Chelating Resins Silica Gel Supported Aminophosphonic Acids

Article pubs.acs.org/IECR

Chelating Resins Silica Gel Supported Aminophosphonic Acids Prepared by a Heterogeneous Synthesis Method and a Homogeneous Synthesis Method and the Removal Properties for Hg(II) from Aqueous Solutions Zengdi Wang,† Ping Yin,*,† Zhi Wang,† Rongjun Qu,*,† and Xiguang Liu †

School of Chemistry and Materials Science, Ludong University, Yantai 264025, P. R. China S Supporting Information *

ABSTRACT: Two kinds of composites silica gel supported by aminophosphonic acids SG-T-P-1 and SG-T-P-2 were prepared successfully by functionalization of silica gel via a heterogeneous synthesis method and a homogeneous synthesis method, respectively, and they were characterized by infrared spectra (IR), scanning electron microscope (SEM), and energy dispersive Xray analysis system spectrum (EDXAS). Moreover, the static adsorption capabilities of these two chelating resins toward transition-metal ions were studied. Our objective was to choose the adsorbent with high adsorption capacities via different synthesis methods, and the results showed that SG-T-P prepared via a homogeneous synthesis method had excellent adsorption capacities and high adsorption selectivity for Hg(II) ions. The Langmuir model was better than the Freundlich model to fit the adsorption isotherms of SG-T-P for Hg(II), and the maximum adsorption capacity was 303.03 mg/g at 15 °C. Thus, the novel chelating resin silica gel supported by aminophosphonic acids SG-T-P prepared by a homogeneous synthesis method is favorable and useful for the removal of Hg(II) ions from aqueous solutions.

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(TCF),8 activated carbon,9,10 dithiocarbamate-anchored polymer/organosmectite composites,11 and so on. Taking the preparation of adsorbents into account, silica gel was widely used as inorganic solid matrix or carrier in inorganic−organic composite materials due to the excellent mechanical and thermal stability, unique large surface area, a firm chemical union with the metals, and well-modified surface properties.12,13 Such kinds of modified silica gel materials have received a great deal of attention recently because of their superior properties and excellent performance in the field of chromatography, adsorption, ion exchange, solid phase extraction, metal ion preconcentration, and catalysis.14−17 Generally, it is difficult for chelating ligands to bond to silica gel because of the relative inertness of the original surface of silica gel. However, bonding of chelating ligands to silica gel surface can be achieved after surface activation. As an amorphous inorganic polymer, silica gel is composed of internal siloxane groups (Si−O−Si) with a large number of silanol groups (≡Si−OH) distributed on the surface. On the surface of active silica gel, silanol groups could react with silane coupling reagents that act as precursors for further immobilization of organic ligands. Chemical modification of the skeleton of silica gel via the covalent coupling of an organic moiety is a promising approach to get such kinds of modified silica gel materials.18 The reaction of silane coupling reagents with surface silanol groups leads to the existence of desirable terminal functional groups on the surface of silica gel, where

INTRODUCTION Water pollution has received increasing attention for several decades because of the higher requirements for the environment and the health of human beings, which are threatened by various contaminants. Heavy metal ions, such as Hg(II), Cd(II), Zn(II), and Cr(III), are the major metal ion pollutants in water, and they can accumulate in the food chain, which poses a severe danger to human health. Mercury, known as a kind of remarkably toxic and nonbiodegradable metal, is ubiquitous in the global environments and derives from both natural sources and human enterprise. The presence of mercury in fish, wastewater, dental amalgams, vaccine preservatives, and in the atmosphere has made this particular toxic metal an increasing focus of health authorities and interest groups.1 Exposure to mercury leads to different toxic effects in the body, including neurological and renal disturbances, inhibition of enzyme activity, and cell damage. The World Health Organization (WHO) has announced a maximum Hg(II) uptake of 0.3 mg per week and a maximum acceptable Hg(II) concentration of 1 μg/L in drinking water.2 Hence, removal of mercury from effluents is very important. In the recent years, attention was focused on the methods for recovery and reuse of metals rather than disposal. Many treatment processes, such as chemical precipitation, electrodialysis, reverse osmosis, ion exchange, adsorption, are currently used. Among these methods, adsorption is highly effective and economical and is a promising and widely applied method.3,4 Therefore, effective adsorbents with strong affinities and high loading capacity for the targeted heavy metal ions have been subsequently studied. For example, a wide range of adsorbents has been used for removing Hg(II) from contaminated water, such as functionalized chitosan,5−7 modified thiol cotton fiber © 2012 American Chemical Society

Received: Revised: Accepted: Published: 8598

January 25, 2012 May 28, 2012 June 10, 2012 June 11, 2012 dx.doi.org/10.1021/ie3002233 | Ind. Eng. Chem. Res. 2012, 51, 8598−8607

