pubs.acs.org/IECR

Mesoporous Zirconium Phosphonate Hybrid Material as Adsorbent to Heavy Metal Ions Yunjie Jia,† Yuejuan Zhang,† Ruiwei Wang,‡ Jianjun Yi,§ Xu Feng,∥ and Qinghong Xu*,† †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, China 100029 State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun, China 130012 § Petrochemical Research Institute, PetroChina Company Limited, Beijing, China 100083 ∥ Shimadzu (China) Co., Ltd., Beijing, China 100020 ‡

S Supporting Information *

ABSTRACT: A new kind of inorganic−organic hybrid zirconium phosphonate material (NTAZP) with mesoporous structure was synthesized using nitrilotris(methylene)-triphosphonic acid (ATMP) and zirconium dichloride oxide octahydrate. The sample possesses a spherical morphology, and the spheres are composed of lobular lamellae. The lobular lamellae have the structure of a worm-like mesoporous (about 2.7 nm) framework and a high surface area (about 160.4 m2/g), which were characterized by SEM, TEM, N2 sorption, XRD, TG-DTA, elemental analysis, TOC (total organic carbon), XPS, and FT-IR spectroscopy techniques. The as-prepared NTAZP was used as adsorbent for the efficient removal of heavy metal ions (e.g., Pb2+, Cu2+, and Cd2+). Our results indicate that the material has good prospects for application as an adsorbent in wastewater processing. layers, were prepared by Alberti and DiGiacomo.19,20 Zirconium phosphonates are very versatile due to the existence of R groups, thereby allowing for the design of inorganic− organic composite materials with tailored properties. Consequently, they have attracted attention in catalysis,21 ionexchange,22 adsorptions,23 and other potentialities.24,25 By using organic phosphonates with surfactant-assisted templates in sol−gel method or intercalations with organic compounds, an interrelated family of mesoporous zirconium phosphates (phosphonates) with molecular formulas of Zr(ROPO3)2 or Zr(RPO3)2 (where R = alkyl, aryl, benzyl, acid radicals, etc.) were synthesized.19,26−28 Owing to their good thermal stability, considerable surface area, adjustable variability of acidity, shapeselective properties, and porous structure on geometric features, the mesoporous zirconium phosphates have good prospects for application in many areas. Here, we report for the first time the synthesis of a mesoporous organic zirconium phosphonate hybrid material (NTAZP) with the formula Zr3O26P6C6H22N2 by a simple autoclaving method in the presence of nitrilotris(methylene)triphosphonic acid (ATMP) and zirconium dichloride oxide octahydrate. The as-prepared NTAZP possesses a spherical morphology, and the spheres were found to be composed of many lobular lamellae, which have the structure of a worm-like mesoporous (about 2.7 nm) framework and a high surface area (about 160.4 m2/g). Compared to a widely used inorganic exchanger, zirconium phosphate (ZrPs) and activated carbon, NTAZP is more efficient in removing heavy metal ions (e.g.,

1. INTRODUCTION Adsorption and separation, as one of the most important applications of the nanoporous materials, has attracted tremendous researching interest, and a large number of adsorbents have been put into practice.1−4 A series of silicabased mesoporous organic−inorganic hybrid materials have recently been developed for removal of heavy metal ions from wastewater, especially cadmium(II), copper(II), and lead(II), which are highly toxic environmental pollutants.5,6 The organic functionalities in these adsorbents typically serve to form complexes with heavy metal ions through acid−base reactions, and the solid support allows easy removal of the loaded adsorbent from the liquid waste.7 Thiols, thiourea, and amines have been used as metal ion binding motifs in mesoporous silicas for efficient removal of toxic heavy metals like Hg(II), Cu(II), and Cd(II).8,9 Mesoporous titaniaphosphonate10 and macroporous titanium phosphonate11 hybrid materials have also been synthesized for the selective adsorption of heavy metal ions. By the combination of supermolecular assembly, a selfgeneration process has also been recently developed for the formation of mesoporous metal oxides,12 phosphated metal oxides,13 and metal phosphates.14 Recently, nanostructured titaniaphosphonate hybrid materials15 and mesoporous titanium phosphonate materials have been successfully synthesized using the self-assembly strategy and have exhibited high adsorption capacities for heavy metal ions in water.16 Layered zirconium phosphates, including α- and γ-zirconium phosphates with the formulas of Zr(HPO4)2·H2O and Zr(PO4)(H2PO4)·2H2O, have been known for a long time since they were first prepared by Clearfield et al.17,18 Shortly thereafter their derivant series products, zirconium phosphonate (Zr(O3PR)2) with organic groups (R) tailored in the © 2012 American Chemical Society

