Functionalized silica for heavy metal ions adsorption

Functionalized silica for heavy metal ions adsorption

Colloids and Surfaces A: Physicochem. Eng. Aspects 221 (2003) 221 /230 www.elsevier.com/locate/colsurfa Functionalized silica for heavy metal ions a...

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Colloids and Surfaces A: Physicochem. Eng. Aspects 221 (2003) 221 /230 www.elsevier.com/locate/colsurfa

Functionalized silica for heavy metal ions adsorption Laurence Bois a,*, Anne Bonhomme´ a, Annie Ribes a, Bernadette Pais a, Guy Raffin a, Franck Tessier b b

a Service Central d’Analyse du CNRS (USR 59), Echangeur de Solaize, BP22, 69390 Vernaison, France Laboratoire «Verres et Ce´ramiques» (UMR CNRS 6512), Institut de Chimie de Rennes, Universite´ de Rennes 1, 35042 Rennes cedex, France

Received 2 December 2002; accepted 27 March 2003

Abstract Heavy metals adsorbents were prepared by co-condensation of tetraethoxysilane and functionalized trialkoxysilane RSi(OR?)3. Functionalized porous silicas with aminopropyl (H2N(CH2)3-), [amino-ethylamino]propyl- (H2N -(CH2)2NH(CH2)3), (2-aminoethylamino)-ethylamino]propyl (H2N -(CH2)2-NH-(CH2)2-NH(CH2)3-), and mercaptopropyl (HS-(CH2)3-) groups were synthesized using dodecylamine as structure directing agent. These materials have been characterized by elemental analysis, powder X-ray diffraction, nitrogen gas sorption, Fourier transform infrared and Raman spectroscopies and thermogravimetric analysis. These organo-silicas were prepared for use in the removal of heavy metal ions from aqueous solutions. Samples synthesized with [amino-ethylamino]propyl- and (2-aminoethylamino)ethylamino]propyl- functions show a high loading capacity for Cu2 , Ni2 , Co2 and the anion Cr(VI). The sample synthesized with a mercaptopropyl function has a high loading capacity for Cd2 . # 2003 Elsevier Science B.V. All rights reserved. Keywords: Mesoporous silica; Non-ionic surfactant; Adsorbent; Copper; Chromate

1. Introduction Mesoporous silica have received considerable attention, because of their unique large surface area, well-defined pore size and pore shape [1,2]. Surfactant molecules are used to form supramolecular assemblies in solution. Metal alkoxides precursors can undergo hydrolytic polymerization

* Corresponding author. Present address: Laboratoire Multimate´riaux et Interfaces, Universite´ Claude Bernard, 69622 Villeurbanne, France. E-mail address: [email protected] (L. Bois).

around these assemblies. Subsequent surfactant removal results in a solid material having ordered porosity. The pores of the hexagonal ordered adsorbent, MCM-41, discovered by scientists of the Mobil Oil Research, can be engineered with ˚ . These large pore sizes diameters from 15 to 100 A make these materials very attractive for applications such as catalytic supports, sensors and sorbents. Applications of mesoporous silicas as heavy metal ions adsorbent have been studied recently [3 /22]. Different surfactant assembly methods have been investigated, involving either electrostatic

0927-7757/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0927-7757(03)00138-9

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surfactant-precursor assembly interactions or hydrogen bonding. Organomodified MCMs using ionic templates have been investigated [23 /30]. Another method, based on the use of neutral templates of polyether’s type has also been proposed [5,31,32]. The use of long chain amines as neutral structural agent has also been suggested, leading to materials designed as hexagonal mesoporous silicas (HMS) [3,6,10,33,34]. The MCM class of materials have hexagonal long range order, while the HMS materials are considerably less ordered and are better described as having wormhole structures [35]. Advantage of the neutral surfactant is that the framework and the template are combined by via hydrogen bonding so the template can be removed by solvent extraction. For environmental applications, the development of functionalized nanoporous materials is necessary, especially for the preparation of heavy metal adsorbents. The functionalization of mesoporous silicas has been mostly studied with a mercaptopropyl group [3 /12,23 /28,31]. For instance, a mesoporous silica (MCM-41) was functionalized with mercaptopropyl groups and its mercury loading was of 2.5 mmol g1 [9,28]. The use of neutral surfactant, which could be removed by solvent extraction, was proposed [3 /6]. The loss of silanol functionality coming from the calcination step was then overcome. A mercury loading capacity of 1.5 mmol g1 was obtained. An other mesoporous silica SBA-15, which is more hydrothermally stable compared with MCM-41 was functionalized with mercaptopropyl groups [17]. The functionalization of mesoporous silica with an aminopropyl group has also been studied [10,13,14,17,19,20,23 /25,36 /38]. Bifunctional silicas containing mercaptopropyl and aminopropyl groups, using dodecylamine as templating agent have been studied [18]. Thiol and amino-functionalized SBA-15 silicas were prepared and used for removing heavy metal ions from waste water. The thiolated SBA-15 adsorbent exhibited a higher complexation affinity for Hg2, while the other metal ions (Cu 2, Zn2, Cr3 and Ni2) showed exceptional binding ability with its aminated analogue [17].