Industrial & Engineering Chemistry Research

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special chelating properties can be finally obtained. This is mainly related to the structure of the organic ligands, the nature of the functional groups present, and the incorporated donor atoms. Furthermore, it has been studied that the behavior of these synthesized chemically modified silica gel used as adsorbent are mainly dependent on the presence of active donor atoms such as O, S, and N of the incorporated organic moieties.16,19,20 Consequently, effective adsorbents with strong affinities and high loading amount for heavy metal ions were subsequently prepared by functionalizing the surface of various substrates, such as activated carbon,21 clay,22 zeolite,23 resin,24 and so on. Among the chelating ligands, people found phosphonic acid groups had chelating ability, and the existence of oxygen atoms in P−O and PO groups makes the group coordinate with a variety of transition-metal ions. If the organophosphonic acid groups are grafted on the solid matrix such as silica gel, this kind of chemical modification can overcome its shortcomings of being toxic and soluble in water and be used in adsorption of metal ions from aqueous solutions. The objective of the present work was to explore novel adsorbents silica gel chemically modified by triethylenetetramine bis(methylene phosphonic acid) SG-T-P using 3aminopropyltrimethoxysilane (APTS) as silane coupling agents via a heterogeneous synthesis method and a homogeneous synthesis method and to get more information on their adsorption behaviors for transition-metal ions, especially for Hg(II) ions, from aqueous solutions. The introduction of the organo phosphonic acid groups onto silica gel can make the material form the stable chelating compounds with transitionmetal ions. In the present work, we prepared SG-T-P-1 and SGT-P-2 adsorbents with both N donor atoms and O donor atoms, which could make the material have excellent coordination properties with transition-metal ions and obtain novel adsorbents with high loading of metal ions and sorption selectivity for Hg(II) ions. The similarities and differences of two resins in structure and property were investigated, and it is expected to find out the relationship between the structures and adsorption properties of resins prepared by different routes, and the research results display SG-T-P prepared by a homogeneous synthesis method was a better adsorbent for Hg(II) removal. Moreover, its adsorption selectivity and adsorption isotherm were further investigated in details.

Infrared spectra (FT-IR) of samples were reported in the range of 4000−400 cm−1 with a resolution of 4 cm−1, by accumulating 32 scans using a Nicolet MAGNA-IR 550 (series II) spectrophotometer, and KBr pellets were used for solid samples. The morphology of the adsorbents was examined on JEOL JSF5600LV scanning electron microscope, JEOL Co., Japan. The EDXAS was performed on a NORAN LEVER-2 EDX analytical instrument. Before observation, the sample was placed on a specimen stub covered with a conductive adhesive tab and provided with a sputtered 15-nm platinum coating. Elemental analyses were performed in the Microanalytical Laboratory of Ludong University. The concentration of metal ions was determined using a 932B-model atomic absorption spectrometer (GBC-932A, made in Australia), equipped with air−acetylene flame. Preparation of Chloro-Modified Silica Gel (SG-C). The introduction of chlorine-containing groups onto the surface of SG was achieved by the treatment of surface silanol groups with CPTS. A sample of 30.0 g of activated SG was suspended in 120 mL of dry toluene, and 30.0 mL of CPTS was added to this suspension. The mixture was mechanically stirred under reflux of the solvent in a nitrogen atmosphere for 12 h. The suspension was filtered, and the solid was transferred to a Soxhlet extraction apparatus for reflux-extraction in toluene and ethanol for 12 h, respectively. The product was dried under vacuum at 50 °C over 48 h, to give the modified silica-gel, named SG-C. Preparation of SG-T-P-1 by Heterogeneous Route. Triethylenetetramine was directly introduced onto the surface of SG-C by heterogeneous route. A mixture of SG-C (20.0 g) and TETA (924 mmol) in ethanol (200 mL) was refluxed for 12 h with continuous stirring under a nitrogen atmosphere. The solid product was then filtered off and transferred to the Soxhlet extraction apparatus for reflux-extraction in ethanol for 24 h. After extraction, the product was dried under vacuum at 50 °C over 48 h, and resin SG-TETA-1 was obtained. SGTETA-1 (15.0 g) was added to 120 mL of ethanol at room temperature for 12 h, and then 3.1 g of paraformaldehyde, 8.4 g of phosphorous acid, and 3.5 mL of hydrochloric acid were added. After being refluxed at 90 °C for 12 h refluxed for 8 h, the product (SG-T-P-1) was filtered off, then washed thoroughly with distilled water, and finally dried under vacuum over 48 h at 50 °C. It was referred to as SG-T-P-1. Synthesis of SG-T-P-2 by Homogeneous Route. Under a nitrogen atmosphere, a mixture of 25.0 mL of triethylenetetramine and 15.0 mL of CPTS was stirred at 80 °C in 150 mL of ethanol solution for 12 h, then the product was distilled until there was no ethanol in it, and then 15.0 g of activated SG was added with 150 mL of toluene as solvent. The mixture was stirred at 110 °C for 12 h, and then the solid was filtered off and transferred to a Soxhlet extraction apparatus for refluxextraction in ethanol for 24 h. The solid product was dried in vacuum at 50 °C over 48 h, and it was referred to as SG-TETA2. SG-TETA-2 (10.0 g) was added to 95 mL of ethanol at room temperature for 12 h, and then 2.5 g of paraformaldehyde, 6.9 g of phosphorous acid, and 2.9 mL of hydrochloric acid were added. After being refluxed at 90 °C for 12 h and refluxed for 8 h, the product (SG-T-P) was filtered off, then washed thoroughly with distilled water, and finally dried under vacuum over 48 h at 50 °C. It was referred to as SG-T-P-2. Computational Details. Theoretical calculations of the modified organic group have been performed with the Gaussian 03 program25 using the B3LYP/6-31+G(d) basis set to obtain