Received: Revised: Accepted: Published: 12266

January 28, 2012 August 16, 2012 September 1, 2012 September 1, 2012 dx.doi.org/10.1021/ie300253z | Ind. Eng. Chem. Res. 2012, 51, 12266−12273

Industrial & Engineering Chemistry Research

Article

Pb2+, Cu2+, and Cd2+) when used as an adsorbent. The maximum adsorption capacity of NTAZP for Pb2+ is 485.3 mg/ g and is much higher than the adsorption capacities of α-ZrP (400 mg/g)29 and activated carbon (17.51 mg/g)30 under similar conditions.

carbon analysis was performed using a Shimadzu TOC-V CPH spectrometer. Scanning electronic microscopy (SEM) and transmission electron microscopy (TEM) images of products were observed on a Shimadzu SS-550 microscope at 15 keV and a JEM-1000 (JEOL Company) at 200 kV, respectively. N2 adsorption−desorption isotherms were recorded on a Quantachrome NOVA 2000e sorption analyzer at the temperature of liquid nitrogen (77 K). The sample was degassed at 150 °C overnight prior to the measurement. The surface area was obtained by the Brunauer−Emmett−Teller (BET) method, and the pore size distribution was calculated using Barret− Joyner−Halenda (BJH) model. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were performed on a HCT-2 thermoanalyzer (Beijing Hengjun Instrument Company) at a heating rate of 10 °C/min, raised from room temperature to 700 °C, using α-Al2O3 as the reference. X-ray photoelectron spectroscopic (XPS) analysis was measured on Shimadzu ESCA-750 and ESCA-1000 spectrometers with Mg Kα X-ray sources. 2.4. Metal Ions Adsorption Test. The metal ions adsorption tests of the mesoporous hybrid material were performed in batch mode. Batch adsorption tests were carried out in 250 mL glass bottles. An aliquot of 0.10 g of the adsorbent was added to 100 mL solution containing different concentrations (from 0.5 to 900 ppm) of Pb(NO3) 2, Cd(NO3)2, Mg(NO3)2, and Ca(NO3)2, respectively. The mixtures were stirred for 24 h at room temperature (25 ± 2 °C) to ensure the sorption equilibrium. The amounts of metal ions adsorbed were monitored by atomic absorption spectroscopy (AAS) analysis using an Agilent 4510 spectrometer. The adsorption capacities were calculated using the different concentrations of solutes before and after the experiments according to the following equation31

2. EXPERIMENT 2.1. Materials. All chemicals used were of analytical reagent grade available from a commercial supplier without further purification. Zirconium dichloride oxide octahydrate (ZrOCl2·8H2O) was obtained from Sinopharm Chemical Reagent Co (Shanghai, China). Nitrilotris(methylene)-triphosphonic acid (ATMP, Scheme 1) was obtained from SigmaScheme 1. Structures of ATMP

Aldrich Company. Lead nitrate, copper nitrate, cadmium nitrate, magnesium nitrate, and calcium nitrate were all obtained from Beijing Chemical Reagent Company. 2.2. Synthesis of the Hierarchical Porous Zirconium Phosphonate. In a typical synthesis procedure, 2.80 g of ZrOCl2·8H2O was added to 30 mL of deionized water in the presence of a hydrofluoric acid solution (40%, about 3 drops) while stirring, followed by the dropwise addition of a saturated solution of ATMP in water (about 10 mL). The pH value during the synthesis remained around 5.0. The mixture was refluxed at 80 °C for 24 h while stirring until a white sol was obtained. The primary product was sealed in a Teflon-lined autoclave and aged statically at 150 °C for 7 days. The final solid product was obtained by centrifuging the sol, washing several times with deionized water and drying at 70 °C. 2.3. Characterization. Crystal structures of the products were examined by using a Rigaku D/MAX powder X-ray diffractometer with a Cu Kα X-ray source (λ = 0.15406 nm, scanning speed is 10°/min). Fourier transform infrared (FTIR) spectra were recorded in the range 4000−400 cm−1 with 2 cm−1 resolution on a Bruker Vector-22 Fourier transform spectrometer using the KBr pellet technique (1 mg of sample in 100 mg of KBr). The chemical compositions of Zr and P were determined by inductively coupled plasma (ICP) with a PerkinElmer plasma 40 emission spectrometer, and C, N, and H were analyzed on a Vario-EL elemental analyzer. Total organic