Mesoporous organosilica functionalized with aminoethylaminopropyl groups has been synthesized, using dodecylamine as surfactant, for the removal of copper ions in aqueous solutions [18]. Another very promising method, involves the use of 1,2-bis(triethoxysilyl)ethane to construct the network. For instance, 1,2-bis(triethoxysilyl)ethane (BTSE) were copolymerized with metal ions complexes of N -[3-(trimethoxysilyl)propyl]ethylenediamine in the presence of supramolecular assembly of cetyltrimethylammonium chloride [7,15,16]. The post-grafting of aminoethylaminopropyl groups on mesoporous silicas, followed by the incorporation of copper ions leads to a very effective anion binding material (arsenate and chromate) [7]. Recently, mono, di- and triamino ligands fixed on SBA-1 mesoporous silica were found to have high adsorption capacities for chromate and arsenate [19,20]. In this work, functional silicas were prepared as heavy metal ion adsorbents. A neutral surfactant was used, the dodecylamine. TEOS, (Si(OEt)4) was used as a silica precursor. Aminopropyl(H2N(CH2)3-) noted (-N), [amino-ethylamino]propyl- (H2N-(CH2)2-NH(CH2)3), noted (-NN), (2aminoethylamino)ethylamino]propyl(H2N(CH2)2-NH-(CH2)2-NH(CH2)3-) noted (-NNN) and mercaptopropyl- (HS-(CH2)3-) noted (-S), were introduced in the silica network using functionalized trialkoxysilane RSi(OR?)3, with R /-N, -NN, -NNN or -S and R?/Me or Et (Table 1). These materials were characterized using elemental analysis, X-ray diffraction, nitrogen gas sorption, Fourier transform infrared and Raman spectroscopies and thermogravimetric analysis. The solids were tested for the removal of heavy metal in aqueous solutions.

2. Experimental section 2.1. Chemicals Dodecylamine (CH3(CH2)11NH2) (Fluka) was used as a template. Tetraethoxysilane (TEOS) was used as a silica precursor. Aminopropyltriethoxysilane (H2N(CH2)3Si(OEt)3), [amino-ethylamino]-propyltrimethoxysilane (H2N-(CH2)2-NH-

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Table 1 Description of the adsorbents

(CH2)3-Si(OMe)3), [(2-aminoethylamino)ethylamino]propyl trimethoxysilane (H2N-(CH2)2-NH(CH2)2-NH(CH2)3-Si(OMe)3) and mercaptotrimethoxysilane (HS(CH2)3Si(OMe)3) were used as chelating agents (Fluka). All chemicals were used as received.

dried. The product was then washed free of surfactant by Soxhlet extraction over ethanol for 6 hours then washed on water and on ethanol and dried at 50 8C.

2.2. Synthesis

C, N and S analyses were performed by the ‘‘Service Central d’Analyse du CNRS (USR 059)’’, Vernaison, France. An X-ray Diffractometer Siemens D500 was used at 30 keV and 40 mA using Cu Ka radiation to measure the crystallinity of the prepared samples. A step size of 0.028 and a time per step of 2 s were used. Nitrogen sorption measurements and BET surface area analysis have been performed using a Flowsorb II 2300 Micrometrics instrument, with the single point B.E.T. method. A mixture of helium with 30% nitrogen was used. Before each measurement, the sample was outgazed under an helium-nitrogen flow for 2 h at 150 8C. Fourier transform infrared and Raman spectroscopies were used to identify the functional groups in the samples. Spectra were recorded with a Nexus Nicolet equipment between 400 and 4000 cm 1 using KBr pellets for infrared spectra and using glass tubes for Raman spectra. Raman