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EXPERIMENTAL SECTION Materials and Methods. Silica gel (SG) of chromatographic grade (80−100 mesh size) was obtained from Qingdao Silicon Create Fine Chemical Co., Ltd. Shandong Province of China. It was activated with nitric acid (HNO3: H2O = 1:1) at refluxing temperature for 3 h, hydrochloric acid (HCl: H2O = 1:1) at room temperature for 6 h, then filtered off, washed thoroughly with distilled water until acid-free, and friendly calcined in muffle at 160 °C for 10 h. Organic solvents toluene was redistilled just before use. 3Chloropropyltrimethoxysilane (CPTS) (Jianghan Chemicals Factory, Jinzhou, China), triethylenetetramine (TETA) (Shanghai Chemical Factory of China), and the other reagents were used without further purification. Stock solutions of Hg(II) (0.1 mol L−1) were prepared by dissolving Hg(NO3)2·H2O in 3% HNO3 to avoid hydrolysis. Ammonium acetate/nitric acid solutions buffer solution were used for pH adjustment, and distilled water was used to prepare all the solutions. 8599

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Figure 1. Optimized geometry of the modified organic group.

mmol/L) and the contact time of 12 h at pH 5.0 and 15− 35 °C.

the optimized molecular structure and Mulliken atomic charges of the modified organic group. Adsorption Experiments for Transition-Metal Ions. Saturation adsorption experiment was employed to determine the adsorption amounts of the adsorbents for different kinds of transition-metal ions. Adsorption capacities of the chelating resins for different metal ions were determined by batch tests according to the following procedure, and they were carried out with shaking 0.02 g of the adsorbents with 10 mL of metal ion solution (2 mmol/L). The mixture was equilibrated for 24 h on a thermostat-cum-shaking assembly at 25 °C. The adsorption amount was calculated according to eq 1 q=

(Co − Ce)V W

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RESULTS AND DISCUSSION Theorecical Calculation of the Modified Organic Group. The introduction of the organic groups onto the activated silica gel can make this material form stable chelating compounds with many transition-metal ions. The aim of chemical modification with aminophosphonic acid, which has both N donor atoms and O donor atoms, is to make the material have excellent coordination properties with metal ions and to obtain a novel adsorbent with a high loading capacity for metal ions. In order to design the title material, we theoretically calculated the modified organic group at the B3LYP/631+G(d) level in advance. The optimized structure of the modified organic group was displayed in Figure 1, and the corresponding bond lengths and bond angles were presented in Table S1 (in the Supporting Information). The P57−O68 and P58−O67 bond lengths were 1.5021 Å and 1.4862 Å, respectively, which agreed with well with those values of phosphonic acid in ref 26 and was slightly longer than the experimental value (1.47 Å). Moreover, the P57−O63, P57− O65, P58−O59, and P58−O61 bond lengths were in the range (1.6131 to 1.6307) Å, comparable to those in phosphonic acid (1.59 to 1.63) Å.26 Table 1 presented the Mulliken atomic charges of the modified organic phosphonic acid group and showed that the oxygen atoms in phosphonic acid groups have more negative charges, and the Mulliken electronic populations of O59, O61, O63, O65, O67, and O68 were −1.045, −0.950, −0.838, −0.825, −0.688, and −0.790, respectively, which made these oxygen atoms chelate with transition-metal ions more easily. Therefore, the designed organic groups might provide a good adsorbent for use in adsorbing transition-metal ions from aqueous solutions. Preparation and Characterization. In order to study the effects of the variation of preparation methods on the structures of chelating resins and the relationship between properties and structures, two kinds of synthetic routes were designed to prepare silica gel supported aminophosphonic acids. The synthetic routes were illustrated in Figure 2; synthesis steps

(1)

where q is the adsorption amount (mmol/g); Co and Ce are the initial and equilibrium concentrations of metal ions (mmol/ mL) in solution, respectively; V is the volume of the solution (mL); and W is the weight of the adsorbents (g). Effect of pH on Adsorption. The effect of pH on the adsorption of Hg(II) was studied by adding 20.0 mg of the adsorbents to 0.2 mL of 0.1 mol/L Hg(II) and 19.8 mL of buffer solution at different pH values in a 100 mL Erlenmeyer flask. The mixture was equilibrated for 12 h on a thermostatcum-shaking assembly at 25 °C, then a certain volume of the solutions was separated from the adsorbents, and the residual concentration of Hg(II) was detected by means of atomic absorption spectrometer. Competitive Adsorption. In order to investigate the adsorption selectivity of the adsorbent for Hg(II), 0.02 g of the adsorbents was added into 10 mL solutions (a binary system containing equal initial concentrations (2.5 mmol/L) of Hg(II) ion and other coexisting metal ions), and the mixture were shaken for 12 h. The initial pH was adjusted to 5.0 with the temperature at 25 °C. Adsorption Isotherms. The isotherm adsorption property of the chelating resins was investigated also by batch tests. In a typical procedure, a series of 100 mL tubes were used. The adsorption isotherms were studied using 0.02 g of the adsorbents with various Hg(II) ion concentrations (1.0−5.0 8600