qe = (c0 − ce)V /W

(1)

where qe is the concentration of the adsorbed solute (mg/g); c0 and ce are the initial and final concentrations of the metal ions in solution (ppm); V (mL) is the volume of the solution; and W (g) is the mass of the adsorbent.

3. RESULTS AND DISCUSSION 3.1. Material Synthesis and Characterization. The synthesis of NTAZP was performed by adding zirconium dichloride oxide octahydrate to a saturated ATMP solution, followed by autoclaving at 150 °C for 7 days. Figure 1A shows the PXRD patterns of the synthesized sample in the 2θ range of 2−70°. As shown in the figure, the material possesses the

Figure 1. XRD patterns of NTAZP (A) and NTAZP delt under different pH values (B). 12267

dx.doi.org/10.1021/ie300253z | Ind. Eng. Chem. Res. 2012, 51, 12266−12273


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the total pore volumes of the synthesized sample are 160.4 m2/ g and 0.5872 m3/g, respectively. Figure 3B shows a representative SEM image of the synthesized zirconium phosphonate and reveals many regular spheres with lobular lamellae and all the pores present in these leaves. The average diameter of each sphere is approximately 6.0 μm. It highlights the interaction between zirconium dichloride oxide octahydrate with ATMP solution owing to the unique structure of ATMP. Figure 4a shows the FT-IR spectra of NTAZP. The strong broadband at about 3400 cm−1 corresponds to the surface-

characteristics of a long-range order, which is similar to normal intercalated or layered zirconium hydrogen phosphonate.32 The peak at about 3.28° in the spectrum indicates that some mesopores with d-values of 2.70 nm exist in the product. That is to say, the mesopores were formed concurrently with the formation of long-range order structure. The d-value of a diffraction peak in the low-angle region for mesoporous materials is equivalent to the space between the repeat unit in the mesoscopic structure, and this spacing is exactly equal to the combined thickness of the pore channel and pore wall.33 The nitrogen adsorption−desorption isotherms of NTAZP and its corresponding pore width distribution are shown in Figure 2. Textural parameters of NTAZP were calculated using

Figure 2. N2 adsorption−desorption isotherms curve (A) and pore width distribution (B) of NTAZP. Figure 4. FT-IR spectra for NTAZP (a) and ATMP (b).

the BJH model and BET method. According to IUPAC classifications, isotherms between II and IV are characteristic of mesoporous materials.34 The amount of nitrogen adsorbed increases sharply at high relative pressures (P/P0 > 0.9) due to capillary condensation producing gaps between the particles. These results suggest the presence of secondary porosity accompanying the normal mesopores.39 The isotherms of the hybrid sample exhibit nonuniform type H3 hysteresis loops due to different adsorption and desorption behaviors, which do not level off at relative pressures close to the saturation vapor pressure. This indicates that the material was composed of aggregates (loose assemblages) of particles forming narrow slitlike pores.35,36 BJH analysis indicated that the maxima pore size distributions centers at approximately 2.70 nm. The welldefined mesoporous structure was further confirmed by TEM observation. Figure 3A shows a representative TEM image of the synthesized sample. It can be clearly seen that most of the wormhole-like pores in NTAZP have a size of about 2.70 nm, with some larger pores more or less regular sizes of 20−30 nm also visible. Together, these pores comprise the mesoporous framework in the synthesized sample. BET surface areas and