Materials, noted N-S, NN-S, NNN-S and S-S were synthesized with a molar ratio of RSi(OR?)3/ TEOS of 15/85 (Table 1). For instance, the N-S adsorbent was prepared with the typical molar composition: 0.22 dodecylamine: 0.85 TEOS: 0.15 H2N(CH2)3Si(OEt)3: 160 H2O: 5 EtOH. The reference adsorbent S was prepared with TEOS as the only silica source: 0.2 dodecylamine: 1.0 TEOS: 160 H2O: 5 EtOH. Dodecylamine (22 mmoles) was dissolved in ethanol (4.6 ml) by stirring and heating at 60 8C. Water (58 ml), heated at 60 8C, was added. The mixture was stirred for 30 min. Then TEOS (17 mmoles) and the aminosilane or the mercaptosilane (3 mmoles) were simultaneously added to the vigorously stirred mixture of dodecylamine, water and ethanol. The mixture was stirred for 24 h at 60 8C. The resulting precipitate was filtered and air

2.3. Characterizations

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spectra were acquired at 4 cm 1 resolution and averaged over 512 scans. A 1 W laser power was used. Thermogravimetric analysis coupled (TA Instruments 2050, TGA Interface AEM) with an Infrared detection (Nexus Nicolet) was used to test the thermal stability of the synthesized samples. About 10 mg of solid was placed in a platinum pan and heated from room temperature to 1273 K at a heating rate of 10 K min 1 in air flow with a flow rate of 90 ml min1. 2.4. Metal ion adsorption Fig. 1. Powder XRD patterns of synthesized silicas: the pattern of S, N-S, NN-S, NNN-S and S-S.

Batch experiments were performed to measure metal ion adsorption capacities of each adsorbent at pH 6. For the preparation of aqueous metal solutions, metal nitrate salts (Cu2, Ni2, Co2, Cd2) were used and a potassium salt for Cr6. Samples of 0.1 g of adsorbent and 10 ml of 10 4 mol l 1 to 3.10 3 mol l 1 metal solutions were mixed at room temperature for 24 h. The samples were centrifugated, filtrated and the residual metal concentrations in the solutions was measured using an Inductively Coupled-Atomic Emission Spectometer (ICP-AES, MAXIM III, Fisons).

which shows that only 80% of -N groups and -NN groups and 60% of -NNN groups are incorporated into the organosilicas, probably because of homocondensation. In the case of -S groups, there is almost a total incorporation in the silica framework. 3.2. X-ray diffraction Powder XRD patterns of absorbents are shown in Fig. 1. The S sample shows a single reflection at 1.78 of 2u and the S-S sample also shows a single XRD reflection at 2.18 of 2u , corresponding to lattice spacings of 5.1 nm and 4.1 nm respectively. No reflection is observed for the amino modified samples. For the S and S-S samples, the diffraction peak at 2u /28 may be due to the d100 reflection in materials with a short-range hexagonal order. Similar patterns have been reported for ordered

3. Results and discussion 3.1. Elemental analysis Elemental analysis of the extracted organosilicas were used to quantify the residual organosilane in the final product. Ligand proportion in each adsorbent (in mmol g1) in the initial mixture and in the final product are listed in the Table 2,

Table 2 Properties of synthesized adsorbents; * ( ) theoretical values Sample

%C*

S 2.2 (0) N-S 7.6 (8.0) NN-S 10.3 (12.1) NNN-S 10.7(15.7) S-S 8.5 (7.7)

Ligand f (mmol g 1)

%N*

%S*

0.1 2.5 4.4 4.7

0 1.8 1.6 1.1 6.8 (6.8) 2.1

(0) (3.1) (5.6) (7.8)

Ligand i (mmol g 1)

Surface area (m2 g 1)

2u (8) d100 (nm)

0 2.2 2.0 1.8 2.1

687 /712 62 /65 120 /220 245 /314 875 /888

2.1 / / / 1.7

Ligand f or ligand i: final or initial ligand quantity in the final product or in the initial mixture.

5.1 / / / 4.1

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mesoporous materials such as HMS [10,31,35,39]. As already noted, these HMS materials do not display the degree of long range order associated with the MCM-41 class of silicates. The mercapto group causes a decrease in the d spacing value (Table 2) [10]. Amino loaded samples are completely amorphous. In this case, surfactant has no influence on the silica structure. Periodicity has also been observed on amino functionalized mesoporous silicas, with a d spacing value larger than that of the silica [10].