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CPTS and TETA onto the surface of silica gel to form resulting chelating resin SG-T-P-2. For the reaction of TETA with chlorinated activated silica gel (SG-C) through route A, cross-linking reactions were inevitable because the N-alkylation reaction could occur at primary amino and secondary amino groups in TETA. These cross-linking reactions might decrease the amount of immobilized ligand (TETA and aminophosphonic acids). Furthermore, complicated cross-linking also could bring about steric restrictions on the formation of transition-metal chelate. In order to minimize the formation of cross-linkings, a homogeneous synthesis method (route B) was designed, and the reaction of CPTS and TETA was carried out in the presence of excessive TETA. The FT-IR analysis is a useful means in identifying the immobilization process by comparing the precursor and modified surfaces,27 then infrared spectroscopy was employed in order to identify the immobilization onto the surface of silica gel, and the results were shown in Figure 3. As seen in Figure 3(a), the infrared spectra of silica gel, SG-C, SG-TETA-1, and SG-T-P-1 displayed important differences. A large broad band at 3433 cm−1 and a weak band at about 1630 cm−1 of the silica gel spectra attributed to the presence of the O−H stretching frequency of silianol groups and also to the remaining adsorbed water. The broad and intense band at 1100 cm−1 was assigned to the siloxane vibration (Si−O−Si), and Si−O bond stretching was detected at 969 cm−1. Moreover, the peaks around 801 cm−1 and 473 cm−1 were due to symmetric stretching and

Table 1. Mulliken Atomic Charges of the Modified Organic Group at the B3LYP/6-31+G(d) Level and the Atom Labels Are According to Figure 1 atoms

charges

atoms

charges

Si1 O2 O3 O4 N26 N34 N42 N50 P57 P58

1.862 −0.615 −0.659 −0.646 −0.414 −0.466 −0.368 −0.160 1.761 1.844

O59 H60 O61 H62 O63 H64 O65 H66 O67 O68

−1.045 0.586 −0.950 0.573 −0.838 0.521 −0.825 0.576 −0.688 −0.790

of SG-T-P-1 included syntheses of SG-C, SG-TETA-1, and the final product SG-T-P-1, and that of SG-T-P-2 included syntheses of CT, SG-TETA-2, and the final product SG-T-P2. The immobilization could be achieved in two distinct ways, that is, a heterogeneous route and a homogeneous route. In the heterogeneous route, the silane coupling reagent CPTS was first grafted onto silica gel and then reacted with TETA, subsequently paraformaldehyde and phosphorous acid to give a final product SG-T-P-1 (Figure S1, Figure of Mannich reaction mechanism of SG-T-P, in the Supporting Information). However, the homogeneous route was based on the immobilization of the new coupling reagent CT prepared by

Figure 2. Different preparation methods of adsorbents SG-T-P-1 and SG-T-P-2 (A: heterogeneous synthesis route; B: homogeneous synthesis route). 8601



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Figure 3. FT-IR spectra of silica gel SG, SG-C, SG-TETA-1, SG-T-P-1, SG-TETA-2, and SG-T-P-2.

Figure 4. SEM images of silica gelSG, SG-C, SG-TETA-1, SG-T-P-1, SG-TETA-2, and SG-T-P-2.

stretching vibration was at 1105 cm−1, and the peaks around 801 cm−1 and 473 cm−1 were due to symmetric stretching and bending vibrations of Si−O−Si, respectively. Moreover, a characteristic stretching band of the free silanol groups centered at 968 cm−1.16 In the spectrum of SG-TETA-2, two new bands appeared at (2925 and 2854) cm−1 that correspond to the typical asymmetric and symmetric stretching vibration of −CH2−, due to the presence of the carbon chain of 3chloropropyltrimethoxysilane and polyamines attached to the silica-gel. A new band appeared at 1401 cm−1 that was assigned to the bending vibration of N−H which transferred to lower frequencies due to the stretching vibration of the remaining Si− O of the silica gelwhich strongly absorbed at 1631 cm−1. The band around 968 cm−1 of the free silanol groups disappeared, because of the reaction with the alkoxide groups of the silylant agent functionalized with polyamines, and the stretching vibration of N−H overlapped in the range of (3725 to 3052) cm−1. In the spectrum of SG-T-P-2, the band at 1401 cm−1 of primary amino group almost disappeared and the strong peak centered at 3426 cm−1 containing the stretching vibration of N−H decreased, confirming the methylene phosphonic acid was successfully introduced onto the amino-terminated chelating resin silica gel. Both of the bonds PO at 1175