adsorbed water, but the absorption of P−OH groups at 1631 cm−1 is not found in the spectrum, indicating that almost all of the P−OHs reacted during the synthesis. Bending and stretching vibrations of C−H in methylenes from alkyl chains of ATMP are found at about 1469 cm−1 and 2988 cm−1, respectively. The sharp band at 1047 cm−1 comes from stretching vibrations of P−O−Zr, which proves the structural formation of NTAZP.34−36 The FT-IR spectrum of ATMP is shown in Figure 4b. The thermal stability of NTAZP was determined by thermal gravimetric analysis (TGA) and differential thermal analysis (DTA), which are shown in Figure 5. The TGA curve demonstrates initial weight loss of 4.0% from room temperature to 200 °C, accompanied by an endothermal peak around 109 °C in the DTA curve, which may be attributed to the

Figure 3. TEM image of NTAZP (A) and SEM image of NTAZP (B).

Figure 5. TGA-DTA analysis for NTAZP. 12268



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3.2. Metal Ions Adsorption. As NTAZP contains organic functional groups that could interact with metal ions, we investigated its metal ion adsorption characteristics. A defined amount of the mesoporous material was dipped into five solutions containing Pb2+, Cu2+, Cd2+, Ca2+, or Mg2+ at a pH value of 4.0. Figure 7 shows the capacity of as-synthesized adsorbent to remove metal ions at different initial concentrations from their homoionic solutions. Changes in concentration of metal ions with time passed are found in the figure. The concentration of metal ions decreased until a certain level, the adsorption equilibrium, and it took approximately 800 min to achieve this state. Figure 8A shows the relationship of maximum adsorptive amount (qe) to the initial concentration of metal ions (50 ppm < c0 < 900 ppm). Notably, qe increases with increased c0; when c0 increases to a certain value, qe tends to level off. qe reaches a maximum adsorption capacity at room temperature (some data are listed in Table 2). The order of the adsorption in terms of maximum amount adsorbed (mg/g) of the five metal ions is Pb2+ (485.3) > Cu2+ (327.1) > Cd2+ (312.0) > Ca2+ (262.4) > Mg2+ (176.2) when c0 is equal to 900 ppm. The maximum adsorption capacities for Pb2+, Cu2+, and Cd2+ ions were higher than those for Ca2+ and Mg2+ ions. The percentages of the five elements adsorbed are also listed in Table 2 (ppm). Similar results were obtained for other initial concentrations, 0.5 ppm < c0 < 1.0 ppm, and are shown in Figure 8B. Table 3 summarizes qe of NTAZP with qe of other adsorbents reported in some articles.37−39 It should be emphasized that the distinction in Pb2+, Cu2+, and Cd2+ adsorption capacities can be ascribed to the difference in physicochemical properties of various adsorbents and the equilibrium condition conducted. Clearly, NTAZP exhibit high efficiency for Pb2+, Cu2+, and Cd2+ removal. In order to assess the effects of NTAZP in its real-world adsorption applications, a series of experiments were conducted on wastewater samples taken from a steel factory (Shougang Group of China). Results indicate that the maximum adsorption percentages of NTAZP for Pb2+, Cd2+, and Cu2+, are approximately 59.04%, 38.55%, and 44.78%, respectively. Therefore, our results suggest that NTAZP may have excellent prospects for application in the field. The results reported here can potentially be explained by coordination theory. For Pb2+, Cu2+, and Cd2+ ions, interactions between the metal ions and the adsorbent exist not only in the form of van der Waals forces but may also exist in coordination forms due to the presence of N atoms in the framework of NTAZP and empty orbitals in Pb2+, Cu2+ ,and Cd2+ ions. It is known that [Pb(H2O)2]2+, [Cu(H2O)2]2+, and [Cd(H2O)2]2+ are initially formed when the metal ions are dissolved in water.

desorption of adsorbed and intercalated water. The weight loss of 22.0% from 400 to 600 °C, accompanied with an exothermic peak at about 403 and 558 °C, can be attributed to the decomposition of organic species and coke combustion. Total weight loss in all the measurements was about 31.50%. The ICP emission spectroscopy was employed to analyze the chemical composition of the resultant solid, and the results are listed in Table 1, with P/Zr molar ratio approximating to 2:1. Combined Table 1. Elemental Analysis of the Synthesized Materiala Anal. Calcd (%) Found