3.3. Nitrogen sorption measurements and BET surface area analysis Analyses were performed on the samples after the surfactant was removed (Table 2). A specific surface area of about 700 m2 g1 was measured on the reference sample S. For the amino-loaded sample (N-S), a drastic decrease of the specific surface area was observed. For NN-S and NNN-S materials, a lower decrease was noted. Surface areas are larger for solids containing the longest amino chain. On the contrary, on the thiol-loaded material (N-S), an increase of the specific surface area (900 m2 g1) was observed. Surface areas are larger for thiol containing solids as compared to amino-loaded solids (60 /300 m2 g1). Similar specific surface area (700 m2 g1) was also noted for silica synthesized with dodecylamine as a template [34]. The surface area decrease with the incorporation of -N groups has also been noted by [10,34]. Specific area of 200 and 300 m2 g1 were also observed on gels resulting from the co-condensation of Si(OEt)4 and aminopropyltriethoxysilane and [amino-ethylamino]-propyltrimethoxysilane, without any template, using a supercritical drying process [40]. Modification of SBA-15 with mercapto or aminopropyl groups also lead to a high decrease of the surface area (from 814 m2 g1 to 460 m2 g1 and 280 m2 g1 respectively) [17]. Most promising way involves the use of 1,2-bis(triethoxysilyl)ethane instead of Si(OEt)4 for the construction of the network, since surface area of 1020 m2 g1 was obtained for a 15% loading with [amino-ethylamino]-propyltrimethoxysilane [14,15].

Fig. 2. Infrared spectra (high wavenumbers 4000 /1300 cm 1) of S (curve a), of N-S (curve b), of NN-S (curve c), of NNN-S (curve d), and of S-S (curve e).

Fig. 3. Infrared spectra (low wavenumbers 1500 /400 cm 1) of S (curve a), of N-S (curve b), of NN-S (curve c), of NNN-S (curve d), and of S-S (curve e).

The X-ray diffraction and nitrogen gas sorption experiments clearly indicate that the amino-loaded solids form disordered and less porous materials and that thiol-loaded solid keeps the silica porous structure. Formation of hydrogen bonding between amino and silanol probably explains this behavior. Aminopropyl groups may be involved within a strong interaction SiO-. . .H NH2-, constituting a cyclic structure, taking more place in the porous structure. In the case of longer amino chain (NN- and NNN-S), cyclic structures may be less favored. In the case of thiol-loaded material,

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Fig. 3 shows the FTIR spectra of samples between 1500 and 400 cm 1. For the S sample (Fig. 3a), a broad peak is noted at 1093 cm 1 with a broad shoulder at 1200 cm 1, due to the siloxane vibrations (SiO)n -. The band at 964 cm 1 is assigned to Si-OH stretching [44]. Others bands at 803 cm 1 and 467 cm 1 are due respectively to Si-O-Si stretching and Si-O-Si bending. For the N-S sample (Fig. 3b), the broad peak has been shifted to lower wavenumbers at 1080 cm 1 while the shoulder is more clearly

Fig. 4. Raman spectra (3500 /1200 cm1) of S (curve a), of NS (curve b), of NN-S (curve c), of NNN-S (curve d), and of S-S (curve e).

there is no hydrogen bonding between thiol and silanol and so silica organization is preserved. 3.4. FTIR and Raman spectroscopies Functional groups were identified using FTIR and Raman spectroscopies (Figs. 2/4). Fig. 2 shows the FTIR spectra of samples between 4000 and 1300 cm 1. For the sample S (Fig. 2a), absorption band at 3738 cm 1 is ascribed to free SiOH groups while the broad absorption band at 3430 cm 1 is assigned to hydrogen bonded SiOH groups [41]. For the N-S sample (Fig. 2b), isolated silanol are not observed. A band is noted at 3630 cm 1 followed by a very broad band at 3150 cm 1. The stretching vibration of NH2 are usually observed at 3250/3450 cm 1 [42]. The lower wavenumbers observed may be explained by the presence of primary amine salt /NH3 [43]. Then vibrations are observed at 1633 cm 1 (d (H2O)), 1600 cm 1 (d (NH2) or d (NH3)), 1550 cm 1 (d (NH3)) [43]. For the NN-S and NNN-S samples (Fig. 2c and d), spectra are similar to that of the N-S sample. For the S-S sample (Fig. 2e), almost no free silanol could be observed at 3738 cm 1, while the hydrogen bonded SiOH groups were observed at 3450 cm 1. A very weak n(S-H) vibration was noted at 2590 cm 1 [42].