bending vibrations of Si−O−Si, respectively. In the spectrum of SG-C, a new band appeared at 2961 cm−1 corresponding to the (C−H) stretching frequency because of the presence of the carbon chain of CPTS attached to silica gel, and a new band at 697 cm−1 was assigned to the C−Cl rocking vibration of SiCH2CH2CH2Cl, whose intensity was observed to decline after the reaction with TETA. The relative intensity of the band at 969 cm−1 for the silanol groups (Si−OH) reduced considerably as expected in such immobilization processed. As to SG-T-P-1, the band at 1400 cm−1 was ascribed to the -NH2 and -NHbending vibration and the strong peak centered at 3432 cm−1 containing the stretching vibration of N−H were both weakened, suggesting the phosphonic acid was successfully introduced onto the amino-terminated chelating resin silica-gel. The bonds PO at 1175 cm−1 and the characteristic sorption peak of P−OH around 930 cm−1 overlapped in the broad band between (1341 and 870) cm−1. As seen in Figure 3(b), it could also be confirmed the presence of amino group and the methylene phosphonic acid bound to the silica surface. A large broad band at 3432 cm−1 and a weak band at about 1633 cm−1 in the IR spectrum of silica gel were attributed to the presence of the O−H bond stretching vibration of the Si−OH group and the adsorbed water. The intense band related to Si−O−Si 8602



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cm−1 and the characteristic sorption peak of P−OH around 930 cm−1 were overlapped in the broad band between 1346 cm−1 and 867 cm−1. Morphologies of silica gel, the intermediates, and the products obtained from the relative synthesis processes were characterized by SEM, and they were shown in Figure 4. SEM was performed on the bare silica and chemically modified particles in order to detect differences in their surfaces. Apparently, the surface of bare silica gel was smooth and became rough after the modification reactions. Moreover, we observed that no clog between particles occurred during the preparation process, and the particles maintained regular lumpy shape. It could be seen that the particle appearance and size of these three samples were similar, demonstrating that the particles of silica gel had good mechanical stability and they had not been destroyed during the whole reaction. Scanning electron micrographs of silica gel, the intermediates and the final products in Figure 4 were obtained at 300× magnification, and their morphologies were displayed to clarify the unagglomeration of the silica gel particles after treatment to support the claiming of regular distribution of the functional group on the whole surface. Moreover, it was evident that the loaded functional groups were distributed on the whole surface that made the surface of the intermediates and the products become rough and had some folds, which could increase the specific area of the products and consequently made the adsorption process faster. The results of EDXAS analysis for SG-TETA-1, SG-TETA-2, SG-T-P-1, and SG-T-P-2 were summarized in Table 2.

phosphonic acid ligands on the surface of silica gel, it can be concluded that the degree of functionalization obtained by a homogeneous route was higher than that obtained by a heterogeneous route. Adsorption of Transition-Metal Ions. The aim of grafting functional aminophosphonic acid groups onto the surface of silica gel is to prepare the modified silica gel with more excellent coordination properties. Saturated adsorption capacities for transition-metal ions were essential parameters for evaluating the ability of modified silica gel to bind and extract different transition-metal ions from aqueous solutions. Figure 5

Figure 5. The static adsorption capacities of SG-T-P-1 and SG-T-P-2 for transition-metal ions (the initial solution concentration of transition-metal ions: 2.0 mmol/L; pH = 5.0; T = 25 °C).

Table 2. Results of EDXAS Analysis for SG-TETA-1, SGTETA-2, SG-T-P-1, and SG-T-P-2 W%

C

O

N

Si

P

F′ (mmol/g)a

SG-TETA-1 SG-TETA-2 SG-T-P-1 SG-T-P-2

20.27 21.81 15.76 22.65

50.22 39.50 57.75 24.81

7.12 10.52 8.18 13.16

21.86 28.16 16.36 32.94

----1.04 6.44

----0.29 1.81

showed the saturation adsorption amounts of SG-T-P-1 and SG-T-P-2 for Hg(II), Cu(II), Ni(II), Co(II), Cd(II), Zn(II), Pb(II), and Cr(III) metal ions. The saturated adsorption experiments for all these transition-metal ions at pH 5.0 were studied at 25 °C, and twenty-four hours of contact time was selected in this study to ensure the transition-metal ions could be completely adsorbed. The research result displayed the static adsorption amounts of SG-T-P-2 for Hg(II) and Cu(II) were 0.780 mmol/g and 0.451 mmol/g, respectively, and those for Ni(II), Co(II), Cd(II), Zn(II), Pb(II), and Cr(III) metal ions were 0.137 mmol/g, 0.115 mmol/g, 0.0762 mmol/g, 0.0599 mmol/g, 0.0516 mmol/g, and 0.0130 mmol/g, respectively. However, the static adsorption amounts of SG-T-P-1 for Hg(II) and Cu(II) were 0.457 mmol/g and 0.284 mmol/g, respectively, and those for Ni(II), Co(II), Cd(II), Zn(II), Pb(II), and Cr(III) metal ions were 0.0985 mmol/g, 0.0658 mmol/g, 0.00686 mmol/g, 0.0476 mmol/g, 0.0497 mmol/g, and 0.00876 mmol/g, respectively. Obviously, two assynthesized adsorbents had a good adsorption amount for Hg(II) and Cu(II) metal ions, especially for Hg(II) ion. Through the amino phosphonic acid groups, SG-T-P-2 and SGT-P-1 can form the stable chelating compounds with many transition-metal ions, especially with Hg(II). According to the theory of hard and soft acids and bases (HSAB) defined by Pearson, metal ions will have a preference for coordinating with ligands that have more or less the same electronegative donor atoms. Chelating agents with N and O donor atoms are highly efficient for the selective sorption of Hg(II) metal ions. As we compared the adsorption amount of SG-T-P-2 and SG-T-P-1, it was clear that the adsorption amount of SG-T-P-2 prepared by a homogeneous route was relatively higher than that of SG-T-