Zr

O

P

C

H

N

27.38 27.42

41.72 41.69

18.37 18.62

7.19 7.22

2.17 2.22

2.77 2.81

a

Calculation shows composition of the material is Zr3(C3H9O12NP3)2·2H2O.

with the conventional elemental analysis of C, H, and N, also listed in Table 1, the synthesized sample could be formulated as Zr3(C3H9O12NP3)2·2H2O. Total organic carbon analysis showed that the percentage of carbon in the product was about 7.22%, a figure very close to the elemental analysis results [TOC analysis of NTAZP and some relative data are shown Figure S1 and Table S1 in the Supporting Information (SI)]. The stability of NTAZP in solutions with different pH values (from 1.0 to 9.0) was also studied. PXRD patterns of the solid are shown in Figure 1B. It was observed that the structure of NTAZP is stable in solutions with pH values of 1.0−5.0 in spite of the decreased intensities of some diffraction peaks. However, stability was decreased in solutions with pH values higher than 7.0. Overall, we determined that NTAZP was generally stable. To examine the sphere-forming process, a series of experiments involving a daily sampling from the autoclave were conducted. Figure 6 shows the tracked progress of morphology formation in the product. It can be clearly seen that the spheres formed gradually with the passage of time. Initially, only a few sporadic small particles were found after the reaction, and there was no evidence of crystallization (Figure 6a). One day later, some irregular small aggregates emerged (Figure 6b), but the appearance of irregular spheres needed four days (Figure 6e). Better spheres were only obtained after seven days had passed (Figure 6h). It is likely that many small micelles of ATMP were formed first in the earliest phase of sphere formation because of the existence of hydrogen bonds among these ATMP molecules. The reactions between ATMP and ZrOCl2 happened on the surface of these micelles, and further interactions between Zr4+ and other ATMP molecules proceeded until the spherical products were formed.

Figure 6. SEM images of the products in different crystallization time: (a) without crystallization, (b) one day, (c) two days, (d) three days, (e) four days, (f) five days, (g) six days, and (h) seven days. 12269



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Figure 7. Change process in concentrations of different metal ions with contact times (adsorbent dosage = 1.0 g/L, temperature is 25 ± 2 °C): (a) 900 ppm, (b) 800 ppm, (c) 700 ppm, (d) 600 ppm, and (e) 500 ppm.

Figure 8. Relation between the equilibrium adsorption amount and the initial concentration (contact time is 800 min, adsorbent dosage is 1.0 g/L, room temperature is 25 ± 2 °C): (A) 50 < c0 < 900 (ppm), (B) 0.5 < c0 < 1.0 (ppm).

Table 2. Adsorption Date (mg/g) and Adsorption Percents (%) (in bracket) of the Adsorbent to Five Metal Ions at Different Origin Concentrations 500 ppm Pb2+ Cd2+ Cu2+ Ca2+ Mg2+

452.28 261.41 201.73 113.40 147.76

600 ppm

(90.45) (52.38) (40.34) (22.68) (29.56)

468.80 297.06 233.23 142.45 151.41

700 ppm

(78.13) (49.51) (38.87) (23.74) (25.24)

469.36 291.80 254.28 163.36 155.23

800 ppm

(67.05) (41.69) (36.33) (23.34) (22.14)

480.10 298.58 278.48 215.82 159.94

900 ppm

(60.01) (37.32) (34.81) (26.98) (19.92)

485.32 312.01 327.13 262.44 176.18

(53.92) (34.67) (36.35) (29.16) (19.58)

Table 3. Comparison of qe of NTAZP for Pb2+, Cu2+, and Cd2+ with Those of Different Types of Adsorbents in References Pb2+ adsorbents qe (mg/g) temperature pH values ref