Fig. 5. Thermogravimetric analysis of S (curve a), of N-S (curve b), of NN-S (curve c), of NNN-S (curve d), and of S-S (curve e).

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the siloxane vibration (1200 cm 1) is due to the SCH2 bending [43]. FT Raman spectra (1200 /3500 cm1) are shown on the Fig. 4. In the case of the S sample (Fig. 4a), almost no signal could be observed. For the N-S sample (Fig. 4b), vibrations bands were noted at 3300 cm 1 and at 2925 cm1 assigned to n(N-H) and nas(CH2). Others bands were also observed at 1450, 1415 and 1320 cm 1, assigned to d (N-CH2) or d(C-CH2), d(Si-CH2) and CH2 in phase twist [45]. Similar spectra were obtained for the NN-S and NNN-S samples (Fig. 4c and d). In the case of the S-S sample (Fig. 4e), a very weak band at 2600 cm 1 due to the S-H stretching mode is also observed. 3.5. Thermogravimetric analysis Thermogravimetric analyses of the extracted organosilicas were performed from room temperature to 1000 8C to test the thermal stability of the samples (Fig. 5). Gas given off were analyzed with a FTIR spectrometer. In the Fig. 6 are represented the sum of infrared intensities versus time during the ATG analysis. For the S material (Fig. 5a/Fig. 6a), a 2 wt% loss at temperature below 150 8C is due to a loss of residual water. The departure of aliphatic chains at 220 8C (3wt%), might be explained by the presence of residual dodecylamine. From 300 8C to 800 8C, another weight loss (5wt%) is due to a CO2 departure. The total weight loss is 10%. For the N-S sample (Fig. 5b /Fig. 6b), a small weight loss (2.5 wt%) occurs below 150 8C, associated with a water removal. Then a second weight

Fig. 6. Infrared intensity versus time for the ATG coupled FTIR experiment of S (curve a), of N-S (curve b), of NN-S (curve c), of NNN-S (curve d), and of S-S (curve e).

observed at 1160 cm 1, because of the vibration ds(Si-CH2). A new band is also observed at 690 cm 1 assigned to the CH2 rocking vibration of SiCH2R [10]. For the NN-S and NNN-S samples (Fig. 3c and d), spectra are similar to the N-S one. For the S-S sample (Fig. 3e), the broad shoulder of

Fig. 7. Adsorption of copper ions on materials at pH 6.

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loss (15 wt%) is noted from 275 8C to 750 8C. Major products formed are NH3, CO2 and CO. Concerning the NN-S and NNN-S samples (Fig. 5c, d, Fig. 6c, d), the same type of thermal degradation is observed. The total weight losses are respectively at 24% and 28%. The water removal is more pronounced for the NN-S and the NNN-S samples compared with the N-S sample. The ammonia removal increases with increasing the amino chain length. In the case of the S-S sample (Fig. 5e, Fig. 6e), an alkane loss (2 wt%), probably due to residual dodecylamine, is observed at 220 8C and followed by a second loss (14 wt%) (CO, CO2, carbonyl sulfide, SO2 and formic acid departures) at 320 8C. The total weight loss is 24%. No water departure has been observed on the thiol-loaded sample, unlike the amino-loaded and the silica samples. Moreover the thermal stability of the S-S sample is higher compared with the amino functionalized samples [10].

mmol g1 of chelating ligands respectively. Then the Cu/chelating ligand ratios are of 0.3 and of 0.4 respectively, which means that a great part of chelating ligands are not accessible to copper. This incomplete binding of Cu2 ions, however, is not surprising for ethylenediamine loaded silica systems. The deficiency in ligand group access is very different of the complete access of mercury ions to binding site of thiol-loaded mesoporous silica [3,4,32]. A capacity of 600 mmol g1 for Hg is not very high [6], and values from about 0.9 mmol g1 to 2.1 mmol g1 have been reported, corresponding to a ratio Hg/SH of 1 [28,32]. The reasons of the low ligand access in such adsorbents are not very clear, they may be related to the poor order and the poor surfaces observed. The efficient separation of copper (II) from aqueous solutions by the NN-S and NNN-S adsorbents lead to study their ability to adsorb other metals.