a F′ = %P × 10/35.5, where %P is the mass percentage of phosphor in product.

Generally, the degree of cross-linking of polyamine depends on the molar ratio of C/N, that is, the higher the molar ratio of C/N, the higher the degree of cross-linking. We could obtain the molar ratio values of SG-TETA-1 and SG-TETA-2 from the calculation of EDXAS results, and they were 3.32 and 2.42, respectively. The above fact demonstrated that the degree of cross-linking of SG-TETA-2 was lower, and the homogeneous route was a more effective route than the heterogeneous route because it could minimize cross-linking reaction. Moreover, the mass percentage of phosphor in SG-T-P-1 and SG-T-P-2 was 1.04% and 6.44%, respectively, which confirmed the phosphonic acid groups were successfully introduced onto the amino-terminated chelating resin silica gel SG-TETA-1 and SGTETA-2. Furthermore, it is possible to calculate the amount of attached functional groups onto the surface of silica gel (F′, mmol/g) from the percentage of phosphor in the functionalized silica-gel. The amounts of F′ for SG-T-P-1 and SG-T-P-2 were 0.29 mmol/g and 1.81 mmol/g. Comparing their relative amount of F′, it was easy to conclude that the amount of attached functional groups for SG-T-P-2 is larger than that for SG-T-P-1. Thus, comparing the amount of functionalized 8603



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Competitive Adsorption. The most important properties of a chelating resin adsorbent material that influence its application are sorption amount in addition to sorption selectivity which is basically an attribute of the functional group of the adsorbent. So, the adsorption selectivity is an indispensable factor for evaluating the capacities of an adsorbent, by which the chelating resin can be used to adsorb a specific transition-metal ion or to separate specific transitionmetal ions from a mixed metal ions solution. In this study, the competitive adsorption experiments by SG-T-P prepared by a homogeneous method were carried out from Hg(II)−Pb(II), Hg(II)−Ni(II), Hg(II)−Zn(II), Hg(II)−Cr(III), and Hg(II)− Co(II) binary systems. The initial Hg(II) concentration as well as other transition-metal ions such as Zn(II), Ni(II), Pb(II), Co(II), and Cr(III) was 2.5 mmol/L. The obtained results for Hg(II) adsorption at 25 °C are presented in Table 3.

P-1 prepared by a heterogeneous route, and different preparation methods might influence the adsorption properties of the adsorbents. Therefore, in the following part, the adsorption of the adsorbents for Hg(II) from aqueous solutions was investigated particularly. Effect of pH on Adsorption of Hg(II). The pH value of the metal ions solution is one of the most important factors influencing the adsorption behavior of transition-metal ions on adsorbents. It not only impacts the surface structure of adsorbents and the formation of transition-metal ions but may also influence the interaction between adsorbents and transition-metal ions. The effect of pH on the adsorption of Hg(II) by the SG-T-P-1 and SG-T-P-2 at 25 °C was illustrated in Figure 6, and the results showed that the adsorption

Table 3. Adsorption Selectivity of SG-T-P Prepared by a Homogeneous Method for Hg(II)a adsorbents SG-T-P

system Hg(II)− Pb(II) Hg(II)− Ni(II) Hg(II)− Zn(II) Hg(II)− Cr(II)

Figure 6. Effect of pH at 25 °C on the adsorption of Hg(II) on SG-TP-1 and SG-T-P-2 at an initial concentration of 1.0 mmol/L.

Hg(II)− Co(II)

capacities at adsorption equilibrium increased with the increase of the solution pH values, particularly in the pH range of 1.0− 5.0. The lower adsorption capacities of Hg(II) ions at low solution pH values was due to the competitive coordination effect of the H+ ions with N donor atoms of −NH− and O donor atoms of ≡PO on the surface of adsorbents. The uptake of Hg(II) beyond the pH (pH >5.0) is doubtful to be attributed only to the interaction of the free Hg(II) cations with the active sites on the adsorbents but also to the formation of metal hydroxide species such as soluble Hg(OH)+ and/or insoluble precipitate of Hg(OH)2.28 Then, the maximum adsorption capacities were achieved at pH 5.0 in the range of 1.0−6.0 examined. Consequently, all the following experiments were performed at pH 5.0. Moreover, the adsorption capacity of SG-T-P-2 was higher than that of SG-T-P-1 in the whole pH range studied, for example, the adsorption capacity of SG-T-P-1 and SG-T-P-2 for Hg(II) ions was 0.930 mmol/g and 0.771 mmol/g at pH 5.0, respectively. According to the above fact, one conclusion could be drawn from the compared experiments, that is, higher N content of silica gel does not ensure a higher utilization ratio of N to form more phosphonic acid groups, and more P content of silica gel could lead to a higher adsorption capacity of transition-metal ions. Then, in the subsequent adsorption research, our work would focus on adsorption kinetics, adsorption thermodynamics, and adsorption isotherm of SG-T-P prepared by a homogeneous synthesis method for Hg(II) from aqueous solutions.