Cu2+

Cd2+

magnetic chelating resin

zwitterionic hybrid polymers

NTAZP

Polycarboxylated starch

ion-imprinted fiber

NTAZP

porous bioadsorbent

571.8 45 °C 6.0

380.2 45 °C 5.0

485.3 25 °C 5.0

128.3 40 °C 7.0

120.0 30 °C 6.0

327.1 25 °C 5.0

278.6 25 °C 5.0

37

38

activated carbon 157.4 25 °C 5.0 39

NTAZP 312.0 25 °C 5.0

The binding energies (BEs) of N1s in NTAZP, NTAZP/ Pb2+, NTAZP/Cu2+, and NTAZP/Cd2+ are shown in Figure 9a−d, respectively. Two states of N atoms, one at 401.19 eV from N−C and one at 398.71 eV from coordination between N atoms and H+ ions that arise from the reaction solution, are found in NTAZP. After the adsorptions were finished, BEs of NN−C in NTAZP/Pb2+, NTAZP/Cu2+ and in NTAZP/Cd2+ were at 401.72, 401.98, and 402.21 eV, respectively. Meanwhile, bonds of N→H in NTAZP were substituted by bonds of N→

One molecule of H2O would potentially detach once these hydrated metal ions enter the pores of NTAZP and coordinate with N atoms that exist in the framework of the adsorbent. Ricordel et al40 indicate that approximately 362 kal/ion-g, 507 kal/ion-g, and 539 kal/ion-g are required for the release of one water molecule from [Pb(H2O)2]2+, [Cu(H2O)2]2+, and [Cd(H2O)2]2+, respectively. In other words, [Pb(H2O)]2+ may be more readily formed and captured by the adsorbent than [Cu(H2O)2]2+ and [Cd(H2O)]2+. 12270



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Mg2+ are shown in Figure 11. It is very interesting that the diffraction peaks (marked with ∗ on the top) in NTAZP/Pb2+,

Figure 11. PXRD patterns of NTAZP/Pb2+ (a), NTAZP/Cu2+ (b), and NTAZP/Cd2+ (c).

NTAZP/Cu2+, and NTAZP/Cd2+ have evident shifts to a larger angle compared to the corresponding peak of NTAZP, indicating that diameter of the pores in the host decreased after the three metal ions were intercalated. The result implies that the existence of coordinations between the metal ions and the host, for the simple adsorption cannot change the host pore diameter. Temperature often plays an important role in any adsorption process. In this paper, the adsorption of metal ions by NTAZP, under different temperatures (from 30 to 80 °C) at a given concentration (c0 = 900 ppm), was studied and the adsorptive curves are shown in Figure 12. It is noteworthy that the

Figure 9. XPS analysis of N1s in NTAZP (a), NTAZP/Cd2+ (b), NTAZP/Cu2+ (c), and NTAZP/Pb2+ (d).

M2+ in the adsorption assemblies, as the BE value of NN→H at 398.71 eV disappears and the BE of NN→M2+ appears in the spectrum. BEs of NN→M2+ in NTAZP/Pb2+, NTAZP/Cu2+, and NTAZP/Cd 2+ are at 399.43, 399.67, and 400.35 eV, respectively. These changes in BEs indicate that coordinative interactions between the metal ions and the adsorbent may exist and that the pore size (about 2.70 nm) in NTAZP is large enough to allow the entry and coordination of [M(H2O)2]2+ with N atoms that exist in structure of NTAZP. Phosphonate groups may also coordinate with metal ions, but no PO groups were detected in the infrared absorption spectra of NTAZP (Figures 4 and 10), as the infrared absorption of PO is at about 1276 cm−1.41

Figure 12. Relations between the equilibrium adsorption amounts and the temperatures: (a) Pb2+, (b) Cu2+, (c) Cd2+, (d) Ca2+, and (e) Mg2+.

adsorbed amount of Ca2+ and Mg2+ increased from 30 to 40 °C, but there was a progressive decrease in the amount adsorbed from 40 to 80 °C (for Mg2+) and 50 to 80 °C (for Ca2+). However, for other heavy metal ions, the processes of adsorption differed at several stages. For example, the growth stages of adsorption were different (e.g., from 50 to 60 °C for Pb2+ and Cd2+, from 40 to 50 °C for Cu2+). Notably, qe increased as temperature rose, but after reaching a certain temperature, qe did not increase further, but decreased instead. Accordingly, the maximum adsorption capacities at suitable temperatures, 50 °C (for Cu2+) and 70 °C (for Pb2+ and Cd2+), were obtained for the adsorbent. pH values of the adsorption system control the surface properties of the adsorbent as well as the speciation of metal ions during the process of adsorption. In this study, the pH

Figure 10. FT-IR spectra of NTAZP and its sorption products: (a) NTAZP, (b) NTAZP/Pb2+, (c) NTAZP/Cu2+, (d) NTAZP/Cd2+, (e) NTAZP/Ca2+, and (f) NTAZP/Mg2+.