3.6. Metal ion adsorption

3.6.2. Others heavy metal ions adsorption Each absorbent was tested for its ability to absorb Co2, Ni2, Cd2 and the anion Cr(VI) at pH 6. Samples NN-S and NNN-S adsorb Co2, Ni2 and the chromate anion. Thiol-loaded sample S-S adsorbs Cd2. Batch tests with nickel solution (contact time 24 h) were used to estimate the quantity of nickel adsorbed (Fig. 8). Control sample S, adsorbs 15 mmol Ni(II) g1. Samples synthesized with thiol SS and amino N-S adsorb 5 and 1 mmol Ni(II) g1 respectively. Samples NN-S and NNN-S adsorb 150 mmol Ni(II) g1. These two samples have a

3.6.1. Copper adsorption Batch tests with copper solution (contact time 24 h) were used to estimate the number of copper adsorbed by organosilicas (Fig. 7). Control sample S synthesized adsorbs 100 mmol Cu(II) g1, as well as samples N-S and S-S. Unloaded S and loaded N-S and S-S absorbents affinity for copper is low. The maximum copper ion adsorption capacities of NN-S and NNN-S materials were 500 mmol Cu(II) g1. The copper adsorption by the NN-S and NNN-S materials seems to follow a Langmuir-type behavior [21]. Cu2 is immobilized on NN-S and NNN-S via complexation with the amino groups, as evidenced by the blue color of the resulting solid [22]. The kinetic of the reaction is very rapid as attested by the immediate coloration of the materials. The initial adsorption curve is quite steep for NN-S until the maximum adsorption capacity is reached. For NNN-S the initial adsorption curve is not as steep as in the case of NN-S one. The maximum adsorption curve is gradually reached from 0 to 103 mol l1 equilibrium concentration. These samples contain 1.6 mmol g1 and 1.1

Fig. 8. Adsorption of nickel ions on materials at pH 6.

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Fig. 9. Adsorption of cobalt ions on materials at pH 6.

Fig. 10. Adsorption of chromate ions on materials at pH 6.

higher nickel adsorption capacity, which is below their copper capacities. Same tests with cobalt solution allow to estimate the cobalt adsorbed quantity (Fig. 9). The S, N-S and S-S adsorbents do not have any affinity with the cobalt ions. Samples NN-S and NNN-S adsorb 60 mmol Co(II) g1 and 100 mmol Co(II) g1 respectively. The affinity of the NNN-S and NN-S adsorbents follow the common order Cu / Co /Ni [46]. These experiments reveal a preference of the NN-S absorbent for the uptake of Cu2 ions compared to that of Ni2and Co2. The NN-S and NNN-S adsorbents also have a preference for the uptake of Cu2, Ni2 and Co2 compared with an unloaded material S. Batch tests with chromate solution (contact time 24 h) were also used to estimate the adsorbed chromate quantity (Fig. 10). For the sample S, an adsorption capacity of 100 mmol CrO2 g1 is 4 observed, as well as for samples synthesized with amino and thiol N-S and S-S. Samples NN-S and

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Fig. 11. Adsorption of cadmium ions on materials at pH 6.

g1 NNN-S adsorb 200 and 250 mmol CrO2 4 respectively. These two samples have a higher chromate adsorption capacity. They contain about 1.6 mmol g1 and 1.1 mmol g1 of ligands respectively. Ratios CrO2 4 /chelating ligand are of 0.1 and of 0.2 respectively. For chromate adsorption, involved mechanism is probably an electrostatic interaction between the chromate anion and the cationated amino sites. Recently, very high chromate adsorption capacities (2500 mmol CrO2 g1) in amino functio4 nalized mesoporous silica SBA-1 were obtained [19]. Adsorbents were impregnated in HCl solution before the metal adsorption to get higher interaction between cationated amino groups and the oxyanions. Batch tests with cadmium solution (contact time 24 h) were also used to estimate the quantity of cadmium adsorbed (Fig. 11). For the sample S, an adsorption capacity of 100 mmol Cd2 g1 is observed and for sample NN-S, an adsorption capacity of 10 mmol Cd2 g1. Sample synthesized with thiol group S-S adsorbs 200 mmol Cd2 g1. The thiol-loaded material have the higher cadmium adsorption capacity, related to the greater affinity of soft ligands such as thiols for soft metals.

Acknowledgements We acknowledge Ruben Vera for X-ray diffractograms (Centre de diffractome´trie Henri Longchambon, Universite´ de Lyon), Nathalie Pontais (Laboratoire Verres et Ce´ramiques, groupe Ni-

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trures, Universite´ de Rennes 1) for the B.E.T. measurements.

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