metal ions

adsorbents capacity (mmol/g)

Hg(II)

0.7014

Pb(II) Hg(II)

0.3304 0.9921

selective coefficient α α

Co(II) Hg(II)

0.0506 0.9122

α

Ni(II) Hg(II)

0.0000 0.8306

α

Zn(II) Hg(II)

0.0000 0.7788

α

Cr(II)

0.0000

Hg(II)/Pb(II)

=

Hg(II)/Co(II)

=

2.12

19.61

Hg(II)/Ni(II)

=∞

Hg(II)/Zn(II)

=∞

Hg(II)/Cr(II)

=∞

a

Hg(II) concentration: 2.5 mmol/L; concentration of coexisting metal ions: 2.5 mmol/L; pH = 5.0; T = 25°C.

The selective coefficients were the ratio of adsorption amounts of metal ions in the binary mixture: The selective coefficient = (q′)/(q″), where q′ is the adsorption amount of Hg(II) ion in the binary mixture and q″ is the adsorption amount of the other metal ions in the binary mixture. The results displayed that SG-T-P prepared by a homogeneous route had excellent adsorption for Hg(II) in the binary ions systems, especially in the systems of Hg(II)− Ni(II), Hg(II)−Zn(II), and Hg(II)−Cr(III). As seen in Table 3, SG-T-P only adsorbed Hg(II) in the binary ions system of Hg(II)−Ni(II), Hg(II)−Zn(II), and Hg(II)−Cr(III), and this indicated that the chelating resin exhibited excellent adsorption selectivity for Hg(II). Moreover, the results shown in Table 3 could provide the possibility of separating Ni(II), Zn(II), and Cr(III) from Hg(II) with the chelating resin at pH = 5.0. Thus, this novel functionalized silica gel chemically modified by triethylenetetramine bis(methylene phosphonic acid)) SG-T-P prepared by a homogeneous method had high adsorption amount and good selectivity for Hg(II), which can be applied for removing this toxic metal element from aqueous solutions. Adsorption Isotherms. The adsorption isotherms were studied in the relationship between equilibrium adsorption capacity and equilibrium concentration at a certain temperature, and the adsorption isotherms of SG-T-P prepared by a 8604



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homogeneous route for Hg(II) at different temperatures were studied and the results are shown in Figure 7. As seen in the

Figure 9. The Freundlich isotherms of SG-T-P prepared by a homogeneous synthesis method for Hg(II) at different temperatures.

where qe is the adsorption capacity, mg/g; Ce is the equilibrium concentration of Hg(II), mg/L; q is the saturated adsorption capacity, mg/g; KL is the Langmuir constant, L/mg; n is the Freundlich constant, and KF is the binding energy constant reflecting the affinity of the adsorbents to metal ions, mg/g. The parameters for the isotherms obtained from Figures 8 and 9 are presented in Table 4. As seen from Table 4, the R2 values obtained from the Langmuir model are much closer to 1 than those from the Freundlich model, suggesting the Langmuir model is better than the Freundlich model to fit the adsorption isotherm of SG-T-P prepared by a homogeneous route for Hg(II). The fact showed that SG-T-P was attributed to monolayer adsorption, and the maximum adsorption capacity of SG-T-P obtained by the Langmuir isotherm for Hg(II) adsorption was 303.03 mg/g at 15 °C. It was clear that the adsorption capacity of SG-T-P prepared by a homogeneous method was relatively high when compared to several other adsorbents such as the modified thiol cotton fiber, dithiocarbamate-anchored polymer/organosmectite composites, silica gel supported diethylenetriamine, GLA (crosslinking with glutaraldehyde)-chitosan, ECH (epichlorohydrin)chitosan, GD (glycidylmethacrylate−divinylbenzene)-EDA, and 2-mercaptothiazoline modified mesoporous silica,8,11,17,29−31 and the adsorption capacities of different adsorbents were listed in Table 5. The above-mentioned research results showed that the novel adsorbent material SG-T-P prepared by a homogeneous method was very useful for the removal of Hg(II). Moreover, we tried to investigate the adsorption mechanism by FT-IR analysis, Figure S2-(1) and Figure S2-(2) showed FT-IR spectra of SG-T-P prepared by a homogeneous method before and after loading with Hg(II) ions. Figure S2 and the relevant explanation are available online in the Supporting Information. Further work is underway to study the adsorption mechanism more deeply and systematically in our subsequent research work. All these adsorption research results showed that the novel adsorbent silica gel chemically modified by triethylenetetramine bis(methylene phosphonic acid) SG-T-P prepared by a homogeneous method is favorable and useful for the removal of Hg(II) from aqueous solutions.