However, in the case of Ca2+ and Mg2+ ions, interaction between metal ions and adsorbent are only van der Waals forces, and there is not coordinative interaction between the metal ions and the host. So their adsorbed amounts were less than those of the heavy metal ions. FT-IR spectra of adsorbent NTAZP and its adsorption product are illustrated in Figure 10. New absorption peaks that contributed to N→M2+ at about 470 cm−1 are observed in NTAZP/Pb2+ (b), NTAZP/Cu2+ (c), and NTAZP/Cd2+ (d), but they cannot be found in NTAZP/ Ca2+ (e), NTAZP/Mg2+ (f). PXRD patterns of NTAZP/Pb2+, NTAZP/Cu2+, NTAZP/Cd2+, NTAZP/Ca2+, and NTAZP/ 12271



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4. CONCLUSIONS A new mesoporous organic zirconium phosphonate was synthesized by a simple autoclaving method in the presence of nitrilotris(methylene)-triphosphonic acid and zirconium dichloride oxide octahydrate without any surfactant. The compound, NTAZP, was tested for its adsorption of heavy metal ions. The morphology of the product was found to be spherical with lobular lamellae, and the presence of mesopores in the lamellae was demonstrated. The large adsorption capacities for the heavy metal ions (Pb2+ and Cd2+) were likely a result of the coordination between the metal ions and the organic groups inside the pore walls. Compared to a widely used inorganic exchanger zirconium phosphate (ZrPs), the newly synthesized material exhibited a better adsorption profile for heavy metal ions in contaminated water.

values of the adsorption system were increased from 1.0 to 5.0. In order to ensure no precipitation during the entire adsorption process and the consistency of the pH environment for each ion, the initial pH value of the solutions did not be exceed 5.0. The solubility product constants of Pb(OH)2, Cu(OH)2, Cd(OH)2, Ca(OH)2, and Mg(OH)2 are 1.42 × 10−20, 6 × 10−20, 7.2 × 10−15, 8 × 10−6, and 1.2 × 10−11, respectively. The effects of pH values on the removal of metal ions by NTAZP from solutions containing the metal ions at a given concentration (c0 = 900 ppm) were examined, and the results are presented in Figure 13. Adsorption capacities for the five

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ASSOCIATED CONTENT

* Supporting Information S

TOC analysis of NTAZP and some relative data is shown Figure S1 and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

Figure 13. Relations between the equilibrium adsorption amounts and the different pH values in solution under room temperature: (a) Pb2+, (b) Cu2+, (c) Cd2+, (d) Ca2+, and (e) Mg2+.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from Project of National Natural Science Foundation of China (Project No. 50602004), the opening Project of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry of Jilin University, and from PetroChina Company Limited.

kinds of ions increased gradually with an increase in solution pH values. There was little adsorption of the heavy metal ions (Pb2+, Cu2+, and Cd2+) in strongly acidic conditions (about pH 1.0) as the environment favors desorption and adsorption is suppressed. Although, overall the adsorption capacities gradually increased, the changes in the adsorbed amounts of Ca2+ and Mg2+ were not significant. In addition, changes in the pH value of the system during the adsorption process (c0 = 900 ppm) were also studied. The relationships between solution pH values and reaction times are presented in Figure 14. No large fluctuations were found in the measurements, and the pH values remained between 4.0 and 5.0. It indicates that the alkalinity/acidity of the solutions remained stable throughout the process of adsorption.

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REFERENCES

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Figure 14. Change of pH values of the solutions during the adsorption process under room temperature (contact time is 1600 min, c0 is 900 ppm). 12272



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Mesoporous Zirconium Phosphonate Hybrid Material as Adsorbent to

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