Figure 7. Adsorption isotherms of Hg(II) onto SG-T-P prepared by a homogeneous synthesis method at different temperatures (pH = 5.0).

figure, the absorption capacity of SG-T-P for Hg(II) increased with the decrease of temperature. At a certain temperature, it was explicit that the adsorption capacity of Hg(II) increased with the increase of the equilibrium concentration. The Langmuir isotherm model and the Fruendlich isotherm model were usually used to interpret the isothermal adsorption experimental data. The Langmuir model assumes that the uptake of metal ions occurs on a homogeneous surface by monolayer adsorption without any interaction between adsorbed ions. The Freundlich model assumes that the uptake or adsorption of metal ions occurs on a heterogeneous surface by monolayer adsorption. In order to well understand the adsorption behaviors, we employed Langmuir eq 2 and

Figure 8. The Langmuir isotherms for the adsorption of Hg(II) onto SG-T-P prepared by a homogeneous synthesis method at different temperatures.

Freundlich eq 3 (see Figures 8 and 9) to fit the experimental data, respectively Ce C 1 = e + qe q qKL

ln qe = ln KF +

■

CONCLUSIONS The heterogeneous synthesis method and the homogeneous synthesis method for aminophosphonic acids functionalized silica gel chelating resins SG-T-P-1 and SG-T-P-2 were developed. The research results showed that SG-T-P-2 had

(2)

ln Ce n

(3) 8605



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Table 4. Isotherm Parameters of Langmuir and Frendlich for the Sorption of Hg(II) Obtained by Using a Linear Method Langmuir

Freundlich

adsorbents

T (°C)

q(mg/g)

KL(L/mg)

R2

KF(mg/g)

n

R2

SG-T-P

15 25 35

303.03 238.10 212.77

0.0219 0.0299 0.0243

0.9976 0.9947 0.9930

27.5720 27.7379 22.6170

2.5853 2.8944 2.8106

0.7664 0.6460 0.6238

■

Table 5. Adsorption Capacities of Different Adsorbents adsorbents

references

adsorption capacities (mg/g)

SG-T-P prepared by homogeneous method TCF (the modified thiol cotton fiber) dithiocarbamate-anchored polymer/ organosmectite composites SG-HO-pD SG-HE-pD SG-HO-dD SG-HE-Dd (where SG means silica-gel; HE means heterogeneous, HO means homogeneous, d means direct, p means protected, and D means diethylenetriamine) GLA (cross-linking with glutaraldehyde)chitosan ECH (epichlorohydrin)-chitosan GD (glycidylmethacrylate−divinylbenzene)-EDA GD- I (polyethylpolyethyleneimine 423 Da) GD- II (polyethylpolyethyleneimine 600 Da) GD- III (polyethylpolyethyleneimine 1800 Da) GD- IV (polyethylpolyethyleneimine 10000 Da) MTZ-MCM-41 (2-mercaptothiazoline modified mesoporous silica)

our work 8 11

303.03 60−70 157.30

17 17 17 17

ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S2, Table S1, and the relevant explanations. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

104.31 36.11 44.13 28.08

*Phone: + 86-535-6696162. Fax: + 86-535-6697667. E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

29

22.3−75.5

■

29 30 30 30 30 30 31

16.3−30.3 212.63 176.52 152.45 132.39 104.31 140.41

■

ACKNOWLEDGMENTS We greatly appreciate the support provided by the National Natural Science Foundation of China (Grant Nos. 51102127 and 51073075), the Nature Science Foundation of Shandong Province (2009ZRB01463), and the Foundation of Innovation Team Building of Ludong University (08-CXB001). REFERENCES

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more excellent adsorption capacities than SG-T-P-1, then it meant that higher N content of silica gel does not ensure a higher utilization ratio of N to form more phosphonic acid groups, and more P content of silica gel could lead to higher adsorption capacity of transition-metal ions. Furthermore, adsorption kinetics, adsorption thermodynamics, and adsorption isotherm of SG-T-P prepared by a homogeneous synthesis method for Hg(II) from aqueous solutions were studied. The adsorption experiment showed that this chelating resin had the highest adsorption for Hg(II) at pH = 5.0, and the adsorption selectivity results of SG-T-P prepared via a homogeneous synthesis method for Hg(II) indicated that it had excellent adsorption for Hg(II) in binary ions systems and could be used to separate Zn(II), Co(II), and Cr(III) from Hg(II) at pH = 5.0. The Langmuir model was better than the hFreundlich model to fit the adsorption isotherms of SG-T-P for Hg(II), and the maximum adsorption capacity was 303.03 mg/g at 15 °C. Thus, this novel adsorbent material silica gel chemically modified by triethylenetetramine bis(methylene phosphonic acid SG-T-P prepared by homogeneous route is favorable and useful for the removal of Hg(II) from aqueous solutions using an adsorption method, and the high adsorption capacity and good selectivity make it a good promising candidate material for the Hg(II) uptake. 8606



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Chelating Resins Silica Gel Supported Aminophosphonic Acids

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